Esterification of oleic acid with ethanol catalyzed by sulfonated cation exchange resin: Experimental and kinetic studies

Esterification of oleic acid with ethanol catalyzed by sulfonated cation exchange resin: Experimental and kinetic studies

Energy Conversion and Management 76 (2013) 980–985 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www...

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Energy Conversion and Management 76 (2013) 980–985

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Esterification of oleic acid with ethanol catalyzed by sulfonated cation exchange resin: Experimental and kinetic studies Yuwang Jiang a, Jie Lu b, Kaian Sun a, Lingling Ma a, Jincheng Ding a,⇑ a b

School of Chemical Engineering, Shandong University of Technology, Zibo 255049, China Department of Resources and Environmental Engineering, Shandong University of Technology, Zibo 255049, China

a r t i c l e

i n f o

Article history: Received 9 May 2013 Accepted 4 August 2013

Keywords: Biodiesel Sulfonated cation exchange resin Esterification Oleic acid Pseudo-homogeneous model

a b s t r a c t This paper investigated the effects of ethanol to oleic acid molar ratio, reaction temperature, catalyst loading, water content and catalyst recycling on sulfonated cation exchange resin in a stirred batch reactor under atmospheric pressure. When the esterification was carried out with an ethanol to oleic acid (42.4 g) molar ratio of 9:1, reflux of ethanol at 82 °C, 20 g of catalyst and 8 h of reaction time, the oleic acid conversion rate reached approximately 93%. A pseudo-homogeneous kinetic model for describing the esterification of oleic acid with ethanol by the sulfonated cation exchange resin was developed on the basis of laboratorial results. The kinetic model can well predict the oleic acid conversion. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Biodiesel, mono-alkyl esters of long chain fatty acids, is an alternative diesel fuel derived from vegetable oils or animal fats by esterification [1,2] and transesterification [3,4] with short chain alcohols. It has many well-known advantages: renewable resource, biodegradability, lower combustion emission profile (especially SOx), lower toxicity, higher security of use and storage, adaptability to engine used, good engine performance and ability to be blended in any proportion with regular petroleum-based diesel fuel [5,6]. Commonly used short chain alcohols include methanol, ethanol, propanol and butanol. Methanol is preferred for commercial use because of its low price and physico-chemical advantages such as polarity and shortest chain [7]. However, ethanol is preferable in esterification and transesterification because it is obtained from agricultural products and is renewable and environmentally acceptable [8]. At present, biodiesel is obtained by reacting triglycerides with lower alcohols in the presence of a strong base used as catalyst. The oils used for alkaline transesterification should contain no more than 1% free fatty acids (FFA) [9,10]. In China, non-edible and waste vegetable oil or animal fat are recommended as raw materials to produce biodiesel [11–14]. If the FFA level exceeds this threshold, saponification reduces the yield and formation rate of ⇑ Corresponding author. Address: College of Chemical Engineering, Shandong University of Technology, 12 Zhangzhou Road, Zibo 255049, China. Tel.: +86 533 2783397; fax: +86 533 2781664. E-mail address: [email protected] (J. Ding). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.08.011

biodiesel. FFA can react with the catalyst rapidly and form long chain soaps, which may bring on serious separation problems. Thus, a pre-treatment step is generally required to decrease the FFA amount to below 1 wt% [15–19]. Close attention has been paid to the esterification of long chain carboxylic acids such as oleic acid (OA) in the context of biodiesel production, as it is present in major oil crops such as soybean, jatropha curcas, sunflower, rapeseed, pongamia, palm and sea mango. FFA may be present in transesterification of vegetable oils as well as recycled feedstocks. Generally, sulfuric acid is used as catalyst for esterification because its acid strength is responsible for the release of more H+ species to protonate the carboxylic moiety of the fatty acid (rate determinant step) [20,21]. However, it has a number of drawbacks such as side reactions, equipment corrosion, additional neutralization of the reaction mass and difficult separation of catalyst from the production stream. In view of cost savings, sulfonated cation exchange resin (SCER) as a heterogeneous catalyst might have greater advantages over sulfuric acid in that it serves as a heterogeneous catalyst for esterification [22–24]. Furthermore, SCER can offer higher selectivity towards the desired product(s) and better reusability in comparison with homogeneous acid catalysts [25]. Some have been done on the esterification of OA with ethanol [26–30]. New heterogeneous catalysts such as (NH4)xCs2.5xH0.5PW12O40 [26], organophosphonic acid-functionalized silica [27], 12-tungstophosphoric acid supported on zirconia [28] were tested in the esterification of OA with ethanol, and desirable results were obtained. Lucena et al. [29] reported that the yield of ethyl ester reached 99.9% by using an adsorption column filled with zeolite

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3A to adsorb the water generated during the esterification of OA with ethanol. Pinnarat and Savage [30] investigated the esterification of OA with excess ethanol under subcritical and supercritical reaction conditions (based on ethanol). However, to the best of our knowledge, the kinetics of esterification of OA with ethanol in the presence of SCER, which is based on the pseudo-homogeneous model, has been rarely reported. Thus, the aim of this study is to use SCER as a heterogeneous catalyst for biodiesel production by esterification of OA with ethanol. The most relevant variables such as ethanol to OA molar ratio, reaction temperature, catalyst loading, reaction time, water content and catalysts recycling for the reaction were examined. The single factor experimental design was applied for the purpose. A pseudo-homogeneous kinetic model for describing the esterification of OA with ethanol catalyzed by SCER was developed according to the experimental data.

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recycling on the reaction conversion were studied. The samples were taken out from the reaction system every one hour and treated by decompression distillation to remove excess ethanol and other impurities to determine the acid value of reaction system so as to calculate OA conversion. The conversion was determined by a standard titration method following China Standard – GB/ T5530-2005. When the reaction was completed, the reaction mixture was cooled to room temperature and settled to separate the liquid phase (ethanol, unreacted OA and biodiesel) and solid phase (catalytic resins). The reused SCER were soaked in anhydrous ethanol to remove remnant OA, whereupon it was washed and kept in a vacuum oven overnight at 80 °C to remove moisture completely. 3. Kinetic model

2. Experimental

The reaction of OA with ethanol under the catalysis of SCER, produces ethyl ester and water. It can be described by the following equation:

2.1. Materials

Oleic acid ðAÞ þ Ethanol ðBÞ $ Ethyl Oleic ðCÞ þ Water ðDÞ

OA (purity > 99.9% w/w), ethanol (purity > 99.7% w/w) and 95% (v/v) ethanol were obtained from Yantai Shuangshuang Chemical Company (Yantai, China). Potassium hydroxide, potassium biphthalate and phenolphthalein were of reagent grade. SCER, CH-A (average size of 0.77 mm) in the H+ form, was purchased from Shandong Dongda Chemical Industry (Group) Company, Zibo, China, and used as the heterogeneous catalyst. Its physicochemical properties are presented in Table 1. Before the experiment, fresh resins were washed with secondary deionized water and kept at 100 °C in a vacuum oven overnight to remove moisture completely [31].

The kinetic model in this investigation was set up on the basis of the following suppositions: (1) The rate of the uncatalyzed reactions was negligible in comparison with the catalyzed reactions; (2) The catalytic sites were all the same on the SCER surface; (3) Pseudo-homogeneous is second-order in the forward and reverse direction [24]. In view of these conditions, the reaction kinetics could be described as:

2.2. Reaction procedure Esterification was carried out in a three-necked batch reactor (250 ml) assembled with a reflux condenser and magnetic whisk at atmospheric pressure. A warmer jacket was used as a heater for the reactor which kept the temperature at an error of ±1 °C and a thermocouple was inserted in the reactor to monitor the reaction temperature. A certain amount of OA was first added into the batch reactor and the speed of magnetic whisk was controlled at 600 rpm [32]. Then the determined amounts of ethanol and catalyst were fed into the batch reactor to start the reaction under desired conditions. The basic reaction conditions were as follows unless otherwise stated, ethanol to OA (42.4 g) molar ratio 9:1, reaction time 8 h, reaction temperature 82 °C, and catalyst amount 20 g. The effects of ethanol to OA molar ratio, reaction temperature, catalyst amount, reaction time, water content and catalyst

Table 1 Characteristics of SCER. Property

CH-A

Structure Matrix Form Total exchange capacity (a) mmol/g (dry) (b) mmol/g (wet) Surface area (m2/g) Average pore diameter (nm) Bead size (%) Moisture content (%) Density (wet) True (g/ml) Apparent (g/ml)

Macroeticular structure Styrene, divinyl-benzen H+ P5.2 P1.5 46 38 0.32–1.25 mm (P95) 610 1.25–1.28 0.75–0.85



dC A ¼ k1 C A C B  k2 C C C D dt

ð1Þ

ð2Þ

where CA, CB, CC and CD denote the concentration of OA, ethanol, ethyl oleate and water, respectively; k1 and k2 were the kinetic constants for the forward and reverse reactions, respectively [33].

C A ¼ C A0 ð1  gÞ

ð3Þ

C C ¼ C D ¼ C A0  g

ð4Þ

C A0 and g referred to the initial concentration of OA and OA conversion. After integrating Eq. (2) became:

d½C A0 ð1  gÞ ¼ k1 ð1  gÞC A0 C B  k2 C 2A0 g2 dt

ð5Þ

Take ethanol and OA in a molar ratio of 9:1 for example.

C B0 ¼ 9C A0

ð6Þ

C B ¼ 9C A0  C A0 g

ð7Þ

Eq. (5) became:

  ðK  1Þg  5K  a  ¼ 2k2 C A at ln  0 ðK  1Þg  5K þ a

ð8Þ

k ¼ A expðEa =RTÞ

ð9Þ

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi where K ¼ k1 =k2 , a ¼ 16K 2 þ 9K . The reaction rate constants k1 and k2 were determined by the trial and error method, using values of the variable K until a plot of the left-hand side of Eq. (8) consisting of a straight line with a negligible intercept was obtained. The esterification reaction rate depended on the value of the activation energy. The lower the activation energy, the higher the reaction rate. Therefore, reduction of the activation energy would effectively promote the reaction. Considering the effect of reaction temperature on the kinetic model, the Arrhenius equation was listed as:

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90 80

80

Oleic Acid Conversion (%)

Oleic Acid Conversion (%)

90

70 60 50 40 30 20

70 60 50 40 30 20 10

0

1

2

3

4

5

6

7

8

0

1

2

Reaction Time (h) Fig. 1. Effect of ethanol to OA molar ratios on the esterification with the ethanol to OA molar ratios: (h) 1:1; (s) 3:1; (4) 6:1; (5) 8:1; (/) 9:1; (.) 10:1; (e) 12:1; (q) 15:1.

The reaction rate constants at varied temperatures could then be utilized to determine the activation energy Ea and pre-exponential factor A in the Eq. (10) converted from Eq. (9):

ln k ¼ Ea =RT þ ln A

3

4

5

6

7

8

Reaction Time (h) Fig. 2. Effect of reaction temperature on the esterification employed reaction temperature: (h) 40 °C; (s) 50 °C; (4) 60 °C; (5) 70 °C; (/) 82 °C.

temperature, the conversion of OA was found to increase rapidly. This was because increase in temperature caused higher molecule motion speed and mass transfer rate which led to higher reaction rate [34].

ð10Þ 4.3. Effect of catalyst loading on the esterification

4.1. Effect of ethanol to OA molar ratios on the esterification For a stoichiometric esterification, the ethanol to OA molar ratio is 1. Generally, the esterification is performed in excess of ethanol to promote the forward reaction since the esterification of OA with ethanol is reversible. In order to study the effect of ethanol to OA molar ratio on esterification, experiments were carried out in ethanol to OA molar ratios of 1:1, 3:1, 6:1, 8:1, 9:1, 10:1, 12:1 and 15:1 as shown in Fig. 1. It can be seen that the ethanol to acid molar ratio markedly affected the esterification. The OA conversion increased as the ethanol to OA molar ratio increased from 1:1 to 9:1, but it decreased when the molar ratio rose from 9:1 to 15:1. Therefore, the optimum molar ratio was 9:1 in this reaction system. In the esterification system, the ethanol concentration played a dominant role. The OA conversion rate increased with addition of ethanol. However, the ethanol adsorbed on the catalyst surface would accumulate when the ethanol concentration increased to a higher level, which offset the increased OA conversion or even deactivated the catalysts whereby the esterification was inhibited [13]. In addition, the amounts of OA and catalyst were fixed in the reaction system. As the ethanol to OA molar ratio increased, the concentration of catalyst in the system decreased, leading to the reduction of conversion. 4.2. Effect of reaction temperature on the esterification Temperature is one of the most important variables affecting the conversion of OA. To optimize the reaction condition, the effect of different reaction temperatures on the esterification was investigated as shown in Fig. 2. The reactions were carried out at room temperature, 40 °C, 50 °C, 60 °C, 70 °C and 82 °C respectively with other conditions fixed as follows: ethanol to OA (42.4 g) molar ratio 9:1, catalyst 20 g, and reaction time 8 h. With the increasing

The effect of different catalyst loading on the esterification was represented in Fig. 3, where the ethanol to OA (42.4 g) molar ratio was 9:1, reaction temperature 82 °C, and reaction time 8 h. The reaction rate increased with an increase in the catalyst loading and the time required for the reaction to reach equilibrium was reduced. The increase in the reaction rate was due to the increase in the total number of acid sites available for the reaction with increasing catalyst loading [32]. Without using catalyst, the OA conversion was 8.18% in 8 h. When the loading amount of SCER reached 20 g, the reaction underwent smoothly till the highest OA conversion rate of 92.75%. It seemed unnecessary to further increase the amount of catalyst for the higher OA conversions. Thus the optimal loading amount of catalyst was 20 g.

100 90

Oleic Acid Conversion (%)

4. Results and discussion

80 70 60 50 40 30 0

1

2

3

4

5

6

7

8

Reaction Time (h) Fig. 3. The effects of the amounts of SCER on the esterification with the catalyst loading: (h) unused; (s) 5 g; (4) 10 g; (5) 15 g; (/) 20 g; (.) 25 g; (e) 30 g.

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90

90

Oleic Acid Conversion (%)

Oleic Acid Conversion (%)

80 80 70 60 50 40 30

70 60 50 40 30 20 10

20

0 0

1

2

3

4

5

6

7

8

1

2

Reaction Time (h)

According to the curves in Fig. 3, the OA conversion increased with the reaction time for a certain catalyst loading. The OA conversion increased rapidly with reaction time at the beginning of the reaction. As the reaction proceeded, the reactant concentration decreased, resulting in reduced reaction rate; meanwhile, the product concentration and the reverse reaction rate increased. The reaction may finally reach equilibrium when the reverse reaction rate is equal to the forward reaction rate [35]. 4.4. Effect of water content on the esterification In order to investigate the impact of the water content on the esterification of OA with ethanol, the reaction was performed using anhydrous and 95% (V/V) ethanol in the presence of the SCER, respectively. The reaction was carried out under the following conditions: ethanol to OA (42.4 g) molar ratio 9:1, reaction temperature 82 °C, catalyst amount 20 g and reaction time 8 h. The results were illustrated in Fig. 4. It showed that the OA conversion using anhydrous ethanol was higher than that using 95% (V/V) ethanol. Nevertheless, the initial reaction rate seemed higher in the presence of a small amount of water. The effect of the water content on OA conversion was very important for a heterogeneous reaction. The presence of water in the reaction medium might have two negative effects. On the one hand, in view of the chemical reaction equilibrium, the water, as a product of esterification, might inhibit the reaction to some extent, leading to a low final conversion [36]. On the other hand, it also led to a better hydrophilicity of the –SO3H active site of the SCER catalyst. Compared with ethanol, it was easier for the hygrogen bond of water to interact with –SO3H owning to the stronger polarity of water. The prohibition of esterification on SCER was because of the poisoning of acid sites by water and the poor attainability of ethanol to active sites in the presence of water [37]. 4.5. SCER reusability Catalytic stability of the SCER was evaluated by performing consecutive batch runs under the same operating conditions: ethanol to OA (42.4 g) molar ratio 9:1, reaction temperature 82 °C, catalyst amount 20 g and reaction time 8 h. The catalytic resins were employed in the esterification for five runs and the results were shown in Fig. 5. It was found that the SCER showed good catalytic activity in the first run, above 92% conversion. However, noticeable

4

5

Fig. 5. The stability of SCER resin in five batch experiments at the same operating conditions: ethanol to OA (42.4 g) molar ratio 9:1, reaction temperature 82 °C, catalyst amount 20 g and reaction time 8 h.

deactivation occurred after the first run. The main reason for conversion decrease was the loss of broken resin and the hydrolysis of sulfonic group in the water. Although no sulfur was present in the OA, a sulfur content of 62.3 ppm was detected in the first product by Micro-coulometric Analyzer (WK-2D, Jiangsu Jiangfen Electroanalytical Instrument CO., Ltd., Jiangsu, China). In the subsequent experiment, conversion decreased slightly, which might result from hydrolysis of sulfonic group. After the fifth run, the SCER became very fragile and broke into fine particles under stirring and could not be used in the repeated experiments. Finally it would lead to decrease in the catalytic efficiency. Certainly, the saturation of absorbing water produced in the esterification might be another reason for the decreased conversion for SCER [38].

4.6. Reaction rate constants The kinetic parameter values were determined by regression analysis of the experimental data with the kinetic model. The fitting of the data of esterification of OA with ethanol at 82 °C with

5.0

ln{[(K-1)η-5K-α]/[(K-1)η-5K+α]}

Fig. 4. Comparison of the effect of water present in the ethanol employed: (s) anhydrous ethanol; (h) 5% water content ethanol.

3

Recycling Times

4.5

4.0

3.5

3.0

2.5 0

1

2

3

4

5

6

7

8

Reaction Time (h) Fig. 6. Determination of the kinetic constants by using Eq. (6) under the conditions of ethanol to OA molar ratio 9:1, catalyst 20 g and temperature 82 °C.

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100 -3 90 -4

Oleic Acid Conversion (%)

lnk 1

lnk

-5

-6

-7

lnk 2

80 70 60 50 40 30

-8 2.8

2.9

3.1

3.0

3.2

20 0

1

-3

Fig. 7. Influence of temperature on the esterification reaction rate constants of OA with ethanol catalyzed by the SCER.

Table 2 Activation energy and pre-exponential factor for the esterification of OA with ethanol catalyzed by the SCER. Reaction rate constant

Ea (Activation energy, kJ/mol)

A (Pre-exponential factor)

RSquare

k1 (Forward reaction) k2 (Reverse reaction)

24.80

114.14

0.9810

23.59

2

3

4

5

6

7

8

Reaction Time (h)

1/T (10 )

2.669

0.9924

Eq. (8) was shown in Fig. 6. Similar plots could also be built to get the reaction rate constants corresponding to other conditions.

4.7. Activation energy and pre-exponential factor As shown in Fig. 7, the Arrhenius equation showed the dependence of the reaction rate constant on temperature in the esterification of OA with ethanol catalyzed by SCER. It clearly indicated that pseudo-homogeneous kinetic model was valid. The plot of ln k vs. 1/T could be illustrated by a straight line and the activation energy Ea was obtained from the slope of the line. As can be seen in Table 2, the activation energy of forward and reverse reaction was 24.80 and 23.59 kJ/mol, respectively [39].

Fig. 8. Comparison of the experimental OA conversion and the theoretical prediction by Eq. (6) under the reaction conditions of ethanol to OA molar ratio 9:1, reaction temperature 82 °C, catalyst amount 20 g and reaction time 8 h, predicted data (h); experimental data (s).

5. Conclusion SCER, being a stronger acid, can be used as potential alternative catalyst for the esterification of OA with ethanol to produce biodiesel. The effect of various reaction parameters such as ethanol/OA molar ratio, reaction temperature, and catalyst loading were studied to optimize the conditions for maximum conversion. As a result of this study, catalyst loading 20 g, reaction temperature 82 °C, ethanol to AO (42.4 g) molar ratio 9:1 and magnetic whisk at 600 rpm were suggested as optimum conditions. Results showed that conversion of AO to ester was 93%. A pseudo-homogeneous kinetic model was presented to describe the esterification reaction. It was found that the calculated activation energy of forward reaction Ea = 24.80 kJ mol1, pre-exponential factor A = 114.14, and activation energy of reverse reaction E0a = 23.59 kJ mol1, pre-exponential factor A0 = 2.67. All the laboratorial data were in agreement with the predictions of the model. This result is valuable in the reactive process analysis for assessing the industrial application potential of the SCER. Acknowledgments The authors thank the support from the Research Excellence Award of Shandong University of Technology and Zibo Technology Research and Development Program of China (Grant No. 2011GG10101).

4.8. Model prediction A suite of experiments were analyzed to identify whether the kinetic model could predict the esterification reaction. The parameters of the kinetic model were derived from the laboratorial data of the esterification of OA with ethanol obtained under the conditions of ethanol to OA molar ratio 9:1, catalyst amount 20 g, reaction temperature 82 °C, and reaction time 8 h. The predictive conversion was carried out according to Eq. (8). The fitness of the laboratorial data with the suggested model was evaluated by comparing the laboratorial OA conversion with the theoretical prediction based on Eq. (8) as illustrated in Fig. 8. It was found that all of the laboratorial data were reproduced with an error of less than 3%. It was obvious that the suggested kinetic model was in agreement with the laboratorial results.

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