Scaba 6SRGT Research Articles

Numerical investigation of hydrodynamic performance and power consumption in stirred reactors with perforated Scaba 6SRGT impellers for non‐Newtonian fluid

AbstractThe present study aims to further understand the effect of perforated Scaba 6SRGT impellers on stirring performance and impeller power consumption by numerically investigating the hydrodynamic properties of non‐Newtonian fluids in a three‐dimensional stirred reactor. After model validation, the fluid velocity, stagnant region dimensionless volume, and impeller power consumption are quantitatively analyzed under the conditions of influencing parameters, including blade perforation pitch, ratio of distance between adjacent perforations, and perforation arrangement. The results reveal that the range of distribution of acceleration and peak velocity of the fluid along the radial appear in regions lower and larger than the dimensionless radius of the impeller (|r/R| = 0.45), respectively. Increasing the perforation pitch from 14 to 20 mm increases fluid velocity from 0.284 to 0.305 m/s and power consumption from 26.99 to 27.25 W, while reducing the stagnant region volume from 16.76% to 14.71%. The fluid velocity and impeller power consumption slightly increase with the ratio of distance between adjacent perforations. Circular‐shaped middle perforations yield higher fluid velocities and vorticity compared to lozenge‐shaped ones. The highest and lowest power consumptions are obtained by circular–circular–circular and lozenge–lozenge–lozenge perforation shape arrangements with values of 27.25 and 26.9 W, respectively.

Newly modified curved-bladed impellers for process intensification: Energy saving in the agitation of Hershel-Bulkley fluids

The power consumption in stirred tanks is a key parameter to determine the efficiency of a rotating impeller. This parameter is highly related to the viscous forces of the working fluid, especially when a complex non-Newtonian behavior exhibits. From this basis, an attempt is made in this paper to reduce the power requirements of Scaba 6SRGT impellers for stirring viscoplastic fluids in cylindrical unbaffled vessels. For this purpose, perforations are created in each blade of the impeller. Effects of the perforation diameter (dp/h = 0, 0.04, 0.07, 0.10 and 0.13), perforation shape (circular, lozenge, rectangular and triangular shapes) and number of perforation (nb = 0, 2, 4, 6, and 9) on the hydrodynamics induced and power consumption are investigated. The obtained results revealed an interesting decrease in power numbers by using the perforation technique. When changing the perforation diameter from 0 to 0.13 and the number of perforations from 0 to 9, Np is reduced by about 3.7 %–21.6 % and 2.6–6.7 %, respectively. In a comparison between the different shapes of perforation, the circular shape has allowed a reduction by about 3.8 % less than that for the other cases, which had almost the same power requirements.

Modified scaba 6SRGT impellers for process intensification: Cavern size and energy saving when stirring viscoplastic fluids

Mixing of a viscoplastic fluid using Scaba 6SRGT impellers in a cylindrical vessel has been studied. Cuts have been introduced in each blade of the impeller in order to reduce the power requirements. Effects of the cut-height (h2/D = 0, 0.015, 0.04, 0.065 and 0.09, respectively), cut-length (l/D = 0, 0.06, 0.12 and 0.18, respectively), and of the number of cuts (nb = 1, 2, 3, 4, and 5) on the hydrodynamics and power consumption were explored. The suggested designs, compared to the classical Scaba 6SRGT impeller, display a reduction in power number by about 20%, 19.9% and 66.6% when the cut-height, cut-length and number of cuts are changed from h2/D = 0 to 0.09, l/D = 0 to 0.18 and nb = 1–5, respectively. However, the increased surface area of cuts is accompanied by a reduction in the size of radial jet of fluid, resulting thus in a weakened axial flow and a decrease in cavern size. As a result, the best trade-off between the reduced power consumption and enlarged cavern size correspond to the case nb = 3, l/D = 0.12. The reduction in power number for this case is estimated to be as about 15%.

Some modifications in the Scaba 6SRGT impeller to enhance the mixing characteristics of Hershel–Bulkley fluids

Mixing in unbaffled cylindrical tanks by open turbines rotating at low Reynolds numbers requires usually a longer mixing time, especially for shear thinning fluids possessing yields stress, where the mixing operation is characterized by the formation of caverns. To overcome this issue, some modifications in the impeller design are introduced. We are interested in mixing of shear thinning fluids with yield stress by curved bladed impellers (known as Scaba 6SRGT) in the laminar and transitional flow regimes. Two types of cuts have been introduced in the impeller blade: one at the middle of blade (referred as Design 2) and the other from the middle until the tip of blade (referred as Design 3). Lengths of cuts in Design 2 and 3 are: l/d2=0.3 and 0.6, respectively, where d2 is the length of blade for the classical impeller. The cuts introduced in the blades allowed a reduction in power consumption by 17.5% and 33% for Design 2 and 3 compared to the classical blade. Also, other modifications are introduced in order to enhance the mixing characteristics. It concerns the impeller eccentricity (e/D=0.12 and 0.25) and blade number (nb=2, 3 and 6) for the case which has given the most reduced power input (i.e. Design 3). The off-centred impeller (case e/D=0.25) with 6-blades has yielded the widest cavern size.

Agitation of yield stress fluids in different vessel shapes

The Agitation of yields stress fluids with a six-curved-blade impeller (Scaba 6SRGT) is numerically investigated in this paper. The xanthan gum solution in water which is used as a working fluid is modeled by the Herschel–Bulkley model. The main purpose of this paper is to investigate the effect of vessel design on the flow patterns, cavern size and energy consumption. Three different vessel shapes have been performed: a flat bottomed cylindrical vessel, a dished bottomed cylindrical vessel and a closed spherical vessel. The comparison between the results obtained for the three vessel configurations has shown that the spherical shapes provide uniform flows in the whole vessel volume and require less energy consumption. Effects of the agitation rate and the impeller clearance from the tank bottom for the spherical vessel are also investigated. Some predicted results are compared with other literature data and a satisfactory agreement is found.

Numerical study of the performance of multistage Scaba 6SRGT impellers for the agitation of yield stress fluids in cylindrical tanks

The 3-D flow fields and power consumption in a vessel stirred by multistage Scaba 6SRGT impeller have been investigated. The Xanthan gum solutions in water were used, which have a shear thinning behavior with yield stress. This study was carried out with the help of a CFD package (CFX 13.0, Ansys Inc.) which is based on the finite volume method to solve the momentum equations. The effects of stirring rate, fluid rheology, impeller number, impeller location and vessel size on the performance of such stirred system are presented. To validate the CFD model, our predicted results have been compared with other literature data and a satisfactory agreement has been found.

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

AbstractMixing 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

Characterisation of the mixing of non‐newtonian fluids with a scaba 6SRGT impeller through ert and CFD

AbstractAn attempt has been made to study the mixing of yield‐pseudoplastic fluids with a Scaba 6SRGT impeller using electrical resistance tomography (ERT) and computational fluid dynamics (CFD). The ERT system with four sensor planes, each containing 16 equispaced stainless steel electrodes, was used to measure the mixing time. The multiple reference frames (MRF) technique and the modified Herschel–Bulkley model were applied to simulate the impeller rotation and the rheological behaviour of the non‐Newtonian fluids, respectively. To validate the model, the CFD results for the power consumption were compared to the experimental data. The validated model was then employed to obtain further information regarding the averaged impeller shear rate, impeller circulation, and pumping capacities. The CFD and ERT data were utilised to investigate the effect of the impeller power, fluid rheology, and impeller size on the mixing time. The mixing time results obtained in this study were in good agreement with those reported in the literature. © 2011 Canadian Society for Chemical Engineering

Numerical study of fluid flow and power consumption in a stirred vessel with a Scaba 6SRGT impeller

Numerical study of fluid flow and power consumption in a stirred vessel with a Scaba 6SRGT impeller The present work deals with agitation of non-Newtonian fluids in a stirred vessel by Scaba impellers. A commercial CFD package (CFX 12.0) was used to solve the 3D hydrodynamics and to characterise at every point flow patterns especially in the region swept by the impeller. A shear thinning fluid with yield stress was modelled. The influence of agitator speed, impeller location and blade size on the fluid flow and power consumption was investigated. The results obtained are compared with available experimental data and a good agreement is observed. It was found that an increase in blade size is beneficial to enlargement of the well stirred region, but that results in an increased power consumption. A short distance between the impeller and the tank walls limits the flow around the agitator and yields higher power consumption. Thus, the precise middle of the tank is the most appropriate position for this kind of impeller.

Improving the dynamic performance of continuous-flow mixing of pseudoplastic fluids possessing yield stress using Maxblend impeller

The strategic approach of this article is to characterize the continuous-flow mixing of pseudoplastic fluids possessing yield stress in a stirred reactor with the Maxblend impeller. Dynamic experiments were carried out through the frequency-modulated random binary input of a brine solution to determine the extent of non-ideal flows. Mixing quality was determined on the basis of the extent of channeling and fully mixed volume. The effects of important parameters such as impeller speed (25–500rpm), absence of baffles, fluid rheology (0.5–1.5%), fluid flow rate (3.20–14.17Lmin−1), and the locations of inlet/outlet on the dynamic performance of the continuous-flow mixing vessel were explored. The performance of the Maxblend impeller was then compared to the performances of various types of impellers such as close-clearance (an anchor), axial-flow (a Lightnin A320), and radial-flow (a Scaba 6SRGT) impellers. It was found when the channeling approached zero and the fully mixed volume approached the total fluid volume in the vessel, the power drawn by the A320 impeller and the Scaba impeller were about 2.9 and 4.3 times greater than that of the Maxblend impeller. Thus, the Maxblend impeller was able to drastically improve the performance of continuous-flow mixing with huge power savings. The mixing quality was further improved by optimizing the impeller speed, decreasing the fluid flow rate, decreasing the fluid concentration, and using bottom inlet- top outlet configuration. The flow non-ideality of the mixing system increased in the absence of the baffles. Thus, better mixing quality and more energy savings can be achieved by employing the findings of this study.

Measuring Mixing Time in the Agitation of Non‐Newtonian Fluids through Electrical Resistance Tomography

AbstractElectrical resistance tomography (ERT), which is a non‐invasive and robust measurement technique, was employed to visualize, in three dimensions, the concentration field inside a cylindrical mixing vessel equipped with a radial‐flow Scaba 6SRGT impeller. The ability of ERT to work in opaque fluids makes this technique very attractive from an industrial perspective. An ERT system with a 4‐plane assembly of peripheral sensing rings, each containing 16 electrodes, was used to measure the mixing time in agitation of xanthan gum solution which is a pseudoplastic fluid with yield stress. An image reconstruction algorithm was used to generate images of the tracer distribution within the sensing zone. In this study, the effect of impeller speed, fluid rheology, power consumption, and Reynolds number on the mixing time was investigated.

Using electrical resistance tomography and computational fluid dynamics modeling to study the formation of cavern in the mixing of pseudoplastic fluids possessing yield stress

Electrical resistance tomography (ERT) provides a non-intrusive technique to examine, in three dimensions, the homogeneity and flow pattern inside the mixing tank. In this study, a 4-plane 16-sensor ring ERT system was employed to study the shape and the size of cavern generated around a radial-flow Scaba 6SRGT impeller in the mixing of xanthan gum solution, which is a pseudoplastic fluid possessing yield stress. The size of cavern measured using ERT was in good agreement with that calculated using Elson's model (cylindrical model). The 3D flow field generated by the impeller in the agitation of xanthan gum was also simulated using the commercial computational fluid dynamics (CFD) package (Fluent). The CFD model provided useful information regarding the impeller pumping capacity, flow pattern, and the formation of cavern around the impeller. CFD results showed good agreement with the experimental data and theory.

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.

Advantages of the hollow (concave) turbine for multi-phase agitation under intense operating conditions

The mixing literature on hollow blade turbines (HBTS), for operation in fully turbulent flow, is reviewed and compared with the results of our own studies. The SCABA 6SRGT is shown to have an almost identical pumping rate to a disc turbine, when compared at the same diameter and specific power. An equation is proposed for the effect of scale and blade geometry on the power number of a range of concave hollow blade agitators. The “flooding–loading” condition is revisited. It is found that, when compared at conditions above the minimum Froude number required to disperse gas, the HBT designs are as energetically efficient as Rushton turbines for dispersing gas. If we compare them on an “ungassed” power basis, as is the usual literature case, then the HBT is more efficient because of their ability to disperse gas without significant loss of power. The much lower power number resulting from the streamlined blade design of the HBTs also ensures that they achieve the minimum Froude number required to disperse gas at a much lower power than a RT. A simple method to avoid “flooding” for radial turbines, based on this work, is proposed. Under fully loaded conditions the hollow blade turbines will handle high gas rates without significant loss of power and this ability is a function of the degree of streamlining. For the suspension of high levels of solids the D = T / 2 hollow blade turbines, at a clearance of T / 4 , are found to be amongst the most efficient agitators especially under gassed conditions, where almost no effect of gassing on the just suspension speeds were noted.

Recent Studies on Agitated Three-Phase (Gas–Solid–Liquid) Systems in the Turbulent Regime

Earlier work using Rushton turbines and down-(MFD) and up-(MFU) pumping, 45°-pitched blade turbines at relatively low concentrations of solids of different size and density in vessels up to 1.8 m diameter enabled correlations to be developed for predicting the minimum speed for solid suspension, NJSg. Further work has been conducted with these impellers and with two modern impellers, a Scaba 6SRGT, a typical hollow-blade radial flow impeller, and a typical up-pumping, axial flow, wide-blade hydrofoil, the Lightnin’ A315, in a vessel of 0.45 m diameter with solids up to 40% by weight. The earlier correlations were validated for the new higher concentration conditions. In all cases, increases in solids concentration increased NJSg and the specific energy dissipation rate required to suspend solids, (ɛT)JSg On the other hand, the large increase (up to two orders of magnitude) in mixing time found at solids concentration >∼15% by weight in two phase systems at NJSg as compared with the case without solids, is essentially eliminated in the three-phase case. At low gassing rates, QGv (vvm), down-pumping impellers achieve suspension and relatively good vertical solids distribution at the lowest (ɛT)jsg-However, with increased QGv UP to 3 vvm, NJSg and (ɛT)JSg increase rapidly, solids maldistribution develops, the flow pattern is very unstable and gross torque fluctuations occur. For the 6SRGT, the operation is very stable and although NjSg and (ɛT)JSg are both high they are insensitive to QGv This high value of (ɛT)JSg may not be a disadvantage if high rates of gas–liquid mass transfer are required. For both the 6MFU and especially the A315(U), the flow pattern, NJSg and (ɛT)JSg are again all very insensitive to QGv and a good vertical solids distribution is maintained. For the A315(U) at high QGv values, (ɛT)JSg is the least amongst the impellers tested, so that wide-blade, up-pumping axial flow hydrofoils are considered to be the optimum impeller when just physical suspension is required or solid–liquid reactions are rate limiting.

Power Consumption Characteristics of Disc Type Impellers.

This study reports on the unaerated power number (Np) and the ratio of aerated to unaerated power consumption (PG/P) for three disc type impellers in water and highly viscous Newtonian and non-Newtonian liquids. Modified disc type impellers (i.e. CBDT—concave bladed disc turbine and Scaba 6SRGT) were found to give lower Np and higher PG/P values than the flat bladed disc turbine or Rushton turbine (DT), in all liquids. Higher PG/P values are generally assumed to imply superior mass transfer potential of a given impeller. However, an examination of gas hold-up (which is closely linked to mass transfer) generated by DT, CBDT and Scaba 6SRGT, suggests that this is not necessarily true and further investigations may be necessary.

The effect of impeller design on gas hold-up in surfactant containing highly viscous non-Newtonian agitated liquids

This paper reports on the effect of impeller design on gas hold-up in high viscosity CMC (apparent viscosity, μ a=0.5–0.8 Pas) and CMC solutions containing polypropylene glycol as an interfacially active additive. Three disc type impellers, namely, flat bladed disc turbine (DT) and two of its modified designs (Concave bladed disc turbine, CBDT, and Scaba 6SRGT) have been used. The relative performance of these impellers is assessed by comparing the values of four key parameters: specific power dissipation (P G/ V), final total gas hold-up ( ε f), tiny bubble hold-up ( ε ft), and τ, a constant reflecting the time dependency of hold-up. It appears that DT demonstrates either superior to or at least comparable performance over its modified designs. In addition, gas hold-up and τ values in following solutions are compared: (i) pure CMC and PPG containing CMC solutions, (ii) PPG containing CMC and Castor oil. All the impellers produced higher gas hold-up and τ values in (i) PPG containing solution than pure CMC solution, whilst dissipating comparable or lower power and, (ii) Castor oil than PPG containing CMC solution. These observations have been explained by considering the differences between the physical and interfacial properties of these liquids and their effects on liquid flow.

Mixing of Three-Phase Systems at High Solids Content (up to 40% w/w) Using Radial and Mixed Flow Impellers.

A liquid-gas-solid system has been studied at higher solids concentration (up to 40% by wt) and gas flow rates (up to 2 vvm) than previously. Two different radial flow impellers (Scaba 6SRGT and a standard Rushton turbine) and a six-bladed mixed flow impeller with pitch angle of 45°, either in its downward (6MFD) or upward pumping (6MFU) mode were used. Power input, mixing time, the speed to achieve complete suspension of the solids (ungassed and gassed) as well as the amount of suspended solids and the height of the liquid-solid interface were measured. In addition, a new technique for measuring the amount of suspended solids in two-phase systems was extended successfully to three phases. When compared to the situation without solids, the mixing time, t m , in the solid-liquid case at the higher solid concentrations was much greater, as previously reported, but for the three-phase case, the increase was relatively small, especially with the Scaba and 6MFU impellers. In general, the power and speed required to suspend the solids increased with increasing solids concentration but once suspended, with the Scaba and the 6MFU impellers, even the highest solids concentration and gas flow rates only required a very small further increase. It has recently been proposed (Pantula and Ahmed, 1997) that by maintaining constant agitator torque on gassing, solids suspension would be sustained. The present work showed this to be broadly valid for the 6SRGT and the 6MFU but not for the Rushton and the 6MFD impellers. Overall, the most stable impeller, requiring the least specific energy dissipation rate for solids suspension and gas dispersion under the most demanding conditions, was the 6MFU.

Influence of impeller type on the flow structure in a stirred reactor

AbstractLarge eddy simulations (LES) of a stirred reactor with a liquid volume of 0.64 m3 were performed. The tank was stirred by either two standard Rushton impellers or two Scaba 6SRGT impellers. The LES can provide details of the flow field that cannot be obtained with so‐called Reynolds‐averaged equations and the corresponding models. The unsteady spatially filtered incompressible Navier‐Stokes equations were solved together with a transport equation for the concentration of an inert additive using a multigrid finite difference code with a staggered grid. The unresolved turbulent scales were modeled using a scale similarity model. The aim of this investigation has been to study the influence of impeller type on the flow structure and scalar transport. The results show that, compared at equal power input, the center plane velocities and the volume flow in the impeller stream do not differ much for the impeller types. Also, for the 6SRGT the periodic fluctuations associated with the blade passing is less pronounced.

Mixing in large-scale vessels stirred with multiple radial or radial and axial up-pumping impellers: modelling and measurements

Mixing phenomena are regarded as one of the major factors responsible for the failure to successfully scale up some bioprocesses. Such phenomena have been investigated within the framework of an EC project ‘Bioprocess Scale-up Strategy’. Mixing in bioreactors depends on energy input, impeller type, reactor configuration and impeller geometry. Here, two different reactors of volumes 12 and 30 m 3 were used, and they were equipped with either multiple Rushton turbines or with a combination of a Scaba 6SRGT radial impeller with multiple 3SHP axial up-pumping hydrofoils above it. Mixing time, power consumption, gas hold-up and liquid velocities were measured at different stirrer speeds and aeration rates in water. At the same total specific power input, aeration did not influence the mixing time much unless it changed the bulk flow pattern. A considerable reduction of mixing time was achieved if the upper impellers were axial instead of radial Rushtons at the same power consumption. The improvement with the axial impellers could be related to the reduction of axial flow barriers due to different circulation flow patterns. The Compartment Model Approach (CMA) was used to develop a flow model based on the general knowledge of the hydrodynamics of both unaerated and aerated stirred vessels. The model was successfully verified for different impeller and reactor configurations and different scales with measured pulse response curves, using either a fluorescent or a hot water tracer. The model can be used for process design purposes.