“A major trucking spill of MHF onto hot highway pavement could vaporize thousands of pounds of hydrofluoric acid — more than twice the largest release of the 1986 Goldfish test.”
Boil-off is one of several paths that highly toxic Hydrogen Fluoride (HF) can go airborne into the community. In the past, the primary focus has been on the release of superheated HF and Modified Hydrofluoric Acid (MHF) from oil refineries’ settler tanks, because flash atomization causes 100% to form a ground-hugging, highly toxic cloud (see: Flash Atomization of HF and MHF).
Mobil’s original intent to suppress flash atomization was to add sufficient additive that the MHF was subcooled, that is, the boiling point is above the operating temperature. (As we now know, an additive level sufficient for subcooling is incompatible with the alkylation process.) However, even subcooled MHF spraying under high pressure from a rupture of a tank will break up into small droplets. While not nearly as fine as droplets from flash atomization, they nevertheless evaporate. Evaluation of how much was the objective of the large-scale MHF release tests conducted by Quest Consultants in 1993 (see: Superheated MHF Excluded from the Only Large-Scale Test Series).
Another significant path for MHF to go airborne is boil-off. This occurs, for example, if the rupture is in the top section of a settler tank. At the release of pressure, the superheated hydrocarbons (typically isobutane with a boiling point of 11F) and MHF (boiling point of 71F at 6-wt% sulfolane) are highly out of thermodynamic equilibrium at the tank process temperature of 106F. The tank’s contents will boil violently and much will be expelled from the tank before plunging in temperature to the low boiling point of the remaining liquid. The volatile hydrocarbons and MHF will then boil off at a rate governed by the transfer of heat from the environment through the tank walls to the cold liquid.
Consider a massive spill from a truck transporting 33,000 lbs of 15 wt% MHF (boiling point 76F) onto a sun-baked highway pavement at 140F. The MHF immediately cools the surface of the pavement to its boiling point of 76F. The temperature response of the pavement with typical parameters is shown in Figure 1.
On the heels of the 1986 “Goldfish” Release Test of hydrofluoric acid (HF), Mobil engineers sought some method to prevent the HF from forming a ground-hugging toxic cloud. In his illuminating presentation at the September 22, 2018 AQMD Refinery Committee Meeting (Watch:https://youtu.be/qwo08BtEQuM?t=5108), Goldfish Test Principal Investigator Dr. Ronald Koopman stated that the affect of the HF release “was much larger than we had expected and the downwind distance was further than we had expected” and “we found that the HF that was released all flashed into an aerosol and a vapor and so there was nothing that ended up on the collection pan or in that tank — nothing was captured — and that was a great surprise to us.”
In their attempt to do something to prevent the formation of a toxic cloud, in the early 1990s, Mobil engineers settled on the additive Sulfolane to suppress the vapor pressure and move HF fluid properties into the subcooled regime, where flash atomization will not occur. But, as seen in the graph above, at least 45% Sulfolane by weight (45 wt%) is needed to achieve subcooled HF at a typical settler-tank temperature of 105F. Although they did not know it at the time, this level of Sulfolane is far higher than the alkylation process can tolerate and still function. In refineries that use MHF, the Sulfolane level is as low as 6 wt%. That’s 1 mole% or one molecule of Sulfolane for every 100 molecules of HF. Continue reading ““Superheated MHF Excluded from the Only Large-Scale Test Series” by Jim Eninger, Ph.D.”
Liquid flowing out of a pressurized tank will flash atomize if the liquid superheat (temperature difference above the boiling point) is large enough. Fthenakis claimed, “The critical superheat typically ranges from 5 to 15K [9 to 27°F] for many fluids of interest.”1 Flash atomization is the shattering of liquid jets into very small (often submicron) aerosol droplets due to the rapid vapor bubble growth of boiling. By contrast, subcooled (below the boiling point) liquid jets will still atomize when exiting an orifice, but then to droplets that are orders of magnitude larger, hundreds of microns in diameter.
“Activists hope they are edging nearer a long-sought ban on a highly-toxic chemical used at refineries in Torrance and Wilmington after the local pollution watchdog for the first time set a deadline — May 19 — for the industry to produce additional safety information about what’s called modified hydrofluoric acid.”
Click the image above to watch TRAA’s short four-minute video that refutes the notion shelter-in-place can protect students from an accidental release of highly toxic MHF from the Torrance or Valero refineries, the only two in California that use it.
“In a weeklong public safety awareness blitz, local activists are distributing 100,000 door hangers to South Bay homes surrounding the Torrance refinery, the same number they say could be affected by a catastrophic leak of a toxic chemical the plant uses to refine gasoline. . . .”