RESEARCHERS at the King Abdullah University of Science and Technology (KAUST), Saudi Arabia have developed a way to convert plastic bottles into porous membranes that could be used for molecular filtration in the chemical industry. The method has the potential to help achieve significant energy savings and “upcycle” plastic waste.
40% of operational energy in the chemical industry is consumed by traditional heat-intensive separation processes. According to Lively and Sholl (2016), membrane-based chemical separation would use 90% less energy than distillation, a thermal separation process which accounts for 49% of the energy consumed by separation processes in the US. According to the paper, energy consumed by separation processes in the US accounts for about 45–55% of total US energy consumption.
However, most conventional membranes are not robust enough to withstand solvents and conditions used in the industry, and alternative ceramic membranes tend to be expensive. Lead researcher Bruno Pulido said that common plastic polyethylene terephthalate (PET) – primarily used in packaging, including plastic bottles – is intrinsically mechanically and chemically robust and could be used in processes requiring harsh chemicals and conditions.
In 2016 about 50m t of PET was produced, accounting for about 9% of plastic production. That same year 29% of PET was recycled in the US, and 52% in the EU. Most recovered PET is “downcycled” and used to create lower-value products.
The proof-of-concept research at KAUST showed that PET could be “upcycled” to produce robust membranes, a product with technological and economic value according to Pulido.
The polymer membranes were produced using the non-solvent induced phase separation method. In the method, the polymer is first solubilised in an appropriate solvent along with a polymer additive. The polymer is then spread (cast) on a surface such as glass or a support – glass in this case – and then submerged in a liquid in which the polymer is not soluble to precipitate the polymer and create a porous polymer film.
The researchers tested a number of production conditions. The desirable membranes – due to their high and relatively uniform porosity – were designated M8, M9, M17, and M18. Most of the other membranes produced could have niche applications depending on the desired pore size.
All the desirable membranes were successfully applied in ultrafiltration in N-dimethylformamide – an organic solvent – at temperatures up to 100 °C, something that most polymeric membranes cannot do. This achieves the main objective of the research which according to Pulido was to enable the use of PET membranes as a platform for organic solvent nanofiltration applications. Specifically, for the recovery of “active pharmaceutical ingredients, rare metal catalysts, and liquid hydrocarbon polishing”.
According to Pulido, there is interest in the use of membranes in organic solvents, “with the promise of having energy savings in industrial separation and purification processes, particularly at the nanofiltration level”.
The developed membranes could be used in thin-film composite membranes, as a support for thin layers of other filtration materials, such as those found in reverse osmosis membranes, said Pulido. They could also be used in applications requiring high chemical and heat resistance.
Additionally, the membranes can withstand acid and bleach, making them useful for filtration and purification processes that require cleaning with these harsh chemicals. Examples include clarification of juices and beverages and purification of whey from milk, said Pulido.
The researchers are currently working to optimise membrane preparation to obtain smaller pore size whilst using hexafluoroisopropanol (HFIP), a non-corrosive solvent alternative to trifluoroacetic acid (TFA). They are also “exploring different techniques and chemistries to prepare thin selective layers on the membranes to separate, with good resolution, molecules of less than 1,000 g/mol molecular weight”.
They are also working to develop hollow fibre membranes. Hollow fibre modules offer higher area-to-volume ratio compared to flat membranes. Additionally, they do not require non-woven supports or spacers – used to provide mechanical stability – required by traditional spiral wound modules that use flat sheet membranes.
Pulido said: “During the 20th Century, membranes went from seldom used, as an analytical tool in the lab, to large-scale industrial installations with a critical economic impact. However, there are still many niche opportunities for membrane technology to expand and contribute in the chemical industry.”
Chemical engineers could facilitate technology transfer and implementation, he said.
ACS Applied Polymer Materials: http://doi.org/dfks
A previous version of this article incorrectly referred to PET as "polyethylene". This should have read "polyethylene terephthalate".
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