PTQ Q4 2025 Issue

400

28.7

Operating Envelope – Skin T: Increased corrosivity

380

360

21.5

340

320

14.4

300

280

7.18

260

240

220

0.0

200

Temperature (˚F)

Figure 5 Transition temperature vs pressure plot, including skin temperatures, indicating heat exchanger still continues

produce water contents ranging from 2.3 wt% to 24 wt% in the residual acid droplets, corresponding to highly corro- sive conditions. The model, therefore, recommended per- forming skin temperature calculations for each exchanger and repeating the surveys at those temperatures to capture boundary layer conditions. Recommendations and lessons from Case 1 Several key lessons emerged from the refinery case study:  Entrained acid measurement is critical : The estimated entrainment of ~2,000 lb/h was an order of magnitude higher than in other HF units. High entrainment increases TT, meaning more heat is required to dissolve the droplets. Fill tests were recognised as prone to human error, and the report recommended installing a flow meter on the acid boot to better measure entrainment and recalculate corro- sion risk. However, care must be taken when selecting and installing these instruments, as the probes may be prone to iron sulphide scaling. v Reducing entrainment lowers TT : Model runs at 0.1 wt% entrainment (a four-fold reduction) lowered the TT by ~20°F. This shift would allow the Feed/Rcy iC 4 exchangers to dissolve the acid droplets at lower temper- atures, reducing exposure to the transition zone. Reducing entrainment can be achieved by optimising settler opera- tion, improving acid-hydrocarbon separation, and minimis- ing turbulence. w Heating alone cannot mitigate corrosion : Simply increasing heat duty may not dissolve all acid droplets at high entrainment. The model showed that heating dissolves HF faster than water, temporarily increasing water content and hydronium concentration. Furthermore, skin temper- atures in the exchangers can be significantly higher than bulk temperatures, creating water-rich droplets near tube walls, even when the bulk fluid is below TT, and leading to

increased corrosion rates. Thus, heating must be combined with entrainment reduction. x Material upgrades may be necessary : If entrainment cannot be sufficiently reduced, the report recommended upgrading preheat exchangers and downstream piping to more corrosion-resistant alloys. The choice depends on expected corrosion rates and cost-benefit analysis. y Operational monitoring is essential : The model rec - ommended implementing online ultrasonic thickness (UT) sensors matched with automated process simulation at locations prone to corrosion. On-stream monitoring would allow operators to correlate thickness loss with predicted (from the simulation) water in acid and temperature and adjust operations accordingly. Overall, this refinery case demonstrates how the thermo - dynamic model can identify high-risk conditions that might not be obvious from bulk temperatures and conventional simulations. By highlighting the role of entrainment and skin temperature effects, the model guides both process improvements and inspection planning. In the next issue of PTQ , Part 2 of this article will further expand on how a new thermodynamic model developed within the HF alkylation JIP marks a significant advance in understanding and managing corrosion in HF alkylation units, demonstrating the ability to identify high-risk condi- tions, such as high entrainment and hot spots, and guide practical actions, such as reducing entrainment, adjusting heat duty, and upgrading materials. Ezequiel Vicent is Director of Consulting at OLI Systems, with 16 years of experience across the refining, petrochemical, and specialty chemical sectors. He leads global consulting projects that combine chemical engineering expertise with digital transformation, helping cli- ents implement predictive models, improve asset reliability, and reduce emissions. Email: ezequiel.vicent@olisystems.com

PTQ Q4 2025