Sulphuric Acid Dilution, strength change over time

[rkrj]

Hiya All

I got a sneaky problem with an inline mixer, mixing 98% sulphuric acid with water, the finished product is 25% sulphuric acid.

The problem is the following: We monitor the temperature and the conductivity of the mix, and they are both stable for a period of time, usually 2-6 days, after which both the temperature and the conductivity start to drift, both rising slightly. When this has been going on for some time, we stop the inline mixer, and flush it with water, and then we can start over with 2-6 days of stable production, after which a new drift start to occur.

As far as I can see, the fact that a flush with water can reset the conditions, means that we got a chemical problem, because a mechanical problem wouldn't be affected by water (a crack, a tiny hole or something).
I just can't see what chemistry is actually involved in this, because I wouldn't think that any solids should be formed in acid as strong as this, neither should valves etc behave like this, with a slow, increasing drift after a period of stable production.

Have any of you experienced something similar, or do you have an idea how I could find the cause of these problems?

 

[kenvlach]

How is the concentrated sulfuric stored? It absorbs moisture from air (can be used as a desiccant).

Another idea is that a layer of sulfate builds up inside the piping prior to or in the mixing chamber. Then, at a certain thickness for the given turbulence, it spalls away, starting a fresh cycle.

More info on materials & conditions needed for more accurate guesses.

 

[rkrj]

The concentrated sulfuric is stored in a 2 m3 steel tank.
The diluted sulfuric acid is stored in a 1 m3 steel tank.

The mixing chamber is made of PTFE, and is basically a T-pipe with purified water and concentrated sulfuric going in, and diluted sulfuric going out. All other piping is steel as well.

Two pumps run the water and sulfuric, I'm not sure of the pressure they apply, but I know that the contra-valve on the sulfuric pump is set to 1,6 bar, after previously being 0,8 bar, but this hasn't solved anything.

If you need more information, please let me know.

 

[ApC2Kp]

rkrj,

The dilution of strong 98% sulfuric acid down to 25% acid must have a lot of heat evolved. I would have expected a cooler to have been included. Hot 25% acid would be extremely corrosive to carbon steel and stainless steel. Typical materials for 25% sulfuric acid would be plastics, fiberglass, graphite coolers, ceramic/ brick or glass. See the NORFALCO site for dilution charts and other information,

http://www.norfalco.com/documents/Norfalco_H2SO4TechBrochure.pdf

Strong 98% sulfuric acid is stored in carbon steel tanks at ambient temperature. Carbon steel piping is used with a strict limit of .75 m/sec (2 ft/sec) to keep passivation film of sulfates on the pipe I.D. Your carbon steel pipe could be running at low enough velocity, except for the more aggressive corrosion of 25% acid. What corrosion rate have your tanks and piping experienced? Does the 25% acid continue to circulate after the tanks are filled?

The drift you have observed might be from corrosion in tanks, or the breathing of tanks (night time cold air in) bringing humidity, as suggested by kenlach. Another possibility could be a small leak-thru at either the 98% acid valve, or at the water valve. For positive shutoff of acid and of water flow the control valves would be configured in a double block and bleed, so that any leak-thru at valves would be detected. The increase of temperature with the drift of concentration would lead to suspect the leak-thru of more 98% acid feeding into mixer. You would want to use a Teflon lined valve and check valve where the acid could see water backflow from mixer.

The incomplete mixing of 98% acid could result in the more dense acid sinking to the bottom of tanks, and lighter 25% acid (or even weaker acid) sitting at the top of tank. A band of wall thinning could occur at the air-liquid face.

Corrosion of carbon steel by sulphuic acid will release hydrogen gas, so be careful working around the tank vents.

 

[ardilesd]

rkrj,
the drift you mentioned in temperature & cond. refers to some on-line measurement in the outlet pipe, during the mixing operation?
Or they are tank conditions ?
Thanks

 

[rkrj]

apc2kp> You are correct, we have a cooler between the mixer and the tank, cooling the mixture from 58-60°C to 13°C using a brine solution. First we got the two pumps, then the t-junction, a pulsation damper, the cooler and then the tank. We got a nitrogen valve on the tank to apply a pressure, in order to supply the costumers inside the factory.

The temperature and conductivity is monitored just before cooling, and from experience we know that the temperature should be in the range specified.

We spot the drift as both temperature and conductivity rising slowly, which indicates a surplus of acid.

The whole factory is relative new, build in 2003, and we haven't as such experienced any corrosion. From the repair-logs dating back to 2003, I can't find any corrosion-related repair. The most common repairs are new nitrogen valves on the tank and replacing the springs in the valves, as they break quite easily.

I had a chat with our former technician, and he think the drift is due to a pressure buildup in the system, that causes the water pump to choke, and not deliver the amount of water needed. I find his theory feasible, but this really doesn't solve the problem, I still need to find the bottleneck.

 

[ApC2Kp]

rkrj,

The 25% acid storage temperature explains why there is little corrosion in the tanks or piping. If the ambient temperature keeps the 25% acid that cold, and the pumps don't have to circulate the acid, then it is understood. The nitrogen pressure is used for pushing the 25% acid out to the supply header to users. The pumps are probably diaphragm positive displacement type that need the pulsation dampener on discharge piping.

If the storage tanks slowly warm up to say 25 to 30 deg C, then the conductivity might be increasing with the temperature. May need to get a chart of conductitvity versus temperature. What is the temperature of the tanks when a wash of the mixer is performed? It could be that fresh stream of cold 13 deg C acid also cools down the storage tanks enough for several days of stable operations. Does the inlet water and the 98% acid get to the mixer with a uniform temperature, as cold as 13 deg C.? It might be good to cross check the concentration / conductivity numbers with a density check for concentration verification, when it is suspected to be at higher concentration.

 

[hacksaw]

pardon me, but it is not clear from the discussion whether or not you are circulating continuously (but with the dilution water shut off, if your conductivity measurement is temperature compensated to a reference temperature or represents the flowing conductivity, etc.

If you are seeing a temperature rise, you need to check for pump heating effects, otherwise you are getting acid (or water) into the system that you have not accounted for.

 

[ApC2Kp]

rkrj,

After some more thought it seems you have a system that makes 25% sulfuric acid by metering 98% sulfuric acid and dilution water into a mixing tee, cooling the 25% acid and taking it to a surge tank pressurized with nitrogen, then delivering the 25% acid to different users through a header pipe running to the plant.

The 98% acid metering pump and the dilution water metering pump might actually be run by a common drive shaft. If so, the dilution ratio would be fixed for various delivery rates. I could see that the metering pumps could be sized for 30-40% duty depending on the user demands versus the surge tank volume of 1 m3. The pumps could run continuous during the day, and be off during the night with no batch operations requiring 25% acid, perhaps.

The high temperature after the acid dilution could result in deposits of minerals from water, depositing in the mix tee and on the conductivity probe. The solids have a reverse solubility with temperature. A similar problem occurs within the acid coolers in a sulfuric acid plant when the water treatment does not limit the the sodium, calcium carbonates, etc. The acid coolers will have silicates foul the exchanger tubes, and require chemical cleaning. The best remedy is to use dilution water that is boiler feed quality, without the boiler corrosion control chemical additives. Check your water chemistry, and perhaps the sodium needs controlling to less than 50 ppm?

 

[rkrj]

First, I would like to thank all of you for valuable ideas to address the problem, it has given me a much better understanding of the whole system.

However, I think we've got the system in control now, as we haven't needed to flush the system for more than 7 days now. We discovered that the bellows in the pulsation damper was broken, and after this was changed, the system has been running smoothly.

When I think of the mechanics of the pulsation damper, I think it could give the problems we have encountered, so I'm crossing my fingers that this was the main reason for the problems we had.