Analysis of Application of Pressure Exchange Device in Thermal Vapor Compression Desalination System

Recent advances in direct fluid-fluid flow induction provide potential for major improvement in performance of thermal distillation systems based on the pressure exchange phenomenon compared to the conventional turbulent mixing controlled ejectors. Pressure exchange devices utilize the work of nonsteady pressure forces acting across moving interfaces. Optimal performances of such devices can be determined through the use of the ideal turbomachinery analog. The analog is configured as a turbine-compressor unit, where the high energy primary fluid expands through the turbine that drives a compressor which compresses the low energy secondary fluid and the two then discharges in a common mixing chamber at a common intermediate pressure. The overall functioning of the turbomachinery analog is similar to the conventional ejector. Thus the turbomachinery analog provides the highest possible performance that an ejector can achieve ideally. An analytical single effect thermal vapor compression (TVC) desalination model is developed. The turbomachinery analog which is the simplest kind of pressure exchange device is simulated in place of the conventional ejector. The objective of the research is to investigate the performance of the system for various ejector efficiencies, so as to achieve the minimum production cost of distilled water. Such a development would make the process comparable with reverse osmosis and mechanical vapor compression desalination system. The system performance is expressed in the form of thermal performance ratio. For similar systems employing conventional steady-state ejectors, thermal performance ratios as high as 2 has been achieved for low compression ratio and low boiling temperature but at a price of high pressure primary steam. This paper reveals that the application of pressure exchange device can achieve even greater performance ratios for lower primary pressure and temperatures, contributing to a significant decrease in production cost. The model is designed for 5m³/day capacity, with an aim of achieving highest possible thermal efficiency. The system is analyzed by varying the critical operating parameters, like compression ratio, top brine temperature, primary pressure and ejector efficiency. The results show that with increase in primary pressure, the required primary temperature goes down. Also the application of pressure exchange device results in a phenomenal 3 fold rise in thermal performance ratio, as compared to conventional ejectors. The results achieved from the simulations are quite encouraging and promising for the future development of more efficient and compact device called the supersonic pressure exchange ejector.