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Reaction Kinetics of Soybean Oil Transesterification at High

Temperature

Shuli Yan, Manhoe Kim, Steve O. Salley, John Wilson, and K. Y. Simon Ng

National Biofuels Energy LaboratoryNextEnergy/Wayne State University

Detroit, MI 48202

Present at AICHe MeetingNov. 16, 2008

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Outline• Introduction

• Experiment

• Catalyst structure

• Kinetic Parameters

• Kinetics of soybean oil to methyl esters

Homogenous catalysis

Heterogeneous catalysis

3

Introduction

• Transesterification of vegetable oil with alcohol for biodiesel production

• Homogeneous catalysis• Heterogeneous catalysis

4

Introduction

• Kinetics of transesterification catalyzed by homogenous catalysts

Dufek studied the kinetics of acid-catalyzed transesterication of 9(10)-carboxystearic acid and its mono- and di-methyl esters.

Freedman et al. reported transesterication reaction of soybean oil and other vegetable oils with alcohols, 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 constants and kinetic order.

Noureddin and Zhu studied the effects of mixing of soybean oil with methanol on its kinetics of transesterication.

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Solid Base Catalysts

Goal

Catalyst T Time(h) Conv.(%) Ref.

ZnO 120°C 24 80 2

HT (Mg-Al) 180°C 1 92 3

SO42-/ZrO2 200°C 4 95.7 4

6

Introduction

• Kinetics of transesterification catalyzed by heterogonous catalysts

Our goal: 1. studying the use of the heterogeneously ZnxLayOz catalyzed

transesterification reaction in batch stirred tank reactors for biodiesel production

2. developing a kinetic model based on the three step ‘Eley–Rideal’ type mechanism to simulate the transesetrification process.

very little information concerning the kinetics of heterogeneously catalytic transesterification at high temperature

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Experiments

Catalyst preparation and characterization Homogeneous-coprecipitation method using urea as precipitant

1. Prepare a mixture solution of Zn(NO3)2 , La(NO3)3 and urea

2. Heat to 100 oC and hold for 6 hr

3. Stirred with magnetic stirrer

4. Filter/unfilter

5. Dry at 150 oC for 8 hr

6. Use step-rise calcination method at 250 (2hr), 300 (2hr), 350 (2hr), 400 (2hr), 450 oC (8hr),

SEM/EDS

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Experiments

• Transesterification

Molar ratio of methanol to soybean oil-----------------38:1

Catalyst dosage----------------------2.3 %(wt)

Stir speed------------------------------490 rpm

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Catalyst structure

• SEM/EDS

10

• SEM/EDS

Catalyst structure

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Effect of mixing

• A picture

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Effect of temperature on methyl esters formation

120 140 160 180 200 220 2400

10

20

30

40

50

Blank

ZnxLa

yO

z

Yie

ld o

f FA

ME

%

Temperature oC

Reaction conditions:

ZnxLayOz, catalyst dosage is 2.3% (wt),

Molar ratio of methanol to oil is 42:1,

Stir speed is about 490 rpm

Temperature was raised by step method. And when getting to the at target temperature point, it was hold for 1min

Fig. 5 Methyl esters yield at different temperature

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Effect of temperature on methyl esters formation

0 100 200 300 4000

20

40

60

80

100

200 oC

Time min

210 oC

180 oC

190 oC

Fig. 6 Effect the temperature on the methyl esters formation

Reaction conditions:

ZnxLayOz, catalyst dosage is 2.3% (wt),

Molar ratio of methanol to oil is 42:1,

stir speed is about 490 rpm.

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Effect of catalyst concentration

• A picture

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Kinetic model

• Assumptions:

1. Only methanol molecule adsorb on the surface of catalyst

2. Surface chemical reaction is the rate-determing step

— pKa (Methanol: 15.54 Natural oil: 3.55 )

— Molecular size (Methanol: 0.33 nm Natural oil: 2 nm)

— Heterolytically dissociate

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Kinetic model

• Eley-Rideal bimolecular surface reactions

CA

fast

RDS

khet

AAB

BCB

An adsorbed molecule may react directly with an

impinging molecule by a collisional mechanism

Fig. 9 Eley-Rideal mechanism

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Kinetic model

• Elementary reactions based on Eley-Rideal-type mechanism

ASSA Where A is methanol molecule and S is an adsorption site on the surface

1. Adsorption

（ 1） 0NCbN AAA

Where is methanol molecule concentration on the surface of catalyst, bA is the adsorption coefficient, is the fraction of surface empty sites, CA is the concentration of methanol.

AN 0N

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Kinetic model

• Elementary reactions based on Eley-Rideal-type mechanism

CDSBAS

Where B is tri-, di-, and mono-glyceride molecule, DS is an adsorpted di-, and mono-glyceride molecule on catalyst surface,

2. Surface reaction

CDBA CNkCNkr 22 （ 2）

Where k2 and k-2 is the reaction rate constants, Cc is the concentration of FAME

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Kinetic model

• Elementary reactions based on Eley-Rideal-type mechanism

SDDS Di-, mono-glyceride and glycerin desorb from catalyst surface

3. Desorption

0NCbN DDD （ 3）

Where is di-, mono-glycerie and glycerine molecule concentration on the surface of catalyst, bD is the adsorption coefficient, CD is the concentration of di-, mono-glycerie and glycerine .

DN

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Kinetic model

According to steps 1 , 2 and 3, we can get （ 4） 0202 NCbCkNCCbkr DDCBAA

DAS NNNN 0Because of

DDAA

S

CbCb

NN

10（ 5）

Then

DDAA

DCDSBAAS

CbCb

CCbNkCCbNkr

122

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Kinetic model

（ 6）DDAA

DCP

BA

CbCb

CCK

CCk

r

1

1

AS bNkk 2D

AP bk

bkK

2

2

Where

（ 7）AA

DCP

BA

Cb

CCK

CCk

r

1

1

Because tri-, di- mono-glyceride and glycerin have low adsorption, AACb DDCb>>

Then

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Kinetic model

BAA

A CCb

kCr

1

B

AA

AS Cb

C

bNk

12 （ 8）

Because the final product glycerine will separate from reaction mixture, we assume that step 2 is unreversible.

BrCkr （ 9）

When methanol concentration is kept constant,

Where

AA

ASr

bC

bNkk

12

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Kinetic model• The rate constant of transesterification reaction

Reaction condition k(s-1)

Temperature oC Pressure Psi

180 ~ 330 0.01299

190 ~ 410 0.01806

200 ~ 450 0.05000

210 ~ 580 0.05220

Table 1 the reaction rate constant of transesetrification

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Kinetic model

• Arrhenius equation

0.0044 0.0046 0.0048 0.0050 0.0052 0.0054 0.0056 0.0058 0.0060-4.5

-4.2

-3.9

-3.6

-3.3

-3.0

-2.7

Ln

k

1/T K-1

ART

Ek lnln

Fig. 10 The temperature dependency of the reaction rate constants

E = 16.4 KJ/mol

25Fig. 11 Mechanism of ZnO-catalyzed transesterification of triglyceride with methanol

O

H2C

HC

H2C

O

O

O

Zn

C R2

C R3

O

O CH3

+O

C O CH3R1

+

O

H2C

HC

H2C

O

O

O

CO

R1

CO

R2

C R3

Zn

O

CH3

O

CH3

O

H2C

HC

H2C

O

O

O

C

O-

C R2

C R3

O

R1

O CH3

Zn O CH3

CH3 OH +O

H2C

HC

H2C

O

O

O

Zn

C R2

C R3

O

O CH3

+O

H2C

HC

H2C

O

O

O

H

C R2

C R3

OZn

O

CH3

O

CH3

ZnOx + Zn(CH3O)2 + OH2CH3OH

+ZnOxZnO O（ 1）

（ 2）

（ 3）

（ 4）

（ 5）

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Conclusion

• A multiporous catalyst

• A kinetic model was developed based on a three-step E-R type of mechanism.

First order reaction as a function of the concentration of triglyceride

E = 16.37KJ/mol

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Future work

Investigate the influence of some kinetic parameters on transesterification such as molar

ratio of methanol to oil, catalyst amount

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AcknowledgementFinancial support from the Department of Energy (DE12344458)

and Michigan’s 21st Century Job Fund is gratefully

acknowledged.

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Thank you!