FIGURE 2. The flow cup test is relatively easy to setup and perform, and the data are used to calculate the Carr index, Equations (1), and Hausner ration, Equation (2)
and then becomes lodged in place. This type of structure is referred to as a “rathole” shown in Figure 1c. The rathole may extend from top to bottom of the bin and may change in diameter of opening as a function of powder depth. Processors prefer mass flow for obvious reasons. Cohesive materials will generally exhibit core flow in plant equipment as originally designed. The hopper wall angle and its material of construction have a direct impact on flow behavior. Therefore the challenge is to manage the problem with the equipment that exists, which means modifying the formulation, or redesigning the bin equipment, if practical.
is needed to allow the powder to discharge from the cup. In a practical sense, this instrument is used as a “go” or “no-go” indicator for powder processing on a regular basis. Angle of repose. This is a simple test method that observes powder in a pile and measures the angle of the pile relative to horizontal. Note that both the angle-of-repose method and the flow-cup test work with powders that are loosely consolidated. They do not attempt to evaluate the powder as it settles, which is what happens when powder is placed in a containment vessel of any kind. This phenomenon, called “consolidation,” is an important distinction to keep in mind because it has direct impact on how flow behavior can change. Tap test. The tap test takes a cylinder of powder and shakes it to determine how much settling will occur. The change in volume of the powder from start to finish is a measurement of the powder’s tendency to consolidate. The “loose fill” density, poured, of the powder at the start of the test is calculated by dividing the cylinder volume into the weight of the
sample. The “tap density,” tapped, is calculated by dividing the reduced volume of powder at the end of the test into the sample weight. The two density values are compared to one another, giving an indicator for the consolidation that can take place over time when the powder settles. Two standard calculations that are typically used by industry to evaluate tap test data are called Carr Index (Carr%) and Hausner Ratio (HR), as defined in Equations (1) and (2):
(1) (2)
Shear cell test for flowability Shear cells measure the inter-particle friction of powder materials. This type of test has direct application to predicting flow behavior in gravity discharge for powders stored in vessels of any kind. Shear cells were
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Traditional tests for flowability As mentioned earlier, there are three common methods for predicting flow: flow cup, angle of repose and the tap test. Flow cup. The most popular testing method is the flow cup, which is quick and easy to use. The cup is basically an open cylinder with a removable disc that is inserted into the bottom (Figure 2). A family of discs, each with a different hole diameter in the middle, is provided with the cup. Once the disc is in place, the cup is filled with powder and the operator observes whether the material discharges through the hole. Processors may know from experience what difficulties they are likely to face depending on the hole diameter that CHEMICAL ENGINEERING
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