Intensification Approaches for Biodiesel Synthesis from Waste

Review pubs.acs.org/IECR

Intensification Approaches for Biodiesel Synthesis from Waste Cooking Oil: A Review Ganesh L. Maddikeri, Aniruddha B. Pandit, and Parag R. Gogate* Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 40019, India ABSTRACT: The use of biodiesel as an alternative fuel has become more attractive recently because of its environmental benefits such as nontoxicity and biodegradability. However, due to the unfavorable economics and other problems for design and operation of large scale reactors, commercialization of biodiesel has not been significantly effective. The specific challenges in the synthesis route based on transesterification include higher separation times, high operating cost, high energy consumption, and low production efficiency due to equilibrium limitations. The present work highlights the potential use of waste cooking oil as a cheap and economical feedstock discussing the advantages of the process and limitations for transesterification reaction. Improvements in the synthesis process based on the different pretreatment methods and process intensifying techniques are discussed with specific reference to transesterification of waste cooking oil. Different physical and chemical pretreatment methods required for the preparation of feedstock include filtration, drying, acidic esterification, adsorption, crystallization, and distillation for the removal of fatty acids and other contaminants. The critical review also highlights the different process intensification techniques such as cavitational reactors, microwave irradiation, microchannel reactor, oscillatory flow reactor, use of cosolvent, and supercritical transesterification process that can be used for biodiesel production process with an objective of enhancing the reaction rate, reduction in the molar ratio of alcohol to oil, and energy input by intensifying the transport processes and overcoming the equilibrium limitations. Guidelines for the selection of optimum operating parameters have also been given with comparative analysis of the different approaches of process intensification. Finally, some recommendations have been made for the possible research that needs to be done for successful commercialization of biodiesel synthesis.

1. INTRODUCTION Increases in population and industrialization have given rise to a significant increase in worldwide fuel consumption, also resulting in significant air pollution which is one of the most serious environmental problems all over the world.1 In addition to the significant air pollution from the coal-fired power stations, heavy vehicles based on diesel engines are also known to release huge quantum of air contaminants such as NOx, SOx, CO, particulate matter, and volatile organic compounds.2 Also, experts suggest that current oil and gas reserves would suffice to last only a few more decades. So, there is a need for an alternative clean fuel for diesel engines to meet the rising energy demand and also to reduce environmental pollution. One of the alternative and viable options for compressionignition engines is use of biodiesel fuel. Biodiesel is a renewable diesel fuel composed of fatty acid alkyl ester that can be synthesized by transesterification, i.e. chemically combining any natural oil or fat with an alcohol. Many vegetable oils, animal fats, and recycled cooking greases can be transformed into biodiesel. Biodiesel has several advantages such as biodegradability, nontoxicity, lower harmful emissions, high flash point, excellent lubricity, and superior cetane number, and can be derived from agricultural surplus, which can also help to improve rural economies.3 Moreover, it is essentially free of sulfur, and engines fueled by biodiesel emit significantly less particulate matter, unburnt hydrocarbons, and carbon monoxide as compared to engines operating on conventional fossil fuels.4 Biodiesel can be used neat or as a diesel additive, but is typically used as a fuel additive to petroleum diesel in compression ignition (diesel) engines because of the differences in the higher heating values (HHVs). The HHVs of biodiesel © 2012 American Chemical Society

(39−41 MJ/kg) are lower than those of gasoline (46 MJ/kg), petro-diesel (43 MJ/kg), or petroleum (42 MJ/kg), but higher than those of coal (32−37 MJ/kg). However, it is estimated that the cost of biodiesel is approximately 1.5−2 times higher than that of diesel fuel. The high price of biodiesel is due to the higher costs of the feed stock and associated technical challenges related to biodiesel production using the generally preferred transesterification route. Use of waste cooking oil, which is much less expensive than pure vegetable oil, as a promising alternative feed stock and the recovery of highquality glycerol as a byproduct can be the primary options that can be considered for favorable economics.5,6 The properties of biodiesel depend on the physicochemical properties of feedstock and the production method. 7 Physicochemical properties of waste cooking oil are dependent on the extent of usage of vegetable oil. Pretreatment of waste vegetable oil is important and essentially required before it is used in transesterification process because the free fatty acid in waste vegetable oil can react with the alkaline catalyst resulting in soap formation. The soap formation during the transesterification reaction prevents the glycerol separation, drastically reducing the ester yield, and can also create operational problems in the reactor/separation units. It has been reported that transesterification is not feasible if the free fatty acid content in the oil is more than 1−2%.8,9 Different Received: Revised: Accepted: Published: 14610

June 25, 2012 October 4, 2012 October 16, 2012 October 16, 2012 dx.doi.org/10.1021/ie301675j | Ind. Eng. Chem. Res. 2012, 51, 14610−14628

Industrial & Engineering Chemistry Research

Review

Figure 1. Graphical representation of financial contribution of the major parameters in production of biodiesel.12

around 7 billion, so around 29 million tons of waste cooking oil is generated per year. Disposal of waste cooking oil creates a significant challenge because of dumping problems and possible pollution of the water and land resources. Previously, waste edible oil was used as an ingredient in animal feed. However, since 2002, the European Union (EU) has banned the use of this oil in fodder making,7 which is likely to be extended worldwide. The raw material cost typically contributes about 70−80% of the total cost of production for biodiesel. A limit of 75% for the cost of raw materials has also been reported12,13 (Figure 1). Waste cooking oil, which is much less expensive than virgin refined vegetable oil, can be used as a possible feedstock. Annually, a total of more than 15 million tons of waste cooking oil is generated from selected countries in the world as shown in Table 1.14

methods that can be used for the pretreatment of waste vegetable oil include esterification, crystallization, adsorption, and distillation, mainly for the treatment or removal of free fatty acids. Today, the majority of biodiesel is produced by transesterification (also called alcoholysis) of vegetable oils in batch reactors (such as stirred tanks) in the presence of alkali, acid, or enzyme as catalyst or under supercritical conditions to form esters and glycerol. These production processes may take 2−24 h to obtain very high oil conversion and fatty acid methyl ester yield. Although it takes only a couple of minutes for the transesterification reaction to be completed under supercritical conditions, high temperature (>300 °C) and high pressure (>40 MPa) have to be used.10 There are some technical challenges related to biodiesel production via transesterification such as (a) lower rates of synthesis typically attributed to mass transfer limitations due to heterogeneous nature of the reaction system, and (b) requirement of high molar ratio (alcohol to oil) as transesterification itself is a reversible reaction. Both these facts result in high operating cost and energy consumption and hence low production efficiency for the biodiesel production. To overcome these problems, different process intensifying approaches can be used to improve the mixing, mass and heat transfer between two liquid phases in transesterification reaction. The objective of the present work is to give an overview of the pretreatment processes for treating waste cooking oil to reduce the free fatty acid content with an objective of making it a suitable feedstock for transesterification reaction. Also, a review of different process intensification approaches which can reduce the production costs of biodiesel synthesis by enhancing heat, mass, and momentum transfer in the physical process is presented.

Table 1. Quantity of Waste Cooking Oil Produced in Selected Countries14 country

quantity (million tons/year)

China Malaysia United States Taiwan Europe UK Canada Japan Ireland

4.5 0.5 10.0 0.07 0.7−10 0.2 0.12 0.45−0.57 0.153

India is a leading consumer of edible oil and the consumption during 2009−2010 was 16.7 million tons.15 It is reported that nearly 10% of edible oil is being thrown out as waste as it cannot be reused.16 If we assume that 10% of the edible oil is consumed in hotels and restaurants, then the total waste cooking oil available in 2009−2010 would be about 0.167 million tons. Assuming that with adequate incentives, 70% of the waste cooking oil could be recovered, the available feed stock could be approximately 0.12 million tons. If India plans a 10% mix in the total diesel consumption (60.14 million tons during 2009−2010),17 6 million tons of biodiesel will be

2. WASTE COOKING OIL AS POTENTIAL FEEDSTOCK FOR BIODIESEL PRODUCTION Waste cooking oil mainly consists of animal and/or vegetable matter that has been used in cooking or preparation of foods. Most of this oil is used for deep-frying process, after which it is no longer suitable for human consumption. Patil et al.11reported that 4.1 kg (9 pound) of waste cooking oil is generated per person per year. Today's world population is 14611

dx.doi.org/10.1021/ie301675j | Ind. Eng. Chem. Res. 2012, 51, 14610−14628


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unsaturated aldehydes, ketones, hydrocarbons, lactones, alcohols, acids, and esters. In the case of hydrolytic reaction, the steam produced during the processing of food containing water causes the hydrolysis of triglycerides, resulting in the formation of free fatty acid, glycerol, monoglycerides, and diglycerides.20 High free fatty acid level, i.e. quantum of free fatty acids greater than 1 mg/g KOH, results in generation of high amounts of undesirable soap simultaneously with transestrification reaction. In the alkali-catalyzed process, the presence of free fatty acid (greater than 1 mg/g KOH) and water in the oil can cause high amounts of undesirable soap formation, also consuming some quantity of alkaline catalyst and reduces the effectiveness, all of which result in a lower conversion. Therefore, for the feedstock with high free fatty acids, it is essential to have a pretreatment stage before subjecting it to transesterification process.21

required. With the available quantity of used cooking oil, nearly about 0.114 million tons of biodiesel can be generated. In India, nearly 1.9% of diesel oil can be replaced by using the biodiesel generated from used cooking oil. In India during 2009−2010, biodiesel production was 1 million tons.18 The available used cooking oil thus can form a useful contributing feedstock for the current biodiesel production. All these data indicate that the used cooking oil can be a potential source for biodiesel production. Waste oils, such as used frying oil, trap grease, and soap stock (byproduct of vegetable oil refineries), which are available cheaply, can also be considered as a sustainable feedstock for the biodiesel synthesis.

3. QUALITY OF WASTE COOKING OIL AND IMPORTANCE OF PRETREATMENT Waste cooking oil is generated from the process of oil frying, i.e. oil is heated in air in the presence of light at a temperature of 160−200 °C for relatively long period of time, beyond which it cannot be used for edible purposes and needs to be disposed. Frying under such conditions results in major physical and chemical changes in the oil altering the physiochemical properties as compared to the virgin oil (Table 2).19 The

4. PRETREATMENT PROCESS FOR WASTE COOKING OIL Pretreatment of waste cooking oil consists of physical treatment for removing the suspended solid contaminants and chemical treatment processes mainly for deacidification. A flow sheet giving the different approaches for pretreatment of waste cooking oil is shown in Figure 2. Issariyakul et al.22 have reported the treatment of waste cooking oil using centrifugation for the removal of solid portions of the oil. Water was removed by mixing used cooking oil (UCO) with 10 wt % silica gel (28− 200 mesh) followed by stirring and vacuum filtration. Dias et al.23 filtered UCO under vacuum after dehydration overnight using anhydrous sulfate, and finally filtered it again under vacuum prior to transesterification. Esterification produces either methyl ester or triglyceride depending on the alcohol used for esterification. If esterification of free fatty acid is carried out with methanol in the presence of acid catalyst, methyl esters are obtained. Similarly, conversion of free fatty acids carried out using glycerol at high temperature (240−280 °C) with or without esterification catalysts (in an inert atmosphere to oppose oxidation) gives mono-, di-, or triglycerides.24 Free fatty acid is removed from waste vegetable oil as soap by treating it with alkali such as KOH or NaOH. Meher et al.25 reduced the acid value to 0.6 by neutralization of free fatty acid content from Karanja oil with alkali. Similarly, Cvengros et al.20 followed a stepwise procedure for the pretreatment of used cooking oil. Free fatty acid was removed by neutralization using alkali (KOH or NaOH) as soaps while high polymer content was treated with activated carbon and removed by adsorption. Adsorption processes for the separation are gaining a wider importance as an efficient and low energy operation for the removal of free fatty acid from waste vegetable oil. Toeneboehn et al.26 used silica hydrogel for the removal of sulfur-containing compounds from fatty materials. Removal of free fatty acids and moisture was carried out by heating the oil under reduced pressure followed by a two stage adsorption with basic-treated activated carbon and a hydrophilic adsorbent such as activated alumina and silica gel, respectively.27 Lee et al.28 used column chromatography containing 50% magnesium silicate and 50% aluminum oxide (basic) to reduce the free fatty acids and water content from waste cooking oil from 10.6 to 0.23 wt % and from 0.2% to 0.02 wt %, respectively. Separation of fatty acids can also be achieved based on the difference in their solidification points. At low temperature, fats and oils that solidify can settle at the bottom of the equipment.

Table 2. Physical and Chemical Properties of Used Frying Oil and Neat Canola Oil19 property

used frying oil

neat canola oil

acid value (mg KOH/g) kinematic viscosity at 40 ° (cSt) fatty acid composition (wt %) myristic (C14:0) palmitic (C16:0) palmitoleic (C16:1) stearic (C18:0) oleic (C18:1) linoleic (C18:2) linolenic (C18:3) arachidic (C20:0) eicosenic (C20:1) behenic (C22:0) erucic (C22:1) tetracosanic (C24:0) mean molecular weight (g/mol)

2.1 35.3

98%) in short reaction times (30 min) even if they are applied at low concentrations (0.5 mol %). Many researchers have succeeded in speeding up the reaction rate by carrying out homogeneous base-catalyzed methanolysis of waste cooking oil in the presence of a low-frequency direct sonication reactor. For instance, Refaat et al.41 studied base-catalyzed transesterification of waste cooking oils at two catalyst loadings (0.5% and 1.0% KOH wt/wt), two reaction temperatures (25 and 65 °C), and three alcohol-to-oil molar ratios (3:1, 6:1, and 9:1) using low-frequency ultrasound (20 kHz) and compared the results with the conventional classical approach. It has been reported that the best yield was obtained using a methanol/oil molar

Table 4. Biodiesel Production by Transesterification Reaction from Waste Cooking Oil Using Ultrasound Methoda reaction condition sr no.

a

alcohol (MR)

T (°C)

power (watt)

MR

performance

ref

60

24

200

65 mL for fatty acid weight of 5g

10

60

conversion = 97%

46

methanol (4:1−7:1)

35−55

25

150−250

6:1

1

45

43

3

methanol (6:1)

KOH(0.5−1.5)

30−70

20

400

6:1

0.75

30

4 5 6

methanol (3:1−9:1) methanol (∼4.6:1) propanol

KOH(0.5−1.0) NaOH(1.0−2.0) Novozyme 435

25−65 60 40−45

20 24 28

100 200 100

6:1 ∼4.6:1 3:1

1 2

65 60

conversion = 89.62% conversion = 98% yield = 90 yield = 98−99 yield = 95 conversion = 94.86

methanol (65 mL for fatty acid weight of 5g)

2

CL

T (°C)

Mg/MCM-41, Mg−Al hydrotalcite, mesoporous K/ZrO2 (10) KOH(0.5−1.25)

1

catalyst (CL in %)

optimized condition freq. (kHz)

42 41 95 47

MR = Molar ratio of alcohol to oil. CL = catalyst loading. T = temperature. freq. = frequency of ultrasound. 14616



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operating pressure (2 and 3 bar). The increase in the rate of reaction may occur due to an increase in cavitation effect when operating pressure is increased from 1 to 2 bar and beyond that there is no significant improvement in the cavitation effect with operating pressure or when the condition of chocked cavitation occurs. The observed results can also be attributed to the fact that due to the complete elimination of mass transfer resistance at higher operating pressure, inherent reaction kinetics becomes rate controlling. At higher pressure, as the transesterification reaction proceeds, the viscosity of reaction mixture reduces rapidly. As viscosity reduces the flow rate increases. The increase in flow rate results in decreasing cavitation number. Up to a certain value of increasing flow rate, there is a considerable decrease in cavitation number, beyond that there is only a marginal decrease in the cavitation number (only a marginal increase in the extent of cavitation will be observed). Therefore, there may be no significant improvement in cavitation effects beyond a certain high value of flow rate. It has been also reported that in the case of the plates having the same flow areas (78.58 mm2), the conversion for the plate 1 (25 holes of 2-mm diameter) which has a larger value of α (ratio of total perimeter of holes to total area of opening) is more as compared to plate 2 (1 hole of 10-mm diameter) which has the lower value of α at constant (1.5 bar) pressure. A higher value of α (by increasing number and minimizing diameter of holes) increases the cavity generating spots (i.e., shear layer area) and frequency of turbulence (i.e., energy released per cavity) resulting in reduced mass transfer resistance due to the generation of better degree of emulsification. In the case of plates having the same value of β, i.e. ratio of diameter of hole to pipe diameter, the conversion is higher for plate 4 (20 holes of 3-mm diameter) having more holes as compared to plate 3 (16 holes of 3-mm diameter) having fewer holes at constant inlet pressure. This is because as number of holes increases, total perimeter increases resulting in larger shear area being available in the system which leads to better conversion due to higher cavitational effects. Overall, it has been reported that a hydrodynamic cavitation reactor gives more than 95% conversion in 10 min of operation time whereas the conventional stirred reactor gives only 60% of conversion in 60 min of operation. Thus, the microlevel turbulence of high intensity generated by cavitating bubbles in hydrodynamic cavitation reactor is very effective in eliminating the mass transfer resistance of the reaction. The oscillation of cavitation bubbles provides more interfacial area for mass transfer and their sudden collapse generates the high intensity turbulence in flowing liquid, enhancing rates of transport processes and hence higher conversion is obtained as compared to the conventional stirred reactor. Since the energy required for biodiesel production represents a significant part of the overall energy input, the use of ultrasonic reactors with better performance than currently employed reactors can reduce the production cost, enhancing the chances for large-scale commercialization of biodiesel production.49 Sonochemical reactor design depends on the objective of obtaining maximum and uniform spatial distribution of cavitational activity with possibly continuous mode of operation. Typically, on a larger scale of operation, it will be a requirement that multiple transducers will have to be used considering the local nature of cavitational events. Typically, flow cell types of arrangements are more suitable for large scale operation as this definitely gives flexibility in terms of the continuous operation and also gives an option of arranging the

fatty acids, such as deep-frying oils from restaurants and food processing. Biodiesel synthesis using solid catalysts could potentially lead to cheaper production because of the reuse of the heterogeneous catalyst,45 which can be easily separated. Georgogianni et al.46 carried out transesterification reaction of soybean frying oil with methanol, in the presence of different heterogeneous catalysts (Mg MCM-41, Mg−Al hydrotalcite, and K+ impregnated zirconia), using low frequency ultrasonication (24 kHz) and mechanical stirring (600 rpm) for the production of biodiesel. It has been reported that the Mg−Al hydrotalcite catalyst showed particularly the highest activity (conversion 97%) and the catalyst activity of ZrO2 in the transesterification reaction increased as the catalyst was enriched with more potassium cations. Use of ultrasonication significantly accelerated the transesterification reaction with a treatment time of 5 h as compared to the reaction time of 24 h in the case of mechanical stirring. Very few research studies are available on the use of ultrasound irradiation in enzyme-catalyzed biodiesel production. Only Novozym 435, a commercial lipase preparation from Candida Antarctica, has been tested in the studies on ultrasound-assisted transesterification reactions of soybean and high acid value waste oil with methanol and propanol, respectively. Wang et al.47 optimized ultrasound-assisted enzyme-catalyzed biodiesel production from high acid value waste oil and 1-propanol by esterification and transesterification with respect to the amount of lipase, propanol-to-oil molar ratio, and frequency and power of ultrasound. Immobilized lipase Novozym 435 was utilized as a biocatalyst. Under optimal conditions, such as 8% loading of Novozym 435, the propanol-to-oil molar ratio of 3:1, the frequency and power of ultrasound of 28 kHz and 100 W, and the temperature of 40− 45 °C, the conversion of 94.86% to propyl oleate was achieved in 50 min. Ultrasound tends to reduce the adsorption of biodiesel and glycerol on the surface of immobilized Novozym 435. As a result, Novozym 435 can be recycled to use with clean appearances, good decentralizations, no agglomeration, easy washing, and higher operational stability. Also, it was noticed that short-chain linear and branched alcohols (C1−C5) showed high conversion to fatty acid alkyl ester. Ghayal et al.48 studied alkali-catalyzed transesterification of waste cooking oil for biodiesel production using hydrodynamic cavitation. They investigated the effect of operating pressure (1, 1.5, 2, 3 bar) and flow geometry (such as hole size, number of holes, and total perimeter of holes on a cavitating orifice plate) of orifice plate on the rate of reaction. It has been reported that the rate of transesterification reaction increases with an increase in the operating pressure and results in a reduction in the reaction time. With an increase in the operating pressure, the flow rate through orifice plate increases. As the amount of liquid passing per unit time through the orifice increases, the number of passes of liquid through cavitating zone increases, due to which liquid experiences the cavitating zone for a longer time and hence results in better conversion. Also, increase in the upstream pressure leads to an increase in the pressure drop across the orifice plate, due to which the collapse intensity of cavity increases. Thus, there is an increase in the magnitude of pressure generated due to cavitation which assists the mass transfer between two immiscible phases of the reaction mixture resulting in higher extents of conversion. It is seen that at lower upstream pressure (1 and 1.5 bar) there is considerable difference in the reaction rate for all geometric configurations but there is no significant difference in reaction rate at higher 14617



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consumption calculations suggest that the continuous-flow microwave methodology for the transesterification reaction is more energy-efficient than using a conventional heated apparatus. Quantitatively, the microwave process has an energy requirement of 26.0 kJ/L of biodiesel as compared to 94.2 kJ/L required for the conventional mode of heating. Similarly, Lertsathapornsuk et al.37 investigated microwave (modified 800-W kitchen microwave oven) assisted continuous biodiesel production (Figure 6) from waste frying palm oil and also

transducers on the wall of the reactor on opposite faces so that standing wave patterns can be generated. Typically, hexagonal or rectangular cross sections have been reported to yield excellent distribution of the cavitational activity and are recommended for large-scale operation.39 The number and location of the transducers also affects the hydrodynamic behavior and the mixing characteristics in the reactor which can be of prime importance especially for biodiesel synthesis, which is limited by mass transfer. While maintaining the geometric similarity for the design of large-scale sonochemical reactors, it is also important to maintain similar conditions of hydrodynamics and mixing characteristics in the reactor so as to achieve the desired objectives. 6.2. Microwave Reactors. In the case of microwaveassisted transesterification, use of microwave irradiation helps in transferring the energy directly to the reactants and accelerates the rate of chemical reactions, reducing the reaction time from hours to minutes or even a few seconds in some cases. Microwaves are basically electromagnetic waves propagating between 300 MHz and 300 GHz containing electric and magnetic components and acting perpendicularly to each other and also to the direction of propagation. In microwave, the electric component is responsible for heating. Microwaves are between radio waves and infrared radiations with wavelengths in the range of about 1 mm to 1 m.50 Heat transfer in the case of conventional heating occurs via conduction and radiation from the source and via conduction and convection within the material. This has significant drawbacks due to the dependency on the thermal conductivity of materials, specific heat, and density.51 Apart from that, conventional heating is rather slow and heat is not distributed uniformly in a reaction vessel, resulting in a higher energy (than the theoretical value) requirement for a particular chemical reaction.52 Microwaves can interact directly with a sample matrix by two mechanisms, viz. dipolar rotation and ionic conduction. When exposed to microwave frequencies, dipoles in the sample align themselves in the direction of applied electric field. As the electric field rapidly oscillates, the dipole tries to rapidly realign to the electric field via rotation. Heat is generated by the frictional forces between the randomly rotating polar molecules and the surrounding media. In ionic conduction, the dissolved charged particles oscillate back and forth under the influence of applied microwave field. When the electric field direction changes, the larger ions dissipate their kinetic energy as heat as they slow down and change direction via friction at molecular levels. Both mechanisms contribute to localized superheating leading to high temperature and pressure gradients.53 In the transesterification process, the mixture contains vegetable oil or fatty acid and alcohol which have polar and ionic components and hence microwave irradiation can play an active role in heating the reactants to the required temperature quickly and efficiently.54 Several reports concerning the usage of microwave irradiation in transesterification are available in recent years and some of them use waste vegetable oil as reaction feed stock. Barnard et al.54 studied the continuous-flow preparation of biodiesel from used vegetable oil using a commercially available multimode microwave apparatus consisting of a continuous microwave power delivery system with an operator-selectable power output from 0 to 1600 W. Experiments were carried out using 2 and 4 L reaction vessels at flow rates of up to 7.2 L/min of methanol and a 1:6 molar ratio of oil/alcohol. Energy

Figure 6. Schematic diagram of continuous microwave reactor.37

evaluated its performance in a 100-kW diesel generator. It has been reported that continuous conversion of waste frying palm oil to ethyl ester was 97% at ethanol to oil molar ratio of 12:1 with catalyst loading of 3.0% NaOH (in ethanol) and 30 s residence time. Also, the energy consumption for microwaveassisted continuous transesterification process was about 269 kJ/L, while the energy consumption for conventional heating process was 799 kJ/L. Engine efficiency of a 100-kW diesel generator powered by biodiesel (B100) (without any modification of the engine) was 0.26 ± 0.03% lower than that powered by conventional diesel, while the specific fuel consumption was 12.73 ± 0.03% higher. Hydrocarbons and carbon monoxide emissions were decreased by 25.11 ± 0.03% and 17.96 ± 0.12%, respectively. Yaakob et al.55 investigated the effect of transesterification reaction parameters such as quantity of the catalyst, reaction temperature, and time on the biodiesel yield and purity from waste frying palm oil using microwave operated at power and frequency of 1250 W and 2.45 GHz, respectively. The obtained results indicate that the maximum yield (88.63%) and purity (99.45%) of biodiesel was observed with the optimized process parameters as reaction temperature of 65 °C, time of 7 min, and catalyst concentration as 1% by weight. In another work,56 ethyl esters were produced from a feed material of esterified crude palm oil (CPO) with 1.7 wt % of free fatty acid content using microwave heating (Sharp model R235 compact microwave oven working at 2.45 GHz with a power of 800 W). It was found that transesterification process using microwave heating resulted in a yield of 85% and its fuel properties were within the limits prescribed by American standards at optimum conditions of molar ratio of oil to ethanol as 1:8.5, 1.5 wt %/wt KOH/oil as a catalyst loading, reaction time of 5 min, and microwave power of 70 W. Koberg et al.57 carried out transesterification of used cooking oil to biodiesel, based on microwave dielectric irradiation as a driving force for the transesterification reaction in the presence of SrO as a catalyst. The transesterification was carried out with and 14618



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production is not easy. The most significant limitation for the 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 not having ready acceptability of microwave reactors in industry. 6.3. Oscillatory Flow Reactors. The oscillatory flow reactor (OFR) is a novel type of continuous flow reactor consisting of a tube containing equally spaced orifice plate baffles where the oscillations are applied in an axial direction within the tube. There is a net flow through the reactor, but superimposed upon this, usually by reciprocating pistons at either end of the reactor, is an oscillatory motion, which is relatively large compared to the net flow. In OFR, mixing due to the oscillating motion of liquid intensifies the process by improving mixing as well as other transport properties, such as heat transfer,60 mass transfer,61 plug flow type residence time distribution,62 and multiphase suspensions.63 When a bulk fluid is introduced into the OFR, radial mixing is enhanced due to the oscillatory motion. The interaction of the orifice plate with the oscillating fluid causes periodic creation and destruction of toroidal vortices above and below each baffle plate. These represent extremely efficient mixing patterns, which also result in significant enhancements in the mass and heat transfer. When there are enough plates in series, the overall residence time distribution is close to that of plug flow. Unlike conventional plug flow reactors in which the plug flow is caused by net flow, the plug flow achieved in an oscillatory flow reactor is due to long residence times because the degree of mixing is not directly dependent upon the Reynolds number of the bulk flow through it, but is mainly related to the oscillatory conditions. This offers the oscillatory flow reactor an important process intensification approach as it allows “long” (e.g., greater than 15 min) reactions to be performed in a plug flow reactor of relatively small length-to-diameter ratio.64 The shear rates in such reactors have been shown to be very uniform and the radial and axial velocities are very similar. Hence the oscillatory flow reactor can be designed with a short length-to-diameter ratio to improve the economy of biodiesel production due to smaller “footprint”, lower capital and pumping costs, and easier control. Harvey et al.65 conducted alkaline transesterification of waste cooking oil and pure rapeseed oil in a continuous oscillatory flow reactor (OFR) to produce biodiesel in a pilot-scale plant. The reaction was performed at a temperature of 20−70 °C, residence time of 10−30 min, and molar ratio of methanol to oil was maintained at 1.5:1. Reaction samples were analyzed for cetane number and glycerides to investigate the effect of temperature and residence time on the conversion. It has been reported that a residence time of 10 min at 50 °C does not produce the required conversion, as unreacted triglyceride remains in the product, and the level of diglyceride is high. It was found that up to 99% conversion of biodiesel was achieved after 30 min at 50 °C. Thus, cetane numbers can be seen to increase with increasing residence time and temperature, attributed to the fact that the degree of conversion increases with these parameters. An in-depth study on OFR in transesterification with heterogeneous catalyst is definitely needed as OFR is ideal for suspending solid catalysts or polymer-supported catalysts. One of the advantages of this technology is the low molar ratio of methanol to oil used in this technology, reducing the operating cost significantly. This can be attributed to reduced backmixing overcoming the

without magnetic stirring. It has been reported that 99.8% conversion of used cooking oil is obtained under MW irradiation of 1100 W output with magnetic stirring in only 10 s. Refaat et al.58 studied the performance of transesterification process carried out by conventional and microwave techniques (using a microwave oven with output microwave power up to 1200 W, controlled via microprocessor) for the production of biodiesel from waste cooking oil and investigated the effects of reaction time, separation time, and yield. They reported that microwave-assisted transesterification showed optimum performance at a temperature of 65 °C, a methanol/oil molar ratio of 6:1, and potassium hydroxide loading as 1%. The application of radio frequency microwave energy decreased the reaction time from 60 to 2 min and separation time from 480 to 30 min and increased yield from 96% to 100% as compared to the conventional technique for biodiesel production. Chen et al.59 have also compared the yield of biodiesel from waste cooking oil using microwave and conventional heating source in the presence of sodium methoxide as catalyst and the important results have been given in Table 5. The maximum Table 5. Comparison of Biodiesel Yields from Waste Cooking Oil under Conventional Heating and Microwave Heating Systems59 method

reaction time (min)

yield (%)

conventional heating

30 45 60 75 90 1 2 3 4 5

86.3 91.5 93.4 94.2 96.6 94.6 95.8 97.9 96.6 92.7

microwave heating

yields of biodiesel from waste cooking oil under conventional heating and microwave heating were 96.6% and 97.5%, respectively. Microwave-assisted heating causes direct absorption of the radiation by the OH group of the reactant which produces a volumetrically distributed heat source within the system. The OH group is directly excited by the microwave radiation, causing the increase in the local temperature around the OH group to be much higher than that of the activation energy needed for the transesterification. This results in microwave heating giving a significantly higher yield of biodiesel in shorter reaction time compared to conventional heating. To summarize, use of microwaves can give significant intensification as compared to the traditional methods for transesterification of waste cooking oil, allowing better yields and conversions into biodiesel in a short time and, consequently, less energy consumption as compared to the conventional heating systems. Also, microwave irradiation requires no stirring and cooling facility and hence also reduces volatilization of alcohol. It is also reported that use of continuous-flow microwave methodology for the transesterification reaction gives a more energy-efficient process than using a conventional heating apparatus. One of the major drawbacks associated with microwave synthesis is that scale-up of the system from laboratory scale operation to industrial scale 14619



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biodiesel typically increases with increasing mean residence time in the microreactor. While the claims of the patents cannot be verified, use of microreactors certainly offers one of the process intensification approaches. Recently, KOHcatalyzed synthesis of biodiesel has been investigated in capillary microreactors, with unrefined rapeseed oil and cottonseed oil as raw materials.70 The influence of the methanol to oil molar ratio, the residence time, the catalyst concentration, the reaction temperature, and the dimension of the capillary (ID 0.25 or 0.53 mm) on the production of biodiesel has been investigated. It has been reported that the yield of methyl esters is 99.4% at the residence time of 5.89 min with KOH concentration of 1% and methanol to oil molar ratio of 6:1 at 60 °C in capillary microreactor with inner diameter of 0.25 mm. The authors also observed that the reaction temperature was not an important factor influencing the yield of methyl ester but the inner diameter of the microchannel reactor had a strong influence on the transesterification reaction because mass transfer area in a microreactor with a smaller dimension is much larger than that with a larger dimension. Thus, it could be concluded that higher methyl ester yield is obtained at even shorter residence time for the microchannel reactor with the smaller inner diameter. Though no study with waste cooking oil was observed, credence to the results can be obtained based on these results with virgin vegetable oil. A microreactor having a single microchannel might be used to make biodiesel at laboratory scale, but increasing output especially at commercial scale, may require using devices having plural microchannels, plural microreactors, or both.69 During scale-up, conventional reactors requires plant designers to increase the size of each reactor unit. This makes scale-up expensive, time-consuming, and sometimes extremely difficult. In contrast, the microchannel reactors can be shop-fabricated, and the microchannel-based plants can be constructed more quickly and easily with guarantees that desired features of basic unit will remain unchanged while increasing the total system capacity. Also, in a microreactor plant, continuous operation is uninterrupted with the replacement of the failed microreactor, while the other parallel units continue production.66 Different types of microstructured mixers with various operating principles and parameters have been designed and successfully applied in microreactor technology for biodiesel production. The modifications are used to decrease diffusion lengths and to increase interfacial area between phases in microchannel. Guari et al.71 studied transesterification of sunflower oil to produce biodiesel using T-type (inner diameter 1 mm; length 160 mm) microstructured mixer. It has been reported that residence time of 112 s is required for complete conversion of sunflower oil to biodiesel. Wen et al.72 used zigzag type microstructured mixer for continuous alkali-based biodiesel production from sunflower oil. This type of reactor was shown to intensify the biodiesel production process by obtaining smaller droplets compared to that of T- or Y-type microstructured mixer. It has been reported that 99.5% yield of biodiesel is obtained in residence time of 25 s using optimized zigzag type microstructured reactor. Sun et al.73 developed a two-step process for fast acidcatalyzed biodiesel production from high acid value oil in a microstructured reactor, which was assembled with a micromixer and connected with a 0.6-mm ID stainless steel capillary. Esterification of oleic acid with methanol was carried out to find suitable reaction conditions. In the first step of esterification, it has been reported that the oleic acid conversion was reduced

equilibrium limitations to a larger extent. The short length-todiameter ratio of this reactor also results in decreased capital costs. The OFR allows these processes to be converted to continuous, thereby intensifying the process. Conversion to continuous processing should improve the economics of the process, as the improved mixing should generate a better product (rendering the downstream separation processes easier), at lower residence time (reduction in reactor volume). These improvements can decrease the price of biodiesel, possibly making it a more realistic competitor to fossil diesel. Skelton et al.64 also studied the development of the oscillatory flow based process for the production of biodiesel from used cooking oil. The synthesis of biodiesel was carried out in a jacketed 25-mm oscillatory flow reactor with a loop length of about 3 m. The processing rate was 3 L per hour at a residence time of 30 min. In the actual process, used commercial cooking oil and methanol are preheated and fed to the loop by metering pumps and temperature is held constant by a hot oil filled jacket. The apparatus can operate at pressures of up to 3 bar g. The product is depressurized and fed to a simple vertical gravity separator with a glass wool coalescence pack. The biodiesel phase then passes to a simple OFR based washing column with settling sections for product and wash water at the top and bottom, respectively. From there, the product passes through a sand-packed coalescer followed by drying in a salt column. To give a reasonable scaleup from the existing apparatus, processing rates between 20 and 30 L/h should be used. While a bigger ratio can be desirable for design purposes it was not considered practicable for operational reasons. Finally, it has been reported that oscillatory flow mixing technology is well suited for the production of biodiesel from used commercial cooking oil. 6.4. Microchannel Reactors. Microreactors and micromixers have gained considerable attention, especially in chemical processes because of their higher transport rates, safer environment for highly exothermic or explosive chemical reactions, compact design, and simpler process control. Microchannel reactors are compact reactors that have channels with diameters in the micro range (typical dimensions of microchannel are in the range of 10−500 μm). Reducing dimensions of the reaction system increases the surface to volume ratio of microreactors (microchannel: 10 000−50 000 m2/m3; laboratory vessel: 1000 m2/m3; and production vessel: 100 m2/m3) which result in high mass and heat transfer.66 Heat transfer coefficients measured in microreactors have been reported to be a maximum of 25 000 W/(m2 K).67 Besides that, microreactors provide excellent mass and heat transfer, shorter residence time, and smaller amounts of reagents, catalyst, and waste products compared to that of macroscale reactors. Also, microreactors are lightweight with compact system design, laminar flow, effective mixing, short molecular diffusion distance, better process control, and small energy consumption.67,68 Jovanovich et al.69 filed a patent on embodiments of methods for using a microreactor to produce biodiesel. Tranesterification can be carried out between oil (which includes soy, inedible tallow and grease, corn, edible tallow and lard, cotton, rapeseed, sunflower, canola, peanut, safflower, and combinations of thereof) and alcohol (typically lower aliphatic which includes methanol, ethanol, amyl alcohol, or combinations thereof) in the presence of catalyst (such as metal oxides, metal hydroxides, metal carbonates, alcoholic metal carbonates, alkoxides, mineral acids, and enzymes). It has been reported that oil conversion to 14620



Review

mixture and allow the reaction to be carried out under milder conditions, enhancing the mutual solubility of the oil−alcohol mixture, reducing the transport limitations, and increasing the reaction rates.79 However, the primary concerns with this method are the additional complexity of recovering and recycling the cosolvent, although this can be simplified by choosing a cosolvent with a boiling point near that of the alcohol being used. Park et al.78 investigated the feasibility of fatty acid methyl ester (FAME) as a cosolvent to increase the mass transfer between soybean oil and methanol. The influence of the initial addition of 0, 5, and 10 wt % of FAME to oil on the FAME production rate was studied. It has been reported that as the amount of FAME added was increased, the time required for the formation of the single-phase system decreased due to the increase in the miscibility of oil in methanol. It has been concluded that introduction of FAME as a cosolvent increases the reaction rate. Thus the production rate of biodiesel can be improved by recycling a portion of FAME as cosolvent into the reaction mixture of transesterification process. Li et al.80 studied the effect of pure organic (which include polar and nonpolar) and cosolvent (mixture of strong and weak hydrophobic solvent) on the yield of biodiesel preparation catalyzed by lipase. It has been reported that polar organic solvents deprive the lipase molecules of their essential water thus deactivating the biocatalyst and making it unable to be reused. In the case of hydrophobic solvents, good dissolution occur between two substrates to accelerate the rate of reaction, but hydrophobic solvents dissolve little methanol, and excessive insoluble methanol deactivates the lipase significantly. It was observed that the biodiesel yields were higher in the presence of cosolvent as compared to other pure organic solvents. This is because cosolvent medium can dissolve both methanol and glycerol to eliminate the negative effects thoroughly. There was practically no loss in the activity of lipase even after being recycled 30 times. Solid acid catalysts are normally used to catalyze the transesterification of oil with high free fatty acid to biodiesel. Lam et al.81 used biodiesel itself as cosolvent for the transesterification reaction of waste cooking oil with methanol catalyzed by SO2− 4 /SnO2SiO2 (solid acid catalyst). It was found that with the use of biodiesel as a cosolvent, a high FAME yield of 88.2% (almost 30% higher than without using cosolvent) can be obtained in a shorter reaction time (1.5 h). In the case of a homogeneous base catalyst, such as KOH, the addition of a cosolvent to the reaction mixture can enhance the reaction rate, due to the promotion of diffusion by homogenization. However, the FAME yields decreased to some extent after the addition of either a liquid cosolvent (THF) or a gas cosolvent (DME) when using a heterogeneous catalyst in the transesterification of waste cooking oil.82 In the presence of heterogeneous catalysts, the glycerol drops formed in the homogeneous reaction mixture readily adhere to the catalyst particles, causing them to agglomerate. As a result at the end of the reaction, the agglomerated catalyst particles adhered to glycerol were found to be deposited on the reactor wall and were found to be deactivated as a result of agglomeration. Similarly, Sabudak et al.83 used tetrahydrofuran (THF) as cosolvent in transesterification reaction of waste frying oil and reported that the highest yield obtained was 90%. From an economic point of view, they conclude that the use of THF in transesterification of biodiesel production is not feasible as

significantly from 99.1 to 10.4% with an increase in the water content from 0.01 to 5 wt % (Table 6), since the presence of Table 6. Effect of Water Content on Oleic Acid Conversiona73 water content (%)

oleic acid conversion (%)

0.01 0.5 1 2 5

99.1 90.3 75.4 55.8 10.4

a

Reactions were carried out with a methanol to oleic acid molar ratio of 30, and a catalyst concentration of 3 wt % at 100 °C in 5 min.

more water leads to a movement of the reaction equilibrium in the reverse direction and loss in acid strength of catalytic protons. The acid value of the oil was reduced from 160 to 1.1 mg KOH/g with methanol to acid molar ratio of 30:1, H2SO4 concentration of 3 wt %, and residence time of 7 min at 100 °C in the first step. The final FAME yield reached 99.5% with a methanol to triglyceride molar ratio of 20:1, H 2 SO 4 concentration of 3 wt %, and residence time of 5 min at 120 °C in the second step. Finally, they concluded that biodiesel production from high acid value oil can be continuously achieved with high yields by acid-catalyzed transesterification in microstructured reactors with total reaction times of less than 15 min. 6.5. Cosolvent. Biodiesel can be produced through transesterification reaction74 in which vegetable oils or animal fats react with short chain alcohol (e.g., methanol). The transesterification reaction can be either catalytic or noncatalytic, but these processes are relatively time-consuming to drive the reaction toward completion. This is because of lower solubility of the reactant (i.e., oil in alcohol) used in transesterification reaction causing mass transfer limitation, especially at the initial phases of the reaction. Cosolvent plays an important role in transesterification process in increasing the yield of biodiesel production. Different cosolvents can be used to accelerate transesterification reaction such as hexane, heptane, ether, cyclohexane, CO2, diethyl ether, toluene, propane, and tetrahydrofuran (THF) which can improve the mass transfer rate between oil and alcohol phase.75−77 Introducing a cosolvent into the transesterification process changes the reaction mixture from a two-phase system to a single-phase system due to an increase in the mutual solubility of alcohol and vegetable oil at low reaction temperatures, based on the concept “like to dissolve like”. The cosolvents form hydrogen bonds with polar compounds (such as alcohols), and also contain sufficient nonpolar hydrocarbon groups to dissolve the high molecular weight oils and fats.78 The mass transfer in the one-phase reaction is superior to that in the two-phase reaction, due to an increase in the contact surface, and hence production of biodiesel can be intensified by the addition of cosolvent into transesterification process. Presence of cosolvent also helps to separate the phases (biodiesel and glycerol) more easily. The separation of the glycerol-rich phase is faster than in the cosolvent-free system. Besides, when hexane is used as a cosolvent, the formation of soap is significantly reduced.77 It was also indicated that the addition of cosolvent could improve both supercritical and subcritical methanol transesterification because the use of cosolvents can decrease the critical point of the reaction 14621



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Scheme 2Hydrolysis (eq 5) and esterification (eq 6) reactions.

take place in the supercritical conditions. Tan et al.86 utilized waste palm cooking oil as the source of triglycerides for biodiesel production using supercritical methanol. The effect of different parameters that influence the reaction, including reaction time (5−25 min), temperature (300−380 °C) and the molar ratio of alcohol to oil (20:1−60:1), were investigated. Results show that 80% yield was obtained under optimum conditions (reaction time 20 min, temperature 350 °C, and molar ratio 40:1) using low-cost oils as a feedstock. It has been reported that the total production costs of biodiesel can be reduced significantly through the supercritical methanol process, provided heat integration is adapted in the process. Demirbas et al.87compared the effects of alkali-catalyzed and supercritical methanol transesterification of waste cooking oil and reported that supercritical transesterification offers great advantage over alkali-catalyzed transesterification to eliminate the pretreatment and operating costs. Conversion of waste cooking oil to biodiesel has been studied by Patil et al.88 using ferric sulfate and supercritical methanol processes. This process resulted in a feedstock to biodiesel conversion yield of about 85−96% in a reaction time of 2 h using a ferric sulfate catalyst, and 50−65% in only 15 min of reaction time using supercritical methanol transesterification method. The results revealed that the supercritical method is probably a promising alternative method to the traditional twostep transesterification process using a ferric sulfate catalyst for waste cooking oil conversion. A conceptual design for supercritical transesterification process for continuous biodiesel production from waste cooking oil has been reported by Van Kasteren and Nisworo89 for three plant capacities (125 000, 80 000, and 8000 TPA). It has been concluded that biodiesel by supercritical transesterification can be scaled up resulting in high purity of methyl esters (99.8%) and almost pure glycerol (96.4%) attained as byproduct with added advantages of elimination of pretreatment related capital and operating costs. Campanelli et al.90 used methyl acetate instead of methanol for supercritical synthesis of glycerol-free biodiesel from edible, nonedible, and waste cooking oils. The results demonstrate that the oil composition does not significantly influence the biodiesel yield because all the oils achieved complete conversion after 50 min at 345 °C, 20 MPa with methyl acetate:oil molar ratio equal to 42:1. Anitescu et al.91 studied the volatility of biodiesel fuel samples produced by supercritical transesterification of triglyceride feedstock of chicken fat using the advanced distillation curve method. They reported that biodiesel fuels produced by supercritical transesterification at ∼400 °C exhibit

THF is applied mainly to increase the solubility of methanol in oil. 6.6. Supercritical Transesterification Process. Supercritical fluid transesterification can be considered an an interesting alternative to the conventional approach (acid- or alkali-catalyzed) since it can be conducted without using any catalyst, avoiding the complications of purification in the downstream processing. In the case of a supercritical fluid process, the reactor pressure and temperature are manipulated to influence the thermophysical properties of solvent (methanol), such as dielectric constant, viscosity, specific gravity, and polarity. Liquid methanol is a polar solvent and has hydrogen bonding between the hydroxide oxygen and the hydroxide hydrogen to form methanol clusters. In supercritical methanol, with increasing temperature, the degree of hydrogen bonding decreases resulting in a reduction in the polarity of methanol, which in turn increases the solubility of oil in methanol. Because of the hydrophobic nature of nonpolar triglycerides, they are well-solvated in supercritical methanol to form a single phase oil/methanol mixture. The higher solubility of oil in methanol increases the rate of formation of methyl esters dramatically in the supercritical state. The ionic product, which is an important parameter for chemical reactions, can also be improved by increasing the pressure. Therefore, in the supercritical methanol treatment of vegetable oil, methanol not only acts as a solvent but also as an acid catalyst, thereby avoiding the requirement of any external catalyst. Among all the supercritical alcohols (methanol, ethanol, 1propanol, 1-butanol, 1-octanol), it has been reported that the rate of transesterification is maximum when supercritical methanol is used.84 The supercritical methanol method is determined to be more water-tolerant than the conventional method using an alkaline catalyst. Tan et al.85 studied the transesterification process under supercritical method in presence of water in oil. It has been reported that yield of biodiesel in supercritical methanol is positively affected by the presence of water in oil. The yield increases steadily with increasing amount of water content. This observation can be best explained by a two-step process, as shown in eqs 5 and 6 in Scheme 2, which consists of hydrolysis and esterification reactions, respectively, instead of conventional transesterification reaction between triglycerides. In supercritical methanol reaction, the presence of water in the reaction mixture induces the hydrolysis of triglycerides, which produces FFA and glycerol, and subsequently leads to the former being esterified with methanol to produce FAME and water. Overall the yield increases steadily as simultaneous reactions of transesterification, hydrolysis, and esterification 14622



Review

characteristics downstream of the constrictions in the fluid flow and the only energy dissipating device in the hydrodynamic cavitation reactor configuration is the pump used for the recirculation of the reaction mixture.93 In the case of microwave-assisted transesterification, use of microwave irradiation helps in transferring the energy directly to the reactants and accelerates the rate of transesterification reactions, reducing the reaction time from hours to minutes. Microwave energy offers numerous potential processing advantages over the conventional heating methods to provide a rapid and volumetric heating to an absorbing medium. Microwaves can interact directly with a sample matrix by two mechanisms viz. dipolar rotation and ionic conduction. Both mechanisms contribute to localized superheating leading to high temperature and pressure gradients. Energy consumption calculations suggest that transesterification reaction assisted with microwave methodology is more energy-efficient than using a conventional heating apparatus. Quantitatively, the microwave process has an energy requirement of 26.0 kJ/L of biodiesel as compared to 94.2 kJ/L required for the conventional mode of heating.54 However, the use of microwaves to produce products at a large scale is a challenging problem. A few studies on continuous reactions under the influence of microwave irradiation have been successful but a perennial problem in the manufacturing of microwave systems is the difficulty to obtain hybrid devices that incorporate distinct materials with different functionalities. In most of the cases, cumbersome prototyping and high investment needed for manufacturing are additional problems that add to the cost of the final product. Exposure to high levels of microwave radiation is also known to cause health problems including cataracts and burns, according to the National Health and Medical Research Council. The thermal effect is almost instantaneous at the molecular level. This effect, however, is limited to a small area near the surface of the dielectric material because microwaves have a penetration depth which depends on the polarity of the compound and are completely absorbed at the boundary. Hence, there are also possibilities of formation of localized hot spots due to uneven heating. It is, therefore, essential that such reactions are studied in controlled conditions of microwave energy supply. This is an important factor for scaling up the reactors for large-scale applications. The oscillatory flow reactor contains combination of the baffles and the oscillatory motion which provide uniform mixing by the formation of periodic vortices in the bulk fluid, resulting in a remarkable increase in mass and heat transfer, while maintaining plug flow. The enhanced mass transfer makes such a reactor highly advantageous when applied to reactions having mass-transfer and equilibrium limitations such as transesterification reaction. Mass transfer limitations in the two-phase biodiesel production can be eliminated if effective mixing methodologies are applied to increase interfacial area by deformation, break-up, and redispersion of droplets into a continuous phase. Similarly reduced backmixing allows the equilibrium limitations to be partially overcome. One of the advantages of using oscillatory flow reactor is that the fluid mechanics are controllable by oscillation conditions (frequency and amplitude) and independent of net flow Reynolds number. In the case of tubular continuous reactor the residence time and net mass flow rate are optimized, which in turn optimizes mixing conditions (i.e., fluid turbulence). On the other hand, in the case of oscillatory flow reactor the residence time and plug flow performance are optimized with the help of oscillations

higher overall volatility and cetane number when compared to commercial biodiesel samples produced by conventional catalytic transesterification. This is attributed to the fact that high temperatures (∼ 400 °C) of the supercritical transesterification process partially decomposed the polyunsaturated fatty acid methyl esters (FAMEs) to lower molecular FAMEs (∼C6−C15) and ∼C10−C17 hydrocarbons. These lighter fuel components shifted the first portion of the distillation curve towards petrodiesel. As compared to base-catalyzed transesterification, supercritical methanol transesterification has the following advantages: (a) no catalyst requirement; (b) no sensitivity to either water and free fatty acid; and (c) free fatty acids in the waste cooking oil are esterified simultaneously. However, the supercritical technology suffers from certain drawbacks including the requirement of high temperature (350 °C) and high pressure (45 MPa). In addition, this method requires a large amount of methanol (1:42 molar ratio of oil to alcohol). In presence of substantial quantity of fatty acid, the material of construction could also be an issue at these extreme operating conditions. 6.7. Comparison of Process-Intensifying Technology for Biodiesel Production from Waste Cooking Oil. Different process intensification techniques for biodiesel synthesis from transesterification reaction have been discussed in the earlier sections based on the working principle, reactor details, operating parameters, and yields in comparison with the conventional technique. Now the comparative analysis of the different process-intensifying techniques has been performed based on advantages and limitations for its use for biodiesel production using transesterification reactions. The use of cavitational reactors in transesterification reaction is not only efficient and time saving but also economically viable as it requires a low quantity of catalyst and only one-third to half of the energy that is consumed by mechanical agitation. Use of sonochemical reactors has proved to be efficient, giving biodiesel yield up to 98−99% and resulting in substantial time and energy saving (dramatic reduction in reaction time accompanied by a simultaneous reduction in static separation time). Sonochemical reactors suffer from disadvantages of scaleup and inefficient operation at large scale. The alternate mode of generating cavitating conditions, i.e. hydrodynamic cavitation, is the phenomenon where cavities are created due to pressure reduction by passing liquid through a constriction (orifice plate or venturi) and this has been found to give higher conversion and cavitational yields (quantum of product per unit energy supplied into the reactor) as compared to the acoustic cavitation. It has been reported that conversion for the transesterification of waste frying oil using acoustic cavitation was 85% after 40 min of the reaction time at optimized conditions which is less than the conversion obtained using hydrodynamic cavitation reactor, i.e. more than 95% within 10 min.48 The cavitational yield has also been reported to be higher (about 10−15 times) for hydrodynamic mode of cavitation (1.53 × 10−4 to 3.1 × 10−4 g/J) as compared to the acoustic mode of cavitation (1.33 × 10−5 to 5.33 × 10−5 g/ J) for biodiesel production through transesterification reaction.92 It has been established that hydrodynamic cavitation is about 40 times more efficient than acoustic cavitation and 160− 400 times more efficient than the conventional agitation/ heating/refluxing method. The scale-up of hydrodynamic cavitation reactors also appears to be easy considering the fact that lot of information is available on the turbulence 14623



Review

with equivalent rates; (b) the homogeneous phase eliminates diffusion problems; (c) the process tolerates higher percentage of water in the feedstock; (d) the catalyst removal step is eliminated; and (e) if high methanol/oil ratio is used, total conversion of the oil can be achieved in a few minutes. Despite having all these advantages, the supercritical methanol method has some serious disadvantages, which include (1) the process operates at very high pressures (25−40 MPa); (2) the high temperatures (350−400 °C) bring along proportionally high heating and cooling costs; (3) high methanol:oil ratios (usually set at 42:1) entails higher costs for the evaporation of the unreacted methanol; and (4) the process does not answer the problem of reduction in free glycerol to less than 0.20% as required according to the international standards. Based on the critical assessment of the literature, few guidelines have been given for the analysis of processintensifying techniques in transesterification reaction for biodiesel production in achieving desired goals in industrial operation. Among all the mentioned process-intensification approaches, individual techniques have inherent limitations when considered for large-scale applications except hydrodynamic cavitation reactor for biodiesel production. In the case of hydrodynamic cavitation equipments, scale-up appears to be much easier due to the fact that knowledge regarding the conditions downstream of the constriction is easily available, scale-up ratios required are lower, and centrifugal pumps operate with higher energy efficiency at larger scales of operation. 6.8. Hybrid Processes Based on Combination of Microwave Irradiation and Cavitational Reactor. The improvement in process-intensifying technology for heterogeneous system (such as oil and alcohol) based on the use of combination or sequential approach of different intensification techniques such as microwave, ultrasound, or hydrodynamic cavitation can further accelerate the chemical reactions and other processing applications significantly. The dramatic acceleration effect may be attributed to a combination of enforced heat transfer due to microwave irradiation and intensive mass transfer at phase interfaces caused by ultrasonic or hydrodynamic cavitation. In transesterification reactions, the rate-determining step is the mass transfer at the interface between two phases i.e. oil and alcohol. With sonication, the liquid jet caused by cavitation propagates across the bubble toward the phase boundary at a velocity estimated at several hundreds of meters per second, and violently hits the surface. The intense agitation leads to the mutual injection of droplets of one liquid into the other, resulting in the formation of fine emulsions. Microwave irradiations compliment these effects by way of enhanced heating as well as by absorption of the molecules resulting in significantly enhanced rates of transesterification reactions. The main advantage of microwave heating is the instantaneous heating of liquids due to dipolar rotation and ionic conduction which contributes to localized superheating leading to high temperature and pressure gradients, which is not the case in conventional heating. The combination of microwave with ultrasound technique can improve the process chemistry, yield, and reduce the processing time by 10−15 fold as compared to the individual operations.39 Hsiao et al.94 employed ultrasonic mixing and closed microwave irradiation to intensify transesterification of soybean oil. Reactions were carried out at methanol/oil molar ratio of 6:1; amount of catalyst as 1.0 wt %, and reaction temperature of 333 K.

hence heat and mass transfer are totally independent of the net mass flow rate through the tubes; this means that ideal mixing conditions can be achieved at very low net mass flow rates. A wider range of operating conditions and residence times can be achieved (high turn-up or turn-down ratios) also maintaining a very small footprint and high throughputs. Oscillatory flow reactor can be applied to both batch and continuous transesterification of waste cooking oil for biodiesel production. The cost of maintenance of oscillatory flow reactor is high due to presence of moving parts such as oscillatory baffles. Microreactors are usually defined as miniaturized reaction vessels fabricated, at least partially, by methods of microtechnology and precision engineering. Microchannel reactors have high volume/surface ratio, short diffusion distance requirement for the reactants, laminar flow, effective mixing, fast response time, easy integration, and small footprint, which are ideal for portable power. In addition, enhanced mass and heat transport properties are also widely recognized as advantages of microreactors. These characteristics have been advantageously used in biodiesel production from waste cooking oil at milder conditions. Recently, the yield of methyl esters of 99.4% was obtained at the residence time of 5.89 min with KOH concentration of 1% and methanol to oil molar ratio of 6 at 60 °C carried out in capillary microreactor.70 A major disadvantage of using microreactors for the biodiesel synthesis is that for the required volumes of biodiesel (bulk chemicals), the capital costs are expected to be significantly higher as compared to the other process intensification approaches. Another problem that may occur in a microchannel reactor when used for transesterification reaction is the clogging of microchannels by either contaminants present in reaction media or formation of small quantity of soap. In case of clogging, the flow distribution inside the microreactor changes and cannot guarantee the adequate transport of the product. Mechanical pumping may generate a pulsating flow which can be another disadvantage in the use of microchannel reactor for transesterification reaction. The noncatalyst options for transesterification are designed to overcome the reaction initiation lag time caused by poor methanol and oil miscibility. Introducing a cosolvent into the transesterification process changes the reaction mixture from a two-phase system to a single-phase system due to increase in the mutual solubility of alcohol and vegetable oil at low reaction temperatures. This is in agreement with a kinetic study that clearly indicates that the reaction rate constant for transesterification increases significantly when the cosolvent is added.78 The presence of cosolvent helps to separate the phases (biodiesel and glycerol) more easily. However, a disadvantage of the use of cosolvent is that it must be separated from the final product after the completion of the reaction. Although it can be distilled together with methanol, further separation between methanol and cosolvent is difficult since they have similar boiling points. It should be noted that methanol is normally purified before it can be recycled for subsequent transesterification reaction. So this factor creates a big economical barrier as the cost of separation of methanol and the cosolvent is significant and it translates into additional production cost while the residual solvent in the biodiesel product can affect the compliance with the international standards. Tranesterification reaction of waste cooking oil by supercritical methanol for biodiesel production has some advantages which include (a) glycerides and free fatty acids are reacted 14624



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Most of the literature related to the use of hydrodynamic cavitation for biodiesel production is based on orifice plate as the cavitating device and not many studies have been reported with the use of sustainable feed stock or a mixed feed stock, which can be a reality in a large-scale production unit. Additional studies are required for the application of hydrodynamic cavitation reactors based on the use of different cavitating devices to increase the cavitation yield. Application to a variety of sustainable feed stocks also needs to be evaluated. Test rigs involving the combination of microwave, sonochemical, and hydrodynamic cavitation reactors can be developed and optimized with respect to the time of operation so that energy efficient operation can be carried out. Considering scaleup and better energy efficiency of hydraulic equipment, hydrodynamic cavitation holds a potential for large-scale applications in cavitationally assisted chemical and/or physical transformations.

Table 7 shows the effects of ultrasonic mixing and closed microwave irradiation on the conversion of soybean oil. The Table 7. Effects of Ultrasonic Mixing and Microwave Irradiation on Conversion Rate94 ultrasonic mixing time (min)

microwave irradiation time (min)

conversion rate (%)

0a

1 2 5 10 0b

11.2 13.1 20.0 21.0 44.1 59.2 72.5 90.2 91.6 97.7

1 2 5 10 1 1

1 2

a

The reaction was assisted by closed microwave irradiation alone and without ultrasonic mixing. bThe reaction was assisted by ultrasonic mixing alone and without microwave irradiation.

8. CONCLUSIONS Depending on the undesired content of the waste cooking oil, different pretreatment methods should be adopted before the transesterification process for biodiesel production. The separation of suspended solids from waste cooking oil can be effectively carried out by filtration whereas the removal of soluble salts by water washing, and moisture removal by drying/adsorption will be required based on the quality of the feed material. Free fatty acid removal by acidic esterification and/or vacuum distillation/stripping and/or adsorption technique can be achieved based on the FFA content. Pretreatment is necessary as the impurities can deactivate catalysts and create problems in the separation of pure products. Scaling up of a laboratory-scale process involving immiscible phases to commercial scale is not an easy task due to mass and heat transfer limitation. Nevertheless, recent advances in technologies such as ultrasonic reactors, microwave irradiation, oscillatory flow reactor, microchannel rector, addition of cosolvent, and supercritical uncatalyzed transesterification have shown high potential in overcoming this limitation. These technologies not only facilitate transesterification reaction in terms of increasing mixing intensity, heat, and mass transfer rate, but also prove to be more energyefficient as compared to the conventional heating and mixing based transesterification process. In cavitational reactors, the collapse of cavity or bubbles produces conditions of high temperature and pressure associated with local turbulence resulting in enhanced reaction rate. Microwave reactors utilize microwave irradiation to transfer energy directly to the reactants and thus accelerate the rate of chemical reaction. Microchannel reactors improve heat and mass transfer rates due to short diffusion distance and high volume/surface area increasing the reaction rates. Oscillatory baffled reactors enhance radial mixing and transport processes by independent and controlled oscillatory motion. Use of cosolvents in transesterification processes changes the heterogeneous nature of the reaction mixture to a homogeneous phase. In the presence of cosolvent, the increase in the mutual solubility of alcohol and WCO at low reaction temperatures results in an increase in the mass transfer due to an increase in the contact surface, and hence the production of biodiesel increases. Supercritical transesterification requires very short time (few min) for the completion of the reaction under supercritical conditions (temperature 350−400 °C and pressure more than 80 bar). It avoids the complications of separation of catalyst in

conversion was significantly high with ultrasonic mixing for 1 min and closed microwave irradiation for 1 (91.6%) and 2 (97.7%) min. It has been reported that ultrasonic mixing causes cavitation near the phase boundary between the soybean oil and methanol. In summary, if the soybean oil and methanol are not thoroughly mixed (without ultrasonic mixing), the conversion rate achieved under the assistance of closed microwave irradiation alone was low. Similarly, for transesterification without closed microwave irradiation, the conversion rate achieved by ultrasonic mixing alone within short reaction time was also low. On the contrary, if the soybean oil and methanol are thoroughly mixed after ultrasonic mixing for 1 min, the conversion rate achieved by 2-min closed microwave irradiation was very high. In short, the optimal procedure was 1-min ultrasonic mixing and 2-min closed microwave irradiation. The discussed effects for pure oil can be equally applicable for the waste cooking oil though no literature reports were observed to the best of our knowledge.

7. SCOPE FOR FUTURE WORK In this section, the most important lacunae of the chosen promising process intensification techniques (possible combined operation of microwave and cavitational reactors) have been discussed highlighting the need for future work required with an objective of successful commercial-scale operation. Microwave reactors provide uneven heating and also the scale-up of the magnetron in the microwave reactor poses a challenge. There is a need to develop designs operating on continuous basis with orientation of magnetrons in such a way that there are not many limitations on the material of construction. Also, the work should be directed in evaluation of pilot-scale continuous reactors for the application of biodiesel synthesis from sustainable feed stocks to establish some degree of confidence in the design engineers. In case of cavitational reactors, hydrodynamic mode of cavitation is found to be more energy efficient as compared to the acoustic mode of cavitation based on cavitational yield which is defined as the yield of product per unit of supplied energy to the system. Unlike acoustic cavitation, the efficiency of the pumps increases with the capacity and hence better cavitational yield can be achieved with larger scale of operation. 14625



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(13) Diyauddeen, B. H.; Abdul Aziz, A. R.; Daud, W. M. A. W.; Chakrabarti, M. H. Performance evaluation of biodiesel from used domestic waste oils: A review. Process Safe. Environ. 2012, 90, 164− 179. (14) Math, M. C.; Kumar, S. P.; Chetty, S. V. Technologies for biodiesel production from used cooking oil - A review. Energy Sustainable Dev. 2010, 14, 339−345. (15) fcamin.nic.in/Dir_Vanaspati/Edible-Oil.doc. (16) Math, M. C. Performance of a diesel engine with blends of restaurant waste oil methyl ester and diesel fuel. Energy Sustainable Dev. 2007, 11, 93−95. (17) http://petroleum.nic.in/reports.htm. (18) http://gain.fas.usda.gov/Recent%20GAIN%20Publications/ General%20Report_New%20Delhi_India_6-12-2009.pdf. (19) Leung, D. Y. C.; Guo, Y. Transesterification of neat and used frying oil: Optimization for biodiesel production. Fuel Proc. Technol. 2006, 87, 883−890. (20) Cvengro, V.; Cvengroova, Z. Used frying oils and fats and their utilization in the production of methyl esters of higher fatty acids. Biomass Bioenergy 2004, 27, 173−181. (21) Charoenchaitrakool, M.; Thienmethangkoon, J. Statistical optimization for biodiesel production from waste frying oil through two-step catalyzed process. Fuel Proc. Technol. 2011, 92, 112−118. (22) Issariyakul, T.; Kulkarni, M. G.; Dalai, A. K.; Bakhshi, N. N. Production of biodiesel from waste fryer grease using mixed methanol/ethanol system. Fuel Proc. Technol. 2007, 88, 429−436. (23) Dias, J. M.; Alvim-Ferraz, M.; Almeida, M. F. Comparison of the performance of different homogeneous alkali catalysts during transesterification of waste and virgin oils and evaluation of biodiesel quality. Fuel 2008, 87, 3572−3578. (24) Hoffman, G. The Chemistry and Technology of Edible Oils and Fats and Their High Fat Product; Academic Press, 1989. (25) Meher, L. C.; Dharmagadda, V. S. S.; Naik, S. N. Optimization of alkali-catalyzed transesterification of Pongamia pinnata oil for production of biodiesel. Bioresour. Technol. 2006, 97, 1392−1397. (26) Toeneboehn, G. J.; Welsh, W. A. Adsorptive removal of sulfur compounds from fatty materials. U.S. Patent 5 298638, 1994. (27) Nakazono, Y. Method of Producing Biodiesel Fuel. U.S. Patent 20100132252, 2010. (28) Lee, K. T.; Foglia, T. A.; Chang, K. S. Production of alkyl ester as biodiesel from fractionated lard and restaurant grease. J. Am. Oil Chem. Soc. 2002, 79, 191−195. (29) Supple, B.; Howard-Hildige, R.; Gonzalez-Gomez, E.; Leahy, J. J. The effect of steam treating waste cooking oil on the yield of methyl ester. J. Am. Oil Chem. Soc. 2002, 79, 175−178. (30) Darnoko, D.; Cheryan, M. Kinetics of palm oil transesterification in a batch reactor. J. Am. Oil Chem. Soc. 2000, 77, 1263−1267. (31) Lee, S.; Posarac, D.; Ellis, N. Process simulation and economic analysis of biodiesel production processes using fresh and waste vegetable oil and supercritical methanol. Chem. Eng. Res. Des. 2011, 89, 2626−2642. (32) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. Possible methods for biodiesel production. Renew. Sust. Energ. Rev. 2007, 11, 1300−1311. (33) Enweremadu, C. C.; Mbarawa, M. M. Technical aspects of production and analysis of biodiesel from used cooking oilA review. Renew. Sustainable Energy Rev. 2009, 13, 2205−2224. (34) Singh, A. K.; Fernando, S. D. Reaction Kinetics of Soybean Oil Transesterification Using Heterogeneous Metal Oxide Catalysts. Chem. Eng. Technol. 2007, 30, 1716−1720. (35) Liu, X.; He, H.; Wang, Y.; Zhu, S.; Piao, X. Transesterification of soybean oil to biodiesel using CaO as a solid base catalyst. Fuel 2008, 87, 216−221. (36) Du, W.; Xu, Y.; Liu, D.; Zeng, J. Comparative study on lipasecatalyzed transformation of soybean oil for biodiesel production with different acyl acceptors. J Mol. Catal. B: Enzym. 2004, 30, 125−129. (37) Lertsathapornsuk, V.; Pairintra, R.; Aryusuk, K.; Krisnangkura, K. Microwave assisted in continuous biodiesel production from waste

the last stages. However, it requires a high alcohol to oil (42:1) ratio and higher capital and operating cost. Overall, among the different process intensifying approaches, cavitational reactors such as hydrodynamic cavitation reactors have very high energy efficiency and also potential for easy scale-up and commercialization for biodiesel production. The fuel properties of biodiesel derived from used cooking oil are in accordance with the ASTM standards. Thus, biodiesel produced from used cooking oil can be used in diesel engines without any engine modifications. Overall, it can be said that the process-intensifying techniques offer considerable promise for the intensification of biodiesel production from sustainable raw materials. Cost of biodiesel can be reduced by using low cost feedstock such as waste cooking oil and incorporating process-intensifying techniques in transesterification processes for biodiesel production. The use of low-cost feedstock and process-intensifying techniques for biodiesel can reduce the production cost significantly.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91-22-33612024. Fax: +91-22-3361 1020. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.L.M. is thankful to University Grants Commission, Government of India for financial assistance. GLM would also like to acknowledge the help of B.N. Rekha in preparation of the manuscript.

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REFERENCES

(1) Karmarkar, A.; Karmarkar, S.; Mukherjee, S. Properties of various plants and animals feedstocks for biodiesel production. Bioresour. Technol. 2010, 101, 7201−7210. (2) Klass, D. L. Biomass for Renewable Energy. In Fuels, and Chemicals, 1st ed.; Academic Press, 1998. (3) Lin, C. Y.; Lin, H. A.; Hung, L. B. Fuel structure and properties of biodiesel produced by the peroxidation process. Fuel 2006, 85, 1743− 1749. (4) Yang, Z.; Xie, W. Soybean oil transesterification over zinc oxide modified with alkali earth metals. Fuel Proc. Technol. 2007, 88, 631− 638. (5) Ma, F.; Hanna, M. A. Biodiesel production: A review. Bioresour. Technol. 1999, 70, 1−15. (6) Zhang, Y.; Dubé, M. A.; McLean, D. D.; Kates, M. Biodiesel production from waste cooking oil: 1. Process design and technological assessment. Bioresour. Technol. 2003, 89, 1−16. (7) Kulkarni, M. G.; Dalai, A. K. Waste Cooking Oil An Economical Source for Biodiesel: A Review. Ind. Eng. Chem. Res. 2006, 45, 2901− 2913. (8) Canakci, M.; Van Gerpen, J. Biodiesel production via acid catalysis. Trans. ASAE 2012, 42, 1203−1210. (9) Canakci, M.; Van Gerpen, J. Biodiesel production from oils and fats with high free fatty acids. Trans. ASAE 2012, 44, 1429−1436. (10) Saka, S.; Kusdiana, D. Biodiesel fuel from rapeseed oil as prepared in supercritical methanol. Fuel 2001, 80, 225−231. (11) Patil, P. D. Biodiesel Production from Waste Cooking Oil Using Sulfuric Acid and Microwave Irradiation Processes. J. Environ. Prot. 2012, 03, 107−113. (12) Ahmad, A. L.; Yasin, N. H. M.; Derek, C. J. C.; Lim, J. K. Microalgae as a sustainable energy source for biodiesel production: A review. Renew. Sustainable Energy Rev. 2011, 15, 584−593. 14626



Review

frying palm oil and its performance in a 100 kW diesel generator. Fuel Proc. Technol. 2008, 89, 1330−1336. (38) Gogate, P. R.; Tayal, R. K.; Pandit, A. B. Cavitation: A technology on the horizon. Curr. Sci. 2006, 91, 35−46. (39) Gole, V. L.; Gogate, P. R. A review on intensification of synthesis of biodiesel from sustainable feed stock using sonochemical reactors. Chem. Eng. Process. 2012, 53, 1−9. (40) Fukuda, H.; Kondo, A.; Noda, H. Biodiesel fuel production by transesterification of oils. J. Biosci. Bioeng. 2001, 92, 405−416. (41) Refaat, A. A.; El Sheltawy, S. T. Comparing three options for biodiesel production from waste vegetable oil. WIT Press 2008, 133− 140. (42) Babajide, O.; Petrik, L.; Amigun, B.; Ameer, F. Low-Cost Feedstock Conversion to Biodiesel via Ultrasound Technology. Energies 2010, 3, 1691−1703. (43) Hingu, S. M.; Gogate, P. R.; Rathod, V. K. Synthesis of biodiesel from waste cooking oil using sonochemical reactors. Ultrason. Sonochem. 2010, 17, 827−832. (44) Marija, M.; Ivana, L.; Olivera, S.; Vlada, V.; Dejan, S. Heterogeneous base-catalyzed methanolysis of vegetable oils: State of art. Hem. Ind. 2010, 64, 63−80. (45) Vyas, A. P.; Verma, J. L.; Subrahmanyam, N. A review on FAME production processes. Fuel 2010, 89, 1−9. (46) Georgogianni, K. G.; Katsoulidis, A. P.; Pomonis, P. J.; Kontominas, M. G. Transesterification of soybean frying oil to biodiesel using heterogeneous catalysts. Fuel Proc. Technol. 2009, 90, 671−676. (47) Wang, J.-X.; Huang, Q.-D.; Huang, F.-H.; Wang, J.-W.; Huang, Q.-J. Lipase-catalyzed production of biodiesel from high acid value waste oil with ultrasonic assistant. Sheng Wu Gong Cheng Xue Bao 2007, 23, 1121−1128. (48) Ghayal, D.; Pandit, A. B.; Rathod, V. K. Optimization of biodiesel production in a hydrodynamic cavitation reactor using used frying oil. Ultrason. Sonochem. 2013, 20, 322−28. (49) Veljković, V. B.; Avramović, J. M.; Stamenković, O. S. Biodiesel production by ultrasound-assisted transesterification: State of the art and the perspectives. Renew. Sustainable Energy Rev. 2012, 16, 1193− 1209. (50) Lidström, P.; Tierney, J.; Wathey, B.; Westman, J. Microwave assisted organic synthesisa review. Tetrahedron 2001, 57, 9225− 9283. (51) Groisman, Y.; Gedanken, A. Continuous Flow, Circulating Microwave System and Its Application in Nanoparticle Fabrication and Biodiesel Synthesis. J. Phys. Chem. C 2008, 112, 8802−8808. (52) Mutyala, S.; Fairbridge, C.; Paré, J. R. J.; Bélanger, J. M. R.; Ng, S.; Hawkins, R. Microwave applications to oil sands and petroleum: A review. Fuel Proc. Technol. 2010, 91, 127−135. (53) Marra, F.; De Bonis, M. V.; Ruocco, G. Combined microwaves and convection heating: A conjugate approach. J. Food Eng. 2010, 97, 31−39. (54) Barnard, T. M.; Leadbeater, N. E.; Boucher, M. B.; Stencel, L. M.; Wilhite, B. A. Continuous-Flow Preparation of Biodiesel Using Microwave Heating. Energy Fuels 2007, 21, 1777−1781. (55) Yaakob, Z.; Ong, B. H.; Satheesh Kumar, M. N.; Kamarudin, S. K. Microwave-assisted transesterification of jatropha and waste frying palm oil. Int. J. Sustainable Energy 2009, 28, 195−201. (56) Suppalakpanya, K.; Ratanawilai, S.; Tongurai, C. Production of ethyl ester from esterified crude palm oil by microwave with dry washing by bleaching earth. Appl. Energy 2010, 87, 2356−2359. (57) Koberg, M.; Abu-Much, R.; Gedanken, A. Optimization of biodiesel production from soybean and wastes of cooked oil: Combining dielectric microwave irradiation and a SrO catalyst. Bioresour. Technol. 2011, 102, 1073−1078. (58) Refaat, A.; El Sheltawy, S.; Sadek, K. Optimum reaction time, performance and exhaust emissions of biodiesel produced by microwave irradiation. Int. J. Environ. Sci. Technol. 2008, 5, 315−322. (59) Chen, K.-S.; Lin, Y.-C.; Hsu, K.-H.; Wang, H.-K. Improving biodiesel yields from waste cooking oil by using sodium methoxide and a microwave heating system. Energy 2012, 38, 151−156.

(60) Stephens, G.; Mackley, M. R. Heat transfer performance for batch oscillatory flow mixing. Exp. Therm. Fluid Sci. 2002, 25, 583− 594. (61) Mackley, M. R.; Stonestreet, P.; Thurston, N. C.; Wiseman, J. S. Evaluation of a novel self-aerating, oscillating baffle column. Can. J. Chem. Eng. 1998, 76, 5−10. (62) Mackley, M. R.; Stonestreet, P.; Roberts, E.; Ni, X.-W. Residence time distribution enhancement in reactors using oscillatory flow. Chem. Eng. Res. Des. 1996, 74, 541−545. (63) Ni, X.-W.; Mackley, M. R.; Harvey, A. P.; Stonestreet, P.; Baird, M. H. I.; Rama Rao, N. V. Mixing Through Oscillations and PulsationsA Guide to Achieving Process Enhancements in the Chemical and Process Industries. Chem. Eng. Res. Des. 2003, 81, 373− 383. (64) Skelton, B.; Bustnes, T. E.; Mackley, M. R. Development of the oscillatory flow based process for the production of biodiesel transport fuel. In 7th World Congress of Chemical Engineering; Glasgow, UK, 2005; 14. (65) Harvey, A. P.; Mackley, M. R.; Seliger, T. Process intensification of biodiesel production using a continuous oscillatory flow reactor. J. Chem. Technol. Biotechnol. 2003, 78, 338−341. (66) Pohar, A.; Plazl, I. Process intensification through microreactor application. Chem. Biochem. Eng. Q. 2009, 23, 537−544. (67) Ehrfeld, W.; Hessel, V.; Lowe, H. Microreactors: New Technology for Modern Chemistry, 1st ed.; Wiley-VCH, 2000. (68) Kobayashi, J.; Mori, Y.; Kobayashi, S. Multiphase Organic Synthesis in Microchannel Reactors. Chem. Asian J. 2006, 1, 22−35. (69) Jovanovic, G. N.; Paul, B. K.; Parker, J.; Al-Dhubabian, A. Microreactor Process for Making Biodiesel. U.S. Patent 8137554, 2012. (70) Sun, J.; Ju, J.; Ji, L.; Zhang, L.; Xu, N. Synthesis of biodiesel in capillary microreactors. Ind. Eng. Chem. Res. 2008, 47, 1398−1403. (71) Guari, G.; Kusakabe, K.; Moriyama, K.; Sakurai, N. Continuous production of biodiesel using a microtube reactor. Chem. Eng. 2008, 14, 237. (72) Wen, Z.; Yu, X.; Tu, S. T.; Yan, J.; Dahlquist, E. Intensification of biodiesel synthesis using zigzag micro-channel reactors. Bioresour. Technol. 2009, 100, 3054−3060. (73) Sun, P.; Sun, J.; Yao, J.; Zhang, L.; Xu, N. Continuous production of biodiesel from high acid value oils in microstructured reactor by acid-catalyzed reactions. Chem. Eng. J. 2010, 162, 364−370. (74) Demirbas, A. Progress and recent trends in biodiesel fuels. Energy Convers. Manage. 2009, 50, 14−34. (75) Muppaneni, T.; Reddy, H. K.; Patil, P. D.; Dailey, P.; Aday, C.; Deng, S. Ethanolysis of camelina oil under supercritical condition with hexane as a co-solvent. Appl. Energy 2012, 94, 84−88. (76) Jiang, J.-J.; Tan, C.-S. Biodiesel production from coconut oil in supercritical methanol in the presence of cosolvent. J. Taiwan Inst. Chem. Eng. 2012, 43, 102−107. (77) Peña, R.; Romero, R.; Martínez, S. L.; Ramos, M. J.; Martínez, A.; Natividad, R. Transesterification of Castor Oil: Effect of Catalyst and Co-Solvent. Ind. Eng. Chem. Res. 2008, 48, 1186−1189. (78) Park, J.-Y.; Kim, D.-K.; Wang, Z.-M.; Lee, J.-S. Fast biodiesel production with one-phase reaction. Appl. Biochem. Biotechnol. 2009, 154, 67−73. (79) Trentin, C. M.; Lima, A. P.; Alkimim, I. P.; de Silva, C.; de Castilhos, F.; Mazutti, M. A. Continuous catalyst-free production of fatty acid ethyl esters from soybean oil in microtube reactor using supercritical carbon dioxide as co-solvent. J. Supercrit. Fluids 2011, 56, 283−291. (80) Li, Q.; Zheng, J.; Yan, Y. Biodiesel preparation catalyzed by compound-lipase in co-solvent. Fuel Proc. Technol. 2010, 91, 1229− 1234. (81) Lam, M. K. Accelerating transesterification reaction with biodiesel as co-solvent: A case study for solid acid sulfated tin oxide catalyst. Fuel 2010, 89, 3866−3870. (82) Guan, G.; Kusakabe, K.; Yamasaki, S. Tri-potassium phosphate as a solid catalyst for biodiesel production from waste cooking oil. Fuel Proc. Technol. 2009, 90, 520−524. 14627



Review

(83) Sabudak, T.; Yildiz, M. Biodiesel production from waste frying oils and its quality control. Waste Manage. 2010, 30, 799−803. (84) Warabi, Y.; Kusdiana, D.; Saka, S. Reactivity of triglycerides and fatty acids of rapeseed oil in supercritical alcohols. Bioresour. Technol. 2004, 91, 283−287. (85) Tan, K. T.; Lee, K. T.; Mohamed, A. R. Effects of free fatty acids, water content and co-solvent on biodiesel production by supercritical methanol reaction. J. Supercrit. Fluids 2010, 53, 88−91. (86) Tan, K. T.; Lee, K. T.; Mohamed, A. R. Potential of waste palm cooking oil for catalyst-free biodiesel production. Energy 2011, 36, 2085−2088. (87) Demirbas, A. Biodiesel from waste cooking oil via base-catalytic and supercritical methanol transesterification. Energy Convers. Manage. 2009, 50, 923−927. (88) Patil, P.; Deng, S.; Rhodes, J. I.; Lammers, P. J. Conversion of waste cooking oil to biodiesel using ferric sulfate and supercritical methanol processes. Fuel 2010, 89, 360−364. (89) Kasteren, J. M. N.; Nisworo, A. P. A process model to estimate the cost of industrial scale biodiesel production from waste cooking oil by supercritical transesterification. Resour., Conserv. Recycl. 2007, 50, 442−458. (90) Campanelli, P.; Banchero, M.; Manna, L. Synthesis of biodiesel from edible, non-edible and waste cooking oils via supercritical methyl acetate transesterification. Fuel 2010, 89, 3675−3682. (91) Anitescu, G.; Bruno, T. J. Biodiesel Fuels from Supercritical Fluid Processing: Quality Evaluation with the Advanced Distillation Curve Method and Cetane Numbers. Energy Fuels 2012, 26, 5256− 5264. (92) Kelkar, M. A.; Gogate, P. R.; Pandit, A. B. Intensification of esterification of acids for synthesis of biodiesel using acoustic and hydrodynamic cavitation. Ultrason. Sonochem. 2008, 15, 188−194. (93) Gogate, P. R.; Pandit, A. B. A review and assessment of hydrodynamic cavitation as a technology for the future. Ultrason. Sonochem. 2005, 12, 21−27. (94) Hsiao, M.-C.; Lin, C.-C.; Chang, Y.-H.; Chen, L.-C. Ultrasonic mixing and closed microwave irradiation-assisted transesterification of soybean oil. Fuel 2010, 89, 3618−3622. (95) Georgogianni, K. G.; Kontominas, M. G.; Tegou, E.; Avlonitis, D.; Gergis, V. Biodiesel Production: Reaction and Process Parameters of Alkali-Catalyzed Transesterification of Waste Frying Oils. Energy Fuels 2007, 21, 3023−3027.

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