Traditional methods of determining powder flow properties |
Many methods of measuring powder flowability exist, yet handbooks of powder flow properties are conspicuously absent alongside those on solids and liquids. The reason is the complexity of powders and the ease with which their bulk properties may change.
Traditional techniques like angle of repose, timed flow through an aperture and even modern instrumentation provide answers for some materials, but generally suffer high variability. The reasons for this are numerous, but the lack of a normalised packing condition is the most important. This issue is discussed below in some detail.
The well-known shear cell technique invented by Jenike is able to provide shear strength data that has long been used for the design of bins and hoppers so that the contained material can be reliably discharged by gravity flow. Indeed, the focus of most powder characterisation instruments has been to assist in the design of powder handling machinery rather than to characterise and classify material flow properties.
How the FT4 methodology evolved |
The first step was to recognise that powder rheology concerns the deformation and flow of a powder bulk and the understanding of the forces needed to cause these displacements. The second was deciding that the flow needed to be dynamic and representative to some extent of real flow conditions. As the basis of a methodology this amounted to the following:
- Establishing a particular flow pattern and flow rate that may be easily reproduced
- Measuring all the forces associated with establishing this flow pattern
- Calculating the work done during the testing cycle and using this energy value as the basis of the flow property assessment
The technique is empirical because it is not yet possible to analyse the complex nature of fluid -particle interactions and predict their rheology.
What are the important user needs? |
The two main user requirements were determined to be:
- The measurement of the flow properties of powders in relation to each of the key factors that affects flowability
- To produce reliable, reproducible data that would be the basis of a reference database for all kinds of powders
The most important functional requirements are:
- Creating a reproducible, dynamic flow pattern representative of moving powders
- Sensitivity - to detect small differences between similar powders
- Reproducibility - of test data and between different instruments
- Operator independence - by providing automated test and analysis procedures
- Simplicity and quickness of operation
- Sample size large (160ml) and small (15ml)
For more information on the operating principles - see FT4 Powder Rheometer.
What is powder? |
Strictly speaking, a powder is a collection of individual particles that make up a given mass. In reality however, this collection of powder particles is usually in combination with air and sometimes also in combination with a liquid such as water.
It is this solids and fluids mixture that we handle and process and therefore it is desirable that we can measure the flow properties of this mixture. Almost always, the amount of entrained air is of great importance and sometimes, moisture adsorption can also be significant.
See The Nature of Powders for more information.
What affects the flow properties of powders? |
The variability of powders arises from the many ways in which their flow properties may be changed. There are a great number of factors and for convenience, they may be grouped under the following headings: The three most important types of influence are:
Physical properties of the powder particles such as their size, size range, shape, hardness, elasticity, porosity, mass, interactions between particles, texture, angularity and so on.
Environmental factors that affect the powder bulk properties such as the air or moisture content, external pressure, vibration, etc. These factors modify the physical distribution and arrangement of the particles in the powder mass.
Individual particle changes caused by factors such as attrition, agglomeration, electrostatic charge and chemical changes.
The bulk properties of a powder will depend on the combined effects of all the above. Whether processing a powder or merely evaluating its flow properties, we need ways of reducing the potential complexity.
See The Nature of Powders for more information.
What powder state do we wish to evaluate? |
Given all these factors, any measurement of flow properties is only of value if we know what was tested. Experience shows that 'identical' samples can produce flow energy values that differ by a factor of 2 or more. A single tap of the sample container can produce such a change. A small amount of aeration can reduce the flow energy value by a factor of 5 or more.
So how do we start and how do we know what it was that was tested?
The starting point when measuring powder flow properties is to evaluate a 'normalised' sample of powder. This means creating a standardised packing condition by controlling and normalising the external variables including factors such as aeration, temperature, vibration and humidity. Then the focus will be predominantly on the physical properties of the powder and this will ensure that reproducibility of measurement is maximised.
This normalised state is achieved by what we will call 'conditioning'. Because conditioning ensures reproducibility it is possible to compare measurements taken at different times, using different instruments at different sites.
Conditioning - how to prepare a powder for evaluation? |
It is essential to produce the standardised packing condition mentioned above as a preliminary to each test cycle. This 'conditioning' process involves gentle displacement of the whole powder sample in order to loosen and slightly aerate the powder. The aim is to disturb and gently drop each particle in order to construct a homogenously packed powder bed. This process removes any precompaction or excess of air.
|
Photo of the end of a conditioning traverse showing the blade moving upwards out of the powder.
(482 KB)
|
 |
A conditioning cycle comprises a traverse of the blade downward and then a traverse upward. The downward traverse would typically use a 5 degree positive helix in order that the blade action is more slicing than compacting. The upward traverse would typically use a 5 degree negative helix that gently lifts the powder and drops it over the blade, each particle coming to rest behind it as in the above photo.
A conditioning cycle is usually completed before each testing cycle in order to remove any residual compaction from any previous test cycle. The exception to this is where a consolidated sample is being evaluated, in which case conditioning is not used.
The basic measurement - basic flowability energy (BFE) |
Ideally we need a single test to measure the rheology of a material reliably and quickly. This would involve a conditioning cycle as described above, followed by the test cycle during which all the forces acting on the blade are measured. Standard settings of blade speed and helix angle would be used during the downward part of the testing cycle - the downward traverse. These would normally be 100mm/s blade tip speed and a 10 degree, negative helix, producing a fairly aggressive compaction at a high flow rate.
If we condition the sample first, this test will provide a reproducible measurement of the work done.
The result is called the basic flowability energy (BFE), expressed in mJ.
The basic flowability energy is therefore the energy required to displace a constant volume conditioned powder at a given flow pattern and flow rate. The test takes typically 1 minute to complete and is independent of the operator.
The BFE assessment provides a quick and reproducible measurement of whether a known material has the expected rheology. It does not provide a comprehensive view of flow properties, because the effect of flow rate, aeration and consolidation is not assessed. For this we need to complete the standard measurements.
Clip here for video clip of standard BFE test. (313 KB)
BFE values may vary greatly for a given type of material depending on the formulation. For example the BFE for Titanium Dioxide might vary from 600mJ to 3000mJ.
A high value of basic flowability energy is not necessarily a bad sign. Such a powder could behave very consistently if the processing conditions suited the powder characteristics. Conversely a low basic flowability energy powder could be difficult to process if it were easily compacted or was sensitive to flow rate.
Sample preparation |
Two methods of measuring out the powder sample are used. These are:
- Conditioned volume
- Constant mass
Conditioned volume procedures:
When evaluating a material for the first time, the sample is sized as a conditioned volume. The procedure is to fill the testing vessel to a volume a little greater than that required for the test. The mass of this sample should be recorded. A conditioning cycle should be run on the sample and the conditioned volume should be recorded. Any adjustment of volume can now be achieved by calculating the required addition or reduction in mass. (For some less stable materials, more than one conditioning cycles may be necessary.)
The volume used is usually 160ml for the larger (200ml) test vessel and 20 or 25ml for the 25ml vessel.
Constant mass method:
Once the mass of a conditioned volume is known, it is usually more convenient to simply measure out the appropriate mass of powder.
Whichever method is used, the end result should be that samples are in fact constant conditioned volumes, thus enabling the results to be directly compared. This is especially important if materials having different bulk densities are to be compared.
Of course, any sample needs to be representative of the powder bulk as far as possible. The reality is that every sample is slightly different to every other. This is due to the variability of powders already discussed at length above. Large differences between 'identical' samples would usually point to a high probability of processing difficulties.
The standard measurements |
The BFE test is the basis of many other measurements that are made to investigate how flow properties are affected by any one of the many variables reviewed above.
The approach is to evaluate how the BFE value is changed for each variable whilst keeping other factors unchanged. Whilst this may appear difficult, the conditioning technique is effective for many of the key variables, allowing reproducibility of data within 2 or 3%, and often within 1%. A powder that is highly prone to segregation is impossible to evaluate without some segregation occurring. However the FT4 principle that moves only a small part of the total powder bulk at any instant, does minimise segregation.
The table below lists the most commonly used flowability parameters along with their definitions. (The values relate to conditioned volume samples of 160ml.)
| Powder State | Defining Parameter | Definition | Range of values - all types of powders. |
| Conditioned | Stability Index | The factor by which the measured energy changes during repeated testing. |
0.2 to 3 Typically 0.8 to 1.3 |
| Conditioned | Basic Flowability Energy (BFE) | The energy needed to displace a conditioned powder at a given flow pattern and flow rate. |
5mJ to 10,000mJ Typically 500mJ |
| Variable flow rates when conditioned. | Flow Rate Index | The factor by which the energy requirement is changed when the flow rate is reduced by a factor of 10. |
0.7 to 10 Typically 3 |
| Compacted or consolidated | Compaction Index | The factor by which the Basic Flowability Energy is increased when the powder is consolidated (by direct pressure, tapping or storage, to specific conditions.) |
1 to 50 Typically 3 |
| Aerated | Aeration Ratio | The factor by which the Basic Flowability Energy is reduced by aeration. (To given air flow rate.) |
1 to 1000 Typically 50 |
The above parameters are derived automatically using the Data Analysis software provided.
These standard tests are reviewed in detail in the following sections.
Stability index (SI) - how BFE varies with repeated testing |
Most powders will stabilise after the initial conditioning cycle. In these cases repeated testing, where each test is preceded by a conditioning cycle, will produce little if any change of measured energy. In other cases, the material continues to change and then stabilises.
The data in the figure below for three different powders shows how energy varies for seven repeat test cycles, each at a 10 degree negative helix and 100mm/s blade tip speed. Each test was preceded by a conditioning cycle.
Referring to the figure, the ratio of test7/test1 is called the stability index and would equal one for stable powders.
Note that the BFE is defined as the stabilised energy value - test7 in the figure.
When characterising powders for the first time, it is recommended that an evaluation of stability be made. Should the powder be stable, it may not be necessary to run the seven-test cycle programme when further testing is required.
A high value of stability index is always significant though the reason may not be obvious. Possible explanations are the slow release of entrained air, agglomeration, moisture adsorption or even segregation or attrition. Usually it is necessary to investigate by further testing to understand the reason.
Similarly, a SI value of less than one indicates instability as the energy requirement reduces with repeated testing. Most likely this is due to loosening of the powder bulk as it becomes more aerated, or by attrition that may cause rounding of particle shape as corners and edges are removed.
Another possible effect results from coating of the wall of the testing vessel. Sometimes this effect will vary as a function of time and so must be allowed to stabilise before the proper testing begins. An example of this is to be seen in an application study Lactose/Magnesium Stearate.
Flow rate index - how BFE varies with the rate of powder flow |
The flow rate index (FRI) is a measure of the extent to which the BFE is changed when the flow rate of the standard test is reduced by a factor of 10.
The graph shows how the BFE varies when the blade tip speed is reduced from 100mm/s to 10mm/s, for three different powder types. Each test involves four individual test cycles, all at a 10 degree negative helix and each following a standard conditioning cycle.
So - why is flow rate important?
For some free flowing materials, such as detergent powders and some polymers (see graph), flow rate index may be close to unity or even less than unity indicating a more Newtonian behaviour, i.e. more energy needed for higher flow rates. Generally, however, powders have a flow rate index greater than 1 and are therefore inherently unstable in the sense that they require less energy once they begin to move faster. This is potentially hazardous behaviour, very apparent in avalanching and indeed in flow from hoppers when the core collapses.
The high flow rate index of most cohesive powders is not fully understood. However the following factors are probably relevant:
At higher flow rates, more air is entrained. This air acts as a lubricant, reduces interparticle contact and also allows the material to be more compressible - all of which reduces the energy required to produce flow
At low flow rates more air is excluded as the material becomes, at least locally, more highly consolidated. It is likely that this consolidated zone extends much further ahead of the blade when the blade is moving slowly so that the total energy consumed is correspondingly higher
-
The interlocking of particles is more likely at low flow rates, where adjacent particles nestle together. This effect is less likely at higher flow rates, where relative particle velocities are greater
The air filled zone behind the moving blade - the "trailing void", is smaller at the lower blade speed, possibly due to the more extended pressure zone and because there is greater time for the material to flow into and fill the void
In summary, a high flow rate index signals a 'potentially difficult to process' material since its flow energies and therefore its flow behaviour, can change significantly during processing.
The standard test - the measurement of SI, BFE and FRI |
In practise a single test programme may be used comprising of usually 11 test cycles.
The figure below shows a typical result of a standard test from which the stability index, the basic flowability index and the flow rate index are derived as follows:
Stability Index = test7 energy / test1 energy
Basic flowability energy = test7 energy
Flow rate index = test11 energy / test8 energy
In practise the type of test programme will vary depending on how well the material is known, its variability and the importance of close control of flow properties.
Standard test programs |
A library of standard test programs is available to the user. Examples include the following:
a) 200ml_1(C+T)_-10@100.pgm
b) 200ml_2C+VFR_-10.pgm
c) 200ml_7STAB+VFR_-10.pgm (used for the data in above graph)
The nomenclature used in these program names means as follows:
- 200ml refers to the use of the standard 200ml testing vessel
- C means a conditioning cycle
- T means a testing cycle
- VFR means a 'variable flow rate' test - used to determine the FRI
- -10 is a 10 degree negative helical path angle, commonly used for testing
- @100 means at a blade tip speed of 100mm/s
- 7STAB means seven stability tests, each comprising a C+T
Some materials may be evaluated using a simple programme such as a) above from which the BFE is determined. This 1 minute test is often sufficient for QC testing of batches of materials on a routine basis.
Test programme b) would be the standard when the powder was known to be stable. This would derive BFE and FRI.
Other materials, perhaps less stable, might use programme c) to derive SI, BFE and FRI.
Information on the available library and the options for creating further programmes are included with the system software.
Further programme types are available for the aeration, compaction and other studies that are described below.
Aeration ratio - how BFE varies with aeration of the powder |
The bulk properties of all powders are affected by air to some extent since the space between the particles is filled with air. The amount of air present determines how the solids interact with each other and this impacts directly upon the flow properties.
Some powders are readily aeratable - they require only a small amount of air to transform the powder bulk into a fluidised bed in which the powder behaves as a fluid and requires only a small amount of energy to produce flow. Cohesive powders that are generally not readily aeratable may nevertheless have their flow properties enhanced by entrained air.
Two key questions that arise are:
- How does the addition of air affect the flow properties?
- How readily does the powder release entrained air?
Both are important in relation to how a material can be handled, stored and processed.
The answer to the first question is determined by measuring the Aeration Ratio (AR)
The aeration ratio is the factor by which the BFE is reduced when a powder sample is fully aerated.
The evaluation of more than 100 different powders has shown that the presence or absence of air in a powder is probably the most important single factor determining its flow performance. Aeration ratios vary from 1.5 to greater than1000. The higher values always relate to powders that fluidise.
Investigating aeration requires an aeration/fluidisation accessory. This comprises a controlled, dry air supply fed through a porous stainless steel disc at the base of the test vessel. The air throughput must be adjustable to allow various levels of air velocity to be established whilst the test is in progress.
The graph shows three very different powder types and how they are affected by being aerated. The catalyst shows the expected dramatic reduction of energy at only 0.25cm/s of air throughput because it readily fluidises. The titanium dioxide is very different, but even this cohesive material shows a significant reduction of energy when aerated. The gypsum is not greatly affected.
A high aeration index usually indicates good flowability once the material has began to flow. On the other hand a powder having an AR of 100 and a BFE of 500mJ will require only 5mJ when it is fully aerated and this could cause processing difficulties. For example, it might flow readily through a stationary screw feeder or flood when discharged from a hopper.
The very different aerated behaviour of different powder types is illustrated in the two photos.
| This photo shows a material (plain flour) that does not aerate and so forms a central channel through which air escapes upwards through the powder. |
 |
|
This photo shows a readily aerated ceramic powder that is able to disperse the air throughout the powder bulk, resulting in fluidisation. Here the air escapes at the surface as a large number of small bubbles.
|
 |
Aeration Testing provides:
- A measurement of how aeratable a material may be
- Indication of the reduction of flow energy that may be achieved by aerating the powder
- Clear indication of how the air passes through a powder mass in relation to channelling or bubbling, (see photos), or in some cases, bulk movement of the powder mass
- Permeability information derived from measurements of the pressure at the base of the powder column
- Indication of the amount of bed disturbance needed (if any), for air to permeate the powder mass.
- Change of volume that occurs on aeration and hence the reduction of bulk density
In summary, air entrainment can radically change flow properties and it is important to understand how a given powder may be affected by aeration and de-aeration processes.
De-aeration characteristics - how readily is air released? |
The ability of a powder to release entrained air is important in relation to its flow performance. At various stages during processing a material may become aerated - for example during discharge from a hopper, or pneumatic transfer. Air retained will greatly affect flowability as will any compaction induced at this time. Powders that are fluidisable usually exhibit a large volumetric expansion. The rate at which entrained air is released, however, once the air or gas supply is removed may vary greatly.
De-aeration testing is intended to quantify this characteristic of a powder.
The graph shows a de-aeration characteristic for two materials. Each of the five tests comprised:
- fully aerate whilst conditioning
- turn off the air
- run de-aeration cycles (0 to 4)
- test and measure BFE
Further de-aeration studies are described in Applications - Aeration and de-aeration.
Compaction index - how BFE is changed by consolidation |
Consolidation of a material usually has a significant effect on flowability. The consolidation process can involve combinations of the following:
- Closer contact between neighbouring particles and the exclusion of some air
- Realignment and interlocking of particles with each other
- Contact stresses and shearing forces resulting in fracture, attrition and the generation of smaller particles and fines
- Elastic and plastic deformations of particles that may permanently change particle shape
- Chemical bonding between contacting particles - especially during long-term storage
These potentially complex combinations can be investigated by consolidating the powder and then testing it in the usual way - except that, in this case the conditioning cycle is omitted. Three methods of consolidation are used:
- Direct pressure consolidation - usually done incrementally to 0.1bar
- Repeated tapping - usually to 100 taps
- Time dependent consolidation resulting from long-term storage
The objective is to determine the increase in energy needed to complete the standard test as a result of the sample being consolidated. The result is expressed as a compaction index:
The compaction index is the factor by which the BFE is increased when a powder sample is consolidated.
Compaction indices vary from 1 to in excess of 10 depending on powder type and condition.
Mechanical tapping often produces higher compaction indices than direct pressurisation to 0.1 bar. However, this is not always the case and for low bulk density, fine powders particularly, such as micronised talcum, mechanical tapping has very little effect. (See graph below)
It is clear that micronised talcum is not affected by tapping but is readily consolidated by direct pressure. In this case the CI = 4.91, the factor by which the BFE value is increased. Note the 40% volume reduction confirming that a great amount of air has been removed by pressurisation.
For all compaction tests, a conditioned volume sample is prepared that is then compacted using one of the three methods described above. The volume after compaction is automatically measured as part of the test programme so that the reduction of volume may be calculated.
A high compaction index indicates a high probability of processing difficulties, especially of course if the powder is subjected to direct pressure, vibration or storage. A powder with a low compaction index is likely to process more consistently, especially if the material experiences a wide range of processing conditions.
Other factors affecting flowability and processability |
- Attrition - determining how vulnerable a material is to attrition
- Moisture adsorption - susceptibility to moisture and how this affects flowability
- Segregation - setting up repeatable disturbance cycles to promote segregation and relating this to flowability
These are described under Applications and in some application studies in Literature.
What the flowability data means |
The various flowability indices above provide information about the powders. Using this information is not always straightforward because the relevance of each parameter depends upon what happens to the powder during handling and processing. For example, if the material is aerated then almost certainly the aeration ratio will be an important indicator of flow performance during processing.
The graph below illustrates how the energy needed to process two powders varies in relation to the degree of aeration or consolidation. It is clear from this that although the particles comprising these powders probably remain unchanged, the blend of solids and fluids being processed may be very different as a result of the conditions imposed upon the powder during processing.
In the case of the food flavouring, the range of energy requirement varies from 3674mJ when consolidated to 29mJ when aerated, a 127:1 reduction. This factor is 7.4 for the plain flour sample.
Some generalisations on using the data are:
- 'Ideal' powders have flowability indices of unity. Some polymers and detergent powders are very stable and are insensitive to compaction or to flow rate, having indices close to one.
- The Stability index (SI) indicates if the material is changing its flow properties as it is worked. This can indicate de-aeration, segregation or agglomeration and is usually an indicator of the need to do more investigation to determine the causes. Stability index values close to unity however, indicate that the material is stable and that the subsequent measured data is likely to be very repeatable.
- In some cases the basic flowability energy (BFE) is the important parameter, suitable for comparing one batch of material with another. This simple 1-minute test could be used for checking the conformance of raw material or intermediates.
- High compaction index (CI) materials are able to change their packing state suddenly, switching from a free flowing powder to a consolidated mass causing a blockage. Whether or not this occurs will depend upon the processing conditions and other flowability indices, especially the aeration ratio.
- Materials having a high compaction index resulting from direct pressure, are likely to give problems if highly compacted in a bin or hopper due to the high amount of energy needed to get them to flow.
- High compaction index powders resulting from vibration are likely to consolidate during storage and transportation. The flowability of some powders is improved by vibration, but others will consolidate and become resistant to flow. The compaction index is a good indicator of which of these is likely to occur.
- Most powders will aerate and some will fluidise. Generally, a high aeration ratio is a sign of good flow performance provided the materials are handled correctly - this means avoiding excessive compaction at one extreme and being able to cope with very free flowing, possibly fluidised powders at the other.
- The overall flow performance of a powder will be dependent largely upon these factors and the conditions prevailing when the material is processed. It is arguable whether or not a high aeration factor is an advantage - this will be very dependent upon the processing conditions. A high flow rate index, or a high compaction index makes inconsistent processing likely. An adverse combination of both could make for real difficulties. This could be compounded further if a material with a high aeration factor was aerated and maybe even fluidised.
- Characterising powders in these terms is the first stage in achieving predictable processing performance. The next stage is to make use of the data….
Making use of the flow properties data |
Compiling a Powders Database
All powders should be evaluated to determine the most important flowability parameters, mostly as described above. The information may be collated in a database of materials to allow:
- Comparison of all materials
- Correlation of material flow performance with processing performance
- Reference for new material development
- Reference for QC information
Optimising productivity and quality
Flow properties information is directly useful for optimising productivity and maintaining the required levels of product quality. There are at least two approaches that may be pursued:
Correlation of flow property data with processing experience. This requires each stage of manufacturing to be ranked for difficulty. Some materials are stable during storage, whilst others consolidate. Some materials will feed consistently from a hopper, whilst others will bridge from time to time causing stoppages. By ranking all materials in relation of each of these aspects and correlating this with the flow data, it is possible to determine the optimum flow properties for each piece of plant. It then becomes possible to predict how a given material will behave at each stage of manufacture or storage.
A more fundamental approach is to examine the processing and handling machinery and to understand the range and types of conditions that are imposed upon any materials processed.
Whatever the powder type, the storage, handling and processing machinery will subject materials to levels of compaction, aeration, vibration, attrition, segregation and other effects that are characteristic of the machinery, even though different powder types will be affected differently. Understanding this will make it easier to match powder and machinery characteristics making it possible to achieve higher operating efficiency and improved product quality.