CFD simulations and X-ray measurements of the local gas hold-up in a stirred tank reactor agitated by a gas-inducing turbine

The experimental and the numerically studies were applied to a non-baffled laboratory-scale stirred tank reactor, mechanically agitated by a gas-inducing turbine. The dispersion of air as gas phase into isopropanol as liquid phase at room temperature under different stirrer speeds was investigated. The X-Ray cone beam tomography measurements were taken at five different stirrer speeds with thresholds of 50 rpm starting from 1000 rpm at which the gas inducement occurs for the given operating conditions.
The cone-beam type X-Ray tomography is a potential method to measure the phase-distributions in stirred vessels. Three-dimensional information can be gathered within only one tomographic scan. The reconstruction of a rotationally symmetric distribution-field is even possible from a single radiographic image. Such an experimental approach was carefully examined and applied to obtain the quantitative measurements of gas-fraction profiles in a stirred tank reactor. Additionally, a moving slit technique was adapted to estimate the inherent scattered radiation offset, which emerges while un-collimated x-rays penetrate the fluid-filled tank. An additional reference measurement was introduced and used to remove beam hardening artefacts. An absolute quantification was possible due to the knowledge of the ratio of the fluids and the reference-materials x-ray absorption coefficients. Phantom-measurements inside the vessel were conducted for performance evaluation. A systematic measurement error of less then 1.5% absolute gas fraction for local gas fractions up to 30% was achieved while maintaining a spatial resolution of better then 1 mm.
The computational fluid dynamics analyses of the stirred tank reactor were performed in 3D with CFX 10.0 numerical software. Five steady state simulations, at stirrer speeds corresponding to the ones at which the measurements were performed, were conducted to be compared with the experimental observations. The tetrahedral mesh with above 1500000 elements was globally refined since a detailed view in the whole geometry is required. The inhomogeneous two-phase flow model with the particle transport model was applied to the system with momentum transfer described by the drag force and turbulence transfer modelled by Sato enhanced eddy viscosity model. The gas phase was modelled as dispersed fluid and the liquid phase as continuous fluid. Different turbulence models and their suitability were considered in the simulations.