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Characterizing Processed Bulk Solids for Accurate Flow

August 14, 2014
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Brookfield powder flow tester with shear cell
Figure 2a
Flow function data showing increased cohesiveness
Figure 2b
Arching dimension data showing increases related to increase in cohesiveness
Figure 3
Chart for percentage addition of flow aid based on arching dimension and unconfined failure strength data

Consider the perfect bulk solid: a material that never jams and never ratholes. Producing the product would be an easy endeavor. Characterizing the material would not be necessary. The material would always flow well. No jams in the feeder system would occur. No production downtime would be incurred. Life would be good. Management would smile.
    If processing bulk solids were only that simple. Other than a product like granulated sugar (which is typically very free flowing and satisfies “perfect bulk solid” criteria), most bulk solids consist of an array of different ingredients. Take, for example, a nutraceutical such as a protein or whey powder found in health stores. Observing the label shows a host of ingredients: whey protein, lecithin, potassium, lactase, flavorings, milk, and more. This complex list of ingredients is blended to create the final product. It has taken months, if not years, to develop, test, and bring to market such a formula. Taken as a whole, this blend of products needs to be characterized for proper flow properties. This ensures that the time and money spent on development pays the proper dividends. When the product goes to the production phase, it will flow properly and maximum profit will be realized.
    To accomplish this, R&D must perform comprehensive testing to characterize the formulation. Then QC has responsibility during production to check incoming materials after blending to ensure that no jams or flow issues occur while creating the final product.

Conventional Tests for Flow Behavior
Most bulk solids are known to have flow issues. Cohesive arching, jamming, particle segregation, and ratholing are all common occurrences during the production cycle. This is due to issues related to the flow pattern of most bulk solids: arching in the hopper occurs during mass flow when the arch builds sufficient strength to support the powder above it; ratholing in the storage vessel occurs during core flow when the powder outside the rathole diameter builds sufficient strength to remain in place and not move. To address and correct these issues, the material needs to be tested and characterized. Common characterization of a bulk solid includes tests for flow function, wall friction, and density. Calculated parameters that come from these tests include values for flow index, arching dimension, and rathole diameter. From these metrics, a specification can be developed to pass or fail the material. During the production cycle, QA/QC checks can be performed and their results compared to the established pass/fail criteria. If the material fails, steps can be taken (such as adding a flow aid) to ensure that improved flow behavior for the product is achieved.
    Too often, the test done to characterize and check a bulk solid is...nothing. How are flow problems fixed with no testing performed? The “hammer” method is a common solution. A hammer is literally used to bash a hopper that has jammed to loosen up the powder. Another common method is the use of flow aid. In many industries, such as foods and pharmaceuticals, the maximum amount of flow aid that can be mixed into a batch of product is 2% by weight. If the product is not flowing well, simply add in the maximum 2% flow aid, run the material again, and expect some degree of improvement in flow.
    So why aren’t flow properties of bulk solids characterized and checked more often? The simple reasons are time and money. Some instruments used to predict common flow properties of powder blends – like particle size analyzers – have been too costly, ranging from $50,000 to $100,000. The time to test a sample is sometimes prohibitive and cannot produce the needed information in time to guide the decision making for each batch. Running the instrumentation can also involve a complicated procedure, requiring not only a trained technician, but also a scientist or engineer to interpret the results.
    There are some simple, inexpensive tests that are run for initial characterization and testing of a bulk solid. These include flow through a funnel, angle of repose measurement, and tapped bulk density test. The problem here is that they are simple “go” or “no go” tests. They give no indication of what may be causing the flow issue with the material or how severe it might become. Is the flow problem due to the humidity level that day? Were some of the blends improperly mixed? Did some of the raw ingredients supplied by an outside vendor change in make-up? None of these potential causes can be gleaned from these simple tests.
    A new and cost-effective solution to these flow behavior problems may now be possible through the use of shear cell technology. Instruments that utilize the annular shear cell design are available at a fraction of the cost of the more sophisticated equipment and can perform tests almost as quickly as the simple instruments mentioned in the last paragraph.

The Shear Cell Solution
Shear cells have existed for decades and have been accepted in the bulk solids industry as per ASTM D1628, which describes the test method using the original Jenike shear cell. Shear cell methodology quantifies inter-particle friction by shearing powder particles against one another. The shear cell simulates the consolidating effect of the powder’s self-weight in a bin by compressing a powder sample, then shearing the particles to measure the amount of force needed to initiate flow or relative movement of the particles. This measurement gives a value for the flowability of the powder as a function of consolidating stress. By incorporating numerous shear cells in an annular ring configuration and applying defined compressive loads to a sample of material, accurate flow function properties can be acquired in a short amount of time, perhaps as little as 25 minutes.

See Figure 1: Brookfield powder flow tester with shear cell
 
    Once a powder is characterized and a production run is initiated, the material should be checked for final quality, namely compliance with a “flowability” benchmark. During a production run, quick, QA/QC tests can be run in as little as 10 minutes to ensure that the material meets the criteria established for the bulk solid to properly flow. If the flow properties change, defined steps can be taken to ensure production downtime is minimized.

Application of the Data
Consider product X, a costly protein mixture, consisting of ingredients such as whey, milk, cocoa, and flavorings. The product is known to be hygroscopic, thus humidity affects its flow. In order to ensure the product is flowing properly, it has been characterized for specific flow properties consisting of a flow function at a final consolidation endpoint and an arching dimension. The consolidation endpoint is the normal stress applied to the sample at a given consolidation level. For the flow function test result, five of these levels are applied to a sample of powder. Mohr Circles are then used to compute corresponding Unconfined Failure Strength and Major Principle Consolidation Stress values and a flow data line is plotted (Figure 2). To simplify characterization of this material, the final consolidation endpoint of 4.8 kPa has been chosen. It is at this level that a corresponding Unconfined Failure Strength is examined. It is known that the product flows well at a consolidation endpoint of 4.8 kPa which has a corresponding Unconfined Failure Strength value at 1.4 kPa and an arching dimension at 53mm. No flow aid is needed at these levels or below them.
    The production run is initiated in the morning when the humidity and temperature levels are low. Production is smooth with no jams or issues. As the day goes on, the humidity level begins to rise. During the production run, the material is carefully gaged using a quick QA/QC flow function test. This test realizes the flow function and arching dimension data required to gage the flow of the product. The flow function graph shows how the material becomes harder to flow. An examination of the Unconfined Failure Strength shows that as this value goes up, due to an increase in humidity, the material moves from an easy flowing material to one that is quite cohesive and poorly flowing.
 
See Figure 2: Flow function data showing increased cohesiveness; arching dimension data showing increases related to increase in cohesiveness

    Thus, as the humidity level rises, a subsequent flow function test shows the product becoming more cohesive; the Unconfined Failure Strength has risen to 3.022 kPa and the arching dimension has increased to 137mm. Based on this data, flow aid is added in at a previously defined ratio of 0.75% by weight. Another QA/QC check using the quick Flow Function test shows that the material is back within specification. Subsequent checks are run and compared to a chart of flow aid percentage based on the flow function and arching dimension data.
    
See Figure 3: Chart for percentage addition of flow aid based on arching dimension and unconfined failure strength data

    Through the implementation of this method, production runs smoothly and the overuse of costly flow aid is eliminated. Rather than just dumping in the maximum 2% flow aid as was previously done, a chart with measured values for flow aid contribution is defined by the flow function and arching dimension data. Thus, maximum production is ensured while regulating the use of costly flow aid.

Conclusion
Once cost prohibitive, instrumentation utilizing shear cell technology is now available to quickly, easily, and cost-effectively analyze and characterize powder blends for flow properties. Easy testing and interpretation of the flow data ensures that production is running smoothly. Thus, the time and cost incurred previously through the use of inaccurate testing is removed and replaced with a defined, proven test method accepted by ASTM and throughout industry.
    Vinnie Hebert is product manager powder flow tester, Brookfield Engineering Laboratories Inc. (Middleboro, MA). For more information call 508-946-6200 email v_hebert@brookfieldengineering.com, or visit www.brookfieldengineering.com.

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