Challenges faced in Chemical Separation
Every year, a massive amount of energy is consumed by chemical manufacturers separating and refining feedstocks to make a wide variety of products including gasoline, plastics, and food. The vast majority of separations out in the field in a variety of industries are thermally-driven systems such as distillation, and because of that, an excessive amount of energy is spent on these separation processes i.e. about 10% of the global energy budget is spent on chemical separations. Avoiding the use of heat and chemical phase changes make separations much less energy-intensive. In practice, using them could produce a 90% reduction in energy cost. Plastic membranes already exist that can be used to separate certain molecules based on size and other differences but these have been limited to processes such as water desalination as they cannot withstand harsh solvent-rich chemical streams while also performing challenging separation tasks. Separation of organic solvents that can be done using inorganic membranes, but they are costly and difficult to scale up.
A solution: A METAL oxide-infused Hybrid Membrane
An attempt was made by researchers at the Georgia Institute of Technology to create membranes that could separate chemicals without using energy-intensive distillation processes. The researchers created a process that infuses a polymer-based membrane with a metal oxide network. The membrane was found to be far more effective at standing up to harsh chemicals without degrading. They used a polymer of intrinsic microporosity, known as PIM-1, which is a rigid polymer with high levels of microporosity. The pre-fabricated membrane was exposed to metal-containing vapors that infuse themselves inside the membrane material. This process is called vapor phase infiltration (VPI), and it creates a uniform network of metal oxide throughout the polymer membrane producing a hybrid membrane. VPI is a process that loads the bulk of polymers with inorganics. It has the advantage over conventional methods such as cross-linking or pyrolysis which doesn’t change the microstructure of the polymer. It also doesn’t require the solvents needed by cross-linking or the high temperatures needed by pyrolysis. The stability of the metal oxide improves while maintaining the structure of the polymer. This prevents it from degrading compared to traditional membranes. This is the first time that VPI has been used to improve membrane stability in harsh chemical environments.
The hybrid membrane can separate solutes from solvents, but can also separate solvents from other solvents without a phase change. Some chemicals that need to be separated are very similar in terms of their size, shape and other properties, which makes them even harder to process using membranes. These new hybrid membranes are much more selective and are able to separate chemicals that are more similar to each other. The membranes were tested using harsh chemicals such as tetrahydrofuran, dichloromethane, and chloroform, and remained stable for three months during the testing phase. The membranes could also separate aromatic molecules that differed in size by as little as 0.2 nm. The process can also be easily translated to an industrial scale.
A polymer-based membrane infused with a metal oxide network may provide an attractive alternative to energy-intensive distillation, say researchers at the Georgia Institute of Technology (Georgia Tech), Atlanta. The infused membrane separates chemicals better than the untreated membrane and can withstand harsh chemicals without degrading, they add. Moreover, incorporating the treatment into standard membrane manufacturing is straightforward, requires minimal investment in new equipment, and adds little to membrane cost, the researchers claim.
“In practice, using them could produce a 90% reduction in energy cost,” notes Ryan Lively, an associate professor in Georgia Tech’s School of Chemical & Biomolecular Engineering.
“After placing the pre-fabricated membrane inside of our reactor, we simply expose it to metal-containing vapors that infuse themselves inside the membrane material,” says Mark Losego, an assistant professor in the School of Materials Science and Engineering. “This process is called vapor phase infiltration, and it creates a uniform network of metal oxide throughout the polymer membrane. We call it a ‘hybrid’ membrane.”
The team tested the membrane using harsh chemicals such as tetrahydrofuran, dichloromethane, and chloroform, and found it remained stable for several months; pure polymer membranes dissolve in minutes.
“Some chemicals that need to be separated are very similar in terms of their size, shape and other properties, which makes them even harder to process using membranes,” Lively explains. “These new hybrid membranes are much more selective. They can separate chemicals that are more similar to each other.” In the tests, the hybrid membranes separated aromatic molecules that differed in size by as little as 0.2 nanometers.
An article in Chemistry of Materials contains more detail.
Future research includes fine-tuning the oxide infusions and making new types of hybrid membranes capable of separating a variety of other chemicals.
“Our infiltration process uses essentially the same chemistries as chemical vapor deposition (CVD). … our initial results are promising, and further optimization of this chemistry would likely be the most productive first step. The Al2O3 chemistry is also amongst the least expensive. However, other chemistries may generate additional value beyond membrane stabilization, including improved separation performance or integrated chemical catalysis,” note the researchers.
Lively and Losego continue: “The current article utilizes well-known test-standards for membrane materials, which enables head-to-head comparisons with other materials. We also explored some proof-of-concept chemical separations, including purifying toluene solvent contaminated with several other alkyl aromatic solvents; we also found that the membranes were effective for alcohol-alcohol separations. As we explore new infiltration chemistries and polymer membrane chemistries, we expect to further refine the separation capabilities of these materials to the point that they can address some of the most challenging molecular separation problems.”
“At its simplest level, the vapor phase infiltration process requires a vacuum pump with a valve, a vacuum ‘chamber,’ and a valved precursor chemical source…While this description is certainly oversimplified, the main point here is that we believe this process has the potential to be directly applied to existing membrane technology without much disruption to upstream membrane module production. It could be a value-added, end-of-the-line add-on process,” they explain.
It’s too early to estimate the cost of the vapor phase infiltration treatment, they say; however, just a few cents in chemical precursors modify these membranes. “Importantly, this technology can be applied to pre-fabricated membrane modules, which we believe will facilitate scale-up (or ‘number up’, in this case) as the technology can be easily added onto the end of existing commercial manufacturing networks,” they add.
The team believes commercialization is possible within 3–5 years. “Optimization of membrane performance, precursor usage, and process time are all still necessary. Longer-term reliability testing also is needed,” they note.
Future research on the membranes will involve looking at how to fine-tune the oxide infusions and make new types of hybrid membranes that are capable of separating a wide variety of chemicals.