High-pressure hydrodesulfurization

The EPA s revised pollution guidelines for on-highway diesel fuels took effect on October 1, 1993, and additional Clean Air Act amendments are pending. As a result, the sulfur content of diesel fuel will have to be reduced from 1 to 2% down to 0.05% as compared with 0.3% conventionally attainable with high-pressure hydrodesulfurization. 

In the two-stage operation, the feed is hydrodesulfurized in the first reactor with partial hydrocracking. Reactor effluent goes to a high-pressure separator to separate the hydrogen-rich gas, which is recycled and mixed with the fresh feed. The liquid portion from the separator is fractionated, and the bottoms of the fractionator are sent to the second stage reactor. 

The studies of ammonia synthesis over Fe and Re and the hydrodesulfurization of thiophene over Mo, described above, illustrate the importance and success of our approach of studying catalysis over single crystal samples at high pressures. The use of surfaces having a variety of orientations allows the study of reactions that are surface structure sensitive 6Uid provides insight into the nature of the catalytic site. Here we have shown that the ammonia synthesis... 

One of the major challenges in the petroleum industry today is the removal of sulfur compounds, especially refractive ones such as 4,6-dimethyldibenzo-thiophene (DMDBT), from petroleum fractions such as diesel to concentrations <5-10 ppm from the current values of 50-500 ppm. The current technology is hydrodesulfurization catalyzed by cobalt-nickel-molybdenum sulfides at high pressures. Reducing sulfur concentratios in diesel fuels below 5-10 ppm... 

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)... 

Toluene hydrogenation (HYD), cyclohexane isomerization (ISOM) and thiophene hydrodesulfurization (HDS) tests have been performed in a high pressure fixed bed continuous flow "Catalest" unit. 

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. 

For materials selection, hydrocrackers are treated the same as hydrodesulfurizers, particularly in the first stage. From a materials standpoint, the demarcation between low-pressure units (hydrodesulfurizers) and high-pressure units (usually hydrocrackers) is 650 psia (4,480 kPa). 

The thiophene hydrodesulfurization and cyclohexene hydrogenation activity of the catalysts were measured in a high pressure reactor. The experimental conditions for the activity tests were a feedstock of thiophene (15000 ppm), cyclohexane (90%) and cyclohexene (10%), flow rate 0.353 ml/min, total pressure = 26 Kg/cm and LHSV=52 l/h. Additional experiences with toluene (90%) and cyclohexene (10%) were also carried out. The operative conditions for the hydrotreating test were selected according to the recent experience about the C0M06 [3]. The products were analysed by gas chromatography by means of a Varian Start 3400 gas chromatograph, with FID detector. 

The thiophene hydrodesulfurization and cyclohexene hydrogenation activity of the catalysts have been measured in a high pressure reactor, the operative conditions having been selected according to the recent experience about the CoMoe and NiMoe based systems because of the common structural and physical-chemical properties [3, 4]. Table 3 shows chemical data and conversion obtained for selected catalysts based on RhMoe. In addition, the data for CoMoe Anderson, CoMo commercial and Rh commercial 7-AI2O3 supported catalysts are included for comparative purposes. 

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). 

An opposite situation happens when the liquid phase is very dilute with B, while gas-phase reactant A is highly soluble (or at high pressures). A common example of such a situation is when slurry systems are employed for hydrodesulfurization of oil fi-actions, in which ppm levels of refractory sulfur compounds (such as dibenzothiophenes and alkyl diben-zothiophenes) may be present in fuel oils like diesel. In comparison, the gas phase is pure hydrogen, which at high pressures behaves like a fairly soluble gas in the hydrocarbon liquid. In such a case, the liquid-phase concentration of the gas-phase reactant is fairly constant throughout (and much higher than the concentration of the liquid-phase reactant B), including inside the catalyst pellets. The effective depletion rate of B (Eq. 6.4) reduces to... 

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. 

A spinning basket reactor has been developed for studying multiphase reactions at high pressure. Studies have been performed on the hydrodesulfurization of various petroleum fractions and a model sulfur compound. Hydrogen and liquid feed are continuously fed to the reactor and contacted with the catalyst which is held in a rotating annular basket. The... 

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