Computational Study of Transport Phenomena at the Microscale: Effects of Washcoat Thickness and Characteristic Length on the Heat and Mass Transfer Processes in Millisecond Reactors for Hydrogen Production

Transport phenomena in millisecond reactors are poorly understood due to extremely short contact times and nonequilibrium states. Numerical investigation of transport phenomena at the microscale was performed for hydrogen production by the steam-methanol reforming reaction. Heat and mass transport analyses were conducted, and dimensionless number analyses were carried out to gain insights into the nature of the transport processes. The effectiveness factor and Thiele modulus were calculated to understand how effectively the catalyzed washcoat is being used. The results indicated that the system is mass transfer limited because significant species gradients exist. The mass transfer rate is about 1 order of magnitude lower than the intrinsic reaction rate. Momentum diffusivity dominates the transport behavior, and convection is more significant compared to conduction and diffusion. Local Nusselt and Sherwood numbers of about 100 and about 55 can be obtained, respectively, with a pressure drop of less than about 200 Pa. Both the washcoat thickness and characteristic length are critically important in determining the catalyst effectiveness and reactor behavior. Large variations in the effectiveness factor must be accounted for to deliver accurate results. The results provide useful guidance in the operation and design of millisecond reactors by quantifying transport process variations and identifying the effectiveness of catalyzed washcoats.