Mathematical Modeling of an Industrial Naphtha Reformer with Three Adiabatic Reactors in Series

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1 Iran. J. Chem. Chem. Eng. Research Note Vol. 28, No. 3, 2009 Mathematcal Modelng of an Industral Naphtha Reformer wth Three Adabatc Reactors n Seres Fazel, Al; Fatem, Shohreh* + ; Mahdavan, Mohammad; Ghaee, Azadeh School of Chemcal Engneerng, Unversty College of Engneerng, Unversty of Tehran, I.R. IRAN ABSTRACT: A mathematcal model for commercal naphtha catalytc reformer of Tehran refnery was developed. Ths model ncludes three sequencng fxed beds of Pt/Al 2 O 3 catalyst at the steady state condton usng detaled knetc scheme nvolvng 26 pseudo-components connected by a network of 47 reactons, n the range of C6 to C9 hydrocarbons. The reacton network conssted dehydrogenaton, hydrogenaton, rng expanson, paraffn and so-paraffn crackng, naphthene crackng, paraffn somerzaton and hydrodealkylaton of aromatcs. The knetc model was fne tuned aganst ndustral plant data usng a feed characterzed by PIONA (Paraffn, Iso-paraffn, Oleffn, Naphthene and Aromatcs) analyss. The fnal outlet results of the reformer such as RON (Research Octane Number), yeld and outlet reformate compostons have shown good agreement wth actual condtons of Tehran Refnery reformng unt. KEY WORDS: Knetc model, Modelng, Naphtha reformng, Fxed bed reactor. INTRODUCTION The catalytc reformng process s one of the most crtcal operatons n petroleum refneres to produce gasolne wth hgh octane number [-6]. To desgn new plants and optmze the exstng ones, an approprate mathematcal model for smulaton the ndustral catalytc reformng process s needed. However usng smple lump knetc models could not show reasonable and approprate results n scaled up reformng processes, therefore ntroducton of new knetc lumps and reacton networks are stll beng the nterested research and development subjects n the feld of modelng and smulaton of reformng processes. The appled reacton and ther knetcs reported n the lterature are dfferent and usually smplfed to the three * To whom correspondence should be addressed. + E-mal: shfatem@ut.ac.r /09/3/97 6/$/2.60 lumps of paraffn, naphtane and aromatc. However these knetc models could not be a proper tool for modelng and smulaton of a commercal process, n the case of Tehran refnery naphtha reformng. In ths study, an approprate knetc model was developed and fne tuned for Tehran refnery catalytc reformer to process the straght run naphtha. The feed was characterzed by naphthenes, soparaffns, paraffns and aromatcs lumps of each carbon number from C6-C9 consstng 26 pseudo-components. The major reformng reactons such as dehydrogenaton, dehydrocyclzaton, somerzaton, hydrocrackng and hydrodealkylaton were consdered n the model. The knetc equatons and ther ntal parameter estmatons were taken from the lterature [3,5,7]. 97

2 T - T- P P T-2 T- 2 Iran. J. Chem. Chem. Eng. Fazel, A., et al. Vol. 28, No. 3, 2009 The knetc parameters were fne tuned by drect search (Nealder-Mead smplex) method. Sutable couplng of optmzaton method wth a fast ODEs solver, choosng proper reactons, knetcs, objectve functon and weght factors for a specal commercal process are crtcal stages of the present knetc modelng. Table :Catalyst characterzaton. Knd of Catalyst Pt/γ-Al 2O 3 Sze and shape Sphercal-.8 mm dameter Partcle densty 690 kg/m 3 Partcle porosty 0.6 MODEL DEVELOPMENT The schematc of the reformng process modeled n ths work s shown n Fg.. The feed to the reformng unt s hydrotreated straght run naphtha. It conssts of naphthenes (Alkyl cyclohexanes (ACH) and Alkyl cyclopentanes (ACP)), paraffns (normal paraffns (np) and soparaffns (P)) and aromatcs (A), contanng lumps of carbon number from C 6 -C 9. The major reformng reactons are endothermc causng temperature decrease of the reacton stream and catalyst along the bed. To prevent reacton rate reducton caused by temperature decrease, the catalyst s dstrbuted n three adabatc reactors n presence of heaters nstalled between the reactors. As shown n Fg., the naphtha feed s mxed wth hydrogen and heated to desred temperature before enterng the frst reactor. In order to delay coke formaton of the catalysts n the fxed bed reformers, the process should work at hgh pressure of hydrogen. In reformng, a bfunctonal catalyst s used n whch an acdc functon provded by a cholornated alumna carrer whch s combned wth a metal functon provded by platnum. [4,7]. Table exhbts the catalyst propertes of Tehran reformng process. The operatonal condtons of Tehran refnery reformer and naphtha feed characterzaton are shown n table 2. The reacton network consdered n ths study s ntroduced by varous reactons such as Dehydrogenaton, Rng expanson, Isomerzaton, Hydrocrackng, Naphthene crackng, Hydrodealkylaton, for C6 to C9 hydrocarbons whch were totally 47 reactons. [3, 5, 7]. The knetc models of the above reactons were ncorporated nto a non-sothermal plug flow reactor model. The partal mass balances of each component were wrtten as follows for 26 components consstng lqud hydrocarbons, LPG and Hydrogen: 47 df = = dw j = ( R j StCo j, ) ;, 2, 3,..., 26 () Reactor Reactor 2 Reactor 3 Flash Naphtha Fresh Feed Feed Net Hydrogen Fg. : Schematc of naphtha reformng process. The partal pressure of each component, n thereacton rate equaton, was related to the molar flow rate by the followng equaton: F P = Pt ; =, 2, 3,..., F = Compressor Fuel Fuel Gas LPG Stablzer Reformate The homogeneous energy balance of each adabatc reactor was wrtten as: In whch the reacton enthalpes and component heat capactes were replaced from Snnott [8]. Ordnary dfferental equatons of mass and heat balances were solved smultaneously by varable order solver based on the numercal dfferentaton formulas (NDFs) method wth MATLAB 7.3 software. Inlet condtons of each reactor were set to outlet condtons of pror reactor n except of temperature that s changed because of ntermedate heaters. The nput to the model were feed flow rate and composton of all lumps n the (2) 98

3 Iran. J. Chem. Chem. Eng. Mathematcal Modelng of Vol. 28, No. 3, 2009 Table 2: Naphtha feed and operatng condtons of Tehran refnery reformng plant. Operatng Condton Inlet temperature of the reactors ( C) Pressure (kpa) Values T 493 T2 493 T3 493 P P P No. of Reactors 3 Total catalyst weght (kg) Catalyst loadng (wt%) R 20 R2 30 R3 50 Feed flow rate (kg/hr) H 2/HC molar rato 5.6 Fresh feed composton (mole %) ACH ACH ACH ACH ACP ACP ACP ACP np np np np P P P P A A A A Feed RON feed, catalyst loadng, recycle gas composton, nlet temperatures and pressures. The output of the model ncluded temperatures and concentraton profles of reformate as a functon of catalyst dstrbuton for each reactor, gas composton and temperature drop. In addton the octane number of the feed and lqud reformate obtaned from a data base of several pure components of naphtha by a subroutne program. The ntal guess values for the frequency factors and actvaton energes were obtaned from the lterature [3] and optmzed usng the commercal plant data of frst few days data, by a drect search algorthm (Nealder- Mead smplex method) due to the mnmzaton of an objectve functon. Ths functon s ntroduced as the sum of absolute relatve devatons of the reactors temperature drop, outlet compostons and lqud reformate yeld as observed n Eq. 4. The weght factors were ntroduced for each term of objectve functon (OF) n order to approach to desred convergence of model and experment. Objectve Functon (OF) 24 Z Z = 00* w Exprmental = Z 20 x x = 00* w + Exprmental = X LY% LY% 00* w2 + Exprmental LY% 3 r= 00* w Tr Tr 2+ r Exprmental Tr RESULTS AND DISCUSSION The pre-exponental Arrhenus constants and actvaton energes were determned after fne tunng wth expermental data. Table 3 gves the predcted and the plant output results of Tehran refnery reformng plant. The predcted temperature drop of the reactors and the reformate yeld, compostons and RON from the last reactor are compared wth actual results of the plant, n ths table. As can be seen, temperature dfferences n all reactors are n good consstency wth realty. Reformate composton, yeld and research octane number are also close to actual values. The maxmum devaton of the tuned model from operatonal values s less than 2 %. 99

4 Iran. J. Chem. Chem. Eng. Fazel, A., et al. Vol. 28, No. 3, 2009 Table 3: Comparson of predcted and expermental results of the reformng unt. Parameters Temperature drop n the reactors ( C) Expermental Tehran Refnery Predcted Absolute Relatve T T T ACH ACH T ( C) Reactor Reactor Reactor ACH Fg. 2: Temperature profles n three reformng reactors. Composton (mole %) of reformate at the outlet of reactor no.3 ACH ACP ACP ACP ACP np np np np P P ACH ACP np Fg. 3: Lumped component molar flow rate versus catalyst percent. A C P P P A A A ACH7 ACH9 ACH8 ACH6 A LY (wt %) RON Fg. 4: Alkylcyclohexanes (ACH) molar flow rates versus catalyst percent. 00

5 Iran. J. Chem. Chem. Eng. Mathematcal Modelng of Vol. 28, No. 3, A8 20 Reactor Reactor 2 Fg. 5: Aromatc (A) molar flow rates versus catalyst percent. The presented model can also predct the trends of the lqud products, LPG, H 2 and other necessary outputs. The present model shows producton of the reformate of 96.8 RON from a feed around 69 RON. The model shows around 79 wt % yeld of reformate aganst 2 wt % LPG whch s consdered n the acceptable range for such a hgh pressure process. The predcted temperature and component profles along the catalyst weght are shown n Fgs. 2 to 6. CONCLUSIONS Nowadays producton of gasolne wth hgh octane number from naphtha produced from crude ol refnng through catalytc reformng reactons, attracts great attenton. In ths research naphtha reformng reactor of Tehran refnery was knetcally modeled, n hgh pressure fxed bed reactors, usng 47 reactons network wth 26 pseudo-components whch enables modelng of hydrogen, C to C5 producton, lqud reformate composton and temperature varatons. The knetcs was fne tuned by drect search (Nelder- Mead smplex) method. Ths study showed that usng fast ODE solver of varable order based on the numercal dfferentaton formulas (NDFs) s sutable for couplng to optmzaton method and can handle large number of components and reactons. The fnal outlet results of the reformer such as RON, yeld and outlet reformate composton have shown good agreement wth actual condtons of Tehran Refnery reformng unt wth global absolute devaton of less than 2 %. The reformng model contans a rch set of model features and the confrontaton of smulaton results wth A7 A9 Reactor A Reactor Reactor 2 Reactor Fg. 6: Hydrogen molar flow rates versus catalyst percent. expermental observatons suggests that the model s well founded and s a good platform for addtonal process specfc refnement. Nomenclatures C p Specfc heat capacty (KJ/kmol.K)) F t Total molar flow rate (kmol/hr) F Molar flow rate of th component (kmol/hr) LY % Reformate lqud mass yeld percent P t Total pressure kpa) P Partal pressure kpa) R Gas constant (J/mol.K) R j j th reacton rate (kmol/(hr.kg cat )) StCo,j Stochometry coeffcent of componet n j th reacton T Temperature (K) W Catalyst weght (kg) w th weght factor X Mole fracton of the th component H j Reacton enthalpy(kj/kmol) T r Temperature dfference across the reactor no. r Receved : 8 th Aprl 2008 ; Accepted : 4 th October 2008 REFERENCES [] Smth, R.B., Knetc Analyss of Naphtha Reformng wth Platnum Catalyst, Chemcal Engneerng Progress, 55, p. 76 (959). [2] Marne, G. B. and Froment, G. F., Reformng of C 6 Hydrocarbons on a Pt-Al 2 O 3 Catalyst, Chemcal Engneerng Scence, 37, p. 759 (982). 0

6 Iran. J. Chem. Chem. Eng. Fazel, A., et al. Vol. 28, No. 3, 2009 [3] Padmavath, G. and Chaudhur, K. K., Modelng and Smulaton of Commercal Catalytc Naphtha Reformers, The Canadan Journal of Chemcal Engneerng, 75, p. 930 (997). [4] Tasker, U. and Rggs, J. B., Modelng and Optmzaton of a Sem Regeneratve Catalytc Naphtha Reformer, AIChE, 43, p. 740 (997). [5] Ancheyta Juarez, J. and Vllafuerte Macas, E., Knetc Modelng of Naphtha Catalytc Reformng Reactons, Energy & Fuels, 4, p. 032 (2000). [6] Hu, S. and Zhu, F. X. X., Molecular Modelng and Optmzaton for Catalytc Reformng, Chemcal Engneerng Communcatons, p. 9, 500 (2004). [7] Ancheyta Juarez, J., Vllafuerte Macas, E., Daz Garca, L. and Gonzalez Arredondo, E., Modelng and Smulaton of Four Catalytc Reactors n Seres for Naphtha Reformng, Energy & Fuels, 5, p. 887 (200). [8] Snnott, R. K., (Edtor), Chemcal Engneerng Desgn, Coulson and Rchardson's Chemcal Engneerng, Vol. 6, 4 th Edton, Elsever, (2005). 02