Performance of the once-through multistage flash desalination process

Abstract

The brine circulation multistage flash desalination process (MSF) dominates the thermal desalination market, while the once-through multistage flash desalination process (MSF-OT) remains to be found on a limited scale. This is because the MSF-OT process has no control on the temperature of the feed sea water. For non-equatorial regions, where the sea water temperature drops to 5 — 15°C during winter operation, the MSF-OT performance deteriorates unless the volume of the low-temperature stages is drastically increased to allow for reduction in the brine reject temperature to lower values. Another approach to solving this problem is to use the brine mixing (MSF-M) technique to control the feed sea water temperature. Irrespective of this, the MSF-OT process should be considered the optimum choice for large-scale thermal desalination in equatorial regions, where the sea water temperature remains constant throughout the year at 28°C. This study focuses on design and performance features of the MSF-OT process. Results are presented in terms of variations in the process thermal performance ratio, the specific heat transfer area and the conversion ratio as a function of the top brine temperature, the number of flashing stages and the feed sea water temperature. The performance of the MSF-OT process is identical to the MSF process as long as the feed sea water temperature remains above 25°C. Evaluation of the MSF-M system and comparison with the performance of the MSF and MSF-OT processes is presented.