Polymer-based membranes selectively separate hydrocarbons

According to the team in Saudi Arabia, their stability and selectivity can be tuned by thermal crosslinking to separate simple hydrocarbon mixtures and complex crude oil fractions.

Separation processes including distillation and evaporation are central to the chemical, pharmaceutical and petrochemical industries, but they are energy intensive, expensive and polluting. According to KAUST, crude oil refineries consume about one per cent of the total energy used worldwide, and some refineries release up to 20 to 35 million tonnes of CO₂ into the atmosphere.

Membranes, with their low carbon footprint and ability to fit in small spaces, offer an attractive alternative to these heat-based processes and can reduce the CO₂emissions of crude oil refineries.

Polymer membranes are cheaper and easier to manufacture and adapt to large-scale processes than inorganic membranes but their low stability under industrial conditions, such as elevated temperature and certain solvents, affects their performance.

The researchers chose the polymer polytriazole, which bears hydroxyl functional groups as a stable backbone, for their membrane. They deposited the polymer dissolved in various solvents onto a glass plate and immersed the support in distilled water to remove the resulting film. Next, they heated the film in a furnace to crosslink the hydroxyl groups and generate a membrane stable in organic solvents as well as in highly acidic and basic media.

Crosslinking is necessary for challenging applications and must provide stability in the broadest range of conditions, said team leader Suzana Nunes. “The key to obtaining membranes that could resist harsh environments like crude oil is the presence of the hydroxyl groups.”

The membranes enriched hydrocarbon mixtures by up to 95 per cent in compounds containing less than ten carbons, the team said. They showed higher selectivity toward paraffins over aromatics, allowing the researchers to target different crude oil mixtures.

“Another crucial factor for the success of these membranes is their asymmetric porous morphology,” Nunes said in a statement.

The top surface of the membranes are said to have presented an ultrathin dense layer consolidated by crosslinking, providing size selectivity. The underlying layers showed a highly porous structure with open interconnected pores that gradually increased with increasing depth to enable permeation.

Stefan Chisca, a research scientist at KAUST and lead author of a paper describing the research in Science, said the team is now scaling up the membranes and manufacturing test modules for pilot plants.

Have your say on this story in the comments box below