Industrial controllers batch crystallization control

The design and operation of industrial crystallizers is where developments in the laboratory are confirmed and their practical significance determined. In recent years, crystallization processes involving specialty chemicals and pharmaceuticals have increased. This has led increased interest in batch crystallization operation, optimization and desigrt At the same time, the advent of powerful computers and their routine avaUabilily has stimulated interest in the area of on-line control of crystallization process (both batch and continuous). Progress in batch crystallization is surrunarized in a number of recent papers and reviews 173-801. In this section I will discuss two areas which I think will have an impact in the next decade. 

In the pharmaceutical industry most solid compounds are crystallized in batch operations, i.e., the crystalline product is isolated at the end of the operating cycle. Many bulk chemicals, such as table sugar, are prepared through continuous processes, in which the product is collected throughout the crystallization cycle. Batch crystallization produces a narrower range of particle size and may afford better control for the efficient crystallization of molecules from complex mixture [14]. 

A critical control aspect common to all batch crystallization is controlling the initial population of crystals. Control action intended to produce large crystals may be insufficient to compensate for the massive generation of nuclei that results from spontaneous nucleation. Industrial experience supports the copious academic documentation that seeding with a defined mass and size range of crystals permits the growth of larger crystals with a narrower size distribution. 

Fines Destruction. In the operation of industrial crystallizers, one would usually want to avoid the fines (i.e., small crystals) since they may cause difficulties in downstream processing equipment (e.g., filtration) and affect both product quality and process economics. Excessive fines may also require a relatively long batch run time to achieve the desired final size of the product crystals. Karpinski (1981) proposed a controlled dissolution of secondary nuclei in order to improve CSD from fluidized bed crystallizers. Jones et al. (1984) first described the application of fines destruction in batch crystallization of potassium sulfate solutions. Their study demonstrated the experimental feasibility of this technology to dramatically reduce the amount of fines in the final product CSD. Their theoretical predictions, obtained from population balance models, agreed with the experimental results. 

There are several common problems encountered in the use of crystallization in the pharmaceutical industry (1) the control of supersaturation (and PSD) in a batch crystallizer (2) the effective use of seed (3) efficient measurement of solubilities in multiple solvent systems to maximize purification and yield and (4) identification and retention of the most stable polymorphic form. 

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. 

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. 

Usually, batch crystallization is used when a relatively low production capacity is required, e.g., below 50 t of product per day. When batch crystallization is equipped with the proper temperature control and seeding system, the crystallization conditions can be adjusted in such a way that the residence times of the crystals, of various sizes, can be kept about the same. Therefore, the CSD can be narrower in batch crystallization than in continuous crystallization, which is one of the significant differences between batch and continuous crystallization without fine removal or a classification method for the product. In practice, industrial continuous crystallization processes contain fines-removal or classification units, such as hydrocyclones, in order to produce crystals of a narrow CSD. 

Figure 23.4 Flowsheet of a typical industrial batch crystallizer, showing concentration and temperature controllers, including cascade control for temperature.

Virtanen, J. 1984. Automatic control of batch evaporative crystallization. In Industrial Crystallization 84. The Hague September 1984. Eds. S.J. Jancic and E.J. de Jong. Elsevier Science. 

Most operations scaled up in the pharmaceutical industry use semibatch (and batch) processing in the general-purpose equipment. Such operations allow for fine control of slow unit operations, for example, reactions needing hours to complete, fermentation, and crystallization, and such fine control may be necessary to ensure high quality and productivity. 

Both batch and continuous crystallization equipment are available at industrial scale, although batch operation is normally favored for pharmaceutical products where batch integrity must be maintained for quality control reasons. Continuous crystallization is suited to higher throughputs and enables more energy efficient operation. 

Having discussed many of the practical issues of operating and controlling continuous and batch industrial crystallizers in Section... 

Frew, J.A. (1973) Optimal control of batch raw sugar crystallization. Industrial and Engineering Chemistry Process Design and Development, 12, 460-467. 

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