Desalination using membrane distillation: Flux enhancement by feed water deaeration on spiral-wound modules

Membrane distillation (MD) is considered a promising technology for desalination applications. In this paper, the main transport phenomena in membrane distillation are analyzed theoretically. One of the main limiting factors is the molecular diffusion resistance caused by stagnant air in the membrane pore volume. A great potential for flux enhancement has been identified to be the removal of air in the membrane pores. Preliminary experiments on water deaeration using a commercial membrane contactor were carried out. The dynamics of membrane and air gap deaeration using deaerated feed water were studied experimentally. A module test facility was equipped with a feed water deaeration setup. Full-scale spiral-wound modules with a membrane area of 5 and 10 m(2) have been used for comprehensive experimental studies on the use of deaerated feed water. The impact of deaeration at different pressures, feed flow rates, feed water salinities and temperature levels is quantified and discussed. A comparison of module operation with and without preceding feed water deaeration is given. The experimental results include module output rate, thermal energy demand, thermal efficiency and an estimation of the electrical energy demand. In all experiments, the beneficial effect of feed water deaeration can clearly be shown for total flux and thermal energy demand simultaneously. For selected operation points, the relative effect was identified to be more than 5000 and the specific thermal energy demand dropped to values of less than 100 kW(th) m(-3). (C) 2012 Elsevier B.V. All rights reserved.