Chemical oxygen generator

 

 

Commercial aircraft provide emergency oxygen to passengers to protect them in case of loss of cabin pressure. Chemical oxygen generators are not used for the cockpit crew, who are typically supplied using compressed oxygen cylinders, also known as oxygen bottles. In narrow body airliners, for each row of seats there were overhead oxygen masks and oxygen generators. In some wide-body airliners, such as the DC-10 and IL-96, the oxygen generators and oxygen masks are mounted in the top portion of the seat backs, since the ceiling is too high above the passengers. If a decompression occurred, the panels were opened either by an automatic pressure switch or by a manual switch, and the masks are released. When the passengers pull down on the mask they removed the retaining pins and triggered the production of oxygen.

The oxidizer core is sodium chlorate (NaClO₃), which is mixed with less than 5 percent barium peroxide (BaO₂) and less than 1 percent potassium perchlorate (KClO₄). The explosives in the percussion cap are a lead styphnate and tetrazene explosive mixture. The chemical reaction is exothermic and the exterior temperature of the generator will reach 260 °C (500 °F). It will produce oxygen for 12 to 22 minutes.^[2][3] The two-mask generator is approximately 63 mm (2.5 in) in diameter and 223 mm (8.8 in) long. The three-mask generator is approximately 70 mm (2.8 in) in diameter and 250 mm (9.8 in) long.

In May 11, 1996, accidental activation of improperly shipped expired generators, mistakenly labeled as empty "oxy canisters",^[4] caused the ValuJet Airlines Flight 592 crash, killing all on board.^[5] An ATA DC-10, Flight 131, was also destroyed while parked at O'Hare Airport, on August 10, 1986. The cause was the accidental activation of an oxygen generator, contained in the back of a broken DC-10 seat, being shipped in the cargo compartment to a repair station. There were no fatalities or injuries because the plane contained no passengers when the fire broke out.^[6][7]

A chlorate candle, or an oxygen candle, is a cylindrical chemical oxygen generator that contains a mix of sodium chlorate and iron powder, which when ignited smolders at about 600 °C (1,100 °F), producing sodium chloride, iron oxide, and oxygen at a fixed rate of about 6.5 man-hours per kilogram of the mixture. The mixture has an indefinite shelf life if stored properly: candles have been stored for 20 years without decreased oxygen output. Thermal decomposition releases the oxygen. The burning iron supplies the heat. The candle must be wrapped in thermal insulation to maintain the reaction temperature and to protect surrounding equipment. The key reaction is:^[8]

2 NaClO₃ → 2 NaCl + 3 O₂

Potassium and lithium chlorate, and sodium, potassium and lithium perchlorates can also be used in oxygen candles.

In the Vika oxygen generator used on some spacecraft, lithium perchlorate is the source of oxygen. At 400 °C (750 °F), it releases 60% of its weight as oxygen:^[9]

LiClO₄ → LiCl + 2 O₂

Advances in technology have provided industrial oxygen generator systems for use where air is available and a higher concentration of oxygen is desired. Pressure swing adsorption (PSA) incorporates a material called molecular sieve for gas separation. In the case of oxygen generation a zeolite-based sieve forces preferential adsorption for nitrogen.^{[citation needed]} Clean, dry air is passed through the sieve beds on the oxygen generator, producing an oxygen-enriched gas. Nitrogen separation membrane equipment is also used.

Chemical oxygen generators are used in aircraft, breathing apparatus for firefighters and mine rescue crews, submarines, and everywhere a compact emergency oxygen generator with long shelf life is needed. They usually contain a device for absorption of carbon dioxide, sometimes a filter filled with lithium hydroxide; a kilogram of LiOH absorbs about half a kilogram of CO₂.

• Self-contained oxygen generators (SCOGs) are used in submarines.
  • They were successfully used by the USS New Hampshire, an American nuclear-powered submarine, in March 2011 when the submarine suffered an oxygen generator failure while under the Arctic ice cap.^[10]
  • An explosion caused by a so-called oxygen candle killed two Royal Navy sailors on HMS Tireless (S88), a nuclear-powered submarine, under the Arctic on 21 March 2007.^[11] The candle had become contaminated with hydraulic oil, which caused the mixture to explode rather than burn.^{[12][failed verification]}
• Self-contained self-rescue devices (SCSRs) are used to facilitate escape from mines.
• On the International Space Station, chemical oxygen generators are used as a backup supply. Each Vika oxygen generator can produce enough oxygen for one crewmember for one day.^[13]

• Oxygen storage
• SolidOx (welding)