Kinetic model for the esterification of oleic acid catalyzed by zinc acetate in subcritical methanol

Kinetic model for the esterification of oleic acid catalyzed by zinc acetate in subcritical methanol

Renewable Energy 35 (2010) 625–628 Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene Kine...

241KB Sizes 0 Downloads 69 Views

Renewable Energy 35 (2010) 625–628

Contents lists available at ScienceDirect

Renewable Energy journal homepage: www.elsevier.com/locate/renene

Kinetic model for the esterification of oleic acid catalyzed by zinc acetate in subcritical methanol Chengcai Song a, b, Yongqin Qi a, Tiansheng Deng a, b, Xianglin Hou a, *, Zhangfeng Qin a a b

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, PR China Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 February 2008 Accepted 16 August 2009 Available online 1 September 2009

The esterification of oleic acid in subcritical methanol catalyzed by zinc acetate was investigated in a batch-type autoclave. The effect of reaction conditions such as temperature, pressure, reaction time and molar ratio of oleic acid to methanol on the esterification was examined. The oleic acid conversion reached 95.0% under 220  C and 6.0 MPa with the molar ratio of methanol to oleic acid being 4 and 1.0 wt% zinc acetate as catalyst. A kinetic model for the esterification was established. By fitting the kinetic model with the experimental results, the reaction order n ¼ 2.2 and activation energy Ea ¼ 32.62 KJ/mol were obtained. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Kinetic model Esterification Oleic acid Methanol Subcritical Zinc acetate

1. Introduction Biodiesel is now considered as an alternative to liquid fuel from petroleum. The production of biodiesel from vegetable oils has been widely researched; however, it is not an economical process because of using valuable vegetable oils. Therefore, waste vegetable oil or animal fat are recommended as raw materials to produce biodiesel. However, the presence of moisture and free fatty acids (FFAs) in these materials may influence the performance and efficiency of such a process. Both water and FFAs can react with the catalyst rapidly and form long chain soaps, which may bring on serious separation problems; a pretreatment step is generally required to decrease the FFAs amount to below 1 wt% [1–5]. A single step process is expected for the production of biodiesel from oils with high FFAs content. Toward this aspect, Hou et al. reported that Lewis acid catalysts are active for both esterification and transesterification and these reactions are enhanced under the subcritical conditions, where oil, FFAs and Lewis acid catalysts are all soluble in the subcritical methanol phase [6]. Moreover, the reaction conditions remain much milder than those under supercritical methanol. The esterification and transesterification via Lewis acid catalysts in the subcritical methanol is then promising for the production of biodiesel from the oil feedstock with high

FFAs content [1–13], in which the esterification of FFAs is a pivotal step that determines the efficiency of whole process. The kinetics for the esterification catalyzed by lipase and/or in supercritical methanol has been studied in several reports. Berrios et al. [9] studied the kinetics of the esterification of FFAs in sunflower oil with methanol in the presence of sulfuric acid; the effect of catalyst concentrations, feed composition, agitation, temperature and reaction time on the esterification reaction was investigated and the activation energies and preexponential factors with different sulfuric acid concentrations were determined. Goddard et al. investigated the esterification of oleic acid and ethanol catalyzed by lipase under supercritical conditions [10]; the reaction was modeled by the Henri–Michaelis–Menten equation and the rate constants were evaluated, but the apparent activation energy was not given. Eiji et al. studied the hydrolysis and methyl esterification for biodiesel production in the two-step supercritical methanol process and found that FFAs acted as acid catalyst [11]; the kinetic models were proposed and fitted with the experimental results. FFAs were supposed to play an important role in the two-step supercritical methanol process. Tesser et al. [12] studied the kinetics of oleic acid esterification with methanol using an acid ion-exchange polymeric resin as catalyst in the presence of triglycerides; the reaction rate was expressed by



* Corresponding author. Tel.: þ86 351 4064152; fax: þ86 351 4041153. E-mail address: [email protected] (X. Hou). 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.08.004

g ¼ kc x A x M 1 

1 xE xW ke x A x M



(1)

C. Song et al. / Renewable Energy 35 (2010) 625–628

where xA, xM, xE, and xW are the molar fractions of the oleic acid, methanol, methyl oleate and water, respectively; kc and ke are the kinetic constant and equilibrium constant, respectively. The apparent activation energy was estimated to be 58.6 KJ/mol. However, the kinetics of the transesterification and esterification of high FFAs oil in subcritical methanol with Lewis acid catalysts were scarcely reported. In this work, the esterification of oleic acid catalyzed by zinc acetate in subcritical methanol in an autoclave was investigated. The effect of temperature, pressure, methanol usage and reaction time on the esterification reaction was examined. A kinetic model was proposed and the kinetic parameters were determined by fitting the model with the experimental results.

100 Oleic acid conversion (%)

626

90

80

70

4

2. Experimental

5

6

7

8

9

10

Pressure (MPa)

2.1. Materials Methanol (purity > 99.9% v/v), oleic acid (purity > 99.9% w/w) and zinc acetate (Zn(CH3COO)2$2H2O, purity > 98.5%) were obtained from Fangzheng Company (Tianjin, China).

Fig. 1. Effect of pressure on the esterification of oleic acid in subcritical methanol. Reaction conditions: 220  C, the molar ratio of methanol to oleic being 4, and reaction time being 30 min.

a moderated pressure, the methanol and oleic acid are mutually soluble and then the esterification is less influenced by pressure.

The reactions were performed in the autoclave (14.8 mL) equipped with an electromagnetic beater. The oleic acid was first introduced, and then the autoclave was sealed and heated to the desired temperature at a fixed heating rate. After that, methanol with zinc acetate (1.0 wt%) dissolved was pumped into the autoclave to the reaction pressure. After reaction for certain time, the mixture in autoclave were discharged and subjected to separation and analysis. Since the critical point of methanol is 239  C and 8.09 MPa, the tests were conducted under 160–220  C and 3–10 MPa. 2.3. Analytical procedure The esterification reaction between oleic acid and methanol can be represented as: catalyst

oleic acid ðAÞ þ methanol ðBÞ 4

methyl oleate ðCÞ

þ water ðDÞ

(2)

The amount of unreacted oleic acid in the product mixture was obtained from its acid value (AV), which can be determined by titration with KOH–C2H5OH solution. The conversion of oleic acid can be calculated according to the equation,

x ¼ ð1  AV1 =AV0 Þ  100%

(3)

where AV0 and AV1 are the acid values of feed and products, respectively. 3. Results and discussion 3.1. Effect of pressure on the esterification reaction As shown in Fig. 1, the conversion of oleic acid increases with the reaction pressure up to 6 MPa and then levels off with further increase of pressure to 10 MPa. At low pressure and relatively high temperature, the reaction mixture may be separated into two phases; the amount of the liquid phase with the catalyst zinc acetate dissolved is restricted, which then results in a low conversion of oleic acid. However, under the subcritical conditions at

3.2. Effect of methanol usage on the esterification reaction The molar ratio of methanol to oleic acid is one of the most important variables affecting the conversion of oleic acid. For a stoichiometric esterification, the molar ratio of methanol to oleic acid is 1. The dependence of oleic acid conversion on the molar ratio of methanol to oleic acid (from 1 to 10) is then shown in Fig. 2. It suggests that the oleic acid conversion increases with the molar ratio of methanol to oleic acid; when the molar ratio exceeds 8, oleic acid in the autoclave is almost completely converted to methyl oleate in 30 min. A high molar ratio of methanol to oleic acid is beneficial to the esterification. 3.3. Effect of temperature on the esterification reaction The oleic acid conversion along with the reaction time for the esterification at different temperatures is shown in Fig. 3. It is obvious that the reaction rate increases significantly with the temperature. At 220  C, the conversion of oleic acid is higher than

100 Oleic acid conversion (%)

2.2. Reaction procedure

80

60

40

2

4

6

8

10

Molar ratio of methanol to oleic acid Fig. 2. Effect of the molar ratio of methanol to oleic acid on the esterification of oleic acid in subcritical methanol. Reaction conditions: 170  C and reaction time being 30 min.

Oleic acid conversion (%)

Oleic acid conversion (%)

C. Song et al. / Renewable Energy 35 (2010) 625–628

80 60

o

220 C o 200 C o 180 C o 170 C o 160 C

40 20 10

20

30

40

50

80

627

experimental points fitted line

70 60 50 40

60

10

Reaction time (min)

20

30

40

50

60

Reaction time (min)

Fig. 3. Effect of temperature and reaction time on the esterification of oleic acid in subcritical methanol. Reaction conditions: 6.0 MPa, the molar ratio of methanol to oleic acid being 4.

Fig. 5. A comparison of experimental points and fitted results with Equation (9) (n ¼ 2.0582) for the esterification of oleic acid in subcritical methanol at 170  C and 6.0 MPa.

80.0% after 20 min reaction and approaches 95.0% after 60 min; this condition is near the critical point of methanol and oleic acid is well soluble in the subcritical methanol. Whereas at 160  C, only an oleic acid conversion of 60.0% is achieved, because the oleic acid can not be well dissolved in methanol at this condition. As suggested by Saka et al. [13], the supercritical methanol has a hydrophobic nature with the lower dielectric constant. As a result, low-polar FFAs oil can be well soluble in supercritical methanol to form a single phase of FFAs oil/methanol mixture, which may promote the esterification reaction of FFAs oil at high temperature.

in this work, kcBb can be treated as a constant. Meanwhile, k is far larger than k0 [12], Equation (4) can be simplified to

3.4. Kinetic model

dc  A ¼ k1 cnA dt

(5)

with x and cA0 refer to the conversion and the initial concentration of oleic acid, we have

cA ¼ cA0 ð1  xÞ

(6)

dx k ¼ 1 ½cA0 ð1  xÞn ¼ k2 ½cA0 ð1  xÞn dt cA0

(7)

where k2 ¼ k1 =cA0 . When n ¼ 1, Equation (7) can be integrated to For the esterification of oleic acid and methanol (as indicted by Equation (2)), the apparent reaction rate can be described as:

(4)

where cA, cB, cC and cD denote the concentrations of oleic acid, methanol, methyl oleate and water, respectively; a, b, g and l refer to their reaction orders. k and k0 are the kinetic constants for the forward and reverse reactions, respectively. Because the concentration of methanol is much higher than that of other components

1.6

ð1  xÞð1nÞ ¼ 1 þ ðn  1Þk2 cnA0 t

(9)

As shown in Fig. 4, the result of esterification reaction fits badly with the first order kinetic equation (Equation (8)), while it fits well with Equation (9), as shown in Fig. 5. The reaction order and rate constant fitted with Equation (9) at different temperatures are then listed in Table 1; the order is found to be 2.2 in average. The rate constants at different temperatures can then be used to get the preexponential factor A and activation energy Ea with the Arrhenius equation

ln k2 ¼ 

1.2

-ln (1-x)

(8)

When n s 1, Equation (7) is integrated to

dc g  A ¼ kcaA cbB  k0 cC clD dt

Ea þ lnA RT

(10)

The reaction order n ¼ 2.2, preexponential factor A ¼ 120.0 and activation energy Ea ¼ 32.62 KJ/mol are then obtained for the

0.8

Table 1 Order and rate constant of esterification reaction estimated at different temperatures.

0.4 0.0

ln ð1  xÞ ¼ k1 t

0

10

20

30

40

50

60

70

t / min Fig. 4. Relationship of ln (1  x) with t fitted with the reaction results of the esterification of oleic acid in subcritical methanol at 170  C and 6.0 MPa.

Temperature ( C)

n

k2

160 170 180 200 220

1.7905 2.0582 2.1628 2.3435 2.6461

0.0107 0.0174 0.0204 0.0303 0.1588

628

C. Song et al. / Renewable Energy 35 (2010) 625–628

esterification of oleic acid in subcritical methanol catalyzed by zinc acetate. The activation energy here is much lower than that of Tesser et al. [12], this may be attributed to the promoting effects of subcritical methanol and zinc acetate catalyst.

4. Conclusions The esterification of oleic acid in subcritical methanol catalyzed by zinc acetate was investigated in a batch-type autoclave. The results suggest that zinc acetate is catalytically active for the esterification of oleic acid. Because all the components can be dissolved in the subcritical methanol, the esterification can be promoted under subcritical conditions. The oleic acid conversion reached 95.0% under 220  C and 6.0 MPa with the molar ratio of methanol to oleic acid being 4 and 1.0 wt% zinc acetate as catalyst. A kinetic model for the esterification was established. By fitting the kinetic model with the experimental results, the reaction order n ¼ 2.2 and activation energy Ea ¼ 32.62 KJ/mol were obtained.

Acknowledgments The authors are grateful for the financial support of the Natural Science Foundation of Shanxi Province (2007011038), the National Basic Research Program of China (2006CB202504) and the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2.YW.H16).

References [1] Zhang Y, Dube MA, Malean DD, Kates M. Biodiesel production from waste cooking oil: 1. Process design and technological assessment. Bioresource Technology 2003;89:1–16. [2] Freedman B, Pryde EH, Mounts TL. Variables affecting the yields of fatty esters from transesterified vegetable oil. Journal of the American Oil Chemists’ Society 1984;61:1638–43. ¨ zbay N, Oktar N, Tapan NA. Esterification of free fatty acids in waste cooking [3] O oils (WCO): role of ion-exchange resins. Fuel 2008;87:1789–98. [4] Canakci M, van Gerpen J. Biodiesel production from oils and fats with high free fatty acids. Transactions of the ASAE 2001;44:1429–36. [5] Kulkarni MG, Dalai AK. Waste cooking oil – an economical source for biodiesel: a review. Industrial & Engineering Chemistry Research 2006;45: 2901–13. [6] Hou X, Qi Y, Qiao X, Wang G, Qin Z, Wang J. Lewis acid-catalyzed transesterification and esterification of high FFA oil in subcritical methanol. Korean Journal of Chemical Engineering 2007;24:311–3. [7] Al-Zuhair S, Ling FW, Jun LS. Proposed kinetic mechanism of the production of biodiesel from palm oil using lipase. Process Biochemistry 2007;42:951–60. [8] Liu K. Preparation of fatty acid methyl esters for gas-chromatographic analysis of lipids in biological materials. Journal of the American Oil Chemists’ Society 1994;71(11):1179–87. [9] Berrios M, Siles J, Martın MA, Martın A. A kinetic study of the esterification of free fatty acids (FFA) in sunflower oil. Fuel 2007;86:2383–8. [10] Goddard R, Bosley J, Al-Duri B. Esterification of oleic acid and ethanol in plug flow (packed bed) reactor under supercritical conditions investigation of kinetics. Journal of Supercritical Fluids 2000;18:121–30. [11] Eiji M, Shiro S. Kinetics of hydrolysis and methyl esterification for biodiesel production in two-step supercritical methanol process. Fuel 2006;85: 2479–83. [12] Tesser R, Di Serio M, Guida M, Nastasi M, Santacesaria E. Kinetics of oleic acid esterification with methanol in the presence of triglycerides. Industrial & Engineering Chemistry Research 2005;44:7978–82. [13] Kusdiana D, Saka S. Kinetics of transesterification in rapeseed oil to biodiesel fuel as treated in supercritical methanol. Fuel 2001;80:693–8.