Compressor Question

[chemeng12345]

When compressing a saturated gas, what happens exactly?

The temperature must rise to keep it at a saturated state - but what keeps the gas from simply condensing instead of the temperature increasing?

 

[dcasto]

Entropy is in charge. Get out a Mollier diagram or P-H Diagram. As you put energy into the gas when you compress it, some energy goes into entropy or a more random state, a gaseous state.

Molecularly, I say it this way, some energy put into a gas makes them move faster, some energy makes the molecule vibrate more. Movement is measured by pressure and vibration by temperature.

Once you compress the gas to a pressure above its criticle pressure, it's always a gas, a dense gas with physical properties close to that of its liquid state under it's criticle pressure, but still a gas.

 

[zdas04]

A little less technical than dcasto.

There is no rule that the gas will stay saturated. As pressure increases, the gas's ability to hold water-vapor decreases (so condensation in the cylinder is possible). To more than offset this, the gas will heat up significantly while being compressed, which increases the water-carrying capacity (way more than the pressure decreased it). Typically you have gas at less than 30% of RH leaving the cylinder.

A positive-displacement compressor (like a recip or screw) will act very much like an adiabatic process (i.e., it happens without transferring heat to or from the environment) so the temperature change will be pretty close to:

T(out) = T(in) * (P(out)/P(in))^((k-1)/k)

Where temperatures and pressures are in absolute units and "k" is the ratio of specific heats.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[chemeng12345]

So is it possible at all to condense a gas by increasing the pressure past the saturation pressure? i.e. Is it physically possible to isothermally compress a gas using standard compressors?

 

[dcasto]

anything is posible, set the whole compressor in liquid N2. But what would really hapen is you'd compress a little and cool, compress and cool. until you compressed it high enough that your cooler would condense the gas, unless the gas criticle temperature is colder than your cooling system. Then nope, not going to get to a liquid that way.

HOWEVER, you will have a broken compressor (well a screw might survive). a pump or a compressor will be damaged by a phase change during compression. These things are not designed to handle the physical properties.

Now what is your application?

 

[chemeng12345]

I'm really just trying to conceptualize vapour compression for use as a reboiler in distillations, and had some mind blocks when it came to compression of the saturated vapour. You guys have been a lot of help!

 

[sshep]

chemeng,

From the standpoint of energy into and out of a compression, the postulated phase change of vapor to liquid is exactly the opposite of what would be expected. Liquification implies a flow of energy (heat) out of the process. Compression is energy (work) into the process.

A phase change seems reasonable only because the concept of vapor pressure can trick us into reverse reasoning. A chemical engineer must never see a phase change without thinking about energy the change that accompanies it- this is the nature of our craft. Example: the above posts (entropy, temp explanations) instictively recognize that adiabatic compression must result in an increase in internal energy.

best wishes always,
sshep

 

[dcasto]

What chemeng is looking for, I believe is a heat pump reboiler. There are lots of propane a ne butane splitter heat pumps in operation.

In a depropanizer with a heat pump the tower is operated at 50 psig. The overhead is a vapor that enters a compressor. The compressor compresses the C3 to 250 psig. The hot propane gas is condened to a liquid at 120 F by reboiling the tower.

Some of the liquid C3 at 120 F and 250 psig is then sent to the top a reflux. Very efficient and the surface tension of C3 is lower making the tower smaller.