Bercic1992%2C MeOH to DME Global Rates

download Bercic1992%2C MeOH to DME Global Rates

of 6

Transcript of Bercic1992%2C MeOH to DME Global Rates

  • 8/3/2019 Bercic1992%2C MeOH to DME Global Rates

    1/6

    Ind. E n g . C h e m . Res. 1992,31, 1035-1040 1035Kautz , K.; Kirsc h, H.; Laufhiitte , D. W. Spu rene leme ntgeh alte inSteinkohlen und den daraus entstehenden Reingassaub en. VGBKraftwerkstech. 1975,55 (lo), 672-6.Knbzinger, H. Benetzung im fes ten Zustand-Ein neuer Weg zurHerstellung uon oxidischen TrEigerkatalysatoren; Dechem a:Frankfurt , June 1,1990.Linnros, B. The Crystal Structure of LiMo02Asz0,. Acta Chem.Scand. 1970,24, 3711-22.Pertlik, F. Structure Refinement of Cubic Asz03 Arsenolithe) withSingle-Crystal Data. Czech. J. Phys. 1978,B B , 170-6.

    Rade mach er, J.; Borgman n, D.; H opfengiirtne r, D.; We dler, G.;Hums, E.; Spitznagel, G. W. X-Ray Photoelectron Spectroscopic(XP S) Study of DeNO, Catalysts after Exposure to Slag Ta pFurnace Flue Gas. Appl. Catal. 1992, in press.Russell, A. S.;Stokes, Jr., J. J. Surface Area in D ehydrocyclizationCatalysis. Ind. Eng. Chem. 1946, 38 , 1071-4.Received fo r review May 13, 1991Revised manuscript received August 1, 1991Accepted October 14 ,1991

    Intrinsic and Global Reaction Rate of Methanol Dehydration over7-A1203PelletsGorazd BerEiEt and Janez Levec*JDepartm ent of Catalysis and Chemical Reaction Engineering, Boris KidriE In stitu te of Ch emistry, andDepartm ent of Chemical Engineering, University of Ljubljan a, 61 000 Ljubljan a, Slovenia, YugoslaviaDehy dration of methan ol on y-Al,O, was studied in a differential f ixed-bed reactor at a pressureof 146 kPa in a temperature range of 290-360 "C. A kinetic equation which describes a Lang-muir-Hinshelwood surface controlled reaction with dissociative adso rption of me than ol was foundto fit the experimental results quite well. Coefficients in the equation follow the Arrhenius and t hevan't Hoff relation. Th e calculated value for the a ctivation energy was found to be 143.7kJ /mol ,while calculated values for the h eat of adsorption of metha nol and water were 70.5 an d 42.1 kJ/mol ,respectively. Th e measured global reaction rates for 3-mmcatalyst particles were compared to thosecalculated by m eans of intrinsic kinetics and tran spo rt processes within th e particles. A reasonableagree men t was found when t he effective diffusion coefficients for reaction components were calculatedusing a parallel-pore model assuming that only Knudsen diffusion is important.

    IntroductionCatalytic dehydration of metha nol over an acidic catalyst(e.g. y-A1203)ffers a potential process for dim ethyl ether(DME)roduction, which is used as an alternative to freonspray propellants. In the M TG process, as has been de-scribed by Chang et al. (1978), he first reactor performssuch a reaction. Th e open literature provides no infor-mation on kinetic equations which can be used successfullyin designing a commercial reactor. From the pate nt lit-erature (Woodhouse, 1935; Brake, 1986) it can be con-cluded tha t reaction takes place on pure y-alumina andon y-alumina slightly modified with phosphates or t i ta-nates, in a tem perature range of 250-400 "C and pressuresup to 1043kPa. The kinetics of meth anol dehy dration onacidic catalysts has been studied extensively resulting indifferent kinetic equations. A sum mary of the pu blishedequations is presented in Table I. Most of the equations,i.e. eqs 4-9, have been derived from the experimentsconducted in conditions not found in an industrial reactor.Th e experim ents were mainly performed with m ixtures ofmethanol, water, and nitrogen at low vapor pressures.Since water produced during th e reaction considerablyretards th e reaction rate, the derived rate equations have,more or less, a semiempirical character and are n ot su itablefor the industrial reactor design, where reaction takes placea t high conversion levels. Th e outlet com ponent concen-tratio ns correspond to the equilibrium values. However,the rate equations (1)-(3) in Tab le I, which were derivedfor an acidic ion exchan ge resin asa catalyst and are basedon th e Langmuir-Hinshelwood (L-H) or the Eley-Rideal(E-R) mechanism, can be used for design purpo ses aftera reversible term is introduced into the driving-force term .The aim of this work wa s to determine an intrinsic rateequa tion which can be used t o model the global reaction

    t Boris KidriE Ins ti tute of Che mistry.* University of Ljubljana.o a a a - ~ a s ~ ~ ~ ~ ~ ~ s ~ ~ - i o ~ ~ $ o ~ . o o / o

    Table I. Summary of the Published Rate Equationsref eauation

    Kallo and CM1I2Knozinger, 1967 -rM = k + k2CwSinicyna et al., 1986; kKM2CM2Gates and -rM = (5)'VbJohanson, 1971 (1 + KMCM + K w C W ) ~Figueras et al., 1971 kKMCM'I2-rM = 1 + KMCM1/' + KwCw

    (7)'han et al., 1972 ~ K M C MSchmitz, 1978 -rM kl + kzCM W bRubio et al., 1980 -rM = klCM1lz kzCwl /z (9Y

    -rM = (1+ K M C M ) ~'Acidic ion excha nge resin as catalyst. bA lumin a or silica-alu-mina as catalyst.

    rates in a pilo t-plant reactor where 3-mm catalyst particleswere used. In order to calculate the global reaction ratethe effectiveness factor m ust be known. Since the intrinsickinetic equation is highly nonlinear the effectiveness factorcan be calculated only numerically.Experimental Section

    Catalyst. A Bayer SAS 350 -pAl,O, catalyst supportin the form of 3-mm spheres was employed as a catalyst.In order to avoid t he intrapa rticle resistances, spheres were0 1992 American Chemical Society

  • 8/3/2019 Bercic1992%2C MeOH to DME Global Rates

    2/6

    1036 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992S u p e r f i c i a l v e lo c it y , cm/s

    JZ T=340C, dP=0.87mm E L50.00 100.00 150.00 200.00 250.003.00 0.50' 1 1 ' 1 1 1 ' 1 1 1 ' 1 1 1 ' 1 ' " ' 1 ' " 1 ' 1 1 1 1 ' 1 " 1 ' 1 1

    \t55wCHsOH> T=360"C< I 10.30

    0.20 E1 ;L

    0.10 ,g8 ;

    I Fo.00 =E 0.00 !0.1 1Par t ic le s ize , mmFigure 1. Determ ination of experim ental conditions where externaland internal transpor t resistances could be neglected.crushed an d sieved. Th ree different particle sizes wereus ed 0.87,0.37, and 0.17 mm. Experimen ts showed th atwithin the particles of 0.17 m m in size the in traparticleresistances are negligible.Reactor. Th e experiments were carried out in a dif-ferential reactor (8-mm i.d.) in a temperature range of290-360 "C. Th e pressure was kept cons tant a t 146 kPa.Th e reactor was operated free of interparticle heat andmass resistances (Figure 1). Th e inlet concentrations ofreactants were varied between 15 and 90 mol % formethanol, and from 0 to 50 mol % for water. In this waywe simulated the conditions which are found at highmethan ol conversions in a commercial reactor. In fact, theproduced water strongly retarded the reaction rate. As aninert gas nitrogen was used. Th e tota l inlet volum etric flowrate wa s kept constant at 65.3 cm3/s. Methanol or amethanol-water m ixture was fed into th e reactor by meansof a Beckman 114M solvent delivery module; nitrogen flowwa s controlled by mean s of an M KS 246 flow controller.Methano l was evaporated in a specially designed evapo-rator t ha t consisted of a 5-m long SS coiled tube (1/4-in.0.d.). Th e temperatures of the ev aporator and reactor ovenwere controlled by microprocessor-based controllers andwere kep t with in *0.3 "C. In order to achieve the desiredconversions, 2-15%, th e mass of the catalyst was variedbetween 0.2 and 2.4 g. A datab ase of more tha n 400 pointswas obtained.Analysis. Th e analysis was performed by me ans of aHP 5890 GC connected to the reactor outlet. Th e GCconditions were as follows: a 230-cm X l/s-in. columnpacked with Porapak T (100/120 mesh). Th e carrier gasHe (the flow rate through th e column was 18mL/min andthrough the reference 25 mL/m in). Th e detector was atherm al conductivity detector (TCD) set in the low posi-tion. Th e oven temp erature was 110 "C for the first 17.5min, increasing a t a rate of 40 "C/min until 130 "C wasreached. Th e duration of the analysis was 25 min. A gassample (0.25 mL) was injected into the GC through asampling valve which was kept a t 110 "C. Th e reactantoutlet-gas compositions were d etermined by th e ca librationcurve for each component.Th e reaction rates were calculated on th e basis of th emeasured conversions, inlet flow rates of methanol, a ndth e mass of catalyst in th e differential reactor. Th eestimatederror for the rate was within &7% (BerEiE, 1990).Th e observed temp erature rise, due th e reaction progress

    in the differentialreactor, aried from 0 to 3 "C, dependen ton the outlet conversion and inlet concentration of m eth-anol. Th e inlet temperature was adjusted 90 that the meantemp erature in th e differential reactor was kept co nstantfor all experiments. Th e maximum systematic error(Massaldi an d M aymo, 1969) can be calculated when a nadiabatic operation of the differential reactor is assumed.A t measured conversions, the calculated systematic errorwas abou t 3%. Since th e reactor was not operated adia-batically, th e tru e error was even smaller.Results and Discussion

    Intrinsic Rate Equation. Th e published r ate equa-tions for dehydration of methanol t o DME over aluminaand acidic ion exchange resins are listed in Table I. It isevident that in almost all rate equations for dehydrationthe reaction rate is proportional to th e square root of themethanol concentration. Th is indicates that the dehy-dration reaction undergoes dissociative adsorption ofmethanol on th e catalyst surface. In the derivation of rateequations for th e dehydration of methano l, the L-H con-cept, which has been further developed by Yang andHougen (1950), was applied. With th e assumption tha tthe surface reaction is a controlling step and that thedissociative adsorption of me thano l on th e surface of y-A1203 s taking place, th e L-H model can be representedin t h e following form (BerEiE, 1990):

    k&M2(CM2- cWcE/K)-rM = (10)(1+ 2(KMCM)l/' + KWcW)4Th e adsorption term for DME in the denominator of eq10 was neglected since the DM E adsorption constant wastoo small compared to th e adsorption constants of meth-anol and water (Gates an d Johanson, 1971).Since the rates of dehydration with pure m ethanol ormethan ol-water m ixtures were measured in the differentialreactor, the concentrations of produced dimethyl etherwere low. Th e reversible term in eq 10 has negligible effecton th e value of driving force; therefore this equa tion canbe compared t o those listed in Table I, which all assumethe reaction is irreversible. Th e reversible term of th edriving-force term (eq 10) consists of three factors: con-centrations of water and DME, respectively, and theequilibrium consta nt which takes values from abo ut 7 to11 (a t conditions employed here). Th e mole fraction ofwater wa s varied from 0 to 0.5, while the mole fraction ofDM E formed during th e reaction course was always in t heorder of a few hun dre dth s (since methanol conversion islow). It was estimated that a t the experimental conditionsthe reversible term might change the value of the driv-ing-force term not m ore tha n 0.5% in a worst case (typi-cally less th an 0.1%). Since the following inequality ap-plies CM2>> CwCD/K, what is implied is that the driv-ing-force term is mainly determined by th e concentrationof methanol in th e feed and does not depend practicallyon the methanol conversion. On th e other hand, this termis importan t when equations are used for th e modeling ofan integrally operated industrial or pilot reactor (whereCw = CD and at the reactor outlet Cw > CM). In that casethe reversible term along th e reactor length increases andconsequently reduces t he driving-force term.T he following criteria were used to distinguish betweenthe kinetic equations shown in Table I: the obtainedconstan ts should be positive numbers, th e equation givesth e smallest values for least squares residuals, th e coef-ficients in the equation must follow the Arrhenius andvan't Hoff relation, and the equation should be applicablefor predicting behavior of an integral reactor.

  • 8/3/2019 Bercic1992%2C MeOH to DME Global Rates

    3/6

    Ind. Eng. Chem. Res., Vol. 31, No. 4, 992 1037Table 11. Residuals Obtained with Nonlinear Regression ofDehydration Rate Equations with Our ExperimentalResults"

    residuals at given T ("C)eq12345678910

    360 "C0.9090.6390.3807.5851.4525.6230.023*0.024*0.006*0.386

    340 "C0.77921.3070.6216.8621.8165.5650.021*0.019'0.00310.690

    320 O C0.4071.4990.5585.1791.9723.6700.037*0.034*0.015*0.567

    290 "C0.021b0.250b0.026bO.17gb0.004b0.013b0.021* b0.013*0.002*b0.02tjb

    "Values marked with an asterisk indicate that only data for aninlet mixture of MeOH-N2 were used. Experime nts were carriedout with mixtures of MeOH-N2 only.Equations 1-10 were compared to he experimental re-sults for each temperature, applying th e M arquardt non-linear regression method (Duggleby, 1984). Since, in th ebeginning, all nonlinear iteration procedures require ap-proximate values of parameters, some of the equationshave to be transformed into linear forms. With th e useof a linear a nd rob ust linear regression (Rousseeuw an d

    Leroy, 1987),we obtained values of parameters which weresubsequently used as starting values of parameters in t heMarqua rdt minimization algorithm. Convergencewa s fastand inde penden t of starting approxim ations of parametersin almost a ll equations; varying starting values of param-eters for a few orders of m agnitude have no t influencedthe results. Reaction rates were calculated with the m eanconcentrations of com ponents in th e differential reactor.Proportional weights were used. T he equations which dono t include th e term of water concentration were testedonly with th e data obtained by the methanol-nitrogen inletmixture. Results are summarized in Table 11. From thistable it is evident tha t eqs 1,3,and 10 have the smallestan d almost equal sums of least squares. Therefore, anadditional test was necessary. From Figure 2 i t can be

    L o ~~~ ' I I I I I V " 3 ~~~*~~ ' 91.56E-003 1 60EL003 1.64EL003 1 .6SEL0031 TFigure 2. Arrhenius plots for constants obtained by nonlinear re-gression for rate eqs 1,3, and 10.concluded tha t th e constants obtained for eqs 3 and 10agree reasonably well with th e Arrhenius an d van't Hoffequation and can therefore be considered as the mostfavorable rate expressions. With respect to the optimi-zation algorithms, eqs 3 and 10 are nondistinguishable,since they represent th e same target function; thu s

    m X I 2 - X 2 X , / K )(1+ 2 ( B x 1 ) ' / 2 + D X & 4 (11)

    In t he case of e q 3, coefficient A is equal to th e productof coefficients A and B obtain ed for eq 10. CoefficientsB an d D have the same values as is evident from Table III.Small differences are obtaine d by neglecting th e reversibleterm in th e case of eq 3. On th e basis of statistica l criteriaalone, we canno t distinguish between th e L-H mechanisman d th e E-R mechanism for dehydration of methanol onyA1203. urtherm ore, when calculated values of th e ac-

    Y =

    Table 111. Optimum Parameter Set for Tested Equations Obtained with Nonlinear Regression (NLR) and Starting Values ofParameters Obtained with Linear (LR) and Robust Regressions (RR)k, kmol(kg/h) KM, 3/kmol Kw, a/kmol

    T,"C eq NLR LR RR N LR LR RR NLR LR RR320 1 5.4 6.0 4.6 52.6 34.2 54.0 45 3.9 384.2 386.93 12 167 11957 13415 1000.4 1077.2 1333.6 455.4 447.6 479.010 11.7 11.1 10.1 884.6 1086.2 1346.0 418.4 449.1 480.1340 1 13.1 25.1 15.8 50.4 10.5 25.3 463.1 286.4 360.43 23 186 14 468 13371 722.2 425.4 359.2 414.1 305.8 298.9 .10 31.9 33.9 37.1 551.3 429.9 362.7 355.4 307.0 299.9360 1 33.6 61.7 30.1 25.3 7.5 25.5 301.6 224.6 283.03 31 753 23 193 23 263 422.9 290.3 290.5 269.4 220.2 221.510 73.5 79.6 78.3 358.3 293.7 305 .5 243.2 221.0 226.0

    Table IV. Summary of Published Values for Activation Energy and Heat of Adsorption for Methanol and Water on VariousCatalystsE,, kJ/mol HM, J /m o l Hw, J/mol T, "C catalyst ref108.3" 160-195 alumina Kallo and Knininger, 1967120.P 300-350 alumina Rubio et al., 1980138.9* 125.5 20.9 150-225 silica-alumina Bakshi and Gavalas, 1975114.2b 127.6 111-150 ion exchan ge resin Than et al., 1972123.0 Th an et al., 1972

    66.6" 289-418 silic a-al umi na Sch mitz , 1978

    This Work117.2c 290-360 alu min a Figu re 3143.7b 70.5 41.1 320-360 alum ina eq 1075.1b 67.0 40.7 320-360 alu min a eq 3a Value obtained by linear regression of initial rat e data. Value obtained by nonlinear regression for selected equation. Value obtainedfrom slope In (-rM) v8 1 / T .

  • 8/3/2019 Bercic1992%2C MeOH to DME Global Rates

    4/6

    1038 Ind. Eng. Chem. Res., Vol. 31, No. 4, 992

    +06\ 'I\0 , -rIIu-nILWI

    10 - I-

    & Concentration o fCH IOH In Inletmixture CHIOH-N2( v o l r )0% CHIOH( u L u 3 M CHIOHOnnno 45% CHIOHUIW 60% CHIOH&&Ab 75% CHIOHA\\\\ -90% CHIOHk+m ' ' ' ' ' 8 ' I I I ' ' 1 I ' ' ' ' ' I ' I 8 r ' ' 1 '7 ' I I I1S7E-003 1.65E-003 1.72E-003 1.80E-0031 / T , 1 / K

    Figure 3. Determ ination of app arent a ctivation energy from initialrate measurements.

    LcC

    UQ)0Ju0 /

    0.1 :-+--0 0.01 4%

    I I I0.01 0.1 1Figure 4. Comparison between experimentally measured and cal-culated intrinsic reaction rates w ith eq 10.tivation energies are compared to the data found in theliterature (Table IV) and to th e results of th e initial ratemeasure ments (Figure 3), eq 10 seems to give more realisticpredictions than eq 3. Th e comparison between the ex-perimentally determined reaction rates and those calcu-lated for the sam e reaction conditions by eq 10 is illus-trated in Figure 4.Global Reaction Rate. The catalyst particles whichare used in commerical fixed-bed reactors are usuallygreater than those used in the kinetic rate equation de-termining experiments. Thes e particles are less reactive,as can be seen from Figure 1, because they exh ibit intra-particle mass- and heat-transfer resistances. Th e reactionrate measured for such particles is called the global reac-tion rate. Th e global reaction ra te is calculated from th eintrinsic rate equation and effectiveness factor by thefollowing equatio n

    M e a s u r e d r a t e

    -RM = q(-rM) (12)where the effectiveness factor is defined as

    ( l /WJ(-rd d V7 = (13)

    In fac t, to calculate th e effectiveness factor a s et of dif-(-rM) IS

    Table V. Sensitivity of Numerically CalculatedEffectiveness Factor on Parameter Variationvariation of produced changeparameter parameter, % in calculated 7, %

    Ae +goo, -90 -0.1; +1.5De f20 f8f10 F8f10 =i4d,P P *l o 7 4-rM

    Table VI. Physical Properties of Catalyst Particles (3-mmGranules of Bayer SAS 350 yA12 0 8 )surface area (BET) 247 m2/gsurface area (a > 100 8, Hg porosimetry) 2.24m2/gbulk density (Hg porosimetry) 1.2846 g/cm3particle de nsity (H e picnometer) 3.268 g/cm3particle porosity 0.607ferential equations describing mass and heat transferwithin a catalyst particle must be solved. Mass nd heattransport within a spherical particle are governed by

    d2C; 2 dCi+ -- = ppyirv (14)dr2 r d r ]with boundary conditionsa t r = Oa t r = R T = Ts; Ci = Cis (17)A n analytical solution of t he above system is possible onlywhen iosthermal conditions within a particle are main-tained. For th e power-law ra te expressions, analyticalsolutions are given in terms of th e Thiele modules (Fro-me nt an d Bischoff, 1979). For more complex rat e equa-tions, a generalized approach or a numerical computationcanbe applied. In th e case of the nonisotherm al conditionswithin a catalyst particle, the effec tiveness factor mus t becalculated numerically.T o solve th e se t of eqs 14-17, th e algorithm proposedby Riggs (1988) was used. Th e approach taken in tha talgorithm converts a boundary value problem w ith ordi-nary differential equations (ODES) nto an initial valueproblem with partial differential equations (PDEs) byadding an appropriate trans ient term to th e differentialequations describing the material and energy balances.Th e obtained set of PD Es is then integrated, accordingtoinitial conditions, using a mathematical package LSODE(Hindmars h, 1986), to the steady state. Th e solution ob-tained at that point is also a solution to the originalproblem.In eqs 14 and 15, two parameters appe ar which describethe transport of heat (Aeff) and mass (Deff) for a particularcomponent in a catalyst particle. From th e results of thesensitivity analysis presen ted in Table V, it is obvious thatthe effective diffusion coefficients for com ponents shouldbe predicted more accurately tha n th e coefficient of ef-fective hea t conductivity. A value of t he effec tive therm alconductivity coefficient was predicted on th e basis of lit-eratu re data and correlations and was found to be 2.7 XkJ /(s m K). From Table VI, where some data of themeasured physical properties of the catalyst particles aresummarized, it can be concluded tha t the majority of thetotal surface arises from the pores w ith a diameter u nder100 A. Thus, it an be assumed tha t for the reaction underthe investigated conditions the Kn udsen diffusion prevails

  • 8/3/2019 Bercic1992%2C MeOH to DME Global Rates

    5/6

    Ind. Eng. C hem. Res., Vol. 31, No. 4, 992 1039Table VII. Comparison between the Experimentally Determined and Numerically Calculated Effectiveness Factors

    V u l Cnlet composition" conversion eq 1 eq 3 eq 1045 10 320 7.2 12.1 0.370 0.413 0.394 0.39445 10 340 9.9 11.8 0.294 0.292 0.275 0.27545 10 360 10.6 10.1 0.236 0.206 0.196 0.19445 0 340 7.9 6.8 0.217 0.205 0.216 0.21845 20 340 6.5 9.1 0.349 0.356 0.327 0.32345 30 340 4.3 7.1 0.415 0.415 0.379 0.37245 40 340 3.2 5.7 0.449 0.469 0.430 0.42115 10 340 15.0 14.2 0.243 0.256 0.236 0.22930 10 340 11.0 12.8 0.277 0.277 0.258 0.25560 10 340 4.8 10.6 0.282 0.303 0.290 0.29075 10 340 4.5 9.6 0.268 0.312 0.302 0.304

    bmo l % CH30H mol % H 2 0 T, C dp = 0.17 mm dp = 3 mm qexpt

    "Mole percent of C H30H and H 20 in N2. q e x p t= [-R,(3 mm)/-r~(0.17m m ) l lT ,~ l .

    T =T =T =T = 290'C360'C340'C3200C

    /

    4 1 1 1 1 1 1 I IM e a s u r e d r a t e0.01 0.1

    Figure 5. Comparison between experimentally measured globalreaction rates for 3-mm catalyst particles and those calculated nu-merically.in th e catalyst particles. For materials like yAl,O, ef-fective diffusivity can be estima ted using a parallel premodel:According to Satterfield (1970), effective diffusivity, w ithina factor of 2, can be predicted if it is assumed that aparallel pore m odel with an experimentally measured po-rosity and a value of 4 for the tortuosity factor is used.Assuming different values of thi s factor, calculations o na trialand error basis were performed until th e value whichprovided a good agreement between the measured andcalculated reaction rates for 3-mm catalyst particles wasobtained. When a value of 3 was taken for the tortuosityfactor in calculating the effective Knudsen diffusioncoefficient and the numerical procedure for calculating theeffectiveness factor, the agreement between the measuredan d calculated reaction rates was fairly good. A compar-ison of the calculated an d experimental me asured globalrates in a differential reactor is shown in Figure 5. InTable VI1 some values are given for the calculated endexperim entally determine d effectiveness factor a t differenttemperatures and compositions of reaction mixtures en-tering the differential reactor. As one can conclude fromth e results in Table W,either one of eq 1, 3 ,or 10 predictsth e effectiveness factor quite reasonable.In Figure 6, comparison between experimentally mea-sured empe rature an d concen tration profiles in an integraladiab atic fixed-bed pilo t reactor is shown (BerEiE, 1990).For operating conditions a s specified in Figure 6, i t wasshown (BerEiE an d Levec, 1991) tha t a one-dimensional

    Deff i= dit/^ (18)

    690

    Y 6 6 5 -

    5 640:c -E :Q) 615-

    ; :2 90-56 5 -

    $

    OPERATINGCONDITIONSTI = 551.15 KP = 2.1 ba r0" = 5.32 kg/hdl = 0.078 rn

    -.__5 4 0 1 , , I , I , , , I I I , , I , I I I I , , , , I I , I I , I , F 0.000.00 0.10 0.20 0.30 0.40Axial c o o r d i n a t e , mFigure 6. Comparison between experimentally measured tempera-ture and concentration profiles in an adiabatic pilot reactor andthose predicte d by a pseudohom ogeneous model.pseudohomogeneous m odel can b e successfully used formodeling of an industria l reactor. From th at figure it isobvious that neglecting the reversible term in a rateequation leads to th e wrong predictions, as demonstratedby the use of eq 1 or 3.ConclusionsFrom t he resu lts discussed above it is obvious that thereexist no significant differences among e q s 1,3 , and 10 whenthey are used for interpretation of th e results ob tained ina differential reactor. Equa tions 3 an d 10 provide th etemperature dependency of th e activation energy and heatof adsorption according to th e Arrhenius and van't Hoffrelation. On the basis of th e criteria applied, it is furtherconcluded th at eq 10 represents the kinetic behavior of th edehydration reaction m ore realistically and is appropriatefor modeling purposes. It is also evident that the globalreaction rate can be successfully predicted with th e pro-posed intrinsic rate equation and the numerical procedurefor th e effectiveness factor calculation. Th e effectivediffusivities for com ponents ca n be effectively predictedusing a parallel pore model and assuming the Knudsendiffusion m echanism.Acknowledgment

    We acknowledge suppo rt from th e Research Council ofSlovenia under G rant No. C2-0541-104and Nafta Lendavafor making i t possible to ru n a pilot reactor a t their fa-cilities for more than 2 months.Nomenclaturec i = pore diameter, 1\

  • 8/3/2019 Bercic1992%2C MeOH to DME Global Rates

    6/6

    1040 Ind. Eng. C h e m . R e s . 1992,31, 1040-1045A , B, D = constants in eq 11,dimensionlessc4 = concentration of component i, kmol/m3dp = particle diameter, mdT = reactor diameter (Figure 6), mDi = diffusivity of com ponent i, m2/sDeff = effective diffusivity of comp onent i, mz/sE , = activation energy, kJ/mo lAH = reaction enthalpy, kJ/molks = rate constant of surface reaction , mol/(g,,,/h)K = equilibrium constan t, dimensionlessK i = adsorption constant of component i, m3/kmolP = pressure, barr = radial coo rdinate, mR = particle radius, m-rM = reaction rate, mol/(g,,/h)-R M = global reactio n rate , mol/(g,./h)Hi heat of adsorption of component i , kJ/molT = temperature, KTI reactor inlet temperature (Figure 6), KV = particle volume, m3x i = mole fraction of component iX = conversion, dimensionlessG re e k Le t t e rsc = catalyst particle porosity, dimensionlessCB = catalyst bed porosity (Figure 6), dimensionless9 = effectiveness factor, dimensionlessA r ~ effective heat conduc tivity of catalyst particle, kJ /( sui = stoichiometric coefficient of component i, dimensionlesspp = particle density, g/cm 3T = tortuosity, dimensionless@JM = methanol mass flow rate (Figure 6), kg/hS u b s c r i p t sD = dimethyl etheri = component i (H20 , CH3)20,CH30H, N,)M = methanolS = conditions at catalysts surfacev = per volumeW = water0 = initial conditions

    = specific heat of component i , kJ/(mol K)

    m K)

    Registry No. M e O H , 67-56-1;pAlz03, 1344-28-1; M E,115-10-6.Literature CitedBakshi, K. R.; Gavalas, G. R. Effects of Nonseparable Kinetics inAlcohol Dehydration over Poisoned Silica-Alumina. AIChE J.1975,21,494-500.

    BerEiE, G. Dehydration of Methanol over yA1208. inetics of Re-action and Mathematical Model of a n Industrial Reactor. Ph.D.Dissertation, The University of Ljubljana, 1990.BerEiE, G.; Levec, J. Reactor Model for the Catalytic Gas-PhaseDehy dration of M ethanol to Dimethyl Ether (DME). Vestn.Slou.Kem. Drus. 1991,38,253-270.Brake, L. D. U.S. Patent 4,595,785, 986.Chang, C. D.; Kuo, J. C. W.; Lang, W. H.; Jacob, S. M. ProcessStudies on the Conversion of M ethanol to Gasoline. Ind. Eng.Chem. Process Des. Deu. 1978,17, 55-260.Duggleby, R. G. Regression Ana lysis of Nonlinea r Arrhenius P lots:An Empirical Model and a Computer Program. Comput. Biol.Med. 1984,14,447-455.Figueras , F.; Nohl, A.; Mourgues, L.; Tra mbouz e, Y. Dehy dration ofMethanol and tert-Butyl Alcohol on Silica-Alumina. Trans.Faraday SOC. 971,67, 155-1163.Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis andDesign; John Wiley & Sons: New York, 1979; 185.Gates , B. C.; Johanson, L. N. Lungmuir-Hins helwood Kine tics of theDehydration of Methanol C atalyzed by Cation Exchange Resin.

    Hindmarsh, A. C. Solving Ordinary Differential Equations on anIBM-PC Using LSODE. LLNL Tentacle Magazine; L L N LLiverm ore, CA, April 1986;Vol. 6,No. .Kallo, D.; Knozinger, H. Zur Dehydratisierung von Alkoholen anAliminiumoksid. Chem.-Ing.-Tech. 1967, 9, 676-680.KlusaEek, K.; Schne ider,P. Stationary C atalytic Kinetics via SurfaceConcentrations from Trans ient Data. Methanol Dehydration.Chem. Eng. Sci. 1982,37, 523-1528.Mas saldi, H. A.; Maymo, J. A. Error in H andling Finite ConversionReactor Data by the Differential Method. J. Catal. 1969, 14,61-68.Riggs, J. M. An Introduction to Numerical Methods f or ChemicalEngineers; Texas Tech University Press: Lubbock, TX , 1988;p406.Rousseeuw, P. J.;Leroy, A. M. Robust Regression and Outlier De-tection; John W iley & Sons: New York, 1987.Rubio, F. C.; Diaz, S. D.; Castillo, D. D.; Trujillo, J. D.; Alvarez, R.A. Deshidratacion Catalitica de Metanol en Fase Vapor. Ing.Quim. (Madr id)1980,12, 13-119.Satterfield, C. N. Mass Transfe r in Heterogeneous Catalysis; M.I.T.Press: Cambridge, MA, 1970;p 42.Schmitz, G. Deshydration du M ethanol Sur Silice-Alumine. Chim.Phys. 1978, 746504355,Sinicyna,0. A.; C umako va, V. N.; Mosk ovskaja, I. F. Kinetika De-gidratacii Metanola do Dimetilovogo Efira na SVK Ceolite. K i-

    net. Katal. 1986,27, 160-1162.Th an , L.N.; Setinek, K.; Beranek, L. K inetics and A dsorption onAcid Catalys ts. IV. Kinetics of Gas-P hase Dehy dration ofMethanol on a Sulphonated Ion Exchanger. Collect. Czech.Commun. 1972,37,3878-3884.Woodhouse, J. C. U.S. Patent 2,014,408, 935.Yang, K. H.; Hougen, 0. H. Determination of Mechanism of Cata-lyzed Gaseous Reactions. Chem. Eng. Bo g. 1960,46, 46-157.Received f o r review May 7, 1991Accepted November 1, 1991

    AIChE J . 1971,17,981-983.

    Kinetics of the Catalytic Hydrochlorination of Methanol to MethylChloride

    Albert0 M. Becerra, Adolfo E. Castro Luna, Daniel E. Ardissone, and Marta I. Ponzi*Facultad de Ingenieria y Administracion, Uniuersidad Nacional de San Luis, INTEQ UI-C ONI CET , Au. 25 deMay o 384, 5730 Villa Mercedes, S an Lu is, ArgentinaTh e intrinsic kinetics of the catalytic hydrochlorination of methanol to methyl chloride on yAl,O,was determined from experiments in a tubular reactor in the tem peratu re range of 513-593 K andat atmospheric pressure,after a catalyst screening and a study of operative conditions. A large num berof detailed reaction m echanisms was considered. A strategy of model discrimination and parameterestimation led to a Hougen-Watson type model with statistically significantand thermodynamicallyconsistent parameters.

    Th e two main technologies used commercially for ob- and chlorination of methane. Methyl chloride is used astainingmethyl chloride are hydrochlorination of methanol an intermediate in the obtainme nt of chlorinated bypro-0888-5885/92/2631-1040$03.00/0 1992 American Chemical Society