Page 90 - 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 67

then combining it with flue gas reduces the latter’s CP and the
corresponding stack heat losses.
Another option for utility placement is to use part of the cooling
demand of a process for generating steam. This is illustrated in
Figure 4.23, in which steam generation is placed below the Pinch.
The GCC can reveal where utility substitution may improve
energy efficiency; see Figure 4.24. The main idea is to exploit heat
recovery pockets that span two or more utility temperature levels.
The technical feasibility of this approach is determined by both the
temperature span and the heat duty within the pocket, which should
be large enough to make utility substitution worthwhile when
weighed against the required capital costs.
Utility cooling below ambient temperatures may be required, a
need that is usually met by refrigeration. Refrigerants absorb heat by
evaporation, and pure refrigerants evaporate at a constant
temperature. Therefore, refrigerants are represented—on the plot of
T (or T*) versus ΔH—by horizontal bars, similarly to the steam levels.
On the GCC, refrigeration levels are placed similarly to steam levels;
see Figure 4.25.
When the level of a placed utility is between the temperatures of
a heat recovery pocket, the Utility Pinch cannot be located by using

T* Pinch
160

140
Point of closest
approach. Not
120 necessarily at the
boiling point
100
Preheat
Superheat
Evaporation
80

60

40
Cooling Water
Q
20 C,min

0 ΔH

FIGURE 4.23 Generating steam below the Pinch.