Freeze crystallization has been used to desalinate seawater, 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; PA1 (b) remove heat from the chamber to form frozen liquid of the desired thickness; PA1 (c) drain the unfrozen liquid containing the concentrated impurities from the chamber; PA1 (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 parameters, 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 consumption 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 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 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 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 depressions formed around air bubbles, and that when air bubbles break free, momentary rapid freezing occurs. With significant 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 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 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 transfer surface area to maximize the quantity of ice produced 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 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 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 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 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 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.
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 minimizes the likelihood of constitutional supercooling producing ice needles growing perpendicular to the heat transfer surface.
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