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# Separation Of Real Gas And Non Condensable Gases (Air)

separation distillation refrigeration

5 replies to this topic
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### #1 rainy143

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Posted 20 July 2018 - 11:41 AM

Hi,

I am relatively new to the forum and and not a ChemE. I am trying to design and model a system for separation of Real Gas and Air by refrigeration and thus condensing the real gas and venting non condensable gas to second stage and  do the same thing there to ultimately exhaust only NCG to atmosphere. The gas mixture is supplied from a ISO tank. I don't know the pressure of the tank. The ambient temperature is 70F. The real gas is condensed at -20F and 20 psig. With this information is there a way I can figure out the molar fraction of Air in the mixture. The mass fraction of air is 10% and Real Fluid is 90%.

### #2 Pilesar

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Posted 20 July 2018 - 01:32 PM

To convert mass fraction of gases to mole fraction of gases you need to know the molecular weight of the component gases. The condensing temperature of the mixture will become colder as the real gas is condensed. Gas mixtures generally follow a law of thermodynamics called Raoult's Law which describes the concept of 'partial pressure'. Using Raoult's Law -- if the total mixture pressure is 20 psig and Real Gas is 50% of the volume, the mixture condensing temperature will be equivalent to what the condensing temperature would be for pure Real Gas at 3 psig. As Real Gas condenses, the condensing temperature of the mixture continues to drop. So practically, you will always have some Real Gas uncondensed.

### #3 Pilesar

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Posted 20 July 2018 - 01:40 PM

Also, a ChemE would consider the complications of water vapor that entered with the air and its relatively high freeze point temperature.

### #4 Bobby Strain

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Posted 20 July 2018 - 02:52 PM

This approach could be dangerous as the oxygen concentration is increased. It's quite strange to have such material in an ISO container. You can't really design any system without knowledge of the gas composition. And, air is a real gas, too. It's not imaginary.

Bobby

### #5 rainy143

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Posted 20 July 2018 - 04:40 PM

Thanks Pilesar and Bobby. By real gas I meant not close to ideal gas. I am not sure if I can approximate air here to ideal gas but I was considering air as close to ideal gas. The other gas is a refrigerant (variable) for this example let's say R22. I am only dealing with preliminary design so approximations are ok.

Pilesar I tried to use Raoult's Law but since air is superheated I cannot find its saturation pressure and hence could not apply to get result.

### #6 Bobby Strain

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Posted 20 July 2018 - 06:59 PM

Again, there is no reason to have air in a refrigerant container.

Bobby