High density gas-solids circulating fluidized bed riser and downer reactors

A systematic and comprehensive study of hydrodynamics and reactor performance was conducted in a 76 mm i.d., 10 m high riser and a 76 mm i.d., 5.8 m high downer reactor under high density/flux operating conditions using fluid catalytic cracking (FCC) catalyst particles. An optical fiber probe was used to obtain a complete mapping of local solids holdup and particle velocity. Catalytic ozone decomposition reaction was employed to study the characteristics of reactor performance in the CFB riser and downer. The superficial gas velocity (Ug) and the solids circulation rate (Gs) were 3-9 m/s and 100-1000 kg/m2·s, respectively. Based on the spatial distributions of catalyst particles and gas reactant in the riser and the downer, hydrodynamics and reactor performance were fully characterized. Solids suspension having a solids holdup of up to 0.2-0.3 could be maintained throughout the entire high flux/density riser. A homogenous axial flow structure was observed at Gs = 1000 kg/m2s. When Gs exceeded about 800 kg/m2s, the axial profile of the particle velocity became more uniform. The axial particle velocity was affected more significantly by high superficial gas velocity especially under high solids flux/density conditions. No net downward flow near the wall was one of the most important advantages of the high flux/density riser over the conventional low flux/density reactor, leading to a reduction of solids backmixing. Radial distributions of the solids holdup were nonuniform with a dilute region and a dense region. When Gs was higher than 700 kg/m2s, the dilute core region shrank to less than 20% of the cross-sectional area. Solids holdups thereafter increased monotonically towards the wall which could be up to 0.59. Moreover, solids holdup remained higher than 0.4 over a wide cross-sectional area (r/R = 0.7-1.0, about 60% of the cross-sectional area) even at the top section of the riser. Radial distribution of solids holdup in the downer was much more uniform than that in the riser. Radial profiles of solids holdup were characterized by a flat value covering a wide region of the cross section and a relatively high value near the wall in the fully developed section. The uniform distribution of solids flow provided a nearly plug flow condition in the downer reactor. As to the ozone reaction in the CFB system, the axial and radial profiles of the ozone concentration were consistent with the corresponding profiles of the solids holdups which indicated that ozone reaction in the CFB reactors was controlled by the gas-solids flow structure. Strong interrelation was observed between the distributions of solids and reactant concentration. Higher solids holdups would give higher ozone conversions. Most conversion occurred in the entrance region, that is, the flow developing zone of the riser and downer reactors. Overall ozone conversions in CFB riser and downer deviated from plug-flow behavior indicating that hydrodynamics affected CFB reactor performance. The extent of the deviation of the conversion could be attributed to the different gas-solids contacting efficiency

Lateral distribution calculation of multi-I beam composite curved bridge with slip effect

This paper presents a modified rigid cross beam method to study the lateral distribution of multi-I beam composite curved bridge with slip effect. First, the effective stiffness expression of single composite curved beam with slip effect were established and calculated by the FEM. Secondly, the lateral load distribution of multi-I beam composite curved bridge is obtained by substituting the effective stiffness of the main composite curved beam into the rigid cross beam method. Finally, The FEM numerical examples show that this method can accurately describe the load distribution characteristics of multi-I beam composite curved bridge with slip effect

Spatio-temporal Joint Modelling on Moderate and Extreme Air Pollution in Spain

Very unhealthy air quality is consistently connected with numerous diseases. Appropriate extreme analysis and accurate predictions are in rising demand for exploring potential linked causes and for providing suggestions for the environmental agency in public policy strategy. This paper aims to model the spatial and temporal pattern of both moderate and extremely poor PM10 concentrations (of daily mean) collected from 342 representative monitors distributed throughout mainland Spain from 2017 to 2021. We firstly propose and compare a series of Bayesian hierarchical generalized extreme models of annual maxima PM10 concentrations, including both the fixed effect of altitude, temperature, precipitation, vapour pressure and population density, as well as the spatio-temporal random effect with the Stochastic Partial Differential Equation (SPDE) approach and a lag-one dynamic auto-regressive component (AR(1)). Under WAIC, DIC and other criteria, the best model is selected with good predictive ability based on the first four-year data (2017--2020) for training and the last-year data (2021) for testing. We bring the structure of the best model to establish the joint Bayesian model of annual mean and annual maxima PM10 concentrations and provide evidence that certain predictors (precipitation, vapour pressure and population density) influence comparably while the other predictors (altitude and temperature) impact reversely in the different scaled PM10 concentrations. The findings are applied to identify the hot-spot regions with poor air quality using excursion functions specified at the grid level. It suggests that the community of Madrid and some sites in northwestern and southern Spain are likely to be exposed to severe air pollution, simultaneously exceeding the warning risk threshold

Towards temporal verification of swarm robotic systems

A robot swarm is a collection of simple robots designed to work together to carry out some task. Such swarms rely on the simplicity of the individual robots; the fault tolerance inherent in having a large population of identical robots; and the self-organised behaviour of the swarm as a whole. Although robot swarms present an attractive solution to demanding real-world applications, designing individual control algorithms that can guarantee the required global behaviour is a difficult problem. In this paper we assess and apply the use of formal verification techniques for analysing the emergent behaviours of robotic swarms. These techniques, based on the automated analysis of systems using temporal logics, allow us to analyse whether all possible behaviours within the robot swarm conform to some required specification. In particular, we apply model-checking, an automated and exhaustive algorithmic technique, to check whether temporal properties are satisfied on all the possible behaviours of the system. We target a particular swarm control algorithm that has been tested in real robotic swarms, and show how automated temporal analysis can help to refine and analyse such an algorithm. © 2012 Elsevier B.V. All rights reserved

Experimental study and mass transfer modelling for extractive desulfurization of diesel with ionic liquid in microreactors

Conventional hydrodesulfurization technology was limited to treat aromatic heterocyclic sulfur compounds in ultralow-sulfur diesel. Extractive desulfurization (EDS) using ionic liquid (IL) exhibited good performance to address these issues, except for its long extraction time (15-40 min). To address this, microreactor was adopted to intensify the IL-based EDS, where dibenzothiophene was extracted from model diesel (MD) as the continuous phase to 1-butyl-3-methylimidazolium tetrafluoroborate as the dispersed phase under segmented flow (which appeared preferably at capillary numbers lower than 0.01). The effects of temperature, residence time and flow rate ratio on the desulfurization efficiency were investigated. The extraction equilibration time could be shortened from more than 15 min in conventional batch extractors to 120 s in microreactors. The extraction process was modeled according to the two-film model applied within a unit cell of the segmented flow, where the mass transfer resistance was considered primarily on the film side of the IL droplet. The mechanism for the improved EDS performance at higher temperatures or larger IL to MD flow ratios was investigated and validated, which was related to the significant increase in the diffusion coefficient or the specific interfacial area. These findings may shed important insights into the precise manipulation of IL-based EDS for a better process design and reactor optimization