Batch crystallization crystallizer types

The concept of programmed operation can also be applied to other types of batch crystallization e.g. precipitation via drowning-out with miscible solvents (Jones and Teodossiev, 1988). 

Example 16.5. Teflon heat transfer tubes that are thin enough to flex under the influence of circulating liquid cause a continual descaling that maintains good heat transfer consistently, 20-65 Btu/(hr)(sqft)(°F). Circulating types such as Figures (d) and (e) of ten are operated in batch mode, the former under vacuum if needed. High labor costs keep application of batch crystallizers to small or specialty production. 

As an alternative to multistage batch crystallization processes with their attendant problems of material handling and losses, several types of continuous column crystallizers have been developed, in which the product crystals are washed with their own melts in countercurrent flow. Those illustrated in Figures 16.14-16.17 will be described. Capacities of column purifiers as high as 500gal/(hr) (sqft) have been reported but they can be less than one-tenth as much. Lengths of laboratory size purifiers usually are less than three feet. 

It is clear that stringent control of batch crystallizers is critical to obtaining a desired crystal size distribution. It is also obvious that the development of a strategy for generating supersaturation can be aided by the types of modeling illustrated above. However, the initial conditions in the models were based on properties of seed crystals added to the crystallizer. In operations without seeding, initial conditions are determined from a model of primary nucleation. 

Curve 4 exhibits very steep solubility. Yield is obtained by cooling the feed solution. To prevent fines formation, the cooling must exactly follow the solubility curve. This is done automatically in batch crystallizers. Continuous crystallizers in series must have the crystallizer stage temperatures selected so as not to cross the solubility curve. Benzoic acid and DMT exhibit this type of solubility. 

The population balance, Eq. (10.3), was then expressed in terms of the new variable y and the crystal size, L. Analytical solutions of the population balance equation were obtained for both the size-independent growth and the linear size-dependent growth. This technique was applied to the analyses of CSDs in different types of batch crystallizers (Tavare et al. 1980). Other solution methods for the population balance equation were discussed by Tavare (1987). 

In this type of batch crystallization, a solute is crystallized from a primary solvent by the addition of a second solvent (antisolvent) in which the solute is relatively insoluble. The antisolvent is miscible with the primary solvent and brings about a solubility decrease of the solute in the resulting binary solvent mixture. 

Continuous, steady-state operation is often regarded as the ideal procedure for many types of process plant equipment, but this is not always true for crystallization processes. Batch operation often offers considerable advantages, such as simplicity of equipment and minimization of encrustation on heat-exchanger surfaces. In many cases, only a batch crystallizer can produce the required crystal form, size distribution, or purity. On the other hand, the operating costs of a batch system can be significantly higher than those of a comparable continuous unit, and problems of product variation from batch to batch may be encountered. 

Determination of the optimal temperature (or supersaturation) trajectory for a seeded batch crystallizer is a well studied problem. This is a dynamic optimization or optimal control problem. The process performance is determined by the crystal size distribution and product yield at the final time. For uniformity of shape and size in the crystals in a seeded batch crystallization process, it is essential to ensure that the nucleation phenomena occurs to the minimum and mostly the seeded crystals grow to the desired size at a certain rate. If nucleation occurs in the initial phase, then there is a possibility that the nucleated crystal will compete with the seeded ones, thus if the phenomena is of late growth, then nucleation in the earlier phase is preferred. Thus, depending upon the process operation, many types of objective functions have been proposed [4]. 

India, Goa and Bombay two plants built ca 1950 batch-type open-hearth roasters ceUs are operated discontinuously, the KMnO is crystallized separately in agitated tanks 1,200 114... 

Double-Pipe Scrapea-Surface Crystallizer This type of equipment consists of a double-pipe heat exchanger with an internal agitator fitted with spring-loaded scrapers that wipe the wall of the inner pipe. The cooling hquid passes between the pipes, this annulus being dimensioned to permit reasonable shell-side velocities. The scrapers prevent the buildup of solids and maintain a good film coefficient of heat transfer. The equipment can be operated in a continuous or in a recirculating batch manner. 

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