(HIGH TEMPERATURE, VAPOR-PHASE CRACKING OF HYDROCARBONS)
Production of Natural Gas Substitutes by Thermal Cracking of Natural Gas liquids HENRY R. LINDEN, CHARLES E. BROOKS',
AND
LOREN N. MILLER
tnstitufe o f Gas Technology, Chicago, 111.
APOR-phase thermal cracking operations under moderate pressure can be conducted in high-alloy, indirectly fired tubes. However, thermal gasification of most normally liquid petroleum fractions in tubes can proceed only for limited periods of time because of the large amounts of carbon and pitch produced. Preliminary studies with natural gas liquids indicated t h a t in the required ranges of 1400" t o 1500" F. temperature and 1- to 10second reaction time, carbon formation up to 5-atm. total pressure was sufficiently low to permit semicontinuous operation of the tube furnace ( 9 , SO). This work has been extended to pilot plant operation by the Institute of Gas Technology using gasor oil-fired alloy tubes, 6 and 8 inches in outside diameter, of 20foot heated length ( I O , $4). The use of limited quantities of feed hydrogen in this pilot plant has resulted in complete conversion of natural gas liquids and petroleum distillates of low molecular weight to satisfactory natural gas substitutes. Katural gasolines appear to be the most attractive feedstock for the production of natural gas substitutes by thermal cracking, as they are normally available a t prices competitive with those for propane and butane and require considerably lower investment costs for storage facilities. However, thermal cracking of propane and butane into gases with heating values, specific gravities, and combustion characteristics approaching those of natural gases may offer sufficient advantages to justify the increased plant investment as compared to propane or butane-air production. I n the work reported here, the influence of the basic variables controlling the conversion of natural gas liquids into natural gas substitutes by high temperature, vapor-phase cracking is developed. The influence of these variables on the yields and compositions of the nongaseous products has also been investigated (46,62), and will be the subject of a separate publication. 8
Present address, New York State Natural Gas Corp., Pittsburgh 22, Pa.
Pyrolysis conditions affect gaseous product yields and compositions
In Figures 7 t o 9 the gaseous product yields and compositions for propane, n-butane, and natu a1 gasoline of 12-pound Reid vapor pressure ( 3 ) (RVP) (see Tables IV and V for analyses) are plotted as functions of reaction time at 1400' and 1500" F. and a t 1- and 3-atm. absolute pressure. Typical cracking data for the 4-t o 5-second reaction time range are given in Table VI. Increases in reaction time resulted in increased formation of methane and decreased formation of higher o l e h s . Hydrogen formation increased slowly and ethane and higher paraffin formation tended t o approach a stationary value, except for the initial distortions due to unreacted feed paraffins. Similarly, ethylene formation tended t o decrease slightly with increasing reaction time, except under conditions where product distribution was affected by the primary decomposition reactions. Increases in reaction temperature had the normal eff'ect of increasing methane and hydrogen formation and decreasing higher paraffin and olefin formation (26-28). At an absolute gasification pressure of 3 atm., the formation of methane and higher paraffins was substantially higher than a t a
1400°F, I ATM.
1 8 r l
I
I
,
4
6
8
16
1400DF, 3 ATM.
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Table IV.
Characterization of Natural Gasoline Feedstock
Specific gravity, 60' F./60° F. Specific gravity API ASTM distillat:on, F. Initial boiling point
a-4
-J
0.768 77.3
-w 2 >-0
20% 30% 40% 50 % 60% 70% 80 %
90%
End point Distillation residue and loss, % ASTM aniline point, C. Carbon-hydrogen weight ratio Gross heating value a t 60° F., B.t.u./lb.
0
rn
100 112 120 128 138 151 166 184 202 231 297 4.0 136.2 5.2 20,500
10%
",
2
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6 *IS
2
4
6
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1500°E, 3 ATM.
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=
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Table V.
Analyses of Propane and Butane Feedstocks
Composition, mole yo n-Butane IBobutane Propane Ethane Gross heating value a t 60" F., B.t.u./lb.
Propane
Butane
...
...
99.7 0.3 21,670
...
... ...
95.8 4.2
21',310
4
2 n
"0
2
4
6
8
IO 12
R EA CTI ON T I M E , SEC.
Figure 7. Gaseous product yields from propane December 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
2475
PRODUCT AND PROCESS DEVELOPMENT pressure of 1 atm. Hydrogen and olefin formation was correspondingly lower. This was the desired result, as the problem was to produce natural gas substitutes-that is, gases of maximum methane and ethane content. The ethane-ethylene-hydrogen system closely approached equilibrium under the more severe cracking conditions for all the three feedstocks used. Under conditions giving incomplete conversion of the feed, particularly at 1300" F. and 3-atm. gasification pressure (Q),deviations from equilibrium were greater, indicating possible influence of the primary cracking reactions. Equilibrium for the propane-propene-hydrogen and butanebutene-hydrogen systems when feeding propane and n-butane, respectively, was not established whenever the primary propane and butane decomposition reactions were incomplete at the shorter reaction times. The secondary reactions were apparently not greatly affected by large concentrations of unreacted propane and n-butane, as shown by the close agreement of propene and butene contents of the product gases at equivalent cracking conditions (Table VI). The limited accuracy of the mass spectrometer analyses for the small concentrations in which species of higher molecular weight were present at the more severe cracking conditions also caused considerable scattering of experimental equilibrium constants. Feed properties affect gasification yields
For specific cracking conditions in the absence of feed hydrogen, the carbon-hydrogen ratio of normally liquid feed hydrocarbons largely determines the weight distribution between gaseous and nongaseous products and thereby the gas-making value of natural gas liquids or petroleum oils. The lower the carbon-hydrogen ratio, the higher the potential gas yield, since the gaseous products consist of hydrocarbons of high hydrogen content and hydrogen, while the nongaseous products consist of hydrocarbons of low hydrogen content and coke (28, 93).
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The gasification yields of propane, n-butane, and 12-pound RVP natural gasoline, unlike feedstocks of higher molecular weight, were greatly affected by feed properties other than carbon-hydrogen ratio. At reaction times from 1 t o 4 seconds, particularly at 3-atm. gasification pressure, the primary cracking reactions of the feed hydrocarbons were incomplete. This resulted in abnormally high concentrations of propane and butane in the product gases for these two feeds. The yields of the major gaseous cracking products were correspondingly lower, as illustrated by the results for 12-pound RVP natural gasoline. For this feedstock, the unreacted hydrocarbons (paraffins) were found in the liquid products (Table VI) and did not affect the gaseous product distribution. The reason for nonconformance of the hydrocarbons of lower molecular weight a t short reaction times with well-established correlations of gaseous product yields and compositions based solely on carbon-hydrogen ratio and pyrolysis conditions ($7, 88, 33) appears to be the relative slowness of the primary cracking reactions-i.e., those specific to the feed hydrocarbon-in comparison with the secondary reactions that normally determine gaseous product composition (16). This is due to a general increase of carbon-carbon and carbon-hydrogen bond strengths with decreasing molecular weights for each homologous series ( 4 6 ) . While the gases produced in high temperature, vapor-phase cracking contain valuable petrochemicals, the utility gas industry normally credits them only with their fuel value, expressed in terms of the heat of combustion recovery as stable product gas per unit quantity of feed, or as percentage of the heat of combustion of the feed. When gasification occurs in the presence of a carrier gas with fuel value, the net recovery of heat of combustion is used. For normally liquid feed hydrocarbons these recoveries a t constant reaction temperature tend t o increase rapidly with reaction time t o a maximum value and then very slowly approach a minimum value determined by the ultimate carbon-hydrogenhydrocarbon (primarily methane) equilibria and the carbon-
14 12
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yields
from
Figure
6
4
REACTION T I M E , SEC. Figure
2476
8.
Gaseous product n-butane
9.
0 2 4 6 81012 8 IO 12 REACTION TIME, SEC
6
Gaseous product yields from natural gasoline
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 12
PRODUCT AND PROCESS DEVELOPMENT
Table Vi.
nipfins - __._ ..-
Effects of Feedstock, Reaction Temperature, and Pressure on Product Composition Propane
Feedstock Reaction temp., a F. Pressure, atm. Reaction time, sec. Gaseous products Wt. % of feed SCF/lb. SCF/lb..ofof feed Composition, mole yo Methane Ethane Propane Butanes Pentanes Ethylene Propene Butenes Pentenes Butadienes Pentadienes Benzene Toluene Hydrogen Acetylene Inerts Liquid products Wt. 97, of feed Compbsition, wt. % Distillate, 0-200° C . Benzene Toluene Xylene Ethylbenzene Co+ aromatics Styrene
0.99G 4.45
3 05 4 23
95.5 15.91
92 8 15 26
80.2 17.56
80 1 17 46
36.4
41.5
47.6 2.6 0.2 0.1
58.4 5.1 0.3
2i :4
16:o 0.9 0.1
7.8 6.4
4.5
6.5 0.1
23:7 7.0 0.3 0.1 0.9 0.2 0.4 0.1
Trace 0.1
22.7 7.2 0.1 0.1 0.4 0.4 0.6
Trace
1.0
1.6
0.5
0.1
..
0.4
18 0
22 2
12.9
17,4
28.8 5.7 2.0 1.5
21.7
7.6 14.6 6.9 5.9 10.0 13.7
24.6 9 7 0.9 1.2
0:2 0.1 0.7
Trace
18.1
8.0 0.8
0.5
5.1
6.1
3.4 7.4 0.6 19.5 5.9 20.1 0.75 99.0
4.8 2.7 0.5 21.5 8.7 24.7 0.55 102 9
hydrogen ratio of the feed. At severe cracking conditions this minimum value is approximately equivalent to the gaseous hydrogen yield obtainable by complete decomposition of the feed hydrocarbon. At increasing reaction temperatures, optimum recoveries are reached more rapidly, and conversely, the approach t o the approximately constant minimum recoveries is also more o rapid. For normally gaseous feed hydrocarbons, 1 0 0 ~ recovery occurs a t zero reaction time and the minimum recovery is approached without the occurrence of an optimum. These trends are illustrated in Figure 10 for propane and natural gasoline feeds with plots of the percentages of gross heat of combustion recoveries a t 60" F. us. reaction time a t various constant gasification pressures and temperatures, The reduction of heat of combustion recovery with increasing gasification pressure corresponded to a decrease of conversion t o gas and an increase in liquid products (Figure 11and Table VI).
0.2 0.4
Trace
1 .o
8.8
1.4 13.3 4.0 13.8 0 31 102 3
73 4 15 70
5.8
0:4
24.8 13.2 1.1 1.6 6.2 5.1 8.0 0.6 19.9 5.7 13.8 0.14 99 7
7.0 3.1 9.0 0.27 99.8
7.6
Trace 1.0
Natural Ga3oline
8.4
4.1 11.8 0.6 18.5 6.4 13.8 0 42 101 4
1400 0.992 3.15 3.91 4.89
1500 0.994 3.11 4.19 4.91
68.0 14.46
69.8 11.89
55.6 9 76
63.2 13.48
57.6 12.38
56.8 4.6
40.7 5.5 0.5 0.1
47.8 8.0 0.3 0.G 1.2 19.1 5.5 0.2
48.2 2.8 0.3 0.2
56.4 5.1
1500 1 02 3.19 4 26 4.62
48 5 2.6 0.3 0.3 0.1 21.0 0.9
46.0 7.9 0.3 0.1 1.3 22.0
1.6
2.6 11.1 12.8 9.0 14.0 9.8
40.2 5.1 0 5 2.2 0.1 26.9
83 6 14 30
0: 1
6.8
19.0
89 1 I4 74
12.7
Trace
4.0
2.7
1400 1.02 3.12 4.21 4.17
0.i 0.3 0.4 0.1 14.2
0:7 0.1 0.6
23.8 0.4 1.4
12.7
0.8
..
0.1
..
16.3
Paraffins Distillate, 200-300" C. Distillate, 300-355' C. Residue and loss Solid products, wt. 3' % Material balance, %
Butane 1500
1400 1.00 3.07 4.21 4.59
0.1 0.5
0:3 0.1 14.9 0.9 0.1 0:3 0.1 0.9
0.6
28.3 6.3 1.3 i:2 0.4 0.5
0.6
0.5 0.2
0.1 0.1 0.2 14.4 1.2 0.1
23:5 0.8
0.1
0:s
0:2 0.1 0.7
13.6
0 :1
..
0.1 0.7 0.1 22.5
..
i:1
0:1
i:6
20 8
26.3
29.8
39.6
31.5
39.6
26 2 4.6 17 0.8 6.4 3.8
24.2 5.7 3.9 0.4 4.5 2.8 3.9 0.5 23.1 10.1 20.9 2.30
9.0 13.8 3.3 2.4 7.8
14.7 5.9 1.3 0.1 2.6
26.0 9.3 1.1
16.2 5.3 0.5 0.5 4.6 2.0 3.5
0.1
0.7 0.1 21.7 1.3 1.8
6.0 0.5
19.8 7.7 22.5 1 37 95 8
Trace
20.9
Trace 14.6
95 6
7.7 7.8
22.5 15.2 2.6 7.9 0 29 99 9
0.3
Trace
1.5
7.1 42.1 10.3 3.2 11.2 0 45 95.7
Trace
19.8
0.8
6.9 6.2 2.0 0.5 21.1 8.3 17.8 0.49 95.2
10.8
18.6 6.3 31.7 2.47 99.7
The effects of pressure when feeds of higher molecular weight are gasified are normally due only to suppression of gaseous olefin formation in favor of gaseous paraffin formation and of liquid and solid polymerization products. I n this study of paraffin feeds of low molecular weight, pressure was also found to suppress primary decomposition reactions. This is illustrated by the higher paraffin contents of the liquid products from natural gasoline a t the higher gasification pressure (Table VI). Product gases approach complete substitutability for natural gas
The percentage substitutability of the product gases for highmethane natural gas was determined on five critical range burners adjusted on the base gas to rated heat inputs a t a gas pressure of 4.4 inches of water column (SI,42). Average port
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1500°F, 3 ATM.
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80 70 60 50
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8 1 0 1 2
0
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R E ACTI ON T I M E , S E C .
Figure 10.
Effect of operating conditions on heat of combustion recovery in gas
December 1955
Figure 11. Effect of carbon-hydrogen ratio of feed on heat of combustion recovery in gas
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
247'1
PRODUCT AND PROCESS DEVELOPMENT FEEDS: P R O P A N E , N BUTANE, NAT. GASOLINE OPER. COND.: 1300-1500"~, 1.4-14 S E C O N D S
I ATM.
3 ATM
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cn
a
80
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v)
a
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1000
1000
1200
1400
1200
1400
GAS
H E A T I N G VPILUE, B I W S C F
1600
1600
Figure 12. Product gas composition as function of heating value and gasification pressure
100
80
a
60
5 loadings ranged from 15,900 to 22,000 B.t.u./hr.-sq. inch. Product gas substitutability was defined as the maximum per cent substitute gas in substitute gas-high-methane natural gas mixtures which gave satisfactory performance on these burners a t primary air shutter adjustments on high-methane natural gas giving moderately soft (-2), normal (0), and moderately hard (4-2) flames in accordance with the American Gas Association code for describing flame appearance (2, 60). Primary aerations corresponding to these adjustments ranged from 28 to 87y0 of theoretical air requirements. The product gases with heating values above 1000 B.t.u. per standard cubic foot were modified by dilution with a simulated combustion gas consisting of 80% nitrogen-20% carbon dioxide, to a heating value and specific gravity t h a t would give the same heat input factor (1) as high-methane natural gas of 1020 B.t.u., per standard cubic foot and 0.6 specific gravity (air = 1). The gases were then tested at the same pressure as the adjustment gas and occurrences of transient flash back on turndown and turnoff, of blowoff and of yellow tipping were noted for each burner a t each of the three primary air shutter adjustments. If necessary, natural gas was then added in 15% increments until satisfactory performance for all burners a t each of the three air shutter adjustments was obtained. I n analyzing the results, the group of burners was treated as a single, very critical, test burner, so when one burner failed the gas was considered unsatisfactory. The compositions of the product gases a t constant gasification pressure could be correlated empirically on the basis of their heating value in the range of 900 to 1600 B.t.u. per standard cubic foot, independent of the feed type and reaction time and temperature (Figure 12). The heating value, in the range where the product gases do not contain very large quantities of unreacted propane and butane, is a more accurate parameter of pyrolysis severity and composition of product gas than the t W ' 8 function used elsewhere in this article for feeds of higher molecular weight. Thus, the only variables which had to be considered in the determination of the combustion performance of the product gases relative t o high-methane natural gas, were the diluent-free heating value and gasification pressure, as the test procedure eliminated all effects other than gas composition (burner heat inputs and primary aerations were maintained constant by dilution with sufficient quantities of inerts). I n Figure 13, the maximum substitutabilities of the product gases for high-methane natural gas are given as functions of these variables for each of the three primary air shutter adjustments. The soft (low primary aeration) adjustment appeared to be most critical, although complete substitutability was approached a t 3-atm. gasification pressure
2478
1100 1300 1500 1700 DILUENT- FREE SUBSTITUTE GAS HEATING VALUE, BTU./SCF
40900
Figure 13. Effect of gasification pressure on maximum substitutability for high-methane natural gas
up to 1000 B.t.u. per standard cubic foot heating value. The substitute gases produced a t the higher pressure gave consistently better performance due t o their higher methane and ethane content (the major natural gas components). At the normal and a t the hard (high primary aeration) adjustments the gases produced a t 3 atm. gave 88% or higher substitutabilities over the entire heating value range tested. The limiting condition a t the soft adjustment was always yellow tipping for one or more of the critical burners, while a t the hard adjustment the limiting condition was always flash back on turndown for one or more of the critical burners. Conclusions
The results of this study of high temperature, vapor-phase cracking of natural gas liquids (propane, butanes, and natural gasolines) indicate that natural gas substitutes can be produced a t a gasification pressure of 3 atm. and a reaction temperature of approximately 1500" F. in the range of 4 to 5-second reaction time, and a reaction temperature of approximately 1400" F. in the range of 9- to 14second reaction time, without excessive carbon formation. Gas yields and heat of combustion recoveries as stable product gas decrease with increasing carbon-hydrogen weight ratio of the feed, and nongaseous product yields increase correspondingly. Gas compositions when correlated on the basis of product gas heating value and gasification pressure are independent of feedstock properties except when significant quantities of unreacted propane and butane are present at short reaction times. Within these limitations, the influence of reaction time and temperature appears to be adequately represented by the product gas heating value, which becomes an index of severity of cracking. Increasing gasification pressure results in improved substitutability of the product gases for natural gas due to the increasing production of methane and ethane and decreasing production of hydrogen and olefins. However, these benefioial results of increasing gasification pressure are partially offset by increased formation of liquid products of high molecular weight and of carbon, and decreased total conversions to gas.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 12
,
