Ethyl alcohol has been produced by fermentation of carbohydrates for many thousands of years, but economic industrial manufacture of synthetic ethyl alcohol began in the 1930s. Although the acid formed can be hydrolyzed directly to ethyl alcohol, the preferred route is by esterification with methyl alcohol followed by hydrolysis of the acetate. Waste liquor from paper pulp manufacture, converted to ethyl alcohol by the sulphate or sulphite process, has provided a useful source in those countries having raw materials. Around 20% of world ethyl alcohol capacity is synthetic, most being produced by the direct hydration route from ethylene. The gases are compressed and recycled to the heat exchangers. Part of the recycle gas is fed to the ethylene purification section to remove impurities and inhibit their build-up. Purified ethylene feedstock is passed through a tower in which 94–98wt% sulphuric acid flows in the opposite direction.