Forced Circulation Crystallizer



Forced Circulation Crystallizer
The Draft Tube Baffle (DTB) Crystallizer
The Draft Tube Baffle (DTB) Crystallizer, used in applications requiring narrow
crystal distribution, and larger average crystal size, has been examined widely in
crystallisation theory. The basic choices of the type of crystallizer for continuous
operation are few: the design engineer has to choose between a mixed tank, a
fluidised bed (also known as "Growth" type or OSLO) crystallizer, and a Draft
Tube Baffle (DTB) crystallizer. The nature of a mixed tank is self-explanatory,
and needs no further elaboration. It is the workhorse of crystallisation
processes, and is best understood, because its simple nature lends itself to
practical modelling and experimentation. The bulk of batch and continuous
crystallisation theory is based on work done with this type of crystallizer, the
Mixed Suspension, Mixed Particle Removal (MSMPR) unit. The OSLO crystallizer
has been in use for most of this century, and has been employed in cases where
a large crystal size is required. It features a crystal bed, which is fluidised by a
supersaturated solution. This allows for crystal growth without any mechanical
mixing, and can generate very large and well-defined crystals. It is very difficult
to model this type of unit in smaller scale, because of geometric limitations;
also, its nature is such that basic design parameters must be based on
assumptions that are difficult or impossible to verify in the field. As a result,
theoretical work on the OSLO is limited, and this raises a severe handicap in
scale-up or troubleshooting operations of such units. The understanding of an
OSLO, therefore, is case-specific and based on empirical evidence. The third
option, the DTB, is a combination of an MSMPR crystallizer and a classifier. It
has been in use for about the last forty years, more or alternative to the OSLO
in fulfilling large-crystal requirements. Although the DTB has been marketed by
very few crystallisation designers, it has been studied well both by its creators
and by Academia. While it also suffers the disadvantage of not being easily
reproduced in small scale, for the same reasons as the OSLO, the design
parameters are easy to define and control accurately. As a result, its
understanding is based on well-proven theoretical work, and this makes the DTB
easy to apply to new crystallisation systems, troubleshoot, and optimise. The
simplest DTB crystallizer comprises two distinct functions and two distinct
regimes : the regime in which crystallisation occurs, and the regime in which
clarification is achieved. The crystallisation zone can be viewed as an MSMPR, in
broad terms of crystallisation kinetics. The clarification zone is employed to
remove particles of a given size from the crystallizer vessel. This is achieved by
an overflow from this zone which is set to carry with it particles of the desired
size, against gravitational settling, while allowing particles of larger sizes to
remain in the crystallisation zone. The disposition of the overflow leaving the
DTB is critical to the crystallizer's operation. In some cases, the overflow is
simply removed from the crystallisation operation, functioning only as a means
to increase the crystallizer's magma density. In other cases, the overflow is
recycled to the DTB crystallizer, after it has been thermally or chemically
modified. The most common objective of such modifications is the "destruction"
(i.e. dissolution) of the crystals in the overflow. The modified overflow is
returned to the DTB's crystallisation zone, where it participates in the
precipitation process, through its increased supersaturation. This has a marked
effect on the crystal size and the size distribution of the product crystals: it
produces larger crystals, and narrows their size distribution.
Figure 1: Draft Tube Baffle
Forced Circulation Crystallizer
Figure 2 shows a continuous forced circulation crystallizer. It is much like a
simple forced circulation evaporator, but it includes specific features to allow
correct crystallization, namely:
an "active volume", designed case by case, to get both required residence
time for crystal growth and mother liquor desupersaturation
a given agitation (recirculation rate) rated to control the extent of
supersaturation arising from the evaporation, and to keep the
temperature difference in the heat exchanger within reasonable limits
a special design of the liquid-vapor separation area to minimize the carry
over losses and avoid the formation of an excessive amount of fines,
which is highly detrimental to crystal growth.
Figure 2: Continuous forced circulation crystallizer
Depending upon specific process requirements, additional devices can be
provided. They are:
internal baffles, used mainly for excess mother liquor overflow and /or
withdrawal of fines when crystal growth is slow or disturbed by impurities
elutriation leg, to improve product purity and to deliver a narrow crystal
size distribution
an internal scrubbing section to reduce to very low value the carry over
losses, or even to provide stripping or absorption devices when a volatile
compound must be recovered
Figure 3: Continuous forced circulation crystallizer
Forced circulation crystallizers are of the (Mixed Suspension Mixed Product
Removal) MSMPR type and operate either on controlled or "natural" slurry
density depending upon process requirements and/or unit material balance.
These systems can be either single or multiple effects and the vapour
recompression concept (either thermal or mechanical) is often applied. Usually,
they operate from low vacuum to atmosphere pressure. As a rule, these units
are used for high evaporation rates and when crystal size is not of the utmost
importance or if crystal grows at a fair rate. Almost any material of construction
can be considered for the fabrication of these crystallizers. It is worth bearing in
mind that the heating element is omitted for vacuum cooling crystallizers.
Typical products are:
NaCl (food or technical grade)
Na2, SO4, K2 SO4
Citric acid
Oslo Type Crystallizer
Oslo type crystallizer also called classified-suspension crystallizer is the oldest
design developed for the production of large, coarse crystals.
The basic design criteria are twofold:
desupersaturation of the mother liquor by contact with the largest crystals
present in the crystallization chamber
keeping most of the crystals in suspension without contact by a stirring
device, thus enabling the production of large crystals of narrow size
The equipment is schematically shown in Figure 4.
Figure 4: Oslo type crystallizer
The classifying crystallization chamber is the lower part of the unit. The upper
part is the liquor-vapor separation area where supersaturation is developed by
the removal of the solvent (water for most applications). The slightly
supersaturated liquor flows down through a central pipe and the supersaturation
is relieved by contact with the fluidized bed of crystals. The desupersaturation
occurs progressively as the circulating mother liquor moves upwards through
the classifying bed before being collected in the top part of the chamber. Then it
leaves via the circulating pipe and after addition of the fresh feed, it passes
through the heat exchanger where heat make-up is provided. It is then recycled
to the upper part.
Additional devices, such as described for the forced circulation crystallizer, are of
course available. The operating costs of the Oslo type crystallizer unit are much
lower than with any other type when both large and coarse crystals are
required. Since crystals are not in contact with any agitation device, the amount
of fines to be destroyed is lower and so is the corresponding energy
requirement. This Oslo type crystallizer (classified - suspension crystallizer)
allows long cycles of production between washing periods. In addition to usual
process operations, the Oslo type crystallizer has also found a number of
interesting applications, e.g. for reaction-crystallization and for separationcrystallization when several chemical species are involved.
Most of the Oslo type crystallization units are of the "close type." However, the
"open" type is worth to be considered when very large settling areas are
required or when the vessel must be fabricated out of high cost alloys or metals.
Figure 5: “OPEN” type Oslo type crystallizer
Typical products are:
(NH4)2 SO4
hydrated mono sodium glutamate
mono ammonium phosphate (MAP)
Induced Circulation Crystallizer
The induced circulation crystallizer design has been recently developed to
provide additional agitation of the active volume of forced circulation
crystallizers with the use of only one pump. Operates similarly to a Draft Tube
Baffle crystallizer but without the internal agitation device. The main
applications are for evaporative crystallization cases. The unit also operates
according to the Mixed Suspension Mixed Product Removal (MSMPR) principle
and all options described for the other designs are of course available for this
concept. The equipment is able to produce a narrow crystal size distribution.
Like other designs, it can be fabricated in almost any material of construction.
Performances and product quality are equivalent to those of a Draft Tube Baffle
unit designed to the same specification, but appear to be limited to nonvisqueous solutions as the induced flow would be quite limited when the mother
liquor exhibits a high viscosity.
Figure 7: Induced Circulation Crystallizer
It has been successfully applied to the following productions:

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