Small steps toward membrane distillation commercialization

Abstract

Membrane distillation (MD)1 is a relatively new process that is under investigation worldwide as a low cost, energy saving alternative to conventional separation processes such as distillation and reverse osmosis. MD has many advantages compared to other, more popular separation processes. It works at room conditions (pressure and temperature), and low-grade, waste, and/or alternative energy sources such as solar and geothermal can be used to power it. MD exhibits a very high level of rejection with inorganic solutions, and the necessary equipment is small. Furthermore, since the process appeared in the late 1960s, proponents have claimed that would be cost effective. As such, it has been studied by academic experimentalists and theoreticians around the world. In industry, however, MD has gained little acceptance and is yet to be implemented. The major barriers to commercialization includeMDmembrane andmodule design, membrane pore wetting, low permeate flow rate and flux decay, and uncertain energetic and economic costs. The driving force in MD processes is the vapor pressure difference across the membrane, which results from an imposed temperature difference. The lower vapor pressure on the permeate side can be set up in various ways: direct contact MD (DCMD),2 osmotic MD,3 sweeping gas MD,4 vacuumMD,5 and air gap MD.6, 7 Our research focuses on the investigation and optimization of DCMD. In this process, a hot solution (feed) is brought into contact with one side of the membrane and a cold solution (permeate) into contact with the other, so that the vapor pressure is different on each side of the membrane. This pressure difference drives the vapor through themembrane pores, and then the vapor condenses in contact with the cold solution on the other side. The hydrophobic nature of themembrane prevents the penetration of the liquid solution into the pores unless a pressure higher than the so-called liquid entry pressure is applied. This Figure 1. Experimental setup. The two double-wall reservoirs contain the feed and permeate solutions. Both variable flow gear pumps drive both solutions. Liquid flow sensors set at the inlet and outlet of each semicell measure the flow rate. Digital pressure transducers and PT100 temperature probes measure the pressure and temperature on both sides of the membrane. Thermostats and heat exchangers connected to the reservoirs controll the temperature.