Flowsheet 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. 

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. 

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. 

Figure 33.4. Flowsheet of biodiesel production by heat-integrated reactive distillation.

Glavic P, Kravanja Z and Homsak M (1988) Heat Integration of Reactors I. Criteria for the Placement of Reactors into Process Flowsheet, Chern Eng Sci, 43 593. 

Fig. 1.23. Flowsheet for a heat-integrated system of an autothermal gasoline reformer (ATR) and a high-temperature shift stage (HTS) interconnected with a heat exchanger [37].

Synthesis is the step in design where one conjectures the building blocks and their interconnection to create a structure which can meet stated design requirements. This review paper first defines chemical process synthesis and indicates the nature of the research problems—to find representations, evaluation functions and search strategies for a potentially nearly infinite problem. It then discusses synthesis research and the most significant results in each of six areas—heat exchanger networks, separation systems, separation systems with heat integration, reaction paths, total flowsheets and control systems. 

Figure 2.8 Generic flowsheet structure of a heat-integrated reactor.

Figure 4.11 present the complete flowsheet together with the control structure. The reaction takes place in an adiabatic tubular reactor. To avoid fouling, the temperature of the reactor-outlet stream is reduced by quenching. A feed-effluent heat exchanger (FEHE) recovers part of the reaction heat. For control purposes, a furnace is included in the loop. The heat-integrated reaction system is stabilized... 

Figure 5.9 presents the flowsheet prior to heat integration. Fresh and recycled phenol is evaporated and mixed with hydrogen in the evaporator (Ev-1) at about 2 bar. The gas mixture enters the catalytic hydrogenation reactor (R-l). The inlet temperature should be kept strictly constant, in this case at 150°C, to avoid the... 

Figure 5.19 Flowsheet with recycles, separation and heat integration.

Figure 7.9 Flowsheet configuration before heat integration.

Light-split reverse. Figure 6.246 shows an alternative configuration where about half of the lighter component is removed in the first column and the other half in the second column. The bottoms from the first column is a mixture of light and heavy components and is fed into the second column. The first column can be run at high pressure and the second at low pressure (direct heat integration). In the flowsheet shown,... 

Prefractionator reverse (ternary). Figure 6.24c shows a third alternative flowsheet that combines heat integration with a complex configuration. This system can be used to separate a ternary mixture. In the system shown, the sidestream column is run at high pressure and the prefractionator at low pressure. 

To illustrate the very large energy savings that are possible with this complex/heat-integrated system, consider the separation of a benzene, toluene, and xylene mixture. A conventional two-column light-out-first separation flowsheet with no heat integration uses twice the energy7 that the prefractionator-reverse flowsheet uses. 

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