Distillation of Ethanol
In general, all bioethanol production methods require a separation of ethanol and dehydration for fuel grade ethanol. The three independent routes for producing fuel grade ethanol from lignocellu — losic biomass discussed in this book are:
1. Aqueous-phase biomass hydrolysis route — fermentation of an aqueous solution of C-5,6 sugars described in Part 2, Chapters 8 and 9.
2. Gasification route — fermentation of syngas described in Part 3, Chapter 12.
3. Gasification route — catalytic conversion of syngas described in Part 3, Chapter 13.
Routes 1 and 2 are aqueous-phase microbial fermentation methods and the end product in these techniques is a mash containing 10% to 12% ethanol in water. In the first route, all non-fermentable solids from the biomass hydrolyzate and yeast cells remain in the fermented solution, which is commonly known as "beer." In the second route, where syngas (a mixture of CO, H2 and CO2) is fermented, buffering salts, and microorganisms used will remain in the solution. In the third route, where syngas is converted to ethanol using a metal-based chemical catalysis system at high temperature, the conversion is usually not as selective as in biochemical routes. This route generally provides ethanol together with methanol, some higher alcohols, ethers, ethylene glycol and a small amount of hydrocarbons. A fractional distillation technique can be used to separate out ethanol from this mixture.