Heat Pinch point analysis

The essential merit of pinch-point analysis is that makes possible the identification of key targets for energy saving with minimum information about the performance of heat-exchange equipment The key results of are ... 

Pinch Point Analysis (PPA) is an extension of the second principle of Thermodynamics to the energy management of the whole plant. PPA deals with the optimal structure of the heat exchange between the process streams, as well as the optimal use of utilities. Among benefits we mention ... 

Pinch Point Analysis starts with the input of data. The first step is the extraction of stream data from a flowsheet simulation, which describes typically the material balance envelope (Reactors and Separators). Proper selection and treatment of streams by segmentation is a key factor for efficient heat integration. The next step is the selection of utilities. Additional information regards the partial heat transfer coefficients of the different streams and segments of streams, and of utilities, as well as the cost of utilities and the cost laws for heat exchangers. 

Figure 10.5 Overall approach in designing a Heat Exchanger Network by Pinch Point Analysis...

Pinch Point Analysis is a systematic process design methodology consisting of a number of concepts and techniques that ensure an optimal use of energy. The Pinch is characterised by a minimum temperature difference between hot and cold streams, and designates the location where the heat recovery is the most constraint. 

Pinch Point Analysis can be used to develop heat Integration schemes. Some could be optimal from the viewpoint of energy saving, but inoperable in practice. We adopt the... 

We will see, however, that this is not a correct analysis if a pinch point exists. In other words, the implicit assumption in the first law analysis is that we always have feasible heat exchange for the given HRAT = 30°C. To illustrate the potential bottleneck, let us consider the graphical representation of the hot and cold process streams in the T - Q diagram shown in Figure 8.1. 

Remark 1 Since we cannot bring the two composite curves closer, the pinch point represents the bottleneck for further heat recovery. In fact, it partitions the temperature range into two subnetworks, one above the pinch and one below the pinch. Heat flow cannot cross the pinch since there will be violations in the heat exchange driving forces. As a result, we need a hot utility at the subnetwork above the pinch and a cold utility at the subnetwork below the pinch. In other words, having identified the pinch point, we can now apply the first law analysis to each subnetwork separately and determine the hot and cold utility requirements. These can be read from the T - Q diagram since they correspond to the horizontal segments AG and CD, respectively. Hence, for our example we have ... 

The second method involves a predictive method that determines VLE and liquid-liquid equihbrium (LLE), which are important for designing evaporators and condensers. Another method is a powerful tool for parameter determination and optimization by Levenberg and Marquardt [28]. This method can be used to develop complex models for process engineering purposes which require an adaptation of parameters to experimental results. The final analysis method deals with the pinch-point method developed by Linnhoff etal. [29]. This technique aims to identify the maximum possible heat recovery and the minimum energy requirement of a thermal or chemical process [30-32]. The apphcation of this method to fuel cell systems is explained in the final part of this section. 

On the cold composite curve, each stream that is to be heated must enter or leave an exchanger at the pinch point. On the hot composite cmwe, each stream that is to be cooled must enter or leave an exchanger at the pinch point. Start the analysis of exchangers in the sink and source sections at the pinch point where all temperatures are fixed. A point of discontinuity on a composite curve indicates the addition or removal of a stream. Added or removed streams must enter or leave an exchanger at the temperature where the discontinuity occurs. 

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