[curiouscat]

Normally with large storage tanks (say, 10 kL) one must vent while pumping liquid in & allow air (or N2) to ingress when pumping liquid out.

Otherwise the ullage inerts might get compressed to a pressure too high or the vacuum created might buckle the tank.

If I am storing a high vapor presssure liquid e.g. Acetaldehyde can one do without these venting procedures? The vapor pressure of acetaldehyde being above 760 mm Hg at all T above 20 C the risk of developing an internal vacuum should never arise, correct?

Conversely, so long at there are no accidental inerts in the tank, the max internal pressure developed can never exceed the vapor pressure of Acetaldehyde, correct? So a tank rated for 5 atm pressure would work fine since at even  50 C the vap pressure of acetaldehyde is only ~3 bar.

Just wanted someone to critique my thinking.

[Corribus]

Can't we take a liquid nitrogen tank (or any liquified cryogenic tank) as an extreme example here?

In general, guess it would depend on the rate of pumping, the vapor pressure of the liquid, and the tank parameters - this would determine how fast (and how strong) a vacuum would form and whether the tank could weather the pressures. I mean, in principle no matter what the substance in the tank is, if you create a vacuum by pumping out, at some point this will cause vaporization of the liquid stored inside, which will mediate the vacuum to some degree - although kinetics will be important. So, what really matters is how much of a vacuum the tank can withstand before it fails. This is more of a mechanical engineering issue than a chemical one, I think.

[curiouscat]

Thanks @Corribus.

I should have clarified. Let's say the tank is not rated to take any vacuum at all. So even 750 mm Hg inside will cause  buckling. That's a conservative assumption.

In which case, if it were holding, say water then pumping water out without an open vent will cause mechanical failure.

Restated, the question is indeed a chemical question and not a mechanical one: "Can the pressure inside such an acetaldehyde tank *ever* drop below atmospheric; no matter how fast you pump out?"

I think it cannot. But I want to throw it out for some critique.

Let's assume ambient T above 20 C and below 50 C always.

[Enthalpy]

I expect the pressure to drop insignificantly below the vapour pressure when you extract the liquid, because bubbles can form, and these are very efficient to vaporize the liquid.

The potential drawback I see (which is a prohibitive annoyance in rockets, maybe not in your application) is that the liquid extracted from the tank is at saturation pressure then, so t's ready to boil at any point and time. Especially, it will cavitate in the pipes. Unless you have meters of hydrostatic pressure between the outlet, the pipes and the boiling surface.

The standard answer against cavitation is an overpressure in the tank, exceeding the vapour pressure, hence obtained by a different gas or by the same gas but heated.

-----

When pumping the liquid in, without venting, you'd need to be absolutely sure that there is zero amount of nonsoluble gas in the ullage. Such gas (air...) staying above the liquid would reach any pressure unrelated with the vapour pressure as it gets compressed.

Cryogenic gasses have it easier there, because most common gasses liquefy or solidify before, for instance, 77K.

[curiouscat]

The potential drawback I see (which is a prohibitive annoyance in rockets, maybe not in your application) is that the liquid extracted from the tank is at saturation pressure then, so t's ready to boil at any point and time. Especially, it will cavitate in the pipes. Unless you have meters of hydrostatic pressure between the outlet, the pipes and the boiling surface.

That's a good point. For even process applications cavitation is a no no.

OTOH, my pump might be ~1 m lower than tank bottom & if I always keep, say, 1m liquid in tank thats a total of ~2m of NPSH. I think low NPSH pumps are available for this duty. I will check.

The standard answer against cavitation is an overpressure in the tank, exceeding the vapour pressure, hence obtained by a different gas or by the same gas but heated.

Indeed. The downside of using a different gas is the venting needed on every cycle (typically). That causes loss of the stored commodity vapors.

What might be interesting is to add enough, say N2, in the ullage to keep a higher pressure. Say 2 atm above vapor pressure (at tank almost empty point). And to let it stay there and compress to maybe 6 atm (at tank full point; reduced ullage).

That would eliminate venting yet keep the liquid above the Vapor Pressure and hence solve all cavitation issues. 

When pumping the liquid in, without venting, you'd need to be absolutely sure that there is zero amount of nonsoluble gas in the ullage. Such gas (air...) staying above the liquid would reach any pressure unrelated with the vapour pressure as it gets compressed.

True. To be safe a Pressure Relief Valve will, of course, be provided. Just that it will be set to a Pressure high enough that it should not activate during normal loading and unloading but only during such eventualities.

[Enthalpy]

To avoid losing your precious vapour when venting, you could build the storage tank like a hydraulic accumulator, where some elastomer (compatible with your aldehyde of course) separates the compound (then completely liquid, not at liquid/vapour equilibrium) from the gas.
http://de.wikipedia.org/wiki/Membranspeicher
http://de.wikipedia.org/wiki/Blasenspeicher
(sorry for ze langueech, bat en.Wiki doesn't hafe it)

Of course, it still needs vents, but in normal operation you expel the nitrogen only.

[curiouscat]

To avoid losing your precious vapour when venting,

The vapor isn't so precious. But the government tells me that people's health is.

you could build the storage tank like a hydraulic accumulator, where some elastomer

Impractical for the tank size I'm dealing with.

[Enthalpy]

I'd try hard at the hydraulic accumulator solution. It has real advantages and I believe it's easy.

The only less usual part is the bladder or membrane, and a supplier can produce it for you. Epdm and others should resist acetaldehyde. The membrane is only a half-sphere or a hat shape, preferably with a rim which must be achieveable by rolling the edge and softening it under some mechanical pressure. Starting from a film, the purposely-made tools are cheap (wood) for
http://en.wikipedia.org/wiki/Thermoforming
just ask a plastic or rubber manufacturer how he would do it.

This one gives as an example just the shape you need:
http://hg-kunststoff.de/en_process.htm
This other delivers 10ft*18ft parts
http://www.rayplastics.com/our-capabilities/large-part-thermoforming/

Some rubbers can just be applied liquid with a brush on a mould, but these may not resist acetaldehyde.

The simplest would be if the membrane can be flat (just with a rim) and extensible enough.

[Enthalpy]

If this is still too small, you might get inspired by gas holders
http://en.wikipedia.org/wiki/Gas_holder
where you store your acetaldehyde instead of gas, and replace water by a liquid denser than 792kg/m³, not miscible, reasonably safe and affordable...

Maybe cis-pinane (hydrogenated by-product of paper factories), or Jp-10 (exo-tetrahydrodicyclopentadiene: hydrogenated nearly-byproduct of refineries), or some phosphate?

The cap's weight relates with the storage pressure, and here the sealing liquid will move more than with natural gas.