Optimal Mixing for Agitated Extraction Columns

The performance of a liquid-liquid extraction (LLE) column can be impacted by the mixing technology. This article provides case studies that help demonstrate the optimal choice between rotational and reciprocating mixing.

Liquid-liquid extraction (LLE), also known as solvent extraction, is a separations technique that leverages the distribution of chemicals between two liquid phases using an immiscible solvent to contact a feed stream, from which a solute is removed or extracted (1). Several factors affect the efficiency and selectivity of this operation, including the ease of solute mass transfer between the two immiscible phases.

The mass transfer rate of the solute from the feed into the solvent can be increased by increasing the interfacial area between the two immiscible liquids. This is accomplished by decreasing the dispersed-phase droplet size and maximizing the contact time. In LLE columns, the continuous phase must preferentially wet internal surfaces to minimize the coalescence of the dispersed phase and maximize the interfacial area. Generally, metal internals should be used if water is the continuous phase, while plastic internals should be used if an organic solvent is the continuous phase. This helps prevent the dispersed phase from preferentially wetting the internals and decreasing the interfacial area.

Agitation is used in LLE columns to minimize the size of the dispersed phase droplets as they flow counter-currently to the continuous phase, thus creating a greater interfacial area. In most cases, greater agitation generally increases the extraction efficiency (i.e., theoretical stages per unit height of the agitated section of the LLE column). However, an excessively high agitation speed can decrease the extraction performance due to back-mixing of the continuous phase or flooding, in which the dispersed phase stops flowing counter-currently to the continuous phase. Thus, the goal is to find the optimal speed using the appropriate mixing technology to achieve the highest separation efficiency.

This article examines rotational and reciprocating mixing in agitated extraction columns and demonstrates how to determine which method of agitation is ideal for a given application...