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Ind. Eng. Chem. Res. 1997, 36, 3520-3527
Coupling of Exothermic and Endothermic Reactions in Oxidative Conversion of Natural Gas into Ethylene/Olefins over Diluted SrO/La2O3/SA5205 Catalyst Vasant R. Choudhary* and Shafeek A. R. Mulla Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
In the oxidative conversion of natural gas to ethylene/lower olefins over SrO (17.3 wt. %)/La2O3 (17.9 wt. %)/SA5205 catalyst diluted with inert solid particles (inerts/catalyst(w/w) ) 2.0) in the presence of limited O2, the exothermic oxidative conversion reactions of natural gas are coupled with the endothermic C2+ hydrocarbon thermal cracking reactions for avoiding hot spot formation and eliminating heat removal problems. Because of this, the process is operated in the most energy-efficient and safe manner. The influence of various process variables (viz. temperature, NG/O2 and steam/NG ratios in feed, and space velocity) on the conversion of carbon and also of the individual hydrocarbons in natural gas, the selectivity for C2-C4 olefins, and also on the net heat of reactions in the process has been thoroughly investigated. By carrying out the process at 800-850 °C in the presence of steam (H2O/NG g 0.2) and using limited O2 in the feed (NG/O2 ) 12-18), high selectivity for ethylene (about 60%) or C2-C4 olefins (above 80%) at the carbon conversion (>15%) of practical interest could be achieved at high space velocity (g34 000 cm3‚g-1(catalyst) h-1], requiring no external energy and also without forming coke or tar-like products. The net heat of reactions can be controlled and the process can be made mildly exothermic or even close to thermoneutral by manipulating the O2 concentration in the feed. Introduction Natural gas is comprised mainly of methane with smaller amounts of C2-C4 alkanes and traces of C5+ hydrocarbons, which all differ widely in their reactivity. For the effective utilization of natural gas, conversion of natural gas into ethylene and other lower olefins in a safe and energy-efficient manner is of great practical importance. Since the past decade, worldwide efforts have been made toward the oxidative coupling of methane (OCM) to ethane/ethylene (Anderson, 1989; Lee and Oyama, 1988; Hutchings et al., 1989; Lunsford, 1990, 1995) and also toward the oxidative dehydrogenation of ethane to ethylene (Argent and Harris, 1986; Morales and Lunsford, 1989; Kennedy and Cant, 1991; Swaan et al., 1992; Desponds et al., 1993; Huff and Schmidt, 1993; Burch and Crabb, 1993; Choudhary et al., 1995). Since the hydrocarbons present in natural gas differ widely in their reactivity, direct oxidative conversion of natural gas under drastic conditions, similar to those employed in the OCM process (i.e., high temperatures with low CH4/O2 ratios), results in a poor selectivity for olefins from C2+ hydrocarbons. In order to avoid this and also to make the natural gas-to-olefins conversion more energy efficient, a novel process called the IFP oxypyrolysis process has been introduced (Mimoun et al., 1990). In this process, C2+ hydrocarbons in natural gas are separated from methane and the catalytic OCM reaction (which is highly exothermic and, hence, highly hazardous) is combined with the endothermic thermal (noncatalytic) cracking of C2+ hydrocarbons (which occurs under oxygen deficient conditions) by allowing these two reactions to occur separately in two different zones of the same reactor. The heat carried by the 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 endother* To whom all the correspondence should be addressed. Telephone: (91) 212-336451 (ext. 2163). Fax: (0212) 333941. Email:
[email protected]. S0888-5885(96)00796-8 CCC: $14.00
Figure 1. Tubular quartz reactor.
mic thermal cracking of C2+ hydrocarbons. Although the heat produced in the OCM reaction is utilized partially in the cracking of C2+ hydrocarbons, 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. © 1997 American Chemical Society
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Figure 2. TPD of CO2, NH3, and O2 on the catalyst.
Figure 4. Influence of temperature on selectivity and alkane/ alkene ratios in the oxidative conversion of natural gas at different NG/O2 ratios in feed (GHSV ) 34 000 cm3‚g-1‚h-1) and steam/ NG ) 1.0). Table 1. Properties of SrO/La2O3/SA5205 Catalyst composition basic sites (µmol‚g-1) acid sites (µmol‚g-1) surface area (m-2‚g-1) particle size pore volume (cm3‚g-1) particle density (g‚cm3) bulk density (g‚cm3) porosity
SrO (17.3 wt %)/La2O3 (17.9 wt %)/SA5205 51.2 24.2 0.5 22-30 mesh 0.28 1.95 1.50 55%
Pore Volume Distribution pore diameter (µm) 500 0.02
energy efficient. Our recent studies on the oxidative dehydrogenation of ethane to ethylene over diluted catalyst in the presence of limited O2 (Choudhary et al., 1995b) has indicated a possibility of direct coupling of the catalytic exothermic oxidative ethane conversion reactions with the endothermic thermal cracking of ethane. Because of the direct coupling of exothermic and endothermic reactions, the process occurs in a most energy-efficient and safe manner. This investigation was undertaken for direct conversion of natural gas (without separating C2+ hydrocarbons) into ethylene/lower olefins with high selectivity at practically important conversion over a supported Srpromoted La2O3 catalyst diluted with inert solid in a most energy-efficient and safe manner. Experimental Section The supported Sr-promoted La2O3 catalyst (SrO (17.3 wt %)/La2O3 (17.9 wt %)/SA5205) was prepared by
3522 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 5. Influence of temperature on conversion and net heat of reactions (∆Hr) in the oxidative conversion of natural gas at different space velocities (NG/O2 ) 12.0 and steam/NG ) 1.0).
impregnating a commercial catalyst carrier SA5205 (a low surface area porous inert catalyst carrier obtained from Norton) having 22-30 mesh size particles, first by lanthanum nitrate and then by strontium nitrate, using the incipient wetness technique. After each impregnation, the resulting catalyst mass was dried at 90 °C for 16 h and then calcined in static air at 900 °C for 4 h. The properties of the catalyst support SA5205 are as follows: surface area ≈ 0.01 m2‚g-1, pore volume ) 0.35 cm3‚g-1, porosity ) 54%, and chemical composition ) 86.1 wt % Al2O3 and 11.8 wt % SiO2. The supported catalyst was characterized for its surface area and temperature-programmed desorption (TPD) of CO2, ammonia, and O2. The surface area of the catalyst was determined by the single-point BET method by measuring the adsorption of nitrogen at its concentration of 30 mol % (balance helium) at liquid nitrogen temperature, using a Monosorb surface area analyzer (Quanta Chrome Corp.). The surface basicity/ base strength distribution on the catalyst was measured by temperature-programmed desorption (TPD) of CO2 (chemisorbed at 100 °C) on the catalyst (1.0g), packed in a shallow bed quartz reactor with low dead volume, from 100 to 950 °C. The CO2 desorbed in the TPD was measured quantitatively by a thermal conductivity detector. The acidity distribution on the catalyst was measured by TPD of ammonia (chemisorbed at 100 °C) from 100 to 950 °C. TPD of oxygen (chemisorbed at 100
Figure 6. Influence of temperature on selectivity and alkane/ alkene ratios in the oxidative conversion of natural gas at different space velocities (NG/O2 ) 12.0 and steam/NG ) 1.0).
°C) was also measured from 100 to 950 °C. In all of the TPD experiments, the linear heating rate was 20 °C‚min-1 and the carrier gas was helium (40 cm3‚min-1), free from traces of moisture and oxygen. Before the TPD, the catalyst was pretreated at 950 °C in a flow of helium (40 cm3‚min-1) for a period of 1 h. In the present case, the chemisorption of CO2, NH3, and oxygen is defined as the amount of the particular sorbate retained on the presaturated catalyst when it was swept with pure helium (40 cm3‚min-1) for a period of 30 min. Before measuring the above surface properties, the catalyst was pretreated in situ in a flow of pure He (2000 cm3‚g-1‚h-1) at 900 °C for 1 h. The oxidative conversion of natural gas to ethylene/ lower olefins was carried out at atmospheric pressure in a continuous tubular quartz reactor (i.d. ) 5 mm) packed with 0.3 g of diluted catalyst (catalyst dilution was done by mixing uniformly 0.1 g of supported catalyst and 0.2 g of catalyst carrier having the same particle size) and having a very low dead volume, at different temperatures, NG/O2 and steam/NG mol ratios in the feed and space velocities (measured at 0 °C and 1 atm of pressure). The reactor is shown in Figure 1. It was kept in a constant temperature zone of a tubular furnace (diameter ) 2.0 cm). The reaction temperature
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3523
Figure 7. Influence of NG/O2 ratio in feed on the oxidative conversion of natural gas at 850 °C (GHSV ) 34 000 cm3‚g-1‚h-1 and steam/ NG ) 1.0).
was measured by a Chromel-Alumel thermocouple located axially in the diluted catalyst bed. The temperature gradient in the catalyst bed was very small (e5 °C). The reactor effluent gases, after removing water by condensation at 0 °C, were analyzed by an online gas chromatograph using Porapak-Q and Spherocarb columns. The C, H, and O balance across the reactor was within an error of 2-6%. Before carrying out the reaction, the catalyst was pretreated in situ in a flow of pure N2 (5000 cm3‚g-1‚h-1) at 900 °C for 1 h. The catalyst showed no significant change in its activity and selectivity when tested for the reaction continuously for about 100 h. Since there were no aging effects on the catalyst, the same catalyst sample was used for all of the experiments, except for those carried out in the absence of steam in the feed. The composition (in mol %) of the natural gas used in the feed was as follows: 87.8% methane, 5.6% ethane, 3.5% propane, 1.6% butane,
