Release Conditions Leading to Aerosol Formation in HF Alkylation

This paper examines typical release conditions leading to aerosol formation in existing HF alkylation technology. The variables affecting aerosol formation in the mechanical and thermal breakup regimes are identified. The properties of these key variables are altered chemically using an additive to reduce aerosol formation for the entire operation envelop without degradation of alkylation efficiency.

Small and large scale tests were conducted to experimentally verify the benefits of the additive in both the mechanical and thermal breakup regimes. This paper summarizes the findings of these experiments and presents a detailed method for estimating the release, droplet formation and dynamics and dispersion behavior of chemically reacting systems. The Texaco/UOP HF alkylation additive technology, aerosolization reduction effects, represents a viable method for reducing the consequences of two-phase hydrogen fluoride releases. 

This method is validated using the small and large scale data and is used to perform sensitivity analysis to assess the impact of storage temperature, pressure, composition, atmospheric and site conditions on aerosol and toxic hazard zone reduction.


Anhydrous hydrofluoric acid (AHF) is used in the petroleum industry as a catalyst for the production of high octane gasoline by the isobutane-isobutylene alkylation reaction. The alkylate product is a key blending component in the production of reformulated gasoline which will meet new federal fuel specifications since it contains virtually no aromatic or olefinic components. Episodic releases of pressurized, superheated anhydrous hydrofluoric acid predominantly produce an aerosol. The aerosol cloud formed may contain concentrations above acceptable exposure criteria. Episodic releases at several refineries have resulted in increasing political pressure to replace existing hydrofluoric acid (HF) alkylation units with sulfuric acid alkylation units. The estimated cost of converting a single HF alkylation unit to sulfuric acid ranges from 50 to more than 100 million dollars, depending upon the unit capacity.

Aerosol Formation

The potential hazard of a liquid toxic or flammable substance when released to the atmosphere is a strong function of source characteristics such as geometry, temperature, density, composition, aerosol fraction, and release rate. Reducing the source emission rate and duration is key to reducing exposure concentrations and durations below established safe threshold values.

Stored liquids which remain in the liquid state after release usually present less hazard because of possible post-release mitigation action. Materials that become airborne immediately upon release, such as HF, chlorine, ammonia, are very difficult to mitigate, especially when large quantities are discharged over short periods of time.

Major factors that influence the amount of a given material which will become airborne upon release to the atmosphere include the material’s vapor pressure and normal boiling point. These properties usually determine whether during discharge a material will be vapor, liquid, or a flashing liquid. The amount of superheated liquid that is converted to vapor upon release, or the flash fraction, can be calculated using thermodynamics. However, this flash fraction does not account for the aerosolized liquid fraction carried with the vapor when sufficient energy is available in the superheated liquid to cause liquid breakup.

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