A T-H method for heat exchanger network synthesis - Industrial

Ind. Eng. Chem. Res. 1987,26, 175-179

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A T - H Method for Heat Exchanger Network Synthesis The temperature-enthalpy diagrams that are useful for visualizing the pinch location in a heat exchanger network can also be used to help visualize the effects obtained with different heat exchanger network alternatives. This visualization is particularly suited for many petrochemical processes where there is not much overlapping between the streams on a temperature-enthalpy diagram.

A t the preliminary stages of a process design, where we are still attempting to fix the structure of a flow sheet, we normally are more interested in seeing what types of alternatives are available to accomplish certain tasks than we are in determining a rigorous estimate of the optimum design conditions. Similarly, when a number of process alternatives have about the same processing costs, we might select the alternative to use in a final design on the basis of simplicity, operability, ease of start-up, safety considerations, etc. We usually consider the costs first, because if the process has little chance of being profitable (less than 1%of the ideas for new designs ever become commercialized)and therefore we decide to terminate the project, we do not want to even consider these secondary factors. For most processes, it is a simple matter to generate a number of heat exchanger network alternatives that all have about the same minimum energy requirements. The temperature-enthalpy, T-H, diagrams developed by Hohmann (1971), Umeda, et al. (1978), and Linnhoff and Flower (1978a,b) provide a clear picture of how the minimum heating and cooling loads in a heat exchanger network change as the approach temperature at the pinch condition is changed. For some plants where there is not much overlapping of the streams on a T-H diagram, which includes many petrochemical processes, the T-H curve8 can also be used as an aid in visualizing how heat loads, the number of heat exchangers, and the driving forces change as various heat exchanger networks are generated. With this approach, each network alternative has its own T-H diagram, and the T-H curves are used in a manner originally suggested by Whistler (1948).

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Figure 1. T-H diagram for the HDA process with a diphenyl byproduct.

Heat Exchanger Network Alternatives Suppose we consider the T-H diagram for an HDA process with a diphenyl byproduct (Douglas, 1985; Terrill and Douglas, 1985a); see Figure 1. The horizontal lines correspond to the condensers and reboilers for the stabilizer, product, and recycle columns, and the other streams correspond to the reactor feed stream, RFS, and the reactor product stream, RPS. Since there is little overlapping of curves for separate streams on the T-H diagram (the stream populations are given at the bottom of Figure l),we can simply associate a heat exchanger with each of the intervals between the breakpoints. The circled numbers on Figure 1then correspond to the heat exchangers shown in Figure 3 (the same numbering scheme is used in all of the figures that follow). For this process, the minimum number of exchangers (Linnhoff, 1982) is 9, whereas there are 13 exchangers in Figure 3. In order to decrease the number of exchangers, we might replace the exchanger that uses part of the product column condenser load on Figure 3 to preheat the reactor feed by an enlarged first section of the feed-effluent heat exchanger (since the heat load of this exchanger is small). In addition, the stabilizer column reboiler can be shifted so that the reactor effluent stream drives the stabilizer and product column reboilers consecutively. In this way we can eliminate another of the feed-effluent exchangers. The T-H diagram is now given by Figure 2, 0888-5885/87/2626-0175$01.50f 0

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0 1987 American Chemical Society

176 Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987 r

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Ind. Engr Chem. Res. Vol. 26, No. 1, 1987 177

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which corresponds to network alternative 6 (Terrill and Douglas, 1986a) shown in Figure 4. If we drive the recycle column reboiler with a utility rather than the reactor product stream, we eliminate another feed-effluent heat exchanger; the T-H diagram is given in Figure 6, which corresponds to alternative 5

(Figure 5 ) . A further modification is to drive the stabilizer reboiler with utility; see Figure 7 for the T-H diagram of alternative 4. The product column reboiler could be driven by a utility instead of the stabilizer; Figure 8 gives the T-H diagram for alternative 3. Yet another alternative is to drive all three reboilers with utilities; Figure 9 gives the

178 Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987

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changing ATmi,, number of units, utilities usage, and driving forces is readily apparent. It is interesting to note that all of these network alternatives have about the same costs at the optimum design conditions (Terrill and Douglas, 1986a). The steady-state operability of the alternatives has also been discussed (Terrill and Douglas, 1986b).

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Acknowledgment We are grateful to the Department of Energy for supporting this work (DOE Contract DE-AC02-8ER10938).

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T-H diagram for alternative 2. The last modification is to remove the pressure shift from alternative 2, which yields alternative 1 with the T-H diagram in Figure 10. The networks not shown here can be found in Terrill ( 1985). For this process, the T-H diagram analysis is particularly simple because each line segment on the general T-H diagram (Figure 1) almost always represents only one stream. When multiple hot or cold streams occur over the same temperature interval, the T-H line segments must be “pulled apart” so that each segment represents only one stream. Then the analysis may continue. Conclusion Thus, we see that T-H diagrams can provide a useful tool for developing alternative networks, simply by shifting line segments on the T-H diagram. The impact of

Douglas, J. M. AIChE J. 1985,31, 353. Hohmann, E. C. Ph.D. Dissertation, University of Southern California, Los Angeles, 1971. Linnhoff, B.; Flower, J. R. AIChE J. 1978a,24, 633. Linnhoff, B.; Flower, J. R. AIChE J. 1978b,24, 642. Linnhoff, B.; Townsend, D. W.; Boland, D.; Hewitt, G. F.; Thomas, B. E. A.; Guy, A. R.; Marsland, R. H. A User Guide on Process Integration for the Efficient Use of Energy; The Institution of Chemical Engineers: Rugby, England, 1982. Terrill, D. L. Ph.D. Dissertation, University of Massachusetts, Amherst, 1985. Terrill, D. L.; Douglas, J. M., submitted for publication in Ind. Eng. Chem. Process Des. Dev. 1986a. Terrill, D. L.; Douglas, J. M., submitted for publication in Ind. Eng. Chem. Process Des. Dev. 1986b. Umeda, T.; Itoh, J.; Shiroko, K. Chem. Eng. Prog. 1978, 74(9), 70. Whistler, A. M. Pet. Refiner 1948,27, 83.

D. L. Terrill, J. M.Douglas* Department of Chemical Engineering University of Massachusetts Amherst, Massachusetts 01003 Received for review August 8, 1985 Revised manuscript received May 22, 1986 Accepted July 8, 1986

A T-H method for heat exchanger network synthesis - Industrial

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