Network pinch method

It is not trivial to determine the best modification options that could feature minimal capital cost and the smallest effect on existing infrastructure and thus achieve maximum energy savings. The search for such options could be very time consuming for a complex heat recovery system. The incentives to find practical yet optimal modifications have been discussed by many other researchers and practitioners. The network pinch method developed by Zhu and Asante (1999) has been proven successful for practical applications, which will be discussed in detail in this chapter. 

In the past, the retrofit methods based on the proeess pineh concept was widely used (Tjoe and Linnhoff, 1986 Policy etal., 1990 Shokoya and Kotjabasakis, 1991). However, the process pinch is developed for grassroots design and is fundamentally irrelevant to the retrofit scenario. The method (Zhu and Asante, 1999) to be discussed here is a two-stage approach identify the network pinch, the true bottleneck of an existing heat recovery system, and determine modihcations with minimum capital costs to overcome the network pinch— that is the reason why this method is called the network pinch method. 

The rationale of the network pinch method is based on the effort and difficulty required for each assessment task. The effort for conducting a revamp study is demanding as it could involve weeks or up to months depending on the complexity of the process. During the revamp evaluation stage, not only itemized costs need to be determined, but effects on piping, pressure drops, and operation will be evaluated as well. To reduce the manpower effort, the network pinch method has been implemented into mathematical... 

In the initial phase of the study, the benchmarking assessment (see Chapter 3) was applied leading to the selection of 17 major process units for a more detailed study. Application of the network pinch method enabled the identification of many opportunities within a 3 month period of the study. The study identified 73 energy modifications, with a cumulative operating cost reduction of 61.3MM/year. The... 

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. 

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 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 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). 

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. 

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. 

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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