Chong Zheng

Interaction of macro- and micromixing on particle size distribution in reactive precipitation

The effects of mixing factors (stirrer speed, feed time, feed location and viscosity) on particle size distribution (PSD) in the precipitation of BaSO4 conducted in a semi-batch stirred tank reactor were investigated. It was found that there exist critical points on the curves of mean size and dimensionless variance versus the mixing factors, at which the mean size or the dimensionless variance is minimal. These results originate from the interaction of macromixing and micromixing. A new mixing-precipitation model including micromixing, turbulent dispersion (mesomixing) and backmixing (macromixing) was proposed to predict PSD. Critical effects of mixing factors, stirrer speed and feed time on PSD were quantitatively predicted by the present model. The results coincided satisfactorily with those of the experiments.

Hydrodynamics and mass transfer in cross-flow rotating packed bed

Abstract Hydrodynamics and mass transfer characteristics in a cross-flow rotating packed bed (RPB) were studied. Models describing the liquid film flowing on the wire gauze packing, the liquid droplet size in the voids, the gas pressure drop, the gas pressure and velocity radial distribution and liquid flow direction have been established. Three different types of mass transfer processes in cross-flow RPB were studied. Mass transfer model was established based on surface renewal model. The calculated results agree well with that of experiments. Experiments show that: The gas flow rate can be as high as 15 m/s and even higher, therefore the diameter of RPB can be greatly reduced compared to that of counter-current RPB; For the liquid side controlling processes, the height of mass transfer unit is 2.5–4 cm, which is close to that of countercurrent flow RPB.

Synchronous visual and RTD study on liquid flow in rotating packed-bed contactor

Rotating packed bed (RPB), a novel multiphase contactor, can greatly intensify multiphase mass transfer processes. However, the real-flow situation in a rotating bed which leads to the high mass transfer rates is still not clear. The first part of this paper describes phenomena and results observed by a video camera mounted on a RPB. The camera rotates synchronously with the rotor so that in situ signal of liquid flow can be acquired, and continuous and stable live images are obtained. In the second part of this paper, two electro-conductivity sensors are mounted on a RPB to pick up signals in situ at different positions during rotating. Real RTD in RPB at different operating conditions are obtained. A mathematical model is developed for simulating the velocity profile, and the film thickness of liquid flow on the packing. The calculated residence time in the packing gives an average relative error of 14% in comparison with the experimental results.

Distributions of flow regimes and phase holdups in three-phase fluidized beds

Abstract Pressure fluctuation characteristics at different axial and radial positions were investigated in a three-phase fluidized-bed Plexiglas column of 285 mm inside diameter and 4100 mm height operated with air, tap water and glass beads under different conditions. It was found that the whole column may display different flow regimes depending on the operating conditions, i.e. total homogeneous bubble regime, total transition regime, total turbulent bubble regime, while the two neighbouring or all three regimes may exist simultaneously at different heights of the column. Schemes of flow regime distribution, which changes with gas velocity, were given, and the criteria for flow regimes correlated with operating variables and axial distance above the gas distributor were established. It was also found that any flow regime was substantially identical at different radial positions across the column. Phase holdups at different heights of the column were measured by means of a combination of pressure drop and sampling methods. Correlations of gas and solid holdup in different flow regimes as a function of operating parameters and axial distance from the distributor were established.

Mass transfer in different flow regimes of three-phase fluidized beds

Abstract The volumetric gas-liquid mass transfer coefficient ( K L a ) and axial dispersion coefficient ( D z ) of liquid phase have been studied in air-oxygen saturated water fluidized beds of 0.52 to 0.755 mm glass spheres in a plexiglass column. The effects of liquid velocity, gas velocity, particle size and loadings on K L a and D z were examined. According to the different correlations of mass transfer, three flow regimes of three-phase fluidized beds were distinguished. It was found that K L a and D z change with different regularities in these different flow regimes. The results have shown that the flow regimes distinguished by different correlations of mass transfer were substantially identical with those distinguished by gas holdup correlations as well as pressure fluctuation analysis as had been found in previous investigations.

Modeling of three-phase fluidized beds based on local bubble characteristics measurements

Abstract Local bubble characteristics in different flow regimes of three-phase fluidized beds were investigated by means of an electro-conductivity probe. Bubble size, bubble rising velocity, bubble frequency and gas holdup were correlated with the operating parameters, axial and radial positions of the column in different flow regimes. Based on the measured results and the analyses of the forces acting on each phase, a one-dimensional three-phase hydrodynamic model was developed. With the radial distribution of liquid turbulent kinematic viscosity, the hydrodynamic model was solved for different flow regimes with parameters accordingly. The results show that the calculations correspond well to the measured results of local gas and liquid velocities.