Thermodynamic Measurements by Gas-Liquid Chromatography

Abstract

The aim of the first part of this work was to provide solubility data for the large scale continuous and semi-continuous gas liquid chromatographs (G.L.C.) in operation in this department by using analytical G.L.C. equipment. A commercial dual-channel chromatograph was modified to enable the partition coefficients of various solutes in polymer stationary phases to be measured at finite concentrations of the solute. The technique chosen was “Elution on a Plateau” in which the chromatographic column is first equilibriated by a carrier gas (nitrogen) containing the solute vapour. The partition coefficient of the solute is then calculated by measuring the elution volume of a small sample of the solute injected. The systems studied were:- 1. a) α-Pinene/Polypropylene Sebacate at 99.8, 120.0, 140.0, 150.0°C. b) β -Pinene/ Polypropylene Sebacate at 100.3, 110.1, 120.0, 150.0°C. 2. a) Dichloro Methane/Silicone Oil MS 200 at 24.8, 29.1, 34.7, 39.8°C. b) 112, Trichloro trifluoro Ethane (Arklone-P)/Silicone Oil MS 200 at 24.8, 29.1, 34.7°C. c) 111, Trichloro Ethane (Genklene-P)/Silicone Oil MS 200 at 34.7, 45.1, 60.5, 74.0°C. A limited amount of ternary data was also obtained whereby the infinite dilution partition coefficient of a third component was measured in the presence of the finite concentrations of the other two components, one of which was the stationary phase. In each case the partition coefficients were found to increase linearly with the gas phase concentration of the solute within the gas phase concentration ranges studied (0-3x10-4 g/cm3 for Systems 1 and 0-5x10-4 g/cm3 for Systems 2). Activity coefficients were calculated from the partition coefficients by using the known molecular weights of the solutes and the stationary phases and the vapour pressures of the solutes. For the binary systems the activity coefficients were correlated by means of the single and two-parameter Flory-Huggins Equation, the parameters being evaluated in this work. In each case the fit was very good, suggesting that this equation is adequate in describing the behaviour of these polymer solutions where polar interactions are moderate or small. The data of Systems 2 was also correlated by the binary Heil Equation with a similar accuracy to that of the Flory-Huggins Equation. The ternary form of the Flory-Huggins equation was fitted to the ternary data and by using the previously calculated binary parameters the solute-solute interaction parameters were estimated. The second part of this work was concerned with studying the separating efficiency of a rotating gas liquid chromatographic unit. This consisted of 44 20cm long, 2.5 cm ID tubes arranged in a circular bundle that rotated in the opposite direction to the flow of the carrier gas. The liquid feed, commercial turpentine with α and β pinenes as the major components, was continuously fed somewhere in the middle and the gas flow rate and the speed of rotation were adjusted so that the least absorbed component (α-pinene) moved with the gas phase, while the other component (β-pinene) was carried along with the packing. This second component was then stripped off in a different section of the bundle. The inlet and off-take operations relied on the seal between stationary graphlon rings and the rotating surface of the bundle. A complete seal was found to be impossible to achieve with a minimum of 5% carrier gas leakage, while up to 50% of the feed was lost. Therefore, only a few runs could be conducted which showed that a steady state operation was impossible under these circumstances. Although this meant a partial defeat of the objective of the first part of the work, some valuable information was gained about the operation of this type of semicontinuous countercurrent equipment.