I
R. C. HIMES,
S. E. MILLER, W.
H. MINK, and
H. L. GOERING
Battelle Memorial Institute, 505 King Ave., Columbus 1 , Ohio
Zone Freezing in Demineralizing Saline Waters The basic information on freezing developed will not only evaluate zone-freezing demineralization, but be of value to all researchers working toward demineralization b y freezing processes, or indeed toward any delicate separations from aqueous systems
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NUMBER of investigators, working on the problem of demineralizing saline water by freezing, have agreed that ice crystallites form, essentially free of salt. However, salt deposits are entrapped among such crystallites and brine adheres to their surfaces. As a solution to this problem, the zonerefining process appears promising. In this method, widely applied in the past few years in the purification of high-melting metals and intermetallic compounds, one end of a long ingot, usually about one tenth of its length, is melted. This molten zone is made to traverse the ingot. Length of the molten zone remains constant, as melting and freezing at the leading and trailing edges. respectively, proceed at the same rate. Solutes are segregated at the freezing interface because of differences in solubility in the liquid and solid phases of the host material; the impurity tends to become more concentrated in one phase) and if the zonerefining operation is conducted near equilibrium, the impurity collects a t one end of the ingot.
Major objective of the present research program is to establish, by means of a detailed engineering analysis: 0
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limits of cost reduction that might be possible on the basis of current knowledge of the zonefreezing process areas in which additional research may affect significant cost reductions
Construction of an experimental engineering model of a continuous zone-purification system has not been started.
part of a closed circulatory system together with centrifugal pump 1 and and another copper coil in reservoir 2. This closed system is filled with ethyl alcohol, which serves as a heat transfer medium. A copper-constantan thermocouple, immersed in the second reservoir, actuates a relay which regulates the temperature in this reservoir by controlling the action of the centrifugal pump. A second closed circulating system consists of a second coil in reservoir 2, a second centrifugal pump, and a cooling jacket in the freezing compartment. This second pump is run continuously. the temperature of the freezing compartment being controlled solely by the reservoir temperature and external conditions. The refrigerating system provided control to within 0.2' C. over a range from room temperature to about -6OOC. The freezing compartment shown, designed for direct visual observations during freezing, consists basically of
Experimental The basic experimental program has been designed to collect data on the dependence of salt segregation efficiencies upon freezing conditions. Such data are obtained satisfactorily by either zone melting or directional freezing under carefully controlled conditions. The experiments were generally conducted by drawing test solutions in boats or tubes through carefully thermostated freezing chambers and melting zones, maintaining a sharp interface by controlled temperature gradients. T o provide the required carefully controlled, low temperature region, special laboratory equipment was designed and constructed. The refrigerant, a dry ice-trichloroethylene mixture, is kept in reservoir 1. Immersed in this bath is a copper coil which forms
i-
Fox boro controller
Keser compartment
Special laboratory equipment maintains the carefully controlled low temperature region VOL. 51, NO. 11
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Triple-tube freezing chamber, showing characteristic types of salt deposit Right.
Upper portion of ingot shows deposit (dome shaped) laid down with no agitation. Lower portion clear except for bubbles was grown under same conditions but with ultrasonic agitation applied. Ice ingot i s grown b y pulling sample holder slowly up into cold zone
three concentric glass cylinders, appropriately mounted and gasketed. The interspace between the two larger tubes is evacuated and continuously pumped to provide insulation, thus preventing deposition of view-obscuring frost. Thermostated coolant is circulated through the interspace between the middle and smallest tubes. The smallest tube is the solution container and is free to move along its axis relative to the other tubes. I n a freezing experiment the remainder of the system is held stationary while the smallest tube is drawn through the chamber, and the fluid, the interface, and the ice ingot may be continuously observed. Clear demarcations among solution, clear ice, and salt deposits could be observed by adding small quantities of sodium fluorescein. I n solution, thi8 dye emits a green fluorescence, but when trapped among the ice crystals, it has a characteristic orange-red color. Both simple sodium chloride solutions and actual sea water (from Battelle’s North Florida Research Station, Daytona Beach, Fla.) were treated. Growth of ingots was observed, crystallite relationships were examined microscopically, and average salt concentrations for small segments of each ingot were measured by conductivity analyses of re-
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melted portions of the ice ingots. The photograph shows a typical ice ingot deposited in the equipment described. T h e water-ice interface may be observed near the bottom of the cell.
Discussion Qualitative results of a large number of experiments may be summarized through several generalizations. Sodium chloride, sodium fluorescein, and natural sea salt exhibit identical segregation characteristics. Clear ice laid down is essentially pure and may be deposited under controlled conditions, offering minimum disturbance of the crystallization process and resulting in growth of large, oriented crystallites. Rates at which clear ice may be laid down decrease with increasing salt concentration, and increase with degree of agitation. Cloudy portions of the ice represent locations in which salt deposits are entrapped among relatively fine crystallites. At least two distinct types of cloudy deposits are observed: one in which deposition is triggered suddenly over the entire solid-liquid interface and which gradually dissipates into clear ice, and one at the core of an ingot of otherwise clear ice. Cloudy
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ice is deposited when supercooled solutions are frozen, when natural crystallization rates are exceeded, and when developing crystallites encroach upon one another, disturbing their smooth propagation. These generalizations lead to several hypotheses on the causes of the cloudy deposits and possible means of their eradication. The first is that natural crystallization rate should be a function of solution concentration. Crystal-growing experiments were conducted by the Czochralski technique, in which a cold seed is dipped into a solution at its freezing point and gradually withdrawn as crystallization proceeds. At low crystallization rates and solution concentrations, sound crystals or ingots of clear ice, consisting of large crystallites extending the length of the ingot, could be pulled. However, as pulling rate and solution concentration are increased, an apparent limiting curve is crossed. I n the region beyond this curve it was not possible to pull clear ice; only cloudy ice consisting of fine crystallites and salt occlusions was grown. At sea water concentration the limiting curve apparently lies below 0.16 inch per hour. At 0.5 inch per hour the curve lies between 7 and 10 grams per liter, and a t 0.91 inch per hour (the highest rate at
D E M l NE R A L I Z IN0 S A L I N E WATERS regation efficiencies upon this shape. I n comparable experiments, ice grown with convex interfaces (as viewed from the solution) have shown segregation efficiencies 20 times as good as those grown with highly concave interfaces. Further investigation of this phenomenon and means of impoing de:irable interface shapes in large-scale, continuous operation would appear to be in order. 4 third hypothesis has been that constitutional supercooling and its occasional breakdown may be responsible for one characteristic type of salt deposition observed. Constitutional supercooling may occur at a freezing interface when removal of impurities, concentrated by segregation a t the interface, is slow relative to the rate of advance of the interface. The result may be a gross suppression of freezing point a t the interface with development of a region of lower concentration, slightly removed from the interface, in which the solution is actually a t a temperature below its freezing point. Breakdown of this condition, either by bridging through of an ice crystal into the supercooled region or by local removal of the more highly concentrated solution from the interface, would trigger rapid growth over the entire interface with resultant entrap-
which it was possible to grow ice ingots by the technique employed) it appears to lie somewhat above 1 gram per liter. Because rather poor segregation efficiencies are generally associated with cloudy ice deposits, single-stage freezing treatments will probably not produce potable water from sea water at useful rates, and a multiple-stage process will be required. A second hypothesis deals with the encroachment of crystallites upon one another during freezing. The large crystallites, which generally constitute the clear ice ingots prepared under favorable conditions, are oriented with the a-6 plane parallel to the tube axis, thus bringing the rapid growth direction parallel to the thermal gradient. Growth rate of these hexagonal crystals is independent of direction in the a-b plane. Therefore, the thermal geometry alone generally determines the shape of the water-ice interface, and the highly concave interfaces, normally developed in cylindrical tubes, result in encroachment of crystallites on one another as they approach the center of the ingot. The resultant breakdown leads to entrapment of dissolved salts. Experiments on modification of interface shape have indicated considerable dependence of seg-
ment such as that observed. Experiments involving continuous monitoring of temperature immediately in advance of the interface and of rate of progress of the interface during formation of the characteristic depozits have given results consistent with the hypothe>ized mechanism. If means of preventing or stabilizing the constitutional supercooling can be devised. freezing processes may be conducted with higher efficiencies and at higher rates. Even within the bounds of the qualitative aspects discussed above, some very encouraging segregations have been obtained in directional freezing and zone freezing experiments. Representative data are prejented in the table. Data from single-ctage freezing experiments indicate the segregations generally obtained in the various concentration ranges noted. The trends with increasing solution concentration and freezing rate are clearly indicated.
Representative Segregation Data from Freezing Experiments with Mechanical Agitation Salt Removed, & : Freezing Kate, In./Hr.__ 0 5 10 4 0
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XaC1 Solution,
G./L.
Sea water in
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Sin gle-St age Freezing
Brine out 0-20 20-30 30-70
99.9 97.5 82
95 270
...
90-60
...
...
Two-Stage Freezing
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Freezing Kate, In./Hr. 0.15 0.75 1 3 15 30 [sea water]
Air in [Sea-water :conversion in a vertical-tube plant was less expensive than in a spiral drum
... ...
99.9+ 99.9+
... ...
90.7 99.97
99.9+
... ... ...
T h e sketchy data thus far obtained in two-stage experiments (directional freeze followed by zone melt) appear to correlate well with the crystal-pulling experiments described above. Freezing rates near the limiting curve from the pulling experiments were chosen for the 3- and 30-gram-per-liter solutions, For the 15-gram-per-liter solution, a rate considerably above the curve was used. Freezing a t a rate well above the free crystallization rate has resulted in a marked depression of the segregation. T h e segregation obtained in two-stage treatment of sea water at 0.15 inch per hour is in line with the other data, but is especially significant in that it demonstrates the technical feasibility of preparing excellent water from sea water. Product water containing as little as VOL. 51, NO. 11
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A spiral drum plant was also studied for sea-water conversion
30 p.p.m. total salts has been obtained a t this low rate. Even a t 0.75 inch per hour, removals have been very good, giving product water below 10 p.p.m. Such results as this last are somewhat surprising in view of the other data: but they point up the need for further understanding of the process, which may open the way to employment of considerably higher freezing rates. Basic Methods for Scale-up
T o demonstrate economic feasibility, basic concepts must be developed for multiple-stage, continuously operating machines. From among seven processes conceived, two were selected for preliminary engineering evaluation. Vertical-Tube Process. The first, a vertical-tube process. might be carried out in a machine such as that illustrated. A number of vertical tubes pass through closely spaced. insulated. horizontal plates. The spaces between the plates are connected. through automatic valves. to both the compression and the expansion parts of a refrigeration system. It is thus possible by proper programming to make any space between plates a melting zone, a freezing zone, or simply a “dead” zone. The bottoms of the tubes are in a reservoir of purified water. Saline water is introduced near the tops of the tubes. By selective operation of the valves, freezing and melting zones are created and moved upward along the tubes. As a plug of ice forms a t the end of the tube in the purified water reservoir and moves upward, air is permitted to enter the tube beneath the plug. At the appropriate time, the air entrance is closed and the plug, followed by an air space and a remelted
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water zone, moves upward; freezing a t the top of the plug and melting a t the bottom proceed a t the same rate. The remelted water from one stage rests on the ice plug of the next. The process continues and results in a n upward movement of a volume of air and downward movement of an equal volume to water with each freezing stage. Near the top of the tube. the ice plug is melted, and saline water is introduced just above the surface of the next ice plug to wash out the brine and refill the tube with saline water. The brine overflows the tube and is discarded. Spiral Drum Process. A drum, constructed with a double wall. is employed. The walls are connected internally with a band positioned spirally so that the annular space formed comprises a single long spiral or coil. The unit in its simplest form may be considered identical to a coiled or spiral tube. The spiral tube rotates in a freezing bath. A plug of ice forms in the bottom of each loop and effectively closes the tube to passage of liquid. Thus, as the tube rotates, water is lifted by the plug and overflows into the next loop, where it eventually is positioned in the freezing zone. As the tube turns, the ice plug leaves the freezing zone and enters the melting zone to become water, which will, in turn, be lifted and will overflow into the next loop. This action continues until the water. after being successively frozen and melted a number of times, overflows from the last loop into the exit line as purified water. When the purified water flows out, it is replaced by air which, because of the rotation of the spiral tube, moves in a direction opposite to that of the purified water. At the other end of the spiral, saline water is fed into the
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tube part-way down the spiral, just ahead of the first ice plug, a t the appropriate time. This washes out the concentrated brine and refills the tube with fresh saline water. The ice plugs may be melted by one of two methods. The air surrounding the drum may be warmed to melt the ice plugs, or a warmed layer of liquid above the freezing liquid may supplythe heat necessary for melting the plugs. The two fluids could be separated by an insulated divider which would fit closely to the drum to prevent mixing. Preliminary cost estimates. made in accordance with procedures laid down by the Office of Saline Water of the United States Department of the Interior, indicate that the vertical-tube system would be much the less expensive of the two processes. I t is estimated that the cost of producing potable water in such a system a t a rate of 100,000 gallons per day would range between $2.50 and 57.00 per 1000 gallons. depending upon the number of stages required. Construction and operation of a small pilot model would be required to firm u p these estimates and to determine whether optimization of some parameters would make possible large-scale production a t lower costs. Acknowledgment
The authors extend appreciation to the Office of Saline Water, U. S. Department of the Interior, the sponsor of this research program. RECEIVED for review May 12, 1958 ACCEPTED July 15, 1959 Division of Water, Sewage, and Sanitation Chemistry, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
