Particle Attrition in Circulating Fluidised Beds System

Particle attrition plays an important role throughout the cycles of a circulating fluidised bed (CFB) and a fluidised bed (FB) process, gradually depriving the bed inventory of valuable mass and changing the bed particle size distribution. The mass loss has to be compensated by a make-up stream. For economic and design purposes, attrition cannot be neglected. Although the particles may be efficient catalysts (or reactants), if the compensation for the lost material amounts to very high expenses, the whole process may become uneconomical. It is then clear that the choice of the solids material should take into account its attrition propensity. The main sources of attrition in fluidised bed systems are the jet region, the bubbling bed and the cyclone. It is common practice to predict particle attrition in industrial scale fluidised bed systems by the population balance method, but is it possible to link that prediction with the breakage propensity of a single particle? This work aims at developing a predictive tool for particle attrition in fluidised and circulating fluidised beds, by attempting to build a path line from the single particle breakage propensity to the attrition occurring in the process. Here, the reference industrial process is the Chemical Looping Combustion (CLC). The CLC is a circulating fluidised bed process under development and as such, the choice of a solids material is critical. A powder of crushed manganese oxide is a candidate material for the CLC process and is used here as test material, as well as its equilibrium equivalent. For simplicity, the two materials are referred to as F-CLC (fresh CLC particles) and E-CLC (equilibrium CLC particles), respectively. The single particle breakability of F-CLC and E-CLC is assessed by impact tests. The experimental results are then used to correlate the extent of breakage upon impact with the particle size and impact velocity, according to the theoretical model of chipping of Zhang and Ghadiri (2002). Further tests are carried out to unveil the effect of impact angle and number of impacts. The results suggest that E-CLC is highly more inclined to attrition than F-CLC. Moreover, the single particle breakage is found to correlate with the magnitude of the impact velocity and the sin of the angle of impact for both materials. Recalling the modelling approach of Ghadiri and co-workers, the single particle breakage model, as derived, and the model of surface wear of Archard and Charj (1953) are coupled with CFD-DEM (Computational Fluid Dynamic-Discrete Element Method) simulations to compute the attrition of F-CLC particles in a Stairmand cyclone. Moreover, the same cyclone is used to characterise attrition of F-CLC particles experimentally as a function of particle size, gas inlet velocity and solids loading. Remarkably, the outcomes of the two approaches are found to agree well. A correlation is eventually derived which expresses the extent of attrition in a cyclone as a function of the variables mentioned above. The analysis revealed that the main source of attrition in the cyclone is given by the particle-wall collisions at the opposite section of cyclone inlet, at any operating conditions. Particle-particle collisions and particle sliding against the wall become significant contributors of attrition at high and low solids loading, respectively. Attrition in the jet region is evaluated at room temperature as the steady state loss rate, using a semi-pilot scale fluidised bed equipped with a porous distributor and a central orifice of variable size. The results of the tests show that jet attrition of F-CLC and E-CLC can be described by two different correlations. The steady state attrition propensity of E-CLC is found to be higher than F-CLC, confirming the outcomes of the impact tests. The analysis on the fines collected on the filter reveals that they are mainly composed by very small particles of about 1 μm. The correlations of cyclone and jet attrition are implemented in a non-dimensional population balance model (PBM) that simulates attrition in a fluidised bed and a circulating fluidised bed. The latter is composed of a fluidised bed where the recycle of solids is provided by a cyclone. The PBM is validated for the fluidised bed configuration against the experimental PSD (Particle Size Distribution) of F-CLC particles after jet attrition in the fluidised bed. The PBM is eventually used to simulate hypothetical cases of a FB and CFB with low and high single particle breakability as well as low and high superficial velocities to assess the dynamic response of the system in terms of material loss, solids circulation rate, requirements for a make-up and PSD in different regions of the system. The simulations allowed to identify the presence of two subsequent regimes where the loss is firstly dictated by the pre-existing fines of the bed inventory and then by attrition. During the two regimes the mean particle size of the bed inventory increases and decreases, respectively. The PBM reveals that the circulation rate is strongly affected by attrition because of the accumulation of entrained particles which are large enough to be captured by the cyclone and recycled. The loss of material and the need for the make-up stream are found to increase using either larger superficial velocities and/or weaker particles

Exploring Stress for Novice Teachers\u27 in Low Socio-Economic Elementary Schools Through Breathing Biofeedback

Teachers who serve children of color in schools that support students from low socio-economic status have turnover rates reported to be as high as 70% (SES; Carver-Thomas & Darling-Hammond, 2017). Numerous approaches to teacher retention in low SES settings have been discussed in the literature (Chester & Beaudin, 1996; Donaldson, 2009; Ingersoll & Kralik, 2004; Jacob, 2007; McKinney, Haberman, Stafford-Johnson, & Robinson, 2008; McLaurin, Smith, & Smillie, 2009; Siwatu, Frazier, Osaghae, & Starker, 2011; Sutcher, Darling-Hammond, & Carver-Thomas, 2016; Taylor & Frankenberg, 2009), but the use of emerging biofeedback technology is an unexplored territory. This type of technology could help novice teachers understand basic changes in stress levels through awareness of the body and mind while teaching. Hence, the potential use of biofeedback strategies to reduce stress levels in teachers, in inclusive settings, that serve students with low SES is explored. To help potentially reduce novice teachers\u27 stress levels (Friedman, 2000; Day & Hong, 2016; Isenbarger & Zembylas, 2006), the researcher explored the effectiveness of biofeedback on breathing rates of teachers in inclusive classrooms serving more than 50% of students from low SES backgrounds. The researcher found, by tracking the rate of stressed breathing of 9 teachers during the instructional day, that teachers reported they were more mindful of their breathing. The researcher found meetings, paperwork, and student behaviors increased their stress and suggested better mentorship as a way to help reduce stress. The paper concludes with implications for practice, and recommendations for future research for teachers is provided

Impact of the COVID-19 Pandemic on Inpatient Antibiotic Consumption in Switzerland.

The aim of this study was to analyze inpatient antibiotic consumption during the first 16 months of the COVID-19 pandemic in Switzerland. The entire period (January 2018-June 2021) was divided into the prepandemic period, the first and second waves, and the intermediate period. In the first year of the pandemic, total overall inpatient antibiotic consumption measured in defined daily doses (DDD) per 100 bed-days remained stable (+1.7%), with a slight increase in ICUs of +4.2%. The increase in consumption of broad-spectrum antibiotics was +12.3% overall and 17.3% in ICUs. The segmented regression model of monthly data revealed an increase in overall antibiotic consumption during the first wave but not during the second wave. In the correlation analysis performed in a subset of the data, a significant positive association was found between broad-spectrum antibiotic consumption and an increasing number of hospitalized COVID-19 patients (p = 0.018). Restricting this dataset to ICUs, we found significant positive correlations between the number of hospitalized COVID-19 patients and total antibiotic consumption (p = 0.007) and broad-spectrum antibiotic consumption (p < 0.001). In conclusion, inpatient antibiotic use during the different periods of the COVID-19 pandemic varied greatly and was predominantly notable for broad-spectrum antibiotics

CFD-DEM Analysis of Particle Attrition in a Jet in a Fluidised Bed

In fluidised bed processes, the solids are in vigorous motion and thus inevitably subjected to mechanical stresses due to inter-particle and particle-wall impacts. These stresses lead to a gradual degradation of the particles by surface wear, abrasion and body fragmentation commonly termed attrition. One significant contribution of attrition comes from the air jets of the fluidised bed distributor. Particles are entrained into the air jet, where they get accelerated and impacted onto the fluidised bed particles. The jet induced attrition only affects the part of the bed which is limited by the jet length, where the mode of attrition is largely collisional. The overall jet attrition rate is therefore the result of the combination of the single particle damage and the flux of particles entering into that region. The attrition behaviour of particles in the jet region is analysed by evaluating their propensity of breakage experimentally and by simulating an air-jet in a bed of particles by CFD-DEM. The frequency of collisions and impact velocities are estimated from which the attrition due to a single air-jet is predicted

Development of a methodology for predicting particle attrition in a cyclone by CFD-DEM

Cyclones are commonly used in the process industry to separate entrained particles from gas streams. Particles entering a cyclone are subjected to a centrifugal force field, driving them to the cyclone walls, where they experience collisional and rapid shearing stresses. Consequently, particle attrition and erosion of the cyclone walls occur, depending on the mechanical properties of the particles and cyclone walls. In this work, the attrition of manganese oxide particles, intended for use in the Chemical Looping Combustion (CLC) process, flowing through a standard design cyclone (Stairmand design) is analysed as an example by considering surface damage processes of chipping and wear. A new methodology is developed, whereby Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations are used to analyse the particle motion and interactions with the cyclone walls. The approach is then coupled with breakage models of chipping and wear to predict the extent of attrition. The impact breakage due to chipping is evaluated experimentally first as a function of particle size and impact angle and velocity. The data are fitted to the chipping model of Ghadiri and Zhang. The model is then coupled with the frequency of collisions and impact velocity, obtained from the CFD-DEM simulation, to work out the particle attrition by chipping. For surface wear the model of Archard is used to account for particle wear by shearing against the walls. The outcome of the work provides a methodology for describing the extent of attrition in different regions of the cyclone

Relationship between Surface Area Coverage of Flow-Aids and Flowability of Cohesive Particles

Poor and inconsistent flow of cohesive powders is a major issue in powder processing. A common solution is to coat the surfaces of the cohesive particles with finer particles, referred to as flow-aids. Such particles adhere to sticky surfaces and act as spacers preventing them from contacting each other and thus reducing the inter-particle forces and bulk powder cohesion. A question which naturally arises is how much flow-aid is needed to enhance the flowability to an optimum level. This work aims to establish a relationship between the degree of Surface Area Coverage (SAC) of flow-aids and the flowability, the latter as determined by a quasi-static shear cell method, as well as the angle of repose test and the FT4 powder rheometer. Glass beads of 90–150 μm sieve cut are made cohesive by silanising their surfaces with a commercial chemical reagent, Sigmacote® and are used as host particles. Two types of zeolite particles are used as flow aids. The mass fraction of the flow aids required to achieve a theoretical SAC of 1, 5, 10, 20, 50 and 100% is first estimated and then the host particles are coated in a pan mixer. The SAC is measured by Scanning Electron Microscopy, coupled with image analysis, and found to correlate well with the estimated value. The optimum surface coverage is found to be when SAC is 10–20%, as this provides the greatest flowability. An increase in SAC beyond this range leads to a gradual reduction in flowability

Aortobronchial fistula and Listeria endograft infection after repeated T/EVAR: a rare combination

Here we present a rare combination of aortobronchial fistula and Listeria endograft infection after repeat endovascular aortic repair. Device retention, debridement and negative pressure wound therapy, in combination with suppressive antimicrobial therapy, led to satisfactory control of infection until the patient died due to another complication. The combination of an aortobronchial fistula and Listeria endograft infection has never been described before. This present case should encourage and show clinicians the importance of an interdisciplinary approach in highly difficult clinical courses

‘Show Me What It’S Supposed To Look Like’: Exploring Mindfulness-Based Support For Early Career Teachers In An Era Of Neoliberal Reform

In an era of top-down mandates and neoliberal reform policies, early career teachers must receive sustained social and emotional support as part of their regular professional development in order to navigate the murky waters of education that can sometimes feel disempowering and disillusioning. Teachers in high-poverty urban schools, who often deal with additional variables like student achievement gaps, disparate funding and high teacher turnover, among myriad other hurdles, are often highly impacted by these policies. This study examined the experiences of a group of early career teachers in an urban school district that participated in a mindfulness-based professional learning intervention. Findings show that participants not only experienced increased self-efficacy in assuaging personal and professional stressors, but also connected newly acquired strategies to the ways they supported the diverse students in their classrooms. Implications from this study suggest that learning about mindfulness-based strategies in an informal group setting can be beneficial for teachers, both personally and professionally, as they navigate the tensions of beginning their careers embedded in “the neoliberal program of reform”

Bacillary angiomatosis and bacteremia due to bartonella quintana in a patient with chronic lymphocytic leukemia

We present a 63-year-old man treated with alemtuzumab for chronic lymphocytic leukemia who developed multiple angiomatous papules and fever. Real-time polymerase chain reaction (RT-PCR) from a skin lesion and blood sample revealed Bartonella quintana as causative agent confirming the diagnosis of bacillary angiomatosis with bacteremia. Treatment with doxycycline, initially in combination with gentamicin, led to complete resolution of the lesions. This case shows the importance of considering bacillary angiomatosis as a rare differential diagnosis of angiomatous lesions in the immunocompromised patient, particularly in chronic lymphocytic leukemia and following lymphocyte depleting treatments as alemtuzumab