Page 140 - Chemical engineering design

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CHEMICAL ENGINEERING
6. Stream 3 requires more heat to bring it to the pinch temperature; amount needed:
H D 2.0 80 20 90 D 30 kW
This can be provided from stream 2, as the match will now be away from the pinch.
The rise in temperature of stream 3 will be given by:
T D H/CP

So transferring 30 kW will raise the temperature from the source temperature to:
Ž
20 C 30/2.0 D 35 C
and this gives a stream temperature difference on the outlet side of the exchanger of:

Ž
90 35 D 55 C
Ž
So the minimum temperature difference condition, 10 C, will not be violated by this
match.
7. Stream 2 will need further cooling to bring it to its target temperature, so a cooler
must be included; cooling required.
H cold D 1.0 90 30 30 D 30 kW

Which is the amount of the cold utility predicted by the problem table.
The proposed network for maximum energy recovery is shown in Figure 3.27.

CP ∆H
90°C 80°C kW/°C kW
180°C 60°C 3.0 360
B C
1 Cooler
150°C 30°C 120
A D 1.0
2
Heater 30 kW
135°C
20°C
A C D 2.0 230
3
140°C 50 kW 60 kW 90 kW 30 kW 80°C
B 4.5 270
4
270 kW Pinch

Figure 3.27. Proposed heat exchanger network T min D 10 ° C
Stream splitting

If the heat capacities of streams are such that it is not possible to make a match at the pinch
without violating the minimum temperature difference condition, then the heat capacity
can be altered by splitting a stream. Dividing the stream will reduce the mass flow-rates
in each leg and hence the heat capacities. This is illustrated in Example 3.16.
Guide rules for stream matching and splitting are given in the Institution of Chemical
Engineers Guide, IChemE (1994).