Research article

Comparative kinetics of esterification of methanol–aceticacid in the presence of liquid and solid catalysts

Mallaiah Mekala and Venkat Reddy Goli*

Department of Chemical Engineering, National Institute of Technology, Warangal 506004, India

Received 18 June 2013; Revised 31 December 2013; Accepted 17 February 2014

ABSTRACT: Esterification of acetic acid with methanol to produce methyl acetate in an isothermal stirred batch reactor hasbeen studied. Sulfuric acid was used as a liquid catalyst, and Indion-180, Indion-190 and Amberlyst-16wet ion exchangeresins were used as solid catalysts. The feed mole ratio was varied from 1 : 1 to 1 : 4. The reaction temperatures were variedfrom 305.15 to 333.15K for sulfuric acid as catalyst and 323.15 to 353.15K for the solid catalysts. The catalystconcentrations were used in the range of 1% to 5%, for the sulfuric acid catalyst, and 0.01 to 0.05 g/cc, for the solid catalysts.The effect of temperature, catalyst concentration, agitation speed, size of catalyst particle and reactant concentration on theacetic acid conversion was investigated. A second-order kinetic rate equation was proposed to fit the experimental data. Forboth forward and backward reactions, the activation energies were estimated from Arrhenius plots. The reaction rateincreased with catalyst concentration and temperature for both the liquid and solid catalysts. The acetic acid conversionwas found to increase with increases in acetic acid to methanol ratio in the feed. The developed kinetic rate equation wasused for the simulation of reactive distillation process, in our laboratory column. © 2014 Curtin University of Technologyand John Wiley & Sons, Ltd.

Keywords: esterification; ion exchange resin; catalysis; kinetics; kinetic model; simulation

INTRODUCTION

Carboxylic acid esters are widely used as softeners,emulsifiers, dispersants, detergents, surfactants andsolvents. Methyl acetate is one of the carboxylic acidester that is an industrially important chemical. It is acolourless liquid, with a mild ester-like odour, and itis miscible with many organic solvents. It is widelyused as a solvent in glues, nail polish removers,perfumery, dye manufacture and chemical reactions,and for extractions. It is also used in the manufactureof a variety of polyesters such as photographic filmbase, cellulose acetate and fast-drying paints, and inthe manufacture of celluloid adhesives from waste film.Methyl acetate is produced by the esterification reaction

of methanol with acetic acid in a reversible reactionaccording to the following reaction scheme, normally inthe presence of either a liquid or a solid catalyst:

CH3COOHþ CH3OH↔CH3COOCH3 þ H2O: (1)

The initiating step in the reaction mechanism isthe protonation of the carboxylic acid. The reaction

rate is very slow without a catalyst even at highertemperatures. So, to enhance the reaction rate, anaddition of a catalyst is required.The kinetics of esterification reaction between acetic

acid and methanol was investigated by Rolfe andHinshel Wood in 1934.[1] The kinetic model proposedby the authors, using hydrochloric acid as the catalyst,was based on the theory of molecular statistics of thereaction. The kinetics of esterification of acetic acidwith methanol reaction in the presence of a hydrogeniodide catalyst was investigated by Ronnback et al.[2]

They proposed the reaction mechanism based on theprotonation of carboxylic acid as the rate-initiatingstep. They observed that hydrogen iodide was esterifiedby methanol and methyl iodide formed as a by-product.The esterification reaction between acetic acid andmethanol using a homogeneous catalyst, H2SO4, inthe reactive distillation column was investigated.[3]

The kinetic rate expression incorporates nonlineardependence on the catalyst concentration. The kineticequation parameters have not been reported for theirkinetic model. A rate expression for the esterificationreaction using a homogeneous sulfuric acid catalystdeveloped,[4] by accounting the linear dependence ofthe catalyst concentration on the reaction kinetics.Elugu et al.[5] proposed a similar dependence andapplied it for the intensification of ester production in

*Correspondence to: Goli Venkat Reddy, Department of ChemicalEngineering, National Institute of Technology, Warangal 506004,India. E-mail: v.reddy@nitw.ac.in

© 2014 Curtin University of Technology and John Wiley & Sons, Ltd.Curtin University is a trademark of Curtin University of Technology

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERINGAsia-Pac. J. Chem. Eng. (2014)Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/apj.1798

a continuous reactor. The kinetic models for acid-catalyzed methyl acetate production using a homoge-neous catalyst in the batch reactor were investigated.[6]

They observed that the rate constant was affected bythe concentration of the catalyst, and the reaction rateincreased with an increase in the catalyst concentration.The similarities and differences between heteroge-neous-catalyzed and homogeneous-catalyzed esterificationof acetic acid with methanol were investigated by Liuet al.[7] The heterogeneous catalyst was commercialNafion/silica nanocomposite (SAC-13), and thehomogeneous catalyst was H2SO4. Casson et al.[8]

studied the comparison of criteria for the prediction ofrunaway reactions in the presence of sulfuric acid-catalyzed esterification of acetic anhydride andmethanol to form methyl acetate and acetic acid. Theauthors was neglected the side esterification reactionof the acetic acid and methanol because it is a slowreaction.Some of the solid catalysts used for esterification

reactions include the following: solid acids and bases,ion exchange resins, zeolites, and acid clay. Ionexchange resins are the most common heterogeneouscatalysts used for esterification reaction. These ionexchange resins not only catalyze the reaction but alsoaffect the equilibrium conversion because of theirselective adsorption of reactants and swellingnature.[9–11] In the heterogeneous catalysis, the forcesactive at solid surface can distort or even dissociatean absorbed reactant molecule and affect the rate ofreaction. But only the cat-ion exchange resins are usedfor the esterification reactions.[9–11] Many of theesterification reactions were investigated using theAmberlyst-15 as a solid catalyst.[12–18] The kinetics ofesterification of acetic acid with methanol using anion exchange solid acid catalyst was investigated,[19]

in an isothermal batch reactor in the temperature rangeof 333–353K. The effects of stirrer speed, reactiontemperature, initial reactant concentration and catalystloading on reaction rate were investigated andoptimized.Thus, a comparative study of both liquid and solid

catalysts for the esterification of acetic acid withmethanol is scarcely available, and studies of Indion-180, an ion exchange resin catalyst for esterification,are not available.In the reactive distillation process, both the reaction

and separation take place simultaneously. For design,simulation and optimization of any reactive distillationprocess, a rate equation, describing the kinetics, isessential. In the present work, esterification of aceticacid with methanol was studied in the presence of both,liquid and solid, types of catalysts. Experiments wereconducted to find the best solid catalyst for theesterification reaction. The effect of various parameterslike catalyst type, agitation speed, reaction temperature,reaction time, catalyst loading and initial reactant

concentration on the esterification were studied forthe selected catalyst.

EXPERIMENTAL

Chemicals

Methanol of purity = 99% w/w, acetic acid of purity99.95% w/w and sulfuric acid of purity = 98% w/wwere purchased from SD Fine Chemicals Ltd (Mumbai,India) and used as supplied without any furtherpurification.

Catalyst

The solid acid catalysts used for the esterificationreaction, Indion-180 and Indion-190, were suppliedby Ion Exchange India Limited, Mumbai, andAmbelyst-16wet by Rohm & Hass, Mumbai. Indion-180 is a new catalyst, and very little literature, asmentioned earlier, is available relating to its suitabilityfor esterification of acetic acid with methanol. It hascross-linked three-dimensional structures of polymericmaterial and is obtained by sulfonation of a copolymerof polystyrene and divinylbenzene (DVB). It is anopaque, dark grey coloured solid spherical bead. Allthe resins used in this study were dried for 2 h in theoven at temperature 363K to remove the moisturecontent. The physical properties of the solid catalystare given in Table 1.

Experimental setup

A schematic diagram of the experimental setup isshown in Fig. 1. The esterification reaction was carriedout in a 500-mL three-neck round-bottom flask, whichwas placed in a heating rota mantle. The rota mantlecontained a heating control knob and a speed controlknob. The rota mantle was maintained at constanttemperature by adjusting the heating knob. The stirringspeed was varied from 240 to 640 rpm using the speedcontrol knob. A mercury thermometer was inserted intothe reactor to measure the reaction mixture temperatureinside the flask. A spiral condenser was connectedto the reaction flask to reduce vapour losses fromthe reactor.

Experimental procedure

In the present study, in the case of esterification with ahomogeneous catalyst, methanol (32 g) and acetic acid(60 g) weighed accurately within an error of ±0.0001 g,using a digital electronic balance. The weighedreactants were charged to the reactor. The desiredamount of catalyst (as weight percentage with respectto acetic acid) was added to the charged reactants,and the reaction mixture was heated to the desired

M. MEKALA AND V. R. GOLI Asia-Pacific Journal of Chemical Engineering

© 2014 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. (2014)DOI: 10.1002/apj

temperature. The time was counted from the momentthe catalyst was added to the reactor, i.e. t= 0. Whereasin the heterogeneous reaction process, the catalyst wasadded, when the reaction mixture reached the desiredtemperature. When the reaction mixture reached thedesired temperature, the time was noted as initial time,i.e. t= 0. The samples were withdrawn at regularintervals of time and analyzed for acetic acidconcentration. The reaction was carried out for asufficient time so that equilibrium conversion wasreached or when no further decrease of acetic acidconcentration was noticed. The temperature wasmeasured with mercury thermometer within anaccuracy of ±0.5K. The withdrawn samples wereanalyzed for the concentration of the acetic acidimmediately, without any time delay. In case of anydelay, the samples were placed in an ice box to preventany further reaction before the analysis.

Chemical analysis

The acetic acid concentration was determined bytitration with a standard solution of NaOH usingphenolphthalein as the indicator. The standard solutionof NaOH was prepared using ultra pure water, withresistance of 18.2MΩ cm, prepared by a purifier systemsupplied by Millipore-Synergy UV System, India.

RESULTS AND DISCUSSION

Two sets of experiments were carried out. In the firstset of experiments, liquid catalyst was used to findout its efficiency in esterification, in comparison withother solid ion exchange resin catalysts. In the secondTa

ble

1.Ph

ysico-chem

ical

properties

ofcatalysts.

Phy

sicalprop

erty

Indion

-180

Indion

-190

Amberlyst-16

wet

Manufacturer

IonExchang

eIndiaLim

ited

IonExchang

eIndiaLim

ited

Roh

m&HassCo.

Shape

Beads

Beads

Beads

Phy

sicalform

Opaqu

e,grey

todark

grey

coloured

Opaqu

e,faintdark

grey

coloured

Opaqu

ebeads

Size(μm)

725

725

600–

800

App

arentbu

lkdensity

(g/cc)

0.55

–0.60

0.55

–0.60

0.98

Surface

area

(m2/g)

28–3

228

–32

30Porevo

lume(m

L/g)

0.32

–0.38

0.32

–0.38

0.20

Max.op

eratingtemperature

(°C)

150

150

130

Hyd

rogenioncapacity

(meq/g)

5.0

4.7

4.8

Matrixtype

Styrene–D

VB

Styrene–D

VB

Styrene–D

VB

pHrang

e0–7

0–7

—Resin

type

Macro

porous

strong

acidic

cat-ion

Macro

porous

strong

acidic

cat-ion

Macro

porous

strong

acidic

cat-ion

Fun

ctionalgrou

p�S

O3�

�SO3�

�SO3�

Ionicform

H+

H+

H+

Figure 1. Experimental setup for kinetic studies. This figureis available in colour online at www.apjChemEng.com.

Asia-Pacific Journal of Chemical Engineering COMPARATIVE KINETICS OF ESTERIFCATION OF METHANOL–ACETIC ACID

© 2014 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. (2014)DOI: 10.1002/apj

set of experiments, three solid ion exchange resincatalysts were studied, and their efficiencies in relationto esterification of acetic acid with methanol werecompared, and the most efficient among them waschosen for further studies. The esterification reactionwas carried out by using sulfuric acid as the liquidcatalyst and Indion-180 as the solid catalyst. Eventhough acetic acid itself acts as the catalyst, its activityfor reaction is slow because of its weak acidic nature.[12]

So, addition of a catalyst will improve the acidic naturethat provides more H+ ions for the reaction. Esterificationreaction with liquid and solid catalysts was investigatedat 1 : 1 mole ratio of acetic acid to methanol. Thetemperature was varied from 305.15 to 333.15K withthe liquid catalyst and 323.15 to 353.15K with the solidcatalyst. The catalyst concentration was varied from 1%to 5%, for the liquid catalyst, and 0.01 to 0.05 g/cc, forthe solid catalyst.

Effect of different operational conditions onthe reaction

Selection of acid catalystThe liquid catalysts like hydrogen iodide, hydrogenbromide, hydrochloric acid and sulfuric acids are usedas the homogeneous catalyst for the esterificationreaction between acetic acid and methanol. Sulfuricacid acts as a better catalyst among them due to itsgreater density[7] and also avoids the side reactionsduring the reaction. Therefore, in the present study,sulfuric acid was selected as the liquid catalyst foresterification. However, sulfuric acid is not a preferredcatalyst in the industry, for reactive distillation pro-cesses, because of its corrosive nature and also theadditional separation step involved.The solid resin catalysts, Indion-180, Indion-190 and

Amberlyst-16wet, were procured to select suitablecatalysts for the esterification of acetic acid withmethanol. The suitability of these catalysts was studied,and the efficiency of the same in relation to acetic acidconversion as function of time has been presented inFig. 2. The temperature was maintained at 343.15K,catalyst concentration was 0.025 g/cc, the mole ratio ofthe reactants was 1 : 1 and the agitation speed was240 rpm for all three catalysts. Figure 2 indicates thatIndion-180 is more efficient than Indion-190 and alsoAmberlyst-16wet. The conversion of acetic acid, after4 h of reaction time, for three catalysts, Indion-180,Indion-190 and Amberlyst-16wet, were found to be68.98%, 67.21% and 61.0%, respectively. The mainreason for Indion-180’s higher efficiency could beits higher pore volume and higher H+ ion capacitywhen compared with the other two catalysts. In viewof its higher efficiency in relation to acetic acidconversion in esterification, Indion-180 has beenfurther investigated.

Comparison of liquid and solid catalyst foracetic acid conversion

Figure 3 shows the conversion kinetics of acetic acid inthe presence of the liquid sulfuric acid catalyst andsolid Indion-180 catalyst at the same catalytic activity(meq/g). In Fig. 3, it is observed that the conversionof acetic acid in the presence of liquid catalyst is higherwhen compared with the solid catalyst. In the case ofthe liquid catalyst, the process is homogeneous, andthere is good mixing and no resistance to mass transfer;therefore, the conversion is better at any given point oftime. However, the equilibrium conversion is the samein both cases.

Effect of temperature

The conversion of acetic acid with time in the presence ofsulfuric acid catalyst, at different temperatures, has beenshown in Fig. 4. The other experimental conditionsmaintained are given in Fig. 4 as legend. The temperaturewas varied between 305.15 and 333.15K. As can be seen

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200 250 300

Con

vers

ion

of a

cetic

aci

d

Time,min

Amberlyst -16wet

Indion- 190

Indion- 180

Figure 2. Conversion of acetic acid for different catalysts at0.025g/cc catalyst concentration and 343.15K temperature.

0

0.1

0.2

0.3

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0.5

0.6

0.7

0.8

0 50 100 150 200 250 300

Con

vers

ion

of a

cetic

aci

d

Time,min

H2SO4

INDION -180

Figure 3. Conversion of acetic acid with time for sulfuricacid and Indion-180 catalyst for the same catalytic activity,meq/g of catalysts at 323.15K. Sulfuric acid concentration:2%(25.01meq/g), Indion-180 catalyst loading: 0.05 g/cc(24.85meq/g).

M. MEKALA AND V. R. GOLI Asia-Pacific Journal of Chemical Engineering

© 2014 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. (2014)DOI: 10.1002/apj

in Fig. 4, the maximum conversion obtained is about 69%at all the temperatures studied. The temperature has nosignificant effect on equilibrium conversion of acetic acid.However, as the reaction temperature increases, the rate ofesterification reaction becomes faster. The presentfindings are in agreement with Ganesh et al.,[6] who alsoreported an increase in the rate of conversion of acetic acidwith an increase in temperature.Figure 5 shows the effect of temperature on the

acetic acid conversion with time, with temperature asthe parameter, for the solid catalyst Indion-180. Otherexperimental conditions maintained were as follows:amount of catalyst of 0.025 g/cc, rpm of 240 andaverage catalyst particle size of 725μm. Thetemperature was varied between 323.15 and 353.15K.In Fig. 5, it can be observed that the maximum conver-sion of acetic acid is about 69%, which is the same asthat for the sulfuric acid catalyst. The rate of conver-sion increases with temperature, indicating that thereaction is controlled by chemical steps.[20] Similar tosulfuric acid catalyst, the rate of conversion of aceticacid is higher at higher temperatures. Both liquid andsolid catalysts provide same conversion, i.e. 69%.However, the sulfuric acid catalyst is more efficienteven at lower temperatures.

Effect of catalyst concentration

Figures 6 and 7 show the conversion of acetic acid vstime, with catalyst concentration as the parameter, forliquid and solid catalysts, respectively. The otherexperimental conditions maintained have been givenas legend in the respective figures. Figures 6 and 7show that the increase in the catalyst concentrationenhances the acetic acid conversion. The maximumconversion obtained, in both cases, is almost the same,that is, about 69%. In the case of the liquid catalyst, therate of conversion increases as the catalyst concent-ration increases, and the maximum conversion of69% was attained within 150min from the start. Inthe case of the solid catalyst (Fig. 7), although the rateof conversion increases with increases in catalystconcentration, and the maximum conversion attainedwas the same as the homogeneous catalyst, the timetaken to attain maximum conversion was more than250min from the start. Moreover, the time taken toattain the maximum conversion increases as thecatalyst concentration decreases. For both types ofcatalysts, the amount of catalyst influences the acetic

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 100 200 300 400

Con

vers

ionn

of a

cetic

aci

d

Time,min

305.15K313.15K323.15K333.15K

catayst=H2SO4catalyst concentraton=2%agitation speed=240rpmmole ratio=1:1

Figure 4. Effect of temperature on reaction kinetics at 2%catalyst concentration.

0

0.1

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0.5

0.6

0.7

0.8

0 100 200 300 400 500

Con

vers

ion

of a

cetic

aci

d

Time,min

323.15 K333.15 K338.15 K343.15 K353.15 K

catalyst=Indion-180catalyst loading=0.025 g/ccagitation speed=240 rpmmole ratio=1:1

Figure 5. Conversion acetic acid at different temperaturesat 0.025 g/cc catalyst concentration.

0

0.1

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0.7

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0 50 100 150 200 250 300 350

Con

vers

ion

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cetic

aci

d

Time,min

1%

2%

3%

4%

5%

catayst=H2SO4agitation speed=240rpmreaction temperature=313.15 Kmole ratio=1:1

Figure 6. Effect of catalyst concentration on reactionkinetics at 313.15K.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200 250 300 350

Con

vers

ion

of a

cetic

aci

d

Time,min

0.01 g/cc

0.025 g/cc

0.03 g/cc

0.04 g/cc

0.05 g/cc

catalyst=Indion-180temperature=333.15 Kagitation speed=240 rpmmole ratio=1:1

Figure 7. Effect of catalyst concentration on reactionkinetics at 333.15K.

Asia-Pacific Journal of Chemical Engineering COMPARATIVE KINETICS OF ESTERIFCATION OF METHANOL–ACETIC ACID

© 2014 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. (2014)DOI: 10.1002/apj

acid conversion because more H+ ions are availablewhen the amount of catalyst in the reaction mixtureincreases. The liquid catalyst sulfuric acid is moreefficient than the solid catalyst indion-180 with respectto the conversion of acetic acid (Figs 6 and 7). How-ever, the liquid catalyst, sulfuric acid, is corrosive andalso needs to be separated from product mixture, whichadds to the product cost of methyl acetate; therefore, itis not a preferred catalyst, in the industry.

Effect of initial reactant mole ratio

The molar ratio of acetic acid to methanol was variedfrom 1 : 1 to 1 : 4, at the catalyst concentration of 2%,reaction temperature of 333.15K and agitation speedmaintained at 240 rpm. Figure 8 shows the acetic acidconversion vs time, with mole ratio as the parameter,for the liquid sulfuric acid catalyst. It can be seen inFig. 8 that the conversion of acetic acid increases withincreases in mole ratio, that is, increases in the proportionof methanol in starting reaction mixture.With an increasein mole ratio of acetic acid to methanol from 1 : 1 to 1 : 4,the conversion of acetic acid increased from 69.1% to90.7%.When the methanol ration in the reaction mixtureis increased, the equilibrium shifts toward the product;therefore, the acetic acid conversion increases.Figure 9 shows the conversion of acetic acid with time,

with feed mole ratio as the parameter, for the solidcatalyst, Indion-180. The molar ratio of acetic acid tomethanol was varied from 1 : 1 to 1 : 4. The other experim-ental conditions maintained were as follows: catalystconcentration: 0.025 g/cc, temperature: 343.15K andstirrer speed: 240 rpm. In Fig. 9, it can be seen that theconversion of acetic acid increases with increases in moleratio. With an increase in mole ratio of acetic acid tomethanol from 1 : 1 to 1 : 4, the conversion of acetic acidincreased from 68.5% to 92.7%. The reason for increasesin acetic acid conversion with mole ratio can be furtherexplainedwith the help of equilibrium equation as follows.The equilibrium constant Ke is given by Eqn (2) as

Ke ¼ CCe½ � CDe½ �CAe½ � CBe½ � (2)

where CAe, CBe, CCe and CDe are the equilibriumconcentrations of acetic acid, methanol, methyl acetateand water, respectively. Because CBe (methanol) is morein the denominator, the concentration of methyl acetate(CCe) also has to be more in order to maintain the sameKe value at a given temperature. Hence, higher conversionof acetic acid takes place as the mole ratio increases.

Effect of agitation speed

The agitation speed in the reaction mixture affects theresistance to external mass transfer, when solid catalystis used. In the present study, a magnetic stirrer wasused to maintain agitation in the reaction mixture.The reaction temperature was 343.15K, and thecatalyst concentration was 0.025 g/cc. The reactionwas conducted at different rpm’s ranging from 240 to640. From the experiments, it was observed that theconversion of acetic acid is unaffected by the stirrerspeed. This indicates that the external mass transferresistance is not a rate-controlling step. As the agitationspeed has no effect on the conversion, all the furtherexperiments were conducted with 240 rpm for theesterification reaction with liquid and solid catalysts.Similar results for the homogeneous catalyst, i.e.

acetic acid conversion vs time, with agitation speed asthe parameter, it is very obvious that the agitation speedhad no effect, whatsoever, on the conversion of aceticacid. This was expected, because the reaction mixturewas homogeneous.

Effect of catalyst particle size

The experiments were conducted using averagecatalyst particle sizes of 425, 550, 725 and 925μm, tostudy the effect of catalyst particle size on theconversion of acetic acid. The average catalyst particle

0

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1

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Con

vers

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aci

d

Time,min

1:01

1:02

1:03

1:04

catayst=H2SO4catalyst concentraton=2%T=333.15 Kagitaton speed=240rpm

Figure 8. Effect of reactant initial mole ratio on the aceticacid conversion at 2% catalyst concentration and reactiontemperature 333.15 K.

0

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1

0 30 60 90 120 150 180 210

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vers

ion

of a

cetic

aci

d

Time,min

1:01

1:02

1:03

1:04catayst=Indion-180catalyst concentraton=0.025 g/ccT=343.15 Kagitaton speed=240rpm

Figure 9. Effect of molar ratio (acetic acid to methanol) onacetic acid conversion.

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size has no significant effect on the acetic acidconversion under experimental conditions. This furtherconfirms that esterification is not mass transfercontrolled. The present results are in good agreementwith literature reports.[21,22]

THE KINETIC MODEL

The experimental data are correlated with the second-order kinetic rate equation for the esterification reactionof acetic acid with methanol using liquid and solidcatalysts. Based on our experimental investigations,the effect of agitation speed and the catalyst particlesize on the reaction rate with solid ion exchange resincatalyst was negligible. The mass transfer resistancesare negligible for the esterification reactions in thepresence of ion exchange resins.[21,22] Hence, a pseudohomogeneous kinetic equation model has been pro-posed, for the esterification reaction between aceticacid and methanol in the presence of solid acidcatalysts. Although the model pertains to reactionmixture mole ratio of 1 : 1 acetic acid to methanol, itis applicable to any mole ratio, with appropriatemodifications.The reaction rate expression (�rA) of the second-

order reaction is

�rA ¼ � dCA

dt¼ kfCACB � kbCCCD (3)

where CA, CB, CC and CD are the concentrations ofacetic acid, methanol, methyl acetate and water ingmol/lit at any given time. kf is the rate constant ofthe forward reaction, and kb is the rate constant of thebackward reaction. Equation (3) can be rearranged asshown in Eqn (4) by assigning Ke = kf / kb, where Ke isthe equilibrium constant of the reversible reaction.

�rA ¼ � dCA

dt¼ kf CACB � CCCD

Ke

� �(4)

The temperature dependency of forward reaction rateconstant and backward reaction rate constants areexpressed by the Arrhenius equation as shown in Eqns(5a) and (5b).

kf ¼ kf0 exp � Ef

RT

� �(5a)

kb ¼ kb0 exp � Eb

RT

� �(5b)

where kf0 and kb0 are the pre-exponential factors, Ef andEb are the activation energies for forward and backwardreaction, R is the gas constant and T is the temperature

in K. Equation (4) is rearranged for the 1 : 1 initialreactant mole ratio in terms of acetic acid conversion(XA) as follows.

dXA

dt¼ kfCA0 1� XAð Þ2 � X2

A

Ke

� �(6)

where XA= (1�CA /CA0) and CA0 is the initialconcentration of acetic acid. At equilibrium, the Ke valueis given by Eqn (7).

Ke ¼ X2Ae

1� XAeð Þ2 (7)

where XAe is the equilibrium conversion of acetic acid.From Eqn (7), it is possible to determine the Ke valuesat different temperatures. The literature reported thatthe equilibrium constant (Ke) values, based onconcentration model, are in the range of 3.9–9.0 forthe esterification of acetic acid with methanol reaction,as given in Ganesh et al.,[6] Jagadeesh babu et al.[19]

and Tsai et al.[22] From the experiments, we obtainedKe as 4.95.Equation (6) can be integrated and rearranged in its

linear form to find the forward reaction rate (kf) asgiven in Eqn (8).

lnXAe � 2XAe � 1ð ÞXAð Þ

XAe � XAð Þ� �

¼ 2kf1

XAe� 1

� �CA0t

(8)

A plot of LHSof Eqn (8) vs reaction timewill yield valueof kf. Figure 10 shows the plot of ln{[XAe� (2XAe� 1)XA] / (XAe�XA)} vs time at different temperatures. InFig. 10, it is observed that at all reaction temperatures,the straight line passes through the origin, and this isa further confirmation that a second-order pseudohomogeneous model is a good fit with experimentaldata.

T=323.15 Ky = 0.011xR² = 0.978

T=333.15 Ky = 0.018xR² = 0.985

T=343.15 Ky = 0.030xR² = 0.990

T=353.15 Ky = 0.038xR² = 0.980

0

1

2

3

4

5

6

7

8

9

0 50 100 150 200 250

ln((

XA

e-(2X

Ae-1

)(X

A)/

(XA

e-XA))

Time,min

catalyst=Indion 180catalyst loading=0.025 g/ccagitation speed=240 rpmmole ratio=1:1

Figure 10. Adopting Eqn (8) for finding forward rateconstants (using solid catalyst).

Asia-Pacific Journal of Chemical Engineering COMPARATIVE KINETICS OF ESTERIFCATION OF METHANOL–ACETIC ACID

© 2014 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. (2014)DOI: 10.1002/apj

Similar method has been adapted to determine theequilibrium and the forward reaction rate constantsfor homogeneous catalytic esterification reactions also.Figure 11 shows LHS of Eqn (8) vs time forhomogeneous catalytic reactions.Backward reaction rate constants can be calculated

by using equilibrium constants and forward reactionrate constants at different temperatures.The influence of temperature on the reaction rate

constants, kf and kb, can be determined by plotting lnkfvs 1/T and lnkb vs 1/T, using Arrhenius Eqns (5a) and(5b), respectively. Figures 12 and 13 show such plotsfor heterogeneous and homogeneous catalyticesterification reactions. For liquid and solid catalysts,kf and kb values are shown in Tables 2 and 3. Fromthe tables, it is observed that as the temperatureincreases, the reaction rate constants increase. For theliquid catalyst, the activation energies for both theforward and backward reactions are found to be62 721 and 62 670 J/mol, and for the solid catalyst,39 732 and 35 359 J/mol, respectively.

For solid catalyst, the effect of the catalyst loading onthe reaction kinetics is studied by analyzing the variationof pre-exponential factor kfo for five different catalystconcentrations for a fixed temperature of 343.15K. Thepre-exponential factor (kfo) was plotted as a function ofthe catalyst concentration as shown in Fig. 14. Amathematical expression for pre-exponential factor as afunction of catalyst loading was obtained from Fig. 14.

kf0 ¼ �26072w2c þ 50674wc þ 2652 (9)

where wc is the catalyst loading in g/cc of initialreaction mixture.

0

5

10

15

20

25

30

35

0 50 100 150 200 250 300 350Time,min

Figure 11. Adopting Eqn (8) for finding forward rateconstants (using liquid catalyst).

y = -4779.3x + 8.1177R² = 0.9725

y = -4342.6x + 5.257R² = 0.9789

-8.5

-8

-7.5

-7

-6.5

-6

-5.5

-5

0.0028 0.00285 0.0029 0.00295 0.003 0.00305 0.0031 0.00315

ln(k

i)

1/T, K-1

kf

kb

catalyst=Indion-180catalyst loading=0.025 g/ccagitation speed=240 rpmmole ratio=1:1

Figure 12. Plot of logarithm of the forward and backwardreaction rate constants as a function of reciprocal oftemperature for solid catalyst.

y = -7544.7x + 18.207R² = 0.9601

y = -7538.3x + 16.577R² = 0.9603

-9

-8.5

-8

-7.5

-7

-6.5

-6

-5.5

-5

-4.5

-4

0.00295 0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033

ln(k

i)

1/T,K-1

catalyst=H2SO4catalyst loading=2%agitation speed=240 rpmmole ratio=1:1

Figure 13. Plot of logarithm of the forward and backwardreaction rate constants as a function of reciprocal oftemperature for liquid catalyst. This figure is available incolour online at www.apjChemEng.com.

Table 2. Reaction rate constants for homogeneouscatalyst at 2% catalyst concentration at differenttemperatures.

Temperature, K kf (lit/mol min) kb (lit/mol min)

305.15 0.001304 0.000261313.15 0.003589 0.000717323.15 0.006346 0.001268333.15 0.010673 0.002133

Table 3. Reaction rate constants for Indion-180 catalystat 0.025g/cc catalyst concentration at differenttemperatures.

Temperature, K kf (lit/mol min) kb (lit/mol min)

323.15 0.001231 0.000283333.15 0.002112 0.000462338.15 0.002288 0.000486343.15 0.00328 0.000676353.15 0.004169 0.000842

M. MEKALA AND V. R. GOLI Asia-Pacific Journal of Chemical Engineering

© 2014 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. (2014)DOI: 10.1002/apj

Similarly, when using the liquid catalyst for theesterification reaction, the frequency factor as a functionof catalyst concentration is found to be

kf0 ¼ �1:0� 106X2c þ 3:0� 107Xc þ 7� 107 (10)

where Xc is the weight% of catalyst with respect to theweight of acetic acid in the initial reaction mixture.

CONCLUSION

Esterification of methanol with acetic acid in thepresence of liquid and solid catalysts has beeninvestigated. Three different types of the solid catalystsIndion-180, Indion-190 and Amberlyst-16wet havebeen investigated. Among the solid ion exchange resincatalyst, Indion-180 was found to be better than the othertwo catalysts studied. The liquid catalyst, sulfuric acid,has been found to be better than all other solid ionexchange resin catalysts. The kinetics of the both liquidand solid catalytic esterification reactions can berepresented by second-order kinetic model. The agitationspeed has no significant effect on the conversion of aceticacid in both types of catalytic reactions. It is observedthat equilibrium conversion increased as the feed molarratio of acetic acid to methanol increased from 1 : 1 to1 : 4. The frequency factor as a function of catalystloading was obtained for both types of catalysts.

NOMENCLATURECA0 Initial acetic acid concentration (gmol/lit)CA Acetic acid concentration (gmol/lit)CB Methanol concentration (gmol/lit)CC Methyl acetate concentration (gmol/lit)CD Water concentration (gmol/lit)CAe, CBe,Cce and CDe

Equilibrium concentration of acetic acid,methanol, methyl acetate and water,respectively (gmol/lit)

Ef Forward activation energy (J/gmol)

Eb Backward activation energy (J/gmol)Ke Equilibrium constantkf Forward reaction rate constant

(lit/gmolmin)kb Backward reaction rate constant

(lit/gmolmin)kf0 Forward frequency factor (lit/gmolmin)kb0 Backward frequency factor (lit/mol min)rA Reaction rate of acetic acid (gmol/lit min)R Gas constant (J/gmol K)T Absolute temperature (K)t Time (min)wc Catalyst loading (g/cc)Xc Catalyst loading (weight% of catalyst

with respect to the weight of acetic acid)XA Acetic acid conversionXAe Acetic acid equilibrium conversion

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kf0= -26072(wc)2 + 50674(wc) + 2652.R² = 0.987

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 0.01 0.02 0.03 0.04 0.05 0.06

Freq

uenc

y fa

ctor

(k fo

)

catalyst loading (g/cc)

T=343.15 Kcatalyst=Indion-180agitation speed=240 rpmmole ratio=1:1

Figure 14. Relation between pre-exponential factor andcatalyst concentration.

Asia-Pacific Journal of Chemical Engineering COMPARATIVE KINETICS OF ESTERIFCATION OF METHANOL–ACETIC ACID

© 2014 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. (2014)DOI: 10.1002/apj