April 02, 2009

Calculate Diffusion Coefficient in Gases

Compared to other physical properties the advantage with diffusion coefficients is that they are fairly uniform for a given state. For gases the value of 10-5 m2/s gives you nearly always the correct order of magnitude. If this rough estimate isn't enough and you don't find tabulated values in the literature and you also don't want to make measurements you can try one of the prediction methods.

Quite a number of different correlations and methods have been proposed over the years but the one semi-empirical equation from Chen and Othmer (J. Chem. Eng. Data 7 (1962), 37) is preferable because

• It is Simple
• Sufficiently Accurate in most of the cases
• Availability of inputs required


According to Chen and Othmer the diffusion coefficient D1,2 for the diffusion of gas 1 in gas 2 at moderate pressures can be calculated from the following equation:

The equation is very simple to use & is given below.

D(1,2) = 6.04 x 10 ^ -9 x (T^1.81 / p) x ( (M1+M2)/M1/M2)^0.5 x (Tc1 x Tc2)^0.1405
x (Vc1 ^0.4 + Vc2 ^0.4)^2

Where
M1, M2 = Mol Wt of both components
Tc1, Tc2 = Critical Temp in K
Vc1, Vc2 = Critical Volume in Cm3/mol
P = System Pressure in bar
T = Temp in K
D1,2 = Diffusion Coeff in M2/sec

I hope it is useful for many of you.

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2 comments:

Sarvesh Bagla said...

Nice Blog! Well most of your content is original and informative.

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James said...

We don't often need these models, but our customers are oftentimes looking for the diffusion modeling equations. Glad I have somewhere to send them now, thanks!

James

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