Heterogeneous Catalytic Mass Transfer Process

Under the reaction conditions, a stable laminar flow layer is formed around the catalyst particle by reactant molecules, product molecules, diluent and other mixture components, and a reactant molecule must pass through this laminar flow layer to reach the outer surface of the catalyst particle. Because the laminar layer impedes this flow, a concentration gradient is formed between the outer surface of the particle and the gas flow layer.
The mass transfer resistance between the fluid and the outer surface of the catalyst is mainly concentrated in the laminar boundary layer around the catalyst particles with a thickness of less than 1 mm. The velocity of the fluid parallel to the particle surface in the layer varies greatly. At the outer surface of the particle, the fluid velocity is zero, while at the junction of the boundary layer and the fluid body, its velocity is equal to the velocity of the fluid body. Therefore, the physical and chemical properties and hydrodynamic properties of the fluid will affect the mass transfer rate, and the particle size will also affect the rate of the transfer process.
The mass transfer coefficient kG increases with the increase of fluid mass velocity, so the transfer rate of component A from the fluid body to the outer surface of the catalyst also increases with the increase of fluid mass velocity. This indicates that the lower the fluid mass velocity, the greater the influence of out-diffusion on the heterogeneous catalytic reaction process. However, in most cases, the mass velocity of the actually produced fluid is relatively large, and there are not many heterogeneous catalytic reaction processes that are controlled by external diffusion. This process is mostly a high-temperature reaction, such as the oxidation of NH3 and CH4 to HCN on the Pt alloy grid, the oxidation of CH3OH to HCHO on the non-porous Ag catalyst, the decomposition of concentrated H2O on the Ag catalyst, and the CO and hydrocarbons in the automobile exhaust gas purifier. Catalytic oxidation, etc., the external diffusion resistance is generally not negligible, even controlled by external diffusion.