Date of Award
Master of Science (MS)
Borg, John P.
Hydrogen is most often produced on an industrial scale by catalytic steam methane reforming, an equilibrium-limited, highly endothermic process requiring the substantial addition of heat at elevated temperatures. The extent of reaction, or conversion efficiency, of this process is known to be heat transfer limited. Scaling the industrial process equipment down to the size required for small, compact fuel cell systems has encountered difficulties due to increased heat losses at smaller scales. One promising approach to effectively scale down the reforming process is to coat the catalyst directly onto the heat exchange surfaces of an integrated reactor/heat exchanger. In this way, heat can be effectively transferred to the catalytic reaction sites and conversion efficiency can be greatly improved.
Optimizing a small-scale integrated reactor requires an understanding of the interactions between the steam reformer reaction kinetics and the heat and mass transfer effects within the heat exchanger. Past studies of these interactions have predominantly focused on highly simplified flow channel geometries, and are unable to account for devices having augmented heat exchange surfaces. Full three-dimensional methods are possible, but require excessive computational resources.
In this work, a mixed-dimensionality modeling approach is developed in order to better address the problems posed by these integrated devices. This modeling approach is implemented using a commercially available thermal finite element code. The solid domain is modeled in three dimensions, while the fluid is treated as a one-dimensional plug flow. The catalyst layer is treated as a surface coating over the three-dimensional surfaces. A subroutine to solve the surface reaction kinetics using a LHHW kinetic model is developed and incorporated into the code in order to address the highly non-linear thermal/kinetic interactions. Validation of the modeling approach is accomplished through comparison of model results to test data obtained from an integrated reactor/heat exchanger test unit. Analysis of the results indicates that the modeling approach is able to adequately capture the complex interactions within the test unit.