Diffusion gas transfer

The shape of polyhedra and the number of faces in them change continuously in the process of foam evolution (as a result of diffusion gas transfer and coalescence). Because of gas diffusion bubbles pass through transformation stages in which a body with five faces (pentahedron), then another body with four face (tetrahedron) [71] are formed and then they disappear. 

The processes of coalescence and diffusion gas transfer accelerate drainage initiation. 

Foam instability is expressed mainly by the processes of excess (referring to equilibrium quantity) liquid outflow, diffusion gas transfer from smaller to larger bubbles and coalescence. If the vapour pressure of the solvent in the surrounding medium of foam is lower than its saturated vapour pressure, then the process of evaporation influences significantly foam collapse. Finally, if a foam is produced from a gas phase, the main component of which is the solvent vapour, condensation of these vapours appears to be the determining process of bubble expansion. 

De Vries [27] has used Eq. (6.11) together with the bubble distribution function proposed by him to derive an expression describing the change in the number of bubbles during the process of diffusion gas transfer. At the initial moment (t= 0) the total number of bubbles is equal to No and at time x it becomes... 

The aqueous stream is at higher pressure than the strip gas (or vacuum) and fast diffusive transport of dissolved gases takes place. Gas transfer membrane technology is suitable for deaeration of boiler feed, building water, and other applications, and produces water with DO levels down to 1 ppb 02. 

Experimental values of diffusivities are given in Table 10.2 for a number of gases and vapours in air at 298K and atmospheric pressure. The table also includes values of the Schmidt number Sc, the ratio of the kinematic viscosity (fx/p) to the diffusivity (D) for very low concentrations of the diffusing gas or vapour. The importance of the Schmidt number in problems involving mass transfer is discussed in Chapter 12. 

In a typical pulse experiment, a pulse of known size, shape and composition is introduced to a reactor, preferably one with a simple flow pattern, either plug flow or well mixed. The response to the perturbation is then measured behind the reactor. A thermal conductivity detector can be used to compare the shape of the peaks before and after the reactor. This is usually done in the case of non-reacting systems, and moment analysis of the response curve can give information on diffusivities, mass transfer coefficients and adsorption constants. The typical pulse experiment in a reacting system traditionally uses GC analysis by leading the effluent from the reactor directly into a gas chromatographic column. This method yields conversions and selectivities for the total pulse, the time coordinate is lost. 

This simplified description of molecular transfer of hydrogen from the gas phase into the bulk of the liquid phase will be used extensively to describe the coupling of mass transfer with the catalytic reaction. Beside the Henry coefficient (which will be described in Section 45.2.2.2 and is a thermodynamic constant independent of the reactor used), the key parameters governing the mass transfer process are the mass transfer coefficient kL and the specific contact area a. Correlations used for the estimation of these parameters or their product (i.e., the volumetric mass transfer coefficient kLo) will be presented in Section 45.3 on industrial reactors and scale-up issues. Note that the reciprocal of the latter coefficient has a dimension of time and is the characteristic time for the diffusion mass transfer process tdifl-GL=l/kLa (s). 

The physical transport of mass is essential to many kinetic and d3mamic processes. For example, bubble growth in magma or beer requires mass transfer to bring the gas components to the bubbles radiogenic Ar in a mineral can be lost due to diffusion pollutants in rivers are transported by river flow and diluted by eddy diffusion. Although fluid flow is also important or more important in mass transfer, in this book, we will not deal with fluid flow much because it is the realm of fluid dynamics, not of kinetics. We will focus on diffusive mass transfer, and discuss fluid flow only in relation to diffusion. 

The driving force for the mass transfer of the solute in the three-phase system can be determined with the solvent/water partition coefficient, just as the partition coefficient for gas/liquid phases, the Henry s Law constant, is used to determine the driving force for the mass transfer of ozone. A solute tends to diffuse from phase to phase until equilibrium is reached between all three phases. This tendency of a solute to partition between water and solvent can be estimated by the hydrophobicity of the solute. The octanol/water partition coefficient Kow is a commonly measured parameter and can be used if the hydrophobicity of the solvent is comparable to that of octanol. How fast the diffusion or transfer will occur depends not only on the mass transfer coefficient in addition to the driving force but also on the rate of the chemical reaction as well. 

Cooper, J.A. and Compton, R.G. (1998) Channel electrodes a review. Electroanalysis, 10, 141. Cussler, E.L. (1984) Diffusion Mass Transfer in Fluid Systems. Cambridge University Press, New York. Dankwerts, P.V. (1970) Gas-Liquid Reactions. McGraw-Hill, New York. 

Under reaction-controlled conditions, the total rate of catalytic oxidation is governed by the rate of surface reaction and is independent of the gas transfer rate from the gas phase to the catalyst surface, so that the CTL intensity is also independent of the flow velocity of sample gas around the sensor. Under diffusion-controlled conditions, the rate of catalytic oxidation is independent of the catalytic activity, but depends on the transfer rate of combustible gas in the gas phase, so that the CTL intensity depends on the flow rate of the gas... 

Membrane contactors are systems in which the membrane function is to facilitate diffusive mass transfer between two contacting phases (liquid-liquid, liquid-gas, etc.) without dispersion of one phase within another [12]. The membrane does not act as a selective barrier, but creates and sustains the interfaces immobilized at the... 

In this chapter, those pumps that are frequently encountered throughout the range of vacuum pressures are dealt with (see Table 3.1). Where necessary, to support the calculations, the operating principles and pump characteristics are reviewed. With gas-transfer pumps operating in the HV/UHV range (typically diffusion or turbomolecular pumps), continuous operation of backing (forevacuum) pumps is required for efficient performance. In such cases, the combination is considered. 

Gas-transfer pumps such as turbomolecular and diffusion pumps are extensively employed in the HV/UHY range. Both types require backing (forevacuum) pumps with the appropriate characteristics to enable their efficient performance. Aspects of the operation of diffusion pumps (Examples 3.10-3.15) and turbomolecular pumps (Examples 3.16-3.19) were considered. 

The various steps in the removal of a gas from air by a porous adsorbent may be confined broadly to the following processes (a) mass transfer or diffusion of the gas to the gross surface (b) diffusion of the gas into or along the surface of the pores of granular adsorbent (c) adsorption on the interior surface of the granules (d) chemical reaction between the adsorbed gas and adsorbent (e) desorption of the product and (/) transfer of the products from the surface to the gas phase. Whether surface reaction or diffusion (mass transfer) to the surface becomes the rate-controlling step will become evident in the analysis of the experimental data with respect to the rate constant. 

At the instant of their creation, free and foam bubble films of thickness not exceeding than 10 pm are formed. The effect of convective diffusion on gas transfer in such films is rather poor [327] and the diffusion equations of Fick can be employed to calculate it. 

Eqs. (3.139)-(3.141) suggest that the rate of diffusion is much lower than the rate of gas dissolution and gas evolution from both film surfaces and the adsorption surfactant layers do not affect gas transfer. However, it is known that monomolecular films from some insoluble surfactants (e.g. cetyl alcohol) considerably decrease the rate of evaporation of the water substrate [204]. At high surface pressures the rate of evaporation can be reduced 5 to 10 times. Lipid bilayers, water and electrolytes can exert a significant effect on gas permeability, as was found in the study of the properties of vesicles (lyposomes) and flat black hydrocarbon films in aqueous medium [479]. 

In such a case the diffusion coefficient is = 7.2-105 cm2 s 1 [487]. This value is higher than the typical diffusion coefficient of air in water but, on the other hand, is considerably lower that the diffusion coefficient in gas medium. Hence, gas transfer through three-vacancy holes is more difficult with respect to air. 

Q10 The transfer of CO across the respiratory surface (TCo) can be used to estimate the efficiency of gas transfer in the lung. A small concentration of CO is added to inspired air it diffuses across the alveolar membranes into the blood. The increase in arterial blood content of CO over a short period of time is measured to estimate the rate of CO transfer. A small concentration of CO must be used as this gas combines strongly with haemoglobin at the same position as oxygen to produce carboxyhaemoglobin. 

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