Sulfur removal Hydrodesulfurization

Natural gas contains both organic and inorganic sulfur compounds that must be removed to protect both the reforming and downstream methanol synthesis catalysts. Hydrodesulfurization across a cobalt or nickel molybdenum—zinc oxide fixed-bed sequence is the basis for an effective purification system. For high levels of sulfur, bulk removal in a Hquid absorption—stripping system followed by fixed-bed residual clean-up is more practical (see Sulfur REMOVAL AND RECOVERY). Chlorides and mercury may also be found in natural gas, particularly from offshore reservoirs. These poisons can be removed by activated alumina or carbon beds. 

Thiophenes continue to play a major role in commercial applications as well as basic research. In addition to its aromatic properties that make it a useful replacement for benzene in small molecule syntheses, thiophene is a key element in superconductors, photochemical switches and polymers. The presence of sulfur-containing components (especially thiophene and benzothiophene) in crude petroleum requires development of new catalysts to promote their removal (hydrodesulfurization, HDS) at refineries. Interspersed with these commercial applications, basic research on thiophene has continued to study its role in electrocyclic reactions, newer routes for its formation and substitution and new derivatives of therapeutic potential. New reports of selenophenes and tellurophenes continue to be modest in number. 

SULFUR REMOVAL FROM TERPENES BY HYDRODESULFURIZATION ON CARBON-SUPPORTED CATALYSTS... 

The hydrodesulfurization process can fall into either the destructive or nondestructive category. However for heavy feedstocks some hydrocracking is preferred, if not necessary, to remove the sulfur. Thus, hydrodesulfurization in this context falls into the hydrocracking or destructive hydrogenation category. The basic chemical concept of the process remains the same, that is, to convert the organic sulfur in the feedstock to hydrogen sulfide. 

In keeping with the known data, the sulfur in polynuclear aromatic systems is difficult to remove in the absence of hydrogen. The heterocyclic rings tend to remain intact and are incorporated into the coke. The presence of hydrogen facilitates the removal of heterocyclic sulfur. Indeed, there is also the possibility the hydrogen prior to cracking or hydrodesulfurization may render sulfur removal and hydrocarbon production even more favorable. 

The near completely random motion of the catalyst bed virtually ensures an isothermal operation, but the efficiency of the hydrodesulfurization reaction tends to suffer because of the back mixing of the product and feedstock. Hence, to effect sulfur removal at over 75% efficiency, it may be necessary to operate with two or more reactors in series. The need for two or more of these units to effectively desulfurize a feedstock may be cited as a disadvantage of the reactor, but the ability of the reactor to operate under isothermal conditions as well as the onstream catalyst addition-withdrawal system and the fact that the reactor size required for an expanded catalyst bed is often smaller than that required for a fixed bed can be cited in support of such a unit. 

The efficiency of the hydrodesulfurization process is measured by the degree of sulfur removal or, in other words, by the yields of sulfur-free products. However, there are several process variables (Table 5-8) that need special attention as any one of these variables can have a marked influence on the course and efficiency of the hydrodesulfurization process. 

In order to accomplish sulfur removal, use is still made of extraction and chemical treatment of various petroleum fractions as a means of removing certain sulfur types from petroleum products, but hydrodesulfurization is the only method generally applicable to the removal of all types of sulfur compounds. 

The objective of the present work is to evaluate the effect of a wide range of process or reaction variables— reaction temperature, hydrogen partial pressure, catalyst loading, and reaction time—on hydrodesulfurization and hydrogenation of filtered liquid product (coal-derived liquid) obtained from the coal dissolution stage in the presence of a commercial presulfided Co-Mo-Al catalyst. The selectivity for desulfurization over hydrogenation (Se) is used to rate the effectiveness of the above mentioned process variables. Se is defined as the fraction of sulfur removal per unit (g) of hydrogen consumed, that is,... 

The catalysts used in the steam reforming process are poisoned by trace components in the hydrocarbon feed - particularly sulfur, chlorine, and metal compounds. The best way to remove sulfur compounds is to convert the organic sulfur species to H2S over a hydrodesulfurization catalyst. The next step is sulfur removal with an absorbent. The same catalyst can usually convert any organochlo-ride species to give HC1 and also act as an absorbent for most problematic metal species. A second absorbent is used for chloride removal.70... 

A critical prerequisite for the use of synthesis gas or CO in most reactions catalyzed by transition metals is that sulfur compounds, such as H2S, thiols, or COS, derived from the sulfur content of crude oil or coal must first be removed (hydrodesulfurization, see Section 21-4). 

Figure 1 Axial profiles of sulfur removal and hydrogen sulfide concentration in the gas for hydrodesulfurization of oil following second-order kinetics in total sulfur.

Figure 1.12 Simplified hydrodesulfurization process diagram three separate fixed-bed catalytic reactors in series reactor 1 is used as a guard bed in which the metals are removed to not poison the HDS catalyst reactors 2 and 3 permit control over temperature and sulfur removal. Adapted from McCulloch [8].

Environmental problems have emphasized the importance of catalysts capable of removing sulfur, and hydrodesulfurization (HDS) has become a central issue in coal and petroleum refining.f " With the same [Mo2Fc2] catalyst on 7-AI2O3, Si02, Ti02, and MgO, activities for removal of sulfur from thiophene were comparable with those of commercial HDS catalysts while showing decreased H2 consumption (less butane formation). 

One method of sulfur removal from refinery streams is by hydrodesulfurization (HDS) in the refineries. This step also directly impacts the characteristics of low sulfur diesel fuels, such as density, aromatics content, cetane number, and cloud point. The magnitude of these changes will depend upon the type and setup of refinery HDS units. However, in the end some refractory compounds in fuel, e.g., 4,6-diraethyl dibenzothiophene, are very resistant to desulfurization, owing to the inaccessibility of the organically bound sulfur atom. Lower pressure HDS units which can work satisfactorily at 350 rag/kg sulfur levels, may have difficulty achieving reduction to 50 or 10 rag/kg sulfur level. 

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]... 

Hydrodesulfurization. Several model compound studies have been undertaken to understand the mechanism of sulfur removal during coal liquefaction. Hydrodesulfurization of thiophene has been reported by Amberg and his co-workers (86-91), Schuit and Gates (92), Lipsch and Schuit (67) and Shah and Cronauer (55). 

Figure 2 shows some experimental results about the effect of both reaction temperatures and LHSV on SRGO hydrodesulfurization using upflow and downflow systems. It can be clearly seen the differences in sulfur removal between these two systems, specially at low temperature and high space velocity. 

Another type of two stage HDS is to remove nitrogen species prior to HDS with silica-gel, silica-alumina or active carbon. The present authors suggest the high efficiency of active carbon for nitrogen species removal. Some refractory sulfur species are also removed by the active carbon, which significantly helps deep hydrodesulfurization. Post removal of sulfur species after hydrodesulfurization can also be used to lower the total sulfiir content below 10 ppm. However, the capacity for sulfur removal is rather limited, compared to the removal of nitrogen species. Hence the application of this approach is restricted to preparation of the ultra clean hydrodesulfurization for fuel cells. 

Hydrotreating refers to a relatively mild operation whose primary purpose is to saturate olefins and/or reduce sulfur and nitrogen content without changing too much the boiling range of the feed. Hydrotreating is applied to a wide range of feedstocks from naphtha to reduced crude oil. If the process is used specifically for sulfur removal, it is usually called hydrodesulfurization or HDS. 

Two approaches for desulfurization, namely, hydrodesulfurization (HDS) and sulfur removal by adsorption, are used with the former being the most mature technology. HDS is a catalytic hydrogenation process that removes sulfur from fossil fuels. In this process, tiie organosulfur compound such as ethanethiol (C2H5SH), a sulfur compound present in some petroleum products, is converted to H2S and sulfur-free organic compounds ethane (C2Hg) by reaction with H2 in the presence of a catalyst as shown below ... 

Each catalyst is designed to be best suited to one type of feed or one type of treating goal. When hydrotreating is done for sulfur removal, the process is called hydrodesulfurization, and the catalyst generally is cobalt and molybdenum oxides on alumina. A catalyst of nickel-molybdenum compounds on alumina can be used for denitrogenation and cracked-stock saturation. 

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