Combined Partial Oxidation and Carbon Dioxide Reforming Process: A Thermodynamic Study

Transcription

1 American Jurnal f Applied Sciences 8 (1): 9-14, 2011 ISSN Science Publicatins Cmbined Partial Oxidatin and Carbn Dixide Refrming Prcess: A Thermdynamic Study Smpp Jarungthammachte Department f Mechanical Engineering, Kasetsart University Si Racha Campus, Sukhumvit Rad, Si Racha, Chnburi, 20230, Thailand Abstract: Prblem statement: CO 2 refrming is ne f the methds t utilize a greenhuse gas t prduce syngas, an imprtant feed fr methanl and Fischer-Trpsch synthesis. Hwever, CO 2 refrming is strng endthermic reactin requiring large amunt f supplied energy. Partial xidatin, an exthermic reactin, is cmbined with CO 2 refrming t serve the energy requirement. Thus, the ptimum ranges f O 2 and CO 2 fed t the prcess crrespnding t feedstck are needed t find. Mrever, ne f the mst imprtant prblems fund in this prcess is slid carbn frmatin. Therefre, the perating range in which the carbn frmatin can be avided is als required t study. Apprach: In this study CH 4 was used as feedstck. The ptimum rage f O 2 and CO 2 fed t the prcess was fund by using thermdynamic equilibrium methd based n minimizatin f Gibbs free energy. The Lagrange multiplier methd was cnducted t frm the equatins and they were slved by the Newtn-Raphsn methd. The slid carbn frmatin zne was als simulated. Results: The simulatin shwed that higher reactin temperature caused higher CH 4 and CO 2 cnversins. Syngas prductin increased with increasing temperature. Operating the prcess with high temperature r high O 2 /CH 4 and CO 2 /CH 4 ratins culd eliminate slid carbn frmatin. Increase f O 2 /CH 4 ratin higher than 0.1 led decreasing syngas while increase f CO 2 /CH 4 ratin caused increasing H 2 and CO. Hwever, when CO 2 /CH 4 ratin was higher than 0.85, increasing CO 2 /CH 4 ratin shwed insignificant change f syngas cncentratin. Cnclusin: The cmbined partial xidatin and CO 2 refrming f methd shuld be perated with reactin temperature f 1050 K. The ptimum range f CH 4 :CO 2 :O 2 fr this prcess is 1: : Key wrds: Partial xidatin, carbn dixide refrming, syngas prductin, slid carbn frmatin, greenhuse gases, increasing temperature, Fischer-Trpsch, thermdynamic study INTRODUCTION CO 2 is ne f the mst imprtant greenhuse gases prduced frm cmbustin prcess (Al-Mutairi and Kushki, 2009; Bundela and Chawla, 2010; Khademi et al., 2009). It can be utilized as reactant fr hydrcarbn refrming t prduce synthesis gas (als called syngas). The syngas, which cnsists f H 2 and CO, is a feedstck fr the methanl and Fischer- Trpsch syntheses (Tsang et al., 1995; Frment, 2000). CO 2 refrming prcess is smetime called dry refrming, which is strng endthermic reactin. Thus, it requires large amunt f heat supplied t the prcess. CO 2 refrming has been widely investigated (Edwards, 1995; Wurzel et al., 2000; Nagaka et al., 200; Li et al., 2004). Bth equilibrium mdel simulatin and experiment were cnducted by Haghighi et al. (2007) t study CO 2 refrming f CH 4. The result has been presented that equilibrium H 2 /CO rati at high temperature was abut 1.0. Hwever, at a specific temperature, higher pressure caused lwer H 2 /CO rati. 9 Mrever, the authrs indicated that a depsited carbn was als ne f the majr prducts btained frm CO 2 refrming. Fr cke (carbn) frmatin, it has been mentined by many researchers that it is an imprtant prblem f the dry refrming reactin, because it leads t catalyst deactivatin (Wurzel et al., 2000; Nagaka et al., 200; Shamsi and Jhnsn, 2003). Partial xidatin f hydrcarbn can be explained as a sub-stichimetric cmbustin prcess and it is an exthermic reactin. The thermdynamic equilibrium f ethanl partial xidatin has been studied by Wang and Wang (2008). At cmplete cnversin, % yield f hydrgen and % cncentratin f CO culd be achieved at K. Partial xidatin f methane t hydrgen and carbn mnxide ver a Ni/TiO 2 catalyst has been experimentally investigated (Wu et al., 2005). Use f fixed-bed reactrs fr the partial xidatin has shwn a drawback f ht spt (Puwant and Muharam, 2006). Ht spt frmatin is a cmmn prblem fund in catalytic partial xidatin (Nait et al., 2008) and it can cause deactivatin f catalysts.

2 Frm the abve infrmatin, the idea t cmbine the partial xidatin and CO 2 refrming prcesses is presented t slve the high energy requirement prblem f CO 2 refrming. Heat prduced frm partial xidatin prcess is supplied t CO 2 refrming prcess. Therefre, the cmbined prcess is a thermally selfsustaining and may nt need t cnsume external thermal energy. The cmbinatin f exthermic and endthermic reactins is called autthermal reactin. The cmbined steam refrming and partial xidatin, which is the well knwn hydrgen prductin prcess, is an example f the autthermal reactin. In this study, the partial xidatin prcess cmbined with CO 2 refrming prcess is investigated thrugh thermdynamic equilibrium mdel. Methane is chsen as feedstck. The effect f reactin temperature n CH 4 and CO 2 cnversins is fcused. Mrever, the reactin temperature is varied t bserve the change f cncentratin f prducts. The frmatin f cke (carbn) is an imprtant issue examined in this study. The effects f CO 2 /CH 4 and O 2 /CH 4 ratis n syngas prductin are als presented. Finally, the ptimum cnditin t syngas yield is suggested. MATERIALS AND METHODS Chemical reactins: The majr gas species cnsidered in this study are CH 4, CO, CO 2, H 2 and H 2 O. Slid carbn is als bserved. The partial xidatin cmbined with CO 2 refrming f methane is invlved with the predminance chemical reactins, which are listed belw: CH4 + 2O2 CO2 + 2H2O (1) 1 CH4 + O2 CO + 2H2 (2) 2 CH4 + H2O CO + 3H2 (3) CH4 + 2H2O CO2 + 4H2 (4) CH4 + C2O 2CO + 2H2 (5) CO + H2O CO2 + H2 (6) 2CO CO2 + C (7) CH4 2H2 + C (8) The chemical equilibrium mdel: The chemical equilibrium mdel can be categrized int 2 types. The first ne is equilibrium mdel based n equilibrium cnstants. It is knwn as the stichimetric mdel. This methd requires the infrmatin f chemical reactins ccurring in the prcess. Anther kind f equilibrium Am. J. Applied Sci., 8 (1): 9-14, mdel is nn-stichimetric mdel, s called the minimizatin f Gibbs free energy methd in which n chemical reactins are invlved. The equilibrium mdel used in this study is develped fllwing the methd discussed in Ref (Jarungthammachte and Dutta, 2008; Jarungthammachte, 2009). At equilibrium state, the ttal Gibbs free energy f the system is minimized. The ttal Gibbs free energy f system is defined as: N N t G = ng i i = niμi i= 1 i= 1 Where: n i and μ i G i (9) = The number f mles and the chemical ptential f species i, respectively = The partial mlar Gibbs free energy f species i If all gases are assumed as ideal gas and all reactins take place at ne atmspheric pressure. The chemical ptential f species i can be presented by: ( ) μ =Δ G + RTln y (10) i f,i i Where: R and T = The universal gas cnstant and temperature, respectively y i = The mle fractin f gas species i and it is the rati f n i and the ttal number f mles in the reactin mixture Δ G f,i = The standard Gibbs free f frmatin f species i Fr slid carbn, the partial mlar Gibbs free energy is: G C(S) Δ G (11) f,c(s) T find the minimum pint, the Lagrange multiplier methd is cnducted with cnstraint f mass balance, i.e.: N aijni = A j,j= 1,2,3,...,k (12) i= 1 Where: a ij = The number f atm f the jth element in a mle f the ith species A j = Defined as the ttal number f atm f jth element in the reactin mixture Frm Eq. 9-12, Lagrangian functin is frmed and partial derivative is applied. The final simultaneus equatins, therefre, can be develped as: ΔG k f,i n λ i j + ln + ai,j = 0, i = 1,..., N (13) RT n tt j= 1RT

3 Am. J. Applied Sci., 8 (1): 9-14, 2011 where, λ j, λ = λ 1,, λ k are Lagrange multipliers. The slutins n i have t be real numbers in the bundary such that 0 n i n tt. Equatin 13 creates the set f nn-linear equatins and thse are slved alng with cnstraint equatins by iteratin technique, the Newtn-Raphsn methd. The data frm the NASA technical memrandum 4513 (McBride et al., 1993) is emplyed t calculate all thermdynamic prperties in this mdel. RESULTS AND DISCUSSION The effect f temperature n reactant cnversins and gas yields: The simulatin f cmbined partial xidatin and CO 2 refrming was cnducted with CH 4 :CO 2 :O 2 feed rati f 1.0:1.0:0.2. The cnversin f reactants affected by reactin temperature is shwn in Fig. 1. The cnversins f CH 4 and CO 2 are defined in Eq. 14 and 15, respectively: ( CH4,in CH4,ut ) CH4 cnversin (%) = 100 (14) CH 4,in ( CO2,in CO2,ut ) CO2 cnversin (%) = 100 (15) CO 2,in higher at lwer temperature. At 800 K, H 2 /CO rati is abut 5, while H 2 /CO rati is 1.09 at 1000 K. It is clearly seen that H 2 /CO rati appraches t 1 at high temperature. This als cnfirms that CO 2 refrming is predminant (reactin (5)) at high temperature. Hwever, at lw temperature, the influence f partial xidatin f methane (reactin (2)) utstandingly Fig. 1: Effect f temperature n CH 4 and CO 2 cnversins where subscripts in and ut represent mle f substance at the inlet and utlet, respectively. Frm Fig.1, it can be bserved that the temperature strngly affects n CH 4 cnversin. It increases when temperature increases. The cnversin f 95% f CH 4 can be achieved at reactin temperature f 1050 K. Fr CO 2 cnversin, it increases with increasing temperature. At higher temperature, CO 2 cnversin dramatically increases because CO 2 refrming is endthermic reactin favring high temperature. This reasn is supprted by increasing CO cncentratin when temperature increases, which is discussed in Fig. 2. The equilibrium prducts frm partial xidatin and CO 2 refrming are exhibited in Fig. 2. Increase f CO mle fractin shws similar trend as CO 2 cnversin. As previusly mentined, high temperature favrs CO 2 refrming and it prduces CO. Fr H 2, its cncentratin increases with increasing temperature. Hwever, at temperature higher than 1000 K, mle fractin f H 2 is quite cnstant. This is because CO 2 refrming and reverse water gas shift reactins are endthermic. High temperature nt nly supprts CO 2 refrming reactin t prduct H 2 but als favrs reverse water gas shift reactin t cnsume H 2. Decreasing H 2 O is clearly bserved at high temperature. That is the effect f reactins (3) and (4), which are strng endthermic reactins. Figure 3 illustrates H 2 /CO rati and syngas (H 2 +CO) prductin frm cmbined partial xidatin and CO 2 refrming. The value f H 2 /CO rati is 11 Fig. 2: Effect f temperature n reactin prducts Fig. 3: Effect f temperature n H 2 /CO ratin and syngas prductin

4 Am. J. Applied Sci., 8 (1): 9-14, 2011 increase in temperature. The slid carbn cannt be fund when reactin temperature is higher than 1025 K. Fig. 4: Effect f temperature n slid carbn frmatin Carbn frmatin: T study mre abut carbn frmatin in cmbined partial xidatin and CO 2 refrming prcess, the simulatin is carried ut at the reactin temperature f 1050K. Figure 5 presents that at CO 2 /CH 4 ratin f 1, higher fed O 2 leads t lwer carbn frmatin because mre O 2 intrduced t the prcess can react mre with carbn and frms CO and CO 2. Supplying O 2 where O 2 /CH 4 rati is higher than 0.15, it causes n carbn frmatin in the prcess. Cmparing with Fig. 4, it can be implied that carbn frmatin can be limited at lwer O 2 supply if the prcess is perated at higher temperature. At O 2 /CH 4 ratin f 0.2 and reactin temperature f 1050 K, carbn frmatin cannt be investigated at CO 2 /CH 4 rati higher than 0.8. Fig. 5: Carbn frmatin affected by O 2 /CH 4 ratin Fig. 6: Carbn frmatin affected by CO 2 /CH 4 ratin presents. Fr syngas, it dramatically increases at lw temperature and slightly changes after 1000 K. Fr Fig. 3, syngas prductin is abut 5.5 ml fr feeding 1 ml f CH 4 if the reactr is perated at temperature higher than 1000 K. The slid carbn frmed in this prcess is als investigated. It is an undesired cmpnent because it deactivates the catalyst and causes pressure drp in reactr. Frm Fig. 4, it elucidates that mle fractin f carbn frmed in the prcess decreases with the 12 Effect f fed CO 2 : The effect f CO 2 fed in t the prcess is investigated here. As shwn in Fig. 1, at 1050 K, the CH 4 cnversin is 95% and Syngas cncentratin nearly reaches the maximum. Frm Fig. 5, H 2 /CO rati is als abut 1 at this reactin temperature. Thus, the reactin temperature is fixed at 1050 K, while CO 2 supplied t the system is varied t study its effect n gas prductin. The ther reactants, e.g., CH 4 and O 2 are als fixed at CH 4 :O 2 f 1.0:0.2. Mrever, at 1050 K, slid carbn may nt be bserved. Even thugh the slid carbn is frmed, its cncentratin is very lw. Therefre, the equilibrium calculatin mits t cnsider slid carbn. Figure 7 exhibits the change f the mle fractin f sme imprtant prducts when CO 2 /CH 4 rati changes. The mle fractin f H 2 increases frm when CO 2 /CH 4 increases frm 0 t Adding mre CO 2 int the prcess, it causes drpping H 2 cncentratin. The similar behavir as H 2 is bserved frm the change f CO. The mle fractin f CO increases with increasing CO 2 fed t the prcess. Hwever, as seen frm Fig. 7, CO starts t decrease when CO 2 /CH 4 is equal t 1.0. Frm the abve discussin, it can be implied that feeding mre CO 2 int the reactr, it may nt prvide gd result. Excess CO 2 cannt be cnverted and it leads t increase f CO 2 in the prduct as exhibited in Fig. 7. The H 2 O cntent increases with increasing CO 2 /CH 4. That is the effect f reverse water gas shift reactin. Figure 8 presents the change f mle f syngas in the prduct cmpared with the change f CO 2 /CH 4 rati. The cncentratin f syngas sharply increases when CO 2 /CH 4 rati increases. Nevertheless, the increase f syngas is hardly bserved after CO 2 /CH 4 rati is higher than 1. Finally, it can be cncluded that at the reactin temperature f 1050 K, the ptimum CO 2 /CH 4 rati range based n syngas yield is and CH 4 cnversin in this range f CO 2 /CH 4 rati is %.

5 Am. J. Applied Sci., 8 (1): 9-14, 2011 Fig. 7: Effect f CO 2 /CH 4 rati n gas cmpsitins Fig. 10: Effect f O 2 /CH 4 rati n syngas prductin Fig. 8: Effect f CO 2 /CH 4 rati n syngas prductin prcess, its cncentratin starts t drp. Hwever, CO 2 cncentratin is bserved that it increases when fed O 2 increases. Fr the changes f CO and CO 2 cncentratin, they can be explained that, mre O 2 added int the reactr causes CH 4 cnversin t CO 2 and H 2 O, that is reactin (1). Mrever, it is als due t CO and H 2 xidatin with O 2 frmed CO 2 and H 2 O, respectively (Amin and Yaw, 2007). nsequently, H 2 and CO cntents reduce because they are cnverted t be H 2 O and CO 2, respectively. This explanatin is cnfirmed by increasing H 2 O and CO 2 illustrated in Fig. 9. As seen frm Fig. 10, the mle f syngas starts t reduce when O 2 /CH 4 rati is abut 0.1. If the maximum syngas yield is cnsidered, the ptimum O 2 /CH 4 rati shuld be 0.1. Hwever, the CH 4 cnversin matching with O 2 /CH 4 rati f 0.1 is abut 94% and 97% f CH 4 cnversin can be achieved at O 2 /CH 4 rati f 0.2. Therefre, the ptimum rage f O 2 /CH 4 rati is Fig. 9: Effect f O 2 /CH 4 rati n gas cmpsitins Effect f fed O 2 : T study the effect f added O 2 n the prductin f cmbined partial xidatin and CO 2 refrming, the rati f CO 2 /CH 4 is fixed at 1 and the reactin temperature is als maintained at 1050 K, while the rati f O 2 /CH 4 is varied frm 0 t 0.8. The results f equilibrium calculatin are shws in Fig. 9 and 10. Figure 9 presents the mle fractin f H 2 O, H 2, CO and CO 2. Fr CO, when mre O 2 is added int the 13 CONCLUSION The thermdynamic equilibrium methd was used t study cmbined partial xidatin and CO 2 refrming prcess. Methane was emplyed as feedstck. The first part f this study was dne based n CH 4 :CO 2 :O2 feed rati f 1.0:1.0:0.2. The results shwed that higher reactin temperature caused higher CH 4 and CO 2 cnversins. CO and H 2 prductin increased with increasing temperature. H 2 /CO rati was abut 1 when the reactin temperature was higher than 1000 K. Fr syngas, it sharply increased at lw temperature and slightly changed after 1000 K. Slid carbn frmatin culd nt be bserved after 1025 K. The next study was carried ut t investigate slid carbn frmatin due t the variatin f O 2 /CH 4 and CO 2 /CH 4 ratins at 1050 K. It was fund that slid carbn frmatin culd be eliminated by increasing O 2 /CH 4 and CO 2 /CH 4 ratins. The effect f fed CO 2 and O 2 n syngas prductin was

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