Sudden vaporization of water

[gte682n]

Hello, I am looking to size a relief valve for a case where we contaminate an organic feed with water. This feed would flow through several heat exchangers until it reaches the bubble point of water. Would this be calculated by simply taking the latent heat of vaporization of water at relief pressure and dividing it into the heat duty of the exchanger where it would reach bubble point?

Thanks,

 

[Latexman]

"Until it reaches the bubble point of water? Not really. In thermodynamics, the bubble point is the temperature (at a given pressure) where the first bubble of vapor is formed when heating a liquid consisting of two or more components. Once you vent off that first bubble, the remaining liquid will be cooled below the bubble point. There could be considerable water left. Plus, a relief valve doesn't separate/discriminate between a vapor and a liquid. When the relief valve reaches set pressure and "pops" open, everything comes out; not just the vapor. I think you want some kind of separation process, don't you?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[LittleInch]

I'm not understanding this post really.

Bubble point to me is when a dissolved gas is released due to low pressure or maybe when some compound vapourises, but this seems to refer to boiling point?

But where does extra pressure come from? As the steam appears, then the pressure rises and the boiling point increases. It takes quite a high rise in temperature for pressure to climb much above the "normal" pressure such that a pressure relief system can operate successfully.

Unless the water generates some kind of reaction which then runs out of control ("organic feed" is a bit vague) then I think you need to actually work out your pressures and temperatures a bit more here. IMO.

A diagram and a bit more description would help.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MortenA]

bubble point=boiling point

Use a steam table to find the correspoing BP at your operating prerssure (e.g. 1 bar (a), BP=100ºC and 15.5 bara BP=200ºC etc)

When a liquid boils, adding more heat will not change temperature (assuming the pressure stays constant) - so a volume of water equivalent to the laten heat/heat input will evaporate. The steam table will give you latent heat at your pressure. On the other hand if your volume stays constant (a closed cylinder) so does your average density and you can use this to solve for pressure/temperature using the increase in enthalpy cause by adding heat.

Best regards, Morten

 

[Latexman]

Kinda, bubble point (more than 1 component) = boiling point (one pure component). The boiling point of a pure component at a given pressure is a single temperature. A multicomponent mixture at a given pressure boils over a range, from the bubble point to the dew point.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[gte682n]

The normal feed stream is a mixture of high boiling organics. They are preheated in this feed train from 140 deg F to 600 deg F prior to a vacuum tower that operates at 60 mmHg. We do have a dehydrator on the pre heat train, basically a flash drum that operates at 235 F and 65 mm Hg. I suppose a better description of the relief scenario would be if a pocket of water were left in the piping or equipment, and the hot organic stream contacted this pocket of water. I am visualizing a pot of hot cooking oil, and dropping some water in it. That sudden expansion of water is what I am looking to relieve.

The equipment and piping is 150# class. Since water boils at 366 deg F at 150 psig, we are concerned with this scenario.

I hope that is a better description, and thank you for the replies so far.

 

[Latexman]

The critical temperature of water is 705 F. Water is susceptible to "rapid phase transition" in the range of about 625 - 705 F. Rapid phase transition is not relievable. It has to be prevented. Your process is close to the danger zone. I suggest some research into RPT. Is the process batch or continuous? A PFD would help. Would 600 F material be transferred through a line that just had maintenance done without drying, or would the temperature be ramped up? We're struggling a bit for information here. I hope this has helped.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[gte682n]

Thank you Latexman. I did a search on this forum for rapid phase transition and found the thread below. The reference to API 521 provides me with the appropriate argument on why a PSV is not suitable for this case.

In general, the equipment would be N2 purged, dead legs blown dry, and slowly heated up as feed is introduced.

Thanks again,

http://www.eng-tips.com/viewthread.cfm?qid=294940

 

[Latexman]

Rapid phase transition has some synonyms, like "explosive boiling" and "phase explosion". You should cast your search net wider than just ET. I bet you already have. I know Wikipedia has a couple small articles on it with at least one promising-looking reference.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.