Entropy generation in multi-stage flash desalination plants

A second-law analysis of two types of MSF desalination plants (once-through OT and with heat rejection section HR) is conducted. Fixing the number of stages to 20, the HR configuration is found to present clearly less entropy generation but more specific area than the OT configuration. The detailed investigation showed that heat transfer is responsible of more than 65% of the irreversibility occurring in a stage. The 30% reduction of the entropy generation number when passing from OT to HR is totally due to the reduction of the heat-transfer irreversibility. The variation of the number of stages for OT configuration has no effect on the entropy generation. Besides, we noted that beyond 10 stages the specific area is not expected to vary noticeably. For the HR configuration, there is a continuous decrease of the entropy generation number accompanied by a continuous increase of the specific area. The comparison between the two plants showed that HR is not necessarily better than OT. The decision of which of them is the most competent depends on the number of stages and on the relative importance we attach to the irreversibility and to the specific area.