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US005555747A
United States Patent [19] [li] Patent Number: 5,555,747
Conlon [45] Date of Patent: Sep. 17, 1996
[54] CONTROL OF CRYSTAL GROWTH IN
WATER PURIFICATION BY DIRECTIONAL
FREEZE CRYSTALLIZATION
[75] Inventor: William M. Conlon, Palo Alto, Calif.
[73] Assignee: Polar Spring Corporation, Menlo
Park, Calif.
[21] Appl. No.: 281,759
[22] Filed: JuL 28, 1994
[51] Int. Cl. 6 B01D 9/04
[52] U.S. Cl 62/532; 62/123
[58] Field of Search 62/123, 532
[56] References Cited
U.S. PATENT DOCUMENTS
3,338,065 8/1967 Ashley 62/123
3,344,616 10/1967 Owen 62/58
4,799,945 1/1989 Chang 62/532
4,954,151 9/1990 Chang et al 62/532
5,032,157 7/1991 Ruff 62/532
OTHER PUBLICATIONS
Heist, Freeze Crystallization, 7 May 1979 Esp. pp. 348-350.
Moyers & Roosseau, Crystallization Operations, in Hand-
book of Separation Proless Tech., 1987, p. 606.
Kuo and Wilcox, Removal of Particles by Solidification
1973, Esp. p. 373, First col.
Burton et al.. The Distribution of Solute in Crystals Grown
From the Melt, Part I Theoretical, 1953, pp. 1987-1989.
Glen, The Physics of Ice, 1974, Chapter Ten pp. 52-55.
Jellinek, The Ice Interface, in Water and Aqueous Solutions,
1972 pp. 95-96.
Taylor, Ice-Water Partition Coefficient for RDX and TNT,
1989, pp. 5-7.
Gill, Heat Transfer in Crystal Growth Dynamics, 1988, pp.
15-16.
Primary Examiner —- Ronald C. Capossela
[57] ABSTRACT
A Directional Freeze Crystallization system employs an
indirect contact heat exchanger to freeze a fraction of liquid
to be purified. The unfrozen fraction is drained away and the
purified frozen fraction is melted. The heat exchanger must
be designed in accordance with a Growth Habit Index to
achieve efficient separation of contaminants. If gases are
dissolved in the liquid, the system must be pressurized.
14 Claims, 5 Drawing Sheets
144 ^
Separation Efficiency
U.S. Patent
Sep. 17, 1996
Sheet 1 of 5
5,555,747
Figure 1
Bulk Liquid Molality = 1 00 Bulk Liquid Molality = 1 000
AT =K f (1 90 - 280) = -90K AT =K f (1 900 - 2800) = -900 K
Figure 2a Figure 2b
U.S. Patent
Sep. 17, 1996
-252 /234
4 -
,232 I I
U.S. Patent
Sep. 17, 1996
Sheet 4 of 5
5,555,747
Figure 5
Growth habit Index (cm/°C)
Figure 6
5 , 555,747
1
CONTROL OF CRYSTAL GROWTH IN
WATER PURIFICATION BY DIRECTIONAL
FREEZE CRYSTALLIZATION
This invention was made with Government support
under contract NAS9- 18843 awarded by NASA and contract
68D30135 awarded by EPA. The Government has certain
rights in this invention.
FIELD OF THE INVENTION
This invention relates to liquid purification methods and
means employing directional freeze crystallization wherein
a liquid is partially frozen, the unfrozen liquid fraction is
extracted, and the remaining crystalline phase is melted.
More particularly, the invention relates to control of crystal
growth to reduce entrapment of contaminants within the
crystal structure to increase separation efficiency.
BACKGROUND OF THE INVENTION
Freeze crystallization has been used to desalinate seawa-
ter, concentrate fruit juices, and separate organic chemicals.
According to Heist, up to about 15% of the fluid mass is
crystallized in a typical application. A variety of methods
have been developed for continuous and batch processes,
incorporating different cooling means, means of separation
of the crystalline phase from the liquid phase, and melting
means. A summary of industrial crystallization practice is
provided by Moyers and Rousseau, but only two paragraphs
are devoted to batch crystallization out of a sixty-five page
article. Batch crystallization techniques for drinking water
treatment have been described Chang and Chang et al. in
two patents described below.
A batch crystallization apparatus for liquid purification
operates in a cycle consisting of the following phases
repeated in sequence, as set forth in U.S. Pat. No. 4,799,945,
issued Jan. 24, 1989 to Chang:
(a) fill a chamber with the liquid to be purified;
(b) remove heat from the chamber to form frozen liquid
of the desired thickness;
(c) drain the unfrozen liquid containing the concentrated
impurities from the chamber;
(d) melt the purified frozen liquid and drain it into a
storage tank or vessel for use.
Because the solubility of impurities is much higher in the
liquid phase than in the solid phase, the impurities become
relatively concentrated in the unfrozen liquid and the ice
sheet is relatively purified. The frozen liquid can be melted
by electric heating, or by applying the heat removed from an
opposing out-of-phase chamber.
The capacity of a batch crystallizer is set forth in U.S. Pat.
No. 4,954,151, issued Sep. 4, 1990 to Chang, Conlon and
Hendricks. The capacity is related to a number of param-
eters, including the Conversion, defined as the volume ratio
of purified liquid to initial liquid. It is generally desirable to
maximize the Conversion to minimize both energy con-
sumption and the volume of unfrozen liquid. The latter is
particularly important when batch crystallization is used for
water treatment in drought prone locations. Chang et al. also
described optimal ice thickness so as to minimize the effect
of re-contamination of the crystalline phase by an adherent
liquid film containing impurities. They also suggested the
use of a thin, thermally insulating layer on the heat transfer
surface to promote more uniform ice growth by increasing
2
the thermal resistance normal to the heat transfer surface
relative to along the heat transfer surface.
The solubility of the dissolved impurities also imposes
limits on Conversion. As the impurity concentrations in the
5 liquid phase increase, one or more of the impurities may
exceed their solubility limits and precipitate, in the case of
dissolved solids, or nucleate a gas bubble, in the case of
dissolved gases. Because the concentration is highest in the
boundary layer at the ice-water interface, that is where they
10 come out of solution. Waters containing carbonates are
particularly prone to have gas bubbles come out of solution,
apparently due to carbon dioxide evolution from changes in
pH.
Kuo observed that particles tend to settle in the depres-
sions formed around air bubbles, and that when air bubbles
15 break free, momentary rapid freezing occurs. With signifi-
cant quantities of carbonate ions in the water, however, I
have observed the crystalline structure is disrupted, and a
opaque matrix of frozen water and bubbles is formed. I
believe that the bubbles restrict the liquid flow paths that
20 would otherwise allow the impurities to diffuse away from
the ice interface. The removal of air bubbles is important in
the production of clear ice by ice making machinery. In the
Vogt tube-ice machine, ice grows inwardly from the walls of
a tube, and water is circulated within the tube of ice to carry
25 away impurities and air bubbles. In triple-point crystallizers
the water is deaerated by the vacuum system, so that bubbles
are not a significant problem.
The prior patents teach the importance of optimizing the
capacity of the refrigeration system relative to the heat
30 transfer surface area to maximize the quantity of ice pro-
duced in a given size apparatus. However, these prior
teachings did not recognize the importance of heat transfer
uniformity on the quality of the ice produced. I have found
that the separation efficiency is determined primarily by the
35 ice crystal growth habit, which is strongly influenced by the
degree of uniformity of the heat transfer surface.
According to Glen, common ice crystals are hexagonal
and grow either parallel (a-axis) or perpendicular (c-axis) to
the plane of the hexagon. Growth is more rapid in the
40 direction of the six a-axes than in the c-direction. According
to Jellinek, when ice forms on a heat exchanger surface, as
in a DFC system, the ice layer spreads rapidly over the
surface, and the morphology and growth rate appear to be
influenced by the polar nature of the substrate. Dendritic
45 growth rapidly skins over the surface and then the crystal
grows perpendicular to the free surface in columns parallel
to the c-axis. Later, some of the crystals are wedged out and
the columnar growth continues, but with the c-axis parallel
to the ice- water interface. The columnar ice is most likely
50 polycrystalline, with parallel grains growing into the liquid.
As the ice thickens, the concentration of contaminants
starts to increase in the boundary layer adjacent to the ice.
Since the freezing temperature decreases as the molality of
contamination increases, the freezing point is lower in the
55 boundary layer than in the bulk liquid. Ice needles can result
from an instability called “constitutional supercooling,”
when an ice crystal penetrates the boundary layer. The tip of
this crystal would be highly supercooled compared to the
relatively pure bulk liquid, so it could grow rapidly.
60 In Directional Freeze Crystallization, cooling :occurs
through the ice layer by conduction, so there are both
concentration and thermal boundary layers. Beyond the
thermal boundary layer the liquid would not be sufficiently
supercooled to support the growth of ice needles. This
65 minimizes the likelihood of constitutional supercooling pro-
ducing ice needles growing perpendicular to the heat trans-
fer surface.
5 , 555,747
3
DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a graph relating contaminant separation
efficiency to a Growth Habit Index.
FIGS. 2A and 2B show a cross-section of ice and con- 5
taminant in a boundary layer, and the effect of bulk liquid
molality on freezing point depression.
FIG. 3 shows a schematic diagram of a Directionally
Freeze Crystallization system employing a horizontally ori-
ented flat freezing surface. to
FIG. 4 shows a schematic diagram of a Directional Freeze
Crysallization system employing a vertically oriented cylin-
drical freezing surface with radially inward ice growth.
FIG. 5 shows a schematic diagram of a Directional Freeze
Crysallization system employing a vertically oriented cylin- 15
drical freezing surface with radially outward ice growth.
FIG. 6 shows a graph relating the Growth Habit Index to
the fraction of liquid that is frozen for different shaped
freezing surfaces. 20
SUMMARY OF THE INVENTION
I have found two types of crystal growth habit — normal
and parallel to the heat transfer surface. The normal mode of
crystal growth exhibits efficient contaminant removal, but 25
the parallel mode is inefficient. I have also found that the
growth habit will change from the efficient normal mode to
the inefficient parallel mode under certain conditions which
are correlated by a Growth Habit Index, G, having units of
(cm 2 C~ 2 ), defined as, 30
G=[D]*[A7/dr/dt-dT/dz],
where
D is the diffusion coefficient, cm 2 s" 1 , of the contaminant;
35
ATj is the freezing point depression, °C., of the unfrozen
liquid;
dr/dt is the ice growth rate, cm s” 1 ;
dT/dz is the heat transfer surface temperature gradient,
°C. cm" 1 . 40
This Growth Habit Index includes four important factors
that influence separation by Directional Freeze Crystalliza-
tion. The first, D, is a property of the contaminant, and
determines the rate at which impurities diffuse away from
the ice front. The second, ATy, is a function of the solution, 45
being determined by the molality of the contaminant. The
third, dr/dt, is the rate at which the ice front advances into
the bulk liquid and was the subject of the patent by Chang
et al., referred to earlier, where the compressor capacity, heat
transfer surface area and refrigerant evaporation temperature 50
were optimized. The fourth factor, dT/dz, is a function of the
heat exchanger on which the ice is grown. In designing a
DFC system, the first two factors are properties of the fluid
to be processed, while the latter two are engineering design
factors. 55
I have found that when the Growth Habit Index G is
sufficiently large, the ice growth habit was entirely normal to
the heat transfer surface, and efficient contaminant separa-
tion was achieved (typically more than 95% contaminant
removal). When the Growth Habit Index G was relatively 60
small, I always found parallel ice growth with correspond-
ingly low separation efficiencies (typically less than 50%).
FIG. 1 shows a range of crystal growth experiments plotted
in terms of the Growth Habit Index on the abscissa and the
separation efficiency on the ordinate. This shows the tran- 65
sition from the normal ice growth habit to parallel growth
occurs for values of G around 20 cm 2 °C" 2 .
4
A possible explanation for the dependence of this growth
habit transition on ATy can be seen by reference to FIG. 2.
FIGS. 2 a and 2b show ice growth into 100 and 1000 micro
molal solutions, respectively, with left and fight icessections.
The left hand ice section has grown one boundary layer
thickness, while the fight hand section has grown two
boundary layer thicknesses, causing a difference in contami-
nant concentration in the boundary layers next to the two
sections. Assuming that 90% of contaminants are rejected
from the ice, and neglecting diffusion of contaminants into
the bulk liquid, a mass balance shows that the freezing point
is lower in the fight hand boundary layer. This freezing point
gradient in the boundary layer is believed to instigate the
growth of ice ridges parallel to the heat transfer surface.
Moreover, the magnitude of the freezing point gradient
increases in proportion to bulk liquid molality, which
explains the dependence on contaminant concentration.
It is important to understand that the value G is not
constant during the batch freeze crystallization process.
First, as the ice grows, the contaminant concentration in the
unfrozen liquid increases, increasing the freezing point
depression ATyin the bulk liquid and in the boundary layer.
Second, the ice growth rate may change, depending on the
geometry of the heat exchanger surface. As pointed out by
Chang et al., the volumetric ice growth rate (that is, cu.cm,
per second) is constant until the ice grows to a critical
thickness sufficient to reduce the rate of heat flow below the
compressor capacity. This critical ice thickness is fixed by
the relation between compressor capacity, heat exchanger
surface area and evaporation temperature. However, the
linear growth rate of ice into the contaminated water, which
is the parameter of interest for the Growth Habit Index, may
vary with ice thickness depending on the radius of curvature
of the ice.
DESCRIPTION OF THE INVENTION
An embodiment of the inventive system is described with
reference to FIG. 3, a schematic flow diagram showing a
sectional view of a horizontally oriented flat plate Direc-
tional Freeze Crystallization system. A conventional vapor
compression refrigeration system is comprised of a Refrig-
erant vapor compressor 110, Refrigerant condenser 112,
Refrigerant expansion device 114, and Refrigerant evapo-
rator 116. A Condenser cooling fan 118 or other suitable
means may be employed to reject heat from the system. The
Refrigerant evaporator circuit 116 is thermally coupled to
the upper side of Freeze plate 122 in a manner chosen to
minimize the temperature gradient along said Freeze Plate.
Around the periphery of the Freeze plate 122 is an Air space
area 148 to which is connected a level sensor 154. A Freeze
tray 120 has an inclined Lower inside surface 150 at the
lowest point of which a Drain port 126 is connected a Liquid
manifold 128. Attached to the Liquid manifold 128 are a
Feed valve 130, a Concentrate valve 132, and a Product
valve 134. Heating means 138 and Temperature sensor 152
are disposed on the bottom of the Freeze tray 120. Controller
144 Sequences the operation of the Refrigerant vapor com-
pressor 110, Condenser cooling fan 118, Feed valve 130,
Concentrate valve 132, Product valve 134, and Heating
means 138 in response to signals from Level sensor 154 and
Temperature sensor 152. Directional Freeze Crystallization
occurs in a batch process comprised of a series of steps. The
first step is to open the Feed valve 130 to admit contami-
nated liquid. When the contaminated liquid is contact with
the Freeze Plate 122 as indicated by level sensor 154, the
second step begins as the Feed valve 130 is closed and the
5,555,747
5
Refrigerant vapor compressor 110 and Condenser cooling
fan 118 are turned on. The third step begins when the desired
quantity of Purified ice 142 has formed on Freeze Plate 122,
as indicated by Temperature sensor 152, Level sensor 154,
or a timer in Controller 144, and the Refrigerant vapor
compressor 110 and Condenser cooling fan 118 are turned
off, and the Concentrate valve 132 is opened to discard the
Concentrated unfrozen liquid 140. After the Concentrated
unfrozen liquid 140 has drained, as indicated by a timer in
Controller 144, the fourth step begins by turning on the
Heating means 138, closing the Concentrate valve 132 and
opening the Product valve 134. When all of the Purified ice
142 has melted, as indicated by Temperature Sensor 152 or
a timer in Controller 144, the Heating means 138 is turned
off, the Product valve 134 is closed, and step 1 is repeated.
An alternate embodiment of the inventive System is
described with reference to FIG. 4, a schematic flow dia-
gram showing a sectional view of a vertically oriented
cylindrical Directional Freeze Crystallization system with
ice growing radially inward. A conventional vapor compres-
sion refrigeration system is comprised of a Refrigerant vapor
compressor 210, Refrigerant condenser 212, Refrigerant
expansion device 214, and Refrigerant evaporator 216. A
Condenser cooling fan 218 or other suitable means may be
employed to reject heat from the system. The Refrigerant
evaporator circuit 216 is thermally coupled to the walls of
the Cylindrical heat exchanger 222 in a manner chosen to
minimize the temperature gradient along said Cylindrical
heat exchanger. At the bottom of the Cylindrical heat
exchanger 222 is a Lower cap 220 at the center of which a
Drain port 226 is connected to a Liquid manifold 228.
Attached to the Liquid manifold 228 are a Feed valve 230,
a Concentrate valve 232, and a Product valve 234. Con-
nected to the Product valve 230 by Fluid conduit means 246
are storage tank 236, Circulating water pump 238, Heating
means 240, and Circulating water valve 242, which is
connected by Fluid conduit means 246 to the Upper Port 250
in the Upper cap 248 at the top of the Cylindrical heat
exchanger 222. Temperature sensor 252 is disposed on the
Liquid manifold 228 and Level sensor 254 is connected by
Fluid conduit means 246 to the Upper Port 250. Controller
244 sequences the operation of the Refrigerant vapor com-
pressor 210, Condenser cooling fan 218, Feed valve 230,
Concentrate valve 232, Product valve 234, Circulating water
pump 238, Circulating water valve 242, and Heating means
240 in response to signals from Level sensor 254 and
Temperature sensor 252. Directional Freeze Crystallization
occurs in a batch process comprised of a series of steps. The
first step is to open the Feed valve 230 to admit contami-
nated liquid and displace air through the Upper port 250 to
the atmosphere through Vent port 256. When the contami-
nated liquid is contact with the entire surface of Cylindrical
heat exchanger 222 as indicated by level sensor 254, the
second step begins as the Feed valve 230 is closed and the
Refrigerant vapor compressor 210 and Condenser cooling
fan 218 are turned on. The third step begins when the desired
quantity of Purified ice 224 has formed on Cylindrical heat
exchanger 222, as indicated by displacement of water due to
volumetric expansion of said Purified ice 224 sensed by
Level sensor 254, or a timer in Controller 244, and the
Refrigerant vapor compressor 210 and Condenser cooling
fan 218 are turned off, and the Concentrate valve 232 is
opened to discard the Concentrated unfrozen liquid 225.
After the Concentrated unfrozen liquid 225 has drained, as
indicated by a timer in Controller 244, the fourth step begins
by closing the Concentrate valve 232, opening the Product
valve 234 and Circulating water valve 242, and turning on
6
the Circulating water pump 238 and Heating means 240 to
circulate previously melted purified water contained in Stor-
age tank 236 over the Purified ice 224. When all of the
Purified ice 225 has melted, as indicated by Temperature
5 Sensor 252 or a timer in Controller 244, the Circulating
water pump 238 and Heating means 240 are turned off. After
an interval of time sufficient to allow melted purified water
to drain into the Storage tank 236, the Product valve 234 and
Circulating water valve 242 are closed, and step 1 is
10 repeated.
Another embodiment of the inventive system is described
with reference to FIG. 5, a schematic flow diagram showing
a sectional view of a vertically oriented cylindrical Direc-
tional Freeze Crystallization system with ice growing radi-
15 ally outward. A conventional vapor compression refrigera-
tion system is comprised of a Refrigerant vapor compressor
510, Refrigerant condenser 512, Refrigerant expansion
device 514, and Refrigerant evaporator 516. A Condenser
cooling fan 518 or other suitable means may be employed to
20 reject heat from the system. The Refrigerant evaporator
circuit 516 is thermally coupled to the walls of the Cylin-
drical heat exchanger 522 in a manner chosen to minimize
the temperature gradient along said Cylindrical heat
exchanger. The Cylindrical heat exchanger 522 is concen-
25 trically mounted in Cylindrical vessel 521 having a Lower
cap 520 and an Upper cap, 548. Connected to the Drain port
526 in the Lower cap 520 is a Liquid manifold 528 to which
are attached a Feed valve 530, a Concentrate valve 532, and
a Product valve 534. Connected to the Product valve 530 by
30 Fluid conduit means 546 are storage tank 536, Circulating
water pump 538, Heating means 540, and Circulating water
valve 542, which is connected by Fluid conduit means 546
to the Upper Port 550 in the Upper cap 548. Temperature
sensor 552 is disposed on the Liquid manifold 528. Pressure
35 sensor 554 is connected to Liquid accumulator 560 which in
turn is connected by Fluid conduit means 546 to the Upper
Port 550. Air accumulator 562 is connected by Fluid conduit
means 546 through Air valve 564 to the Upper Port 550.
Controller 544 sequences the operation of the Refrigerant
40 vapor compressor 510, Condenser cooling fan 518, Feed
valve 530, Concentrate valve 532, Product valve 534, Cir-
culating water pump 538, Circulating water valve 542, Air
valve 564 and Heating means 540 in response to signals
from Pressure sensor 554 and Temperature sensor 552.
45 Directional Freeze Crystallization occurs in a batch process
comprised of a series of steps. The first step is to open the
Feed valve 530 and the Air valve 564 to admit contaminated
liquid and displace air through the Upper port 550 to the Air
accumulator 562, causing the displaced air to be com-
50 pressed. When the contaminated liquid is contact with the
entire surface of Cylindrical heat exchanger 522 as indicated
by Pressure sensor 554, the second step begins as the Feed
valve 530 and Air valve 564 are closed and the Refrigerant
vapor compressor 510 and Condenser cooling fan 518 are
55 turned on. During the second step Purified ice 524 grows
outward from the Cylindrical heat exchanger 522 causing
liquid to be displaced into Water accumulator 560. The
displacement of water into water accumulator 560 is resisted
by resilient bias means, such as a mechanical spring or a
60 trapped volume of air, causing pressure to increase. This
increased water pressure serves to keep gas bubbles in
solution. The third step begins when the desired quantity of
Purified ice 524 has formed, as indicated by Pressure sensor
554 and the Refrigerant vapor compressor 510 and Con-
65 denser cooling fan 518 are turned off. The Concentrate valve
532 is opened to allow liquid to drain from Water accumu-
lator 560 and then the Air valve 564 is opened to allow
5 , 555,747
7
compressed air from Air accumulator 562 to displace the
Concentrated unfrozen liquid 525. After the Concentrated
unfrozen liquid 525 has drained, as indicated by Pressure
sensor 554, the fourth step begins by closing the Concentrate
valve 532 and Air valve 564, opening the Product valve 534
and Circulating water valve 542, and turning on the Circu-
lating water pump 538 and Heating means 540. Previously
melted purified water contained in Storage tank 536 is
heated and circulated through Cylindrical vessel 521 to melt
Purified ice 524. When all of the Purified ice 524 has melted,
as indicated by Temperature Sensor 552, the Circulating
water pump 538 and Heating means 540 are turned off. After
an interval of time sufficient to allow melted purified water
to drain into the Storage tank 536, the Product valve 534 and
Circulating water Valve 542 are closed, and step 1 is
repeated.
FIG. 6 illustrates the use of the Growth Habit Index in the
design of a Directional Freeze Crystallization System. Plot-
ted on FIG. 6 are the calculated Growth Habit indices for
three alternate heat exchangers as the ice fraction is
increased. Curve 610 shows the Growth Habit Index for the
fiat plate heat exchanger; curve 620 shows the Growth Habit
Index for a cylindrical heat exchanger with ice growing
radially inward; and curve 630 shows the Growth Habit
Index for a cylindrical heat exchanger with ice growing
radially outward. Also shown is curve 600, at a constant
Growth Habit Index of 25 cm 2 /C 2 , which might be consid-
ered a lower limit to avoid parallel ice growth.
The freezing point depression, AT y , is identical for each
heat exchanger, and is only a function of the Conversion
fraction. Each Directional Freeze Crystallizer was assumed
to be filled initially with an sodium chloride solution, with
a diffusion coefficient of 1.5xl0 -5 cm 2 /s. The heat transfer
surface temperature gradient was assumed equal for each
heat exchanger at 0.05 °C./cm. Accordingly, the growth rate
was calculated from the heat removal capacity, which was
assumed the same for each heat exchanger. The surface areas
of heat exchangers for curves 610 and 620 and 630 were in
the proportion 4:4:1.
For radial outward growth, curve 630, the Growth Habit
Index is much smaller for low Conversions, increases to a
maximum as the ice growth rate slows, and as the radius of
curvature of the ice increases, approaches the curve 610 for
the flat plate. For radial inward growth, curve 620 always
shows a lower Growth Habit than the flat plate heat
exchanger curve 610 and, due to the combined effects of
higher contaminant concentration and higher ice front veloc-
ity, ultimately shows a lower Growth Habit Index than radial
outward growth curve 630. Accordingly, it may be desirable
to grow crystals radially outward from a cylindrical heat
exchanger, so the decreasing ice growth rate compensates
for increasing contaminant concentration. Moreover, radial
outward growth can be done with one-fourth of the heat
transfer surface area required of the other embodiments,
resulting in substantial cost savings.
It is important to note that the design engineer can
trade-off tighter manufacturing tolerances (that is reduced
temperature gradient) for more heat transfer area (that is
slower growth rate) to achieve the required Growth Habit
Index. To accommodate variations in water quality that
might be encountered in different locations, the system can
be designed for the worst case water quality (that is, the most
contaminated) likely to be encountered. Alternately, the
system can be designed for a typical water quality, and the
fraction of water frozen can be adjusted in the field, so that
ATy does not become large enough to trigger the growth
habit transition.
8
Therefore the present invention will provide a method and
means for efficiently separating contaminants from water.
The present invention avoids unfavorable crystal growth
habits that trap impurities and reduce separation efficiency.
5 Moreover, the present invention provides means for design-
ing a practical batch freeze crystallization system by speci-
fying the allowable heat transfer surface temperature gradi-
ent in terms of the properties of the solute and solvent being
treated, and the refrigeration system being used to remove
to heat from the crystallization system.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a means of
15 increasing the rejection of impurities from contaminated
water by controlling the growth of the crystalline phase.
Another object of the invention is reduce the cost of a
Directional Freeze Crystallization system by reducing the
amount of heat transfer surface area.
20 I claim:
1. A Directional Freeze Crystallization System compris-
ing:
a) a crystallizer means consisting of a heat exchanger,
b) a vessel for containing liquid to be purified
25 c) a cooling means for conveying a heat transfer fluid to
said heat exchanger, said heat transfer fluid being at a
temperature less than the freezing point of said liquid to
be purified
30 d) a filling means for conveying said liquid to be purified
into said vessel,
e) a draining means for conveying unfrozen liquid from
said vessel,
f) a heating means for melting ice frozen on said heat
35 exchanger,
g) a transfer means for conveying melted ice from said
vessel
h) an accumulator connected by a fluid conduit to said
vessel,
40
i) a quantity sensor to determine the volume of unfrozen
liquid in said accumulator
j) a state sensor to determine the absence of frozen liquid
in said vessel
45 j) control means for repeatedly sequentially actuating said
filling means, cooling means, draining means, heating
means and transfer means responsive to said quantity
sensor and said state sensor to fill said vessel with
liquid to be purified, freeze a fraction of said liquid to
50 be purified adjacent to said heat exchanger, drain the
unfrozen fraction of said liquid to be purified, melt the
frozen fraction of said liquid to be purified, and convey
the melted fraction from said vessel.
2. The Directional Freeze Crystallization system of claim
55 1 where the heat exchanger is a flat plate that comprises the
upper boundary of said vessel.
3. The Directional Freeze Crystallization, system of claim
1 where the heat exchanger is a cylindrical surface that
comprises the vertical boundaries of said vessel.
60 4. The Directional Freeze Crystallization process of claim
1 where the quantity sensor is a pressure sensor.
5. The Directional Freeze Crystallization system of claim
1 where the heat exchanger surface temperature is substan-
tially uniform.
65 6. The Directional Freeze Crystallization system of claim
1 where the value of the number formed by dividing the
diffusion coefficient of the unfrozen liquid by the product of
5,555,747
9
the freezing point depression of the unfrozen liquid and the
ice growth rate and the heat exchanger surface temperature
gradient is more than 20 cm 2 /°C. 2 .
7. The Directional Freeze Crystallization system of claim
1 where the control means is adjustable to vary the unfrozen 5
liquid fraction so the value of the number formed by
dividing the diffusion coefficient of the unfrozen liquid by
the product of the freezing point depression of the unfrozen
liquid and the ice growth rate and the heat exchanger surface
temperature gradient is substantially more than 20 cm 2 /°C. 2 10
8. The Directional Freeze Crystallization system of claim
1 where the state sensor is a temperature sensor.
9 . The Directional Freeze Crystallization system of claim
1 where the heating means is conveyed by circulating
purified liquid over the ice. 15
10 . The Directional Freeze Crystallization system of
claim 1 where the accumulator is resiliently biased to effect
an increase in pressure due to volumetric expansion of ice.
11 . A method for liquid purification by Directional Freeze
Crystallization comprising the steps: 20
a) opening a feed valve to admit liquid to be purified into
a vessel and into a water accumulator connected to said
vessel, and opening an air valve to expel air from said
vessel;
b) when the desired volume of said liquid :,to be purified 25
is within said vessel, closing said feed valve and said
air valve and supplying cold refrigerant to the coolant
side of said heat exchanger;
10
c) when the desired volume of ice has formed adjacent to
said heat exchanger, opening a concentrate valve to
drain unfrozen liquid from said water accumulator;
d) after a predetermined time delay, opening said air valve
to drain unfrozen liquid from said vessel, and after a
second predetermined time delay closing said air valve
and said concentrate valve;
e) opening a product valve and a circulating water valve,
and starting a circulating pump and a heater to circulate
purified liquid from a storage tank through said heater
into said vessel to melt the ice;
f) when the ice has melted, turning off the pump and
heater to allow melted ice to drain into said storage
tank;
g) after a third predetermined time delay, closing said
product valve and said circulating water valve.
12 . The method of claim 11 where a pressure sensor in
communication with the vessel indicates the desired volume
of liquid in step b and the desired volume of ice in step c.
13 . The method of claim 11 where the air valve is
connected to an air accumulator for pressurizing the vessel
in step a and to assist in expelling unfrozen liquid from the
vessel in step d.
14 . The method of claim 11 where a temperature sensor is
used in step f to determine that ice has melted.
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