Plait point, solvent extraction

Capacity. This property refers to the loading of solute per weight of extraction solvent that can be achieved in an extrac t layer at the plait point in a Type I system or at the solubihty hmit in a Type II system. 

Thus the extractor column raffinate outlet rate and the solvent inlet rate are approximately equal. This is indeed the minimum solvent rate allowed, since a lower rate will overload the solvent, referencing the plait point. This rate will also set the required minimum extractor column diameter. For some refinery-type extraction operations, such as lube oil extractors, where relatively much larger solvent-raffinate rates apply, this method for determining minimum solvent rate is very economical and desirable. 

A ternary system well suited for solvent extraction is shown in Fig. 6.1-1. The feed consists of substance T (carrier) and a solute B, which has to be removed or recovered in pure form. Both substanees are mutually miscible. The solute B is also miscible with the solvent L whieh is often an organic compound. Substances T and L, however, must have a large miseibihty gap that narrows down with increasing content of solute and, eventually, diminishes at the critical point (or plait point). The boimdaiy of the two-phase region is called binodal curve. The dashed lines within the two-phase region, called tie lines (or conodes), describe the distribution equihbiium of solute B. The dashed-dotted line (eonjugation curve) allows the interpolation between given tie lines. 

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