Catalyst Screening and Kinetic Modeling for CO Production by High

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Catalyst screening and kinetic modelling for CO production by high pressure and temperature reverse Water-Gas Shift for Fischer-Tropsch applications Francisco Vidal Vázquez, Peter Pfeifer, Juha Lehtonen, Paolo Piermartini, Pekka Simell, and Ville Alopaeus Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01606 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017

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Industrial & Engineering Chemistry Research

Catalyst screening and kinetic modelling for CO production by high pressure and temperature reverse Water-Gas Shift for Fischer-Tropsch applications

Title:

Authors: Francisco Vidal Vázquez*1, Peter Pfeifer2, Juha Lehtonen1, Paolo Piermartini3, Pekka Simell1 and Ville Alopaeus4 1

VTT Technical Research Centre of Finland Ltd, Biologinkuja 5, 02150, Espoo, Finland.

2

Karlruhe Institute of Technology, Institute of Micro-Processing Engineering, Campus North Hermann-vonHelmholtz-Platz 1 76344 Eggenstein-Leopoldshafen, Germany. 3

IneraTec GmbH, Nördliche Uferstraße 4-6, 76189 Karlsruhe, Germany.

4

Aalto University, School of Chemical Technology, P.O. Box 11000, FI-00076 Aalto, Finland.

*[email protected]

Abstract In this work, catalyst screening and reaction kinetic modelling is done for two Ni-based and one Rh-based commercial catalysts for reverse Water-Gas Shift (rWGS) reaction under atmospheric and 30 bara pressure. Ni-based catalysts displayed higher activity compared to Rh-based catalyst despite the severe initial deactivation Ni-based catalysts suffered, which increases catalyst selectivity towards CO formation. Ni/Al2O3 catalyst with lower Ni content (2 w-%) exhibited higher selectivity towards CO formation compared to Ni/Al2O3 catalyst with higher Ni content (15 w-%). The Ni/Al2O3 (2 w-% of Ni) catalyst was further tested for kinetic modelling. Three kinetic models were developed based on reaction mechanisms and kinetic models obtained from other publications for rWGS/WGS, methanation and methane steam reforming reactions based on different mechanistic approaches. Model based on mechanistic assumptions originally proposed by Xu and Froment was concluded to be the most suitable to describe the high temperature reaction system of rWGS and methanation over supported nickel catalyst. Based on statistical analysis, model proposed by Xu and Froment was also concluded to be the best for the catalyst and reaction system studied in this work.

1. Introduction Carbon capture and utilization (CCU) is one of the key topics for mitigation of CO2 emissions. There are many different technologies that are applied for the production of diverse chemicals from CO2 such as synthetic natural gas, Fischer-Tropsch products, methanol or even polymers and specialty chemicals. Power-to-Gas and Power-to-Liquids concepts arise as a synergetic solution for storing energy and producing value added products from the intermittent renewable energy sources and CCU. Fischer-Tropsch synthesis (FT) is a well known process which was initially developed in the first half of 20th century for the conversion of gas feedstock containing mainly CO and H2 to hydrocarbons. This process has a long history with periods of relative “inactivity” in the industrial sector. The product distribution from FT consists of linear hydrocarbons of different chain length that obeys the so-called Anderson-Schulz-Flory (ASF) distribution. This product distribution is strictly dependent on operating conditions and catalyst composition. The catalysts used for FT are mainly Fe-based and Co-based catalysts, where Co-based

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catalysts produces mainly paraffinic heavier hydrocarbons and are only active for CO as carbon source. On the other hand, Fe-based catalysts produce mainly olefinic lighter hydrocarbons. Currently, FT has become one of the most relevant topics for CO2 utilization. Even though, Fe-based catalysts are active for CO2, it displays significantly higher activity when CO-rich feedstock is used. For that reason, two-step process combining reverse Water-Gas Shift (rWGS) and FT is one of the most potential processes to value added products when CO2 is the carbon source.1 rWGS reaction converts CO2 into CO according to the reaction equation below: + ↔ +

∆ = 41.5 /

This reaction is thermodynamically favored at high temperatures and its chemical equilibrium is independent on pressure. However, FT using Co-based catalyst is normally operated at around 30 bara. Operating also rWGS at high pressure has both advantages and disadvantages. High pressure rWGS would simplify the two-step process (rWGS+FT) by having the compression stage before rWGS instead of between the rWGS reactor and FT reactor which would require additional cooling, deep water/humidity removal, compression and reheating. Furthermore, gas-phase heterogeneously catalyzed reactions operated at high pressures tend to reduce significantly the size of the reactor. The main disadvantages of high pressure rWGS are more expensive materials for the reactor, higher risk of carbon formation and methane formation. Methanation occurs with two competing reactions, to rWGS, that are thermodynamically favored at low temperature and high pressure: + 4 ↔ + 2 + 3 ↔ +

∆ = −206.1 /

∆ = −165 /

Catalysts for rWGS generally contain Ni, Fe, or even Mo, In and Rh as active metals.2–8 It is generally accepted that on supported metal catalysts, reactions take place on the sites located on the active metal.9 Little amount of literature is found on rWGS at high pressures, or considering simultaneous methane formation. Bustamante et al.10 studied the kinetics of non-catalyzed homogeneous rWGS at atmospheric pressure. They developed a simple power-law kinetic model describing rWGS as an irreversible reaction in gas-phase. Unde4 performed a kinetic analysis for rWGS reaction at atmospheric pressure over Ni/Al2O3 and Al2O3 catalysts. However, no kinetic model was developed in that work. On the other hand, plenty of kinetic models for methanation and steam reforming of methane (SRM) can be found in the literature which can also be applied for rWGS conditions with some limitations.11–16 Xu and Froment14 performed an intrinsic kinetic study for SRM, methanation and WGS reaction at different pressures. This study also included experiments to study rWGS. Fig. 1 graphically explains their simplified reaction scheme for formation of CO, CO2 and CH4.


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Fig. 1: Simplified reaction scheme for formation of CO, CO2 and CH4.

Hou and Hughes15 studied SRM and methanation on supported nickel catalyst and based their reaction mechanism on Xu and Froment work. However, they modified and simplified the mechanism of Xu and Froment. Based on the differences in the mechanism, Hou and Hughes ended up to different rate equations for SRM/methanation and WGS/rWGS reactions. Both Xu and Froment, and Hou and Hughes models assume molecular adsorption of carbon monoxide and carbon dioxide. Instead of these models, mechanisms including dissociative adsorption steps of CO and CO2 are proposed by some authors such as by Alstrup et al.17 for carbon monoxide methanation and by Weatherbee et al.18 for CO2 methanation. There are also models for WGS/rWGS reactions considering molecular adsorption of all reacting compounds (CO, H2O, CO2 and H2) on the active metal of supported metal catalysts proposed e.g. by Hakeem et al.8 for Rh-based catalyst. Furthermore, so-called regenerative models for WGS/rWGS have been proposed e.g. by Temkin19, but typically it is considered that regenerative models are valid for metal oxides and therefore not applicable when reduced supported metals are the active components. This work presents the experimental work on rWGS using Ni- and Rh-based commercial catalysts to assess their performance under different conditions. Methanation reactions of CO and CO2 are also considered. Simplified reaction scheme in Fig. 1 is principally valid for all reaction mechanisms tested in this study. Kinetic modelling is performed using the most potential reaction mechanisms and models found in literature and tested with the experimental data of this study. Two mechanisms considering all three reactions i.e. models by Xu and Froment14 and Hou and Hughes15, are included in the study. Furthermore, SRM/methanation mechanisms based on dissociative adsorption of CO17 and CO218 on the active metal are tested together with a WGS/rWGS model assuming molecular adsorption of all reacting compounds.8 However, this leads to a system where different sites on the active metal are assumed for SRM/methanation reactions and WGS/rWGS reactions, respectively.

2. Experimental part 2.1 Experimental set up Fig. 2 shows a simplified scheme of the experimental set up. The experiments were performed in a quartz tubular reactor of 6 mm inner diameter in a pressurized system. The operating pressure and temperature ranged 1-30 bara, and 500-850°C, respectively. Hydrogen, carbon dioxide and nitrogen were fed separately by different mass flow controllers. A back-pressure controller located downstream allowed to control steadily the pressure in the system. The gases were heated before entering the reactor, and after the reactor as shown in Fig. 2. The quartz tubular reactor was placed inside a fireproof steel pipe with a heating jacket that allowed controlling the temperature in the reactor. The catalyst bed was placed always in the 3 ACS Paragon Plus Environment


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isothermal zone of the reactor. One thermocouple inserted from the top of the reactor measured the temperature at the top of the catalyst bed. The catalyst was always diluted 1:4 (vol.) with SiC of similar particle size. Furthermore, the feed was always diluted with N2 in order to reduce temperature gradients in the catalyst bed.

Fig. 2: Simplified scheme of the experimental set up.

2.2 Catalyst tests Two different sets of experiments were performed. In the first set of experiments, three commercial catalysts (Table 1) were tested under the same experimental conditions and catalyst mass. In these experiments, 0.5 grams of catalysts were tested with a fixed space velocity (WHSV(hr-1)= 102.8), gas composition (N2=42.5 vol-%, H2=38.33 vol-% and CO2=19.17 vol-%), two different pressures (atmospheric and 30 bar) and catalyst bed temperatures between 500 and 800 °C at each of those pressures. Only two different pressure levels were chosen in order to simplify and reduce the number of experiments in the catalyst evaluation. These two pressure level were assumed to be representative of the performance of the catalysts at high and low pressure. The three catalysts were aged during 20 hours under reaction conditions with temperatures up to 850°C at atmospheric pressure before the set of experiments started. In the second set of experiments, the performance of catalyst 2 was further studied for kinetic modelling purposes. In these experiments, following parameters were varied: SV, H2/CO2 ratio, pressure and temperature . In this case, the catalyst was aged during 40 hours under reaction conditions with temperatures up to 850°C at atmospheric and 30 bara pressure. Furthermore, catalyst 2 was tested for kinetic modelling at three different pressure levels, atmpheric, 15 bara and 30 bara, to get better representation of the pressure effect. Table 2 shows a summary of all experimental conditions for both sets of experiments. In these experiments, H2/CO2 ratio was always kept over stoichiometric values (2 or 3) for rWGS reaction because the effluent from rWGS reactor is hypothetically meant to be fed directly to a FT reactor which requires a H2/CO ratio of 2-2,5. All catalysts were reduced in-situ under the same experimental conditions. The H2/N2 flow started with the reactor at ambient temperature and atmospheric pressure. The total gas flow rate and composition was 1 4 ACS Paragon Plus Environment

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Nl/min and 50/50% of H2/N2, respectively. Then, the temperature of the reactor was ramped up to 800°C where it remained stable for two hours. Subsequently, the reactor was flushed with nitrogen and cooled down to reaction temperature. Then, reaction was started at atmospheric pressure with a feed composition of N2=42.5 vol-%, H2=38.33 vol-% and CO2=19.17 vol-%, total volume flowrate 2.087 ln/min and ca. 500°C in the catalyst bed. This set point with these experimental conditions was periodically repeated for each catalyst to check catalyst stability. Table 1: Compositions and particle size of tested catalysts

Catalyst composition (1) Ni/Al2O3

Active metal w-% 15

Particle size (µm) 200-300

2

400-500

unknown

200-300

(2) Ni/Al2O3 (3) Rh/CeO2/Al2O3

Table 2: Summary of experimental conditions of the two different set of experiments.

Exp.

Set

Cat.

mcat (g)

vT (Nl/min)

H2/CO2 ratio

PT (bar)

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

Atm.

1st

All

0.5

30

2.087

2 Atm.

2nd

2

0.25 15

28 29 30 31

30


Temp. (°C) 550 600 650 700 750 800 850 550 600 650 700 750 800 550 600 650 700 750 800 850 550 600 650 700 750 800 850 550 600 650 700


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

32 33 34 35 36

Atm. 3

37 38 39

30

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750 800 550 600 650 700 600 650

Kinetic modelling was not performed under intrinsic conditions of mass transfer due to internal mass transfer limitations of catalyst 2. This was checked a posteriori using Weisz-Prater criterion. Catalyst bed was considered to be isothermal during the experiments for several reasons and thus, no heat transfer limitations were considered. As mentioned above, the catalyst bed was diluted 20/80 vol-% using SiC, and it was placed in the isothermal zone of the tubular reactor (previously tested under N2 flow). The gas feedstock was also diluted with 42.5 vol-% of nitrogen. Furthermore, rWGS reaction is considered to be a mild endothermic reaction with a reaction enthalpy of ca. 35 KJ/mol at 700°C. Blank tests were performed with the tubular reactor packed without catalysts at same gas flowrate, gas composition, temperatures and pressures as the catalyst screening experiments. The results of these experiments exhibited small blank activity mainly towards CO formation at high temperatures probably due to catalytic activity of thermocouple inside the reactor. This blank activity was neglected in the results and discussion part of this article as it was very low compared to the activity exhibited by the catalysts.

2.3 Modelling and simulation Chemical equilibrium calculations were calculated using HSC 6.2 software developed by Outotec.20 This software calculates equilibrium composition of the components applying Gibbs free energy minimization. The parameter estimation of the kinetic models was performed using Matlab by minimization of the sumof-squares of the residues of measured and simulated outlet molar flow rates. The optimization algorithm combined Simplex method and Levenberg-Marquart method. The reactor model used in parameter estimation and reactor simulations was a 1D pseudo-homogeneous plug-flow reactor model. Fugacity coefficients using the gas composition and reaction conditions of the experiments were calculated using Aspen Plus software. These calculations showed that the fugacity coefficients for the different components differed less than 2% from ideal gas in all cases. For that reason, the equations of state were assumed to fit ideal gas behaviour. Four different types of kinetic models were evaluated in this study; one power-law model and three models based on assumed reaction mechanisms on the active sites of a metal catalyst. Table 3 summarizes the kinetic models used for parameter estimation in this study. The kinetic model developed by Xu & Froment14 was applied without any modifications. This model is based on following main assumptions:

o o o o o

All the reactions occur on the similar nickel metal active sites Water is adsorbed dissociatively and hydrogen is formed in this reaction Hydrogen is adsorbed dissociatively Methane is adsorbed molecularly Molecular adsorption of CO and CO2 6 ACS Paragon Plus Environment

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o o

The following carbon containing surface intermediates exist: CH4-S, CH3-S, CH2-S, CH-S, C-S. Coverage of them is assumed to be low. The following carbon and oxygen containing surface intermediates exist: CH2O-S, CHO-S, CO-S, CO2-S.

Then, a kinetic model was also derived and modified for rWGS and methanation reactions from the reaction mechanisms proposed by Hou & Hughes15. The reaction mechanisms for Xu & Froment and Hou & Hughes are found in the Supporting Information Table S1. Differences in the models compared to Xu & Froment can be summarized as follows:

o o

CH2-S species are directly formed from gas phase methane A simpler mechanism than in Xu & Froment. Surface Intermediates CH2O-L, CH3-L, CH4-L missing from Hou & Hughes model

Furthermore, kinetic models were derived from the reaction mechanisms proposed by Hakeem et al.8 for rWGS, and Alstrup17 as well as Weatherbee18 for methanation reactions. This is a different approach where the kinetic equations have different denominator for the rate equations of rWGS reaction and methanation reactions as a consequence of the reaction mechanisms being developed separately for each reaction. The reaction mechanisms for Hakeem et al.8, and Alstrup17 and Weatherbee18 are found in the Supporting Information Table S1 and Table S2, respectively. The Arrhenius and van’t Hoff equations were applied to calculate the temperature dependency of rate constants and adsorption equilibrium constants of the models. Temperature centering was used in these equations to reduce the correlation of activation energies and adsorption enthalpies with kinetic constants and adsorption constants, respectively. The reaction equilibrium constants for the models were calculated using the temperature dependent relationships developed by Swickrath et al.21 for the three reactions. Calculations using these relationships were compared to equilibrium calculations using HSC to check their validity.



1 2 3 4 5 6Table 3: Summary table of kinetic models used for parameter estimation 7 Power-Law model Xu & Froment model Hakeem, Alstrup & Weatherbee model 8Reaction &'(* &)* &' &)* &'( &)*( ! "#$ 9rWGS/WGS ! "#$ 2&'(* &)* − ( ( 3 "#$ =! %&'( &)*( − + = %&'(* &)* − +1",-./0 $ &'( = 10 41 + )* &)* + '( &'( + )*( &)*( + '(* &'(* 5 11 &' * &)'8 &' &)*( &' * &)'8 ! "#$ CO2 ! "#$ 12 7&'( &)*( − ( 9 = ! "#$ 7&'( &)*( − ( 9 = :.6 7&'( * &)'8 − ( 91",-./0 $ : & & & ' * ' methanation/ ( '( ( 13 = &' &)* SRM to CO2 14 %1 + '(* &'( * + '( &' .6 + )* ( ( + ( &'(* 15 : &'(* &)'8 &'(* &)'8 !: "#$ : & & "#$ : ! CO )* ' : ( ( "#$ : = !: %&'( &)* − + 2&'( &)* − 3 16 : = .6 7&'( * &)'8 − 91",-./0 $ : : &':( : &'( methanation/ 17 : = 41 + '( * &'(* + '( &' .6 + )* &)* 5 SRM to CO 18 ( ,-./0 = 1 + )* &)* + '( &'( + )'8 &)'8 19 Denominator + '( * &'(* ⁄&'( 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 8 43 44 45 46 ACS Paragon Plus Environment 47 48

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Hou & Hughes model =

! "#$

&' .6 & . ( '( *

%&'( * &)* −

&'( &)*( +1",-.'' $

=

&'( &)*( ! "#$ 91",-.'' $ .;6 . 7&'( * &)'8 − &'( &'( *

: =

&':( &)*( !: "#$ 91",-.'' $ .6 . 7&'( * &)'8 − : &'( &'( *

,-.'' = 1 + )* &)* + '( &' .6 + '( * &'(* ⁄&'( (

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3. Results and discussion 3.1 Thermodynamic equilibrium calculations Fig. 3 shows the results of the chemical equilibrium calculations based on thermodynamics for 1 and 30 bara (A and B, respectively) conditions when the feed composition is N2=42.5 mol-%, H2=38.33 mol-% and CO2=19.17 mol-%. At both pressures, the equilibrium calculations were obtained for two cases; no CH4 formation is allowed, and CH4 formation is allowed. The purple line represents the CO2 conversion in the equilibrium when the minimization of the free Gibbs energy is performed for CO2, H2, N2, H2O and CO. Thus, this line represents the maximum CO2 conversion or CO yield that can be reached when shift reactions are the only reactions possible. Shift reactions are independent of pressure due to equimolar reactions. CO2 conversion increases with increasing temperature. On the other hand, the brown, pink and orange lines represent the CO2 conversion, CH4 yield and CO yield, respectively, when CH4 is also included in the thermodynamic calculations. In this case, product selectivity switches from CH4 to CO with increasing temperature. Higher pressure has a significant impact on CH4 formation, which considerably increases at high temperatures, compared to atmospheric pressure.

A

1 bar

%

100 90 80 70 60 50 40 30 20 10 0

xCO2 eq (rWGS) xCO2 eq (rWGS+CH4) yCH4 eq (rWGS+CH4) yCO eq (rWGS+CH4)

450

550

650

750

850

Temp. (degC)

B

30 bar

%

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


100 90 80 70 60 50 40 30 20 10 0 450

550

650

750

850

Temp. (degC)

Fig. 3: Equilibrium calculation results at 1 (A) and 30 bara (B) pressure with an initial composition of N2=42.5 vol-%, H2=38.33 vol-% and CO2=19.17 vol-%.



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

3.2 Catalyst screening Fig. 4 presents the experimental results with catalyst 1 at atmospheric (A) and 30 bara (B) pressure with a feed composition of N2=42.5 vol-%, H2=38.33 vol-% and CO2=19.17 vol-%, total volumetric flowrate 2.087 ln/min and at different temperatures. The light-blue circular, green triangular and dark-blue diamond markers are the experimental CO2 conversion, CH4 yield and CO yield, respectively, calculated based on the outlet gas composition of the tubular reactor. The dashed lines are added to the chart to improve visibility of the results. Equilibrium conversion of CO2 was reached with catalyst 1 in all the experimental points. However, the experimental CO yield lays in between the CO equilibrium yield when CH4 formation is considered, and the CO2 equilibrium conversion when no CH4 formation is considered. Thus, equilibrium CO and CH4 yields were not reached, subsequently, the catalyst showed higher selectivity towards CO formation than CH4 formation. One reason for this behaviour could be that the feed composition had a H2/CO2 ratio which was under stoichiometric for CO and CO2 methanation reactions. Fig. 5 presents the experimental results with catalyst 2. This catalyst showed similar behavior as catalyst 1. Equilibrium conversion for CO2 was reached in all the experimental points except for the experimental point having the lowest temperature and pressure. It also exhibited higher selectivity towards CO formation than CH4 formation. Moreover, catalyst 2 showed significantly less activity towards CH4 formation and more activity towards CO formation compared to catalyst 1. The reason of this is probably the lower nickel content of the catalyst 2 compared to catalyst 1. Unde 4 performed experiments for rWGS with bare alumina support as catalyst. Alumina proved to be active for rWGS with high selectivity to CO formation but significantly less active compared to Ni/Al2O3 catalyst. Fig. 6 presents the experimental results for catalyst 3. In this case, the catalyst also exhibited higher selectivity for CO formation than for CH4 formation. Nevertheless, this catalyst was less active than catalysts 1 and 2, resulting in lower yields for both CO and CH4 formation. Catalyst 2 was selected for kinetic modelling based on the results of the catalyst screening.


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A

B

Fig. 4: Experimental results for 0.5 g of catalyst 1 at atmospheric (A) and 30 bara (B) pressure with a feed composition of N2=42.5 vol-%, H2=38.33 vol-% and CO2=19.17 vol-%, total volume flowrate 2.087 ln/min and at different temperatures.



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

A

B



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A

B


3.3 Catalysts stability Each catalysts was tested during the catalyst screening for ca. 60 hours in total during which the first experimental set point was repeated every 20 hours to check the catalyst stability. Catalyst 2 was tested again a total of ca. 130 hrs for kinetic modelling experiments with lower amount of catalyst in the reactor. These total operating times are enough only to make preliminary evaluation of catalyst stability since only initial stabilisation period was observed. Fig. 7 presents the the experimental CO2 conversion, CH4 yield and CO yield for catalyst 2 during 130 hrs. of operation. This chart shows that after 40 hours of initial stabilization of the conversion of CO2 remained stable. Both Ni-based catalysts exhibited significant decrease of activity during initial stabilization period but the selectivity towards CO formation increased in both cases during this initial period. Operating at 30 bara pressure increased the speed of this initial deactivation for both Ni-based catalysts. Catalyst 3 exhibited no initial decrease, exhibiting stable conversion during the 60 hours of testing. 13 ACS Paragon Plus Environment


45 40 35 30 25 %

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20 15 10 5 0 0

20

40

60

80

100

120

Time (hr)

Fig. 7: Stability results for 0.25 g of catalyst 2 at atmospheric pressure with a feed composition of N2=42.5 vol-%, H2=38.33 vol-% and CO2=19.17 vol-%, total volume flowrate 2.087 ln/min and ca. 505°C. The light-blue circular, green triangular and dark-blue diamond markers are the experimental CO2 conversion, CH4 yield and CO yield, respectively.

3.4 Kinetic modelling Internal mass transfer limitation of catalyst 2 was checked using Weisz-Prater criterion22 using calculated effective diffusion coefficients of the chemical components23 and empirical data on Ni/Al2O3 from other publications4,24. These calculations revealed some internal diffusion limitations inside the 400-500 µm particles of catalysts 2 for CO2, CO, H2O and CH4. No diffusion limitations were estimated for H2. These calculations revealed that particle size lower than ca. 200 µm would be optimal to neglect internal diffusion limitations for all the components. For that reason, this study refers to apparent kinetic and adsorption constants. On the other hand, significantly high activation energies obtained for main reactions, particularly for methanation reactions, indicate that reactions are mainly controlled by the reaction kinetics. The kinetic models presented in Table 3 have 6 to 14 parameters (kinetic and adsorption constants, activation energies and adsorption enthalpies) depending on the model. This is quite a large number of parameters to be estimated with 33 experimental points. For this reason, Xu & Froment14 model was initially used for screening of a relevant set and number of parameters to be estimated. The target of this screening was to find out the optimum number of parameters that was necessary for reaching a minimum residuals sum of squares (RSS) between the calculated and experimental outlet molar flow rates and at the same time avoid over parametrization resulting in high parameter variances and correlations between the parameters. In this method, some of the parameters were taken from the literature and remained fixed while the rest of the parameters were optimized by minimization of RSS. During this screening, an activity factor (aF) parameter was used multiplying the kinetic constants of the CO and CO2 methanation reactions. This aF was estimated instead of the two apparent kinetic constant of the CO and CO2 methanation reactions. This helped to reduce the number of the estimated parameters without increasing the RSS. Finally, the number of floating parameters was optimized to four by trial and error. The fixed parameters for Xu & Froment and Hou & Hughes based models were obtained from their respective publication.14,15 The parameters from Xu & Froment publication were also used as fixed parameters for the power-law model and for the Hakeem, Alstrup and Weatherbee based model.


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Table 4: Summary table of the estimated parameters, confidence intervals and RSS for the different kinetic models.

=0,&:,@ !′ ,@ ′)*,@ ′'( *, : RSS F-obs

Power-law

Xu & Froment

6.3E-11 ± 4.08E-08 0.461 ± 0.259 9.72E-08 55.16

2.71E-02 ± 2.14E-02 0.944 ± 0.394 93.8 ± 48 0.597 ± 0.342 3.13E-08 141.02

Hakeem, Alstrup & Weatherbee 0.228 ± 0.367 2.59 ± 2.52 212 ± 233 1.61 ± 1.81 1.01E-07 28.54

Hou & Hughes 2.3E-15 ± 3.61E-16 11.8 ± 11 19.8 ± 16.7 1.33 ± 2.2 3.95E-08 90.6

Table 5: Parameter correlation matrix for kinetic model based on Xu & Froment model.

Xu & Froment =0, &,@ =0,&:,@ 1 !′ ,@ 0.738 ′)*,@ 0.941 ′'(*, : 0.758

!′

,@

1 0.716 0.683

′)*,@

′'( *, :

1 0.637

1

Fig. 8: Parity plots for kinetic model based on Xu & Froment model.

Table 4 summarizes the results from the parameter estimation and parameter analysis for the different models. The derived model from Hakeem, Alstrup and Weatherbee reaction mechanisms provided estimated parameters with wide confidence intervals which include the zero for three of the four parameters. This model displayed a RSS even higher than the power-law model. This poor suitability of the model with experimental data indicates that different denominators in the LHHW rate equations (Table 3) is not suitable approach for simultaneous rWGS and methanation reactions over a metal catalyst. Different denominators are associated with the idea of having different active sites on the active metal surface for rWGS reactions and for methanation reactions. Thus, this modelling result indicate that one type of active metal sites on supported nickel catalyst for all three reactions is the correct approach at least from the modelling point of view.



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Fig. 9: Experimental and calculated results for catalyst 3 at different pressures, H2/CO2 ratios, a total volume flowrate 2.087 ln/min, different catalyst loadings and at different temperatures. The calculated results are obtained using the same input conditions as experiments and using the Xu & Froment model with the own estimated parameters.


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Hou & Hughes model and Xu & Froment model provided both quite similar fits with experimental data. This was expected since the basic mechanistic assumptions of these models were quite similar and the final forms of the models also remained similar. However, estimated parameters of Hou & Hughes model exhibited higher cross-correlations compared to Xu & Froment model. Table 5 shows the correlation matrix between the different estimated parameters of the kinetic model based on Xu & Froment model. In general, the parameters exhibited a low degree of correlation except for the correlation between the apparent adsorption equilibrium constant of CO and the activity factor of the methanation reaction which was moderate. It can be concluded that basic assumptions proposed by Xu & Froment and adopted and modified by Hou & Hughes describe well high temperature system of rWGS and methanation. Thus, it can be assumed that CO and CO2 are adsorbed molecularly and water dissociatively. It can also be assumed that reaction proceeds through some surface intermediates described by Xu & Froment and Hou & Hughes. Table 6 presents the entire kinetic model developed in this study for Ni/Al2O3 (2 w-% of Ni) catalyst based on Xu & Froment model. This model provided the best fit to the experimental data reducing by more than three times the RSS produced by the simple power-law model. The apparent rate constant for rWGS is significantly higher than the apparent rate constants of the methanation reactions at reference temperatures (obtained by multiplying =0, &,@ ∗ !C,@ ) which is in agreement with experimental data. Furthermore, the ratio between the rate constants of rWGS and methanation reactions is higher in our study (for a Ni/Al2O3 with 2 w-% of Ni) than the ratio between these parameters in the original Xu & Froment model (for a Ni/MgAl2O4 with 15 w-% of Ni)14. This underlines again that lower the nickel content in the catalyst leads to higher selectivity towards rWGS. The estimated adsorption constants for CO and H2O also differ from the ones obtained by Xu & Froment14 but in lower extend compared to the kinetic constants. Both adsorption constants increased compared to the original values. In the statistical analysis, the t-values were also calculated for each of the different estimated parameters. These t-obs values confirmed the statistical significance of the estimated parameters. Fig. 8 presents the parity plots comparing the simulation with Xu & Froment model (Table 6), with experimental data. Furthermore, Fig. 9 presents all the experimental results used for the parameter estimation together with calculated results using the developed kinetic model. The model describes well the effect of pressure, H2/CO2 ratio and temperature. The chart for 1 bar, H2/CO2 =2 and 0.5 grams of catalyst shows that the model underestimates a little the activity of the catalyst both for CO formation and CH4 formation. This could be a result of the low number of experiments performed using the lower SV (0.5 grams of catalyst). In the chart for 1 bar, H2/CO2 =2 and 0.25 grams, the kinetic model seems to drop also a little the CO2 conversion at the highest temperature and 1 bar. This happens outside of the studied range of temperatures. This effect vanishes at lower SV. Overall, the model describes well the experimental results. The model shows some limitations when used at high temperature (>800°C), low pressure and high SV.



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Table 6: Kinetic model for catalyst 2 based on Xu & Froment model.

Equation Rate eq. Arrhenius eq. DEN van’t Hoff eq.

rWGS/WGS

&' &)*( !′ "#$ = %&'(* &)* − ( +1",-./0 $ &'( !′ "#$ = !′

,@

∗ exp G−

- 1 1 7 − 9K H # # IJ

CO2 methanation/ SRM to CO2

&' &)*( !′ "#$ = :.6 7&'(* &)'8 − ( 91",-./0 $ &'(

!′ "#$ = =0,&:,@ ∗ !,@ ∗ exp G−

- 1 1 7 − 9K H # # IJ

CO methanation/ SRM to CO : =

&':( &)*( !′: "#$ 91",-./0 $ .6 7&'( * &)'8 − : &'(

!′: "#$ = =0,&:,@ ∗ !:,@ ∗ exp G−

,-./0 = 1 + ′)* &)* + '( &'( + )'8 &)'8 + ′'( * &'(* ⁄&'( C "#$ = C,LMNO ∗ exp G−

Parameters

-: 1 1 7 − 9K H # # IJ

∆C 1 1 7 − 9K H # # IJ

!′ ,@ "PQRS $ =0, &,@ "PQRS $ ′)*,@ "PQRS $ '( ,@ * !,@ * !:,@ * 0.944 (2.59) 93.8(3.99) 2.71 ∙ 10W (4.91) 6.09 ∙ 10W@ 2.96 ∙ 10W 5.12 ∙ 10W6 ∆'( * -* - * -: * ∆)* * 67.13 ∙ 10: 243.9 ∙ 10: 240.1 ∙ 10: −70.65 ∙ 10: −82.9 ∙ 10: *Fixed values in the parameter estimation. Values from Xu & Froment publication were converted to the units of this study PQRS : Observed t-value obtained from the t-test in the statistical analysis of the parameter estimation

18

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)'8, :* 0.1791 ∆)'8 * −38.28 ∙ 10:

′'(*, : "PQRS $ 0.597(3.57) ∆'( * * 88.68 ∙ 10:

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4. Conclusions The catalyst screening of two Ni-based and one Rh-based commercial catalysts under atmospheric and 30 bara pressure was performed to study their suitability for rWGS process having methanation as a side reactions. Ni-based catalysts displayed higher activity compared to Rh-based catalyst but they exhibited significant activity decrease during initial stabilization period, which simultaneously increased catalyst selectivity towards CO formation. After initial stabilization, no deactivation was observed during 130 h for any of the tested catalyst. Ni/Al2O3 catalyst with lower Ni content (2 w-%) exhibited higher selectivity towards CO formation compared to Ni/Al2O3 catalyst with higher Ni content (15 w-%). Three different kinetic models were tested for the reaction system of rWGS and methanation. Based on the parameters estimation, models assuming reactions to take place on the same active sites on nickel gave the best fits of the model (Xu & Froment, Hou & Hughes). It is also obvious that basic mechanistic assumptions of these models including e.g. molecular adsorption of CO and CO2 describe these reactions well. Based on statistical analysis the classical model of Xu & Froment was chosen to be used in further studies. Author information Corresponding author *Tel: +358 40 143 8685, [email protected] Notes The authors declare no competing financial interest. Acknowledgement The Institute of Micro Process Engineering (IMVT) at Karlsruhe Institute of Technology (KIT) is acknowledged for hosting Francisco Vidal Vázquez as guest researcher. The entire experimental work of this study was performed at the IMVT. IneraTec GmbH is also acknowledged for collaboration in performing this work. This work is also part of the NEO CARBON ENERGY project, which is one of the Tekes strategic research openings. The project is carried out in cooperation with VTT Technical Research Centre of Finland Ltd, Lappeenranta University of Technology (LUT) and University of Turku, Finland Futures Research Centre FFRC. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Table S1. Surface reaction steps for Xu & Froment (up left), Hou & Hughes (up right), and Hakeem et al. for rWGS/WGS (below). Table S2: Surface reaction steps for CO and CO2 methanation based on Alstrup et al. and Weatherbee et al.1818



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Nomenclature ∆C

∆ L" $

Enthalpy of adsorption [J/mol] Enthalpy of reaction at reference temperature

=0,&:,L" $

Activity factor for CO and CO2 methanation at reference temperature

DEN

Denominator

-C

Activation energy of reaction i [J/mol]

F-obs

F-observed from F-test

!′C,L" $

Apparent rate constant of reaction i at reference temperature T(K) [mol/(kgcat*s)*barx]

!C,L" $

′C,L" $ C,L" $ &C

Rate constant of reaction I at reference temperature T(K) [mol/(kgcat*s)*barx] Apparent adsorption constant at reference temperature T(K) [barx] Adsorption constant at reference temperature [barx] Partial pressure component i [bar]

C

Reaction rate of reaction i [mol/(kgcat*s)]

R

Ideal gas constant [m3 Pa/(K*mol)]

vT

Volume flowrate [Nl/min]

Acronyms: CCU

Carbon capture and utilization

FT

Fischer-Tropsch synthesis

LHHW

Langmuir-Hinselwood Hougens-Watson

rWGS

Reverse Water-Gas shift

RSS

Residual sum of squares

SV

Space velocity

WHSV

Weight hourly space velocity (hr-1)

WGS

Water-Gas shift


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Table of Contents Graphic


Catalyst Screening and Kinetic Modeling for CO Production by High

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