something about the car

Monolithic structures as alternatives to particulate catalysts for the reforming of hydrocarbons for hydrogen generation Thomas Giroux*,Shinn Hwang,Ye Liu,Wolfgang Ruettinger,Lawrence Shore
Engelhard Corporation,101Wood Avenue,Iselin,NJ08830,USA
Received6February2004;received in revised form28July2004;accepted29July2004
Available online25September2004
Abstract
Currently,many companies are exploring the technical and market potential of distributed hydrogen for use in such varied applications as stationary fuel cell power generation units,on-site generators of hydrogen for industrial uses,and hydrogen fueling stations for fuel cell powered automobiles.
The use of particulate catalyst beds in the industrial production of hydrogen is well known.Engineers conceiving new process designs today for fuel processing can take advantage of the successful experience of environmental catalysis in automotive and stationary applications using monolithic catalysts and other substrates such as heat exchangers.In this paper,the unit operations of hydrogen generation are reviewed with the perspective of describing the opportunities of implementing such new catalyst technologies in small-scale fuel processing.
#2004Elsevier B.V.All rights reserved.
Keywords:Monolith;Reforming;Hydrogen;Fuel cell reformer
1.Introduction
The goal of producing hydrogen in small-scale reactors is receiving increasing attention with the continued interest in the potential widespread application of fuel cells,on-site hydrogen generators,and the establishment of a distributed hydrogen infrastructure.
The industrial plant processes for hydrogen production are well established[1-3]but may not be appropriate for small-scale applications such as residential fuel cells or for unattended operation such as for on-site hydrogen genera-tion.These new applications allow for new process designs that can take advantage of catalyst and engineering advances [4].As engineers consider new process designs for hydrogen production,they can benefit from the experience that monolithic catalyst manufacturers have earned from more than30years of success in stationary environmental and automotive catalysts applications.
This paper discusses the application of catalyzed monoliths and heat exchangers in the use of generating hydrogen with low carbon monoxide for use as a feed stream for fuel cells.
2.Background
2.1.Coated monoliths and heat exchangers as catalyst substrates
The general differences,advantages,and disadvantages of catalyst-coated monoliths over particulate catalysts have been described well previously[5–7].Process design engineers considering a change from the standard packed bed steam reforming process design of hydrogen generation to a new process using monolith-based reactors would do well to familiarize themselves with these differences.
A monolith catalyst support is a thin-walled honeycomb structure made from ceramics like cordierite or metals such as Fecralloy(Fe72.8/Cr22/Al5/Y0.1/Zr0.1,an iron/ chromium alloy with excellent resistance to oxidation at
/locate/apcatb *Corresponding author.
E-mail address:thomas.giroux@(T.Giroux).
0926-3373/$–see front matter#2004Elsevier B.V.All rights reserved.
doi:10.1016/j.apcatb.2004.07.013
elevated temperatures).Examples of metal and ceramic monoliths as well as other potential catalyst substrates such as ceramic and metallic foams and metallic sheets are shown in Fig.1.The active catalyst is deposited as a washcoat onto the geometric surface of the monolithic structure.The washcoat is a mixture of active catalyst components, stabilizers,and a high surface area coating layer based on alumina,for example.
The use of heat exchangers as reactors is marketed today by such companies as Heatric with their Printed Circuit Reactors and Chart Industries with their line of ShimTec reactors[8,9].The practical applicability of such systems as catalyzed chemical reactors has been nicely discussed by Hugill et al.at the Energy Research Centre of the Netherlands[10,11].The recent chemical engineering literature shows interest in the area of‘process intensifica-tion’:the reduction of process volumes by combining and consolidating multiple unit operations into one physical unit [12].Whereas previously a catalyzed exothermic reaction might have been performed by a pellet bed reactor followed by a heat exchanger,today the option exists to have the catalyst deposited on oneflow-side of the heat exchanger while the cooling mediumflows on the opposite side to combine both reaction and thermal management into one process step and into one piece of capital equipment.
From an operational point of view,a catalyzed ceramic monolith can operate approximately adiabatically while a catalyzed heat exchanger can operate closer to isothermally or with a controlled temperature gradient,depending upon the heat exchanger design,control,and operation.This flexibility and breadth of options is helpful for the design engineer looking to optimize the performance of a given chemical unit operation.
Geometric surface area is high for the ceramic and metal monoliths and moderate for heat exchangers.This has implications for which chemical reactions are amenable to monoliths compared to heat exchangers.Rapid reactions requiring short residence times are likely to succeed with catalysts supported on heat exchanger surfaces whereas slower reactions requiring long residence times are more likely to require the higher catalyst loadings obtainable with monoliths.
Ceramic and metal monoliths and metallic heat exchangers offer good mechanical strength while particulate catalysts can face attrition issues.Particulate catalysts under the pressure of the weight of a packed bed,or in motion in a fluidized bed,or in the presence of an aggressive chemical environment such as high temperature and partial pressure of hydrogen and steam,and rapid temperature changes during transient operation can weaken particulates causing attrition, and blockage,thereby decreasing activity and increasing pressure drop.
The substrate material of construction can dictate the thermal behavior of the reactor.Ceramic monoliths act as insulators and behave as nearly adiabatic reactors.Metal monoliths can transfer heat much more readily in axial and radial directions and are not adiabatic.A substrate such as a heat exchanger provides a range of temperature options from isothermal to a temperature gradient from gas inlet to the outlet.
The substrate material of construction affects the coating and adhesion of the catalyst layer.Ceramic monoliths made from cordierite have been proven for more than30years as capable catalyst substrates with few compatibility issues [13].Precious metals usually do not migrate into the cordierite monolith support and components of cordierite do not migrate into the washcoat.This is true even at high steam content,in a reducing atmosphere,and in the presence of sulfur,etc.The porosity of the cordierite material ensures good bonding with the washcoat material.When supporting
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Fig.1.Examples of catalyst substrates(clockwise from upper left):ceramic monolith,corrugated metal foil sheet,small metal monolith,fine metal foam, coarse ceramic foam,large metal monolith.
catalysts on metal structures,care must be taken that the metal and supported catalyst are compatible.Metals that are routinely coated with catalyst washcoat layers are alumi-num,Fecralloy and similar alloys,and stainless steels.Many metals or alloys have components that may be mobile in the reducing atmosphere and elevated temperature and steam of operation.These metals can potentially migrate into the catalytic washcoat and poison the catalyst after a short time of operation.The problem of metal contamination is of particular concern when catalysts are supported on brazed metal parts.In general,it is advisable tofinish all brazing operationsfirst and thoroughly wash the part before coating it with catalyst.Brazing compounds often contain heavy metals like Cd,Sn,etc.,which are known catalyst poisons. Residues of theflux can usually be removed by washing. Adhesion problems can occur if the thermal expansion coefficient of the metal is incompatible with that of the washcoat material and the part is put through thermal cycles. The compatibility of washcoat and substrate may have to be individually evaluated before a part is manufactured.
The substrate material of construction can affect the pressure drop across a monolith metal monoliths can be made with very thin walls compared to ceramic monoliths. This additional open face surface areas at a given cell density means that pressure drop across metal monoliths can be lower than across a ceramic monolith.Regardless of the material of construction,the pressure drop across a coated heat exchanger or monolithic catalyst reactor is significantly lower than that across a packed bed reactor.Heck and Farrauto discuss the calculation of pressure drop across a monolith[13].Farrauto and Bartholomew[3]calculate the pressure drop across a given pellet bed and the pressure drop across a400cells per square inch(cpsi)monolith of comparable volume at a gas hourly space velocity(GHSVor SV)of100,000hÀ1to be0.32MPa for the pellet bed versus only2670Pa for the monolith.
In the comparison of pellet bed and monolith space velocities,an important distinction must be made.The convention is to calculate space velocities of monolithic catalyst reactions using the total gross volume occupied by the monolith substrate.Thus,for the purpose of space velocity calculations,a rectangular monolith catalyst that is 5cm wide,5cm high and40cm long is considered to be1L of catalyst regardless of the cell density,catalyst loading, washcoat thickness,etc.This is not a rigorous description of the catalyst concentration,but this is the convention found in the literature.
In some instances(for example,see Section 3.3), monolithic reactors can be operated at significantly higher space velocities than pellet bed reactors.This reduction in reactor size saves both cost and weight,which has the secondary benefit of having a smaller reactor to heat so that there can be rapid thermal responses to transient behavior. Whether the application is a load-following stationary fuel cell or an on-site hydrogen generator stepping up from standby mode to full operation,the reactors need to be able to respond quickly to changes in temperatures andflow rates.
A monolithic reactor or a catalyzed heat exchanger can withstand such transients.
2.2.Factors to consider in monolith catalyst
reactor design
The use of catalyst monoliths in reactors places some extra engineering demands on the design engineer in the areas of entranceflow velocity distribution,reaction monitoring,and reactor construction.These topics are not discussed in detail but are highlighted and emphasized as issues of critical importance that might not be appreciated or anticipated if previous reactor experience was only with packed bed orfluidized bed reactors.
Entranceflow distribution is an important consideration when designing reactors using monoliths or heat exchan-gers.Due to the segmented channels of monoliths and some heat exchangers,theflow pattern reaching the inlet face of the supported catalyst structure should be uniform,both in terms of velocity and composition,to take best advantage of the coated catalyst performance.
Another consequence of the segmented nature of the ceramic monolith reactor affects how analysis and monitoring of the reaction is done.A thermocouple inserted within one channel of a monolith may not offer a reading that is representative of all channels.A gas sample collected too close to the monolith face may also not be representative of a well-mixed outlet gas composition.This is no worse than the results of non-uniformflow distribution or hot spots occurring in a packed pellet bed though.
In terms of reactor construction,there are some new skills involved in building a reactor with a monolithic catalyst compared tofilling a packed bed.Metallic monolithic catalysts can be welded into a reactor shell and held permanently.However,for the ceramic monolith catalysts, they must be properly canned to hold them inside a reactor shell.For this,a ceramic mat material is used in the gap between the ceramic catalyst skin and the reactor shell.The mat material provides the gripping force to hold the catalyst inside the reactor shell during heating and cooling cycles and forces all gas to pass through the catalyst withoutflowing through the gap(i.e.,flow bypass).One popular ceramic mat used in auto exhaust catalytic converters is Interam TM by 3M TM.This intumescent mat material expands and secures the catalyst tightly within the reactor shell upon heating. Millions of commercial exhaust catalysts have been canned successfully this way each year since1975.However,these mat materials can degrade gradually upon constant exposure to temperatures over8008C.They can lose their gripping force and increased bypass can be observed[14].Therefore, similar to the catalytic converters for utility engines,other high-temperature ceramicfiber material must be used for fuel cell reformers where temperatures over8008C are expected.Several mat materials from various companies are available for high-temperature canning applications,and can
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be used satisfactorily in a fuel cell reformer.For example, the non-intumescent CC-Max1from Unifrax Corporation is used in our laboratory during catalyst aging and evaluation experiments.
This section has served as a discussion of general differences among using monoliths and heat exchangers as catalyst substrates.The particulars in the application of hydrogen generation from hydrocarbons through reforming, water gas shift,and preferential oxidation of carbon monoxide are discussed below.
3.Results and discussion
3.1.Hydrocarbon reforming
The three most widely used catalytic processes to produce
hydrogen(H2)from hydrocarbons(HC)are(1)steam reforming,(2)catalytic partial oxidation,and(3)autothermal reforming.The chemical reactions for each process to produce H2from methane(i.e.the predominant compound in U.S.natural gas)are described in the followings:
(1)Steam reforming(SR):
CH4þH2O!COþ3H2;
D H¼þ206kJ=mol
(1)
(2)Catalytic partial oxidation(CPO):
CH4þ12O2!COþ2H2;
D H¼À36kJ=mol
(2)
(3)Autothermal reforming(ATR):
Combination of CPO and SR reactions(3)
Examples of the application of these approaches can be found in industry and the research literature[3,15].
Since steam reforming generates more hydrogen per mole of methane fed than the other reactions,SR reactors are the standard approach used in industrial hydrogen genera-tion mercially,supported Ni and Ru pellet cata-lysts have been used over the years in hydrocarbon steam reforming reactors in the oil,chemical,and gas industries.
Partial oxidation of hydrocarbons is an attractive route to hydrogen production because of rapid reaction rates. Successful results with partial oxidation of hydrocarbons have been demonstrated in reactors with space velocities as high as1,200,000hÀ1[16–18].
Autothermal reforming is a compromise between the fast reaction of partial oxidation and the higher concentration hydrogen production of steam reforming.Adiabatic ATR reactors can be found in use in the secondary reformer for some commercial ammonia plants.In these reformers, hydrocarbons are combusted catalytically to produce the necessary heat to drive the endothermic steam reforming reaction[3].
Each of these three approaches to hydrocarbon reforming is discussed in more detail below.
3.1.1.Steam reforming
Since steam reforming of hydrocarbons is a highly endothermic process,the steam reforming reactor is required to operate at high temperature to achieve higher conversion.This is especially true when operated at high pressures as illustrated in Fig.2.The steam reforming reaction is favored at lower pressure because of the net increase in the total moles in products.However,due to the requirements of pressure swing adsorption for H2 purification and high-pressure applications for H2,a modern industrial steam reforming plant operates at high pressure(>20bar)to maximize the overall efficiency and economics[15,19].
At the SR temperatures,the reformate compositions readily reach equilibrium as governed by the water gas shift reaction(4),which is moderately exothermic[3]
COþH2O!CO2þH2;D H¼À41kJ=mol(4) The supply of heat energy typically comes both from preheating the feed of steam and hydrocarbons before the reformer inlet and from heat transfer through the reactor walls from external combustion.The energy required for steam reforming of natural gas,i.e.methane at steady state is calculated and shown in Fig.3.Clearly,a heavy duty in heat supply is required for a steam reforming reactor even when the feed is preheated to a very high temperature.For instance,at an inlet temperature of 8008C with a feed of H2O/CH4=3,the energy required to reach equilibrium at8008C is213kJ/mol CH4in the feed.
In industrial-scale H2production,the SR of hydrocarbons takes place inside hundreds of chromium–nickel alloy reactor tubes of70–130mm inner diameter and10–13m long containing pelletized Ni-based catalyst.The tubes are located inside a large direct-fired chamber.In the primary reformer,the outlet temperature is typically required to be
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from800to9008C,which requires theflue gas in the combustion chamber to be in excess of10008C exiting the fired section[20].The severe reaction condition is extremely damaging to the steel reactor[2].
Gas hourly space velocity for steam reforming is typically on order of5000–8000hÀ1on a wet feed basis [20].This means that a large volume of catalyst is required. To reduce the pressure drop across such a large catalyst bed, the pellet size used is relatively large,which results in low effectiveness of the catalyst.
3.1.1.1.Alternatives to pelletized SR bed.Alternatives to a large pelletized catalyst bed for steam reforming must overcome the aforementioned difficulties,especially for smaller-scale applications,such as on-site H2generation and fuel processing for fuel cells,where rapid responses to transient operation is e of monolithic steam reforming catalyst was discussed by V oecks[6]for hexane steam reforming,and comparable performance to pelletized SR catalyst with20wt.%NiO loading was achieved with a straight channel300cpsi monolith with10%NiO loading.
Because of the endothermic nature of the reaction,the SR process is often heat transfer limited.Fast heat transfer to the catalyst,therefore,is critical to accelerate the reaction process.Engelhard Corporation[21]reported that by depositing a precious metal catalyst onto a metal monolithic substrate that facilitates faster heat conduction,a higher temperature was obtained in the catalyst reactor,resulting in higher CH4conversion than with the same catalyst on ceramic particulates.
There are recent reports of new steam reformer reactor designs that supply heat to the SR catalyst more effectively, utilize the catalyst more efficiently,and increase the volumetric efficiency[22,23].A common feature in these designs is to integrate catalytic combustion of hydrogen-containing exhaust gas and/or methane(shown in reactions (5)and(6)),with steam reforming into a heat exchanging unit where heat transfer takes place from the combustion side to the steam reforming side through the conductive metallic substrate.
CH4þ2O2!CO2þ2H2O;D H¼À802kJ=mol(5) H2þ12O2!H2O;D H¼À242kJ=mol(6) In the design by Polman et al.,a steam reformer consists of series of plates that sandwich metal monoliths made from corrugated foils[22].Combustion catalyst and steam reforming catalyst are washcoated on alternating monolith compartments.The heat generated from the combustion compartments transfers through the steel plates to the adjacent steam reforming compartments.In the design by Ismagilov et al.,a heat exchanging tubular reactor comprises a SR metallic foam core inside a steel tube that is surrounded by metallic foam coated with combus-tion catalyst[23].
Robbins et al.[24]employ a reactor configuration of parallelflat plates with alternative passages for CH4steam reforming and H2or CH4combustion.The operability of this integrated combustion and steam reforming design was modeled and experimentally tested.With a supported Pd catalyst for combustion and a supported Rh catalyst for steam reforming,stable operation is achievable at a relatively low inlet temperature(4008C)with H2combus-tion,but a higher inlet temperature is required for CH4 combustion.Activity for steam reforming catalyst is essential in controlling the combustion catalyst temperature.
In a heat exchanging reformer design,the design engineer should pay attention to the geometric surface area required not only for heat transfer but for catalyst coating as well. Heat transfer in such a unit is expected to be very fast since both the heat generation from combustion and the heat removal from steam reforming occur directly at the thin washcoat on metal surface.Heat transfer is through conduction,which is much faster than standard convective heat transfer from gas-phase combustion products.On the other hand,with only a thin layer of catalyst washcoat on the metal surface,a larger coating area is required to contain enough catalyst for the reaction.Attachment offins,metal foam or corrugated foils to the unit will be necessary to increase the surface area for catalyst coating.This is especially important for steam reforming since the reaction is usually reaction kinetic limited,and requires more catalyst.In addition to increasing the coating surface area,it is also helpful to use a more active catalyst or increase the loading,both by increasing active catalytic components in washcoat and increasing the washcoat thickness.For the steam reforming reaction,diffusion through the relatively thin washcoat will have much less impact on the overall process as compared to larger size particulate catalysts.This is supported by the experiment data shown in Fig.4.With the same washcoat loading,the washcoat thickness was increased by1.5times,but performance of the catalyst is essentially the same with decreasing monolith cell density from400to200cpsi(2795m2/m3and channel widths of
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Fig.3.Energy required for steam reforming of1mol methane when the reformate reaches equilibrium at the outlet temperature(H2O/CH4=3.0,P =1bar).
1.1mm for400cpsi versus1890m2/m3and channel widths of1.4mm for200cpsi).
3.1.2.Catalytic partial oxidation over monoliths
CPO reactions are very fast reactions and therefore the reactor volume using this technology to generate hydrogen from various hydrocarbons can be small in size.The CPO reaction across a catalyzed monolith can be performed at space velocities>100,000hÀ1,thus,the reformer size can be many times smaller than an SR reactor where gas hourly space velocities may typically be in the range of3000–8000hÀ1.
Four ceramic monolithic precious metal catalysts have been studied for converting methane to CO and H2by the CPO reaction.The space velocity used was58,000hÀ1and the O2/C ratio was at0.58.As shown in Table1,the percent CH4conversion for all catalysts evaluated was found to be between97.5and98.1%.The percent CH4slip(dry)was between0.5and0.8%.The ratio of CO/CO2was between 93.6and94.3%and the H2(dry)was between36.8and 37.4%.The space–time yield of H2is also presented in Table 1,using the total gross volume of the catalyst monolith in the calculation.Since the activity was not affected significantly by the PM loading,it can be concluded that the CPO reaction of converting methane to CO and H2is basically a diffusion-controlled reaction.The data confirm that very high percent of methane can be converted via this CPO reaction under high space velocity condition(i.e.short residence time).
For fuel cell applications,the CPO reaction has an advantage over the complete combustion reaction because it generates H2and useful CO,which can be used to generate more H2through the water gas shift reaction.The CPO reaction also produces heat that can be used by the fuel processing reformer elsewhere,such as in a hot water heater or in a CPO gas feed preheater.If an oxide catalyst is used as a CPO catalyst,the selectivity toward CO formation will be reduced compared to the PM catalysts of Table1.A larger portion of methane would be fully combusted to produce CO2and H2O,and the total amount of(CO+H2)in the product gas will be lower with an oxide catalyst.In addition, the precious metal catalysts are not irreversibly affected by the presence of any sulfur components in the fuel.Therefore, to increase the thermal efficiency of a reformer,it is preferable that the methane should be converted by the CPO reaction over monolithic PM catalyst instead of the combustion reaction over oxide catalysts.
CPO as the sole pathway of reforming does have limitations though.The use of fuel and air at high temperatures can bring the system into a regime where coke formation is thermodynamically favored.Sufficient coke formation could affect catalyst performance over time. Also,in terms of efficiency,the SR reaction can produce a total of four moles of(CO+H2)from every mole of CH4,as compared to three moles of(CO+H2)per mole of CH4from the CPO reaction.Therefore,the SR reformer can theoretically have higher thermal efficiency than the corresponding CPO reformer.Many system manufacturers are seeking to deliver the promised increased fuel efficiency that are often touted for fuel cells and this efficiency gain starts with the reforming process.A good compromise
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Fig.4.Effect of cell density/geometry surface area for CH4steam reform-
ing.
Table1
Activities of various monolithic CPO catalysts for methane(SV=58,000hÀ1)
Catalyst CPO-A CPO-B CPO-C CPO-D PM loading(g/L)0.9 1.6 2.9 1.4
Pt/Pd/Rh:Mixture A Mixture A Mixture B Mixture C O2/C0.580.580.580.58
Inlet(8C)493490486491 Outlet(8C)787780781772
Dry gas composition
H236.837.237.437.0
O20.00.00.00.0
N242.942.642.442.7
CH40.80.60.50.7
CO18.218.418.618.4
CO2 1.3 1.2 1.1 1.2
CO/(CO+CO2)93.693.894.393.7
%CH4conversion97.297.998.197.5
H2space–time yield(L-H2/h/L-catalyst)27000275002760027200
between the efficiency of SR and the small reactor volumes of CPO is autothermal reforming.
3.1.3.Autothermal reforming
The ATR reformer is a reactor that utilizes the heat generated by the exothermic CPO reaction to supply the heat required for the endothermic SR reaction.With PM,CPO and SR catalysts under the proper operating condition,the SR and water gas shift(WGS)reactions can reach thermodynamic equilibrium and the outlet dry gas composi-tion can be predicted by thermodynamic equilibrium calculations[25,26].Since monolithic Pt/Pd CPO and Pt/ Rh SR catalysts have been demonstrated to be efficient in producing CO and H2from various hydrocarbons fuels such as methane,LPG,kerosene,gasoline,diesel,and jet fuel,the catalyst used in this study were all monolithic Pt/Pd/Rh catalysts.
Thefirst-generation ATR catalysts were designed based on the sequential reaction zone concept.Second-generation catalysts were designed based on the overlapped reaction zone concept[25,27,28].For thefirst-generation catalysts,a CPO-only catalyst monolith was followed immediately downstream by a separate SR-only catalyst monolith.For the second-generation catalysts,the CPO washcoat layer is deposited directly on top of the SR washcoat layer in a single monolith.Separate layers are deposited to separate catalyst components that may negate each other.Another advantage of the two layers is that there is no heat transfer barrier between the two layered reaction zones.The heat generated by the CPO layer is directly and effectively transferred to the SR layer.Furthermore,the higher the CPO catalyst temperature,the higher the SR catalyst temperature,which in turn increases the SR reaction rate and takes more heat away from the CPO layer.For this reason,the maximum gas temperature for the second-generation ATR catalyst was found to be about508C below that of thefirst-generation ATR catalyst under identical operating conditions[27].The second-generation ATR catalyst inherently integrates and improves heat management and can potentially improve its thermal durability and the catalyst life.
Table2compares the hot efficiency for four catalysts under ATR,SR and CPO operation modes at the inlet temperatures of500and5758C.The relative activities of these four catalysts were:Pt/Pd/Rh-ATR>Rh>Pt>Pd. As can be seen by comparing the activities of the ATR,SR and CPO modes of operation,the majority of the hot efficiency for each catalyst was obtained mainly from the CPO reaction.The results reinforce that the CPO reaction has a much faster reaction rate than that of the SR reaction.
3.1.3.1.Effect of sulfur on reforming activity.One concern in the standard industrial production of hydrogen is the exposure of Ni steam reforming catalysts to sulfur species that can irreversibly poison the catalyst.In comparison,here the effect of sulfur present in natural gas on PM monolith ATR activity was studied.As shown in Fig.5for catalyst ATR-A,CP grade methane without sulfur in the feed gas was used for the ATR reaction at O2/C=0.47,H2O/C=2.5,space velocity at55,000hÀ1and inlet temperature of5008C,and the CH4conversion was measured to be about87%.When sulfur-free methane was replaced by commercial natural gas,which contained about2ppm of total sulfur,the CH4 conversion gradually decreased and reached a new steady-state value of80%.When the commercial gas was passed through a sulfur adsorbent trap(Engelhard’s CNG-1)to remove sulfur species except for COS,the CH4conversion gradually recovered and returned to86%.By using or bypassing the sulfur trap,the CH4conversion could be cycled between about80and86%as shown in Fig.5. Therefore,it was concluded that the sulfur in the feed gas would temporarily inhibit the ATR reaction,but the activity inhibition was reversible and could be removed easily by using a desulfurized gas feed.
T.Giroux et al./Applied Catalysis B:Environmental56(2005)95–110101 Table2
Activity data of PM monolith catalysts in ATR,SR,and CPO modes
Catalyst Test mode
ATR SR CPO ATR SR CPO Inlet temp.(8C)500500500575575575 CH4Conversion(%)
1%Pt/Al2O383.70.380.587.90.780 1%Rh/Al2O388.1 4.879.591.39.178.1 1%Pd/Al2O371.8076.376.50.672.7 Pt/Pd/Rh ATR catalyst A917.88492.81483.8 H2space time yield(L-H2/h/L-catalyst)
1%Pt/Al2O320600300161002150090016000 1%Rh/Al2O32160042001610022400730015800 1%Pd/Al2O31460050154001620050014400 Pt/Pd/Rh ATR catalyst A2290060001630023600830016700 CH4conversion=(CO+CO2)/(CO+CO2+CH4+2C2H6);ATR mode=feed of air,CH4,and H2O at GHSVof54,000hÀ1,molar ratios:H2O/C=1.9,O2/C= 0.46;SR mode=feed of CH4and H2O at GHSVof54,000hÀ1,molar ratios:H2O/C=1.9;CPO mode=feed of CH4and air at GHSVof33,000hÀ1,molar ratios: O2/C=0.46.。