Continuous Dialysis of Citric Acid: Solubility and Diffusivity in Neosepta-AMH Membrane

The transport of citric acid through an anion-exchange membrane Neosepta-AMH in a two-compartment continuous dialyzer has been investigated. The basic data obtained were completed by the measurement of the sorption isotherm. Mass transfer rate has been quantified by diffusivity of citric acid in the membrane, which has been determined from the acid concentrations in the streams entering and leaving the dialyzer. For that purpose, a set of ordinary differential equations describing the concentrations profiles in both the compartments of the dialyzer has been numerically solved in the connection with an optimizing procedure. In the mathematical model used, mass transfer resistances in liquid films on both sides of the membrane have been taken into account. All the experiments carried out at steady state (temperature = 25 °C) revealed that first diffusivity of citric acid in the Neosepta-AMH membrane gradually increases with an increasing acid concentration until reaching a weak maximum at an acid concentration of c = 0.464 kmol m–3, then it slightly decreases. In the concentration range of citric acid in the membrane from c = 0 to 0.558 kmol m–3, diffusivity of acid was in the limits from D = 1.79 · 10–12 to 3.33 · 10–12 m2 s–1.