Handbook of conveying and handling of particulate solids

795 Pages·2001·39.595 MB·English

by  A Levy, H Kalman

Preface
The Conveying and Handling of Particulate Solids play major roles in many
industries, including chemical, pharmaceutical, food, mining, and coal power plants.
As an example, about 70% of DuPont's products are in the form of a powder, or
involve powders during the manufacturing process. However, newly designed plants
or production lines produce only about 40+40% of the planned production rate. This
points up clearly the lack of appropriate scientific knowledge and engineering design
skills. Following one's becoming aware of the problem, it should be attacked on three
fronts- research, education and training.
Many new products cannot be manufactured or marketed because of serious
difficulties concerning conveying and handling. That is because in most cases the
mutual effect between handling- and conveying units is neglected during the design of
a new production line. Unlike other states of materials, it is not sufficient just to know
the state of a bulk material in order to determine its properties and behaviour. The
"history" of a bulk material can dramatically affect its properties and behaviour. We
should also keep in mind the fact that an optimal manufacturing line is not necessarily
made by combining individually optimized devices. Therefore, "concurrent
engineering" should be practised in the chemical and related industries.
In order to address both of the problems presented above, an "international
conference" was initiated six years ago, that relates to most processes, units,
equipment and models involving the conveying and handling of particulate solids. In
the conference, researchers, engineers and industrialists working on bulk solids
systems have the opportunity for open dialogue to exchange ideas and discuss new
developments. The present Handbook summarizes the main developments presented
at the last Conference, that took place at the Dead-Sea, Israel in 2000. This Handbook
therefore contains research results from all round the world, and the best scientists
present the state-of-the-art on a variety of topics, through invited review papers. Some
review papers presented at the previous Conference were added. All the papers
presented in this Handbook have been reviewed.
The aim of the handbook is to present a comprehensive coverage of the technology
for conveying and handling particulate solids, in a format that will be useful to
engineers, researchers and students from various disciplines. The book follows a
pattern which we have found useful for tackling any problem found while handling or
conveying particulate solids. Each chapter covers a different topic and contains both
fundamentals and applications. Usually, each chapter, or a topic within a chapter,
starts with one of the review papers. Chapter 1 covers the characterization of the
particulate materials. Chapter 2 covers the behaviour of particulate materials during
storage, and presents recent developments in storage- and feeders design and
performance. Chapter 3 presents fundamental studies of particulate flow, while
Chapters 4 and 5 present transport solutions, and the pitfalls of pneumatic, slurry, and
capsule conveying. Chapters ,6 7 and 8 cover both the fundamentals and development
of processes for particulate solids, starting from fluidisation and drying, segregation
and mixing, and size-reduction and -enlargement. Chapter 9 presents environmental
aspects and the classification of the particulate materials after they have been handled
by one of the above-mentioned processes. Finally, Chapter 01 covers applications
and developments of measurement techniques that are the heart of the analysis of any
conveying or handling system.
We hope that users will find the handbook both useful and stimulating, and will
use the results of the work presented here for further development and investigations.
The Editors
koobdnaH of gniyevnoC dna gnildnaH of etalucitraP sdiloS
.A Levy dna .H Kalman (Editors)
(cid:14)9 2001 Elsevier Science B.V. All rights reserved.
Solids flowability measurement and interpretation in industry
T. A. Bell
E.I. du Pont de Nemours & Company, Inc., Experimental Station, P.O. Box 80304
Wilmington, DE 19880-0304 USA
The practical issue of industrial measurement and description of powder flowability are
discussed from the author's perspective. Common uses of conventional shear testing devices
are described, as are some alternative methods.
.1 INTRODUCTION
The science of soil mechanics was integrated with the related field of powder mechanics
and reduced to industrial practice by Jenike ]1[ in 1964. Since then, it has been possible for
industry to reliably measure the flowability of powders and relate the measurements, in
engineering units, to the design requirements for silo flow. However, Jenike's publication was
neither the first effort to quantify flowability nor the last. New testing methods continue to be
introduced, with varying degrees of success. In many cases these alternative measurement
methods are the result of an industrial necessity and reflect some shortcoming of the Jenike
method. In other cases, they exist because the Jenike method is not known to the people
involved or is not relevant to their problem. Business value can be derived from many
different types of measurements.
2. DESCRIBING FLOWABILITY
2.1. Applications
A surprising amount of time can be spent debating the meaning of flowability, and what
does it really mean if one powder has better flowability than others. From a practical
standpoint, the definition of acceptable flowability is in the eyes of the beholder. A person
accustomed to handling pigments would be delighted if his materials had the handling
properties of cement, while a cement user would wish for the properties of dry sand.
Industries that deal with powders in very small quantities can employ handling techniques of
brute force or human intervention that are not practical in larger scale installations. In many
cases, the chemists developing a new process or powder are completely unaware of the
difficulties in handling powders on an industrial scale, and in some cases the problems are
completely different between the laboratory and the plant. Finally, there are some materials,
such as extremely free flowing granules that may require unconventional descriptive
techniques.
2.2. Clarity and simplicity
Most producers and users of bulk materials do not have the time or interest to study solids
flow and powder mechanics. Many are completely unfamiliar with the field since it is rarely
taught as part of an engineering or science curriculum. A mistake sometimes made by
specialists in the field is the presentation of test results in a form that cannot be readily by the
users. Failure of the specialist to identify and address the key business issues in a way that is
understandable to the intended recipients will severely limit the breadth of application of this
technology.
Silo design studies should show the engineering design outcome first, and the underlying
technical data second. Very few people are interested in yield loci from shear tests, and even
the resulting flow functions often require interpretation in the context of the silo problem.
Flowability measurements for quality control and product development must often be reduced
to one or two numbers as discussed later in this paper. Even with modem statistical
techniques, it is extremely difficult to compare a series of graphs describing the properties of
various bulk material samples. The question then becomes which one or two numbers from
large data sets to use. It could be argued that the difference between a skilled technician and
skilled consultant is the ability of the latter to correctly select which data to work with for a
specific quality control or product development purpose. While there is not a simple answer
to this question, any approach must start with a consideration of the compaction pressures that
the bulk material is exposed to. For free-flowing materials in small bins, the pressures might
be nearly zero. For larger silos, cohesive materials, or those with high wall friction
(see section 3.6, below), calculation of appropriate pressures will be required.
When data is presented in the form of a few numbers, there is inevitably a risk that those
requesting the data will attempt to use it for purposes for which it was not intended. For
example, a measure of the ratholing tendency of a material in silos may not accurately reflect
the uniformity of its delivery in packaging machines. Providing the users with mountains of
data is not a solution, since the same person that will use a simple number inappropriately will
probably also extract the wrong information from a comprehensive collection of data points.
This situation can best be managed by maintaining a dialog with the users on their needs and
the application of the results.
3. GENERAL FORMS OF FLOWABILITY MEASUREMENT
3.1. Free flowing materials - timed funnels
It can be difficult or impossible to measure cohesive strength for highly free flowing
granules. For such materials, the rate of flow is often more important than whether they will
flow at all. In these cases, the time necessary for a pre-determined volume or mass to flow
through a funnel can be the most useful flowability measure. This method is widely used in
the fabrication of metal parts from metal powder ,2[ 3]. The factors influencing the flow time
measurements are numerous and include the particle size distribution, the friction between the
particles and against the wall, the particle density, and gas permeability. Many specialists in
powder mechanics object to the use of such measurements because of the unknown
interactions amongst the factors and the absence of any consideration of solids pressure due to
the self-weight of bulk material. However, in our experience this measurement can be
extremely reproducible and an excellent indicator of the flow behavior in situations that
resemble the test, i.e., rapid flow from small bins.
3.2. Angle of repose measurements
The angle of repose formed by a heap of a bulk material is the best-known method of
describing flowability. Unfortunately, it is quite possibly the worst measure to use. Angles of
repose can be significantly influenced by the test conditions, especially the height that the
material falls to form the heap. There can be pronounced differences in angles of repose for
materials that have similar real-life handling properties. Cohesive materials may form
multiple angles of repose in a single test, and reproducibility may be poor. The measured
angle cannot be directly related to any silo design parameter except the shape of the top of a
stockpile heap.
3.3. Hausner ratio of tapped to loose bulk density
The ratio of the tapped to the loose bulk density has been shown to relate in many cases to
the gain in cohesive strength that follows the compaction of a powder or granular material.
Materials with relatively little gain in bulk density (Hausner ratios below about 1.25) are
considered to be non-cohesive, while increasing values (ratios up to about 2.0) indicate
increasing levels of cohesiveness. However, we have observed that the correlation between
the ratio and more sophisticated measurements is rather poor, and it is unlikely to provide
precise differentiation between generally similar materials. In addition, the test is actually
measuring a form of compressibility, which does not always relate to cohesive strength.
A serious limitation of the Hausner ratio is the elimination of any consideration of bulk
density in the final calculation. As discussed below, two materials with similar Hausner ratios
but different densities are likely to behave much differently in practice. Finally, it has been
shown that tapped bulk density measurements are extremely sensitive to the apparatus being
used and the number of taps. Standardization of these factors is necessary to ensure
consistent and comparable results.
3.4. Properties based on shear testing
In many cases, the most important bulk handling behavior is whether or not the bulk
material will flow reliably by gravity throughout a process. This behavior relates to the
material's arching (doming) and ratholing (piping) propensity, as described by the silo outlet
necessary for reliable flow. Jenike ] 1 [ provides a method of calculating these values that is of
the general form:
(factor H or G) x fc
Arching/rathole diameter = (1)
Bulk Density
In this equation, cf is the unconfined yield strength (also known as ,)cyC a measure of cohesive
strength in response to compaction pressure. The bulk density is measured at the same
compaction pressure as is associated with the cf measurement. The appropriate value of
compaction pressure depends on the situation. As a first approximation, the factors H (for
arching) or G (for ratholing) can be considered to be constants, so the flowability can be
simply described as cohesive strength divided by bulk density. Put in other words, flowability
is the ratio of the cohesive forces holding the particles together vs. the gravity forces trying to
pull them apart.
Since cf and bulk density can both vary with compaction pressure, it is important to make
the calculation of Eq. (1) at the appropriate pressure. This relates largely to the type of flow
pattern in the silo (mass flow or funnel flow) which in turn depends on the friction of the
solids against the walls of the silo. Consequently, a wall friction measurement is usually
necessary to help fix the range of pressures. Since wall friction can also vary with pressure,
the situation can become quite complex. However, many situations can be simplified as
discussed below in section 3.6.
3.5. Flow functions
Flowability is often described on the basis of the flow function (Figure )1 1[ ] derived from
shear testing. The flow function is a graph relating a major principal stress (or1) to the
unconfined yield strength (fc) that it produces in a powder specimen. This graph basically
describes cohesive strength as a function of compaction pressure. Figure 1 shows possible
flow functions for three different materials. It is easy to comprehend and relate to one's own
experience with moist sand or snow, etc. Jenike ]1[ and others have often used the slope of
the flow function as a flowability descriptor. This can obtained by simply dividing the
unconfined yield strength at a particular point by the corresponding value of major principal
stress. This method, while convenient, has several serious drawbacks.
First, a comparison of flow function slopes for different bulk material samples based on
single points presumes that the flow function graphs are linear, and that they pass through the
origin. Neither assumption is necessarily true (see Figure .)1 Second, most shear testing
methods (except Johanson's) used in industry do not directly apply a pressure of lrc to the
sample. A different consolidation pressure is used and the final value of lrc is later calculated
as part of the interpretation of the yield locus generated in the test series. This means that the
person conducting the test cannot pre-select which value of he will test at. Two different
(3" 1
samples, tested at the same consolidation pressure, may produce different values of ,yrc and
hence relate to different points along the flow function. Exact comparison of multiple
samples will require that at least two flow function points for each sample be obtained so that
the comparative values of cf at a particular value of 1~ can be determined by interpolation.
The third drawback of comparisons based on flow functions alone is the fact that such
measurements completely disregard bulk density. Examination of Eq. 1 shows that the bulk
density has equal importance to .cf We have observed cases where the bulk density of a
common material, such as hydrated lime, can vary by up to 50% between suppliers, while the
cohesive strength (fc) varied by 30%. Similarly, in one of our businesses, two products had
D~
~ >. ~..,I ~" ~ oo~176 o
~" ~176176
"0
D~
,.*~176 o o j
O
r
.f
,1~ Stress Principal Major
Fig. .1 Typical flow functions.
identical flow functions but their bulk densities varied by 30%. The flowability in the plant
varied accordingly, and the business people (who had only compared the flow functions) did
not understand why.
3.6. Wall friction measurements
Wall friction measurements are most commonly made with a Jenike shear cell. The force
necessary to push (shear) a sample of bulk material, trapped in a ring, across a wall sample
coupon is measured as a function of the force applied to the top of the sample (Figure 2). The
resulting data is in the form of a graph of shear force versus normal force, known as a wall
yield locus (WYL). For any given point on the graph, the ratio of shear force to normal force
is the coefficient of friction. The arctangent of the same ratio is the wall friction angle. For
some materials the WYL is a straight line that passes through the origin. This is the simplest
case, and wall friction can be described by a single number. In other cases the WYL has a
curvature that causes the wall friction angle to vary inversely with normal force.
The wall friction angle is the primary factor used in determining if a particular silo will
empty in mass flow or funnel flow. Larger values of the wall friction angle correspond to
steeper angles required for the converging hopper at the base of the silo in order to achieve
mass flow. The procedure is described by Jenike [1]. If the wall friction angle exceeds a
certain value, mass flow will not occur. The WYL must be closely examined for the design of
new silos or the detailed evaluation of the behavior of a bulk material in existing silos. If the
wall friction angle exceeds the mass flow limit for a silo installation, the flow pattern in the
silo will be funnel flow and consequently ratholing must be considered. This means that flow
function data in the appropriate silo pressure range is necessary to determine if ratholing can
occur. This can be a different set of testing conditions than that required if only a no-arching
determination is required for a mass flow silo. For quality control or product development
purposes where silo design is not an issue, it is often sufficient to describe the WYL by fitting
a straight line, passing through the origin, to the data set. While this method can produce
errors (especially at low values of normal force) it is adequate as a descriptive tool and greatly
simplifies comparisons.
Fig. 2. Wall friction measurement with Jenike Shear Cell (illustration adapted from Ref. 3).
4. COMMON SHEAR TESTERS
4.1. The Jenike Shear Tester
The Jenike shear tester (Figure 3) was developed as part of the research activities described
by Jenike 1[ ]. It is derived from the shear testers used in soil mechanics, which are typically
square in cross section instead of the circular design used in the Jenike tester. Soil mechanics
tests are usually conducted at compaction (normal) stress levels that greatly exceed those of
imerest in powder mechanics for silo design and gravity flow. At these high normal stress
levels, it is relatively easy to obtain steady state flow in which the bulk density and shear
stress remain constant during shear. This steady state condition is a vital prerequisite for valid
test results. It can also be reached at lower stress levels, but a relatively long shear stroke is
required. With translational shear testers (i.e., those that slide one ring or square across
another) the cross sectional area of the shear zone varies unavoidably during the shear stroke.
The validity of the test becomes questionable if the stroke is too long.
For this reason Jenike devised the round test cell, which permits preparation of the sample
by twisting. A pre-consolidation normal load is applied to the cover, and the cover is twisted
back and forth a number of times. This preparation makes the stress distribution throughout
the cell more uniform and reduces the shear stroke necessary to obtain steady state flow.
After the pre-shear process is completed, a specified pre-shear normal load is applied to the
cover and shear movement is started. Once steady state flow (constant shear force) is
achieved, the initial normal load is removed and a smaller normal load (known as the shear
normal load) is applied. Shear travel resumes and the peak shear force corresponding to the
shear normal load is noted.
The preparation process requires skill and is not always successful on the first attempt.
Different values of the pre-consolidation load or the number of twists may be required to
reach steady state flow. Even with proper preparation, it is only possible to obtain one shear
point from a prepared shear cell. Since several shear points are typically employed to
construct a yield locus, and several yield loci are necessary to construct a flow function, the
cell preparation procedure must be repeated numerous times. It is not uncommon to repeat
the cell preparation process 9 to 25 times per flow function. We typically allow 6 man-hours
for 9 shear tests, so the time investment in this method can be significant.
Fig. 3. Jenike Shear Tester (illustration adapted from Ref. 3).
While the cell preparation procedure is demanding for even conventional powders and
granular materials, it can be nearly impossible to obtain steady state flow with elastic
materials and particles with large aspect ratios, such as flakes and fibers. In these cases, the
allowable stroke of the shear cell may be exceeded before a steady state condition is achieved.
Despite these difficulties, the Jenike cell has remained the best-known and definitive shear
testing method for bulk solids. There are several likely reasons for this. First, the method
was developed first! Second, the method has been validated in industrial use and comparison
to more sophisticated testers. Third, the apparatus is relatively simple and not patented, ideal
for university research and users with limited budgets.
4.2. Peschl Rotational Split Level Tester
Most of the difficulties with the Jenike tester are the result of its limited shear stroke.
Testers that rotate to shear the sample (such as the Peschl) versus the Jenike cell's
translational motion can have a distinct advantage. Shear travel can essentially be unlimited,
as long as there is no degradation of the particles in the shear zone. This unlimited stroke
makes the elaborate Jenike cell preparation process unnecessary, and also makes it possible to
obtain multiple data points from a single specimen. Thus an entire yield locus can be
constructed from a single filling of the test cell. While it is also possible to make repetitive
measurements from a Jenike cell sample, the cell has to be prepared each time. A second
advantage of the rotational cell is that the placement location for the normal force does not
move (translate) during the test. Loads are placed on the centerline of rotation. This makes it
much easier to automatically place and remove loads from the test cell, and can lead to
complete automation of the tester.
Fig. 4. Peschl Rotational Split Level Tester.
01
The Peschl tester (Figure 4) rotates the bottom half of a cylindrical specimen against the
top half, which is stationary. The torque necessary to prevent the rotation of the top half is
measured, and is converted to the shear stress acting across the shear zone. The interior of the
top and bottom of the cell are roughened to prevent the powder from shearing along the top or
bottom ends rather than at the shear plane. It should be noted that the amount of shear travel
varies across the radius of the cell. A particle precisely in the center of the cell sees no shear
travel distance - only rotation about the center line of the tester. Particles at the outside edge
of the cell have the greatest amount of shear travel, with decreasing travel distances as the
radius is reduced. Some researchers have voiced concern about this aspect of the tester, since
the meaning of the individual shear points is somewhat confused. Shear stress values become
averages produced by shearing different regions of the cell different distances. Although
detailed studies have not been conducted, there is some evidence ]4[ that the Peschl tester
produces slightly lower values of unconfined yield strength than the Jenike tester at
comparable values of major principal stress.
The Peschl tester was the first (and for a long time, the only) automated shear cell
available. The volume of the standard cell is relatively small, making it convenient for
expensive bulk materials such as pharmaceuticals and agricultural chemicals. It is widely
used for quality control and product development. Testing times are about 3/1 of that
required with the Jenike cell.
4.3. Sehulze Ring Shear Tester
The issue of non-uniform shear travel in rotational testers can be minimized if the test cell
has an annular ring shape instead of a cylindrical one such as in the Peschl tester. While the
inner radius of the ring still has a shorter shear travel than the outer one, the difference is
relatively small, particularly if the difference between the two radii is small compared to their
average. This concept was developed a number of years ago by Carr and Walker [5]. In our
experience, the early models of the device, while very robust from a mechanical standpoint,
were too massive for delicate measurements. It also was difficult to clean the cell,
particularly the lower ring. This form of ring shear tester never achieved widespread use
when compared to the Jenike cell.
Fig. 5. Schulze Ring Shear Tester.