Ind. Eng. Chem. Process Des. Dev. 1985, 24, 1251-1258
t = time, month V , = wind velocity, mph V, = total volume of pond including solution, salt and ice (if present), ft3 z = elevation of solution or ice or salt layer, ft a1 = solution or ice absorptivityfor direct and scattered solar radiation, dimensionless a2 = solution or ice absorptivity for atmospheric radiation, dimensionless 6 = cumulative thickness of salt deposit on free surface, ft c1 = solution or ice emissivity, dimensionless eac = emissivity of cloudless (clear) sky, dimensionless Xz = heat of dissolution of salt, BTU/lb of salt Xice = heat of fusion ice-salt mixture, BTU/lb X, = heat of vaporization of water, BTU/lb u = Steffan Boltzmann constant p = stoichiometric constant to account for hydrate water 4a = atmospheric radiant energy flux, BTU/(H ft2) 4, = evaporative heat flux, BTU/(H ft2) c $= ~ solar (direct + scattered) radiant energy flux, BTU/(H ft2)
Subscripts 1 = pond solution layer (unless otherwise stated) 2 = salt layer (unless otherwise stated) 3 = air 4 = ice layer fp = freezing point of solution in = streams into pond
1251
out = streams taken out of pond (),, = initial value Literature Cited Barrett, W. T.; O'Neill, J., Jr. I n "3rd Symposium on Salt"; Northern Ohio Geological Society, Inc.: Cleveland, OH, 1970. Calder, R.; Neal, C. Hydro/. Scl. J. 1984,29, 89. Flint, G. I n "Kirk-Othmer's Encyclopedia of Chemical TechnologySupplementary Edltlon"; Wiiey: New York, 1971. Foss, S. D. AIChE Symp. Ser. 1978. 78 (174), 150. Ferguson, J. Aust. J. Sci. Res. l9S2,5 , 315. Helfrich, K. R.; Adams, E. Eric; Godbey, A. L.; Harleman, R. F., MIT Report No. 8-35-82, March 1982. Huii, J. R. Solar Eng. 1980,25, 33. Iaiso, S. 0. Water Resour. Res. 1981, 17, 295. Jlrka, G. H.; Watanabe, M.; Octavio, K. H.; Cero, C. F.; Harleman, D. R. F., MIT Parsons Lab Report No. 238, Dec 1978. Lof, G. 0. G.; Ward, J. C.; Karaki, S.; Dellah, A.; O'Meara, J. W.; Savage, W. F.; Rinne, W. W.; Gransee, C. L. Office of Saline Water R+D Report No. 764, Jan 1972. Meinel, A. B.; Meinel, M. P. "Applied Solar Energy"; Addison-Wesley: Reading, MA, 1976. Pancharatham, S . Ind. Eng. Chem. Process. D e s . Dev. 1972, 1 1 , 287. Rivera, T.; Randolph, A. D. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 182. Sherwocd, T. K.; Plgford, R. L.; Wilke, C. R. "Mass Transfer"; McGraw-Hill: New York, 1975. Vergara-Edwards. L.; Parada-Frederick, N. Paper presented at the 6th Internatlonal Symposium on Salt, New York, 1983. Viessman, W.; Knapp, J. W.; Lewls, G. L.; Harbaugh, T. E. "Introduction of Hydrology"; Harper-Row: New York, 1977. Welt, J. R. "Engineering Heat Transfer"; Wiley: New York. 1974. Received for review August 10, 1984 Accepted December 26, 1984
Pure n-Hexadecane Thermal Steam Cracking Domlnlque Depeyre, Chantal Fllcoteaux, and Chrlstlane Chardalre Laboratoire de a n i e et Informatique Chimiques, Ecok Centrale des Arts et Manufactures, F-92290-Chatenay-Malabry. France
I n the research publications, one cannot easily find experimental data on thermal steam cracking of pure nhexadecane. Thermal decomposition of n-hexadecane in the presence of steam was investigated in a laboratory-scale tubular Incoloy 800 or quartz flow reactor. The temperature range was 600-850 "C,and the pressure was atmospheric pressure. The effluents product distribution was studied as a function of temperature, various amounts of steam, residence time, and wall surface effect of the reactor. At low conversion and low temperature, the overall kinetics of decomposition appeared to be a first-order reaction with a frequency factor of 9.6 X I O l 3 s-' and an activation energy of 57 kcal mol-'. The measured effluent product distribution agreed reasonably wtih the Kosslakoff and Rice theory only at a temperature lower than 650 "C. The optimal yields of C3H, and C2H, were obtained, respectively, at 750 OC and between 800 and 850 OC, 0.5 s, and for a mass ratio of steam to hydrocarbon 2.8:1. The yields expressed in percentage weight of a-olefins lighter than Cg, of paraffins up to C3, and aromatics can be plotted vs. a severity function S, = T,f$03 where T , is the cracking temperature in kelvin and t , the residence time in seconds.
Research on fundamental mechanisms of cracking had led us to study the thermal cracking of a high molecular weight pure hydrocarbon in the presence of steam which acts as a secondary reactions inhibitor. We chose steam as the inert diluent because it is currently used in industry for its ability to decrease the partial pressure of the hydrocarbon and to inhibit the formation of high molecular weight compounds which otherwise would promote the production of coke. Our purpose will be to obtain an optimal production of light a-olefins such as ethylene and propylene and to delay the production of aromatic compounds which are the precursors of coke deposits. The kinetics of cracking of n-hexadecane has been studied by Voge and Good (1949), Tilicheev and Zimina 0196-4305/85/1124-1251$01.50/0
(1956), Panchenkov and Baranov (1958), Fabuss et al. (1962,1964),Panchenkov et al. (1964),Doue and Guiochon (1968,1969), Groenendyk et al. (1970), and Rebick (1979, 1980, 1981) either in a batch reactor or in a flow reactor. The temperature range was 500-700 "C and the pressure range 0.5-100 atm. We must notice that most experimental studies were performed at temperatures lower than 600 "C. Only one study carried out by Hrusovsky et al. (1966) on the steam cracking of technical hexadecane was reported. No investigation on steam cracking of pure n-hexadecane up to 850 "C has been developed. Hexadecane is particularly suitable for investigation of kinetics and mechanisms of cracking, since its number of carbon atoms 0 1985 American Chemical Society
1252
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
Table I. ExDerimental Flow Rates Incoloy 800 surf nature of reactor temp, "C inlet H,O flow rate, g h-' inlet ClBHa4flow rate, g h-l H20/C16H31mass ratio
600-700 23.4 17.9 1.3
49.4 17.9 2.8
67 17.9 3.7
quartz
80.5 17.9 4.5
116.6 17.9 6.5
600-850 23.4 17.9 1.3
39.4 13.4 2.9
49.4 17.9 2.8
82.4 30.4 2.7
103 38 2.7
corresponds to a gas oil charge in the range of petroleum fractions. The purpose of this paper is to show the influence of temperature, amounts of steam, residence time, and nature wall effects upon product distributions of n-hexadecane cracking. Kinetics parameters, Le., frequency factor and activation energy, were determined. Experimental results were compared with product distributions predicted by the theory of Kossiakoff and Rice (1943). Product distributions of a-olefins lighter than cg, paraffins up to Cs, and aromatics (percentage weight) can be plotted vs. a severity function S, = T,t,0.O3.
I. Materials and Methods (a) Basic Component. n-Hexadecane olefin-free for experiments was supplied by Fluka (Switzerland). The purity determined by gas chromatography analysis was 98.42%. The n-hexadecane contained five impurities, the quantities of which were, respectively, 0.17%, 0.12%, 0.13%, 0.30%, and 0.86%. These products were ramified hydrocarbons which were eluted between n-pentadecane and n-hexadecane. Physical characteristics were as follows: melting point, 18-19 "C; boiling point, 283-286 "C; refractive index n20D),1.437; and density d204,0.773. (b) Apparatus. Experiments were carried out at atmospheric pressure in flow tubular reactors, either in Incoloy 800 (composition Ni Co, 30-3590; Cr, 19-2370; C, 0.1%; Al, 0.60%; Ti, 0.15-0.60%; Mn, 1.5%; S, 0.15%; Si, 1%; Cu, 0.75%) without a preheater or in quartz with preheaters of distillated water and hydrocarbon as schematically represented in Figure 1. The Incoloy 800 reactor consisted of two concentric tubes. The outer tube was in Incoloy 800 (60 cm length, i.d. 21 mm). The outside diameter of the inner quartz tube was 15 mm. The outer tube was placed in an electrically heated furnace. Oven regulation was carried out by a modulate temperature regulation connected to a Chromel-Alumel thermocouple. The inlet reactor temperature profile was measured by a moving Chromel-Alumel thermocouple sliding along the wall surface of the inner quartz tube. The quartz reactor had three independently heated and regulated sections. The A section consisted of two concentric quartz tubes. The outer quartz tube was 39 cm in length, i.d. 22 mm. The outside diameter of the inner quartz tube was 15 mm. This section was used for preheating the water steam at the same temperature as the hydrocarbon cracking section. The B section was used for preheating n-hexadecane at 480 "C. Its length was 34 cm. The C section was used for thermal cracking of n-hexadecane. Its length was 40 cm. B and C sections consisted of two concentric quartz tubes. The inner diameter of the outer tube was 22 mm. The outside diameter of the inner tube was 15 mm. The feed water and hydrocarbon were supplied from containers to the inlet reactor either by a peristaltic micropump or by syringes. The flow of nhexadecane varied from 18 to 38 g h-* and that of water from 23 to 116 g h-l. The effluent products were passed through two ice-water traps to condense liquid products and finally cooled to 20 "C. After the solution was quenched, gaseous products were collected in a Mariotte
+
Figure 1. Lower: Incoloy 800 reactor without preheating equipment. Upper: Quartz reactor with preheating equipment. (1) syringes and motor driven unit, (2) peristaltic pump, (3) infrared lamp, (4) oven, (5) thermocouples, (6) temperature controller, (7) temperature recorder, (8) voltage variable transformer, (9) mercury manometer, (10) liquid sampler, (11) gas sampler, (12) quartz tube, i.d. 2.2 cm, (13) quartz tube, i d . 1.5 cm.
flask and gaseous flow rates were measured. (e) Analytical Instrumentation. Effluent products analyses were performed by gas chromatography. H2 and CH4,were analyzed with a chromatograph with a thermal conductivity detector and a stainless steel column packed with a 5-A molecular sieve. The carrier gas flow was nitrogen. A flame ionization detector and a stainless steel column packed with Porasil B were used for the analysis of CHI, C2H6,and C3Hsup to C6. The carrier gas flow was helium. The separation of liquids was operated on a stainless steel column packed with SE 30 on Chromosorb WAW, 60-80 mesh. The carrier gas flow was helium. The identification of the compounds was confirmed by using gas chromatography mass spectrometry coupling analyses carried out in the Mass Spectrometry Center of C.N.R.S. (Centre National de la Recherche Scientifique) and in the Claude Bernard University-Lyons. 11. Experimental Results and Discussion Steam cracking of n-hexadecane was carried out in flow reactors at atmospheric pressure, either in a Incoloy 800 reactor for the range of temperature 600-700 "C, with a 25 "C step and five steam-to-hydrocarbon mass ratios, or in a quartz reactor for the range of temperature 600-850 "C and for two steam-to-hydrocarbon mass ratios. Four runs were made for each temperature and steam dilution. Each run lasted for about 20 min after the steady state was achieved. Experimental conditions are shown in Table I
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
Table 11. Cracking of o-Hexadecane in an Incology 800 Reactor. Influence of Temperature and Steam 17.9 17.9 17.9 17.9 17.9 17.9 17.9 C16H,, inlet flow rate, g h-l H ~ o / C ~ ~ wtH ~ ~ temp, "C
outlet gas wt, g h-'
HZ, wt 70 CH,, wt 70 CZH,, wt CZH4, wt % CSHB, wt % C3H6, wt 70 butenes, wt %
butadiene, wt % C5-CB a-olefins, w t 70 ClO-Cl6 a-olefins, wt % aromatics, wt % cracked C16H34,wt 70 mass balance, % outlet liq wt, g h-' C bal error, % hydrogen bal error, 70 5c
1
8T
1.3 600 2.2 0.04 1.4 1.5 5.0 0.1 2.5 1.3 0.3 12.3 14.2
2.8 600 3.2 0.08 2.0 1.5 7.1 0.3 4.6 2.0 0.5 9.9 11.5
4.5 600 2.0 0.07
1.3 650 7.6
1.1
traces
0.04
0.0
41.8 96.8 15.1 7.6 8.1
38.5
28.3 99.1 15.7 3.4 3.3
4.6 3.2 18.0 0.3 11.4 3.1 1.8 15.1 9.3 2.6 75.2 94.3 9.4 7.8 7.5
0.1
0.5 5.2 0.3 2.5 1.2 0.3 7.3 8.9
101.0 14.8 2.8 4.1
'f
2c
.
2.8 650 7.3 0.2 4.0 2.3 16.9 0.6 11.0 4.2 1.9 9.9 5.8 0.7 61.9 95.6 9.7 7.9 5.1
1.3 700 12.1 6.5 9.8 4.2 25.2 0.2 15.3 3.5 2.9 6.2 4.0 4.0 92.9 88.9 3.8 20.0 30.0
4.5 650 11.2 0.3 5.5 2.6 25.6
1.1 16.1 7.0 4.6 5.8 3.4 0.7 74.2 98.5 6.4 3.2 0.2
17.9 2.8 700 12.8 0.5 7.0 3.7 30.3 1.7 18.6 5.1 4.6 3.2 1.5
1253
17.9 4.5 700 14.0 0.5 7.0 2.1 37.1 1.0 18.9 6.0 5.6 2.8 1.9 1.2 86.5 97.6 3.5 3.7
2.0 79.7 98.5 4.8 3.3 2.3
0.1
F
.
IC
s
xi
D b
-a
15.
-3 ' C .
35 1 T)
>
0
'3:
_ C Y"
1
c
'b
L.
-o"\p-i
2"
Yo
s
53 r.C6m3i
CSP"'^S
q3: 0-
%
?i
Figure 2. Product yields vs. n-Cl6HS conversion. Quartz reactor (-): (X) 600 "C; (0) 625 "C; ( 0 )650 "C; (+) 675 "C; (0) 700 "C; (*) 750 "C; (0)800 "C; (A)850 "C. Incoloy 800 reactor (- - -1: (a) 600 "C; (a) 625 "C; (Et)650 "C; (H)675 "C; (+) 700 "C.
and experimental results are given for 100 g of n-hexadecane feed. (a) Temperature Influence. As expected, for a defined steam-to-hydrocarbon mass ratio, experimental data obtained in both reactors show that gaseous flow increased with temperature while the liquid flow rate decreased. Among gaseous products, hydrogen and propane are present in small quantities, less or equal to 2% weight. The yields of these two gases increase slightly with temperature. However one exception must be noted. As presented in Table 11, in the Incoloy 800 reactor, at 700 "C for a steam-to-hydrocarbon mass ratio equal to 1.3, a large amount of hydrogen and aromatic compounds, 6% and 4% in weight, were detected, and coke deposit was observed on the walls of the reactor. In order to avoid catalytic wall effects of the reactor, other runs were operated in a quartz reactor from 600 up to 850 "C with a stem-to-hydrocarbon weight ratio maintained between 2.7 and 2.9. Figures 2-4 show the plot of the yields in weight percentage vs. n-hexadecane conversion for different temperatures in Celsius and indicates that hydrogen, methane, and aromatics yields increase steadily as the temperature climbs from 600 to 850 "C. The yields of pentene, 1butene, butadiene, ethane, propylene, and ethylene show maxima, respectively, at 675, 700, 750, and between 800 and 850 "C and then decline as the temperature increases. Acetylene occurs near 750 "C and increases up to 2% in weight at 850 "C. No paraffins higher than propane were detected during steam cracking. Among liquid products, all olefins from pentene to pentadecene show optimal yields around 650 "C and tend to disappear at 800 "C. Aromatics were present from 650 "C. The major aromatic products are benzene, toluene, styrene, indene, and
Figure 3. Product yields vs. n-Cl&&d conversion. Quartz reactor (-): (X) 600 "C; (0) 625 "C; (0)650 "C; (+) 675 "C; ( 0 ) 700 "C; (*) 750 "c; (0) 800 "c; (A 850 "c. Incoloy 800 reactor (- - -): (e) 600 "C; (m)625 "C; ( a )650 "C; ( a )675 "C; (+) 700 "C. 2o
T
L
I
i -3 1 0
Arom otic 5
I
2 F
5 .
0
50 n-C
H
16 3 4
100
c o n v e r s i o n 70
Figure 4. Product yields vs. n-C16H34conversion. Quartz reactor
(-): (X 600 "C; (0) 625 "C; ( 0 )650 "C; (+) 675 " C; ( 0 ) 700 "C; (*) 750 "c; (0)800 "c; (A)850 "c. Incoloy 800 reactor (- - -): (til) 600 "C; (a) 625 oC; ( a )650 "c; ( a )675 oC; (+) 700 "C.
naphthalene while alkyled derivatives, cycloalkenes, fluorene, anthracene, or phenanthrene and their alkyled derivates, are minor products. (b) Steam Dilution Influence. Steam-to-hydrocarbon flow rates used in the Incoloy 800 reactor to adjust various steam-to-hydrocarbon weight ratios from 1.3 to 6.5 are shown in Table I. Table I1 illustrates the effect of steam to enhance ethylene, propylene, butene, and butadiene production, while it reduces at high temperature the formation of hydrogen, methane, ethane, and aromatic products. This trend can also be noted in the quartz reactor, for temperatures greater or equal to 750 "C with a flow rate of n-hexadecane of 17.9 g h-l and for steam-tohydrocarbon weight ratios equal to 1.3 and 2.7 as shown
1254
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
Table 111. Cracking of n -Hexadecane in a Quartz Reactor. Influence of Temperature and Steam C16H34inlet flow rate, g h-' H 2 0 inlet flow rate, g h-' temp. "C outlet gass flow rate, g h-l H,, w t 70 CH4, wt 70 CzH6, W t 70 C,H,, wt % CzH2, wt % C3H8, wt % C3&, W t % C4H8, wt % C4H6,wt % CBHi,, wt '70 C&, W t 70 C6H12, wt % CcH,,, W t % C6H8, W t 70 benzene, wt % 1-C7H14, wt % toluene, wt 0'7 1-CsH16,W t % ethylbenzene, wt '70 xylenes, wt 70 styrene, wt % l-CSH18, wt % methylindene + isomers, wt % indene, wt % l-CloH20, wt % ethylindene + isomers, wt 70 naphthalene, wt 70 l-CIiH22, wt % methylnaphthalene + isomers, wt % l-CiZH24, wt % ethylnaphthalene + isomers wt 70 phenylbenzene acenaphthene + isomers, wt % acenaphthalene, wt % fluorene + isomers, wt % anthracene + isomers, wt
+
17.9
17.9
17.9
17.9
17.9 17.9
23.2 750. 15.6 0.8 12.4 3.5 41.3 trace 2.6 18.2 2.2 6.3 trace 0.2 0 0.05 0.2 1.9 0 1.3 0 0.2 0.2 1.1 0 0.5
49.4 750. 16.2 0.5 9.4 2.5 43.6 0.3 1.8 20.9 4.4 7.0 0.6 0 0 . 4 0 2 2.2 0.09 0.8 0.05 0.1 0.09
23.2 800. 14.5 1.0 15.3 2.8 43.8 trace 1.7
49.4 800. 15.3 0.8 13.7 2.1 47.6 0.9 1.8
0.5 4.3 0 0.09 0 0 0.7 3.2 0 1.9 0 0.1 0.2 2.1 0.5 0 . 1 0 0.2 0.6
0 0.8 0 0.1 0.1 0.9 0 0.3
23.2 850. 11.9 1.7 17.2 1.8 39.4 1.1 0 3.3 0 0 2.2 0 0 0 0 3.2 0 1.6 0 0.05 0.3 3.4 0 0.5
49.4 850. 14.2 1.2 16.9 1.4 48.2 2.1 0 5.9 0 3.5 0 0.05 0 0 0.08 3.0 0 1.6 0 0.07 0.4 2.5 0 0.3
0.6
0.3 0.07 0.3
1.1 0 0.5
0.6 0 0.4
1.7 0 0.4
2.0 0 0.5
0.4 0.08 0.3
1.8 0 0.4
1.0 0 0.4
4.6 0 0.4
3.3 0 0.8
0 0.4 0.8 0 0.2
11.1
12.5
0.6 5.6 0.9 0.1 0 0 0.1 2.0
0 a
0 . 2 0
0
0.03
0 0.04
0
a
a
0.06
a
a
0.3
0.2
a
0.7
a a
a a
a
a
0.2 0.3 trace 0.02 0.09 0.1
0.1 0.6 0.7
0.7 0.05 0.4
a
a
trace 0.1
0.8
0.03
a 7.2 99.0 95.9 1.5 1.1 5.4 1.9
a 5.2 96.9 100.3 1.7 0.7 0.5 2.4
trace 12.5 99.2 94.0 2.3 1.0 7.7 0.4
0.3 18.7 98.6 86.7 3.6 0.8 15.1 4.6
0 16.4 99.0 96,7 3.i 0.5
%
methylanthracene + isomers, wt 70 pyrene, wt 70 1 aromatics, wt 70 C16H34cracked, wt % mass balance, % outlet liq wt, g h-' residence time, s carbon bal error, 70 hydrogen bal error, % a
trace 7.4 99.6 94.4 1.4 0.6 7.8 0.9
3c. 3
0.2
Ox
0.6
0.8
residence t i m e
1 SCC.
Figure 5. Quartz reaction: influence of residence time for H,O/ CIBH3 mass ratio 2.8. (+) 675 "C; (*) 750 "C.
over the range of 30-103 g h-', while a steam-to-hydrocarbon weight ratio within 2.7-2.9 was maintained. The flow was laminar, and the temperature profile was registered during cracking. In the calculations of the overall decomposition rate constant from experimentaldata, it was assumed that the reador was in a plug flow. The residence time was determined according to the equivalent reactor volume concept described by Hougen and Watson (1947) and Hirato et al. (1971). The equivalent volume V, is defied as the volume which would give at the temperature of pyrolysis T , the same conversion as the experimental reactor with its temperature profile (eq 1-4). rT,
dV, = rT dV
(1)
dV, = Sa dl,
(2)
v, = Sal,
(4)
4.4
2.0
values not determined
in Table 111. Pyrolysis of propane in a stainless steel reactor in the presence of 0-50 mol % steam was carried out by Herriott et al. (1972). These authors have also found that steam enhances production of ethylene and inhibits production of CHI and C2He,and have mentioned the presence of carbon oxides in the gas effluent. We must indicate that all our experiments were carried out without estimation of carbon oxides. As shown in Tables I1 and 111, steam reduces the partial pressure of n-hexadecane and the residence time and thus delays dehydrogenation, aromatic production, leading to the formation of coke deposit. (c) Residence Time Influence. Different residence times were obtained by varying feed n-hexadecane flow rates over the range of 13.4-38 g h-' and steam flow rates
In the calculation the following values were used: A1 = 2 cm and E = 60 kcal mol-'. T, was represented by the reactor temperature set on the temperature regulator. The residence time was calculated by eq 5 below, including the
t, =
vep
-
SaLeP
(ML-k MG -#- Ms)RT,- (ML+ kf~+ M#T,
(5)
equivalent volume of the reactor and the effluent flow rates of steam gaseous and liquid hydrocarbon products. We have observed that hydrogen, methane, and aromatic formation increases as the residence time increases from 0.3 to 1.2 s. Ethylene production was maximum for temperatures situated between 800 and 850 "C and a residence time of about 0.5 s. The yields of ethane, butadiene, butene, and propylene increase from 600 to 750 "C while the residence time varies from 0.4 to 1 s. Near 750 O C ,
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
10
T
1255
t
8
i c
s 2 6
.
3 T I
35
.
30
-
x4 t
2
0
850
1000
950
1050
1100
5,
Figure 7. Product yields vs. severity function of n-hexadecane steam cracking in the quartz reactor.
20
>
900
15
-
13
-
I850
900
$50
1000
1050
1100 5,
Figure 6. Product yields vs. severity function of n-hexadecane steam cracking in the quartz reactor.
these productions drop quickly as the residence time increases. Liquid a-olefins yields increase between 600 and 650 OC for the residence times varying from 0.4 to 1 s. Beyond this temperature, the production collapses as shown in Figure 5. Finally, it is possbile to enhance either gaseous a-olefins or liquid a-olefins production or aromatic production by acting on both parameters, temperature and residence time. As Kunzru et al. (1972), the product distribution of cracking in percentage weight vs. the severity function has been plotted in Figures 6 and 7, where the severity function is defined as S, = Tet,"
(6)
The value of m which gives the best fit for a-olefins lighter than cg, paraffins up to c3,and aromatics is m = 0.03. This function cannot be applied to olefins from C6 to CI5. As expected, the variation of C2Hs, CzH4, C3H8, C3H6,C4H8,C4H6,and C5HI0with the severity shows a maximum, while Hz, CH4,and aromatics climb steadily as the cracking severity increases (Figures 6 and 7). (d) Comparison of Results Obtained in a n Incoloy 800 Reactor and i n a Q u a r t z Reactor. Since both reactors in Incoloy 800 and in quartz differ by their geometry, it was necessary to compare experimental results and to plot in Figures 2-4 (for the same, one steam-to-hydrocarbon weight ratio 49.4117.9) the yields of products in percentage weight vs. conversion of n-hexadecane percent. For gaseous products and conversion of n-hexadecane inferior or equal to 50%, experimental results obtained with both reactors agree reasonably with the limit of experimental error. One might think that at low temperature, the wall surface effect is not effective. However, beyond n-hexadecane conversion equal to 60%, the yields obtained in the quartz reactor are inferior to those obtained in an
Incoloy 800 reactor. Otherwise quantities of aromatic products formed are higher in the Incoloy 800 reactor than in the quartz reactor. That may be due to the wall surface effect of the reactor which promotes secondary reactions causing especially polymerization of light a-olefins and cycloaddition of olefins by the Diels-Alder mechanism. Albright and McConnel (1979) studied pyrolysis of ethane at 800 OC in the presence of 53% and 52% mol of steam, respectively, in a Vycor glass reactor and in an Incoloy 800 reactor. They did not detect the presence of CO and C02 in the Vycor glass reactor, while 2.3 (% carbon) of CO and 0.6 (% carbon) of COz were estimated in the Incoloy 800 reactor. The amount of coke left on the reactor walls at the end of the run and the production of methane are superior in an Incoloy 800 reactor than in a Vycor reactor. Dente and Ranzi (1983) suggested that the OH- radical formed by the H-abstraction reaction between radicals in the gaseous phase and steam can react with the unsaturated hydrocarbons by addition on unsaturated bonds. R. H2O RH + OH-
+
OH.
+ CzH4
-*
+
OH-CHZ-CH2. OH.
+ CO
-*
-
.O-CH2-CH3 -* CHy + CO + H2
CO2 + Ha
(e) Wall Surface Effect. Numerous studies have been carried out by Crynes and Albright (1969), Tsai and Albright (1975-1976), Brown and Albright (1976), Martin et al. (1976), Albright and Carol Yu (1969),and Albright and McConnel (1979). When steam is used during pyrolysis of light paraffins, these authors have found that metal oxides were formed on the inner wall of the reactor. metal surface + H 2 0
-
(metal oxides) + H2
(7)
In the Incoloy 800 reactor, the main oxides are NiO and Fe304as mentioned by Tsai and Albright (1976). At high temperature when coke was deposited on the walls of the reactor, steam oxidized coke to produce carbon oxides Csurface + HzO -.+ CO + HZ or Csurface + 2H20
-
COZ+
HZ
(8)
(9)
In old reactors, about 8-22% of the inlet steam reacted by reaction 7 to form hydrogen. Carbon monoxide thus formed can reduce the surface oxides and also can disproportionate in coke and carbon dioxide.
2co
-
C(coke)+ co2
(10)
1256
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
Tilicheev and Zimina (1956), Panchenkov and Baranov (1958),Doue and Guiochon (1968),and Groenendyk et al. (1970),where E varied from 55 to 60 kcal mol-' and A from 3 X 1013s-l to 1 X 1015s-l, shows satisfactory agreement with our experimental results. (g) Comparison of Our Results with Those in the Literature and with the Products Predicted by Rice-Kossiakoff Theory. It is well-known that thermal decomposition of paraffin is a monomolecular process by C-C bond split, since C-C bonds are weaker than C-H bonds as proposed by Rice (1931,1933),Rice and Herzfeld (1934), Kossiakoff and Rice (1943), and Benson (1960). P R.1
t
-cs
_.. 0 4 e
" -
I05
40
".5
'1
-
K-2
T
Figure 8. Arrhenius plot for the kinetic constant of overall decomposition of n-hexadecane in the quartz reactor.
Hydrogen can react with coke or metal carbide on the surface of the reactor to produce methane C + 2Hz CH, (11)
-
Albright and Carol Yu (1979) have shown that the coke deposit on the Vycor glass surface or alonized Incoloy 800 surface contained no metal and was inactive, while coke deposited on the nontreated Incoloy 800 surface contained metal particules and was magnetic. The metal particules catalyze coke formation. Other explanations may be considered. According to Bredael (1975) and Shah et al. (1976), the rate of formation of coke depends upon the inner reactor surface-to-volume ratio SL/V. An increase of the SL/V ratio accelerates the rate of coking. Since on one hand the surface-to-volume ratio of our Incoloy 800 reactor is 6.5 cm-l, and other hand the flow is laminar, the wall effect is greater in a laboratory-scale reactor than in an industrial reactor. After each run, our tubular reactor was decoked by air combustion at 600 "C to eliminate carbon deposits. According to Lengelle and Duterque (1972), one can assume that repeated alternation of steam cracking exposures in the same reactor leads to metal carburation. In order to avoid wall effects and metal carburation, Albright and Carol Yu (1979) and Blouri et al. (1981) suggested using walls aluminized or passivated by chromaluminization. (f) Rate Constant. It is well-known that paraffins cracking at low conversion follow first-order kinetics. The rate constants and activation energy were determined by assuming the overall decompositionof n-hexadecane to be fiist order. According to Benton (1931), rate constants K were calculated by eq 13 where K is a function of con-
A
K =
K +
vB
(12)
+[ (L, v In
(v - l ) x ] (13) 1-x version x , residence time t , and the number of moles of product per mole of n-hexadecane cracked Y. Activation energy was determined by the Arrhenius plot K = Ae-EIRT 8
Although values of v determined experimentally were not very accurate since complete analysis of liquids is difficult and each gas is assimilated to a perfect gas, we took into account the variation of v with the advance of the reaction and plotted on a graph logloK vs. 1/T, Figure 8. The values of activation energy and frequency factor were as follows: quartz reactor, E = 57 kcal mol-'; A = 9.6 x 1013s-l. Comparison of the values of activation energy and frequency factor with those of Voge and Good (1949),
Then free radicals formed remove an hydrogen atom from a primary, secondary, or tertiary carbon of the paraffin. When the large radicals thus formed have a carbon skeleton superior to five, prior to rupture by p scission, they may coil around and react upon themselves to produce isomers. The shifting of H atoms in the carbon skeleton to produce isomers may occur in any of the positions from 5 to 16 for geometrical reasons as suggested by Fabuss et al. (1964) R.16l R.16'
-
-
R'1616
R-165to Ra1615
(the upper index gives the place of hydrogen abstraction, the lower index the length of the radical). After isomerization, the radical formed undergoes p scission to produce an olefin and a second generation of shorter free radicals. R.163 C4H, + RSl21
-
R.163
-
C15H30
+ R.1'
Following Fabuss et al. (1964) and Rumyantsev et al. (1975),we assumed that then two positions were available for p scission; the probability of rupture is equal for both positions. However, when p scission gave a large radical, the rate of formation of this radical was 3 times faster than for a methyl radical. In order to calculate the distribution of products for 100 mol of decomposed n-hexadecane, removal of a secondary hydrogen is assumed to require 2 kcal mol-' activation energy less than that of a primary hydrogen. The shifting of a primary to a secondary position is assumed to require an activation energy of 4 kcal mol-l. The different steps of decomposition are R.1
isomerization
R.1' olefins 1
-+
-- -- +
Rs2 isomerization and
Re2
Rs2'
Re5
Ra5'
+P
R6H
scission
+ Ra3 olefins 5 + Rs6 olefins 2
R.l
hydrogen transfer
Termination reactions have been neglected. After five steps of n-hexadecane decomposition,only propyl radicals or shorter radicals are left. In method of Kossiakoff and Rice (1943) and Fabuss et al. (1964), it was assumed that all ethyl radicals produced in each reaction step were converted to ethane via an intermolecular H-atom-abstraction reaction. But at higher temperatures, decomposition of the ethyl radical would occur competitively with the H-abstraction reaction. Pacey and Purnell(l972) took into account in their calculations both H-atom abstraction and ethyl radical decomposition for n-butane pyrolysis. Murata et al. (1973, 1974) suggested a method of calculation based on these two types
Ind. Eng. Chem. Process Des. Dev., Vol. 24,
Table IV. Experimental Values and Product distribution from Rice's Theory Combined To Fabuss et al. and Murata and Saito Methods exDtl theoret exDtl theoret 600 600 650 650 temp, OC press, atm 1 1 19.6 43.7 convers, wt % Product mol per 100 mol of n-Hexadecane Decomposed H2 18.4 17.7 36.2 23.3 CH, 55.4 61.7 79.2 61.5 C2H6 23.0 20.5 27.1 15.1 C2H4 102.4 121.4 161.4 128.3 0.1 6.2 0.2 C3Hs 2.9 C3H6 40.6 49.4 77.7 49.0 33.7 25.8 13.0 25.9 C4Hs 0 11.2 0 C4H6 1.9 16.5 13.4 13.5 C5H10 14.5 19.1 4.4 19.1 CBH12 19.5 15.4 5.1 15.4 C7H14 12.3 12.9 6.3 12.9 C8H16 11.0 11.2 8.5 11.2 CSH18 11.1 7.0 9.6 12.6 9.6 C1oHzo CllH22 10.1 7.8 6.5 7.9 10.1 6.9 4.7 6.9 C12H24 6.5 3.6 6.5 C13H26 7.0 7.1 2.6 7.0 C14H28 5.9 1.6 4 C16H30 4.2 4.0 mass bal 99.4 98.1 5.1 8.4 C bal error, wt % H bal error, wt % 5.0 5.8
of reaction for pyrolysis of n-hexane, n-heptane, and noctane, and n-decane. C2H5. + P CzHG + R.
No. 4, 1985 1257
These authors estimated that the molecular path was 25% in the pyrolysis of dodecene at 600 "C. However, Giraud-Horvilleur and Blouri (1977) in pyrolysis of l-tetradecene indicates that for temperatures superior to 540 "C, the radicalar mechanism is the most effective. In order to explain production of Cn-1H2n-2and Cn-2Hzn-4during cracking of nonene, dodecene, and hexadecene, Rebick (1979) mentioned a third mechanism which is the addition of a small radical to an olefin CnH,,
+
{
H. CH,. C,H,
.
--*
{
CnH,n+,. Cn+lH2n+J.radical addition Cn+2H2n+5.reaction
The new radicals thus formed would decompose in turn as paraffinic radicals. In our experiments, H-abstraction reaction is the most effective. Unstable, large a-olefins decompose to produce shorter a-olefins. Butadiene not predicted by the theory can result from @ scission of a hexenyl radical. C6H12 + CH3.4 CH,=CH-CH. --CH,--CH2-CHs CH4
+
CH2=CH-CH.-CH2-CH2--CH3 CH2=CH-CH=CH2 -+
+ C2H6.
At high temperature superior to 750 "C and high conversion, production of acetylene can be explained by the reaction
+
C2H5 He
+
+P
CzH4
+ Ha
H2
+ R.
+
At high temperatures, the ethyl radicals decomposition is more effective. The results of our calculations shown in Table IV are a combination of the methods of Fabuss et al. (1964) and Murata and Saito (1974). Comparison of product distributions predicted by the theories of Kossiakoff and Rice (1943) and our experimental results at 600 "C and for 20% conversion of nhexadecane agree reasonably with respect to the theoretical values. The largest deviation between the predictions and experimental measurements was the amount of butene. At 650 "C for 44% conversion of n-hexadecane, experimental values do not agree with the theory. C2H4 is 26% too high with respect to the predicted values. One can note that H2, CH4, C2H6, CzH4, C3H6,and C4Hsare in excess while C5+ olefins are deficient with respect to the theoretical values. One might think that at high temperature, large a-olefins are less stable than shorter a-olefins and decompose in turn to produce shorter a-olefins. Different methods have been suggested for the decomposition of a-olefins. Miller (1963) studied the decomposition of octadecene and proposed a molecular mechanism via the cyclic membered ring transition state to produce propene and a-olefin. CnH2, C3H6+ Cn-3H2n-6 retro-ene reaction In order to explain the production of diolefins and shorter olefins, he suggested also a radical mechanism. H-abstraction reaction C,H,, + R. R H + C,H,,-; +
-+
*On CnH2n-1'
olefins + R. {diolefins + R.
Rumyantsev et al. (1975) considered that both mechanisms, molecular and radicalar, can explain the product distributions of 1-heptene, 1-dodecene,and 1-tetradecene.
The formation of benzene, toluene, styrene, and naphthalene not predicted by Rice's theory can result from condensation of short olefins or from addition of a diene and an olefin or a diene and a benzenic ring by Diels-Alder reaction followed by dehydrogenation.
u
+
2H2
It is difficult to compare our results (Table IV) with those of Voge and Good (1949), Fabuss et al. (1962), Doue and Guiochon (1968,1969), and Rebick (1981) (Table V) because the operating conditions are quite different. Most of these authors carried out their experiments at 500 "C and 1 atm. The results of Fabuss et al. (1962) were obtained at about the same temperature and conversion, respectively, 650 "C and 41.6% weight, as our results, but the pressure was 69 atm; that enhances production of paraffins not predicted by the theory of Kossiakoff and Rice. Conclusion This study in two reactors, whose surface nature was different, shows the influence of temperature from 600 to 850 "C, various amounts of steam, and various residence times on the product distribution. It allowed us to obtain experimental data, something which is practically nonexistent in the literature for this temperature range. In the Incoloy 800 reactor, at 700 "C, a low amount of steam, and high conversion, the iron and nickel of the surface reactor
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
Table V. Data of the Literature exptl" theoretb exptlb exutl' theoretC temp, OC. 500 500 502 649 649 press, atm 1 69 convers, wt % 31.5 4.9 41.6 Products mol per 100 mol of n-Hexadecane Decomposed H* CH4 C*H, C2H4 CBH, C3H6
C4Hm C4H8 C4Hs c5 C6
c7
C8 c9
C10 C,, c 1 2
Cl, c 1 4
Cl5
16.7 53.0 52.6 76.9 13.0 47.4 2.8 18.2 1.6 8.6d
24.3 16.2 13.4 10.1 10.9 9.3 7.3 7.7 4.9 0
0 61.2 38.8 99.9 0 50.3 0 27.2 0 14.9 16.5 14.0 12.3 10.9 9.9 8.9 7.8 7.1 7.0 3.5
48.8 48.1 83.6 4.4 43.6 0.0 23.5 0 12.0 13.2 13.0 12.2 11.6 11.7 10.3 9.4 8.2 8.9 2.8
3.7 19.9 40.5 37.4 24.2 26.6 0 0 0 13.3d 21.7d 18.5d 14.4d 12.1 11.3d 9.9d 8.5d 6.5d 0 0
0 56.9 37.5 99.5 4.3 49.1 0.8 25.9 0 13.6d 19.0 15.5 12.0 11.2 9.7 7.7 7.0 6.6 7.0 3.9
Voge and Good. Rebick. Fabuss et al. Olefins + paraffins.
are supposed to act as a catalyst of decomposition of nhexadecane. The high yield of hydrogen and tendency to form coke were registered at this temperature. However, the coke formation can be slowed down by increasing the amount of steam and decreasing the residence time. Our results indicate that as the amount of steam increases, the yields of aromatic products decline and the relative yields of a-olefins C2H4,C3H6,and C4H8increase. In the quartz reactor, high yields of propylene and ethylene were obtained, respectively, at 750 "C and between 800 and 850 "C, for 0.5-s residence time, and a mass ratio of steam to hydrocarbon 2.81. The overall decomposition of n-hexadecane was assumed to be a first-order reaction with a frequency factor of 9.6 X 1013s-l and an activation energy of 57 kcal mol-' for temperature less or equal to 650 "C and low conversion. Rice's theory combined with Fabuss et al. (1962) and Murata and Saito (1974) methods allows us to predict the product distributions for the cracking of n-hexadecane reasonably well, for temperatures less than 650 "C. A severity function can be used to correlate the yield of light olefins lower than C6, paraffins up to C3,and aromatic products for steam cracking of n-hexadecane. These experimental results were obtained in order to develop correlations between product distributions and various cracking parameters. These data would allow us to develop a kinetic model based on the most important free radical reaction steps.
Nomenclature A = frequency factor for Arrhenius rate expression, s-l E = activation energy, kcal mol-' K = first-order rate constant, s-l Le = equivalent reactor length, cm MG = molar flow rate of gas, mol s-l ML = molar flow rate of liquids, mol s-l
Ms = molar flow rate of steam, mol s-l P = atmospheric pressure, cm R = universal gas constant, L cm K-l or cal g-' mo1-l Sa = surface of the annular cross section of the reactor, cm2 S, = severity function SL= sum of lateral areas of the reactor, cm2 t , = residence time, s T , = temperature of cracking, K Ti = temperature of the ith point of measure, K V , = equivalent volume of the reactor, cm3 Y = moles of product formed per mole of n-hexadecane decomposed Registry No. C3Hs, 115-07-1; CzH4,74-85-1; C16H34, 544-76-3. Literature Cited Albright, L. F.; Carol Yu, Y. H. ACS Symp. Ser. 1979, No. 183, 193-203. Albright, L. F.; McConnell, C. F. ACS Symp. Ser. 1979, No. 183, 206-224. Benson, S. W. "The Foundations of Chemical Kinetics"; McGraw-Hill: New York, 1960. Benton, A. F. J . Am. Chem. SOC. 1931, 5 3 , 2984-2988. Blouri. B.; Giraud, J.; Nouri, S.; Herault, D. Ind. Eng. Chem. Process Des. Dev. 1981, 20,307-313. Bredael. P. Ann. Mines Belg. 1975, 1 1 , 1053-1058. Brown, S. M.; Albright. L. F. ACS Symp. Ser. 1976, No. 3 2 , 296-310. Crynes, B. L.;Albright. L. F. Ind. Eng. Chem. Process Des. Dev. 1969, 8 , 25-3 1. Dente, M. E.; Ranzi, E. M. "Pyrolysis: Theory and Industrial Practice"; Academic Press: New York, 1983; Chapter 7. Doue, F.; Guiochon, G. J . Chim. Phys. 1968, 6 4 , 395-409. Doue, F.; Guiochon, G. Can. J . Chem. 1969, 4 7 , 3477-3479. Lait, R. I.; Borsanyi, S. A.; Satterfield, C. N. Ind. Fabuss, 8. M.; Smith, J. 0.; Eng. Chem. Process Des. D ~ v 1962, . 1 , 293-299. Fabuss, B. M.; Smith, J. 0.; Satterfield, C. N. Adv. Pet. Cbem. Refin. 1964, 9 , 157-201. Giraud-Horvilleur, F.; Blouri, B. Inf. Chim. 1977, 164, 113-119. Groenendyk, H.; Levy, E. J.: Sarner, S. F. J . Chromatogr. Sci. 1970, 8 , 115-1 2 1. Herriott, G. E.; Eckert, L. E.; Albright, L. F. AIChE J . 1972, 18 (I), 84-88. Hirato, M.; Yoshioka. S.; Tanaka, M. Hitachi Rev. 1971, 20 (E),326-334. Hougen, 0.A.; Watson, K. M. "Chemical Process Principles"; Wiley: New York, 1947; Vol. 111, pp 884-886. Hrusovsky, M.; Lacko, R.; Foltanova, S.; Bartek, L. Ropa Uhlie 1966, 8 (IO), 292-299. Kosskkoff, A.; Rice, F. J . Am. Chem. SOC. 1943, 6 5 , 590-595. Kunzru, D.; Shah, Y. T.; Stuart, E. B. Ind. Eng. Chem. Process Des. Dev. 1972, 7 7 (4), 605-612. Lengelle, G.; Duterque, J. Rech. Aerosp. 1972, 5 , 249-259. Martin, R.; Nlclause, M.; Scacchi, G. ACS Symp. Ser. 1976, No. 32, 37-49. Miller, D. B. Ind. Eng. Chem. Prod. Res. Dev. 1963, 2 , 220-223. Murata, M.; Saito, S.;Amano, A.; Maeda, S. J . Chem. Eng. Jpn. 1973, 6 , 252-258. Murata, M.; Saito, S. J . chef??. Eng. Jpn. 1974, 7,389-391. Pacey, P. D.; Purnell, J. H. Ind. Eng. Chem. Fundam. 1972, 1 1 , 233-239. Panchenkov, G. M.;Baranov, V. Y. Izv. Vyssh. Uchebn. Zaved. Neft. Gaz 1958, 1 , 103-110. Panchenkov, G. M.; Kuznetosv, 0. I.; Baziievich, V. V.; Bessmertnaya, E. K.; Zhorov, Y. M. Tr. Mosk. Inst. Neftekhim. Gazov. Promsti. im. M . Gubknia 1964 5 1 , 142-147. Rebick, C. ACS Symp. Ser. 1979, No. 183, 1-19. Rebick. C. "Frontiers of Free Radical Chemistry"; Pryor, W. A., Ed.; Academic Press: New York, 1979, published 1980, pp 117-137. Rebick, C. Ind. Eng. Chem. Fundam. 1981, 2 0 , 54-59. Rice, F. 0.J. Am. Chem. SOC. 1931, 53, 1959-1972. Rice, F. 0.J . Am. Chem. SOC. 1933, 5 5 , 3035-3040. Rice, F. 0.; Herzfeld, K. F. J . Am. Chem. SOC. 1934, 5 6 , 284-289. Rumyantsev, A. N.;Nametkln, N. S.; Labrovsky, K. P.; Sanin, P. I.Masaev, I. A.; Vinitsky, 0. M.; Kurashova, E. K. H. R o c . World. Pet. Congr., 9th 1975, 5 , 155-161. Shah, Y. T.; Stuart, E. B.: SmRh, D. K. Ind. Eng. Chem. Process Des. Dev. 1976, 15 (4), 518-524. Tlllcheev, M. D.; Zlmlna, K. I.Khim. Tekhnol. Top/. Masel 1956, 8 , 23-31. Tsai, C. H.; Albright, L. F. Prepr.-Am. Chem. SOC.Div. Pet. Chem. 1975, 20 (I), 154-156. Tsai, C. H.; Albright, L. F. ACS Symp. Ser. 1976, No. 3 2 , 274-295. Voge, H. H.; Good,G. M. J . Am. Chem. SOC.1949, 7 1 , 593-597.
Received f o r review August 4, 1983 Revised manuscript received December 27, 1984 Accepted January 11, 1985
