Determination of minimum fluidization velocity for gas-solid beds by experimental data and numerical simulations

The purpose of this work is to predict the minimum fluidization velocity Umf in a gas-solid fluidized bed. The study was carried out with an experimental apparatus for sand particles with diameters between 310μm and 590μm, and density of 2,590kg/m3. The experimental results were compared with numerical simulations developed in MFIX (Multiphase Flow with Interphase eXchange) open source code [1], for three different sizes of particles: 310mum, 450μm and 590μm. A homogeneous mixture with the three kinds of particles was also studied. The influence of the particle diameter was presented and discussed. The Ergun equation was also used to describe the minimum fluidization velocity. The experimental data presented a good agreement with Ergun equation and numerical simulations. Copyright © 2011 by ASME.

Physical properties and fluidization behaviour of fresh green bean particulates during fluidized bed drying

Changes in the physical properties (such as particle density, bulk density of the bed, shrinkage and bed porosity) of fresh green bean particulates were investigated during drying. Three length:diameter ratios (1:1, 2:1 and 3:1) were considered, using drying conditions of 50 +/- 2 degrees C and 13 +/- 2% relative humidity in a heat pump dehumidifier system. The fluidization behaviour was also evaluated at 10 levels of moisture content. The fluidization experiments demonstrated that the minimum fluidization velocity decreases as the drying proceeds due to the reduced moisture content and changes in the physical properties of the bean particulates. Empirical relationships of the following nature were developed for the change in shrinkage [VR = 1 - Be-kMR], particle density [rho(p) = A + BMR + C (exp)(-D MR)], bulk density [rho(b) = a(1) + b(1)MR + c(1)MR(2)] and bed porosity [epsilon = a(2) + b(2)MR + c(2)MR(2)] with the moisture content during fluidized bed drying.

Fluidization characteristics of moist food particles

Changes in fluidization behaviour of green peas particulates with change in moisture content during drying were investigated using a fluidized bed dryer. All drying experiments were conducted at 50 + 2 0C and 13 + 2 % RH using a heat pump dehumidifier system. Fluidization experiments were undertaken for the bedheights of 100, 80, 60 and 40 mm and at 10 moisture content levels. Fluidization behaviour was best fitted to the linear model of Umf = A + B m. A generalized model was also formulated using the height variation. Also generalized equation and Ergun equation was used to compare minimum fluidization velocity. Copyright ©2006 The Berkeley Electronic Press. All rights reserved.

Evaluation of hydrodynamic parameters of a fluidized-bed reactor with immobilized yeast

The fluidized bed reactor has successfully been used to perform biotechnological processes addressed to the production of high added value. The present work evaluates hydrodynamic parameters of a bench-scale fluidized bed reactor with cells of the yeast Candida guilliermondii immobilized either in calcium alginate beads or in polyvinyl alcohol (PVA). The effects of the following variables on cell immobilization were evaluated at 30 degrees C and feeding a synthetic medium containing 50 g L-1 xylose: total particle density (cells plus support), terminal velocity, particle drag force, minimum fluidization velocity and bed porosity. According to the results obtained, the reactor was shown to operate like a fixed-bed bioreactor at xi < 0.5 and a fluidized bed bioreactor at xi > 0.5. The maximum flow rate needed to obtain maximum bed fluidization in the reactor was equal to the terminal velocity of the immobilized cell particles. Particles of cells immobilized within these supports showed values of drag coefficient lower than those reported for other high-density supports. The evaluation of these hydrodynamic characteristics lead to an adequate bed fluidization inside the reactor, thus improving oxygen transference and availability in the fermentation medium, making the process more viable for future scale-up. (c) 2008 Society of Chemical Industry.

The influence of immobilized biomass and particle size on the fluid dynamics of an anaerobic fluidized bed reactor

Fluid dynamic analysis is an important branch of several chemical engineering related areas, such as drying processes and chemical reactors. However, aspects concerning fluid dynamics in wastewater treatment bioreactors still require further investigation, as they highly influence process efficiency. Therefore, it is essential to evaluate the influence of biofilm on the reactor fluid dynamic behavior, through the analysis of a few important parameters, such as minimum fluidization velocity, bed expansion and porosity, and particle terminal velocity. The main objective of the present work was to investigate the fluid dynamics of an anaerobic fluidized bed reactor, having activated carbon particles as support media for biomass immobilization. Reactor performance was tested using synthetic residual water, which was prepared using the solution employed in BOD determination. The results showed that the presence of immobilized biomass increased particle density and altered the main fluid dynamic parameters investigated.

CFD study of penetration and mixing of fuel in a circulating fluidized bed furnace

The aim of this thesis is to study the mixing of fuel and, also to some extent, the mixing of air in a circulating fluidized bed boiler. In the literature survey part of this thesis, a review is made of the previous experimental studies related to the fuel and air mixing in the circulating fluidized beds. In the simulation part of it the commercial computational fluid dynamics software (FLUENT) is used with the Eulerian multiphase model for studying the fuel mixing in the two and three-dimensional furnace geometries. The results of the three-dimensional simulations are promising and, therefore suggestions are made for the future simulations. The two-dimensional studies give new information of the effects of the fluidization velocity, fuel particle size and fuel density on the fuel mixing. However, the present results show that three-dimensional models produce more realistic representation of the circulating fluidized bed behavior.

Heterogeneous mass transfer in fluidized beds by computational fluid dynamics

The main objective of this research is to estimate and characterize heterogeneous mass transfer coefficients in bench- and pilot-scale fluidized bed processes by the means of computational fluid dynamics (CFD). A further objective is to benchmark the heterogeneous mass transfer coefficients predicted by fine-grid Eulerian CFD simulations against empirical data presented in the scientific literature. First, a fine-grid two-dimensional Eulerian CFD model with a solid and gas phase has been designed. The model is applied for transient two-dimensional simulations of char combustion in small-scale bubbling and turbulent fluidized beds. The same approach is used to simulate a novel fluidized bed energy conversion process developed for the carbon capture, chemical looping combustion operated with a gaseous fuel. In order to analyze the results of the CFD simulations, two one-dimensional fluidized bed models have been formulated. The single-phase and bubble-emulsion models were applied to derive the average gas-bed and interphase mass transfer coefficients, respectively. In the analysis, the effects of various fluidized bed operation parameters, such as fluidization, velocity, particle and bubble diameter, reactor size, and chemical kinetics, on the heterogeneous mass transfer coefficients in the lower fluidized bed are evaluated extensively. The analysis shows that the fine-grid Eulerian CFD model can predict the heterogeneous mass transfer coefficients quantitatively with acceptable accuracy. Qualitatively, the CFD-based research of fluidized bed process revealed several new scientific results, such as parametrical relationships. The huge variance of seven orders of magnitude within the bed Sherwood numbers presented in the literature could be explained by the change of controlling mechanisms in the overall heterogeneous mass transfer process with the varied process conditions. The research opens new process-specific insights into the reactive fluidized bed processes, such as a strong mass transfer control over heterogeneous reaction rate, a dominance of interphase mass transfer in the fine-particle fluidized beds and a strong chemical kinetic dependence of the average gas-bed mass transfer. The obtained mass transfer coefficients can be applied in fluidized bed models used for various engineering design, reactor scale-up and process research tasks, and they consequently provide an enhanced prediction accuracy of the performance of fluidized bed processes.

Effect of apparent viscosity on fluidized bed drying process parameters of guava pulp

Pressure drop and minimum fluidization velocity were experimentally studied in a vibro-fluidized bed of inert particles subjected to different vibration intensities during drying of guava pulp. Maltodextrin was added to the pulp in order to prevent stickiness between particles and the consequent bed collapse. Pulps were initially concentrated, resulting in pastes with different soluble solids content, and a constant fraction of maltodextrin was guaranteed in the final pulp samples. The pulp rheological behavior as affected by temperature and total soluble solids content, including maltodextrin, was evaluated and the effect of pulp apparent viscosity on pressure drop and minimum vibro-fluidization velocity were investigated. Two types of inert particles -3.6 mm glass beads and 3 mm Teflon cylinders were tested and, due to lower pressure drop presented by Teflon cylinders during operation of the dry vibro-fluidized bed, these particles were adopted for pulp drying process. Increasing pulp apparent viscosity caused a considerable increase in the vibro-fluidized bed pressure drop during pulp drying and, as a consequence resulted in a larger value of minimum vibro-fluidization velocity. on the other hand, the negative effect of increasing apparent viscosity could be attenuated by increasing the fluidized bed vibration intensity, which could prevent stickiness between particles. (c) 2006 Elsevier Ltd. All rights reserved.

Effect of apparent viscosity on the pressure drop during fluidized bed drying of soursop pulp

Pressure drop and minimum fluidization velocity were experimentally studied in a vibro-fluidized bed of inert particles subjected to different vibration intensities during drying of soursop pulp. Maltodextrin was added to the pulp in order to prevent stickiness between particles and the consequent bed collapse. Pulps were initially concentrated, resulting in pastes with different soluble solids content, and a constant fraction of maltodextrin was guaranteed in the final pulp samples. The pulp theological behavior as affected by temperature and total soluble solids content, including maltodextrin, was evaluated and the effect of pulp apparent viscosity on pressure drop and minimum vibro-fluidization velocity were investigated. Two types of inert particles -3.6 mm glass beads and 3 mm Teflon cylinders (length and diameter) - were tested and, due to lower pressure drop presented by Teflon cylinders during operation of the dry vibro-fluidized bed, these particles were adopted for pulp drying process. Increasing pulp apparent viscosity caused a considerable increase in the vibro-fluidized bed pressure drop during pulp drying and, as a consequence resulted in a larger value of minimum vibro-fluidization velocity. on the other hand, the negative effect of increasing apparent viscosity could be attenuated by increasing the fluidized bed vibration intensity, which could prevent stickiness between particles. (c) 2006 Elsevier B.V. All rights reserved.

Energetic, ecologic and fluid-dynamic analysis of a fluidized bed gasifier operating with sugar cane bagasse

This work aims to study the thermodynamic, ecological and fluid-dynamic aspects of a circulating fluidized bed gasifier using sugar cane bagasse as biomass, in order to estimate a model of its normal operation. In the initial stage was analysed the composition of biomass selected (sugar cane bagasse) and its lower heating value (LHV) was calculated. The energy balance of the gasifier was done, being the volumetric flow of air, synthesis gas and biomass estimated. Also the power produced by this gasifier was theoretically estimated. Then the circulating fluidized bed gasifier was designed for operation with approximately 100 kg/h of processed biomass. Cross-sectional area of the reactor, feeder size, diameter of the exit zone of the gases and minimum height of the expanded bed were selected. Some bed gasifier hydrodynamic factors were also studied. The minimum fluidization velocity, fluidization terminal velocity, and average fluidizing velocity were calculated, in order to understand the fluid-dynamic behaviour of gasification of this fuel. It was obtained a theoretical model that can support a possible prototype of circulating fluidized bed gasifier biomass. Finally, there were studied the ecological aspects of the gasifier, through an overall methodology. Ecological efficiencies were estimated for two scenarios: first considering the carbon cycle and thereafter disregarding the carbon cycle. In both cases, it can be proved the ecological viability of the project. © 2013 Elsevier Ltd. All rights reserved.

Estudo numérico de modelos de arrasto e do coeficiente de restituição no escoamento gás-sólido em leito fluidizado

Pós-graduação em Engenharia Mecânica - FEG

Prediction of the combustion process in fluidized bed based on physical-chemical properties of biomass particles and their hydrodynamic behaviors

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Estudo dos parâmetros físicos na fluidização em um leito gás sólido

Through measurements of basic parameters for determining the fluidization regime, as particle size, minimum fluidization velocity, bed porosity, etc., This paper analyze the mass distribution of the phases of the bed to be discussed in relation to: the flow gas physical properties of the solid particles and the forces acting on the solid particles circulating within the bed, as the weight force, buoyancy and drag forces (Stokes' Law). Due to the weight force is constant, open up the discussion about which of the other two forces, buoyancy and drag force, influencing the behavior of the bed. We used the photographic method to realize the statistical analyzes. Therefore, we can conclude what changes can be made more convenient in fluidizing the bed to obtain the highest efficiency for a good mixing used in industrial processes

Estudo das características fluidodinâmicas do bagaço de cana de açúcar para aplicações da gaseificação no setor sucroalcooleiro

In this work is studied the fluid dynamic characteristics of sugarcane bagasse for gasification applications and use of syngas in the sugarcane sector. This is an experimental work, which is initially performed the particle size separation of sugarcane bagasse particles, using appropriate equipment. Through the experiment is possible to determine the average diameter of particles pomace typical sugarcane mill. The equipment used for the grading sieve separation assembly consists of several dimensions, arranged vertically and which shows adequate for good separation from the pulp. Later, it makes immediate analysis for the determination of moisture, ash, volatile materials and fixed carbon. The study also determines the gross calorific value and allows analysis of densities of particles of sugar cane bagasse. Also studied the bagasse morphology using electron microscopes, where it was possible to visualize the geometry of the particles of bagasse. The use of Electronic Scanning Microscopy (SEM) provided better understand the morphology and particle size measured by using photography methods. Two methods for determining the sphericity of the particles were also used. The experiments carried out using appropriate standards and specific equations, allowing compare the present results with the values found by several researchers. Subsequently, fluid dynamic simulations were performed for the determination of porosity and minimum fluidization velocity theoretical. It follows that the sphericity and porosity of the bagasse particles influence the minimum fluidization velocity of biomass

Hindered settling of sand grains

The sedimentation rate of sand grains in the hindered settling regime has been considered to assess particle shape effects. The behaviour of various particulate systems involving sand has been compared with the widely used Richardson-Zaki expression. The general form of the expression is found to hold, in as much as remaining as a suitable means to describe the hindered settling of irregular particles. The sedimentation exponent n in the Richardson-Zaki expression is found to be significantly larger for natural sand grains than for regular particles. The hindered settling effect is therefore greater, leading to lower concentration gradients than expected. The effect becomes more pronounced with increasing particle irregularity. At concentrations around 0.4, the hindered settling velocity of fine and medium natural sands reduces to about 70% of the value predicted using existing empirical expressions for n. Using appropriate expressions for the fluidization velocity and the clear water settling velocity, a simple method is discussed to evaluate the sedimentation exponent and to determine the hindered settling effect for sands of various shapes.