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Silicone, also known as polydimethylsiloxane (PDMS), is among the most gas permeable dense polymeric membrane materials available. Gases permeate silicone by a solution / diffusion mechanism, whereby the rate of gas permeation is directly proportional to the product of solubility of the gas, and the rate of diffusion of the dissolved gas in silicone. The permeability coefficient is a parameter defined as the transport flux of a gas (rate of gas permeation per unit area), per unit transmembrane driving force, per unit membrane thickness. The permeability coefficient for various gases and vapors in silicone is presented in the table below.
NOTE: A permeability coefficient listing for a specific gas is not an indication that the membrane is compatible with that gas. Membrane degradation and failure may occur with some gases and liquids. It is the responsibility of the user to determine the suitability of PermSelect® membrane modules in its specific application. Please view the silicone chemical compatibility chart as an initial substance compatibility guide.
|GAS NAME||FORMULA||SILICONE PERMEABILITY COEFFICIENT (Barrer)*|
*1 Barrer = 10-10 cm3 (STP)· cm /cm2 · s · cm-Hg
Therefore the rate of gas transfer across the membrane is proportional to the gas permeability coefficient, the membrane surface area, the trans-membrane gas partial pressure difference, and inversely proportional to the membrane thickness. Thus gas transfer across a membrane increases with increased gas permeability coefficient, increased surface area, increased transmembrane gas partial pressure and decreased membrane thickness.
PermSelect ® silicone membranes are configured into hollow fibers which are small silicone micro-tubes with very thin walls which act as membranes. If there is a gas specie partial pressure difference between the inside and the outside of the hollow fiber, that gas specie can permeate across the thin silicone walls in the direction from high to low gas partial pressure. Thus configuring membranes into hollow fibers enables the packaging large amounts of membrane surface area in very compact volumes. Moreover, hollow fibers constitute a self supported, inherently stable membrane structure that can tolerate high pressure differences between the inside and outside of the hollow fiber.
Hollow fibers are typically packaged in membrane modules in which thousands of hollow fibers are bundled in a very compact volume and sealed or potted within a housing as shown in the figure below. Consequently, the sum of the surface area of each individual hollow fiber constitutes the total membrane area for the module, and it becomes apparent how it is possible to achieve high membrane surface densities with hollow fibers.
Membrane modules will typically have an inlet and an outlet port in fluid communication with the inside of all the hollow fibers (also known as tube side) which are manifolded at both ends of the fiber bundle. Similarly the membrane modules will have one or more ports in fluid communication to the outside of the hollow fibers or the shell side.
PermSelect® silicone membrane modules can be used for liquid contacting and gas separation applications. The side of the hollow fibers (shell or tube) in which a gas or liquid should flow will depend on the specific application for the membrane module, and is selected to maximize membrane module performance. For example, as shown in the figure below, in gas separation a feed mixture of gases enters the membrane module through the inlet port to the tube side and flows through the inside of the hollow fibers. The gas species in the mixture with higher permeability will transfer at a greater rate across the walls of hollow fibers leaving behind the less permeable species. The transferred gas is referred to as the permeate. In the shell side, a vacuum can be applied or a sweep gas (or liquid) can flow therein to carry away the permeate. Exiting the outlet of the tube side is the retentate which constitutes a gas mixture with a higher concentration of the less permeable gas species
Gas Separation. Although the permeability ratio for two gases in a binary gas mixture provides a gross estimate for the ratio of permeate flow for the two gases, actual measurements show that the interaction of a gas mixture on the membrane can affect permeation rates. Moreover feed and permeate pressures can also affect permeation rates as the membrane structure can change under pressure. The separation factor is a membrane property that is useful in determining permeation rates for mixtures of gases under certain operating conditions. We have measured the separation factors for a series of feed gas mixtures using PermSelect® silicone membranes. Click here to see a table of separation factors.