Optimal Mixing in Agitated Extraction Columns

Liquid-liquid extraction columns are designed to increase the interfacial surface area between the two immiscible phases so that the solute to be removed can be efficiently transferred between the phases. The efficiency of an extraction column is generally optimized when the disperse phase droplets are as small as possible and the contact time is maximized. In agitated extraction columns, increased agitation results in higher efficiency (theoretical stages per unit height of the column) up to a certain point. If the agitation speed is too high, this can result in lower performance due to back-mixing of the continuous phase or flooding where the disperse phase stops flowing counter currently to the continuous phase. When designing an agitated column the goal is to find the optimal speed which will produce the best possible efficiency. However, the type of mixing also plays an important role for the best design of an agitated extraction column. Typically there are two methods for providing mixing in agitated column; rotating type impellers, of which there are many different types and reciprocating plates. Experience has demonstrated that the rotating internals work the best for systems that require a high amount of agitation, typically high interfacial tension and/or high density difference between the phases. Conversely, reciprocating internals have been found to be the better choice for systems with low interfacial tension and/or low density difference between the phases. The focus of this presentation will be to focus on these two types of mixing and how each can be used to provide the best column design for different types of systems.