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Fractional distillation of Liquefied air.

Answer Text:
Fractional distillation of
Liquefied air.
- Air is a mixture of gases;
- It can be separated into its constituents by fractional distillation of liquid air.
- During the process air is passed through a series of steps during which it is purified, some components eliminated then it is compressed into liquid prior to fractional distillation.
- The process can be divided into two main stages; Purification and liquefaction;
Fractional distillation of
air;
(a). Purification and liquefaction.
Step 1: Purification:
- The air is purified by removal of dust particles ;
- This is done through the following ways:
Passage through filters; during which air is passed through a series of filters; the dust particles remain within the filters while dust free air passes on to the next stage;
- Electrostatic precipitation where air is passed through charged electrodes which trap oppositely charged dust particles;
Step 2: Removal of carbon (IV) oxide.
- The dust-free air is passed through a chamber containing calcium
hydroxide solution;
- The sodium hydroxide solution dissolves the carbon (IV) oxide present in the air;
- During the reaction, sodium carbonate and water are formed;
- Over a prolonged time; the sodium carbonate absorbs more (excess) carbon (IV) oxide forming sodium hydrogen carbonate;
Equations:
Sodium hydroxide + carbon (IV) oxide → Sodium
carbonate + Water;
`2NaOH(aq) + CO_2(g) to Na_2CO_3(aq) + H_2O(l)`;
In excess;
Sodium carbonate + Water +
Carbon (IV) oxide → Sodium hydrogen carbonate;
`Na_2CO_3(aq) + H_2O(l) + CO_2(g) → 2NaHCO_3(aq)`;
Step 3: Removal of water vapour;
- The dust-free, `CO_2` – free air is then cooled to `-25^oC`;
- This process solidifies the water vapour out as ice;
- This cooling process may be done at temperatures a s low as `-80^0`; so as to solidify any carbon (IV) oxide
(freezing point `-78^oC`) that may have escaped absorption by the sodium hydroxide;
- The removal of water vapour and carbon (IV) oxide are important because it prevents blockage of the pipes in the rest of the system;
Step 4: Liquefaction of air;
- The dry, dust-free and carbon (IV) oxide-free air is compressed to about 100 atmospheres of pressure; causing it to warm;
- The compressed air is cooled by refrigeration;
- The cold compressed air is made to expand rapidly by passage through a nozzle which cools it further;
- The repeated compression, cooling and expansion of air causes it to liquefy at about `- 200^oC`:
Note:
-At this temperature only neon and helium whose boiling points re lower than `-200^oC` remain in gaseous states;
(b). Fractional distillation;
- The liquid air now consists only of nitrogen, oxygen and noble gases (especially argon);
- The liquid air is fed at the bottom of a fractionating
column;
- It is warmed to a temperature of `-192^oC`;
-Nitrogen distils over fast at
-`196^oC` because it has a lower boiling point; and is collected at the top of the fractionating column;
Note:
- Any vapours of oxygen and argon which rise together with nitrogen vapour condense in the column and fall back as liquids;
- The nitrogen collected is 99% pure;
The small amounts of impurities include neon and helium;
- The liquids remaining at the bottom of the fractionating column after vaporization of all nitrogen is mainly oxygen and argon; with traces of krypton and xenon;
- The liquid is again warmed further to a temperature of `- 185^oC`; causing the vapourization of argon whose boiling point is`- 186^oC`;
- This is collected as a gas at the top of the fractionating column;
- The residue liquid is mainly oxygen with minute quantities of krypton and xenon which have even high boiling points;
- The oxygen is drained off and stored as pressurized oxygen in steel cylinders