(691e) Understanding the Impact of Membrane Properties and Transport Phenomena on the Energy Efficiency of a Membrane Distillation Desalination System

Direct contact membrane distillation (DCMD) is a promising thermal desalination process that can treat high salinity waters using low-grade heat. DCMD has several advantages from the perspective of the water-energy nexus. These include the utilization of waste heat (below 100ºC), perfect rejection of nonvolatile solutes, low areal footprint, and high scalability. However, currently, 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 (η_II) of a DCMD desalination system with external heat recovery. We begin by developing a fundamental understanding of mass and heat transport through an MD membrane, before modeling how membrane properties and transport phenomena affect entropic losses in an MD system. In particular, we analyze how the membrane permeability coefficient (B) and thermal conduction coefficient (K) impact MD performance. We show that increasing the B value of a membrane by reducing its thickness, initially leads to an increase in η_II before conductive heat loss through the membrane causes η_II to fall. For a typical MD membrane with a porosity of 0.90, material thermal conductivity of 0.20 W m^-1 K^-1, and a nominal pore diameter of 0.6 μm, we find that the optimal permeability coefficient is 1.59×10^-6 kg m^-2 s^-1 Pa^-1 (572 kg m^-2 h^-1 bar^-1). This value corresponds to an optimal membrane thickness of around 95 μm. Our analysis stresses the importance of effective heat recovery in DCMD. We show that an external heat exchanger with a minimum approach temperature of 10ºC reduces energy consumption by 63%. Finally, we demonstrate that increasing the ratio B/K, 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/K, leads to a 42% increase in η_II 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 1.0 bar to 0.5 bar, results in a 21% increase in η_II 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 1.0 bar and 12% for a bulk membrane pressure of 0.5 bar, given perfect heat recovery.