The HF alkylation process converts low molecular-weight alkenes and iso-paraffins into larger branched-chain paraffinic hydrocarbons with a high-octane number (MON and RON) and low RVP using HF as the catalyst. A common problem in this process is the corrosion caused by the absorbed water to the HF catalyst. The water-rich HF (RHF) corrosion is associated with phase changes in the process fluids. Specifically, the most corrosive environments have been created by the formation of a water-rich RHF phase when condensing out of a vapor or when reaching solubility limit in liquid hydrocarbon stream. Therefore, accurate prediction of phase equilibria in the HF alkylation environment is critical for predicting and mitigating corrosion. Thermodynamic modeling provides a foundation of process simulation to predict phase equilibria and their changes in the mixtures involved in HF alkylation so that optimal operating conditions can be defined to prevent water-rich HF phase changes in locations where corrosion is prone to occur (e.g., outside heat exchangers). OLI’s mixed solvent electrolyte (MSE) thermodynamic model has been applied for this purpose. The MSE model defines properties of each components and interactions between any two components in the HF alkylation mixtures including hydrocarbons, HF, water, selected fluorocarbons and representative acid soluble oils (ASOs). Appropriate definition of these interactions allows accurate prediction of phase equilibria under varying conditions of temperature, pressure, water content in HF, etc. Consistent thermochemical properties of all species / components make it possible to determine chemical speciation that provide insights into corrosion phenomena in the alkylation process. This model can, then, be used to predict corrosion risk in the fractionation train of an HF Alkylation Unit. This includes, but is not limited to, feed lines to the Main Fractionator/Isostripper, depropanizer, HF Stripper, tower flash zones and overhead lines where phase changes are prone to occur.
March 23, 2021 EST