handbook of conveying and handling of particulate solids

Preface The Conveying and Handling of Particulate Solids play major roles in many industries, including chemical, pharmaceutical, food, mining, and coal power plants. As an example, about 70% of DuPont's products are in the form of a powder, or involve powders during the manufacturing process. However, newly designed plants or production lines produce only about 40+40% of the planned production rate. This points up clearly the lack of appropriate scientific knowledge and engineering design skills. Following one's becoming aware of the problem, it should be attacked on three fronts- research, education and training. Many new products cannot be manufactured or marketed because of serious difficulties concerning conveying and handling. That is because in most cases the mutual effect between handling- and conveying units is neglected during the design of a new production line. Unlike other states of materials, it is not sufficient just to know the state of a bulk material in order to determine its properties and behaviour. The "history" of a bulk material can dramatically affect its properties and behaviour. We should also keep in mind the fact that an optimal manufacturing line is not necessarily made by combining individually optimized devices. Therefore, "concurrent engineering" should be practised in the chemical and related industries. In order to address both of the problems presented above, an "international conference" was initiated six years ago, that relates to most processes, units, equipment and models involving the conveying and handling of particulate solids. In the conference, researchers, engineers and industrialists working on bulk solids systems have the opportunity for open dialogue to exchange ideas and discuss new developments. The present Handbook summarizes the main developments presented at the last Conference, that took place at the Dead-Sea, Israel in 2000. This Handbook therefore contains research results from all round the world, and the best scientists present the state-of-the-art on a variety of topics, through invited review papers. Some review papers presented at the previous Conference were added. All the papers presented in this Handbook have been reviewed. The aim of the handbook is to present a comprehensive coverage of the technology for conveying and handling particulate solids, in a format that will be useful to engineers, researchers and students from various disciplines. The book follows a pattern which we have found useful for tackling any problem found while handling or conveying particulate solids. Each chapter covers a different topic and contains both fundamentals and applications. Usually, each chapter, or a topic within a chapter, starts with one of the review papers. Chapter 1 covers the characterization of the particulate materials. Chapter 2 covers the behaviour of particulate materials during storage, and presents recent developments in storage- and feeders design and performance. Chapter 3 presents fundamental studies of particulate flow, while Chapters 4 and 5 present transport solutions, and the pitfalls of pneumatic, slurry, and capsule conveying. Chapters 6, 7 and 8 cover both the fundamentals and development of processes for particulate solids, starting from fluidisation and drying, segregation and mixing, and size-reduction and -enlargement. Chapter 9 presents environmental aspects and the classification of the particulate materials after they have been handled by one of the above-mentioned processes. Finally, Chapter 10 covers applications and developments of measurement techniques that are the heart of the analysis of any conveying or handling system. We hope that users will find the handbook both useful and stimulating, and will use the results of the work presented here for further development and investigations. The Editors Handbook of Conveying and Handling of Particulate Solids A. Levy and H. Kalman (Editors) 9 2001 Elsevier Science B.V. All rights reserved. Solids flowability measurement and interpretation in industry T. A. Bell E.I. du Pont de Nemours & Company, Inc., Experimental Station, P.O. Box 80304 Wilmington, DE 19880-0304 USA The practical issue of industrial measurement and description of powder flowability are discussed from the author's perspective. Common uses of conventional shear testing devices are described, as are some alternative methods. 1. INTRODUCTION The science of soil mechanics was integrated with the related field of powder mechanics and reduced to industrial practice by Jenike [1] in 1964. Since then, it has been possible for industry to reliably measure the flowability of powders and relate the measurements, in engineering units, to the design requirements for silo flow. However, Jenike's publication was neither the first effort to quantify flowability nor the last. New testing methods continue to be introduced, with varying degrees of success. In many cases these alternative measurement methods are the result of an industrial necessity and reflect some shortcoming of the Jenike method. In other cases, they exist because the Jenike method is not known to the people involved or is not relevant to their problem. Business value can be derived from many different types of measurements. 2. DESCRIBING FLOWABILITY 2.1. Applications A surprising amount of time can be spent debating the meaning of flowability, and what does it really mean if one powder has better flowability than others. From a practical standpoint, the definition of acceptable flowability is in the eyes of the beholder. A person accustomed to handling pigments would be delighted if his materials had the handling properties of cement, while a cement user would wish for the properties of dry sand. Industries that deal with powders in very small quantities can employ handling techniques of brute force or human intervention that are not practical in larger scale installations. In many cases, the chemists developing a new process or powder are completely unaware of the difficulties in handling powders on an industrial scale, and in some cases the problems are completely different between the laboratory and the plant. Finally, there are some materials, such as extremely free flowing granules that may require unconventional descriptive techniques. 2.2. Clarity and simplicity Most producers and users of bulk materials do not have the time or interest to study solids flow and powder mechanics. Many are completely unfamiliar with the field since it is rarely taught as part of an engineering or science curriculum. A mistake sometimes made by specialists in the field is the presentation of test results in a form that cannot be readily by the users. Failure of the specialist to identify and address the key business issues in a way that is understandable to the intended recipients will severely limit the breadth of application of this technology. Silo design studies should show the engineering design outcome first, and the underlying technical data second. Very few people are interested in yield loci from shear tests, and even the resulting flow functions often require interpretation in the context of the silo problem. Flowability measurements for quality control and product development must often be reduced to one or two numbers as discussed later in this paper. Even with modem statistical techniques, it is extremely difficult to compare a series of graphs describing the properties of various bulk material samples. The question then becomes which one or two numbers from large data sets to use. It could be argued that the difference between a skilled technician and skilled consultant is the ability of the latter to correctly select which data to work with for a specific quality control or product development purpose. While there is not a simple answer to this question, any approach must start with a consideration of the compaction pressures that the bulk material is exposed to. For free-flowing materials in small bins, the pressures might be nearly zero. For larger silos, cohesive materials, or those with high wall friction (see section 3.6, below), calculation of appropriate pressures will be required. When data is presented in the form of a few numbers, there is inevitably a risk that those requesting the data will attempt to use it for purposes for which it was not intended. For example, a measure of the ratholing tendency of a material in silos may not accurately reflect the uniformity of its delivery in packaging machines. Providing the users with mountains of data is not a solution, since the same person that will use a simple number inappropriately will probably also extract the wrong information from a comprehensive collection of data points. This situation can best be managed by maintaining a dialog with the users on their needs and the application of the results. 3. GENERAL FORMS OF FLOWABILITY MEASUREMENT 3.1. Free flowing materials - timed funnels It can be difficult or impossible to measure cohesive strength for highly free flowing granules. For such materials, the rate of flow is often more important than whether they will flow at all. In these cases, the time necessary for a pre-determined volume or mass to flow through a funnel can be the most useful flowability measure. This method is widely used in the fabrication of metal parts from metal powder [2, 3]. The factors influencing the flow time measurements are numerous and include the particle size distribution, the friction between the particles and against the wall, the particle density, and gas permeability. Many specialists in powder mechanics object to the use of such measurements because of the unknown interactions amongst the factors and the absence of any consideration of solids pressure due to the self-weight of bulk material. However, in our experience this measurement can be extremely reproducible and an excellent indicator of the flow behavior in situations that resemble the test, i.e., rapid flow from small bins. 3.2. Angle of repose measurements The angle of repose formed by a heap of a bulk material is the best-known method of describing flowability. Unfortunately, it is quite possibly the worst measure to use. Angles of repose can be significantly influenced by the test conditions, especially the height that the material falls to form the heap. There can be pronounced differences in angles of repose for materials that have similar real-life handling properties. Cohesive materials may form multiple angles of repose in a single test, and reproducibility may be poor. The measured angle cannot be directly related to any silo design parameter except the shape of the top of a stockpile heap. 3.3. Hausner ratio of tapped to loose bulk density The ratio of the tapped to the loose bulk density has been shown to relate in many cases to the gain in cohesive strength that follows the compaction of a powder or granular material. Materials with relatively little gain in bulk density (Hausner ratios below about 1.25) are considered to be non-cohesive, while increasing values (ratios up to about 2.0) indicate increasing levels of cohesiveness. However, we have observed that the correlation between the ratio and more sophisticated measurements is rather poor, and it is unlikely to provide precise differentiation between generally similar materials. In addition, the test is actually measuring a form of compressibility, which does not always relate to cohesive strength. A serious limitation of the Hausner ratio is the elimination of any consideration of bulk density in the final calculation. As discussed below, two materials with similar Hausner ratios but different densities are likely to behave much differently in practice. Finally, it has been shown that tapped bulk density measurements are extremely sensitive to the apparatus being used and the number of taps. Standardization of these factors is necessary to ensure consistent and comparable results. 3.4. Properties based on shear testing In many cases, the most important bulk handling behavior is whether or not the bulk material will flow reliably by gravity throughout a process. This behavior relates to the material's arching (doming) and ratholing (piping) propensity, as described by the silo outlet necessary for reliable flow. Jenike [ 1 ] provides a method of calculating these values that is of the general form: Arching/rathole diameter = (factor H or G) x fc Bulk Density (1) In this equation, fc is the unconfined yield strength (also known as Cyc), a measure of cohesive strength in response to compaction pressure. The bulk density is measured at the same compaction pressure as is associated with the fc measurement. The appropriate value of compaction pressure depends on the situation. As a first approximation, the factors H (for arching) or G (for ratholing) can be considered to be constants, so the flowability can be simply described as cohesive strength divided by bulk density. Put in other words, flowability is the ratio of the cohesive forces holding the particles together vs. the gravity forces trying to pull them apart. Since fc and bulk density can both vary with compaction pressure, it is important to make the calculation of Eq. (1) at the appropriate pressure. This relates largely to the type of flow pattern in the silo (mass flow or funnel flow) which in turn depends on the friction of the solids against the walls of the silo. Consequently, a wall friction measurement is usually necessary to help fix the range of pressures. Since wall friction can also vary with pressure, the situation can become quite complex. However, many situations can be simplified as discussed below in section 3.6. 3.5. Flow functions Flowability is often described on the basis of the flow function (Figure 1) [1 ] derived from shear testing. The flow function is a graph relating a major principal stress (or1) to the unconfined yield strength (fc) that it produces in a powder specimen. This graph basically describes cohesive strength as a function of compaction pressure. Figure 1 shows possible flow functions for three different materials. It is easy to comprehend and relate to one's own experience with moist sand or snow, etc. Jenike [1] and others have often used the slope of the flow function as a flowability descriptor. This can obtained by simply dividing the unconfined yield strength at a particular point by the corresponding value of major principal stress. This method, while convenient, has several serious drawbacks. First, a comparison of flow function slopes for different bulk material samples based on single points presumes that the flow function graphs are linear, and that they pass through the origin. Neither assumption is necessarily true (see Figure 1). Second, most shear testing methods (except Johanson's) used in industry do not directly apply a pressure of crl to the sample. A different consolidation pressure is used and the final value of crl is later calculated as part of the interpretation of the yield locus generated in the test series. This means that the person conducting the test cannot pre-select which value of (3" 1 he will test at. Two different samples, tested at the same consolidation pressure, may produce different values of cry, and hence relate to different points along the flow function. Exact comparison of multiple samples will require that at least two flow function points for each sample be obtained so that the comparative values of fc at a particular value of ~1 can be determined by interpolation. The third drawback of comparisons based on flow functions alone is the fact that such measurements completely disregard bulk density. Examination of Eq. 1 shows that the bulk density has equal importance to fc. We have observed cases where the bulk density of a common material, such as hydrated lime, can vary by up to 50% between suppliers, while the cohesive strength (fc) varied by 30%. Similarly, in one of our businesses, two products had ~D ~ I, ~ >. "0 ~D O r ~ o ~" oo~176 ~" ~176176 o o ,.*~176 j .f ~1, Major Principal Stress Fig. 1. Typical flow functions. identical flow functions but their bulk densities varied by 30%. The flowability in the plant varied accordingly, and the business people (who had only compared the flow functions) did not understand why. 3.6. Wall friction measurements Wall friction measurements are most commonly made with a Jenike shear cell. The force necessary to push (shear) a sample of bulk material, trapped in a ring, across a wall sample coupon is measured as a function of the force applied to the top of the sample (Figure 2). The resulting data is in the form of a graph of shear force versus normal force, known as a wall yield locus (WYL). For any given point on the graph, the ratio of shear force to normal force is the coefficient of friction. The arctangent of the same ratio is the wall friction angle. For some materials the WYL is a straight line that passes through the origin. This is the simplest case, and wall friction can be described by a single number. In other cases the WYL has a curvature that causes the wall friction angle to vary inversely with normal force. The wall friction angle is the primary factor used in determining if a particular silo will empty in mass flow or funnel flow. Larger values of the wall friction angle correspond to steeper angles required for the converging hopper at the base of the silo in order to achieve mass flow. The procedure is described by Jenike [1]. If the wall friction angle exceeds a certain value, mass flow will not occur. The WYL must be closely examined for the design of new silos or the detailed evaluation of the behavior of a bulk material in existing silos. If the wall friction angle exceeds the mass flow limit for a silo installation, the flow pattern in the silo will be funnel flow and consequently ratholing must be considered. This means that flow function data in the appropriate silo pressure range is necessary to determine if ratholing can occur. This can be a different set of testing conditions than that required if only a no-arching determination is required for a mass flow silo. For quality control or product development purposes where silo design is not an issue, it is often sufficient to describe the WYL by fitting a straight line, passing through the origin, to the data set. While this method can produce errors (especially at low values of normal force) it is adequate as a descriptive tool and greatly simplifies comparisons. Fig. 2. Wall friction measurement with Jenike Shear Cell (illustration adapted from Ref. 3). 4. COMMON SHEAR TESTERS 4.1. The Jenike Shear Tester The Jenike shear tester (Figure 3) was developed as part of the research activities described by Jenike [1 ]. It is derived from the shear testers used in soil mechanics, which are typically square in cross section instead of the circular design used in the Jenike tester. Soil mechanics tests are usually conducted at compaction (normal) stress levels that greatly exceed those of imerest in powder mechanics for silo design and gravity flow. At these high normal stress levels, it is relatively easy to obtain steady state flow in which the bulk density and shear stress remain constant during shear. This steady state condition is a vital prerequisite for valid test results. It can also be reached at lower stress levels, but a relatively long shear stroke is required. With translational shear testers (i.e., those that slide one ring or square across another) the cross sectional area of the shear zone varies unavoidably during the shear stroke. The validity of the test becomes questionable if the stroke is too long. For this reason Jenike devised the round test cell, which permits preparation of the sample by twisting. A pre-consolidation normal load is applied to the cover, and the cover is twisted back and forth a number of times. This preparation makes the stress distribution throughout the cell more uniform and reduces the shear stroke necessary to obtain steady state flow. After the pre-shear process is completed, a specified pre-shear normal load is applied to the cover and shear movement is started. Once steady state flow (constant shear force) is achieved, the initial normal load is removed and a smaller normal load (known as the shear normal load) is applied. Shear travel resumes and the peak shear force corresponding to the shear normal load is noted. The preparation process requires skill and is not always successful on the first attempt. Different values of the pre-consolidation load or the number of twists may be required to reach steady state flow. Even with proper preparation, it is only possible to obtain one shear point from a prepared shear cell. Since several shear points are typically employed to construct a yield locus, and several yield loci are necessary to construct a flow function, the cell preparation procedure must be repeated numerous times. It is not uncommon to repeat the cell preparation process 9 to 25 times per flow function. We typically allow 6 man-hours for 9 shear tests, so the time investment in this method can be significant. Fig. 3. Jenike Shear Tester (illustration adapted from Ref. 3). While the cell preparation procedure is demanding for even conventional powders and granular materials, it can be nearly impossible to obtain steady state flow with elastic materials and particles with large aspect ratios, such as flakes and fibers. In these cases, the allowable stroke of the shear cell may be exceeded before a steady state condition is achieved. Despite these difficulties, the Jenike cell has remained the best-known and definitive shear testing method for bulk solids. There are several likely reasons for this. First, the method was developed first! Second, the method has been validated in industrial use and comparison to more sophisticated testers. Third, the apparatus is relatively simple and not patented, ideal for university research and users with limited budgets. 4.2. Peschl Rotational Split Level Tester Most of the difficulties with the Jenike tester are the result of its limited shear stroke. Testers that rotate to shear the sample (such as the Peschl) versus the Jenike cell's translational motion can have a distinct advantage. Shear travel can essentially be unlimited, as long as there is no degradation of the particles in the shear zone. This unlimited stroke makes the elaborate Jenike cell preparation process unnecessary, and also makes it possible to obtain multiple data points from a single specimen. Thus an entire yield locus can be constructed from a single filling of the test cell. While it is also possible to make repetitive measurements from a Jenike cell sample, the cell has to be prepared each time. A second advantage of the rotational cell is that the placement location for the normal force does not move (translate) during the test. Loads are placed on the centerline of rotation. This makes it much easier to automatically place and remove loads from the test cell, and can lead to complete automation of the tester. Fig. 4. Peschl Rotational Split Level Tester. 10 The Peschl tester (Figure 4) rotates the bottom half of a cylindrical specimen against the top half, which is stationary. The torque necessary to prevent the rotation of the top half is measured, and is converted to the shear stress acting across the shear zone. The interior of the top and bottom of the cell are roughened to prevent the powder from shearing along the top or bottom ends rather than at the shear plane. It should be noted that the amount of shear travel varies across the radius of the cell. A particle precisely in the center of the cell sees no shear travel distance - only rotation about the center line of the tester. Particles at the outside edge of the cell have the greatest amount of shear travel, with decreasing travel distances as the radius is reduced. Some researchers have voiced concern about this aspect of the tester, since the meaning of the individual shear points is somewhat confused. Shear stress values become averages produced by shearing different regions of the cell different distances. Although detailed studies have not been conducted, there is some evidence [4] that the Peschl tester produces slightly lower values of unconfined yield strength than the Jenike tester at comparable values of major principal stress. The Peschl tester was the first (and for a long time, the only) automated shear cell available. The volume of the standard cell is relatively small, making it convenient for expensive bulk materials such as pharmaceuticals and agricultural chemicals. It is widely used for quality control and product development. Testing times are about 1/3 of that required with the Jenike cell. 4.3. Sehulze Ring Shear Tester The issue of non-uniform shear travel in rotational testers can be minimized if the test cell has an annular ring shape instead of a cylindrical one such as in the Peschl tester. While the inner radius of the ring still has a shorter shear travel than the outer one, the difference is relatively small, particularly if the difference between the two radii is small compared to their average. This concept was developed a number of years ago by Carr and Walker [5]. In our experience, the early models of the device, while very robust from a mechanical standpoint, were too massive for delicate measurements. It also was difficult to clean the cell, particularly the lower ring. This form of ring shear tester never achieved widespread use when compared to the Jenike cell. Fig. 5. Schulze Ring Shear Tester. [...]... Methods of Measuring Powder Cohesive Strength, Preprints of the 3rd European Symposium- Storage and Flow of Particulate Solids, PARTEC 95, European Fed of Che Engg, pp 79-88, 1995 7 Maltby, G.G Enstad, Uniaxial Tester for Quality Control and Flow Property Characterization of Powders, Bulk Solids Handling, Vol 13, No 1, pp 135-139, 1993 Handbook of Conveying and Handling of Particulate Solids A Levy and. .. AstraZeneca, Norcem, DuPont and the Norwegian Research Council for financial support during the course of the development of the uniaxial tester, and to Professor Sunil de Silva for help in the preparation of the manuscript Handbook of Conveying and Handling of Particulate Solids A Levy and H Kalman (Editors) 9 2001 Elsevier Science B.V All rights reserved 33 C h a r a c t e r i z a t i o n of p o w d e r flow... University of Technology, The Netherlands, 1996 7 R.J Akers, The certification of a limestone powder for Jenike shear testing, Community Bureau of Reference-BCR, CRM 116, Brussels, 1991 Handbook of Conveying and Handling of Particulate Solids A Levy and H Kalman (Editors) 9 2001 Elsevier Science B.V All rights reserved 39 From discrete element simulations towards a continuum description of particulate solids. .. 18(1995),96/109 Handbook of Conveying and Handling of Particulate Solids A Levy and H Kalman (Editors) 9 2001 Elsevier Science B.V All rights reserved 25 Investigation on the effect of filling procedures on testing of flow properties by means of a uniaxial tester G.G Enstad a and K.N Sjoelystb aTel-Tek, dept POSTEC, Kjoelnes ring, N-3914 Porsgrunn, Norway bTelemark University College, Department of Process... 12(1992),237/240 17 Ploof, D.A and J.W Carson : Quality Control Tester to Measure Relative Flowability of Powders, bulk solids handling 14(1994), 127/132 18 Schwedes, J., Schulze, D and J.R Johanson: Letters to the Editor, bulk solids handling 12(1992),454/456 19 Bell, T.A., Ennis, B.J., Grygo, R.J., Scholten, W.J.F and M.M Schenkel : Practical Evaluation of the Johanson Hang-Up Indicizer, bulk solids handling 14(1994),... REFERENCES 1 A.W Jenike, Storage and Flow of Solids, Bulletin 123, Utah University 1964 2 J Schwedes, Testers for Measuring Flow Properties of Particulate Solids, Proceedings of the International Symposium Reliable Flow of Particulate Solids III, Telemark College, Porsgrunn, August 11 - 13, pp 3 - 40, 1999 3 L P Maltby, Investigation of the Behaviour of Powders under and after Consolidation, Thesis,... properties of bulk solids -which properties for which application J Schwedes Institute of Mechanical Process Engineering, Technical University Braunschweig Volkmaroder Str 4/5, 38104 Braunschweig, Germany To design reliable devices for the handling of bulk solids and to characterize bulk solids the flow properties of these bulk solids have to be known For their measurement a great number of shear and other... Schulze, D and J Schwedes : Determination of the Stress Ratio in Uniaxial Compression Tests, Part 1 and 2, powder handling & processing 6(1994),61/65 & 199/203 15 Schulze, D.: Flowability of Bulk Solids - Definition and Measuring Techniques, Part I and II Powder and Bulk Engng 10(1996)4,45/61 & 6,17/28 16 Johanson, J.R.: The Johanson Indizer System vs The Jenike Shear Tester, bulk solids handling 12(1992),237/240... may be needed for the design of equipment for storage, handling and transport of powders The Jenike method for design of mass flow silos [1] is the most well known example of how flow properties measured in the Jenike tester is used In addition to the Jenike tester, there are many other types of testers [2], some more complicated and more reliable, and some less complicated and less reliable The uniaxial... (Ntimberg) (1986),257/279 7 Maltby, L.P and G.G Enstad: Uniaxial Tester for Quality Control and Flow Property Characterization of Powders, bulk solids handling 13(1993), 135/139 8 Gerritsen, A.H.: The Influence of the Degree of Stress Anisotropy during Consolidation on the Strength of Cohesive Powdered Materials, Powder Technol 43(1985),61/70 9 Peschl, I.A.S.Z.: Bulk Handling Seminar, Univ Pittsburgh, Dec . Quality Control and Flow Property Characterization of Powders, Bulk Solids Handling, Vol. 13, No. 1, pp. 135-139, 1993. Handbook of Conveying and Handling of Particulate Solids A. Levy and H. Kalman. the handbook both useful and stimulating, and will use the results of the work presented here for further development and investigations. The Editors Handbook of Conveying and Handling of Particulate. papers presented in this Handbook have been reviewed. The aim of the handbook is to present a comprehensive coverage of the technology for conveying and handling particulate solids, in a format