Assessment of a subgrid-scale model for convection-dominated mass transfer for initial transient rise of a bubble

The mass transfer between a rising bubble and the surrounding liquid is mainly determined by an extremely thin layer of dissolved gas near the bubble interface. Resolving this concentration boundary layer in numerical simulations is computationally expensive and limited to low Péclet numbers. Subgrid-scale models mitigate the resolution requirements by approximating the mass transfer near the interface. In this contribution, we validate an improved subgrid-scale model with a single-phase simulation approach, which solves only the liquid phase at a highly-resolved mesh. The mass transfer during the initial transient rise of moderately deformed bubbles in the range Re = 72–569 and Sc = 102–104 is carefully validated. The single-phase approach is able to mirror the two-phase flow field. The time-dependent local and global mass transfer of both approaches agree well. The difference in the global Sherwood number is below than 2.5%. The improved subgrid-scale model predicts the mass transfer accurately and shows marginal mesh dependency

DIRECT NUMERICAL SIMULATION OF MASS TRANSFER FROM A SINGLE BUBBLE VIA AN IMPROVED SUBGRID SCALE MODEL

Hydrogenation, oxidation and alkylation are just some of the processes which are performed in bubble columns. One of the reasons to use a bubble column for these processes is the high interfacial mass transfer coefficients. Trying to simulate the mass transfer around the bubbles is however challenging due to the typically high Schmidt numbers of liquids, meaning that the mass boundary layer is very thin compared to the momentum boundary layer. To resolve this thin mass boundary layer, a subgrid scale model can be used. This work focuses on improving the subgrid scale model that we have embedded in our in-house front tracking framework of Claassen et al., AIChe J 2019. In the current implementation the unphysical numerical back diffusion at the grid into the bubble has been prevented with a staircase immersed boundary implementation. A verification has been performed by comparing the simulated, local and global Sherwood number with the analytical solution in creeping and potential flow regimes. Furthermore, the model was validated for 20 free rising bubbles of different shapes at industrial relevant Schmidt numbers (103-105). The model was able to correctly predict the Sherwood numbers.publishedVersio

DIRECT NUMERICAL SIMULATION OF MASS TRANSFER FROM A SINGLE BUBBLE VIA AN IMPROVED SUBGRID SCALE MODEL

Hydrogenation, oxidation and alkylation are just some of the processes which are performed in bubble columns. One of the reasons to use a bubble column for these processes is the high interfacial mass transfer coefficients. Trying to simulate the mass transfer around the bubbles is however challenging due to the typically high Schmidt numbers of liquids, meaning that the mass boundary layer is very thin compared to the momentum boundary layer. To resolve this thin mass boundary layer, a subgrid scale model can be used. This work focuses on improving the subgrid scale model that we have embedded in our in-house front tracking framework of Claassen et al., AIChe J 2019. In the current implementation the unphysical numerical back diffusion at the grid into the bubble has been prevented with a staircase immersed boundary implementation. A verification has been performed by comparing the simulated, local and global Sherwood number with the analytical solution in creeping and potential flow regimes. Furthermore, the model was validated for 20 free rising bubbles of different shapes at industrial relevant Schmidt numbers (103-105). The model was able to correctly predict the Sherwood numbers

Detailed Investigation of Different Diffusion Models for Reactive Catalytic Systems in Isolated Particles and Slender Packed Beds

Packed bed reactors are found abundantly in the chemical industry in applications ranging from product synthesis to effluent treatment and catalytic combustions. While there exists considerable number of studies in terms of flow dynamics, detailed investigations into reactive systems are still scarce. For systems where different components highly interact with each other or with sequential reactions, it is important to accurately represent all species. In this work, we get a step closer to the multi-component modeling of reactive processes in slender packed beds. We will compare different diffusion models like the Dusty Gas Model (DGM) and Fick with Darcy flow (F+D) to investigate when we can make the trade-off between the better accuracy of DGM and lower computational demand of F+D. We will extend the usual comparisons by considering temperature and conjugate transport. The obtained knowledge will be applied to study the CO2 methanation in a slender packed bed where we will vary reactor operating conditions to investigate the effect on mass (and heat) transport and the overall reactor performance. The work presents the models which are necessary to accurately represent reactive, multi-component packed beds and provides insight on the reactor performance as a function of mass transport behavior