Oil removal through activated carbon filter:
Using activated carbon to adsorb oil vapor and oil aerosol is a very common way of compressed air oil removal. Activated carbon, also known as activated carbon black, is a kind of microcrystalline carbon material mainly made of carbon containing materials with black appearance, developed internal pore structure, large specific surface area and strong adsorption capacity. In addition to carbon, the main components of activated carbon also include elements such as oxygen and hydrogen. There are a large number of invisible micropores in the activated carbon material. The micropores in 1g activated carbon material have an expanded area of 800 ~ 1500 m2. The adsorption of oil vapor in compressed air by activated carbon is mainly due to the comprehensive action of van der Waals force, capillarity, mass transfer and differential pressure. Van der Waals force, also known as intermolecular force, is a weak electric attraction between neutral molecules or atoms. When compressed air flows through the surface of activated carbon particles, oil molecules and activated carbon molecules interact and attract each other. When the molecular attraction between activated carbon and oil molecules is greater than that between oil molecules, oil molecules will condense on the surface of activated carbon even if the pressure of oil vapor is lower than the saturated vapor pressure corresponding to the operating temperature. However, this force is relatively weak. If the temperature rises to a certain extent, the molecular attraction between activated carbon and oil molecules will be less than that between oil molecules, and the oil molecules condensed on the surface of activated carbon will vaporize and separate from activated carbon. Therefore, van der Waals adsorption is a kind of physical adsorption.
Capillary action, similar to the capillary condensation of liquid. Activated carbon adsorbent contains a large number of micropores. According to the adsorption potential energy theory of Dubinin and his school, for some adsorption processes, the adsorption in micropores is not adsorbed on the pore wall layer by layer, but volume filling occurs in the micropores of adsorbent. Therefore, due to the effect of adsorption potential energy, micropore filling occurs in the micropores of activated carbon, and oil vapor is continuously adsorbed. And the higher the oil vapor content, the greater the adsorption capacity, and the capillary condensation phenomenon is more obvious.
Mass transfer refers to the transfer of oil molecules from compressed air to the surface of activated carbon. Electron pair sharing or electron transfer occurs between activated carbon molecules and oil molecules, resulting in the formation of new chemical bonds between them, so that the oil vapor stays firmly on the surface of activated carbon. Different from the physical adsorption of van der Waals force, mass transfer is a kind of chemical adsorption.
The principle of differential pressure means that the partial pressure of a gas in two regions is different, and the gas will diffuse from the region with high partial pressure to the region with low partial pressure. The adsorption of activated carbon makes the partial pressure of oil vapor in the activated carbon low, while the partial pressure of oil vapor outside the activated carbon is high. The differential pressure makes the gas have the power to diffuse and promote the diffusion of oil vapor into the activated carbon, so as to be better adsorbed by the activated carbon.
According to the above analysis, the greater the molecular weight of oil, the higher the concentration of oil vapor, and the higher the adsorption capacity of activated carbon. Therefore, for the oil vapor with small molecular chain, the adsorption effect of activated carbon is limited at low concentration. The use of activated carbon filter can remove oil vapor and oil aerosol in compressed air, which can barely meet the grade 1 oil content standard of national standard GB / t13277.1-2016 and ISO 8573-1. However, the oil removal effect is not stable and is greatly affected by external factors. After the adsorption of activated carbon is saturated, it will lose its oil removal capacity, and the service life of activated carbon is affected by gas flow rate, temperature and oil content. Therefore, the service life of activated carbon filter cannot be determined. In order not to affect the back-end gas supply, the oil removal effect can only be ensured by changing the activated carbon frequently, which will cause a certain waste.
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