02-03 Ventilation

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Under the head of "Ventilation" we comprehend all the quotidian processes related to the management of the municipal atmosphere of Luna City. This may call for a change in title.

There are four characteristics of the atmosphere which require management. These are composition, pressure, temperature, & mixing. Composition includes the proportions of oxygen, diluent gas if any, carbon dioxide, water vapour (humidity), trace gases, & airborne solid & liquid matter. Pressure & temperature are largely self-explanatory, although it is to be noted that they will vary in different parts of the city. Mixing is not generally a matter which receives much attention, but is nonetheless important -- & most people realize this when they encounter uncomfortably still air.

Managing these four characteristics will not be a simple or a static task. Both the requirements & the methods used to achieve them will evolve. For example, the settlement must initially rely entirely on "mechanical" methods for its air-handling, while in subsequent phases, as food production by plants becomes more important, so the contribution of green life to atmospheric renewal will increase, as will the special burdens (transpiration, ethylene, &c.) which it places on the system. As the experience of Biosphere 2 showed, a small system cannot rely on biological processes to accomplish what they do in the terrestrial ecosystem, with a truly immense & diverse biological complement, & enough inertia to permit extremely long cycle times. While we expect Luna City to grow towards a more "natural" or "passive" model, in the foreseeable future the artificial components will dominate the system.

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Carbon Dioxide
One minor atmospheric constituent which calls for our attention is carbon dioxide. In small quantities it is not merely unobjectionable but necessary, as dissolved carbon dioxide in the blood helps to govern the breathing reflex. In larger quantities, however, it is troublesome to humans ; while not especially toxic, high concentrations cause a strong feeling of discomfort, even amounting to panic. It is also vital to the growth of plant life. In general, most of our plantings will be enclosed in a separate "greenhouse" atmosphere, with higher levels of CO2 & water vapour than humans will be comfortable with for long periods. We must then prevent the CO2 levels in the municipal atmosphere from rising too high (under ordinary circumstances a strong fall is not to be anticipated) as a result of human & animal metabolism, while preventing the greenhouse atmosphere from experienceing either a drop to excessively low CO2 levels due to photosynthesis, or a rise to excessively high levels due to plant respiration, of particular concern during the night when light is restricted.

Separation

The method commonly used for removing carbon dioxide from spacecraft atmospheres is chemical. The compound CO2 can be considered as carbonic acid anyhydride, & there are various alkaline substances which will combine with it, when present in the atmosphere, to form carbonates. Lithium hydroxide is commonly used in this application, as it will absorb 0.74 kg CO2 per kg LiOH. Ordinarily the absorbent is treated as a consumable, & no attempt is made to renew it, although certain alkaline carbonates can be decomposed by strong heating, restoring the absorptive property (the cement industry depends upon this fact).

A second method, which we may term "physical", has been proposed from time to time, but not (to my knowlede -- corrections?) put into practice. This depends on the fact that, although carbon dioxide does not liquefy at moderate pressures, its freezing point is considerably higher than the boiling points of the other common constituent gasses (save water vapour alone). Accordingly, strongly cooling the air, either by mechanical power (as in a liquid-air machine) or by exposure to a very low temperature heat-sink (such as may be available at a northward radiator), should cause the CO2 to "freeze out" as a solid. This "carbonic snow" can then be separated from the gas just as ordinary particulate matter would be, so long as the processing machinery is held at a sufficiently low temperature to prevent excessive resublimation.

A third method, believed original, is a kind of hybrid. As is well known, due to its property of forming the coordination compound "carbonic acid" (H2CO3), carbon dioxide is highly soluble in water, whereas most of the other constituents of air are slightly soluble. This solubility increases with pressure. Therefore, it appears that, if we compress the air to be processed & inject it into a reservoir of water in such a way as to form finely-divided bubbles, the water will tend to absorb CO2 preferentially to O2 at the large surface area thus provided. As the bubbles rise to the top, the head space above the water surface will be filled with a gas richer in oxygen than that which was introduced ; this gas can be permitted to escape to general circulation through a check valve set to the pump outlet pressure, giving a positive-displacement flow condition. The water is then carried through a second check valve, set the same, into a low-pressure container where it gives up its CO2-rich dissolved gas, which is handled as appropriate, & a force pump then drives the water back into the absorbing chamber.

Disposal

If it is not desired to lose the carbon & oxygen to productive use, & if no industrial demand for carbon dioxide arises, it is necessary to render the CO2 into some usable form. Of considerable importance, of course, is to release the oxygen, but carbon also is necessary for life. Two main methods of disposal, biological & electrochemical, present themselves. The biological method is the well-known process of photosynthesis, wherein water & carbon dioxide are converted, using light energy, into carbohydrate & free oxygen. This is of the utmost importance, as it is the root of all the processes we know of which produce usable human food. At least in the early phase of settlement, however, the available mass of plant life will not be sufficient to convert all of the CO2 produced by human, animal & plant activity, & the rate of photosynthetic conversion will necessarily vary with conditions of lighting, stages of plant growth, &c. The electrochemical method, therefore, should not be neglected. In general, when carbon dioxide & hydrogen gasses are mixed at the proper temperature & pressure, in the presence of the proper catalysts, the oxygen is transferred, producing water H2O & either methane CH4 or solid carbon. Electrolysis of the water then yields back the product oxygen & the reagent hydrogen, making (aside from the inevitable slow loss of hydrogen, which diffuses even through metal pressure vessels) a closed loop. The storage of methane, we may suggest, is an important method for matching biological carbon uptake & CO2 generation rates, & could also be used (in tandem with a capacity for storing unprocessed CO2) as a method of time-shifting energy.