Vapor Pressures and Heats of Vaporization of Primary Coal Tars

Abstract

Executive SummaryThis project had as its main focus the determination of vapor pressures of coal pyrolysis tars. It involved performing measurements of these vapor pressures and from them, developing vapor pressure correlations suitable for use in advanced pyrolysis models (those models which explicitly account for mass transport limitations).This report is divided into five main chapters. Each chapter is a relatively "stand-alone" section. Chapter A reviews the general nature of coal tars and gives a summary of existing vapor pressure correlations for coal tars and model compounds. It is shown that the heterogeneous nature and the complexity of coal tars have made it unrealistic to apply detailed vapor pressure correlations which take into account the variation in the chemical structure of the tars. The significant product of this study is a much improved understanding of the volatility and thermal behavior of coal tars. The volatility was studied by vacuum sublimation and Knudsen effusion experiments. The volatility behavior is considerably more complicated than had been earlier believed.The tars evaporate in a "distillation-like" fashion. More volatile species are lost earlier in the process, leaving behind a progressively less volatile residue. The results suggest that there are three very different classes of compounds, and therefore, at least three different vapor pressure behaviors. The first corresponds to compounds of high molecular weight and significant alkyl character, the second to compounds with significant hydroxyl group content and medium molecular weight, and the third to medium molecular weight aromatic compounds without hydroxyl groups. Hydrogen bonding plays a major role in the determining the tar volatility. vi There has been concern in pyrolysis modeling about how closely Raoult's law is followed in coal tar. It appears from our results that the assumption of ideal mixture behavior could be acceptable for rough models of pyrolysis despite the possibility of strong specific interactions between certain functional groups.The results from the current work show that measuring the vapor pressures of complicated and thermally unstable mixtures is possible at low temperatures. There has also been some concern about condensation-type reactions influencing the results of vapor