Page 111 - Design of Simple and Robust Process Plants

96 Chapter 4 Process Synthesis and Design Optimization
Larger-scale reactor systems Simplification can also be considered from a size per-
spective. Many processes are considered to be restricted by size, and therefore many
reactor systems are doubled, tripled, or even quadrupled. A closer inspection of
these systems shows that different types of constraint are in existence:
. mechanical design;
. mechanical fabrication;
. flow distribution; and
. process-wise, or a combination of these.
Styrene reactors are an example of a mechanical design that is constrained. Histori-
cally, these reactors consisted of a two horizontal bed configuration with a tubular
reactor partly in between, heated by flue gas (Figure 4.10). The mechanical stresses
between the tube sheets and the shell were a constraint on the size, as thermal stres-
ses caused cracks to occur at the connection because of a difference in expansion
during transient operations. The evolution took different paths. Two intermediate
solutions were the development of: (i) a supported thinner tube sheet that resulted
in an increase in reactor diameter from 3 m to 5 m; and (ii) a salt-heated system
that permitted a lower temperature on the shell side. The result was a reactor with a
diameter of >5 m. The next capacity increment was realized by the introduction of a
radial bed reactor (Figure 4.10). The concept included replacement of the shell and

REACTANTS
STEAM
FEED
AIR
FUEL-GAS
FLUE-GAS

STEAM
PRODUCT
ADIABATIC BED - A HEATED TUBULAR PART - ADIABATIC BED
REACTANTS

STEAM

FUEL-GAS
PRODUCT
TWO ADIABATIC BEDS IN SERIES WITH EXTERNAL REHEAT
Fig. 4.10. The evolution of a styrene reactor system to achieve higher capacity.