Abstract
Membrane distillation (MD) is a promising thermally driven membrane separation technique. In MD, water vapor is being separated from the hot feed water solution using a microporous hydrophobic membrane, due to the difference in temperature, and hence vapor pressure, across the membrane. Air gap membrane distillation (AGMD) process is one of the common configurations of applying the MD process for water desalination and other applications. In AGMD, a stagnant air gap is introduced between the membrane and a condensation surface within the membrane module to reduce the conduction heat loss through the membrane. In this review article, design characteristics and operating conditions of AGMD and its modified designs to enhance the productivity and reduce the energy consumption are surveyed and discussed. Previous work on pilot AGMD systems and multi-stage or multi-effect systems with energy saving modules is highlighted. Membrane materials and developments used with the AGMD modules are presented with discussion of membrane fouling and scaling problems. In addition, modeling techniques based on the heat and mass transfer equations and simulation approaches of the AGMD process are presented. The merits of operating the AGMD systems with solar and other renewable energies are discussed along with the economic aspects. The future research directions of AGMD are highlighted in this review. This will help researchers to direct their research without repetition of previous known studies.