Courtesy Penn State Extension Marcellus Education Team
We see it all around, in everything from food packaging, medical equipment, furniture, and vehicles, to toys, computers, and clothing. But most people don't realize that natural gas is where a lot of plastic production starts.
The first stop in the processing of plastic from natural gas is the cracker plant. Crackers turn either naphtha, a crude oil-based product, or ethane, a natural gas liquid, into ethylene, a starting point for a variety of chemical products. In parts of the wet gas-rich Marcellus and Utica shales, ethane is significantly cheaper than naphtha, and crackers using the natural gas liquid have a significant advantage. "Feedstocks make up anywhere from 60 to 70 percent of the cost to manufacture petrochemicals" according to the American Fuel and Petrochemicals Manufacturers.
Ethane is formed the same way other hydrocarbons (e.g. oil and gas) are generated. Hundreds of millions of years ago, organic material such as plankton fell to the bottom of a seabed. Over time, it was trapped in sediment in an anoxic environment (lacking oxygen to break these organic materials down completely). Pressure and temperature converted these materials into hydrocarbons.
These hydrocarbon-bearing formations matured at different rates, even within the same formation, depending on temperature, time, and pressure. Within a formation, one area may produce oil, another area 'wet' natural gas (natural gas mixed with natural gas liquids), and yet another area only 'dry' gas (almost pure methane).
Natural Gas Liquids (NGLs) include ethane, propane, butane, isobutene, and pentane. They also include a small amount of heavier hydrocarbons, such as hexane, heptane, and octane. Ethane is a major component of NGLs, especially in the Marcellus, Utica and Eagle Ford formations. While all of these NGLs can be cracked and used to produce petrochemicals, ethane is often the least expensive to use to create ethylene in places like the Appalachian Basin and the Gulf Coast.
Ethane, like all NGLs, is a liquid underground, but becomes a gas under standard surface pressures and temperatures. Ethane is separated from the gas stream in a processing facility where different pressures and temperatures are applied to draw off each of the gases separately. De-ethanization occurs when the boiling point for only ethane is reached, turning it into a gas.
Gases are captured from the top of stack while liquids remain at the bottom.
Purity ethane (at least 90% ethane, but usually higher) then travels in a pipeline to its destination, an ethane cracker plant. At the cracker plant, which has access to a large energy source, ethane is heated to about 1500 degrees Fahrenheit. This process is called cracking, because heat energy is used to break apart or crack molecules to form new molecules. At that temperature ethane (C2H6) molecules lose two hydrogen molecules, which split off to form a separate, stable hydrogen molecule (H2), leaving molecules which are about 80 percent ethylene (C2H4).
The ethylene formed in the cracking process is next transported by pipeline to another facility to be converted to usable products, the most common of which is polyethylene. Ethylene is at this point still a gas and needs pressure and a catalyst to turn it into polyethylene, a resin. The process by which polyethylene is made from ethylene is known as polymerization.
The term "plastic" suggests one material, but there are actually hundreds of different plastic polymers. Polymerization occurs when a chemical reaction causes molecules to react together to form polymer chains. These polymer chains can be engineered to control the specific physical properties of the resulting plastic resin, thus allowing the product to be designed for many different uses. For example, some plastic products may require extra strength, some require maximum flexibility, and others need to be resistant to solvents. All of these requirements can be accounted for by the polymers used in the process.
Polyethylenes may be labeled as low density or high density polyethylenes (LDPE or HDPE), or other designations that can be seen at the bottom of household containers.
HDPE 2 Container
Polyethylene resin can be transported by truck, barge, or train to a manufacturing facility to make end products. Cracker plants, therefore, usually have access to a lot of transportation options and warehouse facilities to store and ship resins and liquid products.
Ethane is a valuable NGL, which is an important and economical feedstock for plastics. While ethane is produced in numerous shale regions, it has major value nationally and worldwide in its use.