KINETICS OF T H E HOMOGENEOUS PARTIAL OXIDATION OF 0-XYLENE VAPOR B Y AIR CHARLES N. S A T T E R F I E L D A N D JORDAN L O F T U S ' Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Mass.
70)
The rate of homogeneous partial oxidation of o-xylene (0.6 to 1 .O mole in air was determined a t 1 - a h . pressure in a flow reactor over the temperature range of 450' to 550" C. and at residence times of from 2.7 i o 7.2 seconds. Conversion of o-xylene varied from 12.6 to 52.3%. The induction time in this apparatus was less than 0.2 second. The rate of reaction i s first order with respect to o-xylene and has an activation energy of 20,000 cal. per gram mole. Under some industrial conditions used for catalytic oxidation of o-xylene to phthalic anhydride over a vanadium oxide catalyst, a portion of the o-xylene that disappears may b e oxidized homogeneously, a reaction which does not form phthalic anhydride.
of the commercial processes for manufacture of phthalic anhydride is based upon the partial oxidation of o-xylene vapor with air over a vanadium oxide catalyst. Little information has been published about industrial reaction conditions. but one commercial fixed-bed process is reported to use contact times of about 0.1 to 0.15 second and reaction temperatures of about 480' to 620' C . (5'). Yields are about 50 to 60% of theoretical. \\.hich is substantially lo\ver than the 85 to 90YC yields obtained in the competitive process based on catalytic partial oxidation of naphthalene. The o-xylene oxidation process uses a higher reaction temperature and a t least a part of the loss in yield may be caused by homogeneous reaction within the catalyst bed or in void space beyond the bed. Little quantitative information on the rate of the homogeneous partial oxidation of o-xylene has been published and the present investigation was aimed a t determining the conditions under which this homogeneous reaction would become appreciable relative to the catalytic reaction. The study was made in conjunction Ivith an investigation of the partial oxidation of o-xylene in vanadium pentoxide melts ( 4 ) . Only a few previous studies are directly related to our work. In an early investigation: Burgoyne (2) reported activation energies for the oxidation of a variety of substituted aromatics including o-xylene. in which the rate of reaction was followed in a batch rcactor by H manometric method. In a later paper (.?) Burgoyne briefly reported on the nature of the products NE
Present address, H. C. Schutt and Awxiates. 73 Tremont St., Boston, Mas$.
Figure 1 . Schematic diagram of apparatus
102
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
formed in two runs with 570 o-xylene in air a t 4.6 atm. under conditions in which cool flames occurred, or were closely approached A more recent and detailed study by Wright (7. 8) focused on the kind of products formed and mechanism of reaction rather than on its rate. Studies were made in a flow reactor at 1-atm pressure and a t various contact times and ratios of o-xylene to 0 1 and to N2 Wright's analysis of the products of a typical run (o-xylene to 0 2 ratio of 1 to 4, no N2) a t 650' C. in which 7276 of the initial o-xylene reacted showed that about one third of the o-xylene that disappeared went to form CO and (202.about one third to form nonoxygenated aromatic compounds, and about one tenth to form oxygenated aromatic products, while the remainder consisted of a wide variety of organic products from fragmentation of the ring. O u r own studies of the nature of the oxidation products from the homogeneous reaction, which will be discussed in another paper, showed that over the range of 455' to 525" C. o-xylene oxide [CbH4(CHJ20]was the predominant product a t low 10 to 20Y,-but a wide distribution of conversions-e.g.. products developed a t conversions u p to 40Y0,. No significant amount of phthalic anhydride is apparently formed by homogeneous reaction under any condition. Experimental
.4 flow diagram of the experimental apparatus is shown in Figure 1. Liquid o-xylene (Phillips Petroleum Co. research grade, 99.89y0minimum purity) was vaporized and mixed with air by introducing it on the inside walls of a 300-ml. inverted
Figure 2. reactor
Detail
of
/ / / / / Thermobestos pipe insulotion, 2-inch id., 3-inch 0.d.. 14'/2 inches long
I///l
Thermobestos plugs
\ \ \ \ Aluminum pipe, 1.38inch i.d., 1.66-inch o.d., 1 3 inches long b 2 8 - g a g e Cr-AI bore thermocouples covered with glass t a p e
borosilicate glass round-bottomed flask. T h e o-xylene \vas fed a t a constant rate from a 10-ml. hypodermic syringe by a mechanical device which advanced the plunger u p the syringe barrel a t a constant speed. At the low feed rater involved (1 to 2 ml. of liquid per hour) it was necessary to place a thin layer of mercury over the plunger to avoid leakage betlveen the barrel a n d the plunger. Metered air. scrubbed free of carbon dioxide a n d \vater, entered the vaporizer-mixer tangentially a t its mid-plane. ' l h e vaporizer-mixer \.\.as maintained a t 140' to 150°*C. and the exit stream flowed directly to the reactor acsemb1)- (Figure 2). T h e mixture was preheated in the reactor inlet tube, which ivas 3 m m . in inride diameter by 10 c m . long a n d \vas held inside the reactor heater assembly. H e a t transfer calculations indicated th2.t the entering gas !vas heated to within 997c of the tube wall temperature. T h e volume of the preheater \vas less than 1Yc of that of the reactor. .4 reactor of high length-to-diameter ratio is desired for good heat transfer and also so that plug flow may be approached as close1)- as possible, but Batten ( 7 ) has shown that in a reactor of this type a t Reynolds numbers much above 5 a jet will extend into the reactor from the inlet, causing a complex mixing pattern and corresponding uncerta.inty in the interpretation of the kinetics. A borosilicate glass baffle was therefore located in front of the inlet tube to break u p any jetting action of the inlet gas. Reference Byposs ..
T h e glass reactor was enclosed in a n aluminum pipe to reduce axial temperature gradients. 1-hree thermocouples \ v e x placed on the outside \\-all of the reactor, as shoivn in Figure 2 : one a t the junction of the inlet tube and the top of the reactor, the second a t the horizontal mid-plane, and the third a t the junction of the outlet tube and the reactor. '1 hey were wired in place Lvith Sichrorne \\.ire and insulated from the aluminum pipe by glass tape. During the experimental runs, the first (top) thermocouple indicated a temperature 12" to 13' C. lower than the second one, while the bottom thermocouple indicated a temperature 2' C!. loiver than that at the middle. T h e temperature indicated by the mid-plane thermocouple \vas taken as the reaction temperature. Heat was supplied from Chromel' A heating \\.ire enclosed in ceramic beads wound uniformly around the aluminum pipe, and this in t u r n waq covered with Thermobestoq insulation a n d finally a layer of aluminum foil. P p o n cooling, the reactor effluent formed a fog which is difficult to recover in conventional condencers. Because of this, plus the very loiv liquid concentration present---about 5 X 10 - 5 ml. of liquid per cc. of effluent gas-- a n electrostatic precipitator-condenser \vas used. ?'his unit consisted of a 0.069-inch 0.d. high voltage electrode centered within a 22m m . o . d . soda-glass tube 10 inches long. An electrical ground \vas provided by closely Lvrapping 20-gage copper \vire around the outside of the glass tube. Soda glass \vas used instead of borosilicate glass because of itc lo\\er reqistivity--lO" us. 1014 ohms per cm. for borosilicate glass. A 3-inch 0.d. aluminum tube \vas placed around the soda-glass tubing to provide a container for coolant. l ' h e bottom annular space was closed Lvith a large cork stopper. T h e refrigerant was carbon tetrachloride (freezing point -22.6' C.) chilled by solid carbon dioxide. T h e fog formed Xvithin the soda-glasc tube when the reactor efAuent gas \>.aschilled rnigrated to the \valls of the precipitator under the influence of the electrostatic field; liquid products, principal])- oxygenared aromatics plus unreacted o-xy-lene, drained into a 10-ml. buret. T h e effluent gas left the top of the precipitator, and then flowed through a water bubbler and \vet-test meter to the exhaust system. T h e o-xylene concentration varied betkveen 0.6 and 1 .0 mole Yc. .4t 1 mole YL the amount of air present is double that corresponding to complete conversion of o-xylene to COS and Lvater and seven times that correcponding to complete conversion to phthalic anhydride and water. Runs extended over periods of from 4 to 10 hours. 4 constant voltage transformer \vas used to maintain a constant reactor temperature. Analytical Procedures
T\vo qrries of runs were made (ceries Z and series HB). In the first eight runs (series Z) ( 9 ) :concrntration of o-xylene in the reactor effluent gas was determined and the amount of o-x)-lerir that disappeared by reaction \vas calculated from the known air flo\v rate and rate of feed of o-xylrrie to the reactor. 1
Hg Column
COC 12 Column
Helium Bottle
T
3
Four - w o y Stopcocks
H o t Wire Detector
Red F l u i d Column Figure 3. Gas chromatography system
Symbols
R
P T
I
---
R e s t r i c t i o n Tube Pressure Gage Temperature Injection Point E l e c t r i c a l Heofers
VOL. 4
NO.
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JANUARY
1965
103
-
i ; W
J 0.2 v)
5 00
550
1
I
4 50
"C.
I
1 1
0.8
7 0.6
v
W
6 0.4
$ 5c 0.05
0.2 0 0
1
2
3
RESIDENCE
Figure 4.
4
5
6
7
T I M E , SECONDS
D
8 0.0 2
0
Effect of residence time on conversion
A portion of the reactor effluent gas was passed through a 5-mm. i.d. borosilicate glass U-tube, 32 cm. long, filled with 35- to 60-mesh vacuum-dried, commercial detergent (Tide). T h e quantity of gas sampled was measured a t ambient temperature and pressure. T h e amount of o-xylene adsorbed onto the Tide was measured by desorbing it into a Burrell Corp. Kromotog K-1 gas chromatography unit equipped u i t h a differential hot wire thermal conductivity cell. T h e system used is shown in Figure 3. T h e sample U-tube was inserted into a special heater maintained a t 160' C . By manipulating the four-way stopcocks provided on the unit, the materials adsorbed on the Tide were desorbed by passage of helium carrier gas. Water vapor present in this stream was converted to acetylene by passing the mixture through a tube (5-mm. i.d. X 10 c m . long) filled with 20- to 30-mesh calcium carbide. T h e resultant mixture was resolved by passing it through a 5m m . i.d. gas chromatograph U column 84 inches long, filled with Tide. I n the second series of runs (series HB) the amount of oxylene that disappeared was calculated from the amount of liquid collected from the electrostatic precipitator and the concentration of o-xylene in it, corrected for the very small concentration of o-xylene found in the exit gas from the electrostatic precipitator. Twenty weight per cent Dow Corning Co. silicone vacuum grease on 30- to 60-mesh Chromosorb-P firebrick packing was used as the partitioning agent when analyzing the liquid. T h e concentration of o-xylene in the effluent gas was determined by a procedure similar to that used for the series Z runs: except that silicone grease-Chromosorb P packing was used as the adsorbent and as the partitioning agent. Results
Table I shows the results of four runs made to determine the order of reaction with respect to o-xylene concentration, a portion of run series Z. T h e residence time was taken to be equal to the reactor volume divided by the gas volumetric flow rate a t the reaction temperature but neglecting the very small expansion due to chemical reaction. T h e rate of reaction is clearly first order with respect to o-xylene concentration, as shown by the graph in Figure 4 of - h ( l - Y) L'S. time. where x is the fractional disappearance of o-xylene. Extrapolation to zero conversion shows that the induction period a t 542' to 550' C. is less than 0.2 second.
Relation of o-Xylene Concentration and Residence Time to Order of Reaction Run Series Z 5 6 7 8 o-Xylene in reactant feed, vol. yG 0 . 9 9 0 98 0 99 0 62 547 542 547 550 Reactor temperature, C. Fraction of o-xylene reacted 0 523 0 225 0 347 0 278 Residence time, seconds 7 2 2 7 4 1 2 7
Table I.
First order reaction rate constant, second 0 103 0 096
104
w
0 104 0 121
l&EC PROCESS DESIGN A N D DEVELOPMEN1
I20
124
1.28
1.32
1.36
1.40
RECIPROCAL TEMPERATURE, 1000/T IN
Figure
5.
OK:'
Arrhenius graph
Studies over the temperature range of 450' to 550' C . are summarized on Figure 5 by an .\rrhenius graph of the firitorder reaction rate constant. k : zss. the reciprocal of the absolute temperature. For each run k was calculated from the expression : - h ( l - s)
=
k0
(1) where 0 is the residence time in seconds and s is the fractional conversion of o-xylene. T h e six runs comprising run srries HB were all made a t a residence time of 6.3 to 6.8 second5 and the conversion of o-xylene varied from 12.9Yc a t 45S3 C . to 43.57, a t 525' C. It'ithin the run series Z. the four a t 542" to S50" C . are described in more detail in Table I and Figure 4. T h e other three sho\vn correspond to residence times of 6.5 to 7 . 2 seconds and conversions of 12.6 (at 4S1' C.) to 39.7% (at 523' C.). T h e calculated rate constants over this range of temperature and per cent conversion fall closely on a straight line. fitted by the least squares method, which may be expressed as I n k = 10.050 - 20,000 R T
(2)
where k is in set.-' and the activation energy is 20.000 cal. per gram mole. Only one run. that a t the loivest temperature451 ' C.-lies below the Arrhenius line. \vhich implies that the induction period is insignificant above about 455" C . Operating with a 1.0 mole 7cfeed of o-xylene in air. \vhen the reactor temperature was raised above 550' C., oscillations in the differential pressure manometer indicated that smal explosions were occurring. Discussion
Many of the activation energies reported in the literature for hydrocarbon oxidations and other reactions lvere obtained in a batch reactor in which the amount of reaction was follo\ved indirectly by a manometric method. These may be markedly different from activation energies calculated from rates measured in flow systems and ivith the degree of reaction determined directly. In the early study by Burgoyne (2) the time required for the pressure increment over the initial value to rise from 20 to 60yc of the final value \vas taken to be inversely proportional to the reaction rate. L-sing an equimolal ratio of hydrocarbon to oxygen and a total pressure or the order of 100 m m . of Ilg. activation energies \ v e x calculated from studies a t various trmperaturer. Some of these are benzene. 68.0 kcal. pcr mole: toluene. 49.0; ethylbenzene. 4 1 . 5 ; and o-xylene. 38.0. It lvas reported that the pressure increase
t
\vas closely related to oxygen consumption and that similar value, of the activation energ)- \ v u e obtained over a \vide variation of reactant ratios and total pressure. Severtheless the value of 2 0 kcal. per mole for the activation energy of oxylene as at present obtained is about one half that of the earlier one reported. T h e latter. however: \vas based on only three runs at about 425' to 450' C . Induction limes or ignition lags may be defined in different \\-a\-s \vhich are not exactly comparable. Furthermore. they undoubtedly vary ivith the variety of flow and mixing patterns obtdined in diffcrrnt r'eactor configurations. No ignition lag times have been reported for o-xylene. Salooja (6) reported ignition lag rimrs for a number of aromatic and other hydrocarbons. calculated from the minimum residence time a t which, for a given temperature: cool flames or "normal ignition phenornrna" \vcre observed. T h e reactor \vas of a rather complex conical shapcz. including an inner core: in order to maintaiit as near uniform temperature as possible. Using stoichiomrtric and furl-rich m-xyleneeair ratios. the lowest tempcraturc a t Lvhich he could observe an ignition \vas 660' C. a t \\-hich the ignition lag time \vas 4 seconds. T h e insignificantly mmll i~iductioiiperiod in our reactor may n d l be due to the use of the small glass baffle which. beside breaking u p the inlel jrt. provide> an eddy recirculation pattern a t the inlet to the reactor. Beyond that, our system is fuel-lean and our definition of induci.ion period is different. In gay phase reactions catalyzed by a solid there may occur interacrions beriveen homogeneous and heterogeneous kinetic p a t h a ; i n particular. free radicals may be formed on the catalyst ,urface and released to the gas phase where they may propagate a homogeii'eous reaction. O n the other hand the catal? i t surface may cause the recombination or inactivation of radical, \\-hich othenvise might cause chain propagation in the gas phasc. At ~irehent no information is available to indicate ivliether the homogeneous reaction will be increased or decreased in the presence of a packed bed of catalyst. If, lio\vt.vcr. \\'e regard thr n\-o processes as occurring separately,
the above results can be used to estimate the extent of homogeneous oxidation in thc void space in a catalyst bed. For example, a t 550' C. and 0.15-second contact time, which may represent one set of industrial conditions. we estimate that approximately 27, of o-x)-lene which disappear will have reacted homogeneously. Further losses may be expected where local hot spots occur or from reaction occurring in the hot gases leaving the catalyst bed before they can be reduced in temperature. In laboratory units it is common practice to maintain isothermal conditions in the reactor well above and below the catalyst bed. In addition the residence time is often higher than in commercial units. For possibly typical laboratory conditions of 530' C. and residence time of 1.25 seconds, approximately 16y0 of the o-xylene entering will disappear by homogeneous gas phase reaction. These estimates indicate that probably a t least under some reaction conditions in laboratory and plant a portion of the so-called catalytic reaction is instead homogeneous in nature. Acknowledgment
T h e aeries Z runs were performed by J. Zemmella. T h e work \vas supported in part by Grant G-23480 from the National Science Foundation. literature Cited
(1) Batten, J. J., Australian J . Appl. Sa. 12, 11 (1961). (2) Burgoyne, J. H., Proc. Roy. Soc. (London) A161, 48 (1937). (3) Ibzd., A174, 379 (1940). (4) Loftus, J., Sc.D. thesis, Dept. Chem. Eng., Mass. Inst. Technology, 1963. ( 5 ) Petrol. Rejner 42, No. 11, 210 (1963). (6) Salooja, K. C., Combust. Flame 4, 117 (1960). ( 7 ) iVright, F. J., J . Phys. Chem. 64, 1944 (1960). (8) Ibzd., 6 6 , 2023 (1962). (9) Zemmella, J., S. M. thesis, Dept. Chem. Eng., Mass. Inst. Technology, 1963. RECEIVED for review February 26, 1964 ACCEPTED May 8, 1964
OXIDATION OF ACETALDEHYDE T O ACETIC ANHYDRIDE B ENJ A M IN H
.
CA R P ENT
ER,
Graduate Center, West Virginia University, Charleston, Lt' Va
A kinetic model, useful for reactor design, is developed for the catalytic oxidation of acetaldehyde to acetic anhydride in the presence of an organic ester diluent.
The model is based upon a mechanism involving a
sequence of seven reactions established by previous investigations or by this study. Data obtained with tank flow reactors at 56" C. offer new evidence that acetaldehyde monoperacetate is formed as an intermediate which is decomposed, in the presence of cobalt and copper acetate, to form acetic anhydride and water. Acetic anhydride is thus shown to be a precursor of acetic acid rather than a parallel product in the decomposition of acetaldehyde monoperacetate. Acetic acid is produced by subsequent hydrolysis of the anhydride. Estimates of the rate constants involved in the model are presented, together with their standard errors.
anhy-dride i,s produced, together with acetic acid, by reaction of liquid acetaldehyde with dissolved oxygen in the prrsrnce of metal ions. By conducting the reaction in the presence of a n inert cliluent. the yield of acetic anhydride is siibstantially increased ( 3 ) . X mechanism for this reaction \\as 1)ropohcd by Heatley ( i ) . T h e mechanism indicated that &I;n(;
1 Present address, Research and Development Department, C:hrmicak Division. Knion Carbide Corp., South Charleston,
\V. \'a.
peracetic acid \vas formed initially, and subsequently combined with acetaldehyde to give both acetic anhydride and acetic acid directly. Differential equations based on the suggested mechanism Lvere developed and solved by making certain assumptions concerning limiting values of the specific reaction rates. T h e results indicated that some important side reactions must be included to account for observed concentrations of acetic anhydride. Subsequent investigations (6, 8)disclosed the formation of a complex. acetaldehyde monoperacetate, by reaction of perVOL. 4
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JANUARY
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105
