I
C. H. BLOOM, C. W. CLUMP, and A. H. KOECKERT' Lehigh University, Bethlehem, Pa.
Simultaneous Measurement of
,
..
Vapor-Liquid Equilibria and Latent Heats of Vaporization This equilibrium still is a first step toward a complete description of enthal py-cornposition network of a system T H E DETERMINATION of the vaporliquid equilibrium and enthalpy of a mixture is important for both industrial design calculations and theoretical considerations of the thermodynamics of solutions and phase equilibrium. Determination of thermodynamic properties is also of utility to many industrial processes in determining minimum heat or work associated with a given change in the system. For complete definition of the enthalpy-composition network of a system, two types of data are necessary: for the liquid phase-isobaric specific heats of pure liquid components, heats of solution of the components a t a reference temperature and pressure, and isobaric specific heats of the mixture from the reference temperature and pressure to the bubble point of the system; and for the vapor phase-isobaric latent heats of vaporization of the mixture a t the reference pressure and vaporliquid equilibrium data. The still described here measures all the data required to complete the vapor phase structure; however, as a reference saturated liquid (bubble point) line cannot be established, the remainder of the data must be either calculated or experimentally determined by another means. The latter alternative was beyond the scope of the present investigation; therefore, vapor-liquid equilibrium and enthalpy measurements of any utility were restricted to systems for which the bubble point line may be calculated. Data for the ethyl alcohol-water system have proved the applicability of this still to the simultaneous measurements of vapor-liquid equilibrium and
Present address, Redstone Arsenal, Huntsville, Ala.
latent heat. With modifications to this design of the still, it should be possible to obtain data at pressures other than 1 atm. to increase still further the utility of the device.
Design Considerations T o obtain vapor-liquid equilibrium and enthalpy data simultaneously, the equipment is designed so that the mixture in question is a t constant pressure and composition. I n principle, the liquid mixture at the desired composition is brought to its bubble point and is then vaporized and contacted with a liquid mixture of equilibrium composition a t the Same temperature. Appropriate measurements of associated heat contents permit evaluation of the latent heat of the mixture. There are several important design criteria of the still. Recirculating Path. Flow calorimeters should provide for either a single pass or a recirculating path for the vapor phase. A recirculating path offers two important advantages: At steady state the recirculating stream is condensed a t constant composition and pressure, and vapor-liquid equilibrium can be assured. Designs of the single-pass type have been proposed (2,5, 9, 78). They have the advantage of accurate flow rate measurement, but uncertainties in correction for heat leaks precluded confidence in enthalpy measurements. Vaporizer Design. Two types of devices to generate vapor within the still have been proposed: I n one. condensate is vaporized in an external reboiler (7,77), and in the other the condensate is circulated directly to the equilibrium chamber prior to vaporization (72-74).The former type was selected.
Constant Vapor Flow. T o establish steady-state conditions for enthalpy measurements, vapor flow through the system is maintained constant. Measurement of Latent Heat Effects. Latent heat effects are measured from the rise in enthalpy of cooling water necessary to condense the saturated vapor associated with vapor-liquid equilibrium determinations. The major problem was to make sure that a saturated liquid leaves the condenser; then, the cooling water has removed only the integral isobaric heat of condensation. T o meet this requirement, a vapor-liquid contactor was used directly below the condenser. Vapor condensed in the reflux condenser flows down the wall into a short, insulated contacting section. T h e downflowing liquid is exposed to and partially condenses the rising vapor until the liquid is a t its saturation point. A trap at the bottom of the contacting section prevents the liquid from returning to the equilibrium flask. Feed to the condenser is a saturated vapor, and the liquid leaving is saturated; therefore, only the isobaric latent heat of vaporization has been removed by the condenser. Also, this condenser arrangement is able to degas exiting liquid (74). Pressure Control. T h e condenser is connected to a reservoir of confining gas. This system automatically compensates for the changing cooling demands and permits simple measurement and control of pressure. Adiabatic Walls. Heat losses from the equilibrium flask would cause partial condensation of vapor, thus establishing a nonequilibrium condition because of internal reflux. Also, heat losses from the contacting section would produce errors in enthalpy measurement. T o reduce such losses to essentially zero, adiabatic walls were used. VOL. 53, NO. 10
OCTOBER 1961
829
I
Limitations The constructed still for latent heat measurement and vapor-liquid equilibria determination has proved to be a promising device. However, its operation is restricted to 1 a t m . a n d to vapor systems which c a n be condensed with 25' C. cooling water a n d which boil u n d e r 130' C. I n addition, t h e still is not a d a p t a b l e to immiscible systems. It should be possible, however, to modify this device to permit its more general application; data for t h e ethyl alcohol-water system have proved t h e
Experimental
Equipment. The equilibrium still (below) for the simultaneous measurement of vapor-liquid equilibrium and latent heat is based upon the design considerations discussed above. To eliminate contamination problems, the entire set-up in contact with the system under study is fabricated of glass and Teflon. Liquid is vaporized in an external reboiler. Heat to the reboiler is supplied by two 4-foot-long, 250-watt heating tapes wound around the glass tube of the boiler. To vaporize the recir-
applicability of the design to t h e simultaneous measurement of latent h e a t a n d vapor-liquid equilibrium. It is hoped eventually t o incorporate a m e t h o d of measuring heats of mixing a n d isobaric specific heats of mixtures into the scope of this project, so that t h e complete thermodynamic network necessary for t h e construction of enthalpy-composition diagrams c a n be determined. It m a y also b e possible to apply the design t o systems of three or more components.
culating liquid completely without superheating the vapor, about 5 ml. of liquid is permited to accumulate at the elbow the reboiler. This removes any superheat from the vapor and assures saturated vapor. The vapor from the reboiler is directed into the equilibrium flask which contains about 100 ml. of liquid. The equilibrium flask consists of a single bubble cap to provide for effective vapor-liquid contact. The vapor temperature from the reboiler is measured by thermocouple TC2, and the liquid temperature in the equilibrium flask is
CONSTANT PRESSURE TANK
,TO
PRESSURE CONTROL
measured by TC5. A sample tube is provided to withdra\\ equilibrium liquid for analysis. Vapor from the equilibrium flask passes through the countercurrent: vaporliquid contacting section. Heat is removed from the vapor with a standard West-type condenser inserted into the contactor and is measured by the flow rate of water and the temperature difference of the outlet and inlet water This streams (thermocouple T C I ) . enthalpy represents the isobaric latent heat of vaporization of the mixture a t the vapor composition. The tempera-
n
Equilibrium still and auxiliaries are fabricated o f glass and Teflon to eliminate contamination
830
INDUSTRIAL AND ENGINEERING CHEMISTRY
V A PO R- L I QUI D 1Q UIL lBR IUM ture of the rising vapor is measured with thermocouple TC4, and thermocouple TC9 measures the temperature of the condensed vapor in equilibrium with the!iquid in the equilibrium flask. From the bottom of the condenser, the condensate passes into a small Liebig cooler where the temperature is reduced from that of a saturated liquid at the bubble point to 25' to 27' C. The liquid accumulates in a leveler which permits adjustment and observation of the liquid level .in the vaporliquid contactor. The liquid from the leveler represents the feed to the reboiler-Le., recycle back to the equilibrium flask. The liquid feed is metered with a rotameter calibrated at the flowing liquid composition after each run and jacketed with 25.0' C. constant temperature cooling water to ensure accurate determination of the feed rate. Provison is made for sampling the condensed equilibrium vapor. In addition, the cooling water system provides water at 25.0' C. at a constant controlled rate to the calorimetric condenser. A standard Thyratron relay circuit is used for temperature control. Water for enthalpy measurements flows from a constant head tank into the condenser calorimeter. A small surge tank is provided to reduce temperature fluctuations. Measurement of the cooling water flow rate is made by a direct weighing procedure. The adiabatic walls consist of two lengths of aluminum pipe-one around the equilibriumflask and the other around the contactor. These sections of pipe are wound with heating tape, and their temperature is controlled to that within the equipment. Insulation was also placed between the device and the pipes and on the outside of the pipes. Because the regions between the equipment and the enclosures are isothermal, no heat is transferred through these regions and the device operates adiabatically. A Leeds and Northrup Type K-3 potentiometer and a galvanometer are used with copper-Constantan thermocouples for all temperature measurements. The instrument, used with guarded leads and standard cell for the elimination of stray potentials in the thermocouple circuit, is accurate to zt0.5 pv. or 0.02" C. for the equilibrium temperatures. The thermocouples were calibrated against thermometers certified by the National Bureau of Standards. Procedure. At the start of an experimental run, the equilibrium flask, the vapor-liquid contactor reservoir, the leveler, and the recirculating lines were charged with the test mixture. After adjusting the temperature and flow rate of the cooling water to the
condenser and liauid subcooler. heat was applied to the adiabatic walls and to the reboiler. Flow was slowly started until a steady-state boiling was reached and liquid levels could be maintained at steady state. The still was run until constant values of liquid and vapor temperatures indicated equilibrium. The usual length of an entire run was about 5 hours. After equilibrium was attained, the cooling water rate was measured. At the end of the run samples were taken of the liquid from the equilibrium flask and of the condensed vapor. The feed rotameter was then calibrated for the condensed vapor feed by a direct weighing procedure. The level in the leveling device, as well as temperature measurements throughout this system, was used as a gage of steady-state operation. Changes in the level indicated an exchange in mass between the liquid and vapor phases ; thus, enthalpy measurements would not be valid unless the level were constant. Validity Tests. To test the validity of the equilibrium still, the binary system, ethyl alcohol and water, for which complete data are available, was used as the test system. Analyses were made by a modified method of Karl Fischer for water (70). Ethyl alcohol was determined by difference, except in the case of low alcohol concentrations (less than 20 mole %) for which refractive indices were used. A Carl Zeiss Jena interferometer was used to determine the refractive indices. Absolute ethyl alcohol of U.S.P. grade, supplied by U. S. Industrial Chemicals Co., and laboratory distilled water were used without further purification.
Results
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Vapor Liquid Equilibrium Data. The vapor-liquid equilibrium data obtained in this study and activity coefficients, calculated from the following equations:
are presented in Table I. I n computing activity coefficients, the vapor pressures of ethyl alcohol were plotted from data in the International Critical Tables (6) and the vapor pressures of water from the data of Keenan and Keyes (8). These plots were used for the calculations. The vapor-liquid equilibrium data were compared with reliable data of other investigators (4,7, 75). I n the liquid region, the best line through the data points of this work does not deviate from the data of the other investigators by more than k0.2' C., and in the vapor region the deviation is not greater than i10.6" C. This is considered an adequate representation of the data, as the deviations among the comparative data of the other investigators are as much as 0.3' C. in the liquid phase and 0.8" C. in the vapor phase. Calculated values of activity coefficients of ethyl alcohol and water were compared with the values of other investigators (7, 75). The values of activity coefficients do not deviate by more than 2% from these other data. Latent Heat of Vaporization Data. Latent heats were calculated by heat balance as determined by the following equation: m,C,dT
= Am,
I- Q
Q is a heat leak correction. There is an opportunity for heat transfer be-
Table l. Activity Coefficients for Ethyl Alcohol-Water Were Calculated Using Existing Vapor Pressure Data Temp., O C. Liquid Vapor
94.3 91.9 90.0 87.3 86.1 85.2 84.7 81.8 81.6 80.6 80.2 79.5 79.2 79.1 78.6 78.3 78.3 78.2
82.1 82.0 81.2 80.4 80.7 81.2 80.0 79.7 79.7 79.2 78.8 78.8 78.6 78.6 78.4 78.3 78.2 78.2
21
YZ
0.021 0.033 0.050 0.085 0.106 0.125 0.135 0.315 0.321 0.403 0.403 0.556 0.602 0.643 0.689 0.805 0.926 0.987
1.000 1.012 0.999 1.030 1.01 1.05 1.055 1.26 1.27 1.33 1.33 1.60 1.69 1.78 1.88 2.18 2.53 2.64
0.199 0.272 0.353 0.411 0.458 0.488 0.484 0.571 0.572 0.619 0.625 0.675 0.695 0.713 0.741 0.814 0.917 0.985
1430 1260 1180 1070 1020 995 970 870 865 835 825 795 785 782 770 760 760 755
VOL. 53,
625 565 525 475 455 418 430 381 378 365 360 348 345 343 336 332 332
5.03 5.97 4.55 3.41 3.22 2.98 2.81 1.58 1.56 1.40 1.43 1.16 1.11 1.08 1.07 1.01 0.990
331
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NO. 10
OCTOBER 1961
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tween the adiabatic wall (the upper aluminum cylinder) and the condenser jacket water. The heat transferred was found to be dependent only on the temperature of the aluminum wall. Heat gains by the cooling water were found from 2 to 50/, of the total heat measured. The reference enthalpy data for the pure components were taken from the International Critical Tables (6). The reference enthalpy data for the mixtures were determined by Bosnjakovic (7) and by Smith and others (77). These results are compared with the results of this work in Table 11. I t was found that the experimental results are within 291, of the values of Bosnjakovic. Experimental values from this work are plotted on the graph below. An analysis of errors in latent heat measurement indicated that errors in calibration of the rotameter did not exceed 0.5%, errors in the measurement of the cooling water rate were less than O . l % , and errors in measurement of the temperature rise across the condenser did not exceed 0.2%. I t is felt that the latent heat data are accurate to d=2.0%,.
Table II. Comparison of Latent Heat Data for Ethyl Alcohol-Water Shows Good Agreement Latent Heats, B.t.u./lb. Yl
This work
0.675 0.625 0.619 0.572 0.571 0.484 0.411 0.272 0.199 0.985 0.917 0.000
462 482 483 508 493 550 585 694 774 367 379 962
(1) 460 484 486 503 503 548 580 690 756 3 70 382 972
Nomenclature
C,
H rn,
rn,
P Po
spzcific heat of water, B.t.u./lb.C. = enthalpy, B.t.u./lb. = mass of water flowing, lb./min. = mass of material in system flowing, lb./min. = total pressure, mm. of Hg = pure component vapor pressure, mm. of Hg
=
800
m
< 600
3
2
i 400
200
0 04
06
0.8
1.0
MOLE FRACTION ETHYL ALCOHOL
Enthalpy-concentration diagram was constructed using experimental values obtained with the equilibrium still
832
x
= mole fraction in the liquid = mole fraction in the vapor = activity coefficient = isobaric latent heat of vaporization. B.t.u./lb.
y
y
X
Subscripts 1
= ethyl alcohol
2
= water
Acknowledgment
The advice and suggestions of Louis Maus, Jr., during the design stages of the still are greatly appreciated. References
1000
052
= heat leak correction, B.t.u./min. = temperature, “C.
(17) 473 495 497 516 516 563 603 695 753 375 378 972
I200
0.0
Q T
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Bosnjakovic, F., “Technische Thermodynamic,” T. Steinkopff, Leipzig, 1935. (2) Callendar, H. L., Barnes, H. T., Phd. Trans. Roy. Soc. London A199, 55 (1902). (3) Chao, K. C., Hougen, 0.A., Chem. Eng. Sa. 7 , 246 (1958). (4) Cornell, L. W.,- Montonna, R. E., IND.ENG.CHEM.23, 1331 (1933). (5) Dana. L. I., Proc. Am. Acad. Arts Scz. 60, 241 (1925). (6) “International Critical Tables,” McCraw-Hill, New York, 1928. (7) Jones, C. .4.,Schoenborn, E. M., Colburn. A. P., IND. ENG. CHEX 35, 666, 1943. (8) Keenan, J. H., Keyes, F. G., “Thermodynamic Properties of Steam,” Wiley, New York, 1936. (9) McCracken, P. G., Smith, 3 . M., A.I.Ch.E. Journal 2, 498 (1956). (10) Mitchell, J., Kolthoff, I. M., Proshauer, E. S., Weissberger, A., “Organic Analysis,” p. 151, Interscience, New York, 1956. (11) Murti, P. S., Ph.D. dissertation, Univ. of Texas, 1956. (12) Othmer, D. F., Anal. Chem. 20, 763 (1948). (13) Othmer, D. F., IND.EWG.CHEM.20, , ,., , 11928). 741-6 (14) Othmer, ‘ D. F., Ind Eng. Chem., Anal. Ed. 4, 232 (1932). (15) Otsuki, H., Williams, F. C., Chem. En‘np. Progr. Symposium Ser. 49, hTo. 6, 55 (1 553). (16) Redlich, O., IGster, A. T., IND.EXG. CHEM.40, 345 (1948). (17) Smith, D. A,, Kuong, J., Brown, G. G., M7hite, R. R., Petrol. Rejner 24, No. 8, 296 (1945). (18) Tallmadge, J. A., Schroeder, D. W., Edmister, W. C., Canjar, L. N., Chem. En,c. Progr. Symposium Ser. 50, No. 10 (1g54).
RECEIVED for review February 11, 1960 ACCEPTED May 1, 1961 Work supported by fellowship grants from The Texas Co. and Socony Mobil Oil Co.
