Batch crystallization laboratory

In all such laboratory studies, plant conditions and compositions should be employed as far as possible. Agglomeration rates tend to increase with the level of supersaturation, suspension density and particle size (each of which will, of course, be related but the effects may exhibit maxima). Thus, agglomeration may often be reduced by operation at low levels of supersaturation e.g. by controlled operation of a batch crystallization or precipitation, and the prudent use of seeding. Agglomeration is generally more predominant in precipitation in which supersaturation levels are often very high rather than in crystallization in which the supersaturation levels are comparatively low. 

McNeil, T.J., Weed, D.R. and Estrin, J., 1978. A note on modelling laboratory batch crystallizers. American Institution of Chemical Engineers Journal, 24(4), 728-731. 

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. 

The laboratory batch crystallizer used in this study is shown in Figure 2. It consisted of a cylindricd vessel (ID = 155 mm, height = 250 mm) of three litre working capacity agitated by a variable speed 4-bladed (variable-pitch) impeUer. For temperature control, the crystallizer was immersed in a constant temperature water-bath. 

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. 

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. 

Laboratory batch crystallizers have been used successfully to develop crystallization kinetic expressions and to measure the effects of process conditions on the kinetics in realistic crystallization environments approximating those in industrial practice. Laboratory data are needed to help decide what mode of crystallization to use and to determine the features of design that will produce to the greatest degree the crystal properties and yield desired. 

Probably the simplest laboratory batch crystallizer was described by Misra and White (1971). Kinetics of the crystallization of aluminum trihydroxide from caustic aluminate solutions were studied in a 5 1 round-bottomed Bask equipped with a stirrer, thermometer, and a sampling tube (Figure 10.1). The crystallizer was immersed in a constant-temperature bath. 

Figure 10.1 Laboratory batch crystallizer. (Reproduced by permission of the American Institute of Chemical Engineers 1971 AIChE from Kinetics of Crystallization of Aluminum Trihydroxide from Seeded Caustic Aluminate Solutions, Misra, C., and White, E.T., CEP Symposium Series, vol. 67, no. 110, pp. 53-65 (1971).)...

Batch crystallization has several desirable features and advantages in laboratory and industrial applications. Industrial batch crystallizers are commonly used to manufacture a wide variety of crystalline materials with desirable product features and quality. Laboratory batch crystallizers are often used to characterize crystallization kinetics and CSDs and to determine the effects of process conditions on these kinetics and CSDs. 

Figure 4.41. Trend analysis over 12 batches of a bulk chemical. The sieve analysis shows that over time crystals larger than 250 /urn were reduced from a weight contribution in the range of a few percent of the total to about 1% in favor of smaller sizes. Impurity C appears to follow the trend given by the lead compound for the competing side reaction 1. The very low moisture found for sample 3 could be due to a laboratory error because during drying one would expect ethanol to be driven off before water. Methanol is always below the detection limit.

The crystallization step is generally studied quite exhaustively at the laboratory scale and often at the pilot scale. The reaction chemistry should be properly understood to access effects, if any, of the synthesis step on the impurity profile. In batch cooling crystallizers attempts have been made to create optimum conditions by on-line turbidity analysis (Moscosa-Santillan et al., 2000). Physicochemical characterization of the products should be done rigorously (Tanguy and Marchal, 1996). 

Smooth scale-ups from R D laboratory or bench scale to pilot scale and then to commercial size batch-operated, multi-purpose chemical plants are often not easy to achieve for a variety of reasons, often resulting from compromises due to the need to use existing equipment. The consequences of this lack of scalability can be a reduction in product quality and yield, increased by-product formation, longer cycle times, and, in some cases, an inability to reproduce key product properties such as color, size, or crystal structure. These consequences invariably result in an increased use of mass and energy and a production of greater waste per unit mass of product. 

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