Naked Science Forum

General Science => Question of the Week => Topic started by: thedoc on 29/01/2008 19:52:06

Title: QotW - 08.02.03 - Recycled Oxygen
Post by: thedoc on 29/01/2008 19:52:06

How is oxygen made and recycled on the International Space Station?
Asked by Tom Gallard, London

Title: Re: QotW - 08.02.03 - Recycled Oxygen
Post by: thedoc on 29/01/2008 19:52:06

We put this question to Mark Hempsell, Senior Lecturer, University of Bristol:

The oxygen isn’t strictly recycled.  The carbon dioxide that the humans breathe out is filtered out of the air with a molecular sieve then simply dumped overboard.  The oxygen is created from the water that comes from the air and from the washbasins and from the loos.  After that water is cleaned up it is electrolysed.  An electric current is passed through it and it is separated into hydrogen and oxygen.  The hydrogen is dumped overboard; the oxygen is fed into the cabin for the crew to breathe.

Title: QotW - 08.02.03 - Recycled Oxygen
Post by: DoctorBeaver on 31/01/2008 15:14:20

The only way I can think to produce it would be by splitting water into hydrogen & oxygen; but wouldn't it be easier/cheaper to ship pressurised oxygen cylinders from Earth?

As for re-cycling it - a series of biological or chemical filters, maybe?

Title: QotW - 08.02.03 - Recycled Oxygen
Post by: lightarrow on 01/02/2008 08:17:00

If I remember correctly, at least in the past they used compounds like lithium (usually preferred because lighter) or potassium peroxide: it absorbs the CO₂ and the H₂O produced by the astronauts and in doing this it releases O₂:

2Li₂O₂ + 2CO₂ --> 2Li₂CO₃ + O₂

2Li₂O₂ + 2H₂O --> 4LiOH + O₂

Edit: In a 15 y.o. chemistry book I have (Malatesta, Inorganic Chemistry, third edition) is stated: "...Potassium superoxyde KO₂ is mostly used to "transform" CO₂ in O₂ in submarines, space veichles, breath masks:
4KO₂ + 4CO₂ + 2H₂O --> 4KHCO₃ + 3O₂.

Title: QotW - 08.02.03 - Recycled Oxygen
Post by: lyner on 01/02/2008 17:24:30

http://science.nasa.gov/headlines/y2000/ast13nov_1.htm (http://science.nasa.gov/headlines/y2000/ast13nov_1.htm)
This talks about the problem and makes sense.

Title: QotW - 08.02.03 - Recycled Oxygen
Post by: chris on 03/02/2008 10:55:33

The NASA article says something very interesting - that emergency oxygen is produced by "perchlorate candles" and that the same system is used to produce the emergency oxygen supply on aeroplanes. Unfortunately they give no details of this reaction - so what is it?

Chris

Title: QotW - 08.02.03 - Recycled Oxygen
Post by: lightarrow on 03/02/2008 13:06:45

Quote from: chris on 03/02/2008 10:55:33

The NASA article says something very interesting - that emergency oxygen is produced by "perchlorate candles" and that the same system is used to produce the emergency oxygen supply on aeroplanes. Unfortunately they give no details of this reaction - so what is it?

Chris

http://en.wikipedia.org/wiki/Chemical_oxygen_generator:

Quote

A chlorate candle, or an oxygen candle, is a cylindrical chemical oxygen generator containing a mix of sodium chlorate and iron powder. When ignited, the mixture smolders at about 600 °C (1100°F), producing sodium chloride, iron oxide, and about 6.5 man-hours of oxygen per kilogram of the mixture. It releases oxygen at a fixed rate. The mixture has an indefinite shelf life if stored properly; candles stored for 20 years have shown no decrease in oxygen output. The oxygen is released by thermal decomposition. The heat is supplied by the burning iron.

I imagine that with perchlorate it works in the same way.