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A Brønsted ammonium ionic liquid-KOH two-stage catalyst for biodiesel synthesis from crude palm oil
Industrial Crops and Products 41 (2013) 144–149
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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
A Brønsted ammonium ionic liquid-KOH two-stage catalyst for biodiesel synthesis from crude palm oil Zakaria Man a , Yasir A. Elsheikh b , M. Azmi Bustam a , Suzana Yusup a , M.I. Abdul Mutalib a , Nawshad Muhammad a,∗ a b
PETRONAS Ionic Liquid Center, Department of Chemical Engineering, Universiti Teknologi PETRONAS (UTP), 31750 Tronoh, Perak, Malaysia Chemical Engineering Department, College of Engineering, Jazan 45142, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 8 January 2012 Received in revised form 10 April 2012 Accepted 14 April 2012 Keywords: Crude palm oil Triethylammonium hydrogensulfate Esterification Transesterification Biodiesel
a b s t r a c t Palm biodiesel from crude palm oil (CPO) was prepared via transesterification in a two-stage process using acidic ionic liquid as first step catalyst as opposed to using directly the alkaline-catalyzed transesterification which was found to be unsuitable. The esterification of the free fatty acids (FFA) of the CPO was carried out using triethylammonium hydrogensulfate (Et3 NHSO4 ) as the pre-catalyst in the first stage, in which the acid value was reduced from 6.98 to 1.24 mg KOH/g of oil followed by the use of KOH-catalyzed transesterification in the second stage. The effects of molar ratio of methanol to crude palm oil feed, the amount of ionic liquid and the reaction temperature were evaluated for the percent conversion of FFA. The conversion rate of FFA attained was 82.1% when 5.2 wt.% of Et3 NHSO4 was used for the reaction of methanol with CPO at a ratio of 15:1 respectively, and at reaction temperature of 170 ◦ C for 3 h. The second alkali-catalyze step was performed under agitation with stirrer speed of 600 rpm at 60 ◦ C using 1.0% KOH for 50 min. The final biodiesel product was analyzed using gas chromatography (GC) with a reaction yield of 96.9%. The density, kinematic viscosity, acid value, ester content and other properties of the biodiesel sample were also measured and compared to the ASTM and the European biodiesel specifications. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Malaysia has a rich oil palm industry which is currently producing more than sufficient palm oil and its derivatives to fulfill the local demand while serving quite a significant portion of the export market internationally. In view of such excess, there is a possibility that the industry could also channel the palm oil partly or even fully toward producing bio-fuel in order to serve the growing fuel consumption for the future. Palm oil production in Malaysia has increased from 2.57 million metric tons in the year 1980 to 14.96 million metric tons in 2005 where more than 3.79 million hectares of land have been cultivated with the oil palm (Chew and Bhatia, 2008). Palm oil is considered as one of the four leading vegetable oil traded on the world market and it is the cheapest compared to canola, rapeseed and soybean oil. This has made palm oil as one of the potential source for reducing the high cost of feed material for bio-diesel production either as a ready-good substitution or blend for diesel fuel (Crabbe et al., 2001; Kalam and Masjuki, 2002; Tang et al., 2008). In pursuing the strategy for developing renewable energy source for the future, the Malaysian government has
∗ Corresponding author. Tel.: +60 17 4611795; fax: +60 53688151. E-mail address: [email protected] (N. Muhammad). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.04.032
started their palm oil biodiesel project in 1982 (Kalam and Masjuki, 2008; Zhou and Thomson, 2009). The move made by the European Union in setting a target of replacing 20% of the total motor fuel consumption using bio-fuels by 2020 has provided further opportunity for the palm oil. Even in the US market currently, the fossil diesel blended with 20% of soybean biodiesel is already made available and thus can potentially create larger market demand for the use of palm oil in producing bio-fuel due to its cheaper price (Wang et al., 2006, 2007). There were different types of catalysts used for the transesterification reaction of triglycerides (TG) to produce bio-diesel and they could be classified as either homogeneous or heterogeneous catalysts (Dos Santos et al., 2008). The homogeneous catalysts, such as alkaline and acid, have been proven to be more practical in application (Liu et al., 2008). The alkaline catalyst is capable of producing higher yield and purity of bio-diesel with reaction time of between 30 and 60 min (Zhang et al., 2003). Nevertheless, there is a limit of not more than 1.0% FFA content allowed in the vegetable oil feed for the alkaline catalyzed process (Liu, 1994; Leung and Guo, 2006) which made it only suitable for the processed or refined vegetable oil to be used as the feed material due of its low free fatty acid (FFA) and purer TG content compared to the crude vegetable oil (Wang et al., 2007). The presence of high FFA in the feedstock is undesirable as it could lead to side reactions with the alkaline
Z. Man et al. / Industrial Crops and Products 41 (2013) 144–149
(a)
RCOOH + CH3OH
Et3 NHSO4
Table 1 Physical and chemical properties of crude palm oil.
RCOOCH3 + H2O
O C
R1
O O C O
R2
C
R3
O
OH
R1 COO CH3
+
3CH3 OH
KOH
R2 COO CH3 R3 COO CH3
+
CPOa
Property
O
(b)
145
OH OH
Fig. 1. Mechanism of biodiesel preparation via two stage catalyzed process.
catalyst to produce soaps and water causing the downstream recovery and purification of the product to be more difficult (Dorado et al., 2004; Canakci and Van Gerpen, 2001). In addition, the separation and recovery of the alkaline catalyst from the product also becomes significantly more difficult (Kim et al., 2004). As a result of the above, the acid catalyzed process has been preferred to a certain extent (Ma and Hanna, 1999). On the other hand, the acid catalyzed process requires relatively high amount of excess alcohol and high pressure condition for its reaction. Due to its lower yield, a larger size reactor is required with extensive conditioning and purification steps to recover the unreacted alcohol, the valuable by product glycerol, as well as the catalyst from the reactor product. The use of excessive alcohol actually complicates the removal of glycerol due to its high solubility in alcohol (Dubé et al., 2007). Recently, a new combined catalytic process for handling different feedstocks with high FFA has been developed (Wang et al., 2006; Zhang et al., 2003; Canakci and Van Gerpen, 2003). The process involves conducting the acid catalyzed esterification in the first step for the purpose of lowering the FFA content to an accepted range followed by the addition of alkaline catalyst, after the removal of the acid catalyst, to complete the transesterification process. Nevertheless, this process too has drawbacks where acidic waste water is produced along with alkyl ester from the esterification reaction. The problem of managing the highly acidic effluent, the difficulty in catalyst recovery process and the high cost of stainless steel equipment needed for the acidic reaction media became the main limitations for applying this process (Freedman et al., 1984). In another recent development, there has been increasing interest shown in ionic liquids (ILs) as substitutes for conventional catalysts due to their unique properties and features such as extremely low vapour pressure, higher thermal stability and simple recovery process. In particular, the acidic ILs has been demonstrated to be very useful as they show a similar behavior to common acid catalysts in chemical reactions (Duan et al., 2006). In this study, a new two-step catalytic process has been developed to produce bio-diesel from CPO (Fig. 1). Firstly, the FFA in the CPO was esterified with methanol in the presence of Et3 NHSO4 as a catalyst owing to acidic nature. When the FFA content in the CPO has reduced to the level tolerated for the next alkaline catalyst reaction, it was then subjected to next reaction step. Potassium hydroxide (KOH) in the second step was used to catalyze the transesterification of the TG in the CPO with methanol to yield three moles of esters and one mole of glycerol by-product. 2. Materials and methods 2.1. Materials CPO was collected from Felcra Salahuddin Factory – Perak, Malaysia. The chemicals purchased from Sigma–Aldrich company (Malaysia) include 1-triethylamine (≥99.5%), Sulfuric acid (≤98.0%), diethyl ether (≥99.5%), trimethyl-1-pentene (99.9%),
◦
3
Density at 15 C (kg/m ) Kinematic viscosity at 30 ◦ C (cSt) FFA (wt.%) Fatty acid composition (wt.%) Lauric acid (C12:0) Myristic acid (C14:0) Palmitic acid (C16:0) Palmitoleic acid (C16:1) Margaric acid (C17:0) Stearic acid (C18:0) Oleic acid (C18:1) Linoleic acid (C18:2) Linolenic acid (C18:3) Arachidic acid (C20:0) Gadoleic acid (C20:1) a
917.8 38.3 3.49 0.40 1.26 46.90 0.07 0.10 4.59 36.85 9.09 0.20 0.39 0.15
Average of duplicate results.
acetonitrile (anhydrous, ≥99.8%), Methanol (anhydrous, ≥99.8%), and the reference standards, GC grades (>99.0%), also were obtained from Sigma–Aldrich (Malaysia). KOH (analytical grade) was purchased from Merck (Malaysia). All chemicals were used as received and without drying or any further purification. 2.2. Properties of CPO The physicochemical properties of CPO are shown in Table 1. The CPO density was measured using an Anton Paar DMA5000 instrument (meet ASTM D4052-96 (2002)), kinematic viscosity was determined using an Ubbelohde glass viscometer while the FFA content was determined by means of the acid value using the official method Cd 3d-63 (A.O.C.S., reapproved 1997). Agilent Hewlett-Packard 6890 series gas chromatograph was used, which was equipped with flame ionization detector, SP-2340 capillary column (60 m in length, 25 mm of internal diameter, and 0.2 m film thickness) and a split ratio of 100:1 (A.O.C.S., reapproved 1997). 2.3. Preparation of acidic ionic liquid The ionic liquid Et3 NHSO4 was synthesized by mixing 0.4 mol triethylamine and 10 mL anhydrous acetonitrile. Both were charged into a 250 mL round-bottom flask equipped with a reflux condenser, magnetic stirrer and a N2 gas inlet. While the reaction mixture was kept under cold condition and vigorous stirring, equimolar amount of concentrated sulfuric acid (0.4 mol) was gradually added i.e., in drop wise manner, and the mixture was stirred for 4 h at room temperature. The colorless Brønsted ionic liquid was obtained after repeated washing with diethyl ether and trimethyl1-pentene and was dried under vacuum (760 mmHg and 343.15 K) for 6 h, giving Et3 NHSO4 with purity of 98%. The NMR spectroscopic data recorded for the ILs sample is: 1 H NMR (300 MHz, DMSO): ı = 1.069–1.251 (t, 9H), 2.987–3.161 (m, 6H), 6.322 (s, 1H), 8.981 (s, 1H). 2.4. Two-stage catalytic process 2.4.1. Et3 NHSO4 -catalyzed esterification of CPO The esterification of CPO with methanol was performed under reflux condition. In this work, the basic variables such as reaction temperature, molar ratio of methanol to CPO and concentration of Et3 NHSO4 were investigated for the first-step esterification reaction. The experiments were conducted at seven reaction temperatures (120, 130, 140, 150, 160, 170 and 180 ◦ C) and five different ILs concentrations which are 4.6, 4.8, 5.0, 5.2, 5.4 wt.% based on the weight of CPO. The stirring speed was maintained
Z. Man et al. / Industrial Crops and Products 41 (2013) 144–149
100
100
90
90
80
FFA conversion(%)
FFA conversion(%)
146
70 60 50 40 30
80 70 60 50 40 30 20 10
20 4.6
4.8
5
5.2
5.4
0
5.6
Concentration of Et 3 AmHSO 4 (wt.%)
at 600 rpm in line with the recommendation made by Vicente et al. (1998) for sufficient agitation to overcome the mass transfer limitation during the formation of biodiesel. The desired product of the first step reaction was extracted from reaction mixture at different intervals of time (0.5, 1.0, 1.5 and 5 h) followed by washing with ethyl acetate and the different layers (IL and oil layers) obtained were separated carefully. After separation of IL layer, the oil layer containing unreacted TG was further subjected to the alkaline catalyzed hydrolysis. In order to determine the conversion of FFA during the process, the samplings were done manually and the FFA content was analyzed using AOCS Cd 3d-63 titration method (A.O.C.S., reapproved 1997). The conversion of FFA was calculated from the acidity reduction using the following equation:
AVi − AVt AVi
× 100
3:1
6:1
9:1
12:1
15:1
18:1
Molar ratio of methanol to CPO
Fig. 2. Effect of Et3 NHSO4 concentration on FFA conversion (methanol/CPO molar ratio 15:1, temperature 170 ◦ C, rate of stirring 600 rpm).
Conversion (%) =
0
(1)
where AVi is initial acid value of the mixture and AVt is the acid value at any “t” time. 2.4.2. KOH-catalyzed transesterification of CPO For the next reaction step involving alkaline catalyzed transesterification of the extracted oil samples with lower FFA, KOH was used as suggested by several researchers (Sharma et al., 2008; Tomasevic and Siler-Marinkovic, 2003). The temperature of the transesterification reaction was set at 60 ◦ C as recommended by Encinar et al. (2007) with a 6:1 molar ratio of methanol to TG as recommended by Freedman et al. (1984) and Meher et al. (2006). The reaction time was maintained at 60 min (Ghedge and Raheman, 2006). The preheated methanolic KOH mixture was added to the treated CPO in a 250 mL two necked-flask with constant stirring.
Fig. 4. Effect of methanol to CPO molar ratio on FFA conversion at 170 ◦ C; 5.2 wt.% catalyst concentration, and agitation speed of 600 rpm.
After the reaction products had settled, the upper layer mainly consisting of the palm oil methyl ester (POME) was separated and washed with hot water to remove the impurities such as soap, unreacted methanol and residual KOH. The washed palm oil methyl ester which now forms a crude biodiesel was dried under vacuum at 90 ◦ C for 4 h.
2.5. Analysis of the palm biodiesel A Shimadzu Gas Chromatograph (GC-2010) equipped with AOC20i automatic injection and a flame ionization detector (FID) was used for the analysis of the palm biodiesel. The capillary column used was SUPELCO SGE HT-5 with a length of 10 m and internal diameter of 0.32 mm, coated with a 0.1 m film thickness of 100% dimethylpolysiloxane. After 1 min of stabilization at 50 ◦ C, the GC oven temperature was programmed to initially increase by 15 ◦ C/min up to 180 ◦ C, then at 7 ◦ C/min from 180 to 230 ◦ C before finally at 10 ◦ C/min up to 370 ◦ C where it is then held for 5 min, thus giving a total analytical time of about 30 min to ensure the complete elution of the glycerides. The carrier gas used was highpurity helium (≥99.95 mol.%) with a flow rate of 3 mL/min and split ratio of 100:1. The detector temperature was set at 380 ◦ C. A 1.0 L of each sample was injected after dissolving them in n-heptane (GC grade) with methyl heptadecanoate used as the internal standard. The esters and glycerol contents in the samples are analyzed by GC methods according to ASTM D6751 and EN 14214 (Knothe, 2006).
100
FFA conversion (%)
90 80 70 60 50 40 30 20 10 0
120
130
140
150
160
170
180
Temperature (°C) Fig. 3. Effect of reaction time on FFA conversion at 170 ◦ C; 5.2 wt.% IL concentration, 15:1 molar ratios of methanol to CPO and agitation speed of 600 rpm.
Fig. 5. Effect of reaction temperature on FFA conversion at 4.8 wt.% catalyst concentration, 12:1 molar ratios of methanol to CPO and agitation speed of 600 rpm.
Z. Man et al. / Industrial Crops and Products 41 (2013) 144–149
147
Fig. 6. GC chromatograms of (a) Crude palm oil; (b) Transesterified crude palm oil; (c) FAME of crude palm oil; Peaks 1 – methyl laurate, 2 – methyl myristate, 3 – methyl palmitate, 4 – methyl stearate, 5 – methyl oleate, and 6 – methyl linoleate.
3. Results and discussion 3.1. Et3 NHSO4 -catalyzed esterification reaction of CPO (first step)
maximum conversion after the 170 min of reaction time. Furthermore it has been noticed that the increase in amount of ionic liquid ratio beyond 5.4 wt.%, was not effective in the conversion of FFA.
3.1.1. Effect of Et3 NHSO4 concentration on FFA conversion The amount of Et3 NHSO4 used in the process was varied between 4.6 and 5.4 wt.% (based on the weight of CPO). The reaction experiments were carried out using a methanol to CPO ratio of 12:1 and at temperature of 170 ◦ C. As can be seen in Fig. 2, the FFA conversion increases with the increased in IL concentration. The optimum IL amount is found to be 5.2 wt.% as it gives a
3.1.2. Effect of reaction time Fig. 3 shows the effect of reaction time on FFA conversion with Et3 NHSO4 concentration kept at 5.2 wt.% using 12:1 molar ratio of methanol to CPO at reaction temperature of 170 ◦ C. The full experiment was conducted for 5 h with intermediate extraction of samples from the reaction still at specified intervals in order to analyze the product concentration. During the first 30 min, it was
148
Z. Man et al. / Industrial Crops and Products 41 (2013) 144–149
Table 2 Fuel parameters of palm oil methyl ester (POME) as compared to ASTM and European Biodiesel Standards specifications. Property
Unit
Test method
POME in this work
ASTM standard
Test method
European standards
Density, 15 ◦ C Kinematic viscosity, 40 ◦ C Flash point Specific gravity, 15 ◦ C Distillation temperature, 95% Water content Acid value Ester content Free glycerol Total glycerol
kg/m3 mm2 /s ◦ C – ◦ C wt.%, mg/kg mg KOH/g wt.% wt.% wt.%
ASTM D4052 ASTM D445 ASTM D93 ASTM D4052 ASTM D86 ASTM D6304 ASTM D664 – ASTM D6584 ASTM D6584
878 3.56 164 0.895 114.5 0.03 0.44 96.9 0.014 0.11
870–900 1.9–6.0 130 min 0.88–0.90 120 max 0.05 max 0.50 max – 0.020 0.240
EN ISO 3675,EN ISO 12185 EN ISO 3104, ISO 3105 EN ISO 3679 – – EN ISO 12937 EN 14104 EN 14103 EN 14105, EN 14106 EN 14105
860–900 3.5–5.0 120 min – – 500 max 0.50 max 96.5 min 0.020 max 0.25 max
found that 10% of the FFA has been converted to FAME. As the reaction time extended further, more conversion of FFA into FAME was observed until a maximum conversion of about 80% was attained after 210 min when the reaction reached steady state.
3.1.3. Effect of the molar ratio of methanol to CPO In order to study the effect of the molar ratio on FFA esterification, the reaction experiments were conducted with various molar ratios of methanol to CPO within the range of 3:1 to 18:1. The result on FFA conversion obtained versus the molar ratio of the methanol to oil is shown in Fig. 4. During the whole reaction, the Et3 NHSO4 concentration was fixed at 5.2 wt.%, the reaction temperature at 170 ◦ C and the agitation speed at 600 rpm. The reaction time was set at 210 min. The result indicates that the conversion from the esterification reaction was lowest at 3:1 molar ratio of methanol to CPO. As the molar ratio of methanol to CPO increases, the FFA conversion also increases. As the molar ratio increases from 3:1 to 12:1, the FFA conversion to FAME changes from 9.4% to 77.6%. Based on the maximum reaction conversion obtained in the previous section, the reaction can be considered as incomplete for a molar ratio of less than 12:1 due to the lower conversion attained within the same time period allowed for the reaction. The maximum conversion was achieved at the molar ratio of 15:1 beyond which further increase in the molar ratio of the two does not result in any additional conversion.
3.3. Characterization of CPO biodiesel 3.3.1. GC analysis Fig. 6(a) presents the GC chromatogram of CPO sample before starting the transesterification reaction. The GC chromatogram (Fig. 6(a)) shows that the CPO contains small traces of glycerine and high amount of TG. Fig. 6(b) presents the analyzed compounds peaks progression of derivatized glycerine, butanetriol standard, methyl esters, derivatized MG, tricaprin second standard, derivatized DG, and TG. Small peaks of minor components, such as sterols, are also detected by the GC. Fig. 6(C) presents the esters content in the CPO biodiesel. The chromatogram for FAME content in CPO biodiesel indicates the presence of methyl laurate, methyl myristate, methyl palmitate, methyl stearate, methyl oleate, and methyl linoleate in the produced CPO biodiesel. 3.4. Properties analysis To ensure the quality of the produced samples, the important properties of palm oil methyl esters were tested and the results are presented in Table 2. The results were found to be comparable to the biodiesel specifications of the American Standards for Testing Material (ASTM D6751-03) and the European Biodiesel Standards (EN 14214).
4. Conclusion 3.1.4. Effect of reaction temperature The effect of temperature on the conversion of FFA was studied for the temperature range of 120 to 180 ◦ C. The esterification reaction was carried out under the same operational parameters as in the previous part i.e., 5.2 wt.% of Et3 NHSO4 concentration, 15:1 methanol/CPO molar ratio, agitation speed of 600 rpm and reaction time of 210 min. Fig. 5 shows the changes in the conversion of FFA with temperature. The FFA conversion increases from 20% to 82.1% as the temperature was increased from 120 to 170 ◦ C. Further increased in the reaction temperature beyond 170 ◦ C did not result in any increase in the FFA conversions. Therefore, it can be concluded that the optimize reaction temperature is 170 ◦ C.
3.2. KOH-catalyzed transesterification (second step) For the second step reaction, the FFA level in the CPO, after the ILcatalyzed esterification reaction, was maintained at about 0.5 wt.%, which is equivalent to 1.02 mg KOH/g oil. The maximum purity of biodiesel produced was 97.3%, with the optimal concentration of KOH used at 1.0 wt.% of treated CPO. For this reaction, the methanol to TG mole ratio used was 6:1 at a reaction temperature of 60 ◦ C for 60 min duration.
The Brønsted ionic liquid triethylammonium hydrogensulfate has shown reasonably good performance in catalyzing the esterification reaction of the FFA in CPO. Compared to the homogeneous acidic catalysts, the ionic liquids in general, have the prospect to reduce the environmental impact usually associated with chemical processes due to their ease of recoverability and reusability, while generating less waste water. The use of ILs could also lead to less corrosion effect thus making it more favorable in the application for continuous process. The best conversion of FFA in the CPO obtained in this work is 82.1% based on the reaction parameters of; (i) reaction time – 210 min, (ii) quantity of catalyst used – 5.2 wt.%, (iii) methanol to CPO ratio – 15:1 and (iv) reaction temperature of 170 ◦ C. The twostage catalyst reaction provides a simple and economic alternative method to produce biodiesel from CPO. In addition, the separation of glycerol and soap, created by KOH, in this process can be easily isolated from the rich ester layer.
Acknowledgement The financial assistance provided by PETRONAS Ionic Liquid Center, Universiti Teknologi PETRONAS (UTP) is gratefully acknowledged.
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A Brønsted ammonium ionic liquid-KOH two-stage catalyst for biodiesel synthesis from crude palm oil Zakaria Man a , Yasir A. Elsheikh b , M. Azmi Bustam a , Suzana Yusup a , M.I. Abdul Mutalib a , Nawshad Muhammad a,∗ a b
PETRONAS Ionic Liquid Center, Department of Chemical Engineering, Universiti Teknologi PETRONAS (UTP), 31750 Tronoh, Perak, Malaysia Chemical Engineering Department, College of Engineering, Jazan 45142, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 8 January 2012 Received in revised form 10 April 2012 Accepted 14 April 2012 Keywords: Crude palm oil Triethylammonium hydrogensulfate Esterification Transesterification Biodiesel
a b s t r a c t Palm biodiesel from crude palm oil (CPO) was prepared via transesterification in a two-stage process using acidic ionic liquid as first step catalyst as opposed to using directly the alkaline-catalyzed transesterification which was found to be unsuitable. The esterification of the free fatty acids (FFA) of the CPO was carried out using triethylammonium hydrogensulfate (Et3 NHSO4 ) as the pre-catalyst in the first stage, in which the acid value was reduced from 6.98 to 1.24 mg KOH/g of oil followed by the use of KOH-catalyzed transesterification in the second stage. The effects of molar ratio of methanol to crude palm oil feed, the amount of ionic liquid and the reaction temperature were evaluated for the percent conversion of FFA. The conversion rate of FFA attained was 82.1% when 5.2 wt.% of Et3 NHSO4 was used for the reaction of methanol with CPO at a ratio of 15:1 respectively, and at reaction temperature of 170 ◦ C for 3 h. The second alkali-catalyze step was performed under agitation with stirrer speed of 600 rpm at 60 ◦ C using 1.0% KOH for 50 min. The final biodiesel product was analyzed using gas chromatography (GC) with a reaction yield of 96.9%. The density, kinematic viscosity, acid value, ester content and other properties of the biodiesel sample were also measured and compared to the ASTM and the European biodiesel specifications. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Malaysia has a rich oil palm industry which is currently producing more than sufficient palm oil and its derivatives to fulfill the local demand while serving quite a significant portion of the export market internationally. In view of such excess, there is a possibility that the industry could also channel the palm oil partly or even fully toward producing bio-fuel in order to serve the growing fuel consumption for the future. Palm oil production in Malaysia has increased from 2.57 million metric tons in the year 1980 to 14.96 million metric tons in 2005 where more than 3.79 million hectares of land have been cultivated with the oil palm (Chew and Bhatia, 2008). Palm oil is considered as one of the four leading vegetable oil traded on the world market and it is the cheapest compared to canola, rapeseed and soybean oil. This has made palm oil as one of the potential source for reducing the high cost of feed material for bio-diesel production either as a ready-good substitution or blend for diesel fuel (Crabbe et al., 2001; Kalam and Masjuki, 2002; Tang et al., 2008). In pursuing the strategy for developing renewable energy source for the future, the Malaysian government has
∗ Corresponding author. Tel.: +60 17 4611795; fax: +60 53688151. E-mail address: [email protected] (N. Muhammad). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.04.032
started their palm oil biodiesel project in 1982 (Kalam and Masjuki, 2008; Zhou and Thomson, 2009). The move made by the European Union in setting a target of replacing 20% of the total motor fuel consumption using bio-fuels by 2020 has provided further opportunity for the palm oil. Even in the US market currently, the fossil diesel blended with 20% of soybean biodiesel is already made available and thus can potentially create larger market demand for the use of palm oil in producing bio-fuel due to its cheaper price (Wang et al., 2006, 2007). There were different types of catalysts used for the transesterification reaction of triglycerides (TG) to produce bio-diesel and they could be classified as either homogeneous or heterogeneous catalysts (Dos Santos et al., 2008). The homogeneous catalysts, such as alkaline and acid, have been proven to be more practical in application (Liu et al., 2008). The alkaline catalyst is capable of producing higher yield and purity of bio-diesel with reaction time of between 30 and 60 min (Zhang et al., 2003). Nevertheless, there is a limit of not more than 1.0% FFA content allowed in the vegetable oil feed for the alkaline catalyzed process (Liu, 1994; Leung and Guo, 2006) which made it only suitable for the processed or refined vegetable oil to be used as the feed material due of its low free fatty acid (FFA) and purer TG content compared to the crude vegetable oil (Wang et al., 2007). The presence of high FFA in the feedstock is undesirable as it could lead to side reactions with the alkaline
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(a)
RCOOH + CH3OH
Et3 NHSO4
Table 1 Physical and chemical properties of crude palm oil.
RCOOCH3 + H2O
O C
R1
O O C O
R2
C
R3
O
OH
R1 COO CH3
+
3CH3 OH
KOH
R2 COO CH3 R3 COO CH3
+
CPOa
Property
O
(b)
145
OH OH
Fig. 1. Mechanism of biodiesel preparation via two stage catalyzed process.
catalyst to produce soaps and water causing the downstream recovery and purification of the product to be more difficult (Dorado et al., 2004; Canakci and Van Gerpen, 2001). In addition, the separation and recovery of the alkaline catalyst from the product also becomes significantly more difficult (Kim et al., 2004). As a result of the above, the acid catalyzed process has been preferred to a certain extent (Ma and Hanna, 1999). On the other hand, the acid catalyzed process requires relatively high amount of excess alcohol and high pressure condition for its reaction. Due to its lower yield, a larger size reactor is required with extensive conditioning and purification steps to recover the unreacted alcohol, the valuable by product glycerol, as well as the catalyst from the reactor product. The use of excessive alcohol actually complicates the removal of glycerol due to its high solubility in alcohol (Dubé et al., 2007). Recently, a new combined catalytic process for handling different feedstocks with high FFA has been developed (Wang et al., 2006; Zhang et al., 2003; Canakci and Van Gerpen, 2003). The process involves conducting the acid catalyzed esterification in the first step for the purpose of lowering the FFA content to an accepted range followed by the addition of alkaline catalyst, after the removal of the acid catalyst, to complete the transesterification process. Nevertheless, this process too has drawbacks where acidic waste water is produced along with alkyl ester from the esterification reaction. The problem of managing the highly acidic effluent, the difficulty in catalyst recovery process and the high cost of stainless steel equipment needed for the acidic reaction media became the main limitations for applying this process (Freedman et al., 1984). In another recent development, there has been increasing interest shown in ionic liquids (ILs) as substitutes for conventional catalysts due to their unique properties and features such as extremely low vapour pressure, higher thermal stability and simple recovery process. In particular, the acidic ILs has been demonstrated to be very useful as they show a similar behavior to common acid catalysts in chemical reactions (Duan et al., 2006). In this study, a new two-step catalytic process has been developed to produce bio-diesel from CPO (Fig. 1). Firstly, the FFA in the CPO was esterified with methanol in the presence of Et3 NHSO4 as a catalyst owing to acidic nature. When the FFA content in the CPO has reduced to the level tolerated for the next alkaline catalyst reaction, it was then subjected to next reaction step. Potassium hydroxide (KOH) in the second step was used to catalyze the transesterification of the TG in the CPO with methanol to yield three moles of esters and one mole of glycerol by-product. 2. Materials and methods 2.1. Materials CPO was collected from Felcra Salahuddin Factory – Perak, Malaysia. The chemicals purchased from Sigma–Aldrich company (Malaysia) include 1-triethylamine (≥99.5%), Sulfuric acid (≤98.0%), diethyl ether (≥99.5%), trimethyl-1-pentene (99.9%),
◦
3
Density at 15 C (kg/m ) Kinematic viscosity at 30 ◦ C (cSt) FFA (wt.%) Fatty acid composition (wt.%) Lauric acid (C12:0) Myristic acid (C14:0) Palmitic acid (C16:0) Palmitoleic acid (C16:1) Margaric acid (C17:0) Stearic acid (C18:0) Oleic acid (C18:1) Linoleic acid (C18:2) Linolenic acid (C18:3) Arachidic acid (C20:0) Gadoleic acid (C20:1) a
917.8 38.3 3.49 0.40 1.26 46.90 0.07 0.10 4.59 36.85 9.09 0.20 0.39 0.15
Average of duplicate results.
acetonitrile (anhydrous, ≥99.8%), Methanol (anhydrous, ≥99.8%), and the reference standards, GC grades (>99.0%), also were obtained from Sigma–Aldrich (Malaysia). KOH (analytical grade) was purchased from Merck (Malaysia). All chemicals were used as received and without drying or any further purification. 2.2. Properties of CPO The physicochemical properties of CPO are shown in Table 1. The CPO density was measured using an Anton Paar DMA5000 instrument (meet ASTM D4052-96 (2002)), kinematic viscosity was determined using an Ubbelohde glass viscometer while the FFA content was determined by means of the acid value using the official method Cd 3d-63 (A.O.C.S., reapproved 1997). Agilent Hewlett-Packard 6890 series gas chromatograph was used, which was equipped with flame ionization detector, SP-2340 capillary column (60 m in length, 25 mm of internal diameter, and 0.2 m film thickness) and a split ratio of 100:1 (A.O.C.S., reapproved 1997). 2.3. Preparation of acidic ionic liquid The ionic liquid Et3 NHSO4 was synthesized by mixing 0.4 mol triethylamine and 10 mL anhydrous acetonitrile. Both were charged into a 250 mL round-bottom flask equipped with a reflux condenser, magnetic stirrer and a N2 gas inlet. While the reaction mixture was kept under cold condition and vigorous stirring, equimolar amount of concentrated sulfuric acid (0.4 mol) was gradually added i.e., in drop wise manner, and the mixture was stirred for 4 h at room temperature. The colorless Brønsted ionic liquid was obtained after repeated washing with diethyl ether and trimethyl1-pentene and was dried under vacuum (760 mmHg and 343.15 K) for 6 h, giving Et3 NHSO4 with purity of 98%. The NMR spectroscopic data recorded for the ILs sample is: 1 H NMR (300 MHz, DMSO): ı = 1.069–1.251 (t, 9H), 2.987–3.161 (m, 6H), 6.322 (s, 1H), 8.981 (s, 1H). 2.4. Two-stage catalytic process 2.4.1. Et3 NHSO4 -catalyzed esterification of CPO The esterification of CPO with methanol was performed under reflux condition. In this work, the basic variables such as reaction temperature, molar ratio of methanol to CPO and concentration of Et3 NHSO4 were investigated for the first-step esterification reaction. The experiments were conducted at seven reaction temperatures (120, 130, 140, 150, 160, 170 and 180 ◦ C) and five different ILs concentrations which are 4.6, 4.8, 5.0, 5.2, 5.4 wt.% based on the weight of CPO. The stirring speed was maintained
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100
100
90
90
80
FFA conversion(%)
FFA conversion(%)
146
70 60 50 40 30
80 70 60 50 40 30 20 10
20 4.6
4.8
5
5.2
5.4
0
5.6
Concentration of Et 3 AmHSO 4 (wt.%)
at 600 rpm in line with the recommendation made by Vicente et al. (1998) for sufficient agitation to overcome the mass transfer limitation during the formation of biodiesel. The desired product of the first step reaction was extracted from reaction mixture at different intervals of time (0.5, 1.0, 1.5 and 5 h) followed by washing with ethyl acetate and the different layers (IL and oil layers) obtained were separated carefully. After separation of IL layer, the oil layer containing unreacted TG was further subjected to the alkaline catalyzed hydrolysis. In order to determine the conversion of FFA during the process, the samplings were done manually and the FFA content was analyzed using AOCS Cd 3d-63 titration method (A.O.C.S., reapproved 1997). The conversion of FFA was calculated from the acidity reduction using the following equation:
AVi − AVt AVi
× 100
3:1
6:1
9:1
12:1
15:1
18:1
Molar ratio of methanol to CPO
Fig. 2. Effect of Et3 NHSO4 concentration on FFA conversion (methanol/CPO molar ratio 15:1, temperature 170 ◦ C, rate of stirring 600 rpm).
Conversion (%) =
0
(1)
where AVi is initial acid value of the mixture and AVt is the acid value at any “t” time. 2.4.2. KOH-catalyzed transesterification of CPO For the next reaction step involving alkaline catalyzed transesterification of the extracted oil samples with lower FFA, KOH was used as suggested by several researchers (Sharma et al., 2008; Tomasevic and Siler-Marinkovic, 2003). The temperature of the transesterification reaction was set at 60 ◦ C as recommended by Encinar et al. (2007) with a 6:1 molar ratio of methanol to TG as recommended by Freedman et al. (1984) and Meher et al. (2006). The reaction time was maintained at 60 min (Ghedge and Raheman, 2006). The preheated methanolic KOH mixture was added to the treated CPO in a 250 mL two necked-flask with constant stirring.
Fig. 4. Effect of methanol to CPO molar ratio on FFA conversion at 170 ◦ C; 5.2 wt.% catalyst concentration, and agitation speed of 600 rpm.
After the reaction products had settled, the upper layer mainly consisting of the palm oil methyl ester (POME) was separated and washed with hot water to remove the impurities such as soap, unreacted methanol and residual KOH. The washed palm oil methyl ester which now forms a crude biodiesel was dried under vacuum at 90 ◦ C for 4 h.
2.5. Analysis of the palm biodiesel A Shimadzu Gas Chromatograph (GC-2010) equipped with AOC20i automatic injection and a flame ionization detector (FID) was used for the analysis of the palm biodiesel. The capillary column used was SUPELCO SGE HT-5 with a length of 10 m and internal diameter of 0.32 mm, coated with a 0.1 m film thickness of 100% dimethylpolysiloxane. After 1 min of stabilization at 50 ◦ C, the GC oven temperature was programmed to initially increase by 15 ◦ C/min up to 180 ◦ C, then at 7 ◦ C/min from 180 to 230 ◦ C before finally at 10 ◦ C/min up to 370 ◦ C where it is then held for 5 min, thus giving a total analytical time of about 30 min to ensure the complete elution of the glycerides. The carrier gas used was highpurity helium (≥99.95 mol.%) with a flow rate of 3 mL/min and split ratio of 100:1. The detector temperature was set at 380 ◦ C. A 1.0 L of each sample was injected after dissolving them in n-heptane (GC grade) with methyl heptadecanoate used as the internal standard. The esters and glycerol contents in the samples are analyzed by GC methods according to ASTM D6751 and EN 14214 (Knothe, 2006).
100
FFA conversion (%)
90 80 70 60 50 40 30 20 10 0
120
130
140
150
160
170
180
Temperature (°C) Fig. 3. Effect of reaction time on FFA conversion at 170 ◦ C; 5.2 wt.% IL concentration, 15:1 molar ratios of methanol to CPO and agitation speed of 600 rpm.
Fig. 5. Effect of reaction temperature on FFA conversion at 4.8 wt.% catalyst concentration, 12:1 molar ratios of methanol to CPO and agitation speed of 600 rpm.
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147
Fig. 6. GC chromatograms of (a) Crude palm oil; (b) Transesterified crude palm oil; (c) FAME of crude palm oil; Peaks 1 – methyl laurate, 2 – methyl myristate, 3 – methyl palmitate, 4 – methyl stearate, 5 – methyl oleate, and 6 – methyl linoleate.
3. Results and discussion 3.1. Et3 NHSO4 -catalyzed esterification reaction of CPO (first step)
maximum conversion after the 170 min of reaction time. Furthermore it has been noticed that the increase in amount of ionic liquid ratio beyond 5.4 wt.%, was not effective in the conversion of FFA.
3.1.1. Effect of Et3 NHSO4 concentration on FFA conversion The amount of Et3 NHSO4 used in the process was varied between 4.6 and 5.4 wt.% (based on the weight of CPO). The reaction experiments were carried out using a methanol to CPO ratio of 12:1 and at temperature of 170 ◦ C. As can be seen in Fig. 2, the FFA conversion increases with the increased in IL concentration. The optimum IL amount is found to be 5.2 wt.% as it gives a
3.1.2. Effect of reaction time Fig. 3 shows the effect of reaction time on FFA conversion with Et3 NHSO4 concentration kept at 5.2 wt.% using 12:1 molar ratio of methanol to CPO at reaction temperature of 170 ◦ C. The full experiment was conducted for 5 h with intermediate extraction of samples from the reaction still at specified intervals in order to analyze the product concentration. During the first 30 min, it was
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Table 2 Fuel parameters of palm oil methyl ester (POME) as compared to ASTM and European Biodiesel Standards specifications. Property
Unit
Test method
POME in this work
ASTM standard
Test method
European standards
Density, 15 ◦ C Kinematic viscosity, 40 ◦ C Flash point Specific gravity, 15 ◦ C Distillation temperature, 95% Water content Acid value Ester content Free glycerol Total glycerol
kg/m3 mm2 /s ◦ C – ◦ C wt.%, mg/kg mg KOH/g wt.% wt.% wt.%
ASTM D4052 ASTM D445 ASTM D93 ASTM D4052 ASTM D86 ASTM D6304 ASTM D664 – ASTM D6584 ASTM D6584
878 3.56 164 0.895 114.5 0.03 0.44 96.9 0.014 0.11
870–900 1.9–6.0 130 min 0.88–0.90 120 max 0.05 max 0.50 max – 0.020 0.240
EN ISO 3675,EN ISO 12185 EN ISO 3104, ISO 3105 EN ISO 3679 – – EN ISO 12937 EN 14104 EN 14103 EN 14105, EN 14106 EN 14105
860–900 3.5–5.0 120 min – – 500 max 0.50 max 96.5 min 0.020 max 0.25 max
found that 10% of the FFA has been converted to FAME. As the reaction time extended further, more conversion of FFA into FAME was observed until a maximum conversion of about 80% was attained after 210 min when the reaction reached steady state.
3.1.3. Effect of the molar ratio of methanol to CPO In order to study the effect of the molar ratio on FFA esterification, the reaction experiments were conducted with various molar ratios of methanol to CPO within the range of 3:1 to 18:1. The result on FFA conversion obtained versus the molar ratio of the methanol to oil is shown in Fig. 4. During the whole reaction, the Et3 NHSO4 concentration was fixed at 5.2 wt.%, the reaction temperature at 170 ◦ C and the agitation speed at 600 rpm. The reaction time was set at 210 min. The result indicates that the conversion from the esterification reaction was lowest at 3:1 molar ratio of methanol to CPO. As the molar ratio of methanol to CPO increases, the FFA conversion also increases. As the molar ratio increases from 3:1 to 12:1, the FFA conversion to FAME changes from 9.4% to 77.6%. Based on the maximum reaction conversion obtained in the previous section, the reaction can be considered as incomplete for a molar ratio of less than 12:1 due to the lower conversion attained within the same time period allowed for the reaction. The maximum conversion was achieved at the molar ratio of 15:1 beyond which further increase in the molar ratio of the two does not result in any additional conversion.
3.3. Characterization of CPO biodiesel 3.3.1. GC analysis Fig. 6(a) presents the GC chromatogram of CPO sample before starting the transesterification reaction. The GC chromatogram (Fig. 6(a)) shows that the CPO contains small traces of glycerine and high amount of TG. Fig. 6(b) presents the analyzed compounds peaks progression of derivatized glycerine, butanetriol standard, methyl esters, derivatized MG, tricaprin second standard, derivatized DG, and TG. Small peaks of minor components, such as sterols, are also detected by the GC. Fig. 6(C) presents the esters content in the CPO biodiesel. The chromatogram for FAME content in CPO biodiesel indicates the presence of methyl laurate, methyl myristate, methyl palmitate, methyl stearate, methyl oleate, and methyl linoleate in the produced CPO biodiesel. 3.4. Properties analysis To ensure the quality of the produced samples, the important properties of palm oil methyl esters were tested and the results are presented in Table 2. The results were found to be comparable to the biodiesel specifications of the American Standards for Testing Material (ASTM D6751-03) and the European Biodiesel Standards (EN 14214).
4. Conclusion 3.1.4. Effect of reaction temperature The effect of temperature on the conversion of FFA was studied for the temperature range of 120 to 180 ◦ C. The esterification reaction was carried out under the same operational parameters as in the previous part i.e., 5.2 wt.% of Et3 NHSO4 concentration, 15:1 methanol/CPO molar ratio, agitation speed of 600 rpm and reaction time of 210 min. Fig. 5 shows the changes in the conversion of FFA with temperature. The FFA conversion increases from 20% to 82.1% as the temperature was increased from 120 to 170 ◦ C. Further increased in the reaction temperature beyond 170 ◦ C did not result in any increase in the FFA conversions. Therefore, it can be concluded that the optimize reaction temperature is 170 ◦ C.
3.2. KOH-catalyzed transesterification (second step) For the second step reaction, the FFA level in the CPO, after the ILcatalyzed esterification reaction, was maintained at about 0.5 wt.%, which is equivalent to 1.02 mg KOH/g oil. The maximum purity of biodiesel produced was 97.3%, with the optimal concentration of KOH used at 1.0 wt.% of treated CPO. For this reaction, the methanol to TG mole ratio used was 6:1 at a reaction temperature of 60 ◦ C for 60 min duration.
The Brønsted ionic liquid triethylammonium hydrogensulfate has shown reasonably good performance in catalyzing the esterification reaction of the FFA in CPO. Compared to the homogeneous acidic catalysts, the ionic liquids in general, have the prospect to reduce the environmental impact usually associated with chemical processes due to their ease of recoverability and reusability, while generating less waste water. The use of ILs could also lead to less corrosion effect thus making it more favorable in the application for continuous process. The best conversion of FFA in the CPO obtained in this work is 82.1% based on the reaction parameters of; (i) reaction time – 210 min, (ii) quantity of catalyst used – 5.2 wt.%, (iii) methanol to CPO ratio – 15:1 and (iv) reaction temperature of 170 ◦ C. The twostage catalyst reaction provides a simple and economic alternative method to produce biodiesel from CPO. In addition, the separation of glycerol and soap, created by KOH, in this process can be easily isolated from the rich ester layer.
Acknowledgement The financial assistance provided by PETRONAS Ionic Liquid Center, Universiti Teknologi PETRONAS (UTP) is gratefully acknowledged.
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