A jet mixing study in two phase gas-liquid systems

All studies concerned with jet mixing have been focused on liquid phase systems and no studies have been found on jet mixing for gas-liquid two phase systems. In the present study the use of jet fluid as a mixer in gas-liquid systems was proposed. Further by installing an experimental setup, the mixing behavior of liquid phase was studied. Gas flow and jet flow are injected to the mixing vessel countercurrently. In this study, the effect of jet injection, location of the conductivity probe, aeration rate and jet Reynolds number on the mixing time are investigated. The created flow pattern was extracted for each condition and the results often analyzed on the basis of them. It is observed that, for low aeration rates, the injection of jet decreases the mixing time considerably. By increasing the aeration rate, the difference in mixing times between the two cases of jet injection and without jet is reduced. Results also show that the closer the probe is to encounter location of the jet and airflow, the lower the mixing time obtained. Dependence of mixing time on the probe location decreases by increasing the mixing intensity and eliminating dead zones. It is obtained, on the basis of Re_j and the amount of jet travelling in the vessel, increasing the aeration rate has different effects on the performance of mixing. Generally, four different trends for the variation of mixing time with increasing the aeration rate are observed.     

Critical impeller speed for solid suspension in gas inducing type mechanically agitated contactors

Critical impeller speed for solid suspension (N_CS), has been measured in gas inducing mechanically agitated contactors (GIMAC) fitted with two impellers. Five different impeller designs were employed. It was found that the multiple impeller system having both pitched blade downflow turbines was the most energy efficient for the suspension of solid particles. The interimpeller clearance (C₃), and the impeller bottom clearance (C₁) were found to be important parameters. Further, for this optimum combination, the design of the lower impeller was optimized in terms of impeller diameter (D), blade width (W), blade angle (A), and the number of blades (n_b). An empirical model has been proposed based on the mechanism of fluidization.     

Critical impeller speed for solid suspension in multi-impeller three phase agitated contactors

The critical impeller speed for solid suspension in gas-liquid-solid systems has been measured in multi-impeller agitated contractors of 0.15 and 0.30 m and ID and 1.0 m height. Three types of impellers, i.e. disk turbine (DT), pitched turbine downflow (PTD) and pitched turbine upflow (PTU) were used. Air, deionised water and sand particles were used as the gas, liquid and solid phases, respectively. The superficial gas velocity and solid loading were varied in the ranges 0–15 mm/s and 0.5 to 10% w/w, respectively. The effects of impeller type and its diameter, particle size and loading and gas flow rate were studied. Some measurements of gas hold-up and mixing time were also made in order to get some insight of the hydrodynamic behaviour of the reactor. The critical impeller speed for solid suspension in the presence of gas (n_isg) was found to be more than that in the absence of the gas and the increase of critical speed correlated well with the gas flow rate. The influence of particle—liquid parameters on solid suspension speed in the gassed system was similar to but relatively weaker than that in the ungassed condition.     

Gas hold-up, mixing time and gas-liquid volumetric mass transfer coefficient of various multiple-impeller configurations: Rushton turbine, pitched blade and techmix impeller and their combinations

Gas hold-up, mixing intensity of dispersion characterised by exchange flows between adjacent impellers and a volumetric mass transfer coefficient are presented for 18 impeller configurations in triple-impeller vessel of inner diameter . Rushton Turbines, six Pitched Blade impellers pumping down and hydrofoil impellers Techmix 335 (Techmix co., Czech republic) pumping up or down and their combinations were used. aqueous solution was used as a liquid phase, which represents non-coalescent batches. Gas hold-ups and volumetric mass transfer coefficients are presented for individual configurations as functions of specific power dissipated and superficial gas velocity. The regression of the mass transfer coefficients shows large standard deviation (30%). The power number included to the regression to express the impeller configuration effect did not improve the standard deviation significantly (23%). The impeller configurations with low power number (less than unity) provide higher dispersion mixing intensities, while the impeller configurations with high power number provide better mass transfer performance.     

Flow regimes for adiabatic gas-liquid flow in microchannels

The patterns forming during adiabatic gas-liquid flows in single microchannels are reviewed and the parameters influencing pattern transitions are discussed. Six major patterns were identified: the surface tension dominated bubbly and Taylor flows, the transitional churn and Taylor-annular flows, and the inertia dominated dispersed and annular flows. From the various parameters that have been studied in the literature, channel size, phase superficial velocities, liquid phase surface tension, wall wettability and inlet conditions were found to affect the flow pattern formed while channel cross sectional geometry affected the patterns but to a lesser degree. Liquid viscosity and flow orientation with respect to gravity also seemed to play some role but the results were not conclusive. A universal flow regime map does not seem to exist and this is attributed to a lack of consistency in the inlets used in the various studies as well as to the effects of wall properties, such as wettability, contamination and roughness which are not usually varied systematically or reported. From the different flow regime maps suggested, those using U_GS-U_LS as coordinates represented better the transitions between patterns.     

A PIV study of hydrodynamics in gas-liquid high throughput experimentation (HTE) reactors with eccentric impeller configurations

High-throughput experimentation (HTE) is used for the screening of novel catalyst formulations via miniature agitated gas-liquid reactors with vessel volumes of an order of magnitude below the laboratory scale. An investigation of the macroscopic hydrodynamic performance of a miniaturised unbaffled stirred vessel of diameter 45 mm for an air-water system has been carried out at gassing rates of 0.25 and 0.5 vvm (dispersed flow regime) using particle image velocimetry (PIV). Measurements of bubble size and gas hold-up were also made. Eccentric agitation was employed as a means of breaking solid body rotation within the vessel, using a 6-blade, up-pumping pitched blade turbine. The gas phase was introduced via sintered glass panel spargers mounted in the vessel base. Two different configurations were used: firstly where the sparger located directly below the impeller, and secondly where the sparger was located opposite the impeller on the other side of the vessel. Measured distributions of turbulent kinetic energy (TKE) for both gassed configurations were similar and showed significant differences to the ungassed case. Estimates of the gas-liquid interfacial area were at least 2-3 times higher than those found in conventional, lab-scale baffled vessels; this was attributed to the creation of very small bubbles at the sparger ().     

An experimental investigation of gas-liquid two-phase flow in single microchannel contactors

This paper describes two-phase flow pattern and pressure drop characteristics during the absorption of CO₂ into water in three horizontal microchannel contactors which consist of Y-type rectangular microchannels having hydraulic diameters of 667, 400 and , respectively. With the help of a high-speed photography system, flow patterns such as bubbly flow, slug flow (including two sub-regimes, Taylor flow and unstable slug flow), slug-annular flow, churn flow and annular flow were observed in these microchannels. The applicability of the currently available correlations for describing flow pattern transitions in microchannels has been examined. Generally, the predicting performance of these correlations deteriorates as the channel diameter further reduces. Toward solving this discrepancy, an empirical correlation based on the superficial Weber numbers was developed to interpret the transition from Taylor flow to unstable slug flow in three microchannels. Taylor bubble formation process in microchannels was found to be in the squeezing regime at lower superficial liquid velocities (Ca ranging from 0.0019 to 0.029) while the transition to the dripping regime was observed at the highest superficial liquid velocity of 1.0 m/s. Lengths of Taylor bubbles formed in the squeezing regime can be well represented by the scaling relation proposed by Garstecki et al. [Formation of droplets and bubbles in a microfluidic T-junction—scaling and mechanism of break-up. Lab on a Chip, 6, 437-446]. For flow patterns including slug-annular flow, annular flow and churn flow, a simple analysis based on the separated flow model has been performed in order to reveal the observed effect of the superficial liquid velocity on two-phase frictional multiplier in the present microchannels. Then, reasonable correlations for the prediction of two-phase frictional pressure drop under these flow patterns were suggested.     

Design of multiple impeller stirred tanks for the mixing of highly viscous fluids using CFD

The effect of multiple Intermig impeller configuration on hydrodynamics and mixing performance in a stirred tank has been investigated using computational fluid dynamics. Connection between impeller stages and compartmentalisation has been assessed using Lagrangian particle tracking. The results show that by a rotating the Intermig impeller by 45^° with respect to its neighbours, instead of a 90^° rotation as recommended by manufacturers, enables a wider range of operating conditions, i.e., lower Reynolds number flows, can be handled. Furthermore by slightly decreasing the distance between the lower two impellers, fluid exchange between the impellers is ensured down to Re=27.     

运用粒子图像测速仪研究双层Rushton桨搅拌槽内湍流特性

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 liq- uid 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 lo- cations 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.     

Hydrodynamics of gas-liquid counter-current flow in solid foam packings

Solid foam materials combine high voidage and high surface area. These two properties are advantageous for use in chemical reactors due to the low frictional pressure drop and relatively high surface area that may be used for catalyst deposition. Hydrodynamic parameters such as liquid holdup, pressure drop, and flow regimes similar to those for packed beds, have been obtained for the gas and liquid flows through these solid foam packings. The open-celled solid foam packings used were in the range of 5-40 pores per linear inch (ppi). The regimes studied are two high liquid holdup regimes and a low liquid holdup regime (trickle flow regime). Also the flooding points for counter-current flow have been determined.     

Effect of the attack angle on the roll and trailing vortex structures in an agitated vessel with a paddle impeller

The purpose of the present study is to observe the effect of the blade attack angle on the roll and trailing vortex structures in a stirred vessel via laser-Doppler velocimetry (LDV). In this investigation, four-bladed paddle impellers with four attack angles, which were 45^°, 60^°, 75^° and 90^°, respectively, were used. By synchronizing LDV with a rotary encoder coupled to the impeller shaft, angle-resolved measurements of all three velocity components were performed. This experimental method made it possible to capture the details of the vortical structure both behind the impeller blade and discharge region. Our study on the mean flow structure generated by three types of pitched blade turbines (45^°, 60^°, and 75^°, respectively) found that a single trailing vortex was formed around each turbine blade. Roll-up of the vortex sheet issuing from the blade tip was also observed, which indicated a major roll of trailing vortex generation mechanism for each pitched blade turbine.     

Experimental study on geometric structure of isolated mixing region in impeller agitated vessel

Isolated mixing region in agitated vessel with rotated two-bladed paddle impeller and no baffle was visualized experimentally and its structural property was investigated in detail. A set of thin filaments spirally wrapping around the core of the toroidal isolated mixing region is observed under low Reynolds number conditions, which is smaller than 60. Three-dimensional geometrical structure of filament in isolated mixing region depends on the periodical perturbations caused by the rotating impeller. We have succeeded in the determination of three-dimensional geometrical structure of filament in isolated mixing region based on relation between the movement of fluid particle and filament numbers and/or wire turns. Interestingly, the wire turns of filaments are opposite to movements of fluid particles.     

Characterization on the hydrodynamics of a covering-plate Rushton impeller

A modified Rushton impeller with two circular covering-plates mounted on the upper and lower sides of the blades was designed. There are gaps between the plates and the blades. The turbulent hydrodynamics was analyzed by the computational fluid dynamics (CFD) method. Firstly, the reliability of the numerical model and simulation method was verified by comparing with the experimental results from literature. Subsequently, the power consumption, flow pattern, mean velocity and mixing time of the covering-plate Rushton impeller (RT-C) were studied and compared with the standard Rushton impeller (RT) operated under the same conditions. Results show that the power consumption can be decreased about 18%. Compared with the almost unchanged flow field in the lower stirred tank, the mean velocity was increased at the upper half of the stirred tank. And in the impeller region, the mean axial and radial velocities were increased, the mean tangential velocity was decreased. In addition, the average mixing time of RT-C was shortened about 4.14% than the counterpart of RT. The conclusions obtained here indicated that RT-C has a more effective mixing performance and it can be used as an alternative of RT in the process industries.     

Gas recirculation rate and its influences on mass transfer in multiple impeller systems with various impeller combinations

Both the numerical and experimental approaches were used to study the effects of the gas recirculation and non‐uniform gas loading on the mass transfer rate for each impeller in a multiple impeller system. By combining the calculated gas velocity and local gas holdup, the gas recirculation rate around each impeller was estimated. The local mass transfer coefficients for systems equipped with various combinations of the Rushton turbine impeller (R) and pitched blade impeller (P) were determined by using the dynamic gassing out method. It is found that the Rushton turbine impeller has to be served as the lowest impeller in order to have a better gas dispersion and to give a higher overall K_La for a multiple impeller gas‐liquid contactor. The upper pitched blade impeller always enforces the circulating flow around the Rushton turbine impeller just beneath it and gives a higher overall average mass transfer rate. However, the system equipped with only the pitched blade impellers results in a much lower mass transfer rate than the other systems owing to the poor gas dispersion performance of the pitched blade impeller.     

Dispersion coefficient and settling velocity of the solids in agitated slurry reactors stirred with multiple rushton turbines

The feature of solids distribution in tanks stirred with multiple Rushton turbines was investigated. Both transient and steady-state experiments were performed in tanks of two scales with a variety of suspensions. The data were analysed with the axial sedimentation-dispersion model. The axial dispersion coefficient of the solid phase was found not to differ from that of the liquid by more than 20%. The effective particle settling velocity in the stirred medium was then determined. It is confirmed that this parameter is different from the terminal settling velocity. Their ratio exhibits the same dependence on Kolmogoroff microscale and particle size as obtained previously with an indirect, approximate approach.     

假塑性流体在双螺带桨搅拌釜内停留时间分布

采用电导率法研究了羧甲基纤维素水溶液假塑性流体在双螺带搅拌釜内的停留时间分布(RTD)情况,考察了物料黏度、流量、搅拌转速和转向对停留时间分布的影响.发现假塑性流体在釜内的停留时间分布类似于全混釜,但有一定的拖尾现象.物料黏度对平均停留时间和当量全混釜数影响不大;随着流量增大,平均停留时间减小,而当量全混釜数基本不变;...     

Validation of bubble breakage, coalescence and mass transfer models for gas-liquid dispersion in agitated vessel

Bubble breakage and coalescence phenomena and multicomponent gas-liquid mass transfer were studied in a Rushton turbine agitated vessel. Local bubble size distributions (BSD) were measured from air-tap water system at several agitation conditions with capillary suction probe (CSP) technique. The CSP was compared to the digital imaging (DI) and phase Doppler anemometry (PDA) techniques in a stirred vessel. The volumetric BSDs between the CSP and DI were in agreement, but number BSDs showed notable deviation. The limitations of measurement techniques seem to be the main reason.A multiblock stirred tank model with discretized population balances for bubbles and two-film Maxwell-Stefan multicomponent mass transfer between gas and liquid was created for the agitated vessel. The model considers local mass transfer conditions in the vessel and is simple enough for the mathematical optimization of unknown model parameters. Unknown parameters in the mechanistic bubble breakage and coalescence models were fitted against measured local BSDs. After this, a parameter in the liquid film mass transfer correlation was adjusted against absorption and desorption experiments of oxygen. Local gas-liquid mass transfer areas were calculated from the population balance model. The simulations with the validated models show good agreement against experiments. On the other hand, the fitted parameters deviate from the theoretical values, which emphasizes the need of model validation against accurate experiments. Due to their fundamental character and the validation process, the fitted models seem to be useful tools for the design and scale-up of agitated gas-liquid reactors.     

Mixing Time Studies in Multiple Impeller Agitated Reactors

Mixing time studies have been carried in a 0.3m diameter and 0.9m tall vessel equipped with three impellers. Conductivity measurement technique has been used for the measurements of mixing time. Effect of the various parameters i.e. tracer density, tracer volume, speed of rotation and impeller combination on mixing time has been studied for two impeller combinations used viz. PTD‐PTD‐PTD and PTD‐PTD‐DT. A compartment model (with one fitted parameter, the exchange flow rate Q_E) with single compartment per agitation stage has been used to predict the conductivity response and the exchange coefficients are calculated from the model parameter. An attempt has been made to explain the experimental results on the basis of the liquid phase axial dispersion coefficient and cell residence time, calculated from the model parameter Q_E     

Investigation of crystal growth of borax in single and dual impeller batch cooling crystallizer

Abstract

The purpose of this work was to investigate and compare the influence of fluid flow in a single and dual impeller batch cooling crystallizer on crystal growth kinetics of borax decahydrate. Examinations were conducted in a crystallizer of 15?dm³ stirred by a single pitched blade turbine and straight blade turbine as well as their dual configurations. Kinetics parameters of crystal growth determined at applied mixing conditions were correlated with hydrodynamic conditions. In this paper, hydrodynamics was characterized by mixing time, which was experimentally determined, and by fluid flow patterns, which were simulated by the means of computational fluid dynamic (CFD). It was found that although the crystal growth in all systems investigated was controlled by the integration mechanism, the crystal growth rate constant changed significantly with impeller configuration. Regarding the characteristics of the final product, a dependence of the crystal size distribution on the fluid flow pattern was noticed while the number of impellers did not affect the product properties. On the other hand, mixing efficiency differed significantly with the type and number of impellers.     

气液反应双膜论吸收速率及计算应用

根据双膜理论 ,分析了气液反应的吸收机理。提出气液反应的吸收速率的两种计算形式 :( 1)界面式RA=rAiδi-DAldCAdx|x =0 ,该式适用于任意速率的计算 ;( 2 )一般式RA=∫x0 rAdx-DAldCAdx｜x =x ,该式计算过程较繁琐 ,但对反应器设计 ,尤其是对带有反应的气液吸收过程的计算较为适用