Energy-efficient multi-stage reverse osmosis modules: A viable competitor to centrifugal reverse osmosis?

Understanding multistage reverse osmosis (MSRO) and centrifugal reverse osmosis (CRO) operations requires a realistic mass transfer-controlled (MTC) analysis, rather than an idealized thermodynamic equilibrium-controlled (TEC) approach, which assumes infinite membrane permeability. Seawater desalination is studied for feed flow rates of 0.01–0.2 GPM, targeting 50 % recovery in MSRO and single-stage CRO systems. The optimum operating pressure of each stage under the MTC approach is determined by minimizing system excess pressure. CRO operates near the thermodynamic minimum energy based on the TEC approach which does not hold for each flow rate under the realistic MTC approach. Under TEC analysis, CRO consumes 31 % less energy than single-stage RO (Krantz and Chong [1]). This difference decreases to 16 %, 11 %, 8 %, and 7 % for MSRO with 2, 3, 4, and 5 stages, respectively. However, under MTC analysis, assuming a fixed module diameter, the efficiency advantage of CRO diminishes significantly. MSRO with two or more stages outperforms CRO at higher feed flow rates, depending on the permeability, assuming ideal pumps and energy recovery devices. To minimize energy use, the minimum-energy module diameter is determined for both CRO and MSRO, with each MSRO stage having its own minimum-energy module size. The minimum-energy CRO module diameter is larger than the module diameter at each MSRO stage. Higher flow rates require larger module sizes for both CRO and MSRO systems. As each MSRO stage advances, the minimum-energy module size decreases, increasing the permeate flux for that stage. The pressure increase between stages results in higher specific energy consumption at each subsequent stage.