Caustic Tank Vent Issue

Hi All,

I work in a 300 m3/hr ion exchange demin plant which uses HCl 36% and NaOH 47% for resin regeneration, both of these chemicals are stored in 15m3 storage tanks. These are atmospheric thermoplastic tanks which are more or less identical, the main difference being that the HCl tank is vented to a fume scrubber while the caustc tank is vented to atmosphere. Both are topped up via road tanker, HCl is taken off by delivery pump and caustic is delivered by compressed air at approx 2 bar.

One issue we have with the caustic tank is that now and again we get some liquid escaping the vent which then crystallizes on the roof and down the size of the tank. It's definitely not overfilling as we have a high level sensor to shut the delivery valve at 95% and an overflow pipe just above this. It's not a frequent occurence so I've never actually seen at what stage of the delivery this happens but I suspect it's at the end when the barrel is depressuarizing through the tank and out the vent?

Last year we tried to solve this by extending the vent using a 3 meter spool piece. This didn't solve the problem and instead just made it more hazardous, raining droplets over a wider area. Few pictures below...

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The tank has a 2" delivery line entering the top on the tank (splash filling), 4" overflow pipe and 6" vent with cowl.

Looking for some advice on how best to avoid this or at least contain the carryover?

One thought is to make the vent the same as the overflow, having it come down close to ground level where I could have a dustbin and rags to catch any droplets, not solving the problem but at least containing it and not having to decontaminate the tank and bund every year. Others have suggested baffles in the vent or a coarse mesh to knock out any carryover, but to me this just risks blocking the vent and potentially causing the tank to overpressuarize during delivery.

Edited by -G-, 04 June 2019 - 08:08 AM.

Hi ,

I've been using caustic soda up to 48% and the transfer was always by centrifugal pump combined with a  dip tube inside the tank and goose neck for the vent . Because of the weather condition , the lines and storages were traced . 

If your vent get cristallized you have a huge risk to damage your tank during the transfer to users .

My experience with this material .

Attached documents to support your work .

Breizh 

Edited by breizh, 04 June 2019 - 08:52 AM.

Hi breizh,

Thanks for your response. I'm not sure why the plant was designed to pump off only the acid, I thought maybe because a caustic pump would crust up after use.

A dip pipe could be benefitial, I did read this on a website I came across...

Tank inlet should be above the highest anticipated tank level with an inlet dip pipe, which extends below the lowest anticipated tank level to prevent splashing of the caustic soda solution.

Though at the end of delivery there would probably be a lot of bubbling, whether that would be an issue I'm not sure.

We plan on removing the vent extension and cowl next month to clean, like you say you wouldn't want this to block or become seriosly restricted!  

Can I clarify what you mean by "caustic is delivered by compressed air at approx 2 bar."

How does the compressed air "deliver" the caustic from the road tanker to the tank?

Where does the centrifugal pump come in? How is the compressed air of 2bar used to make an electric centrifugal pump deliver caustic from a road tanker to the tank?

What am I not understanding correctly here?

Edited by thorium90, 04 June 2019 - 09:24 AM.

The road tanker has an onboard compressor which pressurizes it's barrel to 2 bar, a delivery hose is connected between the tanker and our delivery point and the caustic solution is simply blown through the delivery line and into our storage tank by the compressed air. The method is described on page 15 of the 'caustic soda handbook' attached by breizh.

It was breizh who mentioned the use of a centrifugal pump in his setup, the plant I work in uses the compressed air delivery method described. Hope that clears things up

I see. Thanks for the clarification.

I normally use sealless magnetic drive pumps for these chemical transfer applications.

You can google around these magnetic drive pumps. There are quite a number of vendors providing these pumps.

It does seem like because air is blown into the tank interior and because the tank interior is air saturated with NaOH, when the air is blown out the top, some of it condenses on the outside.

Normally with pump fill, the amount of displaced air is just the filled volume. But using this "blowing" method probably increases the displaced air by a significant amount, probably causing the condensing and subsequent crystallization of NaOH problem that you are experiencing.

I would think the pump method would reduce the displaced air volume significantly.

If it is still a problem, you can add a fume scrubber using water for scrubbing purposes and direct the vent into the bottom of the scrubber. Then turn on the water for the scrubber during topup events. This should remove the NaOH from the vented air during topup.

Reduce the air pressure to 1 bar_g and see if the problem goes away.  As long as the off load takes less than 1 hour you should not get a complaint from the vendor/truck driver.

We limit our latex trucks to 15 psig and we can off load those in less than 1 hour.  Unless your piping is smaller than our 3" you should have no problem.

Edited by latexman, 05 June 2019 - 07:58 AM.

Greg:

I take this opportunity to state my complete agreement with what Latexman is recommending.  His technique uses common engineering sense and is backed up by empirical field experience.  I urge you to heed what he states.

I also take license to make some comments of my own on this type of unloading operation.  Breizh is another experienced field engineer who is a great source of process information and know-how.  The information on Caustic handling from Dow Chemical is invaluable and you should study it carefully.  Sodium Hydroxide - a.k.a. as “Caustic soda” - should never be taken lightly, especially in transfer operations.

Around 25 years ago, I was handed an assignment to revamp a Phenol transfer operation using pressurized nitrogen as the driving force to push liquid Phenol from a rail tank car to a storage tank.  Several “close call” incidents had happened with the operation in the past and management did not want to take any more chances with a serious accident.  Attached find a copy of some of my work on the project and how I resolved the problem.

I employed the help of the Endress+Hauser company in using an instrument (time has worn my memory, but I think it was a capacitance switch) to detect the presence of the nitrogen gas in the liquid transfer line, thereby giving us the ability to activate a shutdown valve in the transfer line and arresting the compressed nitrogen before it arrived at the storage tank and caused the potential problem of Phenol entrainment to atmosphere as well as a storage tank over-pressure.  Residual liquid Phenol in the line was subsequently hand throttled to the storage tank manually.

The system worked without any problems and as far as I know is probably still in operation - without any tank entrainment, over-pressure or any other problem.  This application is very similar to what you have described - with the exception of the fluid involved.  I have always taken these operations very seriously - and I presume that you do also.  The ramifications of a caustic spillage or a spray or leak on human tissue is a catastrophic event and one I would prefer not to imagine.  Caustic can inflict severe and deadly effects on human tissue and a stray stream exiting a storage tank should not - in my opinion - be a tolerated or accepted event.  Your tank is a relatively small one and the amount of entrainment due to the higher-than-usual vapor velocity inside is a potential problem with pneumatic liquid transfer.  This is one reason why Latexman’s lower pressure recommendation makes a lot of sense.   The cost advantage of a compressed air transfer system over a pump has a draw back ("trade off") in that although the system is simple and low cost, the consequences of compressed air entrance into the target tank and its handling requires additional instrumentation and precautions.

You should strive to employ a lower pressure in the transfer air as well as a larger transfer line to lower the fluid velocities entering your relatively small tank.  A means to safely detect the transfer air within the transfer line and the ability to automatically block the flow is another safety design.  The drawing on Dow Chemical’s Handbook, page 14 is one you should focus on.

 Phenol_tank Rev3.xlsx   53.73KB   69 downloads

thorium90 - I'm not sure why our caustic delivery system was made different to the acid, I'm sure a pump would do away with the issue but hopefully can be avoided with a simpler solution.

latexman - We've had some delivery drivers say that they can't regulate the pressure below 2 bar but I get the feeling this might be BS, as you can imagine they're pretty keen to get offloaded and away asap. Some of the more experienced drivers will knock the compressor off half way through and allow the pressure to drop off for the rest of the delivery. I think I'll get in touch with the transport company and ask them directly about pressure regulation of their tankers.

Art Montemayor - Thanks for your input, sounds like the instrument you describe has the same duty as the one we use in our acid delivery line, it's a sensor that works on vibration which changes with the absence/presence of liquid, this is set up to stop our acid offloading pump and close the delivery valve when the line dries up at the end of delivery. I’m sure this same setup could be applied to our caustic delivery line at relatively low cost. That in combination with the reduced delivery pressure would hopefully do the trick!

The dangers of caustic have become very apparent in recent times, an employee from a different company who works in the same complex got a splash of 47% caustic to the eye. I think he was attending to a caustic bleed valve which was blocked and suddenly cleared, causing caustic to splash up from the ground. He had Diphoterine eyewash administered shortly after but unfortunately wasn't enough to avoid permanent damage to his eyesight, a life changing injury.

Hi,

To me Caustic soda is one of the nastiest products.  I don't see how you address the risk of having your vent line clogged,with its consequences.  Yes, you can reduce the pressure of the compressed air but this is not going to prevent someone to increase it one day !

You should consider to redo the Hazop of your operation, money should not be the driver.

Note : I've seen people burnt with caustic, phenol , I have also seen people modifying the set point on air pressure regulator to speed up the process of transfer , it was in a latex plant !

my view

Breizh

Edited by breizh, 06 June 2019 - 11:46 PM.

Hi breizh,

We have our annual shutdown next month and will be taking the vent off for inspection and cleaning, this will be one year since it was put on new so it's an opportunity to see how much has crystallized during that time. This can be done every year at a minimum to avoid blockage.

You're quite right, all it takes is for one driver to go against instruction and ramp up the pressure, that's why I'm thinking it may be a good idea to swan neck the vent down close to ground level, that way if there is a release it's going to the bund floor and not being released from height? From sodiumhydroxidetank.com...

"Tanks should be vented to atmosphere and the vent line routed to within 3-feet (0.9 meter) of grade"

Hi G, 

I am curious to know if you were able to solve the problem and if so what has been done to remediate the problem.

We have an operating plant that is experiencing something very similar.

I am working on a new plant where the caustic tank is located inside a building with a dike for overflow protection. We will be piping the vent to slightly above ground level, outside the building. However, I am curious to know if you are experiencing the same liquid splashing through the overflow line after the air purging. Since both the overflow and the vent are exposed to the same pressure, i expect to also see some liquid go through the overflow.

To overcome the problem, I am planning on having the overflow pipe have its intake close to the bottom of the tank, so as to impose an additional backpressure for air. The air would take the path of least resistance, ie. the vent. 

Finally, does anyone know if it is required to have an inlet dip pipe that goes to the bottom of the tank? The DOW handbook as well as the oxychem one do not seem to mention a recommendation. I have only found 1 single reference that recommends piping it to the bottom of the tank to prevent splashing.