J.M. Smith

Study of Bubble Formation Under Constant Flow Conditions

Bubble size is one of the key parameters in the design of two-phase gas-liquid bubble column reactors. Accurate knowledge of this parameter is essential for the prediction of gas holdup, heat and mass transfer coefficients. The previousfindings, particularly with respect to the infiuence of orifice size and physical properties of the liquid phase on bubble size, are often of contradictory nature. In this paper, extensive new experimental results are presented for regions where published data are insufficient. The suitability of artificial neural networks for identification of the process variables and modeling is evaluated. The Radial Basis Function (RBF) neural network architecture was used successfully to generate a nonlinear correlation for the prediction of bubble diameter. This correlation predicts the present data and the control data of other investigators with excellent accuracy.

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.

Neural network analysis of void fraction in air/water two-phase flows at elevated temperatures

Radial basis function neural networks have been used to predict cross-sectional and time-averaged void fraction at different temperatures. The data bank contains experimental measurements for a wide range of operational conditions in which upward two-phase air/water flows pass through a vertical pipe of 2.42 cm diameter. The independent parameters are in terms of dimensionless groups such as modified volumetric flow ratio, density difference ratio, and Weber number. A comparison between the experimental and predicted data reveals an overall average error of 3.6% for training and 5.8% for unseen data. In addition, the trend of both predicted results and experimental data are qualitatively consistent.

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.

A numerical study of the interfacial transport characteristics outside spheroidal bubbles and solids

Abstract In this paper, the interfacial transport characteristics to or from spheroidal bubbles are studied numerically and the results of numerical simulations are reported. The numerical modelling uses the SIMPLE method with a non-orthogonal, boundary fitted staggered grid. The aim of this paper is to provide an understanding of the interfacial transport characteristics of inviscid spheroidal bubbles, of different geometric parameters, rising in a stagnant hot or bi-solution liquid. The flow and concentration (or temperature) fields around bubbles and similarly sized rigid spheroids are computed. Detailed analyses of the pressure and vorticity distributions at surfaces of the inviscid spheroidal bubbles are made and compared with those of similar rigid spheroids. Drag coefficients for and wake lengths behind rigid spheroids and inviscid spheroidal bubbles are also presented. Local and mean Sherwood numbers and Nusselt numbers at bubble and rigid spheroid surfaces are compared, as well as their change with the Reynolds and Schmidt numbers and geometric parameters.

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.

Numerical Analysis of Fluid Flows Inside and Around a Liquid Drop Using an Incorporation of Multi-Block Iteration and Moving Mesh

This paper is concerned with a novel procedure for calculating fluid flows in geometries with a time-dependent boundary and its application to a liquid drop problem. The procedure, which employs the SIMPLE algorithm as the solver, can incorporate a multi-block iteration with a moving mesh arrangement in the numerical modelling. For the liquid drop problem, the fluid flows inside and around the interface of the drop are studied numerically. To examine the robustness of the calculating procedure, three cases for different density and viscosity ratios are calculated; two types of returning wake vortex are observed. On this basis, a liquid drop in an industrial process of liquid spraying is studied. The structure of the wake vortex and its variation with exterior Reynolds number of a spherical drop are simulated.

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.

Micromixing in boiling and hot sparged systems: Development of a new reaction pair

The relevance of micromixing in fast reactions can be characterized by measuring the product distribution of a known test reaction. Although several test reaction systems have been published none is suitable for boiling or nearly boiling conditions. This paper presents details of a new test reaction system suitable for sparged aqueous systems at temperatures up to 100°C. This system, which is easy to use, provides the basis for experiments carried out in a standard semi-batch stirred tank reactor (STR) agitated with a Chemineer CD-6 hollow-blade impeller. The quantitative characterization provided by liquid micromixing sheds light on the effect of temperature, and hence vapour pressure, in the operation of gas–liquid reactors.