Safety control of the acetylene production process using wet calcium carbide

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The charging process of acetylene production using warm-method calcium carbide is the process by which calcium carbide enters the reaction system from the atmosphere, and it is the station where explosion accidents are most likely to occur. It introduces the process, design principles, and operating instructions for wet calcium carbide feeding ； Analysis shows that insufficient process conditions (substandard nitrogen, unstable power supply for the bell-and-cap valve, large particle size of raw materials), equipment failures (leaks in the bell-and-cap valve, leaks in flexible connectors, leaks in the equipment itself), and operational errors (inadequate nitrogen displacement or failed displacement procedures, cross-contamination during charging) are the causes of explosions. Specific measures to prevent explosions during the charging process are proposed to address these various causes. 　　In the wet-process production of acetylene from calcium carbide, the main raw material, calcium carbide reacts with water to produce acetylene gas, which possesses a carbon-carbon triple bond structure and thus exhibits highly reactive physicochemical properties; it is prone to addition, substitution, and polymerization reactions. Its flash point is only -17.8°C, and its explosive range when mixed with air is 2.3% to 81%, with the most dangerous range being 7% to 13%. It is classified as a Class A fire hazard material. During the production process, explosions can easily occur during the material feeding operation. Therefore, ensuring safe production during this process and preventing such explosions is an issue that producers in the industry for producing acetylene via wet calcium carbide production must address. 　　1 Introduction to the Design Principles of the Feeding Process 　　1.1 Brief Description of the Process 　　Calcium carbide is delivered to the generator through a feeding funnel, 1# bell-shaped valve, 1# feeding hopper, 2# bell-shaped valve, 2# feeding hopper, a flexible connector, and a feeder chute. The 1# and 2# feeding hoppers are equipped with chlorine exchange pipes and vent pipes respectively; the process flow is shown in Figure 1. 　　1.2 Design Principle Through proper nitrogen displacement and sequential operation of the two bell-type valves, it is ensured that raw calcium carbide enters bin #1 from the atmosphere, and from bin #1 it moves to the feeder before entering the generator. In this way, acetylene gas, oxygen, and a fire source – along with combustible materials and oxidizing agents – are not present at the same time, thereby ensuring safe operation during the feeding process. 　　1.3 Operating Instructions: Calcium carbide is introduced into Bin #1 from the outside atmosphere as follows: Close Valve #1 and Valve #2, open the nitrogen valve for Bin #1 as well as the vent valve for Bin #1; maintain a pressure of around 50 mmHg (6666.1 Pa) in Bin #1 for approximately 5 minutes to completely displace the air inside the bin. After that, close the nitrogen inlet valve for Bin #1 and open Valve #1, after which calcium carbide is poured into Bin #1 using a bucket from the outside. During this operation, container 1# contains only air and sparks generated by the impact of calcium carbide; there is no flammable acetylene gas present, thereby ensuring operational safety. 　　The operation of transferring calcium carbide from Tank 1 to Tank 2 is as follows: The valves on Tank 1 and Tank 2 are kept closed. The nitrogen valve for Tank 1 and the vent valve for Tank 1 are opened to displace any oxygen that may have entered during the addition of calcium carbide to Tank 1. After that, the nitrogen valve and the vent valve for Tank 1 are closed, while the valve on Tank 2 is opened, allowing the calcium carbide in Tank 1 to be transferred into Tank 2. During this process, only sparks are generated by the impact of acetylene gas and calcium carbide, with no oxygen as an oxidizing agent present, thereby ensuring operational safety. 　　Calcium carbide enters the acetylene generator from the discharge chute via a vibrating feeder, coming from hopper No. 2; the amount of discharge is determined by the height of the buffer equilibrium gas tank and the synthesis flow rate. 　　Theoretically, if all process conditions are met, the equipment is in good condition, and the operating procedures are followed correctly, this can fully ensure the safety of the feeding process. However, in practice, explosions can occur during feeding due to unmet process conditions, equipment failures, or operational mistakes. 　2 Analysis of the reasons for potential explosion accidents during the charging process 　　2.1 Explosions caused by inadequate process conditions 　　2.1.1 Substandard nitrogen quality 　　Chlorine, used as gas for safe purging at the charging station, must have a purity of over 99% and a pressure between 0.1 and 0.4 MPa; the presence of water is strictly prohibited. Insufficient purity of nitrogen, along with a high oxygen content, results in inadequate displacement; low pressure increases the displacement time. Water present in the nitrogen causes calcium carbide to react immediately with water to produce acetylene gas. When there are issues with the nitrogen, it is not possible to ensure thorough displacement, which poses a risk of explosion. 　　2.1.2 Unstable power air supply for the bell and cap valve The power air supply for the bell and cap valve can be compressed air, nitrogen, or other gases with a pressure of over 0.2 MPa; too low a pressure can prevent the bell and cap valve from closing properly or cause it to open automatically, rendering the valve ineffective and potentially leading to explosion accidents. 　　2.1.3 Large particle size and high temperature of the calcium carbide raw material The feeding station requires that the particle size of the calcium carbide raw material be between 2 and 50 mm, with less than 10% of the particles being smaller than 2 mm. Excessively large particle sizes can cause the calcium carbide to form bridges, preventing the bell-shaped valve from closing; on the other hand, too small particle sizes can lead to blockages in the feeding chute. The temperature of the calcium carbide raw material is generally required to be below 90°C; excessive temperatures can affect the rubber lining of the feeding hoppers, the flexible connectors, and the sealing rings of the bell-type valves, accelerating their aging and causing leaks in these components, which in turn may lead to explosion accidents. 　　2.2 Equipment failure leading to explosion accidents 　　2.2.1 Leakage from the l# bell cap valve 　　The reasons for leakage from the l# bell cap valve include: detached sealing rings, damaged sealing rings, loss of elasticity in the sealing rings, worn sealing discs, the presence of calcium carbide particles between the sealing disc and the sealing rings, insufficient pressure in the gas supply, and mechanical failures. If calcium carbide leaks from the l# bell cap valve, it can enter the 2# storage tank from the 1# storage tank, resulting in the escape of acetylene gas from the l# bell cap valve, which poses a risk of explosion. 　　2.2.2 Air leakage from Valve 2# The reasons for air leakage in Valve 2# are basically the same as those for Valve 1#. When Valve 2# leaks, when calcium carbide is added to Tank 1#, the escaping acetylene gas, the oxygen brought in by the added material, and the sparks generated by the impact of the calcium carbide all come together, thereby triggering an explosion. 　　2.2.3 Air leakage from flexible joints 　　Drum-type flexible joints are made of rubber, and they are prone to aging and delamination over time, which can lead to acetylene gas leakage. 　　2.2.4 Air leakage from the equipment itself: During the feeding process, actions such as the impact of calcium carbide, vibrations of the feeder, and impacts due to bridging can cause the fasteners to loosen, leading to cracks in the equipment and leaks of acetylene gas, which poses a risk of explosion and fire. 　　2.3 Explosion accidents caused by operational errors 　　2.3.1 Failing to purge the l# storage tank with nitrogen or adding calcium carbide without ensuring proper nitrogen purging 　　Due to an error during the loading process, calcium carbide was added to the 1# storage tank without first purging the l# storage tank or ensuring that the purging was done properly. This violation of the design principles led to the simultaneous presence of combustible materials, oxidizing agents, and a source of ignition during the loading operation, resulting in an explosion accident. 　　2.3.2 Material mixing operation During the feeding process, the 2# bell-shaped valve remains open, and the shared feeder in bins 1# and 2# feeds material into the generator. Over time, with such operations, if the valve on l# leaks, or if valve 1# is opened further, a large amount of acetylene gas will leak out, thereby causing an accident. 　　2.3.3 Opening both bell-shaped valve caps simultaneously When no blind plate is in place at the feed funnel opening, if the operator makes a mistake and opens both #1 and #2 bell-shaped valve caps at the same time, it allows the system to be connected to the outside environment; as a result, large amounts of acetylene gas escape. Even a slight mistake by the operator, such as generating sparks from calcium carbide, can lead to a major explosion. 　　2.3.4 Failure to release the calcium carbide from the hopper or adding more calcium carbide before it has been used up: If the calcium carbide in hopper #1 is not transferred to hopper #2, and more calcium carbide is added to hopper #1 before the calcium carbide in hopper #2 has been used up, or if the calcium carbide from hopper #1 is placed in hopper #2, this can result in the valve in hopper #1 or hopper #2 failing to close properly, leading to air leakage and thus an explosion. 　　3 Measures to Prevent Explosion Accidents During Loading Based on the above analysis, to ensure safety during the loading process and prevent explosion accidents, the following measures should be taken: (1) Ensure an adequate pressure and purity of the nitrogen used for loading, and prevent water from being present. 　　(2) Ensure the pressure of the power air source for the bell cap valve. 　　(3) Strictly control the particle size and temperature of the raw calcium carbide. 　　(4) Ensure that the 1# bell cap valve does not leak air. 　　(5) Ensure that the 2# bell cap valve does not leak air. 　　(6) Ensure the flexible joint is in good condition. 　　(7) Ensure that the feeding equipment does not leak. 　　(8) Carefully carry out the replacement and feeding operations in accordance with the procedures. 　　(9) Cross-contamination is strictly prohibited. 　　(10) It is strictly prohibited to open two bell cap valves at the same time. 　　(11) The 1# hopper must be emptied before calcium carbide is added to it, and the 2# hopper must also be emptied before calcium carbide is placed in it. 　　(12) When the material level in hopper #1 becomes too high and reaches the funnel opening, a gasket can be placed at the funnel opening and a blind flange can be installed; after securing it, valves #1 and #2 should be opened simultaneously. 　　(13) If the power air supply for the bell cap valve is too low, or if the valve is leaking, a blind plate can be placed over the funnel opening or the bell cap valve can be secured in place. 　　(14) The feeding station features a semi-enclosed design to ensure good ventilation, allowing any leaked acetylene gas to disperse rapidly. 　　(15) Strengthen the routine inspection system and shift handover system, enhance the sense of responsibility among operators; when problems are detected, feeding operations should be stopped and such issues addressed promptly. 　　In short, the key to preventing explosion accidents during the process of producing acetylene using wet calcium carbide is to prevent leaks of acetylene gas. As long as operators follow the operating procedures carefully during the feeding process, all process conditions are optimal, and the equipment is in good condition, it will be ensured that acetylene gas, oxygen, and ignition sources do not coexist at the same time. With normal temperatures and pressures in the system, explosion accidents during the feeding process will not occur. (Editor’s note: In the process of producing acetylene via wet calcium carbide method, operating the valves for feeding materials is rather complicated, and since feeding is carried out more than once per shift, mistakes are likely to occur. For greater safety, it is recommended to use computer control – by pressing a feeding button, the computer can handle the complex valve operations, and it can also automatically monitor the oxygen content and nitrogen pressure in tanks #1 and #2; an alarm will be triggered immediately if these values exceed the specified limits.) This post was last edited by shizhiyong001 on 2009-2-5 15:35.]