Multiple pinches

It is rare for there to be two process pinches in a problem. Multiple pinches usually arise from the introduction of additional utilities causing utility pinches. However, cases such as that shown in Fig. 16.18 are not uncommon, where there is, strictly speaking, only one pinch (one place where occurs), but there is a near-pinch. This... 

Figure 16.18 A near-pinch might require the design to be treated as if it had multiple pinches.

The pinch point(s) decompose the overall HEN problem into subproblems corresponding to above and below the pinch, if a single pinch point exists, or above the first pinch, between consecutive pinch point(s) and below the bottom pinch point, if multiple pinch points exist. These subproblems are denoted as subnetworks. It is the existence of pinch points that allows these subnetworks to be treated independently since we have made the implicit assumption that heat cannot cross the pinch point. 

For n = 1, y can be calculated from (5.15). depends on the kinetic parameters (Da, n) as well as on thermodynamic parameters K, a) of the system. Fig. 5.8 illustrates how the minimum reflux ratio varies with the Damkohler number for different reaction orders n. Qualitatively, decreases hyperbolically with increasing Da number. Multiple pinch points are possible for negative reaction orders within a certain Da window due to multiple intersections of the reboUer operating line with the VLF line. At l mm = the critical Damkohler number, Da n, — tP (5.12) is recovered. At Da = the smallest possible reflux ratio R it is obtained, which only depends on thermodynamic parameters besides the product specification xP... 

A low temperature of approach for the network reduces utihties but raises heat-transfer area requirements. Research has shown that for most of the pubhshed problems, utility costs are normally more important than annualized capital costs. For this reason, AI is chosen eady in the network design as part of the first tier of the solution. The temperature of approach, AI, for the network is not necessarily the same as the minimum temperature of approach, AT that should be used for individual exchangers. This difference is significant for industrial problems in which multiple shells may be necessary to exchange the heat requited for a given match (5). The economic choice for AT depends on whether the process environment is heater- or refrigeration-dependent and on the shape of the composite curves, ie, whether approximately parallel or severely pinched. In cmde-oil units, the range of AI is usually 10—20°C. By definition, AT A AT. The best relative value of these temperature differences depends on the particular problem under study. 

Some conditions require breaking up the exchanger into multiple parts for the calculations rather than simply using corrected terminal temperatures. For such cases one should always draw the q versus temperature plot to be sure no undesirable pinch points or even intermediate crossovers occur. 

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 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). 

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. 

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