[Digestion] delivering biogas in cylinders

[AJH]

On Wed, 16 May 2007 22:08:24 +0530, adkarve wrote:

>I was told by an
>energy expert that by not removing the carbon dioxide from the biogas, one
>may be able to accommodate more methane in a cylinder, because methane
>dissolves in liquid carbon dioxide.  Thus, one compresses biogas till the
>carbon dioxide is liquified, and the methane in the mixture automatically
>dissolves in liquid carbon dioxide.

I tried to investigate this on [stoves] some years ago. If we assume a
60:40 split between CH4 and CO2 in biogas and 200 atmosphere as being
an upper practical pressure limit with an ambient temperature of 27C
then from an online calculator at Quest Consultants Inc. I get:

"Input Mixture

  Methane, CH4                               : 0.60000000
  Carbon Dioxide, CO2                        : 0.40000000

Average mixture critical properties

  critical compressibility  :       0.283
  critical density          :      10.308 kgmol/m3
  critical pressure         :    5712.000 kPa
  critical temperature      :     235.880 K
  normal freezing point     :     172.094 K

Mixture properties at 300 K and 20265 kPa

  material phase is 100.00 mole % vapor
  vapor density             :      13.024  kgmole/m3
  liquid density            :      13.025  kgmole/m3
  vapor enthalpy            :    2967.785  kJ/kgmol
  liquid enthalpy           :    2967.242  kJ/kgmol
  vapor heat capacity       :      78.373  kJ/kgmol-K
  liquid heat capacity      :      78.378  kJ/kgmol-K
  feed molecular weight     :      27.230  kg/kgmol
  vapor molecular weight    :      27.230  kg/kgmol
  liquid molecular weight   :      27.232  kg/kgmol
  vapor upper flammable limit: 0.22058824 mole fraction
  vapor lower flammable limit: 0.08333333 mole fraction

vapor compositions

  Methane, CH4                               : 0.60000000
  Carbon Dioxide, CO2                        : 0.40000000

liquid compositions

  Methane, CH4                               : 0.59992503
  Carbon Dioxide, CO2                        : 0.40007497

Isenthalpic flash to a pressure of 101.325 kPa

  temperature after flash   :     170.506 K
  pressure after flash      :     101.325  kPa
  material phase is 93.50 mole % vapor
  vapor density             :       0.073  kgmole/m3
  liquid density            :      29.306  kgmole/m3
  vapor enthalpy            :    4105.391  kJ/kgmol
  liquid enthalpy           :  -13392.434  kJ/kgmol
  vapor heat capacity       :      29.727  kJ/kgmol-K
  liquid heat capacity      :      69.746  kJ/kgmol-K
  feed molecular weight     :      27.230  kg/kgmol
  vapor molecular weight    :      26.086  kg/kgmol
  liquid molecular weight   :      43.682  kg/kgmol
  vapor upper flammable limit: 0.21046018 mole fraction
  vapor lower flammable limit: 0.07801470 mole fraction

vapor compositions

  Methane, CH4                               : 0.64090486
  Carbon Dioxide, CO2                        : 0.35909514

liquid compositions

  Methane, CH4                               : 0.01173579
  Carbon Dioxide, CO2                        : 0.98826421"

Which suggests the CO2 does not significantly liquefy in this mixture
but at this temperature and pressure a sample of pure CO2 would all be
liquid.

Unless I have misinterpreted the result it seems more energy efficient
to remove CO2 before any attempt at compression.

AJH