Membranes as a tool to support biorefineries: Applications in enzymatic hydrolysis, fermentation and dehydration for bioethanol production

The consumption of fossil fuels in excess leads to chronic effect of greenhouse gas (GHG) emissions on the environment. These adverse environmental impacts of GHG have invoked reasonable awareness on renewable energy resources. Bioethanol from lignocellulosic agricultural residue (profusely available renewable raw materials in the tropical areas) exhibits promising alternative to the petroleum based fossil fuel which reduces the net emission of GHGs. But due to certain technological barriers the large scale production of lignocellulosic bioethanol has not been successfully commercialized. To achieve the goal, economically viable bioethanol production technology, which includes pretreatment, enzymatic hydrolysis, fermentation, and dehydration, needs to be developed. Ionic liquid aided pretreatment can recover more than 80% cellulose and 42% lignin from lignocelluloses, which generally contains 30-46% cellulose and 18-25% lignin. Processing of the recovered cellulose towards bioethanol production requires enzymatic hydrolysis, which gives almost 76% reducing sugar yield. Use of ultrafiltration and nanofiltration in hydrolysis concentrates 27% reducing sugar as well as recovers more than 73% enzyme with 50% catalytic activity. Ultrafiltration rejects 100% yeast as well as reveals 15 g/l/h ethanol productivity, which can be subjected to membrane based dehydration by way of pervaporation to produce 99.8 wt% ethanol. The scope of this review focuses on eco-friendly and sustainable method for bioethanol production. A holistic and dedicated approach of this review helps to solve the various technological concerns and realize large scale commercialization of lignocellulosic ethanol.