This Month’s Puzzler
We’ve been operating a soil remediation process for about three months. It takes in chloro-compounds like polychlorinated biphenyls and dioxins. The solids-handling part of the process works flawlessly but biological fouling problems with the solvent extraction circuit (see figure) began to arise about two weeks after start-up.
We take in soil from places a hundred miles away or so. There’s little quality control — we accept almost anything. However, our operators know to reject loads with logs, gravel, concrete and asphalt chunks. One truck delivered a stop sign! Our screw conveyors seem to handle what we take in.
We get our solvent, toluene, from pharmaceutical plants when they’re not burning it in their thermal oxidizer. Originally, we planned to source it from refineries but we can’t compete with the price the aviation industry pays.
We first noticed the problem in the vacuum systems drying the soil; then, it affected the flocculation in our settler and, finally, the extractor itself. I found a similar pond scum in our recycled solvent tank.
Other challenges for me are the rapid decrease in the quantity of recovered solvent as well as odor complaints from our neighbors in the industrial park. I think we may have to put in a scrubber system that we can feed back into our solvent recovery train.
Did we miss a treatment step? What could the residue be? What can be done about this?
Solve The Foulant Problem
You should take a three-pronged approach. Consider:
1. control of foulants at the source;
2. foulant control in transport and in processing steps in equipment; and
3. anti-foulant treatment (e.g., dispersants, defoamers, biocides).
Because soil supply and soil sources are not monitored, it is hard to pinpoint the origin or origins of foulants. Broadly put, there are three major sources of foulants: inorganic materials (e.g., insoluble calcium salts, silica or other insoluble salts), organic compounds (e.g., grease, oil, wax, high-molecular-weight organics), and bio-foulants (e.g., plants, algae, small animals).
See if it is contractually possible (and practical) for you to require suppliers to check soil samples and provide you with analyses or certificates of analysis. You may restrict fouled soil.
Look into the suppliers and the way soil is delivered to you. Transport trucks carrying soil could also leave oil and grease. You could require cleaner trucks.
Check your process equipment (for oil, grease and other sources) and solvents to make sure they are free of foulants.
Options 2 and 3 may not be possible. You will need to consider use of anti-foulants and treatments.
Many anti-foulants, dispersants and defoamers are available on the market. Discuss your specific application with appropriate vendors. Along with cost effectiveness, consider environmental and safety issues.
GC Shah, consultant
Watch Your Bottom Line
The process has several problems; some are manageable, some are not. I don’t even know if the process can be run economically, i.e., in the black.
First, there is the nature of toluene. When exposed to air in your process, the bugs that are present will gradually break down the toluene aerobically. The more oxygen that is present, the faster the bugs eat, leaving a sludge behind. The clue was the odor complaint: if toluene is getting out, air is getting in. Using pharmaceutical toluene also introduces additional nutrients that the aerobic bacteria will eat. It is nearly impossible to completely isolate a process like this one, so take steps to protect it from fouling.
You will want to look more closely at the filtration process. Consider adding a flocculant specific to bugs and a sock filter. Socks are good down to about 1 micron; they can be re-used and are fairly easy to maintain and troubleshoot. A more-expensive more-complex and less-reliable option would be a centrifuge.
Second, dioxins are C-Cl and C-Br compounds. These often are fatal to bugs, so the organisms will convert the compounds into ones they can easily release, like vapors. Unfortunately, those compounds are lethal to other organisms, like humans. If your state environmental protection agency hasn’t caught on to this, you’d better start working on a plan to manage it: you can’t hide the problem forever.
Which brings me to my third point: an odor scrubber. Putting in a vacuum scrubber is an excellent idea but one fraught with its own set of challenges. There are four options: 1) a catalytic thermal oxidizer (TOX); 2) a bleach scrubber (60% removal); 3) a bio-trickle filter (89%); or 4) an activated-carbon filter. The least troublesome method is a TOX. The biggest challenge is providing sufficient fuel for the reaction. You could use natural gas to provide enough fuel to offset the air drawn in from the extractor or you could burn some of the toluene used as the solvent. In addition, the C-halogens require high temperature, perhaps beyond what you can get from gas, and scrubbers themselves for the products. Activated carbon works very well for lean toluene streams but the material only can be regenerated a few dozen times — then a TOX is needed anyway. Bio-filters work very well but are more hands-on than a TOX; again, there is the C-halogen problem with a very toxic sludge and vapor. However, a TOX does contribute carbon dioxide to the atmosphere. All of these issues affect the bottom line.
Dirk Willard, consultant
Our steam stripper isn’t performing as designed. We are seeing 1,200 ppm of bottom contaminants instead of the 400 ppm expected. We anticipated foaming but it is much worse than foreseen, requiring us to cut back the tower rate. We tried raising the bottoms temperature and replaced our anti-foam pump with a larger one, increasing the addition rate.
The reduction in tower capacity, downstream purification of the bottoms and injection of anti-foam was killing our bottom line.
When we still couldn’t meet necessary production and quality, we replaced the upper half of the sieve trays with two beds of random packing, a high-performance glass-filled polyethylene type. (We couldn’t replace the sieve trays entirely because the temperature at the bottom of the tower exceeds the limit for the packing, i.e., 180°F.) We installed a distribution pan at the top of the packing and a single vapor distributor in the bottom bed.
For a week after addition of the packing, the tower almost ran within specifications and we were able to increase capacity by 50% over the initial rate. I think we solved our foaming problem with a 50% reduction in defoamer addition; now, the pump is oversized.
Did we miss a treatment step? What could the bottom contaminants be and what can we do about them?
Send us your comments, suggestions or solutions for this question by December 10, 2021. We’ll include as many of them as possible in the January 2022 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at [email protected] or mail to Process Puzzler, Chemical Processing, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.
And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.