Page 94 - Sustainability in the Process Industry Integration and Optimization

P r o c e s s I n t e g r a t i o n f o r I m p r ov i n g E n e r g y E f f i c i e n c y 71

4.4.2 Heat Integration of Energy-Intensive Processes

Heat Engines
Particularly important processes are the heat engines—steam and gas
turbines. They operate by drawing heat from a higher-temperature
source, converting part of it to mechanical power; then (after some
energy loss) they reject the remaining heat at a lower temperature
(see Figure 4.28). For targeting purposes the energy losses are usually
neglected.
Integrating a heat engine across the Pinch, which is equivalent
to a Cross-Pinch process-to-process heat transfer, results in a
simultaneous increase of hot and cold utility, which usually leads to
excessive capital investment for the utility exchangers. If a heat
engine is integrated across the Pinch, then the hot utility requirement
is increased by Q and the cold by Q − W (in the notation of Figure 4.28).
Heat engines should be integrated in one of two ways.

1. Above the Pinch (Figure 4.29a): This increases the hot utility
for the main process by W, but this extra heat is converted
into shaftwork.
2. Below the Pinch (Figure 4.29b): This offers a double benefit. It
saves on a cold utility, and the process heat below the Pinch
supplies Q to the heat engine (instead of rejecting it to a
cooling utility).
Different heat engines differ in their placement. On the one hand,
steam turbines may be placed either below or above the Pinch because
they draw and exhaust steam. Figure 4.30 shows steam turbine
integration above the Pinch, which has the benefit of cogenerating

T*

Source
T*source
Q

W

Q–W
T*sink
Sink

FIGURE 4.28 Heat engine confi guration.