Condensation from high pressure compression 1

[J19940863]

Hello, I just can't seem to wrap my mind around how to solve this... Any help would be great appreciated! Lets say I am compressing air to 3,300,000 Pa and the temperature of the tank is 90F then I decompress the air and allow it to flow out to standard conditions... What volume of condensation will occur? I would certainly appreciate any help on this! Not sure why I can't think clearly about this.
Thanks

 

[zdas04]

Atmospheric air has the humidity that it has. Nothing in the compression/decompression process changes that. So if you take atmospheric air and increase the pressure to 3.3 MPa, the water content does not change. At all. Relative humidity changes dramatically as you change pressure and temperature, but in opposite directions. Let's say that 100% relative humidity at 16°C and 100 kPa is 13.57 gm/SCm. At 32°C and 3.3 MPa the 100% RH point drops to 1.5 gm/SCm. The missing water vapor would condense in the tank and not be available to condense in the blowdown. Since the ambient air can hold 13 times as much water vapor as the exiting air could have held in the tank, I wouldn't expect any condensation of the tank gas on exit.

On the other hand, depressurizing the tank will create a considerable amount of JT cooling, and the cold air could easily cause condensation in the ambient air that it is entering, so while you won't get any of the water vapor from the tank, the cold air could easily result in condensation on the ground.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[georgeverghese]

At 32.2degC, water vapor pressure =36mmHg, so water content in compressed air, mole fraction, at 34bara = 36/(34x760)=0.0014
Initial JT letdown of air from 34bar abs, 32.2degC to atm results in an exit temp of 292degK = 19degC, at which conditions, water content, mole fraction = 16.5/760=0.022, so no condensation initially.
As the tank depressures, tank contents undergo close to isentropic depressurisation, so tank temp drops considerably as pressure drops. At each pressure point, the corresponding exit temp at 1atm continues to drop below 19degC.
To get the temp / press profile for both the exit and tank as tank pressure drops, model this on a process simulator using the Depressure utility routine - you could use an isentropic eff of say 80-90% to approximate this thermodynamically irreversible process. At some point, you may get the exit temp dropping below water freeze point.

 

[LittleInch]

J19940863,

I'm afraid I don't really understand your question

"What volume of condensation will occur?"

Where?
Under what conditions?
As a liquid volume in the tank or a "mist" on the outlet air?

Compressing to 33 bar at 90F will result in water dropout in the tank which is then normally drained away, which is what Dave is saying. Unless you're taking that water and spraying it back into the low pressure air then it's gone out of the equation.

How humid is the air going in?

Compression packages normally include dryers and water collection / de misters in the process.

Air storage Volumes, relative flowrates would all make a difference in reality as external heat can flow in and out of vessels and pipes.

Think you need access to a Process Engineer or analysis package.

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

 

[J19940863]

Thank you all for your input on this question. Let me explain this in a little more detail as you all made some pretty good points... The storage reservoir that I am using to hold the compressed air is sensitive to water.. So the objective is to compress air that has on average 50% humidity and is at a initial temperature of 80F. This air is then compressed to 3.3 MPa and held in the storage tank that is water sensitive. The air will then be bled off and used. The goal is to be able to use the storage reservoir that is water sensitive so I am wondering how much condensation will form in the reservoir when bleeding the pressure off? Assume a constant reservoir condition of 90F. I know that there will certainly be condensation that forms if I were to bleed the pressure back down to atm, but what if I don't bleed it down to that low of pressure? How can I find that pressure dew point?
Thanks again for all your help! I am struggling to wrap my mind around how to solve this...

 

[zdas04]

Your mixed units bother me a lot. Pick a system. The tank is either at 3.3 MPa and 32.2°C or 478 psig and 90°F. Your answer will be in mass of water per volume of air at standard conditions (either lbm/MMSCF or mg/SCm). Once you've decided on a set of units, get a psychometric chart to find the 100% RH water content of both ambient conditions and pressurized conditions. The numbers will be in the range of what I gave you in my first post. If the 100% RH point for ambient air is 14,000 mg/SCm, and it is at 50% RH, then you have 7,000 mg/SCm going in. At pressure the air can only hold something like 1,500 mg/SCM, so 5.5 gm of liquid water will condense in the tank for every standard cubic meter of air you put into the tank. If the tank volume is 500 m³, then it holds 15,350 SCm and you would expect 84 kg (L) to condense inside the tank.

If the tank is sensitive to water, I would suggest a chiller, water knockout, and dryer on the compressor discharge--once the water is in the tank it is going to become liquid so you have to keep it out if the reservoir is truly water sensitive.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

J1994,

I think you need to read daves very good post more closely.

As you compress the air, it's ability per scf or scm to hold water as water vapour decreases and hence it drops out once you hit 100 RH. this is the opposite of your sentence

" I know that there will certainly be condensation that forms if I were to bleed the pressure back down to atm". No. As you bleed pressure off the main tank its ability to hold water will incresase, not decrease.

I think you're getting confused with something like the JT effect ( cooling) which may drop the air still at some high pressure to below its dewpoint / 100%RH.

what sort of receiver is "sensitive to water"??

Why don't you just drain the water off into a sump and blow it down?

Or as Dave says, dry it on the way in?



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

 

[Compositepro]

When you compress air, you will also compress the water vapor in that air. 50% RH at ambient means that the air contains 50% of the water it can before it cannot hold anymore and condensation will occur. That means that going from 0 psig(15 psia) to 15 psig (30 psia) will result in water starting to condense in your receiver. As others have pointed out, you are misguided in your concerns of condensation during decompression. Your concern needs to be about condensation during compression. Almost all compressed air tanks have drains in the bottom because there is usually water in them, even when there are driers. Drying air so there is never any water in the receiver is not easy, and requires a carefully designed and controlled drying process.

 

[georgeverghese]

You initially have 2phases ( liquid water and compressed air) in the 33barg reservoir at 90degF:
First, bleed all the liquid water out of the drum ( which you can tell when you start to get air spitting out of this bottom drain valve).
Then depressure the drum till you get to say 30-31barg-by the time the air gets back to 90degF, this air will now be superheated in terms of water content, and you wont suffer corrosion at the bottom of this drum. Obviously, if night time temp is less than 90degF, then run this water bleeding operation at night, then depressure to 30barg.
Air at 33barg and any temp T has a higher sat water content than at say 30barg and the same temp T, so any residual water droplets in the tank at 33barg would vaporise at the lower pressure.