Carbon Dioxide as Hydrogen Acceptor in Dehydrogenation of Alkanes Dale B. Fox, Emerson H. Lee,l and Min-Hon Rei Monsanto Polymers and Petrochemicals Co., St. Louis,Mo. 65166
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Reactions CnHzn+2 COZ CnHfn CO HzO (1) and C COZ 2CO (2) were applicable in generating a product mixture containing olefins and CO from a paraffin-COz feed, by use of conventional dehydrogenation catalysts. Conversion to olefins was limited because Reaction 1 was endothermic and equilibrium-controlled. Under favorable conditions for Reaction 1, Reaction 2 was too slow in the presence of the hydrocarbons to prevent coking and catalyst deactivation. However, alternate feeding of paraffin and COZ allowed Reaction 2 to proceed to the right at a reasonable rate and retard catalyst deactivation. Zoned feed of paraffin and COZ in a fluidized-bed reactor also helped Reaction 2 to proceed at a reasonable rate. Continuous addition of air to the paraffin feed gave low conversion and selectivity, but air fed alternately with hydrocarbon or zoned air in a fluidized bed gave satisfactory results.
S t a n d a r d catalytic dehydrogenation of alkanes to form olefins has a number of undesirable features as a commercial process. For example, the usual dehydrogenation catalyst such as Cr20a/A1203 rapidly cokes and deactivates under typical operating conditions (Kearby, 1955). This catalyst may be regenerated with air in a fixed bed or recirculating bed, but the necessary related equipment requires an undesirably large capital investment for a commercial plant. Oxidative dehydrogenation is a n obvious possibility, but it is difficult to achieve high catalytic selectivity from paraffins to desired products when air or oxygen is used (Skarchenko, 1968). As an alternative to these processes, carbon dioxide was investigated as an oxidant to effect dehydrogenation and coke removal by the following reactions :
c + c02 e 2 c o
(2)
Reaction 1 is quite endothermic, and conversion levels are equilibrium-limited. For example, equimolar feeds of CZ, CS, or i-C4 paraffin with C 0 2 a t 650°C and 1 atm give calculated equilibrium conversions of 34, 58, and 69% to the respective monoolefins. Reactions 1 and 2 create the side benefit of a useful coproduct, carbon monoxide. This gives the possibility of generating specific olefin-C0 mixtures for carbonylation processes. Experimental
The laboratory reactors utilized a typical feed system with rotameters and differential flow controllers for maintaining a fixed feed-gas composition. The reactors used were l / 2 to 1-in. diameter quartz, heated by a tube furnace with a temperature controller. The catalyst was normally situated in a central zone of the furnace where the temperature gradient was about 1 5 ° C . The flow was upward so that the reactor could be used as a fixed or fluidized-bed system. 1
To whom correspondence should be addresed.
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I n the tests with zoned feed, the hydrocarbon was injected from the top into the middle of a fluidized bed by means of a dip tube, and the air or C02 entered the bottom of the reactor. Alternate feeding of hydrocarbon and oxidant was achieved by an electrically timed valve which admitted pure hydrocarbon or oxidant (C02 or air) to the reactor inlet. The catalysts used were commercially available types. The fixed-bed catalysts were ground to 20 mesh (US. series) size or smaller to avoid capillary diffusional limitations within the particles during the various tests. The product gases were usually passed through heated exit lines and a sample valve so that the gases could be chromatographically analyzed as a function of time of operation. An exception to this procedure was the use of alternate hydrocarbon and oxidant feeds. I n this case, the products from several total cycles of hydrocarbon and oxidant feed were collected by displacement of water. A syringe sample of the total gas mixture or spot samples from the reactor effluent lines was injected into a chromatograph for analysis. The chromatographic analysis of the products was done with a l/& by 6-ft column of 80-100 mesh Porapak R (Waters Associates, Inc., Framingham, Mass.), by use of a helium carrier gas and dual thermal-conductivity detectors. The temperature was programmed from -78" to 90°C to effect separation and determination of HB,XB,0 2 , CO, CH4, C 0 2 , HzO, and higher hydrocarbons. H2 and HzO were not quantitatively determined in most cases. Results and Discussion
A continuous feed of C02 and various alkanes gave conversions and selectivities shown in Table I. The ratio of olefin to CO in the product was usually about unity, indicating a major amount of the olefin was made through Reaction 1. Although many catalysts were screened for this reaction, the typical commerical alkane dehydrogenation catalysts were best [contrary to the conclusions for ethylbenzene dehydrogenation with C02 (Olson, 1968)l. The best three catalysts tested were commercial Cr203/A1203, V~Oj/.b03, and activated carbon; the optimum operating conditions were about the same as in standard dehydrogenation. These
Table I. Dehydrogenation of Alkanes with COz Pressure, 1 a t m ; feed rates: HC, 30 cc/min; COZ or XZ, 30 cc/min; catalyst wt, 5-6 grams Temp, Catalyst
Alkane
Diluent
O C
Conversion,a
%
Table II. Dehydrogenation of Propane with COZ Catalyst, 5-6 grams CrzO3/.&O3; feed rates: CS, 30 cc/min; COZ,30 cc/min; t = 650°C; pressure = 1 a t m
Selectivity)
%
Continuous feed Producta
Mol
%
co CH, coz
Selectivity,c
%
Alternate feedb Mol % Selectivity,
%
27 89. COz 700 Cr~03/A1203 cz Empty reactor C3 COz 650 5 ,.. 55 68' COz 650 Cr~03/A1~0~ c 3 650 67 70d Nz c 3 Cr~03/A1~03 650 64 44' COz VzOdA1z03 c 3 650 52 65d Activated carbon C3 COz COZ 675 50 37. Crz03/Alz03 n-Cd CrzO3/A1z03 34 84' COS 685 i-C4 a Mol yc alkane converted to products. Mol % of converted alkane to major product. c To ethylene. d To propylene. To Ca olefins; major by-products, methane, ethane, and ethylene.
15.2 4.9 ... 13.7 16.1 9.9 10.9 22.4 ... 27.3 ... CzH4 4.1 9.6 2.3 7.6 Cz& 2.6 6.2 10.8 23.6 1 C A 19.4 68.1 17.6 57.9 C3Hs 22.6 ... 27.3 ... 100.0 100.0 100.0 100.0 Conversion, 55% Conversion, 53y0 a Omitting Hz and H20. Cg, 4 min; COZ, 1 min. Xeglects interconversions between CH, and carbon oxides. Calculated as mol yoof the converted alkane which went to individual products.
facts indicated that Reaction 1 possibly proceeded catalytically by two stages :
Table 111. Alternate Feed with Air or COZ Catalyst, 5-6 grams Crz03/A1~03; t = 650°C; pressure, 1 atm. Timing: C3, 4 min; air or COZ,1 min. Flows: C3, 30 cc/min; air 60 cc/min; Cog, 20 cc/min
(3)
Hz
+ C O z e CO + HzO
. . I
(4)
Reaction 2 proceeded slowly at the normal operating conditions, as indicated by the usual rapid coking and deactivation of the catalyst. The catalysts coked about as fast with COZ as with a n inert diluent such as Kz. From Table I, selectivity t o propylene was somewhat higher with the NZdiluent than with COZ.In our work and others (Van Reijn e t al., 1965), a slightly reduced Cr203/A1~O3was the most active and selective catalyst; therefore, i t was concluded from the various data t h a t COz tends to keep the Crz03/A1203catalyst slightly over-oxidized and prevents highest selectivity. Alternate feeding of propane and COZreduced the rate of catalyst deactivation and also gave different selectivity t o products (Table 11). The alternate feeding of propane and COz gave a relatively lower amount of CO in the product compared t o continuous feed, because the CO came largely from Reaction 2 in the alternate feed system. Thus, the alternate feed process produced both HZ and CO as by-products, whereas the continuous feed of COZconverted practically all the HZto HzO. -4s a result, the ethylene-ethane ratio was higher when COzwas fed continuously and consumed most of the hydrogen. T h a t catalyst deactivation was reduced by the alternating feed indicated that Reaction 2 was inhibited by the hydrocarbons in the system. Thus, the coke was more effectively removed in the absence of the hydrocarbon feed. The relative times and volumes of propane and COZused in t h e alternating feed xere critical. Too much COZ definitely deactivated the catalyst, again indicating the need for a slightly reduced Cr~03/A1~03. Small amounts of COZallowed the catalyst to rapidly coke and deactivate. When small amounts of air were continuously added t o propane feed, the Cr~03/X1~03catalyst was deactivated. However, alternate feeding of air and propane gave good results (Table 111). As with t h e COZ,the time and volume of air treatment were critical; too much air deactivated the catalyst, and too little air allowed coking. Air operation gave a higher ethylene-ethane ratio than COz operation, possibly caused by residual chemisorbed oxygen on the catalyst when air was used; the oxygen would consume hydrogen and shift the ratio in question toward ethylene. An analog to alternate feed operation was tested in the form of a zoned feed in a fluidized-bed reactor. I n this case, the
Air-propane Product p\'Z
%n 23.7 ... 3.9 13.2 3.3 5.1 3.7 19.5 27.6 100,o
Mol
Selectivity,
... ... 4.1 13.8 3.5 10.7 7.6 60.3
COz-propane
%
Mol %b
...
Selectivity,
%
... ...
... 2.1 , . . 19.1 13.6 0.1 ... CzH4 3.1 4.3 CzHe 10.3 14.6 C3H6 31.6 67.5 C3H8 ... 33.7 ... 100.0 100.0 100.0 Conversion, 54% Conversion, 58% a Total sample of both cycles. * Hydrocarbon cycle only. 0 2
co CHa con
propane was fed through the top of the reactor about midpoint in the fluidized bed, similar to fluidized-bed processes for phthalic anhydride production (Graham and Way, 1962). The COz or air was fed into the bottom of the reactor, giving a dehydrogenation zone in the upper part of the bed and a regeneration zone i n the lower part. The natural recirculation of the fluidizing catalyst exposed the catalyst particles alternately to the two zones, establishing a n analog t o the alternate feed system. The distance of the dip tube into the fluidized bed and the relative feed rates of hydrocarbon and oxidant (air or COZ) were critical for good catalyst performance and minimum coking rates. The effectiveness of the alternate feed system and zoned feed is shown in Figure 1; t h e catalysts were able to operate u p to several hours without significant deactivation. On the other hand, continuous feed gave relatively rapid catalyst deactivation. The overall dilution of products was relatively small with alternate feed, since the oxidant was only fed 20Y0 of the time. The 80% on-stream time of pure propane feed is much higher than t h a t achieved in the commercial fixed-bed process with cyclic regeneration. However, the alternate C r a i r feed would probably generate explosive mixtures a t the gas interfaces which would cause problems in industrial use. On the other hand, COZ would cause no explosion hazards and would be relatively easily separated downstream by an alkaline scrubber. Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No.
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Table IV. Products from Dehydrogenation of C2 and C4 Paraffins with C02 Continuous feeda n-CrCO2
C2-CO2 Selectivity,
Mol%
coz
CO CH4 C2H4 CzHs C3He C3Hs C4H8 C4H10
45.4 10.0 2.6 10.4 31.6 ...
... ... ... 100.0
% ... 11.1 ... 88.9 ... ... ... ... 100.0
;-C4-COz
Selectivity,
Mol%
...
-
39.8 13.7 8.9 8.6 7.4 ...
... 6.0 15.6 100.0
%
Selectivity,
Mol%
...
... 13.7 26.5 22.8
...
... 37.0 ... 100.0
26.3 17.9 10.8
...
... ... ... 13.7 31.3 100.0
Alternate feedb n-CrCO2
Cz-co2
%
Mol%
... ... 16.5 ... ... I
.
Selectivity,
.
... 83.5 ... 100.0
%
...
... ... 16.5 ... 78.9 3.4 1.2 ...
... 100.0
100.0
15.2 11.8 6.0 14.3 39.2 0.4 0.1
...
I'-CI-COZ Selectivity,
Selectivity,
Mol%
16.4 19.8 12.6 4.8 8.5
% ... .
.
I
...
12.1 9.2 16.3 ...
... 16.2 21.7 100.0
62.4 ... 100.0
%
%
11.2 21.2 35.9 0.5 0.6
...
Mol
... 35.6
1.o
1.3
... 15.7 14.9 100.0
62.1
... 100.0
Conversion,
% 27 50 35 32 Temp, "C 700 675 665 700 Gas flow rates: HC, 30 cc/min; COZ, 30 cc/min. Timing: HC, 4 min; COZ,1 min.
Figure 1. Conversion of propane as function of time, by use of various modes of operation
The analyses in Table IV show that continuous dehydrogenation of C2 and C4paraffins with C02 gave about 1 mole of CO per mole of olefin in the product. In the alternate feed cases shown, the ratio of CO to olefin is also in the range of unity, but this is considered coincidental since the CO came mainly from the catalyst regeneration cycle (Reaction 2). Obviously, the alternately fed gases are retained on the catalyst surface for a short period, causing a slight overlap of the gases. Ethane was converted t o ethylene fairly selectively since only one major side reaction occurs, cracking to methyl radicals. This gave some methane and C3 hydrocarbons in the product. I n the case of the i-butane feed, the major byproduct was methane; n-butane gave relatively larger
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amounts of ethane and ethylene as one might expect. Table IV shows that the alternate feed systems tended to give higher conversions which apparently resulted from the relatively higher concentration and longer contact time of pure hydrocarbon feeds during the alkane-feed cycle, as compared with the COrdiluted feed in the continuous process. A lower selectivity of n-butane to C4 olefins is shown in Table IV from continuous feed compared with alternate feed. This difference probably resulted from the higher temperature of operation in the continuous process and possibly from an effectively different oxidation state of the catalyst caused by continuous feed of COZ. I n conclusion, the use of COz in alkane dehydrogenation has the advantage of generating a CO-olefin mixture useful for carbonylation processes. The disadvantages result from the alkane-CO2 reaction being endothermic and equilibriumcontrolled. Coking of the catalyst is another problem, b u t this can be minimized by alternate feeding of the alkane and
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c02. Acknowledgment
Thanks are given to K. S. McMahon for suggesting the zoned feed analog to the alternate feed system. literature Cited Graham, J. J., y a y , P. F., Chem. Eng. Progr., 58 ( l ) ,96 (1962). Kearbv, K . K., Catalytic Dehydrogenation,'] P. H. Emmett, Ed.;Vol X, p 453, Reinhold, New York, N.Y., 1955. Olson, D. H. (to Marathon Oil Co.), U.S. Patent 3,406,219 (October 15, 1968). Skarchenko, V. K., Russ. Chem. Rev., 37, l ( 1 9 6 8 ) . Van Reijn, L. L., Sachtler, W. ICI. H., Cossee, P., Brouwer, D. M., Proc. Third I n t . Congr. on Catalysis, Vol 11, p 829, North-Holland, Amsterdam, Netherlands, 1965.
RECEIVED for review May 11, 1972 ACCEPTED September 1, 1972 Presented at the Division of Petroleum Chemistry, 162nd Meeting, ACS, Boston, Mass., April 1972.
