Ind. Eng. Chem. Res. 1993,32, 1549-1554
1549
Thermal Coupling of Methane in a Tubular Flow Reactor: Parametric Study Francis G. Billaud' and Frangois Baronnet Dkpartement de Chimie Physique des Rkactions, ENSIC-INPL, URA No. 328 CNRS, 1, rue Grandville, BP 451,54001 Nancy Cedex, France
Christophe P. Gueret Znstitut Francais du Pktrole, 1-4, avenue de Bois Prkau, BP 311, 92506 Rueil Malmaison Cedex, France
The thermal cracking of methane in the presence of hydrogen (HdCH4 ratio from 1 to 2), was investigated in a tubular flow reactor at lOsPa, in the temperature range 1180-1230 "C, at residence times ranging from 0.2 to 1.5 s. Reaction products were analyzed by gas-phase chromatography: products in the gaseous samples were ethylene and acetylene as major products and ethane, propene, propadiene, propane, butadiene, butenes, and cyclopentadiene as minor products. In the liquid samples (quenched) the products were benzene, toluene, xylenes, and naphthalene. Formation of coke and tar was also observed. Hydrogen dilution increases yields and selectivities of Cz hydrocarbons, avoid carbon formation, but reduces methane conversion. The experimental results show the large effect of short residence times associated with high-temperature levels. The formation of the measured reaction Droducts and the influence of H2 dilution on methane conversion and on the product distribution h e accounted for by a reaction model.
Introduction Recent estimates suggest that world proven reserves of natural gas are increasing regularly and are larger than those of crude oil. Natural gas is prospected and developed in conditions rather similar to those of oil and could have an important role in the energy supply of the next decades. Whereas proven crude oil reserves seems approximately constant, those of natural gas have been regularly growing and could meet demands of consumption for 300 years at the present level (Saint Just et al., 1990). However, it should be stressed that the level of the oil reserves depend on the barrel price, since it could become economical to extract crude oil in more difficult conditions (deep sea fields for instance). The discovery of new fields of natural gas can also modify the situation (an important gas field was recently discovered in the Netherlands; its capacity could reach 20 billion m3). Natural gas is mainly methane (83-97 vol % depending on the origin), and its physical and chemical properties are those of methane. It is liquified at -160 "C and is very stable from a chemical point of view. Temperatures above 1000 "C are necessary to transform CH4 into hydrocarbonsrelatively more stable than CH4 such as benzene, acetylene, and ethene. Moreover, from a thermodynamic point of view, hydrogen and carbon are more stable at a temperature as low as 550 "C. The development of remote gas fields and that of largescale chemical processes based on natural gas require the availability of efficient processes of methane upgrading (Billaud et al., 1989; Khan and Crynes, 1970). In a recent paper, we briefly mentioned the various techniques used or developed to make a rational use of methane as a chemical feedstock. It is worth noting that the oxidative coupling of methane over a catalyst has been largely investigated since Keller and Bhasin reported their first results (Keller and Bhasin, 1982). In this paper we examine an alternative and direct pathway for activating CH4 molecule and to transform it into higher hydrocarbons: the thermal coupling of meth-
* Author to whom correspondence should be addressed. Electronic mail address:
[email protected].
Table I. Carbon Mass Balance between Reactor Inlet and Outlet (Run Conditions: T = 1230 OC, H*/CHd = 1, Ha + CHI = 0.8 nL/min, Residence Time t, = 0.480 s. Reactor 1nlet:Methane = 1.25 mol. Reactor 0utlet:Methane Conversion = 34.8%)
total a
mol of C 0.815 0.096 0.108 0.004 0.007 0.071 0.004 0.003 0.053 0.078
selectivitv. %
1.24
97.4%
22.1 24.8 0.9 1.6 16.3 1.0 0.6 12.1 18.0
Toluene and xylenes.
ane. A parametric study was performed to determine the influenceof various parameters on the pyrolysis of methane and to assess the feasibility of future developments of the thermal coupling reaction. The chosen parameters are residence time, pyrolysis temperature, and hydrogen dilution. In a previous paper, we described the micropilot designed for the thermal coupling of methane, associated with a sampling and analysis setup for all the pyrolysis products (Billaud et al., 1992).
Experimental Section The experimental apparatus was described in an earlier work (Billaud et al., 1992). The experimental procedures and analyticalmethods for gas and liquid products analyses were also similar to those described by Billaud et al. (1992). The reactants used in this investigation and purchased at l'Air Liquide are CH4, N 35 grade, 99.95% (H2 and 0 2 < 10 ppm, C2Hz < 200 ppm); H2, R grade, 99.95% (Hz and 02 < 10ppm); and Ar U grade, 99 % . Much attention was devoted to the reproducibility of the experiments and to the carbon balance. We laid emphasis on the sum of the selectivitiesexpressed in percent of cracked methane which was close to 100%. In Table I, we show an example in 0 1993 American Chemical Society
1550 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 Tc=120O0C
601 50
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200
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reactor length , mm Figure 1. Temperature profile of the reactor. Table 11. Reaction Products Measured by Gas-Phase Chromatography In the Gaseous Sample major products methane ethylene acetylene minor products ethane propene propadiene (C3H4) propane butadiene butenes cyclopentadiene (CsHa) In the Liquid Sample (Quenched) benzene toluene xylenes naphthalene
average conditions (1230 "C, HdCH4 = 1, t, = 0.480 ms) and we can note the satisfactory agreement between the carbon flow at the reactor inlet (1.25 mol) and that at the outlet (1.24), together with a sum of selectivities not too far from 100% (97.4%). The reaction series were performed as follows: (1) for a given temperature and dilution, variation of residence time between 0.4 and 3 s,by adjusting the flow; (2)variation of the assigned oven temperature (1180,1200, and 1230 "C) or hydrogen dilution (HdCH4 = 1and 2). The reaction products measured (except coke and tar) by gas-phase chromatography are listed in Table 11. (Billaud et al., 1992). The major calculated parameters are conversion; selectivities and yields of C2 hydrocarbons (C2H4 + C2H2), benzene, and coke; and ratio (CzHz/CzH4). A typical temperature profile of the reaction tube is shown in Figure 1. From the temperature profile the reaction zone appears to be 20 cm long. Outside this zone,
residence time
, seconds
Figure 2. Experimental selectivities of C2H2,C2&, C&, and coke and methane conversion at 1230OC and HdC& = 1veraua residence time. Pressure 1 atm.
the temperature falls rather steeply from lo00 to 200 O C , which means that the conversion outside the reaction zone would be limited and considered as negligible compared to the conversion in the reaction zone. Therefore we have correlated the conversion, yield, and selectivity data with the maximum reactor temperature. We have calculated the residence time from the volume of the reaction zone and the increasinggas flow a t the temperature and pressure of the reaction, neglecting the gas expansion during the reaction. The relationships used to calculate the selectivity Si and the yield Rj for a product P are as follows: ni(mol of P) s i ( % ) = mol of CH,,, - mol of CH4,0utlet and
Ri(%)
[Si(%11 [C(%)I
100 where ni stands for the number of carbon atoms of P and C (5%) is the methane conversion. Experiments were performed in an alumina tubular reactor (manufactured by Demarquest). 111. Product Distribution and Discussion The selectivity of products is described as a function of the residence time in Figures 2 and 3. Our experimental results show the following: 1. Except C2H4, C2H2, and CsHs (Figure 2), the other hydrocarbons (C2H6, C3H6, C3H4, C4 hydrocarbons, C5H6) are found only in trace amounts (selectivity < 3 % 1, and they have been represented in average conditions of temperature (1230 "C) and dilution (HdCH4 = 1)(Figure 3). It is worth noting that, among theminor hydrocarbons, ethane reaches very rapidly a very low equilibrium value, whereas the other hydrocarbons after having reached a maximum value nearly completely disappear (Figure 3). 2. Likewise, at the investigated temperatures, toluene and xylenes only appear in trace amounts.
Ind. Eng. Chem. Res., Vol. 32,No. 8,1993 1551 6o
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residence time , seconds Figure 3. Experimental selectivitiesof minor products at 1230 O C and Ha/CH, = 1. Pressure 1 atm.
3. C2H4, C2H2, and CeHe are represented by curves showing a maximum, whereas the formation of coke is continuously growing. 4. The maximum methane conversion reached in these experiments is close to 50% (Figure 2). To obtain a better understanding of the overall effect of hydrogen dilution, we have represented our results as yields vs residence time, including in this way selectivity and conversion. Then we observe that dilution by hydrogen despite a negative influenceon the conversionat a given temperature (Figure 4) leads to a positive effect on the product distribution: the C2 yield is increased whereas coke formation becomes less important. For a fixed dilution, it is noted that the C2 yields (C2H4 + C2H2) versus residence time reach a maximum and that this maximum becomes more important when the pvolysis temperature is increased (Figure 5 ) . In the same way, the formation of coke is accelerated by a temperature increase (Figure 6). We also observe that, at a given temperature, an increase of the dilution by hydrogen raises C2 yields and at the same time reduces coke formation (examples given in Figures 7 and 8). We can reach a C2 yield close to 20%, holding the coke yield down at the same time (T = 1230 "C, H2/CH4 = 2, conversion = 35%). The simultaneous variations of the yields of C2H4, C2H2, benzene, and coke for our most severe operating conditions (T, = 1230 "C; HdCH4 = 1) is shown in Figure 9. The shape of the curves shows that the above-mentioned hydrocarbons are genuine intermediate products between CHI and coke. It is also worth noting that the rapid formation of coke is correlated with the decrease of C2H2 yield. The yield of a reaction product is a function of methane conversion; we are therefore able to observe the influence
residence time , seconds Figure 4. Conversion of methane as a function of residence time at different Ha/CH4 ratios. Temperature 1230 OC. Pressure 1 atm. 24 2o
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of the three operating parameters dilution, temperature, and residence time upon conversion and product yields. It appears that, at a fixed dilution, a temperature increase has a positive effect to obtain a given conversion (Figure 10). The effect of this temperature rise is to increase the yield in C2 hydrocarbons and to decrease the yields in benzene and coke. Therefore, it becomes possible to limit the negative effect of the increase of residence time upon the coke formation. Besides that, the positive effect of the dilution by hydrogen at a given temperature can still be observed (increase of the C2 yield and decrease of the coke formation). For a given dilution, it appears that the C2Hd C2H4 ratio increases with temperature. By contrast, the C2HdC2H4 ratio decreases with a rise of the conversion. This leads us to think that acetylene has a more important effect than ethylene in the formation of the heavy fractions found at high conversion.
1552 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 10 1°1
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0 0,O
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Figure 9. Yield of products from methane pyrolysis as a function of time. Temperature 1230 "C. Pressure 1 atm. Feed composition HdCH4 = 1. M-
residence time , seconds Figure 7. Yields of CZfrom methane pyrolysis as a function of residence time at different HdCH4 ratios. Temperature 1230 O C . Pressure 1 atm.
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residence time , seconds Figure 8. Yields of coke from methane pyrolysis as a function of residence time at different Hz/CHd ratios. Temperature 1230 OC. Pressure 1 atm.
IV. Interpretation and Conclusion Experimentally an increase of pyrolysis temperature for a fixed CH4 conversion and dilution can increase CO yields (acetylene) and decrease heavy-product yields. It can be shown, in agreement with the thermodynamics, that the higher the temperature level, the more suitable the conditions of acetylene formation, and the higher the ratio acetylene to ethylene. It is well-known (Figure 11); Chauvel et al., 19851 that hydrocarbons are unstable with respect to carbon and hydrogen whatever the temperature is, except methane and ethane which are more stable than carbon and hydrogen, respectively, at temperatures lower than 800 K
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conversion , K Figure 10. Cz yield from methane pyrolysisas a function of methane conversion at different temperatures. HdCH4 = 1. Pressure 1 atm.
(527 "C) and 400 K (127 "C). Therefore, above 800 K, CH4 has an unpredictable thermal stability in relation to carbon and hydrogen but in fact remains stable up to 1300 K (1027 "C). At this temperature, benzene remains more stable than methane; we can mention that this limiting
Ind. Eng. Chem. Res., Vol. 32, No. 8,1993 1553
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temperature decreases to 850 K (577 OC).if ethane is considered as reactant. The slopes of the straight lines AGOf = f ( T ) corresponding to the aromatic molecules are less steep than those corresponding to paraffinic and olefinic molecules. As a consequence, the stability of the aromatic molecules grows more rapidly as a function of temperature than that of olefins. Therefore, to produce acetylene from methane, a temperature level above 1480 K is required. It should even be higher, approximately 1580K, to produce ethylene;however, at this temperature, the formation of acetylene prevails. This fact can be explained by considering the straight line corresponding to acetylene in Figure 11, above 1480 K; AGOf(CzH2) becomes smaller than the values of the AGOf function for the other hydrocarbons and this comparison explains the formation of acetylene above 1500 K. It appears, taking into account the thermodynamic data and the experimental results that the formation of acetylene and ethylene by the thermal reaction of methane requires a temperature higher than 1500K and a maximum heat transfer. This short discussion accounts for the formation of acetylene above 1500 K. Our results are also in agreement with the mechanism of methane decomposition proposed by Chen et al. (1976) which accounts for the formation of hydrogen, ethane, ethylene, propene, and acetylene. The mechanism has also allowed Roscoe and Thomson (1985) to propose a simulation of the formation of all the products of methane decomposition at 1038 K and 433 torr. To explain the beneficial effect of dilution by hydrogen, we can use the last mechanism proposed by Olsvik and Billaud (1992) accounting for experimental results at 1273 K and 1 atm obtained by the research team from Trondheim (Norway) (Rokstad et al., 1991, 1992). For the purpose of our study the reaction scheme (see Figure 12) has been simplified by considering a “C6H6”term for C6H6 and higher hydrocarbons. By using this mechanism, it is quite possible to explain all the products of the methane reaction in our conditions of temperature and dilution; this mechanism qualitatively explains the in-
Figure 12. Reaction paths for methane pyrolysis.
fluence of hydrogen dilution on the conversion and selectivities in light products (C2). This also shows that hydrogen has at the same time a role of diluant and a chemical effect due to the elementary step:
*
CH, + H’ CH,’ + H, (1) which due to its reversibility leads to a smaller methane conversion and to lower CH3’ concentrations when the hydrogen concentration is raised. As a consequence, the elementary steps involving CH3’ become relatively less important, especiallythe elementary step
+
C,H, CH,’ s CH, + C,H,’ which leads to a smaller CzH4 consumption and an increase of its concentration. It was also shown that CzH3’ leads mainly to CzH2 via the step C,H,’ s C,H, + H’ (3) rather than via a reaction with another CzHz molecule via an elementary step such as C,H, + C,H,’ + C,H,‘ (4) In fact, step 4 is slower than step 2 when the dilution becomes more important. Step 5 is also slower than step C,H,
+ C4H< F? C,H, + H’
(5)
2 when dilution by Hz is increased; as a result and in good agreement with our experimental results, CzHz formation increases and conversely C6H6 formation becomes less important. In our experiments and in agreement with the previous mechanism, the ratio CzHz/CzH4 increases with hydrogen dilution. It is worth noting that this important result was also obtained by Ranzi et al. (1988). This experimental
1554 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993
observation agrees with the overall description of methane pyrolysis by dehydrogenation steps: 2CH4
-
C2He + H2
-
C2H4 + H2
-
C,Hz + H,
-.
2C + H2
At a fixed temperature, the increasing dilution by hydrogen leads to a decrease of the conversion (Figure 4), an increase of the CZselectivity, and a decrease of the coke selectivity. These results are consistent with our first results (Broutin et al., 1991) and also agree with a recent investigation on the thermal coupling of methane in the temperature range 1000-1200 "C (Rokstad et al., 1991) and at higher temperatures (Holmen et al., 1976). This parametric study shows that, in the pyrolysis of methane, it is quite possible to obtain substantial yields of higher hydrocarbons (20%,C ~ H ~ / C = Z 11, H ~with a low yield of coke (3-4 % ) and a conversion close to 35 % (2' = 1230 "C, H2/CH4 E 2). This shows the beneficial effect of high temperatures and dilution by hydrogen that could be explained by our radical mechanism. The reaction of methane pyrolysis then appears as very dependent on the reaction parameters. In particular, there is competition between the CZformation (CzH4+ CzH2) and the formation of a carbon deposit. Acetylene appears as a very important intermediate in the formation of the heavy products. The previous investigation shows that residence time is a very important parameter in the thermal coupling of methane. For a given hydrogen dilution and to maintain a given conversion value, it seems efficient to increase the temperature and to reduce the residence time. In this case, the Cz yields rise and the formation of coke is lowered. Literature Cited Billaud, F.; Baronnet, F.; Freund, E.; Busson, C.; Weill, J. Thermal decompositionof methane-Bibliographic study and proposal of a mechanism. Rev. Znst. Fr. Pet. 1989,44 (6), 813-93. Billaud, F.; Gueret, C.; Baronnet, F.; Weill, J. Thermal coupling of methane in a tubular flow reactor: experimental setup and
influence of temperature. Znd. Eng. Chem. Res. 1992,31,274& 53. Broutin, P.; Busson, C.; Weill, J.; Billaud, F. Thermal Coupling of methane. In Novel Methods ofProducingEthylene, Other Olefins and Aromatics; Albright, L. F., Crynes, B. L., Nowak, S., Eds.; Marcel Dekken: New York, 1992;pp 239-58. Chauvel, A.; Lefebvre, G.; Castex, L. ProckdBs de PBtrochimie. Caractkristiques techniques et Bconomiques;Ed. Technip: Paris, 1985; Tome I, pp 343-71. Chen, C. J.; Back, M. H.; Back, R. A. The thermal decompositionof methane. 11. Secondary reactions, autocatalysis and carbon formation: non-Arrhenius behaviour in the reaction of CHSwith ethane. Can. J. Chem. 1976,54 (20),317544. Holmen, A.; Rokstad, 0.A.; Solbakken,A. High-temperaturepyrolyskt of hydrocarbons. I. Methane to acetylene. Znd. Eng. Chem. Process Des. Deu. 1976,15,439-44. Keller, G. E.; Bhasin, M. M. Synthesis of ethylene via oxidative coupling of methane. Determination of active catalysts. J. Catal. 1982, 73,9-19. Khan, M. S.;Crynes, B. L. Survey of recent methane pyrolysis literature. Znd. Eng. Chem. 1970,62(lo), 54-9. Olsvik, 0.; Billaud, F. Modelling of the decomposition of methane at 1273K in a plug flow reactor at low conversion. J. Anal. Appl. Pyrolysis 1993,in press. Ranzi, E.; Dente, M.; Costa, A.; Bruzzi, V. Thermal decomposition of methane: a mechanistic kinetic scheme. Chim. Znd. 1988,70 (1-2),1-8. Rokstad, 0.A.; Olsvik, 0.;Holmen,A. Thermal couplingof methane. In Natural Gas Conversion; Holmen, A., Jens, K.J., Kolboe, S., Eds.; Stud. Surf. Sci. Catal. 1991,61;633-9. Rokstad, 0.A.;Olsvik,0.;Jenasen,B.; Holmen, A. Ethylene,acetylene and benzene from methane pyrolysis. In Novel Methods of Producing Ethylene, Other Olefins and Aromatics; Albright, L. F., Crynes, B. L., Nowak, S., Eds. Marcel Dekker: New York, 1992;pp 259-71. Roscoe, M.; Thompson, J. Thermal decomposition of methane: autocatalysis. Znt. J. Chem. Kinet. 1985,17,967-990. SainbJust, J.; Basset, J. M.; Bousquet, J.; Martin, G. A. Natural gas: a feedstock for tomorrow. Recherche 1990,21,733-8.
Received for review March 1, 1993 Revised manuscript received April 6, 1993 Accepted April 20, 1993
