Conducted by a team led by William Koros, a professor in Georgia Tech’s School of Chemical & Biomolecular Engineering, the study “Ultraselective glassy polymer membranes with unprecedented performance for energy-efficient sour gas separation” appeared in the May 2019 issue of Science Advances.

Raw natural gas contains many purities, the researchers note, but H₂S and carbon dioxide (CO₂) are arguably the two most important to remove. H₂S can cause eye, nose, and throat irritation in amounts as low as five parts per million (ppm) and instant death when its concentrations exceed 1000 ppm.

Currently an amine absorption-based process dominates most “sour” gas treatment operations, but “it requires huge towers and an enormous amount of energy and expense as well as having environmental concerns,” Koros says.

More than 40 percent of proven raw natural gas reserves in the U.S. are deemed “sour,” requiring treatment to remove excessive CO₂ and H₂S. In parts of the Middle East and Latin America, some reservoirs are presently too difficult to access at all because of high sour gas concentrations, Koros explains.

“Large reserves of natural gas globally remain untapped because of the difficulties involved in processing such low-quality gas,” he adds.

To find solutions, his research team studied a specific type of glassy-ladder polymer of intrinsic microporosity (PIM). PIMs have a highly rigid and contorted backbone structure that doesn’t get compacted so that gas molecules have room to move through the membrane. However, the researchers knew that the conventional PIM wouldn’t be selective enough to filter out H₂S and C_O2.

They found that the key was using a PIM that had been functionalized with amidoxime, which doesn’t hold onto the CO₂ or H₂S as tightly as an amine would, allowing the membrane to both catch and release the impurities.

Koros’ collaborator, Professor Ingo Pinnau at King Abdullah University of Science and Technology, had already found the amidoxime-functionalized (AO) PIM worked well for removing CO₂ and believed it might work well for H₂S. So he sent samples to Koros who tested it in his lab at Georgia Tech, finding that the AO-PIM exceeded their expectations.

“We thought it would be good, but not as good as it turned out to be,” Koros says.

His team found that the AO-PIM membrane provided outstanding separation performance of sour gas streams under challenging feed pressures up to 77 bar.

These membranes could ultimately eliminate the need for thermal energy in the sour gas separation process. But big membranes areas would be required for large-scale separations, so future research would be needed to efficiently scale up the process in terms of space requirements, the researchers note.

One challenge they found relates to aging of the membranes. If they are stored for significant periods while not in use, they can lose some productivity due to settling of the glassy structure. But that loss doesn’t happen while the membranes are in active use, and even if aged, productivity can be restored by immersing them in methanol to swell them back to their original state.

“To continue to expand the impact of this technology, higher separation efficiency (selectivity) and higher productivity (permeability) as well as stable performance of membranes under realistic conditions are needed,” the researchers write.

“Given the global quest for clean energy and sustainable development, the synthesis of such novel gas separation membranes with superior performance opens the door to enormous opportunities for natural gas purification, biogas upgrading, and reducing greenhouse gas emissions.”

In addition to Koros and Pinnau, authors of the study include Shouliang Yi, Bader Ghanem, and Yang Liu.