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author: Nie Hongyuan, Urumqi Petrochemical Company [Abstract] This article introduces the mechanism of explosion when the total hydrocarbon content of the main cooling unit in the air separation unit exceeds the standard, and elaborates on the reasons and preventive measures for the total hydrocarbon content of the main cooling unit in the Urumqi Petrochemical Refinery Area that seriously exceeds the standard. ； It provides an effective reference for how to ensure the safe operation of air separation units in the refinery unit area. keywords: Air separation unit ； main condensing evaporator ； Total hydrocarbon content ； Exceed the standard: explode ； measure: Safe Operation 1 Introduction The most common safety accident in an air separation unit is the explosion of the main condenser due to excessive total hydrocarbon content in the unit. Therefore, the air quality requirements for the air suction port of the air separation unit are relatively high. Generally, the total hydrocarbon content in the air at the air suction port of the air separation unit is required to be less than 8×10-6. The leakage or emission of hydrocarbon gases from various devices in the refinery unit area results in a higher content of hydrocarbon gases in the air. Therefore, the total hydrocarbon content in the air at the suction port of the air separation unit located in the refinery unit area is higher. Therefore, how to ensure the safe operation of the air separation unit in the refinery area is an urgent problem to be solved. There are currently two sets of KDN-2000 full low-pressure high-purity nitrogen devices in the refinery-shantou plant of Urumqi Petrochemical Company. Each set has a nitrogen production capacity of 2000m3/h and is manufactured by Kaifeng Air Separation Equipment Factory. It was put into operation in October 1993. According to production needs, a set of nitrogen is provided for production by refineries, chemical fiber plants and other units. Since the unit was commissioned at the end of 1993, the total hydrocarbon content of the main cooling system has been exceeding the standard (stipulated by Sinopec Corporation).: Alert value 100×10-6 ； If the parking value is 250×10-6 or the individual value is below the warning value, the total hydrocarbon value can be increased to 500×10-6). Later, with the assistance and help of relevant units of the plant, the total hydrocarbon content of the main cooling unit was effectively controlled. The total hydrocarbon content of the main cooling unit dropped from an average of 320×10-6 at the beginning of the test run to the current average of 95×10-6. However, sometimes due to the influence of environmental factors, the total hydrocarbons of the main cooling system are sometimes higher, ranging from 200×10-6 to 250×10-6. 2 The danger of hydrocarbon compounds to the air separation unit. Since the air separation unit was put into operation at the end of 1993, the main reason why the total hydrocarbon content of the main cooling unit has been exceeding the standard is that there are many harmful impurities in the air. The main components are C2H2, CH4, C2H6 and other hydrocarbon compounds (C3H6, C3H8, C4+), etc. NOx and O3 also have a certain content in the air, among which CH4 is difficult to remove. and C2H6, the most dangerous is C2H2. The main sources of these gas impurities are refinery waste gas emissions and run-off, dripping and leaking from various devices. These hydrocarbons accumulate in liquid air (especially oxygen-rich liquid air) and are likely to explode under some detonation triggers. To cause a hazardous hydrocarbon to explode, it must be exposed to a shock or frictional electrostatic discharge. The minimum energy required for such an explosion is called sensitivity. Chemically sensitive substances include O3, nitrogen oxides, and unstable peroxygen compounds. Especially in the presence of O3, explosion sensitivity * * Increase. The cause of industrial cold explosion of air separation equipment is probably caused by hydrocarbons accumulated in liquid oxygen (or liquid string) encountering some kind of ignition source. The source of the ignition is suspected to be from an unstable compound. It is formed by the accumulation of O3 or nitrogen oxides in liquid oxygen and unstable carbon and oxygen compounds. Dangerous impurities in liquid oxygen, if saturated, precipitate as solids with small specific gravity and float near the liquid surface. Especially as the liquid oxygen evaporates, it is easily adsorbed on the liquid surface or on the pipe wall. Therefore, it can easily become the cause of explosion. Among all hydrocarbon impurities, acetylene is generally considered to be the most dangerous to the air separation unit. Under saturated conditions, it is even more dangerous. ； When saturation is not reached, acetylene may exist in the main cooling, which will cause local accumulation and is also dangerous. But acetylene is not the only impurity with explosive hazards. Liquid oxygen (oxygen-rich liquid air) and other hydrocarbon compounds are also explosive hazards. 3. The main reason why the main cooling of the air separation unit in the refinery exceeds the standard. The main reason why the main cooling of the air separation unit in the refinery exceeds the standard is that the total hydrocarbon content in the air at the suction inlet is high. The main reason: 3. 1 Improper site selection of the air separation unit. The air separation unit of our plant is located in the northwest corner of the refinery. It is less than 100 meters away from the nearest normal pressure unit of the refinery and 60 meters away from the purified water gas explosion tank. 3.2 The total hydrocarbon content around the air separation unit is relatively high. Since May 1994, according to the requirements of the refinery, 8 monitoring points have been set up around the air separation unit (see Figure 1). The environmental monitoring center monitors these 8 points every week. According to the monitoring data (see Table 1) It can be seen that the total hydrocarbon content at the highest point No. 6 (the northwest corner of the air separation unit) is as high as 50×10-6, and the average value is 14.78×10-6, which is much greater than the original Petrochemical Corporation’s requirement for the total hydrocarbon content in the air suction port of the air separation unit to be less than 8×10-6. Table 1 Monitoring values of total hydrocarbon content in atmospheric air (×10-6) Sampling point Sampling time September, October, November Average value 1st 2nd 1st 2nd 1st 2nd 1# 20.0 8.75 23.3 10.0 21.1 7.78 15.2 2# 23.0 27.5 12.2 14.2 22.2 7.78 16.08 3# 14.4 6.25 8.89 12.2 14.4 8.89 10.83 4# 15.6 8.75 12.2 8.89 15.6 6.67 11.28 5# 15.6 12.3 36.7 7.78 17.8 8.89 16.5 6# 50 20.0 13.3 10.0 20.0 7.78 20.78 7# 17.8 22.5 7.78 11.1 20.0 10.0 14.86 8# 22.2 10.0 7.78 8.89 20.0 7.78 12.775 Average 14.78 4 Measures to be taken Through the above analysis, we believe that the following two aspects should be taken to control the total hydrocarbon content of the main cooling system. 4.1 Try to control the total hydrocarbon content in the air at the suction port of the air separation unit, and take the following measures in Plan 1 to reduce the total hydrocarbon content in the main cooling unit.: (1) Continue to adhere to the regular discharge of liquid once a day, and the liquid discharge volume each time is 400~500mm (main cold liquid column reading). (2) Modify the sealed door or affix sealing strips. (3) Seal all manhole covers. (4) The compressor inlet and outlet are heightened. Our workshop implements the following step by step. 1) Increase the liquid discharge volume. Change the original liquid discharge volume rule of once every 5 days to once per shift (8 hours). The liquid discharge volume per shift is 200mm~700mm. According to this rule, the total hydrocarbon content is basically maintained between (60~300)×10-6. Later, due to the high frequency of liquid discharge, the drain valve leaked internally, so the workshop modified the drain valve. After the transformation, we increased the liquid discharge volume to 200mm~800mm, and the total hydrocarbon content dropped by (50~27)×10-6. We also explored the relationship between the liquid discharge volume and the total hydrocarbon content. If the liquid discharge volume is greater than 600 mm, the total hydrocarbon content can be kept below 250×10-6. However, if the liquid discharge volume is too high, it will affect the distillation conditions and the purity of the nitrogen product. 2) Sealing door modification or sealing strip. On April 24, we analyzed the total hydrocarbon content of the main cooling unit within a week after sealing the compressor air inlet sealing door. The analysis results showed that the total hydrocarbon content was between (150~330)×10-6 and slightly higher than before sealing. The reason was mainly due to the greater influence of hydrocarbon content in the atmosphere. Secondly, applying sealing strips cannot fundamentally solve the problem of air leakage in the sealed door. Moreover, the sealing strip will fall off within a week after being applied. Must be replaced. 3) Seal all manhole covers. In April 1995, our factory sealed all manhole covers around the air separation unit with cement, and conducted six analyzes within three weeks after the manhole covers were sealed. The analysis results showed that the total hydrocarbons in the air were still between (5 ~ 23) × 10-6, and the effect was not significant. 4) The project of raising the air suction port of the compressor has been analyzed by the scientific research department. The total hydrocarbons at high altitudes from 10 meters to 40 meters change slightly, but there is no pattern. 4.2 Improve the operating system and adopt the following measures in Plan 2 to reduce the total hydrocarbon content of the main cooling unit: (1) The frequency of analysis is once a day. If the total hydrocarbon content is higher than 200×10-6, add an additional analysis 2 to 3 hours after draining to observe the changing trend of the total hydrocarbon content. The liquid is discharged once every shift, and the discharge volume is between 200mm and 700mm. (2) Determine the amount of liquid discharged based on the laboratory analysis results of total hydrocarbon content. The amount of liquid discharged shall be based on the main cold liquid juice. The workshop decides that when the total hydrocarbon content is below 100×10-6, the liquid discharge volume per shift is between 200mm and 400mm. When the total welding content is between (100~250)×10-6, the liquid discharge volume per shift is between 400mm and 700mm. See Figure 2. The liquid drainage time is within 1 hour after taking over, and the continuous liquid drainage time is more than 30 minutes. That is, no more than 15mm per minute. (3) If the total hydrocarbon content is still high after draining, and is greater than 250×10-6, you should contact the factory dispatch in time to drain all the liquid and stop the machine for a large heating treatment. (4) The main cooling adopts high liquid level operation, the main cooling liquid level is maintained between 1800mm and 2800mm, and the main cooling pressure fluctuation should be reduced. Carry out marking stability rate assessment. (5) Shorten the service life of the liquid air adsorber from the original design of 7 to 10 days to 7 days. Completely regenerate the liquid air adsorber, ensuring that the temperature at the blowout outlet, i.e. at the bottom of the bed, reaches 30°C, and then use it after cold blowing to normal temperature. Use nitrogen as the regeneration gas as much as possible to ensure the regeneration effect. (6) When the total hydrocarbon content does not exceed the standard, but monohydrocarbons such as acetylene exceed the standard, corresponding treatment should also be carried out. The warning value of acetylene content is 0.1×10-6, and the parking value is 1×10-6. (7) Replace the compressor inlet filter every 15 days and seal the suction door with a sealing strip. (8) When liquid accumulates, it should be carried out in accordance with the relevant provisions in the operating procedures. The liquid should be drained after each shift, and then the liquid should be accumulated. The liquid accumulation time should not be less than 3 hours, based on the main cold liquid level, which is 133mm ~ 166mm per hour. The principle of liquid accumulation first and then draining should be strictly followed. (9) The discharge of liquid air should be strengthened, and the main cooling liquid air return valve (TG133 valve) should be opened as large as possible while ensuring smooth operation. (10) Ensure the smooth operation of the air cooling tower and prevent impurities from entering the fractionation tower. (11) Check the electrostatic grounding wire of the fractionating tower once a year to ensure that the grounding is intact. (12) When the upstream device is shut down for purging or when there is gas smell in the device, you should contact the dispatching room in time to understand the composition of the discharge medium of the upstream device in a timely manner, increase the number of laboratory analyses, and carefully observe the analysis results. When necessary, a large amount of liquid must be drained or work must be stopped, and ledger records must be kept. (13) Analyze the ambient air regularly (half a year), monitor changes in hydrocarbons in the air, adjust operations in a timely manner to prevent the accumulation of hydrocarbons, and keep good accounting records. (14) Strengthen the operation of the reversible heat exchanger, reduce the temperature difference in the middle, control the temperature difference in the middle to within 10°C, and increase the self-cleaning ability of the reversible heat exchanger. Through the above, the measures in Plan 1 and Plan 2 are implemented. ； The total hydrocarbon content dropped from the original 320×10-6 to 95×10-6. But sometimes it is still above the alarm value. 5. The method to deal with the remaining problems must fundamentally solve the problem. The workshop believes that the problem of excessive total hydrocarbon content in the atmosphere around the air separation unit must be completely solved. Two measures, F, can be taken. 5. 1 Compressor air inlet remote transmission solution: Near the upper air outlet of the air separation unit, such as west of the scientific research institute, the fifth team conducts an environmental impact assessment within the area. Find an area with good air quality (less hydrocarbon components) and is not affected by changes in wind direction as the air inlet of the compressor. Then, it is remotely transmitted to the air separation unit as the raw material wind for the air separation unit, so that the total hydrocarbon content of the air inhaled by the air separation unit meets the requirements and reduces the total hydrocarbon content of the main cooling unit. advantage: (1) It can reduce the total hydrocarbon content of the main cooling system to the maximum extent. (2) The highest degree of feasibility. shortcoming: Because the air inlet pipeline is long, the compressor inlet resistance is large, and the compressor air intake is small and cannot meet production needs. If a blower is added between the inlet pipelines, equipment investment will be increased, and corresponding manpower and material resources will be increased. 5.2 Add a set of air adsorption equipment between the compressor and the air separation cold box. Add a set of air adsorption equipment between the compressor and the air separation cold box to adsorb hydrocarbon components in the air at the outlet of the compressor and reduce the hydrocarbon content in the air entering the cold box to achieve the purpose of reducing the total hydrocarbon content. advantage: High feasibility. shortcoming: Large equipment investment