Superheated MHF Excluded from the Only Large-Scale Test Series

MHF-VaporPressureMap-2
MHF Vapor Pressure vs Temperature at various levels of Sulfolane. Vapor-pressure curves by Dr. George Harpole are anchored to data from U.S. Patent 5,654,251. Other publicly available data from Material Safety Data Sheets (MSDS) and European Patent EP0796657B1 are shown on the graph. (Click graph for full view.)

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.


Even without flash atomization, gasdynamics will break up the pressure-driven, high-velocity HF stream into small droplets, and much of the droplets will evaporate into a toxic vapor before they hit the ground. To experimentally evaluate this process, in 1993 Quest Consultants were contracted by Mobil to perform a large-scale experimental evaluation in Quest’s Oklahoma facility. We learned from Quest’s Principal Investigator, John Cornwell, in his excellent presentation at the September 22, 2018 AQMD Refinery Committee Meeting (Watch: https://youtu.be/qwo08BtEQuM?t=7618) that these were the only large-scale tests conducted with MHF, and Mobil engineers selected all the test conditions. In their 1993 paper “Large-Scale Release Testing of a Modified HF Alkylation Catalyst,” Mobil engineers redacted the Sulfolane concentration levels, but they are easy to figure out by comparing the test summary to U.S. Patent 528,456. The test conditions are shown by black dots in the graph below. As could be anticipated, none of the tests were with superheated MHF. Today’s 6 wt% MHF is shown on the graph with 34oF of superheat — far more than the minimum for flash atomization.

Bottom line: There are no large-scale tests with the low-Sulfolane-level, superheated MHF used in today’s refineries to support the refineries’ false claim that MHF will not flash atomize. In fact, today’s low-Sulfolane-level MHF behaves just like HF.

MHF1993ReleaseTests
The black dots show the conditions for 23 tests in Quest’s 1993 large-scale MHF tests. Some dots represent several tests at the same conditions. Three tests at an unknown concentration greater than 50 wt% Sulfolane are not shown. None of the tests are with superheated MHF. Today’s MHF at 6 wt% and 105F is shown with 34oF of superheat, more than enough to flash atomize. (Click graph for full view.)