Numerical simulation of particle dynamics in different flow regimes in a rotating drum

Flow regimes in a horizontal rotating drum are important to industrial applications but the underlying mechanisms are not clear. This paper investigated the granular flow dynamics in different regimes using the discrete element method. By varying the rotation speed and particle-wall sliding friction over a wide range, six flow regimes were produced. The macroscopic and microscopic behaviour of the particle flow were systematically analysed. The results showed that the angle of repose of the moving particle bed had a weak dependence on the rotation speed in the slumping and rolling regimes, and increased significantly as the flow transited to the cascading and cataracting regimes. The mean flow velocity increased with the rotation speed, but the normalised velocity against the drum speed in the continuous regimes collapsed into a single curve, which can be well described by a log-normal distribution. The particle bed at low rotation speed had a similar density to those of the random loose packing, and became more dilated with the increase of the rotation speed. Similarly, the mean coordination number showed linear dependence on the drum speed. Both the collision energy and collision frequency increased with the rotation speed. However, the normalised collision energy in different regimes can be fitted with a simple scaling law.     

Microdynamic analysis of particle flow in a horizontal rotating drum

The flow of particles in a horizontal rotating drum is studied based on the results generated by Distinct Element Method (DEM). The simulation conditions are comparable to those measured by means of Positron Emission Particle Tracking (PEPT), with a drum being 100 mm in diameter, 35% filled by spheres of 3 mm diameter, and rotating at a speed from 10 to 65 rpm. The simulation method is validated from its good agreement with the PEPT measurement in terms of the dynamic angle of repose and spatial velocity fields. The dependence of flow behaviour on rotation speed is then analysed based on the DEM results, aiming to establish the spatial and statistical distributions of microdynamic variables related to flow structure such as porosity and coordination number, and force structure such as particle interaction forces, relative collision velocity and collision frequency. An attempt has also been made to explain the effect of rotation speed on agglomeration based on the present findings.     

Spray-agglomeration of NPK-fertilizer in a rotating drum granulator

The continuous production of 3 to 4.5 mm NPK-granules is achieved by spraying an appropriate mixture onto recycle or reflux (undersize) product in a rotating drum. Hot air is used to evaporate the water. An extensive study was performed on an industrial granulator to evaluate mass and energy balances, to define the average residence time of the reflux-particles and to develop a model for particle growth.The average residence time was measured from tracer experiments. The mechanism of granulation follows the particle growth principle of drying and layering. Model equations developed by Nienow for a batch fluidized bed granulation were modified to predict continuous operations.The model equation defines the thickness of the coating layer in terms of the initial particle size and mass flow rate of the reflux particles, the mass flow rate of the sprayed mixture, and both the average residence time of the particles and their contact time with the spray. The resulting equation is

     

Investigation of axial segregation in a rotating drum

A binary mixture of particles is used to investigate axial segregation in a rotating drum. The effects of the rotational speed, the fill level, the particle size ratio and the resilience of particles on the segregated band widths are evaluated. When rigid glass particles 0.35 mm or 0.70 mm as the key components are mixed particles with a size ratio of 2, 3, or 4, the dimensionless band width is less affected by the rotational speed, except at fill levels 10% and 20% with 0.35 mm particles as the key components. The dimensionless band width decreases with the increase of the level of fill. Experimental results show that there exists a size ratio where the dimensionless band width has a minimum value using rigid particles. The value is approximately 3. The existence of this value was attributed to the diffusion and inertia mechanisms of the motion of a single particle.A comparison was made on the influence of the fill level and the speed of rotation on the dimensionless band width using rigid glass beads and non-rigid rubber particles. The dimensionless band width increases with the increase of the level of fill using 1 mm rubber particles as the key components with a size ratio 2 at all rotational speeds studied. Whereas the dimensionless band width decreases with increasing level of fill when 1 mm glass particles as the key components with a size ratio 2 are mixed at all rotational speeds studied. When particles of different elastic properties are mixed, segregated bands are formed but are less pure compared to the bands formed due to size segregation. The interfaces between the rubber-rich bands and the glass-rich bands are curved. A remarkable convective motion at the surface of the particle bed is observed.     

Experimental analysis of the dynamic properties of wet granular matter in a rotating drum

We performed experiments to measure the dynamic properties of wet granular matter in a rotating drum device. Four different amounts of liquid and rotation speeds were used in the experiments. The purpose was to quantify the effect of the cohesive force in the granular system. The results show that when only very small amounts of liquid were added, no liquid bridges formed. This is because the liquid was first trapped on the surface of the particles due to the particle roughness. When the volume fraction of the fluid became larger, liquid bridges formed on almost every particle. The results showed that the addition of liquid contents, and changing the rotation speed, both had a significant effect on the dynamic properties of granular matter. This was due to the hysteretic formation and rupturing of liquid bridges and the introduction of inertial force to the device. After the liquid bridges formed between all particles the average energy dissipation due to the hysteretic formation and rupturing of the liquid bridges increased with an increase in the liquid content.     

An experimental study on the effect of liquid content and viscosity on particle segregation in a rotating drum

The segregation phenomenon of wet granular materials was experimentally studied in a quasi-2D rotating drum. The mono-disperse systems and binary-mixture systems (with 4 mm and 2 mm glass beads and 40% filled volume fraction) were used. All the experiments were controlled so the Froude number of the rolling regime was 2.79 × 10^− 4. The effects of the volume and the viscosity of the liquid added to the granular system on the segregation index and angle of repose in the rotating drum were investigated and are discussed in this paper.The experimental results indicate that the volume and viscosity of the added liquid have significant effects on the wet granular flow. The results demonstrate that the segregation index decreases with an increase of the repose angle of the wet granular materials, regardless of the volume or viscosity of the added liquid.     

Gas-solid mass transfer in a rotating drum

Gas-solid volumetric mass transfer coefficients (k_SA) were measured in a rotary drum by evaporation of n-decane into dry air from the surface of porous solids. The effect of drum rotational speed, N (0.09 to 2.0 min^?1), solids volume fraction, η (0.043 to 0.25) and the presence of baffles on k_SA were investigated. In the presence of baffles, k_SA was independent of ηand higher than in the case of a rolling bed where no baffles were present. For the rolling bed case, k_SA increased with increasing η. Mass transfer in the rolling bed was modeled based on the particle motion.     

Analysis of granule breakage in a rotary mixing drum: Experimental study and distinct element analysis

Rotary drums are commonly used in particulate solid industries for mixing, coating and reactions. The process is often accompanied by undesirable breakage of granules. For this reason, a scaled-down version is sometimes used as an attrition testing device. In this work, the attrition of granules inside a rotary drum at 18, 35 and 52 rpm drum rotation speeds for 4000 cycles is studied. The granules used in this study have been produced by extrusion and spheronisation with a size range of 500 to 1000 μm. The rotary drum has an internal diameter of 0.39 m, axial length of 0.3 m and a single baffle. The extent of breakage is quantified by sieving out fine debris which is two sieve sizes smaller than the feed particles. To relate the extent of breakage in the drum to granule characteristics, single granule impact tests have been performed on one type of granule at several velocities. The effects of particle size and impact velocity are analysed and a power-law relationship is fitted between impact velocity and single granule breakage. This information is then used to simulate granule breakage in a rotary drum by Distinct Element Method (DEM). The drum is simulated for 5 rotations at the rotational speeds stated above and the breakage rate is extrapolated to 4000 cycles where it is compared to experimental results obtained. The trends for particle breakage in both experiments (determined by sieving) and extrapolated DEM simulations are in agreement however the orders of magnitudes are different. The comparison shows that the extent of breakage obtained from extrapolated simulations is overestimated at drum speed of 35 and 52 rpm and underestimated at 18 rpm. There is close agreement between experiments and extrapolated DEM simulations for particle breakage at 18 rpm only after 4000. Furthermore, the effect of air drag on the attrition of granules by impact at a drum rotation speed of 52 rpm is investigated, where it is found to significantly reduce the breakage results.     

Experimental study on fluidization of fine powders in rotating drums with various wall friction and baffled rotating drums

Fine powders were found to be fluidized in a rotating drum by internal cycling gas by the drum rotation. It is essentially a fluidized bed without requiring any external fluidizing gas. Such a rotating drum can be regarded as a new gasless fluidized bed for fine powders in contrast to a traditional fluidized bed, possibly leading to a considerable amount of energy savings. In addition, the fluidization quality of fine powders was found to be further improved with the assistance of drum rotation because of the shearing movement among particles that eliminates channeling and cracks and possibly also breaks agglomerates. Five regimes were identified in the rotating drum including slipping, avalanching-sliding, aerated, fluidization and re-compacted regimes. It was also found that drum wall friction plays an important role to fluidize fine powders because the friction carries particles to the freeboard, leading to gas cycling that fluidizes the powders. As well, three types of specially designed baffles were utilized to promote powder fluidization in rotating drums. These baffles effectively bring an early onset of all the regimes in rotating drums by reducing powder-wall slipping, carrying particles and bringing additional gas to the powders.     

Simulation of the granular flow of cylindrical particles in the rotating drum

This study aims at unveiling the effect of particle shape on granular flow behavior. Discrete element method is used to simulate cylindrical particles with different aspect ratios in the rotating drum operating in the rolling regime. The results demonstrate that the cylindrical particles exhibit similar general flow patterns as the spherical particles. As the aspect ratio of the cylindrical particles increases, the active‐passive interfaces become steeper, and the number fraction, solid residence time, and collision force in the active region decreases. The mechanism underlying the difference is the preferential orientation, with particles of greater aspect ratios increasingly orientating their longitudinal axes perpendicular to the drum length. Also, particle alignment in the active region is more uniform than that in the passive region. The results obtained in this work provide new insights regarding the impact of particle shape on granular flow in the rotating drum. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3835–3848, 2018     

Determining the thickness of liquid film in laminar condition on a rotating drum surface using CFD

This is a pioneering paper that deals with the semi-steady-state liquid film thickness on a rotating drum partially submerged in liquid using CFD. The CFD predictions have been compared with both experimental results and analytical solutions. The liquid film thickness and the film rising on a circular curved wall were monitored for various rpm of the rotating drum. The 2D simulations were performed neglecting the end effects (assuming the rotating drum is very long). As the rotating drum speed was increased, the film thickness was found to increase at the film rising side and at the minimum film thickness point (top of the rotating drum). There was never been a steady-state condition reached, the opposite was expected as a wiper is used in the CFD calculation (same as the experimental investigation). The liquid that leaves the domain by the wiper (as outflow boundary condition) is forced to enter to the domain from far end to keep the level of the free surface constant.     

滚筒式煤泥干燥系统的改进

对滚筒式煤泥干燥系统中往复式燃烧炉在生产中出现的问题进行了分析,找出了燃烧炉砌体缝隙生成的原因;采用新型浇注材料烧结整体密封耐磨陶瓷对炉体进行密封后,煤泥干燥系统生产稳定,燃烧炉热工效率大大提高,节约了能源消耗,干燥机的生产能力增加。     

Inter-tablet coating variability: Residence times in a horizontal pan coater

This paper investigates inter-tablet coating variability, specifically, tablet residence times within the spray zone. Discrete element method computer simulations, experiments, and analytical investigations are performed to measure the residence time per pass, the circulation time, and appearance frequency of spherical shaped tablets for a range of pan speeds and tablet loads. In addition, the fractional residence time, defined as the ratio of time spent by a tablet in the spray zone to the total coating time, is measured. The average fractional residence time (averaged over all the tablets in the bed) is found to be equal to the ratio of the time-averaged number of tablets exposed to the spray to the total number of tablets in the pan, a result that is consistent with analyses. The average fractional residence time is observed to be independent of pan speed and total coating time. Furthermore, the fractional residence time is shown to be related to the residence time per pass and circulation time per pass. Appearance frequency is defined as the number of appearances a tablet makes in the spray zone per pan rotation. Simulations and analyses show that appearance frequency decreases with increasing pan speed. Circulation time per pass for a tablet is defined as the average time between successive appearances in the spray zone and residence time per pass is defined as the average time spent in the spray zone per pass. These various measures are all related, but from the standpoint of developing an analytical model for coating variability, fractional residence time is a more useful and intuitive parameter as it determines the fraction of total run time that a tablet spends in the spray. This paper concentrates on determining average fractional residence times and residence time per pass, while the inter-tablet variability is more closely related to the standard deviation of the fractional residence time.     

DEM study of granular flow characteristics in the active and passive regions of a three‐dimensional rotating drum

Three‐dimensional modeling of the solid motion in a lab‐scale rotating drum has been conducted via the discrete element method. After validating the simulated results with available experimental data, the active‐passive interface was identified, following which particle‐scale information in these two regions, in particular the influences of fill level and rotating velocity, were obtained. The results demonstrate that: (1) the total number of particles in the passive region is three times that in the active, (2) the transverse and axial velocities span a wider range in the active region, with the transverse values being greater, (3) the collision force is much higher in the active region, with the greatest magnitudes in the y direction relative to that in the x and z directions, (4) particle displacements are generally lower and have a narrower distribution in the active region, (5) the local solid residence time (SRT) distribution profiles are similar axially in that the highest SRT magnitudes are at the center region of the bed, while the other parts of the bed have uniform SRT magnitudes. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3874–3888, 2016     

Comparison of DEM results and Lagrangian experimental data for the flow and mixing of granules in a rotating drum

This work assesses the accuracy of the discrete element method (DEM) for the simulation of solids mixing using non‐intrusive Lagrangian radioactive particle tracking data, and explains why it may provide physically sound results even when non‐real particle properties are used. The simulation results concern the size segregation of polydisperse granules in a rotating drum operated in rolling mode. Given that the DEM is sensitive to simulation parameters, the granule properties were measured experimentally or extracted from the literature. Several flow phenomena are investigated numerically and experimentally, including the particle residence time, the radial segregation, and the radial variation of the axial dispersion coefficient. An analysis of the DEM model is then presented, with an emphasis on the Young's modulus and friction coefficients. Finally, dimensionless motion equations and corresponding dimensionless numbers are derived to investigate the effect of simulation parameters on particle dynamics. © 2013 American Institute of Chemical Engineers AIChE J, 60: 60–75, 2014     

Investigating the dynamics of cylindrical particles in a rotating drum using multiple radioactive particle tracking

The behavior of granular flows inside rotating drums is an ongoing area of research. Only a few studies have investigated non‐spherical particles despite the fact that particle shape is known to have a significant impact on flow behavior. In addition, the experimental techniques limit the interpretation of the results of these studies. In this work, we compared the flow behavior of cylindrical and spherical particles using the multiple radioactive particle tracking technique to capture the positions and orientations of cylindrical particles simultaneously. We analyzed two important components of the transverse flow dynamics, that is, the boundary between the active and passive layers, and the velocity profile on the free surface. For the cylindrical particles, two general models are proposed to calculate the velocity profiles on the free surface and the effective particle sizes in the active and passive layers. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2622–2634, 2016     

Axial dispersion in the three-dimensional mixing of particles in a rotating drum reactor

Horizontal drum reactors are widely used in industry for the processing of granular material. They are ideally suited for chemical processes that require high temperatures at near-atmospheric pressure. However, the complexities of these reactors have resulted in empirical design procedures that lead to very conservative and costly reactors. This study first reviews critically the extensive literature on experimental results obtained on rotary kilns (without flights) and proposes new design equations for the axial-dispersion coefficient in terms of rotational speed, degree of fill, drum diameter, and particle diameter. A total of 179 data points from the literature, encompassing both the batch and the continuous operational modes, yielded design correlations for slumping, rolling/cascading and cataracting bed behaviours. Additionally, new measurements were made on a pilot-scale rotary drum by tracking a single radioactive particle (emitting gamma-rays) using a battery of nine scintillation counters; these data confirmed the correctness of the proposed design correlations.     

Scaling inter‐tablet coating variability in a horizontal rotating drum

This study investigates how the drum‐to‐particle diameter ratio (D/d) affects the surface speed and interparticle coating variability in geometrically similar coaters. Discrete element method simulations were used to model particle movement in different‐sized, cylindrical drums with identical particle diameters, Froude numbers, fill volume fractions, and spray characteristics. The dimensionless streamwise surface speed profiles become increasingly symmetric as D/d increases, with the maximum speed increasing with D/d. The relationship between the maximum dimensionless speed and D/d is fit well with a power‐law expression. Interparticle coating variability decreases with the square root of the number of drum revolutions after a sufficiently large number of drum revolutions. Increasing D/d increases, in a logarithmic manner, the number of drum revolutions required to reach a given degree of coating variability. A similar logarithmic coating variability trend was observed in simulations using almond‐shaped pharmaceutical tablets, suggesting that the trend is independent of tablet shape. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3743–3755, 2017     

Segregation behavior of binary mixtures of cylindrical particles with different length ratios in the rotating drum

Particle shape impacts the flow behavior of granular material but this effect is still far from being fully understood. Using discrete element method, the current work explores the segregation phenomena of the binary mixtures of cylindrical particles (differing in length but with the same diameter) in the three-dimensional rotating drum operating in the rolling regime, with each cylindrical particle fully represented by the superquadric equation. The important characteristics and the effect of length ratio on the flow dynamics of the binary mixtures are discussed. Some trends are in sync with those of binary mixtures of spherical particles. Unique to nonspherical particles is the orientation of particles, with results indicating that the cylindrical particles align their major axes perpendicular to the drum axis and this behavior becomes more significant for large particles when the length ratio increases. The length-induced radial segregation causes the orientation of large cylindrical particles to be less uniform.