Acetylene decomposition

When acetylene is recovered, absorption—desorption towers are used. In the first tower, acetylene is absorbed in acetone, dimethylformarnide, or methylpyroUidinone (66,67). In the second tower, absorbed ethylene and ethane are rejected. In the third tower, acetylene is desorbed. Since acetylene decomposition can result at certain conditions of temperature, pressure, and composition, for safety reasons, the design of this unit is critical. The handling of pure acetylene streams requires specific design considerations such as the use of flame arrestors. 

FIG. 26-27 Some special arrester designs (a) liquid seal arrester (h) Linde hydraulic seal arrester (c) wetted packed-bed acetylene decomposition arrester. (Howai d, 19S2.)... 

Fig. 1. Carbon filaments grown after acetylene decomposition at 973 K for 5 hours on (a) Co(2.5%)-graphite (b) Fe-graphite (c) Ni-graphite (d) Cu graphite.

Fig. 3. Carbon species obtained after acetylene decomposition for 5 hours at 973 K on the surface of silica-supported catalysts made by pore impregnation (a) Co-SiOj-l (b) Co-Si02-2.

Fig. 5. Acetylene decomposition on Co-HY (973 K, 30 minutes) (a) encapsulated metal particle (b) carbon filaments (A) and tubules of small diameters (B) on the surface of the catalyst.

Fig. 8. Surface carbon species produced in acetylene decomposition on Co-Si02 at 973 K after different reaction times (a) 3 minutes (b) 5 minutes (c) 20 minutes.

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. 

Sutherland, M. E., andWegert, H. W. 1973. An Acetylene Decomposition Incident. Chemical Engineering Progress, 69(4), 48-51. 

Acetylene may propagate decomposition flames in the absence of any oxidant above certain minimum conditions of pressure, temperature, and pipe diameter. Acetylene, unlike most other gases, can decompose in a detonative manner. Among the different types of flame arresters that have proven successful in stopping acetylene decomposition flames are hydraulic (liquid seal) flame arresters, packed beds, sintered metal, and metallic balls (metal shot). 

Sutherland and Wegert (1972) describe the successful use of the Linde hydraulic valve arrester in stopping an acetylene decomposition detonation. As previously noted, these flame arresters are no longer being made by Linde (now Praxair Inc,), but are still available from ESAB Welding Sc Cutting Products of Florence, SC. 

Schmidt, H. 1971. Protective Measures and Experiences m Acetylene Decomposition m Piping and Equipment. IChemE Symposium Series No. 34, pp. 165ff. Institution of Chemical Engineers, Rugby, England. 

Acetylene is widely sold as the fuel for welding torches, and it is stored in large cylinders at high pressures in many welding shops. In fact, this acetylene is mixed with acetone, which has been found to be an effective scavenger of acetylene decomposition, so that these tanks are relatively safe. 

Among the 9 million tons of carbon black which are produced globally per year, only a small fraction of very specific, high-purity conductive carbon blacks can be used as conductive additive in lithium-ion batteries. A traditional conductive carbon is acetylene black, a special form of a thermal black produced by the thermal decomposition of hydrocarbon feedstock.74-75 The particularity of acetylene black to other thermal carbon black production is that the starting hydrocarbon, acetylene, exothermally decomposes above 800°C.75-77 Once the reaction is started, the acetylene decomposition autogenously provides the energy required for the cracking of acetylene to carbon followed by the synthesis of the carbon black ... 

The carbon black is produced by two ways the partial combustion of different hydrocarbons and the exothermic reaction of acetylene decomposition. About 10 million of tons are produced annually in the world, from which 99% for the tire industry. 

Figure 1 indicates an example of how pretreatment of the Incoloy 800 reactor had a very large effect on the acetylene conversion (or on the kinetics of acetylene decomposition). The coke-coated Incoloy 800 reactor (the coke had been deposited on this reactor when butadiene reacted at 500°-700°C.) used in Run 14 resulted in much lower acetylene conversions in the range of 450° to 550°C compared with the same Incoloy 800 reactor after the coke had been burned off with oxygen and after the reactor had been contacted with hydrogen until nearly all surface oxides were eliminated. Most conversion results for the 11 gas samples collected during Run 15 are shown in Figure 1. Gas Samples 1 through 3 at 350°, 400°, and 450°C, respectively, indicated almost no acetylene conversions. A small amount of carbon dioxide was produced at 450°C, indicating some metal oxides had still been present on the surface after the hydrogen pretreatment. The temperature was then increased to 500°C, and the conversions then increased from 66% to 99% during the first 23 min (Samples 4 and 5). Some carbon oxide production was noted in Sample 4 but none in Sample 5 or in any later samples of the run presumably...

Various forms of radiation have been used to produce ions in sufficient quantitites to yield neutral products for subsequent analysis. In principle, it should be possible to use intense beams of UV below ionization threshold for this purpose. To date, however, efforts to collect neutrals from resonant multiphoton ionization (REMPI) have not succeeded. In one experiment, 1 mbar of gaseous -propyl phenyl ether was irradiated at room temperature with a 0.1 W beam of 266 nm ultraviolet (from an 800 Hz laser that gives 8 n pulses) concurrent with a 0.5 W beam at 532 nm. The beams were intense enough not only to ionize the ether in the mass spectrometer, but also to excite it so that it expels propene. After several hours of irradiation < 10% of the starting material remained. Production of carbon monoxide and acetylene (decomposition products of the phenoxy group) could be detected by infrared absorption spectroscopy, but the yield of neutral propene (as measured by NMR spectroscopy) was infinitesimal. 

Because of the large differences in results with hydrogen and argon, other quench gases—helium, nitrogen, and deuterium—were subsequently tested in an attempt to separate chemical effects from physical ones. For example, if gas diffusivity and thermal conductivity are important parameters in preserving acetylene as an intact species, acetylene decomposition in hydrogen, helium, and deuterium would be essentially... 

The acetylene process is a particular form of the thermal process because acetylene thermally decomposes at about 800°C in an exothermic reaction. Once the reaction is started, the acetylene decomposition reaction autogenously provides the energy required for the cracking of acetylene to carbon followed by the synthesis of the carbon black ... 

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