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Powder Flow Q&A

  •   I am currently facing some challenges with our product. We supply a stearic coated calcium carbonate product (d50=2 micron) to the Polypropylene and Polyethylene industry and the bulk density of the product is changing quite drastically. it seems that during when the product is packed at night, it has a higher bulk density then when packed during the day. Is it possible that the humidity in the air could be causing the change in the bulk density? this is affecting the flowability of the product as it hangs in the hopper during extrusion and so compromising the production rate of our customer.

    As I understand it from your question, you handle a very fine powder and you are experiencing bulk density variations during handling.  You have asked if daily relative humidity variations could be causing this variability.

    Bulk density variations can be caused by a variety of factors.  Consider the following scenarios.
    First, if a material is compressible, as most fine powders are, then the level of pressure on the material can induce density variability.  For example, fine confectionary sugar is packed at the bottom of the package, while at the top it remains quite fluffy.

    Second, a fine material can be subject to aeration, and some materials, like fumed silica, can retain air for several hours.  If the fine calcium carbonate is aerated at all from the use of flow aids, like Solimar-type air pads, then this can induce a large bulk density change from a compacted state to an aerated state where the material could behave like a liquid.  Though the flow aid can be helpful in overcoming bridging problems in a converging hopper, the material could then flow like a liquid causing other processing issues.

    Third, the flow pattern for the material in the hopper, along with the requisite flow problems, can induce bulk density variability.  You indicate that the product “hangs in the hopper”, which means the material likely discharges in a funnel flow pattern.  With funnel flow and fine powders that are cohesive, ratholing can result, yielding flow through a small central channel of stagnant material in the hopper.  The powder moving through this small channel can remain quite aerated and flow like a liquid with large swings in bulk density as more deaerated material makes its way in to the moving stream.  Furthermore, if the rathole forms and then collapses, this is a common manner in which density will vary quite dramatically with a fine powder.

    You mention a different packing behavior during the day versus night.  This could be due to operator variability, whereby the shifts may be operating the equipment quite differently.  If the product were dry sand which is free-flowing and incompressible, it is likely the variability is minimal; however, with a fine, cohesive powder, subtle changes in equipment operation can have a profound difference in processing behaviors.

    Fourth, if the material is highly hygroscopic (i.e. prone to absorb moisture), then high relative humidity air exposed to the material could be contributing to poor flow and density changes.  I’d consider the former three scenarios where density variations occur before evaluating if relative humidity air is affecting the material’s behavior.

    It is vital to know the flow pattern of material within the hopper; if mass flow, then ratholing is not possible and no stagnant regions exist.  If funnel flow, many scenarios, as discussed above, can lead to powder density variability.  The key will be to run flow tests on your powder to understand what the flow pattern is in the hopper, then to understand what flow obstructions could result.  Also, knowing the material’s compressibility and permeability (resistance to air flow) will help to resolve the density variability issues.

  •   I have supplied a conveying and cooling system for coated calcium carbonate (CaCO3 + Steric acid 5%). The bag house choking frequency was quite high. I have improved the reverse pulsing, but have not received a satisfactory result. The assumption was steric acid was making a layer on the bag. The melting point is about 89 °C. The temperature at the feed point it was 105 °C and at discharge this was 50 °C. Do you have a solution you can provide?

    Your question is quite involved, and I will introduce several situations that may be the cause of poor system performance.

    1. Improper selection of bag filters. If the material is highly cohesive and adhesive, then the powder will easily pack on many types of filters (e.g., singed polyester, fiberglass). You could consider using a surface filtration product, such as Teflon bags, that has a good dust release tendency, even with sticky powders. Consider also how the material is conveyed in to the bag house – is the stream directed towards the filters? This can increase sticking problems.
    2. Use of pleated vs. bag filters. Though more surface area can be gained with pleated filters, in some cases, powder bridging occurs between the filter pleats due to poor cleaning and high cohesion. With bag filters, the reverse gas pulsing is directed out perpendicular to the bag surface thus providing effective cleaning, whereas with pleated filters, the gas direction will push material back on to the surface in some areas of the pleat. I am not saying that pleated filters are a poor choice – rather, in some cases, pleated filters are not the more effective choice.
    3. Segregation of blend. Do you know if you have a quality blend or if the coated particles remain coated? If high velocity conveying is occurring, this may be allowing removal of the steric acid from the primary particles, thereby allowing separation in the bag house. Consider also the layout of the conveying system – are there many elbows and transitions that maximize turbulent flow and particle separation?
    4. Inadequate cooling. Do you know if your heat exchanger is working adequately? If the feed point temperature is 105°C and the melting point is 89°C, then some melting must be occurring, leading to strong buildup tendencies in the pipeline and bag house. Even if you are below the melting point, you may be operating near the softening point of the powder leading to adherence to the filters. Can vacuum conveying be arranged so that heat-up of the conveying gas does not result with the powder? Also, is humidity an issue? If the powder is hygroscopic (absorbs moisture vapor), then this could be the cause of sticking on the bags.
    5. Proper bag house sizing. Do you have the correct filter area? Is the bag house “can velocity” beyond reasonable limits allowing re-entrainment of the dust?
    6. Proper hopper design for material discharge. Is the hopper on the bag house properly designed to enforce reliable discharge or does bridging and ratholing occur? Selection of the correct hopper outlet size/shape, feeder, and wall surface are vital to enforcing reliable flow.
  •   Presently we are drying powder, micronizing the dried powder, and sifting the same. When a sample is taken at the outlet of the sifter, it meets all particle size specs, but when this sifted powder is pneumatically conveyed to storage silo from where it is bagged into jumbo bags, many times it fails for particle size distribution, when analyzed on a sieve shaker (dry sieve analysis). Can you help us understand what is happening? Also let us know if we can retrofit our existing silo with some mixing device.

    The problem you describe with a changing particle size distribution after pneumatic conveying is not uncommon, though its cause can be from a variety of situations.

    First, you should ensure that your sampling procedure in the jumbo bags is robust and accurate. For example, if your technician is collecting the sample via a scoop from the top of the surface of material in the jumbo bag, then this could be introducing bias towards a finer size distribution. This is due to a fluidization or dusting segregation effect, which are common during bag filling. Fines in the mixture will rise to the top (fluidization) or sides/top with dusting segregation. Therefore, care must be taken in extracting a representative sample (more aptly, samples) from the jumbo bags before making a judgment that particle attrition (size reduction) is resulting during pneumatic transport. You should also take samples across multiple jumbo bags to look at the variability between bags.

    Second, you should evaluate the method of pneumatic transport and determine if dilute phase (full suspension conveying) is resulting. Dilute phase transport can degrade (attrit) particles, especially if they are fragile, such as some granulations. The high velocity transport tends to shift the particle size distribution finer than with a dense phase mode of conveying. Note however that attrition can occur in dense phase, though typically at a much lower rate. The conveying line operation may be able to be fine-tuned to reduce some of the particle damage. Its configuration may also be modified to eliminate excess elbows, which can be a major contributor to attrition.

    Third, you should evaluate the flow pattern through the silo above the jumbo bagger. If the flow pattern is funnel flow, whereby some of the material moves during discharge while some material is stagnant, then this could be introducing segregation during flow. With a mass flow pattern, first-in, first-out flow is achieved through a silo, reducing the likelihood of particle segregation effects during discharge.

    Once these three considerations are quantified and assessed, then proper modifications can be implemented to solve the particle size distribution problem. I would not just plan to modify the silo until it is clear that the problem is caused by the silo!

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  •   We tried conveying fumed silica into a tank with a bucket elevator. No luck coming out the other end. I know this is a very difficult material. Any suggestions?

    You have indicated that you are handling fumed silica, which is a very fine, and low bulk density material. We often see fumed silica with a bulk density of 2-3 lbs/cu ft (30 kg/ cu m), which makes this material highly prone to bridging in hoppers, as well as difficult to flow via gravity discharge from certain conveyors. In fact, you mention that you have difficulties getting material to discharge from a bucket elevator into a tank.

    This is not surprising given the material’s low density and adhesive nature (propensity to stick to surfaces). You may want to consider a different conveyor, such as an aerated system (e.g., pneumatic conveying via vacuum loading the tank or an air slide). The fumed silica will move easily, like water, with an air based conveying technology, though, you will have to separate the air (or gas) at the tank. You may also consider an aero-mechanical conveyor, which uses a steel cable or chain connected to plastic “pucks” contained in a tube. The conveyor runs in a loop, can discharge the fumed silica in the tank, and does so without a lot of air flow, as with pneumatic conveyors or air slides.

  •   We have a 40 ton Nissei. The regrind does not feed like the virgin plastic. Polysulfone has many sizes of regrind and we thought a vibrator in the hopper would shake the regrind and flow into the feed section of the barrel. Any suggestion would be appreciated.

    You have indicated in your question that you have an injection molding machine (i.e., Nissei), and you are having flow issues with reincorporating regrind scrap into the extruder.  This is a common problem, as the regrind particles are often coarse, flaky, and prone to interlocking arches in hoppers with smaller outlets. Using a high frequency vibrator to “shake” the regrind in the hopper may help, but, this approach is based on trial-and-error. If you have control on the particle size of the regrind, it may be better to reduce the size somewhat to improve the flow – just don’t make it too fine that it arches in the hopper due to excessive cohesion. Flow testing on the Polysulfone regrind (either coarse or fine) will explicitly tell you why the material is bridging (arching) in the hopper, and if going to a slightly finer size will work or not through the fixed outlet size on the hopper.

    I have also found in some cases with flaky materials that vibration does not overcome arching, but, in fact tends to pack the material more in the hopper.

    As an aside, you may want to consider adding some of the regrind into the main material, if possible. A blend of regrind, say 10-20% in the main component, may have lower bridging tendencies that if using pure regrind. Again, material testing can predict this per shear testing protocols in ASTM International standard D6128.

  •   What do you recommend for powder flow modeling and simulation?

    You have asked about software for powder flow modeling and simulation. Given there is no additional context to your question, I am left with providing a highly generalized answer.

    If you are looking to model the flow of a dilutely loaded gas/solids mixture, such as in cyclone separators, then companies such as ANSYS Fluent and Creare could be considered. These companies, among others, have experience with computational fluid dynamics (CFD).

    If you are looking to model the flow of contact-bed reactors/purge columns/dryers, then this type of software is usually not commercially available as the companies that have developed the technology for analyzing these applications keeps the information proprietary. Additionally, the interaction of particles to themselves, their boundaries, and with interstitial fluids/gases, is highly complex and many assumptions are made to simplify the analyses. Often the models are continuum-based with gas/solids continuity assumptions and they are not time dependent (in other words, they are steady-state models).

    If you are looking to model larger particle flow (e.g., 20 mm rocks), then discrete element modeling (DEM) technology is commercially available from several software vendors. This technology can be (depending on the model parameters) quite accurate for computationally modeling particle flow through interactions between particles, boundaries, and in some cases, coupled conditions with gas flow using computational fluid dynamics (CFD).

    Using DEM to model particle flow in bins, hoppers, feeders is currently not practical as the number of particles to model is limited to say 1,000,000. The simulations become too computationally intensive as every particle must be advanced through small time-steps and analyzed for their position, velocity, and acceleration while considering their interactions.  In the near future, DEM processing technology will improve, but, as for modeling powder flow with 100 micron-size particles in a hopper, we are just not there yet!

  •   Our application involves discharging rock dust (pulverized limestone) from a 6 ft diameter hopper, only 54 in. total height, using a venturi eductor. Our pitch angle is 45 degrees, and material is UHMW. The hopper is prone to constant ratholing, but due to the height of the space, we cannot increase the pitch angle without sacrificing storage volume. Can switching materials to Tivar solve this problem? Or will it have only a minimal effect compared with increasing the slope angle? Or possibly a compound shaped funnel, with a steeper discharge section?

    You have indicated in your question that ratholing of pulverized limestone is resulting in a conical hopper with a 45 degrees slope and having a venturi eductor at its outlet. The hopper is lined with UHMW plastic, though this does not promote mass flow (flow along the hopper walls), since ratholing is occurring. You have inquired if using a different plastic liner will solve the flow issues. You also mentioned that you prefer to not increase the hopper slope due to space and available volume limitations.

    It sounds like you have two issues occurring. First, no flow along the hopper walls, which when handling cohesive materials, can allow ratholing. Second, the venturi eductor could be exacerbating the ratholing problem as it tends to pull material via a strong vacuum down through the hopper center versus at the hopper walls. We see this behavior frequently with solids-to-liquids mixing devices. Lining the walls with a slippery material allowing mass flow to occur may not alone be sufficient therefore to overcome the ratholing problem.  In addition, finding a liner to allow mass flow at a 45 degree angle will be challenging with fine limestone.

    In this case, the best approach may be to consider a hopper insert that promotes mass flow and avoids aggressive vacuuming of material from the hopper via venturi effects. This insert, in the form of a bullet-shape, can be properly engineered to ensure it has the correct slopes, dimensions, and location to avoid ratholing and promote mass flow. The key to its design is to have material flow test results for the pulverized limestone. Without the correct flow test data, the hopper design will be based on guess-work, which is typically costly and frustrating. Several companies are available to provide powder flow testing services, along with hopper design engineering. Ensure that the testing company uses the shear testing protocols per ASTM International standard D6128.

  •   My bulk material can have variable properties – what factors could be influencing its flow behavior?

    Bulk solids can alter their behavior depending upon a wide variety of factors. Key factors that often affect material properties include moisture content, particle size and distribution, and even particle shape. When evaluating bulk material flow properties, it is vital to replicate actual or expected process conditions to avoid further complications. Conditions that can alter behavior include storage time at rest, pressure, temperature (either constant or cyclic), and relative humidity.

    Think of a simple example with coffee. This can be in the form of a roasted bean, coarse ground, or milled, depending upon how the product is sold or used in a process. The particle size difference alone can result in the coffee either flowing easily (with beans) or having frequent flow problems such as bridging, ratholing, or segregation during handling in hoppers.

    In some cases, a very subtle shift in size, moisture, or chemistry can alter the flow characteristics.

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