ERTC 2025
Clean exhaust gas with extractive oxygen measurement
Michael Ostendorf and Tobias Maillard endress+hauser
European refineries are under mounting pressure. While margins are tightening due to fluctuating feedstock quality, geopolitical supply shifts, and high energy costs, envi- ronmental and safety requirements are only becoming stricter. The European Union’s ‘Fit for 55’ package and ongoing updates to the Industrial Emissions Directive are raising the bar for sulphur content in fuels and demand- ing ever lower emissions of volatile organic compounds and sulphur oxides. For operators, one of the critical battle- grounds is sulphur removal. Mercaptans and hydrogen sulphide must be converted or extracted to produce clean, odourless fuels suitable for European markets. The widely used MEROX ® (mercaptan oxidation) process achieves this by oxidising mercap- tans into disulphides with the help of oxy- gen and a proprietary catalyst. Here lies the operational challenge: con- trolling oxygen precisely. Too little O₂, and mercaptan oxidation is incomplete, leav- ing the product off-spec. Too much, and there is not only wasted reagent but also an increased risk of explosive gas mix- tures forming in downstream handling and vent systems. Accurate and reliable oxygen monitoring is therefore a technical neces- sity – not just for product quality, but for process safety and regulatory compliance. Yet traditional oxygen measurement technologies, such as paramagnetic analys- ers, have significant drawbacks in refinery service. This is where laser-based tunable diode laser spectroscopy (TDLS), as embod- ied in Endress+Hauser’s TRANSIC100LP analyser, offers a step change. MEROX Process and the Role of Oxygen The MEROX process is widely used in refin- eries to ‘sweeten’ light petroleum fractions such as LPG, naphtha, and kerosene. Its operation can be broken down into three main steps: u Extraction/prewash: Hydrogen sul- phide is removed from the hydrocarbon stream using caustic soda. v Oxidation: In the presence of air or oxy- gen and a catalyst, mercaptans (RSH) are oxidised to disulphides (RSSR). w Separation: Disulphides, being insol- uble in caustic, are separated from the sweetened hydrocarbon stream. Oxygen is the limiting reagent in this chemistry. If insufficient O₂ enters the oxi- dation reactor, mercaptans remain uncon- verted, leading to off-spec product and odour issues. Conversely, excess oxygen can migrate with vent gases into down- stream areas where concentrations above ~5 vol% create a flammability hazard. Thus, continuous oxygen measurement downstream of the oxidation step is criti- cal both to verify complete mercaptan oxi- dation and catalyst efficiency as well as to maintain oxygen levels below explosive lim- its in vent systems.
did you know? European refineries, already leaders in adopting cleaner fuel technologies, can benefit significantly from TDLS-based oxygen monitoring Application in European Refineries European refineries, already leaders in adopting cleaner fuel technologies, can benefit significantly from TDLS-based oxy- gen monitoring. Continuous and accurate O₂ measurement ensures that products con- sistently meet EU sulphur and odour stand- ards, strengthening regulatory compliance. At the same time, optimised oxygen dosing helps reduce air consumption, minimise cat- alyst stress, and limit operational variability, improving overall process efficiency. Reliable vent gas monitoring also enhances safety by helping to prevent ignition events – an essential safeguard in densely populated regions. Finally, reduced maintenance needs and longer analyser lifetimes support sustainability goals, align- ing with broader decarbonisation efforts and cost-reduction strategies. One proven application is in light cracked naphtha units, where TRANSIC100LP has been successfully deployed downstream of disulphide separators. Here, its ability to deliver stable readings despite hydrocar- bon-rich environments has validated both its robustness and its economic case. A Technology for the Refinery of the Future For European refiners, the stakes are clear: deliver cleaner fuels while maintaining safe, efficient, and cost-effective operations. The MEROX process, central to desulphuri- sation, depends critically on precise oxygen control. Where paramagnetic analysers struggle with contamination, downtime, and high lifecycle costs, TRANSIC100LP brings laser-based robustness. With TDLS at its core, it provides continuous, selective oxy- gen monitoring that supports both compli- ance and safety while cutting costs. Amidst the transition to more integrated, digitised, and sustainability-driven opera- tions, solutions like TRANSIC100LP repre- sent not just an incremental improvement but a foundational upgrade, enabling the industry to meet tomorrow’s challenges with confidence.
Figure 1 TRANSIC100LP process gas analysers
Limitations of Paramagnetic O₂ Analysers Paramagnetic analysers, which have long been the industry standard, measure oxy- gen by exploiting its magnetic susceptibil- ity. While precise under clean conditions, they are ill-suited to the harsh environment of refinery gas streams: • Contamination sensitivity: Oil mists, aer- osols, and caustic carryover contaminate the sensor cell. Cleaning is often impractical; many units must be replaced outright. • High conditioning demand: Extensive sample filtration, dryers, and scrubbers are required to keep the gas stream analyser- friendly, adding both Capex and Opex. • Maintenance burden: Filters and mem- branes clog, requiring frequent intervention. Analyser downtime is common. • Slow response if bypassed: Some sites sidestep these issues by relying on weekly laboratory analysis. While accurate, this creates a significant lag between process conditions and corrective action, which is unacceptable under modern continuous monitoring expectations. For European refineries – where uptime, compliance, and lean operations are paramount – these drawbacks add up to a technical and eco- nomic liability. TRANSIC100LP and TDLS: A Step Forward Endress+Hauser’s TRANSIC100LP ana- lyser leverages TDLS to directly measure oxygen with high selectivity and resilience. How TDLS works TDLS directs a narrow-wavelength laser beam through a gas sample. Molecules of
oxygen absorb light at a specific wavelength (~760 nm). By tuning the diode laser across this absorption line, the analyser detects the degree of absorption, which correlates directly to O₂ concentration. Key benefits of TDLS over paramagnetics include: • High reliability: O₂ measurement by TDLS is insensitive to dust or flow changes showing virtually no drift. • Fast response: Measurements are near- instantaneous, enabling real-time control. • Non-contact measurement: With no moving parts or magnetic cells, TDLS ana- lysers are inherently more durable. TRANSIC100LP design The TRANSIC100LP analyser adapts TDLS technology to refinery service: • Sample conditioning: A sintered PTFE filter removes aerosols and droplets before the gas enters the optical cell. • Compact footprint: The entire system is lightweight, enabling installation close to the process for reduced lag. • Robustness: Designed for industrial deployment, the unit withstands harsh environments without degradation in performance. • Safety integration: By continuously tracking oxygen in vent gas, it ensures con- centrations remain below 5% – a critical safety threshold for explosion prevention. Lifecycle and Cost Advantages A comparative lifecycle analysis of para- magnetic vs TDLS analysers reveals stark differences: • Installation: With simpler sample condi- tioning, TDLS systems can reduce installa- tion costs by up to 75%. • Maintenance: Filters require minimal intervention, and the optical cell does not degrade with exposure. Annual mainte- nance costs can be cut by more than half. • Operational continuity: With higher uptime and fewer analyser failures, opera- tors avoid costly process interruptions and product quality deviations. In other words, TDLS not only improves measurement reliability but also delivers clear Opex savings, which are crucial for refiners navigating narrow margins.
Contact: sophia.asal@endress.com
Figure 2 Mounted TRANSIC100LP analyser
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