The Thermodynamic Research Of Wax Precipitation In Crude Oils

For the crude oils which contain wax (mixture of long chain alkanes), when the temperature of crude oil drops, the solubility of the heavy hydrocarbons would be sufficiently reduce and they will precipitate in the form of wax. With the more decrease of temperature, the amount of wax precipitated increase and they assemble into large crystal and finally the three-dimensional network structure is constructed. The plug of pipeline caused by wax precipitation has important influence on the production and transportation of crude oils. A model that can predict the conditions under which solids will precipitate and the amount of solids formed under different conditions can be used as a fundamental tool to prevent the solid precipitation.The research work on the wax precipitation principally involves:1. The DSC method for determination of wax precipitation condition has been established. The wax appearance temperature and the amount of wax precipitated under different temperature can be obtained at the same time by DSC method. The method is quickly and reliable and can be done in industry.2. Detail experiments have been conducted to study the best wax precipitation condition under different cooling rate. The wax appearance temperature and the amount of wax precipitated under different temperatures are obtained using DSC method.3. The composition of wax is obtained by gas chromatography, the characterization data are used for the model input data.4. Regular solution thermodynamic theories: a modified thermodynamic solid-liquid equilibrium model which is used to calculate wax precipitation in crude oils has been established in resent work. The researches indicate that when temperature of crude oil lower WAT, the solid paraffin exist in forms of hexagonal , orthorhombic, triclinic and monoclinic phases, with the decrease of temperature, all of the solid alkane become steady orthorhombic phase, so the solid-solid transition is considered in the model. The properties of the pseudo-components are determined by using empirical correlations. New correlations for solid-solid transition enthalpy, solid-solid transition temperature, fusion enthalpy and fusion temperature are also established in this work based on the data from literatures.The calculation results for three crude oil systems from literature and the model crude oil system studied in this research by the modified thermodynamic model present in this work and the regular solution model proposed by Leelavanichkul are compared with theexperimental ones, it can be seen that not only for complicated crude oil systems but also for simple model system, the modified thermodynamic model is much better than the regular solution model proposed by Leelavanichkul for calculating the amount of wax precipitation under different temperatures. For the wax precipitation temperature, the calculation results by the model present in this work are little higher than the experiment ones.