Effect of Water Gas Shift Reaction on the Non-Isothermal Reduction of Wustite Porous Pellet Using Syngas

Transcription

1 Internatonal Journal of ISSI, Vol.8 (211, No.2, pp.9-15 Effect of Water Gas Shft Reacton on the Non-Isothermal Reducton of Wustte Porous Pellet Usng Syngas M. S. Valpour 1 * and M. H. Mokhtar 2 Heterogeneous Reactons Research Group, School of Mechancal Engneerng, Semnan Unversty, P. O. Box: , Semnan, Iran Abstract Effect of water gas shft reacton (CO+H 2 O=CO 2 +H 2 on wustte reducton has been nvestgated by a transent, non-sothermal mathematcal model based on gran model. In ths model, wustte porous pellet s reduced usng Syngas, namely a mxture of hydrogen, carbon monoxde, carbon doxde and water vapor. For ths purpose, governng equatons contanng contnuty equaton of speces and energy equaton have numercally been solved by fnte volume fully mplct method. The model has been valdated by comparng t wth expermental data from lterature. It was found a good agreement between model results and expermental data. Model results have been presented for the pellet reducton ncludng wth and wthout Water Gas Shft Reacton (WGSR. It was found that n pellet scale model, the ect of WGSR s not consderable on the rate of wustte reducton and temperature dstrbuton nsde the pellet. However t affects the dstrbuton of mole fracton of gaseous speces consderably. Keywords: Water Gas Shft Reacton (WGSR, Drect reducton, Wustte pellet, Syngas, Reducton rate, Fnte volume method Introducton 1 Reducton of hematte ron oxde s usually proceeded at three stages,.e. Reducton of hematte to magnette, magnette to wustte and wustte to ron. Among these stages, the reducton of wustte to ron s the slowest stage 1. Therefore, t can be consdered as the rate controllng step n hematte reducton. Many mathematcal models have been reported to understandng the reducton behavor of ron oxdes n drect reducton processes. Most of these models deal wth hematte reducton 2-11 and accordng to the best our knowledge, only two models about wustte reducton has been reported. At the frst one, Usu et al. 12 have developed a mathematcal model for sothermal reducton of wustte pellet usng hydrogen and at the another one, Valpour and Khoshandam 13 have developed a model to nvestgate non-sothermal reducton of wustte porous pellet usng Syngas, namely a mxture of hydrogen, carbon monoxde, carbon doxde and water vapor. A sgnfcant shortcomng of these models s neglectng the ect of WGSR. For typcal operatng condtons n an ndustral reducng shaft furnace usng a mxture of H 2 and CO, water gas shft reacton (WGSR s catalyzed by ron and ts oxdes. Therefore, t s the most mportant sde reacton 14 n drect ron reducton processes. In ths study, the ect of water gas shft reacton on wustte reducton has been nvestgated n pellet scale by a transent, non-sothermal mathematcal model based on gran model. 2. Concept of phenomena and assumptons In order to nvestgate the behavor of a sngle pellet n reducng gas flow, the pellet s assumed as a control volume. It s assumed that the sphercal porous pellet has been made of small dense grans wth almost constant radus. The reducng gas, ncludng manly of hydrogen and carbon monoxde, dffuses through pores of the pellet to react at the nterface and after the reacton wth wustte, products of the reacton dffuses back to the outsde of the pellet and bulk flow respectvely. Fgure 1 shows a schematc confguraton of a porous pellet under the gran model n reducton process. * Correspondng author: Tel: Fax: E-mal: msvalpour@semnan.ac.r Address: Heterogeneous Reactons Research Group, School of Mechancal Engneerng, Semnan Unversty, P. O. Box: , Semnan, Iran. 1. Assstant Professor 2. M.Sc.

2 Internatonal Journal of ISSI, Vol.8 (211, No.2 And overall fractonal reducton of the pellet may be estmated as an ntegraton of f over the entre pellet. 3 F = 3 R P R P r 2 f d r (6 Also, the rate of forward water gas shft reacton s gven by the followng equaton 15 : Fg. 1. Schematc confguraton of a porous pellet under the gran model n reducton process. Man reactons consdered n ths study are the wustte reducton reactons wth hydrogen and carbon monoxde and water gas shft reacton as follows: FeO+H 2 =Fe+H 2 O G T = T FeO+CO=Fe+CO 2 G T = T CO+H 2 O=CO 2 +H 2 G T = T T 57 C T 57 C Accordng to the concept of phenomena, the followng assumptons have been made to obtan governng equatons: The pellet has a sphercal shape wth a constant radus, R P. It has unform and constant porosty, ε. It s made of small dense grans wth constant radus, r g. The reducton of each gran s reversble, frst order and proceeds topochemcally. The Effect of water gas shft reacton s consdered. The Heat of reacton for WGSR s gven to gas phase. Total pressure s the same nsde and outsde of the pellet. There s no change n pellet dameter durng the reducton process and no crack s formed. 3. Governng equatons 3.1. Reacton rate equatons Accordng to the assumptons, the rate of reducton reacton of a gran usng reducng gas s gven as follows: 3(1 ε kr, e n r, = 1 ( 1 f ( C C r + (4 g Ke r, The local fractonal reducton of grans, defned as f =1-(r l /r g 3, s calculated by equaton (5, f = = H2,CO 3kr, ρo rg Ke r, ( 1 f 2 3 e ( C C (1 (2 (3 (5 (7 n 2 = ε k ( 2 wpt yco yh 2O yh y 2 CO2 / Ke w, CO w 3.2. Contnuty equaton of speces Contnuty equaton for each component of the reducng gas s wrtten as a combnaton of dffuson and chemcal reactons as follows: ( εc = ( D ( εc = ( D, C n r, + ν, wn w,, = H,CO, C + nr, + ν, wnw,, = H 2O,CO2 2 (8 (9 where ν,w and ν,w are the stochometrc cocents of -th and -th speces appearng n water gas shft reacton. They are postve for the products and negatve for the reactants of WGSR Energy equaton Energy equaton s wrtten as a combnaton of conducton heat transfer and heat generaton or consumpton due to chemcal reactons as follows: ( ρc T P n r, = H 2,CO = ( H T, r, ( λ T + n ( H 3.4. Intal and boundary condtons w T, w + (1 The governng equatons are subected to the followng boundary and ntal condtons: On pellet surface, r=r P, contnuty of heat and mass fluxes yelds equatons (11 to (13. T t λ = h( Ts t Tb (11 C t D, = km, ( C t Cb, (12 C t D, = km, ( C t Cb, (13 1

3 Internatonal Journal of ISSI, Vol.8 (211, No.2 Symmetry at the centre of pellet, r=, yelds followng boundary condtons: T (, t = (14 C (, t = (15 C (16 (, t = Intal values of tme-dependent varables are defned as follows: T ( r, = T (17 C = (18 ( r, C, C = (19 ( r, C, f ( r, = f (2 4. Parameter estmaton Physcochemcal parameters such as heat and mass transfer cocents, ectve dffusvty, ectve thermal conductvty and ectve heat capacty are estmated by emprcal relatons. These emprcal relatons have been ntroduced by the authors n the prevous paper Frequency factor and actvaton energy Frequency factor and actvaton energy are requred n order to calculate the reacton rate constant wth regards to the Arrhenus law. Frequency factor, actvaton energy and equlbrum constant are lsted n Tables 1 and 2 for reducton by hydrogen, equaton (1, and water gas shft reacton, equaton (3, respectvely. Reacton rate constant for reducton by carbon monoxde s assumed 1/5 of the reducton by hydrogen 16. For water gas shft reacton, when the reducton rate s under 5%, the rate constant s catalyzed by FeO and for the reducton rate above 5%, rate constant s catalyzed by Fe. 5. Numercal soluton Nearly all equatons are nonlnear and coupled. Moreover, physcochemcal parameters such as rate constants, equlbrum constants and etc. depend on temperature. So, the analytcal soluton wll be complcated and the governng equatons are solved by a computatonal method as fnte volume fully mplct method. In ths method, the governng equatons are dscretzed usng control volume approach 17. Dscretzaton process has been dscussed n detal n prevous papers 1, 18. After dscretzaton, the partal dfferental governng equatons are reduced to a large set of coupled lnear algebrac equatons. Ths set of equatons s solved by ndrect teratve procedure as tr-dagonal matrx algorthm (TDMA method 17. The fully mplct method has been appled to ensure numercal stablty and to allow the use of a large tme step, whch s a desrable feature when dealng wth reacton problems Model valdaton Expermental data reported by Usu et al. 12 for the overall fractonal reducton of a reagent wustte pellet has been used for the valdaton of model estmatons. The model s run to smulate the sothermal reducton process n the atmosphere of hydrogen. Fgure 2 shows a comparson between the expermental data and predctons of the present model. As shown n ths fgure, there s a good agreement between model estmatons and expermental data. Table 1. Rate parameters for the reducton of wustte by hydrogen 13 Reacton k,r (m s -1 E a (J mol -1 Ke FeO + H 2 Fe + H 2 O exp ( /T Table 2. Rate parameters for water gas shft reacton 15 Reacton Catalyst k,w (mol s -1 m -3 atm -2 E a (J mol -1 Ke 6 Fe CO + H 2 O = CO 2 + H 2 FeO exp (3863.7/T

4 Internatonal Journal of ISSI, Vol.8 (211, No.2 Fgure 4 shows temperature dstrbuton at three dfferent postons wthn the pellet n WWGSR. As shown n ths fgure, three temperature dstrbutons are very close to each other. Ths fgure shows that temperature dstrbuton wthn the pellet depends mostly on tme and t s not consderably affected by poston nsde the pellet. Fgure 5 shows the nfluence of WGSR on temperature dstrbuton at r=r P /2. Ths fgure shows that the ect of WGSR s very small and t s as lowerng the temperature. Ths small ect may be due to the small reacton rate of water gas shft reacton. Fg. 2. Comparson of the overall reducton rate for a reagent wustte pellet by hydrogen among model results and expermental data 12 (R P =6 mm, ε=.44, T b =1173 K. 7. Results and dscusson Results are presented n two ways. At the frst one, the smulaton s appled for WGSR whch s denoted by WWGSR. In the Second way, smulaton s done wthout WGSR whch s denoted by NWGSR. Pellet characterstcs and reducng gas parameters whch are used n smulatons are ndcated n Table 3. Table 3. Pellet characterstcs and reducng gas parameters used n model estmatons Fg. 4. Temperature dstrbuton versus tme at three dfferent postons wthn the pellet n WWGSR. R P (mm ε T b (K P t (bar y H2,b y H2O,b y CO,b y CO2,b Effect of water gas shft reacton Effect of WGSR on the wustte reducton rate s llustrated n Fgure 3. As shown n ths fgure, ect of WGSR on the rate of wustte reducton s very small. When t s focused n ths fgure, the reducton rate s lowered due to WGSR. Fg. 5. Influence of WGSR on temperature dstrbuton at r=r P /2. Fg. 3. Influence of WGSR on the wustte overall reducton rate. Effect of WGSR on the mole fracton of gaseous speces wthn the pellet s shown n Fgure 6. It s ndcated n ths fgure that: In NWGSR, mole fracton of each component of the reducng gas at all three dfferent postons wthn the pellet tends to be the same value. However, n 12

5 Internatonal Journal of ISSI, Vol.8 (211, No.2 WWGSR model, these mole fractons tend to be dfferent values. Ths ect may be explaned as follows. In NWGSR, when the pellet s completely reduced, there s no more change n gaseous mole fractons by reducton reactons. But n WWGSR, after completon of the reducton, WGSR s stll proceeded and catalyzed by Fe. Therefore t can change the gaseous speces mole fractons. Wth regards to Fgure 4, ncreasng temperature and over tme, the equlbrum constant (Ke of WGSR decreases (see Table 2. Thus, accordng to mole fractons n Table 3 and equaton (7, WGSR proceeds n reverse drecton. Consequently, hydrogen and carbon doxde are consumed and carbon monoxde and water vapour are produced. The ect of WGSR on mole fractons s maxmum at r=, mnmum at r=r P and ntermedate at r=r P / Effect of gas rato and gas utlty In ndustral reducton processes, gas rato (β=h 2 /CO commonly vares n the range of <β<4. and gas utlty (γ=(h 2 +CO/(H 2 O+CO 2 may be changed n the range of 5<γ<49 dependng on the gas reformng system 16. Fgure 7 shows the tme requred for 85% reducton of the wustte pellet usng gas mxtures wth a constant gas utlty and varable gas rato n WWGSR and NWGSR. As shown n ths fgure, n both WWGSR and NWGSR, the tme requred for 85% reducton s decreased as gas rato s rased. The ect of WGSR on ths fgure s very small and t s as lowerng the reducton rate. The tme requred for 85% reducton of the wustte pellet vs. gas utlty s llustrated n Fgure 8. As shown n ths fgure, the rate of reducton s ncreased as the gas utlty s rased, n both WWGSR and NWGSR. Smlar to gas rato, the ect of WGSR on Fgure 8 s very lttle and t s as lowerng the reducton rate. (a (b (c (d Fg. 6. Effect of WGSR on the gaseous speces mole fractons wthn the pellet: (a Hydrogen mole fracton, (b Carbon monoxde mole fracton, (c Water vapour mole fracton and (d Carbon doxde mole fracton. 13

6 Internatonal Journal of ISSI, Vol.8 (211, No.2 Lst of symbols C C P concentraton of the gaseous speces, (mol m -3 specfc heat at constant pressure, (J Kg -1 K -1 D dffusvty of the gaseous speces, (m 2 s -1 E a actvaton energy, (J mol -1 Fg. 7. Tme requred for 85% reducton of the wustte pellet vs. gas rato. f, F local fractonal reducton of grans and overall fractonal reducton, respectvely, (- h convectve heat transfer cocent, (W m -2 K -1 (-ΔH T heat of reacton at temperature T, (J mol -1 k m mass transfer cocent of the gaseous speces through gaseous flm, (m s -1 k r k w rate constant of reducton reactons, (m s -1 rate constant of the water gas shft reacton, (mol s -1 m -3 atm -2 k,r frequency factor of reducton reactons, (m s -1 k,w frequency factor of the water gas shft reacton, (mol s -1 m -3 atm -2 Ke equlbrum constant, (- Fg. 8. Tme requred for 85% reducton of the wustte pellet vs. gas utlty. n rate of chemcal reactons, (mol m -3 s -1 P pressure, (atm 8. Concluson r radal coordnate n the pellet, (m A transent and non-sothermal mathematcal model based on gran model s developed to nvestgate the ect of WGSR on the rate of wustte reducton wth and wthout WGSR. Model results were presented wth and wthout WGSR. The followng results have been obtaned: 1. Effect of WGSR on the wustte reducton rate s very lttle and t s lowered due to WGSR. 2. Temperature dstrbuton wthn the pellet s mostly a functon of tme and t s not affected by the poston nsde the pellet. 3. Effect of WGSR on temperature s very small and the temperature s lowered due to WGSR. 4. Wth consderng WGSR, the mole fractons of carbon monoxde and water vapour are ncreased and the mole fractons of hydrogen and carbon doxde are decreased. 5. The rate of reducton s ncreased as gas rato and gas utlty are rased. However, the rate of reducton decreases due to WGSR. r g, r l R P t T y rad of a gran and reacton nterface wthn a gran, respectvely, (m pellet radus, (m tme, (s temperature, (K mole fracton of components of the reducng gas, (- β=h 2 /CO reducng gas rato, (- ε porosty of the pellet, (- γ = (H 2 +CO/ (H 2 O+CO 2 reducng gas utlty, (- λ thermal conductvty, (W m -1 K -1 ρ mass densty, (Kg m -3 ρ O molar densty of reducble oxygen wthn the grans, (g-atom O m -3 14

7 Internatonal Journal of ISSI, Vol.8 (211, No.2 Subscrpts b related to bulk flow of Syngas ectve parameters related to gaseous reactants n reducton reactons (H 2 and CO related to gaseous products n reducton reactons (H 2 O and CO 2 r, w related to reducton reactons and the water gas shft reacton, respectvely s t related to sold phase (wustte related to total value of parameters related to ntal value of parameters Superscrpts e References related to equlbrum condton [1] M. S. Valpour and Y. Sabooh: Modellng Smul. Mater. Sc. Eng., 15(27, 487. [2] R. H. Sptzer, F. S. Mannng and W. O. Phlbrook: Trans. Metall. Soc. AIME, 236(1966a, 726. [3] R. H. Sptzer, F. S. Mannng and W. O. Phlbrook: Trans. Metall. Soc. AIME, 236(1966b, [4] R. H. Ten and E. T. Turkdogan: Metall. Mater. Trans. B, 3(1972, 239. [5] Q. T. Tsay, W. H. Ray and J. Szekely: AIChE J., 22(1976, 164. [6] Y. Hara, M. Sakawa and S. Kondo: Tetsu-to- Hagane, 62(1976, 315. [7] E. D. Negr, O. M. Alfano and M. G. Chovetta: Chem. Eng. Sc., 42(1987, [8] K. O. Yu and P. P. Glls: Metall. Mater. Trans. B, 12(1981, 111. [9] A. Bonalde, A. Henrquez and M. Manrque: ISIJ Int., 45(25, [1] M. S. Valpour, M. Y. Motamed Hashem and Y. Sabooh: Adv. Powder Technol., 17(26, 277. [11] M. S. Valpour: Scenta Iranca, Transactons C: Chemstry and Chemcal Engneerng, 16(29, 18. [12] T. Usu, M. Ohm and E. Yamamura: ISIJ Int., 3(199, 347. [13] M. S. Valpour and B. Khoshandam: Ironmakng and Steelmakng, 36(29, 91. [14] E. D. Negr, O. M. Alfano and M.G. Chovetta: Ind. Eng. Chem. Res., 34(1995, [15] M. Ishgak, R. Takahash and Y. Takahash: Bull. Res. Inst. Mn. Dressng Metall., 38(1982, 35. [16] Y. Takenaka, Y. Kmura, K. Narta and D. Kaneko: Comput. Chem. Eng., 1(1986, 67. [17] H. K. Versteeg and W. Malalasekera: An ntroducton to computatonal flud dynamcs: The fnte volume method, Addson-Wesley, (1996. [18] M. S. Valpour and Y. Sabooh: Heat and Mass Transfer, 43(27,