Page 103 - Sustainability in the Process Industry Integration and Optimization

80 Cha p te r F o u r

dryers are similarly integrated. This subject has been extensively
studied by Kemp (2007, sections 6.3 to 6.5).

4.4.3 Process Modifications
The basic Pinch Analysis calculations assume that the core process
layers in the onion diagram (Figure 4.2) remain fixed. However, it is
possible—and in some cases beneficial—to alter certain properties of
the process. Properties that can be exploited as degrees of freedom
include: (1) the pressure, temperature, or conversion rate in reactors;
(2) the pressure, reflux ratio, or pump-around flow rate in distillation
columns; and (3) the pressure of feed streams in evaporators.
Such modifications will also alter the heat capacity flow rates and
temperatures of the related process streams for Heat Integration; this
will cause further changes in the shapes of the CCs and the GCC,
thereby modifying the utility targets. The CCs are a valuable tool for
suggesting beneficial process modifications (Linnhoff et al., 1982).
Figure 4.43 illustrates this application of the CCs in terms of the plus-
minus principle. The main idea is to alter the CC’s slope in the proper
direction in order to reduce the amount of utilities needed. This can
be achieved by changing CP (e.g., by mass flow variation). According
to Smith (2005), such decreases in utility requirements can be brought
about by (1) increases in the total hot stream duty above the Pinch,
(2) decreases in the total cold stream duty above the Pinch,
(3) decreases in the total hot stream duty below the Pinch, and/or
(4) increases in the total cold stream duty below the Pinch.
Another guide to modifying processes is the principle of keep hot
streams hot (KHSH) and keep cold streams cold (KCSC). As illustrated in
Figure 4.44, increasing the temperature differences by process

T

+

−
−

+

ΔH
FIGURE 4.43 The plus–minus principle.