Refining filters for greener natural gas

2 min read
A scanning electron microscope image (left) and a high-resolution transmission electron microscope image show an activated, sulphur-containing porous carbon sample. The material created at Rice University can be tuned to balance carbon dioxide sequestration and methane selectivity. (Image courtesy of the Barron Research Group.)
A scanning electron microscope image (left) and a high-resolution transmission electron microscope image show an activated, sulphur-containing porous carbon sample. The material created at Rice University can be tuned to balance carbon dioxide sequestration and methane selectivity. (Image courtesy of the Barron Research Group.)

Natural gas producers want to draw as much methane as possible from a well while sequestering as much carbon dioxide as possible, and could use filters that optimise either carbon capture or methane flow. No single filter will do both, but Rice University scientists say they now know how to fine-tune sorbents for their needs.

Subtle adjustments in the manufacture of a polymer-based carbon sorbent make it the best-known material either for capturing the greenhouse gas or balancing carbon capture with methane selectivity, according to Rice chemist Andrew Barron.

The research has been published in the Royal Society of Chemistry journal Sustainable Energy and Fuels.

"The challenge is to capture as much carbon as possible while allowing methane to flow through at typical wellhead pressures," Barron says. "We've defined the parameters in a map that gives industry the best set of options to date."

Porous filter

Previous work by the lab determined that carbon filters reached their maximum capture ability with a surface area of 2,800 m2/g and a pore volume of 1.35 cm3/g. They also discovered the best carbon capture material didn't achieve the best trade-off between carbon and methane selectivity. The Rice team say they now know how to tune the material for one or the other.

"The traditional approach has been to make materials with ever-increasing pore volume and relate this to a better adsorbent; however, it appears to be a little more subtle," reports Barron.

The lab made its latest filters by heating a polymer precursor and then treating it with a chemical activation reagent of potassium hydroxide (KOH). When the polymer is baked with KOH at temperatures over 500°C (932°F), it becomes a highly porous filter, full of nanoscale channels that can trap carbon.

The ratio of KOH to polymer during processing turned out to be the critical factor in determining the final filter's characteristics. Making filters with a 3-to-1 ratio of KOH to polymer gave it a surface area of 2,700 m2/g and maximised carbon dioxide uptake under pressures of 5-30 bar. 

Filters made with a 2-to-1 ratio of KOH to polymer had less surface area – 2,200 m2/g – and a lower pore volume. This resulted in the optimum combination of carbon dioxide uptake and methane selectivity.

The size of the pores was critical as well. Filters with maximum carbon uptake had the largest fraction of pores smaller than 2 nm. Bigger pores were better for methane selectivity.

"It appears that total pore volume is less important than the relative quantity of pores at specific sizes," Barron says. "Our goal was to create a guide for researchers and industry to design better materials.