Seeding in batch crystallizations

Bohlin, M. and Rasmuson, A.C., 1992. Application of controlled cooling and seeding in batch crystallization. Canadian Journal of Chemical Engineering, 70, 120-126. 

Kohl et al. [8] studied seeding in batch crystallization. The system studied was an aqueous j3-cyclodextrin solution that had a wide metastable zone. They determined a critical size for the seed crystals in this system. When the size of the seeds used was larger than the critical size, secondary nucleation took place. 

Tavare and Garside ( ) developed a method to employ the time evolution of the CSD in a seeded isothermal batch crystallizer to estimate both growth and nucleation kinetics. In this method, a distinction is made between the seed (S) crystals and those which have nucleated (N crystals). The moment transformation of the population balance model is used to represent the N crystals. A supersaturation balance is written in terms of both the N and S crystals. Experimental size distribution data is used along with a parameter estimation technique to obtain the kinetic constants. The parameter estimation involves a Laplace transform of the experimentally determined size distribution data followed a linear least square analysis. Depending on the form of the nucleation equation employed four, six or eight parameters will be estimated. A nonlinear method of parameter estimation employing desupersaturation curve data has been developed by Witkowki et al (S5). 

Myerson (2002), Mullin (2001), and Mersmann (2001) provide excellent descriptions of methods for crystal growth rate measurements. These methods involve measurements of either single crystals or suspensions. Much information can be gained from the traditional technique of measuring ( grab samples or in-line) solute concentration versus time in batch crystallization on a seed bed. Initial and later slopes on such a plot can provide multiple data points of growth rate versus supersaturation. 

In general, in order to obtain a narrow crystal size distribution (CSD), undesired nucleation should be avoided. In batch crystallization, the use of an optimal quantity of seed crystals of an optimal size may be the way to obtain a narrow CSD. According to Kohl et al. [8], the metastable zone of organic compound systems can be quite wide. Therefore, primary crystallization occurs only at a very high level of supersaturation. 

Seeding is commonly used as a control technique, especially in batch crystallization processes. The nucleation rate depends on the total surface area of the crystals. If the total crystal surface is relatively small in relation to the level of supersaturation, small crystals or crystals of undesired shapes may be formed in the early stage of crystallization. Otherwise, if the crystal surface area is sufficient, supersaturation will cause the existing crystals to grow. 

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. 

Broadening of the crystal size distribution in batch crystallizers thus the effect of seeding is particularly important. 

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

Batch Crystallization. Crystal size distributions obtained from batch crystallizers are affected by the mode used to generate supersaturation and the rate at which supersaturation is generated. For example, in a cooling mode there are several avenues that can be followed in reducing the temperature of the batch system, and the same can be said for the generation of supersaturation by evaporation or by addition of a nonsolvent or precipitant. The complexity of a batch operation can be ihustrated by considering the summaries of seeded and unseeded operations shown in Figure 19. 

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. 

Girolami, M.W. and Rousseau, R.W., 1985. Initial breeding in seeded batch crystallizers. Industrial and Engineering Chemistry Research, 25, 66-70. 

Wayne Genck (7 8) has recently published several useful articles about batch crystallization. Often lab filtration after crystallization is done with a thin cake and no problem is observed. But when taken to the plant, this operation takes days to build and wash a cake. To avoid this problem it is best to operate a crystallizer that is properly seeded and cooled according to a profile that follows the equation in reference (7), slow at first and fastest at the end. The other reference (8) discusses the challenges without seeding. Experience by the author confirms that a large amount of seed crystal is required, about 1-2 % wt of the final crystal yield. 

All experiments up to this time employed only minute quantities of seed crystals. In investigating the variables affecting the growth of the dextrose crystals, Newkirk found that the operation could be controlled by using much greater proportions of seed crystals than had hitherto been employed.8 The excessive formation of crystal nuclei too small and numerous to be able to grow to satisfactory size could be avoided by this means. The operation was most economically carried out by leaving in the crystallizer 25 to 30% of a finished batch to act as seed for the... 

In this section, a brief description of the necessary experiments to identify the kinetic parameters of a seeded naphthalene-toluene batch crystallization system is presented. Details about the experimental apparatus and procedure are given by Witkowski (12). Operating conditions are selected so that the supersaturation level is kept within the metastable region to prevent homogeneous nucleation. To enhance the probability of secondary nucleation, sieved naphthalene seed particles are introduced into the system at time zero. 

The Investigation was carried out using a seeded, batch crystallization In the absence of nucleatlon. Supersaturated solutions were prepared, seeded and maintained at a constant temperature while crystallization proceeded. Samples were taken periodically to give a solution for analysis and crystals for size analysis and crystal content determination. 

Figure 1. Change in average crystal size with time for ice crystal seeds grown in 6% lactose under batch crystallization conditions.

Figure 3. Changes in size distribution with time during growth of ice crystal seeds in 6% lactose under batch or semi-batch growth conditions, concentration processes based on suspension growth. In addition, the relation of this observed widening to the heat balance conditions needs to be further explored.

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