An Overview of Hydrodesulfurization and Hydrodenitrogenation

Journal of the Japan Petroleum Institute, 47, (3), 145－163 (2004) 145 [Review Paper] An Overview of Hydrodesulfurization and Hydrodenitrogenation Isao MOCHIDA* and Ki-Hyouk CHOI Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga-koen, Kasuga, Fukuoka 816-8580, JAPAN (Received May 12, 2003) Hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of petroleum products and intermediates are reviewed to provide the basis for developing processes to produce gasoline and diesel oil with very low sulfur content. The reactivity, selectivity and inhibition (susceptibility of substrate molecules to inhibitors) in the catalytic process are very important to develop efficient processes. Recent advances in the understanding of active species, supports and supporting methods are also critically reviewed to suggest the design of catalysts with ade- quate activity to satisfy future regulations on transportation fuels. Details of the structures of the catalysts are not discussed, but the mechanisms of hydrodesulfurization and inhibition are summarized. Catalyst deactivation and reactor design are also briefly reviewed. New approaches to achieve deep hydrodesulfurization are pro- posed. Keywords Hydrotreating, Hydrodesulfurization, Hydrodenitrogenation, Diesel fuel, Gasoline 1. Introduction also reviewed. The preparation, activation, composition and structure of the catalysts in each process are Petroleum refining uses numerous processes includ- discussed along with the associated causes of catalyst ing thermal, catalytic and hydrogenation upgrading deactivation and ultimate catalyst lifetime for each processes as shown in Fig. 1. The hydrogenation process. New and improved catalytic approaches and processes include three major classes, hydrotreating, more active catalysts are also discussed. hydrocracking, and hydrofinishing. Hydrofinishing is Figure 2 illustrates the diversity of composition by really another form of hydrotreating that is used to showing the elemental distribution of some typical achieve the final specifications of fuels. The common petroleum fractions, such as light cycle oil (LCO), features as well as the differences of the various medium cycle oil (MCO), straight run gas oil (SRGO), hydroprocesses will be described. Each process is hydrotreated straight run gas oil (HSRGO), and gaso- individually optimized according to the boiling range line, as determined by gas chromatography equipped and molecular composition of the specific petroleum with atomic emission detection (GC-AED)2),3). Figure fraction to be treated1). Therefore, the process objec- 2 also shows the distributions of specific molecular tives, conditions and configurations, chemistry of fuels species that must be converted into hydrocarbons by and products, catalytic materials, their functions, and hydrotreating. The molecular composition of heavier working mechanisms must be understood for all of the fractions such as heavy VGO (vacuum gas oil), and important hydroprocesses in use today. atmospheric and vacuum residues are not fully under- Products in the refining processes are also stood at present, although high performance liquid hydrotreated, and are basically classified according to chromatography (HPLC) and time of flight mass spec- their boiling ranges. This overview describes the troscopy (TOF-MS) have provided some clues to their detailed chemistry of feeds, products, and their conver- molecular composition4),5). These heavier fractions are sion mechanisms in hydrotreating on a molecular level, believed to be polymeric substances of unit structures including the detailed structures of the reactant, their that are basically similar to those found in the lighter chemical and physical properties, and the mechanisms fractions. Strong molecular associations may be pre- of their conversion. The influences of the detailed sent in the residual fractions6), which causes difficulty molecular interactions on reactivity and inhibition are in both analyses and hydrotreating. One method for characterizing the residue is separation into polar and * To whom correspondence should be addressed. non-polar components by precipitation of the polar * E-mail: [email protected] components with a large quantity of a non-polar solvent J. Jpn. Petrol. Inst., Vol. 47, No. 3, 2004 146 Fig. 1 Stream of Petroleum Refining Process such as heptane into maltene and asphaltene7). tial loss in liquid product yield. The specific impuri- Asphaltenes are believed to be the major contributors to ties depend on the molecular weight of the feedstock to the undesirable features of the residue, such as high vis- be processed. Lower molecular weight feedstocks cosity, coking tendency, metal content, etc. Thus, the such as naphtha, gasoline, intermediate distillates major target of residue hydrotreating is to convert (atmospheric and light vacuum), diesel fuels, and home asphaltenes to lower molecular weight species. The heating oils (kerosene, etc.) contain undesirable impuri- asphaltenes consist of polymeric components contain- ties such as sulfur-containing compounds (S-com- ing polyaromatic rings with long alkyl chains that are pounds), nitrogen-containing compounds (N-com- entangled to form colloidal micelles within the pounds), oxygen-containing compounds (O-com- residue8),9). The polymeric chains also contain some pounds), and polynuclear aromatic compounds (PNA). porphyrins, which include metal components (vanadi- Higher molecular weight feedstocks, such as high vacuum and nickel), in the petroleum. The polyaromatic um distillates, and atmospheric and vacuum residues rings and porphyrins form stacked aggregates and the contain the same impurities as well as significant con- alkyl chains entangle each other. Such intermolecular centrations of metal-containing compounds (M-com- association is schematically illustrated in Fig. 36). pounds). V and Ni are the major metal impurities in GC-AED chromatograms of light and medium cycle petroleum, which are present in the form of porphyrin oils (MCO) in fluid catalytic cracking (FCC) products complexes of V4+ = O and Ni2+ 6). In addition, crude in the gas oil range are illustrated in Fig. 2 as examples oils often contain NaCl, MgCl2, CaCl2, CaSO4, and of cracked oils. Such processed oils can be further naphthenates of some metals such as Ca, Mg and Fe. hydroprocessed to yield high quality fuels. The metal salts can be removed rather easily by wash- ing before distillation. However, small amounts of 2. Hydrotreating Process metal compounds, particularly Fe or derived FeS, often result in operational problems. Naphthenates may dis- The primary objectives of hydrotreating are to solve iron from valves or the reactor vessels and trans- remove impurities, such as hetero-atom and metal-con- fer lines, and become included in the feeds to down- taining compounds, from a feedstock and/or to increase stream processes. In general, the concentration of the hydrogen content of the feedstock, and to lower the these impurities increases with increasing boiling point. molecular weight of the by-products without a substan- Thus, the hydrotreating process of choice depends pri- J. Jpn. Petrol. Inst., Vol. 47, No. 3, 2004 147 SRGO: Straight Run Gas Oil, H-SRGO: Hydrotreated Straight Run Gas Oil, LCO: Light Cycle Oil, MCO: Medium Cycle Oil, VGO (hex- ane soluble fraction): Vacuum Gas Oil. Cn: Paraffin with n carbons, T: Thiophene, BT: Benzothiophene, DBT: Dibenzothiophene, Cz: Carbazole, DM: Dimethyl, EM: Ethylmethyl, TM: Trimethyl. Fig. 2 AED Chromatograms of Various Fuel Oils J. Jpn. Petrol. Inst., Vol. 47, No. 3, 2004 148 In general, sulfur impurity is the major concern because S-compounds are often serious poisons and inhibitors for other secondary upgrading process catalysts. Their combustion products create serious envi- ronmental hazards such as acid rain. Thus, the main processes that have been developed for distillable feedstocks are HDS processes. N-compound impurities are also removed during HDS processes. If succes- sive acid catalysis is important in conversion mechanisms, extensive N-removal is required since the basic N-compounds are both serious poisons and coke pre- cursors on acid catalysts12). Lowering aromatic content through hydrotreating is classified as A: Deagglomeration of asphaltene due to demetallization, B: hydrodearomatization (HDA). HDA reactions occur Depolymerization due to desulfurization6). during HDS and HDN processes, but product quality requirements often require an HDA process after the Fig. 3 Model of Hydrocracking of Asphaltene initial HDS and/or HDN process. Future environmen- tal regulations may emphasize HDA further13). M-compound impurities are found particularly in marily on the boiling range of the feedstock. The high boiling feedstocks, such as atmospheric and vacu- boiling range is dictated by the molecular weight distri- um residues. Thus, HDM processes are tailored for bution of the feedstock. The next most important con- high boiling and very viscous feedstocks. In such sideration in choosing a hydrotreating process is the processes, the removed metals are deposited on the sur- product quality specification, which is predominantly face of the HDM catalyst, so the lifetime of the catalyst related to the total hydrogen content of the product, is of serious concern. As metals accumulate on the which is related to the content of polynuclear aromatics catalyst, the selectivity for the production of desired (PNA). products also decreases14). Thus, HDM processes are O-compounds

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