454
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978
calc . . = calculated t j ... = component, i , j . , . r = reduced c = critical HC = hydrocarbon
Graboski, M. S.,Daubert, T. E., Ind. Eng. Chem. Process Des. Dev., in press, 1978. Peng, D. Y., Robinson, D. B., Ind. Eng. Chem. Fundam., 15, 59 (1976). Soave, G., Chem. Eng. Sci., 27, 1197 (1972). Van Ness, H. C.. "Classical Thermodynamics of Non-Electrolyte Solutions", Pergamon Press, Oxford, 1964.
Literature Cited American Petroleum Institute, "Technical Data Book-Petroleum Refining, Third Edition", Washington, D.C., 1977. Graboski, M. S..Daubert, T. E., Danner, R. P., "Documentation of the Contents of Chapter 8 in Technical Data Book-Petroleum Refining", Xerox University Microfilms, Ann Arbor, Mich., 1978.
Received for review October 25, 1977 Accepted April 27, 1978
partial financial SUPPofi ofthis Work W a s Provided by the Refining Department of The American Petroleum Institute.
Partial Oxidation of Methane in Glass and Metal Tubular Reactors Tse-Chuan Chou and Lyle F. Albright" School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907
Experimental data were obtained in glass tubular reactors for the gas-phase partial oxidation of methane over wide ranges of operating conditions, and these data were correlated with high accuracy using a mechanistic model based on 27 gas-phase reactions and three surface reactions. Experimental data for partial oxidation in aluminum, copper, and packed glass reactors are very different than those in tubular glass reactors because surface reactions are much more important and complicated in these reactors. The mechanistic model developed in this investigation clarifies significantly the gas-phase reactions and to a lesser extent the surface reactions that occur in both metal and glass reactors.
Several mechanisms have been proposed in the past for the partial oxidation of methane (Bauerle et al., 1974; Blundell et al., 1965; Egerton et al., 1957; Enikolopyan, 1959; Hoare and Milne, 1955; and Knox, 1968). Based on a qualitative approach, these mechanisms have helped to explain various phenomena noted during partial oxidations. None of the models which include only gas-phase reactions have, however, yet been shown to represent quantitatively both the kinetics of oxidation and the resulting composition of the product stream. Some of the models have obviously too few chemical steps since they do not predict the production of minor products. Other models fail to predict the complicated kinetic results. The model of Bauerle et al. appears promising but has not yet been tested quantitatively vs. experimental data. Omission of surface reactions in the overall sequence of consecutive and simultaneous reaction steps may in many cases be a serious fault of a model. Mahajan et al. (1977a,b) have shown that several surface reactions are often important. Such surface reactions include oxidation of oxygenated products and even to some extent paraffins, decomposition of oxygenated products, coke formation, and oxidation of coke. These reactions are in general more important in metal reactors than in glass reactors. Whenever glass reactors with small diameters or with glass packing are employed, however, surface reactions tend to be relatively important. Surface reactions are also more pronounced for partial oxidations at lower pressures; in such cases the ratio of surface to mass of reactants is higher. There are currently insufficient data for the partial oxidation of methane over a wide range of operating conditions in order to clarify details of the mechanism and to test mechanistic models. Models employing just gasphase reactions would be expected to be acceptable only for those reactors in which surface reactions are of minor
importance in the overall sequence of consecutive and simultaneous reactions. Perhaps the reactors that fall in this category are limited to fairly large diameter glass reactors. The objectives of the present investigation were to obtain experimental information in several reactors, to identify those reactors in which surface reactions are of minor importance, and to develop mechanistic models for a t least those systems in which gas-phase reactions predominate. E x p e r i m e n t a l Procedures
The experimental procedures employed were similar to those reported earlier by Holtzmeier and Albright (1969) and Mahajan et al. (1977a). In general, the tubular flow reactors employed were 0.40 and 0.62 cm i.d. with internal volumes varying from 17 to 74 cm3. These reactors were immersed in a molten salt bath that could be controlled at any desired temperature in the range of 300 to 550 "C. Calculations indicated that the gas temperatures were always within 1 OC of the bath temperature. Sometimes up to four sample ports were provided along the reactor tube. In such cases, the gas could be sampled and analyzed at intermediate positions, i.e., at various residence times. Flowrates of the methane and oxygen streams were controlled to within 1% of the desired values to obtain residence times of the gases in the reactor from several seconds up to several minutes. Methane used was of ultrahigh purity (over 99.97% purity) and the oxygen had a purity of 99.5% (the remainder was primarily nitrogen). The exit product stream from the reactor was quantitatively analyzed by gas chromatography for all components except peroxides. Peroxides were analyzed by wet analysis. The operating variables investigated were in the following ranges: temperatures, 300 to 550 "c; pressures, 1 to 4 atm; mole ratio in feed of methane to oxygen, 0.5 to 13. 0 1 9 7 8 American
Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978
OXYGEN CONVERSION, 9-
SPACE TIME,SEC
Figure 1. Conversion of oxygen vs. space time in tubular Pyrex glass reactor.
Oxidations in Tubular Glass Reactors. No significant oxidations were noted in glass tubular reactors for runs made a t atmospheric pressure and with residence times of 1 to 2 min until temperatures of at least 470 "C were reached. Once a reaction occurred, the major products obtained were water, carbon dioxide, formaldehyde, and methanol. Hydrogen, hydrogen peroxide, and methyl hydroperoxide were formed in trace amounts. In comparable runs in metal reactors as will be discussed in detail later, both the kinetics of oxidation and the compositions of the products were, however, significantly different. Material and atom balances were made around the glass reactor for each run. Generally, the carbonfhydrogen and carbon/oxygen ratios into and out of the reactor agreed within &5%. A small fraction, generally considerably less than 1% , of the entering carbon atoms was always deposited on the walls of the reactors used. This finding was confirmed by passing oxygen through the reactor at the end of a run; carbon oxides were produced until all of the carbon was removed from the surface. A series of 20 experimental runs was conducted at atmospheric pressure in the range of 470 to 532 "C using a tubular Pyrex reactor having an internal diameter of 0.4 cm. Figure 1 indicates how the oxygen conversion changed as a function of space time for runs at various temperatures when the molar ratio of methane to oxygen was 1.0. Conversions always increased significantly as the temperature increased; in the example shown in Figure 1,the conversions increased from 10% a t 490 "C to over 80% a t 532 "C. Furthermore, the induction period decreased as the temperature increased; the induction period is the time period during which no significant reactions were noted. For the specific run shown at 490 "C, no reactions were noted in the gas samples taken through the first two sample ports. It is estimated that no appreciable reaction occurred in the first third of the reactor; the induction period is estimated to have been about 15 s. The induction period for the run at 532 "C was, however, only about 1 s. Increased temperatures also increased the kinetics of oxidation and decreased the induction periods for runs at feed ratios other than 1.0. An important finding of the present investigation was that surface reactions affected the length of the induction period. As either a new Pyrex or a new Vycor glass reactor was used, the induction period decreased significantly during the first several hours of operation. The relative importance of surface reactions apparently decreased as a new reactor was used during the first several hours of operation. For each run in the tubular glass reactor, the rate of oxidation passed through a maximum at an intermediate oxygen conversion. The conversions at which maxima
455
Figure 2. Reaction rate-oxygen conversion relationship a t different methane-to-oxygen ratio in tubular glass Pyrex reactor a t 520 "C and atmospheric pressure (reaction rate is expressed in pmol/(s cm3)).
I .5
CH,/02
RATIO = I : I
PRESSURE = I ATM
TEMP~RATURE= m MODEL
0 J
w> W
1.0
0c
0.5
0.0 0
IO 20 30 METHANE CONVERSION
40
,%
Figure 3. Yield vs. conversion in tubular glass Pyrex reactor.
I
0
4 METHANE
/
8 OXYGEN RATIO
I2
Figure 4. Effect of methane-to-oxygen ratio on degree of combustion in tubular glass Pyrex reactor (at different conversions, at 500 "C and atmospheric pressure).
occurred varied with the ratio of methane to oxygen in the feed as shown for example in Figure 2. For runs at 520 "C, the maxima occurred between about 15 to 45% conversions for feed ratios varying between 1.0 and 10. The oxygen conversions at which maximum kinetics occurred also varied with temperature; the oxygen conversions for the maxima decreased from about 30 to 15% as the temperature increased from 500 to 532 "C for oxidation runs with a feed ratio of 1.0. Figure 3 shows a plot of yields of major products vs. methane conversion for a run at 500 "C and a feed ratio of 1.0. Similar plots were obtained for runs a t other temperatures and feed ratios. The yields or partial oxidation products formed depended both on the ratio of methane to oxygen in the feed and on the conversion as shown, for example, in Figure 4 which are crossplots based on the results of several runs. In this figure, the degree of combustion was plotted vs. the feed ratio, and plots are
456
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978
Table I. Kinetic Model f o r Partial Oxidation of Methane in Tubular Glass Reactors rate constanta reaction no.
+ HO,. CH,O + OH.
2. 3.
+ 0, CH,. + 0, CH,. + 0,
4.
CH, t OH.
5. 6. 7.
CH, t CH,O,. CH,OOH + CH,. CH,OOH CH,O, + OH. CH, + C H 3 0 . --* CH,OH + CH,.
8. 9.
CH,O CH,O
1.
CH,.
CH,
--*
CH,O,.
* H,O
t CH,.
-+
12.
+ 0, HCO. + HO,, + OH. HCO. + H,O CH,O + HO,. HCO. + H,O, CH,O + CH,O. HCO. + CH,OH H,O, + OH. H,O + HO,.
13.
H,O,
14.
16.
+ HO,. HCO. + 0, * CO, + OH. CO + OH. + CO, + H.
17.
H. t HO,. + 2 0 H .
10.
11.
15.
-+
-+
-
+
-
20H.
HCO. t 0,
-+
H. t HO,.
19.
HO,.
+ HO,,
20.
H,O,
4
wall
CO
* H,
18.
+
t 0,
H,O, t 0 ,
E
reference
45.10 0.00 20.90 0.00 25.80 5.00 31.80 19.70 41.70 9.10
Kondratiev I 1 97 2 ) Seery and Bowman (1970) Kondratiev (197 2) Heicklen (1968)
4.50 x 1013 7.60 x 1013
41.10 3.00
1.10 x 1013
10.90
1.20 x 10"
3.00
LOO x 1013 2.80 x 1013 1.70 x 1014
1.80 32.80 46.80
4.50 x 10"
3.00
HO,.-
4.46 x 2.00 x 1.60 x 7.00 x 2.50 x 1.20 x 2.50 x 5.50 x 2.00 x
10" 10'2 10l2 1015
8.50 81.50 1.07 32.40 1.90 44.10 0.70 57.80
lo1' 1013 1013 1013 lo',
H20+
1/20,
1.00 x 10-1
I/,H,O
+ v40,
1.00
wall
21.
A LOO x 1014 LOO x 1013 5.00 x 1013 3.00 x 1013 1.00 x 1015 2.63 x 1013 6.70 x 1014 1.00 x 10" LOO x 1015 1.25 x 10"
reaction
Drysdale and Lloyd (1970) Heicklen (1968) Neiman and Gal (1968) Kondratiev (1972) Heicklen (1968) Vardanyan e t al. (1974) Kondratiev (1972) Vardanyan e t al. (1974) Kondratiev (1972) Vardanyan e t al. (1974) Heicklen (1968) Kondratiev (197 2) Forrythe and Moler (1967) Kondratiev (1972) Forrythe and Moler (1967) Kondratiev (197 2) Kondratiev (1972) Vardanyan e t al. (1974) Kondratiev (1972) Kondratiev (1972) Forrythe and Moler (1967) Kondratiev (1972) Forrythe and Moler ( 1 9 6 7 ) Kondratiev (1972) Forrythe and Moler (1967) Kondratiev (1972)
wall
1.00 22. OH. 1/zH20 t 1 / 4 0 , A = frequency factor in the Arrhenius expression, s-l or cm3/(g-mol) (s). E = activation energy, kcal/g-mol. ---+
a
shown for 10,20, and 40% oxygen conversions. The degree of combustion is defined for methane as follows. degree of combustion = moles of O2reacted/mole of methane reacted x 100 2.0
,17.50
?
:
YlS.00 _1
r L3 il2.50
The 2.0 in the denominator equals the moles of oxygen that react per mole of methane when combustion is complete; i.e., only carbon dioxide and water are reaction products. The degree of combustion is less as increased amounts of carbon monoxide and especially oxygenated products are formed. More oxygenated products were formed in all cases with higher feed ratios and at lower conversions. The degree of combustion for runs in the tubular glass reactors varied from about 40 to 75. The model shown in Table I represents well all experimental data obtained in this investigation but with the following two restrictions: (a) data obtained in Vycor and Pyrex glass reactors and (b) data obtained after the induction period. The model contains most of the reaction steps in the model proposed by Bauerle et al. (1974). It, however, contains several additional gas-phase reactions; these are the reverse steps for reactions 1, 3, and 6 shown in Table I. The reaction steps for production of formic acid and methyl formate used by Bauerle et al. are not included in Table I since neither compound was detected in the present investigations. These compounds pre-
10.00
,
o.:~
o.Ym
>,&J
u i k
::k.~-~.->.23j
LGC. j r k c -:"'E
!.E:>
SE:;hLS
I.&
2.1ba
Figure 5. Concentration of species vs. space time based on kinetic model.
sumably form only in significant amounts at pressures higher than those employed in the present investigation. Three surface reactions (reactions 20,21, and 22) were also included in the present model. The use of these three reactions in the present model was required in order to get a good fit with the experimental data. The model of Table I was solved using the stiff differential technique proposed by Gear (1967,1969). With the CDC 6500 digital computer at Purdue University, approximately 30 s was required to make predictions a t each set of operating conditions.
Ind. Eng. Chem. Process Des. Dev., Vol. 17,No. 4, 1978 457
Figure 5 shows the computer-printed graph for results at 532 "C and 1:l feed ratio. Predictions of the model agree well as described below with experimental data in the tubular glass reactors following t h e induction period. (a) Yields vs. Conversions. The solid lines of Figure 3 are based on the model; these lines correlate well the experimental data. Equally good fits were obtained for runs a t other feed compositions and temperatures. The model also predicts well the results of Figure 4. (b) Rates of Oxidation vs. Conversion. The solid lines of Figure 2 show typical examples of the good fit between predicted and experimental results. (c) Rates of Oxidation vs. Residence Time after Allowance Is Made for the Induction Period. The solid lines of Figure 1 are the predicted results after the induction period was added in each case; e.g., 15 s was added to the time values used in the model for the run at 490 "C. Although the model reported in Table I predicts with high accuracy the experimental data obtained with glass reactors, such predictions do not automatically prove that the model is essentially a mechanistic or theoretical model. The mechanistic validity of the model is, however, strongly supported by the fact that the A and E values employed for all gas-phase reactions were those reported in the literature. The model is hence primarily an a priori model. Nevertheless, the model still has some features that could be improved. First, there are some uncertainties for several reaction steps as to the best set of A and E values, since more than one set is reported in the literature. In such cases, the different sets were tested in order to determine the one that resulted in the best prediction. In general, A and E values reported in recent literature were the ones finally selected. The model shown in Table I is not applicable whenever surface reactions are of relative large importance such as occurred whenever metal reactors were used in the present investigation or during the induction period in glass reactors. In such cases, more than three surface reactions would undoubtedly be needed in a model. Even for the case of partial oxidations in glass reactors, the terms for these three reactions must be considered to be only semitheoretical in nature. The surface reactions obviously depend on the surface roughness and on the composition of the surface; both are known to vary to some extent with past history or with the accumulated use of the reactor. The values for A for these three surface reactions are thought to be pseudo rate constants that likely vary with past use and with axial position in the reactor (Mahajan and Albright, 1977; Mahajan et al., 1977a,b). Although the model does contain some rather empirical terms, namely the last three terms shown in Table I, the model is considered to be a good representation of the gas-phase steps, and hence can be used for clarifying the overall oxidation and especially the gas-phase reactions. (a) Formaldehyde was the major cause of the chainbranching sequence that resulted in maximum rates of oxidation at intermediate oxygen conversions as is shown in Figure 2. The key branching steps are eq 8,10, and 13. The chain-branching sequence of peroxides (namely reactions 3, 5, and 6) is of lesser (perhaps 10%) importance as compared to the sequence involving formaldehyde. Such a conclusion was reached by calculating the relative amounts of free radicals generated for various time periods using the computed-generated results such as those of Figure 5 in conjunction with the rate constants shown in Table I.
(b) The hydroxyl radical (HO.) was the most important of the several radicals in the gas phase. Over 90% of the methane reacted with HO.. This radical is considerably more reactive than HOz. at the operating conditions investigated. The methylperoxy radical (CH,OO-) was, however, the key radical for production of methanol. At much higher pressures, perhaps 50 atm, CH,OO. may be as important as HO.; higher pressures would result in increased production of methanol. (c) Destruction of a significant fraction of the formaldehyde as a result of surface reactions during the period of low oxygen conversions would explain the induction periods noted in the glass reactors. Further information on such destruction is discussed later in this paper. The rate of production of aldehydes and other chain-branching intermediates was probably only slightly greater during the induction period than the rate of surface decomposition or destruction. Decreased induction periods resulted when the relative importance of the gas-phase reactions increased. Increased temperatures probably increase the rates of gas-phase reactions to a higher degree than the rates of surface reactions. A comparison of the results calculated using the model of Table I with the experimental results helps to clarify those conditions for which the relative importance of surface to gas-phase reactions change. In the tubular glass reactor, the relative importance decreased during the start-up period for the reactors and also whenever temperature increased in the 470 to 540 "C range. Oxidations in Packed Glass Reactor. When the tubular Pyrex glass reactor (with an internal diameter of 0.4 cm) was packed with Pyrex glass beads of 4G50 mesh, the oxidation results were significantly different than those in the unpacked reactor. The surface-to-volume ratios in the unpacked and packed reactors were calculated to be 10 and 538 cm-', respectively. In the packed reactor, the products were exclusively carbon monoxide, carbon dioxide, and water. The kinetics of oxidation in the packed reactor was much lower in all cases. For example, for runs at 540 "C, with the same residence time, and with a feed ratio of 1.0, there was a 0.2% conversion in the packed reactor as compared to 85% in the tubular (or unpacked) reactor; the degrees of combustion in the two reactors were 80 and 55, respectively. The differences in the results can be explained by the increased rates of destruction of formaldehyde in the packed reactor. CH20
surface
CO
+ Hz
Hydrogen produced by the above reaction likely reacted with oxygen to produce water. When an additional term to represent this surface decomposition was added to the model shown in Table I, good agreement between experimental and predicted results occurred when the pseudo-first-order rate constant for it was set equal to lo3 s-l. This agreement of calculated and experimental results is consistent with the conclusion that formaldehyde is of key importance relative to both the kinetics of oxidation and the composition of the product gases. These results are also consistent with the finding that aldehydes decompose on the surface of glass and metal reactors (Mahajan et al., 1977b). Oxidations in Metal Reactors. In the tubular aluminum and copper reactors used, both the kinetics of oxidation and the composition of the product stream were much different than those of comparable runs in tubular glass reactors. Such differences were especially pronounced for atmospheric pressure runs. Figures 6 and 7 show examples of the large differences in the rates of oxidation
458
Ind. Eng. Chem. Process Des. Dev., Vol. 17,No. 4, 1978 I20 CH,/O,
RATIO = 1 . 1
PRESSURE
100
I ATM
I
PYREX;
z
532'C
a
g 80
ALUMINUM
B
COPPER
:
:
E7 525'C
m
METHANE /OXYGEN RATIO I:I
-
501'C
GLASS
60
TEMPERATURE 5 0 3 - 5 2 5 ' C
8
K
PRESSURE I ATM OXYGEN CONVERSION 3 0 - 8 0 9
0 70
z 00
40 V
10
20
K C O , CM/HR
9
Figure 8. Degree of combustion for methane vs. pseudo rate constants in tubular reactors.
20
0 0.0
20 40 60 09 OXYGEN CONVERSION, %
103
Figure 6. Reaction rate-oxygen conversion relationship in tubular Pyrex, aluminum, and copper reactors (reaction rate is expressed in pmol/(s cm3)). $100
t -1
COPPER ( 5 0 0 ° C )
-ALUMINUM
CL, F
(525OC) 00
PYREX (520'C)
in all runs conducted in the copper reactor. Additional information supporting this conclusion was the finding that the rates of oxidation in the copper reactor increased as the reactor was used for oxidations. The change was especially pronounced during the first several hours of operation of the reactor. During this time, the surface roughness increased significantly. Furthermore, the rate of oxidation in the temperature range of about 350 to 450 "C increased by a factor of 2 or 3 as the S / V ratio of tubular copper reactors was changed from 6.5 to 24.2 cm-'; these two reactors had internal diameters of 0.62 and 0.165 cm, respectively. The rate of oxidation was somewhat less than directly proportional to S / V; the fact that it was not in the example given may be caused by variations in the microscopic surface roughness of the two reactors used. If sufficiently high pressures would be employed in the copper reactors, formaldehyde and other oxygenated products would presumably be produced. Clearly, higher pressures act to reduce the relative importance of the surface reactions. Attempts to modify the model shown in Table I in order to correlate the results obtained in metal reactors have, to date, met with only partial success. The major surface reactions in the aluminum reactor probably were the surface decompositions of formaldehyde and other oxygenated compounds. Some surface oxidatiocs likely also occurred, resulting in larger amounts of carbon dioxide. For the copper reactor, surface oxidation steps are probably predominant as indicated by degrees of combustion of 100. An attempt has been made with some success to correlate the degree of combustion for the different reactors vs. the oxidative ability of the inner surface of the reactor. The method used to quantitize this ability is described below. First the inner surface of each reactor was contacted with pure oxygen for about half an hour; the surface was essentially completely oxidized during this period. Then carbon monoxide was passed through the reactor at a steady flow rate, and part of the carbon monoxide was oxidized by the oxidized surfaces to produce carbon dioxide. The fraction of the carbon monoxide that was oxidized decreased as the run progressed because the oxidized surfaces were reduced or depleted. The following equation correlated reasonably well the results In (1 - X,)= 4KL/DV
vxLx-xMETHANE /OXYGEN RATIO = I : I PRESSURE = I ATM
W K W
s0
An
0
20 40 60 80 OXYGEN CONVERSION, %
Figure 7. Oxygen conversion-degree of combustion relationship in tubular reactors.
and in the degrees of combustion, respectively. The rates of oxidation for a run at 525 "C in the aluminum reactor are of interest (see Figure 6). The rates of oxidation remained almost constant for oxygen conversions up to about 7%, and then the rate increased rapidly with increased conversions up to at least 35%. Chain-branching steps involving formaldehyde obviously become important in the conversion range of 7-35 % ; increased concentrations of formaldehyde were also noted with increased conversions. For a run at 500 "C, however, there were no indications that chain-branching steps were ever important. Clearly surface reactions were of major importance in limiting the formaldehyde concentrations a t low conversions. Oxidations began a t lower temperatures in aluminum and especially copper reactors as compared to those in glass reactors. Clearly the reactions at lower temperatures were primarily on the surface. As shown in Figure 6, the rates of oxidation in the copper reactor did not pass through maxima at intermediate conversions such as occurred in the glass reactor and in the aluminum reactor a t 525 "C. Any formaldehyde that may have been produced in the copper reactor was either decomposed or oxidized on the surface. As the operating pressure was increased from 1to 4 atm the degree of combustion decreased for runs in the aluminum reactor. In one comparison, the degree of combustion decreased from 94 to 87; more formaldehyde was detected in the product stream at the higher pressure. The kinetics of oxidation also became more similar to those in the glass reactor as a result of this rise in pressure. Chain-branching reactions involving formaldehyde obviously became of greater importance as the pressure rose. In the copper reactor, the degree of combustion remained at 100 even though the pressure was increased to 4 atm. Clearly surface reactions were of major importance
where Xf is the fraction of carbon monoxide that was reacted by any given time in the reactor, L is the reactor length (cm), D is the reactor diameter (cm), V is the flow velocity of the entering carbon monoxide (cm/s), and K is the pseudo rate constant, (cm/s). The K values were in the following order: copper > aluminum > glass. Figure 8 indicates that the degree of combustion increases as K increases. Although the results of Figure 8 are considered preliminary in nature, the general approach
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978
seems promising. K values of course vary with pressure, temperature, and microscopic surface roughness of the inner surface. K probably oversimplifies the measurement of surface reactions during partial oxidation runs. It is at best a measure of only surface oxidation steps but ignores surface decomposition steps that are of relatively greater importance in aluminum or glass reactors. Discussion of Results The model shown in Table I is considered applicable for partial oxidations of methane at pressures close to atmospheric and when surface reactions are of minor importance. More information is, however, needed to define the limits of applicability of the model, and it should hence be used with caution a t conditions for which the relative importance of the surface reactions are quite different than those in the present investigation. In such cases, the pseudo rate constants for the surface reactions (reactions 20,21, and 22) would probably need to be changed. Such changes may occur in glass reactors a t the following conditions. (a) Subatmospheric Pressures. The surface reactions would be relatively more important since the ratio of surface area to mass of reactants would be greater. (b) High Pressures. Surface reactions would be relatively less important. In addition, reaction steps such as proposed by Bauerle et al. (1974) for production of formic acid and methyl formate should be incorporated into the model. ( c )Glass Reactors with Much Different Diameters. The pseudo rate constants for the surface reactions (reactions 20, 21, and 22) are probably proportional in some manner to the S / V ratio of the reactor. A t high S / V ratios, incorporating a term for the surface decomposition of formaldehyde should be tried. It is not clear, though, whether a single term will always be adequate such as occurred for the packed Pyrex runs. The following equation has been found to predict qualitatively the rate of formation of formaldehyde that is of major importance relative to the kinetics of the overall reaction and also the product composition.
RT = RG - RD - R,, In this equation, RT is the net overall rate of formaldehyde production in the reactor, RG is the net rate of production in the gas phase, and RD and R,, are the rates of formaldehyde disappearance because of surface decomposition and surface oxidation steps, respectively. RD is significantly greater than R,, for oxidations in both aluminum and glass reactors (Mahajan et al., 1977b). In
459
tubular glass reactors, RD and R,, were both very small, approaching zero. In copper reactors, R,, was found to be greater than RD. In the metal reactors and especially in the copper reactors, RT was essentially zero. One of the problems in adding surface reactions to the model shown in Table I is that the rate of each surface reaction is dependent in some manner to the total area of the surface, the composition of the surface, and also the rate constant for the surface reaction. The total area and composition of the surface vary in some complex manner with S / V, microscopic roughness of the surface, the axial position in the reactor, and the composition of the gas phase. Concerning surface compositions, the amount of carbonaceous deposits on the surface and the concentrations of surface oxides at any position in the tubular reactor depend to some extent on the composition of the gas phase. Before a general model for partial oxidations in a l l reactors is developed, considerably more information will be needed concerning the numerous surface reactions identified by Mahajan et al. (197713).
Literature Cited Bauerle, G. L., Lott, J. L., Sliepcevich, C. M., J. Fire Flammability,5 , 190 (1974). Blundell, R. V., Cook. W. G. A., Hoare, D. E.. Milne, G. S.,Symp. (Int.) Combust., Proc., loth, 445 (1965). Drysdale, D. D., Lloyd, A. C., Oxid. Combust. Rev., 4, 157 (1970). Egerton, A. C., Minkoff, G. J., Slaooja, A. C., Combust. Flame, 1, 25 (1957). Enikolopyan, N. S., "7th Symposium on Combustion", pp 157-164, Butterworths Scientific Publications, London, 1959. Forrythe. G., Moler, C., "Computer Solution of Linear Algebraic Systems", pp 27-86, Prentice-Hall, Englewood Cliffs, N.J., 1967. Gear, C. W., Math. Comp., 21, 146 (1967). Gear, C. W., Inf. Process., 68. 187 (1969). Heicklen, J., Adv. Chem. Ser., No. 7 6 , 23 (1968). Hoare, D. E., Walsh, A. D., Symp. (Int.)Combust., Proc., 5th,467-483 (1955). Holtzmeier, L. R., Albright, L. F., Symp. (Int.)Combust.,Proc., 12th, 375-383 (1969). Knox, J. H., Adv. Chem. Ser., No. 76, 1 (1968). Kondratiev, V . N., "Rate Constants of Gas Phase Reactions", NSRDS'COM'72-10014, U S . Department of Commerce (1972). Mahajan, S., Albright, L. F., Id.Eng. Chem. Prccess Des. Dev., 16, 279 (1977). Mahajan, S., Menzies, W. R., Albright, L. F., Ind. Eng. Chem. Process Des. Dev., 16, 271 (1977a). Mahajan, S.,Nickolas. D. M. Sherwood, F., Menzies, W. R., Aibright, L. F., Ind. Eng. Chem. Process Des. Dev., 16, 275 (1977b). Neiman, M. B., Gal, D.. Combust. Flame, 12, 371 (1968). Seery, D. J., Bowman, C. T., Combust. Flame, 14, 37 (1970). Vardanyan, I. A., Sachyan, G. A., Philiposyan, A. G., Nalbandyan, A. B., Combust. Flame, 22, 153 (1974).
Received for review August 23, 1977 Accepted April 17, 1978 The Indiana Gas Association and the National Science Council of The Republic of China supported this research. Paper presented a t the Division of Physical Chemistry, 173rd National Meeting of the American Chemical Society, New Orleans, La,, March 24, 1977.
