Zhengming Gao

Void fraction distribution in sparged and boiling reactors with modern impeller configuration

Vertical void fraction distributions in cold gassed, hot sparged and boiling systems using different agitators with multiple modern hollow blade and up-pumping wide-blade hydrofoil impellers are reported. The void fraction in boiling systems is dramatically different from that in cold gassed or hot sparged systems whether in terms of value or distribution. Under the same gas phase output conditions, the void fraction is much smaller in boiling systems than in cold gassed or hot sparging systems. Hot sparged systems have similar vertical void fraction distributions, with maxima in similar locations but with smaller void fractions overall, like those of cold gassed systems. The results are of particular relevance to the design and operation of reactors with hot sparging or boiling liquids.

Particle Image Velocimetry Study of Turbulence Characteristics in a Vessel Agitated by a Dual Rushton Impeller

Abstract Particle Image Velocimetry (PIV) has been used to investigate turbulence characteristics in a 0.48 m diameter stirred vessel filled to a liquid height (H = 1.4T) of 0.67 m. The agitator had dual Rushton impellers of 0.19 m diameter (D = 0.4T). The developed flow patterns depend on the clearance of the lower impeller above the base of the vessel, the spacing between the two impellers, and the submergence of the upper impeller below the liquid surface. Their combinations can generate three basic flow patterns, named, parallel, merging and diverging flows. The results of velocity measurement show that the flow characteristics in the impeller jet flow region changes very little for different positions. Average velocity, trailing vortices and shear strain rate distributions for three flow patterns were measured by using PIV technique. The characteristics of trailing vortex and its trajectory were described in detail for those three flow patterns. Since the space-resolution of PIV can only reach the sub-grid rather than the Kolmogorov scale, a large-eddy PIV analysis has been used to estimate the distribution of the turbulent kinetic energy dissipation. Comparison of the distributions of turbulent kinetic energy and dissipation rate in merging flow shows that the highest turbulent kinetic energy and dissipation are both located in the vortex regions, but the maxima are at somewhat different locations behind the blade. About 37% of the total energy is dissipated in dual impeller jet flow regions. The obtained distribution of shear strain rate for merging flow is similar to that of turbulence dissipation, with the shear strain rate around the trailing vortices much higher than in other areas.

Hydrodynamic Behavior of a Single Bubble Rising in Viscous Liquids

Abstract The rising behavior of single bubbles has been investigated in six systems with different viscosity and Morton number (Mo) from 3.21×10−11 to 163. Bubbles with maximum equivalent diameter of up to 16 mm were investigated. The bubble Reynolds number (Re) ranged from 0.02 to 1200 covering 3 regimes in which two functions are obtained relating the drag coefficient, CD, with Re and Mo. It has been found that in the high Reynolds number regime the drag coefficient increases until the Reynolds number of about 1200. The classic expression of Jamialahmadi (1994) is improved and extended to high viscosity liquids. A new relationship for the aspect ratio of deformed bubbles in terms of Re, the Eotvos number and Mo, applicable to a wide range of system properties, especially in high viscosity liquids, is also suggested.

Effects of the Blade Shape on the Trailing Vortices in Liquid Flow Generated by Disc Turbines

Abstract Particle image velocimetry technique was used to analyze the trailing vortices and elucidate their relationship with turbulence properties in a stirred tank of 0.48 m diameter, agitated by four different disc turbines, including Rushton turbine, concaved blade disk turbine, half elliptical blade disk turbine, and parabolic blade disk turbine. Phase-averaged and phase-resolved flow fields near the impeller blades were measured and the structure of trailing vortices was studied in detail. The location, size and strength of vortices were determined by the simplified λ 2 -criterion and the results showed that the blade shape had great effect on the trailing vortex characteristics. The larger curvature resulted in longer residence time of the vortex at the impeller tip, bigger distance between the upper and lower vortices and longer vortex life, also leads to smaller and stronger vortices. In addition, the turbulent kinetic energy and turbulent energy dissipation in the discharge flow were determined and discussed. High turbulent kinetic energy and turbulent energy dissipation regions were located between the upper and lower vortices and moved along with them. Although restricted to single phase flow, the presented results are essential for reliable design and scale-up of stirred tank with disc turbines.

GAS DISPERSION AND SOLID SUSPENSION IN A THREE-PHASE STIRRED TANK WITH MULTIPLE IMPELLERS

ABSTRACT Despite much research on gas-liquid-solid systems and their widespread application in industry, gas dispersion with solid suspension in multistage stirred reactors equipped with multiple impellers has received little attention. We report here the critical just-suspension impeller speed for different concentrations of solid particles, gas holdup, and shaft power in a vessel of 0.48 m diameter with four baffles and dished base. Five agitator configurations, each with three impellers mounted on a single shaft, have been used in the experiments. Two novel impeller designs were used, a deep hollow blade (semi-ellipse) disc turbine (HEDT) and four-wide-blade hydrofoil impellers. The hydrofoils were used in both up-pumping (WHU) and down-pumping (WHD) modes. Glass beads of 50 ∼ 150 μm diameter and density 2500 kg · m−3 were suspended at solid volumetric concentrations of 1.5, 3, 6, 9, and 15%. Results show that these suspended solids have little effect on the relative power demand. Agitators using the HEDT radial dispersing impeller at the bottom have a higher relative power demand (RPD = PG/PU) than those with WHD or WHU as the lowest one. For all impeller combinations there is little or no effect on gas holdup with increasing solid concentrations. Of the five different impeller combinations, those with an axial flow bottom impeller have significantly higher just-suspension agitation speeds and power consumption, so mounting the hydrofoil impeller at the bottom is not the optimal configuration for particle suspension. Of these impeller combinations, at a given gas flow rate the arrangement of HEDT + 2WHU has the highest relative power demand, gas holdup, and power input for both the suspension of settling particles and gas dispersion.

Analysis of Turbulence Structure in the Stirred Tank with a Deep Hollow Blade Disc Turbine by Time-resolved PIV

Abstract The turbulence structure in the stirred tank with a deep hollow blade (semi-ellispe) disc turbine (HEDT) was investigated by using time-resolved particle image velocimetry (TRPIV) and traditional PIV. In the stirred tank, the turbulence generated by blade passage includes the periodic components and the random turbulent ones. Traditional PIV with angle-resolved measurement and TRPIV with wavelet analysis were both used to obtain the random turbulent kinetic energy as a comparison. The wavelet analysis method was successfully used in this work to separate the random turbulent kinetic energy. The distributions of the periodic kinetic energy and the random turbulent kinetic energy were obtained. In the impeller region, the averaged random turbulent kinetic energy was about 2.6 times of the averaged periodic one. The kinetic energies at different wavelet scales from a6 to d1 were also calculated and compared. TRPIV was used to record the sequence of instantaneous velocity in the impeller stream. The evolution of the impeller stream was observed clearly and the sequence of the vorticity field was also obtained for the identification of vortices. The slope of the energy spectrum was approximately (5/3 in high frequency representing the existence of inertial subrange and some isotropic properties in stirred tank. From the power spectral density (PSD), one peak existed evidently, which was located at f 0 (blade passage frequency) generated by the blade passage.

Void Fraction Distributions in Cold-gassed and Hot-parged Three Phase Stirred Tanks with Multi-impeller

Abstract Vertical distributions of void fraction in gas-liquid and gas-liquid-solid stirred tanks have been measured in a fully baffled dished base vessel of 0.48 m diameter, using a conductivity probe. The impeller configuration (a hollow half elliptical blade dispersing turbine below two up-pumping wide blade hydrofoils, identified as HEDT+2WH U ) recommended in previous work has been used in this work. The operating temperatures were 24°C and 81°C, identified as cold and hot respectively. The effects of superficial gas velocity, agitator speed and the corresponding power input on the local void fraction in two-phase systems are investigated and discussed. Results show that the increasing of agitator speed or gas flow rate leads to an increase in local void fraction at the majority of measurement points in both cold and hot systems. However, the uniformity of gas dispersion does not always increase as the raising of agitator speed and power input. In either cold or hot sparged conditions, the two- and three-phase systems have similar vertical profiles for void fraction, with maxima in similar locations; however, the void fractions are significantly lower in hot sparging than with cold. In cold operation the presence of particles leads to a lower void fraction at most points, although the local void fractions increase a little with the addition of solid particles at high temperature, in good agreement with the global gas holdup results, and the possible reasons are discussed in this paper. This work can give a better understanding of the differences between cold-gassed and hot-sparged three phase stirred tanks.

Power Demand of Gas Dispersing Impellers Under High Load Conditions

This study has compared the power demand of a traditional Rushton turbine with that of three hollow blade dispersing impellers, the Chemineer CD-6 and BT-6 disc turbines and a design based on the ICI Gasfoil. Some of the large gas loadings were achieved by operating in heated water, so augmenting the sparged gas volume with evaporated water vapour. The results show that the power demand in the flooded regime may be signifcantly higher than in the absence of gas. Furthermore, once flooded, there is a considerable hysteresis that delays the return to loaded operation when the gas rate is reduced. The work has also shown that for a given off-gas rate, the relative power demand of an aerated impeller is almost always greater in a hot system than at ambient temperature, though it is not yet possible to quantify the increase.

The unsparged power demand of modern gas dispersing impeller in boiling liquids

Abstract It is known that impellers operating in boiling or near boiling liquids can develop cavities similar to those observed in gas–liquid systems at ambient temperatures. Considerable reductions in the power demand of traditional impellers operating in unsparged boiling liquids compared with that at ambient temperature have previously been reported and linked to a submergence based agitation cavitation (Smith) number. The performance of high performance gas dispersing impellers operating in boiling liquids has not previously been reported, despite their widespread adoption for mixing and dispersion in chemical reactors. The power demand of selected modern impeller designs (Chemineer CD-6 and BT-6, Lightnin A315 and an impeller based on the ICI Gasfoil design) working in boiling liquids is reported, together with updated information about conventional Rushton and pitched blade turbines. In boiling liquids the power draw characteristics of the new designs are quite different from those of the traditional impeller types. The modern impellers are all efficient at handling high loadings of inert gases. In boiling liquids they maintain high levels of power input — even when operated with high impeller tip speeds that correspond to low cavitation numbers. Such cavitation as may occur clearly does not affect the power demand. The results are of particular relevance to the design and operation of forced circulation crystallisers when secondary nucleation, or the degradation of a particulate product, might be expected to follow cavitation.

Gas Dispersion in Sparged and Boiling Reactors

Gas holdup measurements in cold gassed, hot sparged and boiling systems using different agitators with multiple modern hollow blade and up-pumping wide-blade hydrofoil impellers are reported. In a cold gassed system with given gas flow rates and power input, the different multiple impeller configurations generate very similar gas holdup. The gas holdup in a hot sparged system, between 70°C and 90°C, is only about half that in a cold system with the same total gas flow rate and power input. Moreover, at the higher temperatures there are unexpected and very significant differences between the gas holdup achieved by the different agitators. With most vapour generation occurring near the liquid surface, the vapour holdup in an unsparged boiling system is much lower than that for either cold gassed or hot sparged systems with similar total gas flow rate and power input. These results are of particular relevance to the design and operation of reactors with hot sparging or boiling liquids.