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Continuous production of biodiesel from waste cooking oil using green engineering

Final Report Summary - NOVABIODIESEL (Continuous production of biodiesel from waste cooking oil using green engineering)

The main objective of this project was the development of a complete continuous reaction/separation process for the production of biodiesel (FAME: fatty acid methyl esters) from waste cooking oil (mainly from sunflower, which accounts for most of the edible oil consumed in Portugal) in supercritical carbon dioxide (scCO2). The production of biodiesel was done via an enzymatic reaction which is more selective, avoids the need to neutralize the reaction mixture, and is more amenable to recycling than chemical catalysis. ScCO2 acts both as reaction medium and as the solvent for the separation and purification of biodiesel. The whole process was carried out in a continuous mode, unlike more common processes where reaction and the various separation steps that follow are not integrated. CO2 is non‐toxic, non‐ flammable, abundantly available, and its use is allowed in the food industry. By using only scCO2 as solvent, a cleaner, greener process is obtained. Cleaner chemical processes are both a great challenge and a great priority for European chemists and chemical engineers in the 21st century. By using waste cooking oil as starting material instead of virgin oil, the serious environmental and economic problem of the increasing price of crops is minimized. Through the integration of a diversity of scientific topics such as biocatalysis, supercritical fluids and continuous reaction/separation processes with compressed fluids, the present project offers solutions to the problems that are found in most processes for biodiesel production, such as the low reaction yields and the difficult separation/purification of FAMEs, and can lead to an industrially viable process for the production of biodiesel that is also environmentally attractive.

The project was divided in four main tasks:  

Task 1 evaluated the composition of waste cooking oil (WCO), to assess the influence of the cooking process in the composition of WCO and its influence in the transesterification reaction.

Task 2 evaluated the continuous enzymatic transesterification of virgin and waste cooking oil with methanol and ethanol in scCO2, using different enzymes. In this task, reaction parameters, i.e. temperature, pressure, oil to alcohol molar ration and scCO2 flow rate were optimized for three different enzymes – Novozyme 435, Lipozyme TL IM and Lipozyme RM IM – and two alcohols – methanol and ethanol. 
Task 3 evaluated product fractionation by varying temperature and pressure conditions in a series of gravimetric separators. The separation of the two reaction products, biodiesel and glycerol, can be achieved by changing scCO2 density and with it its solvent power. Given that glycerol is less soluble in scCO2 than biodiesel, glycerol precipitates in a first separator, while biodiesel remains in the scCO2 phase. In cases where the reaction in not complete there will be mono-, di-  and triacylglycerols present in the product stream. Their separation from biodiesel with scCO2 was also evaluated.

Task 4 looked at the economical viability of the complete process.

A high pressure apparatus for the continuous transesterification of oil with methanol and ethanol to yield biodiesel via biocatalysis under scCO2 was developed.

This apparatus consists of two main sections: a reaction section with a high pressure packed- bed enzymatic reactor, and a separation section.

Reactions were carried out with the enzymes Novozyme 435, Lipozyme TL IM and Lipozyme RM IM, and with methanol and ethanol at 200 - 250 bar, 35 – 60 °C, with oil to alcohol molar ratios between 1:6 to 1:56 ,and scCO2 flow rates in the range 5 –  30 ml/min.