242 1
KINETICSOF HIGH-TEMPERATURE ~\IETHAKE OXIDATIOX
The Overall Kinetics of High-Temperature Methane Oxidation in a Flow Reactor1 by Andras Nemeth and Robert F. Sawyer Department of Mechanical Engineering, Division of Thermal Systems, Uniaersitg of Cul$forniu, Berkeley, California 94’730 (Received January 15, 1065)
The rate of methane consumption in an oxidation reaction was measured in a noiiisothermal flow reactor in the The reaction was monitored by mass spectrometric analysis. The followtemperature range of 1180-1282’K. ing overall rate expression was found: d[CHl]/dt = -6 X 1010[CH~]-0.4 [ 0 2 ] 1 , 4 exp(-57,000/RT) [mol ern+ see-’], valid, however, only for temperatures greater than 1200’K. The result is in agreement with data measured by another method in the high-temperature range.
Introduction An adiabatic flow reactor method has been suggested and successfully employed to study the kinetics of several homogeneous gas reactions at high temperatures by Glassman and his ~ o w o r k e r s . ~ -A~ related flow reactor technique was used in the present investigations to study the high-temperature oxidation of methane. Experimental Method The assembly of the experimental apparatus is shown in Figure 1. It consists of four main parts: an oven to heat the carrier gas, a mixing section where small amounts of methane and oxygen, respectively, are added to the hot carrier gas, the reactor, and a sampling probe. The gas flows were monitored by rotameters and pressure gauges; the temperature was measured by Pt-Pt-Rh thermocouples.
PREHEATER
FIRST OVEN
MlXlNQ SECTION
SECOND OVEN
Figure 1. Chemical kinetics flow reactor.
The mixing section was designed so that reaction during the mixing should be negligible compared to the main reaction. Small quantities of reactants, from 0.1 to 6.2% of the carrier flow, were injected a t high velocity for rapid mixing. To minimize catalytic wall effects, the mixer was constructed of nickel-free stainless steel and ceramic. The mixing section and its main dimensions are shown in Figure 2. The reactor was a quartz tube with a length of 34
I CERAMIC
Pz
DUCT, 7 mm
‘STAI’KESS
STEEL
Figure 2. Mixing section.
cm and an inside diameter of 7 mm. The temperature was recorded at three locations along the length of the reactor and at the mixing section. All experiments were a t 1 atm pressure. The gas flow had a Reynolds number from 1000 to 3000 based on the reactor diameter. A water-cooled probe served to sample and quench gases within the reactor (Figure 3). The tip of the probe was designed to provide rapid cooling in an expansive flow through an aerodynamic orifice. Sample pressures within the probe were reduced to about 1 Torr. Samples were analyzed continuously by a Bendix time-of-flight mass spectrometer, Model 17-210. Argon was used as the hot carrier gas. The argon was dry with 99.995% purity; the methane, 99.95%; and oxygen, dry, 99.6%, suppliers’ specifications. Gases were used as received, without further purification. (1) This work supported in part by the U. S. Public Health Service, National Center for Air Pollution Control, under Grant AP-385-02. (2) L. Crocco, I. Glassman, and I. E. Smith, J . Chem. Phys., 31, 506 (1959). (3) I. Eberstein and I. Glassman, “Tenth Symposium (International) on Combustion,” The Combustion Institute, Pittsburgh, Pa.,1965,p 365. (4) R. F. Sawyer and I. Glassman, “Eleventh Symposium (International) on Combustion,” The Combustion Institute, Pittsburgh, Pa., 1967,p 861. (5) B. M. Fabuss, J. 0. Smith, and C. N. Satterfield, Chem. Eng. ( N . Y . ) ,70, (S),153 (1960).
Volume 78, Number 7 July 1569
ANDRASNEMETH ANI) ROBERT B. SAWYER
2422 Table I : Experimental Data and Calculated Rate Constants
Cases
a a a a
= 0%
a a a a a
0% = 0%
=
0 2
= 02 = =
0 2
= 0 2 = 0% = 0 2 = 02 = CH4 = CHI = CHI
a a a a u = CH,
Concn, mole fraction X lo4 zo z a
71 88 68 52 85 105 193 210 33 43 230 92 170 260
29 32 40 17 63 86 182 200 9 22 210 86 140 220
284 284 368 368 340 340 624 624 291 291 550 550 300 300
---------
-
Calcd values for-----------------
-
b/[o*]m
Reaction time, sea
T,OK
n
0.0071 0.0071 0.0093 0.0093 0.0087 0.0087 0.0170 0.0170 0.0060 0.0060 0.0155 0.0155 0.0148 0.0148
1267 1267 1215 1215 1235 1235 1180 1180 1225 1225 1250 1250 1282 1282
1927 X 1767 x 1090 x 1200 X 901 X 861 x 294 X 274 X 1113 X 1134 X
-0.2
10-lo
10-lo 10-lo 10-lo
11
661 660 384 385 336 340 133 126 328 360
-0.4
X X X X X X X X X X
n
-
227 X 237 x 135 x 124 X 125 X 134 x 60 X 66 X 98 X 114 x
lo-" lo-" lo-" lo-" lo-"
lo-" lo-" lo-"
-
L/ [CH4]*--
I___-"
m = 1.2
-0.6
m
m = 1.13
1.4
10-'2 10-12 10-12
lo-'*
10-12
10-l2
1097 97 304 203
2355 2451 6723 4527
50,860 74,193 156,530 96,510
data, reaction orders with respect to methane and oxygen concentrations were -0.4 and 1.4, respectively. Based on these reaction orders, the rate constants were calculated and plotted as a function of the reciprocal of the temperature (Figure 4). With the exception of the lowest temperature point, the data give a straight-line relation from which the overall activa-
20
. .
WATER
:
? I
876-
Figure 3. Sample probe.
The experimental conditions were chosen in such a way that the change of one reactant would be negligible during the reaction in comparison with the other. The temperature change in the reactor was kept within limits for which the concept of an equivalent isothermal temperature could be used. This method was originally suggested for first-order reactions.6 As shown in the Appendix, the concept can be extended to the present case. A series of experiments, in which a fixed temperature profile and concentration of the component in excess were maintained for two different initial concentrations of the other reactant, was conducted. Through a trial and error method based on eq 2 of the Appendix, a reaction order was determined which gave the same reaction rate constant for the two different initial concentrations.
Results and Discussion The experimental data and calculated values are summarized in Table I. The assumed activation energy in the calculation of the equivalent isothermal temperature was 60,000 cal. On the basis of these The Journal of Physical Chemistry
-
5-
'0
::
4-
3-
i
z
I-
2-
fn
a
0 W
I-
c a a
8 0.41 7.6
I
7.8
I
8.0 IOOOO/T
I
8.2
I
8.4
I
OK"
Figure 4. Experimental rate constants: 0 , oxygen in surplus; V, methane in surplus.
tion energy for the reaction was determined by a leastsquares method to be 57,000 cal. The activation energy assumed in the calculation of the equivalent isothermal temperature and reaction order, therefore, was justified. The overall rate expression was found to be -d[CH,]/dt
= 6 X
1010[CH4]-0'4[02]1'4 exp( -57,00O/RT)
KINETICS OF HIGH-TEMPERATUHE METHANE OXIDATION where the concentrations are expressed in mol ern+ and the gas constant in cal mol-l “I
