Boiling-point rankings

Figure 7.2. The gradual change in the process configuration is the direct consequences of two factors increased immiscibility (Fig. 7.3) and the shift of the boiling point ranking of the two products (water and acetate). Table 7.5 gives the optimized design for the production of methyl acetate (MeAc, type I), ethyl acetate (EtAc, type II), isopropyl acetate (IPAc, type II), butyl acetate (BuAc, type IH), and amyl acetate (AmAc, type IE) systems.

TABLE 16.5 Normal Boiling Point Ranking for Pure Components and Azeotropes... 

EFFECTS OF BOILING POINT RANKINGS ON THE DESIGN OF REACTIVE DISTILLATION... 

We use quaternary ideal reactive distillation systems to illustrate the effects of boiling point ranking on conceptual design. Consider the following second-order liquid-phase reversible reaction ... 

For a quaternary system (A, B, C, and D), there are 24 (4 ) possible boiling point rankings (Fig. 17.1). Here, we use LLK, LK, HK, and HHK to denote the component ranging from the lightest boiler to the heaviest boiler. In terms of relative volatilities, these four boilers can be expressed in the following order ... 

Because these two reactants (A and B) are interchangeable and the same appUes to the two products (C and D), this leaves us with 6 (24/2/2) possible configurations. Figure 17.1 shows aU 24 possible boiling point rankings, and they can be further reduced to 6 distinct... 

Note that many real chemical systems have azeotropes associated with quaternary systems, and it may make the neat reactive distillation design infeasible. However, for the real chemical systems illustrated here, the placement of the reactive zone is the same as that of the ideal systems, despite having azeotropes. The boiling point ranking leads to an easy separation between the reactants and products. The two products leave the reactive section from opposite sides of the reactive zone while the two intermediate boilers (the reactants) are kept in the reactive zone. This is the most favorable boiling point arrangement possible for a quaternary reactive distillation system. 

The boiling point ranking clearly indicates that this is a very difficult reactive distillation system because large amounts of reactants A and B can hardly coexist in the liquid phase (one is the LLK and the other is the HHK). This is the worst case scenario for the forward reaction (extremely favorable for the backward reaction). In the production of... 

In terms of reactants and products, the boiling point ranking is just the opposite of type 11 [Eq. (17.15) vs. Eq. (17.14)]. Now the two reactants are lighter than the two products. Thus,... 

The boiling point ranking indicates that if we consume the heavy reactant (HHK component B) before it reaches the bottom of the reactive zone, the heavy product (HK component D) can be obtained from the colunm base. The scenario is simpler toward the top of the colunm because the light product (LLK component C) can be withdrawn from the top while the light reactant (LK component A) can be prevented from leaving the reactive zone. The popular methyl acetate production via acetic acid esterification (Chap. 7) example falls into this category. 

The boiling point ranking suggests that if we consume the light reactant (LLK component A) toward the top of the column, the tight product (LK component C) can be obtained from... 

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