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Mars Academy
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.
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