Concentration of aqueous solutions is a common process in many industries. The technique widely used for the concentration of aqueous solutions is evaporation. However, an efficient evaporation process must be performed under boiling conditions at higher temperatures, which may result in loss and/or damage of certain volatile or heat-sensitive materials in the solutions. Instead of turning water into steam, freeze concentration is a process by crystallizing water into ice at temperatures below freezing point of the solutions, hence, making aqueous solutions concentrated. Purposes of freeze concentration may be obtaining concentrated solutions, purified water or both. Freeze concentration has many advantages over evaporation because of the lower process temperature. At a lower temperature flavors, aromas, nutrients and other valuable components in the original materials can be kept without loss. At a lower temperature, destruction of heat-sensitive substances can be avoided. Therefore, with freeze concentration extremely high quality products can be obtained. In addition, at a lower temperature any escape of volatile hazards from treated solutions or waste waters can be averted. For these reasons, freeze concentration is very attractive to food, beverage, dairy, biochemical, nutriceutical, pharmaceutical, chemical, and environmental industries. Although the total dewatering cost by freeze concentration have until now been higher than that by evaporation, its application has spread in areas where the high quality product is more important or the low temperature of process is necessary.
Theoretically, the latent heat of phase transition from water to ice (about 76 kcal/kg) is only one seventh of that from water to steam (about 540 kcal/kg). It means there is a great potential to save a lot of energy by use of freeze concentration of aqueous solutions. Potential commercial applications in many industries and economy of energy have been an attractive target for scientists and engineers. Efforts have been made for decades to use freeze concentration commercially. Freeze concentration contains, usually, steps of refrigeration of solution, crystallization of ice and separation of ice crystals from the mother liquid. To make a freeze concentration technique commercially feasible, aqueous solutions must be efficiently and economically refrigerated and large, uniform ice crystals, which are easily separated from the solution, must be efficiently obtained. However, ice crystallization is a complex phase transition and control of ice crystallization is very difficult due to the complexity. According to the generally accepted concept that the nucleation rate is proportional to the supercooling square while as growth rate is only linear to the supercooling, supercooling level must be increased in order to promote the efficiency, but this will cause higher nucleation rate resulting in plenty of small ice crystals. In contrast, efficiency will be decreased as lower supercooling level (e.g. &lt;0.4.degree. C.) is used to obtain large and sphere-like ice crystals (Shimoyamada, M. et al., 1997, Nippon Shokuhin Kagaku Kogaku Kaishi, 44(1), 59-61). Therefore, the major difficulties for freeze concentration are in two interrelated aspects. Firstly, it is hard to separate ice crystals from the concentrated solution because of small size of ice crystals obtained. Secondly, process of obtaining large ice crystals proceeds slowly and the efficiency is low.
Since 1970s, a freeze concentration process has been developed and further improved at Grenco Process Technology, B.V., The Netherlands and later Niro Process Technology, B.V., The Netherlands. The technology includes a scraped surface heat exchanger to freeze an aqueous solution and to generate fine ice crystals, a recrystallizer for ripening to form large ice crystals and a wash column to wash and separate ice crystals from the solution. The ripening crystallization (ice crystal growth) and the concentration of solution in the process are performed in multiple stages. Ice crystals are transferred from one stage to another. As claimed, multistage operation is beneficial to higher efficiency. These technologies are described in U.S. Pat. Nos. 4,332,140 to Thijssen et al., U.S. Pat. No. 4,332,599 to Thijssen et al., U.S. Pat. No. 4,338,109 to Thijssen et al., U.S. Pat. No. 4,459,144 to Van Pelt et al. and U.S. Pat. No. 4,557,741 to Van Pelt et al. They are now commercially used in certain areas (ice beer, juice, waste water, etc.). However, in addition to the higher cost for coolant/refrigerant, there are other problems with this existing freeze concentration process that limit its application due to higher costs. These problems include mainly: 1) high energy consumption for ice-scraping in the freezer to obtain sufficient ice nuclei, 2) slow crystal growth by means of ripening in the recrystallizer to produce large ice crystals for efficient separation. The high capital cost of scraped surface heat exchanger also limits the utilization of this freeze concentration process in areas such as food industry.
Ripening is a phenomenon that under certain conditions small ice crystals will melt and large ice crystals will grow when they are co-existing in an aqueous solution. This phenomenon has been discovered for a long time and can be explained by the classical thermodynamic theory based on the relations among surface energy, heat of crystallization and crystal size. The effect of the diameter of spherical isotropic crystals on the lowering of the equilibrium temperature can be interpreted by the Gibbs-Thomson equation. The lowering of the equilibrium temperature of a small crystal in a solution is proportional to the surface energy and the equilibrium temperature of that same solution for a very large crystal, but inversely proportional to its diameter, the density of the crystal and the heat of crystallization. According to this theory, compared to larger ice crystals, smaller ice crystals need higher supercooling level to survive. Therefore, at a certain bulk temperature smaller ice crystals might be melted and larger ice crystals would grow when they are co-existing. However, this transformation process can only occur, generally, in a certain range of small size of ice crystal, within ten micrometers (van der Malen, B. G. M. and van Pelt, W. H. J. M., 1983, in `Progress in Food Engineering--Solid Extraction, Isolation, Purification and Texturization`, C. Cantarelli and C. Peri (eds.), Foster-Varley, Switzlands, 413-434; Thijssen, H. A. C., 1974, in `Advances in Preconcentration and Dehydration of Foods`, Arnold Spicer (ed.), Applied Science Publishers Ltd., London, 115-149). In this size range, the transformation process can proceed very rapidly, even in a moment. But in the larger size range (e.g. larger than 100 micrometers), as happens in the ripening process of the existing freeze concentration technology, it was showed from the previous investigators that the rate of crystal growth was quite limited. In several hours, ice crystal size can reach a mean of only 0.3-0.4 mm (Smith, C. E. and Schwartzberg, H. G., 1985, Biotechnology progress, 1(2), 111-120). And the higher the concentration of solution in the ripening tank, the lower the growth rate of ice crystals. That is why the results were not ideal as efforts were made to apply this theory to the freeze concentration process. Usually, only lower driving force (0.01-0.05.degree. C.) can be used to realize smooth but slow ripening to avoid generation of new fine crystals.
To make ripening process be in progress smoothly, co-existence of small and large ice crystals is necessary under certain conditions. In the existing freeze concentration process, fine ice crystals (nuclei) are generated in a scraped surface heat exchanger by freezing aqueous solutions and scraping the ice layer formed on the heat exchange surface. The work is very energy-consumptive. The energy used for the scraping work and for the refrigeration to remove heat due to friction was estimated to account for two thirds of the total energy consumption (Schwartzberg, H. G., 1988, Potential improvements in food freeze concentration, Presentation at Session 73, AIChE National Meeting, Denver, Colo., August 21-24).
On the basis of scraped surface freezing exchanger and ripening crystallizer of Grenco/Niro process, techniques of freeze concentration were developed on different specific aqueous solutions such as dairy liquids (e.g. U.S. Pat. No. 4,959,234 to Ahmed et al.), coffee extracts (e.g. U.S. Pat. No. 5,736,182 to Jimenez et al.), waste waters (e.g. U.S. Pat. No. 5,443,733 to Mueller et al. and U.S. Pat. No. 5,558,778 to Janssen et al.), etc. In addition to the Grenco/Niro process, other techniques of freeze concentration were also developed. U.S. Pat. No. 4,666,484 to Shah et al. describes a multistage freeze concentrating process that uses screw concentrators and falling film freeze exchangers. Ice crystals are formed in the freeze exchangers and thick orange juices are obtained by use of this technique. The ice crystals are fine and with loose structure due to no further procedure for growth and perfection of the formed crystals to follow. Entrainment of solution in the fine ice crystals brings difficulty to recover the solute. More water is needed to wash the ice crystals so that it increases the load on the freeze exchangers, hence, the energy consumption. The suitability of freeze exchanger to only certain solutions also limits the application of this technique. Ghodsizadeh et al. developed a freeze concentration system and method (U.S. Pat. No. 4,830,645 or European Pat. No. 0360876) different from the Grenco/Niro process mainly in the recrystallization step. Gradient columns are used for growth of ice crystal, separation of ice crystals from the heavy liquor. In the gradient columns a porous bed of agglomerated ice crystals is formed and rising. Ice crystals are removed from the top of ice bed by scraping or cutting devices, with which additional energy is needed. The gradient columns have relatively complex structures and higher capital costs. In 1997, Shirai et al. (Japan Pat. No. 09299704) presented a technique of freeze concentration consisting of three crystallizers and a washing column. Initial ice crystals are obtained by freezing pure water and introduced as seeds into the third crystallizer, where the aqueous solution with higher concentration is present. The original aqueous solution to be concentrated is transported into the first crystallizer. Ice crystals and solutions are transferred from one crystallizer to another in a counter-current manner. Unlike the Grenco/Niro process, the growth of ice crystals and the concentration of solution rely on the jacket cooling of the crystallizers. In addition, heating is applied at the top of the washing column, which is similar to that in Grenco/Niro process, to melt small ice crystals and to promote the growth of large ice crystals in the column. Extra energy consumption for heating the slurry in the column, lower driving force for the ice crystal growth in the crystallizers and limitation of the cooling capacity of the crystallizers are drawbacks of this technology. All the above-mentioned techniques, except the Grenco/Niro process, have not been commercialized due to problems of one kind or another as above-analyzed.
Based on our experimental observations and research findings combined with the fundamentals on ice crystallization, we summarized the phase equilibrium characters, the ice crystallization behaviors, especially, the ice crystal transformation phenomena we discovered. From the angle of taking advantage of energy to a full extent, promoting efficiency and producing large ice crystals, we developed the new freeze concentration technique, in which all the characteristics and behaviors of ice crystallization beneficial to high efficiency and low energy consumption were taken into account and utilized appropriately in the process.
The present invention is on the basis of new principles and means different from those of the existing processes. To overcome the drawbacks of the existing freeze concentration processes, ice scraping and ice crystal ripening are avoided. In our new freeze concentration process the aqueous solution is refrigerated to a suitable, relatively high supercooling level without ice nucleation or without formation of ice layer on the heat exchanger surface and then instantaneous nucleation and catastrophic crystallization are induced for the solution in a nucleator to obtain fine ice crystals. Also importantly, in our new freeze concentration process ice crystal transformation and agglomeration, instead of ripening of ice crystals, proceed at very high rate under controlled endo/exothermal balance conditions to produce large, grain- or sphere-shaped ice crystals.