Critical Rotational Speed for a Floating Particle Suspension in an Aerated Vessel

The critical suspension speeds of floating particles in a gas‐liquid‐floating particle three‐phase system were measured in a multiple‐impeller agitated vessel. Three types of impellers, i.e., simple axial‐flow impeller upflow (SPU) and downflow (SPD), disk turbine (DT) and wing turbine (WT), twelve types of baffles and three kinds of gas spargers were used. The influences of impeller types, baffle configurations, gas spargers, gas superfacial velocity and particle loading on the critical suspension speeds of floating particles were systematically investigated. The optimum regressions of critical suspension speeds were respectively obtained for some better combinations of impellers, bafffles and spargers, such as (a) the 45SPU+45SPD+DT triple impellers, two high‐level baffles and two low‐level baffles (symmetric allocation), gas spargers, (b) the 45SPU+45SPD+DT three‐impeller, standard baffle and small gas sparger. Their errors were smaller than 11 %.     

Dispersion Characteristics of Gas‐Liquid Contactors Agitated by Single and Double‐Stage Rushton Turbines

The study of the loading/complete dispersion transition is of great importance especially in processes with enhanced mixing requirements. In the present work, new data and correlations concerning the dispersion characteristics in gas‐liquid contactors agitated by single and dual Rushton turbine systems are reported. The maximum amount of gas which can be completely dispersed, in the presence of gross, well defined recirculation patterns of gas at a given stirrer speed is predicted. Under these conditions, an increase of flow number with increasing Froude number could always be estimated. With decreasing impeller diameter, d, the same gas amount could be dispersed at higher stirrer speeds. At impeller spacing ΔH = 2 d, for d equal to 0.06 and 0.08 m, and ΔH = 1.54 d, for d = 0.10 m, the complete dispersion conditions of the dual impeller systems were slightly better than the corresponding conditions of the single impeller systems.     

Power Consumption in Gas‐Liquid Contactors Agitated by Double‐Stage Rushton Turbines

The dependence of power consumption on impeller spacing in unaerated and aerated gas‐liquid contactors agitated by dual Rushton turbine systems was studied, and the gas flow rate and viscosity effects were measured in relation to these parameters. The experiments were carried out in a 0.19 m i.d. vessel stirred by two Rushton turbines with a diameter d = 0.10 m; with blade length and blade height 0.25 d and 0.2 d, respectively. In tap water the impellers acted independently for spacings greater than 1.65 d, while in glycerol solutions the two impellers already acted independently at an impeller spacing equal to 1.2 d. In aerated systems, a notable increase in the power consumption with increasing impeller spacing could be detected for small gas flow rates and low viscosities, while a decrease in the Newton number with increasing Froude number could be observed at constant impeller spacing. The Newton number was not affected by flow number at high viscosity values.     

Regional Mean Bubble Diameters and Gas Holdup in Agitated Gas‐Liquid Contactors

Gas‐liquid contacting in mechanically agitated vessels is widely used in the process industry. Bubble size measurements at different flow regions in the vessel provide useful information for the mechanisms of gas dispersion and gas‐liquid flow. In this work, bubble size distributions and distributions of Sauter mean bubble diameters at four different regions in air‐water and air‐NaCl solution systems agitated by a six‐blade disk turbine are measured by using the photographic method. The effects of gassing rate, impeller speed and electrolyte presence in the system have been examined.     

搅拌釜中自浮颗粒悬浮分散的临界搅拌转速

在釜径386 mm的搅拌釜内,考察了搅拌桨型、挡板和气体分布器等对自浮颗粒三相搅拌混合的临界搅拌转速的影响,并回归了实用的关联式,这对工业过程的设计和放大具有一定的指导意义.     

Power Consumption in Agitated Vessels with Dual Rushton Turbines: Baffle Length and Impeller Spacing Effects

The dependence of power consumption on baffle length, L, in vessels agitated by a dual Rushton turbine system was studied within the turbulent regime, and also in relation to the impeller spacing, ΔH. A dependence of varying strength could be observed. The presence of baffles in the agitated systems provided a stabilizing effect with regard to the dependence of the Newton number Ne on the Reynolds number Re. A sharp decrease in power consumption could be detected for baffle lengths L < 0.3 H, with H the liquid height in the vessel. The Newton number was not significantly affected by L in the range 0.3 H < L< 0.5 H. For L > 0.5 H a sharp increase in Ne with increasing L could be observed. The two Rushton turbines act independently at ΔH > 1.65 d, with d being the impeller diameter. As the baffle length decreased, an increased mutual interaction between the two impellers could be observed for a broad regime of ΔH/d values. Ne was not affected by ΔH for the unbaffled agitated systems studied.     

Experimental Study of the Mixing Time in a Jet‐Mixed Gas‐Liquid System

In the present work, the impeller in the conventional gas‐liquid mixed vessels was replaced by a fluid jet as the mixer. Using an experimental setup, the effect of several parameters on the mixing time as a measure of the liquid‐phase mixing intensity, which is one of the required transport characteristics for designing gas‐liquid mixed systems, was studied. The results show that gas injection decreases the mixing time in comparison with the ungassed condition, but the mixing time is not necessarily decreased by increasing the gassing rate. On the basis of the amount of the jet Reynolds number and gassing rate, and thus the created circulation pattern, the mixing time may be decreased or increased. Also, the location of the probe for cases in which there are more dead zones in the vessel have a considerable effect on the measured mixing time. With increasing uniformity of the velocity domain, the influence of the probe location was reduced. Also, by increasing the jet flow rate and decreasing the nozzle diameter, the length of the jet, the amount of entrained bulk fluid, and the intensity of recirculation flow increased, and thus the mixing time decreased.     

Liquid Homogenization in Aerated Multi‐Impeller Stirred Vessel

The macroflow of fluid in a tall cylindrical vessel stirred with multiple stirrers was studied in the case of aeration of a liquid charge. The time of homogenization of the charge (mixing time) was calculated from the time dependency of the tracer concentration measured at various locations. Two types of stirrer were used in the experiments: six‐bladed Rushton turbines and/or pitched‐blade turbines with inclined blades pumping the liquid down or up. Four stirrers of the same type were located on the shaft. Other variables during the experiments were the stirrer frequency and the gas flow rate. It was found that the liquid macroflow in the vessel could be interpreted by the cell model or by the axial dispersion model for unaerated as well as for aerated systems. The influence of the aeration on the macroflow and mixing time was explained by the interaction of buoyancy and radial forces, and equations for the model parameters were proposed containing gas flow numbers and Froude numbers.     

Comparison of Experimental Techniques for the Measurement of Mixing Time in Gas‐Liquid Systems

Measurements of the homogenisation characteristics during the agitation of a liquid and the mixing time by simple in situ conductivity probes are very well established. However, unless special precautions are taken, in the presence of the second phase such as gas, the conductivity trace becomes distorted to a greater or lesser extent, so that it is not possible to follow the transient change of concentration in the liquid phase or estimate the mixing time. In this paper it is confirmed that, without special precautions, simple in situprobes are unsatisfactory. However, by shielding the probe with a “cage”, the ingress of bubbles into the probe region is essentially prevented and satisfactory results can be obtained in situ with responses having as little noise as in the case without gas. A second technique involves elimination of the gas from a small sample stream and measurement of the stream's conductivity transient. By suitable and rather simple treatment of the response, results equivalent to that from the in situ shielded probes can be obtained. The latter technique is especially useful where the placement of in situ probes is difficult. It is also suggested that recent results, which disagree with much of the literature on liquid phase mixing times in gassed systems, arose due to the use of in situ unshielded conductivity probes.     

Particle Holdup Profiles in Horizontal Gas‐liquid‐solid Multiphase Flow Pipeline

Sand holdup is one of the most important hydrodynamic parameters that is needed for performance estimation, design, operation and control of oil‐gas‐sand multiphase production and pipeline transportation systems. The performance of oil‐gas‐sand multiphase flow can be reliably evaluated by measuring the sand holdup in such oil‐gas‐sand multiphase production and pipeline transportation systems. In the present work, a local sand holdup has been measured under conditions analogous to the horizontal oil‐gas‐sand three‐phase slug flow in pipelines. Accurate local sand particle holdup measurements were performed by the digital imaging technique. The results revealed the influence of operating conditions such as gas and liquid velocities and sand particle loading on the distribution of the local sand particle holdup in the horizontal air‐water‐sand multiphase slug flow pipe. Explanations for the observed trends are provided, shedding light on the general structures and mechanisms of the distribution of the local sand holdup in a horizontal oil‐gas‐sand three‐phase slug flow. Such information on the horizontal air‐water‐sand three‐phase slug flow mechanisms are essential to advance the mechanistic approach for predicting local sand holdup distribution and the subsequent effect on sand deposition during multiphase petroleum production and transfer operations.     

Particle Flow Modelling in Slurry‐Fed Stirred Vessels

Despite its importance, experimental information on the Residence Time Distribution (RTD) of solid particles in continuous‐flow stirred vessels is still scant. In this work, experimental data on particle RTD in a high‐aspect‐ratio vessel stirred by three equally‐spaced Rushton turbines, obtained by means of Twin Systems Approach (TSA), are employed to assess the suitability of the well known Axial‐Dispersion Model to describe particle behavior in the investigated system. The data analysis and model parameter assessment are preceded by a discussion on the utility of self‐recirculated systems in carrying out experiments concerning continuous slurry‐fed apparatuses. In particular, the suitability of single recirculated systems is discussed and a way to extract numerical RTD data from the relevant experiments is proposed. The advantages and disadvantages of employing instead a couple of twin systems, as it was actually done to obtain the experimental data employed in this work, is shortly discussed.     

Mass Transfer Coefficients in Mechanically Agitated Gas‐Liquid Contactors

There are a large number of correlations given in literature for the prediction of volume‐related liquid‐side mass transfer coefficients in mechanically agitated gas‐liquid contactors. Significant disagreement can be observed concerning the proposed correlations, so that no single correlation exists representing all of the mass transfer data given in the literature. The observed differences can mainly be ascribed to the differences in the geometry of the system, the range of operational conditions and the measurement method used. On the basis of a comparative study of mass transfer phenomena in agitated Newtonian and non‐Newtonian aerated liquids, a critical discussion of the literature results is presented in this review article, so that final conclusions can be drawn for the k_Lα values in the different single‐ and multiple‐impeller agitated systems studied in the literature.     

Effect of Particle Image Velocimetry Setting Parameters on Local Velocity Measurements in an Agitated Vessel

Although they are obtained under the same conditions, results on the flow field in an agitated vessel achieved using particle image velocimetry (PIV) may vary due to differences in the PIV conditions. The influence on turbulence characteristics of the main PIV setting parameters, i.e., PIV spatial resolution, sampling frequency, and recording time, was investigated. Tests were performed with three different liquids in a developed turbulent field for a Rushton turbine impeller using two‐dimensional time‐resolved PIV. To obtain the relevant velocity gradients, a minimum recording time is needed. No effect of sampling frequency was observed if the sampling frequency was higher than approximately 17 times the impeller frequency, which is about three times the impeller blade frequency.     

A Novel Approach for the Determination of Mechanical Stresses in Gas‐Liquid Reactors

A new method for the determination of mechanical stresses in two‐phase reactors is described. The time‐dependent disintegration kinetics of a clay‐floc system are measured with a laser scanning microscope. By describing the flocs employing fractal geometry and by transforming the disintegration kinetics according to a multifractal‐approach for turbulent flow fields, effective stresses can be calculated for bubble columns in two‐phase operation mode by comparison to the mechanical stress in a turbulent single‐phase couette flow. Results are given for stresses measured in a bubble column at different operating conditions.     

Power Requirement when Mixing a Shear‐Thickening Fluid with a Helical Ribbon Impeller Type

The optimal design of close clearance impellers requires the knowledge of the power demand of the mixing equipment. In non‐Newtonian mixing, this can be readily obtained using the Metzner and Otto concept [1]. In this work, this concept and the determination of the K_s value for an atypical helical agitator (PARAVISC system from Ekato firm) have been revised in the case of shear‐thinning fluids and a shear‐thickening fluid. For poor shear‐thinning fluids, it has been shown that for our mixing system the K_s value does not vary strongly with the flow behavior index, and may be regarded as a constant for the mixing purpose design. By contrast, for the shear‐thickening fluid, power consumption measurements indicate that the relationship between the K_s values and the flow behavior index is much more complex due to a partial solidification of the product around the impeller.     

Mixing and Crystallization Kinetics in Gas‐liquid Reactive Crystallization

A study of gas‐liquid reactive crystallization for CO₂‐BaCl₂‐H₂O system was performed in a continuous flow crystallizer. The influences of mixing on the crystallization kinetics of barium carbonate crystals were investigated. The mixing parameters are stirrer speed, feed concentration, gas‐flow rate, pH of solution, addition rate of NaOH solution, and mean residence time. Under pH‐stat operation, the crystallization mechanism can be assessed by the addition rate of NaOH solution, which acts as an indicator for the absorption rate of carbon dioxide. Assuming a size‐independent agglomeration mechanism, the nucleation rate, growth rate and agglomeration kernel can be obtained, simultaneously, at steady state, by the method of moments. Evidence shows that feed concentration, feed rate, gas‐flow rate, and stirrer speed have a significant influence on the nucleation rates and mean particle sizes. This shows the effect of micromixing. The crystallization mechanism tends to be reaction limited when the feed concentration of barium chloride solution is higher than 5 mM, while at lower stirrer speeds and feed concentrations, the mechanism tends to be both mixing and reaction controlled. The growth rate depends on the mean supersaturation value and the pH of the solution and the mass‐transfer resistance cannot be completely eliminated in this work. For a monodispersal collision model, in the viscous sub‐range of turbulence, the agglomeration kernel can be expressed as β ∝ d^{3 –1/4}, showing a low efficiency of collision. The result is also demonstrated by the agglomeration kernel expression. Comparison with a liquid‐liquid‐mixing reactive crystallization system is also discussed.     

Unsteady Mixing Characteristics in a Vessel with Forward‐Reverse Rotating Impeller

Usually, mixing is carried out in a vessel with four baffles and a single impeller. In some applications, however, the use of a baffled vessel is not recommended. One of the stirring methods used instead is unsteady agitation with forward‐reverse rotating impellers. The aim of this work was to characterize the agitation characteristics in a baffled and an unbaffled vessel with a turbine impeller. Mixing time and mixing power were evaluated in relation to the presence of baffles and the frequency of forward‐reverse rotation. It was found that the frequency of oscillation does not affect either the mixing time and mixing power values or the drag and added mass coefficients. Power requirements and mixing time were higher compared to the steady mixing conditions in a baffled vessel. The results showed that it is not recommended to use baffles because they have no influence on unsteady mixing.     

Engineering Model of a Nonisobaric Heavily Laden Gas‐Particle Jet

An axisymmetrical model of a rapid nonisobaric heavily laden gas‐particle jet is developed. The model is based on the general functions for both axial and radial pressure distributions, which were discovered by a numerical study of rapid gas‐particle jets by the CFD code. The model equations were solved analytically. The results obtained by the model developed are in good agreement with those computed by the CFD code for a wide range of flow parameters.     

A Non‐Conventional Approach for the Make‐Down of Concentrated Suspensions

Concentrated suspensions made of delaminated kaolin clay in water were prepared using two approaches, namely steady stirring and dynamic perturbation operating modes. In the first approach, the impeller was rotated at constant speed in one direction. For the second approach, the flow into the tank was dynamically perturbed by rotating the impeller cyclically in both directions. Two high‐shear impellers were used: a Sevin impeller and a Cowles turbine. During the mixing, the evolution of torque was measured and samples were taken at regular intervals. All the results were compared in terms of the mixing energy consumption, particle average size and distribution, as well as the shear rheology. Suspensions prepared with the dynamic perturbation approach were found to require less energy and exhibited, in general, smaller particles and lower viscosity than with conventional mixing, which indicates that the proposed method could be an efficient alternative to conventional mixing.