Optimizing Blends for Problem-Free Flow

Powders and bulk solids almost inevitably have flow problems during processing. The most common flow problems for cohesive powders with small particle size include arching in the hopper, jamming of the powder in the feeder system, rat-holing, and potential changes in bulk density due to consolidation. While these flow problems are well-known, solving them can be challenging. Complex mixes and blends with variable particle size can only add to this challenge because desegregation is another complication that may come up.
    For this article, we will consider flow problems associated with blended materials, how to characterize these materials so that we can predict their flow behavior, and best practice for solving the problems described above.

The More Complex the Blend, the Greater the Potential for Flow Problems
Many bulk solids in industry consist of a blend or mixture of products. In the nutraceutical and pharmaceutical worlds, these materials can consist of a complex blend of active product and excipients. Some materials can have 20, 30, or more individual components in the mixture. The more cohesive the product mix, the greater the number of flow issues that might be encountered. Additionally, environmental considerations (temperature and humidity variability), coupled with storage conditions before processing (stack height and type of containment vessel), can all contribute to cohesive types of product becoming even more problematic.
    Variable particle size contributes to the complexity of the blend in the following way. Smaller particles tend to be more cohesive; larger particles, more free flowing. Mix the two together for a blended formulation and a different set of issues may arise. In a poorly designed process system with irregular flow behavior, particle segregation can occur with the larger size particles being pushed towards the hopper wall and the smaller particles tending toward the middle of the flow stream. In this case, the blend of smaller and larger particles may no longer remain a blend.
    Agglomeration is another type of problem that may occur when mixing and blending materials. Disparate materials will sometimes tend to agglomerate, or clump together, thus causing jams to occur during the flow process. Costly down time can occur looking for, and breaking up, these agglomerated materials.
    Best practice for industrial processing is to analyze both the final composition of a powder and its individual components for flow properties. Through this characterization work, potentially good and bad performers can be identified early on and benchmarks set for acceptable flow behavior.

Problem Solving Involves Characterization
Characterizing a single type of powder for flow is one thing. Add in the complexity of a blend of powders and the task becomes more complicated. A method needs to be incorporated for not only characterizing the final product, but the individual ingredients used in the mixture. But with perhaps 20 or more ingredients per blend, this needs to be done in a timely manner. The ideal test method would be one that is quick, accurate and based on proven scientific methods.
    The most commonly used test methods in the powder and bulk solids industry are Angle of Repose and Tap Density. While quick, these are subjective tests and supply no indication of flow metrics such as flow function or internal friction angle (both are measures of flowability), potential arching dimension and rathole diameter (these are indicators of the potential problems that may occur). The alternative to simple test methods, like Angle of Repose and Tap Density, are simple trial and error tactics. If a material is proving to be troublesome, a small amount of flow aid may be added to see if the flow process improves. If it does not, a little more is added and the process repeated. This is a time consuming and less scientific approach. If an appropriate flow aid is identified, the flow results can always change yet again with variations in temperature, humidity, and length of storage.
    Clearly, characterization of the product needs to be performed before going to production. This way, flow problems can be addressed before they occur. Problematic powders can be identified before going into the mixture. If these materials are being supplied by a vendor, they can be rejected provided there is an accepted test method for verifying predictable flow behavior.

Shear Cell Testing – a Proven Test Method
Annular shear cell technology is a tried and true method for evaluating powder flowability, and has evolved to become the best accepted practice for defining flow properties. Shear cell technology has been around for decades and is the solution for cost-effective, time-critical, definitive testing. Per ASTM D6128, shear cell testers define powder flow properties by shearing materials at defined consolidation pressures, and most importantly, by achieving critical consolidation of the powder prior to shearing to failure. The annular ring design ensures that the consolidation stresses are uniform across the powder sample’s particles. By slowly rotating the ring and measuring the force required to shear the material, a powder flow curve is established using Mohr Circle analysis to crunch the data.
    The instrument above is one such example of a standalone device utilizing an annular shear cell. Fully software driven, test results are realized in minutes producing data on key powder characterization parameters: Flow Function, Bulk Density, Internal Friction Angle, Wall Friction, Arching Dimension, and Rat-hole Diameter.

Gravity-Feed System
A common type of powder flow system is one that utilizes gravity to discharge the powder from a containment vessel. Simple and inexpensive, powder is poured into the bin and gravity takes over to flow the powder through the hopper. If the powder is a more cohesive type of material, it can arch in the hopper, if not right away, perhaps as the fill level reduces in the bin, thereby causing a flow cessation. To ensure optimum product flow, the material needs to be characterized for common flow properties such as Flow Function, Bulk Density, Internal Friction Angle, Arching Dimension, and Rat-hole Diameter.

    We will examine a powder and define its flow characteristics for Flow Function and Arching Dimension.

Comprehensive Data Results
Taking our example of a gravity-feed system and a powder causing a cohesive arch in the system, let us examine what the data would look like using an annular shear cell for characterizing the sample. By running a flow function test on a small (263 or 43cc) sample of powder, definitive flow results are computed and graphed.
    The Flow Function test will characterize the powder by plotting failure strength vs. consolidating pressure. The resulting graph illustrates flow behavior that can range from Free Flowing to Non-Flowing. The test results will point out defined regions of flow behavior where problems may occur.
    The Arching Dimension result will be calculated automatically via software. The Arching Dimension value is a conservative calculation which defines the capacity of the product to form a cohesive arch of a specific length. The opening of the hopper needs to be larger than this value to ensure that a cohesive arch will not form.
    Assume, for example, that we know the opening of a hopper is 100mm. After running a flow function test, we get the following results for flow function and arching dimension:
    Examining the above graph, we can clearly see the material is of a cohesive nature; its arching dimension has been calculated to be 134mm. The product will clearly form a cohesive arch. Changing the composition of the material by adding flow aid will decrease this value to allow the material to flow properly.
    Flow Function tests are run to characterize the mixture for specific volumes of added flow aid until the best mixture is realized. A flow function graph with associated arching dimension values are computed as follows: examining the graph, again in figure 4, we can see the material, while still a bit cohesive, is now more easy flowing. Its arching dimension has now been computed at 89mm. This product will no longer cause a cohesive arch to form in the hopper. Furthermore, the material has been defined for the amount of flow aid used to enable product to flow properly. This can then be used as a control for future batches of product to ensure the correct amount of flow aid is being used and the product is meeting the specifications of the gravity feed system. Jams and downtime have been eliminated, the product has been properly characterized for flow behavior, and production process is clearly enhanced.

Conclusion
The use of an annular shear cell for powder testing results in comprehensive, accurate characterization of the blend or mixture. Subjective test methods are replaced with definitive scientific data and concrete results. Plus, the ease of shear cell use due to automation of the test method and the recent affordability of this equipment due to strong industrial demand makes it an even more attractive option.
    Through the use of shear cell technology, guess work for flow aids is eliminated. Problematic blends are identified and adjusted before they go to the production phase. Individual ingredients are characterized and tested before being added to the final mixture to ensure proper flow and best practice results.
    Vinnie Hebert is product manager - powder flow tester, Brookfield Engineering Laboratories Inc., Middleboro, MA. For more information, call 508-946-6200, ext 7171, e-mail [email protected], or visit www.brookfieldengineering.com.

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