Separating CO2 from natural gas

[Latsabb]

I have been given an assignment to use a specified distillation column to separate out propene from a gas. The gas consists of propene, propane, ethane and CO2. Separating out the propane has been straight forward, but ethane and especially CO2 are giving me a hard time. To make matters worse, I am constrained to ONLY using pressure and temperature changes. No other variables can be changed, and no other components can be added.

I am using UniSim, which is very similar to HYSYS, to model this column, and after a good 5-6 different case studies, I cannot for the life of me determine how to get the CO2 to condense and separate out. Researching literature from the industry shows that there is almost always a solvent, or amines used to do this, and I cannot find any examples of using just temperature and pressure. Especially not to the purity that the professor is demanding. (98.86% propene in the top product...)

Can anyone tell me if this is even feasible? Surely if CO2 was that easily separated, the industry would be doing that, rather than using MDEA, amines, etc.

Thanks in advance!

 

[Charles Link]

I am not a chemist, but I do know that CO2 will freeze at -78.5 degrees Centigrade(I googled it to find the freezing point). (It makes what is called dry ice, which sublimates to the gas phase at atmospheric pressure). Perhaps this is helpful. editing... Meanwhile methane boils at -161.5 degrees Centigrade according to a google, but I see you are working with ethane and a couple other molecules. Perhaps their boiling points are all below -78.5 degrees C.

 

[Tom.G]

CO₂ liquifies at 11 Atm. and -37C. Density of 68lbs/CuFt ( see page 2 of: http://www.airproducts.com/~/media/files/pdf/company/safetygram-18.pdf)
Also, at STP, specific gravity is 1.5.

Ethane liquifies at 1 Atm and -89C. Density of 34lbs/CuFt. ( see: encyclopedia.airliquide.com
Also, at STP, specific gravity is 1.05.

For others, do a Google search of Specific Gravity followed by name or formula of the gas.

 

[Latsabb]

I am aware of all of these, but due to it being a mixture, the phase envelope is completely different, and a distillation column has multiple trays, each with a different temperature and pressure to increase separation.

After a few more case studies, I contacted the instructor, who then allowed me to add an additional column, which easily separated things as I wanted.