Gas-phase Diffusion Coefficients Research Articles

Simulation and experimental study of gas-phase diffusion coefficient of selenium dioxide

Improving the removal effect of selenium in wet flue gas desulfurization system is a key way to reduce the emission of selenium pollutants from coal-fired power plants. In order to clarify the removal mechanism of selenium pollutants in the desulfurization tower, it is necessary to obtain accurate selenium gas-phase diffusion coefficient. In this paper, molecular dynamics simulations were used to carry out theoretical calculations of gas-phase diffusion coefficients of SeO2 (the main form of selenium in coal combustion flue gas). The gas-phase diffusion coefficients of SeO2 in the range of 393 K–433 K were measured by a self-developed heavy metal gas diffusion coefficient testing device to verify the accuracy of the molecular dynamics calculations. Furthermore, the calculated gas-phase diffusion coefficients of SeO2 under typical binary and ternary components were obtained by correcting on the basis of Fuller's formula. Finally, a single-droplet absorption model for SeO2 was constructed and experiments were carried out to compare the effect of the gas-phase diffusion coefficient on the accuracy of the model calculations. The error of the model calculations was reduced from 8.09 % to 1.96 % after the correction. In this study, the gas-phase diffusion coefficient of SeO2 in the low-temperature range of coal-fired flue gas was obtained. This study can provide basic data for the development of selenium migration mechanism and control technology.

In-situ estimation of water transfer parameters in a proton exchange membrane fuel cell

A reduced model to describe steady state transport of water through a PEM fuel cell is proposed. It includes three rigorously estimated parameters: the effective gas diffusion coefficient of water vapor in hydrogen, of water vapor in air and of water in sorbed phase in the membrane. First, it is proved that the parameters are well decorrelated. Then, uncertainty on estimates is calculated by simulating an experiment with random noise. Finally, the parameters are estimated as a function of the average humidity. It is observed that the gas phase diffusion coefficients do not depend on humidity. The effective diffusion coefficient in sorbed phase decreases slightly with increasing average relative humidity. It is shown that the effect of electrodes on water transport is negligible by comparing the water fluxes through an MEA and a membrane alone. A temperature gradient is imposed between the two plates and allows to obtain the condensation of water on one side of the membrane. A discontinuity of the water flow through the MEA is observed and results are interpreted by an increase of the effective diffusion coefficient in sorbed phase and/or by an increase of the water content of the membrane in contact with liquid water.

A diffusion-based compositionally-extended black oil model to investigate produced gas re-injection EOR in Eagle Ford

In this study, an in-house reduced black oil simulation model is used to investigate cyclic produced gas rejection in Eagle Ford shale formation. Molecular diffusion may play a crucial role in mass transfer in tight shale formations. However, the diffusion has not been incorporated in a black-oil type model in previous studies. In this study, the diffusive term is included into the governing equation and its effect on production performance is examined by using a black oil model. Diffusion coefficients are calculated by using Sigmund correlation, which makes them a function of phase compositions, phase properties, pressure, and temperature. Considering that phase compositions are influenced by large gas-oil capillary pressure nanoconfinement effect, diffusion coefficients are also altered based on this affect. This is the first study that incorporates the large gas-oil capillary pressure effect on diffusion coefficients in nanopores, through using a reduced black oil model. The operation parameters including initial injection time, injection rate, number of cycles are examined. We found that larger injection rates and more number of cycles are efficient in improving the oil recovery. Injection rate is the most crucial parameters in cyclic gas injection approach, followed by cycle numbers. Huff-n-puff results in good efficiency at relatively tight matrix, however, it is not always more efficient for very low permeabilities, as for the very tight formation, the injection gas cannot penetrate to the deeper formation to bring out much more oil from the reservoir in the same “huff” and “soaking” time period. The molecular diffusion plays an important role in gas injection process and cannot be ignored. When nano-confinement effect is included, the diffusion coefficient in gas phase is decreased and the diffusion coefficient in oil phase is increased as pore size decreases, but the effect is not strong especially at relatively high pressures.

Binary Diffusion Coefficients for Methane and Fluoromethanes in Nitrogen.

Binary gas-phase diffusion coefficients, of interest in physical models of atmospheric and combustion chemistry, have been measured in for the homologous series of refrigerant-related (fluoro)methanes: methane (), fluoromethane (), difluoromethane (), and trifluoromethane (). Values have been determined by reverse-flow gas chromatography, which has been previously demonstrated to provide accurate results over a wide range of temperatures. Coefficients were measured at temperatures of (300 to 550) K for all species and extending up to 650 K and 723 K for and , respectively, and down to 250 K for . We also performed measurements for in air at temperatures of (250 to 350) K, obtaining values the same as in within 0.3 %, well within our experimental uncertainty. We report the first measurements for and compare with the limited literature data for the other compounds. Our results agree broadly with earlier measurements in both and air. The greater temperature ranges reported in this work lead to temperature dependences that differ from most previous experiments, although they are consistent with several literature estimates and are similar to temperature exponents found for small hydrocarbons in . Comparison of the present work with a recent study that found different diffusion coefficients for methane when determined in a typical arrested flow apparatus and a novel "twin tube" method unaffected by sample adsorption shows much better agreement with the the arrested flow results over all common temperatures.

Technical note: Determination of binary gas-phase diffusion coefficients of unstable and adsorbing atmospheric trace gases at low temperature – arrested flow and twin tube method

Abstract. Gas-phase diffusion is the first step for all heterogeneous reactions under atmospheric conditions. Knowledge of binary diffusion coefficients is important for the interpretation of laboratory studies regarding heterogeneous trace gas uptake and reactions. Only for stable, nonreactive and nonpolar gases do well-established models for the estimation of diffusion coefficients from viscosity data exist. Therefore, we have used two complementary methods for the measurement of binary diffusion coefficients in the temperature range of 200 to 300 K: the arrested flow method is best suited for unstable gases, and the twin tube method is best suited for stable but adsorbing trace gases. Both methods were validated by the measurement of the diffusion coefficients of methane and ethane in helium and air as well as nitric oxide in helium. Using the arrested flow method the diffusion coefficients of ozone in air, dinitrogen pentoxide and chlorine nitrate in helium, and nitrogen were measured. The twin tube method was used for the measurement of the diffusion coefficient of nitrogen dioxide and dinitrogen tetroxide in helium and nitrogen.

Compilation and evaluation of gas phase diffusion coefficients of halogenated organic compounds.

Organic halogens are of great environmental and climatic concern. In this work, we have compiled their gas phase diffusivities (pressure-normalized diffusion coefficients) in a variety of bath gases experimentally measured by previous studies. It is found that diffusivities estimated using Fuller's semi-empirical method agree very well with measured values for organic halogens. In addition, we find that at a given temperature and pressure, different molecules exhibit very similar mean free paths in the same bath gas, and then propose a method to estimate mean free paths in different bath gases. For example, the pressure-normalized mean free paths are estimated to be 90, 350, 90, 80, 120 nm atm in air (and N2/O2), He, argon, CO2 and CH4, respectively, with estimated errors of around ±25%. A generic method, which requires less input parameter than Fuller's method, is proposed to calculate gas phase diffusivities. We find that gas phase diffusivities in He (and air as well) calculated using our method show fairly good agreement with those measured experimentally and estimated using Fuller's method. Our method is particularly useful for the estimation of gas phase diffusivities when the trace gas contains atoms whose diffusion volumes are not known.

Temperature-Dependent Henry's Law Constants of Atmospheric Amines.

There has been growing interest in understanding atmospheric amines in the gas phase and their mass transfer to the aqueous phase because of their potential roles in cloud chemistry, secondary organic aerosol formation, and the fate of atmospheric organics. Temperature-dependent Henry's law constants (KH) of atmospheric amines, a key parameter in atmospheric chemical transport models to account for mass transfer, are mostly unavailable. In this work, we investigated gas-liquid equilibria of five prevalent atmospheric amines, namely 1-propylamine, di-n-propylamine, trimethylamine, allylamine, and 4-methylmorpholine using bubble column technique. We reported effective KH, intrinsic KH, and gas phase diffusion coefficients of these species over a range of temperatures relevant to the lower atmosphere for the first time. The measured KH at 298 K and enthalpy of solution for 1-propylamine, di-n-propylamine, trimethylamine, allylamine, and 4-methylmorpholine are 61.4 ± 4.9 mol L(-1) atm(-1) and -49.0 ± 4.8 kJ mol(-1); 14.5 ± 1.2 mol L(-1) atm(-1) and -72.5 ± 6.8 kJ mol(-1); 8.9 ± 0.7 mol L(-1) atm(-1) and -49.6 ± 4.7 kJ mol(-1); 103.5 ± 10.4 mol L(-1) atm(-1) and -42.7 ± 4.3 kJ mol(-1); and 952.2 ± 114.3 mol L(-1) atm(-1) and -82.7 ± 9.7 kJ mol(-1), respectively. In addition, we evaluated amines' characteristic times to achieve gas-liquid equilibrium for partitioning between gas and aqueous phases. Results show gas-liquid equilibrium can be rapidly established at natural cloud droplets surface, but the characteristic times may be extended substantially at lower temperatures and pHs. Moreover, our findings imply that atmospheric amines are more likely to exist in cloud droplets, and ambient temperature, water content, and pH of aerosols play important roles in their partitioning.

Compilation and evaluation of gas phase diffusion coefficients of reactive trace gases in the atmosphere: Volume 2. Diffusivities of organic compounds, pressure-normalised mean free paths, and average Knudsen numbers for gas uptake calculations

Abstract. Diffusion of organic vapours to the surface of aerosol or cloud particles is an important step for the formation and transformation of atmospheric particles. So far, however, a database of gas phase diffusion coefficients for organic compounds of atmospheric interest has not been available. In this work we have compiled and evaluated gas phase diffusivities (pressure-independent diffusion coefficients) of organic compounds reported by previous experimental studies, and we compare the measurement data to estimates obtained with Fuller's semi-empirical method. The difference between measured and estimated diffusivities are mostly < 10%. With regard to gas-particle interactions, different gas molecules, including both organic and inorganic compounds, exhibit similar Knudsen numbers (Kn) although their gas phase diffusivities may vary over a wide range. This is because different trace gas molecules have similar mean free paths in air at a given pressure. Thus, we introduce the pressure-normalised mean free path, λP ≈ 100 nm atm, as a near-constant generic parameter that can be used for approximate calculation of Knudsen numbers as a simple function of gas pressure and particle diameter to characterise the influence of gas phase diffusion on the uptake of gases by aerosol or cloud particles. We use a kinetic multilayer model of gas-particle interaction to illustrate the effects of gas phase diffusion on the condensation of organic compounds with different volatilities. The results show that gas phase diffusion can play a major role in determining the growth of secondary organic aerosol particles by condensation of low-volatility organic vapours.

Compilation and evaluation of gas phase diffusion coefficients of reactive trace gases in the atmosphere: volume 1. Inorganic compounds

Abstract. Diffusion of gas molecules to the surface is the first step for all gas–surface reactions. Gas phase diffusion can influence and sometimes even limit the overall rates of these reactions; however, there is no database of the gas phase diffusion coefficients of atmospheric reactive trace gases. Here we compile and evaluate, for the first time, the diffusivities (pressure-independent diffusion coefficients) of atmospheric inorganic reactive trace gases reported in the literature. The measured diffusivities are then compared with estimated values using a semi-empirical method developed by Fuller et al. (1966). The diffusivities estimated using Fuller's method are typically found to be in good agreement with the measured values within ±30%, and therefore Fuller's method can be used to estimate the diffusivities of trace gases for which experimental data are not available. The two experimental methods used in the atmospheric chemistry community to measure the gas phase diffusion coefficients are also discussed. A different version of this compilation/evaluation, which will be updated when new data become available, is uploaded online (https://sites.google.com/site/mingjintang/home/diffusion).

Temperature-Dependent Henry’s Law Constants of Atmospheric Organics of Biogenic Origin

There have been growing interests in modeling studies to understand oxidation of volatile organic compounds in the gas phase and their mass transfer to the aqueous phase for their potential roles in cloud chemistry, formation of secondary organic aerosols, and fate of atmospheric organics. Temperature-dependent Henry's law constants, key parameters in the atmospheric models to account for mass transfer, are often unavailable. In the present work, we investigated gas-liquid equilibriums of isoprene, limonene, α-pinene, and linalool using a bubble column technique. These compounds, originating from biogenic sources, were selected for their implications in atmospheric cloud chemistry and secondary organic aerosol formation. We reported Henry's law constants (K(H)), first order loss rates (k), and gas phase diffusion coefficients over a range of temperatures relevant to the lower atmosphere (278-298 K) for the first time. The measurement results of K(H) values for isoprene, limonene, α-pinene, and linalool at 298 K were 0.036 ± 0.003; 0.048 ± 0.004; 0.029 ± 0.004; and 21.20 ± 0.30 mol L(-1) atm(-1), respectively. The fraction for these compounds in stratocumulus and cumulonimbus clouds at 278 K were also estimated in this work (isoprene, 1.0 × 10(-6), 6.8 × 10(-6); limonene, 1.5 × 10(-6), 1.0 × 10(-5); α-pinene, 4.5 × 10(-7), 3.1 × 10(-6); and linalool, 6.2 × 10(-4), 4.2 × 10(-3)). Our measurements in combination with literature results indicated that noncyclic alkenes could have smaller K(H) values than those of cyclic terpenes and that K(H) values may increase with an increasing number of double bonds. It was also shown that estimated Henry's law constants and their temperature dependence based on model prediction can differ from experimental results considerably and that direct measurements of temperature-dependent Henry's law constants of atmospheric organics are necessary for future work.

Alternative analytical analysis for improved Loschmidt diffusion cell

To measure gas phase diffusion coefficients across porous media, an apparatus called a Loschmidt diffusion cell is often utilized. In previous studies with such an apparatus, an infinite-length assumption is used to simplify the analytical solution. Experimentally, cell lengths must be quite long and measurement time is very brief to fulfill this assumption. In this study, Fick’s second law is applied, and separation of variables with shifted homogeneity technique is performed for data analysis to enable design of a more compact experimental apparatus with extended measurement times and improved precision. The analytical solution is proved by both the inverse-matrix method and finite-volume discretization. Finally, using the new analytical solution obtained, the effective diffusion coefficient is determined for porous media used in fuel cell applications.

Stable Isotopic Fingerprinting for Identification of the Methyl Tert-Butyl Ether (MTBE) Manufacturer

Since the late 1990s, methyl tert-butyl ether (MTBE) contamination in groundwater aquifers has become a serious concern in many countries. This study focused on the carbon and hydrogen isotopic compositions of MTBE reagents produced by six manufacturers from three regions, to investigate the potential of utilizing stable isotopes in tracing the production origin of MTBE. The carbon and hydrogen isotopic compositions (δ13C and δD) of MTBE ranged from −31.0% to −26.4% and −101% to −54%, respectively, and exhibited different signatures according to the manufacturer. Two different supplies of MTBE produced by the same manufacturer, however, had different carbon isotopic compositions, with a deviation of approximately 0.6‰. δ13C values of MTBE were classified according to the area of production (Japan, Europe and the United States), allowing the data from this study to be aligned with those of previous reports. The results suggest that both δ13C and δD values cannot be used to apportion sources of MTBE in detail, but that it is possible to apply a fingerprinting technique, which classifies the country of production of MTBE, using δ13C values alone. Minimal carbon isotope fractionation was observed during the progressive evaporation of MTBE, following the Rayleigh fractionation trend. The impurities of MTBE reagents, in particular the water contents, cause the gas-phase diffusion coefficient to vary and subsequently impact isotope fractionation. These data indicate that although significant loss of MTBE occurs by evaporation, the isotopic composition of the remaining MTBE in contaminated sites can be used to identify its production origin.

Separation and concentration of a low-penetrating impurity by membrane gas separation

The process of concentrating low-penetrating impurities in membrane modules was studied theoretically and experimentally. An expression was derived for the degree of separation in a radial cross-flow module; the degree of separation substantially depends on the gas-phase diffusion coefficient of the impurity. The experimental results obtained for a mixture of carbon dioxide with an impurity of Freon 14 and a mixture of Freon 12 with an impurity of Freon 218 agree with calculated data.

Short Time Scale Dynamics and a Second Correlation between Liquid and Gas Phase Chemical Rates: Diffusion Processes in Noble Gas Fluids

A theoretical formula for single-atom diffusion rates that predicts an isothermal correlation relation between the liquid (l) and gas (g) phase diffusion coefficients, D(T, ρl) and D(T, ρg) is developed. This formula is based on a molecular level expression for the atom’s diffusion coefficient, D(T, ρ), and on numerical results for 1715 thermodynamic states of 25 rare gas fluids. These numerical results show that at fixed temperature, T, the decay time, τDIF, which governs the shortest time decay of an appropriate force autocorrelation function, F(t) F0, is density (ρ)-independent. This independence holds since τDIF arises from the ρ-independent shortest time inertial motions of the solvent. The ρ independence implies the following l−g diffusion coefficient correlation equation: D−1(T, ρl) = (ρl/ρg) D−1(T, ρg) [ρl−1F0,l2/ρg−1F0,g2]. This relation is identical in form to the familiar (isolated binary-collision-like) empirical correlation formula for vibrational energy relaxation rate constants. This is because both correlation relations arise from inertial dynamics. Inertial dynamics always determines short-time fluid motions, so it is likely that similar correlation relations occur for all liquid phase chemical processes. These correlation relations will be most valuable for phenomena dominated by short time scale dynamics.

Investigation of Cyclic Solvent Injection Process for Heavy Oil Recovery

Summary This paper summarizes numerical and experimental simulation results of a cyclic solvent injection process study, which was part of a continuing investigation into the use of solvents as a follow-up process in Cold Lake and Lloydminster reservoirs that have been pressure-depleted by cold heavy oil production with sand (CHOPS). Typically only 5% - 10% of the original oil in place (OOIP) is recovered during cold production; therefore, an effective follow-up process is required. The cyclic solvent injection (CSI) experiment consisted of primary production followed by six solvent (28% C3H8 - 72% CO2) injection cycles. Oil recovery after primary production and six solvent cycles was 50%, which indicates the potential viability of the CSI process. Concurrently with the laboratory physical simulation, a numerical simulation model was developed to represent the physical behaviour of the experimental results. A history match of the primary production portion of the experiment was obtained using an Alberta Innovates - Technology Futures (AITF) foamy oil model. This resulted in the characterization (fluid saturations and pressures) of the oil sandpack at the start of the solvent injection process. The history match of the subsequent six solvent injection cycles was used to validate the numerical model of the CSI process developed at AITF. This model includes nonequilibrium rate equations that simulated the delay in solvent reaching its equilibrium concentration as it dissolves or exsolves in the oil in response to changes in the pressure and/or gas-phase composition. Dissolution of CH4, C3H8 and CO2 in oil and CO2 in water were considered, as was exsolution of CH4, C3H8 and CO2 from oil and CO2 from water. Reduced gas-phase permeabilities resulting from gas exsolution were also included. The history match simulations indicated that: The important mechanisms were represented in the simulations. Significant oil swelling by solvent dissolution occurs during solvent injection periods. This can reduce solvent injectivity and penetration into a heavy oil reservoir during solvent injection periods. Low oil and gas-phase relative permeabilities are required during production periods to match the experimental oil and gas production during solvent cycles. A parametric simulation study showed that the quantity of gas injected in an injection period was relatively insensitive to the oil-phase diffusion coefficients, but was sensitive to solvent solubility in oil, dissolution rates, gas-phase diffusion coefficients, molar densities in the oil phase, gas-phase relative permeability and capillary pressure. It was shown that oil production is highly dependent on how quickly solvent can dissolve in the oil during injection and exsolve from the oil during production.

Gas-phase diffusivity and tortuosity of structured soils

Modeling gas-phase diffusion of volatile contaminants in the unsaturated zone relies on soil-gas diffusivity models often developed for repacked and structureless soil columns. These suffer from the flaw of not reflecting preferential diffusion through voids and fractures in the soil, thus possibly causing an underestimation of vapor migration towards building foundations and vapor intrusion to indoor environments. We measured the ratio of the gas diffusion coefficient in soil and in free air ( D p / D 0) for 42 variously structured, intact, and unsaturated soil cores taken from 6 Danish sites. Whilst the results from structureless fine sand were adequately described using previously proposed models, results that were obtained from glacial clay till and limestone exhibited a dual-porosity behavior. Instead, these data were successfully described using a dual-porosity model for gas-phase diffusivity, considering a presence of drained fractures surrounded by a lower diffusivity matrix. Based on individual model fits, the tortuosity of fractures in till and limestone was found to be highest in samples with a total porosity < 40%, suggesting soil compaction to affect the geometry of the fractures. In summary, this study highlights a potential order of magnitude underestimation associated in the use of classical models for prediction of subsurface gas-phase diffusion coefficients in heterogeneous and fractured soils.

Coefficients of Evaporation and Gas Phase Diffusion of Low-Volatility Organic Solvents in Nitrogen from Interferometric Study of Evaporating Droplets

Evaporation of motionless, levitating droplets of pure, low-volatility liquids was studied with interferometric methods. Experiments were conducted on charged droplets in the electrodynamic trap in nitrogen at atmospheric pressure at 298 K. Mono-, di-, tri-, and tetra(ethylene glycols) and 1,3-dimethyl-2-imidazolidinone were studied. The influence of minute impurities (<0.1%) upon the process of droplet evaporation was observed and discussed. The gas phase diffusion and evaporation coefficients were found from droplet radii evolution under the assumption of known vapor pressure. Diffusion coefficients were compared with independent measurements and calculations (in air). Good agreement was found for mono- and di(ethylene glycols), and for 1,3-dimethyl-2-imidazolidinone, which confirmed the used vapor pressure values. The value of equilibrium vapor pressure for tri(ethylene glycol) was proposed to be 0.044 +/- 0.008 Pa. The evaporation coefficient was found to increase from 0.035 to 0.16 versus the molecular mass of the compound.

Convective diffusion coefficient in the gas phase in evaporation of binary liquids

A semiempirical correlation is proposed to determine the convective diffusion coefficients in the gas phase in the evaporation of binary liquids into an inert gas. The calculated and experimental data on the evaporation of n-butanol-water, ethanol-water, and ethanol-methanol mixtures into argon are shown to be in satisfactory agreement.

Producing chromium carbide using reduction of chromium oxide with methane

AbstractIn the present work, reduction of chromium oxide using methane is investigated. The thermogravimetric method was used to obtain kinetic parameters of the reaction in the temperature range of 870‐975°C under atmospheric pressure. The experimental data was analyzed using the grain model. The reaction product, chromium carbide, was observed to be produced at a temperature of about 140°C lower than by other conventional methods. Carbon deposition, which is highly undesirable in the synthesis of chromium carbide, was negligible in the present experiments. The experiments were conducted in two modes, either under the conditions of reaction control or under the conditions of diffusion control for determining reaction rate constants and diffusion rates to study pore diffusion, respectively. The pore diffusion studies were carried out with and without considering the bulk flow effect. The gas phase diffusion coefficient through the product layer around the grains was calculated with an approximate method. © 2005 American Institute of Chemical Engineers AIChE J, 2006

Membrane gas diffusion measurements with MRI

Gas transport across polymeric membranes is fundamental to many filtering and separation technologies. To elucidate transport mechanisms, and understand the behaviors of membrane materials, accurate measurement of transport properties is required. We report a new magnetic resonance imaging (MRI) methodology to measure membrane gas phase diffusion coefficients. The MRI challenges of low spin density and short gas phase relaxation times, especially for hydrogen gas, have been successfully overcome with a modified one-dimensional, single-point ramped imaging with T 1 enhancement, measurement. We have measured the diffusion coefficients of both hydrogen gas and sulfur-hexafluoride in a model polymeric membrane of potential interest as a gas separator in metal hydride batteries. The experimental apparatus is a modified one-dimensional diaphragm cell which permits measurement of the diffusion coefficient in experimental times of less than 1 min. The H 2 gas diffusion coefficient in the membrane was 0.54 ± 0.01 mm 2/s, while that of sulfur-hexafluoride was 0.14 ± 0.01 mm 2/s, at ambient conditions.