This Month’s Puzzler
The filtration process for liquor leaving a treatment tank at my plant continues to plague us, despite a variety of changes made in the over a decade I’ve been there. It’s now of particular concern to me because I’ve worked my way up from operator to operations superintendent. We started out with a crude filter but went to a cartridge filter, then to a filter press and now to a sock filter (see Figure). Corporate ditched the cartridge filter because the dirty cartridges added to our landfill, and soured on the press because it broke down a lot. A large horizontal pump provided the high pressure we required; that pump hardly ever fouled, although we deadheaded it a lot. We relocated the pH probe from the pump discharge to the treatment tank to ease access for maintenance. The old agitator in the tank was a 45° axial impeller but was replaced with a high-efficiency impeller to increase up/down flow. Corporate also swapped out the old level transmitter — a dP bubbler with a third tube to adjust for density, whose tubes typically fouled at the bottom well before our annual outage — with a radar gauge. It is great except when we steam the tank between batches to help clean the tank; then, we get a lot of high alarms.
The sock filter is a slight improvement over the filter press but takes up a huge amount of real estate. In addition, it poses some operational issues. We must use a lift to get the cages out of the housing; removing the solids is messy and a fire hazard. Corporate had promised that we wouldn’t have to change socks as often as we actually must and that it would be easier than cleaning the filter press. My quality engineer blames the problem on poor agitation in the tank.
How should I approach corporate to reduce the downtime and improve quality control in the process?
Focus On Two Aspects
I see two issues: one with radar level sensor and the other with the sock filter’s maintenance (including safety).
Radar level sensor. The figure seems to indicate use of a “free gas” type device. This radar is sensitive to foaming, steaming and scaling. Steaming can cause erroneous high levels and noisy signals. Consider three options:
1. If this isn’t affecting safety or production, leave things alone — in the short run. Don’t deactivate the alarms, even though they may be a nuisance during steaming. (One risk you run into with the erroneous high level alarms during steaming is that a real low level can cause the inline pump to run dry and could damage seal and pose a potential fire hazard due to leak of flammable particulates.)
2. Install a stilling well, i.e., a suitably sized pipe that surrounds the level radar sensor and keeps steam out. Make sure there’s a very small hole near the top of this pipe for pressure equalization.
3. Change the frequency of radar signal so it’s less sensitive to steaming. Doing this also will change calibration, so talk to the vendor.
Sock filter maintenance issues. The more-frequent-than-expected sock changes could stem from a number of potential causes:
1. Improper pH adjustment (If pH range affects the solubility of reaction products, consider proper pH range — and pH alarms — where products are soluble. You also must assess impact of pH on downstream processing.);
2. An increase in flow rates to the reaction tank;
3. A process change in upstream equipment; and
4. The sock’s pore size differs from that used in the design.
In addition, check the size distribution of solids leaving the treatment tank to see if some solids can be strained or filtered before liquid goes to sock filter system.
Also, do some thinking about long-term environmental strategy. If you currently are handling socks as solid waste (hazardous or non-hazardous), see if the they can be reclaimed or recycled.
GC Shah, Senior Advisor
Get Rid Of The Solids
Engineers feel more comfortable zeroing in on one target when the real problem is the entire process. Consider breaking this up into: 1) agitation; 2) instrument changes; 3) filtration; and 4) design process.
There are reasons why these old 45° axial impellers are still around: shear. The high-efficiency impellers greatly improve liquid/liquid blending but at a sacrifice of shear. Generally, I design agitation with high shear at the bottom and strong blending at the top. For a tall tank, i.e., height/diameter > 2, consider two or perhaps three impellers. If viscosity is < 500 cP, a sparger could help to clean the tank bottom; however, if you must use nitrogen because of the fire hazard, the cost could be high.
Another thing to think about reducing product waste if you can incorporate solids into the solution: try running a side-stream through an inline mixer. There may be a few other ideas useful in dissolving whatever solids are being captured by filtration.
If the material being captured is undesirable, consider precipitation. Unfortunately, this adds to solid waste but could make a sock filter more practical.
Now, consider instruments. Moving the pH probe didn’t help matters, especially if you have fouling or poor mixing: move it back to the pump discharge and order a probe that is self-cleaning — such a probe doesn’t work that well but is better than nothing.
Changing out a bubbler for a radar transmitter seems a poor choice. I assume you’re using a still well to protect the pulse signal from the effects of waves in the tank. This well may pose a potential fouling problem. Also, whereas a bubbler with density compensation can identify true level in foam, radar doesn’t do so well: a light foam slows the pulse; a medium or heavy foam may make it completely unreliable. Remember this if you change the agitation.
Dirk Willard, consultant
We replaced corroded internals and did numerous repairs called for after an inspection of our distillation column. Among the projects, we swapped out pipes and cleaned the feed/product heat exchanger and the reboiler, and put in new control valves and a new feed flowmeter. We’ve now commissioned the unit but are unhappy with the results: product quality from the condensers is marginal; the temperature profile is “unusual” and we can’t increase rate through the tower as expected. I suspect the commissioning work either didn’t follow company guidelines or was poorly managed.
My sleuthing uncovered several issues: 1) pressure readings appear to be lower than expected; 2) the inspector found a number of trays disrupted; 3) that person also discovered several additional broken valves, which apparently has been an on-going problem for the last three years; 4) an inspection by the company responsible for relief valve maintenance detected severe fouling on the relief valve located on the top head — previously, we had cleaned the valves before sending them to the shop, so the inspector didn’t see their as-removed condition; 5) the temperature readings also seem out of whack; 6) the corporate engineer who was responsible reckons the new feed flowmeter is the culprit for the poor tower performance and claims he left its calibration to us. When I asked about hydro-testing and cleaning prior to turning the unit over for commissioning, the supervisor didn’t have any paper work to share; he sent me to the constructors, who said I’d have to get approval from the corporate project engineer. I can’t find much in the files. I talked to maintenance, hoping to find some commissioning reports, but only found a few case files of what they did in the past when they were in on commissioning.
What do you think went wrong? What can be done to get us back on track?
Send us your comments, suggestions or solutions for this question by August 7, 2020. We’ll include as many of them as possible in the September 2020 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.