Hybrid Residue Curves

A composition transformation has been widely recommended to enhance the applicability of reactive residue curves for reactive distillation design (e.g. Ung and Doherty, 1995; Perez-Cisneros et al., 1996). The transformed co-ordinates allow reactive tie-lines to be drawn and interpreted in a manner which parallels conventional distillation design methods. Essentially, the technique relies on producing a set of co-ordinates that are independent of the reaction. Once this is done, standard geometrical methods can be used to predict the distillate product composition from the feed composition and the bottoms product composition (or vice-versa). This is useful for reactive columns which: (a) do not contain any non-reactive sections; (b) approximate perfect fractionation (i.e. the oo/oo case); and (c) can be satisfactorily represented as a ternary or pseudo-ternary system. Thus, there are significant limitations to applying reactive residue curve analysis to the design of multi-component hybrid systems such as ETBE or MTBE synthesis.

Valuable information can be obtained from both reactive and non-reactive residue curve diagrams but neither describes the complete process. A hybrid residue curve can be constructed by overlaying reactive and non-reactive residue curves (in either transformed co-ordinates or with all CA components lumped together). This provides some additional insight but is still not suitable for the design of finite columns.

Figure 5.9 shows ETBE synthesis in a hybrid system at 950 kPag for two cases: no inerts components in the feed; and 95% n-butenes in the feed. No conclusions regarding operating conditions or product compositions can be drawn from this diagram but it shows that both reaction-separation processes are feasible. However, the diagram does provide a priori information regarding the composition profiles, from which operating temperatures can be estimated. Figure 5.9 also highlights a sharp change in composition profiles at the intersection of the reactive and stripping sections for the case with no n-butene in the feed. This is potentially a concern as it might indicate a high sensitivity to some (as yet unknown) design parameter.

Fractionation Column Reboiler

ET BE Mole Fraction

Figure 5.9 - Hybrid Residue Curves for ETBE Synthesis

ET BE Mole Fraction

Figure 5.9 - Hybrid Residue Curves for ETBE Synthesis

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