Process synthesis: The multi-stage flash desalination system

Abstract

The process flow diagram for the multi-stage flash (MSF) desalination process is quiet complex, where it includes several stages for brine flashing, preheaters for feed seawater, two sections for heat recovery and rejection, cooling water stream, and brine recycle stream. The process fundamentals are analyzed in order to have a better understanding of the functions and relations for various elements in the process. The analysis is based on performance characteristics for a number of simplified configurations. These characteristics include the amount of product water per unit mass of heating steam, the specific heat transfer area, the specific flow rates of the cooling and feed seawater, and the salinity, temperature, and specific flow rate of brine recycle and blow down. These characteristics are affected by limitations imposed on the number of stages, the stage temperature drop, and terminal temperature difference in the preheaters. The configurations considered in the analysis include a single-stage flashing unit, a once-through multi-stage flashing system, and configurations with brine recycle. The brine recycle systems include a simple mixer for feed seawater and recycle brine as well as one, two, and three flashing stages in the heat rejection section. A summary of the results show that a single-stage flashing unit has a thermal performance ratio less than one and the once-through system has a very large specific flow rate for the feed seawater. In addition, use of the simple brine recycle mixer results in a high temperature of the rejected brine. The single-stage heat rejection cannot be applied because pinching of the temperature profiles of the feed seawater and condensing vapor. The two-stage heat rejection section is not economical because of the small temperature driving force, which results in large heat transfer areas. This analysis leads to the conventional MSF system, which includes three stages or more in the heat rejection section.