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Methane steam reforming in laboratory scale Farinha, J.F. [email protected] Instituto Superior Técnico, Universidade Técnica de Lisboa Abstract This paper presents experimental results of methane steam reforming, at high temperatures, on a laboratory scale reformer using a sample from a commercial catalyst based on Nickel. Several parameters were changed, such as SCR (Steam Carbon Raio), temperature, SV (Space Velocity) and reactor configuration to optimize the conditions for reforming, including the conversion and selectivity. SCR was changed from 1.5 to 4, with the lower values (below 2) showing carbon deposition, while the best conversion was obtained at 2.5 and 3. The increase of SV led to a reduction of conversion, although hydrogen generation always increased. The selectivity increased with space velocity suggesting the catalyst includes elements favoring the formation of CO 2 . The average reformer temperatures tested ranged from 690 ºC to 813 °C and SV since the 10000 h -1 to 50000 h -1 . The temperature at the inlet was slightly higher than the furnace set point while the outlet value was about 250ºC lower than the inlet. The increase of temperature had an impact in increasing conversion and lowering selectivity as expected. The activation energy of the apparent reaction rate was estimated as 95 kJ/mol in the range of literature values. Tests were performed using two different heights of the reactor (5 and 10 cm) showing a small advantage of the later for a constant SV. Some tests were also carried out for a reactor in layers of sand and catalyst with a total height of about 10 cm leading to the best conversion compared with the other reactors. Using this procedure it is possible to obtain good results by reducing the loading of catalyst by 50% 1 – Introdution Hydrogen has been listed as a strong candidate as energy carrier, contributing to solve the current energy and CO 2 problem. Recently a strong interest has arisen in using H 2 based fuel cells as future source of energy conversion due to the high conversion efficiency of hydrogen energy to electricity in small scale as well as not having emissions of pollutant gases [1]. The production of hydrogen in the long term may be derived from non CO 2 emitting technologies, such as electricity from renewable energy sources or from nuclear power. In the shorter term the introduction of hydrogen relies on the use of the cheaper method to produce it that is conversion from a fossil fuel. There are several processes or a combination of up to five main reactions: four independent: Partial Oxidation (PO) catalytic or not (CPO- Catalytic PO); Steam Reforming (SR) Water Gas Shift (WGS) reaction; PReferential OXidation 1

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Page 1: Methane steam reforming in laboratory scale · Methane steam reforming in laboratory scale Farinha, J.F. zefarinhha26@hotmail.com Instituto Superior Técnico, Universidade Técnica

Methane steam reforming in laboratory scale

Farinha, J.F. [email protected]

Instituto Superior Técnico, Universidade Técnica de Lisboa

Abstract

This paper presents experimental results of methane steam reforming, at high temperatures, on

a laboratory scale reformer using a sample from a commercial catalyst based on Nickel. Several

parameters were changed, such as SCR (Steam Carbon Raio), temperature, SV (Space

Velocity) and reactor configuration to optimize the conditions for reforming, including the

conversion and selectivity.

SCR was changed from 1.5 to 4, with the lower values (below 2) showing carbon deposition,

while the best conversion was obtained at 2.5 and 3. The increase of SV led to a reduction of

conversion, although hydrogen generation always increased. The selectivity increased with

space velocity suggesting the catalyst includes elements favoring the formation of CO2.

The average reformer temperatures tested ranged from 690 ºC to 813 °C and SV since the

10000 h-1 to 50000 h-1. The temperature at the inlet was slightly higher than the furnace set

point while the outlet value was about 250ºC lower than the inlet. The increase of temperature

had an impact in increasing conversion and lowering selectivity as expected. The activation

energy of the apparent reaction rate was estimated as 95 kJ/mol in the range of literature

values.

Tests were performed using two different heights of the reactor (5 and 10 cm) showing a small

advantage of the later for a constant SV. Some tests were also carried out for a reactor in layers

of sand and catalyst with a total height of about 10 cm leading to the best conversion compared

with the other reactors. Using this procedure it is possible to obtain good results by reducing

the loading of catalyst by 50%

1 – Introdution Hydrogen has been listed as a strong candidate as energy carrier, contributing to solve the

current energy and CO2 problem. Recently a strong interest has arisen in using H2 based fuel

cells as future source of energy conversion due to the high conversion efficiency of hydrogen

energy to electricity in small scale as well as not having emissions of pollutant gases [1]. The

production of hydrogen in the long term may be derived from non CO2 emitting technologies,

such as electricity from renewable energy sources or from nuclear power.

In the shorter term the introduction of hydrogen relies on the use of the cheaper method to

produce it that is conversion from a fossil fuel. There are several processes or a combination of

up to five main reactions: four independent: Partial Oxidation (PO) catalytic or not (CPO-

Catalytic PO); Steam Reforming (SR) Water Gas Shift (WGS) reaction; PReferential OXidation

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Page 2: Methane steam reforming in laboratory scale · Methane steam reforming in laboratory scale Farinha, J.F. zefarinhha26@hotmail.com Instituto Superior Técnico, Universidade Técnica

of CO (PROX) and a fifth: metanation [2]. PO has the advantage of being thermally self

sustained but it has a small yield of hydrogen. PO combined with steam injection and WGS

leads to a process called Auto-Thermal steam Rreforming (ATR).

There are several options regarding the fuel to produce hydrogen, the process being simpler

firstly for methanol owing to the lower temperature and then for hydrocarbons and in particular

for natural gas. in this case at high temperature (and pressure).The methane steam reforming is

the leading method of hydrogen production in the world [3], owing to the great volume of natural

gas, consisting mainly of methane, produced annually. Steam reforming is outlined in Eqs. (2)

and (3) [4]:

CH4 + H2O CO + 3H2, ΔH = +206 kJ/mol (1)

CO + H2O CO2 + H2, ΔH = -41.2 kJ/mol (2)

The mechanisms for steam reforming [5] consider that in parallel with reaction (2) the reforming

into carbon dioxide may also occur in parallel:

CH4 + 2 H2O CO2 + 4H2, ΔH = +165 kJ/mol (3)

In this case the water gas shift reaction (3) may occur in both directions.

The reforming process is highly endotermic and usually takes place in a tubular reactor at a

temperature in the range of 750 to 850ºC. For a nickel catalyst, one of the main problems is

coke formation that is avoided by increasing the steam carbon ratio. The control of temperature

in the reactor is another important factor and the removal of sulfur compounds, present for

instance in odorants used for natural gas.

Natural gas on an industrial scale is the most used method for the generation of hydrogen

today. The reformers consist of industrial furnaces where low quality fuels can be burned

generating heat to reformer tubes where the catalyst is. The tubes, are arranged in regions of

the furnace which can produce temperatures of operation close to ideal values around 750 °C.

In order to reduce the scale of the reformers for a decentralized production, several prototypes

and pre-series commercial reformers are currently being developed for productions lower than

50 kg/day. A range of applications can be identified for the application of compact reformers [2].

The purpose of this work is to test a laboratory scale test reactor and to analyze the influence of

the operating conditions and optimize them. The characterization is made for different operating

conditions, calculating the main parameters of reformation, such as selectivity and conversion.

Several test results are available in the literature regarding the influence of the operating

conditions. Lee et al [5] present the variation of conversion with temperature and SCR, showing

that the results follow thermodynamic equilibrium, possibly due to the low space velocity used

(5000 h-1). Results showing the influence of space velocity available from the supplier of the

catalyst used in this work are presented in Ventura [8] showing a decrease of methane

conversion increasing the space velocity from 30000 to 125000 h-1.

The aim of the present work is to test a reformer laboratory reactor and to evaluate and draw

conclusions about the influence of operating conditions, including the SCR, average reformer

temperature and space velocity in the main performance parameters of reforming (selectivity

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Page 3: Methane steam reforming in laboratory scale · Methane steam reforming in laboratory scale Farinha, J.F. zefarinhha26@hotmail.com Instituto Superior Técnico, Universidade Técnica

and conversion). To compare the results with literature the activation energy of the apparent

surface reaction rate is also evaluated.

Following this introduction, section 2 presents the experimental test section and the procedures

used in the experiments. Section 3 presents the results and show the influence of the operating

conditions and the reactor configuration. Section 4 presents the main conclusions.

2 – Experimental reactor A schematic diagram of the SR experimental set-up used in this work is presented in Fig. 1.

This experimental installation is suitable to carry out thermal treatments with controlled gas

composition. To prepare the catalysts, the reactor can be used for drying or calcination and

during catalyst use it can be used to oxidize deposited carbon or to reduce the catalyst.

The reactor is suitable to test any of the reactions used in fuel processing below 1200ºC. For

auto-thermal reforming oxygen can be added in parallel with steam and methane. For methane

steam reforming a nickel based catalyst is used working at a temperature around 750 ºC and

pressure up to ten bar in the test section.

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Figure 1 - Schedule of the installation: 1 - Bomb; 2 - evaporator, 3 - Oven and tubular reactor inside, 4 -

Cooling system (including a condenser and heat exchanger), 5 - balance and deposit of condensed water,

6 to 9 - Controlling flow Gas: 6 - Oxygen, 7 - Natural gas or methane, 8 nitrogen, 9 Hydrogen, 10 -

Controller flow LPG.

The installation allows the control the flow rate of four gas through the flow controllers on the

lines indicated 6 to 9. Three of the flows are mixed upstream of the evaporator (2), while the

flow of oxygen (6) is mixed after the humidifier, before the entry of the reactor. LPG can also be

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Page 4: Methane steam reforming in laboratory scale · Methane steam reforming in laboratory scale Farinha, J.F. zefarinhha26@hotmail.com Instituto Superior Técnico, Universidade Técnica

added through a specific inlet with a drain for excess liquid. To feed liquid fuels at working

pressure these can be mixed with water if compatible with the pump materials.

Water is supplied to the evaporator that operates at 200 °C and the steam is mixed with the gas

mixture (with the exception of oxygen). The mixture with steam may well reach the reactor

without intermediate condensation and then is heated in the oven depending on the operating

conditions. The oxygen is mixed only after the mixture is moistened to avoid the presence of

flammable mixtures in the evaporator or blender.

The reactor tube can be easily removed from inside the furnace where it is positioned through

plugs assembled under pressure. Two thermocouples can be inserted in the reactor tube, one

before and another after the catalyst bed to monitor temperature. The furnace temperature is

controlled by other thermocouples installed in the furnace connected to the control system.

The reaction products are dried by cooling them down to about 20ºC using a Peltier cooler and

the condensed water falls by gravity to a tank in a precision balance. This tank is emptied when

the level reaches a high value. Initially the pump circulates a large water flow to ensure that the

pipes connecting are full of water. The steam water that is dragged with the products can also

be detected in the gas analysis.

The operating pressure of the reactor is governed by a controller installed upstream of the gas

meter. This gas meter is used to monitor the gas products flow rate. There is a sample line

available to connect directly to a gas chromatograph and a main line for the products exhaust.

In the present work gas samples were taken every ten minutes during the tests in a syringe

through a membrane mounted in a T-connection to the main gas products exit line.

Most of the tests were preceded by a reduction treatment of the catalyst and when the previous

case had a small SCR an oxidation was also performed in advance. The furnace was then

heated up to the specified temperature keeping a small nitrogen flow through the reactor. The

experiments were then started by fixing the inlet flows and were carried out during 150 minutes.

For each test 15 gas samples were collected and injected in the gas chromatograph. The

manual procedure introduced some air leakage in the samples and these were detected in the

analysis and removed to normalize the gas composition from the reactor. When the total volume

detected by the gas chromatograph differed more than 10% from the syringe volume the

analysis was disregarded. The values presented in the experimental results are the average of

the analysis validated. Based on the measured gas composition two parameters were defined

that are the conversion (C) and selectivity (S) according to the following equations:

( )COCOCHCH XXXXC ++−=244

1 (4)

( )22 COCOCO XXXS += (5)

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Page 5: Methane steam reforming in laboratory scale · Methane steam reforming in laboratory scale Farinha, J.F. zefarinhha26@hotmail.com Instituto Superior Técnico, Universidade Técnica

3 - Experimental Results A total of 25 tests were performed, most with a reactor height of 5 cm and SCR=3, while

nominal reactor temperature was changed from 800 to 900ºC and SV from 10000 to 50000. The

space velocity was defined based on the inlet flow at normal conditions and the volume of the

catalytic section of the reactor. Besides the height of 5 cm tests were carried out doubling this

height or by keeping the 5 cm of catalyst but putting the catalyst particles divided by

intermediate layers of inert (quartz sand).

Due to the endothermic reaction temperature in the reactor decreases and in general the inlet

temperature was about 30 ºC higher than the nominal and at the outlet about 220 ºC lower than

the nominal, that is there was a temperature reduction of 250 ºC. The average temperature is

therefore about 100 ºC lower than the nominal furnace temperature but as most reactions

should occur in the initial part of the reactor where temperature is higher the data is always

presented as a function of the nominal temperature. These differences in temperature are not

only a result of the reaction zone as well as effects from the furnace design. The reactor is

located in the second half of the furnace and hence there is some temperature gradient due to

end effects.

Figure 2 shows the conversion obtained in tests as a function of the SV, with the results

grouped by different values of SCR, nominal temperature and different constitutions of the fixed

bed reactor.

Figure 2 - Conversion according to SV, with the results grouped into different values of SCR, the oven

temperature and configuration of the reactor

Figure 2 shows as expected that the conversion increases with temperature, because the rate

of reformation of methane has activation energy of very high and therefore is highly affected by

temperature. The values of certain conversion rates for the smallest space velocities (and

longer times of residence) are closer to the conditions of equilibrium.

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Page 6: Methane steam reforming in laboratory scale · Methane steam reforming in laboratory scale Farinha, J.F. zefarinhha26@hotmail.com Instituto Superior Técnico, Universidade Técnica

Changing the SCR from 3 to 2.5 has some influence but within the range of variability of the

results and therefore no firm conclusion can be made. Further tests performed with SCR from

1.5 to 4 keeping the inlet flow rate for methane constant and a constant temperature of 850ºC

showed lower values of conversion for the lower SCR which can be partly explained by the

deposition of carbon in the catalyst particles observed after these tests. For the higher SCR the

conversion also decreased but in this case this may be an influence of the larger space velocity.

Increasing the length of the reactor, keeping the space velocity constant led to a small

improvement of the conversion. This can be attributed to the larger area of heat transfer and an

increase of the heat transfer between the gas and bed, despite the single value of temperature

measured after the reactor being only slightly higher. If the comparison is made for similar inlet

flow rates of course the improvement is much more significant.

Using the reactor with several layers of sand and particles of catalyst, the conversion is higher

and has a smaller influence of SV changed from 24400 to 48900 h-1. The SV was defined based

on the volume of the catalyst, while if it was based on the overall volume SV would be from

9400 to 18800 h-1, showing still an improvement over the other cases. Although only two tests

were carried out for this configuration the results suggest a good improvement in conversion.

This is possibly due to the heating and temperature redistribution when the gases cross the

inert layers, leading to larger temperature at the entrance of the following catalyst layer in

accordance with [5]. In this configuration compared with the catalyst bed (H) of 10 cm it can be

observed that catalysts loads of less than about 50%, can achieve higher levels of conversion.

Figure 3 shows the selectivity as a function of SV, with the results grouped into different values

of SCR, nominal temperature and different constitutions of the fixed bed reactor, as above.

Figure 3 - Selectivity according to SV, with the results grouped into different values of SCR, the oven

temperature and configuration of the reactor

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Page 7: Methane steam reforming in laboratory scale · Methane steam reforming in laboratory scale Farinha, J.F. zefarinhha26@hotmail.com Instituto Superior Técnico, Universidade Técnica

Figure 3 shows that in general the selectivity increases with SV and decreases increasing

temperature, the opposite behaviour of conversion. The effect of temperature can be explained

by thermodynamics that favours the formation of carbon monoxide at higher temperature,

lowering selectivity. Considering an average temperature it can be observed that the value of

selectivity observed in the experimental trials is larger than the equilibrium value. The catalyst

may contain some elements which encourage the formation of CO2 but when the residence time

is longer (smaller SV) this species can form CO, decreasing the selectivity.

Tests conducted at SCR = 2.5 show in figure 2 a slight decrease of selectivity, compared with

tests carried out on SCR = 3, due to the influence of the reaction of displacement of water.

Results obtained for a larger range of SCR keeping the methane flow rate constant show a

much larger increase of selectivity with SCR due to the larger water content as well as the larger

SV that is found to increase selectivity.

The selectivity has a small decrease in selectivity increasing the height of the reactor zone, that

is the opposite tendency observed for the conversion and similar behaviour is observed for the

layered reactor. This may be justified by the increase of the local temperatures.

Figure 4 shows the flow of hydrogen produced as a function of SV and the results were grouped

by nominal temperature of the reactor and SCR as above.

Figure 4 – Flow of hydrogen produced according to SV, with the results grouped into different values of SCR and temperature of the oven

From figure 4 it appears that the production of hydrogen always increases with the increase SV

and consequently with the reactants flow, despite the reduction of conversion because they are

associated with smaller residence times. It can also be observed in figure 4 that the hydrogen

production increases with temperature due to the effect of temperature in the kinetic rates. The

modification of SCR from 3 to 2.5 has a small impact in the production of hydrogen.

Interpretation of results with simplified models

Despite the results do not meet isothermal conditions and the composition of the catalyst is not

known in detail, the results were used to calculate reaction rate constants and the influence of

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Page 8: Methane steam reforming in laboratory scale · Methane steam reforming in laboratory scale Farinha, J.F. zefarinhha26@hotmail.com Instituto Superior Técnico, Universidade Técnica

temperature is analysed., Estimative values of axial diffusion led to the conclusion that even for

the smaller flow rates the reactor is well represented by a plug flow reactor. Using this

simplification and assuming the reforming reactions to be first order in methane, allowed the

calculation of rate constants for the methane consumption (parallel in reactions 1 and 3). In

reality the rates also depend on the concentration of water vapour and the approach to

equilibrium conditions. Since there is always an excess of steam and the conversion is far from

complete, the use of a first order rate was considered a good approximation.

The rate constants determined were compared with mass transfer limitations outside the

particles, leading to the conclusion that the later has a small influence and therefore the rate

constants are characteristic of the apparent kinetics on the outer surface of the particles.

Figure 5 presents the calculated values as a function of the reciprocal of temperature. The

values obtained from the slopes allow to estimate the activation energy of 95 and 240 kJ / mol

for the two sets of values of SCR 3 and 2.5 respectively. Since most data was obtained for

SCR=3 the value of 95 kJ/mol is taken as more representative,, which is of the order of the

value obtained with mass transfer limitations within the particles (66 kJ/mol [6]). The value of the

true activation energy from the intrinsic reforming reactivity is larger 241 kJ/mol [7]..

Figure 5 - ln(K) as a function of the reciprocal of temperature (1/T).

4 – Conclusions

From this study it was found that the best oven temperature for operation is 850 °C, as the it

provides a good conversion and still high selectivity. Larger temperatures although may

increase conversion require more thermal input.

For the reactor with a fixed bed catalyst of 5 cm the best operating conditions are:

• Temperature of the furnace = 850 ° C;

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Page 9: Methane steam reforming in laboratory scale · Methane steam reforming in laboratory scale Farinha, J.F. zefarinhha26@hotmail.com Instituto Superior Técnico, Universidade Técnica

• average temperature of reformation = 750 C (approximately);

• Space velocity = 42000 h-1 (approximately);

• SCR = 3.

The corresponding values for the conversion and selectivity of 78% and 42% respectively.

The influence of the operating conditions as expected show an improvement of conversion,

increasing temperature and reducing space velocity and in general the improvement of

conversion degrades selectivity.

Comparing the values of conversion and selectivity changing the reactor configuration it is

concluded that increasing that there is a marginal increase in conversion doubling the reactor

height, keeping the space velocity. The use of the catalyst in layers compared with the same

amount of catalyst in a continuous reactor shows a large improvement in conversion. Even

comparing similar volume reactors, one in layers partly filled with inert and the other only with

catalyst, the layered reactor increase slightly the conversion, using around 50% of the catalyst,

and continuing to produce high levels of selectivity.

The reaction rate shows a dependence in temperature that can be interpreted reasonably with a

first order kinetic rate, whose activation energy (95 kJ/mol) is found to be of the order of

literature values for apparent kinetic rates.

Nomenclature: SCR Steam Carbon Ratio

SV Space velocity [h-1]

T Temperature [K ]

K Rate of reaction [kg/m2s]

CPO Ctalytic partial oxidation

r Rate of reaction of methane [kg/m2s]

SR Steam reforming

hm Coefficient of mass transfer of [m /s]

WGS Water gas shift

PROX Preferential oxidation of CO

H height of the fixed bed of catalyst

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Page 10: Methane steam reforming in laboratory scale · Methane steam reforming in laboratory scale Farinha, J.F. zefarinhha26@hotmail.com Instituto Superior Técnico, Universidade Técnica

References:

[1] Chen L., Hong Q., Lin J., Dautzenberg F.M., (2007), “Hydrogen production by coupled

catalytic partial oxidation and steam methane reforming at elevated pressure and temperature.”,

Journal of Power Sources 164 (2007) 803-808.

[2] Qi, A.; Peppley, B. e Karan K. (2007), “Integrates fuel processors for fuel cell application: A

review”, Fuel processing technology 88 (2007) 3-22.

[3] Dias J. A. C. e Assaf J. M. (2004), “The advantages of air addition on the methane steam

reforming over Ni/ʏ-Al2O3.”, Journal of Power Sources 137 (2004) 264-268.

[4] Fonseca A., Assaf E. M., (2004), “Production of the hydrogen by methane steam reforming

over nickel catalysts prepared from hydrotalcite precursors”, Journal of Power Sources 142

(2005) 154-159.

[5] S. Lee, J. Bae, S. Lim, J. Park, (2008), “Improved configuration of supported nickel catalysts

in a steam reformer for effective hydrogen production from methane”. Journal of Power Sources

180 (2008) 506-515.

[6] Akers (1955), W.W. and Camp, D.P. AIChE Journal, Vol. 1 (4): 471-475, “Kinetics of the Methane-steam Reaction”.

[7] Xu and Froment (1989) Methane Steam Reforming, Methanation and Water-Gas Shift: I. Intrinsic Kinetics, AIChE Journal 35 (1):88-96.

[8] Ventura, C. (2008), “Modelling of a reformer with integrated burner”, tese de mestrado.

Instituto Superior Técnico, pág. 6:8 e 71:72.

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