Heat integration

Figure 1.46 shows a flowsheet without any heat integration for the different reactor and separation system. As before, this is probably too inefficient in the use of energy, and heat integration schemes can be explored. Figure 1.5 shows two of the many possible flowsheets. 

Figure 1.3 For a given reactor and separator design, there are different possibilities for heat integration. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

Partially vaporized feed reverses these effects. For a given separation, the feed conditions can be optimized. No attempt should be made to do this at this stage in the design, since heat integration is likely to change the optimal setting later in the design. It is usually adequate to set the feed to saturated liquid conditions. This tends to equalize the vapor rate below and above the feed. 

It must be emphasized that it is not worth expending any effort optimizing pressure, feed condition, or reflux ratio until the overall heat-integration picture has been established. These parameters very often change later in the design. 

However, factors such as this should not he allowed to dictate design options at the early stages of flowsheet design because preheating the cold feed hy heat integration with the rest of the process might be possible. 

No attempt should be made to optimize pressure, reflux ratio, or feed condition of distillation in the early stages of design. The optimal values almost certainly will change later once heat integration with the overall process is considered. 

Much work has been carried out to find methods for the synthesis of distillation sequences of simple columns that do not involve heat integration. However, heat integration may have a significant... 

This appears to be a complex problem requiring simultaneous solution of the sequence together with heat integration. 

These heuristics are based on observations made in many practical applications. In addition to being restricted to simple columns, the observations are based on no heat integration (i.e., all reboilers and condensers are serviced by utilities). Difficulties can arise when the heuristics are in conflict with each other, as the following example illustrates. 

Heat Integration of Sequences of Simple Distillation Columns... 

Having found the best nonintegrated sequence, most designers would then heat integrate. In other words, the total problem is not solved simultaneously but in two steps. Moving outward from the center of the onion (see Fig. 1.6), the separation layer is addressed first, followed by the heat exchanger network layer. 

Whether heat integration is restricted to the separation system or allowed with the rest of the process, integration always benefits from colder reboiler streams and hotter condenser streams. This point is dealt with in more general terms in Chap. 12. In addition, when column pressures are allowed to vary, columns with smaller temperature differences are easier to integrate, since smaller changes in pressure are required to achieve suitable integration. This second point is explained in more detail in Chap. 14. 

Having established that there is apparently a mechanism whereby the problems of sequencing and heat integration can be decoupled for simple columns on the basis of energy costs, it is interesting to consider whether there is any conflict with capital cost. A column sequence that handles a large amount of heat must have a high capital cost for two reasons ... 

When the integration of sequences of simple columns was considered, it was observed that sequences with higher heat loads occurred simultaneously with more extreme levels. Heat integration always benefits from low heat loads and less extreme levels, as we shall see later in Chap. 12. Now consider the effect of thermal coupling arrangements on loads and levels. Figure 5.18 compares a... 

It is thus recommended that in a first pass through a design, thermal coupling should not be considered. Rather, simple columns should be used until a first overall design has been established. Only when the full heat-integration context has been understood should thermal coupling be considered. 

As discussed earlier, the application of such techniques should be restricted until later in the design when the full heat-integration context both within and outside the disjtillation system has been established. 

Unless there are constraints severely restricting heat integration, sequencing of simple distillation columns can be carried out in two steps (1) identify the best few nonintegrated sequences and (2) study... 

Stephanopoulos, G., Linnhoff, B., and Sophos, A., Synthesis of Heat Integrated Distillation Sequences, IChemE Symp. Ser., 74 111, 1982. 

Morari, M., and Faith, D. C., The Synthesis of Distillation Trains with Heat Integration, AlChEJ, 26 916, 1980. 

Kakhu, A. L, and Flower, J. R., Synthesi ng Heat-Integrated Distillation Systems Using Mixed Integer Programming, Trans. IChemE ChERD, 66 241, 1988. 

Although the composite curves can be used to set energy targets, they are not a suitable tool for the selection of utilities. The grand composite curve is a more appropriate tool for understanding the interface between the process and the utility system. It is also, as is shown in later chapters, a useful tool for study of the interaction between heat-integrated reactors and separators and the rest of the process. 

Consider again the simple process shown in Fig. 4.4d in which FEED is reacted to PRODUCT. If the process usbs a distillation column as separator, there is a tradeofi" between refiux ratio and the number of plates if the feed and products to the distillation column are fixed, as discussed in Chap. 3 (Fig. 3.7). This, of course, assumes that the reboiler and/or condenser are not heat integrated. If the reboiler and/or condenser are heat integrated, the, tradeoff is quite different from that shown in Fig. 3.7, but we shall return to this point later in Chap. 14. The important thing to note for now is that if the reboiler and condenser are using external utilities, then the tradeoff between reflux ratio and the number of plates does not affect other operations in the flowsheet. It is a local tradeoff. 

It must be emphasi2ed that any energy costs for the separation in the tradeoffs shown in Fig. 10.7 must be taken within the context of the overall heat integration problem. The separation might after all be driven by heat recovery. 

Another way to relate these principles is to remember that heat integration will always benefit by keeping hot streams hot and cold streams cold. ... 

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