Ind. Eng. Chem. Res. 1992, 31, 2748-2753
2748
Hirschberg, E. H.; Bertolacini, R. J. Catalytic control of SO, emissions from fluid catalytic cracking units; Occelli, M. L., Ed.; ACS Symposium Series 375; American Chemical Society: Washington, DC, 1988; Chapter 8, pp 114-145. Koballa, T. E. Sulfur dioxide removal with transition metal oxides supported on alumina, M.Sc. Thesis, Ohio University, 1975. Koballa, T. E.; Dudukovic, M. P. Sulfur dioxide adsorption on metal oxides supported on alumina. Atmospheric Emissions and Energy-Source Pollution. AIChE Symp. Ser. 1977, 73, (No. 165), 199-228. Levenspiel, 0. Noncatalytic Fluid-Solid Reactions. In Chemical Reaction Engineering; Wiley: New York, 1962; pp 338-383. Lowell, P. S.; Schwitzgebel, K.; Parsons, T. B.; Sladek, K. J. Znd. Eng. Chem. Process Des. Dev. 1971, 10, 384-390. McArdle, J. C.; Leshock, D. G.; Williamson, R. R. Sorbent life cycle testing of the fluidized-bed copper oxide process. Presented at the AIChE Spring National Meeting, Houston TX, March 29-April 2, 1987. Ramachandran, P. A.; Doraiswamy, L. K. Modeling of noncatalytic gas-solid reactions. MChE J. 1982, 28, 881-900. Rheaume, L.; Ritter, R. E. Catalytic reduction of SO, emissions from fluid catalytic cracking units. Presented at the 194th National Meeting of the American Chemical Society, New Orleans, LA, Aug 31-Sept 4, 1987. Russell, J. H.; Town, J. W.; Kelly, H. J., ‘Mathematical evaluation of SO2 sorption-regeneration reactions with alkalized alumina”; Report of Investigations 7415; Washington, US. Department of the Interior, Bureau of Mines, 1970.
Snyder, R. B.; Wilson, W. I.; Johnson, I.; Jonke, A. A. “Synthetic sorbents for removal of sulfur dioxide in fluidized-bed coal combustors”; Argonne National Laboratory Report ANL/CEN/ FE-77-1, Argonne, IL, June 1977. Szekely, J.; Evans, J. W.; S o h , H. Y. Reactions of nonporous solids. In Gas-Solid Reactions; Academic Press: New York, 1976; pp 65-107. Tracor. ‘Applicability of metal oxides to the development of new processes for removing SO2 from flue gases”; Final Report 1969, Contract No. PH-86-68, 1, 123. Vasalos, I. A.; Strong, E. R.; Hsieh, C. K. R.; D’Souza, G. J. Amoco’s new ultracat process for SO. control. Presented at the 42th API Refining Meeting, Chicago, IL, May 10, 1977; Preprint 20-77. Vasalos, I. A.; Ford, W. D.; Hsieh, C. K. R. U.S. Patent 4,221,677. Assigned to Standard Oil (IND), 1980. Yang, R. T.; Shen, M. S. Calcium silicates: A new class of highly regenerative sorbents for hot gas desulfurization. AIChE J. 1979, 25, 811-819. Yoo,H. J.; Steinberg, M. ‘Regenerable agglomerated cement sorbent for sulfur removal in circulating fluidized-bed combustion systems. BNL Report 33475, US. Department of Energy, 1983. Yoo, H. J.; McGauley, P. J.; Steinberg, M. ‘Calcium silicate cementa for desulfurization of combustion gases”; BNL Report 33753, U.S. Department of Energy, 1982. Received for review December 23, 1991 Revised manuscript received May 18, 1992 Accepted May 29, 1992
GENERAL RESEARCH Thermal Coupling of Methane in a Tubular Flow Reactor: Experimental Setup and Influence of Temperature Francis G. Billaud,* Christophe P. Gueret, and Franqois B a r o n n e t D6partement de Chimie Physique des R&actions, ENSIC-INPL, URA No. 328 CNRS, 1 , rue Grandville, BP 451,54001 Nancy Cedex, France
Jbr6me Weill Institut Franqais du Pgtrole, 1-4, av. de Bois PrGau, BP 311, 92506 Rueil Malmaison Cedex, France
Thermal coupling of methane was investigated in a tubular flow reactor, made of alumina, in the temperature range 1300-1400 “C. The experimental set-up and the analytical equipment described in the paper has permitted to measure all the liquid and gaseous products and the coke formed with a satisfactory carbon mass balance between reactants and products. This investigation a t several temperatures and for a dilution H2/CH4 = 2 clearly showed the beneficial effect of high temperatures and low residence times upon C2yields. I. Introduction Since the energy crises and the temporary shortages of fossil energy, especially crude oil, which is used as a source of petrochemical bases and for transportation fuels, an important effort has been made to find substitutes. Natural gas fields which are prospected and developed in conditions somewhat similar to those of oil could have a dominant role in the next decades. Whereas the proven world reserves of oil seem approximately constant, those of natural gas are regularly increasing and could meet the world consumption for 300 years at the present level (Saint-Just et al., 1990). However, the point of view must
* T o whom correspondence should be addressed.
be moderated by the fact that the oil reserves could increase with the barrel price, since it becomes economical to extract less conventional oil. Natural gas is essentially methane (83-97 vol % depending on the origin) and therefore difficult to liquefy and quite chemically unreactive. Methane is thermodynamically stable with respect to ita elements. The reactions to make other hydrocarbons, all of which are less stable than methane around lo00 “C,have unfavorable free energies of reaction and are strongly limited by equilibrium. They need a considerable energy input, and therefore temperatures above lo00 “C are required to transform CHI into benzene, acetylene, and ethylene. At 550 “C,hydrogen and carbon are more stable (Parkins, 1990). Therefore it appears that natural gas, or more
oaaa-58~5/92/2~3i-2748$03.~a~~ 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 12,1992 2749 precisely methane, is an interesting source of petrochemicals, but its use and its transformation are difficult and expensive. Methane is relatively costly to transport, several times more than oil (Parkins, 1990). Because natural gas can be completely desulfurized and because the ratio H/C is high, ita combustion does not lead to acid rain and the relatively lower production of COPis quite interesting from the point of view of global warming. Therefore, methane is a very valuable fuel from the environmental point of view and the production of energy is so far ita main use (Saint-Just et al., 1990). However, a more rational use of this resource, especially if we want to make use of the geographically remote gas fields (Sant-Just et al., 1990) and use natural gas as an oil substitute, is that it should also be used to make either petrochemical bases or gasoline components. This objective seems rather difficult to reach because methane is extremely unreactive. If we want to transform methane into valuable hydrocarbons, alcohols, or ethers, the molecule of methane should be functionalized, which means that a very strong C-H bond (104 kcal-mol-') should be broken to introduce chemical functions able to react later on. Until now, the conversion of methane into liquid fuels has been obtained via synthesis gas (SYNGAZ) (C02 + Hz). An important disadvantage of this process is its cost, since it is performed a t very high temperature and is highly endothermic, the energy being supplied by burning gas. In most cases, this process is not satisfactory from an economical point of view and it is why an important research effort has been made in the past 10 years to develop direct conversion processes of methane into alcohols or higher hydrocarbons. We are going to mention briefly the various processes which are used or developed now before describing in more detail the thermal coupling of methane.
11. Indirect Methane Conversion The various processes in use require an identical first step which is the formation of synthesis gas via a highly endothermic steam reforming reaction: CHI + HzO CO + 3Hz AH = 206 kJ.mol-' (1)
-
A nickel catalyst is used at temperature neighboring 900 "C and at 20 bar. The hydrogen formed in the reaction can be used to produce ammonia, mostly used in the fertilizer industry. The production of fertilizers grows in a regular but rather slow way and cannot be an expanding outlet for natural gas industry. The synthesis gas is mainly used in three major types of industrial processes which are the Mobil MTG Process (methanol to gasoline) developed in New Zealand, the Fischer Tropsch process used in South Africa (SASOL), and the catalytic processes of hydroformylation and carbonylation. The economics of these various processes remain the cornerstone of this development.
111. Direct Methane Conversion Investigations aiming at the discovery of new processes of direct methane conversion, avoiding synthesis gas as an intermediate, have been developed in the 1980s, when it appeared that natural gas could have a substantial role in the supply of energy. Before giving more detail on the thermal coupling of methane, we are going to mention briefly the various processes which can be used for the direct conversion of methane. A. Partial Oxidation of Methane. CH4 + f/zOz CHBOH A H = -30.2 kcal.mo1-l (2) The reaction seems quite simple but the reaction products
-
are less stable in the presence of oxygen than methane, Therefore the products tend to be completely oxidized in carbon oxides. A low conversion (8%) is required to maintain a high selectivity for methanol and oxygenated producta (80%)at 450 "C and 50 bar (Hunter et al., 1984). The conversion of methane into methanol will remain limited as long as it will not be possible to remove methanol from the reaction medium as soon as it is produced. An alternative to the flow reactor is a flame CH4/O2 with a quenching by inert gas jets to stop the reaction before the formation of too large amounts of carbon oxides. B. Coupling of Methane with Chlorine. Benson's Process (Benson,1980). Chlorine reacts very readily with methane to give chloromethanes and hydrogen chloride. The pyrolysis of these chloromethanes produces methyl radicals which, at sufficiently high temperature, give reasonable yields of ethylene and acetylene. Senkan's Process (Senkan, 1987). This process is a variant of the previous one; small amounts of oxygen are added to the CH4/Clzmixture to decrease the amount of soot. These two processes have the shortcoming to require high temperatures and to involve very corrosive materials; this hinders their industrial realization. Another variant was proposed by the Pittsburgh Energy Technology Center. Pittsburgh Process, Oxyhydrochlorination of Methane (Taylor and Nocetti, 1989). CH4 + HC1+ f/zO,
-
CH$1+ HzO
(3)
Methane reacts with oxygen and hydrogen chloride in a fluidized bed reactor at 16 bar and 350 OC on a suitable catalyst (copper) to produce a mixture of chloromethanes which are oligomerized on a zeolite catalyst to produce a mixture of hydrocarbons that constitutes a high octane number gasoline. The hydrogen chloride released in the reaction is recycled to the oxyhydrochlorination unit. The moderate temperatures and the high conversion of methane, close to 50%, make this process look rather attractive. Ita commercial development will depend on the feasibility of the handling of very large amounts of hydrochloric acid generated during the reaction. C. Coupling of Methane with Oxygen. The direct conversion of methane into ethane (or ethylene, which is more useful) at intermediate temperatures (600-900 "C) is impossible from a thermodynamical point of view.
--
2CH4 + yZOz 2CH4
+ 02
CzHs + HzO
CzH4 + HzO
(4)
(5) Since ethane and ethylene are less readily oxidized into carbon oxides than methanol, the coupling of CHI with O2 could give better results than the oxidation of CHI into methanol. This reaction has been the focus of a very large amount of work after 1982, when a paper was published by Keller and Bhasin (Union Carbide) (Keller and Bhasin, 1982) in which these authors showed that a C-C bond could be formed on an oxide surface at intermediate temperature. The reaction is performed at temperature ranging from 700 to 900 "C, and there is a wide consensus that it proceeds by the dimerization in the gas phase of methyl radicals produced at the surface of the oxide which is often promoted by an alkaline ion. In the beginning, the reaction was performed by using a cyclic method of operation, methane being introduced alone, and passes over an oxide providing oxygen required to activate the methane molecule. This reaction seems to be more a gas/solid reaction than a true catalytic one. The reduced solid is then re-
-0
2750 Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992
j B j1 ELECTRIC OVEN
MANo-R
-
q-Eq
1
U I
THERMOCOUPLE
4SALYSER
-
]
I
GPC
Ar,CH4
research were made by Egloff (1930) and Kramer and Happel (1955). Summaries of more recent research have been made by Kerkovian et al. (1960), Back (1970), Khan and Crynes (19701, Omar (19821, Back and Back (1983), and Billaud et al. (1989). Before 1928, it was believed that the thermal decomposition of methane was mainly giving carbon and hydrogen and trace amounts of aromatic hydrocarbons. Wheeler and Wood (1928) were among the fist chemists to show that, by controlling the residence time, the major reactions were leading to the formation of gaseous (ethylene, acetylene) and liquid (benzene) higher hydrocarbons and that carbon formation was negligible. Today, direct conversion of methane at high temperature is a problem of interest for a number of academic or industrial research teams. Busson et al. (1990) and Broutin et al. (1990, 1991) described the main results of a parametric study of the pyrolysis of methane obtained in a reactor heated by an oven designed by the Institut Fransais du PBtrole. Rokstad et al. (1991,1992) showed that hydrogen has an important effect on the kinetics of methane pyrolysis, especially that there is a marked decrease of the coke formation. Weill et al. (1990, 1991a,b) carried out an evaluation of the economics of this process considering either the production of ethylene or that of acetylene as a base to produce vinyl chloride. A micropilot plant (capacity 60 L/h) was built in Nancy to carry out a fundamental investigation on the pyrolysis of methane.
GAS FLOWMETER
Figure 1. Experimental apparatus.
oxidized by sending air. This type of reaction has been developed by Arc0 on a predemonstration scale. The conversion yields into C2H6and C2H4can reach 20% for a selectivity to Cz products close to 80-90%. However, after the work done by Ito and Lundsford (1985), most investigations have been done in the "co-feed" mode (continuous instead of batch process), a mixture of methane (around 90%) and oxygen being passed over the catalyst together. A genuine catalytic reaction takes place, but it is difficult to assume the relative importance of the lattice oxygen. Since most reactions take place in the gas phase when the methyl radicals have been formed, undesirable carbon oxides become increasingly abundant when the methane conversion proceeds. This has led Labinger (1988) to suggest that there is an inherent limit to the yield of C2products (30%)because of the gas-phase reactions giving carbon oxides which cannot be kept under control. D. Other Process of Direct Conversion of Methane: Huls and BASF Processes. In some specific circumstances, ancient processes can be maintained under stream. This is the case for the Hiils process, producing acetylene from a flow of methane in an electric arc. It may be still of some commercial value in Canada where natural gas and electricity are abundant and cheap. It is also the case when the capital cost of the equipment has been written off (Huls plant). Another process of acetylene production was also proposed by BASF, but the shortcoming is that the chemical industry perfers ethylene than acetylene as a chemical base. Ethylene produced by steam cracking of naphtha is approximately twice as cheap as acetylene. Thermal Coupling of Methane. This is the type of direct conversion of methane that we have investigated in our laboratory (DCPR, Nancy). The chemical stability of methane (Kamptner et al., (1966) has been known for many years, and a great deal of research has been done on the pyrolysis of methane. Excellent summaries of the early
IV. Thermal Coupling of Methane: Experimental A. Equipment and Experimental Setup (Figure 1). 1. Reactor. This is a tubular reactor made of alumina (Demarquest) (length 500 mm, internal radius 7 mm, external diameter 20 mm). The mnximnl flow across the tube is at the moment 120 L/h, which corresponds to a linear speed u of 0.4 m/s. In these conditions, the Reynolds number is such that a laminar regime (Re < 2000) should be considered, with the associated concentration gradients pud
Re=--
P
- (10-4)(40)(1.45) = 20 300 x 10-6
(6)
where p stands for the voluminal mass (g/cm3),d stands for the reador diameter (cm), and p stands for the viscosity (dynamic). However, the gas diffusion is very important and these concentration gradients are limited so the assumption of plug flow is still valid. Ulrichson and Schmitz (1965) and Cleland and Wilhelm (1956) have published comprehensive analyses of this problem. The former authors have shown that their parameter W/2)2 K=Y
(7)
is characteristic of the reactor length required to reach the laminar flow (k is the overall rate constant of the reaction; Y is the kinematic viscosity). By using reaeonable estimates s-l, u = 300 X of the gas properties (k = cm2/s), K is equal to 1 X which suggests that a laminar flow regime takes place at the entrance of the reactor. The second parameter of Ulrichson and Schmitz D a=(8) K(d/2)2
is a measurement of the presence of concentration gradients (D is molecular diffusivity). If a = 0, there will be no diffusion and the concentration gradients will be im-
Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992 2751 1400 7
v E
loo0i
reaction order 0 1 2
I
m
Q
800-
0
$ a 2
Table I. Variation of the Equivalent Temperature for Various Values of the Reaction Order and of the Activation Energy ( Ein kcal mol-')' calcd average tamp (K)
Tc=l2OO"C
I
E = 50 1439 1440 1442
E = 85 1446 1447 1449
E = 100 1448 1448 1460
'T (set point regulator) = 1473 K.
0
m
3 I-
F'yrolysir zone
0
600-
m
0
Q
"i 00
50
0 0 0
1
150
200
250
3
350
400
DISTANCE, mm
Figure 2. Temperature profile of the reactor.
portant. If (Y > 1, the gradienta will be limited and the assumption of the plug flow will hold. In our case ( D = 1cm2/s), a is >> 1. It is worth noting that Ulrichson and Schmitz have made the assumption of firsborder reactions. It is assumed that the same qualitative consequences will be valid in the case of complex reactions. 2. Oven. This is an electric oven (Vecstar) which can reach a maximum temperature of 1450 "C. The temperature controller is of the proportional integral derivative type. A Pt/Pt-10% Rh thermocouple allows accurate temperature control. The heating tubes have a length of 30 cm, and the high temperature zone is 15 cm long. 3. Measurement of Gas Temperature. The temperature in the reactor is measured by a Pt/Pt-10% Rh thermocouple, the end of which is in the center of the oven. This measurement gives the manimum reactor wall temperature at the center of the heating zone. By translating the thermocouple, we have determined the temperature profile along the reactor for an assigned temperature of 1200 O C . This profile is given in Figure 2, and it clearly appears that the reactor is not really isothermal. Moreover, if we assume that lo00 OC is the minimum temperature at which pyrolysis takes place, only a limitedlength L of the reactor (approximately 20 cm), again nonisothermal, will lead to a substantial conversion. To quantify this approach and to give a better definition of the "pyrolysis temperature", we have used the concept of effective temperature Teffgiven by
where T(0is the absolute temperature in the reactor versus an axial coordinate 1 ranging from 0 to L. From ita definition, it is the temperature of an isothermal reactor which would give the same conversion as the actual reactor. This temperature may be calculated by the same relationship:
The value of Teffis determined for each experiment by integrating the relationship given in eq 10 on the part of the profile corresponding to the pyrolysis zone and by solving the equation for Teff. The length L corresponds to the length of the pyrolysis zone. The first question which arises is the value of the activation energy. It is well-known that the pyrolysis of methane is a very complex reaction and that the choice of an overall activation energy for such a reaction scheme is an important oversimplification. However, the calculation shows that the effective temperature is not strongly related to the value of the activation energy. The second question is linked to the reaction order, since we assume a first order in eq 10. For a reaction order n, the temperature in both denominators of eq 10 would be at the power n. Here again the calculation shows that the effective temperature obtained for the orders 1, -1, or 0 for a given activation energy does not change much (Table I). The third question is about the validity of the assumption of the plug flow regime, since eq 10 holds for this type of regime. If we take into account the value of the Reynolds number in most experiments, it seems that there is a laminar flow in the reactor, but taking into account the corresponding rate and concentration gradients, the diffusion is so important that it cancels this effect which means that, from a practical point of view, we obtain very quickly a plug flow regime. Therefore, eq 10 has been used to calculate the effective temperatures. For an overall activation energy of 85 kcal-mol-', the usual value given for methane pyrolysis in an open reactor (Shantarovich and Pavlov, 1962, 1963; Schneider, 1962; Holmen et al., 1976), the calculated effective temperature Teffis lower by 25-30 OC than the assigned temperature of the reactor. The real temperature should be close to this "average" wall temperature Teffsince the temperature profile, with or without gas (for instance H2), is nearly unmodified. 4. Experimental Setup. Our apparatus can be divided into three main parts: the section for the gas flow control and the preparation of the gas mixtures, the section comprising the reactor + the electric oven, and the section designed for the sampling and the analysis of the reaction products. The incoming gas flows are controlled by Brooks mass flowmeters; the flow ranges are for CHI, H2, N2,and air 0-1.0 L/min and for Ar 0-0.1 L/min. Argon is used as an internal standard. An experiment takes place according to two stages: the pyrolysis itself and the decoking procedure of the reactor by oxidation of the deposit by a mixture N2/air. The reaction mixture CHI + H2 + 2% Ar is directly introduced in the tubular reactor, without preheating. There is a manometer at the inlet of the reador; the reaction pressure is always close to 1 atm. At the reactor outlet, the gases are quenched by cooling at ambient temperature and after going through a filter in a cooled solvent. This device permita the separation into a gaseous fraction ( C & J and
2762 Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992
a liquid fraction at ambient temperature (C5+). The gases after quenching are sampled with a gas syringe for gas chromatographic analysis on a Delsi 200 gas chromatograph equipped with a flame ionization detector. An on-line analysis d o w s the determination of the hydrogen fraction in the reaction gases by using a Delsi gas chromatograph equipped with thermal conductivity detector (TCD). Argon and methane are analyzed on line on a Model 5980 Hewlett-Packard gas chromatograph the detector is again a TCD. The ratio CH4/Ar is used to calculate the conversion. During the reaction itself and the decoking procedure, the total gas flow is measured on a gas flowmeter Schlumberger (volume = 1 dm3). An outline of the apparatus is given in Figure 1. 5. Details on the Analyses. Analysis of the hydrocarbon fraction: Delsi (DI 200 model), flame ionization detection = 250 "C, T a r = 250 "C; capillary c ~ l ~ m n , (FID); TK PONA (I-f$(length 50 m, external diameter 0.5 mm, internal diameter 0.32 mm, thickness of the methylsilicone film 0.52 pm); carrier gas N,; Temperature program, -60 "C for 5 min and temperature increase of 10 "C/min up to 280 "C. Analysis of hydrogen: Delsi (IGC I1 model), thermal conductivity detector (TCD); oven and detector temperature 60 "C; column, molecular sieve 5 A, 40-60 mesh, length 3 m; carrier gas NP. Analysis of argon: Hewlett-Packard (5890 model), TCD; oven temperature 60 "C, detector temperature 250 "C; column, molecular sieve 5 A, 40-60 mesh, length 3 m; column gas flow 30 cm3.min-'; reference gas flow 40 cm3.min-'. B. Mass Balance: Analytical Procedures. 1. Carbon Balance. The carbon balance is obtained as follows. At the reactor inlet, the Brooks flowmeters are first adjusted on the measurements given by the gas meter to allow measurement with a good accuracy (170of the full measurement scale) of the increasing flows at normal pressure and temperature and therefore the total carbon, taking into account the duration of the experiment. At the reactor outlet, the carbonaceous material can be divided into four fractions: 1. The first is the volume fraction of the C1-C5fraction. In the total gas fraction there is also some hydrogen which is measured by gas chromatography (GC). The measurement of the total gas fraction, expressed at normal pressure and temperature, associated with the GC measurements allows a calculation of the carbon content of this fraction. 2. The C6+fraction obtained with the quenching solvent is analyzed by GC, by using an internal standard technique. 3. The Clo+ fraction obtained in the filter is simply weighed and its average molar mass is assumed to be 19 g/mol for an average formula ClsHl2. 4. The coke of molar mass 12 g/mol is determined by the volume of CO and COz obtained by its oxidation with air. Conversion is calculated either indirectly by the results of the gas chromatographic analyses (FID), taking into account the amount of hydrogen, or directly from the TCD measurements using argon as an internal standard: (CH4/Ar)i - (CHJAr), conversion = (CH4/Ar)i
(11)
where CH4/Ar stands for the ratio of the areas for CHI and Ar, with the subscript i for the mixture at the inlet of the reactor and o for the mixture at the outlet.
2. Smoke Trapping. In the experiments at rather high conversion (>15%), we have observed in the gas phase some particles half solid and half liquid at the reactor outlet (this phase cannot be dissolved in conventional solvents). This smoke or aerosol leads to an important dispersal of the reaction products in the lines which has to be kept at a minimum. The solution that we have retained is the use of a filter made of compressed quartz wood. Satisfactory results have been obtained with this device. The lilter is weighed before and after the pyrolysis and the resulting mass of products is called 'soot". 3. Analysis of the Liquids. We have used an internal standard method, the standard being ethylbenzene. For a sampling weight W , of solvent, we add approximately (but accurately known) 10% of standard product We. The percentage P in weight of product (%) is given by
%P=-
area P We -k area e W,
where area P denotes the area of the peak corresponding to the product, area e denotes the area of the peak corresponding to the standard, and k denotes the relative response coefficient of P compared to the standard. C. Temperature Influence on the Pyrolysis of Methane. An experimental investigation of the effect of temperature has been performed to study the possible development of the thermal coupling reaction. The experiments were performed in an alumina tubular reactor (Demarquest). Since it was shown that dilution by hydrogen was decreasing the carbon formation and therefore leading to high selectivities in Cz products (Holmen et al., 1976; Rokstad et al., 1991a,b; Broutin et al., 1991, the temperature influence was investigated for a ratio H2/CH4 = 2. The reactants used in this study and provided by l'Air Liquide were CHI N 35 grade: 99.95% (Hz and O2 10 ppm, CzHz< 200 ppm), Hz R grade 99.95% (Hz and O2 10 ppm), and Ar U grade 99%. The reaction was cond u d as follows: for a hydrogen dilution such as H2/CH4 = 2 and for a residence time equal to 100 ms, the oven temperature was settled between 1300 and 1400 "C. The reaction products (excepted coke and tar)analyzed by gas phase chromatography were as follows: In the gaseous sample, the major products were methane, ethylene, and acetylene; the minor products were ethane, propene, propadiene, butadiene, butenes, and cyclopentadienes. In the liquid sample (quenched), the products were benzene, toluene, xylenes, and naphthalene. The measured amounts are given in Table 11. The main calculated parameters are conversion and selectivities for Cz (CzH4 + CzHz),for benzene and coke. The relationship used for the calculation of the selectivity Si for a product P is ni mol of P Si% = mol of CH4,i- mol of CHI,, (13) where ni denotss the number of carbon atoms of P. The residence times are calculated as follows: t, =
*(radius)*L 293 Q Teq
(14)
where Q = volumic flow (cm38-l) at 1 atm and 293 K at the inlet of the reactor. We assume that there is no chemical expansion (increasing of the number of moles) due to the methane conversion, and assumption which is not correct at high conversion. In Table 111, various values
Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992 2753 Table 11. Results of the Pyrolysis in an Alumina Tubular Reactor at High Temwrature and Low Residence Time oven temperature ("C) residence time (ms) Hz/CHd conversion (%) selectivitv (%)
benzene naphthalene soot coke s u m of selectivities (%)
1400 100 2 37
1380 100 2 30.1
1350 100 2 19.9
1300 100 2 9.3
20.7 56.5 2.8 9.4 1 2.4 6.7 99.6
22.1 58.4 3.85 8.7 0.9 1 4.5 99.45
25 61.3 4.8 6.3 0.7 0.9 1.4 100.4
27.8 55.5 5.2 3.1 3.3 4.7 0.3 99.9
Table 111. Variation of the Residence Time for Various Values of the Reaction Order and of the Activation Energy ( E in koa1 mol-')" reaction order
0 1 2 a
calcd average residence times (8) E = 50 E = 85 E = 100 0.501 0.504 0.502 0.504 0.502 0.501 0.503 0.501 0.5
T (set point regulator) = 1473 K; flow = 0.8 nL/min.
of the residence time for a given flow of incoming reactants are calculated versus the activation energy and the reaction order. It appears that the values obtained are very close in any case. Table I1 shows the influence of temperature on the conversion and on the product selectivities, which can reach respectively 40 and 80% for a residence time of 100 ms.
V. Conclusion The micropilot plant described in this paper and the analytical procedure lead to reproducible carbon mass balances. This technique requires a proper sampling system and the analysis of all the pyrolysis products. This micropilot plant has allowed us to obtain a correct mass balance between reactants and products; for 100 mol of C at the reactor inlet, we generally find between 95 and 105 mol of at the reactor outlet and the sum of the selectivities stands between 90 and 110% of the transformed methane. Our results show that it is feasible to obtain selectivities in ethylene and acetylene around 80% by thermal coupling of methane if the temperature level is high enough. We have clearly shown the beneficial effect on the C2 yields of high reaction temperatures associated with short residence times. Literature Cited Back, M. H. Pyrolysis of Hydrocarbons. Proceedings of the Fourth Materials Research Symposium, Gaithersburg, Oct 26-29; 1970. Back, M. H.; Back, R. A. Thermal decomposition and reactions of methane. In Pyrolysis: Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983;Vol. 1, pp 1-24. Benson, S. F. U.S. Patent 4199533,1980. Billaud, F.; Baronnet, F.; Freund, E.; Busson, C.; Weill, J. Thermal decomposition of methane-Bibliographic study and proposal of a mechanism. Reu. Zmt. Fr. Pet. 1989,44, 813-823. Broutin, P.; Busson, C.; Weill, J.; Billaud, F. Thermal coupling of methane. Abstracts of Papers, 199th National Meeting of the American Chemical Society, Boston, MA, April 22-27,1990;American Chemical Society: Washington, DC, 1990; pp 1,lO. Broutin, P.; Busson, C.; Weill, J.; Billaud, F. Thermal coupling of methane. In Novel Production Methods for Ethylene, Light Hydrocarbons and Aromatics; Albright, L. F., Crynes, B. L., No-
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