Experimental study of air gap and direct contact membrane distillation configurations: application to geothermal and seawater desalination

Abstract

The membrane distillation (MD) is a process for vapour extraction from aqueous solution at temperature which my be much lower than the boiling point of the solution. This operation is realized by means of a micro porous hydrophobic membrane separating a warm solution from a cooler chamber which contains either liquid or gas. The membrane distillation has been used to concentrate at high level different kinds of solutions, to remove salt from seawater in semi-industrial applications and to separate alcohol-water solutions in biotechnological systems. Recently, this process has been used in water desalination and wastewater treatment. So far, essentially two different configurations of membrane distillation processes have been used, namely direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD). Several authors have performed experimental and theoretical analysis of both processes, and desalination equipment based on DCMD and AGMD is commercially available. The large air gap in AGMD allows for the existence of important temperature and partial pressure gradient. This paper objective is to realize DCMD and AGMD experimental tools and to study the effect of the following operating variables: feed concentration, feed flow, feed temperature and the air gap width. The flux up to 3–5 kg/m 2h was achieved and the salt rejection is about 97%. The main advantage of membrane distillation lies in its simplicity and the need for only small temperature differentials. The AGMD configuration is more adapted to desalination of geothermal resources, and the need of energy is lower — only for pumping.