Thermodynamic study of phase transition behavior between the isotropic phase and mesophase using benzene-soluble and benzene-insoluble as phase indicators

Abstract

Preparing carbonaceous mesophase pitch precursor by thermal treatment of isotropic pitch is a feasible and effective method for fabricating various high-performance carbon materials including high-modulus carbon fiber, needle coke, graphite, etc. However, the industrial production of qualified mesophase with desirable properties needs to be further improved in many regions. Especially, during pitch preparation, the transformation from isotropic phase to mesophase is difficult to be controlled since it significantly depends on reaction conditions, raw material and preparation parameters. While the optimization of the production conditions of mesophase with special properties requires time-consuming and costly exploratory experiments due to the lack of the fundamental understanding in the transformation behavior from isotropic phase to mesophase. This paper elaborates a fundamental work seeking a more suitable and accurate thermodynamic model and a parameter estimation method to describe phase transition behavior using benzene-soluble (BS) and benzene-insoluble (BI) fractions of coal tar-based pitch as indicators of isotropic phase and mesophase, respectively. The combined equation of simplified statistical associating fluid theory with anisotropic pseudopotential (SSAFT-APP) is used to describe phase transition, and the related parameters of each equation are estimated based on average molecular weight and carbon aromaticity of BS and BI fractions, respectively. The mesophase content of various samples prepared by annealing the mixtures of BS and BI fractions under different mixing ratios are used to calibrate the computed data. The results show that BS fraction has a restraining effect on the formation of mesophase when BI fraction is lower than 60 wt%, while this effect becomes weak with a higher content of BI fraction. A phase diagram is proposed by substituting the estimated parameters in SSAFT-APP equation with experimental data. Mesophase is able to be generated when the adding amount of BI fraction is increased to 12 wt%, and 100% mesophase could be obtained when the ratio of BI fraction is higher than 94 wt%. This work is expected to shed light on the phase transition behavior between isotropic phase and mesophase during pitch preparation from coal tar. In addition, based on the thermodynamic study optimizing preparation conditions and a novel method for obtaining the products with an optimal proportion between isotropic phase and mesophase is proposed.