Most methods involving distillation for ethanol dehydration utilized in the indus­try comprise at least three steps: (1) distillation of dilute ethanol until it reaches a concentration near the azeotropic point, (2) distillation using a third component

added that allows the ethanol removal, and (3) distillation to recover the third component and reutilize it in the process (Montoya et al., 2005). The azeotropic distillation corresponds to this scheme. This technology consists of the addition of an entrainer to the ethanol-water mixture to form a new azeotrope. The azeo­trope formed is ternary (involves three components) and allows a much easier separation in schemes involving two or three distillation columns. Among the substances most used as entrainers for separation of ethanol-water mixtures are benzene, toluene, я-pentane, and cyclohexane.

In the case of benzene, the process comprises one dehydration (azeotropic) col­umn, which is fed with the mixture containing 90 to 92% ethanol from rectifica­tion column (Figure 8.3). The benzene is added in the upper plate. From the lower part of the azeotropic column, ethanol is removed with water content below 1%, while the overhead vapors in the column top, which correspond to a mixture with a composition equal or near to the composition of the ethanol-water-benzene ternary azeotrope, are condensed and sent to a liquid-liquid separator (decanter). Due to the mixture properties, the ternary azeotrope is located in the immiscibil — ity zone of the ethanol-water-benzene system (Figure 8.4), so once condensed, it is separated into two liquid phases: one phase with high benzene content that is recirculated as a reflux to the azeotropic column, and the another phase with higher water content that is fed to a smaller column for entrainer recovery (strip­ping column). The distillate from the stripping column has a significant benzene concentration and, for this reason, this stream is recycled back to the azeotropic column or to the decanter. The bottoms of the stripping column contain mostly water. If these bottoms have an important amount of ethanol, they are recircu­lated to the concentration column; in this way, the separation of water and ethanol is attained and the entrainer is recovered. As the process is operated in continuous

regime, the benzene is permanently recirculating within the system. Nevertheless, small amounts of this compound leave the scheme along with the ethanol or water streams, thus a make-up stream is required. This latter stream is fed to the first plate (from the top) of the azeotropic column or is mixed with the reflux stream coming from the decanter to this same column.

The phase equilibrium properties are crucial for the design of an azeotropic distillation scheme. This equilibrium can be represented in the ternary diagram shown in Figure 8.4 for the case of benzene. The principles of topologic ther­modynamics can be applied to the analysis of this diagram (Pisarenko et al., 2001). To provide more clarity, molar fractions are employed through the analysis are identical for compositions expressed in mass fractions. The feeding of the starting ethanol-water mixture is indicated by the straight line FB and is accom­plished in such a way that the point M representing the pseudo-starting state of the system is located inside the distillation region I. This region is delimited by the two distillation boundaries, which coincide in the ternary azeotrope with the minimum boiling point. The distillation boundaries define the process constraints because any distillation operation (indicated by straight lines of mass balances) cannot have distillates and bottoms whose compositions are in different regions. When drawing a balance line corresponding to the indirect distillation for the point M (the prolongation of the straight line EM until the distillation boundary represented by the curve AC), bottoms with a composition corresponding to pure ethanol E and distillate with a composition near to the ternary azeotrope repre­sented by the point N are obtained. The composition of this distillate corresponds to the immiscibility zone of the system so it is separated into two liquid phases indicated by the points R and S that are determined following the tie lines of the liquid-liquid equilibrium plot (bimodal plot). The point R represents the liquid phase with higher water content (the raffinate) and the point S represents the liq­uid phase with higher benzene content (the extract) that is evidenced by its higher proximity to the vertex B (pure benzene) compared to point R. The stream with the composition of the point S is recirculated as the reflux to the azeotropic col­umn. The raffinate stream, in turn, undergoes distillation in the stripping column, which is represented by the balance line WRP that is located in the distillation region II. The composition of point P corresponds to the composition of the distil­late stream from the stripping column that is recycled back to either the azeotropic column or the decanter. This type of analysis allows one to predict the behavior of the system without carrying out a rigorous assessment (short-cut method). These short-cut methods allow one to obtain valuable information for the subsequent rigorous modeling of the system. In particular, the application of these methods facilitates the specification of the operating conditions in the distillation columns when commercial process simulators employing rigorous methods are used.

The above-described distillation receives the name hetero-azeotropic distillation considering that the entrainers form azeotropes located within the immiscibility zone of the system. This implies its separation into two liquid phases. The utiliza­tion of n-octane as a co-entrainer along with benzene has been proposed in order to decrease the energy costs of the traditional process (Chianese and Zinnamosca, 1990). The simulation and optimization accomplished based on a mass-transfer model (nonequilibrium model) for this process show that if the values of operat­ing parameters of the column are adjusted to minimize the amount of plates in the azeotropic column, it is possible to reduce the capital costs, but increasing the heat flow rates required implies an increase of the energy costs. In terms of energy costs, the most influencing process parameters are the reflux ratio and flow rate of the stream recirculated from the stripping column to the azeotropic column. For these parameters, their optimum values have been obtained according to economic con­siderations (Mortaheb and Kosuge, 2004). However, the utilization of benzene as an entrainer is not desirable due to its carcinogenic properties. In addition, the azeo­tropic distillation using this compound leads to the appearance of multiple steady states and the occurrence of a parametric sensibility related to small changes in col­umn pressure (Wolf Maciel and Brito, 1995). Taking into account these drawbacks, the use of less contaminant entrainers has been attempted. In particular, some new ethanol-producing facilities in Brazil employ cyclohexane as the entrainer for etha­nol dehydration by hetero-azeotropic distillation.

Significant efforts to reduce the elevated energy consumption of the azeotropic distillation using such entrainers, such as benzene, cyclohexane, diethyl ether, and я-pentane, have been made. Under real conditions, distillation columns are configured in such a way that the dehydration process is operated using the heat recovered from the primary distillation system (concentration and rectification columns). Alternatively, rectification and stripping columns can be operated using the heat released in the azeotropic column. For fuel ethanol industry in the United States, the consumption of thermal energy during the separation and dehydration steps employing the azeotropic distillation is about 4.73 MJ/L ethanol on average (Madson and Monceaux, 1995).