Programmed cooling crystallization

These qualitative considerations of what is now known as programmed cooling crystallization were subsequently described mathematically by MuIIin and Nyvit... 

Although programmed cooling crystallization clearly results in a larger mean crystal size than that from natural cooling it is also evident that some fines i.e. small crystals are also present in the product. Since the solution was seeded these fine crystals must clearly have arisen from crystal attrition or secondary nucleation (see Chapter 5). 

Jones, A.G., 1972. Programmed cooling crystallization. Ph.D. Thesis, University of London. 

Jones, A.G. and Mullin, J.W., 1974. Programmed cooling crystallization of potassium sulphate solutions. Chemical Engineering Science, 29, 105-118. 

Using an analogy with the programmed cooling crystallization, Karpinski and Nyvlt (1983) suggested that the quality of the... 

The resulting numerical prediction for the size-optimal cooling curve is shown in Figure 7.3. It predicts that in order to maximize the final sizes of the S-crystals, the temperature should be held constant for a period at both the start and end of the operation with a convex curve in between. This has the result of reducing both the early and terminal supersaturation levels and so maximizes solute deposition on the S-crystals and their growth rather than that of the A -crystals. Thus, programmed cooling is strictly sub-optimal , but nevertheless remarkably close to the optimum result in this case to be a practical alternative (Jones, 1974). 

Figure 7.4 Microcomputer programming of a hatch cooling crystallizer. A, crystallization vessel, B, control heater, C, control cooler. surrounding the draft-tube), D, contact thermometer, E, discharge plug and conical baffle), F, recorder, G, relay, H, temperature programmer, I, cooling water pump, J, cooling water reservoir, K, water inflow L, water outflow after Jones and Mullin, 1974)...

Jones, A.G., Akers, S.R.G. and Budz, J., 1986. Microcomputer programming of temperature in a batch cooling crystallizer. Crystal Research and Technology, 21, 1383-1390. 

Mayrhofer, B. and Nyvlt, J., 1988. Programmed cooling of batch crystallizers. Chemical Engineering Processing, 24, 217-220. 

Vega, A., Diez, F. and Alvarez, J.M., 1995. Programmed cooling control of a batch crystallizer. Computers and Chemical Engineering, 9, 471-476. 

The objective of programmed cooling is to control the rate at which the magma temperature is reduced so that supersaturation remains constant at some prescribed value, usually below the metastable limit associated with primary nucleation. Typically the batch is cooled slowly at the beginning of the cycle and more rapidly at the end. An analysis that supports this approach is presented later. In size-optimal cooling, the objective is to vary the cooling rate so that the supersaturation in the crystallizer is adjusted to produce an optimal crystal size distribution. 

Gabas, N. Laguerie, C. Batch crystallization of D-xylose by programmed cooling or by programmed adding of ethanol. Chem. Eng. Sci. 1992, 47 (12), 3148-3152. 

Figure 1. Cooling programs for crystal growth by slow cooling I, constant linear growth rate II, constant cooling rate III, optimum allowing for decrease in maximum stable growth rate as crystal becomes larger. (From Ref 1.)...

A batch cooling crystallizer (Figure 10.3) was employed by Mullin and Nyvlt (1971) to study the effect of programmed cooling on the CSD of potassium sulfate and ammonium sulfate crystals. The crystallizer was a cylindrical glass vessel (4 or 301) with a round bottom provided with a discharge outlet. It was fitted with... 

Several techniques, such as the programmed-cooling rate in cooling crystallization and seeding technology, have been developed for controlling the particle size in batch-cooling crystallization processes. The principle of these methods is to inflnence the nucleation rate. 

FIGURE 64.6 A schematic figure of a simple batch crystallizer (a) and one with a programmed-cooling system (b). (Reprinted from Mersmann, A., Ed., Crystallization Technology Handbook, Marcel Dekker, New York, 1995. With permission.)... 

B. Mayrhofer and J. Nyvlt. Programmed cooling of batch crystallizers. Chem. Eng. Process. 24 217-220, 1988. 

Guidelines for the choice of optimum reactant solution concentrations in a precipitation process have been proposed by Lindberg and Rasmuson (2000) who showed that larger crystals are produced when the feed concentration is kept low in the early stages of the process and then allowed to increase with time in a controlled manner. The term programmed feed concentration is introduced for this mode of operation in allusion to the method of programmed cooling for batch crystallizers described in section 8.4.5. 

---