SAR - Review

Executive Summary

This paper summarizes a variety of this author’s industrial experiences with various conditions in size reduction processes which can either positively or negatively influence the narrowness of the product size distributions that can be produced. There are a relatively large number of operating factors, which if not properly selected, can give non first order breakage which unnecessarily broadens the final product size distributions produced by most industrial grinding devices. Key among these types of factors are dry powder aggregation, slurries that exhibit a rheological yield value, mill underfilling or overfilling, non-optimal rotating speeds for certain types of mills, media sizes that are too small for the material size being ground, and performing too large a size reduction ratio with a single grinding device. It is important in any size reduction operation that these potentially negative factors be evaluated (checked) so as to give their best performance. In other words, the best place to start in narrowing particle size distributions at the industrial level is to avoid using non-optimal operating conditions.

On the other hand, there are a number of factors which, when aggressively optimized, can give steeper (narrower) size distributions in a more proactive manner. Chief among these factors are the proper selection of staged grinding devices, the incorporation of appropriate classification, and, when possible, deliberate material selection (including modification in some cases) so as to promote the ability to produce steeper size distribution. The typical response of closed circuits involving classifiers are described in some detail. Also, a useful limiting case analysis method is presented which indicates the most narrow possible size distribution that can be produced with any material when all conditions are ideal. From an experimental viewpoint, the author also strongly recommends and describes a standard test sequence to be routinely run on each new material that an engineer might have to work with in a size reduction process. This rather minimal level of simple laboratory testing can give information which really helps to signal what types of operating responses might be possible on larger equipment with the material in hand.

A tutorial review of flame aerosol technology for manufacture of ceramic powders is presented. In the mid-20th century this field was driven by industrial research and development for production of commodities such as fumed silica and pigmentaty titania. With highly competitive market growth, inexpensive scale-up of existing units is required. In addition, the introduction of this technology to manufacture optical fibers and its potential for cheap synthesis of ultrafine particles (e.g. nanoparticles) has renewed research interest in flame aerosol reactors.

In this review, emphasis is placed in synthesis of particles with controlled size and crystallinity. After an overview of its history, the fundamentals of this technology are summarized, specific applications in the manufacture of fumed silica, pigmentary titania, alumina, composite and non-oxide powders are reviewed and finally research needs are highlighted. With major recent advances in process instrumentation and understanding in both combustion and aerosol science and engineering, this field is ready for a new leap forward.

During the 1995 Annual Meeting of IFPRI at Urbana, IL, USA, the enclosed statement was published and presented. It describes in a short form pressure agglomeration, its subdivision into low, medium, and high pressure techniques, the mechanisms of pressure agglomeration, as well as the author’s opinion as to where need exists for development and research.

To further describe the state-of-the-art of pressure agglomeration, excerpts from the author’s book entitled “Size Enlargement by Agglomeration” are submitted herewith.

It had been suggested to summarize recent research for this report. However, after obtaining “suitable references” from within IFPRI and from searching this author’s files it became clear that such published studies are very specific. They relate mostly to requirements in the pharmaceutical industry or to other specialized applications. They often draw conclusions which are not of general interest and sometimes even misinterpret the basics of the unit operation.

It is not the intent of this report to reconcile the results of such studies. This should be done in a scientific research environment during an interdisciplinary literature search which, again in the reporter’s opinion, is the first and foremost work in this field that should be sponsored by IFPRI. The result of such a study should be general and unified conclusions and a true statement as to what basic information is available. At the same time the many publications must be disregarded which try to explain the influence of specific materials, additives, process modifications, and parameters for very limited applications without connecting this work with a theory that is generally valid.

Introduction

Powders such as food ingredients, pharmaceuticals, dyes, fertilizers, a.s.o., are frequently dispersed and/or dissolved in water or an aqueous liquid in the course of an industrial process or before use. The behaviour of a powder under such treatment is, of course, governed by the forces interacting between the individual particles.

A process in which a dispersion or solution is created involves powder particles in the dry state, in the wetted state, and (a small fraction at any given point in time) particles which are just about to be wetted, i.e., particles penetrating the gas/liquid interface. During wetting, the interaction forces between the particles change significantly. If the material is partly or fully soluble, the properties of the dispersant may also vary. The rate at which a powderous material can be immersed in a continuous or batchwise dispersing process therefore depends on a large number of physical properties. So far, no theory exists which allows the prediction of the powder dispersion rate based on such properties and the type of apparatus employed for the process.

Nevertheless, knowledge of the interaction forces is indispensable for understanding how to influence certain powder properties so that the product is easier to disperse. Quite a lot of work has been performed in the field of powder “instantization”, especially in the food industry and in food related research. Here, certain powder properties termed “instant properties” have been defined, measuring methods have been developed, and technical processes for the instantization of powders exist. All this is closely related to adhesion forces, since agglomeration plays the most important role in this field.

Executive Summary

While phenomena involving fine particles are the subject of considerable interest, the methods employed to observe these phenomena do not always receive equal attention. Because measurements in dense solid suspensions are challenging, it is common to ignore the detailed behavior of instruments and, by focusing on the suspension, to misinterpret their signals.

To address this issue, the present report reviews measurement techniques employed in relatively dense flows of gas and solids, with equal emphasis on successes and difficulties involved in their use. In particular, methods to record the following parameters are discussed:

• pressure
• solid volume fraction (capacitance and optical probes, densitometry, tomography)
• solid flux
• particle velocity (cross-correlation, laser-doppler-anemometry)
• convective and radiative heat fluxes
• diffusion and mixing (sampling probes and tracers)

Wetting and spreading phenomena affect a wide variety of fine-particle processing operations such as the preparation of particle dispersions, the dissolution of solids, wet separation processes for particulate materials, and the drying of solids. This review attempts to highlight the current state of understanding of wetting and spreading phenomena applicable to particulate systems. Aspects of wetting of individual particles, and larger collections of particles are addressed. Following a brief introduction, there view is organized into five main sections.

Section 2

Section 2 describes the general features of wetting and spreading phenomena and introduces the vocabulary associated with the description of wetting. Most of the original analysis and current investigations of wetting dealt with the contacting of liquids onto well defined solid surfaces. Although wetting of particles is far different from this idealized picture, the phenomena that contribute to the wettability of particles are the same. Thus, the results presented in Section 2 form the foundation for the understanding of the wettability of particles and collections of particles.

Section 3

Section 3 discusses the relationship between wettability and the molecular features of both the solid and the contacting fluid. In general, there are both physical and chemical contributions to wettability. Understanding of these effects can lead to useful predictions of wetting tendencies, and also offer insight into how wetting can be modified through the use of dispersants or wetting aids to compatibiliie the solids and liquids.

Section 4

Section 4 reviews the state of understanding of the wetting of individual isolated particles. The various methods that have been developed over the years for the assessment of the wettability of particles are discussed and the difficulties associated with these measurement techniques are presented. Quantification of the influence of wetting on the dispersibility and stability of individual particles are also described.

Section 5

Section 5 reviews wetting processes for large quantities of particles or individual agglomerates of particles. Critical conditions for the spontaneous wetting of collections of particles are discussed. The various stages associated with the wetting of collections of particles are also presented. The problems associated with contacting liquids with large quantities of particles are outlined. The influence of wetting on cohesivity of particle compacts and agglomerates is also described.

Section 6

Section 6 provides a brief review of some practical aspects of wetting phenomena for particle processing technologies. A specific issue addressed is the influence of wetting on the dispersibility of agglomerates.

The summary section of the review identifies some unresolved problems pertinent to particle processing that are recommended for additional research.

The principle conclusion of the review is that while there has been much study of the wetting of particles and large quantities of particles, significant difficulties remain in characterizing the important phenomena. Although much theoretical and experimental effort has been spent in an attempt to quantify wetting phenomena or to predict wetting tendencies in particulate systems, these studies are often thwarted by the lack of readily characterizable surface features of particles at all of the relevant length scales. These important features include both the morphology of the particle surface and the chemical makeup of the particle interface with the fluid.

Since the same physical and chemical phenomena govern the wetting of both ideal (flat, rigid, chemically homogeneous) solids and particle surfaces, it is understandable that the study of the wetting of particles has borrowed concepts originally developed by surface scientists for the study of fluids with idealized surfaces. However, particle technologists must contend with imprecisely defined parameters such as apparent contact angle and inferring wetting behavior based on assumptions about the structure and homogeneity of the particle surface. While the lack of uniformity and detailed knowledge of the particle surface may prevent quantitative a priori predictions about wetting, the theories do allow predictions of relative wetting behavior and experimental methods can be used to assess the effectiveness of treatment strategies designed to improve wettability of particle systems.

The main interests of the fine particle program which is carried out in the research institutes of the Russian Academy of Sciences (RAS) and High School are concentrated around the following topics:

• particle formation;
• nanoscale compacting;
• characterization and measurements;
• applications including based on size-dependent characteristics.

The review presents a quite detailed description of the state of art concerning the above topics involving a general outlook of a scale of the research field as well as of an extended geography of the research performance. Particular attention is paid to the level of readiness of the new material technologies as well as characterisation methods and tools for technological applications.

Single and Twin Screw Extruder Based Continuous Processing Techniques

Single and twin screw extruder based continuous processing techniques offer significant advantages in the processing of pastes in comparison to batch mixers. These advantages include high surface to volume ratio and hence better heat transfer characteristics, tailorable mixing behavior, and ability to work with modular elements i.e., flexible manufacturing capabilities. The extrusion technologies also offer the facility to process and compact solids, melt, mix, deaerate/devolatilize, pressurize, react and shape within the confines of a single machine. In this report the fundamentals of the processing of pastes in extruders are reviewed and elucidated using both single and twin screw extruders. The applications of the extrusion technology in various industries are further outlined by summarizing the technical journal and patent literature available.

Methodology and Mathematical Modeling

The review also includes a methodology for proper characterization of pastes and a mechanism and framework for mathematical modeling of the processing of pastes in single and twin screw extruders. One, two and three dimensional mathematical models of the extrusion processes are covered. Important findings and experimental methods to verify the modeling results are discussed.

Conclusions and Future Directions

The review concludes that the basic failing of the available techniques, both in mathematical modeling and experimental studies, involves the inability to incorporate and characterize the microstructure of the paste as it evolves in the extruder. Future directions of research in this area are provided.

Pneumatic conveying systems have been used by many industries to transport dry bulk particulate materials. The majority of installed systems are so called dilute phase systems where the particles that comprise the material are transported at low concentrations, suspended in the transport gas. Compared with other bulk handling systems they provide a simple means of enclosing the material and great flexibility in routing. The downside to this is that they are often less energy efficient and more hostile to the material being transported. The development of dense phase pneumatic conveying systems was aimed at reducing these disadvantages.

This review examines the state of the art of dense phase pneumatic conveying. The review is divided into five sections:

• Material capability for dense phase conveying.
• Conventional dense phase conveying systems.
• Special systems for low-velocity dense phase conveying.
• Modelling dense phase flow.
• Future research directions.

There has been a certain amount of confusion about the definition of dense phase conveying. The first section of the review examines the nature of dense phase flow, A gas-solids flow in a horizontal pipe is defined as being dense phase when the majority of particles are not suspended in the conveying gas. The possible modes of dense phase flow are identified, and the relationship between the shape of the operating envelope of the pneumatic conveying system and these modes of flow is presented. From this three classes of bulk material can be identified:

• Those only capable of dilute phase flow.
• Those capable of a moving-bed mode of dense phase flow.
• Those capable of a wave-like mode of dense phase flow.

Measurement of the gas diffusion properties (permeability and de-aeration) and cohesiveness of a bulk material provides a means of classifying materials according to the type of dense phase flow that they can achieve. This classification provides a means of assessing the suitability of the material for dense phase transport and the likely benefits in terms of system performance.

If a bulk material is naturally capable of dense phase flow then the selection of system components is similar to that for a dilute phase system. The second section of the review examines those systems that have most commonly employed for naturally dense phase capable bulk materials. Systems using pressure vessels as the solids feed device, blow tank systems, are probably the most common and feature throughout this section. Another important class of system employ so called ‘high pressure’ rotary valves. These permit a V IFPRI Report On Dense Phase Pneumatic Conveying Glasgow Caledonian University continuous solids feed while minimising the air leakage suffered by all rotary valves.

Although many of the feeders employed in these dense phase systems are capable of operating in systems with high pipeline pressure drops, the equation dense phase = high pressure is not always true.

Many users of dilute phase pneumatic conveying systems wish to obtain the benefits of conveying in a dense phase mode, but have materials that are not naturally capable of any mode of dense phase flow. The third section reviews the types of novel systems that have been developed to meet the demands of these applications. These systems may be classified as either:

• special feeders;
• special pipelines - air bypass and air injection system.

Special feeders attempt to introduce the solids into the pipeline in such a manner as to promote &nse phase flow. Special pipelines aim to maintain the correct conditions in the pipeline in order to sustain dense phase flow. From the previous discussion it is clear that the special pipeline systems have the best chance of achieving dense phase flow with a bulk material that is not naturally capable of such flow. When a bulk material is transported at a velocity below its natural minimum it will form a blockage. Most special pipelines permit operation below the natural minimum velocity by providing an automatic means of clearing pipeline blockages.

Air injection systems achieve this by injecting air into the blockage to clear the problem. Air bypass systems achieve this by providing an alternate route for the air flow. This directs air to the downstream end of the blockage to reduce the length of the blockage and hence aid its removal. Air bypass systems have the advantage of being less complex in construction, and do not change the total mass flow rate of gas in the pipeline.

The forth section reviews the types of model that have been developed to describe gas-solids flow in pipes and assesses them in terms of their suitability for predicting kzse phase flows. The aim of any model is to describe the behaviour of a physical process, in this case the model must be able to predict:

• Straight Pipe: A FALL in the pressure drop for a constant solids flow rate as the gas flow rate is increased over the range of velocities normally employed (2-30m/ s)
• Inclined Straigh Pipe: For constant gas and solids flow rates, a maximum pressure drop which occurs at an angle less than 90” (vertically up).
• Bends: A RISE in the pressure drop for a constant solids flow rate as the gas flow rate is increased over the range of velocities normally employed (2-30m/ s)

The modelling techniques developed may be classified as either:

• Inside describe the physical processes that occur inside the Pipe
• Outside describe the behaviour of system components, usually derived from experimental correlations

Outside models have the advantage of simplicity, but are dependent upon experimentally determined parameters. The quality of the experimental data limits the application of these models in terms of scale-up and the range of bulk materials that can be modelled.

The development of inside models provides a means of understanding the physical phenomena that occur during transport. The disadvantage of this type of model is the computational effort required to solve an industrial scale problem.

The final section suggests some areas for further investigation. The key to a successful outcome of this work is the strength of the links between the various strands of the programme (experimental - measurement - theoretical).

David J. Mason

Mixing of particulate materials is widely encountered in the process and related industries. So is segregation. They are competing processes; the one a deliberate act intended to increase the ho- mogeneity of a mixture of different components or sizes, and the other an involuntary process, occurring as a result of the fact that the various forces that may act on the individual components of a particulate mixture may cause them to move in different directions, or to different positions in a bulk, due to their different characteristics. Generally, the most important of these is particle size, although density, shape, surface roughness, resilience, electrostatic properties etc. all can play a role, as can process conditions.

The mechanisms that are mobilised in order to effect mixing are basically three: diffusion (or preferably dispersion), convection and shear. Diffusion with particulate materials only occurs over very short distances and, in order to mobilise the mechanism, the different components need to be brought into the vicinity of one another. This is generally accomplished by convection or shear, the latter being considered by many to be an idealised form of convection. Modern processes of mixing such as hybridisation using mechanical forces etc. are not covered in this review.

Twelve mechanisms have been identified which lead to segregation. Some of these can act within the mixer itself, while others are only encountered in subsequent handling operations. Wherever they are encountered, they reduce the homogeneity of mixtures. Since the three mix- ing mechanisms and several segregation mechanisms can act simultaneously, doubt has been expressed whether a mixing process can be modelled in a deterministic manner. The weight of opinion appears to be that heuristic experimentation must be accompanied by the development of algorithms to create an expert system in order to improve on the state of the art. Suggestions have been made that chaos theory may be applicable. Chemometrics may be another option.

The review includes a description of the classical approaches to characterising mixture quality and concludes that these have inadequate relevance to modern day needs. For the continuous monitoring of mixers and mixtures it is suggested that both the intensity of segregation (as indi- cated by the variance) AND the scale of segregation (as indicated by an autocorrelogram) be used. Such an approach would allow the continuous monitoring of mixing processes. Whatever method is chosen to monitor or evaluate the quality of a mixture, the sample size is critical. It will vary with the purpose for which the mixture is being created, and the demands made a on a mixer will very much depend on the homogeneity it can achieve in a sample of the chosen size. Optical probes offer the possibility of continuously monitoring mixtures, as do combinations of acoustic probes and chemometrics, but these may not be sufficient to determine the quality of a mixture at the microscope level.

The review concludes with a description of models proposed to describe mixing and segregation processes.

Research plans in these two areas at Telemark College and Telemark Technological R & D Centre are described in an appendix.