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Development of special oil for CO2 compressor separation in engineer's field biogas purification Li Sheng, Lu Zhaoxia, Xiao Youcheng, Huang Fuchuan College of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China) The special properties of gas, screening and matching additives. The developed oil has excellent lubricity, emulsification resistance, thermal stability, thermal oxidation stability and viscosity-temperature characteristics, etc., the amount of carbon deposit is small, the volatility is low, and the oil change period is long. After using the compressor oil, the device has good airtightness and high CO2 separation efficiency, and can meet the process requirements for biogas compression and separation of CO2.
Biogas is a kind of clean fuel with excellent performance. It is not only high in combustion heat efficiency, low in cost, but also rich in raw materials, low in price, green, low-emission, recyclable and anti-explosive. It is a kind of high-quality renewable energy. .
Natural biogas has a low calorific value and is extremely small in use, and is usually used mainly for cooking, lighting and heating. In order to increase the calorific value of biogas, expand its use, and improve its economic benefits; at the same time meet the requirements of low carbon and emission reduction, biogas needs to be processed by high-purification separation co2 process. Through research, it is found that after the biogas is compressed, the pressure reaches 20~25MPa, which is much larger than the C2 critical pressure (7.5MP;). The installation of a special gas-liquid separator in the biogas compressor allows the C2 that meets or exceeds the critical pressure to be liquefied and separated from the gas. After pressure research direction: green energy and petrochemical product development.
Tutor Profile: Lu Zhaoxia (1969-), female, associate professor, graduated from Guangxi University in 2006 with a master's degree in chemical technology. She is mainly engaged in the research of petrochemical energy applications and has published many papers.
Corresponding author: Huang Fuchuan (1963-), male, professor, in 2005, graduated from Southwest Petroleum University, Ph.D., is mainly engaged in the research material aspects of lubrication.
The biogas after the C2 separation process is increased, the methane content is increased, the calorific value is correspondingly increased, and the nature is close to that of natural gas. It can be used as a new type of energy source for pipe network energy supply or as a fuel for motor vehicles, greatly improving the commercial use of biogas. And economic benefits.
Compressors are machines used to compress gas to increase gas pressure. According to the process of compressing gas, compressors can be divided into two categories: volumetric and dynamic. A volumetric compressor is used in the biogas compression separation C2 process. Compressor oil is a kind of special equipment lubricating oil, which is also a part of the compressor. It mainly acts on cylinders, pistons, valves, bearings and slides to reduce friction and wear and power consumption, and to compress the gas between the piston and the cylinder wall. , cleaning and cooling. Different from other industrial lubrication, due to the complex composition of biogas, the lubricating oil is directly exposed to the biogas, and is easily affected by other impurities in the biogas, which aggravates the oxidation and aging of the lubricating oil, and the lubrication conditions are more severe. After analysis, the problem of biogas compression and separation of C2 special compressor lubricating oil can be summarized as follows: (1) The temperature of compressed gas in the cylinder piston part is high, the lubricating oil is easy to be oxidized at high temperature, and further condensed into colloid and asphaltene products; 2 The partial pressure of oxygen in the compressor is high, and the unstable components in the compressor oil are mixed with impurities to oxidize into colloid, thereby generating carbon deposit and affecting the airtightness of the valve seat; 3 a large amount of C2 contained in the biogas A small amount of H2S is easily dissolved by water to form acid, which not only affects the lubricity of the compressor oil, but also causes corrosion to the compressor metal; 4 is easily contacted with biogas and is affected by the properties of methane and other impurities; C2 will The seal defect enters the separator or leaks back to the cylinder, affecting the biogas compression separation C2 efficiency.
At present, China's compressor oil standard is still not perfect. At present, domestic air compressor oil is used as natural gas compressor oil. Due to large differences in composition, corrosiveness, and high air tightness requirements, it is not only difficult to solve the above problems, but also easy to appear if the existing common compressor oil or natural gas compressor oil is used on a biogas high-purity separation CO2 compressor. Corrosion, rust, low separation efficiency, etc., resulting in high-purity separation of CO2 compressors can not work properly, affecting the operation of the entire compression and separation process. In order to solve this problem, the author analyzed and developed the biogas high-purity separation CO2 compressor special oil by analyzing the biogas properties and the work of the biogas compressor.
1 Technical requirements for high-purity separation of 2 biogas special oils First, in the process of separating CO2 compressors with high purity of biogas, in order to allow the compressed gas to enter the special separator for gas-liquid separation, the compressor oil must be supplied to the equipment. Provides good sealing performance.
Secondly, during the operation of the biogas compressor, H2S and CO2 are easily dissolved in the condensate to form an acidic liquid, which will cause corrosion to the compressor body. In addition, impurities in the biogas produce deposits, pollute moving parts, increase fuel consumption, and shorten service life. In summary, the developed high-purity biogas separation CO2 compressor oil should not only have suitable flash point, pour point, low carbon deposition trend at high temperature, low flammability, but also meet the following requirements: (1) Suitable viscosity The appropriate viscosity is necessary to ensure lubrication and compression of the cylinder seal. In general, the viscosity needs to be determined according to conditions such as working pressure, operating temperature, piston speed, and flow rate of the conveying gas. Lubricating oil with proper viscosity can ensure the normal operation of the compressor and play a good role in lubrication and cooling. If the viscosity is too large, the oil distribution on the sliding surface will be uneven, resulting in insufficient local oil, resulting in increased wear. 4 In addition, too high viscosity will increase the oil film shear force, resulting in increased liquid flow resistance and reduced coolant efficiency. Easy to produce sludge and carbon deposits. Relatively speaking, if the viscosity is too small, the oil pressure will be too low and the oil film temperature will be insufficient, resulting in increased wear. At the same time, if the viscosity is too small, the sealing effect will be poor or invalid, and the compressor oil will not be diluted. Or it is contaminated by biogas, and it causes gas leakage, which reduces the separation efficiency.
(2) When the excellent oxidation stability biogas compressor is working normally, the exhaust gas temperature is usually 120~200°C, and the exhaust pressure is also 20~25MPa. When the compressor is working, the exhaust valve, valve cavity and pipe are thin. The oil film is heated by the hot metal surface in contact with it and is constantly being purged by the compressed gas discharged from the cylinder. Under such severe oxidation conditions, the lubricating oil is easily oxidized and deteriorated to form carbon deposits. The biogas compressor oil required to be developed should have good oxidation stability.
(3) Good corrosion resistance and rust resistance The biogas composition is complex, containing not only methane and CO2, but also H2S, CO, water vapor, N2, air and other dust impurities. If the biogas is not filtered and dried, it will directly enter the compression. The machine is compressed, in which H2S and O2 are easily dissolved in the condensed water to form an acidic solution, causing corrosion to cylinders, valves, pistons, etc., destroying the lubricating oil film, aggravating the wear of the metal surface, and seriously affecting the safety and normal operation of the compressor. The biogas compressor oil must have good corrosion resistance and rust resistance.
~50% CO2, the main function of the biogas high-purity separation CO2 compressor is to separate the CO2 liquefaction while compressing the biogas. However, during the operation of the compressor, due to the poor airtightness of the device itself, the gas leaks from the valve seat and other components, reducing the CO2 separation rate. Therefore, the development of oil requirements can provide a certain degree of sealing to the compressor equipment.
2 Development of biogas compressor oil 2.1 Biogas compressor oil base oil is selected from the combination of base oil and additives, and base oil is the decisive factor affecting the overall performance of the lubricant. Due to the harsh working conditions of biogas compressors, biogas as a compressed medium has special properties, and traditional mineral lubricants cannot be satisfied. In general, biogas compressor oils use synthetic lubricants that have a low tendency to form carbon deposits. Synthetic lubricating oil has good comprehensive performance, high high temperature performance, good oxidation stability, excellent viscosity and low temperature performance, low volatility, and is more suitable for harsh working conditions than mineral oil.
There are many types of synthetic oils, and the base oils suitable for use as synthetic compressor oils are usually polya-olefins, polyethers, silicone oils, fluorine oils and ester oils. After investigation and experimental analysis, a single type of animal oil, mineral oil or synthetic oil is difficult to meet the increasingly demanding operating conditions of current compressors. In view of the development needs and working conditions of biogas compression and separation CO2 compressors, the developed oil is made of polya-olefin and adipic acid ester.
The adipic acid ester is a kind of ester oil with good thermal oxidation stability, high chemical stability, high viscosity index and biodegradability. In addition, the adipate has good acid stability and is suitable for use in an acidic environment caused by the dissolution of H2S and C2 in biogas to produce acidic substances in water.
Since the biogas has a certain amount of water vapor, during the compression process, the water vapor will be liquefied into condensed water and present in the compressor, and the lubricating oil is inevitably in contact with the water. However, the hydrolysis stability of the ester oil is poor, and hydrolysis and reaction can form acid and alcohol under the action of acid, alkali and enzyme. Moreover, the ester oil tends to be less compatible with the sealing material, resulting in a decrease in the airtightness of the compressor, resulting in gas leakage back to the cylinder, reducing the efficiency of C2 entering the special separator for gas-liquid separation. Therefore, the developed oil is compounded with polya-olefin and adipate to compensate for the lack of performance of the single synthetic oil.
Polya-olefin (PA0) is a kind of long-chain alkane which is a kind of a-olefin which is polymerized by a catalyst. Compared with the same viscosity mineral oil, polya-olefin has better viscosity-temperature characteristics, wide operating temperature range, low pour point, high viscosity index, good sensitivity to additives, good thermal oxidation stability, good low temperature performance, etc. The characteristic gas adopts polya-olefin as the base oil, which can not only reduce the deposits of valves and other components in the biogas compressor, but also make up for the defects that the diester is not suitable for some sealing components, and can extend the oil change period, thereby reducing the compressor. The number of downtimes and repairs extends compressor life. The physical and chemical properties of commonly used PA0 are shown in Table 1. Table 1 Physical and chemical properties of low-viscosity PAO Name Viscosity index Pour point opening flash point volatility After repeated trials and studies, synthetic oils with poly-a-olefin and adipic acid complexes are used as base oils. It can better improve the performance of synthetic oil: (1) the adaptability of the diester to certain plastics and sealing materials is poor, and the base oil added by the polya-olefin compound has less influence on the rubber material. Strengthen the sealing of the sealing element to prevent C2 from leaking back to the cylinder and improve the efficiency of the division. (2 Poly a-olefins have poor abrasion resistance and scratch resistance, and it is necessary to add a peroxydiester having good anti-wear and anti-pressure properties. (3PA0 is added with a certain amount of adipic acid ester to reduce PA0. Friction coefficient, improve lubrication performance. 2.2 Additives Select biogas high-purity separation C2 compressor is special in that C2 liquefaction is achieved by using a special gas-liquid separator installed in the compressor while compressing the biogas. Separated. Because the compressor not only needs good oxidation stability, corrosion resistance, rust resistance, etc. like traditional biogas compressors, it also requires good air tightness to ensure gas-liquid separation efficiency. Ben, due to the characteristics of the base oil, it has been difficult to meet the working conditions of the biogas compressor.
By using a rotary bomb oxygen test, a four-ball test machine, etc., the susceptibility and synergy of the base oil to various additives are analyzed, and suitable additives such as an antioxidant, a metal deactivator, an antifoaming agent, and a clean additive are added to the base oil. Dispersant, friction modifier, viscosity index improver and rust preventive agent, etc., improve and improve the thermal oxidation stability, corrosion resistance, rust resistance and other properties of biogas compressor oil to meet the high purification degree of biogas separation pressure C2 Machine work needs.
2.3 Antioxidants and metal deactivators Since the biogas compressor is operating, the temperature of the metal surface in the cylinder rises, and the metal in the high temperature state is a catalyst for the oxidation of the lubricating oil. Therefore, the oil entering the cylinder is extremely apt to be oxidized after contact with the metal to form a polymer, which causes the oil to deteriorate. The acidic substance formed by the oxidation of lubricating oil will cause serious corrosion of the metal in the compressor. In addition, the sludge and sediment generated by the oxidation of the lubricating oil will cause the oil to thicken, which will easily cause the piston ring to stick and the oil circuit to block, resulting in oil. The performance of the product is seriously reduced.
The rotary oxygen bomb experiment method was used to evaluate the stability of the lubricating oil after compounding the amine antioxidants. The various factors were combined to obtain the mixing ratio. The formulation contains p-, p-diisooctyldiphenylamine, 2-tert-butyl mixed ester, and dithiophosphoric acid triester.
2.4 Antifoam Biogas Compressor In the working process, the circulating oil volume is large, the cycle frequency is high, and the oil product production time exists in the environment of high pressure and high flow rate, which is easy to generate a large amount of foam and accelerate the oxidation speed of the lubricating oil, resulting in Lubricating oil performance deteriorates, compressor operating efficiency decreases, and so on. The antifoaming agent acts mainly to reduce the stability of the foam, inhibit the generation of foam, and increase the speed of eliminating the foam.
The antifoaming agent selected for the developed oil is a polyacrylate-ether. First of all, polyacrylate-ether can make up for the shortcomings of silicone oil antifoaming agent, which is difficult to disperse and unstable in acidic medium during use. Secondly, it can not weaken or lose anti-foaming property when used in combination with acidic additives. Finally, poly The amount of acrylate-ether added is very small, and it is not sensitive to the blending technique, and the defoaming and antifoaming effects are better.
2.5 Extreme pressure anti-wear agent Due to the wide application of precise manufacturing technology, the volumetric compressor widely used in industry is developing in the direction of heavy load, high speed, large temperature range and large displacement. If the anti-wear property of the compressor oil is poor, it will not only cause some rubber seals to shrink and harden slightly, causing biogas to leak back; and it can not reduce the friction and wear between the piston and other components, thus reducing the efficiency of separation C2, Reduce equipment life. Therefore, the development of oil requires the addition of extreme pressure anti-wear agents to improve their anti-wear and extreme pressure properties. According to the requirements of the working conditions of the biogas compressor, the test was carried out by a four-ball test machine and a gear test machine, and an alkyldithiocarbamic acid was selected.
2.6 Anti-rust agent and clean dispersant Because biogas contains a certain amount of H2S and water vapor, in the work of biogas compressor, the compressor components are easily corroded by contact with hydrogen sulfide and water. Corrosion will shorten the life of the transmission components, accelerate the aging of the oil, cause the equipment to operate abnormally or stop running, causing major losses and accidents. Rust inhibitors are substances that block, delay corrosion and rust. Commonly used rust inhibitors include decyl succinate, metal acid salts, imidazoline compounds, amines, and the like. The developed oil is compounded with sodium petroleum sulfonate and ruthenium dinonylnaphthalenesulfonate to inhibit the occurrence of corrosion.
During the working process of biogas compressor, H2S combines with water to produce acidic substances, and mixed with other magazines, it will gradually produce sludge, paint film, carbon deposits and other substances, which not only lead to deterioration of oil, corrosion, and piston ring sticking. Blockage and other phenomena occur, and the resulting deposits can interfere with the seat seal, allowing the critical pressure C2 to leak back into the cylinder. The main function of the detergent dispersant is to prevent the further condensation by the neutralization and solubilization of the mineral acid, the oxidation intermediate and the oxo acid to form a paint film and coke; and also to disperse the already formed paint film in the oil. , to prevent it from sticking to the piston, or to wash off the paint film and coke that began to adhere to the piston. Commonly used detergents are mostly organometallic compounds, and dispersants are mostly metal-free organic polymers, which are collectively referred to as detergent dispersants. Considering the harsh working conditions of the biogas compressor, even if the base oil of the developed oil is a fully synthetic base oil, it is easier to form carbon deposits and sludge. Therefore, the clean dispersing agent added to the developed oil is composed of calcium alkylsalicylate, high alkali petroleum sulfonate and dienyl succinimide, which can not only clean the sludge and asphaltene deposited on the sports pair. It can prevent the formation of colloid and coke; it can also control the oxidation and deterioration of oil, and prolong the oil change period of the oil, thereby improving the safety factor of the compression system and the separation efficiency of C2.
The technical requirements and test methods of the biogas purification and compression filling compressor oil developed in this paper are shown in Table 2. Table 2 Biogas high-purity separation C2 compressor special oil physical and chemical properties Tab.2Physicalandchemicalproperty project test results Experimental method Viscosity index pour point household C flash Point (open / C moisture /% non-corrosion test / grade copper, 100C, 3h) water-soluble acid or alkali no mechanical impurities /% no stability / h carbon residue /% four ball machine test 3 conclusions (1) through analysis Comparing the special working conditions of biogas purification and compression filling compressor, combining the special properties of biogas, using the phthalate and low viscosity PA0 polycomposite as the base oil, and experimenting by rotating oxygen bomb experiment, four ball test machine and gear test machine The scientific and rational selection of additives makes the biogas shrinking machine oil developed to meet the requirements of the biogas compression separation C2 process.
(2) The use of adipic acid ester and low-viscosity PA0 as a base oil solves the problem that a single hexane diester does not adapt to the seal, resulting in C2 entering the separator without reaching the critical pressure.
(3) The developed oil has good comprehensive performance, excellent lubricity, emulsification resistance, thermal stability, thermal oxidation stability and viscosity-temperature characteristics, etc., low coke formation, low volatility, long oil change period, and increased equipment. The air tightness greatly improves the separation efficiency of C2.
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