Gas Dispersion Coefficients Research Articles

Gas Dispersion Coefficient Test System and Dimensionless Inversion Method for Porous Media.

Due to the complex porous media structure of the longwall gob area, it has been difficult to determine the gas dispersion coefficient of oxygen when studying spontaneous coal combustion in the gob area. In this work, we first designed an experimental device for testing the gas diffusion coefficient of porous media. Then, the distribution law of gas concentration in porous media under different particle size conditions was obtained by experiments. Subsequently, we established a dimensionless mathematical model of gas dispersion in porous media and developed a corresponding numerical simulator based on the finite volume method (FVM). The influence of the dimensionless gas dispersion coefficient on the gas concentration distribution was analyzed, and then a dimensionless inversion method of the gas dispersion coefficient was summarized and put forward. Finally, we obtained the values of the gas dispersion coefficient in the experimental device under different particle size conditions by inversion and discussed its effect on the gas dispersion behavior in porous media. The results show that (1) the distribution of gas concentration obtained from the experimental test and numerical simulation is consistent, which verifies the reliability of our work; (2) the dimensionless gas concentration is the highest near the injection point and gradually decreases along the depth and both sides of the test container; (3) with the increase of the dimensionless gas dispersion coefficient, the distance required for uniform gas mixing in the test container is gradually shortened and the gas dispersion coverage is wider; and (4) the larger pore space facilitates the dispersion behavior of the gas, and the gas dispersion coefficient shows a parabolic trend with the increase of porous medium particle size.

Guidance for Measuring the Axial Gas Dispersion Coefficient in Bubble Columns via Gas Flow Modulation

Guidance for Measuring the Axial Gas Dispersion Coefficient in Bubble Columns via Gas Flow Modulation

Uncertainty analysis of gamma-ray densitometry applied for gas flow modulation technique in bubble columns

The gas flow modulation technique is a recently proposed approach for measuring the axial gas dispersion coefficient in bubble columns. This study presents a quantitative analysis of the experimental uncertainty associated with gamma-ray densitometry and ensemble-averaging of the data. The considered uncertainty sources are the statistics of the photon counting process, a mismatch between the modelled and the real radiation propagation due to the spatial extent of the detector, and a potential mismatch between modulation and sampling frequencies. The analysis is based on a numerical gamma-ray propagation model and a Monte Carlo approach to account for statistical uncertainty. The proposed algorithm supports the selection of an optimal total scanning time based on detector size, modulation parameters, involved fluids and column and source parameters. The analysis reveals that a mismatch between the modulation and sampling frequencies is most critical while the impact of the other considered uncertainty sources is rather marginal.

Prediction of intensity of dispersion in surfactant solutions in vertical sparged bubble column

AbstractDispersion of liquid in bubble column is an important process in chemical and allied industries. In the present work, the dispersion characteristics of liquid phase in a bubble column is studied in three different surfactant solutions. A comparative analysis on the dispersion characteristics of liquid phase in different surfactants has been carried out. The gas holdup and mixing time are also analysed. The effect of different physiochemical properties and operating variables on the dispersion coefficient and gas holdup is examined. A novel model for the estimation of dispersion due to bubble motion has been developed which incorporates the effect of gas holdup, liquid viscosity and circulation velocity. The contribution of dispersion coefficient due to the bubble motion in overall dispersion is also examined. The value of velocity distribution parameter in three different surfactant solutions is also compared. Generalised correlations for gas holdup, dispersion coefficient and mixing time are also developed based on the physicochemical properties of liquid. This research may be useful for the selection and application of these three surfactants in industries. This article will also help researchers for designing multiphase flow system where role of dispersion due to the motion of bubbles is significant.

A New Method for Simultaneous Measurements of Gas Dispersion Coefficient and Gas Coefficient of Permeability of Unsaturated Soils

A New Method for Simultaneous Measurements of Gas Dispersion Coefficient and Gas Coefficient of Permeability of Unsaturated Soils

Analysis of the effect of uncertainties in hydrodynamic parameters on the accuracy of the gas flow modulation technique for bubble columns

The gas flow modulation technique has recently been proposed as a novel method for determining the axial gas dispersion coefficient in bubble columns. The approach is based on a marginal sinusoidal modulation of the gas inlet flow rate that acts as a virtual tracer. Axial gas dispersion is then inversely calculated from amplitude damping and phase shift via an analytical solution of the axial dispersion model. The proposed study provides an analysis of the inherent uncertainties related to the assumptions of constant axial gas dispersion coefficient and bubble rise velocity, which are crucial for implementing the method. Besides, the sensitivity of the approach is assessed as a function of the modulation parameters, the bubble rise velocity and the axial gas dispersion coefficient. Eventually, the possibility of tailoring the modulation parameters depending on the expected value of the axial gas dispersion coefficient to increase the sensitivity and to reduce the uncertainty is also assessed.

Eulerian-Lagrangian simulation of air-steam biomass gasification in a three-dimensional bubbling fluidized gasifier

A CFD-DEM (computational fluid dynamics – discrete element method) model, with heat transfer, and homogeneous and heterogeneous chemical reactions accounted for, is implemented here to numerically study the biomass gasification in a three-dimensional bubbling fluidized bed. The results show that the biomass inlet on the right sidewall results in laterally asymmetrical profiles of important bed hydrodynamics and gas concentrations. Increasing the temperature, steam-biomass ratio and equivalence ratio all enlarge the gas voidage, fluid force, collision force and dispersion coefficient of the solid phase, but have different effects on the different homogeneous reaction rates. Vigorous horizontal and vertical solid dispersions occur in the lateral central and sidewalls of the bed, respectively. The results of the current work provide valuable insights regarding the hydrodynamics of the bubbling fluidized bed for biomass gasification, and are expected to be useful for the operation, scale-up and optimization of such systems for sustainable energy production.

Gas phase dispersion/mixing investigation in a representative geometry of gas-liquid upflow Moving Bed Hydrotreater Reactor (MBR) using developed gas tracer technique and method based on convolution/regression

Gas dispersion studies has been executed for the catalyst bed section of a representative geometry of scaled-down industrial Moving Bed Hydrotreater Reactor (MBR). The catalyst bed of MBR is modeled using Axial Dispersion Model (ADM) and its parameters gas dispersion coefficient (Dg) and Peclet number (Pe) are estimated using Residence Time Distribution (RTD) and implementing a methodology based on convolution and regression. Additionally, dimensionless variance (σD2) for the catalyst bed is also measured using RTDs first and second moments to compare with those findings of ADM model. This study is conducted at the varying flow rates of gas and liquid including scaled down operating conditions. The results of Dg,Pe, and σD2 indicate that bed behaves as a packed bed for low liquid flow rate and moves towards three-phase fluidized bed for increasing liquid flow rate. Overall the gas phase behavior is seen to be in plug flow for all the operating conditions, with relatively high dispersion/mixing in packed bed state. Scaled down flow conditions is seen to be best in terms of gas dispersion/mixing and catalyst utilization.

Non-invasive determination of gas phase dispersion coefficients in bubble columns using periodic gas flow modulation

Non-uniform bubble size and liquid velocity distribution in bubble columns lead to gas phase dispersion. This gas phase backmixing is quantitatively modelled in the axial gas dispersion model by the axial gas dispersion coefficient. However, only few gas phase dispersion data are currently available since experimental investigations are expensive and require the application of suitable gas tracers and their reliable detection. In this study a new approach is introduced, which is based on a lock-in measurement of gas fraction modulation. Experiments were carried out in a bubble column of 100mm diameter operated with air/water and air/glycol-water, respectively. Gas holdup was measured via gamma-ray densitometry in synchronization with the modulated inlet flow. The axial dispersion model was adopted to determine the gas phase dispersion coefficient from phase shift and amplitude damping of the gas holdup frequency response. A sensitivity analysis was performed to derive a proper modulation scheme. The calculated gas phase dispersion coefficients show excellent agreement with data from literature.

Horizontal gas mixing in rectangular fluidized bed: A novel method for gas dispersion coefficients in various conditions and distributor designs

Abstract In a rectangular fluidized bed combustor, the tracer gas is injected continuously into the bed from a point source at the center of the distributor plate. In this study, a general governing equation is formulated for tracer gas dispersion in the bed. An analytical solution is derived to estimate the dispersion coefficients, D x and D y , in a horizontal plane. The concentration profiles at different sampling heights with various gas velocities are plotted. Subsequently, to estimate the dispersion coefficients, surface fitting of the obtained analytical solution to the experimental data is performed. The dispersion coefficients obtained from this model are compared with those of a conventional model. Additionally, the effect of walls, bed height and gas injection rate on the dispersion coefficients in a horizontal plane is investigated, and the effect of distributor design on the dispersion coefficients in a horizontal plane is investigated with different tracer positions. It is found that D x and D y are nearly equivalent at a lower tracer gas ratio of the injected gas to the total gas flow rate. It is also demonstrated that the effect of bed height on D x is minor. This model is also able to estimate the dispersion coefficients in the case of a multi-horizontal nozzle distributor.

Effect of wind turbulence on gas transport in porous media: experimental method and preliminary results

We demonstrate a novel experimental arrangement for measuring wind turbulence‐induced gas transport in dry porous media under controlled conditions. This equipment was applied to assess the effect of wind turbulence on gas transport (quantified as a dispersion coefficient) as a function of distance to the surface of the porous medium exposed to wind. Two different strategies for the measurement of wind‐induced gas transport were compared. Experiments were carried out with O2 and CO2 as tracer gases with average vertical wind speeds of 0.02–1.06 m s−1. Oxygen breakthrough curves as a function of distance to the wind‐exposed surface of the porous medium were analysed numerically with a finite‐difference‐based model to assess gas transport. We showed that wind turbulence‐induced gas transport is an important transport mechanism that can be 20–70 times larger than molecular diffusion‐induced transport. Wind conditions and properties of the porous medium had strong controlling effects on this relationship. Importantly, we show that even though wind‐induced gas transport is greatest near to the wind‐exposed surface, it can have marked effects on the variation in gas concentration at much greater depths.Highlights We explored the effect of atmospheric wind turbulence on gas transport in porous media. We measured the depth relation of wind‐induced dispersion in porous media for real wind conditions. Wind‐induced gas dispersion coefficients were 20–70 times larger than molecular diffusion. Wind turbulence can potentially have a considerable effect on gas transport in porous media.

Hydrodynamic behaviors and mixing characteristics in an internal loop airlift reactor based on CFD simulation

This work aims to systematically study hydrodynamics and mixing characteristics in an internal airlift reactor using a computational approach. A 3D transient Eulerian–Eulerian multiphase model is carried out with the standard k–ɛ turbulence model, and the results are used to simulate a virtual tracer. Parameters of importance in the design of airlift reactors, such as the mixing time, circulation time, axial dispersion number, and axial dispersion coefficient, are presented in addition to basic hydrodynamic parameters, such as gas holdup and fluid velocities based on full 3D simulation. The effects of the superficial gas velocity, the downcomer diameter (Ar/Ad), Hl/dr ratio, and scaling up were investigated. The key observations were as follows. With increasing superficial gas velocity, the dispersion number increases, leading to more mixing. Increasing the downcomer area leads to lower dispersion number in the riser and consequently a more plug-flow-like behavior. A greater draft tube height causes a lower dispersion number but a higher dispersion coefficient due to a greater draft tube effect. Using a larger reactor with geometric similarity provides a higher dispersion number with more mixing because of reduced wall effect. In addition, correlations were proposed for gas holdup, circulation velocity, and dispersion coefficients for riser and downcomer.

Adsorption and transport of methane in biochars derived from waste wood

Mitigation of landfill gas (LFG) is among the critical aspects considered in the design of a landfill cover in order to prevent atmospheric pollution and control global warming. In general, landfill cover soils can partially remove methane (CH4) through microbial oxidation carried out by methanotrophic bacteria present within them. The oxidizing capacity of these landfill cover soils may be improved by adding organic materials, such as biochar, which increase adsorption and promote subsequent or simultaneous oxidation of CH4. In this study, seven wood-derived biochars and granular activated carbon (GAC) were characterized for their CH4 adsorption capacity by conducting batch and small-scale column studies. The effects of influential factors, such as exposed CH4 concentration, moisture content and temperature on CH4 adsorption onto biochars, were determined. The CH4 transport was modeled using a 1-D advection–dispersion equation that accounted for sorption. The effects of LFG inflow rates and moisture content on the combined adsorption and transport properties of biochars were determined. The maximum CH4 adsorption capacity of GAC (3.21mol/kg) was significantly higher than that of the biochars (0.05–0.9mol/kg). The CH4 gas dispersion coefficients for all of the biochars ranged from 1×10−3 to 3×10−3m2s−1. The presence of moisture significantly suppressed the extent of methane adsorption onto the biochars and caused the methane to break through within shorter periods of time. Overall, certain biochar types have a high potential to enhance CH4 adsorption and transport properties when used as a cover material in landfills. However, field-scale studies need to be conducted in order to evaluate the performance of biochar-based cover system under a more dynamic field condition that captures the effect of seasonal and temporal changes.

In-depth system parameters of transition flow pattern between turbulent and fast fluidization regimes in high solid particle density circulating fluidized bed reactor

The transition flow between turbulent and fast fluidization regimes in high solid particle density circulating fluidized bed reactor was successfully investigated in this study. In addition, the effects of computational cell and simulation time were compared. Then, the comprehensive explanation and in-depth system parameters were discussed. The results showed the unique system characteristics. For the transition flow between turbulent and fast fluidization regimes, lower range of energy spectrum than the turbulent and fast fluidization regimes was found. As the increasing of gas inlet velocity, the axial and radial solid particle dispersion coefficients increased and decreased, respectively. For the axial and radial gas dispersion coefficients, both of them were increased with the increasing of gas inlet velocity. At the near wall region, the transition flow had no obvious power spectrum peak except the one below 0.2Hz, while, at the center region, the maximum peak at 5.0Hz was found. All the velocities had no significant correlation with solid volume fraction. The solid particle–solid particle interaction or collision then governs the system characteristic. The transition flow between turbulent and fast fluidization regimes will be an alternative choice for applying with circulating fluidized bed reactor in other new physical and chemical processes.

Flow Structures and Mixing Patterns in the Freeboard of Gas-Fluidized Bed Reactors

Flow structures and gas mixing patterns establishing in the freeboard of a bubbling gas-fluidized bed have been investigated by means of a planar laser light scattering (PLLS) technique. Nondiffusive tracing of the flow structures has been accomplished by injection of an aerosol of micrometer-sized scattering particles at a preset axial position in the splash zone of the reactor. A digital image acquisition and analysis technique has been developed and used to characterize the macroscopic and microscopic features of the flow structures. Experiments have been carried out in the freely bubbling regime at gas superficial velocities ranging from incipient fluidization to U/Umf = 2.6. Bed solids consist of glass beads of two different size ranges belonging to group B of the Geldart classification of powders. Results are analyzed with the aim of determining effective gas dispersion coefficients and characteristic time-scales for gas micromixing in the splash zone as a function of the excess gas superficial velo...

Gas–Solute Dispersivity Ratio in Granular Porous Media as Related to Particle Size Distribution and Particle Shape

Measurements of solute dispersion in porous media is generally much more time consuming than gas dispersion measurements performed under equivalent conditions. Significant time savings may therefore, be achieved if solute dispersion coefficients can be estimated based on measured gas dispersion data. This paper evaluates the possibility for estimating solute dispersion based on gas dispersion measurements. Breakthrough measurements were carried out at different fluid velocities (covering the same range in Reynolds number), using O2 and NaCl as gas and solute tracers, respectively. Three different, granular porous materials were used: (1) crushed granite (very angular particles), (2) gravel (particles of intermediate roundness) and (3) Leca® (almost spherical particles). For each material, 21 different particle size fractions were used. Gas and solute dispersion coefficients were determined by fitting the advection–dispersion equation to the measured breakthrough curves and in turn used to calculate gas and solute dispersivities as a function of mean particle size (D m) and particle size range (R) for the 63 particle size fractions considered. The results show that solute and gas dispersivities are related and that their ratio depends on both R and D m. Based on these observations a simple model for predicting the dispersivity ratio from D m and R, was proposed.

Gas Dispersion in Granular Porous Media under Air‐Dry and Wet Conditions

Subsurface gaseous‐phase transport is governed by three gas transport parameters: the air permeability coefficient (ka), gas diffusion coefficient (DP), and gas dispersion coefficient (DH). Among these, DH is the least understood due to hitherto limited research into the relationship between gas dispersion and soil physical characteristics. In this study, a series of advection–dispersion experiments was performed on granular porous media to identify the effects of soil column dimensions (length and diameter), particle size and shape, dry bulk density, and moisture content on the magnitude of gas dispersion. Glass beads and various sands of different shapes (angular and rounded) with mean particle diameters (d50) ranging from 0.19 to 1.51 mm at both air‐dry and variable moisture contents were used as granular porous media. Gas dispersion coefficients and gas dispersivities (α = DH/v, where v is the pore‐gas velocity) were determined by fitting the advection–dispersion equation to the measured breakthrough curves. For all test conditions, DH increased linearly with v. The test results showed that neither soil column length nor diameter had significant effect on gas dispersivity. Under air‐dry conditions, higher gas dispersivities were observed for media with wider particle size distribution and higher dry bulk density. The minor effect of particle shape on gas dispersivity was found under both air‐dry and wet conditions. Under wet conditions, the variations in gas dispersivity were mainly controlled by the air‐filled porosity. An empirical model was also proposed for the prediction of gas dispersivity in granular, unsaturated porous media.

Experimental Investigation of Radial Gas Dispersion Coefficients in a Fluidized Bed

In a fluidized bed boiler, the combustion efficiency, the NOX formation rate, flue gas desulphurization and fluidized bed heat transfer are all ruled by the gas distribution. In this investigation, the tracer gas method is used for evaluating the radial gas dispersion coefficient. CO2 is used as a tracer gas, and the experiment is carried out in a bubbling fluidized bed cold model. Ceramic balls are used as the bed material. The effect of gas velocity, radial position and bed height is investigated.

Circulation of Geldart D type particles: Part II—Low solids fluxes: Measurements and computation under dilute conditions

In Part I of this paper, solids slugging phenomena were studied for high solids fluxes. In this study, the solids slugging was eliminated for extremely low solids fluxes. Here, gamma ray densitometry and PIV techniques were used to measure radially averaged solids volume fractions, and solids laminar and turbulent properties near the wall, respectively. The PIV system measured turbulent properties for solids, such as stresses, granular temperatures, and dispersion coefficients, non-invasively near the wall, in the developing region. This study was the first known measurement of solids dispersion coefficients in the axial and radial directions, using the PIV technique. A two-dimensional kinetic theory based IIT CFD code was used to perform simulations. The measured and computed radially averaged total granular temperatures, axial and radial solids and axial gas dispersion coefficients, reasonably agreed with the literature. This study also showed the capabilities of the kinetic theory based CFD codes to reasonably match the solids dispersion coefficients measured using the PIV technique.

Circulation of Geldart D type particles: Part I – High solids fluxes. Measurements and computation under solids slugging conditions

Solids slugging phenomena were studied in a symmetric IIT riser using gamma ray densitometers. No core-annular regime was observed at high solids fluxes. Computations using the kinetic theory based IIT CFD code confirmed the elimination of the undesirable core-annular regime. The computer code was also used to calculate dispersion coefficients. The computed radially averaged total granular temperatures, axial and radial solids dispersion coefficients and axial gas dispersion coefficients agreed well with the literature. The −5/3 Kolmogorov energy spectrum law was followed at high frequencies. The dimensionless form of the spectral distribution showed good agreement with the Hinze correlation for single phase flow turbulence. The IIT CFD code computed high slip and anisotropy in the fluidized bed system.