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.
From the question, it's apparent that both isohexane and methanol are available in the plant. However, both also appear to have some contaminants in them. No mention was made of the availability of acetone. There is a big advantage in using something already present. Safety data, permitting, training, and experience in handling the solvent are already in place. The steps outlined look reasonable but the specific details will count. Details such as temperatures and hold times and specific steps need to be worked into a true procedure for operations.
The real question that needs to be answered is which solvent works best for the process requirements and gets the equipment clean? A shorter time spent using the more effective solvent could easily give a safer operation than more time required when using a less effective solvent. Without knowing which solvent will work better, you can't come to a decision.
Andrew W. Sloley, principal engineer
CH2M HILL, Bellingham, Wash.
CONSIDER VARIOUS HAZARDS
Isohexane poses an unseen hazard. A typical MSDS shows it contains a mixture of branched butanes and pentanes, e.g., 2, 2 di-methyl butane. Butanes are considered more dangerous than propanes and natural gas because they tend to evaporate easily at room temperature and then recondense somewhere you don't want them to be. That's why the American Petroleum Institute's "Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2" (API-500) recognizes this exceptional hazard by labeling butanes as "highly volatile liquids."
Although API's "Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents" (API-RP-2003) and the National Fire Protection Association's "Recommended Practice on Static Electricity" (NFPA-77) don't provide convenient reference for comparing solvents, An article by Dr. Ebadat of Chilworth Technology (http://www.chilworthpacific.com/pdfs/Vahid_Ebadat_cv.pdf) shows clearly that acetone would be the best choice based on lower minimum ignition energy (MIE) alone: 0.3 micro-Joules (mJ) for hydrocarbons, 0.14 mJ for alcohols and 1.15 mJ for acetone. This is not the whole picture, though. The dangers of static electricity are measured by accumulated energy and voltage. A low MIE and high accumulated energy are the worst for risk of explosion and high voltage is a risk to personnel. Energy is directly proportional to accumulated voltage and the liquid dielectric constant. Methanol has a higher dielectric constant than either acetone or isohexane: 34 vs. 21 vs. 2 at about 77°F. In other words, with methanol you have the lowest threshold with the highest capacity for producing energy. Methanol is a bad choice; however, isohexane appears to have gained in stature because, for a given fluid velocity, the accumulated energy and volts are much lower than for acetone. The auto-ignition temperatures and flash points also favor acetone over isohexane.
Now, let's consider the cleaning method. Fish oil exposed to oxygen forms aldehydes and ketones, so purging and avoiding high temperature is crucial. Operating at a high temperature is a poor idea, so is vacuum. I would lower the temperature of the columns down to about 50°C and increase pressure to atmospheric. The KOH may saponify much above 7–8 pH. Before using the cleaning procedure it would be a good idea to test it in the laboratory.
Dirk Willard, senior process engineer
Middough Engineering, Holland, Ohio
We heat heavy oil with steam before sending it to a reactor. The oil, which is pumped through the shell of the heat exchanger, enters at 100°F and exits at 250°F; 125-psig steam goes through the tubes. The old exchanger had four shell passes and eight tube passes. Someone at corporate engineering mistakenly ordered a heat exchanger with three shell passes and six tube passes. The new shell is rated for 250 psig. The oil/steam overall external heat transfer coefficient is 100 BTU/lb-hr-°F. Is there anything we can do to use this unit so we don't have to order a new heat exchanger and delay production?
Send us your comments, suggestions or solutions for this question by April 15, 2011. We'll include as many of them as possible in the May 2011 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.
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