Simulation investigation on volumetric mixing of the rotary ERD unit and array in the SWRO desalination system

In SWRO desalination system, both low volumetric mixing and high flow capacity of rotary ERD unit is the bottleneck problem at present. Here, a new motor-driven rotary ERD (MD-RERD) is proposed with a large length-diameter ratio rotor and high flow capacity of 110 m³/h. When the length-diameter ratio increases from 14.5 to 18.5 based on unchanged rotor channel diameter, the volumetric mixing of MD-RERD at rotating speed (140 rpm) decreases from 5.503 % to 1.728 % by CFD simulation, which is much lower than that of commercial rotary ERD. Moreover, the MD-RERD shows high energy recovery efficiency of 98.2 %. To further evaluate the mixing performance, an array model composed of 5 MD-RERD units with the flow capacity of 550 m³/h is constructed. The volumetric mixing of the array is 2.339 % under traditional ZZ-array, which is 35.4 % more than that of the single unit due to poor matching degree of volume flowrate between high-pressure route and low-pressure route. Moreover, when the rotor rotates anticlockwise at 140 rpm, the high-pressure route with U-type shows much higher uniformity of flow distribution than Z-type, which is close to that of low-pressure route with Z-type. Therefore, the Z-type is changed to U-type in high-pressure route to construct the UZ-array. Consequently, the matching degree of volume flowrate between the two routes increases, and the volumetric mixing of the array decreases to 1.968 %, which is only 13.9 % more than that of the single unit. It is beneficial to further improve the comprehensive efficiency of the ERD and thus significantly reduce the energy consumption in SWRO desalination system compared to the commercial ERD with a common volumetric mixing of 6 %.