Residuum hydroprocessing catalysts

In Section III, commercial residuum hydroprocessing technology is discussed to establish the role and requirements of hydroprocessing in the overall refinery residuum conversion scheme. Commercial residuum hydroprocessing catalysts and residuum hydrodesulfurization (RDS)-hydrodemetallation (HDM) technology are reviewed briefly. 

This section will focus on the various hydroprocessing technologies that have been commercialized or are in a pilot stage near commercialization. Reactor design characteristics that differentiate the technologies will be highlighted. Included in this section is an overview of the properties and applications of commercial residuum hydroprocessing catalysts. 

The variety of applications and the market growth potential have attracted numerous entries into the residuum hydroprocessing catalyst market, as indicated by the compilation of commercial vendors and catalysts in Table XXIII. Catalysts are available in an assortment of shapes, sizes, and formulations, but detailed information on catalytic metals, support composition, pore size, and pore size distribution is sketchy. 

Vanadium and Nickel Distribution Factors in Residuum Hydroprocessing Catalysts"- ... 

Fig. 40. Typical deactivation curve for residuum hydroprocessing catalyst. Arabian Heavy atmospheric residuum desulfurized to 1.10 wt. % product sulfur with a iV-in. extrudate catalyst (Tamm ei al., 1981).

Fig. 47. Pore size distribution characteristics of typical residuum hydroprocessing catalysts (Howell et ai. 1985).

However, residuum hydrotreating catalysts themselves are susceptible to irreversible deactivation caused by the accumulation of sulfided metal impurities. The gradual buildup of these impurities in the pores of a hydroprocessing catalyst causes plugging and deactivation. 

In Section V, deactivation of catalyst pellets and reactor beds during residuum hydroprocessing is considered. The chemical nature of the metal deposits is described, including a discussion of the physical distribution of these poisons in aged catalysts and reactor beds. Models to predict... 

In the last section, future perspectives for the study of residuum hydroprocessing and the rational design of hydrodemetallation catalysts and processes are offered. 

Based on elemental analyses and microprobe tracing (Dautzenberg et al., 1978), metal deposits appear to be present in sulfide forms and not as adsorbed porphyrin-type compounds or as metals in the elemental or metallic state. Takatsuka et al. (1979) and Rankel and Rollmann (1983) have reported direct linear correlations of the spent catalyst sulfur content with the deposited metal content. The sulfide forms of nickel and vanadium are consistent with expectations based on thermodynamics for the conditions typically encountered in residuum hydroprocessing units (600-800°F, 1000-2200 psig, H2/H2S environment). 

Catalyst selectivity differences have been found for sulfur and metals removal in residuum hydroprocessing (19). Mass transfer limitations are believed to be important (20). The data reported here show that the metal-containing molecules are larger consequently, they should be more subject to diffusion restrictions than the sulfur-containing molecules. Therefore, it will be more difficult for a small pore catalyst to demetallate a residuum than to desulfurize it. 

Size characterization measurements have provided useful information on the importance of the hydroprocessing catalyst pore size distribution and on the effects of visbreaking and hydroprocessing on the residua molecular size distributions. It is apparent that asphaltenes and maltenes are not unique entities, but instead have considerable overlap in their size distributions. A complete study of the effects of processing conditions would require consideration of all components of a residuum. 

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