Ind. Eng. Chem. Res. 1995,34, 1044-1059
1044
Experimental and Modeling Study of the Selective Homogeneous Gas Phase Oxidation of Methane to Methanol Rune Lgdeng,*p+Odd A, LindvAgJ PA1 Soraker,” Per T. Roterud? and Olav T. Onsager“ The Foundation for Scientific and Industrial Research at the Norwegian Institute of Technology (SINTEF), N-7034 Trondheim, Norway, Statoil, Postutlak N - 7004, Trondheim, Norway, Statoil Bamble, 3960 Stathelle, Norway, and Department of Industrial Chemistry, University of Trondheim, N-7034 Trondheim, Norway
The partial oxidation of CH4 to CH30H with air has been investigated experimentally and theoretically. Experimentally, the homogeneous gas phase reaction was studied at pressures in the range 40 x lo5-60 x lo5 Pa, temperatures in the range 723-823 K, and 0 2 concentrations in the range 2.6-4.4% relative to CH4. CH30H selectivities between 35 and 55% were obtained at low conversions, 1.2-2.6%. A kinetic model, based on 61 elementary reactions, was able to reproduce experimental observations, indicating that only moderate selectivities to CH30H (5065%) are obtainable a t low CH4 conversion ( 1000 K). The advantage of a complex mechanistic model, in contrast to an empirical model, is the inherent scientific understanding of the real chemical processes. This foundation opens up for possible reliable model extrapo-
0888-588519512634-1044$09.00/00 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1046 P&uy
-
tooling media outlet
nrthmc inlet
Tertiary N2 / mettune Met
(tHk I
OXrJ 1
Figure 2. The reactor.
lations a t conditions outside the ranges of experimental verification. The model can then be an important tool for choosing new experimental conditions. If the mechanistic model is well founded, it is expected to give a more accurate representation of the response over a wider region of conditions than with a purely empirical function. The model development may also provide crucial knowledge to a future catalyst design process.
Experimental Section The short-term intention of our experimental program was to reproduce the results given in a patent (Chemische Werke Huls AG, 1979). On a longer term, the goal was to obtain fundamental knowledge to enable further improvements in the CH30H yield. The experimental apparatus was for safety reasons located inside an isolated steel compartment with closely controlled environment. A sketch of the apparatus has been published earlier (Onsager et al., 1989). Reactant gases were distributed from the gas storage area to the steel compartment in Autoclave Engineer High Pressure tubes. The experimental procedures were performed and monitored remotely from a Hewlett Packard HP1000-A900 minicomputer. The tubular stainless steel reactor is shown in Figure 2. The basic material of construction is 316 stainless steel. The inner wall of the cylindrical mixing chamber is alsint (Al2O3)(diameter 20 mm). The conical chamber expands from 20 to 36 mm at the level of the cold-finger tip. In principle, the reactor is composed of a tubular mixing section and a conical reaction chamber. The reaction chamber is leading to the water- or air-cooled cold finger. The motivation for the conical section was to stabilize the reaction zone close t o the cooling section and keep the product residence times short. One central intention of the reactor design was t o obtain instant mixing of methane and air. This was considered necessary t o avoid undesired nonselective combustion reactions. It was also considered important to obtain as uniform oxygen concentrations and presicely defined experimental conditions as possible. Air was injected axially into the mixing chamber through a small circular capillary nozzle with diameter 0.27 mm. CH4 was fed through two inlets. The main inlet (the primary inlet) was a small annular slot with diameter 4.0 mm and width 0.1 mm. It was directed at an angle of 20” to the reactor axis. CH4 could also
Table 1. The Analytical Setup GC1 (drygas) molecular sieve 13x, 1 M x 118 in., stainless steel, 60/80 mesh Porapak N, 3 M x 1/8 in., stainless steel, 80/100 mesh carrier Ar components Hz GC2 (drygas) molecular sieve 13x, 1M x 1/8in., stainless steel, columns 60180 mesh Porapak N, 3 M x 1/8in., stainless steel, 801100 mesh carrier He components 0 2 , Nz, CHI, CO, COZ,and CZH6 GC3 (condensate) columns Porapak QS, 4 M x 1/8 in., glass column, 80/100 mesh carrier He components Hz0, CHz0, CHzOH, CHzCHO, CH300CH, CzH50H, HCOOH columns
be fed through two circular passages (“secondary inlets”: diameter 0.5 mm) at an angle of 60” to the reactor axis. The temperature was measured a t five axial locations with 1/16 in. stainless steel sheathed thermocouples. Two thermocouples were located in the mixing chamber and three in the reaction chamber. This arrangement gave a fairly accurate measurement of the temperature profile. Experimental and Analytical Procedure. Initially, preheated CH4 and N2 were fed through the reactor. At stabilized flow, temperature, and pressure conditions, N2 was exchanged by an equal flow of air. The reaction zone was stabilized in the conical part of the reactor. The experiments were normally performed for 30 min or longer at steady-state conditions. The product gases were separated in the condensate tank into a dry gas phase and a liquid phase. The product composition was analyzed on three HP5880A gas chromatographs with thermal conductivity detectors. Two, called GC1 and GC2, were applied for the on-line analyses of dry gas, and one (GC3)was used for the off-line analyses of the accumulated condensate. GC1 was operated with Ar as carrier gas t o provide the best sensitivity for H2. N2 was the internal standard. Table 1 shows the analytical setup. Feed Quality. CH4 (99.99+), N2 (99.991,compressed air (HzO- and CO2-free).
1046 Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995 Table 2. The "Basis-Set"Model: k = AI" exp(-E/RT) (L,cal, s, mol) no.
model reactions
A
E
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61
CH4 + 0 2 'CH3 + H02' CH4 'CH3 + H' CH3 0 2 CH300' CH3 + 'CH3 CzH6 'CH3 + HO2' CH4 + 0 2 'CH3 HO2' CH30' + 'OH 'CH3 HCO CH4 CO 'CH3 'CHzOH CH4 + CH2O CH30' + 0 2 CH2O + HO2 CH30'+ M - CHzO + H + M CH30 'CH3 CH4 + CHzO CH30' CH30' CHzO + CH30H CH30' CH300' CH20 + CH300H CH300' 'CH3 + 0 2 CH3O0' CH2O 'OH CH300' + 'CH3 CH30' + CH30' CH300' + CH300' 2CH30' f 0 2 CH300' + CH300' CH2O + CH30H + 0 2 CH300' HO2' CH300H + 0 2 HO2' + HO2' H202 + 0 2 HO2' M- 0 2 H' + M H2Oz + M--'OH + OH + M HzOz 'CH3 CH4 HOz' Hz02 + CH300' HO2' + CH300H H202 + 'OH H2O + HO2' 'CH3 CH30H CH4 CH30' 'CH3 + CH30H CH4 + 'CH20H CH4 CH300' 'CH3 CH300H CH4 + HO2' 'CH3 + H202 CH4 'OH 'CH3 H2O CHd H'-*CH3 + H2 'OH CH300H CH30' CH30H + 0 2 CH20H + HO2 CH30H + 'CH3 'CH20H CH4 CH30H + 'CH3 CH30' + CH4 CHBOH+ CH30' 'CH20H CH30H 'CHzOH CH300H CH30H + CH300' CH30H + HO2'- 'CH20H + H202 CH3OH 'OH CH30' H2O CH30H + 'OH 'CH20H + H2O 'CH20H + H2 CH30H H' CH30H + H' CH30' + H2 'CH2OH M - CHzO H' M 'CHzOH + 0 2 CH20 + HOz' 'CH20H CH300' CH2O + CH300H 'CHzOH + HO2' CH20 + H202 CH20 0 2 HCO HO2' CH2O + 'CH3 HCO + CH4 CHzO CH30' HCO CH30H CH2O + CH300' HCO + CH300H CHzO 'CH20H HCO + CH30H CH2O + HO2' HCO + H202 CH2O 'OH HCO + H2O CHzO + H'-HCO H2 HCO + 0 2 CO + HO2' HCO + M - co H' M CO + CH30 C02 + 'CH3 CO + HO2' C02 'OH CO + 'OH CO2 + 'H CO + 'H + M HCO + M C2H6 'CH3 + 'CH3
4.035e1On 3.715e15 7.227e08 1.011e12 3.613e09 1.987e10 1.204ell 2.409e09 6.624e07 3.900e34 2.409e10 6.022e10 3.011e08 b 1.000e09 2.409e10 7.829e07 1.265e08 4.637e07 1.807e09 1.204e16 3.546e24 1.204e07 2.409e09 1.746e09 4.725e08 2.168e-02 1.807e08 1.995e08 1.927e02 2.246e01 3.981e15 2.048e10 1.820e08 1.450e-02 3.011e08 1.807e08 9.635e07
56913 103822 0 0 0 0 0 0 2603 33265
---
+
+ + +
+
+
--
+ + +
-- + -+ -+ + + - + -- + + - + + -+
4
-. -
+ +
--
+ +
+ + +
-
-+
+ + + +
+ + + +
- + - -+ + -+ -- + - +
25000 0 0 0 -2583 0 48413 51466 -596 9935 318 8843 16227 18480 14940 2106 8755 43000 44910 9800 6935 4074 13711 12579
0 0 0 0 0 0 -1.18 -2.87 0 0 0 0 3.10 0 0 2.40 3.00 0 0 0 3.10 0 0 0
-4869 -4869 43012 0
2.10 2.10 -8.00 0 0
C
1.696e04 4.240e03 1.144e40 1.204e09 1.204e10 1.204e10 2.048e10 5.54 1.024e08 1.987e09 5.48 1.987e09 3.433e06 2.192e05 5.119e10 1.024e13 1.565e10 1.506ell 4.400e03 6.310e14 3.162e22
4
+
4
+
4
-
0 0 0
C
+
- + - + --
n
0 0 0 -0.64 0 0 0 0 0 -6.65 0 0 0
0 0
38949 5862 2980 11665 5862 11665 -447 3001 1689 20325 11803 23647 -740 3688 91080
0 0
2.81 0 0 2.80 0.00 1.18 1.77 0
-0.11 0.00 0.00 1.50 -1.82 -1.79
high pressure limit rate constants: reactions 2, 4, and 61 bimolecular rate constants: reactions 10, 21, 22, and 56 termolecular rate constants: reaction 60 bimolecular "strong collision limit": reactions 22,43 reaction 45: formulated as CH300 + CH20H CH30 + OH + CH2O in Tsang (1986) "third-body" concentration [MI = 1.0 m o m
-
-
4.035e10 represents 4.035 x 1O*O, etc. CH3 + 0 2 CH300 loglo Kes = ((28510/(4.613)- 5.7 (Khach, 1982); kI4 = k3/Keq. (6.620e01)P5exp(-960/T); h3dk40 = 3.7 exp(-1020/T).
The Mechanistic Model The "basis-set" model is given in Table 2. The molecules accounted for are CH4 (methane), 0 2 (oxygen), CH30H (methanol), CH20 (formaldehyde),CO (carbon
k39+40
=
monoxide], C02 (carbon dioxide), CH300H (methyl hydroperoxide), H202 (hydrogen peroxide), CzHs (ethane), H2 (hydrogen), and H2O (water). The free radical intermediates are CH3 (methyl),CH30 (methoxy),CH2-
Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995 1047 OH (methylhydroxyl), CH300 (methylperoxy), HO2 (hydroperoxy), OH (hydroxyl), H (hydrogen), and HCO (formyl). “Third body” kinetics is illustated by M. M is a carrier of energy. Collisions do not lead to chemical changes of M. Some of the model reactions have limited influence on the predictions. A screening of reactions were performed by a “radical flux” analysis. The reaction was included t o the “basis set” if it contributed to the radical fluxes (formation and consumption of any participating species) by more than 0.1% at any conversion degree. Therefore, the final model includes reactions of different importance. An example of an elementary reaction of minor importance is the unimolecular thermal decomposition of CH4.
-
+
CH4 ‘CH, H’ (1) “his is an initiation reaction, typically of real practical importance at far higher temperatures than 773 K. In this work, it has been identified to be the single source of H radicals in the early stages of the induction period. At a later stage of reaction the bimolecular decomposition of CH30’ and HCO’ radicals takes over the role as the dominant H’ radical producing channels. CH,O’+M-CH,O+H’+M HCO
+ M - co + H + M
(2)
(3)
The exclusion of the decomposition reaction of CHI from the mechanism did not disturb or change the distribution and accumulation rate of any of the molecular products, not even of molecular H2. The reaction was still considered principally important from a view of the detailed fundamental understanding of the microkinetics. The rate constants are mainly taken from the Combustion Data Bases for methane-related components (Tsang and Hampson, 1986) and for methanol-related components (Tsang, 1987). These data bases provide evaluations of the expected accuracy of the rate parameters, termed “uncertainty factors”. Uncertainty factors, typically estimated to be between 1.5 and 10, are given in Table 3. The accuracy is dependent upon various factors, for example the method of determination. Variations of the rate constants within these uncertainty ranges affect the model predictions dramatically for the most sensitive reactions. Strategically, the first stage in the model development was to establish a set of reactions with predicting properties of residence time and product distribution a t a typical pressure, isothermal temperature, and oxygen concentration. Next, the model was revised to obtain a more accurate reproduction of the experimental results with as few and minor modifications of the rate constants as possible. The temperature profile was also considered. Rate constants for the first development stage are given in Table 2. In the revised version of the model, adjusted to reproduce the experimental results, the rate constant recommended by Tsang (Tsang and Hampson, 1986) is used with success for reaction 29. The thermodynamical properties of the CH30 radical have for a long time been difficult to establish. One highly sensitive reaction in the “basis set”, where the
Table 3. Rate Constants Literature Sources of the “Basis-Set”Model and Expected Rate Constant Accuracy reaction no.‘
lit. ref
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
a
uncertainty factor 5 1.5
a C
1.5 5 3 2 5 2 4 5 5 10
a a a a b a a a
a a d e
a
3 large large 5 3 3
a a a a a
f
5 10 2 3 3 10
a a a a b a
g
1.4 1.3
a a
reaction no.‘
lit. ref
32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61
h b i b b b b b b b b b b b b a a a a b a a a a j a a
uncertainty factor 10 1.5 10 3 10 1.5 1.5 3 3 4 2 3 2 2 2 3 10 3 3 1.5 1.5 3 5 3
k a a
3 1.5
a Tsang and Hampson, 1986. Tsang, 1987. Parkes et al., 1977. Khachatryan et al., 1982. e Basevich and Kogarko, 1983. f Tsang and Hampson (1986), p 1120. Rotzoll, 1986. Benson, 1981. Westbrook and Dryer, 1982.J Tsang and Hampson (1986), p 1146. Dagaut et al., 1988. The reaction numbers refer to the model in Table 1.
’
CH3O’ radical is participating, is reaction 26. CH,
+ CH,O’ - ‘CH, + CH,OH
(4) Various rate parameters have been applied in the literature for this step. Vedeneev has applied a rate constant established by Kondrat’ev (Vedeneev et al., 1988a; Kondrat’ev, 1971). A rate constant with lower frequency factor and also a lower activation energy than Kondrat’ev is recommended by the data base of Tsang (Tsang and Hampson, 1986). Another rate constant, from a direct measurement study (Wantuck, 19881, was applied by Durante (Durante, 1989). The model is very sensitive to variations of the rate parameters of reaction 26.
k,,
= 4.725 x
lo8 exp(-8843/RT)
The recommended rate expression (Tsang and Hampson, 1986) with a frequency factor increased 3-fold was adopted by the “basis-set”model. The uncertainty factor is 3. In the revised model a frequency factor closer to the recommended value was used. The adjustment factor was reduced from 3 to 1.91. The reaction mechanism was formulated in terms of differential equations with nonlinear parameters. The differential equations were solved numerically by the 1981 version of LSODE (Hindmarsh, 1980) (Livermore Solver of Ordinary Differential Equations). Restrictions and simplifications were imposed: (a) 0 2 was the only oxidant in the feed. No diluents were accounted for. (b) CH4 was the only hydrocarbon reactant in the feed. (c)The investigation was restricted
1048 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
to the homogeneous gas phase, with emphasis on the C1 participants. Only monoradical intermediates were considered to be of interest at the actual temperature conditions. (d) No wall reactions were included. It was not allowed to adjust the free radical concentrations by introducing speculative wall contributions. This also means that the observed coking on the reactor walls and thermocouple sheats was not a part of the modeling scope. (e) No “third-body”effects were considered. The concentration of M was held constant at 1.0 m o w at all simulated conditions.
Experimental Results Typical experimental conditions and corresponding results are presented in Table 4. The conditions comprise partial pressures of oxygen, nitrogen, and methane, as well as the total pressure. The concentration of oxygen is given relative to methane. The five temperature measurements are given. TI is the thermocouple in the reactor mixing chamber front, T2 is the next, and so on. The residence times in both the mixing chamber and reaction chamber are given. The distribution of the carbon-containing products are given as C-selectivity on single carbon basis. The H2O amount is given as percent 02-selectivity, which is the amount of 0 2 found in water relative to the total 0 2 amount found in all the other products. The amount of molecular hydrogen is given as H2-selectivity, which is the amount of molecular hydrogen relative to the total H2 found in all products. The conversion of CH4 is calculated from the amount of carbon in the products. The 0 2 conversion is calculated from the differences of the analyzed oxygen amounts and the flowmeter amounts. The mass-balance check on “in and out” oxygen was based on the feed-line flowmeter and the analytical quantification of molecular oxygen in the exit gas. The calculated difference is given as “ 0 2 recovered”. The carbonhydrogen ratio of the exit gas is also reported. In CH4, this ratio is 0.25. The C/H ratios of the exit gas should therefore also equal 0.25. Trace amounts of acetaldehyde, methyl formate, ethanol, and formic acid were also detected. These are not included in Table 4.
Modeling Results Modeling results produced with the “basis-set”mechanism and rate constants a t simulated conditions close to typical experimental conditions are presented. Experiment A5 is chosen to represent typical conditions. The temperature profile from experiment A5 is given in Figure 3. A5 is performed at a 36.3 x lo5 Pa, 2.8% 0 2 relative t o methane, and residence time of ca. 1 s. The observed profile reflects the exothermal nature of the reaction. The profile without reaction a t similar conditions is also indicated. The model was applied a t similar conditions as experiment A5; however, the isothermal temperature of the simulation was 723 K.
The Conversion Curve The simulated CH4 conversion curve is illustrated in Figure 4. The time scale in Figure 3 corresponds to the experimental residence time, i.e. 1 s. The “basis-set” model predicts a shorter induction period, ca. 0.23 s, and a higher CHI consumption rate than observed in the experiment. The induction period length and the CH4
$
S y l indrical Chamber
683 0
2
6
4
IO
8
Reoctor Length
Con I co I
Chomber 12
14
1’3
I8
Ccml
Figure 3. Experimental temperature profile for experiment A5. Total pressure = 36.3 x lo5 Pa, 02 content 2.8% relative to CH4. Residence times (cylindricallconical chamber) = 0.5W0.47 s. Thermocouple locations for Tl-T5 are also sketched.
0 0.0
0.1
0.2
0.3
0.4
0.5
0.R
0.7
0.8
0.0
1.0
Residence time [SI
Figure 4. Simulated CHI conversion curve at isothermal temperature 723 K. Total pressure = 36.3 x los Pa. 02 content 2.8% relative to CH4. Residence time = 1 s.
conversion rate differ by a factor of about 2 from the experimental observations. The characteristic S-shaped conversion curve in Figure 4 reflects both an induction period and the selfaccelerating character of the reaction. Both these features are typical for the CHd02 system a t these conditions. A theoretical and kinetic explanation of chain reactions with delayed branching was given already in the 1930s (Semonov, 1935). The problem consisted of describing the oxidation process as a function of time. The solution to this problem is now termed the “Semenov equation”:
q = 141 - exp(-f3)) 0
= fraction converted a t a certain time and 6 = p,(t - tm&. t is time, t,,, is the time for maximal reaction rate, and p, = f - g. f and g are the kinetic coefficients for branching and termination. Semenov’s equation produces the S-shaped conversion curve of Figure 5. The maximum reaction rate occurs at 50% 0 2 conversion. A shift of the maximum reaction rate to lower conversions indicates an increased termination rate. A shift to higher 0 2 conversions indicates that the propagation rate is increased. A comparison of Semen-
1050 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995
I
0
I
2
I
c
Qt..X
e
Figure 5. Theoretical conversion curve from Semenov's equation. Modeling results only IC&
80
calculated as the percentage of H2 relative to the sum of hydrogen (2H) in the products and H2. Figure 6 shows results for CH30H, CH20, and CO. Figure 7 shows results for C2H6, C02, and H2. The model predicts CH2O to be the main primary product at low conversions (
