Composite Curves Module:03 Lecture:06 Module‐03 : Building blocks of PINCH Technology Lecture‐06 : Cold Composite Curves Key words: Composite curve, T‐H diagram, Temperature interval, enthalpy Instead of dealing cold streams individually it is desirable to integrate the energy demand of all the cold streams in appropriate temperature intervals and represent these as a composite cold curve. However, in this composite cold curve the temperature levels of each cold stream and heat demand ( load) should be preserved. The appropriate temperature ranges are detected from the changes in the T‐H plot slopes. If heat capacity flowrate(CP) are constant, then changes will occur only when streams start or finish. Thus the temperature axis is divided into ranges by the supply and target temperature of streams. The temperature‐enthalpy values (slope of line in T‐H diagram) associated with any cold stream can’t be changed, however, the relative position of cold streams can be changed by moving them horizontally( parallel to H axis) relative to reach other. This is possible as the reference enthalpy for a cold stream can be changed independently from the reference enthalpy for the other cold streams. Within each temperature interval, the heat loads of streams( if available) are combined by moving these horizontally to produce a cold composite stream. This cold composite stream, within the said temperature interval, has a CP value that is the sum of the CP values of individual streams present in that temperature interval. Similarly, in a given temperature interval the enthalpy change of the cold composite curve is equal to the sum of the enthalpy changes of all those streams which are present in that temperature interval.The cold composite stream is a virtual single stream that is equivalent to all cold streams present in it, in terms of temperature levels and enthalpy. 3.7 Cold composite curve .
A two stream load integration problem is given in Table 3.5. The scope of cold stream load integration is shown in Fig.3.16. Table 3.5: Two cold stream problem for load integration Name of the stream Supply Temperature Target Temperature CP H Ts, C Tt, C kW/C kW Cold‐1 30 150 2 240 Cold‐2 70 120 1.5 75 Cold stream presence H 150C 60 kW Cold‐1
120C Cold‐1 + Cold‐2 Scope of integration In this temperature interval 175(100+75) kW
70C Cold‐2 80 kW Cold‐1 30C Cold‐1 Fig 3.16 Temperature interval diagram Composite Curves Lecture:06 T No Cold stream
150C Cold‐1 CP=2, Slope=1/2
120C Cold‐1 Cold-1 & 2 Cold‐2 CP=1.5, Slope=1/1.5 70C Cold‐1 30C No cold stream
20C H 75
240
(a)
T No Cold stream
150C Cold‐1 CP=2, Slope=1/2 120C Cold‐1 Cold‐2 CP=1.5, Slope=1/1.5
70C Cold-1 & 2 Cold‐1 30C No cold stream
20C H 75
240
(b)
T No Cold stream
150C CP=2, Slope=1/2
Cold‐1 120C CP=3.5(2+1.5), Slope=1/3.5 70C CP=2, Slope=1/2 30C Cold-1 & 2 Module:03 Cold‐1 No cold stream
20C 215
40 H 60 315
(c)
Fig.3.17(a),(b) & (c) Load integration of two cold stream as given in Table Composite Curves Module:03 Lecture:06 Fig.3.17(a),(b) & (c) shows cooling load integration of two cold streams. Fig.3.17(c) is the required Cold composite curve. In this curve cold duty integration in the temperature interval 50C to 120 C was carried out as in this temperature interval both the cold streams were present. A four stream load integration problem is given in Table 3.6. The scope of cold stream load integration is shown in Fig.3.18. Table 3.6: Two cold stream problem for load integration Name of the stream Supply Temperature Target Temperature Ts, C Tt, C Cold‐1 30 150
Cold‐2 70 120
Cold‐3 35 60
Cold‐4 130 200 CP kW/C 2
4
3
3.5 H kW 240 200 75 245  760 kW Cold‐4 150C 130C 120C Cold‐4 Cold‐1 + Cold‐4 Cold‐1
Cold‐1 + Cold‐2 70C 60C 35C 30C H, kW Cold stream presence 200C Cold‐2 Cold‐1 Cold‐3 Cold‐1 Cold‐1 + Cold‐3 Cold‐1 175 Scope of integration 110(40+70) 20 Scope of integration 300(100+200)
Scope of integration 20 125(50+75)
10  760 kW Fig 3.18 Temperature interval diagram for problem given in Table 3.6 T, C 200C 150C 130C 120C 70C 60C 35C 30C H, kW
100 800 Fig.3.19 Cold composite curve for problem of Table 3.6 Composite Curves Module:03 Lecture:06 Fig.3.19 shows the cold composite curve for problem given in Table 3.6. Further it can be seen that the cold composite curve conserves the heat load of all the individual cold streams( 760 kW) and also maintains the supply and target temperatures of all cold streams. Thus it truly represents Cold‐1 to Cold‐4 streams. Further it can be seen that the slope of the composite curve changes at supply and target temperatures of the individual cold streams. References 1. Linnhoff March, “Introduction to Pinch Technology” Targeting House, Gadbrook Park, Northwich, Cheshire, CW9 7UZ, England 2. Chemical Process Design and Integration, Robin Smith, John Wiley & Sons Ltd.