Estimation of dewatering intensity of the paper web in the wet

Transcription

Aleksandra Judasz
128573
Faculty of Chemistry
Technical University of Lodz
Papermaking and printing
“Estimation of impact of alternative papermaking
additives on paper web dewatering intensity and paper
properties”
Master Thesis
written in
Institute of Papermaking and Printing
under direction of
dr eng. Konrad Olejnik
Lodz 2009
1
“Estimation of dewatering intensity of the paper web in the wet part of paper machine
and selection of alternative additives to improve dewatering intensity”
2009
Table of content ................................................................................................................... 2
Preface .................................................................................................................................. 4
Summary .............................................................................................................................. 5
Objectives and scope of the thesis ...................................................................................... 6
1.
Theoretical introduction ....................................................................................... 7
1.1.
General information and definitions concerning papermaking process,
construction of paper machine and basic functions of its elements ........................ 7
1.1.1. Pulp preparation ............................................................................................ 7
1.1.2. Stock and water systems of paper machine .................................................. 8
1.1.2.1. Stock preparation ............................................................................. 9
1.1.2.2. Stock approach flow system ............................................................. 14
1.1.2.3. Short and long circulation systems ................................................... 14
1.1.3. Wet part of paper machine ............................................................................ 15
1.1.4. Press section of paper machine .................................................................... 20
1.1.5. Dry part of paper machine............................................................................. 23
1.2.
Classification and characteristic of water contained in formed paper web ............. 25
1.3.
Intensity of paper web dewatering and energy consumption
in conventional paper making process .................................................................... 33
1.4.
Dewatering phenomenon ........................................................................................ 39
1.4.1. Mechanisms of dewatering ........................................................................... 39
1.4.2.1. Kozeny-Carman equation ................................................................. 40
1.4.2.2. Hydrodynamic specific surface area assessment .............................. 42
1.4.3. Factors affecting drainage intensity .............................................................. 48
1.4.3.1. Stock preparation ............................................................................. 49
1.4.3.2. Initial solid content and fiber alignment .......................................... 49
1.4.3.3. Stock temperature ............................................................................ 50
1.4.3.4. Permeability of pulp fiber mat ......................................................... 51
1.4.3.5. Fines content .................................................................................... 52
1.4.3.6. Fiber flexibility ................................................................................ 54
1.4.4.7. Retention aids .................................................................................. 56
2.
Research phase ...................................................................................................... 59
2.1.
Characteristic of raw materials and alternative materials used ............................... 59
2.2.
Microscopic pictures ............................................................................................... 62
2.3.
Pulp preparation ...................................................................................................... 66
2.4.
Methodology of analysis ......................................................................................... 67
2.4.1. Methods of pulp dewatering analysis ............................................................ 67
2.4.1.1. Schopper-Riegler freeness ................................................................ 69
2.4.1.2. Water retention value ........................................................................ 70
2.4.1.3. Rapid-Köthen number ...................................................................... 71
2.4.1.4. Hydrodynamic specific surface area ................................................. 72
2.4.1.5. FiberXPress....................................................................................... 74
2.4.2. Methods of paper sheet properties analysis .................................................. 77
3.
Results and discussion .......................................................................................... 79
3.1.
The influence of addition of alternative materials on pulp dewatering .................. 79
3.1.1. Dewatering in the wire section ...................................................................... 79
3.1.2. Dewatering in the press section..................................................................... 84
3.2.
The influence of addition of alternative materials on paper sheet properties .........
4.
Conclusions ............................................................................................................ 157
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Annex 1General definition and symbol list ..........................................................................
List of figures .......................................................................................................................
List of tables .........................................................................................................................
References ...........................................................................................................................
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2009
Preface
The main objective of this thesis is to assess factors affecting dewatering behavior of the
paper web in the wet part of paper machine with a clear focus on the influence and the role of
additives and fibers, and to search for alternative bio-based additives that can reduce the
required drying energy without negatively affecting of paper quality. The aim is to elucidate
the balance between dewatering and water retention value of different raw materials and
additives used during paper production in relation to paper quality. This thesis contains
description of researches carried out in order to find out what is the effect of the addition of
bio-based additives in different ratios on pulp dewatering properties, water retention value and
paper properties. Within Thesis the comparison of influence of additives was done and also
comparison of different dewatering methods used was performed.
Researches were carried out using wide range of dewatering apparatus available, inter
alia, at Institute of Papermaking and Printing at the Technical University of Lodz in Poland
and also abroad.
Hydrodynamic specific surface area measurements were performed according to
Dresden method with usage of the unique analyser, placed in Germany, at Dresden University
of Technology, at Faculty of Mechanical Engineering, Professorship of Paper Technology.
This cooperation was possible within European Union program called Short Term Scientific
Mission during COST E54 Action with title: “Characterization of the fine structure and
properties of papermaking fibres using new technologies”.
Investigations in participating paper mills were carried out with coordination of Centre
of Competence Paper and Board, the Netherlands, and cooperation with experts from paper
mills. Analysis was also performed in the laboratory of Smurfit Kappa Roermond Papier
B.V. in Roermond with the usage of new FiberXPress device, invented and developed by
Voith and Smurfit Kappa.
This thesis has been based on a part of a larger project performed in the framework of
the projects: Briljant, ‘Susprise – Green biorefinery’, ‘Biorefinery program of Energy
Transition’ as well as the ‘Fibre Raw Material program’ of KCPK in pulp and paper industry.
Coordination of the project is performed by Centre of Competence Paper and Board, the
Netherlands with the cooperation from Institute of Papermaking and Printing.
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Summary
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2009
Objectives and scope of the thesis
In every area of engineering, including papermaking, there is long-standing policy to improve
economy of production and quality of products. Sustainable objectives including energy and
raw material savings are the most important aims of every industry. The European paper and
board industry also has set the challenging target to reduce energy consumption with yearly
more than 2%. The Dutch Paper and Board industry has set their targets even further. With
their energy transition initiative they have committed themselves to halve the energy
consumption in the paper chain.
Drying is the most energy consuming unit operation in papermaking. The dryer
section is definitely the largest consumer of thermal energy on a paper machine as steam [1].
In the Netherlands steam is obtained by the combustion of natural gas (in CHP’s), which is
directly related to the oil prices. This makes steam consumption an important parameter to
reduce.
The amount of drying energy required in a paper machine depends significantly on the
dewatering efficiency of the paper web in the wet part of the paper machine. Most drainage on
a paper machine occurs mechanically on the wire and press sections. When the dry solids
content of a web after the paper machine press section is approximately 50% (1 g dry solids/g
H2O) for example, the dryer section removes less than 1% water volume originally received
by the forming section [1]. By changing the intensity of dewatering within the wire and press
section, it is possible to influence the amount of drying energy needed and save the cost of
paper production accordingly. It can be seen that a 5% increase of the consistency at the start
of the drying section can result in a 20% decrease in the energy requirement for drying [29].
Thereby, an increase of 1% in web dryness before the dryer section reduces the amount of
water for evaporation by approximately 4% [1, 30].
The objective of the Master Thesis is to assess the possibility of influencing the
dewatering efficiency in relation to paper and board quality by changing fibers or adding
different alternative additives or fibers. Research work includes processing and analysis of
some hydrophobic (fibrous and/or non-fibrous) bio-based additives to enhance dewatering in
reference to strength properties and choosing the most optimal. Analytical work has been
done to simulate the effect of usage of different furnishes under laboratory conditions and
evaluate dewatering efficiency (during web drainage and pressing). Analysis provides
information about correlation between additives and raw material used, and allows to compare
different dewatering methods available.
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1. Theoretical introduction
1.1.
General information and definitions concerning papermaking
process, construction of paper machine (PM) and basic functions of
its elements
Papermaking is a multidisciplinary technology; all systems of pulp production, paper making
and converting consist of a lot of operations and processes. The complexity of this process
requires understanding of every unit processes.
The aim of theoretical chapter is to provide introduction to papermaking process,
construction of paper machine and basic functions of its elements. Fundamentals about water
contained in formed paper web, dewatering process and factors its affecting are the subject of
the following chapters. Information about dewatering intensity and its relation to energy
consumption are also presented.
1.1.1. Pulp preparation
Papermaking pulp is produced using wood as a primary raw material for the paper and board
industry. Wood is made from cellulose fibers that are bound together by a material called
lignin [2]. At present, wood provides over 90% of the world’s virgin fiber requirement, while
non-wood sources (bagasse, cereal straws, bamboo, etc.) provide remainder. Approximately
one-third of all paper products is recycled as secondary fiber [3].
Wood or other fibrous feedstocks are converted into a papermaking raw material in
process called pulping. The purpose of these processes is to separate the fibers from the wood
and to prepare them for papermaking operations. Fibers can be separated from each other,
without being too damaged, chemically or mechanically.
There are the following pulping types:
 Mechanical pulping – In these processes fibers are separated from each other
by applying the mechanical energy to the wood matrix. Energy causes that the
bonds between the fibers break gradually and fiber bundles, single fibers and
fiber fragments are released. Lignin and also the hemicelluloses are more or
less softened what eases fibers separation. The composition of wood is not
changed.
 Chemi-mechanical pulping – In these processes only small amount of
chemicals is used (and thus only a small amount of lignin is dissolved). The
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pulp obtained from these processes – chemimechanical pulp – is also
considered as belonging to the mechanical pulp family.
 Chemical pulping – In these processes fibers are separated from each other
when lignin and to a large extent, also the hemicellulose is dissolved and
removed. Delignification is done by using chemicals. Product of chemical
pulping is called pulp.
Thus, with regard to the pulping processes, the term “pulp” is collectively used for
chemical, semichemical, chemimechanical, and mechanical pulps [3, 4, 5, 6].
In a pulp mill, after fibers separation, fibers are washed and screened in order to
remove any remaining fiber bundles. The pulp may then be used directly to make unbleached
paper, or be bleached for white paper. In an ‘integrated paper mill’ pulp may be sent directly
to a paper machine section or dried and pressed into bales to be used as a raw material by
paper mills worldwide [2]. Fibrous raw material which is shipped and sold (not processed into
paper in the same facility) is considered as a market pulp [7].
Pulp after pulping operation is processed in different departments in order to obtain
desired papermaking properties. Both chemical and mechanical pulp fibers must be
mechanically treated (refined, beaten) before the fibers become suitable for papermaking. The
difference is that for chemical pulp this is a separate process in the paper mill, while for
mechanical pulps this happens during the mechanical pulping process itself [5].
Chemicals are added either to the stock or to the process water and for this reason it is
important to define stock and water systems of the paper machine (Fig.1).
1.1.2. Stock and water systems of the paper machine
It is possible to distinguish the following areas and systems as part of the entire paper mill
water system:

Stock preparation

Stock approach flow system

Short circulation

Long circulation
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Fig.1. Stock and water systems in the paper machine [x].
1.1.2.1. Stock preparation
When pulp is coming from integrated mill, stock preparation begins with repulping or the
dilution of pulp from integrated mill operations at the pulp storage towers. When paper mill is
not integrated with pulp mill, paper production begins with stock preparation, where baled
pulp treatment runs. Stock preparation contains several unit operations as mechanical
treatment of the stock before the machine chest. Stock preparation consists of:
 disintegration of pulp,
 defibration of pulp,
 refining,
 pulp cleaning,
 proportioning, and blending of the main stock components.
The main stock preparation objectives are:

To add additives: fillers, biocids, chemicals to pulp

In non-integrated mills to pump pulp from pulp mill to the paper mill

To prepare properly the pulp before passing it through the headbox
Slushing operation is realized inside a special device called pulper. During slushing,
water and dry pulp bales are fed into the pulper vat, and the pulper rotor creates strong
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disintegration forces. As a result, a pumpable fiber slurry is received. This fiber slurry is then
pumped to the pulper dump chest.
Objectives of slushing are the following:

to disintegrate bales into pumpable slurry by releasing fiber bonds created in the
pulp dewatering and drying processes.

to disintegrate the fiber slurry so that there are no visible flakes or fiber bundles.

to disintegrate fiber slurry so that fibers are separated, wetted, and flexible before
entering the refining stage.
During slushing, heavy contaminants (e.g. wires, staplers, sand) are also removed.
If there is no defibration/deflaking stage between slushing and refining, the slushing
result must be better than in a system where defibration or deflaking completes slushing.
Figure 2 shows a typical slushing system for dried baled pulps.
Fig.2. Typical slushing system [8].
There are various terms used to describe the action in which pumpable fiber slurry is
further treated so that paper pieces, fiber flakes, fiber bundles, or separated but still dry and
stiff fibers are disintegrated into individual, wetted, and flexible fibers, e.g. "defibration,"
"defibering," "deflaking," and "disintegration". This operation completes slushing if pulper is
not able to produce sufficient defibration. Commonly used terms for expressing the result or
the status of fibers after this stage are degree or result of defibration, defibering, deflaking,
and disintegration. The term "deflaking" is used here to describe the fiber treatment, the
"deflaker" is the machine, and "disintegration degree" refers to the deflaking result.
The object of deflaking is to break the remaining flakes or fiber bundles into separate,
wet, flexible, and externally fibrillated fibers. The effect on pulp properties is mainly seen as
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an increased tensile strength because deflaking has increased the flexibility and produced
some internal fibrillation, thus improving the bonding ability of fibers. Deflaking has
practically no effect on the drainage resistance of the fibers. Only a minor increase in
Schopper-Riegler or decrease in freeness can be observed [8].
Refining is the next step of stock preparation. Refining or beating of pulps is the
mechanical treatment and modification of fibers in water to increase surface area, flexibility
and promote bonding of fibers. Refining is performed mainly to improve the bonding ability
of fibers so that they can be formed into paper or board of the desired properties. Sometimes
the purpose is to shorten too long fibers for a good sheet formation or to develop other pulp
properties such as absorbency, porosity, or optical properties specifically for a given paper
grade.
During refining or beating, fibers in the presence of water are treated with metallic
bars. The plates or fillings are grooved so that the bars that treat fibers and the grooves
between bars allow fiber transportation through the refining machine. Term “refining” is used
to describe the work accomplished with refiners on the fibers [8, 9].
Refining affects fibers in many ways and the most important effects are:
-
Cutting and shortening of fibers
-
Fines production and complete removal of parts from fiber walls, creating debris in
suspension
-
External fibrillation, the partial removal of the fiber wall, leaving it still attached to
the fiber
-
Internal changes in the wall structure, variously described as delamination, internal
fibrillation, or swelling
-
Curling the fiber or straightening the fiber
-
Creating nodes, kinks, slip planes, microcompressions in the cell wall, or removing
those from cell wall
-
Dissolving or leaching out colloidal material into the external liquor
-
Redistribution of hemicelluloses from the interior of the fiber to the exterior
-
Abrasion of the surface at the molecular level to produce a more gelatinous
surface.
Fibers after refining are collapsed (flattened) and made more flexible, and their bonding
surface area is increased. The measurable fiber and sheet properties, when refining chemical
pulps, can be seen as follows:
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
Drainage resistance (water removal resistance) increases.

Tensile strength, tensile stiffness, burst strength, internal bonding strength, and
fracture toughness increases.

Tear strength of softwood fibers might slightly improve at first, but then
decreases, whereas that of hardwood fibers at first significantly increases but then
decreases after prolonged refining.

Air permeability, bulk, absorbency, opacity, and light scattering decreases.

Brightness slightly decreases [8].
The stock is blend of several components in order to reach the desired paper properties
under the most economic circumstances. Stock blending can take place continuously or in a
batch system. In modern papermaking, batch blending is used only for specialty papers
produced on machines with small production rates or even in discontinuous operation,
applying very special furnish components, dyes or chemicals. Figure 3 shows a typical
example for a continuous system.
Fig.3. An example of stock blending and machine chest including sampling station [8].
The number of used pulp components depends on their availability and on the product
properties desired. Accordingly, in stock preparation, the fiber furnish is determined by:

Selection and proportion of the stock components

Improvement and development of the fibers, i.e., beating.
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The consumption of fresh water in stock preparation should be very low if not zero.
Fresh water is used for the dilution of chemicals and eventually as supplement water in startup
situations.
All fiber components are diluted to the same pre-set concentration for blending. Each
pulp typically has a separate pulp chest, the proportioning chest, to ensure a constant supply at
the dosage point. In an integrated mill, pulp is usually picked up at a medium-consistency
storage tower by dilution with water from the main paper machine dilution header. The
concentration in the pulp chest is usually adjusted to 0.2%–0,3% points higher than in the
blend chest. The stock is then diluted to the blending concentration and pumped to blending
via refiners or directly.
The blend chest is also called “mixing chest”, because the aim of this chest is not only
to create complete motion of the stock (mixing) but also to gain complete stock uniformity
(blending).
There are three or more components mixed in the blend chest:
 Primary stock component(s)
 Broke
 Recovered fiber from the saveall.
Broke is paper which is discarded at any point of the manufacturing and finishing
processes inside the paper mill. Broke can be divided into wet broke and dry broke. Wet
broke occurs on a continuous basis as trims from the wire section, press section and partly
from drying section. Dry broke occurs as trims from winders, as e.g., reel slab-offs, in the
finishing room, or during breaks. Usually, all broke is repulped, cleaned, and stored in the
broke system. The processed broke is blended with other components at the blend chest and
thus fed back into the production process. The amount of broke dosed to the furnish depends
on web breaks and the broke line capacity [8, 11].
In specialty paper or dyed paper production, the reuse of broke might be somehow
limited by the required product quality or due to other reason. Depending on the paper grade
and the degree of processing, the broke might be pulped and used at another time or
elsewhere. However, when this broke leaves the mill, it becomes per definition "recovered
paper" or secondary fibers.
The blended pulp is pumped at a constant rate to the machine chest where stock
preparation ends. The stock is diluted by a small concentration decrement, typically about
0.2%-0.3% [8].
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1.1.2.2. Stock approach flow system
The approach flow system extends from the machine chest to the headbox lip. The main
purpose is to meter and dilute the stock including blending with other components like fillers,
chemicals, and additives unless not already added in stock preparation. Then, the lowconsistency stock is pumped and screened before feeding to the headbox. Stock cleaning by
hydrocyclones and deaeration can be included.
The main operations in the approach flow system are:

Dilution to headbox consistency

Removal of product and production disturbing contaminants (solids and air)

Conditioning with chemicals and additives

Feeding the headbox

Supply of additional water for PM cross-profile control in case of a headbox
dilution system.
Thick stock from machine chest is typically diluted at the bottom of the wire pit. The goal of
doing that is to control the basis weight cross-profile at the paper machine. Basis weight
control is more efficient, the lower the solids content of the dilution water is. Headbox
dilution water is usually taken directly from the wire pit. The dilution water is deaerated in a
separate unit or in a separate compartment which is integrated into stock deaeration tank [8].
1.1.2.3. Short and long circulation systems
Short circulation is a compartment in which paper machine wire water is separated from the
stock in web forming and filtrate, which has passed through the wire, is used for the thick
stock dilution prior to entering the headbox. This water is used also as a make-up water for
the beaters. Usage of white water, which contains an abundance of fibrous matter and fillers
used as the raw-material, significantly influences on the total water and fresh water
consumption. Losses of raw materials and additives are also decreased by white water usage.
Short circulation is present in all technological systems of paper machine [8, 10, 11].
Overflow (or excess of white water) from short circulation system is directed (together
with water from suction boxes and wire washing) to long circulation system. This water is
used for applications such as: wire screen showers, felt sprays, seal water for vacuum pumps
and for stock dilution as well. Equipment for fiber recovery and water cleaning (device called
save-all) is installed in the long circulation loop. Long circulation system is usually present in
technological system of the paper machine [8, 10, 12].
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During normal steady state operation of a papermaking machine, an equilibrium
condition develops in the material balance of the short and long stock circulation loops. For
the short circulation loop this means that the fines and filler retention of the paper web are in
equilibrium with the concentration of these materials in the white water circulation; and for
the long circulation this means that the fiber save-all operation, broke filler concentration,
retention chemical concentration and furnish composition are stable [13].
The paper mill stock and water system within the paper machine is responsible for:
 Supplying paper machine with stock with quantity sufficient for the production
capacity of paper machine and possible for reaching a high paper machine
productivity,
 Supplying the stock prepared is such a way that the product at the reel meets given
quality parameters,
 Improving process materials economy, because it increases solids recovery and
recycles water fraction,
 Environmental impact of paper machine water management reduction and increase of
environmental protection [8, 10].
1.1.3. Wet part of paper machine
The papermaking machine is essentially a dewatering, i.e., water removal, system.
Dewatering can be described as a water removal from wet web during paper forming. In the
papermaking art, the term machine direction (MD) refers to the direction that the sheet
material travels during the manufacturing process, while the term cross direction (CD) refers
to the direction across the width of the sheet which is perpendicular to the machine direction
[13].
When stock is prepared accurately, it is pumped through various types of mechanical
cleaning equipment to the paper machine and there paper web is formed.
The term "forming" describes the dilution in the short circulation of the thick stock
flow to a mix flow, the approach flow system, and the CD (cross machine direction)
distribution and jet generation of the mix by the headbox, as well as the creation of a wet web
by dewatering of the mix in the wire section.
The term "mix" is used to denote the thick stock, after dilution with white water to a
fiber concentration low enough to avoid excessive flocculation. The mix is the last link in the
chain:
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
Pulp - original fiber raw material, delivered from pulp production,

Stock - treated pulp including eventual additives,

Thick stock - homogenized stock, delivered from the machine chest,

Mix - thick stock diluted in the short circulation and delivered to headbox and
wire section.
Forming denotes the overall process of paper web generating. The forming of the fiber
web is the crucial stage in building up the paper sheet. During this stage, the basic structure of
paper is created.
The term “formation” exclusively refers to small-scale local basis weight variations in
the final paper web [8]. Besides formation, the distribution of material components such as
fines and fillers in the thickness direction is important.
The receiving of paper web from pulp stock suspension in paper machine is called also
consolidation of paper web. This process is based on dewatering of pulp stock suspension,
consolidation of fibers and fines into wet paper web which after pressing and drying becomes
paper web with specific structure and properties [10].
Mix is fed to wire section through headbox. Headbox is located between short
circulation and wire section (Fig.4) [14]. Headbox main objective is to distribute the mix
evenly in the CD of the paper machine (across the width of the wire section). This means, for
example, that the flow from a pipe with a diameter of 800 mm shall be transformed into a 10
mm thick and 10 000 mm wide jet, with absolutely the same flow rate and flow direction at all
points across the width, as indicated in figure 5.
Fig.4. Headbox location in the paper machine [14].
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Fig.5. Feed pipe for mix and cross-section of jet from headbox (not to scale) [8].
Headbox and the approach piping other aims are the following:

to stabilize pressure variations and pulsation in the infeed flow as well as any
cross directional flow disturbances

to produce a suitable turbulence level in the stock suspension for fibre floc
dispersal

to produce a stock suspension jet in the forming section with a desired consistency
(typically from about 0.5 to 1.0 %), speed and direction [14].
The flow transformation by the headbox from the incoming pipe flow to the delivered
plane jet takes place in mainly three steps:

The cross-direction distributor makes a first distribution of the mix across the
machine width.

Pressure drop elements are introduced to even out the CD flow profile.

A headbox nozzle generates the final jet [8].
There are the following constructions of headbox:

Air-cushion headboxes - are a development of the original, completely open
headboxes, where gravity was the only driving force for the outflow through the
headbox nozzle. Air-cushion headboxes are now used mainly for moderate
machine speeds, for the manufacture of different specialty papers, and for some
kraft paper machines, which require very large jet thickness.

Hydraulic headboxes were designed specifically for twin-wire forming. A main
requirement was small nozzle dimensions to allow a short free jet from the
headbox into the gap between the two wires. Hydraulic headboxes lack the
traditional air cushion and are available either with or without an equalization
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chamber. If disturbing pressure pulsations occur in the approach mix flow, it is
necessary to install a separate air-cushion pulsation damper before the headbox.

Multilayer headboxes - include a separate CD distribution channel for each layer
and flow separation between the different layers throughout the headbox. In the
headbox nozzle, separation vanes are applied, for which bending stiffness as well
as thickness along the vanes is important. Especially the geometrical design of the
downstream ends and the vane length in relation to nozzle length are critical
parameters.
The web is formed by draining water from the mix in the wire section. In this section
different types of formers are used:
 Fourdrinier former
 Hybrid former
 Twin-wire former (also called “Gap former”)
 Cylinder former.
Fig.6. Fourdrinier former [15].
Fourdrinier former is shown in figure 6. Fourdrinier former serves to form one-side
dewatered paper web. It consists of a horizontal wire which is spread between breast roll
(which is under headbox) and couch roll (which is at the end of wire section) and entire raw
of dewatering elements like: forming board, table rolls, foil elements, forming boxes, which
create wire-carrying section. Next section, so called suction boxes section, consists of wet
suction boxes, dandy roll, and dry suction boxes and couch roll for downward dewatering.
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Deckles protect against mix spilling. Dewatering devices are applied below the fourdrinier
wire with the objective of creating dewatering effects and also of providing means for
controlling the degree of fiber flocculation in the sheet formed. Originally, all dewatering
relied on gravity effects and supporting rolls were introduced only to keep the wire horizontal,
while causing a minimum of friction drag [8, 10, 15].
Hybrid former is an improvement of Fourdrinier former, which was complemented
with an upper wire for dewatering upward at the end of the wire section. It is a hybrid form
between fourdrinier former and twin-wire former.
Twin-wire former has a twin-wire forming zone (two continuous wire belts running
very close together) with the stock in between. Stationary elements were mounted on both
sides of the wires, initially opposing each other but later positioned in a staggered mode. The
mix is trapped between two wires what allows symmetrical two-side dewatering (Fig.7) from
the top and bottom of the furnish and prevents two-sidedness.
Fig.7. Basic principle of two-sided dewatering [8].
This principle has the advantage of avoiding the free surface between mix and air and
of increasing dewatering capacity. Since, in each direction, half the dewatering will take place
through half the basis weight, dewatering capacity will in principle quadruple compared to
one-sided dewatering [8].
Since the 1950s paper machines with twin-wire formers have been developed. During
the 1960s, twin-wire blade formers with stationary dewatering elements were developed by
Beloit (Bel Baie) and Black Clawson (Vertiforma). In pure twin-wire formers, the mix jet is
delivered directly into the gap between the two wires, hence the term “gap former” [8]. The
examples of twin-wire formers are shown in figure 8.
Cylinder former is covered with a wire and rotates in a vat of stock. The stock is
picked up onto the cylinder by applying vacuum at point where the cylinder surface exits the
stock. Vacuum is drawn on the stock to drain it until a point at the top of the rotation where a
continuous felt contacts the cylinder. The paper web is released from the cylinder and is
19
2009
carried along on the underside of the felt. The surface then reenters the stock and the process
begins again. Machines with a cylinder former are often used to make multilayer paperboard
[15].
Fig.8. Examples of Twin Wire Formers [16].
1.1.4. Press section of paper machine
After forming and partly dewatering paper web tears away from the wire and is passed
through to press section. On couch roll and hitch roll which are movable wire is changing the
direction of movement and it is turning back towards breast roll. Wire is lead by guide rolls
system and cleaned with showers [10].
The press section is the part of paper machine in which paper web is passed to nip
between two press rolls or a roll and a shoe. Water is expelled from the wet web by
mechanical compression and is partly absorbed by felt or fabric and partly received by voids
in the roll surface. The water removal is often assisted by the application of heat to the wet
web, thus increasing the temperature of the fibrous web and water contained therein [8].
The objectives of press section are:
 To remove a maximum amount of water from the web (further dewatering of paper
web)
20
2009
 To thicken and make even of paper web structure
 To smooth the surface of paper web
 To ensure a sufficiently high wet strength with the press in order to enable web
transferring to the dryer section without any breaks
 To compress paper web in order to enable the formation of strong interfiber bonds
during web drying
 To gain exact joint of specific layers in multilayer papers [10, 14].
Construction and working of the press section is described on the basis of plain press.
The plain press consists of two press rolls, felt, felt conditioning system, systems of felt guide
rolls and felt stretching rolls. The press section usually consists of several presses. The paper
web is transported by felt to press nip between two press rolls. The suction box removing air
which is between paper web and felt should be put before press nip. Water which is drained
from paper web is received outside and absorbed by felt. The felt is washed and partly
dewatered during turning back. Usually the top roll is rubber-covered and bottom roll is made
of granite or cast iron. Modern paper machines posses press sections with 3 to 4 nips and use
new technical developments as vented nips, shoe presses or other types of wide nip presses or
elevated temperature either by using steam boxes or pressing the web against a heated
cylinder or roll.
The paper web after the wire section is a three-phase system, it consists of solid phase
– fibres and fines, liquid phase – water and gas phase – air. The felt cooperates with the paper
web and is also very important in this process. During pressing specific pressure is exerted on
the web due to its compression in the press nip. This pressure (compressive force) is balanced
in each point of the nip by the web structure and water contained in web (water flow
resistance in the fiber network), therefore by a sum of structural pressure and hydraulic
pressure. The structural pressure balanced by the mechanical stiffness of the solid structure
dominates as long as the web is not saturated. When the web becomes saturated, the hydraulic
pressure starts to rise and water flows into the felt, where its movements are determined by
the press design and roll surface structure. Hydraulic pressure is a driving force in pressing
process. In the outgoing nip, there is a reverse flow from the roll structure into the felt and
from the felt into the web. The reverse water flow from the felt to the web is called rewetting
[8, 10, 17].
Analyzing precisely mechanism and proceedings of pressing process, it is possible to
divide pressing area into four phases what can be seen in figure 9.
21
2009
Fig.9. The four phases of the nip process [17].
Phase I extends from the beginning of pressing zone until the place in which paper
web achieves repletion. In this area compressive pressure increases and calipers of web and
felt decrease because of air removing. Minor amount of water is taken away by the air flowing
out of the web very quickly and is passed by capillary forces between web and felt. At the end
of this zone web is saturated but felt still contains water. In phase I there are only slight
changes in web dryness.
Phase II extends from the place in which web achieves repletion until the place where
press pressure reaches the greatest value. This point is reached usually before the line which
joins midpoints of press rolls. In this zone press pressure increases, what causes that hydraulic
pressure also increases and water is drained from the web. Water is channeled to the felt
because of capillary forces and then after achieving repletion by felt water is pressed outside
the press zone. This water slows down bottom press roll in plain presses or is drained to
special dewatering spaces in press roll in modern presses. In this process stage main part of
water is removed from the web.
Phase III extends from the greatest point on pressure curve in press zone to the place
in which web achieves maximum dryness. This point lines up with the greatest point of
structural pressure curve and simultaneously with zero point of hydraulic pressure curve.
Because in this phase hydraulic pressure of felt reaches zero point earlier than paper web,
water flows from the web to felt. Owing to this phenomenon paper web achieves the
22
2009
maximum dryness. Within this zone felt starts to absorb not only water from the web but also
water which is below the felt.
Phase IV extends from the point in which web achieves maximum dryness to the end
of press zone. Within this phase paper web and felt expand taking water which has not been
already drained out of press area. Paper web is here rewetted. Rewetting water comes from
the felt because underpressure caused by expansion is higher in the web than in felt. Moreover
after expansion of web and felt water penetrates from felt to web as a result of capillary forces
acting since capillary tubes in the web are smaller than in the felt and they absorb water more
intensive. [8, 10, 18].
During paper web pressing paper web is thickened what casts light on the most paper
properties causing increase of density, transparency and static strength properties because of
increasing of fiber contact surface, but decrease of web thickness, volume, air permeability,
water absorptiveness and the amount of fines.
1.1.5. Dry part of paper machine
Drying is the last stage in papermaking process. Drying section is the only one part of paper
machine which has remained almost unchanged since their initial development. The main
objective of drying section is to remove water from the web through evaporation. This
phenomenon must go ahead efficiently, economically, evenly and without impairing paper
quality. Dryer section is the larger consumer of in steam energy in paper machine, typically
accounting for 55% of the total machine energy cost [19].
At present drying occurs using predominantly contact drying with steam-heated
cylinders method. A multicylinder dryer section consists of drying set groups having its own
felting and drive systems. Cylinders are arranged in two-tier configuration and the singlefelted or single-tier configuration. Other available configurations are twin-run, tam-run,
putting cylinders in the basement, arranging tier in vertical position, and using more than two
tiers of cylinders. The Yankee dryer for tissue and machine-glazed paper, through-air drying
for tissue, impingement and air flotation drying for coating and sack paper, and infrared
drying for coating and moisture profiling are other commonly used drying methods.
Despite the fact that paper web has got formed construction, it is characterized as
having not enough mechanical strength. Ultimate structure of paper comes from drying
process during which physicochemical processes proceed. They are following:

Evaporation of water from paper web,
23

Creating inter-fiber bonds,

Shrinking of paper web,

Hydrofobization of paper web (paper becomes water-reppelent).
2009
Paper drying is always connected to air that is either the drying medium or surrounds
the drying atmosphere and receives vapor from paper. Evaporation process can be divided
into three stages (Fig.10):

The heating phase - it includes preheating of paper web; during this stage
temperature and drying rate are gradually increased and approach constant rate
conditions.

The constant drying rate phase - drying occurs with the constant drying rate and in
constant temperature, energy input to and consumption by web are in equilibrium.
Evaporation can occur on the web surface or inside the web. The inversion point
between the constant and falling rate phases is the critical moisture content
(CMC).

The falling drying phase – it is characterized by decreasing drying rate and it has
two components called the first and the second falling rate phases. In the first
falling rate phase web has got constant temperature. The second falling rate phase
begins after removal of all free water from the web (89-92% dryness) and
temperature of the web starts to increase gradually because of the diminishing of
the drying velocity [1, 10].
Fig.10. Phases of drying process [1].
24
2009
The finished paper should have a moisture content 4-8%, roughly corresponding to the
equilibrium moisture content of the paper under the humidity conditions at which it will be
used [20].
Paper web after dryer section is very hot and needs to be cooled in order to convert it
further. Paper finishing processes depend on desired paper or board grade and they occur in
finishing end. The main types of finishing processes and their objectives are:

Surface sizing – to improve paper strength properties like internal strength
(bonding strength) or surface strength (low dusting)

Pigmenting – to plug paper pores

Coating – to improve the appearance and printability of the product

Reeling – to render the planiform paper or board produced in a form which is
easier to handle

Calendering – to improve paper surface properties and printing properties and
other factors related to further processing
- Adjusting of paper thickness or caliper to obtain paper of desired
density
- Leveling the paper caliper profile to obtain smooth and even rolls
at the winder
- It may be used for stamping relief designs on the paper with
patterned rolls

Winding – to slit reel into web sections of suitable width and length for customer
and wound up around cores before sending out from the mill

Sheeting – to cut paper into sheets which would be suitable for further processing

Roll handling – to prepare paper rolls or paperboard rolls properly to ensure that
the roll arrives in the right place at the right time [14].
1.2.
Classification and characteristic of water contained in formed paper
web
Water plays very important role in paper forming, converting and using because of its specific
properties. Process of formation and consolidation of papermaking web is dependent on
properties of pulp used. One of the most important property of pulp is the distribution and the
bonding degree of water in formed paper web. Water present in the web fulfils the spaces
25
2009
between and inside the fibers with various distribution of sizes. When the dimension of spaces
is diminishing, the degree of water bonding with fiber inside the web is increasing. Therefore
the water is more difficult to remove in web consolidation process. The degree of water
bonding with fibrous material is one of the most important factors influencing on paper web
forming and dewatering [21].
There are several water classification systems. With reference to these systems in
professional literature there are the following water categories:

Sorption water

Hygroscopic water

Monomolecular sorption water

Bound water

Polimolecular sorption water

Total bound water

Condensation capillary water

Nonfreezing bound water

Adhesive capillary water

Freezing bound water

Inner-fibril water

Retention water

Inter-fibril water

Semi-bound water

Micro-capillary water

Free water

Macro-capillary water

Clarified water

Inaccessible water

Sedimentation water

Accessible water
Usage of all above terms can cause a lot of misunderstandings because some of them can be
treated as synonyms.
This thesis shows one of the water classification systems, which was created in
Institute of Papermaking and Printing at Technical University of Lodz. It takes into account
factors as water distribution and bonding energy of water and fibrous material, but also puts
emphasis on the role and importance of classified water categories in papermaking process.
One of the advantages of above system is the relatively simple water fractions separation
method.
Accepting the criteria which describe the water bonding energy in papermaking pulp
stocks and heading technological aspects, it is possible to divide water categories into the
following (Fig.11):

Clarified water,

Sedimentation water
o Free water

Free gravity water
26
2009
o Free gravity water
o Gravity retention water

Free press water
o Retention water

Semi-bound water

Bound water
Fig.11. General classification of water contained in fibrous materials [22].
Remaining water categories which are mentioned in scientific literature can be
considered as auxiliary and used for more precisely describing of basic water categories. Very
important criterion in classification of basic categories of water included in fibrous material is
the importance of these water categories in technological processes and relatively simple
method of quantitative determining.
Introducing criterion water fraction classification into clarified and sedimentation
water (Fig.12) is reasonable for form and practical sakes. Inter alia, it results from
miscellaneous bonding energy of these water categories with fibrous material. Energy needed
to extract clarified water from suspension equals zero but in case of sedimentation water, it is
higher than zero and according to water location in pulp suspension shows diversity. Fibrous
27
2009
suspension in which the amount of entire water is higher than the amount of sedimentation
water creates not stable system. In this system in stable conditions spontaneous extraction of
clarified water follows. Entire water is then divided into:

clarified water

sedimentation water.
Clarified water is extracted in the way of free sedimentation above sedimentation layer
of fibrous pulp suspension. It consists of water which has not got any connection with fibrous
pulp.
Fig.12. Extracting and measurement principle of sedimentation water content [23].
Sedimentation water accounts for water which is contained in sedimentation layer of
fibrous pulp suspension. This is water, which is embedded in spaces between fibers and inside
them but also in spaces between fibers and fines and inside fines. Sedimentation water content
in fibrous suspension is determined according to PN EN 872-02 standard. Measurement of
sedimentation water relies on placing 1dm2 of tested fibrous suspension in Imhof’s hopper
and definition of the volume of fibrous suspension which gravitated after specific time and
then definition of its dry matter. Sedimentation water content in tested sample is definite as
the amount of water (cm3) which accrues on 1g of dry matter. It usually ranges from 100cm3/g
to 500cm3/g dry matter of fibrous material, what equals 0.2-1% of consistency of pulp
suspension on average 0,5%.
Because of the differences in location and bonding energy of sedimentation water in
pulp suspension, it can be divided into (Fig.13):
o Free water
28
2009
o Retention water
Division into free water and retention water follows exactly during mechanical
dewatering.
Free water is embedded between fibers and fines and is kept by surface tension forces
in fibrous material. It constitutes the main part of sedimentation water. Its content varies
between 100÷500cm3/g. Because of miscellaneous size of spaces between fibrous elements
and also bonding energy, free water can be divided into:

Free gravity water

Free press water
Free gravity water is the water which can be extracted from fibrous pulp in the way of
gravity dewatering on the wire. Free gravity water is extracted spontaneous during filtration
of fibrous materials. After extracting of free gravity water from fibrous suspension,
consistency of fibrous pulp is usually 3÷5%. This category is important in regard of
papermaking industry, where gravity filtration devices are used for thickening pulp
suspensions and extracting these pulps from circulating water and effluents.
During gravity filtration free gravity water is distributed into:
o Free gravity water
o Gravity retention water
Fig.13. Division of sedimentation water [24].
29
1.3.
2009
Intensity of paper web dewatering and energy consumption in
conventional paper making
The paper making process, in general, is a rapid water removal operation in a very large scale.
In conventional paper making (Fig.16), the fibers are mixed with about 100 times their weight
of water and subsequently water is removed. The percentage of dry solids (ds) is called the
consistency/concentration of the sheet [31]. Percentage consistency from the definition is
described as the amount of given substance in relation to the amount of entire mixture and is
denominated in % as a unit [32].
Consistency of the stock flow entering the paper machine headbox is typically 0.2%1.0% (2-10 g fiber per kg water). Pulp stock sheet formed that way is over 99% water by
weight. Pulp is continuously sprayed onto the moving woven mesh brass or bronze cloth
screen at the rear section of the headbox. The water falls through the wire screen and into
drainage trays as the continuous paper sheet is pulled along. At low speeds gravity force
predominates in drainage causing. At higher speeds gravity force is not enough and pumping
action of the drainage elements (i.e., the table rolls or foils) need to be applied. As web
proceeds down the wire a visible change occurs in appearance of the stock. At this point,
when concentration of the web reaches about 2%, its surface ceases to appear mobile because
it loses its liquid sheen and takes on a matte appearance. The consolidation process begins, the
web is formed and drainage elements are no more effective, for water removing. Next, the
sheet travels through a section of suction boxes to physically extract more water. The sheet
then moves onto a couch roll which prepares it to be lifted off the wire screen. At this point,
the paper sheet is barely strong enough to support its own weight (roughly 80-85% water) and
is transferred to a felt. After drainage on the wire or forming section using gravitation,
pulsation, or vacuum (suction), the web consistency increases to 15%-25% [1, 11, 12, 17, 33].
Fig.16. Conventional papermaking process [12].
30
2009
The felt takes the continuous paper sheet on the press section. There mechanical
compression removes water by passing the sheet, supported by a felt, through a series (three
or four pairs) of press cylinders, leaving the sheet with 71-74% water concentration. This
process removes additional free water and some capillary water. The web consistency (now
called dry solids content) then increases to 33%-55% depending on the paper grade and press
section design [1, 12, 31, 33].
Finally, when no more water can be removed mechanically, the web enters the dryer
section where a thermal operation, i.e., evaporation, removes the remaining water, water
which is within the lumen and pores of the fiber wall. The sheet is passed over 40-5- steamheated cylinders (drying section), the final consistency being about 90-95% ds. A small
amount of moisture (5%-9%) remains in the paper even after the dryer section [1, 12, 33].
Relationship between energy use and the amount of water removed is very important.
It is shown in figure 17.
Fig.17. Final energy requirement vs amount of water removed for the three operations
forming, pressing and drying in a paper mill [31].
Removal of water in papermaking process can be divided into three processes
occurring in the paper machine. The bulk of the water is removed in the forming section,
31
2009
using the smallest amount of energy. Drying uses by far the most energy per mt of water
removed.
Pressing involves squeezing water out of the voids and the cell walls. This water
drains away through the felt. At normal pressing temperatures, i.e. 40-50°C, the maximum
consistency after pressing is 40-50% ds, depending on the type of pulp and the density and the
porosity of the sheet. Increased temperatures aid water removal by pressing because the water
viscosity is lowered, fibers are softened and water surface tension is reduced. A 10°C
temperature increase gives a minimum of one percent improvement in consistency [34]. The
influence of the consistency on the steam demand of the drying section is shown in Table 1
[31].
It can be seen that a 5% increase of the consistency at the start of the drying section
can result in a 20% decrease in the energy requirement for drying [35]. Thereby, an increase
of 1% in web dryness before the dryer section reduces the amount of water for evaporation by
approximately 4% [1, 37].
Drying involves evaporation of the remaining water. The fibers and water should be
heated to 100°C. Since water binds chemically to fibers above a consistency of 70% ds, heat
is required for desorption. The water vapor is carried away by pre-heated air. The heat for
heating and evaporation is obtained from saturated low-pressure steam (3-8 bar). The steam
condenses on the inside of the cylinders, transmitting its latent heat to the cylinder shell. Heat
is conducted through the shell to the paper through a thin layer of dirt, rust and air. The
heating efficiency depends on the conductivities of the layers and the mechanism of
evaporation in the sheet [38, 39]. The minimum energy requirement for water evaporation
from paper is 2,55 GJ/mt of water. In practical situations at least 0,15 GJ additional heat per
mt of water evaporated is required to preheat the air and to compensate for condenser and
radiative losses [35]. Values for the steam consumption of newsprint dryers in Canada range
from 3,5-6,7 GJ/mt of paper, with the average value being 4,5 GJ/,mt of paper. In Sweden, the
steam consumption ranges between 2,4 and 5,5 GJ/mt of paper, with an average value of 3,4
GJ/mt [40]. The consistency after the press ranged from 39% to 47% ds. Values for the
Netherlands range from 1,7 to 8,0 GJ/mt of dried paper, the average being about 5 GJ/mt [41].
Minimum energy requirement for:
Heating and evaporation of water,
from 50°C to 100°C
(GJ/mt of paper )
Increasing consistency (%ds)
40%
45%
3.25
2.63
32
(GJ/mt of water evaporated)
2.46
Heating of fibers from 50°C to
100°C
Desorption heat
Total
2009
0.07
0.07
0.07
0.02
3.34
0.02
2.72
0.02
2.55
Tab.1. Minimum energy requirement for water evaporation form paper, expressed in GJ/mt of
water evaporated and GJ/mt of paper [35]. It is assumed that the final consistency of the paper
is 93% ds. In the case of the energy requirement per mt of water evaporated, ingoing
consistency is 45% ds [31].
According to the newest data typical paper machine uses approximately 4GJ of
thermal energy per ton of paper produced as low-pressure steam.
The energy consumption on a paper machine varies with the paper grade produced. On
printing paper machines, the share of steam in total energy consumption is 70%-75%. For
tissue machines, the share is approximately 50%. The dryer section is definitely the largest
consumer of thermal energy on a paper machine as steam.
Most drainage on a paper machine therefore occurs mechanically on the wire and press
sections. When the dry solids content of a web after the paper machine press section is
approximately 50% (1 g dry solids/g H2O) for example, the dryer section removes less than
1% water volume originally received by the forming section [1]. By changing the intensity of
dewatering within the wire and press section, it is possible to influence the amount of drying
energy needed and save the cost of paper production accordingly.
Intensity of dewatering of the paper web in different part of paper machine is defined
as a ratio between total percent of removed water in specific part of paper machine and
overall cost of produced paper. Distribution of percentage removal of water is following:

97%-98% of overall amount of water is removed in wire section

1,2%-2% of overall amount of water is removed in pressing section

0,8%-1% of overall amount of water is removed in drying section [41].
This data agree with simple calculations relating to removed water. Calculations are
done using the definition of consistency/concentration:
Cp 
wp
ws
 100 
wp
w p  ww
where:
Cp – percent consistency/concentration of suspension, [%]
wp – the amount of bone dry pulp [kg]
33
 100 [%] ,
2009
ws – the amount of suspension (what is equal with the amount of bone dry pulp and the
amount of water) [kg]
ww – the amount of water [kg]
By transformation of above formula, it is possible to obtain formula for the amount of
removed water:
ww 
w p  100
Cp
 w p [kg] .
If it is assumed (with reference to above data):

the amount of bone dry pulp, wp = 1kg,

the amount of stock, ws=100kg

the amount of water in the headbox, wE=99kg

consistency in the headbox: 0,5%,

web consistency after the wire section: 25%

web consistency after the press section: 50%,

the final consistency after the drying section: 95%,
The calculations, according to consistency used, are the following:
 The amount of water in the pulp with specific concentrations:
 the headbox, c p  1%:
ww 
1kg  100%
 1kg  99kg
1%
When the consistency is 1%, the pulp composition is: 1kg of bone dry pulp and 99kg of water.
 after the wire section, c p  25%:
ww 
1kg  100%
 1kg  3,00kg
25%
 after the press section, c p  50%:
ww 
1kg  100%
 1kg  1,00kg
50%
 after the drying section, c p  95%:
ww 
1kg  100%
 1kg  0,05kg
95%
34
2009
When the consistency is 95%, the pulp composition is: 1kg of bone dry pulp and 0,05kg of
water.
 The amount of removed water, wR :
 in the wire section: wR  99kg  3kg  96kg
 in the press section: wR  3kg  1kg  2kg
 in the drying section: wR  1kg  0.05kg  0.95kg
 the percentage removal of water, C RW 
wR
 100 [%] ,
wE
 in the wire section:
C RW 
96kg
 100 %  96,97%
99kg
 in the press section:
C RW 
2kg
 100 %  2,02%
99kg
 in the drying section:
C RW 
0,95kg
 100 %  0,96%
99kg
It is important to remember that the results of above calculations depend on the type of
paper produced and the consistencies used.
Analyze the cost of paper production in relation to paper machine it is possible to
observe some characteristic distribution. Taking costs in wire section as 1 unit of value per m 3
or mt of paper, in pressing section this gauge would be 50 units and in drying section it would
be 200 values [41].
Energy consumption is related to energy prices and is an important parameter to
reduce. Because the drying section removes the last part of water from paper web, the amount
of drying energy required depends on the dewatering intensity of the paper web in the wet
part of the paper machine. Changing the dewatering intensity can reduce energy costs and
bring a lot of savings.
1.4.
Dewatering phenomenon
Dewatering is defined as formation of a paper or board web on the wire by removing water at
the paper machine wet end [2, 9] or as removal of water from wet web during formation of
paper sheet [9, 42].
35
2009
Dewatering is a phenomenon which is complex itself and a lot of factors influence on it.
That is why, dewatering can be described from different points of view and it is possible to
distinguish separately mechanisms of dewatering and models of dewatering.
1.4.1. Mechanisms of dewatering
The paper web forming in the wire section is based on changing the diluted pulp stock
suspension in paper web with definite structure. The structure of the paper web formed
depends on the mechanism of dewatering of paper web in the paper machine wire.
Dewatering process can be of two different kinds, filtration or thickening (Fig.18).
Filtration type dewatering is based on successively fibers depositing flat on a wet web
as the suspending water is removed. In the course of water removing, a mix with the same
concentration as in the jet from the headbox is still present above the wet web. As dewatering
proceed, the thickness of filter layer is built up and suspension layer over filter layer is
decreased. Paper web obtained by filtration should have layer structure. This type of
dewatering dominates conventional (fourdrinier) forming.
Thickening type dewatering relies on progressive compressing of a fiber network.
Fiber flocs which have reminded in the mix are also dewatered according to the thickening
principle. During the thickening process, fibrous layer consistency increases concurrently in
whole fibrous layer. Paper web obtained by thickening should have structure in which fibers
order stays similar like in fiber suspension, i.e. fibers are ordered every which way.
Fig.18. Dewatering through filtration (left) and thickening (right) [8].
Filtration type dewatering prevails in the initial forming stage when consistency of
pulp suspension is small. When the consistency increases, possibility of fiber movement is
36
2009
limited and then the thickening type dewatering takes place. It starts to take place when all
free mix has been dewatered. Changing mechanism of dewatering as forming of paper web
proceed is the reason that structure of paper web resolves from layered (at the bottom/wire
side) into having fibers ordered every which way (at the top side) [8, 10].
1.4.1.1. Kozeny-Carman equation
Kozeny-Carman equation is based on permeability principle published by Kozeny and
Carman in 1927 [43]. This principle found also application in liquid permeability method
according to Robertson and Mason, published in 1949 [44]. The background of this theory is
presented beneath. It is strictly related to hydrodynamic specific surface area measurement.
This explanation accounts for the basis of hydrodynamic surface area measurement which
will be presented and described within this Thesis.
Kozeny-Carman equation stems from mathematical equations of Darcy (flow rate
through a porous material with unknown porosity) and Hagen-Poiseuille (flow rate through a
material with known number of circle-cylindrical pores) (Fig.19) [45, 46, 47].
Kozeny-Carman model assumes laminar flow through cylindrical pores perpendicular
to the surface. Kozeny presented that the resistance to flow through packed beds of granular
materials could be explained in terms of the porosity (size and number of pores), specific
surface area and shape factor of the material. In the fiber network both, the porosity and the
fiber orientation have effect on Kozeny factor (K). In the Darcy-equation the unknown
permeability is characterized by a factor, so-called permeability factor K, which is an
important parameter of the hydrodynamic specific surface area method.
37
2009
Fig.19. Flow rate through a porous solid body according to Darcy (left) and Hagen-Poiseuille
(right) [45]
From the Darcy-equation permeability factor K [m2] is defined as:

K

V   L
,
A  p
where:

V - flow rate [
cm 3
],
s
 - dynamic viscosity of liquid [
g
]
cm  s
L – distance between upper and lower screen [cm] (=height of the compressed fiber pad),
A – cross section of material [m2],
p - hydrostatic pressure [Pa].
Permeability factor’s influence on papermaking stock dewatering is described beneath in
permeability of pulp fiber mat description.
Different researches shows that Kozeny-Carman concept originally developed to
predict flow through textile materials also can be used with good precision for predicting flow
through papermaking fiber mats [20, 45, 46, 48].
1.4.1.2. Hydrodynamic specific surface area measurement
38
2009
Hydrodynamic specific surface area assessment is based on the permeability of compressed
water-swollen fibers pad. In a water-swollen fiber pad it is possible to describe internal and
external surface area of a water-swollen fiber wall (Fig.20).
Fig.20. Definition of internal and external surface area of a water-swollen fiber wall [46].
Internal surface area is interpreted as the surface which is not accessible for water molecules
which flow along the external fiber surface. Only the external surface is in contact with the
streaming/flowing water. Hydrodynamic specific surface area of fibers [
m2
] is defined as the
g
surface area of fibers in a compressed water-swollen fiber pad that is accessible for streaming
water molecules.
The relation between Kozeny-Carman equation and hydrodynamic specific surface
area is that the inner surface of cylindrical pores corresponds to the accessible fiber surface in
a compressed fiber pad. Hydrodynamic specific surface area is a function of flow rate through
a compressed fiber pad under defined hydrostatic pressure. Hydrostatic pressure is an
expression of the fiber pad’s resistance against fluid flow. Hydrodynamic pressure gradient
caused by either mechanical compaction or gravity is a driving force in drainage. Streaming
resistance is determined by the pore size within the fiber pad and also by fibers surface
structure, e.g. their external fibrillation. Pore size distribution cannot be measured directly, but
compression forces decrease pore sizes depending on the fiber conformability (the pore
structure within the fiber pad which contains mainly highly flexible fibers will be different
from that of a fiber pad built from stiff and non-flexible fibers).
During hydrodynamic specific surface area measurement for each compression step,
hydrostatic pressure, flow rate, wire screen distance and liquid temperature is measured, and
fiber pad concentration and the permeability factor K are calculated.
39
2009
The permeability behavior of compressed water-swollen fibers pad according to
Roberston and Mason is linear for a certain range of concentration (0.07-0.15
g
) at the
cm 3
given sample amount of 2.5g. Linearity can be expressed by a straight-line equation according
to Kozeny and Carman:
( K  c F2 )1 / 3  (
1 1/ 3
1 1/ 3
)  Vm  c F  (
) ,
2
2  Sm
2  S m2
where:
K – permeability factor [m2],
c F - consistency of the compressed fiber pad [
S m - specific surface area [
Vm - specific volume [
g
],
cm 3
m2
],
g
cm 3
],
g
Specific surface area S m [
m2
] is defined as the sum (Z) of all lateral surface areas of
g
cylindrically shaped pores having the length L and the diameter 2r, related to sample mass m:
Sm 
2r    L
Z
m
cm 3
Specific volume Vm [
] is defined as the sum (Z) of all volumes of cylindrically
g
shaped pores having the length L and the diameter 2, subducted from the total volume of the
fiber pad A*L and related to sample mass m:
Vm 
(A   r2  Z)  L
m
Consistency of the compressed fiber pad c F [
g
] is calculated from sample mass m
cm 3
and the outer dimensions of the fiber pad A and L:
cF 
m
.
A L
The so-called Kozeny-Carman plot is created by putting calculated data in a chart with
Value ( Kc F2 )1 / 3 versus c F .
40
At concentrations smaller than 0.07
2009
g
, fibers can move freely under hydrostatic
cm 3
pressure, and the flow rate increases with increasing pad consistency. At about 0.07
g
,
cm 3
fibers cannot move anymore with hydrostatic pressure, they start to find their “favorite place”
under compression pressure. Flow rate decreases uniformly with increasing pad consistency.
The concentration 0.15
g
is the upper concentration limit for the linear area. It is due to
cm 3
construction limits of the first testing apparatus. At this concentration uniform flow rate
decrease continues and particles move under compression pressure. It was found that the
straight-line equation can be applied also for higher fiber pad compressions. In this case flow
rate decelerates because particles cannot move anymore under compression pressure and start
to collapse.
Total specific surface area of the fibers (not only that part of surface that is not in
contact with their neighboring fibers) is calculated when the straight-line is extrapolated to the
Y-axis. It is due to theoretical assumption that no fiber is in contact with their neighbors when
concentration equals c F  0
g
. According to the theory, all fibers are in contact with each
cm 3
other and no free fiber surface is left in the fiber pad, permeability K is zero, when straightline is extrapolated to the X-axis. At this concentration the specific volume of all fibers can be
calculated [46, 47, 48].
The compressions steps and correspondingly Kozeny-Carman plots are shown in
figure 21.
41
2009
Fig.21. Kozeny-Carman plots for unrefined softwood chemical pulp as an example of pulp
with straight-line assumption between the concentration limits 0.07
g
g
and 0.15 3 ,
3
cm
cm
intercept with Y-axis to calculate specific surface area and intercept with X-axis to calculate
specific volume of the sample [45, 46].
1.4.2. Factors affecting drainage intensity
Dewatering is described in three models, but there are always deviations from even the best
predictions. As it was mentioned earlier, there are a lot of factors determined as affecting the
dewatering of pulp suspension. There are the following:
42
2009
 Stock preparation operations,
 Initial solids content and fiber alignment,
 Stock temperature,
 Permeability of pulp fiber mat,
 Porosity,
 Hydrodynamic specific surface area,
 Fines content,
 Origin of fiber fines,
 Size and shape of particles and fines,
 Distribution of particle sizes within the wet web, homogeneity of paper web,
 Flexibility of wet-fiber/Stiffness of wet fiber
 Recycling of fibers,
 Fibers swelling,
 Retention aids.
The influence of every factors named above on pulp dewatering is described beneath. It
is important to notice that all factors are interrelated and together influence on the
papermaking pulp dewatering and the final paper web appearance.
1.4.2.1. Stock preparation operations
During stock preparation there are three steps which can influence on pulp dewatering. The
first one is deflaking, the second one is refining and the third one is the usage of broke.
Deflaking has practically no effect on the drainage resistance of the fibers. Only a
minor increase in Schopper-Riegler or decrease in freeness can be observed.
The second factor is refining. Refining is performed inter alia to cut fibers and to cause
internal and external fibrillation of fibers. As an effect of refining, the amount of fines in
stock increases. Fibers after refining are collapsed (flattened) and air permeability decreased.
Fibers are made more flexible, their bonding surface area is increased and also their swollen
volume increases. It influences on pulp dewatering and brings about that drainage resistance
increases. As a result the pulp dewatering would be decreased [8].
Usage of broke changes properties of stock. There is almost no difference between wet
broke and stock. Wet broke contains less amount of fines than stock and that is why it can
bring about better dewatering properties. It can be metered into stock without limits. Dry
broke differs from stock significantly. Pulps are irreversibly altered during the first drying,
43
2009
which impairs some paper properties as known from paper made of recycled fibers. The
properties of dry-broke pulp are different when compared to the fresh pulp used. There is no
specific quality difference between dry broke and clean, unprocessed paper, e.g., unprinted
printing shop waste, which is called "wastepaper" or "recovered paper" by definition. Drying
of broke causes decreasing of apparent density, increasing of bulkiness and porosity and also
deteriorating of strength properties.
1.4.2.2. Initial solids content and fiber alignment
Dewatering results can be strongly influenced by the fibrous suspension initial solids content
as well. Researches of Hubbe, Heitmann and Cole have shown that the amount of water
retained in a fiber mat after gravity dewatering is directly proportional to the amount of solids
present. The higher the initially solid content is, the greater is amount of water retained in a
fiber mat. Increasing of solid content causes decreasing of pulp dewatering. This inspection
suggests also that the resistance to flow is increasing with increasing initial consistency and
approves that secondary fines have a greater relative effect than primary fine, what is
discussed further within thesis.
The higher solid content can be understand also as the amount of fibers in suspension,
so higher consistency. The higher the consistency of pulp stock is, the higher is association
between fibers and fine matter. Moreover, fibers are showing increasing tendency to entangle
and forming flocs with the increasing consistency beyond a certain point. Flocculation is
expected to increase the rate of dewatering by gravity [20, 53].
On the other hand, forming paper at relatively high solids content of the suspension
causes that fiber alignment will be chaotic, including a high degree of out-of-plane alignment.
Under these conditions more permeable wet paper mat can be created. Paper formed under
such conditions shown to have a reduced resistance to dewatering [20].
1.4.2.3. Stock temperature
Paper web temperature was changed few times during the last fifty years. When analyzing of
dewatering process, it may be assumed that changes in physical properties of pulp are caused
because of changes in physical properties of fibers carrier, water. It was stated that increasing
of stock temperature may give an increase in pulp dewatering velocity and more rapid
drainage. A 10°C temperature increase gives a minimum of one percent improvement in
consistency. It may be because of changes in water viscosity. Increasing of temperature will
44
2009
cause decreasing of water viscosity. It will aid water removal by pressing because as water
viscosity is lowered, fibers are softened and water surface tension is reduced. The ratio of
force of inertia to internal frictional force (which is defined as liquid viscosity) is out of
proportion high in table roll section. Here decreasing of viscosity forces may do not have any
effect on pulp dewatering. But the effect of decreasing the water viscosity may affect pulp
dewatering on a couch roll and in press section. In these areas the bonding forces between
water and fibers are high in comparison to forces of inertia and there is considerable water
flow resistance in capillary and concentrated paper structure [33, 55, 55].
1.4.2.4. Permeability of pulp fiber mat
Dewatering of pulp depends on the structure of forming mat. Form one hand, it is possible to
assume that in a papermaking furnish fibres are able to slide past each other when they come
into contact and they would form a relatively dense mat. On the other hand, fibres tend to
stick together and not slide past each other and form more bulky, porous mat. Water will flow
easily out of the second presented fibrous mat. The degree to which the furnish components
tend to become packed together is expected to play significant role in determining the formed
mat permeability [20].
Paper is composed of a randomly felted layer of fibres, so its complex structure has a
varying degree of porosity. Paper contains as such as 70% air. Permeability and water
diffusitivity are inherently related to the resistance offered by so highly porous material.
Permeability is the ability of permeate (such as a liquid, gas, or vapor) to penetrate
through a solid. As it was mentioned earlier, researches of Kozeny and Carman and also of
Robertson and Mason present that resistance to flow in the transport channels between the
fibers in the wet web could be explained in terms of the porosity (size and number of pores),
specific surface area and shape factor of the material.
Porosity of paper sample is the ratio of the pore volume to the total sample volume. It
is the measurement of the total connecting air voids, both vertical and horizintal, that exist in
paper. Pores within the cell wall can be divided arbitrary into macropores and micropores.
Larger pores in the cell wall are called “macropores” and relatively smaller pores are called
“micropores”. Macropores are believed to be gaps between microfibrillar lamellae which are
formed in pulping by dissolution of liginn and hemicellulose from the cell wall. The
micropores are spaces within the lamellae. The macropores and micropores are not well
separated nodes on the pore size distribution.
45
2009
Pore size within the fiber pad and also fibers surface structure determine streaming
resistance of penetrate. Pore size distribution cannot be measured directly, but compression
forces decrease pore sizes depending on the fiber conformability. The pore structure within
the fiber pad which contains mainly highly flexible fibers will be different from that of a fiber
pad built from stiff and non-flexible fibers. That is why permeability of a web of previously
dried fiber is higher than that for one from never-dried fiber. Changes in pore size within the
fibre wall is important for the ability of molecules to diffuse in and out of the fibre wall. The
higher the permeability is, the lower is the flow resistance and that is why also drainability is
better. Increasing the permeability in water removal during pressing will lead to increasing of
water removal by pressing.
According to what is written above, in porous structure there is a relationship between
the size and shape of pores and the compressive strength of a material. Drop of pressure
through a porous membrane, such as the cell wall, depends on the size of the pores through
the membrane. The first pores which collapse when a porous material is compressed are the
largest pores in this structure. They are followed sequentially by the smallest. It may be stated
that pores in the cell wall colapse from the macropores to micropores. Larger pores collapse
easier that smaller one. The smaller the pores within a structure the higher is the compressive
strength. It may be expected that dried-pulp are less compressive than never-dried pulp
because drying causes that fiber cell wall pores are closed.
The fiber swelling is also strictly related to the porous structure of fiber mat. During
fiber swelling, fibers changes theirs volume and therefore the volume of pores between fibers
is decreased. Because of that permeability of fiber pad is expected to be lower and drainability
is decreasing. Swelling influence negatively on fiber pad permeability and dewatering [7, 43,
44, 56, 57].
Hydrodynamic specific surface area is defined as the surface area of fibers in a
compressed water-swollen fiber pad that is accessible for streaming water molecules. It is
related to the pore structure of fibers, because the inner surface of cylindrical pores
corresponds to the accessible fiber surface in a compressed fiber pad. The higher the surface
is, the higher is the water flow resistance through fiber pad and therefore the lower is fiber
pad dewatering. The greater pore sizes are, the higher is the surface available for water
streaming and also the higher is hydrodynamic specific surface area and dewatering is lower.
Therefore, dewatering of pulps which are characterized by high specific surface area should
be lower that for those characterized by relatively small hydrodynamic specific surface area.
46
2009
Also dewatering of pulps which have smaller pore size i.e. dried-pulp should be better than
for those which have relatively higher pore size i.e. virgin or swollen pulps.
1.4.2.5. Fines content
Fines is described as pulp fraction, which separated, posses inter alia low dewatering
property, very high external surface (5 to 10 times higher than fiber sample with the same
weight) and high swelling degree. Fines may also contain various range of well-beaten furnish
(from 1% to 30%). That is why, it influences on the pulp drainage. It may be expected that
different types of fines behave differently.
Fines fraction has much higher external surface area per unit mass than typical fibers
in suspension. In this case, the Kozeny-Carman equation can be applied to predict fines
impact on dewatering. Dewatering of fines particles should be slower than of fibers because
of greater frictional resistance expected, when fluid flows through bed of fines (Fig.24).
However, this approach fails to predict increasing fines content effect and increasing basis
weight influence on dewatering.
Fig.24. Typical fiber and typical fine particle comparison. Right side of picture shows
dewatering through a bed of coarse fibers in comparison with fine matter, assuming uniform
packing density [20].
Fines slow drainage as it is unattached to fibers and during pulp flow fines particles
can migrate within the pulp until choke-points and reduce the rate of flow through a fiber mat.
This approach agrees with “choke-point” model. Secondary fines decrease dewatering to a
much greater extent than primary fines [20, 50, 51, 52, 53].
47
2009
The origin of fiber fines has play significant role in pulp dewatering. Primary fines
compared to secondary fines, is considered as negligible to sheet bonding due to its low
specific surface area and mineral particles presence in the fraction. The role of fibers origin in
pulp drainage can be explained not only due to the differences in surface area. Hawes and
Doshi noted that variations in stock dewatering are due to flexibility and conformability
among different kinds of fines. This attitude may follow sealing model [20, 58].
The size and the shape of particles added to or present in papermaking stock also
influence suspension drainage. In regard to fines, fibrillar material (mainly composed of
delaminated call wall material) posses higher surface area per unit mass than fines having
rounded or brick-like shapes (as e.g. parenchyma cells from the wood). That is why fibrillar
material tends to cause greater reductions in dewatering rate. Secondary fibers tend to be long
and slender, so it may be expected that with increasing content of secondary fibers, the mat
can become very effective as a filtration medium [20, 52].
Also distribution of particle size is important. Suspensions which have particles with
wide range of sizes are expected to have higher packing density. As result, smaller particles
can fill in spaces which would occur within suspensions consisting only of larger particles. In
effect, resistance of dewatering has been found to be larger in the case of mixtures [20].
1.4.2.6. Fiber flexibility
Fiber flexibility is also one of factors affecting dewatering. As it was mentioned describing
sealing mechanism, the more flexible fibres are, the greater is the resistance to fibre mat
dewatering. According to other researches, the stiffer the fibres are, the greater is the rate of
dewatering.
This assumption was done also, because stiffness can have impact on density of
packing, so it may also affect rate of dewatering. According to researches performed rapid
drainage is flavored by the presence of relatively stiff fibers. This concept was quantified
using new technique for measurement of wet-fiber flexibility. Also results from researches of
Hawes and Doshi put emphasis on fiber flexibility [20, 58].
Flexibility of fibres changes, the more times fibres are used and recycled. When pulp
is dried and then rewetted, there is a loss of swelling and also porous structure of fiber mat is
changed, most of macropores and a small part of the micropores collapsed. Paper recycling
reduces the swelling of fibres and interfiber bonding and brings a lot more changes. That is
why also recycling of fibres may influence fibre pulp dewatering. The problem occurs with
48
2009
usage of dried market pulp and recycled fibers. The main effect of fibres recycling is loss in
fibre swelling due to hornification. Hornification is a loss in swellability, water uptake and
surface area, of both the long and fines fraction of a pulp. It occurs between 50% and 80% of
solids content. Hornification occurs with heat application (drying) but also with water
removal from pulps through wet pressing. That is why also previously dried chemical pulp is
easier to press then never-dried pulp. Dried pulp has less specific surface area than the neverdried pulp and it has also lower water-holding capacity than virgin fibers. As a result, usage of
recycled fibres should decrease the drainage resistance of pulp, so enhance dewatering, due to
they are stiffer than and cannot absorb the same amount water as virgin fibres. It is important
also to remember that different behaviour of the hornified pulp in static pressure is not
because of effects brought about by changes in fibre flexibility but it is due to the closure of
micropores [25, 28, 29, 51, 56, 59, 60, 61].
Loss in fibre flexibility was noted also as a result of loss in hemicellulose. Loss in
hemicellulose was determined qualitatively and considered as an indicator for loss in fibre
flexibility, although these researches were not conclusive [60].
Absorbed water changes the fibre flexibility, so fibre swelling itself has an impact on
pulp dewatering as well. Swelling is the ability for liquids absorbing by solid with the
simultaneously increasing of solid size, without changes in solid uniformity and with
decreasing of internal cohesion. Water is presented in the wet web in capillary voids between
the fibres and in the porous structure of the fibre wall. Water infiltration to fibres interior has
a great technological impact during paper production, because absorbed water changes the
fibre flexibility and plasticity. Water plasticizes hemicellulose and to a lesser extent than
lignin. Swollen fibres become more resistant for cutting during refining. The closure of the
fibre wall pores upon drying deswells the fibre surface. Fibres swelling affects not only
drainage on wire section, but also wet pressing of fibrous web. In a pressure controlled
situations solid content after pressing is limited by the rate at which water can be removed
from the cell wall. There is a negative correlation between the degree of swelling and the
solids content after pressing. When pressure is exacted on fibre mat, stresses are concentrated
at the contact areas between the fibres, especially at the fibres crossing. High local pressure
removes water from the fibres at these points but water still fulfils some of the voids between
the fibres at a solid content corresponding to fibre saturation point. As a consequence, the
solids content varies along the fibre length and the swelling is inhomogeneous. Press
performance can be improved by pulp deswelling. Removal of swelling water is slower
49
2009
process than the removal of water form interstices between the fibres. In limited impulse
pressure presses, the solid content obtained after pressing is related to the pulp swelling.
Expelling of swelling water appears to be a limiting factor for the attainment of a high solid
content after press section. That is why solids content after the press section is predicted with
the usage of Water Retention Value.
Increased fibre swelling may facilitate the formation of dense, impermeable fibrous
layer. In flow controlled pressing the dewatering of such pulp is expected to decrease [11, 56,
60, 61].
1.4.2.7. Addition of retention aids
To improve pulp dewatering papermakers strive for achieving as high as possible degree of
so-called fine flocculation, i.e. precipitation of fine fraction on the surface of fibers with the
minimal flocculation of fibers themselves. The degree of flocculation chiefly affects pulp
dewatering.
Fibrous material in papermaking suspension (in water system) exhibits negative
charge on its surface. This charge is a result of sorption of hydroxide ions from water or from
dissociation of carboxyl group present on the fiber surface. The charged particles affect the
distribution of ions in the surrounding interfacial region, resulting in an electrostatic double
layer around each particle (inner Stern layer and outer diffuse layer). Electrostatic double
layer consists of adsorptive layer (layer which adheres to the fiber surface, it posses positive
charge) and diffusive layer (layer which extent into water phase, it posses the advantage of
positive charge). Positive charge which exists in the surrounding of fiber compensates the
negative charge of fiber in static conditions. On the border line of adsorptive and diffusive
layer there is so-called electrokinetic potential or potential zeta. This is one of the crucial
factors in papermaking agglomeration or dispersion. Maximal fibers flocculation can be
reached when zeta potential is approaching or equals zero. Electrokinetic potential data allows
to predict how a furnish is likely to respond to the addition of cationic or anionic additives.
The presence of negative electrokinetic potential of fiber in solution causes fibers
repulsion and in case of other particles present in water phase, they remain in stable
dispersion. The surface charge can be reduced with the usage of papermaker's alum or other
more effective retention aids. The main aim of retention aids is to increase the amount of
fibers, fiber fines and fillers retained on wire of papermaking machine. Retention aids settle
on the surface of fibers and fines and neutralize negative charge of fibrous materials. The
repulsion of fibrous materials is weakened and as a result agglomeration of fines, its
50
2009
adsorption onto the fiber surface and flocculation of fibrous material follow. The retention of
fines is increased and that is why the drainability of pulp is improved.
Molecular weight and charge density of a retention polymer are the most important
factors impacting the mechanism of their action. Early retention aid systems were singlecomponent products, most often based on acrylamide chemistry, alum, starch, polyamines,
polyethyleneimines (PEI). Dual-component systems used later were based on combinations of
cationic polymers and anionic inorganic microparticles. Today, multi-component systems,
were long-chain formation aids in conjunction with dual component systems is used, are
applied. It was stated that additives characterized by the highest ability to create flocs bring
about the highest change in drainability.
Retention aids act according to the following mechanisms (Fig.25):
 charge neutralization
 patch formation
 bridging flocculation
 complex flocculation

network flocculation
Fig.25. Schematic illustration of: a) charge neutralization, b) patch formation, c) bridging
flocculation [69]
In papermaking industry chemicals are commonly used in order to improve water
removal on the papermaking machine. Cationic additives were described as chemicals which
increase drainage on the wire section but also they may improve the pressability of a fiber pad
[60].
51
2009
When the amount of retention agent is established, it is important to keep in mind that
it does not only increase retention and pulp dewatering, but also that this addition can cause
deteriorating of formation and strength properties of paper. Tendency to maximal dewatering
of papermaking suspension should not cause excessive impairment of paper web formation
[10, 53, 54, 63, 64, 65, 66, 67, 68].
2. Research phase
This part describes materials and additives used and methods applied to find out what are the
changes brought about with the addition of alternative pulps and what is the behavior of pulp
during dewatering. Because the additives influence on both, pulp and paper properties, the
impact of alternative materials will be distinguished and described separately. This chapter
includes also description of methodology used to perform tested properties.
2.1.
Characterization of raw materials and alternative materials used
There were two pulps which were considered as reference pulps:
 Recovered pulp - Pure recovered paper coming from production of fluting was used to
produce the first reference pulp. Recovered paper (Fig.26) was produced by Smurfit
Kappa Roermond Papier. It was made of 60% of OCC and 40% of Mixed Office
Waste. The recovered paper has 93% of dryness.
 Virgin bleached mixed pulp from hardwood and softwood was used to produce the
second reference pulp. The virgin pulp was coming from production of transfer papers
for screen and digital sublimation printing applications. It was characterized as having
freeness of 30°SR. Before preparation of pulp with alternative materials, virgin pulp
was supplied in a form of pulp cookies (Fig.27).
Before pulps preparation, the reference pulps were stored and conditioned according to
TAPPI T402 [70, 71].
52
Fig.26. Recovered paper used to produce
Fig.27. Virgin bleached mixed pulp used to
the first reference pulp – recovered pulp.
produce the second reference pulp – mixed
pulp.
Cellulose fiber is potentially the most sustainable papermaking biomaterial available.
This raw material may be not enough for future paper production. That is why paper and
board industry is considering other raw materials as useful in papermaking. Preferable
alternative materials are considered as that they might replace conventional raw materials, but
are cheaper; might enhance dewatering or might improve paper or board quality. The
papermaking industry faces also challenge to decrease the amount of energy used, what may
mean that at the same time these alternative additives should help to reach that aim.
As an alternative material for reference pulps 5 different commonly available
hydrophobic (fibrous and/or non-fibrous) substances (Fig.28–Fig.32) coming from the
collaborating (bio-based) industries were used:
 Apple pulp
 Betacal - is one of the by-products of sugar industry, it consists mainly of CaO and
MgO and organic matter,
 Beer fines
 Sawdust with diameter 100µm
 Sawdust with diameter 320 µm
Alternative materials were supplied by three Dutch companies. Information about
chemical composition, dryness, availability and prices of these additives were collected from
companies and sorted in Tab.2. Before pulps preparation, alternative materials were stored in
a laboratory of paper mill in containers (when they were solids) or kept refrigerated to prevent
degradation (in case of wet samples).
53
2009
Alternative materials are characterized by different origin, shape and size of particles.
These factors influence on pulp dewatering behavior. Because of that they may cause
problems and influence diversely on paper machine runnability and general paper production.
Due to variety of raw materials, the dewatering process can be fully characterized and
different method results are expected. The described features of alternative materials may also
have impact on paper sheet properties. It is important to remember that usage of such an
alternative raw materials is worth considering when taking into account the desired pulp and
paper properties.
Alternative materials were added to reference pulp in proportion of 5%, 10% and 15%
in reference to bone dry reference pulps. The way of pulp preparation is described in point
2.3.
Tab.2. Available information about alternative materials.
Additive
Cellulose
[g/kg]
Hemicellulose
[g/kg]
Lignin
[g/kg]
Ash
[g/kg]
Apple pulp
Betacal
-
-
-
43
Beer fines
220
480
60
Sawdust
C100
Sawdust
C320
450-550
100-200
250-300
474
214
246
The
solid
content
[%]
38
64
seasonality
Availability
[ton/year]
1500
100.000
dry
22-27
continuously
produced by
4 months,
available all
year
continuously
5
92
continuously
200
2
92
continuously
200
* also depending on transport (high moisture content)
Fig.28. Apple pulp.
54
500.000
(100.000
dry)
Price
per dry
ton
[€]
5-10
20
120
2009
Fig.29. Betacal.
Fig.31. Sawdust with diameter 100µm.
Fig.30. Beer fines.
Fig.32. Sawdust with diameter 320 µm.
2.2.
Microscopic pictures
Different composition of pulps with alternative materials as addition is visible on microscopic
pictures. Microscopic pictures were taken at Dresden University of Technology, in Germany
during STSM program.
55
Fig.33. Typical recovered pulp.
2009
Fig.34. Eucalyptus vessel in recovered pulp.
Fig. 36. Birch vessel in recovered pulp.
Fig.35. Poplar vessel in recovered pulp.
As it is possible to see from figures 33 to 36, recovered pulp contains a big variety of
particles. Figure 33 demonstrates a typical microscopic picture for recovered pulp. Figure 34
shows a eucalyptus vessel present in recovered pulp. Figure 35 identifies a poplar vessel from
recovered pulp. Figure 36 presents a birch vessel in recovered pulp. This means that the
recovered pulp itself is produced from different raw materials.
56
Fig.38. Recovered pulp with apple pulp piece.
Fig.37. Recovered pulp with apple pulp.
Figures 37 and 38 identify recovered pulp with addition of apple pulp. In these pictures it is
possible to notice apple pulp pieces and liquid dirt agglomerates from impurities of the pulp.
Fig.40. Recovered pulp with betacal seen
Fig.39. Recovered pulp with betacal.
under polarised light.
Figures 39 and 40 depict betacal addition. Also after disintegration, betacal particles are still
much bigger than the normal size of filler particles. Alternative material particles are partly
kept with dirts coming from recovered pulp and part of them is free (Fig.39). Figure 40
indicates recovered pulp with betacal addition visible under polarised light. Betacal particles
contain sugar which can be seen as white spots in polarised light. Smaller particles which are
composed of minerals CaO and MgO are not visible in the microscopic pictures.
57
Fig.41. Beer fines particle.
Fig.42. Recovered pulp with beer fines.
Fig.43. Recovered pulp with addition of
Fig.44. Recovered pulp with beer fines.
beer fines. Eucalyptus vessel comes from
recovered pulp.
Figures 41 to 44 display recovered pulp with addition of beer fines. The big aggregate in
figure 41 is a particle ensemble from the skin of the cereal used for the bear production
(epidermis cells). Figure 42 shows the structure of beer fines. Dirts presented in microscopic
picture seem to be smoother and connected differently than in recovered pulp. Figures 43 and
44 confirm that different raw materials were used to produce recovered pulp. Eucalyptus
vessels are visible in both pictures.
58
Fig.46. Recovered pulp with sawdust C100.
Figures 45 and 46 demonstrate recovered pulp with addition of sawdust with diameter 100µm.
Dark spots visible on both pictures may be from sawdust.
Figures 47 and 48 indicate the presence of sawdust with diameter 320 µm in recovered pulp.
The big particle in figure 47 is a typical shive (wood fibres still connected via ray-cells) which
may come either from sawdust C320 or from recovered pulp. Figure 48 depicts big dark spots
which may certify the presence of sawdust.
2.3.
Pulp preparation
Pulp samples were prepared with the addition of 5%, 10% and 15% of alternative materials in
relation to bone dry reference pulp. Altogether, 32 different pulp samples were prepared for
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2009
testing. Any way of pretreatment was not used to change the size and shape of alternative
materials particles.
Samples can be divided into pulp samples which were prepared and measured almost
immediately after preparation and into pulp samples which were prepared and measured after
at least two hours after preparation. This time was used for pulp swelling. Pulp samples were
used to perform definite researches and after that paper sheets with addition of alternative
materials were made.
Fig.49. Desintegrator.
Fig.50. Prepared pulps placed in containers and left for swelling.
2.4.
Methodology of analysis
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2009
Since alternative materials influence of pulp properties and also on paper sheets appearance,
the methods of analysis of these two effects are separated into:
 Methods of pulp dewatering analysis,
 Methods of paper sheet properties analysis.
In order to study the effect of alternative materials addition on pulp properties, the tests were
performed using non-standardized methods. Information about the methodology of pulp
properties analysis and techniques used to collect data is described beneath in point 2.4.1. The
impact of alternative materials on pulp dewatering process in wire section and on ability of
pulp to being dewatered in press section is also separated.
2.4.1. Methods of pulp dewatering analysis
Due to there is no standard method, which describes dewatering property of papermaking
pulp according to one norm, the following measurements were chosen to investigate and fully
describe dewatering of pulp stock with the usage of alternative pulps:

Schopper-Riegler freeness,

Water Retention Value method,

Rapid-Köthen number,

Hydrodynamic specific surface area according to Dresden method,

FiberXPress.
The impact which alternative materials have on pulp dewatering in the wire section
and in the press section is also separated. The effect of additives on dewatering in the wire
section was tested using:
 Freeness test for both types of pulps after disintegration,
 Freeness test for both types of pulps after disintegration and 2h of swelling,
 Rapid-Köthen number measurement for both types of pulps with addition of
alternative materials after disintegration,
 Hydrodynamic specific surface area measurement for recovered pulp with alternative
materials addition after disintegration,
 Hydrodynamic specific surface area measurement for recovered pulp with alternative
materials addition after disintegration and 2h of swelling,
The effect of alternative additives on pulp dewatering in the press section was tested using:
 Water Retention Value measurement for both types of pulps after disintegration,
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2009
 Water retention Value measurement for both types of pulps after disintegration and 2h
of swelling,
 FiberXPress test for both types of pulps after disintegration and 2h of swelling.
.
Researches were carried out using wide range of dewatering apparatus available.
Schopper-Riegler freeness, Water Retention Value and Rapid-Köthen number tests were
performed at Institute of Papermaking and Printing (IPIP) at the Technical University of Lodz
(TUL) in Poland.
Hydrodynamic specific surface area measurements were performed according to
Dresden method with usage of the unique analyser, placed in Germany, at Dresden University
of Technology, at Faculty of Mechanical Engineering, Professorship of Paper Technology.
This cooperation was possible within European Union program called Short Term Scientific
Mission (STSM) during COST E54 Action with title: “Characterization of the fine structure
and properties of papermaking fibres using new technologies”.
Analysis with the usage of new FiberXPress device, invented and developed by Voith
and Smurfit Kappa, was performed in the laboratory of Smurfit Kappa Roermond Papier B.V.
in Roermond, the Netherlands.
Investigations in participating paper mill were conducted with coordination of Centre
of Competence Paper and Board, the Netherlands, and cooperation with experts from paper
mills.
Since there is no information about the above methods in standards, some information
and method description to present and to make possible by one to be acquainted with the
methods are needed.
2.4.1.1. Schopper-Riegler freeness
Schopper-Riegler tester is the most known instrument which uses the method of filter cake
creating to measure the dewatering possibility of the pulp. The Schopper-Riegler value (°SR)
is a measurement of the rate at which a diluted pulp suspension may be dewatered. It is
specified according to Zellchemig Merkblatt V/7/61. The standards for this measurement are
also: ISO 5267/1, SCAN C19 M3, BS 6035/1.
The industry uses a freeness measurement to characterize the drainage capability of a
pulp on the paper machines. Earlier Schopper-Riegler device was used only to measure pulp
beating degree because there was a correlation between freeness values and a target level of
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2009
pulp refining. Freeness is a measure of how quickly water is able to drain from a fibre furnish
sample. The measurement is based on pouring 1000ml of fibrous suspension with the
concentration of 0,2% (2g bone dry pulp with 998ml of water) to metal container with a built
wire. Pulp is dewatered on the wire with 100cm2 surface and filtrate is collected to calibrated
cylinder. Freeness is determined as the total volume of water discharged from a side orifice of
a specific configuration while the pulp suspension drains freely under gravity. Freeness is
given in Schopper-Riegler [°SR] degrees as a unit. 10ml of water discharged from device
equals 1°SR. The devices are designed so that an operator can judge the speed of dewatering
by observing the volume of liquid collected in a graduated cylinder [69, 72, 73, 74, 75, 76].
The test measurements were performed according to the Zellcheming Merkblatt
V/7/61. The device used is shown in figure 51. The test were carried out two or three (in case
when the difference between results was more than 1°SR) times for each sample and the
average value was calculated. Results of freeness were mass corrected.
Fig. 51. Schopper-Riegler tester.
2.4.1.2. Water Retention Value
Water Retention Value (WRV) is a result of centrifugal test and is expressed as the ratio of
mass (weight) of water retained after centrifugation under specified conditions by a wet pulp
sample to the oven-dry mass (weight) of the same pulp sample. Water Retention Value
accounts for the amount of retention water which is present in fiber after subjecting wet
fibrous sample to a certain amount of centrifugal force. Centrifugal force is a mean of
separating water from the fiber material [28, 78, 79].
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2009
Water retention value is based on an empirical convention, which means that the final
result is dependent on the conditions in testing. There are two methods used to define WRV:
 TAPPI Useful Methods UM 256 – in this method sample is centrifuged at 900g for
30min.
 SCAN-C 102 XE standard proposal – in this method sample is centrifuged at 3000g
for 15min.
The difference is these two ways of test performance in significant. Scan method is preferable
because the higher gravitational field removes more inter-fiber water. This test is less
sensitive to the inter-fiber pore geometry [21, 29, 30, 79, 80, 81].
The result from the Water Retention Value test allows to calculate nominal dryness of
paper web. This is the dryness which corresponds to the amount of retained water in the paper
web and the dryness can be used to assess the dewatering intensity of industrial press sections.
The dryness was calculated with the help of WRV Calculator, application invented and used
at Institute of Papermaking and Printing.
2.4.1.3. Rapid-Köthen number
Rapid-Köthen number (RK number) (drainage time) is described as a time, which takes a
volume of 6l of aqueous fiber slurry to drain in the Rapid-Köthen handsheet former from 8l to
2l under atmospheric pressure. Applied drainage pressure equals 100 kPa (atmosphere).
Drainage time stopped 2l above the screen. The schematic illustration of the method is shown
in figure 53.
The method is was performed according to description from old Zellcheming
Technical Leaflet ZCH V/7/61 [72, 82]. Test was performed only for pulps after
disintegration. Test was repeated twice for each pulp sample and the average from the results
was calculated.
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2009
Fig.53. Illustration of Rapid- Köthen number method [82].
2.4.1.4. Hydrodynamic specific surface area
Hydrodynamic specific surface area measurement is based on permeability principle
published by Kozeny and Carman. The method is described in details in theoretical part of the
thesis in point 1.4.2.2.
After similar pulp preparation as described in point. 2.3. the following activities were
carried out in order to conduct the hydrodynamic specific surface area test:
 Placing the pulp either directly after disintegration or after swelling into a dilution tank
and filling the tank up to a volume of 10 liters
 Taking of the proper amount of pulp corresponding to approximately 2 g bone-dry
material from the dilution tank for performing the hydrodynamic specific surface area
test
 Using the same volume of pulp for exact determination of consistency in the dilution
tank
Simplified testing principle is shown in figure 54. Scheme of the testing device and the
device itself as it is used at Dresden University of Technology is presented in figure 55. The
measurement principle is based on compression of pulp between two wire screens from which
one is moving with a piston and the other is fixed. The measurement is performed in few steps
(Fig.56).
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2009
Fig.54. Simplified testing principle [45].
Firstly, dewatering and compression parts of tank are put together. Water is poured
into the device and it covers the lower screen (1). Earlier prepared pulp is inserted and ready
for drainage (2). Pulp is being dewatered by gravity (3). When drainage is finished, all
thickened fiber material is placed only in the compression part (4). The compression part is
moved to the main device and measurement can start (5). Initially, when the piston is moving
downwards, the system is still without hydrostatic pressure (6). Start of compression is visible
and begins when the piston is moving further downwards and causes sealing between upper
screen and vessel wall (7). Only now the main device container is filled with water. The
overflow system of the main container and the adjustable water outlet (flow rate
measurement) determines the constant hydrostatic pressure drop. The sample is compressed
stepwise, and for each resulting fiber pad concentration, position of the upper screen, flow
rate and temperature are measured.
Additionally to the common procedure to determine specific surface area,
compressibility of pulp can be measured. After performing the six compression steps, the
fibre pad is further condensed by driving the upper piston downwards as far as possible (8).
From the position of the upper piston, the fibre pad concentration is determined, which is
corresponding to the compressibility of the pulp sample at compression pressure of 1.56 kPa
[45, 46].
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2009
Fig.55. Scheme of the testing device and the device itself as it is used at Dresden University
of Technology [45].
During pulps with alternative materials testing, six pressure steps were taken. Also
compressibility of these pulps were tested. This measurement was applied only for recovered
pulp with additives. One measurement was taken for one pulp sample. No tests with virgin
pulp and additives were performed. All researches were carried out according to method
description. The difference between the method description and tests performance was in pulp
samples preparation.
The pulp preparation of all samples was a little different than it is done usually with
hydrodynamic specific surface area method. Normally, samples are swollen and after that
they are disintegrated and deaerated (during deaeration dewatered pulp settles), as air bubbles
in pulp prevent water from streaming through the pad. Swelling prevents fibers from being
too damaged and cut. Within the researches for Short Term Scientific Mission which were
used within this Thesis, the way of investigation was changed to be comparable with the
sample preparation applied on other dewatering tests within the Thesis. Pulp preparation was
done according to what is written in point 2.3. Recovered paper was cut into pieces and
because of that fibers were already more damaged. Pulps were also not deaerated.
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2009
Fig.56. Measurement steps [45].
2.4.1.5. FiberXPress
The new FiberXPress test unit (Fig.57) was developed by Voith Paper Automation. This
device analyzes stock which is usually taken from the mixing chest and gives results in view
of stock water removal capability in the press section. That is why FiberXPress can be used as
a method for estimation of pulp dewatering.
Test of stock is explored by closing stock into sample chamber (2) where it is
pressurized up to 10MPa by pneumatic cylinder (1). The required pressure is built up with
compressed air (4) in the upper chamber of FiberXPress. Water which is pressed out of the
stock passes a wire, which holds back fibers and fines. The screen ensures also a lateral flow
towards the small syringe (diameter 1mm) through which the water flows and is drained into
the weighting scale dish underneath the chamber (5, 6). After the pressing process, fibers are
collected in so-called pellet. The pellet is taken out of the device and dried in an oven. After
drying the mass balance is derived. Stock dewatering performance is assessed by analyzing
the dried fibers and the extracted water.
Fig.57. The FiberXPress test unit [83].
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2009
FiberXPress measurement (Fig.58) is controlled from and recorded with a MS Exel
spreadsheet with embedded VBA code. This device is moreover equipped with computer
aided control of the pressure transducer (3) which allows for the stimulation of arbitrary
pressure profiles over time and potentiometric displacement transducer (8) which allows for
the thickness measurements during the pressing process. Owing to that information about
compressibility and spring-back behavior of furnish can be achieved. The pressing chamber
can be also heated by heating clamp (7).
Conditions of FiberXPress working simulate mechanical dewatering of the mix in a
press. The device was designed taking into account most of the factors influencing on the
maximum dry content possible to achieve at the exit of the press section. It enables one-site
visualization of dry content development as function of time, and analyzes the effects of
varying pressing duration, nip pressure and temperature. The effects of using a different
furnish can be also simulated with the FiberXPress under laboratory conditions and evaluated.
According to inventors, FiberXPress tool is much more advanced than conventional
test methods, based e.g. on water retention capacity of freeness. The test parameters are
controlled more precisely and can be varied over a wide range. This enables a better
approximation to the papermaking process, with more conclusive results accordingly [83, 84,
85, 86].
Fig.58. Photo of FiberXPress in a typical measurement setup (left) and turned for filling
(right). The numbers are reffered to in the text [84].
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2009
The test performance was not different form the typical measurement performance.
The test conditions were established simultaneously with the researches carried out for the
method evaluation [87]. The chosen conditions were accepted as the best conditions that time
allowing for the highest possible dryness achieving and they are the following:
 Pressure 9MPa,
 Time 980s,
 Basis weight 2500
g
,
m2
 Pulp consistency 4%,
 Temperature 21°C.
One measurement for every pulp sample was taken. The result was calculated and given
directly by a computer program. The result can be interpreted as a dryness of paper web
undergoing through ideal press section. Within the thesis this result will be compared with the
result from WRV test.
2.4.2. Methods of paper properties analysis
After pulps preparation and dewatering tests performance, remaining parts of pulps were used
to made paper hand sheets. The assumed basis weight was 110
g
what equals around 3,5g
m2
per sheet (OD weight). The hand sheets were prepared according to TAPPI T205 standard
[88]. The paper sheets were performed with the usage of Rapid-Köthen sheet former (Fig.59).
Fig. 59. Rapid-Köthen sheet former used during investigation.
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2009
To define properties of paper sheets, the following structural-dimensional and strength
tests were carried out in the same laboratory at Roermond:
 Basis weight [g/m2] – according to
 Thickness [mm] – according to
 Air permeance [ml/min] – according to ISO 5636/3
 Breaking strength [kN/m] – according to
 Breaking length [m] – according to
 Elongation [%] – according to
 Stiffness [kN/m] – according to
 Bursting strength [kPa] – according to
 Span Compressive Test (SCT) [kN/m] – according to
Paper sheets tests were performed according to TAPPI T220 standard [89]. The following
devices were used during the investigation:
Fig.60. Caliper gauge.
Fig.61. Air permeance
Fig.62. Bursting strength
tester.
tester.
71
Fig.63. Tensile strength tester.
Fig.64. Compression strength tester.
3. Results and discussion
In chapter 3 results from all pulp and paper tests are presented. Data were collected and sorted
in tables (Tab.3 – Tab.10). Tables are presented in Appendix at the end of the Thesis.
Results of all tests are presented on graphs. Graphs are sorted in turn of the method
description and alternative material used. The trend lines normally visible on graphs can lead to
some relationships which cannot exist or are not possible to achieve in reality. That is why
graphs are released with measurement point results which are connected to show what the
changes of pulp behavior with alternative pulp proportion (5%, 10% and 15%) are and to avoid
misunderstandings.
3.1.
The influence of addition of alternative materials on pulp properties
3.1.1. Dewatering in the wire section
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2009
Fig.65. Freeness as a function of alternative material proportion for all non-swollen pulps made
of recovered paper.
Fig.66. Freeness as a function of alternative material proportion for all non-swollen pulps made
of bleached mixed virgin pulp.
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2009
Fig.67. Freeness as a function of alternative material proportion for all swollen pulps made of
recovered paper.
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2009
Fig.68. Freeness as a function of alternative material proportion for all swollen pulps made of
bleached mixed virgin pulp.
Figure 65 displays the relationship between freeness values tested after disintegration for pulps
made of recovered paper and alternative materials. Figure 66 identifies similar relationship but
for alternative materials added to bleached mixed virgin pulp. Figure 67 points out the
relationship between freeness values tested after disintegration and 2h of swelling for pulps
made of recovered pulp and alternative materials. Figure 68 shows similar relationship but for
bleached mixed virgin pulp and additives.
Diversions in pulp behaviour depend on the shape and size of particles of alternative
material added. Because of that, results of measurements seem to be spread and the changes
brought about by alternative materials addition have not always visible tendency.
For both types of non-swollen pulps (Fig.65, Fig.66) all alternative materials addition
causes decreasing of pulp freeness or that the values of freeness fluctuate. The highest changes
are provoked by sawdust C100, C320 and betacal. This additives are expected to improve pulp
dewatering. In case of swollen pulps (Fig.67, Fig.68), alternative materials addition brings
about mostly freeness increase, so they are expceted to deteriorate the dewatering of
papermaking pulp, especially when using apple pulp and beer fines. When looking at swollen
pulps, the greatest changes in pulp dewatering may be also caused by addition of sawdust
C320, C100 and betacal.
The alternative materials cause that pulp behaviour when using bleached mixed virgin
pulp is similar to recovered pulp. In relation to virgin pulp, the greater changes in pulp
dewatering behaviour are provoked by betacal addition. Usage of apple pulp and sawdust with
diameter 100µm also brings freeness diminishing on.
The relation between freeness results and hydrodynamic specific surface area results is
very strong. Both of them depend on each other.
The relationships between hydrodynamic specific surface area and the proprotion of
alternative material used for all alternative material types are shown in turn in next figures.
Figure 69 points out specific surface area as a function of alternative pulp proportion for all
alternative non-swollen pulps. Figure 70 indicates similar relationship but for swollen pulps.
Both graphs depict that the proportion and the type of alternative pulp used cause
different changes in recovered pulp. Apple pulp is the alternative pulp which increase
hydrodynamic specific surface area. Apple pulp brings about an increase in value with
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2009
proportion of 5% and 10% for both types of pulp. Addition of 15% changes the tendency.
Alternative pulps which decrease hydrodynamic specific surface area are: betacal, sawdust with
diameter 100 µm and sawdust with diameter 320 µm. Alternative pulp such as beer fines can be
treated as an additive which does not have significant effect on pulp behavior when
hydrodynamic specific surface area is compared.
Fig.69. Hydrodynamic specific surface area as a function of alternative pulp proportion for all
alternative non-swollen pulps.
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2009
Fig.70. Hydrodynamic specific surface area as a function of alternative pulp proportion for all
alternative swollen pulps.
3.1.2. Dewatering in the press section
Figure 71 demonstrates the relationship between compressibility of pulp and the usage of
alternative materials for all alternative non-swollen pulps. Figure 71 displays that all alternative
pulps added to recovered pulp changes its compressibility. Only usage of some of additives
shows that it is possible to achieve higher consistency than without adding this material.
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2009
Fig.71. Compressibility as a function of alternative pulp proportion for all alternative nonswollen pulps.
Alternative pulps such as betacal, apple pulp and sawdust with diameter 100µm have
the highest influence on pulp compressibility in case of non-swollen pulp. It means that usage
of these additives may allow achieving higher consistency than for recovered pulp itself in the
same conditions as with recovered pulp. The above additives can be added to paper but only to
some extent.
The behaviour of betacal and apple pulp seems to be similar; the curves are following
almost the same direction. These alternative additives are the best compressed with addition of
5% and 15%. Values of compressibility of recovered pulp with 10% additions are much worse
than with remaining proportions, but still better than with recovered pulp itself. Compressibility
comparisons of apple pulp and betacal usage are presented correspondingly in figures 78 and
97 and described underneath the graphs.
The addition of sawdust with diameter 100µm to recovered pulp in non-swollen state of
pulp causes that pulp is better compressed than pure recovered pulp with addition of 5% and
10% in relation to bone dry recovered pulp. Addition of 15% proportion of sawdust causes that
compressibility of recovered pulp with this alternative material is worse than for recovered
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2009
pulp. The difference between swollen and non-swollen pulp with sawdust C100 addition is
shown in figure 141 and described beneath the graph.
Alternative pulp such as sawdust with diameter 320 µm causes no effect on pulp
compressibility. The lines show that maximal possible consistency does not change
significantly. It may mean that the addition of this pulp is neither here nor there for recovered
pulp or that this alternative material causes that recovered pulp with its addition is not good
compressible. The comparison between swollen and non-swollen pulp with sawdust C320 is
shown in figure 155 and described beneath the graph.
Usage of beer fines as an alternative additive shows that this additive decreases the
consistency when the amount is higher than 5%. Addition may cause the changes of fiber and
the shape of air-containing pores. The comparison between swollen and non-swollen pulp with
this additive is illustrated in figure 122 and described beneath the graph.
Fig.72. Compressibility as a function of alternative pulp proportion for all alternative swollen
pulps.
Figure 72 points out that the behavior of non-swollen recovered pulp with alternative
pulps does not vary so much as for swollen pulps. The highest consistency is possible to get
using betacal, what is similar as for non-swollen pulp. Also beer fines addition for swollen pulp
gives similar results as for non-swollen one. This additive decreases the consistency when the
amount of additive used is higher than 5%. Addition of remaining additives sawdust with
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2009
diameter 100µm and 320 µm samples and apple pulp, causes that recovered pulp is less
compressible.
Figures 72 and 74 display the relationship between dryness achieved by usage of
FiberXPress device and alternative material proportion for both types of reference pulps with
additives. In case of recovered pulp, the highest dryness is possible to reach by using betacal as
an alternative additive. Betacal addition in all proportion gives increase in dryness. The usage
of 15% of betacal gives the highest dryness among applying of all additives in different ratios.
Usage of sawdust C320 gives also rise in dryness as in case of betacal. The tendencies for both
alternative materials have ascending order. Usage of sawdust C100 is limited. 5% proportion in
relation to bone dry recovered pulp allow achieving higher dryness, than with remaining
proportions. Higher ratios than 5% cause dryness decreasing compared to 5%, but the values
are still higher than for recovered pulp itself. It may mean that applying of sawdust C100 may
bring pulp dewatering improvement. Apple pulp and beer fines usage bring about dryness
decreasing. Apple pulp added to recovered pulp in proportion higher than 5% causes that
dryness is diminished compared to reference pulp. Beer fines addition causes that dryness
values fluctuate, but all of them are lower than initial value for recovered pulp.
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2009
Fig.73. Dryness achieved by usage of FiberXPress device as a fucntion of alternative material
proportion for recovered pulp.
Similar behavior is shown for virgin bleached mixed pulp as a reference pulp (Fig.74). Apple
pulp added with all proportions causes that pulp dryness falls down. In case of beer fines, 5%
proportion in relation to bone dry reference pulp gives dryness decreasing, 10% proportion
usage causes that dryness values are approaching values for recovered pulp, and 15%
proportion brings about final dryness diminishing. Dryness values reached with different beer
fines usage seem to fluctuate, similarly as for recovered pulp, and it can be stated generally that
the more additive is used, the lower is the possible achieved dryness for both types of pulps. In
case of virgin bleached mixed pulp the highest dryness can be also reached by addition of
betacal. The highest dryness is achieved when proportion of 5% of betacal is used. The higher
amounts of this mineral additive cause dryness decreasing. 10% addition of betacal brings
about that dryness approaches to the dryness of reference pulp and 15% addition causes that
dryness is lower than for reference pulp. For both sawdust types, dryness values are lower with
addition of alternative materials than value for reference pulp. Sawdust C320 gives better result
than sawdust C100. Sawdust samples added to paper are themselves having very high dry
content. It may mean also that from the mixture of recovered or virgin pulp with sawdust, even
after rewetting, it is not possible to extract more water.
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2009
The comparison between pulp types with different alternative material and the amount
of material used is shown in points 3.1.3.1 to 3.1.3.5 and described beneath the graphs.
Fig.78. Dryness achieved by usage of FiberXPress device as a fucntion of alternative material
proportion for virigin bleached mixed pulp.
3.2.
The influence of addition of alternative materials on paper properties
All graphs are shown as a relationship between tested property and the proportion of alternative
material used. Graphs are divided into graphs showing the alternative materials impact on
reference pulp and on virgin bleached mixed pulp.
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2009
Fig. 50. Thickness index as a function of alternative materials proportion for recovered pulp.
Figures 150 and 151 show the relationship between thickness index and the proportion of
alternative materials used. Both figures display similar relationships. In case of recovered pulp,
thickness index can be increased by the usage of apple pulp, beer fines and sawdust with
diameter 320µm. Application of betacal and sawdust with diamter 100µm does not change
thickness index significantly compared to the reference pulps. In case of virgin bleached mixed
pulp, the addition of apple pulp and beer fines and also sawdust C320 show the greatest
changes as well.
In case of both pulps, sawdust C320 brings about that thickness index ratches
correspondingly to the increase of this raw material proportions. Generaly, the usage of apple
pulp and beer fines causes similar changes, the curves have almost the same shapes. In case of
recovered pulp, the additives seem to be useful until 10%, but 15% proportion brings about
thickness index decreases. In case of virgin bleached mixed pulp, 5% proportion decreases the
thickness index, but the more materials are used, the higher is the thickness index. Usage of
apple pulp and beer fines causes that pieces of these alternative materials were still visible in
paper sheets by the naked eye (Fig. –Fig. ). This may cause errors in paper thickness
measurements. The thickness value given by the caliper gauge could give the thickness of the
alternative material particles present in paper sheets plane instead of thickness of paper sheet.
83
2009
Because of that only sawdust with diamter 320µm can be considered as a material which can
increase thickness index.
Betacal addition with 5% proportion in relation to bone dry recovered pulp causes that
thickness index increases a little, but the more material is used, the lower is the thickness index.
Inverse relationship is shown in case of virgin bleached mixed pulp. 5% proportion brings
about that thickness index increases a little, but the more additive is used, the higher is the
thickness index.
Sawdust with diameter 100µm application with 5% proportion in relation to both
reference pulps brings about that thickness index decreases. The more this additive is used, the
higher is the thickness index. It may mean that the usage of this alternative pulp is worth
considering only then more than 5% proportion in relation to bone dry pulp is applied.
Fig.151. Thickness index as a function of alternative materials proportion for virgin bleached
mixed pulp.
84
2009
Fig.152. Air permeance as a function of alternative materials proportion for recovered pulp.
Fig.153. Air permeance as a function of alternative materials proportion for virgin bleached
mixed pulp.
85
2009
Figures 152 and 153 displays the relationship between the air permenace and the proportion of
alternative materials used. Accorging to the apperance of paper sheets with the usage of apple
pulp and beer fines, it is expected that the results do not show any real tendencies. The results
presented on graphs may be not considered, because the particles of alternative materials are
visible in sheet plane and may cause wrong interpretations of results. Sawdust with diameter
320µm seem to be applicable to both types of pulps with all proportions. Sawdust C320
addition causes that the air permeance is increased the most. Sawdust with diameter 100µm
added to recovered pulp until 10% and to virgin bleached mixed pulp in all proportions also
cause that air permeance is increased. Application of betacal to recovered pulp causes that air
permeance is also increased with all proportion used. The higher changes are brought by
addition of 5%. In case of virgin bleached recovered pulp, betacal can be applied until 10%,
because than it increases the air permeance. 15% proportion seems to be too much and
decreases the flow of air on the paper structure. It may be expected that these additives may be
used in production of papers where higher air permeance is needed, as for dust-absorbing
packagings or filter papers.
Fig.154. Breaking length as a function of alternative material proportion for recovered pulp.
86
2009
Fig.155. Breaking length as a function of alternative materials proportion for virgin bleached
mixed pulp.
Fig.156. Burst factor as a function of alternative material proportion for recovered pulp.
87
2009
Fig.157. Burst factor as a function of alternative materials proportion for virgin bleached mixed
pulp.
Fig.158. Short Span Compression Test as a function of alternative material proportion for
recovered pulp.
88
2009
Fig.159. Short Span Compression Test as a function of alternative materials proportion for
virgin bleached mixed pulp.
Figures 154 and 159 displays the relationship between strength properties of paper and the
proportion of alternative materials used. All figures show that the strength properties of paper
are deteriorated when alternative materials are applied. The strength properties were expected
to decrease in case of apple pulp and beer fines addition, because the particles of these
materials were visible on paper surface. Sawdust C320 addition could have been expected to
give similar results and it did. The usage of betacal and sawdust C100 causes that these
alternative materials were present in the paper matrix, they were not visible in the paper
structure, so they might have acted possitive on paper strength properties until some point (as
fillers do). Unfortunately, also these materials caused falling down of the strength properties:
breaking length, bursting strength (presented as burst factor) and SCT.
89
2009
3.2.1. Pictures of paper sheets
Fig.160. Paper sheet made of 100%
Fig.161. Paper sheet made of 100% virgin
recovered pulp.
bleached mixed pulp.
Fig.162. Paper sheet made
of recovered pulp with 5%
of recovered pulp with
addition of apple pulp.
10% addition of apple
15% addition of apple
pulp.
pulp.
of virgin bleached mixed
pulp with 5% addition of
apple pulp.
apple pulp.
apple pulp.
90
addition of betacal.
10% addition of betacal.
15% addition of betacal.
betacal.
betacal.
betacal.
addition of beer fines.
10%
15%
addition
fines.
of
beer
fines.
91
addition
of
beer
beer fines.
beer fines.
beer fines.
addition of sawdust C100.
10% addition of sawdust
C100.
C100.
sawdust C100.
sawdust C100.
sawdust C100.
92
addition of sawdust C320.
C320.
C320.
sawdust C320.
sawdust C320.
sawdust C320.
93
2009
4. Conclusions
Beer fines
No effect can be seen from S-R value for recovered pulp with addition of non-swollen beer
fines.
C100
Filtration – S-R
Permeation – specific surface area
C320
Plug the screen by particles – increases S-R, drainage goes slower (spec.surf.area goes down =
freeness goes up) –this is for swollen pulp. First it cause plugging of wire, but when we add
more, water flow is similar and it come back to the initial value. Drainability is faster (what we
can see from S-R plot)
Measurement should start when fibers cannot move any more.
Dewatering time with smaller volume (2ml) and the same hydrodynamic pressure is similar as
for higher volume (5ml). It means that dewatering is less intensive.
94
Annex 1
2009
List of abbreviations
CHP – natural gas Combined Heat and Power System. This type of system takes the waste heat
from the burning of fossil fuels and applies it to power another process. For example, a basic
CHP system might generate electricity needed in a particular industrial setting. The excess heat
and steam produced from this process can be harnessed to fulfil other industrial applications,
including space heating, water heating, and powering industrial boilers. [38,39]
MP – paper machine
List of figures
Fig.1. Stock and water systems in the paper machine [x]. ......................................................... 9
Fig.2. Typical slushing system [8]. ............................................................................................ 10
Fig.3. An example of stock blending and machine chest including sampling station [8]. ......... 12
Fig.4. Headbox location in the paper machine [14]. .................................................................. 16
Fig.5. Feed pipe for mix and cross-section of jet from headbox (not to scale) [8]. ................... 17
Fig.6. Fourdrinier former [15]. ................................................................................................... 18
Fig.7. Basic principle of two-sided dewatering [8]. ................................................................... 19
Fig.8. Examples of Twin Wire Formers [16]. ............................................................................ 20
Fig.9. The four phases of the nip process [17]. .......................................................................... 22
Fig.10. Phases of drying process [1]. ......................................................................................... 24
Fig.11. General classification of water contained in fibrous materials [22]. ............................. 27
Fig.12. Extracting and measurement principle of sedimentation water content [23]................. 28
Fig.13. Division of sedimentation water [24]. ........................................................................... 29
Fig.14. Influence of radial acceleration on water content in pulp [25]. .................................... 30
95
2009
Fig.15. Model classification of water contained in papermaking fibrous pulps
a) sedimentation, b) gravity dewatering, c) conventional pressing d) extreme conditions of
pressing [24]. .............................................................................................................................. 32
Fig.16. Conventional papermaking process [12]. ...................................................................... 34
Fig.17. Final energy requirement vs amount of water removed for the three operations forming,
pressing and drying in a paper mill [31]. ................................................................................... 35
Fig.18. Dewatering through filtration (left) and thickening (right) [8]. ..................................... 40
Fig.19. Flow rate through a porous solid body according to Darcy (left)
and Hagen-Poiseuille (right) [45] .............................................................................................. 41
Fig.20. Definition of internal and external surface area of a water-swollen fiber wall [46]. ..... 42
Fig.21. Kozeny-Carman plots for unrefined softwood chemical pulp as an example
of pulp with straight-line assumption between the concentration limits 0.07
and 0.15
g
cm 3
g
, intercept with Y-axis to calculate specific surface area and intercept
cm 3
with X-axis to calculate specific volume of the sample [45, 46]. .............................................. 45
Fig.28. Typical fiber and typical fine particle comparison. Right side of picture
shows dewatering through a bed of coarse fibers in comparison with fine matter,
assuming uniform packing density [20]. .................................................................................... 53
Fig.25. Schematic illustration of: a) charge neutralization, b) patch formation,
c) bridging flocculation [69] ...................................................................................................... 58
Fig.26. Recovered paper used to produce the first reference pulp – recovered pulp. ............... 59
Fig.27. Virgin bleached mixed pulp used to produce the second reference pulp – mixed pulp. 59
Fig.28. Apple pulp. .................................................................................................................... 60
Fig.29. Betacal. .......................................................................................................................... 60
Fig.30. Beer fines. ..................................................................................................................... 60
Fig.31. Sawdust with diameter 100µm. ..................................................................................... 60
Fig.32. Sawdust with diameter 320 µm. .................................................................................... 61
Fig.33. Typical recovered pulp. ................................................................................................ 61
Fig.34. Eucalyptus vessel in recovered pulp. ............................................................................ 61
Fig.35. Poplar vessel in recovered pulp. ................................................................................... 62
Fig. 36. Birch vessel in recovered pulp. .................................................................................... 62
Fig.37. Recovered pulp with apple pulp. .................................................................................. 62
Fig.38. Recovered pulp with apple pulp piece. ......................................................................... 62
96
2009
Fig.39. Recovered pulp with betacal. ........................................................................................ 63
Fig.40. Recovered pulp with betacal seen under polarised light. .............................................. 63
Fig.41. Beer fines particle. ........................................................................................................ 63
Fig.42. Recovered pulp with beer fines. .................................................................................... 63
Fig.43. Recovered pulp with addition of beer fines. Eucalyptus vessel comes
from recovered pulp. ................................................................................................................. 64
Fig.44. Recovered pulp with beer fines. .................................................................................... 64
Fig.45. Recovered pulp with sawdust C100. ............................................................................. 64
Fig.49. Desintegrator. ................................................................................................................ 66
Fig.50. Prepared pulps placed in containers and left for swelling. ........................................... 66
Fig. 51. Schopper-Riegler tester. ............................................................................................... 69
Fig.52. Modified Schopper-Riegler freeness tester [77]. .......................................................... 69
Fig.53. Illustration of Rapid- Köthen number method [82]. ..................................................... 71
Fig.54. Simplified testing principle [45]. .................................................................................. 72
Fig.55. Scheme of the testing device and the device itself as it is used
at Dresden University of Technology [45]. .............................................................................. 73
Fig.56. Measurement steps [45]. ............................................................................................... 74
Fig.57. The FiberXPress test unit [83]. ..................................................................................... 74
Fig.58. Photo of FiberXPress in a typical measurement setup (left)
and turned for filling (right). The numbers are reffered to in the text [84]. ............................... 75
List of tables
Tab.1. Minimum energy requirement for water evaporation form paper, expressed
in GJ/mt of water evaporated and GJ/mt of paper [34]. It is assumed that the final
consistency of the paper is 93% ds. In the case of the energy requirement per mt
of water evaporated, ingoing consistency is 45% ds [31]. ........................................................ 35
Tab.2.
97
2009
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104

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