attached to the column product collector. The gel spheres flow into this vessel by gravity. The vessel is detached, and the solvent is drained off through the fritted glass by vacuum. T h e gel spheres are dried by heated argon and/or steam which is passed u p through the fritted glass while heat is also supplied by a heating mantle around the walls of the drying vessel. The calcination and sintering to give the final, high density product are done in laboratory furnaces with controlled atmosphere and controlled temperature programs.
T h e rates of temperature rise listed for the thoria microspheres were slow enough to be safe for any of the sols we used. Faster rates during either drying or calcining sometimes produced fractured particles, more porosity, larger surface areas, or lower particle crush strengths.
For thoria microspheres, the most commonly used conditions were:
(1) Ferguson, D. E., “Status and Progress Report for Thorium Fuel Cycle Development for Period Ending Dec. 31, 1962,” Oak Ridge Natl. Lab., ORNL-3385 (1963). ( 2 ) Ferguson, D. E., Dean, 0. C., Douglas, D. A., “Sol-Gel Process for the Remote Preparation and Fabrication of Recycle Fuels,” A/Conf. 28/P/237, U. N. 3rd International Conference on Peaceful Use of Atomic Energy, 1964. (3) Ferguson, D. E., Dean, 0. C., Haas, P. A., “Preparation of Oxide Fuels for Vibratory Compaction by the Sol-Gel Process,” CEND-153, Vol. 1,23-38; ORNL-TM-53 (Nov. 20,1961). (4) Kelly, J. L., Kleinsteuber, T., Clinton, S. D., Dean, 0. C., Ind. Eng. Chem. Process Design Develop. 4,212-6 (1965). (5) Merrington, A. C., Richardson, E. G., Proc. Phys. SOG.(London) 59 (331), 1-13 (1947). (6) Wymer, R. G., Douglas, D. A., “Status and Progress Report for Thorium Fuel Cycle Development for Period Ending Dec. 31, 1963,” Oak Ridge Natl. Lab., ORNL-3611 (1965).
Drying O
c.
Argon Argon-steam Argon
25 to 110 110 to 120 120 to 140
Air
100 to 500
Hours 1 8 3 Firing
500 to 1150 1150
100’ C./hr. 300’ C./hr. 4 hours
For thoria-urania sols made with UOz(N03)z or UOS (limited to 10% urania), the urania is reduced to UOn by Ar-4% Hz for 4 hours and then is protected by argon during cooldown. If carbon black is added to the sol, carbide microspheres may be formed by the reaction of the carbon with the oxides a t about 1700’ C. in an inert atmosphere or a vacuum.
Literature Cited
RECEIVED for review December 3, 1965 ACCEPTED April 21, 1966 Division of Nuclear Chemistry and Technology, 150th Meeting, ACS, Atlantic City, N. J., September 1965. Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp.
SURFACE EFFECTS IN THE PARTIAL OXIDATION OF PROPANE L Y L E F. A L B R I G H T A N D E D W A R D M. W I N T E R ‘ Purdue University, Lafayette, Ind. Propane was partially oxidized with oxygen in both flow and batch glass reactors.
The product composi-
tion and the time required for the batch runs varied as the reactors were used (or were aged), indicating that surface changes were occurring as the reactors were used. A rather gradual change was probably caused by a change in the glass itself. More rapid changes between and during the runs were apparently caused by carbonaceous films deposited during the reaction. Acetaldehyde deactivates a reactor, since it apparently attacks these films. Product analyses were made using a four-column gas chromatographic unit.
effects are recognized as being important during the so-called gas-phase partial oxidation of light paraffins. Most of the information concerning these effects for batch reactors has emphasized the length of time required for the reaction. Little information is available, however, concerning the effect of surface on the composition of the product obtained, even though Knox (8) has postulated that minor products are formed on the walls of the reactor vessel. Factors indicated to be important in controlling surface effects include the material of construction for the reactor (79); coating, such as salt, applied to the inner surface of the reactor (6, 73, 77); “aging” of reactor by repeated oxidations (5, 6, 77, 77, 78); and the amount of surface or the surface-volume ratio of the reactor (75, 76). Thin carbonaceous films are reported to form on the reactor surfaces during partial oxidation (4, 72, 79). I n addition, it has been postulated that the surfaces of glass or metal reactors may also change during the oxidations. Some of the terminaURFACE
tion steps for the chains are known to occur on the surface. Several investigators (7, 7, 72) have rather recently postulated that a t least some of the initiation reactions also occur on the wall. Initiation might occur, for example, by the following surface reactions postulated by Semenov (20) :
s + 02‘SOO’ SOO.
+ RH * S O O H + Re
(1)
(2)
Equation 1 indicates that the surface, S, absorbs oxygen to produce some type of a surface peroxy radical, which reacts with a hydrocarbon, R H , to extract a hydrogen atom and to form a radical such as an alkyl radical. Experimental runs were made in both batch and flow reactors to learn more of the effect of aging the ieactor, especially on product composition. A gas chromatographic unit was used for analysis of all major reaction products, except peroxides. Operating Techniques
l
244
Present address, Wayne State University, Detroit, Mich. l&EC PRODUCT RESEARCH A N D DEVELOPMENT
The experimental apparatus was constructed so that the
flow and batch reactors could be connected to much of the same auxiliary equipment. The flow-reactor system is shown in Figure 1. The oxygen and instrument-grade propane (obtained from Phillips Petroleum Co.) were metered and premixed as they passed through a capillary tube, 7, packed with 100-mesh glass beads. The mixture entered the borosilicate glass reactor immersed in a molten salt bath which could be controlled to within 0.5' C. The inlet line of the reactor was constructed of capillary tubing. The reactor proper, 9, had a volume of 311 ml. (about 2.3 cm. in inside diameter and 75 cm. long). The exit line from the reactor was steamjacketed to the gas sampling loop, D, of the gas chromatographic unit. T h e presssure of the reactor was measured by a mercury-filled manometer, 8. The pressure was controlled using a water aspirator and a mercury manostat connected to the exit line of the reactor. Stopcocks A , B , and C were connected to the batch reactor system. Two batch runs were also made in the tubular reactor. The batch reactor was constructed from a 500-ml. roundbottomed flask, 20, as shown in Figure 2. The opening on top of the reactor was joined to a tee. The line, 19, connecting the side opening of the tee to (high-vacuum) stopcock 15 was steam-jacketed; this line with an internal volume of about 7 ml. was connected to the gas chromatographic unit.
The top opening of the tee was connected to a West-type condenser, 18, which had a n internal volume of approximately 20 ml. and was steam-jacketed to prevent condensation of any product pushed out of the reactor as the reaction progressed. Another tee was installed a t the top of the condenser. Its top opening was closed with stopcock 13, which was used to introduce premixed propane and oxygen from a 2liter flask (not shown). The side opening of the tee was connected to a mercury-filled manometer, 16, and to stopcock 14. The mercury height in the manometer was read with a cathetometer. The volume of the lines in the batch reactor system above the condenser was approximately 37 ml. The temperature of the spherical reactor was controlled using a molten salt bath. The gas chromatographic unit was equipped with four columns. Column 1 was 4 meters of p,P'-thiodipropionitrile on Haloport-F (1 to 10 by weight) for separating formaldehyde, acetaldehyde, methylal, propionaldehyde. methanol, ethanol plus 2-propanol (generally as a single peak). acetone, and water. Column 2 was 4 meters of 65/120-mesh silica gel for separating ethane, carbon dioxide, ethylene, propane, and propylene. Column 3 was 2 meters of 65/120-mesh 5-A molecular sieves for separating hydrogen. oxygen. nitrogen, methane, and carbon monoxide. Columns 1, 2. and 3 were connected in series with suitable bypasses, so that one gas sample could be analyzed for all of the main products except for the acids. A separate gas sample was analyzed for the organic acids, ethanol, and the propanols by column 4. This column contained a 85 to 15 (by weight) mixture of sebacic acid and dicyclohexyl sebdcate on 65/130-mesh Celite 545 (1 to 3 by weight). Column 1 was operated a t 70' to 75' C.; the other three columns. at 110' to 120' C. Helium was employed as a carrier gas. The response factor for each of the major components was determined using the pure compound. Ethylene oxide and propylene oxide were found by analysis on another column to be present in only trace quantities in the several product mixtures tested.
C D
Results of Runs in Batch Reactor
TO
Figure 1.
ASPIRATOR
Flow apparatus
REACTANTS
I
II TO GAUGE
STEAM
0 15
"
22
l&23+-+
M
MOLTEN
Figure 2.
SALT
Batch reactor system
PRODUCTS
More than 100 batch runs were made with an equimolar mixture of propane and oxygen. Most runs were made a t a n initial pressure of 145 mm. of Hg and a t 335' C. Between runs, the reactor was usually maintained at 335' C. and pumped down to a pressure of several microns of mercury before adding the feed mixture for the next run. T h e characteristics of the batch runs were similar to those of previous investigators (5, 7). During the first portion of the run, only a relatively small increase of pressure was noted. After the pressure had increased to about 150 mm. of Hg, the rate of pressure increase was rapid until the pressure leveled off a t about 175 mm. of Hg. T h e time required to obtain a pressure rise from 145 to 150 mm. of H g was about three times longer than the time for the pressure to rise from 150 to 175 mm. of Hg. The maximum rate of pressure increase occurred at approximately 58% of the total pressure increase of each run. The time to obtain this increase could be determined for each run to within several seconds and much more precisely than the induction period (time before a significant pressure rise was noted). Hence, the 58y0 time was used in this investigation for comparison of various runs. After 50 runs, the 58y0 times were reproducible within 4%, as indicated by a statistical evaluation. To get such reproducibility, runs had to be made under similar conditions, including the time and conditions between runs. Table I shows results for runs 71 to 74 and 92 to 97. The missing runs in each series (runs 72, 95, and 96) were made one or more days after the previous run, whereas batch runs shown in Table I were made within several hours after the previous run. Table I indicates that all major products for runs 92 to 97, except for water and possibly formaldehyde, were signifiVOL. 5
NO. 3
SEPTEMBER 1966
245
Table 1.
Batch 71, 73, 74
Runs Temperature, C. Propane/oxygen ratio Initial pressure, mm.-Hg Flow pressure. mm. Hg’ 58% time, hr. Residence time, hr. O
Table II.
Reproducibility of Results Tyje of Run
335 0.997
Batch 92, 93, 94, 97 335 0.989
145
145
...
Flow F-11, -15, -25, -26 334-336 0.99-1.02
... ..
300-398
0 . 5 9 =t0 . 0 5 0 . 5 9 =t0 . 0 4
...
...
Max.
Max.
%. Devia-
% Devia-
Av. tion Av. tion Compn., from Compn., from Product Gas 70 Av. % Av. Formaldehyde 1 . 0 2 20 0 . 9 4 16 Acetaldehyde 0.65 2 0.82 1 4.15 9 4.90 3 Methanol 6 0.72 4 Methylal 0.64 6 18.8 14 Carbonmonoxide 1 4 . 2 5 5.7 4 6.6 Carbon dioxide Methane 0.59 7 0.6 13 16 2.0 10 Ethylene 1.5 Propylene 5.7 4 7.1 7 4 30.1 8 Water 36.5
0.055-0.084 Max.
Formaldehyde Acetaldehyde Methanol Methylal Carbon monoxide Carbon dioxide Ethylene Propylene Water
Composition of Product Gases, 70 1.02 1.73 0.79 0.65 0.54 0.39 4.15 3.68 2.47 0.64 1.02 0.86 14.2 18.0 10.8 5.7 7.5 6.0 1.5 1.9 1.4 5.7 8.3 5.7 36.5 32.9 43.0
%.
Deviation Compn., from % Av. 2.54 1.09 8.90 0.25 21.4 4.7 Au.
...
1.0 3.1 34.7
cantly higher than those for runs 71 to 74. Neither a minor repair of the side arm of the reactor system in the time between these two series of runs nor the intermediate runs made to test variations in evacuating the reactor and in heating the side arms would presumably cause these significant differences of composition. Similar trends were also noted in earlier runs, but the earlier comparisons are less reliable, since rather major changes of the equipment or of the operating procedure could have been in part responsible for the differences. The 58% time for each run was found by analysis of covariance to increase linearly as the time between runs increased. I n addition, materials were apparently desorbed from the surfaces of the reactor and/or the connecting lines and condensers in the time between runs. O n several occasions between runs, after the pressure of the system was reduced to 5 to 10 microns, the system was closed by proper stopcocks from the vacuum pump. The pressure slowly rose within 1 0 to 15 hours to about 500 microns and then remained constant. These results imply that the film on the surface was being desorbed (2, 72) or modified in some manner between runs. T h e effect of the time between runs on the composition of the product obtained was apparently fairly small, however. The pump-down procedure and the temperature between batch runs have been reported to affect the induction period (78). Similar results were obtained in this investigation, but the connecting lines and the condensers were also found to affect the 58% time. When a new’ side arm was attached to the “aged” reactor and heated almost to the softening point, the 58y0 time period increased in both cases to about 1.0 hour. After the side arm was aged for several runs, the time leveled off at about 0.6 hour. There was some flow of material in the present equipment as the pressure increased during the batch run, and consideration of such flow seems to be of some importance in the over-all reaction. Effect of Pressure on Reaction. Run 118, made a t 115 instead of 145 mm. of Hg, had a 58y0 time of about 2.7 hours. Comparison of the product composition of this run with the average composition for runs 92 to 97 (see Table I) showed significant differences, particularly for acetaldehyde and 246
Effect of Induction Time on Batch Reactions Au. of 71, Run No. 73, 74 82 37 58% Time, Hr. 0.59 1.66 2.09
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
methanol, which were produced in smaller quantities in the low-pressure run. Differences in yield of these two compounds were presumably caused mainly by variations of the time for the run. Induction Period. Table I1 shows a comparison of batch runs that had large differences of the 58% times of the run. R u n 37 was an early run made with a rather “unaged” reactor. R u n 82 was made after exposing the reactor to air and several days after run 81. Both acetaldehyde and methanol increased significantly in quantity as the time for the run decreased. Water, as a rule, tended to increase with longer times, but the over-all age of the reactor seemed also to be an important factor. Course of Reaction. Runs 99, 101, 103, 106, 114, 116, and 117 were stopped at various degrees of completion by withdrawing a gas sample for analysis. Figures 3 and 4 are plots of the yield of product (moles of product found per mole of propane consumed) against the per cent of the total potential pressure rise [roughly proportional to the oxygen conversion (73)]. Methylal, but surprisingly no methanol, was detected at low conversions. Three explanations were considered : I . Methanol is a secondary product of the over-all reaction ( 74). 2. Methanol is quantitatively absorbed on the surface of the reactor especially during the initial stages of the reaction and is not present in the gas sample analyzed. 3. Methanol is converted to methylal.
The last two explanations are preferred. Extrapolation of the relatively straight-line curves of Figures 3 and 4 to zero pressure rise (also zero conversion) indicates the predicted distribution of the initial product formed. Propylene and water were the major primary products. Knox ( 9 ) also made a similar finding, but did not report the presence of the other products shown here. Figures 3 and 4 were compared with the results of Chernyak et al. (3), who used recovery and analytical techniques that probably are not as accurate as those used here. Their results agree reasonably well, except for the aldehydes, particularly formaldehyde. They obtained about twice as much formaldehyde as was obtained here. Element ratios (H:C and C : O ) for the product gases were compared to those for the feed to the reactor. These ratios agreed (within about 3%) for runs a t higher oxygen conversions, which implies accurate material balances and analyses. For runs a t lower conversions, the element ratios differed by as much as 5 to 15%. These latter discrepancies could be caused by relatively higher analytical errors and DY the rdatively higher adsorption of oxygenated products on the walls of the reactor system. Some adsorption of oxygenated products may have occurred in the sampling line to the chromatographic unit. A heated
““I
L 0.2-
WATER
-
->-
‘
\PROPYLENE
-
0.1 -
f ACETALDEHYDE
,/
ETHYLENE
-0 I
I
I
I
0 20 40 60 80 PERCENTAGE OF TOTAL PRESSURE
L
IO
RISE
60
Figure 4. reactor
80
100
PRESSURE RISE
Production of olefins, CO, and C 0 2 in batch
Production of oxygenated compounds in batch
‘Teflon line was tested and compared to the regular glass line. Formaldehyde appear:, to be the only product that was changed significantly, but because of difficulty in determining it accurately, no positive conclusion is now possible. Surface Changes d u r i n g Batch Runs. Runs 114, 105, 99, 101, and 92 to 97 were stopped a t 9, 13, 33, 72, and 100% oxygen conversion, respectively. Runs immediately after these runs had 58% tiine periods of 0.57, 0.61, 0.70, 0.64, and 0.59 hour, respectively. The 58% time was highest when the previous run had an oxygen conversion of about 50%. Table I11 indicates how the oxygen conversion of the previous run affected the product composition. More methanol, methylal, carbon monoxide, carbon dioxide, ethylene, and propylene were produced when the previous run had high oxygen conversions. No clear trends were noted for formaldehyde and acetaldehyde, but less water seems to have been formed M hen the oxygen conversion for the previous run was high. These differences are greater in all cases than any differences to be expected because of the small changes of the time between runs. A carbonaceous film was probably deposited on the surface during the early stages of a run, and this film caused the .activation defined here as the surface changes that caused a decrease of the time required to obtain Table 111.
40
PERCENTAGE OF TOTAL
