Fossil fuels are known for their sulfur content, which originates from the decomposition of dead organisms over millennia. Mercaptans (thiols) and hydrogen sulfide (H₂S) are two sulfur compounds present in crude oil which contribute to its characteristic odor. In the refining process, these can lead to increased corrosion in distillation equipment at the high temperatures used. Additionally, excess sulfur dioxide (SO_2(g), a pollutant) can be emitted after combustion if sulfur is present in the refined products. Therefore, it is best to remove as much sulfur as possible early on in the refining process.

Sulfur compounds are present throughout the entire boiling range of hydrocarbons in crude oil. Depending on the size and bond strength of these compounds, different desulfurization treatments are available. Lighter impurities (including mercaptans and sulfides) can be removed via hydrotreating in a reactor with a catalyst (generally cobalt molybdenum) and hydrogen under high temperature and pressure. Heavier sulfur compounds can be removed with hydrocracking after the hydrotreating process. Desulfurization (Figure 1) can occur at many points within a refinery, from the crude feedstock to the distillate streams.

Traditionally, the oil and oil product analysis can be monitored by laboratory titration with silver nitrate using a sulfide-coated silver electrode serving as the indicator electrode. However, this methodology does not provide timely results and requires human intervention to implement the results of the laboratory analysis into the process. Online process analysis allows constant monitoring of oil quality without long waiting times in the laboratory, providing more accurate and representative results directly to the control room for quick process adjustments.

By using online process analyzers, operators gain the most representative, up-to-date information they need to accurately identify trends, reduce downtimes, and address operational issues before costly problems arise. In addition, the time of response to corrosion formation is fast and immediate warnings are delivered in case of out-of-specification readings.

The Metrohm Process Analytics 2045TI Ex proof Process Analyzer (Figure 2) with a customized sample preconditioning system could be implemented in many areas within a refinery to ensure that catalysts in the reactors are not exhausted and to limit corrosion further on in the distillation unit. The analyzer fulfills EU Directives 94/9/EC (ATEX95) and is certified for Zone 1 and Zone 2 areas.

The mercaptan and H₂S content in crude oil is determined by a two-endpoint argentometric titration based on ASTM D3227 and UOP163. Endpoint 1 corresponds to H₂S and endpoint 2 to the mercaptans.