Two-Stage Biodiesel Synthesis from Used Cooking Oil with a High Acid Value via an Ultrasound-Assisted Method

Abstract

In this study, ultrasound was used to accelerate two-stage (esterification–transesterification) catalytic synthesis of biodiesel from used cooking oil, which originally had a high acid value (4.35 mg KOH/g). In the first stage, acid-catalyzed esterification reaction conditions were developed with a 9:1 methanol/oil molar ratio, sulfuric acid dosage at 2 wt %, and a reaction temperature of 60 °C. Under ultrasound irradiation for 40 min, the acid value was effectively decreased from 4.35 to 1.67 mg KOH/g, which was decreased to a sufficient level (<2 mg KOH/g) to avoid the saponification problem for the subsequent transesterification reaction. In the following stage, base-catalyzed transesterification reactions were carried out with a 12:1 methanol/oil molar ratio, a sodium hydroxide dosage of 1 wt %, and a reaction temperature of 65 °C. Under ultrasound-assisted transesterification for 40 min, the conversion rate of biodiesel reached 97.05%, which met the requirement of EN 14214 standard, i.e., 96.5% minimum. In order to evaluate and explore the improvement of the ultrasound-assisted two-stage (esterification–transesterification) process in shortening the reaction time, additional two-stage biodiesel synthesis experiments using the traditional mechanical stirring method under the optimal conditions were further carried out in this study. It was found that, under the same optimal conditions, using the ultrasound-assisted two-stage process, the total reaction time was significantly reduced to only 80 min, which was much shorter than the total time required by the conventional method of 140 min. It is worth noting that compared with the traditional method without ultrasound, the intensification of the ultrasound-assisted two-stage process significantly shortened the total time from 140 min to 80 min, which is a reduction of 42.9%. It was concluded that the ultrasound-assisted two-stage (esterification–transesterification) catalytic process is an effective and time-saving method for synthesizing biodiesel from used cooking oil with a high acid value.

Bifunctional Co3O4/ZSM-5 Mesoporous Catalysts for Biodiesel Production via Esterification of Unsaturated Omega-9 Oleic Acid

“…Biodiesel yield (%) = mole of alkyl ester/mole of oleic acid in feed × 100 (5) The experimental data reported herein were an average of three measurements. The standard deviation varied within ±2.5%.…”

“…Natural or synthetic biodiesel has other advantages over mineral, oils such as higher biodegradability and high flash points [3]. Esterification and transesterification reactions are widely applied in biodiesel production via catalytic techniques [4,5]. The conventional biodiesel production process utilizes homogeneous basic catalysts, such as liquid NaOH and KOH, and acidic catalysts, such as H 2 SO 4 , which merge as part of the products at the end of the processes.…”

Enhanced Biodiesel Synthesis via a Homogenizer-Assisted Two-Stage Conversion Process Using Waste Edible Oil as Feedstock

“…Methyl laurate was added to the crude biodiesel as an internal standard, and then the sample was injected under the following conditions: carrier gas nitrogen; injector temperature 280 • C with a split ratio of 1:20; and temperature of the detector 300 • C. The oven temperature started at 210 • C for 4 min, then increased to 240 • C at a rate of 4 • C/min, and finally maintained at 240 • C for 8 min [39]. Equation ( 4) could be used to calculate the conversion rate of the crude biodiesel [16,29,30] The use of a homogenizer at different speeds to facilitate the transesterification reaction did result in different acid value levels, as shown in Table 1 (operation conditions: catalyst (sulfuric acid) amount 3 wt%, methanol/oil molar ratio 12:1, reaction time 10 min, reaction temperature 65 • C). The acid value decreased as the speed increased.…”

“…When the FFA content is less than 0.5% and if the used catalyst is expected to be removed from the remaining oil easily, a solid base catalyst is chosen to complete the transesterification reaction. In general, a combined acid-base two-stage catalytic method can be considered to shorten the reaction time [16][17][18].…”

“…Thus, they are important factors to be investigated. In addition, the process can also be accelerated by using ultrasound [16,27,28] or microwave [29][30][31][32] to promote the reaction. In the previous study [25], the optimal reaction conditions including methanol/oil molar ratio, catalyst dosage, reaction temperature, and reaction time were investigated to achieve the highest conversion of biodiesel.…”