December 1949
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 LITERATURE CITED
Landee, F. A , , and Johns, I. B., Zbid.! 63,2891 (1941).
286'2
(10) Langdon, W.M . , and Keyes, D. B., IND.ENG.CEEM.,34, 938
RECEIVEDSeptember 29, 1948.
Vapor-Liquid Equilibrium Still for Partially Miscible Liquids THOMAS E. SMITH AND ROBERT F. BONNER U. S. Industrial Chemicals, Inc., Baltimore 3, M d .
h simple vapor-liquid equilibrium still is described for use with solutions of partially miscible liquids. The rapidity with which this unit may be set up and operated makes it we11 adapted to the procurement of x-y data needed for the design of plant distillation equipment. Experimental vapor-liquid equilibrium data obtained with this still for the system 1-butanol-water were found to meet the thermodynamic requirements of the differential Gibbs-Duhem equation. The van Laar integrations of this equation, however, were inapplicable. Except at low water concentrations, the data were in general agreement with the values reported by Stoclrhardt and Hull. ANY types of equilibrium stills have been proposed for use with miscible liquids, but for systems of partially miscible .liquids few units have thus far been described in the literature. Determination of vapor-liquid equilibrium data for a partially miscible system such as 1-butanol-water presents a special problem in that the condensate for a given concentration range forms two liquid phases. The ordinary condensate-recirculation type unit cannot be used because the heavier layer will build up in the condensate trap. The type of still used by Stockhardt and Hull (9) on butanolwater systems, although simple t o set up and operate, tends t o produce vapors too rich in the more volatile component. Fractionation is probably caused by the scrubbing action of reflux which passes down the neck of the apparatus prior t o sampling. The apparatus of Colburn, Schoenborn, and Shilling (4) has not been tested by this laboratory, because i t appeared rather difficult to operate. The apparatus described in thLg paper is simple t o operate and has been shown to give thermodynamically consistent results. APPARATUS
The object of all vapor-liquid equilibrium stills is to obtain for analysis samples of a liquid and a vapor which are in equilibrium. The simplest method of obtaining such samples is to distill a relatively small amount of vapor from a flask containing a large quantity of liquid. Theoretically, of course, the vapor sample should be infinitesimal but, in practice, where a measurable sample must be taken, this condition is approached by using a large ratio of boiling liquid to condensate sample. There are
two main disadvantages to this differential distillation method: A large sample is required to charge the still; and unless special precautions are taken, condensation will occur on the neck of the flask aausing fractionation. The still described in this paper (Figure 1) was designed for determining vapor-liquid equilibrium data by the method described in the preceding paragraph. It is similar in principal to the unit of Baker, Hubbard, Huguet, and Michalowski (2)and follows the general design of the Othmer-type still (7). The still consists essentially of a 1-liter distillation flask, a reflux condenser, and condensate return and take-off lines, both of which are provided with coolers. The boiling flask, G, is equipped with standard-taper No. 22 joints, A , for the 0.1 O C. liquid and vapor thermometers and a standard-taper No. 9 plug, C, as a vent for the neck. To he1 prevent fractionation, a va orjacketed tube, E, is providecffor ascension of the vapors, $he ring seal, D,is tilted so that any liquid condensed above this point will not run down the vapor tube, E,but will drain to the liquid return line, J,and hence back t o the still. Heat is supplied by an internal resistance coil, I , consisting of 10 feet of No. 22 Nichrome resistance wire attached to a No. 14 tungsten lead wire, H,which is sealed t o the Pyrex. A standard-taper 24/40 joint, B, is provided for a reflux condenser, which is used instead of a condenser-cooler because the latter tends t o return dissolved air t o the boiling flask. The two coolers, F, are constructed of tubing 4 and 10 mm. in outside diameter. The water outlet of one cooler is connected to the inlet of the other by tubing 8 mm. in outside diameter. The condensate return cooler is connected to the distillation flask by a piece of 2-mm. capillary tubing. A three-way stopcock, K , is provided for sampling the condensate. The entire apparatus, with exception of the heating coil, is made of Pyrex. T o determine the tem rature of the neck walls, an iron-constantan thermocouple is f%d by asbestos ta e against the center of the flask neck. The neck is covered wit! a sheet of asbestos paper 1/16 inch thick, over which are wrapped 10 feet of Nichrome resistance wire. By this means, the walls of the neck can be msintained at any desired tem erature. The whole flask is insulated with 0.5 inch of shreddea asbeatos cement. A magnetic agitator is used to stir the liquid in the boiling flask The Swietoslawski-type ebulliometer used t o obtain boiling point data for the system 1-butanol-water has been proved capable of determining boiling points with a high degree of accuracy. A detailed description of this apparatus is given by Swietoslawski (If). Superheating is avoided by separating thr space in which the liquid is brought to a boil from that in which the boiling temperature is measured.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 12
These runs were made in the folloR-ing manner. Approximately 700 ml. of butanol-water mixture were weighed into the still. All weighings over 25 grams were made on a torsion balance with a 0.05-gram sensitivity, all under 26 grams on an analytical balance. The magnetic agitator was then turned on and about 80 volts were applied t o the internal heating element, When the liquid began to boil, all air was vented from the annular space, thus surrounding the vapor tube with a n insulating blanket of vapor. The standard-taper plug was replaced when the vapor began to condense on the standard-taper joint. A potential of 15 to 20 volts was then applied t o the neck heater and the vapor tem erature was brought up to approximately 8' C. higher than the {quid temperatlurein order to assure no condensation of vapor on the walls of the neck. The temperature of the neck mas usually a few degrees higher than the vapor temperature. The liquid and vapor temperatures were measured by inrans of 0.1 C. thermometers, the neck temperature by a millivoltmeter calibrated directly in degrecs centigrade. The still was allowed to operate for about 1 hour, the 5" C. differential between liquid and vapor temperature being maintained as closely as possible. The temperatures were then recorded and a IO-ml. condensate sample was drawn off through the auxiliary cooler into a weighed 10-ml. graduate immersed in ice to prevent evaporation losses. The still was then operated for 0.5 hour, the temperatures were again recorded, and a second sample was withdrawn after cleaning the auxiliary cooler with pipe cleaners. The two graduates were then weighed on an analytical balance and the weights of the distillates were recorded. Between runs the still was cleaned out by refluxing acetone in it for 15 minutes and then drying it under vacuum. A simple entrainment test was made by distilling 1-butanol dyed an intense color with an organic dye. Even a t a distillation rate many times in excess of that a t which the still waq operated when procuring data, no color could be detected in the distillate. O
AIVALYTICAL PROCEDURE
CENTIMETERS
Figure 1. Vapor-Liquid Equilibrium Still PURIFICATION OF MATERIALS
The 1-butanol used was purified by distillation a t a high reflux ratio through a 20-plate glass bubble-cap column. The portion used for the experimentas boiled a t 117.7' C. (corrected to 760 mm.), had a refractive index of 1.3973 a t 25" C., and contained 0.02 weight % ' yater. These constants agree very well with the values reported in the literature for pure 1-butanol. DETERMINATION OF VAPOR-LIQUID EQUILIBRIUM DATA
Equilibrium vapor compositioras were determined for a series of 16 liquid 1-butanol-water mixtures varying in composition from approximately 5 to 99 mole % water. These data are summarized in Table I. Condensate samples 1, 2, and 3 were homogeneous, whereas all others Rere heterogeneous. All liquid samples were homogeneous a t their boiling points, with the exception of 10, 11, and 12.
TABLE I. EXPERIMENTAL VAPOR-LIQUID EQUILIBRIUMDATA Sample NO.
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16
FOR S Y S T ~~-BUTANOL-WATER M AT 767 MM. Boilinga Mole % Water Activity Coefficient Point, Liquid, Vapor, Water, 1-Butanol
c.
110.95 106.85 106.40 100.85 96.65 96.35 94.00 93.02 93.00 92.70 92.70 92.70 92.80 92.86 95.40 95.80
21
Ill
Yl
Y2
5.0 9.2 9.7 18.1 29.1 30.3 41.7 54.6 55.0 76.2 90.1 90.2 98.0 98.1 99.1 99.2
26.3 38.8 40.2 55.6 66.0 66.6 72.4 75.0 75.3 75.4 75.4 75.4 76.0 76.3 83.9 85.0
3.494 3.352 3.342 3.010 2.585 2.535 2.179 1.789 1.783 1.321 1.103 1.102 1.019 1.020 1.010 1.007
1.020 1.028 1.028 1.052 1,108 1.121 1.222 1.482 1.476 2.708 B 783 6 852 32.64 33.81 43.56 44.82
The samples were analyzed in duplicate by the Karl Fischer method (8). Heterogeneous samples were made homogeneous with a minimum amount of dry methanol, usually about 1 ml. As the amount of methanol added was known, the water content of the sample could be calculated back to a methanol-free basis.
I2C
I IE
I12
104
I0C
9E
I
a Values in this column read from boiling Point-composition aurve (Figure 2) determined with Swietoslawski-type ebulliometer.
92
Figure 2. Boiling Point-Composition Curve for 1-Butanol-Water a t 767 M m .
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1949
2869
thermodynamic consistency. If the vapors obey the perfect gas law, which is usually the case for 'pressures up t o one atmosphere, the data can be evaluated from a thermodynamic standpoint b y application of the GibbsDuhem equation. Although this equation was originally derived assuming isothermal conditions, the following modified form (3) may be used, without serious error, to test isobaric vapor-liquid equilibrium data dlogi = dx
1 -Xdlogyz x
dx
(1)
where
Figure 3.
Vapor-Liquid Equilibrium Diagram for 1-Butanol-Water a t 767 M m .
Consequently, if we plot the log y1 and the log yz against T , the relationship between the slopes of the two curves is given by Equation 1. Moreover, certain qualitative deductions (3)made by inspection of a plot of this type permit ready detection of errors in experimental data. T h e correlation of z-y data by this method requires accurate vapor pressure data for the pure components and accurate knowledge of the boiling points of the liquid mixtures for which the equilibrium vapor compositions have been determined. These data are used t o calculate the denominator of the activity coefficient. The values for y reported in this paper were calculated by using vapor pressure data for water taken from Lange
DETERMINATION OF BOILING POINT DATA
For reasons discussed below, i t was decided t o determine the boiling point data for this system with the Swietoslawski-type ebulliometer previously referred to. Eleven determinations were made covering a concentration range of 0 to 80 mole yo water. The results are recorded in Table 11. The volumes of the mixes had to be such as to bring the liquid level to a point about 15 mm. below the outlet tube when the liquid was raised t o its boiling point. This apparatus oould not be used t o determine boiling points a t high water concentrations (97 to 100 mole yo water) as the condensate oil layer kept building u p in the return line, thus appreciably changing the pot composition. D a t a determined by Stockhardt and Hull were used in this region of the curve.
TABLE 11. EXPERIMENTAL BOILING POINT-COMPOSITION DATA FOR SYSTEM ~ - B U T A N O L - ~ AAT T E767 R MM, Mole % ' Water, XI 0.33 6.60 10.05 16.55 20.66 30.33 40.28 42.20 50.32 60.45 80.11 Data determined with Swietoslawski-type ebulliometer.
When plotted, the data formed a smooth curve (Figure 2) which was in general agreement with the values reported by Stockhardt and Hull. CORRELATION OF DATA
As was pointed out by Carlson and Colburn (S), a plot of experimental data in the form of a n 2-y curve shows the deviation of the points from a smooth curve but gives no idea of their
Figure 4.
Activity Coefficient Curves for 1-Butanol-Water
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. k2
This tabulation shows the experimental data to be thernro dynamically consistent throughout the entire concentration range. Also plotted in Figure 4 are the activity coefficient curves foi Btockhardt and Hull’s data and the theoretical curves calculated from the authors‘ azeotropic composition and boiling point value* using the van Laar solutions (3! 6 ) of the differential Gibbs. Duhem equation. Although Stockhardt and Hull i: data fall on a fairly smooth curve when plotted in the form of an s-y. diagran (Figure 3), the activity coefficient plot reveals obvious discrepancies in the data a t low water concentrations. Not only do thc points fail to fall on a smooth curve, but the values for the lop yz are negative below 10 mole % water in the liquid. This condi tion is theoretically impossible, as the log y1 curve i d alwayb greater than zero and has no maximum or minimum. Because Stockhardt and Hull’s boiling point data agree rather well with the authors’ values, this inconsistency is probably due to a slight Fractionating effect in this region where there is such a large difference in composition between %he liquid and the vapor As was previously pointed out, this enriching effect was probabl? caused by the scrubbing action of reflux which passed down the neck of their apparatus prior to sampling. White (12) has shown that the van Laar solutions of the &bKh Duheni equation may be rearranged in such a manner as to give straight-line plots. Van Laar’s equations as rearranged by Cad eon and Colburn are:
Figure 5. Activity Coefficient Plot for 1-Butanol-Water by White’s Method
These equations may be written h the following straight-liar iorms.
{ 6 ) and values for 1-butanol reported by Stull (10) and Arnold and Lessig ( I ) . The experimental data, when plotted in the form of an z-g diagram (Figure 3), form a good smooth curve. The composition of the liquid in equilibrium with B given vapor was taken to be the average of the pot composition before and after withdrawal of the condensate sample. As the 1-butanol used was very dry, it was unnecessary t o correct for its water content. When an attempt was made t o correlate the data by activity coefficient plots, however, inconsistent results were obtained a t low water concentrations. The fact that the values for both 71 m d y2 were low in this range indicated that the bailing points measured in the still were too high. Boiling point measurements on pure 1-butanol and pure water proved this to be the case. I t was found impossible to eliminate superheating of the liquid m spite of the internal heating coil and magnetic agitator. Comparison of the authors’ liquid temperatures at a later date with a boiling point curve determined with a Swietoslawskitype ebulliometer showed the superheat to vary from 0.1”to L.2 C., depending on the composition of the mix. Using boiling points read from the boiling point-composition curve (Figure 2 ) determined by the ebulliometric method, the authors recalculated the activity coefficients for y1 and 72. The results are plotted in Figure 4. Smooth curves that met the qualitative requirements of the differential Gibbs-Duhem equation were obtained. Quantitative comparison of the slopes of the y curves by application of Equation 1 yielded the following results: O
Mole fraction Theoretical slope ratio Experimental slope ratio
0.25 -3.00 -2.90
0.50 -1.00
-0.97
0.76
-0.38 -0.33
The authors’ activity Coefficients have been plotted b j thae method in Figure 5. The data plotted for f(yl) cover the liquid concentration range 0 to 75 mole yo water, those for f(m)! 18 tci 100 moIe ?& water. It is impossible to include all the data OII one plot, as the mole ratios run from aero to infinity. The function plots as a straight line for the entire miscible regiori (0 to 60 mole % water in the liquid), whereas the yz functioii shows considerable curvature throughout its entire range, in dicating inapplicability of the van Laar equations. 811 the points for both sets of data fall on smooth curves, showing tbr experimental values to be consistent. Stockhardt and Hull’s data are plotted in Figure 5 by thw method for comparative purposes. Here, as with the ordinar) activity coefficient plot, there is evidence of erroneous data BP low water concentrations. The values for f(y1) are low in t h i ~ region; consequently the values for y1 are too high. The corpr sponding values for yz are less than unity, malting log y z ncpptivt and f(y2) imaginary. White, owing to the lack of sufficient experimental data concluded that both functions of y versus the corresponding mol6 ratios form straight-line plots. Also platted in Figure 5 are data calculated by the vau Lam equations from the authors’ azeotropic values. This method of plotting is most useful for correlating data for systems that o h the van Laar solutions of the Gibbs-Duhem equation.
December 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
The calculated &shaped x-y curve (Figure 3) is theoretically unpossible and probably results from the breakdown of this purely mathematical method in the region of two liquid phases. However, this curve can be used in the miscible regions for making theoretical plate calculations accurate enough for engineering purposes. ACKNOWLEDGMENT
The authors wish t o thank B. J. Gaffney and L. W. Bass of U. S. Industrial Chemicals, Inc., for their helpful criticism of this paper. Thanks are also due t o J. C. Mannherz for the drafting of the figures. NOMENCLATURE
arbitrary constant in van Laar equations. Equal to log y1 a t x2 = 0 arbitrary constant in van Laar equations. Equal to log 7 2 at z2 = 0 total pressure, mm. of mercury vapor pressures of pure components, mm. of mercury mole fraction in liquid mole fraction in vapor in equilibrium with x activity coefficient,
2831
Subscripts 1 = low-boiling component (water) = high-boiling component (1-butanol) LITERATURE CITED
Arnold, H. R., and Lessig, E. T., in Perry’s “Chemical hrlm neers’ Handbook,” 2nd ed., p. 377, New York, McGraw-Hil) Book Co., 1941. Baker, E. M., Hubbard, R. 0. H., Huguet, J. H., and Miohalowski, s. s., IND.ENG.CHEM.,31, 1260 (1939). Carlson, H. C., and Colburn, A. P., Zbid.,34, 551-9 (1942). Colburn, A. P., Schoenborn, E. M., and Shilling, D., Zbid., 35 1252 (1943). Laar, J. J. van, 2.physik. Chem., 72, 723 (1910); 83, 599 (1913) Lange, “Handbook of Chemistry,” 6th ed., Sandusky. Ohio Handbook Publishers, 1946. Othmer, D. F., Anal. Chem., 20, 764 (1945). Smith, D.M.,Bryant, W. M. D., and Mitchell, J., Jr., J . Am Chem. Soc., 61,2407-12 (1939). Stockhardt, J. S., and Hull, C. M.,
[email protected].,23,1438-40 (1931). Stull, D. R., Ibid., 39,622 (1947). Swietoslawski, “Ebulliometric Measurements,” Chap. I, New York. Reinhold Publishing Corm. 1945. White, R. R., Trans. Am. Init. Chem. Engrs., 41,539-54 (1945) RECEIVED Msrch 21, 1949.
Phase Equilibria in Hydrocarbon J
Systems J
Phase Behavior in the Methane-n-Butane-Decane
System at 160” F.
H. H. REAMER, J. M. FISKIN, AND B. H. SAGE California Institute of Technology, Pasadena, Calg. The compositions of the coexisting phases in the methane-n-butane-decane system have been investigated at 160’ F. Experimental measurements were made at pressures of 1000, 2000, 3000, and 4000 pounds per square inch absolute. The data obtained permit the evaluation of the compositions of the phases and the equilibrium constants in relation to pressure. The results are presented in tabular and graphical form.
T
HE phase behavior of many binary paraffin hydrocarbon
Y
-
systems has been investigated and the data indicate marked divergences from simple generalizations. These studies have been supplemented by a limited number of measurements of the composition of coexisting phases in ternary paraffin hydrocarbon systems (1, 3, 4). This work emphasized the importance of the nature and amount of the other components upon the distribution of any one hydrocarbon between the liquid and gas phases. In order to investigate more fully the characteristics of ternary systems a study of the volumetric and phase behavior of the methane-nybutane-decane system was undertaken. This work consisted in part of a relatively detailed study of the pertinent binary systems. T h e compositions of coexisting phases and the volumetric behavior of mixtures of methane and n-butane were investigated (11,15, 16). Similar measurements for the methanedecane system were completed (12, 20) and limited information was reported for the n-butane-decane system in the condensed region (19). The pressure-volume-temperature relations of methane (9) and the volumetric and phase behavior of n-butane (&IO) and of decane (1.3,.31)have been established. I n addition the influence of pressure and temperature upon the volume of
several mixtures of methane, n-butane, and decane is known (14). Using all of this information it is possible t o estimate with reasonable accuracy the phase behavior of the coexisting liquid phase throughout the majority of the heterogeneous region of thie ternary system for temperatures above 100’ F. In order t o establish with certainty the location of the combining lines and of the dew-point curve, measurements were taken of the composition of the coexisting phases. Although this work covered several temperatures, the measurements obtained at 160 O F have been presented first in order t h a t the details of the analpt,ica! procedures and equipment employed might be discussed. METHODS AND PROCEDURES
The equipment utilized in this investigation has been described (17, 18). Essentially i t consists of a stainless steel cham. ber within which the hydrocarbons are confined over mercury This chamber is located within an agitated oil bath, the temperature of which is controlled automatically within 0.01” F of the desired value relative t o the international platinum scale The quantity of mercury introduced into the equilibrium vesseE may be varied, thus changing the effective volume of the apparatus. The pressure is measured by means of a‘balance calibrated against the vapor pressure of carbon dioxide at the ice point. Equilibrium is obtained by use of a spiral agitator rotated about a vertieal axis. A movable probe was employed t o ermit the levels of the mercury-hydrocarbon and the gas-liquid Iydrocarbon interfaces t o be ascertained. Sampling ports were provided to permit withdrawal of samples of the liquid and gas phases at equilibrium. T h e connectin tubing between the equilibrium e uipment and the analyticaf apparatus was heated to avoid con%ensation of the less volatile components. The pressure was maintained at a substantially uniform value during
