We occasionally make hydrogen stripper systems where the goal is to strip the hydrogen, and sometimes other gases from a mixed gas leaving everything else in place. We use metal membranes, coiled as shown below, if the gas to be removed is hydrogen. For other gases H2S, siloxanes, we use polymeric membranes. In either case, it’s not a product we advertise, but it’s worthy of a blog post.
A typical customer was a Cambridge researcher using gas chromatography with hydrogen as the carrier gas to separate a series of compounds. He now wanted to recover the separated compounds without the hydrogen.
To minimize the amount of membrane needed, I like to create a plug flow. If the flow is low, or there isn’t much hydrogen, we use a single coiled membrane. If the flow is high or there is a lot of hydrogen, we use several straight membranes in parallel, typically with baffles; we even have a patient for this. Some customers want to recover the hydrogen. If so, we extract it with a vacuum pump. Other customers just want the hydrogen gone. For these oxidation works well. The reaction H2 + 1/2 O2 –> H2O is essentially irreversible, and quite fast on a palladium surface.
A basic analysis is to assume that flux is proportional to hydrogen pressure in the gas, and calculate the hydrogen pressure as total pressure, P, times the fraction of hydrogen in the mixed gas, H/(H + inerts) .
Then, for any small amount of membrane area, dA, the small amount of hydrogen removed per second, dH equals the area, dA, times the permeance of the membrane, P, times P, times H/(H + inerts): dH = -PP (H/(H+Inerts)) dA.
We now rearrange the above: dH(H+Inerts)/H = -PP dA, and integrate:
∫ dH ( H+Inerts)/H = ∫(1 + inerts/H) dH = ∫ -PP dA. It’s found that
Hi – Hf , + inerts • ln (Hi)/(Hf) = PPA. Thus,
A = (Hi – Hf )/PP + (Inerts/PP) ln (Hi)/(Hf).
where A is the total area of membrane needed, inerts = the inert gas flow per second, and and Hi and Hf ate the hydrogen flow rates in the mix at the beginning and end of the extraction. Hi – Hf equals the total amount of hydrogen removed per second.
Note that the first term dominates in the beginning of the extraction process, or where there is a lot of hydrogen content, ln 1 =0), and the pressure of hydrogen is essentially P. The logarithmic term dominates in latter (final) stages, where the inert content is the majority and hydrogen content changes fast. In these stages, it takes very little removal to lower the partial pressure a lot, proportionally, but more membrane is needed since the hydrogen pressure is low.
You derive similar equations for normal distillation, as used to make liquor or refine gasoline. You can also modify the above expression because permeation isn’t always proportional to pressure but to the difference of Pressure to the 1/2 power, or so.
She was only the moonshiner’s daughter, but he lover her still.
Robert Buxbaum, September 2, 2026.