Novel multistage flash reversal Concept: Modelling and analysis

Abstract

In this study, a rigorous modeling and simulation of a novel multi-stage flash (MSF) configuration consisting of reversing the brine circulation, termed MSF reversal (MSF-RV), is developed. Its performance is theoretically investigated and compared with conventional MSF Once Through (MSF-OT) with and without brine mixing. The MSF-RV concept is suitable for treating geothermal streams and can be driven by low grade thermal energy such as solar and geothermal energy and waste heat for direct seawater desalination. Hence, two options of MSF-RV are proposed, i) driven by a direct hot stream (MSF-RVc), and ii) powered by external heat to treat raw seawater (MSF-RVh). The analysis showed that the temperature distribution throughout the stages plays a significant role in the thermal efficiency and heat transfer area requirements for both configurations. Hence, careful selection of the design parameters is necessary to achieve the best performance. For the same recovery ratio, the MSF-RVc was found superior to MSF-OT in terms of gain output ratio (GOR) and specific energy consumption (SEC) by 52% and 60%, respectively. However, the specific area (sA) requirement of MSF-RVc is higher than that of MSF-OT by 50%. Brine mixing by recycling the rejected brine enhances the recovery ratio, GOR, and SEC for both structures. Conversely, the sA requirement increases with brine mixing but marginally for MSF-RV and remarkably for MSF-OT. Moreover, the design parameters of MSF-RVc such as the coolant inlet temperature, the temperature drop on the coolant side, and the coolant to brine ratio affect the overall performance. However, a trade-off between the thermal efficiency (GOR, SEC) and surface area requirement is still observed.