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Marco
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Join Date: Jun 2009
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Will Nitrogen gas remove moisture?

07/02/2009 8:06 AM

Have an application in which a .25 PSI nitrogen blanket will be applied into the head space of a diesel fuel tank. As a side benefit will the nitrogen blanket reduce the fuels 30 PPM moisture content?
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PWSlack
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#1

Re: Will Nitrogen gas remove moisture?

07/02/2009 8:09 AM

What are the temperatures of the diesel and the nitrogen?

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Marco
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#2
In reply to #1

Re: Will Nitrogen gas remove moisture?

07/02/2009 8:19 AM

In advance thank you for your time.. The temperature tank and fuel will be stored between 35 deg. F – 90 deg. F..
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PWSlack
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#3
In reply to #2

Re: Will Nitrogen gas remove moisture?

07/02/2009 8:34 AM

Assuming the nitrogen is dry, and coming from a bulk liquid nitrogen storage tank it will be, the system will reach equilibrium over time where the partial pressure of water in the nitrogen space will be about equal to the saturation pressure of water at that temperature. This data can be found in steam tables, and +0.25psi can be considered to be approximately atmospheric pressure for the purposes of this exercise.

However, the size of the tank, its insulation and temperature swings have to be taken into account as well.

If the tank warms, water (among other things) will evaporate from the diesel into the nitrogen space.

If the tank cools down, the nitrogen with water vapour in it will cool first, and droplets of water will condense on the tank walls in the vapour space. They will then coalesce and run back down into the liquid.

The cycle then begins again.

This will happen with any other volatiles in the diesel as well (though their behaviour is subject to a different set of tables).

So, unless the nitrogen is being swept through the vapour space, and it is usually uneconomic to do this for most installations, the water may not be removed significantly from the diesel. And if it were being swept with nitrogen, then as well as the diesel losing its water, it would also lose other volatiles.

30ppm of water in diesel comes into the domain of "why worry about it?". After all, there is water in the air used for the combustion of it.

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Marco
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#4

Re: Will Nitrogen gas remove moisture?

07/02/2009 9:19 AM

PW, thank you very much for your thoughtful reply. Considering using something like a nitrogen generator so the blanket can be swept through the head space.. I agree with your why worry about it statement for the day-to-day application of diesel fuel, however this application is for the loner term storage of fuel in which moisture > 15 PPM will leach out and settle on the bottom of the tank in the form of free water and become a host for anaerobic algae which feeds on the hydrocarbons thus reducing the C-rating. Additionally, as you know another benefit of using nitrogen is reducing the oxidation process. Again PW thank you very much for your time!
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PWSlack
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#5
In reply to #4

Re: Will Nitrogen gas remove moisture?

07/02/2009 9:42 AM

Nitrogen by definition will have no effect on anaerobic bacteria.

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bhankiii
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#6

Re: Will Nitrogen gas remove moisture?

07/02/2009 10:06 AM

We use dry N2 to remove moisture in temperature chambers - basically by displacing the humid air that's in there.
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Abdel Halim Galala
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#7

Re: Will Nitrogen gas remove moisture?

07/02/2009 11:40 PM

We use dry N2 to remove both O2 and moisture through commissioning and start up of petroleum & petrochemical plants, and we have to check the percentages of both before feeding the product under processing.

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Paddler
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#8

Re: Will Nitrogen gas remove moisture?

07/06/2009 11:27 AM

NO, STOP, THINK!

Using one data point (20 Deg C or 68 Deg F)The vapor pressure of water is 2.8 MM Hg, The vapor pressure of diesel is 0.7 MM Hg.

The MW for water is MW is 18, The MW of Diesel is 500 to 1500.

The vapor pressure of water is 4X diesel but the Molecular weight of diesel is 27 to 83 times water.

The solubility of water in diesel is 0.0005.

Even without my slide rule, counting zeros (watching the decimal point) it would appear that the concentration of water would increase by blowing nitrogen through the vapor space (proportionally more diesel goes to vapor than water)

It is because the very small concentration of water turning to vapor in the liquid is not much compared to the large amount of diesel in the liquid turning to vapor.

Anyone care to check calcs or disagree?
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Users who posted comments:

Abdel Halim Galala (1); bhankiii (1); Marco (2); Paddler (1); PWSlack (3)

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