Heat transport and membrane distillation coefficients in direct contact membrane distillation

This work aims to provide detailed understanding of heat transport in direct contact membrane distillation (DCMD). The influence of mass transfer on heat transfer rates and on the heat transfer coefficient was identified, and the relative significance of each heat transfer mechanism was evaluated. The role of spacers in heat transfer improvement was analyzed. Alternative methods to evaluate the membrane thermal conductivity were also proposed. The heat transfer analysis of the experimental results showed that the effects of mass transfer on the heat transfer rates and on the film heat transfer coefficients were negligible. The heat transfer due to the vapor flow (q(v)) in the membrane was equal to or greater than the heat conduction (q(c)) for the membranes studied and increased with the feed temperature. When the feed temperature was lower than 323 K, the heat loss due to heat conduction across the membrane was the major contribution of the total heat transfer in the membrane. In addition, the temperature distributions in the membranes were closely linear. The membrane distillation (MD) coefficients for each membrane were constant over the flow rates and temperatures studied. The flow pattern in the spacer-filled channel was probably transition flow rather than turbulent flow. The alternative models for calculating the membrane thermal conductivity showed better agreement than the commonly used model. (C) 2002 Elsevier Science B.V. All rights reserved.