Ethylene oxide (EO), also known as ethylene oxide, has the characteristics of flammability, explosiveness, and high toxicity. Its safety management requirements not only share similarities with liquefied petroleum gas but also encompass its unique properties. In the 5th announcement of 2004, the General Administration of Quality Supervision, Inspection, and Quarantine explicitly stated that "Ethylene oxide products that have completed the issuance of industrial product production licenses shall be investigated and dealt with for the unauthorized production and sale of unlicensed products in accordance with relevant laws and regulations." Both the production and utilization enterprises of ethylene oxide are facing increasingly stringent supervision from local regulatory authorities. Ethylene oxide production facilities constructed at the end of the last century are caught in a dilemma of significant capital investment for renovation or elimination due to factors such as regulatory revisions, changing supervisory conditions, new policy requirements, and increased entry thresholds. The safety management status of ethylene oxide is also tense. For instance, on August 19, 2004, an ethylene oxide explosion occurred in the air pollution control device and medical sterilization cabinet of Sterigenics International in Ontario, California, USA. Another incident took place on June 18, 2022, involving an ethylene oxide leakage and explosion at Sinopec Shanghai Petrochemical. These incidents serve as constant reminders for us to remain vigilant against the dangers of ethylene oxide.
Ethylene oxide (EO) or production facilities involving ethylene oxide pose a high level of danger. The design of the process flow is relatively complex, and understanding the physical and chemical characteristics of ethylene oxide in different states is crucial for ensuring the safe operation of the facility. Toxicity, flammability, explosiveness, and polymerization are prominent physical and chemical characteristics of ethylene oxide. Ethylene oxide is a colorless, easily volatile liquid with an ether-like odor that can dissolve in water, alcohols, ethers, and other organic solvents in any proportion. Its chemical properties are extremely reactive, and polymerization reactions can occur at temperatures exceeding 40°C, releasing a considerable amount of heat. At room temperature, it can undergo self-polymerization catalyzed by acids, bases, metal oxides, and chlorides, leading to the blocking of process pipelines by polymer deposits. When ethylene oxide explodes, its temperature can rise from 571°C to 1200°C within an extremely short time (0.002s).
The occupational exposure limit for ethylene oxide is 2 mg/m³. The Occupational Safety and Health Administration (OSHA) in the United States stipulated in 1984 that the 8-hour time-weighted average permissible exposure concentration is 1 ppm, abolishing the previous regulation of a maximum allowable concentration of 50 ppm in the working environment. Ethylene oxide is flammable, toxic, and carcinogenic, classified as a hazardous chemical and subject to strict regulation. When the minimum annual design usage reaches the quantity specified in the "Standard for Quantity of Hazardous Chemicals Used (2013 Edition)" (General Administration Announcement No. 9 of 2013) – 360 t/a, a permit for the safe use of hazardous chemicals must be obtained. Ethylene oxide is a highly reactive combustion agent, irritant, and neurotoxin. Acute poisoning mainly damages the central nervous and respiratory systems. Ethylene oxide exposure can stimulate the respiratory system, causing chemically induced acute tracheobronchitis, bronchopneumonia, and pulmonary edema. Acute ethylene oxide poisoning can also lead to abnormalities in other organ functions, such as myocardial damage and impaired liver and kidney function
The storage system consists of an inlet system, tank body, cooling system, safety release system, nitrogen sealing system, purge system, temperature and pressure detection system, (pneumatic) conveying system, drainage system, analysis and sampling system, and so on. The configuration of these facilities is indispensable for both the safety and functionality of the system. It is worth noting that pneumatic conveying is not only more cost-effective than pump transportation but also safer. The explosive lower limit of ethylene oxide in nitrogen is 75%, significantly higher than the explosive lower limit in air (3%), making it safer in nitrogen. The total pressure of the nitrogen-ethylene oxide mixture generally reaches 0.5 MPa. The nitrogen sealing system can also adopt a three-valve configuration (switching valve).
The unloading process is a critical and accident-prone stage. The unloading procedure for ethylene oxide includes: (1) connecting an electrostatic grounding line to the vehicle to discharge static electricity; (2) connecting the unloading hose to the tank truck discharge pipe, then filling the tank truck with nitrogen to maintain a total pressure of 0.25 MPa for the nitrogen-ethylene oxide mixture, and pressurizing ethylene oxide into the storage tank; (3) after unloading, filling the tank truck with nitrogen to maintain a total pressure of 0.25 MPa to prevent air from entering the tank truck.
The operating temperature for ethylene oxide storage tanks is typically maintained between 0 to 5°C. The purpose is to slow down the polymerization reaction of ethylene oxide and reduce vaporization losses, necessitating the presence of cooling facilities. A 40% ethylene glycol-water solution is chosen for cooling. The operating pressure is set to the saturated pressure at the corresponding temperature, and the design pressure must be greater than the saturated vapor pressure corresponding to the ambient temperature after a loss of cooling, approximately 40°C, with a value of 0.8 MPa. The single-tank loading coefficient is 0.85, and for dual tanks, a loading coefficient of 0.5 is adopted for ease of tank switching when necessary.
Proper material selection is fundamental for ensuring engineering safety.
Tank and pipeline materials are chosen as 304 or 316L; materials such as cast iron, magnesium alloy, and mercury alloy are prohibited.
Double-end mechanical seal pumps are preferred, with shielded pumps, magnetic pumps, or nitrogen pressure delivery being optimal; packing-sealed or single-end mechanical seal pumps are not recommended for use.
Insulation materials must prevent moisture absorption and should not include asbestos, porous mineral wool, magnesium silicate, calcium silicate, etc. Closed-cell insulation materials, such as foam glass wool, are recommended.
The insulation protective layer is preferably made of stainless steel; aluminum protective layers with low melting points are not suitable, and the use of polyvinyl chloride (PVC) is not advised.
Gaskets are made of stainless steel-wound high-purity (98%) flexible compressed graphite with inner and outer rings; materials containing asbestos, asbestos-filled materials, and rubber materials are not suitable.
Valve stem packing materials are selected as corrugated high-purity (98%) flexible compressed graphite or pure polytetrafluoroethylene (PTFE) rings; materials containing asbestos, asbestos-filled materials, or PTFE filled with glass fiber or ceramic are not recommended, as they can absorb ethylene oxide.
Valves are preferably gate valves, globe valves, or high-performance butterfly valves; plug valves and ball valves tend to polymerize and should consider whether the valve cavity structure is prone to ethylene oxide accumulation.
In the ethylene oxide process system, preventing the inclusion of hazardous impurities (such as water, air, rust, etc.) is of utmost importance, along with leak prevention, vaporization prevention, and polymerization prevention. Key points in the design of the ethylene oxide process system include:
Conveying Pipelines
(1) Avoid the use of gas bags and liquid bags, and have measures to prevent material backflow. (2) Choose welding over flange connections to reduce the possibility of leakage. (3) The pipelines for conveying ethylene oxide should be accompanied by cooling and insulation. (4) Set up a nitrogen purge line in the pipeline, and have measures to prevent backflow. (5) Avoid situations where there are closed pipe segments; if unavoidable, install safety valves.
Nitrogen Purge System
(1) The design or operating instructions must clearly state that, before the tank is put into operation, nitrogen must be used for displacement, ensuring that the nitrogen purity is above 99.9%, and it has been analyzed and tested before feeding. (2) The material should be stainless steel. (3) Install check and shut-off valves.
Nitrogen Sealing System
(1) The nitrogen sealing pressure should not be less than 0.5 MPa. (2) Install check and shut-off valves.
Pump System
(1) Pumps and storage tanks should be arranged separately, leaving sufficient safety distance. (2) Implement interlocking measures to prevent idling, no-material conveying, and low-flow conveying. (3) Implement interlocking shutdown measures for high outlet temperature and high inlet-outlet temperature difference.
Unloading System
(1) Design temperature: -10 to 20°C. (2) The loading coefficient should not be greater than 0.79 kg/L. (3) The design pressure is 0.8 MPa, and tank stiffness should be verified at -0.1 MPa; choose the larger value for design wall thickness. (4) For material loading and unloading, use top loading and unloading methods, and the loading and unloading pipes should be made of stainless steel metal corrugated hoses; do not use quick-connect joints with rubber sealing rings. (5) Tank trucks carrying ethylene oxide should be equipped with high-purity nitrogen cylinders and have interfaces connected to the tank body. (6) The nitrogen purity for displacement should not be less than 99.9%, and the oxygen content in the nitrogen seal should not exceed 0.5%. (7) Tank trucks should be equipped with flame arresters.
Safety Release System
(1) Use dual safety valves, in conjunction with rupture discs. (2) The relief pipeline should have nitrogen purging measures not only to dilute the released gas but also to prevent blockages. (3) Release separately, and prohibit direct discharge into the flare system and system venting. (4) There should be measures for collection and treatment, and direct discharge is prohibited.
Safety Instrumented System (SIS)
(1) When the tank volume is greater than 11 m³, an SIS must be installed. (2) The SIS should be independent of the DCS operating system.
(1) Criteria for Setting up the Safety Instrumented System
If the volume of the ethylene oxide storage tank exceeds 11 m³, an independent safety instrumented system must be established according to regulations. According to the requirements of the document No. 3〔2014〕116 from the State Administration of Work Safety, starting from January 1, 2016, newly built chemical installations in chemical enterprises involving "two points and one major" (namely, major regulated hazardous chemicals, major regulated hazardous chemical processes, and major hazardous sources of hazardous chemicals) must be designed with a safety instrumented system in accordance with relevant standards. Starting from January 1, 2018, all newly built chemical installations and hazardous chemical storage facilities involving "two points and one major" must be designed with a safety instrumented system that meets the requirements.
(2) Safety Instrumented Function List
Due to the low critical quantity of ethylene oxide as a major hazard source, a safety instrumented system is generally required.
Ethylene Oxide Storage Tank:
(1) Trigger high-level alarm for the level gauge, interlocking with a high-high-level interlock to cut off the ethylene oxide inlet valve. (2) Trigger high-concentration alarm for the combustible gas alarm, interlocking with a high-high-concentration interlock to open the fire sprinkler switch valve while closing the ethylene oxide inlet valve. (3) Trigger high-limit alarm for the combustible gas alarm (loading and unloading point), interlocking with a high-high-limit interlock to open the fire sprinkler valve while closing the ethylene oxide inlet valve.
Emergency Shutdown: Trigger manual confirmation interlock① close the discharge valve② open the fire sprinkler valve③ close the circulation pump after 5 minutes④ emergency venting⑤ perform the above actions simultaneously.
The design principles, key points, safety facility settings, and safety management points of ethylene oxide storage facilities are based on the physicochemical characteristics of ethylene oxide, incorporating both general engineering design principles and the specific characteristics of the object. Designers and managers should firmly grasp the physicochemical characteristics of ethylene oxide and, in combination with practical work, make flexible judgments and applications to implement safety principles and measures effectively. Only in this way can the design quality be effectively improved, reduce the possibility of accidents from the source, and ensure the safe and smooth operation of production facilities.
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