Ask a chemist - cooling CO2

July 24, 2008 2:51 PM

A question for chemists - I am doing a calculation about the energy requirements of taking CO2 at atmospheric pressure and temperature and compressing it to 2 MPa (2000 kPa, about 300 psi) and cooling it to -31ºC and am a bit uncertain about handling the phase change from gas to liquid.

The 2 MPa, -31ºC is the IPCC's state for transport of CO2 by road or rail tanker.

I've assumed an isentropic two-stage compression to get to 2 MPa.

I've calculated that the gas would be around 140ºC after the compression is complete. So, one can assume that no energy is required to cool to atmospheric temperature. But to cool it further would require refrigeration... and then there is the phase change (heat released going from gas to liquid?).

What do I need to do to calculate the energy required for that 2 MPa at 25ºC to 2 MPa at -31ºC step?
∆h = cp∆T but what about the phase change? (I knew how to handle this, once, long ago)

posted by gspm to science & nature (7 comments total)

In addition to accounting for the heat of vaporization, the specific heat of most fluids tends to become somewhat nonlinear near phase transitions. To compensate for this, you can use tabulated data, such as that provided by NIST for CO₂ and many other fluids.

The enthalpy change for going from 25 C to -31 C is 355 kJ/kg. The minimum (ideal) refrigeration work required to do this is not Δh, but Δh - T_oΔs, which in this case works out to -58 kJ/kg (negative in this case indicating work input to the refrigerator).
posted by yarmond at 3:25 PM on July 24, 2008

Yarmond, according to the link you provided, the enthalpy of CO₂ at 2 MPa is 436 kJ/kg at 25°C and 129 kJ/kg at -31°C - a difference of 307 kJ/kg.
posted by Mapes at 4:35 PM on July 24, 2008

Is this for some kind of carbon capture and storage project?
posted by sindark at 4:46 PM on July 24, 2008

Mapes, you're looking at the internal energy, not the enthalpy.
posted by yarmond at 5:08 PM on July 24, 2008

Maybe this is a dumb question, but CO2 behaves weirdly with regard to phase change. It might want to go right to solid state at 2mpa...?
posted by gjc at 6:23 PM on July 24, 2008

Mapes, you're looking at the internal energy, not the enthalpy.

Ah, right you are, of course.
posted by Mapes at 7:30 PM on July 24, 2008

Thanks yarmond. I really wish I'd come across that tool before getting knee deep in Rivkin's Thermodynamics of Gases and the myriad of complicated equations he uses to assemble his data.

sindark - yes, it is related to some CO2 work. It is easy to find calculations for energy required to compress CO2 for transport in pipelines (at 13.8 Mpa or thereabouts) but I haven't found any calculations for compressing and cooling to the 2 MPa, -31ºC (or thereabouts) condition used for tanker transport. So I took a crack at doing it myself. And only got so far.
posted by gspm at 7:22 AM on July 25, 2008