PTQ Q4 2025 Issue

Settler outlet heated through feed / iC exchangers

Corrosion rate

160F

HF-rich phase (RHF:

) HC phase: and

Why corrosion increases - driving force for corrosivity

Maximum corrosion occurs

98000

100

0.010 0.006 0.004 0.008 0.012 0.002 0.000 0.014 0.016 0.020 0.018 0.024 0.022 0.026

4000

HF in HF-rich phase (3415 0 lb/day)

H0+

3500

97500

HF in HC phase (94185 97600 lb/day)

97000

3000

0 50 40 30 20 10 60 70

96500

2500

HF-rich phase

2000

96000

95500

1500

H0 in HF-rich phase (858 0 lb/day)

H0 in HC phase (16 874 lb/day)

1000

95000

94500

500

94000

Temperature (˚F)

Figure 1 Phase equilibria in HF alkylation process

formation with hydrocarbon species) and phase equilibria (vapour-liquid-liquid). Because corrosion is influenced by the amount and composition of the second liquid phase, the model must accurately calculate each phase’s composition over a wide range of temperatures, pressures, and entrainment levels. Traditional equation of state models used in process sim- ulators treat HF/water as pseudo-components and cannot capture electrolyte behaviour. By contrast, the MSE framework uses fundamental ther- modynamic parameters (activity coefficients, solvation energies, and complexation constants), allowing it to pre- dict the distribution of HF and water between phases and the formation of hydronium ions. The model was param- eterised using data gathered during the JIP and validated against independent experimental solubility measurements. Transition temperature and entrainment An important concept arising from the model is the TT. For a given hydrocarbon composition, pressure, and entrainment of RHF droplets, TT is the minimum temperature required to dissolve all the entrained acid droplets into the hydro- carbon phase. Well below the TT, the system contains two liquid phases (hydrocarbon plus an acid droplet), and the corrosion risk is low because the acid droplets retain high HF content and low water content. As the temperature approaches TT, HF dissolves faster than water, increasing the water/HF ratio in the remaining droplet and raising its corrosivity. Above TT, the mixture becomes a single liquid phase, and the corrosion risk from the ionic liquid (RHF) falls again because no concentrated acid phase is present. Entrainment level is a critical input because higher entrained acid content requires more heat to dissolve com- pletely, raising the TT and exposing equipment to a wider transition zone. Figure 2 depicts a temperature survey for a

(MSE) thermodynamic framework. The model shows that HF solubility increases with temperature and is consistently higher than water solubility. The model captures this trend, reinforcing the conclu- sion that HF dissolves more readily into hydrocarbons than water. Capturing this behaviour is essential because the model will later be used to predict phase equilibria under operating conditions where industry experience has shown a large concentration of corrosion events. Figure 1 summarises model predictions for phase equi- libria in the HF alkylation process. The left panel tracks the distribution of HF and water between the HF-rich phase and the hydrocarbon phase as temperature increases. HF (red markers) decreases in the acid phase and increases in the hydrocarbon phase with heating, while water (blue markers) initially increases in both phases and then col- lapses as the HF-rich phase disappears. The right panel plots mole fraction of hydronium (H₃O + ) in the acid phase. As the water fraction increases, hydronium content rises sharply, explaining why corrosion rates spike when the acid becomes water-rich. The small plot at the top illustrates how the corrosion rate peaks at around 150- 160°F, correspondingo the transition zone identified by the model. Collectively, these plots explain the fundamental chemistry driving phase change corrosion. Development of thermodynamic model Mixed solvent electrolyte framework The thermodynamic model is built upon OLI Systems’ MSE framework, which treats hydrocarbon-HF-water mixtures as complex electrolytes. The model defines chemical interactions among C₃-C₁₂ alkanes and alkenes, representative alkylate molecules (such as 2,2,4-trimeth- ylpentane), representative ASOs, organic fluorides, HF, and water. It simultaneously solves for chemical equilib - ria (dissociation of HF to H + and F - , as well as complex

PTQ Q4 2025