High-pressure hydrodesulfurization process

Under the usual commercial hydrodesulfurization conditions (elevated temperatures and pressures, high hydrogen-to-feedstock ratios, and the presence of a catalyst), the various reactions that result in the removal of sulfur from the organic feedstock (Table 4-3) occur. Thus, thiols as well as open chain and cyclic sulfides are converted to saturated and/or aromatic compounds depending, of course, on the nature of the particular sulfur compound involved. Benzothio-phenes are converted to alkyl aromatics, while dibenzothiophenes are usually converted to biphenyl derivatives. In fact, the major reactions that occur as part of the hydrodesulfurization process involve carbon-sulfur bond rupture and saturation of the reactive fragments (as well as saturation of olefins). 

The wide ranges of temperature and pressure employed for the hydrodesulfurization process virtually dictate that many other reactions will proceed concurrently with the desulfurization reaction. Thus, the isomerization of paraffins and naphthenes may occur and hydrocracking will increase as the temperature and pressure increase. Furthermore, at the higher temperatures (but low pressures) naphthenes may dehydrogenate to aromatics and paraffins dehydrocyclize to naphthenes, while at lower temperature (high pressures) some of the aromatics may be hydrogenated. 

Reactor designs for hydrodesulfurization of various feedstocks vary in the way in which the feedstock is introduced into the reactor and in the arrangement, as well as the physical nature, of the catalyst bed. The conditions under which the hydrodesulfurization process operates (i.e., high temperatures and high pressures)... 

Review of Process Alternatives, Superior Graphite began in 1968 to investigate possible alternatives for desulfurization of petroleum cokes. The methods considered included various chemical treatments and direct thermal purification processes. The chemical treatment methods included hydrodesulfurization and reactions with various alkali metal compounds. Fine grinding of the coke appeared to be required and reaction conditions generally involved high pressure. 

The hydrodesulfurization process operates using high hydrogen pressure, typically 1500-500 psi, and temperatures range from 290°C to 370°C (550-700°F). Several process configurations are used, depending on the feed and the design criteria. ... 

High-pressure/high-temperature phase 67 Hydrodesulfurization process 82 Hydrogen sulfide 157, 164 -, oxidation 154, 162-163 -, removal 181... 

The majorify of sulfur compounds (thioles and sulfides) have been successfully removed from liquid fuel using a hydrodesulfurization process where high temperature and high pressure are required [7-9, 159]. As mentioned in section 2.5 some sulfur species are very resistant to hydrodesulfurization and those include thiophenic compounds, especially dibenzothiophene and 4,6 dimethyldibenzothiophene [148]. Various methods based on extraction and adsorption have been proposed to remove these compounds [7, 8, 13,145, 147-149, 151-158]. In the extraction route, sulfiir species are first oxidized and then extracted using organic solvents as, for instance acetonitrille [13, 149]. On the other hand, an adsorption process is usually tailored to enhance either adsorption forces, selectivity, or to impose a chemical reaction. So fer the enhancement in the removal of thiophenic compounds was reported on materials where n-complexation can occur as on Cu-Y zeolites [151, 153], or on alumina with highly dispersed sodium [147]. In the latter case, mono- and disodium thiophene metallates are formed. Another desulfurization methods use formation and subsequent precipitation of S-alkylsulfonium salts [148]. 

Hydrodesulfurization [HDS, Eq. (1)] is the process by which sulfur is removed from fossil materials upon treatment with a high pressure of H2 (3.5-17 MPa) at high temperature (300-425 °C) in the presence of heterogenexius catalysts, generally transition metal sulfides (Mo, W, Co, Ni) supported on alumina [1]. About 90% of the sulfur in fossil materials is contained in thiophenic molecules, which comprise an enormous variety of substituted thiophenes, and benzo[b]thiophenes, di-benzo[b,d]thiophenes as well as other fused-ring thiophenes, most of which are generally less easily desulfurized over heterogeneous catalysts than any other sulfur compound in petroleum feedstocks (e.g., thiols, sulfide, and disulfides). 

Much of the distillate (furnace oil, diesel oil, kerosene, etc.) produced in a refinery must be hydrodesulfurized. Part of this process involves a low-temperature (about 100°F-140°F), high-pressure (several hundred psig) separation of hydrogen-rich gas and the desulfurized liquid. 

Hydrodesulfurization (HDS) process operates at a pressure highly exceeding the pressure of natural gas available in the existing infrastructure. 

The optimum use of hydrodesulfurization catalysts requires a relatively high hydrogen partial pressure, and it is therefore necessary to introduce with the feedstock quantities of hydrogen that are considerably greater than required on the basis of stoichiometric chemical consumption. In all cases, process economics dictate that unused hydrogen should be recycled after it has been partially (or completely) freed from hydrogen sulfide that was produced in the previous pass. 

High amounts of asphaltenes and resins require high hydrogen partial pressures and may actually limit the maximum level of hydrodesulfurization, or final traces of sulfur in the residuum may only be eliminated under extremely severe reaction conditions where hydrocracking is the predominant reaction in the process. High asphaltene and resin contents are also responsible for high viscosity (Figure 6-7) which may increase the resistance to mass transfer of the reactants... 

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