Estimation of Viscosity of a gas mixture
Estimation of Viscosity of a gas mixture
(OP)
I am an entry level engineer. As an exercise, I was asked to confirm an orifice plate sizing calculation using International Organization for Standardization (ISO) 5167-2. The gas is natural gas with the following composition:
Methane, CH4 98.26%
Nitrogen, N2 .79%
Ethane, C2H6 .63%
Propane, C3H8 .22%
iso-Butane, C4H10 .04%
n-Butane, C4H10 .04%
I-Pentane, C5H12 .01%
n-Pentane, C5H12 .01%
n-Octane, C8H18 .01%
Design temperature and upstream pressure are 365°F and 411.2psi gauge. The Fluid data for the natural gas is very limited; we do not have viscosity of the whole fluid. However, we do have properties for each of the individual gases.
1. How do you calculate the viscosity of the mixture given the composition?
1a. Searching the web, I found the Wilke Method mentioned. Is it good estimation? Where can I find information about it?
2. Is a good estimate to use the viscosity of methane since it is 98.26% of the mixture at 365°F and 411.2psi gauge?
Methane, CH4 98.26%
Nitrogen, N2 .79%
Ethane, C2H6 .63%
Propane, C3H8 .22%
iso-Butane, C4H10 .04%
n-Butane, C4H10 .04%
I-Pentane, C5H12 .01%
n-Pentane, C5H12 .01%
n-Octane, C8H18 .01%
Design temperature and upstream pressure are 365°F and 411.2psi gauge. The Fluid data for the natural gas is very limited; we do not have viscosity of the whole fluid. However, we do have properties for each of the individual gases.
1. How do you calculate the viscosity of the mixture given the composition?
1a. Searching the web, I found the Wilke Method mentioned. Is it good estimation? Where can I find information about it?
2. Is a good estimate to use the viscosity of methane since it is 98.26% of the mixture at 365°F and 411.2psi gauge?
-Jason Nicholson





RE: Estimation of Viscosity of a gas mixture
From the GPSA book, I get a viscosity of .0156 Cp at 14.696 psia, and .0165 Cp at 411 psig
From my similator useing API, I get .0163 Cp
RE: Estimation of Viscosity of a gas mixture
If you want to do a slightly better job - just do a weighted average i.e 0.9825 x visc. CH4 + 0.0079 x visc. N2.....etc.
To go further into it, all the chemicals are going to be relatively non-polar, no H-bonding etc. so I wouldn't expect any interactions that would effect the overall viscosity.
RE: Estimation of Viscosity of a gas mixture
For pure methane NIST gives 0.0161 cP.
RE: Estimation of Viscosity of a gas mixture
µm = {Σi xiµi√Mi}/{Σi xi√Mi}
µm = viscosity of gas mixture
µi = viscosity of the i1 component
xi = mole fraction of the ith component in the mixture
Mi = molecular weight of the ith component
Carr, N.L., "Viscosities of Natural Gas Components and Mixtures." Institute of Gas Technology Research Bulletin No. 23 (1953) June.
RE: Estimation of Viscosity of a gas mixture
RE: Estimation of Viscosity of a gas mixture
-Jason Nicholson