M. Absi-Halabi

Hydroprocessing of vacuum residues: relation between catalyst activity, deactivation and pore size distribution

Abstract Four catalysts with different unimodal and bimodal pore size distributions having different proportions of meso- and macropores were studied. The performance of the catalysts in terms of activity and selectivity as well as deactivation in selected hydrotreating reactions was examined using Kuwait vacuum residue as feedstock. The effect of catalyst pore size was significantly different for different reactions. For sulfur removal (HDS), a unimodal pore catalyst with maximum pore volume in the medium mesopore range (10–25 nm diameter) showed the highest activity. For hydrodemetallation (HDM) and hydrodenitrogenation (HDN) reactions, large-pore catalysts, having a major proportion of their pore volume in 100–300 nm diameter pores, were more effective. Bimodal pore catalysts having large amounts of narrow pores showed a higher rate of deactivation than unimodal pore catalysts with a maximum amount of medium mesopores.

Preparation of a large pore alumina-based HDM catalyst by hydrothermal treatment and studies on pore enlargement mechanism

The changes occurring in the alumina phase during hydrothermal treatment of γ-alumina in the presence and absence of additives such as P, F, phenol and acetic acid, were investigated with a view to understand the mechanism of pore widening in γ-alumina under hydrothermal conditions. The results showed that the formation and growth of boehmite into large crystallites by rehydration of γ-alumina was responsible for pore enlargement. As a part of the study, a wide pore Ni-Mo/γ-alumina was prepared by hydrothermal treatment procedure and its performance in vacuum residue hydroprocessing was evaluated. The catalyst showed a remarkably high activity for hydrodemetallization and asphaltenes conversion reactions in vacuum residue hydroprocessing. Distribution of the deposited metals within the catalyst pellet was more uniform for this catalyst compared to that of a conventional HDM catalyst.

Heavy oil hydrotreating catalyst rejuvenation by leaching of foulant metals with ferric nitrate-organic acid mixed reagents

Abstract Much effort has been devoted in recent years to develop processes for the rejuvenation of metal fouled spent catalysts from residue hydroprocessing units. In this paper we report the influence of adding ferric salts to some organic acids in selective extraction of foulant metals from the spent catalyst prior to decoking. Leaching experiments were carried out using two different organic acids, namely, oxalic acid and tartaric acid with and without ferric nitrate addition. The spent and the treated catalysts were characterized and the improvements in surface area, pore volume and HDS activity as a result of leaching with different reagents compared. The studies revealed that in the absence of ferric salts, the acids showed very poor activity for leaching foulant metals from coked spent catalyst. Addition of ferric nitrate enhanced the leaching efficiency of each acid to a different degree. The selectivity for the removal of the major metal foulant (vanadium) was different for different leaching reagents. The enhanced leaching by the ferric nitrate-organic acid system has been explained in terms of a synergistic mechanism involving oxidizing and complexing reactions. The improvements in surface area and pore volume recovery was found to be related to the extent of vanadium removal from the catalyst. The HDS activity of the catalyst also increased significantly by leaching of the deposited metals. The study showed that oxalic acid-ferric nitrate reagent was superior to other reagent systems in terms of selectivity for vanadium leaching as well as for surface area, pore volume and activity recovery.

Studies on the rejuvenation of spent catalysts: effectiveness and selectivity in the removal of foulant metals from spent hydroprocessing catalysts in coked and decoked forms

Abstract Two routes were used to rejuvenate spent residue hydroprocessing catalysts by leaching foulant metals. In the first, deoiled spent catalyst containing coke and deposited metals in sulfide form was chemically treated to remove the metal foulants. In the second, deoiled spent catalyst was decoked by controlled combustion of coke and the resultant coke-free catalyst containing metals in oxide form was subjected to leaching. Oxalic acid, a chelating agent which is able to form soluble metal complexes, was used for metal leaching in both routes. The influence of adding an oxidizing agent such as H 2 O 2 to oxalic acid in particular its effect on the leaching efficiency was examined in both routes. The spent and treated catalysts were characterized and the improvements in surface area, pore volume and hydrodesulphurization-activity of the catalysts were compared. The selectivity for leaching of the major metal foulant (vanadium) was better, and activity recovery was higher for the catalyst rejuvenated by metal leaching prior to decoking.

Effect of acidic and basic vapors on pore size distribution of alumina under hydrothermal conditions

Abstract The development of wide pore catalysts, with surface areas and mechanical strengths for application in, for example, the hydroprocessing of petroleum residues, has received increased attention in recent years. In the present paper, we have investigated the influence of acidic and basic vapors on pore enlargement of γ-alumina supports under low-temperature hydrothermal conditions. γ-Alumina in the form of 1.5 mm extrudates was subjected to hydrothermal treatment in an autoclave in the presence of acetic acid and ammonia for various time intervals in the temperature range 150–300 °C. The treated catalyst samples were characterized for surface area, pore size distribution and crushing strength. The samples were also examined by scanning and transmission electron microscopes and X-ray diffraction to assess the extent of sintering and possible changes in the alumina phase. The results of the studies revealed that ammonia enhances, whereas, acetic acid suppresses pore enlargement of γ-alumina under hydrothermal conditions. Surface area loss for samples treated under different atmospheres were of the order: ammonia + water > water > acetic acid + water. In the presence of ammonia, selective widening of pores in the 100–250Arange with improved side crushing strength was noticed at low or moderate temperatures (e.g. 150°C). The contrasting effects of ammonia and acetic acid vapors on the sintering and pore enlargement of γ-alumina under hydrothermal conditions is discussed on the basis of the influence of these reagents on hydroxylation of Al-O-Al bonds and the subsequent formation of boehmite and its recrystallization.

Performance comparison of alumina-supported Ni-Mo, Ni-W and Ni-Mo-W catalysts in hydrotreating vacuum residue

Abstract In the present work we report the performance of a Ni-Mo-W γ-Al 2 O 3 catalyst in comparison with conventional Ni-Mo and Ni-W catalysts for promoting various reactions that occur during residual oil hydroprocessing. The performance evaluation tests were conducted in a high pressure fixed bed reactor system using Kuwait vacuum residue as feed. The reactions that were monitored included hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodemetallation (HDM), hydroconversion to distillates (HC), asphaltenes removla, and CCR reduction. The results revealed that the Ni-Mo-W catalyst was more active for various conversions than either the Ni-Mo or Ni-W catalyst. The activity improvement was particularly higher for HDN, HDM, HC, asphaltene conversion and CCR reduction. The addition of W to Ni-Mo Al 2 O 3 enhanced the hydrogenation function of the catalyst.

Effect of thermal treatment on the sintering and structural changes of cobalt—molybdenum/alumina and nickel—molybdenum/alumina hydrotreating catalysts

Abstract The effect of calcination temperature on the Co Mo/Al2O3 and Ni Mo/Al2O3 catalysts in their oxidic form has been studied by X-ray photoelectron spectroscopy, X-ray diffraction, differential thermal analysis and surface area and porosity measurements. Calcination up to 600°C produces no large changes in activity of the catalyst. Calcination at higher temperatures results in loss of surface area, in volatillisation of molybdenum species, in a reduction in the amount of molybdenum on the surface and in the formation of various aluminates. Significant differences in temperature sensitivity are noticed between Co Mo/Al2O3 and Ni Mo/Al2O3 with regard to the above changes. The change in catalytic activity with calcination temperature is related to the changes in the inorganic chemistry of the system induced by calcination. Optimal calcination involves treatment at up to 600°C (Ni-Mo/Al2O3) or 700°C (Co-Mo/Al2O3).

Rejuvenation of residual oil hydrotreating catalysts by leaching of foulant metals: modelling of the metal leaching process

Abstract Increasing emphasis has been paid in recent years on the development of processes for the rejuvenation of spent residual oil hydroprocessing catalysts, which are deactivated by deposition of metals (e.g. vanadium) and coke. As part of a research program on this subject, we have investigated selective removal of the major metal foulant from the spent catalyst by chemical leaching. In the present paper, we report the development of a model for foulant metals leaching from the spent catalyst. The leaching process is considered to involve two consecutive operations: (i) removal of metal foulants along the main mass transfer channels connected to the narrow pores until the pore structure begins to develop and (ii) removal of metal foulants from the pore structure. Both kinetic and mass transfer aspects were considered in the model development, and a good agreement was noticed between experimental and simulated results.

Effect of nickel on the surface acidity of γ-alumina and alumina-supported nickel—molybdenum hydrotreating catalysts

Abstract The influence of nickel on the acid strength distribution of a γ-Al 2 O 3 support and a series of alumina-supported hydrotreating catalysts such as NizMo/γ-Al 2 O 3 and NizP/γ-Al 2 O 3 in their oxidic form in the concentration range close to commercial catalysts were examined by temperature-programmed desorption of ammonia. A substantial increase in the concentration of strong acid sites was observed when nickel was added to γ-Al 2 O 3 . On the other hand, addition of molybdenum to γ-Al 2 O 3 resulted in the neutralization of the strong acid sites with a significant increase in the number of acid sites of medium strength. The influence of phosphorus on the acidity was similar to that of molybdenum. Addition of nickel to MoO 3 /γ-Al 2 O 3 and PO 4 /γ-Al 2 O 3 systems was found to increase the number of strong acid sites considerably, with a corresponding decrease in concentration of the acid sites of moderate strength. A mechanism based on the formation of a nickel aluminate spinel on the surface of alumina and its effect in increasing the net positive charge or the coordinatively unsaturated sites of the surface is proposed in order to rationalize the influence of nickel on increasing the acid strength.

Influence of catalyst pore size on asphaltenes conversion andcoke-like sediments formation during catalytic hydrocracking of kuwait vacuum residues

Abstract A critical factor that limits the maximum attainable conversion of heavy residues to lighter cuts in commercial residue hydroprocessing units is coke-like sediments formation. Suppression of sediments formation is highly desirable to increase distillate yields. As part of a research program on the factors which influence sludge or sediments formation during hydroprocessing of Kuwait vacuum residue for high conversion, we have investigated the relation between catalyst pore size, asphaltene conversion and coke-like sediments formation. Five Ni-Mo/γ-Al 2 O 3 catalysts with different unimodal and bimodal pore size distribution were used in the study. A unimodal pore catalyst with maximum pore volume in medium size mesopore range (100–200dia) showed the highest activity for the overall conversion of the residual oil to distillates. However, a relatively larger percentage of sediments was also observed for this catalyst. Catalysts with a large proportion of macropores, particularly in the 800–3000range produced little or no sediments, but showed poor activity for asphaltene cracking and overall conversion of residual oil to distillates. Molecular weight of the residual asphaltenes in the product increased with decreasing catalyst pore size. The concentrations sulfur and vanadium in the residual asphaltenes were found to be higher for catalysts having only small and meso-pores without macropores. The results have been explained on the basis of the importance of the ratio of feed molecular size to catalyst pore size in determining the diffusion and reaction rates in residue hydroconversion.