The effect of impeller pumping and fluid rheology on solids suspension in a stirred vessel

Pumping velocities of different impellers were measured using laser Doppler velocimetry. It was found that there is a link between impeller pumping capacity and the 5 parameter in the Zwietering correlation. Measurements showed that N_js increases with impeller spacing for dual impeller configurations. A dual‐impeller configuration shows less sensitivity to solids loading change than a single‐impeller configuration, and the change in N_js is larger for small impeller spacing than for large impeller spacing. An increase in slurry cloud height and a reduction in N_js resulted at a small increase of non‐Newtonian viscosity.     

Effect of impeller type on continuous‐flow mixing of non‐Newtonian fluids in stirred vessels through dynamic tests

Mixing of non‐Newtonian fluids with axial and radial flow impellers is prone to a significant extent of nonideal flows (e.g., dead zones and channelling) within the stirred reactors. To enhance the performance of the continuous‐flow mixing of pseudoplastic fluids with yield stress, close‐clearance impellers were utilised in this study. We explored the effects of various parameters such as the type of close‐clearance impeller (i.e., the double helical ribbon (DHR) and anchor impellers), impeller speed (25–500 rpm), impeller pumping direction, fluid rheology (0.5–1.5% xanthan gum solution), fluid flow rate (3.20–14.17 L min^?1) and the locations of outlet (configurations: top inlet–top outlet, top inlet–bottom outlet) on the dynamic performance of the mixing vessel. The performance of the DHR impeller was then compared to the performance of various types of impellers such as axial‐flow (Lightnin A320) and radial‐flow (Scaba 6SRGT) impellers. The dynamic tests showed that the DHR impeller was the most efficient impeller for reducing the extent of nonideal flows in the continuous‐flow mixer among the impellers employed in this study. In addition, the mixing quality was further improved by optimising the power input, increasing the mean residence time, decreasing the fluid yield stress, using the up‐pumping impeller mode and using the top inlet–bottom outlet configuration. © 2011 Canadian Society for Chemical Engineering     

Gas Dispersion in Rheologically‐Evolving Model Fluids by Hybrid Dual Mixing Systems

Gas dispersion experiments (0.18 ≤ Fr ≤ 0.71, 0.02 ≤ F1 ≤ 0.09) were carried out using a hybrid dual mixing system, which included a helical ribbon impeller and either a Smith or a Rushton turbine. Newtonian and non‐Newtonian model fluids were used as rheologically‐evolving fluids to evaluate changes in gas dispersion performance. A motionless helical ribbon agitator was used as a baffle in low‐viscosity Newtonian fluids. Both Smith and Rushton turbines produced a vortex, which was eliminated by the motionless helical ribbon impeller. Gas dispersion in low‐viscosity fluids was enhanced when the helical ribbon agitator and turbine of the dual hybrid mixing system was kept at a rotational speed ratio of 10 (N_T/N_HR = 10), which allowed dispersion at a lower Fr than the turbine alone. For moderate‐viscosity Newtonian fluids, gas dispersion was achieved at Fr ≤ 0.71 and F1 ≤ 0.05. Flow properties of non‐Newtonian fluids played an important role in gas dispersion; transition from dispersing to flooding stages was observed for the fluids that were more shear‐thinning (n ≤ 0.38).     

Predicting Power for a Scaled‐up Non‐Newtonian Biomass Slurry

High‐solids biomass slurries exhibit non‐Newtonian behavior with a yield stress and require high power input for mixing. The goals were to determine the effect of scale and geometry on power number P₀, and estimate the power for mixing a pretreated biomass slurry in a 3.8 million L hydrolysis reactor of conventional design. A lab‐scale computational fluid dynamics model was validated against experimental data and then scaled up. A pitched‐blade turbine and A310 hydrofoil were tested for various geometric arrangements. Flow was transitional; laminar and turbulence models resulted in equivalent P₀ which increased with scale. The ratio of impeller diameter to tank diameter affected P₀ for both impellers, but impeller clearance to tank diameter affected P₀ only for the A310. At least 2 MW is required to operate at this scale.     

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.     

Hydrodynamic Performance of a Ring-Style High-Shear Impeller in Newtonian and Shear-Thinning Fluids

Laminar mixing of Newtonian and shear-thinning fluids induced by a Hockmeyer®-type impeller was investigated. Two unbaffled tanks at three impellers off-bottom clearances (c) were studied. Six geometric combinations, i.e., two d/T and three c/T, were examined where d and T are the impeller and tank diameters, respectively. Determination of the Metzner-Otto constant (K_s) was undertaken. The effects of d/T, c/T, and fluid rheology on K_s, power demand, pumping, shear and viscous dissipation were analyzed. The evaluated geometric ratios and rheology do not significantly affect K_s and power demand, only the rheology had an impact on the remaining hydrodynamic parameters. Pumping was favored with the Newtonian fluid, and shear and viscous dissipation increased with the shear-thinning fluid.     

Mixing of Shear‐Thinning Fluids with Dual Off‐Centred Impellers

Mixing times for inelastic shear‐thinning fluids in stirred tanks have been experimentally investigated using a combination of two off‐centred impellers operating in both co‐ and counter‐rotating modes. A colour‐discolouration technique based on fast acid‐base reaction was used for the determination of the mixing times as well as to reveal the possible presence of caverns and dead regions. A statistical plan of experiments allowed determining the effects of the impeller position, the rotational speed, the flow behaviour index, the impeller type and their mutual interactions. A stronger influence of the impeller position on mixing times was observed for both rotating modes with fluids exhibiting pronounced shear‐thinning. It was also found that segregated regions could be readily destroyed by dual off‐centred impellers as compared with the single centred impeller configuration. Mixed flow impellers were shown to be less efficient in terms of mixing times than radial flow impellers. Results obtained under the best operating conditions were compared to steady stirring experiments showing the potential and drawbacks of the proposed scenarios.     

Assessment of Mixing Performance and Power Consumption of a Novel Polymerization Reactor System

A melt‐phase polymerization reactor with the novel geometry of two helical solid‐tube impellers rotating within a tank, consisting of two intersecting cylinders, was designed and constructed. In order to evaluate the performance of the system, mixing times and power consumption were measured using a viscous Newtonian model fluid (glucose syrup) in place of the polymer melt. The mixing regime was laminar in all runs. The mixing time at various impeller speeds was estimated by injection of a tracer dye (crystal violet), followed by fluid sampling and visible spectrophotometric analysis. A dimensionless mixing time of k_m = 104 ± 36 was obtained. The power draw required to move each impeller through the fluid at various impeller speeds was measured, and a power constant of k_p = 1156 ± 70 was obtained. The system appeared to outperform the conventional single helical ribbon impeller in terms of mixing time, but was less energy‐efficient, as indicated by the larger power constant. The power constant value lies between values previously reported in the literature for conventional helical impellers and values reported for other types of polymerization reactors with different geometries.     

Design rules for suspending concentrated mixtures of solids in stirred tanks

This study examines complete off-bottom suspension conditions for slurries containing mixtures of solids at a wide range of concentrations. Binary combinations of five different particles; bronze, two sizes of glass beads, ion exchange resin, and nickel were tested with two impellers: the Lightnin A310 and the down pumping 45° PBT. The effect of particle size ratio and density ratio of the two solid phases on N_js was investigated. Solids loadings were varied from 3 to 56 wt% (weight percent) with water as the liquid phase. The results showed that the highest particle N_js in a mixture of solids is not a sufficient design specification to ensure suspension of the mixture. For one of the cases studied, the particle–particle interactions became significant at high loadings and resulted in a decrease in N_js. Different behaviours were observed for the other mixtures. The performance of the two impellers was also compared. The A310 impeller consumes significantly less power at N_js than the PBT. Both impellers showed an effect of off-bottom clearance on N_js, but the effect of off-bottom clearance depended on both the solids concentration and on the solids used.     

Prediction of just suspended speed for mixed slurries at high solids loadings

One design heuristic used to determine the just suspended speed, N_js, for mixed slurries assumes that the mixture N_js is dominated by the particle phase with the maximum N_js. This approach does not incorporate the effect of the second solid phase. Two new models are proposed to predict the mixture N_js: the power model and the momentum model. These models determine the mixture N_js using the sum of the power or the sum of the momentum required to suspend the individual solid phases. The models were tested using experimental data for two impellers, a Lightnin A310 impeller and a 45° pitched-blade turbine. A range of off-bottom clearances, and six mixtures of solids up to 27 wt% solids loading completed the data set. The power model accurately predicts mixture N_js for both impellers over the full range of clearances and up to 27 wt% mixtures.     

CFD Investigation of the Mixing of Yield‐Pseudoplastic Fluids with Anchor Impellers

The study was carried out to simulate the 3D flow domain in the mixing of pseudoplastic fluids possessing yield stress with anchor impellers, using a computational fluid dynamics (CFD) package. The multiple reference frames (MRF) technique was employed to model the rotation of the impellers. The rheology of the fluid was approximated using the Herschel–Bulkley model. To validate the model, the CFD results for the power consumption were compared to the experimental data. After the flow fields were calculated, the simulations for tracer homogenization were performed to simulate the mixing time. The effects of impeller speed, fluid rheology, and impeller geometry on power consumption, mixing time, and flow pattern were explored. The optimum values of c/D (impeller clearance to tank diameter) and w/D (impeller blade width to tank diameter) ratios were determined on the basis of minimum mixing time.     

同心双轴复合式搅拌釜用于牛顿流体时的功耗及混合特性

在直径0.48 m的椭圆底搅拌槽内,液位与槽径比(H/T)为0.6,采用不同粘度的牛顿流体糖浆溶液,研究了分别以CBY, 45o四斜叶桨及Rushton涡轮桨作为快速分散桨、锚式桨作为慢速桨构成的同心双轴搅拌系统,在快、慢速轴同向和异向2种旋转方式操作时的功率特性和混合性能. 结果表明,分散桨对锚式桨的功率消耗影响较大. 两轴同向旋转时,分散桨会使锚式桨的功耗降低,转速比RN增加,降低幅度也增大,RN=14时,锚式桨功率可降至单独旋转时的约10%;异向旋转时锚式桨的功率随RN增加而增加,RN=14时,锚式桨功率可增至单独旋转时的2倍左右. 但锚式桨对分散桨的功率消耗影响很小,在±5%以内. 计算同心双轴复合搅拌系统的复合功率准数和复合雷诺数关系时考虑了RN的影响,使在实验条件下不同转向及RN的功率曲线较好吻合. 混合效果同向旋转优于异向旋转,在牛顿流体中,达到相同混合效果时,CBY桨的能量消耗仅为其他2个分散桨的20%~30%.     

Using Tomography to Characterize the Mixing of Non‐Newtonian Fluids with a Maxblend Impeller

The flow field inside a cylindrical mixing vessel was visualized by electrical resistance tomography (ERT), a non‐intrusive measurement technique. Six tomography planes, each containing 16 sensing electrodes, measured the mixing time in the agitation of pseudoplastic fluid exhibiting yield stress. The effects of various parameters such as impeller types, impeller speed, fluid rheology, power consumption, Reynolds number, and absence of baffles on the mixing time were investigated. The Maxblend impeller was able to improve the mixing performance of non‐Newtonian fluids in a batch reactor. The mixing quality could be further enhanced by decreasing the xanthan gum concentration and using baffles in the mixing vessel.     

Experimental Studies on Suspension of Solid Particles in a Low‐Shear Stirred Vessel

A low‐shear stirred vessel was explored. Experimental studies on the suspension of solid particles in solid‐liquid and gas‐solid‐liquid systems were conducted to examine the performance of this new reactor. The method based on the power number curve was modified to determine the critical impeller speeds required for just complete off‐bottom suspension of solids under non‐gassed (N_js) and gassed conditions (N_jsg) in this reactor, and a PC‐6A fiber‐optic probe for the measurement of solid distribution was used to complementarily validate this method. A more homogeneous flow field was gained with a draft tube installed, so that the standard deviations of average shear rate and maximal shear rate are reduced. The modified power consumption method can determine N_js and N_jsg, and the values of N_js with a draft tube are much lower than those without it. N_jsg increases slightly with increasing gas flow rate, and N_jsg with a higher solid weight fraction is larger in this lower‐shear reactor.     

Flow and mass transfer in aerated viscous Newtonian liquids in an unbaffled agitated vessel having alternating forward–reverse rotating impellers

Flow and mass transfer characteristics in aerated viscous Newtonian liquids were studied for an unbaffled aerated agitated vessel with alternating rotating impellers (AAVAI), ie with multiple forward–reverse rotating impellers having four delta blades. The effects of operating conditions such as gas sparging rate, agitation rate and the number of impeller stages, and the liquid physical properties (viscosity) on the gas hold‐up, ?_gD, and volumetric oxygen transfer coefficient, k_La_D were evaluated experimentally. The dependences of ?_gD and k_La_D on the specific total power input and superficial gas velocity differed, depending on the ranges of liquid viscosity. Empirical relationships are presented for each viscosity range to predict ?_gD and k_La_D as a function of the specific total power input, superficial gas velocity and viscosity of liquid. Based on a comparative investigation of the volumetric coefficient in terms of the specific total power input between the AAVAI and conventional aerated agitated vessels (CAAVs) having unidirectionally rotating impellers, the usefulness of AAVAI as a gas–liquid agitator treating viscous Newtonian liquids is also discussed. © 2001 Society of Chemical Industry     

Design and operation of unbaffled aerated agitated vessels with unsteadily forward–reverse rotating impellers handling viscous Newtonian liquids

Design and operation of unbaffled aerated agitated vessels with multiple unsteadily forward–reverse rotating impellers (AJITERs) for viscous Newtonian liquids were studied. The effects of operating conditions such as gas sparging rate, agitation rate and the number of impeller stages, geometrical conditions such as the diameters of vessel and impeller, and the physical properties of liquids on the drag and added moment of inertia coefficients, necessary to predict the average and maximum power consumptions of the impellers in AJITERs, were evaluated and the empirical relationships which estimate values of each of these coefficients are presented. The effects of operating conditions, geometrical conditions and liquid physical properties on the gas hold‐up, ?_gD, and volumetric oxygen transfer coefficient, k_La_D, were evaluated in relation to the total power input which is the sum of the average power consumption of impellers, ie average agitation power input, and aeration power input. Empirical relationships, useful for design and operation of AJITERs, were obtained for each viscosity range, where the dependences of ?_gD and k_La_D on the specific total power input and superficial gas velocity differed, to predict ?_gD and k_La_D respectively as a function of the specific total power input, superficial gas velocity and liquid physical properties. © 2003 Society of Chemical Industry     

Determination of correlations to predict the minimum agitation speed for complete solid suspension in agitated vessels

Experiments were conducted to determine the effects of impeller clearance, impeller diameter, and other operating variables on the minimum agitation speed for off-bottom solid suspension in agitated vessels, N_js, for disc turbines (DTs) and flat-blade turbines (FBTs). Only data for which the impellers produced recirculation flows above and below the impeller (the so-called “double-eight” flow pattern) were considered. Regression equations for N_js were obtained, in which explicit terms for impeller clearance and vessel diameter-to-impeller diameter ratio (T/D) were included. Modified Zwietering equations (Zwietering, 1958) were also obtained, in which Zwietering's parameter S was mathematically expressed as a function of vessel diameter-to-impeller clearance ratio and T/D ratio. When used together with the correlations of Armenante and Uehara Nagamine (1998) for impellers close to the vessel bottom, the equations presented here can be used to calculate N_js for DTs and FBTs for any typical impeller clearance.     

Power Consumption in the Turbulent Regime for a Coaxial Mixer

The power consumption of a new coaxial mixer composed of an anchor impeller and a pitched‐blade turbine impeller, and a series of rods operated in a contra‐rotating mode has been characterized experimentally in the turbulent regime. It is shown that both the power curve and the turbulent power number vary significantly with the speed ratio between the impellers. Likewise with single impeller mixers, the transition regime starts at a Reynolds number above 100 and the turbulent regime between 10³ and 10⁴ irrespective of the definition of the Reynolds number used.     

新型同心双轴搅拌器功率与混合特性的数值模拟

基于同心双轴搅拌器的结构与运行特点,建立了兼顾其流动、混合过程的三维数学模型,并以过程工业应用较多的两种不同尺寸双层组合桨作为内桨、框式桨作为外桨构成的同心双轴搅拌器为研究对象,数值模拟了其在中高黏牛顿流体中同向及反向转动模式的功率特性、流场特性及混合特性。模拟结果表明,同向转动模式下,整个系统的搅拌功耗更小、混合效率更高;外桨功耗受内桨影响较大,一般随内桨转速的增大,恒速外桨的功耗同向转动时会减小、反向转动时会增大;对由桨式搅拌器构成的组合式内桨而言,当内桨直径与釜体直径之比为0.35左右时,相同Reynolds数下的单位体积混合能更小;中高黏牛顿流体中,同心双轴搅拌器的内桨采用上层六斜叶桨+下层六直叶桨的组合形式时更高效节能,仅在体系Reynolds数小于36时,上层二斜叶桨+下层二直叶桨的内桨组合形式才具有相对优势。     

Performance of a New Mixed Down Pumping Impeller

A new mixed down pumping impeller was designed and characterized with both Newtonian and non‐Newtonian fluids in terms of the power consumption and mixing times. A non‐intrusive color‐discoloration technique based on a fast acid‐base reaction was used to determine mixing times and to reveal the presence of both segregated and dead zones at low speed. This new geometry gives similar mixing times to those obtained with a radial turbine but with the power requirements of an axial flow impeller. It was demonstrated that segregated regions formed below the impeller are readily destroyed by the pumping action of this new geometry, while the regions formed above the impeller are destroyed by radial discharge, so that shorter mixing times are obtained. The use of the proposed geometry appears to be a good alternative for mixing applications requiring a good dispersion combined with low power consumption.