biodiesel supercritico

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Kinetics of transesterication in rapeseed oil to biodiesel fuel as treated insupercritical methanol

D. Kusdiana, S. Saka*

Department of Socio-Environmental Energy Science, Graduate School of Energy Science, Kyoto University, Yoshida Honmachi, Sakyo-ku,

Kyoto 606-8501, Japan

Received 28 December 1999; accepted 3 August 2000

Abstract

A kinetic study in free catalyst transesterication of rapeseed oil was made in subcritical and supercritical methanol under differentreaction conditions of temperatures and reaction times. Runs were made in a bath-type reaction vessel ranging from 2008C in subcritical

temperature to 5008C 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 of 3508C was considered as the best condition, with the molar ratio of methanol in rapeseed oil being 42. q 2001

Elsevier Science Ltd. All rights reserved.

Keywords: Kinetics of transesterication; Supercritical methanol; Methyl esters; Biodiesel fuel

1. Introduction

Transesterication of vegetable oils with simple alcoholhas long been a preferred method for producing biodiesel

fuel [13]. Generally speaking, there are two methods of

transesterication 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

purication of the product for catalyst and saponied

products are necessary. The rst problem due to the two

phase nature of vegetable oil/methanol mixture requiresvigorous stirring to proceed in the transesterication 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 than 15 min depending

on the catalyst concentration [4]. Yet, the catalyst problem

cannot be solved for purication. Therefore, this conven-

tional process still requires a high production cost and

energy. The overall process, thus, includes transesterica-

tion reaction, recovery of unreacted methanol, purication

of methyl esters from catalyst and separation of glycerin as aco-product from saponied products.

The latter method involves uncatalyzed transesterica-

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 within 24 min, as described in their previous

work [5]. In addition, because of non-catalytic process, the

purication of products after transesterication reaction is

much simpler and environmentally friendly, compared with

the conventional commercial method in which all thecatalyst and saponied products have to be removed for

biodiesel fuel.

Some researchers have reported kinetics for both acid-

and alkali-catalyzed transesterication reactions. Dufek

and coworkers studied the acid-catalyzed esterication

and transesterication of 9(10)-carboxystearic acid and its

mono- and di-methyl esters [7]. Freedman et al. reported

transesterication 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

Fuel 80 (2001) 693698

0016-2361/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.

PII: S0016-2361(00) 00140-X

www.elsevier.com/locate/fuel

* Corresponding author. Tel./fax:181-75-753-4738.

E-mail address: [email protected] (S. Saka).


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ratio of methanol to Cynara oil rises and that the optimal

ratios for its transesterication result between 4.05 and 5.67.

For its molar ratio less than 4.05, the reaction is reported to

be incomplete, whereas at higher than 5.67, it becomes

difcult to separate glycerin from methanol as a by-product.

Another worker [12] further noted that a 98% conversion ofvegetable oils could be made to the methyl esters at the

molar ratio of 6, but that even higher molar ratio up to 45

was necessary when the oil contained a large amount of free

fatty acids. However, as the molar ratio decreased to the

theoretical value of 3, its conversion was decreased down

to 82%.

In this work, therefore, the effect of the molar ratio of

methanol to rapeseed oil was studied in the range between

3.5 and 42 on the yield of methyl esters formed for super-

critical methanol treatments, assuming that the average

molecular weight of rapeseed oil is 806 as triglycerides.

Fig. 2 shows the obtained HPLC chromatograms of rape-seed oil as treated in various molar ratios for 4 min 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 10 min) are methyl

esteried compounds, while in the longer retention times,

intermediates such as monoglycerides and diglycerides

appeared (1020 min). 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-

esterication reaction of the rapeseed oil. With a higher

molar ratio of methanol applied, the methyl esteried

compounds are increased with a decrease in the intermedi-

ate compounds.

Fig. 3 shows the content of methyl esters produced as

different supercritical treatments were carried out at

3508C. For a molar ratio of 42 in methanol, almost complete

conversion was achieved in a yield of 95% of methyl esters,

whereas for the lower molar ratio of 6 or 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-terication 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, transesterication reactions of rapeseed oil were

carried out with a xed molar ratio of 42 in methanol, the

best condition found in Fig. 3, at various temperatures

ranging from 200 to 5008C. Fig. 4 shows the obtained

HPLC chromatograms of rapeseed oil as treated in various

conditions of temperatures and reaction times, while thecontent 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 of 200 and 2308C, the relatively low

conversion to methyl esters is evident in Figs. 4 and 5 due

to the subcritical state of methanol. In these conditions,

methyl esters formed are at most about 68 and 70% at 200

and 2308C, respectively, at 3600 s (1 h) treatment. These

results are in good accordance with those already reported

[10].

At a temperature of 2708C, the conversion rate is still low

which might be related with the stability of supercriticalcondition. As can be seen in Fig. 1, maximum pressure

reached in this treatment is 14 MPa, still in the transition

between subcritical and supercritical state of methanol.

However, at 3008C, a considerable change in the conversion

rate can be seen with about 80% of methyl esters produced

in 240 s. As observed in the previous study [5], at 3508C,

240 s treatment resulted in a high conversion of rapeseed oil

to methyl esters with its yield of 95%.

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.

D. Kusdiana, S. Saka / Fuel 80 (2001) 693 698 695

Fig. 3. Effect of the molar ratios of methanol to rapeseed oil in trans-

esterication reaction on producing methyl esters, as treated at 350 8C.

Fig. 4. HPLC chromatograms of rapeseed oil as treated at various condi-

tions of temperatures and reaction times with molar ratio of 42 in methanol.


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At even higher temperature of 4008C, the transesterication

reaction is essentially completed for 120 s to convert almost

all rapeseed oil to their methyl esters. However, in such ahigh reaction temperature, new peaks in the shorter reten-

tion time (3 4 min) are dominating in the HPLC chromato-

grams as shown in Fig. 4 and the previous study [5]. This

indicates that decomposition reaction takes place at

temperature above 4008C due to the thermal degradation.

As a result, the transesterication 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 thetemperature 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 be 42,

the concentration of methanol was not taken into account, as

reported by other researchers [10,12].

Diasakov [10] proposed the thermal transesterication

reaction to be divided into 3 steps. 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 transesterication reaction for biodiesel fuel production

are methyl esters with glycerin, we dened a simpler math-

ematical model for this reaction by ignoring the intermedi-

ate reactions of diglycerides and monoglycerides, so the 3

steps can be simplied to be one step as follows:

This reaction is assumed to proceed in the rst order

reaction as a function of the concentration of triglycerides

(TG) and reaction temperature. The rate constant of the

reaction can be determined based on the increased amount

of the product that occurs in some reaction time interval

[13,14], or alternatively, based on the decreased amount

of one reactant. In this work, the decreased amount of one

reactant, that is TG, was chosen. Therefore, the rate constant

of the reaction can be given by Eq. (1)

Rate 2dTG

dt

1

where [TG] refers to the content of vegetable oil used in this

study. In this supercritical methanol method, three species

were dened as methyl esters (ME), glycerin (GL) and

unmethyl esteried compounds (uME) which include trigly-

cerides, diglycerides, monoglycerides and unreacted free

fatty acids. Therefore, Eq. (1) can be modied to be

Rate 2duME

dt2

or

2duME

dt kuME 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, 0 and

that it falls down to uME, t at some later time t, the

integration gives

2

uME;tuME;0

duME

uMEkt

0dt 4

D. Kusdiana, S. Saka / Fuel 80 (2001) 693 698696

Fig. 5. Effect the reaction temperature on the methyl esters formation. The

experimental data are presented by the symbols, whereas the solid lines are

simulated curves based on Eqs. (3) and (6).

Fig. 6. Semilog plot of unmethyl esters content in rapeseed oil during

transesterication reaction. Legends see Fig. 5.


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and

2lnuME;t

uME; 0 kt 5

or

K lnuME;t 2 lnuME; 0

t6

Fig. 6 shows the correlation between the content of

unmethyl esterifed compounds and reaction times. As

mentioned previously, unmethyl esteried compounds are

dened 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 Table1 and the corresponding Arrhenius plot for this method is

presented in Fig. 7. It is evident that at subcritical tempera-

ture below 2398C, the reaction rates are so low but much

higher at supercritical state, with the rate constant increased

by a factor of about 85 at the temperature of 3508C.

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 transesterication

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

transesterication must be valid.

4. Concluding remarks

A highly efcient transesterication process has been

described and the proposed kinetics in transesterication

of rapeseed oil has been proven to t very well with those

of experimental data. A reaction temperature of 3508C withthe molar ratio of methanol being 42 were 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 / Fuel 80 (2001) 693 698 697

Table 1

The rate constant of transesterication reaction

Reaction condition k (s21)

Temperature (8C) Pressure (MPa)

200 7 0.0002

230 9 0.0003270 12 0.0007

300 14 0.0071

350 19 0.0178

385 65 0.0249

431 90 0.0503

487 105 0.0803

Fig. 7. First order reaction rate constant in Arrhenius plot of rapeseed oil in

methanol during transesterication reaction.


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D. Kusdiana, S. Saka / Fuel 80 (2001) 693 698698

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