Synthesis of Optimal Hybrid Membrane Networks for Seawater Desalination Coupled with Salt Production Processes

Seawater Reverse Osmosis (SWRO) has become a leading desalination technology as a result of the global increase in demand for desalinated water. Several technologies have been developed, which entail salt production, to combat brine disposal issues. Integrating these options significantly impacts the economics and overall footprints of desalination systems. In this work, a novel superstructure-based approach has been developed by building upon earlier work. The main motivation of the proposed methodology is to be able to systematically explore different membrane desalination configurations coupled with salt production technologies. All possible design options have been embedded into the proposed superstructure model, whilst deploying optimization techniques that identify economically optimal solutions. Hence, the method can explore reduced water costs by extracting value from concentrates in the desalination system in the form of salt co-products. This work is the first ever attempt to propose a superstructure-based design approach for this problem. This work considers SWRO and Nanofiltration (NF) membranes as the primary synthesis units of the membrane desalination network. NF membranes offer higher rejections of divalent ions over monovalent ions and offer potential opportunity to selectively channel streams containing high value ions to salt production operations. Furthermore, several Salt Production Processes (SPPs) involving desalination brines have also been considered as a third category for potential synthesis units within the network. A case study involving superstructures of multiple membrane units and SPPs is used to demonstrate the proposed method. A 100,000 m3/day production capacity plant, using the membrane modules FilmTec SW30 and NF270 by Dow, is modelled. First of the two SPP options produces calcium carbonate through sodium carbonate softening in a solid contact clarifier. The second SPP produces sodium chloride through the sequential pond evaporation and evaporative crystallization. The cost optimal membrane network designs with salt production are compared against the base case of SWRO desalination without salt production. The results indicate significant reductions in water costs if salts can be co-produced in desalination systems