Hydrogen sulfide and organic sulfides are detectable at a concentration of a few parts per billion by the olfactory sense, so air pollution is always a concern. At low concentrations, total reduced sulfur (TRS) is more of a nuisance than a serious health hazard. They are formed during kraft pulping ( Fig. 15.5) by reaction of sulfides with methoxy groups of lignin via nucleophilic substitution reactions. Reduced sulfur compounds include hydrogen sulfide (H 2S), the mercaptans, including methanethiol (CH 3SH, alias methyl mercaptan or methyl sulfhydrate), methyl sulfide (CH 3SCH 3, alias dimethyl sulfide), dimethyl disulfide (CH 3SSCH 3), and other compounds. Possible reactions in the supercritical phase reaction system were proposed as follows with the reaction Gibbs free energy calculated in Kcal/g-mole at 400☌:
From Fig 2, it was deduced that the reaction system includes some series and parallel reactions with MeSH and DMS as intermediates that further react eventually to hydrogen sulfide. Since typical reaction times for desulfurization of coal with alcohol are in the l/2–2hr range, this explains the lack of significant amounts of H 2S being observed in the previous experiments with coal. Note that very long reaction times (> 4 hr) are required before significant formation of H 2S is observed.
The equilibrium values resulting from the thermodynamic analysis of the system are also given. The total concentrations of these four sulfur compounds in the reaction products and the overall sulfur balance at different reaction times are given in Table 1 the data are plotted in Fig 2.
Very little CH 3SH and H 2S were dissolved in the MeOH solution.
The sulfur compounds in the liquid phase were mainly CH 3-S-S-CH 3 and CH 3-S-CH 3. CH 3-S-S-CH 3, CH 3-S-CH 3, CH 3SH and H 2S were the four sulfur compounds detected in the gas phase. The reactions of the DMDS/MeOH/KOH system were carried out at 400☌.
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