(568a) Liquid-Liquid Extraction: A Simplified View of a Complex Process

Authors: 
Threatt, T. - Presenter, Rice University
Arredondo, J. H., Rice University
Worstell, J. H., Worstell and Worstell, Consultants
The Chemical Processing Industry (CPI) uses liquid-liquid extraction when distillation and rectification are difficult or ineffective. The CPI also uses liquid-liquid extraction to recover temperature-sensitive substances, substances that decompose or form undesired by-products at elevated temperatures. Liquid-liquid extraction is also a common unit operation in petroleum refining. For example, most refineries operate an Edeleanu process, a furfural process, a Duo-Sol process, a phenol process, or a propane process. All these processes involve a counter-current flow of solvent and distilled crude oil through a vertical column. However, some processes use a horizontal extraction vessel.

All liquid-liquid extraction processes are designed based on attainment of phase equilibrium. However, attaining equilibrium in a commercial process is challenging. Why? Because attaining equilibrium requires an infinite time period, which is never available to a commercial process.

This caveat with regard to equilibrium brings into question the procedure for designing a liquid-liquid extraction column. The current method involves using a triangular equilibrium plot, then projecting operating lines through an externally located focal point. The number of lines passing through the external focal point determines the number of stages required to reach a desired combination of raffinate-extract concentrations. In general, this number is not a whole number, which necessitates shifting the focal point empirically to obtain a whole number of lines. The size of the extraction column is then determined using the final number of lines. In summary, liquid-liquid extraction columns are built on an assumption never met industrially, then adjusting that assumption empirically to arrive at an acceptable result.

This presentation applies dimensional analysis to liquid-liquid extraction, which avoids the equilibrium assumption. In this presentation, we derive the pertinent dimensionless parameters necessary to describe a liquid-liquid extraction process and show how to use them to scale from one unit size to another unit size.