Valorization of a high-acidity residual oil generated in the waste cooking oils recycling industries

Abstract

A sludge fraction is obtained from the industries which recycle cooking oil and this sludge contains a large amount of oil with an extremely high acidity ( > 60%). In this work, we propose a scheme for methyl ester production from this residual oil consisting of the esterification of the free fatty acids followed by the transesterification of the remaining triglycerides. Esterifications were carried out with different methanol:oil molar ratios, and various catalysts in different weight ratios. The results revealed that homogeneous catalysts produced higher yields than heterogeneous ones in the esterification reaction. With the aim of improving the process, a previous triglyceride hydrolysis was assayed using lipases from Candida rugosa. Finally, the 3-stage process was performed under the most favorable conditions for each stage obtaining 84% wt. fatty acid methyl esters, which shows the potential of this residual oil as a source of biodiesel.

Determination of the Acidity of Waste Cooking Oils by Near Infrared Spectroscopy

“…The HORECA (hotels, restaurants, and catering) sector produces roughly 400,000 tons of WCO per year in Spain [3]. As a profitable market, many enterprises dealing with the collection and recycling of waste cooking oils for subsequent biodiesel production have been set up [6].…”

“…WCO recycling companies usually collect these oils from clean points or special containers, or buy entire trucks from smaller companies. After filtering in their facilities to remove leftover food, flour, etc., WCO are left to decant to obtain three phases: Clean oil, water, and sludge [6]. This oil is then analysed.…”

Esterification of Free Fatty Acids with Glycerol within the Biodiesel Production Framework

“…Companies responsible for the collection, storage, and treatment of WCO usually have to deal with oils with high acidity, which cannot be used as raw material for subsequent biodiesel production [3,4]. This is because free fatty acids react with the catalyst of the transesterification reaction, rendering 2 of 10 soaps (saponification reaction, Equation (1)).…”

“…The most suitable conditions found by several authors for this procedure when treating oils with high acidity were 5% H 2 SO 4 (wt.) as catalyst [4,7,8], 15:1 methanol to WCO molar ratio [4], 160 • C [4,8] and 2 h reaction time [4,8]. Under these conditions, the acidity of a WCO was reduced from 60.5% to 1% FFA.…”

“…Notwithstanding this, the problem is the further transesterification of the triglycerides, which is not only affected by the presence of water (hydrolysis of triglycerides) but is also inhibited by the presence of high amounts of the product of this reaction, i.e., FAME. As result, the previous esterification drastically reduces the efficiency of the subsequent transesterification, leading to incomplete triglycerides conversion [4].…”