Hydrodesulfurization

We have already discussed some aspects of desulfurization under hydrofinishing. In that section (Chapter 6), mild hydrotreatment of solvent refined stocks was found by Imperial Oil and others to lead to the relatively easy desulfurization of benzothiophenes to alkyl benzenes and dibenzothiophenes to biphenyls and therefore the enhancement of the monoaromatic content of the finished base stock. 

In the production of base stocks by hydrocracking and feed preparation for dewaxing by current hydroisomerization technology, very low sulfur levels are 

As might be expected, ease of desulfurization depends on the type of carbon-sulfur bonds involved. Alkyl sulfides and polysulfides possess weak carbon-sulfur bonds and react rapidly and completely under hydroprocessing conditions, as their uses in catalyst sulfiding agents testify. Sulfur incorporated in thiophene systems have both stronger C(sp2)-S bonds than alkyl C(sp3)-S bonds and the sulfur as well is in the aromatic five-membered thiophene ring system. Table 8.12 

Reactivities of Thiophenic Sulfur in a Range of Ring Systems 

Relative Pseudo First-Order Rate Constant, L/g 

During operation, sour feed is mixed with hydrogen and heated in a fired furnace. The heated mixture is sent to a reactor where the hydrogen combines with the sulfur to form hydrogen sulfide. When the temperature is lowered slightly, the sweet crude condenses, leaving the hydrogen sulfide in a vapor state. This vapor-and-liquid mixture is sent to a separator where the low-sulfur sweet 

It is not usually necessary, however, to presulfide cobalt molybdate catalysts before treating natural gas. Natural gas contains relatively small amounts of simple sulfur compounds that are hydrogenolyzed at a low temperature. Catalysts are sulfided during operation during operation and the eventual sulfur content of the catalyst depends on the sulfur content of the natural gas. Catalyst will normally operate for several years with no loss of activity. The sulfur content of typical feeds is shown in Table 9.4. 

TABLE 9.4. Sulfur Content of Typical Steam Reformer Feedstocks. 

It is not economic to regenerate discharged catalysts for further use following a typical life of several years. 

During HDS, sulfur is extracted from hydrocarbons and released as H2S. Reactivity of S-components can vary greatly depending on the structure of the molecule. In general, HDS reactivity increases according to the hydrocarbon type paraffins naphthenes aromatics. Mercaptans and sulfides are the most reactive species, followed by naphthenic and six-membered aromatic structures 

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H2S is found with the reservoir gas and dissolved in the crude (< 50 ppm by weight), but it is formed during refining operations such as catalytic cracking, hydrodesulfurization, and thermal cracking or by thermal decomposition of sulfur[Pg.322]

Figure 10.9 shows advantages of a hydrodesulfurization unit upstream of an FCC or RCC unit. 

Simple conventional refining is based essentially on atmospheric distillation. The residue from the distillation constitutes heavy fuel, the quantity and qualities of which are mainly determined by the crude feedstock available without many ways to improve it. Manufacture of products like asphalt and lubricant bases requires supplementary operations, in particular separation operations and is possible only with a relatively narrow selection of crudes (crudes for lube oils, crudes for asphalts). The distillates are not normally directly usable processing must be done to improve them, either mild treatment such as hydrodesulfurization of middle distillates at low pressure, or deep treatment usually with partial conversion such as catalytic reforming. The conventional refinery thereby has rather limited flexibility and makes products the quality of which is closely linked to the nature of the crude oil used. 

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). 

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. 

Interestingly, the Mo2S " 4 (Fig- 3f) core stmcture can be viewed as occupying six of the eight vertices of a distorted cube. Reaction of the dinuclear complexes having the Mo2S " 4 core with appropriate metal ions leads to the plaimed assembly of M2M02S4 thiocubane stmctures (19,20). When M = Co (Fig. 3h) the compounds are potential precursors for hydrodesulfurization catalysts (15). 

The composition of a reforming catalyst is dictated by the composition of the feedstock and the desired reformate. The catalysts used are principally platinum or platinum—rhenium on an alumina base. The purpose of platinum on the catalyst is to promote dehydrogenation and hydrogenation reactions. Nonplatinum catalysts are used in regenerative processes for feedstocks containing sulfur, although pretreatment (hydrodesulfurization) may permit platinum catalysts to be employed. 

Sulfur, another inorganic petrochemical, is obtained by the oxidation of hydrogen sulfide 2H2S + O2 — 2H2 0 + 2S. Hydrogen sulfide is a constituent of natural gas and also of the majority of refinery gas streams, especially those off-gases from hydrodesulfurization processes. A majority of the sulfur is converted to sulfuric acid for the manufacture of fertilizers and other chemicals. Other uses for sulfur include the production of carbon disulfide, refined sulfur, and pulp and paper industry chemicals. 

Fig. 14. ICI—LCA process flow sheet PAC, purified air compressor HDS, hydrodesulfurization IP, iatermediate pressure LP, Hquefied petroleum BFW,...

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

Naphthalene sodium prepared in dimethyl ether or another appropriate solvent, or metallic sodium dissolved in Hquid ammonia or dimethyl sulfoxide, is used to treat polyfluorocarbon and other resins to promote adhesion (138—140). Sodium, usually in dispersed form, is used to desulfurize a variety of hydrocarbon stocks (141). The process is most useful for removal of small amounts of sulfur remaining after hydrodesulfurization. 

Reduction and Hydrodesulfurization. Reduction of thiophene to 2,3- and 2,5-dihydrothiophene and ultimately tetrahydrothiophene can be achieved by treatment with sodium metal—alcohol or ammonia. Hydrogen with Pd, Co, Mo, and Rh catalysts also reduces thiophene to tetrahydrothiophene [110-01-0] a malodorous material used as a gas odorant. 

Rigorous hydrogenating conditions, particularly with Raney Nickel, remove the sulfur atom of thiophenes. With vapor-phase catalysis, hydrodesulfurization is the technique used to remove sulfur materials from cmde oil. Chemically hydrodesulfurization can be a valuable route to alkanes otherwise difficult to access. 

The primary determinant of catalyst surface area is the support surface area, except in the case of certain catalysts where extremely fine dispersions of active material are obtained. As a rule, catalysts intended for catalytic conversions utilizing hydrogen, eg, hydrogenation, hydrodesulfurization, and hydrodenitrogenation, can utilize high surface area supports, whereas those intended for selective oxidation, eg, olefin epoxidation, require low surface area supports to avoid troublesome side reactions. 

The breadth of reactions catalyzed by cobalt compounds is large. Some types of reactions are hydrotreating petroleum (qv), hydrogenation, dehydrogenation, hydrodenitrification, hydrodesulfurization, selective oxidations, ammonoxidations, complete oxidations, hydroformylations, polymerizations, selective decompositions, ammonia (qv) synthesis, and fluorocarbon synthesis (see Fluorine compounds, organic). 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

Hydrodesulfurization. A commercial catalyst contains about 4 percent CoO and 12 percent M0O3 on y-alumina and is presulfided before use. Molybdena is a weak catalyst by itself and the cobalt has no catalytic action by itself. 

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