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Dividing crude oil into commodities like gasoline, diesel, and heating oil is a process that requires substantial energy, accounting for about 6 percent of global CO2 emissions. The majority of that energy is utilized for the heat necessary to differentiate the components based on their boiling points.
In a breakthrough that could significantly lower the energy required for crude oil separation, engineers from MIT have created a membrane that sorts the components of crude oil according to their molecular dimensions.
“This presents an entirely new perspective on a separation method. Rather than boiling mixtures for purification, why not distinguish components based on form and size? The primary advancement is that the filters we’ve created can separate extremely small molecules at an atomic scale,” states Zachary P. Smith, an associate professor of chemical engineering at MIT and the principal author of the recent research.
The innovative filtration membrane can effectively segregate heavy and light constituents from oil, and it shows resilience against the swelling that often occurs with other oil separation membranes. This membrane is a thin layer that can be produced using a method that is already commonly employed in industrial processes, potentially enabling it to be scaled for extensive application.
Taehoon Lee, a former postdoctoral researcher at MIT who is currently an assistant professor at Sungkyunkwan University in South Korea, serves as the lead author of the study, which is published today in Science.
Oil separation
Traditional heat-driven techniques for crude oil separation account for about 1 percent of worldwide energy consumption, and it has been predicted that implementing membranes for crude oil separation could decrease the required energy by approximately 90 percent. For this to be successful, a separation membrane must permit hydrocarbons to flow through rapidly while selectively filtering compounds of varying dimensions.
Up to now, most attempts to create a filtration membrane for hydrocarbons have concentrated on polymers of intrinsic microporosity (PIMs), including one known as PIM-1. Although this porous substance allows for quick transport of hydrocarbons, it often excessively absorbs certain organic compounds as they traverse the membrane, resulting in swelling that hinders its ability to sieve based on size.
To develop a superior alternative, the MIT team opted to modify polymers utilized in reverse osmosis water purification. Since their implementation in the 1970s, reverse osmosis membranes have dramatically decreased the energy usage of desalination by about 90 percent — a notable industrial achievement.
The most prevalent membrane for water desalination is a polyamide manufactured using a technique called interfacial polymerization. In this process, a delicate polymer film forms at the interface between water and an organic solvent such as hexane. Water and hexane don’t typically mix, but at their interface, some of the dissolved compounds can react with one another.
In this scenario, a hydrophilic monomer known as MPD, dissolved in water, reacts with a hydrophobic monomer called TMC, dissolved in hexane. The two monomers are conjoined by a linkage referred to as an amide bond, creating a polyamide thin film (designated MPD-TMC) at the water-hexane interface.
While highly effective for water desalination, MPD-TMC lacks the appropriate pore sizes and swelling resistance necessary for hydrocarbon separation.
To modify the material for the separation of hydrocarbons found in crude oil, the researchers first altered the film by switching the connecting bond of the monomers from an amide bond to an imine bond. This new bond is more rigid and hydrophobic, allowing hydrocarbons to pass through the membrane swiftly without causing significant swelling compared to the polyamide version.
“The polyimine material has porosity that develops at the interface, and due to the cross-linking chemistry we’ve incorporated, you now have a material that does not swell,” remarks Smith. “You produce it in the oil phase, induce a reaction at the water interface, and with the crosslinks, it becomes immobilized. Thus, those pores, even when exposed to hydrocarbons, no longer undergo swelling like other materials.”
The researchers also integrated a monomer called triptycene. This shape-stable, molecularly selective material aids the resulting polyimines in forming pores that are optimally sized for hydrocarbons to traverse.
This methodology signifies “a crucial advancement toward minimizing industrial energy consumption,” asserts Andrew Livingston, a professor of chemical engineering at Queen Mary University of London, who was not part of the study.
“This research applies the foundational technology of the membrane desalination industry, interfacial polymerization, and reveals a novel application to organic systems like hydrocarbon feedstocks, which presently consume significant amounts of global energy,” Livingston states. “The innovative concept of utilizing an interfacial catalyst coupled with hydrophobic monomers leads to membranes that demonstrate high permeance and excellent selectivity, and the research illustrates potential applications in relevant separations.”
Effective separation
When the researchers utilized the new membrane to filter a blend of toluene and triisopropylbenzene (TIPB) as a benchmark for assessing separation efficacy, it achieved a concentration of toluene 20 times greater than its initial concentration in the mixture. They also evaluated the membrane with an industrially relevant mixture comprising naphtha, kerosene, and diesel, discovering that it could efficiently differentiate the heavier and lighter compounds based on their molecular dimensions.
If adapted for industrial implementation, a series of these filters could be employed to produce a higher concentration of the desired outputs at each stage, according to the researchers.
“You could envision that with a membrane like this, an initial stage could replace a crude oil separation column. You could segregate heavy and light molecules, followed by utilizing different membranes in a cascade to refine complex mixtures and isolate the chemicals required,” Smith notes.
Interfacial polymerization is already extensively utilized to fabricate membranes for water desalination, and the researchers are optimistic that it can be adapted to mass-produce the films engineered in this study.
“The primary benefit of interfacial polymerization is that it’s a well-established method for creating membranes for water purification, so it’s plausible to simply integrate these chemistries into existing manufacturing scales,” Lee explains.
The research was partially funded by ExxonMobil via the MIT Energy Initiative.
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