INDUSTRIAL AND ENGINEERING CHEMISTRY
746
Vol. 46, No, 4
(5) Lucas, R., J . phus. radium, 10, 151 (1939). (6) Mathien-Sigaud, A., and Levasseur, G., Compt. Tend., 227, 196
tion should also prove to be of practical value in forming and molding processes. The aim of this ivork was a qualitative survey of the behavior of polymer melts in ultrasonic fields. To define the effects of ultrasonic energy upon polymers quantitatively, new and morc refined apparatus will be needed.
(1948).
(7) Pohlman, R., 2 , p h y s , ,107,497 (1937). ( 8 ) Rayleigh, Lord, “The Theory of Sound,” Xew P o r k , Dover
Publications, 1945.
(9) Schmid, G., Phys. Z., 41, 326 (1940). (10) Schmid, G., and Beuttenmuller, E., 2. Elelztrochenz., 49, 3% 11943’1. - - ~ , \
(11) Schmid, G., Beuttenmuller, E., and Rief, A,, 1Cunsfsto.f-
LITERATURE CITED
Ani. Paint J . Contention Daily, p. 20 (Xov, 5 , 1949). Bergmann, L., “Der Ultraschall und seine Anwendung in Wissenschaft und Technik,” 5th ed., Stuttgart, Germany. 6 . Hirzel Verlag, 1949. Krause, R., 2. angew Phys., 2, 370 (19501 Lucas, R., Conapt. rend., 206, 827 (1938).
J
Technik, 13, 65 (1943). (12) Schmid, G., and Rommel, O., Z. phgs. Chem., 185A, 97 (1939). (13) iveissler,A , , J . ~ ~phye,, ~ 21, l 171 (1950). . RECEIVED ior review July 3 , lS53. ACCEPTED.January 6 , 1954 AbEti,art of a thesis prcsented at t h e Teclinische Ilochacliille, Darmstadt. Gernia:i:;,
h i a y 1862.
tudies o
hie
PHILIP F. I W R Z Rattelle Memorial I n s t i t u t e , Columbus I , Ohio
T
HIS paper presents the results of an investigation of the stabi1it)y limits of laminar flames on Bunsen burners shielded to exclude ambient atmospheric air. Two met,hods of excluding ambient air were used: Smithells tubes (8) and an annular shield of constantly flowing inert gas. The effect of the size and configuration of the Smithells tube is discussed. Kith unshielded burners it is possible to observe only three stability limits for laminar flames at any given rate of air input: lean blowoff, lean flash bacli, and rich flash back. In the present x-ork shielding devices were used in order t o study the rich blowoff limit also. This is not possible if ambient air is permitted to circulat,e around the burner port. -4mbient air will stabilize all rich flames, even those containing no primary air, by diffusing into the primary air-fuel mixture or into the fuel and stabilizing the flame on the burner port. The shielding devices were retained in the lean blowoff and flash-back studies, that all the data might be taken under similar conditions. The stability limits of various single fuels, including hydrocarbons and hydrogen sulfide, were observed as a prelude to experiments with binary fuel mixtures. Following this, the behavior of binary hydrocarbon mistures wat! studied on burners with both types of shielding devices. The behavior of mixed hydrocarbon fuels is usually predictable at the lean blowoff and the flash-back limits, but at the rich blowoff limits small inhibiting effects are frequently encountered. The experimental data for binary mixtures can often be correlated Kith an empirical combust,ion equation which permits the prediction of the behavior of fuel mixtures, provided the behavior of the components is known. Likewise. if the behavior of a series of mixtures is known, the stability limits of the individual components can be deduced.
ii witer bath held at room tempcrature maintains the mixing chambers at a uniform temperature and also acts as a sump for the water which is circulated through bhe burner-tube jacket, by means of a jet pump. Figure 2 shows several cross sections of the burner used with the Smithells tubes. Air enters through a 1/8-inch tangenti:ii hole into the vortex-generating chamber at the bottom, from which it pasees through a converging nozzle into a cylindrical throat, which is i/( inch in diameter. The change in cross section accelerates the velocity about fourfold. The fuel in injected radially inward from four l/ls-inch circular ports spaced symmetrically around the periphery of the throat. Thus, mixing of fuel and air is extremely rapid and uniform. When required, an inert gas for quenching the flame after flash back can be injected tangentially in the same plane as the air-injection port’. The fuel-air mixture passes from the vortex miser through :I flow-straightening chamber, consisting of a pipe nipple filled witti lengths of l(,t-inch copper tubing, so that the mixture delivered tci the burner is no longer sn-irling but is essentislly in streamlined flow. h brass burner tube 1.75 em. in inside diameter was used. It was slightly more than 50 tube diameters in length, thus erisuring streamline f l o ~at the burner port. The burner tube is cooled with mater as shown. A special outlet for cooling watrr was installed above the main outlet and as near the burner lip as possible to keep the burner rini adequately cooled. The Smithells tube was concentric with the burnrr t’ubc anti was held in place by a ring of Sauereisen cemrnt, Jyhich also mado an effective seal to prevent leakage of air from the atmospherc: into the flame at the baAe of the Smithells tube.
DESCRIPTION OF APPARATUS
Figure 1 shows a schematic layout of the Bunsen burner and its auxiliary equipment. A11 gas-flow rates are metered by means of critical-flow orifices’ Compressed air from commercial cylinders was used to ensure control over the delivery pressure at all times. The two fuels were mixed in a tee-chamber in the flow system. Each leg of the chamber contains a fixed, four-slot vane which causes the gases to swirl, ensuring rapid and complete mixing. As shown in Figure 1, provision is made for mixing three fuels progressively, if desired.
Figure 1. Schematic Layout of Bunsen Burner and Auxiliaries for Flame-Stability Studies
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1954
747
entering the burner under laminar flow conditions-i.e., Reynolds numbers less than 2000. The fuels used were a t least 99 volume % pure. TREATMENT OF DATA FOR BIN4RY IMIXTURES
For a study of the behavior of two fuels and their mixtures on a comparable basis, the rates of flow are expressed in terms of their relative oxygen requirements for stoichiometric combustion. EXther fuel component of a binary mixture may be designated as a base of reference-for example, if ethylene is used as a reference in a mixture of ethylene and propane, the “ethylene equivalent” of propane is 5 / 3 the volume input of propane, because propane requires 5/3 a8 much oxygen for complete combustion as does ethylene , Accordingly, the fuel input as “equivalent ethylene” in a propane-ethylene mixture is:
L.
Fuel input, cc.
=
CzH4,cc.,
+ 5/3C3Hs,cc.
(1)
Similarly, the ethylene equivalent of isobutane is 6 . 5 / 3 , of methane is 2/3, and of hydrogen sulfide is 1.5/3 BURNER PORT,
kl
VORTEX GENERATINO CHAMBER
--r I
D
D
SECTION D-D
Figure 2. Bunsen-Type Burner INERT GAS TO
____c
Two types of Smithells tubes were used: cylindrical tubes with diameters ranging from 3.8 to 7.6 em., and a divergent conical separator which was made by cutting the bottom from a 300-ml. wide-mouthed Erlenmeyer flask. A thin copper plate with a 3/r-inch hole a t its center is placed on top of the separator tube to eliminate back-diffusion of air. In the experiments in which ambient air was excluded from the burner port by inert gas, the burner was modified by adding a jacket for producing an annular screen of inert gas. Figure 3 shows this modification; all other equipment was the same. The flow of inert shielding gas (nitrogen) was held constant a t a rate sufficient to produce a velocity of 30 em. per second in the annulus. The Bunsen flame was ignited by means of an external flame without altering the flow of shielding gas.
SHIELD FLAME
MIXING CHAMBER
FUEL
\
,
-I
1
EXPERIMENTAL PROCEDURE
To determine the stability limits of a fuel, the air flow is established at a low rate, and an increasing amount of fuel is introduced until a stable, lean flame results. The flow of fuel is then decreased by convenient increments until blowoff occurs. This is the point of lean blowoff. The flow of fuel is then increased by increments until flash back occurs. This is designated as the “lean” flash-back point. A decrease of one increment in the flow of fuel again produces a .table flame. By increasing the flow of fuel further, a region of flash back is traversed and a stable flame is again obtained. Decreasing the flow of fuel one increment causes the flame to flash back. This is the point designated as “rich” flash back. Rich blowoff is then determined by increasing the flow of fuel until the flame lifts from the burner port and travels to the hole in the plate covering the Smithells tube. A decrease of one increment in the fuel flow rate again produces a stable flame. This procedure is repeated over a range of air flow rates. When a mixture of two fuels is being studied, the flow rate of the secondary fuel is held constant a t a predetermined rate throughout the experiment, and the flow rate of the primary fuel is varied, as required, during the course of the experiments. The studies reported here were made with the fuel-air mixtures
”R
FLOW-STRAIGHTENING CHAMBER
FUEL-AIR
FUEL
AIR (TANGENTIAL ADMISSION)
Figure 3.
Schematic Diagram of Inert-Shielded Bunsen Burner and Mixing System
To test the compatibility of mixed fuels a t a particular stability level, or air-flow rate, the experimental data are plotted using the volumetric input rates of the component fuels as coordinates. The fuels are compatible if the plot a t a given stability limit, such as lean blowoff, flash back, or rich blowoff, is linear. If the combustion reactions of the two fuels interfere with each other, the relationship is nonlinear and usually close to parabolic. Two fuels are compatible when the following equation is satisfied:
This equation, the derivation of which is given in detail elsewhere ( 3 ) ,is based on work by LeChatelier ( 6 ) ,Payman (7), and Coward and Greenwald (1). PI and VP are the volume flow rates of
INDUSTRIAL AND ENGINEERING CHEMISTRY
748
Vol. 46, No. 4
30
W
5 r1
25
K
2
2c
v)
a W
t J
I5
I-
2
a
z
5 IC q
5
Figure 4. Stabilitj Limits of Propane Flames over Range of Air Rates 1.75-Cm. Bunsen tuhe with divergent SmitlicIls separator
C
400
800
ETHYLENE
Figure 5 .
1200 INPUT,
1600
2000
2400
CC. PER MINUTE
Stabilit) Limits of Ethjlene Flames over Range of Air Rates
1.75-Cm. Bunsen tube with divergent Smithells reparator TYPE OF SMITHELLS SEPARATOR X CYLINDRICAL A CYLINDRICAL CYLINDRICAL v CYLINDRICAL + CYLINDRICAL 0 DIVERGENT CONICAL
2
DIAM,
CM. 3.81 4.13 5.40 6.51 7.62
-
20 W K
n. In K
c”
I5
i
so
p
IC
E?
4
5
0
, 400
800
1200
ETHYLENE
INPUT,
1600 2000
2400
CC. PER MIN.
Figure 6 . Rich Blowoff Limits of Ethylene-4ir Flames 1.75-Cm. Bunsen tube with divergent and c>lindrical Smithells separators
fuels 1 and 2 in a binary fuel mixture a t a selected stability level. VT and T’Z are the volume flow rates of fuels 1 and 2, each burning alone with air, a t the same selected stability level. EXPERIIlEhTAL RESULTS
STABILITY EXPERI\fEATS MITH SIYGLE FCELS. In the f01lowing sections ale presented the results of flame-stability experiments with typical hydrocarbons and with hydrogen sulfide on shielded 1.75-cni. Bunsen burners and the effects on flame stability of Smithells flame-separator tubes of different diametei s and shapes. 4 shox s STABILITY LIMITSOF PROPANE-.~IR F L ~ I I C SFigure .
the conditions for lean blowoff, flash back, and rich blowoff for propane-air mixtures burning on a 1.75-cm. Bunsen tube with a divergent Smithells separator. The figures beside the data points indicate the Reynolds numbers corresponding to the total flow, fuel plus air, through the burner port for the condition. At air rates higher than 20 liters per minute, flash back did not occur with propane flames. The flash-back region at the lower rates is mostly in the area of rich mixtures. STABILITY LIMITSOF ETHYLENE-AIR FLAUES.Figure 5 s h o w the stability curves for ethylene-air flames on the Bunsen tube with the divergent Smithells separator. The behavior of ethylene is seen to be different from that of propane, shown in Figure 4. There was still a considerable range of flash-back compositions in the maximum air rate of 21.5 liters per minute, and a t this maximum rate there is no evidence of an approach to a region of no flash back. IXFLUESCE OF SIZEAND SHAPEO B SMITHELLS TUBEo s STABILITY LIimrs. Tests were made with a divergent Smithells tube after a 3.8l-cm. cylindrical flame-separator t,ube gave unusual results with ethylene flames at rich blowoff. Further ivork on rich blowoff of et,liylene flames was then donc Tvith a variety of cylindrical Smithells tubes ranging in inside diameter from 3.81 to 7.62 cm. Figure 6 shows the results of these experiments. Only the dat,a obtained wit,hthe two smallest: 3.81- and 4.13-cm., cylindrical Smithells tubes show any dcparture from the st,abilit,y curve vhich is obtained for the divergent Smithells tube and the larger cylindrical Smithells tubes. Furthermore, the departure is appreciable only at the lower air rates, where smoky flames are obtained because of thermal cracking reactions that lead t o soot formation. It may be concluded that, if a divergent Smithells tube or a cyliudrical Smithells tube larger than about 5 cm. in diameter is used, the effect of the tube on cracking react’ions is negligible. STABILITYOF ISOBUTA~E-IZIR F1,ailiEs. lsobutane-air flames were studied also on the 1.75-cm. burner. Figure 7 shows that the stability data obtained with this fuel are not affected b s the shape of the Smithells tube. This is different from the findings with ethylene flames, as shown earlier, where rich blowoff is appreciably affected by the size of the Smithells separator.
1 X,*,D O,*,+
f
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1954
I
2s
'
1
SMITHELLS SEPARATOR 3.81-CM-IO CYLINDER DIVERGENT COME
LEAN
a
2 v)
2c
t -1 1--
a a.
z
I:
E
a
IC
5
C
I
200
400
ISOBUTANE
,
600
800
INPUT,
CC. PER M I N .
1000
X Q
749
ISOBUTANE ETHANE
A PROPYLENE -I?ETHYLENE
* 25
K 6 a 20
r
5
I-- I S
3 P
2
2 IO 5
I
'0 400 800 1200 1600 2000 2 4 0 0 F U E L INPUT AB EQUIVALENT ETHYLENE, CC. PER MIN.
I200
Figure 7. Blowoff and Flash-Back Limits of Isobutane-Air Flames 1.75-Cm. Bunsen tube with two types of Smithells separators
Figure 8. Flash-Back Limits of Isobutane-Air, Ethane-Air, Propylene-Air, and Ethylene-Air Flames 1.75-Cm. Bunsen tube with divergent Smithells separator
Methane-air, ethane-air, and propylene-air mixtures gave re25 sults similar to those for isobutane-air mixtures on 1.75-em. Bunsen burners shielded with cylindrical or divergent Smithells tubes. FLASH-BACK PENIXSULAS FOR HYDROCARBON-AIR FLAMES. 2 20 I t is of interest to compare the flash-back limits of some of the L hydrocarbons which were used in the stability experiments. This is done in Figure 8, using data from the 1.75-om. burner with a divergent Smithells tube. It is apparent that for all the hydrcv) carbons the maximum point of the flash-back curves lies on the 6 I5 t -I rich side of stoichiometric proportions, This is in agreement with the results of flame-speed experiments, which show that the aW maximum flame speeds of hydrocarbon-air mixtures lie on the z a rich side also. 2 10 As a preSTABILITYOF HYDROQEN SULFIDE-AIRFLAMES. requisite to proposed studies of the behavior of hydrocarbonIa hydrogen sulfide mixtures on the Bunsen burner, the stability of a hydrogen sulfide flames was studied on the 1.75-em. burner with I 5 the divergent Smithells tube. E a Figure 9 shows the lean and rich blowoff and flash-back limits of hydrogen sulfide-air mixtures on this burner. The maximum flame speeds of mixtures of hydrogen sulfide and air also lie on 0 the lean side of stoichiometric proportions. AIR IN MIXTURE, PER CENT OF STOICHIOMETRIC In Figures 10 and 11 are compared the results of stability experiments with hydrogen sulfide-air and with ethylene-air mixFigure 9. Stability Limits of Hydrogen Sulfidetures on the 1.75-em. burner with a divergent Smithells separaAir Flames tor. Both figures show the decided difference in the behavior of 1.75-Cm. Bunsen tube with divergent Smithella separator these two dissimilar fuels in the same burner; the ethylene tends toward maximum-stability mixtures on the rich side, whereas the hydrogen sulfide tends toward lean maximum-stability mixtures on the burner with a divergent conical Smithells tube. mixtures. It is apparent that the curve for the binary mixture in which propane is the primary fuel parallels the curve for propane-air STABILITY EXPERIMENTS WITH BINARY HYDROCARBOX flames. Likewise, when ethylene is the primary fuel in the MIXTURES mixture, the stability curve for the mixture parallels the curve Figure 12 BLOWOFF LIMITSOF PROPANE-ETHYLENE FLAMES. for ethylene-air flames. FLASH-BACK LIMITSOF PROPANE-ETHYLENE FLAMES. shows the lean and rich blowoff limits for propane and ethylene Figure 13 shows flash-back curves for propane and ethylene flames and flames and for the flames of two typical propane-ethylene
e
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
750 I
i
Vol. 46, No. 4
I
FUEL 0 ETHYLENE 4- ETHYLENE X HYDROGEN SULFIDE 0 HYOROOEN SULFIDE
STbBlblTY CRlTER ION LEAN BLOW-OFF RICH BLOW-OFF LEAN BLOW-OFF RICH BLOW-OFF
+
ETHYLENE
X HYDROGEN SULFIDE
g 25 w K
a
20
B !
J
I4
a a
z
0 FUEL
FUEL INPUT AS EQUIVALENT ETHYLENE, CC. PER Mlk.
I N W T AS EOUIVALENT ETHYLENE, CC. PER MIN.
Figure 10. Blow-off Limits for Ethylene-Air and Hydrogen Sulfide-Air Flames
Figure 11. Flash-Back Limits for Ethylene-Air and Hydrogen Sulfidchir Flames
1.75-Cm. Bunsen tube with divergent Smithells separator
1.75-Cm. Bunsen tubc with divergent Smithells separator
I
I
I
PRIMARY SECONDARY FLOW RATE FUEL FUEL SECONDARY FUEL, CC. PER MINUTE 0 PROPANE + PROPANE ETHYLENE 180 x ETHYLENE
I FLOW RATE
PRIMARY SECONDARY SECONDARY FUEL, FUEL FUEL CC.PER MIN. PROPANE 3- PROPWE ETHYLENE 180 X ETHYLENE 0 ETHYLENE PROPANE 143 I I I / LEAN RICH 0
-
-
g
2:
-
-
--
K
L 2c
z
-
143
/
w
5
-
PROPANE
25
K
20
ln
i2w
B t
-I
ETHYLENE
-
t -1
I!
ca
sa
z
E E
E IC
a
a
15
IO
! 5
(
0 FUEL
INPUT AS EQUIVALENT ETHYLENE, CC.PER MIN. FUEL
Figure 12. Blowoff Limits of Propane, Ethylene, and Propane-Ethylene Flames over Range of Air Rates 1.75-Cm. Bunsen tube with divergent Smithells separator
€or the flames of two typical binary mixtures of these fuels. The curve for the binary mixture in which propane is the primary fuel parallels the curve for propane-air mixtures, and almost converges a t the maximum air rate of 21.5 liters per minute. When ethylene is the primary fuel in the binary mixture, 110 evidence
400 NPUT AS
800 I200 1800 EO00 2400 EQUIVALENT ETHYLENE, CC. PER MINUTE
Figure 13. Flash-Back Limits of Propane, Ethylene, and Propane-EthyIene Flames over Range of Air Rates 1.75-Cm.Bunsen tube with divergent Smithells separator
of convergence is shown, and the curve3 parallel the curves for ethylene-air flames.
STABILITY LIMITS OF FLAMES SHIELDED BY AN ANSULAR STREAN OF ITERT GAS. Stability experiments were carried out with isobutane-ethylene mixtures on a 1.75-cm. Bunsen burner
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1954
-LEAN
to the maximum rate, the slopes are steep and the relationship between air-fuel composition and air rates is essentially linear. Figure 15 shows the percentage of theoretical air in isobutane flames, in ethylene flames, and in the flames of four isobutaneethylene mixtures a t flash back over a range of air rates. The maxima on the “closed” curves appear to occur a t about 85% of theoretical air. Changes in the composition of the fuel mixture do not seem to influence the location of the maxima. COMPATIBILITY OF BIXARYHYDROCARBON MIXTURES. An indication of the combustion compatibility of binary fuel mixtures may be obtained from a plot using as coordinates the volumetric inputs of the component fuels in all mixtures which exhibit a single stability level. If a linear relationship is found, the fuel components are compatible and the conditions of Equation 2 that V,/V? f V& = l are satisfied. If the relationship is nonlinear, there is either an accelerating or an inhibiting effect of one fuel on the other. No accelerating effects have been Bound with hydrocarbon mixtures. PROPANE-ETHYLENE MIXTURES. Figure 16 shows propaneethylene mixtures a t lean blowoff plotted in terms of volumetric inputs for the component fuels a t six different air rates. The curves all appear to be linear-that is, the fuels are compatible at the lean blowoff limit. Similar linear relationships were found when plots were made with the data for the two flash-back limits and for the rich blowoff limit. Figure 17 shows, for an air rate of 17.2 liters per minute, the relationship between propane and ethylene inputs a t the four criteria of stability: lean blowoff, “lean” flash back, “rich” flash back, and rich blowoff. The designation, lean flash back, in Figure 17 is arbitrary and is not strictly correct for all propane flames and for some binary mixtures with propane aa the primary fuel. I t is so designated merely to indicate that it is the flashback point that is obtained when a lean mixture, burning stably, is enriched to the point of flash back by increasing the rate of fuel
4
FLOW RATE FUEl ISOBUTANE
-
ISOBUTANE ETHYLENE ISOBUTANE ETHYLENE ETHYLENE ETHYLENE ISOBUTANE ETHYLENE ISOBUTANE
0
40
CC PER MIN
-
60
80
100
AIR IN MIXTURE,
87
I20
160
140
180
J
200
PER CENT OF THEORETICAL
Figure 14. Blowoff Limits of Isobutane, Ethylene, and Isobutane-Ethylene Flames over Range of Air Rates 1.75-Cm. Bunsen tube shielded with nitrogen to exclude ambient air
which was shielded with an annular stream of inert gas (nitrogen) to exclude ambient air from the environs of the burner port. Air rates up to about 21.5 liters per minute were used. Figure 14 shows the percentage of theoretical air used with isobutane and ethylene and with four isobutane-ethylene mixtures a t lean and rich blowoff over a range of air rates up to 21.5 liters per minute. Ethylene flames at lean blowoff are much leaner than isobutane flames a t the same air rate. At rich blowoff in the region of air flow from 10 liters per minute
PRIMARY
SECONDARY
TABLEI. COMPATIBILITY ANALYSISFOR PROPANE-ETHYLENE MIXTURESAT FOURSTABILITYLIMITS AND ONE AIR RATE
FLOW RATE SECONDARY FUEL
xA 0
ETHYLENE ISOBUTANE ETHYLENE ISOBUTANE ISOBUTANE
-
ISOBUTANE ETHYLENE
I80 404
751
1
.
(Air rate, 17.2 liters per minute VG*, input rate of unmixed fuel a at limit) Mixture Composition, Co. per Minute ZV,/V: VCzH4/V&Er VCaHs VcaHr VCaHa/ V&E?~ Lean Blowoff Limit 532 = V&) o 1.00 0 1.00
489 403 230 143 55
69 180 404 514 631 712 (= V & H ~ )
0 770 698 597 406 143 55 0
V&Hd
0
0.92 0.76 0.43 0.27 0.10 0
Lean Flash Back 1.00
69 180 404 712 830 875 ( = V&H )
0.91 0.78 0.53 0.19 0.07 0
0.10 0.25 0.57 0.72 0.89 1.00
1.02 1.01 1.00 0.99 0.99 1.00
0 0.08 0.21 0.46 0.81 0.95 1.00
1.00 0.99 0.99 0.99 1.00 1.02 1 .oo
0 0 04 0.10 0.22 0.86 0.95 1.00
1.00 1.01 1.01 1.02 1.01 1.01 1.00
0 0.03 0.08 0.18 0.85 0.94 1.00
1.00 1.00 1.00 0.98 0.98 0.99 1.00
Rich Flash Back 928 900 842 741 143 55 0
‘40
60 80 I00 120 140 160 AIR IN MIXTURE, PER CENT OF THEORETICAL
Figure 15. Flash-Back Limits of Isobutane, Ethylene, and Isobutane-Ethylene Flames over Range of Air Rates 1.7j-Cm. Bunsen tube shielded with nitrogen to exclude ambient air
0 69 180 404 1597 1860 ( = V&H,) 1764
1.00 0.97 0.91 0.80 0.16 0.06 0
Rich Blowoff 1101 ( = VX,”*) 1070 1008 885 143
55
0
0 69 180 404 1878 2088 2211 (= V&ar)
1.00 0.97 0.92 0.80 0.13 0.05
0
752
INDUSTRIAL A N D ENGINEERING CHEMISTRY
I
1 5
I000
"0
1200
AIR INPUT, LITERS PER MINUTE P
6.8
A 0
10.0 12.9
200 400 600 BOO IO00 ETHYLENE IMPUT, CC, FER MINUTE
Figure 16. Relationship between Propane and Ethylene Input for Propane-Ethylene Mixtures at Lean Blowoff 1.75-Cm. Bunsen Tube with divergent Srnithelli separator
t
3i
IO00
Vol. 46, No. 4
ETdYLEWE IN?U?$
ee. PER
b%N.
Figure 15. Relationship between Volumetric Inputs of Component Fuels for Propane-Ethylene Mixtures at Stability Limits Air rate of 17.2 liters per minute QQ I.is5-om. Bunsen tuhe with divergent Smithells segarmtor
AIR imapu?, LITERS PER MI#.
+
89
B 11.4 # 14.3 i 0 17.2
I
a E l ci 0 680
I
! in. 400
z w I-4 3
m
a
200
0
ETHYLENE INPUT, C C . E W MIM
Figure 18. Relationship between Inputs of Component Fuels for Isobutane-Ethylene Mixtures at Lean Blowoff
ETHYLENE INPUT, 6G. PER MlN Figure 19. Relationship between Inputs of Component Fuels for IsobutaneEthylene Mixtures at Flash Back
At
rates on 1.75-om. Bunsen tube with divergent Smithells separator
two air
A t five air rates on 1.75-om. Bunsen tube with divergent Smithells separator
showed a linear relationship, these data meet the requirements of flow at a given air rate. Similarly, rich flash back is arbitrarily Equation 2, ZV,/V; = 1. Thus, for these conditions, the comdefined as the flash-back point that is obtained when a stable bustion processes of tho two fuels are compatible. rich flame is made leaner by a reduction in the fuel rate at a given Figure 20 shows that at rich blowoff the experimental data deviate slightly from linearity. This indicates a small inhibiting air flow. It is true, however, that rich flash-back points lie on the effect caused by the isobutane on the ethylene-that is, thc rich side of stoichiometric. The relationships are all linear, presence of isobutane narrows the rich limit more than woiJd be indicating that the combustion reactions of propane and ethylene are noninterfering, based on any of the four criteria, when the expected if the fuels were entirely compatible at the rich stability two fuels are mixed. limit. These data do not satisfy the empirical equation-i.c., < 1. Table I gives values for V C ~ R ~V,& H ~ , V C ~ R ~ / V & RV~ ,C ~ H ~ zva/v; , ISOBUTAXE-ETHYLCKE FLAVES SHIELDED BV AN z!.NNULAi3 VC,H;. T'C~E,/V&H,, and 2Vu/V*,for the fuel mixtures plotted in Fig, 17. It is apparent from the values for 2V,/V*, given in STREAX O B SITE~OGEN. A series 01 mixtures of the same gnscs, isobutane and ethylene, mas studied at six different air m t v , Table I that the behavior of propane-ethyIene mixtures satisfies using a screen of inert gas t o replace the divergent Smithells tube. Equation 2 a t ail four stability limits. ISOBUTANE-ETHYLENE FLAMES SHIELDED BY A DIVEXGENT Figures 21 and 22 show that linear relationships exist at lean CONICAL SMITIICLLS TUBE. Figure 18 shows that a linear relablowoff and a t dash back; the experimental data are essentially identical to those obtained with the Smithells tube. Accordingly, tionship exists betmen the inputs of the component fuels of isobutaneethylene mixtures at lean blowoff, and Figure 19 shows the behavior of mixtures is not dependent on the protective dethat for flash-back data a similar linear relationship was found. vice used, as long as small Smithell3 Lubes of less than 5-em Since the volumetric fuel inputs a t lean blowoff and flash back diameter are avoided.
April 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
753
1000
f 6
800
n
ETHYLENE INPUT, CC. PER MIN. ETHYLENE INPUT. CC. PER MIN.
Figure 20. Relationship between Inputs of Component Fuels for Isobutane-Ethylene Mixtures at Rich Blowoff A t five air rates on 1.75-cm. Bunsen tube with divergent Smithells separator
STABILITY CRITERION A . *LEAN* FLASH-BACK 0 'LEAN" FLASH-BACK
a
x . 'LEAN" FLASH-BACK
Figure 21. Relationship between Isobutane and Ethylene Inputs at Four Air Rates at Lean Blowoff 1.75-Cm. Bunsen tube shielded with nitrogen t o exdude ambient air
AIR INPUT, LITERS PER MIN.
10.0 14.3
k!
Blp 0 0
I-
I
t z W
s Figure 22. Relationship between Isobutane and Ethylene Inputs at Two Air Rates at Flash Back 1.75-Cm. Bunsen tube ahielded with nitrogen to exolude ambient air
Figure 23 shows that a t rich blowoff isobutane-ethylene mixtures are in slight disagreement with the requirements of Equation 2. Values of XV,/V: for the rich-limit mixtures are slightly less than unity, ranging from 0.93 to 0.99. It has already been shown that identical results were obtained with these mixtures on a 1.75-cm. Bunsen burner fitted with a divergent conical Smithells tube. It is concluded that the deviation is real, and is not caused by influences imposed by the apparatus used. CONCLUSIONS AND COiWMENTS
The data presented permit the following conclusions to be drawn:
A shielded Bunsen burner of 1.75-em. diameter is a useful tool for studying rich and lean blowoff and flashback of flames of fuels with low and moderate burning rates. With this burner, the behavior of flames at blowoff and a t flash back is not d e o t e d by the size or shape of the Smithells
Figure 23. Relationship between VGH,/V*Cd% and VGH,,/ V*C,H,, for Isobutane-Ethylene Mixtures at Rich Blowoff 1.75-Cm. Bunsen tube using nitrogen as shielding gas to ' exolude ambient air
tube, provided the wall of the Smithells tube is a t least 1.5 cm. from the burner rim. The maxima in the flash-back curves for hydrocarbon flames lie on the rich side of stoichiometric proportions. This is in agreement with the results of flame-speed experiments. The maxima in the blowoff and flash-back curves for hydrogen sulfide flames lie on the lean side of stoichiometric proportions. This is also in agreement with the results of flame-speed exoeriments carried out earlier (6). The use of cylindrical Smithells tubes 3.8 and 4.1 cm. in diameter abetted cracking reactions considerably in rich ethylene flames near blowoff a t low Reynolds numbers. Cylindrical tubes 5 om. or greater in diameter and a divergent tube did not abet the thermal-cracking reactions in rich ethylene flames. The behavior of propane-ethylene flames a t lean and rich blowoff and in flash back can be correlated with an empirical combustion equation whose requirements are that a linear relationship exist between the concentrations of the component fuels of a binary mixture a t a specific stability level. Agreement with this relationship indicates that propane and ethylene are compatible fuels in a binary mixture regardless of the over-all
INDUSTRIAL AND ENGINEERING CHEMISTRY
754
composition of the fuel-air mixtures. Furthermore, if the Aamestability limits of propane and of ethylene are known, the behavior of any mixture of these two fuels can be predicted. These findings are in accord Kith other studies in this laboratory of the flame speeds of propane-ethylene mixtures ( 2 ) , and the stability of propane-ethylene flames on vortex-type burners (4). Isobutane-ethylene mixtures are compatible a t lean blowoff and at the two flash-back limits. The data for these stability criteria show that the behavior of the mixtures is in agreement with an empirical combustion equation which requires that I;V,/V$ = 1. Accordingly, it is possible to predict the behavior of these mixtures a t lean blowoff or at flash back, provided the behavior of the component fuels is known. Furthermore, if the behavior of one component and of typical binary isobutane-ethylene mixtures is k n o m , the behavior of the other fuel component may be obtained by extrapolation or calculation. The results of rich-limit studies with the divergent Smithells tube and with an inert gas shield are in good agreement and point to the conclusion that the deviations of the behavior of isobutane-ethylene mixtures a t rich blowoff from the requirements that zV,/V: = 1, though small, are real.
A study of the laminar flames of other binary mixtures and with ternary mixtures is continuing. It is believed that this work will lead to a better understanding of the behavior of multicomponent fuel mixtures.
Vol. 46,No. 4
ACKNOWLEDGMENT
This work was done under the sponsorship of the Flight Research Laboratory, Wright Air Development Center. WrightPatterson ,4ir Force Base, Ohio. The author wishes to express his thanks to R. E, Poling and W. B. Thompson for their able and faithful assistance in carrying out the experimental vork, and to J. F. Foster, division chief at Battelle, for his helpful interest and consideration. LITERATURE CITED
(1) Coward, H. F., and Greenwald, H. P., U. S. Bur. Mines, Tech. Paper 427 (1928). (2) Kurz, P. F., Battelle Technical Rept. 15036-2, to TTright Air Development Center, Wright-Patterson Air Force Base, Contract AF 33(038)-12656 (Sept. 28, 1951). (3) Ibid., 15036-3(April 30,1952). (4) Ibid., 15036-9 (Aug. 25, 1952). (5) Kurz, P. F., ISD.ENC.CHEXI.,45,2361 (1953). (6) LeChatelier, H., Ann. mines, 19 ( 8 ) ,388 (1891). (7) Payman, W., J . Chem.Soc., 115,1436 (1919). (8) Smithells, A,, and Ingle, H., Trans. Chem. Soc. (London), 61, 204 (1892). RECEIVEDfor review August 19, 1963. ACCEPTED December 21, 1953. Presented before the Division of Gas and Fuel Chemistry a t the 124th Meeting of the AYERICAK CHEMICAL SOCIBTY.Chicago, Ill.
Vapor-Liquid Equili m- and p-Xylenes in Different Solvents JU CHIN CHU AND 0.P. KIlARBANDA Polytechnic Institute of Brooklyn, Brooklyn, N. Y .
R. F, BROQBS'
AND S. L. WANG2 Washington University, St. Louis, Mo.
T
HE production of xylenes, which were mainly obtained from coal tar prior t o World War 11,has been greatly increased by
catalytic hydroforming. The separation of a mixture of the p and m-xylenes has presented a difficult problem because of the closeness of the boiling points. A number of methods have been reported ( 1 , 2, 4, 5, 9,13, 14, 16-19, 21-25) for the separation of these two xylene isomers. The original objective of the work reported here 'was t o investigate the possibility of employing extractive distillation for the separation of m- and p-xylenes by determining vapor-liquid equilibrium data of the isomers with different solvents. Two different experimental approaches have been used in this investigation. One is to determine the relative volatility of two xylene isomere directly in the presence of an optimum amount of a solvent. The other is t o determine vapor-liquid equilibrium data of each isomer with a solvent in the complete range of concentration, as suggested by Othmer et al. (31). Vapor-liquid equilibrium data of some systems are represented analytically by the thermodynamic correlations proposed ( 7 , 16, SS, 40). From the results of this study it appears that the extractive distillation is not feasible for the xylene separation. CHOICE OF SOLVENTS
The choice of solvent is affected by both practical consideratione, such as its cost, availability, stability, corrosion characteristics, and toxicity, and by the technical feasibility, which includes 1
Present address, Xfonsanto Chemical C o t Et. Louis, Mo.
a Present address, Kansas State College, Manhattan, Kan.
volatility, solubility, azeotropic formation, and selectivity. The eolvent must be appreciably less volatile than the xylene, in order to save the reboiler duty and to facilitate the subsequent recovery of the solvent. Normally, a difference in boiling points of 20" C. is sufficient. The solvent to be uaed, therefore, should boil above 160' C . The solvent should be a t least completely miscible with the xylene isomere, without the formation of an azeotrope. Empirical correlation ( d 6 ) and experimental investigations (6), as well as literature data, are used to guide the selection in this respect. The selectivity of the solvent is believed to be due to some type of molecular association with one of the isomers to form nonideal solutions in a seIective degree (I, fd). One type of association is so-called hydrogen bonding ( I d ) , the association of two molecules with a hydrogen atom as bridge. If the xylene isomers could be considered t o contain the active hydrogen atom as electron acceptor, compounds classified by Ewe11 et al. (12) as class I1 (alcohols, acids, phenols, primary and secondary amines, oxides, etc.) and Class I11 (ethers, ketones, aldehydes, esters, tertiary amines, nitro compounds, etc.) would be more likely to be promising selective solvents. The second type of association is attributable to the intermolecular forces of attraction due t o electrical moment of the molecules, whether permanent or temporary (94). The dipole moment of p-xylene is 0 (28); that of m-xylene is 0.37 Debye (28). The published correlations of the dipole moment with selectivity, however, are not conclusive enough t o predict or rule out the choice of the solvent. Although a method of preliminary solvent selection by boiling point observation has been reported (14),
