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Mechanistic kinetic modeling of oxidative steam reforming of bioethanol for hydrogen production over Rh-Ni/CeO2-ZrO2 catalyst Tarak Mondal, Kamal K. Pant, and Ajay K Dalai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03828 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 15, 2015

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Industrial & Engineering Chemistry Research

Mechanistic kinetic modeling of oxidative steam reforming of bio-ethanol for hydrogen production over Rh-Ni/CeO2-ZrO2 catalyst Tarak Mondal1, Kamal K. Pant1,* and Ajay K. Dalai2 1,*

Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016 (India) 2 Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon, SK S7N5C5 (Canada) * Corresponding author email: [email protected] Abstract A kinetic study was carried out over Rh-Ni/CeO2-ZrO2 catalyst for the oxidative steam reforming of ethanol (OSRE). Langmuir-Hinshelwood approach based on proposed surface reaction mechanisms was used to develop the kinetic models for OSRE process. Oxidative steam reforming (OSRE), ethanol decomposition (ED) and water gas shift (WGS) reactions were considered as the main reaction pathway to represent the overall OSRE process. Kinetic data were collected in a fixed bed reactor under kinetic-control regime at three different temperatures. The kinetic parameters were estimated using a non-linear regression method. The kinetic model was developed by considering dehydrogenation of adsorbed ethoxy species, decomposition of formate species, and decomposition of acetaldehyde as the rate determining step for OSRE, WGS and ED reactions respectively. The developed model fitted well with the experimental observations at all studied temperatures and contact time. The activation energy for OSRE, WGS and ED reactions obtained were 56.0, 46.1 and 34.8 kJ/mol, respectively. The results revealed that the proposed Langmuir-Hinshelwood mechanistic kinetic model (Model LH-II) is suitable for the OSRE process. Keywords: Kinetic modeling; Langmuir-Hinshelwood mechanism; Oxidative steam reforming; Bio-ethanol; Hydrogen production; Rh-Ni/CeO2-ZrO2 catalyst. 1 ACS Paragon Plus Environment


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1. Introduction Hydrogen is one of the most appropriate energy carrier and is able to produce sustainable energy, which can enormously reduce dependence on fossil fuel energy sources. Hydrogen productions from biomass-derived sources have attracted much attention because of its capability to produce energy with higher efficiency, pollutants-free emission and diverse applications and for that conversion of renewable biomass to H2 is a promising route. Bio-ethanol is one of the most suitable feedstock because of its attractive features viz. high hydrogen content, nontoxicity, growing availability, reliability and low production cost. Various methodologies like steam reforming (oxidative and non-oxidative) and partial oxidation were widely reported in the literature for the production of hydrogen from ethanol. Steam reforming of ethanol (SRE) is the most convenient route among these three processes, as it offers highest hydrogen yield. Whereas, oxidative steam reforming of ethanol (OSRE) provides a balance between the requirement of energy and hydrogen yield as it is a combination of the endothermic steam reforming and the exothermic partial oxidation reactions. The presence of oxygen in the feed also facilitates the removal of carbon deposits and inhibit catalyst deactivation during reforming process1–5. A simplified power law model was widely used for kinetic study of steam reforming of ethanol as reported by various researchers and very few data are available based on mechanistic approach6–8. Akande et al.9 applied a power law model to estimate the reaction order and activation energy (0.43 and 4.41 kJ/mol, respectively) for crude bio-ethanol reforming over Ni/Al2O3 catalyst. Akpan et al.10 also investigated kinetics for catalytic reforming of bio-ethanol using an Eley-Rideal mechanism assuming dissociation of adsorbed ethanol as the rate determining step (RDS). Similarly, Patel et al.11 performed kinetic study of SRE on Ni based catalysts using Eley-Rideal mechanism and obtained activation energy as 66.6 kJ/mol. Mathure 2 ACS Paragon Plus Environment

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et al.12 also performed the kinetic study of SRE over Ni catalyst and calculated activation energy as 21 kJ/mol using an Eiley-Rideal based reaction model. Several studies were carried out using the Langmuir–Hinshelwood (L–H) approach7,10,13–21. Literature suggests that the ethanol steam reforming reaction occurs mainly through dehydrogenation of ethanol to ethoxy species formation on the catalyst surface13,14. Mas et al.15,16 performed a kinetic study on SRE using the Langmuir–Hinshelwood approach. Sahoo et al.17 investigated the kinetic study of SRE over Co/Al2O3 catalysts and developed a Langmuir–Hinshelwood kinetic model considering SRE, WGS and ED reactions. Limited articles have been reported, citing the elementary reactions for SRE9–12,15,17,22,23 and no study has been reported for OSRE. Moreover, the activation energy for SRE process reported in most of the literature is relatively lower, indicating the influence of internal mass transfer resistance; however, in many cases the details are not discussed. Therefore, it can be concluded from literature that more elaborated reaction scheme is required to analyse the complex reaction network occurring during the reforming process. Present work demonstrates the kinetic study of OSRE over 1%Rh-30%Ni/CeO2-ZrO2 catalyst with an effort to develop a mechanistic kinetic model. The elementary reactions considered are similar to SRE system by incorporating the effects of oxygen in the reaction scheme. The main difference between the present study and available literature is in the mechanism (Power law/Eley Rideal/LHHW) and rate determining steps. The LangmuirHinshelwood approach was used to develop the kinetic models. OSRE, ED and WGS reactions were considered as main reaction pathway to represent the overall OSRE process. The kinetic experiments were conducted in a fixed bed reactor at 773, 823 and 873 K under atmospheric pressure, and space times (Wcat/FEtOHo) of 0–2201.3 (kgcats/kmolEtOH) using water-to-ethanol 3 ACS Paragon Plus Environment


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molar ratio in the range of 3.0–9.0. Kinetic data were collected by maintaining plug flow conditions under kinetic control regime. The kinetic and thermodynamic parameters were evaluated using a non-linear least-square regression technique. The nonlinearity of the estimated parameters and any existing correlation among them were checked using a profile plots technique. The confidence intervals of the estimated parameters were calculated based on statistical t-distribution method. Moreover, the significance of the regression coefficient and global regression were evaluated by means of t-test and F-test, respectively.

2. Development of Langmuir-Hinshelwood mechanistic kinetic model The design of commercial ethanol steam reformer requires the development of appropriate kinetic model for hydrogen production from steam reforming. Therefore, present study aimed to obtain a rate equation based on fundamental kinetic studies and compare it with the experimentally observed data. Thus, the overall reaction mechanism for oxidative steam reforming process was described by several mechanistic steps and subsequently the rate equations were derived. H2 and CO2 were formed as the main products, with small amount of CO and CH4 during all the reforming experiments, due to limitation of equilibrium reactions like WGS and ED reactions. Therefore, the variation in the partial pressures of CO and CH4 was attended by considering the WGS and decomposition reactions in the kinetic modelling of OSRE process. Thus, OSRE, ED and WGS reactions (Rxns.(1)-(3)) were taken as the main reaction pathways to represent the overall oxidative steam reforming of ethanol process.

CH3CH2OH + 2H2O + 0.5O2 → 5H2 + 2CO2 ,

0 ∆H298 = 4.4 kJ / mol

4 ACS Paragon Plus Environment

(1)

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CH3CH2OH → CO + CH4 + H2 ,

0 ∆H298 = 49 kJ / mol

( 2)

CO + H2O

0 ∆H298 = m41 kJ / mol

( 3)

CO2 + H2 ,

Graschinsky et al.18 investigated a kinetic study of SRE on RhMgAl2O4/Al2O3 catalyst and formulated different kinetic expressions using elementary reaction steps taking into account the occurrence of two active sites during dissociative adsorption of ethanol in the rate determining steps. In an another study, Ciambelli et al.19 considered the dissociative adsorption of ethanol on the catalysts surface for kinetic evaluation of ethanol steam reforming process. The reaction mechanism of ethanol steam reforming proceeds via dehydrogenation of ethanol first to ethoxy species, and then into acetaldehyde, was proposed by many researchers13,14,17. Thus, kinetic models for OSRE were developed by considering the dissociative adsorption of ethanol on the catalysts surface to form ethoxy species, which subsequently transformed into adsorb acetaldehyde species and then converted to reforming products. For this purpose, two reaction mechanisms have been proposed for OSRE to develop two different Langmuir-Hinshelwood kinetic models. The first surface reaction mechanism (RM-I) for the oxidative steam reforming of ethanol is proposed as follows.

(4)

k

1 O 2 + 2 S ←1 , −→ 2 O .S

k

−2 CH 3 CH 2 OH ( g ) + O . S + S ←2 , → CH 3CH 2 O . S + HO . S

(5)

(6)

k

3,−3 H 2O ( g ) + 2 S ←→ HO. S + H .S

(7)

k

4,−4 HO . S + S ←  → O.S + H . S



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(8)

k

5,−5 CH 3CH 2 O . S + O . S ←→ CH 3CHO . S + HO . S

(9)

k

6 ,−6 CH 3CHO . S + O . S ←  → CH 3 . S + HCOO . S

(10)

k

7 ,−7 CH 3 . S + HO . S ← → CH 3OH . S + S

(11)

k

8,−8 CH 3OH . S + S ←→ CH 3O . S + H . S

(12)

k

9 ,− 9 CH 3O . S + O . S ←→ HCHO . S + HO . S

(13)

k

10,−10 HCHO.S +O.S ← → HCOO.S + H .S

(14)

k

11,−11 HCOOH .S + S ← → HCOO.S + H .S

(15)

k

12,−12 HCOO.S + S ← → CO2 . S + H .S

(16)

k

13,−13 HCOO . S + S ← → CO . S + HO . S

(17)

k

14 ,−14 CO . S + O . S ← → CO2 . S + S

(18)

k

15,−15 CH 3 .S + H . S ← → CH 4 . S + S

(19)

k

16 ,−16 CH 4 . S ← → CH 4 ( g ) + S

(20)

k

17 ,−17 CO . S ← → CO ( g ) + S

(21)

k

18,−18 CO2 .S ← →CO2 ( g )+S

(22)

k

19,−19 2 H .S ← →H2 ( g )+2S


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The elementary reactions were proposed using Langmuir-Hinshelwood (LH) method by assuming uniform adsorption sites which consider the entropy and enthalpy for the adsorption of all the species are equal. These elementary steps consisted of adsorption, surface reaction, and desorption terms, respectively. The rate of each elementary step was assumed at equilibrium. Thus, the site concentration of each intermediate species was expressed as the product of equilibrium constant, vacant cite concentration and partial pressure or concentration of component involve in that reaction. This was performed to eliminate unknown surface concentrations by measurable partial pressures. The overall equilibrium constant for each step were found by lumping some of the intermediate equilibrium constants to minimize the number of parameters and is termed as the equilibrium constant for that particular species17,20. The equilibrium constants thus obtained are written as following equations.

[O.S ] = K1PO1/2 CS = KO PO1/2 CS 2 2

[O.S ] = K 3 K 4 K17

1/ 2 [ HO.S ] = K 3 K17

PH 2 O PH 2 PH 2O PH1/22

( A)

CS = K O

C S = K OH

[CH3CH2O.S ] = KCH3CH2O

[CH3CHO.S ] = KCH3CHO

PH 2O PH 2

CS

PH 2O PH1/22

2 PCO P7/2 2 H2

PO2 PH2O

CS

(C )

CS

PCH3CH2OH PO1/2 2 PH2O

(B)

(D)

CS

( E)

[ HCOO.S ] = KHCOO PCO2 PH1/22 CS

(F )



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[CH3.S ] = KCH3

[CH 4 .S ] =

PCH 4 K14

PCO2 PH7/2 2 PH22O

CS

(G)

C S = K CH 4 PCH 4 C S

(H )

PCO C S = K CO PCO CS K15 PCO2 [CO2 .S ] = CS = K CO2 PCO2 CS K16

[CO.S ] =

[ H .S ] =

PH1/22 1/ 2 K17

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(I )

(J )

C S = K H PH1/22C S

(K )

A kinetic model (LH-I) was developed by considering dehydrogenation of adsorbed acetaldehyde species (Rxn. (9)), decomposition of formate species (Rxn. (16)), and decomposition of acetaldehyde as the rate determining step (RDS) for OSRE, rWGS and ED reactions, respectively. The adsorption/desorption reactions attain the equilibrium state relatively faster compare to surface reactions. Therefore, only surface reactions are considered as RDS to develop kinetic models16,17.

k

OSRE ,− OSRE CH3CHO. S +O. S ← →CH3 . S + HCOO.S

(23)

(24)

k

rWGS , − rWGS HCOO . S + S ←→ CO . S + HO . S

(25)

k

ED , − ED CH 3 CHO . S + S ← → CH 4 . S + CO . S


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The rate of reaction for OSRE reaction using Rxn. (23) as RDS can be written as:

  1 rO SRE = k O SRE  [ C H 3 C H O .S ][ O .S ] − [ C H 3 .S ][ H C O O .S ]  K O SRE  

(26)

 PCH 3CH 2OH PO1/2 2 rOSRE = kOSRE  ( K CH 3CHO K O )−  P H2 

(27)

rOSRE

1 K OSRE

( K CH 3 K HCOO

2 PCO PH42  2 2 )  CS PH22O 

2   2 PCH3CH2OH PO1/2 PCO P5 2 2 H2 1 = kOSRE KCH3CHO KO ( ) 1 − K * ×  CS 1/2 2  OSRE  PH2 P P P CH CH OH O H O 3 2 2 2  

Where,

K OSRE =

kOSRE k − OSRE

* K OSRE =

(28)

KOSRE KCH 3CHO K O K CH 3 K HCOO

Similarly, the rate for reverse WGS reaction using Rxn. (24) as RDS can be written as:

(

)

1 rrWGS = krWGS [HCOO.S]CS − KrWGS [CO.S][HO.S]

(29)

  PH2O 1 (KCO PCOCS )(KOH 1/2 rrWGS = krWGS  KHCOO PCO2 PH1/22 CS CS − KrWGS CS )    PH2  

(30)

 rrWGS = k rWGS K HCOO ( PCO2 PH1/22 )  1 −  

(31)

Where, K * = rWGS

1 * K rWGS

PCO PH 2 O  2  CS PCO2 PH 2 

K rWGS K HCOO KCO K HO

The rate of reaction for decomposition of ethanol Rxn. (25) as RDS can be written as:



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rED = k ED [CH 3CHO .S ]C S − k − ED [CH 4 .S ][CO .S ]

(32)

rED = k ED ([CH 3CHO .S ]C S −

(33)

1 K ED

[CH 4 .S ][CO .S ])

  PO1/2 rED = kED  KCH3CHO PCH3CH2OH 2 CS2 − K1ED × KCH4 KCO PCH4 PCOCS2    PH2O  

rED = k ED K CH 3CHO ( PCH 3 CH 2 OH

PO1/2 2  ) 1 − PH 2 O 

1 * K ED

×

PCH 4 PCO PH 2  2  CS PCH 3CH 2 OH 

(34)

(35)

Where, K ED =

k ED k − ED

* K ED =

K ED KCH 3CHO KCH 4 K CO

The site balanced for active sites was carried out by adding all the adsorption terms of intermediates adsorbed species. CT = CS + [O.S ] + [ HO.S ] + [CH 3 .S ] + [ HCOO.S ] + [CH 3CHO.S ] + [CH 3CH 2O.S ] + [ H .S ] + [CO.S ] + [CO2 .S ] +[CH 4 .S ] (36)


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CS =

1  PH O PCO2 P 1/2 1 + K O PO2 + K O 2 + K CH3 PH 2 PH22O 

7/2 H2

+ K HCOO PCO2 PH1/22 + K CH3CHO

PCH3CH 2OH PO1/2 2 PH 2O

+ K CH3CH 2O

2 PCO P 7/2 2 H2

PO2 PH 2O

+ K H PH1/22 + KCO PCO + KCO2 PCO2

 + KCH 4 PCH 4  

The final rate expressions were obtained by substituting the expression of CS (Rxn. (37)) in Rxns. (28), (31) and (35). The final forms of the rate expressions thus obtained are as follows: kOSRE K CH3CHO K O ( rOSRE =

rrWGS =

 2  CT 

  PH 2O PCO2 PH7/2 PCH 3CH 2OH PO1/2 1/2 1/2 2 2 + K CH 3 + K P P + K + K H PH1/22 + K CO PCO + K CO2 PCO2 + K CH 4 PCH 4   1 + K O PO2 + K OH 1/2 HCOO CO2 H 2 CH 3CHO 2  PH 2 PH 2O PH 2O  

 PCO PH 2O krWGS K HCOO ( PCO2 PH1/22 )  1 − K *1  rWGS P CO2 PH 2 

2

 2  CT 

  PH 2O PCO2 PH7/2 2 PCH 3CH 2OH PO1/2 2 1/ 2 1/ 2 + K CH 3 + K P P + K + K H PH1/22 + K CO PCO + K CO2 PCO2 + K CH 4 PCH 4   1 + K O PO2 + K OH 1/2 HCOO CO2 H 2 CH 3CHO 2  PH 2 PH 2O PH 2O  

k ED K CH3CHO ( rED =

2  PCH 3CH 2OH PO1/2 PCO PH52 2 2 )  1 − K *1 ×  OSRE PH 2 PCH 3CH 2OH PO1/2 2 PH22O 

 PCH 3CH 2OH PO1/2 PCH 4 PCO PH 2 2 )  1 − K1* ×  ED PH 2O PCH3CH 2OH 

 2  CT 

  PH 2O PCO2 PH7/2 PCH 3CH 2OH PO1/2 2 1/2 1/ 2 2 1 + K P + K + K + K P P + K + K H PH1/22 + K CO PCO + K CO2 PCO2 + K CH 4 PCH 4   O O2 OH CH 3 HCOO CO2 H 2 CH 3CHO 1/2 2  PH 2 PH 2O PH 2O  


2

2

(38)

(39)

(40)

(37)


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The second reaction mechanism (RM-II) was proposed assuming the availability of two distinct types of active sites on the catalyst surface for adsorption of different species. Therefore, it was assumed that the first type of site is active for hydrogen adsorption only, while the other site is active mainly for the adsorption of oxygen and carbon containing species. Adsorption sites for species containing oxygen and carbon is designated as S1 (Type 1) and the H adsorbing site is designated as S2 (Type 2) during second kinetic model (LH-II) development. The model LH-II was developed by considering dehydrogenation of adsorbed ethoxy species (Rxn. (43)), decomposition of formate species (Rxn. (52)), and decomposition of acetaldehyde as the rate determining step (RDS) for OSRE, rWGS and ED reactions, respectively. The other conditions were kept same as that of model-1 (LH-I). The following elementary reactions are considered in the second reaction mechanism.

k

1,− 1 CH 3CH 2 OH ( g ) + S1 + S 2 ← → CH 3CH 2 O . S1 + H . S 2

(41)

(42)

k

−2 O 2 + 2 S 1 ←2 , → 2 O . S1

(43)

k

3,−3 CH 3CH 2O . S1 + O . S1 ←→ CH 3CHO . S1 + HO . S1

(44)

k

4,−4 H 2O ( g ) + S1 + S2 ←  → HO. S1 + H . S2

( 45)

k

−5 HO . S1 + S 2 ←5 ,→ O . S1 + H . S 2

(46)

k

6 , −6 CH 3CHO . S1 + O . S1 ←  → CH 3 . S1 + HCOO . S1

(47)

k

7 ,−7 CH 3 . S1 + HO . S1 ← → CH 3OH . S1 + S1


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(48)

k

8,−8 CH 3OH .S1 + S2 ←→ CH 3O.S1 + H . S2

(49)

k

9,−9 CH 3O . S1 + O . S1 ←→ HCHO . S1 + HO . S1 k

10 ,−10 HCHO . S1 + O . S1 + S 2 ← → HCOO . S1 + H . S 2 + S1

(50)

(51)

k

11,−11 HCOO.S1 + S2 ← →CO2 .S1 + H .S2

(52)

k

12 ,− 12 HCOO . S1 + S1 ← → CO . S1 + HO . S1

(53)

k

13,−13 CO . S1 + O . S1 ← → CO2 . S1 + S1

(54)

k

14,−14 CH 3 . S1 + H .S2 ← → CH 4 . S1 + S2

(55)

k

15,−15 CH 4 . S1 ← → CH 4 ( g ) + S1

(56)

k

16,−16 CO . S1 ← → CO ( g ) + S1

k

(57)

k

(58)

,− 17 CO 2 . S1 ←17 → CO 2 ( g ) + S1

18 , − 18 2 H . S 2 ←  → H 2 ( g ) + 2 S 2

The site balanced for active sites S1 and S2 was obtained as CT1 = CS1 + [O.S ] + [ HO.S ] + [CH 3 .S ] + [ HCOO.S ] + [CH 3CHO.S ] + [CH 3CH 2O.S ] +[CO.S ] + [CO2 .S ] +[CH 4 .S ]

CT2 = CS2 + K H PH1/22CS2

(60)


(59)


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Page 14 of 58

The final rate expression for kinetic model LH-II can be written as follows:

rOSRE =

2  PCH3CH 2OH PO1/2 PCO PH52 2 2 1 kOSRE K CH 3CHO K O ( ) 1 − K * ×  OSRE PH 2 PCH 3CH 2OH PO1/2 2 PH22O 

 2  CT1 CT2 

(

rrWGS =

 PCO PH 2O krWGS K HCOO ( PCO2 PH1/22 )  1 − K *1  rWGS P CO2 PH 2 

 2  CT1 

  PH 2O PCO2 PH7/2 2 PCH3CH 2OH PO1/2 1/2 1/2 2 + K CH3 + K P P + K + K CO PCO + K CO2 PCO2 + K CH 4 PCH 4   1 + K O PO2 + K OH 1/2 HCOO CO2 H 2 CH 3CHO 2  PH 2 PH 2O PH 2O  

k ED K CH 3CHO ( rED =

(61)

2

  PH 2O PCO2 PH7/2 PCH3CH 2OH PO1/2 1/2 1/2 2 2 + K HCOO PCO2 PH 2 + K CH3CHO + K CO PCO + K CO2 PCO2 + K CH 4 PCH 4  1 + K H PH1/22  1 + K O PO2 + K OH 1/2 + K CH3 2  PH 2 PH 2O PH 2O  

 PCH 3CH 2OH PO1/2 PCH 4 PCO PH 2 2 )  1 − K1* ×  ED PH 2O PCH 3CH 2OH 

 2  CT1 

  PH 2O PCO2 PH7/2 PCH 3CH 2OH PO1/2 1/2 1/2 2 2 + K P + K + K + K P P + K + K CO PCO + K CO2 PCO2 + K CH 4 PCH 4  1  O O2 OH CH 3 HCOO CO2 H 2 CH 3CHO 1/2 2  PH 2 PH 2O PH 2O  


2

2

)

(62)

(63)

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The kinetic models LH-I and LH-II, both contain 13 parameters (3 rate constants and 10 equilibrium constants). The kinetic model developed by Langmuir-Hinshelwood method generally contains a large number of adjustable parameters24. In addition, strong correlation exists between many of the parameters e.g. heats of adsorption and entropies of adsorption. Therefore, the values of heats of adsorption were fixed with the values reported in literature25.

3. Experimental 3.1. Catalyst preparation and characterization The CeO2-ZrO2 support (1:1 (wt/wt) CeO2/ZrO2) was prepared by using co-precipitation method. The Ni/CeO2-ZrO2 catalyst was prepared by impregnating Ni on CeO2-ZrO2 using the wetness impregnation method. 1wt% Rh was added subsequently by using the wetness incipient method. All the catalysts were characterized by standard methods of surface area analysis using a Micromeritics ASAP 2020 instrument. The crystalline behaviour of the catalysts were analysed by the powder X-ray diffraction (XRD) technique with Shimadzu diffractometer. Temperature programmed reduction (TPR) was carried out on a Chemisorb 2720 instrument. More details about the catalyst preparation and characterization were reported elsewhere26.

3.2. Experimental runs All the catalytic experiments were carried out in a tubular fixed-bed reactor under atmospheric pressure at different temperatures, steam to ethanol (S/E) ratio and space time. Catalysts were reduced in-situ under a reducing atmosphere of H2-N2 mixture (H2 mole ratio of 0.5) at 823 K for 6 h prior to the reactions. The feed (ethanol-water mixture) were pumped into a vaporizer and heated up to 523 K temperature. Then it was mixed with carrier gas (0.5 ml/s N2)



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followed by heating at 523 K in a premixer heater. The mixed feed stream is then passed through the catalyst bed placed in the centre of the reactor. The catalyst bed temperature was measured by a thermocouple with an accuracy of ±2 K. Reaction products were then cooled by passing it through a condenser and separated into a gas-liquid separator. Analysis of the gaseous and liquid products were carried out using a gas chromatograph (GC) equipped with thermal conductivity detector (TCD) and a flammable ignition detector (FID), respectively. The details on experimental set-up and procedure are discussed elsewhere27. For kinetic studies, extensive experiments were conducted to collect sufficient kinetic data. Plug flow conditions (i.e. minimal radial concentration and temperature profile) were ensured during the catalytic experiments, by maintaining the ratio of catalyst bed height to catalyst particle size and the ratio of internal diameter of the reactor to catalyst particle size above 50 and 30, respectively28. To avoid fluid channeling and local temperature gradients, catalyst bed was placed at the centre portal of the reactor and diluted with SiC at a volume ratio of 1:1. The superficial gas velocity and average catalyst particle size (0.5±0.1 mm) were selected from preliminary experiments in such a manner that mass transfer (e.g. film as well as pore diffusion) did not influence the kinetics of the reaction. The operation was done under isothermal condition. The kinetic experiments were conducted at 773, 823 and 873 K under atmospheric pressure, and space times (Wcat/FEtOHo) of 0 – 2201.3 (kgcats/kmolEtOH) over 100 mg of fresh catalyst samples, keeping molar ratio of water-to-ethanol in the range of 3.0-9.0. The kinetic data were collected for a run time of 6 h as there was no deactivation of catalyst in that duration.

3.3. Estimation of kinetic model parameters In each kinetic experiment, the amount of product formed e.g. H2, CO, CH4 and CO2 and amount of unreacted reactants (EtOH, H2O) present in the outlet stream were measured at


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various conditions. The rate of production/consumption of each component was expressed in terms of the rate of three main reactions under consideration, i.e. OSRE, WGS and ED, as rate of production/consumption of individual components are directly related to the stoichiometry of these reactions. The rates of individual components for model 1 and 2 (LH-I and LH-II) are as follows:

rCH 3CH 2OH = − ( rOSRE + rED ) S A

(L)

rH 2O = − ( 2rOSRE + rrWGS ) S A

(M)

rH2 = ( 5rOSRE − rrWGS + rED ) S A

(N)

rCO2 = ( 2rOSRE − rrWGS ) S A

(O)

rCO = ( rED − rrWGS ) S A

(P)

rCH4 = ( rED ) S A

(Q)

rO2 = −0.5 ( rOSRE ) S A

(R)

The pre-exponential factors and activation energies were estimated using Arrhenius equations.

 −E  ki = ki∞ exp  i   RT 

(S)

However, ki∞ and Ei are strongly correlated with each other. Therefore, the rate constants were evaluated with respect to a reference temperature (Tm) (Eq. (T)) to reduce the correlation between ki∞ and Ei

29,30

. In this study, 813 K was used as Tm for the parameter estimation.



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 − E  1 1  ki = ki∞ exp  i  −   R  T Tm 

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(T)

Similarly, the equilibrium constants were estimated using van’t Hoff expression (Eq. (U)) with respect to same reference temperature (Tm) for all the adsorbed species.

K

ads i

 ∆ Siads ∆ Hiads  1 1   = exp  −  −   R R  T Tm   

(U)

The rate expressions for each component (Eqs.(L) –(R)) were expressed in terms of the kinetic model expressions for three main reactions under consideration (e.g. OSRE, rWGS and ED) incorporating the effect of temperature on activation energies and equilibrium constants with the help of modified Arrhenius and van’t Hoff expressions (Eqs. (T) and (U)). The rate expressions thus obtained were substituted into the design equation of integral plug flow reactor. Then, the obtained sets of coupled differential equations were solved by the ODE solver. The non-linear least-square regression was performed to minimize the objective function, i.e. mean residual sum of squares (MRSS) using fmincon as optimization technique to determine the kinetic parameters.

∑(r N

MRSS =

i =1

pred i

− rexp ti

)

2

(V)

N−Z

Where, rpredi and rexpti are predicted and experimental reaction rates, respectively, N and Z are the number of experimental observations and the number of parameters to be estimated, respectively. Equilibrium constants of the adsorbed species were estimated by taking the heat of adsorptions from the literature and their corresponding entropies were calculated by non-linear regression. However, to avoid convergence of the non-linear least square regression at a local minima point 18 ACS Paragon Plus Environment

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during optimization of the kinetic parameters, a proper initial guesses of the parameters are required31,32. Therefore, initial guess of the rate parameters were estimated by fitting the rate vs. space-time data with the kinetic models. The surface site concentrations were kept separate from rate constants in the rate expressions and estimated during optimization process. The initial estimates of Ct were taken from literature17,25. The parameters were estimated by minimizing the sum of squares of differences between experimentally observed values and fitted values. The confidence intervals of the estimated parameters were calculated based on statistical tdistribution method. Furthermore, the significance of the regression coefficient and global regression was evaluated by means of t-test and F-test, respectively33. During parameter estimation process, it is verified that the adsorption entropies and enthalpies for each step are negative, i.e. ∆Hads < 0 and o

o o o ∆Sads = Sads − Sads < 0 while the activation energies of the elementary

steps are positive. The thermodynamics consistency of the estimated model parameters was also verified using Boudart-Mears-Vannice guidelines as 10 < −∆Sads < 12.2 − 0.0014∆Hads during o

o

parameter estimation34. The kinetic parameters thus obtained were then tested using a profiling technique to analyse the extent of nonlinearity of the parameters and models, and to minimize correlation among the parameters31,32.

4. Results and discussion 4.1. Catalyst characterization The surface area and pore volume of the support CeO2-ZrO2, and Rh-Ni/CeO2-ZrO2 catalysts were measured. The surface area of the support (CeO2-ZrO2) was found as 35.8±0.8 m2/g and it was decreased further for metal impregnated catalysts. The result suggests that the surface area and pore volume of the support significantly reduced after the incorporation of 19 ACS Paragon Plus Environment


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metal, due to partial blockage of the pore surface of support by metal oxide particles. The maximum metal dispersion (~2%) was obtained for 1%Rh-30%Ni/CeO2-ZrO2 catalyst, suggesting the presence of Rh improves metal dispersion35. The TPR profile of support exhibited only a broad peak at 833 K corresponding to the reduction of Ce4+ to Ce3+ indicating the incorporation of ZrO2 into CeO2 lattice promotes its reduction by increasing lattice defect36. The reduction profile of Rh-Ni/CeO2-ZrO2 catalyst demonstrated that bulk NiO reduced at lower temperature than that of Ni/CeO2-ZrO2 catalyst, indicating the reducibility of NiO and the support improved after Rh incorporation37,38. The XRD pattern of the CeO2-ZrO2 support demonstrated cubic-fluorite type structure of ceria-zirconia mixed oxide [(Ce0.91Zr0.09)O2]. The diffraction patterns of Rh-Ni/CeO2-ZrO2 exhibited similar phases corresponding to NiO with lesser intensity compared to unpromoted Ni/CeO2-ZrO2 catalyst, indicating the incorporation of Rh improved dispersion of NiO crystallite. No separate phase of Rh was found from XRD analysis, due to either its lower amount in the catalyst or due to high Rh dispersion39.

4.2. Effect of process variables on catalytic activity The effects of various process variables viz. temperature, steam to ethanol ratio, space time, and run time on the conversion of ethanol and product selectivity were investigated over 1%Rh-30%Ni/CeO2-ZrO2 catalyst for OSRE.

4.2.1. Effect of temperature The effect of temperature on product selectivity and catalytic activity on 1%Rh30%Ni/CeO2-ZrO2 catalyst was analysed at a constant space time (W/FAO = 33020 kgcat s/kmol[EtOH]) with molar ratio of EtOH/H2O/O2 equal to 1:9:0.35 (Fig. 1). Ethanol conversion was near to completion and the yield of H2 increased from 3.6 to 4.8 mol/mol with increment in the temperature from 773 to 873 K. Hydrogen yield decreased as the reaction temperature was 20 ACS Paragon Plus Environment

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increased beyond 873 K. This is because of the contribution of the reverse WGS reaction which favours at higher temperature. Maximum hydrogen yield (4.8 mol H2/mol of ethanol fed) was obtained at 873 K. Similarly, hydrogen selectivity reached to a maximum value of 73% as temperature increased up to 923 K followed by a decreasing trends above this temperature. The selectivity to CO2 decreased slightly, while CO selectivity increased from 1 to 4% as temperature increased from 773 to 873 K. Reverse trends between CO and CO2 selectivity were observed above 873 K due to reverse WGS reaction. Methane selectivity was high at lower temperature (773K) due to ethanol decomposition reaction, but almost negligible (873K) due to methane steam reforming reaction (MSR). Therefore, further experiments were carried out at 873 K due to higher H2 selectivity and low CO selectivity.

Fig. 1. Effect of temperature on product selectivity (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35, W/FAO = 33020 kgcat s/kmol[EtOH], Catalyst: 1%Rh-30%Ni/CeO2-ZrO2) 4.2.2. Effect of space time Ethanol conversion and product selectivty in the space time range of 13208 to 33020 kgcat s/kmol[EtOH] was analysed and results are presented in Fig 2. Increment in ethanol conversion observed from 92% to 99% as the value of W/FAO was increased from 13208 to 33020 kgcat s/kmol[EtOH] at 873 K. The yield of H2 also increased significantly from 2.4 to 4.8 (mol H2/mol of ethanol fed) with increasing space time as steam reforming favours at higher space time. The selectivity of H2 increased significantly from 63% to 73% by increasing the space time in the range of space time studied, while CO and CH4 selectivity decreased. The selectivity of CO2 increased from 15% to 21% by increasing space time. The results revealed that the selectivity of H2 and CO2 increased while CO and CH4 selectivity decreased at higher contact time as it favours steam reforming reaction than ethanol decomposion reaction.



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Fig. 2. Effect of space time on ethanol conversion and product selectivity (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35, Catalyst: 1%Rh-30%Ni/CeO2-ZrO2) 4.2.3. Effect of Steam to Ethanol (S/E) ratio The effect of steam to ethanol ratio was also investigated on selected catalyst at 873 K. For this, three different feed stocks with S/E molar ratio of 3:1, 6:1, and 9:1 were used at constant O/E molar ratio of 0.35 and the results are presented in Fig 3. Ethanol conversion increased from 92% to 99% and H2 selectivity also increased from 51% to 73% as water content was increased, because higher partial pressure of steam in feed promotes steam reforming reaction. Accordingly, methane and CO selectivity decreased from 18% to 4% and 13% to 5%, respectively, in the range of S/E studied as the MSR and WGS reactions are favoured at high steam partial pressure. As a consequence, the selectivity of CO2 also increases with increasing water content.

Fig. 3. Effect of S/E ratio on ethanol conversion and product selectivity (T=873K, P=1atm, molar ratio EtOH/O2 = 1:0.35; W/FAO = 33020 kgcat s/kmol[EtOH], Catalyst: 1%Rh-30%Ni/CeO2-ZrO2) 4.2.4. Effect of run time on catalytic activity Experiments were conducted for 24 hours at 873 K with S/E ratio of 9:1 and constant space time to study the catalyst stability and the results are presented in Fig. 4. Complete ethanol conversion (~100%) was achieved throughout the run time. Hydrogen yield remained constant at 4.7±0.1 mol/mol of ethanol fed. The selectivity of product gases (i.e. H2, CH4, CO and CO2) were found approximately 72, 3, 4 and 21%, respectively throughout of run time. There is no obvious decline in conversion and product selectivity which imply that the catalyst did not deactivate during this period. The high stability of the catalyst can be explained by its high activity towards OSRE process and high oxygen storage capacity (OSC) of CeO2ZrO2 support. High OSC of CeO2-ZrO2 support facilitates the removal of deposited carbon over 22 ACS Paragon Plus Environment

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the surface of the catalyst during the reaction and hence prevent the catalyst deactivation. On the basis of the above results, it was concluded that 1%Rh-30%Ni/CeO2-ZrO2 catalyst is highly active and stable for OSRE process. Hence, a detailed kinetics was studied further over this catalyst by varying temperature, steam to ethanol ratio and contact time.

Fig. 4. Effect of run time on product selectivity (T=873K, P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35, W/FAO = 33020 kgcat s/kmol[EtOH], Catalyst: 1%Rh30%Ni/CeO2-ZrO2) 4.3. Kinetic studies 4.3.1. Heat and mass transport limitation The effects of inter and/or intra particle heat and mass transport on the reaction rate were calculated to assure that the data collected during kinetic experiments were not affected by diffusion limitations. The heat transfer resistance inside the pore was calculated using the Prater analysis given by Eq. (W).

 Deff ( C As − C AC ) × ∆ H R   λeff  

(W )

∆Tmax, particle =  Where,

∆Tmax, particle is the maximum possible temperature variation between center position and

surface of a catalyst pellet. The effective diffusivity ( Deff ) is calculated using the formula

Deff = DABε / τ . The effective thermal conductivity ( λeff ) is evaluated using the correlation

λeff / λ = 5.5 + 0.05NRe for packed bed reactors. The maximum temperature variation of 0.4 K was obtained inside a pellet, which indicates that there was negligible temperature variation. The heat transfer limitation across the gas film on catalyst surfaces was estimated using Eq. (X).  L( − rAobs ) × ∆ H i   h  

(X )

∆ Tmax, film = 



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Where,

Page 24 of 58

∆Tmax, film is the maximum possible temperature difference between the bulk gaseous

phase and the surface of catalyst pellet. The maximum temperature difference of 0.5 K was found across the gas film. These results revealed the absence of internal and external heat transport limitation during kinetic experiments40. The Weisz-Prater criterion ( CWP ) was used to determine if internal pore diffusion is limiting the rate of reaction and was calculated by Eq. (Y).

CWP

 ( − rAobs )ρC RC2  =   Deff C AS 

The

CWP was calculated as 0.056, which is much less than 1. This result indicates that there is no

(Y )

significant internal pore diffusion limitations and consequently no concentration gradient exists within the pellet. Mears criterion was used to determine if mass transfer from the bulk gas phase to the pellet surface is limiting the rate of reaction and given as follows.  ( − rAobs ) ρ b RC n    < 0.15 kC C A  

(Z )

It was noticed that the value of the LHS (0.013) of the Eq. (Z) is far less than 0.15. This result indicates that no mass transfer limitation exists in the film during the collection of kinetic data41.

4.3.2. Profile plots The profile technique provides useful information about the nature (linear/nonlinear) of the parameters and the correlation among the parameters. The profile t plots of τ(θp) vs. δ(θp) for each parameter were used to analyse their extent of nonlinearity. The degree of nonlinearity of any parameter is measured by the deviation of the t plots from the 45o line passing through origin. Thus, the need of further reparameterization depends on the nature of the t plot for a


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particular parameter. The profile t plots of

kOSRE , krWGS , and kED are shown in Fig. 5. From Fig.

5, it can be seen that the plots almost coincide with the 45o line, which suggests that these parameters do not possesses nonlinearity.

Fig. 5. Profile t plots of

The profile trace plots between

kOSRE , krWGS , and kED

EOSRE vs kOSRE , ErWGS vs krWGS , and EED vs kED is shown in

Fig. 6. It is observed from Fig. 6, that the trace plots between the Arrhenius parameters do not intersect each other at 90o angle which clearly indicates the existence of correlation between parameters. This suggests that the parameters of this nonlinear model can be linearized to some extent by the temperature centering technique. Similar profile trace plots are also observed between

EOSRE vs krWGS , ErWGS vs kOSRE , and ErWGS vs kED etc. Thus, analysis of profile t and

trace plots revealed that temperature centering technique has helped in reducing the nonlinearity of parameters, however, correlation between parameters still exist as Arrhenius parameters are strongly dependent31.

Fig. 6. Profile trace plots between

EOSRE vs kOSRE , ErWGS vs krWGS , and EED vs kED

4.3.3. Parameter estimation and Model Validation The values of the estimated parameters and their confidence intervals are presented in Table 1 and Table 2. The MRSS values were used to determine the goodness of fit. Further, the MRSS values, statistical tests (t and F tests), and inconsistency of entropies of adsorptions and activation energies in accordance with thermodynamic were compared to discriminate the kinetic models11,17,31,33. The MRSS value for model LH-II is relatively lower compared to LH-I. It



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suggests that the fitting improves by considering dual site mechanism over single active site consideration. The MRSS value of LH-II is lowest (4.3E-3) among these models, as shown in Table 1. Moreover, this model is most preferred amongst the three models as it has the highest F value and lowest t value. Therefore, Model LH-II is considered as a suitable model to predict the kinetic parameters for the OSRE process. From this model, the activation energy for OSRE, WGS and ED reactions were obtained as 56.0, 46.1 and 34.8 kJ/mol, respectively. Activation energies for SRE, WGS and ED have been reported by various authors, are in the order of 50100, 40-110, and 70-100 kJ/mol, respectively. Lower activation energy was obtained for ED reaction, due to the difference in reaction mechanism proposed over selected catalyst in this work.


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1 2 3 Table 1. Estimated kinetic model parameters for different models 4 5 Values at 95% Values at 95% 6 Reaction ki∞ # E Rate i confidence level# confidence level t and F values* 7 -2 -1 -1* Mechanism (mol m s ) MRSS constant (kJ mol ) 8 @873 K -95% +95% -95% +95% 9 and Model 10 0.53; 5640 11RM-I, LH-I 2.87E+13 2.15E+13 3.58E+13 40.9 39.1 42.7 8.5E-2 kOSRE 12 13 4.29E+08 3.43E+08 5.14E+08 30.3 28.2 32.5 krWGS 14 15 16 1.87E+07 1.15E+07 2.58E+07 38.4 36.1 40.6 kED 17 18 19 0.24; 7790 3.29E+14 3.01E+14 3.57E+14 56.0 55.3 56.7 4.3E-3 kOSRE 20RM-II, LH-II 21 22 7.75E+10 7.44E+10 8.05E+10 46.1 45.4 46.8 krWGS 23 24 25 2.49E+08 2.16E+08 2.81E+08 34.8 33.8 35.7 kED 26 27 28 0.86; 2810 21.3E+3 20.2E+3 22.4E+3 48.4 46.6 50.2 1.8E-2 k i∞ 29Power Law 30 n 0.63±0.03 31 32 # The confidence intervals were determined by t-distribution method. 33 * 34 The t and F values, which test for the significance of a regression co-efficient and the global regression were evaluated by t-test and F-test, respectively. 35 36 37 38 39 40 41 42 43 44 27 45 46 ACS Paragon Plus Environment 47 48


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Table 2. Estimated thermodynamic parameters for different models RM-I, LH-I Equilibrium constant

∆Si -1

J mol K

-1

RM-II, LH-II

Values at 95% confidence level# -95%

+95%

∆Si -1

J mol K

-1

Values at 95% confidence level# -95%

+95%

∆Hi kJ mol-1*

KCH3CH2O

-34.6

-38.7

-30.4

-44.0

-46.4

-41.7

-30

KCH3CHO

-19.3

-23.4

-15.2

-58.5

-60.6

-56.5

-30

KHCOO

-45.4

-49.8

-40.9

-53.9

-56.8

-50.9

-100

KO

-17.3

-21.3

-13.3

-40.4

-43.2

-37.5

-29

KH

-81.7

-87.1

-76.2

-63.5

-65.6

-61.4

-50

KOH

-44.5

-49.0

-40.0

-84.3

-86.6

-81.9

-20

KCO

-49.8

-53.9

-45.7

-28.4

-29.0

-27.8

-30

K CO2

-57.4

-60.8

-54.0

-25.7

-28.1

-23.3

-30

K CH 3

-30.6

-35.8

-25.4

-62.6

-64.9

-60.2

-20

K CH4

-57.3

-61.5

-53.1

-24.4

-26.2

-22.5

-20

*

Heats of adsorptions were assumed constant and taken from the literature 24,25.

#

The confidence intervals were determined by t-distribution method.


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The change in experimental and predicted ethanol conversion as a function space time at three different temperatures is shown in Fig. 7. At all temperatures, a reasonably good fitting was obtained for ethanol conversion with space time. These results also revealed that the proposed Langmuir-Hinshelwood kinetic model (LH-II) is suitable for the OSRE process. It can be used to predict the experimental results at higher H2 yield with minimum CO formation.

Fig. 7. Experimental and predicted ethanol conversion as a function of space time at different temperatures. (T=773, 823 and 873K, P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35).

Fig. 8 (partial pressure vs. space time) depicted that, at a fixed temperature (T=873 K) the partial pressures of H2 and CO2 increased, whereas that of water and ethanol decreased with increasing space time. It is noticeable that the decrease in water partial pressure is much more pronounced compared to ethanol because stoichiometrically 3 moles of water reacts with ethanol to form H2 and CO2. The partial pressures of hydrogen produced experimentally during OSRE were compared with the model predicted values with the help of parity plot (Fig. 9). No significant deviation from the line of parity was found as indicated by the correlation coefficient of 0.98. The parity plot of experimental and model predicted reaction rates at all temperatures are shown in Fig. 10. The correlation coefficient value of 0.97 was obtained which indicates the error between experimental and model predicted results is within the tolerance.

Fig. 8. Experimental and predicted partial pressures of products as a function of space time at 873 K (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35). Fig. 9. Parity plot of experimental and predicted partial pressures of hydrogen (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35). Correlation coefficient r2 =0.98. Fig. 10. Parity plot of experimental and predicted reaction rates (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35). Correlation coefficient r2 =0.97.



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5. Conclusions Kinetics of OSRE was investigated over Rh-Ni/CeO2-ZrO2 catalyst which was found to be good catalyst for this process. The mechanistic kinetic models based on the proposed surface reaction mechanisms considering two distinct types of active sites present on the catalyst surface, were developed for the OSRE process using Langmuir-Hinshelwood approach. The LangmuirHinshelwood kinetic model (LH-II) based on dehydrogenation of adsorbed ethoxy species, decomposition of formate species, and decomposition of acetaldehyde as the rate determining steps for OSRE, rWGS and ED reactions, respectively shows a reasonably good agreement with the experimentally obtained results at all temperatures and contact time. The activation energy for OSRE, WGS and ED reactions obtained were 56.0, 46.1 and 34.8 kJ/mol, respectively. The results revealed that the proposed Langmuir-Hinshelwood mechanistic kinetic model (Model LH-II) can be used to predict the kinetic parameters for the OSRE process.

Nomenclature

Deff

effective diffusivity, m2 s-1

DAB

bulk diffusivity of component A in B, m2 s-1

ε

void fraction

τ

tortuosity factor

Ei

activation energy for rate constant of reaction i, kJ mol-1

Hi

enthalpy of species i of reactants or products, kJ mol-1

∆Hi

heat of adsorption for species i, kJ mol-1


Page 31 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


∆Si

entropy of adsorption for species i, J mol-1 K-1

λeff

effective thermal conductivity, kJ m-1 s-1 K-1

λ

molecular thermal conductivity, kJ m-1 s-1 K-1

NRe

Reynolds number based on particle

C As

concentration of ethanol at the pellet surface, kmol m-3

C AC

concentration of ethanol at the centre of the pellet, kmol m-3

− rAobs

observed rate of reaction, kmol m-2 s-1

L

characteristic length of catalyst pellet, m

h

heat transfer coefficient of fluid, kJ m-2 s-1 K-1

ρC

pellet density, kg m-3

ρb

bulk density, kg m-3

RC

radius of the catalyst particle, m

kC

mass transfer coefficient, m s-1

n

reaction order

Ki

equilibrium constant of reaction i or adsorption coefficient for surface species i

ki

rate constant for reaction i, m2 s-1 mol-1

ki∞

pre exponential rate constant for reaction i, m2 s-1 mol-1

ri

rate of reaction i, kmol m2 s-1 or rate of reaction of formation of component i, kmol kgcat-1s-1



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SA CS C Si

CT C Ti

surface area of catalyst, m2 gcat-1 concentration of site S, mol m-2 concentration of site Si, mol m-2 total surface concentration of site S, mol m-2 total surface concentration of site Si, mol m-2

τ(θp)

profile t value for parameter θp

δ(θp)

standardized value of parameter θp

P

operating pressure, atm

Pi

partial pressure of component i, atm

T

temperature, K

Tm

mean temperature, K

Subscripts 1

for active site S1

2

for active site S2

i

reaction or product species

Acronyms BET

Brunauer-Emmett-Teller

LH

Langmuir-Hinshelwood

RM

reaction mechanism

MRSS

mean residual sum of squares

RDS

rate determining step


Page 32 of 58

Page 33 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


OSRE

oxidative steam reforming of ethanol

SRE

steam reforming of ethanol

POX

partial oxidation of ethanol

WGS

water gas shift

rWGS

reverse water gas shift

ED

ethanol decomposition

References

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(2)

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(3)

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(5)

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(7)

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(9)

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(10)

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(11)

Patel, M.; Jindal, T. K.; Pant, K. K. Kinetic Study of Steam Reforming of Ethanol on NiBased Ceria − Zirconia Catalyst. Ind. Eng. Chem. Res. 2013, 52, 15763.

(12)

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Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Methanol–steam Reforming on Cu/ZnO/Al2O3 Catalysts. Part 2 . A Comprehensive Kinetic Model. Appl. Catal. A Gen. 1999, 179, 31.

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Olafadehan, O. A.; Ayoola, A. A.; Akintunde, O. O.; Adeniyi, V. . O. Mechanistic Kinetic Models for Steam Reforming of. J. Eng. Sci. Technol. 2015, 10, 633.

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(35)

Biswas, P.; Kunzru, D. Oxidative Steam Reforming of Ethanol over Ni/CeO2-ZrO2 Catalyst. Chem. Eng. J. 2008, 136, 41.

(36)

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(41)

Fogler, H. S. Elements of Chemical Reaction Engineering; Prentice-Hall: Upper Saddle River, NJ, 2006. 36 ACS Paragon Plus Environment

Page 37 of 58

80 70 H2

60

Product selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CO CH4

50

CO2

40 30 20 10 0 773

873

923

973

Temperature (K)

Figure 1. Effect of temperature on product selectivity (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35 , W/FAO = 33020 kgcat s/kmol[EtOH], Catalyst: 1%Rh30%Ni/CeO2-ZrO2) (The experimental error shown as error bar was calculated by dividing the standard deviation by the square root of number of measurements).



80

100 90

70 H2 selectivity CO selectivity CO2 selectivity

60

CH4 selectivity Conversion

50 20

70 60 50 20

15 10

10

Ethanol conversion (%)

80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 58

5 0 10000

15000

20000

25000

30000

0 35000

W/FAo (kgcat s/kmol EtOH) Figure 2. Effect of space time on ethanol conversion and product selectivity (T=873K, P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35, Catalyst: 1%Rh-30%Ni/CeO2-ZrO2)


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80

100 90

70

80 70

CO CH4

50

60

CO2 40

50 40

30

30 20

Ethanol conversion (%)

H2

60


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 10

10

0

0 3:1

6:1

9:1

S/E ratio Figure 3. Effect of S/E ratio on ethanol conversion and product selectivity (T=873 K, P=1atm, molar ratio EtOH/O2 = 1:0.35; W/FAO = 33020 kgcat s/kmol[EtOH], Catalyst: 1%Rh-30%Ni/CeO2-ZrO2)



70 60


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 58

H2 selectivity

50

CO selectivity CH4 selectivity

40

CO2 selectivity

30 20 10

0 0

5

10

15

20

25

Run time (h)

Figure 4. Effect of run time on product selectivity (T=873 K, P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35, W/FAO = 33020 kgcat s/kmol[EtOH], Catalyst: 1%Rh30%Ni/CeO2-ZrO2)


Page 41 of 58

Tau (Krwgs)

Tau (Kosre)

0.5

delta (Krwgs)

0.5

0.0 -1.0

-0.5

0.0

0.5

1.0

0.0 -1.0

-0.5

0.0

-0.5

-0.5

-1.0

-1.0

Tau (Ked)

0.5

delta (Ked)

delta (Kosre)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 -1.0

-0.5

0.0

0.5

1.0

-0.5

-1.0

Figure 5. Profile t plots of kOSRE, krWGS, and kED


0.5

1.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Eosre Kosre

Page 42 of 58

Erwgs Krwgs

Eosre

Erwgs

1

1

Krwgs

Kosre 0 -2

-1

0 0

1

2

-2

-1

0

-1

-1

-2

-2

Eed Ked

1

2

Eed 1

Ked

0 -2

-1

0

1

2

-1

-2

Figure 6. Profile trace plots between EOSRE vs kOSRE, ErWGS vs krWGS, and EED vs kED


Page 43 of 58

1.0

0.8

Ethanol conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

Expt. X at 773 K Expt. X at 823 K Expt. X at 873 K Pred. X at 773 K Pred. X at 823 K Pred. X at 873 K

0.2

0.0 0

500

1000

1500

2000

2500

W/FAo (kgcat s/kmol EtOH) Figure 7. Experimental and predicted ethanol conversion as a function of space time at different temperatures. (T=773, 823 and 873 K, P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35).



1.0

0.8

H2

0.6

Pi (atm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 58

0.4 H2O

0.2

CO2 0.0

EtOH

0

500

1000

1500

2000

2500

Space time (kgcat s/kmol EtOH)

Figure 8. Experimental and predicted partial pressures of products as a function of space time at 873 K (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35). The geometric symbols represent experimental data whereas dotted lines represent trends of model predicted partial pressures.


Page 45 of 58

0.70

0.60

0.50

2

Predicted PH (atm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.40

0.30

0.20

0.10

0.00 0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

Experimental PH (atm) 2

Figure 9. Parity plot of experimental and predicted partial pressures for hydrogen (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35). Correlation coefficient r2 =0.98.



5E-4

4E-4

Predicted rate (kmol m-2 s-1 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 58

3E-4

2E-4

1E-4

0 0

1E-4

2E-4

3E-4

4E-4

5E-4

-2 -1

Experimental rate (kmol m s )

Figure 10. Parity plot of experimental and predicted reaction rates (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35). Correlation coefficient r2 =0.97.


Page 47 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only

transform to acetaldehyde

ethoxy formation

ads. ethanol

ads. ethoxy sp.

ads. acetaldehyde sp.

RDS : OSRE

ads. methoxy sp. ads. formate sp.

ads. formate sp. active site ads. = adsorbed

sp. = species

catalyst surface

Surface reaction mechanism for oxidative steam reforming of ethanol



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Effect of temperature on product selectivity (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35 , W/FAO = 33020 kgcat s/kmol[EtOH], Catalyst: 1%Rh-30%Ni/CeO2-ZrO2) 217x184mm (300 x 300 DPI)

ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Effect of space time on ethanol conversion and product selectivity (T=873K, P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35, Catalyst: 1%Rh-30%Ni/CeO2-ZrO2) 261x180mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Effect of S/E ratio on ethanol conversion and product selectivity (T=873 K, P=1atm, molar ratio EtOH/O2 = 1:0.35; W/FAO = 33020 kgcat s/kmol[EtOH], Catalyst: 1%Rh30%Ni/CeO2-ZrO2) 264x190mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Profile t plots of kOSRE, krWGS, and kED 230x159mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. Profile trace plots between EOSRE vs kOSRE, ErWGS vs krWGS, and EED vs kED 219x154mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Experimental and predicted ethanol conversion as a function of space time at different temperatures. (T=773, 823 and 873 K, P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35). 218x180mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8. Experimental and predicted partial pressures of products as a function of space time at 873 K (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35). The geometric symbols represent experimental data whereas dotted lines represent trends of model predicted partial pressures. 223x180mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. Parity plot of experimental and predicted partial pressures for hydrogen (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35). Correlation coefficient r2 =0.98. 231x187mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10. Parity plot of experimental and predicted reaction rates (P=1atm, molar ratio EtOH/H2O/O2 = 1:9:0.35). Correlation coefficient r2 =0.97. 237x185mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transform to acetaldehyde

ethoxy formation

ads. ethanol ads. ethoxy sp.

ads. acetaldehyde sp.

RDS : OSRE

ads. methoxy sp. ads. formate sp.

ads. formate sp. active site ads. = adsorbed

sp. = species

catalyst surface

Surface reaction mechanism for oxidative steam reforming of ethanol


Mechanistic Kinetic Modeling of Oxidative Steam ... - ACS Publications

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