Back-extraction process operation and modeling through thermodynamic equilibrium solubility of valeric acid in aqueous and organic phase mixtures

Abstract

Back-extraction is one of the techniques often used in pharmaceutical production processes to remove either products or excess chemical reactants, dissolved in a liquid organic solvent, such as valeric acid (VA) chloride salt compound, which has to be separated from the toluene (T) phase system by a polar aqueous (AQ) solution. In this application-based work, the equilibrium solubility data of VA for the T/AQ solutions at different constant pH values in the range from 5.7 to 10.8 were measured at a calibrated ambient temperature of 23 °C, and at an assigned pH of 8.0 also at a lower temperature of 15 °C. At higher pH of initial aqueous solutions, greater distribution coefficients, mass transfer, and removal efficiencies of VA were found. Solute saturation concentration was modeled by the electrolyte non-random two-liquid (NRTL) thermodynamic model, simulated, and compared with various experimental sets. A comparison showed that predictive physical-chemical description is able to qualitatively describe the VA properties of the separation at differing pH at 23 °C, while the computation with the regressed NRTL binary molecule-molecule parameters for the two binary pairs at a pH of 5.7 slightly better defines the behavior at a pH of 5.7 or 8.0. Raw analysis records, predictions, and optimization could be applied for the partition unit design for splitting the remaining valeroyl chloride after the reaction over the immiscible fluid boundary without thermal energy input. The study is not able to capture the resulting temperature effect on the other thermodynamic relations for VA well enough, probably due to parameterization.