Ind. Eng. Chem. Fundam. 1982,27, 173-175 Higglns, B. G.; Slillman, W. J.; Brown, R. A.; Scrlven, L. E. Ind. Eng. Chem. Fundem. 1977, 16, 393. Higglns, B. 0. Ph.D. Thesis, University of Minnesota, Minneapolls, MN, 1980. Huh, C.: Scriven, L. E. J. Co/bU Interface Sci. 1969, 30, 323. JoseDh. D. D.: Fosdlck. R. L. Arch. Rational. Mech. Anal. 1973. 49. 321. Jose&; D. D.; Sturges, L. J. FIuidMech. 1975, 69. 565. Komar, A.; Yajnlk, K. S. J . FluidMech. 1980, 97, 27. Orr, F. M., Jr.; Brown, R. A.; Scriven, L. E. J. Colloid Interface Sci. 1977, 60. 137.
173
Rwchak, K. J. Ph.D. Thesis, Universlty of Minnesota, Minneapolis, MN, 1974. Salto, H.; Scrlven, L. E. J. Comput. mys. 1981, 42, 53. Silliman, W. J. Ph.D. Thesis, University of Minnesota, Minneapolis, MN, 1979. Wilson, S. J. Fluid Mech. 1969, 38, 793.
Received for review January 23, 1981 Revised manuscript received August 3, 1981 Accepted February 1, 1982
EXPERIMENTAL TECHNIQUES Modified Sealed-Tube Method for the Determination of Critical Temperature Edllberto Mogollon and Webster B. Kay' Department of Chemlcal Engineering, Ohio State University, Columbus, Ohio 432 10
Amyn S. Teja School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Oeorgia 30332
A modified sealed-tube method for the determination of the critical temperatures of pure fluids and fluid mixtures is described. Critical temperatures of ten normal alkanes from n-octane to n-heptadecane have been determined and compared with values found in the literature. The method is capable of yielding data of high precision and is suitable for the study of thermally unstable compounds.
Introduction The determination of the critical temperature of a pure compound or mixture by heating a sample in a sealed glass tube until the liquid-vapor interface is replaced by an opalescent band is commonly referred to as the "sealedtube" method. It is a relatively simple technique which requires only that the amount of sample enclosed be such that ita volume at the critical temperature be equal to the volume of the sealed tube and is capable of yielding data of high precision (Ambrose et al., 1957,1960,1962; Cheng et al., 1962). Modifications to the design of the furnace and of the sealed tube have been made which make it suitable for the study of thermally unstable compounds. In this report, the apparatus and procedure are described and results of the determination of the critical temperatures of the nalkanes from n-octane to n-heptadecane are presented. Experimental Section Apparatus. The furnace was constructed by the Ace Glass Co. and consisted of a quartz tube 5.5 cm in diameter and 46.8 cm long surrounded by a Pyrex glass tube of 9.0 cm diameter. A metallic film of controlled thickness was deposited on the outside surface of the quartz tube. This served as the electrical resistor of the furnace. The current was regulated by means of an autotransformer with a voltage regulator to maintain a constant power supply. 0196-4313/82/1021-0173$01.25/0
With a power input of 750 W, a temperature of 857 K could be maintained. Two diametrically opposed windows 6.5 cm long and 0.32 cm wide and a third window of the same size at 90° to the axis of the other two were provided in order to observe the sample during the heating process. The top of the furnace was closed by a loose-fitting aluminum plug; the bottom was closed by a Teflon plug with an O-ring to ensure a tight fit. The plugs were drilled to take a 0.64 cm diameter stainless steel tube which served as a support for the sealed tube. The furnace was mounted on a trunion and rotated through 90° to provide mixing of the sample during the heating period. The glass ampule for holding the sample is shown in Figure 1. It was constructed from borosilicate glass tubing (0.55 cm i.d. with a 0.2 cm wall thickness) and was provided with a small well about 5.1 cm deep at one end into which an iron-constantan thermocouple was inserted. The thermocouple output was recorded continuously by means of a Leeds and Northrup Speedomax H recorder with a chart speed of 30.5 cm/min and with a sensitivity equivalent to 0.02 K. The thermocouple was calibrated over the range 475 to 975 K by comparison with an NBS certified platinum thermometer. Figure 2 shows the sealed tube and thermocouple assembly. E is the stainless steel tube to which the glass tube G and aluminum cap C are attached by clamps B. The thermocouple A enclosed in a ceramic tube D was inserted 0 1982 American Chemical Society
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Ind. Eng. Chem. Fundam., Vol. 21, No. 2, 1982
.
095-
-_
Table I. Critical Temperatures of the n-Alkanes
.___
t
purity mole, %
exptl T C ,K
99.85 99.68 99.49 99.90 99.70 99.82 99.9 99 99.8 98
568.80 593.61 617.54 f 0.05 638.7 I0.3 657.7 i 0.3 676.2 i 0.5 692.8 * 0.4 709.2 f 0.5 723.0 i 2.0 736.0 i 3.0
Ambrose (1980) TC.K
~
n-octane n-nonane n-decane n-undecane n-dodecane n-tridecane 4THERMOCOWLE I NELL ---L--i=--* 040.
-
,
5Y I
DIMENSIONS IN CfNTIMETERS
(a) AMPOULE - TUBE
~ b SEALE3 ) AI'VIPOL,E
Figure 1. Dimensions and design of sealed tube. TO
RECORDER-
H-4 'L
Figure 2. Tube assembly. through a hole in the aluminum plug with the tip of the couple at rest in the well F. The bottom of tube E was mounted in the Teflon plug so that the tube and thermocouple were held in a fixed position in the furnace in front of the observation windows. A small stream of nitrogen gas was introduced through tube E and the hole H and escaped through a small hole (not shown) in the aluminum plug. This was to provide an inert atmosphere in case the tube burst during heating. Procedure. Prior to starting a critical point determination, the empty furnace, closed at the top by an aluminum plug, was heated to the estimated critical temperature using as a guide time-temperature curves previously obtained on the furnace for a series of constant voltages. The sealed tube was then inserted into the furnace and, simultaneously, a stopwatch was started to record the time of heating. A few minutes later, the temperature recorder was turned on. During the heating, the furnace was rotated through 90° continuously to mix the sample and achieve a uniform temperature. As the temperature approached the estimated critical temperature, the rate of heating was reduced by a decrease in voltage and the liquid-vapor meniscus gradually changed to a distinct band of opalescent fog which was smoke-gray in reflected light and light brown in transmitted light. A t this moment, the time and temperature was recorded. The
n-tetradecane n-pentadecane n-hexadecane n-heptadecane
568.83 594.6 617.7 638.8 658.2 676.0 693.0 722.0
temperature was then reduced by momentarily decreasing the voltage and a redetermination of the critical temperature was made at 3-min intervals thereafter. Approximately 10 min, measured from the time the sample was inserted into the furnace, was required to reach the critical point. Preparation of Sample. In loading a sample for a critical temperature determination, it was necessary to introduce into the tube a quantity of the liquid at room temperature such that its critical volume was approximately equal to the volume of the sealed tube at the critical temperature. To do this, the volume of the tube was determined by measuring the volume of acetone required to fill it. For this purpose, a 1-mL hypodermic syringe with a needle 20 cm long was used, and the average of three determinations was taken as the volume. The amount of sample to be enclosed at room temperature was calculated using a reduced temperature-reduced volume curve for isopentane with an estimated value of the critical temperature of the sample. For hydrocarbons, the critical volume was assumed to be approximately equal to three times the volume of the liquid sample at the normal boiling point. The estimated amount of liquid was then drawn into the syringe and transferred to the tube. Additional samples, slightly more and slightly less in amount, were taken and introduced into separate tubes. Critical temperatures of the three samples were then measured as described above. Noncondensable gas was removed from the liquid sample by a series of operations which involved in situ freezing with a dry ice-acetone mixture and pumping off the residual gas over the solid, followed by melting of the solid sample to release the gas, and refreezing at low pressure. A mechanical oil vacuum pump capable of reducing the pressure to 0.001 torr was used. The degassification cycle was repeated five times or until no gas formed upon melting the sample, before sealing off each tube.
Experimental Results The apparatus and procedures that have been described were used to determine the critical temperature of the n-alkanes from n-octane to n-heptadecane. Table I lists the experimental values that were determined as well as the source and purity of the compounds. For n-decane and higher members of the homologous series, thermal decomposition occurred making it necessary to extrapolate the apparent critical temperature vs. time curve to zero time to get the true value. Figure 3 shows a plot of the data for three different samples of n-tetradecane. The points fall on a single curve and illustrate the high degree of repeatibility of the measurements. A single curve was not, however, obtained for the higher n-alkanes. The uncertainty in the critical temperatures listed in the table represents the maximum variation of the intercept of the temperature-time curves.
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Ind. Eng. Chem. Fundam. 1982, 27, 175-181 i 420
'
l
'
i
'
l
'
N- TETRADECANE
It should be added that samples that decompose at the critical temperature should be heated for a limited time only. As the decomposition progresses gaseous products are formed and pressure builds up which may cause the tube to burst. The prevent this, an insert may be used consisting of a thin-wall metal tube with viewing windows cut to match the windows of the furnace. In our work, the bursting pressures of seven tubes were determined in a separate experiment and were found to range between 4136 and 7584 kPa, with an average of 5612 kPa. Acknowledgment
Grateful acknowledgement is made to the College of Engineering, Ohio State University, for financial aid and to the Phillips Petroleum Company for samples of pure hydrocarbons. TIME
MINUTES
Figure 3. Repeatability testa using three samples of n-tetradecane.
The performance of the apparatus was, in general, satisfactory. Due to the low heat capacity of the furnace it was possible to heat the sample rapidly and by locating the thermocouple in the well surrounded by the sample, the control and measurement of the temperature were superior to that achieved by others.
Literature Cited Ambrose, D. Trans. Faraday Soc.1983, 59, 1988. Ambrose, D. "Vapour-LiquM Critical Propertles"; National Physical Laboratory, Teddington, England, Report Chem. 197, Feb lW0. Ambrose, D.; Cox, J. D.; Townsend, R. Trans. Faraday SOC. 1980, 5 6 , 1452. Ambrose, D.; Grant, D. G. Trans. Faraday SOC. 1857, 5 3 , 771 Cheng, D. C.; McCoubrey, J. C.; Phillips, D. G. Trans. Faraday SOC. 1982, 58, 224.
Received for review June 22, 1981 Accepted December 9, 1981
A Simple Capacitance Sensor for Void Fraction Measurement in Two-Phase Flow Mlng 1. Shu, Charles B. Welnberger,' and Young H. Lee Department of Chemical Englneering, Drexel University, Philadelphia, Pennsylvania 19 104
An inexpensive and simple capacitance sensor has been developed for void fraction measurement of two-phase flow of gases and liquids in cylindrical channels. The sensor causes no flow disturbances and the output is independent of channel diameter. The sensor is especially suitable in terms of sensitivity for void fraction measurement in annular flow. The governing electromagnetic equations are solved to give predictive relationships between void fraction and sensor signal, and the predictions agree wlth the experimental results.
Introduction
For two-phase flow of gases and liquids the gas volume fraction, or void fraction, is one of the primary design parameters. Much effort, therefore, has been devoted to developing techniques for the measurement of void fraction. The available techniques include quick-closingvalves (Hewitt et al., 1961), radioactive attenuation (Isbin et al., 1959; Schrock, 1969), hot wire anemometry (Hsu et al., 1963); Delhaye, 1969), and electrical impedance methods (Gregory and Mattar, 1973; Merilo et al., 1977). The electrical impedance methods, in particular, are attractive and suitable for most investigators since they are simpler to use and are relatively inexpensive compared to the other techniques. Also, the resulting void fraction measurement is time dependent and is averaged over a much shorter distance than that needed for quick-closing valves. These time traces of void fraction are especially useful for flow pattern identification. The electrical impedance of a two-phase mixture depends on void fraction if there exists a difference in dielectric constant or electrical conductivity of each phase. Depending on the electrical characteristics of the two 0196-4313/82/1021-0175$01.25/0
phases and the configuration of the sensing element, impedance can be governed by conductance or by capacitance or by both. Measurements based on capacitance generally provide better reproducibility than those based on conductance because the latter depend on ion concentration and kind, and these can be difficult to control. Besides reproducibility, there are other design considerations for impedance sensors, including flow channel geometry, sensor-induced flow disturbances, flow pattern, and sensitivity. To avoid sensor-induced flow disturbances, the electrode surfaces must either be part of the channel boundary or external to it. For example, in annular channels, Sachs and Long (1961) and Cimorelli and Evangelisti (1967) used the inner and outer walls of the annulus as the two sensor electrodes. For the most common geometry, cylindrical pipes, Gregory and Mattar (1973) tried several types of capacitance sensor geometries, including parallel plates, curved plates, and continuous helices. A major difficulty of impedance sensors concerns the effect of flow pattern on the relationship between sensor signal and void fraction. Cimorelli and Evangelisti (1967) 0 1982 American Chemical Society
