evaporation in tank 3

[packo]

i have tank with liquid(ethylene) on -103degC,on that temp liquid ethylene evaporate an we have gas phase of ethylene wich raise pressure in tank as evaporation continues,i'm trying to somehow connect level of liquid ethylene in tank with pressure that is created by evaporation of liquid,there is one thing more to take in consideration,as pressure goes up from that gas phase evaporation is slower!? is it true? how to sum that all to exactly predict what pressure can i expect for any given level of liquid?
tnx

 

[Goahead]

A similar problem is met with tanks of liquid nitrogen.
The pressure of the gaseous phase is monitored with a manometer, while the level of the liquid is roughly seen with a plunger. The excess nitrogen gas is simply vented to the atmosphere.

http://www.welding-advisers.com/

 

[packo]

it-s ok but i need to find some mathematical background of evaporation-pressure in tank,any helpful ideas ?if i find that in x time evaporate y kg of ethylene how to find resulting pressure of that gas ethylene?

 

[chicopee]

Get a liquid-liquid/vapor-vapor phase diagram for and you will be able to determine pressure and temperature relationship in the saturated vapor region as long as you have no flow in and out of the tank.

 

[Zapster]

Are you trying to determine the liquid level in the tank by the pressure in the tank?

If so, if you know the total mass of ethylene and ensure there is sucifecint space above the liquid phase to hold the evaporating gas. If there is insufficient volume for the evaporating ethylene tank must be vented or it will rupture from over pressurization. Picture this, if you start with a full tank of liquid, any phase change to gas will need to be vented to atmosphere and you will not know the total mass of ethylene. In contrast, if your tank is partially full and sealed off at some known pressure and temperature then you could calculate the increase in the vessel pressure as the liquid changes phase.

One way to keep track of content is with load cells on the tank legs. Then you subtract the empty tank weight from the loaded tank to determine quantity.

Provided you have sucifecint volume in the tank for the evaporated gas without requiring venting then you would have a number of equations to solve simultaneously. Mass balance, Volume balance, liquid phase mass, vapor phase mass, liquid phase vol, and gas phase volume. If you know the physical properties at the range of temp and press you need then write the equations, combine the equations if possible, and then iterate to a solution. This will only work provided difference in physical properties for different temp and pressures is great enough to make the errors in your measurements of temp and pressure minor.

Not sure what you are trying to do; however, I suspect that you will not have accurate enough temp, pressure, total mass and physical properties to calculate a meaningful solution.

 

[timbones]

Unless I missed something, assuming the tank contains only ethylene, the pressure in the tank will be equal to the vapour pressure of the ethylene, which is a function of temperature only. Tank liquid level and vapour space volume are irrelevant.

 

[Ashereng]

how to sum that all to exactly predict what pressure can i expect for any given level of liquid?

The short answer is there isn't a relationship.

If this is a sealed vessel (no in, no out), the pressure above the liquid level = vapour pressure at the temperature.

If this is a purged or maintained vessel (in, out - regulator, PSV, vent, etc)), the pressure above the liquied level = maintained pressure, at all temperatures (it is maintained).



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[packo]

first tnx for all answers,the tank is vented,and i know pressure and level of ethylene in tank but i'm not sure you understood my problem,and problem is -what pressure of evaporation would be if 1.tank is full of liquid eth.
2.tank is half of liquid e.
3.tank is empty(only some gas phase)
for some given heat transfered to tank.is bigest pressure of evaporation for 1,2,or 3?
would evaporation (and resulting pressure) be smaller or bigger if i have full or half tank of ethylene(liquid)?,what evaporation(pressure) will be for some given level ethy. in tank,and how to connect that mathematically?,i hope i somehow clear my concern
is it better to have full or half full tank regarding pressure of evaporation?

 

[Ashereng]

packo,

Check out this web link.

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html

The short answer is, I don't think you can deterine height of liquid based on vapor pressure above the liquid.

I suggest you talk to your process group, and get a more full picture of what is happening above the liquid level in your vented tank.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[djack77494]

packo,
I agree with what Ashereng is trying to tell you. If the tank is full or if it has only 1 mm of liquid in the bottom, the vapor space pressure is ONLY a function of the temperature of the tank's contents. It's straight thermodynamics.
Doug