Electrolytic hydrofluoric acid (HF containing water) continues to be a significant carbon steel corrosion concern in the industry, particularly in the regime where water/HF undergoes phase changes as it is heated and cooled in the fractionation section of the HF alkylation process. This corrosion may be attributed to the formation of a water enriched electrolyte phase. An industry sponsored Joint Industry Project (JIP) was undertaken to better understand this corrosion relationship by developing an electrolyte thermodynamic database and relationships of HF/Water/Hydrocarbon interactions that could be used in process models to evaluate the impact of operating changes on the corrosion potential in these systems. This paper will discuss the creation of the electrolyte thermodynamic model and its application in evaluating the phase transitions that occur in user plants as a function of operating conditions through process simulation. This work identified the complexity of this water/HF phase change corrosion phenomenon which can be significantly affected by changes in operating conditions. It further confirms the difficulty in trying to manage this corrosion by operational controls alone. Examples of user experiences with the modelling tool combined with plant corrosion experiences are used to demonstrate how the modelling helped explain the corrosion observed and how process changes can significantly impact corrosion. This work supported the need for rigorous corrosion monitoring of these HF alkylation processes. It further highlights the value of implementing targeted continuous onstream ultrasonic thickness monitoring sensors and the use of alloy upgrades such as Alloy 400 in water/HF phase change locations.
Hydrofluoric acid (HF) is used as a catalyst in the alkylation process to react isobutane with olefin feeds to manufacture a high octane alkylate product used in gasoline blending. The HF catalyst is added in its anhydrous liquid form (< 400 ppmw H2O) but as it circulates in the reaction system, residual water in the liquid hydrocarbon feed is absorbed by the acid such that the circulating reaction acid builds up a small percentage (0.5 to 2.0 mass%) of water. This water/HF mixture is also referred to as rich HF (RHF). In addition, the alkylation reactions also will generate fluorocarbons and acid soluble oils (ASOs). A portion of the circulating reaction HF acid, containing water and ASO, is sent for regeneration (heating and stripping) to remove a portion of the built up water and ASO to maintain targeted levels of water and acid strength. The circulating reaction emulsion is cooled (reaction is exothermic) and the hydrocarbon and RHF is separated by gravity in a settler vessel where the acid is returned (pumped or convective flow) to further react with incoming hydrocarbons. As separation efficiency is never perfect, some RHF will be entrained in the liquid hydrocarbon withdrawn from the settler. The liquid hydrocarbon effluent from the settler then requires separation of the propane, isobutane (recycled), and normal butane from the alkylate product via fractionation. Products will contain organic fluorocarbons (fluorides) that may be removed in the product streams via a heated defluorination process. A block diagram of the HF Alkylation process is shown in Figure 1.
