Conversion of Saline Water to Fresh Water Using Air Gap Membrane Distillation (AGMD)

Abstract

Membrane distillation (MD) is a combination of evaporation of water from saline solution and diffusion of vapour through a hydrophobic membrane. The driving force is the vapour pressure difference created by temperature difference across the membrane. With development of hydrophobic membranes at a cheaper cost, MD process has been able to draw significant attention in contemporary water research. In this dissertation author has studied various aspects of air gap MD process (AGMD). A one dimensional analysis for the overall transport in an AGMD process has been done in the present study considering all the mass transfer resistances present in the process. A two dimensional analysis of the heat and mass transfer process inside the feed chamber has been considered. Energy and species transport equations were solved numerically using finite difference technique. Application of AGMD for freshwater production for on board ships using waste heat has been proposed utilizing the seawater that is used as coolant for marine engines. For this type of application, the AGMD system was provided with multistage to ensure efficient use of available energy. An AGMD system has been developed and its operation under single stage and multi stage mode has been investigated experimentally. With the same feed and coolant temperature, the multistage rig showed 5.6 times better performance than the single stage in terms of water produced per kWh energy input. A specially designed coolant plate was used to manipulate the mass transfer by enhancing the heat transfer during condensation. For the same equivalent air gap, the production enhanced maximum up to 70% compared to a flat coolant plate for a feed temperature of 55 degree C and coolant temperature of 25 degree C.