Page 113 - Design of Simple and Robust Process Plants

98 Chapter 4 Process Synthesis and Design Optimization
tube exchanger in the reactor by an inter-stage heater. The problem to be solved was
flow distribution at the inlet of the radial reactors, and the engineers were able to
overcome this satisfactorily. By comparing both systems, it was clear that the energy
system of the radial reactors with inter-stage reheat produced less lower-value energy
(steam) and consumed power compared with the flue-gas heating system.
Another example of a mechanical constrained system is the glass-lined, jacket-
cooled reactor. These are not limited by fabrication of the vessels, but by heat
removal. The jacket surface area increases with the square of the diameter, and the
content with the cube of the diameter. The solution was found (process wise) to
involve the application of a reflux condenser (Figure 4.11).
Another variant of the boiling reactor is shown in Figure 4.12. In this example of
simplification, three loop-cooled, agitated reactors are replaced by a boiling reactor
vessel. The vessel is partitioned in three compartments and provided with one reflux
condenser.
The capacity of steam-cracking furnaces is another example which over the years
has increased by factors of 4±5 as a result of improvements in mechanical design.
Combination of reaction with separation This is another step in the consideration of
simpler designs. During the past few decades, the process of reactive distillation ±
sometimes a catalytic distillation ± has matured to a point where it is now conside-
red where distillation of reaction products occurs in the same temperature range as
the reaction. Factors that play a role in reactive distillation include:

. The reaction occurs in the liquid phase; it is often catalytically homogeneous
or heterogeneous, S/L.
. Exothermic reactions have an advantage, as the reaction heat can be used for
the separation. Endothermic reactions can also be applied, in which case the
reaction heat must be added via a reboiler
. Equilibrium reactions have the advantage that conversion can be driven to
higher levels by removing the products.
. Higher selectivity might be achieved by product removal to minimize conse-
cutive reactions.
Several industrial applications of reactive distillation have been installed, for exam-
ple etherifications like MTBE (methyl-tert-butyl ether) production, alkylations like
the cumene process and esterifications like the methyl acetate production.
Other combinations of reaction and separation include: (i) the application of a
zeolite membrane reactor (van de Graaf et al., 1999); (ii) countercurrent gas solid±
solid trickle flow reactor for equilibrium reactions such as methanol synthesis (Wes-
terterp et al., 1987); (iii) inter-stage product removal with absorbers in a methanol
plant (Westerterp et al., 1989); and (iv) oxidative removal of hydrogen between the
reactor stages of a styrene reactor. For more details on simplification, see Chapter 5.
Reactor system synthesis 2 inputs include:

. Results of reactor system synthesis 1,which is a pre-selection of reactor con-
figurations.