Ind. Eng. Chem. Res. 1993,32, 438-444
438
Pyrolysis and Oxidative Pyrolysis of CH3Cl in Steam R. Yildirim and S. M. Senkan' Department of Chemical Engineering, University of California, Los Angeles, Los Angeles, California 90024
Pyrolysis and oxidative pyrolysis of CH3C1 into high molecular weight products such as C2Hz and CzH4 has been studied in a flow reactor in the presence of steam as diluent. The effects of feed composition, in particular the CH4and O2levels, residence time, and temperature on CH3C1conversion and C2 selectivities have been investigated. The presence of small amounts of oxygen in the feed inhibited the formation of carbonaceous deposit such as soot, tar,and coke without decreasing C2 selectivities. It was possible to generate product streams with C2 concentrations in excess of 45 mol % (CzH2 + C2H4 C2H3C1) following the removal of steam, HCI, and unreacted CH3C1, at C2 product yields in the range 2530%.
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Introduction
The chlorine-catalyzed oxidative pyrolysis (CCOP) process was recently developed as a method to convert methane, the major component in natural gas, into more valuable products such as ethylene, acetylene, and vinyl chloride (Senkan, 1987). The CCOP process involves the initial chlorination of methane into CH3C1, followed by the oxidative pyrolysis of CH3Cl into higher molecular products such as C2H2, C2H4, and C2H3C1. The small amount of oxygen introduced with the feed minimizes the formation of carbonaceousdeposits such as soot, tar,and coke which ultimately plug the reactor (Gorin, 1943; Benson, 1980; Weissman and Benson, 1984). The HCl produced can be either converted back to chlorine via the well-known Deacon process or used to oxychlorinate methane directly, thereby completing the catalytic cycle of chlorine. In previous studies, experimental product distributions in the oxidative pyrolysis of CH3C1 in an inert (argon) carrier gas were reported (Granada et al., 1987). In addition, a detailed chemical kinetic mechanism for the CCOP process was also developed and partially verified (Karra and Senkan, 1988). Furthermore, we have experimentally investigated the effects of initial CH4/CH3C1 ratio on product distribution and C2 selectivities (Senkan et al., 1992). The results from the last study also indicated that C2 yields, in particular C2H4, can be improved even at low CH4/CH3C1 ratios and therefore suggested the possibility that excess methane can be replaced with a more separable diluent such as steam. The steam not only can be condensed and therefore separated easily, but the liquid water also readily dissolves HC1 and removes it from the reactor effluents, thereby generating a noncorrosive product mixture. This clearly would be helpful to improve the commercialization potential of the CCOP process. Recently we completed a comprehensive experimental program in which the pyrolysis and oxidative pyrolysis of CH3C1 in steam were studied. In this paper we present results pertainingto the major C1and C2 products formed. The formation of high molecular hydrocarbon trace byproducts will be the subject of a future paper. Experimental Section
The experimental facility used is shown in Figure 1. Specifically, the reactions were studied at atmospheric pressure in a 2.1-cm4.d. and 100-cm-longquartz reactor
* To whom correspondence should be addressed.
which was placed inside a three-zone Lindbergh electric furnace. Mean gas flow velocities in the reactor were in the range of 1-5 m/s during the experiments, suggesting the presence of laminar flow conditions. However, these conditions only result in an uncertainty level of about 10% in concentration profile measurements based on plug flow considerations because of rapid gas diffusion at reaction temperatures. This level of uncertainty is about the same magnitude as other experimental errors (Cathonnet et al., 1981). The steam was generated in the following manner: First, liquid water was injected into a l/4-in. glass lined stainless steel tubing using a pressurized, Le., pulseless, feed system. The temperature of the steel tubing was kept at about 600 "C by using a single-zone electric furnace. A predetermined amount of N2 and/or 02 was also introduced in cocurrent manner with liquid water to aid in the steady generation of steam. The steam/Nz/Ozmixture was then fed into the reactor at the far upstream side and further preheated to the reaction temperature. The CH3C1 (or CH&l/CH4) was then introduced directly into the preheated steam/N2/02 mixture using an air-cooled probe through radially directed injection holes. The water and gas flows were measured using high-accuracy rotameters and controlled with needle valves. The gases were acquired from the following suppliers: CH&1(99.5%) from Liquid Carbonic (Chicago, IL), CH4 (99.0%) from Matheson Gas Products Co. (Cucamongo, CA), 02 (99.0%) from Puritan Bennet (Lenexa, KS), and N2 (99.0%) from Airco Welding Supply (Pomona, CA). Gas sampling was achieved by withdrawing gases through a hot-oil-cooled quartz samplingprobe positioned centrally at the downstream end of the reaction zone through a 100-150-pm-diameter orifice. At this orifice diameter, gas sampling without molecular weight biasing was possible since the mean free path in the reactor was of the order of 1pm. The sampling probe pressure was maintained at about 100 Torr (13.3 kN/m2) to provide rapid removal of gases from the reaction zone and to prevent the condensation of high molecular weight products. Although the samplingprobe alters the temperature field at the downstream, the use of hot oil significantly reduces such effects in the upstream section, thereby allowing the reactor to be treated as an isothermal one. The sample gas stream then passed through an ice trap (about 0 "C) in which the steam condensed, while simultaneously removing the HC1 as well. Our vaporpressure/partial-pressurecalculations, together with the results of experiments in which no steam was condensed prior to gas analysis, indicated the virtual absence of the condensation of all the hydrocarbon products. The gases
0888-5885/9312632-0438$04.00/00 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 439 Sampling Probe
4
Water*HC1
Steam and HC1 Removal
Ouartz Reactor
lnlectlon Probe
1 GC Terminal
m Water
Gas Chromatograph/Mass Spectrometer
Computer U
Nitrogen
U
Oxygen
Figure 1. Schematics of the experimental system.
following the ice trap, i.e., H2O- and HC1-free, were then transferred through glass-lined steel tubing to a computer controlled gas chromatograph/mass spectrometer (GC/ MS) facility (Hewlett-Packard 5890/5971). Samples were introduced to the GC/MS via an automatic valve injection system. The GC was equipped with both a capillary column (50-m X 0.2-mm X 0.5-pm cross-linked methyl silicone gum) for the separation of heavy molecular weight species, as well as packed columns (6-ft Porapak Q and 6-ft Molecular Sieve 13x1 for the separation of low molecular weight species. The capillary column was connected to the MS, and the packed columns were connected to the thermal conductivity detector. For CO, C02, CH3C1, C2H2, C2H4, and C2H3Cl a certified calibration gas mixture (Matheson Gas Products Co.) was used to convert the experimental measurements into absolute concentrations. For 0 2 and N2, a separate calibration mixture was prepared using pure components from gas cylinders. These measurements were expected to result in the determination of species concentrations that were accurate within f10 5%. The concentrations of species in the reactor, i.e., before the removal of steam and HC1, were calculated by ratioing the number of moles of species determined by GC/MS (on a HzO- and HC1-free basis) to the total number of moles in the reactor. The latter was determined by adding the H2O and HCl removed, established by 0-and C1-atom balances, to the total number of moles determined by GCI MS. N2 was used as the internal standard in all the determinations. Carbon balances also were calculated to check the accuracy and consistency of the experimental data using the followingdefinition which takes into account mole changes due to reaction:
carbon balance =
Carbon balances were better than 90% in all of the experiments. Temperatures occasionally were measured using thermocouples; continuous monitoring was impractical due to the rapid degradation of the thermocouples by highly corrosive products. These measurements indicated that nearly isothermal (f10 OC) conditions were present within the central portion of the reactor for about 50 cm. Although sharp temperature gradients normally exist a t the ends, their impact on the results reported here were kept at a minimum because reactants were introduced to and the products sampled from the central, isothermal, section of the reactor. Results and Discussion In Table I the various mixtures considered and the experimental conditions investigated are summarized. First, experiments were conducted with pure CH3Cl (no CH4 or 0 2 ) to establish the effects of steam on conversion and product selectivities unambiguously. Then additional studies were undertaken to systematically explore the impact of temperature, CHI, and 0 2 . In all our studies the major species quantified were CH3C1, CH4,02, C2H2, C2H4, C2HsC1, CO, N2, HzO, and HC1. Some H2 was also produced when high concentrations of CH4 were present in the feed and a t very high reaction temperatures; however, under the experimental conditions reported here the formation of H2 was small. We also detected small amounts of CO2, C2H6, and CH2C12 in some of the experiments.
440 Ind. Eng. Chem. Res., Vol. 32,No. 3, 1993 Table I. Prereaction Compositions of the Mixtures and Experimental Conditions Studied mixture composition (mol %) expt set CH4 CH3Cl 02 NZ HzO 8.89 69.96 21.15 0.00 A 0.00 8.89 69.96 21.15 0.00 B 0.00 8.89 69.96-41.53 21.50 0.00 C 0.00-28.43 9.54 69.96-64.42 D 0.00 20.50 0.00-5.32 9.50 51.49 20.91 3.54 E 14.56 Table 11. Trace Heavy Molecular Weight Products Detected in Experiments concn (ppm) name 0-150 propwe 0-20 propene 50-100 biacetylene 100-200 vinylacetylene 20-50 1,3-butadiene 0-60 cyclopentadiene 100-1200 benzene 0-50 chlorobenzene 20-100 toluene 20-200 ethynylbenzene 100-200 styrene 10-50 naphthalene
In addition to the major C1 and C2 species indicated above, a large number of high molecular weight hydrocarbons were also detected at trace levels. The identities and approximate levels of these trace species are listed in Table 11. The accurate quantitation of these trace species and their relation to the formation of carbonaceous deposits in the CCOP process will be the subject of a future paper. In Figure 2 the mole percent profiles for pure CH3C1 pyrolysis products inside the reactor, i.e., in the presence of steam and HC1, are presented as a function of residence time at 890 "C (experiment set A). In this and subsequent figures lines have been passed through the data points (indicatedby symbols)to show trends. The residence time was determined according to the following relationship: = Al/Q
(2) where A represente the cross sectional area of the reactor, 1 the distance between the point of injection of the reactants, Le., CH3C1, and the sampling probe, and Q the volumetric flow rate of gases through the reactor. The corresponding conversion and selectivity profiles along 7
20
SA= [moles of A X number of C atoms]/ [moles of CH3C1reacted] (3) The amount of CH3C1 reacted, in return, was calculated by appropriately summing up all the major products measured experimentally. That is, direct measurements of CHaCl concentrations at both the inlet and exit of the reactor were not used because the absolute values of the uncertainties in these measurements were larger than the concentrations of the products. It must also be noted that the neglect of high molecular weight species in the determination of CH3C1reacted should not result in errors more than 10% because carbon balances were generally better than 90%. The data presented in Figures 2 and 3 were acquired by simply moving the feed injection probe relative to the sampling probe as described in the Experimental Section. As seen in Figure 2,CH3C1 mole fraction decreases with reaction time accompanied by increases in the mole fractions of C2H2, C2H4, and CH4. However, further increases in residence time eventually decrease the mole fractions of all the C2 products and ultimately lead to soot and coke, which are the most stable thermodynamic products. The mole fraction profile of the internal standard N2 was essentially flat in Figure 2, indicating that the total number of moles remained constant. This however, is not a surprising result because of the presence of significant steam dilution in the system. It is also important to note that substantial levels of CH4 did form in the pyrolysis of CH3C1, a result that is consistent with previous experimental studies (Weissman and Benson, 1984;Karra and Senkan, 1988;Senkan et al., 1992). In
I
Experiment A
I
residence time (s) 0.45-0.89 0.75 0.75 0.75 0.75
the reactor similarly are presented in Figure 3, where the selectivities (8)were defined as a fraction of the CH3C1 reacted according to the following relationship:
. I
temp (OC) 890 850-890 930 890 850-910
Experlm8nl 4
I
CH.CI
'
I 0
04
05
06
07
08
05
Raaidansa T i n r (a)
Figure 2. Effects of residence time on product distributions for experiment set A.
04
05
06
c7
06
c 5
Residence Time ( I )
Figure 3. Effects of residence time on CH3Cl conversionand product selectivities for experiment set A.
Ind. Eng. Chem. Res., Vol. 32,No. 3, 1993 441 Table 111. Mole Percentages of Major Species after Removing Steam, HCI,and Np CZHZ% C2H4% CZH~CI% CO% 0 2% 6.11 1.19 0.93 0.00 0.00 (35.03) (5.84) (5.31) A 890 0.45 85.35 4.64 0.92 1.56 0.00 0.00 (31.64) (6.31) (10.64) 0.75 B 910 72.45 10.10 1.60 0.76 0.00 0.00 (36.66) (5.80) (2.79) Cb 930 0.75 43.32 6.40 2.37 0.52 0.00 0.00 (11.23) (4.17) (0.92) D' 890 0.75 58.65 8.24 1.24 1.09 4.17 13.63 (19.92) (2.99) (2.63) (10.09) (32.96) E 910 0.75 28.03 5.46 2.46 0.26 8.76 4.29 (7.58) (3.42) (0.36) (12.18) (5.96) a Numbers enclosed in parentheses show the mole percentages after removing CH3CI as well. CHI is 7.69% in the feed. 02 is 3.76% in the feed. expt set
A
temp ("C) 890
residence time (8) 0.75
CHI% 8.67 (49.69) 7.00 (47.82) 13.68 (49.66) 41.86 (73.47) 11.67 (28.21) 49.49 (68.77)
CH3C1% 82.56
20
70
Experiment B
Experiment B
CH-CI
C2H,CI
l
o
p
0 850
860
870 Temperature
880
890
900
910
('c)
850
860
870
880 Tempwotura
890
900
