(624b) Energy Cost Optimization in Membrane Desalination and the Thermodynamic Restriction

Modern RO and NF membranes can be operated at remarkably low pressures. However, these pressures are still significantly above the thermodynamic osmotic pressure. Although various studies have advanced a variety of approaches to evaluate the energy cost of reverse osmosis membrane desalination, such studies have not offered a simple mathematical formalism that considered the effect of the lower bound on the feed pressure on energy cost optimization. In the present study, a rigorous theoretical formalism was developed that clarifies the thermodynamic restriction on RO energy cost and provides a basis for RO process optimization. The present approach enables direct analytical solution for the minimum specific energy cost with respect to water recovery, feed and permeate flow rate. The additional impact of pressure drop within the membrane module, energy recovery devices, membrane hydraulic permeability and brine disposal cost were incorporated into the theoretical model. Specific results will be presented to demonstrate the impact of multi-stage RO operation on energy efficiency in relation to membrane cost. Results of the analysis provide direct quantification of the upward shift in product water recovery needed to maintain optimal operation with respect to energy cost minimization under various process constraints. In addition, an analytical approach was developed to enable optimization, with respect to energy efficiency, for multi-pass RO and NF membrane desalting. The implications of the present work to lowering RO desalination cost by optimization of process configuration will be presented with reference to recently developed large-scale process configurations.