Gas Transport Rates Research Articles

Evaluation of local gas transport rate in oscillatory flow in a model human central airways

Evaluation of local gas transport rate in oscillatory flow in a model human central airways

Chemical stability of polyimide membranes at temperatures near Tg

AbstractThe success of polymeric membranes for use in high temperature gas separation applications relies, in part, on the chemical stability of these materials in separation environment. This paper details experimental evaluation of the stability of the membranes prepared using a polyimide as the selective separating layer when exposed to butane/butene/hydrogen mixtures at temperatures near the glass transition temperature of the polymer, 310°C. Further, the effects of small concentrations of sulfur and ammonia compounds in the feed mixture were investigated, as was the influence of a simulated air leak. The gas transport rates and selectivities were unchanged when testing in the hydrocarbon/hydrogen environment for over 30 days. Further, no changes in transport properties were observed when hydrogen sulfide or ammonia was added to the feed mixture. However, degradation due to oxygen exposure was observed. The chemical stability of the polyimide layer appears to be sufficient to be utilized for the separation of high temperature petrochemical streams. © 1995 John Wiley & Sons, Inc.

Investigation of the gas-transport properties of polyaniline

Investigations of the rate and selectivity of gas transport in the electronically conductive polymer polyaniline are described. Both rate and selectivity were found to be dependent on the doping level. Permeability coefficients for all gases studied decreased with increasing doping level, and selectivity coefficients, in general, increased with doping level. The effects of other variables, including the nature of the counterion, temperature and duration of thermal processing of the film, were also explored. Both free-standing polyaniline films (≈25-μm thick) and thin film composite membranes based on polyaniline were studied. The thin film composites were prepared by coating the polyaniline film (≈3-μm thick) onto the surface of a microporous alumina support membrane. The free-standing and thin film composite membranes showed identical rates and selectivities of gas transport. The highest O 2/N 2 and CO 2/CH 4 selectivity coefficients obtained were α O 2/N 2 =15 and α CO 2/CH 4 =55, respectively.

Pressurized gas transport in two Japanese alder species in relation to their natural habitats

AbstractOxygen uptake measurements have shown that pressurized gas transport, resulting from the physical effect of thermo‐osmosis of gases, improves oxygen supply to the roots of the seedlings in two alder species Alnus japonica (Thunb.) Steud. and Alnus hirsuta (Spach) Rupr., which are both native in Japan. When gas transport conditions were established by irradiation of the tree stems the internal aeration was increased to a level nearly equal to the oxygen demand of the root system in leafless seedlings of A. hirsuta, but was higher in A. japonica so that excess oxygen was excreted into the environment. An increase of superoxide dismutase (SOD) activity, which protects plants from toxic oxygen radicals and post‐anoxic injury, has been observed in root tissues of A. japonica when the seedlings were flooded for 3 days. The increase of SOD activity, in concert with high gas transport rates, may enable this tree species to grow in wet sites characterized by low oxygen partial pressure in the soil and by varying water tables. A less effective gas transport, flood‐induced reduction of SOD activity in root tissues, and reduced height growth in waterlogged soil may be responsible for the fact that A. hirsuta is unable to inhabit wettland sites.

Monte Carlo simulation of gas diffusion in regular and randomized pore systems

The rate of rarefied gas transport in clusters of hollow spheres and cylinders is determined using Monte Carlo simulation methods. The spheres and the cylinders, representing large pore bodies and narrow pore throats in actual porous materials, are either placed at the nodes and the branches, respectively, of a cubic network or arranged randomly in space allowed to overlap freely. The Knudsen diffusion coefficient is computed by simulating molecular trajectories and calculating statistical averages of the mean-square displacement of a sample of molecules per unit of travel time. The accuracy of approximating the transport resistance of regular structures of spheres and cylinders by the resistance of a commonly used network of sinusoidal conduits is checked in this work for the first time. It is found that networks of sinusoidal conduits approximate fairly well systems of spheres and cylinders in the low density region when Knudsen diffusivity estimates are pursued. However, at moderate and high densities, pore overlap becomes significant and the sinusoidal conduit geometry fails to represent efficiently the actual geometry during gas diffusion owing to the development of considerable end effects and strong interaction between remote overlapping pores. Regular arrays of spheres and cylinders yield diffusivities that compare well with those in structureless systems with randomized pore positions for the same number density of pores. Sphere-to-sphere, sphere-to-cylinder, and combined size correlations are introduced to regular structures using properly devised algorithms. Comparison of correlated-to-uncorrelated systems for continuous and discrete sphere and cylinder radius distributions reveals that sphere-to-sphere size correlation considerably facilitates the gas transport for any value of the pore system density. On the contrary, sphere-to-cylinder size correlation has a relatively small impact on the diffusivity results, which becomes non-negligible in low-density systems only and for discrete size distributions.

Rate of gas transport in glassy polymers: a free volume based predictive model

Free volume theories of diffusion developed in the literature successfully treat penetrant transport behavior in rubbery polymers. The potential of such a free volume theory to predict the gas transport behavior in glassy polymers is explored here, duly taking into account the excess hole free volume present in the glassy state. The calculations require values for three model parameters appearing in the theory of diffusion which are obtained by fitting experimental gas permeability data in rubbery polymers. The calculations also require an estimate for the hole free volume in the gas-polymer mixture. This is obtained by fitting experimental gas sorption data in rubbery and glassy polymers to a free volume based theory of gas sorption developed here. Thus, the calculation of gas permeability in glassy polymers is strictly predictive in the sense that no use is made of experimental permeation data in the glassy state. I. Introduction Free volume concepts have been successfully used for the quantitative description of equilibrium and transport properties of polymeric systems. The equation of state models of Flory (1965, 1970), Prigogine (Prigogine et al., 1953a-c; Prigogine, 1957), Patterson (Patterson, 1969; Patterson and Delmas, 1970), and Sanchez and Lacombe (1976,1978) are examples of how the free volume concept has been employed in the quantitative treatment of equilibrium behavior. The treatment of viscosity of polymer melts by Williams et al. (1955) (see also Ferry, 1970) and of diffusion by Cohen and Turnbull (1959), Fujita (1961), and Vrentas and Duda (1977a,b, 1979,1986) are some examples of free volume based treatments of transport behavior. Given the success of free volume models in describing diffusion behavior in rubbery polymers, it is natural to explore whether they can be employed also to describe the permeability behavior of gases in glassy polymers. Specifically, we show in this paper that, from our knowledge of sorption behavior in rubbery and glassy polymers and of diffusion behavior in rubbery polymers, we can reasonably predict the permeability of gases through glassy polymer membranes without using any experimental permeability data in the glassy state.

Correlations for Predicting Air Permeabilities and 222Rn Diffusion Coefficients of Soils

The rate of 222Rn gas transport through earthen materials controls 222Rn releases to the atmosphere and to indoor environments. The key soil-related parameters characterizing 222Rn transport in earthen materials are the 222Rn diffusion coefficient and the soil air permeability. Simple correlations have been developed for predicting the 222Rn diffusion coefficient and the air permeability of soils based on fraction of water saturation, total porosity, and arithmetic mean particle diameter. Correlations are based on 1,073 diffusion coefficient measurements and 137 soil air permeability measurements. The geometric standard deviations between the correlation predictions and the measurements are 1.98 for the diffusion coefficients and 2.31 for the soil air permeabilities.

Effect of casting solvent on the permeability of poly(4-methyl-1-pentene)

The permeability of gases in poly(4-methyl-1-pentene) (PMP) films cast from solution depends strongly on the casting solvent used. Permeability coefficients for several gases were measured for PMP films cast from six solvents and for a film prepared from the melt. Sorption isotherms were measured for selected gas-film pairs. Films cast from decalin, xylene and chloroform have lower permeation rates relative to the melt-prepared film. Films cast from cyclopentane, cyclohexane and carbon tetrachloride have permeability coefficients similar to that for the melt-processed film. The differences may be as great as six-fold. X-ray diffraction and thermal analysis were used to characterize the films. It is shown that the casting solvent influences which crystalline modification is formed and the spatial arrangement of the crystallites within the film. The films with very low gas permeability coefficients have a strong preferential orientation of the crystallites with the chain axis normal to the film surface. It is suggested that in these cases, some single crystals were formed during casting which aligned preferentially in the film to give a long tortuous path for permeation of gas molecules. The relative rate of gas transport in the unusually open crystals of PMP is discussed.

The effect of bisphenol monomer structure on the gas permeability of aromatic polycarbonates

AbstractPermeability measurements for oxygen and, to a lesser extent, carbon dioxide were carried out on a series of 24 structurally different aromatic polycarbonates. Polymers were prepared from bisphenols bearing substituents on the aromatic rings and/or the central aliphatic carbon atom. A strong correlation between monomer structure and polymer permeability was found, with gas transport rates varying by over two orders of magnitude between the most and the least permeable materials. In addition, a semilogarithmic relationship was found between oxygen transmission rates and monomer composition for two series of copolycarbonates. Of several methods explored, an excess free volume model was found best able to relate the experimental permeability values to the polymer molecular structure.

Toward improved methods of high frequency ventilation: a study of gas transport mechanisms

Prior studies of gas transport mechanisms are reviewed with the aim of using these results to suggest improvements over current methods of ventilation. These concepts are cast in a simpler framework that more clearly identifies those factors that limit the rate of gas transport under conditions typical of HFV. One simplification introduced simulates the enhancement of molecular diffusion due to the movement of gas back-and-forth through regions of varying cross-sectional area. The region of the lung that currently poses the greatest resistance to gas transport is represented by those airways having diameters in the range of 1.0 to 2.5 mm. This suggests that new efforts should be devoted to better understanding the nature of transport in this zone and to the development of variations in the method of ventilation that have the greatest influence there.

Gas exchange during constant flow ventilation with different gases.

To investigate the mechanisms of CO2 transport during constant flow ventilation, we measured arterial blood gases using air, 80% He-20% O2 (He) or 80% SF6-20% O2 (SF6) as the insufflating gas. At any given flow rate (0.2 to 1.0 L/s), PaCO2 was greatest with He and lowest with SF6. Data for all gases could be described by the equation PaCO2/Pb = 0.044 V-0.64 v0.23, where Pb = barometric pressure, PaCO2 is in mm Hg, V = insufflated flow in L/s, and v = kinematic viscosity (cm2/s). At any given flow rate, the AaPO2 was greater using SF6 than using He. These results are consistent with a 2-zone model of gas transport in which the enhancement of gas transport as V increases may be due to an increase in the turbulent diffusivity in zone I (the region affected by the jet). The decreased gas transport with He compared to air and SF6 at any V may be due to either the decreased penetration depth of zone I caused by the greater kinematic viscosity of He, or the decreased rate of gas transport in the region affected by cardiogenic oscillations (zone II) secondary to the higher molecular diffusivity of He.

Gas transport in branched airways during high-frequency ventilation.

A theoretical model of high-frequency ventilation (HFV) is presented based on the physical convective exchange process that occurs due to the irreversibility of gas velocity profiles in oscillatory flow through the bronchial airways. Mass transport during the convective exchange process can be characterized by a convective exchange length, LE, which depends only on the irreversibility of bronchial velocity profiles and can be measured by the experimental technique of photographic flow visualization in bronchial tree models. Using the exchange length and the molecular diffusivity, a simple model of overall bronchial mass transfer is developed. The model allows a prediction of the mean gas concentration profiles along the airways, the site of maximum mass transfer resistance, and overall flow rate of the gas of interest in or out of the lung as functions of the parameters of HFV. The results predicted by the model agree with the limited experimental data available for animals and humans. For normal unassisted ventilation, total bronchial cross-sectional area around the 15th Weibel bronchial generation is predicted to be the single most important parameter in controlling the total gas transport rate along the airways. For the breathing of room air, values of the respiratory quotient around 0.78 are predicted, which are insensitive to VT and f. The model represents a fruitful combination of fluid mechanical theory and experiment with physiologic data to yield new and deeper insight into the operation of the human respiratory system during HFV and normal breathing.

High-frequency ventilation.

Complete physiological understanding of HFV requires knowledge of four general classes of information: 1) the distribution of airflow within the lung over a wide range of frequencies and VT (sect. IVA), 2) an understanding of the basic mechanisms whereby the local airflows lead to gas transport (sect. IVB), 3) a computational or theoretical model in which transport mechanisms are cast in such a form that they can be used to predict overall gas transport rates (sect. IVC), and 4) an experimental data base (sect. VI) that can be compared to model predictions. When compared with available experimental data, it becomes clear that none of the proposed models adequately describes all the experimental findings. Although the model of Kamm et al. is the only one capable of simulating the transition from small to large VT (as compared to dead-space volume), it fails to predict the gas transport observed experimentally with VT less than equipment dead space. The Fredberg model is not capable of predicting the observed tendency for VT to be a more important determinant of gas exchange than is frequency. The remaining models predict a greater influence of VT than frequency on gas transport (consistent with experimental observations) but in their current form cannot simulate the additional gas exchange associated with VT in excess of the dead-space volume nor the decreased efficacy of HFV above certain critical frequencies observed in both animals and humans. Thus all of these models are probably inadequate in detail. One important aspect of these various models is that some are based on transport experiments done in appropriately scaled physical models, whereas others are entirely theoretical. The experimental models are probably most useful in the prediction of pulmonary gas transport rates, whereas the physical models are of greater value in identifying the specific transport mechanism(s) responsible for gas exchange. However, both classes require a knowledge of the factors governing the distribution of airflow under the circumstances of study as well as requiring detail about lung anatomy and airway physical properties. Only when such factors are fully understood and incorporated into a general description of gas exchange by HFV will it be possible to predict or explain all experimental or clinical findings.

Blood‐gas transfer in an axial flow annular exchanger

AbstractThe rates of hemoglobin saturation and carbon dioxide reduction in blood flowing in an annular‐shaped membrane exchanger were studied theoretically by considering two limiting cases: the fluid‐limited case and the wall‐limited case. The blood is assumed to flow axially with a fully developed laminar profile. In the fluid‐limited analysis the membrane wall is assumed to be infinitely permeable, so the efficiency of the oxygenator depends solely on the rate of gas transport within the blood. In the wall‐limited case the blood is assumed to have infinite transport ability, thus the efficiency of the oxygenator is limited by the diffusion rate through the wall. For realistic radii ratio, the oxygenator is fluid limited, thus suggesting the need for enhanced blood mixing. In the fluid‐limited case the rate of gas transport depends on the radii ratio and a dimensionless length, which is independent of the cylinder sizes. If PO2 = 715 mm. Hg and PCO2 = 0 mm. Hg are maintained at both walls, oxygenation is the slower process. For the same flow rate the length required for complete hemoglobin saturation in the annulus with an inner to outer radius ratio of 0.95 is 1/65 of that required in a circular tube.

Gas Transfer Across the Skin in Man

Any inert component of the inspired gas reaches the skin by the cutaneous perfusion. When part of the body is enclosed in an oxygen-filled chamber, a portion of the inert gas is transported out of the tissues into the oxygen atmosphere. The rates of helium, argon, and nitrogen transport were determined in the forearm and hand of a normal subject at ambient temperatures ranging from 18 to 43 C, analysis being performed by gas chromatography. At any temperature, the ratio Va/Vhe was found to be equal to (αa·Pa/(See PDF)a)/ (αHe·Phe/(See PDF) He) where V is the rate of gas transport, agr the solubility of the gas in blood, P the pressure of the gas in the blood, and mol wt the molecular weight of the gas. This indicates that the gas transport is limited by diffusion through the skin barrier and is affected only indirectly by changes in cutaneous blood flow in individual capillaries. Gas transport is at a minimal value at temperatures below 28 C, increasing in a linear fashion with increasing temperature above 28 C. This increase in transport shows that the increase in cutaneous perfusion at elevated ambient temperatures is accompanied by the opening of additional capillaries, the resulting increase in diffusing capacity being responsible for the increase in gas transport. Submitted on August 6, 1962

Gas transfer across the skin in man.

Any inert component of the inspired gas reaches the skin by the cutaneous perfusion. When part of the body is enclosed in an oxygen-filled chamber, a portion of the inert gas is transported out of the tissues into the oxygen atmosphere. The rates of helium, argon, and nitrogen transport were determined in the forearm and hand of a normal subject at ambient temperatures ranging from 18 to 43 C, analysis being performed by gas chromatography. At any temperature, the ratio Va/Vhe was found to be equal to (αa·Pa/(See PDF)a)/ (αHe·Phe/(See PDF) He) where V is the rate of gas transport, agr the solubility of the gas in blood, P the pressure of the gas in the blood, and mol wt the molecular weight of the gas. This indicates that the gas transport is limited by diffusion through the skin barrier and is affected only indirectly by changes in cutaneous blood flow in individual capillaries. Gas transport is at a minimal value at temperatures below 28 C, increasing in a linear fashion with increasing temperature above 28 C. This increase in transport shows that the increase in cutaneous perfusion at elevated ambient temperatures is accompanied by the opening of additional capillaries, the resulting increase in diffusing capacity being responsible for the increase in gas transport. Submitted on August 6, 1962