The Heat Recovery Pinch

With this rule in mind, divide the process at the pinch as shown in 

Energy Targets for Heat Exchanger Network and Utilities 167 

In choosing to transfer heat, say XP, from the system above the pinch to the system below the pinch, as shown in Fig. 6.8a, then above the pinch there is a heat deficit of XP. The only way this can 

Rgura 6.8 Three forms of crosspinch heat transfer. 

In other words, to achieve the energy target set by the composite curves, the designer must not transfer heat across the pinch by3,4  

These rules are both necessary and sufficient to ensure that the target is achieved, providing that the initialization rule is adhered to that no individual heat exchanger should have a temperature difference smaller than A Tmin. 

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The appropriate placement of the major items of equipment in relation to the heat recovery pinch is as follows ... 

Consider now the consequences of placing simple distillation columns (i.e. one feed, two products, one reboiler and one condenser) in different locations relative to the heat recovery pinch. The separator takes heat Qreb into the reboiler at temperature Treb and rejects heat Qcond at a lower temperature Tcond There are two possible ways in which the column can be heat integrated with the rest of the process. The reboiler and condenser can be integrated either across, or not across, the heat recovery pinch. 

Consider now a few examples of the use of this simple representation. A grand composite curve is shown in Figure 21.2a. The distillation column reboiler and condenser duties are shown separately and are matched against it. The reboiler and condenser duties are on opposite sides of the heat recovery pinch and the column does not fit. In Figure 21.2b, although the reboiler and condenser duties are both above the pinch, the heat duties prevent a fit. Part of the duties can be accommodated, and if heat integrated,... 

If an inappropriately placed distillation column is shifted above the heat recovery pinch by changing its pressure, the condensing stream, which is a hot stream, is shifted from below to above the pinch. The reboiling stream, which is a cold stream, stays above the pinch. If the inappropriately placed distillation column is shifted below the pinch, then the reboiling stream, which is a cold stream, is shifted from above to below the pinch. The condensing stream stays below the pinch. Thus appropriate placement is a particular case of shifting streams across the pinch, which in turn is a particular case of the plus-minus principle (see Chapter 19). 

The appropriate placement of distillation columns when heat integrated is not across the heat recovery pinch. The grand composite curve can be used as a quantitative tool to assess integration opportunities. The scope for integrating conventional distillation columns into an overall process is often limited. Practical constraints often prevent integration of columns with the rest of the process. If the... 

For the low-temperature distillation process shown in the flow diagram in Fig. 7.18, calculate the minimum hot-utility requirement and the location of the heat recovery pinch. Assume that the minimum acceptable temperature difference, ATrma, equals 5°C. The specifics of the process streams passing through the seven heat exchangers appear in the first five columns of Table 7.8, the mass flowrates of the streams being represented within the enthalpy (AH) values. 

Exhaust gases from the gas turbine are used to raise steam in the lower cycle without the burning of additional fuel (Fig. 7.3) the temperatures of the gas and water/steam flows are as indicated. A limitation on this application lies in the heat recovery system steam generator choice of the evaporation pres.sure (p ) is related to the temperature difference (Tft — T ) at the pinch point as shown in the figure, and a compromise has to be reached between that pressure and the stack temperature of the gases leaving the exchanger, (and the consequent heat loss ). ... 

The Pinch principle states that any design where heat is transferred across the Pinch will require more energy than minimum requirements. Consequently, the heat recovery problem is divided into two subsystems, above and below the Pinch. 

Pinch Point Analysis is a systematic process design methodology consisting of a number of concepts and techniques that ensure an optimal use of energy. The Pinch is characterised by a minimum temperature difference between hot and cold streams, and designates the location where the heat recovery is the most constraint. 

What can be learned from the composite curves Briefly, the curves can reveal very important insights for a heat recovery problem maximal process heat recovery, pinch point, and hot and cold utility targets, which can be visualized in Figure 9.7. The explanations for these basic concepts are given as follows ... 

Pinch Point The location of AT is called the process pinch. In other words, the pinch point occurs at AT an- When the hot and cold composite curves move closer to a specified AT, the heat recovery reaches the maximum and the hot and cold utilities reach the minimum. Thus, the pinch point becomes the bottleneck for further reduction of hot and cold utilities. Process changes must be made if further utility reduction is pursued. 

The network pinch identifies the heat recovery limit inherent in an existing heat exchanger network and therefore indicates the requirement of structural changes to the HEN. In other words, when a network reaches its network pinch limit, the only way to overcome it or increase heat recovery is to make stmctural changes if no process changes are considered. 

The pinch analysis method provides the targets for the Heat Exchange Network (HEN) designs of the examined process. This HEN design problem is complex and involves combinatorial problems in the pairing between the hot and cold streams to enhance the heat recovery [28]. HEN design is one of the most fundamental activities in the process design procedure as it is considered as sub-synthesis problem of the overall synthesis problem. 

The appropriate placement of reactors, as far as heat integration is concerned, is that exothermic reactors should be integrated above the pinch and endothermic reactors below the pinch. Care should be taken when reactor feeds are preheated by heat of reaction within the reactor for exothermic reactions. This can constitute cross-pinch heat transfer. The feeds should be preheated to pinch temperature by heat recovery before being fed to the reactor. 

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