enough heat simply results in the reaction slowing up. Ifthe exothermic
reactor is running adiabatically, the control system must prevent excessive
temperature rise through the reactor (e.g., by setting the ratio of
the flowrate of the limiting fresh reactant to the flowrate of a recycle
stream acting as a thermal sink). More details of reactor control are
discussed in Chap. 4.
Heat transfer between process streams can create siguificant interaction.
In the case of reactor feed/effluent heat exchangers it can lead to
positive feedback and even instability. Where there is partial condensation
or partial vaporization in a process-to-process heat exchanger,
disturbances can be amplified because ofheat of vaporization and temperature
effects.
For example, suppose the temperature of a stream being fed to a
distillation column is controlled by manipulating steam flowrate to
a feed preheater. And suppose the stream leaving the preheater is
partially vaporized. Small changes in composition can result in very
large changes in the fraction of the stream that is vaporized (for
the same pressure and temperature). The resulting variations in
the liquid and vapor rates in the distillation column can produce
severe upsets.
Heat integration of a distillation column with other columns or with
reactors is widely used in chemical plants to reduce energy consumption.
While these desigus look great in terms of steady-state economics,
they can lead to complex dynamic behavior and poor performance due
to recycling of disturbances. If not already included in the design, trim
heaters/coolers or heat exchanger bypass lines must be added to prevent
this. Energy disturbances should be transferred to the plant utility
system whenever possible to remove this source of variability from the
process units. Chapter 5 deals with heat exchanger systems.