Conventional HDS Processes

If higher hydrogen pressures could be used, the rates of desulfurization could be substantially increased. However, this is a limited option. As discussed in the beginning of this report, some refineries were able to purchase new high-pressure reactors during a time of low equipment and construction costs. However, new construction will not benefit from this luxury. Many of the presently installed reactors were designed for moderate pressures, less than 5 MPa. It would therefore be desirable to devise new processes around these pressures. 

Increasing temperature is another means to increase reaction rate. This is the lowest-cost process alternative to achieve higher rates as long as no 

As discussed in a later section, H2S is an inhibitor for the catalytic site responsible for direct sulfur extraction. Thus, if the H2S partial pressure could be lowered in the reactor, the desulfurization rate could be increased. The simplest means to achieve this goal is through increased hydrogen recycle rates or increasing the hydrogen/feed ratio. Such changes are expensive and can in some instances lower the overall thoughput of the feed. 

If none of these options are available, new equipment may be necessary. In the last section of this review, a number of novel processes schemes are discussed that have the potential for meeting the new standards while overcoming the limitations presented in this section. 

There are no real thermodynamic limits in the removal of sulfur from any organic sulfur compound by reaction with hydrogen (1, 2, 5). There are, however, limits on the overall rates of conversion that may be achieved by increasing the temperature of the reaction. A classic limitation in rates is the result of the inverse relationship between adsorption on a catalytic surface and temperature. This may be a problem with dialkyldibenzothio-phenes, which have steric limitations for adsorption. 

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In seeking new and improved ways for achieving the ultralow levels of sulfur in the fuels of the future, it is important to understand the nature of the sulfur compounds that are to be converted (especially PASCs), as described in Section III. It is equally important to understand how these transformations occur through interactions with catalytic surface species, the pathways involved during these transformations, and the associated kinetic and thermodynamic limitations. These considerations dictate the process conditions and reactor process configurations that must be used to promote such transformations. In this section, we describe the reactor configurations and process conditions being used today what is known about the catalyst compositions, structure, and chemistry and what is known about the chemistry and reaction pathways for conversion of PASCs in conventional HDS processes. 

Many of these studies utilized noble metals such as Ir, Os, Rh, Ru, or Re, whereas others used more conventional metals such as Mn, Fe, Mo, or Co. The particular metal on which the observations were made is not important at this point. What is important is that all of the important steps required for direct sulfur removal and hydrogenation of thiophene and more condensed derivatives have been shown to occur with soluble metal complexes. Thus, organometallic complex chemistry can be of great value in elucidating the mechanisms involved in conventional HDS processes and perhaps can point the way to improved catalyst formulations. 

The ultradeep desulfurization of the current infrastructure fuels has become a bottleneck in the production of H2 for fuel cell applications. It is urgent to develop a more efficient and environmentally friendly process and technology for the ultradeep desulfurization of the hydrocarbon fuels. Consequently, many approaches have been conducted in order to improve the conventional HDS process and to develop new alternative processes. These approaches include catalytic HDS with improved and new catalysts, reactor and/or process, adsorptive desulfurization,27 oxidative desulfurization (ODS), extractive desulfurization (EDS), and biodesulfurization (BDS) by using special bacteria and others. Some of these works were reviewed recently by Topsoe et al.,15 Whitehurst et al.,28 Kabe et al.,29 Cicero et al.,30 Babich and Moulijn,31 Dhar et al.,32 Song,33 Song and Ma,16,34 Bej et al.,35 Mochida and Choi,36 Hemandez-Maldonado and Yang,37,38 Hemandez-Maldonado et al.,39 Topsoe,40 Brunet et al.,41 Gupta et al.,42 and Ito and van Veen.43... 

Among these blendstocks, the alkyl dibenzothiophenes with alkyl groups at the four- or/and six-positions are typical representatives of the refractory sulfur compounds, which are difficult to remove using the conventional HDS process. Thus, to satisfy the environmental regulations, deep desulfurization and ultra-deep... 

To put these problems into perspective based on conventional approaches for HDS of diesel fuels, for reducing the sulfur level from current 500 ppmw to 15 ppmw (the regulation in 2006) by conventional HDS processing, the volume of catalyst bed will need to be increased by 3.2 times as that of the current HDS catalyst bed. This is consistent in general with the analysis on a t3 ical Co-Mo catalyst by Haldor Topsoe regarding the required increase in catalyst activity and bed temperature for further reduction of sulfur from 500... 

Based on the complementary reactivity of AAT compounds towards oxidation and hydrogenation mentioned above, it seems probable that the EDS processes, when used as post- or pre-treatment in connection with existing conventional HDS processes, will achieve the objectives of lowest combined capital and conversion costs for ULSD production. Preliminary estimates indicate that conversion cost will be below what is expected for high-severity hydrodesulfiirization. 

There are valid chemical reasons for the S5mergism between HDS and EDS. These surround the difficulties of treating alkyl-aryl-thiophene sulfur containing compounds in naphtha and gas-oil by high-severity hydrogenation, while the same compounds are readily oxidized under mild process conditions. It therefore makes sense to remove some of the sulfur via the conventional HDS processing, perhaps to reach 350—750 wppm S, and then to remove the rest via EDS. 

In the conventional HDS process, refiactory sulfur-contining compounds (SCCs) are deprived of the chance to take up the active sites to be hydrogenated because of the higher adsorptivity of nitrogen-containing compounds (NCCs). Therefore, if these NCCs are effectively removed from liquid fuels prior to the HDS process, the limitation of HDS process can be overcome (Fig. 1). 

Hydrodenitrogenation (HDN) processes in current industry are not separated from the other hydrotreatments, viz, HDS, HDO, and HDM. A clear description of the HDS processes for conventional petroleum distillation was... 

The catalytic conversion of thiophenic substrates to the corresponding thiols (hydrogenolysis) (Eq. 3) is a reaction of much relevance in the HDS process as the thiols can subsequently be desulfurized over conventional HDS catalysts with greater efficiency and under milder reaction conditions than those required to ac-... 

The content of sulfur in oil products must be controlled at very low level (mass fraction of about lOx 10 for environmental protection and the follow-up process (such as the reforming process). In petroleum industry, in order to meet this requirement, hydrodesulfurization (HDS) process is mainly adopted with Mo [Mo-Co(Ni)]/Al203 as catalyst. The catalyst must be sulfurized to obtain activity for HDS. The natme of active phase on HDS catalyst has been a hot topic of research. The methods from the complex apparatus, such as EXAFS, XPS, and FTIR to the conventional XRD, TPR, and TPS have been effectively adopted. 

Several new processes have been announced that, when used in connection with the more conventional hydrodesulfurization (HDS) processes, promise to facilitate the achievement of that goal at lower capital and operating costs than required for high-severity HDS. Economic S5mergism is expected in the application of these new processes in tandem with the existing processes for HDS. 

Deep desulfurization of diesel fuels is particularly challenging due to the difficulty of reduce aromatic sulfur compounds, particularly 4,6-dialkyldibenzothiophenes, using conventional hydrodesulfurization processes (HDS). The HDS process is normally only effective for removing organosulfur compounds of aliphatic and alicyclic types. The aromatic sulfur molecules including thiophenes, dibenzothiophenes (DBT), and their alkylated derivatives are very difficult to convert to H2S through HDS. 

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