Polymeric Membranes Used for Cleaning Up Industrial Separations

There are numerous technologies with potential under development that can decrease energy consumption or capture carbon in fields including biotech, nanotechnology, materials science, computer science and more. Not all will prove viable, but with some funding and nurturing, many could help solve Earth’s grand challenge.

One such solution is emerging from new methods of industrial separation processes. At MIT’s Department of Chemical Engineering, Professor Zachary Smith is studying new polymeric membranes that can significantly lessen energy use in chemical separations. He is also undertaking longer range research into optimizing polymeric membranes with nanoscale metal-organic frameworks (MOFs).

Not only are we making and analyzing materials from the fundamental principle of transport, thermodynamics and reactivity, but we’re beginning to take that knowledge to create models and design new materials with separation performance that has never been achieved before. It’s exciting to go from the lab scale to thinking about the big process, and what will make a difference in society.

Professor Zachary Smith, Department of Chemical Engineering, MIT

Smith frequently consults with industry experts who share their expertise on separations technologies. With the Paris climate agreement of 2015 thus far holding together, in spite of the retreat by the U.S., the petrochemical and chemical industries where Smith is mainly focused is beginning to feel the pressure to decrease emissions. The industry is also seeking to lower costs. The cooling and heating towers used for separations require sizable energy, and are expensive to construct and maintain.

Industrial processes used in the petrochemical and chemical industries alone devour a quarter to a third of the total energy in the U.S., and separations account for nearly half of that, says Smith. Nearly half the energy consumption from separations is due to distillation, a process that requires a high level of heat, or in the case of cryogenic distillation, even more energy-hungry extreme cooling.

“It requires a lot of energy to boil and reboil mixtures, and it’s even more inefficient because it requires phase changes,” says Smith. “Membrane separation technology could avoid those phase changes and use far less energy. Polymers can be made defect-free, and you can cast them into selective, 100-nanometer-thick thin films that could cover a football field.”

However, multiple hurdles block the way. Membrane separations are used in only a small fraction of industrial gas separation processes as the polymeric membranes, “are often inefficient, and can’t match the performance of distillation,” says Smith. “The current membranes don’t provide enough throughput — called flux — for high volume applications, and they’re often chemically and physically unstable when using more aggressive feed-streams.”

Many of these performance issues arise from the fact that polymers tend to be amorphous, or entropically disordered.

Polymers are easy to process and form into useful geometries, but the spacing where molecules can move through polymeric membranes changes over time. It’s difficult to control their porous internal free volume.

Professor Zachary Smith

The most challenging separations need size selectively between molecules of only a fraction of an angstrom. To solve this challenge, the Smith Lab is trying to incorporate nanoscale features and chemical functionality to polymers to realize finer-grained separations. The new materials can, “soak up one type of molecule and reject another,” says Smith.

To make polymeric membranes with higher selectivity and throughput, Smith’s team is taking new polymers created at MIT labs that can be reacted to template ordered structure into traditional disordered, amorphous polymers. As he explains, “We then post-synthetically treat them in a way to template in some nanometer sized pockets that create diffusion pathways.”

While the Smith Lab has achieved success with many of these methods, attaining the flux essential for high-volume applications continues to be a challenge. The issue is complicated by the fact that there are over 200 different types of distillation separation processes used by the petrochemical and chemical industry. Yet this can also be a benefit when attempting to add a new technology — researchers can look for a niche rather than trying to change the industry overnight.

“We’re looking for targets where we would have the most impact,” says Smith. “Our membrane technology has the advantage of offering a much smaller footprint, so you can use them in remote locations or on offshore oil platforms.”

Owing to their small weight and size, membranes are already being applied on airplanes to separate nitrogen from air. The nitrogen is then used to coat the fuel tank to prevent explosions similar to the one that blew up TWA Flight 800 in 1996. Membranes have also been employed for carbon dioxide removal at remote natural gas wells, and have found a niche in some larger petrochemical applications such as hydrogen removal.

Smith wants to expand into applications that normally use cryogenic distillation towers, which require vast energy to create extreme cold. In the petrochemical industry, these include nitrogen-methane, ethylene-ethane and air separations. A number of plastic consumer products are made of ethylene, so decreasing energy costs in fabrication could produce huge benefits.

With cryogenic distillation, you not only must separate molecules that are similar in size, but also in thermodynamic properties. The distillation columns can be 200 or 300 feet tall with very high flow rates, so the separation trains can cost up to billions of dollars. The energy required to pull vacuum and operate the systems at -120 degrees Celsius is enormous.

Professor Zachary Smith

Other promising applications for polymer membranes include, “finding other ways to remove CO₂ from nitrogen or methane or separating different types of paraffins or chemical feedstocks,” says Smith.

Carbon capture and sequestration is similarly on the radar. “If there was an economic driver for capturing CO₂ today, carbon capture would be the largest application by volume for membranes by a factor of 10,” he says. “We could make a sponge-like material that would soak up CO₂ and efficiently separate it so you could pressurize it and store it underground.”

One challenge when employing polymeric membranes in gas separations is that the polymers are usually composed of hydrocarbons. “If you have the same type of hydrocarbon components in your polymer that you have in the feed-stream you’re trying to separate, the polymer can swell or dissolve or lose its separation performance,” says Smith. “We’re looking to introduce non-hydrocarbon-based components such as fluorine into polymers so that the membrane interacts better with hydrocarbon-based mixtures.”

Smith is also testing with incorporating MOFs to polymers. MOFs, which are created by connecting together metal ions or metal clusters with an organic linker, may not only solve the hydrocarbon issue, but the entropic disorder problem as well.

“MOFs let you form one, two, or three-dimensional crystal structures that are permanently porous,” says Smith. “A teaspoon of MOFs has an internal surface area of a football field, so you can think about functionalizing the internal surfaces of MOFs to selectively bind to or reject certain molecules. You can also define the pore shape and geometry to allow one molecule to pass while another is rejected.”

In contrast to polymers, MOF structures will not normally change shape, so the pores are far more persistent over time. Moreover, “they don’t degrade like certain polymers through a process known as aging,” says Smith. “The challenge is how to incorporate crystalline materials in a process where you can make them as thin films. One approach we’re taking is to disperse MOFs into polymers as nanoparticles. This would let you exploit the MOFs’ efficiency and productivity while maintaining the processability of the polymer.”

One potential benefit of incorporating MOF-enhanced polymeric membranes is process intensification: bundling various separation or catalytic processes in one step to attain greater efficiencies. “You can think about combining a type of MOF material that could separate a gas mixture and allow the mixture to undergo a catalytic reaction at the same time,” says Smith. “Some MOFs can also act as cross-linking agents. Instead of using polymers directly cross linked together, you can have links between MOF particles dispersed in a polymer matrix, which would create more stability for separations.”

As a result of their porous nature, MOFs may potentially be used for, “capturing hydrogen, methane, or even in some cases CO₂,” says Smith. “You can get very high uptake if you create the right type of sponge-like structure. It’s a challenge, however, to find materials that selectively bind one of these components in very high capacity.”

A similar application for MOFs would be storing natural gas or hydrogen for fueling a car. “Using a porous material in your fuel tank would let you hold more hydrogen or methane,” says Smith.

Smith cautions that MOF research could take several years before it comes to fruition. His lab’s polymer research, however, is a lot further along, with commercial solutions anticipated in the next five to 10 years.

“It could be a real game changer,” he says.