engleski

Modeling of the monolith reactors

INTRODUCTION Monolith catalysts play a key role in improving our atmosphere and reducing pollution1. This type of the so-called structured catalysts (reactors) can simultaneously meet special requirements, e.g. very low pressure drop, excellent mass transfer properties, high surface-to-volume ratio, short diffusion resistance, easy scale-up, etc. It is known, that the removal of NOx from the exhaust gas is one of major issues in environmental protection. Discovery of stable catalytic activity of Cu/ZSM-5 zeolite for the direct decomposition of NO into elements by Iwamoto in 19862, brought intensive research all over the world on catalysis related to deNOx reactions on metal exchanged zeolites and molecular sieves generally. This study is the continuation of our previous investigation of NO decomposition over powder Cu/ZSM-5 catalyst3,4. It deals with the use of a monolithic catalyst, composed of cordierite as an inert carrier and Cu/ZSM-5 zeolite as a catalytic washcoat. The one- (1D) and two-dimensional (2D) heterogeneous models are used for simulation of the monolith reactor with the view to determining the importance of diffusional effects within the washcoat layer. Particular emphasis is put on justifying suitability of such reactor models. EXPERIMENTAL Monolithic catalysts were made of ceramic honeycomb substrate (cordierite) and copper containing ZSM-5 zeolite used as catalytic layer (washcoat). The commercial oval type of honeycomb was cut into square pieces, each with 4 channels. The samples 46-79 mm long were used for coating of the catalytic layer. A detailed description of Cu/ZSM-5 zeolite preparation can be found elsewhere3. To washcoat the monolith, slurry was prepared by suspending Cu/ZSM-5 zeolite in the appropriate solutions. Slurry was introduced into the channels of the monolith by means of vacuum. Then monolith catalyst was dried over 24 hours at room temperature. In some cases the procedure had to be repeated several times in order to increase the amount of the active phase on the monolith walls. Before reaction, the monolith catalyst had been reduced in situ at 773 K for 2 hours under helium flow and cooled to the desired reaction temperature. Reaction of NO decomposition was performed in the temperature range from 573 to 773 K, over the monolith catalysts of various lengths and with various thickness of catalytic layer. Space times were changed by varying total flow rate of the reactant gas (4 % NO/He) over constant volume of the monolith. The catalyst activity for NO removal was evaluated by conversion of NO into N2, when the reaction reached steady state. RESULTS AND DISCUSSION In this study the 1D and 2D heterogeneous models were applied for simulation of the monolith reactor. Several assumption were taken into account, such as steady-state, isothermal conditions, equal conditions within each monolith channel (thus simulation was reduced to the analysis of a single channel), ideal fow and negligible pressure drop along the monolith channel. The change in the geometry of the monolith channels with the increase in the thickness of the catalytic washcoat was also considered. Previously proposed Langmuir-Hinshelwood type of the rate equation was used to describe kinetics of the reaction3,4. Kinetic parameters were assesed using modified differential method and Nelder-Mead method of non-linear optimization. Inlet values used to estimate kinetic parameters were values for the powder Cu/ZSM-5 catalyst. Physical parameters applied to the models were calculated using several correlations proposed in the literature. The reactor models were verified by comparing experimental data with theoretical predictions. Generally, good agreement between experimental data and the values predicted by 1D heterogeneous model was achieved. However, since rate constant increases with the increase in the thickness of the catalytic layer, the reaction occurs within the layer, rather than solely on its surface. In other words, 2D heterogeneous model, which takes into account interphase diffusion, is more suitable for describing the monolith reactor performance. REFERENCES 1. Armor, J.N., Chem. Mater. 6 (1994) 730. 2. Iwamoto, M., Furukawa, H., Mine, Y., Uemura, F., Mikuriya, S., Kagawa, S., J. Chem. Soc., Chem Commun. 16 (1986) 1272. 3. Tomašić, V., Gomzi, Z., Zrnčević, S., Appl. Catal. B: Environ., 18(1998) 233. 4. Tomašić, V., Gomzi, Z., Zrnčević, S., React. Kinet. Catal. Lett., 64(1)(1998) 89.

modeling; monolith reactor; cordierite; washcoat

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