Forward osmosis (FO) has the potential to improve the energy efficiency of membrane-based water treatment by leveraging waste heat from steam electric power generation as the primary driving force for separation. In this study, we develop a comprehensive FO process model, consisting of membrane separation, heat recovery, and draw-solute regeneration (DSR) models. We quantitatively characterize three alternative processes for DSR: distillation, steam stripping, and air stripping. We then construct a mathematical model of the distillation process for DSR that incorporates hydrodynamics, mass-and heattransport resistances, and reaction kinetics, and we integrate this into a model for the full FO process. Finally, we utilize this FO process model to derive a first-order approximation of the water production capacity given the rejected-heat quantity and quality available at U.S. electric power facilities. We find that the upper bound of FO water treatment capacity using low-grade heat sources at electric power facilities exceeds process water treatment demand for boiler water makeup and flue-gas-desulfurization wastewater systems. ■ INTRODUCTIONProposed effluent limitation guidelines at steam electric powergeneration facilities will significantly increase the demand for on-site water treatment. 1 One opportunity to minimize the auxiliary power consumption associated with this treatment capacity is to utilize waste heat available on-site for membranebased water treatment. One potential technology is forward osmosis (FO), where the draw solution is a thermolytic salt (e.g., NH 4 HCO 3 ) ( Figure 1). 2−4 In this two-step process, feedwater is drawn across a semipermeable membrane by a difference in osmotic pressure between the feed solution and the draw solution. The dilute draw solute is then regenerated by thermal decomposition of the thermolytic salt into its constituent gases (i.e., NH 3 and CO 2 ). 5 If waste heat is available, this separation process offers significant electricity savings over reverse osmosis. 6,7 Steam electric power-generation facilities are the largest source of waste heat in the United States, 8 but the feasibility of utilizing this waste heat to drive FO separation processes has yet to be systematically assessed in the peer-reviewed literature. 5,6,9 Past modeling efforts to evaluate the feasibility of waste-heat-driven FO assumed that heat is available at desired quantities and temperatures, 7 whereas experimental demonstrations of FO processes in the peer-reviewed literature utilized electricity or fuel to generate heat. 9−11 This significant gap in the literature exists largely because robust estimates of the quantity, quality, and availability of power-plant waste heat are sparse. 12,13 Our recent work provides estimates of the quantity, quality, and spatial-temporal availability of waste heat for the U.S. power sector over the next 30 years. 14 Demonstrating the feasibility of power-plant waste-heat-driven FO requires the integration of these waste-heat estimates with heat capture, transp...
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