Page 163 - Design of Simple and Robust Process Plants

148 Chapter 5 Process Simplification and Intensification Techniques
5.3.1
Reaction

This combination is exploited in mixed ion-exchange beds for water treatment.
Another application is in the ethylbenzene dehydrogenation reaction, which is
endothermal. In this process ± which is an equilibrium reaction ± the conversion
can be increased by the removal of hydrogen, with such removal being achieved by
selective oxidation between the reaction stages. In this case, the oxidation step
simultaneously generates heat which is used for heating between the stages.

5.3.1.1 Reactive distillation
Reactive distillation has been in use for quite some time in batch processes, al-
though initially it was not known by this name. Esterifications are exothermic equi-
librium reactions that produce reaction water. By removing this water in a rectifica-
tion column or dephlegmator on top of the reactor, see Figure 5.9, it is possible to
shift the reaction to completion if the water is the lightest component (an example
is the preparation of butyl acetate). The application of an esterification and an ether-
ification in both continuous processes are shown in Figure 5.1. The combination of
reaction with the removal of components by distillation is called ªreactive distilla-
tionº. The reaction might be endothermal or exothermal, homogeneous in the liquid
phase, or heterogeneous at a solid catalyst kept under liquid while the liquid is at
bubble point.
The most well-known industrial applications of continuous processes are the
preparation of the octane boosters ETBE (ethyl-tert-butyl ether), MTBE (methyl-tert-
butyl ether) and TAME (tert-amyl-methyl ether) (DeGarmo et al., 1992; Doherty and
Buzad, 1992). Other applications, next to etherification, include de-etherifications,
esterification (as mentioned above), and alkylation of aromatics.

Methyl Acetate MTBE
homogeneous catalyzed heterogeneous catalyzed
C4’s Methanol
ISO butylene
Methyl acetate
Acetic
acid C4’s and
excess
methanol
Sulfuric Pre reactor
acid
catalyst
Methanol

MTBE
Water
Fig. 5.1. Reactive distillation examples; Methyl acetate and MTBE production.