This paper presents a modified hierarchy for the design of chemical processes. The modified hierarchy differs
from the hierarchy of Douglas [Conceptual Design of Chemical Processes; McGraw−Hill: New York, 1988]
in two ways: (i) the design of a chemical reactor is considered as a separate, distinct step in the design
exercise, and (ii) the implementation of a material recycle is considered to be part of the unit integration step,
rather than a predetermined step in the design hierarchy. The advantage of the modified hierarchy is illustrated
on a process to produce benzene by hydrodealkylation (HDA) of toluene (this is a benchmark case in teaching
chemical process design). It is shown that, by applying the modified design hierarchy, a more economical
process can be designed. The economic advantages are achieved mainly due to modifications in the reactor
configuration and the elimination of the gas recycle stream.
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