Acetylene Decomposition - Deflagration

Applied to modem acetylene technology, this results in the following safety principle  

Acetylene imder pressure is only permitted to be enclosed in cavities of hmited volume that can be sealed off from each other or in pipes of limited nominal diameters and limited maximum allowable pressure. The mechanical stability of the casing of these cavities and pipes has to be great enough to safely withstand the possible consequences of decomposition under operating conditions. 

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Fabiano et al. (1999) describe an explosion in the loading section of an Italian acetylene production plant in which the installed flame arresters did not stop a detonation. The arresters were deflagration type and the arrester elements were vessels packed with silica gel and aluminum plates (Fabiano 1999). It was concluded that the flame arresters used were not suitable for dealing safely with the excess pressures resulting from an acetylene decomposition, and may not have been in the proper location to stop the detonation. 

The pressure developed by decomposition of acetylene in a closed container depends not only on the initial pressure (or more precisely, density), but also on whether the flame propagates as a deflagration or a detonation, and on the length of the container. For acetylene at room temperature and pressure, the calculated explosion pressure ratio, / initial > deflagration and ca 20 for detonation (at the Chapman-Jouguet plane). At 800 kPa (7.93... 

Decomposition Flame Arresters Above certain minimum pipe diameters, temperatures, and pressures, some gases may propagate decomposition flames in the absence of oxidant. Special in-line arresters have been developed (Fig. 26-27). Both deflagration and detonation flames of acetylene have been arrested by hydrauhc valve arresters, packed beds (which can be additionally water-wetted), and arrays of parallel sintered metal elements. Information on hydraulic and packed-bed arresters can be found in the Compressed Gas Association Pamphlet G1.3, Acetylene Transmission for Chemical Synthesis. Special arresters have also been used for ethylene in 1000- to 1500-psi transmission lines and for ethylene oxide in process units. Since ethylene is not known to detonate in the absence of oxidant, these arresters were designed for in-line deflagration application. 

Other than the large potential energy, most of the hazards of SCFs are related to the chemical reactivity of the gas itself. Several SCFs, such as scHaO, scHCl and the other acids, corrode standard stainless steel reaction equipment, which could result in catastrophic failure. Explosive deflagration or decomposition is common with acetylene even at subcritical pressures [55]. Perfluoroethylene (SCC2F4) will explode at pressures above 2.7 bar unless inhibitor is added [56]. Even seemingly stable SCFs can explosively decompose at sufficiently high temperatures (e.g. 500 °C for hexane) [57]. Runaway polymerization can be a concern when polymerizable SCFs like SCC2H4 are used [58]. The... 

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