Combined diffusion coefficient for gas mixture

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Thread starter Snow_buggy
Start date Oct 26, 2023
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Plasma physics

Oct 26, 2023

Snow_buggy

TL;DR Summary: Is the diffusion direction from high concentration to low concentration and from high temperature to low temperature?

Hi. I am starting to do a gas mixture simulation. I learn the fundamentals from a paper. (doi: 10.1088/0022-3727/43/2/022001)
For a gas mixture (Fe vapor in Argon), the mole fraction of Fe vapor is calculated by,

and the diffusion flux JFe is given by,

x_ar is the mole fraction of Argon; T is the temperature; P is the pressure; E is the electric field.
It seems like Fe vapor diffuses from high concentration of Fe vapor to low concentration (grad x_Ar) and from high temperature to low temperature (- grad T).

If we calculates the mole fraction of Argon not Fe vapor, this paper (doi: 10.1007/BF01459700) says these coefficients follow this rule,

I don't understand why the temperature diffusion coefficient change the sign? Does it mean the argon diffuses from low temperature to high temperature?

Related to Combined diffusion coefficient for gas mixture

What is the combined diffusion coefficient for a gas mixture?

The combined diffusion coefficient for a gas mixture is a measure of how different gases within a mixture diffuse relative to one another. This coefficient takes into account the individual diffusion coefficients of each gas pair in the mixture and provides a comprehensive understanding of the overall diffusive behavior of the mixture.

How is the combined diffusion coefficient for a gas mixture calculated?

The combined diffusion coefficient for a gas mixture is typically calculated using the Chapman-Enskog theory or other empirical relationships. One common approach is to use the weighted harmonic mean of the binary diffusion coefficients of each gas pair in the mixture. This involves summing the inverse of the binary diffusion coefficients, weighted by the mole fractions of the gases involved.

Why is the combined diffusion coefficient important in gas mixtures?

The combined diffusion coefficient is important because it helps predict how gases will behave when they are mixed together. This is crucial in various applications such as chemical engineering, environmental science, and industrial processes where gas mixtures are common. Understanding the diffusion properties can aid in optimizing processes like combustion, separation, and chemical reactions.

What factors influence the combined diffusion coefficient of a gas mixture?

Several factors influence the combined diffusion coefficient of a gas mixture, including temperature, pressure, the nature of the gases involved, and their respective concentrations. Higher temperatures generally increase the diffusion coefficients, while higher pressures can decrease them. The molecular weights and sizes of the gases also play significant roles in determining the diffusion behavior.

Can the combined diffusion coefficient be experimentally measured?

Yes, the combined diffusion coefficient can be experimentally measured using various techniques such as the Stefan tube method, the diaphragm cell method, or gas chromatography. These methods often involve measuring the rate of diffusion of gases through a medium or across a barrier and then calculating the diffusion coefficient based on the observed data.