Improved modelling and simulation of once-through and reverse multi-stage flash desalination configurations

An improved model for multistage flash (MSF) structures is developed and used to assess the performance of a novel MSF configuration, termed as MSF reversal (RV-MSF) and consisting of reversing the brine circulation stream. The improved model determines the temperature distribution within the stages using heat balance equations while the simplified one, which is commonly used, is based on pre-specified, constant, and equal temperature distribution throughout the stages. The performance of the RV-MSF is investigated and compared with conventional MSF once-through (OT-MSF) with and without brine mixing using simplified and full models. It is found that the simplified model overestimates the required heat transfer specific area for both MSF configurations. Moreover, it underestimates the cooling water and energy requirements for the reversal configuration. Hence, the simplified model may be good for quick analysis but leads to inaccurate design specifications and economic analysis. When brine mixing is utilized, the simplified model still provides erroneous estimates of the heat transfer area for both MSF configurations. Nevertheless, for OT-MSF structure, the simple model can provide comparable predictions with that of the improved model in terms of recovery ratio, performance ratio, and specific energy consumption. For the RV-MSF structure, a mismatch in the two model predictions of surface area, cooling water, and energy requirements is observed. Furthermore, the temperature drop in the cooling system for RV-MSF has a significant influence on the specific surface area and cooling water requirements especially at low values. The different behaviour of the simplified model between the OT-MSF and RV-MSF configurations is attributed to the fact that brine recycling does not affect the feed temperature or the temperature distribution in the system.