MOLECULAR COMPOSITION OF NEEDLE COKE FEEDSTOCKS AND MESOPHASE DEVELOPMENT DURING CARBONIZATION

Open Access
Author:
Wang, Guohua
Graduate Program:
Energy and Geo-Environmental Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 24, 2005
Committee Members:
  • Semih Eser, Committee Chair
  • Ljubisa R Radovic, Committee Member
  • Harold Harris Schobert, Committee Member
  • Alan W Scaroni, Committee Member
  • Jonathan P Mathews, Committee Member
  • A Daniel Jones, Committee Member
Keywords:
  • Mesophase
  • Carbonization
  • Delayed Coking
  • Polyaromatic hydrocarbon
  • Needle Coke
  • FCC decant oil
Abstract:
This study investigates the molecular composition of fluid catalytic cracking (FCC) decant oil and its derivatives that are used as feedstocks for delayed coking to produce needle coke. Needle coke is a premium solid carbon used in manufacturing graphite electrodes for electric-arc furnace. For the first time in the open literature, this study reports data on the molecular composition of the feed that is actually introduced into the coke drum for delayed coking after fractionating the decant oil feedstock together with the liquid products generated from coking. The coker feed (CF), thus, includes the high-boiling fraction of the decant oil and the high-boiling fraction of liquid products (recycle). Hydrotreated fraction of the decant oil samples (HYD) and a vacuum tower bottom (VTB) obtained from decant oils were also analyzed. More emphasis was placed on analyzing the high-boiling fraction of the feedstocks that is not amenable to gas chromatography. Carbonaceous mesophase development from the feedstocks was also studied to seek relationships between the composition of the feedstocks and the needle coke texture. Commercial FCC decant oil (DO) samples and their derivatives (CF, HYD, and VTB) were carbonized in laboratory reactors. DO samples produced semi-cokes that displayed different degrees of mesophase development and CF, HYD and VTB give a higher degree of mesophase development compared to their parent decant oil. Significant differences were observed in the molecular composition of the different decant oil samples and between the decant oils and their derivatives. The principal compounds found in decant oils and their derivatives were found to consist of 3- to 6-ring PAHs (phenanthrene, pyrene, chrysene, benzopyrene, perylene, and benzoperylene). These PAHs are present in the feedstocks as multi-methyl substituted homologues. Compared to the parent DO, CF contains higher concentrations of unsubstituted PAHs, a lower degree of methyl substitution, and a higher proportion of thermally stable alkyl PAH isomers. HYD, on the other hand, contains a significantly higher proportion of hydroaromatics and much lower concentrations of sulfur compounds. The VTB fraction consists almost exclusively of PAH with 4 and higher number of condensed rings. In general, mesophase development from the feedstocks was observed to relate to the overall carbonization reactivity (rate of semi-coke formation) and the relative concentrations of pyrenes and phenanthrenes. This trend confirms a widely accepted notion that the lower the carbonization reactivity, the higher is the degree of mesophase development. Samples of CF and HYD exhibit a relatively low carbonization reactivity (compared with their parent DO) and exhibit a high degree of mesophase development. As one exception to the general trend, VTB fractions exhibited the highest carbonization reactivity, but produced a well developed anisotropic coke texture. The carbonization reactivity of the feedstocks depends on their molecular composition, as exemplified by the comparison of CF with DO. This study shows that the relative distribution of pyrene and phenanthrene compounds plays a significant role in the mesophase development from the needle coke feedstocks. A preliminary quantum chemistry modeling (MNDO level of theory) was conducted using phenanthrenes and pyrenes as models. The results gave a relatively high reaction rate constant for the formation of oligomers with non-planar configuration from phenanthrenes. In contrast, pyrenes produce planar oligomers with lower reaction rate constants.