Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA

Liquid-liquid extraction (LLE) is a separation technique used instead of distillation when either: (a) the relative volatilities of the compounds to be separated are very similar; (b) one or more of the mixture components are temperature sensitive even near ambient conditions; (c) the distillation would require a very low pressure or a very high distillate/feed ratio.¹The driving force for mass transfer is the difference in solubility of one material (the solute) in two other immiscible or partially miscible streams (the feed and the solvent). The feed and solvent streams are mixed and then separated, allowing the solute to transfer from the feed to the solvent. Normally, this process is repeated in successive stages using counter-current flow. The solute-rich solvent is called the extract as it leaves, and the solute-depleted feed is the raffinate. When there is a reasonable density difference between the feed and solvent streams, extraction can be accomplished using a vertical column, although in other cases a series of mixing and settling tanks may be used.

In this experiment, the operational goal is to extract isopropanol (IPA, ~10 - 15 wt. %, the solute) from a mixture of C₈-to-C₁₀ hydrocarbons using pure water as solvent. A York-Scheibel type (vertical mixers and coalescers, one each per physical stage) extraction column is available. Like most extractors, the overall efficiency (number theoretical stages/physical stages) of this column is quite low, especially in comparison to many distillation columns. The low efficiencies arise from both slow mass transfer (two liquid resistances instead of one as in distillation) and often also from maldistribution of the phases. The effect of agitator speed on both the solute recovery in the extract and the overall column efficiency will be evaluated.

In this experiment, the properties of n-nonane are a good approximation to those of the hydrocarbon mixture for equilibrium data purposes. The ternary system water/isopropanol/n-nonane exhibits Type I equilibrium behavior (there is some composition range over which phase splitting will not take place) at room temperature. The equilibrium data for this system can be found in the Appendix.

1. Operating the York-Scheibel Column

1. Fill the extractor with hydrocarbon mixture /IPA feed (if

Figures 3 and 4 show results when both the agitation and feed flow rates were varied over a wide range. The overall efficiency and recovery increase before becoming asymptotic, which is fairly typical of liquid-liquid extractors that are not at or near flooding. At near flooding conditions, the overall efficiency and recovery are expected to sharply decrease. Note that, unlike distillation, flooding can take place in liquid-liquid extraction at either high solvent or high feed rates (or r

Liquid-liquid extraction (LLE) is an alternative to distillation which relies upon solvent-feed immiscibility (or slight miscibility) and favorable solute partition coefficients to attain high solute recoveries in a solvent phase at as low a solvent/feed ratio as practical. Although the range of flows (the "turndown") over which LLE will be effective is often limited, and while stage efficiencies are low such that phase equilibrium is not attained, certain mixtures just cannot be separated using other methods in

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