(112b) HF Alkylation: Novel Thermodynamic Model to Simulate Phase Change Corrosion Zone

Ezequiel Vicent, Peiming Wang, Andrzej Anderko

OLI Systems, Inc. 2 Gatehall Drive, Suite 1D, Parsippany, NJ 07054

email: Ezequiel.Vicent@OLISystems.com

INTRODUCTION

The HF alkylation unit performs a series of specific chemical and physical processes, catalyzed by hydrofluoric acid (HF), to convert isobutane and light olefins (i.e., propylene and butylene) into alkylate, a mixture of larger branched-chain alkanes that are mostly isooctane and isoheptane. Alkylate from this process has a high Research Octane Number (RON of 94), a low Reid Vapor Pressure (RVP of 4 psi), and very little to no sulfur. This allows a typical refinery to improve their gasoline blends to reach premium and mid-range grades, while lowering the overall products’ sulfur content and RPV point.

HF alkylation units, however, are notorious for having significant carbon steel corrosion issues, particularly in the feed lines from the acid settler to the isostripper (or depropanizer, depending on unit configuration), as well as overhead lines. This corrosion has been caused by the formation of a water-enriched acid phase (rich-HF or RHF) that leaves the settler as entrained acid in the liquid hydrocarbon. 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. Corrosion from this water-rich HF can damage the piping systems and equipment, causing dangerous accidents. Thus, a thermodynamic model that can provide accurate prediction of phase equilibria in the HF alkylation environment is essential for predicting and mitigating corrosion.

DESCRIPTION OF THE WORK

An industry sponsored Joint Industry Project (JIP) was formed to enhance our understanding of the relationship between the rich-acid and corrosion through the development of a novel electrolyte thermodynamic model.

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). In this work, OLI’s mixed solvent electrolyte (MSE) thermodynamic model [1] has been applied for this purpose. The MSE model defines properties of each component 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.

RESULTS AND DISCUSSION

The key to assess corrosion risks in the HF alkylation process is to predict how phase equilibria (liquid-liquid and vapor-liquid equilibria) vary as conditions change. Thus, the focus of the model development is to provide accurate prediction of solubilities in the system H2O-HF-hydrocarbons-acid soluble oils. Selected model results and their comparison with literature data will be shown during the presentation for the solubilities of HF in hydrocarbons. Excellent agreement has been obtained between the calculated solubilities and experimental data. These results show how the solubility of HF vary with temperature for a representative list of components found in the settler hydrocarbon fluid. These results can be useful for the effective modelling of HF solubilities in the hydrocarbon phase and to understand the phase equilibria of the two-phase fluid (an electrolytic acid phase and a hydrocarbon phase) that leaves the settler.

More importantly, the solubility effects of HF vs H2O in hydrocarbon drive the corrosivity of the electrolytic solution. The MSE thermodynamic model can predict the solubility differences under variable conditions and the effect of water on HF solubility in hydrocarbons can also be predicted. Model results show of solubilities of HF and water in the a variety of hydrocarbons (comprising the hydrocarbon effluent of an acid settler), showing that HF is more soluble than water in hydrocarbons. Model results suggest that when the liquid hydrocarbon is saturated with HF upon heating, the water-to-HF ratio in the RHF phase will increase because more HF than water will dissolve in the hydrocarbon phase. This results in an increased relative water concentration in the RHF phase, leading to an increasingly corrosive environment. Through the JIP, appropriate interaction parameters between mixture components were developed that allow the MSE model toprovide quantitative prediction on the variation of water content in the entrained RHF phase as temperature increases at any given stream conditions (i.e., composition and pressure).

The model was applied to a JIP member acid settler effluent, simulating the heating process encountered in the feed/rcy-iC4 heat exchangers (preheaters) prior to fractionation, on the main feed line. The model shows that as we increase the fluids temperature, HF will dissolve into the hydrocarbon phase with greater affinity than H2O, increasing the H2O relative concentration in the rich-acid phase. The increased water concentration in the acid causes an increase in the hydronium ion of the rich-acid phase, thus, increasing its corrosivity. The water relative concentration in the RHF will continue to increase (increasing the electrolytic phase corrosivity) until a temperature is reached where all the RHF dissolves into the hydrocarbon phase. We call this temperature the Transition Temperature (TT). The model and its calculation of the TT can help refiners develop Integrity Operating Windows (IOW) to ensure the TT is reached at the heat exchangers to avoid corrosion in the main feed line to the main fractionator.

The new thermodynamic model developed in this work can be combined with process simulation to provide much improved understanding of corrosion caused by RHF phase change in HF alkylation fractionation systems. In addition to the aforementioned application for the feed line to the main fractionator, this model can be applied to the depropanizer, HF stripper, tower flash zones and overhead lines where phase changes are prone to occur. Refiners can use this model to calculate a fluid’s TT and ensure that operations happen above this temperature to avoid corrosion, and create IOWs for the fractionation system of the unit.

This presentation will show the model’s thermodynamic foundation and results obtained in predicting phase equilibria in the HF alkylation environment. An example will be demonstrated on how refiners can make use of this model to develop integrity operating windows to avoid phase-change corrosion in the HF alkylation unit.

REFERENCES

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