Solubility of unsaturated carboxylic acids in supercritical 1,1,1,2-tetrafluoroethane (HFC 134a) and a methodology for the separation of ternary mixtures

Abstract

The solubility of a variety of unsaturated carboxylic acids is determined in supercritical tetrafluoroethane (HFC 134a) at 90 °C using a capacitative method. The solubility is found to decrease in the order crotonic- > 6-methoxytetralone- > methylsuccinic- > α-acetamido-cinnamic- > itaconic acid. In all cases the solubility of the acid in HFC 134a was one to two orders of magnitude greater than that in CO₂ under comparable conditions; equivalent to ca. 20 fold reduction in the energy required to compress the gas needed to dissolve one mole of substrate. The solubility of the ternary system HFC 134a, methylsuccinic and itaconic acids was measured as a function of pressure and the dissolution selectivity for the two acids was characterised. The minimum number of stages required to separate methylsuccinic from itaconic acid is determined using equilateral triangle diagrams.