CAVERN SIZES IN AGITATED FLUIDS WITH A YIELD STRESS

Highly viscous, non-Newtonian Xanthan gum solutions and two transparent model fluids with similar Theological properties have been studied under aerated (up to 1 vvm) and unaerated conditions in a 0.29m diameter agitated vessel. Rushton disc turbines of size 1/3 and 1/2 of the tank diameter have been used alone and also in conjunction with 6-bladed, 45°-pitch axial flow turbines of the same size at speeds up lo 24 rev/s, enabling specific power inputs of up to 15 W/kg to be imparted.

Flow patterns were studied by flow visualisation and hot film anemometry. When the fluids have a yield stress, the fluid divides into a turbulent well-mixed cavern which increases in size with increasing speed with the remainder stagnant. A model for the size of the cavern fits the experimental data well for both aerated and unaerated mixing. Large diameter combinations produce good mixing at about 1 to 2 W/kg which is about 1/3 to 1/4 of that required with small diameter combinations. Single disc turbine impellers are unsatisfactory.     

RAPID SPECTROSCOPIC ASSESSMENT OF REACTION RATES IN PHASE-TRANSFER-CATALYSIS

Power for agitation has been measured under aerated and unaerated conditions in a 0.29 m vessel of Rushton dimensions at specific powers up to 18 W/kg. The fluids studied were water. Newtonian solutions up to 19 m Pas and non-Newtonian shear thinning fluids some of which also exhibited a yield stress and some of which were viscoelastic. For the unaerated case, the power number-Reynolds number plot is in good agreement with the literature. For the aerated case, the result can conveniently be divided into three Reynolds number regions. At Re > ～900, the data obtained for the solutions is not markedly different to that for water except that a higher impeller speed is required to achieve complete gas dispersion. For ～ 10 < Re < ～900. the power drawn is independent of gassing rate and greater levels of elasticity and the presence of a yield stress leads to the lowest power numbers. For Re < ～10, the gassed and ungassed power numbers are equal.     

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.     

A comparison of cavern development in mixing a yield stress fluid by rushton and intermig impellers

Intermig impellers have been postulated as very efficient for mixing highly viscous non-Newtonian fluids (such as xanthan and mycelial broths). However, no formal characterisation has been published and no fair comparisons have been made, based on accurate power drawn measurements and using equal number of impeller stages and equal diameter, if compared (for example) with the performance of Rushton turbines. Characterisation of the shape, size, and evolution of the well-mixed zones or “caverns” were correlated with power drawn, for single and dual Rushton turbines and for one- as well as two-stage Intermig unslotted impellers. Cavern evolution studies were carried out in a mixing tank (diameter=0.205 m, H/T=1.6) equipped with an accurate air bearing dynamometer. Carbopol 940 (0.25 wt.-%) was used as a model, transparent fluid. Impeller to tank diameter ratio was 0.53 for both impellers. Caverns were visualised by injecting methylene blue in the well-mixed zones. A single Rushton turbine developed larger caverns if compared with one-stage Intermig of the same diameter under power drawn below 1.5 kW m^?3. At higher power drawn, both impellers behaved very similarly, reaching a limit in cavern volume of about 40% of the total liquid volume, even at very high (20 kW m⁰³) power drawn. A similar trend characterised dual combinations: below 3 kW m^?3, dual Rushtons gave larger cavern volume if compared with the performance of two-stage Intermigs. In either case, power drawn higher than 3 kW m⁰³ was sufficient to mix more than 90% of the liquid volume. The presence or absence of the slot in the Intermig did not influence cavern development. Experiments with a smaller if compared with those obtained with the larger Intermig (D/T = 0.53).     

Gas—liquid mixing studies with multiple up- and down-pumping hydrofoil impellers: Power characteristics and mixing time

Axial flow impellers with hydrofoil blades, APV-B2, i. e., B2-30 and B2-45 (30° and 45° angle of attack of blades) have recently been designed, mainly for use in the up-pumping mode, and some new results are reported here for single, dual and triple impeller configurations in the turbulent flow regime. The results are compared with dual Lightnin' A-315's pumping downwards as they have traditionally been used and with dual radial flow Rushton turbines. The ungassed power numbers of the APV-B2's are low, allowing these impellers to be used at large impeller-to-tank diameter ratios. The drop in P_g/P is lower with all combinations of APV-B2 impellers, i. e., for single, dual and triple impellers when compared to other configurations tested. Torque fluctuations are less than those with single or dual down pumping axial hydrofoils. Mixing times measured using a decolourisation technique for dual and triple impeller configurations of APV-B2 under unaerated and aerated conditions showed that the mixing times are much shorter than those for the dual Rushton turbine and similar to those found for dual down-pumping A-315's. Detailed mixing characteristics and flow patterns for the dual and triple APV-B2's are also discussed.     

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.     

Liquid phase mixing model for stirred gas-liquid contactor

Impulse response experiments were carried out on the liquid phase of a continuous flow air/water system contained in a 22 cm diameter vessel stirred with a 7.62 cm diameter six-bladed disc turbine impeller. Experimental results were obtained for three impeller speeds, ten air rates and four inlet and outlet combinations of top and bottom.The application of a numerical procedure for Laplace transform inversion and an optimisation routine for curve fitting, allowed a consideration of models of considerable complexity. A digital computer was used to determine the appropriate values of the parameters of the most suitable model which satisfied the experimental responses.The effects of the impeller speed and air flow rate on the values of the various parameters of the model were examined. Good agreement was found between the values of the parameters and similar published data for the unaerated system. Also it was shown that the presence of the gas does not greatly affect the liquid phase mixing characteristics.     

Studies in multiple impeller agitated gas-liquid contactors

Experiments have been performed to study the effect of the density and the volume of the tracer pulse on the mixing time for two impeller combinations in the presence of gas in a 0.3 m diameter and 1 m tall cylindrical acrylic vessel. The tall multi-impeller aerobic fermenters, which require periodic dosing of nutrients that are in the form of aqueous solution, is a classic case under consideration. Conductivity measuring method was used to measure the mixing time. Two triple impeller combinations; one containing two pitched blade downflow turbines as upper impellers and disc turbine as the lowermost impeller (2 PBTD-DT) and another containing all pitched blade downflow turbines (3 PBTD) have been used. Other variables covered during experiments were the density and the amount of the tracer pulse, the impeller rotational speed and the gas superficial velocity. Fractional gas hold-up, Power consumption and mass transfer coefficient have also been measured for both the impeller combinations. Influence of aeration and impeller speed on the mixing time has been explained by the interaction of air induced and impeller generated liquid flows. Three different flow regimes have been distinguished to explain the hydrodynamics of the overall vessel (i.e., multiple impeller system). A compartment model with the number of compartments varying with the flow regimes have been used to model liquid phase mixing in these flow regimes. A correlation for the prediction of the dimensionless mixing time in the loading regime has been proposed in order to account the effect of the density and the amount of the tracer pulse on the mixing time. Correlations have also been proposed to predict fractional gas hold-up and k_La.     

New techniques for measuring and modeling cavern dimensions in a Bingham plastic fluid

A model for predicting the mixing cavern dimensions for Bingham plastic fluids based on the assumption of equal torque is developed. Experimental data has been collected for the purpose of verifying this model, using a novel technique, for both axial and radial flow impellers in a tank of Heinz ketchup. The mixing caverns were found to be the shape of an elliptical torus. The ellipse aspect ratios were determined for both impeller types and are assumed to be constant. The model was able to predict the cavern diameter and cavern height within the experimental uncertainty.     

Measurement of gas and liquid flows in stirred tank reactors with multiple agitators

The gas flow in a 3:1 aspect ratio vessel agitated by triple Rushton turbines has been measured by an ultrasound Doppler probe and by means of residence time studies. Strong recirculation around each impeller is found which fits in well with the compartmentalisation found in earlier liquid mixing studies. Surprisingly, when two axial A315 impellers above a Rushton turbine were used, gas recirculation around each impeller was still found. Study of the liquid phase mixing by a decolourisation technique confirmed that the gas flow essentially destroyed the strong axial liquid flow expected. Indeed, even under unaerated conditions, compartmentalisation was found between each impeller.     

Flow Compartments in Viscoelastic Fluids Using Radial Impellers in Stirred Tanks

The influence of viscoelastic flow properties on fluid dynamics using radial impellers is investigated. The use of transparent model fluids allows for the optical measurement of general flow behavior with a fluorescence dying technique. By varying viscoelastic flow properties, size of agitators and rotational frequency, the impact of these parameters on fluid dynamics is analyzed. Toroidally shaped, cavern‐like flow compartments form around the agitators in all fluids in specific rotational frequency ranges, preventing an efficient mixing. By balancing elastic with centrifugal forces, a simple model is developed with which compartment sizes can be predicted with good accuracy. The results indicate a good suitability of the elasticity number as a scale‐up criterion.     

双层搅拌器组合的气液分散性能研究

系统研究和比较了径流桨和径流桨组合、径流桨和斜叶桨组合以及斜叶桨和斜叶桨组合3类不同的双层搅拌器组合,在气液分散搅拌过程中的优劣。小通气量时径流桨和斜叶桨组合(DT PTD和PTU DT)在相同的单位体积搅拌功率下气含率最高,而在大通气量时,双层上翻式斜桨组合(PTU PTU)气含率最高,并发现大通气量时,下层桨不宜采用下压式斜叶桨。     

Cavern formation in pulp suspensions using side-entering axial-flow impellers

Pulp fibre suspensions display non-Newtonian rheology, including a yield stress. In mixing operations, this creates regions of active motion around the impellers with the cavern size affecting the quality of mixing attained. Due to the opacity of the suspensions, two non-invasive techniques were evaluated for determining cavern dimensions: electrical resistance tomography (ERT) and ultrasonic Doppler velocimetry (UDV), with ERT chosen for most tests due to the speed of data acquisition. Cavern volume as a function of impeller speed is reported for a range of mixing conditions (hardwood and softwood pulp, suspension mass concentrations from one to five percent, two impeller offsets from the wall, and three suspension height-to-chest diameter ratios). A scaled version of a commercial axial flow impeller was used in a standard side-entering configuration. Measured cavern diameters were compared against model predictions available in the literature. The discrepancy between experimental data and model predictions were significant and were attributed to interaction between the developing cavern and the vessel walls. An alternative model was developed for predicting cavern volume taking this interaction into account.     

Viscoelastic Liquids in Stirred Vessels – Part I: Power Consumption in Unaerated Vessels

Different shear‐thinning and elastic fluids (STE fluids) have been stirred under unaerated conditions, in vessels equipped with Rushton disc turbines. Their power consumption has been evaluated over a wide range of stirring rates and their Metzner‐Otto constant (k_s) has been measured. A correlation has then been proposed to predict k_s values for a Rushton turbine operating in non‐Newtonian solutions. Power curves of STE fluids have been drawn and compared with reference curves (Newtonian, shear‐thinning inelastic and elastic with constant shear viscosity fluids). The STE fluids have thus been divided into two categories. The STE fluids of the first category (STE I fluids), which are concentrated viscous solutions of polymers (guar, CMC) reducing the power consumption at the beginning of the transitional region and connecting with the Newtonian reference at higher Reynolds numbers. In contrast, STE solutions of the second category (STE II fluids), which are solutions of drag reducing polymers (PAA), are less viscous and more elastic. They reduce the power consumption at the end of the transitional region and do not connect with the Newtonian reference, at least until Re = 6000. A general correlation has finally been proposed to model the power curve of STE fluids stirred by a Rushton turbine from the laminar to the turbulent regions, as a function of their elasticity.     

Using computational fluid dynamics modeling to study the mixing of pseudoplastic fluids with a Scaba 6SRGT impeller

The 3D flow field generated by a Scaba 6SRGT impeller in the agitation of xanthan gum, a pseudoplastic fluid with yield stress, was simulated using the commercial CFD package. The flow was modeled as laminar and a multiple reference frame (MRF) approach was used to solve the discretized equations of motion. The velocity profiles predicted by the simulation agreed well with those measured using ultrasonic Doppler velocimetry, a non-invasive fluid flow measurement technique for opaque systems. Using computed velocity profiles across the impeller, the effect of fluid rheology on the impeller flow number was investigated. The validated CFD model provided useful information regarding the formation of cavern around the impeller in the mixing of yield stress fluids and the size of cavern predicted by the CFD model was in good agreement with that calculated using Elson's model.     

Tomography images to analyze the deformation of the cavern in the continuous‐flow mixing of non‐Newtonian fluids

Tomography, an efficient nonintrusive technique, was employed to visualize the flow in continuous‐flow mixing and to measure the cavern volume (V_c) in batch mixing. This study has demonstrated an efficient method for flow visualization in the continuous‐flow mixing of opaque fluids using two‐dimensional (2‐D) and 3‐D tomograms. The main objective of this study was to explore the effects of four inlet‐outlet configurations, fluid rheology (0.5–1.5% xanthan gum concentration), high‐velocity jet (0.317–1.660 m s^?1), and feed flow rate (5.3 × 10^?5?2.36 × 10^?4 m³ s^?1) on the deformation of the cavern. Dynamic tests were also performed to estimate the fully mixed volume (V_{fully mixed}) for the RT, A310, and 3AM impellers in a continuous‐flow mixing system, and it was found that V_{fully mixed} was greater than V_c. Incorporating the findings of this study into the design criteria will minimize the extent of nonideal flows in the continuous‐flow mixing of complex fluids and eventually improve the quality of end‐products. © 2013 American Institute of Chemical Engineers AIChE J, 60: 315–331, 2014     

LIQUID PHASE MIXING IN MECHANICALLY AGITATED VESSELS

Liquid phase mixing and power consumption have been studied in 0.3, 0.57, 1.0 and 1.5 m i.d. mechanically agitated contactors. Tap water was used as liquid phase. The impeller speed was varied in the range 2-13.33 r/s. Three types of impellers namely disc turbine (DT), pitched turbine downflow (PTD) and pitched turbine upflow (PTU) were employed. The impeller diameter to vessel diameter ratio was varied in the range of 0.25 to 0.58. The effect of impeller clearance from tank bottom was also studied. Mixing time was measured using the transient conductivity measurement.

The PTD impeller was found to be the most energy efficient for mixing in liquid phase alone. Further, PTD (T/3) was found to be most energy efficient as compared with other impeller diameters. The effect of clearance was found to be design dependent and it was found to be diameter dependent in the case of pitched turbines.

Flow patterns of different impellers have been studied by visual observations (using guide particles). These observations were supported by the measurements using Laser Doppler Velocimetry. A model has been developed for the prediction of mixing time. In the case of all the three impeller designs, a fairly good agreement was found between the predicted and experimental values of mixing time.     

叶片形状对涡轮桨搅拌槽内尾涡特性的影响

Particle image velocimetry technique was used to analyze the trailing vortices and elucidate their rela-tionship with turbulence properties in a stirred tank of 0.48 m diameter,agitated by four different disc turbines,in-cluding 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 λ2-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 ki-netic 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 de-sign and scale-up of stirred tank with disc turbines.     

氨碱法蒸馏系统300 m3预灰桶搅拌桨流场数值模拟分析

氨气蒸馏是氨碱法生产纯碱的重要工序，其中预处理设备预灰桶的搅拌混合效果会直接影响蒸氨效率。因此，系统研究预灰桶内的流场分布，对纯碱工业的发展具有一定现实意义。基于流体力学对预灰桶内流体流动情况进行了计算，分析了工业常用吊链式搅拌桨预灰桶内流场分布。吊链式搅拌中流体以切向流为主，混合效果差。进一步对预灰桶搅拌桨叶进行了选型，分析了斜叶桨、六斜叶圆盘涡轮桨和六直叶圆盘涡轮桨3种桨型以及不同搅拌层数的流场分布，结果表明双层六斜叶圆盘涡轮桨混合效果最好。研究结果对氨碱法制碱工业中预灰桶搅拌设备的选型和设计提供了参考。     

Techniques for visualization of cavern boundaries in opaque industrial mixing systems

In the agitation of complex fluids, the avoidance of caverns is essential for successful blending. Electrical resistance tomography (ERT) and positron emission projection imaging, which can both image within opaque fluids, have been assessed for visualization of cavern boundaries. A vessel of diameter, T = 154 mm, equipped with a single 57 mm diameter six bladed 45° down pumping pitched blade disc turbine formed the test system. The fluid used was aqueous solution of carbopol 940. Both techniques were used to detect and image caverns at Re from 20–86.6 and compared with optical images. Reasonable agreement on the maximum cavern heights and widths were obtained, with the taller and narrower caverns obtained via 3D ERT measurements being attributed to artifacts of the method and interactions between the polymer and tracer. Caverns were also detectable using a robust linear ERT array, which has potential for use within industrial systems. © 2009 American Institute of Chemical Engineers AIChE J, 2009