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Generation of Ethylene Tracer by Noncatalytic Pyrolysis of Natural Gas at Elevated Pressure Yongqi Lu,†,‡ Shiaoguo Chen,*,† Massoud Rostam-Abadi,†,‡ Rodney Ruch,§ Dennis Coleman,§ and Leslie J. Benson§ Illinois State Geological Survey, 615 E. Peabody Drive, Champaign, Illinois 61820, Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, and Isotech Laboratories, Inc., 1308 Parkland Court, Champaign, Illinois 61821 Received July 25, 2004. Revised Manuscript Received October 22, 2004
There is a critical need within the pipeline gas industry for an inexpensive and reliable technology to generate an identification tag or tracer that can be added to pipeline gas to identify gas that may escape and improve the deliverability and management of gas in underground storage fields. Ethylene is an ideal tracer, because it does not exist naturally in the pipeline gas, and because its physical properties are similar to the pipeline gas components. A pyrolysis process, known as the Tragen process, has been developed to continuously convert the ∼2%-4% ethane component present in pipeline gas into ethylene at common pipeline pressures of 800 psi. In our studies of the Tragen process, pyrolysis without steam addition achieved a maximum ethylene yield of 28%-35% at a temperature range of 700-775 °C, corresponding to an ethylene concentration of 4600-5800 ppm in the product gas. Coke deposition was determined to occur at a significant rate in the pyrolysis reactor without steam addition. The δ 13C isotopic analysis of gas components showed a δ 13C value of ethylene similar to ethane in the pipeline gas, indicating that most of the ethylene was generated from decomposition of the ethane in the raw gas. However, δ 13C isotopic analysis of the deposited coke showed that coke was primarily produced from methane, rather than from ethane or other heavier hydrocarbons. No coke deposition was observed with the addition of steam at concentrations of >20% (by volume). The dilution with steam also improved the ethylene yield.
1. Introduction The storage of pipeline gas at underground storage facilities is essential for the natural gas industry to provide additional gas supplies during seasonal and short-term gas demand peaks. In the United States, there are over 400 storage facilities in use, including reservoirs, aquifers and salt caverns.1 Unfortunately, according to estimation by the gas industry, up to $100 million (U.S.) is written off as “lost and unaccountedfor gas” each year. Although some of this loss is due to measurement errors, a significant part of the loss is gas that remains trapped in the geological reservoir structures or that has migrated out of the underground storage facilities. There is a critical need within the pipeline gas industry for an identification tag or tracer that can be added to pipeline gas to improve the deliverability and management of gas storage fields. A suitable tracer would be important for determining communication pathways, inventory control, gas migra* Author to whom correspondence should be addressed. E-mail:
[email protected]. † Illinois State Geological Survey. ‡ University of Illinois at Champaign-Urbana. § Isotech Laboratories, Inc. (1) The Basics Of Underground Natural Gas Storage, Energy Information Administration of DOE (http://www. Eia. Doe. Gov/Pub/ Oil_Gas/Natural_Gas/Analysis_Publications/Storagebasics/Storagebasics), 2002.
tion studies, and in establishing ownership of gas that has leaked or been drawn from storage facilities. Many efforts were made in the past to examine various compounds as tracers in gas reservoirs for gas identification or gas migration studies. Tracers investigated included nitrogen,2 helium,3 hydrogen,4 carbon dioxide,5 sulfur hexafluoride,6 chloropentafluoroethane,6 tritiated ethane,7 tritiated methane,5,7 perfluoromethylcyclopentane (PMCP),7,8 perfluoromethylcyclohexane (PMCH),7,8 1-3-perfluorodimethylcyclohexane,8 and other perfluorocarbon tracers.9 Although there seems to be many tracers developed that can be utilized in gas (2) Cook, T. L.; Brown, L. F.; Meadows, W. R. Tracer Experiments in Eastern Devonian Shale, Report LAUR82543, 1982. (3) Gascoyne, M.; Wuschke, D. M. J. Hydrol. (Amsterdam) 1997, 196, 76-98. (4) Fasanino, G.; Molinard, J. E. NATO ASI Ser., Ser. E 1989, 171, 301-325. (5) Yeh, Y. J.; Kuo, M. C. T. Technical Paper SPE 27042, Society of Petroleum Engineers, Richardson, TX, 1994. (6) Malcosky, N. D.; Koziar, G. Gas Tracer Composition and Method, U.S. Patent No. 4690689, September 1, 1987. (7) Senum, G. I.; Dietz, R. N.; D’Ottavio, T. W.; Goodrich, R. W.; Cote, E. A. Perfluorocarbon Tracer Transport and Dispersion Experiment in the North Sea Ekofisk Oil Field, Technical Report BNL-43811, 1989. (8) Dugstad, O.; Bjornstad, T.; Hundere, I. A. J. Pet. Sci. Eng. 1993, 10, 17-25. (9) Kleven, R.; Hovring, O.; Opdal, S. T.; Bjornstad, T.; Dugstad, O.; Hundere, I. A. Non-Radioactive Tracing of Injection Gas in Reservoirs, Technical Paper SPE 35651, Society of Petroleum Engineers, Richardson, TX, 1996.
10.1021/ef0498216 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/29/2004
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reservoirs studies for various purposes, none are without some limitations. For example, some of these tracers have toxicity problems that may preclude their extensive utilization; some are unstable or less migrationmobile than the natural gas, and some are expensive, in terms of production and transportation. Ethylene is an ideal tracer because (i) it does not exist naturally in the pipeline gas; (ii) it is physically similar to other pipeline gas components and, thus, shows a similar migration mobility; (iii) it is stable and safe during storage; and (iv) it can be detected easily.10,11 However, it would be prohibitively expensive if obtained from an ethylene plant and transported to gas fields. On-site production of ethylene by a low-cost process could potentially be an attractive alternative approach for gas storage. A novel process, known as the Tragen process, is being developed and tested by Isotech Laboratories, Inc., and the Illinois State Geological Survey. This process uses a noncatalytic pyrolysis technology for continuously converting a small fraction of the pipeline gas to ethylene at pipeline pressures and injecting the pyrolysis product gases back into the main gas stream without an additional compression facility. Because the pyrolysis of methane does not occur until the temperature exceeds 1000 °C,12 the 2%-4% of ethane that is present in pipeline gas can be the main feedstock for a process that operates at a lower temperature. Ethane pyrolysis (cracking) has been the major source of ethylene in the petrochemical industry. The industrial process involves thermal dissociation of ethane at >800 °C and at/near atmospheric pressure. The mechanism and kinetics of this reaction have been widely studied since the 1930s.13-20 However, the effects of elevated pressure on the pyrolysis have not been studied in detail. A recent study examined ethane pyrolysis and oxidation at 340 and 613 bar and the temperature range of 777-1177 °C; however, the experiments were conducted in a pulse shock tube.21 Coke formation is a major concern for pyrolysis processes. It is a side reaction that inherently occurs during hydrocarbon pyrolysis (cracking). The two main mechanisms are known as pyrolytic coking, which involves the aromatic ring intermediates in the gas phase, and catalytic coking, which involves catalysts or the presence of catalytic sites on equipment surfaces.22-24 (10) Vogh, J. W.; Cotton, F. O.; Shelton, E. M.; Anderson, R. P. Identification of Injected Storage Gas, National Institute for Petroleum and Energy Report PRT-190-628, September 1987. (11) Vogh, J. W.; Thomson, J. S.; Anderson, R. P. Identification of Injected Storage Gas, National Institute for Petroleum and Energy Research Report GRI900337, 1990. (12) Holmen, A.; Olsvik, O.; Rokstad, Q. A. Fuel Process. Technol. 1995, 42, 249-267. (13) Rice, F. O.; Herzfeld, K. F. J. Am. Chem. Soc. 1934, 56 (2), 284289. (14) Cryder, D. S.; Porter, D. J. Ind. Eng. Chem. 1937, 29 (6), 667673. (15) Storch, H. H.; Kassel, L. J. Am. Chem. Soc. 1937, 59, 12401246. (16) Hepp, H. J.; Spessard, F. P.; Randall, J. H. Ind. Eng. Chem. 1949, 41 (11), 2531-2535. (17) Wall, L. A.; Moore, W. J. J. Am. Chem. Soc. 1951, 73 (6), 28402844. (18) Lee, W. M.; Yeh, C. T. J. Phys. Chem. 1979, 83 (7), 771-774. (19) Sundaram, K. M.; Froment, G. F. Chem. Eng. Sci. 1977, 32, 601-608. (20) Chao, Z. S.; Ruckenstein, E. J. Catal. 2004, 222, 17-31. (21) Tranter, R. S.; Sivaramakrishnan, R.; Brezinsky, K.; Allendorf, M. D. Phys. Chem. Chem. Phys. 2002, 4 (11), 2001-2010. (22) Albright, L. F.; Marek, J. C. Ind. Eng. Chem. Res. 1988, 27, 755-759.
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Various efforts have been made to control the coke formation. These include diluting the feedstock with inert gases, adding gasifying gases (such as steam and CO2), adding trace coking inhibitors (such as chloroplatinic acid25 and sulfur compounds),26,27 or coating dopants onto the metal wall of the reactor.26,27 However, coking kinetics and the effectiveness of these control measures seldom have been reported for pyrolysis at elevated pressures. The overall goal of this study was to evaluate the feasibility of the Tragen process for continuously converting the ∼2%-4% ethane present in pipeline gas into ethylene via the pyrolysis process at the normal pipeline pressure of 800 psi. The experiments were specially designed for this purpose, rather than for a mechanism study. The main effort was focused on investigating the pyrolysis and coking performance of the pipeline gas and examining the effectiveness of steam as a decoking agent. 2. Experimental Section 2.1. Apparatus and Procedures. Pyrolysis experiments were conducted at temperatures of 700-800 °C and at a pressure of 800 psi. Figure 1 shows a schematic diagram of the experimental system. A tubular reactor with a 1/4-in. inside diameter (ID), 3/4-in. outside diameter (OD), and 22-in. length, constructed from a heat-resistant stainless steel (Incoloy 800HT) was used. Two furnaces (Applied Systems, Inc), each connected to a programmable temperature controller (CN2011, Omega Technologies Company), were used to preheat the gas and control the reaction temperature in the pyrolysis reactor. The temperature of the preheater was controlled at 650 °C. The reactor is located in the top furnace and has a heating zone of 30 cm. The transition section between the two furnaces was insulated with glass wool. T1 and T2 are the controlling temperatures of the bottom and top furnaces, respectively. T3 and T4 are the temperatures at two different positions (10 cm apart) inside the reactor. Two thermocouples (model KMQIN062U, Omega Technologies Company) were used to measure T3 and T4. The pressure of the system was controlled using two regulators. The first regulator (at a pressure P1) reduced the natural gas cylinder pressures to the reaction pressure, and the second regulator (at a pressure P3) reduced the postreaction pressure to ∼20 psi. A mass flow controller was installed downstream of the low-pressure regulator, to control the gas flow rate. Product gas passed through the mass flow controller and was split into three streams: one to vent, the second to a gas chromatograph (Agilent 3000 Micro), and the third to a sampling tube IsoTube (Isotech Laboratories, Inc.). In some experiments, water was injected into the reaction gas to produce steam, using a high-pressure diaphragm pump (EVA103D-60-56SSM-EP, BPH Pump and Equipment, Inc.). Water flow rate was controlled by a flow adjuster and could be measured by the dropping rate of the liquid surface in the water supply container. To prevent condensed water from blocking the downstream instruments, an empty stainlesssteel tube was used to cool and condense the steam in the product gas. Condensed water in the tube could be removed through a needle valve (V5). Compressed natural gas was obtained from a gas storage field. Its composition is listed in Table 1. Analytical data for (23) Cai, H.; Krzywichi, A.; Oballa, M. C. Chem. Eng. Process. 2002, 41, 199-214. (24) Froment, G. F. Rev. Chem. Eng. 1990, 6 (4), 293-328. (25) Chan, K. Y. G.; Inal, F.; Senkan, S. Ind. Eng. Chem. Res. 1998, 37, 901-907. (26) Trimm, D. L. Catal. Today 1997, 37, 233-238. (27) Trimm, D. L. Catal. Today 1999, 49, 3-10.
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Figure 1. Schematic flow diagram of the pyrolysis reaction system. Table 1. Composition Analysis of Natural Gasa Composition (ppm)
cylinder number
O2 + Ar
CO2
N2
CO
C1
C2
C2H4
C3
C3H6
H2
1 2 3 4 5
2000 1800 1200 1900 1200
3100 3900 5900 5600 5900
20800 20000 15500 17900 15500
0 0 0 0 0
956300 956800 955200 953400 955200
16900 16800 18700 18000 18700
0 0 0 0 0
823 650 1500 1200 1500
0 0 0 0 0
n/ab n/ab 1200 1200 1200
aThe
sum of components may not be exactly 100%, because not all components are reported. b Not available.
different gas cylinders showed that the methane concentration of this gas was in the range of 95%-96%, and the ethane concentration was in the range of 1.6%-1.9% by volume). Compressed N2 was only used during system preheating, and the apparatus was switched to natural gas after the desired reaction temperature was attained. In some experiments, compressed pure methane was also employed for the purpose of comparison to natural gas. 2.2. Analysis Procedure. The composition of the product gas from the pyrolysis experiments was analyzed by two gas chromatographic procedures. The main components, except for H2, were analyzed online by an Agilent 3000 Microportable gas chromatograph (Agilent Technologies, Europe) that was equipped with a micro thermal conductivity detector (TCD). Helium was used as the carrier gas. Two separation columns were used in each channel. The first column was a molecular sieve (operating at 100 °C) for analyzing O2 + Ar, N2, CH4 and CO, and the second column (operating at 60 °C) was a Plot Q for analyzing CO2, C2H6, C2H4, C3H8 and C3H6. An offline gas chromatograph (Carle 100) that was equipped with a Porapak Q column and a thermal conductivity detector was used to determine the H2 content. Gas composition data were used to calculate the ethane conversion rate, ethylene yield, and selectivity, according to the following relationships:
(
ethane conversion rate (%) ) 1 -
ethylene selectivity (%) )
)
CC2H6 CC0 2H6
CC2H4 CC0 2H6 - CC2H6
ethylene yield (%) )
CC2H4 CC0 2H6
× 100
× 100
× 100
where CC0 2H6 is the ethane concentration in the original
natural gas, CC2H6 the ethane concentrations in the product gas, and CC2H4 the ethylene concentrations in the product gas. To identify reaction pathways of the coking process, the carbon isotopic compositions (13C/12C) of the pipeline gas and the pyrolysis products were analyzed. The conventional offline method was used, which consisted of chromatographic separation, followed by combustion and dual-inlet isotope ratio mass spectrometry (Finnigan MAT Delta S). Values for δ 13C were determined for C1-C3 hydrocarbons as well as CO2. Reproducibility was generally 0.1 per mil or better for the analysis.
3. Results and Discussion 3.1. Pyrolysis without Steam. Experimental data were used to evaluate the effect of temperature and residence time on ethylene yield, ethylene selectivity, and ethane conversion at 800 psi (Figure 2). An optimum residence time, corresponding to the highest ethylene yield, was observed for each test temperature. The peak ethylene yield increased as the temperature increased, and the optimum residence time decreased as the temperature increased (Figure 2a). The optimum residence time at 700 °C was ∼10 s and at 775 °C was ∼2 s. Ethane conversion increased as the residence time increased at all temperatures (Figure 2b). This trend is more obvious at higher temperatures. Longer reaction times and higher temperatures favored the further dehydrogenation of ethane. The selectivity of ethylene was dependent on the dynamic equilibrium between its formation and cracking. At temperatures of 700 and 750 °C, peak selectivities were observed (Figure 2c). However, at 775 °C, ethylene cracking was the dominant reaction, even at short residence times.
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pressure kinetic model.28 For example, at 750 °C, the model predicts a peak ethylene yield of ∼16%, compared to 32% obtained at 800 psi. The high pressure may favor the kinetic rate of ethylene formation. Ethylene could be formed by the following two overall reactions, under the experimental conditions in the study: k1
C2H6 98 C2H4 + H2 k2
CH4 98 0.5C2H4 + H2
(1) (2)
The kinetic data for these first-order reactions, at atmospheric pressure, are, for reaction 1,28
(
A1 ) 1014 exp -
288696 RT
)
(E1 ) 69 kcal/mol)
and, for reaction 2,29
A2 ) 10(7.642-12158/T)
Figure 2. Effects of temperature and residence time on (a) ethylene yield, (b) ethane conversion, and (c) ethylene selectivity. The temperature is specified as T2, which is the control temperature of furnace; the pressure is P ) 800 psi, and no steam was added.
Figure 3. δ 13C isotope analysis of the raw gas and the gaseous products of pyrolysis at 750 °C and a residence time of 5.5 s.
The aforementioned results also revealed a similar dependence of ethylene yield on the residence time to that of ethane pyrolysis at atmospheric pressure. Interestingly, the optimum ethylene yields obtained at 800 psi (the partial pressure of ethane was 14.4 psi) were actually higher than that predicted by the atmospheric-
(E2 ) 56 kcal/mol)
A is the pre-exponential factor and E is the activation energy. The reaction coefficient can thus be determined from the Arrhenius equation. These values were used to roughly estimate the relative contributions of methane and ethane-to-ethylene formation at 800 psi. Accordingly, at 750 °C, the reaction coefficient of reaction 1 is 2 orders of magnitude larger than that of reaction 2. Because the methane concentration in pipeline gas is much greater than the ethane concentration, the ethylene formation rate of reaction 1 was estimated to be ∼50 times larger than that of reaction 2. Therefore, most of the ethylene should be generated by ethane cracking. Figure 3 shows the δ 13C values for the raw gas and the product gas from pyrolysis at 750 °C and a residence time of 5.5 s. The results showed that δ 13C of the ethane in the product stream is comparable to that of the ethane in the raw natural gas, indicating that most of the ethane in the product stream is unconverted ethane in the raw gas. Comparable δ 13C values for the ethane and the ethylene in the product stream also indicate that most of the ethylene was generated from the ethane in the raw gas. Any amount of ethane formed from methane via the combination reaction is not significant. However, this conclusion is not valid for propane and propylene. The δ 13C values of propane in the product stream and in the raw gas are very different, indicating that a significant proportion of propane and propylene in the product stream is generated during the pyrolysis process. Based on their δ 13C values, it can be concluded that propylene and propane are produced by a combination reaction between methane and ethane in the raw gas. The concentrations of the main gas products from pyrolysis at a temperature of 750 °C are displayed in Figure 4. No significant amount of methane was converted during pyrolysis. The propylene concentration showed the same trend as ethylene, but the propane concentration did not present the decreasing trend with (28) Zou, R. J. Fundamentals of Pyrolysis in Petrochemistry and Technology; CITIC Publishing House: Beijing, 1993; pp 83-91. (29) Khan, M. S.; Crynes, B. L. Ind. Eng. Chem. 1970, 62 (10), 5459.
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Figure 4. Concentrations of several hydrocarbon compounds in products from pyrolysis at 750 °C: (a) concentrations of methane and ethane, and (b) concentrations of CO, C2H4, C3H8, and C6+.
Figure 5. Temperature profile along the pyrolysis reactor at 700 and 750 °C.
residence time that was observed for ethane. A small amount of C4, mainly the olefin at a level of up to 30 ppm, was observed but is not displayed here. No C5 was detected; however, total concentrations of C6 and C6+ of >250 ppm were observed in the pyrolysis gases at longer residence times. The previously mentioned residence times were calculated using the value of the gas flow rate at the control temperature T2 and 800 psi. The real residence time could be different, because the temperature along the tubular reactor was not uniform. Gas temperatures T3 and T4 inside the reactor were measured and used to estimate the heat-transfer coefficient and the temperature profiles along the reactor. Figure 5 gives the temperature profiles at flow rates of 1.6 and 3.2 L/min (STP) at several control temperatures. These results indicate that, at higher flow rates (shorter residence times), the real residence time could be even shorter than the value estimated on the basis of the control temperature (T2). 3.2. Coke Formation during Pyrolysis without Steam. Coke deposition was investigated at temperatures of 700, 725, and 750 °C, and residence times of 11.3, 5.5, and 3.5 s, respectively. These conditions correspond to the optimum ethylene yields, as shown in Figure 2. Each experiment was run continuously until the reactor was completely blocked. The reactor was then cooled to room temperature and the coke deposited inside was mechanically removed and weighed. During a normal run, most of the carbon was deposited in a blockage region 2.5-5 cm long inside the
Figure 6. Coking deposition rate during pyrolysis: natural gas pyrolysis at 700 °C and a residence time of 11.3 s, natural gas pyrolysis at 725 °C and a residence time of 5.5 s, natural gas pyrolysis at 750 °C and a residence time of 3.5 s, and pure methane pyrolysis at 800 °C and a residence time of 5.1 s.
reactor. Only small amounts of carbon were observed outside of the plugged region. Very little carbon was observed downstream of the reactor, indicating that no large molecules that could condense in cooler positions were formed during the pyrolysis reactions. This was also supported by the low concentration levels of C6 and larger molecules detected in the product stream. The combined total concentration of C6 and C6+ in the product gases were 56 ppm at 700 °C, 40 ppm at 725 °C, and 26 ppm at 750 °C. These concentration levels are much lower than those reported for the pyrolytic coking mechanism.30 This suggests that the carbon might be deposited primarily through heterogeneous reactions on the reactor surface. Iron and nickel contents that exist in the reactor’s metal material may contribute to this mechanism.31,32 It was also observed that a reduction of the pyrolysis temperature increased the length of the blockage zone. For example, at 750 °C, the blockage was ∼2.5 cm long, and at 700 °C, it was ∼5 cm long. This was expected, because lower gas flow rates were used at lower temperatures, based on their optimum residence times. The gas-phase temperature along the reactor is more uniform at the lower gas flow rate (Figure 5), and the coke (30) Glasier, G. F.; Pacey, P. D. Carbon 2001, 39, 15-23. (31) Blaikley, D. C. W.; Jorgensen, N. Catal. Today 1990, 7 (2), 277286. (32) Kopinke, F. D.; Zimmermann, G.; Nowak, S. Carbon 1988, 26, 117-124.
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Table 2. Results of Steam Pyrolysis of Natural Gas at 750 °C and 800 psi Concentration of Main Gas Componentsa (ppm, dry basis)
Flow Rate (L/min, STP) run run time (h) 1 2 3 4 5 6
6.25 7.5 19.9 9.5 18.0 7.0 a
steam
natural gas, NG
1.63 0.27 0.67 0.61 0.43 0.21
1.92 1.92 1.70 1.94 1.94 1.94
residence steam carbon time (s) content (%) deposit (g) 2.33 3.77 3.49 3.25 3.49 3.85
46.3 12.3 28.4 23.9 18.1 9.80
none 0.67 none none none 0.88
CH4
C2H6
C2H4
C3H8
C3H6
CO
H2
935000 960500 930900 925100 939900 941100
9900 9500 9100 12100 12100 12500
5500 5600 5600 5300 5400 5200
240 300 240 400 400 450
720 1100 930 1000 1100 1100
190 130 160 140 180 790
10500 n/a 12800 10700 11100 14600
The concentrations of argon, O2, CO2, N2, and helium are not listed here.
Figure 7. Result of δ 13C isotope analyses of carbon deposited under several pyrolysis conditions.
deposition would thus be more uniform inside of the reactor. The average coke deposition rate was calculated, based on the total amount of coke deposited, the total experimental time, and the surface area of the plugged zone (Figure 6). Note that the initial coking rate will differ from the coking rate after the surface is covered by the coke. To identify the source of the deposited carbon, a pyrolysis test using pure methane was performed at 800 °C, 800 psi, and a residence time of 5.1 s (see Figure 6). A complete blockage had not occurred, even after almost 8 h. However, significant amounts of carbon had been deposited and the reactor was almost blocked. The reason for this could be that, at higher pressures, methane reactions proceed at higher kinetic rates, resulting in more carbon formation. No C4, C5, and C6 were detected in the product gas during the methane pyrolysis, indicating that the synthesis of heavier hydrocarbons had a small role in the carbon deposition. Results of δ 13C isotope analyses of the deposited carbon, together with those of the raw natural gas, are
shown in Figure 7. The carbon deposits at all test temperatures have a δ 13C value that is comparable to (or slightly more negative than) that of the methane in the raw natural gas. This indicates that the coke deposited on the reactor surface was produced from the gas-phase methane, rather than from ethane or heavier hydrocarbons. This finding is consistent with the results from the pure methane pyrolysis test. 3.3. Effect of Steam Addition. Because coke deposition had occurred at a considerable rate under all anhydrous test conditions, steam was investigated as a potential reactant to reduce or prevent coke deposition. These experiments were conducted at 750 °C, 800 psi, a total flow rate of ∼2.4 L/m (residence time of 3.5 s), and water vapor concentrations in the range of 9.8%46% (by volume). Two experiments (runs 2 and 6), with water concentrations of 9.8% and 12.3%, respectively, revealed that carbon deposition occurred in the hottest reaction zone (Table 2). The coke deposition rates observed in both cases were significantly lower than those without steam addition. Without steam, the reactor was completely blocked within 2 h, whereas, with a water concentration of ∼10%, the reactor was not blocked after more than 7 h (see runs 2 and 6; in both cases, total blockage did not occur). At water concentrations of 18% and above, no carbon deposition was observed at the reactor surface after ∼20 h (see experimental runs 1, 3, and 5). The inside wall of the reactor was very clean and showed no sign of carbon deposition. The condensed water was clear and contained no carbon particles, indicating that pyrolytic carbon formation from gas-phase reactions was also prevented. It was observed that concentrations of ethylene in the product gas (after water was removed) were >5000 ppm and yields of ethylene were ∼30% in all experiments. These yields exceeded those obtained in the pyrolysis
Figure 8. Impact of residence time on ethylene production at 750 °C and 800 psi. The experiment kept the steam volume percentage at ∼20% in the reactor. Residence times of 2.58, 3.46, and 4.88 s correspond to total flow rates of 3.21, 2.39, and 1.70 L/min (STP), respectively.
Noncatalytic Pyrolysis of Natural Gas
process without steam addition, where the ethylene concentration was ∼4000 ppm, under the same conditions. The higher ethylene yield will help reduce the total volume of natural gas that must be treated in industrial applications. However, note that the ethylene concentrations in Table 2 are calculated on a dry basis. Under the given reaction conditions, the ethylene concentrations would be lower, because of dilution by water vapor. For example, for run 1 in Table 2, the actual ethylene concentration in the pyrolysis reactor would be ∼5500 ppm × 1.92/3.55 ) 2975 ppm. Because water vapor favors the thermodynamics of the pyrolysis process, it is expected that the ethylene yield in the steam pyrolysis process will be higher than that of the pyrolysis process without steam addition. The influence of residence time on the concentration of ethylene in the product gas with steam in the reactor also was studied (see Figure 8). As the residence time increased to 3.5 s, the concentration of ethylene increased slightly and then remained at an approximately constant level. The maximum residence time in this study was 4.88 s at 750 °C. It is possible that the optimum flow rate (residence time) for steam pyrolysis may be different from that for pyrolysis without steam. However, it is clear that longer residence time resulted in greater ethane conversion, lower ethylene selectivity, and greater hydrogen concentrations, because of the reforming reactions of natural gas.
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observed for temperatures of 700-775 °C. Isotopic analyses showed that the δ 13C value of the ethylene in the product gas was similar to that of the ethane present in both the raw gas and the product gas, indicating that most of the ethylene was generated from pyrolysis of the ethane in the raw gas. Significant quantities of coke were deposited at the surface of the reactor during pyrolysis without steam. For example, a coking rate as great as 17.1 g m-2 min-1 at 750 °C was observed. Isotopic analyses showed that the coke was mostly formed from methane rather than from ethane or other heavier hydrocarbons in the raw gas. This conclusion was consistent with the observation that coke deposition also occurred when pure methane was run through the reactor. The addition of steam into the reactor during pyrolysis significantly reduced coke deposition. At steam concentrations of >20% (by volume), no coke deposition was observed either at the surface of reactor or at locations downstream of the reactor. The addition of steam during pyrolysis also improved the ethylene yield: the yield was 36% at 750 °C for a residence time of 3.5 s, compared to a yield of 31% for pyrolysis without steam addition. The results presented here indicate that the Tragen process is technically feasible for generating a low-cost tracer for natural gas storage. Future pilot demonstration testing of the Tragen process will provide additional process engineering data needed to advance the technology to a commercial level.
4. Conclusion The pyrolysis of pipeline gas at 800 psi showed a dependence of ethylene yield, with respect to temperature and residence time, that was comparable to that for the reaction at atmospheric pressure. Without steam addition, maximum ethylene yields of 28%-35% and ethylene concentration levels of 4600-5800 ppm were
Acknowledgment. The authors are grateful for the financial support of this work from an Illinois Technology Challenge Grant (No.02-49103) through the Illinois Department of Commerce and Community Affairs (DCCA). EF0498216
