This is how natural gas gets turned into plastics
When Shell Chemical announced earlier this year it would building an ethane cracker in Beaver County, Pennsylvania, it no doubt left many wondering: What exactly is an ethane cracker?
Though an ethane cracker is a pretty high-tech facility, the basic concept is fairly simple: It takes ethane — a gas that’s commonly found mixed in with oil and natural gas deposits — and turns it into the building blocks of plastics.
To learn exactly how that’s done, we called up Goetz Veser, a chemist at the University of Pittsburgh’s Swanson School of Engineering.
First, Veser explained that ethane, which at normal temperatures and pressures is a gas, shares a common origin with oil and natural gas.
“It’s old organic mass,” Veser says. “As we always joke — it’s dinosaurs. It’s plants and animals from geological times.”
There happens to be a lot of ethane in the deep shale formations of western Pennsylvania, which is why Shell wants to build its facility here in the first place.
On a molecular level, ethane is pretty simple: It’s a chain of two carbon atoms surrounded by six hydrogen atoms. Veser says by itself ethane is not all that useful, which is why it has to be processed by a so-called “cracker.”
Before it gets to the plant, the ethane is separated from methane — the main component of natural gas, which is used to heat homes and generate electricity. Then, at the cracker, the ethane gets put into tubes and is heated to around 1,500 degrees. That’s when things start to get interesting.
“As you heat it up, you basically make this molecule wiggle around more and more wildly,” Veser says.
When this happens, the bonds holding its atoms together stretch and get weaker.
“You can almost imagine it like when you have two kids and they hold arms and swing each other around. If they get too crazy and too wild, at some point, they just can’t hold onto each other and they fly apart.”
This “cracking,” or breaking of the molecular bonds, causes the atoms to rearrange themselves. In particular, ethane’s two carbon atoms form a double bond with one another, resulting in a new molecule: ethylene.
Veser says it’s this double bond which makes ethylene so useful. It reacts easily with other chemicals and can be strung together into long molecular chains — sort of like Legos. And when you snap all these ethylene molecules together, you get polyethylene — the ubiquitous plastic found in everything from milk containers to medical devices.
But Veser says the cracking process produces a “whole slew of other side products” in addition to ethylene. The most important of these is coke, which is more or less pure carbon and accumulates inside the cracker’s pipes and has to be cleaned out periodically. To reduce the buildup of coke, Veser says, steam is added to the cracking process.
Needless to say, it’s not exactly a process you should try out in your backyard. Nor would you want to, Veser says: Ethylene is pretty hazardous.
“The property you love about ethylene is that it’s so highly reactive. But that also makes it a molecule you don’t want to transport around very much because being highly reactive also has some negatives: It’s very flammable, very explosive.”
That’s why ethylene is typically converted to polyethylene on site, which is what will happen at the Shell cracker. According to the company, the plant will produce 1.6 million tons of polyethylene pellets annually, which will end up in plastics we use every day.
“It’s almost impossible to imagine modern life without ethylene derivatives,” Veser says.
So in just a few years, Shell’s ethane cracker will be one place where the materials of the modern world will get their start — all from a gas found deep underground that was millions of years in the making.
This report is from The Allegheny Front, an award-winning public radio program covering environmental issues in Pennsylvania and an Innovation Trail partner.