(66l) Biodiesel Dilute n-Alcohol Mixture Characterization by Physicochemical Gas Chromatography

The use of gas chromatography for the measurement of the physicochemical properties of solutions and surfaces is a versatile technique and is frequently referred to as ?inverse gas chromatography.? In this study, we have used a semi-volatile solvent, a commercial biodiesel sample, as the coated stationary phase and characterized its interaction with injected solutes consisting of C1 ? C4 alcohols at finite vapor concentrations corresponding to p/po of 0.1 - 0.5 over the temperature range of 55 ? 85 ˚C. Modifications were made to a Gow MAC Model 550 gas chromatograph (GC) to allow the precise measurement of partition or absorption isotherms of alcoholic solutes and their reduction from skewed chromatographic profiles, with subsequent recording on a laptop computer. Solutes were injected onto columns consisting of 16, 20, and 25 weight percent biodiesel on a high surface area support, while column temperature variation was tracked by data logging the response of various J-type thermocouples placed in the chromatographic oven. Measurements were performed in triplicate for a representative portion of solutes, temperatures, and column loadings and single injections were used for the remainder of the measurements. The resultant chromatographic profiles were relatively symmetrical in the case of methanol and ethanol at lower temperatures (55-65 ˚C) and changed to ?tailing? profiles at temperatures (75-85 ˚C) corresponding to Langmuir?type sorption isotherms. By contrast, n-propanol and n-butanol peak profiles exhibited ?fronting? peaks over the entire temperature range, commensurate with anti- Langmuir sorption behavior. In general as temperature increased, the sorption isotherm shifted to lower uptake values at given pressure and retention times shifted toward shorter times, indicating a reduced level of interaction between the alcoholic solute and the biodiesel substrate. Isotherms consisting of vapor mole or weight fraction uptake in biodiesel versus relative partial pressure (a measure of solute fugacity) were developed by deconvoluting that portion of the peak profile attributed to absorption from the contribution to a Gaussian symmetrical profile contribution due to diffusion of the solute in the chromatographic column. Alcohol sorption isotherm data was measured during the passage of n-alcohol peaks through the calibrated thermal conductivity detector of the GC, and the isotherms were fitted to polynomial equations of appropriate order. The chromatographic peak shapes and resultant isotherm arise from both positive and negative deviations from Raoult's Law depending on the alcohol-biodiesel pair, the temperature, and the vapor state concentration of the alcoholic solute. These data trends will be discussed along with heat of solution data and Henry's Law constants at various p/po values to provide a picture of the intermolecular interactions responsible for the solution behavior of these biodiesel/n-alcohol systems. Such isotherms and derivative thermodynamic data can be used in the design of V-L separation processes for alcohol devolatilization from biodiesel as well as the use of biodiesel (vegetable oil methyl esters) as a ?green? sustainable solvent system.