Segregating sulfur compounds

Physical Separation of Sulfur Compounds. In order to make several types of determinations feasible it is frequently necessary to isolate or to concentrate the sulfur compounds. Various means such as distillation, extraction, and absorption have been employed to affect the separation because these operations are closely related to those processes which may be employed to segregate sulfur compounds on a commercial scale. The discussion of these topics will be undertaken in the section. Processes for Segregating Sulfur Compounds. 

Definite progress is being made in this direction despite the complexity of the problem. It appears that the major accomplishments of the future will be. based on the current research aimed at segregating and identifying the individual sulfur compounds in crude petroleum and in petroleum products. 

The mechanism of the catalyst activation by sulfur is not understood. The amount of sulfur compounds necessary to maintain or increase the catalyst activity depends in some cases on the stability of the heavy-metal sulfide component of the catalyst. Thus molybdenum sulfide seems to require a higher hydrogen sulfide concentration than tungsten sulfide. However, some catalysts that do not contain elements that can form sulfides under reaction conditions also showed an increased activity when sulfur compounds were added to the feed. Hydrogen sulfide in many cases decreases the catalyst sensitivity to nitrogen compounds and thus causes an activity increase. Sufficient data for pure compounds are not available to permit segregation of these effects. 

Sulfur Segregation. Another option is to concentrate sulfur compounds into various streams and selectively treat them. Refiners can undercut product however, such tactics will reduce yields. For diesel, undercutting will lower diesel yield and increase gas-oil products, an undesirable consequence. 

In considering the subject of the sulfur-containing chemicals obtainable from petroleum fractions, the questions that are encountered are what sulfur compounds are present in the various petroleum fractions how can these materials be detected or identified what are the chemical and physical properties and modes of preparation of each of these sulfur compounds or compound types what happens to these sulfur compounds as they are subjected to the various refining processes and lastly, what processes are available for segregating the sulfur compounds ... 

Extraction. Sulfur compounds may be segregated by solvent extraction, by removal of acidic sulfur constituents with alkaline reagents, and by acid extraction. 

Saito et at.130 studied the salts of TMTSF and the sulfur analogue tetra-methyltetrathiafulvalene, TMTTF, with a polycyano dianion. Although the conductivity of both compounds was low (crrt = 10-5 Scm-1 for TMTSF vs. 10-7 Scm-1 for TMTTF) the conductivity of the Se-donor salt was improved by two orders of magnitude. Optical absorption spectroscopy was also used to assess the materials. The electronic transition between radical cations within the segregated donor columns occurred at considerably lower energy (8800 cm-1) in the TMTSF salt than in the TMTTF (11500 cm-1). A concurrent improvement... 

In the case of multi-component alloys and compounds, the surface composition may also change in addition to surface relaxation and reconstruction. For instance, the first layer of (100) plane on the surface of a nickel-aluminiim alloy enriches itself with aliuninum whose atomic size is larger than nickel. Such an enrichment of some constituents on the soUd surface is called surface segregation [Van Hove, 1993]. It is abo known that surface active minor impurities of oxygen, phosphorus and sulfur in metallic iron segregate to the clean siirface of iron [Nii-Yoshihara,... 

Carbon-sulfur groups are extremely stable surface compounds which cannot be removed by thermal treatment up to 1470 K. Only by a reductive treatment with hydrogen it is possible to clean carbon from sulfur adsorbates. One source of sulfur is the fuel used for the generation of the carbon from which about 90% are covalently bonded to the carbon and 10% are segregated as adsorbate which can be removed by solvent extraction. The abundance of sulfur can amount to several wt%. Removal of the structural sulfur is possible by hydrogen reduction to H2S at about 1000 K. A collection of references on this subject is found in the literature [33]. In activated carbons sulfur can also be present in an oxidized form as sulfate or as C-S-O compounds. 

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