Generation of static electricity in polypropylene powder and measures to eliminate it

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The polypropylene production facility of Jinzhou Petrochemical Company was built in 1984; it employs a liquid-phase batch polymerization process, using efficient catalysts to facilitate chain initiation, growth, transfer, and ultimately the directional polymerization to produce high-purity polypropylene powder. After capacity expansion and renovation, the current production capacity is 25,000 t/year. Due to issues such as long operation cycles of the equipment, outdated packaging processes for finished products, aging machinery, high labor intensity for employees, and a high risk of flash explosions during the discharge of finished products, the company divided the entire polypropylene production facility into 6 evaluation units for assessment, based on the characteristics of this type of facility. 　　Evaluation result: The significant risk lies in the possibility of flash explosions occurring at the discharge outlet when discharging the finished product; the impact and evaluation level are 7.0 (using the matrix method). The main factor contributing to this risk is static electricity. In polypropylene production facilities, large amounts of polypropylene powder are generated during the production and packaging processes. The relatively insulating nature of polypropylene is also a major factor that contributes to the generation of static electricity. Therefore, to ensure safety during production, it is essential for employees to have a better understanding of static electricity and to take measures to mitigate its hazards, which is crucial for the safe production of polypropylene. 　　I. Generation of static electricity in polypropylene powder The processes and locations where static electricity easily occurs in polypropylene powder are shown in Figure 1. http://www.hbsafety.cn/article/UploadPic/2006-9/20069281702248174.gif 1. Discharge pipe of the polymerization reactor: When the reaction and recovery process are complete, the pressure inside the polymerization reactor is 1.2–1.4 MPa. Under such high pressure, the polypropylene powder passes at high speed through a discharge pipe with a diameter of 100 mm, which is equipped with two valves, and enters a flash drum with a diameter of 2800 mm. During this process, the polypropylene powder travels along a tortuous path, and it comes into contact with various materials; as a result, a large amount of static electricity is generated in the polypropylene powder. Additionally, due to electrostatic induction, a significant amount of static electricity is also produced within the reactor vessel. 　　2. Stirring process 　　Due to the shearing action of the stirrer blades, these blades come into contact with the polypropylene powder as well as with each other; based on the principle of the double electric layer, a large amount of static electricity is generated. 　　3. Flash drum discharge pipe: The polypropylene powder falls freely from a height of over 3 meters, entering the flash drum with a diameter of 2800 mm and then passing through a discharge pipe with a diameter of 150 mm; it reaches the ground after passing through two valves. Its speed is very high, and its path is also tortuous. As a result, a large amount of static electricity is generated. 　　4. Weighing scale The polypropylene powder that comes out of the feed port carries a large amount of static electricity, and due to electrostatic induction, the weighing scale also accumulates a significant amount of static electricity. 　　II. Measures to Eliminate the Hazards of Static Electricity in Powder The hazards caused by static electricity in polypropylene powder during production include: first, the risk of flash explosions and fires ； Second, it causes electric shock to the human body. 　　Conditions under which static electricity can cause flash explosions and fires: First, a flammable substance (flammable gas or flammable powder) mixes with air to form an explosive mixture, whose concentration is within the explosive limit range ； Secondly, in areas with explosive mixtures, there must be sufficient discharge energy. If either of the two is damaged, no explosion or fire will occur. 　　To eliminate the hazards posed by polypropylene powder, the following countermeasures should be taken for the aforementioned areas (processes) prone to static electricity. 　　1. Perform oxygen analysis in the polymerization reactor before discharging material to the flash tank. Due to process requirements, combustible gases and static electricity are inevitable during the discharge of material from the polymerization reactor to the flash tank as well as during mixing. Therefore, to prevent accidents, it is necessary to control the oxygen level in the flash drum so that it does not form an explosive mixture with the flammable gases. According to production requirements, the oxygen content in the flash drum must be less than 3% before discharge is allowed. Otherwise, continue using nitrogen displacement until the oxygen content meets the requirements. 　　2. Grounding of live conductors To prevent electric shock to people and avoid accidental operations, the flash evaporator and weighing scales are grounded to direct the large amount of static electricity generated into the ground. This is the simplest and most effective way to eliminate the hazards of static electricity. 　　3. Analyze the combustible gas content in the flash drum before discharging the product. When discharging and packaging the product from the flash drum, oxygen and static electricity are inevitable; therefore, to prevent accidents, it is necessary to control the combustible gas content in the flash drum so that it does not form an explosive mixture with oxygen. Per production requirements, the combustible gas content in the flash drum must be less than 0.8% before discharge is allowed. 　　However, another combustible material—polypropylene powder—cannot be ignored, as it cannot be eliminated or controlled. In other words, inside the discharge port that is completely exposed to the air, even if the concentration of flammable gases