Ind. Eng. Chem. Res. 1993,32, 796-799
796
Free-Radical Kinetic Model for Homogeneous Oxidation of Methane to Methanol Jin-Woo Chun and Rayford G. Anthony’ Department of Chemical Engineering, Kinetics, Catalysis and Reaction Engineering Laboratory, Texas A&M University, College Station, Texas 77843
A free-radical reaction mechanism for homogeneous oxidation of methane to methanol was used to investigate the effect of oxygen concentration, residence time, and reaction temperature on the product distribution. The results from the simulation are compared with data for reactor feeds of 2.30-4.35% oxygen and 95.65-97.70% methane and pressure of 50 atm. Methanol selectivities decreased from 55 % to 34% as the reaction temperature increased. Oxygen and methane conversions were 100% and approximately equal to the oxygen concentration in the feed, respectively. Helton’s free-radical model with a change in the enhancement factors was used for the simulations. The enhancement factors were determined using several temperatures, oxygen conversions of 20-99 % , and a residence time of 2 s. For 8- and 17-s residence times the model predicts the performance for the lowest temperature. A simulation is also presented, which suggests a method of operation to achieve methanol selectivities and yields of 70% and 7 % , respectively. Introduction Free-radical models for homogeneousmethane oxidation have been developed by Chou and Albright (1978), Vardanyan et al. (19811,Onsager et al. (19891,Durante et al. (1989),Droege et al. (1989), and Helton (1991). With the exception of Helton, the authors used the proposed models for isothermal simulation of their data. However, in the oxidation of methane to methanol with a methane and oxygen feed the reactions are very exothermic and a nonisothermal temperature profile exists in the reactor. Onsager et al. (1989) simulated partial oxidation of methane with 1.5-3.5% oxygen feed concentrations and demonstrated that the free-radical model predicted and explained the observed data at low methane conversions. Helton (1991) constructed a free-radical reaction mechanism for oxidation of methane and ethane to methanol by adding elementary steps describing reactions of CHd and CzHs and their products. He used actual temperature profiles for his simulations and showed that the proposed kinetic model was capable of predicting the homogeneous reaction of methane and ethane with molecular oxygen. Onsager et al. (1989) and Helton (1991) determined the importance of each reaction channel for their models by using the extent of oxygen consumed by each reaction path. To investigate the effect of oxygen concentration, residence time, and reaction temperature on oxygen and methane conversions and product distributions for methane oxidation to methanol, a modified version of Helton’s (1991)modelwasused. Actualtemperature profiles (Chun, 1992; Chun and Anthony, 1993b) and the plug flow assumption were used. The feed contained 2.30-4.35 % oxygen and 95.65-97.70% methane and the pressure was 50 atm. The temperatures were 660-720 K. Experimental Methods Reactor System. Autoclave Engineers’ microscale bench-top reactor with a 900 control system was used for this study. The tubular reactor and high-temperature heaters surrounding the reaction tube were located in a reactor oven maintained at 423 K. The process line from the reactor oven to the gas chromatograph (GC) was maintained above 453 K to prevent condensation of
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methanol and water. A 46-cm Pyrex tube inserted inside a 316 stainless steel tube was used as the reactor, and the inside diameter of the Pyrex reactor tube was 10 mm for most experiments. A Pyrex tube was used as a sheath over a 316 stainless steel thermowell, and the outside diameter of the thermowell with the sheath was 6 mm. For some runs two Pyrex tubes were used as liners to decrease the inside diameter of tube from 10 to 7 mm and a different Pyrex tube with a 5-mm outside diameter was used as the sheath for the thermowell. By using this technique experiments could be conducted with residence times of 2 s or less. The length of the heating jacket for the reaction tube was 16.5 cm, and was divided into three heating zones. The reactants passed through a mixer/ vaporizer assembler, an eight-port switching valve, and then to the fixed bed reactor. The temperatures of the furnaces for the reaction zone were 623-773 K, and pressures of 50 atm were used. The reactor effluent flowed through a 2-pm filter, a back-pressure regulator, and the switching valve prior to exiting the reactor oven. The products were analyzed by on-line GC. An axial temperature profile was measured for each reaction condition by moving a K-type thermocouple up and down inside the thermowell. The temperature profiles were measured for feeds of methane and methane and oxygen to the reactor. When oxygen was added to the methane feed, a difference in the temperature profiles occurred, and this difference was used to determine the location of the reaction zone. The average temperature used in the figures is the arithmetic average of seven equally spaced temperature measurements within the reaction zone. Detailed temperature profiles and methods of analysis of the products were reported by Chun (1992). Chemicals. Methane from two sources,Specialty Gases and Equipment Co. and Airco, were used in this study. The methane from Specialty Gases and Equipment Co. was CP grade with the specificationof 99.0 % methane, 50 ppm oxygen, 2000 ppm nitrogen, 100 ppm COz, 50 ppm CO, 500 ppm ethane, 500ppm ethylene, 300 ppm propane, 300 ppm propylene, 100 ppm C4+, and 10 ppm H20. The purified methane from Airco had specificationsof 99.0 % methane, 0.6% nitrogen, 0.5% oxygen,0.2% CO and COZ, and 0.127% ethane. Oxygen with 99+% purity was purchased from Airco. 1993 American Chemical Society
--I
CH31
CH30)J---,
b
Ind. Eng. Chem. F&., Vol. 32, No. 5,1995 797 Onecouldartificial)yaeaumeaninitialpooloffr~rcbdicale, which are produced by impurities in tbe feed or other unknown source, or multiply the pmexponential of the above reaction by an enhancement fador. For this study an enhancement factor was determined using data colI d for space times of 2 s and different @mperaturee to obtain oxygen conversions of 1 W W . The preexponential factor published by Taang and Hampton was multiplied by theenbancement factor of85 X 10% obtain the desired match of model simulationsand emperimental data. The eee-radical model ale0 predicted 7-16% selectivities for formaldehyde, but in our experimental work, selectivitiesof formaldehydewere only 2-7 % Therefore, an enhancement factor af l.QX 106was used for the reaction of CH,O + 0, HCO + HO, to consume the formaldehyde produced by the reactions listed below. The HCO reacte rapidly with other free radicals to produce CO. The model containe t h e following reaction paths for the decomposition of CH30 to CH20
.
0.0001 0.001 0.01
0.1
1
10
100
Oxygen Conversion (96) pylnve 1. Comparisonof simulationwith a free-radical model with data for methane oxidation to methanoI. (Preaaure = 60 atm; feed: 3.9% oxygen, 96.1% methane; residence time = 2 a. Solid line, simulation. Data, ( 0 ) CH&H ( 0 ) CO (A)COz. Experimental temperatureprofiieswere used, but averagetemperatures range from 680 to 710 K.)
Result8 and Discussion Free-RadicalKinetic Model. The free-radical kinetic model used in this study is a modification of the model developed by Helton for oxidation of feeds of methane and ethane to methanol. T h e reactor simulations were conducted by aesuming the reactor could be approximated with the plug flow reactor model and a kinetic model, which contains 178 free-radical reactions. The set of ordinary stiff differential equations were integrated using MODE (Livermore Solver of Ordinary DifferentialEquations), which uses a variation of Gear’s method for stiff differential equations. This algorithmwas combined with the Yale Sparae Matrix Solver. Each simulation took approximately 6 minon a 48633-MHz personal compuhr. Activation energies and pre-exponential factors for t h e free-radical reactions were compiled and published by Tmng and Hampton (1986). In thiesystem the initiation step is hydrogen abstraction from methane by molecular oxygen, i.e., CH,
+ 0,
-
CH,
+ HO,
with an Arrhenius equation of kl = 4.0345 X 107 exp((56913/1.9872)/T) ms/(s mol). However, when the set of free-radical reactions were u88d in a simulation with the measured temperature profile for a residence time of 2 s and an average temperature of 682 K,no reaction was predicted to OCCUI. Experimentally, the methane converoionwas 0.8 % and oxygen conversion was 18%. T w o @ble reasons for the difference inthe model prediction and the experimental observations could be due to generation of free radicals at thie temperature due to impurities in the feed, or to initiation of free radicals by the reactor walls. Chun and Anthony (1903a) show that no significant difference in conversions and selectivities occur at equal reaction times with an empty Pyrex tube (surface/volumeratio = 10 cm-1) and a tube packed with 1.41/2 mm Pyrex be& (surfam to volume ratio = 32.3 cm-9. Hence, signifhnt generation of free radicals by the reactor walls seem to be unlikely. Two procedures could be wid in the modeling to enhance the rate of methane and oxygen consumption.
-
CH30 + M- CH,O
+H +M CH,O + HO, CH,O + H, CH,O + OH
+ -
C q O + 0,
CH30 H CH30 + 0
-
(1) (2)
(3) (4)
CH30 + OH
CH,O
+ H,O
(5)
+ HO,
CH,O
+ H,O,
(6)
CH,O
All of these paths result in an increase in formaldehyde in the product. To further decrease the formation of formaldehyde, which also decreases the amount of CO produced, reaction 1 was removed from the frete-radical model. This reaction was chosen because it increases rapidly with increasing temperature and is a strong function of pressure relative to reactions 2+, Le., the concentration of M in the model is equal to the molar density of the reaction mixture. T h e need for an enhancement factor for formaldehyde consumption indicates that something may be miming from the kinetic model or that significant errors exist in the measurement of formaldehyde concentrations. Closure of the oxygen atom balances for 100 consecutive experiments with 4.35% oxygen in the feed was 100 f 6.7 % ,and for seven experiments with 2.3 % oxygen in the feed the closure was 109 f 5 % Carbon and hydrogen atom balance closures were 99.9 i 0.6% for both feed compoeitions of oxygen. Considering them excellent closures on the atom balances, and the effort expended to ensure accurate determination of formaldehyde and the other componentsin the producte, suggests that very little error exists in the experimental data. Therefore,the need to use an enhancement factor is due to the poor understanding of the kinetics for the formationand consumption of formaldehyde. Figure 1 illustrates model predictions for the product distributions. The importance of formation of the intermediate species CHsOOH and CHaOO is ale0 illustrated. Theee species were postulated to be produced in the preheating zone of the reactor. They are converted to
.
798 Ind. Eng. Chem. Res., Vol. 32, No. 6,1993
6o
CHsOH and HCHO by reaction with other free radicals. T h e conversion of CH3OH and HCnO to carbonmonoxide by reaction with oxygen and other free radicals is ale0 illustrated. The other key free radical for methane oxidation is CHsO. Simulatiom indicate that CHsOreacta with metham to produce about 50% of the methanol. It also indicates that this free radical r e a h with many free radicals to prodwe methanol and formaldehyde. H e b n (1991)showed that the concentration of CH30 was lo00 times leas than that Of CH300. Therefore, Figure 1 does not show the CHaO. Carbon dioxide selectivitieswere &lo%, but dculated carbon dioxide selectivities were less than 1%. T h e simulationsindicatethat modit carha dioxide is produced from carbon monoxide by the fallowing key reactions:
CO + CH,O
-
CH,
CO + OH -* CO, CO + H,O
-.
CO,
+ CO,
+H + OH
If one changes the kinetic parameters for these three reactions, Le., uses enhancement factors, then the carbon dioxide selectivities can be increased. T h e simulations indicate that methanol selectivity decreases from 55 to 45% for oxygen conversions of 18100% and methane conversions of 0.7 to approximately the mole fraction of oxygen in the feed, which in the caee of Figure 1is 3.9 % But after all of the oxygen isconsumed, the model predicts very little loss of methanol. This later result agrees with experimental data obtained by setting the feed composition and feed rate and increasing the furnace temperatures to obtain a curve of methane conversion versus temperature. At the initial furnace temperature eeminge no reactions occur. As the temper& r e is increased,methane conversions less than 1% and oxygen mnversiom less than 20% will be obtained. T h e next increment in furnace temperature will result in 100% conversion of oxygen and a methane conversion equal to themole~onofoxygenhthefeed.Themodelpredicts the~datapointasilluetratedinFigurea2-4fordifferent residence times and oxygenconversions. However, as the temperature is increased methanol selectivity decreases, but the eimulation does not predict this decrease in resulta are particularly innominal residence thei of 2 e enhancement factom, and there is an overlap in the temperature ranges. Also, for the reaction conditione presented in Figures 2-5, the simulations predict 100% oxygen conversions in the first 2 s of the reaction zone. Figure 2 shows that selectivity of methanol decreases from approximately 45 to 38% for residence times of 8 and 17 8. But even though the simulation predicts the n residence time, the decline in asnotpmdhted. Figure~3illuetrates r carbon monoxide selectivity with the change in residence time; Le., CO selectivities increase with increaeh temperature at the same residence time and the trend$ for 8 and 17 s do not change eigniticantly with inmasing temperature. Obviously, the model does not adequately predict the loss of methanol to CO at the ~r.tempep$kueeandreeidencetimesgreaterthanth~ required to obtain 100% conversion of oxygen, the limiting reactant. Figurea 4 and 5 illustrate that methanol selectivity decreases and carbon monoxide selectivityincreaseswhen
.
I
660 870 680 690 700
710
720
Average Temperature (K) Fmre 2. Effect of midonce time on methanol selectivities. (Presslue = 60 atm, feed: 4% oxygen, 96% methane. Actual temperature profiles were used in the aiulations. Solid line b the simulation.) 17 sec
8 sec A
aR Y
40
.-E
l
8
1
I
f
i60
670
EXD
680
40
- Sirn
690
700
710
72;
Average Temperature (K) Figure 8. Effect of residencetime on carbon monoxide selectivities. (Pressure = 60 atm; feed: 4% oxygen, 96% methane. Actual temperature profies were used in the eimulationa. Solid line ie the simulation.)
the oxygen concentration in the feed is increased. These results indicate that further reaction of methanol to carbon monoxide increased by increasing the oxygen feed concentration. Simulation with N Series Plug Flow Reaetma. Simulation and experimental data on methanol oxidation (Chun, 1992)show that a significant amount of methanol oxidizes to carbon monoxide and carbon dioxide. The data and simulationsalso indihte that at low oxygen feed concentzatiom the methanoi selectivity incmms. Tkmfore, a series of simulations were conducted allowing for interstage feeding of oxygen and intershge removal of methanol, formaldehyde, and water. For the simulations presented in Mgive 6, atotaloxygen feed of 10% was uead at the reaction temperatare Of 673 K e.g., 0.6 % of oxygen was fed at each redctor inlet with 20 plug flow reactors in series, and in some cases methanol, formaldehyde, and water were removed betwmn stages. Figure 6 shows that the selectivity of methanol and conversion of methane can be increased significantlywith N plug flow reactors in seiriea, if the liquid producta from each reactor are collectedbefore entering the next reactor. When 20 plug flow reactors are connected, 70% of
Ind. Eng. Chem.Res., Vol. 32, No. 6, 1993 799 76
50
50
P
A
70 %
*= > 66 .c - 30
-8
60
a
65
Q,
- 20
1
0
CH3OH Actual
CHSOH Actual 880
870
890
700
710
720
0
Average Temperature (K) Figure 4. Effect of oxygen f e d concentration on methanol sblNvitia. (Solid linea are hulatione. Pressure = 50 atm; oxygen conversion = 100%;residence time = 8 8. Experimental temperature
profilee were uaed in the simulations.)
A
ap
.-E 5
--g_g__x__
40t
6
8
10
12
CH4 Conversion (%) Figure 6. Simulation of methane oxidation to methanol with N plugflowreactoreinseriea. (Total02= lO%;N=numberofrea&rs. (*) CHaOH, HzO, and HCHO were collected before entering the next
2.3% 0 2 - 30
0
4
reactor. Pressure 50 atm; temperature 673 K.) decrease significantlywith increases in temperature. The results show that the methanol selectivity decreases significantly by increasing the oxygen to methane feed ratio because of secondary oxidation of methanol to carbon monoxide.
3.9% 08 v
2
20-
- Slm
10-
- Slm
0
0
X
- 20
Actual
- 10
Actual
0' 670
'0 600
690
700
710
720
Average Temperature (K)
-
FigureI. Effect of oxygen feed concentration on carbon monoxide selectivitiea (Solid lineaare simulations. Pressure = 50 atm, oxygen conversion 100% ;residence time = 88. Experimental temperature
profides were uaed in the simulations.)
methanol selectivityand 13% of methane conversion are obtained. T h e selectivitiesof methanol without collecting the liquid products were 385 with 10 or 20 plug flow r m r a in eeri6s. These res$ts illqatrate the need for a new catalyst whicbhactiveand selectivity at temperatures I& than 673 K to prevent the loss of methanol by freeradical reactions. At the higher temperatures free-radical readions are dominant and it is unlikely that a selective catalyst can be developed in the range where free-radical reactions are the dominating reactions.
Conclusions The free-radical kinetic mechanism is capable of predicting the homogeneous reactions for methane oxidation to methanol. The results from simulationand experiments show that methanol selectivities and carbon monoxide selectivitiesda not change aigaificantly by changing the residence time or reaction temperature at 100% oxygen convera@nand residencet h e e greater than 8 s;Le., at the same temperature the eelectivities would be about the same. However,for a given residence time the selectivities
Acknowledgment This researchwas supported in part by a grant from the Texas Advanced Research Program (No.4026). Literature Cited Chou, T. C.; Albright, L. F. Partial Oxidation of Methane in G h and Metal Tubular Reactors. Ind. Eng. Chem.Process Des. Dev. 1978,17 (4),464.
Chun, J. W. Direct Oxidation of Methane to Methanol. Ph.D. Dissertation, Department of Chemical Engineering, Texas A&M University, 1992. Chun, J. W.; Anthony, R. G. Catalytic Oxidations of Methane to Methanol. Ind. Eng. Chem. Res. 1993a, 32,259. Chun, J. W.; Anthony, R. G.Partial Oxidation of Methane, Methanol, and Mixtures of Methane and Methanol, Methane and Ethane, and Methane, Carbon Dioxide and Carbon Monoxide. Id. Eng. Chem. Res. 199313, p r d i g paper in this h u e . Droege,M. W.;Hair,L.M.;Pib,W.J.;Weatbroolt,C.K.TheThermal Gas Phase Reactions of Methane and Oxygen: A Cornpariaon of Model Calculations and Experimental Reeults. Pseaented et the AICHE Spring Meeting 1989, Houston, TX,1989;paper no. 62E. Durante, V. A.; Walker, D. W.; Seitzer, W. H.; Lyons, J. E. Vapor Phase Hydroxylation of Methane. Preprinta of 3B Symposium on Methane Activation, Conversion, and Utilization; 1989 International Chemical Congress of Pacific Basin Societies; Honolulu, HI, 1989. Helton, T. E. Methanol and Carbon Monoxide Production from Natural Gas. Ph.D. Dieeertation, Department of Chemical Engineering, Texae A&M University, 1991. Reeearch Adviser: R. G. Anthony. Onsager, 0. T.; Soraker, P.; Lodeng, R. Experimental Inveatigation and Computer Simulation of the Homogeneous Gae Phaw Oxidation of Methane to Methanol. Preprints of 3B Symposium on Methane Activation, Conversion, and Utilization; 1989 International Chemical Congress of Pacific Baein Societies; Honolulu, HI, 1989. T a g , W.; Hampeon, R F. Chemical Kinetic Data Base for Combustion Chemistry. Part 1. Methane and Related Compounds. J. Phys. Chem. Ref. Data 1986,15 (3), 1087. Vardanyan, I. A.; Yan, S. Mechanism of the Thermal Oxidation of Methane. Kinet. Katal. 1981.22 (4), 846. Receiued for review July 2, 1992 Revised manuscript receiued December 28, 1992 Accepted February 1,1993
