Recycling Air

The basic requirement of the atmosphere is to provide oxygen at a partial pressure sufficient for metabolic needs.  The temperature and relative humidity (RH) must be appropriate and the levels of carbon dioxide and trace contaminants must be sufficiently low to avoid adverse affects.  Life support gas delivery systems have a singular purpose - to deliver oxygen and its inert carrier at a pressure and quantity that will sustain nominal, productive functioning of the human crew.  

It is essential to control the levels of oxygen and carbon dioxide as an excess of either of them can produce unfavourable results.  For example when the partial pressure of oxygen is as low as 13.75kPa, virtually everyone will experience altitude sickness including shortness of breath, headaches, insomnia, impaired ability to concentrate or perform complex tasks, and sometimes nausea and vomiting.  The physiological effects of high levels of carbon dioxide include increased respiration rate, heart rate, and blood flow to the brain; hearing losses; mental depression; headaches; dizziness; nausea and ultimately unconsciousness.

Gases can be recycled and produced in several ways.
1.  oxygen can be generated by reduction of carbon dioxide
         Decomposition of carbon dioxide:    CO2 --> CO (g) + O2

2.  oxygen can also be generated by the electrolysis of water
         Electrolysis of water:  2H2O --> O2 + 2H2
(Both of these methods are very energy intensive and will not work for a long-duration mission)

3.  It might be possible to generate oxygen from lunar soils and there is abundant data from the Viking probes to suggest that oxygen may be available from superoxides in the Martian soils or it can be generated from the thin CO2 in the Martian atmosphere.  All of these processes will require energy.  It is estimated that 75 kilowatts of energy would be required to extract 22 pounds of liquid oxygen from the Martian air.

4.  Carbon dioxide can be removed by lime-water absorption, but this method is not very effective.

5.  Carbon dioxide can be reacted with lithium hydroxide (solution absorbing CO2)

6.  In a system called the Solid Amine Water Desorbed (SAWD) system, an Amine is attached to a resin and CO2 binds to the resin and is eventually released into vacuum.

7.  Another process of CO2 removal is  called Ion Exchange Electrodialysis, which is a complex process where CO2 is passed over ion exchange resins reacting with the CO2 to form carbonate ions.  At the same time, the resin is regenerated by an electrical field and by a system of counterflow, the carbonate ion concentrated in an adjacent cell.

8.  In the Sebatier process CO2 is reduced in combustion with hydrogen to form methane and oxygen.  But this process has many disadvantages mainly due to the fact that it requires complex equipment requirements, a large mass of equipment and very high energy requirements and cooling down.  

9.  The Bosch process is a CO2 combustion process which produces solid carbon and oxygen by the equation:           
CO2 + 2H2 --> 2H2O + C    
This process has its disadvantages as it also requires large equipments and a large input of energy along with cool down requirements.  

Replenishment gas is necessary in any habitat system because all systems leak to some extent.  A replenishment mixture is used to restore cabin pressure when it falls below a certain level.  Nitrogen, helium, and sometimes even argon or neon can be used as carrier gases.  The fundamental principal is that a carrier gas contributes to the partial pressures and mediates the level of oxygen.

Two methods of gas removal, production and recycling are proposed for our manned-mission to Mars.

Choice 1
The nitrogen and oxygen composition of the atmosphere will be controlled by signals from air an composition monitor and a special computer software.  This atmosphere Revitalization Subsystem demonstrates the capability of providing a healthy working environment for the crew and achieves a power saving of up to 200 watts over other kinds of operating modes.  These savings are significant and represent additional electrical power available for science experiments onboard our space-mission.  (Note: other additional testing is still going on in order to determine the capability of various types of subsystems to remove other trace contaminants)  Breathing air will be cleansed of deadly carbon dioxide in three steps.  First the carbon dioxide will be filtered and accumulated in a container.  Then it will be combined with hydrogen in a catalytic converter to produce methane gas and water.  The methane will be vented and the water will be broken down into new oxygen for breathing and hydrogen for reuse in the process.  This is one of the most efficient ways of recycling breathable air.

Choice 2
In another process, called a Regenerable Absorption Process, CO2 is loosely absorbed onto ion exchange materials and then released under carefully controlled conditions.  The most commonly used regenerable absorption process is called a Solid Amine Water Desorbed (SAWD).  The SAWD consists of weak-base solid ion exchange resin. The resin is produced in a process where an amine chemical, for eg. diethylenetriamine, is attached to the resin (porus polystyrene divinylbenzine).  The CO2 then passes over the resin and a chemical reaction takes place, loosely binding to the CO2 resin.  The ion exchange bed is used until it cannot bind any more CO2.  The resin bed is then exposed ot a vacuum, which is of course very plentiful in a space envrionment, and the CO2 is released from the SAWD.  This relatively fast process of regeneration frees the ion exchange column so that it can be used again.