Mass transport mechanisms and module design aspects in the recovery of dilute volatile organic compounds by hydrophobic pervaporation

Abstract: Hydrophobic pervaporation has gained a lot of attention as an alternative process for mild aroma recovery, but the real commercial applications of this process are still in their infancies. Better understanding of mass transport phenomena in composite pervaporation membranes is essential from the point of view of membrane manufacturing, while process simulation is an important tool for predicting pervaporation module performance. With this in mind, the aim of this thesis was to investigate both the composite membrane performance and the module design aspects for pervaporative aroma recovery. When evaluating the composite membrane performance, the resistance of the membrane was divided into two parts, the active layer resistance and the support layer resistance. Pervaporation experiments were performed on homologous series of alcohols and esters in order to evaluate the influence of component properties, such as molecular size, volatility, chemical nature and polarity on the performance of the composite membrane. Additionally, the sorption experiments on the active layer polymer material were performed and the average diffusivity coefficients were calculated from a steady-state pervaporation flux. Both solubility and diffusivity studies show that there is a tendency towards the clustering of water and small polar molecules, which in turn causes the decrease in diffusivity of the polar permeating species. Regarding the influence of the support layer, a suitable compromise between the nature and geometric characteristics of the support structure and the thickness of the active layer should be made in order to improve the mass transfer characteristics, especially for pressure-sensitive compounds. Process simulation was performed using a modified version of an existing pervaporation simulation tool for aroma recovery, in order to investigate module design aspects of pervaporation. By applying the simulation to four aroma compounds, two alcohols and two esters, the influence of major process and module design parameters on the performance of a single module has been investigated. The results of the simulation show that detailed modelling of single module behaviour is an important aspect of the optimisation of pervaporation plant performance.