The Evolution of Powder Technology
Powder Technology developed from the union of civil engineers, concerned about the desirable stability of soils, and the industrial engineers concern with the undesirable stability of bulk solids in order to obtain reliable flow. Structural engineers joined this coalition, by needing to design storage economic containers to contain the stresses of bulk material in static and flow conditions. Flow problems with coal and iron ore in the US steel industry stimulated a study of the parameters that influence the tendency for bulk materials to arch in storage bins. From this single work flowed a torrent of further research on other topics of associated industrial interest. The implied deliverance of industry from the scourge of bulk handling problems was found embryonic. Both the human and application backlog was formidable and never thoroughly tackled. The testing and design process was also demanding in skill and experience. Consequently, some disillusionment set in and the impetus for radical progress was lost, research funds being directed to more glamorous or promising technical marvels.
However, threads of common interest provided the cohesion to steadily bind together the disparate segments of manufacturing industry. As one active research organisation withdrew from the field others entered, to provide continuous support for a background core of enthusiasts toiling in this morass of technical complexity. It is now recognised that it is not possible to reliably predict the behavior of a given bulk material from knowledge only to its particle of composition. The name of a product cannot reflect the multitude of attributes that influence how a bulk material behaves therefore a data bank of material flow characteristics based upon generic descriptions is a very fallible asset. The inherent complexity of the subject lies in the host of variable factors that may be relevant to any specific application. However, the state of the art is such that most solids handling applications can be designed to work satisfactorily. Unfortunately, it will be some time before there are sufficient qualified engineers to apply this technology at the scale required. The greatest task lies in widespread basic education in the subject.
Solids Handling Technology is a multi-discipline science, with roots in civil and mechanical engineering, and stimuli from structural and chemical engineering. Its emergence as a single, comprehensive field of study followed a long gestation period in soil mechanics and a belated recognition that particulate solids represented a fourth, or multi-state of matter. The technology combines features from liquids, solids and gases, but embraces infinitely greater variation due to the interaction of a host of factors. The effect of mechanical, chemical, thermal, electrostatic and molecular features in a two or three phase media brings in further complications and the situation is made even more complex by the influence of scale, ambient conditions, industrial equipment factors and operating sequences. No wonder that accumulated technical knowledge in the subject took a long time to mature to a coherent structure. Any attack on a subject of this complexity must involve simplifying assumptions. Treating the media as a continuum is appropriate under certain conditions and allows significant advances to be made.
History
Bulk solids have been handled and stored for thousands of years. A famous engineer Archimedes, calculated the number of grains of sand that would fill the then known universe as 1051, displaying a basic understanding of the packing characteristics of bulk solids. His perception of density being mass divided by displacement, regardless of container shape, is also applicable to particulate solids and their variable void condition.
His invention of using helical screws to pump water evolved to ubiquitous applications for handling of bulk solids. Evidence of grain stores in Babylon, ancient Egypt and through the Roman Empire shows that relative large volumes of bulk materials have been shipped, stored and handled from antiquity. However, it is only in recent times, with the growth of cities and the industrial revolution, that concentrations of a wide range of loose solids have been held in large gravity flow structures other than by manual handling. Whereas grain is comparatively free flowing, many other primary and processed products display difficult flow and handling characteristics. ’Solids Handling’ remains a mature industry with an immature technology.
Modern solids handling technology has its roots in soil mechanics. The stability of bridges, buildings, earthworks, dams and military fortifications has long attracted the attention of engineers. The work of Coulomb, (1) and Rankine’s work on friction, (2) and of Reynolds, (3) who observed dilatancy effects on sand during deformation, are especially relevant to bulk solids flow. The one and only known paper of an obscure German engineer in Hamburg in 1895 was a landmark in solids handling technology. His theory explained the findings of an English engineer, Isaacs Roberts, (4) of the effect of wall friction on silo wall pressures. By way of wood and glass models and simple calculus Janssen, (5) developed a theory of pressure distribution in grain silos that remains today the most widely used method of assessing forces on silo walls. To quote from Alan Roberts, who’s most detailed review of this work and the subsequent 100 years, (6), is condensed below ’One excellent paper that withstands a century (and no doubt more) of scrutiny and application is worth more than 100 papers having a short ’half life, passing into oblivion soon after publication’.
Bulk technology progressed slowly over the next half century. Verification and refinements to Janssen followed, mainly being concerned with structural aspects of silo construction. Airy, (7) Prante, (8) Toltz (9) Ketchum, (10) Jamieson, (11) Lufft, (12) Pliessner (13) Bovey, (14) and many others, constructing flow experiments and calculations. Interestingly, the findings of Janssen were re-discovered by Shaxby in a joint paper with Evans, (15). Hvorslev, (16) examining the stability of cohesive soils, introduced the important concept of ’critical state’ to the study of the failure characteristics of bulk material. He showed the peak stress at failure to be a function of the effective normal stress and the void ratio, and independent of the stress history of the material. Dallavalle described the interdisciplinary nature of the technology in his 1943 book on ‘Micromeritics,’ which embraced a wide range of topic relating to the science of particles. Hans Rumpf and his team at Karlsruhe, made major contributions to powder technology. Despite these, increases in the scale of production and growing automation highlighted serious limitations in solids storage and handling technology. This was clear from the empirical nature of equipment design, a thriving ’flow aids’ industry and by the number and scale of silo failures reported by Theimer (17).
The Break Through
A young, Polish army officer stood on a hill in 1939, with Germans advancing up one side and Russian forces up the other. It was time to pack up fighting and seek refuge in England. Andrew Jenike studied for his engineering degree in London after the war and later settled in America, working for US steel. He decided to examine a range of industrial problems and collected all the information available on about thirty subjects, collecting all the information in separate boxes that he rooted through systematically. One day he suddenly made the crucial decision that the flow of solids was one of the most important problems of the day and, being a man of very positive action, immediately scrapped all the boxes of papers collected on other topics. He approached various universities before agreeing with Utah University to research on bulk solids flow for one dollar per year. This must have been the best scientific bargain of the century. His knowledge of Russian was fortunate in that he came across a publication by a little known engineer called Sokolovskii, (18), that provided the key mathematical procedure for analyzing converging flow of a plastic medium. With the aid of a young student named Jerry Johanson, he developed a solids flow theory, an instrument for measuring flow related properties of powders and a design methodology for flow in bulk storage containers.
These tools offered a solution to the age-old problem of designing a silo that would guarantee the reliable discharge of non-free flowing materials. His classical Utah Engineering Experimental Station thesis, (19), published in 1964, burst upon the academic world and suddenly changed the subject of bulk solids from a black art to a respectable topic of study. The influence of filling and discharge sequences, definitive flow patterns, transient and switch stresses, all fell into an understandable and predictable structure. Problems experienced in the handling loose solids, long the scourge of industry, appeared to be banished to history.
Australia instituted a combined attack on solving bulk solids storage and handling problems with the Universities of Newcastle and Wollongong combining to form a centre of Bulk Solids excellence. Through the prolific and words and deeds of Alan Roberts and Peter Arnold they made a strong contribution to performance of the continents large-scale iron and steel export industry, and spread the word internationally. Roberts, Arnold and Mclean’s publication (20) of a more illegible interpretation of Jenike’s work, put the theory within reach of industry.
The Golden Years
In UK in the late 60s this science formed a part of Harold Wilson’s ‘White Heat of technology’ that was going to transform British industry. The work of Roscoe, Schofield and Wroth at Cambridge University, on critical state soil mechanics, (21) fell into place. The Government set up Warren Spring Laboratories, with a major section under Dr. Fred Valentin devoted to bulk technology development. Bradford University formed a School of Powder Technology, headed Dr. John Willams. Prof. Scarlet formed a School of Particle Technology at Loughborough University. Rumpf’s energetic team at Karlsruhe Technical University in Germany, produced sterling work. The exciting days of theory, testing and development flowed fast and furious.
Devices were designed at Warren Spring Laboratories for measuring the tensile and cohesive strength of compacted bulk materials, and they conducted much research into powder and paste behavior. The use of injected air for controlling the state of powders was put on a scientific basis and the work broadened into investigating various forms of solids handling equipment. Walker, at the South East Electricity Generating Board, developed the more user friendly Annular Shear Cell and supporting theory, (22), and Williams and Birks introduced the Unconfined Failure Tester, surely the ultimate tool for measuring arching potential.
The inter-disciplinary nature of the bulk solids industry was firmly established when Abraham Goldberg organised the first PowTech exhibition. This was quickly followed by a proliferation of similar events in UK and internationally. Specialised trade journals for the bulk solids industry were started, and more continue to be introduced. The most notable of these were the journals ‘Bulk Solids Handling’ and ’Powder Handling and Processing’, introduced by Reinhard Wohlbier through Trans Tech Publications. Their contribution to the international diffusion of the technology merits the highest praise. These stirrings of a coherent industrial discipline have steadily strengthened, as shown through the formation of the U.K. trade organisation SHAPA, ( Solids Handling and Processing Association ), which now has over 100 members selling equipment to this market.
The British Materials Handling Board was set up by the government to assist the co-ordination of research into powder technology and aid the dissemination of information. Despite sterling work by its secretary Peter Middleton in organising meetings, publishing reports and books, with only limited, erratic resources it was only able to scrape the surface of the task. Relative to the scale of the industrial importance of bulk solids, the number of Universities and research establishments involved throughout the world has always been surprisingly small, even though their contribution has been magnificently significant.
The Struggle to Apply the Technology
Somehow, however, British Industry never secured the extent of benefits expected. These truly golden years of promise for powder technology are not as yet fulfilled. As a ‘new’ subject in its consolidated form across industry, the scale of the training problem was immense and never caught up with the backlog. Bulk technology remained a little taught subject in the syllabus of either mechanical or chemical engineering degree courses, tending to fall between the two stools of the mechanical and chemical engineering professions. This is despite the fact that over half of all the products used or consumed by humans are at some stage in a particulate form and are handled many times from source to use.
Many groups previously active have retracted or become extinct. The School of Powder Technology at Bradford University was assimilated into the Chemical Engineering department with the retirement of Williams. The Loughborough School of Particle Technology never reached the same heights when Scarlet departed to Delft. The initial Government backing faded as the ’breakthrough’ of efficiency was sluggish, culminating in Michael Heseltine selling the Warren Spring Laboratory site for a car park. This situation partially explains the sad reading of the Rand reports (23), (24), on the lack of improvement in the solids handling industries from the 1960s onwards. The dismal record of efficiencies in plants that handle loose solids is compared with industries that handle gasses and liquids. The real reason for the lack of technical progress is that powder technology is a vast subject, greatly complicated by the host of interacting variables that influence the behavior of a mass of particulate solids.
The equipment and operator sensitivity of the apparatus, the time consuming process and the expertise needed to interpret the results of the Jenike method tended to confine application to major installations. It did not help that the theory was complex and written in a style difficult to understand. The academic world pursued even more complicated mechanical devices to reflect ‘true’ shear conditions, as the basic theory was considered solved, taking the subject further from widespread industrial utility.
It took about ten years to develop a written procedure and provide a reference test material to give a basis for achieving consistent results, and it is still a very dubious question as to whether that objective has been achieved. (25). No wonder that the technology virtually ground to a halt, with limited shining exceptions such as at British Steel under Herbert Wilkinson and Harold Wright. The Jenike technique remains the only internationally recognised method of establishing the critical arching dimension of a bulk material in a mass flow silo to determine a ‘safe’ outlet size. As an example of the slow progress in the industry, the adoption the Jenike method as a standard for ASTM in USA, the country of its origin, only gained approval during last year.
A handicap to the promotion of the technology was that it is in few people’s interest to publicise that the value of wall friction is arguably the most important solids parameter for design requirements. This is a simple measurement to secure and is the major material factor influencing mass flow in hoppers. However, even this measurement must be undertaken with a robust preparation and operating procedure, as was brought out by a series of test conducted by the E.F.Ch.E. Working Party on the Mechanics of Particulate Solids, (26). It is not widely promoted that static stresses are crucially important to problems of initiating flow from a first fill condition, as emphasis on dynamic stresses for flow channel analysis tended to dominate theoretical considerations. The pragmatic requirements of industry came a poor second to the intellectual challenge of understanding the failure complexities of particulate solids in different stress conditions.
J. Schwedes, Brian Scarlet, and Gisle Enstad of Tel-Tek (POSTEC) in Norway, separately sought the Holy Grail of powder testing, through the development of inevitably sophisticated, bi-axial test devices. Peschl and Schultz market versions of an annular shear tester for automatic conduction of shear tests. Michael Rotter, at Edinburgh University re-invented the Uni-axial tester for coal applications, addressing the problem of sample delicacy by a tri-wall cell form and countering wall friction by double-ended compaction in the preparation cylinder, using an elastic support for the walls. Alan Roberts in Australia and Andy Matchett of Teeside University have studied the effect of vibration on wall friction. An alternative approach to the failure properties of cohesive powders has been made by Molerus through fluid dynamics and Enstad has proposed a new theory of arching in mass flow hoppers. Colin Thornton at Aston University is pursuing the interaction of hundreds of thousands of particles in two dimensions, by means of computer simulations.
The task of replicating this analysis in three dimensions is many orders of magnitude more difficult. Discrete Element Modeling, (DEM) is held as future tool for relating bulk behavior to particle attributes. A spoonful of micron size particles contains in the region of 5,000,000,000 particles, and individual interactions are significantly more complicated than the simple models used. The number of particle factors pertaining to solids flow is also great. Realistic replication and prediction of bulk material behavior on these lines may therefore be expected to lie far in the future. A composite rheological model, constructed of the basic elements of elasticity, plasticity and yield, (27), has ten components, each of which are potentially transient and stress dependent, so classification on these lines is also too complex to contemplate at the current state of the art. Positron particle tracking identifies the route taken by particles in a complex field of motion. This knowledge gives the engineer a better understanding of mixing, flow patterns and drying process, but this technique must be further developed to provide equipment design data.
Where does that leave us today in terms of solving current problems?
Researchers are using powerful and sophisticated techniques, such as positron tracking, DEM simulation, mathematical models and finite element analysis, to examine the behavior of loose solids. In UK, two major bulk solids centres, based respectively at Greenwich University and the Glasgow Caledonian University, provide consultation service and training for industry. Extensive programs are under way to investigate caking properties and pneumatic handling related parameters. Teeside, Cambridge, Surrey, Edinburgh, Birmingham, Bristol, Bath, and a few other Universities have sections active in different areas of bulk technology. A center for Bulk Solids technology has been formed at the University of Florida, and many other US universities are taking an interest in the discipline. The work of the New South Wales group in Australia has been recognized as representing a major industry by the formation of a Key Centre for the subject of bulk handling technology. The scope of test instruments for measuring powder properties cover an extensive range. Schwedes, (28), identifies 27 types of shear testing devices alone.
Jerry Johanson is back on the scene with a set of instruments to measure wall friction, bulk porosity and a form of shear strength. This latter device is presented with limited underlying theory, and hence is not intellectually acceptable to the scientific community. His vast experience has brought him to the conclusion that the Jenike method is not appropriate to industry and that a simpler approach is needed. The author has used a similar form of ‘vertical shear cell’ for some years as a simple tool to measure static failure conditions of a uni-axially loaded compact, a formation and failure condition reflecting incipient collapse over an opening in a non-mass flow container. An elementary relationship the mass that stimulates shear against the strength opposing failure, leads to the prediction of a critical size of opening at which self-weight collapse will take place. Not an elegant solution, but a conservative guide to the size of outlet needed to commence flow compared with a situation of wall slip and of re-developing flow after a dynamic stress field has been established. A wall friction device and this tool suffice to address many solids handling problems. Test devices are being introduced to explore stresses in dynamic conditions and simpler annular cells made available. Solids storage equipment is being introduced, that pragmatically exploits planar and converging/diverging flow channels to give reliable flow through smaller outlets. The use of inserts, a subject rich with potential – and hazards, remains relatively unexploited.
A plethora of flow aid techniques and devices are promoted for stimulating flow. These are rarely classified according to their suitability for specific applications, or even for cost effectiveness. The use of air in many forms, vibration and mechanical agitators via for attention, without potential users having a clear structure of fitness for purpose or any distinction between a scientific or brute force approach. There is obviously much work to be done to provide industry with systematic selection criteria. The methodology for design of a mass flow hopper section to ensure reliable flow is well established. The headroom requirement for a mass flow section of an expanded flow type of hopper, or other means to stimulate discharge can usually be accommodated. By contrast, the provision of discharge facilities to overcome potential rathole problems tends to incur disproportional large costs. Means to calculate the critical rathole diameter more precisely, and counter rathole problems, merit close attention. This is an area where a database of unsatisfactory installations may be useful for research purposes.
The physical properties of some type of bulk solids present special flow difficulties. Fibrous, flaky and dendretic shaped particles defy conventional shear testing procedures. Products that degrade, ‘cake’ or sinter require a different method of testing to quantify their flow difficulties. In the first stages of product/application evaluation, there is a need for a systematic characterization matrix that identifies potential flow and handling difficulties before the equipment selection and design process commences. Segregation remains a complex subject for many reasons. There are many mechanisms of particle separation and their scale and intensity is very application interactive and sensitive. The significance of their varied consequences is also highly variable, in some cases subjective or known only to the user or, to often, the plant operator. Design techniques to mitigate or restore to some extent segregation that has taken place are not widely known or applied. To this end, a better understanding of velocity contours within different types of flow channel would be useful.
Dense phase conveying technology is largely empirically, and new installations are usually based upon practical trials. The boundary between powder mechanics and fluid mechanics tend to blur and/or have separate consideration in this field. The crying need of industry is for the degree syllabus of all chemical and mechanical engineers to include the basics of bulk technology, not exotic, ethereal techniques that are the domain of specialists. An understanding of elementary stress mechanics, friction and shear mechanisms, flow regimes, two-phase effects in compaction and dilatation and the elements of segregation processes are needed by engineers in industry. These would enable the coming generation of equipment designers, plant and process engineers to avoid many of the pitfalls that still bedevil the industry. There are also new problems, such as ‘silo quaking’, where large periodic forces are imposed on the structure by intermittent motion of the stored contents. Most performance shortfalls are not due to major deficiencies in the process, mechanical or structural engineering field, but arise because of apparently minor and comparatively low cost plant items do not receive the attention to function that is vital for reliable performance. The cost of equipment, such as chutes and bin outlets, has no relationship to costs of production failure.
And what of the future?
The formulation of a universal data bank for bulk material properties appears superficially attractive but carries serious, if subtle hazards. There is a need for a characterisation structure, to quantify key flow-related features of a new product and predict its potential behavior nature by correlation with a substance of known manner. Wider appreciation of the importance of wall friction and different shear strength values would be useful. More textbooks and guide documents are required on both theoretical and applied topics, to which the writer has made a small contribution, (29 - 31). Tim Bell, in his review of industrial needs in solids flow for the 21st century, (32), emphasizes the continuing research required for quantifying flow and pneumatic handling behavior. He makes the point that the credibility of the technology rests with the ability to provide numerical solutions. Most of all, cheap, simple and user friendly test devices need to be widely available, to provide quantified values for design and contract specifications, accompanied by undergraduate training that leads to an understanding of the key principles for their application.
The challenges of new materials such a nano-particles, for increased performance reliability, better industrial efficiency, improved working conditions, cost savings and all the benefits associated with more predictable designs, place a heavy demand on engineers to further develop bulk technology. In the words of Churchill, when England barely survived the darker days of World War II, ‘This is not the end, it is not even the beginning of the end. It is more like the end of the beginning’.
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