Ind. Eng. Chern. Res. 1992,31, 1985-1988
1985
Kinetic Analysis and Modeling of the Esterification of Oleic Acid and Oleyl Alcohol Using Cobalt Chloride as Catalyst N. Sbnchez, A. Coteron, M. Martinez, and J. Aracil* Departamento de Ingenieria Quimica, Facultad de Ciencias Quimicas, Uniuersidad Complutense de Madrid, Ciudad Uniuersitaria, Madrid 28040, Spain
The kinetic of the homogeneous liquid phase synthesis of an analog of jojoba oil in a stirred tank reactor using cobalt chloride as catalyst has been studied a t different temperatures, concentrations of the catalyst, and acid/alcohol molar ratios. A mathematical model that predicts quite well the experimental results has been developed.
Introduction The continuous increase of the number of industrial applications of high molecular weight esters as lubricants for high-speed machinery and for pharmaceutical uses, cosmetics, food additives, etc., has increased the importance of the synthesis of analogs of this kind of natural esters such as sperm whale oil, carnauba wax, jojoba oil, etc. (I). Jojoba oil is unique for seed oil because it is a wax ester instead of a typical triglyceride. Jojoba wax esters are similar to those in sperm whale oil, orange roughy, black oreo, and small-spored ore0 and can be used in those areas where sperm whale oil has been used in the past (2). The potential uses of jojoba oil range from skin-care products and shampoos to food and high-pressure lubricants, which has made the demand for jojoba oil grow considerably in the past decade (3). Nevertheless, there are two problems in using jojoba oil. The first is that we cannot get a great amount of this oil, and the second is that it is not economical. Therefore, we intend to develop a very easy synthesis process of an analog of jojoba oil based on cheaper raw materials (4). The esters’ obtention methods are based in the reaction between an alcohol and acid to form the ester and water. The classic method is usually carried out in the presence of a mineral acid. Sulfuric acid is more commonly used, but it has various problems: formation of undesirable products, by-products, and polymerization products due to secondary reactions with the double bonds of the molecule (5),as well as problems in the separation of the esters from the catalyst. In the beginning of the past decade new heterogeneous catalytic processes were researched and developed that mainly avoid the problems given by the classic method and significantly reduce production costs. They are catalytic systems based upon cobalt formulations that allow high yields (6). Information about the analysis and kinetic modeling is very rare and limited to particular conditions. Despite the industrial interest of the compound, there are no references about the kinetic study of the process using cobalt chloride as catalyst. Nevertheless, there is some information about the kinetic model using other catalysts. Previous workers (7)studied the kinetics of the reaction of ester%cation, mostly in the presence of excess of alcohol, and came up with a suggestion that the reaction may be second order with respect to the acid. Leys and Othmer (8) proposed a second-order kinetic equation for the esterification process using sulfuric acid. However, this equation is not based on the acid concentration and does not consider the reversible nature of the process. These authors found that the specific reaction rate was related to various process variables by a complex empirical equation that has been attributed to the influence of the side reaction between alcohol and sulfuric acid to form alkylsulfuric acid.
Later, Chandalia et al. (9) considered that both sulfuric acid and alkylsulfuric acid act as catalysts and the reaction rate is considerably different in the presence of these two substances. The kinetic analysis of a wide range of esters, in vapor and liquid phase has been described using a simplified second-order power model and hydrous zirconium oxide as catalyst (IO). Zhou et al. (11) described a model for the esterification in the vapor phase using low boiling point alcohols and acids over p-toluenesulfonic acid considering the chemical species, reactants and products, as pure compounds. Esterifications using cation exchange resins as catalysts are widely described; depending on the working conditions kinetic models vary from power function to Langmuir-
Hinshelwood-Hougen-Watson (12). Santacesaria et al. (13)proposed a pseudo-second-order model for catalytic esterification of alcohol and acids with Y-zeolites as catalysts, though it had to be modified for low acidlalcohol ratios to the Rideal model or the bimolecular surface model, 80 it describes the experimental field treated. However, the above models do not reflect the catalytic esterification of high molecular weight compounds, products that are of industrial interest. For this reason, the aim of this research is to study the kinetic analysis of the esterification between oleic acid and oleyl alcohol in a pseudohomogeneous liquid phase based in our experimental data using different quantities of cobalt chloride in a temperature range of industrial interest, and considering a range of acid/alcohol molar ratios, working in batch operation in the absence, by continuous removal, of water from the system.
Experimental Section Equipment. The esterification experiments were carried out in a slurry glass reactor (continously stirred tank reactor) of dimensions 500.cm3semispherical bottom, 7 cm high, and 5 cm in diameter. The reactor was equipped with a stationary baffles attached along the circumference. A marine-type propeller was employed. The impeller speed was set at 700 rpm to maintain the catalyst particles in a good condition. A temperature recorder and and controller and a speed controller were provided. The reactor that was immersed in a constant-temperature bath was capable of maintaining the reaction temperature to within fO.l OC of that desired for the reaction. The samples to follow the reaction were withdrawn each 10 min at the beginning, and one sample was withdrawn every 30 min at the end. Materials Used. The commercial catalyst, cobalt chloride by Merck, of constant activity, was used in our experiment. Oleic acid, purity better than 99.0%, and oleyl alcohol, purity better than 98.0%,were both supplied by Henkel-Iberica.
0888-588519212631-1985$03.00/00 1992 American Chemical Society
1986 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 X.
08-
m
.
0.7 060504-
030.2 1
.
I
8
0.5 04
0.1 -
Oh 0
. . . . . . ::I .. 0.6
8
O.,}
I 50
100
150
200
260
300 350
400
450
500
550
800
Tirnebnin)
Figure 1. Acid conversion versus time. T = 164 OC; CcAT= 0.85%; acid/alcohol molar ratio = 0.6.
Analytical Method. The technique to be used had to be capable of monitoring the reaction components. These products were determined by gas chromatography/mass spectrometry (GC/MS) and quantitatively by capillary column gas chromatography. The GC/MS data were obtained on a 5992B Hewlett-Packard instrument. Gas chromatographywas performed on a fused silica capillary column, Hewlett-Packard OV-l(12-m length, 0.31-mm i.d., 0.17-pm film). The gas chromatograph was equipped for split-splitless injections (30 8). The operating conditions of the instruments were as follow: ionization energy, 70 eV; scan speed, 1100 a m u d ; m888 range, 40-400 amu; data treated with an HP 9825. A computer was connected on line with the GC/MS system. For the separation of the products, the oven temperature was programmed at 150 O C for 1min and then raised 15 deg min-' and maintained at 275 OC until all components had eluted. The quantitative gas chromatography analyses were performed on a 5790 A Hewlett-Packard instrument using the column and conditions described above in the GC/MS analysis. The detector was a flame ionization detection type and the injection system was splitless. Also we employed the parameters defining the quality of jojoba oil, viz., solidifying point, saponification value, iodine value, and viscosity, to compare the quality of the product synthesized with that of the natural oil. Procedure. The following operation procedure was adhered to: The reactants and catalyst were added to the reactor fitted with a reflux condenser. When the working pressure (16 mmHg) was reached by using a vacuum pump, the reaction mixture was heated to the desired temperature. Then the stirring began. The reactants were well mixed for 8 h Analysis of the samples were performed at 30-min intervals by gas chromatography. During the experiment,the following variables remained constant: reactor temperature, impeller speed, and working pressure. Before starting the reactor, nitrogen was passed through the reactor. Mechanism of the Reaction The first step of all mechanisms observed for the acidcatalyzed esterification involves a fast protonation of the acid, followed in subsequent steps by the nucleophilic attack of the alcohol (14). There is a mechanism proposed for the reaction between oleic acid and methanol using p-toluenesulfonic acid (11). In the vapor phase over hydrous zirconium oxide, a complex intermediate was formed between the acid and the catalyst during the esterification process (IO),but there is no information about the reaction when reactants are
-0
50
100
150
ZOO
250
300
350
400
450
500
550
600
Time(min) Figure 2. Acid conversion versus time. T = 116 O C ; CCAT= 0.45%; acid/alcohol molar ratio = 0.6. Table 1. Experimental Conditions reaction temp ("C) 116,140,164 catal concn (mol/L) 0,0.01164,0.01748,0.02532, 0.03316,0.03910 acid/alcohol molar ratio 0.6,1, 1.5 init alcohol concn (mol/L) 1.9836,1.5879,1.2710 init acid concn (mol/L) 1.1902, 1.5879, 1.9066 16 oper press. (mmHg) stirrer speed (rpm) 700 catal cobalt chloride Commerical Quality of Joioba Oil and of Analon viscosity I2 (25 "C) solidifying saponification point ("C) value value (cP) jojoba oil 8 92 a2 35.2 jojoba oil 6 94 94 29 analog ~~
~~
~
high molecular weight unsaturated compounds. According to a previous work (15),an intermediate complex between the cation of the metallic salt and the reactants, stabilized by the unsaturated bonds, is involved in the process.
Experimental Results Seventeen experiments were done to determine the influence of temperature, catalyst concentration, and acid/alcohol molar ratio, and to determine the kinetic constants and activation energy of the process. Figures 1 and 2 show typical experimental conversion of acid, XA,,versus reaction time curves for various initial ratio concentrations, temperatures, and catalyst concentrations. A summary of experimental conditions and the commercial quality of the product synthesized is given in Table I. Before preceeding to a detailed kinetic analysis of the results, the effects of the independent variables, temperature, catalyst concentration, and initial molar ratio, of the reactants are described. The stirrer speed was fmed at 700 rpm; no limit to material transfer had been found between 500 and 1200 rpm. The work pressure was set at 16 mmHg. Effect of Temperature. The effect of temperature on the esterification is shown in Figure 3. It can be observed that the rate of reaction increases with temperature. This is due to the high activation energy of these reactions with these catalysts (14). The effect of the temperature is the same for all catalyst concentrationsand initial molar ratios. Effect of the Catalyst Concentration. Catalyst concentration influence on the reaction rate is the expected as shown in Figure 4: reaction rate increases with the
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1987 Table 11. Kinetic Constants
0.6
1
*
I
0.6
*
*
*
*
tamp C°Cj 116 116 116 140 140 140 140 164 164 164
a
a
0.4 1
0.3 0.2-
8
I
0.1 0.
-
0
8 I
.
8
-.
8
- _-
SO
100
8 --
160
8
8
.--.
K ' = K1
CCAT
+ K2CcA;
K1
0 0.01748 0.033 16 0 0.01164 0.02532 0.039 10 0 0.01748 0.033 16
0.01532 0.03582 0.05567 0.03990 0.014 17 0.23167 0.2468 0.08201 0.38500 0.42882
0.0151 0.0151 0.0151 0.0461 0.0461 0.0461 0.0461 0.0927 0.0927 0.0927
K2 1.2160 1.2160 1.2160 5.9225 5.9225 5.9225 5.9225 11.5697 11.5697 11.5697
regression coeff 0.998 0.998 0.998 0.973 0.973 0.973 0.973 0.961 0.961 0.961
--
200 260 300 360 400 460 600 650 600
Time(min)
Figure 3. Acid conversion versus time. CcAT= 0.85;acid/alcohol molar ratio = 1:l.
Therefore, we can consider the system as pseudohomogeneous, where the overall reaction rate is the rate of the catalyzed reaction plus the noncatalyzed reaction rate. r -dCAc/dt = KlCfiCAC + KZCCATCfiCAC (1)
r
-dCAc/dt = (K1 + K2CCAT)CfiCAC = K'CfiCAC (2)
Taking into account that the amounts of acid and alcohol that react at any time, t , are the same and are described as C d f i = CACOXAC 0.4
*
1 I
*
we can write eq 2 as
r = -dCAc/dt = K'CACO(~ - XAC)(~ALO - C A C ~ A C(4) )
"
r = -dCAc/dt = K'CAco'(1 0
60
X)O
1SO
200 250 300 360 400 460 600 660 600
Tindmin)
Figure 4. Acid conversion versus time. T = 140 "C;acid/alcohol molar ratio = 1:l.
I
k
*
*
1-
100
160
(5)
r = -dCAc/dt
K'CACo'(1
CACO
- XAC
- XAC)
= K't
,
0.1 L .t
60
- XAC)(M- XAC)
where M = Cm/CAcp If M = 1:
-1 XAC X. 1,
0
(3)
200 260 300 360 400 460 600 660 600
Time(min)
Figure 5. Acid conversion versus time. T = 164 OC;CCA, = 0.85%.
catalyst concentration. This influence is always observed, independentof the working temperature and the reactant molar ratio considered. Effect of the Initial Concentration Ratio of the Reactants. The initial concentration ratio of the reactanta has an influence on the reaction rate, especially the 3@ and 60-min accumulative conversion data. When the ratio acid/ alcohol is decreased, the conversion increases, as shown in Figure 5, whatever the other variables are. Kinetic Analysis. From experimental curves, and from earlier works (151, it is expected that the kinetics of the esterification between oleic acid and oleyl alcohol can be described by an irreversible second-order power function model, considering that water is continuously eliminated from the reaction system.
where K' = K1 + K2CcAT. The kinetic parameters K', K1, K,, and EA were estimated. The weighted s u m of mean relative deviations of experimental data against calculated data were used here as the objective function. The minimization of this function was carried by a direct search method.
Discussion The comparison of experimental data with those calculated on the basis of the proposed kinetic model is presented in Table I1 and Figures 6 and 7. In Table I1 are collected the values of the experimental values of the specific reaction rate of the forward reaction. It was calculated by applying regression methods to the individual experiments, series of the same catalyst concentration, and finally the same temperature runs. Applying the Arrhenius equation and plotting In K' versus -1/RT, as shown in Figure 6, the activation energy shows a value of 16 kcal/mol and the preexponential factor is 1.18 X loB,for the esterification reaction between oleic acid and oleyl alcohol. Therefore, we can conclude that the noncatalyzed esterification reaction between oleic acid and oleyl alcohol
1988 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
the facts reported in the Introduction. The deviation between the experimental and predicted results becomes progressively larger for slower reaction rates.
Lnk
31
I " 100
110
120
130
140
1/RT (1E-5)
Nomenclature CAC= acid concentration, mol/L CACo= initial acid concentration, mol/L CAL= alcohol concentration, mol/L CAu) = initial alcohol concentration, mol/L CCAT = catalyst concentration, mol/L EA = activation energy, kcal/mol K1= kinetic constant, L mol-' h-' K 2= kinetic constant, L mol-' h-' CCAT-' K' = overall rate constant, L mol-' h-' r = rate of reaction, mol L-' h-' T = absolute temperature, K t = time, h XAc = acid conversion Xu = alcohol conversion Y , = calculated conversion Ye = experimental conversion Registry No. Wl,7646-79-9;oleic acid, 112-80-1;oleic alcohol, 143-28-2;oleic oleate, 3687-46-4.
Figure 6. Arrhenius plot.
Literature Cited
(1-Yo I 'IC).lo0
(1) Bhatia, V. H.; Gulati, I. B. Chemistry & Utilization of Jojoba.
Chem. Era 1981,17, 137-144. (2) Buisson, D. H.;Body, D. R.; Dougherty, G. J.; Eyes, L.; Vlieg, ۤ P. Oil from Deep Water Fish Species as a Substitute for Sperm 1982.59 Whale and Joioba Oil. JAOCS. J. Am. Oil Chem. SOC. (91,390-395. 10 (3) . . Seventh International Conference on Joioba and its Uses, Meeting Abstracts. JAOCS, J. Am. Oil Cheh. SOC. 1988,65 (1): 0 (4) Martinez, Mercedea; Torrano, Emilio; Aracil, Jose. An Analogue of Jojoba Oil. A StatisticalApproach. Ind. Eng. Chern. Res. 1988, - 10 27, 2179-2182. (5) Al-Saadi, A. N.;Jeffreys, G. V. Esterification of Butanol in a Two-Phase Liquid-Liquid System. AIChE J. 1981, 27 (5), -20 754-772. 0 (6) Sokolov, V. P.;Baidin, I. I.; Boravchuk, Yu. P.; Drozdov, A. S.; -90 Kogan, V. A. PAT USSR 702,000 (C1C07c69/24),Dec 5,1979; 0 0.2 0.4 0.6 0.8 1 I t 1A 11 1A 2 O.oldlOmkohoi Apl2429 779 13,December 1976. (7) Mc Cracken, D. J.; Dickson, P. F. Reaction Kinetics of CycloFigure 7. Residual analysis plot for experimental resulta. hexanol-AceticAcid Esterification. Znd. Em. Chem. Process Des. Dev. 1967, 6 (3),286-292. (8) Leyes, C. E.; Othmer, D. F. Esterification of Butanol and Acetic does not contribute to the global reaction rate, and the Acid. Znd. Eng. Chem. 1945, 37 (10). 968-977. reaction is well described by the following model: (9) Chandalia, S.B.; Dhanuka, V. R.; Malshe, V. C. Kinetics of the l' = 1.18 x lo9 ~ X ~ ( - ~ ~ ~ ~ ~ / R ~ ~ C * T C A CLiquid C A Phase L Esterification of CarboxylicAcids with Alcohols in the Presence of Acid Catalysts. Chem. Eng. Sci. 1977, 32 (5), The conformity between experimental and computed 551-556. (10)Takabashi,K.; Shibagaki,K.; Matshusita, H. The Esteflication values seems to be quite satisfactory. The maximum deof Carboxylic Acid with Alcohol over Hydrous Zirconium Oxide. viation was about 15% and the average error was about Bull. Chem. SOC. Jpn. 1989,62, 2353-2361. 5 % as shown in the residual analysis plot (Figure 7). (11) Zhou, M.; Gilot, B.; Domenech, S. Modelisation d'un Reacteur d'esterification. Partie 1: Esterification de l'acide oldique par le Conclusions mdthanol. Determination des donndes thermodinamiques et cindtiques. Entropie 1984, 120, 3-10. In this work we have shown that esterification in the (12) Gimdnez, J.; Cbta, J.; Cervera, S. Vapor-Phase Esterification liquid phase of unsaturated reactants of high molecular of Acetic Acid with Ethanol Catalyzed by a Macroporow Sulfoweight can be carried out using cobalt chloride as catalyst. nated Styrene-Divinylbenzane(20%) Resin. Ind. Eng. Chem. Res. No by-producta resulting from addition reactions involving 1987,26, 198-202. double bonds have been obtained using this catalyst. (13) Santacesaria, E.; Geloea, D.; Danise, F.; Cerra, S. Vapor-Phase Esterification Catalyzed by Decationized Y Zeolites. J. Catal. The production rate of oleic acid ester can be well de1983,80,427-436. scribed by a second-order kinetic model, first order in relation to the acid and first order in relation to the alcohol. (14) March, J. Advanced Organic Chemistry: Reactions Mechclnipm & Structure, 3rd ed.; Wiley: New York, 1985. It is also observed that the noncatalyzed reaction does not (15) Urteaga, Laura Eetudio de la CinQica de la Reaccibn de significantly contribute to the global rate of reaction. Esterificacibn entre el Acido Octanoic0 y el Alcohol N-octanol. As expected, the experimental results show that inM.S. Thesis, Universidad Complutense Madrid, 1986.
**
1
1
creased catalyst concentration and temperature enhance the reaction rate. The conversion of acid is higher as the acid:alcohol ratio is decreased. This in accordance with
Received for review July 15, 1991 Accepted May 2, 1992
