Molecular Assemblies in Asphaltenes and Their High-Temperature Coke Products. 2. Levels of Molecular Organization

Abstract

An initial electron crystallographic investigation of the molecular organization of coke, described in a previous report, attempted to understand how long- or short-range order (i.e., mutual orientation beyond or below approximately 10 μm) found, respectively, in sponge and shot coke, might be detected. Signs of local nucleation were found at very small selected area diameters (e.g., 0.25–1.25 μm) with a test case based on the microcarbon residue test (MCRT), made on a sponge coke-forming sweet vacuum tower bottoms (VTB) feed. In MCRT experiments on sweet VTB, a 490 °C annealing temperature was used instead of the 500 °C employed in our earlier study. At 5 min annealing time, electron diffraction patterns revealed the formation of small nuclei within a 0.25 μm diameter. The smallest detected arcing of (002) reflection, found at 5 min annealing time, persists for all longer annealing times; only the distribution of these angles decreased. At a 5-fold increase of selected area diameter (1.25 μm), and at suitably small annealing times (≤15 min), some regions were found to include two competing nuclei. At longer annealing times only singly oriented nuclei were found within this sampled area. These electron diffraction results show a good correlation with the polarized-light optical microscope images of 5, 10, 15, and 30 min annealing times in which small mesophase spheres form, grow, and coalesce into large mosaics and domains. At another nearly 5-fold increase of selected area diameter (5.8 μm), shot and sponge coke could finally be distinguished from one another. The former is much more disordered (average (002) arc angle 134(60)°) than the latter (average (002) arc angle 40(13)°). Thus, nuclei for the sponge coke can grow to larger lateral limits with approximately the same orientation of stacked rings. This result has been repeated in a time sequence based on MCRT trials carried out at 490 °C on a Maya crude vacuum residuum, known to produce shot coke in coker units. Also, polarized light microscopy results are in accord with the electron diffraction observations, revealing the overall distribution of smaller coherent paracrystalline domains. Tests of other commercial cokes, including those from various parts of a Flexicoker unit, and material from a steam cracker, reveals that the molecular ordering is roughly dependent on the H/C atomic ratio of the resulting product. In agreement with earlier observations, long-range ordering of coke, i.e., the sponge coke product, is favored by the least amount of aromatic ring heteroatoms. Both types of coke appear first as small nuclei, but only the lateral growth of the sponge coke is relatively unconstrained.