Hydrodesulfurization of Fluid Catalytic Cracking Decant Oils for the Production of Low-sulfur Needle Coke Feedstocks
Open Access
- Author:
- Wincek, Ronald Thomas
- Graduate Program:
- Energy and Geo-Environmental Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 01, 2013
- Committee Members:
- Semih Eser, Thesis Advisor/Co-Advisor
- Keywords:
- Decant Oil
Needle Coke
Hydrodesulfurization
Delayed Coking
Mesophase - Abstract:
- Needle coke, produced by the delayed coking of fluid catalytic cracking decant oils, is the primary filler used in the production of graphite electrodes. The processing of increased amounts of sour crude oils has yielded higher levels of sulfur in the available decant oil feedstocks. High levels of sulfur in decant oils (DO), typically greater than 0.5 weight percent, can result in unacceptable levels of sulfur (>0.5 wt%) in the needle coke. During subsequent graphitization of the needle coke for electrode production, the sulfur is released through a process called “puffing” which reduces the quality of the electrode. The overall objective of this thesis study was to investigate the possible use of hydrodesulfurization (HDS) for decreasing the sulfur content of the starting decant oils, including the effect of hydrodesulfurizing conditions and catalyst type on sulfur removal, chemical composition of the DO, and the effect on mesophase formation and final coke properties. Three decant oils containing different concentrations of sulfur were chosen to represent low-, medium-, and high-sulfur feedstocks. These DO were analyzed for their elemental composition. The GC amenable portion of the low- and medium-sulfur DO was also quantitatively analyzed by GC/MS to determine their molecular composition. The GC/MS analysis revealed that the DO consisted of normal alkanes and polycyclic aromatic hydrocarbons (PAH). The PAH included 2-ring (naphthalens), 3-ring (phenanthrenes), and 4-ring (pyrenes, and chrysenes) PAH and their alkylated analogs. Significant differences were observed in the molecular composition between the low- and medium-sulfur decant oils. The medium-sulfur DO contained greater amounts of n-alkanes, 2- and 3-ring PAH, while the low-sulfur DO contained greater amounts of 4-ring PAH. A detailed analysis of the sulfur-containing PAH was also performed on the GC amenable portion of each DO using the GC/MS. Approximately equal amounts of dibenzothiophenes (DBT) and benzonaphthothiophenes (BNT) were contained in the low-sulfur decant oil. However, roughly 2/3 of the sulfur compounds contained in the medium-sulfur decant oil were heavier BNT. The medium-sulfur DO also contained low concentrations of benzothiophenes (BT). The HDS of DO was performed in a bench-scale reactor using different types of catalyst, catalyst bed temperatures, H2 pressures, and liquid hourly space velocities (LHSV). Using the high-sulfur DO as feedstock, the HDS results clearly show that a traditional CoMo catalyst achieved greater sulfur reduction than a NiCoMo catalyst containing larger pores. Additional HDS experiments demonstrated that the goal of 0.5 wt.% sulfur could be achieved in the low-sulfur decant oil at relatively low temperatures and H2 pressures at a LHSV of 1.0 h-1. While the target of 0.5 wt.% sulfur could also be achieved in the medium-sulfur DO, higher catalyst bed temperatures and/or H2 pressures were required. Under relatively mild reaction conditions the HDS catalyst showed significant reactivity for BT, DBT, and BNT. The most refractory sulfur species to the CoMo catalyst are the dimethyl-, trimethyl DBT and methyl-, dimethyl- and trimethyl- BNT. These alkylated sulfur compounds are assumed to be those with methyl group-induced steric hindrance to the catalyst such as 4,6-DMDBT. The catalyst also demonstrated activity for the hydrogenation of aromatic compounds and hydrocracking of alkanes in the decant oils. Slight changes were exhibited in n- alkanes, while more significant hydrogenation of PAH were observed in both DO. Reductions of naphthalenes, phenanthrenes, and chrysenes were observed, with chrysenes being the most prone to hydrogenation. Pyrene was observed to be the most resistant PAH to hydrogenation during HDS. Some evidence was also seen for the hydrocracking of the HDS hydroaromatic compounds. Subsequent carbonization of the parent decant oils and several HDS products clearly showed that HDS of decant oils can result in an improvement in the quality of mesophase development, as measured by an optical texture index (OTI), with increasing severity of HDS. However, with additional sulfur reduction, the quality of mesophase was observed to decrease. This decrease may be attributed to the presence of additional hydroaromatics and biphenyls formed during HDS.