Design of Crystallizers

The heat transfer areas of cooling or evaporation crystallizers must be large enough to remove the heat (cooling) or to transfer the heat of evaporation. Let us have a look on the heat transfer area and the temperature profile in the solution/suspension very close to the solid strrface because the minimttm and maximum temperature is decisive for incrustation and fouling. The heat fltrx derrsity q = a - AT or the temperature difference AT = q/a between the suspension and the siuface has to be limited. 

The suspension density in industrial crystallizers can assume values up to 500 kg/ia or to volumetric crystal holdups between p = 0.2 and 0.25. 

Fimdamentals of fluid dynamics of stirred vessels and fluidized beds eanbe found in Chap. 3. In small vessels it is difficult to suspend especially large crystals with a high density. On the other hand, blending can be insufficient in large vessels with the result that there are great differences of the supersaturation and suspension density. In the case of continuously operated ciystallizers a great ratio of the residence 

In industrial batch crystallizers the ratio of the volume V based on the internal volumetric flow rate Fcirc of circulation (Fcirc = N -n-d or V/Vciic is often between 30 and 120 s (Mersmann 2002) or, in other words, the circulation time is short in comparison to the batch time Tgatch ( circ/ Batch .01)). In very large crystallizers (V 100 m ) it can happen that the supersaturation is remarkably reduced especially close to heat transfer areas or in the vicinity of feed points with the result that in a big volume fraction of the apparatus the supersaturation tends to zero. This can be avoided if the rotor speed is above the minimum speed according to 

Here x is the mean residence time of crystals in a continuously operated cooling crystallizer. According to (8.5-2) a low dimensionless supersaturation Ac/p leads to high speed of rotors in ciystallizers. 

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Coefficient of Variation One of the problems confronting any user or designer of crystallization equipment is the expected particle-size distribution of the solids leaving the system and how this distribution may be adequately described. Most crystalline-product distributions plotted on arithmetic-probability paper will exhibit a straight line for a considerable portion of the plotted distribution. In this type of plot the particle diameter should be plotted as the ordinate and the cumulative percent on the log-probability scale as the abscissa. 

Cisternas, L.A., 1999. Optimal design of crystallization-based separation systems. American Institute of Chemical Engineers Journal, 45, 1477-1487. 

Nyvlt J., 1992. Design of crystallizers. Boca Raton CRC Press. 

The design of crystallization processes for the manufacture of Active Pharmaceutical Ingredients is a significant technical challenge to Process Research and Development groups throughout the Pharmaceutical and related industries. It requires an understanding of both the thermodynamic and kinetic aspects of crystallization, to ensure that the physical properties of the product will consistently meet specification. Failure to address these issues may lead to production problems associated with crystal size, shape and solubility, and to dissolution and bioavailability effects in the formulated product. 

This chapter provides a framework for the design of crystallization solvents for pharmaceutical products. The design part involves application of a... 

Mersmann, A. and Kind, M. Inti Chem. Eng. 29 (1989) 616-626. Modeling of chemical process equipment the design of crystallizers. 

The crystallization process can be illustrated by a phase diagram that shows which state (liquid, crystalline, or amorphous solid [precipitate]) is stable under a variety of crystallization parameters. It provides a means of quantifying the influence of the parameters such as the concentrations of protein, precipitant(s), additive(s), pH, and temperature on the production of crystals. Hence phase diagrams form the basis for the design of crystal growth conditions (McPherson, 1999 Ducruix and Giege, 1992 Ducruix and Giege, 1999 Chayen et ah, 1996 and references therein). 

The design of crystallizers is based on knowledge of phase equilibria, solubilities, rates and amounts of nuclei generation, and rates of crystal growth. Each system is... 

The electronic structure of donor and acceptor components is more or less predictable and the general relationship between, for example, structure and donor (acceptor) strength or structure and stability of ion radicals, is quite familiar to organic chemists. The design of new electronic structures is not only quite possible but represents the main trend of current research activities in the field. On the other hand, only preliminary steps have been made toward predicting and designing of crystal structure [15-17]. 

FR may vary considerably from face to face in the same crystal. Consequently, mean values of FR or F0 are of interest in the design of crystallization apparatus, whereas experimental studies with single crystals may measure mass transfer rates at specifically oriented surfaces. McCabe and Smith (M3) suggest a procedure for estimating the contribution of individual surface rate coefficients to mean values and a method for calculating the increase in linear crystal dimensions from the latter. 

The design of crystallizers is based on knowledge of phase equilibria, solubilities, rates and amounts of nuclei generation, and rates of crystal growth. Each system is unique in most of these respects and not often predictable. The kind of information needed for design of a continuous crystallizer is indicated by the data supplied for Example 16.1 and as listed in greater detail below. 

Values of the exponent have been found of the order of 1.5, but again no correlation of direct use to the design of crystallizers has been achieved. The sucrose growth data of Figure 16.6(b) are not quite log-log linear as predicted by this equation. 

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