Page 108 - 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 85

T

PINCH

ΔH

FIGURE 4.50 Pinch design principle.

(2) no process-to-process heat transfer may occur across the Pinch;
and (3) no inappropriate use of utilities should occur.
At the Pinch, the enthalpy balance restrictions entail that certain
matches must be made if the design is to achieve minimum utility
usage without violating the ΔT constraint; these are referred to as
min
essential matches. Above the Pinch, the hot streams should be cooled
only by transferring heat to cold process streams, not to utility
cooling. Therefore, all hot streams above the Pinch have to be matched
up with cold streams. This means that all hot streams entering the
Pinch must be given priority when matches are made above the
Pinch. Conversely, cold streams entering the Pinch are given priority
when matches are made below the Pinch.
Now recall the example from Table 4.2. Figure 4.49 shows the
scaled grid diagram, indicating the hot and cold Pinch temperatures.
The part above the Pinch requires essential matches for streams 2
and 4, since they are entering the Pinch. Consider stream 4. One
possibility is to match it against stream 1, as shown in Figure 4.51.
Stream 4 is the hot stream, and its CP is greater than the CP for cold
stream 1. As shown in the figure, at the Pinch the temperature
distance between the two streams is exactly equal to ΔT . Moving
min
away from the Pinch results in temperature convergence because the
slope of the hot stream line is less steep owing to its larger CP. Since
ΔT is the lower bound on network temperature differences, the
min
proposed heat exchanger match is infeasible and thus is rejected.
Another possibility for handling the cooling demand of stream 4
is to implement a match with stream 3, as shown in Figure 4.52. The
CP of stream 3 is larger than the CP of stream 4, resulting in divergent