Researchers distill facts of a chemical separation process

An insight that could significantly improve this rapidly emerging technology.

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The chemical separation process of organic solvent nanofiltration (OSN) has been a subject of significant interest since its development a few decades ago. This innovative technology has the potential to revolutionize crucial industries such as fuel, food, and pharmaceuticals.

Now, a team of researchers from Yale and the University of Wisconsin-Madison has uncovered groundbreaking insights that challenge the existing understanding of how this technology functions. This discovery holds the promise of significantly enhancing the effectiveness of OSN.

Organic solvents are versatile liquids containing carbon and oxygen, such as alcohols, ethanol, benzene, and ketones. Utilizing membranes with nanosized holes, OSN effectively separates various chemicals. Its applications encompass hydrocarbon separation in oil refining and pharmaceutical ingredient purification.

OSN offers a cost-effective and chemical-additive-free alternative to traditional technologies like distillation, evaporation, and solvent extraction. Despite its effectiveness, the underlying mechanism of OSN has remained ambiguous. The prevailing theory suggests that OSN operates through a solution-diffusion mechanism, wherein solvent molecules dissolve and diffuse through the membrane from areas of high concentration to regions of lower concentration.

In their study, Menachem Elimelech of Yale and a team from the University of Wisconsin-Madison conducted experiments and computer simulations that challenged a longstanding theory about the physics of reverse osmosis, a method used to desalinate seawater.

The chemical separation process of organic solvent nanofiltration.
The chemical separation process of organic solvent nanofiltration. Credit: Yale University

The traditional view attributed reverse osmosis to the solution-diffusion mechanism, but Elimelech’s research revealed that changes in pressure within the membranes were responsible for pushing water through the membranes, known as the pore-flow model.

Building on their findings, Elimelech and his research team also questioned the conventional thinking on organic solvent nanofiltration (OSN). Through similar experiments and simulations, they demonstrated that the OSN process is driven by changes in pressure rather than the concentration of solvent molecules, much like reverse osmosis.

Elimelech highlighted that one key piece of evidence is the ability of the membranes for organic solvents to allow the passage of numerous solvent molecules held together due to the large pores.

“So it’s a cluster,” said Elimelech, the Sterling Professor of Chemical and Environmental Engineering. “And once you have a cluster solvent, it cannot move by diffusion. You need to have a driving force like pressure.”

The study also revealed that organic solvents, which are more complex than water, interact differently with polymer membranes. The friction between the solvent and the membrane has a significant impact on the flow rate of the solvent through the membrane. The researchers noted that the strength of the solvent’s binding to the material is closely related to the speed at which the solvent passes through the membrane. In some instances, certain organic solvents can cause the polymer material to stretch or compress by interacting with it.

This advanced understanding of the physics of organic solvent nanofiltration (OSN) will enable scientists to enhance OSN technology. For example, Elimelech suggested that these findings could be used to optimize the design of filtering membranes, leading to a wider range of separations and advancements in clean energy technologies.

Ying Li, the lead investigator of the computer simulations and an associate professor of mechanical engineering at UW-Madison, stressed the “critical role of the disparity between solvent size and membrane pore size in determining solvent permeance” during their research.

“Fine-tuning the pore size of various dense polymer membranes to approximate the molecular size of unwanted permeants, coupled with the use of a solvent possessing a smaller molecular size, holds the potential for achieving highly selective separations in OSN,” Li said.

Journal reference:

  1. Hanqing Fan, Jinlong He, Mohammad Heiranian, Weiyi Pan, Ying Li, Menachem Elimelech. The physical basis for solvent flow in organic solvent nanofiltration. Science Advances, 2024; DOI: 10.1126/sciadv.ado4332

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