restored to the level of the original catalyst, as shown b) the improvements in h>drogen factor from 18.0 to 7.4 a n d in carbon factor from 3.3 to 1.6. From these results it is apparent that such a chemical treatment can largely overcome the detrimental effects metal contaminants have on cracking catalysts. i\'ith h\ drodesulfurization catalysts. the results are also attractive. T o determine the effects of the washing procedure in this case. a nickel oxide-cobalt oxide-molybdenum oxidealumina (NiCoMo) \vas tested in residue hydrodesulfurization before and after processing a very high metals-content stock and then re-evaluated following a chemical treatment with aqueous g1)colic acid (Table 1-111). While the fresh catalvst gave 87% desulfurization of a Kuwait residue and the contaminated catalyst gave 04y0,the washed catalyst gave 81%. .4gain the results indicate a definite beneficial effect caused by the washing procedure. Similar results have been obtained with nickel oxide- tungsten oxide-alumina hydrodesulfurization catalysts.
Acknowledgment
T h e authors thank 1M. M. S m i a r t . who participated in the early stages of this work. Literature Cited .\PI Preprint, (1 ) Grane. H. K.. Conner. .J. E.. Masologites. G. P., 26th Midyear Meetin%. Division of Refining. Houston, Tes.,
Mav 1961.
( 2 ) Gulf Research 8r Dei.elopment Co.. Pittsburgh. Pa.. un-
published data. 1959. 1 3 ) Herman. .J.. Scafe. E. T. fto Soconv-Mobil Oil C o . ) . U. S. Patent 2,380,731 (July 31. 1945). ( 4 ) Leum. L. N.,Connor. J . E.. Ind. En?. Chrm. Prod. R P S .Drwlofi. 1, 145. (1962). ( 5 ) McAfee. J.. Montgomerv. C. I$'.. Hirsch. J . H.. Horne. IV. .I., Summers. .Jr.. C. R.. Prtrol. R e f . 34, No. 5. 156-62 (1955). ( 6 ) Sanford. R. A . Erickson. H . Rurk. E. H.. Gossett, E. C., Van Petten. S. L.. Ibid..41, No. 7. 103 (1962). ( 7 ) il'hitaker. .A. C.. Kinzer. A. D.. Ind. En?. Chem. 47, 2153 (1955).
RECEIVED for review October 4, 1962 ACCEPTEDDECEMBER 20, 1962 Division of Petroleum Chemistry. 142nd Meeting. ACS. .4tlantic City. N. J . ? September. 1962.
E N D OF SYMPOSIUM
OXIDATION OF BUTANE T O MALEIC ANHYDRIDE T. C. BISSOT A N D K . A. BENSON Electrochemicais Department, E. I. du Pont de .Vemours 3 Co., Inc., .Vzaqara Falls, S. Y.
Kinotics of maleic anhydride formation via partial oxidation o f n-butane over CoMo04 catalyst i s controlled b y two consecutive first-order reactions: dehydrogenation of n-butane and decomposition of maleic anhydride. The intermediate oxidation of butene to maleic anhydride i s so r a p i d that it has no influence on the kinetics of the over-all reaction.
the lowest cost four-carbon molecule, is potentially one of the best raw materials for production of maleic anhydride. Most commercial processes, however, are based on the catalytic oxidation of benzene, over vanadium oxide cata1y:ts. T h e benzene process does not utilize all of the carbon atoms, as would Oxidation of a molecule [vith only four carbon atom:. I t is no surprise, then. to find many patents on the catalytic oxidation of butane, butene. and butadiene to maleic anhydride (7, 4: .5. 6, 8. 70. 7 7). Some academic work has also been published on the subject (2, 9 ) . These efforts have recently led to the commercial production of maleic anhydride from butene ( 3 ) . Very few of the above references touch on the partial osidation of butane. and only Hartig (4) has reported appreciable yields of maleic anhydride from this starting material. H e employed a n unsupported cobalt or nickel molybdate catalyst in a fluidized solids reactor. Carbon dioxide and carbon monoxide were the only by-products formed in greater than trace quantities. The work reported here was undertaken to extend the discoveries of Hartig by investigating the effect of reaction variables o n conversion of butane to maleic anhydride and, if possible, to define the mechanism and kinetics of the reaction. ORMAL B U T A N E ,
Apparatus and Catalyst
T h e reaction \vas studied in fluidized solids reactors. Initial experiments \yere made in a column 1 inch in diameter and 3 feet long. .4 column 2.5 inches in diameter and 8 feet long \vas used for more precise data. Both were made of borosilicate glass and heated Lcith closely spaced windings of Nichrome ribbon. -4 thermoiiell was centered throughout the reactor. It was studded a t close intervals with baffles of slass rod which cstended from the thermoivell to the \call of the reactor. T h e reactants. butane of 99Yc purity and air. \cere metered by dry gas meters and by rotameters and \\-ere premised just before entering the fluidized solids bed. T h e off-gases from the reactor \cere firs1 filtered to remove catalyst fines and then passed into a Lvater scrubber to remove the product as maleic acid. X portion of the off-gas \vas dried and analyzed at this point and the remainder metered and ventcd. Per cent oxygen \cas measi:red continuously \vith a Pauling oxygen analyzer. Butane and carbon dioxide \cere determined by vapor chromatography usirie; a Perkin-Elmer Model 154B L'apor Fractometer ivith a 1-meter silica %el column a t 100' C . Periodic Orsat analysrs for 0,. CO.. and CO were made? to obtain complete carbon and oxyyen balances. T h e catalyst was a n unsupported cobalt or nickel molybdate modified by addition of boric acid. Its prrparation was based o n the method of Hartig (-7). I n a typical preparation of the cobalt molybdate catalyst, VOL. 2
NO. 1
MARCH
1963
57
1 mole (144 grams) of molybdenum trioxide was dissolved in 130 ml. of 28% ammonium hydroxide and diluted with 2400 ml. of distilled water. After addition of 75 grams of boric acid, the solution was heated to 95' C . in a 4-liter beaker. A solution containing 1 mole (291 grams) of cobaltous nitrate hexahydrate, Co(N0a) 2 . 6H20, in 600 ml. of distilled water, was prepared in a 2-liter beaker and heated to 95' C. The cobalt nitrate solution was rapidly poured into the ammonium molybdate solution. The resulting purple precipitate was immediately filtered on a Buchner funnel without washing, dried a t 110' C., and then ground to pass a 40-mesh screen. The catalyst was treated by roasting a t 525' C. overnight. I t was screened to obtain the 40- to 200-mesh fraction for the fluidized solids reactor. The nickel molybdate catalyst modified with boron was prepared in a similar manner, except that a 1.5 to 1 molybdenum-nickel ratio in the precipitating solution was necessary to obtain a selective catalyst.
W
z
220@ 3
m 5 Figure 1. catalyst
l5
15
IO CONTACT
20
TIME
(SEC.1
Reaction of butane over cobalt molybdate
Experimental
The catalyst bed was preheated to 10' to 15" C. below the desired reaction temperature with air and nitrogen in place of butane. At the start of the run the N2 was shut off and butane feed turned on by means of a three-way plugcock. The airbutane ratio was generally controlled at 20 to 1. The stoichiometric ratio for the reaction
t
CIHla
% BUTANE
DISAPPEARANCE
Figure 2. Butans conversion to maleic anhydride 0 Z'/,-inch reactor 1 -inch reactor
+ 3.50,
-L
+ 4H2O
C4H20~
is 17.5 to 1. A typical run \vas 40 to 90 minutes in length, depending on the flow rates used. Analysis by vapor chromatography was started when the oxygen concentration in the off-gas became constant. At the end of the run nitrogen was again substituted for butane. The scrubber was drained and rinsed when the system had been flushed free of butane. Total acid content was determined by titrating an aliquot with standard base and calculating as maleic anhydride. Periodic checks for maleic acid content were made by evaporating an aliquot to dryness, redissolving the solid residue in tetrahydrofuran, and analyzing by infrared, using known concentrations of maleic acid in this solvent as standards. The maleic acid determined by this procedure agreed to within 10% of the quantity based on total acidity. The difference is probably due to small amounts of other acids formed in the reaction, such as acetic and formic, as well as some maleic acid lost in the isolation procedure. Air-butane ratios and linear velocity were calculated from dry gas meter readings. Catalyst bed depth was noted visually. Per cent CO? and butane in the off-gas were taken from the average of the vapor chromatograph determinations. Per cent 0 2 in the off-gas was taken as the average of periodic readings on the continuous analyzer. Butane disappearance was calculated from feed and off-gas analyses. Conversion to maleic anhydride was defined as the moles of maleic acid, based on total acidity. in the scrubber divided by the moles of butane feed. Yield was defined as conversion divided by fraction butane disappearance in the reactor. Discussion of Resulk
1
2 3 CONTACT
4 TIME
(
5 6 SEC. 1
7
Figure 3. Decomposition of maleic anhydride over cobalt molybdate catalyst 0 58
NBas carrier gas
Air as carrier gas
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
The effect of contact time o n butane disappearance was measured at 425', 450°, and 475' C. with a 5-foot bed of cobalt molybdate catalyst in the 2.5-inch-diameter reactor at an airbutane ratio of 20 to 1. Contact times were varied by changing the linear velocity of the gas mixture. The apparent rate of reaction followed first-order kinetics, as shown by Figure 1. T h e increase in the rate constant with temperature gives an energy of activation of 20,000 cal. per gram mole. The initial and rate-controlling step in this reaction of butane is believed to be dehydrogenation. The ability of cobalt molybdate catalyst to dehydrogenate hydrocarbons is well known. When a mixture of nitrogen and butane was passed over cobalt molybdate catalyst in the 1-inch reactor at 475', u p to 8y0 of the hydrocarbons were converted to mixed butenes and butadiene.
~~
Oxidation of Butane in Multistage Reactors with Interstage Recovery of Maleic Anhydride Two-StaEe Reactor Three-Stage Reactor Stage 1st 2nd Ow-all 1st 2nd 3rd ... 1 1 1 1 1 Reactor diam., inch ... COMOO~ COMOO~ CoMoO4 COMOO~ CoMoOi Catalysto ... 430 430 475 47s 475 Temp., C. ... 24.8 ... ... Air-butane ratio 21.4 ... ... 0.57 0,60 0.69 0.51 0.53 Linear vel., ft. /sec. 34 24 24 24 30 24 Bed depth, inches 8.7 3.5 3.3 2.9 Contact time, sec. 4.9 3.8 6.1 1.8 4.0 6.1 3.0 6.1 Off-gas COi. % ' 1.7 13.9 7.7 2.0 10.5 1.7 Off-gas 0 2 , % ' 12.1= 24.8 11.7 11.9 15.2 1 8 . 5a Conver., yo 60. O a 47 55a 76 30.6 33.5a Butane disappear., % 20.1 34.3 32 34 33 38.2 Yield, % a Conwrsion and butane disappearance calculated from composition of gases entering stage and not on feed composition. Table I.
To optimize the conversion of butane to maleic anhydride, variations of temperature, linear velocity, bed depth, reactor diameter. and air-butane ratio \\ere explored in detail. The maximum conversion to maleic anhydride \\as significantly dependent on only two variables: per cent butane disappearance and selectivity of the catalyst. In Figure 2 the conversion to maleic anhydride is plotted against butane disappearance for two batches of cobalt molybdate catalyst. \Vith these and all other cobalt molybdate catalysts, the maximum conversion to maleic anhydride occurred between 50 and 60% butane disappearance. These maxima indicate that decomposition of the product \\as occurring. This was found to be the case. Small measured amounts of maleic anhydride vapor \cere passed over the catalyst a t 500' C . with both air and nitrogen used as carrier gases. Contact time of the maleic anhydride with the CoMo04 was varied by changing linear velocity. Decomposition followed first-order kinetics (Figure 3). Identical results for air and nitrogen as the carrier gas showed the decomposition to be independent of oxygen concentration. In the experiments with nitrogen the catalyst turned black, indicating partial reduction of cobalt molybdate to cobalt molybdite. To minimize product decomposition, a multiple reactor system with intermediate product recovery was used for some experiments. Over-all conversion of 24.3% was obtained with the apparatus, which consisted of a second and a third 1-inch-diameter fluidized solids reactor complete with filters and water scrubbers set u p in series with the first reactor. Provisions were made for sampling and analyzing the gas stream between reactors. In Table I the experimental results are given for each individual reactor and for the over-all case, The per cent butane oxidized in each reactor was controlled by adjusting the temperature. In a similar experiment with a more selective lot of cobalt molybdate catalyst, an over-all conversion of 24.8% was obtained with two stages. Within the range investigated temperature was not a controlling factor in maximizing conversion to maleic anhydride. Single-stage conversions of 15y0were obtained a t both 400' and 500' C., depending on contact time and activity of the catalyst. Therefore, gross dilution of the expensive cobalt or nickel molybdate catalyst with inert solids did not significantly decrease conversion. However, because of the lower catalyst activity due to dilution, it was necessary to raise the temperature to obtain the maximum conversion which always occurred a t 50 to 60% butane disappearance. With fluidized beds of equal volume, substitution of 35- to 200-mesh firebrick for 90%
c3
Over-all
... ... ... ... ... 72 9.7 6.1 1.7 24.3 84.4 28.8
-1
I O 20 30 40 50 60 70 80 90 7- BUTENE DISAPPEARANCE Figure 4. anhydride
of
Oxidation
to
maleic
Conversion
Yield
0
X
1 -Butene cis-2-Butene
0
butene
+
30
W
>
Z 0 0
20
IO
COMPOUND
A
,
'% DISAPPEARANCE
Figure 5. Theoretical recovery of intermediate for consecutive first-order reactions KI K, A
-
-
VOL. 2
t
B
+
NO. 1
C MARCH 1963
59
of a nickel molybdate catalyst only slightly decreased maximum conversion? from 15 to 14%, as operating temperature was increased from 400' to 475' C. Attempts to use an inert solid, such as firebrick? as a support for cobalt and nickel molybdate have not yielded selective catalysts for the oxidation of butane to maleic anhydride. Ca olefins oxidized a t a significantly loiver temperature than n-butane and gave higher conversions to product. For example. a mixture of cis- and trans-2-butene gave a maximum conversion of 24% at a reaction temperature of 450' C.: while n-butane with the same catalyst and flow rates gave a maximum conversion of 12% a t 500' C. Pure cis-2-butene gave even higher maximum conversion (317,), indicating that the oxidation is stereospecific. 1-Butene was also used as a hydrocarbon source, because it is commercially available. The double bond isomerization in 1-butene has recently been shoivn to favor the cis-2-butene isomer (7) kinetically. Figure 4 shows that 1-butene and cis-2-butene behave similarly in the reactor.
\+'hen cB is a maximum, dc,/dt = 0 and klc, = k2cB. The follo\ving identit) holds when dc,ldt = 0:
x le -kit
(6) (71
= k2e-k,t
Thus, knowledge of the cA at which cB is a maximum is enough to define the ratio of k l to k~ and the maximum mlue of cB. The cB corresponding to any other value of c, can be calculated. Figure 5 shows the family of curves calculated when the maximum cB occurs a t 40 to 80% disappearance of A. The experiments on the oxidation of butane with cobalt molybdate catalysts showed maximum conversion to maleic acid between 50 and 60y0 butane disappearance (Figure 2). Maximum theoretical conversion (see Figure 5) is between 25 and 33%, whereas experimental single-stage conversion was only 15y0 maximum. The difference between experiment and theory is attributed to the butene oxidized directly to carbon dioxide and water without going through maleic an. hydride (reaction b', Equation 1). The percentages of the theoretical conversion which are observed experimentally:
Kinetics of the Reaction
15/33 X 100 to 15/25 X 100
This investigation led to the following kinetic analysis : ka
Butane + butene
kl, -+
k, '
L
'
k,
(1)
CO??C 6 >H?O
The initial step governed by the rate constant, k,, is the dehydrogenation of butane to a mixture of the butene isomers. The rate of this reaction is first order with respect to butane concentration and is independent of oxygen concentration. The mixture of butene isomers is then partially oxidized to maleic anhydride, while the remainder is oxidized completely to carbon dioxide, carbon monoxide, and water. The stereo configuration of the 2-butene isomers is an important factor in the ratio betlveen these competitive reactions identified by kb and kh'. Beach has reported higher yield of maleic anhydride from the oxidation of cis-2-butene than from the mixture of the cis- and trans- isomers ( 7 ) . This fact was also observed in this investigation. The oxidation of butenes was found to be a considerably faster reaction than the dehydrogenation of butane. The rate constants, kh and kh'? are therefore much greater in magnitude than k, and can be neglected in a consideration of the over-all kinetics. The rate of decomposition of the product, maleic anhydride, was shown to be first order with respect to its Concentration. No dependence on oxygen concentration was found. Considering only the butane reacting to form product, the problem is reduced to the classical case of two consecutive first-order reactions :
where A is butane: B is maleic anhydride, and C is COS. The concentrations of .4 and B at any time t are given for such a system by c,
= a+
45 to 60C;
represent the range of maximum yields of maleic anhydride that could be attained if no maleic anhydride decomposed.
maleic anhydride I
\
=
(3)
Conclusions
These kinetic studies show that butane can be oxidized to maleic anhydride in practical yields if conditions are designed to minimize product decomposition. Single-pass conversion can be exceeded by lowering the butane disappearance per pass and either recycling the unreacted butane or passing it through a series of low conversion reactors. The latter approach. as was taken in these laboratory studies. is considered to be the most favorable economically. lt'ith a two- or threestage reactor system this potential route to maleic anhydride would give raw material costs of only 2 to 2.5 cents per pound, which is less than commercial processes. The principal disadvantage is the necessity for multistage reactors with intermediate product recovery? which entails a high investment. The next step in evaluating this process would be pilot plant studies to determine catalyst life. optimum reactor design, and product recovery systems. The temperature independence of maximum conversion in the range of 400' to 500' C. suggem that fixed-bed reactors operating Lvithin this range may be as effective as the isothermal fluidized solids reactors. literature Cited
(1) Beach, L. K. (to Standard Oil Development C o . ) , U. S. Patent 2,537,568 (Jan. 9,1951). (2) Bretton, R. H., \Van, S. \V., Dodge, B. F., Znd. Eng. Chem. 44, 594 (19521. (3) C/&. Eig. 'Vews 38,40 (July 11, 1960). (4) Hartig, J. P. (to E. I. du Pont de Nemours Br Co.), U . S. Patents 2,625,519 (Jan. 13, 1953). 2,691,660 (Oct. 12, 1954). (5) Jacobs, D. I. H. (to Distillers Co.). Zbid..2,649,477 (Aug. 18, 1053)
(6) kyantz, K. \V. (to E. I. d u Pont de Nemours & C o . ) . Ibzd.. 2,605,238 (July 29, 1952). (~, 7 ) Lucchesi. P. J., Baeder, D. L.. Long-well. J. P., J . A m . Chem. Sot. 81, 3235 (1959). 18) Reid. J. C. (to Atlantic Refininz co.). , , U . S. Patent 2.773.838 , , \
,
C.
\\!here a is the initial Concentration of A and cA and cB are the concentrations of A and B a t time t . It is also true that:
(Dec. 11, 1956). (9) Skinner, W.A,, Tieszen. D.. Znd. Eng. Chem. 53, 557 (1961). (10) Slotterbeck, 0. C . (to Standard Oil Development C o . ) , I.:. S. Patent 2,260,409 (Oct. 28, 1941). (11) \t'alters, C . H. (to Union Carbide and Carbon Carp.), Zbid..2,097,904 (Nov. 2 > 1937).
(5)
RECEIVED for review July 18; 1962 ACCEPTEDDecember 5 , 1962
60
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
