Interplay of Kinetics and Thermodynamics in Catalytic Steam Methane

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The Interplay of Kinetics and Thermodynamics in the Catalytic Steam Methane Reforming over Ni/MgO-SiO2 Naoki Kageyama, Brigitte R. Devocht, Atsushi Takagaki, Kenneth Toch, Joris W. Thybaut, Guy B Marin, and S. Ted Oyama Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03614 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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Industrial & Engineering Chemistry Research

The Interplay of Kinetics and Thermodynamics in the Catalytic Steam Methane Reforming over Ni/MgO-SiO2 Naoki Kageyamaa, Brigitte R. Devochtb, Atsushi Takagakia,c, Kenneth Tochb, Joris W. Thybautb*, Guy B. Marinb, S. Ted Oyamaa,c,d* a

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan

b

Laboratory for Chemical Technology, Ghent University, Tech Lane Ghent Science Park – Campus A, Technologiepark 914, B-9052 Ghent, Belgium c

d

College of Chemical Engineering, Fuzhou University, Fuzhou 350116, China

Environmental Catalysis and Nanomaterials Laboratory, Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0211, United States

*Corresponding authors: STO [email protected], JWT [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT The steam methane reforming (SMR) reaction was studied on a Ni/MgO-SiO2 catalyst at 923 K (650 o

C) and 0.40 MPa in a tubular packed-bed reactor. The partial pressures of CH4 and H2O were varied

between 20 and 140 kPa and 80 and 320 kPa respectively. Measurements were carried out without mass and heat transport limitations, as verified by the Weisz-Prater and Mears criteria. Experimentally, the CH4 conversion increased with the inlet partial pressure of H2O and decreased with the inlet partial pressure of CH4. However, at low CH4 inlet partial pressures, i.e., at 40 and 60 kPa, the conversion passed through a maximum. Rate expressions were derived based on a simple two-step sequence. A statistical analysis led to a globally significant, weighted regression and resulted in a good agreement between the model and the experimental data, as indicated by a low F value of model adequacy of 2.84. The rate and equilibrium coefficient parameters were statistically significant as indicated by narrow confidence intervals. The model was able to correctly describe the experimentally observed maximum in the methane conversion and allowed to relate this behavior to CH4 and H2O surface coverages. The model was able to capture the increasing selectivity of CO2 with the H2O inlet partial pressure and increasing methane conversion. The effect of changing the total pressure and H2O/CH4 ratio on the CH4 conversion as a function of the space velocity was simulated and corresponded to both the experimental and literature data.

A major finding of the modelling is that as flow rate is increased there is a

crossover in the order of conversion with pressure due to a transition from thermodynamic to kinetic control. Although the SMR equilibrium conversion decreases with pressure, away from equilibrium at high flow rates, conversion is higher at higher pressures due to enhanced adsorption rates. Keywords: steam methane reforming; Ni/MgO-SiO2 catalyst; integral kinetic analysis; kinetics and thermodynamics


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1. INTRODUCTION

Hydrogen plays an important role in today’s chemical industry, especially in the refining of petroleum and the synthesis of chemical products. Recently, hydrogen has received attention as a potential energy source for fuel cells because of its high energy conversion efficiency 1. The majority of hydrogen is produced by fossil fuel reforming 2 , especially via steam reforming of natural gas, whose main component is methane.

Steam Methane Reforming (SMR) is a strongly endothermic reaction (Eq. 1)

which is generally accompanied by the moderately exothermic water-gas shift reaction (Eq. 2).

ΔHo298 = +206 kJ mol-1

Eq. 1

ΔHo298 = -41 kJ mol-1

Eq. 2

Both reactions contribute to the production of H2. methane (CH4 + CO2

In contrast, the much studied dry-reforming of

2 CO + 2H2) cannot be used for commercial hydrogen production

because the reverse water-gas shift reaction (CO2 + H2 produce water. 3

CO + H2O) consumes hydrogen to

The SMR process is operated industrially at 0.3-2.5 MPa total pressure and at high

temperature, i.e., typically between 900 and 1300 K. The energy requirement is considerable and is commonly supplied by the combustion of natural gas, c.q., methane, in a furnace in which the SMR reactor is placed. In general, this results in a high energy consumption and large CO2 emissions. The simultaneously occurring water-gas shift reaction 4 produces additional H2 and CO2. There have been 3 ACS Paragon Plus Environment


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many attempts to improve the process efficiency. In a study by Zheng et al., the energy required for the endothermic reforming was supplied using solar energy as a renewable energy source 5 . In an investigation by Hafizi et al., chemical looping was employed by cycling fine metal powders between oxidation and reduction steps for hydrogen production from methane 6. In addition to these attempts, efforts have been made to develop new catalysts. Kinetic modeling bridges the gap between phenomena occurring at the conceptual and practical scale. It provides essential information for reaction design and corresponding process scale-up 7 as well as for understanding catalyst deactivation 8. It can also be used to better understand the catalyst activity so as to allow its improvement. As such a better understanding is pursued in this work with a model accounting for the most kinetically relevant steps. Although some studies aim for increasingly more detailed descriptions of the reaction 9,10,11 this work takes an opposite approach to produce the simplest possible analysis.

The significance of the work is two-fold.

First, it presents a minimal kinetic model

with just two equilibrium parameters and two rate parameters to describe the MSR and WGS reactions that can be used by other researchers as a metric for comparison.

Second, it demonstrates that the

conversion falls with increasing pressure at low space velocity (high space time) as expected from thermodynamics, but increases with increasing pressure at high space velocity (low space time) from kinetic control.

As far as we know, this interplay between thermodynamics and kinetics mediated by

pressure has not been noted before. 4 ACS Paragon Plus Environment

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The catalyst employed in this research is Ni-based, which is the most commonly used metal, but employs a MgO-SiO2 support that has not been studied widely.

Magnesium silicate can be made in

high surface area and has been used as a catalyst [12] as well as a catalyst support [13].

It is strong,

low cost, and offers sulfur tolerance, which is a desirable property with streams derived from natural gas.

In addition, Ni on this support is easier to reduce than on Al2O3-based supports because

aluminates are not formed.

In this research a series of steam methane reforming measurements was

conducted by varying the ratio of the inlet gas components, methane and water vapor as reactants and nitrogen as inert dilution gas at a pressure of 0.40 MPa.



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2. 2.1

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EXPERIMENTAL SECTION Experimental

The gases used in this study were CH4 (99 % Toatsu Yamazaki Co., Ltd.), H2 (99.99 % Showa Denko Gas Products Co., Ltd.) and N2 (99.99 % Toatsu Yamazaki Co., Ltd.).

A conventional Ni/MgO-SiO2

catalyst (JGC Catalysts and Chemicals Ltd.) consisting of 53.0-58.0 wt% NiO, 8.1-11.1 wt% MgO and 24.2-28.2 wt% SiO2 (supplier information) was selected. This commercial catalyst is used for naphtha reforming and is, hence, chemically sufficient stable enough at high pressure and temperature. The BET area was determined by N2 adsorption at 77 K using a volumetric apparatus (BELSORP-mini, Microtrac BEL Corp.). The H2 chemisorption uptake was measured using a pulse flow instrument (CHEMBET3000, Quantachrome Instruments) at 313 K after 12 h reduction at 923 K in 5 % H2/Ar.

Steam methane reforming was studied in a vertical concentric tubular reactor of a geometry suitable for future membrane reactors studies. The reactor consisted of an inner dense alumina tube, a quartz sleeve and an outer stainless steel shell and was heated by an external electric furnace, see Fig.1. An amount of 0.1 g of Ni/MgO-SiO2 catalyst diluted with 3.9 g SiO2 particles (Wako Pure Chemical Industries Ltd.) formed the catalyst bed of volume 3.0 cm3. For optimal heat transfer, the catalyst bed was placed in the outer annular region between the quartz tube and the dense alumina tube. It was held in the middle of the reactor by extra SiO2 particles filling the lower half of the reactor, see Fig. 1. The outer tubular 6 ACS Paragon Plus Environment

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stainless steel tube covered the quartz tube for mechanical strength and rigid sealing, but an opening allowed pressure equalization between its inside and the outside of the quartz sleeve. The reactor pressure was controlled by a backpressure regulator connected at the reactor outlet and the reactor temperature was monitored by a thermocouple placed at the bottom of the catalyst bed. A total pressure of 0.40 MPa and a temperature of 923 K was used for the measurements. The catalyst particles were sieved to 400-630 μm in diameter and the SiO2 particles used for dilution of the catalyst to 600-850 μm. Upstream of the reactor, CH4 and N2 were mixed and introduced to a vaporizer heated at 473 K. Liquid water was fed using a pump (Hitachi L-7100) to the vaporizer and immediately vaporized and mixed with the N2/CH4 flow. The mixed gas flowed to the reactor in a heated tube to prevent water condensation in the transfer lines. The inlet gas composition was varied by keeping the total flow rate at 270 μmol s-1 (400 cm3 (NTP) min-1), corresponding to a constant gas-hourly space velocity of 2000 h-1. The weight-hourly space velocity (WHSV) and space time (W/F0CH4) was 12000-72000 Ncm3 gcat1

h-1 and 1.12 - 6.72 kgcat s mol-1 respectively with respect to CH4. The inlet flow composition varied in

the range of 5-35 vol% for CH4 and 20-70 vol% for H2O, diluted with nitrogen. The space time is inversely proportional to the weight-hourly space velocity, see Eq. 3, with Tn and pn the normal temperature and pressure of 273.15 K and 101325 Pa, W the catalyst mass [kg], F0CH4 the inlet flow rate of methane [mol s-1] and R the universal gas constant [J mol-1 K-1].



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𝑊𝑊

0 𝐹𝐹𝐶𝐶𝐶𝐶 4

=

𝑇𝑇𝑛𝑛 ∙ 𝑅𝑅

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Eq. 3

𝑝𝑝𝑛𝑛 ∙ 𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊

The outlet gas flow was analyzed by a gas chromatograph (GC-8A, Shimadzu Co.) equipped with a thermal conductivity detector (TCD) using a combination of Porapak-Q, Shincarbon and Molecular sieve 5A columns. However, it was difficult to accurately detect the precise amount of H2O with the TCD due to its large amount and the high affinity between H2O and the GC columns. The amount of H2O was calculated from the mass-balance averages of hydrogen and oxygen individually. For interpreting the dataset, the experimental data was normalized by closing the carbon balance, ensuring that the total carbon flow rate at the reactor inlet equals that at the reactor outlet.

Fig. 1. Reactor used in the steam methane reforming studies


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2.2 Intrinsic kinetics regime verification

Prior to the kinetic modelling, the Weisz-Prater and the Mears criteria were calculated to assess the absence of mass and heat transfer limitations 14. The Mears criterion,

�

−∆𝑯𝑯𝑹𝑹𝑪𝑪𝑯𝑯𝟒𝟒 𝒅𝒅 𝒉𝒉𝒉𝒉

∙

𝑬𝑬𝒂𝒂

𝑹𝑹𝑹𝑹

� < 0.15

Eq. 4

was calculated to be equal to 0.049 at the reaction condition which gave the highest CH4 conversion 58 % and the Weisz-Prater criterion, 2 𝑅𝑅𝐶𝐶𝐶𝐶4 𝜌𝜌cat 𝑑𝑑𝑝𝑝

4𝐷𝐷e 𝐶𝐶CH4 ,b

< 0.3

Eq. 5

was calculated as 0.032 at the same reaction conditions. It can, hence, be concluded that there were no internal nor external heat or mass transfer limitations in the present study.

Definitions and actual values for the quantities in Eqs. 4 and 5 are given in Table 1 below.

The highest

CH4 conversion of 58% was obtained with 40 kPa CH4, 200 kPa H2O and 160 kPa N2., W = 100 mg, vtot = 400 cm3 (NTP) min-1.



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Table 1. Quantities in the Mears and Weisz-Prater Criteria Quantity

Definition

Values

Units

ΔH

Heat of the reaction of SMR

-2.04x105

J mol-1

RCH4

Net rate of formation of CH4

0.169

mol kgcat-1 s-1

d

Specific length of the reactor

2.5x10-3

m

h

Heat transfer coefficient

770

J m-2 K-1 s-1

T

Temperature

923

K

Ea

Activation energy of SMR

1.5x105

J mol-1

R

Gas constant

8.31

J mol-1 K-1

ρcat

Catalyst density

5180

kg m-3

dp

Catalyst pellet diameter

5.16x10-4

m

n

Reaction order

1

kc

Mass transfer coefficient

0.173*

m s-1

CCH4,b

Bulk gas concentration of CH4

5.51

mol m-3


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Parameter estimation

The Iteratively Reweighted Least Squares (IRLS) method has been used as regression procedure for model parameter estimation 15. This procedure consists of minimizing the sum of squares of the residuals e, i.e., the difference between the experimental observations and model simulations, which approximate the experimental error ε with respect to the model parameters β in which the variance-covariance matrix D is taken into account, see Eq. 6. This matrix is required because of the heteroscadisticity of the experimental error, i.e., its variance depends on the operating conditions and weighs each of the elements within the sum of squares in a statistically justified manner. S (β ) = ε D (ε ) ε  → Min T

−1

β

Eq. 6

The response weights, i.e., the elements of the inverse of the variance-covariance matrix are automatically determined within the modelling software used, i.e., the MicroKinetic Engine, and updated after each regression until convergence in the weights was achieved. Typically, a few iterations sufficed to reach this convergence. The approximation of the true model parameters β with the most optimal parameter estimates b yields the final residuals e resulting in the minimization of the objective function S, see Eq. 7. ε = y− Xβ  → e = y − X b


Eq. 7


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The tubular reactor is simulated as an ideal plug flow reactor, operated in steady state. The set of continuity equations for all components forms the reactor model, shown by Eq. 8 with Ri the net rate of formation of component i and W the catalyst mass. Eq. 8

dFi = − Ri dW

The independent variables that serve as an input for the model during the regression procedure are the space time, the temperature, the total pressure and the inlet molar flow rate of the five components, i.e., methane, water, hydrogen, carbon monoxide and carbon dioxide. The five model responses are the corresponding outlet molar flow rates of the same components. In total, 37 experiments, including 14 repetition experiments, were taken into account for the regression. A model is deemed to be adequate if the difference between the experimentally observed responses and the model calculated responses can be mainly attributed to the experimental error 16.

The model

adequacy is assessed via an F test, in which an Fa value is defined as the ratio of the lack-of-fit sum of squares and the pure-error sum of squares (Eq. 9). The former is calculated as the difference between the residual sum of squares and the latter as the pure-error sum of squares of the variance of repeated observations 17. The test is based on the null hypothesis that the model is adequate and states that Fa is distributed to F with dfLOF degrees of freedom in the numerator and dfPE degrees of freedom in the denominator. If the calculated F value is smaller than the corresponding tabulated F value for the selected confidence level, the null hypothesis is accepted and the model is considered to be adequate. In this work 12 ACS Paragon Plus Environment

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a confidence level of 0.95 is selected, corresponding with a tabulated F value in the range of 2 – 5, the exact value being determined by dfLOF and dfPE. It should be noted that the adequacy test is a severe test which is rarely fulfilled. SSQ LOF df LOF Fa = SSQ PE df PE

Eq. 9

In order to assess the global significance of the regression, the hypothesis that all parameters would be equal to zero simultaneously is verified, via an F test. By means of this test it is determined whether the actual model performs better than a model which takes the average of the observed experimental values as response values. The Fs value is calculated as the ratio of the regression sum of squares and the residual sum of squares (Eq. 10). If the calculated F value exceeds the tabulated at the selected confidence level, the null hypothesis is rejected and the regression is seen as meaningful and globally significant. In practice, the null hypothesis is easily rejected and the calculated F values should at least be of the order of magnitude of 100. SSQ REG df REG Fs = SSQ RES df RES

Eq. 10



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2.4 Software The model in this work was regressed to the experimental data and analyzed with the MicroKinetic Engine (µKE) 18. The µKE is a software tool for the simulation and regression of chemical kinetics and has been developed at the Laboratory for Chemical Technology at Ghent University. Two deterministic regression routines are implemented in the software, i.e., the Rosenbrock and Levenberg-Marquardt algorithm which are consecutively used during the process. The µKE is mainly characterized by its user-friendliness, robustness and flexibility.


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3. RESULTS AND DISCUSSION

3.1 State-of-the-art and catalyst choice

This study employed a Ni/MgO-SiO2 catalyst. uptake, Vads, was 90 µmol g-1.

The BET surface area was 105 m2g-1 and the hydrogen

The hydrogen uptake (Vads) can be used to calculate the particle size 19

from the formula, d = 6c/ρσVadswNA where c is the supported Ni metal weight % (43.2wt%), ρ is the bulk Ni density (8.91 g cm-3), σ is the surface Ni atom density (0.0649 nm2/atom), w is the weight of catalyst, and NA is Avogadro’s number. The calculated particle size was 41 nm.

The dispersion of the

catalyst can be obtained from the hydrogen uptake and the Ni content of the catalyst, D = VadsMWNi/c, and is found to be 1.2%, which is consistent with the large particle size.

Measurements on the catalyst

were carried out for more than two weeks, while verifying reproducibility of points.

This sets the

stability of the catalyst at least 200 h. Fresh catalyst was loaded as needed for different measurements.

Recent developments in the area of catalytic SMR were described in a 2013 review 20. Here, the latest catalysis studies since then are summarized. Using conventional SMR reactors 21, most of the papers report Ni as the principal component but use different supports such as CaO-Ca5Al6O14 22, γ-Al2O323, K2TixOy-Al2O3 24, α-Al2O3 25, NiAl2O4 26, TiO2 27, SiO2 28, ZrO2 29,Al2O3 (Ca co-loaded as Ca-Ni/Al2O3)30, SBA-15 31. Ni has been reported to change the distribution of certain co-loaded metals and improve their catalytic activity for other reactions too, apart from SMR 32 . The performance of those catalysts is 15 ACS Paragon Plus Environment


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summarized in Table 2 at reaction conditions that are similar to those used in the present investigation. As can be seen, some of the experimental results are at or close to equilibrium. The present studies are carried out away from equilibrium in order to obtain data in the kinetic regime.

Table 2. Summary of recent results in steam methane reforming WHSV (space

Catalyst

Ni content

T

P

K

MPa

time) H2O/CH4

Ncm3CH4 h-1 gcat-1

(kgcat s

XCH4 Equil.XCH4 %

%

CO2/CO Stability Ref.

molCH4-1) Ni/CaO-Ca5Al6O14 15 wt% Ni 923 0.10

4

230 (350.83)

95

95

0.48

NR

22

Ni/γ-Al2O3

20 wt% Ni 928 0.10

3

15000 (5.38)

88

98

NR

NR

23

Ni/K2TixOy-Al2O3

11 wt% Ni 973 0.10

2.5

15000 (5.38)

86

96

NR

>100 h

24

Ni/α-Al2O3

20 wt% Ni 973 0.10

3

35000 (2.31)

84

97

1.6

NR

25

33 wt% Ni 923 0.10

3

38000 (2.12)

80

96

0.70

NR

26

10 wt% Ni 773 0.10

1

6000 (13.45)

27

29

2.4

> 96 h

27

0.5

12000 (6.72)

20

83

NR

NR

28

1

39000 (2.07)

20

71

1.2

4h

29

873 0.10

4

NR

85

95

0.36

NR

30

25 wt% Ni 823 0.10

2

NR

21

48

NR

NR

31

2

48000 (1.68)

40

61

2.1

> 200 h

*

NiAl2O4 (spinel structure)

Ni/TiO2 Ni/SiO2 Ni/ZrO2 Ca-Ni/Al2O3 Ni/SBA-15 Ni/MgO-SiO2

NR: Not reported,

10 wt% NiO

973 0.10

15 wt% Ni 873 0.10 64 wt% NiO

56 wt% NiO

923 0.40

* This work 16 ACS Paragon Plus Environment

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A more extended range of operating conditions has been investigated as part of the present work. Of course, some repetition experiments have been performed, i.e., 10 experiments with feeds consisting of CH4 20 %, H2O 40 %, N2 40 %, 2 experiments with CH4 20 %, H2O 60 %, N2 20 % and 2 experiments with CH4 25 %, H2O 60 %, N2 15 %. The relationship between the methane conversion and the WHSV is plotted in Fig. 2 for the studies reported in Table 1. As can be seen, the data can be categorized in two regions, each exhibiting a decreasing conversion with increasing WHSV. The measurements with a H2O/CH4 ratio exceeding 2 resulted in a higher CH4 conversion than those with a H2O/CH4 ratio below 2. Variation of the points from the trend lines is expected as there is some variation in temperature and H2O/CH4 ratio, even though the reported studies all used a pressure of 0.10 MPa. The results obtained as part of the present work with a H2O/CH4 ratio equal to 2 and with a higher pressure of 0.40 MPa are situated in the middle of the two regions. The filled square shows a simulated result, which will be discussed later, see section 3.



Space time / kgcat s mol-1CH 100

--

8.07

4.03

80

4

2.69

(20)

(19)

CH4 conversion / %

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Page 18 of 42

2.02

1.61

(22)

(21) (23) H2O/CH4 = 2.5 ~ 4

60

This work H2O/CH4 = 2

40 20 0

(19)

0

H2O/CH4 = 0.5 ~ 1 (26)

(25)

10000

20000

30000 3 CH4

WHSV / Ncm

-1

40000 -1 cat

50000

h g

Fig. 2. Relationship between the conversion of methane and WHSV/space time in recent studies on steam methane reforming with Ni-based catalysts. Reference numbers are indicated by each point and correspond to Table 1. The lines are a guide for the eye. ○: H2O/CH4 > 2, □: H2O/CH4 < 2, ×: this work (H2O/CH4/N2 = 2/1/2, total inlet flow 400 Nml min-1, 0.4 MPa, 923 K), ■: simulated result (H2O/CH4/N2 = 2/1/2, total inlet flow 400 Nml min-1, 0.10 MPa, 923 K), by solving the set of Eq. 8 with the net rates of formation given by Eqs. 19 to 23. The rate and equilibrium coefficients are estimated by a weighted regression and are shown in Table 2. 3.2 Reaction description and model construction

In this study, SMR is described with a simple model consisting of four steps, which are defined as (1) CH4 adsorption on the catalyst surface, (2) H2O adsorption on the catalyst surface, (3) the oxidation of CH4 towards CO and H2 and (4) the water-gas shift reaction.

This combination of steps was chosen

as the simplest possible set from a kinetic standpoint, and is not meant to represent an actual mechanism 18 ACS Paragon Plus Environment

with elementary steps. In fact it is unlikely for methane and water to adsorb molecularly, and the adsorbed species shown are just representations of species such CHx, C, CO, or H

As such the

parameters of the reaction will be called equilibrium coefficients and rate coefficients, rather than equilibrium constants and rate constants.

Simple Model

Reaction (1)

Reaction (2)

Reaction (3)

Reaction (4)

We have also considered a case where Reaction (4), the water-gas shift reaction, is reversible.

This

will be discussed below and the results presented in the Supplementary Information. The asterisk indicates adsorbed species or empty sites. In this model the products CO, CO2 and H2 are assumed to desorb from the catalyst instantaneously, i.e., no adsorption is taken into account for the products. The model combines a surface reaction

for the reforming step with a gas-surface reaction

for the water-gas shift reaction. The latter does not imply a mechanism and is formulated simply to 19


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Page 20 of 42

avoid an explicit term containing adsorbed CO.

Aside from the equilibrium adsorption steps, the other steps are considered to be irreversible, with the reverse reactions not being involved. fitting parameters in this model.

This is a simplification, made in order to reduce the number of

Since much of the data are at relatively low conversion, this is

permissible. The effect of the reverse reaction is incorporated into the reaction rate parameters, which will have lower values than if the back reaction were explicitly considered.

This will be discussed

later.

The equilibrium coefficients for Reaction (1) is K1 and for Reaction (2) is K2. Eq. 11 and 12 show the net reaction rate of step (3) and (4) with k3 and k4 the corresponding rate coefficients. The reactant concentrations are expressed in terms of surface concentrations because, contrary to the products, CH4 and H2O react from the adsorbed state.

r3 = k 3CCH * C H' O*

Eq. 11

r4 = k 4 pCO C H O*

Eq. 12

4

2

2

When two surface species are involved in a reaction step as in Reaction (3), the surface species cannot occupy just any sites, but these sites need to be next to each other for the reaction to take place. This adjacent site requirement has been treated in a textbook 33, and is accounted for in the rate expression of the reaction step, using the number of adjacent neighbors z and Ctot the total concentration of 20 ACS Paragon Plus Environment

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active sites as shown in Eqs. 13 and 14.

When properly accounting for the dual site mechanism in the

rate expression of the reforming step, the reaction rate varies linearly with the total concentration of active sites, and not quadratically. 34 The new rate expressions r3 and r4 , shown in Eq. 14 and Eq. 15, are obtained by writing the concentration of the surface species in terms of the surface coverage 35.

C H O*

= zθ H 2O

Eq. 13

r3 = k 3 zCCH *θ H 2O = k 3 zCtotθ CH 4 θ H 2O

Eq. 14

r4 = k 4 pCO Ctotθ H 2O

Eq. 15

C H' O* = z 2

2

Ctot

4

After substituting the surface coverages with observable quantities, i.e., the partial pressures of the components, the final expression of the reaction rate equations is obtained. The site balance (Eq. 16) and the expressions for quasi equilibrium of the adsorption/desorption steps (Eq. 17 and 18), allow to write the rate expression in terms of the partial pressure of methane, water and carbon monoxide (Eq. 19 and 20). 1 = θ * + θ CH 4 + θ H 2O

Eq. 16

θ CH =

k1+ pCH 4 θ * = K1 pCH 4 θ * k1−

Eq. 17

θH O =

k 2+ p H Oθ * = K 2 p H 2Oθ * k 2− 2

Eq. 18

4

2



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

r3 =

r4 =

k 3 K 1 K 2 zC tot p CH 4 p H 2O

(1 + K

1

p CH 4 + K 2 p H 2O

)

k 4 K 2 C tot p CO p H 2O 1 + K 1 p CH 4 + K 2 p H 2O

2

=

=

k 3' K 1 K 2 p CH 4 p H 2O

(1 + K

1

p CH 4 + K 2 p H 2O

k 4' K 2 p CO p H 2O 1 + K 1 pCH 4 + K 2 p H 2O

Page 22 of 42

)

2

Eq. 19

Eq. 20

In order to solve the plug flow reactor model, see Eq. 8, the rate expressions are substituted in the net rate of formation Ri for each component (Eqs. 21 to 5).

3.3

RCH 4 = −r3

Eq. 21

RH 2 O = − r3 − r4

Eq. 22

RCO = r3 − r4

Eq. 23

RCO2 = r4

Eq. 24

RH 2 = 3r3 + r4

Eq. 25

Model and parameter estimates assessment and discussion

Fig. 3a,b) show the same data of CH4 conversion as a function of the H2O inlet partial pressure for CH4 inlet partial pressures amounting to 40 kPa, 80 kPa, and 120 kPa. The lines represent the results of the weighted regression. As expected, the CH4 conversion was higher when its inlet partial pressure was lower. The CH4 conversion increased with increasing H2O partial pressure, although at high H2O inlet partial pressure the CH4 conversion tended to decline. This behavior can be explained by the CH4 inhibition on the catalyst surface by adsorbed H2O.

The numbers in Fig. 3a represent the equilibrium quotients for the MSR reaction, defined as 22 ACS Paragon Plus Environment

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𝑝𝑝𝐻𝐻2 3 𝑝𝑝𝐶𝐶𝐶𝐶

𝑄𝑄𝑆𝑆𝑆𝑆𝑆𝑆 = 𝑝𝑝

𝐶𝐶𝐻𝐻4 𝑝𝑝𝐻𝐻2 𝑂𝑂

1

∙ 𝑝𝑝

𝑜𝑜

Eq. 26

2

At equilibrium at 923 K, QSMR is equal to the equilibrium constant, K = 2.76.

All the points are in a

state far away from equilibrium. The numbers in Fig. 3b represent the equilibrium quotients for the WGS reaction, defined as 𝑝𝑝𝐻𝐻2 𝑝𝑝𝐶𝐶𝐶𝐶2

𝑄𝑄𝑊𝑊𝑊𝑊𝑊𝑊 = 𝑝𝑝

𝐶𝐶𝐶𝐶

𝑝𝑝𝐻𝐻2 𝑂𝑂

.

At equilibrium QWGS is equal to the equilibrium constant, K= 2.04. state, but some were relatively close.


Eq. 27 No point was in the equilibrium


65 CH4 conversion / %

a)

0.0054

0.0053 0.0033

0.0083 55

0.012 0.0006 0.0004

45 35

Numbers: QSMR

0.015 0.012

0.0093 0.0067

0.011 25 40

80

b)

1.31

1.17

1.24 55

0.011 0.013

120 160 200 240 280 H2O inlet partial pressure / kPa

65

CH4 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

Page 24 of 42

360

1.21 Value of QWGS

1.15 1.56

45 1.16

35 25 40

320

0.96 1.06

80

1.17

1.37

1.43

1.06 1.02

120 160 200 240 280 H2O inlet partial pressure / kPa

320

360

Fig. 3. Simple model results for CH4 conversion as a function of the H2O inlet pressure. Points: experimentally observed, lines: calculated by solving the set of Eq. 8 with the net rates of formation given by Eqs. 19 to 23. Results are for at a total pressure of 0.40 MPa and a temperature of 923 K. The rate and equilibrium coefficients are obtained by a weighted regression and are shown in Table 2. ♦, full line, black - 40 kPa CH4 inlet partial pressure (p0CH4) or a space time of 3.36 kg s mol-1, ●, dashed line, red - 80 kPa p0CH4 or 1.68 kg s mol-1, ■, dotted line, blue - 120 kPa p0CH4 or 1.12 kg s mol-1. a) The numbers shown are the equilibrium quotients for the methane steam reforming, QSMR, with a value at equilibrium of 2.76. Conditions are far from equilibrium. b) The same data are shown but now the numbers show the equilibrium quotients for the water-gas shift reaction, QWGS, with a value at equilibrium of 2.04. Conditions are closer to equilibrium.


Page 25 of 42

Because the data for the WGS reaction were relatively close to equilibrium, calculations were carried out to ascertain whether the reverse reaction could influence the results of the fitting of the present data. Details are reported in the Supplementary Information.

The analysis indicates that there was no

significant influence on the results. Calculated values for the kinetic parameters are shown in Table 3, to be discussed later. Fig. 4 shows the selectivity to CO and CO2 as a function of the H2O inlet partial pressure for a CH4 inlet partial pressure amounting to 80 kPa. The model is able to simulate the correct trend for the selectivities with varying H2O partial pressure. Nevertheless there is a systematic deviation between the experimentally

35

85

30

80

25

75

20

70

Selectivity to CO2 / %

observed and calculated data.

Selectivity to CO / %

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


65

15 160

200 240 280 H2O inlet partial pressure / kPa

Fig. 4. Selectivity to CO and CO2 as a function of the inlet partial pressure of H2O at a total pressure of 0.4 MPa, a temperature of 923 K, a space time of 1.44 kg s mol-1 and conversions between 40 % and 44 %. Points: experimentally observed, lines: calculated by solving the set of Eq. 8 with the net rates of formation given by Eqs. 19 to 23. The rate and equilibrium coefficients are estimated by a weighted regression and are shown in Table 2. ♦, full line, black - selectivity to CO, ●, dashed line, red - selectivity to CO2. 25 ACS Paragon Plus Environment


Fig. 5 shows the CO and CO2 selectivity as a function of CH4 conversion at a H2O inlet partial pressure of 160 kPa. Most of the data only exhibit a minor dependence on the CH4 conversion. The two empty symbols deviate more strongly from the simulated lines and are considered as outliers. They were obtained with a CH4 inlet partial pressure of 20 or 40 kPa which were the lowest pressures used, so produced the lowest amounts of CO and CO2. Therefore, these points were retained in this figure, but omitted in the regression analysis.

100

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

Page 26 of 42

80 60 40 20 0 30

40 50 CH4 conversion / %

60

Fig. 5. Selectivity to CO and CO2 as a function of the conversion for the simple model at 0.40 MPa and 923 K, a catalyst mass of 0.1 g, a H2O inlet partial pressure of 160 kPa and a CH4 inlet partial pressure between 20 and 100 kPa. Points: experimentally observed, lines: calculated by solving the set of Eq. 8 with the net rates of formation given by Eqs. 19 to 23. The rate and equilibrium coefficients are estimated by a weighted regression and are shown in Table 2. ♦, full line, black - selectivity to CO, ●, dashed line, red - selectivity to CO2. 26 ACS Paragon Plus Environment

Page 27 of 42

Fig. 6 shows a parity diagram for each of the responses for the simple model. The weighted regression shows an excellent agreement between model simulated and experimentally measured CH4 outlet molar flow rates, a good agreement for H2O, CO, and CO2, and moderate agreement for H2. The closure of the carbon balance of the experimental data, as explained in the procedures, has led to a slight but consistent overestimation of the H2 outlet molar flow rate and corresponding underestimation of the CO2 outlet molar flow rate in the experimental dataset. The non-zero average value of the error of two out of five model responses, i.e., the outlet flow rate of H2O and H2, is attributed to the non-closure of the hydrogen and oxygen balance in the experimental data.

80 60

2

40

Predicted FH O / 10-6 mol s-1

220

4

Predicted FCH / 10-6 mol s-1

100

20 0 0

20

40

60

80 -6

100

180 140 100 60 20 20

140

180

220

-1

2

10 8

2

6

Predicted FCO / 10-6 mol s-1

24

-1

Predicted FCO / 10 mol s

100

Observed FH O / 10 mol s

4

4 2 0

60

-6

-1

Observed FCH / 10 mol s

-6

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

2

4

6

8 -6

10 -1

22 20 18 16 14 12 10 10

12

14

16

18

20 -6

Observed FCO / 10 mol s

22 -1

Observed FCO / 10 mol s 2


24


140

2

Predicted FH / 10-6 mol 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 28 of 42

120 100 80 60 40 40

60

80

100

120

Observed FH / 10-6 mol s-1

140

2

Fig. 6. Parity diagram for the simple model of the molar outlet flow rate FCH4, FH2O, FCO, FCO2 and FH2 for the weighted regression at a temperature of 923 K and a total pressure of 0.4 MPa. The estimated rate and equilibrium coefficients are shown in Table 2.

Table 3 compares results of the simple model and a model that considers the reversibility of the WGS reaction.

Shown are the parameter estimates and the corresponding 95 % confidence intervals; K1 for

CH4 adsorption, K2 for H2O adsorption, k3’ for steam methane reforming and k4’ for the water-gas shift reaction. The K1, K2, and k3’ parameters are close in both models, indicating no major deviations in their interpretation. The k4’ values are necessarily different because of the assumption of reversibility, which introduces the equilibrium constant for the WGS in the calculations ( K 4 =

k 4+ K S = ). k 4− K 2

For the simple model all parameters are statistically significant as shown by the narrow confidence intervals. For the results of this paper, the tabulated Fa value for model adequacy is 1.48. The calculated Fa value of 2.39 is very close to the tabulated one, indicating that the model is on the edge of being adequate. 28 ACS Paragon Plus Environment

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The tabulated F value with respect to the global significance is 2.42. The calculated Fs value of the weighted regression of 1.02x104 greatly exceeds this value and hence the performed regression is globally significant. The maximum binary correlation coefficient amounts to -0.9043 between the equilibrium coefficient K1 and the rate coefficient k3’. For the simple model with consideration of the reverse WGS reaction the tabulated Fa value for model adequacy is 1.48. The calculated Fa value of 2.84 is very close to the tabulated one, indicating that the model is also on the edge of being adequate. The tabulated F value with respect to the global significance is 2.42. the calculated Fs value of the weighted regression of 7.81x103 greatly exceeds this value and hence the performed regression is globally significant. The maximum binary correlation coefficient amounts to -0.93 between the equilibrium coefficient K2 and the rate coefficient k3’. Overall, both models are adequate.

The simple model has narrower parameter confidence intervals while

the simple model with the reverse WGS reaction has a larger Fs value.



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Page 30 of 42

Table 3. Calculated model parameters at 923 K for the weighted regressions with 95 % confidence interval

Simple model

Simple model with reversible water-gas shift

K1

30.3 ± 0.3

28.8 ± 4.8

MPa-1

K2

13.5 ± 0.1

12.5 ± 2.5

MPa-1

k 3' = k 3 zC tot

1.7 ± 0.1

1.63 ± 0.17

mol kgcat-1 s-1

k 4' = k 4 C tot

74.0 ± 0.5

1.11 ± 0.03

mol MPa-1 kgcat-1 s-1

Fs

1.02x104

7.81x1043

-

Fa

2.39

2.84

-

Units

Fig. 7 shows the calculated coverages of the catalyst surface obtained with the simple sequence by species derived from CH4 (θCH4), H2O (θH2O), and empty sites (θ*), as well as the product of θCH4 and θH2O as a function of the H2O partial pressure at a CH4 partial pressure amounting to 80 kPa and calculated from K3 and K4 obtained from the weighted regression, see Table 2, and Eqs. 16-18. With increase of the partial pressure of H2O, the catalyst surface coverage by CH4 decreased, as the coverage by H2O increased. As was discussed in the description of the reaction sequence, the coverages denote species associated with methane (θCH4) and species associated with water (θH2O), not necessarily the reactants themselves, as these would probably not be on the surface at high coverages. Consideration of actual species would greatly complicate the analysis, introducing many additional parameters, that is unwarranted from the data.

The

product of both coverages reaches a maximum with increasing H2O inlet partial pressure. This reflects the behavior of the CH4 conversion versus the H2O inlet partial pressure, see Fig. 3. Indeed, Eq. 14 shows that the CH4 consumption rate in SMR (r3) is proportional to the product of the coverage of CH4 and H2O. 30 ACS Paragon Plus Environment

Page 31 of 42

Therefore, the model follows this trend, Fig. 3. It can be noted that the fraction of free sites on the surface decreases as the H2O partial pressure increases, but falls slightly less than the coverage in CH4 derived products.

0.20

1.0

θCH x θH O 0.15

2

θH O 2

θ*

0.4 0.2

0.10

4

0.6

2

θCH x θH O / -

4

0.8

Coverage / -

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


0.05

θCH

4

0.00

0.0 100

200

300

H2O partial pressure / kPa Fig. 7. Coverage of catalyst surface by species related to H2O (θH2O), CH4 (θCH4), empty sites (θ*) and the product of θH2O and θCH4 as a function of the partial pressure of H2O. Partial pressure of CH4: 80 kPa, Temperature: 923 K, Total pressure: 0.40 MPa. The coverages are calculated with Eqs. 14 to 16. The equilibrium coefficients are estimated by a weighted regression and are shown in Table 2. Full line, black - θH2O x θCH4, dashed line, red - θH2O, dotted line, green - θ*, dash-dot line, blue - θCH4.

The kinetic model with the model parameters estimated by the weighted regression is subsequently used to simulate the CH4 conversion at a WHSV of 48000 Ncm3 h-1 gcat-1, a H2O/CH4 ratio of 2 and a total pressure of 0.1 MPa, see filled square in Fig. 2. By decreasing the total pressure from 0.4 MPa to 0.1 MPa, the conversion drops as indicated by the arrow in Fig. 2. This is an interesting result, as the 31 ACS Paragon Plus Environment


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Page 32 of 42

equilibrium conversion decreases with increasing pressure due to the stoichiometry of the SMR reaction, see Eq. 1. The increase in conversion with pressure is due to enhanced kinetics, with rates increasing with higher coverages of reactants as given by Eqs. 11 and 12. This will be discussed subsequently.

An important result from the model developed in this work is presented in Fig. 8 which illustrates the effects of total pressure and space time on the CH4 conversion.

The currently developed model did

not include thermodynamic constraints as it assumed an irreversible reaction, hence, an additional software package (NASA, Chemical Equilibrium with Applications) was used to determine the equilibrium conversion. As expected, the CH4 conversion decreases with increasing inlet CH4 flow rate. At low space velocity the conversion is lower at higher pressure because of thermodynamic equilibrium. This is, of course, because the increase in moles in the SMR dictates that conversion will be lower at higher pressure by Le Chatelier’s principle.

However, at higher space velocity a transition is made to

the kinetic regime. Thus, a higher total pressure leads to a higher reaction rate, because adsorption rates are higher, as long as the reaction is not close to thermodynamic equilibrium.

These results are not

due to the suppression of the back reaction, as the model used here assumes an irreversible reaction step. Those results rationalize why SMR is practiced industrially at high pressure, despite the unfavorable equilibrium. The decrease in conversion with space velocity is reported in other studies24,25,36,37, but the crossover has not been noted. 32 ACS Paragon Plus Environment

Page 33 of 42

Space time / kgcat s molCH -1 100

--

8.07

4.03

2.69

2.02

4

1.61

0.10 MPa (Xeq= 86.7%) CH4 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


80

0.20 MPa(Xeq= 74.3%) 0.40 MPa(Xeq= 60.6%)

60 40 20 0

Thermodynamic control 0

10000

20000

Kinetic control 30000

3 CH4

WHSV / Ncm

40000 -1 cat

-1

50000

g h

Fig. 8. Simulation result of CH4 conversion change as a function of the WHSV/space time of CH4 at various pressures. The points are the result of the calculation with Eq. 19 and Eq. 20 and the lines are drawn by B-spline through the points including the equilibrium points, temperature: 923 K, inlet gas composition: H2O/CH4/N2 = 2/1/2, total inlet flow of 400 Nml min-1. The dashed lines show the equilibrium CH4 conversion for each reaction condition. The rate and equilibrium coefficients are estimated by a weighted regression and are shown in Table 2. Full line, black – 0.1 MPa, dash-dot line red, 0.2 MPa, dashed line, blue – 0.4 MPa. Fig. 9 shows the effect of changing the H2O/CH4 ratio on the CH4 conversion as a function of WHSV. The CH4 conversion is higher for higher H2O/CH4 ratios and decreases with increasing WHSV. These trends are similar to those seen in Fig. 2 for both higher and lower H2O/CH4 ratio and can be understood based on the analysis of the surface coverages as shown in Fig. 7. Thus, the model developed in this work is able to describe general trends found from analysis of literature data. 33 ACS Paragon Plus Environment


Space time / kgcat s molCH -1 16.14 13.45 11.53 4

100

10.09

3.0 80

CH4 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

Page 34 of 42

60

2.0 1.5 1.0

40

H2O/CH4 = 0.5

20 0

5000

6000 7000 8000 WHSV / Ncm3CH h-1 gcat-1 4

Fig. 9. Comparison of simulated CH4 conversions as a function of WHSV/space time of CH4 varying H2O/CH4 ratios at 923 K, 0.10 MPa, total inlet flow of 400 Nml min-1, inlet flow CH4 80 Nml min-1. The lines are drawn by B-spline through each points. The rate and equilibrium coefficients are estimated by a weighted regression and are shown in Table 2. Full line, magenta - H2O/CH4 = 3.0, green - H2O/CH4 = 2.0, blue- H2O/CH4 = 1.5, red - H2O/CH4 = 1.0, black - H2O/CH4 = 0.5.


Page 35 of 42

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4.


CONCLUSIONS

A Ni/MgO-SiO2 catalyst was used to study the catalytic steam methane reforming in a tubular reactor at 0.40 MPa and 923 K.

The CH4 and H2O partial pressure effects were measured and analyzed by a

combined simple reaction model that considered the adsorption of the reactants and the surface reaction of their derived species, as well as the water-gas shift reaction . Values for the rate and equilibrium coefficients in this model were obtained by regression and resulted in a good agreement with experimental results as well as generally reported trends in the literature.

Consideration of the

reversibility of the water-gas shift reaction did not significantly improve the simple model. Particularly interesting was that the space velocity (space time) effect on the methane conversion revealed a cross-over with total pressure due to transition from thermodynamic to kinetic control. Due to the molar expansion during reaction, increasing the total pressure will lead to a decrease in conversion when operating in the thermodynamic controlled regime. In the kinetic controlled regime, the reactant surface coverages monotonically increase with total pressure, resulting in higher methane conversions. Model predictions at lower total pressures correspond to experimental observations reported in the literature, confirming that the developed model, although extremely simple, conveys the most essential features for SMR.



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Page 36 of 42

NOMENCLATURE Roman A b CA CA* Ctot C’ dp De e Fa Fs FA ∆H0 i.d. k kc K n o.d. p r RA R T W X X y z

matrix A estimated model parameter concentration of A concentration of A adsorbed on surface total concentration of active sites dimensionless concentration catalyst pellet diameter effective gas-phase diffusivity residual F value for adequacy F value for significance molar flow rate of A standard reaction enthalpy inner diameter reaction rate coefficient mass transfer coefficient equilibrium coefficient reaction order outer diameter pressure specific reaction rate specific production rate of A universal gas constant temperature catalyst mass independent variable conversion measured quantity number of neighbors Greek 36 ACS Paragon Plus Environment

mol m-3 mol kg-1 mol kg-1 m m2 s-1 mol s-1 mol s-1 kJ mol-1 m reaction dependent m s-1 Pa-1 m Pa mol kgcat-1 s-1 mol kgcat-1 s-1 J mol-1 K-1 K kg % mol s-1 -

Page 37 of 42

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


β ε θA ρ

model parameter experimental error surface coverage of A density

-

mol s-1 kg m-3

Subscripts b cat eq

bulk catalyst equilibrium

-

Superscripts 0 *

standard, inlet active site


-


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 42

AUTHOR INFORMATION Corresponding authors S. Ted Oyama Email [email protected] Joris Thybaut

Email [email protected]

Notes The authors declare no conflicting interests.

ACKNOWLEDGEMENTS:

This work was supported by a doctoral fellowship from the Fund for Scientific Research Flanders (FWO), the European Research Council under the European Union’s Seventh Framework Programme (FP7/2014-2019) / ERC grant agreement n° 615456 and the ‘Long Term Structural Methusalem Funding by the Flemish Government’.

NK is grateful for support from Kagaku Jinzai Ikusei Program.

STO received support from the director, National Science Foundation under grant CHE-1361842. and STO also received assistance from Fuzhou Univ., College of Chemical Engineering.

The catalyst

was kindly supplied by JGC Catalysts and Chemicals Ltd.

SUPPORTING INFORMATION Brief description of the simple model fit augmented by reversibility of the water-gas shift reaction. Comparison of results of the simple model and the model with reversibility of the WGS reaction. Details of the calculation of the Mears Criterion for lack of heat transfer limitations. 38 ACS Paragon Plus Environment

AT

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Weckhuysen, B.M.; Bruijnincx, P.C.A. Role of magnesium silicates in wet-kneaded silica–magnesia 39 ACS Paragon Plus Environment


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Crossover of CH4 conversion curves with WHSV/space time due to a transition between thermodynamic to kinetic control.

Space time / kgcat s molCH -1 100

--

8.07

4.03

2.69

2.02

4

1.61

0.10 MPa (Xeq= 86.7%) CH4 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

80

0.20 MPa(Xeq= 74.3%) 0.40 MPa(Xeq= 60.6%)

60 40 20 0

Thermodynamic control 0

10000

20000

Kinetic control 30000

3 CH4

WHSV / Ncm

40000 -1 cat

-1

g h

ACS Paragon Plus Environment

50000

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Interplay of Kinetics and Thermodynamics in Catalytic Steam Methane

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