Transesterification and Hydrotreating Reactions of Rice Bran Oil for Bio-Hydrogenated Diesel Production

Abstract

Two different methods of production of bio-hydrogenated diesel (BHD), simply called green diesel from rice bran oil (RBO), were performed. In the first route, a direct hydrotreating reaction of RBO to BHD catalysed by Pd/Al₂O₃ was performed in a high-pressure batch reactor. Operating conditions were investigated as follows: catalyst loading (0.5 to 1.5% wt.), temperature (325 to 400 °C), initial hydrogen (H₂) pressure (40 to 60 bar) and reaction time (30 to 90 min). The optimal condition was obtained at 1% wt catalyst loading, 350 °C, 40 bar H₂ pressure and 60 min. Yields of crude/refined biofuels and BHD achieved were approximately 98%, 81.71% and 73.71%, respectively. In another route, transesterification together with hydrotreating reactions of rice bran methyl ester (RBME) to BHD was performed using the optimal conditions obtained from the first route. The amount of 98% crude biofuel was obtained and was equivalent to production yields of refined biofuel (85.71%) and BHD (68.51%). Furthermore, physical and chemical properties of both RBO/RBME green diesel were also considered following ASTM standard methods. In summary, both catalytic reactions were achieved in the range of a low-speed industrial diesel and were further recommended for BHD or green diesel production from RBO.

1. Introduction

The petroleum crisis, affecting many areas around the world, is still a major fuel problem in Thailand, with over 50.1% of total energy consumption rely on fuel. More than 60% of the petroleum used was imported from abroad. This has increased since last year, especially in both the transportation and industrial sectors [1]. Nevertheless, the source of fossil fuels being continuously depleted is a main reason to increase more serious changes concerning environmental problems. However, there are many efforts to find renewable energies, which could be sustainable resources and more environmentally friendly.

Because Thailand is an agro-industrial country, there are many kinds of agricultural raw materials such as corn, soil bean, palm, sunflower, etc. However, rice, a reasonable widely planted crop, is the most important product when compared among other crops. It is an export product as well as being the main course for Thai people and even in many other countries around the world. In 2020, a large amount of Thai rice (5,724,660 MT) was totally exported worldwide [2]. In the rice milling process, rice bran is always generated as a by-product. Recently, rice bran (more than 640,000 T) has mainly been used as a low-cost material for animal feed applications [3]. Moreover, it is also defined as desirable second-generation source of oil (16–32% wt. oil content). In a rice bran oil (RBO) processing plant, foreign matter is firstly eliminated from rice bran. Subsequently, lipase in rice bran is deactivated by steam pressure at 100 °C and rice bran further goes through a drying process to remove water from the previous step. Then, RBO is extracted from rice bran via solvent extraction. Lastly, RBO is distillated to obtain high-purity rice bran oil. Furthermore, RBO creates a higher benefit for rice farming and upgrading rice from a by-product to a value-added product [3,4,5]. In addition, Thailand also has a high potential for RBO production (140,000 T per year) [3]. Thus, the development of renewable fuels using RBO as a raw material could be considered as an alternative approach. Moreover, there were few research works that studied the production of various biofuels from RBO [6]. Currently, there are two main catalytic conversion processes of triglycerides (or free fatty acids) to diesel fuel; (i) transesterification/esterification processes, the product obtained is called “Biodiesel”, and (ii) hydrotreating processes to produce “Biohydrogenated diesel” (BHD), or simply called “Green diesel”.

Typically, biodiesel, one of the renewable fuels, has gained much attention in recent years as an environmentally friendly alternative to diesel fuel due to its high cetane number, good lubricity, biodegradability and non-toxicity. Generally, biodiesel or fatty acid methyl ester (FAMEs) is produced via transesterification of triglycerides or esterification of fatty acids. It has been used as a component in diesel blending [7]. However, the use of FAMEs in current diesel engines could lead to several problems such as filter plugging, corrosion of metal parts and deposits on fuel pumps. Moreover, the high oxygen content of FAMEs causes uncompetitive energy densities and oxidation stabilities compared to petroleum diesel [8,9,10]. Therefore, it has been converted to deoxygenated fuel (green diesel) or BHD via hydro-processing to improve its properties [11].

However, as mentioned above, bio-hydrogenated diesel (BHD) or green diesel, a second-generation diesel-like hydrocarbon fuel with 15–18 carbon atoms, provides many good properties compared to biodiesel. It is suitable for use in diesel engines because of its high cetane number, good oxidative stability, low viscosity, fewer NOx emissions and improved cold flow property [11]. It is also suitable for blending with petroleum diesel [12]. It can be produced by the hydro-processing of triglyceride. Typically, hydro-processing consists of two processes: (i) the hydrotreating process, which can remove undesirable impurities such as oxygen sulfur, nitrogen and metals from raw material by reacting with hydrogen in the presence of a catalyst, and (ii) hydro-cracking, which is a process of breaking down hydrocarbon molecules to lighter hydrocarbon compounds under hydrogen pressure [12]. There are many catalysts used for hydro-processing such as nickel (Ni), niobium phosphate (NbOPO₄), zeolite, rhodium (Rh), platinum (Pt) and palladium (Pd). A previous study [13] reported that Pd/C has high efficiency for deoxygenation of soybean oil with a total hydrocarbon content of 99%. Recently, Pd/Al₂O₃ was also reported as a high potential catalyst for bio-hydrogenated kerosene (BHK) production from palm oils [14,15].

Therefore, in this work, bio-hydrogenated diesel (BHD) production from rice bran oil (RBO) and rice bran methyl ester (RBME) via a hydrotreating reaction catalysed by 0.3% Pd/Al₂O₃ was performed. Firstly, production of green diesel from RBO was conducted in a high-pressure batch reactor. Different main parameters including catalyst loading, reaction temperature, initial hydrogen pressure and reaction time affecting green diesel production from RBO were investigated. Subsequently, the optimal conditions for BHD or green diesel production from RBO were further used as prototype to produce green diesel from RBME. Finally, some properties of BHD from RBO/RBME were characterised and compared to industrial diesel.

2. Results

2.1. Characterisation of Rice Bran Oil (RBO)

Some RBO properties are shown in

Table 1. Characterisation of RBO.

Parameters	Methods	Value	Commercial Vegetable Oil
Acid value (mg KOH/g of oil)	AOAC 1990	0.4	≥0.6
Specific gravity (ρ substance/ρ water) at 15.6 °C	ASTM D1298	0.9	0.9
Viscosity (cSt: mm2/s) at 40 °C	ASTM D445	44.10	30–60

. It was indicated that RBO showed a low acid value, in the range where the transesterification reaction was able to exist directly and it was unnecessary to prepare the RBO by an esterification reaction. Meanwhile, the RBO-specific gravity result was similar to previous studies [16,17]. Moreover, it was found that the RBO viscosity showed a very high value at 44.10 mm²/s.

2.2. Bio-Hydrogenated (BHD) Production from RBO

2.2.1. Effect of Catalyst Loading

The results showed that the variation of catalyst loading from 0.5, 1.0 to 1.5% did not have any effect on the percent crude biofuel yield; it was 97.80, 97.00 and 98.00%, respectively. With an increase in the catalyst loading from 0.5 to 1.0% wt., refined biofuel yield increased from 73.71 to 81.43%. The maximum catalyst loading (1.5%) presented biofuel of 81.71%. Similarly, the lowest BHD yield (59.41%) was obtained at 0.5% catalyst loading. However, BHD yields of 1% and 1.5% catalyst loadings were obtained as approximately 81.43% and 81.71%, respectively. It was indicated that a higher catalyst loading did not show any considerable refined biofuel yield and BHD yield changes (See Figure 1). Accordingly, the catalyst loading of 1% was selected to further investigate other parameters. Moreover, the results obtained were in agreement with previous studies [18,19], which reported that higher catalyst loading did not significantly affect biofuel production.

The decomposition profile of BHD samples from TGA data is shown in Figure 2. All three samples were degraded between 150 °C and 500 °C. The maximum temperature of degradation for 0.5% wt., 1% wt. and 1.5% wt. catalyst loading was 250 °C, 280 °C and 280 °C, respectively. In the case of 1% wt. catalyst loading, a large amount of the BHD sample was decomposed at a temperature in the diesel range (250–360 °C). On the other hand, the results obtained from other catalyst loadings presented two main fractions. The first fraction indicated the composition of biofuels (180–380 °C). Another fraction referred to triglycerides (380–500 °C) [13]. It was shown that using 1% wt catalyst loading could convert RBO to green diesel effectively.

2.2.2. Effect of Reaction Temperature

The results were found when the reaction temperature between 325 °C and 400 °C was changed. It resulted in similar percentage of crude biofuel yield (96–98%). In the same way, the percentage of refined biofuel yield was in the range from 81.43 to 81.71%. At the lowest reaction temperature (325 °C), a 67.43% BHD yield was obtained. This also increased by 8.28% as the reaction temperature was increased from 325 to 375 °C. Otherwise, higher reaction temperature (400 °C) led to a lower BHD yield (68%) as shown in Figure 3. This is described by the enhancement of cracking and isomerization reactions at higher temperatures. In a previous work, it was also reported that production of green diesel from palm oil by hydrotreating at a high temperature (≥360) caused a lower product yield [20]. The highest BHD yield (75.71%) was achieved at 375 °C. It was higher than the yield at 350 °C (0.2%). Therefore, the reaction temperature of 350 °C was selected as the optimal temperature due to preferable economics. Moreover, a previous study [13] also reported that the optimal temperature for BHD from the hydro-processing of soybean oil was 350 °C.

The thermogravimetric curves of BHD samples from TGA data are shown in Figure 4. The weight loss occurred when the reaction temperature was between 150 °C and 500 °C. The maximum temperatures of weight loss for 325 °C, 350 °C, 375 °C and 400 °C were 400 °C, 400 °C, 220 °C and 150 °C, respectively. Bio-hydrogenated oil was degraded in gasoline and jet fuel (50–250 °C) at high temperatures of 375 °C and 400 °C. The lowest triglycerides conversion was observed at 325 °C. The largest fraction of diesel in the range of 250–360 °C was obtained when the reaction temperature reached 350 °C. Therefore, it was chosen for use in further experiments.

2.2.3. Effect of Initial Hydrogen Pressure

The results showed that the percentage of crude biofuel yield (98%) was obtained at three different initial hydrogen pressures. For the percentage of refined biofuel yield, initial hydrogen pressures of 30, 40 and 50 bar led to obtaining 77.72, 81.72 and 78.86% refined biofuel yields, respectively. In between the initial hydrogen pressure of 30 and 40 bar, the percentage of BHD yield increased significantly from 66.29 to 73.71%. However, it decreased down to 70.86% at the high initial hydrogen pressure (60 bar), as shown in Figure 5. This was due to hydrotreatment process consisting of four reactions; reduction, hydrodeoxygenation, deoxygenation, and decarboxylation. Both reduction and hydrodeoxygenation reactions are preferable at high hydrogen pressures. However, a higher H₂/oil ratio is favourable for cracking and hydrogenation reactions. Patil and Vaidya (2018) explained that these side reactions resulted in low efficiency of hydrotreating reactions [21]. Furthermore, a previous study [6] also reported that the optimal hydrogen pressure for biofuel production via hydrotreating of rice bran oil was 50 bar.

The thermogravimetric curves of BHD samples are shown in Figure 6. The mass loss occurred between 150 °C and 500 °C. The maximum temperatures of degradation for 30, 40 and 50 bar were 250 °C, 350 °C and 350 °C, respectively. The largest diesel proportion was obtained at 40 bar. For initial hydrogen pressures of 30 bar and 50 bar, there were two major fractions of gasoline and jet fuel and triglycerides. It could be concluded that 40 bar of hydrogen pressure was the best condition in this study.

2.2.4. Effect of Reaction Time

The results were found that the variations of reaction times did not influence the percentage of total liquid yield. The similarity results of crude biofuel yields were obtained at 98%. In the case of a reaction time of about 30 min, an 84.57% refined biofuel yield was obtained. This slightly increased to 85.71% at a reaction time of 90 min. In term of percentage of BHD yield, an increase in reaction time from 30 to 60 min affected BHD yields minimally, rising from 72.57 to 73.71%. However, yields decreased to 68.57% as the reaction time increased up to 90 min (Figure 7). It could be implied that a higher yield of light hydrocarbon was obtained with increasing reaction time [22]. A slightly decrease of the BHD yield (C15-C18) was observed at 90 min. The results obtained were in line with previous work [23], which reported that a longer reaction time had a negative effect on BHD yield.

The degradation profile of BHD samples is shown in Figure 8. The weight loss occurred between 50 °C and 500 °C. The maximum temperatures of degradation for 30 min, 60 min and 90 min were 400 °C, 320 °C and 350 °C, respectively. At a reaction time of 60 min, the highest diesel fraction was observed. On the other hand, the lowest triglycerides conversion was obtained at 30 min. However, it was found that favourable jet fuel and diesel ranges were shown when the reaction time of 90 min was used. It indicated that the optimal time for BHD production from RBO was 60 min.

2.3. Transesterification of RBO and Characterisation of RBME

A biodiesel yield of 98.5% was obtained. Approximately 1.5% of the solid form of glycerol remained after the reaction. This means that the reaction was quite completed. The RBME composition was analysed by GC-MS. The results revealed that the product of biodiesel mainly contained oleic acid methyl ester (56.48%), which was similar to previous studies [16], lignoceric acid methyl ester (10.98%), myristic acid (8.02%) and also other components as shown in

Table 2. Fatty acids composition of RBME obtained from direct transesterification.

Composition	Quantity (% wt)
Capric acid (C10:0)	3.16
Myristic acid (C14:0)	8.02
Palmitic acid (C16:0)	2.95
Palmitoleic acid (C16:1)	5.37
Stearic acid (C18:0)	1.34
Oleic acid (C18:1)	56.48
Linoleic acid (C18:2)	1.14
Linolenic acid (C18:3)	0.67
Arachidic acid (C20:0)	2.45
Gadoleic acid (C20:1)	0.02
Behenic acid (C22:0)	7.01
Erucic acid (C22:1)	0.41
Lignoceric acid (C24:0)	10.98
Total	100

.

In addition, oleic acid is an unsaturated fatty acid in a type of monounsaturated fatty acid, and 56.48% oleic acid was found. It would be beneficial to the biodiesel quality in terms of viscosity. Furthermore, the presence of a high ratio of mono-unsaturated fatty acids was reported, which could decrease oxidative stability and cold flow problems of biodiesel [17].

2.4. Production of Bio-Hydrogenated Diesel (BHD) from RBME

The RBME obtained from the experiment in the Section 3.5 was used as a raw material for BHD production. Hydrotreating of RBME was carried out under the optimal conditions of RBO including catalyst 1%wt, a 350 °C reaction temperature, 40 bar initial hydrogen pressure and reaction time at 60 min. The results showed that the percentage crude biofuel yield, percentage refined biofuel yield and percentage BHD yield were 98%, 85.71% and 68.51%, respectively, as shown in Figure 9.

Comparing BHD production from two different routes, the direct method of RBO to BHD was 73.71%, while another route of RBME to BHD reached about 68.51%. This only shows a 5.21% difference. It might be stated that the direct route to convert RBH to BHD via the hydrotreating reaction would be the better and more practical method to choose. The cost for production should be considered.

2.5. Properties of Biodiesel (RBME) and RBO/RBME Green Diesel

Biodiesel (RBME) and bio-hydrogenated diesel (BHD) obtained from RBO/RBME were characterised by some physical and chemical properties including specific gravity, viscosity meter and distillation according to the standard of ASTM and the results are shown in

Table 3. Characterisation of biodiesel (RBME) and BHD from RBO/RBME.

Parameters	Methods	Diesel *		RBME	BHD from RBO	BHD from RBME
		High Speed	Low Speed
Specific gravity at 15.6 °C	ASTM D1289	0.81–0.87	Max. 0.92	0.89	0.95	0.88
Viscosity (cSt) at 40 °C	ASTM D445	1.80–4.10	Max. 8.0	5.55	7.53	4.40
Distillation	ASTM D86	90%	-	90%	89.6%	92%

* The commercial grade diesel standard in Thailand [24].

. The properties of RBME and RBO/RBME green diesel were in good agreement with the standard quality and specifications of low-speed diesel engines. Only the specific gravity of RBO BHD was minimally higher than the standard value. It should be noted that these biofuels are suitable for use with low-speed diesel engines.

2.6. Potential of RBO/RBME for BHD Production with Other Feedstocks

Based on the data in

Table 4. Comparative green diesel production from different feedstocks.

Feedstocks	Catalysts	Conditions	%BHD Yield	References
Rice bran oil (RBO)	0.3% Pd/Al2O3	350 °C, 40 bar H2, 1 h	73.71	This work
Soybean oil	5% Pd/C	350 °C, 10 bar N2, 5 h	38	[13]
Crude palm oil	5% Pd/C	400 °C, 40 bar H2, 3 h	51%	[22]
Degummed palm oil	5% Pd/C	400 °C, 40 bar H2, 1 h	70%	[22]
Palm fatty acid distillate	5% Pd/C	375 °C, 40 bar H2, 0.5 h	81%	[22]
Coffee ground oil	5% Pd/C	400 °C, 40 bar H2, 2 h	22.3	[23]
Rice bran methyl ester (RBME)	0.3% Pd/Al2O3	350 °C, 40 bar H2, 1 h	68.51	This work
Fatty acid methyl esters (FAMEs)	10% Ni/HZSM-5	280 °C, 8 bar H2, 80 h, LHSV of 4 h−1	47.7	[18]
Methyl oleate	3.8% Pd/SBA-15	270 °C, 60 bar H2, 6 h	70	[26]

, RBO illustrated high potential for BHD production under lower reaction temperatures. Although, using low catalyst loading (0.3% Pd), BHD yield from RBO was higher than other vegetable oils. However, it remained less than palm fatty acid distillate. Considering RBME BHD production, its yield was also higher than mixed methyl ester and similar to methyl oleate. It was clearly shown that both RBO and RBME can be used as raw materials for BHD production. Furthermore, biofuel production via transesterification to biodiesel followed by hydrotreating should be applied with impure oil. The main problem of unrefined oils is too high a viscosity. This could be figured out by this alternative route [25].

3. Materials and Methods

3.1. Raw Materials

Rice bran oil (RBO) was kindly obtained from Thai Edible Oil Co., Ltd., Bangkok, Thailand. A commercial-grade 0.3% Pd/Al₂O₃ catalyst used throughout the study was purchased from Heze Development, Zone Dayuan Chemical Co., Ltd., Heze, China. It was rod-shaped with a pore dimeter of 7.5 nm [14]. The chemical reagent of hydrogen gas with 99% purity, purchased from Kassidis Trading Co., Ltd., Khon Kaen, Thailand was used for one of the initial feedings during the reaction. Methyl heptadecaoate was purchased from Sigma Co., Ltd., Bangkok, Thailand. Potassium hydroxide and methanol (99.8%) were analytical-grade, while helium gas was industrial-grade.

3.2. Characterisation of RBO

Some properties of RBO including acid value, specific gravity and viscosity were investigated, following the standard methods set by AOAC, ASTM D1298 and ASTM D445, respectively.

3.3. Catalyst Characterisation

A commercial-grade 0.3% Pd/Al₂O₃ catalyst was then characterised in terms of specific surface area by the Brunauer–Emmett–Teller (BET) method, while a crystalline structure was determined by an X-ray diffractometer (XRD) using a PAnalytical X’pert Pro instrument (Malvern Panalytical Ltd., Royston, UK). In addition, scanning electron microscope (SEM) observations were examined by a FEI quanta 200 (Thermo Fisher Sciencetific, Waltham, MA, USA). It should be noted that the details of each method were already explained by our group [14]. Figure 10 shows the white colour, rod shape and 7.5 nm pore dimeter of the Pd/Al₂O₃ catalyst.

3.4. Bio-Hydrogenated Diesel (BHD) Production from RBO

The production of BHD from RBO via hydro-processing was carried out in a high-pressure batch reactor with a 200 mL working volume (P.T. Scientific Co., Ltd., Bangkok, Thailand). A schematic of the reactor is shown in Figure 11. Firstly, RBO was used as a raw material to investigate the effects of four parameters (catalyst loading, reaction temperature, hydrogen pressure and reaction time) on the hydro-processing reaction. It should be noted that each parameter was examined in triplicate for reproducibility.

3.4.1. Effect of Catalyst Loading

Both Pd/Al₂O₃ catalyst and RBO (50 mL) were loaded into the reactor. Hydrogen gas was then flushed 3 times to remove oxygen. Catalyst loading was varied from 0.5 to 1.5% wt. Initial hydrogen pressure was increased to 40 bar. Reactor content was heated up to 375 °C and the propeller was stirred at 450 rpm for 1 h.

3.4.2. Effect of Reaction Temperature

Firstly, 50 mL of RBO and Pd/Al₂O₃ catalyst were added into the reactor followed by hydrogen flushing. The effect of temperature was investigated by variation from 325 to 400 °C. Other parameters were set as follows: initial hydrogen pressure (40 bar), agitation rate (450 rpm) and reaction time (1 h).

3.4.3. Effect of Initial Hydrogen Pressure

The reactor was filled with catalyst (Pd/Al₂O₃) and RBO (50 mL) and flushed with hydrogen gas 3 times. The variation of initial hydrogen pressure was performed from 30 to 50 bar. The temperature was increased to the best condition obtained previously. The reaction mixture was stirred at 450 rpm for 1 h.

3.4.4. Effect of Reaction Time

At the beginning, reactant catalyst (Pd/Al₂O₃) and 50 mL RBO were loaded into the reactor. Hydrogen gas was flushed 3 times to reach deoxygenation. To investigate the effect of reaction time, it was varied between 1, 2 and 3 h. The agitation rate was set at 450 rpm. The reaction was conducted with the best conditions of catalyst loading, reaction temperature and initial hydrogen pressure.

3.5. Transesterification of RB

The production of biodiesel from RBO via the methanolysis reaction was performed in a 1000 mL beaker. The process’s conditions were described in a previous work [17] as a 6:1 methanol to RBO molar ratio, KOH catalyst 0.85% wt., 40 °C reaction temperature, 600 rpm agitation rate and 1 h reaction time. After that, a product of RBME and a by-product of glycerol were separated by a separating funnel. Subsequently, the RBME phase was washed with distilled water and then the distilled residue water and methanol were removed from RBME via vacuum distillation. Biodiesel yield (%) was calculated as shown in Equation (1). The RBME sample was taken for analysis as described in the section of analytical methods. Finally, RBME was used as one of the substrates for green diesel production.

3.6. Production of Bio-Hydrogenated Diesel from RBME

Firstly, RBME (50 mL) was loaded into the reactor followed by the catalyst (Pd/Al₂O₃). Hydrogen gas was flushed 3 times to remove oxygen. The hydro-processing of RBME was carried out according to the optimal conditions of RBO.

3.7. Analytical Techniques

3.7.1. Characterisation of RBO

Some properties of RBO including acid value, specific gravity and viscosity were investigated, following the standard methods set by AOAC, ASTM D1298 and ASTM D445, respectively.

3.7.2. Distillation following the Standard Method of ASTM D86

Total liquid products obtained after the hydrotreating reaction were calculated as shown in Equation (1) and the analysis of samples of the quantity of each product type by distillation followed the ASTM D86 standard. These were distilled to separate gasoline, kerosene and green diesel by different boiling temperatures, in the ranges of 50–150 °C, 150–250 °C and 250–360 °C, respectively. After that, the results were used to calculate the percentage refined biofuel yield and percentage BHD yield as shown in Equations (2) and (3), respectively.

$$\% \mathrm{Crude}\text{-}\mathrm{biofuel} \mathrm{yield}=\frac{ \mathrm{Total} \mathrm{liquid} \mathrm{product} \left(\mathrm{mL}\right)}{\mathrm{Raw} \mathrm{material} \left(\mathrm{mL}\right)} \times 100\%$$

(1)

$$\% \mathrm{Refine}\text{-}\mathrm{biofuel} \mathrm{yield}=\frac{\mathrm{Total} \mathrm{biofuel} \mathrm{obtained} \mathrm{from} \mathrm{distillation} \left(\mathrm{mL}\right)}{\mathrm{Total} \mathrm{liquid} \mathrm{product} \mathrm{used} \mathrm{in} \mathrm{distillation} \left(\mathrm{mL}\right)} \times 100\%$$

(2)

$$\% \mathrm{BHD} \mathrm{yield}=\frac{\mathrm{BHD} \mathrm{obtained} \mathrm{from} \mathrm{distillation} \left(\mathrm{mL}\right)}{\mathrm{Total} \mathrm{biofuel} \mathrm{obtained} \mathrm{from} \mathrm{distillation} \left(\mathrm{mL}\right)} \times 100\%$$

(3)

3.7.3. Thermogravimetric Analysis (TGA)

Total liquid products obtained from hydrotreating of RBO were analysed via TGA. The changes in weight of the substances were investigated by thermal properties with TGA. After that, TGA profile results were used to calculate and convert a derivative graph of weight against temperature by the formula shown in Equation (4).

$$\mathsf{\Delta }\mathrm{Weight}/\mathsf{\Delta }\mathrm{Temperature}=\mathrm{X}\text{-}\mathrm{axis};\frac{\mathrm{Temp}{.}_1+\mathrm{Temp}{.}_2}{2} \mathrm{and} \mathrm{Y}\text{-}\mathrm{axis};\frac{{\mathrm{Weight}}_2-{\mathrm{Weight}}_1}{{\mathrm{Time}}_2-{\mathrm{Time}}_1}$$

(4)

3.7.4. Biodiesel Yield

Biodiesel yield (%) was calculated as shown in Equation (5) [17].

$$\mathrm{Biodiesel} \mathrm{yield} (\%)=\frac{\mathrm{Biodiesel} \mathrm{product} \left(\mathrm{mL}\right)}{\mathrm{Raw} \mathrm{material} \left(\mathrm{mL}\right)} \times 100\%$$

(5)

3.7.5. Characterisation of RBME

Some properties of RBME including acid value, specific gravity and viscosity were investigated, following the standard methods set by AOAC, ASTM D1298 and ASTM D445, respectively. Furthermore, the RBME sample was analysed for fatty acids composition according to EN 14214 [14,15].

3.7.6. Properties of Biodiesel/Green Diesel Characterisation

Biodiesel and BHD products from the best conditions were characterised by some physical and chemical properties including specific gravity, viscosity meter and distillation following the standard methods set by ASTM, ASTM D1298, ASTM D445 and ASTM D86, respectively.

4. Conclusions

BHD production from RBO via two different routes was successfully carried out. The optimal conditions for direct hydrotreating of RBO were 1% wt. catalyst loading, 350 °C reaction temperature, 40 bar initial H₂ pressure and reaction time of 60 min. Under optimal conditions, a high BHD yield of 73.71% was obtained. The production of BHD from RBME was also investigated under the best conditions of RBO. It provided a high BHD yield (68.51%). Moreover, RBO/RBME BHD were characterised by some properties according to ASTM standard methods. The results showed that both BHD types meet the standard of low-speed diesel. There is only specific gravity of RBO-BHD which showed higher than the standard value. It was clearly shown that these processes could be used as prototype for BHD production from refined/unrefined oil.