Flow Properties of Cohesive Powders Tested by a Press Shear Cell

The most important design parameters for roller presses can be referred to flow and compression characteristics of bulk materials. Usually the flow properties are measured in the low stress range 1–50 kPa at the shear rate of about 1 mm/min. But this does not fit the stress regimes in the roller press. Therefore, the compression and flow behavior of the powder have to be investigated at higher pressures, shear rates, and shear displacements. These properties of bulk materials in the so-called medium pressure range 50–1000 kPa can be analyzed using a press shear cell. Tests were implemented with limestone, bentonite, and microcrystalline cellulose at average 23°C powder bed temperature using shear rates from 0.00042 to 0.042 m/s and a more realistic preshear displacement from 0.1 to 2 m for practical applications in powder compaction. Physical observation based compression functions were developed for the low and medium pressure range, which include simple equations for the compression rate and specific compression work.     

Compressibility and flow properties of a cohesive limestone powder in a medium pressure range

The most important design parameters for roller presses can be referred to flow characteristic of bulk materials. Usually the flow properties are measured in the low stress range 1–50 kPa at the shear rate about 1 mm/min. But this does not fit the stressing conditions in the roller press. Press shear cell was used for shear tests with cohesive limestone powder from Gummern in the so-called medium pressure range 50–1000 kPa.     

MEASUREMENT OF COHESION AND ANGLE OF INTERNAL FRICTION USING CUBICAL TRIAXIAL TESTER AND COMPARISON WITH COMPUTER CONTROLLED SHEAR CELL

A flexible boundary-type cubical triaxial tester (CTT) was used to measure the flow parameters, i.e., cohesion and angle of internal friction, of Mohr-Coulomb model. The three test materials used in this study were BCR limestone, ground silica and wheat flour. Flow parameters for these materials have been reported in literature using different shear testers. A comparison between the CTT and published computer controlled shear cell (CCSC) results showed that for: 1) BCR limestone -- at low consolidation loads, the results were comparable; however, there were differences in cohesion at 12.5 kPa and cohesion and angle of internal friction at consolidation stress of 6.6 kPa, 2) ground silica -- flow parameter values were comparable at 2.8 and 8.4 kPa, and 3) wheat flour — cohesion values were different, however, angle of internal friction values were comparable. The differences between the CTT and published shear cell results were attributed to the differing initial bulk density values and lack of knowledge of the shear plane location in the CTT.     

Rate-Dependent Elasto-Viscoplastic Constitutive Model for Industrial Powders. Part 1: Parameter Quantification

ABSTRACT

The rate-dependent mechanical behavior of a dry industrial powder (MZF powder) was studied using a cubical triaxial tester (CTT) within the context of a new elasto-viscoplastic model (PSU-EVP model). The compression and shear properties of the powder were quantified at compression rates of 0.62, 6.21, and 20.7 MPa/minute with pressures up to 11 MPa. Test results demonstrated that the compression and shear responses of the powder were nonlinear, consistent, and reproducible (coefficient of variation or COV ≤ 15%). Also, MZF powder exhibited varying elastic and plastic deformation at different pressure levels that were quantified using statistical correlations (R² > 0.90). For example, the average bulk modulus and shear modulus values for MZF powder increased linearly with pressure (R² > 0.90) at all compression rates. The failure stress values also increased with the increase in mean pressure. For instance, at a compression rate of 0.62 MPa/minute, failure stress increased from 5.0 to 13.3 MPa as the confining pressure increased from 2.2 to 8.5 MPa. Similar effects were noted at compression rates of 6.21 and 20.7 MPa/minute. Overall, failure stress decreased with increasing compression rate. From the data collected, it was demonstrated that compression rate does have substantial effect on the compressibility and shear behavior of powders that can be quantified using the CTT and is suitable for use in the PSU-EVP model.     

Shear testing of lactose powders: The influence of consolidation stress and particle size on bulk density and estimated cohesion

The shear testing protocol developed by Jenike in 1964 is commonly used to characterize powder flow in the food and pharmaceutical industries. The protocol requires the measurement of consolidated bulk density and the stress required to shear a powder bed under a series of normal consolidation stresses. In this work, the influence of preconsolidation stress and surface–volume mean particle diameter on the bulk density of 13 milled lactose powders consolidated in an annular shear cell below 5 kPa is examined. Five empirical correlations that relate bulk density to preconsolidation stress are tested. Each correlation contains two fitting parameters and their values are determined by regression; the parameters are further correlated with particle diameter. A new correlation that simultaneously relates bulk density to preconsolidation stress and particle diameter is proposed. The correlation is valid for milled lactose powders of ∼29–223 μm and preconsolidated at 0.31–4.85 kPa, and can estimate bulk density to ±10% of the measured bulk density. It is a convenient tool for the estimation of the bulk density term in an earlier correlation that links the cohesion of milled lactose powders to particle surface area per unit volume and preconsolidation stress.     

CHAOS IN ROTATING LACTOSE POWDER BEDS

The flow properties of organic powders are of great significance for the transfer, sampling and mixing of materials in the fine chemical, pharmaceutical, cosmetic, food, and agricultural fields. The following studies were performed to describe the complex flow behavior of lactose powders, a common pharmaceutical excipient. Pharmaceutical powder flow is a crucial element of successful dosage form design and manufacture. Three lactose samples, bulk (median diameter = 52.3µm, σ_g = 1.9), 45-75µm and 125-180µm, were studied. Their flow patterns were analyzed and irregular oscillations of the dynamic angle of repose around the mean were observed. Broad band frequency distributions were observed in the power spectra. Phase-space plots revealed complex non-overlapping trajectories. Return maps exhibited grouping of data points with a well defined structure indicative of a chaotic system. The combined observations are consistent with underlying order, and were observed in all lactose samples. These studies are among the first demonstrating chaos in organic powders. It is anticipated that powder specific chaotic inputs may be used to manipulate and improve powder flow at various stages in the pharmaceutical manufacturing process.     

Measurement of Bulk Mechanical Properties and Modeling the Load-Response of Rootzone Sands. Part 1: Round and Angular Monosize and Binary Mixtures

The bulk mechanical properties of two different types of rootzone sands (round and angular) were measured using a cubical triaxial tester. Two monosize sands (d 50 = 0.375 mm and 0.675 mm) and their 50:50 binary mixtures (d 50 = 0.500 mm) were studied. The compression, shear, and failure responses of the above-mentioned six compositions were analyzed, compared, and modeled. Two elastic parameters (bulk and shear moduli) and two elastoplastic parameters (swelling and consolidation indices) of the six sand compositions were also calculated and compared. The angular sand was more compressible than round sand during isotropic compression. In addition, the angular sands tended to have lower initial bulk density and high porosity values. Among the three different size fractions, the 0.375 mm mixture was least compressible for both sand shapes. The failure strength and shear modulus of the angular sand were higher than the round sands. In addition, due to their simplicity, phenomenological models were developed to predict the compression and shear behavior of the sands. The prediction models were validated using subangular and subround sands. Average relative difference values were calculated to determine the effectiveness of the prediction models. The mean average relative difference values for compression profiles, i.e., volumetric stress vs. volumetric strain, were from 16 % to 39 %, except for the initial load-response portion (< 1 % volumetric strain). The predictive models were effective in reproducing the failure responses: at 17.2 kPa confining pressure, the mean of average relative difference was 23 %; at 34.5 kPa , the mean difference was 24 %.     

The Effect of Agglomeration Methods on the Micromeritic Properties of a Maltodextrin Product, Maltrin 150™

Maltrin M150 is a fine powder of maltodextrin which is a carbohydrate product made by controlled hydrolysis of corn starch. Agglomerated Maltrin was prepared using a fluidized bed granulation process and a roller compaction method, respectively. The micromeritic properties of these two granular products were compared. Three different sizes of granules (20/30, 40/50 and 80/100 mesh size) were used in the evaluation. Granules produced by the fluidized bed method showed a relatively low bulk density as compared to the roller compacted granules. As the granule size was reduced, the roller compacted granules showed a decrease in bulk density while an increase in bulk density was seen in the fluidized bed granulated product. A better flowability of the roller compacted granules was demonstrated by a higher flow rate and a lower compressibility index. For a given compression pressure, roller compacted granules produced compacts with a lower tensile strength. A significant work-hardening effect was exhibited by the roller compacted product.     

PARTICLE CONCENTRATION MEASUREMENTS BY LASER IMAGING FOR A TURBULENT DISPERSION

A laser imaging system has been developed which can be used for investigating the particle concentration variation in explosive test apparatus such as the Ciba-Geigi and Hartmann Bomb during turbulent dispersions of air-particle mixtures. The pulsed UV (337 nm) laser imaging system using a 500X optical and electronic magnification system has a measurement volume of 900 µm by 675 µm and an in-focus depth of field of 780 µm for a 32 µm diameter particle. Particles in the measurement control volume are imaged every 33 ms during the dispersion process and viewed in real time but stored for later analysis on a video tape system.

This paper presents the results of investigating the lycopodium particle concentration variations during the dispersion process of 0.200 grams of lycopodium particles in the Hartmann Bomb explosive test apparatus. Data were taken at the center line and at a radius ratio of 0.5 at a height of 0.102 m (4 inches) above the base of the Hartmann lucite tube. Twenty-five separate dispersions were made at each radius ratio and were based on a reservoir pressure of 103 kPa (15 psig) and 0.200 g of lycopodium powder. The average number of lycopodium particles based upon 25 dispersions at 33 ms intervals in the 473.9 × 10⁶ m³ control volume are reported for a total elapsed time of 15 seconds. The maximum average particle concentration observed was 6.4 particles at 133 ms for r/R = 0.0 and 6.5 particles in 333 ms for r/R = 0.5. Based upon uniform dispersion model for 0.200 g of lycopodium powder, 6.8 particles per control volume, would be expected. The time averaged data followed a Poisson Distribution for each time increment after 0.73 s for both radius ratios of r/R = 0.0 and 0.5 (based upon 95% confidence interval and Kolmogorov-Smirnov test). Data from 0 to 0.73 seconds could not be assigned confidence levels as the data did not follow a Poisson Distribution or any other known statistical distribution. No significant particle agglomeration was observed for the dispersion of lycopodium particles. In any one dispersion the number of lycopodium particles in the control volume was observed to vary widely during each 033 second measurement cycle.

To further investigate the particle dispersion, the flow pattern characteristics in the Hartmann dispersion apparatus were studied using flow visualization techniques based upon a matched Reynolds number (3.13 × 10⁶) dispersion of fluorescent dye by turbulent water injection. The matched Reynolds number flow visualization work further indicated the Hartmann Bomb dispersion method produces local pockets of nonuniformly-mixed mixtures during initial stages of the dispersion process, and this work further points out the shortcomings of integrating optical probes.     

SPINNING FIBERS FROM POWDER SUSPENSIONS

Green ceramic fibers from Al₂O₃, Si₃N₄, Ce-ZrO₂, SiC, and other ceramics can be prepared by dry spinning of powder-loaded suspensions. The green fibers contain 54 to 64 vol. % ceramic powder in an ethyl methacrylate polymer base. Continuous fibers with an average diameter of 60 to 175 µm were spun at rates of 10 to 40 meters/minute. Spinnable dope compositions are defined for simple ternary powder + polymer + solvent systems, and are compared for several powders in MEK-based solvents. Spinnability is related to extrusion pressure, and drying conditions, and the shear rheological behavior of the dope. Fiber preparation by suspension dry spinning is compared with melt spinning of powder-loaded thermoplastics.     

Simultaneous Deposition of Powder in Three Parallel-Oriented Cylindrical Dies

The inhomogeneity of bulk density distribution created during the die filling process might cause quality problems for powder compacts, such as distortion, lamination, and cracking. To avoid these problems, understanding the die filling process and ensuring a uniform pre-compaction powder deposition are necessary. The second-generation pressure deposition tester (PDT-II) was developed to investigate simultaneous deposition of powder into multiple dies. Its design requirements and new features were proposed through evaluating the main strength and limitations of the mass deposition tester (MDT). The operation of the PDT-II and analysis of its data showed that it generates real-time deposition profiles of the entire process for multiple locations. PDT-II data can be used to study the effects of various filling-related parameters (such as die shape, powder flowability, and feed shoe speed) on the deposition process and final pressure distribution. For cylindrical dies filled with a granulated powder with d₅₀ = 600 μm (1) at low feed shoe speeds (20 and 100 mm/s), the half circle close to the leeward end had higher final pressure values than the forward half circle; (2) at high feed shoe speed (500 mm/s), the final pressure distribution was more uniform than at lower feed shoe speed; (3) the final within-die pressure distribution at the bottom of the dies was not always symmetrical about the center line of the feed shoe movement direction, even though sometimes it was quite symmetrical; (4) the overall trend was that pressure decreases with increasing radial distance for lower feed shoe speeds; and (5) higher feed shoe speed (500 mm/s) resulted in higher final pressure values (774.5 to 1424.5 Pa) than lower feed shoe speeds (20 and 100 mm/s) (235.2 to 1136.0 Pa) at most of the locations. The results proved that feed shoe speed does have an effect on pressure distribution and its uniformity.     

BUBBLE FLOW IN PRESSURIZED GAS/SOLID FLUIDIZED BEDS

This contribution presents measurements of bubble behaviour for A, B and D powder of Geldart classification in a semi-industrial scale pressurized fluidized bed. Using capacitance probes local measurements on bubble flow were taken at static pressures of 0.2, 1.0 and 2.5 M Pa. In a small “two-dimensional” transparent equipment, quartz sand with mean particle diameter of 157 µm was fluidized with Frigen® R 115. In addition, pressure fluctuations in the bulk of the fluidized bed with inner diameter of 0.4 m have been studied experimentally.     

Numerical Study of Air Compressibility During Horizontal Pneumatic Transport

Using numerical simulations, the effect of the compressibility of air on the flow pattern of particles and pressure drop in the presence of particles during horizontal pneumatic transport operating under negative pressure was examined. The length and inside diameter of the pipeline were 30 m and 40 mm, respectively, and the chosen particles (4 mm in diameter) had densities of ρ_p = 1000 and 2000 kg/m³. The mean air velocities at pipe the inlet were U_inlet = 19, 22, and 28 m/s, and the range of the mass flow rate ratios of particle to air, μ, was varied up to 2.0. For a given inlet air velocity, the difference in the flow pattern between compressible and incompressible flow calculation is generally small. For ρ_p = 1000 kg/m³ particles the additional pressure drop in compressible flow increases when μ is above 0.5 and U_inlet is 28 m/s, μ is above 1.3 and U_inlet is 22 m/s, and μ is above 1.5 and U_inlet is 19 m/s. In these cases, the particle flow pattern is homogeneous. For ρ_p = 2000 kg/m³ particles, the pressure drop increases only when μ is above 1.5 and U_inlet is 28 m/s. The difference is not noticeable when the particle flow pattern is heterogeneous. Also, the difference in the additional pressure drop is much larger during homogeneous flow than heterogeneous flow.     

Investigation of flow properties of powders by means of a uniaxial tester, in relation to direct tablet compression

Unexpected poor flowability during commercial production of a direct compression tablet formulation initiated an investigation of the flow properties of the powder mixture and its components by means of a uniaxial tester. The failure function—a curve describing the strength of the powder bed as a function of the maximum main stress that has consolidated the bed—of the powder mixture and its components was determined. The drug was more cohesive than the filler, which was somewhat more cohesive than the powder mixture. Three excipients—a binder, a glidant and a lubricant—constituting 3.5 w/w% of the formulation improved the flowability of the mixture of active ingredient and filler. The failure function discriminated powder mixtures with poor flow from mixtures with medium or good flow. However, it was not possible to discriminate medium from good flow by means of the failure function. Attempts to correlate univariately the flow property parameters of the powder mixtures with particle size data or flow property data of included active ingredient and filler batches failed. Therefore a multivariate approach was tested. Principal component analysis (PCA) and projection to latent structures by means of partial least squares (PLS) were employed. An excellent PCA model was obtained with the flow properties of the powder mixture. A good PCA model of tableting performance—based on tablet weight variation and tablet machine speed—was obtained.     

Talc functionality as lubricant: texture, mean diameter, and specific surface area influence

Talc is widely used as a glidant (flow regulator) for powders. This study highlights the characteristics that confer to talcs new end use properties in improving the lubrication function during compression. We studied the contribution of texture, mean diameter (D50), and specific surface area on the residual die pressure, the ejection pressure, the lubrication index, and the tablet hardness. Different textures were studied: microcrystalline, macrocrystalline, and moderately macrocrystalline talc grade. The compression parameters were improved according to the texture. D50 varies from 0.62 to 15 µm. As D50 decreases, the lubrication performance is improved. Finally, the specific surface area of talcs was studied. This last characteristic of talcs was shown as the most relevant parameter in determining lubrication ability.     

Measurement of Bulk Mechanical Properties and Modeling the Load Response of Rootzone Sands. Part 3: Effect of Organics and Moisture Content on Continuous Sand Mixtures

The interactions between organics and sand particles at different moisture contents are important in understanding the general mechanical behavior of rootzone sand mixtures. Towards this end, eight rootzone sand mixtures (4 shapes ×2 moisture contents) used in golf green construction were tested using the cubical triaxial tester (CTT). These eight mixtures consist of sphagnum peat as the organic source and four sands of varying particle shape (round, subround, subangular, and angular). The sand-peat mixtures were tested at two moisture contents (air-dried and 30 cm tension). Of all the test samples, air-dried round sand with peat had the highest initial bulk density (IBD) value (1.49 g/cc), while moist angular sand with peat had the lowest IBD value (1.23 g/cc). These values influenced the compression behavior of samples, for example, the air-dried round sand with peat was least compressible while moist angular sand with peat was most compressible. Generally, moisture enhanced the compressibility of test specimens. At an isotropic pressure of 100 kPa, the volumetric strain value of moist round sand with peat was 47% higher than the volumetric strain value of the air-dried round sand with peat. Consequently, moisture and peat in bulk sand samples act as lubricants and assist in the compression process. In addition, bulk modulus values decreased with moisture. Due to the dominant effect of peat, there were no large differences between bulk modulus values of different particle shapes. The shear and failure responses of the above-mentioned eight compositions were also analyzed, compared, and modeled. Of all sand mixtures tested, air-dried angular sands with peat had the highest brittle-type failure stress value, 181 kPa at 34.5 kPa confining pressure, and moist subangular sand with peat had the lowest ductile-type failure stress value, 141 kPa at the same confining pressure. Shear modulus values increased with the increase of mean pressure, but in the case of sands containing both moisture and peat, shear modulus values increased gradually. Overall, peat and moisture content have a dominant effect on the compression and failure behavior of the rootzone sands.

rootzone sand mixtures moisture effect particle shape effect organics effect mechanical behavior compression response shear/failure response prediction models     

Development of a Medium Pressure Flexible Boundary Cubical Triaxial Tester

ABSTRACT

A medium pressure flexible boundary cubical triaxial tester has been designed and fabricated. In this tester, air pressure up to 70 MPa can be applied to all six surfaces of a 50X50X50 mm cube-shaped powder specimen via flexible rubber membranes. Pressure in a vertical direction (top-bottom faces of the powder specimen) and the pressure in a horizontal direction can be controlled independently. This tester can handle displacements as large as 50 mm in each of the three principal directions. Hydrostatic triaxial compression (HTC) tests. conventional triaxial compression (CTC) tests, and mean effective stress (MES) tests will be conducted on three powders, including a pharmaceutical powder, a ceramic powder, and an aluminum oxide powder. HTC tests will be conducted at 0 to 20 MPa, with 3 loading-unloading cycles. CTC and MES tests will be conducted at several pressure levels from 0 to 20 MPa.     

Cohesion of lactose powders at low consolidation stresses

The flow characteristics of a powder system are known to be influenced by particle size distribution, particularly the content of fine particles, and interparticle forces. This paper reports an investigation that has identified and quantified links between physical properties, viz size distribution, bulk density and particle density, and cohesion in compacted beds of powder. An annular shear cell was used in the determination of the cohesion of cohesive and free-flowing milled lactose powders at low consolidation stresses in the range 0.31–4.85 kPa and under ambient conditions. Following consideration of the compaction and shearing processes, it was postulated and confirmed that cohesion could be expressed as a function of powder surface area per unit volume and dimensionless preconsolidation stress. It was shown that care is needed in the measurement of surface–volume mean diameter when applying correlations developed from the experimental data.     

Comparison of mechanical properties of ground corn stover,switchgrass, and willow and their pellet qualities

Mechanical and physical properties of ground corn stover, switchgrass, and willow were measured and compared in addition to the quality of pellets. Biomass was size-reduced with two different screen sizes (3.175 and 6.35?mm) and conditioned to obtain samples at two different moisture contents (17.5 and 20% on wet basis). Ground switchgrass had the smallest and willow had the highest D₅₀ when size-reduced with the same screen size. Hydrostatic triaxial compression tests were performed using the cubical triaxial tester to determine the bulk modulus, compression index, and spring-back index at specific unloading pressures (20, 45, 70, and 95?kPa). The trends of pressure vs. volumetric strain and void ratio vs. natural log of pressure were similar for all three materials; however, the magnitudes were different. Willow, size-reduced with 3.175?mm screen size at 17.5% wet basis, had the highest bulk modulus among different conditions of all the three biomass. Pellet durability values for all the three materials were higher than 80%. Corn stover pellets formed with 3.175?mm screen size at 20% wet basis had the highest diametral tensile and axial compressive strengths among different conditions for all the three biomass, however the values were not significantly different (p?>?0.05).