Ootober 1950
INDUSTRIAL AND ENGINEERING
and further study might reveal a more optimum temperature for the reaction. The isopropylphenol waa more stable to cracking action than was the thymol. The results for this reo-ETHYLPHENOL TO B~NZOFURAN. action are summarized in Table 11. A run, not recorded in the table, made at 550' C. and space velocity of 1970 yielded no benzofuran. I n comparing the ease of dehydrocyclization of the various phenols, the most easily cyclized is the unsaturated o-allylphenol. A run with the o-propenylphenol, With the double bond next to the benzene ring, gave essentially the same results. Since these compounds are unsaturated to start with, they cannot be compared directly with the saturated phenols. Of these, thymol was the most easily dehydrocyclired, then isopropylphenol, and finally ethylphenol. From this limited group, it might seem that electron-releasing groups promote this reaction. Other phenols are being investigated with respect to this point.
CHEMISTRY
2117
ACKNOWLEDGMENT
This work was supported in part by a Frederick Gardner Cottrell grant-in-aid from the Research Corporation, 406 Lexington Ave., New York, N. Y.,and in part under Oflice of Naval Research contract NR45B149. LITERATURE CITED
(1) Gresham and Bruner, U. 8.Patent 2,409,676(Oat. 22, 1946). (2) Hansoh and Blondon, J . Am. C h .Sa.,70,1661 (lSaS), (3)Hansoh, Saltonetall,and Settle, Zbid., 71,943(1949). (4) Hoog, Verheus, and Zuiderweg, Trans. Farudau Soc., 35, 996 (1939). (6)Koelsoh,J . Am. C h .Soc., 67,672(1948). (6)Orohin, Reggel, and Friedel, Zbid., 71,2743 (1949). (7) Rosenmund and Schnurr, Ann., 460,66(1928). (8) Stoermer,Zbid., 312,274 (1900). (9) Ibid., p. 290. RECEIVED Ootober la, 1949.
Low Temperature Calcination Rates of Limestone ARCHIE WAKEFIELD, JR.',
AND MACK TYNER
University of Florida, Gainasville, Fla. T h e calcination of partioles of natural limestone from 0.02 to 0.08 inch in diameter at temperatures between 1400' and 1700' F. has been investigated. The effects on cate of calcination of partiole size, temperature, and rarbon dioxide concentration in the furnace atmosphere were measured. An equation which represents the data was developed empirically. The data indicate that, in the range of conditions studied, rate of calcination is controlled by diffusion rate of carbon dioxide from particle.
T
L
H E calcination of limestone to form "quick" or unslaked lime is a very simple process consisting of heating the limestone to a temperature 8bove 1700' F. and holding i t a t that temperature until calcination is complete. Unfortunately, however, no general statement may be made about the time required to burn a given sample of limestone a t a given temperature. Few investigations have been made into this problem in this country, but much work has been done in other countries (usually on pure calcium carbonate); however, little agreement can be noted among the various investigators ( 1 , B , 7 , 9 , ID, 14, 16, 17). Most of these workers attempted to apply physical-chemical concepts to the reaction, and have determined reaction orders which vary from zero to about two. Also, some investigators (8,11) have noted that limestone, as is to be expected, is only qualitatively similar to pure calcium carbonate in the calcination reaction. It seems unlikely that the concept of reaction order has any bearing on the case, because the rate at which limestone particles calcine is mote likely to depend on the rate of some physical process, such as heat or material transfer, than on the velocity of the purely chemical portions of the process. Even fewer investigations have been made into this phase of the problem than into the chemical aspects ( 3 , 4 ) . Furnaa' (9)work indicates that heat transfer plays an unimportant role in the low temperature calcination of particles of limestone a few centimeters in 1
Preaant addreas. Wakefield Optical Company Charlotte, N. c
diameter. He heated pieces of limestone in a furnace for varying lengths of time and then cut them open to determine, if possible, the reaction mechanism, It was clear that the calcination proceeded along a concentric zone which advanced into the stone. Furnas was able to derive an equation for the rate of advance of this zone at any temperature, and he found this rate to be independent of particle size and external carbon dioxide concentration-Le., per cent carbon dioxide in the furnace atmosphere. Further, he found that at furnace temperatures below about 1700' F. the center of the body rose to the temperature of the furnace long before calcination was complete, while at furnace temperatures above 1700" F. the center temperature rose to a value of 1700' F. regardless of the furnace temperature and remained at that value until calcination was complete. In this second case, the center of the particle was in an un. stable condition but could not decompose until the zone of calcination penetrated to it. Because no heat went past the calcining zone to raise the center temperature, the calcining zone must have been utilizing heat as fast as it could be supplied; thus, the rate of penetration of this zone depends on the rate of heat transfer to this point. When furnace temperatures were below 1700' F., there was an excess of heat being supplied to the body above and beyond that which could be used for calcination because the internal temperature was the same as that of the furnace; consequently, heat transfer was not the deciding factor in these low temperature calcinations. I n the present investigation, since the particles used were much smaller than those used by Furnaa and a slightly lower temperature range was used, i t was thought that Furnas' data might not apply. However, it seemed entirely reasonable to assume that small particles would calcine in the same manner larger ones-i.e., along a concentric zone. If the rate of calcination depends on the rate of diffusion of the carbon dioxide released from the particle, the logical approach toward correlation of the experimental data would be to express this rate as a function of the partial pressure driving forceLe., the difference between the equilibrium disaociation pressure of carbon dioxide above limestone, and the external, or atmoepheric, partial prmure of carbon dioxide.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2118
t
i
:1
n I400
Is00
1 W
1700
FURNACE TEMPERATURE
le00
OK
Figure 1. Differential Thermal Analysis Curves for Florida Limestone Differential oDuple had e.m.f. shsracteriatic of 0.0236
rnv./"
F.
Inasmuch as the dissociation pressure of limestone increases with temperature, and the rate of diffusion increases with increase in partial pressure driving force, it would be expected that higher temperatures would cause an increase in the rate of diffusion and, consequently, the rate of calcination. ,Barring its influence on the diffusivity, and assuming that heat transfer plays no part in the reaction, the only effect of temperature should be that occasioning the increase in dissociation pressure. In order to take into account the possible effect of carbon dioxide in the furnace atmosphere, correlation was made on the basis of the differeqce between the dissociation pressure of the limestone and the partial pressure of carbon dioxide in the furnace. This necessitated determining the dissociation pressures of limestone, for which data are lacking in the literature. It might be argued that the dissociation pressures of calcium carbonate would approximate those for high calcium limestone, but several investigations (9,8, 1 1 ) seem to indicate that dissociation pressures of natural limestones are generally lower than those of pure calcium carbonate and vary considerably with the amount and type of impurity. Consequently, it was deemed necessary to make some measurement of dissociation pressure on the sample used in these experiments, This waa done by means of a differential thermal analyzer, which consisted of a small electric furnace oontainin two recep tacles in which were placed, respective1 ,a standard t&mina sample and a sample of the limestone. Tie saplples were about 0.5 gram of plus 100-mesh material. A dserential thermocouple measured the temperature difference between the two samples and a second thermocouple measured the furnace temperature. Provision waa made for varying the voltsge a plied to the furnace heatin coils 80 that the furnace could be [eated a t any desired rate. 5 0 t h thermocouples were Of Chromel-Alumel and the eleotromotive force was measured with two otentiometers. Th! furnace waa heated at a rate which wae hepd constant to h0.5 The temperature was measured every 3 minutes alF+g%Lpolation for furnace temperature at any time to
Vol. 42, No. 10
T w o runs were made with the furnace heating at the rate of 7' and 8' F. per minute. Figure 1shows the electromotive force of the differential thermocouple plotted against the furnace temperature. In each case, at the minimum point shown, the limestone was below the furnace temperature by the temperature difference indicated by the differential couple. The two values for the dissociation obtained are 1692' and 1697' F. The average was taken aa 1695' F. Plots of the dissociation pressure of calcium carbonate against the reciprocal of the absolute temperature (6, 18, 19, 16) are ahown in Figure 2. The value of the dissociation temperature of the limestone aa determined with the differential thermal analyzer was plotted as indicated by the circle in the figure. Because the apparatus was not suited for measurements at other carbon dioxide concentrations, a line was drawn through the single experimental point parallel to the other lines on the plot. In effect, this procedure assumes only that the heat of calcination of limestone is the same as for calcium carbonate, and in the absence of other data or equipment to obtain that data, it is felt that this curve will suit the present purpose. This curve defines values only for the particular limestone used; application of the generalizations derived herein to other limestones whose dissociation pressures are not the same necessitates a new determination of the curve.
46
48 50 52 54 RECIPROCAL DEGREES RANKINE x 105
56
Figure 2. Equilibrium Dissociation Pressurea of Calcium Carbonate and Florida Limestone
TABLE I. DATASUMMARY FOR CALCINATION OF FLORIDA LIMESTONE Partiate Size No.
Tzrnz.,
9 10 9 10 11 9 11 12 11 11 12
1650 1650
No. of COa in Runs Furnace, % Made
APcot,
Atm.
Slope of S ua. Time Line
Because a part of the heat received by the limestone is used
to supply the heat of reaction while the limestone is calcining, its temperature will lag behind that of the alumina until the reaction is completed, and then suddenly rise. If the heating rate is sufficiently high and the limestone is enclosed, the concentration of carbon dioxide around the sample will be 100% as soon aa an appreciable amount of the sample decomposes. Consequently, the maximum temperature difference between the limestone and the alumina will correspond to the temperature at which the dissociation pressure of limastone is 1 atmosphere.
1680 1680 1680
1700 1700 1700 1690 1690 1690
0 0 20 20 20 75 65 60
75 80
80
2 2 2 2 2 4 3 2 1 2 2
0.77 0.77 0.72 0.72 0.72 0.31 0.41 0.46 0.23 0.18 0.18
0.340 0.470 0.340 0.427 0.601 0.154 0,264 0.408 0.149 0.093 0.144
2119
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1950
The balance was zeroized before each run. After the furnaoe reached the desired temperature and the carbon dioxide concentration was set, the sample was placed on the pan by means of a “salt shaker’’ arrangement consistin of a short piece of 0.25-inch pipe with small holes drilled in the sijes. One end of the ipe was plug ed and the other end was screwed,to a steel rod whicg served as a%andle. It was found that consistent results could not be obtained unless the sample was distributed widely enough 80 that each particle was isolated. This salt shaker gave the desired scattering effect. As soon as the sample was on the pan, a timer was started and weight determinations were begun. Each time the balance was brought back to the original rest point the time and the balance reading were recorded. Readings were taken in this manner until no further change in weight was noted, a t which point the final weight was recorded opposite “infinite time.” From this weight and the per cent loss on ignition as measured in the analysis, the weight of the original sample could be determined. Sample weights were in the range of 100 to 200 mg.
>.
m
FURNACE
200 400 800 TIME IN S E C O N D S
Figure 3. Original Data for Size 11 Runs at 1690’ F. with 80% Carbon Dioxide Atmosphere PROCEDURE
The a paratus for measuring the calcination rates of limestone consiste8 of an ordinary analytical balance resting on a table top beneath which was slty a small electric furnace. A platinum pan was suspended insde the furnace by platinum supporting wirea assing through the table top and balance floor and attache8to the balance m place of the left-hand pan. The balance was converted to Chainomatic operation for greater speed in weighin The furnace temperature was measured by means of a ChromeflAlymel thermocouple in the same horizontal plane and about 0.25 inch to one side of the platinum pan. The temperature was controlled by a Leeds & Northrup recorder-controller which was set for “on-off” control of the power to the furnace. The temperature in the furnace never varied more than 10’ F. from the control point during these runs using this method of control. Carbon dioxide was introduced from a cylinder, through a rotameter, into L ceramic tube leading through one end of the furnace. It was found that the carbon dioxide content in the furnace was r p o r t i o n a l to the rotameter reading. Holes and leaks in the urnace necessitated a high carbon dioxide rate in order to maintain sufficient concentration in the furnace. After each series of runs, the carbon dioxide content was determined with an Orsat apparatus, and after each such determination, the rotameter reading was held constant. The maximum error in the percentage of carbon dioxide noted during a check run was 5%.
Table I shows the runs made, the limestone particle size numbers, furnace temperatures, a n d carbon dioxide atmosphere concentrations (columns 1, 2, 3, and 4). For illustration, Table I1 gives the original data obtained for the runs at 0 20 40 60 Bo 100 1690’ F. and 80% carPERCENT COz REMAINING bon dioxide for size 11. Figure 4. Relation between S Figure 3 shows these (Percentage of Radius Penedata plotted as percentrated) and Degree of Calcinatage of the radius penetion for Spheres trated by the calcining zone, S, us. time. The value of the per cent of the radius penetrated was found from the following easily derived equation:
This equation is plotted in Figure 4. Figure 5 shows a plot of the curves for size 9 runs under dilTerent conditions; Figure 6 is a plot of the lines for several different sizes at constant temperature and constant carbon dioxide concentration (these lines have been shifted slightly so that all will
The sample of limestone selected for these tests was analyzed with the following results: CaO MgO Si02
RaOs
Lo- on ignition
% 55.76 0.67 i.6a 0.47 5 m 43.35
101.88
The material had a density of 2.70 grams per cc. and was soft enough to be easily crushed between the fingers. About 2 pounds of this material were sieved in small batches. The particles resting on each sieve were given a size number as follows: Partiole Size No. 10
11 ia 13 14
Tyler Screen Numbers Paaging Retained on
Av. Opening of
Screens, Inch 0.0555 0.0394 0.0280 0.0198 0.0140
The diameters of the particles in any given size were taken as the average of the screen openings above and below, as shown. These particles were treated in the correlation as spheres and upon examination under low magnification this appeared to be the best approximation. The particles were smooth and rounded.
TIME IN SECONDS
Figure 5. Correlation of Data for Size 9 Runs under Various Conditions Curve A B C
D
E
co, in Furnaoa, %
T%mp
0 20 0
1650 1680 1560 1700
75 0
1460,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2120
pass through the origin). These plots indicate that the rate depends on particle size and temperature aa well as external carbon dioxide concentration, but is a constant for any one run. There is some doubt that the straight lines as plotted could be extrapolated to 100% penetration-indeed, several of the runs seem to indicate a gradual decrease in slope at the upper ends. As can be seen from Figure 4, however, the approximation of a straight line is justified for the practical case, because when the penetration has reached a value of 60% of the radius, calcination is more than 90% complete.
Let R
Vol. 42, No. 10
ori inal radius of particle = racks of uncalcined interior portion of particle at any
T
time
W
= weight of Cot present in prtrti:le at any time P = density of limestone 0.4335 = fraction of COZin original limestone
Then the weight of carbon dioxide present will be given by
W
-
'/,ma 0.4335~
(5)
and the loss in weight per unit time dr
= 4*r8 0.4335 p
or more simply
de
=Cr* dr
de
where C is a constant. But from the experimentally derived equation
L and since r = R
P
A P C O ~ D12' e
1490
-
A P C O ~ R ' /e~
(8)
1054
- L, r = R -
TIME IN SECONDS
Figure 6. Correlation of Data for All Sizes at 1560' F. and 0% Carbon Dioxide
-
APcozR'/'
B
1054
0.8
Table I gives the values of the slopes of all the plots of per cent penetration against time, as well as the partial pressure driving force, APCOZ(columns 5 and 6). The values of dissociation pressure for the limestone were read from curve E, Figure 2, obtained as indicated previously, and the partial pressure in the furnace was subtracted therefrom to obtain these figures. For the runs made without the addition of carbon dioxide, the partid pressure driving force is merely the dissociation pressure. If the slopes of the per cent of penetration against time linw were multiplied by the square root of the average particle size and plotted against the partial pressure driving force, all the data would fall along an approximately straight line as shown in Figure 7. The slope of this last line was measured as 7.45,which in combination with the plots of the original data yields an equation:
UI W
a W
E OA 0
H
c 4
8"
a
a
0
0
0.05
6.10
x D)4 where S
-
Figure 7. Correlation of All Runs for Different Particle Sizes and Various Carbon Dioxide Atmospheres
-
0 runsmade with no CO, i n furnace atmosphere
per cent of radius penetrated bv calcining zone
= run# made in COrcontaining atmospheres
h p c o ~= dissociation pressure of limestone minus external COn
pressure
6
D
= time in seconds =
mean particle diameter in inches
This equation may be put into a more convenient form by considering that S = L / R X 100, where L is the distance of penetration ill inches and R is the particle radius. (3)
Thus, the rate of penetration is given by: Inches penetrated per minute
-
APcozD' --__ 1490
1'
(4)
TABLE 11. CALCINATION OF SIZE 11 LIMESTONE AT 1690' F. AND 80% CARBON DIOXIDEATMO~PHERE Time, eec. 28 74
100
160 202 227 280 330 380 472 515
00
Run 1 Weight, rng. 148.3 140.8 138.0 128.7 123.6 118.8 111.3 104.0 99.1 94.3 91.7 89.2
% radius penetrated 4.5 8.0 11.3 18.2 20.0 23.8 26.0 39.7 47.0 57.8 66.3
...
CONCLUSIONS
The form of the derived equation may be investigated to determine whether it suits the assumption of diffusion mechanism.
Orig. sample = 89.2
o,5665 = 157 rng.
Time,
Run 2 Weight,
% radius
mg. penetrated 138.0 2.8 128.7 7.2 123.6 10.3 118.8 13.7 111.3 19.7 28.5 104.0 31.9 99.1 35.2 98.6 38.2 94.3 52.0 86.8 59.2 84.4 89.5 81.9 80.2 80.2 0rig.sample = 0.6665 142 mg. 880,
- ..
-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
October 1950
average or mean value of the severs1 properties involved in this function. Consequently, the form of the derived equation satisfies the assumption that diffusion is the controlling factor in the low temperature calcination of Florida limestone particles in the diameter size range of 0.02 to 0.08inch.
and from this equation dr
&
-APcorR1fa 1054
Putting Equation 10 into 7:
8
(11)
If diffusion is the controlling factor, the rate of loss in weight must equal the rate of diffusion. The diffusion rate will, in general, be given by a driving force divided by a resistance, or: Rate of diffusion =
KAPcol driving force resistance
(12)
Q
1 ma
(13)
The resistance will be inversely proportional to the cross section of the diffusion path and directly proportional to several properties such as length of diffusion path, number of each type of gas molecule, and total number of molecules in the diffusion path:
Resistance
diffusion function croas-section area
LITERATURE CITED
Aabe, V. J., Rock Products, 47,No.9,68,704,100-2 (1944). Furnas, C.C., IND.ENO.CHEM.,23,634-8 (1931). Gilkey, W. A., Ibid.. 18,727(1926). Haslam, R.T., and Smith, V. C.. Ibid., 20,170 (1928). Htittig, G.F.,and Kappel, H., Angm. Chem., 53,67-9 (1940). Johnson, J., J. A m . Chem. Soc., 32,938(1910). Khomyakov, K. G.,Yavorovskaya, S. F., and Arbuzov, V. A., Sci. Repts. Moscow State Univ., 1936,No.6,7747.
Comparing Equations 11 and 12, the driving force appears aa it should, and the resistance is indicated as being proportional to: Resistance
2121
(14)
I n this case, r4 is a measure of the cross-sectional area of the diffusion path and appears in the correct place in Equation 13. The mean value of the "diffusion function" in Equation 14 is indicated in 13 as R-'Ia, which does not appear illogical as the
Linzell, H. K., Holmes, M. E., and Withrow, J. R., Trans. Am. Inst. Chem. Enws., 18,249-81 (1926). Maskill, W., and Turner, W. E. S., J. SOC.Glaaa Technol., 16, 80-93 - _ _ _ f1932). I Orosco, E :; Ministerio trabalho id. e com. Inst. nucl. tech. (Rio de Janeiro), 1940 (Separate). (11)Perederii, I.A., Stroilel. Materialy, 1937,No. 11,56-61. (12)Smyth, F. E., and Adams, L. H., J . A m . Chem. SOC.,45, 116784 (1923). (13) Southard, J. C.,and Royster, P. H., J . Phys. Chem., 40,436-8 (1936). (14) Splichal, J., Skramovsky, S.,and Goll, J., Collection Czecho8bv. Chem. Oommun., 9,302-14 (1937). (16)Tamsru, S., Siomi, K., and Adati, M., 2. physik. Chem., A157, 447-67 (1931). (16) Whiting, 0.H., and Turner, W. E. Sa, J . SOC.G h s Technol., 14T,409-24 (1930). (17)Zawadski, J., and Bretsznajder, S., Compt. rend., 194, 1160-2 (1932). RECEIVED March 24, 1950. Abstracted from a thesis presented in June 1949 to the Graduate Suhool of the University of Florida in partial fulfillment of the requirementa for the degree of m t e r of science in engineering.
Vapor-Liquid Equilibria at Subatmospheric Pressures TETRADECANE-HEXADECENE SYSTEM R. R. RASMUSSEN' AND MATTHEW VAN WINKLE University of Texas, Austin, Tex. Experimental vapor-liquid equilibrium data for the system tetradecane-1-hexadeceneat absolute pressures of 760, 400, 200, 100,50,20, and 10 mm. of mercury are presented. In addition, activity coefficient-composition relations are included for the components at the various pressures studied. The experimental data were smoothed by use of the modified Duhem equation and are presented in the form of conventionalx y plots.
T
HE separation of higher boiling hydrocarbons from their
d n
mixtures by fractionation requires the uw of subatmospheric pressures, in most instances, to prevent thermal decomposition. Thus it is necessary to have available vapor-liquid equilibrium data at subatmospheric pressures for equipment design and operation considerations. At the present time, few vapor-liquid equilibrium data are available on the higher molecular weight compounds. The difficulty of obtaining the pure higher boiling campounds has been 1 Present addrw, Carbide & Carbon Chemicals Corporation, Texas City, Tex.
the greatest single factor in delaying investigations in this field of study. PURITY OF COMPOUNDS
The tetradecane and hexadecene used were obtained in a relatively purified form. The refractive index of the hexadecene was n$ = 1.4435 compared to 1.4441 for the pure hexadecene (specified by supplier, 1.4425 to 1.4450), and for the tetradecane n2$ = 1.4298compared to 1.42886 for the pure tetradecane (sperified by supplier, 1.4280 to 1.4310). The hexadecene was checked for the degree of unsaturation by use of the io2ine value test, in accordance with a modification of the procedure given by Prescott (9). An average iodine value of 111.5 was obtained for the hexadecene, compared to the theoretical value of 113. Based on pure hexadecene, this indicates 98.7% of theoretical unsaturation. A series of carbon and hydrogen determinations waa made on a micro scale to obtain further checks on the purity of the compounds. The composition of the hexadecene was indicated to be
