Page 92 - 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 69

(a) T* (b) T
HP steam
HP steam
LP steam LP steam
LP steam Pinch

Process Pinch Process Pinch LP steam
Pinch

CW
CW
ΔH ΔH
Grand Composite Curve Balanced Composite Curve
FIGURE 4.26 Locating the LP-steam Utility Pinch.

BCCs are a useful additional tool for evaluating heat recovery,
obtaining targets for specific utilities, and planning HEN design
regions.

4.4 Extended Pinch Technology

4.4.1 Heat Transfer Area, Capital Cost, and
Total Cost Targeting
In addition to maximizing heat recovery, it is also possible to estimate
the required capital cost. The expressions for obtaining these
estimates are derived from the relationship between heat transfer
area and the efficiency of a heat exchanger. Methods for targeting
capital cost and total cost were initially developed by Townsend and
Linnhoff (1984) and further elaborated by others (e.g., Ahmad,
Linnhoff, and Smith 1990; Colberg and Morari, 1990; Linnhoff and
Ahmad, 1990; Zhu et al., 1995).
The HEN capital cost depends on the heat transfer area, the
number of the heat exchangers, the number of shell-and-tube passes
in each heat exchanger, construction materials, equipment type, and
operating pressures. The heat transfer area is the most significant
factor, and assuming one-pass shell and tube exchangers it is
possible to estimate the overall minimum required heat transfer
area; this value helps establish the lower bound on the network’s
capital cost. Estimating the minimum heat transfer area is based on
the concept of an enthalpy interval. As shown in Figure 4.27, an
enthalpy interval is a slice constrained by two vertical lines with fixed
values on the ΔH axis. This interval is characterized by its ΔH