section and its control system. The task sometimes falls through the
cracks to ensure that these two sections operate effectively when coupled
together. Thus it is important that we perform the Downs drill.
We must ensure that all components (reactants, products, and inerts)
have a way to leave or be consumed within the process. The consideration
ofinerts is seldom overlooked. Heavy inerts can leave the system
in the bottoms product from a distillation column. Light inerts can be
purged from a gas recycle stream or from a partial condenser on a
column. Intermediate inerts must also be removed in some way, for
example in sidestream purges or separate distillation columns.
Most of the problems occur in the consideration of reactants, particularly
when several chemical species are involved. All of the reactants
fed into the system must either be consumed via reaction or leave the
plant as impurities in the exiting streams. Since we usually want to
minimize raw material costs and maintain high-purity products, most
of the reactants fed into the process must be chewed up in the reactions.
And the stoichiometry must be satisfied down to the last molecule.
Chemical plants often act as pure integrators in terms of reactants.
This is due to the fact that we prevent reactants from leaving the
process through composition controls in the separation section. Any
imbalance in the number ofmoles ofreactants involved in the reactions,
no matter how slight, will result in the process gradually filling up
with the reactant component that is in excess. The ternary system
considered in Chap. 2 illustrated this effect. There must be a way to
adjust the fresh feed flowrates so that exactly the right amounts ofthe
two reactants are fed in.