Nucleation and crystal growth in batch crystallizers

Nucleation and crystal growth in batch crystallizers

Janse, A.H.

De Jong, E.J. (promotor)

Mechanical, Maritime and Materials Engineering

1977-09-14

The aim of the present work is to gain knowledge of the mechanism of formation of the crystal size distribution in batch crystallizers in order to give directives for design and operation of batch crystallizers. The crystal size distribution is important for the separation of crystals and mother liquid, the subsequent washing and drying of the crystals, and the product properties. Cumulative mass distributions and population density distributions are used side by side. The average crystal size and the spread in sizes, determined from the cumulative mass distribution, are used to correlate with the relevant process parameters, whereas the time-dependent population density distributions are used to interpret the formation of the distribution in terms of nucleation and growth. A schematic description of a cycle of a batch crystallizer and the main design choices are given in chapter 2. The nucleation and growth mechanisms are reviewed in chapter 3. There, it is shown that the usually applied interpretation of the width of the meta-stable zone gives misleading information about the primary nucleation rate because the nuclei have to grow to a detectable size before detection. An explanation is given for the experimentally often reported relation between the width of the meta-stable zone and the cooling rate based on the fact that the nuclei have to reach a minimum size before detection. Further, the mechanisms of secondary nucleation are briefly reviewed. The discussion of the growth kinetics leads to the conclusion that the growth rate can vary from crystal to crystal when the surface integration step partly or completely determines the growth rate ; growth dispersion can occur. The development in time of the crystal size distribution was simulated for assumed models for the primary and secondary nucleation rate and the growth rate. The results of the simulations, discussed in chapter 4, show that the population density distribution of the primary crystals exhibits a maximum, which is formed at the instant of maximum supersaturation. A unique relation exists between the maximum population density and the final average crystal size when primary nucleation occurs exclusively. This relation is independent of the primary nucleation kinetics and the cooling rate. The population density of the secondary crystals increases with decreasing crystal size. The total distribution is a superposition of the distribution of the primary crystals, mainly formed in the period of high supersaturation, and the distribution of secondary crystals, mainly formed in the final phase of the crystallization period. The horizontal distance between two population density distributions is independent of the crystal size when the growth rate is size independent. The horizontal distance increases with crystal size when the growth rate increases with size or when growth dispersion occurs. Thus, the time-dependent development of the population density distribution gives information about nucleation and growth. The literature data on crystal size distributions from unseeded and seeded batch crystallizers are reviewed in chapter 5. Only a small number of references deals with time-dependent distributions, most references deal with the effect of the process parameters on the final average crystal size. In general the average size decreases with increasing stirrer speed and cooling rate. Relatively little information is gained about the mechanism of distribution formation. Crystallization experiments were carried out in a 48 1 unseeded batch crystallizer with aqueous solutions of potassium dichromate and potassium alum. The experimental equipment and procedure are described in chapter 6; the results are presented and discussed in chapter 7. For potassium dichromate, the average crystal size increases with decreasing stirrer speed and is independent of the cooling rate, whereas for potassium alum the average crystal size increases with decreasing stirrer speed and decreasing cooling rate. The thermal treatment of the solution (thermal history) does not affect the crystallization kinetics of both substances. Attrition of large potassium dichromate crystals occurs at high stirrer speed; attrition of potassium alum crystals does not occur. The time-dependent population density distributions show that the crystal growth of both substances is not size independent; growth experiments reveal that growth dispersion occurs. It is concluded from the population density distributions that the primary nucleation rate is not only a function of the supersaturation, but also of the stirrer speed and the cooling rate. The primary nucleation rate is by approximation proportional to the stirrer speed to the power t2 to +4 and the cooling rate to the power -2 to -5. The exact relation between the primary nucleation rate, the supersaturation, the stirrer speed and the cooling rate was not determined since this relation is not necessary for the design of batch crystallizers. The number of small, secondary potassium dichromate crystals increases with increasing stirrer speed and decreasing cooling rate, which is consistent with the experimentally determined secondary nucleation rate in a continuous crystallizer. The number of small, secondary potassium alum crystals increases with increasing stirrer speed and increasing cooling rate; these trends are in agreement with known secondary nucleation kinetics. The different trends for both substances are caused by a different dependence of the secondary nucleation rate on the supersaturation (cooling rate). The implications of the results for the method of operation and the design of batch crystallizers are discussed in chapter 8. A design procedure is outlined which takes into account the production rate specification, the product properties specification and the possible formation of incrustation.

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doctoral thesis

(c) 1977 Janse, A.H.