Mass transfer effects in SCR reactor for NOx abatement in diesel engines

Abstract

In this study, mass transfer limitations in a monolith reactor for selective catalytic reduction (SCR) of NOx were investigated. The reactor channels were washcoated with a commercial copper chabazite (CHA) catalyst. SCR experiments with different space velocities (20,000–80,000 h−1 (STP) range) were carried out. The feed to the reactor contained 300 ppm NH3, 300 ppm NO, 10% CO2, 8% O2, and 5% H2O with the balance being N2. NO conversion at steady state was measured between 100 and 590°C. Results revealed that the overall reaction rate is controlled by surface reactions up to 210°C but between 210 and 380°C both chemical reaction, internal and external mass transfer resistances are important. The maximum NO conversion was obtained for the highest flow rate above 350°C, therefore, external mass transfer rate dominates the overall reaction rate in this zone while the reactor operates in the kinetic regime at high flow rates and at low temperatures. Kinetic parameters for standard SCR reaction were determined using the data in the kinetically controlled region. Intrinsic rate constants were combined with the internal diffusion parameters to determine the apparent rate constants and the region where internal mass transfer limitations occur. Overall and apparent reaction rate parameters were used to extract the external mass transfer coefficients in the interphase diffusion controlled region. Various correlations for mass transfer coefficient for monolith reactors were compared with the external mass transfer coefficients found in this study.