Page 110 - Sustainability in the Process Industry Integration and Optimization

P r o c e s s I n t e g r a t i o n f o r I m p r ov i n g E n e r g y E f f i c i e n c y 87

of a hot stream if a feasible essential match is to result. Generalizing
Eq. (4.9) shows that the CP of the stream entering the Pinch must be
less than or equal to the CP of the stream leaving the Pinch:

CP entering pinch d CP leaving pinch (4.10)

The Pinch Design Method incorporates a special tool for handling
this stage: CP tables (Linnhoff and Hindmarsh, 1983). Here the
streams are represented by their CP values, which are sorted in
descending order. This facilitates the identification of promising
combinations of streams for candidate essential matches. Sizing the
matches follows the so-called tick-off heuristic, which stipulates that
the heat exchange match should be as large as possible so that at least
one of the involved streams will be completely satisfied and then
“ticked off” from the design list; see Figure 4.53.
Completing the Design
The HEN design above the Pinch is illustrated in Figure 4.54. The
design below the Pinch follows the same basic rules, with the small
difference that here it is the cold streams that define the essential
matches.
Figure 4.55 details the design below the Pinch for the considered
example. First the match between streams 4 and 1 is placed and sized
to the duty required by stream 4. The other match, between streams 2
and 1, formally violates the Pinch rule for placing essential matches.
However, since stream 1 already has another match at the Pinch, the
current match (between 4 and 1) is not strictly termed essential. Up

CP Q STREAM
[kW/ºC] [kW]
150ºC
203.3ºC 250ºC
2 15 1500
150ºC 200ºC
4 25 1250

140ºC 180ºC
1 20 800
800 kW
140ºC T X = 181.7ºC 230ºC
3 30 2700
1250
1250 kW T X =140+ =181.7ºC
30
FIGURE 4.53 The tick-off heuristic.