ICE Topics: Process Control by Design
Lecture Notes 3: the Heat Exchanger Network for Heat Recovery
revised 2004 Dec 17
Dr. Barry S. Johnston, Copyright 2004.
chemical plant designs use heat integration to reduce energy costs
Suppose you have a hot process stream that needs cooling.
Perhaps you could use a utility
stream, such as cooling water.
The energy transferred to the cooling water would then be
discarded to the plant environment (via cooling tower or river discharge).
that you have another process stream that needs heating.
IF the desired enthalpy changes of the
two process streams are similar, IF the temperature differences support practical heat transfer
rates, IF there is no safety problem routing these streams through a common piece of equipment,
IF the two streams are in reasonable proximity, etc., THEN it may pay to recover the energy of
the hot stream into the cold one, and thus save utility costs.
saving utility costs is good, but operation becomes more complicated
We have given up flexibility by tying two process streams together; it may be that starting up,
shutting down, or performing maintenance will affect larger portions of the plant.
operating disturbances that affect one process stream may now propagate into the other.
complicated plant structures motivate more sophisticated process control to cope with these new
use steady-state process simulators for flowsheet development
Codes such as Aspen Plus, Chemcad, and Hysys are routinely used to develop chemical process
They calculate the thermodynamic state of each stream in the flowsheet.
stream properties must, of course, satisfy the steady-state material and energy balances.
complete the calculations requires models for performance of the flowsheet equipment, as well
as physical properties.
We will not use these tools in this module, but we will remain mindful of how they would be
In what follows, therefore, we can anticipate (1) writing material and energy balances (2)
describing the performance of equipment (3) using physical property data.
we present an example of heat integration and classify the variables
Consider the following heat recovery network, based on an example presented by McAvoy (2):
To rationalize this example, imagine that we care about all the exit temperatures – this would be
the case if we were processing each stream separately in heat exchangers with utility streams.
, and T
are classified as controlled variables
(system outputs that we wish to
maintain at a target value).
If the primary purpose of the network is to cool stream 1, then mass
and inlet temperature T
are regarded as determined by another process; for our purpose