Cooling Crystallizer

The elements of design for a vacuum cooling crystallizer are the same as for the evaporative crystallizer except a heat exchanger is not required. The operating features are also similar. In this case, the heat for evaporation is supplied by the sensible heat of the feed and the heat of crystallization. 

If it is desired to operate at a temperature which results in the solution having a vapor pressure below the vapor pressure of the available coolant, a steam-jet booster may be used in the vacuum system. 

By eliminating the evaporation, the vaporization chamber is eliminated and the vessel is now designed to operate at atmospheric pressure. 

To keep the supersaturation of the solution in the metastable region, the temperature drop through the heat exchanger must be comparatively small. To prevent scaling of the heat exchanger surface, the temperature difference between the mother liquor and the coolant must be kept small. 

---

Anilides, (a) To 1 ml. of aniline in a small conical flask add very slowly and carefully about i ml. of acetyl chloride. A vigorous reaction occurs and a solid mass is formed. Add just sufficient water (about 15 ml.) to dissolve the solid completely on boiling. On cooling, crystals of acetanilide separate out filter and determine the m.p. 

The reaction with ethyl iodide is less rapid and it is necessary to warm the mixture gently until cloudy. On cooling, crystals of the ethiodide are formed, and after recrystallisation from methylated spirit have m.p. 84 . 

Cool flames Cooling crystallizers Cooling towers... 

Supercritical fluids can be used to induce phase separation. Addition of a light SCF to a polymer solvent solution was found to decrease the lower critical solution temperature for phase separation, in some cases by mote than 100°C (1,94). The potential to fractionate polyethylene (95) or accomplish a fractional crystallization (21), both induced by the addition of a supercritical antisolvent, has been proposed. In the latter technique, existence of a pressure eutectic ridge was described, similar to a temperature eutectic trough in a temperature-cooled crystallization. 

Mass and Energy Balances. The formulation of mass and energy balances follows procedures outlined ia many basic texts (2). The use of solubihties to calculate crystal production rates from a cooling crystallizer was demonstrated by the discussion of equations 1 and 2. Subsequent to determining the yield, the rate at which heat must be removed from such a crystallizer can be calculated from an energy balance ... 

The specific enthalpies ia equation 9 can be determined as described earUer, provided the temperatures of the product streams are known. Evaporative cooling crystallizers operate at reduced pressure and may be considered adiabatic (Q = 0). As with of many problems involving equiUbrium relationships and mass and energy balances, trial-and-error computations are often iavolved ia solving equations 7 through 9. 

FIG. 18 66 Forced-circulation baffle surface-cooled crystallizer. (Swenson Process Equipment, Inc.)... 

An Oslo surface-cooled crystallizer is illustrated in Fig. 18-71. Supersaturation is developed in the circulated liquor by chilling in the cooler H. This supersaturated liquor is contacted with the suspension of ciystals in the suspension chamber at E. At the top of the suspension chamber a stream of mother hquor D can be removed to be used for fines removal and destruction. This feature can be added on either type of equipment. Fine ciystals withdrawn from the top of the suspension are destroyed, thereby reducing the overall number of ciys-tals in the system and increasing the particle size of the remaining product ciystals. 

The crystallization kinetics defines the open time of the bond. For automated industrial processes, a fast crystallizing backbone, such as hexamethylene adipate, is often highly desirable. Once the bond line cools, crystallization can occur in less than 2 min. Thus, minimal time is needed to hold or clamp the substrates until fixturing strength is achieved. For specialty or non-automated processes, the PUD backbone might be based on a polyester polyol with slow crystallization kinetics. This gives the adhesive end user additional open time, after the adhesive has been activated, in which to make the bond. The crystallization kinetics for various waterborne dispersions were determined by Dormish and Witowski by following the Shore hardness. Open times of up to 40 min were measured [60]. 

Thermoplastic urethane adhesives may be processed into an adhesive film. I,amination of two substrates can, in theory, be done immediately, but the film is often extruded onto one substrate, covered by a release liner, and allowed to cool. Crystallization follows to create a non-tacky film that may be cut into specific shapes. The release liner is then removed, and the shaped adhesive can be heat-activated on one substrate, using infrared lamps. The second substrate is then nipped under pressure, followed by a cooling press to speed crystallization. Once the backbone has crystallized, the bond should be strong. 

Computational fluid dynamics (CFD) is the numerical analysis of systems involving transport processes and solution by computer simulation. An early application of CFD (FLUENT) to predict flow within cooling crystallizers was made by Brown and Boysan (1987). Elementary equations that describe the conservation of mass, momentum and energy for fluid flow or heat transfer are solved for a number of sub regions of the flow field (Versteeg and Malalase-kera, 1995). Various commercial concerns provide ready-to-use CFD codes to perform this task and usually offer a choice of solution methods, model equations (for example turbulence models of turbulent flow) and visualization tools, as reviewed by Zauner (1999) below. 

The shape of the equilibrium line, or solubility curve, is important in determining the mode of crystallization to be employed in order to crystallize a particular substance. If the curve is steep, i.e. the substance exhibits a strong temperature dependence of solubility (e.g. many salts and organic substances), then a cooling crystallization might be suitable. But if the metastable zone is wide (e.g. sucrose solutions), addition of seed crystal might be necessary. This can be desirable, particularly if a uniformly sized product is required. If on the other hand, the equilibrium line is relatively flat (e.g. for aqueous common salt... 

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

Figure 7.2 Batch cooling crystallization, (a) Uncontrolled rapid cooling without seed crystals, (h) Controlled cooling with seed crystals added after Griffiths, 1928)...

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)...

Figure 7.5 Experimental, (a) transient supersaturatiom, and (h) consequent product crystal size distributions from hatch cooling crystallizations after Jones and Mullin, 1974)...

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). 

Figure 9.4 Population density distribution of potassium sulphate crystals from continuous cooling crystallization Jones and Mydlarz, 1989)...

Doki, N., Kubota, N., Sato, A., Yokota, M., Hamada, O. and Masumi, F., 1999. Scaleup experiments on seeded batch cooling crystallization of potassium alum. American Institution of Chemical Engineers Journal, 45(12), 2527-2533. 

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

Jones, A.G., 1974. Optimal operation of a batch cooling crystallizer. Chemical Engineering Science, 29, 1075-1087. 

Jones, A.G., Chianese, A. and Mullin, J.W., 1984. Effect of fines destruction on batch cooling crystallization of potassium sulphate solutions. In Industrial Crystallization 84. Eds. S.J. Jancic and E.J. de Jong, Amsterdam Elsevier, pp. 191-194. 

Monnier, O., Fevotte, G., Hoff, C. and Klein, J.P., 1997. Model identification of batch cooling crystallizations through calorimetry and image analysis. Chemical Engineering Science, 52, 1125-1139. 

Ottens, E.P.K and de Jong, E.J., 1973. A Model for Secondary Nucleatioii in a Stirred Vessel Cooling Crystallizer. Industrial and Engineering Chemistry Eundamentals, 12, 179-184. 

Roliani, S. and Paine, K., 1991. Feedback control of CSD in a continuous cooling crystallizer. Canadian Journal of Chemical Engineering, 69, 165. 

---