To make biodegradable plastics, manufactures are increasingly turning to polylactic acid (PLA). Demand for PLA is expected to reach one megaton a year by 2020.
Currently, most PLA is produced through sugar fermentation, but researchers say they’ve developed a cheaper and cleaner way to produce it: using glycerol, a waste by-product of biodiesel production.
Glycerol is a by-product in the manufacturing of first-generation biofuels, and as such is not high-grade but contains residues of ash and methanol.
“Nobody knows what to do with this amount of waste glycerol,” says Merten Morales, a PhD student at ETH Zurich and part of a research team led by Professor Konrad Hungerbühler.
This waste substance is becoming more and more abundant, with 3 megatons in 2014 expected to increase to over 4 megatons by 2020. Because of its impurity, glycerol is not suitable for the chemical or pharmaceutical industry.
It also is not a good energy source because it doesn’t burn well.
“Normally, it should go through waste water treatment, but to save money and because it is not very toxic, some companies dispose of it in rivers or feed it to livestock. But there are concerns about how this affects the animals.”
20 Percent Less CO2
The new procedure reduces the overall CO2 emission by 20 percent compared to fermentation: per kilogram of lactic acid produced, 6 kilograms of CO2 are emitted with the new method compared to 7.5 kilograms with the conventional technology.
Also, by lowering the overall cost of the process, the researchers calculated a 17-fold increase in potential profits.
“Our calculations are even rather conservative,” says Morales. “We assumed a glycerol feedstock of relatively good quality. But it also works with low-quality glycerol, which is even cheaper.”
How The Process Works
Glycerol is first converted enzymatically to an intermediate called dihydroxyacetone, which is further processed to produce lactic acid by means of a heterogeneous catalyst.
A team led by Javier Pérez-Ramírez, a professor at the Institute for Chemical and Bioengineering, designed a catalyst with high reactivity and a long life span. It consists of a microporous mineral, a zeolite, whose structure facilitates chemical reactions within the pores.
Pérez-Ramírez’s team collaborated with Hungerbühler’s group to improve the catalyst step by step, while at the same time performing the life-cycle assessment of the procedure as a whole.
“Without the assessment and comparison with the conventional method, we might have been happy with an initial catalyst design used for our study, which turned out to be less eco-friendly than fermentation,” explains Pierre Dapsens, a PhD student in the Pérez-Ramírez group.
By improving several aspects of the catalyst design, the researchers were finally able to surpass sugar fermentation both from an environmental and an economic point of view.
Industrial processes are often turned “sustainable” simply by switching to a renewable resource.
“However, taking the whole process into account—from the source of the feedstock to the final product and including waste management—you will often find that a supposedly sustainable production method is not necessarily more sustainable than the conventional one,” adds Cecilia Mondelli, a senior scientist in the Advanced Catalysis Engineering group, who is also involved in the study.
Read more about the method in the journal Energy and Environmental Science.