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A new apparatus for total-pressure measurements by the static method: Application to the vapour pressures of cyclohexane, propan-2-ol and pyridine
FIuid Phase Equilibria, 12 (1983) 143-153 Elsevier Science Publishers B.V.. Amsterdam
143 -
Printed in The Netherlands
A NEW APPARATUS FOR TOTAL-PRESSURE MEASUREMENTS BY THE STATIC METHOD: APPLICATION TO THE VAPOUR PRESSURES OF CYCLOHEXANE, PROPAN-2-OL AND PYRIDINE JAN WECLAWSKI Znstytut
and ANDRZEJ
Chemii FizyczneJ
(Received November
PAN,
BYLICKI
Knsprzaka
44/52,
01-224
Warsaw
3, 1982; accepted in final form February
(Poland)
3, 1983)
ABSTRACT Weclawski, J. and Bylicki, A., 1983. A new apparatus for total-pressure measurements by the static method: application to the vapour pressures of cyclohexane, propan-2-01 and pyridine. Fluid Phase Equilibria, 12: 143- 153. The construction of a new static assembly is described in this paper. The apparatus is an improved modification of that proposed by McGlashan and Williamson (196 1). Owing to the symmetrical construction of the oil bath and to efficient heat exchange, good temperature and pressure stabilities are obtained. The use of greaseless valves in the assembly containing the filling apparatus facilitates the filling operation and eliminates absorption of grease-derived substances. Supplementary experiments have been performed in order to verify the temperature and pressure readings: the apparatus was employed for measurements of the vapour pressures of cyclohexane, propan-2-01 and pyridine in the temperature range 298.15-348.15 K.
INTRODUCTION
A static apparatus offers several advantages in vapour-liquid equilibrium (VLE) measurements. It makes possible operation over a wider temperature range than in accessible using dynamic methods, allows measurements on systems whose components differ highly in volatility, and permits operation with highly hygroscopic substances. Recent years have seen increasing use of static apparatuses for measurements of VLE data; the work of Gibbs and Van Ness (1972), Tomlins and Marsh (1976) and Aim (1978) may be cited as examples. The method requires thorough degassing of the substances which constitute the mixture investigated, and unless this is accomplished properly, the measured pressures will be incorrect. Degassing means the removal of highly volatile components from relatively nonvolatile liquid. The only difficulty arises in that the volatile components are present in small amounts, and their concentration must be reduced to near zero. Several procedures, such as alternate freezing and pumping to high vacuum, vacuum sublima0378-3812/83,‘$03.00
0 1983 Elsevier Science Publishers B.V.
144
tion, and vacuum distillation with periodic withdrawal of vapour, are employed. Usually these procedures take several days and are tedious. Recently a procedure for rapid degassing of liquids was published by Van Ness and Abbott ( 1978). Their degassing apparatus is a rectifying column connected to a vacuum system through a fine capillary, which is the key to successful operation of the apparatus. The capillary which these authors used was - 0.1 mm in internal diameter and - 0.5 cm in length. The capillary should be as fine as possible; unless it is very fine, significant loss of liquid may be observed. The other factors which give rise to slow data production by static methods are the filling procedure and the time required to achieve the steady state. The most popular filling procedure is loading of glass ampoules containing degassed liquid, which are then connected to the equilibrium cell; after breaking of the glass seal, the substances investigated are distilled into the cell. Mixtures of required composition are prepared by weighing. A very rapid method for static measurements was proposed by Gibbs and Van Ness (1972). In their method, pure substances are introduced by means of piston injectors filled with degassed liquid. In this manner a new mixture can be obtained without disturbing thermal equilibrium in the apparatus. The usefulness of this method is limited strongly by problems which arise in constructing piston injectors of sufficient tightness. Mercury manometers are usually employed for pressure measurements within a moderate pressure range. Several corrections should be considered when using mercury manometers, and moreover attention should be paid to their construction to avoid erroneous readings. The other device used is a precision pressure gauge equipped with a fused-quartz Bourdon tube, which when calibrated gives direct pressure readings. The disadvantage of this method is that the gauge must be calibrated from time to time to eliminate erroneous measurements resulting from hysteresis. Also, as the sensing element of commercial gauges is thermostated at 317.15 K, VLE measurements must be done at temperatures below 317.15 K, to avoid condensation of vapour; otherwise, a cut-off manometer must be used. In most cases temperature is measured using a platinum resistance thermometer connected to a Miiller bridge, or by means of other thermometers calibrated against a platinum resistance thermometer. APPARATUS
The purpose of constructing the present static assembly was to design an apparatus which enables rapid and precise measurements. In order to achieve good and rapid temperature stability, a small liquid-bath was con-
145
strutted in which an axially placed metal pipe divides the oil container into two parts. In this manner, the liquid circulating outside the pipe constitutes thermal insulation for the liquid circulating inside. A rotary pump placed at the bottom of the vessel pumps silicone oil into the pipe; the oil subsequently circulates outside of the pipe where a naked kanthal-wire heater ( - 20 FJ at 298.15 .K) is, spread on teflon pegs. A temperature controller (UNIPAN, Poland; Type 650) equipped with a 100 Q platinum resistance sensor (1 cm length, 1.3 mm diameter) is additionally employed. The sensor is placed in a copper shield, to give protection against oil activity. The temperature stability and distribution were checked at 324.15 K using two similar YSI thermistors, and the measured temperature differences in the part containing the equilibrium cell and cut-off manometer were found to be less than 0.003 K (the sensitivity of the measuring system). The low thermal inertia of the heater and sensor allow thermal stability to be achieved in a relatively short time. Temperature is measured using a platinum thermometer (Tinsley) calibrated by the National Physical Laboratory (U.K.). The thermometer is connected to a 6-decade Mtiller bridge (Tinsley). The pressure measuring system is more complicated. The pressure exerted by the vapour is equilibrated in a cut-off manometer against a pressure of nitrogen. In turn, the nitrogen pressure is measured using an external mercury manometer enclosed in a wooden box to protect it from temperature changes in the laboratory. The internal manometer is illuminated by dispersed yellow light; the menisci in the external manometer mercury are illluminated using movable lamps provided with narrow diaphragms. The external manometer is made of constant-diameter glass tubing with an internal diameter of 20 mm, constructed such that the two arms of the manometer are placed co-axially; this is why it was not possible to estimate the zero-pressure error, but the arrangement facilitates pressure readings. Both manometers were filled under vacuum with twice-distilled mercury. The mercury levels are measured using a cathetometer (Spindler and Hoyer) with an accuracy of k 0.02 mm. To avoid optical error, the windows in the thermostat and in the manometer box are constructed of a special flat glass. Degassing is carried out by slow distillation of liquid cooled to near its freezing point (propan-2-01, pyridine) or by vacuum sublimation (cyclohexane). This procedure is repeated two to four times, changing the direction of distillation. The degassed substances are distilled into glass ampoules which are then sealed and weighed. The amount of each substance is known from the mass balance. At the time when the measurements were done, the paper of Van Ness and Abbott (1978) had not yet been published; otherwise an attempt would have been made to follow these authors’ experiences and recommendations. The degassing apparatus was constructed after numerous trials employing cold-traps of vari-
nitrogen
I
Fig. 1. Schematic diagram of static assembly: A, ampoules; C, equilibrium cell; CT, cold trap; D,, D,, degassing apparatus; LB, liquid bath; IM, internal manometer; EM, external valves; V,, V,, V,, Vs, glass manometer; S, gas purge; V,, V,, V,, V,, VP-V,,, g lass-teflon vacuum valves; WB, wooden box.
ous shapes and sizes, and after using various cooling media. The best results were obtained employing a glass cylinder (13 cm length, 9 cm external diameter) provided with a wide well into which liquid nitrogen is poured (see Fig. 1, D, ). The substances (initial volume - 50 cm3) could be degassed in 24 h. A schematic diagram of the static assembly is shown in Fig. 1. METHOD
The new static apparatus required verification of design and operation by comparison of vapour-pressure measurements with literature values. Tests should also be done to check each part of the measuring system. The first step was to investigate the temperature distribution in the thermostat, as described in the preceding section. Further, we verified the measurements of absolute temperature. Although the platinum resistance thermometer had been calibrated at the National Physical Laboratory (U.K.) and there was no reason to doubt its calibration, it was necessary to estimate the resistance and thermal electroforces introduced by the connecting line. A large Dewar flask was filled with a mixture of ice and water, in which the thermometer was placed. The temperature was measured over 2 h, connecting the thermometer to the resistance bridge either directly or by the connecting line. Within system precision, no differences were observed. Since there was no possible way to estimate the heights of the mercury levels in the external
147
manometer when both arms were connected to vacuum, it was necessary to measure the pressures of standard substances in order to estimate the accuracy of the pressure system. We chose water and cyclohexane, which are easy to purify and to degas; moreover, cyclohexane can be dried easily. Water vapour pressures were measured at temperatures of 323, 338, 348 and 358 K; for cyclohexane, measurements were made at 15 temperatures from 297 to 348 K. The latter results are presented below in Table 2. The vapour pressures of water and cyclohexane agree well with the literature data of the American Petroleum Institute (1964) Wexler ( 1976), Willingham et al. (1945) and Zwolinski and Wilhoit (1971). Systematic measurements were performed for cyclohexane, propan-2-01 and pyridine, and for their binary mixtures. The degassed substances were distilled into glass ampoules connected to the degassing device, and when each ampoule had been filled it was sealed. The amounts of the substances were deduced from the mass balances. Each ampoule was then sealed to the filling line, air was evacuated, the glass line connecting the ampoule and equilibrium cell was heated, and then, after opening the break seal, the sample was distilled into the equilibrium cell, cooled by liquid nitrogen. After complete transfer of a sample into the cell, the valve V, was closed, the substance was thawed, and the vapour pressure was equilibrated by the pressure of nitrogen. A similar procedure was employed in preparing the mixtures. First, component A was introduced, and then one or more ampoules containing component B were used to prepare successive mixtures. To avoid large errors ‘in mass transfer, no more than three ampoules of component B were used to make a mixture. An effort was made to estimate the completeness of mass transfer, but reliable tests require entire reconstruction of the filling line. For this reason it was necessary to give up the trials, and we believe that the data of McGlashan and Williamson (1961) may instead be taken into consideration. In the present case the mass transfer should be even better, because greaseless valves are employed in the filling apparatus. The time needed to achieve the thermal steady state depends on the initial and final temperatures as well as on proper adjustment of the regulator parameters. In most cases the thermal steady state was achieved after 2 h, and pressure was well stabilized within the next 0.5 h. The readings of temperature and pressure were executed two or three times at intervals of 15 min, and then the thermostat was heated to reach the next temperature level. In general, measurements were done in the temperature range 298-348 K at temperature intervals of 5 K. The pressure readings were corrected for local gravity, mercury vapour pressure and mercury temperature, but’ the hydrostatic pressure of nitrogen was neglected. We believe that the temperatures measured differ by _t 0.003 K from IPTS-68; pressure was measured with an accuracy of f 13 Pa, and mass with an accuracy + 5 x 10m5 g.
TABLE
1
Experimental 298.15 K
and literature
Compound
kp
values of densities
(s cme3)
Cyclohexane
0.77372 0.77376
Propan-2-01
0.78101 0.78102
Pyridine
0.97799 0.97800
Plit
(&? cm-3)
0.77389 0.7737 0.7739 0.78093 0.7812 0.7809 1 0.97806 0.97824
of cyclohexane,
propan-2-01
and pyridine
at
Reference American Petroleum Institute Timmermans ( 1965) Nagata and Yamada (1974) Hales and Ellender (1976) El-Yafi et al. (1976) Timmermans (1965) Biddiscombe et al. (1954) Helm et al. (1958)
(1964)
MATERIALS
All materials were research-grade reagents (ZD Chemipan, Poland); they were sealed in glass ampoules for protection against contamination Certificates from GLC analysis and water analysis by the Fischer method were attached to each ampoule. The purities of the cyclohexane and propan-2-01 were 99.98 mol%; that of pyridine was 99.95 mol%. After breaking the seals, the substances were stored over molecular sieves in a desiccator. The purity and water content were rechecked for each series of degassed materials. In every case the water content was lower than the sensitivity of the analytical method applied (0.005% by mass). There does not exist a reliable method which allows a check of the thoroughness of degassing prior to vapour-pressure measurements. The only criterion of thorough degassing employed here was provided by measurements of the vapour pressures of the pure substances. In the present static apparatus, in which a teflon-glass valve is used to close the equilibrium cell, it is possible to study the vapour pressures of pure substances after stepwise withdrawal of the vapour phase. This method was of great importance in estimating the purities of the materials considered. It is obvious that this test cannot be employed for mixtures. In addition, the densities of the pure substances were measured and compared with literature values. The measured densities of cyclohexane, propan-2-01 and pyridine are presented in Table 1. The experimental technique for the density measurements will be described by Weclawski (1983) in a subsequent paper. RESULTS
AND
DISCUSSION
Vapour pressures for the pure substances were measured in the following temperature ranges: for cyclohexane, 297-348 K; for propan-2-01, 298-345
149
K; and for pyridine, 298-353 K. The vapour pressures were fitted to the Antoine equation. Accurate and reliable values of vapour pressures are very important for a description of the properties of binary mixtures. For this reason, for each substance measurements were carried out several times to prove that the results were reproducible. In addition, two further tests were done: a certain amount of substance was flushed out and the measurements were repeated, and also a pure substance (or mixture) was stored in the equilibrium cell for several days and measurements were repeated. The latter test was done to check on the tightness of the valve V, and to verify whether the substances had been contaminated by impurities coming from the mercury or teflon. To describe well the vapour pressures of the pure substances, experiments were done at temperature intervals of 5 K and even of 1 IS. The measured vapour pressures of cyclohexane, propan-2-01 and pyridine and the differences between the experimental and calculated values of these pressures are listed in Table 2. Values of the Antoine-equation parameters and the root-mean-square deviations obtained for cyclohexane, propan-2-01 and pyridine are listed in Table 3. The experimental data fit the Antoine equation very well, which allows comparison of the present results with literature data. There are many literature data for vapour pressures of cyclohexane measured over various temperature ranges and using different methods. Considering the tempera-
TABLE
2
Vapour pressures of pure substances fitted to Antoine equation
T (W 297.519 298.511 299.506 303.525 304.526 308.537 313.555 314.570 318.578 323.604 328.616 333.65 1 338.657 343.657 348.660
Pclp (Pa) 12640 13230 13844 16509 17233 20406 25035 26074 30483 36864 44248 52798 62562 73726 86369
Pyridine
Propafl-2-01
Cyclohexane AP (Pa)
T(K)
PeXp (Pa)
-11 -4
298.641 303.572 304.540 308.591 313.604 318.611 323.609 325.608 328.605 333.647 338.646 340.658 343.658 345.669 348.669
5933 8018 8509 10739 14252 18668 24171 26744 31007 39457 49703 54380 62063 67704 769 10
7 4 3 -1 0 5 -4 3 5 -7 -15 12 0
AP (Pa) 1
1 16 - 17 -1 0 -8 5 4 -8 16 -3 -4 -4 0
T(K)
PeXp (Pa)
298.528 304.882 313.573 318.590 323.616 328.683 328.701 333.719 338.739 343.654 348.682 353.730
2850 3992 6142 7791 9805 12237 12244 15156 18589 22558 27327 32730
AP (Pa) 6 9 -5 -4 4 -6 -9 5 -6 -20 -6 -11
150 TABLE
3
Values of Antoine-equation Comnound
Cyclohexane Propan-2-01 Pyridine
parameters
and root-mean-square
deviations
Parameter
(RSD) RSD (Pa) a
A
B
C
8.984068 9.970700 9.217380
1211.570 1416.663 1406.090
- 49.338 - 70.057 - 54.564
8 9 10
a RSD = [ 2 AP,2/( n - 3)]“2. i--l
ture range in which they were measured and the quality of the data, the most useful literature data for the present purposes are those recalculated by Zwolinski and Wilhoit (197 1) and those of Willingham et al. (1945). Table 4 lists the present data, the values calculated for the Antoine-equation parameters estimated by Zwolinski and Wilhoit (1971), and their differences. For propan-2-01, comparison had to be made in a different manner, since the literature data of Ambrose and Spralce (1970) were reported for the temperature range 328-372 K. For the joint temperature range 328-349 K we calculated vapour pressures of propan-2-01 using the present Antoineequation parameters. Table 5 lists the experimental data of Ambrose and
TABLE
4
Vapour pressures of cyclohexane as measured from literature data, and their differences
in present
experiments,
values
T (K)
peXp (Pa)
Pcalc (Pa)
AP (Pa)
297.519 298.511 299.506 303.525 304.526 308.537 313.555 3 14570 318.578 323.604 328.616 333.65 1 338.657 343.657 348.660
12640 13230 13844 16509 17233 20406 25035 26074 39483 36864 44248 52798 62562 73726 86369
12650 13224 13831 16500 17227 20404 25035 26067 39488 36865 44250 52812 62582 73717 86366
- 10 6 13 9 6 2 0 7 -5 -1 -2 - 14 -20 9 3
recalculated
151 TABLE
5
Vapour pressures of propan-2-01 measured by Ambrose and Sprake (1970), pressures recalculated from the present Antoine-equation parameters, and their differences T(K)
Pexp (Pa)
Pcalc (Pa)
AP (Pa)
325.473 329.929 333.948 337.241 340.237 342.854 345.281 347.522 349.604
26540 33044 40017 46612 53372 59931 66601 73286 79997
26559 33061 40026 46612 53370 59923 66588 73277 79981
- 19 -17 -9 0 2 8 13 9 16
Sprake (1970), the pressures recalculated using these parameters, and their differences. It was even more difficult to relate the vapour pressures measured for pyridine to literature data, because the latter have been measured over various temperature ranges and moreover are not consistent. Finally, after numerous trials, we decided to adopt the experimental data of Herington and Martin (1953) and to compare them with values recalculated from the data of McCullough et al. (1957), the data of Rogalski (1978), and from the present data. The experimental data, the values recalculated from the litera-
TABLE
6
Vapour pressures of pyridine measured by Herington and Martin (1953) (P,), values recalculated from the data of McCullough et al. (1957) ( P2) and from the data of Rogalski (1978) ( P3), and from the present data (Pa), and their differences
Z’(K)
Pressure (Pa)
p, 303.234 3 14.678 320.477 325.860 331.499 341.553 348.920 355.580 355.878 361.609
8506 10820 13806 20805 27592 35246 35646 43472
p*
p3
p4
20805 27607 35222 35600 43511
3652 6481 8499 10826 13808 20806 27600 35206 35584 43487
3656 6482 8503 10827 13806 20793 27576
p, - p.3
pz-
p, - p4 -4 -1 -4 -1
3 -7 0 12 16
p4
12 31
2 13 24
152
ture and from the present data, and their differences, are listed in Table 6. Each set of recalculated vapour pressures of pyridine is referred to the temperature range in which the original measurements were made. The values for higher temperatures are listed to show the differences. In conclusion, the agreement between the vapour pressures of the pure substances as obtained in the present experiments and the literature data is good. The agreement is best for cyclohexane and worst for pyridine, for which the literature data are not consistent. This difference may be supposed to result from impurities contained in the latter material, as it is much easier to purify cyclohexane than pyridine. SUMMARY
The results obtained in the present experiments for cyclohexane, propan2-01 and pyridine have confirmed the usefulness of the static assembly for vapour-pressure measurements. This apparatus has been used for investigations of the binary mixtures formed by these three substances. The results obtained for these mixtures will be presented in a subsequent paper. ACKNOWLEDGEMENTS
The authors wish to thank Drs. S. Malanowski and M. Rogalski for their kind assistance in the computer calculations. This work was done within the Polish Academy of Sciences Research Project 03.10. REFERENCES Aim, K., 1978. Measurement of vapour-liquid equilibrium in systems with components of very different volatility by the total-pressure static method. Fluid Phase Equilibria, 2: 119-142. Ambrose, D. and Sprake, C.H.S., 1970. Thermodynamic properties of organic oxygen compounds. XXV. Vapour pressures and normal boiling temperatures of aliphatic alcohols. J. Chem. Thermodyn., 2: 631-645. American Petroleum Institute, 1964. Selected Values of Properties of Hydrocarbons and Related Compounds. Project 44, Thermodynamics Research Center, Texas A&M University, College Station, TX. Biddiscombe, D.P., Coulson, E.A., Handley, R. and Herington, E.F.G., 1954. The preparation and physical properties of pure pyridine and some methyl homologues. J. Chem. Sot., 1957- 1967. El-Yafi, A.H., Martinez, H., Newsham, D.M.T. and Vahdat, N., 1976. Vapour-liquid equilibria for ethanol + t-butanol+ water and methanol + isopropanol + water at atmospheric pressure. J. Chem. Thermodyn., 8: 1061-1073. Gibbs, R.E. and Van Ness, H.C., 1972. Vapour-liquid equilibria from total pressure measurements: a new apparatus. Ind. Eng. Chem. Fundam., 11: 410-413.
153 Hales, J.L. and Ellender, J.H., 1976. Liquid densities from 293 to 490 K of nine aliphatic alcohols. J. Chem. Thermodyn., 2: 1177-l 184. Helm, R.V., Lanum, W.J., Cook, G.L. and Ball, J.S., 1958. Purification and properties of pyrrole, pyrrolidine, pyridine and 2-methylpyridine. J. Phys. Chem., 62: 858-862. Herington, E.F.G. and Martin, J.F., 1953. Vapour pressures of pyridine and its homologues. Trans. Faraday Sot., 49: 154-162. McCullough, J.P., Douslin, D.R., Messerly, J.F., Hosselopp, LA., Kincheloe, T.C. and Waddington, G., 1957. Pyridine: experimental and calculated chemical thermodynamic properties between 0 and 1500 K; a revised vibrational assignment. J. Am. Chem. Sot., 79: 4289-4295.. McGlashan, M.L. and Williamson, A.G., 1961. Thermodynamics of mixtures of n-hexane + nhexadecane. Part 2. Vapour pressures and activity coefficients. Trans. Faraday Sot., 57: 588-600. Nagata, I. and Yamada, T., 1974. Correlation and prediction of excess thermodynamic functions of strongly nonideal liquid mixtures. Ind. Eng. Chem., Proc. Des. Dev., 13: 47-53. Rogalski, M., 1978. Ph.D. Thesis, Instytut Chemii Fizycznej PAN, Warsaw, Poland. Timmermans, J., 1965. Physicochemical Constants of Pure Organic Compounds. Elsevier, Amsterdam. Tomlins, R.P. and Marsh, K.N., 1976. A new apparatus for measuring the vapour pressure of liquid mixtures: excess Gibbs free energy of octamethylcyclotetrasilixane + cyclohexane at 308.15 K. J. Chem. Thermodyn., 8: 1185-1194. Van Ness, H.C. and Abbott, M.M., 1978. A procedure for rapid degassing of liquids. Ind. Eng. Chem. Fundam., 17: 66-67. Weclawski, J., 1983. Excess functions in the binary mixtures pyridine+cyclohexane and pyridine+ propan-2-01. Fluid Phase Equilibria, to be published. Wexler, A., 1976. Vapor pressure formulation for water in range 0” to 100°C: a revision. J. Res., Natl. Bur. Stand. (U.S.), Ser. Phys. Chem., 80A: 775-785. Willingham, C.B., Taylor, W.J., Pignocco, J.M. and Rossini, F.D., 1945. Vapor pressures and boiling points of some paraffin, alkylcyclopentane, alkylcyclohexane, and alkylbenzene hydrocarbons. J. Res., Natl. Bur. Stand. (U.S.), 35: 219-244. Zwolinski, B.J. and Wilhoit, R.C., 1971. Handbook of Vapor Pressures and Heats of Vaporization of Hydrocarbons and Related Compounds. American Petroleum Institute, Res. Project 44. Thermodynamics Research Center, Texas A&M University, College Station, TX.
143 -
Printed in The Netherlands
A NEW APPARATUS FOR TOTAL-PRESSURE MEASUREMENTS BY THE STATIC METHOD: APPLICATION TO THE VAPOUR PRESSURES OF CYCLOHEXANE, PROPAN-2-OL AND PYRIDINE JAN WECLAWSKI Znstytut
and ANDRZEJ
Chemii FizyczneJ
(Received November
PAN,
BYLICKI
Knsprzaka
44/52,
01-224
Warsaw
3, 1982; accepted in final form February
(Poland)
3, 1983)
ABSTRACT Weclawski, J. and Bylicki, A., 1983. A new apparatus for total-pressure measurements by the static method: application to the vapour pressures of cyclohexane, propan-2-01 and pyridine. Fluid Phase Equilibria, 12: 143- 153. The construction of a new static assembly is described in this paper. The apparatus is an improved modification of that proposed by McGlashan and Williamson (196 1). Owing to the symmetrical construction of the oil bath and to efficient heat exchange, good temperature and pressure stabilities are obtained. The use of greaseless valves in the assembly containing the filling apparatus facilitates the filling operation and eliminates absorption of grease-derived substances. Supplementary experiments have been performed in order to verify the temperature and pressure readings: the apparatus was employed for measurements of the vapour pressures of cyclohexane, propan-2-01 and pyridine in the temperature range 298.15-348.15 K.
INTRODUCTION
A static apparatus offers several advantages in vapour-liquid equilibrium (VLE) measurements. It makes possible operation over a wider temperature range than in accessible using dynamic methods, allows measurements on systems whose components differ highly in volatility, and permits operation with highly hygroscopic substances. Recent years have seen increasing use of static apparatuses for measurements of VLE data; the work of Gibbs and Van Ness (1972), Tomlins and Marsh (1976) and Aim (1978) may be cited as examples. The method requires thorough degassing of the substances which constitute the mixture investigated, and unless this is accomplished properly, the measured pressures will be incorrect. Degassing means the removal of highly volatile components from relatively nonvolatile liquid. The only difficulty arises in that the volatile components are present in small amounts, and their concentration must be reduced to near zero. Several procedures, such as alternate freezing and pumping to high vacuum, vacuum sublima0378-3812/83,‘$03.00
0 1983 Elsevier Science Publishers B.V.
144
tion, and vacuum distillation with periodic withdrawal of vapour, are employed. Usually these procedures take several days and are tedious. Recently a procedure for rapid degassing of liquids was published by Van Ness and Abbott ( 1978). Their degassing apparatus is a rectifying column connected to a vacuum system through a fine capillary, which is the key to successful operation of the apparatus. The capillary which these authors used was - 0.1 mm in internal diameter and - 0.5 cm in length. The capillary should be as fine as possible; unless it is very fine, significant loss of liquid may be observed. The other factors which give rise to slow data production by static methods are the filling procedure and the time required to achieve the steady state. The most popular filling procedure is loading of glass ampoules containing degassed liquid, which are then connected to the equilibrium cell; after breaking of the glass seal, the substances investigated are distilled into the cell. Mixtures of required composition are prepared by weighing. A very rapid method for static measurements was proposed by Gibbs and Van Ness (1972). In their method, pure substances are introduced by means of piston injectors filled with degassed liquid. In this manner a new mixture can be obtained without disturbing thermal equilibrium in the apparatus. The usefulness of this method is limited strongly by problems which arise in constructing piston injectors of sufficient tightness. Mercury manometers are usually employed for pressure measurements within a moderate pressure range. Several corrections should be considered when using mercury manometers, and moreover attention should be paid to their construction to avoid erroneous readings. The other device used is a precision pressure gauge equipped with a fused-quartz Bourdon tube, which when calibrated gives direct pressure readings. The disadvantage of this method is that the gauge must be calibrated from time to time to eliminate erroneous measurements resulting from hysteresis. Also, as the sensing element of commercial gauges is thermostated at 317.15 K, VLE measurements must be done at temperatures below 317.15 K, to avoid condensation of vapour; otherwise, a cut-off manometer must be used. In most cases temperature is measured using a platinum resistance thermometer connected to a Miiller bridge, or by means of other thermometers calibrated against a platinum resistance thermometer. APPARATUS
The purpose of constructing the present static assembly was to design an apparatus which enables rapid and precise measurements. In order to achieve good and rapid temperature stability, a small liquid-bath was con-
145
strutted in which an axially placed metal pipe divides the oil container into two parts. In this manner, the liquid circulating outside the pipe constitutes thermal insulation for the liquid circulating inside. A rotary pump placed at the bottom of the vessel pumps silicone oil into the pipe; the oil subsequently circulates outside of the pipe where a naked kanthal-wire heater ( - 20 FJ at 298.15 .K) is, spread on teflon pegs. A temperature controller (UNIPAN, Poland; Type 650) equipped with a 100 Q platinum resistance sensor (1 cm length, 1.3 mm diameter) is additionally employed. The sensor is placed in a copper shield, to give protection against oil activity. The temperature stability and distribution were checked at 324.15 K using two similar YSI thermistors, and the measured temperature differences in the part containing the equilibrium cell and cut-off manometer were found to be less than 0.003 K (the sensitivity of the measuring system). The low thermal inertia of the heater and sensor allow thermal stability to be achieved in a relatively short time. Temperature is measured using a platinum thermometer (Tinsley) calibrated by the National Physical Laboratory (U.K.). The thermometer is connected to a 6-decade Mtiller bridge (Tinsley). The pressure measuring system is more complicated. The pressure exerted by the vapour is equilibrated in a cut-off manometer against a pressure of nitrogen. In turn, the nitrogen pressure is measured using an external mercury manometer enclosed in a wooden box to protect it from temperature changes in the laboratory. The internal manometer is illuminated by dispersed yellow light; the menisci in the external manometer mercury are illluminated using movable lamps provided with narrow diaphragms. The external manometer is made of constant-diameter glass tubing with an internal diameter of 20 mm, constructed such that the two arms of the manometer are placed co-axially; this is why it was not possible to estimate the zero-pressure error, but the arrangement facilitates pressure readings. Both manometers were filled under vacuum with twice-distilled mercury. The mercury levels are measured using a cathetometer (Spindler and Hoyer) with an accuracy of k 0.02 mm. To avoid optical error, the windows in the thermostat and in the manometer box are constructed of a special flat glass. Degassing is carried out by slow distillation of liquid cooled to near its freezing point (propan-2-01, pyridine) or by vacuum sublimation (cyclohexane). This procedure is repeated two to four times, changing the direction of distillation. The degassed substances are distilled into glass ampoules which are then sealed and weighed. The amount of each substance is known from the mass balance. At the time when the measurements were done, the paper of Van Ness and Abbott (1978) had not yet been published; otherwise an attempt would have been made to follow these authors’ experiences and recommendations. The degassing apparatus was constructed after numerous trials employing cold-traps of vari-
nitrogen
I
Fig. 1. Schematic diagram of static assembly: A, ampoules; C, equilibrium cell; CT, cold trap; D,, D,, degassing apparatus; LB, liquid bath; IM, internal manometer; EM, external valves; V,, V,, V,, Vs, glass manometer; S, gas purge; V,, V,, V,, V,, VP-V,,, g lass-teflon vacuum valves; WB, wooden box.
ous shapes and sizes, and after using various cooling media. The best results were obtained employing a glass cylinder (13 cm length, 9 cm external diameter) provided with a wide well into which liquid nitrogen is poured (see Fig. 1, D, ). The substances (initial volume - 50 cm3) could be degassed in 24 h. A schematic diagram of the static assembly is shown in Fig. 1. METHOD
The new static apparatus required verification of design and operation by comparison of vapour-pressure measurements with literature values. Tests should also be done to check each part of the measuring system. The first step was to investigate the temperature distribution in the thermostat, as described in the preceding section. Further, we verified the measurements of absolute temperature. Although the platinum resistance thermometer had been calibrated at the National Physical Laboratory (U.K.) and there was no reason to doubt its calibration, it was necessary to estimate the resistance and thermal electroforces introduced by the connecting line. A large Dewar flask was filled with a mixture of ice and water, in which the thermometer was placed. The temperature was measured over 2 h, connecting the thermometer to the resistance bridge either directly or by the connecting line. Within system precision, no differences were observed. Since there was no possible way to estimate the heights of the mercury levels in the external
147
manometer when both arms were connected to vacuum, it was necessary to measure the pressures of standard substances in order to estimate the accuracy of the pressure system. We chose water and cyclohexane, which are easy to purify and to degas; moreover, cyclohexane can be dried easily. Water vapour pressures were measured at temperatures of 323, 338, 348 and 358 K; for cyclohexane, measurements were made at 15 temperatures from 297 to 348 K. The latter results are presented below in Table 2. The vapour pressures of water and cyclohexane agree well with the literature data of the American Petroleum Institute (1964) Wexler ( 1976), Willingham et al. (1945) and Zwolinski and Wilhoit (1971). Systematic measurements were performed for cyclohexane, propan-2-01 and pyridine, and for their binary mixtures. The degassed substances were distilled into glass ampoules connected to the degassing device, and when each ampoule had been filled it was sealed. The amounts of the substances were deduced from the mass balances. Each ampoule was then sealed to the filling line, air was evacuated, the glass line connecting the ampoule and equilibrium cell was heated, and then, after opening the break seal, the sample was distilled into the equilibrium cell, cooled by liquid nitrogen. After complete transfer of a sample into the cell, the valve V, was closed, the substance was thawed, and the vapour pressure was equilibrated by the pressure of nitrogen. A similar procedure was employed in preparing the mixtures. First, component A was introduced, and then one or more ampoules containing component B were used to prepare successive mixtures. To avoid large errors ‘in mass transfer, no more than three ampoules of component B were used to make a mixture. An effort was made to estimate the completeness of mass transfer, but reliable tests require entire reconstruction of the filling line. For this reason it was necessary to give up the trials, and we believe that the data of McGlashan and Williamson (1961) may instead be taken into consideration. In the present case the mass transfer should be even better, because greaseless valves are employed in the filling apparatus. The time needed to achieve the thermal steady state depends on the initial and final temperatures as well as on proper adjustment of the regulator parameters. In most cases the thermal steady state was achieved after 2 h, and pressure was well stabilized within the next 0.5 h. The readings of temperature and pressure were executed two or three times at intervals of 15 min, and then the thermostat was heated to reach the next temperature level. In general, measurements were done in the temperature range 298-348 K at temperature intervals of 5 K. The pressure readings were corrected for local gravity, mercury vapour pressure and mercury temperature, but’ the hydrostatic pressure of nitrogen was neglected. We believe that the temperatures measured differ by _t 0.003 K from IPTS-68; pressure was measured with an accuracy of f 13 Pa, and mass with an accuracy + 5 x 10m5 g.
TABLE
1
Experimental 298.15 K
and literature
Compound
kp
values of densities
(s cme3)
Cyclohexane
0.77372 0.77376
Propan-2-01
0.78101 0.78102
Pyridine
0.97799 0.97800
Plit
(&? cm-3)
0.77389 0.7737 0.7739 0.78093 0.7812 0.7809 1 0.97806 0.97824
of cyclohexane,
propan-2-01
and pyridine
at
Reference American Petroleum Institute Timmermans ( 1965) Nagata and Yamada (1974) Hales and Ellender (1976) El-Yafi et al. (1976) Timmermans (1965) Biddiscombe et al. (1954) Helm et al. (1958)
(1964)
MATERIALS
All materials were research-grade reagents (ZD Chemipan, Poland); they were sealed in glass ampoules for protection against contamination Certificates from GLC analysis and water analysis by the Fischer method were attached to each ampoule. The purities of the cyclohexane and propan-2-01 were 99.98 mol%; that of pyridine was 99.95 mol%. After breaking the seals, the substances were stored over molecular sieves in a desiccator. The purity and water content were rechecked for each series of degassed materials. In every case the water content was lower than the sensitivity of the analytical method applied (0.005% by mass). There does not exist a reliable method which allows a check of the thoroughness of degassing prior to vapour-pressure measurements. The only criterion of thorough degassing employed here was provided by measurements of the vapour pressures of the pure substances. In the present static apparatus, in which a teflon-glass valve is used to close the equilibrium cell, it is possible to study the vapour pressures of pure substances after stepwise withdrawal of the vapour phase. This method was of great importance in estimating the purities of the materials considered. It is obvious that this test cannot be employed for mixtures. In addition, the densities of the pure substances were measured and compared with literature values. The measured densities of cyclohexane, propan-2-01 and pyridine are presented in Table 1. The experimental technique for the density measurements will be described by Weclawski (1983) in a subsequent paper. RESULTS
AND
DISCUSSION
Vapour pressures for the pure substances were measured in the following temperature ranges: for cyclohexane, 297-348 K; for propan-2-01, 298-345
149
K; and for pyridine, 298-353 K. The vapour pressures were fitted to the Antoine equation. Accurate and reliable values of vapour pressures are very important for a description of the properties of binary mixtures. For this reason, for each substance measurements were carried out several times to prove that the results were reproducible. In addition, two further tests were done: a certain amount of substance was flushed out and the measurements were repeated, and also a pure substance (or mixture) was stored in the equilibrium cell for several days and measurements were repeated. The latter test was done to check on the tightness of the valve V, and to verify whether the substances had been contaminated by impurities coming from the mercury or teflon. To describe well the vapour pressures of the pure substances, experiments were done at temperature intervals of 5 K and even of 1 IS. The measured vapour pressures of cyclohexane, propan-2-01 and pyridine and the differences between the experimental and calculated values of these pressures are listed in Table 2. Values of the Antoine-equation parameters and the root-mean-square deviations obtained for cyclohexane, propan-2-01 and pyridine are listed in Table 3. The experimental data fit the Antoine equation very well, which allows comparison of the present results with literature data. There are many literature data for vapour pressures of cyclohexane measured over various temperature ranges and using different methods. Considering the tempera-
TABLE
2
Vapour pressures of pure substances fitted to Antoine equation
T (W 297.519 298.511 299.506 303.525 304.526 308.537 313.555 314.570 318.578 323.604 328.616 333.65 1 338.657 343.657 348.660
Pclp (Pa) 12640 13230 13844 16509 17233 20406 25035 26074 30483 36864 44248 52798 62562 73726 86369
Pyridine
Propafl-2-01
Cyclohexane AP (Pa)
T(K)
PeXp (Pa)
-11 -4
298.641 303.572 304.540 308.591 313.604 318.611 323.609 325.608 328.605 333.647 338.646 340.658 343.658 345.669 348.669
5933 8018 8509 10739 14252 18668 24171 26744 31007 39457 49703 54380 62063 67704 769 10
7 4 3 -1 0 5 -4 3 5 -7 -15 12 0
AP (Pa) 1
1 16 - 17 -1 0 -8 5 4 -8 16 -3 -4 -4 0
T(K)
PeXp (Pa)
298.528 304.882 313.573 318.590 323.616 328.683 328.701 333.719 338.739 343.654 348.682 353.730
2850 3992 6142 7791 9805 12237 12244 15156 18589 22558 27327 32730
AP (Pa) 6 9 -5 -4 4 -6 -9 5 -6 -20 -6 -11
150 TABLE
3
Values of Antoine-equation Comnound
Cyclohexane Propan-2-01 Pyridine
parameters
and root-mean-square
deviations
Parameter
(RSD) RSD (Pa) a
A
B
C
8.984068 9.970700 9.217380
1211.570 1416.663 1406.090
- 49.338 - 70.057 - 54.564
8 9 10
a RSD = [ 2 AP,2/( n - 3)]“2. i--l
ture range in which they were measured and the quality of the data, the most useful literature data for the present purposes are those recalculated by Zwolinski and Wilhoit (197 1) and those of Willingham et al. (1945). Table 4 lists the present data, the values calculated for the Antoine-equation parameters estimated by Zwolinski and Wilhoit (1971), and their differences. For propan-2-01, comparison had to be made in a different manner, since the literature data of Ambrose and Spralce (1970) were reported for the temperature range 328-372 K. For the joint temperature range 328-349 K we calculated vapour pressures of propan-2-01 using the present Antoineequation parameters. Table 5 lists the experimental data of Ambrose and
TABLE
4
Vapour pressures of cyclohexane as measured from literature data, and their differences
in present
experiments,
values
T (K)
peXp (Pa)
Pcalc (Pa)
AP (Pa)
297.519 298.511 299.506 303.525 304.526 308.537 313.555 3 14570 318.578 323.604 328.616 333.65 1 338.657 343.657 348.660
12640 13230 13844 16509 17233 20406 25035 26074 39483 36864 44248 52798 62562 73726 86369
12650 13224 13831 16500 17227 20404 25035 26067 39488 36865 44250 52812 62582 73717 86366
- 10 6 13 9 6 2 0 7 -5 -1 -2 - 14 -20 9 3
recalculated
151 TABLE
5
Vapour pressures of propan-2-01 measured by Ambrose and Sprake (1970), pressures recalculated from the present Antoine-equation parameters, and their differences T(K)
Pexp (Pa)
Pcalc (Pa)
AP (Pa)
325.473 329.929 333.948 337.241 340.237 342.854 345.281 347.522 349.604
26540 33044 40017 46612 53372 59931 66601 73286 79997
26559 33061 40026 46612 53370 59923 66588 73277 79981
- 19 -17 -9 0 2 8 13 9 16
Sprake (1970), the pressures recalculated using these parameters, and their differences. It was even more difficult to relate the vapour pressures measured for pyridine to literature data, because the latter have been measured over various temperature ranges and moreover are not consistent. Finally, after numerous trials, we decided to adopt the experimental data of Herington and Martin (1953) and to compare them with values recalculated from the data of McCullough et al. (1957), the data of Rogalski (1978), and from the present data. The experimental data, the values recalculated from the litera-
TABLE
6
Vapour pressures of pyridine measured by Herington and Martin (1953) (P,), values recalculated from the data of McCullough et al. (1957) ( P2) and from the data of Rogalski (1978) ( P3), and from the present data (Pa), and their differences
Z’(K)
Pressure (Pa)
p, 303.234 3 14.678 320.477 325.860 331.499 341.553 348.920 355.580 355.878 361.609
8506 10820 13806 20805 27592 35246 35646 43472
p*
p3
p4
20805 27607 35222 35600 43511
3652 6481 8499 10826 13808 20806 27600 35206 35584 43487
3656 6482 8503 10827 13806 20793 27576
p, - p.3
pz-
p, - p4 -4 -1 -4 -1
3 -7 0 12 16
p4
12 31
2 13 24
152
ture and from the present data, and their differences, are listed in Table 6. Each set of recalculated vapour pressures of pyridine is referred to the temperature range in which the original measurements were made. The values for higher temperatures are listed to show the differences. In conclusion, the agreement between the vapour pressures of the pure substances as obtained in the present experiments and the literature data is good. The agreement is best for cyclohexane and worst for pyridine, for which the literature data are not consistent. This difference may be supposed to result from impurities contained in the latter material, as it is much easier to purify cyclohexane than pyridine. SUMMARY
The results obtained in the present experiments for cyclohexane, propan2-01 and pyridine have confirmed the usefulness of the static assembly for vapour-pressure measurements. This apparatus has been used for investigations of the binary mixtures formed by these three substances. The results obtained for these mixtures will be presented in a subsequent paper. ACKNOWLEDGEMENTS
The authors wish to thank Drs. S. Malanowski and M. Rogalski for their kind assistance in the computer calculations. This work was done within the Polish Academy of Sciences Research Project 03.10. REFERENCES Aim, K., 1978. Measurement of vapour-liquid equilibrium in systems with components of very different volatility by the total-pressure static method. Fluid Phase Equilibria, 2: 119-142. Ambrose, D. and Sprake, C.H.S., 1970. Thermodynamic properties of organic oxygen compounds. XXV. Vapour pressures and normal boiling temperatures of aliphatic alcohols. J. Chem. Thermodyn., 2: 631-645. American Petroleum Institute, 1964. Selected Values of Properties of Hydrocarbons and Related Compounds. Project 44, Thermodynamics Research Center, Texas A&M University, College Station, TX. Biddiscombe, D.P., Coulson, E.A., Handley, R. and Herington, E.F.G., 1954. The preparation and physical properties of pure pyridine and some methyl homologues. J. Chem. Sot., 1957- 1967. El-Yafi, A.H., Martinez, H., Newsham, D.M.T. and Vahdat, N., 1976. Vapour-liquid equilibria for ethanol + t-butanol+ water and methanol + isopropanol + water at atmospheric pressure. J. Chem. Thermodyn., 8: 1061-1073. Gibbs, R.E. and Van Ness, H.C., 1972. Vapour-liquid equilibria from total pressure measurements: a new apparatus. Ind. Eng. Chem. Fundam., 11: 410-413.
153 Hales, J.L. and Ellender, J.H., 1976. Liquid densities from 293 to 490 K of nine aliphatic alcohols. J. Chem. Thermodyn., 2: 1177-l 184. Helm, R.V., Lanum, W.J., Cook, G.L. and Ball, J.S., 1958. Purification and properties of pyrrole, pyrrolidine, pyridine and 2-methylpyridine. J. Phys. Chem., 62: 858-862. Herington, E.F.G. and Martin, J.F., 1953. Vapour pressures of pyridine and its homologues. Trans. Faraday Sot., 49: 154-162. McCullough, J.P., Douslin, D.R., Messerly, J.F., Hosselopp, LA., Kincheloe, T.C. and Waddington, G., 1957. Pyridine: experimental and calculated chemical thermodynamic properties between 0 and 1500 K; a revised vibrational assignment. J. Am. Chem. Sot., 79: 4289-4295.. McGlashan, M.L. and Williamson, A.G., 1961. Thermodynamics of mixtures of n-hexane + nhexadecane. Part 2. Vapour pressures and activity coefficients. Trans. Faraday Sot., 57: 588-600. Nagata, I. and Yamada, T., 1974. Correlation and prediction of excess thermodynamic functions of strongly nonideal liquid mixtures. Ind. Eng. Chem., Proc. Des. Dev., 13: 47-53. Rogalski, M., 1978. Ph.D. Thesis, Instytut Chemii Fizycznej PAN, Warsaw, Poland. Timmermans, J., 1965. Physicochemical Constants of Pure Organic Compounds. Elsevier, Amsterdam. Tomlins, R.P. and Marsh, K.N., 1976. A new apparatus for measuring the vapour pressure of liquid mixtures: excess Gibbs free energy of octamethylcyclotetrasilixane + cyclohexane at 308.15 K. J. Chem. Thermodyn., 8: 1185-1194. Van Ness, H.C. and Abbott, M.M., 1978. A procedure for rapid degassing of liquids. Ind. Eng. Chem. Fundam., 17: 66-67. Weclawski, J., 1983. Excess functions in the binary mixtures pyridine+cyclohexane and pyridine+ propan-2-01. Fluid Phase Equilibria, to be published. Wexler, A., 1976. Vapor pressure formulation for water in range 0” to 100°C: a revision. J. Res., Natl. Bur. Stand. (U.S.), Ser. Phys. Chem., 80A: 775-785. Willingham, C.B., Taylor, W.J., Pignocco, J.M. and Rossini, F.D., 1945. Vapor pressures and boiling points of some paraffin, alkylcyclopentane, alkylcyclohexane, and alkylbenzene hydrocarbons. J. Res., Natl. Bur. Stand. (U.S.), 35: 219-244. Zwolinski, B.J. and Wilhoit, R.C., 1971. Handbook of Vapor Pressures and Heats of Vaporization of Hydrocarbons and Related Compounds. American Petroleum Institute, Res. Project 44. Thermodynamics Research Center, Texas A&M University, College Station, TX.