Natural Gas Liquids
The natural gas we use in our homes is mostly methane, the simplest form of hydrocarbon. But natural gas at the well may include other hydrocarbons, such as ethane, propane, butane and pentane. Each of these so-called "natural gas liquids" - or NGLs - has its own unique properties that make it suited to a specific use: For example, butane is used in lighters, while propane is used in backyard grills and home heating systems. Processing facilities remove NGLs so they can be recovered and used separately.
Most NGLs can be removed using an absorption process that passes the mixed gases through a pool of gas-absorbing oil that catches the heavier hydrocarbons but allows the methane to move through. However, light hydrocarbons such as ethane are tougher to recover. Small amounts of these light hydrocarbons can be left in natural gas with no ill effects. If larger quantities are present, however, they can be removed using a process called cryogenic expansion.
A "cryogenic" process is one that takes place at very low temperatures. In cryogenic expansion, a powerful fan blows cooled gas from a narrow pipe into an expansion chamber. This reduces the vapor pressure of the gas, which has the effect of further reducing the gas temperature to approximately -120F. At this temperature, methane remains a gas while other hydrocarbons become liquid and can be removed. The collected NGLs are then separated from one another using another temperature-based process. As discussed in the "Refining", each type of hydrocarbon has a unique boiling point - the temperature at which it converts from liquid to vapor. Recovered NGLs pass through a series of progressively warmer chambers, each heated to a temperature that will vaporize a specific hydrocarbon from the remaining liquid.»next