I
HOLGER C. ANDERSEN, ALFRED
J.
HALEY, and WALTER EGBERT
Research and Development Division, Engelhard industries, Inc., Newark, N. J.
Purifying Olefins by
...
Catalytic Hydrogenation
OLEFINS
PRODUCED by cracking processes invariably are contaminated with appreciable amounts of acetylenes and diolefins. These impurities are also associated with ethylene occurring in coke oven gas and in other streams, such as methanol-synthesis gas produced by the Sachsse process. Whenever the ethylene from such sources is to be used in chemical processing, these impurities must be removed. The necessity to purify may arise because of potential hazards, such as accumulation of solid acetylene in low-temperature process equipment, or because of chemical reasons. Of the several methods proposed or used for acetylene removal, the most elegant is catalytic hydrogenation. Under some conditions, the impurity is not only removed, but the yield of olefin increased, via
CzHz
+ Hz = CzH4
(1)
Under other circumstances, saturated hydrocarbons, generally innocuous, are formed : CzHa
+ H Z = CZH6
(2)
Removal of acetylene and propadiene is accompiished with
b
Increased olefin yield Simpler temperature control
b
Less than 10 p.p.m. remaining
T h e theoretical aspects of acetylene hydrogenation have been reported (2-6). T h e experimental work described in this report relates specifically to the practical problem of removing acetylenes to the parts-per-million level, under conditions applicable to industrial processing. Ethylene a n d Cracked Gases Acetylene may be removed at any of a number of points in an ethylene plant. T h e practice in plants constructed several years ago was to remove the acetylene in the cracked gas before hydrogen removal ( 9 ) ; one of the
principal arguments in favor of this location is that the hydrogen is already present. More detailed consideration shows that this location is not necessarily the best. The data of Table I were obtained from both published literature, and experimental research, and may not duplicate actual field operation. For acetylene removal from the raw, high-hydrogen gas, base metal catalysis have adequate activity and requisite selectivity, although runaway conditions are possible. When acetylene is removed by addition of limited hydrogen to the C-2 or product stream, platinumgroup metal catalysts, in particular palladium, are superior to any other thus far discovered. As shown in Table I, the catalyst requirement and metal concentration can be very low, compensating for the inherently high cost of these metals. Location of the acetylene removal facility at these points also permits complete recovery of butadiene from the cracked gas. This part of the experimental work, therefore, refers to investigations on supported palladiumcontaining catalysts with compositions
Table 1.
Acetylene Can Be Removed from a Number of Points in an Ethylene Plant Demethanize+ Deethanize FinalHeavy+ Acid Gas RemovalEnds Drying Depropanize Stripper Removal Raw Gas C-2 Stream Product 20 Typical compn., mole yo HZ 25 CHI CtH2 0.3 0.6 1 CtH4 30 59.7 98-99 CzHs 20 40.3 C Y 4.7 Gas flow, SCFH/1000 SCFH 3330 1670 1000 product Applicable catalyst type Ni-Co-Cr ( 1 4 ) Palladium or Modified Palladium (1.8. 10) Co-Mo. (9.11. 1 4 ) None Required 1.2 3 1.2 3 Hydrogen addition, moles/mole acety500-1000 (9) 1000-3000 450 4500 450 4500 lene 1800-2600 (11) Space velocity, SCFH/CF 2.2 0.22 1.5-4.4 1.7 3.7 0.37 Cubic ft. catalyst required/1000 SCFH producta - 1 to -3 0 to -1 + 0 . 7 to 4-1 0 to -1 -1 +0.7 to 1 Ethylene gain ( f ) or loss ( - ) % of product 5-10 0 0 0 0 Steam required, yo of stream Optional Steam-air or inert gasSteam-air followed Steam-air followed Steam-air or inert gasRegeneration procedure air, 1022O F. no hydroair, 1022O F. no by H2 at 750'by € I 2 at 700'800' F. 840° F. hydrogen gen >2 mo. > 3 mo. Time between regenerations 15-40 wk. 3-6 mo. > 2 mo. >3 mo. 1-2 yr. 2-3 yr. Life, estimated > I yr. >1 yr. > I yr. >1 yr. 0 Based on middle of space velocity range. +Hydrocarbon Cracker+
+
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901
in which the hydrogen-acetylene ratio is limited. Experimental. The arrangement consisIed of a simple train of high-pressure stainless steel piping connected to a stainless steel reactor, heated either by resistance wire or by immersion in a bath of electrically heated peanut oil. Generally improved results were obtained when linear flow was high. This was achieved by using a "single file" reactor in which a single row of catalyst pellets was contained in a stainless tube. The pellets, l / 8 inch in diameter, \ v e x charged individually to a tube having an inside diameter o€ 0.152 inch. For a 50-liter-per-hour stream passing through the standard (temperature and pressure, NTP) reactor a t 100 p.s.i.g. and 212'F., this corresponds to a linear flow of 40 feet per minute, empty reactor basis. Bypassing a t the reactor walls would result in these laboratory studies being conservative, compared with industrial reactors which are essentially without wall effect. Pressures up to 550 p.s.i.g. were studied, but the flows were always measured a t atmospheric pressure in glass capillary flowmeters calibrated with wet-test meters. Temperatures used ranged from 200' to 400'F.: although equilibrium calculations showed that a 10 p.p.m. acetylene level could be attained a t temperatures as high as 572' F. The feed gas was prepared by pressuring into an evacuated cylinder the required amounts of Matheson C.P. acetylene, after passage through water and then through a dry-ice trap, Matheson commercial ethylene, and commercial electrolytic hydrogen. After gas addition, the cylinders were usually rolled €or 1 hour to ensure uniformity, and then analyzed. Agreement between calculated and analyzed values was generally within 10%. Gas was analyzed for acetylene by passing it through 5% silver nitrate in methanol and titrating liberated " O s . T h e limit of detectability was 1 to 3 p.p.m. Hydrogen was determined calorimetrically (7) or chromatographically( 73). T h e calorimetric method was estimated to have a sensitivity of approximately 0.02 to 0.1 % H,, and the chromatographic method ap roximately 25 p.p.m. bolymers were taken to be those substances condensible in silica gel at dry-ice temperature and atmospheric pressure. Typically, the sensitivity of determinations was about O . O l ~ oby weight of polymer in the gas. Catalysts employed were commercial preparations of Engelhard Industries Inc. Types I and I1 consisted of palladium, 0.001 to 0.03570 on l/*-inch alumina pellets, whereas Types I11 and I V contained a promoter metal in addition (8). Results. A series of differential reactor-type experiments over a wide range of pressures, temperatures, and composition gave results which did not
902
fit any simple kinetic expression. However, for one set of conditions (Type I catalyst; 1 atm. a t 212' F.; inlet gas, 0.1% CzH2 and 0.3'% H Zin commercial ethylene), the conversion us. space velocity relationship was pseudo first order, when plotted according to Hougen and Watson (72). By interpolation, conversions of 9970 were found a t 15,000 standard cubic feet per hour per cubic foot and 99.9% a t 10,000. PRESSURE.Each type of catalyst promotes acetylene hydrogenation over a broad range of pressures, the removal being rather insensitive to pressure. This result, obtained a t low conversion, cannot Is: applied uncritically to gas purifications where complete removal is required ; however, a single NTP space velocity is generally usable over a wide pressure range. Where both pressure and per cent hydrogen are low, the removal rate may become strongly dependent on hydrogen partial pressure. I n such cases, it is often advantageous to employ a modified catalyst composition. TEMPERATURE. The temperatures quoted refer to the catalyst bed, most experiments having been made under essentially isothermal conditions. I n a full-scale plant with near-adiabatic reactors, a temperature rise of between 52' and 131' F. would occur in the bed for 1% CzHz in ethylene containing 3% 1 3 2 a t 392' F., depending on the extent to which the excess hydrogen reacts with the ethylene. Careful temperature control is required in catalytic processing with the raw gas, to prevent excessive hydrogenation of ethylene, but such control is unnecessary in the type of process described here. Temperature rise is limited by the gas composition itself. T h e generally useful range of temperatures for acetylene removal over platinum metals catalysts is 212' to 392' F., although temperatures as low as 150' F. are often effective with fresh catalyst. Catalyst temperature may be increased to compensate for gradual loss in activity with age. The effect of temperature on polymer formation is not known, although it appears likely that higher temperature favors polymer formation. SPACEVELOCITY.The effect of increasing space velocity is, of course, to decrease the extent of hydrogenation. Inasmuch as the objective in most of this work was to achieve acetylene removal of 99 to 99.9%, space velocities were chosen to provide such removal with an ample safety factor. For a H Z to CzHz ratio of 3, space velocity of 4500 standard cubic feet per hour per cubic foot satisfies this condition ; accordingly, this value was used in most of the work reported here. For a Hz to CzH2 ratio of 1.2, of interest where increase of ethylene content is attractive, a space
INDUSTRIAL AND ENGINEERING CHEMISTRY
velocity of 450 sufficed. At this lower HZto CzHz ratio, a different catalyst is used €rom that operating on the high ratio. Combinations of space velocity and HZ to CzHz ratio intermediate between these values may also be employed. INLETGAS COMPOSITION. For one type of catalyst under generally fixed conditions, catalyst life was inversely related to acetylene content. This appears to be because polymer formation rate is roughly porportional to the acetylene concentration. This polymer formation is equal to approximately 5y0 of the input acetylene, a t a 3 to 1 hydrogen to acetylene ratio. However, it is not certain that this relationship holds for other catalyst types. T h e nature of the polymer has not been completely ascertained, but infrared analyses show hexenes as major constituents. The polymer can be removed from the gas stream by activated carbon. I n a commercial plant, it may be preferable to remove the polymer by distillation or absorption processes. This can often be accomplished by locating the acetylene removal step in the C-2 stream (Table I): after which ethane and heavier components normally are removed in any case. Lower space velocities are required as the Hz to C Z H Zratio is decreased. I n addition, side reactions become of relatively greater significance. I n practice, therefore, i t was found that a different type of catalyst was required for loiv ratios than for high ratios. At least three combinations of ratio and space velocity result in satisfactory purification, a t high influent acetylene level (1.4 to 270):
Ha/ CzHz
Preferred Catalyst Type
Space VelocityU
IIIb IV"
4500
2
1.2
IVb
450
3
1300
For removal to less than 10 p.p.m. G H ? . Table IV. Tested for 1129 hr. At the lower ratios, there is a tendency toward increase in polymer formation. which varies from about 5o/c of the input acetylene for the highest ratio shown, to 10 to 28y0, for the lowest. The lower ratios are of course attractive in that net ethylene loss by hydrogenation is not possible. At the lowest ratio shown, ethylene content is increased. Some comparative features of using the two extremes in ratios and space velocities are shown in Table I. An economic choice of one mode of operation over the other can be made only on the basis of a detailed study. Obviously, however, the possibilities for ethylene loss or gain increase as the acetylene level increases, so that the
CAT A LY T I C HYDROGENAT I O N low-ratio process becomes more attractive a t higher acetylene levels. T h e high-ratio, high space-velocity operation appears to be favored up to acetylene concentrations of about 1%; a t higher levels, detailed cost studies are required to permit logical choice. At high ratios, the excess hydrogen will report in the effluent if it is not consumed by ethylene hydrogenation. T h e extent to which it appears in the effluent generally increases with catalyst age. I n many cases, such hydrogen is unobjectionable, or removable. Where its presence cannot be tolerated, removal by catalytic means can be effected. T h e poisons which must be considered in a n actual ethylene plant are CO, HtS, and organic sulfur (COS and CS2). Sulfur poisoning might be expected to occur by selective adsorption of the sulfur compound a t or near the site active for acetylene hydrogenation. I n certain instances, NHa and CEIaOH are also possible contaminants, but they are in these cases present in the stream for only limited times. Short tests a t 100 to 200 p.s.i.g. showed no poisoning with 100 p.p.m. CO. At 60 p.s.i.g., however, removal was imperfect and the C O tolerance was between 50 and 100 p.p.m. CO. This means that hydrogen containing less than 0.3% and preferably less than 0.1% CO would be required for such an application. Much higher carbon monoxide levels are tolerable in streams richer in hydrogen. I n such cases, in fact, the CO is beneficial in that it confers selectivity on the catalyst for acetylene us. ethylene removal (Table 11). T h e compositions given in Table I1 are sufficient for good operation, but are not necessarily limiting. T h e data also show that selectivity is influenced not only by CO and Hz, but also by C Z H ~ concentration or total pressure, or both. Sensitivity to sulfur can be decreased by increasing hydrogen, temperature or pressure. T h e sulfur levels shown in Table I11 are somewhat greater than those normally encountered in C-2 or ethylene product streams. Accordingly, it is quite practical to employ the precious metal catalysts in commercial
Table 111.
Sensitivity to Sulfur Can Be Decreased b y Increasing Hydrogen, Temperature, or Pressure to 10 p.p.m. or less for test period (Space veloc., 4400 SCFH/CF; conditions for removing CZHZ up to 187 hr.) HzS, P.P.M. 6 10-40 COS. 23-30 P.P.hI
I1
I I
2 0.06 0.06
450 275 0
392-428 392 248
3
gases containing these impurities a t low concentrations. At high hydrogen levels, much higher sulfur concentrations can be tolerated, analogous to the case of carbon monoxide. I n the coke-oven gas example of Table 11, satisfactory acetylene removal was found over a period of a t least 4.5 months, with 150 p.p.m. COS in the feed. An interesting observation of poisoning with the catalysts of this study is that the effect is reversible, high activity returning when the poison is omitted from the stream. LIFE. I t is desirable to determine operating life of each of the several catalysts as a function of each of the important variables, but limitations of time prohibit accumulation of such complete data. Fortunately, some generalizations can be employed to limit the data required for life evaluation. As noted earlier, life for one catalyst type is inversely related to acetylene content. T h e amount of polymer formed is directly related to acetylene content. I t appeared that loss of catalyst activity was generally due to accumulation of polymer deposits. Where the inverse relationship of life with acetylene content applies, catalysts having satisfactory life in high-acetylene streams will also be satisfactory in low-acetylene gas. Therefore life tests a t the 1 to 2yc acetylene level have been adopted as a general criterion of usefulness. Useful information on life can be obtained by making relatively short-term experiments a t accelerated velocity and assuming that life a t normal design space
0 37.4a 14.4
Activity recovers when HzS
Life Test at 4500 SCFH/CFb
Operating Time, Hr. 6
Residual CzHz, P.P.M.
Temp., F. 284 374 392 397 388 385 388 383
25 50
100 1000 1400 1800 2200
100
16 3 2 1 1 1 1
Accelerated Aging at 54,000 SCFH with Regenerations
% CzHz Removal a t 392' F.
Time, Hr.
86 90
(0) 150 200 250
70 50
After 1st Regeneration, 1% OZ/NZ,1022O F., Followed by Air 1022O F.
100 300. 500
88 92 93 94
650
50
(0)
After 2nd Regeneration (0) 150 300 500 1128 Type IV Catalyst
92 92 92 85 84
Life Test at 450 SCFH/CFC
Operating Time, Hr. 44 200 300 400 500
Temp., F.
Residual CzHz, P.P.M.
221 298 306
311 302 293 302 320 338
600 700
873
Slight CZH4loss C~Haunchanged C2H4 increases C2H4increases
3 6
Table IV. Life Data for Two Acetylene-Removal Catalysts Were Obtained Type I11 catalysta
Higher Carbon Monoxide Levels Are Tolerable in Streams Richer in Hydrogen (CzH2 removal, good in all cases) Cat- Press., Inlet Compn., % ' CZH4 Type of Gas alyst P.S.I.G. HP CzHz C2H4 CO Other Change
Ethylene 111 100-200 3 1 96 0.01 Coke oven gas I 350 52 0.15 2.4 8 Acetylene off-gas I 350 48 0.1 8.5 29 Methanol-synthesis gas I loo 6o O e 5 29 Methanol-synthesis gas V b 0-300 4 0.1 0.5 94 a Includes 150 p.p.m. COS. 0.5% Pt on alumina pellets.
392-428 392
a
r = Hz/CzHz in commercial CzH4. Not good at 392' F. or COS, respectively, is removed from stream.
Table II.
1. 2. 3. 4. 5.
3 6
1670
5
1 1 1 1 1 1 1 4
a 2% CPHZ, 6% Hz, 92%-commercial CZH4; press., 280-450 p.s.i.g. Experiment stopped by choice. 1.4% CzHz, 1.7% Hz, balance commercial CzHa; press., 275 p.s.i.g.
VOL. 52,
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NOVEMBER 1960
903
Table V.
Removal of Methylacetylene and Propadiene from Propylene
(CaHI in Matheson commercial propylene, with
Application Trace impurity clean-
I.
Space Viloc., Cata- SCFH/ lyst CF I11 4500
Hz/ CaH4 10
N2;
Temp.,
lmpurityi
CHaCCH CHzCCHz 50 0
OF. 336-363
UP
IIa.
High impurity
IIb. High impurity 111. a
I11
2700
3
10,000
0
I
1350
3
5,000
5,000
390-417
10,000
0
264-392
I 1350 1.5 High impurity-low H2 addition 10 p.p.m. for CH,CCH; 40 p.p.m. for CHzCCHz.
velocity can be calculated by multiplying the “accelerated” life by the space velocity ratio. For the first type of operation in Table IV, it can be concluded that a life of one year or more can be obtained with the type of catalysts used, a t the highest input acetylene levels anticipated in commercial ethylene plants. REGENERATION. I n accordance with the previous observation that catalyst deterioration occurs by deposition of polymers, it has been found that spent catalysts can be regenerated by a simple combustion process. T h e catalyst is treated with 1% 0 2 in Nz or steam a t 932’ to 1022’ F. for several hours, until the bulk of the combustible is removed. The oxygen content is then gradually raised until the regeneration gas consists of air. Good results have also been obtained a t 842’F., but 932” to 1022’ F. appears to produce consistently higher and more permanent activity in the regenerated catalyst. At these relatively short periods of temperature exposure, little change in surface area occurs (Table IV). REMOVAL O F HYDROGEN. Unreacted hydrogen may appear in the treated stream, particularly a t high H2 to CtHz ratios in the feed, and a t advanced catalyst age. Where such hydrogen cannot be tolerated, and where hydrogen stripping facilities are not available, a two-step catalytic process can be employed : after the first stage in which acetylene is removed, a second is provided containing a different catalyst, for ethylene hydrogenation. An effective catalyst for this purpose is 0.5% palladium on alumina. Propylene With the current growth of the propylene industry, the need for purifying propylene and C-3 streams has arisen, paralleling in many respects the requirements in ethylene purification. This development is more recent, and
904
press., 100 p.s.i.g.)
400
Impurity removed to 1-8 p.p.m. for at least 2544 hr. Test stopped by choice Catalyst effective about 1400 hr./cycle. Regenerated twice. Does not remove allene thoroughly Both impurities removed to limit of detectiona for 4100 equivalent hr. After regeneration, impurities removed at 297O F. At 1500 equivalent hr., good removal at 264O F.
both laboratory work and pilot evaluations are more limited than for the corresponding ethylene cases. Two compounds, rather than one, are of practical interest in this case: methylacetylene and propadiene (allene). Both of these occur in the C-3 fraction of cracked hydrocarbon streams normally serving as feeds to processes for manufacture of polymer, oxide, glycols, and other derivatives. Thermodynamically, temperatures u p to 464’ to 572’ F. can be used before a limit of 10 p.p.m. impurity is exceeded, just as was found in the ethylene case. Experiments were, therefore, made in the temperature range of 212’ to 392’ F . These experiments were less extensive than those for acetylene removal from ethylene, as it was soon found that similar conditions were applicable to the two cases. Experimental. The experimental studies were similar to those described previously. T h e amounts of mixture which could be made without risking propylene condensation, and hence nonuniformity, were too small for continuous running. A concentrate of impurity and hydrogen in nitrogen was prepared and a metered quantity of this mixture was then blended with a larger metered flow of Matheson C . P . propylene. Analyses upstream of the reactor confirmed the composition calculated from the flows of master mix and propylene. Methylacetylene was determined by passing the gas through silver nitrate and titration of the liberated nitric acid. Propadiene (Caribou Chemical Co., Columbus, Ohio) was determined chromatographically, usually with a sensitivity of 40 p.p.m. Most experiments were made a t a pressure of 100 p.s.i.g. Results. The general pattern of results was similar to that reported for acetylene removal from ethylene. The type I catalyst series appeared to be of most general value; type I11 was effective in methylacetylene removal, but did not respond well to propadiene.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Accordingly, most long-term experiments were made with the type I catalyst, especially a t high inlet impurity levels (Table V). Good purification and regenerability were demonstrated for the type I catalyst operating a t a hydrogen to impurity ratio of 3 to 1 and an hourly space velocity of 1350. A much lower ratio is possible when methylacetylene is the only impurity, and further work may show this ratio to be practical when both impurities are present. A full-scale plant has been operating for a t least five months with type I catalyst, essentially under condition I I b of Table V, except that the operating pressure is 300 p.s.i.g. As in ethylene purification, excess hydrogen present in the purified stream could be removed by 0.57, palladium catalyst. References (1) Andersen, H. C., Haley, A. H. (to Engelhard Industries, Inc.1, U. S. Patent 2,909,578 (Oct. 20, 1939). (2) Bond, G. C., “Catalysis,” Vol. 111, Reinhold, New York, 1955. (3) Bond, G. C., J . Chem. SOC.2705 (1958). zd 4288 1958). $id:: 4738 [1958). (6) Bond, G. C., Dowden, D. A., Mackenzie, N., Trans. Faraday SOC.54, 153746 (1958). (7) Cohn, G. C., Anal. Chem. 19, 832-5
!:{
(I 047) \-- ”/.
(8) Cohn, G. C., Haley, A. H., Shields, H. A. (to Engelhard Industries, Inc.), U. S. Patent 2,927,141 (March 1 , 1960). (9) Fleming, H. W., Keely, W. M., Gutmann, W. R., Petrol. Rejner 32, 138-43 (1953). (10) Frevel, L. K., Kressley, L. J. (to The Dow Chemical Co.), U. S. Patent 2,802,889 (Aug. 13, 1957). (11) Girdler Catalyst Data Sheet, “G-54 Catalyst,” 1959. (12) Hougen, 0. A , , Watson, K. M., “Chemical Process Principles,” Pt. 3, John Wiley, 1947. (13) Kryacos, G., Boord, C. E., Anal. Chem. 29,787 (1957). (14) Reitmeier, R. E., Fleming, H. W., Chem. Eng. Progr. 54, 48-51 (1958).
RECEIVED for review February 15, 1960 ACCEPTED July 21, 1960
