Tubular and packed flow reactors

As a matter of convenience the loading on a flow reactor is expressed as 

Various units for both these quantities are in common use. Some of these are stated in problem P4.05.03 as well as in P3.01.04 and P3.01.05. How to find the actual contact time of a given operation is illustrated in problem P4.05.02 as well as in P3.01.02 and P3.01.03. 

Material and energy balances on plug flow and packed bed reactors are summarized in Tables 2.5 and 2.6. They are formulated on a differential 

In a rate equation the concentrations are replaced by ratios of molal and volumetric flows, 

A typical power law rate equation in these units is, 

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As discussed in Section III, for small deviations from plug flow such as those occurring in tubular and packed-bed reactors, a model consisting of a series of tanks can be used to represent the fluid mixing. The conversion predicted by the model can be found from the equations discussed in the section on conversion in ideal stirred tanks. Figure 29 shows the ratio of reactor volume needed with stirred tanks to the volume needed... 

In the preceding sections, we discussed the operation of plug-flow reactors with gas-phase reactions under the assumption that the pressure does not vary along the reactor. However, in some applications, the pressure significantly changes and, therefore, affects the reaction rates. In this section, we incorporate the variation in pressure into the design equations. For convenience, we divide the discussion into two parts tubular tube with uniform diameter and packed-bed reactors. 

To conclude, packed reactors are the most effective means of promoting radial mass transfer in a flowing stream. They have, however, much lower volume/surface ratio than tubular and 3-D reactors, which is a disadvantage if interactions between solutes and surfaces are undesirable. The smaller the dp is, the larger N, surface area, and pressure drop will be. In general, short reactors (L up to 25 cm) are practical beyond that length, the use of packed reactors becomes progressively more difficult. 

Flow Reactors Fast reactions and those in the gas phase are generally done in tubular flow reaclors, just as they are often done on the commercial scale. Some heterogeneous reactors are shown in Fig. 23-29 the item in Fig. 23-29g is suited to liquid/liquid as well as gas/liquid. Stirred tanks, bubble and packed towers, and other commercial types are also used. The operadon of such units can sometimes be predicted from independent data of chemical and mass transfer rates, correlations of interfacial areas, droplet sizes, and other data. 

The general question of whether or not plug flow can be attained is discussed in Volume 3, Section 1.7. (Tubular Reactors) and the special case of Plug-Flow (Fermenters) is considered in Chapter 5, Section 5.11.3. A more detailed consideration of dispersion in packed bed reactors and those effects which enhance and invalidate plug flow is given in Chapter 3, Section 3.6.1. 

In any real situation, reactants only flow through the reactor because there is a difference in pressure between the inlet and the outlet. Methods for calculating the pressure drop in pipes and packed beds have been outlined in Chap. 1. Often, the pressure drop is negligible compared with the total pressure and it is usual to assume that a tubular reactor with plug flow operates at constant pressure. 

Packed Bubble Bed Reactor (BBR) This is a tubular flow reactor with concurrent up-flow of gas and liquid (Figure 3.11). The catalyst bed is completely immersed in a continuous liquid flow while gas rises as bubbles. Some applications of BBR are the catalytic denitrification of aqueous nitrate solutions and the hydrogenation processes. 

Let us now consider a catalytic packed bed reactor , i.e. a tubular reactor filled with a grained catalyst through which the gas mixture flows. With the particle diameter of the catalyst, dp, an additional dimensionless number dp/d is added to the pi-space the Reynolds number is now expediently formed with dp. The reaction rate is related to the unit of the bulk volume and characterized by an effective reaction rate constant ko,eff = k . The thermal conductivity (k) also has to be valid for the gas/bulk solids system and diffusion can be considered as being negligible (Sc is irrelevant). The complete pi-space is therefore ... 

Various laboratory reactors have been described in the literature [3, 11-13]. The most simple one is the packed bed tubular reactor where an amount of catalyst is held between plugs of quartz wool or wire mesh screens which the reactants pass through, preferably in plug flow . For low conversions this reactor is operated in the differential mode, for high conversions over the catalyst bed in the integral mode. By recirculation of the reactor exit flow one can approach a well mixed reactor system, the continuous flow stirred tank reactor (CSTR). This can be done either externally or internally [11, 12]. Without inlet and outlet feed, this reactor becomes a batch reactor, where the composition changes as a function of time (transient operation), in contrast with the steady state operation of the continuous flow reactors. 

Catalytic reaction nethod. The methanol-conversion reaction was carried out in a ordinary flow reactor under atmospheric pressure. A 0.5 ml portion of the catalyst was packed into a Pyrex tubular reactor of 6 mm inner diameter. The reaction gas, composed of 20 100% MeOH balanced with N2, was then allowed to flow through the catalyst bed at a temperature in the range 24-0 360°C and a space velocity (SV) in the range 4-00 4-000 liter"liter 1,h 1. The olefin-conversion reaction was carried out in a flow reactor of 8 mm inner diameter. The reaction gas, composed of an olefin (CgH, C3H6 or C,Hg) and N2 mixed at various ratios, was then allowed to flow through the catalyst bed at a temperature in the range 260 360°C and a space velocity in the range 900 4500 h-1. 

It is the purpose of this chapter to discuss presently known methods for predicting the performance of nonisothermal continuous catalytic reactors, and to point out some of the problems that remain to be solved before a complete description of such reactors can be worked out. Most attention will be given to packed catalytic reactors of the heat-exchanger type, in which a major requirement is that enough heat be transferred to control the temperature within permissible limits. This choice is justified by the observation that adiabatic catalytic reactors can be treated almost as special cases of packed tubular reactors. There will be no discussion of reactors in which velocities are high enough to make kinetic energy important, or in which the flow pattern is determined critically by acceleration effects. 

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