PTQ Q1 2026 Issue

HF Alkylation Corrosion Monitoring

32˚C

Figure 2 HF alkylation corrosion monitoring

about HF alkylation corrosion and the use of thermody- namic modelling to manage it. HF solubility drives phase behaviour The fundamental reason for phase change corrosion is that HF dissolves far more readily in hydrocarbons than water. As entrained acid droplets are heated, HF rapidly parti- tions into the hydrocarbon phase while water remains in the separate acid phase. The droplets become water-rich, raising the hydronium concentration and corrosion poten- tial. When the acid droplets eventually dissolve or when the hydrocarbon cools downstream, the water-rich phase can condense, causing sudden and severe corrosion at spe- cific locations. Any modelling or monitoring strategy must account for this differential solubility. One very important point is that, in other studies, a refin - ery had calculated TTs using the old HF-iC₄-nC₄ solubility curves and calculated a TT ~135ºF. The plant was running the main fractionator feed at 170ºF to try to avoid transi- tions zones, but they were still seeing corrosion in the line, the exchangers, and the feed nozzle to the tower. Proper ionic modelling through OLI simulation showed that the actual TT based on their level of entrainment was ~190ºF. The plant did not have enough heat to get to above this temperature and resorted to upgrading metallurgy in the exchanger bundles, main fractionator feed line, and feed nozzle. Transition zones are narrow but dangerous The model identifies three regimes: fully soluble, liquid-liq - uid (entrained acid) and transition. In the soluble zone, the mixture is a single liquid phase, and corrosion is minimal. In the entrained zone (well below TT), droplets persist but remain HF-rich and relatively benign. The transition zone, which greatly depends on rich HF (RHF) composition in the settler and pressure, occurs when droplets are partially dis- solved and increasingly become water-rich.

Corrosion rates peak in this zone (up to 1,000+ mpy in field cases), and even small changes in temperature or pressure can move the operating point into or out of it. Operators must avoid the transition zone by either stay- ing well below TT (cold operation) or heating above TT to dissolve droplets completely. However, heating may be limited by exchanger duty or skin temperature constraints. Also note that cold operations may lead to corrosion inside the tower, as entrainment may increase due to higher feed rates to the unit. Operators must avoid the transition zone by either staying well below transition temperature (cold operation) or heating above TT to dissolve droplets completely Entrainment level is a critical parameter High entrainment of acid droplets increases the TT, extend- ing the transition zone. The first refinery case showed that 0.5 wt% entrainment led to a TT of ~230°F, making it impos- sible for the preheat exchangers to dissolve the acid without excessive heat duty. Reducing entrainment to 0.1 wt% low- ered the TT significantly for the main fractionator feed. The second study found that increasing entrainment from 160 to 500 lb/h impacted the formation of acid phases inside the tower, creating free acid phases below the feed tray and driving corrosion inside the tower. These studies underscore that entrainment effects depend on system composition and pressure and must be evaluated for each unit. Accurate measurement of entrain- ment is therefore essential; instrumentation upgrades such as flow meters can improve the reliability of this calculation.

PTQ Q1 2026