ETHANE PYROLYSIS

-

THERMOCOUPLE r- WELLS

I\

I

II

DRYING TUBE

FLOW METER

INLET PRESSURE

COLUMN

CRACKING COIL AND FURNACE WET TEST METER

T DLIVJI ITY

SAMPL.ER

A

OUTLET

Figure 1. Diagram of Apparatus

ETHANE PYROLYSIS H. J. HEPP, F. P. SPESSARD, AND J. H. RANDALL Phillips Petroleum Company, Bartlesville, Okla. Ethane was cracked in a copper-lined, alloy steel tube a t temperatures in the range 1700' to 1900" F. Hydrogen and ethylene were the principal products, under the experimental conditions, but acetylene and butadiene were also formed in appreciable amounts. The concentration of ethylene in the cracked gas increased with increased ethane cracking to a maximum value of 37% a t 1800%F. and 83% destruction of ethane. Acetylene concentration also increased with increased ethane destruction, the per-

centage in the cracked gas reaching 3.4Vo under the niost drastic conditions employed. Butadiene reached 8 steady-state value of about 1% of the cracked gas. The yields of hydrogen, methane, and ethylene plus acetylene are correlated with the temperature, pressure, and reaction time variables on the basis of the equilibrium dissociation of ethane. First-order reaction velocity constants, calculated from the data, lie on an extrapolation of the plot of the Steacie and Shane ethane rate equation.

E

region 1700" to 1900" F. since the literature data in this region are scanty. I n addition to information on products composition, some cracking velocity data were also obtained. On the basis of these and literature data, a simple correlation of the yields of hydrogen, methane, and ethylene with cracking conditions i s developed for temperatures ranging upward from about llOOo F.

THYLENE is an important raw material in the manufacture of synthetic rubber, plastics, glycol, alcohol, and numerous other chemical products, as well as in the manufacture of highquality aviation fuels such as diisopropyl. Ethylene is a chief product formed in the cracking of hydrocarbons, and is obtained predominantly by recovery from refinery cracked gas streams or by cracking ethane or propane at rela ively high temperatures. Propane ha;; been a preferred charging stock for ethytene manufacture since relatively low temperatures are required and perpass yields are high. However, the price of propane has increased to a point where in many cases it is more economical to use other feed stocks. Ethane is a particularly attractive feed stock for ethylene manufacture. High ethylene yields may be obtained by proper choice of cracking conditions, and ethane is readily freed of sulfur impurities which might contaminate the product. I n addition, may make it possible the recently introduced pebble heater (1,8) to operate ethane cracking processes at higher temperatures than are practical for metal-tube cracking furnaces, with resultant high per-pass as well as high ultimate yields. I n the present investigation ethane cracking was explored in the temperature

APPARATUS AND PROCEDURE

A flow method was chosen to obtain the desired information, Since reaction rate data were wanted as well as product composition data, it was necessary to know gas temperature with reasonable accuracy. Direct measurement is difficult, and an indirect method was chosen. A long, small-bore metal tube was heated in close proximity to a large heat reservoir held at the desired temperature for the reactor. The favorable surface-to-volume ratio of such tubes, combined with high turbulence, gives some assurance that gas temperature will approach the tube-metal temperature. A large reserve of heat was provided by a heavywalled container surrounding the cracking coil, and the coil

2531

INDUSTRIAL AND ENGINEERING CHEMISTRY

2532

Vol. 41, No. 11

The temperature of the heat distributor and of the core was R u n No. 16 15 14 17 8 10 9 12 11 measured by Chromel-Alumel Cracking tube Copper-Lined Steel -Quartz Tube Packed with--. thermocouples placed in MuIlite Zircon Alumina Mullite Temperature, O F . 1790 1785 1803 1902 1802 1700 1702 1702 1700 inch 11-ells (Figure 1). The Pressure, mm. Hg thermocouples mere checked Inlet, av. 1066 1301 767 754 1008 1097 944 746 747 against the melting point of Outlet, av. 743 737 738 746 754 748 725 739 739 Sverage . 904 1019 873 921 834 743 743 756 751 sodium chloride. The maxiFlow rate a t 25' C mum temperature difference 760mm.,qc./sec:) 4.23 6.16 3.36 4.52 1.54 ,.. . . ~ ... ... Vol. of cracking zone, observed between the core and cc. 0.152 0.286 0.285 0.147 0.325 ... ... .0.. .3 5 ... wall was 20' F. and usually Time, see. 0,009 0.013 0.022 0.008 0.052 0.26 0.25 0.30 Vol. expansion 1.38 1.65 1.77 1.88 1.92 1.78 1.79 1.82 1.80 was about l o o F* The E t h a n e reacted, % ' 43.3 71.8 83.0 90.7 94.0 83.0 86.5 89.7 90.0 peratures listed in Table I are % of equilibrium 46.7 77.1 88.8 92.5 100 92.6 96.6 102 101.5 k , sec.-I 61.2 99 81.8 293 54.4 an average of the core temperatures during the run. Composition of cracked gas, mole % H2 27.1 38.4 42.3 45.1 46.5 42.7 42.1 48.4 45.2 I n the last four runs of Table CHI 2.2 a. 1 6.7 8.4 10.9 9.4 10.4 11.9 13.1 I the cracking coil and core CzHz 0.4 1.0 2.0 3.2 3.4 1.4 1.3 0.8 1.7 CZHI 27.9 36.6 37.0 36.1 33.4 34.8 35.9 32.2 32.1 were replaced by an 0.8-inch 41.2 17.1 9.6 4.8 3.0 9.5 7.6 5.0 4.8 CzHa inside diameter quartz tube 0.5 0.5 0.4 0.4 0.3 0.5 0.4 0.4 CaHa 0.4 0.2 0.2 0.1 0.1 ... CaHa 0.3 0.1 0.1 0.1 packed with about twenty-five CaHs 0.2 0.7 1.0 1.0 1.0 0.8 0.9 0.8 ClH8 0.1 0.1 0.1 0.3 0.1 0.1 0.1 .0...7 0.1 3/8-inch c e r a m i c p e b b l e s . C4H10 0.1 0.1 0.1 ... ... ... 0.1 ... 0.1 There is considerable uncerCs+ 0.1 0.2 0.5 0.6 1.2 1.0 1.0 0.5 1.6 Total 1Oo.b 100.0 1oo.o l0o.b ioo.0 1oo.o l0o.b 1oo.o 1oo.o tainty as to the actual gas temperature and contact time obYield of products, moles/100 mole ethane reacted H2 91.0 91.0 92.5 95.1 97.0 94.0 89.4 110.4 95.4 tained with this arrangement. CH4 7.3 12.0 14.6 17.6 22.7 20.8 21.6 27.2 27.7 Consequently these data were CzHz 1.3 2.4 4.4 6.8 7.1 3.1 2.9 1.8 3.7 CzIh 89.6 84.6 79.2 74.8 68.0 74.9 74.2 73.4 66.2 not employed in cracking-rate C3H3 1.3 1.2 1.1 0.8 0.8 0.7 1.1 0.9 0.8 evaluation. C3Hs 1.2 0.5 0.4 0.2 0.2 ... 0.2 0.2 0.2 C4H6 0.7 1.7 2.2 2.1 2.1 1.8 1.9 1.7 1.7 I n a typical run nitrogen CIHB 0.2 0.2 0.2 0.6 0.2 0.2 ... 0.2 CIHIO 0.5 0.2 0.2 ... ... .0.. .2 0.2 ... 0.2 was passed through the feed 5 5 1 1.3 2.5 2.2 2.1 1.2 3.3 cs - 0 ._ _ 0 ._ _ 1 .system and coil, and vented Total 193.6 194.3 196.9 109.3 200.6 197.j 19a.8 216.8 s 4 a t the coil outlet while the a Composition of feed in mole 70': CHI, 0.01; C2H4, 1.54; CzHa, 97.78; CaHs, 0.67; total, 100.00. furnace was being brought to temperature. The lines beyond the vent and the anaitself was threaded into a central core which presented a sublytical fractionating column were evacuated. IThen furnace temperature was satisfactory, the nitrogen flow )$-as st,antial area for heat pickup. This heat was transferred by conduction to the back side of the coil. It is believed that the stopped and ethane flow was begun. The cracked products tube metal temperature was very near the temperature reported were vented while final temperature adjustments were made. except a t the inlet end. However, the error in reaction zone When conditions were steady, the vent was closed and the volume introduced by uncertainty here is thought to be less effluent stream divided, part of it going to the Podbielniak serious than temperature measurement errors. If it is assumed Heligrid-packed analytical column and the remainder to a meter that all of the sensible heat required in run 8, for example, is and vent. The run was terminated by venting the cracked gas at the coil outlet, and flushing the feed system and cracking added in a 1-inch length of tubing, the heat transfer rate through the outer wall of the a/,,-inch tube employed is about 24,000 coil with nitrogen. B.t.u. per square foot per hour. This is a reasonable heat The samples were analyzed by fractional distillation suppletransfer rate a t the temperature level employed and indicates mented by absorption-combustion analyses of the various an uncertainty of no more than 2 or 3 inches in the 25 inches used fractions. Carbon dioxide was determined by absorbing in in the reaction zone in run 8. caustic, oxygen in Oxsorbent, hydrogen and carbon monoxide by combustion over copper oxide, methane by combustion over Figure 1 shows the apparatus employed. Heat was supplied by two 750-watt concentric coils of Chromel-A wire, wound on platinized silica gel, olefins by absorption in sulfuric acid of the appropriate concentration, butadiene by absorption in maleic Alundum cores, to a 10-inch cup of 3.75-inch inside diameter, made of 18-8 chrome-nickel steel. Inside this cup was placed a anhydride, and acetylene and methylacetylene by reaction with heavy-walled 18-8 alloy steel vessel equipped with a lid for disalcoholic silver nitrate. tributing heat uniformly over the cracking coil and providing The analysis given for run 12 was made on the mass spectroan isothermal cracking zone. The walls and lid were 1inch thick graph. Mass spectrograph analyses were also obtained for runs and the bore 1.5 X 4 inches. 9, 10, and 11, and good checks with the fraotional distillations The cracking coils used in the first five runs of Table I were were obtained. The mass spectrograph results for hydrogen were made of copper-lined 18-8 alloy steel tubing, of 4.8-mm. outside 41.9, 41.8, and 46.4 and for ethylene, 36.4, 35.5, and 31.3, reand 0.8-mm. inside diameters. Simila runs in an unlined 18-8 spectively, for the three runs. Carbon was not determined. However, because of the small alloy tube shut down in a f m minutes owing to coke deposition in the coil. No plugging was experienced with the copper-lined bore of the tubing used, any appreciable coke formation would tubing, and it is believed that catalytic effectswere absent. The have plugged the cracking tube and terminated the run. This tubing was formed into a spiral which was threaded into a spiral did not happen, all runs being voluntarily terminated. groove on a 1-inch-diameter core of 18-8 alloy ste 1. The length DISCUSSION AND CORRELATION OF DATA of tubing in the cracking coils used in the individual runs was The smallest coil varied, depending on operating conditions. The data obtained are presented in Table I and yield data used contained 12 inches of tubing in the reaction zone, and derived from the lit,erature in Table 11. The ethane was obtained from the Blatheson Company and contained about 98y0 ' the largest, 25 inches.

TABLE I.

b

+

ETH.4NE PYROLYSISa

INDUSTRIAL AND ENGINEERING CHEMISTRY

November 1949

2533

TABLE 11. LITERATURE DATA Citation

% % Pf Exptl. Temp., CZH8 EquilibMethod F. Cracked n u m

(1)

Flow

(3)

Static Static

40

(17)

20

Flow

Yield, M o l e d l O O Mole5 He C& C2Hz CIH,

+

1525 1770 1066 1066 1066 1094 1094 1094 1094 1094 1094 1094 1094 1094

81.8 88.4 2.64 4.29 4.40 6.2 6.7 7.3 7.7 11.0 11.2 11.0 11.9 14.3

107 95.5 21 34 25.9 47.3 51.1 55.9 58.9 84.0 85.1 84.0 91.0 109.0

104 96.4 97.6 102.5 100.0 84.5 96.0 80.0 86.6 87.4 91.5

29.4 21.7 3.9

84.6 79.4

26.3 42.3

65.6 74.8 97.6 100.0 100.0 95.0 95.5 93.0 91.5 91.0 84.8 81.6 82.0 68.9

2552

69.3

69.3

84.8

18.9

81.8

...

...

...

14.5 9.5 9.4 14.6 15.2 24.7

...

be reacted per pass a t the same ethylene yield. This suggested that ethylene yield could be correlated with the temperature, pressure, and reaction time variables on the basis of the ethaneethylene-hydrogen equilibrium. Such a correlation is presented in Figure 2. Equilibrium constants for the reaction

0

CZ"3 +C2H4

4- H Z

are plotted in Figure 3. The values defined by the solid line were used in this correlation, because the larger number of points permitted a better extrapolation to the higher temperatures, and X

EASTWOOD h POTAS

A

o

FREY 8 SMITH HAGUE e WHEELER TRAVERS & HAWKS

0

PRESENT INVESTIGATION

I 20

1 40

I 60

I

80

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100'

100)

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I

120

PER CENT OF EQUILIBRIUM

Figure 2. Correlation of Product Yield us. Per Cent of Equilibrium Decomposition

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1

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10 I

ethane (Table I). Hydrogen, methane, and ethylene were the principal products formed under the conditions used. Small amounts of Ca and CC olefins were also f0rm.d along with somewhat larger amounts of butadiene. The relatively constant yield of butadiene over the rather wide range of conditions suggests that this may be a steady-state concentration. Acetylene was formed in appreciable amounts, the yield increasing with increasing severity of cracking. The maximum acetylene yield obtained was approximately 7 moles per 100 moles of ethane reacted. Traces of Ca and C4 acetylenes and probably allene were also present. The Cs and heavier products were not analyzed beyond determining that they were highly unsaturated and contained cyclopentadiene and benzene. The reaction times given in Table I were calculated by dividing the average of the gas flows into and out of the cracking tube in cubic centimeters per second, corrected to reaction temperature and pressure, into the volume of the cracking zone, measured from the point a t which the cracking tube entered and left the heavy-walled heat distributing vessel. Tables I and I1 show that ultimate ethylene yield is mainly dependent on the per cent of ethane reacted. The major effect of increasing temperature is to permit a greater fraction of ethane to

1.0

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820

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Figure 3.

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Equilibrium Constants (CtHs

1800

2000 2 2 0 0

+ GH( + H,)

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

Vol. 41, No. Pi

(7), Pease (9, IO), Sachsuo ( I I ) , Steacie and Shane ( I d , IS), anri Storch and Kassel ( I d ) . Pease showed that the reaction waA of the first order, and t h i h has been confirmed by later workers. However, there has been much disagreement on the activation energy. Marek and McCluer found a value of about 77,000 calories, and later Kuckler and Thiele arrived a t similar value. However, the value as determined by Sachsse was 69,700. The careful work of Steacie and Shane, who used quartz reactors of up to 17-liter capacity, confirmed this value. As Figure 5 shows, the value of 59,700 agrees well with data from the present investigation. Figure 5 is a plot of the values of the fir&order reaction velocity oonstant calculated from the data of Eastwood and Potas ( 1 ) and of the present investigation Also shown is an extrapolation of +heplot of the Steacie and Sham crhane equation, log k = 14.02

- (69,700/2

3 RI')

This lints rtrikcs a fair avcrrtgc tlirough tho points shouar

(2) % y , F. E., and Huppke, %V. F ~IWU. , ENG.@HEM., 25,64 (1933, (3) Erey, F. E., and Smith, I), Fa,I h i d . , 20,948 (1928). (4) Hague, E. N., and Wheeler, K. lr , J . Chenz. S o r . , 1929,378-93 (51 Ri&akowslry, G. B., Romeyn, W,, Ruhoff, J. E., Smith, H A , , a n d Vaughan, W. E., J . Am. .LTha,i S o z . . 57, 6.4 (1935)

Figure 4.

Equilibrium Dissociation of Ethane Constant fsessura TS. K I P

st

use of the dashed-line values would ninkc no c.ssentia1 change in the wrrelation. Figure 4,relating the pcr cent of ethanr dissuciatctl a i cquiihrtum, nnd the ratio of the equilibrium constant, t o reaction pressui*r, for flow type (constant pressure) experimonts, was derived from the equilibrium constants of Figure 3 This chart may be used to estimate the equilibrium dissociation of ethane for constant pressure systems a t temperatures ranging upward I ~ Q about I ~ 1100" F. and the existing pressure. A siniil;tr chart mag readily be constructed for constant volume systems, I n developing Figure 2, the yield of hydrogen, methane, x o d ethylene plus acetylene for each experiment was plotted againrt ethane reacted, expressed as per cent of equilibrium. The 1x1 cent of equilibrium was calculated by dividing the obserT-ed per cent of ethane cracked by the ethanc dissociated at equilihrimii xt the same pressure and temperature. The curves of Figure 2 were drawn through the points lruiri the present investigation (Table I) and do not necessarily strike an average with the literature data. However, the correlation with literature data is considered good in view of the cxpcriniental difficulties involved in this type of experimentation and the multiplying factor involved in calculating ultimate yields from low conversion experimenh. The yields of hydrogen, methane, and ethylene plus acetylene. P T Q ethane ~ cracking may be estimated from Figure 2 over il wide range of cracking conditions, including temperatures at least up to 2000" F., ethane destructions up to, or somewhat beyond, the equilibrium dissociation, and over at least a moderate presure range. Additional data are needed to determine whether the correlation holds for pressures substantially higher than covered in the experiments.

100

U m Y'

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530

KINETICS

The kinetics of ethane cracking have been studied by a number of workerq notably Kuckler and Thiele (e), Marek and McCluer

lSb0

Figure 5.

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500

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2000 ' 1800 TEMPERATURE, ' E

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First-Order Reaction Velocity Constants us. Temperature

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

November 1949

(6) Kuckler, L., and Thiele, H., 2. physik. Chem., B42, 359-79 (1939). (7) Marek, L. F., and McCIuer, W. B., INn. ENQ.CHEW,23, 878 (1931). (8) Norton, C. L., Jr., J . Am. Cerum. SOC.,29,187 (1946). (9) Pease, R. N.,J. Am.C h .SOC.,50,1779(1928). (IO) Pease, R.N., ttnd Durgan, E. S.,Ibid., 50,2715(1928). (11) Sacheae, Hans, 2. physik. Chen~.,B31,79-86,87-104 (1935). (12) Steaoie, E.W.R., C h m . Rev., 22,311 (1938). (13) Steacie, E. W.R., and Shane, G., Can. J. Research, 18B,203-16 (1940).

2535

(14) Storoh, H.H., and Kassel, L. S., J. Am. Chem.Soc., 59, 124&6 (1937). (15) Travers, M. W.,and Hawks, J. A., Tr5w. Faraday Soc., 35, 864 (1939). (16) Travers, M.W.,and Pearce, T. J. P., J . SOC.C h m . Ind., 53, 321T (1934). (17) Tropsch, H., and Egloff, G., IND. ENG.GIIEY.,27,1063 (1935). I~BICEIYED January 7, 1940. Presented before the fourth Southwest Nngional Meeting of the AMERICAN C A ~ M I C ASOCIETY, L Shreveport, La.. December 1948.

Low Temperature Behavior of Butadiene-Styrene Copolymers

x

EFFECT OF COMPOUNDING VARIABLES R. D. JUVE AND J. W. MARSH The Goodyear Tire & Rubber Company, Akron, Ohio

S

YNTHETIC rubbers and natural rubber increase in stiffness at low temperature and tend to lose their elastic characteristics. This stiffening and hardening phenomenon occurs in varying degrees with various elastomers. Natural rubber and certain synthetic rubbers crystallize during extended exposure at lorn temperature, whereas other synthetic rubbers such as G R S remain amorphous (7). I n a general review of the low temperature properties of synthetic rubber, Liska (6) has shown that decreased styrene in butadiene-styrene copolymers improves the flexibility at low temperature. The low temperature flexibility of vulcanized articles made from any particular rubber or synthetic rubber is influenced by the compounding ingredients admixed with the elastomer. In this paper results are shown of some studies of the effect of these compounding ingredients upon the low temperature serviceability of butadiene-styrene copolymers. Somewhat similar work on the effect of a large number of plasticizers in GR-S has been conducted at the Rubber Laboratory, Mare Island Naval Shipyard (8) with particular emphasis on compression set at low temperature. EXPERIMENTAL PROCEDURE

For the evaluation of plasticizers in GR-S,the following campounding recipe was used: GR-S EPC black Zinc oxide Steario acid Sulfur Mercaptobenzothiazole Diphgny lguanidine Plasticizer

Parts by Weight 100.00 50.00

3.00 1 .oo 1.60

reducing the tendency for butadiene-styrene copolymer vulcanizates to stiffen during service at low temperature. Compounding variables which affect the behavior at low temperature include type and amount of plasticizer, particle size of the carbon black, and extent of vulcanization.

Tensile and elongation values at 27 ' C. were measured on the Goodyear autographic machine (I), tests at 93 C. were made on a standard L-2 Scott tester equipped with a heating jacket. The same machine was used for the tests at -57" C. A standard Bcott L-13 tester was placed in a -41 ' C. room for tests at that temperature. Standard A.S.T.M. dumbbells, 1 X 0.25 inch, were used on the Scott tester. Rebound values were determined on thn Goodyear-Healy rebound machine. Hot cut flex-life determinations were made on the Goodyear fleving machine using a rectangular sample 5 1 / ~inches long and 0.90 inch wide cut from a standard test sheet. A 6/32-inchcross wire cut wa8 made at the center. During the flexing, the samples are stretched to 20% elongation. The method for measuring dynamic properties has been described by Gehman et al. (3). Compression set test was conducted by a modification of A.S.T.M. D 395-471' method B. The per cent compression set was taken as the per cent set remainin after release from 30% compression for 168 hours at -57" Volatility is recorded ae the weight loss after 48 hours in an air oven at 100' C . O

6.

0.62

0.78 As shown

In the study of the effect of carbon black particle size on low temperature properties, the recipe which follows was used: Polvbutadiene Paraflux Zinc oxide St.aric acid Sillfur Mrrcsptobensothiaeole DIr)hnnyluiianidine Carbon black

Elastomers lose elasticity and tend to become stiff at

low temperature. A study has been made of means of

Parts by Weight 100.00 6.00 3.00

1 .oo

2.10

0.83 1.04

As shown

The test for low temperature flexibility has been described by Gehman et al. (4). The tc~mrsraturesat which the relative moduli are 2, 5, 10, and 100 times the modulus at 25' C. are designated as Tt,Ts,T,o, and Ttw.

EFFECT O F PLASTICIZERS UPON LOW TEMPERATURE FLEXIBILITY

Twenty-six plasticizers, including those shown in the literature to have very low freezing points, were studied in G R S . Both 10 and 20 parts of each per 100 parts of GR-S were tested. GR-S was selected for the plasticizer study because it stiffens a t a higher temperature than butadiene-styrene copolymers of lower styrene content, thereby allowing a more critical examination of the plasticizing effect. Typical data from which the stiffness values shown on Table I were taken are plotted on Figure 1 for GR-S containing Flexol TOF (trioctylphosphate) and Para Flux (saturated polymerized hydrocarbon). The relative modulus curves clearly describe the flexibility of the stocks.