Obtaining ethane from the chemical ethylene involves a chemical reaction which takes place inside a long, heated tube. This tube is called a thermal cracking unit which is a type of plug flow reactor. A picture of a plug flow reactor is shown below.

The large tube is a furnace which heats the ethane as is flows through the tube. The heat added to the chemicals in the tube allow the chemical reaction to happen. As you can see in the picture, ethane is the molecule entering the plug flow reactor, and after a series of chemical reactions, ethylene appears as it exits the tube. The C stands for carbon and the H stands for hydrogen. The lines connecting the C's and H's together show that a chemical bond exists between the two atoms.

A pictures of the entire plant can be seen below. Within this plant is where the plug flow reactor lies and the chemical reactions occur.