Enzymatic esterification of acid oil from soapstocks obtained in vegetable oil refining: Effect of enzyme concentration

Significance Statement

Presently, many countries are still persistently using fossil fuels despite the high environmental pollution risk they pose. Currently, the effects of excessive emission of the oxides of carbon, nitrogen and sulphur are evident through acid rain and global warming. To counteract this, the world is turning to biofuels mainly due to the lower carbon dioxide emissions. Biodiesel, consisting of a mixture of fatty acid methyl esters, is mainly obtained by transesterification of triglycerides found in vegetable oils and animal fats. Due to its economic feasibility, homogeneous alkaline transesterification is widely used in the biodiesel production industry. However, this technique becomes inapplicable when the raw material has a high free fatty acids content, as it results in the consumption of the catalyst, yield reduction and soap production.

Soapstock, a low cost and highly acid raw material, mainly obtained as a residue from chemical processes used in the refining of vegetable oils, seems to be a promising and sustainable material for biodiesel production. Conversely, due to its high free fatty acids content, an alternative production route is required to convert such fatty acids in methyl esters. The use of chemical and biological heterogeneous catalysts in such process seems promising. Even though, the use of acid oil from soapstock for biofuels production, and more specifically the applicability of bio-catalysts for its esterification, are still poorly studied.

A research study, led by Joana Maia Dias at LEPABE-Laboratory for Process Engineering, Environment, Biotechnology and Energy (Faculty of Engineering, University of Porto) in Portugal, evaluated the enzymatic esterification of an acid oil from soapstocks obtained in vegetable oil refining using a commercial lipase in batch reactors under specific conditions. Her team established as main objectives to characterize the acid waste oil aiming enzymatic esterification and evaluate the need for pretreatment, study the effect of enzyme concentration during esterification using selected reaction conditions and perform complementary studies for assessing the effect of other key variables. This research work is now published in the Renewable Energy journal.

The researchers started their empirical work by conducting a pretreatment on the acid oil to reduce its mineral acidity before enzymatic esterification could be undertaken. They then undertook the esterification reactions under batch conditions where vigorous magnetic stirring was maintained. The methyl ester content in the final product was determined by gas chromatography.

They observed that the amount and type of alcohol utilized had minor influence in the reaction conversion and that the fractionated addition of methanol had only expressive effect for lower catalyst concentrations, with final conversions being still unsatisfactory. They noted that the best conditions were 4 wt.% of enzyme, 35 °C, 24 h, and 1:1.5 molar ratio of acid:alcohol, which afforded an 80% reduction of acidity.

Herein, enzymatic esterification of acid oil of soapstock from vegetable oil refining has successfully been demonstrated. It has been seen that the enzyme concentration deeply influences the reaction at the range of conditions studied. More so, the enzymes allow not only the esterification of the fatty acids but also the transesterification of various glycerides present, thereby obtaining a final product having a good concentration of methyl esters, which is highly valued in the biofuels industry. This technique is very adaptable and will enable the future production of biodiesel in a more sustainable manner.



Mariana Cruz, Sílvia Cardinal Pinho, Ricardo Mota, Manuel Fonseca Almeida, Joana Maia Dias. Enzymatic esterification of acid oil from soapstocks obtained in vegetable oil refining: Effect of enzyme concentration. Renewable Energy (2017) 1-7

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