Crude oil is sticky stuff and often clogs filters membranes and other equipment used in the oil and gas industry. To address this problem, scientists at Argonne National Laboratory have developed a novel approach, which will prolong the lifetime of key industrial equipment.
The new invention consists of a type of coating that produces thin films of water-loving, oil-repelling molecules on the surface of filter membranes. These metal oxide molecules grab onto any loose water atoms while resisting oil.
“One of the best ways to clean oily water is with membranes,” said Seth Darling, director of the Institute for Molecular Engineering at Argonne. “The problem is that the oil sticks on the membrane and clogs the holes until the membrane stops working. Today, if people have an oil-fouled membrane, either they replace it or they try to clean it with harsh chemicals to wash away the oil.”
The scientists used a method that uses chemical vapors to deposit a very thin coating of the metal oxide on all of the filter membrane surfaces. They experimented using different metal oxides on off-the-shelf commercial polymer membranes to find which ones worked the best. The team recently published the results in ACS Nano.
The process, called atomic layer deposition, isn’t new, but it’s never been used this way to modify membranes before, Darling said.
“It’s kind of cutting-edge,” Darling said. “The coating is just a few nanometers in thickness. If the coating were thicker than this, it would close off the tiny pores. What you want is a minimal change of the pore structure, but you want to change the chemistry of the substance lining those pores.”
To create this layer in the past, people tried to attach nanoparticles to a membrane by flowing them through it or growing them on it. But particles tend to get ripped off as water flows through those systems. Atomic layer deposition is different because the metal oxide film, in this case, forms strong chemical bonds with the polymer to which it is adhered. In the atomic layer deposition process, the membrane is exposed to a sequence of vapors that stick molecules together, forming covalent bonds with the polymer.
“Some polymers bind more easily than others, and some repel oil while others do not,” Darling said of his group’s process working with a variety of metal oxides. “At this point, we have a pretty good sense of which ones work and why.”
Tin oxide and titanium oxide formed the tightest bonds with water molecules, capturing them and layering them across the surface.
“When oil contacts the membrane, it will stay separate because it flows over the water layer,” said Hao-Cheng Yang, a postdoctoral researcher working on the project.
Fouled membranes can be a costly hassle for the oil and gas industry. For instance, when oil companies replace clogged filters during the hydraulic fracturing process, they have to shut down their equipment to make the change. Oil-resistant membranes like this could significantly reduce the need for both filter replacement and the downtime it creates, said John Harvey, Argonne’s business development executive handling the technology.
“Just from my knowledge of the oil and gas sector, if we could make a membrane that performs to even a fraction of what we’ve seen in lab testing, it will be a phenomenal improvement over what’s available now. That represents a huge savings,” said Harvey.