Effect of transfer in the vapour phase on the extraction by pervaporation through organophilic membranes: experimental analysis on model solutions and theoretical extrapolation

Mass transfer in pervaporation is usually regarded as limited by the solution-diffusion step inside the dense selective polymer layer. In the case of pervaporation for the extraction of volatile organic compounds through organophilic membranes, especially at low feed temperature (about 300 K), the influence of the downstream pressure cannot be neglected. A contribution to the study of the operating parameters on the vapour side in a pilot plant — from the membrane to the condenser — to the overall mass transfer is presented.

A “convection-diffusion” model has been established to calculate the partial pressure gradients in the vapour phase up to the downstream face of the membrane. This equation has been combined with a relation for the mass transfer inside the membrane with a driving force expressed as a difference in fugacities.

The partial permeate pressures and the pervaporate fluxes obtained first with a pure compound (water) and secondly with binary mixtures (water-ethanol) pervaporated through membranes of polydimethylsiloxane (PDMS) on a pilot plant scale are well predicted by the model. Moreover, on the permeate side, the effects of unavoidable non-condensable gases, of the condenser temperature and of the distance between the module and the condenser on the flux and on the selectivity have been established for different total permeate pressures (300–3000 Pa). At high pressure, the pervaporation selectivity towards ethanol exhibits a minimum value as a function of the permeate circuit design.