4.1.2 Ethanol

4.1.2.1 Ethanol as Chemical and Fuel

Ethanol, C2H5OH, is one of the most significant oxygenated organic chemicals because its unique combination of physical and chemical properties make it suitable as a solvent, a fuel, a germicide, a beverage, and an antifreeze; its versatility as an intermediate to other chemicals and petrochemicals also contributes to its significance. Ethanol is one of the largest bulk-volume chemicals used in industrial and consumer products. The main uses for ethanol are as an intermediate in the production of other chemicals and as a solvent. As a solvent, ethanol is second only to water. Ethanol is a key raw material in the manufacture of plastics, lacquers, polishes, plasticizers, per­fume, and cosmetics. The physical and chemical properties of ethanol are primarily dependent upon the hydroxyl group which imparts the polarity to the molecule and also gives rise to intermolecular hydrogen bonding. In the liquid state, hydrogen bonds are formed by the attraction of the hydroxyl hydrogen of one molecule and the hydroxyl oxygen of another molecule. This makes liquid alcohol behave as though it were largely dimerized. Its association is confined to the liquid state, whereas it is monomeric in the vapor state.

Another important property of ethanol in its fuel application is that the ethanol-water binary system forms an azeotrope at a binary concentration of 95.63 wt% ethanol and 4.37 wt% water and this azeotropic mixture boils at 78.15°C, which is lower than ethanol’s normal boiling point (NBP) of 78.4°C. Therefore, straight distillation cannot boil off ethanol at a concentration higher than this azeotropic concentration. Most industrial grade ethanol has 95 wt% ethanol and 5 wt% water (190 proof). Therefore, fuel-grade ethanol is commonly produced by a combinatory process between distillation and zeolite-based absorption/adsorption in order to overcome the azeotropic concentration barrier encountered in the distillation separation process.