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Acid base catalyzed transesterification kinetics of waste cooking oil
Fuel Processing Technology 92 (2011) 32–38
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Acid base catalyzed transesterification kinetics of waste cooking oil Siddharth Jain ⁎,1, M.P. Sharma 2, Shalini Rajvanshi Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India
a r t i c l e
i n f o
Article history: Received 11 March 2010 Received in revised form 7 August 2010 Accepted 29 August 2010 Keywords: Waste cooking oil (WCO) Transesterification kinetics Methyl ester (ME) Fatty acid methyl ester (FAME)
a b s t r a c t The present study reports the results of kinetics study of acid base catalyzed two step transesterification process of waste cooking oil, carried out at pre-determined optimum temperature of 65 °C and 50 °C for esterification and transesterification process respectively under the optimum condition of methanol to oil ratio of 3:7 (v/v), catalyst concentration 1%(w/w) for H2SO4 and NaOH and 400 rpm of stirring. The optimum temperature was determined based on the yield of ME at different temperature. Simply, the optimum concentration of H2SO4 and NaOH was determined with respect to ME Yield. The results indicated that both esterification and transesterification reaction are of first order rate reaction with reaction rate constant of 0.0031 min− 1 and 0.0078 min− 1 respectively showing that the former is a slower process than the later. The maximum yield of 21.50% of ME during esterification and 90.6% from transesterification of pretreated WCO has been obtained. This is the first study of its kind which deals with simplified kinetics of two step acid–base catalyzed transesterification process carried under the above optimum conditions and took about 6 h for complete conversion of TG to ME with least amount of activation energy. Also various parameters related to experiments are optimized with respect to ME yield. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The nation imported 121.672 million tons of crude oil for $67.988 billion in 2007–08 as opposed to 111.502 million tons imported for $48.389 billion in 2006–2007, according to the latest data released by the petroleum ministry in New Delhi. As our oil import dependence has reached 80%, it is important to undertake optimal exploitation of domestic energy resources with a view to increasing our country's energy security [1]. This fact coupled with the recent hike of crude oil prices up to 135US$ per barrel (2008) has forced the search for alternative fuels based on renewable energy. Since the diesel is being heavily used in diesel engines, therefore, the search for alternative to diesel fuel become inevitable. Biodiesel from vegetable oil resources can substitute diesel in considerable proportion and therefore, vegetable oil resources, particularly, the non-edible one, deserve due consideration. Biodiesel, the monoalkyl esters of long chain fatty acids derived from a renewable lipid feedstock such as vegetable oil or animal fat, is becoming popular as substitute of or additive to diesel in developing as well as developed countries [2,3]. In India, fuel ethanol and biodiesel have been acquiring special importance from energy security and environmental concerns point of view as they can offer large scale employment in the growing and processing of resource, particularly in
⁎ Corresponding author. Tel.: + 91 9456382050; fax: + 91 1332 273517. E-mail addresses: [email protected] (S. Jain), [email protected] (M.P. Sharma). 1 Tel.: + 91 1334 232694; fax: + 91 1332 273517. 2 Tel.: + 91 1332 285836; fax: + 91 1332 273517. 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.08.017
rural areas [4]. The main advantages of biodiesel is its renewability, better quality of exhaust gas emissions, its biodegradability and its contribution to the reduction in CO2 emissions[5]. The biodiesel can be prepared by transesterification process which combines vegetable oils with alcohol in the presence of the catalyst to form fatty acid alkyl esters (i.e., biodiesel) and glycerol [5–22]. Methanol is the most commonly used alcohol for transesterification because of its low cost [3,5]. Numerous studies have been carried out on various aspects of biodiesel production for use in diesel engine. Most of the current challenges concern with the reduction of production cost which is still higher than petrodiesel owing to higher cost of non-edible oil resources. The WCO available from restaurants, food processing industries, fast foods industries has been found to be one of the resources that can be used to produce biodiesel with much less required for its production except collection and refinement. Boocok and co-workers [6–8] have reported that one step base catalyzed methanolysis of soybean oil using tetrahydrofuran as co-solvent, has been found suitable if oil has free fatty acid (FFA) less than 1%. Only well refined vegetable oils with less than 1% FFA were transesterified by Zhang et al. [12,16] who compared different processes of biodiesel production and found that alkalicatalyzed process is the simplest one but had a higher raw material cost compared to other processes. For the oils with high FFA contents, either the acid catalyzed or two step acid–base catalyzed process is preferred which requires excess of methanol. In the later case, the first acid catalyzed step is used to reduce FFA to b1% (w/w) followed by base catalyzed transesterification for the conversion of TG to ME [12–16]. Dufek et al. [23] have studied the acid catalyzed esterification and transesterification of 9-(16)-carboxystearic acid and its mono and di-
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methyl esters and reported unequal chemical reactivity for different carboxyl and carboxyl methyl groups. Freedman et al. [24] reported the transesterification of soyabean and other oils with methanol and butanol to examine the effect of alcohol type on the transesterification process. Noureddini and Zhu [25] studied the effect of mixing of soyabean oil with methanol on the kinetics of reaction using one phase transesterification process and found that the mixing had profound effect on the ME yield. Separate acid catalyzed, alkali catalyzed, enzyme catalyzed, or supercritical transesterification of different oils including WCO has been studied by many researchers (26–30). Diasakov et al. [26] investigated the kinetics of unanalyzed transesterification reaction of soyabean oil. Further, several authors have used the methods involving costly chemicals and requiring much time and efforts for the analysis of intermediate reaction products during the course of kinetics study of transesterification but Kusdiana and Saka [27] have used % yield of methyl ester as the only parameter to monitor the rate of reaction and that 3 step conversion from TG-DG, DG-MG and MG-ME has been simplified in terms of conversion of TG to ME. Barnwal and Sharma [28] has carried out the techno-economic analysis of biodiesel production from different oil feedstocks and found that pongamia, a non-edible oil, can yield biodiesel @ Rs.10.50/ l compared to diesel (Rs.22/l) at that time and sesame oil gave the costliest biodiesel @ Rs.54/l. The land requirement for growing Jatropha plants for meeting the requirement of different blends of biodiesel with diesel like B5, B10 and B20 were calculated for the buses of Uttar Pradesh State Road Transport Corporation (UPSRTC) and reported in our earlier paper [22,29]. Khan et al. [30] have developed the quantitative analysis of the product mixture formed during transesterification reaction. Mittelbach [31] described a simple and reliable method, which allows the determination of the overall content of tri-, di-, and monoglycerides in fatty acid methyl esters (FAME) by isocratic liquid chromatography using a density detector. Saiffudin and Chau [32] studied the transesterification of used frying oil with 0.5% NaOH in ethanol using microwave irradiation and found that there is considerable enhancement in the reaction rates. The consumption of edible oil is very high in the country and still the indigenous production does not meet the consumption and considerable amount of edible oil is imported and it is therefore, not advisable to divert these sources for biodiesel production. On the other hand, the non-edible oil resources can be a solution for biodiesel production. The seed oil producing plants can be grown on waste/ semi arid lands under National Biodiesel Programme of Govt. of India and potential availability of non-edible oils in India is about 1 million ton per year. Presently, Jatropha, Pongamia, Sal, Mahua, Neem, waste cooking oil (WCO) etc. have been identified as the main non-edible oil resources, out of which WCO has been selected for the present study. Commercially available crude oils and fats contain considerable amount of free fatty acids (FFA) that react with the alkaline catalyst and form saponified products during base catalyzed transesterification that also needs exhaustive purification of the products. The saponification not only consumes the alkali catalyst but also the
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resulting soaps can cause the formation of emulsions which create difficulties in downstream recovery and purification of the biodiesel. This puts significant limitations on the conversion of WCO to biodiesel by base catalyzed transesterification process. Since, the FFA in WCO is more than 2.0% (w/w), it is prudent to reduce FFA by esterification using methanol and acid catalyst. The resulting oil phase having a low FFA (less than 0.5% w/w) can then be subjected to the base alkalicatalyzed transesterification for biodiesel production [33]. There are several routes to obtain biodiesel using oilseeds, edible oils and waste oils as feed stock. Transesterification of triglycerides with low molecular weight alcohols catalyzed by homogeneous catalysis is the most used one [34–39]. In view of the above, it is seen that little work is reported on the kinetics of two step transesterification of high FFA oils. Previous authors mainly concentrated on low FFA vegetable oils but in the present work, high FFA waste cooking oil, is used for study purpose. The present paper reports the results of simplified kinetics of transesterification carried out with respect to ME yield only which is not only simple but also less time consuming and less costly compared to separate acid catalyzed process which takes much longer time for its completion as evidenced by limited reports available in the literature. It is possible to convert high FFA waste cooking oil to biodiesel with least amount of activation energy. Also various parameters related to conversion of waste cooking oil into biodiesel have been optimized with respect to ME yield. This is the first study of its kind which uses simple 2 step acid–base transesterification of high FFA waste cooking oil to convert it into biodiesel. As on storing, the FFA contents of oil increased significantly, the present technology will be very useful to convert high FFA stored oils to biodiesel. 2. Material Waste cooking oil was collected from hostel messes of Indian Institute of Technology Roorkee (India) campus. The availability of WCO is about 160 l per month which may further increase with hostel capacity. The entire supply of refined soybean oil to all the messes is from one supplier and therefore the oil quality has been assumed to be same throughout the year. All the chemicals like H2SO4, KOH, methanol, anhydrous Na2SO4, etc. used were of analytical grade and 99% pure. Raw WCO was filtered to remove all insoluble impurities followed by heating at 100 °C for 10 min to remove all the moisture. The fatty acid compositions of refined soybean oil and its WCO as determined by Gas Chromatography are shown in Table 1. The fuel properties of refined WCO and its biodiesel were determined as per standard method and reported in Table 2. The above table indicates that about 93.1% FFAs are unsaturated and therefore WCO has high saponification value, i.e., 198.10 mg KOH/ g oil which can be computed from the following equation [40]. SV = 268–ð0:418⁎P Þ–ð1:30⁎SÞ–ð0:695 OÞ–ð0:77⁎LÞ–ð0:847⁎LLÞ where, SV = saponification value, P = Palmitic acid, S = Stearic acid, O = Oleic acid, L = Linoleic acid and LL = Linolenic acid.
Table 1 Fatty acid composition of refined soybean oil and its WCO collected from messes. Fatty acid
Formula
Palmitic acid (P) Palmitolileic acid Stearic acid (S) Oleic acid (O) Linoleic acid (L) Linolenic acid (LL) Arachidice acid Gadolic acid
C16H32O2 C16H30O2 C18H38O2 C18H34O2 C18H32O2 C18H30O2 C20H40O2 C20H36O2
CH3(CH2)14COOH CH3(CH2)5CH CH–(CH2)7–COOH CH3(CH2)16COOH CH3(CH2)7–CH CH–(CH2)7COOH CH3(CH2)4 CH CH–CH2–CH CH–(CH2)7 COOH CH3(CH2)4CH CH–CH2–CH CH–CH2–CH CH–(CH2)4 COOH CH3–(CH2)18 COOH
Systematic name
Structure
% FFA in edible oil
% FFA in WCO
Hexadecanoic acid Cis-9 hexadecnoic acid Octadecanoic acid Cis9-Octadecanoic acid cis-9-cis-12-Octadecadeneoic acid cis-6-cis-9-cis-12 Octadecatrienoic acid Eicosanoic acid
C16 C16:1 C18 C18:1 C18:2 C18:3 C20 C20:1
10.4 0.35 4.7 24.8 52.5 6.5 0.75 –
4.1 2.4 1.4 40.5 38.0 10.6 0.8 1.6
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Table 2 Fuel properties of waste cooking oil and its biodiesel. Properties
Waste cooking oil
Biodiesel from waste cooking oil
Density (kg/m3 @ 15 °C) Viscosity (cSt, @ 30 °C) Flash point (°C) FFA contents (%) Gross calorific value (MJ/kg) Acid value (mg KOH/g)
937 50 235 21.84 38.27 21.84
892 4.2 178 0.67 37.82 0.38
The experiments were conducted in a batch reactor of 1.5 l capacity equipped with condenser, stirrer, inlet and outlet and temperature measurement instrument. The whole setup was put in constant temperature water bath (±0.5 °C). The schematic of the experimental set up is shown in Fig. 1.
2.1. Experimental procedure The experiments were performed at different temperatures using different concentrations of H2SO4 and NaOH in acid and base catalyzed reactions with respect to ME yield. Due to high FFA contents of WCO (21.84%), a two step process was selected for converting oil into methyl ester. The acid catalyzed esterification was used to reduce the FFA to b1% using H2SO4 as catalyst at an optimum temperature. The resulting oil having b1% FFA was finally subjected to base catalyzed transesterification at optimum temperature to produce ME. The procedure is described in the next section.
2.1.1. Acid pretreatment step The refined and moisture free oil was poured into the reactor (Fig. 1) and heated at different temperatures (20, 30, 40, 50, 60, 70 and 80 °C) to optimize the temperature for maximum yield and maximum FFA reduction. The mixture of conc. H2SO4 (0.5%, 1%, 1.5%, 2% and 3% w/w) with methanol (30% v/v) was separately heated at same temperatures (20, 30, 40, 50, 60, 70 and 80 °C) and then added to heated oil in the reactor. The mixture was heated at that temperature for 3 h to complete the esterification. The samples were withdrawn at pre-determined time intervals to calculate % ME yield. After acid catalyzed transesterification step, the mixture was cooled and allowed to settle overnight. This mixture was finally subjected to base catalyzed reaction, where all the residual acid is neutralized and as such, the acid removal is not required in the above step.
2.1.2. Base catalyzed transesterification The esterified oil (b1% FFA) was poured into the reactor and heated at different (20, 30, 40, 50, 60, 70 and 80 °C) to optimize the temperature of reaction for maximum yield. A mixture of NaOH (0.5%, 1%, 1.5%, 2% and 3% w/w) in methanol was heated at the same temperature (20, 30, 40, 50, 60, 70 and 80 °C) for 5 min and added slowly to the heated oil. The reaction mixture was heated, refluxed and stirred at 400 rpm for about 3 h. The samples were withdrawn at pre-determined time intervals to determine % ME formed. After 3 h, two distinct layers were formed and the mixture was allowed to settle for 2 h or overnight. The heavier glycerol layer was separated from the lighter ME layer by separating funnel. 2.2. Sample treatment The ME layer was separated, washed with water, heated to remove moisture and dried over anhydrous Na2SO4 and checked for its purity using GC, thin layer chromatography and flash point. The rate of reaction was calculated using % ME yield data with respect to time and reaction rate constants and order of reaction was also evaluated along with activation energy of transesterification step and the results are reported in the Results and discussion section. 2.3. GC analysis The samples were analyzed for ME formation at a predetermined interval of time by Gas Chromatograph (Netal make) equipped with a flame ionization detector and a capillary column for injecting the sample. The GC oven was kept at 230 °C (5 °C/min) and a total analytical time was 30 min. Nitrogen was used as carrier gas. Quantitative analysis of % ME was done using European standard EN 14103:2003 [41]. The % ME yield was calculated using Eq. (1). Free fatty acids in the samples were determined using stock solution (Methyl heptadecanoate and n-heptane). % of ME = ∑A AEI CEI VEI m
∑A−AEI C ⁎V × EI EI × 100 AEI m
ð1Þ
Total peak area from the methyl ester in C14 to that in C24:1; Peak area corresponding to methyl heptadecanoate; Concentration of the methyl heptadecanoate solution (mg/ml); Volume of the methyl heptadecanoate solution (ml); Mass of the sample (mg).
3. Results and discussion 3.1. Determination of optimum reaction parameters 3.1.1. Effect of reaction temperature The results of variation of ME yield with temperature (20, 30, 40, 50, 60, 70 and 80 °C) using different concentration of H2SO4 as acid and NaOH as base catalyst (0.5%, 1%, 1.5%, 2% and 3% w/w) for esterification and transesterification using optimum amount of methanol, i.e., 30% of the total volume (v/v) are shown in Fig. 2(a) and (b) which shows that during esterification, the maximum yield of ME (21.5%) was obtained at 65 °C at 1% (w/w) H2SO4 concentration, while during transesterification, 90.6% yield of ME was obtained at 50 °C using 1% NaOH (w/w). As the value of reaction parameters increases beyond this value FAME yield decreases due to reversible nature of reaction [42–44].
Fig. 1. Schematic diagram of experimental setup.
3.1.2. Effect of catalyst amount Effect of catalyst concentration is shown in Fig. 2(a) and (b) for esterification and transesterification respectively. According to this as the amount of catalyst increases, the ME yields also increases and at 1% (w/w) catalyst concentration the yield will be highest in both the
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was over 3:7, glycerol separation becomes more difficult due to emulsion of glycerol with ME, thereby decreasing the yield. This observation was also reported in the literature [42,44]. 3.1.4. Effect of reaction time The variation of ME yield with time are shown in Fig. 4(a) and (b) which indicates that during esterification, the yield of ME increased with time at a faster rate within first 100 min owing to the faster rate of FFA reduction and thereafter it becomes constant indicating that further conversion of FFA to ME has almost completed within 180 min. While during the base catalyzed transesterification, % ME yield has increased from 21.5% to 90.6% within 180 min. Variation of ME yields vs time (Fig 4b) show that no further conversion of TG to ME takes place after this period. Further, the graph of % FFA reduction with time at various methanol to oil ratio (Fig. 5) indicates that 98% conversion of FFA has taken place during 180 min in first step. The variation of % ME yield vs % FFA reduction as shown in Fig. 6 shows a linear relationship between FFA reduction and ME yield during acid esterification step with a slope of 0.9223 indicating that during acid catalyzed esterification as the FFA content decreases from 21.84% to less than 1%, ME yield increases up to 21.5% linearly with no further increase in the same. 3.2. Kinetics of transesterification of waste cooking oil
Fig. 2. % ME yield vs. temperature (a) acid esterification and (b) base catalyzed transesterification at different catalyst concentration.
The kinetic studies of transesterification of WCO have been carried out at the optimum conditions of temperature, catalyst concentration and 400 rpm of stirrer (the later has not been optimized in the present study). Diasakov et al. [26] has reported that the transesterification reaction to proceed in 3 steps in which triglyceride (TG) reacts with methanol to produce diglycerides (DG), which further reacts with methanol to yield monoglycerides (MG) that finally reacts with methanol to produce methyl ester and glycerol. Since one mole of methyl ester is generated per mole of methanol reacted at each step,
cases. However, if the catalyst concentration goes beyond this value then the ME yield will not increase. This observation is in agreement with literature also [25,27,42,44]. 3.1.3. The effect of methanol/oil ratio The variation of different amount of methanol as % of oil has been studied with respect to % ME yield under the above optimum conditions and the results are reported in Fig. 3 which shows that a maximum yield of 90.6% was obtained at methanol to oil ratio of 3:7 and therefore this optimum ratio has been used during the experimentation. As the % methanol is increased up to 30% (v/v), yield increases up to 90.6%. However, when the amount of methanol
Fig. 3. %ME formed vs. % volume of methanol of the oil for transesterification step.
Fig. 4. % ME yield vs time (a) acid esterification and (b) base transesterification.
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respect to % ME yield as a function of time. Therefore the first order rate constant of the reaction can be expressed by Eq. (6). Rate = −
dðMEÞ dt
ð6Þ
where ( ) represents the %ME yield. Rate equation of first order may also be written as −Rate = k(ME). It can be modified as dðMEÞ = kðMEÞ dt where (ME) refers to the % ME yield. Assuming that the initial concentration of ME at time t = 0 is ME0 and that it increases to MEt at time t. The integration of this equation gives: ME
∫MEt0 Fig. 5. Variation of FFA vs time for different methanol to oil ratio (v/v) during acid catalyzed step.
in all, six rate constants are reported in the literature for the whole reaction from TG to methyl ester (biodiesel) as shown by the equation given below in Eqs. 2, 3 and 4. k1
0
TG þ ROH ⇄ DG þ R CO2 R
ð2Þ
k4
k2
0
DG þ ROH ⇄ MG þ R CO2 R
ð3Þ
k5
k3
0
MG þ ROH ⇄ GL þ R CO2 R
ð4Þ
k6
According to the above equation, whole transesterification results ultimately in the production of methyl ester and therefore, all the intermediate reaction products (e.g. DG and MG) can be ignored and simple mathematical model expressing the whole conversion as one step has been developed (Eq. 5).
→3R CO R þ GL Catalyst
TG ¼ 3ROH
0
2
ð5Þ
The whole reaction is assumed to precede as first order reaction as a function of concentration of ME [26,27]. The rate constant of the reaction can be determined based on the increased amount of the product that occurs in some reaction time interval [5,17]. In this work, the increased amount of one reactant that is ME was choosen. Accordingly, the kinetics of transesterification has been studied with
t dðMEÞ = k ∫ dt ðMEÞ 0
and ln
ðMEt Þ = kt ME0
or k=
InðMEt Þ−InðME0 Þ t
ð7Þ
If the slope of ‘ln k’ vs ‘1/T’ is a straight line then the assumption that have been taken for the reaction, i.e., 1st order will be correct. Using Eq. (7), the reaction rate constant under optimum conditions of temperature, catalyst concentration, methanol and rpm of stirring, has been computed as 0.0031 min− 1 and 0.0078 min− 1 for esterification and transesterification respectively. This indicates that the rate of acid catalyzed esterification is slower than base catalyzed transesterification perhaps due to the presence of considerable amount of intermediate reaction products interfering in the transformation to methyl ester [45,47 and 48]. This observation is in contrast to the faster rate of esterification reaction compared to slow transesterification under supercritical transesterification conditions [46]. Due to this reason the conversion of high FFA oil takes longer time to convert to ME compared to oil with low FFA. Further, a plot of dME/dt vs yield using log graph gives straight line for both esterification and transesterification reaction as shown in Fig. 7(a) and (b) which indicates that both the reaction are of first order. This also indicates that the rate of reaction varies linearly with yield. The slope of straight line for esterification and transesterification respectively are given by Eqs. 4and5 which indicates that rates of reaction varies linearly with yield. For acid transesterification: ln ðdME = dtÞ = −0:9524 ln ðMEÞ + 2:9288
2 R = 0:9988
ð8Þ
2 R = 0:9577
ð9Þ
For base transesterification: ln ðdME = dtÞ = −0:8713 ln ðMEÞ + 2:4369
Fig. 6. Variation of % ME yield (%) with % to FFA reduction during acid transesterification.
The slope of the straight line in Fig. 7a and b was found as −0.9524 and −0.8713 with a value of R2 of 0.9988 and 0.9577 indicating that the data collected are 99.88% accurate for esterification and 95.77% accurate for transesterification reaction. Further, the transesterification was carried out at various temperature for different time and ME yield was calculated. The results of the same are shown in Fig. 8, indicates that maximum yield of ME (90.6%) occurred at 50 °C compare to other temperatures. Above 50 °C, the ME
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(a)
37
3.3. Determination of activation energy Activation energy is calculated through Arrhenius equation which include activation energy ‘Ea’ and frequency factor ‘A’, given below [48]: −Ea = RT
ð10Þ
k = Ae
Where, R the universal molar gas constant and T is the temperature (K). Since the activation energy is dependent on temperature and therefore the rate constants at any temperature (within the validity of the Arrhenius equation) [19] can be computed using Eq. (11):
(b)
Fig. 7. Plot of reaction rate vs yield on logarithmic graph (%) for (a) acid esterification and (b) base transesterification of WCO.
yield decreases to a very little amount due to reversible nature of reaction [47]. The results of variation of % ME yield with times at various temperatures as given in Fig. 8 indicates that during base catalyzed transesterification, the maximum yield of 90.6% ME was obtained in 180 min when reaction temperature was 50 °C as the reaction time or temperature increases beyond this value, the FAME yield decreases due to reversible nature of the reaction [42,44].
Fig. 8. Variation of FAME yield with time for various temperatures.
ln k = ln A−Ea = RT
ð11Þ
Eq. (11) is a linear equation and therefore a plot of lnk and 1/T is given in Fig. 9 also validates the first order of the reaction. Activation energy (Ea) has been calculated using Fig. 9 which is 88,764.53 J/mol. Based upon the above results, optimum conditions for biodiesel production from WCO are summarized in Table 3 which shows that a maximum of 90.6% yield of ME was achieved within 180 min. Further, the reaction rate constants of 0.0031 and 0.0078 min− 1 were obtained for esterification and transesterification respectively indicating that rate of esterification is slower than the transesterification and that FFA present in the oil are completely transformed into ME during transesterification [45,49]. The above findings can be well applied to high FFA oils for the conversion to biodiesel. A number of investigators studied the kinetics of supercritical transesterification at various temperatures and pressures and reported that reaction rate constants increase with increase in temperature [27] while the present study was carried out at an optimum temperature of 65° and 50 °C for esterification and transesterification respectively under optimum conditions of acid and base concentration, methanol to oil ratio, temperature, rpm, etc. under atmospheric pressure conditions. This is first study of simple transesterification kinetics carried out at pre-determined temperature and atmospheric pressure the rate of reaction of esterification as well as transesterification were studied with respect to % ME yield. It is not only simple but also less costlier and less time consuming compared to that involves the estimation of six different rate constants involving complex reactions from TG–MG, DG–MG and MG–ME at different temperatures and pressures as reported by Fukuda et al. [50], Marchetti et al. [51] and Karmi et al. [52]. The simple acid–base transesterification of WCO for biodiesel production has also been reported for the first time contrary to the work on separate acid or base or enzyme catalyzed processes.
Fig. 9. Plot of Arrhenius equation (ln k v/s 1/T).
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Table 3 Optimum conditions for biodiesel production from WCO. Parameters
Esterification
Transesterification
Methanol to oil ratio Catalyst concentration (w/w) Temperature (°C) RPM Time (min) Yield (%) Reaction rate constant (min− 1)
3:7 1% H2SO4 65 ± 0.5 400 180 21.5 0.0031
3:7 1% NaOH 50 ± 0.5 400 180 90.6 0.0078
4. Conclusions The present study deals with the kinetics of two step acid–base catalyzed transesterification of high FFA containing WCO under optimum conditions of methanol to oil ratio of 3:7(v/v), temperature of 65 °C and 50 °C for acid and base transesterification respectively, 400 rpm and catalyst concentration of 1% (w/w) for H2SO4 and 1% (w/w) for NaOH. The kinetics has been studied with respect to ME yield directly from TG by ignoring the complex reactions determining 6 different reaction rate constants which is not only time consuming but costlier and needs much efforts. The rate constant of both the reaction are found as 0.0031 min− 1 and 0.0078 min− 1 respectively indicating that the former is slower than the later reaction. The yield of ME from esterification and transesterification are found as 21.5% and 90.6% respectively. This also represents that the energy required for the transesterification to occur is also very small which is calculated as 88764.53 J/mol. Such low energy process can be very useful for ‘on farm’ biodiesel production from high FFA oils and offers opportunities for energizing remote areas through rural electrification. The process can find wide application for the conversion of high FFA oils, especially, non-edible oils which may have more FFA during long term storage of the resource due to their poor oxidation stabilities. References [1] Ministry of Petroleum and Natural Gas (2007), http://petroleum.nic.in/ speeches/26-11-08.pdf. [2] F. Ma, M.A. Hanna, Biodiesel production: a review, Bioresource Technology 70 (1999) 1–15. [3] S. Jain, M.P. Sharma, Prospects of biodiesel from Jatropha in India: a review, Renew Sustain Energy Rev 44 (2010) 763–771. [4] MNRE (Ministry of New and Renewable Energy), Government of India (2008). [5] A. Demirbas, Biodiesel fuels from vegetable oils via catalytic and noncatalytic supercritical alcohol transesterifications and other methods: a survey, Energy Convers Manage 44 (2003) 2093–2109. [6] D.G.B. Boocock, S.K. Konar, J. Lau, Ethyl esters from the single-phase basecatalyzed ethanolysis of vegetable oils, Journal of the American Oil Chemists Society 80 (4) (2003) 1558–9331. [7] D.G.B. Boocock, S.K. Konar, V. Mao, H. Sidi, Fast one-phase oil-rich process for the preparation of vegetable oil methyl esters, Biomass and Bioenergy 11 (1) (1996) 43–50. [8] D.G.B. Boocock, S.K. Konar, V. Mao, C. Lee, S. Buligan, Fast formation of high purity oil methyl ester, Journal of the American Oil Chemists Society 75 (9) (1998) 1167–1172. [9] P.R. Muniyappa, S.C. Brammer, H. Noureddini, Improved conversion of plant oils and animal fats into biodiesel and co-product, Bioresource Technology 56 (1996) 19–24. [10] M.P. Dorado, E. Ballesteros, F.J. Lo'pez, Optimization of alkali catalysed transesterification of brassica carinata oil for biodiesel production energy, Fuel 18 (2004) 77–83. [11] G. Antolı´n, F.V. Tinaut, Y. Bricenˇo, Optimization of biodiesel production by sunflower oil transesterification, Bioresource Technology 83 (2002) 111–114. [12] Y. Zhang, M.A. Dube´, D.D. McLean, M. Kates, Biodiesel production from waste cooking oil: process design and technological assessment, Bioresource Technology 89 (2003) 1–16. [13] M.I. Al-Widyan, A.O. Al-Shyoukh, Experimental evaluation of the transesterifation of waste palm oil into biodiesel, Bioresource Technology 85 (2002) 253–256. [14] G.M. Tashtoush, M.I. Al-Widyan, A.O. Al-Shyoukh, Experimental study on evaluation and optimization of conversion of waste animal fat into biodiesel, Energy Conversion and Management 45 (2004) 2697–2711. [15] M. Canakci, J. Van Gerpen, A pilot plant to produce biodiesel from high free fatty acid feedstocks, Transaction of ASAE 46 (4) (2003) 945–954. [16] Y. Zhang, M.A. Dube, D.D. McLean, M. Kates, Biodiesel production from waste cooking oil: economic assessment and sensitivity analysis, Bioresource Technology 90 (2003) 229–240. [17] M. Canakci, J.V. Gerpen, Biodiesel production from oils and fats with high free fatty acids, Transactions of the ASAE 44 (6) (2001) 1429–1436.
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[38] RF Sickels and R Teall, US Patent 6,979,426, 2005—freepatentsonline.com [39] M.P. Sharma, Biodiesel production from cottonseed and pongamia oil, Journal of Indian water resource society 29 (1) (2009) 49–58. [40] A. Gopinath, G. Puhan Sand Nagarajan, Theoretical modeling of iodine value and saponification value of Biodiesel fuels from their fatty acid composition, Renewable Energy 34 (2009) 1806–1811. [41] Determination of ester and linolenic acid methyl ester contents, English version of DIN EN 14103 (2003). [42] L. Xuejun, H. Huayang, W. Yujun, Z. Shenlin, Transesterification of soybean oil to biodiesel using SrO as a solid base catalyst, Catalysis Communications 8 (2007) 1107–1111. [43] A. Yusuke, M. Kouji, K. Hidetoshi, F. Keisuke, Theoretical study of transesterification of triglycerides to biodiesel fuel, Fuel 88 (5) (2009) 786–791. [44] K. Hak-Joo, K. Bo-Seung, K. Min-Ju, M.P. Young, K. Deog-Keun, L. Jin-Suk, L. 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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Acid base catalyzed transesterification kinetics of waste cooking oil Siddharth Jain ⁎,1, M.P. Sharma 2, Shalini Rajvanshi Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India
a r t i c l e
i n f o
Article history: Received 11 March 2010 Received in revised form 7 August 2010 Accepted 29 August 2010 Keywords: Waste cooking oil (WCO) Transesterification kinetics Methyl ester (ME) Fatty acid methyl ester (FAME)
a b s t r a c t The present study reports the results of kinetics study of acid base catalyzed two step transesterification process of waste cooking oil, carried out at pre-determined optimum temperature of 65 °C and 50 °C for esterification and transesterification process respectively under the optimum condition of methanol to oil ratio of 3:7 (v/v), catalyst concentration 1%(w/w) for H2SO4 and NaOH and 400 rpm of stirring. The optimum temperature was determined based on the yield of ME at different temperature. Simply, the optimum concentration of H2SO4 and NaOH was determined with respect to ME Yield. The results indicated that both esterification and transesterification reaction are of first order rate reaction with reaction rate constant of 0.0031 min− 1 and 0.0078 min− 1 respectively showing that the former is a slower process than the later. The maximum yield of 21.50% of ME during esterification and 90.6% from transesterification of pretreated WCO has been obtained. This is the first study of its kind which deals with simplified kinetics of two step acid–base catalyzed transesterification process carried under the above optimum conditions and took about 6 h for complete conversion of TG to ME with least amount of activation energy. Also various parameters related to experiments are optimized with respect to ME yield. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The nation imported 121.672 million tons of crude oil for $67.988 billion in 2007–08 as opposed to 111.502 million tons imported for $48.389 billion in 2006–2007, according to the latest data released by the petroleum ministry in New Delhi. As our oil import dependence has reached 80%, it is important to undertake optimal exploitation of domestic energy resources with a view to increasing our country's energy security [1]. This fact coupled with the recent hike of crude oil prices up to 135US$ per barrel (2008) has forced the search for alternative fuels based on renewable energy. Since the diesel is being heavily used in diesel engines, therefore, the search for alternative to diesel fuel become inevitable. Biodiesel from vegetable oil resources can substitute diesel in considerable proportion and therefore, vegetable oil resources, particularly, the non-edible one, deserve due consideration. Biodiesel, the monoalkyl esters of long chain fatty acids derived from a renewable lipid feedstock such as vegetable oil or animal fat, is becoming popular as substitute of or additive to diesel in developing as well as developed countries [2,3]. In India, fuel ethanol and biodiesel have been acquiring special importance from energy security and environmental concerns point of view as they can offer large scale employment in the growing and processing of resource, particularly in
⁎ Corresponding author. Tel.: + 91 9456382050; fax: + 91 1332 273517. E-mail addresses: [email protected] (S. Jain), [email protected] (M.P. Sharma). 1 Tel.: + 91 1334 232694; fax: + 91 1332 273517. 2 Tel.: + 91 1332 285836; fax: + 91 1332 273517. 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.08.017
rural areas [4]. The main advantages of biodiesel is its renewability, better quality of exhaust gas emissions, its biodegradability and its contribution to the reduction in CO2 emissions[5]. The biodiesel can be prepared by transesterification process which combines vegetable oils with alcohol in the presence of the catalyst to form fatty acid alkyl esters (i.e., biodiesel) and glycerol [5–22]. Methanol is the most commonly used alcohol for transesterification because of its low cost [3,5]. Numerous studies have been carried out on various aspects of biodiesel production for use in diesel engine. Most of the current challenges concern with the reduction of production cost which is still higher than petrodiesel owing to higher cost of non-edible oil resources. The WCO available from restaurants, food processing industries, fast foods industries has been found to be one of the resources that can be used to produce biodiesel with much less required for its production except collection and refinement. Boocok and co-workers [6–8] have reported that one step base catalyzed methanolysis of soybean oil using tetrahydrofuran as co-solvent, has been found suitable if oil has free fatty acid (FFA) less than 1%. Only well refined vegetable oils with less than 1% FFA were transesterified by Zhang et al. [12,16] who compared different processes of biodiesel production and found that alkalicatalyzed process is the simplest one but had a higher raw material cost compared to other processes. For the oils with high FFA contents, either the acid catalyzed or two step acid–base catalyzed process is preferred which requires excess of methanol. In the later case, the first acid catalyzed step is used to reduce FFA to b1% (w/w) followed by base catalyzed transesterification for the conversion of TG to ME [12–16]. Dufek et al. [23] have studied the acid catalyzed esterification and transesterification of 9-(16)-carboxystearic acid and its mono and di-
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methyl esters and reported unequal chemical reactivity for different carboxyl and carboxyl methyl groups. Freedman et al. [24] reported the transesterification of soyabean and other oils with methanol and butanol to examine the effect of alcohol type on the transesterification process. Noureddini and Zhu [25] studied the effect of mixing of soyabean oil with methanol on the kinetics of reaction using one phase transesterification process and found that the mixing had profound effect on the ME yield. Separate acid catalyzed, alkali catalyzed, enzyme catalyzed, or supercritical transesterification of different oils including WCO has been studied by many researchers (26–30). Diasakov et al. [26] investigated the kinetics of unanalyzed transesterification reaction of soyabean oil. Further, several authors have used the methods involving costly chemicals and requiring much time and efforts for the analysis of intermediate reaction products during the course of kinetics study of transesterification but Kusdiana and Saka [27] have used % yield of methyl ester as the only parameter to monitor the rate of reaction and that 3 step conversion from TG-DG, DG-MG and MG-ME has been simplified in terms of conversion of TG to ME. Barnwal and Sharma [28] has carried out the techno-economic analysis of biodiesel production from different oil feedstocks and found that pongamia, a non-edible oil, can yield biodiesel @ Rs.10.50/ l compared to diesel (Rs.22/l) at that time and sesame oil gave the costliest biodiesel @ Rs.54/l. The land requirement for growing Jatropha plants for meeting the requirement of different blends of biodiesel with diesel like B5, B10 and B20 were calculated for the buses of Uttar Pradesh State Road Transport Corporation (UPSRTC) and reported in our earlier paper [22,29]. Khan et al. [30] have developed the quantitative analysis of the product mixture formed during transesterification reaction. Mittelbach [31] described a simple and reliable method, which allows the determination of the overall content of tri-, di-, and monoglycerides in fatty acid methyl esters (FAME) by isocratic liquid chromatography using a density detector. Saiffudin and Chau [32] studied the transesterification of used frying oil with 0.5% NaOH in ethanol using microwave irradiation and found that there is considerable enhancement in the reaction rates. The consumption of edible oil is very high in the country and still the indigenous production does not meet the consumption and considerable amount of edible oil is imported and it is therefore, not advisable to divert these sources for biodiesel production. On the other hand, the non-edible oil resources can be a solution for biodiesel production. The seed oil producing plants can be grown on waste/ semi arid lands under National Biodiesel Programme of Govt. of India and potential availability of non-edible oils in India is about 1 million ton per year. Presently, Jatropha, Pongamia, Sal, Mahua, Neem, waste cooking oil (WCO) etc. have been identified as the main non-edible oil resources, out of which WCO has been selected for the present study. Commercially available crude oils and fats contain considerable amount of free fatty acids (FFA) that react with the alkaline catalyst and form saponified products during base catalyzed transesterification that also needs exhaustive purification of the products. The saponification not only consumes the alkali catalyst but also the
33
resulting soaps can cause the formation of emulsions which create difficulties in downstream recovery and purification of the biodiesel. This puts significant limitations on the conversion of WCO to biodiesel by base catalyzed transesterification process. Since, the FFA in WCO is more than 2.0% (w/w), it is prudent to reduce FFA by esterification using methanol and acid catalyst. The resulting oil phase having a low FFA (less than 0.5% w/w) can then be subjected to the base alkalicatalyzed transesterification for biodiesel production [33]. There are several routes to obtain biodiesel using oilseeds, edible oils and waste oils as feed stock. Transesterification of triglycerides with low molecular weight alcohols catalyzed by homogeneous catalysis is the most used one [34–39]. In view of the above, it is seen that little work is reported on the kinetics of two step transesterification of high FFA oils. Previous authors mainly concentrated on low FFA vegetable oils but in the present work, high FFA waste cooking oil, is used for study purpose. The present paper reports the results of simplified kinetics of transesterification carried out with respect to ME yield only which is not only simple but also less time consuming and less costly compared to separate acid catalyzed process which takes much longer time for its completion as evidenced by limited reports available in the literature. It is possible to convert high FFA waste cooking oil to biodiesel with least amount of activation energy. Also various parameters related to conversion of waste cooking oil into biodiesel have been optimized with respect to ME yield. This is the first study of its kind which uses simple 2 step acid–base transesterification of high FFA waste cooking oil to convert it into biodiesel. As on storing, the FFA contents of oil increased significantly, the present technology will be very useful to convert high FFA stored oils to biodiesel. 2. Material Waste cooking oil was collected from hostel messes of Indian Institute of Technology Roorkee (India) campus. The availability of WCO is about 160 l per month which may further increase with hostel capacity. The entire supply of refined soybean oil to all the messes is from one supplier and therefore the oil quality has been assumed to be same throughout the year. All the chemicals like H2SO4, KOH, methanol, anhydrous Na2SO4, etc. used were of analytical grade and 99% pure. Raw WCO was filtered to remove all insoluble impurities followed by heating at 100 °C for 10 min to remove all the moisture. The fatty acid compositions of refined soybean oil and its WCO as determined by Gas Chromatography are shown in Table 1. The fuel properties of refined WCO and its biodiesel were determined as per standard method and reported in Table 2. The above table indicates that about 93.1% FFAs are unsaturated and therefore WCO has high saponification value, i.e., 198.10 mg KOH/ g oil which can be computed from the following equation [40]. SV = 268–ð0:418⁎P Þ–ð1:30⁎SÞ–ð0:695 OÞ–ð0:77⁎LÞ–ð0:847⁎LLÞ where, SV = saponification value, P = Palmitic acid, S = Stearic acid, O = Oleic acid, L = Linoleic acid and LL = Linolenic acid.
Table 1 Fatty acid composition of refined soybean oil and its WCO collected from messes. Fatty acid
Formula
Palmitic acid (P) Palmitolileic acid Stearic acid (S) Oleic acid (O) Linoleic acid (L) Linolenic acid (LL) Arachidice acid Gadolic acid
C16H32O2 C16H30O2 C18H38O2 C18H34O2 C18H32O2 C18H30O2 C20H40O2 C20H36O2
CH3(CH2)14COOH CH3(CH2)5CH CH–(CH2)7–COOH CH3(CH2)16COOH CH3(CH2)7–CH CH–(CH2)7COOH CH3(CH2)4 CH CH–CH2–CH CH–(CH2)7 COOH CH3(CH2)4CH CH–CH2–CH CH–CH2–CH CH–(CH2)4 COOH CH3–(CH2)18 COOH
Systematic name
Structure
% FFA in edible oil
% FFA in WCO
Hexadecanoic acid Cis-9 hexadecnoic acid Octadecanoic acid Cis9-Octadecanoic acid cis-9-cis-12-Octadecadeneoic acid cis-6-cis-9-cis-12 Octadecatrienoic acid Eicosanoic acid
C16 C16:1 C18 C18:1 C18:2 C18:3 C20 C20:1
10.4 0.35 4.7 24.8 52.5 6.5 0.75 –
4.1 2.4 1.4 40.5 38.0 10.6 0.8 1.6
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Table 2 Fuel properties of waste cooking oil and its biodiesel. Properties
Waste cooking oil
Biodiesel from waste cooking oil
Density (kg/m3 @ 15 °C) Viscosity (cSt, @ 30 °C) Flash point (°C) FFA contents (%) Gross calorific value (MJ/kg) Acid value (mg KOH/g)
937 50 235 21.84 38.27 21.84
892 4.2 178 0.67 37.82 0.38
The experiments were conducted in a batch reactor of 1.5 l capacity equipped with condenser, stirrer, inlet and outlet and temperature measurement instrument. The whole setup was put in constant temperature water bath (±0.5 °C). The schematic of the experimental set up is shown in Fig. 1.
2.1. Experimental procedure The experiments were performed at different temperatures using different concentrations of H2SO4 and NaOH in acid and base catalyzed reactions with respect to ME yield. Due to high FFA contents of WCO (21.84%), a two step process was selected for converting oil into methyl ester. The acid catalyzed esterification was used to reduce the FFA to b1% using H2SO4 as catalyst at an optimum temperature. The resulting oil having b1% FFA was finally subjected to base catalyzed transesterification at optimum temperature to produce ME. The procedure is described in the next section.
2.1.1. Acid pretreatment step The refined and moisture free oil was poured into the reactor (Fig. 1) and heated at different temperatures (20, 30, 40, 50, 60, 70 and 80 °C) to optimize the temperature for maximum yield and maximum FFA reduction. The mixture of conc. H2SO4 (0.5%, 1%, 1.5%, 2% and 3% w/w) with methanol (30% v/v) was separately heated at same temperatures (20, 30, 40, 50, 60, 70 and 80 °C) and then added to heated oil in the reactor. The mixture was heated at that temperature for 3 h to complete the esterification. The samples were withdrawn at pre-determined time intervals to calculate % ME yield. After acid catalyzed transesterification step, the mixture was cooled and allowed to settle overnight. This mixture was finally subjected to base catalyzed reaction, where all the residual acid is neutralized and as such, the acid removal is not required in the above step.
2.1.2. Base catalyzed transesterification The esterified oil (b1% FFA) was poured into the reactor and heated at different (20, 30, 40, 50, 60, 70 and 80 °C) to optimize the temperature of reaction for maximum yield. A mixture of NaOH (0.5%, 1%, 1.5%, 2% and 3% w/w) in methanol was heated at the same temperature (20, 30, 40, 50, 60, 70 and 80 °C) for 5 min and added slowly to the heated oil. The reaction mixture was heated, refluxed and stirred at 400 rpm for about 3 h. The samples were withdrawn at pre-determined time intervals to determine % ME formed. After 3 h, two distinct layers were formed and the mixture was allowed to settle for 2 h or overnight. The heavier glycerol layer was separated from the lighter ME layer by separating funnel. 2.2. Sample treatment The ME layer was separated, washed with water, heated to remove moisture and dried over anhydrous Na2SO4 and checked for its purity using GC, thin layer chromatography and flash point. The rate of reaction was calculated using % ME yield data with respect to time and reaction rate constants and order of reaction was also evaluated along with activation energy of transesterification step and the results are reported in the Results and discussion section. 2.3. GC analysis The samples were analyzed for ME formation at a predetermined interval of time by Gas Chromatograph (Netal make) equipped with a flame ionization detector and a capillary column for injecting the sample. The GC oven was kept at 230 °C (5 °C/min) and a total analytical time was 30 min. Nitrogen was used as carrier gas. Quantitative analysis of % ME was done using European standard EN 14103:2003 [41]. The % ME yield was calculated using Eq. (1). Free fatty acids in the samples were determined using stock solution (Methyl heptadecanoate and n-heptane). % of ME = ∑A AEI CEI VEI m
∑A−AEI C ⁎V × EI EI × 100 AEI m
ð1Þ
Total peak area from the methyl ester in C14 to that in C24:1; Peak area corresponding to methyl heptadecanoate; Concentration of the methyl heptadecanoate solution (mg/ml); Volume of the methyl heptadecanoate solution (ml); Mass of the sample (mg).
3. Results and discussion 3.1. Determination of optimum reaction parameters 3.1.1. Effect of reaction temperature The results of variation of ME yield with temperature (20, 30, 40, 50, 60, 70 and 80 °C) using different concentration of H2SO4 as acid and NaOH as base catalyst (0.5%, 1%, 1.5%, 2% and 3% w/w) for esterification and transesterification using optimum amount of methanol, i.e., 30% of the total volume (v/v) are shown in Fig. 2(a) and (b) which shows that during esterification, the maximum yield of ME (21.5%) was obtained at 65 °C at 1% (w/w) H2SO4 concentration, while during transesterification, 90.6% yield of ME was obtained at 50 °C using 1% NaOH (w/w). As the value of reaction parameters increases beyond this value FAME yield decreases due to reversible nature of reaction [42–44].
Fig. 1. Schematic diagram of experimental setup.
3.1.2. Effect of catalyst amount Effect of catalyst concentration is shown in Fig. 2(a) and (b) for esterification and transesterification respectively. According to this as the amount of catalyst increases, the ME yields also increases and at 1% (w/w) catalyst concentration the yield will be highest in both the
S. Jain et al. / Fuel Processing Technology 92 (2011) 32–38
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was over 3:7, glycerol separation becomes more difficult due to emulsion of glycerol with ME, thereby decreasing the yield. This observation was also reported in the literature [42,44]. 3.1.4. Effect of reaction time The variation of ME yield with time are shown in Fig. 4(a) and (b) which indicates that during esterification, the yield of ME increased with time at a faster rate within first 100 min owing to the faster rate of FFA reduction and thereafter it becomes constant indicating that further conversion of FFA to ME has almost completed within 180 min. While during the base catalyzed transesterification, % ME yield has increased from 21.5% to 90.6% within 180 min. Variation of ME yields vs time (Fig 4b) show that no further conversion of TG to ME takes place after this period. Further, the graph of % FFA reduction with time at various methanol to oil ratio (Fig. 5) indicates that 98% conversion of FFA has taken place during 180 min in first step. The variation of % ME yield vs % FFA reduction as shown in Fig. 6 shows a linear relationship between FFA reduction and ME yield during acid esterification step with a slope of 0.9223 indicating that during acid catalyzed esterification as the FFA content decreases from 21.84% to less than 1%, ME yield increases up to 21.5% linearly with no further increase in the same. 3.2. Kinetics of transesterification of waste cooking oil
Fig. 2. % ME yield vs. temperature (a) acid esterification and (b) base catalyzed transesterification at different catalyst concentration.
The kinetic studies of transesterification of WCO have been carried out at the optimum conditions of temperature, catalyst concentration and 400 rpm of stirrer (the later has not been optimized in the present study). Diasakov et al. [26] has reported that the transesterification reaction to proceed in 3 steps in which triglyceride (TG) reacts with methanol to produce diglycerides (DG), which further reacts with methanol to yield monoglycerides (MG) that finally reacts with methanol to produce methyl ester and glycerol. Since one mole of methyl ester is generated per mole of methanol reacted at each step,
cases. However, if the catalyst concentration goes beyond this value then the ME yield will not increase. This observation is in agreement with literature also [25,27,42,44]. 3.1.3. The effect of methanol/oil ratio The variation of different amount of methanol as % of oil has been studied with respect to % ME yield under the above optimum conditions and the results are reported in Fig. 3 which shows that a maximum yield of 90.6% was obtained at methanol to oil ratio of 3:7 and therefore this optimum ratio has been used during the experimentation. As the % methanol is increased up to 30% (v/v), yield increases up to 90.6%. However, when the amount of methanol
Fig. 3. %ME formed vs. % volume of methanol of the oil for transesterification step.
Fig. 4. % ME yield vs time (a) acid esterification and (b) base transesterification.
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S. Jain et al. / Fuel Processing Technology 92 (2011) 32–38
respect to % ME yield as a function of time. Therefore the first order rate constant of the reaction can be expressed by Eq. (6). Rate = −
dðMEÞ dt
ð6Þ
where ( ) represents the %ME yield. Rate equation of first order may also be written as −Rate = k(ME). It can be modified as dðMEÞ = kðMEÞ dt where (ME) refers to the % ME yield. Assuming that the initial concentration of ME at time t = 0 is ME0 and that it increases to MEt at time t. The integration of this equation gives: ME
∫MEt0 Fig. 5. Variation of FFA vs time for different methanol to oil ratio (v/v) during acid catalyzed step.
in all, six rate constants are reported in the literature for the whole reaction from TG to methyl ester (biodiesel) as shown by the equation given below in Eqs. 2, 3 and 4. k1
0
TG þ ROH ⇄ DG þ R CO2 R
ð2Þ
k4
k2
0
DG þ ROH ⇄ MG þ R CO2 R
ð3Þ
k5
k3
0
MG þ ROH ⇄ GL þ R CO2 R
ð4Þ
k6
According to the above equation, whole transesterification results ultimately in the production of methyl ester and therefore, all the intermediate reaction products (e.g. DG and MG) can be ignored and simple mathematical model expressing the whole conversion as one step has been developed (Eq. 5).
→3R CO R þ GL Catalyst
TG ¼ 3ROH
0
2
ð5Þ
The whole reaction is assumed to precede as first order reaction as a function of concentration of ME [26,27]. The rate constant of the reaction can be determined based on the increased amount of the product that occurs in some reaction time interval [5,17]. In this work, the increased amount of one reactant that is ME was choosen. Accordingly, the kinetics of transesterification has been studied with
t dðMEÞ = k ∫ dt ðMEÞ 0
and ln
ðMEt Þ = kt ME0
or k=
InðMEt Þ−InðME0 Þ t
ð7Þ
If the slope of ‘ln k’ vs ‘1/T’ is a straight line then the assumption that have been taken for the reaction, i.e., 1st order will be correct. Using Eq. (7), the reaction rate constant under optimum conditions of temperature, catalyst concentration, methanol and rpm of stirring, has been computed as 0.0031 min− 1 and 0.0078 min− 1 for esterification and transesterification respectively. This indicates that the rate of acid catalyzed esterification is slower than base catalyzed transesterification perhaps due to the presence of considerable amount of intermediate reaction products interfering in the transformation to methyl ester [45,47 and 48]. This observation is in contrast to the faster rate of esterification reaction compared to slow transesterification under supercritical transesterification conditions [46]. Due to this reason the conversion of high FFA oil takes longer time to convert to ME compared to oil with low FFA. Further, a plot of dME/dt vs yield using log graph gives straight line for both esterification and transesterification reaction as shown in Fig. 7(a) and (b) which indicates that both the reaction are of first order. This also indicates that the rate of reaction varies linearly with yield. The slope of straight line for esterification and transesterification respectively are given by Eqs. 4and5 which indicates that rates of reaction varies linearly with yield. For acid transesterification: ln ðdME = dtÞ = −0:9524 ln ðMEÞ + 2:9288
2 R = 0:9988
ð8Þ
2 R = 0:9577
ð9Þ
For base transesterification: ln ðdME = dtÞ = −0:8713 ln ðMEÞ + 2:4369
Fig. 6. Variation of % ME yield (%) with % to FFA reduction during acid transesterification.
The slope of the straight line in Fig. 7a and b was found as −0.9524 and −0.8713 with a value of R2 of 0.9988 and 0.9577 indicating that the data collected are 99.88% accurate for esterification and 95.77% accurate for transesterification reaction. Further, the transesterification was carried out at various temperature for different time and ME yield was calculated. The results of the same are shown in Fig. 8, indicates that maximum yield of ME (90.6%) occurred at 50 °C compare to other temperatures. Above 50 °C, the ME
S. Jain et al. / Fuel Processing Technology 92 (2011) 32–38
(a)
37
3.3. Determination of activation energy Activation energy is calculated through Arrhenius equation which include activation energy ‘Ea’ and frequency factor ‘A’, given below [48]: −Ea = RT
ð10Þ
k = Ae
Where, R the universal molar gas constant and T is the temperature (K). Since the activation energy is dependent on temperature and therefore the rate constants at any temperature (within the validity of the Arrhenius equation) [19] can be computed using Eq. (11):
(b)
Fig. 7. Plot of reaction rate vs yield on logarithmic graph (%) for (a) acid esterification and (b) base transesterification of WCO.
yield decreases to a very little amount due to reversible nature of reaction [47]. The results of variation of % ME yield with times at various temperatures as given in Fig. 8 indicates that during base catalyzed transesterification, the maximum yield of 90.6% ME was obtained in 180 min when reaction temperature was 50 °C as the reaction time or temperature increases beyond this value, the FAME yield decreases due to reversible nature of the reaction [42,44].
Fig. 8. Variation of FAME yield with time for various temperatures.
ln k = ln A−Ea = RT
ð11Þ
Eq. (11) is a linear equation and therefore a plot of lnk and 1/T is given in Fig. 9 also validates the first order of the reaction. Activation energy (Ea) has been calculated using Fig. 9 which is 88,764.53 J/mol. Based upon the above results, optimum conditions for biodiesel production from WCO are summarized in Table 3 which shows that a maximum of 90.6% yield of ME was achieved within 180 min. Further, the reaction rate constants of 0.0031 and 0.0078 min− 1 were obtained for esterification and transesterification respectively indicating that rate of esterification is slower than the transesterification and that FFA present in the oil are completely transformed into ME during transesterification [45,49]. The above findings can be well applied to high FFA oils for the conversion to biodiesel. A number of investigators studied the kinetics of supercritical transesterification at various temperatures and pressures and reported that reaction rate constants increase with increase in temperature [27] while the present study was carried out at an optimum temperature of 65° and 50 °C for esterification and transesterification respectively under optimum conditions of acid and base concentration, methanol to oil ratio, temperature, rpm, etc. under atmospheric pressure conditions. This is first study of simple transesterification kinetics carried out at pre-determined temperature and atmospheric pressure the rate of reaction of esterification as well as transesterification were studied with respect to % ME yield. It is not only simple but also less costlier and less time consuming compared to that involves the estimation of six different rate constants involving complex reactions from TG–MG, DG–MG and MG–ME at different temperatures and pressures as reported by Fukuda et al. [50], Marchetti et al. [51] and Karmi et al. [52]. The simple acid–base transesterification of WCO for biodiesel production has also been reported for the first time contrary to the work on separate acid or base or enzyme catalyzed processes.
Fig. 9. Plot of Arrhenius equation (ln k v/s 1/T).
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S. Jain et al. / Fuel Processing Technology 92 (2011) 32–38
Table 3 Optimum conditions for biodiesel production from WCO. Parameters
Esterification
Transesterification
Methanol to oil ratio Catalyst concentration (w/w) Temperature (°C) RPM Time (min) Yield (%) Reaction rate constant (min− 1)
3:7 1% H2SO4 65 ± 0.5 400 180 21.5 0.0031
3:7 1% NaOH 50 ± 0.5 400 180 90.6 0.0078
4. Conclusions The present study deals with the kinetics of two step acid–base catalyzed transesterification of high FFA containing WCO under optimum conditions of methanol to oil ratio of 3:7(v/v), temperature of 65 °C and 50 °C for acid and base transesterification respectively, 400 rpm and catalyst concentration of 1% (w/w) for H2SO4 and 1% (w/w) for NaOH. The kinetics has been studied with respect to ME yield directly from TG by ignoring the complex reactions determining 6 different reaction rate constants which is not only time consuming but costlier and needs much efforts. The rate constant of both the reaction are found as 0.0031 min− 1 and 0.0078 min− 1 respectively indicating that the former is slower than the later reaction. The yield of ME from esterification and transesterification are found as 21.5% and 90.6% respectively. This also represents that the energy required for the transesterification to occur is also very small which is calculated as 88764.53 J/mol. Such low energy process can be very useful for ‘on farm’ biodiesel production from high FFA oils and offers opportunities for energizing remote areas through rural electrification. The process can find wide application for the conversion of high FFA oils, especially, non-edible oils which may have more FFA during long term storage of the resource due to their poor oxidation stabilities. References [1] Ministry of Petroleum and Natural Gas (2007), http://petroleum.nic.in/ speeches/26-11-08.pdf. [2] F. Ma, M.A. Hanna, Biodiesel production: a review, Bioresource Technology 70 (1999) 1–15. [3] S. Jain, M.P. Sharma, Prospects of biodiesel from Jatropha in India: a review, Renew Sustain Energy Rev 44 (2010) 763–771. [4] MNRE (Ministry of New and Renewable Energy), Government of India (2008). [5] A. Demirbas, Biodiesel fuels from vegetable oils via catalytic and noncatalytic supercritical alcohol transesterifications and other methods: a survey, Energy Convers Manage 44 (2003) 2093–2109. [6] D.G.B. Boocock, S.K. Konar, J. Lau, Ethyl esters from the single-phase basecatalyzed ethanolysis of vegetable oils, Journal of the American Oil Chemists Society 80 (4) (2003) 1558–9331. [7] D.G.B. Boocock, S.K. Konar, V. Mao, H. Sidi, Fast one-phase oil-rich process for the preparation of vegetable oil methyl esters, Biomass and Bioenergy 11 (1) (1996) 43–50. [8] D.G.B. Boocock, S.K. Konar, V. Mao, C. Lee, S. Buligan, Fast formation of high purity oil methyl ester, Journal of the American Oil Chemists Society 75 (9) (1998) 1167–1172. [9] P.R. Muniyappa, S.C. Brammer, H. Noureddini, Improved conversion of plant oils and animal fats into biodiesel and co-product, Bioresource Technology 56 (1996) 19–24. [10] M.P. Dorado, E. Ballesteros, F.J. Lo'pez, Optimization of alkali catalysed transesterification of brassica carinata oil for biodiesel production energy, Fuel 18 (2004) 77–83. [11] G. Antolı´n, F.V. Tinaut, Y. Bricenˇo, Optimization of biodiesel production by sunflower oil transesterification, Bioresource Technology 83 (2002) 111–114. [12] Y. Zhang, M.A. Dube´, D.D. McLean, M. Kates, Biodiesel production from waste cooking oil: process design and technological assessment, Bioresource Technology 89 (2003) 1–16. [13] M.I. Al-Widyan, A.O. Al-Shyoukh, Experimental evaluation of the transesterifation of waste palm oil into biodiesel, Bioresource Technology 85 (2002) 253–256. [14] G.M. Tashtoush, M.I. Al-Widyan, A.O. Al-Shyoukh, Experimental study on evaluation and optimization of conversion of waste animal fat into biodiesel, Energy Conversion and Management 45 (2004) 2697–2711. [15] M. Canakci, J. Van Gerpen, A pilot plant to produce biodiesel from high free fatty acid feedstocks, Transaction of ASAE 46 (4) (2003) 945–954. [16] Y. Zhang, M.A. Dube, D.D. McLean, M. Kates, Biodiesel production from waste cooking oil: economic assessment and sensitivity analysis, Bioresource Technology 90 (2003) 229–240. [17] M. Canakci, J.V. Gerpen, Biodiesel production from oils and fats with high free fatty acids, Transactions of the ASAE 44 (6) (2001) 1429–1436.
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