Ribbon Mixers Research Articles

DEM study of mixing performance of superquadric particles in an industrial-scale ribbon mixer

Horizontal ribbon mixers with a U-shaped vessel are widely used to mix non-spherical particles, yet the accurate characterisation of particle morphology and in-depth elucidation of the mixing mechanisms are still lacking. In this study, a superquadric DEM model is developed to study the mixing behaviours of non-spherical particles in an industrial U-shaped ribbon mixer. The model of superquadric particle mixing is first validated against experiments. Then the influences of two key particle shape-related parameters (i.e., aspect ratio and surface blockiness) on the mixer performance are comprehensively evaluated in terms of Lacey mixing index, velocity distribution, dispersion coefficient and force analysis. The results show a weak dependence of mixing behaviours on the variation of particle aspect ratio. The case of aspect ratio of 1.0 indicates the minimum degree of mixing, followed by the cases of aspect ratios of 0.5, 2.0 and 4.0. On the other hand, with the increase of surface blockiness from 2 to 7, the degree of mixing at the quasi-steady state decreases from 0.886 to 0.875, and the proportion of particles having larger contact forces also slightly decreases. This work provides a cost-effective tool to evaluate the superquadric particle mixing in an industrial U-shaped ribbon mixer.

Framework for choosing energy efficient mixing equipment whatever motions it induces: Example of application for planetary and helical ribbon mixers

Energy savings in industrial processes have become one of the main goals of the current society. This paper aims to propose a systematic approach for the assessment of mixing equipment energy performance for highly viscous fluids. The framework is illustrated and applied by performing a rational comparison of the homogenization efficiency of classical mixing systems (involving an agitator revolving around a vertical axis centred in the tank) with that of planetary mixers when mixing highly viscous fluids. For that purpose, the pioneering framework of Zlokarnik is first reminded and its limitations with regard to non-traditional mixing equipment are discussed. Then, the theoretical background to extend this approach to these latter is presented. Experimental data on power consumption and mixing time are provided to demonstrate the applicability of the present frame. The results illustrated that a planetary mixer can be an economic solution for performing highly viscous fluids homogenization.

Discrete particle simulation for mixing of granular materials in ribbon mixers: A scale-up study

This work presents a numerical study of particle mixing and scale-up of ribbon mixers with four sizes (mixer A - 5.62 L, mixer B - 49.02 L, mixer C - 142.91 L, and mixer D - 482.32 L). The results show that the mixing performance becomes worse with the mixer size increasing. A correlation is proposed to predict the mixing rate as a function of the scale-up ratio and Froude number. The top-bottom initial loading condition of granular materials mix much faster than the side-side and front-back loading conditions. With the mixer size becoming larger, the total mixing time increases significantly. The power consumption per unit mass for the four mixers is in this order: mixer A > mixer D > mixer C > mixer B. The results suggest that scale-up is necessary to save mixing time and power consumption and reduce additional operating costs such as labour and down-time.

DEM study of effects of particle size and density on mixing behaviour in a ribbon mixer

Ribbon mixers are used in various industries for the mixing of particle matter. The discrete element method (DEM) is used to investigate the effects of the particle size and density on particle mixing in a laboratory-scale ribbon mixer. The mixer consists of a ribbon impeller with spokes and a horizontal cylindrical vessel. The particle size varies from 5 to 15 mm, and density from 417 to 2500 kg/m3. A particle-scale index is used to analyse the mixing state, and the impeller torque is used to evaluate the mixer's mechanical performance. The results show that the mixing rate generally becomes slower at a given density when the particle size is reduced with the fill-level fixed. For small particle sizes, the density effect was negligible, and the particle-size effect was described by an empirical equation, which was also extrapolated for much smaller particles. Effects of particle size and density on the impeller torque were also investigated.

DEM investigation of mixing indices in a ribbon mixer

Mixing index is an important parameter to understand and assess the mixing state in various mixers including ribbon mixers, the typical food processing devices. Many mixing indices based on either sample variance methods or non-sample variance methods have been proposed and used in the past, however, they were not well compared in the literature to evaluate their accuracy of assessing the final mixing state. In this study, discrete element method (DEM) modelling is used to investigate and compare the accuracy of these mixing indices for mixing of uniform particles in a horizontal cylindrical ribbon mixer. The sample variance methods for mixing indices are first compared both at particle- and macro-scale levels. In addition, non-sample variance methods, namely entropy and non-sampling indices are compared against the results from the sample variance methods. The simulation results indicate that, among the indices considered in this study, Lacey index shows the most accurate results. The Lacey index is regarded to be the most suitable mixing index to evaluate the steady-state mixing state of the ribbon mixer in the real-time (or without stopping the impeller) at both the particle- and macro-scale levels. The study is useful for the selection of a proper mixing index for a specific mixture in a given mixer.

Insights into granular mixing in vertical ribbon mixers

AbstractIn this research, granular mixing in vertical ribbon mixers was studied by means of discrete element method (DEM) simulations. Time‐averaged velocity distributions, granular mixing, mean square displacement of tracer particles, diffusion coefficients, and Peclet number in axial and azimuthal directions were used to find the flow pattern of particles and the dominant mixing mechanisms. Strong azimuthal motion of particles was observed, and it was found that by decreasing the height in the mixer, this azimuthal motion becomes stronger. The effects of rotating speed, fill level, and filling method were studied on the quality of mixing, where the quality of mixing was assessed using relative SD mixing index. It was found that by increasing the rotation speed from ωs = 60 rpm‐120 rpm, the mixing quality improves in a linear trend. Decreasing the fill level from 12 cm‐11 cm did not change the mixing quality, while with further decrease of the fill level to 10 cm, the mixing quality was improved. The mixing quality was also much better when the powder was inserted side‐to‐side instead of in axial layers.

Discrete element analysis of the particle mixing performance in a ribbon mixer with a double U-shaped vessel

In this study, a discrete element method is employed to simulate the mixing process of solid particles in a horizontal ribbon mixer with a double U-shaped vessel. A mixing index, i.e. the so-called Lacey index, is adopted to evaluate the mixing quality of particles. The effects of the operational and geometrical parameters including initial loading, particle size, impeller rotational speed, and inner blades on the mixing quality of particles have been investigated. Results suggest that the initial loading and the impeller rotational speed have significant effects on the mixing quality of particles, while the other two parameters have relatively small effects. Moreover, the effect of each parameter on the mixing quality has been explained by utilizing the relative velocity components between the centroids of particles after collision, and this ribbon mixer provides much more intense relative movements of particles along the vertical direction than the axial and side–side directions. Finally, the mixing performance between the ribbon mixers with respective single and double U-shaped vessels is compared. Results show that the ribbon mixer with a double U-shaped vessel shows better mixing performance under top–bottom and front–back initial loadings, however, worse mixing performance under side–side initial loading.

Optimisation of mixing performance of helical ribbon mixers for high throughput applications using computational fluid dynamics

The work presented focuses on the optimisation in a 1L vessel using an anchor with a helical ribbon design using CFD and a learning algorithm, optimised by minimisation of the torque output of the mixer and the homogeneity of the mixture in the vessel after a defined mixing time. The results were successfully validated experimentally using Electrical Resistance Tomography (ERT) and direct torque measurements.The study determined that the height of the mixer is a key factor in the performance of the mixer, with other significant factors present, but with a lower impact. For the case of torque, all design features of the mixer which increase the size, i.e., surface area acting against motion, were found to be significant in increasing the modelled torque response. The Auger screw was found to have no significant impact on either mixing and torque response.The results illustrate the capability of optimisation algorithms to achieve results comparable to those achieved experimentally, while assessing a significantly larger number of design options and optimising for several performance indicators simultaneously.

Mixing performance of ribbon mixers: Effects of operational parameters

Ribbon mixers are widely used in practice because they are capable of providing high speed convective mixing. Here, the discrete element method (DEM) is used to investigate the effects of impeller speed and fill level on the mixing behaviors of mixtures of particles with different cohesion in two-bladed and four-bladed ribbon mixers, each having a horizontal cylindrical vessel. The mixing behaviors are characterized by a particle-scale mixing index. Simulations show that the mixing rate increases with the impeller speed for both the cohesive and non-cohesive particle mixtures up to a certain speed, beyond which it shows a reduction. The mixing rate becomes poorer at higher impeller speeds for mixing of cohesive particles in the 2-bladed mixer. Inspection of velocity fields shows that many localized recirculation flows exist when the mixing non-cohesive particles, preventing the overall mixing. By contrast, when mixing cohesive particles, there exist circumferential flow about the shaft axis and convective flow in the horizontal axial direction, improving the particle mixing. The mixing rate deteriorated with an increase of the fill level in both the two-bladed and four-bladed mixers. The mixing rate of the particles is higher in the four-bladed mixer compared to the two-bladed mixer. With the increase of fill level, the particle flow changes successively from the sliding type of flow to recirculation flow and then to cascading flow for non-cohesive particles. The four-bladed mixer performs better for mixing at high fill levels and stronger cohesion, consolidating its advantage for mixing cohesive particles.

Influence of mixing protocol on fresh and hardened properties of self-compacting concrete

Mixing procedure as well as type of mixing highly influence the fresh and hardened properties of self-compacting concrete (SCC) besides other parameters such as mix proportions, water to powder ratio, and mineral and chemical admixtures. This study attempts to investigate the effect of different types of mixing on the properties of SCC for providing a guideline in choosing the appropriate mixer type. Further, the influence of time of addition of superplasticizer on the properties of concrete is also studied. Four different types of mixer such as pan, Eirich, drum, and ribbon mixers are employed. It is found that the compressive strength and horizontal slump of mix obtained through Eirich mixer is higher compared to all other mixes. Further, no significant influence of time of addition of superplasticizer is noticed on hardened properties whereas marginal increase in flow is observed with delayed addition.

3D/2D hybrid model for ribbon impellers operating in laminar regime

In the study a 3D/2D hybrid model for ribbon mixers, for a laminar range of mixing was verified. Distributions of the experimental and model values of velocity components and the experimental and model values of mixing power for this type of mixers were compared. Further on, based on the model solutions the optimum parameters of ribbon agitators due to the time of homogenization were established.

Flow and mixing performance in helical ribbon mixers

This paper explores the effect of impeller design on the chaotic mixing in a helical ribbon mixer. Numerical simulations using the smoothed particle hydrodynamics (SPH) method are used to solve for the fluid flow around the moving impellor. The mixing flow is visualised by extracting the hyperbolic Lagrangian coherent structures (LCS) of the flow from the finite-time Lyapunov exponent (FTLE) field, which provides qualitative information about the topology of the mixing structures and highlights the major barriers to transport. This is combined with a mixing measure that calculates the degree of mixing between different regions. A single helical ribbon mixer (SHR) was found to create an axially symmetric circulation cell that moves fluid down the outside of the tank and upward near the centre. Smaller circulation cells near the inside edge of the ribbon superimpose a chaotic mixing flow over this primary flow, which stretches and folds fluid volumes around the ribbon according to the shape of the LCSs. The horizontal struts supporting the ribbon generate strong circumferential mixing, but this is only within a narrow horizontal plane with thickness comparable with the strut diameter. The addition of an extra ribbon, 180 degrees out of phase (DHR) doubles the number of smaller circulation cells while decreasing their efficiency in mixing the fluid between the inner and outer regions of the tank. Overall the mixing rate is improved over the other ribbon geometries. The addition of a central screw (CSR) creates a zone of low mixing immediately surrounding the screw. The mixing processes surrounding the helical ribbon are qualitatively identical to the SHR, but the overall mixing rate is slightly increased.

CFD investigation of new helical ribbon mixers bottom shapes to improve pumping

AbstractIt is well known that helical ribbon (HR) impellers exhibit poor bottom pumping properties. Three different HR impeller geometries, varying in their bottom design, are compared in terms of their mixing performance. Particular emphasis is given to the improvement of pumping capabilities. The selected bottom geometries were a standard bottom termination, an anchor‐type termination and a paddle‐type termination. The investigation was based on the use of 3D finite‐element simulation to assess the hydrodynamics and mixing performance using various distributive and dispersive criteria. Analysis of Poincaré maps showed that an HR with a paddle‐type termination operating in a down‐pumping mode offered the best performance. Copyright © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.

Basic trends in the improvement of domestic equipment for mixing particulate materials

Area I: Establishing the Field of Use of Standard Mixers. In a particulate material there is a very complex relationship between the particles that are in contact. This relationship is governed by the properties of the particles (their material nature, shape, relative configuration, size) and the external conditions (humidity, temperature, the occurrence of electric charges on the particles, the degree of compression, etc.). As a result it is virtually impossible to predict the behavior of a particulate material when a mixing element acts on it without conducting experiments. At the present time it is only possible to identify the class of mixer according to the physicomechanical properties of the particulate material to be mixed and the volume of a single portion. A definite model can only be selected from several designs by experimental studies, which require a suitable stock of laboratory mixing equipment. In terms of their process mechanism, standard batch mixers can be divided into two classes: circulatory and bulk mixing. In circulatory mixers the material being mixed is displaced by the operating element along a given circuit, directly acting on the particles in a relatively small area of the internal volume of the mixing compartment. They are only effective for materials that flow well (according to the classification of the Severodonetsk Scientific-Research Institute of Chemical Engineering for noncohesive and cohesively flowing bulk materials [1]). Centrifugal paddle mixers of type TsL and planetary screw mixers of type PSh can be assigned to this class [2, 3]. In bulk mixers the particulate material is displaced by the working surfaces of the agitator in a random manner around the internal volume of the mixing chamber. In this case the blades of the mixing element cover virtually all the internal zones of the mixer's body. Mixers for cohesive particulate materials are also recommended, but cohesively flowing particulate materials can also be mixed in them. Bulk mixers also include LN and GN ribbon mixers, PZh plow mixers, ZL Z-shaped mixers, and ZSh Z-shaped mixers with discharge screw [2, 3]. The choice of the class and type of mixer can be made initially according to the required linear rate of rotation of the mixer operating element, which is calculated from an empirical equation taking into account the required homogeneity of the mixture, the weight fraction of the key component, and the tensile strength of the particulate material [4]. Area II: Choice of Optimum Operating and Design Parameters for Mixers with an Established Design, In most studies in this area the limiting value of the coefficient of heterogeneity of the mixture and the time for attaining this value, the optimum values of the coefficient for filling the mixing chamber with material, the rate of rotation of the operating element, and the energy consumption are determined experimentally on the selected mixer (with respect to a specific mixture of particulate material). While there is an overall benefit, the general disadvantage of these experiments should be noted. The recommendations or formulas obtained from the experiments can only be applied to a given model of mixer over the studied range of operating conditions and for a specific particulate material. Mixer optimization using mathematical models of the mixing process has unfortunately not yet become a sufficiently well established method amongst the developers of mixing equipment. In order to determine the parameters of the models

Heat transfer to viscous Newtonian and non-Newtonian fluids for helical ribbon mixers

Temperature distributions and heat transfer rates have been measured for helical ribbon impellers in vessels of diameter 0.15 and 0.4m. Viscous Newtonian and non-Newtonian liquids have been used covering a Reynolds number range of 10 −2 < Re < 10 3. The critical impeller speed required to eliminate gross temperature gradients within the bulk of the vessel has been established and has been shown to be influenced critically by agitator geometry and fluid properties. The measured rates of heat transfer from the wall of the vessel to the agitated liquid indicate that the film heat-transfer coefficient is dependent upon agitator speed in the creeping flow region. Further, the effect of impeller geometry upon rates of heat transfer has been shown to be significant in this region. General correlations have been provided for design purposes. Theoretical models that fail to allow for the effect of impeller geometry on the film heat-transfer coefficient can give rise to large errors in the creeping flow region.

The mixing of aqueous carboxymethylcellulose solution with Loch Aline sand

The mixing of 1% aqueous carboxymethylcellulose solution with Loch Aline sand has been investigated in lab-scale Z-blade and ribbon mixers. Liquid was added in the range 0 – 15% dry weight basis by pour, sparge or spray methods. Liquid distribution in the bulk material (excluding cake) was investigated and a mixing index I was defined which at steady state was independent of the type of machine, composition or operating conditions. This observation has been qualitatively explained by a simple model. Rates of mixing were studied and shown to consist of three regions: an initial solid—liquid contacting period in which mixing is very rapid; an intimate mixing period; and a steady state condition. Pour liquid addition gave the most rapid mixing. The ribbon mixer was most effective for 5% liquid while the Z-blade was superior at 15% liquid.

Power consumption of helical ribbon mixers in viscous newtonian and non-newtonian fluids

Power input measurements are reported for helical ribbon impellers for two scales; a 0.15 m diameter and a 0.4 m diameter tank. Data for viscous Newtonian and non-Newtonian fluids are brought together by use of the average apparent viscosity concept and the following equation: ▪ where k s is the shear rate constant, c is the clearance between vessel wall and impeller with diameter D. Power measurements from this work combined with relevant information extracted from the published literature indicate that impeller geometry has a profound effect upon power requirement, particularly in the laminar region, where the reported data can be described by: ▪ where K p is a geometric constant and all the other symbols have their usual significance. Theoretical models which fail to allow for system geometry and fluid properties give values which may be seriously in error.

Power requirements of helical ribbon mixers

The aim of this paper is to present an equation which enables mixing power to be calculated for helical ribbon mixers within the laminar region of mixing (Re < 100).

Power Requirement of Vertical Ribbon Mixer

The effects of various operating conditions on the rotational torque of vertical ribbon mixers have been studied.The rotational torque, T[Kgm] of the mixers increases with increasing the height of powders bed, H[m], the bulk density, ρa[kg/m3] and the internal friction factor μi[-] of the powders, and the diameter of the ribbon, D0[m]. In the range of the rotating speed, N, of 30≤N≤100r.p.m., the torque remains almost constant and has a tendency to decrease as the ribbon width increases.The torque data are well correlated with the experimental conditions by the relation T=0.15ρa1.0μi0.7Rp1.2Rb-0.8H1.5D03.0, where, Rp [-] and Rb [-], respectively, are the ratios of the pitch and the width of the ribbon to the ribbon diameter.

Power Requirement of Horizontal Ribbon Mixers

Power requirement of solid mixers is influenced in a complicated manner by various factors, such as the operating conditions, the properties of the powders, the dimension of the mixers, and so on.In the present work, the relation between the torque of the horizontal mixers and those factors has been studied.The torque increases with increasing the charged ratio, f, the bulk density, ρa and the internal friction factor, μi of the powders, and the dimension of the mixer. On the other hand, the torque is not influenced by the rotating speed, N, in the range of 30≤N≤200r.p.m. The shear force has been found to be a dominant factor in the total power requirement of the mixer.The power data are well correlated with the experimental conditions by the relation Pgc=1.3μiρadf(φD02L)deNs de=[πL/4{2/P0(D02-(D0-2b0)2)+1/Pi(Di2-(Di-2bi)2)}]1/2 where D: diameter, L: length, b: width, Pi, 0: pitch, Ns: rotating speed, all in M. K. S. units, and suffices “0” and “i” refer to the outer and inner ribbon.