05 the production of butyl acetate and methanol via reactive and extractive distillation. i....

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The Production of Butyl Acetate and Methanol via Reactive and

Extractive Distillation. I. Chemical Equilibrium, Kinetics, and

Mass-Transfer Issues

Laureano J imenez,*, Alfonso Garvn, and J ose Costa-Lopez

Depart m ent of Chemi cal E ngi n eeri ng and M et al l ur gy, Un i versi t y of B arcel ona, c/ Mar t i F r a n q u e s 1,08028 B ar cel ona, S pai n

We studied the kinetic and chemical equilibrium of the transesterification of methyl acetatew i t h n-butanol in a batch st irred-tank reactor with a polymeric acid resin as a catalyst . Theaim of this work was to determine whether reactive and extractive dist illat ion is a promisingalternative for managing a byproduct from the poly(vinyl alcohol) process. The experiments wereperformed in concentration and temperature ranges similar to those predicted for operation.The entra iner (o-xylene) was observed to ha ve no influence on t he kinetics. Interna l a nd externa lma ss-tra nsfer resista nces were found to be negligible under t he operat ing conditions considered.The experimenta l result s w ere best described (a vera ge error of 2%) by a pseudohomogeneousmodel with a first-order dependency on the reactants. The influence of the temperature wasmodeled wit h t he Arrhenius equa tion. The forwa rd a nd reverse kinetic constan ts w ere consistentwith the chemical equilibrium va lues.

Introduction

The use of unit operat ions separa tely a nd consecu-tively to achieve cost-effective conversion requires highreflux rat ios a nd prohibitive recycle flow ra tes. Reactiveseparation processes such as reactive distillation, reac-t iv e abs o rpt ion , an d re a ct iv e c ry st a l l iza t io n h av e a t -tracted growing interest as promising alternat ives inboth industrial applications and scientific research. Thispaper focuses on reactive distillat ion, a technology th atis particularly suited for reversible reactive systems inwhich chemical equilibrium limits the conversion. Other

adva nta ges are tha t side react ions can be bypassed, thelimitations of azeotropic mixtures can be overcome, hot-spot problems can be avoided, and the heat of reactioncan be u s ed for s elect iv e p rodu ct re mov a l .1 Thesesynergist ic effects mean that react ive dist il lat ion canhave significant economic advantages over a conven-t ion al de sig n . Alt h ou g h re act iv e dist i l la t io n is n otadvantageous in every case (e.g. , react ion rates mustbe s imilar t o t h o s e in a re a ct o r a t p res s u res s u i t ablefor distillation), it is commonly used for etherificationand esterification reactions, although it has been fruit-fu l ly ap p l ie d t o a lk y la t io n , n i t ra t io n , an d a midat io nprocesses. Reactive distillation has been proposed formethy l a ceta te (MeAc) hydr olysis,2,3 an d it is used as a

model for research. A very useful designers checklisthas been published by Hoshang a nd Fa ir.4 In a ddit ion,a c omp le t e l is t of re fe ren ce s , c la s s if ie d as p at e n t s ,t h e rmody n amic p ro pe rt ie s, an d cas e s t u die s c an befou n d in t h e w ork by H a u an . 5

Problem Statement

We developed a new process to recover MM20, abyproduct from the poly(vinyl alcohol) industry. On onehand, a byproduct processing plant is more elaboratean d cos t ly t oday t h an e ve r be fore , be cau s e q u al i t yrequirement s ar e severe. On the other ha nd, a r ecoverysystem is a business opportunity.

MM20, a mixture of MeOH a nd MeAc rich in MeOH(30 wt % MeAc), is first converted into MM80 (80wt % MeAc, 0.05 wt % maximum water content , and0.05%ma ximum a cidity). The objective is to process theMM80 by r eact ive dist illat ion w ith n-butanol (BuOH)

an d obtain h igh-purity MeOH a nd buty l acetat e (B uAc).Despite volat ile organic carbon legislat ion, B uAc con-sumption is expected to grow in th e near future.6 Th eoverall process can be represented by

The problem is that the system simultaneously has alow chemical equilibrium extent and several azeotropes.T o e l imin at e aze o t ro p e s , t wo t e c h n iq u e s a re wide lyu s ed: p res s u re s win g dist i l la t io n , wh ic h c h a n g e s t h esystem composition and moves the distillation bound-ar ies, or entra iners, which modify the relative volat ility.I n ou r cas e , t h e s o lve n t g o al a ims t o bre a k t h e a ze o-tropes a nd improve conta ct between the reacta nts in t he

rea ctive section of the rea ctive and extra ctive distilla tion(RED ) unit .

No kinet ic or chemical equilibrium da ta were a vail-able in t h e l it e ra t u re , an d a l t h o u gh t h e y c a n be e st i-mated from group contribution methods,7,8 experimentaldat a are s t ron g ly re comme n ded, p a r t icu la r ly wh e nproduct specifications are based on maximum impuri-ties.

The literat ure9,10 shows tha t heterogeneous cata lystssuch as sulfonated macroporous ion-exchange resinsaccelerate esterificat ion. Resins provide products ofcon s t an t q u al i t y a n d min imize wa s t e wat e r an d corro-

* To whom correspondence should be addressed. Tel.: +34-977-559617. Fa x: +34-977-559667/21. E -ma il: ljimenez @etseq.urv.es.

Pr esent a d d r ess: Depar t m ent of Chem ical Engineer ing,ETSE Q, Un iversity Rovira i Virgili, Av. dels Pa sos Cat alans26, 43007 Tarr agona , S pain.

MeAc + BuO H T BuAc + MeOH

6663Ind. Eng. Chem. Res. 2002, 41 , 6663-6669

10.1021/ie0107643 CC C: $22.00 2002 American C hemical SocietyP ubl ish ed on Web 11/16/2002


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sion problems. The ma in dra wba cks ar e the low therma ls t abi l it y , t h e n e ed for c a t a ly s t co n t a in ers t o imp rov emechanical properties, and the possibility of diffusionproblems. Resins are also susceptible to both short-termpoisoning and long-term deactivation.

Th e f irs t p ar t of t h is p a p er focu s es on ch e micale q u il ibr ium an d k in et ic as p e ct s , u s in g a s imilar ap -proach to t ha t of Popken et a l.11 The d esign of rea ctivedist il lat ion is currently based on expensive and t ime-con s u min g s eq u e n ce s of la bora t ory an d p ilot -p lan texperiments. The second pa rt 12 presents th e RED unitand the solvent recovery system design and dynamicmodeling. Recently, Castor et al . 13 an d Po dre barac e ta l. 14 ha ve used similar approaches for va rious a pplica-tions.

Experimental Work

Chemicals. E . M e rc k ( D a rms t adt , G e rma n y ) s u p -p l ie d t h e c h e mic a ls . M e O H a n d B u O H we re U v as o lspectroscopy-grade (purity > 99.9 wt %), and MeAc an dBuAc were high-purity (purity > 99.8 w t %). o-Xylene(purity > 99.0 wt %) a n d N,N-dimethylformamide(purity > 99.0 wt %) were dist illed tw ice in a packedcolumn, a nd t heir f inal purit ies were 99.6 an d 99.8 wt%, respectively. The subst a nces were dr ied w ith U nionCa rbide 3 m olecular sieves, provided by Fluka AG(Buchs, Switzerland). All purities were checked by gaschromatography.

Catalyst. Amberlyst 15,13 a sulfonic ion-excha ngeresin (exchang e capa city of 4.81 molH+kg-1), manufac-t u re d by R oh m & H a as (P h i la de lph ia , PA), w as u s edas c a t a ly s t . B e fore t h e e xp erimen t s , t h e ca t a ly s t w asdried a t 95 C for 24 h t o re mov e t h e wa t e r f rom t h epores. Under these conditions,15 the equilibrium mois-ture is less tha n 1 w t %, a nd t herefore, no hydrolysisreaction occurs.

Apparatus. Figure 1 shows a schemat ic diagra m ofthe setup. The experiments were conducted in a stain-less steel st irred-tank reactor (FC-3 model, 300 mL)manufa ctured by P ressure Products Industries (Warm -ister, PA). The unit was equipped with a turbine motorand a digital ta chometer speed controller (Dyna /Magmodel MM-016-06). The temperature was measuredwith PT-100 thermometers (accuracy of (0.1 K) a n dcontr olled by a P ID w ithin (0.2 K w ith a potentiometer.The pressure indicat or wa s a digital ma nometer ((0.1

kPa). A filter (45-m) was u s ed t o p rev en t an y of t h ecatalyst from being dragged into the external recycle.The external pump was a Tuthill series D model (Alsip,IL).

Analysis. The ana lyses were car ried out in a Hewlett-P ackard (P alo Alto, CA) 5890 Series II P lus gas chro-ma tograph, equipped w ith a n FI D a nd electronic pres-sure control. The capilla ry column w a s a Supelco 2-4159column with a 1-m P TE-5 film (30 m, 0.0032-mm i.d.)wi t h h e l ium a s t h e c arr ier g a s .

Procedure. The experiments were conducted in athermostated batch reactor that was overpressurized (10at m) to mainta in all of the chemicals in the liquid phase.

To fix th e st art ing point of t he kinet ic experimentsprecisely, the following procedure was used. First, thereactor (A) was filled with the solvent, the nonreactivemixture, an d the cata lyst . Then, the system wa s heatedto the temperatur e set point, a nd the pressure wa s fixedat a rou n d 3-4 at m under t he working condit ions withnitrogen (B ) by C1. When th e required temperat ure wa sreached, the three-wa y va lve (D) wa s switched, and thepressure increased; the nitrogen passed through C2 a ndpushed t he rea ctant (E) into the reactor. The externalrecycle stream was cooled with a heat exchanger (F)

before it entered the pump (G) to prevent any flashingat t h e s amp le p o in t ( H ) . T h e re s ide n c e t ime in t h eexternal recycle loop (line F-G-H ) was les s t h a n 1 .5min, and this influence was neglected compared withthe long kinetic and chemica l equilibrium experiments.Th e in it ia l comp os i t ion wa s k n own from t h e f irs ta n a l ys is . S a m pl es w e re t a k en ev er y 5 m in a t t h ebe g in n in g o f t h e e x p e rime n t s an d e v e ry 20-30 minthereafter (8-12 s a m p le s p er t e st ) t o m on i t or t h eprogress of the react ion. The amount of sample wasaround 1-2 mL. TIC, P I , TI, a nd S I (I) controlled a ndmonitored the systems opera t ing varia bles.

Experimental Plan. Various experiments were car -ried out st ar ting from different compositions: (a ) a lco-

hol, ester, and entrainer mixtures, because forward andbackward reactions occur simultaneously; (b) mixtureswit h MM80 a s the feed; and (c) mixtur es with differento-xylene concentrations, because of the possible influ-ence of o-xylene on catalyst activity.

T h e t h e rmal s t abi l i t y o f t h e c a t a ly s t re s t r ic t s t h eoperating temperature to 90-95 C. P reliminary simu-lat ion results predicted that temperatures in the REDunit should be higher tha n 50 C to achieve measur a blereact ion ra tes.

The kinetic experiment s la sted for 5-6 h (1.5 wt %cata lyst), w hereas th e equilibrium experiments lastedfor 9-12 h (6.5 w t % cata lyst). Over 40 equilibriuman d 25 kinetic experimental run s were ma de to evalua tet h e e ffect s of ca t a ly s t con ce n t ra t ion , s t i rrer s p ee d,catalyst size, temperature, and chemical concentration.

Results

During the experiments, no loss of cata lyt ic act ivityor dam a ge by sw elling forces wa s observed. A set of fiveexperiments wa s performed wit h the same cata lyst, andno difference in activity was observed. Nevertheless, thecata lyst w as discar ded after each experiment. The molaryields calculated from the a na lytical results for replicat ee xp erimen t s a lwa y s ag re ed t o w i t h in a few p erce n t((3%). E quilibrium a nd kinetic expressions were de-rived separately, and their coincidence was used as aglobal consistency test.

Figure 1. Schematic diagram. A, batch st irred-tank reactor; B,nitrogen cylinder; C1 and C2, f low paths; D, three-way valve; E,container; F, heat exchanger; G, pump; H, sample point; I , controlmonitor.

6664 Ind . En g. Ch em. Res., Vol. 41, No. 26, 2002


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Estimation of Physical Properties.The activitycoefficients were estimated by NRTL,16 wit h t e mpe ra-ture-dependent interact ion parameters (eqs 1 and 2).Vapor-phase nonidealit ies were calculated using H ay-den and OConnell s method.17 An y o t h e r p arame t e ru s ed w as re t r ie ve d f rom t h e As pe n P L U S D a t a ba s e.18

No VL E dat a wa s fou n d in t h e l i t era t u re fo r t h e k e ysystems (except MeAc + B u O H ), a n d T-x-ymeasure-

ments were performed.19,20 The para meters ar e shownin Tables 1 and 2.

wh e re Gij, Ri j, i j, ai j, a n d bi j are the interact ion param-e t ers in t h e NR TL mode l. We fou n d e xp erimen t a levidence of pseudoaz eotrope behavior for t he meth a nol+ o-xylene system at high methanol composition (notpredicted by UNI FAC). This a spect is a ddressed in m oredetail in part I I of this paper. 12

ResidueCurve Map Analysis.Residue curve ma ps(RCMs) ha ve been successfully applied to complexnonideal separat ion systems and provide valuable in-sights an d design assista nce for a variety of separ at ionp roce ss es . R CM s a re bas e d s olely on t h e s y s t emsphysical properties: vapor-liquid equilibrium, liquid-liquid equilibrium, an d solubility dat a. The number an dtype of singular points is unknown a priori . The tem-perature always increases along a residue curve line,an d t h e s in g u lar p o in t s are e i t h e r n o de s ( s t able o runsta ble) or sad dles. The role of th e singular points canbe assigned using Doherty a nd P erkins rules,21 an d t h etopology for t he w hole composition spa ce ca n be st a ted.This makes RCM a promising technique in the earlypha se of development of a ny project. Aspen SP LIT22 w a sused to compute the RCM.

An accurate analysis of the quaternary nonreact iveR C M d ia g r a m (F i gu r e 2 ) r ev ea l s t h a t t h er e a r e t w odistilla tion r egions. The MeOH + MeAc a zeotrope a ctsa s a n u n s t a b l e n o d e , a n d B u A c a n d B u O H a r e b o t hs t able n ode s, w h e rea s M eO H a n d t h e B u O H + B uAcaze ot ro pe are bot h s addles . An y of t h e fou r feas ibledist i l la t io n s e q ue n ce s de t ect e d lea d t o t h e de siredsepar at ion. These two a spects show tha t i t is advisa blet o u s e e i t h e r a bou n dary -cros s in g s t ra t e g y or an e n -tra iner to separa te the products.

Extractive Distillation.Solvent selection is the keyfactor in RED , as th e entr a iner objective is to break t heazeotropes and t he solvent recovery syst em ha s consid-

era ble influence on t he process performa nce.23 The huge

number of possible solvents has led to some previousselections being made on the basis of heuristics.2,24

The most commonly accepted parameter for solventselection is selectivity.25 The higher t he selectivity, thebetter the solvent . Different entrainers are comparedby con s ide r in g t h e s i t u at ion a t in fin it e d i lu t ion , a ss t a t e d in e q 3

Experimenta l work using headspace gas chromatogra-phy was done for alcohol + ac e t a t e + e n t ra in er s y s -tems.26 The S

m, S

criterion is useful for clustering thesolvents into different gr oups, but a definitive selectioncriterion cannot be stated. The experimental resultss h o w t h at t h e be s t e n t ra in e rs are a lk y lbe n ze n e s a n dalkanes. Also, the importance of peripheral properties(e.g., sa fety, cost , density), th at is , propert ies tha t ar eof interest when select ing a solvent but that often donot directly affect the separation, was discussed. Whenal l o f t h e s e c o n s ide ra t io n s an d p arame t e rs h ad be e nweighed, o-xylene w as selected a s the best alterna t ive.Figure 3 shows th e MeOH + MeAc + o-xylene pseudo-binary dia gram . This diagra m w as used to compute theminimum entra iner concentra tion required to break th ebinary azeotrope. Moreover, an analysis of the spacecomposition of th e five-component syst em resid ue curve

Table 1. Binary Parameters for the NRTL ActivityCoefficient Model

system b12(K ) b21(K ) R12

MeAc + B u O H -3897.4 1567.7 0.30MeOH + o-xylene 447.17 516.01 0.30B u O H + o-xy lene 130.32 387.12 0.30BuAc + o-xylene -18.438 63.604 0.30

Table 2. Solvation and Association Parameters for theHayden and OConnell Method

MeOH MeAc B uOH B uAc o-xylene

MeOH 1.63MeAc 1.30 0.85B uOH 1.55 1.30 2.20B uAc 1.30 0.53 1.30 0.53o-xylene 0.00 0.60 0.00 0.60 0.00

Gi j) exp(-Ri ji j) (1)

i j) ai j+bi j

T (2)

Figure 2. Nonreact ive RCM for the MeAc + B u O H + BuAc +MeOH system at 101.3 kPa.

Figure 3. I nfluence ofo-xylene on th e vapor-liquid equilibriumof the MeOH + MeAc system 101.3 kPa . xi

/

) xi/jxj, i, j) MeOH,

MeAc (jxi/

)1).

Si j,S

)i,S

j,S

(3)

Ind. Eng. Chem. Res., Vol. 41, No. 26, 2002 6665


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map reveals that there is just one dist il lat ion region,wh e re o-xylene a nd the MeOH + MeAc azeotrope arethe st able a nd unsta ble nodes, respect ively.

Entrainer Influence. Amberlyst 15 wa s selected a sthe cata lyst because of its capa city t o work in a nhydrousand nonpolar conditions. The sulfonic groups give theresin a strong aff inity for polar molecules, which arepreferentially adsorbed, and inhibit the adsorption ofo-xylene. Moreover, a s MM80 is wa ter -free, th e possiblereduction of the ava ilable cat alyst act ive sites is mini-mized. Neither the chemical equilibrium nor the kineticresults shows dispersion depending on the initial com-position.

External and I nternal Mass Transfer Effects.The reacta nts a nd products must be tra nsported th rough-out the catalyst pores and inside the gel beads where95%of the acid sites a re loca ted.27 Diffusion phenomena,therefore, can ha ve a n enormous effect on t he results .The reactions are slightly endothermic, and t he cata lystpart icles ca n be considered to be essentially isotherm al.T h e h e at o f mix in g was a ls o n o t fo u n d t o h av e an yinfluence.

Three full sets of experiments were carried out withdifferent a mounts of cat alyst (Figure 4), different a gita -tion velocities (Figure 5) a nd d ifferent sieved fra ctionsof the catalyst (Figure 6). All experiments were per-forme d a t 80 C a n d 10 bar . F ig u re 5 sh ows a min orinfluence of the st irring speed, but the discrepancy isin t h e ran g e o f t h e e rro r o f t h is t e c h n iq u e . F ig u re 6shows tha t , for high o-xylene concentrations (thus, lowconversion), no diffusion problems were found. The mainconclusion is tha t the external an d/or interna l ma ss-

tra nsfer diffusion exerts no influence. As a secondaryconclusion, we can st at e tha t t he reproducibility is veryhigh. A qualitat ively theoret ical verif icat ion of mass-transfer resistance was made using the dimensionlessB io t n u mber an d t h e Th iele mo du lu s .28 The resultsprovided by the Biot number confirm tha t the tra nsport

ac ro s s t h e l iq u id-s ol id in t er fac e is ov er 1 orde r ofmag n it u de larg e r t h a n t h e t ra n s port in s ide t h e p ore sof the catalyst . Also, the low Thiele modulus values( be t we e n 0.058 a t 60 C an d 0 .363 a t 90 C) h ad acorresponding effect iveness factor that is virtually 1(>0.95). In a ccorda nce with t he results fr om this section,in the subsequent experimental w ork, we used unsievedcata lyst par t icles and a st irring speed of 1000 rpm.

Side Reactions. Th e v a lu es of t h e e q u il ibr iumconstants were est imated using Aspen PLUS (AspenTechnology Inc., Cambridge, MA), by minimizing theG ibbs free energy. As all chemicals w ere wa ter-free, theconversions caused by hydrolysis were expected to beinsignifican t . G as chroma tographic ana lysis confirmedtha t th e forma tion of such byproducts a s dimethyl ether

by dehydration of MeOH or acetic acid by hydrolysis wasnegligible (less th a n 0.01 wt %).

Chemical Equilibrium Experiments. For a homo-g e n e o u s s y s t e m wit h an e q u i l ibr iu m re ac t io n in t h eliquid phase, the equilibrium constant can be writ tena s28

Therefore, the equilibrium constant can be expressedas the product of the activity coefficient constant (K),the liquid molar fraction constant (Kx), and t he fugacityconstant (Kf). Although the entra iner has a significan t

effect on the activity coefficients and equilibrium tem-peratures, its influence on the activity coefficient con-sta nt is negligible (Ta ble 3). As expected, the Kf v a r i a -t ion w a s i ns ig ni fi ca n t , b ut i t w a s s t il l t a k en i nt oconsideration in the calculations.

The chemical equilibrium constant under standardcondit ions can be derived from the Gibbs free energy,yielding t he equa t ion

Equation 5 is valid assuming that the heat of react ionis constant over the temperature range (experiments

Figure 4. In fluence of the am ount of cata lyst on the B uAc rateof formation.

Figure 5. E f f e ct of t he s t irr ing s p e ed on t he B u Ac ra t e offormation.

Figure 6. I mp a ct of t he c a t a ly s t p a r t ic le s iz e d ia me t er on t heBuAc rate of formation.

Ka )iixi

i[fi

fio]

i

) K

KxKf (4)

ln Ka )S

o

R -

Ho

R

1T

(5)



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we re p e rfo rme d e v e ry 5 C be t we e n 50 a n d 90 C) .Analyses with liquid temperature-dependent heat ca-pacities were performed (the para meters w ere retrieved

from t h e As pe n P L U S dat a ba s e ). U s in g t e mpe rat u re -dependent heat capacit ies or consta nt heat capacit ies,the model predictions fit the da ta very well (r2 ) 0.9968a n d r2 ) 0.9993, respectively), because the heat capaci-t ies of the reacta nts a nd products a re almost consta nt .Ho values (10 510 ( 141 an d 11 570 ( 149 J mol-1,respectively) a nd So va lues (29.25 ( 0.49 a nd 31.22 (0.51 J mol-1, respect ively) were coincident , and thesimpler expression wa s preferred. F igure 7 shows t heKa dependence on temperat ure. For exam ple, the theo-retical standard heat of reaction is 4800 J mol-1,18 aboutthe sam e order of ma gnitude as t he experimenta l value,although the experimental error associated with thistechnique can ha ve values around 1 kJ mol-1.

Eq uilibrium conversion sta rt ing from different feedrat ios was completed. An iterat ive method that com-puted calculat ions with Aspen P LUS in the inner loop(ac t ivi t y an d fu g aci t y coe ff icien t s ) w a s u s ed, wh i leconcentra tion calculations w ere performed in the outerloop. The equ ilibrium conversion for th e MeOH + MeAcaz eotrope and stoichiometric BuOH ra nged between 31and 36% over t he t empera ture ra nge st udied. Theseresults confirms that RED can be used to circumventthe chemical equilibrium constra int .

Kinetic Experiments. We evaluated the effects oft h e t e mp e rat u re , t h e re a g e n t c o n c e n t ra t io n , an d t h eentra iner on the rea ction ra te (Ta ble 4). Figure 4 showstha t t he react ion ra te is proport ional to the a mount ofca t a ly s t an d t h a t t h e re a c t ion ra t e c a n be a f fe ct e d by

u s i n g t h i s a m o u n t a s t h e b a s i s . T h e S P S S s o f t w a r epackage capabilities29 were used to car ry out th e regres-sions.

The tools and regression techniq ues used (generalizedreduced gradient) were not able to simultaneously f i ta l l o f t h e p arame t e rs o f t h e L an g mu ir-Hinselwood-Hougen-Watson (LHHW) equat ion, no matter whichadsorption mechanism was selected.28 Different sets ofabsorption kva lues w ith different orders of magnitudeexhibit approximately the same error. This situation canbe part ially justified for coupling effects in t he differentadsorption terms.

The pseudohomogeneous model with a second-ordere xp res s ion (e q 6) was in g ood ag re eme n t wi t h t h eexperimental data. The power-law model did not sig-nificant ly improve the predict ive capabilit ies, as theupgrade does not just ify the need for four addit ionalparameters. Some authors 3,4,6,14,27 h av e a lre ady dra wn

similar conclusions for the hydrolysis, esterification, andtransesterification of alcohols and acetates. We assumedan Arrhenius-type temperature dependence for both thedirect an d reverse constants.

Experimenta l profiles were obta ined (Table 4) for eachexperiment. For th e sake of illustra tion, Figure 8 showst h e r u n s a t 8 0 C w i t h e x p e r i m e n t s i n w h i c h b o t hforw ar d a nd reverse reactions prevail. At 50 C , becauseof the small absolute value of the react ion rate, slightdeviat ions lead to high relat ive errors, and therefore,these dat a sets w ere not used. Figure 9 shows t he direct

Table 3. Activity Coefficient Equilibrium Constant atDifferent T emperatures at 10 atm

MeAc B uOH B uAc MeOH o-xylene

xeq (m ol) 0.1320 0.1976 0.0461 0.3698 0.2545eq 1.3156 1.1641 1.2959 1.4387 1.9687k 1.2174Teq(K ) 338.15xeq (m ol) 0.1682 0.1428 0.0637 0.2049 0.4204eq 1.1702 1.5234 1.0424 2.1177 1.3892k 1.2382

Teq(K ) 323.15xeq (m ol) 0.0896 0.0884 0.0532 0.1096 0.6592eq 1.2555 2.0367 1.0354 3.1378 1.1224k 1.2705Teq(K ) 353.15xeq (m ol) 0.2792 0.1716 0.1368 0.2954 0.1169eq 1.1891 1.2191 1.2101 1.4614 1.9686k 1.2198Teq(K ) 363.15

Figure 7. Dependence of the chemical equilibrium consta nt ontemperature for the t ransesterif icat ion of MeAc with BuOH .

Table 4. Experimental Runs for the KineticExperimentsa

set ca ta lyst (g) T ( C ) in it ia l m ixt ur e

1b 0.451 90 rea gent -r ich2 1.428 90 product -r ich3b 2.048 90 MM804 1.349 80 rea gent -r ich5 2.003 80 product -r ich6b 2.028 80 MM807b 2.323 80 MM80

8b

0.451 70 rea gent -r ich9 2.136 70 product -r ich10b 3.542 70 MM8011b 0.541 60 rea gent -r ich12 1.555 60 pr oduct -r ich13b 2.026 60 MM80

a C ond it ions : 10 b a r , 1000 rp m, a nd c omme rcia l c a t a ly s t .b Triplicate experiment.

Figure 8. B u Ac c onc e nt ra t ion p rof i le a t 80 C f or t he t ra ns -esterificat ion of MeAc with BuOH .

r ) k cMeAccB uO H - kcBuAccMeOH (6)



6/7

and reverse kinet ic f i t t ing for the set of experimentsconducted a t 90 C.

The regressed kinetic pa ra meters for the pseudohomo-g en e ou s mode l ( eq 6) a n d t h e de pe n den ce of t h ispara meter on tempera ture a re shown in Ta ble 5. Thesevalues follow the Arrhenius model, and the results areprovided in Table 6. It is notew orthy th a t, a lthough th eforwa rd and reverse react ions had similar coefficientsof determination (over 0.99 in both cases), the Fisherdistribution va lues (F) were significan tly different (13 517and 45 331, respectively). The F value is an indicatorthat considers the variance taken into account by themode l an d t h e n u mbe r of p a rame t e rs : h ig h F values

indicat e bet ter models.29

This factor a nd t he inherentproblem of regressing logarithmic va lues lead to highrelative errors in some parameter estimates (see Table7) . T h e v a lu e s fo r t h e e q u i l ibr iu m c o n s t a n t an d t h ekinetic parameters are similar to those found for thetra nsesterif icat ion of MeAc w ith ethanol.30

Conclusions

Reaction kinet ics, chemical equilibrium, and mass-transfer issues have been evaluated for the transesteri-ficat ion of MeAc with B uOH, w here the yield is stronglylimited by t he equilibrium conversion. The a dvant ageof using an a cid resin cat alyst is tha t very few byprod-ucts are formed. o-Xylene was identif ied as a suitable

extra ctive agent, a nd it w as used a s the reaction solvent.The opera ting conditions wer e such th a t th e control stepwa s the react ion at the cata lyst surface, an d therefore,the kinetic expression was considered to be essentiallyrepresentat ive of th e intrinsic rat e of react ion a nd freeof mass-transfer effects. The pseudohomogeneous ki-netic model proved to have very good predictive capa-bilit ies (average errors ar ound 2%). Equilibrium an dkinetic results were consistent (Ka k/k). The effectsof temperature and composit ion were studied so thata n expression could be developed to model the RE D un itin part I I of this paper. 12

Acknowledgment

The authors than k DGI CYTa nd CIRIT for providingthe necessary facilit ies and A. Destro (UTN, Panama)and R. P eis (UTM, G ermany) for car rying out some ofthe experiments. S ome of t he a uthors (L.J . a nd A.G.)gra tefully acknowledge the financial support from Fun -da cion Ca ja de Ma drid.

Notation

aij ) interaction parameter in the NRTL modelA i )preexponential factor, L molmin gca t-1

bij ) interaction parameter in the NRTL model, K-1

ci )molar concentrat ion, molL-1

Ea ) activation energy, kJ mol-1

F )Fisher distribution va lue (significance )95%)fi )fugacity , P aGi j) interaction parameter in the NRTL modelG)Gibbs free energy of formation, J mol-1

H )enthalpy, J mo l-1

Ka )chemical equilibrium consta ntMM20 )byproduct from the poly(vinyl alcohol) processMM80 ) azeotropic mixture of MeOH + MeAc from the

poly(vinyl alcohol) processr ) reaction rate, molL-1min-1gca t-1

RC M)

residue curve ma pRE D ) reactive and extractive distil lationrpm )revolutions per minut eSi )selectivit yS)entropy, J mol-1K-1

T ) temperature, K or Cxi, yi ) liquid/va por pha se molar fr a ctions

xi/, yi

/

) l iquid/vapor phase pseudobinary molar fractions

Greek Symbols

Rij ) interaction parameter in the NRTL modeli) liquid-phase activity coefficienti j) temperature-dependent interaction parameter in the

NRTL modeli )st oichiometric coefficient

Subscripts and Superscripts

i, j ) it h a n d jth components, respectivelym )multicomponento )st an dar d sta te/conditionsS ) in the presence of solvent ) infinite dilutions ) mean value ) reverse reaction* )pseudobina ry basis

Literature Cited

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Figure 9. Fit of forward and reverse kinetic constants a t 90 Cfor the tra nsesterificat ion of MeAc with BuOH .

Table 5. Temperature Dependence of the Forward andReverse Kinetic Constants

T

( C )k

(Lmol-1min-1g ca t-1)k

(Lmol-1min -1gca t-1)

60 1.044 10-3 ( 4.55 10-6 1.145 10-3 ( 2.49 10-5

70 2.283 10-3 ( 1.59 10-4 2.457 10-3 ( 2.74 10-5

80 4.563 10-3 ( 6.98 10-5 5.040 10-3 ( 3.13 10-5

90 8.989 10-4

( 6.58 10-5

1.001 10-2

( 1.30 10-4

Table 6. Arrhenius Constants for the Forward andReverse Reactions

A (L mol-1min -1g ca t-1) Ea (k J mol-1)

k 2.018 108 ( 2.1 107 71.96 ( 2. 7k 2.839 108 ( 4.6 107 72.67 ( 0.46

Table 7. Err or in the Estimation of theTransesterification Reaction Rate

T ( C) r (%) rma x(%)

60 1.13 2.4570 2.31 3.9280 2.43 5.6390 1.37 3.51



7/7

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Received for review September 12, 2001Revised man uscript received August 21, 2002Accepted September 19, 2002

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05 the production of butyl acetate and methanol via reactive and extractive distillation. i....

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