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Methane Steam Reforming over LaCrl-,Ni,03 Perovskite Catalysts
by
Ram Chandra Paul
A thesis in confomity with the requirements for the Degree of Master of Applied Science
Depariment of Chernical Engineering and Applied Chernistry University of Toronto
O Copyright by Ram Chandra Paul 2000
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Methane Steam Reforming over LaCrl-,Ni.03 Perovskite Catalysts
By: Ram Chandra Paul
A thesis in conforrnity with the requirements for the Degree of Master of Applied Science
Department of Chernical Engineering and Applied Chemistry University of Toronto
C Copyright by Ram Chandra Paul 2000
ABSTRACT
The kinetics of the stearn reforming of methane over perovskite catalysts with the
formula LaCr,-,Ni,O; (x = 0.25 & 0.40) have been exarnined over a wide range of
temperatures (500 '~ to 1 0 0 0 ~ ~ ) and atmospheric pressure. These catalysts work well
from 700 to 9 0 0 ' ~ for methane conversion (space velocity = 13.500 hr-'. and gas
composition is Cl& : HI : H?O : Ar = 3 : 3 : 6 : SO), the higher nickel content (Ni=0.40)
catalyst is more active than the lower nickel content O\ii=0.25) catalyst. The water-gas
shif t reaction is close to equilibrium, while the stearn reforming reaction is not at
equilibrium for the conditions studied. Activity behaviour is in agreement with Mar-ko 's
methane oxidation result (1). However, these catalysts are unstable in the reforming
environment, as they partial1 y reduce to elemental nickel at high temperatures (worked up
to 1 000'~). Structural changes (frorn orthorhombic to rhornbohedral) occur at lower
temperatures (investigated up to 750'~). The higher nickel content catalysts are less
stable than the lower nickel content catalysts.
ACKNOWLEDGEMENTS
1 am grateful to my supervisor Professor Charles A. Mims for the great supervision,
inspiration and funding that made it possible to complete this work.
1 am also indebted to many of my colleagues (Chris, David, Greg, Nader, Nathan, Rishi
and Xiaûha) in WB 103 for the usefùl discussions and help offered whenever 1 needed
it. 1 would specially like to thank Chris for his technical and writing help and Dr.
Xiaoi~ua Chen for his technical hetp.
I would like to thank my wife Pratima Rani Pm1 for her patience and continuous
support, and for the many hours she spent alone during her severe sickness with Our
newborn daughter Reethi P m / .
Last of all, E would like to thank rny boss Ali A b d Jama at the Qatar Fertilizer Company.
Qafco, and a former student of my supervisor, who encouraged me to work with
Professor Chdes . A. Mims.
Isothermai maintenance catalyst activity ................................. 49 Space velocity experiments ............................................... 52
......................................................... Kinetic experirnents 54 4.5.1 Effects on activity in different compositions &
temperatures ...................................................... 54 4.5.1.1 Cornparison in activity between LaCro.6Ni~.~03
......................... & LaCr0.7~Nio.2503 catalysts 62 4.5.2 Reaction orders ........................................... 64 4.5 -3 Activation energies ........................... .. .. ... ... . . . . 68
Catat yst characterization by X-ray di =action (XRD) ................ 4.6.1 X-ray diffraction of LaCro.sNi0.40~ ..................... ... . 4.6.2 X-ray diffraction of LaCr~. rsNi~.~sO~ .........................
Chapter-5: Discussion ........................................................................ 5.1 Activity of perovskite catalysts ........................................... 5.2 Change in activity of perovskite catalysts ............................... 5 -3 Inabitity to reform original catalysts .....................................
....................................................... 5.4 Change of catalysts ......................... ........................... 5.5 Kinetic parameters ,..
--.-....-.-.-...... ....................................*..**... Chapter-6: Conclusions ... .............................................................. Chapter-7: Recommeadations
Appendix-A Calculation procedure for methane conversion, water analysis ................................................. and reaction quotients
Appendix-B Calculation procedure for time normalization of reaction rate ..................................................... with two examples
Appendix-C Calculation procedure of kinetic parameters ...................... Appendix-D Two tables of methane conversion data ...........................
....................................................... Appendix-E Reaction orders ............................................. Appendix-F XRD results of cataiysts
................................................... Appendix-G Activation energies Reference list .................... .. .............. .... ....................................
List of Figures
Figure 2.2.1.1 Figure 2.2.1.2 Figure 2.2.2.1 Figure 3.1 Figure 3.2.2.1 Figure 3.2.3.1 Figure 3.2.3.2 Figure 4.1.1.1
9 . Figure 4.1.2.1 10 . Figure 4.1.3.1
1 1 . Figure 4.1.3.2
12 . Figure 4.2.1.1
13 . Figure 4.2.2.1
14 . Figure 4.2.2.2
1 5 . Figure 4.2.2.3
16 . Figure 4.2.3.1 1 7 . Figure 4.3.1 1 8 . Figure 4.3.2
1 9 . Figure 4.4.1 20 . Figure 4.5.1.1
2 1 . Figure 4.5.1.2
22 . Figure 4.5.1.3 23. Figure 4.5.1.4
Perovskite structure ..................... ... ..................... Lattice parameters and unit ceil volumes of LaCri.,Ni,03 ... Thermogravimetric reduction of LaCrl.,Nia3 ................... Simplified process flow diagram of the experimental ........... Schernatic of steam generation ..................................... Schematic of reactor flow ........................................... Schematic of reactor ................................................ Comparison of methane conversion in temperature screening experiments using two different methods ........................ Product distribution in temperature increasing sequence ...... Comparison of equilibnum coefficient and reaction quotients for stem reforming ................................................ Comparison of equilibrium coefficient and reaction quotients for water-gas shift reaction ........................................ Methane conversion dependence as a fùnction of temperature ............................................................ Catalyst activity increases at each set with respect to previous set and maximum temperature of exposure
................................... (temperature increasing sequence) Catalyst activity increases at each set with respect to previous set and maximum temperature of exposure (temperature decreasing sequence) .................................. Catalyst activity increases with successive number of exposures at same maximum temperature ........................ Oxidation effect on used and fiesh catalyst ........................ Isothermal maintenance catalytic activity at 6 7 5 ' ~ and 700 '~ .. Isothermal maintenance catalytic activity in two different time segments .......................................................... Influence of products (CO, ) on reforming reaction ............... Activity increases with nurnber of runs and regardless of gas composition (Ni = 0.40). ............................................ Activity of composition- l increases with other compositions and temperature (Ni = 0.40). ...........................................
........... Change in activity after time normalization (Ni = 0.40). ........................ Activity changes with the number of runs 60
24 . Figure 4.5.1.1.1 LaCr0.6Ni~.~0~ catalyst is more active than LaCro.7sNio.2503 catalyst ................................................................. 63
25 . Figure 4.5.2.1 Methane orders for methane conversion (Ni = 0.40) .............. 66 26 . Figure 4.5.3.1 Activation energy for methane conversion at different nins
and samples ........................................................ - 69 ... 27 . Fig E-4.5.2.1 H20& ratio has rninor effect on methane orders ............... 87 28 . Fig E-4.5.2.2 Hydrogen orders for methane conversion, first mn, batch-F .... 88 29 . Fig E-4.5.2.3 Stem orders for methane conversion, first run, batch-F ......... 89
30 . Fig E-4.5.2.4 3 1 . Fig E-4.5.2.5 32 . Fig E-4.5.2.6 33 . Fig E-4.5.2.7
34 . Fig E-4.5.2.8 35 . Fig E-4.5.2.9 36 . Fig E-4.5.2.10 3 7 . Fig E-4.6.1.1 3 8 . Fig E-4.6.1.2 3 9 . Fig E-4.6.1.3 40 . Fig E-4.6.2.1 4 1 . Fig G4.5.3.1
1 . Table 2.4.2.1
2 . Table 3.3.1 3 . Table 3.3-4.1
4 . Table 4.5.2.1
5 . Table 4.5.2.2
6 . Table 5.5.1
Methane orders for methane conversion, second run, batch-F .... 90 Hydrogen orders for methane conversion. second run. batch-F .. 9 1 Steam orders for methane conversion, second run, batch-F ...... 92 Methane. hydrogen and steam orders for methane conversion, specialrun,batc h.1 .................................................... 93 Methane orders for rnethane conversion, first run, batch-H ...... 94 Hydrogen orders for methane conversion, first mn, batch-H ... 95 Steam orders for methane conversion, first run, batch-H ......... 96 XRD of batch-A and fiesh catalyst (Ni = 0.40) .................... 97 XRD of batch-C and eesh catalyst (Ni = 0.40) ..................... 98 XRD of batch-1 and fkesh catalyst (Ni = 0.40) ..................... 99 XRD of batch-H and fiesh catalyst (Ni = 0.25) .................. 100 Activation energîes for temp . increasing & decreasing sequence .. 10 1
List of Tables
Relationship between reforming activity and nickel content of precipitated catalyst ................... .. ............... .... .........
................................................. List of experiments Partial pressure of different components in five different gas
.......................................................... compositions List of methane orders for methane conversion at different
......................................................... Hz0/H2 ratios List of orders for methane, hydrogen and steam at different
.................................................................... runs List of activation energies .......................................
7 . Table D-4.1.2.1 Methane conversion (%) at different temperature in two di fferent methods ....................................................
..................... 8 . Table D.4.2.1.1 Methane conversion at different temperature
List of Appendices
Appendix-A Calculation procedure for methane conversion. water analysis and reaction quotients .................................................
Appendix-B Calculation procedure for time normalization of reaction rate with two examples .............. .. .....................................
Appendix-C Calculation procedure of kinetic parameters ................... ... Appendix-D Two tables of methane conversion data ........................... Appendix-E Reaction orders ....................................................... Appendix-F XRD results of catalysts ............................................. Appendix-G Activation energies ...................................................
INTRODUCTION
The production of synthesis gas ( H z + CO) and hydrogen by the steam reforming of
methane (4, which is the main component of natural gas (up to 95%) (3), is an important
and well-known industrial process. Methane steam reforming is important because it
provides the raw materials (hydrogen and carbon monoxide) for many industrial
processes, such as the production of methanol and oxo-alcohols, and the Fischer-Tropsch
synthesis (5). Also, the hydrogen can be used for refineries, hydrogenation reactions, &el
cells and ammonia synthesis; the reducing gas for the direct reduction of iron ore; and
carbon dioxide for urea production (3).
The methane steam reforming is usually camied out at pressures up to 35 bar and
temperatures of 8 0 0 ~ ~ or higher due to its endothermic nature (3); the reaction is
represented by equation (1 ):
CtCI + Hz0 - 3H2 + CO AH = 206 W/mol (1)
The CO produced may react fùrther with water in the water-gas shift reaction:
CO + H 2 0 + COz + Hz AH=-41 kJ/mol
CO2 may also fonn directly fiom methane and water (36):
C& + 2Hz0 - CO2 + 4H2 AH= 165 kJ/mol
Nickel has been recognized as the most suitable metal for aeam reforming of
hydrocarbons. Other metals such as cobalt, platinum, palladium, iridium, ruthenium and
rhodium can be used. These precious metals are considerably more active than nickel but
nickel is much cheaper and nifficiently active to enable suitable catalysts to be produced
economically (3).
It is well esiablished that, in the steam reforming of saturated hydrocarbons over
supported nickel catalysts, the catalytic activity progressively decreases with usage.
Three main causes of loss in activity have been identified, namely, (i) catalyst sintering;
(ii) poisoning by sulhr-containing organic compounds present in the hydrocarbon
feedstock, and (iii) the formation of surface carbonaceous residues, so called "catalyst
carbiding" (6).
Some metal oxides are k n o m to be active catalysts for oxidation catalysis, and it has
been shown that the combination of metal oxides can be better catalysts than the single
component metal oxide (7). Sometimes metal oxides are as good as metals; cornplex
oxides are more stable and active, and provide more flexibility. In addition, complex
oxides have mobile oxygen ions (oz-) which are suitable for other uses such as membrane
reactors and fiel ce11 electrodes. The research effort into complex metal oxide catalysts
with perovskite structure (LaCr1-~Ni,03 where x = O to 1) is concentrated in these areas:
energy generation using solid oxide fùel cells (materials for electrodes), environmental
application (destruction of CO and NO), electrochemical oxygen membranes and sensors,
superconducting applications and integrated optics (8). Perovskite catalysts have mobile
oxygen which may contribute to the reforming reaction. Previous studies show
reforming conditions exist during methane oxidation with oxygen over perovskite (LaCri.
,Ni,03) catalysts and catalysts are themally stable at high temperature ( 5 0 0 ~ ~ ) during
methane oxidation (8).
Our intention is to investigate the uses of perovskite (LaCr1-,Ni,03j catalysts to perfonn
the reforming reaction even though the existing nickel catalysts are working well but with
a few difficulties such as smail carbon deposition, sintering, poisoning and deactivation
(6, 9, 10, 38). Previous studies on the oxidation of hydrocarbons indicate that perovskite
catalysts have comparable activity to other catalysts (1 1, 12). Perovskite catalysts with
the formula LaNii.rC~,03 (where x = 0.2 tol) show very good performance in methane
steam reforming (13). Specific compositions (perovskite) with optimized properties have
been investigated for application as elements in solid oxide fuel cells and as potential
catalysts for environmental applications (12, 15-17). Our focus on methane steam
reforming over perovskite cataiysts (LaCrl-,Ni,03 where x = 0.25 & 0.40) is important
because of the abundant quantity of methane available in natural gas (3, 18). The
industrial importance of methane steam reforming and the excellent catalytic properties
of perovskite catalysts are the motivation for the present research work. Our objective is
the kinetic study of methane steam reforming reaction over perovskite catalysts.
This thesis is organized in the foilowing manner:
Chapter Two, Literaîure Review, gives the general motivation and the objective of the
thesis, background information on perovskite catalysts, chemistry of methane steam
reforming and design of steam reforming catalysts.
Chapter Three, Experimentui, covers expenment procedure, materials and components
used in this work, and an overview of the experiments.
Chapter Four, Resufts, covers the results found in the experiments and analysis.
Chapter Five, Discussion, covers the expIanation of the obtained results and comparison
with previous authors' results.
Chapter Six, Conciusions, is the summary of the new information obtained by this study.
Chapter Srven, Recomnren~om, contains proposais for follow-up work.
CHAPTER TWO
LITERATURE REVIEW
This chapter presents the background information and the motivation for this work, as
well as general information on perovskite catalysts, chernistry of methane steam
reforming and design of steam reforming catalysts.
2.1 General Information and Motivation for this Work
The production of synthesis gas (Hz + CO) and hydrogen by the steam reforming of
methane (4, the main component of natural gas (up to 95%) (3), is an important and
well-known industrial process. The objective of the catalytic methane steam reforming
process is to extract the maximum quantity of hydrogen held in water and hydrocarbon
feedstock, and to produce carbon monoxide and/or carbon dioxide simultaneously.
Methane stearn reforming is an important industrial reaction because it provides the raw
materials for many synthesis processes in the modern chernical industry. The most
common uses are: synthesis gas for methanol and 0x0-alcohols, and the Fischer-Tropsch
synthesis (5); hydrogen for refineries, hydrogenation reactions, fiiel cells and ammonia
synthesis; reducing gas for direct reduction of iron ore; and carbon dioxide for urea
production. The methane steam reforming is carried out usuaily at pressures up to 35 bar
and temperatures of 800'~ or higher due to its endothermic nature (3,39).
Nickei has been recognized as the most suitable metal for steam reforming of
hydrocarbons. Other metals such as cobalt, pIatinurn, palladium, iridium, ruthenium and
rhodium can be used. These precious metals are considerably more active than nickel but
nickel is much cheaper and suficiently active to enable suitable catalysts to be produced
economically (3). The activity and deactivation patterns of these catalysts (Ni) have been
wideiy studied (18, 19). The reforming reaction takes place on the nickel surface, thus the
catalyst must have the maximum stable nickel surface area available to the reactants (3).
Meta1 oxide catalysts are important in the catalytic reaction. These catalysts are normally
more resistant during high temperature reforming reaction while metal catalyst suffers
coke formation problem. It has been shown that a combination of metal oxides can be
better catalysts than the single component metal oxide (7, 18). The early lanthanide
perovskites have received the most attention for oxidation catalysis, particularly in
catalytic combustion for environmental applications (destruction of hydrocarbons, CO,
NO), although their use in other reactions is also important (8). Some studies on the
oxidation of hydrocarbons indicate that perovskite oxide catalysts are comparable in their
activity to other catalysts (18). Specific compositions (perovskite oxide) with optimized
properties have been investigated for application as elements in solid oxide fùel cells and
as potential catalysts for environmental applications (12, 14-17). The perovskite catalyst
LaNil-,Co,O3 (where x = 0.2 tol) shows very good performance in methane steam
reforming (13). By choosing the appropriate perovskite composition, it is possible to
combine in one material good oxygen ion and conductivity along with the high activity
provided by nickel sites. The reason for Ni substitution for Cr in LaCr03 is to increase the
stability of the mixture relative to LaNi03, and to enhance the conductivity and the rate of
reaction by the presence of Ni atoms (relative to LaCr03) (8).
The activities per unit nickel surface area, the specific activities, are within the same
range for a great number of catalysts irrespective of the nickel surface are* support
material, preparation o r activation procedure. Use of supports, such as zirconia and
carbon, results in very poor specific activities, whereas some decrease of the specific
activity is observed when using silica-alumina and titania supports. Addition of alkali in
various ways results in a significant drop in the specific activity, the effect of potassium
being larger than that for sodium (20).
It is well established that, in the steam reforming of saturated hydrocarbons over
industrially used supported nickel catalyst, the cataiytic activity progressively decreases
with usage. Three main causes of loss in activity have been identified, namely, (i) catalyst
sintering; (ii) poisoning by sulfur-containing organic compounds present in the
hydrocarbon feedstock, and (iii) the formation of surface carbonaceous residues, so called
"catalyst carbiding" (6).
The kinetics and the specific activity for reforming reactions Vary for different catalysts
and depend strongly on the method of catalyst preparation and composition (20). The
major differences in the kinetics are found in the influence of steam partial pressure. This
is related to the ability of the support material to adsorb steam. Active magnesia or the
presence of alkali enhances s t e m adsorption. The kinetics cannot be described in a broad
temperature range by a simple power law, as the powers Vary with temperature (20).
The operation with minimum steam-carbon ratios can lead to the formation of carbon
either by decomposition of methane or carbon monoxide (19, 21). Metal catalysts are
more susceptible to this problem than metal oxide catalysts. Carbon deposition may
decrease the aaivity of the catalyst and cause severe problems in the reformer. Various
methods have been used to prevent carbon deposition, namely, the use of additives, and
the control of the size of the metallic particles of the catalyst and the operational
conditions (19).
Carbon-fiee steam reforming can be obtained on a partly sulfur-passivated nickel catalyst
under conditions that, without the presence of sulfk, would result in the formation of
whisker carbon. Sulfùr inhibits the rate of carbon formation more than the rate of the
reforming reaction (4).
COz methane reforming has been studied extensively with different catalysts to produce
synthesis gas. Bdrov and Apel'bnirm suggested that CO2 shifis to CO and Hz0 through
the reverse water-gas shifi reaction (WGS), and then H20 reacts with C f i (steam
reforming) to produce synthesis gas (17). CO2 methane reforming has two distinct
drawbacks: (1) This reaction (247.3 kJ/mol) is more endothermic than methane steam
reforming (206 kJImo1) and (2) the H2/C0 (11) ratio is lower than required for
subsequent processes (22). Some industries use hydrogen (at arnmonia plants) and COz
(at urea plants) as a raw matenal. The steam reforming of methane extracts hydrogen
from both methane and steam, which has economical advantages because methane is
more expensive than steam.
Mims and his group have worked on perovskite catalysts particularly with the formula
LaCrl,,Ni,03 (where x = O to 1) (1, 2). They have already looked at methane oxidation
over this perovskite catalyst. Their studies show that the reforming condition exists
during methane oxidation at high temperature (>SOO~C), perovskite catalysts have
excellent catalytic properties and these catalysts are themally stable at high ternperature
(<500°c) (1). Furthemore, these catalysts perform the reforming reaction in the
membrane reactors and at the anode of Solid Oxide Fuel Cells (SOFC). In addition,
methane steam reforming very important because of its wide industrial use, and the
abundant quantity of methane (up to 95%) is available in natural gas. The industrial
impoxtance, the abundant quantity of methane in Our natural gas and the excellent
catalytic property of perovskite catalyst are the motivation for the present research work.
Our attention is focused on the kinetic midy of methane steam reforming over perovskite
catalysts (LaCr, -,Ni,03 where x = 0.25 & 0.40).
2.2 General Information on Perovskite Catalysts
2.2.1 Perovskite Materials and Physical Properties of LaCrl-.NiXO3
"The mineral perovskite (CaTiO,) has a structure in which the oxide ions and the large
cation (ca2-) form a ccp (cubic close packed) array with the smaller cation (T?)
occupying those octahedral holes formed exclusively by oxide ions. The generic formula
for the materiat can be represented by AB03 (A = Lanthanide or aikaline earth metal and
B = Transition metal). The A cation is comparable in size to the 02- ion and the B cation
is much smaller" (8). A schematic of the perovskite oxide crystal lattice (23) is shown in
Figure 2.2.1.1. "The crystal structure of perovskite materials is very tolerant to changes
in the radii of the A and B cations and can accommodate large concentrations in cation
vacancies at the A-sites". This allows great flexibility in substituting cations on A and B-
sites to obtain the desired physico-chernical properties. In general, doping on the A-site
modifies ionic conductivity, while B-site doping detemines electronic conduaivity and
cataiytic activity (8).
Perovskites have been the subject of several reviews (24, 25) relating catalytic activity to
the bulk composition, especially linking it to the B site cation. Arai and McCarty (12, 16)
have recently studied LaB03 systems where B represents Co, Mn, Fe, Cu, Ni and Cr, and
also some other partially substituted systems.
McCarty and Wise (16) found that LaCr03 is much more stable ta reduction than LaNi03
at SOFC operating temperatures. Actually, LaNi& decomposes to La2Ni04 and Ni0 at
temperatures above 8 5 0 ' ~ (26). In essence, site reducibility correlates with activity. but
anticorrelates with stability. The reason for partiai substitution of Cr by Ni (reducible
cation) in LaCrOs (non-reducible lattice) was to increase the stability of the mixture
Cr, Ni
Figure 2.2.1.1 : Perovskite structure
relative to LaNiOs and to enhance the conductivity and rate of reaction by the presence of
Ni atoms (relative to LaCrO3) (8).
89 C O rhombohedral
1 orthorhombic
orthorhombic
Figure 2.2.1.2: Lattice parameters and unit ce11 volumes of LaCri,,Ni,03. [Adapted fiom Mims el al (2) J
Information about the bulk structure was obtained fiom references (27, 28) LaCr03 is
orthorhombic at room temperature (a t b t c and a = P = y = goo), while LaNi03 is
rhombohedral (a = b = c and 120' > a = f3 = y t 90') (29). The intermediate LaCrl.,Nix03
compositions have either the orthorhombic LaCr03 structure for - 0.6 or the
rhombohedral L a i 0 3 structure for x 2 0.7 (see Figure 2.2.1.2) (2). LaNi03 is a
conductor due to the nature of ~ i ~ - ions, while p-type conductivity in L~CI-O3 is
generated by positive holes. The electrical conductivity of LaCr03 increases when Cr is
substitute by Ni.
2.2.2 Reduction Sta bility
"The LaCri-sNi,03 samples show one or more distinct reduction stages when heated at
5OUmin in 5% HZ saturated with water vapor at ambient temperature. LaNi03 (x = 1)
reduces in two steps. The first, between 1 8 0 ' ~ and 400°c, corresponds to the reduction of
M3- to ~ i ~ ' . The second reduction step, between 4 5 0 ' ~ and 550°c, corresponds to
complete reduction to Ni metal and La203. Al1 of the temary perovskites show the
reduction of ~ i ) - to ~ i ~ - below 400'~. For x > 0.5, this reduction to M2- is complete.
Further reduction of these matenals to Ni metal occurs as the temperature increases. The
reduction occurs continuously over a range of temperature with a mechanism in which Ni
metal is formed together with a more chromium-rich perovskite phase according to the
reaction
100 300 5 0 0 700 9 0 0
Temperature OC
Figure 2.2.2.1 Themogravimetric reduction of LaCr,.,Ni,03 at 5'Urnin in 5% Hz saturated with water vapor at ambient temperature. [ Adapt ed from Mims el al (t)]
The extent of this process depends strongly on composition and is govemed by the
stability of the residual temary perovskite phases as well as by cation mobility. For
material with O < x < 0.5, the reduction to M2- is not complete in the first wave at 4 0 0 ' ~
(see Figure 2.2.2.1). The remaining hIi3- does reduce to ~ i ~ ' by 800'~ in two subsequent
poorly resolved processes. The LaCri,Ni.03 compounds with x < 0.5 do not reduce to
nickel rnetal under these conditions at s 900°C" (2).
2.3 Chemistry of Steam Reforming
2.3.1 Thermodynamics
The objective of the catalytic methane steam reforming process is to extract the
maximum quantity of hydrogen held in water and hydrocarbon feedstock, and to produce
carbon monoxide and/or carbon dioxide simultaneously. Methane steam reforming is an
important industrial reaction because it provides the raw material for many synthesis
processes in the modem chemical indu-. The moa common uses are: synthesis gas for
amrnonia and methanol production; hydrogen and carbon monoxide for 0x0-alcohols and
the Fischer-Tropsch synthesis; hydrogen for retineries, hydrogenation reactions and fuel
cells; reducing gas for direct reduction of iron ore and carbon dioxide for urea production
(3).
The reforming of methane utilizes two simple reactions: the reforming reaction (1) and
the water-gas shifi reaction (2) - Ct-4 + Hz0 P. 3Hz + CO AH = 206 kJ/mol ( 1 )
CO + HzO COz + H2 AH=-41 kJ/moI (3)
The reforming reaction is strongly endothermic, so the forward reaction is favoured by
high temperature as well as by low pressure because of increase in moles, while the shifl
reaction is exothermic and is favoured by low temperature but is largely unaffected by
changes in pressure because the number of moles are constant. To maximize the overall
efficiency (and hence economics) of the conversion of carbon to carbon dioxide and the
production of hydrogen, refonners in ammonia plants are operated at high temperature
and pressure. For methane, the stoichiometic requirement for s tem per carbon atom is
1.0. However, it has been demonstrated that this is not practical because al1 catalysts
developed so far tend to promote carbon fonning reactions under steam reforming
conditions. These reactions can only be suppressed by using excess steam, with the result
that the minimum ratio is in the region of 1 -7. However, the reforming reaction itself is
also promoted by an excess of steam and hence some advantage is denved fiom this
necessity. In practice, ratios of 3.0-3.5 are commonly used, but there can be econornic
attractions in using lower steam ratios, and there is now a trend to move in this direction
(3)-
2.3.2 Kinetics
There is general agreement that the reaction is first order in methane, but there is less
agreement with other kinetic parameters. This is due in part to the use of different
catalysts and experimental conditions, but offen it has resulted from a lack of appreciation
of diffision and heat transfer limitations. Thus, reported activation energies span a wide
range of values caused by varying degrees of diffision limitations. Indeed, the apparent
activity increases as the particle size decreases, but this increases the pressure drop across
the reformer, thus restricting the size of the catalyst (3).
Russian researchers (30) used a recirculation laboratory reactor to reforrn methane over
nickel foi1 in order to obviate any pore diffusion limitations. From their results obtained
at 800-900~~ and one atmospheric pressure, they concluded that the water-gas shift
equilibrium was always established. The derived activation energy over this temperature
range was 130 kJ/mol. They also concluded that the decomposition of methane to carbon
on the nickel surface was very much slower tham the reaction of methane with steam. The
same researchers (31) also examined the kinetics using a conventional catalyst
comprising nickel on alumina, and the detived activation energies were 77 kT/mol
between 8 0 0 - 9 0 0 ~ ~ and 100 kJ/mol between 700-800'~. These results clearly
demonstrate the effect of diffusion on the kinetic parameters.
There has been some debate about which products are formed first during the steam
reforming reactions (32), and it appears that the relative concentrations of carbon
monoxide and carbon dioxide leaving the catalyst surface depend on the efficiency of the
catalyst in the water-gas shifl reaction. With rhodium-based catalysts, the CO/COt ratio
of the initially formed carbon oxides is relatively high (in keeping with poor shifi
activity), whereas with nickel catalysts, the amount of carbon monoxide is lower.
2.4 Design of Steam Reforming Catalysts
2.4.1 Pbysical Properties
The catalyst must be strong enough to withstand handling, from manufacture to charging
in the reformer, as well as the stress generated by the process conditions, such as thermal
cycles arising fiom plant start-up and shutdown. The catalyst also must have a suitable
physical shape to provide sufficient surface area to give an acceptable activity per unit
volume of packed bed whilst possessing acceptable low pressure-drop characteristics.
The support must not be aEected by water condensing on it, and it should produce a
small quantity of dust and matenal carryover, which could fou1 heat exchangers and other
catalysts downstream (3). The pellet size and geometry, porosity and effective radius are
also very important (33).
2.42 Nickel as a Steam Reforming Catatyst
For many years nickel has been recognized as the most suitable metal for the steam
reforming of hydrocarbons, although other materials can be used too. Some precious
metals are considerably more active per unit weight than nickel. but nickel is much
cheaper and sufkiently active to enable suitable catalysts to be produced economically.
The reforming reaction takes place on the nickel surface, so the catalyst must be
manufactured in a form which produces the maximum stable nickel surface area available
to the reactants. This is done by dispersing the nickel as small crystallites on a refractory
support which must be sufficiently porous to allow access by the gas to the nickel
surface. This is usually achieved by precipitating nickel as an insoluble compound, fiom
a soluble salt, in the presence of a refractory mppon such as mixtures of aluminium
oxide, magnesium oxide, calcium oxide and calcium aluminate cernent. Altematively, the
nickel can be incorporated by impregnating performed catalyst support, such as alumina
or an aluminate, with a solution of a nickel salt which is subsequently decomposed by
heating to the oxide. In either case, the nickel oxide is reduced to the metal by hydrogen
supplied from another plant, or by cracking a suitable reactant gas over the catalyst as the
reformer is being started up (3).
Catalytic performance and strength are determined by the catalyst formulation (3).
Impregnated catalysts are generally stronger than precipitated catalysts, and this is one of
the reasons for their widespread use. When comparing the activity of different catalya
types, it is necessary to take into account their nickel contents. The activity of a steam
reforming catalyst in service is closely related to the available surface area of the nickel
metal and the access the reactants have to it. Moa commercial natural gas catalysts are
now of the impregnated type and they give a relatively high nickel surface area when first
reduced, but under normal reforming conditions, the surface area decreases due to
sintering of the nickel crystallites. The higher the temperature, the more rapidly the
sintering proceeds. It is not surpnsing that the activity is a function of the overall nickel
content. However, it has been demonstrated that with both impregnated and precipitated
catalysts, there is an optimum beyond which an increase in nickel content does not
produce any further significant increase in activity. Typically, the optimum nickel
contents are approximately 20% for precipitated catalysts and up to about 15% for
impregnated catalysts, but this depends on the nature and the physical propenies of the
actual support (3). Table 2.4.2.1 shows the results of two series of experiments with
precipitated catalyst where this effect is clearly demonstrated (3).
Table 2.4.2.1 Relationship between reforming activity and nickel content of precipitated catalyst in laboratory tests (3)
Nickel Methane Nickel Methane contents (%) conversion (%) contents (%) conversion ( O h )
10.6 10.3 19.3 15.5
Feed: methane and steam (stem ratio 3 .O)
Temperature: inlet 450°c, exit 6 0 0 ' ~
Pressure: 26 bar
Space velocity: 35000 h f l
2.4.3 Supports for Nickel Steam Reforming Catalysts
The design of a steam reforming catalyst support must reflect the need for it to be robust
at high temperature and pressure. It also must be suitable for the dispersion of nickel
crystallites and allow access of the reacting species, but it must not interfere with their
activity. If possible, it should promote, or at least sustain, the activity of the nickel, but it
must not catalyse side-reactions, particularly those which produce carbon deposits (3).
CHAPTER THREE
EXPERIMENTAL
This chapter covers the materials, reactor syaems, expenmental procedure and conditions
used within the study. Reports on individual experiments in succeeding chapters contain
specific information on the experimentai procedure and conditions. Figure 3.1 shows a
simplified process flow diagram of the expenmentai set-up.
~eac io r for ~ e a d o r for steam generation steam reforming
Figure 3.1 : Simplified process flow diagram of the ejrperimentai set-up
The steam was produced by reacting hydrogen and oxygen over a supponed platinum
catalyst at 2 1 0 ~ ~ . This steam was used as a feed for s t em reforming reaction (see Section
3.2.2 for details). Steam and excess hydrogen were then mixed with methane and argon, and
entered the reforming reactor. After reaction, the product gases were analyzed by gas
chromatograph (GC) (see Sections 3.2.3 & 3.2.4 for details).
3.1 Materials
Materials, which include catalysts and gases, were used in this study. More details for
materials are found in subsequent sections.
3.1.1 Catalysts
Catalysts were used for two different purposes; (i) for steam generation from hydrogen and
oxygen; and (ii) for methane steam reforming. To produce steam fTom hydrogen and
oxvgen, a catalyst of platinum supported on alumina (0.35 g) was used. Two different
catalysts LaCro.sNio.403 and LaCr~.îsNio.~~O~ were used for methane steam reforming, which
were calied as Ni = 0.40 & Ni = 0.25 respectively. Soth perovskite catalysts were obtained
from the University of Houston. Catalysts were received as fine powders. Al1 the catalyst
powders were pressed into pellets, cmshed and then sieved to size 150-300 micron particle
size. In each batch, the catalyst amount was 0.28 to 0.33 g.
3.1.2 Gases
Five different gases, such as oxygen, hydrogen, methane, argon and helium, were used in the
experiments. Oxygen, hydrogen and methane were used as reactants, argon for dilution of
the reactants and helium for the GC carrier gas. Different gases were obtained from different
companies in different grades. FoIlowing is a list of the gases, grades and suppliers:
Oxygen, grade W, Matheson Gas Products
Hydrogen, grade UHP, BOC Gases
Methane, grade UHP, Matheson Gas Products
Argon, grade UHP, Matheson Gas Products
Helium, grade UHP, Matheson Gas Products
-411 gases came in cylinders, and were used without fiirther purification. Each cylinder was
equipped with a single-stage high pressure regulator to protect the mass controllers fiom the
cylinder high pressure. Gases coming fkom regulators went through in-line filters and were
then introduced to the mass flow controllers where the flows were measured. After
measuring flows, these gases were then mixed and passed to the reaction section.
3.2 Reactor Components
Components include flow controllers to control flows, reactor systems for steam generation
and methane steam reforming, and gas chrornatograph for products analysis. More details
for components will be found in the subsequent sections.
3.2.1 Flow Controllers
Four h K S 1159B controllers controlled individual gas flows; each had a different maximum
flow rate (20, 20, 30 and 50 cc/min N2 STP). The flow controllers were calibrated using a
mini-Buck M-5 soap caiibrator. The largest capacity flow controller was for the diluent
stream (Ar).
3.2.2 Reactor System for Steam Generation
T o produce steam, platinum supported on alumina catalyst was placed in a reactor made
fiom a Swagelock SS port connecter with two SS tnts on each side (Figure 3.2.2.1). This
reactor placed was inside an insulated box where, it was heated to 2 10°C to ensure complete
conversion of oxygen and to prevent steam condensation. Hot au fiom an electric air gun
was passed over cataiyst bed to heat it up.
Flow fiom controllers
4 ,Hot air supply H 2 v A * Thermocouple
,- v Insulated box
Ptfaiumina catalyst bed
Stem + hydrogen
Figure 3.2.2.1: Schematic of steam generation
To monitor the approximate cataly st temperature, a 1 /16" sheathed K-type thermocouple
was placed close to the catalyst bed. The gas flow always contained 50% excess hydrogen to
ensure complete conversion of oxygen. Complete conversion of oxygen produced a desired
steamlhydrogen mixture. Actual feed gases from flow-controllen were CHI, Hz, Oz and Ar
but desired H20M2 was obtained fkom stoichiometry assuming complete conversion of
oxygen.
3.2.3 Reactor System for Reforming
Reactions were performed in a single pass, down-flow, fixed bed reactor. The catalyst was
loaded on a quartz frit housed inside a quartz tube reactor.
Methane --rn Thennocouple Flow 1 1 from controller
Argon
Stearn t Hz
Furnace Temperature Conti
Reactor Catalyst Furnace
.aller
bed
- CiC
Figure 3.2.3.1 : Schematic of reactor flow
The reactor design allowed great flexibility in gas flow patterns. The gases could either enter
through top and exit through the bottom or enter through the top and exit through the top
without mixing. This was possible by having an annular reactor configuration where gases
could pass on both sides of the annulus. Teflon seals separated the gas flows at the top and
the bottom annulus. In our case, gases entered through top and exited through the bottom.
The reactor material was INCONEL, and al1 the fittings attached to it were Swagelock
SS3 16. Reactant gases entered at the top of the reactor (see Figure 3.2.3.2), then flowed
down around the k-type thermocouple which was placed directly above the catalyst to
monitor catalyst bed temperature, and then reached the preheat zone. Eventually, the
reactant gases reached the catalyst bed where the reaction occurred. The product gases
passed through the quartz frit that supported the catalyst bed. Finally, the product gases
exited through a 1/8" tube at the bottorn of the reactor. -4 sealed quartz tube was used in the
bottom part of the reactor to fil1 the dead volume in order to prevent unwanted gas phase
reactions and to reduce down-strearn residence time and holdup.
The reaction temperature was maintained by an electric fimace. To control the fumace
temperature, an Omega CN2OI 1 temperature controller was used with a K-type
thermocouple embedded in the fùrnace refiactory wall. One trap was installed at the lowest
part of the gas line to separate condensed water fiom the reactor effluent in order to prevent
GC damage by water. To prevent contamination and back flow of the system, four filters (2
prn) and four one-way valves were installed at four different gas Iines in front of each flow
controller. Pressure was monitored using a pressure gauge on each line, and a pressure drop
(< i 3 H a ) was generated by the quartz frit and catalyst bed.
Reactor hiet (Hz. H20,CH4 & Ar)
&
Quartz Tube
Evacuated Quartz Tube
Teflon Seals
Reactor Ourlet e*
Quartz Reactor
Inconel Reactor WW' Cataiyst Bed Quartz Frit
DCMENSIONS
i,
Quartz Reactor:
l
Quartz Tube:
Evacuated Quartz Tube
OD=9mm
Fig 3.2.3.2: Schernatic of reactor
3.2.4 Gas Chromatograpb
Product gas analysis was performed using an MT1 M 2 O O GC equipped with a dual
column~detector (TCD) system that shared a single the injection pon. The following
conditions were used:
- Molsieve 5A (4m) for analysis of inert, Hz, CO and C&; Temperature = 35OC
- Poraplot U (4m) for analysis of C& & CO2; Temperature = 3S°C
- Sampling time = 20 sec
- Injection time = 20 rnsec
- Analysis time = !60 sec
- Camer gas was Helium (99.9999%)
These parameters gave reasonable separation and retention times for al1 the gas species.
Calibration of the GC was carried out using a standard gas mixnire (Scotty N Calibration
Gas) with the following compositions (volume %), HI = 4.04%, Oz = 4.97%, Nz = 4.98%,
CO = 4.99%, CO2 = 5.03% and CHi, = 4.00% and accuracy &2%.
3.3 Overview of Experiments
Four main types of experirnents were done using the LaCro.ooNi0-~03 catalyst. The
LaCro.7sNio.zs03 catalyst was used only for the kinetic experiments. Expenments were as
follows:
Temperature screening experiments at one standard gas composition over a wide
temperature range (500°C to 1000°C) to observe catalyst stability and activity at high
temperature.
Isothermal expenments to investigate catalyst stability over time at one standard gas
composition at two different temperatures ( 6 7 5 ' ~ and 700'~).
Space velocity (product dependence) experiments to estimate the effects of products and
extemal mass transfer limitations on the kinetics at one temperature (700'~).
Experiments over a range of temperatures (675 to 750 '~) and gas compositions to
determine the kinetic parameters (power rate law), and stability and activity at lower
temperature for five different compositions.
The list of experiments is given in Table 3.3.1. This list gives information about catalyst
batch, data set, and a short description of experiments. These experiments are described in
detail in following sections. At the beginning of each new experiment set, the reactor loop
was evacuated to 3 .5~10" torr to remove al1 gases fiom the system. In addition, steady state
was established at al1 data collection temperatures.
Table 3.3.1: List of Experiments
Catalyst batch
Data set Lacr0.sNb.403
Description of expenments
Series of temperature screening (increasing & decreasing
sweep, same max. temperature) experiments; temperature range
is 500 to 1 0 0 0 ~ ~ by an increase of 2 5 ' ~ in standard gas 1 composition
Series of temperature screening (increasing & decreasing
sweep, same max. temperature) experiments; temperature range
is 500 to IOOOOC by an increase of 2 5 ' ~ in standard gas
composition
Series of temperature screening (increasing & decreasing
sweep) expenments at different maximum temperatures (750,
850, and 1 0 0 0 ~ ~ ) by an increase of 2 5 ' ~ in standard gas
composition
Catalyst was oxidized at 1 0 0 0 ~ ~ with pure oxygen, and cooled
to 500'~. Then data were collected from 500 to 1 OOO'C by an
increase of 2 5 ' ~ in standard gas composition.
Fresh catalyst was preheated to 1 0 0 0 ~ ~ in oxygen and held two
hours at 1000°~, then cooled to 5 0 0 ~ ~ . Data was collected from
500 to 1000 '~ by an increase of 2 5 ' ~ in standard gas
composition.
. . . . . . . continued
List of Experiments - -
Data set Description of experiments
Five different compositions, different methane, steam and
hydrogen partial pressures, and different H20/H2 ratios at four
different temperatures (725, 750, 775 and 800'~) with constant
argon flow (30 cchin).
Five diflerent compositions, different methane, steam and
hydrogen partial pressure, and different HzO/H2 ratios at four
different temperatures (675, 700, 725 & 750'~) with constant
total flow (41 cc/rnin).
Five different compositions, different methane, steam and
hydrogen partial pressure, at four different temperatures (675,
700, 725 & 750'~) with constant total flow (41 cc/min).
Isothermal time maintenance experiments at 675 & 7 0 0 ~ ~ .
Space velocity experiments at 700'~.
Kinetic experiments (for power law) at 700'~. -
LaCro.dio.&3 Five different compositions, different methane, steam and
G-l 1 hy drogen partial pressures, at four different temperatures (675,
700, 725 & 750'~) with constant total flow (41 cdmin).
Five different compositions, different methane, steam and
H-l I hydrogen partial pressures, and at four different temperatures
(675, 700,725 & 750 '~ ) with constant total flow (41 cdrnin).
3.3.1 Temperature Screening Experiments
To observe effects of the temperature on reaction rate and catalyst stability, experiments
were camied out fiom 500°C to 100VC with a single gas composition at atmospheric
pressure, and data were collected at every 25°C after establishing steady state (data sets are
A-1-5, B-1-4, C-2-8 & D-1). The gas composition was C& : H2 : HzO : Ar = 2 : 3 : 6 :
30 (cclmin) and the space velocity was 13,500 hf'. This gas composition is considered as
the "standard" gas composition. More temperature screening investigations were done (data
sets are E-2, F-1, F-2 & 1-1) at lower temperatures (tiom 675°C to 750°C) for five different
compositions which will be described in more detail in Section 3.3.4.
3.3.2 lsothermal Catalyst Stability Measuremeat Experimeats
To investigate catalyst stability and the change of reaction rate with the length of time on
Stream, two sets of data were obtained at 675°C and 700°C for LaCr0.6Ni0.403- The duration
of exposure was 25 and 30 hours for 6 7 5 ' ~ and 7 0 0 ' ~ respectively. The feed pas
composition was CH;I : Hz : Hz0 : Ar = 2 : 3 : 6 : 30.
3.3.3 Space Velocity (Product Dependence) Experiments
To investigate the change of reaction rate with the product partial pressures (especially
CO,), four different space velocity experiments were done at 700°C. Gas composition was
CI-L : H2 : Hz0 : Ar = 2 : 3 : 6 : 20. Space velocities were 9500, 19000, 285000 and 3800
(hi1). The space velocity data were obtained using 0.1 8 cc (batch-1) catalyst . The same
batch of catalyst was used for isothermal catalyst stability measurements, space velocity and
kinetic experiments successively.
3.3.4 Kinetic Experiments
Reaction orders to fit a power law rate expression (r = k(PcH4)'(PH2)y(Pm~)z) were
determined. To obtain the reaction orders, experiments were camied out from 675 to 750°C
with different gas compositions, and data were collected at every 2S°C. Table 3.3.4.1 shows
the partial pressure of different components for five different compositions.
I I Composition 1 Pcea (atm.) 1 Pm (atm.) 1 PA~O (atm.) ( P* (atm.)
Table 3.3.4.1: Partial pressure (atm.) of different components for five different compositions; Comp-l was "standard" composition; H2 was doubled in Comp-2, Hz0 was doubied in Comp-3, both Hz & Hz0 were doubled in Comp-4 and C h was doubled in Comp-5 with respect to standard composition.
The reaction order with respect to methane was obtained by varying the methane partial
pressure fiom 0.049 to 0.098 atm while rnaintaining the hydrogen pressure at 0.0732 atm
and the steam pressure at 0.146 atm. The reaction order with respect to hydrogen was
obtained by varying the hydrogen partial pressure from 0.073 to 0.146 atm while
maintaining the methane pressure at 0.049 atm and the steam pressure at 0.146 atm.
Conversely, the reaction order with respect to stearn was obtained by varying the s t e m
partial pressure from 0.146 to 0.293 atm while maintaining the hydrogen pressure at 0.0732
atm and the methane pressure at 0.049 atm. These are minimal experiments (two points for
each parameter) for kinetic data. More details are not justified because, as will be shown in
the Results and the Discussion sections, the catalyst changed with time.
To investigate the temperature effect on the reaction rate and catalyst stability at lower
temperatures ( 6 7 5 ' ~ to 750°c), five different compositions were chosen (data set F- 1, F-2,
G-1, H-1 & 1-3). The first composition was C& : Hz : Hz0 : Ar = 2 : 3 : 6 : 30 (cdmin).
The hydrogen partial pressure was doubled with respect to the standard composition in
second composition and argon flow was adjusted to get constant flow (CH, : Hz : Hz0 : Ar
= 2 : 6 : 6 : 27). In the third composition, the steam partial pressure was doubled with
respect to the standard composition and argon flow wzs adjusted to get constant flow (CI% :
H? : HzO : Ar = 2 : 3 : 12 : 24). Both hydrogen and steam were doubled with respect to the
standard composition and argon flow was adjusted to get constant flow (CI& : H2 : Hz0 :
Ar = 2 : 6 : 12 : 21) in the fourth composition. In the fifth composition, methane was
doubled with respect to the standard composition and argon flow was adjusted to get
constant flow (CH4 : Hz : Hz0 : Ar = 4 : 3 : 6 : 28). Total flow was 41 cc/min for al1
compositions. The catalyst heating rate was 1 SO°C/hr up to 300°C, 1 OO°C/hr up to 450°C and
40°C/hr afterwards. Fifieen minutes were allowed to stabilize the reaction (steady statej at
each temperature. Experimental conditions were different for different batches; these are
explained in more detail in the Resuits section. The results of these experiments wi11 be
described in Chapter Four.
CHAPTER FOUR
RESULTS
This chapter presents the results of the various experiments, which are listed in Section-
3.3. The overview of the experiments is given in Section 3.3 and more specific
experimental conditions are mentioned with specific results.
4.1 Initial Temperatures Screening
4.1.1 lMass Balance
The methane conversion was calculated using two different methods (described in
Appendix-A). Both methods are based on mass balances; one uses a carbon balance and
the other uses argon as a tracer. Fig 4.1.1.1 shows the consistency of the two different
methods. Al1 of the other results presented are based on a carbon balance. Data are
presented in Appendix-D (Table 4.1.2.1). Analysis accuracy is 9% to t 5 % in two
successive data. For example, if we consider methane conversion (based on carbon
balance) at 8 0 0 ~ ~ (61.25% at 8 0 0 ~ ~ and 40.55% at 775 '~ ) in Appendix-D (Table
4.1.2.1), the maximum error is (61 -25 - 40.55)X(&0.05) = kl.04. So the conversion at
800'~ is between 60.21% to 62.29%. But the conversion value has to be between O to
100 including analysis error because it can not be negative and above 100. Analysis error
for al1 expenments is between +3% to k5% in two successive data.
Fig 4.1.1.1: Cornparison of methane conversion in temperature screening expenments using two different rnethods; details in Appendix-A; catalyst batch-A and feed flow C&: Hz: H20: Ar = 2:3:6:30; temperature increasing sequence; space velocity 13,500 hr-l
4.1.2 Product Distribution
The GC analyzed al1 the product gases except for water. Water concentration was
determined by an H & O balance, as described in Appendix-A. CHJ and Hz0
concentrations decrease and the hydrogen concentration increases with increasing
temperature. CO2 is the primary product of COx below 650 '~ . The water-gas shifi
reaction favours this product at low temperature. CO produced from steam reforming is
convexted to CO2 by the water-gas shifi reaction at low temperatures (below 650'~); thus,
the CO concentration is very low at this temperature. CO2 concentration decreases above
800°c, even though methane conversion increases with temperature. The CO
concentration increases with temperature.
................... W........
Fig 4.1.2.1 : Product distribution in temperature increasing sequence (wet basis %); cataiyst batch-A and feed flow Ca: Hz: H20: Ar = 2:3:6:30; space velocity 13,500 hi'; details in Appendix-A
4.1.3 Reaction Quotients
Reaction quotients for s t em reforming reaction and water-gas shift reaction were
determined for al1 temperature screening data using these following equations:
3 Q r e r h n g = Pm *Pc&H~*PHzo; Reforming reaction: CI& + Hz0 = CO + 3H2
Q w . ~ ~ ~ - ~ ~ r b f i = PHZ*P~02/pc0*P~20; Water-gas shifi reaction: CO + Hz0 = CO2 -t Ht
Fig 4.1.3.1 shows the reaction quotients for the steam reforming reaction for one set of
data (temperature increasing sequence). The reaction quotient data approach equilibrium
vaIues at high temperature. The difference between the equilibrium constant and the
reaction quotient is greater at lower temperature. Thus, the reaction is kinetically limited
at low temperature. In other words, steam reforming reaction is not at equilibrium for this
catalytic nin.
Reaction quotients for the water-gas shift reaction are presented in Fig 4.1.3.2. The
reaction quotients are very close to the equilibrium values at high temperature ( > 8 0 0 ~ ~ ) .
But at lower temperature (<800°c), the reaction quotients are greater than the equilibrium
constants. In addition, we have seen that CO2 concentration is greater than CO
concentration at Iow temperatures (see Fig 4.1.2.1). Therefore, these data show that CO2
is the primary product of CO, (CO, = CO + CO2). At low temperatures the kinetics are
too slow to establish equilibriurn. Thus, the reaction quotients are greater than the
equilibrium constants at low temperatures. Lf CO is pnmary product of COx, then the
reaction quotients would smaller than the equilibrium constants at low temperatures.
Fig 4.1.3.1: Cornparison of equilibrium constants and reaction quotients for aearn reforming reaction; (CH, + H20 = CO + 3H2); catalyst batch-A (Ni =
0.40); space velocity 13,500h.f' and feed flow C I t : Hz: HZO: Ar = 2:3:6:30
Fig 4.1.3.2: Cornparison of equilibrium constants and reaction quotient for water- gas shift reaction; (CO + Hz0 = CO2 + Hz); cataiyst batch-A (Ni =
0.40); space velocity 13,500 hr" and feed flow Cs: Hz: H20: Ar = 2:3 :6:3O
4.2 Activity Changes Durhg Screening
4.2.1 Activity Changes at Same Maximum Temperature
Methane conversion increases drastically above 700% and conversion is almost
completed within 9 0 0 ~ ~ at standard gas composition for a space velocity 13,500 hr-', and
about 94% conversion is in this temperature range (see Fig 4.2.1.1). Conversion is very
low below 700'~ because steam reforming reaction is slow at lower temperature due to
its strong endothermic nature. Conversion is low above 900% because methane is almon
gone.
The oxygen feed was stopped at 9 5 0 ' ~ after obtaining temperature-increasing data, and
catalyst was in H2/C& environment for half an hour while it was cooling to room
temperature. Then H2/CW flows were stopped. To obtain the temperature-decreasing
sweep conversion data, the catalyst was heated up to 1000'~ and held there for an hour.
An attempt was made to reproduce data in the temperature decreasing sweep (on the
down cycle) using the same catalyst (batch-A). It was found that the used catalyst
(reproduced data) was more active than the fiesh catalyst (initial data). For example, if
we look at temperature 7 5 0 ~ ~ ~ the increase in conversion is about 28%. The change in
activity for the different sets makes it necessary to investigate the reason. One possibility
is that this may be due to the experiment procedure because the oxygen feed was stopped
at a high temperature (950'~) after data collection, and possibly the catalyst reduced to
Ni or NiO. To confirm the problem tiom the expenment procedure, we used catalyst
batch-B for the same type (temperature increasing and decreasing) of experiments, and
gas flow was stopped at low temperature (<400°c), where no reaction should occur. But
the results of batch-B are simiiar to the results of catalyst batch-A. Thus, it is unlikely
that the increase in conversion is due to the experirnent procedure. We have seen that the
results of both batches are similar, so we have presented one set of data fiom the
temperature increasing sweep and one set of data fiom the temperature decreasing sweep
for catalyst batch-A in Fig 4.2.1.1. Data are presented in Appendix-D (Table 4.2.1.1).
Figure 4.2.1.1 shows the increase in conversion about 28% at 7 5 0 ' ~ for two sets of data
(initial and reproduced). It indicates that the catalyst changes afier high temperature
exposure
Temp. increasing
Tem p. decreasi ng
500 600 700 800 900 1000
Temperature [Celsius]
Fig 4.2.1.1: Methane conversion dependence as a fiinction of temperature; conversion is higher at temperature decreasing sequence than temperature increasiq; cat&t batch-A (Ni = 0.40); space velocity 13,500 hi'; feed flow C&: Hz: H20: Ar = 2:3:6:30
4.2.2 Activity Changes at Differeot Maximum Temperatures
We have seen in Section 4.2.1 that the activity increases in successive sweeps with
respect to the initial nveep; what causes this increase in activity? Does this change occur
at a particular temperature, or after a particular temperature? To find this answer, we
used catalyst batch-C (Ni = 0.40) for screening expenments with three different
maximum temperatures.
Eight different sets of data (data set C-1-8) were obtained using catalyst batch-C at three
different maximum temperatures (750°c, 8 5 0 ' ~ and 1 0 0 0 ~ ~ ) . Two sets (temperature
increasing and decreasing sweeps) were obtained for each maximum temperature 7 5 0 ' ~
and 850°c, and four sets (temperature increasing (two sets) and decreasing (two sets)
sweeps) were obtained for maximum temperature of 1000'~. Experiments were carried
out from lower maximum to higher maximum temperature in tum, and data collection
was started at 5 0 0 ~ ~ . The reactor was on-line during the data collection of al1 eight sets
of data, and it was at 2 3 0 ' ~ (where no reaction should occur) to collect the n e a set of
data d e r two sacs of data (the temperature increasing and decreasing sweeps) collection
at each maximum temperature. The standard gas composition (CH< : HZ : HtO : Ar = 2 : 3
: 6 : 30) was used for al1 eight sets of data.
The results of four sets of data for temperature-increasing sweeps are presented in Figure
4.2.2.1 and four sets of data for temperature-decreasing sweeps are presented in Figure
4.2.2.2. In these figures, the activity increases for each successive data set with respect to
the previous data set for the increasing maximum temperature and increase in activity is
greater at higher maximum temperature. Also, the activity increases in the next data set
with respect to the initiai data set even if the same maximum temperature is reached. So
we can conclude that a progressive change occurs in catalyst activity at ail temperatures
(investigated up to 1000°C) in the reforming reaction zone; and the increase in activity is
greater at higher maximum temperature.
Teperabae increasing =quem
-7th set (1000 C )
Fig 4.2.2.1: Catalyst activity increases at each set with respect to previous set and maximum temperature of exposure; Feed gas was C h : H2 : Hz0 : Ar = 2 : 3 : 6 : 30; catalyst batch-C (Ni = 0.40); space velocity 13,500 hr- ' temperature increasing sequence
6abh-C
- Temperature decreasing
swep 8th set (1 000 C)
Sth set (1000 C) P
4th set (850 C) -
7nd set (750 C )
I
450 550 650 75 0 850 950 1 OS0
T em perature [Celsius]
Fig 4.2.2.2: Catalyst activity increases at each set with respect to previous set and maximum temperature of exposure; Feed gas was C& : HZ : H20 : Ar = 2 : 3 : 6 : 30; catalyst batch-C (Ni = 0.40); temperature decreasing sequence
Batch-C
Temperature increasing 8 decreasing sequence
1 st cycle
cytk
450 550 650 750 850 950 1050
Tem perature [Celsius)
Fig 4.2.2.3: Catalyst activity increases with successive number of exposures at same maximum temperature; feed gas was C h : H2 : HzO : Ar = 2 : 3 : 6 : 30; catalyst batch-C; temperature increasing (' 1 ') and decreasing ('2') sequence
The last four sets of data are cailed respectively the first cycle and the second cycle in
Figure 4.2.2.3. Each cycle has temperature increasin~ and decreasing data starting fiom
500'~ to 1000'~ and vise versa. The first cycle data were collected before the second
cycle data. Number ' 1 ' is for temperature increasing sweep and number '2' is for
temperature decreasing sweep.
Figure 4.2.2.3 shows that catalyst activity increases with a successive number of
exposures at the same maximum temperature. It is clear that catalysts are changing
(structural changes) dunng high temperature reforming. This change is iikely due to the
partly reduction of the catalyst or the complete reduction to elemental nickel. More
details of this change are discussed in Section 4.6.
4.2.3 Influence of Oxidation on Activity over Used and Fresh Catalysts
We have seen in Section 4.2.2 that activity increases with temperature and number of
exposures at the same temperature. What are the probable reasons for this change? This
may be due to the reduction of the perovskite catalysts, or there may be other reasons (see
Section 4.6). If the catalyst is reduced, can it go back to its original state after oxidation?
To investigate this query, catalyst batch-C was oxidized at 1000'~ in pure oxygen for
two hours d e r eight sets of data collection (see Section 4.2.2), and then cooled down to
5 0 0 ~ ~ . One set of data was collected in a temperature-increasing sequence (up to 1000~~)
at the standard gas composition. Catalyst batch-D was used for an additional
investigation to check the oxidation effect on a fkesh catalyst. This batch was preheated at
~ O O O O C in oxygen and held for an hour, and cooled down to 5 0 0 ~ ~ . One set of data was
then collected in a temperature increasing sequence (up to 1 0 0 0 ~ ~ ) at the standard gas
composition.
The results of the oxidation effect for both batches (C&D) are presented in Figure 4.2.3.1.
For batch-C, the activity is higher afier oxidation than before oxidation from 7 2 5 ' ~ to
900°c, and activity is higher before oxidation than afier oxidation up to 7 2 5 ' ~ and both
are very close above 900 '~ . It is hard to determine whether or not the catalyst has partly
gone back to its original state after oxidation. But it is clear from Figure 4.2.3.1 that the
catalyst did not go back completely to its original state (see Section 4.6). The activity for
batch-D is close to that of batch-C above 9 0 0 ~ ~ and the activity is lower below 9 0 0 ~ ~ for
batch-D. In other words, data for batch-D is less active. Thus oxidation has no effect on
the activity of the eesh catalyst, Le. catalyst does not change in oxidizing environment.
Temperature increasing sequence data
450 550 650 750 850 950 1 OS0
Temperature [Celsius]
Fig 4.2.3.1: Oxidation effect on used (reduced, batch-C) and fiesh (batch-D) catalysts and feed gas was C& : Hz : H20 : Ar = 2 : 3 : 6 : 30
4.3 Isothermal Maintenance Catalyst Activity
We have seen that the activity increases with increasing temperature and number of
exposures at the same temperature. Do the activity changes depend only on these
parameters? We want to check whether the activity is stable over time at a constant set of
conditions. Catalyst batch-1 was used to investigate isothennal maintenance of activity.
This batch was at 6 7 5 ' ~ and 700'~ for 25h and 30h, respectively in standard gas
composition.
Fig 4.3.1: Isothermaî maintenance catalytic activity at 675'~ and 700'~; feed gas was C h : HZ : H20 : Ar = 2 : 3 : 6 : 30; catalyst batch-1
f irst 6 hours
O 1 2 3 4 5 6
Hours
Batch-l m - m
700
Last 8 hours (b)
22 24 26 28 30
Hours
Fig 4*3*2: Isothemal maintenance catalytic activity in two different time segments; (a) for firçt 6 hours and (b) for last 8 hours
The results of isothermal time effeas are presented in Figure 4.3.1. Catalyst activity
increases more rapidly in the first few hours (>6 hours). The increasing rate of reaction is
0.000473/hr (1 8.93%/hr) and 0.00 138fhr (5.15%/hr) at temperatures of 6 7 5 ' ~ and 700'~
respectively for the first 6 hours, and SE-OSh (0.32%/hr) and 6 E - 0 6 h (O.OSYo/hr) at
temperatures of 6 7 5 ' ~ and 7 0 0 ' ~ respectively for the last few hours (see Figure 4.3.2).
Calculation is based on the first data at two different time segments.
In the last few hours, the increase of activity is much slower, which indicates that the
catalyst activity is fairly stable. So our conclusion fiom this investigation is that the
activity of catalyst has a time effect for the first few hours (up to 25 hours) at a particular
temperature ( 6 7 5 ' ~ and 700°c), and then a stable activity is reached. In other words,
catalyst activity is fùnction of time for few hours.
4.4 Space Velocity Experiments
We have seen in previous sections that the activity depends on temperature, the number
of exposures at the same temperature and the length of time of exposure. In addition, it
may depend on the products. To investigate the products (CO,) influence on methane
steam reforming, space velocity experiments were done. Dependence on the reactants
(CH4, H 2 0 and H2) is discussed in the next section.
The increase in activity is much slower after longer exposure tirnes (previous Section).
Therefore the catalyst was kept at a particular temperature for a long time (30 h) before
space velocity data collection in order to minimize the time effect on activity. Catalyst
batch-1 was used to collect space velocity data. This batch was on-line for 25 hours at
675'~ and 30 hours at 7 0 0 ' ~ before space velocity experiments. The gas composition
was CI& : Hz : HzO : Ar = 2 : 3 : 6 : 20, space velocities were 9500, 19000, 28500 and
3 8000 (hr") and the temperature was 700'~. The results of the space velocity (data set I-
2 ) expenments are presented in Figure 4.4.1. The activity increases slightly up to a
certain space velocity (-19000 hr") above which the activity remains constant. This
means that there is no product ef%ect on the reforming reaction or there is little effect on
lower space velocity. In addition, CO2 dependence result is in agreement with Rostup-
Neilsot1 and Bak Hansen 's results (9).
Gas composition:
C H4: H2:H20:Ar=2:3:6:20
OOE+O 1 E+4 2E+4 3E+4 4E+4
Space velocity (l/hr)
Fig 4.4.1: Influence o f products (CO,) on reforming reaction; catalyst batch-1, feed flowwasC~:H~:H20:Ar=2:3:6:20;Ni=0.40
4.5 Kinetic Experiments
4.5.1 Effects on Activity in Different Compositions and Temperatures
Two different types of experiments were done to investigate the effects of gas
composition and temperature on activity. The first type of experiments had constant
argon flow and different methane, hydrogen and steam flows, and also different HzO/H2
ratios at four different temperatures (700, 725, 750 & 8 0 0 ~ ~ ) using catalym batch-E. As
the argon flow was constant in al1 experiments, the partial pressure changed for al1
reactants with changing flow of a particular reactant. Thus, these conditions were not
controlled enough to observe the effects on methane, steam, hydrogen and H20/Hz ratio
over steam reforming kinetics, but are usefùl to get a rough idea.
The second type of expenment was with five different gas compositions, different
methane, hydrogen and steam flows, and different H20/Hz ratios with constant totat flow
(4 1 cc/min) at four different temperatures. Composition-1 had CI& : Hz : H 2 0 : Ar =
2 2 6 3 0 , and was considered to be the "standard" gas composition. Compositions-2, 3, 4
and 5 had double hydrogen, double steam, double both hydrogen and steam and double
methane respectively with respect to the standard gas composition (see Section 3.3.4).
Data were obtained at four different temperatures (67s0c, 700°c, 725 '~ and 7 5 0 ~ ~ ) ~
from lower temperature to higher temperature. Two different types of catalysts,
LaCr0.6Ni~.~0~ (batch-F) and LaCr0.75Ni0.2503 (batch-G and H), were used. hiring the
first run (data set F-1) in batch-F, data were obtained for five different compositions at
four different temperatures, and data for Composition-1 were repeated &er each
composition. The reactor was cooled down to ambient temperature after the first mn.
During the second run (data set F-2) in batch-F, data were obtained for five different
compositions at four different temperatures. M e r the second run, the catalyst was cwled
down to 6 7 5 ' ~ and one more set of data (data set F-3) was obtained at 6 7 5 ' ~ for five
different compositions (called the third run). The reactor was on-line up to completion of
data collection, and it was set to 2 3 0 ' ~ after data collection. The results of three different
runs for batch-F are shown in Figure 4.5.1.1 without time normalization and each
temperature has five different compositions starting from Composition-1 to Composition-
5. The results of repeatzd Composition-1 after different compositions are presented in
Figure 4.5.1.2. In figures 4.5.1.1 & 2, Compositions-1 to 5 are presented in tum at each
temperature for different runs.
The activity of Composition-1 increased with al1 other compositions at different
temperatures (see Fig 4.5.1.2). The activity increased 1.07 to 1.3 times with each
successive analysis and 1.47 times overall (first to last) at 675'~, 1 .O1 to 1.18 times with
each successive analysis and 1.37 times overall at 700°c, 1.04 to 1.25 times with each
successive analysis and 1.55 tirnes overall at 725'~, 1.09 to 1.14 times with each
successive analysis; and 1.51 times overall at 7 5 0 ' ~ for Composition-1 in first run for
batch-F. Overall increased activity is 4.72 times in the first mn, and the activity increased
from the first to second run 4.45 times at 675'~; these are very close. Activity increased
4.45 and 1.72 times from the first to second run and the second to third run respectively
at 675'~ Le., increasing rate decreases with the number of mns. Activity increased 3.62,
3.41 and 2.31 times h m 6 7 5 ' ~ to 700°c, 7 0 0 ' ~ to 7 2 5 ' ~ and 7 2 5 ' ~ to 7 5 0 ' ~
respectively during the first run at standard gas composition. In the second run, activity
increased 2.08, 2.26 and 2.6 times from 6 7 5 ' ~ to 700°c, 7 0 0 ' ~ to 7 2 5 ' ~ and 7 2 5 ' ~ to
750'~ respectively (see Figures 4.5.1.1 & 2) at standard gas composition. So increase in
activity at different temperature ranges and at different temperatures is diffèrent.
Experimental sequence
Fig 4.5.1.1: Activity increases with the number of nins and regardless of gas composition; batch-F; Ni = 0.40; each temperature set consists of five different compositions starting fiom Composition- 1; numbers indicate the the number of composition; data without time normalization
- - - -
Experimental sequence
Fig 4.5.1.2: Activity o f composition- l increases with other compositions and at different temperatures (OC); batch-F; Ni = 0.40; first run; number indicates the composition numbers; data without time nonnalization
We have seen in Figures 4.5.1.1 and 4.5.1.2 that the activity increased with the number of
successive runs, increasing temperature and regardless of compositions. The increase in
activity is in agreement with the results of the temperature screening experiments, which
are discussed in Sections 4. t . l& 2
As mentioned in Section 4.3, catalyst activity is fiinction of time. We have to time
normalize the data with respect to the initial data (t = O) to get the actual activity at a
particular temperature for a particular composition. The assumption was made that the
increase in activity is linear during data collection at a particular temperature but this
assumption is not quite true because increase in activity is fast in first few hours (see
Section 4.3). It is challenging to normalize these data for a changing cataiyst; and that
assumption was made for simplicity but the reliability of this normalization will be less.
The procedure to calculate the actual rate is described in Appendix- B. Data collection
was repeated for Composition-1 after collecting data for all compositions at al1 different
temperatures dunng the second and third runs for batch-F, and the first run for batch-H to
do time normalization. The results afier time normalization for the first and second runs
in batch-F are presented in Figure 4.5.1.3.
Experimental sequence
Fig 4.5.1.3: Change in activity after time normalization; batch-F, first and second mns; each temperature set consists of five different compositions starting fiom Composition- 1 ; Ni = 0.40
Using catalyst batch-G & H, data were obtained similarly for five different compositions
and four different temperatures Iike the second and third runs in batch-F. These two
batches were used to test reproducibility, and the results are close to each other. The
results of batch-H are presented in Figure 4.5.1.4 withuut time effect correction.
2nd run
1st run
Fig 4.5.1.4: Activity increases with number of mm; batch-H; first and second mns; each temperature set consists of five different compositions starting from Composition- 1; data without tirne norrnalization
The increase in activity is less for batch-H compared to batch-F with the increasing
number of runs. The activity for Compositions-2, 3 & 4 is lower than Composition-1,
which indicates that Hz and H20 inhibit reforming reaction.
The foliowing conclusions can be made from the different compositions and the different
temperatures expenments for two different catalysts (LaCr0-6Nio.~O3 and
LaCr0.7sNi0.2~03):
(a) The activity increases with increasing temperature regardless of gas composition.
(b) The activity increases with the increasing number of mns.
(c) The increase in activity is higher for LaCr0.6Ni0.40~ than for LaCr0.7~Ni0-2s03 i.e.,
the higher nickel content catalyst has a higher change in activity in the same
reforming environment.
(d) Methane enhances the reforming reaction at ail conditions mentioned in this section,
and both hydrogen and steam enhancehnhibit reforming reaction in different mns for
different catalysts.
(e) Increase in activity is less for lower nickel content catalysts.
4.5.1.1 Corn parison in Activity Between LaCro.6Nio.403 Catalyst (Ni = 0.40) and L a C r 0 . , ~ N i ~ 0 ~ Catalyst (Ni = 0.25)
Two different catdysts @ACr0.6Ni0.403 & LaCr0.75Ni0.2503) were used to determine the
effects on activity at different compositions and temperatures. We have seen in previous
sections that catalyst activity changes with time, increasing with the temperature
regardless of gas compositions and the number of runs for both catalysts; the increase in
activity is higher for LaCr0.6Ni0,~03 than for LaCro.r5Ni0.2503. But still we do not know
which catalyst is more active. To investigate this, a cornparison was done between the
first run of batch-F (Ni = 0.40) and the first nin of batch-H (Ni = O Z ) , which are
presented in Figure 4.5.1.1.1. Time normalization was done (see Appendix-B) for these
two mns because catalyst activity is a function of time, as shown in Section 4.3.
Figure 4.5.1.1.1 shows clearly that the Ni = 0.40 catalyst is more active than the Ni =
0.25 catalyst only except for the Composition-1 & 2 for the Ni = 0.25 cataiyst. However,
the difference in activity is not the same at al1 temperatures and for different gas
compositions. This difference is higher at higher temperature (725 '~ and 750'~). The
reason for this difference is a greater increase in activity for Ni = 0.40 (see Section 4.5.1).
But the activity for Compositions-2, 3 & 4 are in opposite trend for these two catalysts.
For the Ni = 0.25 catalyst, the activity decreased for these Compositions (2, 3 & 4) with
respect to Composition- 1, but did not for Ni = 0.40 catalyst. In both catalysts, the rate for
Composition-5 was significantly greater than for composition-1 . At low temperature
(675'~), the activity is almost equal for Compositions-1 & 2 in the first nin for both
catalysts. But in the second nin, the activity for al1 compositions is greater for the Ni =
0.40 catalyst than that of the Ni = 0.25 catalyst. This is due to the higher change of Ni =
0.40 catalyst.
The reforming reaction takes place on the nickel surface, and the catalyst is more active
as it has a higher nickel surface area (3). The higher nickel content catalyst (Ni = 0-40)
has a greater nickel surface area than the lower nickel content catalyst (Ni = 0.25) (2), so
the first catalyst shouid have higher activity. The obtained result is consistent with this
t heory.
Fig 4.5.1.1.1: LaC~0.6Nio.403 cataiyst (Ni = 0.40) is more active than LaCr0.7sNi0.2503 (Ni = 0.25); first mn for batch-F & H; each temperature set has five different compositions starting tiom Composition- 1 ; numbers indicate the composition number; afler time normalization
4.5.2 Reaction Orders
We have seen in previous sections that the catalyst activity changes with increasing
temperature, maximum temperature of exposure, number of exposures and length of time
of exposure. It is difficult to determine the kinetic parameters using such a changing
catalyst. There wiil be doubt about the reliability of the kinetic parameters for this
catalyst due to the changing behavior. An attempt was made to determine the kinetic
parameters using two different catalysts LaCr0.6Ni0.403 and LaCr0.7sNi0.2~03. The
measured reaction rate was normalized with respect to initiai data (t = O) to avoid the time
effect on activity; the procedure of data normalization is in Appendix-B. Minimum
experiments (two points for each parameter) were done to obtain the reaction orders.
More detail was not justified, because of the catalyst changes with time that were
discussed in Section-4.3.
The reaction orders for Ca, Hz and Hz0 for methane conversion were calculated to fit
following rate equation:
Rate = k (C&)' (Hz)" (&O)' . . . . . . . . . . . . . . . . . . . ... . . . ... . . . . . -. . . - .. (4.1
where
k is the reaction rate constant, and x, y, and z are the reaction orders for methane,
hydrogen and steam, respectively. A plot of In(rate) vs In(conc) of the reactant being
varied will give a straight line with a dope equal to the order of that reactant. The
intercept is equal to Ink', where k' is the product of reaction rate constant (k) and the
concentration (partial pressure, in atm.) terni of the reactant at constant concentration.
Experimental conditions were described in Section 3.3.4. The plots of methane, hydrogen
and steam dependencies for methane conversion are presented in Figure 4.5.2.1 and in
Appendix-E (Figures E-4.5.2.2 to 11). The obtained orders are presented in Table 4.5.2.2.
These plots show la(rate) of rnethane conversion vs ln pp (pp for partial pressure) of the
reactant being varied, and cover temperatures of 675 '~ , 700°c, 7 2 5 ' ~ and 750 '~ . These
dopes represent the reaction order of rnethane conversion piven on the ordinate with
respect to the concentration (partial pressure, in atm.) of the reactant in the abscissa.
Reaction orders were calculated for the fira and second mns from catalyst batch-F, a
special run from catalyst batch-l and the first mn from catalyst batch-H after time
normalization. Data were obtained using catalyst batch-1 afler the isothermal catalyst
stability measurement and space velocity dependence experiments, whic h were
considered as a special mn. The effects of the H20Mz ratio (mole) on methane orders
were investigated using catalyst batch-F during the first mn. Three different Ham2
ratios (1, 2 and 4) were chosen and the results are presented in Figure E-4.5.2.1
(Appendix-E) and Table 4.5.2.1. Ali orders are close to first order. Lower H20/H2 ratio
(1) has higher methane orders but these orders are close to higher H20/H2 ratios (2 and
4). A H 2 0 N 2 ratio = 2 was used to obtain nethane orders in different nuis for different
catalysts. Analysis accuracy is +3% to 3%. For example, if we consider methane order
(1.14) at 750'~ for H20/H2 = 3 in Table 4.5.2.1, the maximum error is 1.14 X (kû.05) =
0.06. So the methane order is between 1 .O8 to 1.20.
Fig 4.5.2.1: Methane orders for methane conversion; tirst run; batch-F; hydrogen and aeam are in constant partial pressure; Temperatures 675, 700, 725 and 750'~; Ni = 0.40
Table 4.5.2.1 : List of methane orders for methane conversion at different EtO/a2 ratios (without time normalization)
3 0.99 1 .O2 1 .O4 1.14 * Analysis accuracy is +3% to +5%.
Table 4.5.2.2 : L is t of orders for methane, hydrogen and steam at different runs (after time normalization)
Catalyst & Component ~ e r n ~ . ( * ~ ) no. of mn 675 700 725 750
Batch-F CH4 0.95 1.17 0.92 1 .O6 Nickel = 0.40 Hz 0.04 0.1 1 0.08 0.06 First run Hz0 0.17 0.1 1 0.22 0.09 Batch-F CH4 0.69 0.64 0.83 1 .O0 Nickel = 0.40 H2 -0.04 -0.1 O -0,003 -0.02 Second mn H20 -0.0 1 -0.04 -0.14 -0.06 Batch-I c h 0.85 Nickel = 0.40 Hz -0.1 1 Special run H20 -0.13 Batch-H CH4 0.8 1 0.89 1.16 1 .O9 Nickel = 0.25 H2 -0.13 -0.19 -0.20 -0.02 First r u H20 -0.57 -0.48 -0.27 -0.27
* Analysis accuracy is S% to +5%.
The apparent reaction orders are almon consistent for different mns and catalysts except
steam orders for the Ni = 0.25 catalyst (batch-H). In the first (batch-F), al1 orders are
positive and methane orders are close to first order. But in the second run (batch-F),
methane orders are positive, and hydrogen and steam orders are negative. Special run
(batch-1) and batch-H have trends similar to that of the second run. Methane orders are
smaller for the second mn, special run and batch-H than for the first run of batch-F.
Steam orders (negative) are higher in batch-H than in other mns Le., steam inhibition is
observed in this reaction.
4.5.3 Activation Energies
The reaction rate constants for methane conversion were calculated fiom k =
rate/(C&)s(H~)y(H20)z where C& H2 and H 2 0 represent the partial pressure (atm.) of
C a , Hz and H20 respectively after the reaction orders had been evaluated. The Arrhenius
plots of the denved reaction rate constant for methane conversion are presented in Figure
4.5.3.1, where In of rate constant is plotted against 1/T (T is in Kelvin) for w o different
catalysts. The calculation procedure for the estimation of kinetic parameters is given in
Appendix-C. The apparent activation energies of methane conversion are 389 Wmol
(93.1 7 kcal/mol), 3 70 kJ/mol (95.1 2 kcal/mol), and 4 1 8 kJ/mol ( 1 00.27 kcavmol) for the
first run (batch-F), the second run (batch-F) and the first mn (batch-H) respectively. The
derived activation energies in this way are similar for the first and the second mns in
batch-F. This procedure may not reliable for a wide range of pressure because two points
were used to detennine the reaction orders. So the reliability of the obtained activation
energies will be less. The activation energies were determined fiom Figure 4.5.3.1. Time
normalized data were used to estimate activation energy. Activation energies were
determined at standard gas composition fiom temperature increasing and decreasing
sequences data (Temperature Screening Expeximents) at standard gas composition where
ln of reaction rate is plotted against lm (T is in Kelvin). Activation energies are 259
kJ/mol (62.7 kcal/mol) and 142 kUmol (34 kcal/rnol) for temperature increasing and
decreasing sequence respectively (see Appendix-G). These values are more reliable
because six data points were used to obtain them. Cornparison of activation energies are
done in Section 5.5 (Table 5.5.1).
Fig 4.5.3.1: Activation energy for methane conversion at different runs and sarnples; (a) is the first run for batch-F; (b) is the second run for batch-F and (c) is the first nin for batch-H
4.5 Catalyst Characterization by X-ny Diffraction (XRD)
We have already discussed the change in catalyst activity in earlier sections. Still, we are
not sure about the reasons for this change, but it seems likely that the catalyst reduced to
Ni or NiO. Most of the used catalysts (batches-& C, H and I) and the two fiesh samples
(LaCro.6Nio.403 and LaCro.75Ni0.2503) were characterized by XRD to investigate stxucturai
changes.
4.6.1 X-ray Diffraction of LaCro.aio.r03
The XRD results of batches-& C and 1 are presented with a fiesh LaCr~.&o.& sample
in Figures F-4.6.1.1, 2 & 3 (Figures F-4.6.1.1 to 3 are in Appendix-F) to compare the
change which occurred during the experiments. There are some rninor phases that appear
as impurities (LaiCrOs and other non-identified phase) in the fiesh catalyst. But these
minor phases are gone fiorn batches-A, C and 1 afler the expenments. This means that the
catalyst surface becomes clean when it is exposed to feed gas at high temperatures This
clean surface should have higher activity than the initial dirty surface.
Figure F-4.6.1 shows another important change, which is the decomposition (reduction)
of the catalyst to elemental nickel. The XRD results show that the catalyst reduces to
elemental nickel, which we considered in Section 4.2.3. Elemental nickel constitutes 5 to
6 % (W.) of the sample in batch-A after screening expenments. We can conclude fiom
this observation that LaCr0.6Ni0.403 decomposes to elemental nickel during the reforming
reaction at high temperature (observed up to ~OOO~C), and the surface becomes clean.
The XRD results of batch-C are different fiom batches-A & 1 even though al1 these
batches were the same type of catalyst (LaCro-sNi0.40~); however, this batch was oxidized
after screening expenments. There is no change in the used catalyst with respect to the
fresh catalyst except for rninor phases impurities.
The XRû results of batch-1 are different f?om batches-A & C even though al1 these
batches were the same type of catalyst (LaCro.sNio.~O& however, the operating
maximum temperatures were different for these batches. The maximum operating
temperatures were 7 0 0 ' ~ for batch-1 and IOOOOC for batches-A & C. Figure F-4.6.1.3
shows that the catalyst batch-1 has distorted to rhornbohedral structure fiom orthorhombic
structure and this cataiyst has split peaks, which are different fkom batches-A & C and the
fiesh sample, and the minor phases are gone. But there is no visible elemental nickel or
nickel oxide.
4.6.2 X-ray Diffraction of LaCro..rsNi~.~~O~
The XRD result of batch-H is presented with the fresh LaCr0.75Ni0.2~03 sample in Figure
F-4.6.2.1 (Appendix-F) to compare the change that occurred during the kinetic
experiments. Maximum operating temperature was 7 5 0 ' ~ for the kinetic experiments.
This fresh sample also had slight impurities, and these impurities were partly removed
during the experiments. Both tiesh and used catalysts were simiiar in stnicture, but the
used catalyst had some wider peaks.
DISCUSSION
5.1 Activity of Perovskite Catalysts
Perovskite catalysts work well for the methane steam reforming reaction, but these
catalysts are not stable in a reforming environment. The Ni = 0.40 catalyst is more active
than Ni = 0.25 catalyst (see Figure 4.5.1.1.1) because the first catalyst has more active
nickel sites (2). This result agrees with Marko's methane oxidation result (8). Methane
conversion increases drastically (4.3% to 99%) fiom 700 '~ to ~OO'C, and there is only a
1 % increase from 9 0 0 ~ ~ to 1000'~ at standard gas composition (see Figure 4.1.1).
5.2 Change in Activity of Perovskite Catalysts
The catalyst with different nickel contents has different activity and the increase in
activity is also different. Increase in activity for catalyst with higher nickel contents (Ni =
0.40) is greater than that with lower nickel contents (Ni = 0.25) (see Figures 4.5.1.3 & 4)
because the change in catalyst sites is greater for higher nickel contents (Ni = 0.40).
The increase in activity was measured at standard gas composition (see Figure 4.5.1.2)
with respect to initial activity at the sarne gas composition. The catalyst activity at the
standard gas composition increased with al1 other compositions at different temperatures.
The increase in activity was 1 .O7 to 1.3 times with each successive analysis and 1.47
tirnes overall at 675'~; 1.01 to 1.18 times with each successive analysis and 1.37 times
overall at 700'~; 1.04 to 1 .ZS times with each successive analysis and 1.55 times overall
at 725'~; 1.09 to 1.14 times with each successive analysis and 1.5 1 times overall at
7 5 0 ' ~ in the first run for batch-F. Overall increase in activity was 4.72 times in the first
run and from the first to second run was 4.45 times at 675'~. They are very close.
Increase in activity was 4.45 and 1.72 times fiom the first to second run and the second
run to third respectively at 675'~ Le., the increasing rate decreases with the number of
mns. Activity increased 3.62, 3.41 and 2.3 1 times fiom 6 7 5 ' ~ to 700°c, 700'~ to 7 2 5 ' ~
and 7 2 5 ' ~ to 750 '~ respectively during the first run In the second activity increased
2.08, 2.26 and 2.6 times from 6 7 5 ' ~ to 700°c, 7 0 0 ' ~ to 7 2 5 ' ~ and 7 2 5 ' ~ to 7 5 0 ' ~
respectively (see Figures 4.5.1.1 & 2) at standard gas composition. So the increase in
activity at different temperature zones (from one temperature to another temperature) and
at different temperatures is different. As the change in activity is different in two mns at
the same temperature and it does not follow any general pattern, it is difficult to
generalize the change in activity and the reliability of this generalized relation will be
less.
In addition, the kinetic behaviour is different at different gas compositions and similar at
different temperamies for different nickel content cataiysts. Hydrogen and steam enhance
the reforming reaction over higher nickel contents catalyst initially (first run), but inhibit
the reaction over the lower nickel contents catdyn. But this behaviour for higher nickel
contents changes later on (after the first mn) i-e., hydrogen and steam inhibit reforming
reaction (see Figures 4.5.1.3 & 4.5.1.1.1).
Moreover, time has additional effects on activity change. Investigation was done at two
different temperatures (675% & 700'~) at standard gas composition for 25 to 30 hours.
The activity increases rapidly in the first few hours (>6 hours). In the last few hours,
increase in activity is much slower, which indicates that the catalyst aaivity is fairly
stable. Pretreatment of catalysts before conducting kinetics expenments may help to
avoid time effect on reaction rate. There must be changes with temperature.
We have seen that activity depends on many factors, such as temperature, gas
composition, time and catalyst nickel content. To investigate the probable reasons for the
change in activity, the catalyst was characterized by X-ray diffraction. Fresh Ni = 0.30
sample has minor phases as impurities (La2Cr06 and other non-identified phases). These
minor phases are partIy gone, and the perovskite decomposes to elemental Ni, and Ni is 5
to 6% (W.) afier screening at 1000'~ (see Figures 4.6.1.1 & 2) Le., this catalyst reduced
to Ni. This Ni is the probable reason for increased activity. During kinetic experiments at
750'~ or below, the perovskite structure distons to a rhombohedral structure fiom a
orthorhombic structure, the minor phases are gone, and some of the peaks are split but
there is no visible Ni or Ni-oxide. There could be a small amount of Ni (see Figure
4.6.1.3). Catalytic properties are affected by the structure even though the chernical
compositions are the same. The increased activity in kinetic experiments is due to the
distortion of stmcture. As well, the fiesh Ni = 0.25 sample also has slight impurities.
These impurities are partially gone after lonetic experiments at 7 5 0 ' ~ or below. The
structures of both eesh and used perovskite are very similar but used perovskite has some
wider peaks (see Figure 4.6.2.1). The distortion of the structure depends on temperature
and Ni contents. The distortion is greater in the higher nickel contents cataiyst and the
increase in activity is higher for higher nickel contents catalyst (see Figures 4.5.1.1 &
4.5.1.4) Le., the more Ni in the catalyst, the less stable it is. This is consistent with TGA
data (2).
5.3 Inability to Reform Original Catalyst
We have seen that the cataiyst is reduced during screening at 1 0 0 0 ~ ~ . An attempt was
made to reform the original catalyst from this reduced (used) catalyst by oxidation. But
this reduced catalyst did not go back to its original condition (see Figure 4.2.3.1) as per
catalyst activity i.e., this is consistent with the inability to make these perovskite catalysts
by "heat and beat". But the XRD result is different fiom the previous result. The XRD
result of this catalyst shows that the reduced catalyst has retumed its original state (see
Fig F-4.6.1.3). The reason for this disagreement is not clear.
We have seen that cataiyst changes in reducing environment. Does this change occur in
the oxidizing environment? To investigate this, fiesh catalyst was oxidized in pure
oxygen. This oxidized catalyst showed similar activity to the non-oxidized catalyst. So
the catalyst does riot change in oxidizing environment.
In addition, catalyst structure changes or is reduced to Ni partly during reforming. This
could be a new way of making a reforming catalyst that may be usefùl, because industrial
catalysts are prepared in such a way that they can be reduced to Ni fiom Ni-oxide during
reforming, and reforming reaction takes piace on the nickel surface (3).
5.4 Change of Catalysts
Perovskite catalyst partly reduces by creating oxygen vacancies above 3 0 ~ ' ~ at H20/HZ
environment. In addition, structure changes fiom orthorhombic to rhornbohedral. This
changed catalyst back to its original state afler oxidation, i.e. catalyst is reproducibie. The
partly reduced catalyst fûrther reduces and produces elemental nickel at higher
(investigate up to 1 0 0 0 ~ ~ ) in HI environment, not in H20/H2 environment. Once catalyst
is reduced to elemental nickel, it does not back to original state even afier oxidation., i.e.
catalyst is not reproducible.
5.5 Kinetic Parameters
To obtain kinetic parameters, minimal experiments were done (two points for each
order). More details are not justified, because we have seen that the activity changes with
time. The figures of reaction orders are presented in Appendix-E and Figure 4.5.2.l.The
apparent orders are almost consistent in different runs and in different sarnples afier time
normalization. The assumption is made that the change in activity is linear at each run
during data collection. In the first run for Ni = 0.40 catalyst, methane, hydrogen and
steam orders for rnethane conversion varied fiom 0.92 to 1.17, 0.03 to O. 1 1 and 0.09 to
0.22 respectively. But in the second run for the same catalyst, methane, hydrogen and
steam orders varied fkom 0.64 to 1 .O, -0.003 to -0.035 and -0.008 to -0.144 respectively.
For the Ni = 0.25 catalyst, methane, hydrogen and steam orders varied fiom 0.8 1 to 1.16,
-0.1 O4 to -0.192 and -0.008 to -0.144 respectively. One special run for batch-1 (Ni = 0.40)
was done where the activity change was much slower afier (2 X 30 hours) 60 hours of
exposure; and methane, hydrogen and steam orders were 0.854, -0.107 and -0.126
respectively. Methane orders were in a wide range fiom 0.64 to 1.17. in general, the first
run had higher orders for methane, hydrogen and steam with respect to other nins. The
orders are close for other nins.
Moreover, hydrogen and steam have small positive orders in the first run and small
negative orders in the rest of the nins which means that hydrogen and steam slightly
enhance reforming reaction in the first run and slightly inhibit it in the rest of the mns.
In agreement with previous investigations of the methane steam reforming reaction, the
rate of reaction is found to be close to the first order with respect to P CHJ- ROSS and Steel
( 1 972) found that rnethane and steam orders were 1 and - 0 . 5 respectively, and they used
coprecipitated Niialumina containing 75% Ni in its reduced form (34). The dependence
(slightly positive and negative value) of the rate on steam and hydrogen partial pressure
implies that the s tem and hydrogen compete slightly with methane for the active
catalytic sites. The obtained orders for steam are close to those of Ross and Steel (1972)
(31) especially for Ni = 0.25 catalyst. But these orders are also close to those of Phillips,
Mrrihall and Tzmer (37) except for Ni = 0.25 catalyst. The activation energies obtained
by present author (see Figure 4.5.3.1 and Appendix-G), Jianpo and Gilbert (35),
Remhack and Xeinisch (35), Meyer and Kopsel(35) are presented in Table 5.5.1. Meyer
m d Kopsel (35) used fine catalyst particles (0.25-0.63 mm). Jianguo and Gilbert (35)
used fine catalyst particles (0.18-0.25 mm). The derived activation energies are similar
for the first and the second mns in batch-F but greater with respect to other authors'
results. The activation energy for batch-H is also close to that of batch-F. The reliability
of the obtained activation energies will be less because two points were used to determine
the reaction orders. The activation energies obtained from temperature increasing and
decreasing sequences data are more reliable because enough data points (six) were used
to obtain them. Activation energy for temperature increasing sequence is significant with
respect to other authors results.
Table 5.5.1: List of activation energies
1 Author 1 Experiment 1 Ni contents in 1 Activation energy
1 I 1 catalyst (wt %) / (kJ/rnol)
l / Templ. increasing; Ni=0.40 1 9.72 I 259
Present 1 First run; batch-F; Ni=0.40 1 9.72 1 389
I I I
1 1 First mn; batch-H; Ni=0.25 1 6.10 1 418
2nd run; batch-F; Ni=0.40
1 Jianguo & Gilbert 1 1 15.2 1 240
9.72
L 1 1
* This catalyst had some elemental nickel; ~ernp'. presents temperature; Accuracy = fi%
370
Rennhack & Heinisch I
248
CHAPTER SIX
CONCLUSIONS
Perovskite catalysts (LaCro.6Nio.40; & LaCro,6Nio.~O;) have good methane steam
reforming.
The higher nickel content catalysts are less stable than the lower nickel content
catalysts.
The higher nickel content catalyst has higher activity in methane steam reforming.
The increase in activity is greater in higher nickel content catalysts.
The catalyst reduces at high temperature in the reforming environment, and changes
structure (orthorhornbic to rhombohedrel) at lower temperature during long time
exposure.
These perovskite catalysts may be the potential catalysts for methane stearn reforming
even though they reduce because the existing reforming catalysts are produced in
oxide fonn but are then intentionally reduced to perform methane stearn reforming.
COz appears as a primary product of CO, in reforming reaction.
The perovskite catalysts reduced to nickel do not retum to its original state even afier
oxidation.
CEIAPTER SEVEN
1. Further investigation should be done to establish these catalysts as potential catalysts
for methane steam reforming reaction-
2. The reason(s) for perovskite reduction and/or structural change shouid be
investigated.
3. An atternpt should be made to improve the catalyst's property to avoid the reduction
during methane stram reforming.
4. An examination should be made of steam reforming with natural gas (mixture of
methane, ethane, propane etc.) over the same perovskite catalyst and methane stearn
reforming over other perovskite catalysts.
5 . Coke formation on this catalyst should be checked.
6. Thermal stability and surface area change of perovskite catalysts should be checked.
Appendix A: Calculation procedures for methane conversion, reaction rate, water analysis and reaction quotients for steam reforming and water-gas shift reaction
&Methane conversion:
1. Based on carbon balance:
Conversion (%) = 1 00* (xco + woz )/(xcH~ i- xco f xcoz )
where x indicates percentage composition
7 -. Based on argon as a tracer:
Conversion (%) =100*(Ac~j,in/A,\,i~ -ACH~.~U~/A:Z~.~U~)/(ACH-I.~~/A.-\~.~~)
kvhere A indicates the area of GC analysis
Reaction rate:
Reaction rate (micro- mol/grn.sec) = Conversion(%) * methane flow rate (micro-moVsec)
/ (W. o f catalyst (&m) * LOO)
Reaction quotients:
Stearn reforming reaction = P C O * ~ H ~ * * ~ / P C H ~ * P W ~ O
Water-gas shifi reaction = pcol*pccd PCO*PH~O
where "p" indicates the partial pressure o f the subscnpt component
Appendix B: Calculation procedure for time normalizing of reaction rate
As we have seen in Section-4.3 that catalyst activity changes with time. So the measured
reaction rate has included the increase in activity with time. We need to exclude this
increased activity for time to get the actual reaction rate. An attempt was made to
normalize the measured reaction rate (micro-mo1dg.s) with respect to the initial reaction
rate. The activity factor was measured at standard gas composition (Composition-1). To
do that, data collection for Composition- l was repeated afier each different composition,
and aRer five different compositions in some cases. Fint, 1 tned to find out the catalyst
activity for the Composition-1 at different gas compositions using repeated Composition-
1 data. Then the activity factor was measured with respect to the initial activity for
Composition- 1. Using this activity factor, measured data were normalized to get the
actual reaction rate. Similar procedure was used for different temperatures. See Figure
4.5.1.2 for data collection procedure.
rmsaqursdn (t) = Activity factor (t) * r (t = 0)
rno-iizcan (t = 0) = rmeasurrdn (t) 1 Activity factor (t) .......................... 03-11
To get the activity of Composition-1 at different gas compositions:
................................................ rc1.n = rcl + Td*ri/Tt (B-2)
To get the activity factor:
Activity factor (t) = rCiqn rcl,i
From (B-1)
where
r, . i : Initial reaction rate (micro-mole/,am.sec) for Composition- 1
rnomalized,n : Nonnalized reaction rate for Composition -n (n = 1, 2, 3, 4 & 5 )
rrnesisurcd.-, : Reaction rate measured for Composition -n (n = 1, 2, 3,4 & 5 )
rcl ,, : Reaction rate for Composition- 1 at Composition-n ( n = 1,2,3.4 & 5)
r;, : Reaction rate for Composition- 1 before different Compositions
ri : Increase in reaction rate for Composition-1 in two successive data
Td : Time difference (lu) in data collection beîween Composition- 1 and Composition-
n ( n = 1,2,3,4 & 5 )
Tt : Total time required (hr) for Composition-n (n = 1,2,3, 4 & 5) and Composition- 1
repeating, or five different compositions and Composition- 1 repeating
Two exarnples are given to show the calculation procedure.
Example-1: (see Fig 4.5.1.2 for data collection. the fint run at 675%. batch-F)
r,, ,, = 0.003603
r,, = 0.003972
ri = 0.004383 - 0.003972 = 0.000416
Td = 0.5 hr
Tt = Ihr
Using equation (B-2)
rcie3 = rci + Td*ïi/Tr = 0.003972 + 0.5*0.000416/1 = 0.00418
Example-2: (data fkom the second run at 675 '~ . batch-F)
Appendix C: Calculation procedure of Kinetic Parameters
1. Dependence (order) of reaction
Expression used for kinetic parameter estimation:
Rate = k.(CH4)X.(H2)S.(H20)z ................................................... ( 1
where k is the reaction rate constant, and x, y, and z are the reaction orders for methane,
hydrogen and steam respectively, and Rate, in (micro-rnole/gm.sec), is calculated from
the GC data. CH4, Hz and H z 0 stand for partial pressure (atm.) for C&, HI and H 2 0
respectively.
Taking In on both sides on equation (l), the reaction orders x, y and z can be determined
graphically as follows:
In(rate) = Ink +- x.ln(CH4) + y.ln(H2) +z,ln(HzO)
In(rate) = x. ln(CH4) + Ink' ................................................................ ( 2 )
where k' = k.(Hz)'.( H20)"
In(rate) vs ln(CH4) plot gives x as dope and Ink' as an intercept (1).
similar procedure for y and z
2. Activation Energy (E):
Reaction rate constant, k is calculated from equation (2)
Activation energy (E) is then determined graphically as the dope of the Ink vs 1iT plot. (Temperature T is in Kelvin)
E = ((lnk)? - (lnk) )/((UT)? -(l/T) i)*0.00 1 98 (kcaYmo1)
Table 4.1.2-1: Methane conversion (%) at different temperature in two different methods (Analysis accuracy is +3% to f 5%)
Temperature Conversion (%) Temperature Conversion (%) OC Carbon Argon as a OC Carbon Argon as a
balance tracer balance tracer
Table 4.t.l.l: Methane conversion at different temperature (Analysis accuracy is S% to 15%)
Temperature Conversion (%) Temperature Conversion (%) OC Temperature Temperature OC Temperature Temperature
increasing decreasing increasing decreasing
Appendix-E: Reaction Orders
-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 Ln PP(atm.) of methane
-4 .O0 -3.50 -3.00 -2.50 -2.00 -1.50
Ln PP(atm.) of methane
Fig E-4.4.2.1: H20H2 ratio has minor effect on methane orders; batch-F; without tirne norrnalization
Appendix-E : Reaction Orders
Fig E-4.5.2.2: Hydrogen orders for methane conversion; first run; batch-F; methane and steam are in constant partial pressure
Appendix-E: Reaction Orders
Ln PP (am.) af s m
Fig E-4.5.2.3: Steam orders for methane conversion; first mn; batch-F; methane and hydrogen are in constant partial pressure
Appendix-E : Reaction Orders
Fig E-4.5.2.4: Methane orders for methane conversion; second run; batch-F; steam and hydrogen are in constant partial pressure
Appendix-E : Reaction Orders
Fig E-4.5.2.5: Hydrogen orders for methane conversion; second run; batch-F; steam and methane are in constant partial pressure
Appendix-E : Reaction Orden
Fig E-4.5.2.6: Steam orders for rnethane conversion; second mn; batch-F; methane and hydrogen are in constant partial pressure
Appendix-E: Reaction Orders
1
-1 -4 -1 -2 -1 .O -0 -8
Ln PP (atm.) of methane
-1 -2 -1 .O -0 -8
Ln PP (atm.) of hydrogen
-0 -8 -0.6
Ln PP (atm.) of steam
Fig 4.4.2.7: Methane, hydrogen and s t e m orders for methane conversion at 700°c in batch-1, Ni = 0.40
Appendix-E : Reaction Orders
Ln PP (atm) of methane
-3.5 -3.0 -2.5 -2.0
Ln PP (atm.) of methane -3.5 -3.0 -25 -20
Ln FF (atm) of mthane
Fig E-4.5.2.8: Methane orders for methane conversion; batch-H; steam and hydrogen are in constant partial pressure
Appendix-E: Reaction Orders
-3.0 -2.5 -20 -1 -5
Ln PP (atm) of hydrogen
-3.0 -2.5 -2.0 -1.5
Ln PP (atm.) of hydrogen
Ln PP (atm) of hydrogen
-3.0 - 2 5 -20 -1.5
Log of PP (atm.) of hydrogen
Fig E-4.5.2.9: Hydrogen orders for methane conversion; batch-H; steam and methane are in constant partial pressure
Appendix-E: Reaction Orders
-2.0 -1 -5 -1 .O
Ln PP (atm .) of steam
-2.0 -1 -5 -1 .O
Ln PP (atm.) of steam
Ln PP (atm.) of steam
-2.0 -1 -5 -1 .O
Ln FP (atm.) of steam
Fig E-4.5.2.10: Steam orders for methane conversion; batch-H; methane and hydrogen are in constant partial pressure
Appendix-F: XRD Results of Catalysts
900 1
N = 0.40
Fres h cataiyst 600 -
Figure F-4.6.1.1: XRD of batch-A and fresh catalyst (Ni = 0.40)
Appendix-F: XRD Results of Catalysts
-
I
Ni = 0 . 4
Fres h catalyst
Figure F-4.6.2.2: XRD of batch-C and fiesh catalyst (Ni = 0.40)
Appendix-F: XRD Results of Catalysts
Ni = 0.40
Fresh catalyst
Figure F4.6.1.3: XRD of batch-1 and fiesh catalyst (Ni = 0.40)
Appendix-F: XRD Results of Catalysts
Ni = 0.25
Fresh cataiyst
Figure F4.6.2.1: XRD of batch-H and fiesh catalyst (Ni = 0.25)
Appendix-G: Activation Energies
1.1
1 O O O / l (K)
1.1
1 ooorr (K)
Fig G-4.5.3.1: Activation energy for methane conversion at standard gas composition during temperature screening experiments;
(a) temperature increasing sequence; (b) temperature decreasing sequence
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