Design of Fixed Bed Reactors

The design problem can be approached at various levels of sophistication nsing different mathematical models of the packed bed. In cases of industrial interest, it is not possible to obtain closed-form analytical solutions for any but the simplest of models under isothermal operating conditions. However, numerical procedures can be employed to predict effluent compositions on the basis of the various models. In the subsections that follow, we consider first the fundamental equations that must be obeyed by all packed-bed reactors under various energy transfer constraints, and then discuss some of the simplest models of reactor behavior. These discussions are limited to psendo steady-state operating conditions (i.e., the catalyst activity is presumed to be essentially constant for times that are long compared to the fluid residence time in the reactor). 

The units of ry are moles converted/(volume time), and ry plays a role similar to the rates employed in homogeneous reactor design. Consequendy, the design equations developed earher for homogeneous reactors can be extended for use in obtaining estimates of fixed bed reactor performance. Two dimensional pseudo homogeneous models can also be developed to allow for radial dispersion of mass and energy. 

To simplify a preliminary reactor design analysis, engineers often assume that the pressure drop across a reactor is negligible and has little induence on reactor size requirements. 

Several correlating equations for the friction factor have been proposed for both the laminar and turbnlent dow regimes, and plots of (or functions thereof) versus Reynolds number are frequendy presented in standard duid dow or chemical engineering handbooks (e.g., 74, 97). Perhaps the most useful of the correlations is that represented by the Ergun equation (98)  

Notice that the superdcial mass velocity G is constant throughout the bed but that p will vary for compressible duids. When the pressure drop is small relative to the absolute pressure, equation (12.7.4) may be used for gases by employing the arithmetic average of the inlet and outlet densities. This approximation is useful in solving design problems via machine computation. 

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The reactions are highly exothermic and very rapid. Consequently conventional practice in the design of fixed bed reactors for phthalic anhydride production has been based on the use of multitube reactors to ensure good heat transfer and good temperature control. These are required to ensure good selectivity. Often a thousand or more small diameter tubes may be... 

A fixed-bed reactor normally consists of a cylindrical tube of assembly of tubes into which catalyst particles are loaded and contained in a fixed position. The tube, filled with the catalyst particles, may be either vertical or horizontal, depending on process circumstances and can be heated or cooled by a suitable gas or liquid medium circulated through a shell enveloping the tubes. Various designs of fixed-bed reactor are found in practice and some of these will be described briefly. The mode of... 

The ebullated, expanded, and slurry-bed reactors utilize a fluent catalyst zone unlike the stationary catalyst design of fixed-bed reactors. This design overcomes several of the problems encountered when processing residua in fixed-bed catalytic reactors. The commercial H-Oil process (Eccles et al., 1982 Nongbri and Tasker, 1985) employs the ebullated-bed, whereas the... 

Figure 6.5 Three different designs of fixed bed reactors for gasification...

Factors Involved in the Preliminary Design of Fixed Bed Reactors ... 

In discussing the preliminary design of fixed bed reactors in Sec. 11.3 we mentioned that adiabatic operation is frequently considered in industrial operation because of the simplicity of construction of the reactor. It was also mentioned why straight adiabatic operation may not always be feasible and examples of multibed adiabatic reactors were given. With such reactors the question is how the beds should be sized. Should they be designed to have equal ATs or is there some optimum in the AT s, therefore in the number of beds and catalyst distribution In Section 11.3. this problem was already discussed in a qualitative way. It is taken up in detail on the basis of an example drawn from SOj oxidation, an exothermic reversible reaction. To simplify somewhat it will be assumed, however, that no internal gradients occur inside the catalyst so that the effectiveness factor is one. 

FACTORS INVOLVED IN THE PRELIMINARY DESIGN OF FIXED BED REACTORS 495... 

Froment G F., "Progress in fundamental design of fixed bed reactors", International Chemical Reaction Engineering Conference, Pune (India) 9-11th January (1984), Vol 1,... 

The design of fixed-bed reactors eventually involves solving the conservation equations (Eqs. 9.6 through 9.15). It is rare that analytical solutions are obtained because of the nonlinearity involving the Arrhenius expression and intrinsic kinetics. Therefore, the design invariably requires numerical solutions of the equations. This is dealt with in the next section. 

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