Heat exchanger network pinch

Example 16.1 The process stream data for a heat recovery network problem are given in Table 16.1. A problem table analysis on these data reveals that the minimum hot utility requirement for the process is 15 MW and the minimum cold utility requirement is 26 MW for a minimum allowable temperature diflFerence of 20°C. The analysis also reveals that the pinch is located at a temperature of 120°C for hot streams and 100°C for cold streams. Design a heat exchanger network for maximum energy recovery in the minimum number of units. 

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

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. 

The use of the pinch technology method in the design of heat exchanger networks has been outlined in Sections 3.17.1 to 3.17.6. The method can also be applied to the integration of other process units such as, separation column, reactors, compressors and expanders, boilers and heat pumps. The wider applications of pinch technology are discussed in the Institution of Chemical Engineers Guide, IChemE (1994) and by Linnhoff et al. (1983), and Townsend and Linnhoff (1982), (1983), (1993). 

Determine the pinch temperatures and the minimum utility requirements for the streams set out in the table below, for a minimum temperature difference between the streams of 20°C. Devise a heat exchanger network to achieve the maximum energy recovery. 

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... 

Figure 18.35 The network pinch limits the energy recovery in the existing heat exchanger network. (From Asante NDK and Zhu XX, 1997, Trans IChemE, 75A 349, reproduced by permission of the Institution of Chemical Engineers.)...

Consider now how the network pinch might be overcome. There are four ways in which the network pinch can be overcome and the performance of the existing heat exchanger network improved beyond that for the pinched condition14. 

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

Other established attempts on heat integration of batch plants are based on the concept of pinch analysis (Linnhoff et al., 1979 Umeda et al., 1979), which was initially developed for continuous processes at steady-state. As such, these methods assume a pseudo-continuous behaviour in batch operations either by averaging time over a fixed time horizon of interest (Linnhoff et al., 1988) or assuming fixed production schedule within which opportunities for heat integration are explored (Kemp and MacDonald, 1987, 1988 Obeng and Ashton, 1988 Kemp and Deakin, 1989). These methods cannot be applied in situations where the optimum schedule has to be determined simultaneously with the heat exchanger network that minimises external energy use. 

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

In the following section, we will discuss the approach proposed by Ciric and Floudas (1991) for the synthesis of heat exchanger networks without decomposition. Note that we will present the approach for the pseudo-pinch case (which is the most general). The approach for the strict-pinch case (which is a constrained scenario of the pseudo-pinch and as such features more structure) can be found in Ciric and Floudas (1991). 

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