[ permeation ]
Fig . 1 . Test setup used for permeation measurements : a black polymeric pipe is fitted with red end-fittings and a metal jacket pipe is placed around the polymeric pipe .
Higher temperature equals higher permeation rate
One key factor that needs to be emphasized is temperature . The movement of molecules is highly dependent on the energy available to provide for this movement . This energy is most often in the form of heat . A higher temperature will result in a higher permeation rate . Temperature cannot be captured in the equation mentioned on the previous page . This requires a different approach , for instance using the equation of Arrhenius . This will not be discussed further in this article but remember that a higher temperature increases the permeation rate .
How to determine permeation ?
There are several approaches to measure the permeation rate of a product . The approaches intrinsically do not vary much . A pipe material separates two volumes . This could , for instance , be a sheet of pipe material separating two chambers or polymeric pipe separating the inner volume from the outer volume ( which is
enclosed by a special jacket pipe ). The permeate is applied in one volume and the accumulation of the permeate in the second volume is measured over time via gas measurements or pressure measurements . The method is adjusted to best suit the question that needs to be answered . In case of the measurement of permeation through pipes , the accumulation of the permeate in a metal jacket pipe around the actual pipe is an applicable method ( see Figure 1 ). Furthermore , complete objects can be put in confined spaces where the amount of accumulated permeate can be determined by means of gas chromatography . This could be used for permeation measurements of complete hydrogen fuel tanks .
Because of the predictable behavior of permeation , one can estimate the loss of hydrogen of complex systems , such as an entire gas grid , by determining the permeation rate of small representative segments , such as individual pipes taken from the grid . Also , once the
Hydrogen Tech World | Issue 8 | February 2023 19