meoh storage vessel

[oil198]

A quick question regarding methanol and vapor pressure. I am a little confused regarding the relationship between nitrogen padding and vapor pressure. We are designing a storage tank for methanol with a pad/de-pad system. I understand we need to keep a blanket to prevent o2 and air from entering the system for corrosion reasons.

Secondly, nitrogen will pad the vessel during thermal in-breath. But I don't understand, why or how the really low vap. pressure of methanol can somehow lead to a vacuum scenario which is why we have to maintain a positive pressure. At extremely cold temperatures vap. pressure is below 0 gauge. Can anyone clear this concept up for me? Thanks.

 

[Montemayor]

Oil:

Nitrogen padding in a Methanol tank is not done to avoid corrosion. It is primarily to comply with environmental regulations and safety hazards. The intent of the application is to keep the methanol (vapors) in the tank and pressurized so that air can't invade the vapor space and create a combustible mixture.

The vacuum case can appear in a such a tank through the results of pump out (or drainage) while the N₂ makeup supply valve is inoperative due to whatever reason. For this hazard scenario you have the appplication of the usual conservation vent - a relief device that is a combination pressure/vacuum relief valve mounted on the roof of the tank. You can also apply a separate, independent vacuum relief device as well.

The nitrogen vent system should work to maintain the tanks vapor space with a positive N₂ pressure, so there is no vacuum produced by thermal cooling - as long as the N₂ pad is working. There is no "vapor pressure" to fear - just a sudden vacuum caused by lack of sustaining a positive N₂ pressure.

The above only deal with operating conditions. You can "suck in" a tank for other reasons, but you don't mention those.

 

[sshep]

The vacuum from thermal inbreathing will come from the density change of the N2 in the vapor space even without any vapor condensation.

A tank which never had hydrocarbons in it would still buckle if cooled just from the contraction of N2. Use the ideal gas law and calculate the volume difference of the starting tank mass of N2 as it is cooled from 20C to 0C (as is happening where I am every day/night). That volume difference is how much the tank would have to "contract" if you had no pad.

best wishes,
sshep

 

[dcasto]

I'll disagree with Montemayor on the corrsion aspect. In 1975 we didn't have air regulations and we padded the methanol tanks with natural gas to keep the water/O2 from eating up the tanks. Methanol is very hydroscopic and if left in the air it will draw in the water and O2 and rust a tank in no time. Ethanol the same, this why ethanol as a fuel is a problem unless the fuel system is made of stainless steel or other corrosion resistant parts.

 

[oil198]

Thanks all for your excellent replies. So vapor pressure is not really taken into consideration when desiging the nitrogen pad system?

 

[sshep]

You should size the vacuum/vent with the tables in API2000, which will also set N2 pad sizing. You must provide atmospheric pressure vacuum safety vents as a backup to the N2. As I recall the heat flux (to convert volume to rate) and other basis are given.

best wishes,
sshep