Ž .Journal of Molecular Catalysis A: Chemical 159 2000 353–357www.elsevier.comrlocatermolcata

The influence of the isomerization reactions on the soybeanoil hydrogenation process

ˇ ˇ )Dusan Jovanovic, Zeljko Cupic , Miroslav Stankovic, Liljana Rozic, Branislav Markovicˇ ´ ´ ´ ˇ ´ ´Institute of Chemistry, Technology and Metallurgy, Department of Catalysis and Chemical Engineering, UniÕersity of Belgrade,

NjegoseÕa 12, 11000 Belgrade, YugoslaÕia

Received 25 October 1999; accepted 4 February 2000

Abstract

Soybean oil has been partially hydrogenated in the presence of a synthesized nickel catalyst, in an industrial reactor,under the standard industrial working conditions. The GC measurements of fatty acids in the investigated oil andhydrogenated products have been performed. The total trans isomers content has been determined by the IR spectroscopy.

The model is established, which could be useful for the characterization of hydrogenation and isomerization reactions ofpoly-unsaturated acids. Iteration procedure, incorporating numerical simulation in each step is used to calculate rateconstants of the proposed model from the comparison of the experimental and calculated kinetic runs. By the simultaneoustreatment of the kinetic runs for the fatty acids composition and trans-isomer content, the 12 rate constants of individualhydrogenation and isomerization reaction steps are obtained. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Partial hydrogenation; Nickel catalyst; Soybean oil; Reaction mechanism; Numerical simulation

1. Introduction

Partial hydrogenation of the carbon-to-carbondouble bonds in triglycerides, especially of veg-etable oils, is a major industry in many parts ofthe world. Principal products, obtained by cat-alytic hydrogenation, include oleomargarines,shortening, soap stock and industrial greasesand oils. Despite its importance, hydrogenationmechanism is not completely clarified, untilnow. The chemistry of triglycerides partial hy-

) Corresponding author. Tel.: q381-11-630-213; fax: q381-11-637-977.

ˇ ˇŽ .E-mail address: zcupic@helix.chem.bg.ac.yu Z. Cupic .´

drogenation process is complicated. Both themono- and poly-unsaturated acid groups in theoil hydrogenate at various rates, depending onthe operating conditions. Furthermore, the geo-metrical and positional isomerization of thedouble bonds in mono- and poly-unsaturatedacid groups occurs in significant numbers dur-ing the hydrogenation.

Attempts have been made in modeling oftriacylglycerols hydrogenation process, with areaction scheme allowing for hydrogenation and

w xcis–trans isomerization 1 . The model used inthis attempt was based on the reaction mecha-nism of mono-unsaturated acids. Therefore, it isnot useful in the modeling of the consecutivehydrogenation steps, which lead to the hydro-

1381-1169r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S1381-1169 00 00154-0

( )D. JoÕanoÕic et al.rJournal of Molecular Catalysis A: Chemical 159 2000 353–357´354

genation of poly-unsaturated acids. A more ade-quate model is needed to describe the hydro-genation of the soybean oil.

The authors have already done some prelimi-nary investigation of the soybean oil catalytichydrogenation in the presence of a synthesizedcatalyst NICAT-2000 in an industrial reactorw x2,3 . Concentrations of the fatty acids werebeing measured in the course of reaction timerequired for obtaining the commercially accept-able characteristics of a partially hydrogenatedmass. The C fatty acids mass extent in the18

investigated oil and hydrogenated products havew xbeen performed by GC measurements 2 . The

total trans isomers content during soybean oilhydrogenation has been determined through IR

w xspectroscopy 3 . The hydrogenation and theisomerization rate constants have been esti-mated in separate steps, in a way, as thoseprocesses are independent of each other.

At first, only the hydrogenation rate constantswere estimated in order to characterize the se-lectivity of the investigated catalyst. As it is ausual procedure in the characterization of theindustrial process, the mechanism of the soy-bean oil catalytic hydrogenation reaction was,regarded as a series of consecutive stages:

k k k3 2 1

C ™C ™C ™C 1Ž .18:3 18:2 18:1 18:0

where C , C , C , and C are linolenic,18:3 18:2 18:1 18:0

linoleic, oleic and stearic acids, respectively,and k , k , and k are the corresponding rate1 2 3

constants. In order to apply a simple first ap-proximation mathematical model, all the geo-metric and positional isomerization processeshave been neglected. The rate constants k , k ,3 2

and k obtained from this model represent the1

cumulative rate constants of the process thatincorporate a sum of all corresponding rates atwhich the particular geometric and positionalisomers are being hydrogenated. Such cumula-tive rate constants for the unsaturated acidshydrogenation reactions have been determinedand they are k s2.88=10y2 miny1, k s3 2

1.17 = 10y2 miny1 and k s 1.98 = 10y41

miny1. According to these values, the usedcatalyst NICAT-2000 has demonstrated high ac-

w xtivity and selectivity rates 2 .Further, the isomerization rate constant was

evaluated. At the beginning of the hydrogena-tion process, almost all of the fatty acids arepresent in cis form. The cis isomer concentra-tion logarithm exhibits a linear dependence onthe reaction time. From its slope the cumulativerate constant of cis™ trans transition is deter-

Ž ct y3 y1.mined k s7.73=10 min . This cumula-tive rate constant incorporates a sum of allcorresponding rate constants of the reactions atwhich particular fatty acids isomerize. Rate con-stant for only one direction of the isomerizationprocess is obtained this way.

The aim of these further efforts was to obtainthe best fitting values for the isomerization rateconstant of trans™cis transition, k tc. Since ithas not been possible to measure the individualisomeric composition of each fatty acid, thefollowing two assumptions have been accepted

Ž .previously as the necessary approximation: athat the isomerization reaction for each of these

Ž .fatty acids is going on at the same rate; and bthat both the cis and trans isomers of any fattyacid participate in the hydrogenation process atequal rates. According to the accepted assump-tions, the isomer form of molecules is notchanged by the hydrogenation process and thisis what additionally simplifies the model asrelated to a complex real system. For such asimplified model, a numeric simulation with theuse of rate constants for a reversible isomeriza-

Ž y1tion reaction in a wide value interval 10 toy8 y1.10 min has been chosen, using the cumu-

lative hydrogenation constants. The results ofthe numeric simulation are compared with theexperimental measurements of the total transisomer content. Only the upper limit for k tc was

Ž tc -3 -1.obtained, k -1.0=10 min . However, theobtained excellent agreement of the numericresults and the experiment is making it possibleto use the obtained values for the characteriza-tion of the hydrogenation process selectivity in

w xrelation to a parallel isomerization 3 .

( )D. JoÕanoÕic et al.rJournal of Molecular Catalysis A: Chemical 159 2000 353–357´ 355

Table 1Starting values of the rate constants

y1Ž .i min 3 2 1ct y3 y3 y3k 7.73=10 7.73=10 7.73=10itc y3 y3 y3k 1.00=10 1.00=10 1.00=10ic y2 y2 y4k 2.88=10 1.17=10 1.98=10it y2 y2 y4k 2.88=10 1.17=10 1.98=10i

The obtained values for the isomerizationrate constants represent some approximation.Further enhancements are possible through animprovement of the fitting process, which wouldopen the possibility of using a more precisemodel with fewer approximations.

Ž .2

In this model, only one approximation is stillpreserved, which states that the hydrogenationreactions, by themselves, do not change theisomer composition. It seems that this last ap-proximation does not leave any important con-straints on the kinetic behavior, because theisomerization reactions with appropriate rateconstant values could compensate for the effectof possible hydrogenation from trans to cisforms.

The present model involves 12 rate constants.To quantitatively analyze such a complex sys-tem and obtain reliable values of rate constantsis an almost unsolvable task. However, the

w xmethod used in Ref. 1 does not directly con-strain the number of rate constants that could beevaluated. The object of this paper was to eval-uate all 12 rate constants from the comparisonof the experimental and calculated kinetic runs.

2. Experimental

Ž .Soybean oil Yugoslav soy-bean variety , inthe presence of a synthesized catalyst NICAT-

w x2000 2 , has been partially hydrogenated in anŽ .industrial reactor capacity: 5000 l at the Fac-

tory of Edible Oils and Fats ‘‘VITAL’’ in Vr-bas, under the standard industrial working con-ditions. The GC measurements of the fatty acidsin the oil and hydrogenated products understudy, have been performed on the modelHP5890 series Plus. The total trans isomerscontent during the soybean oils hydrogenationŽ .with NICAT-2000 catalyst in an industrialreactor has been determined through IR spec-

Ž .troscopy PE-983G .The set of differential equations, correspond-

ing to the investigated model, is solved nu-merically by the ODE45 algorithm. The rateconstants are obtained from kinetic runs byminimizing the sums of squares of deviationsbetween the calculated and experimental con-centrations of fatty acids and trans isomers.The minimization is carried out by the simplexmethod. The minimization parameters are thevalues of the rate constants. It was necessary toapproximately calculate the isomeric composi-tion of the soybean oil, which corresponds toinitial values for the numerical simulation.Therefore, the proportional distribution of thetrans isomers between fatty acids of the rawsoybean oil was assumed.

3. Results and discussion

The rate constants of all reactions involved inthe complex hydrogenation process are opti-mized iteratively, to get the best fit of theexperimental data with numerical simulation.The starting values of the rate constants were

Ž .identical to published ones Table 1 .

Table 2Isomerization rate constants, after the first iteration step

y1Ž .i min 3 2 1ct y3 y4 y2k 3.74=10 8.32=10 2.04=10itc y3 y4 y6k 6.38=10 2.70=10 4.07=10i

( )D. JoÕanoÕic et al.rJournal of Molecular Catalysis A: Chemical 159 2000 353–357´356

Table 3Hydrogenation rate constants, after the second iteration step

-1Ž .i min 3 2 1c -2 -2 -4k 4.83=10 1.16=10 1.47=10it -2 -2 -7k 1.85=10 2.30=10 1.50=10i

In all tables, k ct denotes the rate constant fori

cis™ trans transition of the C fatty acid, k tc18:i i

stands for trans™cis transition of the sameacid, k c and k t are the hydrogenation rate con-i i

stants of cis and trans forms for the fatty acidwith ‘‘i’’ double bonds.

Preliminary investigations implied that theefficiency of the iteration procedure is sensitiveon the initial values of the rate constants. Largenumber of minimization parameters result indecreasing its efficiency. Therefore, rate con-stants are calculated in several steps. In the firststep, only the isomerization constants have been

Ž .determined approximately Table 2 .In the next iteration step, the hydrogenation

Ž .constants are refined Table 3 .Finally, both the hydrogenation and isomer-

ization rate constants are refined simultaneouslyto get the best fitting values.

The numerical simulation with this set of rateŽ .constants Table 4 obviously gives better re-

sults for the total trans isomer content, thanŽ .with initial ones, Fig. 1a . At the same time,

fatty acids composition is also well simulatedŽ .Fig. 1b .

The obtained isomeric distribution of the fattyŽ .acids Fig. 2 is significantly different than the

w xapproximate calculation done previously 3 .This difference is a simple consequence of thefact that the present model allows the fact that

Table 4Rate constants, after the final iteration step

-1Ž .i min 3 2 1ct -3 -4 -2k 3.91=10 8.06=10 1.98=10itc -3 -4 -6k 6.09=10 2.82=10 3.92=10ic -2 -2 -5k 5.00=10 1.17=10 9.74=10it -2 -2 -8k 1.92=10 2.10=10 9.62=10i

Ž .Fig. 1. Experimental results symbols and the numerical simula-Ž .tion with initial dashed line and final values of rate constants

Ž . Ž . Ž .solid line . a Total trans isomer content in % wt. b Fatty acidscomposition in % wt.

different fatty acids could isomerize with differ-ent reaction rates.

Fig. 2. Calculated isomeric distribution based on the asumption ofproportional distribution of trans isomers between the fatty acidsŽ .symbols connected with thin lines and the numerical simulation

Žwith final values of rate constants for different fatty acids lines.without symbols .

( )D. JoÕanoÕic et al.rJournal of Molecular Catalysis A: Chemical 159 2000 353–357´ 357

For the isomerization rate constants, compa-rably small values are obtained in all casesexcept for rate constant k ct. The small values1

obtained imply that isomerization is proceedingthrough the catalytic hydrogenation process andis dependent on the availability of active site.The strong influence of the isomerization reac-tion is obtained in the case of oleic acid, whichis significantly transformed in trans isomer.This process improves the catalyst selectivitytoward the oleic acid since the trans isomer ofthis acid seems to be less sensitive to further

Ž .hydrogenation Table 4 . The isomerization rateconstants obtained for linoleic acid indicate thatthis process is slow and directed toward themore reactive trans isomer. Therefore, the iso-merization should have partially promotionaleffect on this step of the hydrogenation process.

4. Conclusion

The iteration procedure, which incorporatesnumerical simulation in each step, enables us touse a more realistic model. By the simultaneoustreatment of the kinetic runs for the fatty acidscomposition and trans-isomer content, the rateconstants of individual reaction steps are ob-tained.

The results gained in this procedure are veryinformative on the reaction pathways, which

could possibly dominate in a complex mecha-nism. According to the values of the rate con-stants obtained, only oleic acid is subjected tointensive isomerization and linoleic acid has anespecially slow isomerizing stage during hydro-genation in the investigated catalyst.

The influence of the catalyst and processparameters on hydrogenation and isomerizationrates will be a subject for future work.

The new experiments are needed now tocheck for the detailed isomer distribution, whichis predicted by the model. There is a leak inexperimental and theoretical efforts in the fieldof the reactor dynamics, which could have stronginfluence on the reaction rates.

Acknowledgements

This work was partly supported by the Ser-bian Ministry of Science and Technology.

References

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