Kinetics of transesteri?cation in rapeseed oil to biodiesel fuel as treated in
supercritical methanol
D.Kusdiana,S.Saka*
Department of Socio-Environmental Energy Science,Graduate School of Energy Science,Kyoto University,Yoshida Honmachi,Sakyo-ku,
Kyoto606-8501,Japan
Received28December1999;accepted3August2000
Abstract
A kinetic study in free catalyst transesteri?cation of rapeseed oil was made in subcritical and supercritical methanol under different reaction conditions of temperatures and reaction times.Runs were made in a bath-type reaction vessel ranging from2008C in subcritical temperature to5008C at supercritical state with different molar ratios of methanol to rapeseed oil to determine rate constants by employing a simple method.As a result,the conversion rate of rapeseed oil to its methyl esters was found to increase dramatically in the supercritical state, and reaction temperature of3508C was considered as the best condition,with the molar ratio of methanol in rapeseed oil being42.q2001 Elsevier Science Ltd.All rights reserved.
Keywords:Kinetics of transesteri?cation;Supercritical methanol;Methyl esters;Biodiesel fuel
1.Introduction
Transesteri?cation of vegetable oils with simple alcohol has long been a preferred method for producing biodiesel fuel[1±3].Generally speaking,there are two methods of transesteri?cation reaction.One is the method using a cata-lyst and the other is without the help of a catalyst.The former method has a long story of development and now biodiesel fuel produced by this method is in the market in some countries such as North America,Japan and some west European countries.
However,there are at least two problems associated with this process;the process is relatively time consuming and puri?cation of the product for catalyst and saponi?ed products are necessary.The?rst problem due to the two phase nature of vegetable oil/methanol mixture requires vigorous stirring to proceed in the transesteri?cation reac-tion.To solve this problem,Boocock et al.reported that the use of a simple ether such as tetrahydrofuran can make this two phase nature into one phase of its mixture and that methyl esters can be produced in less than15min depending on the catalyst concentration[4].Yet,the catalyst problem cannot be solved for puri?cation.Therefore,this conven-tional process still requires a high production cost and energy.The overall process,thus,includes transesteri?ca-tion reaction,recovery of unreacted methanol,puri?cation of methyl esters from catalyst and separation of glycerin as a co-product from saponi?ed products.
The latter method involves uncatalyzed transesteri?ca-tion of vegetable oil in supercritical methanol as recently reported by Saka and Kusdiana[5].The supercritical state of methanol is believed to solve the two phase nature of oil/ methanol mixture to form a single phase due to a decrease in dielectric constant of methanol in supercritical state[6].As a result,the reaction was found to be complete in a very short time within2±4min,as described in their previous work[5].In addition,because of non-catalytic process,the puri?cation of products after transesteri?cation reaction is much simpler and environmentally friendly,compared with the conventional commercial method in which all the catalyst and saponi?ed products have to be removed for biodiesel fuel.
Some researchers have reported kinetics for both acid-and alkali-catalyzed transesteri?cation reactions.Dufek and coworkers studied the acid-catalyzed esteri?cation and transesteri?cation of9(10)-carboxystearic acid and its mono-and di-methyl esters[7].Freedman et al.reported transesteri?cation reaction of soybean oil and other vegeta-ble oils with alcohols[8],and examined in their study were the effects of the type of alcohol,molar ratio,type and amount of catalyst and reaction temperature on rate
Fuel80(2001)693±698
0016-2361/01/$-see front matter q2001Elsevier Science Ltd.All rights reserved. PII:S0016-2361(00)00140-X https://www.doczj.com/doc/5c18937585.html,/locate/fuel
*Corresponding author.Tel./fax:181-75-753-4738.
E-mail address:saka@energy.kyoto-u.ac.jp(S.Saka).
constants and kinetic order.Noureddin and Zhu applied the effects of mixing of soybean oil with methanol on its kinetics of transesteri?cation[9].Recently,Diasakov et al. reported kinetics on uncatalytic transesteri?cation reaction of soybean oil[10].However,the kinetic study of vegetable oil in supercritical medium without catalyst has not yet been presented.
Therefore,in this study,kinetics of transesteri?cation of rapeseed oil to biodiesel fuel was studied as treated in super-critical methanol without using any catalyst.We reported the effects of molar ratio and reaction temperature on methyl ester formation followed by a proposed simple method for the kinetics of the transesteri?cation reaction.
2.Materials and methods
The rapeseed oil from Nacalai Tesque was used in this work as a vegetable oil.In addition,various methyl esters from fatty acids such as palmitic,stearic,oleic,linoleic and linolenic acids were also obtained from Nacalai Tesque and used as a standard.
The supercritical methanol biomass conversion system employed in this work is described in a previous work[5]. In brief,the rapeseed oil and methanol with a different molar ratio was fed to the reaction vessel made of Inconel-625and it was shaken to mix.Subsequently,the entire reaction vessel was quickly immersed into the tin bath preheated at a designated reaction temperature and kept for a set time interval for subcritical and supercritical treatments of methanol(200±5008C).It was,subsequently,moved into the water bath to stop the reaction.The temperature and pressure inside the vessel were monitored during the treat-ment to con?rm that a subcritical or supercritical condition was achieved.For a subcritical treatment,the temperature was too low for tin bath to be melt.Therefore,oil bath was used for the treatments at200and2308C with a temperature controller.The treated liquid was then removed from the reaction vessel and allowed to settle for about30min.After the product was separated to be two portions,the upper and lower,each portion was evaporated at908C for20min to remove methanol.
The obtained products were analyzed on its composition by using the high performance liquid chromatography (HPLC)(Shimadzu,LC-10AT)which consists of the column(STR ODS-II,25cm in length￡4.6mm in inner diameter,Shinwa Chem.Ind.Co.)and refractive index detector(Shimadzu,RID-10A)operated at408C with 1.0ml/minˉow rate of methanol as a carrier solvent.The sample injection volume was20m l and peak identi?cation was made by comparing in the retention time between the sample and the standard compound as in a previous work [5].
3.Results and discussion
The supercritical experiments of methanol with rapeseed oil were carried out with a batch-type of reaction vessel. Therefore,the temperature and the pressure inside the reac-tion vessel are different in different reaction conditions.Fig. 1shows a typical relationship between the maximum reac-tion temperature and pressure inside the reaction vessel during the treatment.Since a critical point of methanol is 2398C(T c)in temperature and8.09MPa(P c)in pressure, supercritical state of methanol can be achieved in the shadowed zone in Fig.1.
3.1.Effect of molar ratio of methanol to rapeseed oil The molar ratio of methanol to rapeseed oil is one of the most important variables affecting the yields of methyl esters converted.The stoichiometry of the transesteri?ca-tion of rapeseed oil requires three molecules of methanol to react with one molecule of rapeseed oil.Encinar[11] stated in the conventional commercial process with alkaline catalyst that the yield of methyl esters increases as the molar
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ratio of methanol to Cynara oil rises and that the optimal ratios for its transesteri?cation result between4.05and5.67. For its molar ratio less than4.05,the reaction is reported to be incomplete,whereas at higher than5.67,it becomes dif?cult to separate glycerin from methanol as a by-product. Another worker[12]further noted that a98%conversion of vegetable oils could be made to the methyl esters at the molar ratio of6,but that even higher molar ratio up to45 was necessary when the oil contained a large amount of free fatty acids.However,as the molar ratio decreased to the theoretical value of3,its conversion was decreased down to82%.
In this work,therefore,the effect of the molar ratio of methanol to rapeseed oil was studied in the range between 3.5and42on the yield of methyl esters formed for super-critical methanol treatments,assuming that the average molecular weight of rapeseed oil is806as triglycerides. Fig.2shows the obtained HPLC chromatograms of rape-seed oil as treated in various molar ratios for4min under supercritical conditions.In the previous study[5],it was demonstrated that the intensive peak in the chromatogram observed in the short retention times(3±10min)are methyl esteri?ed compounds,while in the longer retention times, intermediates such as monoglycerides and diglycerides appeared(10±20min).Therefore,from Fig.2,it is apparent that the conversion state of rapeseed oil is different as various molar ratios of methanol were applied to the trans-esteri?cation reaction of the rapeseed oil.With a higher molar ratio of methanol applied,the methyl esteri?ed compounds are increased with a decrease in the intermedi-ate compounds.
Fig.3shows the content of methyl esters produced as different supercritical treatments were carried out at 3508C.For a molar ratio of42in methanol,almost complete conversion was achieved in a yield of95%of methyl esters, whereas for the lower molar ratio of6or less,incomplete conversion was apparent with the lower yield of methyl esters.These lines of evidence,therefore,indicate that the higher molar ratios of methanol result in the better transes-teri?cation reaction,due perhaps to the increased contact area between methanol and triglycerides.
3.2.Effect of temperature on methyl esters formation
To determine the effect of temperature on methyl esters formation,transesteri?cation reactions of rapeseed oil were carried out with a?xed molar ratio of42in methanol,the best condition found in Fig.3,at various temperatures ranging from200to5008C.Fig.4shows the obtained HPLC chromatograms of rapeseed oil as treated in various conditions of temperatures and reaction times,while the content of methyl esters obtained is shown in Fig.5,in which the obtained experimental data are shown by the symbols,whereas the simulated curves are shown by the lines as discussed later.
At temperatures of200and2308C,the relatively low conversion to methyl esters is evident in Figs.4and5due to the subcritical state of methanol.In these conditions, methyl esters formed are at most about68and70%at200 and2308C,respectively,at3600s(1h)treatment.These results are in good accordance with those already reported [10].
At a temperature of2708C,the conversion rate is still low which might be related with the stability of supercritical condition.As can be seen in Fig.1,maximum pressure reached in this treatment is14MPa,still in the transition between subcritical and supercritical state of methanol. However,at3008C,a considerable change in the conversion rate can be seen with about80%of methyl esters produced in240s.As observed in the previous study[5],at3508C, 240s treatment resulted in a high conversion of rapeseed oil to methyl esters with its yield of95%.
An important result here is that the composition of methyl esters yielded is very similar with that prepared by the conventional commercial process with alkaline catalyst.
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At even higher temperature of4008C,the transesteri?cation reaction is essentially completed for120s to convert almost all rapeseed oil to their methyl esters.However,in such a high reaction temperature,new peaks in the shorter reten-tion time(3±4min)are dominating in the HPLC chromato-grams as shown in Fig.4and the previous study[5].This indicates that decomposition reaction takes place at temperature above4008C due to the thermal degradation. As a result,the transesteri?cation reactions of rapeseed oil to methyl esters proceed appropriately at temperature of 3508C under supercritical condition of methanol without any catalyst used.
3.3.Kinetics of rapeseed oil to methyl esters
To correlate experimental data and to quantify the temperature and reaction time effects observed above,the experimental results were analyzed further in terms of the kinetics of rapeseed oil to methyl esters.As mentioned earlier,the model is based on overall reaction.Since the molar ratio of methanol to rapeseed oil was?xed to be42, the concentration of methanol was not taken into account,as reported by other researchers[10,12].
Diasakov[10]proposed the thermal transesteri?cation reaction to be divided into3steps.Triglycerides react with methanol to produce diglycerides,and then diglyc-erides react to produce monoglycerides.Finally monoglyc-erides react with methanol to give glycerin as a by-product. At each reaction step,one molecule of methylated compounds is produced for each molecule of methanol consumed.As a result,six different rate constants of the reaction are reported for the whole reaction.
Due to reality that?nal products for the whole reaction in the transesteri?cation reaction for biodiesel fuel production are methyl esters with glycerin,we de?ned a simpler math-ematical model for this reaction by ignoring the intermedi-ate reactions of diglycerides and monoglycerides,so the3 steps can be simpli?ed to be one step as
follows:
TG
d t
1
where[TG]refers to the content of vegetable oil used in this study.In this supercritical methanol method,three species were de?ned as methyl esters(ME),glycerin(GL)and unmethyl esteri?ed compounds(uME)which include trigly-cerides,diglycerides,monoglycerides and unreacted free fatty acids.Therefore,Eq.(1)can be modi?ed to be
Rate 2d
uME
d t
2 or
2d uME
d t
k uME 3
where[uME]refers to the content of the species,excluding methyl esters and glycerin,that result or remain after the supercritical treatment was carried out.Assuming that the initial concentration of uME,at time t 0;is uME,0and that it falls down to uME,t at some later time t,the integration gives
2
uME;t
uME;0
d uME
u ME
k
t
d t 4
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and
2ln uME;t
uME;0
kt 5
or
K ln uME;t 2ln uME;0
t
6
Fig.6shows the correlation between the content of unmethyl esterifed compounds and reaction times.As mentioned previously,unmethyl esteri?ed compounds are de?ned as other compounds obtained from the upper portion excluding?ve types of methyl esters,such as methyl palmi-tate,methyl oleate,methyl stearate,methyl linoleate and methyl linolenate.The straight line was determined to?t the data in order to adopt the?rst order rate equation. Based on the results in Fig.6,the rate constant was obtained for each reaction temperature as shown in Table 1and the corresponding Arrhenius plot for this method is presented in Fig.7.It is evident that at subcritical tempera-ture below2398C,the reaction rates are so low but much higher at supercritical state,with the rate constant increased by a factor of about85at the temperature of3508C. Liquid methanol is a polar solvent and has hydrogen bondings between OH oxygen and OH hydrogen to form methanol clusters.Because the degree of hydrogen bonding decreases with increasing temperature,the polarity of methanol would decrease in supercritical state.This means that supercritical methanol has a hydrophobic nature with the lower dielectric constant.As a result,non-polar triglycerides can be well solvated with supercritical metha-nol to form a single phase of vegetable oil/methanol mixture.This phenomenon with the high temperature con-ditions seems to be likely to promote transesteri?cation reaction of rapeseed oil.
The simulation was made on a relationship between the formation of methyl esters and reaction times,based on Eqs.
(3)and(6)to examine the?tness of the experimental results, as shown in Fig.5.In this?gure,the simulated curves are shown by lines and the experimental data are represented by symbols.In the subcritical temperature,simulated curves are somewhat different from those of experimental data. This would be because at the longer treatment,the conver-sion rate is low due to the equilibrium reaction approached. However,at the supercritical state,the simulated curves?t well with the experimental results in all cases.Therefore,a simple method proposed to determine the rate constants in transesteri?cation must be valid.
4.Concluding remarks
A highly ef?cient transesteri?cation process has been described and the proposed kinetics in transesteri?cation of rapeseed oil has been proven to?t very well with those of experimental data.A reaction temperature of3508C with the molar ratio of methanol being42were considered as the best condition for a free-catalyst process of biodiesel fuel production.The supercritical methanol method,therefore, offers a potentially low cost method with simpler technol-ogy for producing an alternative fuel for compression igni-tion engines.The considerable yield of methyl esters by the environmentally friendly method renders this technique ideally suited for industrialization.
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D.Kusdiana,S.Saka/Fuel80(2001)693±698697 Table1
The rate constant of transesteri?cation reaction
Reaction condition k(s21)
Temperature(8C)Pressure(MPa)
20070.0002
23090.0003
270120.0007
300140.0071
350190.0178
385650.0249
431900.0503
487105
0.0803
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