The effect of increased top brine temperature on the performance and design of OT-MSF using a case study

A mathematical model of a Once-Through Multi-Stage-Flash (OT-MSF) desalination system is developed. This study shows the impact of top brine temperature (TBT) of up to 160 °C on both the design and performance characteristics of MSF systems. Such a high TBT can be achieved by nanofiltration pretreatment to remove scale-forming compounds. System performance is evaluated by the thermal performance ratio (PR) and the required specific area (sA). For a fixed brine reject temperature (Tend) and inter-stage temperature drop (ΔT), adding stages results in the TBT increasing by ΔT for each stage added and the PR increases monotonically with the TBT. On the other hand, the required sA decreases and then increases again beyond a certain TBT. The Sirte desalination plant in Libya is taken as a case study. It is found that by increasing the TBT to 161 °C from a typical value of 118 °C keeping Tend and ΔT fixed; the PR can be increased by 41.5%, reaching a value of 14.6 while the required sA increases by 0.9%. Although there is a penalty in terms of the increased number of stages required to achieve this arrangement, there is a clear advantage in terms of PR, with a relatively small compromise in sA.