Effect of inner-layer thermal conductivity on flux enhancement of dual-layer hollow fiber membranes in direct contact membrane distillation

The thermal conductivity of hydrophobic/hydrophilic hollow fiber membranes is a key factor affecting flux in direct contact membrane distillation processes. In this paper both experimental work and mathematical modeling are performed to investigate the effect of the hydrophilic layer's thermal conductivity on vapor flux. Modeling predicts a significant increase in vapor flux from 31.4 to 78.5kgm^-2h^-1 under the feed and distillate temperatures of 80 and 20°C, respectively, when the thermal conductivity of the inner hydrophilic layer is varied from 0.2 to 1.4Wm^-1K^-1. To investigate this experimentally, graphite particles and multiwall carbon nanotubes (MWNT) were embedded for the first time into the hollow fiber hydrophilic layer to improve its thermal conductivity. It was found that incorporating graphite alone led to only a minor improvement in thermal conductivity. However, by incorporating both graphite and MWNT the thermal conductivity was increased from 0.59 to 1.30Wm^-1K^-1. This improvement is attributed to the conducting network formed by the MWNT which bridges the polymer nodules. The improved thermal conductivity led to a significant increase in vapor flux from 41.2 to 66.9kgm^-2h^-1, under the inlet feed and distillate temperatures of 80.4 and 15.3°C, respectively, in general agreement with the mathematical modeling.