Flame-Stability Limits of Ethylene, Propane, Methane, Hydrogen, and Nitrogen Mixtures JOSEPH GRUMER, MARGARET E. HARRIS, AND HAROLD SCHULTZ U.S . B u r e a u of Mines, P i t t s b u r g h , P a .
U
primary air us. B.t.u./hour/sq. inch ( 1 ) . Diagrams such as Figure 1 can be obtained for any gas or combination of gases by measurement in the laboratory. However, it would be most time-consuming to make such diagrams for all possible mixtures of fuels. Simple means of representing all possible combinations of fuels are obviously needed. The flame-stability diagram for propane (98.6’3,), and incidentally for fuels consisting of propane plus air, is given in Figure 2. The flame-stability diagram for hydrogen ( 99.7Y0) can be constructed from Figure 4 for flash back and Figure 5 for blowoff. The flame-stability diagram for 100% methane is given by Grumer and Harris ( 4 ) and that for natural gas by Grumer, Harris, and Schultz (6). The diagram for pure carbon monoxide has not been determined, because that fuel behaves very differently when absolutely pure and in mixtures. Carbon monoxide is rarely available in the pure state. The gases listed above are the only single-component combustible fuels needed for the present discussion.
NDER a cooperative agreement with the American Gas Association, the Bureau of Mines has been measuring and systematizing the combustion characteristics of fuel gases used by gas utilities, and, in particular, the flash-back and blowoff characteristics of single-component gases (hydrogen, carbon monoxide, methane, ethylene, etc.) and all likely mixtures of these gases. (The yellow tipping of fuel gases is currently being investigated.) The result is a body of useful information that can guide the selection of exchangeable communitywise fuel gases and aid in the design of burners. The first gases considered were methane, hydrogen, and carbon monoxide and mixtures of the coke-oven type. In an earlier paper (4)a concise graphic method was presented for recording experimentally determined flame-stability limits of binary mixtures of methane, hydrogen, and carbon monoxide, and a semiempirical method was given for calculating flame-stability limits of their tertiary mixtures. The present paper extends this work to flash-back and blowoff data for ethylene, propane (propane plus air), and hydrogen; the binary mixtures of ethylene, propane, hydrogen, and nitrogen; and multicomponent mixtures of the oil-gas type.
BINARY MIXTURES
SINGLE-COMPONENT FUELS
The flash-back and blowoff limits of a fuel burning in free air may each be represented by a single curve of “critical boundary velocity gradients,” g p or gB ( 7 , 8), versus fuel-air composition in the burner, expressed as the fuel-gas concentration “fraction of stoichiometric,” F (9,3, 6). These two curves comprise the flame-stability diagram of the fuel. For example, Figure 1 is the flame-stability diagram of ethylene (99.7%). The two curves of the flame-stability diagram separate three regions of flame behavior on b u r n e r s o n e in which flames flash back, a second in which flames are stable, and a third in which flames blow off. Such a diagram is characteristic of the fuel gas and, within the wide applicability of the theory, does not depend on any test burner or group of burners. Earlier papers have described the theory underlying this type of diagram and its application in predicting burner performance with exchanged gases (2,6,6,IO),as well as its use in predicting flash-back and blowoff limits of a particular burner, expressed in the gas-industry units of per cen
Interchangeable fuel gas and burner elem ents
. , .command
much gas industry a n d community interest
These studies on flash back and blowoff
will provide information to guide selection of
suitable
gas
and
burner elements
The simplest method of representing binary mixtures assumes that the flame-stability limits correspond t o weighted averages of the single components making up the mixture. g Y + L + ~ + , = n,gu
+ ?Lagl + naycr+ . . .
(1)
where g is equal to the flash-back or blowoff gradient and n is equal to the fractional volumetric concentrations of each coniponent. Values of gl/, gar etc., can be read from the representative flame-stability diagram of the type shown in Figure 1. Mixtures of alkanes and alkenes, such as methane, ethane, propane, butane, and ethylene, follow this rule. For example, Figure 3 is the flame-stability diagram of a mixture containing 79.4% methane, 2O.6Y0 ethylene. Comparison of experimental points in Figure 3 with curves calculated from Equation 1 and the flame-stability diagrams for methane and ethylene shows excellent agreement. A second mixture containing 78.6’% ethylene and 21.4% methane showed equally good agreement between experimental data and predicted curves based on Equation l. However, not all binary mixtures follow Equation 1. Explanations for the exceptions must be sought in the kinetics and mechanisms of combustion. Some of these exceptions have been discussed in previous papers (4,9, 2 1 ) . In these instances, the graphic method presented by Grumer and Harris ( 4 ) is applied. This method consists essentially in the following: Flame-stability diagrams were measured for a number of mixtures of two single gases, ranging from 0 to 100% of each. The data obtained were used to construct “composite flame-stability diagrams” for binary mixtures such as those shown in Figures 4 and 5, which summarize the flash-back and blowoff gradients,. respectively, for all mixtures of ethylene and hydrogen. Each of these two graphs consists of a family of curves along which fuelair composition F , expressed as a fraction of stoichiometric fuelgas percentage, is constant. Each curve is a pIot of critical
1760
I
September 1955
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
1 Blowoff
1761
Stable flame region ~.
6,000
___-
A’
I I
I
Y’
i
I
I
2
6000
5
4000
>.
9
LLI >
&
< n z
2,000
3
g
1,000
-I
E
0
600 400
200
n Figure 1.
4 8 12 16 20 24 28 GAS CONCENTRATION. FRACTION OF STOICHIOMETRIC
32
Flame-stability diagram for YY.7% ethylene, 0.2q;’, butene, and 0.1% propene
boundary velocity gradients for either flash back or blowoff versus ratios of ethylene to hydrogen and is entirely comparable to the composite flame-stability diagrams given by Grumer and Harris ( 4 ) . From 0 to 50% hydrogen, the ratio plotted as the abscissa is Hz/CzHd, and from 50 to 100% hydrogen, it is C2HdHz. This is done to avoid a value of infinity. Figures 4 and 5 can be used to draw the flash-back and blowoff curves of a particular ethylene-hydrogen fuel or of pure ethylene or pure hydrogen, by taking the ordinates on each F curve corresponding to the desired hydrogen-ethylene ratio and plotting these ordinates (gradients for flash back or for blowoff) against the F values. Similarly, Figures 6 and 7 are for the binary system of propanehydrogen and Figures 8 and 9 are for the binary system nitrogenhydrogen. This graphic method is applicable to any binary system of gases. ADDITION OF NITROGEN TO BINARY MIXTURES
The simplest concept of the addition of nitrogen to a binary mixture is to consider it as a diluent. This procedure has proved very successful, probably because of the high nitrogen content of all air flames. For example, Figure 10 is the flamestability diagram for a mixture of 62.5y0 methane, 22.2% hydrogen, and 15.3y0nitrogen and compares experimental points with predicted curves. This is equivalent to treating nitrogen as one of the fuds in Equation 1 and assigning a value of aero to the critical flame-stability gradient for nitrogen. In this instance, the fuel is assumed to consist of two identities-the binary complex (hydrogen plus methane) and (nitrogen). The flashback and blowoff gradients for the hydrogen-methane complex were taken from Figures 2 and 3 of Grumer and Harris (4). Experimental flash-back gradients of fuels consisting only of carbon monoxide, hydrogen, and nitrogen were considerably lower than predicted on the assumption that the fuel consists of the complex (hydrogen plus carbon monoxide) and (nitrogen), or the complex (hydrogen plus nitrogen) and (carbon monoxide). Experimental blowoff gradients were adequately matched by .values calculated on the basis of the first of the two alternatives. This exception does not impose a severe operating limitation as
100
.4
8 12 16 20 24 28 GAS CONCENTRATION, FRACTION OF STOICHIOMETRIC
0
32
Flame-stability diagram for Y8.6yo propane and 1.4% butene
Figure 2.
I
B0,OCO
1
+- , Blowoff
10,000 8000
I
1 /
,.
, ;
I Stable flame region ~
1
I
Tube diameter cm
4 35a
2,000
294
1,000
800 600 400
200
100
0
Figure 3.
4
.8 12 16 20 24 GAS CONCENTRATION FRACTION OF STOICHIOMETRIC
28
32
Flame-stability diagram for 79.4y0 methane and ZO.6Y0 ethylene
Comparison of calculated curves and experimental points
gases consisting of only carbon monoxide, hydrogen, and nitrogen, which are of the producer and blue gas type and are generally mixed with other fuels before going into the gas lines. Tests have shown that, in these more complex mixtures, nitrogen behaves as a simple diluent. Wherever nitrogen is a constituent, it has been treated as a simple diluent. However, carbon dioxide, when present in excess of about 10 to l5%, depresses flame-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1762
I
0
I
H 2, percent
P '7"
100
78.4
446
C
H
55.3
4,
percent
Vol. 47, No. 9
H * , percent 44.6
21.6
66.8
80 91.4 30
I
66.8
80 91.4100
33.1
20 8.5.0
I Ill
100 0
0.2 0.4
0.6
0.8
1.0
0.8 0.6 0.4
0.2
Hz
Figure 4. Composite diagram of critical boundary velocity gradients for flash back of ethylene-hydrogen fuels
Figure 5. Composite diagram of critical boundary velocity gradients for blowoff of ethylene-hydrogen fuels
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
September 1955
I
0
I
17.4
H 2, percent 44.6
74.5
I
H , percent
17.4
74.5 a9 100
44.6
c 0
1763
1
H
8,
percent
I
89100
2oo
(F= -fraction
100 0
Figure 6. Composite diagram of critical boundary velocity gradients for flash back of propane-hydrogen fuels
0.2
G ~ cokenttation,' S
of stoichiometric)-
I
F=
I
0.4 0.6 0.8 1.0 0.8 0.6
/ 0.4 0.2
0
Figure 7 . Composite diagram of critical boundary velocity gradients for blowoff of propane-hydrogen fuels
INDUSTRIAL AND ENGINEERING CHEMISTRY
1764
H 2 , percent 62.4
I
H * , percent
‘1
I
N 2 , percent
59.8
49.6
37.3
Vol. 47, No. 9
0
59.8
62.4
1
571N 49.6
2,
perynt
37.3
I
IT 0
2,000,000
1
Figure 8. Composite diagram of critical boundary velooity gracients for flash back of nitrogen-hydrogen fuels
Figure 9. Composite diagram of critical boundary velocity gradients for blowoff of nitrogen-hydrogen fuels
MULTICOMPONENT FUELS
stability gradients more strongly than the same percentages of nitrogen. This ma be attributed to the greater heat capacity of carbon dioxide. G o attempt has been made to cover the range of fuels containing more than 10 to 15% carbon dioxide because such mixtures are rarely supplied to consumers of piped gas. When present in small percentages, carbon dioxide may be treated in the same manner as nitrogen.
The task of measuring the critical boundary velocity gradients of all possible multicomponent mixtures is a forbidding one. An alternative and simpler course was found and reported ( 4 ) . To restate, although taking weighted averages of the flash-back and blowoff gradients of single gases did not satisfactorily yield those of binary mixtures, it seemed likely that the gradients of binary
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
September 1955
1765
1w,aoo 80,000 60,000 40,000
z 8
I10000 +5 8,000
9 LL
6,OW
4000
s Y
s
2000
3
P 2
10W 800
8
600 400
200
1
100 4
Figure 11.
Figure 10. Flame-stability diagram for 62.5y0 methane, 22.2% hydrogen, and 15.301,nitrogen
Composition,
%
Methane Ethane ... Ethylene 33.4 Propane Propylene Butane Butylene Hydrogen 15 2 Carbon monoxide Nitrogen 14 0 Carbon dioxide Stoich. fuel gas 10 3 Specific gravity 0 689 Heating value, B.t .u./ cu. ft, 967 0 Treated as methane. Treated as ethylene.
Fuel 2 29.1
... 28.4 ...
Fuel 4 42.6 2.2a 18.1 1.90
....
...
0.2
11.8 ....
12.5
10.6
27.0
....
26 2 22 1 0.26
...
....
7.6 0 875
13 3 0 . 0
Fuel 3 32.1
...
.
I
0.28 O.lb
.
... ...
11.8 0.726 825.0
17.0 9.1 3.4 5.2 10.8 0.686 911.0
FFe
Table 11.
Flame-stcibility diagram for oil-gas 1
+
0
37.5
...
... ...
...
...
17.5 3 9 13 3 7 4 12.5 0.699 778.0
The authors wish to acknowledge the able assistance of Valeria
R. Rowe in gathering some of the data in this paper.
Listing of Complexes for Use in Equation 1
++
+
ACKNOWLEDGMENT
Flash-Back Blowoff Figures Complexes Complexes Used (CZH4 HI) and (CH4) (CHI HI) and (C2H4) ( 1 , 4 ) U 3 (1, 3)b 2 (C2H4 f H P )and (CH4) and (C3Hs) (CHI Hz) and (CsHs Hz) and (CZH4) (1, 4, 6, 7 ) a , (1, 3)b 3 (CZH4 Hz) and (CH4) (CH4 4- He) and (CZH4) ( 1 , 4)=, (1, 3)b 4 (CeHa He) and (CH4 CO) (CHI HI) and (CHI CO and ( C I I I ~ ) (1, 4)a, (3, 6, 7 ) a 5 (CeH4 He) and (CH4 CO) (CHa Hz) and (CH4 CO] and (CZH4) (1, 4Ia, (3, 6 , 7 ) b I n all cases nitrogen and carbon dioxide are treated as diluents. a Figures in this paper. b Figures in ( 4 ) .
+ ++ +
+
+
20.4
fuels and some single fuels could be combined to give the gradients of multicomponent fuels. Designating the single constituents of the fuel as a, j3, y , etc., and possible binary complexes for the fuel as y, z, etc., a limited number of combinations exist for use in Equation 1. Emh of these combinations has been tried and compared with experimental data for multicomponent fuels. The proper choice h a become known empirically in this manner. Knowing the suitable complexes, the flash-back and blowoff gradienb for the binary complex of interest are obtained from the appropriate composite flame-stability diagram, and the flash-
Fuel Gas No. 1
32
back and blowoff gradients of single co-nponent constituents of interest are obtained from their proper flame-stability diagrams. [In the case where a component 01 is to be distributed between components 13 and y to yield 01 B = y and a y = z, the disy ) and tribution of 01 is made on the basis of the ratios f f / ( f f y/(P y ) . These values are substituted in Equation 1 for Bu, g*.l A number of multicomponent fuels have been used t o develop and test the above procedure over a range of likely compositions. Some of these fuels are listed in Table I. Pertinent data bearing on the selection of complexes to be used in Equation 1 are given in Table 11. Figures 11, 12, and 13 indicate the accuracy of these calculations for fuels of the oil-gas type-namely fuels 1, 2, and 3 of Table I. Experimental points and calculated curves are shown in Figures 11 through 13. The adequacy of these calculations for fuels of the coke-oven type is shown by Grumer and Harris ( 4 ) . The results with more complicated fuel mixtures are shown in Figures 14 and 15. Reference should be made to fuels 4 and 5 in Tables I and I1 when examining these figures. [For fuels consisting mainly of ethylene (5oyOor more), other complexes are needed for flash back and blowoff-namely, (ethylenehydrogen) and (methane) or (methanecarbon monoxide) as the case may be.]
Composition of Fuels
Fuel 1 37.4
28
37.4 % methane, 33.4% ethylene, 15.2 ?hhydrogen, 14.0% nitrogen Comparison of calculated curves and experimental points
Comparison of calculated curves and experimental points
Table I.
8 12 16 20 24 GAS CONCENTRATION, FRACTION OF STOICHICMETRIC
++ + +
+ + +
1766
Vol. 47,No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
z = ; -
80 OCO
AI
60 000
40 OW
I
20000
0"
E
.:
z
10000
80W 6000
d
E+
4000
& 2
2000
" s 2 3
: 1000 5 800 c
5
600 400
200
100
0
Figure 12.
4
8 12 16 20 24 GAS CONCENTRATION FRACTION OF STOICHIOMETRIC
Flame-stability
diagram for oil-gas 2
1
I
I
1
1
I
I
Figure 13.
8 12 16 20 24 GAS CONCENTRATION FRACTION OF STOICHIOMETRIC
28
32
Flame-stabili t y diagram for oil-gas 3
32.1% methane, 28.4% ethylene, 12.5% hydrogen, 27.0% nitrogen
29.1 % methane, Z6.2y0 ethylene, 22.1 % propane, 11.8% hydrogen, 0.2% propene, 10.6% nitrogen Comparison of calculated curves and experimental points
1,000,000 1
4
32
28
Comparison of calculated curves and experimental points
I
600,000
400,O00 200,000
40,000 20,000
.
I
Blowoff
e m , '1 / I'. i
10,000 ---c
4,000
4,ooo~---$---~j
-
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+----{++{
2,000
~
Tube diameter
1,000
600 400 200
200
100 0 .4 .8 1.2 1.6 2.0 2.4 2.8 32 GAS CONCENTRATION, FRACTION OF STOICHIOMETRIC
100
Figure 14. Flame-stability diagram for fuel 4 42.6% methane, 18.1% ethylene, 17.0% hydrogen, 9.1 9% carbon monoxide, 2.2% ethane, 1.9% propane, 0.2% propene, 0.2% butane, 0.1 % butene, 5.2 9%carbon dioxide, 3.4% nitrogen Comparison of calculated curve8 and experimental points
!
!"--.-
I
Flashback
'
1
1
1 1
1 1
0 4 8 12 16 20 2.4 2.8 32 GAS CONCENTRATION, FRACTION OF STOICHIOMETRIC
Figure 15. Flame-stability diagram for fuel 5 37.SYc methane, 20.470 ethylene. 17.5Yc hydrogcn, 3.970 carbon monoxide, 13.37~nitrogen, 1.451, carbon dioxide Comparison of calculated curves and evperimental points
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1955
NOMENCLATURE
g = boundary velocity gradient, second-’
F = fuel-gas concentration, fraction of stoichiometric n = fractional concentration of a component in a mixture SUBSCRIPTS y = component in a mixture z = component in a mixture F = flash back B = blowoff LITERATURE CITED
Grumer, J . , Gas, 30, 41 (1954). (2) Grumer, J., IND.ENG.CHEM.,41, 2756 (1949). (3) Grumer, J . , “Study of Combustion Chracteristics of Fuel Gases,” Interim Report 1, AGA Project PDC-3-GU, October
1761
(4) Grumor. J., and Harris, A I . E., 1x0. ENG.CHEM.,44, 1547 (1952). (5) Grumer, J., Harris, M. E., and Schultz, H., Ibad., 44, 1554 (1952). (6) Lewis, E., and Grumer, J., Gas A g e , 105, 25 (LIay 11, 1950). (7) Levis, B., and von Elbe, G., J . Ckem. Phys., 11, 75 (1943). (8) Lewis, B., and yon Elbe, G., Trans. Am. SOC.Mech. Engrs., 1948, 307. (9) Reiter, S. H., and Wright, C. C., IKD.ENG.CHEM.,42, 691 (1950). (10) von Elbe, G., and Grumer, J., Ibid., 40, 1123 (1948). (11) Walker, P. L., Jr., and Wright, C., Division of Gas and Fuel Chemistry, 117th Meeting, ACS, Houston, Tex., 1950.
(1)
1951.
RECEIVED for review October 27, 1954. A C C E P l h D February 9, 1955. Resented before the Division of Gas and Fuel Chemistry a t the 126th 1\Ieetlng of the A M E R I C A S CHEMICAL SOCIETY, New York, N. Y., 1954. Research supported by the American Gas Association, Project PDC-3-GU.
Industrial Design D a t a
Critical Properties and Vapor Pressures of Some Ketones KENNETH A. KOBE, HORACE R. CRAWFORD, AND ROBERT W. STEPHEXSON L’niversity of Texas, Austin, Tez.
T
HE chemical engineer frequently finds it necessary to esti-
mate design data from meager or nonexistent information on various important compounds or groups of compounds. The theory of corresponding states, first pointed out by van der Waals, represents a firm basis for estimation and extrapolation of data to give values within reasonable limits of error that are satisfactory for estimation or design purposes. The best known correlation based on the law of corresponding states is the compressibility chart for gases that shows the deviation of the behavior of the gas from ideality. Various workers have presented compressibility charts, the most recent of which (11) has a maximum deviation of about 1% below the critical point, except for the gases hydrogen, helium, ammonia, and water. Up t o T, = 10, the maximum deviation is about 2.5’%. Further use of the compressibility chart and initial properties of the compound has been made to correct various thermodynamic properties, such as enthalpy, entropy, heat capacity, and JouleThomson coefficient for the effect of pressure (6). Fugacities and fugacity coefficients are also expressed as functions of the reduced pressure and temperature of the compound (18). The reduced properties of a compound are being used to estimate from generalized curves such transport properties as thermal conductivity (4) and gaseous diffusion coefficients ( 3 ) . Physical properties such as viscosity (16), liquid thermal expansion coefficient ( I S ) , and latent heat of vaporization (14) can be represented by generalized curves. A survey of critical properties (9) showed that such data are larking or are discordant for many industrially important compounds. A program of work has been started to determine these data and to provide correlations that can be used for groups of compounds containing like functional groups. The first group of compounds selected for study is the aliphatic ketones. DETERMINATION OF CRITICAL CONSTANTS
Kobe and Lynn (9) have discussed the important methods for determining the critical constants of elements and compounds.
Some of these are mentioned here, so a comparison can be made with the method used in this work. P-V-TProperties. A number of laboratories in this country are engaged in determining the P-V-T properties of compounds, particularly hydrocarbons (1). In the course of such work a thorough study usually is made of the critical region, so that the critical values are known with relatively high precision. Such work is time-consuming and these precise values are known for only a relatively small number of compounds. IPATIEFF AND MONROE. The point of deviation of the vapor pressure-temperature relationship from its smooth curve, or the two-phase boundary, was used by Ipatieff and Monroe ( 7 ) to indicate the critical point. When a weighed sample is placed in a constant-volume bomb and heated, the pressure indicated on the attached gage is the vapor pressure as long as two phases are present. When the vapor pressure line reaches the pressurevolume envelope, the line may change slope, depending on whether the isometric is to the left or right of, or on the critical .volume. If the isometric is to the left of the critical volume, the pressure rises rapidly because the bomb has become completely filled with liquid. If the isometric is to the right of the critical volume, the bomb is filled only with vapor, and the pressure rises slowly. If the isometric is directly on the critical volume, the liquid-vapor contents pass directly into the “fluid” region and there is no discontinuity in the vapor pressure curve. Ipatieff and Monroe used a pressure-temperature curve to represent their data. An accuracy of 1 2 ’ C. was obtained for the critical temperature, but the critical volume was indeterminate by this method. GRIFFIN. The method used by Griffin ( 6 ) was essentially that of Ipatieff and Monroe with refinements to give greater accuracy, and allows the determination of critical temperature, pressure, and volume. Griffin plotted the logarithm of the vapor pressure against the reciprocal of the absolute temperature and obtained straight lines which broke sharply a t the two-phase envelope. His critical values for nitromethane were t , = 315’ f
