Impact of osmotic and thermal isolation barrier on concentration and temperature polarization and energy efficiency in a novel FO-MD integrated module

Abstract

In this study, a novel integrated forward osmosis - membrane distillation (FO-MD) module equipped with an isolation barrier carefully placed between the FO and MD membranes is experimentally investigated, and its performance is compared with a conventional hybrid module. The function of the isolation barrier is to osmotically and thermally separate the FO draw solution (DS) and MD feed channels. A systematic approach is adopted to compare the flux through both modules under (i) different and similar hydrodynamic conditions, (ii) different DS concentrations and temperatures, and (iii) different feed solution concentrations. All experiments were performed for 9 h each in batch mode using a custom-made compact module. New FO and MD membrane sheets were mounted for each experiment to ensure similarity in operating conditions. The proposed module design increased the flux by 22.1% using the same module dimensions but different hydrodynamic conditions. The flux increased by 16.6% using the same hydrodynamic conditions but different module dimensions. The FO/MD energy ratio reduced from 0.89 to 0.64 for the novel module, indicating better utilization of energy (primarily from MD). The gain output ratio (GOR) increased on average by 15.8% for the novel module compared to the conventional module, with a maximum increment of 20.7%. The temperature and concentration polarization coefficients in the MD operations showed improvements of 17.4% and 2.6%, respectively. The presence of the isolation barrier inside the integrated module indicated promising improvements of the flux and internal heat recovery, and further significant enhancements are expected for larger scale modules. Additionally, the novel module design offers unprecedented process integration opportunities for FO-MD as well as other membrane hybrid systems.