An apparatus has been developed as a means of controlling the partial pressure of CO2 (pCO2) in air in a closed or semiclosed environmental system. The apparatus takes CO2 from the air in a source chamber and supplies the CO2, at a regulated partial pressure, to the air in a sink chamber. In the original intended application, the chambers would be located aboard a spacecraft: the source chamber would be inhabited by crewmembers, and the sink chamber would be a plant-growth chamber. Apparatuses like this one could also be used to control pCO2 in research plant-growth chambers on Earth.
In the case of an inhabited source chamber, the pCO2 in that chamber can generally be expected to exceed the pCO2 required for a sink chamber in which plants are to be grown. In such a case, CO2 can be gathered periodically from the source chamber to maintain the required CO2 loading of the solution while the required pCO2 in the plant-growth chamber is maintained through equilibrium exchange. Because of the large CO2 capacity of the alkanolamine, stable control of the pCO2 in the plant-growth chamber is easily achieved.
When the pCO2 in the source chamber is less than the pCO2 required in the sink chamber, temperature-dependent solubility is used to pump CO2 against its concentration gradient: Inasmuch as the equilibrium pCO2 above the alkanolamine solution increases with temperature, CO2 is gathered through the membrane on the source-chamber side at a temperature lower than that at which it is released through the other membrane on the plant-growth-chamber side.
The absorption of CO2 in an alkanolamine solution takes place through reversible ionization reactions that change the relative abundance of electrically conductive species in the solution. As a result, there is a relationship between the electrical conductance of the solution and its CO2 load. This relationship is exploited in the present apparatus: The electrical conductance measured at a given temperature is taken as an indication of the CO2 loading of the solution and, by extension, of the equilibrium pCO2 of the atmosphere in contact with the solution.
This work was done by Jeffrey L. DeHart, James R. Akse, and James E. Atwater of Umpqua Research Co. for Johnson Space Center. For more information, contact the Johnson Commercial Technology Office at (281) 483-3809.
MSC-22911

NASA Tech Briefs Magazine
This article first appeared in the November, 2001 issue of NASA Tech Briefs Magazine.
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