The Progressive Mode of Multistage Crystallization

Abstract

Continuous multistage separation schemes classically incorporate a reflux to take advantage of the multiplicity of stages available. Without this reflux, the total possible separation under steady state conditions is no better than that which could be obtained with a single stage. The maximum possible separation is obtained at total reflux, in which case, each stage is able to perform a full stage separation. If the separation factor per stage is expressed by s, then the total separation factor S for N stages is given by s^N . However, at total reflux there can be no product, and practical multistage operations are carried out at some partial reflux which, in conjunction with the appropriate number of stages, can be optimized for the specific separation required.

The following is a comparative analysis of two types of continuous multistage crystallization schemes. The first represents a reflux process in the classical sense, whereas the second is analogous to zone refining (and includes continuous zone refining as an example). The latter scheme, designated the progressive mode because the charge essentially progresses along an infinite series of stages, is shown to be up to twice as efficient as the classical reflux mode.