“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 visible, 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.
A second example of boil-off is the primary focus of this post as well as of item III from the post Five Points TRAA Science Advisory Panel Members Would Have Liked to Make at the Rule 1410 Refinery Committee Meeting. Except for a 15 wt% sulfolane additive used for transportation, which increases the boiling point of HF modestly from 67F to 76F, MHF trucks have none of the other refinery-based mitigation systems. With an air temperature of 87F, highway surfaces can reach 143F. The stored solar heat in a hot highway will vaporize MHF. In addition, the direct heat from the sun over the duration of the spill will add significantly to the boil-off.
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.
The classical mathematical transient heat-conduction solution predicts an initial infinite heat transfer to the MHF, where, in reality, a short period of film boiling will transition into the nucleate boiling regime. The rate of boiling falls off as temperature is conducted from deeper into the pavement, as shown in Figure 2 (blue curve). The cumulative amount of hydrofluoric acid vaporized is shown by the red curve.
Consider, for example, a realistic hypothetical scenario where it takes emergency responders two hours to reach the site of a massive MHF spill and complete the hazardous task of neutralizing it. By that time, Figure 2 shows that 5 lbs of hydrofluoric acid would be vaporized by boil-off for every square foot of pavement. For a characteristic pavement area, consider a square with sides equal in length to four standard highway lane widths of 12 feet, which has a surface area of 2304 square feet (4 × 12 feet)². For this case, 11,520 lbs of hydrofluoric acid would be boiled off.
In addition, direct solar heating of the spill will contribute significantly to the boil-off. In the example above, direct solar heating at a rate of 1 kW/m² will alone boil off 2.2.lbs/hr per square foot or 10,000 lbs in two hours, which brings the total with conduction from the pavement to 21,520 lbs of MHF vaporized.
This is an alarming amount that would be blown by a breeze into a roadside neighborhood. For comparison, this is more than twice the 8,300 lbs released in the 1986 Goldfish test in the Nevada dessert.
And, this doesn’t include HF evaporation from the surface of the spill, which will happen even without an elevated pavement temperature. The drop in the evaporating MHF temperature towards wet-bulb pulls heat out of even ambient-temperature pavement and enhances the heat transfer from elevated-temperature pavement. This will be the subject of another blog post. For a preview, watch this experiment on the thermodynamics of evaporation of a highly volatile liquid like MHF.
Notes about the Mathematical Model and the Solution
Two-dimensional transient heat conduction in a slab is one of the prime examples used in college-level teaching of partial differential equations. The curves presented in Figures 1 & 2 were calculated on an Excel spreadsheet from solutions presented in the lecture notes for the MIT undergraduate class “Linear Partial Differential Equations” taught by Dr. Matthew Hancock.
The Pavement parameters used were:
- Thickness: 11 inches
from USGS Materials in Use in U.S. Interstate Highways
- Density: 150 lbm/ft³
Thermal Conductivity: 1.25 Btu/hr-ft-°F
Heat Capacity: 0.28 Btu/lbm-°F
from CalTrans Highway Design Manual Chapter 620 – Table 622.1
- Hydrogen Fluoride Heat of Vaporization: 146 Btu/lbm
from Solvay Anhydrous Hydrogen Fluoride Technical Data Sheet