A review on FAME production processes

Fuel 89 (2010) 1–9

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

Review article

A review on FAME production processes Amish P. Vyas *, Jaswant L. Verma, N. Subrahmanyam Chemical Engineering Department, Nirma University, Ahmedabad 382481, India

a r t i c l e

i n f o

Article history: Received 5 June 2009 Received in revised form 6 August 2009 Accepted 6 August 2009 Available online 27 August 2009 Keywords: Bio-diesel Transesterification Ultrasound Microwave Algae

a b s t r a c t Among the options explored for alternative energy sources, bio-diesel is one of the most attractive. This paper discussed about the various production processes, few of which are applied at industrial level also, to produce basically FAME (later can be utilized as bio-diesel after purification) and will be termed as biodiesel in this paper. Transesterification of vegetable oils/fats and extraction from algae are the leading process options for bio-diesel production on large scale. This paper reviews briefly the literature on transesterification reaction using homogeneous, heterogeneous and enzyme catalysts. Employing also ultrasound, microwave and supercritical alcohol techniques and also algae based bio-diesel. Ó 2009 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vegetable oil as diesel fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bio-diesel production via transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Homogeneous alkali (base) catalyzed transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Homogeneous acid-catalyzed transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Heterogeneous acid and base-catalyzed transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Enzymatic transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Supercritical and subcritical alcohol transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Microwave assisted transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. Ultrasound assisted transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bio-diesel from algae oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Renewable energy sources are developed worldwide, owing to high oil prices and to limit greenhouse gas emissions. Petroleum is the largest single source of energy consumed by the world’s population, exceeding coal, natural gas, nuclear, hydro, and renewable. Limited crude petroleum reserves and other sources are on the verge of reaching their peak production. The depletion of known

* Corresponding author. Tel.: +91 2717 241911; fax: +91 2717 241917. E-mail address: [email protected] (A.P. Vyas). 0016-2361/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2009.08.014

1 2 2 2 2 2 3 4 4 6 7 7 8

petroleum reserves will make renewable energy sources more attractive. Bio-diesel is a renewable fuel that is produced mainly from vegetable oils and animal fats. Named by the National Soy Diesel Development Board (presently National Bio-diesel Board) which has pioneered the commercialization of bio-diesel in the US during 1992 [1]. Stringent environmental regulations created huge interest in bio-diesel as an alternative fuel aiming major reduction of vehicular emissions. Bio-diesel is safe, renewable, non-toxic, and biodegradable in water (98% biodegrades in just a few weeks), contains less sulfur compounds and has a high flash point (>130 °C). Table 1 shows the average bio-diesel emissions compared to conventional diesel [2].

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A.P. Vyas et al. / Fuel 89 (2010) 1–9

Table 1 Average bio-diesel emissions compared to conventional diesel [6]. Emission type

B20 (%)

B100 (%)

Total unburned hydrocarbons CO CO2 Particulate matter NOx SOx Polycyclic aromatic hydrocarbons (PAHs) Nitrated PAHs

20 12 16 12 +2 20 13 50

67 48 79 47 +10 100 80 90

2. Vegetable oil as diesel fuel It is generally known that vegetable oils and animal fats were investigated as diesel fuels well before the energy crises of the 1970s and the early 1980s sparked renewed interest in alternative fuels. It is also known that Rudolf Diesel (1858–1913), the inventor of the engine that bears his name, had used peanut oil as fuel in his invention [3]. High fuel viscosity in compression ignition is the major problem associated in use of vegetable oils as a fuel. Viscosities of vegetable oil are ranging 10–20 times greater than diesel fuel [4,5]. Four major techniques (dilution, microemulsion, pyrolysis, and transesterification modification techniques) as well as the direct use of the oil are used for viscosity reduction. Microemulsions with alcohols have been prepared to overcome the problem of high viscosity of vegetable oils. Pyrolysis/cracking, defined as the cleavage to smaller molecules by thermal energy, of vegetable oils over catalysts has been investigated [5,6]. Transesterification process has been widely used to reduce the high viscosity of triglycerides [7]. Esterification is the sub category of transesterification. This requires two reactants, carboxylic acids (fatty acids) and alcohols. Esterification reactions are acid-catalyzed and proceed slowly in the absence of strong acids such as sulfuric acid, phosphoric acid, organic sulfonic acids and hydrochloric acid [8]. Transesterification (also called alcoholysis) is the reaction of a fat or oil with an alcohol (with or without catalyst) to form esters and glycerol. Because the reaction is reversible, excess alcohol is used to shift the equilibrium to the products side [9]. Under Transesterification reaction with alcohol the first step is the conversion of triglycerides to diglycerides, which is followed by the subsequent conversion of higher glycerides to lower glycerides and then to glycerol, yielding one methyl ester molecule from each glyceride at each step [10]. 3. Bio-diesel production via transesterification 3.1. Homogeneous alkali (base) catalyzed transesterification Transesterification reaction can be catalyzed by both homogeneous (alkalies and acids) and heterogeneous catalysts. The most commonly used alkali catalysts are NaOH, CH3ONa, and KOH [11]. The reaction mechanism for alkali-catalyzed transesterification was formulated as three steps. The alkali-catalyzed transesterification of vegetable oils proceeds faster than the acid-catalyzed reaction. The mechanism of the base-catalyzed transesterification of vegetable oils was discussed by Demirbas [12]. In the alkali catalytic methanol transesterification method, the catalyst is dissolved into methanol by vigorous stirring in a small reactor. The oil is transferred into a bio-diesel reactor and then the catalyst/alcohol mixture is pumped into the oil. The final mixture is stirred vigorously for 2 h at 340 K at ambient pressure. A successful transesterification reaction produces two liquid phases: ester and crude glycerol [13].

Gemma et al. [11] selected four different alkaline catalysts i.e. NaOH, KOH, CH3ONa, CH3OK for alkali-catalyzed transesterification of Sunflower oil. The bio-diesel purity was near 100 wt.% for all catalysts. High bio-diesel yields were obtained by using the sodium or potassium methoxide (99.33 wt.% and 98.46 wt.%, respectively), because they only contain the hydroxide group, necessary for saponification, as a low proportion impurity. However, when sodium or potassium hydroxides were utilized as catalysts, biodiesel yields decreased to 86.71 wt.% and 91.67 wt.%, respectively. This is due to the presence of the hydroxide group that originated soaps by triglyceride saponification. Owing to their polarity, the soaps dissolved into the glycerol phase during the separation stage after the reaction. In addition, the dissolved soaps increased the methyl ester solubility in the glycerol, an additional cause of yield loss [11]. Joana et al. reported higher yields (reaching 97%) using virgin oils as compared to waste frying oils (reaching 92%) [14]. The base-catalyzed reaction is reported to be very sensitive to the purity of the reactant. Free fatty acid (FFA) content should not exceed beyond a certain limit. If FFA content in the oil were about 3%. It has been found that the alkaline-catalyzed transesterification process is not suitable to produce esters from unrefined oils [15]. In order to prevent saponification during the reaction, FFA and water content of the feed must be below 0.5 wt.% and 0.05 wt.%, respectively. Because of these limitations, only pure vegetable oil feeds are appropriate for alkali-catalyzed Transesterification without extensive pre-treatment [16]. 3.2. Homogeneous acid-catalyzed transesterification The liquid acid-catalyzed transesterification process is not much popular as the base-catalyzed process. Homogeneous acidcatalyzed reaction is about 4000 times slower than the homogeneous base-catalyzed reaction. However, the performance of the acid catalyst is not strongly affected by the presence of FFAs in the feedstock. In fact, acid catalysts can simultaneously catalyze both esterification and transesterification. Thus, a great advantage with acid catalysts is that they can directly produce bio-diesel from low-cost lipid feedstocks, generally associated with high FFA concentrations (low-cost feedstocks, such as used cooking oil and greases, commonly have FFAs levels of >6%) [17]. The mechanism of the acid-catalyzed transesterification of vegetable oils was discussed in detailed by Ulf et al. [18]. For acid-catalyzed systems, sulfuric acid [19,20,16,21], HCl, BF3, H3PO4, and organic sulfonic acids, have been used by different researchers [17]. Freedman et al. [16] compared the transesterification of soybean oil with methanol, ethanol and butanol using 1% concentrated sulfuric acid based on the weight of oil. In preliminary experiments with 6:1 M and 20:1 M ratios at 3 h and 18 h, respectively, conversions to ester were unsatisfactory. A molar ratio of 30:1, however, resulted in a high conversion to the methyl ester. Each alcoholysis was conducted near the boiling point of the alcohol. The number of hours needed to obtain high conversions to the ester were 3, 22, and 69, respectively, for the butyl, ethyl, and methyl esters [16]. Mohamad et al. reported H2SO4 as superior to HCl commonly used as acid catalyst for transesterification of used vegetable oils [19]. Table 2 shows the work carried out for bio-diesel production from various feedstocks under different conditions using homogenous acid and base catalyst. 3.3. Heterogeneous acid and base-catalyzed transesterification Homogeneous catalysts showed greater performance toward transesterification to obtain bio-diesel. The problems associated with the homogeneous catalysts are the high consumption of energy, form unwanted soap byproduct by reaction of the FFA, expensive separation of the homogeneous catalyst from the reaction

3

A.P. Vyas et al. / Fuel 89 (2010) 1–9 Table 2 Homogeneous acid and base-catalyzed transesterification. Oil

Catalyst

Catalyst amount (%)

Alcohol wt.% oil

Oil to alcohol molar ratio

Reaction conditions

Ester yield (%)

Ester conversion (%)

Ref.

Waste cooking oil (WCO) Karanja oil

H2SO4

4

Methanol

1:20

95 °C, 10 h

–

P90

[21]

KOH

1

Methanol

1:6

98

Pongamia pinnata Rapeseed oil

KOH KOH

1 1

Methanol Methanol

1:10 1:6

Sunflower oil

NaOH

1

Methanol

1:6

Used frying oil (UFO)

1.1 1.5 1.3 3 1

Methanol

1:7.5

Soybean oil Soybean oil

NaOH KOH CH3ONa H2SO4 H2SO4

65 °C, 2 h, 360 rpm 60 °C, 1.5 h 65 °C, 2 h, 600 rpm 60 °C, 2 h, 600 rpm 70 °C, 30 min

1:6 1:30

Soybean oil

H2SO4

0.5

n-Butanol Methanol Ethanol Butanol Methanol

120 °C, 60 min 65 °C, 50 h 78 °C, 18 h 117 °C, 3 h 100 °C, 3.5 bar,8 h

1:9

mixture [30] and generation large amount of wastewater during separation and cleaning of the catalyst and the products [31]. The use of heterogeneous catalysts could be an attractive solution. Heterogeneous catalysts can be separated more easily from reaction products [32]. Undesired saponification reactions can be avoided by using heterogeneous acid catalysts. They enable the transesterification of vegetable oils or animal fats with high contents of FFAs, such as deep-frying oils from restaurants and food processing [33]. Bio-diesel synthesis using solid catalysts could potentially lead to cheaper production costs because of reuse of the catalyst and the possibility for carrying out both transesterification and esterification simultaneously [34]. Satoshi et al. carried out the transesterification of soybean oil with methanol to fatty acid ester over the solid super acid catalysts of WZA (tungstated zirconia–alumina), SZA (sulfated zirconia–alumina), and STO (sulfated tin oxide) at 200 °C–300 °C in fixed bed reactor under atmospheric pressure and reported WZA as a promising solid acid catalyst for the production of bio-diesel from soybean oil (conversion over 90%) [35]. Table 3 shows the work carried out for bio-diesel production from various feedstocks under different conditions using heterogeneous acid and base catalyst.

[22]

– 96

92

[23] [24]

97.1

[25]

85.3 86.0 89.0 – – – – –

[26]

>95 >99 >99 >99 99

[27] [28,16]

[29]

3.4. Enzymatic transesterification Enzymatic transesterification using lipase looks attractive and encouraging for reasons of easy product separation, minimal wastewater treatment needs, easy glycerol recovery and the absence of side reactions [48]. Practical use of lipase in pseudohomogenous reaction systems presents several technical difficulties such as contamination of the product with residual enzymatic activity and economic cost. In order to overcome this problem, the enzyme is usually used in immobilized form so that it can be reused several times to reduce the cost and also to improve the quality of the product [49]. When free enzymes are used in a bio-diesel process, the enzymatic activity can be partially recovered in the glycerol phase. However, the build-up of glycerol limits the possible number of reuses [50]. Several studies of lipase-mediated transesterification for biodiesel production in solvent-free system were proposed [51,52]. In such systems, methanol has poor solubility in oil feedstocks, and too much methanol existing as drops in a system would have some negative effect on lipase activity [51,53]. To overcome this problem Yuji et al. recommended the stepwise addition of metha-

Table 3 Transesterification of vegetable oil by using various heterogeneous catalysts. Catalyst

Catalyst amount wt.% of oil (%)

Oil

Alcohol

Molar ratio

Optimum reaction condition

Ester conversion (%)

Ester yield (%)

Ref.

Mg/La (magnesium–lanthanummixed oxide) Mg/La (magnesium–lanthanummixed oxide) S–ZrO2 sulfated zirconia (Calcium ethoxide) Ca(OCH2CH3)2 (Calcium ethoxide) Ca(OCH2CH3)2 Li/CaO KF/Al2O3 KNO3/Al2O3 KNO3/Al2O3 KF/Eu2O3 Eu2O3/Al2O3 KI/Al2O3

5

Sunflower

Methanol

53:1

65 °C, 30 min

–

100

[30]

5

Sunflower

Methanol

53:1

–

100

[30]

5 3 2 4 6.5 6.0 3 10 2.5 3

Soybean Soybean Soybean Karanja Palm Soybean Jatropha Rapeseed Soybean Soybean Waste oil

Methanol Methanol Ethanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol

20:1 12:1 12:1 12:1 12:1 15:1 12:1 12:1 6:1 15:1 9:1

Room temperature, 2.2 h 120 °C, 1 h 65 °C, 1.5 h 75 °C, 3 h 65 °C, 8 h 65 °C, 3 h 65 °C, 7 h 70 °C, 6 h 65 °C, 1 h 70 °C, 8 h 65 °C, 8 h 200 °C, 5 h

– – – – – 87 84 92.5 63.0 96.0 92.0

98.6 95.0 91.8 94.8 90.0 – – – – – –

[33] [36] [36] [37] [38] [39] [40] [41] [42] [31] [43]

7.5 3

Soybean WCO

Methanol Methanol

15:1 18:1

65 °C, 9 h 200 °C, 10 h

67.0 –

– 98.0

[44] [45]

3 8

Soybean Soybean

Methanol Methanol

10:1 12:1

65 °C, 8 h 65 °C, 3 h

85.6 –

– 95

[46] [47]

SO24 /TiO2–SiO2 Mg–Al hydrotalcites (ZS/Si) zinc stearate immobilized on silica gel KOH/NaX zeolite CaO

4

A.P. Vyas et al. / Fuel 89 (2010) 1–9

nol, since the solubility of methanol in the alkyl esters is greater than in the oil, and consequently limits enzyme deactivation [54]. In addition, the liberated glycerol can also inhibit the reaction by limiting substrate and product diffusion, due to its insolubility in oil or organic solvent [55]. The enzymatic alcoholysis of triglyceride was studied in petroleum ether, hexane and gasoline solutions. However, the solubility of methanol and glycerol in these solvents is low and the above problems probably persisted [56]. To solve this problem, tert-butanol was used as an ideal solvent. With a certain amount of tert-butanol as the reaction medium, both methanol and byproduct glycerol are soluble, so the negative effect caused by methanol and glycerol on lipase catalytic activity could be totally eliminated [56,57]. Different acyl acceptors have been studied for enzymatic catalyzed bio-diesel production. Alcohol has been chosen as the acyl acceptors by the majority of the researchers. Several alcohols like methanol [57–59] ethanol [60–62], 2-propanol [63] and 2-butanol [64] have chosen as acyl acceptors for lipase-catalyzed transesterification. Apart from alcohols methyl acetate [65,66] and ethyl acetate [67] were also used as acyl acceptors. Jech et al. used different types of alcohols to test the deactivation effect on the enzyme. Both linear alcohols such as methanol, ethanol, propanol and butanol and branched alcohols such as isopropanol, 2-butanol and isobutanol. Jech et al. reported that all of the linear alcohols were toxic to the immobilized enzyme. The degree of deactivation was found to be inversely proportional to the number of carbon atoms in the linear lower alcohols. In case of branched alcohols the degree of deactivation was to be lower than that by the linear alcohols [68]. The effect of water content on the production of bio-diesel from soybean oil using lipases from R. oryzae, C. rugosa and P. fluorescens, Novozym 435 and B. cepacia have all shown that enzyme activity was low in the absence of water, which supports the fact that a minimum amount of water is required to activate the enzyme. With increased addition of water there was a considerable increase in ester production, showing the enhancement in the activity of the enzyme [49]. On the other hand Yuji et al. has reported that with the addition of water that ester production decreased [51]. The amount of water to be maintained in bio-diesel production using immobilized lipase depends on the feedstock (the water content of feedstock differs for waste oil to that of refined oil), source of lipase (some commercial lipases are in a powder form which must be dissolved in coupling media before immobilization process), immobilization technique (some immobilization techniques involves the use of water) and the type of acyl acceptor (analytical grade or reagent grade). Thus, it was recommended to optimize the water content depending on the reaction system used [48]. Compared to chemical approach enzymatic approach for biodiesel production offers more advantages but cost of lipase is the major issue for the industrialization of lipase-mediated bio-diesel production. Du et al. reported that there are two ways to reduce the lipase cost. One is to reduce the production cost of the lipase, which can be realized through new lipase development, fermentation optimization, and downstream processing improvement. Another way is to improve/extend the operational life of the lipase, and this can be achieved through enzyme immobilization, alcoholysis reaction optimization, etc. [53]. Table 4 shows the comparison of various lipase-mediated bio-diesel production. 3.5. Supercritical and subcritical alcohol transesterification Transesterification of vegetable oil with non-catalytic supercritical methanol provides a new way of producing bio-diesel. Transesterification reaction in supercritical conditions was completed in minutes, while the conventional catalytic transesterification takes several hours [72]. Transesterification of triglycerides (non-polar

molecules) with an alcohol (polar molecule) is usually a heterogeneous (two liquid phases) reaction at conventional processing temperatures because of the incomplete miscibility of the non-polar and polar components. Under supercritical conditions, however, the mixture becomes a single homogeneous phase, which will accelerate the reaction because there is no interphase mass transfer to limit the reaction rate. Another positive effect of using supercritical conditions is that the alcohol is not only a reactant but also an acid catalyst [73,74]. A reaction mechanism of vegetable oil in supercritical methanol is presented by Kusdiana and Saka [74]. It is assumed that an alcohol molecule directly attacks the carbonyl atom of the triglyceride because of the high pressure. In the supercritical state, depending on pressure and temperature, hydrogen bonding would be significantly decreased, which would allow methanol to be a free monomer. The transesterification is completed via a methoxide transfer, whereby the fatty acid methyl ester and diglycerides are formed. In a similar way, diglyceride is transesterified to form methyl ester and monoglycerides which is converted further to methyl ester and glycerol in the last step. The presence of water in the reaction system does not affect the yield of methyl esters under supercritical alcohol transesterification [74]. Methanol, ethanol, 1-propanol, 1-butanol, or 1-octanol were used to study the transesterification of rapeseed oil at temperatures of 350 °C (rapeseed oil to alcohol molar ratio 42:1) and reported >90% yield of methyl esters was achieved [75,76]. On the other hand, it took 8 min for ethanol, 1-propanol, and 1-butanol to obtain the same yield of the corresponding alkyl esters, and even longer for 1-octanol. In the case of ethanol, 1-propanol, and 1-butanol about 8–14 min of supercritical treatment was necessary to achieve almost complete conversions of triglycerides to fatty acid alkyl esters, while for 1-octanol, 20 min was required to obtain the same yield. Synthesis of bio-diesel by supercritical methanol has a drawback with the high cost of apparatus due to the high temperature and pressure, which are not viable in the large scale practice in industry [77]. So, researches have focused on how to decrease the severity of the reaction conditions. Co-solvents, such as carbon dioxide [75,76,79], hexane [77,78], propane [80] and calcium oxide [81] and subcritical alcohol [79,82] with small amount of catalyst, added into the reaction mixture can decrease the operating temperature, pressure and the amount of alcohol. The supercritical methanol method with co-solvents like hexane and condensed CO2 can improve the product yield [77]. A 98% yield of methyl esters was observed in 20 min at the subcritical condition (160 °C) with 0.1 wt.% potassium hydroxide and methanol to oil ratio of 24 [78]. Table 5 shows the work carried out for bio-diesel production from various feedstocks under different conditions using supercritical alcohol. 3.6. Microwave assisted transesterification The use of microwave heating as a tool for preparative chemistry is continuing to grow. By using microwave irradiation it is often possible to reduce reaction times significantly as well as improve product yields. An alternative energy stimulant, ‘‘microwave irradiation’’ can be used for the production of the alternative energy source, bio-diesel [90]. Microwave irradiation activates the smallest degree of variance of polar molecules and ions such as alcohol with the continuously changing magnetic field. The changing electrical field, which interacts with the molecular dipoles and charged ion, causes these molecules or ions to have a rapid rotation and heat, is generated due to molecular friction [90]. The preparation of bio-diesel using microwave offers a fast, easy route to this valuable biofuel with advantages of a short reaction time, a low oil/ methanol ratio, an ease of operation a drastic reduction in the

5

A.P. Vyas et al. / Fuel 89 (2010) 1–9 Table 4 Comparison of various lipase-mediated bio-diesel production. Carrier used

Source of enzyme

a (%)

Oil

b

Solvent

c

d

Conversion (%)

Yield (%)

Ref.

– –

Candida antarctica –

1.6 4

Methanol Methanol

t-Butanol t-butanol

4:1 4:1

50 °C, 24 h 40 °C, 12 h

95 –

– 88

[56] [57]

Polypropylene ep100 -

10

Methanol

Hexane

4.5:1

40 °C, 48 h

91

–

[59]

15

Soybean

Ethanol

–

7.5:1

96

–

[60]

10

Jatropha

Ethanol

–

4:1

31.5 °C, 7h 40 °C, 10 h

92

–

[61]

20

Palm

Ethanol

–

18:1

–

[62]

10

Propan-2-ol

–

4:1

Acrylic resin

Candida antarctica

30

Jatropha Karanja Sunflower Soybean

58 °C, <24 h 50 °C, 8 h

98

Acrylic resin

Pseudomonas fluorescens Thermomyces lanuginosus Chromobactrium viscosum Pseudomonas fluorescens Candida antarctica

Cotton seed Waste cooking palm oil Sunflower oil

–

12:1

40 °C, 10 h

– 92

[63] [66]

MacroporousAcrylic resin – Toyonite-200 M

Candida antarctica

10

Jatropha

Methyl acetate Ethyl acetate

92.8 91.7 93.4 -

–

11:1

40 °C,12 h

91.3

–

[67]

Candida antarctica Pseudomonas fluorescens Candida antarctica

3 9.4

Rapeseed Sunflower

Methanol 1-Propanol

t-Butanol –

4:1 3:1

35 °C, 12 h 60 °C, 20 h

95 91

–

[69] [70]

2

Soybean

Methanol

[Emim][TfO]

4:1

50 °C,12 h

–

80

[71]

Celite-545 POS-PVA

Acrylic resin

Amount of enzyme used (wt.% of oil): a, acyl acceptors: b, alcohol to oil molar ratio: c, optimum reaction conditions: d.

Table 5 Supercritical alcohol transesterification reaction conditions. Vegetable oil

Molar ratio

Alcohol

Temperature and pressure

Reaction time

Reactor type

Conversion (%)

Yield (%)

Ref.

Sunflower oil Rapeseed oil Hazelnut kernel oil

40:1 42:1 41:1

Methanol Methanol Methanol

350 °C, 200 bar 350 °C, 45 MPa 350 °C, NA

40 min 240 s 300 s

96 95 95

– – –

[6] [83] [84]

Jatropha oil Soyabean oil Coconut oil, palm kernel oil Cottonseed oil

40:1 40:1 42:1

Methanol Methanol Methanol

350 °C, 200 bar 310 °C, 35 MPa 350 °C, 19.0 MPa

40 min 25 min 400 s

8 mL SS reactor 5 ml reaction vessel made of Inconel-625 100 ml cylindrical autoclave made of 316 stainless steel 11 mL reactor of SS 316 75 ml tube reactor Tubular flow reactor

>90 – 95–96

– 96 –

[85] [86] [87]

41:1

Methanol

230 °C, NA

8 min

Autoclave

–

[88]

300 °C, NA

60 min

8.8 ml SS reactor

98 (Methanol) 75 (Ethanol) 60.30

–

[89]

Ethanol Palm oil

45:1

Methanol

NA: not available.

quantity of by-products, and all with reduced energy consumption. Table 6 shows the comparison of energy consumption for the preparation of bio-diesel using conventional and microwave heating. Several examples of microwave irradiated transesterification methods have been reported using homogenous alkali catalyst [92,93,90], acid catalyst [91] and heterogeneous catalyst [94,95]. Nezihe et al. [92] reported 93.7% (for 1.0% (w/w) KOH) and 92.2% (for 1.0% (w/w) NaOH) yield of bio-diesel at 313 K temperature within 1 min under microwave heating. Michael et al. [96] used continuous-flow microwave methodology for the transesterification and reported continuous-flow microwave methodology for the transesterification reaction is more energy-efficient than using a conventional heated apparatus. Microwave assisted transesterification of castor bean oil was carried out in the presence of methanol or ethanol, using a molar ratio alcohol/castor bean oil of 6:1, and 10% w/w basic alumina (in relation to the oil mass) as catalyst. A 95% conversion were obtained under basic conditions (Al2O3/50% KOH) using methanol at conventional (60 °C, stirring, 1 h) or microwave conditions (5 min) [95]. Aside from the great advantages of microwave-assisted reactions, there are also a few drawbacks. Microwave synthesis is not

easily scalable from laboratory small-scale synthesis to industrial multi kilogram production. The most significant limitation of the

Table 6 Comparison of energy consumption for the preparation of bio-diesel using conventional and microwave heating [96].

a

Reaction conditions

Energy consumption (kJ/L)a

Conventional heatingb Microwave, continuous-flow at a 7.2 L/min feedstock flow Microwave, continuous-flow at a 2 L/min feedstock flowc Microwave heating, 4.6 L batch reactione

94.3 26.0 60.3 (92.3)d 90.1

Normalized for energy consumed per liter of bio-diesel prepared. On the basis of values from the joint US Department of Agriculture and US Department of Energy 1998 study into the life cycle inventory of bio-diesel and petroleum diesel for use in an urban bus. c Assuming a power consumption of 1700 W and a microwave input of 1045 W. d Assuming a power consumption of 2600 W and a microwave input of 1600 W. e Assuming a power consumption of 1300 W, a microwave input of 800 W, a time to reach 50 °C of 3.5 min, and a hold time at 50 °C of 1 min. b

6

A.P. Vyas et al. / Fuel 89 (2010) 1–9

Table 7 Microwave assisted transesterification. Oil/triolein

Catalyst

a (%)

Alcohol

b

Microwave conditions

Reaction conditions

c (%)

d (%)

Ref.

Cottonseed oil

KOH

1.5

Methanol

1:6

7 min, 333 K

92.4

–

[90]

Rapeseed oil

KOH

1

Methanol

1:6

Reaction mixture irradiated using 21% of an exit power of 1200 W Reaction mixture irradiated using 67% of an exit power of 1200 W

5 min, 323 K

93.7

–

[92]

1 1 1 5

Methanol Ethanol Methanol Methanol

1:6 1:6 1:6 1:6

40 W 220 W 40 W 25 W

3 min, 313 K 30 min 25 min 05 min 1 min, 323 K

93.7 – – – –

[95]

5

Methanol

1:12

200 W

60 min, 338 K

94

95 95 95 98 98 –

NaOH SiO2/50% H2SO4 SiO2/30% H2SO4 Al2O3/50% KOH KOH NaOH H2SO4/C

Castor oil

Triolein Castor oil

[97] [98]

Catalyst amount (wt.% of oil): a, oil to alcohol molar ratio: b, ester yield: c, ester conversion: d.

scale up of this technology is the penetration depth of microwave radiation into the absorbing materials, which is only a few centimeters, depending on their dielectric properties. The safety aspect is another reason for rejecting microwave reactors in industry [91]. Table 7 shows the work carried out for bio-diesel production from various feedstocks under different conditions using microwave irradiation.

3.7. Ultrasound assisted transesterification Ultrasound has proven to be a very useful tool in enhancing the reaction rates in a variety of reacting systems. It has successfully increased the conversion, improved the yield, changed the reaction pathway, and/or initiated the reaction in biological, chemical, and electrochemical systems [99]. Ultrasound is defined as sound of a frequency beyond that to which the human ear can respond. The normal range of hearing is between 16 Hz and about 18 kHz and ultrasound is generally considered to lie between 20 kHz to beyond 100 MHz [100] like any sound wave, ultrasound alternately compresses and stretches the molecular spacing of the medium through which it passes, causing a series of compression and rarefaction cycles. If a large negative pressure gradient is applied to the liquid so that the distance between the molecules exceeds the critical molecular distance necessary to hold the liquid intact, the liquid will break down and voids (cavities) will be created, i.e., cavitation bubbles will form. At high ultrasonic intensities, a small cavity may grow rapidly through inertial effects. As a result, some bubbles undergo sudden expansion to an unstable size and collapse violently, generating energy for chemical and mechanical effects [101]. The collapse of the cavitation bubbles disrupts the phase boundary and causes emulsification, by ultrasonic jets that impinge one liquid to another [102].

A low frequency ultrasonic irradiation could be useful for transesterification of triglyceride with alcohol. Ultrasonication provides the mechanical energy for mixing and the required activation energy for initiating the transesterification reaction [103]. Ultrasonication increases the chemical reaction speed and yield of the transesterification of vegetable oils and animal fats into bio-diesel [104]. Ultrasonic assisted transesterification method presents advantages such as shorter reaction time and less energy consumption than the conventional mechanical stirring method [105], efficient molar ratio of methanol to TG, and simplicity [106]. For the transesterification of 1 kg soybean oil conventional mechanical stirring method and ultrasonic cavitation method consume 500 and 250 W/kg of energy, respectively [105]. Carmen et al. [102,107] reported that conversion of vegetable oil (no further information on the nature of the oil was provided) to methyl esters was the highest for a 1.0% (w/w) NaOH concentration (i.e., 95% after 10 min at room temperature using Ultrasonication (28 kHz). Hoang et al. [108] studied effects of molar ratio, catalyst concentration and temperature on transesterification of triolein with ethanol under ultrasonic irradiation and reported optimum conditions for the formation of ethyl ester under ultrasonic irradiation at 25 °C were E/T (ethanol to triolein) molar ratio of 6:1, base catalyst (NaOH or KOH) concentration of 1 wt.%, and reaction time of less than 20 min. Lifka and Ondruschka [109] studied the effect of ultrasonication versus mechanical stirring on the alkaline transesterification of rapeseed oil using NaOH at a concentration of 0.5% w/w at 45 °C. A conversion of 80–85% was obtained for both ultrasonicated and mechanically stirred reactions after 30 min. Carmen et al. (2007) [110] used ultrasonically driven continuous process for palm oil transesterification and reported >90% conversion at 20 min residence time in reactor with 6:1 methanol to oil

Table 8 Ultrasound assisted transesterification. Oil/ triolein

Catalyst

Catalyst wt.% of oil

Alcohol

Oil to alcohol molar ratio

Ultrasonic frequency (kHz)

Source of ultrasound

Reaction conditions

Ester yield (%)

Ester conversion (%)

Ref.

Na

NaoH

0.5

n-Propanol

1:6

28

Ultrasonic cleaner (1200 W)

25 °C, 20 min

92

–

[102]

Triolein

NaOH

1

Ethanol

1:6

40 40

25 °C, 20 min 25 °C,<20 min

88 –

98

[108]

Triolein

KOH

1

Methanol

1:6

40

25 °C, 10 min

–

>90

[111]

Soybean

NaOH

1.5

Methanol

NA

24

60 °C, 20 min

97

–

[112]

Frying oil Fish oil

C2H5ONa

0.8

Ethanol

1:6

20

60 °C, 60 min

98.2

–

[113]

NA: not available.

Ultrasonic cleaner (1200 W) Ultrasonic cleaner (1200 W) Ultrasonicator (200 W) Ultrasonic probe

7

A.P. Vyas et al. / Fuel 89 (2010) 1–9

molar ratio. Table 8 shows the work carried out for bio-diesel production from various feedstocks under different conditions using ultrasound irradiation.

4. Bio-diesel from algae oil Algae are another source of triglycerides. Microalgae are photosynthetic microorganisms that convert sunlight, water and carbon dioxide to algal biomass. Algae are more productive than corn or soybeans, as every cell are a factory. Unlike corn, algae need not be grown on arable soil and also it can be grows in water bodies like ponds, lakes and even seas and oceans. Therefore, there are no food related issues with algae. Algae not only reduce a plant’s global warming gases, but also consume other pollutants [114]. Microalgae are producing 15–300 times more oil for bio-diesel production than traditional crops on an area basis. Furthermore compared with conventional crop plants which are usually harvested once or twice a year, microalgae have a very short harvesting cycle (1–10 days depending on the process), allowing multiple or con-

Table 9 Comparison of some sources of bio-diesel and oil content of some microalgae [116].

a b

Crop

Oil yield (L/ha)

Corn Soybean Canola Jatropha Coconut Oil palm Microalgaea Microalgaeb -Botryococcus braunil -Chlorella sp. -Crypthecodinium cohnii -Cylindrotheca sp. -Dunaliella primolecta -Isochrysis sp. -Monallanthus salina -Nannochloris sp. -Nannochloropsis sp. -Neochloris oleoabundans -Nitzschia sp. -Phaeodactylum tricornutum -Schizochytrium sp. -Tetraselmis sueica

172 446 1190 1892 2689 5950 136,900 58,700

tinuous harvests with significantly increased yields (Table 9). Greater light capture and conversion efficiencies ultimately lead to reduced fertilizer and nutrient inputs and so resulted in less waste and pollution [115]. Advantages, other than mentioned above, of deriving bio-diesel from algae include biofuel contains no sulfur, is non-toxic, and is highly biodegradable. Depending on species, microalgae produce many different kinds of lipids, hydrocarbons and other complex oils. Not all algal oils are satisfactory for making bio-diesel, but suitable oils occur commonly [116]. Table 9 shows the oil content of some microalgae. Harvesting algae and extracting oil present technical and cost hurdles. The dominant algal species found in a pond could range from small unicellular to large colonial or filamentous species. Harvesting of the algae for biomass conversion would require a universally applicable harvesting technology, such as centrifugation or chemical flocculation, to enable the recovery of any algal type. However, these processes are very expensive. Both methods are expensive when applied in large scale commercial production. Researchers are seeking the best of a variety of approaches. Table 10 shows that some companies have just finished R&D on expansion of production areas through location of new farming sites [117].

Oil content (dry wt.%)

5. Conclusions

25–75 28–32 20 16–37 23 25–33 >20 20–35 31–68 35–54 45–47 20–30 50–77 15–23

70% oil (by wt.) in biomass. 30% oil (by wt.) in biomass.

Due to the concern on the availability of recoverable fossil fuel reserves and the environmental problems caused by the use those fossil fuels, considerable attention has been given to bio-diesel production as an alternative to petro diesel. Bio-diesel fuel comes from renewable sources as it is plant- not petroleum-derived and as such it is biodegradable and less toxic. In addition, relative to conventional diesel, its combustion products have reduced levels of particulates, carbon oxides, sulfur oxides and, under some conditions, nitrogen oxides. Bio-diesel is made by transesterification reaction, transesterification is a chemical reaction between triglyceride and alcohol in the presence of catalyst or without catalyst which gives bio-diesel and by product glycerol. Transesterification can be carried by using homogeneous (sulfuric acid, sodium hydroxide, and potassium hydroxide) catalyst, heterogeneous catalyst (sulfated zirconia, MgO, CaO, etc.), enzymatic (lipase) catalyst and non-catalytic supercritical alcohol. Apart from conventional techniques for bio-diesel production, application of microwave and ultrasound are explored for better results compared to conventional methods.

Table 10 Leading the algae-based fuel industry, laboratories, and universities in the US [117]. Company

Research affiliation

Status

Funding

Bioreactor

Ohio State University Pilot Project

Bioreactor

Massachusetts Institute of Technology

Finished R&D, finalizing specs

PetroAlgae (www.petroalgae.com) Solix Biofuels (www.solixbiofuels.com) LiveFuel (www.livefuels.com)

Bioreactor Bioreactor

Arizona State University University of Colorado

Pond

Infinifuel (www.infinifuel.com)

Pond

Algae Biofuels (www.petrosuninc.com) Energy Farms (www.tgoiltech.com)

Pond

NREL, Sandia National Laboratory University of Nevada at Reno and the Desert Research Institute Undisclosed

R&D R&D, looking into Fischer–Tropes processing R&D

Subsidiary of Green Shift Corp. and Veridium Corp., Public company $22 million in venture capital from Draper Fisher Jurvetson, Polaris, Axis Partner Subsidiary of XL Tech Group Inc. $500,000 University of Colorado

Pond

Nanoforce Technologies

GS Clean Tech (www.gscleantech.com) GreenFuel Technologies (www.greenfuelonline.com)

Bioreactor or pond?

R&D; final construction phase of geothermal bio-diesel processing plant Final stage of field testing R&D

Private funded by Morgenthaler family Private investors

Subsidiary of PeteroSun Drilling Inc., Public Company Subsidiary of TransGlobal Oil Corp., Public Company

8

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