Transport studies in pervaporation.

Abstract

In the theoretical part of this work, mathematical equations were derived for describing steady state pervaporation transport considering the chemical potential gradient as the driving force for the flow of penetrant. The membrane is split perpendicular to the penetrant flow direction into small segments in which an imaginary liquid (or vapor) phase is in thermodynamic equilibrium with each membrane segment. The mathematical equations obtained for pure penetrant permeation are the same as derived by the pore flow model. Based on the analysis of binary mixture system, the possibility of concentration polarization phenomena occurring inside the membrane was pointed out. In the analysis of binary mixture system, coupling was considered in the liquid-filled region of the membrane but no coupling was considered in the vapor-filled region of the membrane. The theoretical prediction of concentration polarization occurring inside the membrane was substantiated by experimental data. In the experimental part of the work separation of acetic acid/water mixture by pervaporation in the entire composition range was investigated using symmetric and dense aromatic polyamide membrane. Aromatic polyamide is a highly hydrophilic material. The effect of downstream pressure on the pure component permeation was studied at 25$\sp{\circ}$C. First, the penetrant concentration profiles of pure water inside the membrane were established at different downstream pressures by performing steady state pervaporation experiments. Secondly, the profiles of the binary penetrant mixture (acetic acid/water) inside the membrane were established. The process variables studied were; feed temperature (25$\sp{\circ}$C, 35$\sp{\circ}$C and 40$\sp{\circ}$C), downstream pressure (467 Pa, 1200 Ps and 2666 Pa) and feed composition covering the complete range of the binary mixture composition. These experiments were performed by using a stack of identical membranes during steady state pervaporation, stopping the pervaporation experiment, dividing the stack into substacks, desorbing and analyzing the penetrants sorbed from each substack. Sorption experiments were performed from liquid phase and vapor phase for the binary mixture of acetic acid-water. The sorption experiments from the liquid phase were performed at 25$\sp{\circ}$C, 35$\sp{\circ}$C and 40$\sp{\circ}$C for the entire binary mixture composition range. Vapor sorption isotherms were also established at these temperatures. The value obtained for the amount of penetrant in the membrane during steady state pervaporation in some cases was higher than the corresponding equilibrium sorption value. This seems impossible from the thermodynamic point of view. It was concluded that there are two different equilibria. The first one is the static equilibrium achieved during sorption experiments and the second one is the dynamic equilibrium achieved during pervaporation experiments. The structure of the polymeric membrane is different under these two circumstances. Based on the experimental data a novel design for the pervaporation membrane has been proposed. State of permeant (penetrant) study for acetic acid-water-polyamide system was performed using differential scanning calorimetry technique. Acetic acid present in the membrane did not show any response in the thermogram corresponding to the phase change. It was concluded that acetic acid present in the membrane was in non-freezable bound state. Some spectroscopic studies were also undertaken using FTIR-ATR (Attenuated Total Reflection) technique to study the penetrant interaction with the polymeric membrane. Penetrant concentration profiles predicted from the newly developed transport model were compared with the experimental penetrant profiles.