Posted by Diffusion in Polymers Platform on November 18, 2004 at 12:13:27:

In Reply to: CH4 or H2 diffusion through schedule 40 PVC pipe posted by Charles Barker on May 14, 2004 at 18:40:59:

Dear Charles,

Based on your input, the total diffusion surface is calculated and is 0.07 m2.
The diffusion figures are as follows:
-------------------------------------------------------------
The time it takes for Hydrogen to diffuse through a 5 mm wall is 1 DAY (time lag).
The time it takes for Methane to diffuse through a 5 mm wall is 1 YEAR (time lag).

1. Hydrogen permeability in Poly Vinyl Chloride @ 25 degrees Celsius

The permeation in stationary state of pure Hydrogen through plastic PVC will be
1.3 E-12 [m3 stp / m3 x bar ] x [ m2/s];

Permeation of pure Hydrogen at 1 bar through a wall of 5 mm gives:
2.6 E-10 [m3 stp / m3] x [ m/s];

Assuming that the average gas pressure is: (1.35 bar - 1 bar)/2 = 0.18 bar* and that the mixture contains 50% Hydrogen (=0.09 bar), the mass of 1 cubic meter Hydrogen is: 7.3 gram/m3;

So the Permeation of the Hydrogen considered, in grams through the wall of 5 mm is:
1,9 E-9 [gram Hydrogen/ m3 PVC] x [ m/s];

Based on a surface of 0.07 m2 the Hydrogen diffusion through PVC becomes:
1,3 E-10 [gram Hydrogen / s] = 11 micro gram / day stationary permeation, settled after 2 days.

2. Methane permeability in Poly Vinyl Chloride @ 25 degrees Celsius

The permeation in stationary state of pure Methane through plastic PVC will be
2.2 E-14 [m3 stp / m3 x bar ] x [ m2/s];

Permeation of pure Methane at 1 bar through a wall of 5 mm gives:
4.4 E-12 [m3 stp / m3] x [ m/s];

Assuming that the average gas pressure is: (1.35 bar - 1 bar)/2 = 0.18 bar* and that the mixture contains 50% Methane (=0.09 bar), the mass of 1 cubic meter Methane is: 58,1 gram/m3;

So the Permeation of the Methane considered, in grams through the wall of 5 mm is:
2,5 E-10 [gram Methane/ m3 PVC] x [ m/s];

Based on a surface of 0.07 m2 the Methane diffusion through PVC becomes:
1,8 E-11 [gram Methane / s] = 1,5 micro gram / day stationary permeation, settled after 2 years of hydrogen exposure of the material.

------------------------------------------------------------------

If you need further assisstance please do not hesitate to contact us.

Regards,
Diffusion Polymers
:
: In studies of methane content in coalbeds, a fresh gas bearing coal
sample is put in a water-filled pressure tight container of made of 5
mm thick schedule 40 PVC pipe. Another method is to put the same type
of coal sample in a water-filled container made of 5 mm wall thickness
aluminium pipe. the desorbed CH4 or H2 accumulates in a small headspace
volume above the water fill. volume measuremenets are at first made
every few minutes, but after gas content and pressure declines, the
measurements are made daily. Hence gas pressure inside the sample
container is not allowed to rise much. Typically the initial pressure
inside the cylinder <5psi (35KPa) above ambient atmospheric pressure.>
The desorbed gas volume measurements can go on for days to months
depending on the physical diffusion/desorption character of the coal
sample. Typically the pressure of the desorbed gas constantly declines
and the experiment ceases when the internal pressure equals
atmospheric.
: Those workers that use PVC have been attacked because of conjectured
diffusion of CH4 or H2 through the schedule 40 pipe versus the
supposed rates of diffusion through aluminum pipe. Does anyone have an
grasp on how diffusion of CH4 or H2 in PVC pipe would effect these
measurements?

------ www.diffusion-polymers.com ------
* For the sake of this estimation a slight deviation from
Raoult's law is chosen. Raoult's law would state:
[(1.35-1.00) / 2] / 1.175 = 0.15 bar CH2/CH4 / System Pressure