The building blocks of the petrochemical world often take a back seat to the stars of the energy and manufacturing world. Let's give these unsung heroes a closer look!
Much less is known about ethylene and propylene. They are commodities that have a cult-like status, as they are used as raw materials in nearly every petrochemical supply chain, most of which end up as manufactured goods that are in our homes.
Following the manufacturing chain backwards, take a look around you. Are the buttons on your clothing made of plastic? Is your clothing made with synthetic fibers? Is there any vinyl in your shoes? How about that television, Xbox, laptop and phone of yours?
At some point in time, all of these items were petrochemicals known as ethylene and propylene. Like yin and yang, ethylene and propylene are similar but not the same.
What is Ethylene?
Ethylene has been around as long as plants have existed, as it is a gas emitted by plants as they mature and ripen. Synthetic ethylene has existed for centuries, as it occurs as a component of ethanol. It was christened "ethylene" in the 1700s by European scientists as a literal mash up of "daughter of ethyl" and classified as an "olefiant" which meant "oil-making" at the time. Ethylene is colorless and nearly odorless. It is poisonous and combustible.
Ethylene is also one of the most useful chemicals known to humans. It is relied upon at manufacturing sites that produce thousands of other chemicals and products used in nearly every industry imaginable: paints, coatings, labels, packaging, cleaners, fibers, alcohols, plastic goods – just to name a few.
So, does it really just come from the air around us?
Not quite. To produce the billions of tons of ethylene to feed the manufacturing chains that the modern world has come to depend upon, ethylene is created by a chemical process called steam cracking.
Steam cracking is exactly as it sounds. A liquid raw material coming from the natural gas or crude oil stream and known as a "natural gas liquid" hydrocarbon is "cracked." The molecules are split open through a heating process that turns them from a liquid into gas.
Steam and catalysts are introduced to the ethylene production process and in the production unit known as a "steam cracker," ethylene is created.
Ethylene most easily (and safely) travels by pipeline. As it is a gas, it leaves the plant in a pipeline and is pumped into a storage site, which is almost always an underground salt cavern.
In North America, salt caverns occur naturally in parts of the U.S. Gulf Coast area and can be simulated and built in places where they do not naturally occur, such as the Midwestern U.S.
As a molecule, ethylene most closely resembles ethane. One of ethylene's early aliases was "ethene" and global customs agencies still use this term to describe the commodity. In chemical shorthand, it is written as C2= with the equals sign representing the double strand hydrocarbon bond. As hydrocarbons come from the earth, ethylene is also sometimes described as an organic chemical.
The most common use for ethylene is in the production of polyethylene ("many daughters of ethyl"), also known as plastic or plastic resin or just PE. The gaseous ethylene is fed into a plant that is usually next door to it that contains production lines known as "trains." These trains cool the ethylene and combine it with catalysts during a chemical process known as polymerization. At the end of the process, the ethylene has become a solid form and is chopped up into millions of small grains known as "pellets" that can be easily stored in railcars and transported to warehouses, cargo ships or other manufacturing sites.
PE itself has many forms, and different combinations of acronyms that describe its properties such as tensile strength and melt flow temperatures. The polyethylene used to make sheets, liners and plastic bags is known as either "low density" (LDPE) or "linear low density" (LLDPE). Polyethylene used to make hollow forms such as a trash can or flower pot is known as "high density blow mold" (HDPE BM). Polyethylene used to make solid items such as toys or appliances is known as "high density injection" (HDPE Inj).
As a pellet, polyethylene is useless. But when the pellets of the varying grades (LDPE, LLDPE, HDPE BM or HDPE Inj) are melted, everything is possible. Phones, cars, bottles, bags, flooring, clothing, pipes, toys ... it's everywhere. Polyethylene is not combustible (it melts) but it does not disintegrate easily and therefore faces disposal challenges as trash heaps mount all over the world. Recycling ameliorates the trash issue, but only to a certain extent.
Ethylene is also the main ingredient in other major manufacturing market sectors, such as vinyls, acrylics, solvents and glycols. More commonly, these chemicals are used in cosmetics, cleansers, paints, inks and common solutions such as antifreeze.
In other words, ethylene is everywhere.
What is Propylene?
A literal sister chemical to ethylene is propylene, the "daughter of propyl" aka “propene” and C3= ... but this sibling is the more complicated child, filled with nuance, multiple identities and varied origins.
Firstly, there are three types of propylene. Whereas ethylene is ethylene is ethylene, a 99% purity gas, propylene comes in three varieties:
- Refinery grade (RGP), which is a liquid that is a byproduct of propane production with a purity of 65% and higher. To learn about the world of propane, click here.
- Chemical grade (CGP), which is a gas that is the product of upgraded RGP or propane, with a purity of 94% and higher.
- Polymer grade (PGP), also a gas that is the product of upgraded RGP or propane, with a purity of 99%.
Let's get to know these three faces of propylene a little better.
Refinery Grade Propylene
Refinery grade propylene is the only liquid form of propylene. It is part of a shipment of propane. It is created alongside propane at either a refinery or an NGLs fractionation plant. RGP has three very different uses:
- To upgrade into better grades of propylene.
- As a raw material for a gasoline additive called alkylate, which boosts octane levels in common gasoline.
- As a raw material for a petrochemical called cumene, which is the raw material for a petrochemical called phenol, which is used to produce phenolic or epoxy resins (think laminates, fiberglass, particle board). Fun fact about cumene: when it was first “discovered” it was used as aviation fuel for planes fighting in World War II.
In addition to the gasoline and phenol markets, the producers of the "finer" grades of propylene also fight over this useful gem of a feedstock.
Chemical Grade Propylene
Chemical grade propylene is a very versatile petrochemical, used in the manufacture of many common household items, including acrylic yarns, chemical solvents, cleansers, shampoos and soaps, contact lenses and foam padding.
Polymer Grade Propylene
Polymer grade propylene has one mission: to make polypropylene. Like ethylene and polyethylene, PGP is polymerized and turned into plastic pellets that are similar to polyethylene but have different uses. Most carpets that are made of synthetic fibers use polypropylene. Most plastic bottle caps are made of polypropylene. Automobile dashboards are usually made of polypropylene. There is polypropylene all around you during most moments of your life.
As "the champagne of propylenes," polymer grade can also be used in any application that uses chemical grade propylene – it'll just cost you more.
And if this wasn't getting complicated enough – there's another twist. Propylene is also created during the steam cracking process. That's right, ethylene makers also make a wee bit of propylene.
The propylene yield of a steam cracker varies depending on what raw material is being used. Ethane creates almost no propylene byproduct. Propane and butane, however, do. And if the price of propylene is high, that is what ethylene producers will turn to – the feedstock that gets them the most money for their cracking.
However, with the "Shale Gale" that has gripped the country – and inspired not just the first new ethylene plants to be built since 2004, but the initiation of more than a DOZEN new ethylene plants – ethane has been the feedstock of choice for most ethylene producers, because the price is so low that sometimes it feels like it's free.
So, companies that had been cracking propane and heavier feedstocks like gasoil for decades were accustomed to producing a certain amount of propylene when they made ethylene. And switching to ethane made the propylene output nearly disappear.
Would the U.S. have to begin IMPORTING propylene? Egad!
No! Science came to the rescue and a petrochemical process known as dehydrogenation could turn ordinary propane straight into chemical grade OR polymer grade propylene. These plants, unsurprisingly called propane dehydrogenation units, are far smaller than a refinery. In 2015, there was one PDH unit in the U.S., and it was viewed for many years as an experimental technology. By 2018, there were three – and more are on the way.
Another way to make chemical grade or polymer grade propylene is to upgrade good old RGP in a type of plant called a "splitter" – it literally splits the RGP molecules and keeps the ones that create the higher-purity propylenes, which industry folks call "finished grade propylenes."
Having so many aspects to it, one can now see why propylene is a more complex commodity than dear old ethylene.
And now we bring it all back to the central topic of the manufacturing world....
Plastics made from ethylene and propylene are solids. And they are measured in solid measurements, in tons or more commonly (in the U.S.) in pounds. Millions of pounds. A single railcar can hold up to 200,000 pounds. And what manufacturer is ordering just one railcar? It's more like 50 railcars a pop. That's a lot of plastic pellets.
The standard pricing of most commodities is based on how it is stored or transported. Oil is priced in barrels. Plastics are priced in pounds. Ethylene and propylene are also priced in pounds.
Wait, what? Wouldn't trying to hold a pound of ethylene or propylene be like trying to catch a moonbeam?
Yes and no. Ethylene and propylene are measured by their flow, by the pace of their racing through pipelines per hour. But "flow" doesn't really translate into dollars so easily. And so, a system was developed in the U.S. to measure how many equivalents of a pound could "flow" at a set time measurement. The ethylene and propylene markets are so tightly bound up with the prices of polyethylenes and polypropylenes that pricing them all per-pound creates a pretty sturdy supply chain in terms of wrangling costs.
As the world at large becomes more obsessed with data, information, risk management and "tools" of all kinds, you may one day see the words “ethylene” and “propylene” crawl across the tickers on financial television shows alongside such luminaries as WTI and Brent.
And now, you can say you knew them before they became famous!