Comments on: Ask a chemist - cooling CO2
http://ask.metafilter.com/97439/Ask-a-chemist-cooling-CO2/
Comments on Ask MetaFilter post Ask a chemist - cooling CO2Thu, 24 Jul 2008 15:25:39 -0800Thu, 24 Jul 2008 15:25:39 -0800en-ushttp://blogs.law.harvard.edu/tech/rss60Question: Ask a chemist - cooling CO2
http://ask.metafilter.com/97439/Ask-a-chemist-cooling-CO2
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. <br /><br /> The 2 MPa, -31ºC is the IPCC's state for transport of CO2 by road or rail tanker.<br>
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I've assumed an isentropic two-stage compression to get to 2 MPa.<br>
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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?).<br>
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What do I need to do to calculate the energy required for that 2 MPa at 25ºC to 2 MPa at -31ºC step?<br>
∆h = cp∆T but what about the phase change? (I knew how to handle this, once, long ago)post:ask.metafilter.com,2008:site.97439Thu, 24 Jul 2008 14:51:12 -0800gspmchemistryscienceCO2gascompressionBy: yarmond
http://ask.metafilter.com/97439/Ask-a-chemist-cooling-CO2#1419917
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 <a href="http://webbook.nist.gov/chemistry/fluid/">NIST</a> for CO<sub>2</sub> and many other fluids.<br>
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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<sub>o</sub>Δs, which in this case works out to -58 kJ/kg (negative in this case indicating work input to the refrigerator).comment:ask.metafilter.com,2008:site.97439-1419917Thu, 24 Jul 2008 15:25:39 -0800yarmondBy: Mapes
http://ask.metafilter.com/97439/Ask-a-chemist-cooling-CO2#1420013
Yarmond, according to the link you provided, the enthalpy of CO<sub>2</sub> at 2 MPa is 436 kJ/kg at 25°C and 129 kJ/kg at -31°C - a difference of 307 kJ/kg.comment:ask.metafilter.com,2008:site.97439-1420013Thu, 24 Jul 2008 16:35:42 -0800MapesBy: sindark
http://ask.metafilter.com/97439/Ask-a-chemist-cooling-CO2#1420030
Is this for some kind of carbon capture and storage project?comment:ask.metafilter.com,2008:site.97439-1420030Thu, 24 Jul 2008 16:46:21 -0800sindarkBy: yarmond
http://ask.metafilter.com/97439/Ask-a-chemist-cooling-CO2#1420059
Mapes, you're looking at the internal energy, not the enthalpy.comment:ask.metafilter.com,2008:site.97439-1420059Thu, 24 Jul 2008 17:08:07 -0800yarmondBy: gjc
http://ask.metafilter.com/97439/Ask-a-chemist-cooling-CO2#1420133
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...?comment:ask.metafilter.com,2008:site.97439-1420133Thu, 24 Jul 2008 18:23:06 -0800gjcBy: Mapes
http://ask.metafilter.com/97439/Ask-a-chemist-cooling-CO2#1420193
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Mapes, you're looking at the internal energy, not the enthalpy.</em><br>
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Ah, right you are, of course.comment:ask.metafilter.com,2008:site.97439-1420193Thu, 24 Jul 2008 19:30:39 -0800MapesBy: gspm
http://ask.metafilter.com/97439/Ask-a-chemist-cooling-CO2#1420554
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.<br>
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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.comment:ask.metafilter.com,2008:site.97439-1420554Fri, 25 Jul 2008 07:22:44 -0800gspm