Engineers explore membrane distillation technologies

Using membrane distillation to purify water presents a challenge: membranes must be designed to purify water efficiently while ensuring zero contamination of the clean water. Engineers from Colorado State University (CSU) have investigated this challenge and offer insight into why certain membrane designs work better than others.

Global water scarcity means that scientists and engineers are investigating new ways to harvest purified water from unconventional sources, like sea water or even wastewater.

Along with a team of research engineers, Tiezheng Tong, assistant professor in the Department of Civil and Environmental Engineering at CSU, is studying an emerging technology called membrane distillation.

Membrane distillation involves a thin, water-repellent membrane that exploits vapour pressure differences between hotter impure liquid, called ‘feedwater’, and colder purified water called ‘permeate’. During the process, water vapour passes through the membrane and is separated from the salty or dirty feedwater. According to Tong, membrane distillation works better than other technologies like reverse osmosis, which can’t treat extremely salty water such as desalination brines or produced water from hydraulic fracturing.

Tong and materials scientist Arun Kota in the Department of Mechanical Engineering joined forces to uncover the science behind designing the ideal membrane. In experiments described in Nature Communications, the CSU researchers investigate why certain membrane designs used in membrane distillation work better than others.

“The fundamental knowledge from our paper improves mechanistic understanding on the water–vapour transport within microporous substrates and has the potential to guide the future design of membranes used in membrane distillation,” Tong said.

In membrane distillation, feedwater is heated to separate the pure and impure components by differences in volatility. The microporous membrane is a key component of the set-up because it allows water vapour through, but not the entire impure liquid. Typically, the membrane is made of a hydrophobic material in order to let only the water vapour pass through but maintain a barrier for the feedwater.

However, these hydrophobic membranes can fail because the feedwater, such as shale oil-produced water, can have low surface tension. This low surface tension allows the feedwater to leak through the membrane pores, contaminating the pure water on the other side — a phenomenon called membrane wetting.

Hydrophobic vs omniphobic membranes

Previous research found that using omniphobic membranes — membranes that repel all liquids, including water and low surface tension liquids — keep the vapour/water separation intact. But, omniphobic membranes typically slow down the rate and amount of water vapour passing through the membrane, dramatically reducing the efficiency of the entire process.

The CSU researchers set out to explain the tradeoff between hydrophobic and omniphobic membranes. Through systematic experiments, led by postdoctoral researchers Wei Wang in Kota’s lab, and Tong’s graduate student Xuewei Du, they found that conventional hydrophobic membranes create a larger liquid–vapour interfacial area. This increases the amount of evaporation taking place. With the omniphobic membranes, they saw a much smaller liquid–vapour interface. This explains the difference between the membranes’ performances.

The omniphobic membranes used in the experiments were made without depositing extra particles so the researchers were able to determine that their observations weren’t the result of structural changes to the membranes.

Despite no solution to the tradeoff, the researchers’ insights reveal the core challenge around making membrane distillation a successful technology.

“If you understand the problem thoroughly, then there is scope for solving it,” Kota said. “We have identified the mechanism; now we have to solve the tradeoff problem.”

For example, smart membranes with exceptional omniphobicity and a simultaneously large liquid–vapour interfacial area can render membrane distillation a robust and cost-effective process for water purification. More collaborative research has been initiated by the team to design such smart membranes, with the goal of increasing the efficiency of membrane distillation.

Tong added that the research happened at the interface of two disciplines: surface science and membrane technology.

To read the research paper, click here.

Source: Colorado State University.

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