3D modelling by computational fluid dynamics of local interactions of momentum, mass and heat transfers with catalyst deactivation in gas-solid catalytic reactors of low aspect ratios

Packed beds of gas-solid systems are extensively used as reactors, separators, dryers, filters, heat exchangers and combustors. The design of packed beds requires a detailed knowledge of local dynamics of flow, composition and temperature. Unfortunately, investigations for the development of 3D modelling codes by computational fluid dynamics are still not sufficiently mature compared with those relying on 2D modelling or simplified pseudo-homogenous models. This project investigates non-uniform catalyst deactivation in packed bed reactors of low aspect ratios under steady-state and dynamic operations. Low aspect ratio packed beds were selected as they are known to generate non-uniform distribution of local flow. Detailed knowledge of flow dynamics in terms of local structure of the packed bed, pressure drops, interstitial flow, heat and mass rate distributions was examined. The discrete element method was used to generate various packing configurations and the results of profiles of porosity were in a good agreement with the semi-analytical models, especially, in the vicinity of the wall. Similar oscillation trends with damping profiles towards the centre of the packed beds were observed. Flow heterogeneity was assessed by tests of mass transfer dispersion through a Lagrangian approach. Interactions of fluid flow, mass and heat transfers, and local deactivation of alumina catalyst Al2O3 of CO oxidation were investigated under design and operating conditions. An increase in the activation energy of deactivation promoted the deactivation by accelerating the reaction rate and releasing additional thermal energy, which in turn accelerated the deactivation. The 3D modelling allowed observation of local catalyst deactivation at packing pore level which is typically not accessible by the 2D modelling or pseudo-homogeneous models. In addition, the deactivation was quite asymmetrical along axial and radial directions, leading to uneven rates of thermal expansion and contraction and causing local deactivation associated with temperature runaways

A study of differential and integral operators in linear viscoelasticity

This thesis identifies and explores a link between the theory of linear viscoelasticity and the spectral theory of Sturm-Liouville problems. The thesis is divided into five chapters. Chapter 1 gives a brief account of the relevant parts of the theory of linear viscoelasticity and lays the foundation for making the link with spectral theory. Chapter 2 is concerned with the construction of approximate Dirichlet series for completely monotonic functions. The chapter introduces various connections between non-negative measures, orthogonal polynomials, moment problems, and the Stieltjes continued fraction. Several interlacing properties for discrete relaxation and retardation times are also proved. The link between linear viscoelasticity and spectral theory is studied in detail in Chapter 3. The stepwise spectral functions associated with some elementary viscoelastic models are derived and their Sturm-Liouville potentials are explicitly found by using the Gelfand-Levitan method for inverse spectral problems. Chapter 4 presents a new family of exact solutions to the nonlinear integrodifferential A-equation, which is the main equation in a recent method proposed by Barry Simon for solving inverse spectral problems. Starting from the A-amplitude A(t) = A(t, 0) which is determined by the spectral function, the solution A(t, x) of the A-equation identifies the potential q(x) as A(0, x). Finally, Chapter 5 deals with two numerical approaches for solving an inverse spectral problem with a viscoelastic continuous spectral function. In the first approach, the A-equation is solved by reducing it to a system of Riccati equations using expansions in terms of shifted Chebyshev polynomials. In the second approach, the spectral function is approximated by stepwise spectral functions whose potentials, obtained using the Gelfand-Levitan method, serve as approximations for the underlying potentia

Solutions of Some Difference Equations Systems and Periodicity

In this article, analysis and investigation have been conducted on the periodic nature as well as the type of the solutions of the subsequent schemes of rational difference equations with a nonzero real numbers initial conditions

Synchronization of decentralized event-triggered uncertain switched neural networks with two additive time-varying delays

This paper addresses the problem of synchronization for decentralized event-triggered uncertain switched neural networks with two additive time-varying delays. A decentralized eventtriggered scheme is employed to determine the time instants of communication from the sensors to the central controller based on narrow possible information only. In addition, a class of switched neural networks is analyzed based on the Lyapunov–Krasovskii functional method and a combined linear matrix inequality (LMI) technique and average dwell time approach. Some sufficient conditions are derived to guarantee the exponential stability of neural networks under consideration in the presence of admissible parametric uncertainties. Numerical examples are provided to illustrate the effectiveness of the obtained results.&nbsp

Deactivation of the preferential oxidation of CO in packed bed reactor by 3D modelling and near-infrared tomography

Scaling up the results on catalyst deactivation to industrial operations, where transport phenomena are of significance, is often not straightforward. The operations of industrial reactors are judiciously focused on the dynamics of the deactivation along the axial length of the reactors, which are generally known approximately. Processes of strong energy release or fast chemical kinetics, such as oxidation reactions, cracking, etc., are associated with a deactivation where the time characteristics of the flow and transports are of magnitudes of the deactivation time-on-stream. Local deactivation of the preferential oxidation of CO was investigated by three-dimensional modelling of flow, mass and heat transfers inside a packed-bed reactor and validated by near-infrared tomography. The profiles of deactivation were sensitive to the rates of deactivation, heat transfer by dispersion and intra-particle mass transfer. At pore scale of the packing, pronounced deactivation was revealed near the wall due to a preferential flow circulation. The deactivation progressed at the exteriors of the catalytic particles, particularly over the regions in contact with the convective flow. Unlike the mass dispersion, the heat dispersion promoted the deactivation by shifting the moving waves of deactivation upstream, leading to asymmetrical maps inside the catalytic particles

Gas flow visualisation in low aspect ratio packed beds by three-dimensional modelling and near-infrared tomography

Nonuniform local flow inside randomly porous media of gas-solid packed beds of low aspect ratios ranging from 1.5 to 5 was investigated by three-dimensional modelling and near-infrared tomography. These beds are known to demonstrate heterogeneous mixing and uneven distributions of mass and heat. The effects of the confining wall on flow dynamics were found nonlinear, particularly for aspect ratios lower than 3. High velocities were mainly observed in regions near the wall of aspect ratio value of 1.5 and those of values higher than 3, owing to high local porosities in these zones. Mass dispersion characterised by both experimental near-infrared imaging and by particle tracking showed discrepancies with literature models, particularly for aspect ratios lower than 3. Uncertainties were more significant with the radial dispersion due bed size limits. Beyond this value, the wall affected more the axial dispersion, confirming the nonlinear impact of the wall on global hydrodynamic

Room-Temperature Quantum Emitter in Aluminum Nitride

A device that is able to produce single photons is a fundamental building block for a number of quantum technologies. Significant progress has been made in engineering quantum emission in the solid state, for instance, using semiconductor quantum dots as well as defect sites in bulk and two-dimensional materials. Here we report the discovery of a room-temperature quantum emitter embedded deep within the band gap of aluminum nitride. Using spectral, polarization, and photon-counting time-resolved measurements we demonstrate bright ($>10^5$ counts per second), pure ($g^{(2)}(0) < 0.2$), and polarized room-temperature quantum light emission from color centers in this commercially important semiconductor

Examine the Perceived Risk of Falls Among Patients Receiving Acute Care

Purpose: In an effort to lower the number of falls that occur among hospitalized patients, several facilities have begun introducing various fall prevention programs. However, the efficacy of fall prevention programs is diminished if patients do not consider themselves to be at risk for falls and do not follow recommended procedures. The goal of this study was to characterize how patients in four different acute care specialist services felt about their risk of falling while in the hospital. Methods: One hundred patients admitted to the study hospital with a Morse Fall Scale score of 45 or higher were given the Patient Perception Questionnaire, a tool designed to assess a patient's perception of their own fall risk, fear of falling, and motivation to take part in fall prevention efforts. Scores on the Morse Fall Scale were gathered through a historical assessment of medical records. Descriptive statistics, Pearson's correlation coefficients, and independent sample t tests were used to examine the data. Results: The average age was 65, and around half (52%) were men and half (48%) were women. Based on their ratings on the Morse Fall Scale, all 100 participants were classified as being at high risk for falls. However, only 55.5% of the individuals agreed with this assessment. The likelihood that a patient would seek assistance and the degree to which they feared falling both declined as their faith in their mobility improved. Patients hospitalized after a fall exhibited considerably lower confidence scores and greater fear scores than patients who had not been injured in a fall. Conclusions: Patients who have a high fall risk assessment score may not believe they are at risk for falls and may not take any steps to reduce their risk. The prevalence of falls in hospitals might be mitigated by the creation of a fall risk assessment technique that takes into account both objective and subjective factors

Room-temperature quantum emitter in aluminum nitride

A device that is able to produce single photons is a 5 fundamental building block for a number of quantum technologies. 6 Significant progress has been made in engineering quantum emission in 7 the solid state, for instance, using semiconductor quantum dots as well 8 as defect sites in bulk and two-dimensional materials. Here we report 9 the discovery of a room-temperature quantum emitter embedded deep 10 within the band gap of aluminum nitride (AlN). Using spectral, 11 polarization, and photon-counting time-resolved measurements we 12 demonstrate bright (>105 counts s−1), pure (g2(0) < 0.2), and polarized 13 room-temperature quantum light emission from color centers in this 14 commercially important semiconductor