The Influence of the Hydrodynamic Conditions on the Performance of Membrane Distillation

Abstract

Membrane distillation (MD) is an evaporation/condensation process of volatile components through a hydrophobic porous membrane. The maintenance of gas phase inside the membrane pores is a fundamental condition required to carry out the MD process. A hydrophobic nature of the membrane prevents liquid penetration into the pores. Membranes having these properties are prepared from polymers with a low value of the surface energy, such as polypropylene (PP), polytetrafluoroethylene (PTFE) or polyvinilidene fluoride (PVDF) (Alklaibi & Lior, 2005; Bonyadi & Chung, 2009, Gryta & Barancewicz, 2010). Similar to other distillation processes also MD requires energy for water evaporation. The hydrodynamic conditions occurring in the membrane modules influence on the heat and mass transfers, and have a significant effect on the MD process efficiency. The MD separation mechanism is based on vapour/liquid equilibrium of a liquid mixture. For solutions containing non-volatile solutes only the water vapour is transferred through the membrane; hence, the obtained distillate comprises demineralized water (Alklaibi & Lior, 2004; Gryta, 2005a; Schneider et al., 1988). However, when the feed contains various volatile components, they are also transferred through the membranes to the distillate (ElBourawi et al., 2006; Gryta, 2010a; Gryta et al., 2006a). Based on this separation mechanism, the major application areas of MD include water treatment technology, seawater desalination, production of high purity water and the concentration of aqueous solutions (El-Bourawi et al., 2006; Drioli et al., 2004, Gryta, 2006a, 2010b; Karakulski et al., 2006; Martinez-Diez & Vazquez-Gonzalez, 1999; Srisurichan et al., 2005; Teoh et al., 2008). A few modes of MD process are known: direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD), vacuum membrane distillation (VMD) and osmotic membrane distillation (OMD). These variants differ in the manner of permeate collection, the mass transfer mechanism through the membrane, and the reason for driving force formation (Alklaibi & Lior, 2005; Gryta, 2005a). The most frequently studied and described mode of MD process is a DCMD variant. In this case the surfaces of the membrane are in a direct contact with the two liquid phases, hot feed and cold distillate (Fig. 1). The DCMD process proceeds at atmospheric pressure and at temperatures that are much lower than the normal boiling point of the feed solutions. This allows the utilization of solar heat or so-called waste heat, e.g. the condensate from turbines or heat exchangers (Banat & Jwaied, 2008; Bui et al., 2010; Li & Sirkar, 2004).