538 Fermentation and Biochemical Engineering Handbook in the solution. This requires the proper orientation of several molecules at the moment of a random collision. Since the number of possible orientations increases with ing complexity of the molecule, considerably higher degrees of supersaturation can be obtained for solutions of chemicals with complex molecules 2.2 Formation of a Supersaturated Solution If a solution is to have only a slight degree of supersaturation, then a cyclic system in which large quantities ofliquor are supersaturated uniformly is required. The solution must then be brought back to saturation before feed liquor is allowed to enter the system and the mixture is again supersaturated in the next cycle The removal of the metastable supersaturation is a slow process. A large amount of crystal surface is required to allow for the large number of random collisions necessary to remove the supersaturation generated during the cycle. The proper orientation of both the molecules in solution and the molecule on the crystal surface is required for deposition, and the increased complexity of the molecule increases the number of collisions required for proper orientation If the supersaturation generated during the cycle is not completely removed, the level of supersaturation attained during the following cycle is increased. This increase from cycle to cycle will continue until the supersatu ration level of the solution exceeds the metastable region and enters the labile region,where spontaneous nucleation occurs. The occurrence of spontane ous nucleation means loss of control of crystal size Supersaturation is clearly the most important single consideration any crystallization process. By giving proper attention to the degree of supersaturation generated during each cycle and its proper release during the designstage, half the battle will be won. Supersaturation should be controlled by making certain only small changes in temperature and composition occur in the mass of mother liquor 2.3 Appearance of Crystalline nuclei Usually the crystallization equipment is charged with a clear feed solution. As this solution is saturated, it is important to control the increase in supersaturation as the labile region is approached. This is important since the formation of an excessive number of nuclei will cause a continuous crystallizer system to have an extremely long period before desired crystal538 Fermentation and Biochemical Engineering Handbook in the solution. This requires the proper orientation of several molecules at the moment of a random collision. Since the number of possible orientations increases with increasing complexity of the molecule, considerably higher degrees of supersaturation can be obtained for solutions of chemicals with complex molecules. 2.2 Formation of a Supersaturated Solution If a solution is to have only a slight degree of supersaturation, then a cyclic system in which large quantities of liquor are supersaturated uniformly is required. The solution must then be brought back to saturation before feed liquor is allowed to enter the system and the mixture is again supersaturated in the next cycle. The removal of the metastable supersaturation is a slow process. A large amount of crystal surface is required to allow for the large number of random collisions necessary to remove the supersaturation generated during the cycle. The proper orientation of both the molecules in solution and the molecule on the crystal surface is required for deposition, and the increased complexity of the molecule increases the number of collisions required for proper orientation. If the supersaturation generated during the cycle is not completely removed, the level of supersaturation attained during the following cycle is increased. This increase from cycle to cycle will continue until the supersatu￾ration level ofthe solution exceeds the metastable region and enters the labile region, where spontaneous nucleation occurs. The occurrence of spontane￾ous nucleation means loss of control of crystal size. Supersaturation is clearly the most important single consideration for any crystallization process. By giving proper attention to the degree of supersaturation generated during each cycle and its proper release during the design stage, half the battle will be won. Supersaturation should be controlled by making certain only small changes in temperature and composition occur in the mass of mother liquor. 2.3 Appearance of Crystalline Nuclei Usually the crystallization equipment is charged with a clear feed solution. As this solution is saturated, it is important to control the increase in supersaturation as the labile region is approached. This is important since the formation of an excessive number of nuclei will cause a continuous crystallizer system to have an extremely long period before desired crystal