Gas Diffusion Theory Research Articles

Theory and Simulation of Gas Diffusion in Cholesteric Liquid Crystal Films

Theory and simulation of the dynamical phenomena that characterizes gas diffusion in a cholesteric film are presented. A classical mass transfer theory for cholesteric liquid crystals was used to construct a model that describes the one dimensional transient gas diffusion in a film. The boundary conditions that describe the concentration and orientation conditions in a gas-liquid crystal surface were obtained using the Euler-Lagrange equations for surface reorientations. Numerical solutions to the coupled mass transfer-orientation equations are presented and used to develop a comprehensive view of the phenomena. The main governing parameters that control gas diffusion in a cholesteric film are identified. Two different regimes are identified: (i) diffusion limited regime, and (ii) orientation limited regime. The diffusion limited regime is characterized by strong concentration-orientation couplings, enhanced mass transfer, and up-hill diffusion. The orientation limited regime is characterized by weaker concentration-orientation couplings, and weaker mass transfer enhancements. Conditions that lead to enhanced gas absorption are identified, characterized, and explained in terms of the orientational contribution to the mass flux. Conditions that lead to the uncoiling of the cholesteric helix into a nematic ordering are identified, and the kinetics of the phase transformation is characterized.

Carbon dioxide efflux and pCO2 in soils of threeQuercus ilex montane forests

Soil CO2 efflux and pCO2 in the soil atmosphere were measured during one year at three montane sites of Mediterranean sclerophyllous forests in NE Spain. Two sites were located in the upper and lower slopes of a small catchment in the Prades mountains (mean precipitation 550 mm year−1), and a third site was located on a lower slope in the Montseny mountains (mean precipitation 900 mm year−1). The three sites were similar in bedrock and vegetation, but differed in soil characteristics and water availability. Seasonal variation of CO2 efflux and soil pCO2 were affected by soil temperature and, to a lesser extent, by soil moisture. Annual mean soil CO2 efflux (considered as soil respiration) was similar at Montseny and at the comparably located site at Prades (83 ± 18 S.E. vs. 75 ± 9 mg CO2 m−2 hour−1 , respectively), and was highest at the Prades upper slope site (122 ± 22 mg C02 m−2 hour−1 ). Despite those relatively similar CO2 effluxes, mean soil pCO2 was much higher at both Prades sites than at Montseny. Soil pCO2 always increased with depth at Prades while maxima pCO2 at Montseny were often at 20–30 cm depth. A model based on gas diffusion theory was able to explain why soil pCO2 was much higher at Prades than at Montseny, and to reproduce the shape of the vertical profile of pCO2 at the Prades soils. Nevertheless, the model failed to simulate the soil pCO2 maximum found at 20–30 cm depth at the Montseny site. Model simulations using a time-variable CO2 production rate suggested that pCO2 maxima at intermediate depth could be the result of a transient situation instead of an equilibrium one.

Magnetizing Reduction of Iron Ores

The efficiency of reduction-roasting/magnetic separation of hematite, goethite and any extremely finegrained iron ore, which is non-responsive to conventional processing techniques, is reviewed. Roasting equipment, roasting mechanism, superior results over conventional techniques, and certain advantages are highlighted. Previous works on solid state diffusion theory and gas diffusion theory, in the direction of explaining the mechanism of iron ore reduction are indicated. The transformation of hematite to magnetite, wustite, and metallic iron, with a reductant, at elevated temperature, is established by XRD analysis.

Atom confinement in a laser open atomizer

We have investigated the effects of a buffer gas (air) on the vapor plume produced in a laser open atomizer using atomic absorption spectroscopy. The absorption signal is related to the concentration of absorbing species by a modified Beer-Lambert equation in which the buffer gas pressure and temperature and the diameter of the diffusing species appear explicitly. Our experimental results show that the observed maximum of absoprtion as a function of gas pressure is well explained by the classical gas diffusion theory. Furthermore, our measurements indicate that the plume created by the laser has a conical shape.

On the stomatal frequency in barley : IV. Estimating mesophyll resistance taking account of respiration during photosynthesis.

This research was undertaken to explain why high stomatal frequency (low stomatal resistance) Ieaf had high photosynthetic capacity by gas diffusion theory. Mesophyll resistance was estimated by electrical analogues taking account of respiration during photosynthesis. The mesophyll resistance was estimated from the data of photosynthesis and transpiration in barley differing in stomatal frequency or stomatal resistance. The assumptions were that C02 concentration in the atmosphere, C02 concentration at the chloroplasts and mesophyll resistance were constant. The results showed that mesophyll resistance was much smaller than was reported previously and stomatal resistance had an important role in controlling photosynthesis.

Diffusion of gases from hydrocarbon deposits

Diffusion dispersion of hydrocarbon deposits is used in oil and gas prospecting geology, hydrogeologic investigations, well logging and for gas surveying. This paper reviews the theory of gas diffusion and gives mathematical formulas for determining the migration of hydrocarbons. The paper goes on to discuss the use of gas surveying to locate concentrations of natural gas. A description of advantages and disadvantages to this method is also given.

The Absorption and Diffusion of Water in Rubber

Abstract A new differential equation of diffusion is put forward to represent the movements of water vapor through rubber in accordance with the osmotic theory of absorption. Experiments are described which, together with the evidence quoted from existing knowledge, confirm a number of predictions as to differences between the absorption of a gas and of water vapor by rubber. These predictions deal with the shapes of the absorption- and desorption-time curves between narrow and wide limits of humidity, the relative times required for absorption and desorption, and the variation of absorption and desorption periods with thickness of the rubber sheet. The results are definitely inconsistent with the simple gas-diffusion theory, which is often assumed to apply to water in rubber. It is established that the shape of the humidity-absorption curve is a dominant factor in determining time-water content relations. Except for changes over very restricted ranges of humidity, the absorption- and desorption-time curves are of different shape, and desorption is much more rapid than absorption. The periods required for absorption and desorption are greater, the higher the humidity and the higher the hygroscopicity of the rubber. The law that the time required to reach a given stage of saturation varies as the square of the linear dimensions of the sample (the shape being constant) is equally true for diffusion, according to the old and new equations.