How to Purge Tank to Remove Hydrocarbon ?

You really want to discuss this with the Process Engineer at your facility, Uncle, otherwise you may find yerself out on yer a£$e like I did if things go wrong. Cor! Imagine the repercussions through CR4 if someone did get it wrong! The'd be calling themselves "Guest" for the rest of their naturals!! <Wheeze>.

I've seen dry ice used effectively. My lawyers won't let me call this a suggestion, though. :)

Dear Guest,

The boiling point of cyclohexane is 80,7 °C. (Check it here: http://chemfinder.cambridgesoft.com/ ) I thing it will be enough to blow inert gas (eg. N2) throught the tank near temperature of 70 °C. I should use hydrocarbon trap to catch cyclohexane from outlet gases.

Best whises!

Tibor Nagy

PS: Don't forget I am only a chemist not a chemical engineer!!! (and I speak only my native language, sorry.)

I've never had to do that before, could you fill the tank with water and drain, removing all residue of the cyclohexane and then purge the space above and below the floating head with nitrogen and out to atmosphere? If you can't vent it, how about erecting a temporary flare to burn it off?

What is the reason for wanting to purge the tank? Is this for product purity or for welding saftety?

There are too many unanswered questions. If you can answer a few of these you may get some help.

What have they done in the past?

Is all the liquid removed? (how do you know?) Displaced with water?

How large is the tank? (Diameter and height at the time of cleaning)

What level to clear? (TLV 300 ppm. Typical OEL 200 ppm) I assume you need to be below 100 PPM for entry? Welding? inspection? demolition?

The typical method is to fill and empty the tank several times with an inert gas (nitrogen) and measure he outlet with GC or appropriate instrument. Purging the tank will take many tank volumes of you count on dilution.

Some suppliers are now using very cold nitrogen gas (from a vaporizer truck) and displacing the warm vapors from the bottom to the top in one sweep. There is a clean break in large LNG spheres (and VCM Spheres where less than 1.0 PPM is required). Do you have on line analysers to track the concentration?

How do you plan testing all areas of the tank?

What country are you in?

Register and get alerted if there are solutions for you.

Thank you friends for your inputs. The tank is already in service for last 3 - 4 yrs. Leak is suspected at its bottom. So it is to be inspected internally and carry welding if required. So this is the reason for asking about safe purging plan for tank. Now you all can add more on this.

pump it out first. Then air purge until dry. It has a high boiling point of 80C so at room temp it will dry slowly as you send air through it until all the puddles have evaporated. This is inflammable, and if it has leaked at the bottom there will be a large area of saturated soil beneath it (unless it was properly built in the first place, in which case it will have a gravel bed beneath it.).

If hydrocarbon wetted soil is heated by welding, it may catch fire, so you need high volume fresh air purge at all times to keep it safe.

Once you find the leak you can assess. It may be where the liquid line comes in and can be caused by subsidence of the bottom on weak bedding which allowed the pipe to poke up and come loose?

Again, Where are you?

I have put false bottoms in tanks before. It is not an easy process.

In the US, OSHA has regulations that control these operations.

If you are purging the inert gas and residual cycle the a flare stack, when was the last time the flame arrester was inspected and cleaned?

In the US you need a confined entry permit to proceed including extraction devices and trained rescue team. All connections have to be disconnected and blinded. You will need oxygen analyzer, LEL or explosion meter, and toxic gas meter for Cyclohexanol.

You need a hot work permit and all the details that go with that. Fire watch, barricades, dedicated fire extinguishers and the like.

You will need a lockout/tag out permit process for any agitators, blowers, valves pumps disconnected to prepare the vessel.

Based on what you have told me I would:

1. Get a variance for air and water permit excursions or arrange treating water and nitrogen.

2. Fill the tank with water to the highest level possible.

3. Purge the vapor space with hot nitrogen to vaporize liquid remaining on the top.

4. Measure the nitrogen in the exhaust every 3 tank volumes and track how it drops.

5. When it drops to <100 PPM, sample the water and compare to your permit amount.

6. Drain the water to treatment and prepare for entry.

7. Confirm the leak did not saturate the ground and make an explosive or flammable situation.

Continuously monitor the interior vapor space while entry is in progress. Don't assume it won't change.

My friend u are EXACTLY RIGHT . All these things are considered and tank is purged out of Hydrocarbon. But on the Tank floor there are some holes penetrating completely through, (Point: 7) which while putting false bottoms by welding it can liberate cyclohexane trapped below tank floor i.e. in the foundation of the tank. It is sure that leak has saturated the foundation.

Now can u tell the way / steps to carry out welding SAFELY to fix small plates to cover holes) on tank floor HOLES ?

If I may add, once you have done the cleaning as outlined in previous posts, Carefully inspect any internal structures. We had a floating roof than that had roof supports and guides made of 6" pipe that had cracked at weld about 6' from the floor. There were two supports that had filled with Hydrocarbons to the level of the crack. The initial test showed a clear atmosphere inside the tank. Two days later the supports were being removed and just by luck the problem was discovered before disaster struck.