Transport Processes and Separation Process Principles (Includes Unit Operations) Fourth Edition

12.7. CONTINUOUS MULTISTAGE COUNTERCURRENT EXTRACTION

12.7A. Introduction

In Section 12.5 single-stage equilibrium contact was used to transfer solute A from one liquid to another liquid phase. To transfer more solute, single-stage contact can be repeated by bringing the exit L₁ stream into contact with fresh solvent V₂, as shown in Fig. 12.5-6. In this way a greater percentage removal of solute A is obtained. However, this is wasteful of the solvent stream as well as giving a dilute product of A in the outlet solvent extract streams. In order to use less solvent and to obtain a more concentrated exit extract stream, countercurrent multistage contacting is often employed.

Figure 12.7-7. Extraction-tower flows: (a) process flow and material balance ...