Transesterification of castor oil under ultrasonic irradiation conditions. Preliminary results

Transesterification of castor oil under ultrasonic irradiation conditions. Preliminary results

Fuel Processing Technology 103 (2012) 9–15 Contents lists available at SciVerse ScienceDirect Fuel Processing Technology journal homepage: www.elsev...

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Fuel Processing Technology 103 (2012) 9–15

Contents lists available at SciVerse ScienceDirect

Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Transesterification of castor oil under ultrasonic irradiation conditions. Preliminary results J.M. Encinar a,⁎, J.F. González b, A. Pardal c a b c

Department of Chemical Engineering and Physical Chemistry. Extremadura University, Av. de Elvas s/n. 06071-Badajoz, Spain Department of Applied Physics. Extremadura University, Av. de Elvas s/n. 06071-Badajoz, Spain Department of Technologies and Applied Sciences, ESAB, IPBeja. Rua Pedro Soares s/n. 7800-Beja. Portugal

a r t i c l e

i n f o

Available online 18 January 2012 Keywords: Biodiesel Ultrasonic conditions Ultrasound bath Transesterification Castor oil

a b s t r a c t In this work, the preliminary results obtained in the transesterification of castor-oil with methanol in the presence of ultrasonic irradiations, are presented. The fundamental objective was to study the acceleration that the ultrasound causes in the reaction rate. The effect of different operating variables, such as frequency and supplied power dissipation, catalyst (KOH) concentration and methanol oil molar ratio were also investigated. The evolution of the process was followed by gas chromatography, determining the concentration of the methyl esters at different reaction times. The biodiesel was characterized by its density, viscosity, saponification value, iodine value, acidity index, water content, flash point, combustion point, cetane index and cold filter plugging point (CFPP), according to ISO norms. Between these properties, viscosity, the number of cetane and the CFPP do not attain the values established by the standard EN 14214. These circumstances would force to use the castor biodiesel oil mixed with diesel fuel oil, or to add additives in order that the previous properties reach the specified values. High methyl ester yield and fast reaction rate could be obtained, for castor oil transesterification, under ultrasonic conditions, even if the reaction temperature was relatively low. In general, the variables of operation tested exercise a positive effect on the reaction rate, but the obtained final yield (equilibrium) is very similar in all cases. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Biodiesel has become very attractive as a biofuel due to its environmental benefits. It has less air pollutants than diesel and it is nontoxic and biodegradable and, because it is produced from renewable sources with high energetic efficiency, biodiesel does not produce greenhouse effects, because the balance between the amount of CO2 emissions and the amount of CO2 absorbed by the plants producing vegetable oil is equal [1–3]. A variety of biolipids can be used to produce biodiesel: virgin vegetable oil feedstock; waste vegetable oil; animal fats and non-edible oils such as jatropha, neem oil, castor oil, and tall oil [4]. Biodiesel has been mainly produced from edible vegetable oils all over the world, which are easily available on large scale from the agricultural industry [5]. However, there are concerns that biodiesel feedstock may compete with food supply in the long-term [5]. Recently, environmentalists have started to debate on the negative impact of biodiesel production from edible oil on our planet especially deforestation and destruction of ecosystem. In order to overcome this devastating phenomenon, suggestions and research have been

⁎ Corresponding author. Tel.: + 34 924 289672; fax: + 34 924 289385. E-mail address: [email protected] (J.M. Encinar). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2011.12.033

conducted to produce biodiesel using alternative or greener oil resources such as non-edible oils [6]. Different authors have studied the use of castor oil for biodiesel production. Sousa et al. evaluated the production of methyl esters from castor oil and methanol after neutralization of castor oil with glycerol. The reaction was carried out under atmospheric pressure and ambient temperature in a batch reactor, employing potassium hydroxide as catalyst [7]. Ramezani et al. investigated the parameters affecting castor oil transesterification reaction, applying four basic catalysts. Using the optimum results, they proposed a kinetic model which resulted in establishing an equation for the beginning rate of transesterification reaction [8]. Zieba et al. studied the methanolysis of castor oil to methyl esters, with homogeneous catalysts (KOH, H2SO4 and 12-tungstophosphoric acid) and also the presence of solid salts [9]. Santana et al., in their work, designed and simulated a continuous biodiesel plant in HYSYS simulator, using castor oil as feedstock. The developed process was capable of producing biodiesel at high purity using an alkali catalyst [10]. Biodiesel is produced chemically, reacting a vegetable oil or animal fat with an alcohol, usually by a transesterification reaction. The mechanism of the base-catalyzed transesterification of vegetable oils is shown in equations [1] to [4]. The first step (Eq. (1)) is the reaction of the base with the alcohol, producing an alkoxide and the protonated catalyst. The nucleophilic attack of the alkoxide at the

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carbonyl group of the triglyceride generates a tetrahedral intermediate (Eq. (2)), from which the alkyl ester and the corresponding anion of the diglyceride are formed (Eq. (3)). The latter deprotonates the catalyst, thus regenerating the active specie (Eq. (4)), which is now able to react with a second molecule of the alcohol, starting another catalytic cycle. Diglycerides and monoglycerides are converted by the same mechanism to a mixture of alkyl esters and glycerol [11]. −

þ

KOH þ B⇌RO þ BH :

ð1Þ

(2) R'COO

R'COO CH2 R''COO CH

-OR

+

R''COO

CH2 CH

H2C O

H2C OCR''' O

2. Experimental OR C

R'''

O-

(3) R'COO R''COO

CH2 CH

H2C

OR O

C

R'COO

CH2

R''COO

CH

H2C

R'''

+

ROOCR'''

O-

O-

(4) R'COO

CH 2

R''COO

CH

H 2C

+ BH +

R'COO

CH 2

R''COO

CH

O-

H 2C

+ B OH

The presence of water gives rise to hydrolysis of some of the produced esters, with consequent soap formation (Eqs. (5)–(6)). These reactions lower the yield of esters, cause emulsions and make hard the recuperation of glycerol [11].(5) O

O + R'

OR

H2O

R'

OH

+ ROH

(6) O R'

O OH

+ KOH

R'

OK

+

The objective of this preliminary study is the application of ultrasonic irradiation in the basic transesterification reaction of castor oil with methanol. The variables affecting the methyl ester yield during the transesterification reaction, such as, amount of catalyst, methanol oil molar ratio and ultrasound power and frequency, were investigated to optimize the reaction conditions. In this previous study we used a Fisher Bioblock Scientific Transonic TI-H Cleaner, like other authors [12,15,16], to study the effect of ultrasound in the transesterification reaction. Further results about a study that we are developing with an ultrasonic sound, will be presented.

H2O

The transesterification reaction is initially heterogeneous and relatively slow, since alcohol and oil phases are not miscible. Hence, initially the reaction only takes place in the interphase alcohol–oil and the process is controlled by the mass transfer. The ultrasonic field is known to produce unique chemical and physical effects that arise from the collapse of the cavitation bubbles. A low frequency ultrasonic irradiation can be used to produce emulsions from immiscible liquids and, in consequence, can facilitate the transesterification reaction [12]. The ultrasonic irradiation of heterogeneous systems increases the interaction between the phases due to the collapse of cavitation bubbles and the ultrasonic jet that impinges one liquid to another, disrupting the phase boundary and causing emulsification. The ultrasound, in the chemical processing, enhances both the mass transfer and chemical reactions, offering the potential for shorter reaction times, cheaper reagents and less extreme physical conditions [13]. The lower rates of synthesis have been typically attributed to mass transfer limitations due to heterogeneous conditions, existing during the reaction. The use of cavitational reactors can favor the chemical reaction and propagation leading to the enhanced mass transfer and interphase mixing between the phases and they also can lower the requirement of severity of the operating conditions, in terms of temperature and pressure [14].

The castor oil was provided by the Research Center “La OrdenValdesequera” (Badajoz, Spain) Section of Non-Food Crops. This castor oil was characterized, analyzing his fatty acid content and measuring its density, viscosity, water content, acid value, iodine value and saponification value (Table 1). Potassium hydroxide (KOH) was supplied by Merck (pellets GR for analysis), methanol, 96%, was purchased from Panreac. All other chemicals were obtained commercially and are of analytical grade. Reaction of transesterification was carried out in a 500 mL spherical reactor, provided with a thermostat, sampling outlet and condensation systems, within a Fisher Bioblock Scientific Transonic TI-H Cleaner (25/45 kHz, with a maximum power of 2400 W), as shown in Fig. 1. The evolution of the process was followed by gas chromatography. The reactor was initially charged only with oil, placed in the bath and asserting an initial temperature of 20 °C. Different amounts of catalyst were dissolved in different amounts of methanol and the resulting solution was added to the reactor. At this point, the bath was placed at the different frequency and power and started running the time. The reaction was carried out during 60 min. In Table 2 the different reaction conditions are specified. After reaction time, the mixture was neutralized with the required amount of sulfuric acid and it was placed in one separatory funnel and allowed to stand overnight to ensure that the separation of methyl esters and glycerol phase occurred completely. Glycerol phase (bottom phase) was removed and left in a separate container. Methyl esters (biodiesel) were heated, at 85 °C, to remove methanol. Remaining catalyst was extracted by successive rinses with distilled water. Finally, water present was eliminated by heating at 110 °C. The methyl ester content was assayed by gas chromatography in a VARIAN 3900 chromatograph, provided with an FID, employing a silica capillary column of 30 m length, 0.32 mm ID, and 0.25 mm film thickness. Heptane was used as solvent, and the carrier gas was helium at a flow rate of 0.7 mL/min. The injector temperature was kept at 270 °C, and the detector temperature, 300 °C. Temperature ramp started with 200 °C, and then went 20 °C/min up to 220 °C. The calibration curve of peak area and the quantity of biodiesel Table 1 Chemical and physical properties of commercially available castor oil used in this study. Property Density at 15 °C Viscosity at 40 °C Water content Acidity index Iodine value Saponification value Palmitic acid (C18:0)a Stearic acid (C16:0)a Oleic acid (C18:1)a Linoleic acid (C18:2)a Linolenic acid (C18:3)a Ricinoleic acid (C18:1)a a

Unit

Average −3

kg m cSt wt.% −1 mgKOHgoil wt.% −1 mgKOHgoil wt.% wt.% wt.% wt.% wt.% wt.%

Carbon atom number: double bond number.

961.2 258.0 0.2 1.2 76.6 174.0 1.20 1.30 3.50 4.5 0.5 88.20

J.M. Encinar et al. / Fuel Processing Technology 103 (2012) 9–15

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compared to other vegetable oils and makes of it a prime candidate as an additive for diesel fuel [18]. 3. Results and discussion The reactions presented in this study were performed with an initial temperature (bath and oil) of 20 °C and without temperature control. It was observed, during experimentation, that the temperature of the reaction mixture increased, between 10 and 15 °C, and the bath temperature increases around 5–10 °C, as a result of ultrasound radiation. Additionally, some experiments were done with a high initial temperature (40 °C) and it was found that the conversion rate decreased. An increase in the operating temperature result in an enhanced solubility of methanol in the other phase, thereby increasing the extent of conversion initially, but the extent of cavitational effects is dampened at higher operating temperatures [19]. 3.1. Effect of ultrasound frequency

Fig. 1. Schematic representation of the experimental setup. 1: control panel (ultrasound frequency, ultrasound power, time and bath temperature); 2: cleaning tank stainless steel; 3: water; 4: spherical glass reactor; 5: thermostat; 6: condensation systems; 7: sampling outlet.

were linear. Samples were taken out from the reaction mixture, neutralized and heated to remove methanol, centrifuged, 5 min at 6000 rpm, and then analyzed by gas chromatography. The analytical methods used to determine the characteristics of the biodiesel were basically those recommended by the European Organization for Normalization (CEN). This organization specifies the criteria that should be satisfied by a biodiesel of high quality, or diesel and biodiesel mixtures, for its use in motor vehicles [17]. In this work the raw material mainly used was castor oil which, as other vegetable oils, is mainly constituted by triglycerides. Castor oil is viscous, pale yellow non-volatile and non-dry oil. It has a good shelf life, and it does not turn rancid unless subjected to excessive heat. Due to this particular chemical composition castor oil is a raw material in great demand by the pharmaceutical and chemical industries. However, its use, as fuel for internal combustion engines, can become complicated because of its extremely high viscosity and high water content. Thus, a better perspective is offered by its transesterification. Castor oil also possesses a hydroxyl functionality that is rare in vegetable oils. The presence of such a functional group adds extra stability to the oil and its derivatives by preventing the formation of hydroperoxides. The presence of ricinoleic acid, which is a complex fatty acid that contains both a double bond and a hydroxyl group, can increase lubricity to the castor oil and its derivatives, as

Fig. 2 shows the influence of the frequency of the ultrasound bath on the reaction conversion. The two experiments were carried out at a constant power of 80%. It can be seen that at 45 kHz, the induction period is smaller and the reaction was completed faster than at 25 kHz. However the final yield was similar for both frequencies. In consequence, the frequency of the ultrasound affects the process from one point by sight kinetic, so that the reaction accomplished with bigger frequency is faster and the status of equilibrium is attained before, although the yield is similar in both reactions. The frequency of the ultrasonic waves affects the critical size of the bubble. An increase in the frequency leads to a decrease in the volume and implosion time of the cavitation bubble. The higher frequencies produce smaller cavitation bubbles with increased surface area/volume ratios, leading to increased transport activities across the bubble interface. The net effect is to produce a greater enhancement of emulsification and biodiesel reaction rate. A shorter implosion time causes the collapse to proceed almost adiabatically because the exchange of the released energy with the surroundings is limited. As frequency increases further, the resonant bubble size is not large enough to produce enough energy upon collapse, to effect significant emulsification and reaction rate [20]. 3.2. Effect of catalyst concentration The effect of potassium hydroxide concentration on the transesterification of castor oil was investigated with its concentration varying from 0.3 wt.% to 1.0 wt.% (based on the weight of castor oil). The obtained results have been depicted in Fig. 3. It can be seen, from the figure, that an increase in the catalyst concentration from 0.3 wt.% to 0.7 wt.% results in an increase in the conversion from 69.2% to 92.2%.

Table 2 Experimental used conditions. Properties of biodiesel produced from castor oil and comparison with the standard EN 14214. Run number

Ultrasound frequency, kHz

Ultrasound power, %

MeOH:Oil

%KOH

Yield, %

Density15ºC, kg m− 3

Viscosity40ºC, cSt

1 2 3 4 5 6 7 8 9 10 11 12 EN-14214

45 25 45 45 45 45 45 45 45 45 45 45

100 100 80 80 80 80 100 60 40 80 80 80

6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 12:1 9:1 3:1

1.0 1.0 1.0 0.7 0.5 0.3 0.7 0.7 0.7 0.7 0.7 0.7

91.7 90.8 92.9 92.2 89.4 75.4 93.3 90.0 35.0 92.0 91.7 54.2 96.5

910 914 912 909 909 923 915 912 940 912 909 930 860–900

16.7 16.8 15.4 15.2 17.2 25.2 15.8 17.9 55.0 15.3 15.5 41.5 3.5–5.0

J.M. Encinar et al. / Fuel Processing Technology 103 (2012) 9–15

100 90 80 70 60 50 40 30 20 10 0

3.3. Effect of ultrasound power

25kHz 45kHz 0

20

40

60

Time (min) Fig. 2. Effect of ultrasound frequency on the extent conversion (6:1 MeOH:oil, 1%KOH, 100% ultrasound power).

Conversion (%)

Initially, insufficient amount of KOH results in incomplete conversion of triglycerides into the esters as indicated from its lower ester content. However, above a catalyst concentration of 0.7%, there isn't a significant extent of the reaction conversion. The observed results can be attributed to the side reactions, like saponification reaction, that occur simultaneously with transesterification and reduce the conversion of triglycerides to desired ester. It is possible to see two zones. In the first, which can extend up to 20–25 min, we see a positive influence on the amount of catalyst on the rate of formation of methyl esters. Indeed, as the amount of catalyst is greater, it reaches peak production sooner, which is to say, the asymptotic zone of the curve. Thus, in experiment with 1.0% the maximum is reached in just 15 min, whereas in the experiment carried out at 0.3% this maximum is not reached until 50 min. The second zone of the curve (the asymptote) is very similar in all cases. It is also noted an increased induction period in the experiments with lower rates of catalyst. Given the results obtained, the rest of the experiment was conducted with a catalyst concentration of 0.7 wt.%. This results in catalyst savings, which is positive from an economic standpoint, and also facilitates the processing of the final product, which is positive from technical and environmental standpoints. By increasing the amount of catalyst, emulsions are formed in the washing step thus hindering the purification. During washing the soap present in the ester phase has the tendency to accumulate at the interfacial region between the liquids. Due to its structure (one hydrophilic end and a long hydrophobic chain) the soap molecules are oriented perpendicular on the interfacial region between the two immiscible phases: water and esters. The soap molecules collect inside esters and water molecules forming emulsion, thus making the separation difficult [21].

100 90 80 70 60 50 40 30 20 10 0

1.0% KOH 0.7% KOH 0.5% KOH 0.3% KOH 0

10

20

30

40

50

60

Time (min) Fig. 3. Effect of catalyst concentration on the extent of conversion (45 kHz, 80% ultrasound power, 6:1 MeOH:oil).

The optimization of the amount of energy supplied to the reaction mixture allows achieving the highest rates of biodiesel formation and maximum yields at the lowest possible energy input, which reduce the cost of production of biodiesel. Hence, the effect of ultrasound power on the biodiesel formation was studied for 45 kHz. The power was varied as specified, while other experimental conditions were the same for all of the reactions in this set of experiments (Fig. 4). Using a lower power (40%), despite the evolution of the samples did not show any conversion, however a final product was obtained with 35% conversion (probably due to the difficulty in taking homogenous samples). When the power was increased, extent of conversion also increased. However, from 60% to 100%, there are no significant differences on the conversion. This can be attributed to the fact that at higher power levels, cushioning effect is usually observed which results in decreased transfer of energy into the system and hence lower cavitational activity [19]. At the beginning of the reaction, there is very low interfacial area available for the reaction to occur (heterogeneous system). Hence, the rate of the reaction is very low, and the curves are initially flat, with induction periods observed in some cases. As time increases, the amount and size of the emulsion formation vary because of ultrasonic cavitations, which result in a larger increase in the interfacial area available for mass transfer. This increase, in the interfacial area, causes the corresponding rate increase in the formation of the biodiesel. The curve virtually flattens out when significant conversions are achieved as the reaction approaches its equilibrium. These observations can be easily explained by noting that ultrasonic parameters positively influencing cavitations in liquids, in general, improve emulsifications in terms of smaller droplet size of the dispersed phase after disruption. When the intensity (i.e., ultrasonic power/irradiation area) is increased, the acoustic amplitude increases and a more violent collapse of the cavitation bubble will occur. The harsher the collapse of the cavitation bubble, the higher the jet velocity and micromixing at the phase boundary between the oil and methanol phases. This results in finer emulsion formation and, hence, a higher mass-transfer coefficient and thus higher biodiesel formation [20]. 3.4. Effect of molar ratio of methanol to oil According to the stoichiometric reaction [22], 3 mol of alcohol per mole of triglyceride (oil) is necessary to produce 3 mol of methyl esters and 1 mol of glycerin. However, considering that the transesterification reaction is a chemical equilibrium, we can move this equilibrium to the right using methanol: oil relations over the stoichiometric. The effect of alcohol amount on yield of the transesterification experiments was conducted with different ratios of methanol to oil in

Conversion (%)

Conversion (%)

12

100 90 80 70 60 50 40 30 20 10 0

100% 80% 60% 40% 0

10

20

30

40

50

60

Time (min) Fig. 4. Effect of ultrasound power on the extent of conversion (45 kHz, 6:1 MeOH:oil, 0.7%KOH).

Conversion (%)

J.M. Encinar et al. / Fuel Processing Technology 103 (2012) 9–15

100 90 80 70 60 50 40 30 20 10 0

12:1 9:1 6:1 3:1

0

10

20

30

40

50

60

Time (min) Fig. 5. Effect of molar ratio of methanol to oil, on the extent of conversion (45 kHz, 80% ultrasound power, 0.7%KOH).

the range of 3:1 to 12:1. Fig. 5 shows the changes in percentage of methyl esters formed with the different molar ratios of methanol to oil. It can be seen that with the increase of the molar ratio of methanol to oil from 3:1 to 9:1, the methyl ester yield increased from 54% to 92%. However, the yields were slightly reduced when the ratio of methanol to oil was higher than 9:1. Therefore, increasing the alcohol amount beyond the optimal ratio will not increase the yield, but it will increase the cost for alcohol recovery, hence, an optimum operating ratio should be selected on the basis of overall economics and the equilibrium conversion. It can also be observed that there is a significant increase in the rate of reaction: with 6:1, it took about 40 min to reach equilibrium conversion, whereas with 9:1 molar ratio the equilibrium conversion was achieved in only 20 min. With an increase in the feed molar ratio, the quantity of methanol in the reaction mixture increases, which mainly affects the cavitational intensity. Excess of methanol provides additional cavitation events in the reactor, leading to formation of enhanced emulsion quality (smaller drop sizes), providing additional area for the reaction and hence increasing conversion. Excess of methanol also favors the removal of water of reaction as aqueous phase, thereby not hindering in the progress of reaction [14].

3.5. Fuel specifications Table 2 shows the yield, density and viscosity obtained for all experiments. Table 3 shows the influence of operating variables on other parameters of the biodiesel correspondent to the experiment with the higher yield in methyl esters, which reaction conditions were: 45 kHz, 80% ultrasound power, 6:1 MeOH:oil and KOH 0.7 wt.%. These parameters are very important since the quality of

Table 3 Properties of biodiesel produced from castor oil under optimal conditions (45 kHz, 100% ultrasound power, 6:1 MeOH:oil and [KOH] 0.7 wt.%) in this study. Parameter

This study

EN-14214

Water content, % −1 Saponification value, mgKOHgoil Iodine value, wt.% −1 Acidity index, mgKOHgoil CFPP, °C Flash point, °C Combustion point, °C Cetane index Monoglyceride, % Diglyceride, % Trigliceryde, % Fatty acid methyl esters, %

0.08 174.1 81.1 0.49 18 215 220 34.9 0.5 0.2 0.4 93.3

b 0.05 – ≤120 ≤ 0.5 ≥120

≤0.8 ≤0.2 ≤0.2 ≥96.5

13

the final product (biodiesel) is strongly conditioned by them. The fuel properties of the biodiesel were determined with the help of standard tests, and it was found that the biodiesel properties were very close to diesel fuel specifications, according to EN-590, and biodiesel European Standard draft. For comparison, in Tables 2 and 3, standard EN-14214 values have been enclosed. Density is one of the most important properties of fuels, because injection systems, pumps and injectors must deliver the amount of fuel precisely adjusted to provide proper combustion [23]. It is known that biodiesel density mainly depends on its methyl ester content and the remaining quantity of methanol; hence this property is influenced primarily by the choice of vegetable oil and, in some extent, by the applied purification steps [24]. In this study, the density ranges between 909 and 940 kg.m − 3. In general, as conversion increases the density decreases as shown in Table 2, the value obtained in some experiments are justified by the low yield. According to EN 14214 biodiesel density should be between 860 and 900 kg.m − 3. Therefore, castor biodiesel would be outside those limits, the density can be corrected by the use of blends with petro diesel. The viscosity is a very important property related to the biodiesel utilization. Fuel viscosity impacts on both injection and combustion efficiencies. Higher viscosity leads to a higher drag in the injection pump, causing higher pressures and injection volumes, especially at low engine operating temperatures. As a direct consequence, the timing for fuel injection and ignition tends to be slightly advanced for biodiesel, which may, in turn, lead to higher NOx emissions due to higher maximum combustion temperatures [25]. The viscosity of castor oil, studied in this work, is 258.0 cSt and the best value for biodiesel achieved with the transesterification from that oil was 15.2 cSt. EN 14214 sets the limits of viscosity between 3.5 and 5.0 cSt. As with density, castor biodiesel break this rule. The viscosity should be corrected by mixing with mineral diesel. In Table 2 it can be observed that the viscosity decreases with conversion. Biodiesel contaminated with water can cause engine corrosion or reaction with glycerides to produce soaps and glycerol. Therefore, the EN standard restricted the water content in biodiesel setting the maximum allowable content of 0.05% (i.e. 500 mg/kg). On the industrial scale, dewatering is usually carried out by distillation under vacuum (5 kPa) at temperatures of 30–40 °C, leading to the more pronounced decrease of the water content [24]. As it can be observed in Table 3, the saponification value was 174.1 mg of KOH per gram of sample. The saponification value is related to the average molecular weight of the sample. But the acids that are present in the glycerides or in the methyl esters are the same. Only the change of glycerol by ethanol is produced. In consequence, the average molecular weight does not change significantly [17]. The number of double bonds of fatty acids is related to the iodine value. This parameter describes the content of unsaturated fatty acids and it is only dependent on the origin of the vegetable oil. In consequence, the biodiesel obtained from the same oil should have similar iodine values. A limitation of unsaturated fatty acids may be necessary because the higher heating of unsaturated fatty acids results in polymerization of glycerides. Results reveal that high iodine values have been linked with low oxidation stability, causing the formation of various degradation products, which can negatively affect engine operability by forming deposits on engine nozzles, piston rings, and piston ring grooves. This effect increases with the number of double bonds in the fatty acid chain. Therefore, it is better to limit the content of higher unsaturated fatty acids, like linolenic acid, than to limit the degree of unsaturation with the iodine number [17,25,26]. In the standard EN-14214, both parameters are limited. The maximum iodine value is 120 and the maximum linolenic acid ester is 12 wt.%. In our case the iodine value, 81.1, was inferior to 120 in all cases. On the other hand, as it has been indicated previously, the linolenic acid contained in the oil was only 0.5 wt.%.

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The acidity index, expressed as mg KOH/g of sample, is in accordance with the maximum required limits given in the EN 14214 biodiesel standard norm (0.5 mg KOH/g). The highest temperature at which the fuel, when cooled under defined conditions, will not flow through a filter of a defined wire mesh, within a certain time, is called cold filter plugging point (CFPP). One of the major problems associated with the use of biodiesel is poor flow properties at low temperatures. Partial solidification in cold weather may cause blockages of fuel lines and filters, leading to fuel starvation and problems during engine start-up. Long-chain saturated fatty esters increase significantly CP and PP, reducing saturated fatty acid content of vegetable oils and can improve cold temperature flow properties of biodiesel. To improve cold temperature flow characteristics of biodiesel, several proposals have been suggested, including winterization, additives, esterification with branched alcohols, and modification of oil chemical composition [25,26]. In the standard EN 14214, the CFPP value is not specified, since it is different in each country. The biodiesel from castor oil has a high CFPP having although a lower content of saturated fatty acids. This limitation is mainly caused by the viscosity, which increases as the temperature decreases and, therefore, flows more slowly. The filter does not get clogged, but biodiesel does not flow in the time required for analysis. Flash point is the temperature at which the fuel ignites when exposed to a flame. The flash point of biodiesel is higher than the conventional fuel which makes it safer. Low flash points will also indicate the presence of methanol and therefore it is a very important parameter to be determined considering handling, storage and safety of the fuel [27]. The combustion point is the temperature at which a flame causes ongoing inflammation of a fuel and, usually, is higher than the flash point. The minimum value required by the standard EN 14214 is 120 °C for the flash point, without establishing a value for the combustion point.The biodiesel analyzed has a flash point of 215 °C, quite above the required value and other vegetable oils, and a combustion point of 220 °C. Compared with diesel no. 2 in which typical valuesof flash and fire points are 85 and 95 °C respectively [17], this biodiesel has a guarantee safety. The cetane number is an important indicator of the quality of the fuel and it is usually measured using a standard engine test (ASTM D613). However, it is relatively difficult to measure and has been rarely determined for vegetable oils and fatty acid esters [17]. In this work, the ASTM Standard D976 was applied, using the boiling point and density for the calculation of the cetane index. This parameter guarantees that there will be a good control of the combustion, increasing performance and improving cold starts, which gives rise to less exhaust gases. The standards EN 14214 and EN 590 specify that the cetane number must have a minimal value of 51. A quantitative relation between the cetane index and the cetane number does not exist. The value of the cetane index is similar of a diesel fuel. As an example, according to EN 590, a typical value for no. 2 diesel is about 46 [22]. The cetane index obtained for the biodiesel synthesized from castor oil has a value of 34.9, lower than mineral diesel and required by the standard. Finally, the amount of mono-, di- and triglycerides is little. In the case of the monoglycerides, it is smaller than the values recommended by the standard EN 14214. The presence of these products, in high concentration, increases the formation of carbon residues. Hence, in our case this problem would be of little account. 4. Conclusions The castor oil transesterification, under ultrasonic conditions, presents itself as a good alternative for the production of biodiesel, since high methyl ester yield and fast reaction rate can be obtained even if reaction temperature is relatively low. This would be able to imply an energetic saving, but it would be necessary to consider the energy

consumed in the generation of the radiation. Hence, an energetic balance should be carried out considering both options. The methyl esters derived from castor oil has properties that generally satisfy the limits in EN 14214. The properties, such as viscosity, number of cetane and CFPP, which do not meet the values established by the standard (although very close to these), could be corrected by mixing with commercial oil or by adding certain additives. Consequently, the use of this oil is an effective way to reduce the raw material cost. In addition, the pollution problems could be reduced. On the other hand, in general, the variables of operation tested, frequency and supplied power dissipation, catalyst (KOH) concentration and methanol oil molar ratio, exercise a positive effect on the reaction rate, but the obtained final yield (equilibrium) is very similar in all cases

Acknowledgment The authors express their gratitude to the “MICINN” and the “Junta de Extremadura” for the financial support received to perform this study by means of Projects ENE2009-13881, PRI09B102 and PDT 09A037 respectively.

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