(432a) Vacuum-Enhanced Direct Contact Membrane Distillation: Modeling and Effect of Membrane Support Layer On Mass Transfer

G. Rao¹, A. Achilli ², and A. E. Childress¹

¹Department of Civil and Environmental Engineering, University of Nevada Reno, Reno, NV 89557

² Environmental Resources Engineering Department, Humboldt State University, Arcata, CA 95521

Membrane distillation (MD) is a promising technology to produce drinking water because of its lower pressure and membrane mechanical strength requirements and almost complete rejection of salts and organics compared with pressure driven membrane processes such as reverse osmosis (RO). Practical application of MD is limited by its relatively low water flux. Vacuum-enhanced direct contact MD (VEDCMD) could be an attractive alternative because it shows 2-30 times higher water flux than direct-contact MD (DCMD) [1]. This study aims to develop a model for water flux prediction in VEDCMD, evaluate the value of vacuum enhancement, and investigate the effect of membrane support layer on mass and heat transfer.

Two mathematical models (Model 1 and Model 2) were developed for VEDCMD. Model 1 assumes that viscous flow and Knudsen diffusion are combined in parallel, and then they are combined with molecular diffusion in series. Model 2 assumes that Knudsen diffusion and molecular diffusion are combined in series, and then they are combined with viscous flow in parallel. Water fluxes predicted by both models using different temperatures and membranes were compared with experimental results. The experiments were performed using four different single-layer membranes under feed temperatures of 40 and 60 ^oC and distillate temperature of 20 ^oC in a bench-scale VEDCMD system.

The effects of vacuum-enhancement were also evaluated. In cases of low fouling solutions, vacuum-enhancement is desirable as it results in higher flux and does not significantly increase the rate of membrane fouling. The cost of vacuum enhancement was also compared with other methods to increase flux.

The effect of support layer on mass transfer was investigated as well by comparing experimental results of seven intact membranes (active layer plus support layer) with experimental results obtained with active layers only from each of these membranes. It was found that water fluxes of the membranes with active layers only were up to 60% higher than water fluxes of the intact membranes. Membrane characteristics were linked to water flux and results will be presented.

Reference:

[1] T.Y. Cath, V.D. Adams, Amy E. Childress, Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement. Journal of Membrane Science, 2004. 228: 5 -16.