9 tests to better know your bulk solid
A behind-the-scenes look at how Solex Thermal Science’s work in the lab puts your bulk solid under the microscope.
Author: Jamie Zachary
There is more than meets the eye with your urea granules. Or your sugar fines. Or your powders, pellets and particles. Each one has a different story, from how it’s made to where it’s made.
The characteristics and properties of bulk granular solids can and will vary because of a whole host of reasons – sometimes between the same type of material, sometimes at the same facility.
Moisture content, void fraction, particle shape and particle size distribution all play a role in determining the flowability and thermal characteristics of your product. This means any heating, cooling and drying solutions will be unique to your bulk material.
Yet there is no textbook definition of many properties that are essential in bulks solids heat exchange – properties such as bulk thermal conductivity. Rather, it’s a practical assessment that’s influenced by many material and operational factors for a given application
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This is where lab testing can provide better insight into your material, and then lead to an ideal heat exchange solution for your application.
We caught up with Devon Robinson, Application Engineer, who took us behind the scenes of what takes place during lab testing of the samples sent to us by our customers.
Solex: What are some tests you typically run?
Robinson: The main ones are bulk density, particle size distribution and angle of repose:
- Bulk density: This involves putting it into a known volume, weighing it and repeating the process to ensure a representative result. For structural reasons, we also look for the worst-case bulk density. For example, if the product sat over the weekend or has been vibrated/shaken, is there a concern with consolidation?
- Particle size distribution: We use a riffler to get an unbiased sample, and then pass that sample through a series of stacked sieves. The coarser material will stay at the top, while the finer stuff will settle closer to the bottom. This gives us the data points we need to determine the actual distribution.
- Angle of repose: This is important for vertical heat exchangers because it defines the design of the inlet hopper, and informs the ideal location for the level probe. We start by making a few representative piles about eight to 10 inches in diameter. We then measure the steepest angle relative to the surface the material sits on.
Solex: Are there other tests you consider?
Robinson: If it’s a material we’re less familiar with, we’ll look at measuring properties such as flow, moisture content and various thermal properties:
- Flow testing: We pick the most appropriate discharge feeder and attach it to a flow test rig. Small-scale exchanger plates are spaced at various gaps and representative sample is passed between them. After several runs where we change the settings of the rig, we’ll be able to determine the closest spacing that safely allows for uniform mass flow of the sample.
- Moisture content: If needed, we do this as soon as the sample arrives at the lab to avoid any further moisture loss that might have occurred in transit. We weigh a few samples and dry them in an oven over 12 to 24 hours. The difference in weight before and after drying represents the moisture content.
- Thermal properties: This involves determining a material’s specific heat and thermal conductivity. We have a proprietary method where we measure how well a bulk material conducts heat under anticipated operating conditions.
Solex: Are there any tests you conduct that are a bit out of the ordinary?
Robinson: Sometimes we need additional information that either other tests won’t tell us or it’s a material that’s new to us.
- Gas pressure drop through the bulk solid: This is normally something we can estimate based on the particle size distribution. Yet if the particles are irregularly shaped, for example, a separate pressure drop test will provide more accurate information. During the test, we pass known quantities of air through a closed bed of sample, measuring pressure drop for various flow rates.
- Drying curve: We add product to a test rig that represents the volume between two heated exchanger plates. We then blow dry air at a known quantity and known temperature through the rig, again at representative operating conditions (e.g., non-fluidizing). The entire unit sits on a sensitive scale that measures the loss-in-weight with time. This allows us to produce a drying curve for a sample at a specific set of conditions.
- Critical relative humidity (CRH): This helps us understand the highest ambient humidity that can exist in the voids of the product before the sample starts to absorb moisture. We start by placing a small amount of product on a laser-balanced scale. We then very accurately measure the change in weight as we add different levels of warm, humid air over a certain length of time.
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Solex: Do you test everything?
Robinson: Because we've done a lot of testing over the past 30 years, we have a solid understanding of many materials and can often interpolate based on our proprietary material properties database. That includes different types of fertilizers, sugars, oilseeds and minerals. So instead of testing everything, we ask questions to determine if a customer's application involves a material we haven't seen before. And then we determine if it we need to test it.
That being said, we still offer the ability to test anything at a customer's request or if we believe there is a need to do so in order for us to confidently support our performance guarantee.
Did you know?
Solex also conducts on-site pilot testing around the world? Under realistic process conditions, our test units closely duplicate full-size equipment, and is tailored to analyze and validate your specific application.
Ready to talk specifics? Contact a Solex team member today.
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May 26, 2021
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