Application Note using:
Highlighting reproducibility & Comparison vs Flodex
Granular materials and fine powders are widely used in industrial applications. To control and to optimize processing methods, these materials have to be precisely characterized. The characterization methods are related either to the properties of the grains (granulometry, morphology, chemical composition, …) and to the behaviour of the bulk powder (flowability, density, blend stability, electrostatic properties, …).
However, concerning the physical behaviour of bulk powder, most of the techniques used in R&D or quality control laboratories are based on old measurement techniques.
During the last decade, we have updated these techniques to meet the present requirements of R&D laboratories and production departments. In particular, the measurement processes have been automatized and rigorous initialization methods have been developed to obtain reproducible and interpretable results.
Moreover, the use of image analysis techniques improves the measurements precision.
A range of measurement methods has been developed to cover all the needs of industries processing powders and granular materials.
However, in this application note, we will be focused on the GranuFlow instrument.
The GranuFlow is an improved laboratory silo compared to the ancient Hall Flow Meter (ASTM B213, ISO4490) and compared to the “Flow Through An Orifice” method described in the Pharmacopeia (USP1174).
The GranuFlow is a straightforward powder flowability measurement device composed of a silo with different apertures associated with a dedicated electronic balance to measure the flowrate. This flowrate is computed automatically from the slope of the mass temporal evolution measured with the balance. The aperture size is modified quickly and easily with an original rotating system. The measurement and the result analysis are assisted by software. The flowrate is measured for a set of aperture sizes to obtain a flow curve.
Finally, the whole flow curve is fitted with the well-known Beverloo theoretical model to obtain a flowability index (Cb, related to the powder flowability) and the minimum aperture size to obtain a flow (Dmin) (for theoretical background, user can refer to Appendix 1).
The whole measurement is performed easily, fastly and precisely.
In this paper, we used a complete set of hole diameters: 4, 6, 8, 10, 12, 14mm and 16mm.
The main purpose of this application note is to provide information about the measurements reproducibility with the GranuFlow and to show some examples about what is it able to offer. In a second part, a comparison between Hall Flowmeter and the GranuFlow is presented in order to show the advantage of using the GranuFlow.
The product FlowLac 100 provided by Meggle Pharma is used in this application note. It is produced by spray-drying a suspension of fine milled alpha-lactose monohydrate crystals in a solution of lactose. When lactose in solution is spray-dried, a rapid removal of water is taking place, whereby amorphous, non-crystalline lactose is formed in addition to crystalline lactose.
Due to the spray-drying process, this powder has a spherical shape, consisting of small alpha-lactose monohydrate crystals bound by amorphous lactose.
The GranuFlow analysis were performed at 20.6°C and 34.6%RH. Mass Flowrate was investigated for different hole size (from 4mm to 16mm). Measurements were repeated three times.
F is the powder flowrate (in g/s) and Cb the Beverloo parameter (in g/cm3). Dmin is the minimum aperture size to obtain a flow (for more information about the Beverloo model, please refer to Appendix 1).
5 min are needed to run one complete measurements (with every hole size, cleaning and with Beverloo’s Law calculation).
Flodex analysis were performed at 21.2°C and 34.3%RH.
The Mass flowrate was measured for the same aperture size than those used with the GranuFlow (from 4 to 16mm). Measurements were repeated two times.
30 min are needed to run all measurements (with every hole size, cleaning, but without plotting the Beverloo’s Law).
GranuFlow versus Flodex
The following figure allows comparison between GranuFlow and Flodex.
All the error bars are calculated using the standard deviation obtained for reproducibility measurements (S is the average sum of squared residuals, calculated with the experimental and Beverloo mass flowrates).
The flowability of FlowLac 100 powder was investigated three times with the GranuFlow and two times with the Flodex:
The first observation is related to the ease of use of the GranuFlow in comparison with Flodex. Indeed, many time is wasted to change Flodex’s disks and to clean all the workplan between two experiments.
Moreover, Flodex instrument does not allow the Beverloo law determination (calculations were done after experiment using the excel software).
Regarding the average sum of squared residuals, it is possible to conclude that the Beverloo law regression is more accurate with the GranuFlow (S = 2.70g²/s²) than the Flodex instrument (S = 9.99g²/s²).
If we consider the error bars (especially with an aperture of 16mm), we can see that the reproducibility is better with the GranuFlow than Flodex. This fact is explained by the complete automatic procedure for the GranuFlow, while the time measurement is achieved manually (chronometer) with the Flodex instrument.
Finally, the GranuFlow and Flodex result are slightly different, some issues with the Flodex instrument may explain this fact: powder aeration/electrostatic charges during measurement and porous medium height dependency.
Triboelectricity and powder aeration
For the Flodex experiment, the powder is used to analyse mass flowrate versus aperture size.
However, despite this protocol allows to use a small powder quantity, it also leads to electrical charges build up inside the powder (cf. Figure 3).
Therefore, at the end of experiment the powder mass flowrate will be erratic.
Moreover, using the same powder will aerate it, and therefore, it will modify the powder flowing behaviour.
Powder height dependency
Contrary to the fluids, when a silo is discharged by gravity, the flow rate does not depend on the height of the granular layer.
Indeed, when this value is greater than 1.2 times the diameter of the silo, the pressure at the bottom of the silo saturates due to the Janssen effect and hence, the flow rate remains constant (Mankoc et al., 2007).
However, due to the small height of the Flodex instrument (7.5cm), the powder height dependency is still observed at the end of its tank discharge.
Thus, this instrument will be only useful to have an idea about the minimum aperture for the powder to flow.
An experiment with the GranuFlow is extremely faster than with the Flodex (5 min with the GranuFlow and 30 min with the Flodex).
The GranuFlow allows to plot the full Beverloo mass flow rate curve, while the Flodex only allows experimental data measurements.
The GranuFlow provides powder flow ability measurements with Beverloo Law (i.e. Cb coefficient, with an error close to 2.4%) and an estimation of the Cohesive Index with Dmin parameter (minimum diameter for the powder to flow in silo configuration).
However, the Flodex provides powder flow ability with a slightly worse accuracy (3.1%) and no information about the Beverloo law is given (the calculation needs to be carried out with Excel).
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