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February 13, 2018

A roadmap for making plastics from natural gas

Written By: Silke Schmidt

Natural gas—primarily composed of methane—has been booming for the past 15 years, and its dominance over coal as an energy source has reduced carbon emissions in the United States. The majority of natural gas is currently used for electricity generation, heating and cooking, and only 1.5 percent is converted into the chemicals that make plastics, adhesives and other consumer products.

As more methane is converted into these commodity chemicals, less carbon is released into the atmosphere, further reducing the energy sector’s contribution to global warming.

And chemical engineers at the University of Wisconsin-Madison have now developed a framework for evaluating less complex and costly methane-to-chemicals conversion strategies that help realize these environmental benefits.

“Recent findings have sparked an interest in simpler reactions that don’t require oxygen and generate less carbon dioxide as a byproduct,” says Christos Maravelias, the Vilas Distinguished Achievement Professor and Paul A. Elfers Professor in Chemical and Biological Engineering at UW-Madison. “Our study provides a roadmap that highlights the technology gaps our research community has to fill so that companies will consider putting these newer non-oxidative methane conversion processes into practice.”

The study, published online Jan. 23, 2018, in the journal Joule, goes well beyond studying reaction and separation systems in the lab. It considers the entire series of steps—and their associated costs—that make up a commercial process, from the initial methane reaction to separating and storing the desired products to recycling unreacted methane.

For example, an initial reaction may only convert 25 percent of the methane feedstock into a high-demand building block for plastics, such as ethylene. But the remainder isn’t simply lost. Instead, multiple effective separation and recovery loops can increase a facility’s overall methane conversion rate to more than 75 percent. Critical players in the process are catalysts: chemicals that trigger a chemical reaction without being involved in it themselves.

“Modeling a multi-step process that reflects the complexity of a real system was challenging, but we wanted our results to be useful for other researchers,” says Kefeng Huang, a postdoctoral fellow in Maravelias’ lab who is the paper’s first author. “Our model shows them how their specific lab result, such as the discovery of a more selective catalyst, fits into the bigger picture of a commercially appealing technology.”

The model also illustrates exactly how changes in the market price for natural gas (the input) or ethylene (the output) affect the overall economics of the conversion process.

One of the study’s main findings is that future research should focus on two key areas: a first-pass methane conversion rate above 25 percent and a catalyst “coking” percentage below 20 percent.

“Coking is the formation of carbon residues on the catalyst,” Huang says. “It reduces its chemical activity, which slows down the reaction of interest, and reduces the fraction of carbon that is converted into ethylene or other desirable end products.”

Achieving these kinds of process improvements is no small feat, but the first step is an awareness of the biggest knowledge gaps.

“We have outstanding researchers in the area of catalysis, including my colleagues and co-authors,” Maravelias says. “That’s why I’m confident that scientific advances, whether they involve reduced manufacturing costs of catalysts, better performance or both, will allow us to use a greater percentage of natural gas as a feedstock for commodity chemicals, which will further reduce our country’s carbon emissions.”