Mixing Characteristics of Three-Cylinder Valve-Controlled Energy Recovery Device Based on Liquid Piston

The isobaric energy recovery device can significantly reduce the energy consumption of the seawater reverse osmosis system by recycling the residual pressure energy of high-pressure concentrated brine. Three-cylinder valve-controlled energy recovery device (TC-ERD) solves the fluid pulsation of traditional two-cylinder devices, but the use of a “liquid piston” exacerbates the mixing between brine and seawater. Herein, the evolutionary law of “liquid piston” and the relationship between volumetric mixing degree and operating conditions are explored. The results show that the “liquid piston” first axially expands and then gradually stabilizes, isolating the brine and seawater. Additionally, as long as the volume utilization ratio (UR) of the pressure exchange cylinder remains constant, there will not be much difference in the volumetric mixing degree after stabilization of the “liquid piston” (Vm-max) regardless of changes in the processing capacity (Q) and cycle time (T0). Therefore, the equation for Vm-max with respect to the operating parameters (Q, T0) is derived, which can not only predict the Vm-max of the TC-ERD, but also provide an empirical reference for the design of other valve-controlled devices with “liquid piston”. When the Vm-max is 6%, the efficiency of the TC-ERD at design conditions (30 m3/h, 5.0 MPa) is 97.53%.