Minimization of energy consumption for a two-pass membrane desalination: Effect of energy recovery, membrane rejection and retentate recycling

The energy optimization of two-pass membrane desalination, at the limit imposed by the thermodynamic restriction, was investigated and compared with a single-pass membrane desalting operation at the equivalent targeted overall salt rejection and permeate product recovery. The analysis considered the effect of pump and energy recovery efficiencies, membrane rejection, and retentate recycling from the second to the first-pass. The optimization results suggest that when the desired overall salt rejection can be achieved via a single-pass, then this process configuration will result in a lower specific energy consumption (SEC) relative to the two-pass membrane desalting process. However, if the desired overall rejection (or specific ion rejection, e.g., boron) cannot be achieved with the highest available rejection membrane in a single-pass, then a two-pass configuration is a feasible solution. In the latter case, the lowest energy consumption will be attained when a membrane of the highest available salt rejection is used in the first-pass. In cases in which desalting is accomplished at recoveries below the critical water recovery (i.e., the optimal recovery for a single stage), an operational sub-domain may exist in which the SEC for a two-pass process can be lower than for a single-pass counterpart, but only if the single-pass process is not operated at its globally optimal state. Retentate recycling from the second-pass to the first-pass feed can reduce the SEC for the two-pass process. However, optimization solution of the two-pass process is always a single-pass process as the optimal desalination process.