HDS process

Fig. 26. Mixed-phase, tridde-bed distributor used for hydrodesulfurization in the Unicracking—HDS process (98).

Catalysts used in hydrotreatment (hydrodesulfurization, HDS) processes are the same as those developed in Germany for coal hydrogenation during World War II. The catalysts should be sulfur-resistant. The cobalt-molybdenum system supported on alumina was found to be an effective catalyst. 

This chapter follows the organization used in the past. A summary of the electronic properties leads into reports of electrocyclic chemistry. Recent reports of studies of HDS processes and catalysts are then summarized. Thiophene ring substitution reactions, ring-forming reactions, the formation of ring-annelated derivatives, and the use of thiophene molecules as intermediates are then reported. Applications of thiophene and its derivatives in polymers and in other small molecules of interest are highlighted. Finally, the few examples of selenophenes and tellurophenes reported in the past year are noted. 

Although neither the enzymes, nor the genes were identified or isolated by the time the patents were filed, their use was claimed as part of one of the patents [239], It should be noted that the type of sulfur compounds (aromatics) to be removed were predicted and a divisional patent was obtained [238], In the subsequent patent, [241] the type of sulfur compounds to be removed were mentioned, and additionally the pre- or post-BDS stage was discerned to be a mild-HDS process (MDS). 

The hydrogenolysis of thiophenes to thiols is a relevant reaction in the HDS process (Equation (16)). 

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

In a practical HDS process for gas oil, both aromatic species existing in the feed and various types of sulfur compounds compete for the active sites on the catalyst surface. Moreover, H2S and some other hydrocarbons produced in the early stages of the desulfurization appear to inhibit the HDS of the less reactive sulfur species. The reactivities of refractory sulfur compounds and the effects of inhibitors in gas oils need to be fully understood for the development of an improved economical desulfurization process. 

In describing catalytic activities and selectivities and the inhibition phenomenon, we will use a common format, where possible, which is based on a common reaction pathway scheme as outlined in Scheme 1. In contrast to the simple one- and two-ring sulfur species from which direct sulfur extrusion is rather facile, in the HDS of multiring aromatic sulfur compounds such as dibenzothiophene derivatives, the observed products are often produced via more than one reaction pathway. We will not discuss the pathways that are specific for thiophene and benzothiophene as this is well represented in the literature (7, 5, 8, 9) and, in any event, they are not pertinent to the reaction pathways involved in deep HDS processes whereby all of the highly reactive sulfur compounds have already been completely converted. 

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. 

Most of today s distillate HDS processes consist of fixed-bed, down-flow reactors configured in a manner similar to that shown in Fig. 8 (7). It should be noted that hydrogen is used in excess and is recirculated after scrubbing out the H2S byproduct. Care must be used in the scrubbing operation as it is necessary to maintain a low but optimum level of H2S in the recycle stream to maintain catalyst stability and activity. The consequence of this H2S requirement when hydrotreating PASCs to extinction is discussed in more detail in later sections, but at this point it should be mentioned that H2S is a strong inhibitor of HDS for PASCs. 

Having established reliable values for all of the important rate constants as a function of alkyl substitution on dibenzothiophenes, it is now possible to examine critically how these rate constants (and associated changes in product selectivity) are affected by other components of commercial gas oils and by the H2S that is produced during the HDS process. It is also possible to evaluate how these various rate constants are affected by changes in catalyst composition and by process conditions. Knowledge of the details of these effects can lead to novel catalyst modifications and process configurations that may be able to reach the new stricter standards of 0.05% S. These topics are discussed in later sections. However, for perspective, we will first summarize what is known about present-day catalyst compositions and catalytic mechanisms that bring about the transformations observed in 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 reactivity of an organic sulfur compound in an HDS process depends on several factors. The compound must first be adsorbed onto the catalyst... 

Obtaining adsorption constant data in complicated reaction systems, such as HDS processes, is difficult as can be seen from the preceding discussion. It is often more instructive to determine the relative adsorption behaviors for competing materials in binary mixtures. This has been done by many authors and this approach is discussed next. 

The least strongly sorbed reactant in HDS processes is dihydrogen. It could well be that the major reason for rate reduction by inhibitors is prevention of dihydrogen adsorption by competing molecules. This would include the sulfur-containing reactant as well. This would explain the phenomenon of self-inhibition in HDS reactions. 

More research in the area of inhibition of the reactions of alkyldibenzothi-ophenes is strongly recommended to aid in finding the means to overcome inhibition limitations and provide suggestions for new catalysts and/or processes that will be able to meet the new stricter low-sulfur specifications. Some novel concepts for improved HDS processes that address these specific problems are discussed in the next section. 

Inhibition of desulfurization reactions by H2S produced as a byproduct in the HDS process... 

Temperature limitations in HDS processes imposed by thermodynamic limitations on concentrations of intermediates... 

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