Reactor, isothermal monolithic catalyst

A simple isothermal pseudo-homogeneous, single-channel, ID model is typically adopted to model a monolith SCR reactor [27, 30, 38, 40-50], which implies uniform conditions over the entire cross-section of the monolith catalysts and accounts only... 

The basic equations that describe fixed-bed reactors have been presented in Section 3.6.2. In the present Section Isothermal, Adiabatic and Non-isobaric fixed bed operations as well as the case of Monolithic catalysts are presented. 

Lie et al. [35] modeled the CO oxidation by O2 on a Pt/Al203 catalyst with an isothermal monolithic converter in order to assess the effect of cyclic feeding on the performance of the reactor. The kinetic model of Herz and Marin [59] was used, which consists of a closed sequence of elementary steps. The reactor model is essentially as described by Eqs. (25)-(27), but now includes accumulation terms for all three phases the gas phase, the pores of the washcoat, and the catalyst surface. 

Foams were proved to be highly suitable as catalytic carrier when low pressure drop is mandatory. In comparison to monoliths, they allow radial mixing of the fluid combined with enhanced heat transfer properties because of the solid continuous phase of the foam structure. Catalytic foams are successfully used for partial oxidation of hydrocarbons, catalytic combustion, and removal of soot from diesel engines [14]. The integration of foam catalysts in combination with microstructured devices was reported by Yu et al. [15]. The authors used metal foams as catalyst support for a microstructured methanol reformer and studied the influence of the foam material on the catalytic selectivity and activity. Moritz et al. [16] constructed a ceramic MSR with an inserted SiC-foam. The electric conductive material can be used as internal heating elements and allows a very rapid heating up to temperatures of 800-1000°C. In addition, heat conductivity of metal or SiC foams avoids axial and radial temperature profiles facilitating isothermal reactor operation. 

The model of Reference (67) was later applied to evaluate the performance of an SCR catalyst with proprietary composition (124). Koebel and Elsener also compared, on a fully predictive basis, a similar model to experimental data of NO conversion and NH3 slip obtained on a diesel engine test stand (125). In this case, while the model was shown to describe qualitatively the performance of the SCR monolithic reactor, specifically with reference to the NO conversion versus NH3 slip relationship, an exact quantitative match was found impossible. According to the authors, the reasons for the discrepancies may include unaccovmted kinetic effects of the contaminants present in the diesel exhaust gases, vmcertainties due both to the extrapolation of the kinetic parameters and to the measurement of the intraporous diffusivities, and the excessive simplification involved in the assumption of a pure Langmuir isotherm for NH3 adsorption. 

A parametric study on the effects of axial heat conduction in the solid matrix has shown that i) such effects are negligible in ceramic monoliths (cordierite, kj = 1.4 w/m/K) but expectedly significant in metallic monoliths (Fecralloy, k i = 35 W/m/K) when a constant heat flux is imposed at the external matrix wall ii) however, the influence of axial conduction in metallic monoliths is much less apparent if a constant wall temperature condition is applied, since the monolith tends to an isothermal behavior. Metallic matrices exhibit very flat axial and radial temperature profiles, which seems promising for their use as catalyst supports in non-adiabatic chemical reactors. 

Catalytic activity data herein reported were collected over state-of-the-art commercial vanadium-based, Fe- and Cu-promoted zeolite SCR catalysts. The original monolith samples were crushed to powder, sieved, and loaded in a quartz microflow reactor (60-80 mg) consisting of a quartz mbe (6 mm i.d.) placed in an electric oven. This experimental setup affords isothermal operation of fast transients in a chemical regime, free of any diffusional intrusions. He as carrier gas enables evaluation of N-balances. 

The second stage of the scale-up approach involves monolith reactor experiments over small catalyst samples with a volume of a few cubic centimeters. The data obtained from this intermediate stage serve either as a primary validation of the intrinsic reaction kinetics or for kinetic parameter estimation in case microreactor experiments have been omitted. Monolith reactor experiments are able to reproduce more accurately the phenomena prevailing in real full-scale converters taking into account the catalyst s geometry, the flow dynamics along the channel, and the intraporous diffusion over the washcoat. At the same time, the experiments are performed under controlled laboratory conditions, involving isothermal operation and the use of synthetic gas mixtures. 

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