(578g) Solvent Design for Acid Extraction

Liquid-liquid extraction of carboxylic acids has been widely investigated. Several solvents, including low boiling esters ethers and ketones as well as high boiling reactive solvents have been proposed. The extraction capacity of low boiling solvents is poor, and economics of downstream processing of laden low boiling solvents fails because of disastrous energy consumption. Trialkylphosphane oxides and trialkylamines provide excellent partition properties. Trialkylphosphane oxides may be applied without dilution. They may also be mixed with hydrocarbon based diluents to adjust the viscosity, density and interface tension. Interaction of trialkylphosphane oxide based solvents with carboxylic acids is of Lewis acid – Lewis base character. Distillative downstream processing of laden trialkylphosphane oxide based solvents is simple, and it does not need excessive energy. Lately, trialkylamine based solvents seem to become fashionable again. Trialkylamine based solvents interact with carboxylic acids by ammonium salt formation. The laden solvent forms greasy salts. The physical properties of the basic reactive constituent must therefore be severely corrected by admixing modifiers and diluents. Distillative separation of carboxylic acids from laden trialkylamine based solvents demands high temperature (to break up the salt) and it needs an entrainer for separation. Carboxylic acids form heterogeneous acetotropes with hydrocarbons. In distillative separation of carboxylic acids from trialkylamine based solvents we may therefore make use of the diluent. However, these technological limitations do not really suggest to make trialkylamine based solvents the first choice in solvent selection for carboxylic acid separation. In conclusion, trialkylphosphane oxide based solvents offer several advantages in carboxylic acid isolation from aqueous feed over low boilers and over trialkylamine based solvents. Trialkylamine based solvents may show interesting potential in acid extraction with downstream processing by reextraction.