Page 165 - Design of Simple and Robust Process Plants

150 Chapter 5 Process Simplification and Intensification Techniques
Modeling of these systems has led to extensive progress being made, based on
the development of a steady-state reaction extrusion interaction diagram, the reac-
tion kinetics, heat removal, and description of the mixing parameters.

5.3.1.3 Reactive extraction
This is one way to shift the equilibrium of a reaction (Sharma, 1988). This method
is also applied to avoid further degradation of a component through in-situ removal.
Selection of the solvent is crucial, and not a straightforward task. Next in importance
to the distribution coefficients is the solubility in both phases, while the solvent
should be inert to the reaction. Some examples reported in literature include:

. the bromination of dialcohols HO(CH 2 ) 6 OH to HO(CH 2 ) 6 Br with aqueous
HBr. The problem is to prevent formation of the dibromo product, and this is
realized by using a hydrocarbon solvent that forms the basis of a high selec-
tivity. In this specific case, the hydrocarbon extracts the mono bromide and
not the di-hydroxy compound, and so prevents the dibromo component
being formed.
. the epoxidation of olefinic compounds with the production of metachloroper-
benzoic acid in an aqueous phase. This may result in undesirable side reac-
tions, but by introducing the solvent dichloromethane, the product is
extracted and thus not available for reaction with the epoxy compound in the
aqueous phase. This results in a high yield of the desired product.

5.3.1.4 Reaction with integrated feed effluent exchange
This concept resulted in the development of commercial reverse-flow reactors for
the purification of polluted air by catalytic combustion (Matros and Noskov, 1988;
see also Figure 4.9 in Chapter 4). Catalytic combustion systems are often exposed to
a wide variation of the inlet concentration. At low concentration the reaction might
starve, while at high inlet concentration the reaction will over-heat the reaction
mass. In the conventional design as well as the reverse-flow reactor design provi-
sions have to be made to cope with these situations. The dynamics of the reverse-
flow reactors were studied by van de Beld and Westerterp (1996); the synthesis gas
production is another application, as described by Blanks et al. (1990).
The main benefits for this technique are capital savings and the resultant very
compact units. The limitations are the dynamic understanding of the system which
requires dynamic simulations that are capable of handling discrete operations.
The integration between reaction and heat exchange is an option which has been
practiced for many years in shell and tube heat exchangers, although in these cases
the integrated heat exchanger network designs have often made the system rather
complicated from a control perspective.

5.3.1.5 Reactive adsorption
A gas-solid-solid trickle flow reactor for equilibrium reactions was developed to a
large extent at the University of Twente in The Netherlands, and was concentrated
mainly on the synthesis of methanol (Kuczsynski et al., 1987; Westerterp et al., 1987