Pyrolysis from acetylene, butadiene

Production of Coke and Other Pyrolysis Products From Acetylene, Butadiene, and Benzene in Various Tubular Reactors... 

The pattern of commercial production of 1,3-butadiene parallels the overall development of the petrochemical industry. Since its discovery via pyrolysis of various organic materials, butadiene has been manufactured from acetylene as weU as ethanol, both via butanediols (1,3- and 1,4-) as intermediates (see Acetylene-DERIVED chemicals). On a global basis, the importance of these processes has decreased substantially because of the increasing production of butadiene from petroleum sources. China and India stiU convert ethanol to butadiene using the two-step process while Poland and the former USSR use a one-step process (229,230). In the past butadiene also was produced by the dehydrogenation of / -butane and oxydehydrogenation of / -butenes. However, butadiene is now primarily produced as a by-product in the steam cracking of hydrocarbon streams to produce ethylene. Except under market dislocation situations, butadiene is almost exclusively manufactured by this process in the United States, Western Europe, and Japan. 

Thermal reactions of acetylene, butadiene, and benzene result in the production of coke, liquid products, and various gaseous products at temperatures varying from 4500 to 800°C. The relative ratios of these products and the conversions of the feed hydrocarbon were significantly affected in many cases by the materials of construction and by the past history of the tubular reactor used. Higher conversions of acetylene and benzene occurred in the Incoloy 800 reactor than in either the aluminized Incoloy 800 or the Vycor glass reactor. Butadiene conversions were similar in all reactors. The coke that formed on Incoloy 800 from acetylene catalyzed additional coke formation. Methods are suggested for decreasing the rates of coke production in commercial pyrolysis furnaces. 

Novel industrial applications may evolve from continuous wave (CW) CO 2 laser-driven homogeneous pyrolysis of hydrocarbons (e.g. C4Hlo n-heptane % which seems to be advantageous over conventional pyrolysis methods For example, the CW CO2 laser SF -sensitized decomposition of cyclohexane in the gas phase yields mainly ethene and 1,3-butadiene (equation 7). The product distribution is almost invariable with the extent of conversion and different from conventional pyrolysis No acetylene is formed... 

By-products from EDC pyrolysis typically include acetjiene, ethylene, methyl chloride, ethyl chloride, 1,3-butadiene, vinylacetylene, benzene, chloroprene, vinyUdene chloride, 1,1-dichloroethane, chloroform, carbon tetrachloride, 1,1,1-trichloroethane [71-55-6] and other chlorinated hydrocarbons (78). Most of these impurities remain with the unconverted EDC, and are subsequendy removed in EDC purification as light and heavy ends. The lightest compounds, ethylene and acetylene, are taken off with the HCl and end up in the oxychlorination reactor feed. The acetylene can be selectively hydrogenated to ethylene. The compounds that have boiling points near that of vinyl chloride, ie, methyl chloride and 1,3-butadiene, will codistiU with the vinyl chloride product. Chlorine or carbon tetrachloride addition to the pyrolysis reactor feed has been used to suppress methyl chloride formation, whereas 1,3-butadiene, which interferes with PVC polymerization, can be removed by treatment with chlorine or HCl, or by selective hydrogenation. 

Pyrolysis of poly(1,4-butylene terephthalate) is in some respects similar to that of poly(ethylene terephthalate) and generates compounds such as benzoic acid, terephthalic acid dibutylene ester, probably terephthalic acid, etc. Also the formation of CO, CO2, benzene, biphenyl, etc. is similar. On the other hand, the percentage distribution of different compounds is rather different for poly(1,4-butylene terephthalate). A relatively high level of butadiene is generated from this polymer, white in the case of PET, formation of acetylene would not be thermodynamically possible and acetaldehyde is formed. Also, the esters of terephthalic acid with C4 alcohols are at considerably higher levels in the pyrolysate of PBT, which seems to indicate that they are more stable than the corresponding esters with C2 alcohols in PET pyrolysate. 

Fig. 3.8. Laser pyrogram of phenanthrene. 1 = Methane, ethylene, acetylene (main compounds in the light products), butadiene 2 = benzene 3 = naphthalene 4 = methylnaphthalene 5 = dimethyl-naphthalene 6 = phenanthrene (initial product). The laser pyrolysis products were separated on a column containing Apiezon L. From ref. 86.

The 1,3-butadienyl radical is primarily a by-product of butadiene pyrolysis in this system but results from vinyl addition to acetylene in flames of other aliphatic fuels. In aromatic flames 1,3-butadienyl may be produced by oxidative and pyrolytic decomposition of aromatic species, as suggested in a study of benzene flames (10) ... 

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