Tensile strength of cohesive powders

Measurement and prediction of cohesive powder behaviour related to flowability, flooding or arching in silos is found to be very challenging. Previous round robin attempts with ring shear testers did not furnish reliable data and have shown considerable degrees of scatter and uncertainty in key measurements. Thus studies to build a reliable experimental database using reference materials, are needed in order to evaluate the repeatability and effectiveness of shear testers and the adopted methodologies. In this paper, first we study the effect of particle size on the yield locus for different grades of limestone (calcium carbonate). We use the nonlinear Warren Spring equation to obtain the values of cohesion C, tensile strength T, and the shear index n. We recover linear (n = 1) yield loci for  μm with respectively small C. For smaller fractions (m), the powder is strongly cohesive with considerable non-linearity (). Then we compare the values of the parameters and n obtained from two different shear testers (Schulze and Brookfield PFT), thereby demonstrating the validity of the Warren Spring equation. The differences encountered are fortunately not leading to a great deviation in terms of cohesion C and tensile strength, T although further experiments with a variety of cohesive powders are needed to confirm or rebut this point. Finally, we compare the values of the tensile strength obtained by a tranverse experimental method using the Ajax tensile tester and found a very good agreement

Application of layers of protection analysis to prevent coronavirus infection

LOPA methodology is applied to an encounter with the SARS-COV-2 infection as an initiating event and subsequently independent protection layers (namely health safeguarding protocols), such as social distancing, ventilation, hand hygiene, face masks and vaccinations. LOPA is applied considering numerical quantification of the COVID fatality index in order to manage the transmission risk to a tolerable level, namely the fatality risk due to seasonal flu. This measurement tool quantifies the ratio of the annual death rate due to the SARS-COV-2 infection, to the annual death rate of the common flu and it is applied to a chemical plant. The lower this quantified value is, the more the COVID-19 infection death rate approaches to that of the common flu. Thus, any improvement in safeguarding protocols should reduce this index. The input data is based on public domain COVID-19 infection statistical data and websites accessible in the United Kingdom. The COVID-19 transmission rate is statistically analyzed with random number sampling, to simulate the random pattern of the virus’ person to person infection in the community. The success of the COVID-19 protection protocols is probabilistic and depends on the public’s compliance which are modelled by observational surveys

3D numerical simulation of upflow bubbling fluidized bed in opaque tube under high flux solar heating

Current solar Heat Transfer Fluids (HTF) only work below 600°C. We proposed to use air-fluidized Dense Particle Suspensions (DPS), also called Upflow Bubbling Fluidized Bed (UBFB), in tubes as a new HTF and storage medium in the frame of the so-called CSP2 FP7 European project. UBFB can operate up to the solid sintering temperature (1400 °C for SiC particles), thus improving the plant efficiency and cost of produced kWh. The DPS capacity to extract heat from a tube absorber exposed to concentrated solar radiation was demonstrated and the first values of the tube wall-to-DPS heat transfer coefficient were measured. A stable outlet temperature of 750 °C was reached with a metallic tube, and a particle reflux in the near tube wall region was evidenced. In this paper, the UBFB behavior is studied using the multiphase flow code NEPTUNE_CFD. Hydrodynamics of SiC Geldart A-type particles and heat transfer imposed by a thermal flux at the wall are coupled in 3D numerical simulations. The convective/diffusive heat transfer between the gas and dispersed phase, and the inter-particle radiative transfer (Rosseland approximation) are accounted for. Simulations and experiments are compared. The temperature influence on the DPS flow is analyzed

Investigating reverse osmosis membrane fouling and scaling by membrane autopsy of a bench scale device

In response to the escalating world water demand and aiming to promote equal opportunities, reverse osmosis desalination has been widely implemented. Desalination is however constantly subjected to fouling and scaling which increase the cost of desalination by increasing the differential pressure of the membrane and through decline in permeate flux. A bench-scale desalination equipment has been used in this research to investigate the mitigation of fouling and scaling. This study also involved the performance of membrane autopsy for fouling characterisation with special attention to flux decline due to sulphate precipitation and biofouling. Visual inspection, scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and microbiology tests (API) were performed. Results obtained showed the presence of diatoms, pseudomonas and polysaccharides as the main foulants causing biofouling. Analysis revealed sulphate deposits as well as aluminium, calcium and silica as the main elements contributing to inorganic scaling. Findings pointed out that the pre-treatment system of the small-scale reverse osmosis water treatment was inefficient and that selection of pre-treatment chemicals should be based on its compatibility with the membrane structure. The importance of characterization for the verification of fouling mechanisms is emphasized

Particle Attrition Mechanisms, their characterisation, and application to horizontal lean phase pneumatic conveying systems: A review

Understanding particle attrition is vital to the optimisation of a wide range of industrial processes. Lean phase pneumatic conveying is one such process, whereby the high energy particle impacts can cause undesirable loss in product quality or change in bulk behaviour. The attrition process is resolved into a material function and a process function; the combination of these functions dictates the attrition mechanism present, and the magnitude of failure observed. Subsequently, the forces applied to the particles are examined within the context of lean phase pneumatic conveying. Finally, empirical and numerical models are reviewed along with comments on experimental method. To summarise some of the findings of this review: the requirement of standardised test equipment is recognised in order to compare the wide variety of particulate materials under comparable loading conditions; stronger correlation between the results obtained from different particle attrition test methods is required; and finally, seldom are the manufacturing conditions (where applicable) linked to the particulate attrition behaviour

Long-term dust generation from silicon carbide powders

Most dustiness studies do not measure dust release over long durations, nor do they characterize the effect of dust release on bulk powders. In this study, we tested the dustiness of two different samples of silicon carbide (SiC) powders (referred to as F220 and F320) over six hours using a vortex shaker. Additionally, we characterized the bulk sample for change in shape and size distribution due to the testing. Both powders release respirable fractions of dust particles but differ in their dust generation behavior. The numbers of released respirable particles for powder F220 are more than two times higher than those of powder F320. The dust generation mechanism might include the release of aerosols due to the attrition of particles owing to inter-particle and particle-wall impaction. This study emphasizes the need for long duration dustiness tests for hard materials like SiC and characterization for change in bulk material properties due to dust generation and release. Furthermore, the results can aid in selecting the bulk material for long-term applications based on dustiness

Role of particle size on the multicycle calcium looping activity of limestone for thermochemical energy storage

The calcium looping process, based on the reversible reaction between CaCO3 and CaO, is recently attracting a great deal of interest as a promising thermochemical energy storage system to be integrated in Concentrated Solar Power plants (CaL-CSP). The main drawbacks of the system are the incomplete conversion of CaO and its sintering-induced deactivation. In this work, the influence of particle size in these deactivation mechanisms has been assessed by performing experimental multicycle tests using standard limestone particles of well-defined and narrow particle size distributions. The results indicate that CaO multicycle conversion benefits from the use of small particles mainly when the calcination is carried out in helium at low temperature. Yet, the enhancement is only significant for particles below 15 μm. On the other hand, the strong sintering induced by calcining in CO2 at high temperatures makes particle size much less relevant for the multicycle performance. Finally, SEM imaging reveals that the mechanism responsible for the loss of activity is mainly pore-plugging when calcination is performed in helium, whereas extensive loss of surface area due to sintering is responsible for the deactivation when calcination is carried out in CO2 at high temperature

Hydrodynamics and particle motion in upward flowing dense particle suspensions: application in solar receivers

Dense gas–solid suspensions have the potential to be applied as heat transfer fluids (HTF) for energy collection and storage in concentrated solar power plants. At the heart of these systems is the solar receiver, composed of a bundle of tubes which contain the solid suspension used as the thermal energy carrier. In the design investigated here, the particles form a dense upward-flowing suspension. Both density of the suspension of these particles and their movement have a strong influence on the heat transfer. An apparatus was designed to replicate the hydrodynamic and particle motion in the real solar energy plant at ambient temperature. The governing parameters of the flow were established as the solid feeding flow rate, the fluidisation velocity, the solids holdup, the freeboard pressure and the secondary air injection (aeration) velocity. In the case studied, aeration was applied with air introduced into the uplift transport tube some way up its length. This study finds that the amount of this secondary air injection is the most important parameter for the stability and the uniform distribution of the solids flow in the tubes. Solids motion was measured using the non-invasive positron emission particle tracking (PEPT) technique to follow the movement of a 60 µm tracer particle, onto which was adsorbed the positron emitting 18F radioisotope. Analysis of the resulting three-dimensional trajectories provides information on solids flow pattern and solids velocity. Results show the overall behaviour of the bulk material in detail: small step-wise movements associated with bubble motion superimposed on a general trend of upward flow in the centre and downward flow close to the walls. These findings suggest that this particular type of flow is ideal for transporting energy from the walls of the solar receiver tubes

Evaluation and control of the adhesiveness of cohesive calcium carbonate particles at high temperatures

Understanding the adhesiveness of fine particulate materials at high temperatures is important to achieving the stable, economical operation of various industrial systems. In the present research, two types of calcium carbonate (CaCO3) particles having different mean particle sizes (often used as heat carriers in energy systems) were evaluated. The tensile strengths of beds of these materials were determined at various temperatures by tensile strength measurement tester. The adhesiveness was found to increase greatly at 500 °C even without chemical reactions or sintering, and X-ray diffraction analyses showed thermal expansion of the CaCO3 crystals at 500 °C. Pure alumina (Al2O3) and silica (SiO2) microparticles did not exhibit the same pronounced increases in tensile strength or crystal expansion at this same temperature. Because the surface distances between these primary particles were presumably small, it is proposed that van der Waals forces between the particles greatly increased at high temperatures. The addition of Al2O3 nanoparticles to the CaCO3 decreased the tensile strengths of the powder beds both at ambient temperature and at 500 °C. The experimental data confirm that the surface distances between primary particles were increased upon incorporating the nanoparticles, such that the tensile strength decreased during heat treatment