Crystallization multistage process

In the multistage process described on Fig. 20-14 feed enters one of several crystallizers installed in series. Crystals formed in each crystallizer are transferred to a hotter stage and the liquid collected in the clarified zone of the crystallizer is transferred to a colder stage and eventually discharged as residue. At the hot end, crystals are transferred to a vertical purifier where countercurrent washing is performed by pure, hot-product reflux. TSK refers to this multistage process as the countercurrent cooling crystallization (CCCC) process. In... 

Figure 16.13. MWB (Metallwerk Buchs) batch recirculating crystallizer, with freezing on and melting off insides of thin film heat exchanger tubes adaptable to multistage processing without external solids handling [Miitzenberg and Saxer, 1971).

A characteristic feature of non-chromatographic separations utilizing cyclodextrins is that they are aimed at preparative separations. Unfortunately only incomplete separations or enrichments can be attained. By repeating the separations in multistage processes, the required component can be enriched on preparative, and even industrial, scale. Many examples have been published both for partial separation of compounds, isomers, or enantiomers through selective crystallization of their complexes (3). 

Even at quiescent conditions the fundamental mechanisms of polymer crystallization, especially at an early stage, are still poorly understood [6-16]. For many years, nucleation and growth as a stepwise process has dominated the discussion [11,12]. In contrast to this view Strobl [13] proposed a multistage process to explain polymer crystallization, while others concluded on the basis of X-ray scattering data to a spinodal-assisted process [17-29]. Common to both views is that the crystallization is preceded by an ordered precursor (so-called pre-ordering). Clear structural information about such possible precursors - necessary to verify these hypotheses - is still scarce. As a resiJt, during recent years an important and still open debate has been going on about polymer crystallization. 

In the Kterature there are suggestions that the observation of the p form during slow crystallization results from the retardation of one of the substages of a multistage process leading to the a form [66]. The p a transition observed at 145°C [67] seems to confirm that hypothesis. 

The exact order of the production steps may vary widely in addition, some parts of the process may also vary. Metal formate removal may occur immediately after the reaction (62) following formaldehyde and water removal, or by separation from the mother Hquor of the first-stage crystallization (63). The metal formate may be recovered to hydroxide and/or formic acid by ion exchange or used as is for deicing or other commercial appHcations. Similarly, crystallization may include sophisticated techniques such as multistage fractional crystallization, which allows a wider choice of composition of the final product(s) (64,65). 

Figure 10-14. The SNIA BPD process for producing caprolactam (1) toluene oxidation reactor, (2) fractionator, (3) hydrogenation reactor (stirred autoclave), (4) multistage reactor (conversion to caprolactam), (5) water dilution, (6) crystallizer, (7) solvent extraction, (8) fractionator.

All freeze separation processes depend on the formation of pure solvent crystals from solution, as described for eutectic systems in Section 15.2.1. which allows single-stage operation. Solid-solution systems, requiring multistage-operation, are not usually economic. Several types of freeze crystallisation processes may be designated according to the kind of refrigeration system used as follows . 

As an alternative to multistage batch crystallization processes with their attendant problems of material handling and losses, several types of continuous column crystallizers have been developed, in which the product crystals are washed with their own melts in countercurrent flow. Those illustrated in Figures 16.14-16.17 will be described. Capacities of column purifiers as high as 500gal/(hr) (sqft) have been reported but they can be less than one-tenth as much. Lengths of laboratory size purifiers usually are less than three feet. 

Sulzer-MWB [MetalWerk AG Buchs] An obsolete process for recovering naphthalene from a coal-tar fraction by multistage fractional crystallization from the melt. Formerly operated by Rutgerswerke at Castrop-Rauxel, Germany. 

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