November 6 2012 3 06 /11 /November /2012 19:47
The creation of an artificial atmosphere is necessary for manned underwater and space travel. Humans breathe in oxygen and exhale carbon dioxide and water. Air contains 78% nitrogen, 21% oxygen, 0.04% carbon dioxide, and a trace amount of water vapor. Humans exhale about 4% carbon dioxide and 17% oxygen. 15% oxygen is the minimum that can be breathed safely. Mental impairment starts at 14%, and unconsciousness starts at 10%. Oxygen deprivation causes disorientiation but is not particularly painful. A pig can show that well. (Video clip from the BBC documentary "How to Kill a Human Being")
Carbon dioxide levels below 2% are not harmful, while above that level causes an increased rate of breathing and a sense of suffocation, as carbon dioxide concentration and not oxygen concentration is used to produce the feeling of asphyxiation. Greater than 10% causes "a strangling sensation". (ref) Excess water vapor is not harmful to humans, but it can damage equipment.
Ideally, an artificial atmosphere will add oxygen and remove carbon dioxide and water, keeping them at a normal level. There are several ways to add oxygen. One is a simple oxygen tank. Oxygen is separated from liquefied air and placed in a tank under high pressure. These tanks are readily available in welding supply stores and medical supply companies. However, they are not compact and require storage of a compressed gas. This is not an option for nuclear submarines which stay underwater for months. Instead, electrolysis of water is used. This process runs off the nuclear power from the submarine and produces pure oxygen from a solution of an electrolyte in water. This is much more compact and can produce a huge amount of oxygen. Sea water cannot be used because electrolysis of salt water produces a toxic mixture of chlorine and oxygen. Another method of oxygen generation is using dry chemicals. A mixture of sodium chlorate with a little iron powder generates a reliable stream of oxygen, while leaving only non-toxic byproducts behind. Some of the sodium chlorate helps burn the iron, while the rest is decomposed by the heat of the burning iron, releasing oxygen and sodium chloride. Other chlorates and perchlorates can be used similarly. Commercial airliners use barium peroxide and sodium chlorate, which react when ignited by a percussive igniter, producing a stream of oxygen gas.
An all-around chemical for creating an artificial atmosphere is potassium superoxide. This chemical reacts with water vapor, releasing oxygen and potassium hydroxide. The potassium hydroxide reacts with exhaled carbon dioxide, forming potassium carbonate and releasing water. The water starts the cycle again, and the reaction goes on until all of the chemicals are exhausted. However, not enough carbon dioxide is absorbed by this reaction, and it is dangerously reactive with liquid water.
Carbon dioxide absorbers are primarily alkalis. The most common is soda lime, which consists of a mixture of sodium and calcium hydroxides. The sodium hydroxide readily absorbs carbon dioxide, forming sodium carbonate. This then reacts with calcium hydroxide, precipitating insoluble calcium carbonate (which drives the reaction forward) and producing sodium hydroxide. On a space station, the more expensive but lightweight lithium hydroxide is used. Due to its smaller atomic mass, a smaller mass of lithium hydroxide can absorb an equal amount of carbon dioxide (compared to soda lime).
Water absorbers can be such common substances as silica gel. Calcium chloride and magnesium sulfate are excellent water absorbers and are quite cheap. Dehumidifiers condense water on cold coils, removing it from the atmosphere by electricity without the need for a chemical treatment. These are better for large artificial atmospheres.
Soda lime and oxygen canisters are not difficult to get hold of, but producing an apparatus with enough surface area and circulation to absorb exhaled carbon dioxide at home can be challenging. Still, it would be an interesting project to create an artificial atmosphere. Firefighters, hazmat crews, and space technicians use them all of the time to get pure breathing air in an unbreathable atmosphere.