Engineered Osmosis for Energy Efficient Separations: Optimizing Waste Heat Utilization FINAL SCIENTIFIC REPORT DOE F 241.3

The purpose of this study is to design (i) a stripper system where heat is used to strip ammonia (NH{sub 3}) and carbon dioxide (CO{sub 2}) from a diluted draw solution; and (ii) a condensation or absorption system where the stripped NH{sub 3} and CO{sub 2} are captured in condensed water to form a re-concentrated draw solution. This study supports the Industrial Technologies Program of the DOE Office of Energy Efficiency and Renewable Energy and their Industrial Energy Efficiency Grand Challenge award solicitation. Results from this study show that stimulated Oasys draw solutions composed of a complex electrolyte solution associated with the dissolution of NH{sub 3} and CO{sub 2} gas in water can successfully be stripped and fully condensed under standard atmospheric pressure. Stripper bottoms NH{sub 3} concentration can reliably be reduced to < 1 mg/L, even when starting with liquids that have an NH{sub 3} mass fraction exceeding 6% to stimulate diluted draw solution from the forward osmosis membrane component of the process. Concentrated draw solution produced by fully condensing the stripper tops was show to exceed 6 M-C with nitrogen-to-carbon (N:C) molar ratios on the order of two. Reducing the operating pressure of the stripper column serves to more » reduce the partial vapor pressure of both NH{sub 3} and CO{sub 2} in solution and enables lower temperature operation towards integration of industrial low-grade of waste heat. Effective stripping of solutes was observed with operating pressures as low as 100 mbar (3-inHg). Systems operating at reduced pressure and temperature require additional design considerations to fully condense and absorb these constituents for reuse within the Oasys EO system context. Comparing empirical data with process stimulation models confirmed that several key parameters related to vapor-liquid equilibrium and intrinsic material properties were not accurate. Additional experiments and refinement of material property databases within the chosen process stimulation software was required to improve the reliability of process simulations for engineering design support. Data from experiments was also employed to calculate critical mass transfer and system design parameters (such as the height equivalent to a theoretical plate (HETP)) to aid in process design. When measured in a less than optimal design state for the stripping of NH{sub 3} and CO{sub 2} from a simulated dilute draw solution the HETP for one type of commercial stripper packing material was 1.88 ft/stage. During this study it was observed that the heat duty required to vaporize the draw solution solutes is substantially affected by the amount of water boilup also produced to achieve a low NH{sub 3} stripper bottoms concentration specification. Additionally, fluid loading of the stripper packing media is a critical performance parameter that affects all facets of optimum stripper column performance. Condensation of the draw solution tops vapor requires additional process considerations if being conducted in sub-atmospheric conditions and low temperature. Future work will focus on the commercialization of the Oasys EO technology platform for numerous applications in water and wastewater treatment as well as harvesting low enthalpy energy with our proprietary osmotic heat engine. Engineering design related to thermal integration of Oasys EO technology for both low and hig-grade heat applications is underway. Novel thermal recovery processes are also being investigated in addition to the conventional approaches described in this report. Oasys Water plans to deploy commercial scale systems into the energy and zero liquid discharge markets in 2013. Additional process refinement will lead to integration of low enthalpy renewable heat sources for municipal desalination applications. « less