(282b) Factors Affecting H2S Scavenger Performance in Asphalt

Vapor phase hydrogen sulfide (H₂S) concentrations in heavy oils such as asphalt and residuum vary based upon source and handling conditions. This work will examine and document the conditions that influence the phenomena of H₂S generationâ??for example, the cracking of other indigenous sulfur compoundsâ??and generate quantitative data on the H₂S removal performance of metal-based H₂S scavengers.

The vapor phase presence of H₂S presents a number of challenges for the industry that are exacerbated when dealing with hot oils such as asphalt. The primary concern is the personnel exposure risk presented by the toxicity of H₂S. Heavy oils are a particular concern because of the inherently high concentration of H₂S in the liquid phase of the oil and the tendency for the H₂S to partition strongly to the vapor phase. From a mechanical integrity perspective, H₂S is a weak acid that, when in the vapor phase, can readily combine with moisture in a storage tank and condense to a liquid phase, resulting in increased corrosion and shorter asset life.

Industry-representative samples of asphalt were heated to various temperatures at varying vessel liquid levels. The H₂S concentrations in the vapor space of the samples were compared to examine the impact of temperature and vessel liquid level on partitioning of H₂S, cracking of the other sulfur-bearing species, and performance of metal-based H₂S scavengers. The testing confirmed that vapor phase H2S concentrations are dependent upon the liquid level of the vessel and the temperature at which the heavy oil is stored, due to both increased vapor/liquid partitioning and increased cracking of other sulfur-bearing species. The metal-based scavenger was not negatively impacted by increasing oil temperature and furthermore, the higher temperatures improved performance of the scavenger, presumably due to improved mixing and contact efficiency.