If the extraction system is relatively concentrated but still immiscible (this is almost a contradiction), we can extend the ratio unit mass balances developed in Chapter 12 for a single solute to extraction. Assumption 5, Eq. (13-2), no longer needs to be valid. This ratio-unit analysis is easily extended to cross-flow systems.
When the diluent and solvent are immiscible, weight ratio units are related to weight fractions as
where X is kg solute/kg diluent and Y is kg solute/kg solvent. Note that these equations require that the phases be immiscible.
The operating equation can be derived with reference ...