(675d) An Improved Closed-Circuit Reverse Osmosis (CCRO) for Water Desalination: Cyclic Dynamics and Energy Performance

The Closed-Circuit Reverse Osmosis (CCRO) developed by DuPont/Desalitech enables high recovery desalination with a small footprint. It is consists of CC filtration and flushing steps that are alternated periodically. There is one drawback with the original CCRO - the reflux (recirculation) introduces mixing in such a separation process, which is unfavored by Thermodynamics laws [1]. The energetics of CCRO may be improved by adding a reciprocating-piston cylinder [2]. However, the size of the cylinder may not be adjusted in real time during operation, thus limiting the maximum stroke length of the piston [3]. In this work, the effect of the actual-to-ideal ratio of cylinder sizing on the performance of the improved CCRO is analyzed. If the cylinder is under-sized, the CC filtration step may start with the original CCRO and transition to the improved CCRO (see Figure below for the operation steps). If over-sized, only the improved CCRO is implemented, while the CC filtration and flushing periods are extended.

To study the cyclic dynamics of the proposed improved CCRO system, a mathematical model is developed, which consists of partial differential equations (PDEs) for the RO and ordinary differential equations (ODEs) for the cylinder. The coupled mathematical model is discretized using orthogonal collocation and solved using Matlab. Detailed breakdowns of energy consumption at the cyclic steady state (CSS) are then tabulated.

For 95% ultra-high recovery applications studied in this work, under-sizing the reciprocating-piston cylinder by 70% or over-sizing by 50% revealed only 2% or less extra energy consumption than the ideal case (where the cylinder is perfectly sized). If the cylinder is completely removed, the system reverts to the original CCRO, and the energy consumption may increase by about 30%. This work confirms the superior energy performance and operational flexibility of the improved CCRO.

References:

[1] Li, M. Cyclic Simulation and Energy Assessment of Closed-Circuit RO (CCRO) of Brackish Water, Desalination, 545, 116149, 2023.

[2] Li, M. An Improved Closed-Circuit RO (CCRO) System: Design and Cyclic Simulation, Desalination, 554, 116519, 2023.

[3] Li, M. Effect of Cylinder Sizing on Performance of Improved Closed-Circuit RO (CCRO), Desalination, 561, 116688, 2023.