Sustain. Environ. Res., 20(6), 417-422 (2010) (Formerly, J. Environ. Eng. Manage.)

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COMPARISON OF BIODIESELS PRODUCED FROM WASTE AND VIRGIN VEGETABLE OILS Yi-Shun Hung,1 Yi-Hung Chen,1,* Neng-Chou Shang,2 Cheng-Hsin Chang,3 Tsung-Lung Lu,4 Ching-Yuan Chang2 and Je-Lueng Shie5 1

Department of Chemical Engineering and Biotechnology National Taipei University of Technology Taipei 106, Taiwan 2 Graduate Institute of Environmental Engineering National Taiwan University Taipei 106, Taiwan 3 Department of Civil Engineering Tamkang University Taipei 251, Taiwan 4 Retech Environmental Solutions Co., Ltd. Hsinchu 302, Taiwan 5 Department of Environmental Engineering National I-Lan University I-Lan 260, Taiwan.

Key Words: Biodiesel, waste vegetable oil, esterification, transesterification, fuel properties ABSTRACT In this study, a two-step process (acid catalyzed esterification followed by alkali catalyzed transesterification) was employed to convert waste vegetable oil (WVO), which showed the free fatty acids of 1.33 wt%, acid value of 2.66 mg KOH g-1, and the kinematic viscosity at 40 °C of 39.5 mm2 s-1, into the biodiesel. The WVO biodiesel was synthesized under the following conditions: the methanol to oil molar ratio of 6, the H2SO4 catalyst dosage of 0.5 wt% based on the oil weight, and the reaction temperature of 45 °C for the acid catalyzed esterification; the methanol to oil molar ratio of 9, the KOH catalyst dosage of 2 wt% based on the oil weight, and the reaction temperature of 60 °C for the alkali catalyzed transesterification. The acid values of the esterified WVO and the WVO biodiesel were determined as 1.42 and 0.18 mg KOH g-1, respectively. In addition, a single-step process (alkali catalyzed transesterification) was used to convert virgin vegetable oil (VVO) to biodiesel for comparison. These two oils were successfully converted to biodiesel while the ester contents of the WVO and VVO biodiesels were 98.8% and 99.3%, respectively. As a result, the WVO biodiesel had less polyunsaturated fatty acid methyl esters, higher kinematic viscosity, and lower oxidation stability compared to those of the VVO biodiesel. Furthermore, the fuel properties of two biodiesels can satisfy most biodiesel standards except the cold filter plug point and oxidation stability. INTRODUCTION In recent years, the need for energy resources has increased with the increase of human population. Due to the depletion of world petroleum reserves and increasing environmental concerns, the interest in using renewable energy, hydroelectricity, or nuclear energy as alternative sources for petroleum-based fuels has remarkably risen [1,2]. Furthermore, biodiesel is con*Corresponding author Email: [email protected]

sidered as one important renewable fuel [3,4]. Biodiesel is a mixture of methyl esters with long chain fatty acids and made from vegetable oil, animal fats, or even waste vegetable oil (WVO). Biodiesel as fatty acid methyl esters (FAMEs) has many advantages such as biodegradability and non-toxicity [5]. Biodiesel also has a favorable combustion-emission profile, producing much less carbon monoxide, sulfur oxides, nitrogen hydride, particulate matter, and un-

Sustain. Environ. Res., 20(6), 417-422 (2010)

418

burned hydrocarbons compared to the petroleum-base diesel [6]. Therefore, it is beneficial to reduce air pollution and minimize the emission of greenhouse gas by using biodiesel. These properties make biodiesel a good alternative fuel to substitute the petroleum-based diesel [7]. It is believed that large-scale production of biodiesel from edible oil may bring about global imbalance in the food supply, thus increasing the prices of edible oils. Since more than 95% of the current biodiesel is made from edible oils, many problems have been arisen accordingly [8]. Hence, the accessible utilization of WVO may be a feasible way to decrease the use of edible oils [9]. WVO is relatively cheap and considered as a potential feedstock for biodiesel production. In addition, the improper disposal of WVO frequently causes the environmental problems. However, the use of the WVO probably leads to more complex procedure and worse fuel properties in comparison with the biodiesel produced from virgin vegetable oil (VVO) because its high content of free fatty acids (FFAs). For example, alkali catalysts are generally preferred in the conventional biodiesel industry because of their high transesterification efficiency. However, the alkali catalyst would be consumed by the FFAs to form the emulsions that make the subsequent separation process more difficult. Thus the significant saponification phenomenon results in the low biodiesel yield when the feedstock contains high FFA amount. For feedstocks with acid value higher than 2 mg KOH g-1, a pre-treatment step is recommended to esterify the FFAs to generate esters [10]. Therefore, WVO are usually converted into biodiesel via an esterification reaction as Eq. 1 and followed by a transesterification reaction as Eq. 2 to benefit the biodiesel yield. O

O R

C

OH

+

CH3OH

Fatty acid

(acid catalyst)

R

esterification

C

Methanol

O

CH3

+

H2O

Water

Biodiesel (FAMEs)

O

(1)

MATERIALS AND METHODS

O

R1

C O

O

CH2

R2

C O

O

CH

R3

C

O

+

3 CH3OH

CH2

Triglyceride (vegetable oil)

Methanol

(alkali catalyst) transesterification

WVO biodiesel, the nature of the WVO would remarkably depend on the feedstock source. Therefore, the detailed production procedure and fuel properties using the WVO as the biodiesel feedstock in Taiwan still need to be investigated. This study presents a process to convert the WVO, which was the waste collected from a restaurant in Taipei, into biodiesel via a two-step process. Moreover, the fuel properties of the WVO and VVO biodiesels including acid value, cold filter plugging point (CFPP), density, ester content, iodine value (IV), kinematic viscosity (KV), and oxidation stability were determined and compared with the biodiesel specifications of CNS 15072 in Taiwan. Fuels with low CFPP values exhibit beneficial low-temperature flow properties for vehicle engines in cold-weather climates [15,16]. A high IV value has been linked to poor oxidation stability, resulting in the formation of various degradation products, which can negatively affect engine operability by forming deposits on engine nozzles, piston rings, and piston-ring grooves [17]. Note that the lower KV is the primary reason why biodiesel should be used as an alternative fuel instead of neat vegetable oils or animal fats. A fuel with a high KV can lead to undesired consequences such as engine deposits [18]. The KV of biodiesel is approximately one order of magnitude lower than that of the parent oil or fat, leading to better atomization of the fuel in the combustion chamber of the engine. The storage of biodiesel over extended periods may lead to the oxidative degradation of the FAMEs that can compromise fuel quality [19]. The oxidation stability of biodiesel depends on the FAME composition and the quantity of natural and synthesized antioxidants in the finished fuel. Oxidation can lead to the formation of corrosive acids and deposits that may cause the increasing wear in engine fuel pumps. The oxidative instability is a major barrier to increase the acceptance of biodiesel by engine and fuel injection equipment manufacturers [20].

R1

C O

O

CH3

R2

C O

O

CH3

R3

C

O

CH3

Biodiesel (FAMEs)

+

HO

CH2

HO

CH

HO

CH2

Glycerol

1. Materials

(2)

Table 1 summarizes the literatures about the experimental conditions for converting WVO to produce biodiesel. The experimental conditions in the two-step procedure (esterification and transesterification reactions) includes the molar ratio of methanol to WVO (nM/nO), the reaction time (min), the stirred speed (ω, rpm), the reaction temperature (T, °C), and the catalyst dosage based on the oil weight (Wcat, wt%). Although some studies have been conducted on synthesizing the

The WVO free of meat was collected from one restaurant in Taipei, Taiwan. The WVO was used in frying foods for about several days before being discarded. The VVO as the source of the WVO, originally a mixture of soybean oil and palm oil, was purchased from Ttet Union Co. (Tainan, Taiwan). The ACS-certified methanol was obtained from Mallinckrodt Chemicals (Phillipsburg, NJ, USA). The KOH pellets (85%) were purchased from Riedel-deHaën (St. Gallen, Switzerland). The sulfuric acid (95-97%) was purchased from Showa (Tokyo, Japan). Anhydrous magnesium sulfate was obtained from Showa (Tokyo,

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Table 1. Conversion of WVO to biodiesel in the literatures Reference Zheng et al. [6] Phan and Phan [11] Komintarachat and Chuepeng [12]

Wang et al. [13,14]

Present work

Methods Acid-catalyzed transesterification Alkali-catalyzed transesterification Solid acid catalyzed transesterification

Acid-catalyzed esterification Alkali-catalyzed transesterification Acid-catalyzed esterification Alkali-catalyzed transesterification

Initial acid value (mg KOH g-1) 12

ω

T

Wcat

(-) 74-250

Reaction time (min) 240

(rpm) 100-600

(°C) 70-80

H2SO4

0.67-3.64

7-8

20-120

NA

30-50

30

1.5-3.5a

30-120

100

90-120

75.92 ± 0.04

3-10

15-240

NA

95

2.10 ± 0.04

18

60

NA

65

2.66

6

60

300

45

1.42

9

120

600

60

nM/nO

0.75 wt% KOH 0.25-1.25 wt% Synthesized solid acid catalysts 0-4 wt% Fe2(SO)4 1 wt% KOH 0.5 wt% H2SO4 2 wt% KOH

Yield or conversion (%) 99 ± 1 88-90 83-97.5

91.6-97.2 97.0 46.6 98.8

NA: not available

Japan). 2. Biodiesel Production from WVO and VVO

One may notice that the acid and alkali catalysts are commonly applied in the esterification and transesterification reactions, respectively. In this study, an acid-catalyzed esterification followed by an alkalicatalyzed transesterification process was conducted to convert WVO into biodiesel. On the other hand, the VVO was directly converted into biodiesel via an alkali-catalyzed transesterification process. The appearance of the WVO was clear, transparent, and in dark yellow in which some degradation products and water may be present. Before the acid-catalyzed esterification, the WVO was heated to about 120 °C for 60 min to remove the water and other volatile impurities as the pretreatment step. In the acid-catalyzed esterification, the methanol and H2SO4 were added into the WVO in a flask to carry out the esterification reaction for another 60 min in the conditions of the nM/nO ratio of 6 and Wcat of 0.5 wt% while the ω and T were maintained at 300 rpm and 45 °C, respectively. The treated WVO was separated from the resulting mixture in a separation funnel and then washed two times with saturated salt solution (three times the volume of the treated WVO) to remove excess methanol, sulfuric acid, and impurities. The treated WVO retrieved from the acid esterification step was added with KOH-containing methanol while the nM/nO ratio and the Wcat were 9 and 2 wt%, respectively. The ω and T were controlled at 600 rpm and 60 °C, respectively, for the reaction time of 120 min. After the alkali-catalyzed transesterification

reaction, the reaction solution was left overnight to settle, a process that resulted in phase separation. The ester layer was washed two times with saturated salt solution (three times the volume of the ester phase) to remove any residual methanol, KOH, or glycerol that may be present. Finally, trace amounts of water were removed by adding anhydrous magnesium sulfate, followed by filtration [15]. Notice that the experimental conditions of the transesterification reaction of VVO were the same as those of the treated WVO. 3. Fuel Property Analyses

An attempt has been made to compare the fuel properties of the WVO and VVO biodiesels. The acid value, CFPP, density, IV, and KV of the samples were determined according to EN 14104, EN 116, EN ISO 3675, EN 14111, and EN ISO 3104, respectively. Moreover, the ester content and FAME compositions of the WVO and VVO biodiesels were analyzed according to EN 14103, using an Agilent 6890N gas chromatograph (Santa Clara, CA) with a flame ionization detector (GC-FID). The capillary column was a VB-WAX column (Western Analytical Products, Murrieta, CA) with a length of 30 m, a film thickness of 0.25 μm, and an internal diameter of 0.32 mm. Helium was used as carrier gas and also employed as an auxiliary gas for the FID. A standard method of determination of oxidation stability of FAMEs using accelerated oxidation test has been established with a Rancimat apparatus based on European standard EN 14112 [21,22]. Rancimat test, applied to FAMEs, determines the induction period of a sample aged at 110 °C under a stream of air by monitoring the conductiv-

Sustain. Environ. Res., 20(6), 417-422 (2010)

ity of deionised water where the outlet vapours are collected. Rapid production of volatile acids such as formic acid at the end of the induction period induces an increase of the water conductivity. The end point, expressed in hours, is determined by a graphic method in which tangents are drawn [23]. RESULTS AND DISCUSSION 1. Properties of WVO and VVO

The properties of the feedstocks about the WVO and VVO are shown in Table 2. The acid value of the WVO of 2.66 mg KOH g-1, which corresponds to the FFA content of about 1.33 wt%, was significantly higher than that of the VVO of 0.25 mg KOH g-1. It was caused by the frying and cooking operations at high temperature in which some triglycerides were oxidized to generate FFAs, resulting in an increase of acid value. The KV of the WVO of 39.5 was higher than that of the VVO of 35.6 mm2 s-1. The increase in KV was reported due to possible polymerization, which led to in the formation of higher molecular weight compounds [24]. On the contrary, the IV of the WVO of 89 was smaller than that of the VVO of 93 g I2 100 g-1. The IV is related to the number of double bonds of fatty acids [25]. One may infer that the FFAs mainly came from the oxidation of unsaturated fatty acids due to their poor oxidation stability, thus accounting for the decrease of IV. 2. Acid Value Variations during Biodiesel Production and FAME Compositions

Figure 1 depicts the variations of acid value in different reaction stages of biodiesel production from the WVO and VVO. The acid value of WVO was significantly reduced to 1.42 mg KOH g-1 after the acidcatalyzed esterification reaction, indicating that about 47% FFAs in the WVO has been converted. Compared with the results in the literatures (Table 1), the conversion of the FFAs in the esterification reaction of the present work was lower because of the lower nM/nO and T. Nevertheless, it already satisfied the common criterion for the practicability of the alkalicatalyzed transesterification (acid value ≤ 2 mg KOH g-1) [10]. Therefore, an alkali-catalyzed transesterification process is applied to convert the esterified WVO with methanol into FAMEs and glycerol. As a result, the acid value of the WVO biodiesel was only Table 2. Properties of WVO and VVO Property Acid value Density at 15 °C IV KV at 40 °C

Unit mg KOH g-1 kg m-3 g I2 100 g-1 mm2 s-1

WVO 2.66 921.2 89 39.5

VVO 0.25 919.0 93 35.6

3 original

2.66

after esterification

2.5

Acid value (mg KOH g-1)

420

after transesterification 2

1.5

1.42

1

0.5 0.18 0

WVO

0.25 0.10

VVO

Fig. 1. Histogram of acid value in different reaction stages using WVO and VVO for biodiesel production.

0.18 mg KOH g-1 which was even lower than that of the original VVO (0.25 mg KOH g-1, Table 2). Nevertheless, the VVO biodiesel had the lowest acid value of 0.10 mg KOH g-1. The FAME compositions of the WVO and VVO biodiesels are shown in Table 3. The primary FAMEs in the biodiesels were methyl palmitate, methyl oleate, and methyl linoleate. For instance, the WVO biodiesel had about the methyl palmitate of 27 wt%, methyl oleate of 37 wt%, and methyl linoleate of 27 wt% while the saturated and unsaturated FAMEs were about 33 and 67 wt%, respectively. By comparison, the VVO biodiesel consisted of higher content of methyl linoleate and methyl linolenate which contain polyunsaturated carbon-carbon double bonds. It is because that the fatty acids with polyunsaturated carboncarbon double bonds have poor oxidation stability and are relatively easy to be decomposed in use [8]. 3. Fuel Property of WVO and VVO Biodiesels

The fuel properties of the WVO and VVO bioTable 3. FAME compositions of WVO and VVO biodiesels FAME composition (wt%) WVO biodiesel VVO biodiesel Methyl myristate (C14:0) 0.6 0.6 Methyl palmitate (C16:0) 27.3 26.2 Methyl stearate (C18:0) 4.4 4.2 Methyl oleate (C18:1) 37.1 35.9 Methyl linoleate (C18:2) 27.0 28.9 Methyl linolenate (C18:3) 2.9 3.6 Methyl arachidate (C20:0) 0.4 0.4 Methyl eicosenoate (C20:1) 0.2 0.2 Saturated FAMEs 32.7 31.4 Unsaturated FAMEs 67.2 68.6

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Table 4. Fuel properties of WCO and VVO biodiesels and compared with ASTM D6751, CNS 15072, and EN 14214 specifications Property Unit WVO biodiesel Acid value mg KOH g-1 0.18 CFPP °C 8 Density at 15 °C kg m-3 881.1 Ester content wt% 98.8 IV g I2 100 g-1 90 KV at 40 °C mm2 s-1 4.4 Oxidation stability, 110 °C h 0.2 a Not specified; depends upon the location and time of year.

diesels are given in Table 4 in comparison with biodiesel standards. Because the fuel properties of the biodiesel are mainly determined by their major components, the particular alkyl esters, biodiesel would have distinct fuel properties due to dissimilar FAME compositions. The CFPP value of both the WVO and VVO biodiesels was 8 °C which was in excess of the requirement of CNS 15072 due to the high content of the saturated FAMEs. Thus the addition of antifreeze or the blending with other biodiesels of low CFPP values was necessary to improve the low temperature properties of the WVO and VVO biodiesels. In addition, the WVO and VVO biodiesels have high ester content, moderate density, IV and KV, which satisfied biodiesel standards. Moreover, the WVO and VVO biodiesels revealed the similar IVs compared to those of their parent oils (Table 2). It indicated that the acid-catalyzed esterification and the alkali-catalyzed transesterification reactions slightly change the IV. The WVO biodiesel had lower IV of 90 than that of the VVO biodiesel (93 g I2 100 g-1) because of its lower polyunsaturated FAME content. The KV of the WVO and VVO biodiesels were 4.4 and 4.3 mm2 s-1, respectively, which were one-ninth and one-eighth those of their parent oils (Table 2). Higher KV of the WVO biodiesel was due to its higher saturated FAME content. Consequently, the FFAs and triglycerides in the WVO and VVO can be efficiently converted into FAMEs based on the present experimental processes, resulting in the remarkable decrease of the KV. One should note that the oxidation stability of the VVO biodiesel was 2.6 h which can not satisfy the requirement of the biodiesel standards because of the abundant amount of polyunsaturated carbon-carbon double bonds. The FAMEs containing the polyunsaturated carbon-carbon double bonds show higher oxidative reaction rates compared to the saturated or monounsaturated one [8,24]. Furthermore, the oxidation stability of the WVO biodiesel was only 0.2 h. It was because that the natural or added antioxidants in the VVO have been consumed during the frying and cooking operations at the high temperature. One may note that the VVO would contain natural antioxidants such as α-tocopherol in the concentration range of

VVO biodiesel 0.10 8 879.4 99.3 93 4.3 2.6

ASTM D6751 0.5 max a – – – 1.9-6.0 3.0 min

CNS 15072 0.5 max 0 max 860-900 96.5 min 120 max 3.5-5.0 6.0 min

EN 14214 0.5 max a 860-900 96.5 min 120 max 3.5-5.0 6.0 min

several hundred ppm and present a significant lose of α-tocopherol of about 36-63% after the frying process [26]. Therefore, the addition of antioxidants, which is a common procedure to provide acceptable oxidation stability, is necessary for the WVO and VVO biodiesels. CONCLUSIONS The acid value of WVO collected from the restaurant in Taipei was 2.66 KOH mg g-1 which required a two-step process (esterification reaction followed by transesterification reaction) for biodiesel production. The acid value of the WVO would decrease to 1.42 KOH mg g-1 after the esterification reaction where the methanol to oil molar ratio (nM/nO), reaction temperature, H2SO4 catalyst dosage were 6, 45 °C, and 0.5 wt% based on the WVO weight, respectively. Then the acid value remarkably decreased to 0.18 KOH mg g-1 after the transesterification reaction where the nM/nO ratio, reaction temperature, KOH catalyst dosage were 9, 60 °C and 2 wt% based on the WVO weight, respectively. As a result, the acid value, density, ester content, IV, and KV of the obtained WVO and VVO biodiesels satisfied the biodiesel standards. On the other hand, the WVO biodiesel contained lower polyunsaturated FAME content compared to the VVO biodiesel, resulting in lower IV and higher KV. Due to the abundant saturated FAMEs of about 32 wt%, the cold filter plug point of the WVO and VVO biodiesels was 8 °C and required to be decreased by adding antifreeze or blending with other biodiesels of low CFPP values to satisfy the biodiesel standard. In addition, the distinct oxidation stability of the WVO and VVO biodiesels were 0.18 and 2.6 h, respectively, which indicated the requirement of the addition of antioxidants to improve the oxidation stability. ACKNOWLEDGEMENTS This study was supported by the instant technical assistance program of small and medium-sized enterprises (SME), Industrial Development Bureau, Ministry of Economic Affairs.

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Discussions of this paper may appear in the discussion section of a future issue. All discussions should be submitted to the Editor-in-Chief within six months of publication. Manuscript Received: August 16, 2010 Revision Received: October 27, 2010 and Accepted: October 28, 2010