Page 138 - Chemical engineering design

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CHEMICAL ENGINEERING
Network design for maximum energy recovery
The analysis carried out in Figure 3.22, and Figure 3.23, has shown that the minimum
utility requirements for the problem set out in Table 3.3 are 50 kW of the hot and 30 kW
of the cold utility; and that the pinch occurs where the cold streams are at 80 and the
Ž
hot 90 C.
The grid representation of the streams is shown in Figure 3.25. The vertical dotted lines
represent the pinch and separate the grid into the regions above and below the pinch.
CP
(kW/°C)
90°C 80°C
180°C 60°C
3.0
1
150°C 30°C
1.0
2
135°C 20°C
2.0
3
140°C 80°C
4.5
4
Figure 3.25. Grid for 4 stream problem

For maximum energy recovery (minimum utility consumption) the best performance is
obtained if no cooling is used above the pinch. This means that the hot streams above
the pinch should be brought to the pinch temperature solely by exchange with the cold
streams. The network design is therefore started at the pinch; finding feasible matches
between streams to fulfil this aim. In making a match adjacent to the pinch the heat
capacity CP of the hot stream should be equal to or less than that of the cold stream. This
is to ensure that the minimum temperature difference between the curves is maintained.
The slope of a line on the temperature-enthalpy diagram is equal to the reciprocal of the
heat capacity. So, above the pinch the lines will converge if CP hot exceeds CP cold and as
the streams start with a separation at the pinch equal to T min , the minimum temperature
condition would be violated.
Below the pinch the procedure is the same; the aim being to bring the cold streams
to the pinch temperature by exchange with the hot streams. For streams adjacent to the
pinch the criterion for matching streams is that the heat capacity of the cold stream must
be equal to or greater than the hot stream, to avoid breaking the minimum temperature
difference condition.

The network design above the pinch

CP hot CP cold
1. Applying this condition at the pinch, stream 1 can be matched with stream 4, but
not with 3.