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 Production of 1,3-Butadiene from C4 Alkanes and Alkenes
  • Production of 1,3-Butadiene from C4 Alkanes and Alkenes
  • Butane and butene mixtures from natural gas and refinery waste gases are feedstocks for pure dehydrogenation or for dehydrogenation in the presence of oxygen. Historically they were particularly available in the USA, and for this reason, industrial processes were developed almost exclusively there. Today, butane/ butene dehydrogenation has lost much of its former importance due to high costs. The last Petro-Tex dehydrogenation plant in the USA is currently idle, though several plants are still operating in the CIS. In Japan, the last Houdry dehydrogenation plant was shut down in 1967 after eight years of operation.

    By 1985, worldwide dehydrogenation of n-butane and n-butenes accounted for less than 3% of the total butadiene volume.

    Dehydrogenations of n-butane and n-butenes are endothermic processes requiring large amounts of energy:


    Relatively high temperatures (600-700 °C) are necessary to achieve economical conversions. To attain the same rate of reaction, n-butane requires a temperature roughly 130 °C higher than the n-butenes. At these temperatures, side reactions such as cracking and secondary reactions involving the unsaturated compounds become important. Therefore, a short residence time and a selective catalyst must be used.

    Dehydrogenation involves an increase in the number of moles of gas, and so is favored by an addition of steam as in the case of steam cracking. The lowering of the partial pressure of the hydrocarbons achieved in this way decreases coke deposition, isomerization and polymerization.

    If catalysts unstable in the presence of steam are employed, the dehydrogenation process is usually operated under reduced pressure. This will shift the equilibrium of dehydrogenation towards butadiene.

    The Houdry single-step process for the dehydrogenation of butane (Catadiene process from ABB Lummus CRSt used in 20 plants in 1993) is one of the most important processes commercially, and also one of the oldest. It is also the basis for the Catofin alkane dehydrogenation process. A H2O-sensitive Cr-, Al-oxide catalyst is introduced at 600-620°C and 0.2-0.4 bar. The catalyst must be regenerated after a few minutes by injecting air to burn off the coke layer. With a butane conversion of 30-40%, butadiene yields of up to 63% can be reached. Between the dehydrogenation and regeneration periods, the catalyst is evacuated to remove the reaction mixture. The three operations take place in cycle in separate reactors which are part of a single unit. The high energy requirement for the dehydrogenation is met by the oxidation of the coke layer on the catalyst (adiabatic method).
     
    The Dow process is a butene dehydrogenation method which takes place with the addition of steam. It operates at 600-675 °C and 1 bar over a Ca-Ni-phosphate catalyst stabilized with Cr2O3. The heat of dehydrogenation is provided by the addition of superheated steam (H2O: butene ratio of 20: 1) to the reaction, analogous to the dehydrogenation of ethylbenzene to form styrene. The conversion of butene is about 50%, with a selectivity to butadiene of about 90%. After a reaction period of 15 minutes, the catalyst must be regenerated for 11 minutes. In practice, parallel reactors, alternately regenerated and utilized for hydrogenation, are employed. The butadiene is isolated from the reaction mixture by extractive distillation. Similar processes have been developed by Shell using a Fe- Cr-oxide catalyst with K2O additive, and by Phillips Petroleum with a Fe-oxide-bauxite catalyst.

    Besides the dehydrogenation of C4 hydrocarbons to butadiene, another dehydrogenation method (in the presence of oxygen) has gained in importance. In this process, known as oxidative dehydrogenation, the dehydrogenation equilibrium between butenes and butadiene is displaced by the addition of oxygen towards greater formation of butadiene. The oxygen not only removes H2 by combustion, but also initiates dehydrogenation by abstracting hydrogen from the allyl position. At these high temperatures (up to 600°C), oxygen also acts to oxidatively regenerate the catalyst.

    In industrial operation, a sufficient quantity of oxygen (as air) is introduced so that the heat supplied by the exothermic water formation roughly equals the heat required for the endothermic dehydrogenation. In this way the butene conversion, the selectivity to butadiene, and the lifetime of the catalyst can be improved. By using an excess of air, the maximum temperature can be controlled by addition of steam. Mixed oxide catalysts based on Bi/Mo or Sn/Sb are most often used.

    The Phillips O-X-D process (oxidative dehydrogenation) for the manufacture of butadiene from n-butenes is an example of an industrially operated dehydrogenation process. n-Butenes, steam and air are reacted at 480-600°C on a fixed-bed catalyst of unrevealed composition. With butene conversions between 75 and 80%, the butadiene selectivity reaches roughly 88-92%. This process was used by Phillips until 1976.

    Petro-Tex also developed a process for the oxidative dehydrogenation of butenes (oxo-D process) that was first used in the USA in 1965. The conversion with oxygen or air is performed at 550-600 °C over a heterogeneous catalyst (probably a ferrite with Zn, Mn or Mg). By adding steam to control the selectivity, a selectivity to butadiene of up to 93% (based on n-butenes) can be reached with a conversion of 65%.

    A new oxidative dehydrogenation process for butane/butene has been developed and piloted by Nippon Zeon. Details of the catalyst composition and process conditions have not yet been disclosed.

    Another method for removing H2 from the dehydrogenation equilibrium involves reacting it with halogens to form a hydrogen halide, from which the halogen is later recovered by oxidation. For a time, Shell employed iodine as the hydrogen acceptor in the Idas process (France) for the dehydrogenation of butane to butadiene.


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