FREEZE CRYSTALLIZATION: IMPROVING THE ENERGY

ESL-IE-81-04-18
FREEZE CRYSTALLIZATION: IMPROVING THE ENERGY
EFFICIENCY OF A LOW-ENERGY SEPARATION PROCESS
James A. Heist
Consulting Chemical Engineer
Irlilmington, NC
i
i
constant temperature and pressure on an enthalpy~
temperature-concentration diagram. Anyone of t~e
colligative properties of the solution can also Qe
measured and the thermodynamic properties calculated
from that data. The result is always the same. 'A
reversible process would require less than I BTU per
pound of water to separate it from the salt. Th~
actual energy requirements of the various separation
processes - membrane, crystallization, and evapora­
tive - are shown in Table 1.
ABSTRACT
.
,,\
Freeze crystallization is an efficient separa­
tion process that can potentially be used in any
application now using fractional distillation or
evaporation. Since most solvent extraction process­
es use distillation, it can also be substituted for
that process. Freeze crystallization is a high
energy efficiency separation process that can be
applied to a wide variety of industrial requirements.
It is demonstrated here that membrane processes are
the only separation technology that can approach
freezing for energy efficiency. Two versions of the
basic freeze crystallization process are discussed
that reduce energy consumption even further. In
achieving the lower energy consumption they also
provide other benefits that reduce costs. The
various benefits are quantified and several applica­
tions are discussed as illustrations of the capabil­
ity of the two versions of the process.
I
Several conclusions can be derived from this com~ar­
ison. The significantly higher efficiency of thJ
membrane processes indicates the importance thatl
they will play in future industrial separations.
Reverse osmosis and electrodialysis have technic~l
limitations that are not necessarily inherent, b t
merely reflect the infancy of the technology. Elec­
trodialysis can be used effectively only on electro­
lytes. Reverse osmosis is limited in applicability
by the state-of-the-art of membrane technology; I
commercial membranes are relatively fragile devi4es.
The separation factor for small to intermediate
sized non-electrolytes is relatively poor and little
fractionation is possible between molecules of s~mi­
lar structure. The rapidly expanding range of a pli­
cations is proof that research in this area is ,
effective and profitable, as the efficiency would
indicate.
I
INTRODUCTION
The Problem
Separation processes are a generally inefficient unit
operation. When seawater desalting became a national
priority nearly thirty years ago the technology
developers in that fledgling industry realized that
existing separation processes were inadequate for
the task. The past decade has seen many of the
desalting principles applied to numerous industrial
applications. With the spiraling cost of energy and
the conservation needs that it creates, all areas of
industrial activity must be examined. One of the
greatest energy use areas in the process industries
is for separations, which makes improvements in this
area a high priority research objective. When con­
sidering areas for research in separation processes
it is useful to examine the capabilities of the
various technologies, looking for those processes
that provide the greatest utility at the lowest cost.
i
The evaporative processes, the backbone of indus+
trial separation processing, are relatively ineffi­
cient. With seawater, the evaporative separatio~
factor is very high and only one equilibrium state
is needed to produce a high quality product. Wh¢n
fractionation of components with similar volatili­
ties is required, the energy consumption of evap¢r­
ative processes increases still more. Fractionai
distillation consumes at least the amount of thel
single effect evaporator and often many times that
amount. The principles of the higher efficiency!
evaporative processes can be applied to fraction~l
distillation in some cases, but even then it cantt
approach the efficiency of the non-evaporative Pfocesses.
!
A useful method of examining the alternative separa­
tions is to compare them on an application where they
are equally capable of performing the separation.
Seawater desalting is a useful example because its
properties and those of the product, water, are
familiar to engineers. The minimum energy required
to make this separation is the difference in the
free energy between the pure component and the solu­ ution. The difference in energy can be determined
directly by reading the difference in enthalpy at
Freeze Crystallization
ii
I
crystalli~a­
The other process in Table I is freeze
tion. While not as low in energy consumption asithe
membrane processes, it has other advantages. Th~
first advantage is that crystallization is usually a
single equilibrium stage process. Since it operates
at lower temperatures and the latent heats of crys­
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Proceedings from the Third Industrial Energy Technology Conference Houston, TX, April 26-29, 1981
ESL-IE-81-04-18
TABLE 1
SEPARATION PROCESSES
PERFORMANCE COMPARISON
ENERGY CONSUMPTION
·SEPARATION FACTORS 3
eTectrical thermal equivalent
organic
electro­
BTUILBl
KW-HR
BTUILB
volatile non-vola­
lytes
1000 GAL
tile
REVERSE OSMOSIS-energy recovery
22
REVERSE OSMOSIS-no energy recov'y
30
FREEZE CRYSTALLIZATION
50
-
residue
impurity
concen­
tration 2
25
l.
varies
50
10%
35
l.
varies
50
10%
60
1000
1000
1000
20%
90
-
110
varies
varies
10,000
t;VAPORATION
15
120
140
varies
varies
10,000
unlimited
MULTIEFFECT EVAPORATION
5
225
230
varies
varies
10,000
unlimited
SINGLE EFFECT EV_ilPORATION
1
1100
1100
varies
varies
10,000
unlimited
VAPOR COMPRESSION EVAPORATION
~lULTI-STAGE
l.
2.
3.
20%
Electrical conversion: 10,000 BTU/KW-HR performance on seawater at 35% conversion.
Maximum concentration of impurity in the unconverted solution.
Separation factor ~ ratio of concentration in product and mother liquor as conversion
approaces 0.
tallization are always less than vaporization, the
entropy change is smaller for this process than for an
evaporative process. The lower temperatures also
lessen corrosion effects so that less expensive
materials of construction are required. Very high
separation factors are the rule with crystallizing
processes, so the purity of the product is excellent.
When mother liquor is drained off there is still a
layer of impurity coating the crystal surface. In
addition to removing the bulk of the liquid, the
crystal separation device must also wash the adher­
ing layer from the surface. A device that does this
very effectively and efficiently, the wash column,
has been devised and adopted in seawater conversion
pilot plants. The principles of this device are
also adopted in the multi-stage crystallization
equipment now being marketed.
FREEZE CRYSTALLIZATION PROCESS TECHNOLOGY
General Process Description
The crystals are melted either in direct contact
with condensing refrigerant, or through a heat
exchange surface, depending on the requirements of
the process. Compressor work on the refrigerant is
minimized by compressing it to the lowest possible
equivalent condensing temperature. Direct condens­
ing on melting crystals is thus used when the melt­
ing temperature of the crystal is less than the
temperature of cooling water. If the opposite is
the case, energy is minimized by rejecting all heat
from the refrigeration system to cooling water.
All freeze cryst~llization processes, whether batch
or continuous, operate in the same manner, perform­
ing the following functions:
-crystallization from the solution, producing
discreet crystals that are free from occlusions
and produced to minimize bridging between indi­
vidual crystals.
-separation of the crystals from the remaining
mother liquor containing the concentrated impuri­
ties by a combination of draining and washing.
-recovery of the refrigeration effect in the
crystals by condensing refrigerant vapor and
using the latent heat in the crystals to absorb
the latent heat of the condensing vapor.
-refrigeration, to remove heat from the crystal­
lizer and increase its pressure to condense in
contact with the product crystals.
A general process flow diagram is presented in Fig­
ure 1 illustrating how these functions fit together.
The refrigeration system is adapted to fit the re­
quirements of the crystallization device. Indirect
contact processes work with a closed refrigeration
system where there is no problem with refrigerant
contamination. Direct contact processes require
either that the refrigerant be compressed directly
or absorbed in an absorbent. The absorption refrig­
eration cycle trades heat transfer surface for the
simplicity of a closed refrigeration cycle. A waste
heat absorption cycle replaces the refrigeration
system with a source of waste heat and cooling water.
The required temperature difference often is less
than 50 0 • Further description of each of the two
low energy consumption processes will be presented
in a discussion of each. A general description of
all freeze crystallization technologies can be found
in reference (1).
The crystallizer is a device that removes heat from
the process liquid, converting part of that flow to
crystals. The heat removal mechanism can be either
by direct contact with a refrigerant or through a
heat exchange surface. Since the driving force is a
major source of irreversibility in the process, the
direct contact processes are much more efficient.
Both of the process variations discussed in this
paper are direct contact, one using the solvent in
the process solution as the refrigerant and the other
using a secondary refrigerant.
THE WASTE HEAT ABSORPTION PROCESS
This process contains all of the components common to
freeze crystallization systems - crystallizer, wash­
ing device, refrigeration system, crystal melter,
The crystals formed in the crystallizer are discreet
particles and are pure, containing no occlusions.
98
Proceedings from the Third Industrial Energy Technology Conference Houston, TX, April 26-29, 1981
ESL-IE-81-04-18
6­
melted product
~
vapor refrigerant
I
~:~~ctionl
--­
,
Maln
Compresso
Compresso
I
'Zolliiensed
'ref'
I
WASH
COLUMN
PRODUC "-------I
FEED
CONCEN.Lll.t1..J..L_---I
FEED
HEAT
EXCHANGERS
feed
recycle
Concentr~te
Pump
CRYSTALLIZE
crystal slurry
Slurry
Pum
FIGURE 1
,
"
con
FREEZE CRYSTALLIZATION PROCESS SCHEMATIC
illustrate the procedure for evaluating potenti~l
absorbents for this process and predicting the ~e­
quired waste heat qualities that will be requir~d.
An aqueous system is used because of the availabil­
ity of accurate temperature, composition, and v~por
pressure data for both the process solution and: a
suitable absorbent, in this case a caustic soda:
solution. With pure water the triple point is at
32 0 F. and 4.6 ~n Hg absolute pressure (2). MOft
solutes depress the freezing point by 2 to 40 F,. per
molal and decrease the partial pressure of the water
vapor by .1 to .2 ~n Hg (3). The effects are s6all­
er per molal concentration in strong solutions,' but
at saturation of many materials in water the vapor
pressure will be 3.0 mm Hg or less and the free~ing
point will be depressed by 100 F. or more. To
create the refrigeration effect, the vapor pressure
of the absorbent must be about .3 mm Hg lower than
that of the solution. The example in the figur~
uses a concentrated caustic solution with a vappr
pressure of 3.0 mm Hg. and a 'cold' end operatipn of
95 0 F. As the absorbent performs i t is dilutedl by
the water vapor, raising its vapor pressure and,
making it unsuitable for further absorption. ~t is
regenerated by raising its temperature to the ~egree
needed to boil off the absorbed solvent. The rlegen­
eration temperature is determined by the vapor
pressure at which the generated vapor will condense
on cooling medium. Most of the generated vapor is
condensed on the pure ice, with a vapor pressure of
and energy recovery heat exchangers. The refrigera­
tion system is an absorption cycle using waste heat
to regenerate the absorbent, shown schematically in
Figure 2. Water vapor is the refrigerant, boiling
from the process solution, producing ice crystals at
the triple point of the (water) solvent. Note that
all absorption freeze crystallization processes
operate at the triple point of the solvent. Heat
leaked into the system from ambient surroundings and
mechanical energy put into the system must be removed
by the refrigeration system. Most of this heat
results in excess vapor boiled from the process
solution. The excess vapor as well as that used to
make ice in the crystallizer must be boiled from the
absorbent. The excess portion will have no ice to
condense upon, and must be rejected to an ambient
receiver, usually cooling water. This receiver has
a higher vapor pressure than does the crystal, and
the heat required for regeneration must have a higher
temperature than the bulk of the waste heat. This
heat rejection loop is shown in Figure 2 with the
rest of the refrigeration cycle.
Most absorption systems have been designed for opera­
tion with aqueous systems, using water as the solven~
As will be discussed in the Research Needs section
on this process, there is no reason that an absorp­
tion process can't use an organic as the triple point
solvent/refrigerant.
Cycle Analysis - An example is used here to
99
Proceedings from the Third Industrial Energy Technology Conference Houston, TX, April 26-29, 1981
ESL-IE-81-04-18
1% salt
30.5
UlW
H~
85° F
densing temperature on the Haste heat absorbing
medium, in this case a 10,000 mg/l coolin8
water. The vapor pressure is 30.5 rr~ Hg, and
with a .5 mm driving force gives a minimum re­
generation vapor pressure for this portion of
the absorbent of 31 Elm Hg. absolute. This is
shoHn by the horizontal line marked C in the
figure.
D.T~e minimum absorbent concentration is esti­
mated from absorbent temperature and the re­ quired vapor pressure. The temperat~re of the
absorbent is determined by a suitable temper­ ature rise in the cooling Hater (10 0 F. above the
80 0 F. cooling Hater here) and an approach
temperature across the heat exchanger (50 F.
here). Point A on the Figure shoHs that the
minimum solution of caustic soda that will
l.Jork is 49%.
E.As the absorbent is heated it is also concen­
trated during regeneration. A final concentra­
tion is chosen that compromises betHeen waste
50% NaOH
31.0 nun Hg
I 5° F
Cooling
wafer
:;>
CONDITIONS
50% NaOH .01% salt
5.0 mm Hg
mm Hg
II
°
4'8
F
34
F
aste
eat
"'.
CONDITIONS
49% NaOH
10~ Galt
3.0 mm fig
3.3 nun Hg
95° F
20° F
FIGURE 2
Q)
+J
~
WASTE HEAT ABSORPTION PROCESS
REFRIGERATION SCHEMATIC
.--l
0
1lro
g'
4.6 m~ Hg. Absorbent circulation is determined by a
trade-off of pumping energy and hig~er drivi~g force
requirements in the generator, and here a 1% increase
in absorbent concentration across the generator was
arbitrarily set. The condensing vapor pressure and
strong absorbent concentration set the temperature
requirements needed in the waste heat source, in
0
this case l13 F. The steps to perform this analysis
are shown graphically on Figure 3.
A.The required water partial pressure of the
absorbent is set .3 IPlll Hg beloH the triple
point of the solvent in the process solution.
B.The condensing vapor pressure is set .4 nm Hg
above the triple point of the pure crystal, or
at the cooling water temperature, which ever is
lowest. Here the crystal is ice with a va90r
pressure of 4.6 mm hg and a driving force of .4
rrun Hg, giving the minimum vapor pressure for
regeneration of 5.0 mm Hg. This is represented
by the line B in the figure.
C.The vapor pressure required for rejection of
heat fron the system is determined by the con­
~
,
Q)
3
'j)
U)
(j)
~
H
8­
~
20
30
40
50
'l'eI!'percl ture,
60
°C.
FIGURl, 3
h'l\S'I'l': HEAT ABSORPI'ION FP!·:r:Zr:
PROCESS CYCLf. N. l\LYSIS
100
Proceedings from the Third Industrial Energy Technology Conference Houston, TX, April 26-29, 1981
70
uo
~'.
ESL-IE-81-04-18
I
I
about 8% of the cost of a conventional absorptipn
process. This cost is eliminated with the wast~
heat version and there is no incremental cost ip its
place. Hhile this savings is not significant b~tween
the various freezing processes (see reference (~)),
it is sufficient when combined with the energy I
savings to show a clear advantage for this over! the
conventional absorption process where the wastelheat
is available and process conditions allow the higher
temperature operation.
I
heat quality and pumping requirements for
circulation of the absorbent between the absorb­
er and generator.
F.The absorbent regeneration temperature is
determined by locating the strong absorbent con­
centration and the required vapor pressure for
regeneration, determined in step B. At 50%
caustic and 5.0 mm Hg, the required temperature
is 113 0 F., as shown by point B and line F on
the figure.
G.The excess vapor absorbed in the crystallizer
must be rejected to ambient, as demonstrated
along the strong absorbent line G in the
figure. The end point, C, is determined by the
vapor pressure of the medium that condenses the
excess vapor, in this case cooling water.
H.The temperature for regeneration of the portion
of the absorbent associated with the heat rejec­
tion loop is determined by constructing a ver­
tical line from point C. Usually this vapor is
absorbed directly into cooling water so there
is no requirement for a heat transfer driving
force ab~ve the 50 F. in the heat rejection
generator. The overall requirement in this
example is for 175 0 F., or 6 psia steam.
Research and Development Activity
The absorption process is under development in
several government sponsored programs in pilot
plants with capacities up to 400,000 pounds of ~ce
per day. Several process problems have been id~nti­
fied that would benefit from increased absorbent
temperature, even with the conventional absorpt~on
process. The existing electrolytes that are prpven
absorbents have disadvantages ranging from thermo­
dynamic inefficiencies to corrosion characteristics
requiring expensive heat transfer materials. He are
presently conducting a study for the Office of
Hater Research and Technology in the U.S. Department
of the Interior which will define organic absorbents
that eliminate many of these disadvantages. Antici­
pated results will be improved efficiency and better
operations in conventional absorption systems and
lower regeneration temperatures in waste heat
systems.
The principal advantage of the waste heat absorption
process is the energy savings over similar processes
using conventional refrigeration for cooling the
absorbent. The energy requirements for the two
alternatives are shown in Table 2 for applications
with a 10 and 35 0 F. freezing point depression.
Note that the advantages of a waste heat process are
ever greater as the freezing point depression of pro­
cess solution over the pure solvent increases. The
ability of any absorption process to achieve the very
low vapor pressures associated with the upper freez­
ing point depression is somewhat questionable and is
discussed in the next section.
The absorption process is difficult even at an
operating pressure of 3.5 rum HG. For applicability
to high freezing point depression systems, \<Ihere the
triple point vapor pressure is below 2.5 to 3.0 mm
Hg, the process would benefit significantly fro. use
of a secondary refrigerent that would increase the
pressure of operation but still use the waste heat
absorption refrigeration cycle. A computer program
that will identify suitable secondary refrigerent ­
absorbent combinations is being developed at this
time. The same program will be capable of defi~ing
absorbents for systems with non-aqueous solvents.
The results should be available i~ the fall of this
year.
Haste Heat
Conventional
Absorption
Absorption
Freezing
Freezing
Power Requirement
Freezing Point Depression Vi<'
Kw-Hr/1000 Gal H2O 10
32
10
32
Pumps
8
8
7
7
Refrigeration
-
-
28
93
Non Condensibles
Removal
5
5
5
5
5
18 to
21
5
5
18
45 to
60
5
110
!'lechanical
Total
TABLE 2
HASTE HEAT ABSORPTION FREEZE PROCESS
ENERGY REQUIREMENT ADVANTAGES
Another advantage of the waste heat process is that
a major item of capital cost is removed. The refrig­
eration system, exclusive of the heat exchangers
that must remain for a waste heat system, represents
THE HYDRATE FREEZING PROCESS
I
In this process the phenomenon of hydrate cryst?l
formation is used. A hydrate crystal contains mole­
cules of both water and the refrigerant. The ctystal
is a clathrate; that is, the refrigerant molecuies
are not chemically bonded to the water in thecnystal
The guest (refrigerant) molecules stabilize theiice
crystal, allowing it to exist at a higher tempeta­
ture. Most gases will form hydrates with water!in
this manner but relatively few occur at conditions
that make them appropriate for this process. C~ys­
tallization temperatures 50 0 F. above the normal
freezing point of water are possible in this ma¢ner.
The gaseous hydrates discussed here are a specia!l type
of clathrate and the discussion that follows applies
strictly to water solutions. Clathrates do exi$t in
organic solutions and could conceivably be used 'in
the manner that will be described for aqueous appli­
cations (4). An extension of the concepts of this
process will be discussed below in the Process
Extension sub-heading.
101
Proceedings from the Third Industrial Energy Technology Conference Houston, TX, April 26-29, 1981
ESL-IE-81-04-18
This process also contains the normal elements of
the freeze crystallization process, plus some auxil­
iary items to meet the process needs unique to it.
The process uses a vaporizing refrigerant to remove
the heat from the process solution necessary to make
the crystals. The refrigerant interacts with the
solvent allowing crystal formation at a temperature
above the normal freezing point of the solvent
(water in the case of desalting). The crystal is
melted by compressing the vaporized refrigerant to
the point at which it will condense on the crystal
and in giving up the latent heat of condensing will
melt it. The refrigerant is chosen for minimum
solubility in the solvent so that the two phases can
be separated in a decanter. The liquid refrigerant
is recycled to the crystallizer, but the product and
rejected mother liquor must be stripped to recover
the dissolved refrigerant. Non-condensibles also
must be bled from the system to prevent vapor block­
ing of any coupled mass··hea t transfer surfaces.
Cycle Analysis -The mechanics of this process
are best understood by following the cycle on Figure
4. Phase equilibrium of the solution and the re­
frigerant are shown on this diagram. Line A-B repre­
sentsthe vapor liquid equilibria of pure refrigerant.
Line C-D shows the temperature-pressure dependence
of hydrate formation. Where the two intersect is
called the hydrate critical point. It is significant
because it represents the lowest temperature and
pressure where compressed refrigerant will condense
on the hydrate crystals to melt them. Of the four
quadrants formed by the two equilibria lines in the
diagram, hydrate crystals exist in two of them,
regions I and II. In region I the refrigerant exists
as vapor and in region II it is a liquid. The
refrigeration cycle operates along line A-B, evapor­
ating to produce more crystal as the pressure is
lowered and condensing to stop crystal formation if
it is raised above the equilibrium line. Line C-D
separates the regions of hydrate crystal formation
from the regions with no solid phase. The critical
point represents the conditions at which hydrate is
formed from a pure water solution. In an organic
clathrate system it would represent the conditions of
f or'mation from pure solven t . The exis tance of a
solute in the solvent that depresses the freezing
point of the solvent also depresses the formation of
the clathrate approximately equally. Thus, the
dashed lines in the figure represent various levels
of impurities in the solution with associated crystal
temperature formation represented by the difference
between the critical point and the intersection of
the appropriate impurity level line with line C-D.
The impurity lines represent tie-lines between the
two equilibrium curves, with the intersection with
the A-B line representing the refrigerant boiling
conditions existing during crystallization.
the compressor are determined directly from the
refrigerant curve, line A-B. Suction and discharge
pressures should allow vaporizing and condensing
driving forces of 1 0 C. Compressor power is calcu­
lated using these conditions and the properties of
the refrigerant and from mass flowrates determined
from a mass and energy balance around the proposed
process. Pumping costs can be estimated from the
breakdown in Table 5 that follows in the discussion
of advantages of this process. Heat rejection power
requirements are calculated using the hydrate crit­
ical conditions as the suction conditions and cool­
ing water for discharge conditions. The quantity of
heat to be rejected can be determined by converting
the mechanical and pump power requirements into a
thermal equivalent (3413 BTU per hour per kw load)
and an estimate of the ambient heat leakage into the
process. The ambient leakage will vary with the
process conditions and the climate, and can be
estimated from the main compressor power, since
higher power there relates to both process size and
crystallizing temperature. Use 20% of the main
compressor load for systems whose main compressor
power is 25 HP or lower and 10% if over 500 HP, with
linear interpolation for points between. Again, for
svstem costs and further discussion. see Reference 1.
D
IV
II
i
kE
liquid refrigerant
process liquid
crystal
III
A
vapor refrigerant
process liquid
no crystal
S
C
TEMP.
..
FIGURE 4
HYDRATE PROCESS PHASE DIAGRM1
The most difficult part of analyzing this process is
deriving the equilibrium data for the hydrate. The
information exists for many gaseous clathrates in
saline waters (5,6). This can be extended to other
electrolytes in water. Extension to solutions con­
taining other organics is an uncharted area. Suit­
able results for a preliminary analysis can probably
be obtained by predicting freezing point depressions
from activity coefficient effects on the water by the
dissolved solutes. Once the equilibrium curves are
established the suction and discharge conditions of
Advantages
The difference in energy consumption between a hy­
drate process and its nearest physical correlary, the
secondary refrigerant process, are demonstrated in
Table 3 for two different conditions. The table
illustrates the effect of freezing point depression
on the power consumption of freeze crystallization
processes. Operation at the higher temperatures of
102
Proceedings from the Third Industrial Energy Technology Conference Houston, TX, April 26-29, 1981
ESL-IE-81-04-18
The total reaction rate of hydrates remains constant
as the solute concentration increases.
Secondary
Refrigerant
Process
Power Requirement Freezing Point Depression of
10
32
30
KW-Hr/lOOO gal H2O 10
12
8
6
Pumps
6
Hydrate
Process
18
Refrigeration
50
20
2
2
2
2
Mechanical
1
1
1
2
27 to
35
61
30 to
40
The solubility of component 1 in component 2 for a
binary organic system is given by the expression'
79
Non-Condensibles
Removal
Total
Process Extension
ln (l/alp'X) = (del H fus
where alp
X
del H fus
95
R
TABLE 3
T
Tm
HYDRATE FREEZE PROCESS
ENERGY REQUIREMENT ADVANTAGES
R T) x (1 - Tm/T)
the activity coefficient of component 1
the mol fraction of component 1
the latent heat of fusion of component 1,
cal per gram mol.
the ideal gas constant, 1.987 cal per
gram mole _ oK.
the system temperature, oK.
the melting temperature of component 1,
oK.
This equation shows that the crystallizing temper­
ature of component 1 dissolved in component 2 is
increased at a given concentration by increasing: the
activity coefficient of component 1. While thisiis
not the mechanism of clathrate formation in the QY­
drate process with aqueous systems the effect of: the
gaseous refrigerant is the same. In organic syste:lls,
where clathrates have not been investigated or if
the clathrating agent is not a suitable refrigerant,
this phenomenon can be used to reduce the energy
required for secondary refrigerant freeze crystalliz­
ation in much the way the host agent functions in
the hydrate process. By adding a small fractioniof
a component that increases the activity of cryst~l­
lizing compound, the temperature at which it canibe
crystallized from the solution is raised. Tailoting
the compound would then allow its recovery from the
concentrated mother liquor.
the hydrate process over a non-hydrating secondary
refrigerant reduces energy in three ways:
-operation at the higher temperature reduces the
ambient heat leakage into the system, vaporizing
less refrigerant and creating less load for the
main refrigeration compressor. Depending on the
ambient temperature and the crystallizing temper­
ature, the main compressor power can be reduced
by at least 5% in this manner.
-the lower ambient leakage results in less load
on the heat rejection compressor, at least 60% of
whose power requirements are created from this
source. This can create as much as a 20% reduc­
tion in total process power requirements.
-by operating at the higher temperature the vis­
cosity of the process fluid is lower and the
pumping requirements for operation of the wash
column are less. About 25% of the total pumping
power is used for slurry transfer to the wash
column at 1.5 poise. The total power required
at varying viscosities is linearly proportional
to the viscosity, so a reduction of the viscos­
ity by one half will produce a power savings of
about 5% of the total process requirement. In
some applications a four-fold reduction is
possible with a 30 0 F. temperature change,
resulting in a power reduction of over 15%.
Research and Development Activity
Two hydrate desalting processes were developed
through pilot plant demonstration. Laboratory
evaluation of numerous hydrating agents has been:pe~
formed in the seawater conversion program. Recently
interest has increased in the clathrating phenomenon,
as evidenced by presentations at technical meeti~gs
and in the literature. While not directly addre$sed
to development of freeze crystallization process~s,
the activity will undoubtedly benefit this area ~s
well. The energy and capital cost benefits of t~is
process suggest that it should receive more attenhon
than that justified by the desalting program. Ttle
applications discussion to follow will demonstrate
that it is the one freeze crystallization proces~
that might be technically capable of replacing e~ap­
orative crystallizers in many industrial separat~on
applications.
'
Another advantage created by the higher operating
temperature is in capital cost reductions. It can be
shown that wash column costs are approximately pro­
portional to the viscosity. The economy of scale
exponent of .55 to .67 for tanks and process vessels
is offset by the fact that wash column volume in­
creases with the 1.5 power of viscosity. Fabrication
and installation costs of the wash column account for
about 10% of the cost in a desalination plant, where
the viscosity is about 1.5 poise. At high concentra­
tions and large freezing point depressions where a
four-fold reduction in viscosity is feasible the
effect on cost would be to reduce total plant capital
requirements by as much as 25%. Further capital cost
reduction is realized by the difference in crystal
growth rates between the hydrate and conventional ice
crystallization processes. The growth and nucleation
rates are about equal at low freezing point depres­
sions, but as the solution becomes more concentrated
the growth rate of ice crystals decreases substan­
tially, requiring longer residence times to achieve
the necessary crystal size for economical washing.
Application of the activity coefficient enhancement
correlary in organic systems requires that fast 4nd
inexpensive methods be derived for identifying the
material that will effect the activity change.
Addition of solid-liquid equilibrium predicit0nsto
the data bases and physical property computer pro­
grams subscribed to by much of the process industry
is making this a more realistic task. A great deal
of work has been done on secondary refrigerant freeze
crystallization that will be directly applicable to
its use in this type of application.
103
Proceedings from the Third Industrial Energy Technology Conference Houston, TX, April 26-29, 1981
ESL-IE-81-04-18
APPLICATIONS
Developing a list of all of the applications in all
process industries for freeze crystallization tech­
nology would require a great deal of time and re­
search and take longer to present than allowed here.
Rather, the variety of applications will be discussed
generically by industry. Generally, with the
extended capabilities of the two processes discussed
above, freeze crystallization can be considered for
any application that now uses an evaporator or
fractional distillation process for separating and
purifying one or more components from a solution.
Conditions that are less favorable for freeze crys­
tallization include:
-operations increasingly removed from ambient
temperatures, which increase the power required
for heat rejection.
-applications with large freezing point depres­
sions, where the power required in the main
compressor increases.
Of course, the viability of any application is
determined as much by the ability of the altern­
atives as by the capability of the freeze cyrstalliz­
ation process. Areas of application in various
industries are discussed in the following paragraphs.
Organic Chimicals Industry - There is a great
deal of waste heat available in this industry at a
quality of 150 to 350 0 F. that could be used to
operate a waste heat process. The key to successful
use of this process will be finding suitable absor­
bents for operation in each situation. There are
many distillation columns that run with high reflux
rates to separate material with similar volatilities.
In many of these applications the energy use for
distillation is so large that freeze crystallization
can effect a 95% reduction even when the crystalliz­
ing point is depressed by 40 to 50 0 F. In other
applications freeze crystallization can produce a
higher quality product than distillation by
excluding materials of similar volatility. Waste­
waters from many chemical processes that are conven­
tionally treated by biological oxidation or adsorp­
tion can be economically recovered by freeze crys­
tallization. One clJemical company is developing a
freeze process for recovery of acetic acid from a
waste stream. They found that freezing uses less
energy tlJan aerating ponds and tlJat the value of the
recovered acetic acid will provide a positive payout
on the equipment.
and non-citrus fruit juices and tomato sauces and
pastes are examples. Other applications require
concentration and sometimes crystallization of food
products, such as in sugar refining. The industry
lJas a great deal of waste lJeat available from steril­
izing and packaging operations. An example of how
these two freeze crystallization processes could be
used is presented by the beet sugar industry. There
a waste heat process would be used to concentrate
the 15% juice initially obtained to 40% sugar.
About two tlJirds of the water is removed here. At
this point the freezing point depression becomes too
great for further operation of a triple point
process and the viscosity is increasing rapidly.
The hydrate process could then be used to remove the
rest of the water and to crystallize the sugar
directly. The advantage of this process in this
industry is enhanced by the fact that single effect
evaporative crystallizers are often required to
minimize temperature effects on the food product.
Power Industry - Many areas of the country
require zero discharge of all liquid wastes from all
industrial sources. For the power industry in the
soutlJwest U.S. this has meant changes to much more
efficient use of water and ponds for storing the
dissolved residuals, mostly in the cooling tower
blowdown. Evaporation from the storage ponds has
been much less tlJan was originally predicted and
environmental requirements have increased tlJe costs
of the ponds. Waste water management has thus
become a very expensive operation. The dissolved
solids can be stored in a minimum volume when
converted to mineral crystals and saturated brine.
The waste heat process is especially well adapted to
the requirements of this application because it can
use heat from the boiler feed pump turbine drive to
operate the system. This source is not large enough
for an evaporative crystallizer because of the
difference in heat requirement for the same flow of
wastewater. An evaporative crystallizer that ties
into the steam condenser directly is reduced in
capacity when the plant goes to partial load,
rapidly becoming ineffective. Since the boiler feed
pump turbine runs with relatively constant steam
flow, regardless of plant load, the freeze crystal­
lization system is not crippled by plant load
reductions.
Pulp and Paper Mills - This industry has both
large quantities of waste heat and large flows of
process fluids. The most concentrated of the streams,
black liquor from digestion of the lignin in the
wood, is concentrated for chemical recovery in most
operations. A waste heat process has beeen demon­
strated to economically preconcentrate this flow to
enhance evaporator operation, but becomes limited by
viscosity. Thy hydrate process is uniquely suited
for further concentration as the viscosity at all
concentrations is very sensitive to temperature.
Other process fluids that contain recoverable
organics that are presently treated for BOD and
color reduction include bleach plant effluents,
evaporator condensates, and paper machine white
water. Concentration of these streams with a waste
heat process would be relatively inexpensive and
would allow economical recovery of those chemicals
by crystallization or extraction.
Inorganic Chemicals Industry - Evaporation and
crystallization are commonly used in this industry.
In many of the applications a waste heat freeze
crystallization process could be substituted,
achieving the energy reduction indicated by Tables 1
and 2. In some cases the reduced operating temper­
ature improves the yield of product from solution.
In potash and soda ash production, for example, the
relative solubility of product is less at lower
temperatures. An evaporative crystallizer starts to
precipitate impurity along with the product crystals
Where freeze crystallization recovers another 5 to
10% product. The waste heat freeze process is also
attractive for concentrating wastewaters from this
industry for product recovery or ultimate disposal.
Food Industry - This industry has many applica­
tions that require concentration of juices. Citrus
104
Proceedings from the Third Industrial Energy Technology Conference Houston, TX, April 26-29, 1981
ESL-IE-81-04-18
BIBLIOGRAPHY
Energy Industries - Petroleum refining and the
emerging synfuels industry both produce process
condensates that contain recoverable chemical values.
Phenolics materials are the most abundant and valu­
able chemical. An evaluation of a typical waste at
just over 1000 mg/l phenols showed that freeze crys­
tallization was the only process capable of treating
the waste for discharge with a profit. The value of
the chemical was great enough to pay for concentra­
tion and recovery. These industries also generate
large amounts of waste heat that is dissipated in
cooling towers. In some parts of the country they
will face zero discharge requirements. The petro­
leum refining industry has numerous process conden­
sates containing sufficient organic content to make
recovery feasible.
1. Heist, J.A., "Freeze Crystallizai ton," Chern.·
Eng., Vol 81 (10) P 72, May 1979.
2. Perry, R.H. & C.H. Chilton, eds, Chemical
Engineers Handbook, Ed 5, Table 3-275, p 3-205,
McGraw-Hill (New York), 1973.
3. Barrow, G.M., Physical Chemistry, p 656, McGtaw­
Hill (New York), 1966.
4. Grayson, M. (ed), Kirk Othmer Encyclopedia of
Chemical Technology, 3rd Ed. Vol 6, p 179, Wile~
New York (1979).
5. Barduhn, A.J., H.E. Towlson, & Y-C Hu, "The
Properties of Gas Hydrates and Their Use in
Demineralizing Seawater," OSW R&D Report No -44,
NTIS No PB 171031, U.S. Dept. of Commerce, (1960k
6. Barduhn, A.J., N. Klausutis, R.W. Collette, ~
J.R. Kass, "Further Properties of Hydrates &
Hydrating Agents," OSW R&D Report No 88, NTIS
No 181583, U.S.Dept. of Commerce (1964).
.
Primary and Secondary Metal Production - Pick­
ling operations produce a liquor that has been
depleted of its acid value. As this progressively
occurs the capacity of the pickling line decreases.
The industry has found it profitable to install
recovery processes that evaporate the excess water
and precipitate metal salts so the remaining acid can
be recycled and capacity of the line maintained. The
evaporative processes used for this purpose are
single effect and quite expensive because of the
corrosive nature of the liquor. A waste heat freeze
process, since it would operate with 10% or less
acid, would be very effective. Since it is a direct
contact process it would eliminate the problem with
tube plugging when temperatures aren't strictly
maintained in the evaporative process. Coke produc­
tion condensates fall in the same category as the
process condensates from the petroleum and synfuels
industry discussed above. If this industry is forced
to go to the extensive water recycle measures pro­
posed by the EPA there will also be a need for
concentration devices there, with the waste heat
freeze crystallization process ideally suited.
105
Proceedings from the Third Industrial Energy Technology Conference Houston, TX, April 26-29, 1981