BUTANES (C₄H₁₀)

Like propane, butanes are obtained from natural gas liquids and from refinery gas streams. The C₄ acyclic paraffin consists of two isomers: nbutane and isobutane (2-methylpropane). The physical as well as the chemical properties of the two isomers are quite different due to structural differences.

   For example, the vapor pressure (Reid method) for n-butane is 52 lb/in.2, while it is 71 lb/in.2 for isobutane. This makes the former a more favorable gasoline additive to adjust its vapor pressure. However, this use is declining in the United States due to new regulations that reduce the volatility of gasolines to 9 psi, primarily by removing butane. Isobutane, on the other hand, is a much more reactive compound due to the presence of a tertiary hydrogen.

  Butane is primarily used as a fuel gas within the LPG mixture. Like ethane and propane, the main chemical use of butane is as feedstock for steam cracking units for olefin production. Dehydrogenation of n-butane to butenes and to butadiene is an important route for the production of synthetic rubber.  Due to its higher reactivity, isobutane is an alkylating agent of light olefins for the production of alkylates. Alkylates are a mixture of branched hydrocarbons in the gasoline range having high octane ratings.

    Dehydrogenation of isobutane produces isobutene, which is a reactant for the synthesis of methyl tertiary butyl ether (MTBE). This compound is currently in high demand for preparing unleaded gasoline due to its high octane rating and clean burning properties.

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