Thermodynamic characterisation and modelling of coal liquids

Abstract

For the description of the temperature-pressure relationship of coal oils, a model is required that requires as input only data from analytical chemical techniques. Coal oils are obtained during liquefaction and differ in properties from crude oils in their contents of aromatic hydrocarbons. As aromatic hydrocarbons have stronger interactions, the modelling of fluids with high contents of aromatic hydrocarbons is more difficult than that of aliphatic fluids. The thermodynamic characterisation and modelling of these liquids plays an important role in reactor design. Before a chemical reactor for upgrading and/or cracking of these fluids can be constructed, the energy required by these separation and conversion processes should be estimated. The technology of coal liquefaction, that is the production of coal oils from coal, is not limited to coal: other solid feedstocks, such as municipal waste and biomass may serve as a feedstock as well. The basis for the modelling was Van Laar's equation of state. This equation of state was developed in 1924 as one of the first modifications of the Van der Waals equation of state. The use of an equation of state as a tool for the modelling of the temperature- pressure relationship of organic fluids has been reported earlier in literature. Equations of state are usually used for calculations at high temperatures and/or pressures, where little or no experimental data are available. The thermodynamic modelling work requires a stepwise process. At various levels, data are required to ~upport the model development. Firstly, the data commonly available for these liquids should be obtained. These data may characterise the fluid in terms of major and minor components, elemental composition, molecular weight distribution, or relative concentrations of functional groups. For the trade in these fluids, these data are required anyway. A second source of data is the literature, where from many pure substances experimental thermodynamic data are available. As for substituted naphthalenes data were lacking, vapour pressure measurements 0 several substituted naphthalenes have been carried out. After verification of the capabilities of the equation of state with pure substances, binary mixture data were used in order to investigate the influence of empirical corrections on the performance of the equation of state. Furthermore, experimental work on binary systems provides useful information on the dependency of the vapour pressure on the composition of the mixture. The final stage of the model development is the regression of the coefficients of the equation of state to parameters that can be obtained for coal oils as well. This regression is carried out on a database of pure substance data, and then the regression formulae can be tested with experimental data on coal oils. In this work, an oil of the Point of Ayr facility and 7 of its fractions, each with a boiling range of about 30 K, have been used for this purpose.