96% H2SO4 Tank storage - Nitrogen purge or Dried Air - vent dryer ? 4

[mrtangent]

Dear Sirs,

I would appreciate your feed back on the following.

We have a large hundred + tone tank which stores concentrated cold H2SO4. It is run under atmospheric conditions - atmospheric vent. We use nitrogen to purge the tank. At present we are pushing to drop our site nitrogen usage. One thing we are considering is using a

Vent dryer instead of the nitrogen to the tank. I would appreciate anyone’s thoughts comments if this is practical, would be uneconomic (ie it would cost us so much to install a dryer v N2 saving etc)

Has anyone used a vent dryer are they reliable ?

Does it need focus to get it to be reliable ?
Do they have operational risks ?
How often have you used them ?
How expert are the suppliers?

 

[BenThayer]

There are some design details lacking with regards to your post and the nitrogen usage.

Have you identified the amount you are using at this tank?

Do you just have a valve partially open and an open goose neck on the top of the tank?

Do you meter the nitrogen through a rotameter?

Do you have a pad/de-pad regulator system with a conservation breather vent relief device to protect the tank?

Is the tank mild carbon steel where moisture will increase the corrosion of the tank?

I am not a fan of using plant air and local dessicants for this type of tankage. If you are using low pressure regulators where the nitrogen only enters if the tank level is dropping and the nitrogen exits only when filling the tank, there should be practically no usage at this point.

The design should dictate if you need improvements, not a policy decision by someone that does not know what the nitrogen is providing.

If you get significant moisture in the tank and have corrosion problems and the tank fails, they will likely have a push at the site to reduce acid spills.

 

[Montemayor]

James:

Here are my responses--
1) A vent dryer can be reliable, depending on the amount of maintainence and upkeep that it receives; you don't describe what you mean by a "vent dryer" (it can mean several things) so I'll elaborate: your vent dryer is nothing more than a static bed of adsorbent (such as silica gel, activated alumina, etc.; it can also be made of a hydroscoptic substance -like a salt- that captures atmospheric water by hydration and drains the aqueous salty residue away). The drying substance (adsorbent or hydroscopic salt) doesn't regenerate by itself; it requires manual replacement on a timely and constant basis. This is a pain in the nalgas, as I've heard it described in Arizona. You have to furnish manual servicing and drying media to it constantly - and the quantity depends on the variations of atmospheric humidity where you are located. Additionally, I should warn you that the unit only "drys" the atmospheric air it comes in contact with -that which is the breathing air quantity that enters and exits due to thermal or filling effects in the tank(s). But the sad news is that the amount of "drying" it does can never compete with cryogen-sourced nitrogen. The vent dryer will dry to a very inefficient level as compared with any other type of dryer - so comparing it with nitrogen blanketing (not purging) is not an engineering logical alternative. There will be water vapor getting through the vent dryer and into the tank's vapor space allowing the Sulfuric Acid to slowly do its work in capturing it.

2) I don't have the slightest idea what you mean by "focus"; it doesn't work like a camera;

3) The only operational risks I have seen in the field are lack of operability due to little or no maintenance and subsequent severe corrosion from aqueous salt drainage due to using hydroscopic salt. This latter effect can be a mess.

4) I've never "used" them; I've found them in use (or rather, dis-use) where I've gone; I've stayed around long enough to see them eventually replaced - with a formal, engineered nitrogen blanketing system.

5) The suppliers don't claim to be experts; they'll sell you what you want. You have to specify to them what you want and they'll build, fabricate and furnish what you want. Actually, all it is is a static cannister of adsorbent sitting on a tank vent nozzle. If you correctly engineer your application based on the normal breathing and displacement needs (as well as the tolerable water content) you will, I believe, discover that the static cannister will be very large and almost impossible to safely maintain at an elevated height. This will force you to make a logical engineering decision and locate it at grade with large vapor piping, structural steel, ladders, platform, lights, OSHA handrails, etc. (see where I'm going?).

Unless you have the facilities to engineer it yourself and regenerate the adsorbent on an intermittant basis, I wouldn't recommend this system as a reliable, operative one.

Now, let's go back to something you wrote: "We use nitrogen to purge the tank." I wouldn't "purge" a Sulfuric Acid tank; I'd employ a constant, steady Nitrogen blanket on top of the fluid. That way, the only N2 consumption that I have to tolerate is the inventory displacement, breathing effects, and N2 leaks (if I allow them). If I balance my Nitrogen blanket between two or several tanks by filling into one while pumping out of another -while they're both connected to the same blanket header - I can reduce some of the displacement and breathing effects. I would not continuously run Nitrogen through the tanks and allow it to exit to atmosphere (called "purging"). I want to make perfectly sure that we understand the terms used to describe a system - otherwise, we'll find ourselves going around in circles trying to understand each other.

I hope this experience helps you out.


Art Montemayor
Spring, TX

 

[hacksaw]

we've used nitrogen and instrument (dried air) for this purpose. they both work; it all depends on your precise requirements. agree with above comments, you need to engineer a solution, not just purchase one on the basis of cost.

 

[rmw]

Art,

And, by the way, nice to have you back on board. You must have found a computer out there in Arizona.

I went back and re-read his post carefully, and by focus, I think he means attention, or care, or maintenance, which you already covered in your answer. In other words, I think he is asking is it a pain in the nalgas.

rmw

 

[mrtangent]

Thank you for your responces. I can see some strong views.

Montemayor when I mean focus - I've often inherited lots of half finished projects. The solution been found, implemented but not maintained. The initator had'nt planned that with any new installation you need to monitor and ensure that it fully works.

The nitrogen header idea is one I will look for applicatons.

Many thanks

Jim

 

[Montemayor]

rmw:
Thanks for the welcome; it's nice to join in with you guys whenever I can - even out here in the Sahuaro National Forest area. However, it hasn't been easy the last 3 weeks - the site hasn't been available for entry many times and I'm working with my son-in-law's Mac model while my granddaughters are in school. We're hitting the trail back to Texas with the girls as soon as school's out and we can saddle up the horses.

James:
I believe you've indicated you're already piped up to employ an N2 blanket; however, I'd caution you as one Chem Eng to another: wrap yourself together with a sharp Mech Eng and make sure that your H2SO4 tank(s) are mechanically and structurally up to an engineered blanket.

Depending on the amount of vapor pressure you need to impose on the vapor space, you may need reinforcement and anchoring. Hopefully, this has already been done - this is an area of mechanical expertise where you can profit a lot and you should get very focused with your M.E. and fully overlap on all the process-mechanical features of a blanketing system. It is simple in concept - but it must be SAFE, especially since you're dealing with anhydrous sulfuric. The more you understand about the mechanical workings and underpinnings of a tank, the better you will understand and dominate the safety requirements and proper operations. I have prepared several Excel workbooks dealing with this application and they are intended to serve as a teaching tool in applying safe pressure/vacuum relief to blanketted tanks and they walk you through the calculations and the documentation portions. Do not forget about the detailed documentation and MOC procedures that are required by OSHA. If you are interested, feed me your email address and I'll send you a WinZip file when I get back to Texas in 3 weeks.
Have a good day.

Art Montemayor
Spring, TX

 

[unclesyd]

We used dry N2, pressure controlled by a conservation valve, to blanket our H2SO4 tanks.

With H2SO4 + O2 + a very small amount of H2O strange things can happen to your equipment, especially the piping.

 

[cawse001]

A concept you may wish to consider, is to use the hygroscopic properties of the acid by using it as the absorbent fluid in the vent "dryer".

Use a small, dedicated pump to recycle the acid out of the tank, through the dryer and back into the tank.

Particularly important is acid consumption and hence fresh makeup rates, such that the acid concentration does not decrease significantly over time.

Cheers.

 

[pmureiko]

Does this project have an attractive payout? This needs to be checked before any engineering resources are applied.

Management comes up with all kind of ideas which then becomes your job to challenge and justify. Don't assume they know what they are doing. Because they have few cost control variables, they regularily target nitrogen use. Let's see what your situation is.

Making some assumptions on tank volume, turnover, nitrogen price and usage, I think you have a 12,000 gal tank that uses around $200 per year of nitrogen. The maintenance cost of a vent dryer could easily be more than the savings. Under these assumptions the effort does not have a positive payout and could actually cost you because of increased risk of corrosion damage.

 

[smckennz]

Hi James

of the three 98% sulphuric acid plants I do work at, all use atmospheric vents with no desiccants or blanketing.
The plants have storage tanks ranging from 200T to 2000T, all are of mild steel construction and are around 40 years old. There is a little loss of tank wall thickness around the "high water level" but they are still quite serviceable.

If your turn over your tank contents regularly, and climate is not too severe. I think all you need do is to fit a PVC weatherproof vent, arranged so any dribbles land either in the liquid in the tank (extend the PVC pipe slightly into the tank so the weak acid doesnt run down the tank roof/walls). The vent pipe gets a hard time because that is where the air moisture combines with the acid fume.

With a desiccant filter there is always the possibiliy it will block, and the acid discharge pump can then suck in the tank roof, unless you fit and maintain a flap valve in parallel.

Mild steel acid tanks give off a little hydrogen when they are empty. Make sure you thoroughly purge (I use CO2) the tank prior to any hot work, and ensure the tank is well earthed.

Cheers

Steve

 

[JoeTank]

As a storage tank engineer engineer and one involved in the inspection and repair of many sulfuric acid tanks, I would like to offer comments and observations from the outside looking in.

I have seen very few ASTs for concentrated H2SO4 storage that have had anything but an open vent to the atmosphere. Most have shown varying degrees of corrosion in the upper portions of the tank. I've heard a few tank owners refer to this area where the liquid level fluctuates as the tidal zone.

I suspect that many of these tanks would have benefitted from some type of control on the moisture that is allowed to enter the tank.

The pattern of corrosion is sometimes easy to detect visually, especially when the tank level has remained static for long periods (several months). A groove of corrosion loss at the air-liquid interface is often evident in this case.

The other case is much more difficult to quantify. When the acid level cycles up and down a lot, I often observe a general (but not uniform) corrosion loss that is visually diffucult to detect and identify. For that reason, we have often been called in on tank problems that have gone undetected, even after multiple tank inspections.

The moral of this story is that these types of tanks must be inspected by those that know what to look for and how to look for it. All to often NDE firms are asked to conduct an inspection, but are not qualified or experienced to do so. Most owners cannot discern which firms are capable of doing it right. Too often, they end up making the selection based on hourly rates. I will put my soap box away now.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[MedicineEng]

James:
Just my oppinion:
Go for the N2 blanketing system. In our site we also have N2 blanketing systems, not for corrosion issues but for safety (flammables). Our system is quite simple and effective:
We have a inlet diaphgram valve set for 25 mbar and then a vent valve set on 50 mbar.
You have is to assure that you tanks support the over pressure.

PR

 

[mrtangent]

Monty

Could you please email me your spreadsheet
james.bruce@dowcorning.com