Mixing in crystallization processes

Crystallization Basic Concepts (md Industrial Applications, First Edition. Edited by Wolfgang Beckmann. 

Especially during a fast precipitation process, a short blend time is of an outstanding importance, as nudeation and growth processes compared to cooling crystallization are very fast. Here, insufficient blending together with dead zones may have a dramatic and disadvantageous impact on product quality. The objective for such a process must be to find a suitable impeller system to prevent these effects and therefore to prevent uncontrolled particle formation. 

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Chen, Jianfeng, Lu, Ying and Chen Gantang (1994). Mixing-reaction crystallization process—(I) Experiments. J. of Chem. Indus. Eng. (China), 45(2) 176-182 (in Chinese). 

Adapa, S., Dingeldein, H., Schmidt, K.A., Herald, T.J. 2000. Rheological properties of ice cream mixes and frozen ice creams containing fat and fat replacers. J. Dairy Sci. 83, 2224-2229. Adleman, R., Hartel, R.W. 2002. Lipid crystallization and its effect on the physical structure of ice cream. In Crystallization Processes in Fats and Lipid Systems (N. Garti, K. Sato, eds.), pp. 381-427, Marcel Dekker, New York. 

The number density n is given as a fimction of the particle variables such as diameter, volume, or other properties, denotes the particle velocities in 3D external space in a completely back-mixed system this term vanishes. Vi is related to internal convective particle variables in crystallization processes it defines the growth rate. The B- and D-terms denote sources (birth) and sinks (death) for nucleation, a omeration, and breakage. The gradient of the flux j denotes exchange across the boundaries of the balancing volume. In Section 5.3, we will discuss particular solutions of this equation. 

In another process, hypochlorite filtrate is treated with lime slurry to precipitate dibasic crystals that are filtered. The filtrate is mixed with strong caustic, chlorinated, and filtered to remove NaCl crystals. The filtrate containing Na and Ca hypochlorite is mixed with dibasic crystals and chlorinated producing a slurry of Ca(OCl)2 is filtered the cake goes to a dryer and the filtrate to the dibasic crystallizer (195). 

Almost all flows in chemical reactors are turbulent and traditionally turbulence is seen as random fluctuations in velocity. A better view is to recognize the structure of turbulence. The large turbulent eddies are about the size of the width of the impeller blades in a stirred tank reactor and about 1/10 of the pipe diameter in pipe flows. These large turbulent eddies have a lifetime of some tens of milliseconds. Use of averaged turbulent properties is only valid for linear processes while all nonlinear phenomena are sensitive to the details in the process. Mixing coupled with fast chemical reactions, coalescence and breakup of bubbles and drops, and nucleation in crystallization is a phenomenon that is affected by the turbulent structure. Either a resolution of the turbulent fluctuations or some measure of the distribution of the turbulent properties is required in order to obtain accurate predictions. 

Common pollutants in a titanium dioxide plant include heavy metals, titanium dioxide, sulfur trioxide, sulfur dioxide, sodium sulfate, sulfuric acid, and unreacted iron. Most of the metals are removed by alkaline precipitation as metallic hydroxides, carbonates, and sulfides. The resulting solution is subjected to flotation, settling, filtration, and centrifugation to treat the wastewater to acceptable standards. In the sulfate process, the wastewater is sent to the treatment pond, where most of the heavy metals are precipitated. The precipitate is washed and filtered to produce pure gypsum crystals. All other streams of wastewater are treated in similar ponds with calcium sulfate before being neutralized with calcium carbonate in a reactor. The effluent from the reactor is sent to clarifiers and the solid in the underflow is filtered and concentrated. The clarifier overflow is mixed with other process wastewaters and is then neutralized before discharge. 

Zeolites are formed by crystallization at temperatures between 80 and 200 °C from aqueous alkaline solutions of silica and alumina gels in a process referred to as hydrothermal synthesis.15,19 A considerable amount is known about the mechanism of the crystallization process, however, no rational procedure, similar to organic synthetic procedures, to make a specifically designed zeolite topology is available. The products obtained are sensitive functions of the reaction conditions (composition of gel, reaction time, order of mixing, gel aging, etc.) and are kinetically controlled. Nevertheless, reproducible procedures have been devised to make bulk quantities of zeolites. Procedures for post-synthetic modifications have also been described.20 22... 

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