Ind. Eng. Chem. Res. 1997, 36, 2075-2079
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Noncatalytic Oxypyrolysis of C2+-Hydrocarbons from Natural Gas to Ethylene and Propylene in a Most Energy-Efficient and Safe Manner Vasant R. Choudhary,* Shafeek A. R. Mulla, and Amarjeet M. Rajput Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
Noncatalytic oxypyrolysis of C2+-hydrocarbons from natural gas at 700-850 °C in the presence of steam and limited oxygen yields ethylene and propylene with appreciable conversion and high selectivity but with almost no coke or tarlike product formation. In this process, the exothermic oxidative hydrocarbon conversion reactions are coupled directly with the endothermic cracking of C2+-hydrocarbons by their simultaneous occurrence. Hence, the process operates in a most energy-efficient and safe (or nonhazardous) manner and also can be made almost thermoneutral or mildly endothermic/exothermic, thus requiring little or no external energy for the hydrocarbon conversion reactions. Introduction Energy-efficient conversion of natural gas into ethylene (which is a keystone to the petrochemical industry) is of a great practical importance. Natural gas contains hydrocarbons (methane as the main constituent, ethane, propane, and butanes as minor constituents, and C5+-hydrocarbons in traces) which differ widely in their reactivity. Activation of methane is very difficult, but that of higher hydrocarbons is relatively easier. In the IFP (Institute Francais du Petrole) oxypyrolysis process for the conversion of natural gas to olefins (Mimoun et al., 1990), the catalytic oxidative coupling of methane (OCM) reaction (which is highly exothermic and hence highly hazardous) is combined with the endothermic thermal (noncatalytic) cracking of C2+hydrocarbons (separated from the natural gas) occurring under oxygen-deficient conditions by allowing these two reactions to occur separately in two different zones (connected in series) of the same reactor. The heat carried by product stream of the exothermic OCM reaction in the first zone is then partially utilized in the second zone (i.e., the post OCM catalyst bed zone) for the endothermic thermal cracking of C2+-hydrocarbons. Although, the heat produced in the OCM reaction is utilized partially in the cracking of C2+-hydrocarbons, yet this process is highly hazardous, less energy efficient, and very difficult to operate/control as there is no direct coupling of the exothermic and endothermic reactions and also this process requires separation of C2+-hydrocarbons from natural gas. Our earlier studies (Choudhary et al., 1991) showed appreciable conversion of methane (of practical interest) to ethylene in the noncatalytic (homogeneous) oxypyrolysis of methane at 750-900 °C. We have also shown earlier (Choudhary et al., 1989, 1994a) that ethylene in high yields can be obtained from natural gas by its noncatalytic (homogeneous) oxypyrolysis. This investigation was undertaken with an objective of studying the noncatalytic oxypyrolysis of natural gas in the presence of steam and limited O2, thus allowing the endothermic thermal cracking of C2+-hydrocarbons to occur simultaneously with the exothermic hydrocar* To whom all correspondence should be addressed. Telephone: (91) 212-336451 (ext. 2163). Fax: (91)-212-333941/ 330233. E-mail:
[email protected]. S0888-5885(96)00373-9 CCC: $14.00
bon oxidative conversion reactions for making the overall process thermoneutral or mildly exothermic/ endothermic and thereby making the process highly energy efficient and the process operation nonhazardous and simple. Experimental Section The noncatalytic oxypyrolysis of natural gas (NG) was carried out at atmospheric pressure in an empty quartz reactor (i.d., 15 mm; volume, 5.2 cm3; L/D ratio ≈ 2.0), having a low dead volume, at different temperatures (700-850 °C), NG/O2 mole ratios (6.0-18.5) in the feed, and space velocities (1000-3000 h-1), using a mixture of natural gas (composition, mol %: methane, 87.8%; ethane, 5.6%; propane, 3.5%; butanes, 1.6%; CO2, 1.2%; N2, balance), oxygen, and steam as a feed. The reactor has mixed-flow characteristics. It is described elsewhere (Choudhary et al., 1991). It was kept in a constant-temperature zone of a tubular furnace (diameter: 19.0 mm). The reaction temperature was measured by a chromel-alumel thermocouple located axially in the center of the reactor. The gas hourly space velocity (GHSV) was measured at 0 °C and 1 atm. The reactor effluent gases, after removing water by condensation at 0 °C, were analyzed by an on-line gas chromatograph using Porapak-Q and Spherocarb columns. The C, H, and O balance across the reactor was within 5%. The results were reproducible within 3-5%. The conversion and selectivity (which is based on the carbon content of the hydrocarbons present in the natural gas) are defined as follows: carbon (in NG) conversion (%) ) carbon in all the products (e.g., CO, CO2, and olefins) × 100 carbon in hydrocarbons from NG product selectivity ) carbon in a particular product × 100 carbon in all the products formed conversion of individual hydrocarbons (%) ) hydrocarbon in feed - hydrocarbon in product stream × 100 hydrocarbon in feed
Results and Discussion Results showing the influence of temperature, space velocity, and natural gas/oxygen mole ratio in the feed © 1997 American Chemical Society
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Figure 1. Effect of temperature on the oxypyrolysis of C2+hydrocarbons from natural gas [GHSV (gas hourly space velocity measured at 0 °C and 1 atm) ) 3000 h-1, NG/O2 ) 11.5, H2O/NG ) 1.0, Carbon (in NG) ) carbon content of all the hydrocarbons present in the natural gas].
Figure 3. Effect of NG/O2 mole ratio in the feed on the oxypyrolysis of C2+-hydrocarbons from natural gas (temperature, 800 °C; GHSV ) 2000 h-1; H2O/NG ) 1.0).
Figure 2. Effect of space velocity on the oxypyrolysis of C2+hydrocarbons from natural gas (temperature, 850 °C; NG/O2 ) 11.5; H2O/NG ) 1.0).
on the conversion of carbon (i.e., carbon content of all the hydrocarbons in the natural gas) and also of the individual hydrocarbons (viz. methane, ethane, propane, and butanes) present in the natural gas, on the selectivity for CO, CO2, ethylene, and propylene, and on the net heat of reactions in the oxypyrolysis of C2+hydrocarbons from natural gas in the presence of steam (NG/steam mole ratio ) 1.0) and limited O2 are presented in Figures 1-3. The influence of steam/NG ratio in the feed on the conversion, net heat of the reaction, and product selectivity at two different temperatures (750 and 850 °C) is shown in Figures 4 and 5. The negative conversion of methane indicates the formation of methane from the higher hydrocarbons. The net heat of reactions (∆Hr, expressed in kilocalories per mole of carbon converted) in the oxypyrolysis of C2+-hydrocarbons from natural gas is obtained by subtracting the heat of formation (at the reaction temperature) of the components in the feed from that of the components
Figure 4. Effect of H2O/NG ratio in the feed on the conversion of carbon and individual hydrocarbons from natural gas and net heat of reaction in the oxypyrolysis of C2+-hydrocarbons from natural gas (NG/O2 ) 12.0; GHSV ) 2000 h-1).
present in the product stream. The heat of formation data (Stull et al., 1965) used in the estimation of the net heat of reactions are given in Table 1. The overall process is exothermic and endothermic when the net heat of reactions (∆Hr) is negative and positive, respectively. The product selectivity is based on the conversion of carbon (in NG). The influence of the various reaction parameters on the oxypyrolysis process is found to be as follows. Effect of Temperature. Figure 1 shows a strong influence of temperature on the conversion of carbon (in NG), O2, ethane, and propane, on the selectivity for CO and propylene, on the ethylene/ethane and propylene/
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Figure 5. Effect of the H2O/NG ratio in the feed on the product selectivity in the oxypyrolysis of C2+-hydrocarbons from natural gas (NG/O2 ) 12.0; GHSV ) 2000 h-1). Table 1. Data for the Heat of Formation (∆Hf) of Feed and Product Components at Different Temperatures heat of formation, ∆Hf (kcal‚mol-1) feed or product
700 °C
750 °C
800 °C
850 °C
methane ethane propane butane ethylene propylene CO CO2 H2O O2 H2
-21.82 -26.07 -32.03 38.89 +8.47 -1.12 -27.19 -94.88 -59.29 0.0 0.0
-21.93 -26.19 -32.14 -38.99 +8.38 -1.23 -27.25 -94.90 -59.37 0.0 0.0
-22.04 -26.30 -32.23 -39.08 +8.28 -1.35 -27.32 -94.92 -59.45 0.0 0.0
-22.13 -26.40 -32.32 -39.18 +8.18 -1.46 -27.40 -94.99 -59.52 0.0 0.0
propane ratios, and also on the net heat of reaction (∆Hr). However, there is little or no influence of the temperature on the conversion of butane and methane and also on the selectivity for CO2 and ethylene. With an increase in temperature from 700 to 850 °C, (a) the conversion of carbon (in NG), O2, ethane, and propane is increased from 7.3 to 19.3%, 20.0 to 80%, 12.1 to 54.2%, and 42.8 to 90.2%, respectively, (b) the selectivity for CO is increased from 5.4 to 14.3%, whereas that of propylene is passed through a maximum, (c) the C2H4/ C2H6 and C3H6/C3H8 ratios are increased exponentially from 0.5 to 2.7 and 0.3 to 5.2, respectively, and (d) the net heat of reaction (∆Hr) is increased from -7.8 to +3.9 kcal‚mol-1 (i.e., the mildly exothermic process becomes a mildly endothermic one). Effect of Space Velocity. Results in Figure 2 indicate that the space velocity (i.e., contact time) has a very significant effect on the conversion of O2, ethane, and propane, on the selectivity for CO, ethylene, and propylene, on the olefin/alkane ratios, and also on the net heat of reaction (∆Hr) but has little or no influence on the conversion of methane and butane and also on the CO2 selectivity. When the GHSV is decreased (i.e., contact time is increased) from 3000 to 1000 h-1, (a) the
conversion of carbon (in NG), O2, ethane, and propane is increased from 19.3 to 20.9%, 38.0 to 80.0%, 54.2 to 85.3%, and 90.2 to 100%, respectively, (b) the selectivity for CO is increased from 14.3 to 30.2%, that of propylene is decreased from 23.9 to 5.4%, but that of ethylene passed through a maximum, (c) the C2H4/C2H6 and C3H6/C3H8 ratios are increased from 2.7 to 9.3 and 5.2 to 18, and (d) the net heat of reaction (∆Hr) is decreased from +3.9 to -10.0 kcal‚mol-1 (i.e., the endothermic process becomes an exothermic one). Effect of NG/O2 Ratio. The NG/O2 ratio in the feed also has a strong influence on the conversion (except that of butane), selectivity (except for CO2), olefin/alkane ratios, and net heat of reactions in the oxypyrolysis of natural gas (Figure 3). With an increase in the NG/O2 ratio from 6.0 to 18.5, (a) the conversion of carbon (in NG), O2, ethane, and propane is decreased from 21.1 to 13.1%, 60 to 30%, 62.1 to 40.1%, and 93.0 to 75.3%, respectively, (b) the selectivity for CO is decreased from 32.4 to 7.1% but that for ethylene and propylene is increased from 58.9 to 74.5% and 7.3 to 17.6%, respectively, (c) the C2H4/C2H6 and C3H6/C3H8 ratios are decreased from 3.4 to 1.7 and 2.5 to 1.0, respectively, and (d) the net heat of reaction (∆Hr) is increased from -18.6 to +2.0 kcal‚mol-1 (i.e., the exothermic process becomes an endothermic one). Effect of H2O/NG Ratio. The results in Figures 4 and 5 show that the influence of H2O/NG in the feed on the net heat of reaction and product selectivity is complex. When the H2O/NG ratio is increased from 0 to 2.0, the conversion, selectivity, and net heat of reaction are influenced as follows. At 750 °C, the conversion of carbon (in NG) and ethane is decreased from 20 to 10.6%, and 47.2 to 19.0%, respectively, and that of propane and butanes is increased from 57.0 to 80% and 49.8 to 100%, respectively, at low H2O/NG ratio, but remained almost constant at high H2O/NG ratio. The net heat of reaction (∆Hr) is decreased from -10.6 to -15.4 kcal‚mol-1 (at low H2O/ NG ratios), passed through a minimum at a H2O/NG ratio of about 0.3, and then increased to 10.6 kcal‚mol-1. The selectivity for ethylene is increased from 54.0 to 65.6% (at low H2O/NG ratio), passed through a maximum at a H2O/NG ratio of about 0.3, and then decreased to 54.1%. The selectivity for propylene is decreased from 26.4 to 15.1% (at low H2O/NG ratio), passed through a minimum at a H2O/NG ratio of about 0.3, and then increased to 21.1%. The CO selectivity is decreased from 18.5 to 16.0%. However, there is little or no change in the CO2 selectivity. At 850 °C, the conversion of carbon (in NG) and ethane is decreased from 23.6 to 18.8% and 76.5 to 69.5%, respectively, and that of propane and butane is increased from 93.8 to 100% and 63.1 to 100%, respectively, at low H2O/NG ratio, but remained almost constant at high H2O/NG ratio. The net heat of reaction (∆Hr) is decreased from -5.9 to -10.9 kcal‚mol-1 (at low steam/NG ratios), passed through a maximum at a H2O/NG ratio of about 0.3, and then increased to +2.5 kcal‚mol-1. The selectivity for ethylene is increased from 52.7 to 74.1%, and propylene selectivity is decreased from 24.0 to 6.7% (at low H2O/NG ratio) and remains almost constant at high H2O/NG ratio. The CO selectivity is increased from 22.7 to 26.7% (at low H2O/ NG ratio), passed through a maximum at a high H2O/ NG ratio of about 0.3, and then decreased to 17.7%. There is little or no change in the CO2 selectivity. The
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increase in the H2O/NG ratio has little or no effect on the formation of H2 in the process. Although the conversion of butane at all the process conditions was almost 100%, there was no formation of butenes or other C4 alkenes. A significant conversion of methane was observed only for the high concentration of oxygen in the feed. No coke or tar formation is observed when steam is present in the feed. However, in the absence of steam, brownish tarlike products are found to be deposited on the reactor outlet tube walls. The above results indicate that natural gas can be converted to ethylene with minor amounts of propylene with high selectivity at appreciable conversion (which is of practical interest) of carbon in the natural gas by its thermal (or noncatalytic) oxypyrolysis in the presence of steam and limited O2, without formation of coke or tarlike products. This process, however, involves a complex network of a large number of exothermic hydrocarbon conversion (e.g., oxidative dehydrogenation of C2+-hydrocarbons) and combustion reactions and endothermic thermal cracking of C2+-hydrocarbons. Since the conversion of O2 is much less than 100%, the oxidative conversion reactions are expected to occur throughout the reactor. Thus, both the oxidative conversion and thermal cracking or decomposition reactions occur simultaneously. A very large number of freeradical reactions (Geisbrecht and Daubert, 1975) are expected to occur in the oxypyrolysis of natural gas. The important chain initiation and olefins and carbon oxides forming reactions expected to occur in the process are as follows:
Ethane Conversion Reactions C2H6 + O2 f C2H5 + HO2 (endothermic)
(1)
C2H6 f C2H5 + H (highly endothermic)
(2)
C2H5 + O2 f C2H4 + HO2 (exothermic)
(3)
C2H5 f C2H4 + H (endothermic)
(4)
C2H5, C2H4 f CO, CO2, and CH4 (highly exothermic) (5) Propane Conversion Reactions C3H8 f C3H6, C2H4, CH4, and H2 (endothermic) (6) C3H8 f CO, CO2, H2O, CH4, C2H4, and C3H6 (highly exothermic) (7) Butane Conversion Reactions C4H10 f C3H6, C2H4, CH4, and H2 (endothermic) (8) C4H10 f CO, CO2, H2O, CH4, C2H4, and C3H6 (highly exothermic) (9) The activation energy (∆E) for the chain initiation reactions (1) and (2) and ethylene forming reactions (3) and (4) in the ethane conversion is in the following order (Chen et al., 1991): reaction (2) (∆E ) 89.0 kcal‚mol-1) . reaction (1) (∆E ) 51 kcal‚mol-1) > reaction (4) (∆E ) 41.1 kcal‚mol-1) . reaction (3) (∆E ) 6.4 kcal‚mol-1). At lower temperatures and/or higher concentrations of O2, reactions (1), (3), and (5) become predominant over
reactions (2) and (4), making the overall conversion process exothermic. However, at lower concentrations of O2, reaction (4) (which is endothermic) also becomes important even at lower temperatures, causing a decrease in the process exothermicity, whereas, at higher temperatures and lower concentrations of O2, reactions (2) and (4) become very predominant, making the process endothermic. Thus, in general, the endothermic cracking reactions are predominant at higher temperatures and lower concentration of O2 and the exothermic oxidative conversion reactions become more important at higher concentrations of O2 and lower temperatures. The observed changes in the net heat of reactions (Figures 1 and 3) are consistent with it. The overall process can be made almost thermoneutral or mildly exothermic/endothermic by manipulating the temperature and NG/O2 ratio in the feed. In this process, the exothermic and endothermic reactions occur simultaneously. Because of the direct coupling of the exothermic and endothermic reactions, the heat produced in the exothermic reactions is used instantly by the endothermic reactions which are increasingly favored at higher temperatures. This has two important advantages: one, the process occurs in a most energy-efficient manner, and, second, the possibility of hot spot formation leading to reaction runaway is totally avoided as result of bufferlike action on the reaction temperature. Therefore, although the process is very complex, its operation/control is very simple as there are no serious heat-transfer problems. Direct coupling of exothermic and endothermic reactions having similar advantages of great practical importance has also been investigated by us for oxy-steam reforming (Choudhary et al., 1994b), oxy-CO2 reforming (Choudhary et al., 1995a), and simultaneous oxy-steam and oxy-CO2 reforming (Choudhary et al., 1994c) of methane to syngas and also for catalytic oxidative conversion of ethane to ethylene in the presence of limited O2 using a diluted catalyst bed (Choudhary et al., 1995b). In the later process, the energy input to the reactor furnace relative to the energy input required in the absence of any reaction to maintain the same temperature was found to be quite consistent with the net heat of reaction (∆Hr) at the different reaction conditions (Choudhary et al., 1995b). In the present case also, a similar observation is made. Apart from the above advantages, unlike the IFP process (Mimoun et al., 1990), this process does not require separation of C2+-hydrocarbons from natural gas and can also be operated at lower temperatures (750850 °C). Conclusions By the noncatalytic oxypyrolysis of C2+-hydrocarbons from natural gas in the presence of steam and limited oxygen, ethylene and propylene with high selectivity at a carbon (from all the hydrocarbons in NG) conversion of practical interest and also with almost no coke or tarlike product formation can be produced in a most energy-efficient and safe manner, requiring little or no external energy. This process involves a direct coupling of the exothermic oxidative conversion and endothermic thermal cracking reactions (of C2+-hydrocarbons in NG) due to their simultaneous occurrence, thus making the oxypyrolysis process almost thermoneutral or mildly exothermic/endothermic, depending upon the process conditions and also avoiding the hot spot formation and reaction runaway conditions. Hence, although this
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process involves a complex network of a large number of hydrocarbon conversion reactions, its operation is very simple. Acknowledgment S.A.R.M. is grateful to the Council of Scientific and Industrial Research, New Delhi, India, for the award of Senior Research Fellow. Literature Cited Chen, Q.; Hoebink, J. H. B. J.; Marin, G. B. Kinetics of the Oxidative Coupling of Methane at Atmospheric Pressure in the Absence of Catalyst. Ind. Eng. Chem. Res. 1991, 30, 2088. Choudhary, V. R.; Chaudhari, S. T.; Rajput, A. M. A Process for Conversion of Natural Gas to Ethylene. Indian Patent Appl. 988/DEL/89, 1989. Choudhary, V. R.; Chaudhari, S. T.; Rajput, A. M. Oxidative Pyrolysis of Methane to Higher Hydrocarbons: Effects of Water in Feed. AIChE J. 1991, 37, 915. Choudhary, V. R.; Sansare, S. D.; Rajput, A. M. A Two-Step Process for Production of Liquid-Hydrocarbon from Natural Gas. U.S. Patent 5,306,854, 1994a. Choudhary, V. R.; Rajput, A. M.; Prabhakar, B. Coupling of Catalytic Partial Oxidation and Steam Reforming of Methane to Syngas. 207th Annual ACS Meeting (Symposium on Methane and Alkane Conversion), San Diego, March 13-18, 1994b. Choudhary, V. R.; Rajput, A. M.; Prabhakar, B. NiO/CaO-Catalysed Formation of Syngas by Coupled Exothermic Oxidative
Conversion and Endothermic CO2 and Steam Reforming of Methane. Angew. Chem., Int. Ed. Engl. 1994c, 33, 2104. Choudhary, V. R.; Rajput, A. M.; Prabhakar, B. Energy Efficient Methane-to-Syngas Conversion with Low H2/CO Ratio by Simultaneous Catalytic Reaction of Methane with CO2 and Oxygen over NiO-CaO. Catal. Lett. 1995a, 32, 391. Choudhary, V. R.; Uphade, B. S.; Mulla, S. A. R. Coupling of Endothermic Thermal Cracking With Exothermic Oxidative Dehydrogenation of Ethane to Ethylene Using Diluted SrO/ La2O3. Angew. Chem., Int. Ed. Engl. 1995b, 34 (6), 665. Geisbrecht, R. A.; Daubert, M. E. Chemical and Physical Processes of Hydrocarbon Combustion: Chemical Processes. Ind. Eng. Chem. Process Des. Dev. 1975, 14, 159. Mimoun, A. R.; Bonnaudet, S.; Cameron, C. J. Oxypyrolysis of Natural Gas. Appl. Catal. 1990, 58, 269. Stull, D. R. Janaf Thermochemical Tables; Clearinghouses Federation of Scientific and Technical Information, U.S. Department of Commerce/National Bureau of Standards: Washington, DC, 1965.
Received for review July 1, 1996 Revised manuscript received January 28, 1997 Accepted February 3, 1997X IE9603732
X Abstract published in Advance ACS Abstracts, April 15, 1997.
