proper orientation of the molecules, rather than by film diffusion to the surface; the crystal growth rate approaches zero order with increasing beretained in the crystallizer for a sufficient amount oftime to allow it to grow to the desired size The growth type crystallizers maintain the crystals in a fluidized bed (thereby providing both agitation and size classification of the crystals). The supersaturated solution flows through the fluidized bed and releases the supersaturation to the crystal surface Not all crystals will remain in the crystallizer the calculated retention time. This is only a statistical average. Since there will be a range of growth times, there will be a distribution of crystal sizes. The more narrow the range of actual retention times, the more narrow the crystal size distribution 3.0 CRYSTALLIZATION EQUIPMENT The type of equipment to be used in a crystallization process depends primarily upon the solubility characteristic of the solute. Solutions from fermentation processes can be classified as follows 1. Chemicals where a change in solution temperature has little effect on the solubility. An example is he thyl enetetramine as shown in Fig e supersaturated solution is produced by evaporation of the solvent. The equipment needed here is called an evaporative crystal lizer(see Fig. 4) 2. Chemicals, e.g, fumaric acid, which show only a moder- ate increase in solubility with increasing temperature. a combination of evaporation and cooling may be used to produce the supersaturated solution. Depending upon the yield required, this operation may be carried out in either a vacuum cooling crystallizer or an evaporative crystal- 3. Chemicals, e.g, adipic acid which show a large increase in solubility with increasing temperature. Cooling the solution can be an effective way to produce the supersatu rated solution, although a combination ofevaporation and poling can also be employed In addition to the two types of crystallizers mentioned above, a cooling crystallizer may be used(see Fig. 6)Crystallization 541 proper orientation of the molecules, rather than by film diffusion to the surface; the crystal growth rate approaches zero order with increasing driving force. Since growth becomes a function of time only, the crystal must be retained in the crystallizer for a sufficient amount oftime to allow it togrow to the desired size. The growth type crystallizers maintain the crystals in a fluidized bed (thereby providing both agitation and size classification of the crystals). The supersaturated solution flows through the fluidized bed and releases the supersaturation to the crystal surface. Not all crystals will remain in the crystallizer the calculated retention time. This is only a statistical average. Since there will be a range of growth times, there will be a distribution of crystal sizes. The more narrow the range of actual retention times, the more narrow the crystal size distribution. 3.0 CRYSTALLIZATION EQUIPMENT The type of equipment to be used in a crystallization process depends primarily upon the solubility characteristic of the solute. Solutions from fermentation processes can be classified as follows: 1. Chemicals where a change in solution temperature has little effect on the solubility. An example is hexamethyl￾enetetramine as shown in Fig. 3. The supersaturated solution is produced by evaporation of the solvent. The equipment needed here is called an evaporative crystal￾lizer (see Fig. 4). 2. Chemicals, e.g., fumaric acid, which show only a moder￾ate increase in solubility with increasing temperature. A combination of evaporation and cooling may be used to produce the supersaturated solution. Depending upon the yield required, this operation may be carried out in either a vacuum cooling crystallizer or an evaporative crystal￾lizer (see Fig. 5). 3. Chemicals, e.g., adipic acid, which show a large increase in solubility with increasing temperature. Cooling the solution can be an effective way to produce the supersatu￾rated solution, although a combination of evaporation and cooling can also be employed. In addition to the two types of crystallizers mentioned above, a cooling crystallizer may be used (see Fig. 6)