Gas-side particulate fouling in biomass gasifiers

Particulate fouling is defined as the accumulation of particles on a heat exchanger-surface forming an insulating powdery layer. Particulate fouling in biomass gasifiers is a major problem that may lead to inefficient operation. As observed in a large-scale biomass gasifier the character of the fouling layer is related to the local gas temperature and velocity. At high gas-side temperatures, the fouling layer structure changes from a fragile powdery form to a coherent sintered structure. Particulate fouling of heat exchangers shows an asymptotic growth rate with a levelling off increase of the thickness. The fouling growth rate is determined by the difference between the deposition rate and the removal rate of particles on and from the fouling layer respectively. Till now, most attention has been given to the deposition process. The objective of this research project is to study and model removal of particles from particulate fouling layers as a function of the fouling layer structure. By integrating the removal rate and the depositing rate a complete perspective can be given about the fouling process. Two mechanisms contribute to the removal of particles from fouling layers: the transfer of kinetic energy from an incident particle to bed particles or the forces exerted on the bed of particles by a shear flow. To study the removal of particles from fouling layers by shear flow, fouling experiments in a heat exchanger set-up have been done with particles of different sizes and different materials running under different gas speeds. It is found that the smallest particles in the flow deposit first on the tubes of the heat exchanger at areas of minimum flow velocities. Then the larger particles deposit and the fouling layer starts to build up. The fouling layer thickness and growth over the heat exchanger tube is influenced by the flow speed. As the flow speed in the heat exchanger increases, the thickness and the surface area of the fouling layer deposited over the heat exchanger tube are reduced. There is a limiting flow speed above which fouling is avoided. This limiting speed appears to be related to the critical flow velocity required to roll a particle resting on a flat surface. To prevent fouling, the gas speed in a heat exchanger should be larger than the critical flow velocity that corresponds to the particle size most likely to stick on the heat exchanger tube. It appeared that the removal of particles from fouling layers by shear flow becomes important at high gas speeds. The second mechanism, which is the removal of particles by an incident particle impact, is the dominant removal mechanism at low gas speeds. The removal of particles from powdery fouling layers due to an incident particle impact is investigated numerically and experimentally. A numerical model is developed to simulate the interaction between a particle hitting a bed of particles. The numerical model is based on the discrete element method. The forces during collision are based on the concepts of contact mechanics. The numerical model can predict the particle velocity at which an incident particle starts either to stick, rebound or remove other particles from the impacted bed of particles. The critical sticking and removal velocities of a particle hitting a powdery layer become independent of the layer thickness, if the thickness is larger than a certain limit. This limit is of a thickness of 2 particles in case of a monodisperse particulate layer and when the particles in the layer are arranged in an orthorhombic structure. It is found that the ejection time of particles from a bed of particles due to an incident particle impact is proportional to its diameter and to the square root of the number of bed layers. To validate the numerical model, experiments are carried out in a vacuumed column. In the experiment, incident particles drop on a bed of particles and the sticking, bouncing and removal behaviour is measured as a function of the incident particle impact speed. The numerical model predictions regarding the critical sticking and removal velocities are in agreement with the experimental results. During operation of heat exchangers, particulate fouling layers may sinter due to the high gas-side temperature. The influence of the gas-side temperature on the fouling layer structure and consequently on the removal and deposition of particles are investigated. The fouling layer structure is dependent on the gas-side temperature in relation to the minimum sintering temperature. Impaction experiments are carried out to determine the sticking and removal velocities for an incident particle hitting a bare tube surface, a powdery fouling layer and a sintered layer. The change in the heat exchanger surface from a bare tube to a powdery one increases the critical sticking velocity with at least one order of magnitude, which consequently speeds up the fouling process. The further change in the heat exchanger surface from a powdery form to a sintered one lowers significantly the ability of an incident particle to stick on the fouling layer or to remove particles out of the fouling layer. Particles that are still able to deposit on the sintered fouling layer will not sinter immediately and can therefore be removed, due to an incident particle impact. Sintering reduces the fouling rate of heat exchangers by lowering the deposition rate of new particles and increasing the removal rate of newly deposited particles, such that the fouling process becomes as slow as the formation of a single layer on a bare tube during the initiation period. When the initiation period is longer than the characteristic sintering time, the newly deposited particles become sintered and we revert again to the sintered case, which leads to a very slow fouling process known as the asymptotic behaviour of particulate fouling layers. The numerical model is extended to account for an incident particle hitting a sintered layer. Summarizing, the removal and deposition of particles from particulate fouling layers as a function of the fouling layer structure have been numerically modelled and the model is experimentally validated. By combining the numerical model together with a CFD-particle transport model, the growth rate of the particulate fouling layer around a heat exchanger tube can be quantitatively predicted

Sress and strain concentration factors for plate with small notch subjected to biaxial loading – Three dimensional finite element analysis

AbstractThe through-thickness variations of stress and strain concentration factors for plate with small central notch, circular notch or double U-notch, subjected to uniaxial and biaxial loading have been systematically analyzed by using three dimensional finite element method (3D FEM). It is found that the maximum stress and strain concentration factors occur on the mid plane of plate only in the case of thin plate. However, in the case of thick plates, the sites of these maximum values are found near the plate surface. Furthermore, this site is more close to the plate surface in the case of small notch radius and/or large plate thickness. The stress and strain concentration factors increase with decreasing the biaxial ratio at the plate interior, while, the opposite trend is found at the plate surface

GARANTIR O DIREITO DAS CRIANÇAS À EDUCAÇÃO: EQUILÍBRIO ENTRE RESPONSABILIDADE LEGAL E A ARBITRARIEDADE DA AUTORIDADE PARENTAL

In Saudi Arabia, the right to education for children is considered of utmost importance through the official education system, which provides a legal framework to protect them. This system mandates the provision of basic education for all children without any discrimination. and it is essential for parents to actively participate in this field by cooperating with educational authorities and providing a supportive environment for children's learning. Despite these legal guarantees, there remain challenges related to effectively implementing these policies, and there may be instances of arbitrariness or negligence in providing education by parents. Therefore, establishing effective mechanisms for monitoring implementation and holding responsible parties accountable is necessary. Additionally, emphasis is placed on raising awareness about children's rights and the importance of obtaining a good education within society. Consequently, ensuring this right requires integrated efforts between the government and civil society to improve the quality of education and its provision.  Na Arábia Saudita, o direito à educação das crianças é considerado da maior importância através do sistema de ensino oficial, que proporciona um quadro jurídico para as proteger. Este sistema obriga à oferta de ensino básico a todas as crianças sem qualquer discriminação e é essencial que os pais participem ativamente neste domínio, cooperando com as autoridades educativas e proporcionando um ambiente favorável à aprendizagem das crianças. Apesar destas garantias legais, continuam a existir desafios relacionados com a aplicação efectiva destas políticas, podendo haver casos de arbitrariedade ou negligência na prestação de educação por parte dos pais. Por conseguinte, é necessário estabelecer mecanismos eficazes de controlo da aplicação e de responsabilização dos responsáveis. Além disso, a tónica é colocada na sensibilização para os direitos das crianças e para a importância de obter uma boa educação na sociedade. Consequentemente, a garantia deste direito exige esforços integrados entre o governo e a sociedade civil para melhorar a qualidade do ensino e a sua oferta

Fault tolerant radiation monitoring system using wireless sensor and actor network in a nuclear facility

In nuclear facilities, the reading of the sensors is very important in the assessments of the system state. The existence of an abnormal state could be caused by a failure in the sensor itself instead of a failure in the system. So, being unable to identify the main cause of the “abnormal state” and take proper actions may end in unnecessary shutdown for the nuclear facility that may have expensive economic consequences. That is why, it is extremely important for a supervision and control system to identify the case where the failure in the sensor is the main cause for the existence of an abnormal state. In this paper, a system based on a wireless sensor network is proposed to monitor the radiation levels around and inside a nuclear facility. A new approach for validating the sensor readings is proposed and investigated using the Castalia simulator

Evaluation of peripheral lymphocyte subsets' alteration and IL6 serum level correlated with Severity and outcome in Corona virus disease 2019 (COVID-19)

Background: Coronavirus disease 2019 (COVID-19) has rapid spread worldwide and its pathogenesis is still not well understood. It's critical to identify the key immune inflammatory markers that may be correlated with COVID-19 severity. This study aimed to study the association of the peripheral lymphocyte subsets alteration and IL-6 serum level with disease severity and outcome in COVID-19. Methods: Samples from 30 COVID-19 patients were collected; one is EDTA anticoagulated for flowcytometric analysis of different lymphocyte subsets and the other for Interleukin-6 (IL6) serum level assessed by ELISA technique. Results: Absolute lymphocytic count (0.9 (0.5 - 1.4)× 103/µL) , CD4+ T cells (217 (135.6 - 445.5) cells/µL) , CD8+ T cells (160 (112 - 338) cells/µL) and natural killer (NK) cells (33.3 (18.2 - 99.5) cells/µL)  were significantly reduced in severe COVID-19 patients with significantly elevated IL-6 serum levels 90 (70-120) (pg/mL) in severe patients. Lower T lymphocytes and NK subset counts with higher IL-6 levels were significantly associated with higher mortality. However, B cell count was not associated with severity or mortality. Il-6 levels, CD4+ and CD8+ T cells counts were considered best predictors of disease severity and mortality according to ROC curve analysis (with AUC 0.842, 0.884 and 0.773 respectively). Conclusion:Peripheral lymphocyte subsets as CD4+ T cells, CD8+ T cells and NK cells were significantly reduced in severe COVID-19 patients. CD4+ T cell count was the most significant biomarker for disease severity .Serum IL-6 levels were higher in severe illness. So, IL-6 can serve as a significant predictor of COVID-19 severity. As regard mortality and relation with lymphocytic count and lymphocytic subsets, total lymphocytic count and all T lymphocyte subsets CD4+, CD8+ and CD56+ cells count can be used as a significant predictor of death in COVID-19 patients. However, CD19+ cells counts had no relation with death

Fault tolerant radiation monitoring system using wireless sensor and actor network in a nuclear facility

In nuclear facilities, the reading of the sensors is very important in the assessments of the system state. The existence of an abnormal state could be caused by a failure in the sensor itself instead of a failure in the system. So, being unable to identify the main cause of the “abnormal state” and take proper actions may end in unnecessary shutdown for the nuclear facility that may have expensive economic consequences. That is why, it is extremely important for a supervision and control system to identify the case where the failure in the sensor is the main cause for the existence of an abnormal state. In this paper, a system based on a wireless sensor network is proposed to monitor the radiation levels around and inside a nuclear facility. A new approach for validating the sensor readings is proposed and investigated using the Castalia simulator

OPTIMIZATION OF FLOW DIRECTION TO MINIMIZE PARTICULATE FOULING OF HEAT EXCHANGERS

The influence of flow direction with respect to gravity on particulate fouling of heat exchangers is investigated experimentally to determine the optimal flow direction to minimize fouling. Four orientations of flow have been investigated, horizontal flow, upward flow, downward flow and a flow under an angle of 45°. It is observed that fouling starts at the point of stagnation irrespective of the flow direction, and also at the top of the heat exchanger tubes. Particulate fouling grows from these two points till they meet and the fouling layer covers the whole surface of the heat exchanger tube. Fouling at the upper half of the tubes is much faster than the lower half of the tubes, and the fouling rate is faster at the bottom tubes of the heat exchanger section than at the upper tubes. The best orientation for lingering particulate fouling is the downward flow, where the flow stagnation point coincides with the top point of the heat exchanger tubes and the growth of the fouling layer only starts from one point

The effect of biventricular pacing on cardiac function after open heart surgery

Background: Temporary postoperative pacing could enhance recovery of the cardiac function. The right ventricular pacing (RV) is commonly used, but it can cause dyssynchronous contraction of both ventricles. Biventricular pacing (BV) could improve the systolic function by synchronizing the ventricular contraction. The aim of this study is to evaluate the effectiveness of biventricular pacing in improving the hemodynamics in the early postoperative period compared to other pacing modes. Methods: This is a clinical crossover trial including 50 patients who underwent open cardiac surgery in the period from September 2017 to September 2018. Mean age was 46.78± 12.09 years, and 50% were males. Temporary pacing leads were attached to the anterior wall of the right ventricle 1-2 cm paraseptally and the lateral wall of left ventricle 1-2 cm paraseptally. Each patient was paced for 3 minutes in the first 1-4 postoperative hours with 20 minutes washout period between different pacing modes. Study endpoints included cardiac output, ejection fraction (EF) and wall motion abnormality. Results: Biventricular and right ventricular pacing increased postoperative cardiac output (6.31± 1.28 and 5.2±0.72 L/min; respectively), but BV pacing was superior to RV pacing (P-value <0.001). The effect of BV pacing was more evident in patients with EF < 50% (7.27± 0.895 vs. 5.26 ± 0.634 L/min; p< 0.001). The postoperative EF improved during BV pacing (53.16± 4.71%) compared to RV pacing (49.4± 4.07%; P-value <0.001). Both BV and RV pacing were associated with less paradoxical septal wall motion abnormality (P-value <0.001). Conclusions: Temporary postoperative biventricular pacing improves hemodynamics compared to right ventricular and no pacing. Routine BV pacing is recommended especially in patients with low ejection fraction

Fetal adrenal biometry, cervical assessment and inflammatory cytokine for prediction of preterm labor: Comparative study

Background: Preterm birth is a multifactorial problem with a confounding management. Latent interval (Time to delivery) always shows wide variation and presence of a valid, reliable and applicable predictor is a controversial issue.Objectives: to evaluate the role of fetal adrenal biometry (corrected-total gland volume c-TGV and fetal zone enlargement FZE), cervical length and IL17 serum level (maternal inflammatory biomarker) in prediction of preterm that could help in proper management and decreased morbidities.Patient and Methods: Observational comparative study for 100 case (28 to 36 weeks gestation) showing clinical signs of threatened preterm labor. Corrected fetal adrenal gland volume, fetal zone enlargement, cervical length and IL17 serum level were measured then all candidates were followed up till delivery to classify the results into two groups according to the (latent interval). Diagnostic performance for all variables was done to detect the best cutoff value that can predict impending preterm delivery within 7 days using univariate analysis and receiver operating characteristic (ROC) curve.Results: The studied predictors were showing best cutoff, sensitivity%, specificity% and accuracy% as follow: c-TGV (≥400 mm3/kg, 80, 75, 85), FZE(≥ 50%, 90, 80, 92), cervical length (≤16 mm, 70, 67, 65), IL17 serum level (8 pg/ml, 80, 95, 83) respectively.Conclusion Utility of corrected fetal adrenal gland volume, fetal zone enlargement in addition to the IL17 (inflammatory marker) as a non-invasive predictors for impending preterm birth can guide a proper decision