The danger of the mixture of acetylene and ethylene is mostly due to their chemical differences and potential for violent reaction. Acetylene has a lower explosive limit (LEL) of 2.5% (volumetric concentration) and an upper limit (UEL) of 81%, while ethylene has a LEL of 2.7% and UEL of 36%. When two are combined, the explosion range can be extended to 2.5%-81% (according to Le Chatelier’s rule), and when the concentration of the mixture is within this range (e.g. 5% acetylene + 10% ethylene), explosion can be initiated by static electricity (energy ≥0.02mJ) or an open fire (temperature ≥305°C acetylene spontaneous ignition temperature). The maximum explosion pressure can be 1.2MPa (6 times that of common air explosion). A valve failure at a 2008 chemical plant, by the U.S. Chemical Safety Board (CSB), saw acetylene ethylene be combined with ethylene, producing an explosion which caused three deaths and direct financial losses of $23 million.
Thermodynamic instability is another danger, with acetylene being able to self-degrade at pressures of >0.2MPa (releasing 8.8kJ/g of energy) and ethylene polymerizing spontaneously at high pressure (producing low-density polyethylene, releasing 12kJ/g of heat). If the pressure of a mixture of gases in a closed vessel (storage tank or pipeline) is raised to 1.5MPa, the vessel will fail due to a chain reaction (the design limit for the pressure vessel is generally 1.0MPa). According to statistics of Industrial and Engineering Chemistry Research, the decomposition rate of acetylene ethylene mixture at 30°C and 0.5MPa is 300% higher than pure single gas, and the reaction activation energy is as low as 65kJ/mol (120kJ/mol for pure acetylene), which greatly increases the risk of runaway.
Toxic additive effect cannot be ruled out with IDLH (immediate life and health threatening concentration) of 1,700ppm for acetylene and 8,000ppm for ethylene, but co-toxicity is possible when in combination (e.g., 20% more effective in inhibiting the central nervous system). In inadequately ventilated workshops (air exchange rate <5 times/hour), when the leakage concentration is 500ppm acetylene + 2,000ppm ethylene, exposed workers of 15 minutes duration will suffer from dizziness and nausea (85% chance), and the 1-hour fatality rate is approximately 12% (animal models).
In terms of storage and transport risks, acetylene needs to be dissolved in acetone (a porous medium in the cylinder) to be stored safely, while ethylene can be stored under pressure as a liquid. If the two mix due to cross-contamination of the pipeline (e.g. 0.1%/year likelihood of valve failure), an explosive mixture (acetylene/ethylene ratio >2.5%) will be formed. In 2015, a German chemical plant was contaminated with a mixture by corrosion in the pipeline, which led to a flash explosion, ruptured 200 meters of pipe, and cost 4.8 million euros to repair.
Detectors and protection problems include:
Accuracy of gas detectors: Routine catalytic combustion sensors respond to acetylene ethylene mixture with ±15% error (±5% error for single gas), which leads to an alarm delay (e.g. alarm at 50% LEL).
Cost inerting: Nitrogen injecting (purity ≥99.999%) to reduce the percentage of oxygen content to <10%, and 1.8 times more than single gas as the treatment price is increased from $0.5 to $0.9 per cubic meter.
Industry standards (e.g., OSHA 1910.101) explicitly prohibit acetylene from being kept in the same building as ethylene, and violators would face a one-time fine of as much as $14,502 (US 2023 norm). Engineering controls (e.g., split lines, double-check valve arrangement) can reduce the likelihood of leakage from 0.01% to 0.001%, but with an additional upfront investment of 15%-20%.