What are the methods for removing oil from compressed air?

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With the development of the economy and industries, more and more enterprises are beginning to pay attention to the quality of compressed air, especially its oil content. In industries such as electronics, semiconductors, high-precision manufacturing, food, and pharmaceuticals, the demand for oil-free compressed air of high quality is also on the rise. Therefore, the demand for oil-free compressed air is becoming increasingly urgent. Types and sources of oil in compressed air To obtain oil-free compressed air, we must understand the types and sources of oil in compressed air. ISO 8573 defines oil as a mixture of hydrocarbons consisting of 6 or more carbon atoms (C6+). Hydrocarbons are mainly various types of hydrocarbons, such as saturated hydrocarbons, unsaturated hydrocarbons, aromatic hydrocarbons, etc. Some examples of saturated alkanes are shown in Table 1. https://pic3.zhimg.com/80/v2-9ba4fedae88e63c1fe07807e15d691e6_720w.jpg During the production of compressed air, various types of lubricants remaining in the air compressor, pipes, and valves inevitably get mixed in. High-boiling-point, high-molecular-weight hydrocarbons are the main components of lubricating oils; therefore, the oil in compressed air is a mixture of such hydrocarbons, for which there is no single chemical formula. Theoretically, oil-free air compressors do not introduce any additional oil when producing compressed air, but this does not mean that there is no oil in the compressed air. Oil is present in our atmosphere; the more industrialized an area is, and the more polluted it is, the higher the level of oil in the environment. In some steel and petrochemical plants, the oil content at the inlet of air compressors can reach as much as 5 mg/m3. In such areas, even when oil-free air compressors are used, the oil content in the compressed air still does not meet the Class 1 standard specified by ISO8573-1. Therefore, using an oil-free air compressor can minimize the oil content in compressed air, but it does not guarantee that the resulting compressed air is completely oil-free. Furthermore, from the compressor outlet to the point where the air is used, compressed air passes through numerous pipes, valves, and connectors. As long as any one of these pipes, valves, and connectors is not cleaned properly, it will contaminate the compressed air in the entire piping system. Even if the compressed air at the compressor outlet has an acceptable oil content, its oil level will increase by the time it reaches the points where it is used, resulting in the compressed air becoming unusable. Therefore, to meet the demands of end-user gas points, it is necessary to take appropriate measures to reduce the oil content in compressed air. Methods for removing oil from compressed air: In compressed air, oil exists in three forms: liquid, gas, and aerosol. (Strictly speaking, the oil in oil aerosols is in liquid form, only its volume and size are within certain limits.) Different methods are required to remove oils of different forms. 1. Oil removal using filters can be achieved for liquid oils and the vast majority of oil-gas aerosols, by employing coagulative filters and utilizing the filtration principle. The filtration principle is as follows: First stage: direct interception. As shown in Figure 1, light oil droplet particles larger than the gaps in the filter material are directly intercepted on the surface layer of the filter material. https://pic4.zhimg.com/80/v2-70803d1da6432b8dffb360038bc77c4b_720w.jpg Phase 2: Impact separation, as shown in Figure 2. Impact separation relies primarily on the inertia of the oil droplets; after passing through the gaps in the filter material, the solid particles and liquid droplets strike the fibers of the filter material and adhere to them. https://pic2.zhimg.com/80/v2-7b2bc3e99ad27da2f0de38bd7b63ebb9_720w.jpg Stage 3: Brownian motion, as shown in Figure 3: Tiny droplets move along the flow direction and attach to the fibers of the filter material, gradually accumulating to form larger droplets ； At the same time, the irregular Brownian motion of the tiny oil droplets also causes them to adhere to the fibers of the filter material. https://pic2.zhimg.com/80/v2-01c841bf3515029ebd12538309756dd1_720w.jpg Phase 4: Liquid drainage, as shown in Figure 4: In the outermost liquid collection layer, the droplets gradually coalesce and grow larger; then, under the effect of gravity, they flow to the bottom of the filter and are drained away. https://pic4.zhimg.com/80/v2-07c3e14cbab7e117bb8d8fce292914fb_720w.jpg Through the step-by-step filtration in these 3 stages, the compact filter can reduce the residual oil content to as low as 0.01 mg/m3. However, no compact filter can completely remove the impurities contained in compressed air ； At this point, the measure of the filtering effectiveness of a compressed air filter is the “filtration efficiency”. The percentage of the number of impurity particles separated by the filter, out of the total number of such particles, is referred to as the “filtration efficiency”. For example, if the compressed air at the filter inlet contains 100,000 particles of 1 μm in size, and after passing through the filter only 1 particle of 1 μm in size remains at the outlet, then the filtering efficiency of this filter is 99.999% (for 1 μm particles). Since the filtration efficiencies of the above three filtration stages are not identical, the filtration efficiency of the filter is the sum of the filtration efficiencies of these three stages, as shown in Figure 5. https://pic3.zhimg.com/80/v2-6f1bdccb294fec37fee2e13e136ce56a_720w.jpg As can be seen from the above image, the filtration efficiency at different stages is closely related to the size of the impurity particles. After filtration, the filter medium is filled with various impurities. Filtering through coalescing filters can remove the vast majority of liquid oils and oil-air aerosols, but such filters are unable to remove oil vapors; therefore, using coalescing filters, the highest achievable oil content level is that specified in standards GB/T 13277.1 -2016 and ISO 8573-1, namely Level 2. Removing oil using an activated carbon filter involves employing activated carbon to absorb oil vapors and oil aerosols; this is a very common method for removing oil from compressed air. Activated carbon, also known as activated charcoal, is a type of microcrystalline carbon material primarily made from carbon-containing materials. It appears black in color, has a well-developed porous internal structure, a large specific surface area, and strong adsorption capabilities. In addition to carbon, the main components of activated carbon include elements such as oxygen and hydrogen. Activated carbon materials contain a large number of micropores that are invisible to the naked eye; the total surface area of these micropores in 1 gram of activated carbon material can reach as much as 800–1500 m2. Activated carbon can adsorb oil vapors in compressed air, primarily as a result of the combined action of van der Waals forces, capillary action, mass transfer, and pressure differences. VanderWaals force, also known as intermolecular force, is a weak electrostatic attractive force that exists between neutral molecules or atoms. As compressed air flows over the surface of the activated carbon particles, interactions occur between oil molecules and activated carbon molecules, resulting in an attraction between them. When the molecular attraction between the activated carbon and the oil molecules is greater than the attraction between oil molecules themselves, the oil molecules will condense on the surface of the activated carbon, even if the pressure of the oil vapor is lower than the saturated vapor pressure corresponding to the operating temperature. However, this force is relatively weak; when the temperature rises to a certain level, the molecular attraction between the activated carbon and the oil molecules becomes weaker than the attraction between the oil molecules themselves. As a result, the oil molecules that are attached to the surface of the activated carbon vaporize and separate from it. Therefore, van der Waals adsorption is a type of physical adsorption. Capillary action is similar to the capillary condensation phenomenon of liquids. Activated carbon adsorbents contain a large number of micropores. According to Dubinin and his school’s theory of adsorption potential energy, in certain adsorption processes, the adsorption within these micropores does not occur in layers on the pore walls; rather, volume filling takes place inside the micropores of the adsorbent. Therefore, within the micropores of activated carbon, due to the effect of adsorption potential energy, a filling of these micropores occurs, and oil vapors are continuously adsorbed ； Furthermore, the higher the oil vapor content (i.e., the greater the partial pressure of the oil vapor), the greater the adsorption amount, and the capillary condensation phenomenon becomes more pronounced. Mass transfer refers to the transfer of oil molecules from compressed air to the surface of activated carbon. The active carbon molecules and oil molecules share electron pairs or undergo electron transfer, resulting in the formation of new chemical bonds between them; this allows the oil vapor to remain firmly attached to the surface of the active carbon. Unlike the physical adsorption of van der Waals forces, mass transfer is a form of chemical adsorption (i.e., a chemical reaction occurs between the two). The principle of pressure difference states that when the partial pressures of a certain gas are different in two regions, the gas will diffuse from the region with higher partial pressure to the region with lower partial pressure. The adsorption effect of activated carbon results in a low vapor pressure of oil vapors inside the activated carbon, while the vapor pressure outside it is high. This pressure difference provides a driving force for diffusion, facilitating the diffusion of oil vapors into the activated carbon and thus enabling them to be absorbed more effectively by it. From the above analysis, it can be seen that the higher the molecular weight of the oil, the higher the concentration of oil vapor, and the greater the adsorption capacity of activated carbon. Therefore, for oil vapors with small molecular chains, at low concentrations, the adsorption effect of activated carbon is relatively limited. The use of activated carbon filters can remove oil vapors and oil aerosols from compressed air, and it is possible to meet the oil content standards specified in standards GB/T13277.1-2016 and ISO 8573-1 Class 1; however, the oil removal efficiency is not stable and is greatly affected by external factors. Once activated carbon becomes saturated with oil, it loses its ability to remove oil. Moreover, the service life of activated carbon is influenced by factors such as gas flow rate, temperature, and oil content; therefore, it is impossible to determine the service life of an activated carbon filter. To avoid affecting the gas supply at the backend, the oil removal efficiency can only be maintained by frequently replacing the activated carbon, which results in some waste. The catalytic cracking method using a conversion-type oil removal machine has various drawbacks when it comes to oil removal, whether filter or activated carbon adsorption is employed; the oil removal efficiency is not stable, and for industries with strict requirements, these methods are insufficient. The currently more advanced method is to use the principle of catalytic cracking to remove oil from compressed air. Catalytic cracking, as the name implies, involves using chemical methods to break down the molecular chains of oil through fragmentation, thereby removing the characteristics of oil molecules and achieving the purpose of oil removal. Products such as oil-free catalytic compressors make use of this principle, as shown in Figure 6(a), to render the compressed air they process oil-free. The working principle of this device is as follows: The compressed air, which needs to have its oil removed, is preheated in a heat exchanger before entering a tank equipped with a catalyst. The tank has its own temperature control system that maintains the reaction temperature at around 150°C, as shown in Figure 6(b). Under the action of a catalyst and oxygen, oil molecules (hydrocarbons) in compressed air have their long molecular chains broken down into shorter ones; these shorter chains are further broken down into even shorter chains (Figure 7). The inner surfaces of the micropores in the catalyst particles can bind oxygen atoms. When oil-containing compressed air enters these micropores, oxygen molecules, upon approaching the inner surfaces of the micropores, are split into two oxygen atoms, which then adhere to those surfaces. When the oil molecule chains come into contact with the inner surface of the pores, one of their carbon atoms combines with two oxygen atoms to form a carbon dioxide (CO2) molecule, while two hydrogen atoms combine with one oxygen atom to form a water molecule (H2O). Consequently, the oil molecule chains break apart and, together with the carbon dioxide and water molecules, detach from the inner surface of the pores. After breaking, the oil molecule chains continue to undergo the same chemical reactions. After breaking, the oil molecule chains continue to undergo the same chemical reactions until all of them are completely broken and react with oxygen atoms. In the end, only carbon dioxide and water remain as reaction products. After being cooled by the heat exchanger along with the compressed air, the reactants enter the downstream piping (Figure 8). https://pic3.zhimg.com/80/v2-4155e9edfe4fc2b7a01701f761501132_720w.jpg For example, as shown in Figure 9: In reality, the breakdown of oil molecules occurs almost “instantly”; the reaction rate is extremely fast. Therefore, after going through this series of steps, the oil present in compressed air can be completely removed. Since the catalyst is only used to catalyze the cracking reaction and does not participate in the reaction itself. https://pic1.zhimg.com/80/v2-b2e721bbd0ceac200edf26ef09072a9c_720w.jpg Therefore, the catalyst itself has a very long service life. At the same time, it can ensure efficient catalytic cracking reactions. Moreover, the oil removal efficiency does not decline over time, ensuring stable oil removal performance throughout the entire lifespan of the equipment. Moreover, the oil content in the compressed air after oil-free catalytic cracking can meet or exceed the Class 1 oil content standard specified by ISO 8573-1. At the same time, due to the high temperature of 150°C in the oil-free catalytic cracking process, the compressed air processed by the oil-free catalytic unit is free of live bacteria, cocci, and viruses. Catalytic cracking can be used to remove oil vapors, oil aerosols, and even liquid oil droplets from compressed air. Since this is a chemical method for oil removal, the speed and efficiency of oil removal are very high; it is almost possible to eliminate all oil from the compressed air, thereby fully meeting the oil content standards specified in **GB/T 13277.1-2016 and ISO 8573-1 Standard Class 1.