Page 38 - Sustainability in the process industry

P r o c e s s I n t e g r a t i o n 15

and targets for HEN performance. This procedure is possible
thanks to thermodynamics. The hot and cold streams for the
process under consideration are combined to yield (1) a Hot
Composite Curve representing, collectively, the process heat
sources (the hot streams); and (2) a Cold Composite Curve
representing the process heat sinks (the cold streams). For a
specified minimum allowed temperature difference ΔT , the two
min
curves are combined in one plot (see Figure 4.7), providing a clear
thermodynamic view of the heat recovery problem.
The overlap between the two Composite Curves represents the
heat recovery target. The overlap projection on the heat exchange
axis represents the maximum amount of process heat being internally
recovered. The vertical projection of the overlap indicates the
temperature range where the maximum heat recovery should take
place. The targets for external (utility) heating and cooling are
represented by the nonoverlapping segments of the Cold and Hot
Composite Curves, respectively. The methodology is described in
more detail in Chapter 4.

2.5 Supertargeting: Full-Fledged HEN Targeting
After obtaining targets for utility demands of a HEN, the next logical
step is to estimate targets for capital and total costs. Capital costs in
HENs are determined by many factors, of which the most significant
is the total heat transfer area and its distribution among the heat
exchangers. Townsend and Linnhoff (1984) proposed a procedure for
estimating HEN capital cost targets by using the Balanced Composite
Curves, which are obtained by adding utilities to the Composite
Curves obtained previously (see Figure 4.7). The HEN heat transfer
area target is computed from the enthalpy intervals in the Balanced
Composite Curves by using the heat transfer coefficients given in the
HEN problem specification (assuming “vertical heat transfer” and
“spaghetti-type topology”). Improvements to this procedure that
have been proposed involve one or more of the following factors:

1. Obtaining more accurate surface area targets for HENs
that exhibit nonuniform heat transfer coefficients (Colberg
and Morari, 1990; Jegede and Polley, 1992; Zhu et al., 1995;
Serna-González, Jiménez-Gutiérrez, and Ponce-Ortega,
2007).
2. Accounting for practical implementation factors, such as
construction materials, pressure ratings, and different heat
exchanger types (Hall, Ahmad, and Smith, 1990).
3. Accounting for additional constraints such as safety and
prohibitive distance (Santos and Zemp, 2000).