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Pre-reforming of natural gas on a Ni catalyst
Applied Catalysis A: General 282 (2005) 195–204 www.elsevier.com/locate/apcata
Pre-reforming of natural gas on a Ni catalyst Criteria for carbon free operation Thomas Sperle a,1, De Chen a, Rune Lødeng b, Anders Holmen a,* a
Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway b SINTEF, Applied Chemistry, N-7465 Trondheim, Norway Received 20 July 2004; received in revised form 8 December 2004; accepted 10 December 2004
Abstract Pre-reforming of natural gas has been studied on a nickel catalyst at 480–550 8C and 20 bar using a TEOM (Tapered Element Oscillating Microbalance) reactor. The focus has been on carbon formation and the main objective was to study the influence of C2–C3 hydrocarbons in the methane feed on carbon deposition on the catalyst. Coking thresholds for different mixtures of hydrocarbons were determined by varying the steam to carbon (S/C) ratio at various temperatures. The steady-state coking rate decreases with increasing S/C ratio, and increases with increasing carbon number of the hydrocarbon. Unsaturated hydrocarbons show a strong effect on coking rates and on carbon thresholds. For mixtures of methane and propane/propene, the steady-state coking rate as well as the coking threshold decreased with a decrease in the mole fraction of propane/propene and with an increase in the hydrogen mole fraction. The coking rate revealed a complicated temperature dependency, and a minimum in the coking rate and in the coking threshold was detected at 500 8C. A relationship between the critical steam to carbon ratio and the thermodynamic carbon activity is also developed based on a suggested reaction mechanism, which can properly predict the role of hydrocarbon, hydrogen and water in carbon formation. # 2004 Elsevier B.V. All rights reserved. Keywords: Carbon deposition; Steam reforming; TEOM; Carbon threshold; Nickel catalyst; Natural gas; Pre-reforming conditions
1. Introduction Steam reforming on Ni catalysts in tubular reformers is the main process for production of synthesis gas and hydrogen from natural gas. Although methane is the main component of natural gas, the gas usually also contains higher hydrocarbons [1]. The higher hydrocarbons can be converted to methane and carbon oxides over a nickel-based catalyst in adiabatic pre-reformers at lower temperatures, normally 400–550 8C [1–3]. The potential for carbon formation of the feed to the primary reformer is thereby decreased. Traces of sulphur can also be selectively adsorbed in the pre-reformer. Feed flexibility is improved in the plant, the temperature can be raised and the steam to * Corresponding author. Tel.: +47 735 941 51; fax: +47 735 950 47. E-mail address: [email protected] (A. Holmen). 1 Present address: Statoil R&D Centre, Postuttak, N-7004 Trondheim, Norway. 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.12.011
carbon (S/C) ratio can be lowered in the inlet gas to the tubular reformer. In pre-reforming, light hydrocarbons are converted to methane, hydrogen and carbon oxides (1). The carbon monoxide is hydrogenated to methane (2), and the water-gas shift equilibrium (3) is usually established: m Cn Hm þ nH2 O ! nCO þ n þ < 0 (1) H2 DH298 2 ¼ 206 kJ=mol (2) CO þ 3H2 ¼ CH4 þ H2 O; DH298 CO þ H2 O ¼ CO2 þ H2 ;
DH298 ¼ 41 kJ=mol
(3)
Steam reforming of hydrocarbons (1) higher than methane is irreversible reactions [2,4]. Pre-reforming catalysts have a high risk of carbon formation and of catalyst deactivation. The catalyst deactivation is mainly caused by poisoning, carbon deposition (including gum formation) and also by sintering. Sulphur is the most serious poison in the hydrocarbon feed and it should be removed to a low level in a desulphurisation unit before the pre-reformer. Poisoning by
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sulphur has been extensively studied [5–9]. Sintering of nickel crystallites has generally not been considered as a severe problem at pre-reforming conditions, although sintering of nickel is reported to take place [10,11]. Carbon deposition is then the most critical parameter for selecting operating conditions for pre-reforming. Carbon formation on the catalyst will take place by the following reactions: CH4 ¼ C þ 2H2 ; DH 298 ¼ 75 kJ=mol m Cn Hm ¼ nC þ H2 2 2CO ¼ C þ CO2 ; DH 298 ¼ 172 kJ=mol
(4) (5) (6)
Cracking of hydrocarbons by reaction (4) and (5) and the Boudouard reaction (6) produce adsorbed carbon atoms on the catalyst surface. Surface carbon may form coke by encapsulating the surface, or by producing carbon filaments [12]. A selvedge with high carbon concentration is formed at the front of the nickel particle. Carbon then diffuses through the nickel, and precipitates on the support side of the particle. The process is driven by a concentration gradient, caused by higher solubility of carbon on the gas side than on the support side of the nickel particle. Filamentous carbon will eventually break down the catalyst, but it does normally not strongly affect the number of active nickel sites. Catalyst activity can be lost when filamentous carbon is blocking pores on the catalyst [13]. The feed composition can also change with time in industrial production of synthesis gas from natural gas. A slight increase in the content of higher or unsaturated hydrocarbons can change the conditions for carbon free operation significantly. High steam to carbon ratios can be used to suppress carbon formation. However, pressurising and heating a large excess of water is expensive, and knowledge of the window for carbon free operation is economically meaningful. Criteria based on kinetics and on thermodynamics [14,15] are used for predicting the operating window. Significant differences in thermodynamic properties of carbon have been found for filamentous carbon, for graphite and for nickel carbide [3,16]. The equilibrium constant for filamentous carbon is between the one for graphite and the one for nickel carbide. The difference in energy between whisker carbon and graphite is ascribed to surface energy for the carbon cylinder and to structural defects. Alstrup et al. [17] use the term equilibrium for the gas composition where carbon formation starts or stops. Snoeck et al. [12] prefer the term coking threshold for this condition, because diffusion cannot be considered a normal reversible surface reaction step. Thermodynamics are often derived from experiments at the coking threshold [12]. However, experimental results show that the actual critical steam to carbon ratio during steam reforming is normally higher than the one predicted by thermodynamics [18]. It has therefore been concluded that carbon formation is a question of kinetics and the local approach to the reforming equilibrium [18]. The critical steam to carbon ratio will then be a
function of the catalyst, the composition of the inlet gas and the operation conditions. Different techniques have been developed to evaluate the coking thresholds for reforming catalysts such as pseudoadiabatic fixed-bed reactors [1,5,11] and conventional microbalances [3,13,21]. In pseudo-adiabatic reactors, the temperature profile is measured as a function of the time on stream and deactivation is studied by measuring the movement of the temperature profile. However, the interpretation of the experimental data is not straightforward, since deactivation during steam reforming shows a complex behaviour. The conventional microbalance suffers from the insuperable problem of reactant bypass and kinetic data for both the main reaction and the carbon forming reaction is difficult to obtain [20]. Filamentous carbon will fill-up the void between the catalyst particles and gradually increase the bypass in the conventional microbalance as the experiment progresses [20]. However, the TEOM (Tapered Element Oscillating Microbalance) reactor has proved to be a powerful tool for studying deactivation and carbon formation simultaneously [20,21]. In a TEOM reactor all the reactants are forced to flow through the catalyst bed, thus eliminating the bypass problem and the TEOM reactor can be described as a fixed-bed reactor. The objective of this work has been to study the coking rates and the coking thresholds at pre-reforming conditions using a TEOM reactor. In the present study, steam is fed with methane and methane mixed with ethane/ethene or propane/ propene. The effect of temperature on the coking threshold and the potential for reducing the coking threshold by the addition of hydrogen is also addressed.
2. Experimental A model catalyst (UE200X) supplied by Haldor Topsøe was used in all experiments. The catalyst contains 18 wt.% Ni and is supported on an alumina–magnesia spinel. The catalyst properties are listed in Table 1. This study was performed in a TEOM reactor at a total pressure of 20 bar, and temperatures ranging from 480 to 550 8C. The TEOM set-up is described in detail elsewhere [20]. Seven to 15 mg of catalyst with particle size of 0.4– 0.6 mm was loaded into the TEOM reactor together with about 60 mg of MgAl2O4 spinel particles of the same size as diluent. The use of a diluent will reduce possible temperature gradients in the catalyst bed and also agglomeration of growing filamentous carbon, thus reducing Table 1 Properties of the UE200X model catalyst from Haldor Topsøe BET surface area (m2/g) ˚) Average pore diameter (A Ni dispersion (%) Metallic surface area (m2/g) 18 wt.% Ni supported on a alumina–magnesia spinel.
64.8 124 9.6 11.9
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Table 2 Reaction conditions, measured coking thresholds (S/Cmin), and calculated thermodynamic carbon activities (aC) based on Eq. (22) Mole ratio
Phydrocarbon (bar)
PH2 (bar)
T (8C)
S/Cmin
aC
Effect of carbon number 1 CH4 2 CH4/C2H6 3 CH4/C2H6/C2H4 4 CH4/C3H8
1 1/0.1 1/0.08/0.008 1/0.035
3.9 3.9 3.9 3.9
0.12 0.12 0.12 0.12
500 500 500 500
0.74 1.18 2.19 1.3
118.0 450.4 1570.6 589.6
Effect of C3/C1 ratio 6 CH4/C3H8/C3H6 7 CH4/C3H8/C3H6 8 CH4/C3H8/C3H6
1/0.007/0.003 1/0.032/0.01 1/0/0
3.84 3.96 3.85
0.12 0.11 0.12
510 510 510
2.5 3.4 0.86
640.1 3339.1 118.0
Effect of hydrogen 9 CH4/C3H8/C3H6 10 CH4/C3H8/C3H6 11 CH4/C3H8/C3H6
1/0.03/0.01 1/0.03/0.01 1/0.03/0.01
3.84 3.96 3.85
0.12 0.58 1.1
510 510 510
3.4 2.5 1.5
640.1 3339.1 118.0
No.
Mixture
the potential for pressure drop. The catalyst was reduced in H2/Ar = 1/1 at 550 8C after being heated from ambient to 550 8C at a rate of 2 8C/min. The maximum temperature was held overnight. During start-up, steam was added to the carrier flow of Ar in the presence of hydrogen to keep the catalyst from being oxidised. The hydrocarbons were then introduced and the argon flow was adjusted to obtain the desired partial pressure of hydrocarbons. The experiments started by using a relatively low steam to carbon ratio to reduce the induction period [12]. Thereafter, the steam to carbon ratio was increased stepwise, while the partial pressure of hydrocarbons was kept constant by adjusting the Ar flow. In order to obtain constant partial pressure of hydrocarbons when varying the steam to carbon ratio, the dilution by argon was varied slightly in this study. The coking studies were performed at pre-reforming conditions for feeds consisting of methane, methane/ethane, methane/propane, methane/ethane/ethene and methane/ propane/propene. In most experiments, the H2 level was 3.0% relative to CH4 although the effect of H2 was studied at 510 8C for a feed consisting of methane/propane/propene. The ethane/methane ratio was 0.1, the propane/methane ratio was 0.035, the ethane/ethene/methane ratio was 0.08/ 0.008/1 and the propane/propene/methane ratio was 0.03/ 0.01/1. The effect of carbon number on coke deposition was studied at a space time of hydrocarbons of 0.11 gcat h/mol, while the effects of propene to propane ratio as well as the effect of hydrogen were studied at a space time of 0.06 gcat h/mol. However, the space-time of hydrocarbons other than methane is quite large and varied for different mixtures due to only relatively small amount added into the mixtures. The partial pressure of hydrocarbons was kept constant at approximately 3.9 bar in most of the experiments, where Ar was used as balance. Experimental conditions are listed in Table 2. The reactor effluent was analyzed by online gas chromatography [20]. The conversion of methane was about 50% of equilibrium composition in most of the experiments. During pre-reforming methane is
both a product and a feed component, and the term methane conversion will refer to the relative amounts of methane in the feed gas and product gas, and is calculated as if methane was only a reactant.
3. Results and discussion Carbon formation from methane and methane containing propane and propane/propene has been studied at 20 bar, in the temperature range of 480–550 8C and at different steam to carbon ratios. Some experiments with methane containing ethane and ethane/ethene have also been performed. Based on previous work and experience an induction period is to be expected in the formation of filamentous carbon [13,21–23]. The initiation of carbon formation was provoked by using low steam to carbon ratios, the reason for this was to obtain a uniform and comparable initiations of filaments on the surface. Thereby, carbon whisker growth is obtained with relatively short induction periods. An example of a start-up period for the TEOM experiments is shown in Fig. 1. After an induction period lasting for about 2–3 h, there is a high affinity for carbon giving a very rapid carbon formation lasting for about 20 min. After the initial period with rapid
Fig. 1. Weight increase during the initial phases of an experiment with carbon formation from methane at 550 8C and 20 bar. Steam to carbon ratios for the first and second straight lines are 0.83 and 1.1, respectively.
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Fig. 2. Rate of carbon formation from methane containing propane and propene. O represents a fresh catalyst and X a precoked catalyst containing 100 wt.% carbon. Reaction conditions: T = 500 8C, Ptot = 20 bar, W/Fhydrocarbon = 0.06 gcat h/mol, pCH4 ¼ 3:8 bar and a ratio of CH4/C3H8/ C3H6 of 1/0.032/0.01.
carbon build-up, the reaction was studied using two different steam to carbon ratios, both giving straight lines for the weight increase with time on stream and no deactivation of the carbon formation was observed. Linear regression lines are calculated, and shown as thin lines in Fig. 1. Previous studies using TGA have shown that coking at 500 8C can proceed without deactivation [13] in the presence of C1 to C3 alkanes. This has been verified in the present work by measuring the rate of carbon formation at 510 8C on a fresh catalyst and on a pre-coked catalyst containing 100 wt.% carbon. The results are given on Fig. 2. The experimental data for steam to carbon (S/C) ratios of 1.5 and 2.0 reported in Fig. 2, are recorded at equal partial pressure of all gases. In spite of the large amount of carbon deposited on one of the catalysts, the agreement is very good indicating that carbon formed at these conditions is mainly filamentous carbon. For the experiments with pre-coked catalyst the partial pressure of methane was reduced as the rate of carbon formation approached zero. For S/C ratios of 3.3, 3.6 and 3.9, the partial pressures of methane were 3.45, 3.48 and 3.28 bar, respectively. For the fresh catalyst, the partial pressure of methane was held constant and equal to 3.8 bar. Fig. 3 shows the coking rate and the carbon thresholds at 500 8C for all the different feeds. The line for the methane/ propane/propene mixture in Fig. 3a is fitted from several data points as shown in Fig. 3b. The critical steam to carbon ratio at zero net coking rate is defined as the coking threshold in the present work. The values were obtained by extrapolation of the steady-state coking rate to zero. Fig. 3 shows clearly that the rate of carbon formation and the carbon threshold at 500 8C increase with the carbon number of the hydrocarbon added to the methane feed. However, when comparing these rates, it should be kept in mind that the ratio between the partial pressure of higher hydrocarbons and methane is not the same for C2 and C3, although the total pressure of hydrocarbon is identical for all the experiments. The experimental ratios of C2/C1 and C3/C1
Fig. 3. (a) Rate of carbon formation from different hydrocarbon mixtures. T = 500 8C, Ptot = 20 bar, Phydrocarbon = 3.9 bar, W/Fhydrocarbon = 0.11 gcat h/ mol, C1 (^), C1/C2 (&), C1/C3 (), C1/C2/C2=(~) and C1/C3/C3=(*); (b) rate of coke formation from methane/propane/propene (C1/C3/C3=(*)). Same conditions as in Fig. 3a.
are 0.1 and 0.035, respectively. The ratio has been selected to simulate the typical natural gas composition. Moreover, it has to be noted that the mole fraction of carbon atoms is different for each mole of the hydrocarbons with different carbon numbers. The corresponding mole ratios for C2/C1 and C3/C1 on a carbon atom basis are 0.2 and 0.105, respectively. Although the C3/C1 is much lower than C2/C1, the measured coking rate and coking threshold in the presence of C3 are higher than the ones in the presence of C2. It clearly indicates that the potential for carbon formation increases significantly with increasing carbon number. The presence of small amounts of olefins leads to a large increase in the coking rate and in the coking threshold and in particular the addition of propene increased the coking rate and the carbon threshold significantly (Fig. 3a and b). The effect of varying the ratio between C3 (propane/ propene) and methane was also studied. As shown in Fig. 4, the coking rate increased very much with an increase in the ratio between C3 (propane/propene) and methane. The ratio between propane and propene was held constant for the experiments reported in Fig. 4. Pre-reforming of methane gave conversions in the range of 3–5%. The selectivity to CO was in the range of 4–7% for most of the experiments. The GC analysis showed that even
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Fig. 4. Rate of coke formation from methane/propane/propene as a function of the amount of propane/propene added to methane. C3/C1 ratios varied at 510 8C, pCH4 ¼ 3:8 bar, steam to carbon (S/C) ratio = 2.5 and constant propene/propane ratio = 1/3.
traces of C2 could not be observed in the product gas when feeding propane/propene together with methane. For the experiments with C2 (ethane/ethene) in the feed, traces of C2 was always found in the reactor effluent. The concentration of C3 has not been measured. The conversion of C2 is generally larger than 80% and the conversion of C3 is expected to be even larger. It was impossible to operate at differential conditions, due to the fact that only small amount of C2 and C3 were fed into the reactor, giving a very large residence time for them. A large concentration gradient of C2 and C3 is expected, making it difficult to interpret kinetic data. Therefore, the reported coking rates are just average values. Based on the experimental results in the present work, it can be concluded that the carbon growth potential is the largest at the inlet, due to the high concentrations. From this argument, it is expected that the measured coking threshold be determined by the conditions at the inlet of the reactor. The reforming reactions are endothermic. Even though the amounts of C2 and C3 hydrocarbons are low relative to methane, their conversion is high. They will therefore definitely contribute to the heat consumption in the reactor. Temperature gradients over the film surrounding the catalyst pellets, and concentration gradients inside the catalyst pores may cause problems in the experiments. A worksheet, provided by Eurokin [24], employing heat- and massbalance equations and rules of thumb regarding external and internal concentration as well as temperature gradients was used. No significant gradients were found when only methane was fed, indicating the absence of external and internal transport limitations. For reforming of methane mixed with other hydrocarbons, a temperature drop over the film surrounding the pellet was calculated. Steam reforming of methane mixed with propane and propene gave the maximum value for this temperature gradient, almost 6 8C. This possible temperature gradient was not accounted for in the interpretation of the experimental data. Figs. 5 and 6 illustrate the effect of hydrogen. The effect of adding hydrogen to the reactant mixture on the resultant
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Fig. 5. Rate of coke formation from methane/propane/propene (1/0.03/ 0.01) at 510 8C as a function of H2/C at a steam to carbon (S/C) ratio of 1.5, Ptot = 20 bar, pCH4 ¼ 3:8 bar and W/Fhydrocarbon = 0.06 gcat h/mol.
carbon formation from methane/propane/propene mixtures was studied by varying the partial pressure of hydrogen at 510 8C. Two different approaches were used, either the partial pressure of H2 was increased at constant steam to carbon ratio or the steam to carbon ratio was increased at constant partial pressure of H2. Fig. 5 shows the coking rate and the carbon threshold as a function of the hydrogen to carbon (H2/C) ratio at a steam to carbon ratio of 1.5. In Fig. 6 the rate of carbon formation and the carbon threshold are shown as a function of the steam to carbon ratio at hydrogen to carbon mole ratios of 2.7 and 13.4. The results presented in Figs. 5 and 6 clearly show that adding hydrogen reduces the coking rate and the coking threshold significantly. These results are consistent with previous findings, where increased H2 content in the feed has been found to reduce the carbon formation [16,22,25–27]. The thermodynamic importance of hydrogen through reactions (4) and (5), affects the gasification of carbon and the carbon activity. Snoeck et al. [12] found the rate of gasification by hydrogen to pass through a maximum, suggesting that the hydrogen molecules compete for the same active sites as the carbon atoms segregating from the metal to the surface. It is likely that hydrogen also reduces
Fig. 6. Rate of coke formation from methane/propane/propene (1/0.03/ 0.01) at 510 8C as a function of steam to carbon (S/C) ratio at H2/C (mol/mol) = 2.7 (*) and 13.4 (^), Ptot = 20 bar, pCH4 ¼ 3:8 bar and W/Fhydrocarbon = 0.06 gcat h/mol.
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Fig. 7. Rate of coke formation from methane as a function of the steam to carbon (S/C) ratio for different temperatures. T = 480 (~), 500 (&) and 550 8C (^) . Ptot = 20 bar, pCH4 ¼ 3:9 bar and W/Fhydrocarbon = 0.11 gcat h/mol.
catalyst deactivation by gasifying encapsulants [25]. In a study by Lødeng et al. [21] dealing with decomposition of CH4, small amounts of hydrogen or water were found to increase the carbon formation. The effect of H2 was studied both in a conventional microbalance and in the TEOM. It was concluded [21] that H2 availability was rate limiting in the initial phase, and that moderate amounts of H2 increased the rate of carbon formation by lowering the apparent activation energy for carbon formation. Sidjabat and Trimm [25] observed carbon formation at 400 8C when adding less than 5% H2 to the hydrocarbon feed. At this temperature no carbon was formed without hydrogen in the feed. In Snoeck et al. [16] experiments, H2 enhanced carbon formation at very low hydrogen partial pressures and pCH4 ¼ 1:5 bar. The effect of low partial pressure of hydrogen on carbon formation has lead to the suspicion that hydrogen has a complex role in the formation of filamentous carbon. Nolan et al. [28] found closed carbon structures when no H2 was present in the formation of Boudouard carbon on nickel at 500 8C. H2 was essential for filament production. They suggested that the open-edged carbon structure of filamentous carbon was energetically unfavourable unless an agent, such as hydrogen, could satisfy the valences at the edges of the filaments. As the amount of hydrogen in the reaction gas was reduced from 1 to 0.03%, the morphology of the carbon
Fig. 8. Rate of coke formation from methane/propane/propene (1/0.03/ 0.01) as a function of the steam-to-carbon (S/C) ratio at different temperatures. T = 480 (), 490 (^), 500 (&), 510 (*) and 520 8C (~). Ptot = 20 bar, pCH4 ¼ 3:8 bar and W/Fhydrocarbon = 0.06 gcat h/mol.
Fig. 9. Coking thresholds in hydrocarbon pre-reforming: effect of temperature for methane (~) and for the mixtures of C1/C2/C2 = (1/0.08/0.008) (*) and C1/C3/C3 = (1/0.03/0.01) (^).
changed from predominantly filamentous to encapsulating. However, in the present work a sole decrease in the coking rate was observed with increasing amount of H2. This might simply be due to the relatively high amount of hydrogen used. In a study performed in a conventional microbalance [29], water was the most effective carbon-gasifying agent at 550–650 8C on nickel foils and on supported nickel. H2 was more effective than CO2. The total conversions of the hydrocarbons have been controlled at a relatively low level (<10%) in the present work. Therefore, the gasification from CO2 is less important. However, the effect of external addition of CO2 on carbon formation has to be investigated in the future in detail. The superiority of steam compared to hydrogen in carbon gasification is confirmed by other researchers [23]. The rates of carbon growth during methane reforming at different temperatures are shown on Fig. 7. As expected, the rate of carbon formation as well as the coking threshold increased with increasing temperature. Reforming of methane with ethane/ethene added to the feed was also performed at 480 and 500 8C. A larger growth rate was found at the lower temperature, but a high coking threshold was found at the high temperature. Carbon formation in mixtures of methane with propane/propene was studied at temperatures from 480–520 8C, and the results are given in Fig. 8. A complicated temperature dependency was observed as the results indicate negative apparent activation energy at temperature <500 8C. A minimum in the coking threshold and also in the rate of carbon formation from methane/propane/propene mixtures was observed at about 500 8C. The carbon thresholds for pre-reforming of methane and for mixtures of methane and C2 or C3 are plotted as a function of the reciprocal temperature in Fig. 9. A minimum is clearly observed at 500 8C for the methane/propane/ propene mixtures, while an increase in the coking threshold with temperature was observed for methane. The mixture of methane/ethane/ethene was not studied extensively at
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Fig. 10. Coking thresholds in hydrocarbon pre-reforming: effect of the partial pressure of propane/propene relative to methane at 510 8C. Ptot = 20 bar, pCH4 ¼ 3:8 bar and W/Fhydrocarbon = 0.06 gcat h/mol.
different temperatures, no conclusion can therefore be drawn. The effect of the amount of C3 relative to methane on the coking threshold was also studied and the results at 510 8C are shown in Fig. 10. The presence of small amounts of propane/propene increases the coking threshold quite dramatically. There are several reports in the literature of maximum rates of carbon formation with temperature [13,25,27]. Rostrup-Nielsen [13] observed that a temperature of 550– 575 8C gave a maximum rate of carbon formation when studying steam reforming n-heptane in a thermogravimetric system. Sidjabat and Trimm [25] observed a maximum in carbon formation from 1-butene and hydrogen at about 530 8C. Acetylene and toluene were also studied, and temperatures for maximum carbon formation were located at about 500 and 600 8C, respectively. It has been proposed that the apparent negative activation energy, and the selectivity to carbon for the different types of hydrocarbons, can be explained by the existence of surface intermediates containing more than one carbon atom [13]. At high temperatures, these intermediates will react to adsorbed
Fig. 11. Calculated carbon activities from Eq. (18) plotted to the observed minimum steam to carbon (S/C) ratio at 500 8C for all hydrocarbons involved in the study. Olefins (*) and paraffins (~) give two different straight lines.
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CHx. Rostrup-Nielsen and Trimm [27] stated that negative apparent activation energy could be adsorption effects, coupled with minor influence of gasification and selfpoisoning. The rate of carbon formation in these studies passes a minimum when the temperature was increased further and at these temperatures, gas phase carbon formation starts [25]. It is interesting that the minimum rate of carbon formation observed in the present study for reforming of methane mixed with propane and propene, is at a much lower temperature, about 500 8C. Some other explanation might therefore also exist for explaining the complex temperature dependency in the present work. Carbon formation by gas phase routes is very slow at low temperatures unless the feedstock has very high carbon affinity [4] or the gas phase residence time is high. It seems unlikely that a dramatic increase in carbon formation when the temperature is raised at 500 8C is caused by gas phase carbon. High total pressure will favour gas phase carbon, but dilution by argon and substantial amounts of water in the feed gas leads to the conclusion that carbon is formed on the catalyst. This is also supported by other studies [13]. A study on hydrogenolysis of propane, where propane reacts with H2 to form ethane and methane has revealed that the conversion of propane passes a maximum of about 400 8C on a Ni/MgAl2O4 catalyst [30]. Primary hydrogenolysis, yielding ethane and methane passed a maximum in ethane production at about 350 8C. These effects can influence the rate of carbon formation in mixtures containing higher hydrocarbons and hydrogen. Hydrogenolysis will lower the carbon number in the carbon forming hydrocarbons. If hydrogenolysis is important, a maximum in the rate of hydrogenolysis can lead to a minimum in the rate of carbon formation and coking threshold. Reported temperatures for the maximum rates and yields in hydrogenolysis [30] do not coincide precisely with the minimum for carbon formation in the present study. Feed composition, total pressure and catalyst formulation affect the rates and probably the location of maximum and minimum rates with temperature. However, hydrogen assisted cracking could have a significant effect on carbon formation from higher hydrocarbons and this is reflected in the proposed reaction scheme presented below (Eqs. (7)–(15)). Prediction of coking threshold for natural gas steam reforming has attracted much attention. Rostrup-Nielsen [15,19] has suggested a simplified reaction mechanism for whisker carbon formation from hydrocarbons, where hydrogen mainly influences the gasification of the surface carbon. Based on the mechanism, a steady-state activity has been derived [15], which was found to explain quite well the steam effect, but not the effect of hydrogen on carbon formation. The model predicts a positive effect of hydrogen. The reaction scheme is therefore modified, where the chain degradation by cracking of adsorbed hydrocarbons is substituted by hydrogen assisted cracking (Eq. (8)). At low temperatures, cracking of hydrocarbons (Eq. (5)) will be of less importance. In this simplified mechanism gasification
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of adsorbed carbon species takes place by reaction with adsorbed OH. The reversibility of the cracking of methane (Eq. (4)) is also taken into account in the scheme presented below. The role of hydrogen is described in Eqs. (8)–(10): k1
Cn Hm þ 2 !2 Cn Hy k2
2 Cn Hy þ H! CHx þ 2 Cn1 Hz k3
2 Cn Hy !n C þ y=2H2 k4 x CHx , C þ H2 2 k4 k5
C!½C; Ni bulk ! whisker carbon k6
CHx þ OH!gas k7
C þ OH!gas
(7) (8) (9) (10) (11) (12) (13)
K8 1 H2 O þ , OH þ H2 (14) 2 K9 1 H2 þ , H (15) 2 Reaction (14) is a simplified reaction step, where the equilibrium steps of steam and hydrogen adsorption/desorption, steam decomposition/recombination are combined. As mentioned above, carbon gasification from CO2 is relatively less important in the present work, which was not taken into account in the reaction mechanism. When deriving the steady-state carbon activity, it is assumed that carbon formation from adsorbed species with more than one carbon atom proceeds much faster than from adsorbed CHx: k3 k4, in accordance with the work by Rostrup-Nielsen et al. [19]. It is also assumed that hydrogenolysis is much faster than carbon formation from adsorbed carbon species: k2 k3. The last assumption is probably only valid at low temperatures and close to the carbon threshold, where the rate of carbon accumulation is very slow. The resulting steady-state carbon activity becomes: k 1 k 3 p Cn H m asc / ½C ¼ (16) ðxþ1Þ=2 k2 K 9 ½k7 K 8 pH2 O þ k4 pH2
Where [*C] is the concentration of surface carbon on Ni surfaces, pCn Hm , pH2 O and pH2 are the partial pressure of hydrocarbons, steam and hydrogen, respectively. This expression gives a steady-state carbon activity, which increases with the partial pressure of hydrocarbons ðpCn Hm Þ, and decreases with the partial pressures of water and hydrogen. If r1 > r2, there is a potential for accumulation of adsorbed CmH y. Eq. (16) is derived for a situation with no gum formation, r1 = r2. From a kinetic point of view, partial pressures of the hydrocarbon, water, and hydrogen affects the tendency for carbon formation through Eq. (16). It is well in line with the observed coking thresholds observed in the TEOM that H2 and H2O lower the carbon production, and that high content of the hydrocarbon increases the carbon potential. Olefins and aromatics adsorb easily, giving high rate constants for adsorption, k1, and
thereby increase the steady-state activity for carbon in Eq. (16). The equation derived from the present work for the steady-state activity can be rearranged to: pC n Hm (17) asc / ½C ¼ 0 k pH2 O þ k00 pxH2 The rate constants are lumped, with k0 ¼ k2 kk71Kk38 K 9 and k00 ¼ k2 kk74 K 8 K 9 , with rate constants from Eqs. (7–15). The thermodynamic carbon activity for a hydrocarbon can be defined according to Eq. (18), assuming that reactions (9) and (11) are at pseudo-equilibrium. 0 11=n p Cn H m aCn Hm ¼ n@K eq m=2 A (18) pH2 The potential for carbon formation from a hydrocarbon is an activity calculated on carbon basis. Keq is the equilibrium constant for cracking of the hydrocarbon by reaction (5), and n is the number of carbon atoms in the hydrocarbon. Higher hydrocarbons can produce more carbon per mole of gas, and this is taken into account by the term n in Eq. (18). In this study, the carbon potential for the mixture, or a total thermodynamic carbon activity, is calculated by summarising the individual activities: X aC ¼ yi aCn Hm ;i (19) i
The term yi, is not the molar fraction of component i in the reactant mixture, but should be treated as a hydrocarbon fraction: pC H y Cn H m ¼ P n m (20) i pCn Hm ;i In Fig. 11, the calculated activities for the different hydrocarbon mixtures at 500 8C are plotted to the experimental steam to carbon ratio at the carbon threshold. The olefins have a higher affinity to carbon relative to the paraffins than what is calculated from Eq. (18). The critical steam to carbon ratio can be expressed as: pH2 O ¼ aaC þ b (21) p Cn H m The value of a for the gas mixture containing olefins is higher than for the paraffin mixture. Thermodynamically, it is hard to account for the observed coking tendency of olefins compared to paraffins. The equilibrium constants for carbon formation from paraffins and olefins are of the same order of magnitude. From a kinetic point of view, it is easier, especially the rate of adsorption of olefins, k1 in Eq. (16), is much higher than that of paraffins. Considering Eq. (21), the constants a and b are hydrocarbon and catalyst specific. It is therefore possible to calculate a factor when calculating the thermodynamic potential for carbon formation from olefins. The equation is modified by a factor of kolf in Eq. (22) to give a corrected carbon activity. kolf is 20 obtained by best fitting with experimental data. The corrected carbon activities at different conditions are listed in Table 2.
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4. Conclusions
Fig. 12. Calculated carbon activities from Eq. (22) based on the proposed mechanism to observed critical steam to carbon S/C ratio for coke formation. k0 = 0.023, k00 = 4.94, and z = 2. The numbers on the figure refer to numbers in Table 2.
aC ¼
X
yi kolf aCn Hm ;i
(22)
i
It was found that Eq. (21) predicted well the effect of the carbon number and the partial pressure of hydrocarbons on the coking threshold, after using the corrected carbon activity in Eq. (22). However, it cannot take into account the effect of hydrogen and the C3/C1 ratio on the critical steam to carbon ratio. A more robust expression for the relationship between the coking threshold and thermodynamic carbon activity can be derived from the kinetic expression in Eq. (17). At the carbon threshold, the carbon activity equals unity [15,19], and Eq. (17) can be rearranged to express the critical steam to carbon ratio, an expression including the thermodynamic carbon potential from Eq. (19): pH2 O pH2 pz ¼ k 0 aC k00 þC p Cn H m pC n Hm p Cn H m
(23)
The term z ¼ xþ1 2 , where x is the coefficient for hydrogen from Eq. (10), m is the amount of H in the hydrocarbon and C is the constant, which equals critical S/C ratios at zero pressure of hydrogen. Observed carbon thresholds are plotted against the right hand side of Eq. (23) in Fig. 12. The single point deviating strongly from the straight line stems from the experiment where the C3/C1 ratio is varied. The results clearly indicate that Eq. (23) can well predict the coking threshold in different gas mixtures with variations in carbon number and hydrocarbon concentrations. On the other hand, Fig. 12 shows that the derived mechanisms (9)–(17) is quite suitable for describing the carbon thresholds observed with the current mixtures of hydrocarbons. The expression linking thermodynamic carbon activity and partial pressures is valid at least for the studied mixtures of C1–C3. It is also only fitted for the temperature region of the present work. Further studies where all the hydrocarbons used in the present work are studied at several temperatures can be done to refine the equations for steady-state carbon activity (18) and the expression for critical steam to carbon ratio to thermodynamic activity (24).
Carbon formation on a Ni/MgAl2O4 catalyst is studied at 20 bar and temperatures in the range 480–550 8C in a tapered element oscillating microbalance. Hydrocarbon mixtures containing CH4, CH4/C2H6, CH4/C2H6/C2H4, CH4/C3H8 and CH4/C3H8/C3H6 diluted in Ar was steam reformed at different steam to carbon ratios to find the threshold for carbon formation. At 500 8C the coking rate and carbon threshold increased with the carbon number and a dramatic increase in coking was observed for olefins. Hydrogen lowered the rate and the threshold for coking. Reduced partial pressure of C3 in the mixture of propane/propene and methane gave lower coking rates and carbon thresholds. Methane showed increased carbon formation rate with temperature. A minimum in rate of carbon formation and coking threshold at 500 8C was observed in steam reforming of methane/propane/propene. Gas phase reactions are not responsible for the observed increase in coking between 500 and 520 8C. Possible chain degradation, lowering the carbon potential of the hydrocarbon mixture was proposed to provide an explanation for the complex temperature dependency observed for methane/propane/propene. Carbon potentials based on thermodynamic carbon activities from cracking of hydrocarbons are calculated for the different mixtures. A mechanism is proposed, where chain degradation of the hydrocarbon takes place by hydrogen assisted cracking. A steady-state carbon activity, expressing the local kinetic carbon potential is derived. The steady-state carbon activity is based on the proposed reaction mechanism, from which a relationship between the critical steam to carbon ratio and the thermodynamic carbon activity is also developed. It can properly predict the role of hydrocarbon, hydrogen and water in carbon formation, and is well in accordance with the observed critical steam to carbon ratios observed in the TEOM.
Acknowledgements The support of this work from the Norwegian Research Council and Statoil is greatly acknowledged. Haldor Topsøe A/S is acknowledged for providing the catalyst.
References [1] T.S. Christensen, Appl. Catal. A. 138 (1996) 285. [2] S.E. Nielsen, I. Dybkjaer, Ammonia Plant Saf. Relat. Facil. 37 (1997) 125. [3] J.R. Rostrup-Nielsen, J. Catal. 27 (1972) 343. [4] D.L. Trimm, Catal. Today 37 (1997) 233. [5] T.S. Christensen, Rostrup-Nielsen, J. Am. Chem. Soc. (1995) 433. [6] O. Yamazaki, K. Tomishige, K. Fujimoto, Appl. Catal. 136 (1996) 49. [7] M. Clerici, R. Zennaro, Chim. Ind. 79 (1997) 47.
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Pre-reforming of natural gas on a Ni catalyst Criteria for carbon free operation Thomas Sperle a,1, De Chen a, Rune Lødeng b, Anders Holmen a,* a
Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway b SINTEF, Applied Chemistry, N-7465 Trondheim, Norway Received 20 July 2004; received in revised form 8 December 2004; accepted 10 December 2004
Abstract Pre-reforming of natural gas has been studied on a nickel catalyst at 480–550 8C and 20 bar using a TEOM (Tapered Element Oscillating Microbalance) reactor. The focus has been on carbon formation and the main objective was to study the influence of C2–C3 hydrocarbons in the methane feed on carbon deposition on the catalyst. Coking thresholds for different mixtures of hydrocarbons were determined by varying the steam to carbon (S/C) ratio at various temperatures. The steady-state coking rate decreases with increasing S/C ratio, and increases with increasing carbon number of the hydrocarbon. Unsaturated hydrocarbons show a strong effect on coking rates and on carbon thresholds. For mixtures of methane and propane/propene, the steady-state coking rate as well as the coking threshold decreased with a decrease in the mole fraction of propane/propene and with an increase in the hydrogen mole fraction. The coking rate revealed a complicated temperature dependency, and a minimum in the coking rate and in the coking threshold was detected at 500 8C. A relationship between the critical steam to carbon ratio and the thermodynamic carbon activity is also developed based on a suggested reaction mechanism, which can properly predict the role of hydrocarbon, hydrogen and water in carbon formation. # 2004 Elsevier B.V. All rights reserved. Keywords: Carbon deposition; Steam reforming; TEOM; Carbon threshold; Nickel catalyst; Natural gas; Pre-reforming conditions
1. Introduction Steam reforming on Ni catalysts in tubular reformers is the main process for production of synthesis gas and hydrogen from natural gas. Although methane is the main component of natural gas, the gas usually also contains higher hydrocarbons [1]. The higher hydrocarbons can be converted to methane and carbon oxides over a nickel-based catalyst in adiabatic pre-reformers at lower temperatures, normally 400–550 8C [1–3]. The potential for carbon formation of the feed to the primary reformer is thereby decreased. Traces of sulphur can also be selectively adsorbed in the pre-reformer. Feed flexibility is improved in the plant, the temperature can be raised and the steam to * Corresponding author. Tel.: +47 735 941 51; fax: +47 735 950 47. E-mail address: [email protected] (A. Holmen). 1 Present address: Statoil R&D Centre, Postuttak, N-7004 Trondheim, Norway. 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.12.011
carbon (S/C) ratio can be lowered in the inlet gas to the tubular reformer. In pre-reforming, light hydrocarbons are converted to methane, hydrogen and carbon oxides (1). The carbon monoxide is hydrogenated to methane (2), and the water-gas shift equilibrium (3) is usually established: m Cn Hm þ nH2 O ! nCO þ n þ < 0 (1) H2 DH298 2 ¼ 206 kJ=mol (2) CO þ 3H2 ¼ CH4 þ H2 O; DH298 CO þ H2 O ¼ CO2 þ H2 ;
DH298 ¼ 41 kJ=mol
(3)
Steam reforming of hydrocarbons (1) higher than methane is irreversible reactions [2,4]. Pre-reforming catalysts have a high risk of carbon formation and of catalyst deactivation. The catalyst deactivation is mainly caused by poisoning, carbon deposition (including gum formation) and also by sintering. Sulphur is the most serious poison in the hydrocarbon feed and it should be removed to a low level in a desulphurisation unit before the pre-reformer. Poisoning by
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sulphur has been extensively studied [5–9]. Sintering of nickel crystallites has generally not been considered as a severe problem at pre-reforming conditions, although sintering of nickel is reported to take place [10,11]. Carbon deposition is then the most critical parameter for selecting operating conditions for pre-reforming. Carbon formation on the catalyst will take place by the following reactions: CH4 ¼ C þ 2H2 ; DH 298 ¼ 75 kJ=mol m Cn Hm ¼ nC þ H2 2 2CO ¼ C þ CO2 ; DH 298 ¼ 172 kJ=mol
(4) (5) (6)
Cracking of hydrocarbons by reaction (4) and (5) and the Boudouard reaction (6) produce adsorbed carbon atoms on the catalyst surface. Surface carbon may form coke by encapsulating the surface, or by producing carbon filaments [12]. A selvedge with high carbon concentration is formed at the front of the nickel particle. Carbon then diffuses through the nickel, and precipitates on the support side of the particle. The process is driven by a concentration gradient, caused by higher solubility of carbon on the gas side than on the support side of the nickel particle. Filamentous carbon will eventually break down the catalyst, but it does normally not strongly affect the number of active nickel sites. Catalyst activity can be lost when filamentous carbon is blocking pores on the catalyst [13]. The feed composition can also change with time in industrial production of synthesis gas from natural gas. A slight increase in the content of higher or unsaturated hydrocarbons can change the conditions for carbon free operation significantly. High steam to carbon ratios can be used to suppress carbon formation. However, pressurising and heating a large excess of water is expensive, and knowledge of the window for carbon free operation is economically meaningful. Criteria based on kinetics and on thermodynamics [14,15] are used for predicting the operating window. Significant differences in thermodynamic properties of carbon have been found for filamentous carbon, for graphite and for nickel carbide [3,16]. The equilibrium constant for filamentous carbon is between the one for graphite and the one for nickel carbide. The difference in energy between whisker carbon and graphite is ascribed to surface energy for the carbon cylinder and to structural defects. Alstrup et al. [17] use the term equilibrium for the gas composition where carbon formation starts or stops. Snoeck et al. [12] prefer the term coking threshold for this condition, because diffusion cannot be considered a normal reversible surface reaction step. Thermodynamics are often derived from experiments at the coking threshold [12]. However, experimental results show that the actual critical steam to carbon ratio during steam reforming is normally higher than the one predicted by thermodynamics [18]. It has therefore been concluded that carbon formation is a question of kinetics and the local approach to the reforming equilibrium [18]. The critical steam to carbon ratio will then be a
function of the catalyst, the composition of the inlet gas and the operation conditions. Different techniques have been developed to evaluate the coking thresholds for reforming catalysts such as pseudoadiabatic fixed-bed reactors [1,5,11] and conventional microbalances [3,13,21]. In pseudo-adiabatic reactors, the temperature profile is measured as a function of the time on stream and deactivation is studied by measuring the movement of the temperature profile. However, the interpretation of the experimental data is not straightforward, since deactivation during steam reforming shows a complex behaviour. The conventional microbalance suffers from the insuperable problem of reactant bypass and kinetic data for both the main reaction and the carbon forming reaction is difficult to obtain [20]. Filamentous carbon will fill-up the void between the catalyst particles and gradually increase the bypass in the conventional microbalance as the experiment progresses [20]. However, the TEOM (Tapered Element Oscillating Microbalance) reactor has proved to be a powerful tool for studying deactivation and carbon formation simultaneously [20,21]. In a TEOM reactor all the reactants are forced to flow through the catalyst bed, thus eliminating the bypass problem and the TEOM reactor can be described as a fixed-bed reactor. The objective of this work has been to study the coking rates and the coking thresholds at pre-reforming conditions using a TEOM reactor. In the present study, steam is fed with methane and methane mixed with ethane/ethene or propane/ propene. The effect of temperature on the coking threshold and the potential for reducing the coking threshold by the addition of hydrogen is also addressed.
2. Experimental A model catalyst (UE200X) supplied by Haldor Topsøe was used in all experiments. The catalyst contains 18 wt.% Ni and is supported on an alumina–magnesia spinel. The catalyst properties are listed in Table 1. This study was performed in a TEOM reactor at a total pressure of 20 bar, and temperatures ranging from 480 to 550 8C. The TEOM set-up is described in detail elsewhere [20]. Seven to 15 mg of catalyst with particle size of 0.4– 0.6 mm was loaded into the TEOM reactor together with about 60 mg of MgAl2O4 spinel particles of the same size as diluent. The use of a diluent will reduce possible temperature gradients in the catalyst bed and also agglomeration of growing filamentous carbon, thus reducing Table 1 Properties of the UE200X model catalyst from Haldor Topsøe BET surface area (m2/g) ˚) Average pore diameter (A Ni dispersion (%) Metallic surface area (m2/g) 18 wt.% Ni supported on a alumina–magnesia spinel.
64.8 124 9.6 11.9
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Table 2 Reaction conditions, measured coking thresholds (S/Cmin), and calculated thermodynamic carbon activities (aC) based on Eq. (22) Mole ratio
Phydrocarbon (bar)
PH2 (bar)
T (8C)
S/Cmin
aC
Effect of carbon number 1 CH4 2 CH4/C2H6 3 CH4/C2H6/C2H4 4 CH4/C3H8
1 1/0.1 1/0.08/0.008 1/0.035
3.9 3.9 3.9 3.9
0.12 0.12 0.12 0.12
500 500 500 500
0.74 1.18 2.19 1.3
118.0 450.4 1570.6 589.6
Effect of C3/C1 ratio 6 CH4/C3H8/C3H6 7 CH4/C3H8/C3H6 8 CH4/C3H8/C3H6
1/0.007/0.003 1/0.032/0.01 1/0/0
3.84 3.96 3.85
0.12 0.11 0.12
510 510 510
2.5 3.4 0.86
640.1 3339.1 118.0
Effect of hydrogen 9 CH4/C3H8/C3H6 10 CH4/C3H8/C3H6 11 CH4/C3H8/C3H6
1/0.03/0.01 1/0.03/0.01 1/0.03/0.01
3.84 3.96 3.85
0.12 0.58 1.1
510 510 510
3.4 2.5 1.5
640.1 3339.1 118.0
No.
Mixture
the potential for pressure drop. The catalyst was reduced in H2/Ar = 1/1 at 550 8C after being heated from ambient to 550 8C at a rate of 2 8C/min. The maximum temperature was held overnight. During start-up, steam was added to the carrier flow of Ar in the presence of hydrogen to keep the catalyst from being oxidised. The hydrocarbons were then introduced and the argon flow was adjusted to obtain the desired partial pressure of hydrocarbons. The experiments started by using a relatively low steam to carbon ratio to reduce the induction period [12]. Thereafter, the steam to carbon ratio was increased stepwise, while the partial pressure of hydrocarbons was kept constant by adjusting the Ar flow. In order to obtain constant partial pressure of hydrocarbons when varying the steam to carbon ratio, the dilution by argon was varied slightly in this study. The coking studies were performed at pre-reforming conditions for feeds consisting of methane, methane/ethane, methane/propane, methane/ethane/ethene and methane/ propane/propene. In most experiments, the H2 level was 3.0% relative to CH4 although the effect of H2 was studied at 510 8C for a feed consisting of methane/propane/propene. The ethane/methane ratio was 0.1, the propane/methane ratio was 0.035, the ethane/ethene/methane ratio was 0.08/ 0.008/1 and the propane/propene/methane ratio was 0.03/ 0.01/1. The effect of carbon number on coke deposition was studied at a space time of hydrocarbons of 0.11 gcat h/mol, while the effects of propene to propane ratio as well as the effect of hydrogen were studied at a space time of 0.06 gcat h/mol. However, the space-time of hydrocarbons other than methane is quite large and varied for different mixtures due to only relatively small amount added into the mixtures. The partial pressure of hydrocarbons was kept constant at approximately 3.9 bar in most of the experiments, where Ar was used as balance. Experimental conditions are listed in Table 2. The reactor effluent was analyzed by online gas chromatography [20]. The conversion of methane was about 50% of equilibrium composition in most of the experiments. During pre-reforming methane is
both a product and a feed component, and the term methane conversion will refer to the relative amounts of methane in the feed gas and product gas, and is calculated as if methane was only a reactant.
3. Results and discussion Carbon formation from methane and methane containing propane and propane/propene has been studied at 20 bar, in the temperature range of 480–550 8C and at different steam to carbon ratios. Some experiments with methane containing ethane and ethane/ethene have also been performed. Based on previous work and experience an induction period is to be expected in the formation of filamentous carbon [13,21–23]. The initiation of carbon formation was provoked by using low steam to carbon ratios, the reason for this was to obtain a uniform and comparable initiations of filaments on the surface. Thereby, carbon whisker growth is obtained with relatively short induction periods. An example of a start-up period for the TEOM experiments is shown in Fig. 1. After an induction period lasting for about 2–3 h, there is a high affinity for carbon giving a very rapid carbon formation lasting for about 20 min. After the initial period with rapid
Fig. 1. Weight increase during the initial phases of an experiment with carbon formation from methane at 550 8C and 20 bar. Steam to carbon ratios for the first and second straight lines are 0.83 and 1.1, respectively.
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Fig. 2. Rate of carbon formation from methane containing propane and propene. O represents a fresh catalyst and X a precoked catalyst containing 100 wt.% carbon. Reaction conditions: T = 500 8C, Ptot = 20 bar, W/Fhydrocarbon = 0.06 gcat h/mol, pCH4 ¼ 3:8 bar and a ratio of CH4/C3H8/ C3H6 of 1/0.032/0.01.
carbon build-up, the reaction was studied using two different steam to carbon ratios, both giving straight lines for the weight increase with time on stream and no deactivation of the carbon formation was observed. Linear regression lines are calculated, and shown as thin lines in Fig. 1. Previous studies using TGA have shown that coking at 500 8C can proceed without deactivation [13] in the presence of C1 to C3 alkanes. This has been verified in the present work by measuring the rate of carbon formation at 510 8C on a fresh catalyst and on a pre-coked catalyst containing 100 wt.% carbon. The results are given on Fig. 2. The experimental data for steam to carbon (S/C) ratios of 1.5 and 2.0 reported in Fig. 2, are recorded at equal partial pressure of all gases. In spite of the large amount of carbon deposited on one of the catalysts, the agreement is very good indicating that carbon formed at these conditions is mainly filamentous carbon. For the experiments with pre-coked catalyst the partial pressure of methane was reduced as the rate of carbon formation approached zero. For S/C ratios of 3.3, 3.6 and 3.9, the partial pressures of methane were 3.45, 3.48 and 3.28 bar, respectively. For the fresh catalyst, the partial pressure of methane was held constant and equal to 3.8 bar. Fig. 3 shows the coking rate and the carbon thresholds at 500 8C for all the different feeds. The line for the methane/ propane/propene mixture in Fig. 3a is fitted from several data points as shown in Fig. 3b. The critical steam to carbon ratio at zero net coking rate is defined as the coking threshold in the present work. The values were obtained by extrapolation of the steady-state coking rate to zero. Fig. 3 shows clearly that the rate of carbon formation and the carbon threshold at 500 8C increase with the carbon number of the hydrocarbon added to the methane feed. However, when comparing these rates, it should be kept in mind that the ratio between the partial pressure of higher hydrocarbons and methane is not the same for C2 and C3, although the total pressure of hydrocarbon is identical for all the experiments. The experimental ratios of C2/C1 and C3/C1
Fig. 3. (a) Rate of carbon formation from different hydrocarbon mixtures. T = 500 8C, Ptot = 20 bar, Phydrocarbon = 3.9 bar, W/Fhydrocarbon = 0.11 gcat h/ mol, C1 (^), C1/C2 (&), C1/C3 (), C1/C2/C2=(~) and C1/C3/C3=(*); (b) rate of coke formation from methane/propane/propene (C1/C3/C3=(*)). Same conditions as in Fig. 3a.
are 0.1 and 0.035, respectively. The ratio has been selected to simulate the typical natural gas composition. Moreover, it has to be noted that the mole fraction of carbon atoms is different for each mole of the hydrocarbons with different carbon numbers. The corresponding mole ratios for C2/C1 and C3/C1 on a carbon atom basis are 0.2 and 0.105, respectively. Although the C3/C1 is much lower than C2/C1, the measured coking rate and coking threshold in the presence of C3 are higher than the ones in the presence of C2. It clearly indicates that the potential for carbon formation increases significantly with increasing carbon number. The presence of small amounts of olefins leads to a large increase in the coking rate and in the coking threshold and in particular the addition of propene increased the coking rate and the carbon threshold significantly (Fig. 3a and b). The effect of varying the ratio between C3 (propane/ propene) and methane was also studied. As shown in Fig. 4, the coking rate increased very much with an increase in the ratio between C3 (propane/propene) and methane. The ratio between propane and propene was held constant for the experiments reported in Fig. 4. Pre-reforming of methane gave conversions in the range of 3–5%. The selectivity to CO was in the range of 4–7% for most of the experiments. The GC analysis showed that even
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Fig. 4. Rate of coke formation from methane/propane/propene as a function of the amount of propane/propene added to methane. C3/C1 ratios varied at 510 8C, pCH4 ¼ 3:8 bar, steam to carbon (S/C) ratio = 2.5 and constant propene/propane ratio = 1/3.
traces of C2 could not be observed in the product gas when feeding propane/propene together with methane. For the experiments with C2 (ethane/ethene) in the feed, traces of C2 was always found in the reactor effluent. The concentration of C3 has not been measured. The conversion of C2 is generally larger than 80% and the conversion of C3 is expected to be even larger. It was impossible to operate at differential conditions, due to the fact that only small amount of C2 and C3 were fed into the reactor, giving a very large residence time for them. A large concentration gradient of C2 and C3 is expected, making it difficult to interpret kinetic data. Therefore, the reported coking rates are just average values. Based on the experimental results in the present work, it can be concluded that the carbon growth potential is the largest at the inlet, due to the high concentrations. From this argument, it is expected that the measured coking threshold be determined by the conditions at the inlet of the reactor. The reforming reactions are endothermic. Even though the amounts of C2 and C3 hydrocarbons are low relative to methane, their conversion is high. They will therefore definitely contribute to the heat consumption in the reactor. Temperature gradients over the film surrounding the catalyst pellets, and concentration gradients inside the catalyst pores may cause problems in the experiments. A worksheet, provided by Eurokin [24], employing heat- and massbalance equations and rules of thumb regarding external and internal concentration as well as temperature gradients was used. No significant gradients were found when only methane was fed, indicating the absence of external and internal transport limitations. For reforming of methane mixed with other hydrocarbons, a temperature drop over the film surrounding the pellet was calculated. Steam reforming of methane mixed with propane and propene gave the maximum value for this temperature gradient, almost 6 8C. This possible temperature gradient was not accounted for in the interpretation of the experimental data. Figs. 5 and 6 illustrate the effect of hydrogen. The effect of adding hydrogen to the reactant mixture on the resultant
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Fig. 5. Rate of coke formation from methane/propane/propene (1/0.03/ 0.01) at 510 8C as a function of H2/C at a steam to carbon (S/C) ratio of 1.5, Ptot = 20 bar, pCH4 ¼ 3:8 bar and W/Fhydrocarbon = 0.06 gcat h/mol.
carbon formation from methane/propane/propene mixtures was studied by varying the partial pressure of hydrogen at 510 8C. Two different approaches were used, either the partial pressure of H2 was increased at constant steam to carbon ratio or the steam to carbon ratio was increased at constant partial pressure of H2. Fig. 5 shows the coking rate and the carbon threshold as a function of the hydrogen to carbon (H2/C) ratio at a steam to carbon ratio of 1.5. In Fig. 6 the rate of carbon formation and the carbon threshold are shown as a function of the steam to carbon ratio at hydrogen to carbon mole ratios of 2.7 and 13.4. The results presented in Figs. 5 and 6 clearly show that adding hydrogen reduces the coking rate and the coking threshold significantly. These results are consistent with previous findings, where increased H2 content in the feed has been found to reduce the carbon formation [16,22,25–27]. The thermodynamic importance of hydrogen through reactions (4) and (5), affects the gasification of carbon and the carbon activity. Snoeck et al. [12] found the rate of gasification by hydrogen to pass through a maximum, suggesting that the hydrogen molecules compete for the same active sites as the carbon atoms segregating from the metal to the surface. It is likely that hydrogen also reduces
Fig. 6. Rate of coke formation from methane/propane/propene (1/0.03/ 0.01) at 510 8C as a function of steam to carbon (S/C) ratio at H2/C (mol/mol) = 2.7 (*) and 13.4 (^), Ptot = 20 bar, pCH4 ¼ 3:8 bar and W/Fhydrocarbon = 0.06 gcat h/mol.
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Fig. 7. Rate of coke formation from methane as a function of the steam to carbon (S/C) ratio for different temperatures. T = 480 (~), 500 (&) and 550 8C (^) . Ptot = 20 bar, pCH4 ¼ 3:9 bar and W/Fhydrocarbon = 0.11 gcat h/mol.
catalyst deactivation by gasifying encapsulants [25]. In a study by Lødeng et al. [21] dealing with decomposition of CH4, small amounts of hydrogen or water were found to increase the carbon formation. The effect of H2 was studied both in a conventional microbalance and in the TEOM. It was concluded [21] that H2 availability was rate limiting in the initial phase, and that moderate amounts of H2 increased the rate of carbon formation by lowering the apparent activation energy for carbon formation. Sidjabat and Trimm [25] observed carbon formation at 400 8C when adding less than 5% H2 to the hydrocarbon feed. At this temperature no carbon was formed without hydrogen in the feed. In Snoeck et al. [16] experiments, H2 enhanced carbon formation at very low hydrogen partial pressures and pCH4 ¼ 1:5 bar. The effect of low partial pressure of hydrogen on carbon formation has lead to the suspicion that hydrogen has a complex role in the formation of filamentous carbon. Nolan et al. [28] found closed carbon structures when no H2 was present in the formation of Boudouard carbon on nickel at 500 8C. H2 was essential for filament production. They suggested that the open-edged carbon structure of filamentous carbon was energetically unfavourable unless an agent, such as hydrogen, could satisfy the valences at the edges of the filaments. As the amount of hydrogen in the reaction gas was reduced from 1 to 0.03%, the morphology of the carbon
Fig. 8. Rate of coke formation from methane/propane/propene (1/0.03/ 0.01) as a function of the steam-to-carbon (S/C) ratio at different temperatures. T = 480 (), 490 (^), 500 (&), 510 (*) and 520 8C (~). Ptot = 20 bar, pCH4 ¼ 3:8 bar and W/Fhydrocarbon = 0.06 gcat h/mol.
Fig. 9. Coking thresholds in hydrocarbon pre-reforming: effect of temperature for methane (~) and for the mixtures of C1/C2/C2 = (1/0.08/0.008) (*) and C1/C3/C3 = (1/0.03/0.01) (^).
changed from predominantly filamentous to encapsulating. However, in the present work a sole decrease in the coking rate was observed with increasing amount of H2. This might simply be due to the relatively high amount of hydrogen used. In a study performed in a conventional microbalance [29], water was the most effective carbon-gasifying agent at 550–650 8C on nickel foils and on supported nickel. H2 was more effective than CO2. The total conversions of the hydrocarbons have been controlled at a relatively low level (<10%) in the present work. Therefore, the gasification from CO2 is less important. However, the effect of external addition of CO2 on carbon formation has to be investigated in the future in detail. The superiority of steam compared to hydrogen in carbon gasification is confirmed by other researchers [23]. The rates of carbon growth during methane reforming at different temperatures are shown on Fig. 7. As expected, the rate of carbon formation as well as the coking threshold increased with increasing temperature. Reforming of methane with ethane/ethene added to the feed was also performed at 480 and 500 8C. A larger growth rate was found at the lower temperature, but a high coking threshold was found at the high temperature. Carbon formation in mixtures of methane with propane/propene was studied at temperatures from 480–520 8C, and the results are given in Fig. 8. A complicated temperature dependency was observed as the results indicate negative apparent activation energy at temperature <500 8C. A minimum in the coking threshold and also in the rate of carbon formation from methane/propane/propene mixtures was observed at about 500 8C. The carbon thresholds for pre-reforming of methane and for mixtures of methane and C2 or C3 are plotted as a function of the reciprocal temperature in Fig. 9. A minimum is clearly observed at 500 8C for the methane/propane/ propene mixtures, while an increase in the coking threshold with temperature was observed for methane. The mixture of methane/ethane/ethene was not studied extensively at
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Fig. 10. Coking thresholds in hydrocarbon pre-reforming: effect of the partial pressure of propane/propene relative to methane at 510 8C. Ptot = 20 bar, pCH4 ¼ 3:8 bar and W/Fhydrocarbon = 0.06 gcat h/mol.
different temperatures, no conclusion can therefore be drawn. The effect of the amount of C3 relative to methane on the coking threshold was also studied and the results at 510 8C are shown in Fig. 10. The presence of small amounts of propane/propene increases the coking threshold quite dramatically. There are several reports in the literature of maximum rates of carbon formation with temperature [13,25,27]. Rostrup-Nielsen [13] observed that a temperature of 550– 575 8C gave a maximum rate of carbon formation when studying steam reforming n-heptane in a thermogravimetric system. Sidjabat and Trimm [25] observed a maximum in carbon formation from 1-butene and hydrogen at about 530 8C. Acetylene and toluene were also studied, and temperatures for maximum carbon formation were located at about 500 and 600 8C, respectively. It has been proposed that the apparent negative activation energy, and the selectivity to carbon for the different types of hydrocarbons, can be explained by the existence of surface intermediates containing more than one carbon atom [13]. At high temperatures, these intermediates will react to adsorbed
Fig. 11. Calculated carbon activities from Eq. (18) plotted to the observed minimum steam to carbon (S/C) ratio at 500 8C for all hydrocarbons involved in the study. Olefins (*) and paraffins (~) give two different straight lines.
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CHx. Rostrup-Nielsen and Trimm [27] stated that negative apparent activation energy could be adsorption effects, coupled with minor influence of gasification and selfpoisoning. The rate of carbon formation in these studies passes a minimum when the temperature was increased further and at these temperatures, gas phase carbon formation starts [25]. It is interesting that the minimum rate of carbon formation observed in the present study for reforming of methane mixed with propane and propene, is at a much lower temperature, about 500 8C. Some other explanation might therefore also exist for explaining the complex temperature dependency in the present work. Carbon formation by gas phase routes is very slow at low temperatures unless the feedstock has very high carbon affinity [4] or the gas phase residence time is high. It seems unlikely that a dramatic increase in carbon formation when the temperature is raised at 500 8C is caused by gas phase carbon. High total pressure will favour gas phase carbon, but dilution by argon and substantial amounts of water in the feed gas leads to the conclusion that carbon is formed on the catalyst. This is also supported by other studies [13]. A study on hydrogenolysis of propane, where propane reacts with H2 to form ethane and methane has revealed that the conversion of propane passes a maximum of about 400 8C on a Ni/MgAl2O4 catalyst [30]. Primary hydrogenolysis, yielding ethane and methane passed a maximum in ethane production at about 350 8C. These effects can influence the rate of carbon formation in mixtures containing higher hydrocarbons and hydrogen. Hydrogenolysis will lower the carbon number in the carbon forming hydrocarbons. If hydrogenolysis is important, a maximum in the rate of hydrogenolysis can lead to a minimum in the rate of carbon formation and coking threshold. Reported temperatures for the maximum rates and yields in hydrogenolysis [30] do not coincide precisely with the minimum for carbon formation in the present study. Feed composition, total pressure and catalyst formulation affect the rates and probably the location of maximum and minimum rates with temperature. However, hydrogen assisted cracking could have a significant effect on carbon formation from higher hydrocarbons and this is reflected in the proposed reaction scheme presented below (Eqs. (7)–(15)). Prediction of coking threshold for natural gas steam reforming has attracted much attention. Rostrup-Nielsen [15,19] has suggested a simplified reaction mechanism for whisker carbon formation from hydrocarbons, where hydrogen mainly influences the gasification of the surface carbon. Based on the mechanism, a steady-state activity has been derived [15], which was found to explain quite well the steam effect, but not the effect of hydrogen on carbon formation. The model predicts a positive effect of hydrogen. The reaction scheme is therefore modified, where the chain degradation by cracking of adsorbed hydrocarbons is substituted by hydrogen assisted cracking (Eq. (8)). At low temperatures, cracking of hydrocarbons (Eq. (5)) will be of less importance. In this simplified mechanism gasification
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of adsorbed carbon species takes place by reaction with adsorbed OH. The reversibility of the cracking of methane (Eq. (4)) is also taken into account in the scheme presented below. The role of hydrogen is described in Eqs. (8)–(10): k1
Cn Hm þ 2 !2 Cn Hy k2
2 Cn Hy þ H! CHx þ 2 Cn1 Hz k3
2 Cn Hy !n C þ y=2H2 k4 x CHx , C þ H2 2 k4 k5
C!½C; Ni bulk ! whisker carbon k6
CHx þ OH!gas k7
C þ OH!gas
(7) (8) (9) (10) (11) (12) (13)
K8 1 H2 O þ , OH þ H2 (14) 2 K9 1 H2 þ , H (15) 2 Reaction (14) is a simplified reaction step, where the equilibrium steps of steam and hydrogen adsorption/desorption, steam decomposition/recombination are combined. As mentioned above, carbon gasification from CO2 is relatively less important in the present work, which was not taken into account in the reaction mechanism. When deriving the steady-state carbon activity, it is assumed that carbon formation from adsorbed species with more than one carbon atom proceeds much faster than from adsorbed CHx: k3 k4, in accordance with the work by Rostrup-Nielsen et al. [19]. It is also assumed that hydrogenolysis is much faster than carbon formation from adsorbed carbon species: k2 k3. The last assumption is probably only valid at low temperatures and close to the carbon threshold, where the rate of carbon accumulation is very slow. The resulting steady-state carbon activity becomes: k 1 k 3 p Cn H m asc / ½C ¼ (16) ðxþ1Þ=2 k2 K 9 ½k7 K 8 pH2 O þ k4 pH2
Where [*C] is the concentration of surface carbon on Ni surfaces, pCn Hm , pH2 O and pH2 are the partial pressure of hydrocarbons, steam and hydrogen, respectively. This expression gives a steady-state carbon activity, which increases with the partial pressure of hydrocarbons ðpCn Hm Þ, and decreases with the partial pressures of water and hydrogen. If r1 > r2, there is a potential for accumulation of adsorbed CmH y. Eq. (16) is derived for a situation with no gum formation, r1 = r2. From a kinetic point of view, partial pressures of the hydrocarbon, water, and hydrogen affects the tendency for carbon formation through Eq. (16). It is well in line with the observed coking thresholds observed in the TEOM that H2 and H2O lower the carbon production, and that high content of the hydrocarbon increases the carbon potential. Olefins and aromatics adsorb easily, giving high rate constants for adsorption, k1, and
thereby increase the steady-state activity for carbon in Eq. (16). The equation derived from the present work for the steady-state activity can be rearranged to: pC n Hm (17) asc / ½C ¼ 0 k pH2 O þ k00 pxH2 The rate constants are lumped, with k0 ¼ k2 kk71Kk38 K 9 and k00 ¼ k2 kk74 K 8 K 9 , with rate constants from Eqs. (7–15). The thermodynamic carbon activity for a hydrocarbon can be defined according to Eq. (18), assuming that reactions (9) and (11) are at pseudo-equilibrium. 0 11=n p Cn H m aCn Hm ¼ n@K eq m=2 A (18) pH2 The potential for carbon formation from a hydrocarbon is an activity calculated on carbon basis. Keq is the equilibrium constant for cracking of the hydrocarbon by reaction (5), and n is the number of carbon atoms in the hydrocarbon. Higher hydrocarbons can produce more carbon per mole of gas, and this is taken into account by the term n in Eq. (18). In this study, the carbon potential for the mixture, or a total thermodynamic carbon activity, is calculated by summarising the individual activities: X aC ¼ yi aCn Hm ;i (19) i
The term yi, is not the molar fraction of component i in the reactant mixture, but should be treated as a hydrocarbon fraction: pC H y Cn H m ¼ P n m (20) i pCn Hm ;i In Fig. 11, the calculated activities for the different hydrocarbon mixtures at 500 8C are plotted to the experimental steam to carbon ratio at the carbon threshold. The olefins have a higher affinity to carbon relative to the paraffins than what is calculated from Eq. (18). The critical steam to carbon ratio can be expressed as: pH2 O ¼ aaC þ b (21) p Cn H m The value of a for the gas mixture containing olefins is higher than for the paraffin mixture. Thermodynamically, it is hard to account for the observed coking tendency of olefins compared to paraffins. The equilibrium constants for carbon formation from paraffins and olefins are of the same order of magnitude. From a kinetic point of view, it is easier, especially the rate of adsorption of olefins, k1 in Eq. (16), is much higher than that of paraffins. Considering Eq. (21), the constants a and b are hydrocarbon and catalyst specific. It is therefore possible to calculate a factor when calculating the thermodynamic potential for carbon formation from olefins. The equation is modified by a factor of kolf in Eq. (22) to give a corrected carbon activity. kolf is 20 obtained by best fitting with experimental data. The corrected carbon activities at different conditions are listed in Table 2.
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4. Conclusions
Fig. 12. Calculated carbon activities from Eq. (22) based on the proposed mechanism to observed critical steam to carbon S/C ratio for coke formation. k0 = 0.023, k00 = 4.94, and z = 2. The numbers on the figure refer to numbers in Table 2.
aC ¼
X
yi kolf aCn Hm ;i
(22)
i
It was found that Eq. (21) predicted well the effect of the carbon number and the partial pressure of hydrocarbons on the coking threshold, after using the corrected carbon activity in Eq. (22). However, it cannot take into account the effect of hydrogen and the C3/C1 ratio on the critical steam to carbon ratio. A more robust expression for the relationship between the coking threshold and thermodynamic carbon activity can be derived from the kinetic expression in Eq. (17). At the carbon threshold, the carbon activity equals unity [15,19], and Eq. (17) can be rearranged to express the critical steam to carbon ratio, an expression including the thermodynamic carbon potential from Eq. (19): pH2 O pH2 pz ¼ k 0 aC k00 þC p Cn H m pC n Hm p Cn H m
(23)
The term z ¼ xþ1 2 , where x is the coefficient for hydrogen from Eq. (10), m is the amount of H in the hydrocarbon and C is the constant, which equals critical S/C ratios at zero pressure of hydrogen. Observed carbon thresholds are plotted against the right hand side of Eq. (23) in Fig. 12. The single point deviating strongly from the straight line stems from the experiment where the C3/C1 ratio is varied. The results clearly indicate that Eq. (23) can well predict the coking threshold in different gas mixtures with variations in carbon number and hydrocarbon concentrations. On the other hand, Fig. 12 shows that the derived mechanisms (9)–(17) is quite suitable for describing the carbon thresholds observed with the current mixtures of hydrocarbons. The expression linking thermodynamic carbon activity and partial pressures is valid at least for the studied mixtures of C1–C3. It is also only fitted for the temperature region of the present work. Further studies where all the hydrocarbons used in the present work are studied at several temperatures can be done to refine the equations for steady-state carbon activity (18) and the expression for critical steam to carbon ratio to thermodynamic activity (24).
Carbon formation on a Ni/MgAl2O4 catalyst is studied at 20 bar and temperatures in the range 480–550 8C in a tapered element oscillating microbalance. Hydrocarbon mixtures containing CH4, CH4/C2H6, CH4/C2H6/C2H4, CH4/C3H8 and CH4/C3H8/C3H6 diluted in Ar was steam reformed at different steam to carbon ratios to find the threshold for carbon formation. At 500 8C the coking rate and carbon threshold increased with the carbon number and a dramatic increase in coking was observed for olefins. Hydrogen lowered the rate and the threshold for coking. Reduced partial pressure of C3 in the mixture of propane/propene and methane gave lower coking rates and carbon thresholds. Methane showed increased carbon formation rate with temperature. A minimum in rate of carbon formation and coking threshold at 500 8C was observed in steam reforming of methane/propane/propene. Gas phase reactions are not responsible for the observed increase in coking between 500 and 520 8C. Possible chain degradation, lowering the carbon potential of the hydrocarbon mixture was proposed to provide an explanation for the complex temperature dependency observed for methane/propane/propene. Carbon potentials based on thermodynamic carbon activities from cracking of hydrocarbons are calculated for the different mixtures. A mechanism is proposed, where chain degradation of the hydrocarbon takes place by hydrogen assisted cracking. A steady-state carbon activity, expressing the local kinetic carbon potential is derived. The steady-state carbon activity is based on the proposed reaction mechanism, from which a relationship between the critical steam to carbon ratio and the thermodynamic carbon activity is also developed. It can properly predict the role of hydrocarbon, hydrogen and water in carbon formation, and is well in accordance with the observed critical steam to carbon ratios observed in the TEOM.
Acknowledgements The support of this work from the Norwegian Research Council and Statoil is greatly acknowledged. Haldor Topsøe A/S is acknowledged for providing the catalyst.
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