Fluid Transient Analysis for Enhanced Performance of an Energy Recovery Device for a Small-Scale Reverse Osmosis Desalination Unit

Abstract

This article presents a comprehensive study of a double-acting cylinder (DAC) energy recovery device (ERD). The DAC was specifically designed, manufactured, and experimentally tested within a small-scale 5 m3/day brackish water reverse osmosis (RO) unit. The distinctive advantage of the DAC lies in its ability to operate without an extra booster pump, thereby reducing initial costs and streamlining system complexity. A comparative analysis was conducted between the station operating without any ERD and the station equipped with a DAC. For both scenarios, a parametric study was carried out to analyze the relationship between specific energy consumption (SEC) and recovery ratio at varying recovery percentages (10%, 15%, 20%, 25%, and 30%) for different salinity levels. This analysis was conducted across various feed flowrates, with the percentage reduction in SEC calculated for each case. The results show the DAC's ability to effectively reduce the SEC by up to 40%. Additionally, the study investigated brine-feed stream mixing within the DAC, highlighting its capability to prevent undesirable mixing despite internal leakage. However, its widespread adoption has been hindered by realizable pressure fluctuations associated with its implementation, which can lead to rapid fatigue failure. To address this issue, a direct-contact air vessel was integrated into the system to minimize pressure fluctuations and enhance the performance of the DAC. Its optimal size was determined through numerical analysis using the method of characteristics, with detailed design equations presented for future reference. The results affirm the indispensable function of the air vessel in attenuating unsteady effects.