Double Helical Ribbon Research Articles

Development of new correlation for the prediction of power number for closed clearance impellers using machine learning methods trained on literature data

AbstractThe accurate estimation of the power number for closed clearance impellers holds significant importance in industries such as chemical, biochemical, paper and pulp, as well as paints, pigments, and polymers. Existing state‐of‐the‐art correlations for predicting power numbers, however, are inaccurate for impeller Reynolds number . In this study, we compiled a dataset of 1470 data points from 15 research articles in the open literature, covering five types of impellers: (i) anchor; (ii) gate; (iii) single helical ribbon; (iv) double helical ribbon; and (v) helical ribbon with screw. Six machine learning models, namely artificial neural networks (ANN), CatBoost regressor, extra tree regressor, support vector regressor, random forest, and XGBoost regressor, were developed and compared. The results revealed that ANN emerged as the most efficient model, demonstrating the highest testing R2 value of 0.99 and the lowest testing MAPE of 7.3%. Further, we used the ANN model to develop a novel set of process correlations to estimate impeller power numbers for the industrially important anchor and double helical ribbon impellers: which significantly outperform the existing state‐of‐the‐art correlations available in literature.

Flow-accelerated polycondensation reaction to prepare rigid rodlike poly(p-phenylene-cis-benzobisoxazole)

Under the conditions specified in this work, the polycondensation reaction to prepare rigid rodlike poly(p-phenylene-cis-benzobisoxazole) (PBO) is diffusion-controlled. This work aims to study the flow-driven polycondensation reaction of PBO. The effect of mixing efficiency through changing the stirring speed on the polycondensation reaction of PBO was first investigated in a paddle impeller reactor (PIR). In the initial stage of polymerization, the increase of stirring speed is obviously conducive to accelerating the polycondensation reaction. However, the promoting effect of stirring speed on the polycondensation reaction is greatly reduced in the middle and late stages of polymerization. The effect of the flow pattern with different proportions of shear flow, elongational flow and rotational flow on the polycondensation reaction of PBO was further investigated by comparing three reactors including Couette-Taylor reactor (CTR) with a complete shear flow, PIR with strong shear and elongational flows, and double helical ribbon reactor (DHR) with strong shear and rotational flows. Result shows that the improvement of proportions of rotational flow and elongational flow is beneficial to accelerate the polycondensation reaction of PBO in the middle and late stages of polymerization, especially for the rotational flow.

Design optimization and scale-up characteristics of a double-helical ribbon reactor for biomass catalytic pyrolysis

Pyrolysis characteristics are closely related with the inner mixing performance of biomass and biochar particles in agitated biomass catalytic pyrolysis reactors. In this study, a novel double-helical ribbon reactor was designed for biomass catalytic pyrolysis. DEM (Discrete Element Model) simulations were performed to optimize the structure and operating conditions. Model calibration and validation experiments were firstly conducted. The effects of structural parameters and rotation speed on mixing performance were further explored. The results showed that the mixture homogeneity was closely related to the impeller outer diameter and rotation speed, while the mixing rate could be optimized by adjusting the inner diameter and pitch. The optimal operation condition showed adequate mixing in the axial and radial directions. Besides, the scale-up analysis demonstrated that similar mixing performance was presented at the same Froude number. Finally, the correlations of mixing characteristics provided a theoretical reference for industrial reactors design.

Which impeller should be chosen for efficient solid–liquid mixing in the laminar and transitional regime?

AbstractThe vast majority of solid–liquid mixing studies have focused on high Reynolds number applications with configurations and impeller geometries adapted to this type of regime. However, the mixing of particles in a viscous fluid is an essential element of many contemporary industries. We used the computational fluid dynamics‐discrete element method model previously developed in our group to investigate solid–liquid mixing with close‐clearance impellers in the laminar regime of operation. We compared different geometries, that is, the double helical ribbon, anchor, Paravisc™, and Maxblend™ impellers. We investigated the impact of fluid viscosity and compared the results with those obtained with the pitched blade turbine, a more commonly used impeller, based on power consumption for equivalent mixing states. This study highlights that the higher the viscosity of the fluid, the more interesting it is to use close‐clearance impellers for their ability to generate a strong shear stress and a strong bulk flow in the entire vessel.

Mixing effects on decentralized high-solid digester for horticultural waste: Startup, operation and sensitive microorganisms

This work studied the use of a horizontal high-solid digester for the decentralized anaerobic treatment of horticultural waste (fallen leaves), where the effect of intermittent mixing by a modified double helical ribbon impeller was investigated. Before experimental verification, the flow pattern and theoretical mixing time were first characterized by CFD simulation. Subsequently, three mixing time intervals (i.e., 3 min/3 hr; 18 min/3 hr; 108 min/3 hr) and one control setup (i.e., without mixing) were compared for their performance during start-up and semi-continuous operation. It was found that minimal mixing was necessary for an efficient digester’s start-up but increased mixing intensity for semi-continuous operation. The results were further interpreted by correlating the digester performance and microbial communities. Those microorganisms sensitive to increased mixing intensity were highlighted and analysed.

CFD-DEM simulations of solid-liquid flow in stirred tanks using a non-inertial frame of reference

Mixing applications operating in the laminar regime are used in numerous industrial processes in the pharmaceutical, chemical, and food industries. The aim of this paper is to introduce a numerical model adapted to solid-liquid mixing situations in stirred tanks. The method presented herein is based on a Euler-Lagrange approach using the CFD-DEM method. This method couples computational fluid dynamics (CFD) for the fluid with the discrete element method (DEM) for the solid particles. We introduce a rotating frame of reference approach, which is the first of its kind for CFD-DEM. In this paper we discuss the main issues related to the modeling of complex rotating impeller geometries, we explain the various issues involved in conducting a CFD-DEM simulation in a non-inertial frame, we compare our model with experimental results obtained with a pitched blade turbine and, lastly, we use our model to study solid-liquid mixing with a double helical ribbon.

On the Use of the Coaxial Cylinders Equivalence for the Measurement of Viscosity in Complex Non-Viscometric, Rotational Geometries

The rheology of macroscopic particle suspensions is relevant in many industrial applications, such as cement-based suspensions, synthetic and natural drilling fluids. Rheological measurements for these complex, heterogeneous systems are complicated by a double effect of particle size. On the one hand, the smallest characteristic length of the measuring geometry must be larger than the particle size. On the other hand, large particles are prone to sediment, thus calling for the use of rotational tools that are able to keep the suspension as homogeneous as possible. As a consequence, standard viscometric rotational rheometry cannot be used and complex flow geometries are to be implemented. In this way, however, the flow becomes non-viscometric, thus requiring the development of approximate methods to translate the torque vs. rotation speed raw data, which constitute the rheometer output, into viscosity vs. shear rate curves. In this work the Couette analogy methodology is used to establish the above equivalence in the case of two complex, commercial geometries, namely, a double helical ribbon tool and a square-shaped stirrer, which are recommended for the study of relatively large size suspensions. The methodology is based on the concept of the reduction of the complex geometry to an equivalent coaxial cylinder geometry, thus determining a quantitative correspondence between the non-standard situation and the well-known Couette-like conditions. The Couette analogy has been used first to determine the calibration constants of the non-standard geometry by using a Newtonian oil of known viscosity. The constants have been subsequently used to determine the viscosity curves of two non-Newtonian, shear thinning fluids, namely a homogeneous polymer solution and two heterogeneous concentrated suspensions. The results show that the procedure yields a good agreement between the viscosity curves obtained by the reduction method and those measured by a standard viscometric Couette geometry. The calibration constants obtained in this work from the coaxial cylinder analogy are also compared with those provided by the manufacturer, indicating that the calibration can improve the accuracy of the rheometer output.

Complementary methods for the determination of the just-suspended speed and suspension state in a viscous solid–liquid mixing system

A poor understanding of mixing dynamics may lead to major economic losses in numerous industries due to poor yields and waste of raw materials. Then processes can be optimized through a better understanding of solid–liquid mixing dynamics.Previous studies have characterized the suspension of solid particles in a viscous medium using a standard high shear impeller (a pitched-blade turbine or PBT) and have measured the fraction of suspended particles using the pressure gauge technique (PGT). Since the PGT cannot be applied in a straightforward way to close-clearance impellers, we developed a novel technique based on the PGT to determine the fraction of suspended particles in a system using a close-clearance impeller such as a double helical ribbon (DHR).We studied a solid–liquid suspension in the laminar and transitional regimes with high particle loadings and a DHR. Since the dynamic pressure cannot be subtracted from the total pressure determined by the PGT, we developed two alternative methods. The first was the pressure difference method, which we used to determine a range of speeds where Njs is located. The second was the sedimentation-based method which we used to accurately quantify Njs and effectively eliminate the dynamic pressure. Their pertinence and applicability were demonstrated.

Effects of rotational speed and fill level on particle mixing in a stirred tank with different impellers

Abstract The particle mixing was studied in a cylindrical stirred tank with elliptical dished bottom by experiments and simulations. The impeller types used were double helical ribbon (HR) + bottom HR, pitched blade ribbon + bottom HR, inner and outer HR + bottom HR, and pitched blade ribbon + Pfaudler + bottom HR labeled as impellers I to IV, respectively. The quantitative correlations among the rotational speed, fill level and power consumption for impeller I and impeller II were obtained by experiments to validate the discrete element method (DEM) simulations. The particle mixing at different operating conditions was simulated via DEM simulations to calculate the mixing index using the Lacey method, which is a statistical method to provide a mathematical understanding of the mixing state in a binary mixture. The simulation results reveal that as the rotational speed increases, the final mixing index increases, and as the fill level increases, the final mixing index decreases. At the same operating conditions, impeller III is the optimal combination, which provides the highest mixing index at the same revolutions.

Impact of shear stress and impeller design on the production of biogas in anaerobic digesters

Today, intensification of anaerobic digestion is still a scientific and technical challenge. The present study proposed combined experimental and computational fluid dynamics simulations to characterize the impact of shear stress and impeller design on the biogas production after sequential additions of substrate. Liquid phase (cattle manure digestate) rheological law was experimentally determined and input in numerical simulations. The results showed that the original use of a double helical ribbon in digester allowed a significantly faster dispersion of fresh substrate than the use of a classical Rushton turbine, leading to a 50% higher methane production rate. However, with both impellers, too high agitation rates entailed a clear slow-down of production rate and a decrease in CH4 content. To avoid this loss of productivity, it was shown that the maximal value of shear stress, determined by numerical simulations, was a consistent parameter to set the upper agitation conditions in digesters.

Energy efficiency of different impellers in stirred tank reactors

The flow energy efficiency of different impellers for stirring rheologically complex fluids (yield stress fluids) in cylindrical tanks has been investigated in this paper. Four impellers have been used: a Maxblend, an anchor, a gate and a double helical ribbon impeller. Our investigations were achieved via numerical simulations with the help of a CFD (computational fluid dynamics) computer program CFX 13.0. It was found from the predicted results that the Maxblend impeller gives the best performance. The effects of some design parameters on the flow energy efficiency and the power consumption have been also studied; it concerns the Maxblend impeller: its grid size and its paddle curvature.

RENDIMIENTO DE DOS SISTEMAS DE MEZCLADO EN LA PRODUCCIÓN DE YOGUR BATIDO

Two mixing devices used for stirred yogurt production were evaluated. One system had two coaxial impellers, an anchor and a modified anchor; the other had a double helical ribbon agitator. The performance of the mixing systems was evaluated using cooling time, syneresis, and sensory analysis. Cooling time results for both mixing systems showed that the double helical ribbon system needed shorter cooling times than the coaxial anchor-modified anchor one. There were no statistical differences in syneresis of the yogurts made in the different mixing vessels. Sensory analysis was evaluated considering difference-from-control test, quantitative descriptive analysis, and preference test. These experiments showed that the best quality was obtained using a double helical ribbon device at 45 rpm. In light of the findings, it is possible to suggest stirred yogurt production in small tanks with a double helical ribbon mixing system at 45 rpm.

Power consumption, local and average volumetric mass transfer coefficient in multiple-impeller stirred bioreactors for xanthan gum solutions

Mass transfer and mixing performances are very critical for xanthan gum fermentation process. Power consumption, local and average volumetric mass transfer coefficient (kLa) were compared for six impeller combinations in a 50L perspex tank with xanthan gum solutions. Impellers used in various combinations can be distinguished as two categories: “small-diameter” impeller, which include Rushton turbine, hollow blade turbine and wide-blade hydrofoil impeller and “large-diameter” including ellipse gate impeller, Intermig and double helical ribbon. The results show that in order to gain the same power input, the rotating speed of “small-diameter” impeller combinations increases as the concentration of xanthan gum increases, while it decreases for “large-diameter” impeller combinations. The two categories also show distinguished mass transfer rates. For the “small-diameter” impeller combinations, the kLa values near the wall region drop faster than that in other areas as the concentration of xanthan gum increases. While for the “large-diameter” impeller combinations, the distribution of kLa is homogenous except in the bottom area but with poor gas dispersion capabilities as concentration of xanthan gum increases. The averaged kLa for each impller combination was correlated well with the specific gassed power input, superficial gas velocity and effective viscosity. The obtained correlation shows that the kLa strongly depends on specific power input and viscosity, but is less influenced by the gas flow rate.

Hydrodynamics in a vessel stirred by simple and double helical ribbon impellers

Abstract The present paper treats the agitation of shear thinning fluids in a cylindrical unbaffled vessel with a flat-bottom. Two kinds of impellers have been used: simple and double helical ribbons. This work is achieved with the help of CFD package (CFX-12.0), which is based on the finite volume method to solve the continuity and momentum equations. The effect of impeller rotational speed, fluid rheology, impeller size, impeller clearance from the tank bottom on the flow fields and power consumption have been investigated. Finally, a comparison between the two impellers is made. It was found that the best performance is obtained with double helical ribbon impeller placed at the middle of the tank, with moderate blade size and operating at Re g > 20. Our results have been compared with those of other literatures and a satisfactory agreement is observed.

Hydrodynamics Characterization of an Ellipse Gate Impeller by Experimental and Numerical Studies

AbstractHydrodynamics characteristics like flow pattern, shear rate distribution, power consumption, axial pumping capacity, mixing time, and mixing efficiency of an ellipse gate (EG) impeller were investigated by experimental and numerical methods. The numerical simulation results were validated by experimental data of power consumption and mixing time. Results indicate that the axial pumping number of the EG impeller is larger than that of any other reported large‐scale impeller under laminar regime, and that the shear rate formed by this impeller is less sensitive to Reynolds numbers. In‐depth analysis reveals the different function of each part of the EG impeller under different flow regimes. This impeller provides an almost similar mixing efficiency like the double‐helical ribbon impeller under laminar regime, but much higher mixing efficiency both under transitional and turbulent flow regimes.

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

Finite element modeling of the laminar and transition flow of the Superblend dual shaft coaxial mixer on parallel computers

Abstract The macro-mixing mechanisms of the Superblend coaxial mixer consisting of a Maxblend impeller and a double helical ribbon agitator mounted on two independent coaxial shafts rotating at different speeds are numerically investigated. The simulations are based on the resolution of the Navier–Stokes equations with help of a parallel three-dimensional finite element solver exploiting the capabilities of high performance computers. To model the rotation of agitators a hybrid approach based on a novel finite element sliding mesh and fictitious domain method is used. The power consumption, the flow patterns, the shear rate distribution, the pumping capacity and the mixing time of the Superblend mixer are calculated from the simulated hydrodynamics. The simulations allow observing the flow as it evolves from deep laminar (Re=0.1) to transition (Re=520) regime. As Reynolds number increases, several recirculation zones above and below the middle of the tank are formed. It is found that operating the agitators in co-rotation mode requires less power consumption and exhibits equal or shorter mixing time than counter-rotation mode. The larger power consumption in counter-rotating mode is caused by the presence of high shear vortices generated between the two coaxial agitators. Furthermore it is shown that the shear distribution throughout the Superblend coaxial mixer operating in co-rotation mode is almost homogenous, which is highly desirable for shear sensitive products. In view of the results obtained in this work, the Superblend coaxial mixer is found as a good alternative for tough mixing applications.

Calculation of Metzner Constant for Double Helical Ribbon Impeller by Computational Fluid Dynamic Method

Using the multiple reference frames (MRF) impeller method, the three-dimensional non-Newtonian flow field generated by a double helical ribbon (DHR) impeller has been simulated. The velocity field calculated by the numerical simulation was similar to the previous studies and the power constant agreed well with the experimental data. Three computational fluid dynamic (CFD) methods, labeled I, II and III, were used to compute the Metzner constant k s. The results showed that the calculated value from the slop method (method I) was consistent with the experimental data. Method II, which took the maximal circumference-average shear rate around the impeller as the effective shear rate to compute k s, also showed good agreement with the experiment. However, both methods suffer from the complexity of calculation procedures. A new method (method III) was devised in this paper to use the area-weighted average viscosity around the impeller as the effective viscosity for calculating k s. Method III showed both good accuracy and ease of use.

Self-assembled All-inorganic Chiral Polyoxovanadate: Spontaneous Resolution of Nitrate-incorporated Octadecavanadate

Abstract A chiral octadecavanadate has been isolated and optically resolved into enantiomers by spontaneous resolution. From the partial oxidation reaction of a reduced decavanadate [V10O26]4− with nitric acid, a chiral octadecavanadate encapsulating a nitrate template was formed. A nitrate anion was incorporated at the center of the spherical polyoxovanadate cage. The structure may be regarded as a double helical V8 ribbon between two square pyramidal VVIO5 units, consisting of half-pitch helix with ca. 11.0 Å diameter and a half pitch length of 8 Å.

Power and Mixing Time Study Involving a Maxblend ® Impeller with Viscous Newtonian and Non-Newtonian Fluids

Physical investigations have been carried out to characterize the power consumption and the mixing time in a vessel equipped with a Maxblend ® impeller (Sumitomo Mechanical Equipment, Japan). The laminar, transition and turbulent flow regimes were investigated using viscous Newtonian and non-Newtonian shear-thinning fluids. For the determination of the mixing times, a discolouration method based on a fast acid-base reaction was used in conjunction with image analysis to generate mixing evolution curves. Three vessel sizes were used from which the influence of the scale on the mixing efficiency and the power consumption could be determined. With the geometrically similar mixers used here, it was clearly shown that power and efficiency do not depend on the scale, under certain conditions. The power consumption was found almost identical as that of an anchor but far lower than that of a double helical ribbon impeller. The power and shear constants, K p and K s, were found very sensitive to small variations in the bottom clearance. The K s value remained almost constant for a wide range of flow indices. The dimensionless mixing time, Nt m, decreased with the reciprocal of the Reynolds number in the laminar flow region. Based on data from the literature, a comparison with many different large impellers also indicated that the Maxblend technology makes better use of the mixing power in the upper part of the laminar regime and in the low turbulent regime.