Cost Optimization of Osmotically Assisted Reverse Osmosis

Abstract

We develop a nonlinear optimization model to identify minimum cost designs for osmotically assisted reverse osmosis (OARO), a multi-staged membrane-based process for desalinating high salinity brines. The optimization model enables comprehensive evaluation of a complex process configuration and operational decision space that includes nonlinear process performance and implicit relationships between membrane stages, saline sweep cycles, and make-up, purge, and recycle streams. The objective function minimizes cost, rather than energy or capital expenditures, to accurately account for the tradeoffs in capital and operational expenses inherent in multi-staged membrane processes. Generally, we find that cost-optimal OARO processes minimize the number of stages, eliminate the use of saline make-up streams, purge from the first sweep cycle, and successively decrease stage membrane area and sweep flowrates. The optimal OARO configuration for treating feed salinities of 50-125 g/L total dissolved solids to a water recovery of 30-70% results in process costs less than or equal to $6 per m3 of product water. Sensitivity analysis suggests that future research to minimize OARO costs should focus on minimizing the membrane structural parameter while maximizing the membrane burst pressure and reducing the membrane unit cost.

Mathematical and optimization modelling in desalination: State-of-the-art and future direction

“…They optimized operating and design variables and found that the hybrid system has a higher overall recovery and lower operating cost than the MSF system and better product water quality than RO alone. Bartholomew [157] developed a cost optimization model for osmotically assisted RO, in which they investigated the relationship between membrane stages, saline sweep cycles, and makeup, purge and recycle streams for high-salinity feeds in the range of 50,000 to 125,000 ppm TDS. The optimized design resulted in costs less than $6/m 3 water with water recoveries between 30-70%.…”

“…The optimized design resulted in costs less than $6/m 3 water with water recoveries between 30-70%. They studied 3 cases: (1) feed TDS of 75 g/L and 50% water recovery, (2) feed TDS of 75 g/L and 70% water recovery, and (3) feed TDS of 125 g/L and 40% water recovery [157]. They found that cost-optimal unit water cost was the lowest for case 1, as shown in Figure 38A.…”

“…Figure 38B shows the normalized costs of the various components (membrane capital and replacement costs, capital costs of pumps, pressure exchanger, electricity costs and other operating expenses). They used a nonlinear optimization model with the objective of minimizing the cost of the OARO system and found that OARO can be economically feasible for feed salinities of up to 125 g/L and water recoveries of up to 70% [157]. of appropriate renewable energy technology for supplying electricity to the RO plant, energy balances of the integrated system, and the cost of water production and electricity [173].…”

“…Cost-optimal unit water costs (A) and normalized component costs (B) for the three cases studied by for OARO of high-salinity brine[157] 3. Future direction3.1 Modelling of renewable energy systems with desalination Despite reduced energy consumption, energy still makes up the largest portion of overall costs in desalination systems.…”

Pathways for minimal and zero liquid discharge with enhanced reverse osmosis technologies: Module-scale modeling and techno-economic assessment

“…Mechanical vapor compression (MVC) brine concentrators are currently the state-of-theart technology for concentrating hypersaline brine, typically up to ≈250 g/L total dissolved solids (TDS), which can then be passed to crystallizers for zero-liquid-discharge [9,10]. While MVC possesses a high recovery of latent heat and can achieve a specific energy consumption in the range of 20-40 kWh e /m 3 of feedwater [9][10][11], the technology remains energy-intensive, primarily uses electrical energy for vapor compression, and requires stainless steel and titanium materials for avoiding corrosion, resulting in high capital cost. Furthermore, based on estimates of second-law efficiencies for zero-liquid-discharge, MVC brine concentrators were shown to be less than half as efficient (8.5% and 11.6% second-law efficiency for single-and two-stage MVC) as multi-effect evaporators used for crystallization (24.4% second-law efficiency) [12], indicating that efforts should focus more on improving the energy efficiency of the brine concentration step in zeroliquid-discharge applications.…”