6 replies to this topic

[Conchubhar]

Hello everyone,
A question on blanketing. We obviously use blanketing to maintain inert atmospheres, and for inbreathing, as a layer of protection against vacuum in a tank. Another reason commonly cited is reduction in evaporation losses form a tank containing a volatile liquid.

Indeed, it has been mentioned repeatedly on these forums to make sure your vapor pressure is lower than your tank operating pressure, when specifying your blanket and conservation vent setpoints.

This confuses me, as vapor pressure pressures are always lower than atm - they have to be for storage of a liquid at ambient temperatures! - so as long as your nitrogen blanket is above atm (10mbarg let's say) you're fine.

This leads me to my confusion. Estimate losses of vapors from your tank by the equations - mol fraction = vapor pressure / tank operating pressure, flowrate of vapor out = mol fraction of vapor in headpsace * total flowrate of vapor+non-condensibles leaving the tank.

Looking at these equations, use of the blanket increases the pressure in the tank, and hence reduces the mol fraction of vapor in the headspace, but not by much - and hence losses of vapors are going to be pretty much the same whether you have blanketing or not.

Is my logic correct? I've always been told blanketing greatly reduces evaporation losses!

[Qalander (Chem)]

Dear Conchubhar Hello/Good Afternoon,

Although I could not exactly understood what you tried to convey/ communicate! in your OP.

Still I give a very 'raw' &'crude' explanation here;
Probably you might have heard or used Pressure cookers;work at elevated pressures,accordingly Boiling Point of water gets increased and Foods get cooked faster at higher than normal (atmospheric pressure) water boiling point(s) temperature.

Almost in a somewhat similar pattern the increased pressure(s) on the surface of stored liquid shifts(elevates) required Boiling Range temperature(s); thus consequentially indirect reduction in the rate of stored liquid vaporisation at storage temperature(s).

Probably it gives you some point to ponder& the way forward.

[Conchubhar]

Hi Qalander,
Thanks for your help.
The main point I was trying to convey was how does blanketing a storage tank reduce evaporation losses - my logic suggests the decrease is negligible.

Also, when increasing the pressure over a tank using blanket gas, you're usually increasing it by ~10mbar, not a lot!

I was of the opinion vapor pressure varied only with temperature e.g. Antoine. And the equations I use to estimate the loss of vapor from a tank during outbreathing only have vapor pressure, operating pressure and total outbreathing flowrate as variables.

[Qalander (Chem)]

Still, In all fairness my suggestion holds since the pressure inside the Food Cooking under pressure is not that great and similar suppression activity helps in reduced evaporation rate(s) of blanketed liquids.

1) Moreover the partial pressure concept is not really the controlling case here I assume.Other forum giants may correct me;if I am inaccurate.

2) The gases/inerts occupying the vapour spaces of blanketed liquid storage containments are first to out-breathe;thus indirectly direct loss of stored liquids vaporisations is comparatively lesser if without anything except the stored liquids own vapours filled this volume.

3) For pumping out in-breathing first the inerts/gas expands initially to support pump-out and

subsequently if at all any atmospheric air needed comes in contact with blanketing inerts/ gas instead of stored liquid's vapors directly are minimized through the presence of Inerts or Gases other than the stored liquids vapors.

However there might be others more learned people around to guide further on the issue.

[djack77494]

You might first identify the alternative against which blanketing will reduce losses. If it is an open vented atmospheric tank (e.g. gooseneck), then I think it's obvious that the blanketed tank will have reduced losses. If you're comparing against a "bottled up" atmospheric storage tank, then it's less obvious. I'd say that the blanketed tank will operate at some positive pressure, and that inbreathing and outbreathing will be at a minimum compared to any alternatives. In the same breath, I will admit that the effect will be quite small vis-a-vis a bottled up tank where breathing occurs through a conservation vent.

[Qalander (Chem)]

I believe 'Paul' May guide,Throw light on the issue in a much more thorough& Comprehensive way.

[proinwv]

Actually I like djack's answer. It is all relative.

Further, the purpose of blanketing is not necessarily to reduce losses. It primarily is used to protect.

Protect the product from oxidation.
Protect the product from contamination by atmospheric air and impurities.
Protect from fire.
Protect the atmosphere from contamination due to evaporation.