INFLUENCE OF TECHNOLOGICAL PARAMETERS ON COMPRESSION AND FLOW BEHAVIOUR OF COHESIVE POWDERS

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1 INFLUENCE OF TECHNOLOGICAL PARAMETERS ON COMPRESSION AND FLOW BEHAVIOUR OF COHESIVE POWDERS M. Stasiak 1, J. Tomas 1, M. Molenda, L. Grossmann 1 and P. Müller 1 1. Department of Mechanical Process Engineering, Otto-von-Guericke University, Magdeburg, GERMANY. Department of Physical and Technological Properties of Agricultural Materials Institute of Agrophysics, Polish Academy of Science, Lublin, POLAND. Abstract Research was conducted to examine the influences of some technological factors on properties of two powders - microcrystalline cellulose (MCC) and potato starch (PS). A low pressure range (up to 1 kpa) was applied in direct shear test to determine flowability and friction properties. A medium pressure range (5-1 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 (3-6 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 3ºC to ºC resulted in a 1% increase of the internal friction angle value determined in the press shear cell in the range of preshear displacement from.1 to 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. 1. 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]. 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 []. 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 some technological factors on flow and compression behaviour of two cohesive powders commonly used in industry. 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). Tests were performed in three ranges of applied loads. Low pressure range (up to 1 kpa) was applied in a direct shear test to determine flowability and friction properties [3]. Medium pressure range (5-1 kpa) was applied in press shear cell to evaluate friction properties of MCC. Compression behaviour of MCC was determined in uniaxial compression tests at high pressure range (3-6 MPa).

2 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 6 mm in diameter. Tests were performed following the standard procedure with consolidation reference stresses σ r of, 6, and 1 kpa and shear rate v of.33 mm s -1 [,5]. Medium pressure tests were performed in the press shear cell [6. The experiments were conducted under average 3 C and C powder temperature. In order to ensure this temperature, special equipment was installed in the test instrument [7]. High pressure tests were performed in a hydraulic press machine, using a cylindrical probe compressed by piston [8]. The cylindrical probe was mm in diameter. Tests were performed for two different temperatures of powder ( C and C), for two values of pressure (3 and 6 MPa). Densities of the agglomerates were determined for three different deformation speeds in a range from. to 8.7 mm s -1 and for three heights of the probe (h =, and 6 mm). Low pressure range tests were fulfilled in IA PAS Lublin, medium and high pressure tests in OvGU in Magdeburg. 3. RESULTS 3.1 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 - h were higher from values obtained for samples sheared without time consolidation - h. Values of τ increased with an increase in consolidation stress from approximately to 6 kpa. Also for MCC the maximal shear stress were higher for h of consolidation. For potato starch the highest increase in τ with time of consolidation reaching 1% was obtained for the lowest value of consolidation pressure. For other consolidation stress values of τ increased in a range from 15 to % with increase of time of consolidation. For PS values of flow index i (i=σ c /σ 1 ) were found characteristic for easy flowing and cohesive powders (Fig. 1). Values of i for MCC were characteristic for cohesive and strong cohesive powders. Flow functons FF were found increasing with an increase in time of consolidation. hours consolidation resulted in 6% up to 5% increased FF values. Unconfined yield strenght σc in kpa PS h PS i= Major consolidation stress σ 1 in kpa i=.5 i=.1 3

3 Unconfined yield strenght s c in kpa MCC h MCC Major consolidation stress σ 1 in kpa Fig. 1: Effect of storage time on low functions FF of potato starch PS and microcrystalline cellulose MCC (immediately shearing h and shearing after two hours of consolidation time h). In the case of PS a strong slip stick effect was noted. Oscillations of experimental curves of τ against s are presented on figure. Oscillations were observed for σ r of 6 and 1 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 1 kpa. With an increase in addition of talcum powder the amplitude of oscillations τ A decreased and for 6% addition were.53 kpa at σ r of 6 kpa and 1.7 kpa at σ r of 1 kpa. In case of 8 and 1% addition no oscillations of experimental curves were observed. 1 i=.5 i=.5 i=.1 Shear stress in kpa 8 6 σ r =1 kpa V= mm/min potato starch potato starch + Talcum powder % % 6% 8% 1% Preshear displacement s in mm

4 Amplitude of oscillations τ A in kpa σ r =6 kpa σ r =1 kpa R =.917; r = -.95; α =.1; τ A = *addition of Talcum powder R =.9; r = -.96; α =.1; τ A = *addition of Talcum powder Talcum powder addition in % POTATO STARCH Fig. : Influence of addition of talcum powder on experimental curves τ=f(δl) and on amplitude of oscillations τ A for PS with different additions of talcum powder. Addition of 1% of talcum powder resulted in a reduced slip stick effect, but at the same time approximately 5% decrease in FF values at 1 kpa of consolidation stress was obtained. Values of FF for PS were characteristic for free and easy flowing materials. 3.1 Medium pressure range Tests were determined for MCC in a press shear cell for speed of deformation. mm s -1 and for preshear displacement from.1 to m. There is a strong influence of the powder bed temperature on values of the internal friction angle φ (Fig. 3). No significant influence of preshear displacement on φ was observed. 55 Angle of internal friction φ in deg T=º T=3º Preshear displacement s in m Fig. 3: Influence of powder temperature on angle of internal friction φ for three preshear displacements

5 An increase in temperature of MCC from 3ºC to ºC resulted in an increase of φ. The highest increase from 36º to 1 º was obtained for 1 m of preshear displacement whereas the lowest for the minimal value of preshear displacement. 3.1 High pressure range Experiments for high pressure range were performed for MCC, a material used in production of tablets. Density of agglomerates ρ in kg/m T = º C v k =. m/s h = mm Pressure p in MPa Fig. : Density of MCC agglomerates for 3 and 6 MPa of consolidation pressure. An increase in pressure from 3 to 6 MPa resulted in % increase of agglomerate density ρ. The value of ρ ranged from 83 kg m -3 for 3 MPa of consolidation pressure up to 13 kg m -3 for 6 MPa (Fig. ). There were no strong influences of temperature, deformation speed and initial sample height on ρ.. CONCLUSIONS Values of flow functions obtained for microcrystalline cellulose and potato starch were found increasing with an increase in consolidation time from to hours. 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. Increasing temperature from 3ºC to ºC resulted in increase of the internal friction angle determined in press shear cell. 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. 5. NOMENCLATURE D powder probe diameter, mm FF flow function h powder probe height, mm

6 i flow index (σ c /σ 1 ) p consolidation pressure, MPa s preshear displacement, m T powder bed temperature, ºC v 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 σ 1 major consolidation stress, kpa σ c unconfined yield strength, kpa σ r consolidation stress, kpa 6. REFERENCES 1. Bell A., Ennis B.J., Grygo R.J., Scholten W.J.F., Schenkel M.M.: Practical evaluation of the Johanson hang-up indicizer. Bulk Solids Handling 1(1): , Stasiak M., Molenda M., Horabik J.: Determination of modulus of elasticity of cereals and rapeseeds using acoustic method. Journal of Food Engineering, 8, 51-57, Standard Shear Testing Technique for Particulate Solids Using the Jenike Shear Cell, Institution of Chemical Engineers Molenda M., Stasiak M., Moya M., Ramirez A., Horabik J., Ayuga F.: Testing Mechanical Properties of FoodPowders in Two Laboratories Degree of Consistency of Results. International Agrophysics, (1), 37-5, Stasiak M., Molenda M.: Direct shear testing of flowability of food powders. Research in Agricultural Engineering, 5: 6-1,. 6. Reichmann B., Tomas J.: Expression behaviour of fine particle suspension and the consolidated cake strenght. Powder Technology 11(-3): , Grossmann L., Tomas J.: Flow properties of cohesive powders tested by press shear cell. Particulate Science and Technology : , Grossmann L., Tomas J., Csöke B.: Compressibility and flow properties of a cohesive limestone powder in a medium pressure range. Granular Matter 6: 13-19,.