Optimising thermal efficiency of direct contact membrane distillation by brine recycling for small-scale seawater desalination

A technique to optimise thermal efficiency using brine recycling during direct contact membrane distillation (DCMD) of seawater was investigated. By returning the hot brine to the feed tank, the system water recovery could be increased and the sensible heat of the hot brine was recovered to improve thermal efficiency. The results show that in the optimal water recovery range of 20 to 60% facilitated by brine recycling, the specific thermal energy consumption of the process could be reduced by more than half. It is also noteworthy that within this optimal water recovery range, the risk of membrane scaling is negligible — DCMD of seawater at a constant water recovery of 70% was achieved for over 24 h without any scale formation on the membrane surface. In contrast, severe membrane scaling was observed when water recovery reached 80%. In addition to water recovery, other operating conditions such as feed temperature and water circulation rates could influence the process thermal efficiency. Increasing the feed temperature and reducing the circulation flow rates increased thermal efficiency. Increasing the feed temperature could also mitigate the negative effect of elevated feed concentration on the distillate flux, particularly at a high water recovery. Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.

A reduction of over 50% in the specific thermal energy consumption of DCMD of seawater could be achieved by brine recycling. The experimental results reveal an optimal water recovery range of 20 to 60% with respect to thermal efficiency. A high water recovery beyond this optimal range led to an increase in the risk of membrane scaling. Indeed, severe membrane scaling was observed at 80% water recovery. On the other hand, DCMD of seawater at water recovery of up to 70% was operated for over 24 h without any observable membrane scaling. Results reported here suggest that the risk of membrane scaling within the water recovery range for an optimal thermal efficiency is negligible. In addition to water recovery, feed temperature and water circulation rates could also influence the process thermal efficiency. Elevating feed temperature and reducing circulation rates increased thermal efficiency. Increasing feed temperature also helped reduce the negative effect of increased feed salinity on water flux at a high water recovery.