Temporary Divergence: Diffusivity: That Smell-Methyl Mercaptan (Methanethiol), Odorless Natural Gas, Odorless Propane, and Even Flatulence
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Diffusivity of Methyl Mercaptan Versus Methane and Propane
Methyl mercaptan[3-6], “methanethiol”, is the byproduct of many natural processes. Flatulence is one example. Because of its odor threshold, 1 ppb has been reported, methanethiol is also added to odorless natural gas, methane, and odorless propane for detection purposes. Apparently, it is used as a communication warning system in mining operations as well.
In this blog post, I will be comparing the diffusivity of the polar chemical methanethiol to the non-polar chemicals methane and propane in air. I have heard reports that the diffusivity of methanethiol is significantly greater than methane and propane. See bottom of post for diffusivities.Since reference has tabular values for methane and propane in appendix B, I will use the tabular values and the Chapman-Enskog equation to calculate diffusivity values for methane and propane. For methanethiol, I will use Fuller, et al equation and tabular values for the atoms making up methanethiol,
Chapman-Enskog Equation. From reference, the average absolute error of this “theoretical equation” is 7.9%
If is chosen as unity and “n” expressed by ideal-gas law
For Non-polar gases: Methane and Propane
Methane in air at 25C and 1 atmosphere (atm)
Neufield, et al.:
From appendix B
Diffusivity of Methane in Air:
Propane in air at 25C and 1 atmosphere (atm)
Neufield, et al.:
From appendix B
Diffusivity of Propane in Air:
For polar molecule , will use Fuller, et al. equation. From reference, the absolute relative error of this equation is 5.4%. Authors report an average absolute error of about 4% when using
Fuller, et al.
T = 25C = 298 K; P = 1 atm
Summation of “Atomic and Structural Diffusion Volume Increments from table 11-1
Diffusivity of methanethiol in air
Diffusivities in air in decreasing order
At a detection threshold of 1 part per billion (ppb) and the above diffusivities, one might detect methanethiol prior to experiencing propane. In truth, there is an equation that takes “mixture” into account but I do not know the percent mixture of each component.
Equation for mixture
is the mole fraction of component “n” in the gas mixture evaluated on a component-1-free basis
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• Wait until 2nd attempt
 Poling, Bruce E.; Prausnitz, John M.; O’Connell, John P. (2001) The Properties of Gases and Liquids, Fifth Edition. New York: Mcgraw-Hill.
 Welty, James R.; Wicks, Charles E.; Wilson, Robert E. (1984) Fundamentals of Momentum, Heat, and Mass Transfer, third edition. New York: John Wiley & Sons.
 ScienceBlogs. Molecule of the day. Methanethiol (They put that in, you know), March 2009. scienceblogs.com[online]. 2013. Available from: http://scienceblogs.com/moleculeoftheday/2009/03/18/methanethiol-they-put-that-in/
 Wikipedia. Methanethiol. Also known as methyl mercaptan. en.wikipedia.org[online]. 2013. Available from: http://en.wikipedia.org/wiki/Methanethiol
 NCBI.PubChem Substance. Methanethiol-Substance Summary (SID 3699). Also known as Methylmercaptan (CAS: 74-93-1). pubchem.ncbi.nlm.nih.gov[online]. 2013. Available from: http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?sid=3699
 US National Institute of Standards and Technology. NIST. Methanethiol. webbook.nist.gov[online]. 2013. Available from: http://webbook.nist.gov/cgi/cbook.cgi?ID=74-93-1&Units=SI
 Wikipedia. Flatulence. en.wikipedia.org[online]. 2013. Available from: http://en.wikipedia.org/wiki/Flatulence