Use of Crude Glycerol for Glycerolysis of Free Fatty Acids in Crude Palm Oil

The glycerolysis of crude palm oil (CPO) using potassium hydroxide was studied. This work aimed to evaluate free fatty acid (FFA) conversion in CPO through glycerolysis using crude glycerol. The effect of molar ratios of FFA to crude glycerol, reaction temperature, stirring speed, and catalyst loading was investigated. The best conditions for the highest FFA conversion of 95.45% were found: 1:4 of FFA to crude glycerol molar ratio, 75°C of reaction temperature, 250 rpm stirring speed, and 0.75% (w/w of oil) of catalyst loading. These conditions have decreased the FFA of CPO from 6.15% to 0.28% in 90 minutes. A kinetic model of the second-order reaction rate was used to quantify the influence of process parameters. It can be concluded that the crude glycerol as a side product of transesterification can be utilized in glycerolysis to lower the cost of biodiesel production.

Crude glycerol; Crude palm oil; Free fatty acid; Glycerolysis

The increasing demand for fuel as an energy source is a challenge due to the depletion of petroleum reserves. Another aspect that has become a concern in recent years is pollution using fossil fuels, which causes global warming—an increase in the earth's temperature. Biodiesel—fatty acid alkyl esters—is a renewable, non-toxic, and biodegradable fuel that can replace petroleum diesel fuel (Ibrahim et al., 2020). Biodiesel has some advantages compared to petroleum diesel, such as renewable, clean-burning fuels, and biodegradable ones that produce fewer toxic pollutants and greenhouse gasses (Hidayat et al., 2018; Amin, 2019).

Several methods have been developed to produce biodiesel, such as esterification, transesterification, pyrolysis, supercritical reaction, and lipase-catalyzed (Kottititum et al., 2014). The most common method is transesterification. In this method, the vegetable oil reacts with an alcohol (e.g., methanol or ethanol) in the presence of an alkaline catalyst to produce biodiesel and glycerol as a by-product. Transesterification is considered more efficient and can produce biodiesel with a high yield and purity. However, this process requires high purity oil as a raw material. Oil as a raw material for transesterification reaction should contain FFA <0.5% or acid number <1 mg KOH/g (Kara et al., 2018). The presence of FFA in oil can cause side reactions through alkaline-catalyzed transesterification to form soaps and prohibit ester product and glycerol separation. As a result, the quantity and quality of biodiesel obtained will be reduced if it does not use an appropriate production process (Aparamarta et al., 2019). Esterification is generally the initial treatment of raw materials with high FFA content and is carried out before transesterification. However, esterification requires relatively high temperatures, large amounts of alcohol, and acid catalysts. Berrios et al. (2010) used a molar ratio up to 1:60 of FFA to methanol to reduce the FFA of waste cooking oil from 2.14% to 0.34%.

Glycerolysis is an alternative method that can be carried out to reduce FFA content in oil as biodiesel feedstock before the transesterification reaction. This process is usually applied when FFA content is higher than 5% (Mamtani et al., 2021). Additionally, glycerolysis is more profitable as no excessive amount of methanol is used during the process like in acid esterification (Elgharbawy et al., 2021). Glycerolysis is considered as a potential method because the use of glycerol as a byproduct of transesterification process; therefore, it is expected to reduce the price of biodiesel production (Supardan et al., 2017).

Wang et al. (2011) reported the application of a superacid solid catalyst for the glycerolysis of FFA in waste cooking oil. However, the use of superacid solid catalysts has several disadvantages, such as high catalyst prices, excess glycerol needs, catalyst separation difficulties, and high reaction temperatures. Many studies reported that base catalysts could be used to speed up a glycerolysis reaction. Kombe et al. (2013) and Kombe (2015) have examined the use of NaOH base catalysts for glycerolysis of castor and jatropha oils. Based on their research, the FFA content can be reduced from 4.54% and 6.50% to 0.07% and 0.06%, with temperatures of 65°C and 56°C, respectively. Supardan et al. (2017) used co-solvent to enhance the mass transfer in glycerolysis of waste cooking oil. Recently, the intensification of the glycerolysis reaction using hydrodynamic cavitation was reported by Satriana et al. (2018).

        Currently, crude palm oil (CPO) is the major vegetable oil source for biodiesel production in Indonesia. However, the FFA content in CPO is relatively sensitive to the environment. There is potential for the utilization of CPO with high FFA to lower biodiesel production cost. In this work, CPO with high FFA content reacted with crude glycerol as a low-cost reactant obtained from transesteri?cation using a base catalyst. This research is expected to contribute to the development of the biodiesel manufacturing process. The objective of this work was to evaluate the effect of crude glycerol to FFA molar ratio, reaction temperature, stirring speed, and catalyst loading on the performance of the glycerolysis process. The effect of process parameters is quantified through the study of glycerolysis kinetics. 

        In the present study, free fatty acid in CPO was reduced through glycerolysis using crude glycerol. The base catalyst, KOH, can efficiently catalyze glycerolysis of FFA in CPO using crude glycerol. In the range of experiments studied, the initial FFA of 6.15% was reduced to 0.28% under the following conditions: 1:4 of FFA to crude glycerol molar ratio, 75°C of reaction temperature, 250 rpm of stirring speed, and 0.75% w/w of catalyst loading. Therefore, glycerolysis is potential to be applied because it would decrease the FFA in CPO to be lower than 0.5% as biodiesel feedstock for a transesterification reaction. The kinetic study showed that the second-order reaction rate could be used to describe the glycerolysis reaction. It can be concluded that the utilization of crude glycerol as a by-product of transesterification for reducing the cost of biodiesel production has the potential to be implemented. However, there is still a need to develop an intensification process for glycerolysis to lower the temperature and enhance the immiscibility of oil and glycerol phases.

        We would like to acknowledge the contributions of Universitas Syiah Kuala, which has funded this research through Program Riset Unggulan Universitas Syiah Kuala Percepatan Doktor (PRUU-PD) research grant No. 292/UN11/SPK/PNBP/2020.