What is the separation mechanism of gas separation membrane?
Issuing time:2023-11-29 15:18
The working principle of gas separation membranes is determined by factors such as the size, shape, polarity and chemical interaction of gas molecules. These characteristics determine the speed at which each gas molecule passes through the membrane. Due to different types and shapes of gases, the membrane used They are also different, because each type of membrane works based on different separation mechanisms. Here are some commonly used gas separation mechanisms and corresponding membrane types:
1. Solution diffusion mechanism. This mechanism refers to how gas molecules dissolve in the membrane material and then diffuse through the membrane. This mechanism is determined by the solubility and diffusion rate of the gas molecules. The gas mixture contacts one side of the membrane, and the gas molecules Selectively dissolves in the polymer matrix based on its solubility. Once dissolved, the gas molecules diffuse through the polymer chains to the other side of the membrane. The diffusion rate is determined by the concentration gradient, solubility and diffusion of each gas component in the polymer. Rate-determined. Commonly used polymer gas separation membranes, such as membranes made of materials such as polyimide, polysulfone or cellulose acetate, work on solution diffusion. This mechanism is particularly effective for separating gases with different solubilities, such as carbon dioxide and methane.
2. Knudsen diffusion mechanism, which is based on the operation of different diffusion rates of gas molecules through narrow pores or channels in the membrane. It is named after the Danish physicist Martin Knudsen. In Sen diffusion, gas molecules pass through a membrane by colliding with the walls of a narrow pore or channel. The rate of diffusion depends on the size of the gas molecules and the size of the pore or channel. Smaller gas molecules are more likely to diffuse quickly through the pore or channel. While larger gas molecules experience more frequent collisions and slower diffusion, the Knudsen diffusion mechanism is particularly relevant for membranes with nanoporous structures. These membranes are called "Kudsen membranes" or "nanoporous membranes", usually Made from materials such as zeolites, metal-organic frameworks (MOFs) or silica, the pore sizes are in the range of a few nanometers or less. The Knudsen diffusion mechanism is very effective for separating gases with significantly different molecular sizes, such as separating light gases such as hydrogen or helium from larger molecules such as hydrocarbons.
3. Molecular sieving mechanism. The membrane pores have a certain size, allowing small gas molecules to pass through the membrane, but large gas molecules cannot. These mechanisms depend on the pore size of the membrane and the size of the gas molecules. Such as microporous filtration membranes and zeolite membranes, which separate gases according to size and shape through a molecular sieving mechanism. They have a specific size distribution, allowing smaller gas molecules to pass while blocking larger molecules. This selectivity is based on molecular size. Capable of separating gas mixtures.
4. Selective mechanism of molecules. For example, some composite membranes combine a polymer matrix with a selective layer containing specific transport promoters or carriers. These carriers chemically react with target gas molecules to enhance their permeability through the membrane. The promoted transport mechanism is particularly suitable for separating gases that react chemically with a carrier, such as carbon dioxide from natural gas or flue gas, where the carrier selectively binds carbon dioxide molecules and promotes their smooth passage through the membrane.
5. Ion transport mechanism, that is, ion exchange membranes. Ion exchange membranes are usually made of materials such as perfluorinated polymers and separate gases according to their ionic properties. These membranes allow selective transmission of ions while blocking neutral gas molecules. As used in applications such as hydrogen fuel cells, the membrane selectively transports protons (H+ ions) while separating hydrogen from other gases.
Most gas separation membranes can use a combination of these mechanisms to achieve the desired separation performance. The choice of specific membrane and separation mechanism depends on the target gas mixture, desired selectivity, operating conditions and other factors. Although these cannot be seen or touched, We sometimes don’t care about intangible substances, but almost everything is inseparable from them, and they are of great scientific importance in all fields that require them.