Carbonization studies of petroleum vacuum distillation residua have been carried out on a laboratory scale with the objective of developing a better understanding of the causes and mechanisms of the incidence of shot coke formation in delayed coking. The rates of carbonization of the residua are found to inversely relate to the extent of mesophase development in the resulting semicoke. The asphaltenic fractions of the petroleum stocks carbonized most rapidly and produced cokes containing small anisotropic regions (fine mosaics), similar to that of commercially produced shot coke and consistent with arrested mesophase development. The addition of aromatic petroleum materials enhanced mesophase development commensurate with their reducing the rate of carbonization. While the microstructural features of shot coke are reproducibly produced in the laboratory, the characteristic spherical morphology is not and appears to be determined by the conditions obtaining in large-scale operation. It is suggested that this morphology may arise from the segregation of regions of rapidly carbonizing constituents from a less reactive matrix.
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