Thermodynamics and phase relationship of carbonaceous mesophase appearing during coal tar pitch carbonization

Abstract

Carbonaceous mesophase appears as an intermediate product in production of electrical and mechanical carbon materials, needle coke for graphite electrodes used in electric-arc furnaces, pitch-based fibers and aluminum-smelting prebaked electrodes through carbonization process. A thermodynamic approach for phase behavior estimation of carbonaceous mesophase is presented to provide more insights and to lead to semi-quantitative modeling of this process. Phase behavior of binary, ternary and multi-component systems are described by minimization of the Gibbs free energy function proposed by Hu and Hurt for mesophase pitches which exhibits orientational energy contribution of molecules in mesophase. The model enables estimating the reversible phase transition of mesophase upon temperature cycling as suggested by Lewis. The capability of the model to estimate the molecular weight distribution of species in the phases in equilibrium and variation of mesophase content with thermal soaking time has been discussed. Estimated ternary isothermal sections showing isotropic-mesophase and iso-iso immiscibility are presented for the first time. The phase behavior of ternary systems with average molecular weight in the range of average molecular weight of coal tar pitches are described. The application of the model to phase diagram prediction of the specific systems which exhibit the miscibility gap has been studied.