Skin temperatures matter Bulk process temperatures may not reflect the temperature at the tube wall, where corrosion occurs. Heat exchanger tubes can have skin temperatures tens of degrees higher than the bulk fluid, pushing the local fluid into the transition zone even when the bulk temperature is below TT. Models should incorporate skin temperature calculations and per - form surveys at those temperatures to capture boundary layer conditions. Operators should measure or calculate skin temperatures and adjust heat duty accordingly. In some cases, redesigning baffles or altering flow distribution can reduce hot spots. Pressure is a lever to shift TT The T-P heat maps show that increasing pressure lowers the temperature at which the acid phase dissolves. In gen - eral, raising pressure shifts the operating envelope to lower temperatures, helping to keep the system within the solu - ble or entrained zone. Conversely, pressure reductions can raise TT and increase corrosion risk. As HF alkylation units continue to produce high-octane fuels for cleaner transportation, embracing such modelling and monitoring approaches will be key to sustaining reliability, safety, and environmental stewardship Material selection remains important Even with optimised operations, some units may still expe - rience corrosion due to unavoidable transitions. Material upgrades can provide a margin of safety. The first study recommended evaluating the cost-benefit of retrofit - ting preheat exchangers with corrosion-resistant alloys. However, upgrading only part of the system can shift corrosion downstream; therefore, a holistic assessment is required. On-stream ultrasonic thickness sensors, corrosion coupons, and acid composition analysers can provide real-time data to validate model predictions. Integrating MSE-based mod - els with plant data into a digital twin can help operators adjust plant parameters in real time, ensuring operations remain within safe boundaries. OLI is currently involved in work that will combine the OLI models with machine learn - ing algorithms to predict corrosion events and schedule maintenance proactively (see Figure 2 ). Continuous monitoring and digital twins enhance reliability The acid regenerator may be key to unit reliability The previous two studies focused on entrainment from the settler and trying to reduce said entrainment to lower
TTs and avoid liquid acid phases forming below the main fractionator’s feed tray. Another option is to increase the removal of water and ensure water in the RHF is minimised. Therefore, the acid regenerator, which recovers the spent HF acid and attempts to remove water and acid-soluble oils (ASOs), is crucial in this operation. Conventionally, it is thought that a cooler feed to the regeneration tower will cool the tower and send more acid, water and ASOs down the column. If we increase the water down the column, then we decrease the water in the acid returning to the reactor. However, a better approach to per - form this ‘cooling’ is to lower the iC₄ stream temperature coming from the depropaniser into the regenerator (reduce heating). The temperature difference will ‘slam’ the water down, along with acid and ASOs. However, this comes at a great cost, both in the form of make-up acid and in caustic use. OLI is currently helping refiners perform acid regeneration optimisation to cost-ef - fectively reduce the amount of water returning to the reac - tor, while minimising make-up acid and caustic costs. Conclusion The new thermodynamic model developed within the HF alkylation JIP marks a significant advance in understanding and managing corrosion in HF alkylation units. By recog - nising that HF solubility is orders of magnitude higher than water solubility and by explicitly modelling the electrolyte behaviour of HF water–hydrocarbon mixtures, the model predicts when and where a water-rich acid phase will form. Its integration into OLI flowsheet ESP and OLI Studio: Stream Analyser enables refineries to quantify entrained acid carryover, calculate transition temperatures, generate operating envelopes and evaluate mitigation strategies. The case studies shown in this article and Part 1, pub - lished in PTQ Q4 2025 , demonstrate how the model can identify high-risk conditions, such as high entrainment and hot spots, and guide practical actions, such as reducing entrainment, adjusting heat duty, and upgrading materials. Several lessons emerge for practitioners: entrainment measurement is crucial; transition zones are narrow but dangerous; skin temperatures can drive corrosion; water reduction from the acid regenerator is possible and can be made cost-effective; and pressure can be used as a lever to shift transition temperatures, in some cases. Most importantly, thermodynamic modelling must be complemented by accurate plant data and continuous monitoring to create a feedback loop that ensures oper - ations remain within safe boundaries. As HF alkylation units continue to produce high-octane fuels for cleaner transportation, embracing such modelling and monitoring approaches will be key to sustaining reliability, safety, and environmental stewardship. Ezequiel Vicent is Director of Consulting at OLI, with 16 years of experience across the refining, petrochemical, and specialty chemi - cal sectors. He leads global consulting projects that combine chemi - cal engineering expertise with digital transformation, helping clients implement predictive models, improve asset reliability, and reduce emissions. Email: ezequiel.vicent@olisystems.com
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