Crystallization heat removal rate

These results show that it may be more efficient to depectinize fruit juices before their concentration by freezing because this would give minimum losses at lower heat removal rates and thus at conditions of more economical operation. The implication of these results for the design of a scraped surface crystallizer are currently being examined. 

Although many commercial crystallizers operate with some form of selective crystal removal, such devices can be difficult to operate because of fouling of heat exchanger surfaces or blinding of screens. In addition, several investigations identify interactions between classified fines and course product removal as causes of cycling of a crystal size distribution (7). Often such behavior can be rninirnized or even eliminated by increasing the fines removal rate (63,64). 

Quenching the vapour with cold air in the chamber may increase the rate of heat removal although excessive nucleation is likely and the product crystals will be very small. Condenser walls may be kept free of solid by using internal scrapers, brushes, and other devices, and all vapour lines in sublimation units should be of large diameter, be adequately insulated, and if necessary, be provided with supplementary heating to minimise blockage due to the buildup of sublimate. One of the main hazards of air-entrainment sublimation is the risk of explosion since many solids that are considered safe in their normal state can form explosive mixtures with air. All electrical equipment should therefore be flame-proof, and all parts of the plant should be efficiently earthed to avoid build-up of static electricity. 

Combination of Equations 1 and 2 allows calculation of the rate of heat transfer from the growing crystal surface to the bulk solution. Under heat balance conditions, this rate of heat generation must be balanced by the amount of heat removed from the crystallizer by convection and conduction. This will be determined by the overall heat transfer coefficient, U, between the bulk solution and the refrigerant including convective resistances between the fluid and both sides of the crystallizer wall (refrigerant side and product side) as well as the conductive resistance across the crystallizer wall. 

Ice Crystal Growth. In order to quantify these results for the production of large disc and spherical crystals, seversd batch experiments on 6% lactose solutions were undertaken. The experimental conditions and results are shown in Table II. In these experiments, nuclei were generated at -2.5 C (except for Run Sa at -4.0°C) and input to the batch crystallizer controlled at various refirigerant temperatures. As these crystals grew, the total crystal surface area was controlled manually in order to maintain a heat balance for a constant value of the refrigerant temperature. Slurry removal rate for these experiments... 

Figure 3.3 shows a typical freezing curve which results when heat is removed at a continuous rate from foodstuffs and the temperature decreases. The curve has three zones first, the removal of sensible heat from the food between the initial temperature and the freezing temperature second, the removal of the latent heat of fusion leading to a change of state and the formation of ice crystals and third, further sensible heat removal down to the required storage temperature. A number of features of the freezing curve require explanation. Whilst... 

The scraped surface, or close-clearance exchanger, illustrated in Fig. 10 is required for a few very difficult situations. An example is purification by fractional crystallization, in which a refrigerant boiling in the annulus cools a solution of various substances and certain species selectively crystallize out on the surface of the inner pipe. The crystalline deposit must be continuously scraped off of the surface in order that the heat transfer rate be maintained. The crystals are eventually removed from the remaining liquid by filtration. 

Calculate the energy removal An energy balance around the crystallizer (see Fig. 10.5) gives Q = LHl + SHs — FHf, where Q is the heat added (or the heat removed, if the solved value proves to be negative) L, S, and F are the flow rates for mother liquor, solid product, and feed, respectively and Hl, Hs, and Hp are the enthalpies of those streams relative to some base temperature. Select a base temperature Tr of 70°C, so that Hp = 0. 

During a freezing process, ice crystallizes out of solution at a rate largely dependent on the rate of removal of the heat of crystallization. If this rate is faster than the diffusion of solutes away from the growing ice front, then the ice may entrap areas of concentrated solution. If the solute interacts with / adsorbs to the growing ice front, it may partition itself between the solution and the grain boimdaries of the ice. 

In practice, the rate of ice formation during freezing of foods is nearly always limited by the rate of heat removal. Even at a few degrees of undercooling, a considerable portion of the water can freeze, and this produces as much as 334 J of heat per g water frozen. Moreover, the system will soon become more or less solid, preventing convection and agitation. Hence the undercooling at the ice crystal surface is reduced to small values (often much below one kelvin). Linear crystallization rate then is rarely more than some micrometers per second this is, however, still fast compared to the rate obtained for crystallization of most substances from solution. 

In common with other crystallisation processes the size and number of crystals formed has a direct relationship to the rate of cooling. Rapid cooling will tend to favour the formation of large quantities of crystals. Under these conditions many centres of crystallisation occur giving rise to a multitude of small crystals. Lower rates of cooling tend to favour larger crystals with fewer centres of crystallisation. Low rates of heat removal will provide an opportunity for the formation of large uniformly packed clusters of crystals. 

A batch evaporative crystallizer (Figure 10.2) was used by Baliga (1970) to study the crystallization kinetics of potassium sulfate crystals. The crystallizer was equipped with a reflux condenser and a controlled distillate splitter so that the net solvent removal rate could be controlled closely. Heating of the crystallizer... 

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