Desulfurization distillation

Catalytically desulfurized distillates or kerosene from West Texas raw stocks have smoke points that are a direct function of per cent hydrogen (smoke point == 25.5H — 317), and severe treatment results in a smoke point of 48. 

Hydrofining usually involves only minor molecular changes of the feed with hydrogen consumption in the range of about 100 to 1,000 cu.ft./bbl. Typical applications include desulfurization of a wide range of feeds (naphtha, light and heavy distillates, and certain residua) and occasional pretreatment of cat cracker feeds. 

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. 

Although desulfurization is a process, which has been in use in the oil industry for many years, renewed research has recently been started, aimed at improving the efficiency of the process. Envii onmental pressure and legislation to further reduce Sulfur levels in the various fuels has forced process development to place an increased emphasis on hydrodesulfurization (HDS). For a clear comprehension of the process kinetics involved in HDS, a detailed analyses of all the organosulfur compounds clarifying the desulfurization chemistry is a prerequisite. The reactivities of the Sulfur-containing structures present in middle distillates decrease sharply in the sequence thiols sulfides thiophenes benzothiophenes dibenzothio-phenes (32). However, in addition, within the various families the reactivities of the Substituted species are different. 

Products from coking processes vary considerably with feed type and process conditions. These products are hydrocarbon gases, cracked naphtha, middle distillates, and coke. The gas and liquid products are characterized by a high percentage of unsaturation. Hydrotreatment is usually required to saturate olefinic compounds and to desulfurize products from coking units. 

Desulfurization of other diesel feedstocks from Total Raffinage was also reported by EBC. In these studies, different engineered biocatalysts were used. Two different middle distillate fractions, one containing 1850 ppm sulfur and other containing 650 ppm sulfur, were tested. R. erythropolis sp. RA-18 was used in one experiment and was reported to desulfurize the diesel from 1850 to < 1200ppm sulfur within 24 hours. On the other hand, it removed sulfur from a middle distillate with 650ppm sulfur to below 200 ppm sulfur [222], Various Pseudomonas strains were also tested in this study and reported to remove less amounts of sulfur. A favorable characteristic of the Pseudomonas strains is their inability to form stable emulsions, which can be useful trait for product recovery. 

Folsom, B. R. Schieche, D. R. DiGrazia, P. M., et al., Microbial Desulfurization of Alkylated Dibenzothiophenes From a Hydrodesulfurized Middle Distillate by Rhodococcus Erythropo-lis 1-19. Applied and Environmental Microbiology, 1999. 65(11) pp. 4967-4972. 

Lee, M. K. Prince, R. C. Pickering, I. J., et al., Microbial Desulfurization of Crude Oil Distillate Fractions Analysis of the Extent of Sulfur Removal and the Effect on Remaining Sulfur. Abstracts of Papers of the American Chemical Society, 1997. 213 p. 11-FUEL. 

The overall rate of desulfurization for enabling commercialization has to be greater than 1.2mmol/g dcw/h [1,12,13], The rates of desulfurization of model compounds such as DBT is usually higher than that achievable for real feedstocks such as diesel and other middle distillates. Thus, it is estimated that a more than 100-fold improvement in rates for removal of total sulfur (not just DBT) over wild-type organisms is necessary to achieve rates suitable for commercialization. 

DODD A process for the deep desulfurization of middle petroleum distillates. Introduced by Exxon in 1989. 

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. 

Irradiation of matrix-isolated imidazole-2-carboxylic acid gave the 2,3-dihydro-imidazol-2-ylidene-C02 complex (31) characterized by IR spectroscopy and calculated to lie 15.9 kcal mol above the starting material. A series of non-aromatic nucleophilic carbenes (32) were prepared by desulfurization of the corresponding thiones by molten potassium in boiling THF. The most hindered of the series (32 R = Bu) is stable indefinitely under exclusion of air and water and can be distilled without decomposition. The less hindered carbenes slowly dimerize to the corresponding alkenes. Stable aminoxy- and aminothiocarbenes (33 X = O, S) were prepared by deprotonation of iminium salts with lithium amide bases. The carbene carbon resonance appears at 260-297 ppm in the NMR spectrum and an X-ray structure determination of an aminooxycarbene indicated that electron donation from the nitrogen is more important than that from oxygen. These carbenes do not dimerize. 

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. 

Alternative desulfurizations can be achieved using tributylstannane or hydrazine. Reaction with the former reagent is carried out by heating the mercaptole for 1.5 hours at 80° with 3-4 equivalents of tributylstannane and azobis(isobutyronitrile) as a catalyst and distilling the product and the byproduct, bis(tributyltin) sulfide, in vacuo. Yields are 74-95% [798],... 

---