Natural gas flows underground through porous rock and is mined and brought to the surface for processing. Uses of natural gas are for fuel and cooking gas, however; the hydrocarbons which comprise natural gas may be separated and used for other purposes. In the first part of this story, we will see how ethane, which is used to make polyethylene, can be separated from natural gas.

Some sources of natural gas are found underground on the tops of oil beds. While underground, natural gas and oil are often at high temperature and extremely high pressure, for instances, a reservoir found in Louisiana contained a volatile oil at a temperature of 246 degrees F and a pressure of 4836 psia. A breakdown of the composition of the reservoir oil is as follows: methane (60%); ethane (8%); propane (5%); butanes (4%); pentanes (3%); hexanes (1%); heptanes and heavier (15%); nitrogen (2%); carbon dioxide (2%). At this high pressure and temperature, the reservoir oil is at its bubble point. In order to separate ethane from the volatile oil, the oil must first be separated into light and heavy hydrocarbons. Light hydrocarbons or C1-C4 constitute natural gas and exist in the gas phase at room temperature and ambient pressure. The heavier hydrocarbons, C5-C24, used for solvents for organic compounds, gasoline, and lubricating oil are mostly in the liquid phase at room temperature and ambient pressure. After the light and heavy hydrocarbons are separated, ethane may then be obtained from the natural gas stream.

One method to separate the heavy and light hydrocarbons in the volatile oil is to isothermally flash the oil at a lower temperature and pressure once it reaches the surface of the earth. This method produces a vapor stream of light hydrocarbons and a liquid stream of mostly heavy hydrocarbons with some residual light hydrocarbons also. The liquid stream is then isothermally flashed again at atmospheric pressure in order to obtain a vapor stream of the residual light hydrocarbons.

Once the light hydrocarbons have been separated from the heavier hydrocarbons and are in the vapor phase, ethane may then be separated from the vapor stream. To do this, the two vapor streams from the two flashes are sent to a distillation column that separates the vapor streams into components. In this way, ethane is removed from the natural gas stream.

For this example, a chemical engineer's job is to separate the highest percentage of ethane from the volatile oil using the most cost effective methods. The two processes, Isothermal flash and Distillation, are analytically solved and represent typical problems that chemical engineers solve to find the best and most cost effective solution available.