The Influence of the Isomerization Reactions on the Soybean Oil Hydrogenation Process

Transcription

1 Unversty of Belgrade From the SelectedWorks of Zeljko D Cupc 2000 The Influence of the Isomerzaton Reactons on the Soybean Ol Hydrogenaton Process Zeljko D Cupc, Insttute of Chemstry, Technology and Metallurgy Avalable at:

2 . Journal of Molecular Catalyss A: Chemcal The nfluence of the somerzaton reactons on the soybean ol hydrogenaton process ˇ ˇ ) Dusan ˇ Jovanovc, Zeljko Cupc, Mroslav Stankovc, Lljana Rozc, ˇ Branslav Markovc Insttute of Chemstry, Technology and Metallurgy, Department of Catalyss and Chemcal Engneerng, UnÕersty of Belgrade, NjegoseÕa 12, Belgrade, YugoslaÕa Receved 25 October 1999; accepted 4 February 2000 Abstract Soybean ol has been partally hydrogenated n the presence of a syntheszed nckel catalyst, n an ndustral reactor, under the standard ndustral workng condtons. The GC measurements of fatty acds n the nvestgated ol and hydrogenated products have been performed. The total trans somers content has been determned by the IR spectroscopy. The model s establshed, whch could be useful for the characterzaton of hydrogenaton and somerzaton reactons of poly-unsaturated acds. Iteraton procedure, ncorporatng numercal smulaton n each step s used to calculate rate constants of the proposed model from the comparson of the expermental and calculated knetc runs. By the smultaneous treatment of the knetc runs for the fatty acds composton and trans-somer content, the 12 rate constants of ndvdual hydrogenaton and somerzaton reacton steps are obtaned. q 2000 Elsever Scence B.V. All rghts reserved. Keywords: Partal hydrogenaton; Nckel catalyst; Soybean ol; Reacton mechansm; Numercal smulaton 1. Introducton Partal hydrogenaton of the carbon-to-carbon double bonds n trglycerdes, especally of vegetable ols, s a major ndustry n many parts of the world. Prncpal products, obtaned by catalytc hydrogenaton, nclude oleomargarnes, shortenng, soap stock and ndustral greases and ols. Despte ts mportance, hydrogenaton mechansm s not completely clarfed, untl now. The chemstry of trglycerdes partal hy- ) Correspondng author. Tel.: q ; fax: q E-mal address: zcupc@helx.chem.bg.ac.yu Z. ˇ Cupc ˇ.. drogenaton process s complcated. Both the mono- and poly-unsaturated acd groups n the ol hydrogenate at varous rates, dependng on the operatng condtons. Furthermore, the geometrcal and postonal somerzaton of the double bonds n mono- and poly-unsaturated acd groups occurs n sgnfcant numbers durng the hydrogenaton. Attempts have been made n modelng of tracylglycerols hydrogenaton process, wth a reacton scheme allowng for hydrogenaton and cs trans somerzaton wx 1. The model used n ths attempt was based on the reacton mechansm of mono-unsaturated acds. Therefore, t s not useful n the modelng of the consecutve hydrogenaton steps, whch lead to the hydro r00r$ - see front matter q 2000 Elsever Scence B.V. All rghts reserved.. PII: S

3 354 D. JoÕanoÕc et al.rjournal of Molecular Catalyss A: Chemcal 159 ( 2000) genaton of poly-unsaturated acds. A more adequate model s needed to descrbe the hydrogenaton of the soybean ol. The authors have already done some prelmnary nvestgaton of the soybean ol catalytc hydrogenaton n the presence of a syntheszed catalyst NICAT-2000 n an ndustral reactor w2,3 x. Concentratons of the fatty acds were beng measured n the course of reacton tme requred for obtanng the commercally acceptable characterstcs of a partally hydrogenated mass. The C18 fatty acds mass extent n the nvestgated ol and hydrogenated products have been performed by GC measurements wx 2. The total trans somers content durng soybean ol hydrogenaton has been determned through IR spectroscopy wx 3. The hydrogenaton and the somerzaton rate constants have been estmated n separate steps, n a way, as those processes are ndependent of each other. At frst, only the hydrogenaton rate constants were estmated n order to characterze the selectvty of the nvestgated catalyst. As t s a usual procedure n the characterzaton of the ndustral process, the mechansm of the soybean ol catalytc hydrogenaton reacton was, regarded as a seres of consecutve stages: k k k C C C C 1 18:3 18:2 18:1 18:0. where C 18:3, C 18:2, C 18:1, and C18:0 are lnolenc, lnolec, olec and stearc acds, respectvely, and k 1, k 2, and k3 are the correspondng rate constants. In order to apply a smple frst approxmaton mathematcal model, all the geometrc and postonal somerzaton processes have been neglected. The rate constants k 3, k 2, and k obtaned from ths model represent the 1 cumulatve rate constants of the process that ncorporate a sum of all correspondng rates at whch the partcular geometrc and postonal somers are beng hydrogenated. Such cumulatve rate constants for the unsaturated acds hydrogenaton reactons have been determned and they are k s2.88=10 y2 mn y1 3, k2s 1.17 = 10 y2 mn y1 and k s 1.98 = 10 y4 1 mn y1. Accordng to these values, the used catalyst NICAT-2000 has demonstrated hgh acwx 2. tvty and selectvty rates Further, the somerzaton rate constant was evaluated. At the begnnng of the hydrogenaton process, almost all of the fatty acds are present n cs form. The cs somer concentraton logarthm exhbts a lnear dependence on the reacton tme. From ts slope the cumulatve rate constant of cs trans transton s deter- ct y3 y1 mned k s 7.73 = 10 mn.. Ths cumulatve rate constant ncorporates a sum of all correspondng rate constants of the reactons at whch partcular fatty acds somerze. Rate constant for only one drecton of the somerzaton process s obtaned ths way. The am of these further efforts was to obtan the best fttng values for the somerzaton rate constant of trans cs transton, k tc. Snce t has not been possble to measure the ndvdual somerc composton of each fatty acd, the followng two assumptons have been accepted prevously as the necessary approxmaton: a. that the somerzaton reacton for each of these fatty acds s gong on at the same rate; and b. that both the cs and trans somers of any fatty acd partcpate n the hydrogenaton process at equal rates. Accordng to the accepted assumptons, the somer form of molecules s not changed by the hydrogenaton process and ths s what addtonally smplfes the model as related to a complex real system. For such a smplfed model, a numerc smulaton wth the use of rate constants for a reversble somerza- y1 ton reacton n a wde value nterval 10 to y8 y1 10 mn. has been chosen, usng the cumulatve hydrogenaton constants. The results of the numerc smulaton are compared wth the expermental measurements of the total trans somer content. Only the upper lmt for k tc was tc -3-1 obtaned, k = 10 mn.. However, the obtaned excellent agreement of the numerc results and the experment s makng t possble to use the obtaned values for the characterzaton of the hydrogenaton process selectvty n relaton to a parallel somerzaton wx 3.

4 D. JoÕanoÕc et al.rjournal of Molecular Catalyss A: Chemcal 159 ( 2000) 355 Table 1 Startng values of the rate constants y1 mn ct y3 y3 y3 tc y3 y3 y3 k 7.73= = =10 k 1.00= = =10 k 2.88= = =10 c y2 y2 y4 t y2 y2 y4 k 2.88= = =10 The obtaned values for the somerzaton rate constants represent some approxmaton. Further enhancements are possble through an mprovement of the fttng process, whch would open the possblty of usng a more precse model wth fewer approxmatons.. 2 In ths model, only one approxmaton s stll preserved, whch states that the hydrogenaton reactons, by themselves, do not change the somer composton. It seems that ths last approxmaton does not leave any mportant constrants on the knetc behavor, because the somerzaton reactons wth approprate rate constant values could compensate for the effect of possble hydrogenaton from trans to cs forms. The present model nvolves 12 rate constants. To quanttatvely analyze such a complex system and obtan relable values of rate constants s an almost unsolvable task. However, the method used n Ref. wx 1 does not drectly constran the number of rate constants that could be evaluated. The object of ths paper was to evaluate all 12 rate constants from the comparson of the expermental and calculated knetc runs. 2. Expermental. Soybean ol Yugoslav soy-bean varety, n the presence of a syntheszed catalyst NICAT wx 2, has been partally hydrogenated n an ndustral reactor capacty: 5000 l. at the Factory of Edble Ols and Fats VITAL n Vrbas, under the standard ndustral workng condtons. The GC measurements of the fatty acds n the ol and hydrogenated products under study, have been performed on the model HP5890 seres Plus. The total trans somers content durng the soybean ols hydrogenaton wth NICAT-2000 catalyst. n an ndustral reactor has been determned through IR spectroscopy PE-983G.. The set of dfferental equatons, correspondng to the nvestgated model, s solved numercally by the ODE45 algorthm. The rate constants are obtaned from knetc runs by mnmzng the sums of squares of devatons between the calculated and expermental concentratons of fatty acds and trans somers. The mnmzaton s carred out by the smplex method. The mnmzaton parameters are the values of the rate constants. It was necessary to approxmately calculate the somerc composton of the soybean ol, whch corresponds to ntal values for the numercal smulaton. Therefore, the proportonal dstrbuton of the trans somers between fatty acds of the raw soybean ol was assumed. 3. Results and dscusson The rate constants of all reactons nvolved n the complex hydrogenaton process are optmzed teratvely, to get the best ft of the expermental data wth numercal smulaton. The startng values of the rate constants were dentcal to publshed ones Table 1.. Table 2 Isomerzaton rate constants, after the frst teraton step y1 mn ct y3 y4 y2 tc y3 y4 y6 k 3.74= = =10 k 6.38= = =10

5 356 D. JoÕanoÕc et al.rjournal of Molecular Catalyss A: Chemcal 159 ( 2000) Table 3 Hydrogenaton rate constants, after the second teraton step -1 mn c t k 4.83= = =10 k 1.85= = =10 In all tables, k ct denotes the rate constant for cs trans transton of the C fatty acd, k tc 18: stands for trans cs transton of the same acd, k c and k t are the hydrogenaton rate con- stants of cs and trans forms for the fatty acd wth double bonds. Prelmnary nvestgatons mpled that the effcency of the teraton procedure s senstve on the ntal values of the rate constants. Large number of mnmzaton parameters result n decreasng ts effcency. Therefore, rate constants are calculated n several steps. In the frst step, only the somerzaton constants have been determned approxmately Table 2.. In the next teraton step, the hydrogenaton constants are refned Table 3.. Fnally, both the hydrogenaton and somerzaton rate constants are refned smultaneously to get the best fttng values. The numercal smulaton wth ths set of rate constants Table 4. obvously gves better results for the total trans somer content, than wth ntal ones, Fg. 1a.. At the same tme, fatty acds composton s also well smulated Fg. 1b.. The obtaned somerc dstrbuton of the fatty acds Fg. 2. s sgnfcantly dfferent than the approxmate calculaton done prevously wx 3. Ths dfference s a smple consequence of the fact that the present model allows the fact that Fg. 1. Expermental results symbols. and the numercal smulaton wth ntal dashed lne. and fnal values of rate constants sold lne.. a. Total trans somer content n % wt. b. Fatty acds composton n % wt. dfferent fatty acds could somerze wth dfferent reacton rates. Table 4 Rate constants, after the fnal teraton step -1 mn ct tc c k 3.91= = =10 k 6.09= = =10 k 5.00= = =10 k 1.92= = =10 t Fg. 2. Calculated somerc dstrbuton based on the asumpton of proportonal dstrbuton of trans somers between the fatty acds symbols connected wth thn lnes. and the numercal smulaton wth fnal values of rate constants for dfferent fatty acds lnes wthout symbols..

6 D. JoÕanoÕc et al.rjournal of Molecular Catalyss A: Chemcal 159 ( 2000) 357 For the somerzaton rate constants, comparably small values are obtaned n all cases except for rate constant k ct 1. The small values obtaned mply that somerzaton s proceedng through the catalytc hydrogenaton process and s dependent on the avalablty of actve ste. The strong nfluence of the somerzaton reacton s obtaned n the case of olec acd, whch s sgnfcantly transformed n trans somer. Ths process mproves the catalyst selectvty toward the olec acd snce the trans somer of ths acd seems to be less senstve to further hydrogenaton Table 4.. The somerzaton rate constants obtaned for lnolec acd ndcate that ths process s slow and drected toward the more reactve trans somer. Therefore, the somerzaton should have partally promotonal effect on ths step of the hydrogenaton process. 4. Concluson The teraton procedure, whch ncorporates numercal smulaton n each step, enables us to use a more realstc model. By the smultaneous treatment of the knetc runs for the fatty acds composton and trans-somer content, the rate constants of ndvdual reacton steps are obtaned. The results ganed n ths procedure are very nformatve on the reacton pathways, whch could possbly domnate n a complex mechansm. Accordng to the values of the rate constants obtaned, only olec acd s subjected to ntensve somerzaton and lnolec acd has an especally slow somerzng stage durng hydrogenaton n the nvestgated catalyst. The nfluence of the catalyst and process parameters on hydrogenaton and somerzaton rates wll be a subject for future work. The new experments are needed now to check for the detaled somer dstrbuton, whch s predcted by the model. There s a leak n expermental and theoretcal efforts n the feld of the reactor dynamcs, whch could have strong nfluence on the reacton rates. Acknowledgements Ths work was partly supported by the Serban Mnstry of Scence and Technology. References wx 1 P. Smon, A. Celkova, S. Schmdt, J. Am. Ol Chem. Soc wx 2 D. Jovanovc, A. Zujc, ˇ M. Stankovc, N. Jovanovc, B. Markovc, T. Novakovc, Proc. 7th Nordc Symposum on Catalyss, TurkurAbo, 2 4 June, paper P46., wx 3 Z. ˇ Cupc, ˇ D. Jovanovc, Lj. Mares, ˇ M. Stankovc, Lj. Rozc, ˇ D. Guzna, Z. ˇ Jovanovc, 4th Internatonal Conference on Fundamental and Appled Aspects of Physcal Chemstry Physcal Chemstry 98, Book of papers, Belgrade, September, 1998, pp