EFFECT OF WIND ON BUOYANT DIFFUSION FLAMES hitid Correlation O M E R A. P l P K l N AND C E D O M I R M . SLIEPCEV I C H Drpartmmt of Chemical Engineering, The L7imersity of Oklahoma, S o r m a n , Okla.
Buoyant diffusion flames of natural gas were observed in wind tunnel experiments to determine the extent of bending b y wind. The experimental results were correlated through use of a simple physical model for the gross flame structure. Flame buoyancy was varied through changes in diameter of the natural gas burner nozzle. Nozzle velocity was held constant a t a value comparable to that expected above pools of freeburning liquids. Nozzle diameters were sufficiently large to make the momentum contribution of the fuel source negligible in comparison with buoyancy. A flame d r a g coefficient, C, i s introduced in the flame momentum balance. A single straight-line correlation of In C,(Re) vs. In Re i s obtained after extracting the influence of flame angle of tilt and applying an empirical correction to account for increasing flame roughness at larger diameters.
and unconfined, buoyant diffusion flames even under calm conditions. L-nder the influence of wind. such flames have not been studied sufficiently to provide the minimum requirements for intelligent specification of structure fireproofing. proper separation of tanks. minimum distance of dikes from property lines. and useful location of fire-protection equipment. Flames of this type arising from burning pools of liquid are of considerable practical significance. ‘I‘hey occur frequently in industry and are responsible for extensive property damage each year. \‘L’ind probably contributes more to fire spread than any other element in nature. yet little is known about its quantitative influence on the structure of large-scale flames. T h e literature has not a single systematic study of wind effect on uncontrolled buoyant diffusion flames from burning pools of liquid. [Hirst and Sutton (70) observed such flames from a pool of liquid recessed into an airfoil. but their primary concern was the effect of reduced pressure on extinction velocities.] I n fact. until 1957 (3).very little attention had been given to w c h flames, even under calm conditions. ‘ f h e paper by Blinov and Khudiakov (.3) has been a center of interest because of the three burning regimes reported : laminar and fully developed turbulent regimes, with a n unstable transition region bet\veen them. I n relation to the present study. the most significant point is that. under calm conditions. fully developed turbulent motion cannot be expected in flames from burning liquids. unless the pool diameter is greater than approximately I .3 meters. Since 1957, three laboratories have made yirnilar studies (5.S: 9). where interest has been primarily in liquid burning rate. Several important questions concerning the influence of Tvind on buoyant diffusion flames need to he answered quantitatively. To \vhat extent are flames tilted by Lvind? \Vhat rliect does wind have on flame length and intensity of radiation? How can the temperature field he predicted downwind YcosiTRoLLm:
U.are not kvell understood,
of large scale flames? W h a t influence does flame tilting by wind have on the radiant heat transfer to a n object downwind? How does wind affect the fuel burning rate? This paper is devoted primarily to the first question for controlled buoyant diffusion flames of natural gas. A more comprehensive study is now being planned, in which uncontrolled (free-burning) buoyant diffusion flames from burning pools of liquid will be studied in a much larger bvind tunnel. Thomas (20)published what appears to be the first pertinent article related to the effect of wind on flame length. H e correlated the effect of wind on the length to diameter ratio for flames from burning wood cribs. but did not report on the bending effect. I n a very recent report on the interaction effects of mass fires, Putnam and Speich (74)analyzed wind effect on single-source natural gas flames. but their region of experimental investigation is different from that of the present paper. Several Bureau of Mines reports and articles (-1-6. 27) mention that wind reduces the intensity of flame radiation, but d o not present a quantitative statement of this phenomenon. Probably the most difficult question is how to predict the temperature field downwind of a large scale flame. This is a problem of scaling with a three-dimensional heat source involving radiation. T h e most successful related study appears to be that of Rankine (75). in Lvhich scaling relationships were applied to the t\vo-dimensional temperature field downwind of a line source of heat. infinite in extent (approximated by a row of butane burners in a large Lvind tunnel). Rankine’s results have been discussed by Taylor (79). T h e question of effect of flame tilting on radiant heat transfer can he answered from purely geometrical considerations, if it is assumed that the flame acts as a black body radiator of known temperature and a suitable model representation of its shape is made. If one considers a differential object, this involves VOL. 3
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evaluation of configuration factors as defined by the equation 051
.''
0.
04-1 0 4-
cos
cos
(22
dAz (1)
rz W
Z
a
Figure 1 is a graphical representation of the solution of Equation 1 for a vertical differential object located directly downwind of a cylindrical flame with a constant L I D of 3. This figure illustrates the practical significance of flame bending by wind.
03-
J
U
t
02-
W
0 I-
Practical Significance of Flame Angle of Tilt o
05
IO
I5
2 0
2 5
D X
0.8
07
0.6
W
5
05
A U
t
04
Consider a differential element of wood, insulated on the back face and sides. located 10 feet above the top of a tank dike, 40 feet wide. which contains burning gasoline. We compare the minimum allowable distances a t which the wood element may be located from the tank dike without spontaneous ignition for the case of (1) a vertical flame and (2) a flame tilted 60' from the vertical by wind. To demonstrate the purely geometrical aspect of the problem. and simplify the illustration. let us assume that the flame L I D and intensity of radiation remain unaffected by wind. Changes in the convective field downwind of rhe flame will be ignored and the convection heat transfer coefficient assumed to remain constant a t 3.0 B.t.u./hr.-sq. ft.-" R. A steady-state energy balance for the wood element is simply
9 0.3
where dA1 is the differential area and qi is the energy flux for component i, which may be due to either radiation or convection, and is positive or negative depending on whether the flux is directed toward or away from the element. The expanded form of Equation 2 in terms of all the contributions is
0.2-1
0.1
c q i dA1
uTf4
= [ 6 f F d A I+ A ,
+ %(I -
F d A l - + A , ) U T~~
1
e,(i)u~,4 - h ( ~ , T , ) I ~ A=~ o
(3)
Since the flame is thick and gasoline fires are luminous because of carbon particles, the flame emissivity, e/. should approach unity (2, p. 123). Likewise, the emissivity of wood! e,. in its highly charred state (near the spontaneous ignition temperature) should be approximately unity. Black body radiation is assumed from the surroundings; hence is also unity. 'Thus. upon solving Equation 3 for the configuration factor, one obtains FdAI-
Ai =
u(TW4- To4)f h ( T , - To) - U ( T f 4- To4)
(4)
The flame temperature, 7'1, for conservative spacing, can be estimated from the maximum radiation power of gasoline flames reported by Burgess and Zabetakis (6)p-that is? 72 kw. per square foot of liquid surface, which corresponds to 7.64 kw. per square foot of cylindrical flame surface. Hence, the effective flame temperature is calculated as : 4
TI 0
0.5
15
10
2.0
X
Figure 1. Configuration factors for a vertical differential object and a tilted cylindrical Aarne with L/D = 3 l&EC FUNDAMENTALS
4
,(7.64) ((3417) = 1975' R. 0.1712 x 10-8
2 5
D -
148
=
I-
From the work of Simms (77) it is known that practically all cellulosic materials can be heated to a maximum equilibrium temperature of approximately 975' F. without spontaneous ignition; therefore, this temperature is used for T,. From Equation 4. the maximum permissible configuration factor is
a n important role in the design of facilities to be located where the potential source of such fires is present. In this preliminary investigation, only buoyant flames of natural gas have been considered, in order to avoid the COLIIplications introduced in the uncontrallcd fire, where the rate of fuel consumption is determined by the feedback of energy from the flame. Experimental
Figure 2. Photograph of wind tunnel and accessories for flame experiments
calculated to he 0.375. Since y / D is 0.25, from Figure 1 it 1s seen that the permissible value of D / x is 2.0 for the calm case, where B = 0, and 0 775 for the tilted flame, where B = 60'. Thus, under calm conditions the woad element may be located 20 feet from the tank dike without ignition; when the flame is tilted 60' from the vcrtical by wind (at this point we have not considered what wind velocity is required to produce this amount of tilting), the wood will ignite when less than 52 feet from the dike. Hence, for this illustration, the bending effect of wind has increased the minimum spacing by a factor of approximately 2.5. T h e practical implications of this example are obvious And indeed, consideration of flame tilting by wind should play
Table 1.
Results of Wind Tunnel Ter
(7
Run No. 2 3 5 6 8
ID, wind), In. Ff./Sec.
us
11 12
0.375 0 . 4 5 0.375 2.00 0.475 0.634 0.475 1 . 9 2 0.610 0.834 0.610 2.17 0.740 0.517 0.7401.583
0.206 4.00 0.402 3.69 0.695 4.72 0.267 2.51
1.000 0,630 0.630 0.428 0.428 0.338 0.338 0.338 0.243 0.243
14.69 2.44 5.92 1.237 4.54 1.361 5.43 1.180 0.503 3.43 1.805
83 371 149 452 252 655
0.203 0,203 0.203
5.51 0.581 0.656
338 1014 1352
1.oon
189
580 916 226 693
,."_/i
21.46 2.29 2.17
0.791 0.800
0.672
,.
J"1
1.320
1 _jUU
2850
Yafion 73.
VOL. 3
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MAY
1964
149
wind (velocity of 150 feet per minute) gas flame from 0.740-inch nozzle
8 satisfying mathematical and theoretical elegance, even under calm conditions, the task of performing a similar theoretical analysis, taking into account the influence of wind, appears formidable. However, through the use of suitable models, one can salvage more than the minimum amount of information available from dimensional analysis alone: Such is the basis of the present analysis, in which a
Figure 4.
Cross section of cylindrical flame model
orifice with a Reynolds number greater than 5000; in the present study, the maximum fuel gas Reynolds number was approximately 425. Their fuel gas velocity a t thc orifice ranged from approximately 40 to 100 feet per second, which is high compared with the cnnstant value of 0.5 foot per second used in all the runs of this study. Further, the flames of this study were, relatively, far more buoyant, as they permitted a
where the only forces involved are those due to buoyancy and wind drag. T h e assumption of uniform air inducement is probably the most severe assumption employed in the analysis. However, based on the appearance of the flames of this study-for example, see Figure 3-a more complex assumption does not appear to be justified. T h e rate a t which fuel is supplied to the flame is ( r D z ,4) w,p,. Thus. the first term in Equation 7 , representing the momentum contribution of the fuel in the direction of the normal. becomes ( I ~ L ‘ )=~ (7rD2/’4) ~ ~ ~ w,p,
(ZL~,sin
6)
The forces acting on the flame in the direction of the normal are given by the expressions
F (buoyancy)
F (drag)
=
=
- ( D L cos
( r D 2 / 4 )L cos 6 ( p a
e)
7
-
Equation 8 has been written in this form to show the explicit appearance of the various dimensionless groups. All of the dimensionless groups predicted by dimensional analysis are present. except the Reynolds number. Thus, C f is expected to be at least a function of this variable. The Reynolds number of this analysis is defined as Du/u,, where D is the characteristic flame dimension (diameter for cylindrical flames and width for channel flames), u is thr wind velocity, and Y, is the kinematic viscosity of surrounding air. Considering the nature of the model, one would expect the Reynolds and Froude numbers to be significant dimensionless groups, as both arise in the process of rendering the NavierStokes equation of motion ‘nto dimensionless form. By altering the form of Equation 8 , the possibility of eliminating the term representing the momentum contribution of the fuel is explored.
g sin B gc
pj) -
cos 6
In the expression for drag force, a drag coefficient, Cfi is defined which is to be evaluated experimentally. Upon substituting the various terms into Equation 7 and solving for 6’: one obtains
The comparable expression derived from turbulent plume theory for point source of heat by Putnam and Speich (though
From Table I it is seen that, since cos e is in all cases greater than approximately 0.25, the second term in the denominator is small in comparison with the first term, which represents the contribution of buoyancy. A decline in the significance of the second term is apparent as diameter is increased. Thus, for large scale flames from burning pools of liquid, the second term will be negligible, and one can write
In more general terms, an expression for 6 which applies for all flame shapes is
not explicitly in this form) is 0 = tan-’ -
(F)’ ’}.
.
Such an expression was unsatisfactory for
.
.
correlation of the present data. where f is a shape factor which depends upon flame geometry.
A simplified form of this equation, 0
= tan-’
cut -, was proposed D
by Churchill and Sliepcevich (7)
1:
Values off for various flame shapes are: Flame Type Cylindrical Channel Cone Pyramid Parallelepiped
I
k
a-
-i 0
f */4 1/ 2 */6 2/3 1
Correlation of Results
4
‘Lo
1 10
Equation 10 rearranged to be explicit in terms of the drag coefficient is
LEGEND BURNER 10 (In )
0.74
.‘ &
1.365 1.610 I
, ,,,,,,,
REYNOLDS
,
,
, , , , I’1
loa
IO0
NUMBER
All of the variables necessary for the calculation of CY were measured in the 20 experimental runs reported in Table I . except average flame gas temperature which is required for The density ratio may evaluation of the density ratio. p,’pa. be approximated by the expression
Figure 5. Variation of drag coefficient Cj (Re, 8, E ) with Reynolds number for buoyant diffusion flames of natural gas VOL. 3
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From this expression one observes the expected influmce of decreasing flame temperature as diameters become larger (Appendix .4). Drag coefficients calculated through use of Equation 12 and the density ratio approximation are listed in Table I and plotted in Figure 5. I t is apparent from Figure 5 that the drag coefficient is a function of variables other than Reynolds number. In fact: some function of diameter is strongly indicated as a parameter. since, for each diameter. plotting In C j us. In R e yields a straight-line relationship. and each line of correlation has approximately the same slope. From the nature of the flame model, one would expect the functional form of the drag- coefficient in Equation 12 to be Cf(Re. 8. 5 ) . where E is a flame roughness factor and 6' is the angle of tilt. T h e effect of 8 can be estimated from the known influence of angle of attack (complement of angle of tilt) on the drag coefficient for cylinders-that is, one can calculate a "normalized" drag coefficient
from ivhich the contribution of 0 has been extracted. p is a ratio of drag coefficients (the drag coefficient of a cylinder at a particular angle of attack divided by the drag coefficient for the same cylinder Lvith a n angle of attack of 90'). Values of p are allvays equal to or less than unity and may be obtained from several sources. Those values shown in Table I have been dcrived from Sweeter's Figure 1211.7 (78) where their ratio of minimum to maximum elliptic cylinder diameter, 7 !L, is equivalent to cos 8. \+'hen plotting the values of Cj(Re, f ) listed in Table I in the same manner as for Cj(Re. 8. () in Figure 5, the variation of drag coefficient kvirh diameter for a given Reynolds number is significantly reduced : an elevenfold variation in C,(Re, 0, () is reduced to approximately a fivefold variation in C,(Re, F ) . As \vas the case for Cj(Re, 8, E ) , Cj(Re, [), values are greater a t larger flame diameters. Qualitatively, a n increase in C, (Re. 5 ) with increasing diameter is in agreement with the experimentally observed increase in flame roughness a t large diameters. Quantitatively. however, a fivefold variation in Cj(Re. E ) ow'ng to f is not readily assessed. Nevertheless, a good correlation of the drag coefficient C,(Re), independent of (1 and 5. can be obtained through a n empirical expression for the roughness factor as a function of diameter alone. T h a t is, one can express Cf(Re) as CARe)
=
0.1945 for all runs and o( is evaluated to be 0.614 per inch. The values of Cr(Re) calculated from Equation 13 for the 20 experimental runs are listed in Table I and plotted in Figure 6. Since the ultimate objective is to find a method for predicting the behavior of large flames. it is of interest to explore the suitability of Equation 13, expressed explicitly in terms of tan 8, in the limit as diameter becomes large. One finds that tan 0 ( D large) =
2pu2Cj(Re) 0.144 ae-'.*Dg
where use has been made of the fact that. as diameter increases,
L / ' D approaches a constant value for buoyant diffusion flames from burning pools of liquid. In limited observations (by Sliepcevich) this ratio is found more nearly to approach 3.0. rather than 1.7 reported by Blinov and Khudiakov ( 3 ) . Though the correlating expression of Equation 13 at large diameters reduces to the rather simple expression of Equation 14. the use of Equation 14 is limited by the uncertainty in extrapolation of Cj(Re) values from Figure 6 for large Reynolds numbers. In fact, from almost casual observations of large scale flames, one can say with certainty that an extrapolation is not valid. For this reason further experimentation ivith flames in a larger wind tunnel is being planned to explore the functional form of C,(Re) for Reynolds numbers greater than 2000. Some limited observations of uncontrolled flames up to 20 feet in diameter suggest that at large Reynolds numbers Cj(Re) may approach unity, so that tan 8 is approximated by the following expression derived directly from Equation 14 : tan 6'
(Dlarge)
0.24 @u2 =
~
D
In Equation 15, pool diameter, D. is expressed in feet and Lvind velocity, u, in feet per second. Putnam (73) found that the experimental results presented in Figure 6 may also be correlated with the expression
CARe, E ) E @ )
where
in which D* is the dimensionless diameter D/Daer. and k is an empirical constant evaluated to give the best fit of the data ( k = --2.2). T h e reference diameter is the smallest used in the experiments. PC,4Re) Observing that Cf- C,(Re, 8 . f ) = ___ , and applying
+ LEGEND BURNER IO (In 1 0
f(D)
A Q .
Equation 10. the correlating expression for Cf(Re) becomes
-
+ #
&'
In .Appendix A . 1.32
KT, is sho\&n to have a constant value of
l&EC FUNDAMENTALS
0 373 0 475 0 61 0 74 I05 I365 I610
( u ” D ~ ) - ~tan * 0
=
R(Re)-’’4
O n e also encounters a problem in extending the results to large diameters. as apparently R cannot remain unchanged and still give satisfactory results for these data and large scale flames as Lvell.
From Equation 14-4. the density ratio for these experiments is found to vary from 0.206 to 0.320. ivhich corresponds to a maximum flame temperature of 2160’ F. and a minimum of 1230’ F . Flame emissivity varies from 0.206 for the smallest flame diameter to 0.628 for the largest. Acknowledgment
Future Program
A s an extension of the present work: experiments are planned to study unconfined buoyant diffusion flames from various combustible liquids in a 6- X 6- X 25-foot \vind tunnel. using both cylindrical and channel-type burning trays; the former ranging u p to 12 inches in diameter. Flame temperature and intensity of radiation w d l be measured. the temperature field downwind of the flames closely monitored, and fuel burning rate measured. Appendix A. Method for Approximation of Density Ratio, p , / p a
One Lvould suspect that the average flame gas temperature \vould decrease as diameter increased? and hence p , ‘pa would increase. This is showii to be true in the following development for calm flames. T h e added complexity of considering the \vind influence as \vel1 is not undertaken. T h e fraction of flarnc: energy released by radiation from a cylindrical flame can be expressed as follows: Q(radiation) 11 =
-(total)-
-
a D L u(Tr4 - Tu4) (1 w0
( a D 2 ) pg -, AHc (net) 4 M,
(‘4-1)
~~
T h e authors are indebted to S. \V. Churchill of the University of Michigan for valuable suggestions regarding the theoretical aspects of this study. The experimental data were obtained in the Mind tunnel of the School of Aeronautical Engineering, Cniversity of Oklahoma. under the direction of B. V. Ketcham and with the assistapce of J. 0. Steele and D. Finn Nomenclature
surface area? sq. f t . flame surface area, sq. ft. B = empirical constant in correlating expression by Putnam. dimensionless C = constant in equation by Churchill and Sliepcevich, f t ./(miles/‘hr .)’ = flame drag coefficient, Equation 10, C, = C, x (Re! 8. E ) , dimensionless C,(Re, [) = “normalized” drag coefficient, effect of 8 extracted, dimensionless C,(Re) = “normalized” drag coefficient, after empirical correction for flame roughness, Equation 13, dimensionless D = flame diameter, inches D* = D IDref, dimensionless diameter = reciprocal of dimensionless horizontal projection, D/X Figure 1. dimensionless = flame shape factor. Equation 1 1 dimensionless f F = force. Ib. force = body force, Ib. force Fh = field force: Ib. force F, Fd.4, - A * = configuration factor, defined by Equation 1, dimensionless P = gravitational acceleration, ft.Jsec.2 = conversion constant, Ib. mass ft./lb. force-sec.2 Pc h = convection heat transfer coefficient, B.t.u./hr.s q . ft.-’ R . AHc (net) = net energy of combustion. B.t.u./lb. mole k = empiricJ’constant. Equation 13. dimensionless K = constant in Equation A-4, dimensionless L = flame length. inches L* = calm flame length, ft. LID = flame length to diameter ratio, dimensionless M = fluid mass, Ib. mass M = fluid mass rate. lb. mass/sec. M’ = molecular weight n = jqdication of normal direction = heat flux, B.t.u. ’hr.-sq. ft. 4 = volumetric fuel rate, cu. f t . :set. QO = absolute value of radius vector from differential r object to flame surface, f t . = Reynolds number. Du ’ha. dimensionless Re U = wind velocitv. ft. ‘sec. ZL’ = vertical flow velocity. f t . !'set. 7u” = vertical velocitv of fuel at source. f t . ‘sec. X = distance from horizontal projection of differential object to flame base. inches = vertical distance of differential object above Y flame base. inches = dimensionless vertical distance of differential j’D object above flame base. Figure 1 = arbitrary vertical height of point in flame above z source. f t . (or other consistent units) T = temperature. ’ R . = velocity, f t . sec. V P = pressure. Ih. force sq. f r . R = gas law constant. consistent units A Al
=
=
c,
~
Some writers object to expressing the energy released by radiation as some function of the net energy of combustion (2. pages 210 through 212). For the present analysis, we find the definition of Equation A-1 to be a useful one. Since Ta4\vi11 be srriall in comparison with Tr4: and the assumption of ideal gas behavior is a very good one. so that PM,’ = p/RT,. Equation ‘4-1 becomes
One obtains an expression for P/,’pu by solving Equation A-2 for p f and dividing the I-esult by p a :
Further, M,’ is approximately equal to M a f ; thus the prefix to the first term in pa-entheses may simply be replaced by T,. Also. the first term in parentheses is constant for these experiments, provided q is constant. Thus. the final result is (‘4-4)
Experimental work of the Bureau of Mines (4)> for a number of both gaseous and liquid fuels. indicates that 7 is not a strong function of diameter. Thus. for the relatively narrow range of burner diameters considered in this study, the assumption of a constant value for q should not introduce significant error. Based on a value of q = 0.25 for these natural gas flames. T R K is found to be 0.1945. ’\Vith a reference temperature of 2000’ F. for the flime of run 3: the vdlue of a is 0.614 per inch.
GREEK a
=
radiation absorption coefficient. in. VOL. 3
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153
= drag coefficient ratio, dimensionless = emissivity, dimensionless = fraction of flame energy released by radiation,
dimensionless bending angle of flame from the vertical, angle of tilt, dimensionless; also, time variable in momentum balance, sec. = kinematic viscosity, sq. ft., sec. = flame roughness factor, dimensionless = density, lb. mass, cu. ft. = Stefan-Boltzmann constant, B.t.u., hr.-sq. ft.-" R4. = wind stress, Ib. force,'sq. ft. = angle between surface normal and radius vector. dimensionless =
SUBSCRIPTS 1, 2
=
surface 1, 2
a
= surrounding air
b
= =
f
=
4
=
I
= = = =
0 0
LC
body field type force or signifies flame when used with drag coefficient and density gas ith contribution in out property a t source or surroundings wood element
Literature Cited
(3) Blinov. V. I.: Khudiakov, G. N.. .4cad. Nauk L7SSR 113, 1094-8 (1957). (4) Burgess. D. S., et al., Fire Research Abstr. Rev. 3, 177-92 (1961). (5) Burgess, D. S.. et al., Natl. .\cad. Sci. -Natl. Res. Council, NAS-NRC Pub. 786 68-75 (1961). (6) Burgess, D. S..Zabetakis, M. G., U. S. Bur. Mines, Rept. Invest. 6099 (1962). (7) Churchill. S. LV.. Sliepcevich, C. M.. private communication. Aug. 3, 1961. (8) Cmmons, H. LV., Natl. Acad. Sci.-Natl. Res. Council, NASN R C Pub. 786, 50 67 (1961). (9) Fons. I V . L., et al.. Conrbust. Flame 5 , 283-7 (1961). (10) Hirst. K., Sutton, D., I h d . 5 319-30 (1961). (11) Priestley. C. €1. B., Quart. J . Roy. Meteorol. SOC.82, 1 6 5 7 6 (1956). (12) Priestley, C. H. B., Ball, F. K., Z62d., 81, 144-57 (1955). (13) Putnam. A. A , . private communication, Aug. 14, 1963. (14) Putnam. A. A , Speich. C . F., NBS Contract CST-717, Battelle Memorial Institute, Summary Rept. 2, 3-2'7 (1963). (15) Rankine, 4 . 0.. "F.I.D.0. Investipations," Petroleum Lt'arfare Department, Great Britain, 1945. (16) Rouse. H.. Yih. C:. S.. Humphreys. H. LV.. 7ellus 4, 201 -10 (1952). (17) Simms, U. L.. Combust. Flame 4, 293-300 (1960). (18) Streeter, V. I,.. "Handbook of Fluid Dynamics," McCkawHill, New York, 1961. (19) 'Iaplor, G. I., "Fire Under the Influence of Natural Convection." Natl. Acad. Sci.- Natl. Res. Council. NAS-NKC Pub. 786, 10--31 (1961). (20) 'I'homas. P. H.. Insf. Fire Eng. Quart. 21, 197-219 (1961). (21) Zabetakis, M. G.. Burgess, I). S.. U.S. Bur. Mines, Rept. Invest. 5707 (1961).
(1) Ball. F. K . , Quart. J . Roy. Meteoroi. SOC.84, 61-5 (1958). (2) Berl. \V. G.. ed.. "Use of Models in Fire Research," Natl. Acad Sci. Natl. Res. Council. NAS-NRC Pub. 786 (1961).
RECEIVEDfor review July 8. 1963 ACCEPr m February 4, 1964
MAGNETIC STUDY OF COBALT MOLYBDENUM OXIDE CATALYSTS J A M E S 1. R I C H A R D S O N FlumblP Oil €3 Rejiiirring Co , Baytown. T e x a s
Magnetic susceptibility measurements were used to determine the nature and composition of active and inactive components in cobalt molybdate-alumina desulfurization catalysts containing 1 0% Moo3, with initial Co:Mo ratios from 0.1 to 1.0, and heat treated in the range of 538" to 816" C. Below an initial Co:Mo ratio of 0.3, the fresh catalysts contain no CoA1204 and less than 10% of the cobalt exists as COO,the remainder forming an active complex with molybdena. Above an initial ratio of 0.3,the final composition depends on both heat treatment and cobalt content. In the desulfurization of West Texas gas oil, an intrinsic rate constant correlates with the active Co:Mo ratios, passing through a maximum at an active Co:Mo ratio of 0.1 8. Several commercial catalysts also fit this correlation.
N CATAI YST
terminology a "cobalt molybdate" catalyst is a
I mixture of cobalt and molybdenum oxides dispersed on some support. such as alumina, and heat treated a t a n elevated temperature. Usually the exact stoichiometry and structure of the active catalytic species are unknown. especially under reaction conditions. A considerable amount of experimentation has been aimed a t rstablishing the most desirable cobalt and molybdenum concentrations. Commercial preparations cover a wide range, with 5 to 13% Mo, 1 to 6% Co, and Co : Mo ratios from 0.2 to 1.0. Nahin and Huffman (7) advocate a C o : M o ratio of 1 .O for cobalt molybdate desulfurization catalysts. whereas Engel and Hoog (4).Sulimov et 01. (72). and Porter ( 8 ) find optimum results with 0.2. Beuther. Flinn, and McKinley. 154
l&EC FUNDAMENTALS
investigating the hydrodesulfurization activity of promoted molybdenum oxide -alumina catalysts, find maximum activity of 0.35 ( 3 ) . T h e formation of cobalt-molybdenum and cobalt-aluminum compounds plays a n important role in the establishment of catalytic properties (5). However, the components of' these catalysts have not been identified, nor the reason for an optimum Co: Mo ratio established. 'This paper presents the results of a magnetic study of various laboratory and cornmercial cobalt molybdate catalysts. 'I he magnetic measurements are used to identify and isolate the active and inactive components of the catalysts. l ' h e Co: Mo ratios in the active component are then correlated with activities for a typical process-type application.
