Highlighting reproducibility & Comparison vs Flodex using GranuFlow

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Highlighting reproducibility & Comparison vs Flodex

Highlighting reproducibility & Comparison vs Flodex nomenclature


Theoretical Framework

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.

Experimental setup


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.

figure 1 FlowLac 100, SEM Picture and particle size distribution (manufacturer data)

Experimental protocol


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

Experimental results

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:

figure 2 Mass flowrate versus aperture size - comparison between GranuFlow and 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.

Flodex issues

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.

figure 3 Beaker photography after experiments with Flodex - Highlighting the electrostatic effect

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).


Cascade of granular flows for characterizing segregation, G. Lumay, F. Boschin, R. Cloots, N. Vandewalle, Powder Technology 234, 32-36 (2013).

Combined effect of moisture and electrostatic charges on powder flow, A. Rescaglio, J. Schockmel, N. Vandewalle and G. Lumay, EPJ Web of Conferences 140, 13009 (2017).

Compaction dynamics of a magnetized powder, G. Lumay, S. Dorbolo and N. Vandewalle, Physical Review E 80, 041302 (2009).

Compaction of anisotropic granular materials: Experiments and simulations, G. Lumay and N. Vandewalle, Physical Review E 70, 051314 (2004).

Compaction Dynamics of Wet Granular Assemblies, J. E. Fiscina, G. Lumay, F. Ludewig and N. Vandewalle, Physical Review Letters 105, 048001 (2010).

Effect of an electric field on an intermittent granular flow, E. Mersch, G. Lumay, F. Boschini, and N. Vandewalle, Physical Review E 81, 041309 (2010).

Effect of relative air humidity on the flowability of lactose powders, G. Lumay, K. Traina, F. Boschini, V. Delaval, A. Rescaglio, R. Cloots and N. Vandewalle, Journal of Drug Delivery Science and Technology 35, 207-212 (2016).

Experimental Study of Granular Compaction Dynamics at Different Scales: Grain Mobility, Hexagonal Domains, and Packing Fraction, G. Lumay and N. Vandewalle, Physical Review Letters 95, 028002 (2005).

Flow abilities of powders and granular materials evidenced from dynamical tap density measurement, K. Traina, R. Cloots, S. Bontempi, G. Lumay, N. Vandewalle and F. Boschini, Powder Technology, 235, 842-852 (2013).

Flow of magnetized grains in a rotating drum, G. Lumay and N. Vandewalle, Physical Review E 82, 040301(R) (2010).

How tribo-electric charges modify powder flowability, A. Rescaglio, J. Schockmel, F. Francqui, N. Vandewalle, and G. Lumay, Annual Transactions of The Nordic Rheology Society 25, 17-21 (2016).

Influence of cohesives forces on the macroscopic properties of granular assemblies, G. Lumay, J. Fiscina, F. Ludewig and N. Vandewalle, AIP Conference Proceedings 1542, 995 (2013).

Linking compaction dynamics to the flow properties of powders, G. Lumay, N. Vandewalle, C. Bodson, L. Delattre and O. Gerasimov, Applied Physics Letters 89, 093505 (2006).

Linking flowability and granulometry of lactose powders, F. Boschini, V. Delaval, K. Traina, N. Vandewalle, and G. Lumay, International Journal of Pharmaceutics 494, 312–320 (2015).

Measuring the flowing properties of powders and grains, G. Lumay, F. Boschini, K. Traina, S. Bontempi, J.-C. Remy, R. Cloots, and N. Vandewall, Powder Technology 224, 19-27 (2012).

Motion of carbon nanotubes in a rotating drum: The dynamic angle of repose and a bed behavior diagram, S. L. Pirard, G. Lumay, N. Vandewalle, J-P. Pirard, Chemical Engineering Journal 146, 143-147 (2009).

Mullite coatings on ceramic substrates: Stabilisation of Al2O3–SiO2 suspensions for spray drying of composite granules suitable for reactive plasma spraying, A. Schrijnemakers, S. André, G. Lumay, N. Vandewalle, F. Boschini, R. Cloots and B. Vertruyen, Journal of the European Ceramic Society 29, 2169–2175 (2009).

Rheological behavior of β-Ti and NiTi powders produced by atomization for SLM production of open porous orthopedic implants, G. Yablokova, M. Speirs, J. Van Humbeeck, J.-P. Kruth, J. Schrooten, R. Cloots, F. Boschini, G. Lumay, J. Luyten, Powder Technology 283, 199–209 (2015).

The flow rate of granular materials through an orifice, C. Mankoc, A. Janda, R. Arévalo, J. M. Pastor, I. Zuriguel, A. Garcimartín and D. Maza, Granular Matter 9, p407–414 (2007).

The influence of grain shape, friction and cohesion on granular compaction dynamics, N. Vandewalle, G. Lumay, O. Gerasimov and F. Ludewig, The European Physical Journal E (2007).

Appendix one for Highlighting reproducibility & Comparison vs Flodex app note

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