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?
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