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Nov 13, 2024

Methyl ester production process from palm fatty acid distillate using hydrodynamic cavitation reactors in series with solid acid catalyst | Scientific Reports

Scientific Reports volume 14, Article number: 27732 (2024) Cite this article Metrics details This study aims to optimize the reduction of free fatty acids (FFAs) in palm fatty acid distillate (PFAD)

Scientific Reports volume 14, Article number: 27732 (2024) Cite this article

Metrics details

This study aims to optimize the reduction of free fatty acids (FFAs) in palm fatty acid distillate (PFAD) using hydrodynamic cavitation reactors (HCRs) in series and a solid acid catalyst for biodiesel production. Hydrodynamic cavitation is used to accelerate the esterification of FFAs using a heterogeneous acid catalyst. There are three HCRs units, and each HCR composed of a 3D-printed rotor and stator, is separated by flanges and equipped with a basket for holding Amberlyst-15 catalyst. Through response surface methodology (RSM), the esterification process is optimized by adjusting its optimal parameters, namely, methanol (2–12 wt%), circulation time (30–170 min), and rotor speed (1000–3000 rpm). The optimal conditions for achieving a maximum methyl ester purity of 89.76 wt% in converting FFA in first-step esterified oil are 9 wt% methanol (molar ratio of methanol to oil of 4:1), 133 min of circulation time, and 2000 rpm of rotor speed. An 82.48 wt% biodiesel yield is achieved from the HCRs in series under the optimal conditions. Scanning electron microscope images reveal that after the esterification process, there are minor cracks and defects on the catalyst’s resin surface, indicating the presence of residual reactants. Further examination of the catalyst after the esterification process, reveals an average absorption pore diameter of 341.41 Å and BET surface area of approximately 41.68 m2/g. Although there were slight physical changes in the catalyst, HCRs technology offers a viable FFA reduction process that could enhance biodiesel production efficiency. Moreover, the optimized conditions achieved in this study contribute to the advancement of biodiesel production processes and provide insights into the performance of the catalyst used.

The global production of biodiesel has continued to increase. By interfering with global supply and demand, the conflicts in Israel, Palestine, Russia, and Ukraine have had significant impacts on the global petroleum business and fuel prices1. In addition, a variety of other factors, including government policies, environmental concerns, and technological advances, have also contributed to the rise in demand for biodiesel as an alternative. Government policies are necessary for promoting the production of biodiesel, and several nations have established renewable energy goals and incentives to promote its widespread usage. Such policies include tax breaks, subsidies, and standards for blending biodiesel with regular diesel fuel2. First and foremost, biodiesel plays a crucial role in decreasing greenhouse gas emissions. In comparison with petroleum-based diesel, biodiesel produces lower levels of carbon dioxide, particulate matter, and sulfur compounds3,4. Reducing emissions not only helps combat climate change, but also enhances air quality, thereby improving public health5. By adopting biodiesel, nations can improve their energy security and advance sustainable development. Various industries are increasingly using biodiesel fuel owing to its flexibility and eco-friendliness6. In the transportation industry, biodiesel can serve as a direct substitute for diesel fuel in current diesel engines without requiring any modifications to engines7. Blending biodiesel with petroleum diesel in various proportions offers the flexibility to comply with specific emission standards while achieving the desired performance requirements for diesel engines8. Furthermore, several industries, including power generation and other operations, have adopted biodiesel to a certain extent. Stationary diesel engines can use it to generate electricity and heat, thereby reducing reliance on conventional fossil fuel power plants9.

For biodiesel production, Thailand relies primarily on feedstocks derived from crude palm oil (CPO)10. O. Farobie et al. investigated the biodiesel production from CPO using the enzyme-catalyzed transesterification process. The highest methyl ester purity of 97.91% was obtained under the optimal condition of 40 °C reaction time, 0.3 wt% enzyme loading, and the methanol to oil molar ratio of 7:1 within 48 h reaction time11. However, CPO, when refined, is an edible oil, and much of its production is dedicated for use in foodstuffs. Therefore, if the proportion of biodiesel in petroleum diesel is to be further increased, there is a need to consider the use of non-edible oils as raw materials for the production of biodiesel. One such oil is palm fatty acid distillate (PFAD), a byproduct of the refining of CPO12. PFAD is already used in the manufacture of soap and detergent13, as well as in the animal feed industry14. However, PFAD has a very high free fatty acid (FFA) content15, and if it is to be used as a basis for the production of biodiesel, it is necessary to subject these fatty acids to a process known as esterification16. To obtain an acceptable methyl ester purity from PFAD, the esterification process needs to be accelerated through the use of a suitable catalyst17.

Both homogeneous and heterogeneous catalysts are used for the esterification in biodiesel production18. Sulfuric acid is a common homogenous acid catalyst for this process. However, as discussed by Oni et al.19 and Jomy et al.20, one common issue with this type of catalyst is corrosion of metal equipment by sulfuric acid21. Biodiesel production from high FFA oil frequently involves a two-step procedure. During the first step, an acid catalyst facilitates the conversion of FFAs in oil22. The second step uses an alkaline catalyst, such as potassium hydroxide (KOH) or sodium hydroxide (NaOH), to convert the glycerides and residual FFA in the oil from the first step into biodiesel23. One drawback of the two-step procedure is the possibility of leftover homogenous acid catalyst remaining in the processed oil that serves as the raw material for the second step. It is therefore necessary to neutralize any such residual acid catalyst before proceeding to the second step24. This requires the addition of an excess of alkaline catalyst. This increases the process cost and might be considered a disadvantage in comparison with the use of a solid acid catalyst.

Employing a renewable solid acid catalyst in the first processing step reduces the need for neutralization in the second step, resulting in cost savings and less of an environmental effect25. Moreover, a simple physical procedure, namely, filtering, can remove heterogeneous catalyst from the liquid reaction phase. In the esterification process, a solid acid catalyst is used to promote the formation of esters from alcohol and acid. Heterogeneous catalysts are solid materials that are typically insoluble in the reaction mixture and remain in a separate phase. Amberlyst-15 is one of the most well-known and widely used solid acid catalysts for esterification reactions. It is a sulfonic acid-functionalized polystyrene resin with a large surface area and strongly acidic sites for catalytic activity26. Moreover, heterogeneous acid catalysts like Amberlyst-15 have a number of desirable properties, including nonvolatility, nontoxicity, temperature stability, uniformity, dependability, reduced corrosion, regenerability, and reusability27. Hasanudin et al. used esterification with a sugar cane-derived zeolite-sulfonated carbon catalyst to reduce FFAs in sludge palm oil (SPO). Fourier-transform infrared spectroscopy (FTIR) confirmed the successful generation of ester compounds via this esterification process, with 94.19% FFA conversion being achieved. The optimal conditions for the reaction were found to be 78.98 °C, 119.97 min, and 2.97 g of catalyst. This research demonstrated the potential of the catalyst derived from sugar cane for use in biodiesel production from SPO28. Gupta and Deo studied the effects of preheating the Amberlyst-15 catalyst on biodiesel production using Karanja oil, with the aim of improving the catalyst’s efficiency and reusability by optimizing the preheating temperature. Their study found an optimal preheating temperature of approximately 200 °C, which led to a higher surface area and conservation of sulfonic acid groups on the catalyst. However, the catalyst lost its effectiveness on repeated use, with a decrease in biodiesel yield from 85 to 27% after four cycles. The preheated Amberlyst-15 catalyst samples were characterized by analyzing the surface area, pore volume, and content of sulfonic acid groups through scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and FTIR spectroscopy. This study provided valuable insights into the activation of catalysts for biodiesel production by preheating and into the reusability of heterogeneous catalysts29.

There have been a number of studies of the efficiency and yield of a variety of methods for enhancing the esterification reaction in biodiesel production, including hydrodynamic cavitation, mechanical stirring, microwave irradiation, and ultrasonic cavitation. Ultrasonic cavitation has several advantages in biodiesel synthesis compared with other methods, including better reaction kinetics, improved mass transfer, decreased catalyst usage, and enhanced product quality30. However, ultrasonic cavitation systems are costly in comparison with alternative cavitation techniques. The equipment necessary for producing and regulating ultrasonic waves can be expensive, limiting accessibility for certain applications or industries. Scaling up ultrasonic cavitation processes can be challenging31. As the quantity of raw material increases, it becomes increasingly difficult to maintain constant cavitation conditions throughout the system. Scaling up to large-scale industrial applications requires significant ultrasonic power. An increase in ultrasonic power will lead to a higher cost, mainly associated with the more powerful ultrasonic transducers required to convert electrical energy into high acoustic frequencies32.

Another cavitation approach for biodiesel production is hydrodynamic cavitation, which uses high-speed fluid flow to create and implode cavitation bubbles33. The rotor–stator type hydrodynamic cavitation reactor (HCR) has many advantages, including improved mass transfer, adaptability, cost efficiency, scalability, and reduced environmental impact34. Samani et al. studied biodiesel production from safflower oil using an HCR. These reactors have advantages such as reduced energy consumption and quicker reaction rates in comparison with traditional reactors owing to the large amount of energy released in the reactor by the formation and subsequent collapse of the cavitation bubbles. HCRs can thereby substantially accelerate the heat and mass transfer rates involved in chemical processes35. The optimal conditions for the highest biodiesel yield of 89.11% in the study by Samani et al. was a reaction time of 63.88 s, a catalyst concentration of 0.94 wt%, an alcohol-to-oil molar ratio of 1:8.36, and a separation of 15.3 mm between the rotor and stator. This study thus indicated that HCRs have the potential to improve the efficiency of biodiesel production and encourage the utilization of renewable fuels35. Sun et al. presented a critical analysis of recent improvements in HCR technology for biodiesel production. They demonstrate the potential of HCRs to improve acid- and alkali-catalyzed production processes, increase methyl ester purity (to as much as 99%), and satisfy the biodiesel quality standards EN 14,214 and ASTM D675136. Table 1 shows the purity of esters, the percentage of yields, the molar ratio of methanol to oil, the reaction time, the reaction temperature, and the type of catalyst, allowing for a comparison of the different reactors applied in the solid-catalyzed esterification process. The following Table 1 summarizes the various reactors used in the biodiesel synthesis process. There is currently no study that employs three HCR in series with stationary baskets designed to contain solid catalysts that decrease FFA from PFAD raw material.

In the present study, a novel type of HCR is developed that shares the advantages of continuous stirred tank reactors (CSTRs), namely, the ability to operate continuously, improved control over reaction conditions compared with batch reactors, high conversion efficiency, simple scaling up, safe operation, adaptability, and suitability for steady state operation42. In a CSTR, reactants are uniformly mixed, and this uniformity of reaction conditions helps to ensure consistent product quality43. However, for a single HCR to operate in the same way as a CSTR, a lengthy rotor and mechanical shaft would be required, resulting in the risk of misalignment and difficulty in controlling the reactor at high rotational speeds. Therefore, the HCR in this study is composed of three HCR units in series. The units are separated by flanges coupled with stationary baskets containing catalyst. Each HCR unit consists of a 3D-printed rotor and stator. The proposed reactor is applied to the reduction of the FFA content of PFAD using Amberlyst-15 catalyst, and the optimal process parameters are determined. The optimizing crucial parameters such as methanol (2–12 wt%), circulation time (30–170 min), and rotor speed (1000–3000 rpm) via the circulation esterification process was performed. Finally, the yield of the biodiesel product is evaluated after purification, providing valuable insights into the efficacy of this innovative approach.

PFAD was acquired from the refined palm oil refining process. At room temperature (32 °C), PFAD is a yellow wax phase with fatty acid compositions of 39.82 wt% palmitic acid, 44.70 wt% stearic acid, 11.41 wt% oleic acid, 1.04 wt% linoleic acid, 0.36 wt% of capric acid, 0.83 wt% of lauric acid, 1.51 wt% of myristic acid and contents, 0.29 wt% of palmitoleic acid and 0.05 wt% of arachidic acid44. It served as a raw material in the esterification process, which resulted in the reduction of FFA in PFAD by the utilization of heterogeneous catalysts. The other compositions of PFAD are FFA content of 98.36 wt%, triglyceride (TG) of 0.01 wt%, diglyceride (DG) of 1.32 wt%, monoglyceride (MG) of 0.31 wt%, and 0.866 kg/L density at 60 °C. In the esterification procedure, two commercial chemical reagents, namely, methanol with a purity of 99.7% and an Amberlyst-15 acid catalyst, were used. Amberlyst-15 in the form of black spherical beads was acquired from Sigma Aldrich in the United States. Following the esterification process, the catalyst exhibited a color change, transitioning from a darker to a lighter shade of black compared to the fresh catalyst. Amberlyst-15 has a porosity of 0.32 ± 0.01, a hydrogen ion-exchange capacity of 5.1 meqH+/g, and an average pore diameter of approximately 300 Å, and it is thermally stable up to 120 °C. Thin layer chromatography with flame ionization detection (TLC/FID, Iatroscan MK-6(s), Mitsubishi Kagaku Latron Inc., Tokyo, Japan) was used to analyze the percentages of methyl esters (MEs), triglycerides (TGs), diglycerides (DGs), monoglycerides (MGs), and free fatty acids (FFAs) in PFAD and esterified oil after the esterification reactions. The mobile phase for the development process was prepared by mixing hexane-diethyl ether-formic acid (80:20:0.2 vol%) in the first tank and hexane-benzene (1:1 vol%) in the second tank. A microdispenser was used for dropping a 1 µL hexane-diluted sample onto the origin (zero level of the rod holder) of the chromarods. The chromarods are submerged in the development tank, which contains the first mobile phase, until the solvent reaches 80 on the rod holder. The chromarods are then submerged in the development tank with the second mobile phase, to ensure the solvent level does not reach 100 on the rod holder. Following the development procedure, the chromarods’ mobile phase is removed using a rod drier (Model: TK-10) at 110 °C for 10 min. The chromarods are scanned with FID to evaluate MEs, TGs, DGs, MGs, and FFAs under regulated conditions of 165 mL/min hydrogen flow rate and 2.0 L/min air flow rate45. Ionization and vaporization occur as the chromarods pass through the hydrogen-air flame, enabling the FID to identify the components. The percentages of each compound in the sample (MEs, TGs, DGs, MGs, and FFAs) are determined using Eq. (1) with TLC/FID analyzer. However, the final samples of biodiesel have been carefully validated to analysis the purity of ME in the sample using the gas chromatograph–flame ionization detector (GC–FID, GC 7890; Agilent Technologies, USA) to verify the quality of ME. This sample was analyzed by GC–FID following the EN 14,103 test method. To determine product yield, the amount of product from each step was compared with the 100 wt% of PFAD. The product yields of the first-step esterified oil yield after soaking and the biodiesel yield after second-step esterification utilizing HCRs are determined using Eq. (2).

The production of MEs from high-FFA oil differs significantly from that in the case of low-FFA oil, because the high levels of FFAs result in lower methyl ester purity and have an effect on the quality of the final product. However, one method to overcome this difficulty is by soaking Amberlyst-15 resin in high-FFA oil together with methanol46. This procedure helps to reduce the FFA content and facilitates the next step, namely, the production of MEs. In the research of Kansedo et al., they described that the soaking dry Amberlyst 15 with methanol for 2–3 h was to allow the active sites of the ion exchange resins saturated with methanol. The porosity of Amberlyst-15 enables efficient transport of ions and molecules in and out of the resin, ensuring better access to active sites and promoting optimal reaction kinetics47. Deboni et al. reported good efficiencies during the removing of FFAs from miscellas composed of soybean oil and isopropanol using Amberlyst A26 OH resin in a fixed bed48. Cuevas et al. reported similar results for the deacidification of miscellas composed of palm oil and n-propanol using the same resin49. Therefore, in this study, a first step involving reduction of the FFAs in PFAD using the soaking method with Amberlyst-15 was used to enhance the effectiveness and efficiency of the subsequent esterification process in the HCR. The porosity of Amberlyst-15 enables efficient transport of ions and molecules in and out of the resin, ensuring better access to active sites and promoting optimal reaction kinetics50. Furthermore, Amberlyst-15 has a well-defined and uniform pore size distribution, which ensures consistent diffusion and adsorption properties. The porous structure of Amberlyst-15 also provides a large surface area for ion exchange, enhancing the catalytic activity and adsorption capacity of the resin51,52. Thus, Amberlyst-15 plays a crucial role in improving the performance of the biodiesel production process. Table 2 lists the properties of Amberlyst-15.

In the first stage of the soaking process, an electric heater was installed inside the soaking tank to warm the PFAD to 60 °C and thereby reduce its viscosity. The temperature of the soaking tank was measured by an electrical thermocouple. When the temperature of the PFAD reached 60 °C, methanol was poured gradually into the tank. A specified amount of Amberlyst-15 catalyst was prepared in the stainless steel sieve, which was wrapped with a #500 sieve with a sieve size of 25 μm to prevent the catalyst from leaking into the mixture in the tank. The fixed proportions used for the soaking process were 44.7 wt% of methanol and 38.6 wt% of Amberlyst-15. The Amberlyst-15 in the stainless-steel sieve was immersed in the mixture of PFAD and methanol. Analysis showed that the FFA content in the oil after this first step of esterification had been reduced from 98.36 wt% to 40.99 wt%, while 49.92 wt% ME was obtained after a four-hour soaking period. Following soaking, the catalyst was separated from the mixtures using a stainless-steel sieve. After filtration, the first-step esterified oil was used as a raw material in the second step of esterification using HCRs in series.

A Flashforge Creator 3 high-performance 3D printer with Flashprint software version 4.5.1 was utilized to fabricate the rotors from ABS filament. Three rotors were constructed using a high-performance option with 0.05 mm layer resolution and 100% filled filament. The layer thickness of the infill filament was set at 0.0025 mm. The rotor infill design included a hexagonal pattern for strengthening the filling filament. The printed rotors were installed in the individual HCR units. The schematic in Fig. 1 shows the HCRs installed in series. In each HCR, the rotor was 92 mm in height and 120 mm in diameter, and the stator had an inner diameter of 140 mm, so there was a 10 mm space between rotor and stator. There were 160 spherical indentations of diameter 8 mm and depth 12 mm on the surface of each rotor. Perpendicular holes were drilled into the rotor’s surface at 22.5° intervals from its center line. This study utilized 3D printing for rotors, highlighting its high-speed prototyping capacity, ability to create complex geometries, and flexibility in creating innovative design. Although 3D printed parts are not suitable for long-term industrial applications due to plastic material limitations, they are useful as concept demonstration tools and process development. The findings suggest that 3D printing technology can help design more robust and resilient reactors, reducing prototyping costs compared to traditional CNC machining processes. The three HCR units were separated by flanges coupled with stationary catalyst baskets, each of which could contain a maximum of 100 g Amberlyst-15 catalyst. However, the weight of Amberlyst-15 was set at 80 g to avoid a large pressure drop in the HCRs while taking into account the maximum pressure and flow rate of the chemical dosing pump. Fig. 2 shows the preparation of the solid acid catalyst basket assembly. The assembly process for the solid acid catalyst basket follows these steps: (1) the flanges for preparation stationary basket were cleaned and prepared; (2) a stainless steel sieve (#500 sieve with a sieve size of 25 μm) which is enough to protect the leak of Amberlyst-15 (300 μm) was put as a basement layer; (3) the specific amount of Amberlyst-15 was filled; (4) a stainless steel sieve (#500 sieve with a sieve size of 25 μm) was covered again over the Amberlyst-15; (5) a stainless steel sheet with a circular hole was supported to hold the basket to be strong; (6) the stainless-steel clamp ring was used to lock the slot of the catalyst basket to be secure, strong to hold the basket and protect the leakage of solid catalyst during the experiment. Four bolts were used on the top and bottom flanges to seal the reactor and prevent any leakage. An inlet valve was located at the bottom flange to feed the mixture into the reactor, and an outlet valve was located at the top flange to collect samples and provide a link to the circulating tank.

One of the challenges encountered during investigations with hydrodynamic cavitation and solid acid catalysts is cavitation erosion. Extended exposure to strong cavitation can lead to erosion of reactor components such as the catalyst basket and rotor surface, limiting the reactor’s long-term and performance. The stability of solid acid catalysts under extreme conditions of hydrodynamic cavitation (high shear forces, temperature variations) must be carefully investigated. To regulate the temperature of the reaction, a circulation pump was installed in the collection tank, and a thermocouple was set up to continuously monitor the temperature. Furthermore, a variety of factors, such as thermal cracking, leaching, and poisoning, can deactivate solid acid catalysts, leading to a gradual decrease in catalytic activity over time. As a result, this study preserved a constant catalyst loading to prevent excessive pressure drop in the reactor, and the filtration difficulty of small resins could lead to catalyst leaks during experiments. It could compromise cavitation intensity and mass transfer. As a result, the limited catalyst loading (80 g) may restrict the reaction rates, especially for processes that require a high catalyst demand. The HCR rotors were driven by a variable-speed electric motor through a flexible coupling that compensated for any misalignment between the motor shaft and the rotor shaft. With increasing rotor speed, the velocity of the liquid on the rotor surface also increased, leading to the formation of a zone of low pressure on the lip of each spherical indentation as liquid left the indentation at high velocity. A cavitation phenomenon occurred when the fluid velocity near the edge of an indentation became sufficiently high that the pressure locally dropped to or below the vapor pressure of the liquid.

For considering the scaling up for the reactor, the dimensionless number calculation was needed to concern. In this study, the dimensionless number was cavitation number (CN). The dimensionless number CN have to be fixed and control the variation of the velocity which can be calculated based on the geometric ratio of the reactor. A dimensionless number known as CN has been used to relate the flow conditions with the cavitation intensity as expressed by Eq. (3)53.

where P2 is the downstream pressure at the outlet of reactor, Pv is the vapor pressure of flowing liquid, \(\:\rho\:\) is the density of flowing liquid, and v is the surface velocity of the liquid on the surface of rotor. For this study, CN for one HCR unit under the optimal condition was calculated as (101325–84403.4)/(0.5 × 755 × 12.542) = 0.3 using Eq. (3), when P2 = 101,325 Pa (in this case), Pv, methanol = 84403.4 Pa at 60 °C, ρmethanol = 755 kg/m3 at 60 °C, and v = 12.54 m/s at 2000 rpm. In addition, the surface velocity at n = 2000 rpm (under optimal condition) was calculated as v = rω, = 0.06 × 209 = 12.54 m/s, when ω = 2πn, = 2π(2000/60) = 209 rad/sec, when radius of rotor (r) = 0.12/2 = 0.06 m. The size of the rotor-stator hydrodynamic cavitation reactor can be increased calculating based on the diameter of the stator (Ds). For example, diameter of the rotor (Dr) can be increased based on the optimal condition of this study by using this relationship Dr = 0.8571Ds, when Ds = 140 mm, and Dr = 120 mm.

Reduction process of FFA in first-step esterified oil using HCRs in series with heterogeneous acid catalyst. RO, 3D printed rotor; ST, stator; F, flanges with solid catalyst baskets; T, circulation tank; HT, immersed heater; TC, digital thermocouple; S, sampling port; C, coupling; P, circulating pump.

Steps for preparation of solid acid catalyst basket assembly.

For the second step of esterification, which used HCRs in series, used whole mixtures of first-step esterified oil mixed with methanol in the homogenous phase after the soaking process as a raw material. The circulating pump continuously pumped the mixtures of first-step esterified oil and methanol through the HCRs at a constant flow rate of 65 L/h after pouring them into the circulation tank (“T” in Fig. 1). Therefore, the mixture circulated through the HCRs in series and returned to the circulation tank with the circulating pump. The first-step esterified oil was used as feedstock for the esterification process using the HCRs in series with a heterogeneous acid catalyst. The experiment was run according to the experimental design conditions by RSM. Each condition has its own specific amount of methanol. For the esterification process with HCRs, the respective amount of methanol was mixed with first-step esterified oil in the collector tank and fed into the reactor at a constant flow rate of 65 L/h using the circulating pump (“P” in Fig. 1; Grundfos Alldos DME 60–10B). To maintain a constant temperature of the first-step esterified oil, it was heated to a temperature of 60 °C in the collector tank (“T” in Fig. 1) using the immersed heater. The temperature of the collector tank was measured using an electrical thermocouple. The temperature of the esterified oil from outlet port was around 64 °C. Therefore, a circulating pump (SANSO, model: PMD-37 1) was also used in the collector tank to regulate the temperature of esterified oil from outlet port of HCRs to be ensure a stable temperature (60 °C). Therefore, all the experiments were carefully controlled every condition from the experimental design. A fixed amount of 80 g Amberlyst-15 was placed in the catalyst basket attached to each flange. The flanges were then covered with a #500 sieve with a fine mesh sieve and enclosed with rings to prevent the catalyst from passing through the basket. The details of steps to set up solid catalyst basket will be described in the previous session. As the oil–methanol mixture flowed through the catalyst basket, the FFAs present in the oil underwent an ion exchange reaction with the catalyst and the FFA molecules became adsorbed on the catalyst surface, as a consequence of which the FFA level in the oil decreased. The various parameter ranges for methanol, circulation time, and rotor speed were examined to obtain the optimal values to minimize the FFA in the esterified oil.

The reaction temperature of the mixture was carefully kept below the boiling point of methanol, 64.7 °C. The esterified oil and methanol mixture was circulated through the Amberlyst-15 baskets to investigate the effect of circulation time on reduction in FFA levels. For each group of experimental settings, a graduated cylinder and a timer were used to calibrate the flow rate of esterified oil. Three 25 mL samples were taken at a specific circulation time at the HCR output valve, which corresponded to the setting ranges from the design of experiment (DOE), which will be addressed in the next section. Each sample was immediately frozen in an ice box to halt the chemical reaction. In addition, the crude biodiesel was cleaned with water to eliminate any remaining impurities and methanol. Finally, a TLC/FID analysis was performed to determine the ester, FFA, MG, DG, and TG contents of the purified esterified oil. A scanning electron microscope (SEM) (Quanta 400, Thermo Fisher Scientific, Brno, Czech Republic) was used at a magnification of 50× to examine the catalyst surface morphology both before and after the esterification process. After esterification, the catalyst’s surface area and pore diameter were determined using an ASAP 2460 analyzer (Micromeritics, Norcross, GA, USA) according to the Brunauer–Emmett–Teller (BET) method.

Response surface methodology (RSM) was used to optimize the reduction of FFA levels by esterification with a heterogeneous catalyst and HCRs in series. Multiple regression analysis with the following second-order polynomial was used to predict the FFA reduction:

For the reduction of FFA levels from those in the first-step esterified oil obtained after soaking, the three independent variables in this experiment are methanol content (2–12 wt%), circulation time (30–170 min), and rotor speed (1000–3000 rpm). The response Y represents the weight% of MEs. In this study, central composite design (CCD) response surface methodology (RSM) was used to optimize the reduction of FFA levels by esterification with a heterogeneous catalyst and HCRs in series. By estimating the precision of surface responses, the value of α can be determined, where star design is ± α. There are three types of CCD; (1) Circumscribed design (CCC); (2) Inscribed design (CCI); and (3) Face cantered (CCF). This study used inscribed design (CCI) under CCD. The CCI design utilized the factor setting as star points and created a factorial design within those limits. By applying five levels to each independent variable, the axial parameter αx was found to equal 1 for rotatable CCI54. The five code levels for the three independent variables were obtained as − 1, −0.595, 0, + 0.595, and + 1. Table 3 shows these code levels for each independent variable in this study.

In regression analysis, it is important to determine the statistical significance of the findings. Microsoft Excel provides a useful tool to evaluate the statistical significance of the regression coefficients. In this case, the p-value plays a crucial role. The p-value is a measure of the probability that the observed data is due to chance alone. In the context of regression analysis, it helps determine whether the relationship between the independent variables and the dependent variable is statistically significant. In this study, three different factors were considered to affect the conversion of FFAs in first-step esterified oil to MEs, namely methanol M (wt%), circulation time T (min), and rotor speed S (rpm), and 18 experimental procedures were carried out with different values of the three independent variables M, T, and S. The response was taken to be the methanol purity ME (wt%). The ranges of the three independent variables were methanol 2–12 wt%, rotor speed 1000–3000 rpm, and circulation time 30–170 min, as shown in Table 3. These three variables were studied to determine the optimal conditions for ME production from the first-step esterified oil using the HCRs in series. Table 4 shows the results of the esterification process according to the DOE. The RSM was used to enhance ME purity using HCRs by fitting the model by multiple regression. Every regression coefficient in the model was determined to be statistically significant based on the p-value. The findings were considered to be statistically significant if the p-value was less than 0.05 at a 95% confidence level. Equation (5) is full form of second order polynomial of predicted model from first regression run and Table 5 shows the respected p-value of every coefficient of Eq. (5).

In general, a p-value less than 0.05 is considered statistically significant at a 95% confidence level. When running a regression analysis in Microsoft Excel, the software automatically calculates the p-value for each independent variable. If the p-value for an independent variable is less than 0.05, it suggests that the relationship between that variable and the dependent variable is statistically significant. In addition to the p-value for individual variables, it is also important to consider the significance of interaction terms in regression analysis. Interaction terms are created by multiplying two or more independent variables together. These terms capture the possible combined effects of the variables.

According to Table 5 for Eq. (5), the p-values of some coefficients are more than 0.05. The p-value of coefficient β5, β6 and β8 are greater than 0.05 which is related to the interaction term of β5MT, β6MS and β8TS. Therefore, the effect of interaction terms: methanol and circulating time; methanol and speed; and circulating time and speed on responses ME were low significant for this study. In a regression analysis, if the p-value for an interaction term is not statistically significant (greater than 0.05), the software automatically eliminates the interaction term from the regression model. This means that the interaction term does not contribute significantly to explaining the variation in the dependent variable. After automatically elimination, the new second order polynomial Eq. (6) of predicted model was obtained. By automatically eliminating non-significant interaction terms, it helps simplify the regression model and focus on the variables that are most relevant and statistically significant. This helps to ensure that the regression analysis provides meaningful and interpretable results. The prediction model for the heterogeneous catalytic process used to produce MEs for this study was then as follows:

For the heterogeneous esterification reaction for Eq. (6), the linear term β1M exhibited the highest level of significance among the coefficients, indicating that the methanol content plays an important role in influencing ME purity. Following closely, the term β2T emerged as the second most significant, representing the influence of circulation time, as evidenced by its low p-values in the correlation prediction model. The quadratic terms β4M2 and β5T2 representing the influences of methanol and circulation time ranked third and fourth, respectively. The term with the lowest level of statistical significance in the analysis was identified as β3S. Hence, it can be inferred that the rotor speed was the least prominent independent variable. The coefficients of the prediction model and the ANOVA for Eq. (5) are presented in Tables 6 and 7, respectively. ANOVA was used to fit a quadratic response surface model with the least squares method and evaluate the goodness of the fit to the collected data. The assessment of the model’s fit to the experimental data was conducted at a confidence level of 95%, as represented in Table 7. When the F-test is used to remove the null hypothesis of each model, the calculated F-value from the model, F0, must be greater than the critical value Fcritical. In this study, the F0 values were found to be greater than the Fcritical values. Furthermore, the number of experiments conducted was sufficient to examine the impact of the variable factors on the production of higher ME. Therefore, it is reasonable to assume that a high ME can be produced from esterified oil pretreated by soaking via heterogeneous esterification using HCRs in series, as predicted by the fitted models. The correlation between the ME purities anticipated by the empirical model and those acquired from practical experiments is illustrated in Fig. 3. In addition, the accuracy of the prediction models was evaluated utilizing the adjusted determination coefficient \(R_{{{\text{adjusted}}}}^{2}\)and the determination coefficient R2. The values of R2 and \(R_{{{\text{adjusted}}}}^{2}\) for the heterogeneous esterification reaction using HCRs in series were 0.986 and 0.978, respectively. These large values confirm the model’s high significance and indicate a strong correlation between the independent and dependent variables. These statistical tests show that the selected model can accurately predict the ME results across all the experimental variables.

Predicted purities of ME versus experimental ME content from heterogeneous esterification process using HCRs in series.

The contour plots in Fig. 4 show the relationship between the dependent variable of ME purity and the independent variables of methanol, circulation time, and rotor speed for heterogeneous esterification from first-step esterified oil. The key variable for heterogeneous catalytic esterification, according to the statistically significant findings from previous section, is the methanol content. Therefore, the contour plots relating to methanol are the first point of discussion here. The effect of methanol and rotor speed on ME production is shown by the contour plot in Fig. 4a. In regions where the methanol content ranges from 7 to 10 wt% and the rotor speed ranges from 1500 to 2500 rpm, the results demonstrate that it is possible to derive more than 85 wt% ME purity. Farvardin et al. obtained similar findings indicating that the production of ME from waste cooking oil can be achieved using combined hydrodynamic and ultrasonic cavitation systems. To achieve a high level of ME purity, they employed RSM to optimize key factors, including residence time, methanol, and KOH. Their statistical analysis indicated that the methanol-to-oil ratio had a significant impact in increasing the ME purity through transesterification55. A study conducted by Kolhe et al. showed that biodiesel could be produced from waste frying oil using hydrodynamic cavitation. The findings of that study indicated that the molar ratio of methanol to oil played a crucial role in achieving high production of ME. For 3:1, 4.5:1, and 6:1 molar ratios of methanol to oil, the resulting ME purities were 54, 93.86, and 95.4 wt%, respectively56. Fig. 4b shows how the relationship between methanol content and circulation time affects ME production. The highest ME purity of more than 85 wt% was obtained for methanol contents between 6 and 11 wt% and a circulation time range of 100–165 min, The correlation between rotor speed and circulation time affects ME purity, as shown in Fig. 4c. When the circulation time is between 110 and 160 min and the speed between 1500 and 2500 rpm, the ME purity exceeds 85 wt%.

Contour plots of optimized conditions for ME production from first-step esterified oil using heterogeneous esterification process with HCRs in series: (a) effects of methanol and rotor speed; (b) effects of methanol and circulation time; (c) effects of rotor speed and circulation time. SigmaPlot v14, https://grafiti.com.

The optimal conditions for reduction of FFA levels by esterification were considered using the predictive model in Eq. (5). RSM was used to obtain the optimum conditions from a set of 18 experimental conditions. Table 8 presents the details of physical properties, compositions, and product yields of PFAD, esterified oil, and biodiesel. To proceed with biodiesel production through base-catalyzed transesterification, it is necessary for the FFA content to be below 2 wt%. In the actual experiment, an 89.76 wt% maximum purity of ME and an FFA content of 1.25 wt%, adequate to proceed the next step of the transesterification process, were achieved under optimal conditions of 9 wt% methanol, 133 min circulation time, and 2000 rpm rotor speed. The percentage difference between the predicted and actual ME values is 0.22%. Moreover, this final sample was analyzed using GC–FID analysis method to verify the ME purity. The results showed that the methyl ester of 87.77 wt% was detected at the sampling port of HCRs in series under the optimum condition, as shown in Table 8.

The product yields from the esterification process with HCRs in series were calculated based on the weight of initial PFAD, as listed in Table 8. To provide a clear comparison and analysis of the product yield, calculation of the biodiesel yields from soaking process and the esterification process with HCRs in series were compared with the 100 wt% of PFAD. The first-step esterified oil yield after 4 h soaking process was 91.68 wt% (calculated based on the 100 wt% of PFAD). After this step, the crude biodiesel was purified by washing it with warm water to remove methanol and impurities from the oil. After the purification process, the amount of purified biodiesel was calculated to determine the average biodiesel yield by weight%. The results showed that the circulation heterogeneous esterification process using HCRs in series gave a 82.48 wt% biodiesel yield after washing (calculated based on the 100 wt% of PFAD).

SEM can be used to obtain detailed information about the morphology, topography, and composition of a material under investigation, and SEM images can reveal the size, shape, and distribution of particles, as well as any surface modifications that may have occurred during a chemical reaction. Therefore, the morphology of both fresh Amberlyst-15 catalyst and the catalyst after it had been used in the esterification process under the optimal conditions was examined by SEM at 50× magnification. The resulting images are shown in Fig. 5. The SEM image of the fresh resin catalyst reveals a smooth surface, as shown in Fig. 5a. The image of the catalyst after its use in the esterification process reveals some cracks on the resin surface. Any tiny hole on the surface creates an active site within the structure, which serves as a flow channel. The esterification procedure led to a few insignificant defects on the surface. The use of the catalyst in the HCRs led to reactant deposition on its surface. However, there were no apparent changes in the microstructure of the catalyst. Similar results were obtained by Lamba for Amberlyst-15 used in the esterification of decanoic acid with ethanol. It was found that the surface of the catalyst showed no changes after its first use; however, a small defect was discovered on the surface of a catalyst that had been reused for a fourth time. This defect was caused by the deposition of some reactant on the surface of the catalyst while the process was run in a small batch reactor57.

SEM images of Amberlyst-15: (a) fresh catalyst; (b) after the esterification process using HCRs under optimal conditions.

Brunauer-Emmett-Teller (BET) analysis is a method used to determine the surface area of materials by measuring the adsorption of gas molecules onto a solid surface. BET analysis of nitrogen absorption and desorption in Amberlyst-15 is crucial for understanding its surface properties, which directly impact its performance in ion exchange applications. For Amberlyst-15, a widely used ion exchange resin, BET analysis can provide valuable insights into its nitrogen absorption and desorption characteristics58. The absorption analysis of Amberlyst-15 was performed under N2 gas adsorption/desorption isotherm at 77 K with equilibration interval 5 s as shown in Fig. 6. Nitrogen Absorption was performed to assess the surface area and porosity of Amberlyst-15 through nitrogen gas adsorption. Nitrogen gas is introduced at various pressures, allowing it to adsorb onto the surface. The amount of nitrogen adsorbed at each pressure is measured. The highest absorption of 210 (cm3/g STP) was found at relative pressure of 1 (p/po). The BET surface area of the average adsorbent was estimated as 18.65 (cm3/g STP). For desorption analysis, nitrogen gas is released from the Amberlyst-15 surface, providing insights into the material’s stability and regeneration capability. After nitrogen adsorption, the pressure is decreased to allow the nitrogen to desorb. The amount of nitrogen released is measured at various pressures. The BET surface area of the average adsorbent was estimated as 27.03 (cm3/g STP). The pore size distribution of Amberlyst-15 was shown in Fig. 7. Pore size distribution describes the range and frequency of different pore sizes within the resin. This can be visualized through a graph generated from BET analysis, typically showing the volume of pores against pore size. The average pore volume of Amberlyst-15 was 0.297 cm3/g.

N2 adsorption–desorption isotherms for BET analysis of Amberlyst-15.

Pore size distribution of Amberlyst-15 (pore diameter vs. pore volume).

The characteristics of the Amberlyst-15 catalyst were determined from the concentration of acid sites and the BET surface area for both fresh catalyst and catalyst that had been used in the esterification process under the optimal condition. The BET surface area of fresh Amberlyst-15 was approximately 42.87 m2/g, with an average absorption pore diameter of 325.1 Å. Following the esterification process using HCRs, the BET surface area of the catalyst was approximately 41.68 m2/g and it had an average absorption pore diameter of 341.41 Å. The BET surface area of the catalyst decreased after the esterification process owing to catalytic activity occurring during the reaction50. As a similar result, Burmana et al. conducted a study of heterogeneous catalyst using Amberlyst CM-4 catalyst with pre-treatments involving methanol washing and hydrochloric acid (HCl) reactivation in biodiesel production, utilizing methanol and hydrochloric acid. In the step of HCl washing process, they reported that there is necessary to activate the highly acidic surface of catalyst with HCI to be ensure to proceed next step of esterification. The results of Amberlyst catalyst washed by HCI gave high methyl ester purity. Washing with HCl led to a 12% increase in the conversion. The purpose of this HCl washing step was to remove sulfur levels present in the catalyst, which then formed chlorosulfonic acid and was separated during filtration. Moreover, the reusability of Amberlyst 15 was also studied. They stated that the Amberlyst catalyst can be reused up to seven times, as the results continue to methyl ester purity a conversion rate exceeding 80%. Once this catalyst fails to achieve a conversion rate exceeding 80%, it is rinsed with methanol and activated with HCl. In their result, BET surface area decreased after esterification reaction. After the catalyst has been utilized, the product residues attach to the catalyst, resulting in a reduction in the size of the pores59.

However, an increase in average absorption pore diameter area was observed, because some cracks which may inhibit due to cavitation effect during esterification process in the reactor were observed on the surface of the catalyst as shown in Fig. 5b. A comparable outcome was observed in the investigation conducted by Fan et al., who employed Amberlyst 15 to evaluate the synthesis of cellulose acetate. Their results described that Amberlyst 15 exhibits a lowered mechanical strength when subjected to vigorous agitation, which results in surface cracking during the reaction60. Moreover, the stability of solid acid catalysts under the harsh conditions of hydrodynamic cavitation (high shear forces, temperature fluctuations) needs to be carefully evaluated. Ammen et al. studied the effect of ultrasound irradiation on solid acid catalyst. They described that at these frequencies, 20 kHz to 10 MHz, molecules react quickly to the intense ultrasound radiation. Through the process of cavitation, these radiations cause surface materials to undergo chemical or physical changes. They reported the BET analysis of solid catalyst of Ni loading on γ-Al2O3 with sonification. The results showed that the pore size and pore volume were observed to decrease in conventional methods as metal loading increased. However, the pore size and pore volume slightly increased when the ultrasound irradiation method was employed. The activity of catalysts is also influenced by the increase in pore size and pore volume. The results indicate that ultrasound irradiation increases the average absorption pore diameter area. In addition, solid acid catalysts are susceptible to deactivation as a result of a variety of reasons, including coking occurring as a result of thermal cracking, leaching, and poisoning, which results in a gradual decrease in catalytic activity over time61. Another study of Putra et al. analyzed the BET results of a CaO/SiO2 catalyst generated from eggshell and palm empty fruit bunch waste. In their study, the transesterification process was performed in a three necked glass flask. The molar ratio of methanol to oil of 14:1, heterogeneous catalyst loading of 8 wt%, reaction time and temperature of 90 min and 60 °C was fixed. The BET analysis of fresh and used CaO/SiO2 was performed. Their results showed that BET surface area of fresh CaO/SiO2 is higher than used one after transesterification process because oil covered the utilized CaO/SiO2 catalyst, making it difficult to detect the CaO peaks. It has been observed that catalysts with a high surface area exhibit a significant increase in catalytic activity during the production of biodiesel. The low surface area observed in the CaO/SiO2 catalyst can be attributed to the presence of oil, which tends to cover and block the catalyst surface62. Furthermore, in the research of Boz et al. who studied the esterification and transesterification of waste cooking oil over Amberlyst 15 catalyst, they explained that in general, the process of esterification is usually faster than transesterification in the batch reactor. In addition, this is exemplified by water being formed as a side product during an esterification reaction. Consequently, the reduction of biodiesel yield was noted with increased FFA concentrations. This arises from larger quantities of water produced during FFA esterification and which hindered the advanced form of transesterification reaction. Triglycerides are hydrolyzed by water that results from esterifying reactions. The water molecules are absorbed on the surface and within the Amberlyst 15 pores. A protective layer is formed as a result of an adsorption process, which prevents oil from reaching catalyst surfaces. Thus, there is a probability that Amberlyst 15 catalyst may be affected negatively by water resulting in decreased efficiency of biodiesel production63.

The energy demand for the entire system of HCRs in series listed in Table 9. The average electricity usage was measured by the digital power meter. For the first step soaking process, the 1 L of PFAD was preheated and maintained to temperature of 60 °C within 240 min by an immersion heater (model: SANGI, SG-252) and circulating pump (model: SANSO, model: PMD-371). The electricity consumption for preheating PFAD up to 60 °C and controlling the stable temperature in the tank were 2.247 and 0.08 kW h, respectively. For the second step esterification process with HCRS, the preparation for the preheating the mixture of methanol and first step esterified oil from soaking process to 60 oC and controlling the stable temperature in the tank consumed 0.627 and 0.034 kW h. The hydrodynamic cavitation reactor in series (HCRs) required the electricity consumption of 0.273 kW h to operate a rotor on the process. The mixture of methanol and first step esterified oil was supplied into the hydrodynamic reactor by using one digital pump, which required the electricity usage of 0.011 kW h. The total electricity consumption for HCRs in series of circulation esterification processes was 0.398 kW h after stable temperature in the tank of pretreatment process by soaking Amberlyst-15 (excluding the preheating process). Therefore, the average electrical energy consumed to produce crude biodiesel was 0.007 kW h/L.

This study has investigated the use of HCRs in series to reduce the levels of FFAs in PFAD as part of the biodiesel production process. To reduce the FFAs in PFAD to give an ester purity of 89.76 wt%, the optimal conditions according to RSM were found to be 9 wt% methanol, 133 min circulation time, and 2000 rpm rotor speed. Following purification of this crude biodiesel with warm water to eliminate residual impurities, an average product yield of 82.48 wt% was obtained. Following the esterification process, the BET surface area of the Amberlyst-15 was approximately 41.68 m2/g and its average absorption pore diameter was 341.41 Å. The BET surface area of the catalyst decreased after the esterification process owing to catalytic activity occurring during the reaction. However, an increase in average absorption pore diameter was observed, which may inhibit the esterification process. The results of this study provide interesting insight into the potential use of HCRs in series to reduce FFA levels in PFAD by esterification using a heterogeneous acid catalyst. Further research into the use of HCRs in series could lead to the development of techniques for biodiesel synthesis that use heterogeneous base catalysts such as calcium oxide to convert remaining TGs, DGs, and MGs to high purity of MEs via second-step transesterification. In addition, the catalyst’s long-term stability and reusability under HCR conditions will be investigated, as well as the possible benefits of other catalytic supports or changes.

The authors declare that the data supporting the findings of this study are available within the paper. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request.

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This research was supported by a Postdoctoral Fellowship from the Prince of Songkla University and the National Science, Research, and Innovation Fund (NSRF) and Prince of Songkla University (Grant number [ENG6701121S]).

Department of Mechanical and Mechatronics Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla, 90110, Thailand

Ye Min Oo, Panupong Juera-Ong & Krit Somnuk

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Y.M.O.; Conceptualization; methodology; validation; formal analysis; writing, original draft; visualization. P.J.-O.; methodology; investigation; data curation; SEM analyses. K.S.; Supervision; writing, review and editing; conceptualization; project administration; funding acquisition. All authors contributed to the final version. All authors have read and agreed to the published version of the manuscript.

Correspondence to Krit Somnuk.

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Oo, Y.M., Juera-Ong, P. & Somnuk, K. Methyl ester production process from palm fatty acid distillate using hydrodynamic cavitation reactors in series with solid acid catalyst. Sci Rep 14, 27732 (2024). https://doi.org/10.1038/s41598-024-78974-3

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Received: 05 July 2024

Accepted: 05 November 2024

Published: 12 November 2024

DOI: https://doi.org/10.1038/s41598-024-78974-3

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