Comparative study of direct-contact and air-gap membrane distillation processes

Direct-contact and air-gap membrane distillation (DCMD and AGMD, respectively) processes have been modeled as a two-dimensional conjugate problem in which a simultaneous numerical solution of the momentum, energy, and diffusion equations of the feed and cold solutions have been carried out. The results were validated in comparison with available experimental results The two processes have been compared in terms of their sensitivity to the main parameters and the mass transfer resistances of the common domains. The results show, among other things, that the process thermal efficiency of AGMD is higher than that of DCMD by about 6% due to the presence of the air gap. The permeate flux of DCMD is higher than that of AGMD by about 2.3-fold and 4.8-fold for T-hi = 80 and 40 degrees C, respectively. Increase of the thermal conductivity of the membrane material (k(m)) improves the DCMD process by mainly improving the process thermal efficiency and improves the AGMD process by mainly improving the permeate flux.