The analyses of the esterification of acetic acid with five alcohols ranging from MeOH to AmOH (C1-C5) are intended to gain insight into the design of reactive distillation by varying the chemical species discretely. As the carbon number in the alcohol increases, the flowsheet changes from type I to type II and then to type III (Fig. 7.2). The determinant factors in the flowsheet selection are the ranking of the pure component/azeotrope temperatures (Table 7.4) and the size of the LL envelope (Fig. 7.1). This implies that the structure of the flowsheet can be determined once the VLLE data become available. The flowsheets may look different in the arrangement, but they all include the following units: a stripping section, a reactive zone, and a rectifying section. Once the flowsheet structure is determined, the design can be carried out in a sequential manner by minimizing the TAC. It is interesting that most of the dominant design variables are associated with the feed: the feed tray locations (for type I and III) and the feed ratio (for type II). This can be understood because the reactants' composition distribution is important for kinetically controlled reactive distillation. We also observed that the function of the reactive zone goes beyond providing necessary conversion. The reactive section also facilitates separation by reacting away the heavy reactant toward the lower part of the reactive zone and by consuming most of the light reactant toward the upper part.
The TAC comparison in Figure 7.16 shows that the type II (EtAc and IPAc) flowsheet is the most expensive process, followed by the type III flowsheet (BuAc and AmAc). The type I flowsheet is the most economical process. This is because the type II flowsheet boils up both products to the top of the reactive distillation column (Fig. 7.2) and recycles an almost ternary azeotropic composition back to the decanter from the stripper. Thus, this flowsheet is energy intensive and requires significant capital investment in the heat transfer area. The difference in the TACs between EtAc and IPAc comes from the VLLE advantage of the IPAc system, which has a larger LL envelope (Fig. 7.1). The type III flowsheet, despite not boiling up both products to the top, needs to boil up some acetate/alcohol along with H2O to get into the LL envelope and then recycles the organic phase totally back into the column. We would expect larger energy consumption for this type of system compared to systems without a decanter. Again, the difference between BuAc and AmAc is that the AmAc system has a larger LL envelope (Fig. 7.1). Finally, the MeAc system (type I) only boils up components that are necessary to achieve desired product purity and therefore much smaller energy consumption is expected. Another reason for the MeAc to have the lowest TAC is that the purity level of the acetate typically seen in the literature is 0.98 (mol fraction), compared to 0.99 in the other four cases. The reason for the difference is that higher purity levels at both ends (e.g., xD MeAc = xBH2O = 0.99) are not feasible for
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