System, crystal nonideal

Since the recovery of neutral Lrlle may be performed after crystallization, redissolution and recrystallization, the concentrations of impurities in the solution were reduced by an order of magnitude in an additional series of experiments. Figure 8 shows the results. Once again L-VaJ is relatively unimportant and Puu appears constant, but note that the data do not go through the origin. Moreover, close examination shows that Puu > 1 which means that purification by crystallization has not occurred. Figure 9 shows that the purification factor for L-Leu is not constant and, therefore, the system is nonideal. 

Experimental determination of the surface composition in nonideal systems, in which the gradients extend over several layers inwards the crystal is as difficult as the exact calculations. Therefore, one has to make again rather unpleasant assumptions. 

Salting-out crystallization operates through the addition of a nonsolvent to the magma in a crystallizer. The selection of the nonsolvent is based on the effect of the solvent on solubility, cost, properties that affect handling, interaction with product requirements, and ease of recovery. Adding a nonsolvent to the system increases the complexity of the process it increases the volume required for a given residence time and produces a highly nonideal mixture of solvent, nonsolvent, and solute. 

Many subjects of great importance to foods are not included. In order to keep the theory simple, the treatment is often restricted to gases and crystals and dilute, homogeneous, ideal solutions, whereas most foods are concentrated, inhomogeneous, highly nonideal systems. In particular, coverage of colloid and surface science is insufficient. 

Given the limited data base from which solubility correlations can be drawn, it is essential to measure the solubility directly for the system of interest during process development. Since process conditions often favor operation with high concentrations of solute, such systems are often thermodynamically nonideal. It is necessary to measure the solubility in the solvent system(s) of interest in order to optimize the yield and the purity. To accomplish the latter relies upon the ability to measure the solubility of the key impurities as well as the product of interest. This requires the availability of both the key impurities and product however, the impurities often are not available as isolated solids. In that case, the solubility of impurities must be deduced from the purity profile of mother liquors taken from crystallizations of the actual process stream. It is often simplest and always fastest to measure the solubility and carry out crystallizations in a single-solvent system. However, working in multiple-solvent systems increases the likelihood of improving the yield, the separation factor, and the prospects of observing more of the possible crystal forms that may exist. 

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