362 Ind. Eng. Chem. Fundam., Vol. 17, No. 4, 1978
Apparatus for Accurately and Rapidly Measuring the Solubility of Gases in Liquid Mixtures David A. Armitage, Roger G. Llnford, and David G. T. Thornhill' School of Chemistry, Leicester Polytechnic, Leicester LE 1 9BH, England
A continuousdilutionapparatus has been developed for measuring the solubility of gases in liquid mixtures across the full composition range. Degassed liquids are added to a calibrated gas-filled volume from measuring burets, and the gas solubility is determined from the pressure of the undissolved gas at equilibrium. The entropy of solution is obtained by re-eauilibriationat a series of temperatures. The apparatus can yield solubilities accurate to within 1%.
A new continuous-dilution apparatus is described which can both rapidly and accurately yield gas solubilities in liquid mixtures across the full composition range. Measuring burets are utilized to add the degassed liquids to a calibrated volume containing the gas. The rate of equilibrium is enhanced by continuously circulating the liquid mixture through the gas using a magnetically operated glass pump. This pump is similar to the design described by Dymond and Hildebrand (1967) in their gas solubility apparatus, and subsequently used by Cukor and Prausnitz (1971). Equilibrium is adjudged to have been achieved when the pressure of the undissolved gas remains constant. From this latter pressure the amount of dissolved gas can be calculated. The solubility apparatus, made of glass, is shown in Figure 1. In order to avoid contamination of the liquids, greaseless vacuum taps incorporating Teflon and Viton O-ring seals, made by J. Young Ltd. (London, England), were utilized throughout the apparatus. Cell C, whose volume is approximately 230 cm3, is directly connected to a side arm, P, inside which is contained a metal-in-glass plunger. The plunger is moved up and down by a horseshoe magnet attached to a rod, which in turn is joined to an eccentric wheel driven by an electric motor. Movement of the plunger causes liquid to be pumped from the base of C and to flow down over a glass-blown cheek D. Inside the plunger there are two glass valves which reduce the tendency of liquid to backflow within the pump. Also directly connected to C is a mercury manometer, M, constructed of Veridia precision bore (20 mm) tubing with one fiducial mark, F3, on the left limb. Calibration (by nitrogen compression) of the volume enclosed by C, P, and the manometer tubing down to F3 enables the gas volume to be calculated. Two measuring burets, B1 and B2, also made of Veridia precision bore (20 mm) tubing can communicate with C via taps T7 and T9, respectively. The capacity of each buret is about 230 cm3. The lower ends of the burets dip into mercury reservoirs, and by connecting taps T4 and T12 to nitrogen or taps T 5 and T11 to vacuum the mercury levels in the burets can either be raised or lowered. The dotted line in Figure 1 represents a water-filled tank thermostatted to rt0.03 K, between 10 and 45 "C. The vessel in which the solvents are degassed consists of two 500-cm3globes, G1 and G2, joined together by wide bore tubing, and it has two Viton O-ring ball joint outlets, J1 and 52. Joint J1 is connected to vacuum via socket joint S1, while 52 can be connected to either of the socket joints, S3 or S4, by means of two different sections of capillary tubing. These lengths of tubing have a Viton O-ring ball joint at one end and a socket joint at the other end, and
enable solvent to be transferred from G2 to either of the measuring burets without contamination by grease. For the sake of clarity only the connection between the degassing vessel and B1 is shown in Figure l. High vacuum for the apparatus is provided by sorption pumps. These consist of wide bore glass tubing, sealed at the lower end and filled with 13X molecular sieve. Pressures of less than Torr can be reached by cooling these pumps in liquid nitrogen. Experimental Section Operation. Degassing of the solvents and filling of the measuring burets is carried out as follows: about 250 cm3 of solvent is poured into G1 through J1, and initially degassed by a combination of boiling some away under vacuum and then pumping on the frozen solid. Next the solid is sublimed into G2 by cooling the latter with liquid nitrogen and warming G1. When this is complete T1 and T2 are opened to evacuate any degassed air. The sublimation process is repeated between G1 and G2 until on opening T1 and T2 no pressure rise is registered by the Pirani gauge E. Sublimation causes the entire solvent to pass through the gas phase and ensures thorough degassing. Finally the solvent is thawed when it is contained in G2, and the measuring buret which is to be filled, together with the section of tubing back to T3, is evacuated. Following this either T7 or T9 is closed, G2 warmed, and T3 opened to allow solvent to be impelled by its own vapor pressure into the buret. During the filling procedure nitrogen, at a pressure exceeding the vapor pressure of the solvent, is applied to T4 or T12 to prevent liquid escaping from the base of the buret. This degassing procedure is thorough but slow; the time penalty is not severe, however, as sufficient solvent is stored in the buret for several runs. Having filled the burets, the volumes enclosed by C, P, and M are evacuated and filled with purified gas to such a pressure that, when solvents are added to C, the capacity of the manometer relative to vacuum will not be exceeded. Taps T13 and T15 are now closed and the right limb of M re-evacuated to allow the pressure in the apparatus (and hence the amount of gas there) to be determined. By applying nitrogen pressure to T4 and T12 degassed solvent can be injected into C through T7 or T9, respectively. The volume of degassed solvent injected into C can be calculated by measuring the change in height of the mercury menisci in the burets. Aliquots of the degassed solvents remaining in the burets can be subsequently added for solubility measurements at other liquid compositions. After an injection the pump is switched on, causing solvent to trickle down over D and continually expose a
0019-7874/78/1017-0362$01.00/00 1978 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 17, No. 4, 1978
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Figure 1. The gas solubility apparatus.
thin film of fresh solvent to the gas. This speeds the attainment of equilibrium and avoids supersaturation. Pumping is continued until the total pressure in the apparatus remains static. Alternatively, the apparatus can be used in a "semi-wet" mode. In this method, volumes C, P, and the left limb of M are evacuated before injecting the degassed solvents into the solubility cell. Then an amount of gas, which can be subsequently calculated, is added to the cell from the precision bore right limb of M through T13 and the capillary tube, w. This capillary tubing prevents diffusion of solvent vapor out of the cell when transferring the gas. In order to obtain the partial pressure of the gas from the total pressure measurements, it is necessary to allow for the vapor pressure exerted by the liquid mixture. In this work the required mixture vapor pressures were directly measured in this apparatus; these results will be reported in detail at a later date. The amount of solvent used in an experiment depends on the degree of solubility of the gas. For moderate to high solubility gases, about 0.3 mol of solvent suffices. Because this apparatus is of the dilution type, only relatively small amounts of solvent are needed to cover the complete composition range. It is not necessary to make up and subsequently discard new solvent mixtures for each compositon. After completing a series of measurements the majority of gas is pumped away and then the solvent mixture is distilled back into G1, via taps T8, T1 and T2, by cooling G1 with liquid nitrogen. The liquid burets are emptied in a similar manner by opening, in addition, taps T 7 and T9. Using this apparatus, we found the mole fraction solubility of ethane in benzene at 28.36 "C to be 141.5 X compared with the value of 142.5 x calculated from
the data of Horiuti (1931). This indicates that an accuracy of within 1% can be attained. Materials. Benzene and 1,1,2-trichlorotrifluoroethane were obtained from British Drug Houses (England) and both had minimum stated purities of 99.8 mol %. The benzene was dried with sodium wire prior to use. Ethane of a minimum stated purity of 99.0 mol% was purchased from Cambrian Chemicals Ltd. (England). All the materials were used without further purification, their purities being in accord with the recommendations of Clever and Battino (1975).
Results and Discussion The solubility of ethane has been measured in several mixtures of benzene and 1,1,2-trichlorotrifluoroethaneat 298.11 K. Measurements have also been made of the entropy of solution of this gas in the pure halocarbon and one of the mixtures. Results were found to be reproducible to within 170.In Table I the mole fraction solubilities of ethane at 1 atm partial pressure, x2, as a function of the mole fraction of benzene, xl, at 298.11 K are compared with the values of Linford and Hildebrand (1969). From a plot of our values of In xz against xl,we were able to interpolate our solubilities at the same mole fractions studied by Linford and Hildebrand, and these values are denoted with an asterisk in Table I. The anomalously high solubilities at low values of x1 will be discussed elsewhere. Table I1 shows the variation of solubility with temperature for ethane in the pure halocarbon and one benzene-halocarbon mixture. This table also shows the values of entropy of solution, Sz - Szo/cal mol-l K-l, calculated from the slope of a plot of In x z against In .'2 As can be seen, the agreement for the mixtures is within experimental accuracy, but it is poor for the solubility and entropy of solution of ethane in the pure halocarbon.
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Table I. Mole Fraction Solubility of Ethane, x , , at 1 Atm Partial Pressure and 298.11 K, in Mixtures of Benzene, Mole Fraction, x , , with 1,1,2-Trichlorotrifluoroethane. Values Denoted with an Asterisk Are Interpolated
lo%,
Xl
0.000 0.104 0.203 0.260 0.308 0.404 0.502 0.510 0.602 0.701 0.756 0.776
104x, (this work) 268.4* 276.8 271.0 267.8* 261.4 248.4 239.2 238.1* 223.9 207.5 199.6 195.5*
(Linford and Hildebrand)
285.8 265.4
238.0
193.3
Table 11. Mole Fraction Solubility of Ethane, x2,at 1 Atm Partial Pressure at Several Temperatures in Pure 1,1,2-Trichlorotrifluoroethaneand in One Mixture of Benzene, Mole Fraction, x,, with 1,1,2-Trichlorotrifluoroethane; Also the Entropy of Solution, S, - S,O/cal mol-’ K-’ S, - Szo/
x,
T/K
0.000 0.000 0.000 0.000
284.01 287.71 291.01 291.06 294.01 298.09 298.08 287.71 293.09 298.11 303.02 308.04
0.000 0.000 0.000 0.756 0.756 0.756 0.756 0.756
104x2 cal mol-’ K-l (this work) (this work) 348.4 323.9 306.1 307.9 288.1 267.1 269.7 231.8 214.8 199.6 187.7 175.9
-10.7
s, - S Z 0 I cal mol-’ K’’ (Linford and Hildebrand)
-8.5
-8.0
Nevertheless, we are confident that our measurements in the pure halocarbon are correct. This is because one aliquot of halocarbon was successively diluted with benzene to cover the mole fraction range. A lower solubility for pure halocarbon (as observed here) would seem to imply that our halocarbon was not completely degassed, but this cannot be true because our mixture solubilities would then also have been less than those of Linford and Hildebrand. In fact, the two sets of mixture measurements agree very closely with each other. When the apparatus was operated in the “semi-wet” mode equilibrium between the gas and solvent, mixtures occurred in about 2 h. However in the “dry” mode, when degassed solvent was added to dry ethane, or when degassed benzene was added to pure 1,1,2-trichlorotrifluoroethane which was already in equilibrium with ethane, the attainment of final equilibrium was much slower. This delay was caused by the solvent or solvent mixture vapor being slow to diffuse and “wet” the gas in the left manometer limb. We were able to overcome this problem by leaving a Teflon-covered stirrer bead inside vessel C at the
start of a run, and after injecting solvent into the vessel a magnet was used to draw the solvent covered bead up and down the left manometer limb. This movement agitated the gas and allowed it to become thoroughly wetted by the solvent vapor in a very short time. Nevertheless, for this study, after each addition of solvent and also after every change in temperature the gas-solvent mixtures were left thermostatting for 24 h to confirm that equilibrium had been achieved. Conclusions The apparatus is inexpensive, easy to construct, and simple to use. When operated with care it can yield results accurate to f l % . It cannot be used with gases such as hydrogen sulfide which react with mercury, and also cannot be used at temperatures where the vapor pressure of mercury is not negligible. Using the apparatus, in a continuous-dilution mode, the solubility of one gas at one temperature in eight binary mixtures of two solvents can be determined in about 10 days. This time includes 2 days which are required to degas the two solvents and fill the burets. This compares favorably with the time and labor required to measure the same number of mixture solubilities using apparatuses based on the batch principle (Clever and Battino, 1975). With a batch apparatus every measurement entails degassing the mixture, loading it into the apparatus, adding a new aliquot of gas, attaining equilibrium, and finally emptying the apparatus. In our apparatus, the solvents can be stored in the measuring burets for many months and still remain thoroughly degassed. Because of this feature the reproducibility of results is very good, and the solubility of different gases in various compositional mixtures of two solvents can be easily measured. With a batch method the efficiency of degassing each mixture can vary. The versatility of this apparatus could be increased by incorporating further measuring burets, which would enable gas solubilities in multicomponent solvent mixtures to be studied. The addition of an independent gas buret would facilitate measurements in the “semi-wet” mode, because the use of the reference side of the manometer as a gas buret makes the calculation of the amount of gas added somewhat tedious. Acknowledgment We would like to dedicate this work to Professor J. H. Hildebrand, whose ideas have aided us greatly. D.G.T.T. would like to acknowledge Leicestershire County Council for their financial support.
Literature Cited Clever, H. L., Battino, R., in “Solutionsand Solubilities Part l ” , M. R. J. Dack, Ed., Vol. VI11 of “Techniquesof Chemistry”,A. Welssberger, Ed. WileyInterscience, New York, N . Y . , 1975. Cukor, P. M.. Prausnitz, J. M., Ind. Eng. Chem. Fundarn., 10, 638 (1971). Dymond, J., Hildebrand, J. H., Ind. Eng. Chem. Fundam., 6, 130 (1967). Horiuti, J., Sci. Pap. Inst. Phys. Chem. Research, Tokyo, 17,No. 341, 125 (1931). Linford, R. G.,Hildebrand,J. H . , J. Phys. Chern.. 73,4410 (1969).
Received for reuiew December 6,1977 Accepted June 19, 1978
