- Email: [email protected]
Studying carbon formation at elevated pressure
Applied Catalysis A: General 206 (2001) 231–236
Studying carbon formation at elevated pressure John N. Armor∗ , Daniel J. Martenak Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA 18195, USA Received 25 January 2000; received in revised form 21 April 2000; accepted 30 April 2000
Abstract This manuscript describes the use of the tapered element oscillating microbalance (TEOM) to monitor the formation of elemental carbon under extreme reaction conditions, specifically the steam reforming of methane at 650◦ C and operating pressures as high as 200 psig. Since the TEOM reactor passes the entire feed stream over all of the catalyst bed under a variety of process conditions at high temperatures and high pressures, one can get an accurate assessment of the relative change in catalyst mass, and therefore, carbon deposition with time on stream. In particular, the studies clearly show the accelerated rate of carbon deposition on Ni catalysts which occurs at elevated pressures during steam methane reforming. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon formation; Microbalance; TEOM; SMR; Ni catalysts
1. Introduction The formation of elemental carbon on equipment and catalysts is a serious issue and concern to those running carbon based feedstocks for the production of merchant H2 [1]. Often over the course of years of reactor operation, the conversion of methane or higher hydrocarbons by use of steam reforming or CO2 reforming can produce carbon as a byproduct. Significant accumulations of carbon can arise with time which can lead to reactor fouling and/or blocking of the active sites on the catalyst, thus decreasing catalyst activity and eventually catalyst life while also creating hot spots which become self-accelerating carbon deposition regions. Carbon which accumulates in the reactor can comprise several different forms: whisker coke (typified by Ni atoms atop growing carbon fila∗ Corresponding author. Tel.: +1-610-481-5792; fax: +1-610-481-2989. E-mail address: [email protected] (J.N. Armor).
ments); pyrolytic carbon (from free radical processes); and ‘encapsulating’ coke [2–4]. Carbon accumulation can also cause spalling and pulverization of catalysts, which can also lead to poor heat distribution. By understanding and minimizing carbon formation, one can achieve greater operating life in commercial reactor systems. Today, the average life of steam methane reforming (SMR) catalysts is 3–5 years, and at a typical charge of 20 m3 per plant replacement, the catalyst can cost US$ 150,000–250,000. Several other major commercial processes suffer from serious carbon deposition, such as steam reforming (SR) of higher hydrocarbons, CO2 reforming of hydrocarbons, production of olefins, production of styrene via ethylbenzene, and the synthesis of NH3 (since NH3 synthesis plants are really H2 plants). In SR, carbon is believed to derive from the decomposition of methane or higher hydrocarbons (into H2 and carbon) or from CO disproportionation: 2CO= CO2 +C. Carbon formation is minimized in today’s commercial SMR operations by using a relatively high
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 6 0 8 - 6
232
J.N. Armor, D.J. Martenak / Applied Catalysis A: General 206 (2001) 231–236
steam/hydrocarbon feed ratio of ∼3/1, the use of alkali ion modifiers and by the addition of low levels of H2 to the feed [5]; however, carbon deposition can result from process upsets or catalyst poisoning. The problem continues to persists today in steam reforming processes, and an on-going challenge within the industry is to design improved process schemes that permit for carbon free operation [6,7]. In a few operations, the addition of 2–10 ppm of sulfur compounds can be used to poison highly active Ni sites which are believed to give rise to rapid carbon formation [8]. A more common approach involves the use of alkali ion modifiers [9], while recent patents disclose the use of more exotic metals such as Ag, Ge, and Bi [10], the addition of gold [11], and the use of precious metal promoted catalysts [12,13]. A large number of publications are now appearing in the scientific literature on CO2 reforming of CH4 to produce syngas, H2 /CO. A number of new catalysts are being described which are reported to effectively catalyze the CO2 reforming of CH4 reaction without the formation of carbon at 1 atm for hundreds of hours. From the large number of recent publications claiming improved catalyst performance based only on testing at 1 atm, there seems to be the impression that this is sufficient to access catalyst acceptability. The formation of carbon is very dependent on the system pressure, and the industry seeks high pressure H2 /CO, not low pressure product [14]. Within the industry, it is known that if one operates these reforming reactions at elevated pressures (∼12 atm), dramatic increases in the rates of coke formation are observed over most types of Ni based catalysts. Formation of carbon on a catalyst can be difficult to follow at elevated pressure running in a continuous mode. In this manuscript, by use of the tapered element oscillating microbalance (TEOM), comparative studies at pressures ranging from 1 atm to 100 psig will show the dramatic increase in carbon formation when operating at high pressures using CO2 /CH4 feeds. Since its introduction as a tool to characterize minute mass changes on solid materials, the TEOM has found limited uses in catalysis, principally in following olefin oligomerization reactions over a variety of catalysts [15,16]. There are brief reports about its use in measuring adsorption and kinetics associated with coke formation typical of catalytic cracking reactions [17,18].
2. Experimental All the microbalance studies were performed on a unique unit identified as a TEOM Series 1500 Pulse Mass Analyzer from Rupprecht & Patashnick Co., Albany, NY. This analyzer incorporates an inertial balance that measures the mass of a sample bed by monitoring the oscillating frequency changes of the catalyst contained within a tapered element. The feed flow is metered through upstream blending hardware that can be customized for a variety of process configurations. The blended feed is then passed down the center of the tapered element and through the entire sample bed, where the gaseous feed interacts with the solid sample, simulating plug flow operations. Mass increases result in a decrease of the tube’s natural oscillating frequency. A direct relationship exists between the tube’s change in frequency and mass of the sample bed. This technique enables highly precise and accurate mass determinations, providing more resolution than conventional microbalances. We used the high pressure/high temperature option of the analyzer which is capable of operating at temperatures up to 700◦ C and pressures to 900 psig. Three Brooks mass flow controller heads were used to deliver process gases, and an ISCO high pressure syringe pump was used to deliver water (which was converted to steam in the lengthy preheat zone prior) to the oscillating glass unit. Catalysts were evaluated at standard conditions (650◦ C, up to 200 psig, 0–2:2:1=H2 O:CO2 :CH4 feed with 2% added H2 and total gas flow of 100 cm3 /min). The high temperature version of the TEOM is limited to 700◦ C. Both the 0.10 and 0.20 cm3 sample elements were employed. The TEOM reactor was charged with 30–100 mg samples of catalyst, sized to 18/25 mesh, mixed with 150 mg of quartz chips (to minimize the chance of reactor rupture during rapid catalyst coking) and held in place with quartz wool plugs. Each sample was pre-reduced in the reactor with a 10% H2 in N2 blend at 100 cm3 /min as the reactor temperature was ramped from ambient to 650◦ C. Once at 650◦ C, the sample was held in the reducing gas blend for an additional 30 min, or until catalyst weight, as determined by the TEOM, stabilized. Feed to the reactor was then switched from reducing gas blend to the reforming feed described above, and the balance mass was re-zeroed. Data was collected at 0.8 s intervals as the reforming reaction progressed and coke
J.N. Armor, D.J. Martenak / Applied Catalysis A: General 206 (2001) 231–236
deposition occurred. Precautions need to be taken to stop the reaction if large amounts of carbon are deposited, which will cause the glass sample tube to crack (due to expansion). Stable baseline performance should be attained before switching gas compositions, and one must account for adsorption/desorption occurring on the samples. A conventional Ni/alumina steam reforming catalyst (15% Ni on alumina, Harshaw NI-0302) crushed and sieved to 18/25 mesh was used for most studies. The Ni/magnesia catalyst was prepared according to the description provided by Hu and Ruckenstein [19]. Ni(NO3 )2 ·6H2 O was dissolved in a minimum amount of de-ionized water and was continuously added with stirring to magnesium oxide (Aldrich 34,279-3) contained in a 100 ml beaker. The resulting green paste was dried over 72 h period at room temperature and then calcined in air at 800◦ C for 3 h and then crushed and sieved to 18/25 mesh.
3. Results and discussion As part of an ongoing effort to understand coke formation on reforming catalysts, experiments aimed at relating SR parameters to coke formation were initiated. It is known from plant experience that higher operating pressure enhances coke formation on commercial Ni reforming catalysts. Coke formation is also favored by operating at temperatures below ∼900◦ C, particularly in the range of 600–700◦ C. In an attempt to quantify the rates of coke formation as a function of process parameters such as pressure, temperature and feed composition, a series of experiments utilizing the unique capabilities of the TEOM reactor were undertaken. Due to the lack of information in the scientific literature regarding the effect of operating pressure on catalyst coking during SMR, operating pressure was the initial parameter investigated and the results were benchmarked against commercial reactor performance. More traditional methods of measuring carbon deposition involve a microbalance or a TGA. With the typical hanging pan microbalance, much of the process gas bypasses the catalyst and catalyst temperature can be difficult to control, hence this is not a very accurate way to measure carbon deposition under reaction conditions. Alternatively, a spent catalyst
233
can be recovered from a reactor and then subjected to atmospheric TGA analysis to determine the residual level of carbon on the catalyst. Obviously this approach suffers from being only a one point assay, long after the reaction itself is over. The TEOM on the other hand offers a high pressure, in-situ analysis with real process gases in a real-time mode. 3.1. Control runs A control run was carried out with a reactor charged entirely with quartz chips to determine the base carbon deposition rate on an inert-filled reactor with the gold-plated reactor cap in place. The carbon deposition rate for the quartz filled reactor was negligible; the observed mass increase is most likely due to carbon build-up on the reactor cap and slight thermal coking on the inner reactor wall. Before proceeding with high pressure studies, the performance of the TEOM was established by benchmark runs at 1 atm with the standard Ni/alumina catalyst. Here we observed that CO2 addition does repress CH4 cracking to carbon; addition of larger amounts of CO2 were even more effective at reducing CH4 cracking. Also, the addition of water vapor substantially reduced carbon formation; however, some carbon formation was still evident at 1 atm. Although the amount of carbon may seem small, after running such a reaction for years (as demanded in a commercial operation), continual accumulation of even small amounts of carbon would be unacceptable, but difficult to assess with short term reactor studies. 3.2. Impact of higher pressure In the commercial operation of steam reformers, the high operating pressures which are required for most commercial operations accelerate carbon deposition. Most of the literature work on SMR and CO2 /CH4 catalyst development has been done at 1 atm total pressure. This is much lower that commercial operating conditions, and any realistic assessment of catalyst performance should be made at higher pressures. The impact of total pressure upon carbon formation is amply demonstrated in Fig. 1. Commercial reforming units often run at ∼300 psig. Rates of carbon deposition were very high even at 100 psig, so for these studies we limited our high pressure operation to 100 psig;
234
J.N. Armor, D.J. Martenak / Applied Catalysis A: General 206 (2001) 231–236
Fig. 1. Effect of total pressure upon carbon deposition; conditions: 650◦ C, 2:1=CO2 :CH4 , natural gas as CH4 , H2 pre-reduction, 200 mg sample holder (filled circles — quartz chips at 0 psig; inverted triangles — with increasing pressure at 100 psig). The maximum mass on the Ni catalyst profiles corresponds to where the feed flow was halted and a N2 purge of the catalyst bed was initiated to prevent reactor rupture.
however, there is a clear trend that carbon deposition rates increase dramatically with increasing pressure. 3.3. Use of MgO supported catalysts Recent literature on CO2 /CH4 reforming has pointed to the improvement in the use of a MgO support in place of alumina. This was amply demonstrated below (Fig. 2) illustrating the carbon deposition rate for a NiO/MgO versus a commercial Ni/Al2 O3 catalyst evaluated at 650◦ C and 100 psig processing a 2:1=CO2 :CH4 (natural gas) feed. The NiO/MgO operated with minimal carbon deposition for >30 min at 1 atm. The Ni/Al2 O3 however, rapidly accumulated 5 mg of carbon in ∼6 min. Experiments employing NiO/MgO for reforming of CH4 indicate the ability to operate with lower carbon deposition rates than those attainable when operating with a traditional Ni/Al2 O3 reforming catalysts. There have been a number of reports in the literature recently regarding the use of the solid solution of NiO/MgO for SMR and mixed reforming of CH4 . Knowing that dramatic differences in carbon deposi-
tion can occur at high total pressures, this material has been evaluated in the TEOM to assess the effect of operating pressure on carbon deposition rates. While the carbon deposition rates for NiO/MgO are much less than carbon deposition rates for commercial Ni/Al2 O3 reforming catalysts, the rates remain very sensitive to operating pressure. Fig. 3 below illustrates carbon deposition rates versus pressure for NiO/MgO, operating at 650◦ C. We have carried out similar studies on other reforming catalysts to 200 psig at 650◦ C with similar dramatic increases in carbon deposition on other Ni based catalysts. The Table 1 displays the Table 1 Carbon deposition rate as a function of operating pressure during CO2 reforming of CH4 a Catalyst
Pressure (psig)
Carbon deposition rate (g carbon/mg catalyst/min)
NiO/MgO NiO/MgO NiO/MgO
0 50 100
0.052 0.11 1.2
a Conditions: 650◦ C, 2:1=CO :CH , natural gas as CH , H 2 4 4 2 pre-reduction, 30 mg, catalyst diluted in quartz chips.
J.N. Armor, D.J. Martenak / Applied Catalysis A: General 206 (2001) 231–236
235
Fig. 2. Comparison between alumina and magnesia as supports; conditions: 650◦ C, 100 psig, 2:1=CO2 :CH4 , natural gas as CH4 , H2 pre-reduction. The maximum mass on the Ni/Al2 O3 profile corresponds to where the feed flow was halted and a N2 purge of the catalyst bed was initiated to prevent reactor rupture.
Fig. 3. Effect of total pressure with Ni-magnesia catalyst; conditions: 650◦ C, 2:1=CO2 :CH4 , natural gas as CH4 , H2 pre-reduction, 30 mg catalyst diluted in quartz chips.
236
J.N. Armor, D.J. Martenak / Applied Catalysis A: General 206 (2001) 231–236
carbon deposition rates for the three experiments illustrated in the plot. While the rates appear low, given operation of such reformer units over years, this level of carbon deposition would be unacceptable. Here one sees the real value in measuring rates of carbon deposition at much higher pressures. 4. Conclusion The formation of carbon from the reaction of CO2 with CH4 is rapidly accelerated by raising reactor pressure. The TEOM reactor is an effective instrument to probe for catalyst activity in situ with regard to SMR and the resulting carbon deposition that often accompanies that process. It clearly shows dramatic and significant differences between catalyst performance at 1 atm and commercial operation pressures approaching 100 psig. Thus, one cannot assume that the absence of detectable carbon formation at 1 atm extends to operation under process conditions exceeding 100 psig.
References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Acknowledgements We thank Air Products and Chemicals, Inc., for the permission to publish this work.
[17] [18] [19]
J.N. Armor, Appl. Catal. A 176 (1999) 161. D. Trimm, Catal. Today 37 (1997) 233. J.R. Rostrup-Nielsen, Catal. Today 37 (1997) 225–232. J.R. Rostrup-Nielsen, J. Catal. 85 (1984) 31–43. J.R. Rostrup-Nielsen, in: H. de Lasa (Ed.), Chemical Reactor Technology for Environmentally Safe Reactors and Products, Kluwer Academic Publishers, Dordrecht, 1993, p. 249. J.R. Rostrup-Nielsen, I. Alstrup, Catal. Today 53 (1999) 311. D. Trimm, Catal. Today 49 (1999) 3–10. J.R. Rostrup-Nielsen, Stud. Surf. Sci. 68 (1991) 85. I. Alstrup, B. Clausen, C. Olsen, R. Smits, J.R. Rostrup-Nielsen, Stud. Surf. Sci. 119 (1998) 5. I. Ul-Haque, D. Trimm, US Patent 5,595,719 and 5,595,717. J. Hyldtoft, J. Norskov, B. Clausen, European Patent 0 884 274 A1. J.E.S. Hegarty, A.M. O’connor, J.R.H. Ross, Catal. Today 42 (1998) 225. A. Nicolaas, J. van Keulen, M.E.S. Hegarty, J.R.H. Ross, P.F. van den Oosterkamp, Stud. Surf. Sci. Catal. 107 (1997) 537. J.N. Armor, Res. Chem. Intermed. 24 (1998) 105. D. Chen, H.P. Rebo, K. Molijord, A. Holmen, Ind. Eng. Chem. Res. 36 (1997) 3473–3479. De Chen, A. Gronvold, H.P. Rebo, K. Moljord, A. Holmen, Appl. Catal. A: General 137 (1996) L1. S.C. Fung, C.A. Querini, K. Liu, D.S. Rumschitzki, T.C. Ho, Stud. Surf. Sci. 88 (1994). F. Hershkowitz, P.D. Madiera, Ind. Eng. Chem. Res. 32 (1993) 2969. Y. Hang Hu, E. Ruckenstein, Catal. Lett. 43 (1997) 71.
Studying carbon formation at elevated pressure John N. Armor∗ , Daniel J. Martenak Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA 18195, USA Received 25 January 2000; received in revised form 21 April 2000; accepted 30 April 2000
Abstract This manuscript describes the use of the tapered element oscillating microbalance (TEOM) to monitor the formation of elemental carbon under extreme reaction conditions, specifically the steam reforming of methane at 650◦ C and operating pressures as high as 200 psig. Since the TEOM reactor passes the entire feed stream over all of the catalyst bed under a variety of process conditions at high temperatures and high pressures, one can get an accurate assessment of the relative change in catalyst mass, and therefore, carbon deposition with time on stream. In particular, the studies clearly show the accelerated rate of carbon deposition on Ni catalysts which occurs at elevated pressures during steam methane reforming. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon formation; Microbalance; TEOM; SMR; Ni catalysts
1. Introduction The formation of elemental carbon on equipment and catalysts is a serious issue and concern to those running carbon based feedstocks for the production of merchant H2 [1]. Often over the course of years of reactor operation, the conversion of methane or higher hydrocarbons by use of steam reforming or CO2 reforming can produce carbon as a byproduct. Significant accumulations of carbon can arise with time which can lead to reactor fouling and/or blocking of the active sites on the catalyst, thus decreasing catalyst activity and eventually catalyst life while also creating hot spots which become self-accelerating carbon deposition regions. Carbon which accumulates in the reactor can comprise several different forms: whisker coke (typified by Ni atoms atop growing carbon fila∗ Corresponding author. Tel.: +1-610-481-5792; fax: +1-610-481-2989. E-mail address: [email protected] (J.N. Armor).
ments); pyrolytic carbon (from free radical processes); and ‘encapsulating’ coke [2–4]. Carbon accumulation can also cause spalling and pulverization of catalysts, which can also lead to poor heat distribution. By understanding and minimizing carbon formation, one can achieve greater operating life in commercial reactor systems. Today, the average life of steam methane reforming (SMR) catalysts is 3–5 years, and at a typical charge of 20 m3 per plant replacement, the catalyst can cost US$ 150,000–250,000. Several other major commercial processes suffer from serious carbon deposition, such as steam reforming (SR) of higher hydrocarbons, CO2 reforming of hydrocarbons, production of olefins, production of styrene via ethylbenzene, and the synthesis of NH3 (since NH3 synthesis plants are really H2 plants). In SR, carbon is believed to derive from the decomposition of methane or higher hydrocarbons (into H2 and carbon) or from CO disproportionation: 2CO= CO2 +C. Carbon formation is minimized in today’s commercial SMR operations by using a relatively high
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 6 0 8 - 6
232
J.N. Armor, D.J. Martenak / Applied Catalysis A: General 206 (2001) 231–236
steam/hydrocarbon feed ratio of ∼3/1, the use of alkali ion modifiers and by the addition of low levels of H2 to the feed [5]; however, carbon deposition can result from process upsets or catalyst poisoning. The problem continues to persists today in steam reforming processes, and an on-going challenge within the industry is to design improved process schemes that permit for carbon free operation [6,7]. In a few operations, the addition of 2–10 ppm of sulfur compounds can be used to poison highly active Ni sites which are believed to give rise to rapid carbon formation [8]. A more common approach involves the use of alkali ion modifiers [9], while recent patents disclose the use of more exotic metals such as Ag, Ge, and Bi [10], the addition of gold [11], and the use of precious metal promoted catalysts [12,13]. A large number of publications are now appearing in the scientific literature on CO2 reforming of CH4 to produce syngas, H2 /CO. A number of new catalysts are being described which are reported to effectively catalyze the CO2 reforming of CH4 reaction without the formation of carbon at 1 atm for hundreds of hours. From the large number of recent publications claiming improved catalyst performance based only on testing at 1 atm, there seems to be the impression that this is sufficient to access catalyst acceptability. The formation of carbon is very dependent on the system pressure, and the industry seeks high pressure H2 /CO, not low pressure product [14]. Within the industry, it is known that if one operates these reforming reactions at elevated pressures (∼12 atm), dramatic increases in the rates of coke formation are observed over most types of Ni based catalysts. Formation of carbon on a catalyst can be difficult to follow at elevated pressure running in a continuous mode. In this manuscript, by use of the tapered element oscillating microbalance (TEOM), comparative studies at pressures ranging from 1 atm to 100 psig will show the dramatic increase in carbon formation when operating at high pressures using CO2 /CH4 feeds. Since its introduction as a tool to characterize minute mass changes on solid materials, the TEOM has found limited uses in catalysis, principally in following olefin oligomerization reactions over a variety of catalysts [15,16]. There are brief reports about its use in measuring adsorption and kinetics associated with coke formation typical of catalytic cracking reactions [17,18].
2. Experimental All the microbalance studies were performed on a unique unit identified as a TEOM Series 1500 Pulse Mass Analyzer from Rupprecht & Patashnick Co., Albany, NY. This analyzer incorporates an inertial balance that measures the mass of a sample bed by monitoring the oscillating frequency changes of the catalyst contained within a tapered element. The feed flow is metered through upstream blending hardware that can be customized for a variety of process configurations. The blended feed is then passed down the center of the tapered element and through the entire sample bed, where the gaseous feed interacts with the solid sample, simulating plug flow operations. Mass increases result in a decrease of the tube’s natural oscillating frequency. A direct relationship exists between the tube’s change in frequency and mass of the sample bed. This technique enables highly precise and accurate mass determinations, providing more resolution than conventional microbalances. We used the high pressure/high temperature option of the analyzer which is capable of operating at temperatures up to 700◦ C and pressures to 900 psig. Three Brooks mass flow controller heads were used to deliver process gases, and an ISCO high pressure syringe pump was used to deliver water (which was converted to steam in the lengthy preheat zone prior) to the oscillating glass unit. Catalysts were evaluated at standard conditions (650◦ C, up to 200 psig, 0–2:2:1=H2 O:CO2 :CH4 feed with 2% added H2 and total gas flow of 100 cm3 /min). The high temperature version of the TEOM is limited to 700◦ C. Both the 0.10 and 0.20 cm3 sample elements were employed. The TEOM reactor was charged with 30–100 mg samples of catalyst, sized to 18/25 mesh, mixed with 150 mg of quartz chips (to minimize the chance of reactor rupture during rapid catalyst coking) and held in place with quartz wool plugs. Each sample was pre-reduced in the reactor with a 10% H2 in N2 blend at 100 cm3 /min as the reactor temperature was ramped from ambient to 650◦ C. Once at 650◦ C, the sample was held in the reducing gas blend for an additional 30 min, or until catalyst weight, as determined by the TEOM, stabilized. Feed to the reactor was then switched from reducing gas blend to the reforming feed described above, and the balance mass was re-zeroed. Data was collected at 0.8 s intervals as the reforming reaction progressed and coke
J.N. Armor, D.J. Martenak / Applied Catalysis A: General 206 (2001) 231–236
deposition occurred. Precautions need to be taken to stop the reaction if large amounts of carbon are deposited, which will cause the glass sample tube to crack (due to expansion). Stable baseline performance should be attained before switching gas compositions, and one must account for adsorption/desorption occurring on the samples. A conventional Ni/alumina steam reforming catalyst (15% Ni on alumina, Harshaw NI-0302) crushed and sieved to 18/25 mesh was used for most studies. The Ni/magnesia catalyst was prepared according to the description provided by Hu and Ruckenstein [19]. Ni(NO3 )2 ·6H2 O was dissolved in a minimum amount of de-ionized water and was continuously added with stirring to magnesium oxide (Aldrich 34,279-3) contained in a 100 ml beaker. The resulting green paste was dried over 72 h period at room temperature and then calcined in air at 800◦ C for 3 h and then crushed and sieved to 18/25 mesh.
3. Results and discussion As part of an ongoing effort to understand coke formation on reforming catalysts, experiments aimed at relating SR parameters to coke formation were initiated. It is known from plant experience that higher operating pressure enhances coke formation on commercial Ni reforming catalysts. Coke formation is also favored by operating at temperatures below ∼900◦ C, particularly in the range of 600–700◦ C. In an attempt to quantify the rates of coke formation as a function of process parameters such as pressure, temperature and feed composition, a series of experiments utilizing the unique capabilities of the TEOM reactor were undertaken. Due to the lack of information in the scientific literature regarding the effect of operating pressure on catalyst coking during SMR, operating pressure was the initial parameter investigated and the results were benchmarked against commercial reactor performance. More traditional methods of measuring carbon deposition involve a microbalance or a TGA. With the typical hanging pan microbalance, much of the process gas bypasses the catalyst and catalyst temperature can be difficult to control, hence this is not a very accurate way to measure carbon deposition under reaction conditions. Alternatively, a spent catalyst
233
can be recovered from a reactor and then subjected to atmospheric TGA analysis to determine the residual level of carbon on the catalyst. Obviously this approach suffers from being only a one point assay, long after the reaction itself is over. The TEOM on the other hand offers a high pressure, in-situ analysis with real process gases in a real-time mode. 3.1. Control runs A control run was carried out with a reactor charged entirely with quartz chips to determine the base carbon deposition rate on an inert-filled reactor with the gold-plated reactor cap in place. The carbon deposition rate for the quartz filled reactor was negligible; the observed mass increase is most likely due to carbon build-up on the reactor cap and slight thermal coking on the inner reactor wall. Before proceeding with high pressure studies, the performance of the TEOM was established by benchmark runs at 1 atm with the standard Ni/alumina catalyst. Here we observed that CO2 addition does repress CH4 cracking to carbon; addition of larger amounts of CO2 were even more effective at reducing CH4 cracking. Also, the addition of water vapor substantially reduced carbon formation; however, some carbon formation was still evident at 1 atm. Although the amount of carbon may seem small, after running such a reaction for years (as demanded in a commercial operation), continual accumulation of even small amounts of carbon would be unacceptable, but difficult to assess with short term reactor studies. 3.2. Impact of higher pressure In the commercial operation of steam reformers, the high operating pressures which are required for most commercial operations accelerate carbon deposition. Most of the literature work on SMR and CO2 /CH4 catalyst development has been done at 1 atm total pressure. This is much lower that commercial operating conditions, and any realistic assessment of catalyst performance should be made at higher pressures. The impact of total pressure upon carbon formation is amply demonstrated in Fig. 1. Commercial reforming units often run at ∼300 psig. Rates of carbon deposition were very high even at 100 psig, so for these studies we limited our high pressure operation to 100 psig;
234
J.N. Armor, D.J. Martenak / Applied Catalysis A: General 206 (2001) 231–236
Fig. 1. Effect of total pressure upon carbon deposition; conditions: 650◦ C, 2:1=CO2 :CH4 , natural gas as CH4 , H2 pre-reduction, 200 mg sample holder (filled circles — quartz chips at 0 psig; inverted triangles — with increasing pressure at 100 psig). The maximum mass on the Ni catalyst profiles corresponds to where the feed flow was halted and a N2 purge of the catalyst bed was initiated to prevent reactor rupture.
however, there is a clear trend that carbon deposition rates increase dramatically with increasing pressure. 3.3. Use of MgO supported catalysts Recent literature on CO2 /CH4 reforming has pointed to the improvement in the use of a MgO support in place of alumina. This was amply demonstrated below (Fig. 2) illustrating the carbon deposition rate for a NiO/MgO versus a commercial Ni/Al2 O3 catalyst evaluated at 650◦ C and 100 psig processing a 2:1=CO2 :CH4 (natural gas) feed. The NiO/MgO operated with minimal carbon deposition for >30 min at 1 atm. The Ni/Al2 O3 however, rapidly accumulated 5 mg of carbon in ∼6 min. Experiments employing NiO/MgO for reforming of CH4 indicate the ability to operate with lower carbon deposition rates than those attainable when operating with a traditional Ni/Al2 O3 reforming catalysts. There have been a number of reports in the literature recently regarding the use of the solid solution of NiO/MgO for SMR and mixed reforming of CH4 . Knowing that dramatic differences in carbon deposi-
tion can occur at high total pressures, this material has been evaluated in the TEOM to assess the effect of operating pressure on carbon deposition rates. While the carbon deposition rates for NiO/MgO are much less than carbon deposition rates for commercial Ni/Al2 O3 reforming catalysts, the rates remain very sensitive to operating pressure. Fig. 3 below illustrates carbon deposition rates versus pressure for NiO/MgO, operating at 650◦ C. We have carried out similar studies on other reforming catalysts to 200 psig at 650◦ C with similar dramatic increases in carbon deposition on other Ni based catalysts. The Table 1 displays the Table 1 Carbon deposition rate as a function of operating pressure during CO2 reforming of CH4 a Catalyst
Pressure (psig)
Carbon deposition rate (g carbon/mg catalyst/min)
NiO/MgO NiO/MgO NiO/MgO
0 50 100
0.052 0.11 1.2
a Conditions: 650◦ C, 2:1=CO :CH , natural gas as CH , H 2 4 4 2 pre-reduction, 30 mg, catalyst diluted in quartz chips.
J.N. Armor, D.J. Martenak / Applied Catalysis A: General 206 (2001) 231–236
235
Fig. 2. Comparison between alumina and magnesia as supports; conditions: 650◦ C, 100 psig, 2:1=CO2 :CH4 , natural gas as CH4 , H2 pre-reduction. The maximum mass on the Ni/Al2 O3 profile corresponds to where the feed flow was halted and a N2 purge of the catalyst bed was initiated to prevent reactor rupture.
Fig. 3. Effect of total pressure with Ni-magnesia catalyst; conditions: 650◦ C, 2:1=CO2 :CH4 , natural gas as CH4 , H2 pre-reduction, 30 mg catalyst diluted in quartz chips.
236
J.N. Armor, D.J. Martenak / Applied Catalysis A: General 206 (2001) 231–236
carbon deposition rates for the three experiments illustrated in the plot. While the rates appear low, given operation of such reformer units over years, this level of carbon deposition would be unacceptable. Here one sees the real value in measuring rates of carbon deposition at much higher pressures. 4. Conclusion The formation of carbon from the reaction of CO2 with CH4 is rapidly accelerated by raising reactor pressure. The TEOM reactor is an effective instrument to probe for catalyst activity in situ with regard to SMR and the resulting carbon deposition that often accompanies that process. It clearly shows dramatic and significant differences between catalyst performance at 1 atm and commercial operation pressures approaching 100 psig. Thus, one cannot assume that the absence of detectable carbon formation at 1 atm extends to operation under process conditions exceeding 100 psig.
References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Acknowledgements We thank Air Products and Chemicals, Inc., for the permission to publish this work.
[17] [18] [19]
J.N. Armor, Appl. Catal. A 176 (1999) 161. D. Trimm, Catal. Today 37 (1997) 233. J.R. Rostrup-Nielsen, Catal. Today 37 (1997) 225–232. J.R. Rostrup-Nielsen, J. Catal. 85 (1984) 31–43. J.R. Rostrup-Nielsen, in: H. de Lasa (Ed.), Chemical Reactor Technology for Environmentally Safe Reactors and Products, Kluwer Academic Publishers, Dordrecht, 1993, p. 249. J.R. Rostrup-Nielsen, I. Alstrup, Catal. Today 53 (1999) 311. D. Trimm, Catal. Today 49 (1999) 3–10. J.R. Rostrup-Nielsen, Stud. Surf. Sci. 68 (1991) 85. I. Alstrup, B. Clausen, C. Olsen, R. Smits, J.R. Rostrup-Nielsen, Stud. Surf. Sci. 119 (1998) 5. I. Ul-Haque, D. Trimm, US Patent 5,595,719 and 5,595,717. J. Hyldtoft, J. Norskov, B. Clausen, European Patent 0 884 274 A1. J.E.S. Hegarty, A.M. O’connor, J.R.H. Ross, Catal. Today 42 (1998) 225. A. Nicolaas, J. van Keulen, M.E.S. Hegarty, J.R.H. Ross, P.F. van den Oosterkamp, Stud. Surf. Sci. Catal. 107 (1997) 537. J.N. Armor, Res. Chem. Intermed. 24 (1998) 105. D. Chen, H.P. Rebo, K. Molijord, A. Holmen, Ind. Eng. Chem. Res. 36 (1997) 3473–3479. De Chen, A. Gronvold, H.P. Rebo, K. Moljord, A. Holmen, Appl. Catal. A: General 137 (1996) L1. S.C. Fung, C.A. Querini, K. Liu, D.S. Rumschitzki, T.C. Ho, Stud. Surf. Sci. 88 (1994). F. Hershkowitz, P.D. Madiera, Ind. Eng. Chem. Res. 32 (1993) 2969. Y. Hang Hu, E. Ruckenstein, Catal. Lett. 43 (1997) 71.