Effects of Catalyst Properties on Hydrodesulfurization on of Fluid Catalytic Cracking Decant Oils as Feedstock for Needle Coke

Open Access
Author:
Abrahamson, Joseph Peter
Graduate Program:
Energy and Mineral Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 08, 2015
Committee Members:
  • Semih Eser, Thesis Advisor
Keywords:
  • Decant Oil
  • Hydrodesulfurization of Decant Oil
  • Needle Coke
Abstract:
This thesis investigates hydrodesulfurization of fluid catalytic cracking decant oils and its impact on carbonaceous mesophase development during carbonization. Delayed Coking of decant oils leads to the development of carbonaceous mesophase, a liquid crystalline phase, that is responsible for the structural anisotropy of needle coke. Because of its anisotropic microstructure, needle coke is used as the filler along with a coal tar pitch binder for manufacturing graphite electrodes for electric-arc furnaces. When sulfur content of the filler needle coke is high (>0.8 wt. %) an irreversible volume expansion (puffing) takes place when extruded electrodes are subjected to graphitization heat treatment. Puffing in needle coke results primarily from the evolution of sulfur in the form of CS2 and H2S. Puffing causes the formation of mico-cracks reducing the quality of electrodes. Most practical and efficient process to control the sulfur levels in needle cokes is catalytic hydrodesulfurization of the feedstock decant oil prior to coking. The overall objectives of this thesis study were to investigate the effects of catalyst properties on hydrodesulfurization activity, hydrogenation activity, and to investigate the effect of hydrogenation on the subsequent mesophase development from the hydrotreated decant oil. Two decant oils with relatively low and high sulfur concentrations were used as feedstocks. Decant oils were fractionated by vacuum distillation into three cuts representing the light fraction, middle fraction, and vacuum bottoms. The hydrodesulfurization of the middle fraction and parent oils were performed in a bench-scale fixed bed flow reactor. Reaction variables of interest include temperature, hydrogen pressure, and catalyst properties. Overall, increasing reaction temperature suppressed the hydrogenation of the constituent polyaromatic hydrocarbons, but promoted sulfur reduction. Increase in hydrogen pressure, on the other hand, did not appear to have much impact on desulfurization, but significantly increased the extent of hydrogenation. Four catalysts (with varying average pore sizes, promoter atoms, and supports) were prepared with sequential incipient wetness impregnation to evaluate their activities for hydrodesulfurization and hydrogenation of decant oils. The desirable outcome of decant oil hydrodesulfurization is to achieve the required sulfur removal particularly from large polyaromatic ring systems with minimum hydrogen consumption. Removing sulfur from larger ring systems in decant oils is the most effective way of reducing the needle coke sulfur content because large aromatics end up in the coke product obtained from Delayed Coking. Hydrodesulfurization experiments showed that the catalysts with large pores in the support (14 nm average diameter) had a higher activity for removing sulfur from large polycyclic aromatic rings compared to the catalyst with smaller pores (7 nm average diameter) in the support. Catalysts with larger pores also showed a lower overall activity for hydrogenation, thus, achieving both objectives of the hydrodesulfurization process. A reduction in hydrogenation activity is desired because it lowers hydrogen consumption during hydrodesulfurization and preserves the hydrogen aromaticity of decant oil which is important for mesophase development. Molybdenum (Mo) based hydrodesulfurization catalyst are typically promoted by either Co or Ni. Using Co as promoter in the prepared catalysts showed higher overall hydrodesulfurization activity than those with Ni promoter. Using a TiO2 support for a CoMo catalyst instead of Al2O3 with essentially the same pore size distribution increased the hydrodesulfurization activity, while decreasing the hydrogenation activity. Therefore, using TiO2 as a catalyst support could also prove to be useful for the objectives of decant oil hydrodesulfurization. To observe the effects of hydrotreatment on mesophase development, untreated and treated parent oils, distillate fractions, and treated middle fractions blended with vacuum bottoms were carbonized. The extent of mesophase development in the resulting semi-cokes was gleaned from an optical texture analysis using polarized-light microscopy. A qualitative measure of semi-coke texture by Optical Texture Index showed that hydrotreatment of the feedstocks to reduce sulfur content did not have a significant effect on mesophase development upon subsequent carbonization. It is concluded, therefore, that hydrodesulfurization of decant oil can effectively reduce the sulfur content in decant oils, and therefore, it could achieve the desirable sulfur level in the needle cokes to be produced from the hydrotreated decant oils. This can be done without compromising the anisotropy of the resulting needle coke microstructure. Using catalysts with larger pore sizes can remove sulfur selectively from large aromatic ring systems for most effective use of hydrogen in decant oil hydrodesulfurization.