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 Production of 1,3-Butadiene from C4 Cracking Fractions
  • Production of 1,3-Butadiene from C4 Cracking Fractions
  • C4 fractions with an economically isolable Butadiene content are available in countries where ethylene is manufactured by steam cracking of naphtha or higher Petroleum fractions. The C4 fraction amounts to about 9 wt% of the cracked product from conventional high-severity cracking of naphtha. The C4 fraction contains 45-50 wt% of butadiene.

    Western Europe and Japan are the main areas utilizing this raw material base for butadiene. In the USA, the usual cracking of natural and refinery gases supplies only very small amounts of butadiene compared to naphtha or gas oil cracking. The butadiene content obtained by cracking various feedstocks in ethylene plants at different cracking severities is summarized in the following table:

    However, naphtha and gas oil cracking has increased in importance in the USA. In 1976, 36% of the total butadiene capacity was based on isolation from these cracked products. During the 1990s, this has remained stable at about 45-50%. In Western Europe, the opposite is occuring; i.e., a shortage of butadiene is being caused by the increasing use of natural gas in cracking processes to manufacture olefins.

    Separation of butadiene from a mixture of C4 hydrocarbons is not possible by simple distillation as all components boil within a very close temperature range, and some form azeotropic mixtures. Consequently, two isolation processes based on a chemical and a physical separation have been developed:
    1. The older, chemical separation process exploits the formation of a complex between butadiene and cuprous ammonium acetate, [Cu(NH3)2]OAc. This process was developed by Exxon as an extraction procedure for processing C4 fractions with low butadiene content. These fractions can contain only small amounts of acetylenes, or the extraction process will be disturbed by foam formation. Another disadvantage is the relatively involved regeneration of the extractant.
    2. All modern processes for butadiene isolation are based on the physical principle of extractive distillation. If selective organic solvents are added, the volatility of particular components of a mixture is lowered (in this case butadiene). They then remain with the solvent at the bottom of the distillation column, while the other impurities, previously inseparable by distillation, can be removed overhead.

    Acetone, furfurol, acetonitrile, dimethylacetamide, dimethyl-formamide and N-methylpyrrolidone are the principal solvents employed in this extractive distillation.

    Extractive distillations are particularly suitable for the presently available butadiene-rich C4 cracking fractions with a relatively high share of alkynes such as methyl-, ethyl- and vinylacetylene, as well as methylallene (1,2-butadiene). In modern processes with solvents such as dimethylformamide (Nippon Zeon, VEB Leuna), dimethylacetamide (UCC), or N-methylpyrrolidone (BASF, ABB Lummus Crest), alkyne separation is a stage in the operation of the process. In the older processes developed and operated in the USA with solvents such as acetone, furfurol (Phillips Petroleum) or acetonitrile (Shell, UOP, Arco), removal of C4 alkynes, for example by partial hydrogenation, prior to distillation was essential to avoid problems arising from resin formation.

    The basic principle of solvent extraction of butadiene from a C4 cracking fraction can be described as follows. The vaporized C4 fraction is introduced at the bottom of an extraction column. The solvent (e. g., dimethylformamide or N-methylpyrrolidone) flows through the gas mixture from the top, and on the way down becomes charged with the more readily soluble butadiene and small amounts of butenes. Pure butadiene is introduced at the bottom of the extraction column to drive out as much of the butenes as possible. The butenes leave the separating column overhead. In another column, the degasser, the butadiene is freed from solvents by boiling and is subsequently purified by distillation. In the BASF N-methylpyrrolidone process, butadiene is obtained in approximately 99.8% purity. The butadiene yield is 96% relative to the original butadiene content in the C4 cracking fraction.

    The BASF process for the extraction of butadiene with N-methyl-pyrrolidone (NMP) was first operated commercially in 1968 by EC-Dormagen. By 1990, there were 24 production facilities using the BASF process in operation or under construction worldwide with a combined capacity of 2.25×106 tonnes per year.

    This is one of the leading butadiene extraction processes, exceeded only by the Nippon Zeon GPB process (Geon Process Butadiene) which was still used in more than 30 plants in 1985.

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