Investigation on the development and orientation of mesophase microstructure during the two-stage pyrolysis of FCC decant oil

Abstract

The two-stage pyrolysis was proposed to effectively form uniaxially fibrous mesophase during pyrolysis of FCC decant oil. Detailed investigation focuses on main factors of regulating mesophase orientation and corresponding changes of mesogen stacks. The first-stage treatment under low temperature of 440 °C and high pressure of 2 MPa for 8 h ensures formation of bulk mesophase with well-coalesced texture and good molten fluidity; subsequently, fracture of residual alkyl side-chains together with devolatilization of trapped light molecules under high temperature (450–480 °C) and low pressure (0.2–1.0 MPa), mainly contributes to volatile evolution at solidification of two-stage pyrolysis. Thereby, the overlap between sufficient volatile evolution that provides enough shear force on mesophase and appropriate viscosity of bulk mesophase, high enough for maintenance of mesophase deformation but not too high to prevent penetration of gas bubbles, is readily achieved and then significantly promotes the transition from mesophase domain into acicular flow texture. Moreover, mesophase orientation at solidification is followed by transformation of mesogen stacks from columnar short-range order into nematic order, leading to decrease of stacked height and number of layer per stack. And further cracking and dehydroaromatization reactions at solidification of two-stage pyrolysis facilitate elimination of steric hindrance, relaxation of structural strains and heal of defects within the planar aromatic sheets, thus resulting into a more homogenous distribution of stacked mesogens and a smaller degree of disorder within the 2D mesogen lattice.