A by-product of the production of refined palm oil (RPO) is a low-cost product known as palm fatty acid distillate (PFAD) [
1]. PFAD is not edible and is commonly used as a feedstock for the production of soap, animal feed, and oleochemical products [
2]. Moreover, vitamin E, which has a high market value, is extracted commercially from PFAD, so it can also be used as a feedstock for health foods and in the pharmaceutical industry [
3]. PFAD comprises mainly free fatty acids (FFA) and is a promising biofuel feedstock [
4]. On average 72.53 million tons of crude palm oil (CPO) production could be produced in 2020 [
5]. In addition, there is no official production data of PFAD, however, the estimated PFAD production was estimated based on the approximately 4% of FFA in CPO [
6]. Thus, approximately 2.9 million tons per year of PFAD was produced. For the largest CPO production countries such as Indonesia, Malaysia, and Thailand, the CPO production was 43.5, 19.9, and 3.1 million tons, and the approximate production of PFAD was 1.74, 0.796, and 0.124 million tons, respectively. The use of ultrasound (US) can assist the mixing of immiscible liquids in biodiesel production by inducing sonochemical reactions [
7]. US treatment has been applied to biodiesel production to decrease the reaction time, decrease the consumption of chemical reactants, and increase the product yield [
8,
9]. The dramatic effect of US stems from cavitation, which involves the rapid collapse of ultrasonically generated bubbles, causing localized pressure and temperature spikes as well as strong microscale mixing. US treatment is frequently used in various applications such as oil emulsification, food production, and homogeneous mixing for chemical reactions [
10]. In particular, US treatment has been used for homogeneous esterification in the production of biodiesel from high-FFA feedstocks. For example, Hayyan et al. [
11] studied the US treatment of low-grade palm oil (LGPO). They found that sonication induced esterification reactions, indicating that US treatment is an appropriate and beneficial route for the production of biodiesel. The initial 20% FFA in the LGPO was decreased to less than 3% under the optimum conditions, which involved 2 wt.%, H
2SO
4, a methanol-to-LGPO ratio of 10:1, and a reaction time of 300 min at 50 °C [
11]. Mohod et al. [
12] studied the application of acoustic cavitation for methyl ester production from a high-FFA karanja oil. The acid value (AV) of the karanja oil was reduced from 14.15 mgKOH g
−1 to less than 2.7 mgKOH.g
−1 after processing with a methanol-to-oil molar ratio of 5:1 and 2% H
2SO
4 loading at ambient temperature [
12]. Andrade-Tacca et al. [
13] used US treatment to achieve auto-induced temperature increases and mechanical mixing to reduce the AV of jatropha oil (36.56 mgKOH.g
−1) by acid-catalyzed esterification pretreatment. The esterification conversion efficiencies were 56.73% and 83.23% after US treatment for 10 and 30 min, respectively. These FFA conversion efficiencies were much higher than those achieved with mechanical mixing (i.e., 36.76 and 42.48% after 10 and 30 min reaction, respectively). Furthermore, the AVs of the jatropha oil esters were 15.82 and 23.12 mgKOH.g
−1 when US and stirring were used, respectively, after 10 min reaction. Thus, clearly US has a significantly better effect than mechanical mixing in reducing the AV of jatropha oil [
13]. Trinh et al. [
14] carried out biodiesel production from rubber seed oil (RSO) using an US homogenizer at a frequency of 18 kHz and power of 500 W. After esterification, the high FFA content of the RSO was reduced from 40.14% to 0.75% under the optimal conditions, which were a methanol-to-oil molar ratio of 23:1, 7.5 wt.% H
2SO
4, and 30 min sonication time at 50 °C [
14]. Joshi et al. [
15] also investigated the esterification of karanja oil using US treatment. The esterification process was optimized by selecting operating parameters for an US horn and an US flow cell. The US horn reactor reduced the AV from 10.5 to less than 2.34 mgKOH.g
−1 after treatment with a methanol-to-oil molar ratio of 6:1, 1.5 wt.% H
2SO
4, 70% duty cycle, and 90 min reaction time. In contrast, the US flow cell reduced the AV of the karanja oil to less than 2.77 mgKOH.g
−1 under the optimal conditions: 1.5 wt.% H
2SO
4, methanol-to-oil molar ratio of 6:1, 70% amplitude, and 0.33 L.min
−1 flow rate. Maximum AV conversions of 77% or 74% were obtained using the US horn and US flow cell, respectively, making the former superior to the latter [
15].
The use of US treatment in multi-stage biodiesel production has been previously investigated. Gole and Gogate (2013) optimized the methanol molar ratio and the sulfuric acid concentration for two-step methyl ester production from high AV Nagchampa oil using US, microwaves (MW), and microwaves followed by US (MW/US). Esterification was employed to decrease the AV of Nagchampa oil from 18.9 to 1.7 mgKOH.g
−1 to prevent saponification reactions. The optimal methanol-to-oil molar ratios were 4:1, 3:1, and 2:1 and the optimal sulfuric acid concentrations were 3, 3, and 2 wt.% for US, MW, and MW/US treatments, respectively. For the transesterification reaction, the optimum amount of KOH required in all three methods was 1 wt.%, and the optimal methanol-to-oil molar ratios were 6:1, 6:1, and 4:1 for the US, MW, and MW/US treatments, respectively. They concluded that microwaves combined with US improved the rate of biodiesel processing, and also allowed the use of less methanol and energy [
19].
In Thailand, biodiesel is commonly produced from crude palm oil [
20]. Methanol is used as the alcohol in the esterification and transesterification reactions [
21]. However, methanol is toxic and dangerous and can be converted into formaldehyde and, further, into formic acid. Generally, the methanol used in conventional biodiesel production is produced from petrochemicals. In contrast, ethanol is non-toxic, and its production in Thailand is based on the fermentation of cassava, molasses, and sugarcane raw materials. Moreover, ethanol contains more carbon than methanol, giving it a higher heating value on combustion [
22]. Currently, ethanol is widely used in transesterification to prepare ethyl ester from low-FFA oil with a biodiesel production process in batch mode. However, a few studies have attempted to extend the use of ethanol to esterification reactions for the production of biodiesel from high-FFA oil using US treatment. Hanh et al. [
23] studied biodiesel production from oleic acid by US-assisted esterification with ethanol, butanol, and propanol. They varied the alcohol-to-oleic-acid ratio, concentration of acid catalyst, and sonication time. The optimum conditions for operation were an alcohol-to-oleic-acid molar ratio of 3:1, 5 wt.% H
2SO
4, and 120 min irradiation time at 60 °C [
23]. Kumar et al. [
24] used US treatment to produce ethyl esters from coconut oil. The optimum conditions were an ethanol-to-oil molar ratio of 6:1, 0.75 wt.% KOH, and 7 min reaction time. The best yield (98.2%) was obtained at an ethanol-to-oil molar ratio of 6:1, whereas the same result was achieved with an ethanol-to-oil molar ratio of 8:1 in a conventional batch process [
24]. From the above literature review, homogeneous catalysts have been most widely investigated in the transesterification and esterification of oils to produce the biodiesel. The advantages of homogeneous catalysts are that they have a faster reaction, cheaper catalyst, and require lower catalyst loading than heterogeneous catalysts. To date, the continuous production of ethyl esters using US clamps on a tubular reactor with homogeneous catalysts of KOH and H
2SO
4 has been rarely reported. Thus, in the current study, we performed two-step continuous esterification using H
2SO
4 followed by continuous transesterification using potassium hydroxide (KOH) as the base catalyst in US-modified tubular reactors. The raw materials, PFAD, the first-step esterified oil, and the second-step esterified oil, were used in the first, second, and third steps, respectively. The key component in this novel process is the US-assisted tubular reactor for the continuous production of palm fatty acid distillate ethyl ester (PFADE as biodiesel). The US treatment improves the physical and mass transfer effects of the raw materials and, thus, enhances chemical reactions via the shock waves produced by cavitation [
25]. An assessment of the parameters influencing PFADE production in our US-modified tubular reactor was also carried out, and we optimized the ethanol content, H
2SO
4 content, KOH loading, and length of US clamps based on the purity of the end-product PFADE over the three processing steps. To achieve this optimization, we used a five-level, three-factor central composite design (CCD) of experiments using the response surface methodology (RSM). Finally, the total average electricity and chemical consumption for the US-assisted process are reported for the treatment of PFAD at a flow rate of 25 L h
−1.