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ETH team develops catalytic process to make lactic acid from glycerol biodiesel byproduct; 20% lower CO2 than fermentation pathway

Researchers at ETH Zürich developed an eco-friendly cascade process to make large amounts of lactic acid from glycerol, a waste by-product in the production of biodiesel. Polylactic acid (PLA) is a promising alternative for making plastics, as it is biodegradable and made from renewable resources. Manufacturers use PLA for disposable cups, bags and other sorts of packaging. The demand for PLA is constantly rising and has been estimated to reach about one megaton per year by 2020.

The research groups of ETH professors Konrad Hungerbühler and Javier Pérez-Ramírez at the Institute for Chemical and Bioengineering are now introducing a new method to produce lactic acid. The process is more productive, cost-effective and climate-friendly than sugar fermentation, which is the technology currently used to produce lactic acid. The new method’s greatest advantage is that it makes use of a waste feedstock: glycerol.

Glycerol is a by-product in the manufacturing of biodiesel, and as such is not high-grade but contains residues of ash and methanol. This waste substance is becoming more and more abundant, with 3 megatons in 2014 expected to increase to more than 4 megatons by 2020. Because of its impurity, glycerol is not suitable for the chemical or pharmaceutical industry. Moreover, it does not burn well and is thus not a good energy source.

Nobody knows what to do with this amount of waste glycerol. 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.

—Merten Morales, a PhD student in the Safety and Environmental Technology group of professor Hungerbühler.

In the ETH cascade procedure, glycerol is first converted enzymatically to an intermediate called dihydroxyacetone, which is further processed to produce lactic acid by means of a heterogeneous catalyst.

The researchers of the Advanced Catalysis Engineering group of professor Pérez-Ramírez designed a tin-containing MFI zeolite catalyst with high reactivity and a long life span. The close collaboration between the two research groups allowed the catalyst to be improved step by step while at the same time performing the life cycle assessment of the procedure as a whole.

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.

Taking into account the energy saved by using the waste feedstock glycerol and the improved productivity, the new procedure reduces the overall CO2 emission by 20% 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 of the profit possible by using the new process.

Resources

  • Morales M, Dapsens PY, Giovinazzo I, Witte J, Mondelli C, Papadokonstantakis S, Hungerbühler K, Pérez-Ramírez J (2014) “Environmental and economic assessment of lactic acid production from glycerol using cascade bio- and chemocatalysis,” Energy & Environmental Science, doi: 10.1039/C4EE03352C

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