Hydrodesulfurization of Gasoline

Recent regulation forces deeper hydrodesulfurization. While HDS of FCC gasoline is rather difficult without hydrogenation of olefin and aromatic components, which are major sources for high octane number, sulfur species in gasoline are reactive forms of thiols, thiophenes and benzothiophenes, which are readily desulfurized. Selective HDS without olefin hydrogenation is being extensively explored at present.  

Such a selective hydrodesulfurization must clarify the active sites of CoMo and NiMo sulfides supported on alumina for hydrodesulfurization and hydrogenation. CoMo is certainly more selective for hydrodesulfurization with limited hydrogenation activity than NiMo. Hence, cobalt is applied for the present purpose. Coordinatively unsaturated Co sulfide on M0S2 is often postulated as the active site for both reactions (see Fig.7). The metal with unsaturated valence is proposed as the hydrogenative site in cooperation with a Mo-S-H group while the metal with the unsaturated valences must be the hydrodesulfiiiization site. H2S concentration in HDS can be reduced to enhance the hydrodesulfiiiization selectivity. 

Several patents are issued for selective HDS of FCC gasoline by poisoning the hydrogenation site more than the hydrodesulfurization one over CoMoS/alumina. Amines, alkali metal ions, and carbon deposition have been proposed in the patents to increase the selectivity for hydrogenation. 

The strong acidity of zeolites is sometimes postulated to desulfurize through hydrogen transfer, however, its contribution must not be exaggerated. Thus, sulfur balances in FCC process are being carefiilly described to discover the origins of S-compounds in FCC products.  

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Trickle-bed reactors are widely used in hydrotreating processes, i.e., hydrodesulfurization of gasoline and diesel fuel, in petroleum refining, chemical, petrochemical, and biochemical processes. The knowledge of hydrodynamic parameters is vital in the design of a TBR because the conversion of reactants, reaction yield, and selectivity depend not only on reaction kinetics, operating pressure, and temperature, but also on the hydrodynamics of the reactor. Special care is also required to prevent flow maldistribution, which can cause incomplete catalyst wetting in some parts... 

Hydrodesulfuration of gasoline and diesel fuels Pt, Pd, and Pt-Pd mesoporous ZSM-5 zeolite (total metal content of 0.5 wt%) Higher sulfur removal efficiency than metal/microporous zeolite or metal/y-Al203 1175]... 

Hydrodesulfuration of gasoline and diesel fuels Pd/mesoporous Beta Better catalytic performance than Pd/Al-MCM-41 (51% vs 35%) because of the higher acidity of the zeolite and than Pd/conventional Beta because of its larger mesopore volume (176,177]... 

Saturation of olefins other than reactive olefins usually is not desired. The added hydrogen is often expensive or useful elsewhere, and it does not provide any real improvement in product quality. Acmally, product quality may be reduced in the case of gasolines. Research octane number losses may be correlated with increasing olefin saturation. So in many cases, hydrodesulfurization conditions are selected with an eye toward minimizing olefin saturation over and above that needed for product quality improvement. There is one exception saturation of certain olefins shows substantial improvements in Motor octane number. This is true for iso- and n-pentenes and to a lesser extent for higher boiling isoolefins. The higher n-olefins show octane losses upon saturation. 

Hatanaka, S. Yamada, M., and Sadakane, O., Hydrodesulfurization of Catalytic Cracked Gasoline. 2. The Difference Between HDS Active Site and Olefin Hydrogenation Active Site. Ind. Eng. Chem. Res, 1997. 36 p. 1510. 

Hatanaka, S., Yamada, M., and Sadakane, O. Hydrodesulfurization of catalytic cracked gasoline. 3. Selective catalytic cracked gasoline hydrodesulfurization on the Co-Mo/gamma-Al203 catalyst modified by coking pretreatment. Industrial Engineering Chemistry Research, 1998, 37, 1748. 

Hatanaka, S., Sadakane, O., and Okazaki, H. Hydrodesulfurization of catalytic cracked gasoline—Selective catalytic cracked gasoline hydrodesulfurization using C0-M0/7-AI2O3 catalyst modified by pyridine. Journal of the Japan Petroleum Institute, 2001, 44, 36. 

The role of catalysis in the petroleum industry has been equally revolutionary. Meta I-supported systems (e.g. of Topsoe and Shell) for catalytic reforming, hydrodesulfurization and hydrodenitrification, alkylation catalysts and shape selective systems (e.g. zeolites and pillared clays) for catalytic cracking (FCC) and production of gasoline from methanol (Mobil MTG) all represent significant technical and commercial achievements. 

As described above, catal5dic deep hydrodesulfurization of FCC gasoline must make ultra low sulfur gasoline while maintaining octane number. Hence, novel catal5dic and non-catal5dic processes have been proposed and evaluated. 

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). 

Thiophene is the typical model compound, which has been extensively studied for typifying gasoline HDS. Although, some results are not completely understood, a reaction network has been proposed by Van Parijs and Froment, to explain their own results, which were obtained in a comprehensive set of conditions. In this network, thiophene is hydrodesulfurized to give a mixture of -butenes, followed by further hydrogenation to butane. On the considered reaction conditions, tetrahydrothiophene and butadiene were not observed [43], The consistency between the functional forms of the rate equations for the HDS of benzothiophene and thiophene, based on the dissociative adsorption of hydrogen, were identical [43,44], suggesting equivalent mechanisms. 

The DS7 strain is characterized for its activity on the representative sulfur groups of the molecules present in fuel producing cuts, both gasoline and diesel. Examples given include straight-run gas oils, gas oils from hydrodesulfurization and the main streams coming from the atmospheric distillation of petroleum (cuts 70-160°C, 160-230°C and 230-350°C.),... 

Liberated gasses are drawn off at the top of the tower with the naptha. The gas is recovered to manufacture refrigerated liquefied petroleum gas (LPG). The naptha is condensed at a temperature of about 52 °C (125 °F). Part of the condensed naptha is normally returned to the top of the tower. The naptha product stream is split into light naptha for gasoline blending and heavy naptha for further reforming. Inside the tower, kerosene is withdrawn at a temperature of about 149 °C (300 °F). Diesel is withdrawn at a temperature of 260 °C (500 °F). These middle distillates are usually brought up to specification with respect to sulfur content with hydrodesulfurization. The heavy oil... 

Optimum catalysts in use today for hydrodesulfurization typically consist of Mo or W metals promoted with Co and supported on weakly acidic alumina supports. Increasingly stringent regulatory requirements have pushed sulfur specifications for transportation fuels to les s than 10 ppm for diesel fuels and 30 ppm for gasoline... 

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