|Posted by Rohvannyn on February 14, 2015 at 6:00 PM|
February 13, 2015
This is the beginning of what will hopefully be a series on life support and bioastronautics. The main object is the formulation of possible alternative means of providing low-mass alternatives for private space transports with human crews or colonies in space or on other celestial bodies.
As is well-known, Lithium Hydroxide (LiOH) is commonly used aboard space craft for scrubbing carbon dioxide from cabin air. This is accomplished through the reaction 2(LiOH)+CO2 > LI2CO3+H2O. The lithium compound therefore absorbs carbon dioxide but moistens the air of the cabin. When I was looking at early designs for the Biodyne environment/propulsion system (mentioned elsewhere on this site) during high school and college It seemed obvious that if Lithium metal could be brought on an extended voyage instead of LiOH, it should be possible to assist in the removal of water vapor as well as CO2 from cabin air and the reactions might constitute a power source in the same way that hydrox fuel cells do double duty as water suppliers and generators of DC power. Originally the biodyne was conceived as a set of double-duty reactions which furnishing life support and generating power, could become a propulsive mechanism by using excess power to accelerate reduced end-products out electric rocket engines of some description.
Lithium with a molar mass of 6.941 is about 29 % as massive as LiOH in terms of unit amount needed to absorb a mol of CO2. Lithium burned with oxygen (2Li)+(.5O2) > Li2O can form 2(LiOH) with the addition of 1 H2O molecule. The combination of Lithium metal with pure oxygen however is one of the most powerful reactions known and even LI2O with water is “explosive.” If we could carry forward both reactions safely in a confined space, Metallic Lithium in a pure state might be an optimum reagent on which to base CO2 removal and some drying of atmosphere.
We’ll imagine three chemical reactors which we’ll call A, B and C. B and C are equivalent. In A lithium hydroxide is contained and it removes CO2 from moist cabin air, producing lithium carbonate and additional water vapor (see above reaction.) From A the moister, scrubbed air goes either to B or C in which lithium oxide is contained. Water from air reacts with Li2O to form LiOh and each molecule of LiOH can also absorb an additional molecule of water to form hydrated LiOH. Once the LI2O in B. has been saturated with sufficient water to reach LIOH potential and hydrate same, the flow from A will be routed to C. while B is brought to a temperature of 109 C (228 F) which will drive of the water of hydration from the LiOH. This can be captured and recycled for further use in the life support system. Nonhydrated LiOH would then be transferred to A for further DO2 capture. Of course When C. has been saturated, the flow from A. will shunt back to B.
In this way we can potentially use metallic Lithium for removing CO2 and some of the excess moisture from bacin air and if we’re clever, we might gain an additional power source. The amount of power to be gained from a lithium-Oxygen reaction matched to Human CO2 output is only about 20 % of that forthcoming from H2-O2 fuel cells generating water, assuming 20 liters per day for a 6-person crew. This assumes very high efficiency of energy conversion. When I was in college some success was being had with lithium fuel cells using air as electrolyte. If we had a need for very high temperature on board for some reason, possibly processing sewage, it might be reasonable to burn lithium in pure oxygen in a closed reaction vessel to generate Li2O in a batch which could be transferred to B. or C. or perhaps the reactor could be designed to serve as the B. or C. reactors, needing only a rotating of modules. In the same way A. might be built in the same configuration, removing the need to actually handle the various chemicals except when carbonate must be removed and new lithium inserted.
All of this suggests some interesting areas of small-scale research and as the same reactions work with sodium, it need not be overly expensive.