Step 7: Check component balances
Identify how chemical components enter, leave, and are generated or
consumed in the process.
Component balances can often be quite subtle, but they are particularly
important in processes with recycle streams because of their integrating
effect. They depend upon the specific kinetics and reaction
paths in the system. They often affect what variable can be used to set
production rate or reaction rate in the reactor. The buildup of chemical
components in recycle streams must be prevented by keeping track of
chemical component inventories (reactants, products, and inerts) inside
the system.
We must identify the specific mechanism or control loop to guarantee
that there will be no uncontrollable buildup of any chemical component
within the process (Downs drill).
What are the methods or loops to ensure that the overall component
balances for all chemical species are satisfied at steady state? We can
limit their intake, control their reaction, or adjust their outflow from
the process.
As we noted in Chap. 2, we can characterize a plant's chemical
components into reactants, products, and inerts. We don't want
reactant components to leave in the product streams because of the
yield loss and the desired product purity specifications. Hence we
are limited to the use of two methods: consuming the reactants by
reaction or adjusting their fresh feed flow. Product and inert components
all must have an exit path from the system. In many systems
inerts are removed by purging off a small fraction of the recycle
stream. The purge rate is adjusted to control the inert composition
in the recycle stream so that an economic balance is maintained
between capital and operating costs.
We recommend making a Downs drill table that lists each chemical
component, its input, its generation or consumption, and its output.
This table should specify how the control system will detect an imbalance
in chemical compone