Many products from the pharmaceutical, food or chemical industry are sold in the form of powders.
These powders are susceptible to caking, which affects their usage and quality. In most of the cases, caking of powders is caused by absorption of moisture from the environment.
By using powder rheometers, we can study the caking rate in powders and find out the factors that contribute to caking at various stages.
Causes of Powder Caking
Environmental factors that cause caking of powders are temperature and humidity; changes in the former cause condensation of powder particles while changes in the latter causes dissolution of the powder.
Also, consolidation of powders during the testing process can cause mechanical caking of the powder. The powders need to be subjected to thorough testing in order to study the onset and progression of caking. Modern powder testing methods, such as the rheometer, are easy to operate and provide quick and useful information pertaining to powder characteristics.
The data obtained speaks about the impact of consolidation on the rate of caking, which can be useful in deciding upon the most appropriate manufacturing and storage conditions.
Rheometers are very useful in determining the dynamic properties of a sample and also enable direct measurement of the powder’s flowability.
The most important baseline characteristic is the basic flow energy (BFE); this is the amount of energy that is needed to rotate a blade that is immersed in the powder at a specific rotational and vertical speed.
Figure 1 represents the axial and rotational forces acting on a blade.
Figure 1. Measuring BFE with a powder rheometer
The value of BFE can be measured by automated test methods and is the most important parameter that gives a good idea of the cohesive properties of the powder sample.
Caking of powders influences the value of BFE greatly for the reasons described below. The inter-particle bonding increases with caking; therefore more energy is needed to rotate the blade. if the caked sample is stiff, it will offer more resistance to movement.
Caking due to moisture absorption and increases the bulk density of the sample, which results in higher values of BFE.
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Effect of Consolidation on Caking
In order to study the effect of consolidation on caking, a few samples were subjected to compacting stress of 9kPa, while an equal number of samples were left without consolidation. The test results of all the samples with the corresponding BFE values are shown as a function of time in Figure 2.
Figure 2. Investigating the impact of consolidation on caking by tracking changes in BFE as a function of time
It was observed that for both samples BFE increased initially for a period of four days. After four days the consolidated sample showed a prominent increase in BFE.
Five and a half days later, the resistance shown by the consolidated sample was double that of the initial value. On the other hand, the unconsolidated powder offered this amount of resistance only after eight days. The value of BFE, however, steadily increased for both the samples.
Thus, it can be concluded that it is better to store the powder after subjecting to low stress and restricting the storage period to four days.
Practical Aspects to Controlling Caking
Practical solutions to control caking are discussed below. One way is to run the bin with a low level of powder and topping up often with small volumes. This technique reduces the pressure acting on the material in the hopper and also minimises the amount of time the powder is kept in the bin.
The time the powder spends in the bin depends on the flow in the bin; for powders running in a mass flow it reduces uniformly but that is not the case for funnel flow. Mass flow is characterised by all the powder particles moving in the bin and the powder is taken out of the bin in the first in, first out order.
A hopper having steeper walls than the limiting value would result in mass flow. When such a hopper is not used, funnelling will occur and the residence time of the powder becomes non-uniform. Figure 3 depicts both the flows.
Figure 3. Comparing powder transit through a hopper under A) mass flow and B) funnel flow conditions
Caking is therefore influenced by the flow in the bin and also by static storage of the powder.
From the above discussion, it is clear how caking affects the performance of the powder and brings down its quality by a great extent.
Using effective testing methods to determine the BFE, which gives an idea about the extent of caking and effect of consolidation, chemical and process engineers can make better decisions on the right manufacturing and storage methods.
This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.
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