Explosion-proof safety measures for the condensation evaporator in air separation equipment

Thread Content

Air separation units play a very important role in the coal chemical industry; therefore, extra attention must be paid to their safe operation. According to incomplete statistics, at the end of the 1970s and the beginning of the 1980s, there were over 30 explosions of large and medium-sized air separation units in China, as well as over 100 explosions of small air separation units. Starting from the mid-1990s, explosions of the condenser-evaporators (hereinafter referred to as main coolers) in large air separation units occurred repeatedly both domestically and internationally, resulting in severe losses. In 1996, an explosion occurred in the 6000 m3/h main cooler at Liaoning Shunyiethylene Chemical Company, resulting in damage to the air separation tower and 4 deaths ； In 1997, an explosion occurred in the 80,000 m3/h main cooling unit of Shell Petroleum’s plant in Bintulu, Malaysia, resulting in the complete destruction of the air separation equipment and 12 injuries. It can be seen that main cooling explosion protection is key to the safety of air separation equipment. 　　Mechanism of primary cryogenic explosion 　　1. Hazardous substances 　　a. Flammable components: mainly hydrocarbons such as acetylene; acetylene is the most hazardous, with a very low solubility in liquid oxygen (5.6×10-6 mg/L), allowing it to precipitate in solid form easily and trigger an explosion. 　　b. Clogging components: mainly carbon dioxide, moisture, and nitrous oxide; nitrous oxide in particular is attracting increasing attention. When these substances crystallize and precipitate, they block the main cooling channels, leading to \"dry evaporation\" and \"dead-end boiling\" in the main cooler. This results in the concentration, accumulation, and precipitation of hydrocarbons, which can trigger explosions in the main cooler. c. Strong oxidizers: Liquid chlorine is a strong oxidizer. 　　2. Several detonation factors a. Mechanical impact detonation by solid impurity particles (friction of acetylene particles, shock from liquid oxygen). 　　b. Static electricity: When carbon dioxide particles reach (200–300)×104 ppm, static electricity can be generated, with a voltage of up to 3 kV. 　　c. Substances with extremely high chemical sensitivity (such as ozone and nitrogen oxides). 　　d. Pressure pulses caused by airflow impact, pressure shocks, and cavitation lead to temperature increases, which in turn trigger explosions. 　　Explosion prevention measures for the two main cooling systems: 1. Strengthen control of the quality of raw material air. The oxygen production area should be located in the upwind direction all year round, at a distance of more than 300 meters from the acetylene generation station, and away from sources of harmful gases. It is necessary to strengthen the control of the quality of raw material air; if contamination becomes severe, appropriate measures must be taken. 　　2. Remove harmful substances and prevent the accumulation of hydrocarbons, etc. The main factors contributing to such accumulation are as follows: a. Make full use of the liquid-air and liquid-oxygen adsorbers to remove hydrocarbons such as acetylene; strictly replace the adsorbers on schedule and control the temperature for heating and regeneration in order to improve the adsorption efficiency. 　　b. Discharge 1% of the product liquid oxygen from the main cooler to remove hydrocarbons. 　　c. Periodically perform thorough heating of the air separation unit to remove residual carbon dioxide and hydrocarbon impurities accumulated in the heat exchangers and distillation columns. d. For liquid oxygen pumps that operate continuously and use molecular sieves for adsorption, the adsorption efficiency for nitrous oxide is not good; a layer of 5A molecular sieve can be added inside the molecular sieve adsorber. 　　3. Use high-precision, state-of-the-art testing instruments to enable both online and offline monitoring. This process should be carried out regularly and in a systematic manner; if the environmental conditions deteriorate, effective measures must be taken promptly to keep harmful substances within specified limits: acetylene at 0.5, methane at 120, total carbon at 155, carbon dioxide at 4, and nitrous oxide at 100 (on the order of 10-6). 　　4. Control of the operating liquid level: a high liquid level and a large circulation ratio prevent carbon dioxide and hydrocarbon compounds from accumulating and concentrating. The gas plant at Wuhan Iron and Steel Group adopts full submersion operation. After years of safe operation, all process parameters remain the same as before submersion; there is still sufficient separation space, the heat exchange area meets the requirements, and no gas-liquid entrainment occurs in the oxygen produced. Therefore, full submersion operation for primary cooling is beneficial and harmless. 　　5. During temporary shutdowns and restarts for explosion prevention purposes, it is inevitable that there will be a period of time during which the liquid level is low; during this phase, local concentration of hydrocarbons can occur. Moreover, when restarting, the plate heat exchanger does not operate properly for a while, resulting in poor self-cleaning capabilities and carbon dioxide buildup. Combined with air flow impacts, this can lead to micro-explosions in the main cooler. Therefore, the number of temporary shutdowns should be minimized, or full drainage should be avoided. The main cooler should be heated separately, and if possible, it should be heated thoroughly. 　　6. Regular cleaning: After 2 years of operation or longer, the distillation tower and liquid oxygen circulation system should be cleaned and degreased. The main cooling unit must be soaked for 8 hours; after cleaning, it should be thoroughly blown out with air at sufficient pressure, and then fully heated and dried. 　　7. Preventing static electricity accumulation 　　Liquid oxygen has a high specific resistance, which makes it prone to the generation of static electricity; without grounding, static voltages of several thousand volts can be generated. Therefore, it is necessary to regularly check the grounding of air separation units. 　　8. Preventing oil from entering 　　If oil gets into the air separation unit, it will contaminate the adsorbent and affect its ability to adsorb acetylene; therefore, rotary blowers that can easily introduce oil into the air should be eliminated, and more attention should be paid to the inspection and maintenance of the expanders. 　　9. Strengthen the management of calcium carbide slag. The residual acetylene in calcium carbide slag causes significant air pollution, especially on rainy days; strict control is necessary, and it is best to bury it deep underground. 　　10. Strengthen operation, maintenance, and management. In terms of operation, careful attention must be paid to the steps involved in removing harmful impurities, such as temperature control of plate heat exchangers, stable control of the main cooling system, and monitoring of harmful substances. 　　For maintenance, the monitoring instruments and meters need to be calibrated regularly to ensure the accuracy of the test results ； Super-cycle operation should be handled with caution; stop the machine in time for heating and purging. 　　In terms of management, it is necessary to strictly adhere to procedural rules, strengthen equipment management, prevent illegal operations, maintain the integrity rate of equipment, and strictly implement the principle of \"not letting anything go unchecked.\" 　　11. Strengthen technical training to improve skill levels 　　Conduct regular and ad-hoc training on an annual basis to enhance awareness of explosion prevention and improve operational skills. This article is from China Compressor Network