Supercritical Water Desulfurization of Organic Sulfides Is Consistent with Free-Radical Kinetics

Abstract

Supercritical water desulfurization (SCWDS) has potential as a technique for removing sulfur from feedstocks such as heavy oil and bitumen. However, a fundamental understanding of SCWDS (such as the underlying chemical mechanisms, relative rates of desulfurization, and the role of SCW and hydrocarbons) is limited. In the present work, we have gained molecular-level insights into this process by measuring the kinetics of decomposition of a variety of organic sulfides in the presence of hydrocarbons and supercritical water in a continuously fed stirred-tank reactor (CSTR). The results are consistent with a free-radical mechanism, with hydrogen abstraction from the sulfide as the rate-determining step. The decomposition rates of the aliphatic and aromatic sulfides varied depending on their molecular structure, with conversions after 31 min at 400 °C ranging from less than 3% (our detection limits) to more than 90%. These differences in the reactivity correlate with the estimated heats of reaction for the critical hydrogen abstraction. The decomposition rates of the sulfides were affected by the presence of hydrocarbon carriers, with the rates being higher in the presence of alkanes than in the presence of toluene, as expected for a free-radical process. Product distributions and rates of radical-induced alkane cracking during this process were likewise affected by the presence of different sulfides. The decomposition of several different sulfides is consistent with 3 / 2 power kinetics, providing further evidence that the reaction proceeds via a radical mechanism. The knowledge developed in the current work provides a fundamental basis for further improvements in SCWDS.

A Review of Laboratory-Scale Research on Upgrading Heavy Oil in Supercritical Water

“…It is well known that the combustion of hydrocarbon compounds containing N or S will release a great deal of air pollutants (NOx, SOx) [12,94], whereas the existence of Ni and V in heavy oil will enhance the viscosity of crude oil and deactivate the cracking catalyst. In addition, the Ni and V in refined oil will accelerate the wearing and corrosion of the turbo wall and the continuous emission of their oxidation have potential carcinogenic risk [95][96][97].…”

“…Extensive studies showed that the introduction of SCW could effectively lower the sulfur content of heavy oil [12,59,75,100]. However, it is noteworthy that the removal of sulfur was closely related to the location of sulfur in compounds [12,94]. Usually, the aliphatic sulfur (in mercaptanes, sulfides and disulfides) cleaves easily in SCW, while the aromatic sulfur (in thiophene) is difficult to be reduced because of their steric hindrance [12,94,101].…”

“…Patwardhan et al [94] reported that the reactivity of several model organosulfur compounds in SCW obeyed the following order: Dibenzyl sulfide ≈ benzyl phenyl sulfide > isopropyl phenyl sulfide ≈ hexyl sulfide ≈ tetrahydrothiophene >> thiophene. Ates et al [12] inferred that aliphatic sulfur could be effectively removed by SCW alone, while the efficient removal of sulfur from thiophene compounds was not possible without a suitable desulfurization catalyst.…”

Conversion of brown coal continuously supplied into the reactor as coal–water slurry in a supercritical water and water–oxygen mixture

“…The amount of oxygen in CO 2 corresponds to 50.3 wt.% of oxygen in OM. Hydrogen sulfide appeared in the conversion products of OM supplied from II fuel cylinder (Sample 2, Table 2) as a result of thermolysis and hydrolysis of the aliphatic C S bonds [33,34]. Approximately as little as 3% of sulfur from initial COM evolved as H 2 S because the bulk of COM sulfur is located in the composition of thiophene-like structural fragments.…”

Energy from Waste Materials and Unconventional Sources

“…The decomposition rates of the aliphatic and aromatic sulfides varied depending on their molecular structure, with conversions after 31 min at 400°C ranging from less than 3 % to more than 90 % (Patwardhan et al 2013). Supercritical water upgrading (SCWU) of crude oils reduces sulfur content and decreases average molecular weight, without rejecting carbon as coke products.…”