Process Modeling of Forward Osmosis and Pressure Retarded Osmosis Integration with Seawater Reverse Osmosis
pressure retarded osmosis
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PublisherThe University of Arizona.
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AbstractOsmotically driven membrane processes, like forward osmosis and pressure retarded osmosis, may hold key advantages when integrated with reverse osmosis for seawater desalination. The spiral-wound membrane platform in which these processes are applied has inherent disadvantages that need to be explored. Maintaining proper operating pressure in both of the fluid channels of a spiral-wound membrane requires the feed and draw streams to be operated at different flow rates, often as drastic as a 1:10 ratio. This affects the thermodynamic equilibrium of the system and drastically affects potential water and energy recovery. In this work, a model was created to rigorously represent spiral-wound membranes to increase modeling accuracy. A process configuration that features periodic recharging of the stream inside of the envelope is proposed to mitigate the effects of the flow rate difference. The model is used to compare the multi-stage design to single-stage configurations for both forward osmosis and pressure retarded osmosis by testing various feed and draw flow rate ratios, between 1:10 to 1:1, operated by each process as well as important membrane characteristics such as channel height and water and salt permeability. The multi-stage design shows an increase in wastewater utilization from 62.6% to 90% when compared to the single-stage designs for forward osmosis. Additionally, the multi-stage configuration increases the pressure retarded osmosis specific energy recovery from 0.13 kWh/m3 to 0.55 kWh/m3. However, the increased effectiveness of these multi-staged designs comes with a reduction in average water flux and power density, which leads to the requirement of more membrane area and capital investment for potential system implementation.
Degree ProgramGraduate College
Degree GrantorUniversity of Arizona
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Forward osmosis extractors : theory, feasibility and design optimizationMoody, Charles Donald,1949-; Kessler, J. O.; Rasmussen, William O.; Gay, Lloyd W. (The University of Arizona., 1977)Osmosis occurs when two solutions of differing osmolar concentrations are separated by a membrane permeable to the solvent but not to the solutes. In osmosis, water flows spontaneously from the low concentration source solution to the high concentration driving solution. This dissertation examines forward osmosis as a low-technology, lowenergy use process for hydration and dehydration of aqueous solutions. The fundamental mechanical device is a continuous counterflow extractor which incorporates a semipermeable membrane separating the source and driving solutions. The counterflow design permits maximum water recovery from the source solution and maximum dilution of the driving solution. The nonlinear differential equations describing the water and solute flows in the extractor are solved using analytical and numerical techniques. The resulting mathematical models contain design equations which can be used to determine the optimum membrane transport characteristics, optimum membrane size, and the asymmetric membrane orientation which minimizes concentration polarization. Theoretical and experimental results compare well. Two applications discussed in detail are the production of potable water from seawater using human nutrients, and fertilizer-driven forward osmosis (FDFO) for converting saline water to irrigation water. In these applications, the final desalted product is not pure but contains the human or plant nutrient used to drive the process. For extracting drinking water from seawater, 1 kilogram of nutrient powder can extract 6 kilograms of fresh water from one osmolal seawater, thus reducing the storage weight for food and water aboard a lifeboat by a factor of seven. The product water contains approximately 14 weight percent nutrients. Other driving solutions can be used as well. One kilogram of ethanol can extract approximately 20 kilograms of drinking water from seawater with the alcohol concentration of the resulting drinking water product being four to five weight percent. For converting saline water to irrigation water, FDFO can economically extract 80 kilograms of water per kilogram of fertilizer from 3200 mg/1 (0.1 osmolal) brackish water and 14 kilograms of water per kilogram of fertilizer from seawater. For open greenhouses, these quantities of water represent 24 and 4 percent of the total irrigation requirements, respectively. A final evaluation of the economic feasibility of FDFO requires more information on low-pressure membrane transport properties, costs, and lifetimes. For pure water production from seawater, the forward osmosis extractor can employ an easily removable and recyclable driving solute such as sulfur dioxide. The ten kilocalories per kilogram of water low temperature (100 °C) energy for removing and recycling the sulfur dioxide can be supplied by waste heat, by solar heating, or by burning crop wastes.
Applications of Direct Osmosis: Design Characteristics for Hydration and DehydrationKessler, J. O; Moody, C. D.; School of Renewable Resources, University of Arizona, Tucson; Department of Physics, University of Arizona, Tucson (Arizona-Nevada Academy of Science, 1975-04-12)In direct (normal, forward) osmosis water automatically flows through a semipermeable membrane from a "source" solution of low concentration to a "driving" solution with higher solute content. The process requires a membrane which is impermeable to the solutes; hydrostatic pressure differences are not directly involved and can be set equal to zero. In principle, direct osmosis is a low -technology, low-power consumption method for reducing the water volume of industrial effluents or liquid agricultural products, and for reclaiming brackish irrigation water. In the latter application the driving solution may utilize fertilizer as a solute; the source solution is drainage that contains harmful salt components. This type of operation has been experimentally demonstrated. This paper summarizes basic physical principles and introduces some quantitative design factors which must be understood on both a fundamental and on an applications level.