Figure 6.25 Exploiting a path to save energy.
In a grassroots design, the skilled engineer would try to eliminate the loops, as they are always caused by having more than the minimum number of heat exchangers and indicate some redundancy in the network. By breaking a loop, the number of heat exchangers, and hence investment cost, will reduce (Figure 6.26).
In a retrofit design, however, paths are more important, and they form the basis of the network pinch, a relatively recent development in pinch analysis. Network pinch addresses the additional constraints imposed by the specific configuration of an existing facility. It is aimed specifically at finding the best energy savings for the least investment cost.
The Network Pinch
Existing networks can usually be improved by using paths to shift the loads between exchangers, but eventually a design will be reached from which no further improvement is possible, although it is still far from the pinch target. The starting network configuration imposes a constraint that hinders further improvement.
Network pinch analysis identifies the heat exchanger forming the bottleneck to increasing heat recovery and provides a systematic approach for removing this bottleneck. It is a step-by-step method for implementing energy savings in a series of consecutive projects.
Once the offending exchanger is identified, following four options can be considered for removing the constraint .
• re-sequencing-reversing the order of exchangers to improve heat recovery;
• re-piping-changing the matched streams to improve heat recovery;
• adding a new exchanger-to change the load on the offending exchanger;
• stream splitting-to reduce the load on a stream in the offending exchanger.
This is a software-led process, so that all possible paths in the network are explored and new ones identified by, for example, placing one new exchanger. Each path is then tested to see how much energy can be economically squeezed from that path, and the various paths are ranked in terms of their potential energy saving. It has been shown that these paths are independent of each other and a series of paths can be successively analyzed to produce a cumulative saving. The
network pinch revamp method is therefore sequential, but it examines various configurations in a systematic way, at the same time allowing the designer to interact with the software-led design procedure.
The network retrofits tend to fall into two different categories because there are, generically, two types of heat exchanger networks: the 'oil refining' type and the 'petrochemical' type (Figure 6.27).
The 'oil refining' type consists of sloping composite curves displaying a long tubular region like an extended pinch point and single hot and cold utilities. Refining networks are usually quite complex.
The 'petrochemical' type displays horizontal segments in the composite curves (change of phase), multiple utilities and fewer exchangers per stream.
In the 'oil refining' type, the paths are used first and the loads are shifted to improve heat recovery, usually by area enhancement on selected exchangers. Once the network pinch is reached, the four options presented above are exploited. The retrofit design often spans a large section of the process, and the projects may affect a number of exchangers.
The analysis of the petrochemical type of network usually involves successive removal of small numbers of exchangers from the network (two or three at a time), and re-targeting each time to identify the potential saving. Petrochemical revamps tend to cover smaller sections of the process and produce more or less independent projects, each affecting few exchangers only.
Other Applications of Pinch Technology
Previous sections have discussed the application of pinch analysis to individual process units. The technique for extending the analysis to the overall manufacturing site, total site analysis, is also described.
In certain cases, pinch analysis can be applied to whole areas, usually exclusively industrial but sometimes integrated with communities .
An area-wide pinch technology analysis was applied to Kashima chemical complex in Japan . This case study showed that although the individual plants in the complex were quite efficient, there was large energy saving potential through energy sharing among the various sites.
A similar study was carried out for the Rotterdam industrial area, with over 60 manufacturing sites, including four oil refineries and several petrochemical, chemical and food processing plants and a municipal incinerator. The study identified around 2000 MW of recoverable waste heat from these companies, of which 400 MW could be shared between them, 500 MW could be used for power generation and 1100 MW exported to heat the nearby greenhouses .
Notice that while area-wide integration is technically quite straight forward, its commercial aspects may sometimes be challenging. A detailed pinch analysis would require the sharing of sensitive process information. The success of a project will depend on good selection of data requirements so that sufficient data could be obtained to produce a meaningful analysis without endangering commercial confidences.
Water pinch is a systematic technique for analyzing water networks and identifying projects to improve the use of water in industrial processes. It identifies improvement opportunities in water re-use, regeneration (the partial treatment of process water that allows its re-use) and effluent treatment.
Like energy, water can be quantified in terms of quality and quantity, and similarly looking composite curves can be produced. Figure 6.28 shows the profile of the available sources arising within the process, and the various sinks or demands
Internal Water Snnrr«*
Figure 6.28 The water pinch.
for water. As for energy, the overlap region identifies where the internal wastewater streams can be re-used.
The one major difference between energy and water is that energy has only one dimension of quality and that is temperature. Water, on the other hand, can have several values, one for each contaminant (e.g., conductivity, dissolved solids, organics, etc). This would require that the composite curves for each impurity are developed. If each impurity created the same pinch point within the process, the analysis would not be too difficult to carry out manually. In reality, each contaminant creates a pinch at a different point and water pinch analysis has to rely on mathematical programming to identify the improved designs and optimize the trade-offs.
Hydrogen systems can also be analyzed by pinch techniques, again employing the concept of quality versus quantity. It is largely confined to the oil refining industry where the demand for hydrogen is continually increasing due to legislative pressure on low-sulfur products and the processing of increasingly sour crude oils.
The analysis is conceptually similar to the water pinch, as both hydrogen and water networks can be defined in terms of flowrate and purity. Their composite curves are hence alike. The scope for hydrogen re-use is defined by the overlap of the source and the sink composites, which is limited by the pinch point, as before. The target for minimum pure hydrogen make-up (from a hydrogen plant or from import) is given by the horizontal difference between the curves at the high purity end. The minimum purge rate is defined by the horizontal difference between the curves at the low purity end. In the case of hydrogen systems, the purge is however not wasted, but is fed to the site fuel gas system.
Similarly to water systems, hydrogen flows have multiple contaminants, but these are often approximated to a single ' impurity' since the different contaminants do not normally exclude re 'use of hydrogen streams in other processes. However there are some factors specific to hydrogen systems:
• Cost- because of the high cost of hydrogen and hydrogen generation equipment, systems are usually highly integrated with significant re-use of purge gas.
• Pressure-as the compression costs are high, pressure is an important parameter in the overall economics of the system.
• Effect of purity on production-hydrogen purity influences the economics of refinery unit operation in terms of throughput, yield or run length.
To analyze hydrogen systems, which is a complex problem, computerized algorithms are used , and they incorporate these additional parameters. The software will look for an overall optimum, maximizing the plant's profitability by simultaneously addressing hydrogen system operating costs and the process benefits, and minimizing the investment required to achieve them.
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