Dramatic Large-Scale Demonstration that Sulfuric Acid Is Vastly Safer than HF for Alkylation

Background

In 2014, there was a very large release of sulfuric acid at a refinery in Martinez, California that demonstrated its safety in the community compared to what would have happened if the refinery had used hydrogen fluoride. Before going into details of the Martinez release, first some background on the two chemicals. It is widely acknowledged in the industry that sulfuric acid alkylation is vastly safer than hydrogen fluoride (HF) with respect to offsite consequences. An additive refineries uses in a failed attempt to make HF safer is at such a low level it hardly makes a difference.

Sulfuric acid is vastly safer because its molecules are strongly bound in the liquid state. Even at elevated  temperature, there is no tendency for them to go airborne as an aerosol or vapor. One measure of sulfuric acid’s extremely low volatility is its high boiling point of 639F. Sulfuric acid in a beaker placed in a kitchen oven at 500F will remain as a liquid and not boil. Compare this to HF, whose molecules are only weakly bound in the liquid state. Considered a “fuming liquid,” HF boils at 67F. An open beaker at room temperature on the kitchen counter will spontaneously spew a white vaporous cloud. Another measure of volatility is vapor pressure. At room temperature, HF’s vapor pressure is a million times higher than sulfuric acid’s.

Definitive field-scale experiments on the release of sulfuric acid from an alkylation unit were conducted in 1991 by Quest Consultants in Oklahoma and documented in Sulfuric Acid Release Report. Thirty-six releases of sulfuric acid — alone and mixed with hydrocarbons — were conducted. In every case, virtually all of the released sulfuric acid was collected on the floor of the test cell downstream of the release.

Quest Consultants repeated the experiments in 1993 with hydrogen fluoride. In their paper Effectiveness of Mitigation Systems in Reducing Hazards of Hydrogen Fluoride Leaks, Quest’s assessment is that when mixed with hydrocarbons, 100% of the hydrogen fluoride will go airborne, even with 50% sulfolane added to the HF to suppress its volatility.

The United States Environmental Protection Agency (EPA) requires facilities that use extremely hazardous substances to develop a Risk Management Plan (RMP). Hydrogen fluoride is on the list of extremely hazardous substances that require one. Sulfuric acid, so long as it’s not mixed with sulfur trioxide, which makes it fume, is not on the list. Thus the EPA explicitly acknowledges the stark difference in the threat to the community between sulfuric acid and HF.

The 2014 Sulfuric Acid Release at the Tesoro Refinery in Martinez CA

The U.S. Chemical Safety Board (CSB) reported on an accidental release of 84,000 pounds of sulfuric acid over two-and-a-half hours from an alkylation-unit settler tank at the Tesoro Refinery in Martinez, California, on February 12, 2014. The release was onto the refinery grounds and into a process sewer system. There was no vapor cloud or offsite consequences to the community. The spill burned two workers, who were transported to the nearest hospital burn unit by helicopter. They survived and returned to work after five months.

A Modified Hydrofluoric Acid (MHF) release of this magnitude would have had cataclysmic consequences not only for refinery workers, but also for the surrounding community. A ground-hugging toxic cloud would be lethal for more than eight miles downwind. (For comparison, there are 50,000 pounds of MHF in one Torrance refinery settler tank.) This full-scale incident is further dramatic evidence that sulfuric acid alkylation is vastly safer than MHF alkylation, contrary to the claims of Exxon-Mobil and the Torrance Refining Company.

In addition to sulfuric acid, there are two other alkylation processes commercially available: Solid-Acid and Ionic-Liquid Alkylation.