Reactor heat integration

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. 

G. Kolios, B. Gloclder, A. Gritsch, et al., Heat-Integrated Reactor Concepts... 

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

The model is solved numerically. Figure 9.12 presents the reaction temperature and conversion versus heat-transfer capacity of the reactor, UA. From Figure 9.12, we find that we need to provide UA = 95 kW/K heat-transfer capacity, if we want to reach the operating point T = 268 K, xA = 0.4. Figure 9.12 also shows that the combined FEHE-reactor system does not show multiple steady states, which should always be a concern when designing heat-integrated reactors [16]. 

The control performance of the heat-integrated reactor-column system shown in Fig.. 5.9 deteriorates as the auxiliary rehoiler provides less and less heat to the column. The reason is that uncontrolled variations in the steam pressure of the waste heat boiler affect the heat supplied to the column. When these variations are of the same order of magnitude as the total heat supplied by the auxiliary reboiler, the latter cannot compensate properly for the variations. Part of the prob-... 

Kolios, G., Gritsch, A., Morillo, A., Tuttlies, U., Bernnat, J., Opferkuch, E, and Eigenberger, G. Heat-integrated reactor concepts for catalytic reforming and automotive exhaust purification. Applied Catalysis. B, Environmental, 2007, 70 (1—4), 16. 

Heat-integrated reactors Reactive separations Reactive comminution Reactive extrusion Fuel cells... 

Before performing a controllability analysis, ensure the stability of the plant. The first step is to close all inventory control loops, by means of level and pressure controllers. Then, check the stability, by dynamic simulation. If the plant is unstable, it will drift away from the nominal operating point. Eventually, the dynamic simulator will report variables exceeding bounds, or will fail due to numerical errors. Try to Identify the reasons and add stabilizing control loops. Often a simple explanation can be found in uncontrolled inventories. In other situations the origin is subtler. Some units are inherently unstable, as with CSTR s or the heat-integrated reactors. The special case when the instability has a plantwide origin will be discussed in Chapter 13. 

Similarly with the recycle of mass, the recycle of energy can induce non-linear phenomena that affect the whole plant. Practical solutions can be obtained by examining these aspects at the level of sub-systems. The next sections present two examples of practical significance heat integrated reactors and heat integrated distillation columns. 

Table 13.1 Design alternatives of the HDA heat-integrated reactor...

This example demonstrates that the controllability of the heat-integrated reactors must be investigated in more detail before other design steps are undertaken. Nonlinear behaviour puts serious constraints on conversion range and operating parameters, particularly on the temperature. The optimal conversion is not only a matter of trade-off between separation and recycle costs, but also a problem of control and operation. 

Figure 20.3 Two configurations for an exothermic reactor requiring feed preheating (a) Reactor with independent preheat (b) Heat-integrated reactor.

Perform a degrees-of-ffeedom analysis for the noninteracting exothermic reactor shown in Figure 20.3a. Suggest an appropriate control structure. Carry out the same exercise for the heat-integrated reactor shown in Figure 20.3b. Compare the results. 

Luyben et al. (1999) Chapter 4 discusses the design of control systems for reactors in general. The design of heat-integrated reactor systems is discussed in Chapter 5. 

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