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Storage and Discharge of Bulk Solids
Abstract
In most industrial situations, powders and bulk solids have to be stored in and discharged from bins and silos. This can result in – often unexpected – problems, e.g., flow obstructions or segregation. Most of these problems can be avoided by proper design of equipment with respect to the flow properties of the bulk material to be stored. Especially important is the design of the hopper and all downstream equipment (e.g., feeder) having an influence of the flow in the hopper.
In the present chapter the typical problems occurring in bins and silos are addressed, and the design of hoppers and related equipment with the goal to avoid these problems is outlined. Most important point is the determination of the hopper slope required to avoid stagnant zones, and the outlet dimensions to avoid flow obstructions like stable arches or ratholes. These quantities are calculated based on the flow properties of the respective bulk solid. The flow properties are quantities like, e.g., compressive strength and internal friction, and have to be measured with appropriate shear testers.
... Deposition of the material part to the hopper walls due to adhesion, freezing, etc. Other particles of material can interact with each other to form an arch [1], [2]. Material development can occur relatively quickly. ...
The features of bulk materials hopper storage in the metallurgical and mining industries in the Far North are determined. Possible options for the material hanging, its clumping, and sticking to the walls of the hoppers are indicated. Methods for eliminating the materials hanging in hoppers are examined and their effectiveness is evaluated. In the article it is considered the use of air-impact devices to solve this problem. The process of pneumatic collapse of hangs in the hopper based on these devices is considered. An algorithm for automatic control of pneumatic caving systems based on the use of fuzzy logic methods is proposed. A description of the structural support of this algorithm is given. The solution is based on the use of a sensor that controls the level of material on the conveyor through which material is removed from the hopper. The Mamdani algorithm is used as an algorithm for fuzzy inference. For modeling fuzzy systems was used the Fuzzy Logic Toolbox package of the MATLAB computing environment. The results are presented in the form of graphs and tables and allow take into account the effect of low temperatures on the process of pneumatic collapse. The proposed work algorithm also allows to adapt the accumulated human experience in the management process.
Bulk material handling operations are key operations in process industries such as biorefineries, chemicals and pharmaceuticals. To ensure a successful production operation, a consistent and reliable flow of the materials from storage vessels without dust generation and flow obstruction are required. The objective of this study was to investigate the effect of particle size and moisture content on the theoretical estimation of silo design parameters for fractionated loblolly pine grinds. Loblolly pine grinds were fractionated into sizes using screen apertures: 1.40, 1.00, 0.71, 0.50, 0.25 mm and pan. The fractionated loblolly pine grinds were adjusted to the moisture level of 4.78%, 8.68%, 16.53%, 22.21% and 25.53% (w.b). There was a reduction in the hopper half angle with increase in moisture content and reduction in the fraction size. The hopper half angle also decreased with increase in angle of wall friction. The adjusted hopper outlet sizes varied between 1.20 and 28.56 mm. The wall normal and vertical pressure acting on the cylindrical section of the silo increased from 9.35 to 45.42 kPa and 15.34 to 48.91 kPa, respectively, with increase in fraction size and decrease in moisture content. The initial fill and flow induced pressures acting on the hopper section of the silo increased from 15.34 to 48.91 kPa and 24.71 to 78.79 kPa, respectively, with increase in fraction size and decrease in moisture content. Results of this study will be helpful in designing effective storage vessels and understanding the pressures exerted by stored biomass in the storage vessel.
Since publication of the fundamentals of silo design in the 1960s [1,2,3], those methods have been used more and more for practical applications. The reasons for the increasing acceptance of scientific methods of silo design are probably the inclusion of the fundamentals of bulk solids technology in the education of young engineers and the exchange of information between engineers employed in industry. Another important reason could be the necessity to store increasing amounts and new kinds of bulk solids with unknown flow behaviour in automated plants where silo storage and transport must be undisturbed. Since in many industries more and more new kinds of bulk solids with unknown flow properties are produced, the application of the measuring and calculation methods of bulk solids technology is essential. Examples of products with increasing importance are filter dust, sewage sludge and FGD-gypsum in the area of environmental technology, or ceramic powders. In the present paper three examples will be presented in order to illustrate the range of application of silo design methods: the design of a silo for storage of moist limestone, the dimensioning of a silo for storing a product with large time consolidation, and the reduction of vibrations caused by discharge of a cement clinker silo.
The book concentrates on powder flow properties, their measurement and applications. These topics are explained starting from the interactions between individual particles up to the design of silos. A wide range of problems are discussed - such as flow obstructions, segregation, and vibrations. The goal is to provide a deeper understanding of the powder flow, and to show practical solutions. In the first part the fundamentals of bulk solids flow are outlined: particle-particle interactions, stress, friction, strength, density, flow properties and methods of measurement, shear testers and other test procedures, specific properties of powders and how they are influenced by moisture, temperature, and flow agents. The second part deals with the application of the flow properties in the design of properly working bulk solid containers like bins, silos, and hoppers. Other topics include: the technical design of silo walls, feeders, discharge aids, inserts, the assessment of stresses in silos, segregation and how to avoid it, and vibrations or shocks occurring in silos.
To date the application of computerized mass flow channel design procedures has been restricted by the need to evaluate numerically the critical flow factors. This restriction can be eliminated by adopting the empirical expressions presented for both axisymmetric and plane flow critical channels. The intrinsic accuracy and convenience of the expressions presented will widen the use of computerized procedures on both personal computers and hand-held calculators.
The uncertain stress distribution in annular shear cells is the reason for the lack of confidence in shear test results obtained with ring shear testers. However, theoretical investigations on the stress-strain conditions in an annular shear cell and experiments reveal that annular shear cells may be used to obtain reliable results for silo design if a certain test procedure is used.
The shear tester RO-200 QC AUTOMATIC is an automated robust machine intended for on-line measurement of the mechanical properties of powders. This machine specifies the flowability index and other mechanical properties of powders or granular materials. Potential applications include quality control, classification of powders, specification for the purchasing department and sales department, design of handling equipment, safety of installations and technological calculations.
This paper describes a design procedure which has been developed from an approximate mathematical description of the operation of air blasters. The model was checked using experimental data obtained from a test rig using copper concentrate and coal. A comparison of the sizing and placement strategy derived from the design procedure with that derived from commercial suppliers of air blasters indicates that the commercial strategies can be somewhat conservative.
The ratio of horizontal to vertical stress at uniaxial compression (the lateral stress ratio) is an important parameter for the calculation of stresses acting in the cylindrical part of a silo. For the determination of the stress ratio with laboratory tests an appropriate device was built - the so-called Lambdameter. With this device experiments were carried out to study the influence of the measurement procedure and of several other parameters (e.g. wall roughness). On the basis of the results of these experiments a proposal of a standardized measurement procedure was laid down.
One of the most important mechanical properties related to the flowability of a bulk solid is the internal shear strength. Various types of shear testing equipment have been developed over the past 20 years and are reviewed in this paper starting from the original Jenike flow tester and concluding with a new rotational shear tester recently developed in The Netherlands.
With the data gathered from an experimental silo, operating with two different kinds of bulk solid (cohesive limestone powder and free-flowing plastic pellets), it is shown to what great extent the geometry of the transition from hopper to feeder influences flow profile and thus the stress pattern within the hopper. The optimal geometry for achieving mass flow and even withdrawal of a bulk solid over the entire outlet area depends on the bulk solid's flow properties and, in the case of the cohesive limestone powder, must be closely adhered to, as the flow pattern of this particular bulk solid is very sensitive to any deviations. Furthermore, it is shown how the load acting on the feeder can be ascertained. While the power required of the feeder for bulk solid removal in the discharging condition can be satisfactorily calculated from Jenike's theory, some well-known analytical methods available for the filled condition of the silo provide results which are too imprecise. The assumption of hydrostatic conditions in the hopper leads to stress values which are several times greater than those actually measured in the experimental silo. With the help of the empirically based equations, DIN1055, Part 6, one can find acceptable solutions for the order of magnitude and for the tendency of the stresses acting on the hopper walls. How one might determine the load on the feeder from these solutions needs further looking into. Through basic physical considerations, the dependency of the force required for removal of a bulk solid from under the outlet on the vertical load acting on the feeder, was found to be of a simple nature, from which an upper limit can be calculated. This was confirmed by measurements made on the experimental silo.
To design reliable devices for the handling of bulk solids, knowledge of the flow properties of these bulk solids is essential. For the measurement of flow properties, numerous types of shear testers are available. This paper reviews developments in the design of new shear testers. Some new results of the experimental work carried out with shear testers are presented. After a short description of the measured flow properties of bulk solids and the basic principles of shear testers, the design and/or results of some devices (Jenike shear tester, unconfined yield strength test, triaxial tester, True Biaxial Shear Tester, ring shear tester, wall friction tester) are presented. The developments and investigations reported were made with two considerations in mind. On the one hand, knowledge of the behaviour of bulk solids should be enlarged. For this goal, very specialised shear testers, e.g. the True Biaxial Shear Tester or the triaxial tester, are required. On the other hand, simpler testers have been developed to permit straightforward measurement of the flow properties used for practical applications, e.g. silo design. In addition, it is shown that the results obtained with a shear test are dependent on the kind of shear tester and the shear procedure.
The authors have compared the results of their own investigations on the flow of granular solids (a.o. a number of seeds) with results published by others. The flow of all the seeds tested through the circular orifices used in the experiments, is represented by the equation RésuméLes auteurs comparent les résultats de leurs propres recherches concernant l'écoulement de matériaux granulaires (e.a. un nombre de semences) aux résultats publiés par d'autres auteurs. Le courant de toutes les semences examinées par les auteurs à travers les orifices circulaires employées dans la recherche est décrit au moyen de la formule: ZusammenfassungDie Verfasser vergleichen die Ergebnisse eigener Versuche über das Ausströmen von Schüttgut (u.a. einige Samen) mit einer Anzahl von anderer Seite veröffentlichten Befunden. Der Durchfluss aller von den Verfassern untersuchten Samen durch kreisförmige Öffnungen konnte durch die Gleichung: erfasst werden.
In 2008 a round robin project on ring shear testers was initiated in order to determine a range of results for a defined bulk solid (limestone powder CRM-116), similar to a round robin project with Jenike shear testers carried out in the 1980s. All users of automatic ring shear testers RST-XS or RST-01.pc were invited to take part. Twenty-seven labs supplied results. Finally, per stress level up to 60 yield loci (21 labs) measured with the RST-XS, and up to 19 yield loci (10 labs) measured with the RST-01.pc have been received.Compared to the above mentioned tests on Jenike-type shear testers, both ring shear testers provide clearly smaller standard deviations. Thus, despite different ambient conditions, the range of results is quite narrow.With the mean values and standard deviations calculated from the ring shear tester results a kind of “reference range” is defined. This range should be matched with a certain probability with the results of future tests with the standard powder if the instructions regarding stresses and ambient conditions are followed. Thus, the results provide orientation to all users of ring shear testers RST-XS and RST-01.pc.
Air is entrained when fine powders are loaded from a height into a container. The air takes some time to dissipate, which gives rise to problems in bag filling, silo loading, charging of bulk tankers etc. An analysis of the percolation of air through a granular bed is presented, together with an analytical solution via consolidation theory. Although the solution is approximate, the mathematics is straightforward and highlights the effect of different boundary conditions on deaeration behaviour. The mathematics also enables the prediction of large-scale deaeration behaviour from bench-scale tests on a powder. Measurements of air pressures in the interstices of the powder (pore air pressures), and their variation with time, in a large-scale hopper are presented, and compared with predictions from the theory.
An experimental and theoretical study has been made of the rate with which fine particles undergo gravity discharge in air from a hopper. The particles studied were small enough for their motion to be significantly retarded by air resistance but large enough to be substantially free from cohesion (50 μm ⩽ d ⩽ 500 μm). A simple flow system was chosen so that particle and air properties varied in one dimension only and could be measured easily. The theoretical analysis yields inter-related equations for particle flow rate and for the spatial distributions of particle stress, bed voidage and interstitial air pressure. Particle flow rates predicted from basic material properties and hopper geometry are about twice as large as measured values but show the correct dependency on particle size.
When a fine powder is filled into a silo the gas trapped by the particles will take an appreciable time to escape and will cause an increase in the lateral pressure on the walls of the silo. A mathematical analysis of the problem is presented and numerical solutions are provided. The theoretical results are compared with model measurements and the implications for hopper design are discussed.
In an earlier paper published in Vol. 1, No. 4. of this journal one of the authors gave a brief account of an annular shear cell apparatus which had been developed for measuring the strength characteristics of granular materials as part of a project whose aim is to establish a basis for the design of bunkers to handle these materials. The big advantage of the apparatus over direct shear boxes is that the unlimited travel available makes it possible for the materials to be readily prepared to any chosen degree of compaction prior to the subsequent testing. (The purpose of this paper is to give a more detailed account of the apparatus so as to allow other workers to construct and use it. Ed.)
Experiments have been performed to test the deaeration analysis proposed by Rathbone and Nedderman [1]. It has been found that this analysis predicts the deaeration rate accurately for Geldart type-A materials though it underpredicts the deaeration rate for type-C materials. The prediction of the analysis for the initial degree of aeration was, however, not confirmed by the experiments.Measurements have also been made of the discharge rate of aerated materials and it was found that the rate of discharge increases with the degree of aeration. For fully aerated materials, the flow rate was found to be comparable with that of an inviscid fluid.
Fine powders often exhibit significant two- phase (solid/gas) interactions when they are handled in bins, hoppers, and processing vessels. As a result, numerous and more complex flow problems can occur which are not observed with larger particle bulk solids. The powder's flow pattern has a major effect on the development of certain flow problems, especially flooding. This can usually be overcome by using mass flow designs. Unfortunately, when mass flow is achieved the powder's discharge rate through a hopper outlet may be severely restricted.
The behaviour of solids fluidized by gases falls into four clearly recognizable groups, characterized by density difference (ϱs–ϱf) and mean particle size. The most easily recognizable features of the groups are: powders in group A exhibit dense phase expansion after minimum fluidization and prior to the commenment of bubbling; those in group B bubble at the minimum fluidization velocity; those in group C are difficult to fluidize at all and those in group D can form stable spouted beds. A numerical criterion which distinguishes between groups A and B has been devised and agrees well with published data. Generalizations concerning powders within a group can be made with reasonable confidence but conclusions drawn from observations made on a powder in one group should not in general be used to predict the behaviour of a powder in another group.
La pratique croissante de la recherche payee au sein de l'industrie des moteurs de recherche du Web remet en cause des valeurs ethiques tenues pour vitales dans une societe democratique : la liberte par rapport aux partis pris, la vie privee et la confiance. La protection de ces valeurs n'est pas incompatible avec les objectifs de la recherche payee et doit etre integree des la conception.
The work in this paper develops a theoretical model for predicting the interstitial air pressure distribution for bulk solids flowing in a conical mass flow bin. The theoretical model is based on continuum mechanics theory. The boundary conditions are consistent with air pressure and bulk density continuity in the vertical direction. Close agreement between theoretical and experimental air pressure distribution is obtained. Both the theoretical model and experimental results indicate that the magnitude of air pressure increases with increasing the hopper outlet size, the surcharge level and/or decreasing the powder permeability.
Karlsruhe, Univ., Fak. für Chemieingenieurwesen, Diss., 1976.
Thesis (doctoral)--Rheinisch-Westfälische Technische Hochschule Aachen, 1987.
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