Monolith velocity profiles

Laminar flow is the usual flow regime met in monolith reactors, given that the typical Reynolds number has values below SOO. The radial velocity profile in a single channel develops from the entrance of the monolith onward and up to the position where a complete Poiseuille profile has been established. The length of the entrance zone may be evaluated from the following relation [3] ... 

In addition, reactors equipped with monolithic bodies show a total absence of radial stream components. Inhomogeneities in concentration, temperature, and velocity profiles over the reactor cross section are thus propagated throughout the length of the reactor. 

With honeycombs (monoliths), pressure drop could be reduced by a factor of 100 or more, with corresponding reductions in equipment and operating costs. The total absence of radial flow in such structures, however, precludes their use in multitube reactors the bulk heat generated by the reaction would not be transported to the tube walls, the selectivity of the process would decrease dramatically, and prevention of reaction runaway would prove difficult. Furthermore, the lack of radial vectors means that inhomogeneities in radial velocity profiles would be maintained these inequalities in residence time would reduce selectivity and result in poor utilization of the catalyst in many of the channels. 

Monolithic honeycomb have found major application in the selective catalytic reduction process (Fig. 16). The pressure op is about 250-1000 Pa. The SCR reactor asks for a flat velocity profile of the flue gas. Therefore, a layer of dummy honeycombs is installed for flow-straightening purposes. 

After extracting the kinetic parameters, selected results for CO oxidation over were used to analyze the effect of non-uniform temperature and velocity distributions on the conversion of CO. In order to determine the optimum number of multiple CSTR s to capture the behavior of a PFR, the rate law of Oh and Carpenter (14) for the NO+CO reaction was used to model a monolith channel as a CSTR in series. The results indicated that it was sufficient to use 5 reactors in series to capture the performance of the PFR behavior in the NO+CO reaction The cells of a monolith reactor were taken as independent parallel reactors ignoring the mass transfer and diffusion through the ceramic pores. The axial and radial temperature and velocity profiles collected from the literature(4,5) are used to calculate the... 

Figure 1. Subsections of the quarter monolith. The radial divisions were decided based on the shape of the velocity profile (4). The azimuthal dimension was treated as uniform.

A major simplification in the modeling of monolithic channels is to decouple fluid mechanics from the problem by assuming a well-defined velocity profile. In this respect, the laminar flow assumption is likely to be valid since the timescale for viscous diffusion over a transverse length a is much smaller than the one for convection through a channel with length L ... 

Figure 2.25 shows isoplots of the velocity in the -direction in the yz plane through the center of the channels including a region of 2 mm upstream of the monolith for three different z positions of the probe tip. The velocity profile in the channels is completely developed after a distance of approximately 0.5 mm. As expected, the smaller the velocity in the channel with the probe, the deeper the probe is launched into the channel. It should be noted that even if the probe tip is at Xprobe = 0 mm, the volume flux in channel with the probe is already reduced by 4%. It is furthermore observed that the ratio of the volume fluxes in the channel with probe and in the reference channel is rather independent from the inflow velocity and the wall temperature. This means that for a larger flow rate at the inlet, the velocities in all channels increase, but the ratio remains the same. Also, the suction at the probe tip has a negligible influence, even if the volume flux sucked is increased to 5 ml/min. The reason for this is that only a small portion of the volume flux of the entire channel is sucked in. 

Physical assumptions. Early models assumed that the radial velocity profile is uniform across the monolith (Oh and Cavendish (1981), Heck et al. (1976), Young and Finlayson (1976a,b)). In other words, the monolith is assumed to behave as a single channel when heat loss is neglected. 

Studying converter behaviour for a single reaction at steady state helps to understand the main features of the process. For the sake of simplicity, we will assume that the monolith is adiabatic, that there is neither heat conduction, nor internal diffusion limitations, and that the radial velocity profile is uniform. The kinetic rate expression is taken from Heck et al. (1976) (See also (2.28 -30)). 

Figure 7 Effects of space velocity on axial bed-iemperature profiles and product composition for steam reforming of -hexane on the hybnd monolith (From Ref 8 )...

Figure 11 Axial (left) and radial (right) temperature profiles of the metal monolith bed during steam reforming of methane and nitrogen flow conditions at gaseous-inlet hourly space velocities of 8400 hr. (From Ref. 10.)...

Figure 3. (a) The space velocity distribution and (b) the axial and radial conversion profiles in a cylindirical monolith. The conversion results presented here are determined with a feed composition representative of net oxidizing conditions. 

Selected kinetic information was used to analyze the reactor performance in a monolith. The temperature and velocity distributions in a monolith geometry leads to non uniform conversion profiles. The present model results can be used to tailor the metal loading as a function of radial coordinate to improve the conversion non-uniformities and the precious... 

A microstructured monolith for autothermal reforming of isooctane was fabricated by Kolb et cd. from stainless steel metal foils, which were sealed to a monohthic stack of plates by laser welding [73]. A rhodium catalyst developed for this specific application was coated by a sol-gel technique onto the metal foils prior to the sealing procedure. The reactor carried a perforated plate in the inlet section to ensure flow equi-partition. At a weight hourly space velocity of 316 L (h gcat). S/C 3.3 and O/C 0.52 ratios, more than 99% conversion of the fuel was achieved. The temperature profile in the reactor was relatively flat. It decreased from 730 °C at the inlet section to 680 °C at the outlet. This was attributed to the higher wall thickness of the plate monolith compared with conventional metallic monolith technology. The reactor was later incorporated into a breadboard fuel processor (see Section 9.5). 

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