Page 134 - Chemical engineering design

114
200 CHEMICAL ENGINEERING
180 Streams 1
160 Stream 1 CP = 3.0
Temperature, °C 120 Stream 2 Streams 1 + 2
140
100
CP = 3.0 + 1.0 = 4.0
80
60
40 Stream 2 CP = 1.0 kW/°C
20
0
0 100 200 300 400 500 600 0 100 200 300 400 500 600
Enthalpy, kW Enthalpy, kW
(a) (b)
Figure 3.21. Hot stream temperature v. enthalpy (a) Separate hot streams (b) Composite hot streams

Hot utility
50 kW
200
180
160 Hot streams
140
Temperature, °C 120 Pinch Cold streams ∆T = 10°C

100
80
60 min
40
20 30 kW
Cold utility
0
0 100 200 300 400 500 600
Enthalpy, kW
Figure 3.22. Hot and cold stream composite curves

As for the two-stream problem, the displacement of the curves at the top and bottom
of the diagram gives the hot and cold utility requirements. These will be the minimum
values needed to satisfy the target temperatures. This is valuable information. It gives the
designer target values for the utilities to aim for when designing the exchanger network.
Any design can be compared with the minimum utility requirements to check if further
improvement is possible.
In most exchanger networks the minimum temperature difference will occur at only
one point. This is termed the pinch. In the problem being considered, the pinch occurs at
Ž
Ž
between 90 C on the hot stream curve and 80 C on the cold stream curve.