The flammability of ethylene acetylene is caused by the chemical structure and energy properties: Acetylene (C₂H₂) contains a triple bond energy of 835 kJ/mol, which is lower than the 615 kJ/mol double bond of ethylene (C₂H₄), so the decomposition activation energy is just 190 kJ/mol (ethylene 260 kJ/mol). According to NFPA 68, the lower explosive limit (LEL) of acetylene in air is 2.5% (volume fraction) and the upper limit (UEL) is 81%, while the LEL of ethylene is 2.7% and UEL is 36%, and when mixed together, the explosion range is extended to 2.5-81% (acetylene rich) or 2.7-36% (ethylene rich). When the mixing ratio deviation exceeds ±5%, the deflagration risk probability increases to 2.8 times that of pure gas. According to OSHA Accident Statistics in 2021, acetylene ethylene accidents accounted for 31% of industrial gas accidents, 68% of which were caused by decomposition explosions due to uncontrolled pressure (i.e., tank pressure >15 psi) or high temperature (>30°C).
Risk of increased decomposition reaction characteristics: Acetylene itself decomposes at 1.5 bar pressure, and the decomposition rate increases exponentially with temperature – 0.01%/min at 30°C and 12%/min at 100°C, with heat release being 58 kJ/mol. 2020 Yeosu Chemical plant explosion in South Korea, due to poor valve material (copper content <65%), acetylene ethylene mixed gas flow rate >8 m/s, friction-electrostatic accumulation up to 45 mJ (30 mJ ignition threshold), resulting in a chain explosion, direct loss of $23 million. Although ethylene itself is very stable, its lower ignition energy in mixture with air is as low as 0.07 mJ (acetylene is 0.02 mJ), and it is more readily ignited by small sparks on leakage.
It requires special design for storage and transport: acetylene ethylene mixture must be dissolved in acetone (98% or more concentration) and stored in cylinders containing porous materials such as calcium silicate (92% or more porosity). Acetone possesses a solvent capacity for acetylene of 27 L/L (ethylene 9 L/L), inhibiting the decomposition reaction. ASME code requires ≥6.35 mm (usual gas tank is 4.5 mm) tank wall thickness and should be equipped with bursting disc (burst pressure is 1.3 times design pressure). In 2022, the EU REACH regulation update requires mixed gas cylinders to be labeled with UV blocking coatings (light transmission <0.5%) to prevent free radical chain reactions triggered by light, and such measures have reduced the decomposition accident rate by 89%.
Detection and monitoring technology is the most important: Infrared spectroscopy (IR) detectors must be sensitive to ≤1 ppm (acetylene) and ≤5 ppm (ethylene) with a response time of <2 seconds. The German BASF plant adjusts the mixing ratio in real time through PID control algorithm (precision ±0.5%), compressing the concentration fluctuation from ±5% to ±0.8%, and reduces the risk of leakage by 94%. The 2023 ISO 10156 revision makes it obligatory for cylinders to be equipped with NFC chips, 30 parameters such as storage pressure, mixing ratio, and filling date (error ±0.05%), and reduces the emergency response time by 40%.
Economic balance and compliance expenses: The capital outlay of acetylene ethylene safe processing systems is 220% higher than that of conventional gases (i.e., 120,000 yuan per unit of explosion-proof ventilation systems), but can reduce annual accident losses by 450,000 yuan. Air Products’ inerting system (nitrogen purity ≥99.999%) pushes out leaking gas at 0.5 m³/min, which results in an oxygen concentration of <10% and 35% less maintenance than conventional solutions. China 2022 chemical plant safety transformation data show that the accident rate of compliant enterprises dropped 76%, insurance costs decreased 42%, and the return on investment time is 2.3 years.
These facts illustrate that the special treatment requirements of acetylene ethylene are an unavoidable choice to manage its molecular instability, wide explosion range and decomposition risk, and that a balance of safety and usefulness can be achieved only through a combination of materials science, intelligent monitoring and stringent regulation.