Simulation of Particle Mixing and Separation in Multi-Component Fluidized Bed Using Eulerian-Eulerian Method: A Review

In practical engineering applications, the mixing and separation behavior of multi-component particles is importance to the fluidized bed operation. The development of many practical processes is inseparable from the knowledge of particle mixing and separation, such as material processing of ash-soluble coal gasification, multi-phase flow in boilers, and petrochemical catalytic processes. In recent years, due to the obvious advantages of the Eulerian–Eulerian model, many researchers at home and abroad have used it to study the mixing and separation behavior of particles. The paper reviews the use of Eulerian–Eulerian model to study the mixing and separation of multi-component particles in fluidized beds. The Eulerian–Eulerian model describes the gas-phase and each of the individual particles as continuums. The mechanism of particle mixing and separation, the influence of different factors on the particle mixing and separation including differences in particle size and density, the differences in apparent air velocity, the differences in model factors are discussed. Finally, an outlook for the use of Eulerian–Eulerian model to study the mixing and separation behavior of three component particles and related research on the drag model between particles

Development of filtered Euler–Euler two-phase model for circulating fluidised bed: High resolution simulation, formulation and a priori analyses

Euler–Euler two-phase model simulations are usually performed with mesh sizes larger than the smallscale structure size of gas–solid flows in industrial fluidised beds because of computational resource limitation. Thus, these simulations do not fully account for the particle segregation effect at the small scale and this causes poor prediction of bed hydrodynamics. An appropriate modelling approach accounting for the influence of unresolved structures needs to be proposed for practical simulations. For this purpose, computational grids are refined to a cell size of a few particle diameters to obtain mesh-independent results requiring up to 17 million cells in a 3D periodic circulating fluidised bed. These mesh-independent results are filtered by volume averaging and used to perform a priori analyses on the filtered phase balance equations. Results show that filtered momentum equations can be used for practical simulations but must take account of a drift velocity due to the sub-grid correlation between the local fluid velocity and the local particle volume fraction, and particle sub-grid stresses due to the filtering of the non-linear convection term. This paper proposes models for sub-grid drift velocity and particle sub-grid stresses and assesses these models by a priori tests

Numerical and experimental study of hydrodynamics in a compartmented fluidized bed oil palm shell biomass gasifier

Numerical and experimental studies of hydrodynamic parameters of fluidized beds formed by either a single component system or a binary mixture in a pilot plant scale model of a Compartmented Fluidized Bed Gasifier (CFBG) have been performed. The numerical study is carried out with an Eulerian-Eulerian description of both gas and particle phases and a standard drag law for multiphase interaction. The numerically simulated results are then compared with the experimental results.The 2D and 3D flow patterns of the combustor and the gasifier are first generated from the numerical study to observe the bubble formation, possible channeling behavior and the binary mixing patterns in the bed.For a single component system, detailed 3D numerical analyses and experimental studies are done to investigate the bed expansion ratio, bubble diameter, bed pressure drop, and fluidization quality in CFBG. Two types of Geldart B inert particles namely river sand and alumina are used in the study.All trends of the aforementioned studies are well-predicted with the numerical values not greater than 15% of the recorded experimental values. Good fluidization is attainable in the combustor side, while the pressure drop behaviour seen for the gasifier with river sand shows that channelling occurs in the bed. The channelling behaviour becomes more severe with alumina bed.The solid circulation rate (SCR) is numerically simulated in this study as well. Solid circulation rate (SCR) increases with the increase in bed height while the main bed aeration does not affect the SCR which is consistent with the experimental data.For a binary mixture system with palm shell and river sand as the second fluidizing material, detailed 3D numerical analysis of the bed expansion ratio is done in parallel with the experimental study. The results of numerical predictions of overall mixing quality and local mixing index are verified by comparing with the experimental results. The actual trends of the studies are modestly captured by the numerical model with under-predicted values of less than 20%. The overall binary mixing quality is enhanced with the smaller palm shell size and larger palm shell weight percent. In addition, increasing the superficial gas velocity increases the local binary mixing index in the experiment.From the studies on bed expansion, bubble formation, steady equilibrium state and overall binary mixing quality, the 2D model provides well over-predicted values compared to the 3D flow model. Also, the local mixing index of the binary system is not captured by the 2D model. The numerical values predicted by 3D model are closer to the actual values.The key findings from the aforementioned studies are used as a guide to develop and operate the pilot plant scale CFBG with 0.5 ton/day of palm shell feed for fuel gas production

Subgrid models for heat transfer in multiphase flows with immersed geometry

Multiphase flows are ubiquitous across engineering disciplines: water-sediment river flows in civil engineering, oil-water-sand transportation flows in petroleum engineering; and sorbent-flue gas reactor flows in chemical engineering. These multiphase flows can include a combination of momentum, heat, and mass transfer. Studying and understanding the behavior of multiphase, multiphysics flow configurations can be crucial for safe and efficient engineering design. In this work, a framework for the development and validation, verification and uncertainty quantification (VVUQ) of subgrid models for heat transfer in multiphase flows is presented. The framework is developed for a carbon capture reactor; however, the concepts and methods described in this dissertation can be generalized and applied broadly to multiphase/multiphysics problems. When combined with VVUQ methods, these tools can provide accurate results at many length scales, enabling large upscaling problems to be simulated accurately and with calculable errors. The system of interest is a post-combustion solid-sorbent carbon capture reactor featuring a solid-sorbent bed that is fluidized with post-combustion flue gas. As the flue gas passes through the bed, the carbon dioxide is exothermically adsorbed onto the sorbent particle’s surface, and the clean gas is passed onto further processes. To prevent overheating and degradation of the sorbent material, cooling cylinders are immersed in the flow to regulate temperatures. Simulating a full-scale, gas-particle reactor using traditional methods is computationally intractable due to the long time scale and variations in length scales: reactor, O(10 m); cylinders, O(1 cm); and sorbent particles, O(100 um). This research developed an efficient subgrid method for simulating such a system. A constitutive model was derived to predict the effective suspension-cylinder Nusselt number based on the local flow and material properties and the cylinder geometry, analogous to single-phase Nusselt number correlations. This model was implemented in an open source computational fluid dynamics code, MFIX, and has undergone VVUQ. Verification and validation showed great agreement with comparable highly-resolved simulations, achieving speedups of up to 10,000 times faster. Our model is currently being used to simulate a 1 MW, solid-sorbent carbon capture unit and is outperforming previous methods in both speed and physically accuracy.2017-06-21T00:00:00

Development of a Simulation Model for Fluidized Bed Mild Gasifier

A mild gasification method has been developed to provide an innovative clean coal technology. The objective of this study is to developed a numerical model to investigate the thermal-flow and gasification process inside a specially designed fluidized-bed mild gasifier using the commercial CFD solver ANSYS/FLUENT. Eulerain-Eulerian method is employed to calculate both the primary phase (air) and secondary phase (coal particles). The Navier-Stokes equations and seven species transport equations are solved with three heterogeneous (gas-solid), two homogeneous (gas-gas) global gasification reactions. Development of the model starts from simulating single-phase turbulent flow and heat transfer to understand the thermal-flow behavior followed by five global gasification reactions, progressively with adding one equation at a time. Finally, the particles are introduced with heterogeneous reactions. The simulation model has been successfully developed. The results are reasonable but require future experimental data for verification

Computationsl modelling of dimethyl ether separation and steam reforming in fluidized bed reactors

This study presents a computational fluid dynamic (CFD) study of Dimethyl Ether (DME) gas adsorptive separation and steam reforming (DME-SR) in a large scale Circulating Fluidized Bed (CFB) reactor. The CFD model is based on Eulerian-Eulerian dispersed flow and solved using commercial software (ANSYS FLUENT). Hydrogen is currently receiving increasing interest as an alternative source of clean energy and has high potential applications, including the transportation sector and power generation. Computational fluid dynamic (CFD) modelling has attracted considerable recognition in the engineering sector consequently leading to using it as a tool for process design and optimisation in many industrial processes. In most cases, these processes are difficult or expensive to conduct in lab scale experiments. The CFD provides a cost effective methodology to gain detailed information up to the microscopic level. The main objectives in this project are to: (i) develop a predictive model using ANSYS FLUENT (CFD) commercial code to simulate the flow hydrodynamics, mass transfer, reactions and heat transfer in a large scale dual fluidized bed system for combined gas separation and steam reforming processes (ii) implement a suitable adsorption models in the CFD code, through a user defined function, to predict selective separation of a gas from a mixture (iii) develop a model for dimethyl ether steam reforming (DME-SR) to predict hydrogen production (iv) carry out detailed parametric analysis in order to establish ideal operating conditions for future industrial application. The project has originated from a real industrial case problem in collaboration with the industrial partner Dow Corning (UK) and jointly funded by the Engineering and Physical Research Council (UK) and Dow Corning. The research examined gas separation by adsorption in a bubbling bed, as part of a dual fluidized bed system. The adsorption process was simulated based on the kinetics derived from the experimental data produced as part of a separate PhD project completed under the same fund. The kinetic model was incorporated in FLUENT CFD tool as a pseudo-first order rate equation; some of the parameters for the pseudo-first order kinetics were obtained using MATLAB. The modelling of the DME adsorption in the designed bubbling bed was performed for the first time in this project and highlights the novelty in the investigations. The simulation results were analysed to provide understanding of the flow hydrodynamic, reactor design and optimum operating condition for efficient separation. Bubbling bed validation by estimation of bed expansion and the solid and gas distribution from simulation agreed well with trends seen in the literatures. Parametric analysis on the adsorption process demonstrated that increasing fluidizing velocity reduced adsorption of DME. This is as a result of reduction in the gas residence time which appears to have much effect compared to the solid residence time. The removal efficiency of DME from the bed was found to be more than 88%. Simulation of the DME-SR in FLUENT CFD was conducted using selected kinetics from literature and implemented in the model using an in-house developed user defined function. The validation of the kinetics was achieved by simulating a case to replicate an experimental study of a laboratory scale bubbling bed by Vicente et al [1]. Good agreement was achieved for the validation of the models, which was then applied in the DME-SR in the large scale riser section of the dual fluidized bed system. This is the first study to use the selected DME-SR kinetics in a circulating fluidized bed (CFB) system and for the geometry size proposed for the project. As a result, the simulation produced the first detailed data on the spatial variation and final gas product in such an industrial scale fluidized bed system. The simulation results provided insight in the flow hydrodynamic, reactor design and optimum operating condition. The solid and gas distribution in the CFB was observed to show good agreement with literatures. The parametric analysis showed that the increase in temperature and steam to DME molar ratio increased the production of hydrogen due to the increased DME conversions, whereas the increase in the space velocity has been found to have an adverse effect. Increasing temperature between 200 oC to 350 oC increased DME conversion from 47% to 99% while hydrogen yield increased substantially from 11% to 100%. The CO2 selectivity decreased from 100% to 91% due to the water gas shift reaction favouring CO at higher temperatures. The higher conversions observed as the temperature increased was reflected on the quantity of unreacted DME and methanol concentrations in the product gas, where both decreased to very low values of 0.27 mol% and 0.46 mol% respectively at 350 °C. Increasing the steam to DME molar ratio from 4 to 7.68 increased the DME conversion from 69% to 87%, while the hydrogen yield increased from 40% to 59%. The CO2 selectivity decreased from 100% to 97%. The decrease in the space velocity from 37104 ml/g/h to 15394 ml/g/h increased the DME conversion from 87% to 100% while increasing the hydrogen yield from 59% to 87%. The parametric analysis suggests an operating condition for maximum hydrogen yield is in the region of 300 oC temperatures and Steam/DME molar ratio of 5. The analysis of the industrial sponsor’s case for the given flow and composition of the gas to be treated suggests that 88% of DME can be adsorbed from the bubbling and consequently producing 224.4t/y of hydrogen in the riser section of the dual fluidized bed system. The process also produces 1458.4t/y of CO2 and 127.9t/y of CO as part of the product gas. The developed models and parametric analysis carried out in this study provided essential guideline for future design of DME-SR at industrial level and in particular this work has been of tremendous importance for the industrial collaborator in order to draw conclusions and plan for future potential implementation of the process at an industrial scale

Particle Attrition with Supersonic Nozzles in a High Temperature Fluidized Bed

Fluidized beds are widely used for a variety of processes such as food, pharmaceutical, petrochemical and energy production. As a typical application of fluidized beds, the fluid coking process uses thermal cracking reactions to upgrade heavy oils and bitumen from oil sands. In order to maintain a well fluidized bed and a satisfactory operation, a series of supersonic nozzles are used to inject high pressure steam in the bed to maintain the coke particle within an optimal range. Currently, the attrition nozzles consume a large florwrate of high pressure and superheated steam, which accounts for about 40 % of the total energy consumption in fluid coking reactors. Improving the efficiency of the attrition process would increase energy efficiency and reduce sour waste water production, reducing the environmental impact of heavy oil upgrading. Therefore, the main objective of the present thesis is an experimental and numerical study of particle attrition with supersonic nozzles in high temperature fluidized beds. The specific objective is to improve particle grinding efficiency and reduce the steam consumption in the fluid coking process. To achieve the research objective, the primary investigations focused on the solids entrainment and penetration of jets issuing from supersonic nozzles, which have significant effects on particle attrition. Novel measuring techniques, therefore, were developed to accurately measure the flowrate of solids entrained into the jet and its penetration length. The numerical and experimental studies reveal that the jet penetration lengths are related to the two-phase Froude number. A new correlation was developed to predict the penetration length of jets issuing from supersonic nozzles in high temperature fluidized beds, based on Benjelloun’s correlation and the Froude number. The attrition experimental results demonstrate that larger scale nozzles, operating with a high flowrate of a low molecular weight gas at high temperature provide the highest grinding efficiency. A jet-induced attrition model in fluidized beds at high temperature has been proposed and developed. The model is a coupled Eulerian-Eulerian multiphase model with a population balance method. The particle-particle interactions are described with the kinetic theory of granular flow. Experimental results were used to determine and modify the critical parameters of the model. The best prediction was obtained using the Ghadiri breakage kernel, generalized daughter size distribution function, and discrete solution method. Finally, the research focused on the enhancement of jet-induced attrition in fluidized bed. A twin-jet nozzle gave a grinding efficiency that is about 35% higher than with a single nozzle. The benefits of the twin-jet nozzle seem stronger at higher nozzle pressures and high temperature. It is likely that the twin-jet nozzle entrains more solids into the jets when compared with a single nozzle with the same gas flowrate