THIS MONTH'S PUZZLER
At our batch specialty chemicals plant the production manager wants us to use waste isohexane as a cleaning solvent. He proposes pumping the isohexane, which is available from a nearby process, into our columns prior to cleaning for startup. The commissioning engineer opposes using it on the grounds that a safer cleaning solvent, such as spent methanol or acetone, could be available. He suggests using methanol because it's cleaner. We're cleaning two knockout pots and two distillation columns that have been used for several months in the production of fish oil (eicosapentaenoic acid and docosahexaenoic acid). The material safety data sheet for fish oil shows the following: closed-cup flash point, 149°C; auto-ignition point, unknown; and conditions to avoid, "oxygen." The columns each contain one 8-ft. bed of structured packing. Normally, the columns operate at a maximum of 100-torr with electric thermosiphon reboilers. The condensers use chilled glycol. (The cleaning process proposed by the production manager appears online at http://www.chemicalprocessing.com/articles/2011/vanquish-vexing-venting.html.) What do you think of the commissioning engineer's objection? Is there a better approach for cleaning the process?
SCRUTINIZE THE SOLVENT
Check the Kb [Kari-butanol] value and check the toxicity [for the solvent] before you go further.
Guy Weismantel, president
Weismantel International, Kingwood, Texas
Isohexane is very harmful to the environment and health. Methanol is also very toxic. So, I suggest using acetone for flushing as it is clean and less hazardous to the environment and health.
Wikipedia notes: "According to a report by the Cornucopia Institute, hexane is used to extract oil from grains as well as protein from soy, to such an extent that in 2007, grain processors were responsible for more than two-thirds of hexane emissions in the United States. The report also pointed out that the hexane can persist in the final food product created; in a sample of processed soy, the oil contained 10 ppm, the meal 21 ppm and the grits 14 ppm hexane. The adverse health effects seem specific to n-hexane; they are much reduced or absent for other isomers. Therefore, the food oil extraction industry, which relied heavily on hexane, has been considering switching to other solvents, including isohexane."
Some notes about acetone usage from Wikipedia: "Acetone has been studied extensively and is generally recognized to have low acute and chronic toxicity if ingested and/or inhaled. Inhalation of high concentrations (around 9,200 ppm) in the air caused irritation of the throat in humans in as little as 5 min. Inhalation of concentrations of 1,000 ppm caused irritation of the eye and throat in less than 1 hr; however, inhalation 500 ppm of acetone in the air caused no symptoms of irritation in humans even after 2 hr of exposure. Acetone is not currently regarded as a carcinogen, a mutagenic chemical or a concern for chronic neurotoxicity effects. Acetone can be found as an ingredient in a variety of consumer products ranging from cosmetics to processed and unprocessed foods. Acetone has been rated as a GRAS (Generally Recognized as Safe) substance when present in beverages, baked goods, desserts, and preserves at concentrations ranging from 5 to 8 mg/L. Additionally, a joint U.S-European study found that acetone's 'health hazards are slight.'"
Amr Hatem Rashed, production engineer
Abu Qir Fertilizers & Chemical Industries, Alexandria, Egypt
WHICH SOLVENT WORKS BEST?
For safety, the case that methanol or acetone is "safer" is weak. Commonly accepted values for flash point, lower explosion limit and upper explosion limit are: isohexane, -9°F, 1.0 v%, 7.4 v%; acetone, -4°F, 2.5 v%, 12.8 v%; methanol, 54°F, 6.0 v%, 36 v%. All are heavier than air and vapor clouds will tend to travel along the ground (though methanol is just a little bit heavier). All will be used at temperatures far in excess of the flash point. Methanol has a higher flash point and higher LEL but a much wider range for potential explosive mixtures. Isohexane has the lowest flash point and the lowest LEL but also the narrowest explosive range. Acetone lies in between on all of the criteria here. Toxicity and exposure data are also mixed. It's difficult to say which is the safer solvent here.