Effects of net-type spacers on heat and mass transfer in direct contact membrane distillation and comparison with ultrafiltration studies

The study of heat and mass transfer enhancement in direct contact membrane distillation (DCMD) by the net-type spacers was intended to develop the heat transfer correlation for the spacer-filled channels. DCMD experiments performed with 20 spacers different in voidages and hydrodynamic angles were used to determine the spacer performance in heat and mass transfer enhancement. The effect of high mass transfer rates on heat and mass transfer coefficients was also investigated to explain the experimental results compared with ultrafiltration (UF) with the spacers. The results showed that spacers enhanced mass fluxes up to 60% and increased heat transfer coefficients by approximately two times over the empty channels. The optimum spacer geometry was found at the voidage and hydrodynamic angle of 0.6 and 90°, respectively. The heat transfer correlation for spacer-filled channel was obtained. The analysis of high mass transfer rate convection showed that the effect of high mass transfer rates on mass transfer coefficients was remarkable for ultrafiltration but was insignificant for membrane distillation.