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Thread: Bulk Powder Density

  1. #1
    Author Guest

    Bulk Powder Density

    This is the discussion on the article
    Bulk Powder Density
    by Lyn Bates

  2. #2
    Lawrence K. Nordell

    Lawrence K. Nordell

    President and CEO

    Conveyor Dynamics Inc.

    Conveyor Dynamics Inc.

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    Bulk Density Measurements


    I read your articles with keen interest. They are always quite insightful. It is apparent that you know your subject well. As a respected docent, I beg your theosophy and references on the matter of measurements influenced by:

    1. Particle size distribition - filling of voidages
    2. Particle shape - ideal consolidation of round verses slabby nested particle groups and others of interest
    3. Plasticity and elasticity rheology and time dependency
    4. Moisture by degree - light, critical (massive flow property change), and saturated
    5. Multiple properties of particle groups as separtely measured and then integrated as a mass system.

    Wishing You A Merry Christmas and Cheer Filled Holdiays,

    Lawrence Nordell
    Conveyor Dynamics, Inc.

  3. Powder and Bulk Properties

    Hi Lawrence,

    Thanks for your interest, it is encouraging to learn that my contributions as useful.

    Relating to your queries, the degree of voidage formed with different particle size distributions depends to some extent upon the behaviour of the finer fractions. There is an excellent publication by W.A.Gray in the powder technology series edited by J.C.Williams, entitled ‘The packing of solid particles’, published by Chapman and Hall. It is when inter-particle forces become strong, say at less than 50 micron sized particles, that geometric theory breaks down and the state of the powder and its condition of aeration becomes significant.

    The behaviour of rounded and nodular grains are more easy to predict than flakes and highly irregular shapes because the condition of isotropy, or alignment structure, then has a strong influence on the nature of the bulk material by how the particles nest together. A bed of plate-shaped particles can suffer a huge change if vibration disturbs the structure to give closer alignment. The phenomena of ‘clay slips’ and ‘liquifraction’ has caused many landslides and unstable cargoes where the moisture content is able to fill the voids to excess if the particles align to a closer packing when vibrated. This has lead to the specification of a ‘safe transportable moisture content’ for ship cargo when handling damp minerals.

    Plasticity and elasticity have a massive influence on flow behaviour. I have constructed a chart of a universal rheological model, with notes on the characteristics of the various elements, that I can send by post if you forward your address. This highlights the complexity of particulate solids behaviour under these various influences. Elastic granules, such as cork and rubber, form small flats on points of contact that are difficult to disturb. Plastic materials have a similar tendency under time consolidation.

    Moisture has a variable effect. A small amount gives increasingly more difficult flow until the stage that some local regions become saturated, above which value the lubrication effect starts to outweigh surface tension and flow conditions ease. With mixed coal this poorest flow condition occurs in the region of 10 to 12 % moisture content.

    So far as the considering the relationship between individual particle properties and those of the bulk assembly I think it is generally best to consider those most appropriate to the conditions of interest in any given situation. I also constructed a chart to outline the various aspects and relation of measurable attributes to application. I will send a copy of this also if of interest. Some features, such as segregation and degradation, need to take account of the operation form, scale and sequence to develop a clear understanding of effects.

    I recently completed a ‘Glossary of terms in Powder and Bulk Technology’ that has been published by the British Materials Handling Board. This is more than a dictionary in that it includes much explanatory material and background information and standards. If of interest, I can send contact details. I hope that you find these notes useful.
    Lyn Bates

  4. #4

    I need help!


    My name is Nickolas Seyve, I am a french Student in an Food Science and Nutrition school in Dijon and I have to do a research work on mixing different sort of powders.

    I read through your article "Bulk powder density", which I found very interesting, and I was wondering if you could give me some piece of advice or give me some references about the density of powder such as wheat flour, corn flour, soybean flour, and animal protein flour... I would like to know if it is also hard to mix such componants and what are the best blenders to do this kind of mix.

    The purpose of my work is to be able to find the theory to be able to calculate the dimension of a blender to produce a well mixed powder.

    Thanks in advance, looking forward to hearing from you.


    Nickolas Seyve

  5. Bulk Densities

    A comprehensive file of typical bulk densities is available on the web site if you care to submit the materials of interest. Regarding mixing of the fine dry powders you mention, these can be well mixed by batch or continuous type ribbon blenders. The choice of mxer in this case is usually determined by the scale and nature of the following process, except where there are a large number of ingredients to be mixed in variable proportions or btch tracability is required, in either of these cases a batch type machine is more usually needed.

  6. #6
    ellenqilu Guest
    Dear Lyn Bates:

    I am pleased to read your essay in this forum. I'm drom China and not clear of the bulk density. Now, I have a problem to be solved so wish you can help me. How to reduce the tappen bulk density to a range. We are a manufacturer of pharmaceuticals and need to manufacure bulk material as the customer's requirement.

    Looking forward to hearing from you.

    Thank you.

  7. increasing bulk density

    Most bulk density problems arise with fine powders because of their tendency to entrain air in dilated flow or agitated conditions. One approach is to try and prevent the material dilating in the handling process. In general this requires an examination of the flow route and maximise the confinment of the flow. For example, transfer down a medium steep vee chute with a radius underside or fill a container through a full retracting chute that has a probe on the top to lift the bottom of the chute slowly, but stop lifting if the probe is uncovered. It is sometimes practical to pre-compact the feed by means of a suitably designed screw feeder. (This should only be used with powders that are compressible, not firm granular products).

    If the powder is held in a highly dilated form it is necessary to accelerate the escape of air from the voids. The normal path through a deep bed is tortuous and the rising air replaces air that is moving from the upper levels. By-pass channels can be created by the use of long vertical rods that are vibrated to whirl, so forming open paths through which air can rise without resistance or re-aerating the upper regions.

    Another technique for a robust vessel is to seal the container amd connect to a vacuum cylinder sized to reduce the void pressure to an acceptable level. This works best with virtually full containers. To avoid a dust problem, the vacuum vessel should be reduced in pressure before connecting to the storage unit and have a mass flow discharge shape, so that any dust can settle and be drawn off before the next vacuum cycle. This connection will generate an increased pressure differential to draw out air from the bed and, when exposed to ambient pressure the bed will compact. Note that powders prone to adopt a fluidised condtion are also prone to have very poor flow properties when compacted.Excess air content in a powder bed is more difficult to disperse in warm or hot conditions because the air is more viscous at elevated temperatures.

    Each application has to be considered on its individual merits for cost and effectiveness. Ajax is always prepared to review problem areas associated with product flow and bulk state control via

  8. #8

    optimising packing density by choice of particles size distributions


    I was hoping you might be able to shed some light on a particular matter.

    I am interested in maximising the packing density of some crystalline powders (typical particle sizes range from about 20-300 micron). One option is to optimise the packing through choice of particle size distributions. I have done a bit of reading of some related papers (Sohn/Moreland, Yu/Standish etc) which propose a number of packing models, but none appear to provide what I'm after.

    This is not my area of expertise and as a result I'm a little lost when it comes to determining the state of play of various packing models. For example, which is the most applicable to small scale (say 1dm3) packing of multiple sized crystallline particles?

    Also I was thinking of using an ultrasonic bath to "close-pack" the material but have seen relatively little on this subject.

    If you have any ideas - references to models, techniques or software that could help me in this situaton it would be appreciated. I am now chasing down a copy of Gray (thanks to your article) but wont have it for a while.



  9. Close Packing


    A basic difficulty associated with calculating a theoretical packing array is that it does not address the mechanics of bed formation, which is influenced by a host of factors. Looking at the overall situation it is considered important to distinguish between freely orienting particle structures and beds where particle cohesion restrains the relative motion of the constituents. There are also different considerations to take into account when the bulk is slow to de-aerate. If all the particles are inert and larger than 100 microns the mass is usually free flowing, i.e. the particles will re-orient easily, and the bulk is relatively quick to settle to a stable state. A bed with a significant proportion of 20 micron particles may be expected to inhibit the diffusion of air in the voids and be cohesive and non-free flowing when settled, but potentially offer a more dense condition. Intermediate compositions may be expected to show roughly proportional behaviour.

    Homogenisation of a mixture containing fines may be achieved by an agitated, tumbling device, such as a rotating drum fitted with blades that disturb the bulk. The container should be ‘dump-loaded’ if practical, rather than have the contents loosely poured. The application of vibration will initially tend to disassociate the load path in the particle structure and allow the mass to reform with a closer order of packing. However, sustained vibration will tend to cause migration of the finer particles under the larger members to ultimately ‘float’ larger particles to the surface of the bed. An initial compaction will therefore be followed by a reduction in bulk density as voids are created between the elevated coarse fractions.

    A tapping motion mobilises gravity and inertia to accelerate close packing. An effective mechanism suggested for the volume indicated is to impart vertical shocks equivalent to dropping the container about 25mm about 50 times onto a hard surface. The effect will be enhanced if there is a weight uniformly distributed over the bed surface during this process. The operation will be even more effective if it can be carried out in an atmosphere of reduced pressure that is suddenly released to atmospheric pressure when the tapping ceases. The pressure differential between normal atmospheric pressure and that prevailing in the voids will apply an additional consolidating load to give further compaction.

    You may find that experimenting with different compositions and techniques is more productive, and interesting, than conducting a deep theoretical analysis that suggests an optimum composition that is hard to attain in practice. Good luck, and please let me know how you go on.


  10. #10
    Donna A. Guest

    Powder Flowability

    Dear Lyn,

    Im with the R&D Group of one noodle seasoning manufacturing company. I would like to know if there are standard methods on how to measure/quantify the flowability of a certain seasoning powder mix. The mix generally contains granulated sugar and salt with particle size of 180 microns which is about 50%, 45% are fine powders and the remaining 5% are dehydrated vegetables with particle size of about 2-4mm.

    What are other elements of powder morphology that we may measure so we can establish standards for consistent packing?

    thanks very much!

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