Significant parameters and technological advancements in biodiesel production systems

Fuel 250 (2019) 27–41

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Review article

Significant parameters and technological advancements in biodiesel production systems M. Erdem Günaya, Lemi Türkerb, N. Alper Tapanc,

T

⁎

a

Department of Energy Systems Engineering, Istanbul Bilgi University, 34060 Eyup, Istanbul, Turkey Department of Chemistry, Middle East Technical University, 06800 Çankaya, Ankara, Turkey c Department of Chemical Engineering, Gazi University, 06570 Maltepe, Ankara, Turkey b

A R T I C LE I N FO

A B S T R A C T

Keywords: Catalyst type Reaction temperature Calcination temperature Mechanical stirring Supercritical transesterification Microwave transesterification Hydrodynamic cavitation Ultrasound technology Future directions

Biodiesel is a mixture of fatty acid esters formed by transesterification of vegetable oil, animal fat, algae oil or waste oil with an alcohol like methanol (CH3OH), ethanol (C2H5OH) or higher alcohols. There are many important catalytic variables like catalyst type and composition, support type and pretreatment conditions (i.e. calcination temperature and time) which are utilized to achieve high yields for the transesterification reaction. In addition, operational conditions such as reaction temperature, alcohol type, alcohol to oil molar ratio and stirring speed have also quite high significance. Moreover, all these variables can be optimized under supercritical conditions by novel techniques like ultrasonic and microwave irradiation or hydrodynamic cavitation. In this work, significant catalytic and operational variables for biodiesel production are reviewed. In addition, dominant parameters together with their limitations during the application of advanced technologies are investigated in detail. Then, it has been concluded that, for better control and higher yields of biodiesel production, future research works should focus on addition of co-solvents, use of longer chain alcohols, bulky structures or ionic liquids, adjustment of mode of irradiation and modification of the instrumentation or the equipment.

1. Introduction The world fuel consumption has been increasing since the industrial revolution and this increase is even sharper especially in the recent years due to the increased movement of goods, services and technology. Although most of the fuel demand for transportation is supplied by petroleum-based fuels, their resources are depleted year by year. Moreover, the recovery of petroleum from new reservoirs in extreme locations is sometimes also too difficult being too costly. Besides, excessive use of petroleum-based fuels causes air and water pollution leading to global warming. However, biofuel production from renewable sources is a sustainable way to maintain the ever-growing fuel demand, causing negligible harm to the environment due to the fast bioenergy cycle. As a result, the world biofuel production has an increasing trend in the recent years as shown in Fig. 1; such that, annual bioethanol production increased from 13.7 billion metric ton in the year 2000 to 78.8 billion metric ton in the year 2016 (multiplied by a factor of 5.8) while annual biodiesel production increased from 0.77 billion metric ton to 29.8 billion metric ton in the same period of time (multiplied by a factor of 38.6) [1].

⁎

Although biodiesel is an alternative fuel that can be produced from any edible plant-based oil or animal-based fat, this kind of 1st generation feedstocks are not preferred anymore due to the requirement of large farm areas, competition with the food market and thus high costs in the raw material [2]. Instead, biodiesel is recently preferred to be made from non-edible plant-based oils with (high oil yield), waste cooking oils and algae oil (2nd or 3rd generation feedstocks) [3]. The basic pictorial representation of biodiesel production is shown in Fig. 2, where the main steps for the process are given. It should be noted that the process may have some minor (or sometimes major) differences especially in the case of application of advanced techniques. The main reaction in biodiesel production is transesterification of any oil with a short chain alcohol like methanol or ethanol. In fact, vegetable oil can straightly be used as a fuel but its low fuel quality due to high viscosity, incomplete combustion, coking etc. make it insufficient for combustion engines. Therefore, transesterification of these types of oils by lower boiling alcohols convert them to high quality fuels by lowering viscosity, boiling and flash point. The transesterification of oil (Eq. (1)) is an endothermic equilibrium reaction requiring 3 mol of a short chain alcohol per 1 mol of oil in

Corresponding author. E-mail address: [email protected] (N. Alper Tapan).

https://doi.org/10.1016/j.fuel.2019.03.147 Received 27 September 2018; Received in revised form 26 March 2019; Accepted 28 March 2019 0016-2361/ © 2019 Published by Elsevier Ltd.

Fuel 250 (2019) 27–41

80

Bioethanol

Biodiesel

2.1. Catalyst design variables 2.1.1. Catalyst type and composition The transesterification reaction of oil can be catalyzed by a homogenous alkaline catalyst like sodium hydroxide (NaOH), sodium methoxide (NaOCH3) or potassium hydroxide (KOH), or by a homogenous acid catalyst like sulfuric acid (H2SO4) or hydrochloric acid (HCl). Although alkaline type homogeneous catalysts are preferred more frequently; transesterification reaction by this type of catalysts cause saponification of triglyceride or fatty acid methyl ester product. Moreover, saponification leads to the consumption of the catalyst (Eqs. (2) and (3)), decrease of the biodiesel yield and complication of the separation processes after biodiesel production. Acid catalysts, compared to alkaline ones, do not lead to saponification; however, the reaction rate is slower. Therefore, the reaction needs higher temperatures and pressures to achieve the desired conversion. On the other hand, transesterification by heterogeneous catalysts do not lead to saponification, as well as the separation and purification of the product is easy, hence the reaction is economically more feasible and the catalyst can be reused by simple filtration process [11,12]. Some commonly applied heterogeneous catalysts are alkali-metal carbonates (Na2CO3, K2CO3), alkaline earth metal carbonates (CaCO3), alkaline earth metal oxides (CaO, MgO, SrO, BaO) and other oxides such as ZnO [13].

60 40 20 0

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Biofuel production (million metric ton)

M. Erdem Günay, et al.

Year Fig. 1. Change of world bioethanol and biodiesel production through years [1].

stoichiometric terms as applied by various researchers [4–6]; however, to ensure a higher conversion, alcohol to oil ratio should be kept in excess [7–10]. Triglyceride which is the main constituent of these oils is not soluble with these short chain alcohols and emulsions form during the course of the reaction therefore mass transfer should be enhanced by mechanical means. These emulsions are caused by the intermediates like mono and diglycerides which contain hydroxyl groups from alcohols and nonpolar groups from oil. When these emulsions break, high density glycerol and low-density ester phases form. Of course, in order to transfer oil into esters, a very reactive catalytic alcohoxy phase is needed which is formed by the dissolution of a strong base like NaOH or KOH with alcohol phase. These alcohoxy groups attack ester groups of triglycerides to convert them to glycerine. Transesterification reaction mechanism described above requires the control of kinetics and mass transfer by catalytic variables and mode of reaction conditions.

(2)

(1)

(3)

Considering the fact that the feedstock that is used to produce biodiesel have a large variety of fatty acid and free fatty acid compositions, the process of biodiesel production may sometimes become too complicated. In addition, the differences in the operational conditions and catalytic systems complicate the process even more. Thus, excessive amount of laboratory research is needed before coming up with a commercialized biodiesel production process. In this work, significant catalytic and operational parameters and recent technological advancements in biodiesel production are extensively reviewed as a guideline to the active researchers as well as the new researchers in the field.

In addition to alkaline heterogeneous catalysts, nowadays, green biocatalysts also attract the scientific community due to their energy efficient enzymatic processes. It was shown through many sources that, through immobilization or in liquid phase, high yields can be achieved with robust lipase enzymes. Moreover, the enzymatic process can even be combined with the latest technologies like ultrasonic and can be operated in continuous mode by different reactor designs or life times of enzymes can be improved to take further steps towards industrial production. Last but not least, the application of metagenomics in enzyme technology opens vast perspectives for the development of stable and solvent tolerant biocatalysts for biodiesel production [14–16]. In order to achieve higher transesterification rates, one of the factors that should be optimized is the catalyst loading in the oil phase. Catalyst loading depends on other reaction variables and the optimum loading may vary accordingly. For instance, during the transesterification of rapeseed oil by methanol with Zn/Al catalyst, Jiang et al. [17] increased the catalyst loading from 0.5 wt% to 2.0 wt% and determined the optimum catalyst loading as 1.4 wt%, above, however no significant effect was observed. On the other hand, Wen et al. [18] investigated the catalyst loading for the transesterification of soybean oil by methanol with Li doped MgO catalyst by increasing the catalyst amount from 3 to 15 wt%. They found the optimum catalyst loading as

2. Significant variables for biodiesel production In order to achieve a high biodiesel yield, there are many catalytic variables that needed to be optimized such as the type of the catalyst and its composition, support type and pretreatment conditions (like calcination temperature and time). Moreover, there are also many operational conditions requiring to be adjusted such as reaction temperature, alcohol type, alcohol to oil molar ratio and stirring speed. 28

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M. Erdem Günay, et al.

Reactor (transesterification) Separation heater

alcohol+ catalyst

feedstock (oil)

Post treatment (washing and drying)

crude biodiesel

filtration

crude glycerine

stirrer

Pretreatment

Refining

(i.e. acid catalyzed pretreatment for high free fatty acid feedstock)

(i.e. removal of residual organic matter, water, salt, methanol and odor)

water and other impurities

refined biodiesel

refined glycerine

Fig. 2. Basic pictorial representation of biodiesel production process.

9% wt; further increase of the catalyst amount was reported to have negative effect which was attributed to increased saponification rate. The same optimum catalyst loading (9 wt%) was determined by Yu et al. [19] for the transesterification of pistaciachinensis oil by methanol with CaO–CeO2 mixed oxides. The authors observed an increase in biodiesel yield from 3 wt% to 9 wt% due to the increase of the active sites of the catalyst while they observed a decrease at higher catalyst loadings due to the low diffusion rate and bad mixing in the catalystmethanol-oil system. The same argument was also given by Kotwal et al. [20] for the transesterification of sunflower oil by methanol with flyash based catalyst (optimum loading: 15 wt%) and by Vyas et al. [21] for the transesterification of jatropha oil by methanol with KNO3/Al2O3 solid catalyst (optimum loading: 6 wt%). Moreover, Eevera and Pazhanichamy investigated the transesterification of cotton seed oil by methanol with NaOH, and they found out the optimum catalyst loading as 1.5 wt%. They observed incomplete conversion of oil to biodiesel when the catalyst concentration was low, and found a high degree of saponification if catalyst loading was higher than the optimum [22]. Muthukumaran et al. [23] also tested the biodiesel process on Madhucaindica oil by methanol with KOH, and found the optimum catalyst loading as 1 wt%. In their work, they observed that a higher catalyst loading than the optimum decreased the biodiesel yield due to the increased saponification rate (Eq. (2)) that shifts the equilibrium reaction (Eq. (1)) from right to left.

Table 1 Effect of calcination temperature on the transesterification of rapeseed oil with Ca/Al composite catalyst (methanol-oil molar ratio 15/1, reaction temperature 65 °C, reaction time 3 h, catalyst loading 6 wt% w.r.t. oil, calcination time 8 h) [24]. Calcination temperature (°C)

Specific surface area (m2/g)

Biodiesel yield (%)

120 400 600 800 1000

2.33 5.14 27.36 9.80 3.27

60 89 94 92 88

Likewise, Yu et al. [19] investigated the effect of calcination temperature on the transesterification of chinensis oil with methanol using CaO–CeO2 mixed oxide catalysts, and they found that 700 °C was the optimum calcination temperature leading to the maximum biodiesel yield (Fig. 3). They observed an improved activity with the increase of the calcination temperature whereas the activity decreased as the temperature increased above 700 °C which is due to the decrease of the specific surface area and pore volume [19]. Another example is the work of Xie and Wang-2013 [25], in which the influence of calcination temperature on transesterification of soybean oil with WO3/SnO2 catalyst was studied. They found that the conversion increased significantly by increasing the calcination temperature from 200 °C to 900 °C due to the increase of acidity of the catalyst (as discussed by the authors). They also observed that a further increase of temperature caused a decrease in conversion as shown in Fig. 4.

2.1.2. Calcination conditions A calcination procedure is frequently applied to heterogeneous catalysts to improve its activity. There are also many studies in the literature about the application of calcination as a pretreatment of transesterification catalysts. For instance, Meng et al. [24] analyzed the effect of the calcination temperature ranging from (120 °C to 1000 °C) for the transesterification of rapeseed oil with methanol using Ca/Al composite oxide-based alkaline catalyst. They found that the specific surface area of the catalyst increased with the increase of calcination temperature from 120 °C to 600 °C, then decreased when the calcination temperature was raised from 600 °C to 1000 °C. This fact was attributed to the agglomeration of the catalyst particles at high temperatures due to sintering. They also found that the optimum calcination temperature was 600 °C giving a biodiesel yield of 94% as shown in Table 1.

2.2. Operational variables 2.2.1. Type of alcohol and alcohol to oil ratio The types of alcohols commonly used for the transesterification reactions are methanol, ethanol, propanol, butanol, pentanol and amyl alcohol [26]. Since methanol is the shortest chain alcohol with high polarity it is the most frequently used alcohol followed by ethanol. Both methanol and ethanol have advantages and disadvantages; the reaction with methanol is reported to be faster at a lower optimal temperature 29

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M. Erdem Günay, et al.

alcohol to oil=15 alcohol to oil=9 alcohol to oil=3

100

100

Fatty acid conversion %

Biodiesel yield%

80

60

40

20

0 500

550

600

650

700

750

80 60 40 20 0

800

0

Calcination temperature (oC)

1

2

3

4

Reaction time (h)

Fig. 3. Effect of calcination temperature on the transesterification of chinensis oil with CaO–CeO2 mixed oxide catalysts (methanol-oil molar ratio 30, reaction temperature 100 °C, reaction time 6 h, catalyst loading w.r.t oil 9 wt%, calcination time 5 h) [19].

Fig. 5. Effect of alcohol (methanol) to oil molar ratio on fatty acid conversion for jatropha oil at T = 65 °C by K2SiO3/Al-SBA-15, catalyst wt% with respect to oil is 3 [6].

than 30 was even detrimental due to the increased solubility of the glycerine product in the reaction mixture. However, in the case of transesterification reaction being carried out in high pressure autoclaves and at high temperatures, higher alcohol to oil ratio was observed to be beneficial as seen during the transesterification of rapeseed oil by methanol with Zn/Al catalyst [17].

100

80

Fatty acid conversion %

alcohol to oil=12 alcohol to oil=6

60 2.2.2. Reaction temperature One of the prominent variables that affects the rate of transesterification reaction is the reaction temperature. Although the reaction can even be successfully performed at room temperature [30], based on reaction kinetics, the rate of reaction increases with the increase of reaction temperature. Moreover, for the case of transesterification of oils, higher temperatures also lead to the decrease of viscosity of the oil and a better mixing of the reactants [31]. However, there is an optimum point for the temperature and further increase beyond this point causes the decrease of biodiesel yield due to increased saponification (in case of the alkaline type homogenous catalysts) and due to rapid evaporation of the alcohol used (i.e. methanol boils at 64.6 °C at 1 atm pressure) [32]. Hence, the reaction temperature is generally kept below the boiling point of the alcohol; however, it is still possible to perform the reaction at higher temperatures under reflux conditions [32] or under high pressure [17]. There are numerous works in the literature searching for an optimum temperature for transesterification; some of which are summarized in Table 2. For example, Leung and Guo [33] investigated the effect of changing the reaction temperature from 30 °C to 70 °C on the transesterification of canola oil while using methanol with NaOH catalyst. They found the optimum temperature for the maximum biodiesel yield as 45 °C, whereas increasing the reaction temperature from 45 °C to 70 °C caused the biodiesel yield to decrease from 93.5% to 90.4% due to accelerated saponification at higher temperatures (Eqs. (2) and (3)); however, the reaction time for the completion of the reaction decreased from 60 min to 15 min due to the increase of rate of reaction. This saponification reaction may be caused either by irreversible reaction of triglyceride or unremoved biodiesel with alkaline catalyst as shown in Eqs. (2) and (3). Likewise, Abbah et al. [32] studied the effect of changing the temperature from 30 °C to 65 °C on the transesterification of neem seed oil by methanol with KOH catalyst and by holding the reaction time constant at 1 h; the optimum temperature for the maximum biodiesel yield was determined to be 55 °C. Lower biodiesel

40

20

0 200

400

600

800

1000

Calcination temperature (oC) Fig. 4. Effect of calcination temperature on the transesterification of soybean oil with WO3/SnO2 catalyst (methanol-oil molar ratio 30, reaction temperature 180 °C, reaction time 5 h, catalyst loading w.r.t oil 5 wt%, calcination time 5 h) [25].

compared to the reaction with ethanol [27] and also reaction with methanol may lead to a higher biodiesel yield as in the case of transesterification of sunflower oil [28]. However, ethanol obtained by renewable methods is less toxic compared to methanol [29] and in some cases, such as the transesterification of soybean oil, the rate of reaction may even be higher compared to that of methanol [8]. Although, there is almost no limit for alcohol to oil ratio [26], increasing this ratio is simply not effective beyond a certain point. For instance, as shown in Fig. 5, it is seen that increasing the methanol/oil molar ratio in the range of 9–15 has no significant effect on fatty acid conversion in the case of transesterification of jatropha oil [6]. In addition, Wen et al. stated that during the transesterification of soybean oil by methanol over Li doped MgO catalyst, alcohol to oil ratio greater than 15 makes the heating process and the post separation process more difficult, and they concluded that the optimum ratio is between 12 and 15 [18]. Moreover, Xie et al. [25] found that alcohol to oil ratio greater 30

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3 10 1.4 9 9 5 6h 0.75 1 1.3 2.5 h 6h 3h 2h 6h 5h 2h 1h 1h 80 min

94.7 98.0 83.6 93.9 93.0 79.2 90.8 94.0 93.5 95.0

yields below this temperature was attributed to the slow reaction rates at low temperatures while lower yields above this temperature was attributed to increased evaporation of the methanol and increased saponification. The drop in biodiesel conversion due to the increased evaporation rate at high temperatures were also reported during transesterification reactions of soybean oil by methanol with Li doped MgO catalyst [18], pistaciachinensis oil by methanol with CaO–CeO2 mixed oxides [19], soybean oil by ethanol with NaOH catalyst [34] and silybummarianum oil by methanol with zirconia modified with KOH [31]. Biodiesel production in high pressure autoclaves and at high temperatures were also investigated by various researchers. For instance, Jiang et al. [17] increased the reaction temperature from 170 °C to 220 °C for the transesterification of rapeseed oil by methanol with Zn/ Al catalyst. Although they observed a significant increase in biodiesel conversion up to 200 °C, no significant increase beyond this temperature was observed. The work of Xie et al. [25] also confirmed this result for transesterification of soybean oil by methanol with WO3/SnO2 catalyst; in this work they found out the optimum reaction temperature as 180 °C and observed that the reaction mixture turned into black at high temperatures due to the carbonization of the reactants. 2.2.3. Mechanical stirring Most often alcohols and oils form immiscible mixtures and initially the transesterification reaction between them is a two-phase reaction. However, as the transesterification proceeds and alkyl ester is formed, it acts as a solvent for both phases and the reaction continues as a single phase reaction [35]. Therefore, agitation is quite important especially during the initial state of the reaction when the reaction is mass-transfer controlled, and a poor mass transfer in this state causes the reaction rate to be slow [36]. Stirring the alcohol-oil mixture provides the dispersion of the two phases by increasing the contact area between them and hence increasing the mass transfer rate [17,37]. There are various studies in the literature investigating the effect of stirring on the biodiesel production. The optimum stirring speeds and the corresponding experimental conditions leading to maximum biodiesel conversion for some of those studies are summarized in Table 3. For instance, as seen in the table, Noureddini and Zhu [35] investigated the effect of stirring on the transesterification of soybean oil with NaOH catalyst. They proposed a reaction mechanism which is initially masstransfer controlled then kinetically controlled. Ma et al. [38] stated in their work that the agitation speed was quite critical during the initial states of the reaction while it was not that important after a stable mixture was formed. On the other hand, Kim et al. [39] investigated the effect of stirring on the reaction of soybean oil with Na/NaOH/γ-Al2O3 catalyst by changing the stirring speed from 300 rpm to 1500 rpm and they did not find any significant effect (the biodiesel conversion was reported to increase from about 76 to 78%). Moreover, Vicente et al. [40] investigated the effect of the stirring speed on the transesterification of sunflower oil with KOH catalyst. They concluded that kinetics of the reaction mechanism which initially involved a mass transfer-control then followed by a kinetically controlled reaction. They also found that a minimum of 600 rpm stirring speed was needed to overcome the mass transfer limitation in the initial states of the reaction. In addition, Berrrios et al. [41] also confirmed the positive effect of the increase of stirring speed for the transesterification of refined lard. Lakshmi et al. [37] investigated the effect of stirring speed on the transesterification of rice bran oil and karanja oil with KOH catalyst. They found that when the stirring speed was low, the top and bottom of the reactor vessel had unmixed dead zones which disappeared by increasing the stirring speed. They also concluded that increasing the stirrer speed more than a certain value had no significant effect on the rate of the reaction. One recently published study by Dhawane et al. [42] investigated the effect of agitation speed for the transesterification of refined rubber seed oil with iron doped carbon catalyst. They

Jatropha oil Soybean oil Rapeseed oil Soybean oil Pistaciachinensis oil Soybean oil Silybummarianum oil Neem seed oil Canola oil Soybean oil K2SiO3/AlSBA-15 cerium (III) trisdodecylsulfate trihydrate Zn/Al catalyst Li doped MgO CaO–CeO2 mixedoxides WO3/SnO2 zirconia modified with KOH KOH NaOH NaOH Wu [6] Ghestiet al.[8] Jiang et al. [17] Wen et al. [18] Yu et al. [19] Xie et al.[25] Takase et al. [31] Abbah et al. [32] Leung and Guo[33] Silva et al. [34]

Methanol Ethanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Ethanol

9 6 42 12 30 30 15 7 6 9

65 100 200 65 110 180 60 55 45 40

Catalyst loading to oil wt % Reaction time Reaction temperature (°C) Alcohol /oil mole ratio Alcohol type Oil type Catalyst Reference

Table 2 Optimum temperatures and other experimental conditions reported in the literature leading to maximum biodiesel conversion.

Biodiesel conversion (%)

M. Erdem Günay, et al.

31

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78 72 99.8 89.3 89.3

∼100

70.8

1500 600 600 600 1250

600

800

Mechanical stirrer (anchorshaped) Mechanical stirrer 60

2h

0.79

In the history, the extraordinary properties of supercritical fluids (SCF) was first realized in the beginning of 20th century [45]. But the application of supercritical fluids on biodiesel production began in the 21st century [46]. Recently, non-catalytic pathways are regarded as promising solutions for high biodiesel yields from different oil sources. Non catalytic transesterification by SCF is carried out at much higher temperatures and pressures compared to conventional biodiesel production methods and therefore lead to higher esterification, transesterification rates and easier separation of the products from the reaction mixture [47–51]. In fact, at these conditions, high miscibility between oil and supercritical solvent phase increases mass transfer rates of the transesterification reaction [47–49]; in addition, supercritical fluids exhibit extraordinary physical properties (like density) and solvation power [50,51]. Another advantage of SCF is that the presence of water has a positive effect on the yield of transesterification product in contrast to the ordinary transesterification of waste cooking oil in the presence of alkaline catalysts. In the conventional alkaline transesterification, due to the free fatty acid (FFA) and water content of the oil, saponification causes a drop in catalyst effectiveness and fuel quality increasing the operational and pretreatment costs [47–49,52]. On the other hand, in the supercritical region, since the dielectric constant of water is much lower (lower dielectric constant means lower polarity), subcritical and supercritical water behave as organic solvents exhibiting extraordinary solubilizing power toward organic compounds having large nonpolar groups [47–49]. Therefore, higher tolerance to FFA and water contents in the oil feedstock lead to esterification of FFA and transesterification of triglycerides concurrently [50,51]. Apart from the advantages, super critical conditions have some drawbacks. For instance, batch mode operation is not favorable since control of reaction during heating up and cooling down of the reactor is not easy [49,53–57]. There is also high equipment cost and high energy consumption due to high temperatures and pressures during the operation. This limits the applicability of supercritical transesterification process for large scale industrial use. This problem can be solved by the addition of co-solvents like hexane, carbon dioxide or propane into the reaction mixture. These co-solvents help to decrease the extreme process conditions to milder ranges by decreasing the critical conditions or phase equilibrium condition making the process more practical [58,59]. As its name implies, supercritical conditions are achieved above the critical pressure and temperature of reaction mixture together with a solvent. For instance, in the case of ethanol solvent, supercritical medium is above the critical temperature and pressure of alcohol (above Tc = 514 ± 7 K and Pc = 6.3 ± 0.4 MPa). Supercritical conditions for different alcohol/oil ratios (500:1 to 6:1) should be determined before performing experiments by using computer programs like VMGsim program and an equation of state like Peng Robinson that can define the phase behavior of triglyceride and alcohol accurately. After computation of phase behavior of different alcohol/oil mixtures, the reaction conditions for the supercritical mixture and reaction conditions can be set properly [60]. Moreover, experiments can be

Soybean oil Ca(C3H7O3)2/CaO Li et al. [44]

15

1 6h 60 CH3ONa Alcantara et al. [43]

7.5

Soybean oil Sunflower oil Sunflower oil Refined lard Refined rubber seed oil Soybean oil Na/NaOH/γ-Al2O3 KOH KOH KOH Fe/C Kim et al. [39] Vicente et al. [40] Vicente et al. [40] Berrios et al. [41] Dhawane et al. [42]

6 6 6 6 9

60 65 65 60 50

2h 1 min 1h 20 min 1h

1 g catalyst 1 1 0.9 4.5

46.3 330 Beef tallow NaOH

6

80

10 min

0.3

97.4 550 Karanja oil KOH

6

60

1h

1

86.5 98.4 600 700

Mechanical stirrer Mechanical stirrer (4 bladed flat turbine) Mechanical stirrer (4 bladed flat turbine) Mechanical stirrer (with turbine type impeller) Mechanical stirrer Helix stirrer Helix stirrer Magnetic stirrer Not given Soybean oil Rice bran oil

Noureddini and Zhu [35] Vijaya Lakshmi et al. [37] Vijaya Lakshmi et al. [37] Ma et al. [38]

6 6

50 60

1.5 h 1h

0.2 1

82.8 400 Not given Rapeseed oil

Zn/Al complex oxide NaOH KOH

Oil type

The catalyst design and operational variables can also be optimized under supercritical conditions or during the application of advanced techniques like ultrasonic and microwave irradiation or hydrodynamic cavitation. In this section, dominant parameters and limitations during the application of these advanced techniques and the critical conditions of reaction mixtures are investigated. 3.1. Supercritical transesterification

Jiang et al. [17]

42

200

3h

1.4

Biodiesel conversion (%)

3. Technological advancements to increase biodiesel yield

Catalyst

Alcohol /oil mole ratio

Reaction temperature (°C)

Reaction time

Catalyst loading to oil wt%

Type of stirrer

Stirring speed (rpm)

concluded that a higher agitation speed provided good mixing of the reactants due to formation of turbulence which minimized the external mass transfer resistance.

Reference

Table 3 Optimum stirring speeds and the corresponding experimental conditions leading to maximum biodiesel conversion reported in different publications in the literature.

M. Erdem Günay, et al.

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designed and statistically tested by advanced statistical tools for the significance of important reaction conditions like temperature or cosolvent to alcohol mass ratio [61]. 3.1.1. Solvents In general, experiments under supercritical conditions are performed between 270 and 400 °C at 20 MPa as seen in the literature. C1 to C4 type alcohols and ethers like methyl tert-butyl ether (MTBE) [62] can be used as solvents in continuous or batch mode [53] for biodiesel production. It was observed that among these solvents, methanol has the highest performance at residence times of 3–30 min and 270 °C–400 °C with an alcohol to oil ratio of 40 at 20 MPa. The highest activity of methanol is due to its smaller size and rather higher dipole moment compared to other alcohols which altogether provides easy extraction of fatty acids by its oxygen atom and stable ester molecules [53,62]. The lower reactivity of higher carbon chain alcohols could be related to lower dipole moments [63]. Although methanol has the highest activity, its toxicity prevents researchers to work with it in favor of other alternatives possessing higher carbon numbers like ethanol, 1-propanol, 1-butanol due to health, environmental and fuel quality concerns [59,63,64]. For instance, ethanol is a renewable, plant based and environmentally friendly product without disturbing carbon cycle. In addition, diesel quality of ethyl esters are better than methyl esters in terms of cetane number, oxidation stability and cold flow property [59]. Even more, fatty acids can dissolve better in ethanol-cosolvent mixture (like CO2) which is due the closer solubility parameter of ethanol to fatty acid than methanol [61]. Lower carbon numbered alcohols, like methanol, also attract water easily and lead to corrosion of the reactor equipment. In order to avoid negative effects of these solvents, 1-propanol can be used as an alternative. By using 1-propanol at the optimum conditions, up to 94% yields can be achieved at 25–30 min residence times at 350 °C. The residence times even drop to 10 min when the reaction temperature is increased to 400 °C. 1-propanol can also be produced by chemical and environmentally friendly biochemical pathways [63,64]. Like 1-propanol, the use of 1-butanol is also advantageous since it is also renewable and can be produced from agricultural wastes like rice straw. Moreover, it has higher cetane number, heating value and higher solubility in diesel than those of lower carbon alcohols. Previous studies indicate that equilibrium yields can be achieved at 400 °C with 14 min residence times [65,66]. The molecule size and bulky structure of solvents (MTBE > 1propanol > ethanol > methanol) have also an important effect on the operational conditions of biodiesel production. Although residence time increases with the molecule size, interestingly at certain temperatures (like 270 °C) by domination of polarity on the steric effect, some bulky structures like MTBE has higher conversions compared to smaller molecules like ethanol due to increased solubility. Therefore, in addition to the size of molecule, the bulky structure of molecule and the critical reaction temperature have also important implications on biodiesel production [63]. Even more, useful fuel additives like glycerol tert-butyl ether can be produced besides biodiesel with MTBE solvent [67]. The optimum conditions for most of the solvents are 350 °C and 20 MPa since dramatic changes are observed in the yields at short residence times as seen in Fig. 6 [53,68,69]. In general, non-catalytically, the increase in supercritical temperatures (475–560 K) increases fatty acid conversion at different scales of residence times (50–300 s) [62,70–72]. As the supercritical temperatures are increased furthermore (close to 400 °C), the biodiesel yields become almost equal and independent of the size or the structure of solvent molecule [47–49,69]. There could be a tradeoff between residence time, temperature of SCF and conversion of fatty acid in terms of operating cost of reactor and the quality of the biodiesel. Higher temperatures or longer residence times lead to complete conversion or equilibrium conversion

Fig. 6. Effect of temperature on biodiesel yield in supercritical methanol (SCM), supercritical ethanol (SCE), and supercritical MTBE (SCMTBE), supercritical butanol (SCB), supercritical propanol (SCP) (experimental conditions: 20 MPa, 15 min, canola oil-to-reactant molar ratio of 1:40) [68,69].

values sacrificing for operating cost of reactor. At longer residence times close to 30 min the performance gap between supercritical propanol (SCP) and supercritical ethanol (SCE) is also closed [53,54]. For the optimization of biodiesel production at the supercritical conditions, artificial neural networks (ANNs) [11] or advanced statistical tools [61] can be used to determine the relative importance or the variance of process variables. For instance, it was shown that for the biodiesel production from crude mahua oil, the four variables; namely temperature, ethanol/oil molar ratio, time, and initial CO2 pressure with relative importance of 39.24, 19.61, 28.57 and 12.58, respectively have strong effects on the fatty acid ethyl ester (FAEE) content. The degree of effectiveness of variables was found to be in the order of temperature > reaction time > ethanol/oil molar ratio > initial CO2 pressure [73]. In another study, the significance of process variables with different alcohols were compared; such that, in the case of supercritical esterification of Spiriluna oil, it was observed that although temperature and CO2 co-solvent ratio were found to be statistically significant for methanol, these parameters were found to be statistically insignificant for ethanol [61]. 3.1.2. Use of ionic liquids and CO2 in combination with SCF To close the performance gap between higher and lower chain alcohols, there are other options, like the use of ionic liquids with SCF [70,74,75] by which complete conversions and very high yields (∼98%) can be achieved at short time scales and mild operating conditions close to 520 K and 10 MPa [46]. The distinctive features of ionic liquids comprise high thermal and electrochemical stability under extreme temperatures and pressures, their extraordinary physical properties like low melting point (∼373 K), low viscosity, low vapor pressure, acidic nature and non-toxicity. The ionic liquids can be designed for the specific purpose by selecting different cations (1,3-dialkylimidazolium etc.) and anions (PF6−, BF4−, CF3SO3− etc.). These compounds have very high catalytic activity since they can form stable ionic complexes (like carbocations, carbanions etc.) for longer life times leading to lower activation energies. Moreover, they can be reused several times without any chemical wastes and without significant loss in yields (97–93.5%) [46], soap formation or emulsions [76,77] and product mixture can be easily removed from the ionic liquid by an organic solvent since ionic liquids are only soluble in an ionic phase [74,76–80]. Data from gas chromatography (GC) and (proton nuclear magnetic resonance) 1H NMR proved that esters were formed in greater quantities with almost complete conversion of waste oil when alcohol solvent was used in combination with ionic liquids (IL). In this reaction mixture, the effect of supercritical conditions can be observed clearly during the transition from sub to super critical conditions (reaction time > 0 min) (0 min means that the moment when the system reached supercritical condition of alcohol) by reaching to the equilibrium faster [46]. In addition to the use of ionic liquids, as mentioned before co33

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Table 4 Kinetic parameters and optimum conditions for different SCF during transesterification of Canola oil. Reference

Meloni et al. [77] Meloni et al. [77] Farobie and Matsumura [53] Martins et al. [80] Meloni et al. [77]

Supercritical fluid

A(dm3.mol−1.min−1)

Optimum conditions

Ea for TGDG (kJ/ mol)

Ea for DG-MG (kJ/mol)

Ea for MG-GL (kJ/mol)

Optimum yield (%)

TG-DG

DG-MG

MG-GL

P (MPa)

T (°C)

Residence time (min)

alcohol oil molar ratio

Methanol Ethanol 1-Propanol

6.85 * 107 7.03 * 1010 9.69 * 107

1.47 * 105 4.09 * 105 2.69 * 105

5.50 * 105 7.29 * 105 8.03 * 103

20 20 20

350 350 400

5 20 10

40 40 40

100.99 141.28 111.39

71.77 80.31 78.99

77.28 81.64 60.96

97.7 90.0 94.4

1-Butanol MTBE

1.18 * 108 3.39 * 107

6.31 * 104 1.78 * 105

5.54 * 103 7.76 * 105

20 20

400 350

15 20

40 40

114.67 103.68

73.31 73.28

61.01 79.84

94.7 80.0

including FFAs, DGs, and MGs [82–84]. Either decreasing the ratio of methanol/oil or increasing water content up to 2.5 wt% (on the basis of the weight of the oil) does not have a significant effect on the fatty acid methyl ester (FAME) yield from the HT oil [85]. Although excessive temperatures above 573 K may cause a drop in FAME yields due to thermal decomposition and isomerization reactions, it was not observed in hydrothermal liquefaction of the extracted oil [81].

solvents like CO2 can also be employed to decrease the supercritical temperature and pressure of alcohols. For instance, in the literature, the optimum conditions were reported for the conversion of mahua oil into biodiesel as 304 °C, 29:1 (alcohol to oil ratio), 36 min (reaction time) and 40 bar [73]. When the reactor pressure was controlled by the pressure of CO2, it was observed that biodiesel yield increased by CO2 pressure due to the increase in the density of the reaction mixture, but above 40 bar excessive pressures were found to be ineffective [55].

3.1.5. Combined solid basic catalysts and ultrasonic emulsification with supercritical fluids Solid basic catalysts like SrO with effective oxide supports like Al2O3 or CaO can also be used with supercritical fluids (at the conditions of 623 K, 30 MPa, SCE (12:1) alcohol-oil ratio and optimum 2 wt% catalyst) in order to reduce alcohol-oil ratio, increase the biodiesel yields (up to 97.46%) and improve the biodiesel quality (like viscosity standards from 3.5 to 5 mm2/s). From the point of economic and environmental aspects, promising solid basic catalyst should not leach or dissolve during these extreme conditions. For this reason, proper oxide catalysts like ZnO, MgO, SrO and supports (like Al2O3 and CaO) should be selected and effective techniques like impregnation should be applied for preparation of these basic catalysts. In addition, the catalyst content in different reaction mixtures for different oxide materials should be optimized which may range between 2 and 5 wt% [60]. In order to enhance miscibility of oils with alcohols and increase the speed of transesterification, ultrasonic emulsification together with solid catalysts can also be applied in combination with SCF. In fact, the application of ultrasonic emulsifier may increase the contact surface area between the phases and consequently increases the mass transfer coefficient [60].

3.1.3. Kinetics Like in the case of normal operating conditions (close to the boiling point of alcohol and atmospheric pressure), transesterification reaction under supercritical conditions follows three reversible steps from TG to DG (triglyceride to diglyceride), DG to MG (diglyceride to monoglyceride) and MG to glycerine. After parameter estimation by nonlinear regression, kinetic analyses show that methanol has the highest forward rate constants and lowest activation energies among MTBE and ethanol which indicates the most stable activated complex [53]. The order of kinetic activities and optimum conditions of different supercritical solvents with Canola oil can be seen in Table 4. The table shows that the order of activities of supercritical fluids is SCM > SCE > SCMTBE > SCP > SCB approximately. It is seen that for different types of oils, the activation energy of transesterification for SCP and SCB is higher compared to lower alcohols although different experiments give different activation energies but comparable magnitudes due to distinct type of apparatus used (batch and continuous) [49,53–57,63,68]. Of course, higher activation energies of higher carbon-contained alcohols are due to lower forward and backward transesterification rate constants in the three steps, namely TG to DG, DG to MG and MG to glycerine mechanisms.

3.2. Microwave transesterification 3.1.4. Effect of prior oil extraction on supercritical process Extraction of oil from plant-based sources (like microalgae, other wild plant or agricultural crops) is an important process before transesterification since it determines FA content of the oil and therefore affects the transesterification rate. For this purpose, sub and supercritical conditions for the extraction and transesterification can be combined. Before reaching the supercritical temperatures, extraction process from the source is started at subcritical conditions (like 573 K and 10 MPa) and then oil can be converted to transesterification products by increasing the temperature and pressure to supercritical temperatures [81]. Type of extraction process can also affect the yields from the supercritical processes. Different extraction processes also lead different FFA contents, and higher FFA content means higher yields and higher transesterification rates compared to thermal decomposition and/or isomerization rates in contrast to lower transesterification rates. Therefore, selection of a better extraction process like hydrothermal (HT) method is an important criterion for achieving high quality biodiesel [81]. The reason why HT process produces higher FA content oil is that during the HT process, some of the lipids (TGs) in microalgae could have been hydrolyzed to form free fatty acids (FFAs). Therefore, the reaction medium could be a mixture of partially hydrolyzed lipids,

Microwave radiation is an advanced heat source which can be used for thermal pretreatment and reaction, and combines them in a single step [86–88]. The microwave assisted process is a very effective technique which improves both the extraction process (from wild and domestic plant based sources) [89,90] and the transesterification process [91–93]. In addition, microwave assisted process is energetically efficient since the microwave radiation can be adjusted only to heat the reactants without heating the reactor (otherwise causing reverse temperature gradient and therefore heat transfer from the reactant to the vessel wall) [89]. It is also economical compared to conventional route with the major advantages of attaining high yields with reduced amount of solvent and catalyst use, shorter reaction time and lower temperatures [67,86,94–96]. For instance, as seen in Fig. 7, transesterification by direct microwave radiation of triglycerides with potassium hydroxide catalyst could achieve 95% conversion in 5 min. The measured microwave energy consumption was only 87% of the calculated heat requirement for both the reactants and the vessel. Although the degree of energy efficiency of the microwave heating and the inefficiency of the conventional heating may vary, depending on the production process and reactor shape, the 34

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solvent-solvent-oil mixture. By this technique, it was succeeded to achieve lower energy consumption compared to conventional mechanical stirring like in the case of transesterification of kitchen waste cooking oil [67]. In addition to temperature control, the residence time under microwave heating should also be optimized not to exceed the point where water is extracted from plant source and when soap formation starts which has detrimental effects on biodiesel yield and not to exceed the point that cause excessive solvent evaporation. Optimum residence time can be detected by proton NMR spectroscopic technique showing the limits of biodiesel yield [86,87,102]. In addition, statistical models like Taguchi model can be used to determine optimum reaction time for the highest yields [67]. Interestingly during the microwave heating, there is a gradual rise in the product yield (almost 1% rise) as the methanol/oil ratio is decreased. However, as the methanol ratio is increased further, the product yields may exhibit an opposite trend like in the case of transesterification of Chinese tallow tree seed oil in the presence of sodium hydroxide catalyst. The probable reason could be the high polarity of methanol which leads the absorption of microwave radiation even at low concentrations. On the other hand, when alcohol concentration is increased excessively, higher amounts of absorbed microwave energy by the alcohol blocks the absorption of microwave by the plant source, thus decreases the extraction efficiency and increases separation cost during single step process as mentioned in the beginning of this section [86]. Although microwave technology has some significant advantages, there are some drawbacks related with safety and scaling up for industrial production. The penetration depth of microwave into reaction mixture is limited when large columns are used [103] and there is high microwave instrumentation cost at industrial scale [96]. Therefore, in order to solve scale up problems and irradiation efficiency, microwave instrumentation can be modified by direct injection of microwave power into reaction mixture by using coupling rods clamped to a pressured microwave window built on the reactor. However, if additional coupling rods are required in the reactor, they should be installed properly in order to prevent malfunctioning due to interference of different microwave phases with each other emitted from the coupling rods. These advanced microwave systems can be modeled and are based on computer simulations. It was observed that approximately 99% of the microwave power is absorbed by the reaction mixture by using couplers and dosing microwave directly into the reaction mixture [97].

Fig. 7. Profiles for % FAME during transesterification of soybean oil at 60 °C and comparison of energy consumed by microwave heating and calculated energy requirement for reactor wall and reaction mixture heating to maintain the desired reaction temperature [97].

energy consumption difference between the microwave and conventional heating definitely increases as the scale of the reactor increases due to higher temperature gradient between the wall and reaction mixture as indicated by the simulation results [97]. The heat requirement of microwave assisted processes and conventional process can be calculated by using a simple formula (Eq. (4)) as shown below:

Q = m ·Cp ΔT

(4)

where Q is the amount of energy consumed, m is the mass of reaction mixture, Cp is the specific heat capacity of the mixture and ΔT is the temperature difference after applying microwave power, conventional heating or simultaneous cooling and microwave heating procedure (SCMH) [89]. The mechanism of microwave interaction with the oil source and solvent is that the rapid generation of heat and pressure build-up by dipolar rotation and ionic conduction within biological matrices significantly affect microstructure of the plant. On the other hand, the rapid increase of temperature above boiling point leads to a supercritical state of the solvent; and tri-glyceride and FFA molecules colliding with each other cause frictional heating and form different isomers [86,93,98–100]. These unstable isomers if exposed to microwave irradiation by frequent pulses can be transesterified to FAME with lower activation energy without reverting back to the glycerides [98].

3.2.2. The effect of pulse width, intensity and frequency of microwave on biodiesel yield The efficiency of biodiesel production can be increased by adjusting microwave power intensity and pulse. In general, pulses are generated by an apparatus consisting of microwave oven radiating 2.45 GHz magnetron. The procedure for pulse technique involves ramping and holding periods. Within the ramping period, microwave power is adjusted to reach the desired reaction temperature. During the holding period, power is adjusted again to keep the reaction temperature at the desired level. Maximizing or minimizing microwave during the ramping period does not mean higher FAME yield. The ramping period power level should be optimized to give uniform pulse frequencies during these two periods. For instance, during the production of biodiesel from Chlorella sp. via transesterification, it was observed that intermediate power settings exhibited (like 500 or 250 W) the highest FAME yields. When a higher power setting was used, the system automatically lowered the pulse rate despite 20% duty cycle (percentage of irradiation period in one cycle) and led to lower FAME yields [104].

3.2.1. Critical issues during microwave heating Although increasing the reactor temperature favors transesterification, it has to be well controlled since regions of hot spots can occur due to non-uniformity during microwave irradiation. In order to avoid this problem, simultaneous cooling and microwave heating (SCMH) procedure or stirring can be applied to create an ideal reaction mixture and extract the maximum amount of lipids from the plant source [89,97]. In fact, the higher efficiency of SCMH is due the cooling effect which allows the plant source to absorb higher doses of microwave radiation and trigger higher oscillations among the molecules [101]. It was observed that during the application of this operational procedure, microwave heating consumed only one third of the conventional heating energy and the cooling effect promoted more microwave penetration to the reaction mixture. The results revealed that during the treatment of microalgae, one step SCMH yielded higher biodiesel production (75%) than water bath (13.46%) and microwave method (15.39%). Biodiesel properties like cetane number achieved by SCMH method were also better than the other two methods [89]. Microwave assisted transesterification can be also energetically improved by advanced blending techniques to achieve single phase reaction mixtures before microwave irradiation. Advanced shakers can be used to create turbulent regime with high shear for one phase co-

3.2.3. Catalysts with microwave power Like in the case of supercritical fluids, well prepared supported base catalysts (like SrO on silica) can be used together with microwave assisted system. Repeated reactions with these catalysts can be performed 35

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successfully since strongly supported (like by silica beads) metal oxides facilitate higher biodiesel production in a continuous mode [105]. These metal oxides when impregnated at nanoscale with lower loadings on the support could even shorten residence time of the reaction and microwave irradiation time with high conversions, almost to completion, due to very high surface areas and larger active sites on the catalyst [106]. In addition, solid acid catalysts like SO3H-ZnAl2O4, with advanced physical properties like high surface area (376.26 m2/g, pore volume 0.16 cm3/g, average pore diameter 3.55 nm and acid density of 2.10 mmol.g−1) can also be applied with microwave radiation giving nearly to 95% methyl ester yields and high-quality biodiesel conforming to EN 14214 and ASTM D6751 standards. 3.3. Hydrodynamic cavitation Hydrodynamic cavitation is formed by the expansion and the collision of bubbles. This oscillating behavior of bubbles creates highly energetic local regions by eliminating the mass transfer limitations between two different phases (like oil and alcohol) in the flow system [107,108]. During the biodiesel production, like other advanced techniques, hydrodynamic cavitation can be used to increase the efficiency of pretreatment process and transesterification by decreasing the reaction times [109,110]. For instance, the study by Bokhari et al. [109] showed that the reaction time, transesterification and energy efficiency of the hydrodynamic cavitation were up to six, eight and two times superior, respectively compared to the mechanical stirring. In addition to these, during the biodiesel production the hydrodynamic cavitation compared to the mechanical stirring is also more economical since up to 5-fold lower feedstock is used per produced product [109–112]. The reason for the large difference in energy efficiency between hydrodynamic cavitation and mechanical stirring is due to lower heat loss to the surroundings in shorter reaction times. For instance, during the transesterification of waste cooking oil, it was found that the hydrodynamic cavitation consumed 57% less energy compared to the mechanical stirring for 15 min reaction time [111]. Exergy analysis also shows that hydrodynamic cavitation is advantageous compared to conventional mechanical mixing. Process simulation composed of transesterification reactor, separation and purification units indicate that overall exergy (maximum theoretical work) efficiency for hydrodynamic cavitation integrated process can go up to 98% which is 6% higher than processes using mechanical stirring. This result was linked to low raw material requirement, lower alcohol to oil ratio, lower number of process equipment used and low amount of waste streams due to high conversions [113].

Fig. 8. Schematic representation of hydrodynamic cavity assisted transesterification system.

Fig. 9. Effect of plate geometry on FA conversion. Pipe diameter = 20 mm. Transesterification of frying oil at 60 °C [107].

which is detrimental to biodiesel yield [114]. On the other hand, lower number of holes with large hole diameters (like the geometrical change in the plates from 2 mm diameter 25 holes to 3 mm diameter 20 holes) decrease cavitational effect; therefore, biodiesel yields occur as seen in Fig. 9 [107]. Therefore, plate geometry optimization is a critical factor for high ester yields and scaling up the reactor system [107,110]. For instance, in a hydrodynamic cavitation reaction system, an optimized plate with 21 holes of 1 mm diameter with an inlet pressure of 3 bar can give a reaction time and transesterification efficiency 3-fold less and 4fold more than the mechanical stirring. The transesterification or yield efficiency in a hydrodynamic cavitation reactor system is based on the energy consumed by the double diaphragm pump as given in Eq. (5) [110]. This equation is very important since it is a criterion for optimization of plate geometry [109].

3.3.1. Reactor system In general, a hydrodynamic cavitation reaction system mainly consists of a jacketed tank and double diaphragm pump to create cavity effect in the flow line [107,112]. The pump outlet is divided into bypass and main lines with valves in order to regulate the desired pressure before the orifice where cavitation takes place. The velocity through the orifice is monitored by measuring upstream flow rate of the reaction mixture. The reactor tank is heated by hot liquid glycerine flowing through the jacket. Generally, from 1 to 3.5 bar inlet pressures are exerted before the orifice plate to create the cavitation effect [109]. Beyond 3.5 bar, the ester yield is not affected by the hydrodynamic cavitation due to the dramatic rise in cavitation effect and coalescence of cavities which cause blockage in the reactant stream [107]. The schematic representation of the reactor system is shown in Fig. 8.

Esterification efficiency =

3.3.2. Plate geometry Cavitation conditions are dramatically affected by different plate geometries like hole spacing and distribution of hole on the plate since the geometry changes the regime of upstream flow [109]. Small hole diameter with close distances between holes causes merging of cavities

(initial FA value) − (final FA value) 2 × 100

× (mass off treated product)

(pump power) × (reaction time)

(5)

Not only the plate geometry but the operating parameters of the hydrodynamic cavitation reactor can also be optimized by using a computational tool like Design Expert. For instance, it was seen that during the transesterification of rubber seed (Heveabrasiliensis) oil, 36

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famous recently due to its potential for lower residence times, production cost and successful applicability in acidic or basic medium and also in the presence of an enzymatic catalyst. Especially ultrasonics is more effective in the case of enzymes, since slow enzymatic reaction steps can be enhanced even with the application of low amplitudes [14]. The ultrasound technology is also a versatile technology which can be applied both to oil extraction from different sources and esterification of aliphatic acids [112,118–120]. The main advantage of ultrasound technology compared to the conventional methods is that it can improve both the mass transfer rate and the reaction rates in contrast to the mechanical agitation which is mostly effective in increasing the interfacial surface area [121]. When the performances of the ultrasonic irradiation and the mechanical stirring are compared, it is observed that the ultrasonic method is more efficient leading to a higher biodiesel conversion. The main reason beyond this performance is that ultrasonic irradiation is exerted in a more continuous fashion compared to the mechanical stirring which provides necessary energetic environment for the transesterification [122]. During the ultrasound irradiation, sound waves create pressure gradient in the fluid by expansion and compression. If the ultrasound power is amplified, the cavitation effect is observed causing periodic growth and explosion of micro bubbles, micro jets and shockwaves in the fluid. These processes occur adiabatically increasing local temperature and pressure and hence increase the heat and mass transfer rates [123,124]. By forming micro turbulence by radial motion (while the cavitation bubbles enhance emulsification of immiscible phases) free radicals formed by the periodic explosion of bubbles which increase the reaction rates [125]. In order to maximize the physical effect of ultrasonic radiation, the optimization of ultrasonic parameters (amplitude, cycle and pulse) is critical to achieve the highest conversions and fast reaction kinetics. For instance, during enzyme catalyzed esterification it was seen that the forward reaction rate constants are affected significantly by the amplitude. Ultrasonic amplitudes close to 200 W were seen to lead to enzyme–TG complex having an optimum binding strength, therefore can be reliably applied [126]. After this enzyme-TG complex release the first ester product, this step is repeated until the free enzyme and glycerine product are formed. The final reaction step involving the intermediate enzyme complex and free enzyme is shown in Eq. (7) [127–129].

optimum values were determined to be as (6:1) methanol to oil ratio, 8 wt% catalyst loading, 30 min reaction time and 55 °C reaction temperature [109]. The efficiency of the hydrodynamic cavitation reactor can be determined by a dimensionless cavitation number as given by Eq. (6), which gives a quantitative measure of the cavitational intensity [115].

Cv =

Pf − Pv 1 ρU 2 2

(6)

In Eq. (6), Pf (Pa) is the downstream pressure after the orifice and Pv (Pa) is the vapor pressure of the reaction mixture, ρ (kg/m3) is the density of reaction mixture and U (m/s) is the velocity of the stream through the orifice which can be calculated by the diameter of the orifice. Cavitation number, Cv drops by an increase in the inlet pressure, because the increase in the pressure drop increases the reaction mixture flow rate and hence the velocity through the orifice plate. Hence, a lower cavitation number means longer conversion times for fatty acid molecules in localized cavitation zones, low mass transfer resistance between the oil/alcohol phases and shorter residence times to reach desired FA conversions [107,109]. From the point of a pseudo first order transesterification kinetics the hydrodynamic cavitation was observed to have up to 7-fold higher rate constants compared to the mechanical stirring in the temperature range of 50–60 °C. Of course, this is due to the micro turbulence caused by expansion, compression of cavities at high local temperature and pressure enhancing the reaction rate and mass transfer rate exponentially. This is a situation contrast to lower mass transfer area limited by the propeller during mechanical stirring [110–112]. The fuel quality from the hydrodynamic cavitation reactor also meets the standards of diesel such that it has lower volatile compounds, it is cleaner and safer than the regular diesel fuel [110,116]. Since the hydrodynamic cavity has also some limitations, it can also be combined with other technological advancements like the ultrasonic irradiation keeping in mind that each of these techniques possesses some superior advantages. When the hydrodynamic cavitation and ultrasonic techniques are compared, the hydrodynamic cavitation seems to be superior to the ultrasonic technique in terms of scaling up and high bubble density, reaction time and yields. On the other hand, the ultrasonic technique creates higher temperature and pressure environment that is necessary for kinetics and ester yield [115,117]. A hybrid hydrodynamic acoustic cavitation reaction system can be constructed with the installation of ultrasonic generator in the reactor tank after the inlet line including the orifice plate. In these hybrid systems, for instance in the case of rapeseed oil transesterification, complying with European standards, very high yields (∼97%) in the time scale of seconds can be achieved which is an European record. Although two different technologies are combined, the orifice geometry is crucial in the ester yield because without an optimum orifice plate, percent yield drops approximately by 20%. In order to visualize the positive effect of the ultrasound technology, low catalyst concentrations and low ultrasound amplitudes are necessary. The reason is that at high ultrasound amplitudes and temperatures, formation of fine micro emulsions of alcohol/oil/catalyst multiphase mixture after transesterification occurs which needs more time for the separation of ester and glycerine. By lowering the temperature after the reaction, separation time can be reduced. At high temperatures (≥45 °C), the negative effect of high ultrasound radiation can be reversed by the determination of optimum alcohol to oil ratio. For instance, it is 4 to 1 in the case of rapeseed oil transesterification at 45 °C and 125 μm amplitude of irradiation wave [117].

Kf1

Kf2

E′ + M ⟷ [E′M ] → E + G Kb1

(7)

It was calculated that the backward rate constant (kb1) from the intermediate complex was negative under ultrasonic irradiation which meant that the backward reaction from the intermediate was slowed down. During ultrasonic irradiation, not just only one parameter may affect the transesterification performance; but also, both the individual ultrasonic parameters and the interaction of these parameters may have different significance during the biodiesel production. For instance, during waste lard transesterification, by the use Tukey’s honest significance (honestly significant difference) test it was find out that although manipulation of ultrasonic cycle rate (cycle/s−1) does not have a significant effect on the biodiesel production, amplitude of radiation and the interaction between amplitude and cycle rate may affect the transesterification dramatically [126]. Other than the cycle rate (or frequency) and amplitude, the duration of pulsing, duty cycle, and the duration of cycle are also very important and should be optimized for high biodiesel yield and energy efficiency [130,131]. Although continuous ultrasonic irradiation enhances the emulsification of alcohol and oil phases dramatically, energy efficiency drops and corrosion problems arise by the time on the horn tip of the sonicator [132]. On the other hand, due to the necessary interrupted energy supply to the reactor, pulsing mode increases the energy efficiency in addition to the mass transfer even in heterogeneous catalytic systems [133,134]. As seen in Table 5, up to 50% energy saving is possible by changing the

3.4. Ultrasound technology and ultrasonic parameters The application of ultrasound in biodiesel production has been 37

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Table 5 Biodiesel yield, energy consumption and savings for different pulsing modes (solvent: methanol) [131]. On/off time ratio

Total on time during reaction (s)

Total off time during reaction(s)

Power (W)

Consumed energy (kJ)

Saved energy (kJ)

Duty cycle (%)

% Energy saving

% Biodiesel yield

3/3 3/2 5/3 7/3 5/2 3/1 9/3 7/2 9/2 5/1 7/1 9/1

45 54 57 63 65 68 69 70 74 75 79 81

45 36 33 27 25 22 21 20 16 15 11 9

150 150 150 150 150 150 150 150 150 150 150 150

6.75 8.10 8.55 9.45 9.75 10.20 10.35 10.50 11.10 11.25 11.85 12.15

6.75 5.40 4.95 4.05 3.75 3.30 3.15 3.00 2.40 2.25 1.65 1.35

50 60 63 70 71 75 75 78 82 83 88 90

50.0 40.0 36.7 30.0 27.8 24.4 23.3 22.2 17.8 16.7 12.2 10.0

94 96 95 97 95 95 93 98 93 95 94 92

4. Conclusions

duty cycle and cycle time. These two parameters may have different significance on the biodiesel production. For instance, by changing the duty cycles (or pulsing mode) without keeping the cycle time constant, it was observed that after a certain value of the duty cycle the biodiesel yield drops. Whereas, by keeping the duty cycle constant, manipulation of the cycle time significantly affects the biodiesel yields. This result indicates that selected cycle time (as seen in Table 5) is more effective on the biodiesel production than the duty cycle during irradiation and should be optimized. Pulsing mode can be adjusted by changing the on/ off period in one cycle or the duty cycle. Percent duty cycle is defined in Eq. (8) [131]. Table 5 also shows that there is a tradeoff between the high biodiesel yield and energy saving at different pulse modes [131].

Duty cycle (%) =

time on (s) × 100 total time(on + off) (s)

All the advanced techniques discussed in this article have many advantages but should be applied with caution taking into account of their limitations, effective parameters and critical conditions for which negative effects are accompanied. In the case of supercritical fluids, continuous mode is preferred since the heating and cooling periods after the discharge of the product lead to high energy consumption throughout the biodiesel production. During the continuous production, in order to shift the high temperatures to milder ranges and change the phase behavior of the reaction mixture, co-solvents should be used. Although, in general, smaller molecule size of supercritical alcohol (e.g., methanol) means high conversions and low residence times, they should be replaced by nontoxic, higher-carbon-content alcohols. Even bulky structures can be selected for supercritical transesterification since their polarity and steric effect (which is strongly affected by the reaction temperature) could dramatically change their activities and could make them superior with respect to lower chain alcohols. In the case of microwave heating, the excessive temperature rises caused by irradiation create some local hot spots and non-uniform temperature profile in the reactor. In order to control the reactor temperature during irradiation, microwave heating can be combined by cooling or should be carefully monitored. More advanced temperature programmed heating procedures should be studied for the effectiveness of microwave technique. Other than temperature control, the duration of microwave heating is another issue since longer residence times during extraction lead to the removal of water from plant tissue and cause saponification. Therefore, if the product composition can be continuously monitored by spectroscopic techniques, point of water extraction can be avoided. The alcohol ratio should also be optimized since high polarity of alcohol may block the absorption of microwave radiation by plant source during extraction and therefore drops the extraction efficiency. Scaling up issues about microwave assisted transesterification can be solved by using direct application of microwave into the reaction mixture. This could be done by coupling rods. However, these instruments should be carefully located to avoid interference of microwaves at different phases. Mere application of microwave is not sufficient to increase the biodiesel yield. Additionally, tuning of the microwave pulse mode during heating is also very important. Uniform power pulses during both the heating and holding periods independent of the duty cycle generally give higher ester yields. In the case of hydrodynamic cavitation, pressures beyond 3.5 bar is not effective because of the coalescence of cavities. The occurrence of the same phenomenon can be observed due to the uneven plate geometry. This can be prevented by the optimization of distribution of holes and hole spacing. It was observed that when the hole spacing is too small or when large hole diameters are used, the cavitational effect

(8)

The duty cycle can also be used to determine the dissipated energy. During ON period, the effective irradiation intensity or flux can be calculated by Eq. (9) which is based on the horn tip diameter or the reactor diameter (d) [133,134]. For instance, based on the reactor diameter, the optimum ultrasound intensity is close to 8.64 W/cm2 for the biodiesel production from the used vegetable oil [134].

input power kW I⎛ 2 ⎞ = d2 ⎝ cm ⎠ π 4

( )

(9)

The intensity could also be based on the volume of the reaction mixture (for instance, the studied ultrasound radiation densities were: 13.84 W/mL for ethanol, 15.40 W/mL for methanol, and 14.58 W/mL for ethanol–methanol mixture (50 wt% methanol, 50 wt% ethanol) during waste cooking oil esterification) [131]. Like the duty cycle and cycle time, the intensity (or amplitude) of irradiation should also be optimized, because if certain threshold is exceeded, due to the excessive irradiation intensity high temperatures arise. Rapid temperature increase may cause the evaporation of alcohol phase and the occurrence of side reactions which would decrease the transesterification efficiency [135,136]. Other than the evaporation of alcohol, high temperatures also weaken the cavitation effect by super saturation of the cavitation bubbles. Alcohol vapor decreases the collapse of bubbles and hence the mass transfer. In the literature, different studies suggest different temperature limits ranging between 40 and 60 °C for ultrasonically assisted transesterification [132,137,138]. The intensity of ultrasonics is especially critical in enzymatic transesterification since higher intensities cause higher acoustic pressures which can damage the structure of the enzyme. Therefore, suitable conditions for frequency (i.e. lower than 100 kHz) and duty cycles (i.e. 50%) should be selected for to achieve a high biodiesel yield [14].

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drops. Based on the previous studies, hole to pipe diameter ratio could be taken as 20 as an optimum value. Like microwave technique the pulsing mode is also important during the ultrasonically assisted transesterification. For instance, the amplitude threshold for alcohol evaporation should not be exceeded or suitable amplitude or frequencies should be selected for enzymatic processes not to damage the molecular structure of the enzyme. In general, the amplitude should be controlled to keep the reaction temperature below 60 °C. It was observed that not only the ultrasonic amplitude but also the interaction of amplitude and cycle rate (or frequency) play a key role to the high biodiesel yields. It was also observed that cycle time is more important than the duty cycle. Therefore, complex relationships between two or three parameters should be carefully studied for the advancement of ultrasonic method. We believe that all these research and development efforts for biodiesel production will lay a strong foundation for the expansion of biodiesel consumption for the countries directing their plant-based resources for renewable fuels. If the advancements in biodiesel production keep its pace, in the future biodiesel will overcome yield, quality and production rate issues smoothly and fossil fuels will have to face more stronger competition with second generation biofuels.

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