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A. J. deLange | all galleries >> Galleries >> miscellaneous > partition.jpg
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This chart assumes that a container pressurized with pure nitrogen is partially filled with beer from another container in which it is in equilibrium with CO2 at 12 psig at 40 F. Another way of achieving this same situation is to assume that a container which is partially filled with beer at 40 F and pressurized with CO2 to 12 psig has the CO2 swept out by nitrogen.

As the partial pressure of CO2 in the headspace is 0 CO2 will move from the beer to the headspace until equilibrium is established. At this point the chemical potentials of CO2 are the same in the headspace and the beer and no further exchange of CO2 occurs. The left axis shows the equilibrium headspace pressure and the right hand axis the volumes of CO2 in the beer vs the fraction of the container's volume which is headspace. Thus is the container is filled (headspace fraction a very small number) the pressure in the headspace is 12 psig and the volumes of CO2 in the beer about 2.5. If the container is only half filled, the equilibrium partial pressure of CO2 in the headspace would be about 1 psig and the volumes about 1.4.

Note that this chart does not represent what would happen if the container were filled and then the beer drawn off. The math for that would be appreciably more difficult

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