Achieving uninterrupted and controlled flow is a challenge that powder processing plants face often. The interaction between the powder particles and the vessel walls is determined by the particle properties and the nature of the wall surface.
Changing the powder characteristics once the product has been fixed is not a commercially viable option. Hence we need to exercise a lot of thought before choosing the construction material of the plant equipment. Wall friction angle (WFA) is the parameter by which we can determine the friction offered by the equipment to the powder particles.
Tools for Measuring Wall Friction Angle
Conventional techniques of measuring WFA involve shearing a powder bed against a disc of the potential material for construction.
Over the years, the technique has been improvised to provide more accurate and repeatable results, even for low friction materials. The powdered materials differ in their properties with their aerated counterpart; hence establishment of a steady baseline is vital to ensure repeatable results. Freeman Technology has come up with FT4, a versatile powder tester that helps achieve this steady baseline.
A normal vertical force is applied to force the sample through the material and the rotational torque is measured in order to measure WFA. The ability of FT4 to ptoduce reliable data is shown through the case studies on 15 different materials, as discussed in the subsequent sections.
In this case study, we investigated the WFA on 15 different construction materials using the Respitose ML001 powder, which has a broad particle distribution and exhibits low to medium cohesiveness and high compressibility. Figure 1 shows the powder characterization for Respitose ML001.
Figure 1. Powder characterization data for Respitose ML001 [For full details of measurement methodologies see reference 2]
For WFA testing, a pre-shear stress is applied on a compressed powder until steady condition is reached. After this, the rotation is stopped and the normal force is adjusted and the first measurement is noted down.
Rotation is resumed and the value of shear stress increases until the peak value is reached. Rotation is stopped again and the second measurement is taken by adjusting the value of normal stress. The procedure is repeated for various values of consolidating stress.
Figure 2 shows the shear stress versus normal stress. The data collected for all the 15 materials is shown in Table 1.
Figure 2. Raw data from WFA measurement showing the initial build-up of shear stress to a steady-state condition and the data collected during the following tests at reduced normal stresses
Table 1. WFA data for Respitose ML001 and different materials of construction/surface finishes
*SS = Stainless Steel
The observation from the values in the table is that the surfaces that offer low resistance exhibit extremely good repeatability for small values of WFA.
Chromium-Nickel-plated 316 L stainless steel offers the maximum resistance while milled polyoxymethylene copolymer offered least resistance. Figure 3 gives the shear stress versus normal stress plot for data obtained from testing stainless steel
Figure 3. Shear stress vs. normal stress data for stainless steel showing the impact of surface roughness on the level of friction between the powder and metal [Surface friction is quantified by RA, a higher RA value equating to a rougher surface]
The WFA values for stainless steel surface increased proportionally with the roughness, except for electro polished stainless steel. Therefore, stainless steel, which is electro polished, does not offer resistance to Respitose ML001.
The case study also revealed another important factor that surface roughness alone does not determine WFA; other properties of the wall material also influence WFA.
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Designing the Hopper based on WFA values
Measurement of WFA is done mainly to assist the designing of hoppers, the other parameters that determine hopper design are half angle, the angle that the hopper makes with the vertical, and the outlet opening. Figure 4 shows the WFA values for Respiritose ML001, using software package designed by Freeman Technology.
Figure 4. The impact of WFA on hopper half angle and outlet size for a uniaxial hopper operating in a mass flow regime
This method works with the largest value of half angle by maintaining constant mass flow which sets the entire powder bed moving. The test results reveal that complete mass flow is possible when the friction between powder and the wall surface reduces. Therefore, a low value of WFA implies that the hopper can be designed with larger hopper half angle.
Considering the outlet size, it is large for materials that offer low resistance. The size of the outlet is decided based on the force balance situation of flow/no-flow formed by the stable powder arch at the outlet of the hopper.
In conclusion, many factors affect the selection of the best material for construction. A balance should be struck between cost and performance.
Accurate measurement of the friction between surface and powder is key as it influences controlled and smooth flow through the plant. The above case study and the data obtained are mainly useful in designing the hopper and also to do a comparative study of the advantages and disadvantages offered by various materials.
This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.
For more information on this source, please visit Freeman Technology.