Antioxidant Consumption Kinetics and Shelf-Life Prediction for

Nov 8, 2016 - Kinetic studies on the oxidation of biodiesel and the consumption of antioxidants were performed in this work, using the standard Rancim...
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Antioxidant Consumption Kinetics and Shelf-Life Prediction for Biodiesel Stabilized with Antioxidants Using the Rancimat Method Jian Zhou,* Yun Xiong, and Yonggang Shi Department of Fuel Chemistry, Logistical Engineering University, Chongqing 401331, People’s Republic of China ABSTRACT: Kinetic studies on the oxidation of biodiesel and the consumption of antioxidants were performed in this work, using the standard Rancimat method within the temperature range of 100−150 °C. On the basis of the length of induction period, antioxidant efficiency was ranked in the descending order of tert-butylhydroquinone (TBHQ) > pyrogallol (PY) > propyl gallate (PG) > butylated hydroxyanisole (BHA) > butylated hydroxytoluene (BHT). The natural logarithm of the initial antioxidant concentration varied linearly with respect to the induction period. Kinetic information for the consumption of the antioxidant could be obtained by pseudo-first-order reaction assumptions. TBHQ had the lowest consumption rate, followed by PG, PY, BHA, and BHT; the lower consumption rate seemed to lead to a longer induction period. The critical antioxidant concentration increased in the ascending order of PY < PG < BHA < BHT < TBHQ, basically in accordance with the order of antioxidant efficiency, except TBHQ. The apparent activation energy for the consumption of the antioxidant was determined using the Arrhenius equation: TBHQ (97.85 kJ/mol), PG (96.91 kJ/mol) > BHT (91.74 kJ/mol) > PY (82.76 kJ/mol) > BHA (80.08 kJ/mol), which agreed well with the order of activation energy for the oxidation of biodiesel. TBHQ, PG, and BHT, more temperature sensitive than PY and BHA, showed higher susceptibility to oxidative degradation at a higher temperature. The shelf life of biodiesel at ambient temperature was predicted with the extrapolation method and ranked in the descending order of TBHQ > PG > PY > BHT > BHA, basically in line with the order of the reaction constant of antioxidant consumption.

1. INTRODUCTION As a result of the growing concern on environmental protection and energy security, biodiesel has received increasing worldwide attention as a renewable alternative biofuel. Biodiesel consists of long-chain fatty acid esters derived from vegetable oils, animal fats, and waste cooking oils and offers many advantages over conventional petroleum diesel, such as inherent biodegradability, enhanced lubricity, improved safety, and reduced exhaust emissions.1,2 The study of biodiesel in recent years is primarily focused on the following aspects: feedstock selection, production technology, engine performance, oxidation stability, and cold flow behavior. The resistance of biodiesel against oxidation has always been a priority concern to suppliers, distributors, and users. As a result of the presence of unsaturated methyl esters of oleic, linoleic, and linolenic acids, biodiesel has high susceptibility to oxidative degradation, which could result in the deterioration of fuel quality and engine performance as well as exhaust emissions.3,4 The oxidation stability of biodiesel is usually characterized by the length of induction period by the Rancimat method. According to the newest EN 14214:2014 standard, a minimum induction period of 8 h is required to ensure biodiesel quality.5 The use of antioxidants is the most widely accepted practice for improving the oxidation stability of biodiesel. The efficiency of antioxidants to inhibit the oxidation of biodiesel depends upon many factors, including their chemical structures, interactions with the substrate, and type of stability test methods used to evaluate antioxidant performances.6 From the economical point of view, antioxidant concentrations should be within the range of 200−1000 ppm. The following synthetic antioxidants are generally more preferred than natural antioxidants because of their higher effectiveness when they are added to biodiesel: tert-butylhydroquinone (TBHQ), © XXXX American Chemical Society

pyrogallol (PY), propyl gallate (PG), butylated hydroxyanisole (BHA), and butylated hydroxytoluene (BHT).7 These phenolic compounds could break the chain propagation by donating hydrogen atoms to active free radicals, and the stable antioxidant radicals do not initiate another radical reaction or propagate the oxidation process.1,2,4,6,7 It is known that antioxidants can be gradually consumed by scavenging free radicals during the antioxidative process. When antioxidants are consumed to critical levels, the induction period will be over and the oxidation will proceed at the same rate as that in the absence of antioxidants.8,9 The testing temperature has significant influence on the kinetic behavior of antioxidant consumption and biodiesel oxidation. A higher testing temperature leads to a faster reaction rate and shorter analysis time.10 Richaud et al.11 investigated the oxidation kinetics of methyl oleate, methyl linoleate, and methyl linolenate at 90, 110, 140, and 150 °C using the chemiluminescence method. Induction time and maximal steady-state intensity seemed to obey the Arrhenius law in the temperature range studied, and a lower temperature resulted in a longer induction period and lower maximal intensity. Kinetic parameters, such as reaction order, reaction rate, and apparent activation energy, could be obtained by subjecting fuel samples to heat treatment at different temperatures. Pereira et al.10 examined the effect of the temperature on the oxidation behavior of soybean biodiesel at room temperature, 60, and 110 °C, by monitoring the increase of peroxide values as well as Received: August 31, 2016 Revised: November 7, 2016 Published: November 8, 2016 A

DOI: 10.1021/acs.energyfuels.6b02199 Energy Fuels XXXX, XXX, XXX−XXX

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the consumption of antioxidants, and to predict the shelf life of the biodiesel at ambient temperature.

parallel loss of natural tocopherols. The results indicated that the evolution of hydroperoxides during the induction period followed zero-order reaction kinetics. A similar kinetic study for hydroperoxide formation in sunflower/rapeseed biodiesel at 80, 85, 90, and 95 °C was carried out by Dinkov et al.12 using the modified ASTM D2274 method. The average reaction order for hydroperoxide formation was about 0.47, and the shelf life of the studied biodiesel via extrapolation was between 1.17 and 1.27 years. Borsato et al.13 evaluated the oxidation kinetics of soybean biodiesel stabilized with BHA, BHT, and TBHQ by submitting fuel samples to an accelerated oven test at 30, 50, and 80 °C, in which the biodiesel oxidation presented a firstorder reaction in the presence and absence of antioxidants. The Rancimat method specified in the EN 14214 standard uses a temperature of 110 °C to determine the oxidation stability of biodiesel. If possible, the temperature can be further raised to 130 °C to shorten the analysis time.14 However, many previous studies have been conducted at temperatures lower than 120 °C, partly as a result of concerns that some antioxidants may volatilize and decompose at a high temperature.8,9,15 Because temperatures have different effects on the antioxidant activity in inhibiting the oxidation of biodiesel, antioxidants may have different reaction mechanisms under different experimental conditions.16,17 As far as we know, there is little knowledge about the kinetic behavior of antioxidants at temperatures higher than 130 °C using the Rancimat method and only limited information is currently available regarding how the antioxidant performances are influenced by abusive temperatures. It is important to establish whether oxidation stability determined at one temperature could be correlated with that obtained at another temperature.18 Dunn19 investigated the effect of the block temperature (T) on the oil stability index (OSI) of soybean (60−80 °C) and used cooking oil (75−95 °C) biodiesels. Two mathematical models demonstrated good linear correlations for both biodiesels studied, ln(OSI) versus T (°C), and ln(OSI) versus T−1 (K), and the second model could be employed to calculate the apparent activation energy of the first-order reaction of biodiesel oxidation. Xin et al.8 studied the oxidation kinetics of safflower biodiesel stabilized with propyl gallate at temperatures from 100 to 120 °C using the Rancimat method. The reaction rate of propyl gallate consumption fitted well with the Arrhenius law, and the activation energy for propyl gallate consumption was calculated as 97.02 kJ/mol. Although there are reports dealing with the effect of the temperature on the oxidation stability of biodiesel, few conclusive results associated with the detailed kinetics of biodiesel oxidation and antioxidant consumption have been reached. Most previous studies involving biodiesel oxidation stability were focused on ranking the antioxidant efficiency based on the length of the induction period. To the best of our knowledge, few attempts were made to reveal more kinetic information, such as the reaction rate and activation energy, about the temperature dependence of antioxidant consumption and biodiesel oxidation during the antioxidative process. On the basis of this background, the effects of the temperature on the oxidation of biodiesel stabilized with most often used antioxidants, BHA, BHT, PY, PG, and TBHQ, were investigated in this study, at temperatures ranging from 100 to 150 °C using the standard Rancimat method. The purpose of this work was to determine the kinetic data for the oxidation of biodiesel, to investigate the antioxidative characteristics for

2. EXPERIMENTAL SECTION The biodiesel used in this study was obtained through the transesterification of waste cooking oil and provided by Chengdu Hengrun Hi-Tech Co., Ltd., Sichuan, China. The fresh biodiesel was known to contain no antioxidant additives, and its main physiochemical properties as well as fatty acid compositions were given in Table 1. The Agilent 7890A gas chromatography (GC)

Table 1. Main Physiochemical Properties of the Waste Cooking Oil Biodiesel and Its Fatty Acid Profile property

value

viscosity at 40 °C (mm2/s)

4.20

density at 15 °C (g/cm3)

0.878

CFPP (°C)

5

flash point (°C)

159

cetane number sulfur content (ppm)

54.0 PY > PG > BHA > BHT, in agreement with earlier observations by Mittelbach et al.2 Obviously, this order of antioxidant efficiency was consistent in the temperature range of 110−140 °C. The differences in the antioxidant efficiency could be largely attributed to their chemical structures.2,3,7,9,23 TBHQ, PY, and PG have at least two active hydroxyl groups on their benzene rings, making them more easily donate hydrogen atoms to free radicals than BHA and BHT to interrupt the chain propagation. The antioxidant potencies of the monohydric BHA and BHT may not be fully realized during the experiments as a result of possible decomposition and volatilization losses.24 During the antioxidative process of TBHQ, the derivative compounds formed may also retain antioxidant properties, and some of them were even reported to have higher activity than TBHQ itself.25,26 3.3. Antioxidant Consumption Kinetics. The dependence of the oxidation stability of biodiesel upon the antioxidant concentration is clearly illustrated in Figure 1. These antioxidants showed a sharp increase in the induction period at concentrations lower than 250 ppm, followed by a slight increase at concentrations higher than 500 ppm. The increase in the induction period with an increasing antioxidant concentration appears to be nonlinear and possibly exponential, which was also observed in previous studies.8,9 Considering that the consumption of the antioxidant depends upon its initial concentration, the pseudo-first-order reaction kinetics may be used to describe the antioxidant consumption mechanism: dC/ dt = −kC.27 When the antioxidants were gradually consumed by free radicals from initial concentration C0 to critical concentration Ccr at the end of the induction period, they were assumed to be no longer effective to retard the oxidation of the biodiesel. After integration within the concentration (from C0 to Ccr) and time (from IP0 to IP) limits, the above equation can be written as ln C0 = k(IP − IP0) + ln Ccr, where C0 and Ccr are the initial and critical concentrations (×106, ppm) of the antioxidant in the biodiesel, IP and IP0 are the induction periods (h) of the biodiesel treated and untreated with antioxidant, and k is the reaction constant (h−1) of the antioxidant consumption. As seen from Figure 2, there is a good fit for all antioxidants between the logarithm of the initial concentration ln C0 (×106, ppm) and the induction period IP (h) at each temperature, with the coefficient of determination R2 exceeding 0.9116. The reaction constant k (h−1) for the consumption of antioxidants can be determined from the slope of these fitted lines. After substitution of the reaction constant k (h−1) and the initial concentration C0 (ppm) into the equation ln C0 = k(IP − IP0) + ln Ccr, the critical concentration Ccr (ppm) can be obtained as well, as shown in Table 3. We can see clearly from Table 3 that both reaction constant k and critical concentration Ccr increased with an increasing temperature. The reaction constant almost doubled for a 10 °C rise in the temperature. A higher k indicated a faster rate of consumption and, possibly, a lower antioxidant efficiency. The consumption of BHT was slightly slower than BHA at lower temperatures (120 °C). The difference between the consumption rates of BHT and BHA increased markedly with

Scheme 1. Chemical Structures of the Five Synthetic Antioxidants Used in This Study

3. RESULTS AND DISCUSSION 3.1. Biodiesel Characterization. The biodiesel used in this work was derived from waste cooking oil, which is one of the major feedstocks for biodiesel production in China.17 Biodiesel made from waste cooking oil usually has high contents of unsaturated fatty esters and, therefore, shows noticeably poor oxidation stability in many previous studies.21 As seen from Table 1, this biodiesel fulfilled the EN 14214:2014 quality specifications with the exception of the oxidation stability, which could be ascribed to its high unsaturated fatty ester contents. This biodiesel had 23.57% of saturated fatty esters, 46.51% of monounsaturated fatty esters, and 28.88% of polyunsaturated fatty esters. The predominant fatty ester was methyl oleate (C18:1, 41.53%), followed by methyl linoleate (C18:2, 27.78%) and methyl palmitate (C16:0, 15.53%). The composition profile of this biodiesel agreed well with previous reports that also chose used frying oil as raw material.2,14,22 3.2. Rancimat Induction Period. The Rancimat induction period values obtained at 100−150 °C for biodiesel treated with antioxidants are presented in Table 2. A good repeatability of this method could be confirmed with a coefficient of variation of lower than 5% in all cases. The untreated biodiesel showed only 3.01 h induction period at 110 °C, which failed to meet the minimum 8 h oxidation stability requirement prescribed in the EN 14214:2014 specification. As seen in Table 2, the addition of antioxidants could noticeably extend the induction period. The longer the induction period, the better the oxidation stability. Both the testing temperature and antioxidant concentration had significant influences on the length of the induction period. The lower the temperature, the longer the induction period. In addition, the higher the concentration, the better the oxidation stability. PY gave the highest induction period value at 100 ppm, followed by TBHQ, PG, BHA, and BHT. At concentrations greater than 250 ppm, the effectiveness of antioxidants was not in the same order: TBHQ showed the best oxidative stability, followed by PY, PG, BHA, and BHT. From the table, we can see that BHA was slightly better than BHT at all concentration levels, while TBHQ, PY, and PG were far more efficient than the former according to their induction period values. A total of 100 ppm of TBHQ and PY could match 1000 ppm of BHA and BHT; almost 500 ppm of BHA and BHT were needed to satisfy the requirement of the specification. It should be noted, C

DOI: 10.1021/acs.energyfuels.6b02199 Energy Fuels XXXX, XXX, XXX−XXX

D

1000 ppm

750 ppm

500 ppm

250 ppm

blank 100 ppm

TBHQ PY PG BHA BHT TBHQ PY PG BHA BHT TBHQ PY PG BHA BHT TBHQ PY PG BHA BHT

PY TBHQ PG BHA BHT

22.52 ± 0.33 22.87 ± 0.40

21.12 ± 0.21 20.74 ± 0.23

19.22 ± 0.27 18.16 ± 0.22

15.53 ± 0.09 14.32 ± 0.29

1.45 1.75

1.00 1.11

1.41 1.21

0.56 2.06

0.72 0.51

3.83

6.62 ± 0.25

11.35 ± 0.08 10.67 ± 0.05

CV (%)

100 °C

mean ± SD

110 °C

22.30 20.07 15.10 7.24 6.27 33.45 22.70 19.57 9.18 8.29 36.82 24.34 22.73 10.23 9.63 51.12 25.89 24.51 11.10 10.83

3.01 16.11 12.36 10.07 4.94 4.55

0.03 0.30 0.26 0.24 0.07 0.02 0.41 0.74 0.42 0.10 0.08 0.41 0.38 0.21 0.14 0.03 0.50 0.53 0.24 0.08 0.14 0.12 0.39 0.47 0.21 0.12

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

mean ± SD

1.83 3.71 2.79 1.32 1.29 1.23 1.67 1.10 1.56 0.42 1.36 2.17 1.04 0.79 1.45 0.23 1.50 1.93 1.92 1.14

0.94 1.87 2.07 2.37 1.48 0.40

CV (%) 1.54 ± 7.07 ± 5.42 ± 4.29 ± 2.35 ± 2.25 ± PY > TBHQ 9.92 ± 8.81 ± 6.53 ± 3.41 ± 2.87 ± 15.00 ± 10.48 ± 8.64 ± 4.33 ± 3.75 ± 17.10 ± 11.45 ± 10.24 ± 5.02 ± 4.46 ± 22.78 ± 11.61 ± 11.01 ± 5.43 ± 5.03 ± TBHQ > PY

CV (%)

130 °C mean ± SD

0.01 0.92 0.61 ± 0.01 0.14 1.94 3.07 ± 0.04 0.10 1.82 2.22 ± 0.02 0.11 2.59 1.94 ± 0.01 0.02 0.92 1.08 ± 0.01 0.07 3.12 0.95 ± 0.02 > PG > BHA > BHT 0.06 0.56 4.31 ± 0.08 0.14 1.64 4.19 ± 0.01 0.16 2.38 2.83 ± 0.06 0.02 0.63 1.60 ± 0.03 0.05 1.72 1.30 ± 0.02 0.05 0.35 6.90 ± 0.06 0.20 1.93 5.06 ± 0.22 0.20 2.33 3.85 ± 0.10 0.04 1.04 2.16 ± 0.02 0.09 2.42 1.74 ± 0.04 0.33 1.91 7.86 ± 0.34 0.15 1.33 5.46 ± 0.05 0.23 2.26 4.65 ± 0.04 0.04 0.84 2.44 ± 0.04 0.05 1.15 2.07 ± 0.06 0.27 1.18 10.79 ± 0.15 0.05 0.40 5.71 ± 0.12 0.23 2.08 5.13 ± 0.05 0.07 1.28 2.78 ± 0.00 0.06 1.19 2.36 ± 0.03 > PG > BHA > BHT

mean ± SD

120 °C

Rancimat induction period (h)

Table 2. Oxidation Stability of Biodiesel Stabilized with Antioxidants Determined at 100−150 °C

1.77 0.31 2.10 1.61 1.15 0.85 4.36 2.54 0.79 2.5 4.29 0.96 0.88 1.47 2.68 1.43 2.14 0.99 0 1.40

0.95 1.21 0.93 0.42 0.76 2.11

CV (%)

140 °C

1.93 2.03 1.39 0.82 0.63 3.33 2.50 1.87 1.13 0.86 3.89 2.74 2.23 1.35 1.01 5.19 2.91 2.45 1.47 1.17

0.31 1.45 1.16 0.86 0.56 0.46 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

± ± ± ± ± ± 0.01 0.01 0.02 0.01 0.01 0.03 0.04 0.01 0.01 0.01 0.08 0.03 0.02 0.01 0.02 0.15 0.10 0.01 0.01 0.01

0.01 0.02 0.02 0.02 0.02 0.00

mean ± SD

0.50 0.64 1.25 1.54 2.24 0.79 1.55 0.51 1.15 0.95 2.14 0.91 1.00 0.96 1.70 2.81 3.58 0.51 0.96 0.70

3.11 1.65 1.30 2.03 3.45 1.08

CV (%)

150 °C

2.60 ± 0.04 1.54 ± 0.02 1.27 ± 0.01

1.81 ± 0.04 1.45 ± 0.02 1.12 ± 0.01

1.69 ± 0.08 1.32 ± 0.02 0.98 ± 0.02

0.97 ± 0.01 1.08 ± 0.02 0.70 ± 0.01

0.74 ± 0.01 0.57 ± 0.01 0.44 ± 0.01

mean ± SD

1.38 1.53 0.76

2.08 1.18 0.86

4.87 1.68 1.78

0.84 1.39 1.86

1.10 2.20 1.86

CV (%)

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DOI: 10.1021/acs.energyfuels.6b02199 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Influence of the antioxidant concentration on biodiesel oxidation stability determined at 100−150 °C.

Figure 2. Natural logarithm of the initial antioxidant concentration versus Rancimat induction period determined at 100−150 °C.

Table 3. Reaction Constant k (h−1), Critical Concentration Ccr (ppm), and Coefficient of Determination (R2) for Consumption of Antioxidants at Various Temperatures TBHQ

100 110 120 130 140 150

°C °C °C °C °C °C

PY 2

PG 2

k

Ccr

R

k

Ccr

R

0.0606 0.1351 0.2720 0.5607 1.1210

79.03 81.14 90.17 91.69

0.9305 0.9476 0.9395 0.9331 0.9116

0.2401 0.4779 0.8563 1.5635 2.8579

4.42 6.99 11.02 16.29

0.9984 0.9918 0.9970 0.9995 0.9991

BHA 2

k

Ccr

R

0.1574 0.3353 0.7029 1.4305 2.7720

33.66 40.61 43.76 48.11

0.9962 0.9931 0.9811 0.9936 0.9906

E

BHT 2

k

Ccr

R

0.2044 0.3741 0.7417 1.3585 2.4387

38.72 50.40 56.99 61.79 59.73

0.9992 0.9993 0.9949 0.9884 0.9850

k

Ccr

R2

0.1872 0.3622 0.7984 1.5982 3.1981

53.30 68.20 73.02 71.86 79.95

0.9868 0.9798 0.9579 0.9685 0.9643

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consumption of the antioxidant can be calculated from the slope of the line (−Ea/R) and ranked in the following order: TBHQ (97.85 kJ/mol), PG (96.91 kJ/mol) > BHT (91.74 kJ/ mol) > PY (82.76 kJ/mol) > BHA (80.08 kJ/mol). The Ea value of propyl gallate was confirmed with the activation energy of PG consumption (97.02 kJ/mol) in safflower biodiesel reported by Xin et al.8 The activation energy is equal to the energy barrier that must be overcome for a chemical reaction to occur. The faster rate of consumption of PY was in line with its lower activation energy than PG. According to Tan et al.28 and Hashemi et al.,29 a higher Ea indicates a smaller temperature change needed to induce a certain change in the rate of oxidation. The reactions with higher Ea are more temperaturesensitive, and reactions with lower Ea are less temperaturesensitive.30 Despite the faster rate of consumption, BHT showed a higher temperature sensitivity than BHA. Therefore, the biodiesel stabilized with PY and BHA demonstrated a lower temperature dependence with respect to oxidation compared to samples with TBHQ, PG, and BHT, because the latter displayed higher susceptibility to oxidative degradation at a higher temperature than the former. 3.4. Shelf-Life Prediction. To verify the temperature dependence of the induction period, Figure 4 shows the natural logarithm of the induction period (ln IP) as a function of the temperature (in °C) for biodiesel stabilized with antioxidants. A high linear relationship (R2 > 0.9963) exists between the decrease in the induction period with an increasing temperature. A 10 °C rise in the temperature decreased the induction period by a factor of 2.10−2.20 for TBHQ and PG, 2.11−2.19 for BHT, 2.02−2.17 for PY, and 1.98−2.13 for BHA. Both antioxidant type and antioxidant concentration affected the temperature coefficients. If the mechanism of antioxidant action in biodiesel does not change at room temperature,31 the straight lines in Figure 4 could be extrapolated to ambient temperature (25 °C) to obtain the shelf life of biodiesel, as shown in Figure 5. Apparently, TBHQ, PY, and PG presented a longer shelf life than BHA and BHT at ambient temperature. A half year of shelf life is now recommended for biodiesel under common market conditions.8,9,32 A total of 250 ppm of TBHQ, PG, PY, BHT, and BHA could have a guarantee of approximately 25.0, 13.9, 13.2, 6.8, and 5.4 months of storage, respectively. The extrapolated induction period of BHT at a lower temperature was much higher than that of BHA, probably as a result of lower volatilization and decomposition losses. At concentrations greater than 250 ppm, the shelf life of PY and BHA seemed to be independent of the antioxidant concentration but dependent upon the antioxidant type. Over the whole concentration range, a longer shelf life would be expected with a higher antioxidant concentration in the cases of TBHQ, PG, and BHT. Overall, the shelf life for these antioxidants could be ranked as follows: TBHQ > PG > PY > BHT > BHA, basically in agreement with the order of the reaction constant of antioxidant consumption. According to the results obtained, a lower reaction rate of antioxidant consumption seemed to lead to a longer shelf life of biodiesel. It should be mentioned that the Rancimat method has rarely been applied in realistic storage conditions to obtain the shelf life of biodiesel. As pointed out by Reynhout,33 the Rancimat method has its own limitations: long induction times may not be accurately determined with this method as a result of water evaporation in the conductivity cells. According to Xin et al.,8 the overall storage stability of biodiesel cannot be directly

the increasing temperature, which could be due to volatilization and decomposition losses of the former at a high temperature employed. Considering the better antioxidant efficiency of PY than PG based on the length of the induction period, the latter presented an unexpected slower rate of consumption and the difference between the consumption rates of PY and PG decreased with the increasing temperature. Overall, the reaction constant increased in the following order: TBHQ < PG < PY < BHA < BHT. On the basis of the assumption of the first-order decay, the lower critical concentration could be associated with higher antioxidant efficiency. PY had the lowest critical concentration, followed by PG, BHA, BHT, and TBHQ, basically in line with the order of antioxidant efficiency, except TBHQ. TBHQ showed the best oxidation stability and lowest consumption rate, while it also required the highest critical concentration. The possible reason is that the reaction mechanism of TBHQ with the free radicals is quite different from that of the other four compounds: TBHQ can act as not only a radical scavenger but also a radical producer.4 Given the fact that the oxidation products of TBHQ may be also effective in retarding the oxidation, it would be difficult to determine when the critical concentration is reached.25,26 Therefore, the prediction of the antioxidant efficiency based only on the critical antioxidant concentration can be misleading when the critical antioxidant concentration does not take the antioxidative effects of the oxidation products into consideration. In the case of TBHQ, it was assumed that the oxidation products formed during the antioxidative process as well as the TBHQ itself contributed to the improvement of oxidation stability.25,26 The Arrhenius equation, k = A exp(−Ea/RT), was employed to describe the temperature dependence of the reaction constant k, where A is the pre-exponential factor, Ea is the apparent activation energy, R is the ideal gas law constant (8.314 J mol−1 K−1), and T is the absolute temperature in kelvin. In Figure 3, it can be seen that ln k versus 1/T (in K) follows a strong linear relationship over the whole temperature range studied, with the coefficient of determination R2 being 0.9998, 0.9996, 1, 0.9993, and 0.9986 for THBQ, PY, PG, BHA, and BHT, respectively. The activation energy Ea for the

Figure 3. Temperature dependence of the reaction constant for antioxidant consumption. F

DOI: 10.1021/acs.energyfuels.6b02199 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 4. Natural logarithm of the Rancimat induction period as a function of the temperature (°C) for biodiesel stabilized with various concentrations of antioxidants.

of peroxide values could be a valuable index to verify the usefulness of the extrapolation fit of the Rancimat method. On the basis of the induction period values determined at elevated temperatures (70−110 °C) using the Rancimat apparatus, Pereira et al.10 obtained an estimated shelf life of 186−376 days at room temperature (15−25 °C) with the extrapolation method, which agreed with the induction period value of 314 days, determined by monitoring the rate of hydroperoxide formation. The only challenge with realistic shelf-life experiments is that it could take months or years to obtain the induction period values, which could explain why the shelf life of biodiesel has been determined in most cases by the extrapolation method. 3.5. Biodiesel Oxidation Kineitcs. The oxidation of biodiesel generally proceeds according to the free-radical chain reaction mechanism characterized by an induction period. A pseudo-first-order reaction was often employed to simplify the kinetics of biodiesel oxidation before the induction period.12,19 The temperature dependence of the induction period could be expressed with an Arrhenius-like equation: IP = A′ exp(Ea/RT). According to Dunn,19 this model provides a means for quantifying the kinetic parameters of the biodiesel oxidation reaction. As seen from Figure 6, a strong linear relationship (R2 > 0.9985) with a positive slope (Ea/R) could be observed between the natural logarithm of the induction period (ln IP) and the inverse of the temperature (1/T). The addition of antioxidants in biodiesel presented different temperature dependences. The apparent activation energy Ea for the oxidation of biodiesel stabilized with antioxidants was shown in Figure 7. The untreated biodiesel reported an Ea of 99.18 kJ/mol, which could be increased to 103.74, 104.52, 106.21, and 100.52 kJ/mol for biodiesel treated with 100 ppm of TBHQ, PY, PG, and BHT, respectively. A further increase in the concentration led to a decrease in the activation energy for the oxidation of biodiesel, following an opposite trend than expected: higher concentrations generally give increased values of Ea. Of these antioxidants, PY and BHA showed the most

Figure 5. Shelf life of biodiesel at ambient temperature stabilized with antioxidants by the extrapolation method.

measured with the Rancimat method, because many factors could affect the fuel quality during storage, such as the presence of water, microbial contamination, and storage conditions. On the basis of the assumption that the mechanism of the oxidation reaction does not change at high (above 100 °C) and low (close to storage) temperatures, the extrapolation method, widely used in many studies, could provide a useful practice for predicting the shelf life of biodiesel at low temperatures. To verify the extrapolated induction period determined by the standard Rancimat method, realistic shelf-life experiments are of great importance. In previous studies, the shelf life of biodiesel at low temperatures is often determined by monitoring the increase of primary oxidation products (peroxide values) during the oxidative deterioration.12,13 The pioneering work by Pereira et al.10 convinced that the evolution G

DOI: 10.1021/acs.energyfuels.6b02199 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 6. Temperature dependence of the Rancimat induction period for biodiesel stabilized with various concentrations of antioxidants.

sensitivity of BHT, which demonstrated a higher protective effect at a lower temperature than BHA. It should be mentioned that these phenolic compounds, possibly acting as prooxidants at higher concentrations, are more easily oxidized than the biodiesel substrate during the antioxidative process. Care must be taken when interpreting the apparent activation energy for the oxidation of biodiesel: the participation of these antioxidants in other side reactions, especially at higher concentration levels, may change the course of the oxidation reaction as well as the mechanism.17,34

4. CONCLUSION Kinetic studies on the oxidation of biodiesel and the consumption of antioxidants were performed in this work. An increase in stability of biodiesel could be observed with the addition of antioxidants. These antioxidants, differing in their antioxidative characteristics, could be ranked in different orders according to different parameters: antioxidant efficiency (TBHQ > PY > PG > BHA > BHT), consumption rate (TBHQ < PG < PY < BHA < BHT), critical concentration (PY < PG < BHA < BHT < TBHQ), apparent activation energy (TBHQ, PG > BHT > PY > BHA), and predicted shelf life (TBHQ > PG > PY > BHT > BHA). The lower consumption rate of antioxidant seemed to lead to a longer induction period and predicted shelf life of biodiesel. The determination of the critical concentration would be difficult when the oxidation products of the antioxidant contributed to the improvement of oxidation stability. More work is needed in the future to confirm the antioxidative effects of the oxidation products of TBHQ formed during the antioxidative process of biodiesel oxidation. Of these antioxidants, PY and BHA did not show an increase of projected shelf life but instead a marked decrease of activation energy with an increasing concentration. The apparent activation energy Ea for the consumption of the antioxidant agreed well with the order of Ea for the oxidation of biodiesel; TBHQ, PG, and BHT, more temperature sensitive than PY and

Figure 7. Apparent activation energy for the oxidation of biodiesel stabilized with antioxidants obtained from the Arrhenius-like equation.

marked decrease of the activation energy with an increasing concentration. Further addition of PY and BHA to untreated biodiesel decreased the temperature dependence of the oxidation reaction to a large extent. Overall, the apparent activation energy Ea for the oxidation of biodiesel agreed reasonably well with the order of Ea for the consumption of antioxidant: TBHQ, PG > BHT > PY > BHA. The oxidation of biodiesel stabilized with TBHQ, PG, and BHT was more temperature-sensitive than that with PY and BHA; increasing the temperature had more significant effects on the oxidation of biodiesel treated with TBHQ, PG, and BHT than the latter. Although PY presented higher antioxidant efficiency than PG at all test temperatures, the lower activation energy of PY than PG indicated that the latter was more susceptible to oxidative degradation at a higher temperature than the former. The same held true for the higher temperature H

DOI: 10.1021/acs.energyfuels.6b02199 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

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BHA, demonstrated higher susceptibility to oxidative degradation at a higher temperature. The kinetic data of this work provided a better understanding of the antioxidative characteristics of these phenolic compounds in stabilizing the biodiesel from autoxidation. The acting mechanism of the oxidation products of TBHQ as well as the temperature sensitivity of PY and BHA need to be further investigated in future studies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jian Zhou: 0000-0001-9357-2929 Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.6b02199 Energy Fuels XXXX, XXX, XXX−XXX