Ethanol concentration by forward osmosis with solar-regenerated draw solution

Production of fuel-grade ethanol requires removing excess water following fermentation, which is typically performed by an energy intensive distillation process. This paper proposes a simple, low-cost, and scalable alternative method of removing water using forward osmosis. The draw solution is an aqueous brine that is regenerated by solar evaporation. Separation limits of this approach are calculated using the van Laar equation for the non-ideality of the ethanol/water mixture, and experimentally measured activities for aqueous solutions of NaCl, CaCl2, K2HPO4, and K4P2O7. We focus on production of 95%, 50% and 30% (w/w) ethanol solutions as fuel for cooking stoves and homogeneous charge compression ignition (HCCI) engines, both of which tolerate larger amounts of water in the product. Evaporation-limited production rates are computed using long-term meteorological data for sites in Ethiopia, Thailand, Brazil, and India. These calculations can be used to determine the size requirements necessary for a given daily production rate for any desired ethanol concentration.

► Distillation is an energy-intensive part of ethanol fuel production.
► Forward osmosis into a brine can be used to remove water from ethanol solutions.
► Solar evaporation can be used to regenerate the brine draw solution.
► Ethanol production rate is described using a thermodynamic model.