Estimation of Diffusion Coefficients in Gases

At low to moderate pressures, binary diffusion coefficients vary inversely with pressure as can be seen in Equation 3.37. At high pressures, the product Dx/ isno longer constant but decreases with increasing P. Also, as indicated earlier, at low pressures, the binary diffusion coefficient is independent of composition. At high pressures, where the gas phase may deviate significantly from an ideal gas, small effects of composition have been noted. 

Although Equation 3.37 shows that DAB is proportional to T1-75, the power to which T should be raised depends on the actual temperature. Thus DAB varies as Tin to T1 [8]. The value of 1.75 represents an average and may be not appropriate if wide temperature ranges are to be considered. 

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