Dehydration to Fuel Grade Ethanol
Ethanol leaving the rectification column in the distillation plant is the ethanol-water azeotrope mixture. Azeotropes are also known as constant-boiling mixtures, and this ethanol-water mixture is a minimum-boiling azeotrope at a composition of 89.4 mol% ethanol and 10.6 mol% water, as shown in Figure 14.1. This product is not suitable as a fuel, since it contains a fairly high percentage of water; therefore a special process for removal of the remaining water is required for the production of anhydrous fuel grade ethanol. This step is known as dehydration or drying of ethanol, and will concentrate hydrous ethanol up to at least 99.5% ethanol required for fuel applications. The distillation and drying to fuel grade ethanol are mature technologies, as corn — and sugarcane-based first generation bioethanol is in wide use as a blend in fuel in the United States, Brazil and a few other countries. These techniques developed for first generation ethanol are generally applicable to cellulosic ethanol as well. The systems used in the distillation are presented in Chapter 14 and drying or dehydration to fuel grade ethanol will be discussed in this chapter. This dehydration step is discussed only in a few review articles [1, 2], but most of the technology is patented.