(652a) Theoretical Comparison of Direction Contact Membrane Distillation (DCMD) and Liquid Gap Membrane Distillation (LGMD)

Traditional membrane distillation processes include Direct Contact Membrane Distillation (DCMD, Fig.1.a), Air Gap Membrane Distillation (AGMD, Fig.1.b), Vacuum Membrane Distillation (VMD, Fig. 1.c), and Sweep Gas Membrane Distillation (SGMD not shown in the Figure). Each has its own advantages and disadvantages. For example, DCMD produces high flux, and therefore productivity. AGMD offers high energy efficiency, and VMD is suitable for the recovery of valuable volatiles. In all membrane distillations, hot feed stream is introduced on one side of the membrane, and cool distillate (in DCMD), an air gap with a condensing surface (AGMD), or a vacuum of sweeping gas (VMD and SGMD) is introduced on the opposite side. In the presentation, the Liquid Gap Membrane Distillation (LGMD), which combines the operational simplicity of a DCMD and the energy recovery in AGMD, is introduced and a theoretical study of DCMD and LGMD will be the focus of the presentation. Transport modeling approaches will be discussed. The results from simulation using Engineering Equation Solver (EES) for DCMD and LGMD systems will be presented. Modeling results of typical DCMD systems and LGMD systems for desalination application under different operation conditions will be compared. The normalized water vapor flux of a membrane module (i.e. water vapor permeability of a membrane module) for DCMD and LGMD are graphed against thermal efficiency (Figure 2). For applications powered by sufficient, low cost heat, DCMD systems offer higher normalized vapor flux up to a Gained Output Ratio of ~ 3. For applications asking optimum thermal efficiency, therefore, a GOR higher than 3.0, LGMD offers higher normalized flux. This methodology of comparing normalized flux provides a theoretical guidance on the selection of technologies based on DCMD or LGMD.