Pinch design

Turning now to the cold-end design, Fig. 16.6a shows the pinch design with the streams ticked off. If there are any cold streams below the pinch for which the duties eu e not satisfied by the pinch matches, additional process-to-process heat recovery must be used, since hot utility must not be used. Figure 16.66 shows an additional match to satisfy the residual heating of the cold streams below the pinch. Again, the duty on the unit is maximized. Finally, below the pinch the residual cooling duty on the hot streams must be satisfied. Since there are no cold streams left below the pinch, cold utility must be used (Fig. 16.6c). 

This design procedure is known as the pinch design method and can be summarized in five steps... 

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.

The pinch design method developed earlier followed several rules and guidelines to allow design for minimum utility (or maximum energy recovery) in the minimum number of units. Occasionally, it appears not to be possible to create the appropriate matches because one or other of the design criteria cannot be satisfied. 

Solution Figure 16.16a shows the streeun grid with the CP tables for the above- and below-pinch designs. Following the algorithms in Fig. 16.15, the... 

The network can now be designed using the pinch design method.The philosophy of the pinch design method is to start at the pinch and move away. At the pinch, the rules for the CP inequality and the number of streams must be obeyed. Above the utility pinch and below the process pinch in Fig. 16.17, there is no problem in applying this philosophy. However, between the two pinches, there is a problem, since designing away from both pinches could lead to a clash where both meet. 

The design method used so far, the pinch design method, creates an... 

The pinch design method is a step-by-step approach which allows the designer to interact as the design progresses. For more complex network designs, especially those involving many constraints, mixed equipment specifications, etc., design methods based on the optimization of a reducible structure can be us d. ... 

Linhoff, B. and Hindmarsh, E., 1983. The pinch design method for heat exchanger networks. Chemical Engineering Science, 38, 745. 

The pinch design method, as discussed so far, has assumed the same A Tmin applied between all stream matches. In Chapter 16, it was discussed how the basic targeting methods for the composite curves and the problem table algorithm can be modified to allow stream-specific values of A Tmin. The example was quoted in which liquid streams were required to have a A Tmin contribution of 5°C and gas streams a ATmin contribution of 10°C. For liquid-liquid matches, this would lead to a ATmin = 10°C. For gas-gas matches, this would lead to a ATmin = 20°C. For liquid-gas matches, it will lead to a ATmin = 15°C 2. Modifying the problem table and the composite curves to account for these stream-specific values of ATmin is straightforward. But how is the pinch design method modified to take account of such A Tmin contributions Figure 18.9 illustrates the approach. Suppose the interval pinch temperature from the problem table is 120°C. This would correspond with hot stream pinch temperatures of 125°C and 130°C for hot streams with ATmin contributions of 5°C and 10°C respectively. For... 

The pinch design method creates a network structure based on the assumption that no heat exchanger should have a temperature difference smaller than ATmin. Having now created a structure for the heat exchanger network, the structure can now be subjected to continuous optimization. The constraint that no exchanger should have a temperature... 

Linnhoff B and Hindmarsh E (1983) The Pinch Design Method of Heat Exchanger Networks, Chem Eng Sci, 38 745. 

See also Heat exchanger networks factors in, 13 219 Pinch design problem example,... 

Pinch design shortcomings, 13 211-212 Pinch Design Method, 13 197-203 practical design considerations for,... 

Pinch analysis, targets of, 13 217 Pinch design. See also Pinch Design Method... 

Most recent synthesis algorithms are also based upon the principles of the thermodynamic pinch (Linnhoff et al., 1979 Umeda et al., 1978). Recognition of the pinch provided great physical insight into the problem of HEN synthesis. The reader is assumed to be familiar with the principles of the pinch and with general methods for HEN synthesis [e.g., pinch design method (Linnhoff et al., 1982 Linnhoff and Hindmarsh, 1983), structural optimization methods for selection of a minimum set of stream matches (Papoulias and Grossmann, 1983), and determination of the most economical network structure (Floudas et al., 1986) from the predicted matches]. 

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