Multibed reactor

TABLE 23-2 Multibed Reactors, Adiabatic Temperature Rises ... 

Figure 17.19. Reactors for the oxidation of sulfur dioxide (a) Feed-product heat exchange, (b) External heat exchanger and internal tube and thimble, (c) Multibed reactor, cooling with charge gas in a spiral jacket, (d) Tube and thimble for feed against product and for heat transfer medium, (e) BASF-Knietsch, with autothermal packed tubes and external exchanger, (f) Sper reactor with internal heat transfer surface, (g) Zieren-Chemiebau reactor assembly and the temperature profile (Winnacker- Weingartner, Chemische Technologie, Carl Hanser Verlag, Munich, 1950-1954).

Figure 17.20. Control of temperature in multibed reactors so as to utilize the high rates of reaction at high temperatures and the more favorable equilibrium conversion at lower temperatures, (a) Adiabatic and isothermal reaction lines on the equilibrium diagram for ammonia synthesis, (b) Oxidation of SOz in a four-bed reactor at essentially atmospheric pressure, (c) Methanol synthesis in a four bed reactor by the ICI process at 50 atm not to scale 35% methanol at 250°C, 8.2% at 300°C, equilibrium concentrations.

The catalyst beds are mounted in a single reactor vessel because it is more economical than using multiple vessels. The spacing between beds is set at 1 m. The length-to-diameter aspect ratio of the vessel is 10. Because a multibed reactor must have internal piping, flow distributors, and bed supports, a multi-bed reactor vessel is more expensive than a simple vessel. We assume that each additional bed increases reactor capital cost by about 25%, as shown in Table 5.2. 

In the EBMax process, benzene is fed to the bottom of the liquid-filled multibed reactor. Ethylene is co-fed with the benzene and also between the catalyst beds. Polyethylbenzenes, which are almost exclusively diethylbenzenes, undergo transalkylation with benzene in a second reactor. Mobil-Badger offers both liquid phase and vapor phase transalkylation processes. The vapor phase process removes benzene feed coboilers such as cyclohexane and methylcyclopentane as well as propyl and butylbenzenes. Because the EBMax process produces very low levels of propyl and butylbenzenes, for most applications, the more energy efficient liquid phase process is preferred. Worldwide, there are currently ten licensed EBMax units with a cumulative ethylbenzene production capacity of five million metric tons per year. 

OCTG AIN A hydrofinishing process that reduces the sulfur and olefin content of gasoline without reducing its octane number. A proprietary zeolite catalyst is used in a fixed bed, multibed reactor having an intermediate cooling system. Developed by ExxonMobil and first commercialized in 1991. 

To find this maximum it suffices to choose the conditions in the first of the two beds, since the optimal policy of the second has been calculated already. Equation (11.5.d-2) is Bellman s optimum principle. Consider now the last three beds. These can be decomposed into a first bed (in the direction of process flow) and a pseudo stage consisting of the last two beds, for which the optimal policy has already been calculated for a series of inlet conditions. The procedure is continued in the same way towards the inlet of the multibed reactor. 

This type of reactor contains several separate, adiabatically operated catalyst beds, allowing defined temperature control. Several methods of coohng are possible internal or external heat exchangers or direct cooling by introduction of cold gas (quench reactor). The multibed reactor is particularly suitable for high production capacities. 

Single-bed reactor Tubular reactor Multibed reactor ShaUow-bed reactor Fuidized-bed reactor ... 

In fixed beds, cooling and partial recycling of the exit stream of the reactor is possible but this affects the residence time distribution. To overcome this difficulty and obtain a very high conversion level, a second reactor in series could be required. Cold injections of gas or liquid in a multibed reactor are another technique but it is thermodynamically very inefficient (Figures 1 and 2). 

GAMS Polymers reactor (semibatch) Multibed reactor for SO3... 

The thermodynamic limitation of SO2 oxidation requires an adiabatic multibed reactor with intermittent cooling in external heat exchangers to achieve nearly full conversion. Alternatively, cooling is possible by stepwise quenching with cool air. For today s SO2 oxidation catalysts, typically four stages with intermittent cooling are needed to reach more than 98% conversion. 

What is characteristic for a multibed reactor is that several, often adiabatic catalyst beds are coupled in series. Heat exchangers are placed in between the beds heat should be supplied in the case of an endothermic reaction (so that the reaction is not extinguished in the following step), and heat should be removed in the case of an exothermic reaction (to... 

This kind of multibed reactor system is utilized in reforming processes (Figure 5.7). Catalytic reforming is an endothermic process the Pt catalyst is placed in packed beds coupled in series, and between the beds, heating elements are present at an elevated temperature, since the temperature falls in the reaction zone due to the occurrence of endothermic reactions. 

A multibed reactor system that is used in the oxidation of SO2 to SO3, on a V2O5 catalyst, is illustrated in Figure 5.8. The reactors operate at atmospheric pressure, and the heat of the reaction is removed by external heat exchangers, which are also used to preheat the feed into the reactor. In modern sulfuric acid factories, at least four catalyst beds in series... 

In the synthesis of ammonia (NH3), a multibed reactor is used. The reaction takes place on an iron catalyst, and ammonia is synthesized from hydrogen and nitrogen. Extraordinary... 

FIGURE 5.8 Multibed reactor for oxidation of SO2 to SO3. (Data from Froment, G. and Bischoff, K Chemical Reactor Analysis and Design, 2nd Edition,Wiley, New York, 1990.)... 

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