Gases are captured from top of stack wile liquids remain at the bottom
Rarely a day goes by without coming into contact with items made of plastic, a synthetic material made from a wide range of organic similarly bonded units, or polymers. 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.
Next week, the process of making ethylene and polymerization. How Plastic is Made from Natural Gas - Part 2
Dan Brockett, Extension Educator, Penn State Extension