PTQ Q4 2025 Issue

Identify corrosive regimes in hydrofluoric alkylation units: Part 1

How thermodynamic modelling can identify high-risk conditions and guide practical actions to reduce entrainment, adjust heat duty, and upgrade materials

Ezequiel Vicent OLI Systems

H ydrofluoric (HF) acid alkylation units are workhorses of modern refineries. They convert isobutane and light olefins into high-octane alkylate, a gasoline blend stock valued for its clean burning and high research octane number (RON). The process employs HF acid as both catalyst and solvent, and consequently carries a unique set of safety and corrosion challenges. Hydrocarbon effluent leaving the reactor enters a settler where dense acid and lighter hydrocarbons are separated. The acid is recycled to the reactor, while the ‘sweet’ hydrocar- bon product is routed through preheat exchangers and a main fractionator (isostripper or depropaniser, depending on unit design) before further downstream processing. Experience across the industry shows that these post-settler streams are prone to rapid corrosion, leaks, and unplanned shutdowns. Loss of containment events have been attributed to phase change corrosion, a phenomenon where the rich HF (RHF, enriched with soluble hydrocarbon, water, and ASOs) that is entrained/carried over in the hydrocarbon stream starts to solubilise in the hydrocarbon liquids as temperature increases. The remaining acid at the various temperatures becomes more corrosive as the relative composition of water in the remaining RHF increases. Traditional process simula- tors were not able to predict the composition of this second liquid phase or the conditions under which it changes sol- ubility, making it difficult to define integrity operating win- dows (IOWs) for safe operation. A joint industry project (JIP) involving refiners and OLI Systems, therefore, focused on developing a rigorous ther- modynamic model to predict the solubility of HF, water, and acid soluble oils (ASOs) in hydrocarbons, to calculate when and where the corrosive phase forms, and to integrate this knowledge into simulation tools for unit design and operation. This article summarises lessons learned from that model- ling work and describes how the resulting model has been applied using OLI Flowsheet: ESP and OLI Studio: Stream Analyser to identify corrosive regimes in several HF alky- lation units. Two recent studies illustrate how the model enables refineries to quantify entrained acid carryover, cal- culate transition temperatures (TTs), and adjust operations or materials accordingly. The objective is not only to report case-specific results, but to extract general principles that practitioners can apply to their own units.

Fractionation train and phase change corrosion Process overview The HF alkylation process begins in a reactor, where HF acid catalyses the alkylation of isobutane with ethylene, propylene, or butenes. Effluent from the reactor is a liq- uid-liquid mixture consisting of a RHF acid phase and a hydrocarbon phase composed of isobutane, alkylate, and dissolved ASO. In the settler, these phases separate. However, separa- tion is not perfect, and the hydrocarbon leaving the settler generally carries small droplets of acid. This entrained RHF contains HF, water, and ASO in proportions similar to the reactor acid. Immediately after the settler, the hydrocarbon stream is at low temperature, and the entrained acid has a high HF content and low water content, resulting in low corrosivity. The mixture is then preheated in exchangers before entering the fractionator, where lighter hydrocar- bons are removed, iC₄ is recycled back to the reactor, and heavier alkylate is recovered. Unfortunately, heating the fluid also triggers phase and compositional changes that drive corrosion. HF is far more soluble in hydrocarbons than water, and this difference increases the corrosivity of RHF as it heats up on its way to the tower. As the entrained acid and hydrocarbon mixture is heated, HF dissolves quickly into the hydrocarbon phase, while water dissolves much more slowly. Consequently, the residual acid droplets become enriched in water and depleted in HF. This water-rich HF phase has a higher concentration of hydronium ion (H₃O + ) and is far more corrosive to carbon steel than the original acid. At the same time, the fractionation system experi- ences vapour-liquid-liquid transitions, as hydrocarbon, HF, and water redistribute across phases. Without an accurate model, operators cannot tell whether they are operating in a fully soluble zone (no second liquid), an entrained acid zone (two liquids due solely to entrained droplets), or the more dangerous transition zone, where it reaches its maximum corrosivity (maximum water vs lowest HF composition). Experimental validation To predict these phenomena, the JIP team assembled sol- ubility data from literature and participating refineries and incorporated them into the OLI Mixed Solvent Electrolyte

PTQ Q4 2025