N2 flow needed for blanketing

[Orsiz]

Dear all,

I'm trying to calculate the nitrogen flow needed for purging a hopper. This hopper (with a volume of 0.40 m3) is a bin in which different solids (polymer, additives, ..) are collected and via which they are discharged in a mixer (here all the components are mixed together at high temperature), this is a continuous process.

To ensure a safe operation, the oxygen concentration in the hopper should be reduced to 5 vol%. The polymer and other additives take air in the hopper and this air flow was calculated as 4 m3/hr. My question is: how do I calculate the nitrogen flow needed to purge this continuous air flow and have an oxygen concentration of 5 vol%?

The N2 flow shall be continuous as well and going from the bottom of the hopper to the top (where the polymer and feed enters) and shall be extracted via an outlet on the hopper together with other fumes.

I attached a schematic drawing.

Best regards,
Orsiz

 

[EdStainless]

I would supply the purge flow near the material discharge, you don't want to risk pockets of air trapped in the feed portion.
Do you believe the estimate of 4cum/hr?
If so, then you need 3-4 times that for purge flow.

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P.E. Metallurgy, Plymouth Tube

 

[IRstuff]

oxygen content of air is 21%

4:1 dilution results in 4.2% oxygen, assuming complete mixing.

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[Orsiz]

@EdStainless and @IRstuff: I believe the estimate of 4 m3/h is correct. I calculated this based on the throughput, bulk polymer density and air density. What's your opinion of the 4 m3/h flow rate?

Don't you have to take the volume of the hopper into account? Will I immediately reach the needed oxygen concentration of 5 vol% when I start up then? Isn't there a certain time needed in order to purge the oxygen? The concept of nitrogen purging is relatively new to me so I still have to learn a lot about it.

 

[IRstuff]

Hopper volume is 2% of flow rate, so 1.2 minutes to clear.

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[EdStainless]

If you are starting from empty I would purge 8 volumes (10 min) at 16-20 m3/hr before adding any material, and then maintain the flow during operation.

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P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

For continous purging of the hopper, the rate can be computed by setting up a N2 balance equation between the 2 feeds and the exit air.

If y m3/hr is the N2 purge rate, and N2 purity is say 99%, and say we add a 20% safety margin to this estimated 4m3/hr of free air getting into the hopper, we will have

0.99y + (4 x 1.2 x 0.79) = ((4 x 1.2) + y) x 0.95, which gives y = 25.2m3/hr.
If you dont include a safety margin, y = 21m3/hr.

This is the continous rate.

 

[Orsiz]

Thank you georgeverghese for the balance equation, this seems a good way for approaching these kind of problems.

 

[gelsi]

I'm aggree with georgeverghese.

Only for further evaluation, Orsiz:
if I'm not wrong, from 0.99y + (4 x 1.2 x 0.79) = ((4 x 1.2) + y) x 0.95
y = 16 (19.2) Actual m3/h

that happens without fumes and if all streams with gas/vapour are at the same temperature:
1) fumes with low or neglected O2 content reduce nitrogen request;
2) if nitrogen is colder than air feeded, y becomes lower and if hotter, y becomes higher (but rarely the last cond. happens)