An integrated approach to modelling of chemical transformations in chemical reactors

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1 Euean Symsium n mute Aded Aided Pcess Engineeing 15 L. Puigjane and A. Esuña (Edits) 005 Elsevie Science B.V. All ights eseved. An integated aach t mdelling f chemical tansfmatins in chemical eacts Tai Salmi, Dmity Yu. Muzin, Jhan Wänå, Matias angas, Esa Tukniitty, Ville Nieminen Åb Akademi, Pcess hemisty ente, Labaty f Industial hemisty FIN-0500 Tuku/Åb Finland, fax , Jhan.Wana@ab.fi Abstact An integated aach t the mdelling f chemical eacts, aticulaly catalytic nes is esented. The mdelling aach stats fm quantum-chemical calculatins, mechanistic hythesis, deivatin f kinetic exessins in de t achieve an aiate kinetic mdel. The mdel aametes ae detemined by egessin analysis and the cmlex behaviu f fixed bed eacts, including catalyst deactivatin is descibed in an adequate manne. A geneal flwsheet f the cedue is sed. eywds: kinetics, eact, mdelling, quantum chemisty 1. Intductin Mathematical mdelling f chemical eacts is ne f the mst demanding tasks in chemical engineeing because f the inteactin f seveal simultaneus henmena, such as chemical kinetics, mass and heat tansfe as well as fluid dynamics. In ecent yeas, a lt f attentin has been aid n detailed mdelling and calculatin f fluid dynamics (FD). Hweve, the cucial int in the descitin f a chemical eact is the chemical tansfmatin itself. In additin, a majity f industially eating chemical eacts invlve the esence f tw thee hases, which emhasizes the le f intefacial mass and heat tansfe. F hetegeneusly catalysed cesses, the mdelling f intaaticle mass and heat tansfe is included. The tendency in chemical eactin engineeing eseach is nwadays t mve me fm bulk chemicals t fine and secialty chemicals. Thus the system cannt be descibed by few eactins, but a cmlex eactin netwk aeas. The kinetics f the incated eactins is usually exeimentally measuable, but the develment f ate equatins equies a dee-ging insight n the eactin mechanism. The geneal chemical intuitin can give insiatin t mechanistic cnsideatins, but me igus calculatins vided by quantum chemisty ae needed t cnfim/eject the kinetic hytheses. In the cuent ae, we esent an integated aach t the mdelling f chemical eacts. The gal is t achieve as gd but as simle as ssible mdel. The integated mdelling aach was alied n seveal catalytic systems, such as thee-hase hydgenatin f aldehydes and ketnes as well as hydcabn tansfmatins (i.e.

2 skeletal ismeizatins f alkenes) ve mesus and micus catalysts. The chaacteistic featue f the systems cnsideed is that a simlistic, ugh mdelling aach discading the detailed eactin mechanisms leads t inaiate ate equatins, which ae nt able t descibe the gess f the eactins and the develment f the duct selectivities cectly.. ase study Skeletal ismeizatin f linea alkanes t banched cunteats has attacted attentin t a lage extent, since inceasing the degee f banching f alkanes can bst the ctane quality f a gasline factin. The alicatin f banched hydcabns is an envinmentally me accetable altenative cmaed with the techniques, such as blending with amatics xygenates (Etl et al 1997, Guillaume et al 00, Hužvik et al. 1997, Nieminen et al. 004, Oun 00). inetics f n-butane ismeizatin ve bifunctinal Pt-H-Mdenite was studied in a catalytic fixed-bed eact by vaying eactant atial essue and temeatue. The main ducts wee isbutane, ane and entanes. The state f adsbed secies inside the catalyst es (channels) was investigated by quantum-chemical calculatins, which suggested that alkxy secies ae fmed inside the channels an examle f the calculatins is dislayed in Fig. 1. The eactin ate shwed cmlex deendence n the eactant atial essue. Thee kinetic mdels wee develed based n the cuent undestanding f the eactin mechanisms including hydgenatin and dehydgenatin stes n the metal sites, skeletal ismeizatin n the acid sites and deactivatin due t the cke fmatin. A enabled mnmlecula isbutane fmatin ath, mdel B bimlecula and mdel bth eactin aths. Fig.1 1-butene in the zelite channel accding t quantum chemisty. A kinetic mdel based n the exeimental esults was develed. Only the eactins sed in the liteatue t duce the main ducts, ane, isbutane and entanes, wee taken int accunt while the eactin aths, such as hydgenlysis and mnmlecula butane cacking wee mitted. The easn f this is that the amunt f by-ducts is min and neglecting them kees the mdel easnably simle cntaining less estimated aametes while still being able t catue the main kinetic featues f the eactin. The entanes, isentane and n-entane, wee lumed tgethe,

3 because bth f them ae (hydgenated) cacking ducts f eactin netwk n the catalytic sites is dislayed in table 1.. n 5 Table 1 Reactin netwk n the catalytic sites N (1) N () N () N (4) N (5) is (1). n 4,, () A. B.. D., 5,, 5, n 4,,,, is () (4) (5), n is 5, is, n is 5, n (6) (7) 1 1 () (9) The On the ight hand side f the equatins (1)-(10) abve, the stichimetic numbes (N) the f stes alng indeendent utes ae esented. A cesnds t mnmlecula isbutane fmatin and included ute N (1) as the sle ath t isbutane. Rutes N (4) and N (5) descibe byduct fmatin. In mdel B isbutane is fmed bimleculaly (N () ) and the mnmlecula ath f isbutane fmatin is neglected. The valid utes ae N () -N (5). includes utes N (1), N () -N (5). Thus, in mdel isbutane can be fmed eithe mnmleculaly bimleculaly. The ate equatins f Langmui-Hinshelwd tye wee deived using the assumtin that atelimiting stes ae the suface eactins n the catalyst 1 k1 4, H 4 i4, H í k 4, H 4 (1) k, H 4 k 4 5, H 5 4, H 4 6, 6 H k ()

4 whee and 1 i, H i, H i, lefin i, H, H The ate cnstants and thei temeatue deendencies wee mdelled with the Ahenius equatin, mdified in de t imve simultaneus estimatin f eexnential fact and activatin enegy. Since catalyst deactivatin is a fund featue, it was included in a geneal way in the mdel. The ate f the eactins ae given by equatin belw, whee 0,i is the initial eactin ate f eactin i, and a dentes the elative activity: i 0, i a whee the activity fact is calculated fm 1 1 a k cp,0 t 1 ex k c t 1,0 a P (6) Reactin and deactivatin wee assumed t be unifm thughut the eact bed and the catalyst aticles. The cmnent mass balance is witten as (=atial essue). d j mcatja j, 4, i 4, d whee is the sace time f the fixed bed. 5 The veall geneatin ates f alkanes ae detemined by the ismeizatin f lefins n acid sites giving geneatin ates (ane) i (isbutane) (n-butane)... H 0 LH 4 (entane) The eact mdel equatins wee slved numeically by a stiff ODE-slve duing the aamete estimatin which was caied ut by a Levenbeg-Maquadt algithm imlemented in the sftwae Mdest (Haai 001). Examles f the fit f the mdel t the exeimental data ae vided by Figs -. The figues eveal that the descitin f the cnvesin alne including the catalyst deactivatin is nt enugh, but a detailed analysis f the duct distibutin is needed, as evealed by the selectivity analysis (Fig. ). The detailed kinetic mdeling V () (4) (5) (7)

5 enables us t judge, which mechanism is evailing unde secified cnditins (essue, temeatue). The mdel, which enabled bimlecula eactin ath f isbutane fmatin, had a gd fit n the selectivity t isbutane at high eactant essues but was incaable t edict the incease in selectivity t isbutane with deceasing n-butane essues. At the same time, the abve mentined tendency was vey well edicted by the mdels enabling mnmlecula mechanism f isbutane fmatin. The kinetic mdelling als suted the sal that excess f ane cmaed t entanes is due t cnsecutive cdimeizatin f fmed with t 9 fllwed by cacking t thee secies. 5 4 nvesin, mle-% , n-butane:h 40: TOS /min Figue. Examles f cnvesin as a functin f time n steam at 67 by the kinetic mdels cmaed t the exeimental values. Selectivity t isbutane, mle-% n-butane atial Figue. Selectivity t isbutane at TOS = 10 min as a functin f n-butane atial essue at 67 by the kinetic mdels cmaed t the exeimental values.

6 . nclusins The aach alied is biefly summaized in the flwsheet sketched belw. Successful mdelling f catalytic eacts equies a stngly integated aach. Due t the gess f alied quantum chemisty it is ssible t get ideas and insiatin f mechanistic hythesis, which ae bught t kinetic equatins including catalyst deactivatin. Futheme, mdels f heat and mass tansfe as well as flw mdels ae incated. Efficient and bust numeical algithms ae used t slve the kinetic and eact mdels. The aach shuld nt have a single missing link, since the final gal is a eliable design tl f chemical eacts integated t suunding cess units. nstuctin f stichimetic scheme Quantum chemical calculatins Deivatin f ate equatins Hythesis n eactin mechanism inetic exeiments heck f veall themdynamics f test eact Estimatin f kinetic aametes Mass and heat tansfe celatins and exeimentatin Exeimental veificatin f flw cnditins inetic mdel Mass and heat tansfe mdel Flw mdel REATOR MODEL Refeences Sie, S. T. Handbk f Hetegeneus atalysis, eds. Etl, G.; nözinge, H.; Weitkam, J. VH/Wiley, 1997, Guillaume, D.; Sula,.; Galtie, P. Fm single events they t mlecula kinetics alicatin t industial mdelling. hem. Eng. Sci. 00, 5, 461. Nieminen, V.; uma, N.; Salmi T.; Muzin, D. Yu. n-butane ismeizatin ve Pt-H-MM-41. atal. mm. 004, 5, 15. Haai, H. Mdest Usess Guide 6.0, PfMath Oy, Helsinki, 001. Hužvika, J.; Pnec, V. Skeletal ismeizatin f n-butene. atal. Rev. Sci. Eng. 1997, 9, 19 On, Y. A suvey f the mechanism in catalytic ismeizatin f alkanes. atal. Tday 00, 1,. Acknwledgements This wk is at f the activities at the Åb Akademi Pcess hemisty ente within the Finnish ente f Excellence Pgamme ( ) by the Academy f Finland