The esterification of acetic acid with ethanol in a...

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The esterification of acetic acid with ethanol in a pervaporation membrane reactor Ayça Hasanoğlu*, Yavuz Salt, Sevinç Keleşer, Salih Dinçer Chemical Engineering Department, Yıldız Technical University, Davutpaşa Campus, 34210 Esenler-Istanbul, Turkey Email: [email protected] Received 21 July 2008; revised 01 December 2008; accepted 09 February 2009 Abstract Pervaporation membrane reactors are the systems in which the separation and reaction are carried out simul- taneously in order to increase conversions by removing one or more of the products formed during equilibrium reactions. In this study the esterification reaction of acetic acid and ethanol to produce ethyl acetate and water was investigated using a batch pervaporation membrane reactor. The experiments were carried out in the tem- perature range of 50–70°C. The ratios of ethanol concentration to acetic acid concentration were chosen as 1 and 1.5. Amberlyst 15 and sulfuric acid were used as the catalysts. Polydimethylsiloxane (PDMS) prepared in our labs was used as the membrane material, permselective to ethyl acetate formed by reaction. In this way, con- versions were increased by continuous removal of ethyl acetate from the reaction media. Conversions are found to increase with an increase in both molar ratios of reactants and temperature. Temperature has a strong influence on the performance of the pervaporation membrane reactor because it acts on both the esterification kinetics and pervaporation. Keywords: Pervaporation membrane reactor; Esterification; Ethyl acetate; Catalyst 1. Introduction The use of membranes in chemical reaction processes has attracted much attention. Since sep- aration membranes permit selective permeation of a component from a mixture, they help to enhance the conversions of thermodynamically or kinetically limited reactions through controlled removal of one or more product species from the reaction mixture. Using membranes to separate products in a reversible reaction is an effective method for producing esters. By applying a hybrid process, such as esterification–pervapora- tion, it is possible to shift the equilibrium towards Desalination 245 (2009) 662–669 *Corresponding author. Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and Modelling From the Nano to the Macro Scale (EWM 2008), May 2528, 2008, Vale do Lobo, Algarve, Portugal. 0011-9164/09/$– See front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.02.034

Transcript of The esterification of acetic acid with ethanol in a...

The esterification of acetic acid with ethanol in apervaporation membrane reactor

Ayça Hasanoğlu*, Yavuz Salt, Sevinç Keleşer, Salih Dinçer

Chemical Engineering Department, Yıldız Technical University, Davutpaşa Campus,34210 Esenler-Istanbul, TurkeyEmail: [email protected]

Received 21 July 2008; revised 01 December 2008; accepted 09 February 2009

Abstract

Pervaporation membrane reactors are the systems in which the separation and reaction are carried out simul-taneously in order to increase conversions by removing one or more of the products formed during equilibriumreactions. In this study the esterification reaction of acetic acid and ethanol to produce ethyl acetate and waterwas investigated using a batch pervaporation membrane reactor. The experiments were carried out in the tem-perature range of 50–70°C. The ratios of ethanol concentration to acetic acid concentration were chosen as 1and 1.5. Amberlyst 15 and sulfuric acid were used as the catalysts. Polydimethylsiloxane (PDMS) prepared inour labs was used as the membrane material, permselective to ethyl acetate formed by reaction. In this way, con-versions were increased by continuous removal of ethyl acetate from the reaction media. Conversions are foundto increase with an increase in both molar ratios of reactants and temperature. Temperature has a strong influenceon the performance of the pervaporation membrane reactor because it acts on both the esterification kinetics andpervaporation.

Keywords: Pervaporation membrane reactor; Esterification; Ethyl acetate; Catalyst

1. Introduction

The use of membranes in chemical reactionprocesses has attracted much attention. Since sep-aration membranes permit selective permeationof a component from a mixture, they help to

enhance the conversions of thermodynamicallyor kinetically limited reactions through controlledremoval of one or more product species from thereaction mixture. Using membranes to separateproducts in a reversible reaction is an effectivemethod for producing esters. By applying ahybrid process, such as esterification–pervapora-tion, it is possible to shift the equilibrium towards

Desalination 245 (2009) 662–669

*Corresponding author.

Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring andModelling – From the Nano to the Macro Scale – (EWM 2008), May 25–28, 2008, Vale do Lobo, Algarve, Portugal.

0011-9164/09/$– See front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.desal.2009.02.034

higher reaction yields. Considerable savings canalso be made in the amount of reactants required(as there is hardly any need for excess amount ofone of the starting components) and the reactiontime. Pervaporation, itself, has become in recentyears a promising technology, potentially usefulin applications such as the dehydration andremoval/recovery of organic compounds fromaqueous solutions, as well as the separation oforganic mixtures. Pervaporation is often appliedin combination with another technology in ahybrid process; among these, pervaporation-dis-tillation systems and PMR (pervaporation mem-brane reactor) are already finding industrialapplications. In PMR, the membrane eitherremoves the desired product or the undesiredproduct [1–2].

Ethyl acetate (EAc) is an important raw mate-rial for many applications in chemical industryincluding coatings, adhesives, perfumes, andplasticizers [3]. With the consciousness of theimportance of the mixture behaviour and separa-tion of ternary or quaternary mixtures consistingof ethyl acetate, water, ethanol (EOH), and aceticacid (AAc) in industrial applications; pervapora-tion assisted esterification reaction of acetic acidand ethyl alcohol, yielding ethyl acetate andwater was investigated in this study. In the liter-ature several studies of pervaporation coupledesterification based on water removal have beenreported, while ester removal was not muchinvestigated. In this study, the focus is on theremoval of ethyl acetate formed by the reactionbetween ethanol and acetic acid using PMR in thetemperature range of 50–70°C. The catalysts usedwere sulfuric acid and Amberlyst 15. The influ-ence of parameters such as temperature and ratiosof the reactants on the reactor performance wereanalyzed [4].

2. Materials and methods

Acetic acid, ethanol and sulfuric acid usedwere analytical grade and purchased from J.T.Baker. Amberlyst 15 was purchased from Acros.

The membrane used in this work was a cross-linked hydrophobic membrane, polydimethyl-siloxane (PDMS) prepared in our labs [5]. PDMS(RTV 615 A) and its crosslinking agent (RTV 615B) were purchased from GE Silicone representa-tive in Turkey. A solution of PDMS and itscrosslinking agent (10 wt.%) was degassed undervacuum, then cast on membrane plates using afilm applicator, and crosslinked for 1 h at 100°Cin an oven by heat treatment. The thickness of theresulting membranes was ~ 200 µm. Thinnermembranes were not used because they deformedduring PMR applications.

Reactions were carried out in batch reactorsboth with and without membrane under the sameconditions. Thus, the effect of membrane on theconversions was determined. Initial molar ratios ofethanol/acetic acid were selected as 1 and 1.5.Batch reactor without membrane and batch perva-poration membrane reactor used are illustrated inFig. 1. The membrane cell was maintained at con-stant temperature by a heating jacket. Reactionmixture was stirred by a mechanical stirrer duringthe pervaporation. The membrane was supportedon a perforated stainless steel disk with a holediameter of 5 mm. Two pairs of teflon o-ringsbetween flanges provided the vacuum seal. Thepressure at the downstream side was kept atapproximately ≤ 1 kPa. During the pervaporationruns product samples were taken from the collec-tion tubes, and analyzed every 50 min. Permeatecompositions were determined by using a Shi-madzu GC 9A model gas chromatograph equippedwith a TCD detector, using Poropak T80/100 col-umn with dimensions of 6′ × 1/8″. The oven tem-perature was kept at 180°C. Helium was used asthe carrier gas. Effective membrane area was 13.25cm2, while the feed mixture was 100 mL giving amembrane area/volume ratio of 0.1325 cm–1.

3. Results and discussions

The esterification reaction was performed bothwith and without membrane at two differentethanol/acetic acid ratios (M = 1 and 1.5) and three

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different temperatures (50, 60 and 70°C) in thepresence of Amberlyst 15 catalyst, where 5 gAmberlyst 15 /100 g acetic acid was used. Fig. 2compares the variation of conversions with time fordifferent temperatures at equimolar condition (M =1) in the presence of Amberlyst 15 catalyst. It canbe seen that the conversions determined using the

batch pervaporation membrane reactor are slightlyhigher than the conversions obtained using thebatch reactor without membrane. Fig. 2 shows thatconversions increase with temperature. Tempera-ture has a strong influence on the performance ofPMR because it acts on both the kinetics of esteri-fication and pervaporation [6]. An increase in

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Fig. 1. (a) Batch reactor without membrane; 1–2: cold water outlet and inlet of the condenser, 3: condenser, 4: temper-ature sensor, 5: sample inlet, 6: circulator, 7: water bath, 8: magnet, 9: magnetic stirrer (b) Batch pervaporation membranereactor; 1–2: cold water outlet and inlet of the condenser, 3: condenser, 4: stirrer, 5: temperature sensor, 6: sample valve,7-8: hot water inlet and outlet of the membrane reactor jacket, 9: membrane, 10: vacuum gauge,11: vacuum pump,D1,D2,D3: Dewar containers [4].

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temperature induced accelerations of both esterifi-cation reaction and pervaporation separation. Fur-thermore, the ester content increased much fasterat the higher temperature, however the ethyl acetatecontent in the reaction mixture decreased abruptlybecause of membrane permeation flux increase,thus accelerating the esterification.

Fig. 3 compares the conversions at differenttemperatures when the ratio of ethanol/acetic acid

(M) was increased to 1.5 in the presence ofAmberlyst 15. The conversions obtained at M =1.5 were higher than the conversions obtained atM = 1. It is well known that an excess amount ofone reactant leads to increased conversions. Thusincreasing M to a reasonable value in PMRimproves the reactor performance.

Fig. 4 shows the fluxes of each componentthrough the membrane for different M values at

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 100 200 300 400t (min)

x

50°C M = 1 without membrane50°C M = 1 membrane reactor60°C M = 1 without mebrane60°C M = 1 membrane reactor70°C M = 1 without membrane70°C M = 1 membrane reactor

Fig. 2. The variation of conversion with time at different temperatures in the presence of Amberlyst 15 (M = 1).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 100 200 300 400

t (min)

x

50°C M = 1.5 without membrane50°C M = 1.5 membrane reactor60°C M = 1.5 without membrane60°C M = 1.5 membrane reactor70°C M = 1.5 without membrane70°C M = 1.5 membrane reactor

Fig. 3. The variation of conversion with time at different temperatures in the presence of Amberlyst 15 (M = 1.5).

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60°C in the presence of Amberlyst 15. As can beseen, the fluxes of ethyl acetate through PDMSmembrane are much higher than those of othercomponents. This is not unexpected because thesolubility parameter of PDMS (δ

PDMS= 8.1

(cal/cm3)0.5) is closer to ethyl acetate (δEAc

= 9.1(cal/cm3)0.5) than the other components [7]. Thus,PDMS is more selective to ethyl acetate than othercomponents. The solubility parameter is a meas-ure of the affinity between polymer and penetrant,and can give at least a qualitative informationabout interaction between polymer and penetrant.As the affinity between permeant and polymerincreases, the amount of liquid inside the polymerincreases, and consequently the flux through themembrane increases [8]. As can be seen in Fig. 4,ethyl acetate fluxes at M = 1 are higher than at M= 1.5. Since the fluxes increase with feed concen-tration in a pervaporation process, the EAc fluxesat equimolar condition are higher because ethylacetate concentration at M = 1 is much higher thanat M = 1.5 at a given time.

Experiments were also carried out in thepresence of sulfuric acid at 60°C, where 1 g sul-furic acid/100 g AAc was used. Fig. 5 shows thevariation of conversions with time at two differ-ent molar ratios in the presence of sulfuric acidcatalyst at 60°C. As expected, conversionsincrease with increasing molar ratios. The con-

versions obtained with sulfuric acid are higherthan the conversions obtained with Amberlyst15 at a given time, as shown previously in Figs.2 and 3, indicating that the reaction is morerapid with sulfuric acid. It is well known thathomogeneous catalysts are usually more effi-cient than heterogeneous catalysts. Despite astrong catalytic effect, the use of homogeneouscatalyst, e.g., sulfuric acid, suffers from draw-backs, such as the existence of possible sidereactions, equipment corrosion, and having to

Fig. 4. Partial fluxes of each component through the membrane at 60°C in the presence of Amberlyst 15: (a) M = 1, (b)M = 1.5.

Fig. 5. The variation of conversion with time at 60°C inthe presence of sulfuric acid.

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Fig. 6. Partial fluxes of each component through the membrane at 60°C in the presence of sulfuric acid: (a) M =1, (b)M = 1.5.

Fig. 7. The variations of F with time at different temper-atures in the presence of Amberlyst 15 (M = 1.5).

deal with acid-containing waste. The use of ionexchange catalyst such as Amberlyst 15 holdsfollowing distinct advantages over homoge-neous catalysts in catalysis — the purity of theproducts is higher, as the side reactions can becompletely eliminated or are significantly less;the catalyst can be easily removed from thereaction mixture by filtration, and the corrosiveenvironment caused by the discharge of acid-containing waste is eliminated [9].

Fig. 6 represents the partial fluxes of compo-nents through the membrane at 60°C using sulfu-ric acid catalyst. The ethyl acetate fluxes aremuch higher than those obtained in the presenceof Amberlyst 15. Since the EAc production isfaster with sulfuric acid catalyst, EAc concentra-tions in the feed mixture are higher than thoseobtained with Amberlyst 15 catalyst. Thereforean increase in the production rate causes anincrease in the fluxes.

As for the esterification–pervaporation cou-pling process, the ratio (F) of rates of EAcremoval to EAc production is found to be the keyfactor defined as [10]:

(1)F J S Vc t

= ×EAc

EAcd d

/

/

Here, F is a dimensionless parameter thatstands for the interaction between EAc removaland production during the coupling process, andwhen F > 1, the rate of EAc removal is largerthan the production rate, indicating that the conversion could attain 100%, and limited bythe EAc production rate. Fig. 7 represents thevariations of F with time for the reaction in thepresence of Amberlyst 15 at different tempera-tures for M = 1.5. Fig. 7 shows that F increaseswith increasing temperature indicating that the

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Fig. 8. The comparison of the variations of F with time inthe presence of two different catalysts at 60°C (M = 1.5).

rate of EAc permeation is faster than the rate ofEAc production. Although both the EAc pro-duction rate and permeability coefficientincrease with temperature, EAc production ratechanges lesser as temperature increases, whilethe rate of EAc removal changes more as tem-perature increases.

Fig. 8 represents the variation of F with timein the presence of two different catalysts at thesame conditions (60°C and M = 1.5). The F val-ues obtained with sulfuric acid are higher thanthose obtained with Amberlyst 15. EAc produc-tion rate is higher with sulfuric acid than withAmberlyst 15 indicating that sulfuric acid is amore efficient catalyst than Amberlyst 15. EAcproduction rate increases with the efficiency ofthe catalyst, thus resulting in the increased EAcpermeation flux. When sulfuric acid was used asthe catalyst, the maximum EAc content in themixture was achieved in a shorter time. The per-meation of EAc was proportional to EAc contentin the mixture, and increased with the increasingconcentrations of EAc.

4. Conclusions

Based on the experimental results it can beconcluded that the PDMS membrane can be used

to remove selectively the ethyl acetate formed inthe esterification reaction to obtain relativelyhigher conversions by using PMR. Temperaturehas a strong influence on the membrane reactorperformance because it acts on both the kineticsof esterification and pervaporation. Although useof sulfuric acid instead of a hetereogenous cata-lyst such as Amberlyst 15 enhances the conver-sions, its corrosive action on the system andremoval from the reaction media present chal-lenging problems.

Acknowledgement

The financial support of YTUAF (25-07-01-06) is appreciated. The scholarship of TUBITAK-BİDEB for A.Hasanoğlu is also acknowledged.

Nomenclature

AAc acetic acidc

EAcconcentration of EAc (mol m−3)

EAc ethyl acetateEOH ethanolF ratio of the rate of EAc removal to

EAc productionJ

EAcpermeation flux of EAc (mol m−2 h−1)

M ratio of EOH concentration to AAcconcentration

S membrane area (m2)t time (minute)V volume of reaction mixture (m3)x conversion (AAc is the limiting reactant)

Greek

δ solubility parameter (cal/cm3)0.5

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