Page 79 - Sustainability in the Process Industry Integration and Optimization

56 Cha p te r F o u r

T
Q H,min Q UC,above XP

Q UH,below XP

PINCH

XP Q UH,below Q C,min Q UC,above ΔH

FIGURE 4.12 More in, more out.

violated. This could also happen if the external utilities are placed
incorrectly. For example, any utility heating below the Pinch will
create a need for additional utility cooling in that part of the system
(Figure 4.12). Conversely, any utility cooling above the Pinch will
create a need for additional utility heating. The implications of the
Pinch for heat recovery problems can be distilled into the following
three conditions, which must hold if the minimum energy targets for
a process are to be achieved.
1. Heat must not be transferred across the Pinch.
2. There must be no external cooling above the Pinch.
3. There must be no external heating below the Pinch.
Violating any of these rules will lead to an increase in energy utility
demands. The rules are applied explicitly in the context of HEN
synthesis by the Pinch Design Method (Linnhoff and Hindmarsh,
1983) and also before a HEN retrofit analysis to identify causes of
excessive utility demands by a process. Other HEN synthesis
methods—if they achieve the minimum utility demands—also
conform to the Pinch rules (though sometimes only implicitly).

4.3.3 Numerical Targeting: The Problem Table Algorithm
The Composite Curves are a useful tool for visualizing heat recovery
targets. However, they can be time-consuming to draw for problems
that involve many process streams. In addition, targeting that relies
solely on such graphical techniques cannot be very precise. The
process of identifying numerical targets is therefore usually based
on an algorithm known as the Problem Table Algorithm (PTA). Some