Reactor length, effect

In a tubular reactor system, the temperature rises along the reactor length for an exothermic reaction unless effective cooling is maintained. For multiple steady states to appear, it is necessary that a... 

For a more detailed analysis of measured transport restrictions and reaction kinetics, a more complex reactor simulation tool developed at Haldor Topsoe was used. The model used for sulphuric acid catalyst assumes plug flow and integrates differential mass and heat balances through the reactor length [16], The bulk effectiveness factor for the catalyst pellets is determined by solution of differential equations for catalytic reaction coupled with mass and heat transport through the porous catalyst pellet and with a film model for external transport restrictions. The model was used both for optimization of particle size and development of intrinsic rate expressions. Even more complex models including radial profiles or dynamic terms may also be used when appropriate. 

For packed bed reactors, Carberry and Wendel (1963), Hlavacek and Marek (1966), and Carberry and Butt (1975) report that axial dispersion effects are negligible if the reactor length is sufficient. These and other researchers (Young and Finlayson, 1973 Mears, 1976) have developed criteria based on the reactor length for conditions where axial dispersion can safely be neglected. The criterion shown in Table V is a classic criterion for neglecting axial mass dispersion. The works by Young and Finlayson (1973) and Mears (1976) provide more detailed criteria to predict when axial dispersion is unimportant in nonisothermal packed bed reactors. 

Since detailed kinetic data for the deposition of Si from SiH are scarce and somewhat contradictory, the kinetic parameters are evaluated from experimental data in ref (31) Figure 3 shows the experimental data and the predicted growth rates. The model predicts the decrease in growth rates along the reactor length due to depletion of SiH4 as well as the inhibiting effect of increased H2 concentration ... 

The boundary conditions are dx/dz = Px at z =0 and dx/dz = 0 at z = 1. Here, P is the axial Peclet number, defined as the product of velocity and reactor length, divided by the effective axial diffusivity. 

Thus diffusion limitations decrease the yield twofold. These results may be generalized to include interface and intraparticle diffusion for bidisperse catalysts [5]. The effect of diffusion limitation on the concentration distribution over the reactor length can be calculated from Equations 8.24, 8.29 and 8.33. An example of such calculations is shown in Figure 8.4 for the case CM = 0, DtA = D and kjkx = 0.1. Although the rate of... 

Figure 8.4 Effect of diffusional limitations on the concentration distributions of A and B over the reactor length for kfkx = 0.1, a = 5 (dashed lines).

Meats23-2 showed that the catalyst bed-length effect observed during de-nitrogenation of gas oils in pilot-scale reactors can be correlated on the basis of an axial dispersion effect on the reactor performance. Montagna and Shah29 showed that the bed-length effect observed in desulfurization reaction with 22 percent KVTB and 36 percent KATB (see Fig. 4-4) can also be explained on the basis of an axial dispersion effect on the reactor performance. 

US dramatically increases dispersion of an injected volume in the carrier, which adds to the effects of, especially, the flow-rate, reactor length, inner diameter of the transporting tubes, viscosity and temperature. 

The effect of geometric parameters on the adiabaticity of a test reactor can be deduced from Table V. It can be seen that for improperly designed laboratory reactors the axial and radial heat flows can be quite appreciable even when the net heat loss is zero. From this table it follows that the radial heat flow is reduced as the bed diameter is increased, whereas the axial heat flow diminishes as the reactor length is increased. Hence, long pilot plant reactors of wide diameter will perform best as adiabatic reactors even with suboptimal design of compensation heaters. 

In Chapter 8, axial dispersion in tubular reactors was discussed. Typical industrial reactors have sufficiently high flow rates and reactor lengths so the effects of axial dispersion are minimal and can be neglected. A rule of thumb is that axial dispersion can be neglected if ... 

To ensure the absence of axial dispersion [not included in Eq. (1)], the reactor length, Az, should be at least 50 particle-diameters long, typically about 1 cm for the small particles needed to avoid intraparticle diffusion effects. The bed diameter can be about 4 or 5 mm. 

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