Electrostatic hazards in polypropylene production by batch liquid-phase bulk method and their elimination

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Electrostatic Hazards in the Production of Polypropylene via Batch Liquid-Phase Bulk Method and Their Mitigation: This paper analyzes the causes and hazards of electrostatic charges generated during the batch liquid-phase bulk method for polypropylene production, and proposes measures to eliminate these electrostatic hazards through process and equipment improvements, installation of electrostatic eliminators, and enhanced management. 　With the development of industrial production, static electricity also poses many hazards. Especially in the petrochemical industry, where raw materials and products are flammable and explosive, the electrostatic hazards associated with polyolefin powders are increasingly becoming a new serious problem. In a petrochemical plant, explosion accidents caused by static discharge occurred on multiple occasions during the packaging of polypropylene powder using the batch liquid-phase bulk process (hereinafter referred to as the small-bulk process). Within one year in 1997, there were 6 fire incidents caused by static discharge at the flash evaporation unit of the polypropylene plant using this small-bulk process. The small-scale polypropylene plant at Jinzhou Petrochemical and Chemical Plant No. 3 has conditions for the generation and accumulation of static electricity during production, transportation, and packaging processes. Flash explosions and fires caused by static electricity have occurred on multiple occasions, and the presence of static electricity poses a serious threat to the safe operation of this polypropylene plant. How to prevent and eliminate static electricity in order to ensure the safe operation of production equipment is an issue that deserves in-depth exploration and urgent resolution. 1 Generation of static electricity in polypropylene powder 111 Brief introduction to the production process Propylene with a purity of over 99.5% from the propylene purification unit is purified through desulfurization, dearsenification, deoxidation, and dehydration before entering the propylene metering tank. After measurement, it is added to the polymerization reactor in four batches, after which stirring is started. The activator, silane, catalyst, and hydrogen are added to the polymerization reactor in measured amounts respectively. After the material has been added, hot water is introduced into the jacket to raise the temperature. When propylene undergoes polymerization, the jacket switches to cold water to control the reaction temperature. Once the reaction is complete, open the valve on the reactor to recover the unreacted high-pressure propylene. Then, the material is sprayed into the flash tank via the discharge valve of the polymerization tank. The material entering the flash drum is agitated and subjected to vacuum flashing to separate the unconverted propylene. Then, it is purged with N2 gas until the combustible gas content in the flash tank is less than 0.18%; after that, the discharge valve of the flash tank is opened to release the contents for packaging. Its process flow is shown in Figure 1. 112 Electrostatic potential of polypropylene powder: Polypropylene powder has poor water absorption, a dry surface, and a high resistivity; its resistivity is greater than 10148 ·cm. The polypropylene powder is stirred in the polymerization reactor and then sprayed into the flash vessel at high speed. Due to the shearing action of the mixing blades, the repeated contact and separation between the blades and the powder result in a large amount of static charge being generated on the surface of the powder. When the material is sprayed at high speed into the flash drum, friction and impact between the powder and the tube walls as well as the drum cause an further increase in the surface charge of the powder. Polypropylene powder has good insulation properties; the charge it carries is not easy to dissipate and tends to accumulate. When the static electric potential exceeds a certain limit, discharge occurs, leading to accidents. The static electric potential of the polypropylene powder at the discharge port is shown in Table 1. 2　Electrostatic hazards in production: Since its commissioning, the polypropylene plant has experienced multiple instances of flash fires inside the flash vessels, as well as flash fires and ignition at the packaging points, flash fires inside the packaging bags, and spark discharges when switching valves. When opening or closing the valves, phenomena such as an electric shock sensation occur. 211 Spark discharge occurs when the discharge ball valve is opened. After the polymerization reaction comes to an end, the discharge ball valve of the polymerization reactor is opened at a pressure of 112–114 M Pa (gauge pressure) in the polymerization reactor, with the gauge pressure in the flash tank being almost zero, in order to feed material into the flash tank. The gasket between the ball core and the housing of the ball valve is made of insulating material, and the sealing filler between the valve stem, which drives the ball core to rotate, and the housing is also made of insulating material. Therefore, when polypropylene powder passes through the ball core at high speed, friction and impact cause both the powder particles and the ball core to acquire static charges, which gradually increase. A strong electric shock sensation is felt when turning on and off the valves. Sparking discharge occurs at the moment the valve-opening wrench makes contact with the equipment or is not in proper contact. 212 Explosion in the flash vessel: During the flash deactivation of polypropylene powder, continuous stirring is carried out, which generates static electricity of high potential. There have been instances of explosions inside the flash drum caused by static discharge during deactivation operations involving the introduction of air into the drum. The surface of the powder inside the kettle turned black; some of the powder melted and formed clumps, and the paint on the drainage pipes was burned. 213 Flash explosion and fire during packaging: Due to incomplete vaporization, the propylene adsorbed on the powder does not vaporize completely. Abnormal polymerization reactions and issues such as sticky materials make it difficult for the flammable gases contained in the powder to vaporize. During packaging, combustible gases are released around the discharge port; as the concentration of these gases increases, the static electricity generated during discharge can lead to flash explosions or fires. Such accidents have occurred 214 times during production: a flash fire inside the packaging bag. Once, at the end of packaging, there was not enough material for one bag, so that bag was placed under the discharge port and left there. When taking the shovel out of the bag, it flashed and ignited, burning my hand. 3 Measures to eliminate the hazards of static electricity. As mentioned above, the areas where static electricity poses the greatest risk are the flash drum and the discharge port. The hazards of static electricity include causing flash explosions and fires, as well as inducing an electric shock sensation in humans. Therefore, the following measures are taken. 311　Modification of the discharge port and forced exhaust: To prevent the accumulation of flammable gases and reduce the concentration of polypropylene dust, the discharge port was modified, and an exhaust duct was installed. The combustible gases and polypropylene dust from the discharge opening are quickly expelled outdoors, preventing them from reaching an explosive concentration. 312　Grounding of weighing scales: To eliminate static electricity on the weighing scales, the weighing instruments—ordinary scales—are placed on a well-grounded copper plate. 313 Eliminating static electricity in discharge ball valves: To eliminate the static electricity on the valve stem, two layers of lead gaskets are inserted into the gland box, or conductive sealing materials are used as gaskets; this allows the static electricity on the valve stem to be discharged quickly, thereby preventing static discharge and electric shock. 314 Improvement of the flashing process: When purging the flashing vessel with N2, N2 was previously introduced from the upper part of the vessel and removed through the vent pipe at the upper part. To prevent dead zones, it was changed to introduce N2 from the bottom of the kettle. To make the flashing operation safer, in addition to strictly controlling the content of flammable substances, N2 is used for pressure maintenance during discharge and packaging, thereby reducing the oxygen level in the tank to a safe range and enhancing the safety of operating the flashing tank. 315 Installation of an electrostatic eliminator at the discharge port: An AJS22 type fully automatic powder electrostatic eliminator was installed at the discharge port of the flash tank. An electrostatic eliminator consists of a controller and an ion generator. The controller is an explosion-proof electrical device, while the ion generator is a positive-pressure ventilation type of explosion-proof device. This device generally has two operating modes: computer-feedback control ion implantation mode; and static electric field automatic balance ion implantation mode. It automatically switches between the two operating modes based on the specific configuration, with the system setup determined by factors such as the dielectric material, process conditions, and intended use. The working principle of the computer-controlled feedback ion implantation method is shown in Figure 2. After the air supply and power source are connected, the corona needle inside the ionizer discharges, ionizing the air into charged ions, which are then emitted in sufficient quantities to neutralize the static charge carried by the polypropylene powder passing through the ionizer. After charge removal, it essentially reaches an electrically neutral state. An extremely small amount of uneliminated static charge is detected by a static electricity sensor. The controller readjusts the amount of ions output based on the detected remaining charge. The entire electrostatic elimination process is a cycle that starts with detecting the remaining charge of the powder and ends with adjusting the amount of ions output. Since the cycle time is only a few dozen milliseconds, the tracking speed is high, which allows for the maximum reduction of static electricity in the powder. After installing the static eliminator, the static potential of the polypropylene powder at the discharge port was significantly reduced, as shown in Table 2. It effectively controlled the flash explosions and fires that occurred at the packaging station due to static discharge. 316 Strengthen safety production management and strictly enforce safety production rules and regulations. Strictly control the combustible gas content during the packaging of flash vaporized material; packaging is allowed only when the combustible gas content is below 0.18%. Due to the batch operation mode, and to avoid mistakes, a summons and ticket system has been established: the operation ticket is sent to the process where the material arrives. All process parameters, analysis data, and operational procedures must be clearly recorded on the operation ticket; operations can only be carried out after being approved and signed by the operator. The operators must be properly dressed, and the packagers must touch the zero-potential rod before starting work to eliminate any electric charge on their bodies. 4 Conclusion: In every stage of small-volume polypropylene production, there are conditions for the generation and accumulation of static electricity in the polypropylene powder. The hazards of static electricity represent a significant risk in manufacturing. After implementing some of the above measures to eliminate static electricity hazards, fires and flash explosions caused by static electricity have been basically prevented, as well as incidents of electric shock to people. In addition to being capable of igniting explosive mixtures of propylene and air through electrostatic discharge sparks, polypropylene powder can also ignite explosive mixtures of polypropylene dust and air. Therefore, in production, it is necessary to strictly control the combustible gas concentration around the discharge port, as well as the concentration of polypropylene dust, in order to prevent dust explosions and fires. 5 References 1 Guan Xiaosheng, et al. Static Electricity Disasters and Their Prevention. Shenyang: Liaoning Science and Technology Press, 1986. 2 Yang Youqi. Static Electricity Safety Technology. Beijing: Chemical Industry Press, 1983. 3 Safety Technology Research Institute of Sinopec Corporation. User Manual for Model AJS22 Fully Automatic Powder Static Electricity Eliminator. 1996