AN EXPERIMENTAL COMPARISON OF PERFORMANCE BETWEEN FLAT SHEET AND HOLLOW FIBER MEMBRANE MODULES IN DIRECT CONTACT MEMBRANE DISTILLATION SYSTEM FOR DESALINATION OF SEAWATER

Abstract

Membrane distillation desalination is an emerging technology with a promise to considerably reduce environmental impacts associated with traditional desalination technologies such as thermal and reverse osmosis. Various membrane module configurations have been reported in the literature. The most popular configuration reported in the literature was the direct contact and can be used for both flat sheet and hollow fiber membranes. The literature study has shown that the bulk of studies involved either flat sheet or hollow fiber modules separately and no single study involved both configurations in a single research work. This investigation was therefore aimed at comparing the flux performance of flat sheet and hollow fiber direct contact modules under similar conditions using real seawater from the Qatar coastal area of the Arabian Gulf. Our work has shown that the flat sheet direct contact membrane distillation module gave consistently higher fluxes than the hollow fiber module. The highest flux measured for the flat sheet module at 65 C and 3 L/min was 37.1 L/m2.h compared to 5.2 L/m2.h for the hollow fiber module under identical experimental conditions. The large difference in flux between the flat sheet module and the hollow fiber module was explained in terms of difference in flow regime and convective heat transport in the flow compartment of the two modules. The flow channel Reynolds numbers ranged between 283 to 770 for the flat sheet module while the Reynolds numbers ranged between 106 to 287 for the hollow fiber module. Our work also indicated that the Nusselt number Nu can be as much as 11 times higher for the flat sheet module than for the hollow fiber. This clearly indicates that the convective heat transfer coefficient in the hot side is much greater in the flat sheet module than in the hollow fiber module. The maximum hot side convective heat transfer coefficient for the flat sheet module was estimated to be 17044 W/m2.h compared to 2163 W/m2.h for the hollow fiber module. The values of thermal coefficients obtained in this work were consistent with literature values. The observed salt rejection for all used membranes is above 99.8% thus confirming the ability of DCMD desalination to produce very pure water. This work has shown that more work needs to be done in hollow fiber module design to enhance flow turbulence and therefore improve convective thermal coefficients which will lead to higher permeate fluxes.