Understanding the impact of membrane properties and transport phenomena on the energetic performance of membrane distillation desalination

Abstract

Direct contact membrane distillation (DCMD) is a thermal desalination process that is capable of treating high salinity waters using low-grade heat. As a water treatment process, DCMD has several advantages, including the utilization of waste heat (below 100℃), perfect rejection of nonvolatile solutes, low areal footprint, and high scalability. However, the energy efficiency of DCMD is relatively low compared to other work-based and thermal desalination processes. In this study, we aim to quantify how membrane properties and process conditions affect the exergy or second-law efficiency (ηII) of a DCMD desalination system with external heat recovery. In particular, we analyze how the membrane permeability coefficient (B) and thermal conduction coefficient (K̅) impact MD performance. We show that increasing the B value of a membrane by reducing its thickness, initially leads to an increase in ηII before conductive heat loss through the membrane causes ηII to fall. For a typical MD membrane with a porosity of 0.90, material thermal conductivity of 0.20Wm−1K−1, and a nominal pore diameter of 0.6μm, we find that the optimal permeability coefficient is 1.59×10−6kgm−2s−1Pa−1 (572kgm−2h−1bar−1). This value corresponds to an optimal membrane thickness of around 95μm. Our analysis stresses the importance of effective heat recovery in DCMD. We show that an external heat exchanger with a minimum approach temperature of 5℃ reduces energy consumption by 72%. Finally, we demonstrate that increasing the ratio B/K̅, rather than just the B value, is key to increasing the exergy efficiency of DCMD desalination. For example, increasing membrane porosity from 0.70 to 0.90, which yields a 160% increase in B/K̅, leads to a 42% increase in ηII from 5.3% to 7.6%. The advantages of reducing the bulk pressure (P) in the membrane pores are also explored. For a typical membrane, halving P from 1.0bar to 0.5⁢bar, results in a 21% increase in ηII from 7.0% to 9.2%. We conclude by identifying that the maximum exergy efficiency achievable as membrane porosity tends to unity is 10% for a bulk membrane pressure of 1.0bar and 12% for a bulk membrane pressure of 0.5bar, given perfect heat recovery.