Hydrodesulfurization of residuals

Three-phase reactors are widely used in hydroprocessing operations and for oxidation reactions. Trickle bed reactors have been widely used for hydrodesulfurization of residue oils, hydrodesulfurization, and hydrocracking of gas oils and in numerous oxidation reactions. Three-phase fluidized bed reactors are also used in coal liquefaction and in Fischer-Tropsch synthesis. It is in these and similar examples that the review presented in this monograph can most pertinently be applied. 

The model was basically developed for hydrodesulfurization of residual oil with a high level of catalyst contaminants causing pore mouth plugging of catalyst. 

Figure 1. Conversion and chemical hydrogen consumption for hydrodesulfurization of residuals with H-Oil...

Satori, H, and S, Nishizaki, "Studies on the Hydrodesulfurization of Residual Oil using a Test Apparatus of Larger Size. II, On the Theoretical Analysis of the Fixed Bed Reactor", Int, Chem, Eng. (1971) 339-344. 

Hydrodesulfurization of residual feedstocks with a high metals content is a commercial proposition today (J.j ) - Based on previous work of many investigators (e.g. ref. 3 through 9) Shell s major efforts were focussed on the development of a suitable catalyst, for which a better understanding of the ageing phenomena occurring in the catalyst particle was needed. 

A major problem in the catalytic hydrodesulfurization of residual oils is the deactivation of the catalyst by metal-containing asphaltenic species in the feed. As can be seen from the results of a typical desulfurization experiment presented in Fig. 1, the catalyst shows a rapid initial decline which is attended with a fast build-up of coke on the catalyst. At a relatively low catalyst age 0, as defined in Section IV, a stationary coke level is reached. In contrast, the deposition of the inorganic remnants of the hydro-cracked asphaltenes (mainly vanadium and nickel sulfides) continues and gradually clogs the pores in the outer zone of the catalyst particles, as confirmed by electron microprobe analyses of spent catalyst samples (see Fig. 2). This causes a slow further loss in desulfurization activity over a longer period of time. Ultimately, the catalyst becomes totally inactive for desulfurization because the - still active - inner core has become completely inaccessible to the sulfur-bearing molecules. 

Simple conventional refining is based essentially on atmospheric distillation. The residue from the distillation constitutes heavy fuel, the quantity and qualities of which are mainly determined by the crude feedstock available without many ways to improve it. Manufacture of products like asphalt and lubricant bases requires supplementary operations, in particular separation operations and is possible only with a relatively narrow selection of crudes (crudes for lube oils, crudes for asphalts). The distillates are not normally directly usable processing must be done to improve them, either mild treatment such as hydrodesulfurization of middle distillates at low pressure, or deep treatment usually with partial conversion such as catalytic reforming. The conventional refinery thereby has rather limited flexibility and makes products the quality of which is closely linked to the nature of the crude oil used. 

Burning of sulfur to produce SO can create both burner system corrosion problems as well as atmospheric air emission concerns. About 1% to 5% of the fuel sulfur burned is converted to S03 and the remainder is converted to S02. If a system operates below its dew point, the SO, can react with condensed water to form sulfuric acid. Much work is being done through hydrodesulfurization, neutralization, and engineering to reduce the amount of sulfur oxides produced through burning of residual fuel. 

Frost, C.M., and Cottingham, P.L. 1971. Hydrodesulfurization of Venezuelan Residual Fuel Oils. Report of Investigations RI 7557. U.S. Bureau of Mines, Washington, DC. 

The Shell residual oil hydrodesulfurization process is broadly defined as a process to improve the quality of residual oils by removing sulfur, metals, and asphaltenes... 

A Catalyst Deactivation Model for Residual Oil Hydrodesulfiirization and Application to Deep Hydrodesulfurization of Diesel Fuel... 

Feed properties and operation conditions determine catalyst life in the residue hydrodesulfurization. In a high conversion operation of vacuum residue, catalyst deactivation due to coke is as important as the one due to metals. Though many researchers have worked on understanding and modelling deactivation of residue hydrodesulfurization catalysts, there has still been a controversy in a coke deactivation mechanism [2, 3]. Very few publications are available discussing an effect of a bed temperature profile on catalyst deactivation in large scale adiabatic commercial reactors. Most of the studies on deactivation of residue hydrodesulfiirization catalysts have been done with small-scale isothermal reactors [2,3,4,5]. The activity tests of the used catalysts were conducted to study the catalyst deactivation in the commercial reactors. This paper also describes an effect of a bed temperature profile on coke deactivation, which was tested in the commercial reactors. 

The catalyst which has a larger pore diameter tends to show a lower deactivation rate, as well as lower HDS activity. Figure 1 shows one example of the results of residual hydrodesulfurization experiments testing three kinds of catalysts which have different pore diameters. The micro-reactors were operated under the same conditions, as shown in Figure 1. Catalyst A, Catalyst B and Catalyst C were the test catalysts which have the same properties with different pore diameters ( C > B > A). The activity and deactivation rate of each catalyst were shown to depend strongly on pore diameter, as shown in Figure 1. 

In a further work, Tang and Curtis26 have studied the influence of the tyre components by replacing coal with model compounds 4-(l-naphthylmethyl)-bibenzyl (NMBB), dibenzothiophene and 5-methyl-8-(l-methylethyl)dibenzo-thiophen-4-ol (MMDH). Carbon black was active for the NMBB hydrocracking, whereas little activity was found with SBR, waste tyres and waste tyre liquefaction residues. However, when the latter residues were previously heat treated to remove organic coatings and recover the carbon black component, significant NMBB conversions were observed. Moreover, both carbon black and heat-treated residues were active for the hydrodesulfurization of dibenzothiophene and MMDH, particularly when combined with Mo naphthenate and S. These results confirm the catalytic properties of carbon black in coal-tyre coliquefaction, provided that its surface area is accessible. 

This differs from conventional hydrodesulfurization by the small amounts of sulfur compounds initially present and the even lower contents required in the product, both of sulfur (I ppm for the 60 to 150 C cut) and olefins (50 ppm). Moreover, the components to be removed are thiophenic, entailing relatively severe operating conditions (elevated temperature and high hydrogen pressure) due to their poor reactivity. However, the existence in the feed of residual amounts of certain diolefins which resist dedienization, and especially of olefins, tends to make the medium extremely reactive. 

Trickle bed reactors have grown rapidly in importance in recent years because of their application in hydrodesulfurization of naphtha, kerosene, gasoil, and heavier petroleum fractions hydrocracking of heavy gasoil and atmospheric residues hydrotreating of lube oils and hydrogenation processes. In trickle bed operation the flow rates are much lower than those in absorbers. To avoid too low effectiveness factors in the reaction, the catalyst size is much smaller than that of the packing used in absorbers, which also means that the overall void fraction is much smaller. 

Papyannakos, N. and J. Marangosis. Kinetics of Catalytic Hydrodesulfurization of a Petroleum Residue in a Batch-Recycle Trickle Bed Reactor. Chem. Eng. Sci. 39 (1984) 1051-1061. 

Figure 1. Hydrodesulfurization of a Caribbean long residue (feed I)...

Figure 3. Vanadium removal during hydrodesulfurization of Caribbean and Middle East long residues...

Various simplified approaches based on kinetics have been reported in the literature. Papayannakos and Georgiou (1988) developed a kinetic model for hydrogen consumption during catalytic hydrodesulfurization of a residue in a trickle-bed reactor. The atmospheric residue of Greek petroleum deposits in the Aegean Sea was used as feedstock. They found that the kinetic model correlated well with the experimentally measured hydrogen consumption. 

Marafl et al. (2007,2008) from Kuwait has been working very hard on studies to determine the maximum metal capacity of different catalysts used in hydrodesulfurization of atmospheric residue by accelerated aging tests. 

Papayannakos, N., Geoigiou, G. 1988. Kinetics of hydrogen consumption during catalytic hydrodesulfurization of a residue in a trickle-bed reactor. J. Chem. Eng. Jpn. 

Thermal Cracking. In addition to the gases obtained by distillation of cmde petroleum, further highly volatile products result from the subsequent processing of naphtha and middle distillate to produce gasoline, as well as from hydrodesulfurization processes involving treatment of naphthas, distillates, and residual fuels (5,61), and from the coking or similar thermal treatment of vacuum gas oils and residual fuel oils (5). 

Natural gas contains both organic and inorganic sulfur compounds that must be removed to protect both the reforming and downstream methanol synthesis catalysts. Hydrodesulfurization across a cobalt or nickel molybdenum—zinc oxide fixed-bed sequence is the basis for an effective purification system. For high levels of sulfur, bulk removal in a Hquid absorption—stripping system followed by fixed-bed residual clean-up is more practical (see Sulfur REMOVAL AND RECOVERY). Chlorides and mercury may also be found in natural gas, particularly from offshore reservoirs. These poisons can be removed by activated alumina or carbon beds. 

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