Applicability of the Perturbed Hard Chain Equation of State for Simulation of Distillation Processes in the Oleochemical Industry. Part I: Separation of Fatty Acids

Abstract

Fatty acids constitute an important group of chemicals with extensive end-use markets, either for their direct uses or for the intermediate uses of their numerous derivatives. These oleochemical products are currently purified or fractionated using distillation at subatomspheric pressures. Due to the increasing interest of the pharmaceutical industry in some fatty acids with high purity, supercritical fluid extraction followed by appropriate fractionation steps are also employed. It would therefore be convenient to utilize a single thermodynamic model that is capable of predicting all thermodynamic properties needed, and covering the whole pressure range which is currently applied in both industries. Different equations of state and activity coefficient models were scanned, and the perturbed hard chain equation of state (PHC-EOS) provided results which are as good as those obtained using activity coefficient models, and in some cases better. Since there are very scarce experimental data on the behavior of binary mixtures containing species with associative tendency, an attempt was also made to incorporate a correlation for the dimerization constants to account for chemical association of these higher carboxylic acids. Using a comprehensive and thermodynamically screened vapor pressure data base, pure component parameters for the C₆-C₂₀ fatty acids were computed. The fitted parameters were incorporated in the PHC-EOS and resulted in good reproduction of binary vapor-liquid equilibria. Calculations of multicomponent mixtures were performed using these binary parameters. A flow-sheeting program was utilized to simulate a distillation process consisting of two integrated columns, operated at subatmospheric pressures, for the purification of different hydrogenated fatty acid feedstocks. The simulated flow rate, concentration, and temperature profiles were compared with some real operating data obtained from a major oleochemical plant. Analysis of this comparison revealed that the PHC-EOS is very well suited for simulating different distillation processes for the purification and fractionation of C₆-C₂₀ fatty acids. Furthermore, the need for more accurate thermodynamic information describing the chemical association of these compounds was also accentuated.