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INDUSTRIAL AND ENGINEERIXG CHEMISTRY
1. The hydrogen consumption us. time curves are straight lines whose slope shom a temperature coefficient of 1.2 per 10" C. It is suggested that the diffusion of hydrogen through a liquid film surrounding the catalyst surfaces is the factor controlling the rate of hydrogen consumption. This is supported not only by the low-temperature coefficient and the approximately constant rate of consumption xith varying time, but also by the lack of dependence of the hydrogen-consumption rate on other essential factors, such as rate of oxygen removal and rate of liquefaction. 2. A study of the oxygen-elimination rates at various temperatures shows that the main reaction is a thermal decomposition of the coal substance that results in the rapid elimination of about 60 per cent of the oxygen. This reaction has a relatively high activation energy. A second reaction with a considerably lower activation energy is responsible for the slower elimination of the remaining 40 per cent of the oxygen. This reaction is probably a contact catalytic process. A third reaction is the condensation of the liquefied coal substance to form more stable molecules. This process becomes appreciable at temperatures beyond 400 C. and results in apparent negat.ive temperature coefficients for the second oxygen-elimination reaction. 3. From a similar study of the rates of liquefaction of the coal at various temperahres, at least four processes appear to be involved : a. Solution or extraction of the coal substance. This is practically the only process occurring below 370" C. Its temperature coefficient is about 1.2 per 10" C., and it is probably a diffusion of dissolved material away from the surface of the coal. b. A thermal decomposition of the coal substance above 370" C. involving the elimination of about 60 per cent of the oxygen of the coal.
353
c. The production of hydrocarbon gases (mainly methane and ethane), probably by destructive catalytic hydrogenation of dissolved coal in the range 370-415 O C. and mainly by thermal decomposition of dissolved coal above about 415' C. d. The condensation of dissolved coal substance to form molecules more stable than the original coal. These molecules subsequently lose hydrogen, largely by methane formation. Above 440' C. this process results in coke formation, even in the presence of 180 atmospheres of hydrogen.
dcknoivledgment The authors are grateful to H. M. Cooper, R. P'.Abernethy, and other members of the Coal Analysis Section for analyses of the coals, pitches, and insoluble residues.
Literature Cited (1) Fisher, C. H., and Eisner, Abner, IND. ESG. CHEM.,29, 1371-6 (1937). (2) Graham, J. I., and Skinner, D. G., J. SOC.Chem. I n d . , 48, 12936T (1929); Shatwell, H. G., and Graham, *J. I.,Fuel, 4 , 25-30 75-81, 127-31 (1925). (3) Hirst, L. L., et al., IND.ERQ.CHEM., 31. 869-77 (1939). (4) Landau, H. G., and Asbury, R. S., Ibid., 30,117 (1938). (5) Storch, H. H., I b i d . , 29, 1367-70 (1937). PRESENTED before the Division of Gas and Fuel Chemistry at the 97ch Meeting of the American Chemical Society, Baltimore, Md. Published b y permission of t h e Director, C . S. Bureau of Mines. (Not subject t o copyright.)
Liquid-Vapor Equilibrium Relations in Binary Systems J
The P-I'-T-x relations at the liquid- and vapor-phase boundaries in the ethanen-butane system were worked out from measurements on a series of mixtures varying in composition from pure ethane to pure n-butane. The T-x diagrams of the coexisting liquid and vapor at constant pressure were constructed, and a comparison was made with those computed by means of the solution laws. The results indicate definitely that the solution laws are inadequal e for calculating these relations at high pressures. Vapor-liquid equilibrium constants were obtained for ethane and n-butane. Information on the effect of composition as well as temperature and pressure on the equilibrium constants of ethane is given by a comparison of the constants for ethane dissolved in n-butane and for ethane dissolved in n-heptane.
d
THE ETHANE12-BUTANE SYSTEM W. B. KAY Standard Oil Company (Indiana), Whiting, Ind.
I
S -4PREVIOUS paper (3)the need for accurate preasure-
volume-temperature-composition (P-V-2'-z) data as an aid in the study of the phase-equilibrium composition relations in the critical region of hydrocarbon mixtures was pointed out; and the results of a determination of the P-VT-2 relations for the ethane-n-heptane system mere presented. These same relations have been determined for the ethane-n-butane system and are reported here. The data, in combination with those obtained on the ethane-n-heptane system, have made possible a comparison of the phaseequilibrium constants of ethane dissolved in n-butane and ethane dissolved in n-heptane and thus have yielded information o n the effect of composition on the equilibrium constants.
Apparatus and Preparation of Mixtures For the determination of the P-V-T-2 relations for the ethane-n-butane system the border curves of five different mixtures of known composition and the volumes of their saturated liquid and vapor were determined. The apparatus
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VOL. 32. NO. 3
and experimental technique were the same as those employed in the ethane-n-heptane study (3). M i x t u r e s of known composition of ethane and nbutane were prepared by first filling the experimental tube with a known quantity of pure nbutane gas and then adding the necessary amount of ethane gas to make a mixture of the desired comp o s i t i o n . When possible, the succeeding mixture containing a greater p e r c e n t a g e of ethane was prepared merely by adding more ethane t o the previous FIGURE 1. METHODOF FILLING Exmixture. PERIMENTAL TUBE WITH BUTANE The apparatus for filling the tube with pure-n-butane gas was constructed of Pyrex glass and is shown in Figure 1. Experimental tube A was supported on a glass tripod, B , which rested on the bottom of cup C. The upper part of the tube was enclosed in section D to which C was attached by means of a ground joint. Two side arms with ground joints were fused to D; one led to bulb G which contained about 100 cc. of mercury, the other to the high-vacuum pump and to the supply of pure n-butane in steel bomb F . In the wall of the expanded section of the experimental tube near the open end, a small pocket, H , was blown in which rested the steel ball, to be used later with an electromagnet for stirring the sample. The apparatus was first thoroughly evacuated and flushed several times with pure butane gas before a sample was taken. The amount of the sample admitted to the tube was controlled by regulating the pressure in the apparatus which was measured on the combined mercury pressure manometer and safety valve J . The open end of the
I
I
20
' 4b 1 6 0 MOL PERCENT ETHANE
i
dJ
'
FIGURE 3 (above). RELATION BETWEEN COMPOSITION AND CRITICAL PRESSURE, PRESSURE AT POINT OF MAXIMUM PRESSURE, AND PRESSURE AT POINT OF MAXIMUM TEMPERATURE FOR MIXTURES OF ETHANE AND %BUTANE FIGURE 4 (below). RELATION BETWEEN COMPOSITION AND TEMPERATURE AT CRITICALPOINT,POINT OF MAXIMUM PRESSURE, AND POINTOF MAXIMUM TEMPERATURE FOR MIXTURESO F ETHANEAND n-BnTmE tube was then closed with mercury by rotating bulb G, at ground joint E , through 180" and thus causing the mercury to be spilled into C. The pressure was always taken less than atmospheric so that when the apparatus was opened to the air, mercury was forced into the tube and the gas sample was compressed. The apparatus, up t o point K , was then removed from the vacuum line. Section D was removed from cup C, and the stuffing box assembly was put in place on the tube; the open end of the tube was always kept below the surface of the mercury in C. The tube was finally transferred to the compressor, the open end being closed securely with the forefinger during the transfer. The purity of the butane sample was tested by determining the pressure change during the condensation of the vapor when the tube was heated in the vapor bath with boiling chlorobenzene vapors-i. e., at a temperature of about 268" F. A change of pressure of 1 pound or less was considered satisfactory. Since the vapor pressure of n-butane at 268" F. is about 390 pounds per square inch, this amounts to an error of not over 0.3 per cent. The Tyeight of the sample was determined by calculation, either from the measured volume of the condensed vapor and the density of the liquid (which was determined in a separate experiment) or from the volume of the vapor at a known pressure and temperature by means of the compressibility factors for n-butane as determined by Jessen and Lightfoot (2). The method of adding ethane to the butane in the experimental tube was the same as that employed in the work on the ethanelzheptane system. Briefly, the amount of ethane to make a mixture of the desired composition was measured in a gas microburet and injected into the experimental tube.
Purification of Materials
FIGURE 2.
PRESSURE-TEMPERATURE DIA-
GRAMS AT CONSTANT COMPOSITION OF MIXTURES OF ETHANE AND %BUTANE
The ethane used was from the supply prepared for the work on the ethane-n-heptane system ( 3 ) . The starting material for the preparation of pure n-butane was a sample of c. P. n-butane obtained from Phillips Petroleum Corporation. According to analysis the material was
MARCH, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
355
composed of 99.4 per cent n-butane and 0.6 per cent isobutane. It was further purified as follows: The gas was first passed through fuming sulfuric acid and then through concentrated potassium hydroxide solution, dried over calcium chloride and phosphorus pentoxide, and finally condensed in the boiling flask of the distilling apparatus. The distilling column was an unsilvered vacuum-jacketed column about 2 feet in length, filled with small capillary glass helices. About 350 cc. of liquid butane were charged to the still, and the distillation was run a t a slow rate using a high reflux ratio. A middle cut of about 100 cc. was collected in the high-vacuum degassing apparatus in a trap cooled by a mixture of carbon dioxide snow and acetone. The liquid was freed from traces of air and other noncondensable gas by distilling it several times back and forth from one trap to another, the residual gas being pumped off after each distillation. Finally, the degassed liquid was distilled into a highpressure steel storage bomb.
Results and Discussions
d !
4
The experimental pressure, temperature, and density data from several hundred equilibrium measurements on the five mixtures and data for ethane (3) and butane (4)were plotted on large-scale plots, and smooth curves were drawn through the points. Table I is a summary of the data as read from the smoothed curves. I n Figure 2 the border curves of the mixtures and the vapor pressure curves of ethane and n-butane are shown. The points are those determined experimentally. The broken line is the envelope curve which is tangent to the border curves a t their critical points and ends in the critical points of pure ethane and n-butane. The composition of the mixtures and the temperatures, pressures, and densities a t the critical point, the point of maximum pressure, and the point of maximum temperature on the border curve are given in Table 11. The combination of the border curves of the mixtures and the vapor pressure curves of the pure constituents form a space diagram similar to that for the ethane-n-heptane and other binary systems that have been studied. As compared to the ethane-n-heptane system, the effect of composition on the critical constants of the mixtures (Figures 3 and 4) is not so marked. For example, the maximum critical pressure is 843 pounds per square inch for a mixture composed of 68.9 mole per cent ethane, whereas in the ethane-heptane system it amounted to 1263 pounds per square inch for a mixture composed of 77.1 mole per cent ethane. Likewise, the maximum temperature range of retrograde condensation is about 15' as compared to 91" F. These results, however, are in agreement with the general behavior of mixtures of compounds belonging to the same homologous series; i. e., the extention of the liquid-vapor region into the high pressure region becomes smaller, the smaller the difference in molecular veights of the components. The temperature-density curves of the saturated liquid and vapor of the five mixtures studied are given in Figure 5. The corresponding curves of ethane ( 1 ) and n-butane have been added. As noted first in the ethane-n-heptane system, there are two points of inflection in the curves; they are barely noticeable for mixtures containing small amounts of ethane but quite apparent for mixtures rich in ethane. The
ISDUSTRIAL ,4XD ENGINEERIKG CHEMISTRI
356
L'OL. 32, NO. 3 ~~
TABLE 7 - Compn.-W t . % Cz Mole % CZ
11.
Av. Mol. Wt.5
T.
PC
do
Lb./sq. in.
Lb./cu. ft.
550.1 646.1
14.24 14.83 15.04 15.24 14.67 14.05 13.736
306.0 283.1 237.4 191.2 147.6 108.4 90.1
,
---3Iex.
Point-
-Critical O F .
0 58,078 17.49 53.17 45.10 45.43 65.77 39.64 70.46 82.18 35.04 90.26 94.72 31.53 100 100 30.047 0 Calculated from t h e composition. 0 9.88 29.82 49.85
COMPOSITION AND CRITICAL CONSTANTS OF ETHhUE-n-BVThNE
780.8 841.6 827.2 759.2 712
NQ I
WT X ETHAN[ rrEYJTANE
w
e
5 E
w
a 2
irl -I
lo
1
10
20 30 DENSITY LBS/CU FT
FIGURE5. DENSITYOF SATURATED LIQUID AND VAPOROF MIXTURESOF ETHANE AND n-BUTANE critical point of the mixture is located, within experimental error, a t the second point of inflection or point having the greater density. The relations between the temperature and the composition of the coexisting liquid and vapor phases in equilibrium a t constant pressure (Figure 6) were determined graphically
FIGCRE 6.
RELATIOKS IS BUTANE SYSTEM
TEMPER.4TURE-COMPOSITIOK
TCC O F .
285:o 244.6 204.0 158.7 112.0
Temp. Point-dcc
PCC
Lb./sq. in. Lb./eu. ft.
...
ii:s
634 732 779 793 753
10.1 9.2 9.1 10.0
~IIXTZIRES --Max. Tmax.
Pressure Point-p
OF.
zSi:5
233.3 191.5 153.3 110.5
Prnax. p
dmax. p
Lb./sq. in. Lb./cu. ft.
...
64s 785 841.5 832 764
16:3 16.7 15.5 12.4 12.1
from cross plots of Figure 2 . Points read from these T-s diagrams when drawn to a large scale are given in Table 111. It is observed qualitatively that, as the pressure is increased, the region where the liquid and vapor phases coexist diminishes. Below 550 pounds per square inch, the critical pressure of n-butane, this region is limited only by the temperature; above 550 pounds per square inch it is limited by the composition of the mixture as well. For example, a t GOO pounds per square inch the diagram terminates with a mixture containing 8.8 mole per cent ethane. At 800 pounds per square inch, which is above the critical pressure of either component, the liquid and vapor phases can coexist only in mixtures containing between 49.0 and 90.0 mole per cent ethane while a t 843 pounds per square inch the two phases can coexist only for a mixture containing 68.9 mole per cent ethane. The extent to which the equilibrium relations between the liquid and vapor can be calculated by the modified solution laws, in which fugacity is used instead of vapor pressure, is shown in Figure 7. At 100 pounds per square inch the deviations of the calculated from the experimental values are not bad but a t 400 pounds per square inch they are quite large. These results are in agreement with the results on other mixtures. From the diagrams in Figure 6 data were obtained for calculating the vapor-liquid equilibrium constants for ethane and %-butane. The relation of these constants to pressure a t various temperatures is shown graphically in Figures 8 and 9. For comparison of these experimental values with those calculated from the fugacity of the hydrocarbons, curves constructed from tables given by Sherwood (6) were added. I n Figure 8 the data for ethane dissolved in n-heptane were also added. The curves in Figure 8 show plainly that the values of the equilibrium constant depend not only on
FIGURE 7. THE ETHAKEIZ-
AND
COMP9RISON O F CALCULATED
EXPERIMENTAL EQUILIBRIUM CURVES
. 4 100 ~ AND
400 POUXDS PER
SQUARE
INCH
MARCH, 1940
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
TABLE111. TEMPERATCRE-COMPOSITIOS CCRVES AT CONSTAST PRESSURE -LiquidTemp.,
--
O F .
Mole
% Cz
-Vapor----. Tzmp.,
Mole
F. % C? 100 Lb. per Sa. In.-146.0 138 129 120 110 99
146 135.6 125 114.5 97 76 58.5 30.5 10 -4.9 - 17 - 28 -36 - 47
0 2 4 6 10 15 20 30 40 ~. 50 60 70 80 100
50 38 21.5 13.5 4 -8 - 23 -32.5
0 10 20 30 40 50 60 70 80 85 90 92 94 96 98 99
--300 241.2 233.5 225 217 202 183.5 166 134.5 108.5 87 69.5 55 42 30 21.5
Lb. per Sq. In.0 229 2 217 4 204 6 190.5 10 175.5 15 15s 20 137 30 113 40 98 50 80 60 71.5 61 70 80 50 90 37.5 30 100
10 20 30 40 50 60 70 80 85 90 92 94 96 98 99
--500 295
Lb. per Sq. In:0 279 2 263.5 4 247.5 6 231 10 213.6 15 194 20 172 30 145 40 128.5 50 110 60 101.5 70 92.5 80 82.5 90 71.5 100 66
283 289 277 264.5 247.5 230.5 199 170 144.5 123 104.5 88 73 GO --700 268 266 262.8 255.5 238 221 5 192 167 145 125 106.5 88.5
8B 69.5
-.-
Lb. per 27.8 27.2 28.0 30 35 40 50 60 70 80 90 100
Sq. In.---267 265 252.5 236 215 191 162 146.5 127 112 104 96
820 Lb. per 215 65.7 55 213 56 209.5 202.5 58 196 60 182 65 169 70 158 73 148 80 144.5 82 141.5 84 141 84.5
Sq. In.--213.8 213 211.5 204 193.5 180.5 165.5
--
151
146.5 141.3
-Liquid-VaporTemp., Mole Tynp.. Rlole O F . % CY F. % Cz --ZOO Lb. per Sa. In.--202.7 0 193 10 2 193.5 1 8 2 . 5 20 4 171 30 184.5 158.5 6 176 144 8 167.5 159.5 10 60 129 70 111 15 140.5 20 122.5 88.5 Q3 _74.5 30 40 69 57 49.5 .5 0 48.5 34 60 39 21 70 27 80 13 '0I 5 90 -6 100
4;
---400 270.9 264 257 250 235.5 217.5 200 169 141.5 117.5 98.5 81.5 67 53.5 42.5
Lb. per Sq. In.0 257 2 242.5 4 228 6 213 10 197.5 15 178.5 20 156.5 30 131 40 115.5 50 97 60 88 70 78.5 80 67.5 90 56 100 49.5
--600
Lb. Der . Sa.. In. 10.2 9.1 294 8.8 286.5 15 10 278.5 zn 12 262 30 14 244.5 40 16 226 .50 60 20 205.5 30 182.5 70 40 155 80 50 138 85 60 119.5 90 70 103 94 80 94.5 96 90 85 98 99 100 80
IO 20 30 40 50 60 70 80 85 9 '3 92 91 98 93 9'3
295 294 289.5 283 276.5 '770 257 '726 196 169 145.5 125 107 90 i5.j
311 32 411 511 GO 711 80 8.5 90 94 96 98
r--800 228 225.5 221 205 190 164.5 143 139 135 131.5 128 127.5
10 20 30 40 50 60 70 80 85 90 92 94 96 98 99
-
Lb. per Sq. In.50.0 227 47.0 224.5 50 215.5 55 193.5 60 165.5 70 149 80 137 82 128.0 84 86 88 89
52 54 60 70 80 85 88 89.8
in
lNU?
EQUILIBRIUM CONSTANT K =Y/X FIGURE8. COMPARISON OF EXPERICALCULaTED PRESSUREEQOILIBRIOM CONSTAXTCURVESAT CONSTANTTEMPERATURE FOR ETHANE IN %-HEPTANE BND ETHANE I N nBUTANE MENTAL AND
a given compound iS dissolved, the greater the temperature and pressure range over vhich the mixtures approximate perfect solutions.
---
EQUILIBRIUM CONSTANT K = Y/X
---
840 Lb. Der Sq. In.-196 64.3 195 194 63.8 191 65 190 186.5 68 181.5 181.5 176.5 70 175.5 171.8 72 169 73 169.7 167.5 74 168 167 75.2
ciated with the fact that mixtures of e t h a n e i n butane have lower critical temperatures and pressures and therefore a r r i v e earlier a t the critical state where the laws of ideal solutions fail altogether. Generalizing on these f a c t s , we m a y say that qualitatively the higher the molecular weight (i. e., the higher t h e crit;cay temperature) o f t h e m a te i al
357
66 68 70 72 74 76 76.3
the temperature and pressure but on the nature of the material in which i t is dissolved. U p t o 100" F. the agreement of the curves for ethane in butane arid in heptane with the theoretical curves is fairly good. The small deviations that do occur are greater in the case of ethane in heptane than of ethane in butane, as would be expected since ethane and heptane are more dissimilar than ethane and butane. With increasing temperature and pressure, however, the deviations increase a t a proportionally greater rate for ethane in butane than for ethane in heptane. Therefore a t 200" F. the values of the equilibrium constants for ethane in butane deviate widely from the theoretical whereas those for ethane in heptane are .;till in moderately good agreement. This behavior is asso-
FIGORE 9. COMPARISON OF EXPERIMENTAL AND CALCULATED PRESSURE-EQUILIBRIUM TEMPERACOXSTANT CURVESAT CONSTANT TORE FOR n-BOTkYE I N ETHANE
In Figure 9 the agreement of the n-butane data with the theoretical curves is excellent for all temperatures and for pressures up to 200 pounds per square inch. At the lower temperatures and a t pressures above 200 pounds per square inch the deviations increase rapidly with pressure, while a t the higher temperatures the deviations are relatively small even up to 500 pounds per square inch.
Literature Cited (1) International Critical Tables, Vol. 111, p. 230, New York McGraw-Hill Book Co., 1928. (2) Jessen and Lightfoot, IND. ENQ.CHEM., 28, 870 (1936). (3) Kay, I b i d . , 30,459 (1938). (4) Ibid., 32, 358 (1940). ( 5 ) Sherwood, "Absorption and Extraction", p. 105, New York, McGram--Hill Book Co., 1937. PRESENTED (in combination with t h e paper on page 358) before t h e Divisiori of Petroleum Chemistry a t t h e 97th Meeting of t h e Amerioan Chemica: Society, Baltimore, Md.
