Page 117 - Sustainability in the Process Industry Integration and Optimization

94 Cha p te r F o u r

understanding of the thermodynamic limitations imposed by the set
of process streams, and then it exploits this knowledge to design a
highly energy-efficient HEN. However, another approach has also
been developed: the superstructure approach to HEN synthesis,
which relies on developing a reducible structure (the superstructure)
of the network under consideration. An example of the spaghetti-
type HEN superstructure fragments typically generated by such
methods (Yee et al., 1990) is shown in Figure 4.64.
Typically, this kind of superstructure is developed in stages (Yee
et al., 1990) or blocks (Zhu et al., 1995), each of which is a group of
several consecutive enthalpy intervals (discussed previously). Within
each block, each hot process stream is split into a number of branches
corresponding to the number of the cold streams presented in the
block, and all cold streams are split similarly. This is followed by
matching each hot branch with each cold branch. Once developed,
the superstructure is subjected to optimization. The set of decision
variables includes the existence of the different stream split branches
and heat exchangers, the heat duties of the exchangers, and the split
fractions or flow rates of the split streams. The objective function
involves mainly the total annualized cost of the network, although
the function may be augmented by some penalty terms for dealing
with difficult constraints. Because the optimization procedure makes
structural as well as operating decisions about the network being
2
designed, it is called a structure-parameter optimization. Depending

Enthalpy Interval

Hot
Streams

Cold
Streams

FIGURE 4.64 Spaghetti superstructure fragment.

2 This should not be confused with the “parameter” entities from Mathematical
Programming.