Page 119 - Sustainability in the Process Industry Integration and Optimization

96 Cha p te r F o u r

applies Pinch Analysis to obtain a picture of the thermodynamic
limitations of the problem; but then, instead of continuing on to
direct synthesis, it builds a reduced superstructure. At this point
the method follows the route of the classical superstructure
approaches, including structure-parameter optimization and
topology simplifications. The cycle of optimization and simplification
is usually repeated several times before the final optimal network
is obtained. All resulting networks feature a high degree of heat
recovery, though rarely the maximum possible. A key component
of this technique is avoiding the addition of unnecessary features
to the superstructure, and this is an area where Pinch Analysis can
prove helpful. A good example of a hybrid method for HEN
synthesis is the block decomposition method (Zhu, 1997).

4.5.4 Key Features of the Resulting Networks
The networks obtained by the different synthesis methods have
distinct features, which influence their total cost and their properties
of operation and control. Because the Pinch Design Method
incorporates the tick-off heuristic rule (Figure 4.53), the networks
synthesized by this method tend to have simple topologies with
few stream splits and feature minimum number of units (Linnhoff
et al., 1982). Both the tick-off rule and the Pinch principle dictate
that utility exchangers be placed last, so they are usually located
immediately before the target temperatures of the streams.
However, the tick-off rule may also result in many process streams
not having utility exchangers assigned to them, which may reduce
control efficiency. The Pinch Design Method may reduce network
flexibility because it relies on the Pinch decomposition of the
problem (Figure 4.50) and so, to a large degree, fixes the network
behavior.
Both the pure superstructure approach and the hybrid approach
tend to produce more complex topologies. Their distinctive feature is
the greater number of heat exchangers and stream splits, a result of
how the initial superstructure is built. Spaghetti-type subnetworks
also present a significant challenge to control.

4.6 Total Site Energy Integration

The concept of the Total Site was introduced by Dhole and Linnhoff
(1993b). Figure 4.65 shows a typical industrial Total Site. Refinery and
petrochemical processes usually operate as parts of large sites or
factories. These sites have several processes serviced by a centralized
utility system involved in steam and power generation. The two
major components of Total Site integration are closely related: heat
recovery (through the steam or utility system) and power
cogeneration.