Page 206 - Design of Simple and Robust Process Plants

5.7 Simplification and Ranking per Unit Operation 191
The status of knowledge at any one time determines into which category the reactor
design will fall. Thus, the selection of category is seldom an option at the design
stage, but will be made at the research stage, when three general rules to simplify
the development must be followed:
1 ®Reactions within a gas phase should, preferably, be converted to liquid phase
reactions, the advantages of which include:
. better mixing;
. higher concentrations;
. easier heat conditioning; and
. smaller equipment dimensions.

An exception is that of reactions which take place inside the catalyst particle, where
diffusion plays an important role. In such cases, the high diffusion coefficients of
gases offer a major advantage.

2 ®Heterogeneous reactions should, preferably, be converted to one phase, either
liquid or gas. The advantages of homogenous reactions are:
. no mass and heat transfer limitation between phases; and
. flow conditions are more easily implemented.
This also explains why reactor intensification concentrates on heterogeneous reactor
systems, to minimize these limitations (see Section 5.5).
3 ®Integration of multiple functions in one reactor system should also be con-
sidered as an option, especially for equilibrium reactions, where shifting of
the reaction will lead to higher conversions and selectivities (see Section
5.3.1)
The reactor category is predetermined at the design stage, and will fall into one of the
above categories. The macro design of the reactor within the selected category will
be determined by:
. catalyst selection;
. energy household; and/or
. mixing/flow distribution.
For priority ranking in terms of simplicity of the different categories, the chemistry
(as well as the catalyst selection) is beyond this discussion. However, the types of
reactor employed in the different categories will be discussed briefly, and a priority
ranking will be made. In the examples given, the related heat transfer is seen as
heat removal, although it is also applicable to heat addition (in which case the termi-
nology must be adapted).