Pinch design stream splitting

The introduction of multiple stream splits is often cited as a drawback of the pinch method. Stream splits can be problematic in process operation. For example, when an oil or other multicomponent stream is heated and partially vaporized, then the stream is a two-phase mixture. It is difficult to control the splitting of such streams to give the required flow rate in each branch. Experienced designers usually constrain the network to avoid multiple stream splits whenever possible, even if this leads to designs that have higher than minimum utility consumption. 

Clearly, in designs different from those in Figs. 16.13 and 16.14 when streams are split to satisfy the CP inequality, this might create a problem with the number of streams at the pinch such that Eqs. (16.3) and (16.4) are no longer satisfied. This would then require further stream splits to satisfy the stream number criterion. Figure 16.15 presents algorithms for the overall approach. ... 

The cold-utility target for the problem shown in Fig. 16.22 is 4 MW. If the design is started at the pinch with stream 3, then stream 3 must be split to satisfy the CP inequality (Fig. 16.22a). Matching one of the branches against stream 1 and ticking off stream 1 results in a duty of 8 MW. 

Solution Figure 18.17a shows the stream grid with the CP-tables for the above- and below-pinch designs. Following the algorithms in Figure 18.16, a hot stream must be split above the pinch to satisfy the CP inequality, as shown in Figure 18.17b. 

Once the initial network structure has been defined, then loops, utility paths and stream splits offer the degrees of freedom for manipulating network cost in multivariable continuous optimization. When the design is optimized, any constraint that temperature differences should be larger than A Tmin or that there should not be heat transfer across the pinch no longer applies. The objective is simply to design for minimum total cost. 

The above rules can be put together into a general design procedure at Pinch, as illustrated by the Fig. 10.33. Firstly, the stream count rule is checked. If not fulfilled, a first stream split is performed to balance streams, cold stream above the Pinch, or hot stream below the Pinch. Then the CP s rule is checked for matches close to the Pinch. If not fulfilled, again stream splitting is executed, this time opposite to the first. Note that the above rules might be not respected away from Pinch. 

When designing a HEN to meet its MER targets, stream splitting must be employed if (a) the number of hot streams at the pinch, on the cold side, is less than the number of cold streams or (b) the number of cold streams at the pinch, on the hot side, is less than the number of hot streams. In this way, parallel pairings that fully exploit the temperature differences between energy sources and sinks are possible. Moreover, stream splitting helps to reduce... 

Figure 10.29 shows the designs for three HENs, having minimum utilities, at three values of Ar j 30, 25.833, and 16.9 K. For the first two, in Figures 10.29a and 10.29b, using the method of Linnhoff and Hindmarsh (1983), stream splitting is required below the first pinch. Stream Cl is split into two streams between the pinches because just streams HI and H2 are present. Below the second pinch, stream Cl is split into three streams because all three hot streams are present. As shown in Table 10.6, both the purchase and utility costs are lower at The former is reduced because the cooling water exchanger is no longer needed. At Ar, = 16.9 K, the HEN is much simpler because no pinches exist, as shown in Figure 10.29c. Hence, the purchase cost is lower and the cost of utilities is equal to that at...

Similar to the design of HENs, when matching rich and lean streams at the pinch, on the rich side, it is necessary that the number of rich streams be less than or equal to the number of lean streams. When this is not the case, lean streams must be split until the number of rich and lean streams is equal. Also, on the lean side, it is necessary that the number of lean streams be less than or equal to the number of rich streams. Here, rich streams must be split until the number of rich and lean streams is equal. The development of MENs above and below the pinch, including the need for stream splitting, is illustrated in the next example. 

By contrast, now consider part of a design below the pinch (Fig. 16.12a). Here, hot utility must not be used, which means that all cold streams must be heated to pinch temperature by heat recovery. There are now three cold streams and two hot streams in Fig. 16.12a. Again, regardless of the CP values, one of the cold streams cannot be heated to pinch temperature without some violation of the constraint. The problem can only be resolved by splitting a hot (a)... 

Another above-pinch problem is shown in Fig. 18A. This time, there is one hot stream and there are two cold streams. Thus, as far as the number of streams is concerned, there should be no need to split a stream. However, the CP inequality also needs to be satisfied above the pinch the hot stream, with a CP of 0.5, requires a cold stream with a greater CP. Neither of the two cold streams in Fig. 18A is large enough to satisfy the CP inequality. The design problem can be solved by splitting the hot stream into two branches, as shown in Fig. 18B. Splitting arbitrarily into two branches with a CP of 0.3 and 0.2 allows... 

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