Threshold problem

Not all problems have a pinch to divide the process into two parts. Consider the composite curves in Fig. 6.10a. At this setting, both steam and cooling water are required. As the composite curves are moved closer together, both the steam and cooling water requirements decrease until the setting shown in Fig. 6.106 results. At this setting, the composite curves are in alignment at the hot end, 

In some threshold problems, the hot utility requirement disappears, as in Fig. 6.10. In others, the cold utility disappears, as shown in Fig. 6.11. 

Considering the capital/energy tradeoff for threshold problems, two possible outcomes are shown in Fig. 6.12. Below the threshold 

Flgum 6.9 A design that achieves the energy target. 

Energy Targets for Heat Exchanger Network and Utilities 171 

To design the heat exchanger network for a threshold problem, it is normal to start at the most constrained point. The problem can often be treated as one half of a problem exhibiting a pinch. 

Threshold problems are encountered in the process industries. A pinch can be introduced in such problems if multiple utilities are used, as in the recovery of heat to generate steam. 

The procedures to follow in the design of threshold problems are discussed by Smith (1995) and IChemE (1994). 

The use of multiple utilities can lead to more than one pinch in a problem. In introducing multiple utilities the best strategy is to generate at the highest level and use at the lowest level. For a detailed discussion of this type of problem refer to Smith (1995) and IChemE (1994). 

Considering the capital-energy trade-off for threshold problems, there are two possible outcomes as shown in 

In design, the same rules must be obeyed around a utility pinch as a process pinch. Heat should not be transferred across it by process-to-process transfer and there should 

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Figure 6.11 In some threshold problems, only a hot utility is required below the threshold value of AT i .

It is interesting to note that threshold problems are quite common in practice, and although they do not have a process pinch, pinches... 

Figure 6.13 Threshold problems are turned into a pinch problem when additional utilities are added.

In Sec. 6.3 it was mentioned that some problems, known as threshold problems, do not have a pinch. They need either hot utility or cold utility but not both. How should the approach be modified to deal with the design of threshold problems ... 

The philosophy in the pinch design method was to start the design where it was most constrained. If the design is pinched, the problem is most constrained at the pinch. If there is no pinch, where is the design most constrained Figure 16.9a shows a threshold problem that requires no hot utility, just cold utility. The most constrained part of this problem is the no-utility end. Tips is where temperature differences are smallest, and there may be constraints, as shown in Fig. 16.96, where the target temperatures on some of the cold... 

Figure 16.10 shows another threshold problem that requires only hot utility. This problem is different in characteristic from the one in Fig. 16.9. Now the minimum temperature difference is in the middle of the problem, causing a pseudopinch. The best strategy to deal with this type of threshold problem is to treat it as a pinched problem. For the problem in Fig. 16.10, the problem is divided into two parts at the pseudopinch, and the pinch design method is followed. The only complication in applying the pinch design method for such problems is that one-half of the problem (the cold end in Fig. 16.10) will not feature the flexibility offered by matching against utility.

Figure 16.9 Even though threshold problems have large driving forces, there are still often essential matches to be made, especially at the no-utility end.

Figure 16.10 Some threshold problems must be treated as pinched problems requiring essential matches at both the no-utility end and the pinch.

Problems that show the characteristic of requiring only either a hot utility or a cold utility (but not both) over a range of minimum temperature differences, from zero up to a threshold value, are known as threshold problems. A threshold problem is illustrated in Figure 3.29. 

It is interesting to note that threshold problems are quite common in practice and although they do not have a process pinch, pinches are introduced into the design when multiple utilities are added. Figure 16.13a shows composite curves similar to the composite curves from Figure 16.10 but with two levels of cold utility used instead of one. In this case, the second cold utility is steam generation. The introduction of this second utility causes a pinch. This is known as a utility pinch since it is caused by the introduction of an additional utility4. 

A problem table analysis of the data indicates that it is a threshold problem requiring only cold utility. The threshold value of ATmin is 117°C, corresponding with a cold utility duty of 10,100 kW. It is proposed to use steam generation as cold utility for which ATmin = 10°C. 

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