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Experimental and numerical assessment of the phase distribution in a stirred tank reactor agitated by a gas-inducing turbine
Hristov, H. V.; Boden, S.; Hampel, U.; Kryk, H.; Schmitt, W.; Hessel, G.
Over the last century two-phase flow mixing, engineered to take place in mechani-cally agitated tank reactors, has become one of the most common operations in the industry. Traditionally the gas-phase is supplied via a single pipe or a ring sparger mounted beneath the rotating impeller. The gas-inducing impellers provide an alter-native gas injection, in which case the gas is fed directly in the stirrer region.
A non-baffled laboratory-scale tank reactor mechanically agitated by a gas inducing turbine was experimentally and numerically studied. Above a certain impeller speed, at the investigated experimental conditions, the stirrer becomes able to induce gas and consequently disperse it into the bulk liquid phase. The two-phase system under examination comprises air as gas phase into isopropanol as liquid phase at room temperature. The gas-phase distribution was assessed at five different impeller speeds starting from 1000 rpm, at which gas inducement occurs.
The cone-beam type X-Ray tomography, which can provide three-dimensional infor-mation on the gas-phase distribution, was employed to experimentally study the two-phase system. The reconstruction of a rotationally symmetric distribution-field is pos-sible from a single radiographic image. Such an experimental approach was applied to obtain the quantitative measurements of gas-fraction profiles. Additionally, a mov-ing slit technique was adapted to estimate the inherent scattered radiation offset, which emerges while un-collimated X-rays penetrate the fluid-filled tank. An addi-tional 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.
The computational fluid dynamics analyses of the stirred tank reactor were performed with CFX 10.0 numerical software. The numerical predictions at 1000 rpm used pre-vious simulation results at lower impeller speed as an initial guess. Starting from 1000rpm, five simulations were performed at stirrer speed thresholds of 50 rpm to be compared with X-Ray cone beam tomography experimental observations. The tetra-hedral 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 de-scribed by the drag force and turbulence transfer modelled by Sato enhanced eddy viscosity model. The gas phase was modelled as dispersed fluid with a mean diame-ter of 1 mm and the liquid phase as continuous fluid.
The results demonstrate the X-Ray cone beam tomography and the CFD capabilities to capture the two-phase flow in detail, which can provide valuable information for the industry. In particular the special gas phase distribution can have a crucial impact on the reactor performance. This can be in detail predicted by the computational fluid dynamic software, which can prove to be an essential tool for the reactor optimisation and scale-up.
DECHEMA 2006 - GVC/DECHEMA-Jahrestagungen 2006, Rhein-Main-Hallen, Wiesbaden, 26.-28.09.2006, Wiesbaden, Germany
Abstract in refereed journal
Chemie Ingenieur Technik 78(2006)9, 1274