Mixing Control in an Isobaric Energy Recovery Device of Seawater Reverse Osmosis Desalination System

Abstract

Mixing phenomena in an isobaric energy recovery device (ERD) of a seawater reverse osmosis (SWRO) desalination system are investigated experimentally and numerically using Particle Image Velocimetry (PIV) and Computational Fluid Dynamics (CFD). The ERD, which recovers energy from high-pressure brine discharged from RO membranes, is one of the most important mechanical devices in a SWRO desalination system. In this ERD, seawater is introduced into a vertical chamber from the top, and then high-pressure brine is introduced into the chamber from the bottom. The high-pressure brine pressurizes the seawater through direct liquid-to-liquid contact, transferring high-pressure energy of the brine to the seawater. This enables a sharp reduction in the electric energy consumption, typically 50%, of high-pressure pumps used to elevate seawater pressure for RO membranes. The energy recovery efficiency of the present ERD is over 98%, which is extremely high compared to a conventional turbine-type energy recovery device, such as a Pelton turbine, which has a system energy recovery efficiency of 60 to 80%. The possible weakness of the present ERD is the amount of mixing between brine and seawater around the direct contact surface, because mixing phenomena increase the salinity of seawater supplied to RO membranes. A higher pressure is required to keep the same amount of permeate from the membrane, which results in an energy loss in the system. To minimize mixing, a set of unique flow distributors was invented and placed at both ends of the pressure exchange chamber, which stabilizes the contact surfaces and suppresses excessive mixing. Mixing phenomena in the pressure-exchange chamber are investigated experimentally in detail with PIV and numerically with CFD, and the effectiveness of the flow distributors is clarified.