Azeotropic three-column alternatives

Fig. 32. Alternative three-column flowsheet. Again, note mixing of the azeotropic mixture. By with the feed.

A pioneering paper by Ryan and Doherty explored several alternative heterogeneous azeotropic configurations using benzene as the entrainer. They examined two- and three-column flowsheets and concluded that the three-column flowsheet with a preconcentrator (beer still), an azeotropic column, and a recovery column was the economic optimum. They used approximate ternary diagram methods of analysis. 

From the above discussion it can be observed that the initial diluted feed can be sent to the water-column and not to the ethanol recovery column (azeotropic distillation). In consequence, this alternative has only two columns, and has been occasionally presented as innovative. Doherty Malone (2001) discusses in detail all the alternatives discovered along the time using benzene as entrainer. The twO-column sequence is not the most economical because of large entrainer recycle and expensive columns. It seems that the two-column sequence with prefractionator, in total three units, offers the best compromise between investment and operation (solvent recycle) costs. 

In Chapter 8 we explored the steady-state design of the TAME reactive distiUalion system. The reactive column is part of a multiunit process that includes other columns for recovery of the methanol reactant. The recovery is necessary because the presence of methanol/C5 azeotropes unavoidably removes methanol from the reactive column in the distillate stream. The economics of two alternative methanol recovery systems were evaluated in Chapter 8. In this chapter the dynamic control of the process is studied, and an effective plantwide control structure is developed. The process has three distillation columns one reactive column, one extractive distillation column, and one methanol/water separation column from which methanol and water are recycled. 

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