INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1950
29s
age of a monomer on polymerization and its equivalent volume of revolution. The equivalent volumes of revclution have been calculated by two methods.
40
30
ACKNOWLEDGMENT
The authors wish to thank Florence Durkee of this laboratory for the diallyli phthalate prepolymer analysis and Mary O’Hearn, also of this laboratory, for numerous specific gravity measurements.
I,
rp
10 r
LITERATURE CITED
Branch, G. E. K., and Calvin, Melvin, “Theory of Organic Chemistry,” pp. 106-110, 122, New York, Prentice-Hall, 1941. (2) Staudinger, H., Trans. Fccradag SOC.,
(1) l@
200
300
400
SW
8Qo
700
Born
D ) v f V A W MLUMEOP WyoWTloli tN
Figure 4.
SOD
fooo
UDD
1200
1300
CUBIC A*
29,28 (1933). (3) Stuart, H. A., “Molek~lstruktur,” p. 48, Berlin, Julius Springer, 1934. (4) Tobolsky, A. V., Leonard, F., and Roeser, G. P., J. Polymer Sci., 3,604 (1948).
Relation between Shrinkage and Equivalent Value of Revolution SUMMARY
+
The equation xy 15y = 3750 describes the general relationship which has been found to exist between the per cent shrink-
RDCEIVED August 18.1949.
Catalytic Dehydrogenation of Butenes K. K. KEARBY Esso Laboratories, Standard Oil Development Company, Linden, N. J .
d
Catalysts have been developed which dehydrogenate butene to butadiene and ethylbenzene to styrene in the presence of diluent steam. Ultimate recycle yields of butadiene of 70 to 85% are obtainable at butene conversion levcls of about 40 to 20%. Of major importance in the composition of these catalysts is the promotion of iron oxide with potassium carbonate. The catalyst originally used in the Standard Oil Company of New Jersey butadiene process also contained oxides of magnesium and copper. The performance characteristics of this catalyst and a number of other active compositions are discussed.
T
HE “Jersey” process of catalytic butene dehydrogenation
was adopted by the government during the war for the production of butadiene. The plants using this process made butadiene a t only a fourth of the cost of butadiene produced from alcohol. This cost differential resulted in tJhe shutting down of the alcohol plants in 1945 and 1946, and since that time most of the butadiene produced in the United States and Canada has been made from butenes. While the conversion of alcohols to butadiene is a relatively old process, the butene dehydrogenation process was a wartime commercial development. One of the major factors in making
this process feasible was the development of a satisfactory catalyst by the Standard Oil Development Company. In addition to its use for making butadiene, this oatalyst was also used extensively for the production of styrene from ethylbenzene. The properties of this catalyst and of various alternate catalysts which were developed are described in this paper. A description of the general features of the “Jersey” process has been published previously by Russell, Murphree, and Asbury (19). Prior to this work the dehydrogenation of butenes a t reduced pressures over alumina-chromia catalysts had been studied by Grosse, Morrell, and Mavity (7). The use of reduced pressures resulted in less carbon being formed and made higher equilibrium yields of butadiene possible. However, their technique of operating with a partial vacuum appeared to present several difficulties as compared with dilution with an inert gas to obtain reduced butene pressure. Operation on a large scale at reduced pressure would involve appreciable costs for compressors and would require precautions t o prevent air from leaking into the hydrocarbon stream a t high temperatures. The use of an inert diluent would overcome these disadvantages and in addition could carry heat for the reaction to such an extent that a simple adiabatic reactor could be used. Of the possible inert diluents which might be used, steam offered several advantages. It could be separated by simple condensation from the products, and it would be expected to
INDUSTRIAL AND ENGINEERING CHEMISTRY
296 I oc
1
I
I
I
I
I4 VOL. H 2 O / V O L . BUTENE
80 ID
7 VOL. H20/ VOL. BUTENE 1150-12OO'F.; 5OOV./WHR.
r
P
0
60
tion dehydrogenation process appeared to center on finding a catalyst which would be operable in the presence of steam in the lowest temperature range dictated by equilibrium considerations (about 1100" to 1250" F.). With this objective catalyst, development studies were carried out which produced catalysta subsequently used extensively for t,he production of butadiene and styrene. DEHYDROGENATIONS OVER 1707 CATALYST
>.
k
2
I-
3 -I
Vol. 42, No. 2
40
W
m
ap 20
% BUTENE REACTED
Figure 1. Butene Dehydrogenation 1707 catalyst, atmospheric pressure, Llir. periods
reduce carbon deposition. Also steam could be used for removal of deposited carbon from the catalyst, and expensive provisions for air regeneration could be eliminated. These factors were the bask for a research program carried out by the Standard Oil Development Company to find a butene dehydrogenation process which could operate in the presence of diluent steam. The same considerations probably led the government to distribute a large proportion (331,200 short tons per year as compared with 33,000 short tons per year from butane) of its dehydrogenation plants to the steam dilution dehydrogenation process as compared to allocations to the partial vacuum process. PREVIOUS STATUS O F STEAM DILUTION PROCESSES
The performance of a catalyst for diolefin production may be judged by its activity and selectivity. Activity is measured by the per cent of butene converted per pass, and selectivity is defined by the ratio-conversion to butadiene divided by the total conversion. The selectivity represents the ultimate yield obtainable by recycling unreacted butene. Alumina-chromia catalysts of the type used by Grosse et al. ( 7 ) a t reduced pressures are not satisfactory in the presence of diluent steam. This is shown by the data in Table I. A catalyst Containing 6% chromic oxide was poisoned by steam and gave yields per pass of butadiene of only 1.4 and 1.6 mole 7, in the presence of diluent steam as compared to 17 and 27.5% in the absence of steam. The activity of a catalyst containing 437, chromic oxide was less sensitive to steam, but the selectivity t o butadiene decreased from 63 to 37%. These catalysts gave lorn yields of carbon monoxide plus carbon dioxide, indicating relatively low activity for catalyzing the water gas reaction between deposited carbon and steam. The relatively high conversions to butadiene of 27.5 and 29% obtained in the absence of diluent steam Tere accompanied by conversions to carbon of 8.1 and S.9%, respectively. The removal of this carbon by air regeneration is an expensive operation, and requires careful control in the partial vacuum process. A wide range of catalyst compositions for dehydrogenating butene to butadiene and ethylbenzene to styrene has pi eviously been reported in the patent literature (a-4,6, IO, 18, $0). A number of these compositions were evaluated in the present investigation but relatively low yields of butadiene were obtained. iiccordingly, the problem of developing a satisfactory steam dilu-
At the time the rubber program was frozen, a catalyst having the composition 72.4 magnesium oxide-18.4 ferric oxide-4.6 cupric oxide-4.6 potassium oxide (designated 1707 catalyst) was selected as the most promising then available for the steamdilution butene dehydrogenation process. This selection was made when maximum production of butadiene was a major consideration as compared with such factors as butene utilization and steam consumption. The most important feature of 1707 catalyst was that it combined a potassium promoter with a satisfactory dehydrogenating compound. The dehydrogenating properties of iron '\T ere utilized under conditions which maintained it in a partially oxidized form which had relatively low carbon-forming properties, and the potassium promoter accelerated the removal of deposited carbon by the water gas reaction ( I S ) . The net carbon deposition was sufficiently small to allow good catalyst activity and selectivity to be maintained over reasonable operating periods. Catalyst 1707 is effective for producing butadiene and isoprene from butene and 2-methyl-2-butene, respectively, and is also efficient for converting ethylbenzene to styrene It is apparent in Table I1 that 2-methyl-2-butene gives slightly higher conversions and selectivities to diolefin than does butene. Ethylbenzene is even more reactive, giving about the same conversion and selectivity as 2-methyl-2-butene but a t a higher feed rate and lower steam dilution ratio. Many of the styrene plants adopted the use of 1707 catalyst when it became available and were able t o increase yields of styrene from ethylbenzene considerably. Because operation a t less severe conditions was possible, these plants normally obtained longer catalyst life than the butadiene plants.
DEHYDROGENATION OVER XLUAIISA-CHROMIA TABLE I. BUTENE CATALYSTS
[Butene, 500 vol./vol./hr.
(S.T.P.) : butene partial pressure, 95 mm. H g , 1-hr. periods] 67 h1?03-43 Crz03 94 81203-6 CrzOa 1100 1100 1200 1200 1200 1200 None Steam S o n e Steam None Steam
Catalyst, wt. Temperatuie, F. Diluent Conversiona, % 24.5 1 4 . 6 44 12 46 42 Total TOC4H6 17 1 . 4 27.5 1.6 29 15.5 T o CO COz 0.8 0.3 0.5 0.9 0.35 3.3 T o carbon 1.8 0 8.1 0.2 8.9 7.6 Selectivity to CaHs, % b 71 10 63 13 63 37 a Conversions represent moles of butene converted t o each product per 100 mole? of butene in feed (on bask of carbon balance). b 100 (conversion to CaHs/total conversion).
+
TABLE 11. DEHYDROGESATIONS OVER 1707 CATALYST (1200O F.; 1-hr. periods; atm. pressure)
2-JIethylEthyl2-butene benzene Hydrocarbon feed Butene __ Feed rate, vol. /vol./hr.Q 500 b 500 680 Vol. steam/vol. feeda 14/1 . . 14/1 9..5/1 Conversion, yo 1otal 38 50 50 49 T o diene 28 32 35.5 350 T o CO f COz 3.5 ... 2.4 3.7 0.1 I.ld To carbon 0.1 . Selectivity to diene, Yo 74 6'7 71 71C a Volumes of gas at 0,"C. and 760 mm. per volume of catalyst per hour. b D a t a from correlation curves. C Styrene. d Yalue is high because temperature is higher than required for ethylbenzene.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1950
TABLE111. PRODUCT DISTRIBUTIONIN BUTENE DEHYDROGENATION
[ 1707 catalyst; 1200° F.; butenea, 800 vol./vol./hr.
TO C4H6 T o carbon To CO TO COz T o CH4 To CzH4 T o CiHe TO CsHe T O CsHa TO
cS+
Total
(S.T.P.); 7/1 steam dilution; atm. pressure] Deactivated Catalyst6 Active Catalyst Conversion, % Selectivity, % Conversion, % Selectivity, % ' 22 76.0 29 74.2 0.03 0.1 0.12 0.3 0.08 0.2 0.04 1.4 2.1 7.3 2.50 6.4 0.8 2.8 1.3 3.3 1.0 0.24 0.6 0.08 0 2 10.7 3.3 8.4 1.3 3.3 0.2 0.7 1.2 3.1 29 100 39 100
[:::
0 97.2% C4- (O,O% isoC4'); 0.2% Ca; 0.3% Ca-; 2.2% Ca: 0.1% Cs+. b Catalyst activity lowered by preliminary calcination a t high temperature.
TABLEIv. REACTIONS OF BUTENES, 2-PENTENE, 72-BUTANE (1707 catalyst; atm. pressure) Mixed n-butenes 1-Butene 2-Pentene 1170 1170 1200 500 500 500 7 7 14
Hydrocarbon F. Temperature Feed rate vdl./vol./hr. Vol. stea&vol. feed Conversion, % Total T O ClH6 To CbHs To CO COz Selectivity to diolefin, %
+
_I
27 22
25 21
2.1 82
1.8 84
..
..
42 14 11 4.6 60
AND
n-Butane 1200 500 9.4 9.6 1.2
..
0.5 13
Accompanying the dehydrogenation reaction, small amounts of carbon are deposited on the catalyst. Considerable variation is observed from cycle to cycle, but the conversion t o water gas is normally 10 t o 30 times that to deposited carbon. The production of carbon dioxide is about tenfold greater than that of carbon monoxide. The selectivity of the butene dehydrogenation reaction over 1707 catalyst decreases with increasing conversion level. This is illustrated for averaged data in Figure 1. It is apparent that the highest ultimate yields of butadiene can be obtained by operating a t low conversion levels with recycle of unreacted butene. For example, to obtain an 80% ultimate yield, operation a t conversions below 27 t o 29% are required. The selectivity of the reaction is not very sensitive t o moderate changes in operating conditions, but, as shown in Figure 1, tends t o be higher in operations at increased steam dilution and at lower temperatures. The use of impure feed stocks results in lower selectivities. Actual plant operations with 1707 catalyst have utilized impure butene streams and have not, in general, attained selectivities as high as those shown in Figure 1. It is apparent that comparisons of the selectivities obtainable from various catalysts should be made with the same feed stock, a t the same steam dilution ratio, and should be corrected to the same conversion level. Decreased selectivities to butadiene are caused by increased selectivities to degradation products such as cracked gases and carbon dioxide. The relative amounts of these reactions obtained a t two conversion levels are summarized in Table 111. It is apparent that degradation to cracked gases is about double that to water gas, the majer cracked product being propylene. The selective conversion over 1707 catalyst of olefins to corresponding diolefins is independent of the position of the double bond in the olefin. The results in Table IV for pure 1-butene and for a mixture of 1-butene and %butene are similar, and i t has been found that the unreacted butenes from either of these normal butenes are present in the reaction products in about equilibrium proportions. Olefins containing more than four carbon atoms in a chain undergo cracking as well as dehydrogenation. Guyer has
297
found that 2-pentene undergoes more extensive conversion to butadiene than t o pentadiene (9). Table IV shows a butadiene yield of 14 mole % ' as compared t o 11 mole 70 of pentadiene. The results in Table IV also show that n-butane gives a low conversion with a poor selectivity to butadiene. The low conversion of butane would be expected to cause it t o build up in a recycled C4 feed stream. However, in admixture with butenes, butane reacts to an appreciably greater extent. A special study of a normal butene feed containing 13.2% of n-butane showed that the butane reached a maximum concentration of 32% in the recycle stream. CARBON DEPOSITION AND REGENERATION CYCLES
The dehydrogenation of butene over 1707 catalyst is accompanied by the formation of small deposits of carbon and larger quantities of carbon monoxide and carbon dioxide. Minimum carbon deposition is obtained by maintaining a n adequate potassium content in the catalyst and by the use of high steam to butene ratios. The carbon monoxide and carbon dioxide formed in dehydrogenations over 1707 type catalysts may be regarded as a product of the reaction of diluent steam with deposited carbon. Carbon deposition results in shorter operating cycles or in longer regeneration periods, during which steam containing no butene is passed over the catalyst. Although the laboratory investigations were usually based on 1-hour reaction and 1-hour steaming periods, it appeared that a 10-minute steaming period out of each hour was sufficient when operating a t 7 t o 1 steam t o butene ratio and 1200" F. With the 10-minute regenerations only 0.9% carbon formed on the catalyst after 402 hours of operation, and this was believed t o have been deposited during the later cycles when water gas formation was observed to decrease. Other experiments a t a 14 to 1 steam dilution ratio showed that continuous operation without regeneration was possible for more than 100 hours. I n this test it was necessary t o increase the temperature from 1200" t o 1225" F. in order to maintain a constant conversion level. At 1200" F. the conversion dropped from 41 to 30% in 17 hours, while a t 1225" F. a conversion level of 32 to 3470 was maintained for 101 hours. Under operating conditions the potassium carbonate in 1707 catalyst is sufficiently volatile t o be gradually stripped from it. Unless this effect is counteracted by addition of potassium carbonate to the catalyst, carbon deposition increases and results in a decrease in catalyst activity and selectivity. However, such deactivation is temporary and can be corrected by replenishing the promoter. The marked effect of intermittent addition of potassium carbonate is shown in Table V. It is apparent that placing 5 weight % potassium carbonate pills ahead of the catalyst enabled it t o be operated for about 800 hours a t 30% con-
TABLE V.
EFFECT OF ADDED POTASSIUM CARBONATE ON LIFE OF 1707 CATALYST (S.T.P.); 10/1 steam dilution; 1-hr. periods] Temperature for 30% Conversions, Conversion, Selectivity % to C4H6, % F. 0 5 0 5 0 5
[Butene, 400 vol./vol./hr.
Added K z C O ~ '%,b . Hour 14 562 724 800C
5
b C
Estimated from correlations. Grams of KzCOa pills/lOO grams of catalyst, placed ahead of catalyst. Catalysts had decreased from original 4.6% to 0.15% and 1.75% KzO.
INDUSTRIAL AND ENGINEERING CHEMISTRY
298
version before a temperature of 1250' F. was required. In the absence of the added potassium carbonate pills, operation for less than 562 hours was possible. Both catalysts responded to the addition of 5% potassium carbonate after the 800th hour, the temperature requirement for 3070 conversicn dropping to below 1200" F. The potassium content of a used catalyst varies considerably with its position in the reactor. The endothermic dehydrogenation reaction causes a decreasing temperature gradient through the catalyst bed and results in the catalyst at the feed inlet side of t h e reactor being deficient in potassium carbonate even though t h e average content of the bed is sufficient. The gradient in the catalyst beds a t the 800th hour for the catalysts shown in Table V are illustrated in Table VI. This effect has been observed to result in an actual increase in potassium content a t the exit end of the reactor during initial hours of operation, Apparently, the potassium carbonate is able to migrate with considerable ease. This is also indicated by the fact that simple mixing of the promoter-rich and promoter-depleted portions of a catalyst frequently results in a marked increase in catalyst activity.
TABLZ VI. VARIATIONSIN POTASSIEM OXIDE CONTENTOF USED1707 CATALYST
(After 800-hour operation: original KzO content was 4.6%) Position of catalvst in bed. ' ~~.% distance inlet tb outlet 12 31 50 75 100 yo KaO in portion of bed indicated No KGOs addition 0.035 . . . 0.042 0.092 0.46 5% KzCO:i addition 0.46 1..l 1 . 6 2.5 3.1 ~
~
~~
,I
dv. 0.16
1.75
With regard to the ultimate life of the catalyst when operating with potassium carbonate addition, an extended laboratory test was still giving a 27% conversion a t 1200" F. with a 77% selectivity when it was terminated after 7 months. However, a catalyst life of this length was not realized in commercial plants under less ideal operating conditions, and the butadiene plants normally replaced 1707 catalyst after a few months. PHYSICAL AND CHEMICAL NATURE OF 1707 CATALYST
Catalyst 1707 has a bulk density of about 1.0 to 1.2 grams per cc. and has a relatively low surface area as determined by nitrogen adsorption. Although a sample of freshly prepared catalyst had a n area of about 30 square meters per gram, this value decreased t o 7.3 square meters per gram after 350 hours of use. Electron microscope and x-ray diffraction studies indicated t h a t the loss in surface area was accompanied by a n increase in crystal 4ze of the catalyst. X-ray studies of a catalyst which had been in use for 2000 hours did not indicate formation of different crystalline components although line sharpening indicative of crystal growth was observed. Catalysts as normally taken from a reactor after being regenerated with steam gave strong patterns of magnesium oxide and ferroso-ferric oxide and weak patterns of copper. However, no distinction between ferroso-ferric oxide and magnesium ferrite spinel could be made by the technique used. After reduction with hydrogen at 1200' F. the catalyst gave x-ray patterns of magnesium oxide, iron, and copper. The reduced catalyst, when operated with a 7 t o 1 steam to butene feed for periods of 6 to 8 hours and quenched in pure nitrogen, gave a pattern of magnesium oxide and copper with no pattern of any iron compound being apparent. This result indicates the eyistence under actual operating conditions of a different iron structure (possibly a solid solution of magnesium oxide and ferrous oxide) than that which is normally stable in steam a t reaction temperature. Possibly this form of iron is the active catalyst. The formation of an intermediate iron compound under actual operating conditions was also indicated by studies of its reduction
Vol. 42, No. 2
with hydrogen. The catalyst quenched in pure nitrogen from reaction conditions in the presence of 7 volumes of steam and 1 volume of butene contained oxygen corresponding to the proportions FeOl.o t o FeOo.7, whereas regenerated (steamed) catalyst corresponded to the proportions FeOl.a. These results are summarized in Table VII. The degree of reduction attained under dehydrogenation conditions is not necessarily limited by equilibrium with the gas phase and may be determined by the relative reaction rates of the catalyst with the gas phase as conipared with that between the catalyst and the small amount of deposited carbon on its surface. The latter reaction may also have occurred to a limited extent during the 2- to &minute period required to cool the above samples. Additional evidence that a partially reduced oxide of iron is the active component of the catalyst was obtained in an experiment in which air was added to the regeneration steam. Lower activities and selectivities were obtained until the use of air was terminated. I t appears probable that the air may have resulted in a greater degree of oxidation of the ircn, and that a longer period on butene and steam was required before it reached the partially reduced state. The reduction studies also indicated that steamed catalyst probably contains magnesium ferrite spinel since oxygen in excess of that required for ferroso-ferric oxide was present. The stability of 1707 catalyst to heating a t high temperatures varies considerably with its method of preparation. Catalysts prepared from ferric nitrate and calcined a t 1200' F. shrink 15 t o 25% when heated a t 1400" F., whereas catalysts prepared from ferric sulfate shrink only 1 to 2%. The shrinkage of the two catalysts under actual operating conditions differ, by about the same amount. While it is possible to preshrink catalysts prepared from ferric nitrate, catalysts made from ferric sulfate are generally preferred because of their lower cost and greater stability.
TARLE srII.
OXIDATIOX STATE O F
(Catalyst compcsition, Catalyst Treatment Steamed at 1200° F. Steam butene, 3 hr. Steam butene, 8.78 hr.
+ +
1707 C A T 4 L Y S T
75 XgO-20 FelOa-8 I
