Impact of catalyst particles on the hydrodynamics in slurry bubble columns
Most gas-liquid reaction processes in bubble columns are operated with catalysts, which are suspended as solid particles. A prominent example is the Fischer-Tropsch synthesis for the production of liquid fuels from synthesis gas.
The optimization of these energy-intensive processes requires the best possible utilization of the compression energy in terms of gas-liquid interfacial area and phase interactions considering the impact of suspended catalyst particles. The in-house developed ultrafast X-ray tomography was applied to visualize the dynamic gas structures in slurry bubble columns operated with particle-laden liquid phases.
Figure 1: Visualization of gas-slurry flow structures via ultrafast X-ray tomography.
In this study, experiments were performed at various particle concentrations and evolving gas structures, gas holdups and bubble size distributions were extracted. Thereby, opposing particle effects were encountered depending on the particle concentration. This was verified by transition points from coalescence regime to breakup regime by means of the bubble size distribution.
Figure 2: Proof of opposing particle effects in slurry bubble columns in terms of flow stabilization and destabilization.
At low particle concentration, particles attach to the spherical bubble surface and stabilize it. At complete coverage of the surface (particle concentration of approx. 5 %), further addition of particles results in a slurry of increasing particle-induced apparent viscosity, which promotes bubble coalescence and the transition into the heterogeneous flow regime. At particle concentrations of 20 % and higher, interactions between suspended particles and gas bubbles increase and breakup of bubbles is promoted. Accordingly, the gas-slurry interfacial area increases, which enhances the gas-liquid mass transfer.
These dual hydrodynamic effects indicate that the flow behavior of slurry bubble columns cannot be mimicked by two-phase systems with enhanced molecular viscosity. Instead three-fluid models are required.
References: S. Rabha, M. Schubert, U. Hampel (2014). Regime transition in viscous and pseudo viscous systems: A comparative study. AIChE Journal 60, 8, 3079-3090.
S. Rabha, M. Schubert, U. Hampel (2013). Hydrodynamic studies in slurry bubble columns: Experimental and numerical study. Chemie Ingenieur Technik 85, 7, 1092-1098.
S. Rabha, M. Schubert, M. Wagner, D. Lucas, U. Hampel (2013). Bubble size and radial gas hold-up distributions in a slurry bubble column using ultrafast electron beam X-ray tomography. AIChE Journal 59, 5, 1709-1722.
S. Rabha, M. Schubert, U. Hampel (2013). Intrinsic flow behavior in a slurry bubble column: A study on the effect of particle size. Chemical Engineering Science 93, 401-411.