Channel reactors honeycomb monoliths

The SCR catalysts are used in the form of honeycomb monoliths or plates to guarantee low pressure drops in view oflarge frontal area with parallel channels, high external surface area per unit volume of catalyst, high attrition resistance and low tendency for fly ash plugging. The SCR monoliths and plates are assembled into standard modules and inserted into the reactor to form catalyst layers. 

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. 

The modeling of mass transfer and reaction in catalytic filters can be compared, in a first approximation, with the twin problem concerning honeycomb catalysts. The pores of the filters will have as counterparts the channels of the monolith, whereas the catalyst layer deposited on the pore walls of the filter will be related to the wall separating the honeycomb channels, which in general are made exclusively of catalytic material. Considering, for example, the DeNOx reaction. Fig. 9 shows schematically the NO concentration profiles within the channels/pores and the catalyst wall/layer of the two reactor configurations. 

Catalyst - The catalyst herein under study is a commercial Ti-W-V catalyst with 9-weight % of WO3 and about 0.6 % of V2O5. Monolith channels had an opening of 0.415 cm and a wall thickness of 0.08 cm. Sample honeycomb monoliths were cut with a cross section of 3x3 channels and length of 15 cm, calcined for 2 hours at 500, 750 and 800°C and loaded in the reactor for DeNOx testing. The monoliths were instead crushed into granules of 1-1.5 mm diameter and tested in a separate rig for SO2-SO3 activity measurements. 

Both reactor types R3 and R4 use the segmented flow (Taylor) principle. They are divided into two categories R3 has very small channels (<1 mm) and R4 are monolith reactors (honeycomb), well developed on the laboratory scale with at least one example of industrial application. Category R3 includes single-channel and multi-ple-channel reactors [10], etched in silicon [10] or glass [10,11], with wall-coated or immobilized catalysts in the case of gas-liquid-solid additions [12], and capillary microreactors for gas-liquid-liquid systems [13]. 

A simple model has been developed to describe the temperature fields inside honeycomb monolithic matrices suitable for use as catalyst supports in non-adiabatic reactors. The model Includes realistic features such as axial heat conduction and non-uniform flow distribution in the channels, securing a satisfactory match with avail2d)le data. 

In Fig. 2.16, a close-up picture of the reactor with the capillary for temperature profiles is displayed. The FHS, the capillary inserted into one channel and the ceramic fiber paper covering FHS, catalyst, and BHS are visible. The setup can be used for in situ measurements, where axial temperature and concentration profiles can be detected from one channel of the monohthic catalyst. Figure 2.17 shows the micro volume tee mounted onto the motorized linear stage used for moving the capillary. The setup is similar to the one introduced for foam monoliths by Horn et al. (2006a,b) and then adapted to honeycomb monoliths by Donazzi et al. (2011a,b). A novel feature of our... 

Monolith reactor model—For validation purposes, the kinetic models of the SCR catalyst [6] and of the PGM catalyst [18] were used to simulate catalytic activity runs over honeycomb monoliths coated with the SCR and the PGM component, respectively, of the studied ASC system. In the case of the SCR catalyst, the kinetics were implemented in a heterogeneous dynamic ID -I- ID model of a single monolith channel, accounting both for external (gas-solid) and internal (intra-porous) mass transfer resistances [12, 25, 26]. Model simulations... 

Honeycomb monolith reactors are commonly used in automobile exhaust emission control and for NOx reduction in power-plant flue gases by catalytic reduction, but they also can be used for photocatalytic reactions in the gas phase (see Fig. 7.2c). This type of reactors contains certain number of channels of circular or square cross section. The photocatalyst is coated onto the inner walls of channels... 

Another synthesis process proposed to receive benefits from operating with monolith catalysts is the conversion of methanol for gasoline production [16,17J. The catalyst used was the ZSM-5 zeolite. However, rather than binding the catalyst onto the wall by use of a washcoat, it was uniformly crystallized on the cordierite honeycomb (62 cells/cm ) wall surfaces (up to 30% by weight), similar to the method described in the patent assigned to Lachman and Patil [18]. The effects of methanol partial pressure on conversion and temperature on hydrocarbon selectivity were determined. Three regimes of mass transfer resistances are experienced in this reaction reactant transfer to the reactor walls within the monolith channels through the laminar flow, diffusion resistance at the surface between zeolite crystals on the walls, and diffusion into the zeolite molecular-size pores to the active sites within the crystals, where the reaction rate limit is anticipated. 

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