Esterification of palm fatty acid distillate (PFAD) in supercritical methanol: Effect of hydrolysis on reaction reactivity

Esterification of palm fatty acid distillate (PFAD) in supercritical methanol: Effect of hydrolysis on reaction reactivity

Fuel 88 (2009) 2011–2016 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Esterification of palm fatty ...

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Fuel 88 (2009) 2011–2016

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Esterification of palm fatty acid distillate (PFAD) in supercritical methanol: Effect of hydrolysis on reaction reactivity Duangkamol Yujaroen a, Motonobu Goto b, Mitsuru Sasaki b, Artiwan Shotipruk a,* a b

Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Patumwan, Phayathai, Bangkok 10330, Thailand Department of Applied Chemistry and Biochemistry, Kumamoto University, Kumamoto 850-8555, Japan

a r t i c l e

i n f o

Article history: Received 3 December 2008 Received in revised form 24 February 2009 Accepted 25 February 2009 Available online 16 March 2009 Keywords: Esterification Supercritical Biodiesel Palm fatty acid distillate Fatty acid methyl esters

a b s t r a c t This study demonstrated the potential use of local palm fatty acid distillate (PFAD) as alternative feedstock for fatty acid methyl esters (FAMEs) production and the possibility to replace the conventional acid-catalyzed esterification process (with H2SO4), which was industrially proven to suffer by several corrosion and environmental problems, with non-catalytic process in supercritical methanol. At 300 °C with the PFAD to methanol molar ratio of 1:6 and the reaction time of 30 min, the esterification of PFAD in supercritical methanol gave FAMEs production yield of 95%. Compared with transesterification of purified palm oil (PPO) in supercritical methanol, the production of FAMEs reached the maximum yield of only 80% at 300 °C with higher requirement for methanol (1:45 PPO to methanol molar ratio). Compared with the conventional acid-catalyzed esterification of PFAD, only 75% FAMEs yield was obtained in 5 h. The presence of water in the feed (between 0 and 30% v/v) was found to lower the yield of FAMEs production from PFAD significantly. This negative effect was proven to be due to the further hydrolysis of FAMEs, which nevertheless can be minimized when high content of methanol was used. Ó 2009 Published by Elsevier Ltd.

1. Introduction Nowadays, Thailand’s governmental agencies have been attempting to promote the research programs on alternative fuel developments in order to replace or substitute the exhausting petroleum-based fuels. Among them, biodiesel is one of our promising types of alternative fuel for substitution of conventional diesel oil in transportation section. Currently, it is mainly produced from the transesterification of local purified palm oil (PPO), which is achieved from the refinery of crude palm oil. Typically, crude palm oil always contains high amount of free fatty acids (FFAs) and the presence of too high FFAs easily results in the soap formation during the transesterification reaction [1]. To avoid this reaction, most of FFAs in crude palm oil is removed (as called palm fatty acid distilled or PFAD), then PPO with low content of FFA is used as main feedstock for biodiesel production. Recently, several works have reported the possible conversions of several FFAs (e.g. oleic acid, palmitic acid) to fatty acid methyl ester (FAMEs) via the esterification reaction [2–9]. Compared to the expensive PPO (0.74 USD per liter), PFAD by-product of palm oil refinery costs only 0.37 USD per liter. Thus, the utilization of PFAD for FAMEs production via esterification reaction would reduce the production cost of biodiesel, and would consequently enable it to compete economically with petroleum-based fuel. * Corresponding author. E-mail address: [email protected] (A. Shotipruk). 0016-2361/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.fuel.2009.02.040

Typically, the esterification reaction is carried out by acid-catalyzed process. Conventional acid-catalysts are sulfuric acids (H2SO4) and hydrochloric acid. It is well established that the acid-catalyzed transesterification requires a long time to complete the reaction. Furthermore, extensive washing is required to remove all acid compounds from the product, causing large amount of wastewater. Enzymes catalyzed reaction is another alternative procedure, which is more environmentally friendly. Nevertheless, the main drawback of this process is the high cost of enzymes which makes the process unattractive for industrial scale. Alternatively, Saka and Kusdiana proposed a method for biodiesel production via non-catalytic transesterification of vegetable oils in supercritical methanol [10]. The reaction was found to be completed in shorter time than the conventional catalytic processes, furthermore, purification of the products is much simpler and the process is more environmentally friendly. In the present work, we aimed at the development of a process for esterification reaction of local PFAD feedstock. This work is in collaboration with local palm oil refinery industries since they are attempting to utilize their own PFAD as another feedstock for biodiesel production. The acid-catalyzed esterification process (with H2SO4) seems to be inappropriate according to their previous report on the massive corrosion in the system equipment; we thereby proposed the non-catalytic supercritical methanol process as an alternative procedure. The study here was carried out in order to determine the effect of operating conditions (i.e. reaction time, temperature, and methanol to PFAD molar ratio) on the yield

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of FAMEs produced by esterification of PFAD in supercritical methanol. Importantly, the effect of water content in the feed on the yield of FAMEs was investigated. In addition, the yield of FAMEs from the non-catalytic esterification was compared to that observed from the conventional acid-catalyzed esterification and from the transesterification of PPO in supercritical methanol. It is noted that this work was performed within the conditions close to the practical application i.e. PFAD was supplied from local palm oil refinery industry and methanol used here was commercial grade (95%).

2. Experimental 2.1. Chemicals PFAD sample (MW = 287) used in this study was provided by Burapha Munkong Co. Ltd, Thailand. It consists of 93 wt% free fatty acid (FFA) (45.6% palmitic, 33.3% oleic, 7.7% linoleic, 3.8% stearic, 1.0% myristic, 0.6% tetracosenoic, 0.3% linolenic, 0.3% ecosanoic, 0.2% ecosenoic, and 0.2% palmitoleic acid) and the rest elements are triglycerides, diglycerides (DG), monoglycerides (MG) and traces of impurities. PPO sample (MW = 974) used in this study was supplied by Chumporn Palm Oil Industry Public, Co, Ltd, Thailand. FAMEs standard (methyl palmitate, methyl stearate and methyl oleate) were obtained from Wako Chemicals, USA. Commercial grade methanol (95%) and analytical grade hexane (99.9%) were purchased from Fisher scientific, UK. 2.2. Esterification in supercritical methanol A batch type reactor was applied to study the esterification reaction in supercritical methanol. The system employed for this experiment consists of an electric furnace, a 8.8 ml stainless steel reactor (having the maximum allowable reaction pressure of 200 MPa (AKICO, Japan)) and a temperature controller, as schematically shown in Fig. 1. Prior to the experiment, the furnace temperature at the location of the furnace adjacent to the reactor (T1) was controlled at 250–300 °C. The reactor, with another thermocouple inserted at the center of the reactor to measure the reaction temperature (T2), was firstly charged with the mixture of methanol and PFAD with various molar ratios from 1:1 to 1:12, and then placed in the furnace. It is noted that, initially when the reactor

was placed in the furnace, T1 was dropped by 1–2 °C and took 1– 2 min to reach the set point again. After reaching the set point, T2 was then measured; it was found that the differences between T1 and T2 were around 8 °C. Thus, the actual reaction temperatures were 242 and 292 °C when the furnace temperatures were set at 250 and 300 °C, respectively. At these conditions, the pressures of the systems were measured to be between 10 and 15 MPa. After a period of reaction (10–80 min), the vessel was removed from the heater and placed into a water bath to stop the reaction. The reaction products were discharged from the reactor and were allowed to settle into three phases. The top phase was unreacted methanol which was removed by evaporation. The middle phase was FAMEs (biodiesel), from which the FAMEs product was drawn for gas chromatography (GC) analysis, and the bottom phase was water. It is noted that the effect of water content on the rate of esterification was also studied by two sets of experiments. In the first experiment, the effect of water content (between 0 and 30 water/PFAD wt%) on the conversion of PFAD was determined at a selected conditions. Secondly, the hydrolysis of FAMEs (produced from esterification of PFAD as described earlier) under subcritical water was investigated. To do so, the known amount of FAMEs from supercritical methyl esterification of PFAD was hydrolyzed with distilled water at various percentages of water/FAMEs (from 0 to 30 wt%) and various reaction times (0–90 min). After the reaction, the products were analyzed by GC. 2.3. Conventional esterification For comparison, the conventional esterification reaction was also studied in a batch type reactor. PFAD was firstly melted at 60 °C and mixed with methanol (with PFAD to methanol molar ratio of 1:6). The mixture was charged into the vessel (1,000 ml) that was connected with a condenser. H2SO4 (at 5 wt% of PFAD) was then added to the mixture and the reaction was allowed to take place for a specified reaction time. It is noted that the addition of the acid caused the temperature to rise to 67 °C. After the reaction, the product was cooled and allowed to settle into three phases. The upper phase was the remaining methanol, the middle phase was FAMEs; and the bottom phase was water. Water was removed from the product by a separatory funnel, whereas methanol was removed by evaporation. Lastly, FAMEs product was neutralized by washing repeatedly with water in a separatory funnel and the remaining water was finally removed by a rotary evaporator.

Fig. 1. Schematic diagram of apparatus for biodiesel production in supercritical methanol (P = 10–15 MPa).

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Analysis of FAMEs in products was carried out by gas chromatography (GC) (Shimadzu 14B, USA) with a flame ionization detector (FID). The GC consists of a column (Rtx 5, 30 m, 0.25 mm ID, 0.25 lm). The parameters for oven temperature was programmed to increase from 150 °C (with 2 min holding time) to 250 °C (with 5 min holding time) at the ramping rate of 5 °C min1. Sample were prepared by adding 0.1 ml of oil to 6 ml of n-hexane and ecosane was used an internal standard (2 ml of the samples were injected into column). The analysis of FAMEs allowed the determination of the percentage yield of FAMEs, which was defined as follows:

% yield of FAMEs ¼

W FAMEs  100 W raw material

where WFAMEs and Wraw material are the weight of FAMEs obtained and the weight of starting material (PPO or PFAD), respectively. 3. Results and discussion 3.1. Esterification of PFAD in supercritical methanol The esterification of PFAD in supercritical methanol was carried out under several operating conditions, i.e. reaction temperature, inlet PFAD to methanol molar ratio, reaction time, and inlet water content. Details of these experimental results are presented and discussed below. 3.1.1. Effect of reaction temperature The reaction was carried out at the system temperature (T1) between 250 and 300 °C with inlet PFAD to methanol molar ratio of 1:6 and reaction time of 30 min. The results in Fig. 2 indicate that the yield of FAMEs slightly increased (from 64% to 73%) when the system temperature increased from 250 °C to 280 °C. At 290 °C, the yield rose noticeably to 86% and reached 95% at 300 °C. Theoretically, the increase in temperature reduces the polarity of methanol due to the breaking down of hydrogen bonding, which results in the higher solubility of PFAD in methanol. It is noted that the complete solubility occurs as the temperature approaches the mixture critical temperature, at which point the reaction mixture became homogeneous and the reaction took place rapidly. By using Lydersen’s method of group contribution with application of Lorentz– Berthelot-type mixing rules [11], the critical temperature for the mixture of PFAD and methanol with molar ratio of 1:6 is approximately 282 °C. This is specifically agreeable with the result in Fig. 2, in which the sharp increase in product yield was observed at the system temperature (T1) of 290 °C, while the actual reaction temperature at the center of the vessel (T2) was 282 °C.

3.1.2. Effect of PFAD to methanol molar ratio According to the stoichiometry of the esterification reaction, one mole of alcohol and one PFAD are required to produce one mole of FAMEs and water. Practically, excess amount of alcohol is always applied in order to shift the equilibrium to the right-hand side. However, the use of too high amount of alcohol could also increase the cost of FAMEs and/or biodiesel production. Thus, the effect of PFAD to methanol molar ratio on the yield of FAMEs was investigated by varying the molar ratio from 1:1 to 1:12 (at the reaction time of 30 min and reaction temperatures of 250 °C and 300 °C). As seen in Fig. 3, the yield of FAMEs increased as the PFAD to methanol molar ratio increased from 1:1 to 1:6 for both reaction temperatures (250 °C and 300 °C). On the other hand, the yield significantly reduced when the PFAD to methanol molar ratios further increased to 1:9 and 1:12. The increase in the yield with increasing PFAD to methanol molar ratio (from 1:1 to 1:6) is due to the fact that the larger amount of methanol provides greater potential to interact with the molecules of palm fatty acids. Furthermore, theoretically, the critical temperature of the reactant mixture decreases with increasing amount of methanol (as summarized in Table 1); therefore, it is expected that the yield of FAMEs would increase with increasing methanol content. Nevertheless, when the PFAD to methanol molar ratios further increased from 1:6 to 1:9 and 1:12, the yields were found to be lower. This result could possibly be attributed to the higher water content presented in the system, since methanol used in the present work was commercial grade methanol (95%). Water could react readily with FAMEs under subcritical water condition, thus inhibit the overall yield (this will be later explained in Section 3.1.4). The most suitable molar ratio was found at 1:6, in which the maximum yield of FAMEs was 74 and 95% at 250 °C and 300 °C, respectively. It should also be noted that the trend at 250 °C was similar to that at 300 °C but the yield of FAMEs was significantly lower; this is due to the fact that this reaction temperature was below the critical temperature of mixture for all ranges of PFAD to methanol ratio studied.

100

% Yield of FMAEs

2.4. Product analysis

80

300C 250C

60 40 20 0 1:1

1:2

1:3

1:6

1:9

1:12

Molar ratio

%Yield of FAMEs

100

Fig. 3. Effects of molar ratios on methyl esterification of PFAD at 250 and 300 °C, reaction time 30 min (P = 10–15 MPa).

80 60 Table 1 The critical properties of PFAD and methanol mixtures at various compositions.

40 20 0 250

260

270

280

290

300

Temperature (C) Fig. 2. Effects of temperature on methyl esterification of PFAD at 250–300 °C, reaction time 30 min and molar ratio of PFAD to methanol of 1:6 (P = 10–15 MPa).

Molar ratio

Tc (K)

Tc (oC)

Pc (atm)

Vc (L/mol)

1:1 1:2 1:3 1:6 1:9 1:12 1:48

606.48 587.18 575.02 555.17 545.12 538.96 520.73

333.48 314.18 302.02 282.17 272.12 265.96 247.73

24.24 32.34 38.36 49.66 55.98 60.01 72.43

0.48 0.35 0.28 0.21 0.18 0.16 0.13

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100

%Yield of FAMEs

%Yield of FAMEs

100 80 60

250C 300C

40 20

80 60 40 20 0

0 0

10

20

30

40

50

60

70

80

90

Time (mins) Fig. 4. Effect of reaction time on methyl esterification of PFAD at 250 and 300 °C, reaction time 30 min and molar ratio of PFAD to methanol of 1:6 (P = 10–15 MPa).

3.1.3. Effect of reaction time The effect of reaction time was then determined by varying the reaction time from 10 to 80 min at two different temperatures (250 °C and 300 °C), while keeping the PFAD to methanol molar ratio constant at 1:6. As shown in Fig. 4, the result indicated that at 300 °C the yield of FAMEs increased steadily with increased reaction time up to 30 min, providing the yield of 95%. After that, the conversions remained nearly constant. At the system temperature of 250 °C, the yield of FAMEs significantly increased during the first 30 min then slowly increased and reached the maximum yield of FAMEs at 92% in 70 min. Thus, we suggest here that the esterification of PFAD can complete within about 30–70 min, depending on the reaction temperature. It should be noted the system employed in this study required the heating time of about 15 min for the reaction temperature at the center of the vessel to reach the steady temperature. This implied that the actual reaction time to obtain the desired yield should be shorter if preheating of the reactants and the reaction vessels was provided. More efficient preheating might be achieved with use of continuous process with preheating of the reactants. It is noted that, for comparison, the conventional acid-catalyzed esterification of PFAD with H2SO4 at various reaction times was also performed. With the same PFAD to methanol molar ratio of 1:6 and reaction temperature of 67 °C, only 75% FAMEs yield was obtained in 5 h. In order to produce FAMEs up to 90%, longer reaction time (up to 9 h) was required. 3.1.4. Effect of inlet water content It is well established that the presence of water provides negative effect on the biodiesel production process via alkali- and acidcatalyzed reactions, since water interferes with the catalyst and reduces catalyst performance [12]. For the alkaline-catalyzed process, the conversion was reported to reduce slightly when water was present in the system. As for acid-catalyzed reaction, only as little as 0.1% of water added to the reaction could lead to 6% reduction of the production yield [13]. According to the transesterification in supercritical methanol, Kusdiana and Saka demonstrated that in the presence of water up to 30% w/w, triglycerides still transesterified to FAMEs efficiently with very minimal loss in conversion [14]. Nevertheless, for the esterification in supercritical methanol, Kusdiana and Saka reported approximately 2–5% reduction in conversion of oleic acid to methyl oleate in the presence of water (up to 30%) at 350 °C with the oleic acid to methanol molar ratio of 1:42 [15]. In this study, the effect of the presence of water between 0 and 30% of water was determined on the supercritical methyl esterification of PFAD at the system temperature of 300 °C and reaction time of 30 min, and with the PFAD to methanol molar ratio if 1:6. The result in Fig. 5 shows that the presence of water indeed lowered the percent yield of FAMEs. Water between 5 and

0

5

10

15

20

25

30

Water(%v/v) Fig. 5. Effect of water on percent yield of FAMEs at 300 °C, 30 min (P = 10–15 MPa).

25 vol% of water to PFAD lower the yield of FAMEs to only 64%, while at 30 vol% of water to PFAD, the yield dramatically decreased to 57%. The significant decrease in FAMEs yield compared to that observed by Kusdiana and Saka [15] could be due to the lower methanol content and reaction temperature of the present work. Moreover, at prolong reaction time employed, the presence of water might further react with FAMEs by hydrolysis reaction and converted FAMEs back to fatty acids. Therefore, these results suggested that, in order to maximize the yield of FAMEs produced from PFAD, care must be taken to remove water from the feed and to prevent extended reaction time. 3.2. Hydrolysis of FAMEs in supercritical condition From the result observed in Section 3.1.4, water was found to reduce the yield of FAMEs. The hydrolysis of known amount FAMEs, produced from the esterification of PFAD, was performed to prove that the inhibitory effect of water is due to the further hydrolysis reaction. Experiments were conducted to determine the effect of water content between 0 and 30% (v/v) and the hydrolysis time from 5 to 30 min on the amount of FAMEs hydrolyzed. Fig. 6 shows the relation between the amount of FAMEs remained (unhydrolyzed), the reaction time, and inlet water content. It can be seen that the degree of hydrolysis increased with increasing the amount of water and reaction time. This result therefore confirmed the significant impact of hydrolysis on esterification reaction under supercritical condition. The system temperature of 300 °C is between boiling point temperature and critical temperature, thus water is considered to be in subcritical state. At this condition, water has high ion product (Kw), which means water readily dissociate into hydronium and hydroxide ions. The increased in these ions would play an important role in hydrolysis reaction, which breaks the large molecule of FAMEs back to fatty acid. As the next step, since esterification reaction is typically performed with excess amount of methanol, the effect of methanol presented in the system on the hydrolysis reactivity was carried out in order to examine the effect of hydrolysis in the system that has greater resemblance to the actual process. In this experiment, the hydrolysis in the presence of methanol was carried out at 300 °C for 30 min. The amount of FAMEs, methanol, and water used in the hydrolysis reaction in this experiment was calculated from the ratio of final mixture obtained after complete the esterification reaction. For example, the products from esterification with PFAD to methanol molar ratio of 1:6 (at 100% conversion) would consist of 1 mol of FAMEs, 1 mol of water, and 5 mol of the remaining methanol, giving FAMEs: methanol: water molar ratio of 1:5:1. It is noted that this ratio was chosen for the hydrolysis study here. It was found that, in the presence of methanol, only 8% of FAMEs was hydrolyzed, which is significantly lower than that observed without the presence of methanol (20%). This result con-

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1vol%

10vol%

10

15

20vol%

30vol%

% of FAMEs unhydrolyzed

100

80

60

40

20

0 5

20

25

30

Time(min) Fig. 6. Effect of various water on hydrolysis of FAMEs for various reaction time 30 min at 300 °C (P = 10–15 MPa).

firms that the presence of methanol lower the degree of hydrolysis, furthermore, the less inhibitory affect of water reported by Kusdiana and Saka [15] (2–5% reduction in oleic acid conversion in the presence of water up to 30%) can be understood, since the oleic acid to methanol molar ratio in their work was as high as 1:42.

30 min and molar ratio 1:6 was as high as 74%. Hence, it can be concluded from this study that the production of biodiesel from PFAD requires milder reaction conditions and shorter period of time than transesterification of PPO.

3.3. FAMEs production from PFAD vs. PPO

FAMEs could be efficiently produced from the non-catalytic esterification of PFAD in supercritical methanol. The most suitable conditions were i.e. PFAD to methanol molar ratio of 1:6, reaction temperature of 300 °C and reaction time of 30 min, in which 95% FAMEs was produced. Compared to the conventional acid-catalyzed esterification of PFAD, with the PFAD to methanol molar ratio of 1:6 and reaction temperature of 67 °C, only 75% FAMEs yield was obtained in 5 h. Cautiously, the presence of water in the feed was found to lower the yield of FAMEs due to the further hydrolysis of FAMEs; this negative effect can be minimized when high content of methanol was used. Compared with the transesterification of PPO in supercritical methanol, the production of FAMEs from PFAD requires milder reaction conditions and shorter period of time. We concluded here that locally produced PFAD can be used as an alternative feedstock for biodiesel production, and the esterification in supercritical methanol has a great potential to replace the conventional acid-catalyzed esterification process.

%Yield of FAMEs

It was proven from the previous sections that PFAD can be efficiently used as feedstock for biodiesel production via esterification in supercritical methanol. Its reactivity was then compared to the transesterification of PPO in supercritical methanol within the same reactor system. It is noted that, for transesterification reaction, the PPO to methanol molar ratios were varied from 1:6 to 1:45; and the experiment was conducted at 250 °C and 300 °C with the reaction time of 50 min. As seen in Fig. 7, the yield of FAMEs increased with increasing methanol content for both temperatures and reached the highest yield of 80% at 300 °C with the PPO to methanol molar ratio of 1:45. Compared between both reactions, the results here indicated that FAMEs can be efficiently produced from the esterification of PFAD with considerably less amount of methanol and reaction temperature required. Specifically, the amount of the methanol required to produce 100 g of biodiesel by methyl esterification at 300 °C was only 75 g while supercritical transesterification of PPO required as high as 198 g of methanol. Considering the transesterification at 250 °C (50 min), the yield was much lower than that obtained with esterification of PFAD at the same operating temperature. Transesterification at 250 °C gave less than 15% FAMEs yield in 50 min, whereas the yield of esterification of PFAD at 250 °C for

100

PFAD 1:6, 300 °C

80

PFAD 1:6, 250 ° C

300C

40

250C

20 0 1:12

1:15

Acknowledgments The authors thank Burapa Munkong Co. Ltd for providing PFAD. Financial supports from Thailand Research Fund and The Commission of Higher Education, and Chulalongkorn Graduate School are greatly acknowledged. References

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1:6

4. Conclusions

1:25

1:35

1:45

Molar ratio Fig. 7. Effect of the molar ratios of methanol to palm oil on the percent conversion of FAMEs at 250 and 300 °C (P = 10–15 MPa).

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