fnd. Eng. chem. Process Des. Dev. lQ86,25, 722-728
722
4
w : wake region
D, : axial dispersion coefficient, cm2 s D,: radial dispersion coefficient, cm s
ED : energy consumption rate, cm2/s g : gravity acceleration, cm/s2 h : heat-transfer coefficient, W/(m2 K) H : bed height, cm Z : turbulent intensity, cm/s Le : eddy length, cm m : ratio of the liquid phase in the wake-to-liquid phase in the fluidized region M : weight of solid, g n : Richardson and Zaki’s index AP : pressure drop in the bed, N/mz r : radial coordinate, cm R : radius of the column, cm U : fluid superficial velocity, cm/s U, : eddy velocity, cm/s Ut : terminal velocity, cm/s X : dimensionless radial coordinate 2 : axial coordinate
Literature Cited Balrd, M. H. 1.; Rice, R. 0. Chem. Eng. J . 1975, 9 , 171-174. Bhatle, V. K.; Epstein, N. Roc. Int. Conf. FIuM. Its Appllcat. 1974, 380-392. Clift, R.: Grace, J. R.; Weber, M. E. “Bubbles, Drops, and Particles”; Academic Press: New York, 1978; Chapter 7. Darton, R. C.; Harrison, D. Chem. €ne. Scl. 1975, 3 0 , 581-586. El-Temtamy, S. A.; ECSharnoubi, Y. D.; ECHalwagl. M. M. Chem. Eng. J . 19798, 78. 151-159. ECTemtamy. S. A.; ECSharnoubi, Y. D.;Ei-Halwagl, M. M. Chem. Eng. J . 1979b, 78, 161-168. Henrlksen, H. K.; Ostegaard, K. Chem. Eng. J . 1974, 7 , 141-146. Hlnze, J. 0. “Turbulence”; McGraw HIii: New York, 1958. Joshi. J. 8. Trans. Inst. Chem. Eng. 1980, 58, 155-165. Joshi, J. B. Chem. Eng. Res. Des. 1983, 67, 143-161. Kang, Y.; Suh, I. S.; Klm, S. D. Proc. P A C K IIZ 1983, 2 , 1-8. Kang, Y.; Suh, I . S.; Kim, S. D. chem. Eng. Commun. 1985, 3 4 , 1-13. Kang, Y.; Lee, H. K.; Kim, S. D. Proc. APCCHE 111 1984, 7 , 159-164. Kato, Y.; Uchlda, K.; Kago, T.; Morooka. S. Powder Technol. 1981, 28, 173-179. Khang. S. J.; Schwartz, J. 0.; Buttke, R. D. AIChE Symp. Ser. 1983, 79, 47-54. Kim, S. D.; Baker, C. G. J.; Bergougnou, M. A. Can. J . Chem. Eng. 1972, 50. 895-701. Kim, S. D.; Baker, C. 0. J.; Beraouanou, M. A. Can. J . Chem. Ens. 1975, - 53, 134-139. Kim, S. D.; Baker, C. 0. J.; BergoUgnou, M. A. Chem. Eng. Scl. 1977, 32, 1299- 1306. Kim, S. D.; Kim, C. H. J . Chem. Eng. Jpn. 1983, 76,172-178. Kllnkenberg, A. A.; Krabnbrlnk, H. J.; Lauwerler, H. A. Ind. Eng. Chem. 1953, 4 5 , 1202-1208. Michelson, M. L.; Ostergaard, K. Chem. Eng. J . 1970, 1 , 37-46. Muroyama, K.; Fan, L. S. A I C M J . 1985, 37, 1-34. Richardson, J. F.; Zaki, W. N. Trans. Inst. Chem. Eng. 1954, 32, 35-53. Riquarts, H. P. Ger. Chem. Eng. 1980, 3 , 286-295. Riquarts. H. P. Ger. Chem. Eng. 1981, 4 , 18-23. Schllchtlng, H. ”Boundary Layer Theory”; McGraw-Hill: New York, 1968. Schugerl, K. Proc. Int. Symp. NuM. 1987, 782-796. Veil, Yu. K.: Manokov, N. Kh.; Manshllin, V. V. Int. Chem. Eng. 1968, 8 , 293-296.
Greek Letters : viscosity, mPa s p : density, $cm3 : phase hol up : modified Peclet number { : dimensionless axial coordinate jt
Subscripts 2 : two phase 3 : three phase a : axial direction f : fluidized region g : gas bubble region 1 : liquid phase max : maximum o : average value r : radial direction s : solid phase
Received for review April 22, 1985 Revised manuscript received October 28, 1985 Accepted November 18, 1985
Kinetics of Methyl Oleate Catalytic Hydrogenation with Quantitative Evaluation of Cis-Trans Isomerization Equilibrium Rlcardo J. Grau,’ A l M o E. C8ssano,t and Mlguel A. Baltanls’’ f N E C , 3000-ante
Fe, Argentha
A precise quantitative determinetion of the relative values of hydrogenation and isomerization rates of methyl oleate during catalytic hydrogenation at 398-443 K and 370-647 kPa is given. The proposed reaction model and mathematical methodology of solution also yleld accurate estimates for the cis-trans isomerization equilibrium, in good agreement with the experimental resub generally found by the practitioners in the refinlng and oil processing industry.
and, in many cases, the melting point and texture of the reaction products. Thus, edible oil, margarines, and shortenings are obtained, as well as stable fatty acids suitable for paint formulations or other industrial applications that take advantage of their interfacial activity. Geometric and positional isomerizations occur simultaneously with a reduction in the degree of unsaturation during hydrogenation reactions. Most often, a limitation in the extent of cis-trans isomerization is as important as the control of the hydrogenation itself, given the deleterious effect of trans isomers on the physical properties of fatty compounds (Duncan, 1984; Beckman, 1983; Stingley
Partial hydrogenation of vegetable oils and fatty acids are typical heterogeneous gas-liquid-solid catalytic processes developed mainly to enhance the chemical stability
* To whom correspondence should be addressed.
t Research Assistant from CONICET. *Memberof CONICETs Scientificand TechnologicalResearch Staff and Professor at U.N.L. 11 Instituto de Desarrollo Tecnoldgicopara la Industria Quimica. Universidad Nacional del-Litoral (U.N.L.) and Consejo Nacional de Investigaciones Cientificas y TBcnicas (CONICET).
O796-4305/86/ 1125-0722$OI.50/0
0
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and Wrobel, 1961). It is therefore important to quantify both the rate and the cis-trans chemical equilibrium constants of unsaturated fatty acid as a means to establish properly the reaction conditions that lead to desirable products. Oleic acid and its derivatives have been usually chosen as the model compounds for studying both hydrogenation and cis-trans (oleic-elaidic) isomerization. Albright and Wisniak (1962) reported that the trans/& equilibrium ratio was approximately equal to 2. Litchfield et al. (1963, 1965) catalyzed the isomerization reaction of oleic acid with nitrous acid generated in situ by reacting sodium nitrite with mineral acids and found that the equilibrium mixture contained 75430% elaidic acid (at 303-363 K) instead of the 66% generally accepted before. Albright (1967) found later that the trans-cis equilibrium ratio varied between 2 and 4. Recently, Gut et al. (1979) reported new values of the cis-trans isomerization equilibrium constant for 453-483 K, but unfortunately two different values for it were determined at each temperature, each value being originated in the fitting of a proposed kinetic model. The purpose of this work is to offer a precise quantitative determination of the cis-trans isomerization equilibrium during the catalytic hydrogenation of methyl oleate. The method also grants the evaluation of the relative values of isomerization vs. hydrogenation reaction rates via a simple linear regression. An extension to the more detailed network including linoleic and/or linolenic acid methyl esters is in progress in our laboratory and will be reported in the future. The terms “oleate” and “elaidate” refer to cis- and trans-octadecenoic methyl esters, respectively.
Model Formulation The effect of hydrogen pressure on the reaction rate constitutes an important (macroscopic) source of kinetic information from which it is possible to postulate plausible reaction mechanisms for the hydrogenation of unsaturated hydrocarbons and related compounds. Thus, several research teams have reported the order of reaction with respect to hydrogen (rhydrogenation a pH,”) for the heterogeneous catalytic hydrogenation of methyl oleate using diverse catalysts and operating conditions. Hashimoto et al. (1971) used the experimental results of Eldib and Albright (1957) and Wisniak and Albright (1961) for the hydrogenation of cottonseed oil in the liquid phase with Ni catalyst at low (40-140 psig) and high (150-1500 psig) pressures, respectively. They found n = 1for oleate and elaidate hydrogenation, 0.5 for linoleate hydrogenation, and 0.5 for oleate/elaidate isomerization, respectively. Given the existence of these less than 1orders of reaction, they concluded that Hz dissociation was an important step. Pihl and Schoon (1971) found that n increased with temperature for the oleate hydrogenation in cottonseed oil, being 1.32 a t 463 K. Cordova and Harriott (1975) used a Pd catalyst for the liquid-phase hydrogenation of oleate at 394 K and 44.7-90 psia. They concluded that n was less than 1 and that n = 0.5 could acceptably approximate their experimental results. Vapor-phasehydrogenationof oleate in a continuous and perfectly mixed reactor was studied by Lidefelt et al. (1983), at 421-483 K. The values of n increased with temperature and were always larger than 1. Two different mechanisms of reaction were proposed to explain this behavior: (a) enhanced oleate adsorption in the presence of H2and (b) hydrogenation of semihydrogenated oleate as the rate-determining step. Both mechanisms led to the
04l 383
I
398
I
413
I
428
I
443
T
453 [OK1
Figure 1. Reaction order with respect to liquid-phase hydrogen concentration at different temperatures (95.5%confidence limits are shown).
same final kinetic expression, and they could not to discriminate between a and b. Preliminary scoping reaction studies were performed in our laboratory for the liquid-phase catalytic hydrogenation of oleate using a commercial supported Ni catalyst in a batch slurry reactor. Using a power-law expression for stearate production rate, we found that the values of n increase with temperature and are definitively less than 1 (Figure 1). Details of the reaction conditions, feedstocks, etc., are indicated below in the Experimental Section. Special care was put into the determination of n because (i) the reaction rate must be determined indirectly from concentration vs. time curves and (ii) the pseudo-first-order constant in the rate expression includes the methyl esters’ concentrations. Consequently, the influence of PH2on the reaction rate must be evaluated always for the same composition. This was assured by means of a “cup and cap” device (Grau et al., 1985) that wholly eliminated induction times and granted precise initial rate values. The n values indicated in Figure 1confirm that a simple description of the reactions involved may be attained by using the steps sequence proposed by Allen and Kiess (1956) based mainly in the semihydrogenated radical theory usually known as the Horiuti and Polanyi (1934) mechanism. However, whereas this mechanism is now generally accepted for double bond hydrogenations (Draguez de Hault and Demoulin, 1984; Larsson, 1983; Puri, 1980; Van der Planck et al., 1972; Hashimoto et al., 1971; etc), it has not been always stated clearly which are the hypotheses leading to the formulation of the macromodels used for the process modeling. Herein, the following set of hypotheses is put forward and analyzed subsequently: (a) Hydrogen adsorption is “independent” of the concentration of the unsaturated species. (b) Hydrogen adsorption is an equilibrium reaction. (c) Unsaturated species adsorb under nonequilibrium conditions. (d) Saturated species are not adsorbed. (e) The net rate of reaction of adsorbed intermediate species is nearly zero (steady-state approximation). (f) There is only one type of active sites. (g) The total concentration of the active sites is constant. An ”independent” adsorption of hydrogen (hypothesis a) is meant to indicate that the amount of surface-adsorbed H2 is not modified during the hydrogenation reaction as long as moderate conversion levels are kept. This does not imply that the methyl esters do not influence Hzadsorption but that the fraction of active surface sites effectively accessible for the double bond adsorption of the methyl esters is a very small number (due to steric hindrances) despite the well-known perferential adsorption of the olefinic portion of the molecules. Consequently, the following distinction can be made
[a]= (1- n[a]+ f [ a ] = [8]+ [a]
(1)
where f is the fraction of free active sites effectively ac-
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Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
cessible for methyl esters adsorption. If f
