Azeotropic Distillation Method
Azeotropic distillation is another method that can be used for concentration of ethanol to 99.5%, and this is the method used in many early fuel ethanol plants. In this technique a third component is added to the water-ethanol mixture. This third component is called the entrainer, and this compound selectively interacts with one of the components in the azeotrope mixture, allowing the azeotrope to be broken and the components separated. Several compounds such as benzene [17], cyclohexane [18], hexane [19, 20], normal heptane [20], isooctane [21, 22], normal pentane [17, 23], acetone [21], diethyl ether [23], and polymers [24] can be used as an entrainer to produce anhydrous ethanol from water-ethanol azeotrope mixture. However, cyclohexane and benzene are the most commonly used entrainers. Presently, benzene is very rarely used due to its carcinogenic nature, although it is still being employed in some countries. The process using и-pentane is to be operated under high-pressure conditions due to the relatively low boiling point of pentane. The added component, or entrainer, being present in the liquid phase can alter the activity coefficient of various components, and unless the components already present are identical in the physical and chemical properties, the change in activity coefficient will be different for each component, thereby altering their relative volatility. This technique of adding an entrainer to alter the volatility is effective only when the component in the original mixture does not obey Raoult’s law. In general, deviation from Raoult’s law makes it easier to significantly alter the relative volatility by the addition of the entrainer component. The added entrainer is recovered after the azeotropic distillation dehydration of fuel ethanol, and the recovered entrainer must be recycled or reused for a practical process. The recovery of entrainer is usually carried out by decantation, or separation of the phases, and then the entrainer is returned back to the azeotropic distillation column [2].
A schematic diagram for the dehydration of ethanol using azeotropic distillation using an entrainer like benzene or cyclohexane is shown in Figure 15.1. Ethanol is first concentrated in a conventional distillation column to near the binary azeotropic composition as described in Chapter 14 to approximately 90% ethanol, with 10% water. This 90% concentrated ethanol is fed into the azeotropic column shown in Figure 15.1. A secondary feed, which is rich in entrainer, is introduced on the top tray. The bottom product from this tower is at least 99.5% or nearly pure ethanol. The vapor at the top of the azeotropic distillation column approaches ternary azeotrope composition and is fed into a decanter [2]. This decanter works similar to large liquid-liquid separatory funnel, which separates the heavier water phase from lighter organic phase rich in entrainer. The entrainer-rich organic phase of the decanter and a small entrainer make-up stream comprise the secondary feed and are recycled to the azeotropic column. The aqueous
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Figure 15.1 Heterogeneous azeotropic distillation of ethanol-water mixtures. (Reprinted with permission from reference [2]; copyright 2010 Elsevier). |
phase from the decanter is sent to a second column called the stripping column as shown in Figure 15.1, where it is processed further to recover ethanol and entrainer. This recovered ethanol and entrainer is retuned back to the azeotropic distillation column and the water removed is drained from the bottom of the stripper [2].
This process requires quite a large amount of energy because it is necessary to maintain and recirculate large quantities of entrainer throughout the column to achieve the desired effect. In addition, pure ethanol must be adequately stored to prevent water from the atmosphere being absorbed by it. It is interesting to note that now it is possible to directly attain a "dry" mixture of ethanol plus hydrocarbon, utilizing less energy, instead of obtaining anhydrous ethanol. In this case, high concentrations of entrainer necessary to circulate throughout the column are achieved by a new input stream of the hydrocarbon and not by its vaporization-condensation. The ethanol plus hydrocarbon mixture thus obtained may be employed as an additive to gasoline without the need of subsequent distillation. This technique is possible because many of the constituents in gasoline may be used as entrainers in the dehydration of ethanol by azeotropic distillation. In one experiment using gasoline
components in the direct dehydration of ethanol, Gomis et al. studied the viability of an azeotropic distillation process using isooctane as an entrainer to dehydrate ethanol and obtain a dry mixture of ethanol plus isooctane [21]. The experimental results indicate that azeotropic distillation allows obtaining mixtures of isooctane plus ethanol with water concentrations lower than 50 ppm. The results point out that the most critical parameter for this process is the reboiler heat duty. Low values of this parameter (< 2.2 kJ/g of feed ethanol) produce mixtures of ethanol plus isooctane with excessive water contents. At high-heat duty values (> 3.6 kJ/g of feed ethanol) the azeotropic distillation column does not function properly, as the top stream condenses, giving only one liquid phase. High capital cost, high energy consumption, reliance on toxic chemicals like benzene and sensitivity to feedstock impurities are some disadvantages of the azeotropic distillation process range [2].
15.5 Extractive Distillation Methods
Extractive distillation is another method for concentration of water — ethanol mixture to nearly anhydrous fuel grade ethanol. This type of dehydration can be carried out using either a high boiling solvent or an inorganic salt.