Forward osmosis extractors : theory, feasibility and design optimization

Abstract

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.