Refinery desulfurization

In oil and gas refinery appHcations, titanium is used as protection in environments of H2S, SO2, CO2, NH, caustic solutions, steam, and cooling water. It is used in heat-exchanger condensers for the fractional condensation of cmde hydrocarbons, NH, propane, and desulfurization products using seawater or brackish water for cooling. 

All refining operations may be classed as either conversion processes or separation processes. In the former, the feed undergoes a chemical reaction such as cracking, polymerization, or desulfurization. Separation processes take advantage of differences in physical properties to split the feed into two or more different products. Distillation, the most common of all refinery separation processes, uses differences in boiling points to separate hydrocarbon mixtures. 

Sulfur in cmde oil is mainly present in organic compounds such as mercaptans (R-SH), sulfides (R-S-R ) and disulfides (R-S-S-R ), which are all relatively easy to desulfurize, and thiophene and its derivatives (Fig. 9.2). The latter require more severe conditions for desulfurization, particularly the substituted dibenzothiophenes, such as that shown in Fig. 9.2. Sulfur cannot be tolerated because it produces sulfuric add upon combustion, and it also poisons reforming catalysts in the refinery and automotive exhaust converters (particularly those for diesel-fueled cars). Moreover, sulfur compounds in fuels cause corrosion and have an unpleasant smell. 

Can you think of reasons why substituted dibenzothiophenes are more difficult to desulfurize than thiophene or simple thiols (see Fig. 9.2) Depending on the choice of catalyst, hydrodesulfurization can be accompanied by hydrogenation to various extents. In which of the product streams in the refinery would you choose hydrogenative HDS and in which would you not ... 

As one more common example of liquid fuels present reference may be drawn to liquified petroleum gas (LPG) or bottled gas or refinery gas. This fuel is obtained as a by-product during the cracking of heavy oils or from natural gas. It is dehydrated, desulfurized and traces of odours organic sulfides (mercaptans) are added in order to identify whether a gas leak has occurred. Supply of LPG is carried out under pressure in containers under different trade names. It consists of hydrocarbons of great volatility such that they can occur in the gaseous state under atmospheric pressure, but are readily liquifiable under high pressures. The principal constituents of LPG are n-butane, iso-butane, butylene and propane,... 

Baco, F., Debuisschert, Q. Marchal, N., et al., Prime G+ process, desulfurization of FCC gasoline with minimized octane loss, in Fifth International Conference on Refinery Processing, AICHE 2002 Spring National Meeting. 2002. New Orleans, LA, March 11-14. 180-188. 

In this section, we will begin by discussing overall process designs and process alternatives. Most of the designs come from processes patented by EBC, although other players have contributed recently as well. The alterations to processes come from variations in the raw material to be desulfurized, (diesel vs. crude oil), or from point of application perspective (before or after HDS, in oil field vs. in refinery, etc.) or from changes to reaction schemes (complete desulfurization vs. stopping at an intermediate... 

Shell s microbiological desulfurization process is carried out by mixing coal with an aqueous biocatalyst solution [158], The coal considered in this invention concerns bituminous coal containing inorganic sulfur (pyritic).This process seems to be applicable to refinery pet-coke, which contains sulfur in the form of inorganic sulfides. Nowadays, when coke has become one of the major products of heavy oil and bitumens refining, such desulfurization processes might have potential uses. 

Litol Also called Houdry-Litol. A process for making benzene by dealkylating other aromatic hydrocarbons. It is a complex process which achieves desulfurization, removal of paraffins and naphthenes, and saturation of unsaturated compounds, in addition to dealkylation. The catalyst contains cobalt and molybdenum. Developed by the Houdiy Process and Chemical Company and Bethlehem Steel Corporation. First installed by the Bethlehem Steel Corporation in 1964. Subsequently used at British Steel s benzole refinery, Teesside, England. 

Assuming that demand for petroleum continues to increase at a rate of 1.2% per annum to 2010,37 and that all gasoline and diesel produced by U.S. refineries will have a sulfur content of less than 30 ppm, desulfurization of gasoline and diesel to these low levels will require extensive hydrotreating of both catalytic cracker feed and product of distillate. 

Many processes in a refinery use steam as a stripping medium in distillation and as a diluent to reduce the hydrocarbon partial pressure in catalytic or thermal cracking [37]. The steam is eventually condensed as a liquid effluent commonly referred to as sour or foul water. The two most prevalent pollutants found in sour water are H2S and NH3 resulting from the destmction of organic sulfur and nitrogen compounds during desulfurization, denitrification, and hydrotreating. Phenols and cyanides also may be present in sour water. 

E6er, Wasserscheid, R, Jess, A., Deep desulfurization of oil refinery streams by extraction with ionic liquids, Green Chem., 6,316-322,2004. 

Energy demand, the implementation of sulfur oxide pollution controls, and the future commercialization of coal gasification and liquefaction have increased the potential for the development of considerable supplies of sulfur and sulfuric acid as a result of abatement, desulfurization and conversion processes. Lesser potential sources include shale oil, domestic tar sands and heavy oil, and unconventional sources of natural gas. Current supply sources of saleable sulfur values include refineries, sour natural gas processing and smelting operations. To this, Frasch sulfur production must be added. 

In some instances, where the liquid stream is a small by-product it may not be desulfurized onsite. Rather, that material might be sold as feedstock to a petroleum refinery. 

The recovered sulfur industry exists primarily as a result of the necessity of removing sulfur values from hydrocarbon fuels before combustion so that sulfur emissions to atmosphere are reduced. In the case of sour gas, the principal source of recovered sulfur, the product that results from recovery of the sulfur is clean-burning, non-polluting methane. In the case of refineries handling high sulfur crude the product is low sulfur gasoline and oils. Thus every ton of sulfur recovered is a ton that is not added to the atmosphere. The recovery process itself however, is also the subject of optimization and recent developments in recovery efficiency have further ensured that the environmental impact in the immediate vicinity of these desulfurization facilities will be minimized. 

In 1979 sulfur obtained as a by-product from petroleum refining accounted for 19.7 percent of total sulfur produced in the U.S. The requirement to desulfurize residual fuels or alternatively to refine them to finished transportation fuels will result in a substantial increase in sulfur produced at refineries even if medium sweet crudes continue to be the primary refinery feedstock. However, most experts predict that crudes will become sourer in the future. The contribution from natural gas is an additional uncertainty. Conventional wisdom predicts that natural gas demand will maintain current levels or possibly decline over the next 20 years. The combination of these factors may increase conventional by-product sulfur from petroleum and natural gas by a factor of three or more by the year 2000. This would bring its sulfur contribution up to approximately 12 million tons by 2000, the same as that predicted by the MITRE estimate for synthetic fuels sulfur production. Thus, a possible total contribution of 60 percent of projected sulfur demand could be met by the combination of these by-product sources of sulfur. 

On the supply side, sulfur production is now controlled more by the demand for energy through the desulfurization of fuels than by the demand for sulfur per se, and this tendency is increasing. In 1965 involuntary byproduct recovered sulfur amounted to less than 20% of total elemental sulfur production in the United States and Canada, but by 1980 over 60% of all elemental sulfur resulted from refinery and natural gas processing operations. Many future hydrocarbon energy sources (coal, deep gas, heavy oil, shale, etc.) contain considerably more sulfur compared with conventional hydrocarbon fuels, and thus their exploitation will add to the ever increasing supply of by-product sulfur. 

To meet the demand for low-sulfur gas oil, guidelines for the design of new catalysts for such extensive desulfurization while maintaining product quality and minimizing the increase in costs to refineries are urgently needed. 

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. 

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