[ permeation ]
Hydrogen permeation through polymers
Permeation is a mechanism that describes the transport of gas through a material . Permeation is often considered for materials that function as a barrier to separate two environments . Think for instance of a pipe separating the transported gas or liquid from the environment , or a tank containing a fuel . With a growing interest in use of hydrogen , and also carbon dioxide ( for carbon capture and storage ), the permeation properties of these gases are important to quantify . Knowing the permeation properties helps to comprehend possible safety , environmental , and economical issues . This article explains the basic nature of permeation and different approaches to determine the permeation rate .
By Ernst van der Stok , Sjoerd Jansma and Suzanne van Greuningen , Kiwa
What is permeation ?
Permeation is a natural process via which a gas , liquid , or vapor ( a permeate ) moves through a solid material . Permeation is often erroneously referred to as leakage . However , when we talk about leakage , the barrier layer is punctured and therefore ( locally ) no barrier material is present to separate both environments . The difference between the two is easily explained by imagining a balloon filled with air : a leak will deflate the balloon within seconds , while permeation will cause the balloon to slowly shrink in weeks ’ time .
To explain the mechanism of permeation in more detail , we will use a homogeneous polymeric pipeline transporting hydrogen as an example . Polymeric pipelines made from polyethylene ( PE ) or polyvinyl chloride ( PVC ) are currently widely used all over the world to transport gaseous fluids such as natural gas to households and industries . Knowing the permeation properties of these pipes is relevant when hydrogen transport is considered . Please note that the permeation mechanism for other permeates is not different from this example with hydrogen . The pipe material is able to absorb hydrogen at the inner pipe wall . This is possible because in almost all polymeric materials some ‘ free volume ’ is present in between the polymeric chains . This free volume can be occupied by the hydrogen molecules which over time move through the free volume of the material . The movement of the molecules is driven by diffusion and can be described by Fick ’ s laws of diffusion . This is a concentration-driven process , as explained in the box on the following page . Since hydrogen is a gas , the difference in hydrogen concentration is directly correlated to the partial pressure difference via Henry ’ s law . This allows us to describe the permeation behavior of hydrogen via the difference in partial pressure between either side of the pipe wall .
The rate at which hydrogen moves through a material is dependent on a variety of material characteristic factors , such as the amount of free volume available for the hydrogen molecules within the material , and the affinity of the material with hydrogen . Therefore , each material has a different resistance to the permeation of hydrogen . Metals , for instance , have in general a much higher resistance to permeation compared to polymers , but there is also a significant difference in resistance among the various polymers . This material-dependent property is expressed as the permeability coefficient of the material for a certain permeate .
The dimensions of the pipe influence the permeation rate , too . If the pipe wall is twice
Hydrogen Tech World | Issue 8 | February 2023 17