DYNAMIC MODELING AND SIMULATION OF BIOETHANOL STEAM REFORMING REACTOR FOR FUEL CELL PURPOSES

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1 DYNAMIC MODELING AND SIMULATION O BIOETHANOL STEAM REORMING REACTOR OR UEL CELL PURPOSES 1 Eugeno Turco Neto and Dr. Sed Imtaz 1 Scence wthout Borders Scholarshp holder CNPq/Memoral Unverst of Newfoundland, Exchange Student n Ol and Gas Engneerng Assstant Professor n Process Engneerng/Memoral Unverst of Newfoundland 1, Memoral Unverst of Newfoundland. St. John s, A1C 5S7, P.O. Box, Canada e-mal: smtaz@mun.ca ABSTRACT- Boethanol and hdrogen as a new source of energ are beng strongl consdered b ndustres and scentfc communt for causng less polluton and beng cheaper when compared wth fossl fuels. In ths context, a new knd of engne, called fuel cells, that uses electrochemstr, thermodnamcs and transport phenomena (flud mechancs, heat transfer and mass transfer) concepts to convert hdrogen n water, and generate electrcal energ, has been desgned and tested b researchers and bg companes over the world. However, hdrogen s consdered dangerous e expensve due to the storage requrements, whch s usuall made n sphercal contaners under hgh pressure. In order to overcome these drawbacks, on-board boethanol reformng reactor (ESR), whch converts boethanol n hdrogen, s beng ntegrated on board fuel cell sstems to produce clean energ effcentl and safel. Moreover, the desgn of the control sstem for the reactor s fundamental once temperature and converson profles must be kept n proftable range front eventual dsturbance. Therefore, a dnamc model s requred n order to predct the behavor of the boethanol steam reformng reactor when such dsturbance takes place and to evaluate the controller parameters. Ths work has as purpose to develop a dnamc model for the ESR; to predct the dnamc behavour front to usual ndustral dsturbance as well as to consder the pressure drop along the reactor; create a SIMULIN model that represents the process as a block dagram common used n process control loops, and to develop a Gude User Interface (GUI) n order the user can run the smulaton outsde the programmng envronment. ewords: Boethanol, Hdrogen, uel Cells INTRODUCTION As an energ vector, hdrogen has a great potental for becomng a domnant fuel n a wde varet of applcaton due to ts hgh energ denst, hgh heat capact and non-pollutant products after oxdaton processes (erold et al., 1). Even though hdrogen can also be used n heat engnes, fuel cells have been beng consdered the deal processor sstem for producng electrct va hdrogen, whch can be defned as a devce n whch converts electrochemcal energ n heat and electrct whle fuel s suppled (erold et al., 1). Some advantages of fuel cells when compared wth the conventonal heat engnes, such as, hgher effcenc (Holland et al., 1) and converson of chemcal energ of hdrogen n electrct and heat wthout combuston processes (Bose et. al, ), are the motvaton for man studes and publcatons about the desgn of uel Cells n a wde of resdental and ndustral envronments. However, uncertantes are found about the development and economc feasblt of enablng technologes, such as use of renewable energ sources, advanced producton processes, storage routne, safet and rsk, and nfrastructure (erold et al., 1). In terms of converson of lqud fuels n useful power, nternal combuston engne has acheved a maxmum effcenc of 5% whle fuel cells have acheved 85%. Ths s the man motve n whch fuel cells have been beng ponted out as a powerful alternatve for energ producton. Also, due to the lack of nfrastructure as mentoned before, onboard processng sstem for producng hdrogen s beng proposed as complement for the fuel cells sstem. In ths wa, man hdrogen sources has been studed, but renewable fuels has been focused b researchers, once the objectve for fuel cell operaton s guarantee a sustanable process (Degluomn et al., 1). In

2 ths vew, Boethanol has become an deal renewable fuel due to several motves, such as nontoxct, hgh H/C rato, net postve energetc balance, and cheap and ease producton (erold et. al, 1). In order to understand and desgn the fuel cell sstem, a dnamc model s requred once t shows the nteracton between process varables when such sstem are submtted to common ndustral dsturbances. Also, the dnamc model for the reactors nvolved n fuel cell power plants are mportant to predct the transent performance for the reactor, whch has a strong mpact on the effcenc of the fuel cell sstem (Degluomn et al., 1). As ths work focus on the boethanol fuel cell sstems, ts man purpose s to develop a dnamc model for the Ethanol Steam Reformng Reactor to predct the effects of common ndustral dsturbances on ts performance. Also, a full mplementaton n MATLAB/SIMULIN, wll be done for smulatng the reactor performance, once t s one of the most used software s to desgn process control sstems. PROCESS MODELING The man reactons occurrng n Ethanol Steam Reformng Reactor (ESR) are provded b Souza (1) followng b the Eq. 1,,, and. C H O (1) CH CO 6 H HO CO H CO () CH HO CO H () CH H O CO H () These reactons take place n a presence of N/Al O nsde the ESR. In order to develop the governng equatons for the reactor operaton, the reacton rate expressons must be known. These expressons are provded b Souza (1), whch are shown b Eq. 5, 6, 7, 8, and 9. r1 k1 P C H6O (5) r r k P DEN CO CO H O H.5 H k DEN P CH HO CO H H P (6) 7) k CO H P r (8).5 CH H O DEN P H H O H O DEN 1 P CO CO H H CH CH (9) H In other to appl the materal and energ balance n the ESR, some assumptons have been made n order to smplf the model and reduce the computatonal effort n solvng the equatons, whch are: 1- deal gas behavor; - pressure drop do not depend on tme; - plug flow reactor; - constant overall heat transfer coeffcent along the reactor jacket. or a Plug low reactor, the dnamc energ and materal balance gven b ogler () are shown b the Eq. 1 and j C j j k rk (1), A z t Ua c 1 T T a T C p k rk H R k C Cp,, (11) A z c T t These equatons are partal dfferental equatons (PDEs) n terms of tme t and axal poston z, whch soluton ma be dffcult to obtan and the are not sutable for control desgn. In order to smplf these equatons, a dscretzaton n axal poston z s requred, whch transforms the PDEs n Ordnar Dfferental Equatons n terms of tme for each element of volume comprsed n Δz. The mesh has been bult consderng the dscretzed ponts are evenl spaced b a dstance Δz. Ths procedure can be made knowng that a PR s represented as an nfnte seres of Contnuous Strred Tanks Reactors (CSTRs). Such modelng s shown b Degluomn (1) and used n ths work. Overall Materal Balance Another assumpton s made related wth the overall materal balance, whch the accumulaton of total mass nsde the reactor s neglected. Therefore, a pseudo-stead state overall materal balance dscretzed equaton s shown b the Eq. 1. r1 r r wcat r r 1, (1) Momentum Equaton (Pressure Drop) Assumng neglected effect of tme n the pressure profle along the reactor, a pseudostead state for the Momentum Equaton can be used to predct the pressure profle along the reactor. The Ergun s equaton has been consdered n ths work and t s shown b Eq. 1.

3 dp G(1 ) 15 1 P 1.75G dz gcdp D p P (1) Therefore, a dscretzed Ergun s Equaton s used for predctng the pressure n each element of reactor. Usng backwards fnte dfferences dscretzaton, the dscretzed equaton s shown b Eq.1. P T 1 r 1 P P 1 z (1) P T Where 1 r G(1 ) G (15) gcdp Dp or smplct, mass flux (G), porost (Φ) and vscost (μ) are consdered constant along the reactor. Component Materal Balance T T r The dscretzed component materal balance s smlar to that for a dnamc CSTR for each element of reactor, whch s shown b Eq. 16 and 17. d dt j r Energ Balance dt dt Where r 1 c C j, 1 r j, j, k k1 p1 T 1 r n t C T ( a p n t C k1 Q UA z T T ) (18) p r k, k, w cat, r H R, k, w cat, (16) Q (17) r Table 1 Dmensonal and Operatonal Parameters for ESR Parameter Value L 11.1 m D.116 m U 1 W/m Φ.65 ρ c 55. kg/m T a D p μ 98 ΔH r,1 98 ΔH r, 98 ΔH r, 98 ΔH r, (W c /W cat,total )x x1 - m.7951x1-5 Pa.s 8.7 kj/mol kj/mol 5.9 kj/mol 16.7 kj/mol The governng equatons have been mplemented n MATLAB/SIMULIN, and the ntal condtons for each ODE have been set up as the stead state condton for the process varables lsted n the Table. Table Inlet Process Varables Varable Value r 1 mol/s G.97 kg/m s T 98 P x1 5 Pa CH6O. HO.8 In order to obtan the stead state values for the process varables n each pont of the reactor, the dervatves related wth the tme have been neglected and the set of dfferental equatons have become a set of nonlnear algebrac equatons. The stead state results have been obtaned solvng those equatons usng the functon fsolve n Matlab. The g. 1, and show the stead state profles for temperature, ethanol and hdrogen compostons. RESULTS In order to smulate the conservaton equaton developed above, some process parameters for the ESR must be provded. The goal of ths work s predct the dnamc behavour of an Ethanol Steam Reformng Reactor that has ndustral dmensons, once most of research n ths feld has been appled for small reactors. Therefore, the reactor dmensons proposed b Nummedal et al. () have been used n ths work. Such parameters are lsted n the Table 1.

4 1 Dsturbance n the Inlet eed low Rate Temperature () The g. and 5 show the dnamc behavour of outlet temperature, ethanol and hdrogen compostons of the ESR, respectvel, when the feed flow rate s ncreased from 1 mol/s to 1. mol/s and when t s decreased to 1 mol/s to.5 mol/s, n t=s z/l % gure 1 - Stead State Temperature Profle Outlet Temperature () rom 1 to 1. mol/s rom 1 to.5 mol/s Ethanol Composton tme (s) gure - Dnamc behavour of Outlet Temperature after Dsturbance n eed low Rate z/l % gure - Stead Stead Ethanol Composton Profle.8.7 Outlet Hdrogen Composton rom 1 to 1. mol/s rom 1 to.5 mol/s Hdrogen Composton z/l % gure - Stead State Hdrogen Composton In order to analze the dnamc behavour of the ESR, the stead state condtons have been set as ntal condtons and dsturbances n nlet feed flow rate and temperature have been made tme (s) gure 5 - Dnamc behavour of Outlet Hdrogen Composton after Dsturbance n eed low Rate It can be observed that after ncreasng the feed flow rate, both outlet temperature and hdrogen composton decrease due to the decrease on the reactants resdence tme. Once t happens, the reactants spend less tme nto the reactor, hence, the converson s dmnshed and the flud s submtted to the heat transfer from the hot gases for less tme and the temperature decreases. These effects are nverted when feed flow rate s decreased.

5 Dsturbance n the Inlet Temperature The g. 7 and 8 show the dnamc behavour of outlet temperature, hdrogen and ethanol compostons of the ESR, respectvel, when the nlet temperature s ncreased from 98 to 1 and when t s decreased from 98 to 8, n t=s. After ncreasng the nlet temperature, an nverse response s observed for the ext temperature. As the overall ethanol steam reformng reactor s endothermc, based on the Le Chateler s prncple, when the temperature ncrease the reacton dslocates to the endothermc wa, whch explan the ncreasng on the hdrogen composton and the frst decreasng on the ext temperature. As the reactons have most effects at the begnnng of the reactor, the heat losses due to the endothermc reactons has no longer effect on the temperature profle, and the heat transfer from the hot gases make the ext temperature ncrease. Tme delas are observed for all the cases; however, t s so small that cannot be observed n the prevous graphs. SIMULIN MODEL AND MATLAB GUIDE US- ER S INTERACE (GUI) Outlet Temperature () rom 98 to 1 rom 98 to 8 In ndustral envronment, the usage of nterfaces for the equpment s mportant for control, smulaton and tranng purposes; moreover, the codng ma be extremel dffcult to the user understand ts usage. A SIMULIN nterface provdes good representaton for process control desgn and, moreover, a good representaton for the process once the block dagram shows the equpments and the sgnals for each stream wthn the process. The g. 8 shows the SIMULIN model developed n ths work tme (s) gure 6 - Dnamc behavour of Outlet Temperature after Dsturbance n the Inlet Temperature.81 Outlet Hdrogen Composton rom 98 to 1 rom 98 to tme (s) gure 7 - Dnamc behavour of Outlet Hdrogen Composton after Dsturbance n the Inlet Temperature gure 8 - Smulnk Model or Ethanol Steam Reformer The block representng the ethanol steam reformer has been creatng usng an S-functon codng, whch enables the user to create ts own block contanng the solver and dervat. Also, the parameters for the model are drectl nserted n the smulnk model b maskng the ethanol steam reformer block. The g. 9 shows the wndow at whch the parameters are set up.

6 The man purpose of the GUI s plot the dnamc behavour of outlet temperature and hdrogen composton once gven the value of the dsturbance n one of the nputs, whch are nlet feed flow rate, pressure and/or temperature. Also, the number of dscretzed ponts can be gven b the user dependng on the tme demand and precson of the results. The ntal condtons are set b the user and after clckng start, the SIMULIN starts the smulaton wthout the user beng n ts platform. CONCLUSION gure 9 Parameters Wndow created for the Smulnk Model The smulaton s run after the ntal condtons are set up at the MATLAB envronment and pressng the pla button. The graphcs are automatcall shown and updated for each step of tme durng the smulaton. A GUI has been developed for the ESR reactor n order to execute the smulaton and get the results outsde the SIMULIN s programmng envronment, whch wll facltate the usage for the controller development and to smulate ndustral cases. The fg. 8 shows the GUI bult b the authors. gure 6 - MATLAB GUIDE USER'S INTERACE APPLIED TO THE ESR Observng the results obtaned n the smulatons for each tpe of dsturbances, the dnamc behavour obtaned for the ESR operaton has followed the expected behavour provded b chemcal reactons engneerng theor. The stead state smulaton have demonstrated that the ethanol s rapdl consumed near at the reactor entrance and the temperature drops down due to endothermc reacton effects, whch s reduced some ponts farl far from the entrance and the temperature ncreases due to the heat transfer from the hot gases n the jacket. Also, Analzng the dnamc smulaton, the after step changes have been done on the nlet condtons the reactor behavour has matched wth what was expected b the theor. Once the nlet temperature s ncreased, both outlet temperature and hdrogen composton ncrease due to the Le Chateler s law, whch endothermc are favored b the ncrease of the temperature. Also, f the nlet flow rate s ncrease b a step change, the outlet temperature and hdrogen composton decrease due to the fact the resdence tme decrease and the reactants spend less tme nsde the reactonal envronment. The MATLAB Gude User s Interface can be consdered an mportant step to make the smulaton more realstc and sutable for process control analss, once t can represent a real ndustral nterface. The dnamc stud for the boethanol steam reformng reactor s mportant once t provdes the hdrogen for the fuel cell and ts operaton can be quckl affected due to the fast response front dsturbances n the reactor condtons. The valdaton of the model has been done usng data from other smulatons; however, the authors are workng on to fnd expermental data, whch s beng challenge due to the fact ths technolog s new. NOMENCLATURE

7 nlet ndex node ndex j component ndex k reacton ndex r k reacton rate of reacton k (mol/g s) k k rate constant of reacton k k equlbrum constant of reacton k j adsorpton constant of component j j molar composton of component j P total pressure (Pa) r total molar flow rate (mol/s) T temperature () Δz dscretzed element of space (m) G mass flux (kg/m s) L reactor length (m) D reactor dameter (m) D p catalst dameter (m) U overall heat transfer coeffcent (W/m ) C p mxture molar heat capact (J/mol ) ΔH R,k reacton heat of reacton k (J/mol) A c reactor cross-sectonal area (m ) w cat mass of catalst (g) Hot Gases Temperature () T a Processng: Process Intensfcaton, (), 9-. ogler, H. S. (). Elements of chemcal reacton engneerng. Upper Saddle Rver, N.J: Prentce Hall PTR. Greek Smbols µ vscost (Pa.s) Φ porost j,k stochometrc coeffcent of component j n reacton k ρ mxture denst (kg/m ) ρ c catalst denst (kg/m ) REERENCES erold, D., Basualdo, M. S., & Outbb, R. (1). PEM fuel cells wth bo-ethanol processor sstems: A multdscplnar stud of modellng, smulaton, fault dagnoss and advanced control. London: Sprnger. Holland, B. J., Zhu, J. G., & Jamet, L. (7). uel cell technolog and applcaton. Unverst of Technolog, Sdne. Bose, T. & Bénard, P. (). Hdrogen as an energ vector. In: Proceedngs of the power engneerng, large engneerng sstems conference. Degluomn, L. N., Bset, S., Lupp, P., & Basualdo, M. (1). Control orented dnamc model of the bo-ethanol processor sstem., do:1.17/ _1 Nummedal, L., Røsjorde, A., Johannessen, E., & jelstrup, S. (5). Second law optmzaton of a tubular steam reformer. Chemcal Engneerng and