George Karaiskakis

Measurement of diffusion coefficients by reversed-flow gas chromatography instrumentation

Abstract A method is reported for measuring mutual diffusion coefficients in gases. It is a gas chromatographic method, but not a broadening technique. Two short empty columns are placed perpendicular to one another: a diffusion column through which no carrier gas flows, and a chromatographic colum. The carrier gas which flows through the latter carries over to the detector the diffusion flux established in the diffusion column. Analysis of the concentration-time curve recorded gives the value of the diffusion coefficient of an injected solute into the carrier gas. More reliable results are obtained by the so-called “chromatographic sampling”, i.e. reversing the direction of flow of the carrier gas at definite known times. The analytical mathematical expression, describing the elution curves when the gas flow is reversed, is derived and used to determine diffusion coefficients for fifteen gas pairs. The results are of high precision, and comparison with the theoretical values shows that they have also high accuracy.

Measurement of activity coefficients by reversed-flow gas chromatography

Abstract The determination of activity coefficients for various two-component liquid—vapour systems was accomplished by reversed-flow gas chromatography. By means of derived mathematical equations, the gaseous equilibrium concentration of each component under study, in the pure state and in the binary mixture, was determined. From these equilibrium concentrations, activity coefficients for the alcohol component in the binary mixtures methanol—water, ethanol—water and 1-propanol—water were calculated. The coefficients found depend on the nature of the alcohol component, its molar fraction and temperature. From the activity coefficients measured and their variation with temperature, excess partial molar thermodynamic functions of mixing were calculated. The activity coefficients found are compared with those at infinite dilution, calculated by an empirical equation.

Time-resolved determination of surface diffusion coefficients for physically adsorbed or chemisorbed species on heterogeneous surfaces, by inverse gas chromatography.

A new simple method is developed for measuring surface diffusion coefficients Ds of gases adsorbed on heterogeneous surfaces, using the reversed-flow version of inverse gas chromatography. The Ds values are found in a time-resolved way, together with the corresponding adsorption energy values, the local adsorbed concentrations, and the local adsorption isotherm values. A relative dynamic adsorption rate constant, an adsorption/desorption rate constant, and a surface reaction rate constant are also found in the same experiment, together with the total diffusion coefficient of the gas in the solid bed. The method has been applied for carbon monoxide, oxygen gas, and carbon dioxide as adsorbates on 75% Pt+25% Rh catalyst supported on SiO2, at 593.8 K.

Measurement of diffusion coefficients in multicomponent gas mixtures by the reversed-flow technique

SummaryThe reversed-flow method for measurement of gas diffusion coefficients in binary mixtures is now extended to simultaneous determination of effective diffusion coefficients for each substance in a multicomponent gas mixture. The method is applied to six ternary mixtures, each consisting of two gaseous hydrocarbons and H2, He or N2. The results are in agreement with a limiting case of the Stefan-Maxwell equations.

Gas chromatographic kinetic study of carbon monoxide oxidation over platinum-rhodium alloy catalysts.

The kinetics for the oxidation of carbon monoxide in the presence of excess oxygen over Pt-Rh alloy catalysts were studied by using the reversed-flow gas chromatography technique. Suitable mathematical analysis equations were derived by means of which the rate constants for the oxidation reaction of carbon monoxide, as well as for the adsorption and desorption of the reactant CO on the catalysts pure Pt, 75 atom% Pt+25 atom% Rh, 50 atom% Pt+50 atom% Rh, 25 atom% Pt+75 atom% Rh and pure Rh supported on SiO2 were determined. All the catalysts show a maximum rate constant for the production of CO2 at a characteristic temperature close to that found in the literature. The rate constants for the adsorption of CO increase generally with increasing temperature, while those for the desorption decrease with increasing temperature. From the variation of the rate constants with temperature activation energies for the oxidation reaction and adsorption of CO were determined, which are sensitive to the composition of the catalytic surface. The appearance of CO2 and carbon, when introducing pure CO into the column with the catalysts, verified a partial dissociative adsorption (e.g., disproportionation) of CO on the catalysts used. The latter indicates a mechanism for the CO oxidation through a partial dissociative adsorption of CO followed by the reaction of adsorbed CO molecules with adsorbed O atoms.

Temperature variation of gas diffusion coefficients measured by the reversed-flow sampling technique

Abstract The temperature dependence of mutual diffusion coefficients in the binary gas mixtures C 2 H 6 + He, C 2 H 4 + He, C 3 H 6 + He, C 2 H 6 + N 2 , C 2 H 4 + N 2 and C 3 H 6 + N 2 has been studied experimentally, using the reversed-flow gas chromatography sampling technique reported earlier. An improved sampling procedure was developed theoretically and applied, giving reversal-peaks twice the height of those obtained previously, thus increasing the sensitivity and precision of the method. The 43 diffusion coefficients determined for the above six binary gas mixtures at various temperatures show an average difference of 4.4 % from those calculated using the Fuller-Schettler-Giddings equation. The mean exponents in T n , giving the temperature dependences of gas diffusion coefficients, were in accord with results given in the literature.

Sedimentation Field-Flow Fractionation of Colloidal Particles in River Water

Abstract Sedimentation field-flow fractionation (sedimentation FFF) is a new technique that has been used to separate and characterize a variety of submicron particles. With the aid of a newly developed on-channel concentration procedure, sedimentation FFF was used to study the colloidal particles contained in samples collected from the Colorado, Green, and Price Rivers in eastern Utah during the spring run-off season. Samples from the different rivers displayed distinctly different fractograms, which could serve to “fingerprint” and characterize the colloidal content. The fractions obtained, when analyzed, displayed distinct differences in chemical composition. Each fraction re-emerged at its original elution volume when re-injected into the apparatus, indicating that the sedimentation FFF procedure had successfully concentrated then fractionated the colloidal particles in the river water.

Flux of gases across the air–water interface studied by reversed-flow gas chromatography

In the present work the reversed-flow gas chromatographic technique was applied for the study of flux of gases across the air-water interface. The model system was vinyl chloride-water, which is of great significance in food and environmental chemistry. Using suitable mathematical analysis, equations were derived by means of which the following physicochemical quantities were calculated: diffusion coefficient of vinyl chloride (VC) into water, partition coefficient of VC between the water (at the interface and the bulk) and the carrier gas nitrogen, overall mass transfer coefficients of VC in the gas (nitrogen) and the liquid (water), gas and liquid film transfer coefficients of VC, gas and liquid phase resistances for the transfer of VC into the water, and finally the thickness of the stagnant film in the liquid phase, according to the two-film theory of Whitman. From the variation of the above parameters with temperature, as well as the volume and the free surface area of the water, useful conclusions concerning the mechanism for the transfer of VC into water were extracted. These are discussed in comparison with the same parameters calculated from empirical equations or determined experimentally by other techniques.

Determination of adsorption equilibrium constants by reversed-flow gas chromatography

Abstract A method for determining adsorption equilibrium constants of gases or vapours between a solid adsorbent and a gas phase is described. The method is based on a technique reported earlier for measuring diffusion coefficients by reversed-flow gas chromatography. The adsorbing gas is introduced at the one end of a diffusion column, which is partly filled with the solid adsorbent, and is connected perpendicularly to an empty chromatographic column. Through the latter helium is flowing as a carrier gas, carrying over to a flame-ionization detector the solute entering at the junction of the two columns by the diffusion current. The mathematical expression describing the elution curves of the extra peaks produced beacause of the gas flow reversals is derived and is used to calculate the adsorption equilibrium constants of methane, ethane, n -butane, ethene, propene and but-1-ene between helium and aluminium oxide. From the variation of these constants with temperature, differential enthalpies and entropies of adsorption of the above gases on aluminium oxide have been calculated. They are discussed in comparison with the same parameters as determined from gas chromatographic retention data.

Study of the sorption of carbon monoxide, oxygen and carbon dioxide on platinum-rhodium alloy catalysts by a new gas chromatographic methodology

Abstract The mechanism of the CO oxidation reaction over Pt–Rh alloy catalysts was extracted from the adsorption of CO, O 2 and CO 2 on these catalysts. Reversed-flow gas chromatography was applied to the study of these important sorption processes. Using suitable mathematical analysis, equations were derived by means of which rate constants for the adsorption and desorption of CO, O 2 and CO 2 on and from the Pt–Rh bimetallic catalysts were determined. Temperature dependence of the rate constants for the sorption processes were also determined and the Arrhenius parameters for the adsorption and desorption were calculated. The dependence of activation energies for adsorption, desorption and disproportionation reaction on the catalyst Pt content indicates that for the CO oxidation reaction the most active is the Pt-rich catalyst, while for the CO disproportionation reaction the most active is the Rh-rich catalyst.