Symposium on the Chemistry of Gaseous Hydrocarbons (Continued from August Issue) Prenented before the Joint Meeting of the Division8 of Petroleum Chemistry and of Gas and Fuel Chemistry at the 89th Meeting of the
American Chemical Society, New York, N. Y., April 22 to 26,1935.
Cracking and Polvmerization of Low Molecular Weight Hvdrocarbons J
J
OK'SIDER-
time of contact the cracking and The thermal decomposition of methane, p o l y m e r i z a t i o n may be conABLE inethane, propane, n-butane, the correducted as separate operations, terest has sponding olefins, and acetylene is discussed been shown lately in the polyor the two steps may be carried from the point of view of maximum olefin out together i n o n e p r o c e s s . merization of gaseous olefins for and liquid fuel production. Propane has Both of these m e t h o d s h a v e t h e production of a n t i k n o c k been studied extensively. fuels. The writers have already been studied in greatest detail: (1) Simulp r e s e n t e d s o m e results of a A large variety of catalysts taneous cracking and polymerization at and contact materials have been general nature on the subject atmospheric pressure, (2) cracking at atinvestigated in connection with (26). The object of this paper is mospheric pressure for maximum unsatuthe cracking and polymerization to present additional data and rates followed by polymerization at high reactions. These will be taken to submit a comprehensive outup under a separate heading. line of the more important work pressure and lower temperature, and (3) donein thisline by other workers. In judging t h e r e s u l t s r e an intermediate step introduced in (2) to Cracking is discussed mainly ported, it mag be well t o note remove hydrogen by selective oxidation are from the point of view of gaseous that 4.7 gallons (17.8 liters) of discussed. olefin formation, and polymerizabenzene is the theoretical maxiThe removal of hydrogen increased the tion is limited as much as posmum yield (assuming complete conversion on a carbon basis) sible to motor fuel production. yield of liquid products almost fourfold. Paraffins will be d i s c u s s e d obtainable per 1000 cubic feet The introduction of air into heated olefins first, then the olefins, and acety(28.3 cubic meters) of methane, initiates polymerization at much lower l e n e ; e a c h hydrocarbon or while ethane should be capable temperatures and pressures. hydrocarbon m i x t u r e b e i n g of yielding 9.4 g a l l o n s (35.6 taken up separately. Theeffect liters), and propane 14 gallons of t h e r m a l t r e a t m e n t will (j2.9 liters). Although these PER K. FROLICH AND PETER J. WIEZEVICH be dealt with first with respect theoretical yields, which do not Standard Oil Development Company, t o ole& production, and then take into account any c a r b o n t o polymerization, Cycloparafloss whatsoever, m i g h t c o n Elizabeth, N. J. fins will not be i n c l u d e d i n ceivably be amroximated if the this article since a detailed survey of the Fork in this field corresponding pure ole6ns were subjected ^tb polymerization, has already been given by Egloff, Bollman, and Levinson (13). due allowance must necessarily be made for inevitable losses resulting in the cracking of the paraffins. According to Frolich (21) all petroleum hydrocarbons, Saturated Hydrocarbons with the exception of methane and possibly ethane, are inThe conversion of paraffin hydrocarbons of low molecular herently unstable, thermodynamically speaking, a t temperaweight into liquid products takes place in two steps. The tures above about 200" C. Below this point, however, t h e first step consists of cracking of the paraffins to unsaturates; rate of reaction is so slow that practically no decomposition the latter are then polymerized to compounds of higher occurs even of the most complex and unstable molecules. molecular weight. By proper choice of temperature and As shown by the data of Parks and Huffman (64)in Figure 1, 1055
QC
INDUSTRIAL AND EKGINEEHIKG CHEMISTRY
1056
the instability of the hydrocarbons increases with increase in temperature but the temperature coefficients vary for the different homologous series. In this chart the paraffin hydrocarbons are represented by solid lines, the unsaturates by broken lines, and the cyclic hydrocarbons by dots and dashes.
400 so0 6W
mo am
300 1000 1100 TEMP, OK.
VOL. 27, NO. 9
of hydrogen, and 25 cubic feet (7.1 cubic meters) each of ethylene and acetylene. This residue gas had a calorific value of 775 gross B. t. u. per cubic foot a t 60" F. and 30 inches of mercury (15.6" C. and 760 mm.). The presence of small amounts of ethane increased the yield to a large extent. Inert diluents such as nitrogen and steam (7f)were found to have no adverse effect unless present in large amoimts, while hydrogen tended to depress the yield. Addition of acetylene and ethylene to the methane caused an increase in liquid products. Table I summarizes results reported in the literature on maximum obtainable yields of unsaturated gases and light oils from methane. The temperatures a t which these results were obtained varied considerably, but this can be accounted for by the different times of heating employed. According to Boomer (4), recycling the exit gas doubles the yield of liquid product in four passes. An increase in pressure from 2 to 10 atmospheres decreased the yield of unsaturated hydrocarbons and had little effect on the yield of hydrogen (75). At much higher pressures the yield of hydrogen was also found to decrease.
1200 BOO 1400 ISDO
FIGURE1. THERMODYNAMIC STABILITY OF HYDROCIRBOKS (54) Thus, while at low temperatures aromatics are as a rule less stahle than paraffins, the order is reversed in the range 300' to 500' C . From somewhat below 400" C. and up, there is, consequently, an increasing tendency for the formation of aromatics from paraffins, although any aromatics thus produced are themselves unstable and tend to decompose further into methane, carbon, and hydrogen, as well as into polynuclear homologs (14). The only simple hydrocarbon whose stability increases with rising temperature is acetylene (and probably its homologs). Such considerations lead to the conclusion that, under the conditions of temperature and pressure employed commercially, the cracking of a hydrocarbon into two smaller hydrocarbon molecules is, for practical purposes, a nonreversible process. On the other hand, reactions involving only liheration of hydrogen are of the reversible type. Examples are the formation of aromatics by removal of hydrogen from certain naphthenes and the liberation of hydrogen from a paraffin to form an olefin with the same number of carbon atoms. Another conclusion is that the temperature theoretically required for hydrogen to split off decreases with increasing molecular weight in a given series of hydrocarbons. It is also a general rule that, in the case of paraffins, the carbonhydrogen bond is stronger than the carbon-carbon bond so t h a t the tendency to crack generally is greater than the tendency to dehydrogenate.
@
I
I
I
I
i
'/
I
I
IO
s 1000
1050
1100
1150
1100
1250
TEMPERATURE 'C.
Methane Since it is very resistant to chemical change, methane requires high temperatures for thermal dissociation. When this occurs, hydrogen, ethylene, acetylene, light oil, tar, and carbon are produced. Figure 2 illustrates the yields of these products a t various temperatures and constant rate of flow as determined by Smith, Grandone, and Rall (66). The best average yields obtained by these investigators per 1000 cubic feet (28.3 cubic meters) of methane are approximately 0.3 gallon (1.1 liters) of light oil (largely benzene) and 2.5 pounds (1.1 kg.) of liquid tar containing about 35 per cent naphthalene and anthracene and 65 per cent of unknown hydrocarbons. I n addition, there were obtained about 1200 cubic feet (34 cubic meters) of gas containing 710 cubic feet (20.1 cubic meters) of methane, 440 cubic feet (12.5 cubic meters)
FIGURE2.
THERMAL DECOMPOSITIOX OF M E T H A Y E (62)
With a gas analyzing 89.7 per cent methane, 4.8 ethane, 3.4 propane, and 1.5 butanes, Cambron (5) observed a maximum ethylene yield of 14.3 per cent a t 993" C. corresponding to 8.3 per cent in the exit gas, and a 7.65 per cent yield of
acetylene a t 1200" corresponding to 3.5 per cent in the exit gas. Figure 2 shows that the yield of liquid products begins to drop off a t about 1220' C. while the amount of ethylene and acetylene formed continues to increase. In fact, a t much higher temperatures (2000' to 3000" C.), such as those obtained in the electric arc (23, 44, 77), very little if any liquid products are recovered, but the yields of acetylene and ethylene are high (17, 24). The maximum ethylene yield
INDUSTRIAL AND ENGINEERING CHEMISTRY
SEPTEMBER, 1933
TABLEI. M.~XIMUM YIELDS OF GNSATURATED GASES A N D LIGHTO I L FROM METHAKE Investigator
Max. Unsaturate Yield %, H? % ill in exit gas Temp. gas
c. Hague and Kheeler ( 2 7 , Vysoky (74; Padovani ( 0 3 ) Boomer (41 Stanley and Nash (71) Smith, Grandone, and Rall ifi2) \Varren ( 7 5 ) Dunstan, Hague, and I T heeler
4.6
...
... ... 3.7 4.4 3.8
950
iiio
1240 1100
Max. Light Oil TieIda GaL,/M CU.fl.
31 5
32 8 39 6 33 0
0 0 0 0 0 0
26 29 40 20
30
OC. 1000 1050
The writers have made experiments on the simultaneous cracking and polymerization a t about 950" C. of a gas containing ethane and methane in equal amounts. This mixture yielded a maximum of 0.5 gallon (1.9 litera) of light oil and 0.5 gallon (1.9 liters) of tar per 1000 cubic feet (2.3 cubic meters). Chamberlain and Bloom ( 7 ) worked with a gas containing 71.2 per cent methane, 23.3 per cent ethane, 4.7 per cent nitrogen, and 0.8 per cent carbon dioxide, obtaining
1100
1 i50 1200
0 25
4.0
(11)
18
1037
0 27 4.1 34 2 Average a, No attempt made t o correlate time-temperature relationship in comparing the results reported by different workers.
(4 per cent) is claimed to be obtained a t 1200" C. and 100 mm. pressure, a t 1500' C. and 50 mm. no ethylene is obtained although the yield of acetylene is 14.5 per cent (60). High-temperature cracking of methane is employed for the preparation of carbon black by the thermatoniic process. Theoretically, 1000 cubic feet (28.3 cubic meters) of methane should yield 31.82 pounds (14.5 k g ) of carbon as well as 2000 cubic feet (56.6 ciibic meters) of hydrogen. This yield, lion-ever, is never realized in practice. By eniploping the blow-and-run process, high yields of carbon are nevertheless obtained, and the comparison between the gas analyses before and after treatment (900" to 1400" C.) as reported by Moore (49) are as follow (in per cent by volume): Natural Gas Carbon dioxide Illuminants Hydrogen Carbon monoxide Methane Sitrogen
Resultant Gas
0.4 0.7
0 9 3
1 85 1 5
... ...
92.8
5,l
4
1 0
6.3
Ethane There iq a difference of 200" C. or more between the thermal decomposition temperatures of methane and ethane. Hence, if mixtures of methane and its homologs are subjected to temperatures below approximately 900" C., it is reasonably safe to aisume that very little of the methane has undergone any change for the short times of heating genwally employed in the cracking work under discussion (&PA, ,$SA, 5 $d). A compilation of the yields of liquid products obtained by Dunstan, Hague, and Wheeler with various saturated hydrocarbons at constant rate is given in Figure 3 (11). From these data the maximum yield of total liquids from ethane is about 1.83 gallons (6.9 liters) per 1000 cubic feet (28.3 cubic meters), the light oil yield being in the neighborhood of 0.92 gallons (3.5 litera). The maximum ethylene concentration in the exit gas appear. to be about 24.5 per cent a t 750" C., while the higher olefin yield IS about 4.5 per cent at the same temperature. More recent data olitainecl by Cambron ( 5 ) with a gas analyzing 95.01 per cent ethane, 3.78 per cent methane, and 1.21 per cent nitrogen at substantially constant rate of flow are shown in Table 11.
Temp. OC. 969 989
1000
io07
TCMPERPTURL
FIGURE
3.
'c.
P R O D U C T I O V O F LIQUIDPRODUCTS FROM G4sEOUs HlDROC4RBONS ( i f )
a maximum yield of about 0.29 gallon (1.1 liters) per 1000 cubic feet of light oil and about 0.1 gallon (0.4 liter) of tar< a t 1040" C. A large-scale plant gave a yield of about 1.0 gallon (3.8 liter.) per 1000 cubic feet a t 760" C. before the unit plugged with carbon.
Propane Since it is the cheapest gaseous paraffin of high molecular weight in a large number of locations ( 2 5 ) ,propane offers the greatest opportunity of giving promising returns. For that reason considerable research has been carried out with this hydrocarbon.
Olefin Production
Some of the earliest experiments on the thermal decornposition of ethane-propane fractions of natural gas condensates were carried out by Zanetti and Leslie (78, 79). The rewlts obtained by the writers and their associates with a quartz reactor are given in Figure 4. The maximum concentration of propylene in the exit gas (13.2 per cent) occurs a t a lower temperature (810" C.) than the maximum concentration of ethylene (29.4 per cent) which occurs a t about 890". The ratio of ethylene to propylene in the cracked gas is therefore roughly 2 to 1, increasing slightly with temperature, presumably because the propylene initially fornied enters more readily into secondary reactions than does the ethylene. Howerer, if the cracking is carried nearly to completion, the sum of ethylene and propylene i n t h e e x i t g a s i s close to 40 per cent over a wide temperature range. TABLE11. THERMAL DECOXPOSITION OF ETHASE (5) yield of C ~ H ~ Since the volume of gas is practically doubled by the ExpaiiGas Analysis - Based on C2Hs cracking process, this means that the yield of ethylene won GH: CIH, H? CzH6 CHn Entering Cracked plus propylene is of the order of 80 per cent on the basis 5% 5% %; % % % % % 37, o , 69 2i, 2s, 34, 39, 79, of entering propane. Podbielniak (M), in semi-conimer56.8 1.43 28.8 32.i 28.7 6 0 47.2 90.0 cia1 cracking of a gas mixture containing 83.4 per cent 57.0 1.6 28.2 34.9 21.6 11.0 46.7 12.6 57.5 2.0 28.0 35.5 21.1 11.6 46.5 ,1.2 propane, 10.3 ethane, 48 methane, 1.4 butanes, and 0.1 pentanes in alloy tubes, reports the maximum yield
-
,
,
1058
VOL. 27. NO. 9
INDUSTRIAL AND EKGINEERING CHEMISTRY
of ethylene (28.2 per cent in the exit gas) to have been obtained a t 815" to 840" C. By recycling the gas four times, t h e yield of ethylene was calculated to be raised from 39.5 cubic feet (1.1 cubic meters) per 100 cubic feet (2.8 cubic meters) of raw gas (for the first pass) to 63.66 cubic feet (1.8 cubic meters). Ebrey and Engelder (12) obtained a maximum unsaturate concent r a t i o n from propane of 33 per cent a t 760" C. at which the propylene maximum of 27 per cent was also reached. The maximum propylene concentra30 tion in the exit gas reached about 11 per cent a t 660".
Cracking in Presence of Oxygen Although it is true that in the case of p r o p a n e the carbon-carbon bond LI is normally weaker than the c a r b o n - h y d r o g e n bond, the w r i t e r s h a v e 10 been able to improve the olefin yield by s e l e c t i v e dehydrogenation u s i n g I the m e t h o d of Hopkins (65,66). A study of the crack700 800 900 IWO ing of propane in a copper FIGURE 4. CRACKING OF PROPANEtube in the presence of about 70 per cent air in I n quartz tube 0.25 inch X 2 feet (6.4 mm. ,X 80.8 cm.) in size: rrtte of the inlet gas has indicated flow,150 litera per hour; contact time, that it is possible to secure 0.4 second. a conversion of 75 to 80 per cent of the inlet propane into unsaturated compounds. The unsaturates formed consist of ethylene and propylene (with a little acetylene) in the ratio of 3 parts of ethylene to 2 of propylene. These optimum yields were obtained with a copper reactor a t 675" * 15" C. and a time of heating of about 30 seconds. The results may be compared with a 60 to 70 per cent optimum conversion by straight cracking a t 700" C. using a copper-coated steel reactor. The material constituting the reactor was found to have some effect upon the course of the reaction. At lower temperatures (below 560" C.) a mild steel reactor gave higher yields of unsaturates than one of copper. At 600" C. and above, copper was much more satisfactory since a t these temperatures the iron apparently catalyzed the polymerization of the unsaturates into liquid and solid tars. Glass (in the presence of air) seemed to exert a slight inhibiting action upon the cracking. Oxide catalysts, in general, had but little effect upon the reaction. Most of them, such as iron oxide, zinc oxide, chromic oxide, and copper oxide, acted as slight inhibitors. Nickel oxide was a strong decomposition catalyst producing considerable carbon. However, when a mixture of nickel oxide, zinc oxide, and chromic oxide was used, some positive catalytic effect was observed.
Simultaneous Cracking and Polymerization By allowing the cracked gas to remain in the reactor long enough for secondary reactions to take place, most of the propylene and part of the ethylene polymerize to liquid products. The cracking and polymerization may also be carried
out in separate steps. The maximum yield obtained by the writers in one-step operation (at 880" C.) was 4.4 gallons (16.7 liters) of liquid products per 1000 cubic feet (28.3 cubic meters) of propane of which 1.52 gallons (5.8 liters) were light oil. Boomer (4) reports about 2 gallons (7.6 liters) of light oil and 1.5 to 2 gallons of tar obtained with stabilizer gas, but these higher yields might be due to t h e presence of small amounts of higher homologs which exert a considerable effect. The writers' results are substantiated by Dunstan, Hague, and Wheeler (Figure 3). By means of their chart it is possible to calculate the yields of oils obtained with mixtures of saturates. This is done by multiplying the yields with each constituent a t the given temperature by the percentage of the constituent in the gas and adding the resulting products. A stabilizer overhead gas treated in theqe laboratories had the following percentage composition: Methane and fixed gas Ethane Propane Butane
4.2 18 6 60 1 1 2
Ethylene Propylene Butylene
2.2 13.4
0 3
At 900" to 960" C. (outside tube temperature) a yield of 1.7 to 1.8 gallons (6.6 liters) of oil per 1000 cubic feet (28.3 cubic meters) was obtained, of which 70 to 75 per cent boiled below 220" C. and slightly less than 50 per cent of the total product was benzene. The exit gas increased in volume 60 to 70 per cent and contained 30 to 40 per cent unsaturates. Podbielniak (55) obtained a yield of 1.8gallons of light oil (6.8 liters) per 1000 cubic feet with a heavier gas while employing alloy tubes. By use of a two-stage treatment with baffled tubes, Cambron and Bayley (6) were able to obtain a yield of light oil corresponding to 2.57 gallons (9.7 liters) per 1000 cubic feet of propane put through. I n this case the liquids formed in the first stage were removed before the gas entered the second furnace. Approximately the same yield of light oil was obtained when the propane was recycled under pressure of one atmosphere through a 28 per cent chromium nickelfree alloy steel tube, although the rate of formation of t h e liquids was obviously increased markedly.
Two-step Pressure Process I n view of the low yields of liquid products produced b y simultaneous cracking and polymerization a t atmospheric pressure, attempts were made to raise the efficiency of t h e process by carrying out the cracking reaction as a separate operation a t atmospheric pressure followed by polymerization a t higher pressure. It was felt that the use of pressure would reduce the reaction temperature required for polymerization to take place, and that in this way it might be possible t o control the polymerization reactions and avoid formation of tar. The experimental work showed that this was actually t h e case (67). The best yield, 1.95 gallons (7.4 liters) of liquid per 1000 cubic feet (28.3 cubic meters) of original propane, was obtained by polymerizing the olefins in the cracked propane a t 650" C., 700 pounds per square inch (45 atmospheres) pressure and 17 seconds time of heating, about 1.29 gallons (4.9 liters) of this product boiling below 200" C. The product consisted of a mixture of olefins, aromatics, naphthenes, and paraffins. Pressure greatly lowered the reaction temperature and decreased t a r formation but increased the formation of gaseous saturates. The work of Frey and Hepp (19) did not show much advantage in a two-stage pressure polymerization of ethane-propane mixtures, when cracking is carried out under polymerization pressure. T o promote the formation of aromatic products, i t was found desirable to keep the pressure moderately low in most of this work (22, 61). For example, a combination of 500Ato 800 pounds per square inch (32 to 52 atmospheres) and 600" C.
SEPTEMBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
was found t o give satisfactory results. I n other experiments the pressure was brought u p as high as 3000 pounds (200 atmospheres), thereby making it possible to polymerize a t temperatures as low as 400" C. But it was noted that increasing pressure caused a change in chemical composition of the products from the aromatic type obtained a t atmospheric pressure to a decidedly nonaromatic material resulting from operation at the highest pressure studied.
Two-step Process with Intermediate Hydrogen Removal -4s already mentioned, propane is readily cracked to a mixture containing ethylene and propylene in a ratio somewhat higher than 2 to I, these two olefins constituting about 40 per cent of the cracked gas. This corresponds to some 80 per cent conversion of the propane into olefins. The polymerization of this cracked mixture under pressure showed that the hydrogen present had a decided tendency to react with the olefins to form ethane and propane. Since this materially reduced the amount of olefins available for polymerization, Reid (9) and Frolich (68) independently proposed to remove t h e hydrogen in the cracked gas. White and Frolich (69) developed a simple method of accomplishing this removal by selective oxidation with oxygen or air in the presence of materials such as copper oxide with practically no siniultaneous oxidation of hydrocarbon constituents or loss of olefins.
00
400
500 boo 700 800 RE4CTOR WALL TEMPERATURE -'C
FIGURE 5 . CRACKING OF
900
n-BLTmE
I n quartz tube ' / 8 inch X 1 5 feet (2 22 X 30.4 c m ' In size; rate of flow, 40 liters per hour.
Pressure could then be applied to good advantage in the polymerization step. The best yield was obtained at 400" C. and 2500 pounds (170 atmospheres) pressure, corresponding to 7.75 gallons' of total product (29.4 liters) per 1000 cubic feet (28.3 cubic meters) of original propane. About 5.7 gallons (21.6 liters) of this material boiled below 225" C. I n another experiment a t 600 pounds (41 atmospheres) and $00" C., the total yield of liquid was 4.56 gallons (16.1 liter-) boiling below 225' C. These yields could be increased by employing a recycle system. The product boiling below 400" F. (204" C.) had a knock rating of 60 to 85 per cent benzene equivalent and a gum content of only 5 mg. per 100 cc. The single-pass yield represented a 78 per cent conversion efficiency. By 1 The yield of 10 gallona (37.85 liters) previously reported for cracked propane by Frolich and Wiezevich ( 2 5 ) is in error. Actually it u a s obtained wzth a gae of higher olefin content.
1059
adding the recycle yield, the efficiency could be raised to above 90 per cent, based on the olefins present in the cracked propane. The great advantage obtained by intermediate hydrogen removal is shown in Table 111. The first method involving simultaneous cracking and polymerization a t 880' C. and atmospheric pressure gave a fairly high yield of tar and low yield of light oil. The use of a two-step process involving cracking at atmospheric pressure immediately follov-ed a t 840" C. by polymerization a t high pressure (700 pounds or 48 atmokpheres) considerably reduced the amount of tar formed without improving the light oil yield. In the third case the hydrogen n-as removed by copper oxide after cracking the propane at 840" C., and the dehydrogenated mixture was subjected to 2300 pound3 pressure (170 atmospheres) a t 400" C. The yield of light oil was more than doubled while the proportion of tar was considerably le35 than in the first case. TABLE 111. CRACKIXG AND POLYMERIZATIOX OF PROPAXE TO
PRODCCE LIQUIDPRODUCTS
Method
Best Conditions for Polymerisation Beet Yields, Temp. Pressure Total Liquid
Light Oil Cal ,/M, cu. ft.
c. Lb./sq. i n . Simultaneous 880 0 4.4 1.52 Two-stepa 650 700 l.Q5 1.29 Intermediate Hz removala 400 2500 7.75 5.73 a Propane originally cracked a t 840' C. and atmospheric pressure.
The introduction of air prior t o the polymerization of cracked propane did not cause any preferential removal of hydrogen, the oxygen appearing almost completely in the exit gas as oxides of carbon. This was also observed by Vysoky (74)t,o be the case with methane. In the presence of copper, benzene formation under such conditions was found to be completely suppressed. The hydrogen removal may he carried out by firat cracking the gas to the desired unsaturate content, followed by passing it over copper oxide or similar material a t 300" to 350" C. and 2 to 5 seconds contact time. By this means it is possible to remove over 90 per cent of the hydrogen with a very low loss of olefins. The reduced copper may be reoxidized by contacting with air. The addition of theoretical amounts of oxygen or air to react with the hydrogen makes the process continuous and does not appear t o induce the combustion of the other const'ituents. Some experiments made to study the behavior of the individual olefins are of interest since they throw light on the mechanism of t'he polymerization reactions. The results reported above shov that the yields of benzene are only a small fraction of the theoretical. Whenever att,empts were made to boost the yield of polymerization products, in operation a t low pressure, the increase would always appear as tarry material while the benzene yield remained approximately constant. Since this pointed to benzene as an intermediate product in the formation of tar, the following experiments were made: In one case, hydrogen containing a knoivn concentration of benzene vapor \vas passed through the reaction chamber at atmospheric pressure but otherwise under conditions favorable for polymerization. Practically all the benzene could be recovered from the exit gas, showing that no appreciable reaction took place. On the other hand, when ethylene was added to the mixture of hydrogen and benzene vapor, both ethylene and benzene disappeared with the formation of large amounts of tar. In other words, the benzene initially formed combined with additional amounts of olefins, probably Tvith simultaneous dehydrogenation, forming compounds of higher molecular weight. Be-
1060
IXDUSTRIrlL ASD ENGINEEKISG CHEMISTRY
cause of this "snowball" effect, it was impossible to increase the benzene yield simply by increasing the time of contact at the high temperature required for polymerization in the absence of catalysts.
n-Butane The effect of temperature on the product3 obtained from n-butane as determined by the w i t e r s and their associates with a quartz tube are submitted in Figure 5 . As has been shown to be the case with propane, the temperature (650' C.) at which maximum propylene concentration occurb (11.1 per cent) is somewhat lower than the temperature (730" C.) for maximum ethylene (29 per cent). The highest concentration of butylene (8 1 per cent) was reached a t about 670" C. For maximum total unsaturates (42 per cent), a temperature of about 690" C. would be required. Frey and Hepp (19) carried out some large-scale experiments with butane in which the charge n a s first cracked a t 750" to 770" C. under 2 atmospheres pressure to a maximum content of unsaturates of 43 per cent by volume, followed by release of the gaseous mixture into an insulated chamber without further introduction of heat and over a much longer period of time. The temperature was found to rise 50" C. during the passage through the chnniber and a maximum yield of benzene was claimed to be obtained with a minimum time of contact.
Catalysts and Contact Materials As far back as 1916 Zanetti and Leslie (78, 79), while working on the thermal decomposition of natural gas fractions, found that iron and nickel strongly catalyzed the decomposition of the product to coke and hydrogen. Copper was shown not to have this effect. Ebrey and Engelder (12) noted that chromium and cobalt also favored the production of large quantities of hydrogen. Platinum also behaves similarly t o iron and nickel (28). By placing a cylinder of copper gauze between the heating rod and the cold walls of his reaction tube, Cambron ( 5 ) was able to cut down the heat loss through radiation, thereby reducing the current required to keep the rod a t the desired temperature. A marked reduction in gas volume increase was also noted and was attributed to the formation and condensation of liquid products on the walls of the tube. Cambron likewise obberved that reduced copper catalyzed the dehydrogenation of ethane, This was also found to be the case with reduced copper-tin gauze. Since these metals were found to corrode rapidly, they were treated with molybdic oxide (Moo3) which is known to be a strong dehydrogenation catalyst. This treatmrnt appeared to remedy the corrosion without appreciably affecting the activity of the contact. In order to obtain turbulence and therefore improve the heat transfer, Cambron and Bayley (6) passed the paraffinic gases through tubes containing copper baffles. The results obtained were markedly different as shown by the data in Table IV.
VOL. 2 7 , NO. 9
this purpoqe because the nickel catalyzed the formation of elementary carbon, but nickel-free alloys containing over 20 per cent chromiuin TTere found satisfactory. At 950" C. a 50 per cent increase in the yield of liquids Tvas obtained by carrying out the polymerization stage of the reaction under conditions of turbulent flow. Chamberlain and Bloom ( 7 ) tried other contact materials such as fused qjlica, clay, and monel metal. They found that copper gave the highest yields a t the lowest temperatures, but the metal disintegrated rapidly (probably clue to oxidation). These investigators concluded that the carbon formed has a high selectivity in the conversion of paraffins to aromatics, but this activity is easily destroyed by partial oxidation or by the formation of carbon-metal compounds. Graphitic carbon was the final result of this loss in activity. Stanley and ?;ash (71)have shown that graphitic carbon seriously suppresses the formation of higher hydrocarbons. Work carried out in these laboratories in accordance with certain I. G. Farbenindustrie patents indicates that externally heated silicon tubes (@) shorn a decided promise in resisting the formation of carbon under properly controlled conditions. Podbielniak (57), contrary to the observationq of Cambron and Bayley ( 6 ) , claims that carbon is prevented in semi-commercial cracking by the use of nichrome tubes. Other contact materials which have been proposed have been aluminum, tin, zinc (40),yariouq oxides of zinc, magnesium, copper, uranium, and 4 v e r ( 3 1 ) , iron oxide (40, 59), molten metals o r salts (51), salt> of metaphosphoric acid (36), and various nickel and copper catalysts (20). Frolich and Boeckeler (70) disclose a reduced zinc oxidechromium oxide catalyst for the production of olefins from paraffin hydrocarbons higher than methane. h catalyst of 30.2 mole per cent zinc and 69.8 per cent chromium (reduced with methanol), when employed in the cracking of propane a t 635' C. (13.4 volumes of propane vapor per minute per volume of reaction space), produced a gas containing 14.3 per cent propylene, 0.2 per cent ethylene, 26.1 per cent hydrogen, and 59.4 per cent residual propane without appreciable carbon formation. Ferrous sulfate and sulfide catalyzed carbon deposition, while others such as potassium borate and calcium pyrophosphate gave only small amounts of carbon. The type of reactor material employed appears to have a decided effect upon the initial yields of liquid products. Quartz, copper, metalloids, and lustrous carbons were quite effective in retarding carbon formation. I n his study of the cracking of propane, Boeckeler ( 3 ) ranked the materials studied as catalysts in the following order: copper gauze, zinc-copper on pumice, nickel gauze, and stainless steel. Sone of the materials showed marked activity, and they did not influence the character of the reaction but merely lowered the temperature a t which a given extent of reaction may be obtained.
Polymerization of Olefins
At low temperatures, in the absence of catalysts, the olefins require a long time of heating to produce good yields of liquid products. At higher temperatures the TABLE IV. CRACKING OF PROPANE IN EMPTY A K D IN BAFFLED rate of polymerization is much greater, but a differTUBES AT HIGHTEMPERATURES (6) ent type of product is obtained. Type of Gas ExpanExit Gas Analysis-Olefins I n Figure 3 Dunstan, Hague, and Wheeler (11) Tube Temp. Rate sion C2H2 C2Ha CaHs H2 Produced give yields of liquid products obtainable with the a C. L./hr. % % % % % G./hr. Empty 1032 412 aO,d 1,1 d,% 12,0 161 various olefins at high temperature in a flow procBaffled 1035 404 58.3 0.6 19.1 12.6 14.3 304 ess. By means of this chart the authors claim that it is possible to determine the yield of liquid product a t atmospheric pressure with any mixture of By means of baffles these authors (6) claim to have brought paraffins and olefins. Under pressure, with 2 to 3 hours contact time, they have been able to obtain 80 to 92 per cent the temperahre down within the range of usefulness of special alloy steels. Heat-resistant alloys of the 18-8 type (18 per yields of liquid products from ethylene. For instance, in cent nickel, 8 per cent chromium) were found unsuitable for an enameled autoclave, a t 1325 pounds (90 atmospheres)
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SEPTEMBER, 1935
IWDLSTRIAL AND EUGINEERIUG CHEMISTRY
pressure and 385" to 390" C., after a 3-hour period a 92 per cent conversion to liquid products was realized. Theqe and others reported (47A, 63, 64) are in agreenirnt with results obtained bv the writers. In one case a yield of 10 gallons (37.85 1iters)bf liquid p r o d u c t s p e r 1000 cubic feet (28.3 c u b i c m e t e r s ) was obtained by the miters a t 400" C. and 2000 pounds pressure (135 atmospheres), 68 per cent of the liquid boiling below 2%" C. In another case! a yield of 11 gallons (41.6 liters) of liquid product per 1000 cuhic feet was obtained a t 400" C., 2700 pounds (183 atmospheres) pressure, and 8 to 12 hours heating time. the conversion being 97 per cent. With a contact time of 19 seconds, the 400 800 best yield obtained wit,h ethylene FIGURE6. EFFECT OF was a t BOO" C. and 800 pounds INITIALPRESSURE (BEFORE ADDITIOK OF A I R ) (54 atmospheres) pressure, and o s POLYMERIZATION OF the amount of oil obtained was OLEFINS 3.7 gallons (14 liters) per 1000 At 300' t o 305" C.; ratio of cubic feet, 91 per cent boiling beoxygen t o olefin. 0 06 t o 0.08 tween 30" and 225" C. Wiezevich and Whiteley (76) found that the addition of a small amount of air or oxygen (10 per cent) into the hot reactor helps to set off the polymerization reaction a t a lower temperature and pressure. For instance, a yield of 10.9 gallons (41.4 liters) of liquid per 1000 cubic feet of reacting ethylene mas realized by introducing air into the gas kept a t 900 pounds (61 atmospheres) pressure and 310" C., whereas without, the oxygen no measurable polymerization took place under these conditions. With air the conversion of ethylene per pass was about 70 per cent. As noted by Dunstan et al. (11), propylene required more severe conditions of temperature and pressure for the same yield of liquid, and butylene even more severe conditions t'han propylene. Figure 6 illustrates the data obtained in these laboratories when air is employed for initiating the polymerization reaction. The best yields obtained wit'h propylene and butylene under these conditions mere 10 gallons (37.85 liters) and 9 gallons (34.1 liters), respectively, per 1000 cubic feet bal;ed on the olefin reacting. However, these yields are not t'he maximum obtainable. With 1 to 2 hours heating time, the products are fairly lo~vboiling (80 to 90 per cent off a t 225" C.); a t higher times of heating (8 to 12 hours) the boiling point tends to increase (50 to 60 per cent off a t 225" C.). ,4n interesting method for improving the antiknock value of olefin polymers is disclosed by Wietzel and Pfaundler (SS) These investigators first polymerize the gaseous olefins under pressure a t 300" to 500" C. to produce liquid products which are subject'ed to catalytic dehydrogenation a t temperat'ures above 500" C. A detailed survey of the work done on the thermal decomposition and polymerization of olefins is given by Egloff, Schaad, and Lowry (16). lZO0
I
Catalysts with Olefins -4large number of catalysts and contact materials have been reported for the polymerization of olefins. Silicon ( 3 4 , copper and noble inetals (8, S7), boron fluoride (35, 52), zinc chloride (47), boron fluoride with hydrogen halide (SS), aluminum chloride n-ith zinc (&), and others have been shown to have high activity. By means of boron fluoride accompanied with finely dkided nickel (SO), the polymeriza-
1061
tion of ethylene may be controlled to give high yields of butylene. Silica gel (48) was found to catalyze the formation of aromaticsfrom propyleneat 650" C. Clay (58)at 170" to 370" C. at 600 to 1500 pounds (41 to 100 atmosphere?) pressure has also been found effectire in polymerizing gaseous olefins. Gayer (26) prepared a synthetic alumina-silica catalyst which is more active in polymerizing propylene at atmospheric pressure than Floridin clay. At 350" C. a 30 per cent yield per pass of lowboiling liquid product. was obtained which had a high octane number (87 after steam distillation) and boiled mostly in thr hexane range. Phosphoric acid (32) and its salts (29*46) have also been found effective in polymerizing gaseous olefin?. Ipatiev ('73) eaiployed orthophosphoric or orthophosphorus acids for polymerizing a refinery gas. At 60" C. and atmospheric pressure, a yield of 8 galloiis (31.0 liters) per 1000 cubic feet (28.3 cubic meters) of liquid product of 94 octane number was obtained. By this means it was possible to increase the over-all yield of gasoline from the cracking stock by about 12 per cent. Other catalyiti which have been employed for this purpose are basic zinc chromite (SO), metals of the eighth series (-$a), ammonium molybdate (41), magnesiiim oxide (39),and bauxite (72). An advantageous method of operation consiyts in polymerizing the cracked gas catalytically (to convert most of the olefins to niotor fuel) and then cracking the resiclual gas a t a temperature higher than the previous cracking temperature to convert the more resistant Daraffinic hydrocarbons into olefins; the latter are likewise r I p o l y m e r i z e d catalytically to motor fuel. This stepwise treatment may be continued until the olefin yield on cracking becomes too small to justify further treatment. Removal of the hydrogen before polymerization (68, 69) is found to be of value in many cases. Such a process yields a superior blend of polymers suitable for aviation fuel. As shown by Figures 4 and 5 , the temperature required for cracking to obtain maximum unsaturates raries with each hydrocarbon constituent. If this temperature is exceeded, some of the more valuable olefins of higher molecular weight FIGURE 7 . THEHhlAL will be destroyed. Hence, espeDECOMPOSITION OF cially in the case of mixed gases, it ACETYLENE (18) is often more desirable to convert the componeiits to uiisaturates in more than one stage, employing intermediate polymerization betn-een the cracking stage
