Reactor/heat exchanger systems autothermal reactors

Some Reactor/Heat-Exchanger Systems 6.2.1 Autothermal Reactors... 

The essential feature of an autothermal reactor system is the feedback of reaction heat to raise the temperature and hence the reaction rate of the incoming reactant stream. Figure 1.6 shows a number of ways in which this can occur. With a tubular reactor the feedback may be achieved by external heat exchange, as in the reactor shown in Fig. 1.6a, or by internal heat exchange as in Fig. 1.6b. Both of these are catalytic reactors their thermal characteristics are discussed in more detail in Chapter 3, Section 3.6.2. Being catalytic the reaction can only take place in that part of the reactor which holds the catalyst, so the temperature profile has the form... 

There are reactions where the heat of reaction can be employed to preheat the feed when an exothermic reaction is operated at a high temperature (e.g., ammonia N2 + 3H2 <-> 2NH3 or methanol CO + 2H2 CH3OH synthesis, water-gas shift reaction CO + H20 <-> H2 + C02). These processes may be performed in fixed-bed reactors with an external heat exchanger. The exchanger is primarily used to transfer the heat of reaction from the effluent to the feed stream. The combination of the heat transfer-reaction system is classified as autothermal. These reactors are self-sufficient in energy however, a high temperature is required for the reaction to proceed at a reasonable rate. 

Under autothermal operation the hot effluent of a fixed-bed reactor is used to heat up the cold feed to the ignition temperature of the catalytic reaction. Since no other addition or removal of heat takes place, autothermal operation is restricted to reaction systems which all together are exothermic. The conventional reactor design consists of an adiabatic packed-bed reactor coupled with a countercurrent heat exchanger (Fig. 23A). 

Figure 6.12 An autothermal reactor system with details on external heat exchange from product to feed. [After J.M. Smith, Chemical Engineering Kinetics, 3rd ed., with permission of McGraw-Hill Book Co., New York, NY, (1981).]...

Figure 6.13 General form of multiplicity in an autothermal reactor system with heat exchange between feed and product. [After H.H. Lee, Heterogeneous Reactor Design, with permission of Butterworth Publishers, Boston, MA, (1985).]...

The heat integration system consists of a catalytic burner reactor (BUR) and three heat exchangers (HXl, HX2, HX3). In the catalytic burner reactor, the unconverted hydrogen is combusted at 370 °C to deliver the heat necessary for the vaporizer and the reformer. The whole FP-FC system operates autothermally. 

The system consisting of the reactor and the F/P heat exchanger is said to be autothermal if the desired outlet conversion can be achieved without addition of heat from an outside source. In other words, the system is autothermal if the Stream leaving the reactor is hot enough to heat the feed Stream to the temperature that is necessary to achieve the desired conversion, at steady state. 

---