The High Temperature Annular Shear Cell: A modified ring shear tester to measure the flow properties of powders at high temperature

Although changes of cohesive behaviour of powders is observed at high temperature in several industrial process units, conventional testers and procedures are still not suited for testing powder flow properties at high temperature. In this work a High Temperature Annular Shear Cell was designed, built and set-up in order to directly measure the flow properties of powders up to 500°C. A temperature control system was also developed to establish a uniform temperature inside the powder sample. Yield loci at room temperature and 500°C were measured for samples of fluid catalytic cracking catalyst (FCC powder), fly ashes, natural corundum, synthetic porous alumina and glass beads. Experimental evidences did not reveal a univocal effect of temperature in the tested range. Finally, shear tests on glass beads mixed to with some high-density polyethylene (HDPE) (1% of the total weight) confirmed that a significant increase of the cohesive behaviour occurs at high temperature when liquid bridges form due to the melting of one of the solid phases

A theoretical framework for the interpretation of the effects of temperature on interparticle interactions

A High Temperature Annular Shear Cell was used to directly measure yield loci up to 500°C and to evaluate the effect of temperature on the macroscopic flow properties of powders. A theoretical framework was developed according to the particle-particle approach of Rumpf and Molerus. In particular, the tensile strength of the powder experimentally evaluated for fluid cracking catalyst, corundum and glass beads was related to the van der Waals forces acting between particles assuming alternatively elastic and plastic deformation at contact points. Both the assumptions provide correct order of magnitude results in terms of tensile strength if plausible value of the local curvature at contact points of particles is taken into account. Furthermore, both the increasing cohesive consolidation and the slight increase of the cohesive behaviour with the temperature suggest the occurrence of the plastic deformation of the contact points and, therefore, that the plastic deformation assumption should be adopted to explain the effect of the temperature on the interparticle interactions

A rheological model for the flowability of aerated fine powders

A mechanically stirred fluid-bed rheometer (msFBR) was used to study the rheology of powders aerated below the fluidisation threshold. Glass ballotini (group B) and silica powders (group A) with different fine contents were tested. The torque necessary to rotate an impeller immersed in a bed of aerated powders was measured for different impeller depths and aeration rates. A model was developed: (a) to estimate the state of stress at the impeller depth, following Janssen’s approach for the evaluation of stresses in silos, and (b) to evaluate the torque, with the hypothesis that it is determined by the powder shear on a flat cylinder surface around the impeller. The model uses some powder properties, such as the dynamic and the wall yield loci of the powder used, which were estimated with a Peschl shear cell modified for small loads. The reasonable prediction of the torque at impeller depth larger than 3 cm provided by the model supports the hypothesis that the torque is defined by the plastic deformation of powders and can be explained within a simple Mohr–Coulomb approach to powder flow. The passive stress distribution that appears to set up during the shearing experiments leaves open some fundamental questions regarding the limiting conditions determining such behaviour. As in previous experiments found in the literature, aeration does not affect the rheology of powders but modifies the stress distribution within the bed. The content of fines turns out to be a key factor in the determination of powder rheology as measured both with the shear cell and with the fluid-bed rheometer

On the role and the origin of the gas pressure gradient in the discharge of fine solids from hoppers

The Brown and Richards (Principles of Powder Mechanics, Pergamon Press, Oxford, UK, 1970) correlation for the discharge rate of fine powders from a hopper was modified to account for the gas pressure gradient near the outlet. According to Dons*+ et al. (Chem. Eng. Sci. 52 (1997) 4291) there is a transition between a granular floow region and a suspended floow region near the hopper outlet. Brown and Richards (1970) stated that the particle discharge rate depends on the 9ow conditions just above this transition surface. In the modified equation that is developed to account for the gas pressure, a term including the gas pressure gradient at this surface appears. The gas pressure gradient is evaluated from the literature experimental results by considering the Donsì et al. (1997) finding that a significant part of the gas pressure gradient near the hopper outlet is due to the suspended motion. Furthermore, a simplified analysis is carried out to evaluate from the experimental results the voidage variation within the solids phase that is responsible for the onset of the gas pressure gradient

Dust release from aggregative cohesive powders subjected to vibration

Dust is released from a mechanically vibrated bed of cohesive powders aerated close to the minimum for fluidization. The materials tested were silica and potato starch. The effect of both vibration intensity and vibration frequency on dust formation are verified, and a model to account for these two variables has been developed. A dimensionless dust release parameter has been defined and calculated from the experimental results. This parameter is able to explain the role of the acceleration due to vibration and of cohesive interparticle forces, also accounting for the magnification effects of acceleration intensity close to resonance conditions. However, the same parameter is unable to fully account for changes in dust emission induced by fluidized bed bubbling promoted by vibration frequencies close to resonance. Without considering these effects, frequency determines a reduction of dust release that should be explained by introducing some effects other than those considered in our model

The measurement of powder flow properties with a mechanically stirred aerated bed

This paper re-examines a set of experimental data published by Bruni et al. (2007a, 2007b) [Bruni, G., Barletta, D., Poletto, M., Lettieri, P., 2007a. A rheological model for the flowability of aerated fine powders. Chem. Eng. Sci. 62, 397–407; Bruni, G., Lettieri, P., Newton, D., Barletta, D., 2007b. An investigation of the effect of the interparticle forces on the fluidization behaviour of fine powders linked with rheological studies. Chem. Eng. Sci. 62, 387–396] carried out on a mechanically stirred fluid-bed rheometer (msFBR), which was developed to study the rheology of aerated and fluidized powders. The use of aeration below fluidization allowed to carry out experiments with powders at very low consolidation levels. Two mathematical models, based on the Janssen approach to evaluate stresses in powder containers, were developed in order to relate the torque measurements in the Fluidized Bed Rheometer to the flow properties of the powders measured with standard powder flow testers. Results indicate that the models were able to satisfactorily predict the torque measured by the msFBR. The larger complexity of the Walker (1966) [Walker, D.M., 1966. An approximate theory for pressures and arching in hoppers, Chem. Eng. Sci. 21, 975–997] and Walters (1973) [Walters, J.K., 1973. A theoretical analysis of stresses in silos with vertical walls, Chem. Eng. Sci. 28, 13–21] stress analysis adopted in one of the two models did not introduce significant improvements in the evaluation of the stress distribution to justify its use. A procedure for the inverse application of the model was developed and applied to estimate the powder flow properties starting from msFBR data. The application of this procedure provided good results in terms of effective angle of internal friction and is promising for the ability of the system to explore powder flow at very low consolidation states

The Effect of Vibrations on Fluidized Cohesive Powders

The fluidization of a cohesive silica powder has been tested with the help of mechanical vibration. The experiments showed how the effectiveness of vibrations changed with the vibrational acceleration and frequency. The aggregative behavior of powders has been highlighted and a model procedure is proposed to predict the aggregate size starting from the measurement of powder flow properties with conventional shear testers

Measurement and prediction of CO2 solubility in sodium phosphate monobasic solutions for food treatment with high pressure carbon dioxide

Two experimental systems were designed and tested to measure the CO2 solubility in sodium dibasic phosphate solutions (0.276, 2.76 and 5.52 g/100 g water) at different pressures (7.5 and 15.0 MPa) and temperatures (35, 40 and 50°C). The results were compared with those for pure water. Three thermodynamic solubility models were tested using the Aspen Simulation PlusTM software: 1) Peng-Robinson equation of state (EOS), where the a and b parameters were evaluated with the Wong and Sandler mixing rules (PRWS) and the activity coefficients were defined using the functional groups with the modified UNIFAC method 2) Electrolytic non-random two liquids (ELECNRTL) with the Redlich-Kwong equation of state for aqueous and mixed solvent applications 3) The completely predictive Soave-Redlich-Kwong (PSRK) equation of state. CO2 solubility was a strong function of sodium dibasic phosphate concentrations. The predictions of the PRWS EOS agreed well with the experimental data in the pressure and temperature ranges tested. A higher difference between the experimental and predicted results was observed for conditions close to the CO2 critical point and for low sodium dibasic phosphate concentrations. Thermodynamic models 2 and 3 predictions had a much higher deviations from experimental data

Analysis of industrial reactive powders flow properties at high temperature

The Chemical Industry Association reports that most of chemical products currently in production involve the use of particles at some stage in the manufacturing process. Particle science and technology are essential to the improvements of many types of consumer products (including polymers, paints, food and healthcare, to mention just a few) and tackle most contemporary grand challenges, such as in advanced manufacturing, sustainable energy, waste management, and food preservation. To this end, fluid bed reactors are usually used. They are particularly useful in high temperature systems where the fluidization behaviour allows particulate materials to be handled at a higher temperature than in a static system. However, fluid bed operations at elevated temperatures are usually limited by a tendency of the particles to form agglomerates, thus causing the bed to defluidize. This may occur at temperatures well below the normal particles’ melting point, and has been encountered in different processes, including the carbo-chlorination reaction involving titaniferous materials. This phenomenon is a result of a strong particle-particle interaction causing more rapid rate of sintering. Several studies have highlighted that the phenomena involved are extremely complex, and that a direct quantification of the particle-particle interactions and their changes with temperature remains a challenge. The objective of this paper is to report preliminary experimental observations on the effect of temperature on the flow behaviour of various titanium ore powders. To this end, both fundamental fluidization and rheological measurements will be performed to assess indirectly the particle-particle interactions at high temperature. The former will be performed using a unique high power pulsed X-ray Imaging technique, while a High Temperature Annular Shear Cell available at the University of Salerno will be used to characterize the rheological properties of such particles

Characterisation of flow properties of coal-petcoke-biomass mixtures for co-firing

Solid biomass is often mixed with coal for cofiring in power plants to reduce greenhouse gases emissions. The resulting powder mixtures can give rise to handling and feeding problems due to the occurrence of intermittent flow or blockage. In this paper the flow properties of mixtures of a coal-petcoke powder and two types of biomass materials, dried olive husk (OH) and grape seed meal (GSM), were studied. The most significant effects on the cohesive properties of some powder mixtures were found for low biomass content mixtures due to their higher fraction of fine particles