Multiple utilities

Note that there are eight heat-capacity flow rates, four temperatures, and nine equahty constraints, and hence, three decision variables (e.g., fi, F, and Fg). Also, Ar ,i is set at 10 K. In a more general formulation, it could be adjusted as an optimization variable. 

See the CD-ROM that accompanies this book for the GAMS input file, COST.l, which obtains a solution using the Chen approximation to the log-mean temperature difference.  

Thus far, multiple sources of hot and cold utilities have not been considered. For example, steam is normally available at several pressures, with its cost a function of the pressure (and temperature) level, as discussed in Section 17.1 (Table 17.1). To reduce the cost of utilities, 

Designing HENs Assisted by the Grand Composite Curve 

The grand composite curve helps to better utilize utility resources when designing HENs, as illustrated in the following example. 

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Aichele, P., Sequencing Distillation OperationBeing Serviced by Multiple Utilities, M.Sc. dissertation, UMIST, U.K., 1992. 

Threshold problems are encountered in the process industries. A pinch can be introduced in such problems if multiple utilities are used, as in the recovery of heat to generate steam. 

The use of multiple utilities can lead to more than one pinch in a problem. In introducing multiple utilities the best strategy is to generate at the highest level and use at the lowest level. For a detailed discussion of this type of problem refer to Smith (1995) and IChemE (1994). 

Aichele P (1992) Sequencing Distillation Operations Being Serviced by Multiple Utilities, MSc Dissertation, UMIST, UK. 

It is interesting to note that threshold problems are quite common in practice and although they do not have a process pinch, pinches are introduced into the design when multiple utilities are added. Figure 16.13a shows composite curves similar to the composite curves from Figure 16.10 but with two levels of cold utility used instead of one. In this case, the second cold utility is steam generation. The introduction of this second utility causes a pinch. This is known as a utility pinch since it is caused by the introduction of an additional utility4. 

Based on the above, a Model Test should be inexpensive, use simple equipment, have high resolution, and test for properties of multiple utility. 

Linnhoff March Ltd.), and UniSim ExchangerNet (Honeywell Inc.) allow the design engineer to plot composite curves, optimize AT in, set targets for multiple utilities, and design the heat exchanger network. 

Figure 10.23 Multiple utilities 10.3.3 Variable temperature utilities...

When there are multiple utility options available, the question is which utihty is to be selected to reduce overall utihty costs. This involves setting appropriate loads for the various utihty levels by maximizing cheaper utihty loads prior to use of more expensive utilities. The grand composite curve is an elegant tool for accomplishing this purpose. 

The GFR with related fuel cycle is aiming at the sustainable utilization of fuel resources by recycling, as well as multiple utilization and high thermal efficiency, and it may develop new-generation nuclear energy systems that serve as an efficient use of nuclear energy resource and... 

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