Flow Characterization of Laundry Detergent Powders

Several brands of laundry detergents are manufactured in powder form. While popular with consumers for their cleaning power, one question of interest to manufacturers is the challenge required to process the powder. The final stage in manufacturing may include storage of the powder for a time interval prior to packaging. One possible unknown is whether there will be difficulty in discharging the powder from the storage vessel to the fill lines.
    
Three major brands were evaluated in the Brookfield Powder Rheology Laboratory for flow behavior with an instrument that simulates test conditions for gravity discharge of powders through a converging hopper. This article presents the test results and gives an analysis of findings that could impact future consideration for how these brands are formulated and the storage equipment design.
    
Traditional tools used to predict powder flow behavior in gravity discharge have included the flow cup, angle of repose measurement, and tap test. The flow cup is filled with powder which discharges out a hole in the bottom; observation leads to a basic go/no-go assessment on whether powder will flow out the hopper on the storage vessel. Angle of repose is the evaluation of powder in a loosely consolidated pile; measurement of the angle relative to horizontal is the method. Correlation of this measured angle with powder flow in gravity discharge is empirical. Nonetheless, many companies chart and rank powders for “flowability” using this method. Tap test takes a cylinder of powder open at the top and filled to the brim. The sample is thumped up and down a fixed number of times to determine how much settling takes place in the cylinder. Comparison of the tapped volume to initial volume gives a compressibility ratio which is correlated with degree of difficulty in processing. In all of the above measurements, assessment of “flowability” is essentially a matter of conjecture.
    
Shear cells are well-known instruments that measure powder flow behavior using established scientific principles to determine powder failure strength (the capacity for powder particles to slide/move relative to one another with sufficient force, such as gravity) as a function of consolidating stress. The basic concept is that storage bins can have variable fill levels of powder. The self-weight of the powder in the bin presses down on powder particles in the hopper section. The higher the fill level in the bin, the greater the consolidating stress in the hopper. Compacting force due to the powder self-weight pushes powder particles in the hopper closer together, thereby creating increased powder strength with potential to build an arching bridge over the hopper opening. Shear cell data can be analyzed and used to predict the potential length of the arching bridge.
    
Figure 1a shows an annular shear cell containing a powder sample. Figure 2b shows the lids that attach to a compression plate on the instrument (1c) and are brought down into contact with the sample. One lid has small pockets and is referred to as the “vane lid”; the other has a flat surface made of a material similar to the wall of the hopper. The lid in either case presses down on the powder sample and applies a defined pressure to the powder particles. With the powder sample in this state of pressurized consolidation, the shear cell rotates on a turntable through a small angle. The lid is attached to a torque sensor and rotates with the cell at first. Powder particles at the intersection of lid with cell suddenly start to slide against one another after sufficient torque has been applied. This torque value where sliding commences is equated with the failure strength of the powder for that particular consolidating stress. This same test is then repeated at higher consolidating stresses to simulate increased fill levels in a bin.
    
Flow Functions for the three laundry detergents evaluated in the Brookfield Powder Lab are illustrated in Figure 2. In the graph x-axis is consolidating stress and y-axis is powder failure strength. Industry has agreed upon regions of flow behavior ranging from “Free Flowing” along the x-axis to “Non-flowing” along the y-axis. Product A is generally speaking a “cohesive” powder in contrast to Product B which is clearly on the border between “free flowing” and “easy flowing”. Product C lies between the other two. Relative ranking of the three detergents for gravity discharge based on visual observation of Flow Function graphs places Product A as most challenging and Product B as easiest.
    
Figure 3 shows the density curves for these same powders as a function of consolidating stress. Product A has the largest change in density across the range of consolidating stresses that are applied, increasing by over 150 kg/cu m from its loose fill condition at 525 kg/cu m. This is an increase of almost 30%. Product B changes from loose fill at 500 to 565 kg/cu m, which is a 13% increase. Change in density can sometimes be an indicator of potential flow issues. Powders that increase in density by 35% or more have problems during gravity discharge from the bin and almost always exhibit core flow behavior. This is a type of flow where the powder material on top cascades toward the center and down the middle while the material at lower levels waits to move until the fill level reduces to that location. See Figure 4. The “core” or “funnel” in the middle may be referred to as a “rathole” because, should flow stop before the bin empties, the channel down the middle of the bin from top to bottom remains vacant of powder. The area outside the rathole is filled with powder that has consolidated to a degree that movement of particles under gravity alone is not possible.
    
Potential arching dimensions, which are related to mass flow behavior, as shown in Figure 4, can be calculated for each powder from the Flow Function data. Figure 5 shows the graph that describes how the potential arching dimension will vary depending on fill level of powder in the bin. Relative ranking of powders is identical to the Flow Function graph. Product A will pose a challenge while Products B and C may be “free flowing” at best and “easy flowing” at worst.
    
Potential Rathole diameter appears to be more significant for all powders should core flow prevail. Calculations provide the following values for a bin that is 8 m high and 2 m in diameter:

Product A: 1522mm
Product B: 652mm
Product C: 1326mm

Both Product A and C appear to have more significant challenge for steady flow behavior in gravity discharge compared to Product B. One possible conclusion from the above data is that Product A will definitely move in core flow while Product C has good potential to exhibit mass flow. Product B flow behavior may not be predictable and therefore requires feedback from plant personnel as regards processing issues during gravity discharge.

Shear cell measurements provide a quantifiable way to benchmark formulations and perform QC checks on daily batches prior to processing. This can lead to predicting problems before they occur. A future article will address how shear cell data can be used to address hopper design and the choices available to the plant manager to enhance flow behavior with the existing equipment.
    
    Brookfield Engineering Laboratories has been a leading manufacturer of viscometers and rheometers for laboratory and on-line process control applications for more than 80 years. For more information, visit www.brookfieldengineering.com.

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