We neutralize a waste product with calcium carbonate prior to processing for disposal. The neutralization set-up consists of a weigh-feeder, twin-impeller agitator in a pressure vessel, recirculation pump, heat exchanger (cooler) and centrifugal exhaust fan. The weigh-feeder adds the carbonate to the baffled agitated tank. The reaction is complete when the spike in solution temperature dissipates and the volumes of hydrogen gas decrease. The process poses several problems: the slurry butterfly feed valve normally operates at 30% open but was designed to operate at 50%, leading to severe erosion; the pump often fails because of either gas blockage or suction fouling; the reaction frequently is incomplete, causing safety problems from the erupting hydrogen (the agitator is designed for about 0.5 hp/1,000 gallons with a impeller-diameter/tank (D/T) ratio of only about 0.4); the heat exchanger isn't performing well, although it checked out during commissioning. What can we do to make the process run smoothly? How might we limp by until the next available outage? Can you explain possible causes of the equipment failures?
MAKE EQUIPMENT CHANGES
Let's start with two easy-fix items.
Get a smaller butterfly valve or, better yet, a diaphragm valve suitable for erosive, clumpy calcium carbonate. (The carbonate likely is clumpy because of the water converted to vapor from the waste solution.) A sifter might reduce clumping.
Also, improve the removal of the hydrogen safely. If gas is erupting, the rupture discs typically usually used to protect such tanks may suffer spider-web failure, which comes from operating too close to the rupture point. You may want to over-size the exhaust fan system to prevent hydrogen from accumulating overhead.
Now, let's consider one of the key problems for this reaction: poor agitation. This leads to the exothermic reaction occurring in corners near the baffles where heat can accumulate. Such tanks usually are lined — and the liner likely has failed or soon will because the heat of reaction is poorly dissipated by the agitation. Horsepower/gallon is a poor measure of agitation, particularly for slurries. The cited 0.5 hp/1,000 gallons is 1/20th of what it should be for a slurry reaction. So, one solution is a bigger agitator with more shear. Increase the D/T ratio, trim the impellers you have or install new ones; check to make sure flow is downward and don't sacrifice too much pumping capacity so that the solids are suspended.
This brings me to my next point: solid suspension. Clumped solids may be settling out and burning holes in the liner.
What long-term fixes could improve reliability? Replace the shell-and-tube heat exchanger with a U-tube, which can handle slurry better. Put in a different agitator. Check the baffles to ensure no hot spots are being created in the dead spaces. Raise the pump suction and install a vented duplex strainer. Consider an axial pump such as those used in crystallizers; these can handle slurry more easily.
As for limping through until the next outage, try a nitrogen sparger, if the exhaust system can handle the extra load. It's expensive but may keep the clumps from fouling the pump and will help disperse the hydrogen.
Dirk Willard, senior process engineer
Middough Consultants, Holland, Ohio
Every 3–8 months we burn through an O2 serpentine coil made of chromium-lean nickel alloy. Data show the maximum O2 temperature exceeds 1,000°C, the maximum allowable by the alloy mill, for almost an hour on average during startups and shutdowns. Engineers in the past have advised us to change material or switch to a helical coil to address the problem. Our new engineering manager thinks that better control might extend the life of the coil. Currently, we use a cascade control system: a temperature controller handles fuel while a slave flow control loop regulates combustion air to maintain a 14% excess air ratio. Could better tuning or changes in the control system do the trick? Are there any other ideas worth exploring? Do we know enough about the root cause?
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