FTIR measurement of monomer fractions in dilute alcohol-acetone systems for the evaluation of the sPC-SAFT EoS

[摘要] ENGLISH ABSTRACT: The simplified Perturbed-Chain Statistical Associating Fluid Theory (sPC-SAFT) is characterised by thedual advantages of decreased computational intensity, while remaining accurate for a variety ofsystems. Vapour-liquid equilibrium data are used to generate equation of state parameters.However, incorporating monomer fraction data into the parameter regression has long beenadvocated as a good, or even preferred, practice. Therefore, the monomer fraction data of dilutealcohol-acetone systems were analysed in this study. A small stainless steel sample vessel wasconstructed with temperature control, manual pressure control and a mechanism for liquid phaseanalysis via infrared spectroscopy.The performance of the spectrometer was verified by comparison with the ethanol – n-hexane dataof von Solms et al. (2007), after which new monomer fraction data were obtained for dilutesolutions (between 0.01 and 1.5 mol%) of methanol, ethanol, 1-propanol and 2-propanol in acetonenear 23 °C.For dilute alcohol-acetone systems it was found that the propanols had the highest monomerfractions, and methanol the lowest. With increasing alcohol concentration, the monomer fractionsdecreased exponentially to values of 0.4 and 0.1 for methanol and the other alcohols respectively.The excess availability of hydrogen bond acceptors in the mixtures explains the equivalencyobserved for ethanol, 1-propanol and 2-propanol.For dilute acetone-alcohol systems it was found that, especially for methanol and ethanol, there wasa pronounced trend towards acetone monomer fractions of 1 at infinite dilution. For the acetone –2-propanol system, a previously unrecorded monomer peak was observed and quantified. Acetonemonomer fractions tended to decrease as alcohol chain-length increased, showing that acetonecould more easily penetrate the hydrogen bond network of the solvent when the solvent-solventbonds were weaker.Monomer fraction data were compared to predictions for the sPC-SAFT scheme and parameterscombinations published in the literature. The experimental data were accurately modelled usingmodified association parameters such that the solute associates strongly (εAB≈103 κ≈1), while thesolvent parameters were decreased (εAB≈102 κ≈10˗3) to give a weakened solvent association effect.The difficulty for the dilute solute in penetrating the solvent bonding network appeared to be similarto the hydrophobic effect.Two new association schemes were proposed for acetone, assigning a single (N) or two (2N)negative association sites to represent the oxygen valence electron pairs. These schemes showedrelative success in modelling acetone as the solvent in the mixture, while not being able to predictacetone monomer fractions when acetone was the solute. For dilute acetone-alcohol systems, thedata were best described using the 2B model for acetone, while the best choice of scheme for thealcohol varied from system to system.For dilute alcohol-acetone mixtures it was generally found that a 2B-N model (with modifiedassociation parameters) provided the best fit to those experimental data. Accurate modelling below0.1 mol% was difficult to attain with average errors decreasing to the order of 10% when this areawas excluded. In this highly dilute region, not one of the models could describe the rapid change in(monomer fraction) gradient sufficiently while simultaneously offering accurate predictions over theentire experimental range.

[发布机构] Stellenbosch University