Nitrogen Purging and Air Changes

It has to be mathematical and that too very complex.

Are you going to vacuum it before charging N₂ ?

Else it might be only by trial and error.

The first change will make it more than 85% N2 (Theoretically 85% but since the N2 will move more of air and not mixed uniformly before displacing hence >85% after first change)

The second will bring to >93%

The third > 96% and so on.

If you suck out the air, may be the second change will bring the non-N₂ to ppm level.

we will not vacum the tank before purging, just the tanks will be gass free for the inspection purpose and after successfull inspection and OK to load from the surveyor the terminal will blow the N2 in the tanks what i understand from your answer is that we will have 3 atmosphere changes of nitrogen before loading rite???

The thing is that MEG(mono ethylene glycol) is a non flammable cargo , so its very less likely that we will do purging for the O2 contents , the purpose of purging nitrogen is to maintain the standard of MEG considering the heavy cost of cargo, rougly how much nitrogen to be purged of a tank of 1000 M3 tank???

How does MEG deteriorate in contact with air, then?

As far as the quality of MEG is concerned, air contains a certain level of moisture and that moisture can effect the quality of the MEG, the one reason behind the purging is to remove the moisture contents from the tank, does that answer your question sir????? kindly can you answer mine???

Ambient air can contain a wide range of water content. I suggest you use a moisture or dew point monitor to measure the moisture in your tank and compare the measured amount to an established limit. If you need to dry the air in your tank , you can do it with N2 purge as you are planning. There are alternate strategies for gas drying such as purging with dry air and desiccating the air already in the tank.

It might be easier to remove the moisture from the air than to purge the vessel with nitrogen.

Here's a thought - instead of purging with nitrogen, it might be more economic to purge with -40degC dewpoint instrument air instead.

It all depends on how dry he needs the interior and he has not disclosed that. Minus 40 degreesC is the same as -40 degrees F and is about 120 ppmv at 1 atmosphere. Purging with this air would eventually get you close to -40 degree dew point, but would never get you below that. If he needs the interior to be below -40 degrees dew point he will need to purge with drier gas. If the tank will take a 1 atmosphere pressure differential, he could evacuate it with a pump and backfill with dry gas. This will save expense and time.

Use a dewpoint meter as suggested, relying on calculations will get you nowhere. No need to be angry at PW, he is just trying to help. Could you heat the N2 to get the tank dry more quickly?

NO one of you guys have untill now answered my initial question, i will write my question again, how many air changes are required to be done to a tank of 1000M3 capacity with nitrogen for it to be inerted, we are talking outside the initial question,

But we have answered (At least partly)

I assume (of course you are talknig about only removel of humidity) but you do have to replace Oxygen too.

Unfortunately you have to do it by trial and error and I assume that by about 3rd or Fourth replacement it will be OK.

Since the replacement quantity is large, why don't you

Dry it usin desiccant

vacuum it (need not be very high degree) that case you will save a lot in replacement.

But finally you do have to check the O2 content to ensure.

hello Thanks for the answers and the matter is now very clear, i am thankfull to MR. GURU from india, and MR WELDERMAN , and mr. BETOMACHINE from USA, i really appreciate the answers posted, MR Betomachine u asked soem questiions , actually what we re doing is that its a small chemical tanker going to load MEG, the vessel will prepare the tanks for loading and that includes cleaning , mopping & drying to a acceptable level, In the loading port after the tank has been examined by a surveyor and declared fit for loading then the shore facilities will flow the N2 through a proper hose, the quantity of nitrogen to be purged will be like 3 to 4 times the volume of the tank, i found mr. GURU`s answer most appropriate in this regard, rest of the answers were perfect but what i was looking for i got from that answer, that 3 air changes are quite similar to the BALLAST WATER EXCHANGE in merchant vessels, Thanks anyways for all of you guys Cheers

The answer is infinite. This is especially true since the nitrogen is lighter than dry air though it may be heavier than moist air depending on mosture content. As I recall you are interested in removing the moisture. You need to decide what level moisture is acceptable and measure it. Sorry.

Okay, I'll spell it out for you. Assume you have air in the tank and a water vapor content of, let's say, 10,000 ppmv. After one complete purge with dry nitrogen, you would have 50% of the original O2 or about 8.5% and 50% of the original water vapor, or about 5000 ppmv. After a second complete purge, you would have 50% of what you had after the first purge, etc. You could write the differential equation for water v/s purge, but I think you get the idea.

This works assuming that you get good mixing in the tank, correct? I think there are a lot of issues with using a set number of volumes and not conducting any sort of analysis.

Is the tank at atmospheric pressure? Can you pressurize it with N2? Are you pulse purging or just flow purging? How do you know that you are purging the entire tank or getting good mixing?

This may be overly detail-oriented, but I think that they are relevant questions.

The actual mixing of the gas is independant of any assumptions one makes. We frequently purge tanks to lower internal moisture content. Fans can be used to promote mixing, but are not required for these gasses. Once you have proper instrumentation to measure gas concentration, you can purge and measure to optimize the process. We normally use analysis to verify the data, not the other way around.

The original posting was about purging the vessel with nitrogen. Now, the agenda has changed to the topic of removing moisture from the air that is present. That would explain the forum's confusion.

As per MSDS

http://users.bigpond.net.au/tuscany/deltrex/products/s134.htm

FLAMMABILITY

Combustible liquid.

STORAGE AND TRANSPORT

Store in a cool place dry place in closed containers. Store in well ventilated area. Store away from oxidising agents.

Perhaps you could drop a chunk of "dri ice" into the container. CO2 being heavier than air would purge the container.

....though it wouldn't do what the poster now wants and that is to remove moisture from the tank vapour space. Nitrogen, it seems, might be a 'red herring'.

some terminal purge volume to volume i.e. ship tank 1000M3, purge with 1000M3 of nitrogen. Some other terminals have completely eliminated the N2 purging prior to MEG loading.

This is the latest practice for ship tanks preparation proir to MEG loading.

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