Cohesive Bulk Solids Research Articles

A modified flowability classification model for moist and cohesive bulk solids

The Jenike flowability classification is designed to indicate the handleability of bulk solids in material handling processes. However, for moist, cohesive bulk solids, it may fail to represent the true flowability/handleability at high consolidation stresses. The study presented in this paper introduces a modified flowability classification model aiming to improve the flowability/handleability prediction across a wide range of consolidation pressures. The evolution of the flow function of bulk materials under increasing moistures is initially discussed, based on which the Jenike flowability classification is reviewed. The modified flowability classification is then derived by considering the flow/no flow condition for mass flow hopper designs under both low and high consolidation stress states. The influence of the bulk material type and general operational conditions are also taken in account in deriving the modified classification method. A suite of bulk solids with a wide range of material properties are selected in an experimental program to validate the modified flowability classification. Results suggested the modified flowability classification is able to more accurately indicate the overall flowability/handleability of the bulk solids independent of the applied major consolidation stress.

Simulation study of the discharge characteristics of silos with cohesive particles

In many industrial applications the silo for bulk materials is an important part of an overall process. Silos are used for instance to buffer intermediate products to ensure a continuous supply for the next process step. This study deals with the discharging behaviour of silos containing cohesive bulk solids with particle sizes in the range of 100-500 μm. In this contribution the TOMAS [1,2] model developed for stationary and non-stationary discharging of a convergent hopper is verified with experiments and simulations using the Discrete Element Method. Moreover the influence of the cohesion of the bulk solids on the discharge behaviour is analysed by the simulation. The simulation results showed a qualitative agreement with the analytical model of TOMAS.

Modelling of the Flow of Streams of Cohesionless and Cohesive Bulk Materials in a Conveyor Discharge Point of a Descending Belt Conveyor

The paper presents the analysis of the condition of the flow of cohesionless and cohesive bulk solids in a conveyor discharge point of a descending belt. The analysis was carried out for stationary flows at high velocities. It presents mathematical methods of description of the velocity of materials leaving a conveyor discharge point with a descending conveyor belt, as well as final equations which enable the determination of the velocity of material after it has left the discharge point, with the accuracy sufficient for practical use. Next, the obtained mathematical descriptions for the mentioned material groups have been compared. The usefulness of the equations in determining the velocity of the material beyond the point, which enable to omit the indirect equations, has been demonstrated. Finally, the difference between the values of the velocities obtained with the proposed and indirect equations as well as the relative error for the proposed method have been calculated.

Anisotropic behaviour of bulk solids and its effect on silo design

The anisotropic material behaviour is a characteristic property of bulk solids. Prior research work proved that the measured unconfined yield strength of cohesive bulk solids is lower when the direction of the collapsing stress and the consolidation stress does not coincide. A new powder tester has been developed to measure both the unconfined yield strength in the uniaxial direction, which provides consolidation and collapse stresses in the same direction, as well as the anisotropic unconfined yield strength, which means consolidation and collapse stresses are in orthogonal directions. The friction between the walls and the bulk solid is minimised by the use of rubber membranes which deform uniformly with the bulk sample. The uniaxial and the anisotropic unconfined yield strengths have been measured for a limestone powder at different consolidation stresses. Experimental results as well as their application on the design for funnel flow silos are discussed. To avoid piping two design criteria exist for the design of a funnel flow silo: Jenike assumes a passive state of stress to prevail close to the silo outlet (Jenike, 1961 [1]); Johanson assumes an active state of stress (Johanson, 1969 [2]). While the approach of Jenike leads to good results, the estimation of the critical outlet diameter in a silo with an active state of stress is not satisfying. The pipe dimensions for an active state of stress were measured using a silo centrifuge. The application of the design criteria gives reasonable and safe design value only if the anisotropic behaviour of the bulk solid is considered which was measured by the new powder tester.

A Predictive Model for Arch Destruction in Hopper-Bottom Bins for Cohesive Bulk Solids

A theoretical model is developed to predict arch destruction by vibration in hopper-bottom bins for cohesive bulk solids. The bulk solids material in the bin is divided into layers of equal mass, and the layers are connected by elastic springs and frictional dampers. The Lagrange equations are derived for the bin system and solved numerically to predict the vibration characteristics of individual layers. A theoretical criterion is also established from the force equilibrium to determine the required acceleration for arch destruction. The predicted resonant frequencies and accelerations of vibration for arch destruction are in close agreement with the experimental values reported in the literature for a model bin filled with ground feed. The numerical results show that each layer of material resonates at certain excitation frequencies, but not all layers resonate at the same excitation frequencies. The peak acceleration in each layer occurs only at resonant excitation frequencies. The model predicts the lowest excitation frequency (the least energy input) for arch destruction. Theoretically, required acceleration for arch destruction decreases from the hopper bottom to top, but the lowest excitation frequency for arch destruction varies randomly in the hopper.

Application of model tests on cohesive bulk solids using silo centrifuge technique

Model tests under reduced scale can only be used for silo design with free flowing bulk solids. Model tests with cohesive bulk solids are not possible under gravity forces due to the dependency of the bulk solids strength on the consolidation stresses. To perform experiments with silos under reduced scale and using cohesive bulk solids centrifugal forces must be applied. In this paper, experiments under reduced scale using a silo centrifuge are shown. The scale-up from model size to full size scale is possible by calculating the ratio of centrifugal acceleration to gravitational acceleration. An overview of centrifuge studies according silo design is given. The results from experiments with cohesive and fine bulk solids regarding the stress state and the critical outlet dimension are presented in this paper. The critical outlet dimension from silo centrifuge testing is compared with calculated results investigated by the Jenike-method.

DEVELOPMENT OF A PREDICTIVE MODEL FOR ARCH DESTRUCTION BY VIBRATION IN STORAGE BINS FOR COHESIVE BULK SOLIDS

A predictive model is developed for arch destruction during vertical vibration in storage bins for cohesive bulksolids. The model is based on the equilibrium of static and inertial forces on an arch. The cohesive bulk material is simulatedas thin layers connected by elastic and damping elements during vibration. The model predicts the minimum accelerationamplitudes and thrust forces required for destructing arches at certain locations in storage bins. A small cylindrical bin of178 mm diameter and 120 mm high was used to conduct an experiment to validate the prediction model. Tests were conductedfor vibration frequencies from 18 to 90 Hz. It is theoretically proven that the optimal vibration frequencies for arch destructionare among the natural frequencies of the cohesive material storage system. The model predictions compared favorably withexperimental results. The average difference between the model predictions and measured acceleration amplitudes for archdestruction was 7%.

Improvements in Silo Design for Moist and Cohesive Bulk Solids by Using a Silo Centrifuge

AbstractThe Jenike‐method is state of the art in silo design for trouble‐free flow, even for cohesive bulk solids. However, for some bulk solids, especially very cohesive and moist bulk solids, even the correct application of the Jenike‐method can lead to an over‐design of the silo outlet. If a reliable estimation of the critical outlet dimension is possible the dimension as well as the machinery costs can be reduced. Model testing can help to reduce the over‐design but reasonable test equipment and flexibility are only possible with silo centrifuge testing. Results, regarding the state of stress and the critical outlet dimensions, from experiments using a silo centrifuge with cohesive and moist bulk solids are presented. The influence of creeping effects on silo design is investigated using the silo centrifuge with moist gypsum.

Consolidation and flow of cohesive bulk solids

The strength and the flow properties of bulk solids can be determined by performing shear tests. A shear test always consists of two parts, consolidation and measurement of strength. The measured strength depends on the way and degree of consolidation. During consolidation a stress history is impressed on the bulk solid sample which only vanishes after steady state flow. Since the consolidation is normally done nonhydrostatically, the strength will depend on the direction of stress application (anisotropy). Thus, the strength of a bulk solid sample strongly depends on its stress history and it can show anisotropic behaviour. These effects have to be considered when comparing available shear testers.

Extension of elastoplastic constitutive models with respect to cohesive bulk solids : Hohl, H W; Schwedes, J Powder TechnolV70, N1, April 1992, P31–42

Extension of elastoplastic constitutive models with respect to cohesive bulk solids : Hohl, H W; Schwedes, J Powder TechnolV70, N1, April 1992, P31–42

Extension of elastoplastic constitutive models with respect to cohesive bulk solids

A simple general method is presented to extend elastoplastic constitutive models formulated for coarse sand with respect to cohesive bulk solids. A model given in the literature is treated as example. The developed method permits the calculation of the deformation characteristics by means of numerical methods even when tensile stresses or free surfaces occur. The tensile strength is the only new model parameter which depends linearily on the compressive strength of the powder. Using literature data for limestone from a biaxial tester, the relation between compressive strength and a dimensionless pressure parameter characterizing the consolidation state is calculated. The procedure to actualize the status variables of the constitutive model during the hardening process is explained and an iterative method is given to reduce stress states on limiting surfaces.