Gas Diffusion Rate Research Articles

Characteristics of CO gas adsorption on modified natural zeolite

The characteristics of CO gas adsorption on chemically treated natural zeolite from the Kampo area of Korea were investigated. Acid treatment was more effective in liquid adsorption than alkali treatment because the exchanged H + ion attributed to the enlargement of pore size. However, in gas adsorption, a different tendency was observed. In HCl treated zeolite, 80% of the pore area, which was derived from a higher portion of micropores around 10 Å, led to hindrance of the gas diffusion rate. Though the pore and surface area decreased in NaOH treatment, an increase in the gas adsorption amount was found, and it could be supposed that the exchanged Na + ion located on the surface served as an adsorption site. The NaOH treated natural zeolite showed a higher value and 0.017 g of the adsorption amount at 5 N NaOH compared with 0.011 g at 5 N HCl.

Measurement of Net Photosynthetic and Transpiration Rates of Spinach and Maize Plants under Hypobaric Condition.

An environmental control system for measuring net photosynthetic and transpiration rates was constructed to study the feasibility of growing plants under hypobaric condition in order to achieve crop production for a controlled ecological life support system (CELSS) in space. The total pressure of the system's assimilation chamber was reduced to 10kPa, with the rates for spinach and maize plants being measured under total air pressures of 10, 40, 70, and 100kPa at the same CO2 partial pressure condition. Photosynthetic rates of spinach and maize increased until a total pressure of 10kPa. The rate of increase in the photosynthetic rate under low total pressures was distinctive in maize. In addition, the rate of increase was found to be affected by the CO2 gas diffusion rate. Photosynthesis is considered to be enhanced by a decrease in boundary layer resistance and stomatal resistance to CO2 transfer due to an increase in the CO2 diffusion coefficient. The transpiration rate of spinach slightly increased under low total pressures, whereas that of maize also increased, though much less; both plants had lower rates than those simulated using water vapor diffusion theory.

Steady-state model of a zirconia oxygen sensor in a simple gas mixture

The theory of gas sensors developed previously is used to analyze in detail the response of a zirconia oxygen sensor exposed to a mixture containing oxygen and a single reducing gas such as CO or hydrogen. Results for changes in the sensor e.m.f. as a function of the inverse redox ratio (defined as twice the ratio of the partial pressure of oxygen to that of the reducing gas in the cases cited) due to variations in gas diffusion, adsorption, desorption, and Pt electrode surface-reaction rates are presented. The response of the sensor to a fully equilibrated gas mixture is also described.

Direct demonstration of gas diffusion into the middle ear.

The gas composition of the middle ear differs from that of air, and resembles the gas composition of mixed venous blood. This observation suggests the existence of a bi-directional route for gas diffusion between the middle ear and blood. In an attempt to demonstrate this route in a direct way, we tracheotomized guinea pigs in such a way that they breathed freon-22 directly into the distal part of their tracheostomy. The proximal part of the trachea was sealed so that air by-passed the oropharynx and nasopharynx, thus preventing freon-22 from making contact with the eustachian tube orifice. At the same time middle ear gases were monitored with a mass spectrometer, through a measuring probe which was inserted into a hole in the bulla. The appearance of freon-22 in all middle ears--after 8 min on the average--demonstrated direct gas diffusion from the blood into the middle ear, since freon-22 could reach the middle ear only from the blood stream, i.e., by diffusion. Differences in gas diffusion rates into and from the middle ear may therefore play a role in regulating middle ear gas economy, and therefore in middle ear pressure.

Generation of microcellular polymeric foams using supercritical carbon dioxide. II: Cell growth and skin formation

AbstractWe have generated microcellular polymeric foam structures using a pressure induced phase separation in concentrated mixtures of supercritical CO2 and poly(methyl methacrylate). The process typically generates a microcellular core structure encased by a nonporous skin, the thickness of which decreases with increasing saturation pressure. This trend can be described by a model for skin formation that is based on the diffusion rate of gas out of the sample. Significant density reductions on the order of 30 to 70% can be achieved by changing the pressure and temperature conditions in the foaming process. There are several ways in which the saturation pressure affects the average cell size, with the net effect that cell size decreases sharply with increasing pressure above 2000 psi, leveling out at higher pressures. Cell size increases with increasing temperature from 40°C to 70°C. A model for cell growth, based on a cell model of Aremanesh and Advani, modified to include the effect of CO2 on model parameters, reproduces these trends.

Hot‐stage Microscopy of Cake Batter Bubbles during Simulated Baking: Sucrose Replacement by Polydextrose

ABSTRACTA hot‐stage video microscopical technique was developed to study the effects of sucrose replacement by polydextrose (weight for weight basis) on the foam characteristics of cake batters, before and during heating. Polydextrose substitution increased the mean size of air‐bubbles in the cake batter and introduced a larger variation in bubble size distribution than the sucrose batter. The differences between sucrose and polydextrose batters on initial bubble size distribution were reflected in expansion rates of bubble populations during heating. The lack of uniformity in the bubble distribution of batters containing polydextrose increased the rate of gas diffusion from small bubbles to larger ones. On heating, the bubble population of sucrose batters expanded more than that of polydextrose substituted batters.

Reducing gas exchange of fruits with surface coatings

Surface coatings can increase a fruit's skin resistance to gas diffusion, modify its internal atmosphere composition and depress its respiration rate; effects on transpiration tend to be comparatively small. A steady state mathematical model which considered diffusion through cuticle and pores separately was used to investigate two possible mechanisms by which coatings achieve these effects: either by acting as a film wrap or by blocking pores. It was concluded that coatings mainly exert their effects on skin resistance to diffusion of the permanent gases by blocking a greater or lesser proportion of the pores on the fruit surface. Both covering of the cuticle and blocking pores were important for resistance to water vapour diffusion. Coupled with a proposed differential resistance of the skin to oxygen, carbon dioxide and water vapour, this accounts for the observed effects of coatings on internal atmosphere modification, respiration and transpiration rates. Coating treatments which achieved substantial reductions in transpiration rates were associated with the fruit becoming anaerobic. Inherent variability in skin resistance to gas diffusion and fruit respiration rate, and differing proportions of pores blocked by coating, appear to be responsible for the highly variable response of individual fruit to a given coating treatment. Surface coatings may therefore be better suited to treating fruits destined for processing, in which fruit to fruit variability may be less critical, than to fruits destined for the fresh market, in which uniformity of individual fruit quality is of paramount importance.

Gas solubility and diffusion in a polystyrene–polybutadiene block copolymer

AbstractOver the temperature range of 20 to 90°C, the equilibrium solubility of methane or argon in a star‐branched polystyrene–polybutadiene (SB) block copolymer can be reliably estimated from a volume‐fraction‐weighted average of the homopolymer solubilities; the value of heat of solution for SB block copolymer lies between the two values associated with the constituent homopolymers. Diffusion coefficients for gases in the block copolymer are bracketed by the homopolymer values over the range of temperature employed, but the activation energies for diffusion in SB are higher than either of the homopolymer values. The anomalous temperature dependence and the inability of a computer simulation to reproduce experimental diffusion coefficients supported the concept of a temperature‐dependent restriction on chain mobility in the polybutadiene regions of the block copolymer. This β factor embodies the relative reduction in the rate of gas diffusion through the B regions of the copolymer compared to B homopolymer; β has a value of about 2–3 at room temperature but decreases to unity when the polystyrene chains become mobile at the polystyrene Tg.

An amalgamation between hormone physiology and plant ecology: A review on flooding resistance and ethylene

Both distribution of terrestrial plants and species composition in flood plain communities are strongly influenced by flooding (waterlogging, partial submergence, or submergence). The interaction between a plant's flooding resistance and the seasonal timing, duration, depth, or frequency of flooding often determines plant distribution in flood plains. Flooding may be accompanied by marked physical changes in light, carbon availability, diffusion rate of gases, and density of the environment. Various physiological processes may be affected by these flooding-induced physical changes, including aerobic respiration, photosynthesis, and processes in which light acts as a source of information (e.g., phytochrome photoequilibrium). Certain plant species acclimatize and adapt to these physical changes to relieve the constraints imposed by the flooded environment. Underwater photosynthesis, enhanced shoot elongation, adventitious roots, and aerenchyma formation are typical adaptive responses which are believed to improve the oxygen status of submerged plants. Ethylene and other plant hormones play a central role in the initiation and regulation of most of these adaptive responses, which permit “escape” from anaerobiosis. Mechanisms of direct tolerance of anaerobic conditions, such as a vigorous fermentative respiratory pathway, are of particular importance when the plant is very deeply submerged, or during the night and when the water is sufficiently turbid to exclude light.Studies on the cosmopolitan genus Rumex, distributed in a flooding gradient on river flood plains, have integrated plant hormone physiology with plant ecology. Rumex species showed a high degree of interspecific variation in ethylene production rates, endogenous ethylene concentrations, ethylene sensitivity, and ethylene-mediated growth responses. The field distribution of Rumex species in flooding gradients is explained in terms of a balance between endogenous ethylene concentrations and sensitivity towards this growth regulator (“ethylene economy”). Much data has been gathered using a recently developed laser-driven photoacoustic detection technique capable of detecting six parts of ethylene in 1012 parts air flowing continuously over the plant.

Estimated Diffusion Rates of Inorganic Gases from Southeastern Colorado Reservoirs

Helium, argon, and nitrogen are small, chemically unreactive molecules with relatively large effective diffusion coefficients compared to most hydrocarbons. If these gases have existed in gas fields for even the shortest geologic time spans, steady-state diffusion must be at least approximated, and the diffusional flux of these gases through a reservoir cap rock may be estimated using Fick's First Law of Diffusion. Diffusional loss represents a minimum loss rate since mass transfer along fractures and faults would be faster. Under a steady-state condition, the rate of diffusional loss most be balanced by an equal influx of the gas into the reservoir. Using a natural gas field's estimated reserves, natural gas composition, area, and the estimated flux of a given gas through the field's cap rock, the turnover time of the gas is estimated. Southeastern Colorado gas fields producing from the Morrow Formation often contain anomalously high concentrations of nitrogen (to 70%), helium (to 5%) and yield turnover times for these gases of generally less than 100,000 years. Unless the N{sub 2}, He, and Ar were emplaced within the last 100,000 years, there must be a continuing large flux of gas into these fields to balance the diffusional loss. The largemore » fluxes of inorganic gases required to maintain their concentrations in natural gases raises questions about the age and longevity of gas fields. Extension of these calculations to light hydrocarbons implies that catagenesis is a more recent and ongoing process than is often believed.« less

A simple apparatus to demonstrate differing gas diffusion rates (Graham's law)

The apparatus described in this note dramatically demonstrates relative diffusion rate effects for common gases like methane and carbon dioxide.

The role of surface viscosity in gas diffusion in aqueous foams. II. Experimental

The rate of gas diffusion in aqueous foams is amongst others determined by the gas composition in- and outside the bubble and the rheological behavior of the bubble surface. In an earlier paper a model was presented to emphasize the influence of both parameters. A power law was used to incorporate the surface and rheological behavior of the bubble into the model. Power law parameters were determined using a Langmuir trough equipped with a caterpillar belt. By means of the observation of the size of bubbles situated at a beer surface, the bubble shrinking rate was determined. Experiments show that the model is adequately describing the actual shrinking or growing rate of bubbles.

The role of surface viscosity in gas diffusion in aqueous foams. I. Theoretical

Disproportionation is one of the processes that can determine foam stability. The rate of disproportionation is not only governed by differences in the Laplace pressure, topology of the bubble, or other well-known phenomena, but also by the gas composition and the rheological behavior of the bubble surface. The importance of surface rheological aspects such as surface viscosity is evident since the surface tension is one of the driving forces of this process. A new model is introduced describing the rate of gas diffusion in foams. In this model a power law is embodied describing the dependence of the surface viscosity on the relative surface compression or expansion rate. Also embodied in the model is the presence of gases with different solubility. The results show that the rate of disproportionation depends mainly on the partial pressure differences of the gases present. However, when gas composition throughout the foam is uniform the rheological behavior of the bubble surfaces is the main parameter determining the disproportionation rate.

The formation and properties of wheat flour doughs

Among the cereal flours, only wheat flour will form a viscoelastic dough when mixed with water. The viscoelasticity appears to be because the gluten proteins are water compatible and thus will swell and interact. The gluten protein's large molecular size and low charge density appear to allow them to interact by both hydrogen and hydrophobic bonds. Wheat flour doughs are also unique in their ability to retain gas. This property appears to result from a slow rate of gas diffusion in the dough. The third major unique property of wheat flour doughs is their ability to set in the oven during baking, and thereby to produce a rigid loaf of bread. Although not clearly understood, this appears to be a heat-induced crosslinking of the gluten proteins.

Measurements of the Rate of Gas Diffusion in Rigid Cellular Plastics

This paper describes a method for measuring gas diffusion through cellular plastics A specimen is placed in a chamber, which is then scaled and pressur ized with the test gas. The pressure increase causes some gas to enter the specicmen, and this entry results in a reduction of the chamber pressure By following the change in chamber pressure with time an appropriate transport characteristic for the material is determined An application of this method to a core specimen of an extruded polystyrene board is presented.

Measurement of the sound speed in air by sing-around velocimetry

The sing-around velocimeter [M. Greenspan and C. Tschiegg, Rev. Sci. Instrum. 28, 897–901 (1957)] is capable of great precision and is an elegant measurement technique. It is also relatively easy to implement with low-cost electronics. Besides the obvious experiments on the dependence of sound speed in gases on temperature and molecular weight, it is also possible to measure gas diffusion rates or demonstrate the adiabatic nature of acoustic oscillations. Absolute measurement of sound speed within 1% is easily done. These demonstrations were developed for a graduate-level thermodynamics course given at the Pennsylvania State University Great Valley Graduate Center.

Enhancement of stability of aqueous foams by addition of water-soluble polymers—measurements and analysis

It is experimentally shown that the stability of aqueous foams can be significantly enhanced by the addition of a small amount of water-soluble polymers. Such enhancement is attributed to the retardation both in the rate of drainage and in the rate of interbubble gas diffusion. Addition of water-soluble polymer increases the bulk viscosity and thereby decreases the rate of drainage. It also decreases the gas permeability of liquid lamella and as a result the rate of interbubble diffusion is retarded. A simple analysis incorporating both lamella drainage and interbubble diffusion has been developed. The qualitative predictions of this analysis are found to be consistent with our experimental observations.

Kinetics of the formation of nickel aluminide coatings on pure nickel by chemical vapour deposition

A chemical vapour deposition process was used to deposit aluminium onto a pure nickel substrate which was not in contact with the powder mixture. After deposition for about 1 h the aluminium content at the surface reached a high value and then remained almost constant with time. The kinetics of the process is governed by a combination of solid and gas diffusion rates, and a qausi-steady-state appears to exist at the gas-coating interface. The experimental results can be explained satisfactorily by a combination of a stagnant film model for gaseous diffusion and solid diffusion rates.

Aging of polyurethane foams

The degradation of foam thermal properties due to the diffusion of the air into the foam, termed the foam aging, is a major drawback of polyurethane foams. An analytical model was developed to predict the effective diffusion coefficient of the foam and from it the rate of gas diffusion and foam aging. The model requires a measurement of the wall permeability and foam internal geometry. An experimental steady-state technique was developed to measure the permeability of the cell walls. To check the models an experimental technique was developed to measure the foam effective diffusion coefficient. The model predictions of the effective diffusion coefficient were found to be within 29% of the foam data or better.

CVD aluminide nickel

A gas phase aluminum deposition was developed for Ni by CVD process. The specimen surface quickly came to equilibrium and remained close to equilibrium for the duration of the coating runs. The kinetics of the process was governed by a combination of solid and gas diffusion rates, and a steady-state appeared to exist at the gas phase-coating interface. By combining Levine, Caves, Sivakumar, and Seigle’s model for gaseous diffusion and calculation of solid diffusion rates, some success has been achieved in explaining the results.