Reactor stagnant zones

FIGURE 15.7 Effect of a stagnant zone in a stirred tank reactor according to the side capacity model. 

At present, the chief value of RTD studies is for the diagnosis of the performance of existing equipment, for instance maldistribution of catalyst in a packed reactor, or the presence of bypassing or stagnant zones in stirred tanks. No correlations have been achieved for cr2(tr) or njrriang in terms of operating conditions, and only limited correlations for De. 

The scale-up of trickle beds presents many difficulties mainly due to maldistribution of fluids, which leads to different routes for the liquid and gas, stagnant zones, and hot spots. In trickle-bed reactors, the particle diameter and residence time are the same for all scales. The consequence is that in different scales we have different Reynolds numbers and velocities. 

The mass balances [Eqs. (Al) and (A2)] assume plug-flow behavior for both the gas/vapor and liquid phases. However, real flow behavior is much more complex and constitutes a fundamental issue in multiphase reactor design. It has a strong influence on the reactor performance, for example, due to back-mixing of both phases, which is responsible for significant effects on the reaction rates and product selectivity. Possible development of stagnant zones results in secondary undesired reactions. To ensure an optimum model development for CD processes, experimental studies on the nonideal flow behavior in the catalytic packing MULTIPAK are performed (168). 

For high recycle ratios, the residence time distribution of a recycle reactor tends towards that of a completely mixed flow reactor, regardless of the flow mode inside the reactor vessel provided that there is no stagnant zone (Buffham and Naum an, 1984). 

Due to the consumption of reactants and the production or consumption of heat, concentration and temperature profiles can develop in the stagnant zone around and in the particle itself (Fig. 11). In the following paragraphs, criteria are derived to ensure that the effect of these gradients on the observed reaction rate is negligible [4, 27, 28]. In gas/liquid/solid slurry reactors, the mass transfer between the gas and liquid phase has to be considered, too (see Refs 9 and 29). 

The reactor is built to accomplish a horizontal flow of gases through the beds in such a way that the top and bottom of the reactor are stagnant zones and the gases exiting from the beds do so at a sufficiently low velocity to prevent lofting and entrainment of fines. 

Since mean residence time and the volumetric flow rate are known, value of V (the active volume of the fluid in the reactor) is readily calculated. If the calculated value of V is smaller than the reactor volume, it indicates that a stagnant zone (not available to the flowing fluid) exists in the reactor. In heterogeneous fluid-fluid reactors, measuring the mean residence time of each fluid provides the holdup of each in the reactor. Comparing the RTD curve to that of CSTR and plug-flow reactor provides an indication on the deviations of the actual flow patterns from those of idealized flows. 

We probably don t have to, at this point, say that the overall model here, or even considerable simplifications of it, are best left to numerical solution. The axial dispersion model seems to work pretty well, at least for cases where the holdup/ wetting does not vary much with position in the reactor. For large changes in this factor, or for nonideal flows involving stagnant zones or liquid/gas bypassing, some version of one of the combined models will be required. It will be understood that these will be very specific to the particular design under consideration. 

Cabaret et al. (2008) and Gagnon et al. (1998) concluded that better mixing and higher product conversion can be achieved if a close clearance impeller, such as the helical ribbon, is used in conjunction with a radial flow impeller such as the RT in a highly viscous system. The Rushton-type turbine provides proper gas dispersion, while the close clearance impeller attempts to contact most of the reactor volume and provides proper bulk mixing, shear distribution, lower apparent viscosity, and minimal stagnant zones (Tecante and Choplin, 1993). These effects also lead to higher reactor utilization and can decrease power requirements. 

I 7 Microstructured Reactors for Fluid-Fluid Reactions Internal circulations Stagnant zone... 

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