Hydrodesulfurization applications

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

The Rijnmond area is that part of the Rhine delta between Rotterdam and the North Sea. The Commission for the Safety of the Population at large (COVO) commissioned the study for six chemicals and the operations associated with them acrylonitrile, liquid ammonia, liquid chlorine, LNG, propylene, and part of a separation process (diethanolamine stripper of a hydrodesulfurizer). The study objectives were to evaluate methods of risk assessment and obtain experience with practical applications of these methods. The results were to be used to decide to what extent such methods can be used in formulating safety policy. The study was not concerned with the acceptability of risk or the acceptability of risk reducing measures. 

Thiophenes continue to play a major role in commercial applications as well as basic research. In addition to its aromatic properties that make it a useful replacement for benzene in small molecule syntheses, thiophene is a key element in superconductors, photochemical switches and polymers. The presence of sulfur-containing components (especially thiophene and benzothiophene) in crude petroleum requires development of new catalysts to promote their removal (hydrodesulfurization, HDS) at refineries. Interspersed with these commercial applications, basic research on thiophene has continued to study its role in electrocyclic reactions, newer routes for its formation and substitution and new derivatives of therapeutic potential. New reports of selenophenes and tellurophenes continue to be modest in number. 

These metals form chalcogenolate complexes in several oxidation states, and from the application-oriented point of view manganese compounds have been synthesized as models for hydrodesulfurization processes and rhenium and technetium derivatives as models for radiopharmaceuticals. 

LEIS has been applied to study the surface composition of Co-Mo and Ni-Mo hydrodesulfurization catalysts [46-48], Fe-based Fischer-Tropsch [49] and ammonia synthesis catalysts [50], and model systems such as Pt evaporated on Ti02 [51]. The review of Horrell and Cocke [52] describes several applications. 

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

The first large scale application of trickle bed reactors was to the hydrodesulfurization of petroleum oils in 1955. The temperature is elevated to enhance the specific rate and the pressure is elevated to improve the... 

M. M. Piwetz, J.S. Larsen, T.S. Christensen, "Hydrodesulfurization and Pre-reforming of Logistic Fuels for Use in Fuel Cell Applications," Fuel Cell Seminar Program and Abstracts, Courtesy Associates, Inc., November 1996. 

It should also be noted that polysulfido complexes are not only interesting because of their structures and reactivity, but also because of their possible applications. They can, for example, be used to prepare sulfur rings of predetermined size and they are also suspected to play a role in catalysis (particularly in hydrodesulfurization). 

Among these sulfides, only the ordinary cobaltfll) sulfide, CoS has commercial applications. It is used as a catalyst for hydrogenation or hydrodesulfurization reactions. Cobalt(II) sulfide is found in nature as the mineral syco-porite. The mineral linneite is made up of C03S4, tricobalt tetrasulfide. 

Trickle-bed reactors usually consist of a fixed bed of catalyst particles, contacted by a gas liquid two-phase flow, with co-current downflow as the most common mode of operation. Such reactors are particularly important in the petroleum industry, where they are used primarily for hydrocracking, hydrodesulfurization, and hydrodenitrogenation other commercial applications are found in the petrochemical industry, involving mainly hydrogenation and oxidation of organic compounds. Two important quantities used to characterize a trickle-bed reactor are... 

Whitehurst, Isoda, and Mochida write about catalytic hydrodesulfurization of fossil fuels, one of the important applications of catalysis for environmental protection. They focus on the relatively unreactive substituted di-benzothiophenes, the most difficult to convert organosulfur compounds, which now must be removed if fuels are to meet the stringent emerging standards for sulfur content. On the basis of an in-depth examination of the reaction networks, kinetics, and mechanisms of hydrodesulfurization of these compounds, the authors draw conclusions that are important for catalyst and process design. 

The chapter Fundamental Studies of Transition-Metal Sulfide Catalytic Materials by Chianelli, Daage, and Ledoux reviews current understanding of the relationship between structural and other properties of these catalysts and their catalytic activity and selectivity in hydrodesulfurization. In view of increasing environmental demands, this field has been heavily researched. The authors show how systematic studies and applications of novel methods can provide considerable understanding of these important catalysts. 

The structural differences between the various sulfur-containing molecules make it impractical to have a single rate expression applicable to all reactions in hydrodesulfurization. Each sulfur-containing molecule has its own hydrogenolysis kinetics that is usually complex because several successive equilibrium stages are involved and these are often controlled by internal diffusion limitations during refining. 

Application of the second-order rate equation to the hydrodesulfurization process has been advocated because of its simplicity and use for extrapolating and interpolating hydrodesulfurization data over a wide variety of conditions. However, while the hydrodesulfurization process may appear to exhibit second-order kinetics at temperatures near 395°C (745°F), at other temperatures the data (assuming second-order kinetics) does not give a linear relationship (Figure 4-9) (Ozaki et ah, 1963). 

While the definitions of the various hydroprocesses are (as has been noted above) quite arbitrary, it may be difficult, if not impossible, to limit the process to any one particular reaction in a commercial operation. The prevailing conditions may, to a certain extent, minimize, cracking reactions during a hydrotreating operation. However, with respect to the heavier feedstocks, the ultimate aim of the operation is to produce as much low-sulfur liquid products as possible from the feedstock. Any hydrodesulfurization process that has been designed for application to the heavier oils and residua may require that hydrocracking and hydrodesulfurization occur simultaneously. 

Hydrodesulfurization, through the application of hydroprocesses is linked to the relevant environmental regulations as well as to the desired distribution of products. Achieving these goals is compounded when heavy oil or residua are the feedstock because of the molecular complexity of the constituents. 

However, there are more chances of localized heating in the catalyst bed and (in addition to the more expensive reactor design per unit volume of catalyst bed) it may be more difficult to remove contaminants from the bed as part of the catalyst regeneration sequence. For this reason alone, it is preferable that this type of reactor is limited to hydrodesulfurization of low-boiling feedstocks such as naphtha and kerosene and application to the higher-boiling heavy oils and residua is usually not recommended. 

In summary, the hydrodesulfurization of the low-, middle-, and highboiling distillates can be achieved quite conveniently using a variety of processes. One major advantage of this type of feedstock is that the catalyst does not become poisoned by metal contaminants in the feedstock since only negligible amounts of these contaminants will be present. Thus, the catalyst may be regenerated several times and onstream times between catalyst regeneration (while varying with the process conditions and application) may be of the order of 3 to 4 years (Table 6-6). 

Combined with hydrodesulfurization, the process is fully applicable to the feed preparation for fluid catalytic cracking and hydrocracking. The process is capable of using a variety of feedstocks including atmospheric and vacuum residues derived from various crude oils, oil sand, visbroken tar and so on. 

In order to accomplish sulfur removal, use is still made of extraction and chemical treatment of various petroleum fractions as a means of removing certain sulfur types from petroleum products, but hydrodesulfurization is the only method generally applicable to the removal of all types of sulfur compounds. 

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