Pharmaceutical
Lactose Powders Flowability Classification using Granuflow
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...
Introduction
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.
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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.
GranuFlow
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.
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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). 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, 14 and 16mm. The main purpose of this application note is to provide information regarding lactose analysis for the Pharmaceutical field.
Lactose Analysis
Material
The powders used in this application are provided by Meggle Pharma. All these samples are made of lactose. They are called by the manufacturer Tablettose 70, Tablettose 80, Flowlac 90 and Flowlac 100. According to supplier’s data, the physico-chemical properties of these powders are described by the following table:
| Bulk Density (g/l) | Tapped Density (g/l) | Hausner Ratio | |
|---|---|---|---|
| Tablettose 70 | 530 | 640 | 1.21 |
| Tablettose 80 | 620 | 770 | 1.24 |
| Flowlac 90 | 560 | 670 | 1.20 |
| Flowlac 100 | 590 | 710 | 1.20 |
Table 1: Lactose psycho-chemical properties
SEM pictures have been made in order to have an information of the particle size distribution and particles shape:
The first observation concerns the particles shape, indeed, all Flowlac samples have spherical shape, while Tablettose samples have irregular one. Then, with the help of ImageJ Software, the granulometric analysis of the four samples have been carried out (dpp is the mean primary particle diameter and σ the standard deviation):
| dpp (µm) | σ (µm) | |
|---|---|---|
| Tablettose 70 | 90.95 | 75.61 |
| Tablettose 80 | 39.17 | 48.80 |
| Flowlac 90 | 42.55 | 50.72 |
| Flowlac 100 | 47.28 | 33.71 |
Table 2: Lactose granulometric analysis
GranuFlow Analysis
The GranuFlow analysis were performed at 26°C and 40.0% RH (w = 8.5gH20/kgDryAir). The Mass Flowrate was investigated for different hole size (from 4mm to 16mm). F is the powder flowrate (in g/s) and Cb the Beverloo parameter (in g/mm3). Dmin is the minimum aperture size to obtain a flow:
| Powder Flowrate F (g/s) | ||||||||
|---|---|---|---|---|---|---|---|---|
| D (mm) | Flowlac 90 | Beverloo | Flowlac 100 | Beverloo | Tablettose 70 | Beverloo | Tablettose 80 | Beverloo |
| 4 | 0.78 | 0.89 | 0.73 | 0.73 | 0.68 | 0.68 | 0.54 | 0.41 |
| 6 | 2.34 | 2.45 | 2.08 | 2.01 | 2.01 | 2.06 | 1.47 | 1.35 |
| 8 | 4.92 | 5.02 | 4.30 | 4.13 | 4.47 | 4.42 | 3.23 | 3.00 |
| 10 | 9.21 | 8.77 | 7.75 | 7.21 | 8.26 | 7.92 | 5.68 | 5.49 |
| 12 | 14.34 | 13.84 | 12.13 | 11.37 | 13.08 | 12.71 | 8.74 | 8.93 |
| 14 | 20.52 | 20.35 | 16.51 | 16.71 | 18.92 | 18.92 | 13.02 | 13.42 |
| 16 | 27.07 | 28.41 | 23.53 | 23.34 | 27.21 | 26.67 | 19.97 | 19.05 |
| Cb (g/mm3) | 2.80E-04 | 2.30E-04 | 2.80E-04 | 2.10E-04 | ||||
| Dmin (mm) | 0 | 0 | 0.4 | 0.7 | ||||
Table 3: Raw data obtained with the GranuFlow instrument for the four lactose samples
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These results are really interesting, indeed by the look of Hausner ratio (cf. Table 1), we can see that the classical tap density test (“Densitap”) is unable to make differentiation between one sample to another (despite the high heterogeneity in terms of samples physico-chemical composition).
However, the GranuFlow allows to its user to make powder classification with great accuracy (with the help of Cb and Dmin parameters). Although Flowlac 90 and Tablettose70 have the same Cb parameter, Dmin information allows us to affirm that Flowlac90 has the best flowability from all samples and its followed by Tablettose70. Flowlac100 comes in third position, while Tablettose80 has the lower flowability.
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To prove these assumptions the following figure shows the mass flowrate according to hole diameter:
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Figure 5: Mass Flowrate versus aperture size for all lactose samples. Lines represent the Beverloo Law
This graph shows the good correlation between experimental data and modeled values (with Beverloo law).
This fact is highly important, because with the Beverloo model, user can make data interpolation, and thus predicts the mass flowrate for different hole sizes.
Conclusions
- The GranuFlow allows to plot the full mass flowrate curve
- The GranuFlow gives information about the Beverloo law (i.e powder flowability and minimum diameter for the powder to flow in silo configuration).
- The GranuFlow allows to classify powders in terms of flowability, even if the classical tap density test is unable to see the Hausner ratio difference.
References
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