Atmospheric Air mainly consists of Oxygen and Nitrogen gases along with
small quantities of water vapour, Carbon Dioxide, Argon, Helium, etc. Oxygen
and Nitrogen from the Air are separated due to difference in boiling points
by distillation through a fractional column.
Atmospheric Air is sucked in by a multi stage Compressor through a filter
and is compressed to the design pressure. The compressed Air is then passed
through inter-coolers, Industrial Refrigerator, Moisture Separators, and
then to the Molecular Sieve Battery for removal of Carbon Dioxide,
Hydrocarbons and Moisture from the process Air. This pure Air then passes
through the 1st Heat Exchanger, where it is cooled by the out going Nitrogen
and Oxygen. Part of this cooled Air is passed through Expansion Engine and
the other part through the 2nd Heat Exchanger. Both the Expansion Engine and
2nd Heat Exchanger help in further cooling down the Air, which is finally
released to the bottom of the column through an expansion valve. The Air
becomes liquid at this stage.
The column consists of two parts. The lower column and upper column. In
between the lower and upper columns there is a condenser, which acts as a
reflux for the lower column and as a re-boiler for the upper column. The
liquid Air at the bottom of lower column separates through the trays to give
crude Oxygen at the bottom and approximately 90% pure Nitrogen at the top.
Crude Oxygen termed as rich liquid is then expanded through an Expansion
Valve from the lower column to the middle of the upper column. Crude
Nitrogen termed as poor liquid is expanded through another Expansion Valve
from the top of the lower column to the top of the upper column. Due to
difference in the boiling points, the pure Nitrogen boils over and
accumulates at the top of the upper column and Oxygen, which accumulates at
the bottom of the upper column.
Both Nitrogen and Oxygen are removed through separate paths in Heat
Exchangers, for cooling the incoming air. Oxygen is compressed to a
prescribed settled pressure by a liquid pump and is directly filled into
cylinders. Nitrogen is however available at a pressure of approximately
O.5kg/cm2 and the same can be compressed into cylinders with help of an
independent high- pressure compressor.
Normally the Nitrogen available is of 98.5% purity only and is let out into
the atmosphere as waste product. However, when Nitrogen is to be filled,
into the cylinders for commercial use, the plant is controlled by a change
in the valve setting to make the mixed Air chamber operative to achieve the
required Nitrogen purity. (Facility of filling Nitrogen gas (PPM Grade) and
withdrawal of liquid Nitrogen is also possible.)