Catalyst honeycomb

ActivatedL yer Loss. Loss of the catalytic layer is the third method of deactivation. Attrition, erosion, or loss of adhesion and exfoHation of the active catalytic layer aU. result in loss of catalyst performance. The monolithic honeycomb catalyst is designed to be resistant to aU. of these mechanisms. There is some erosion of the inlet edge of the cells at the entrance to the monolithic honeycomb, but this loss is minor. The peUetted catalyst is more susceptible to attrition losses because the pellets in the catalytic bed mb against each other. Improvements in the design of the peUetted converter, the surface hardness of the peUets, and the depth of the active layer of the peUets also minimise loss of catalyst performance from attrition in that converter. 

H. Yamamoto and co-workers, Warm-Up Characteristics of Thin Wall Honeycomb Catalysts, SAE 910611, Society of Automotive Engineers,... 

Although mote expensive to fabricate than the pelleted catalyst, and usually more difficult to replace or regenerate, the honeycomb catalyst is more widely used because it affords lower pressure losses from gas flow it is less likely to collect particulates (fixed-bed) or has no losses of catalyst through attrition, compared to fiuidized-bed and it allows a mote versatile catalyst bed design (18), having a weU-defined flow pattern (no channeling) and a reactor that can be oriented in any direction. 

Fig. 3. Schematic of the flow through (a) a honeycomb catalyst stmcture and (b) a cross section of a honeycomb channel.

The increasing use of sihconized coatings for weather durabiUty caused severe masking problems for the all-metal, filter mesh-like catalyst elements available in the 1970s. Interest in catalytic afterburners increased when dispersed-phase precious metal—alumin a-on-ceramic honeycomb catalysts offered economically attractive results. 

DENOX (Shell DENOX) A low-temperature, add-on SCR system that operates at between 120 and 350°C. The honeycomb catalyst contains vanadium and titanium. 

Laboratory data collected over honeycomb catalyst samples of various lengths and under a variety of experimental conditions were described satisfactorily by the model on a purely predictive basis. Indeed, the effective diffusivities of NO and NH3 were estimated from the pore size distribution measurements and the intrinsic rate parameters were obtained from independent kinetic data collected over the same catalyst ground to very fine particles [27], so that the model did not include any adaptive parameters. 

The single cylindrical pore is of course not the geometry we are interested in for porouS catalysts, which may be spheres, cylinders, slabs, or flakes. Let us consider first a honeycomb catalyst of thickness It with equal-sized pores of diameter cfp, as shown in Figure 7-14. The centers of the pores may be either open or closed because by symmetry there is no net flux across the center of the slab. (If the end of the pore were catalytically active, the rate would of course be sHghtly different, but we will ignore this case.) Thus the porous slab is just a collection of many cylindrical pores so the solution is exactly the same as we have just worked out for a single pore. 

Ceramic honeycomb monoliths are porous macro-structured supports consisting of parallel channels. On the walls a thin layer of active material can be applied (Figure 1). Honeycomb catalyst supports were originally developed for use in automotive... 

The up-scaling from microreactor to small monoliths principally deals with the change of geometry (from powdered to honeycomb catalyst) and fluid dynamics (from turbulent flow in packed-bed to laminar flow in monolith channels). In this respect, it involves therefore moving closer to the conditions prevailing in the real full-scale monolithic converter, while still operating, however, under well controlled laboratory conditions, involving, e.g. the use of synthetic gas mixtures. 

Validation at intermediate scale was first performed by comparing the results of kinetic runs over small honeycomb catalyst samples (volumes in the range... 

Kinetic runs over small monolithic honeycomb catalyst samples were performed in two different rigs at Politecnico di Milano and in the Daimler... 

A third transient experiment is shown in Fig. 47, where again experimental results (symbols) are compared with model simulation (solid lines). At time = 0 s, 1,000 ppm of NH3 and 1,000 ppm of NO were simultaneously fed to the honeycomb catalyst (5 cm3) in a stream of 10% F120, 10% 02, balance nitrogen, with an SV = 72,000 h-1. Temperature was set at 250 °C. 

In Section 17.8.3 we discussed the catalytic combustion of methane within a single one of the tubes in a honeycomb catalyst, illustrated in Fig. 17.18. The high velocity, and thus the dominance of convective over diffusive transport, makes the boundary layer approximations valid for this system. We will model the catalytic combustion performance in one of the honeycomb channels in this problem. 

So far the multiple steady-state phenomena in the flow of reactants through channels with catalytic walls have been described only by Hlavacek et al. (55, 58). The behavior of the honeycomb catalyst is similar to that of the packed bed however, the domain of multiple steady state... 

FIGURE 7 Optical photomicrograph of a cross section of a honeycomb catalyst. The thickness of the granular material, the washcoat, can be measured with a calibrated reticle in the microscope eyepiece. 

Figure 1.5 Rhodium honeycomb catalyst microstructure device. The microchannels have been manufactured by wire erosion.

B. Angele, K. Kirchner, and E. G. Schlosser (1980) The poisoning of noble metal catalysts by phosphorus compounds. III. The deposition of catalyst poisons in honeycomb catalysts, Chem. Eng. Sci 35 2101-2105... 

Monolith (or honeycomb) catalysts are prepared by covering the surface of the channels with a suspension... 

Nippon Shokubai developed a process for catalytic wet air oxidation and implemented it in at least 10 industrial plants in Japan and elsewhere 152,214). As reported by Liick 152), the Nippon Shokubai process involves a Pt-Pd/Ti02-Zr02 honeycomb catalyst that is not sensitive to deposition of solids on the catalytic surface. Typical operating conditions of the process are a temperature of 493 K, a pressure of 4 MPa, and a space velocity of 2 h (153). It has not been disclosed beyond any doubt whether monoliths are indeed applied in these processes. 

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