Chemical Engineering November 2011 - 37

Process only unshifted composite curve
Unoptimized utilites unshifted composite curve
200
250
300
100
150
50
100
200 300 400 500
Heat duty, MBtu/h
600 700 800 900 1,000
FIGURE 3. When the CCs for the hot and cold streams are
plotted and aligned so that the desired pinch point is achieved,
the utility requirements can be easily determined
Process only shifted composite curve
200
250
300
100
150
50
100
200 300 400 500
Heat duty, MBtu/h
600 700 800 900 1,000
FIGURE 5. Shifting the hot composite curve down by 1/2 of
the pinch temperature and the cold composite curve up by 1/2
of the pinch temperature creates a shifted composite curve
Shifted optimized utilities curve
200
250
300
100
150
50
100
Cold utility
at 65°
200 300 400 500
Heat duty, MBtu/h
600 700 800 900 1,000
FIGURE 7. Adding the utility streams into the shifted composite
curve shows how the optimum energy solution can create
additional pinch points
articles and resources available that
go into detail on creating the heat exchanger
networks themselves [1-3].
Pinch analysis example
Figure 3 shows two composite curves
that depict the hot and cold utility
requirements as 42 MBtu/h and 114
MBtu/h, respectively. It has a 20°F
pinch temperature. One thing to note is
that only the process streams are plotted.
The reasoning is that we want to
make adjustments to the process, by
adding or rearranging heat exchangProcess
pinch
points
Hot utility
at 159°
Utility pinch
points
200
250
300
100
150
50
10
20
30 40 50 60 70
Heat duty, MBtu/h
80 90 100
110 120
FIGURE 8. Replotting the grand composite curve shows the
utilities' impact on the curve
ers, in order to exploit the energy inventory
in the process streams before adding
energy (hot or cold) via utilities.
There are two features of this example
process worth noting. The hot
stream enters warmer than the desired
cold stream outlet, and the cold stream
enters colder than the desired hot
stream outlet. The implication of this is
that it is not necessary to have the hot
utility above the desired cold-streamoutlet
temperature and conversely, the
cold utility does not need to be below the
desired hot-stream-outlet temperature.
Pinch temperature selection
A rarely addressed point that needs
to be discussed is how the pinch temperature
was determined. Where does
that number come from? In the training
materials I have read, the choice
of pinch temperature is usually rather
vague. Materials often mention that it
is the best temperature that the heat
exchanger network can obtain for this
type of process. In order to help quantify
the pinch temperature, it is useful
to take the following relationships
into account. The basis of the pinch
CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2011 37
200
250
300
100
150
Pinch point
50
20
40
60
80
100 120 140 160
Heat duty, MBtu/h
FIGURE 6. Subtracting the hot CC from the cold CC, at each
temperature, creates a grand composite curve, which can be
used to determine the temperatures required for the utilities
Process only grand composite curve
Cold utility duty =
114 MBtu/h
Hot utility duty
42 MBtu/h
200
250
300
Pinch = 20°F
Cold utility duty
114 MBtu/h
100
150
50
100
200 300 400 500
Heat duty, MBtu/h
600 700 800 900 1,000
FIGURE 4. Adding the utilities to the ends of the respective
hot and cold composite curves can sometimes lead to an unoptimized
solution
Process only grand composite curve
Hot utility duty =
42 MBtu/h
Cold utility duty
114 MBtu/h
Hot utility duty
42 MBtu/h
Hot utility
at 159°F
Pinch points
Cold utility
at 65°F
Temperature, °F
Temperature, °F
Temperature, °F
Temperature, °F
Temperature, °F
Temperature, °F
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Chemical Engineering November 2011

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