Butane is an alkane hydrocarbon with the chemical formula \(text{C}_4text{H}_{10}\). It is a colorless and highly flammable gas at standard room temperature and pressure, but it is easily converted into a liquid under moderate pressure or cooling. This characteristic of being readily liquefiable makes it a versatile and efficient energy source for portable applications. Butane is not manufactured synthetically but is instead extracted as one component from the complex mixtures found in fossil fuels, specifically natural gas and crude oil.
Natural Occurrence
Butane is a naturally occurring component found within the two primary sources of fossil fuels: natural gas and crude oil. In raw natural gas, butane exists alongside lighter hydrocarbons like methane (the main component) and ethane, and heavier ones such as propane and pentane. Butane is considered one of the Natural Gas Liquids (NGLs).
In a typical gas stream, butane is a minor constituent, requiring dedicated industrial processes to isolate it from the much larger volume of methane. Butane is also dissolved within crude oil and is recovered during the initial stages of petroleum refining. The butane separated from these two sources comes in two structural forms, normal butane (\(text{n-butane}\)) and isobutane (\(text{i-butane}\)), which have the same chemical formula but different atomic arrangements.
Separating Butane From Raw Materials
The process of obtaining pure butane involves isolating it from the other hydrocarbons in the raw natural gas liquids stream or the crude oil refinery stream. This physical separation is achieved primarily through fractional distillation, which exploits the differences in the boiling points of the various hydrocarbons. The mixture of Natural Gas Liquids, which includes ethane, propane, butanes, and natural gasoline, is fed into a series of tall distillation towers known as a fractionation train.
In these industrial columns, the liquid mixture is heated, causing the components to vaporize and then condense at different points within the tower, depending on their boiling temperatures. Butane has a boiling point around 31 degrees Fahrenheit (\(text{-0.5}\) degrees Celsius), which is higher than the lighter gases like ethane and propane, but lower than the heavier natural gasoline components. This distinct boiling point allows it to be separated from the lighter gases that rise to the top of the column and the heavier liquids that remain at the bottom.
The fractionation process involves a sequential separation in a series of columns. Separation often starts with a de-ethanizer to remove ethane and methane, followed by a de-propanizer to remove propane. The remaining stream, enriched with butanes and heavier components, is then sent to a de-butanizer column. In this final stage, the butanes (both normal and iso-butane) are collected overhead, separating them from the heavier natural gasoline.
Converting Butane to Isobutane
Once normal butane (\(text{n-butane}\)) has been separated, it is often chemically modified into isobutane (\(text{i-butane}\)), which is a more valuable isomer for certain applications. Both normal butane and isobutane share the same four carbon and ten hydrogen atoms. However, \(text{n-butane}\) has a straight chain structure, while \(text{i-butane}\) features a branched structure. This difference in structure gives isobutane a lower boiling point, which affects its performance in various industrial processes.
The conversion from \(text{n-butane}\) to \(text{i-butane}\) is accomplished through a refinery process known as isomerization. In an isomerization unit, the \(text{n-butane}\) feed is mixed with hydrogen and passed over a specialized catalyst, often containing platinum or a combination of aluminum chloride and hydrogen chloride. The reaction takes place under moderate temperatures, typically below 482 degrees Fahrenheit (250 degrees Celsius), and pressure.
The catalyst facilitates the rearrangement of the carbon skeleton, converting the \(text{n-butane}\) molecules into the branched \(text{i-butane}\) form without changing the overall chemical formula. This conversion is a deliberate step taken to increase the supply of isobutane, as the demand for it in high-octane fuel production often exceeds the amount available in the raw feedstocks. The resulting mixture is then typically sent to a deisobutanizer to separate the new isobutane product from any unconverted \(text{n-butane}\).
Common Commercial Uses
Butane finds widespread use in both its normal and isomerized forms across various sectors. Normal butane (\(text{n-butane}\)) is a common fuel source, notably used in portable lighters, camping stoves, and as a component in Liquefied Petroleum Gas (LPG) mixtures. It is also blended directly into gasoline during the cooler winter months to increase the fuel’s volatility and ensure easier engine starting.
Beyond its use as a fuel, \(text{n-butane}\) serves as a feedstock in the petrochemical industry, where it is used to manufacture ethylene and butadiene, which are precursors for synthetic rubber and plastics. Isobutane (\(text{i-butane}\)) is highly valued in petroleum refining because it is a precursor for alkylate, a high-octane gasoline component that improves engine performance. It is also employed as a refrigerant (R-600a) in domestic cooling systems, and as a propellant in aerosol spray products.