Crystallizers multistage vacuum

Cyclohexanone oxime is converted quantitatively to caprolactam by Beckmann rearrangement in the presence of oleum, which is of sufficient strength to consume the several percent water in the molten oxime. The reaction mass is neutralized with aqueous ammonia to a cmde caprolactam layer and a saturated solution of ammonium sulfate. Approximately 1.5 kg of the total 4.4 kg ammonium sulfate per kilogram of caprolactam is produced in this step. Purification is by multistage vacuum crystallization from aqueous solution in neatly quantitative yield. 

The Standard Messo multistage vacuum crystallizer Figure 8.52) provides a number of cooling stages in one vessel. The horizontal cylinder is divided into several compartments by vertical baffles that permit underflow of magma from one section to another but isolate the vapour spaces. Each vapour space is kept at its operating pressure by a thermocompressor, which discharges to a barometric condenser. 

Figure 8.52. Standard Messo multistage vacuum crystallizer...

Figure 8.2-8 shows a horizontal multistage crystallizer, appropriate for vacuum cooling crystallizatioa Through several walls the steam of the different stages is separated the suspension flows from one stage to the next Fresh solution is added... 

Crystallization from a solution is a thermal separation process. A solution of one or more solids molecularly dispersed in a solvent is usually concentrated by multistage evaporation of the solvent. Since the concentrated solution becomes supersaturated by cooling ( cooling crystallization ), evaporation of solvent ( evaporation crystallization ) or flash evaporation ( vacuum crystallization a combination of cooling and evaporation) causes crystals to form and grow. To reduce the degree of supersaturation, the surplus forms a solid which... 

Vacuum crystallization needs no heat transfer area in the solution, and therefore no incrustation problem occurs. To increase economic efficiency with evaporation crystallization, with respect to steam input, the process is designed for multistage operation with steam heat recovery. The first stage operates under conditions close to ambient pressure, and the final stage under a vacuum, such that the steam is condensed without compression at the temperature of the available cooling water. 

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