Mixing batch crystallization

As mentioned above batch crystallizers are usually simple vessels provided with some means of mechanical agitation or particulate fluidization. These have the effect of reducing temperature and concentration gradients, and maintain crystals in suspension. Baffles may be added to improve mixing and heat exchange or vacuum systems may be added, as appropriate. Various design combinations are available and some are illustrated in Figure 7.1. 

Jones, A.G., 1984. The Design of Well-Mixed Batch Crystallizers. SPS DR17. (Harwell/ Warren Spring Separation Processes Service), 40pp. 

In batch operations, mixing takes place until a desired composition or concentration of chemical products or solids/crystals is achieved. For continuous operation, the feed, intermediate, and exit streams will not necessarily be of the same composition, but the objective is for the end/exit stream to be of constant composition as a result of the blending, mixing, chemical reaction, solids suspension, gas dispension, or other operations of the process. Perfect mixing is rarely totally achieved, but represents the instantaneous conversion of the feed to the final bulk and exit composition (see Figure 5-26). 

Phenomena, methods of operation, etc. have been studied extensively for the use of crystallization in separation processes. Although much remains to be learned about such processes, relatively little attention has been given to the other functions and the purpose of this work was to examine the role of various process variables in determining the purity of crystals recovered from a batch crystallizer. The system studied experimentally was a model system of amino acids, and the key variables were the composition of the liquor from which a key amino acid was crystallized, the rate at which supersaturation was generated by addition of an acid solution to reduce solubility, and the degree of mixing within the batch unit. 

The first-principle method used in the present study will be outlined below. Population balance analysis on a perfectly mixed batch crystallizer with negligible crystal breakage and agglomeration yields the familiar nucleation rate equation used by Misra and White (5)... 

Perhaps the most troublesome aspect of batch crystallizers is the difficulty associate ciystal size distributions in going from one batch to the next. This may be overcome and control of mixing conditions. In general, however, the development of methods for design and analysis of batch crystallizers lags those for cortinuous systems. 

Often in the laboratory and in batch crystallization, the feed is dribbled onto the top suspension surface of the crystallizer. Usually this is a poor way to feed because the top surface is often not very well mixed into the bulk. The result is that the feed solution pools on the surface and a high supersaturation envelope forms around it, leading to high nucleation and encrustation rates. As a general rule, it is usually far preferable to introduce the feed well beneath the top surface of the suspension. 

The quality, productivity, and batch-to-batch consistency of the final crystal product can be affected by the conditions of the batch crystallizer. Several factors considered here include batch cycle time, supersaturation profile, external seeding, fouling control, CSD control, growth rate dispersions, and mixing. 

Mixing is an important factor affecting batch crystallization. On one hand, sufficient mixing is required to maintain crystals in suspension, to assure an adequate rate of energy transfer, and to achieve uniformity of suspension properties throughout the crystallizer. On the other hand, the effect of mixing on batch crystallization is largely system-dependent. 

The remaining seven chapters deal with individual topics important to industrial practice, such as design, mixing, precipitation, crystallizer control, and batch crystallization. In addition, topics that have become important in recent years, such as melt crystallization and the crystallization of biomolecules are also included. Each chapter is self-contained but assumes that the reader has knowledge of the fundamentals discussed in the first part of the book. 

Experiments were done, using a batch crystallizer apparatus as shown in Fig.l, varying the Mg/P molar ratio from 1-4. The experimental conditions are shown in Table 2. During the process of batch crystallization, the mixed suspension was sampled at the determined crystallization time. The samples were filtered through 0.45 fjL m membrane filter, and the concentrations in the filtrate were measured. Also sampled crystals were obtained when the concentration was observed to be constant, to be observed by SEM and be analyzed by the Xray diffractometer. The composition of crystals was analyzed after dissolution by a pH 2 HCl solution. 

The study started with a batch crystallization experiment using seeded method. The purpose of this batch experiment was to deteimine the parameters needed for the subsequent experiment, i.e. the seeded continuous crystallization experiment using an MSMPR (mixed-suspension-mixed-product-removal) crystallizer. These parameters were levels of supersaturation, residence time, stirring rate, and concentration of additives, respectively. 

In industrial production, the temperature distribution in a batch mixing tank is sometimes a critical factor in process control for instance, in batch crystallization and in batch chemical reaction systems. In some processes, the solution temperature should follow a certain cooling or heating program, while in some other cases, heat removal should proceed in such a way that the temperature in the tank be kept constant in order to prevent temperature runaway. Therefore, heat transfer control is usually required during the batch and plays a crucial role in optimising the operation of the process in order to obtain the desired end-product and improve operational safety. 

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