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Particle technology
Pergamon
Chemical En#incerino Science, Vol. 50, No. 24, pp. 4081- 4089, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0009-2509/95 $9.50 + 000
0009-2509(95)00225-1
PARTICLE T E C H N O L O G Y J. BRIDGWATER Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, U.K.
(Received 1 July 1995) Abstract--Powders and granular materials are in common use in the chemical and allied industries yet traditional research and teaching in chemical engineeringdo not reflect this significance.Quite apart from the desire to advance knowledge for traditional processes and to enhance competitiveness,demands arise from the extension of chemical engineering ideas to a much broader range of products and processes. Progress in particle technology is now possible from advances in theory, from instrumentation, and from computer methods. The subjects of elasticity, plasticity and frictional flow of powders are central to an understanding of behaviour. In general, particles have a complicated internal structure and the design and engineeringof this structure is a critical feature, with packing and colloidal effects, to name but two, being critical. Stress is not uniformly transmitted, this passing along preferred paths with breakage being initiated from the tips of cracks in the paths. There is now a wide range of accessible and important problems that can and should be solved; particle technology does, after all, constitute a good half of the discipline. INTRODUCTION Particle technology is a term that has been coined to describe that part of engineering which is concerned with the production and processing of powders and granular materials. It is perhaps alarming to discover that the education of chemical engineering undergraduates contains little or no coursework on particle technology yet we know that various studies have shown that about 60% of the products of the conventional chemical industry alone are in solid form. Many of the other products require use of a solid during processing. The spread of chemical engineering skills into industries such as pharmaceuticals, minerals processing, biotechnology, food, new materials including ceramics and cement are ones in which solids can be even more dominant. The chemical engineering discipline lacks a structured approach to particle technology, a situation that has arisen from the scientific complexity underlying the requirements of this part of chemical engineering practice. The case has been made repeatedly in many countries in the recent past because of its enormous impact in the chemical industry. In what follows it is argued that, in certain areas, ways ahead are now becoming clearer due to enhanced interest from the industry in its endeavours to make production safer, more efficient and more reliable. Industry also needs to find quick and economic ways of manufacturing new products that are more sophisticated chemically or biologically than those in current production. Conceptually, there are signs of theoretical approaches becoming available and hence certain regions of particle technology can develop a rigour comparable to that of established parts of the discipline of chemical engineering. In many cases these concepts are becoming available due to significant advances in instrumentation and data acquisition. This is quite distinct from significant insights
arising from the use of computers to simulate processes in particle technology. Quite apart from developing quantitative methods, in many cases simulation would seem to be the only way available to assess which parts of the basic physics are critical and which are not. The chance to write on a subject in which the author had long been active comes unexpectedly owing to a timetabling clash that took off the young Turk lined up to deal with this area to the Particle Technology meeting in Niirnberg in March 1995 which overlapped with the meeting in Cambridge, Massachusetts. The area reviewed, however unfashionable and difficult it may seem to some, is a central one. The sheer breadth of it can be seen from the Niirnberg meeting and from the first meeting of the Particle Technology Forum of the American Institute of Chemical Engineers at Denver in the summer of 1994. The approach adopted here is one that relies on personal interests and little attempt has been made to assess the state of knowledge of the whole area. The discussion is personal and selective. A great number of themes are appearing in particle technology with many able workers within these. However, some key areas and key approaches are illustrated. Debate on the future shape of research in chemical engineering has been stimulated recently by a number of workers, not least by Mashelkar in his Danckwerts lecture of 1994 (Mashelkar, 1995). How the discipline interacts with the changing character of science and of processing and its implications for professional practice, teaching and research in chemical engineering has been analysed recently (Bridgwater, 1995). This last discourse identifies a number of themes. One is the development of an understanding in chemical engineering of the relationship of products to processing, it being argued that the discipline has become much too obsessed by the latter. Indeed life cycle
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analysis has become a necessary part of thinking. It was argued that the business and scientific climate is led by three simultaneous industrial revolutions, these being in: • information technology - - computing, • biology, • chemistry, materials science and physics.
Solids Storage I Solids Flow I liquid
Dry Mixing
Wet Mixing
I High Shear Mixing
There are knock-on effects relevant to the traditional bounds of chemical engineering, these being identified
gas Degassing
as"
Paste Flow
• instrumentation, • tackling old problems in classical areas, • tackling new problems in classical areas. Five of these six areas, the sole exception being biology, feature in what follows. Particle technology is illustrated principally by a case study followed by comments on certain other areas. A CASE STUDY
The process followed is manufacture of a honeycomb catalyst as might be used to make a car exhaust. It provides a means of discussing the state of knowledge in a broad section of the discipline. Figure I shows a ceramic honeycomb of approximately 100 mm maximum external dimension. It has in it an array of regular square passages of approximately 2 by 2 mm cross-section each. One way of manufacturing such monoliths is shown in Fig. 2. The various processing stages are as follows. The solid components are stored in a number of hoppers and
Extrusion
Extrudate Handling Cutting Drying i Thermal Processin8
vaporised liquid
oxidised binder
Product Fig. 2. Stages in the manufacture of a ceramic honeycomb extrudate.
then fed in appropriate amounts by a flow system into a powder mixer. There these components are mixed in the dry state, this being done in the absence of liquid in order to promote an initially rapid rate on the macroscopic scale. Liquid components are then added into the same piece of equipment and the mixing
Fig. 1. Ceramic honeycomb extrudate, maximum dimension of tO0 mm.
Particle technology process continued. In general it is found that powder mixers are not sufficiently effective in producing a uniform material and thus a high shear mixing stage is then employed. The purpose of this is to enhance mixing on the microscopic scale which probably often includes the breakdown of particle agglomerates. Before the material can be subject to further processing, it is necessary to remove any gas that may have been held in the system. The paste is then passed through a flow system to an extruder. This will generally be a single or twin screw system comprising a feed storage system at inlet, a screw to convey material into the system and further, probably different, screws to ensure compaction before passing the material through a die plate to form product of the right shape in cross-section. The material so produced is said to be in the green state. Here it has sufficient strength to retain its shape but is delicate and thus the design of the extrudate handling system is important. Thereafter the extrudate has to be cut and this must be carried out without damaging the individual membranes of the monolith. It is then ready for final processing. Here this is shown to be drying which must be conducted in a manner to avoid the introduction of unwanted imperfections into the structure. During this stage the liquid solvent is removed. The extrudate is then generally subject to thermal processing during which organic binders that may have been introduced into the system are removed and product strength is developed either by solid state diffusion causing sintering or by the presence of further solid materials in the bulk, such as clay, that weld the larger principal particles together. The product from this thermal processing stage is indeed the final product, Certainly this is not a unique processing system for this product. Indeed, the sequence of stages is quite complex and good process design in future will turn on finding physical or chemical methods of avoiding one or more of these stages. Nonetheless, the flowsheet is fairly typical for a solids processing plant and provides a useful vehicle for assessing the state of knowledge.
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the carrying out force balances on bodies of materials bounded by such surfaces. This allows the position of slip planes occurring to be determined. To this has to be added the concepts of active failure and passive failure. The former is a solution corresponding to horizontal extension and the latter a solution to one of horizontal compression. Janssen (1895) obtained a first solution to the stresses developed in a cylindrical bunker. Work allied to theories of plastic flow was carried out by Sokolovskii in the Soviet Union in the 1940s and he obtained methods of determining the stress distributions in moving powder, it being supposed that at every point the material followed some frictional yield criterion. These ideas were taken up by Jenike who took these and provided a means of analysis for solid storage systems. Although difficulties were encountered in a completely rigorous analysis, he nonetheless gave a procedure which, moderated in the light of experience in the selection of appropriate safety factors, has provided a means of design for many storage systems. The approach can be illustrated by the analysis of a two-dimensional planar system. The force balance on an element of powder is as given in Fig. 3. The first
Frictional Methods
(
stationary annulus
gore
k,~ Core Flow
Mass Flow J
Oc
z +
~z
c~x ~
ao~
=pg
Ox
• +
~x
Solids storage and f l o w
o x = °0(1
Various types of flow pattern may arise during solids flow out of the storage system. It can happen that powder has frictional properties such that it is impossible to extract it from an outlet at the bottom of the hopper. Secondly, it is possible that there may be a central moving channel of material surrounded by stationary material giving rise to what is termed core flow (Fig. 1). Thirdly, if the design so permits, all particles in the storage system can be in motion when the outlet is opened and the system is then said to be in mass flow although this does not imply that all particles at a given vertical level are moving at the same speed. Procedures for the analysis of such systems can be taken back to the work of Coulomb (1776) who, working on the stability of embankments as a military engineer, developed the idea of slip surfaces through
o,
+ sine
Ox
,~
-0
Oz
cos 20)
= t~ 0(1 - s i n ¢ c o s 2 0 )
x xz = - x ~x = o 0 s i n ¢ s i n 2 0
g t bt Xzx + ~ - - ~ dz ~Z
a x
t-
0o
o z +
Z dz Oz
Fig. 3. Flow patterns in a storage system; equations for a two-dimensional stress analysis; force balance on an element of powder in two dimensions.
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J. BRIDGWATER
equation says that the weight of an element is balanced by a change in normal stress on the horizontal plane crz and shear stress on the vertical plane r~z. The second equation says that the net horizontal force is zero with the change in normal stress of a vertical plane trx being balanced by change in the shear stress on a horizontal plane zzx. By the principle of moments z2~ = z~z. The equations can be coupled with a MohrCoulomb yield criterion which include expressions for the horizontal stress tr~ and the vertical stress trz in terms of the average normal stress ao, ~b the measure of friction of the material, and 0 the angle of rotation of the system shown from the orientation of the principal planes, those on which the shear stresses are zero. This yield criterion argues for a linear relationship on a plane between the normal stress and shear stress at some orientation at a point in a flowing powder. The procedure for determining ~b is quite complex requiring consolidation of the powder at a number of stresses and testing at lower stresses. The interpretation of the data can prove difficult and very considerable effort has been made to ensure that precise experimental procedures are followed. The issues raised by the design procedure are, however, far from settled. For instance, if an outlet is situated other than at the centre of the base, design for flow can be problematical. Codes have been developed to describe the stress distribution in the walls of bunkers but these can again be seriously in error if the offtake is offcentre. In solving these equations by the method of characteristics, it is found that the solution does not necessarily penetrate all the regions of space that it should and the appropriate transformation of coordinates or utilisation of a more fundamentally based failure condition remains uncertain. The characterisation of the frictional properties of the powder are not yet related in any scientific way to properties of individual particles such as size, size distribution, shape or inter-particle forces. Nedderman (1992) gives a thorough account of the application of continuum mechanics to powder storage problems. Significant contributions have come, inter alia, from Jackson (1983) and Savage and Sayed (1979). The problems facing this area of the discipline are still considerable but there is clearly plenty of scope for mathematical analysis on the appropriate description of flow, the nature of the failure criterion, and the relationship of this to the fundamental properties of the particles. It must be added that the conventional design procedure stemming from the ideas of Jenike includes safety factors. There are also arbitrary steps in the procedure which are necessary to give a solution as these are not sufficiently Sustained scientifically. Some recent work has focused on the study of solids how during cascade down a free surface, the so-called avalanching flows, say by using molecular modelling techniques. However, it has to be said that these problems are a quite rare practical occurrence and remain really rather intractable since there is an enormous variation in properties from packed bed below
the flowing avalanche of approximately five particle diameters of width to a region entirely free of particles above (Campbell, 1990). There is certainly significant scope for computer simulation to describe these properties and behaviour and, indeed, it seems almost certain that such probing to discover the key particle and particle interaction variables will be essential to gain further physical insight. This has been the thrust of work stemming from Cundall and Strack (1979). Walton (1982) shows the potential of full computer simulation, and Hogue and Newland (1994) that of analysis of elements of the powder structure. The subject is now becoming popular with physicists, see for example the surveys in the book by Mehta (1994). Even in this comparatively developed area there is thus ample opportunity for modelling that is analytical through to that based on computer simulation. Mixing
Mixing is common throughout chemical processes yet the study of powder mixing remains extremely rudimentary. In part this stems from difficulties of simply seeing what is happening within the equipment. These devices have until recently proved opaque; it has simply not been possible to understand what has been happening. However with the availability of instrumentation and computing, it is clear that once information becomes available the rate of progress will necessarily be rapid. Another issue causing difficulties in the understanding of mixing has been that of sampling. It has proved extremely difficult to withdraw a reliable sample and to analyse it promptly. This one sample in itself is insufficient to describe the mixture structure and normally up to 40 or so samples are necessary and the variance of the sample concentrations used, normally linked into some arbitrary function often including the variance of the unmixed material and/or the variance of the system if it were to become entirely random. Taking and analysing 40 samples is normally an impossible task. Added to this, the size of the sample necessarily has an influence on the magnitude of the results. These simple facts have been easily overlooked in the past. Excellent surveys of past work come from Fan et al. (1990), and Poux et al. (1991). The physical processes affecting mixing of cohesionless systems, ones in which the particle-particle force is much less than the particle weight, have been studied in fundamental terms. Here one mechanism has been identified as inter-particle percolation, a separation occurring in the slip zones of failing materials in which a smaller particle moves through the zone, which is some ten particles or so in width, under the action of gravity. Particle migration is another mechanism; it is the motion of a particle in the direction of the highest rate of strain. This arises since a large particle can make better use of holes appearing on one side rather than another and accordingly moves in the direction of increasing rate of strain. Thirdly, there is free surface segregation, which is segregation due to
Particle technology size or density which occurs while cascading down a free surface. It is rather in contrast to inter-particle percolation since now the particle weight can contribute significantly in relative terms to the local stress and accordingly influences findings. With respect to cohesive materials, the colloidal inter-particle forces described elsewhere by Zukoski (1995) must indeed be the dominant ones to understand. It will often happen that there is indeed a liquid present and then liquid development of the strength of the liquid bridges and the distribution of the liquid throughout the system would be significant. This is indeed part of many granulation processes as deployed in the pharmaceutical industry for instance. Liquid added in the granulation process needs to be dispersed through the whole of the system and then pressed into the pores between all the particles. The very process of granulation is in urgent need of greater understanding in a mechanistic way. Mixing does indeed provide an instance of the application of modern instrumentation technique to unravelling the behaviour. Figure 4 shows the results of studies on the pattern of flow in a powder mixer using positron emission particle tracking using a mixer described by Bridgwater et al. (1993). In this technique a particle containing a positron-containing radionuclide is followed and the velocity distribution determined. It is possible to track particles moving up to 2 m/s in equipment up to 300 mm dimension. We see for the first time something of the flow patterns that can arise. The results shown in Fig. 4 are for a trough-shaped mixer mixed by an impeller consisting of a number of blades mounted on a horizontal shaft. There is one at each end and four in the middle giving five "compartments" within the vessel. Data have been recorded on the position of a radioactive tracer, there being over 1000 determinations of its position in each second. The mixer is 225 mm long with an internal diameter of the semi-dysfunctional base of 150 mm. The bed material is rice of ca 2 mm diameter and 4-5 mm length. The tracer is a silica glass cylinder of 2 mm diameter and 2 mm length. It is immediately evident from the axial coordinates that the tracer particle undergoes movement between a number of defined zones within the system. It remains within one local region moving about some central position but then shifts to another general axial position. At present it is not known what causes this behaviour to occur but it must be remembered that the tracer is of different properties to the bulk material and further work needs to be done in which the properties are precisely matched. At the same time, the data also reveal that the circulation patterns evident when a section is taken through the mixer and the velocities computed. The particles are drawn up at the end faces by the action of the end blade and pulled down about a third of the distance along the mixer. The tracer is relatively absent from axial positions low in the bed, these corresponding to blade positions. We can see that the tracer tends to exist on the free surface or is biased towards one end. The latter is found to be CE$ 50-24-P
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common but its origins are unclear. For a long mixer it is uncertain as to what the behaviour will be; does the central section give a true depiction of the central part of a much longer system? Such are the questions that we are now able to address. At first sight, however, it would appear that seeking to model the system as a number of well stirred units in series might be appropriate. A means of unravelling behaviour and describing it in chemical engineering terms is now possible.
High shear mixing Although there is little quantitative information, it is known that it is essential to shear the mixture from the wet mixing stage considerably in order to promote uniformity. This is believed to be linked to promoting the agglomeration and uniform wetting of all particles formerly within agglomerates. It is much related to the quality of many products and is a fertile area for further research.
De-gassing Although the subject has received no quantitative study for pastes so far as can be ascertained, it is generally necessary to remove gas that has become entrained in earlier stages of the process prior to further processing. Failure to do so can lead to breakage of extrudate or to damaged surface due to bubble rupture. The subject is well recognised from polymer processing but the nucleation process remains hard to understand.
Paste flow and extrusion The sheared de-gassed paste can now be passed to an extrusion system. A means of describing such flows has been developed recently and is illustrated in Fig. 5. Here the paste flows out of a barrel of diameter Do into a dieland of diameter D and length L. In the dieland the mean velocity is V. By supposing that the extension that occurs to the paste as it flows from the barrel to the dieland is equivalent to that during the drawing of a plastic wire, the pressure drop could be related to the other variables as shown in which the terms Co, ~, ~o and fl are deemed to be properties of the paste, the first two relating to paste extension in the entry and the second two to paste shear in the dieland. This simple formulation has been exploited recently by Benbow and Bridgwater (1993)~ It is certainly far from a complete picture of the behaviour of a paste but it has served as a useful staging post in the analysis of these systems. Thus, for example, it is possible to vary the solid properties and liquid properties and liquid amount per unit mass of paste and to relate changes in such variables to the extrusion parameters rationally. Similarly it is possible to take the extrusion parameters and use these for predicting the design of dieplates; application of the thinking to single screw design or to die-injection moulding to form discs is proving possible although it is still unclear under what conditions predictions will break down.
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Occupancy and velocity profiles for a tracer particle within a ploughshare mixer containing 12.5% v/v at 2Hz rotation frequency. Experiment time, 1300s. End u
50 mm/s 0.80%-
0.72-0.80% ~ 0.64-0.72% 0.56-0.64% I 0.48-0.56% 0.40-0.48% 0.32-0.40% 0.2t41.32% 0.16-0.24% 0.08-0.16% 0.0-0.08%
50 mm/s 0.30%0.27-0.30% 0.24-0.27% i 0.21-0.24% 0.18-0.21% 0.15-0.18% i 0.12-0.15% 0.09-0.12% 0.0~.0.09%
~ o.o3-0.o6% 0.0-0.03%
Axial displacement of the tracer as a function of time. 440 410 380 350 320 290 260 230 200 0
l l i l l i l l I l l l l I i l l !
25
50
75
i
100
l
,
I
125
'
'
'
'
I
I
150
,
i
i
I
175
time - s e c o n d s
Fig. 4. Flow patterns in a powder mixer obtained by positron emission tomography.
'
'
,
I 2OO
Particle technology
Plastic Methods Lubricated Flows Ram
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table that this is an area that the chemical engineer should also seek to understand. The extrudate may be passed onto a moving conveyor or may hang under gravity and be periodically sliced. It should be possible to relate the proper design of the system for handling the extrudate, which keeps further deformation within tolerable limits, to be determined by the elastic and plastic properties of the paste systems being created.
Bar
Cutting
Die E~
Generally speaking it will be desirable to cut the extrudate although breakage under its own weight is also employed. Cutting this can be effected by a fine wire or by a cutting blade at a die face. This is a subject which has received relatively little attention although it has been demonstrated that the cutting force on a wire is linearly related to the plastic yield strength of the paste. It can happen that the very process of cutting can cause pores in the system to become blocked. This is another fertile unexplored area.
Drying
I P = 2(~ ° + o~V)In D0 + 4L ('~0 + [$V) 1 D D Fig. 5. Flow of a paste from a barrel into a die land; equation to describe the behaviour by plastic flow.
It has been found for many systems including ceramics used for making catalysts that if the flow is observed through a transparent wall, the velocity in the dieland is identical to the average exit velocity of the paste. There is thus substantial evidence that the flow is occurring through a lubricated layer at the wall. To regard such systems as a power law polymer would not be appropriate. From this we see that the elements of plastic flow are a feature warranting attention; the work is still in its infancy.
Extrudate handling Once the material has been extruded, considerable practical problems can arise in the handling of this material before it is converted into a useful product. It would be easy to regard such an area as the province of the mechanical engineer or production engineer but such individuals would lack the knowledge of the physics of flow and the chemistry of the components. It thus seems rational and inevi-
Once the extrudate has been formed into its final shape, it is necessary to make it into a final solid object. It is necessary that the material be dried in a way that does not promote the development of defects in the structure. Despite the widespread commercial significance, scientific methods for dryer design as related to properties of the material created remain difficult. Certainly for the modelling to become satisfactory, it is necessary to understand the size distribution of the pores it has created as well as the influence of surface properties on the behaviour of the liquid therein. The energy demands of the drying process are considerable so means of minimising or avoiding the costs altogether are certainly pertinent topics. Despite its importance and many studies, this subject is complex and in need of greater understanding; N M R imaging should prove invaluable.
Thermal processing The final stage will be to take the dried product (which will generally be in a somewhat fragile state) and to heat it to a higher temperature in order to convert it to a stronger material. In the thermal processing stage, any organic material added to help lubricate the flow will be lost. The solid matrix that is left behind can, if the temperature is high enough, undergo solid state diffusion around the points of contact to create a three-dimensional network which is intrinsically strong. It often happens that partide-particle bonds are formed by the use of binders such as clay that are strong and cause strength to develop at lower temperatures. The techniques are well known from the ceramics literature but chemical engineering modelling has been little deployed to date.
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J. BRIDGWATER an agglomerate
ment
Development
o f Stress Fabric
es
crystallites I
yer
How can we control the structure of the agglomerate? Fig. 6. A depiction of an agglomerate structure. SOME OTHER TOPICS We now turn to examine a number of other important areas. One area of great significance concerns the means by which we choose to form particles. These are frequently complex structures. The notional particle is depicted in Fig. 6 which shows an agglomerate comprised of two particle components held together by a glue. There are pores between the particles and a skin or coating around it. The way in which a particle is designed will generally have an effect on its performance in practice; this is central to its quality and marketability. Food, pharmaceuticals and detergents are but three examples of industries in which complex particle structures need to be created reliably and there is also a need to be able to modify these structures in response to changes in public demand in a reliable manner. The particle is an extremely complex mechanical structure and we need to learn how to engineer it. If a particle agglomerate is put into compression and if it were to have a uniform internal structure, a stress distribution would be given by solution of the elastic equations arising due to Hertz. In practice (Fig. 7) it is found that the stress is carried through preferred paths and thus within the agglomerate as a whole certain particles carry much greater loads than others. It is evident that if the agglomerate is to fail, this will occur on one of the stress paths in the stress fabric where a crystallite reaches a stage where it cannot further withstand the local load. Once failure of this crystallite occurs, the stress fabric readjusts itself and the process is repeated. An exactly similar mechanism is believed to be at work in the crushing of many layers of particles such as occurs in crushing and grinding equipment. How can we predict the stress fabric and the rates of breakage associated with it either in agglomerate or in beds of particles? Computer simulations are now being performed (Potapov
Fig. 7. A depiction of a set of paths carrying the imposed stress in a polycrystalline agglomerate.
Elastic - Plastic Effect8
t
Oefect
/~art
of crystallite
t/ / / ;/~
k~~~c~J.~--./ / ~
crack
development
Fig. 8. Stress concentration at a crack tip.
and Campbell, 1994). This is an important area in which simulation and stress distributions are showing some value. If finally one looks at the breakage of material on a much smaller scale, we can see the effects as depicted in Fig. 8. The crystallite will not be entirely uniform and if a defect exists in the crystal, be it a crack or an intrusion of another solid, the particle is subject to strain and there is a stress concentration at the tip of the defect. If the applied force to the material should be exceeded at the tip, then the crack can propagate further. However, the analysis is in fact complicated by the occurrence of a plastic region around the tip. Indeed if the plastic region is so large that it exceeds
Particle technology the dimension of the particle, it becomes questionable whether the particle can be effectively broken in the crushing and grinding process. Thus an understanding of the breakdown of particles, be it deliberate as in comminution or accidental as in attrition, raises profound questions about mechanical strength of particles that hinge on the crack size and energy necessary to advance the defect into the crystallite (Mullier et al., 1987). It is another key area calling for attention.
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the availability of new experimental techniques. While some of the subject areas do not look like conventional chemical engineering, these offer exciting challenges for the improved design of traditional products or the design of the products and processes of the future. There should certainly be an impetus from industry to help grapple with the difficult issues since those companies first gaining an effective understanding of a process to make a product will have an opportunity to gain a significant advantage.
CONCLUSION
It can be seen that within this field of endeavour the state of knowledge is not yet strong. While in some cases mathematical descriptions are available, it is easy to see that these can lack generality. On the other hand we do not have the experimental tools and the theoretical insight to use mathematical formulations for yield predictions of general validity. It is becoming clear that some of the mechanisms of flow can be elucidated by use of modern instrumentation. In other instances, we are still left with little more than what has been used in the past; effective design and operation are then reliant on the skills of the practitioner to make the product effectively by the appropriate process route. On many occasions, equipment design will proceed by prior knowledge and guesswork and the ability even to carry out small scale trials to give an effective guidance to large scale performance can prove elusive. There is thus very considerable scope to improve our knowledge of behaviour from the merely descriptive, by understanding the physical and chemical mechanisms, to the mathematical that is suitable for design and which is grounded on scientific principles. From this discussion we can see that there are components of knowledge that have origins in material science or mechanical engineering that we are going to need to embrace within chemical engineering in order to master the ability to design and control processes for solid products which must include the design of the solids products. Thus an understanding of the frictional properties of the materials, the plastic flow of the lubricated solids, colloidal interactions, packing theory and microscopic modelling of packing is needed be these through the equivalent of molecular simulators or through comprehensive modelling of inter-particle interactions including full allowance for frictional and colloidal effects. There is a need to grasp the developments of defects in solid materials. This material provides the basis of coursework for chemical engineers. The focus is significantly different from allied disciplines. Indeed other engineering disciplines have shown a remarkable lack of interest in this field; it is thus natural that it should be taught by chemical engineers in a chemical engineering context. Even with modern techniques and ideas, it is clear that the progress will occur patchily being much dependent on germination of new theoretical ideas or
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Benbow, J. J. and Bridgwater, J., 1993, Paste Flow and Extrusion. Clarendon Press, Oxford. Bridgwater, J., 1995, Fifty Years Young. Products and Processes - the Future of Chemical Engineering. Cambridge University Press (in press). Bridgwater, J., Broadbent, C. J. and Parker, D. J., 1993, Study of the influence of blade speed on the performance of a powder mixer using positron emission particle tracking. Trans. Instn Chem. Engrs A 71, 675~81. Campbell, C. S., 1990, Rapid granular flows. Ann. Rev. Fluid Mech. 22, 57-92. Coulomb, C. A., 1776, Essai sur une application des rcgles de maximis /~ qudques problemcs de statique, rdatifs ~. l'architecture. Memoires de Mathbmatique de rAcademie Royale des Sciences (Paris) 7, 343-382. Cundall, P. A. and Strack, O. D. L., 1979, A discrete numerical model for granular assemblies. G~otechnique 29, 47~5. Fan L. T., Chen, Y. M. and Lai, F. S., 1990, Recent developments in solids mixing. Powder Technol. 61, 255-287. Hogue, C. and Newland, D. E., 1994, Computer simulation of moving granular particles. Powder Technol. 78, 51-66. Jackson, R., 1983, Some mathematical and physical aspects of continuum models for the motion of granular materials, in Theory of Dispersed Multiphase Flow (Edited by R. E. Meyer), pp. 291-337. Academic Press, New York. Janssen, H. A., 1895, Versuche fiber Getreidedruck in Silozellen. Zeitschrift des Vereines Deutscher lngenieure 39, 1045-1049. Jenike, A. W. and Shield, R. T., 1959, On the plastic flow of Coulomb solids beyond failure. J. Appl. Mech. 26,
599-602. Mashelkar, R. A., 1995, Seamless chemical engineering; the emerging paradigm. Chem. Engng Sci. 50, 1-22. Mehta A. (Ed.), 1994, Granular Matter. An Interdisciplinary Approach. Springer, New York. Mullier, M. A., Seville, J. P. K. and Adams, M. J., 1987, A fracture mechanics approach to the breakage of particle agglomerates. Chem. Engng Sci. 42, 667~77. Nedderman, R. M., 1992, Statics and Kinematics of Granular Materials. Cambridge University Press, Cambridge. Poux, M., Fayolle, P., Bertrand J., Bridoux, D. and Bousquet, J., 1991, Powder mixing; some practical rules applied to agitated systems. Powder Technol. 68, 213-234. Savage, S. B. and Sayed, M., 1979, Gravity flow of cohesionless granular materials in wedge-shaped hoppers, in Mechanics Applied to the Transport of Bulk Materials (Edited by S. C. Cowin), pp. 1-24. American Society of Chemical Engineers, New York. Walton, O. R., 1982, Explicit particle dynamics model for granular materials. Proceedings of the Fourth International Conference on Numerical Methods in Geomechanics, pp. 1261-1268. Edmonton, Canada. Zukoski, C. F., 1995, Particles and suspensions in chemical engineering: accomplishments and prospects. Chem. Engng Sci. 50, 4073-4079.
Chemical En#incerino Science, Vol. 50, No. 24, pp. 4081- 4089, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0009-2509/95 $9.50 + 000
0009-2509(95)00225-1
PARTICLE T E C H N O L O G Y J. BRIDGWATER Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, U.K.
(Received 1 July 1995) Abstract--Powders and granular materials are in common use in the chemical and allied industries yet traditional research and teaching in chemical engineeringdo not reflect this significance.Quite apart from the desire to advance knowledge for traditional processes and to enhance competitiveness,demands arise from the extension of chemical engineering ideas to a much broader range of products and processes. Progress in particle technology is now possible from advances in theory, from instrumentation, and from computer methods. The subjects of elasticity, plasticity and frictional flow of powders are central to an understanding of behaviour. In general, particles have a complicated internal structure and the design and engineeringof this structure is a critical feature, with packing and colloidal effects, to name but two, being critical. Stress is not uniformly transmitted, this passing along preferred paths with breakage being initiated from the tips of cracks in the paths. There is now a wide range of accessible and important problems that can and should be solved; particle technology does, after all, constitute a good half of the discipline. INTRODUCTION Particle technology is a term that has been coined to describe that part of engineering which is concerned with the production and processing of powders and granular materials. It is perhaps alarming to discover that the education of chemical engineering undergraduates contains little or no coursework on particle technology yet we know that various studies have shown that about 60% of the products of the conventional chemical industry alone are in solid form. Many of the other products require use of a solid during processing. The spread of chemical engineering skills into industries such as pharmaceuticals, minerals processing, biotechnology, food, new materials including ceramics and cement are ones in which solids can be even more dominant. The chemical engineering discipline lacks a structured approach to particle technology, a situation that has arisen from the scientific complexity underlying the requirements of this part of chemical engineering practice. The case has been made repeatedly in many countries in the recent past because of its enormous impact in the chemical industry. In what follows it is argued that, in certain areas, ways ahead are now becoming clearer due to enhanced interest from the industry in its endeavours to make production safer, more efficient and more reliable. Industry also needs to find quick and economic ways of manufacturing new products that are more sophisticated chemically or biologically than those in current production. Conceptually, there are signs of theoretical approaches becoming available and hence certain regions of particle technology can develop a rigour comparable to that of established parts of the discipline of chemical engineering. In many cases these concepts are becoming available due to significant advances in instrumentation and data acquisition. This is quite distinct from significant insights
arising from the use of computers to simulate processes in particle technology. Quite apart from developing quantitative methods, in many cases simulation would seem to be the only way available to assess which parts of the basic physics are critical and which are not. The chance to write on a subject in which the author had long been active comes unexpectedly owing to a timetabling clash that took off the young Turk lined up to deal with this area to the Particle Technology meeting in Niirnberg in March 1995 which overlapped with the meeting in Cambridge, Massachusetts. The area reviewed, however unfashionable and difficult it may seem to some, is a central one. The sheer breadth of it can be seen from the Niirnberg meeting and from the first meeting of the Particle Technology Forum of the American Institute of Chemical Engineers at Denver in the summer of 1994. The approach adopted here is one that relies on personal interests and little attempt has been made to assess the state of knowledge of the whole area. The discussion is personal and selective. A great number of themes are appearing in particle technology with many able workers within these. However, some key areas and key approaches are illustrated. Debate on the future shape of research in chemical engineering has been stimulated recently by a number of workers, not least by Mashelkar in his Danckwerts lecture of 1994 (Mashelkar, 1995). How the discipline interacts with the changing character of science and of processing and its implications for professional practice, teaching and research in chemical engineering has been analysed recently (Bridgwater, 1995). This last discourse identifies a number of themes. One is the development of an understanding in chemical engineering of the relationship of products to processing, it being argued that the discipline has become much too obsessed by the latter. Indeed life cycle
4081
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J. BRIDGWATER
analysis has become a necessary part of thinking. It was argued that the business and scientific climate is led by three simultaneous industrial revolutions, these being in: • information technology - - computing, • biology, • chemistry, materials science and physics.
Solids Storage I Solids Flow I liquid
Dry Mixing
Wet Mixing
I High Shear Mixing
There are knock-on effects relevant to the traditional bounds of chemical engineering, these being identified
gas Degassing
as"
Paste Flow
• instrumentation, • tackling old problems in classical areas, • tackling new problems in classical areas. Five of these six areas, the sole exception being biology, feature in what follows. Particle technology is illustrated principally by a case study followed by comments on certain other areas. A CASE STUDY
The process followed is manufacture of a honeycomb catalyst as might be used to make a car exhaust. It provides a means of discussing the state of knowledge in a broad section of the discipline. Figure I shows a ceramic honeycomb of approximately 100 mm maximum external dimension. It has in it an array of regular square passages of approximately 2 by 2 mm cross-section each. One way of manufacturing such monoliths is shown in Fig. 2. The various processing stages are as follows. The solid components are stored in a number of hoppers and
Extrusion
Extrudate Handling Cutting Drying i Thermal Processin8
vaporised liquid
oxidised binder
Product Fig. 2. Stages in the manufacture of a ceramic honeycomb extrudate.
then fed in appropriate amounts by a flow system into a powder mixer. There these components are mixed in the dry state, this being done in the absence of liquid in order to promote an initially rapid rate on the macroscopic scale. Liquid components are then added into the same piece of equipment and the mixing
Fig. 1. Ceramic honeycomb extrudate, maximum dimension of tO0 mm.
Particle technology process continued. In general it is found that powder mixers are not sufficiently effective in producing a uniform material and thus a high shear mixing stage is then employed. The purpose of this is to enhance mixing on the microscopic scale which probably often includes the breakdown of particle agglomerates. Before the material can be subject to further processing, it is necessary to remove any gas that may have been held in the system. The paste is then passed through a flow system to an extruder. This will generally be a single or twin screw system comprising a feed storage system at inlet, a screw to convey material into the system and further, probably different, screws to ensure compaction before passing the material through a die plate to form product of the right shape in cross-section. The material so produced is said to be in the green state. Here it has sufficient strength to retain its shape but is delicate and thus the design of the extrudate handling system is important. Thereafter the extrudate has to be cut and this must be carried out without damaging the individual membranes of the monolith. It is then ready for final processing. Here this is shown to be drying which must be conducted in a manner to avoid the introduction of unwanted imperfections into the structure. During this stage the liquid solvent is removed. The extrudate is then generally subject to thermal processing during which organic binders that may have been introduced into the system are removed and product strength is developed either by solid state diffusion causing sintering or by the presence of further solid materials in the bulk, such as clay, that weld the larger principal particles together. The product from this thermal processing stage is indeed the final product, Certainly this is not a unique processing system for this product. Indeed, the sequence of stages is quite complex and good process design in future will turn on finding physical or chemical methods of avoiding one or more of these stages. Nonetheless, the flowsheet is fairly typical for a solids processing plant and provides a useful vehicle for assessing the state of knowledge.
4083
the carrying out force balances on bodies of materials bounded by such surfaces. This allows the position of slip planes occurring to be determined. To this has to be added the concepts of active failure and passive failure. The former is a solution corresponding to horizontal extension and the latter a solution to one of horizontal compression. Janssen (1895) obtained a first solution to the stresses developed in a cylindrical bunker. Work allied to theories of plastic flow was carried out by Sokolovskii in the Soviet Union in the 1940s and he obtained methods of determining the stress distributions in moving powder, it being supposed that at every point the material followed some frictional yield criterion. These ideas were taken up by Jenike who took these and provided a means of analysis for solid storage systems. Although difficulties were encountered in a completely rigorous analysis, he nonetheless gave a procedure which, moderated in the light of experience in the selection of appropriate safety factors, has provided a means of design for many storage systems. The approach can be illustrated by the analysis of a two-dimensional planar system. The force balance on an element of powder is as given in Fig. 3. The first
Frictional Methods
(
stationary annulus
gore
k,~ Core Flow
Mass Flow J
Oc
z +
~z
c~x ~
ao~
=pg
Ox
• +
~x
Solids storage and f l o w
o x = °0(1
Various types of flow pattern may arise during solids flow out of the storage system. It can happen that powder has frictional properties such that it is impossible to extract it from an outlet at the bottom of the hopper. Secondly, it is possible that there may be a central moving channel of material surrounded by stationary material giving rise to what is termed core flow (Fig. 1). Thirdly, if the design so permits, all particles in the storage system can be in motion when the outlet is opened and the system is then said to be in mass flow although this does not imply that all particles at a given vertical level are moving at the same speed. Procedures for the analysis of such systems can be taken back to the work of Coulomb (1776) who, working on the stability of embankments as a military engineer, developed the idea of slip surfaces through
o,
+ sine
Ox
,~
-0
Oz
cos 20)
= t~ 0(1 - s i n ¢ c o s 2 0 )
x xz = - x ~x = o 0 s i n ¢ s i n 2 0
g t bt Xzx + ~ - - ~ dz ~Z
a x
t-
0o
o z +
Z dz Oz
Fig. 3. Flow patterns in a storage system; equations for a two-dimensional stress analysis; force balance on an element of powder in two dimensions.
4084
J. BRIDGWATER
equation says that the weight of an element is balanced by a change in normal stress on the horizontal plane crz and shear stress on the vertical plane r~z. The second equation says that the net horizontal force is zero with the change in normal stress of a vertical plane trx being balanced by change in the shear stress on a horizontal plane zzx. By the principle of moments z2~ = z~z. The equations can be coupled with a MohrCoulomb yield criterion which include expressions for the horizontal stress tr~ and the vertical stress trz in terms of the average normal stress ao, ~b the measure of friction of the material, and 0 the angle of rotation of the system shown from the orientation of the principal planes, those on which the shear stresses are zero. This yield criterion argues for a linear relationship on a plane between the normal stress and shear stress at some orientation at a point in a flowing powder. The procedure for determining ~b is quite complex requiring consolidation of the powder at a number of stresses and testing at lower stresses. The interpretation of the data can prove difficult and very considerable effort has been made to ensure that precise experimental procedures are followed. The issues raised by the design procedure are, however, far from settled. For instance, if an outlet is situated other than at the centre of the base, design for flow can be problematical. Codes have been developed to describe the stress distribution in the walls of bunkers but these can again be seriously in error if the offtake is offcentre. In solving these equations by the method of characteristics, it is found that the solution does not necessarily penetrate all the regions of space that it should and the appropriate transformation of coordinates or utilisation of a more fundamentally based failure condition remains uncertain. The characterisation of the frictional properties of the powder are not yet related in any scientific way to properties of individual particles such as size, size distribution, shape or inter-particle forces. Nedderman (1992) gives a thorough account of the application of continuum mechanics to powder storage problems. Significant contributions have come, inter alia, from Jackson (1983) and Savage and Sayed (1979). The problems facing this area of the discipline are still considerable but there is clearly plenty of scope for mathematical analysis on the appropriate description of flow, the nature of the failure criterion, and the relationship of this to the fundamental properties of the particles. It must be added that the conventional design procedure stemming from the ideas of Jenike includes safety factors. There are also arbitrary steps in the procedure which are necessary to give a solution as these are not sufficiently Sustained scientifically. Some recent work has focused on the study of solids how during cascade down a free surface, the so-called avalanching flows, say by using molecular modelling techniques. However, it has to be said that these problems are a quite rare practical occurrence and remain really rather intractable since there is an enormous variation in properties from packed bed below
the flowing avalanche of approximately five particle diameters of width to a region entirely free of particles above (Campbell, 1990). There is certainly significant scope for computer simulation to describe these properties and behaviour and, indeed, it seems almost certain that such probing to discover the key particle and particle interaction variables will be essential to gain further physical insight. This has been the thrust of work stemming from Cundall and Strack (1979). Walton (1982) shows the potential of full computer simulation, and Hogue and Newland (1994) that of analysis of elements of the powder structure. The subject is now becoming popular with physicists, see for example the surveys in the book by Mehta (1994). Even in this comparatively developed area there is thus ample opportunity for modelling that is analytical through to that based on computer simulation. Mixing
Mixing is common throughout chemical processes yet the study of powder mixing remains extremely rudimentary. In part this stems from difficulties of simply seeing what is happening within the equipment. These devices have until recently proved opaque; it has simply not been possible to understand what has been happening. However with the availability of instrumentation and computing, it is clear that once information becomes available the rate of progress will necessarily be rapid. Another issue causing difficulties in the understanding of mixing has been that of sampling. It has proved extremely difficult to withdraw a reliable sample and to analyse it promptly. This one sample in itself is insufficient to describe the mixture structure and normally up to 40 or so samples are necessary and the variance of the sample concentrations used, normally linked into some arbitrary function often including the variance of the unmixed material and/or the variance of the system if it were to become entirely random. Taking and analysing 40 samples is normally an impossible task. Added to this, the size of the sample necessarily has an influence on the magnitude of the results. These simple facts have been easily overlooked in the past. Excellent surveys of past work come from Fan et al. (1990), and Poux et al. (1991). The physical processes affecting mixing of cohesionless systems, ones in which the particle-particle force is much less than the particle weight, have been studied in fundamental terms. Here one mechanism has been identified as inter-particle percolation, a separation occurring in the slip zones of failing materials in which a smaller particle moves through the zone, which is some ten particles or so in width, under the action of gravity. Particle migration is another mechanism; it is the motion of a particle in the direction of the highest rate of strain. This arises since a large particle can make better use of holes appearing on one side rather than another and accordingly moves in the direction of increasing rate of strain. Thirdly, there is free surface segregation, which is segregation due to
Particle technology size or density which occurs while cascading down a free surface. It is rather in contrast to inter-particle percolation since now the particle weight can contribute significantly in relative terms to the local stress and accordingly influences findings. With respect to cohesive materials, the colloidal inter-particle forces described elsewhere by Zukoski (1995) must indeed be the dominant ones to understand. It will often happen that there is indeed a liquid present and then liquid development of the strength of the liquid bridges and the distribution of the liquid throughout the system would be significant. This is indeed part of many granulation processes as deployed in the pharmaceutical industry for instance. Liquid added in the granulation process needs to be dispersed through the whole of the system and then pressed into the pores between all the particles. The very process of granulation is in urgent need of greater understanding in a mechanistic way. Mixing does indeed provide an instance of the application of modern instrumentation technique to unravelling the behaviour. Figure 4 shows the results of studies on the pattern of flow in a powder mixer using positron emission particle tracking using a mixer described by Bridgwater et al. (1993). In this technique a particle containing a positron-containing radionuclide is followed and the velocity distribution determined. It is possible to track particles moving up to 2 m/s in equipment up to 300 mm dimension. We see for the first time something of the flow patterns that can arise. The results shown in Fig. 4 are for a trough-shaped mixer mixed by an impeller consisting of a number of blades mounted on a horizontal shaft. There is one at each end and four in the middle giving five "compartments" within the vessel. Data have been recorded on the position of a radioactive tracer, there being over 1000 determinations of its position in each second. The mixer is 225 mm long with an internal diameter of the semi-dysfunctional base of 150 mm. The bed material is rice of ca 2 mm diameter and 4-5 mm length. The tracer is a silica glass cylinder of 2 mm diameter and 2 mm length. It is immediately evident from the axial coordinates that the tracer particle undergoes movement between a number of defined zones within the system. It remains within one local region moving about some central position but then shifts to another general axial position. At present it is not known what causes this behaviour to occur but it must be remembered that the tracer is of different properties to the bulk material and further work needs to be done in which the properties are precisely matched. At the same time, the data also reveal that the circulation patterns evident when a section is taken through the mixer and the velocities computed. The particles are drawn up at the end faces by the action of the end blade and pulled down about a third of the distance along the mixer. The tracer is relatively absent from axial positions low in the bed, these corresponding to blade positions. We can see that the tracer tends to exist on the free surface or is biased towards one end. The latter is found to be CE$ 50-24-P
4085
common but its origins are unclear. For a long mixer it is uncertain as to what the behaviour will be; does the central section give a true depiction of the central part of a much longer system? Such are the questions that we are now able to address. At first sight, however, it would appear that seeking to model the system as a number of well stirred units in series might be appropriate. A means of unravelling behaviour and describing it in chemical engineering terms is now possible.
High shear mixing Although there is little quantitative information, it is known that it is essential to shear the mixture from the wet mixing stage considerably in order to promote uniformity. This is believed to be linked to promoting the agglomeration and uniform wetting of all particles formerly within agglomerates. It is much related to the quality of many products and is a fertile area for further research.
De-gassing Although the subject has received no quantitative study for pastes so far as can be ascertained, it is generally necessary to remove gas that has become entrained in earlier stages of the process prior to further processing. Failure to do so can lead to breakage of extrudate or to damaged surface due to bubble rupture. The subject is well recognised from polymer processing but the nucleation process remains hard to understand.
Paste flow and extrusion The sheared de-gassed paste can now be passed to an extrusion system. A means of describing such flows has been developed recently and is illustrated in Fig. 5. Here the paste flows out of a barrel of diameter Do into a dieland of diameter D and length L. In the dieland the mean velocity is V. By supposing that the extension that occurs to the paste as it flows from the barrel to the dieland is equivalent to that during the drawing of a plastic wire, the pressure drop could be related to the other variables as shown in which the terms Co, ~, ~o and fl are deemed to be properties of the paste, the first two relating to paste extension in the entry and the second two to paste shear in the dieland. This simple formulation has been exploited recently by Benbow and Bridgwater (1993)~ It is certainly far from a complete picture of the behaviour of a paste but it has served as a useful staging post in the analysis of these systems. Thus, for example, it is possible to vary the solid properties and liquid properties and liquid amount per unit mass of paste and to relate changes in such variables to the extrusion parameters rationally. Similarly it is possible to take the extrusion parameters and use these for predicting the design of dieplates; application of the thinking to single screw design or to die-injection moulding to form discs is proving possible although it is still unclear under what conditions predictions will break down.
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J. BRIDGWATER
Occupancy and velocity profiles for a tracer particle within a ploughshare mixer containing 12.5% v/v at 2Hz rotation frequency. Experiment time, 1300s. End u
50 mm/s 0.80%-
0.72-0.80% ~ 0.64-0.72% 0.56-0.64% I 0.48-0.56% 0.40-0.48% 0.32-0.40% 0.2t41.32% 0.16-0.24% 0.08-0.16% 0.0-0.08%
50 mm/s 0.30%0.27-0.30% 0.24-0.27% i 0.21-0.24% 0.18-0.21% 0.15-0.18% i 0.12-0.15% 0.09-0.12% 0.0~.0.09%
~ o.o3-0.o6% 0.0-0.03%
Axial displacement of the tracer as a function of time. 440 410 380 350 320 290 260 230 200 0
l l i l l i l l I l l l l I i l l !
25
50
75
i
100
l
,
I
125
'
'
'
'
I
I
150
,
i
i
I
175
time - s e c o n d s
Fig. 4. Flow patterns in a powder mixer obtained by positron emission tomography.
'
'
,
I 2OO
Particle technology
Plastic Methods Lubricated Flows Ram
4087
table that this is an area that the chemical engineer should also seek to understand. The extrudate may be passed onto a moving conveyor or may hang under gravity and be periodically sliced. It should be possible to relate the proper design of the system for handling the extrudate, which keeps further deformation within tolerable limits, to be determined by the elastic and plastic properties of the paste systems being created.
Bar
Cutting
Die E~
Generally speaking it will be desirable to cut the extrudate although breakage under its own weight is also employed. Cutting this can be effected by a fine wire or by a cutting blade at a die face. This is a subject which has received relatively little attention although it has been demonstrated that the cutting force on a wire is linearly related to the plastic yield strength of the paste. It can happen that the very process of cutting can cause pores in the system to become blocked. This is another fertile unexplored area.
Drying
I P = 2(~ ° + o~V)In D0 + 4L ('~0 + [$V) 1 D D Fig. 5. Flow of a paste from a barrel into a die land; equation to describe the behaviour by plastic flow.
It has been found for many systems including ceramics used for making catalysts that if the flow is observed through a transparent wall, the velocity in the dieland is identical to the average exit velocity of the paste. There is thus substantial evidence that the flow is occurring through a lubricated layer at the wall. To regard such systems as a power law polymer would not be appropriate. From this we see that the elements of plastic flow are a feature warranting attention; the work is still in its infancy.
Extrudate handling Once the material has been extruded, considerable practical problems can arise in the handling of this material before it is converted into a useful product. It would be easy to regard such an area as the province of the mechanical engineer or production engineer but such individuals would lack the knowledge of the physics of flow and the chemistry of the components. It thus seems rational and inevi-
Once the extrudate has been formed into its final shape, it is necessary to make it into a final solid object. It is necessary that the material be dried in a way that does not promote the development of defects in the structure. Despite the widespread commercial significance, scientific methods for dryer design as related to properties of the material created remain difficult. Certainly for the modelling to become satisfactory, it is necessary to understand the size distribution of the pores it has created as well as the influence of surface properties on the behaviour of the liquid therein. The energy demands of the drying process are considerable so means of minimising or avoiding the costs altogether are certainly pertinent topics. Despite its importance and many studies, this subject is complex and in need of greater understanding; N M R imaging should prove invaluable.
Thermal processing The final stage will be to take the dried product (which will generally be in a somewhat fragile state) and to heat it to a higher temperature in order to convert it to a stronger material. In the thermal processing stage, any organic material added to help lubricate the flow will be lost. The solid matrix that is left behind can, if the temperature is high enough, undergo solid state diffusion around the points of contact to create a three-dimensional network which is intrinsically strong. It often happens that partide-particle bonds are formed by the use of binders such as clay that are strong and cause strength to develop at lower temperatures. The techniques are well known from the ceramics literature but chemical engineering modelling has been little deployed to date.
4088
J. BRIDGWATER an agglomerate
ment
Development
o f Stress Fabric
es
crystallites I
yer
How can we control the structure of the agglomerate? Fig. 6. A depiction of an agglomerate structure. SOME OTHER TOPICS We now turn to examine a number of other important areas. One area of great significance concerns the means by which we choose to form particles. These are frequently complex structures. The notional particle is depicted in Fig. 6 which shows an agglomerate comprised of two particle components held together by a glue. There are pores between the particles and a skin or coating around it. The way in which a particle is designed will generally have an effect on its performance in practice; this is central to its quality and marketability. Food, pharmaceuticals and detergents are but three examples of industries in which complex particle structures need to be created reliably and there is also a need to be able to modify these structures in response to changes in public demand in a reliable manner. The particle is an extremely complex mechanical structure and we need to learn how to engineer it. If a particle agglomerate is put into compression and if it were to have a uniform internal structure, a stress distribution would be given by solution of the elastic equations arising due to Hertz. In practice (Fig. 7) it is found that the stress is carried through preferred paths and thus within the agglomerate as a whole certain particles carry much greater loads than others. It is evident that if the agglomerate is to fail, this will occur on one of the stress paths in the stress fabric where a crystallite reaches a stage where it cannot further withstand the local load. Once failure of this crystallite occurs, the stress fabric readjusts itself and the process is repeated. An exactly similar mechanism is believed to be at work in the crushing of many layers of particles such as occurs in crushing and grinding equipment. How can we predict the stress fabric and the rates of breakage associated with it either in agglomerate or in beds of particles? Computer simulations are now being performed (Potapov
Fig. 7. A depiction of a set of paths carrying the imposed stress in a polycrystalline agglomerate.
Elastic - Plastic Effect8
t
Oefect
/~art
of crystallite
t/ / / ;/~
k~~~c~J.~--./ / ~
crack
development
Fig. 8. Stress concentration at a crack tip.
and Campbell, 1994). This is an important area in which simulation and stress distributions are showing some value. If finally one looks at the breakage of material on a much smaller scale, we can see the effects as depicted in Fig. 8. The crystallite will not be entirely uniform and if a defect exists in the crystal, be it a crack or an intrusion of another solid, the particle is subject to strain and there is a stress concentration at the tip of the defect. If the applied force to the material should be exceeded at the tip, then the crack can propagate further. However, the analysis is in fact complicated by the occurrence of a plastic region around the tip. Indeed if the plastic region is so large that it exceeds
Particle technology the dimension of the particle, it becomes questionable whether the particle can be effectively broken in the crushing and grinding process. Thus an understanding of the breakdown of particles, be it deliberate as in comminution or accidental as in attrition, raises profound questions about mechanical strength of particles that hinge on the crack size and energy necessary to advance the defect into the crystallite (Mullier et al., 1987). It is another key area calling for attention.
4089
the availability of new experimental techniques. While some of the subject areas do not look like conventional chemical engineering, these offer exciting challenges for the improved design of traditional products or the design of the products and processes of the future. There should certainly be an impetus from industry to help grapple with the difficult issues since those companies first gaining an effective understanding of a process to make a product will have an opportunity to gain a significant advantage.
CONCLUSION
It can be seen that within this field of endeavour the state of knowledge is not yet strong. While in some cases mathematical descriptions are available, it is easy to see that these can lack generality. On the other hand we do not have the experimental tools and the theoretical insight to use mathematical formulations for yield predictions of general validity. It is becoming clear that some of the mechanisms of flow can be elucidated by use of modern instrumentation. In other instances, we are still left with little more than what has been used in the past; effective design and operation are then reliant on the skills of the practitioner to make the product effectively by the appropriate process route. On many occasions, equipment design will proceed by prior knowledge and guesswork and the ability even to carry out small scale trials to give an effective guidance to large scale performance can prove elusive. There is thus very considerable scope to improve our knowledge of behaviour from the merely descriptive, by understanding the physical and chemical mechanisms, to the mathematical that is suitable for design and which is grounded on scientific principles. From this discussion we can see that there are components of knowledge that have origins in material science or mechanical engineering that we are going to need to embrace within chemical engineering in order to master the ability to design and control processes for solid products which must include the design of the solids products. Thus an understanding of the frictional properties of the materials, the plastic flow of the lubricated solids, colloidal interactions, packing theory and microscopic modelling of packing is needed be these through the equivalent of molecular simulators or through comprehensive modelling of inter-particle interactions including full allowance for frictional and colloidal effects. There is a need to grasp the developments of defects in solid materials. This material provides the basis of coursework for chemical engineers. The focus is significantly different from allied disciplines. Indeed other engineering disciplines have shown a remarkable lack of interest in this field; it is thus natural that it should be taught by chemical engineers in a chemical engineering context. Even with modern techniques and ideas, it is clear that the progress will occur patchily being much dependent on germination of new theoretical ideas or
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