COMPRESSION AND FLOW BEHAVIOUR OF COHESIVE POWDERS

Mateusz Stasiak¹, Jürgen Tomas², Marek Molenda¹, Peter Müller^2
1 Department of Physical and Technological Properties of Agricultural Materials, Institute of Agrophysics, Polish Academy of Science, Lublin, Poland
² Department of Mechanical Process Engineering, Otto-von-Guericke University, Magdeburg, Germany

ABSTRACT

Research was conducted to examine the influences of consolidation time, consolidation pressure, temperature, addition of lubricant, speed of deformation and dimension of the probe on properties of two powders – microcrystalline cellulose (MCC) and potato starch (PS). A low pressure range (up to 10 kPa) was applied in direct shear test to determine flowability and friction properties. A medium pressure range (50–1000 kPa) was applied in the press shear cell to evaluate friction properties of MCC. The compression behaviour of MCC was determined in uniaxial compression tests at high pressure range (30–60 MPa).
A strong influence of consolidation time on flow function was observed. In the case of PS a strong slip stick effect was also noted. An increase in temperature from 23°C to 40°C resulted in a 10% increase of the internal friction angle value determined in the press shear cell in the range of preshear displacement from 0.1 to 2 m. There were no strong influences of temperature, speed of deformation and initial sample height on compression behaviour determined in uniaxial compression tests at high pressure range.

Key words: powder, flowability, compressibility, slip-stick effect.

NOMENCLATURE

D – powder probe diameter, mm
FF – flow function
h – powder probe height, mm
i – flow index (σ_c/σ₁)
p – consolidation pressure, MPa
s – preshear displacement, m
T – powder bed temperature, °C
ν – shear rate, mm·s^-1
φ – angle of internal friction, deg
ρ – agglomerate density, kg·m^-3
τ – shear stress, kPa
τ_A – amplitude of shear stress oscillations, kPa
σ₁ – major consolidation stress, kPa
σ_c – unconfined yield strength, kPa
σ_r – consolidation stress, kPa

INTRODUCTION

Handling of powders still remains as one of the least understood areas associated with solid processing plants. Predictable processing, increase of quality and reduction of losses of products are still the main issues [1,5,7]. Process design and optimization determine the properties and their quality. With increasing scale of industrial operations, the design of reliable processes and efficient equipment requires more precise information about physical product properties and how to change them at different process conditions [8]. There is a strong need to determine the compression and flow behaviour of cohesive powders.

The objective of this project was to examine the influence of consolidation time, consolidation pressure, temperature, addition of lubricant, speed of deformation and dimension of the probe on properties of two powders – microcrystalline cellulose (MCC) and potato starch (PS) on flow and compression behaviour. Experiments were effectuated in the frame of cooperation between Department of Mechanical Process Engineering at Otto-von-Guericke University in Magdeburg and Department of Physical and Technological Properties of Agricultural Materials at Institute of Agrophysics Polish Academy of Science in Lublin.

MATERIALS AND METHODS

Research was performed for microcrystalline cellulose (MCC) and potato starch (PS). Particle size distributions and microscope pictures of experimental materials are presented on the Fig. 1. The size of single particle for MCC is ten times higher than for PS. The shape of PS is globular with smooth surface during MCC is fibrous with rough surface. Tests were performed in three ranges of applied loads. Low pressure range (up to 10 kPa) was applied in a direct shear test to determine flowability and friction properties [5,7,10]. Medium pressure range (50–1000 kPa) was applied in press shear cell to evaluate friction properties of MCC according to [2]. Compression behavior of MCC was determined in uniaxial compression tests at high pressure range (30–60 MPa).

Fig. 1. Particle size distributions and microscope pictures of experimental materials

The parameters were determined under loads corresponding to those encountered in practical operation regime. Low pressure range experiments were performed in a direct shear tester 60 mm in diameter. Tests were performed following the standard procedure with consolidation reference stresses σ_r of 4, 6, and 10 kPa and shear rate v of 0.033 mm·s^-1 [4,9].

Medium pressure tests were performed in the press shear cell [6]. The experiments were conducted under average 23°C and 40°C powder temperature. In order to ensure this temperature, special equipment was installed in the test instrument [2] (Fig. 2).

Fig. 2. Experimental setup and method of determination powders parameters in press shear cell

High pressure tests were performed in a hydraulic press machine, using a cylindrical probe compressed by piston [3]. The cylindrical probe was 40 mm in diameter. Tests were performed for two different temperatures of powder (20°C and 40°C), for two values of pressure (30 and 60 MPa). Bulk densities were determined for three different deformation speeds in a range from 4.2 to 8.74 mm·s^-1 and for three heights of the probe (h = 20, 40 and 60 mm). Low pressure range tests were fulfilled in IA PAS Lublin, medium and high pressure tests in OvGU in Magdeburg.

RESULTS

Low pressure range. Experimental results from direct shear tests demonstrated strong influence of change in time of consolidation for experimental powders. The values of the maximum shear stresses τ after two hours of consolidation – 2h were higher from values obtained for samples sheared without time consolidation – 0h. Values of unconfined yield strength are higher for MCC powder.

Values of τ increased with an increase in consolidation stress from approximately 2 to 6 kPa. Also for MCC the maximal shear stress were higher for 2h of consolidation. For potato starch the highest increase in τ with time of consolidation reaching 100% was obtained for the lowest value of consolidation pressure. For other consolidation stress values of τ increased in a range from 15 to 20% with increase of time of consolidation.

For PS values of flow index i (i = σ_c/σ₁) were found characteristic for easy flowing and cohesive powders (Fig. 3). Values of i for MCC were characteristic for cohesive and strong cohesive powders. Flow functions FF were found increasing with an increase in time of consolidation. 2 hours consolidation resulted in 6% up to 50% increased FF values. This is probably the result of rough surface and nonregular shape of single particles. The reason for the influence of the consolidation time on values of FF could be the inelastic deformation behaviour of experimental powders.

Fig. 3. Effect of storage time on flow functions FF of potato starch PS and microcrystalline cellulose MCC (immediately shearing 0h and shearing after two hours of consolidation time 2h); middle points denotes mean values and horizontal and vertical bars standard deviations

In the case of PS a strong slip stick effect was noted. Oscillations of experimental curves of τ against s are presented on Fig. 4. Oscillations were observed for σ_r of 6 and 10 kPa. Amplitudes of oscillations τ_A for initial material were 1.8 kPa at σ_r of 6 kPa and 3.7 kPa at σ_r of 10 kPa. With an increase in addition of talcum powder the amplitude of oscillations τ_A decreased and for 6% addition were 0.53 kPa at σ_r of 6 kPa and 1.47 kPa at σ_r of 10 kPa. In case of 8 and 10% addition no oscillations of experimental curves were observed. Single particles of talcum powder used in experiments as a lubricant are much more smaller than PS particles that is why PS particle is covered by talcum powder particles and this makes flow and moving between particles easier.

Fig. 4. Influence of addition of talcum powder on experimental curves τ = f(Δl) and on amplitude of oscillations τA for PS with different additions of talcum powder

Addition of 10% of talcum powder resulted in a reduced slip stick effect, but at the same time approximately 50% decrease in FF values at 10 kPa of consolidation stress was obtained. Values of FF for PS were characteristic for free and easy flowing materials.

Medium pressure range. Tests were determined for MCC in a press shear cell for speed of deformation 4.2 mm·s^-1 and for preshear displacement from 0.1 to 2 m. There is a strong influence of the powder bed temperature on values of the internal friction angle φ (Fig. 5). No significant influence of preshear displacement on φ was observed.

Fig. 5. Influence of powder temperature on angle of internal friction φ with vertical bars denotes standard deviations for three preshear displacements

An increase in temperature of MCC from 23°C to 40°C resulted in a significant increase of φ. The highest increase from 36° to 41° was obtained for 1 m of preshear displacement whereas the lowest for the minimal value of preshear displacement.

High pressure range. Experiments for high pressure range were performed for MCC, a material used in production of tablets.

Fig. 6. Bulk densities of MCC with standard deviations for 30 and 60 MPa of consolidation pressure

An increase in pressure from 30 to 60 MPa resulted in 20% increase of bulk density ρ. The value of ρ ranged from 830 kg·m^-3 for 30 MPa of consolidation pressure up to 1030 kg·m^-3 for 60 MPa (Fig. 6). There were no strong influences of temperature, deformation speed and initial sample height on ρ. Probably reason for such behaviour is the range of deformation which reached 60%.

CONCLUSIONS

1. Values of flow functions obtained for microcrystalline cellulose and potato starch were found increasing with an increase in consolidation time from 0 to 2 hours.

2. A slip stick effect was noted for potato starch. An increase of talcum powder addition resulted in a decrease of the slip stick effect and in flow function values.

3. Increasing temperature from 23°C to 40°C resulted in increase of the internal friction angle determined in press shear cell.

4. There were no strong influences of temperature, speed of deformation and initial sample height on compression behaviour determined in uniaxial compression tests in high pressure range.

REFERENCES

1. Bell A., Ennis B.J., Grygo R.J., Scholten W.J.F., Schenkel M.M., 1994. Practical evaluation of the Johanson hang-up indicizer. Bulk Solids Handling 14(1), 117–125.

2. Grossmann L., Tomas J., 2006. Flow properties of cohesive powders tested by press shear cell. Partic. Sci. Technol. 24, 353–367.

3. Grossmann L., Tomas J., Csöke B., 2004. Compressibility and flow properties of a cohesive limestone powder in a medium pressure range. Granular Matter 6, 103–109.

4. Molenda M., Stasiak M., Moya M., Ramirez A., Horabik J., Ayuga F., 2006. Testing mechanical properties of food powders in two laboratories – degree of consistency of results. International Agrophysics, 20(1), 37–45.

5. Molerus O., 1975. Theory of yield of cohesive powders. Powder Technology 12, 259–275.

6. Reichmann B., Tomas J., 2001. Expression behaviour of fine particle suspension and the consolidated cake strenght. Powder Technology 121(2–3), 182–189.

7. Schwedes J., 2002. Consolidation and flow of cohesive bulk solids. Chem. Eng. Sci. 57(2), 287–294.

8. Stasiak M., Molenda M., Horabik J., 2007. Determination of modulus of elasticity of cereals and rapeseeds using acoustic method. J. Food Eng. 82, 51–57.

9. Stasiak M., Molenda M., 2004. Direct shear testing of flowability of food powders. Res. Agric. Eng. 50, 6–10.

10. Standard shear testing technique for particulate solids using the Jenike Shear Cell, Institution of Chemical Engineers. 1989.

Accepted for print: 14.05.2009

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Mateusz Stasiak
Department of Physical and Technological Properties
of Agricultural Materials, Institute of Agrophysics,
Polish Academy of Science, Lublin, Poland
Doświadczalna 4,
P.O. Box 201, 20-290 Lublin 27, Poland
Phone: (+48) 81 743 85 58
Fax: (+48) 81 744 50 67
email: mstasiak@ipan.lublin.pl

Jürgen Tomas
Department of Mechanical Process Engineering,
Otto-von-Guericke University, Magdeburg, Germany


Marek Molenda
Department of Physical and Technological Properties
of Agricultural Materials, Institute of Agrophysics,
Polish Academy of Science, Lublin, Poland
Doświadczalna 4,
P.O. Box 201, 20-290 Lublin 27, Poland

Peter Müller
Department of Mechanical Process Engineering,
Otto-von-Guericke University, Magdeburg, Germany

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