Continuously operated crystallizer principles

Mathews and Rawlings (1998) successfully applied model-based control using solids hold-up and liquid density measurements to control the filtrability of a photochemical product. Togkalidou etal. (2001) report results of a factorial design approach to investigate relative effects of operating conditions on the filtration resistance of slurry produced in a semi-continuous batch crystallizer using various empirical chemometric methods. This method is proposed as an alternative approach to the development of first principle mathematical models of crystallization for application to non-ideal crystals shapes such as needles found in many pharmaceutical crystals. 

Suspension crystallization is capable of producing very pure crystals mostly in a continuous operating mode, which is an advantage compared to the most batch solid layer crystallization processes. Another positive feature compared to solid layer crystallization is the better purification per process step and hence a less number of process steps usually with respect to crystallization. Therefore, suspension crystallization plants need in principle less energy compared to solid layer processes. Whether the investment costs of such plants are smaller as well depends on the complexity of the moving parts in suspension plant concepts compared to solid layer concepts (no moving parts, except pumps). 

Today, chemical reactors are used for the industrial conversion of raw materials into products. This is naturally facilitated by chemical reactions. Raw material molecules are referred to as reactants. Industrial reactors can be operated batchwise or in a continuous mode. In the batchwise operation mode, the reaction vessel is loaded with reactants, and the chemical reaction is allowed to proceed until the desired conversion of reactants into products has taken place. A more common approach is the continuous operation of a chemical reactor. Reactants are fed continuously into the reaction vessel, and a product flow is continuously taken out of it. If the desired product purity cannot be achieved in the reactor—as is often the case—one or several separation units are installed after the actual reactor. Common separation units include distillation, absorption, extraction, or crystallization equipment. A chemical reactor coupled with a separation unit constitutes the core of a chemical plant, as illustrated in Figure 1.1. The role of the chemical reactor is crucial for the whole process product quality from the chemical reactor determines the following process steps, such as type, structure, and operation principles of separation units [ 1 ]. 

Systems that exhibit behavior of the type illustrated in Fig. 4 cannot be purified in a single crystallization stage. They represent situations in which multiple stages or continuous-contacting devices may be useful. The principles of such operations are analogous to those of other countercurrent contacting operations—for example, distillation, absorption, and extraction. 

This type is similar in principle to the tank type, but the cooling surfaces are continually scraped or agitated to prevent the fouling by deposited crystals and to promote heat transfer. They are suitable for processing high-viscosity liquors. Scraped-surface crystallisers can be operated batchwise, with recirculation of the mother liquor, or continuously. A disadvantage of this type is that they tend to produce very small crystals. A typical unit is the Swenson-Walker crystalliser shown in Volume 2. 

The transfer of mass as well as heat from one material phase to another is quite commonly encountered in chemical process flow sheets. The same physical laws, rate equations, and design principles can be applied to mass-transfer operations as occurring in absorption, adsorption, crystal-lization, distillation, drying, extraction, jluidization, and humidification Equipment is designed to obtain intimate contact between phases, in either a stagewise or continuous manner, and many special types of equipment have been developed for any given operation. This discussion will be limited to the conventional types of equipment. 

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