Integration of vacuum and sweep gas pervaporation to recover organic compounds from wastewater

Abstract

Pervaporation with hydrophobic membranes has been widely recognised as a possible process to recover organic compounds from wastewater. Compared to vacuum pervaporation, on which many researchers have focused, sweep gas pervaporation has received little attention. The aim of this study was to analyse opportunities for integrating and optimising both process layouts for the treatment of wastewater. The focus was on hollow fibre modules. Two module configurations of hollow fibre modules were considered: (1) shell-side and (2) tube-side feed flow. An advanced simulation program based on a phenomenological/semi-empirical model was used. The influence of (1) process parameters such as permeate pressure and the size of the sweep stream per module, and of (2) module design parameters such as void fraction or module configuration was determined for two model substances pyridine and phenol. Based on the simulations, guidelines for the optimisation of pervaporation are presented. These include the observation that for vacuum pervaporation shell-side feed flow is superior, whilst for sweep gas pervaporation tube-side feed flow should be selected. In the former case and for a given feed rate per module, the void fraction within the module should be selected as low as possible to reduce the effect of concentration polarisation. This approach is, however, limited by the pressure resistance of packed fibres causing an increasing pressure gradient on the feed side. For hydrophobic pervaporation of wastewater, sweep gas pervaporation should be combined with a moderate vacuum (of around 0.1 bar) to improve the pervaporation performance; the performance at atmospheric pressure for the conditions selected leads to excessive membrane areas. Similar to vacuum pervaporation the void fraction should be selected as high as possible for tube-side feed flow, and as low as possible for shell-side feed flow.