Diffusion effects gaseous

A second type of apparatus based on the pressure diffusion effect is the separation nozzle. Pressure gradients in a curved expanding jet produce an isotopic separation similar to that in a centrifuge. The separation effect obtained with a single jet is relatively small, and separation nozzle stages, similar to gaseous diffusion stages, must be used in a cascade to realize most of the desired separations. 

The decrease in molar volume during calcination of CaC03 to CaO has the effect of increasing the porosity of the sorbent which is important to the efficient utilization of limestone. The gas-solid reaction between S02 and limestone sorbent consists of a number of steps (2.) diffusion of gaseous S02 through the pores of the calcined limestone, reaction of 02 with CaO to form calcium sulfate (CaSO,), and diffusion of S02 through the calcium sulfate product layer to react with additional CaO in the particle. Unfortunately, the sulfation reaction ... 

De - effective diffusivity of gaseous reactant in the catalyst pellet... 

If powders are used for the sample material, the packing of the particles will have an effect on the rate of reactions involving gases. Tighter packing inhibits free diffusion of gaseous species in and out of the reaction zones. Hence, the decomposition... 

A simple geometric model will demonstrate the nature of gaseous diffusion effects on the action of catalytic surfaces. Figure 6a shows a catalytic surface S located at a distance Xl from a point in the gas phase... 

Magniiitde of gaseous diffusion coefficients. For the purpose of estimating gas diffusion effects, Table II shows some numerical values of gaseous diffusion coefficients. 

Diffusion in gas space between catalyst affecting experimental activation energy. In the case of gaseous diffusion across a finite volume of space before molecules reach the catalyst, the consequences of diffusion effects on such an interpretation is seen from inspecting formula (14), keeping in mind that k increases exponentially as the temperature is raised and thus kL/D increases in the same manner, since the variation of D with temperature (%-power) can be neglected compared to the exponential temperature dependence of k. 

When a reaction follows first-order kinetics, is diffusion-limited, but is operating with ordinary-type diffusion, a reduction of apparent order below unity will result and may indicate an order as low as In such cases it is possible to provide conditions under which the effective gaseous diffusion coefficient of the reactant experiences a negligible change with reactant concentration, such as by adding an excess of another inert gas component or by maintaining the total pressure equal by the addition of an inert gas. Corrections can be made if the degree of diffusion limitation 7/ is known. 

Figure 14.40 shows the most accurate measurements of the thermal diffusion effect in UF vapor at low pressure, by Kirch and Schutte [K2]. Results are plotted both as k, for comparison with other gases in Fig. 14.39, and as the thermal diffusion constant y. The very low values, under 0.00005, explain Nier s [N3] inability to detect a thermal diffusion effect in UF5 vapor. The thermal diffusion coefficient 7 is so much smaller than the analogous parameter in gaseous diffusion, Qq 1 = 0.0043, that vapor-phase thermal diffusion caimot compete economically with gaseous diffusion for uranium enrichment. 

It is widely accepted that mass transfer effects are determinant for the FTS consequently, the selectivity toward certain olefins depends on the path of the species dixring the process, making difficult the establishment of unequivocal relationships between the diffusion resistances and the selectivity. One of the principal features of this complex process is that the diffusion of gaseous species H2 and CO through the pores of the catalysts filled with liquid waxes... 

Diffusion within the pellet is either equimolar counterdiffusion or at low concentration of the diffusing species, and the effective diffusivities of gaseous reactant and product are equal and uniform throughout the pellet. 

In effect, the n gaseous species present in the diffusing mixture are... 

The differential material balances contain a large number of physical parameters describing the structure of the porous medium, the physical properties of the gaseous mixture diffusing through it, the kinetics of the chemical reaction and the composition and pressure of the reactant mixture outside the pellet. In such circumstances it Is always valuable to assemble the physical parameters into a smaller number of Independent dimensionless groups, and this Is best done by writing the balance equations themselves in dimensionless form. The relevant equations are (11.20), (11.21), (11.22), (11.23), (11.16) and the expression (11.27) for the effectiveness factor. 

Successful operation of the gaseous diffusion process requires a special, fine-pored diffusion barrier, mechanically rehable and chemically resistant to corrosive attack by the process gas. For an effective separating barrier, the diameter of the pores must approach the range of the mean free path of the gas molecules, and in order to keep the total barrier area required as small as possible, the number of pores per unit area must be large. Seals are needed on the compressors to prevent both the escape of process gas and the inflow of harm fill impurities. Some of the problems of cascade operation are discussed in Reference 16. 

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