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Ind. E n g . C h e m . Res. 1990, 29, 539-544 Chen, I. W.; Shiue, D. W. Resistivity to Sulfur Poisoning of Nickel-Alumina Catalysts. Ind. Eng. Chem. Res. 1988,27,1391-1396. Chen, I. W.; Lin, S.Y.; Shiue, D. W. Calcination of Nickel/Alumina Catalysts. Ind. Eng. Chem. Res. 1988, 27, 926-929. Corella, J.; Monzon, A. Modeling of the Deactivation Kinetics of Solid Catalysts by Two or More Simultaneous and Different Gases. Ind. Eng. Chem. Res. 1988, 27, 369-374. Cullity, B. D. Element of X - r a y Diffraction; Addison-Wesley: Reading, MA, 1978. De Bokx, P. K.; Wassenberg, W. B. A.; Geus, J. W. Interaction of Nickel Ions with a r-Al,O, Support during Deposition from Aqueous Solution. J . Catal. 1987, 104, 86-98. Ertl, G.; Weiss, U.; Lee, S. B. The Role of Potassium in the Catalytic Synthesis of Ammonia. Chem. Phys. Lett. 1979, 60, 391-394. Gardner, D. C.; Bartholomew, C. H. Kinetics of Carbon Deposition during Methanation of CO. Ind. Eng. Chem. Prod. Res. Dev. 1981,20, 80-87. Hegedus, L. L.; Summers, J. C. Improving the Poison Resistance of Supported Catalysts. J . Catal. 1977, 48, 345-353. Kelley, R. D.; Candela, G. A,; Madey, T. E.; Newbury, D. E.; Scheli, R. R. Surface and Bulk Analysis of a Deactivated Raney Nickel Methanation Catalyst. J. Catal. 1983, 80, 235-248. Laine, J.; Brito, J.; Gallardo, J.; Severino, F. The Role of Nickel in the Initial Transformations of Hydrodesulfurization Catalyst. J . Catal. 1985, 91, 64-68. Magnoux, P.; Cartraud, P.; Mignard, S.; Guisnet, M. Coking, Aging, and Regeneration of Zeolites. J . Catal. 1987, 106, 235-241. Martin, G. A.; Primet, M.; Dalmon, J. A. Reactions of CO and CO, on Ni/Si02 above 373K as studied by Infrared Spectroscopic and
Magnetic Methods. J . Catal. 1978,53, 321-330. McClory, M. M.; Gonzalez, R. D. The Role of Alkali Metal as Promoters in the Methanation and Fischer-Tropsch Reaction: As in Situ Study. J. Catal. 1984, 89, 392-403. Mirodatos, C.; Praliaud, H.; Trimet, M. Deactivation of Nickel-Based Catalysts during CO Methanation and Disproportionation. J. C a m / . 1987, 107, 275-287. Proliaud, H.; Dalmon, J. A.; Mirodatos, C.; Martin, G. A. Influence of Potassium Salt Addition on the Catalytic Properties of SilicaSupported Nickel. J . Catal. 1986, 97, 344-356. Richardson, J. T. SNG Catalyst Technology. Hydrocarbon Process. 1973, Dec, 91-95. Richardson, J. T.; Crump, J. G. Crystallite Size Distributions of Sintered Nickel Catalysts. J . Catal. 1979, 57, 417-425. Taskalis, K. S.; Tsotsis, T. T.; Stiegel, G. J. Deactivation Phenomena by Site Poisoning and Pore Blockage: The Effect of Catalyst Size, Pore Size, and Pore Size Distribution. J. Catal. 1984,88, 188-202. Van Den Berg, F. G. A.; Glezer, J. H. E.; Savhtler, W. M. H. The Role of Promotors in CO/H, reactions: Effects of MnO and Moop in Silica-Supported Rhodium Catalysts. J . Catal. 1985, 93, 340-352. Viswanathan, B.; Gopalakrishnan, R. Effect of support and Promotor in Fischer-Tropsch Cobalt Catalyst. J. Catal. 1986,99,342-348. Zielinski, J. Morphology of Nickel/Alumina Catalysts. J . Catal. 1982, 76, 157-163. Received for reuiew March 24, 1989 Revised manuscript receiued November 6, 1989 Accepted November 22, 1989
Formation of C2+Hydrocarbons in CH4/02 Diffusion Flames with C12, CH,Cl, or HC1 Additives R6mi Le Bec, Paul-Marie Marquaire, and Guy-Marie C6me* Ddpartement de Chimie Physique des RPactions, CNRS, I N P L - E N S I C e t Uniuersite Nancy I , 1 rue Grandville, 54000 Nancy, France
T h e reactions of C H 4 / 0 2 mixtures with Cl,, CH3C1, or HC1 have been investigated by means of diffusion flames quenched by jets of a n inert cold gas. Under the reaction conditions studied, acetylene appears t o be the most important valuable product. Adding C12t o CH4/02 mixtures results in a n increase of methane conversion, C, selectivity, a n d soot formation. Similar effects occur on addition of CH3C1. Adding HC1 t o CH,/O2 mixtures does not noticeably alter the results. Our results are qualitatively interpreted by means of a free-radical reaction mechanism. Because of the large amounts of natural gas feedstocks available in the world, it is a great research challenge to succeed in converting methane, its major constituent, into higher molecular weight hydrocarbons, such as ethylene or acetylene. Thus, natural gas could substitute crude oil as a raw material for the industrial production of many basic chemicals. Various processes have been designed in order to convert methane: electric arc, plasmas, heterogeneous catalytic reactions with oxygen or oxygen and chlorine, as well as homogeneous reactions such as pyrolysis, reactions with oxygen, with chlorine, or with oxygen and chlorine. Among these processes, BASF-type processes (production of acetylene in rich methane-oxygen flames) are particularly attractive, being essentially autothermal, but they are often affected by soot formation and cabonaceous deposits. In 1980, Benson patented a process of ethylene and ethane production in a flame of mixtures of methane and chlorine. Later, Weissman and Benson (1984) published results on the study of the pyrolysis of CH3CI. This re-
* To whom correspondence should be addressed. 0888-5885/90/2629-0539$02.50/0
action, producing C2+hydrocarbons, could be the second step of a two-step chlorine-catalyzed process, the first step being the chlorination of methane into chlorinated methanes. The HC1 formed could be recycled by methane oxychlorination or converted into C12 via a Deacon-type reaction (2HC1+ '/202Clz + H,O). However, in these processes, the formation of heavy products, tars and soot, could be a very important drawback. Senkan (1987) recently patented the so-called chlorine-catalyzed oxidative process (CCOP), which is basically an improvement of the second step mentioned above. Senkan claims that the addition of small quantities of oxygen to methyl chloride during the pyrolysis decreases the formation of carbonaceous deposits while maintaining high yields of C2 hydrocarbons. Taking the previous comments into account, it clearly appears that the direct reaction, Le., the oxychlorination of methane, could be an interesting way of producing Cz+ hydrocarbons. We report here a study of the reactions of various mixtures containing methane, oxygen, and a chlorinated compound, e.g., Cl,, CH,Cl, or HC1, in a flame. For this study, we have designed a special burner with
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G 1990 American Chemical Society
540
Ind. Eng. Chem. Res., Vol. 29, No. 4, 1990
I I
/ / I
20
Flame zone *int ' D
T
The distances are i n n
, I
1
Figure 1. Burner and quenching systems.
operating conditions based on the same principles used for well-known autothermal processes, such as BASF. Partial combustion of the methane is achieved by means of a diffusion flame followed by rapid quenching with jets of inert gas. The influences of chlorinated additives and of several other parameters (reactant concentrations, conditions of admission of the reactants, flame length, etc.) on the product distribution have been pointed out, particularly concerning the methane conversion, the C2 selectivity, and the formation of soot.
Experimental Section The reactor, shown in Figure 1, is made of three coaxial tubes allowing the input of three different gases. In fact, in many experiments, only two tubes (a and b) were necessary. The lengths L , and L2 are adjustable, thus defining two space times. The diameter D , is also adjustable. The quenching of the reacting mixture is carried out at the outlet of the burner by means of four turbulent jets of argon, perpendicular to the axis of the burner. Each turbulent jet widens into a conical shape with an angle of 22" (Hinze and Hegge Zijnen, 19491, and this quenching system has been designed in order to cover the whole outlet section of the burner by the four turbulent jets. The quenching efficiency of such a device was estimated to reach rates of up to lo6 K / s (Led6 et al., 1983). The reaction time in the diffusion flame can thus be very short, in the range 1-10 ms. The burner is put in a casing cooled by the surrounding air, allowing a secondary quench. Special care in the use of materials has been taken with regard to corrosion and safety problems. The reaction is carried out at atmospheric pressure, the flow rates being regulated and measured by means of mass flowmeters. The flow rate of each reactant is in the range 100-300 cm3/min (NTP). A typical flow rate of quenching gas is 1500 cm3/min (NTP). Gases are supplied by the Air Liquide Company with the following purities: CH,,
99.995%; 02,99.5%; Ar, 99.5%; Cl,, 99.7%; CH3Cl, 99%; HCI, 99.995%. At the output of the casing, sampling is achieved under steady-state conditions, and the analysis of gases is carried out by gas chromatography. CH4,C2H2,C2H4,and C2H6 are well separated on a 5-ft X 1/8-in.column packed with 80-100-mesh Carbosphere (oven temperature: 120 "C) and measured by a flame ionisation detector. C3 and C4 compounds and chlorinated hydrocarbons are separated on a I-ft X 1/8-in. precolumn packed with 80-100-mesh Spherosil XOB 075 followed by a 6-ft X 1/8-in. column packed with 80-100-mesh Porapak Q (oven temperature: 120 "C), and detected by a FID. COPand H 2 0 are separated on a 3-ft x 1/4-in.column packed with 80-100-mesh Porapak Q (oven temperature: 50 "C). CO is analyzed on a 7-ft X 1/4-in.column packed with 1.5% squalane on 60-80-mesh activated charcoal (oven temperature: 50 "C). Hz is analyzed on a 13-ft x 1/4-in. column packed with 60-80-mesh activated charcoal (oven temperature: 50 "C). For these gases, the detection is carried out by using a catharometric detector. The carrier gas is nitrogen except for CO and COz analyses in which hydrogen is used. Calibrations were made with pure reference gases. The inlet and outlet molar flow rates of the jth compound, Fojand F j , respectively, are deduced from measurements of the volumetric flows and the molar fractions determined by the GC analyses. During the experiments involving only CH4 and O2 reactants, the carbon balance is generally better than 95%. For some of these experiments, the reaction products were passed in a gas-washing bubbler, allowing the formaldehyde formed to be absorbed in water and then titrated by a polarographic method (Whitnack and Walters, 1955). During the experiments involving a chlorinated compound, NaOH scrubbers are placed a t the outlet of the casing and absorb both the HCl and CO, formed and the residual C1,; therefore, these gases are not quantitatively measured, and the corresponding chlorine and carbon balances cannot be achieved.
Results The influence of various parameters on the methane conversion, products, and soot selectivities, as defined below, has been studied. The methane conversion is defined by the following relationship: 'CH4 - FCH4
conversion =
'CH4
The selectivity (Sj)of formation of the jth product is defined as the fraction of converted methane that is transformed in that product:
where u. is the number of carbon atoms contained in the jth product. For example, the CzH2selectivity is given by 2FCzH2 SC2H2
=
OCH4 - FCH4
The H2 selectivity is defined by the following relationship, taking the hydrogen balance into account: S H ~=
FH2 2(FoCHl
-
FCH4)
Ind. Eng. Chem. Res., Vol. 29, No. 4, 1990 541
15t
v I :
lo
/
,
m7*-*-+,
C2H4 Sel. C2H6 S I .
-*
