TONNAGE OXYGEN PLANT(Pak. Steel).
Since
the LD-Converter can not produce steel without Oxygen and for a LD-Converter
shop to become an economically tonnage steel producer, high purity oxygen
should be readily available in large quantities. So Oxygen generating plant
affiliated with LD-Converter(BOF) must have high degree of operating continuity
and reliability.
The
composition of dry air is approximately 78% nitrogen, 21% oxygen, and 1% argon (by volume) plus small amounts of carbon dioxide,
neon, helium, krypton, hydrogen, and xenon. In addition, variable amounts of
water vapors are present (depending upon humidity) plus other gases produced by
natural processes and human activities.
The portions of the air separation process
that operate at very low temperatures (the distillation columns, heat
exchangers and cold interconnecting piping sections) must be well insulated to
minimize energy consumption and avoid operating problems. To accomplish this,
these components are located inside insulated, sealed (and nitrogen purged) structures called "Cold boxes” or Air separating unit.
Appearance of cold boxes of Tonnage Oxygen
Plant.
In
process of O2 production, the air is compressed, cooled and much of the water vapors
and CO2 in the incoming air is condensed and removed. The
Nitrogen, Oxygen and Argon are produced
by the fractional distillation of liquid air in an air separation unit; a
process that separates liquid nitrogen which boils at 77.3 K, from argon which
boils at 87.3 K and liquid oxygen which boils at 90.2 K.
The distillation system uses two
distillation columns in series. Nitrogen leaves the top of each distillation
column; oxygen leaves from the bottom.
Argon has a boiling point similar to that of oxygen and is preferentially
stay with the oxygen product. As high purity oxygen is required, crude argon must be separated
&removed from the distillation system.
Pakistan
Steel has an Oxygen Plant, manufactured by L'Air Liquide of France.The
main consumer of oxygen is the Steel Making Department where oxygen is blown
into the LD-Converters to convert molten pig iron into raw steel. Oxygen is
also supplied to the Central Maintenance department and other departments where
it is used in welding and cutting processes.
The
booster compressors (02 air
compressors 42,500 nm3/hr each) pump the gaseous oxygen produced by the oxygen
plant into the storage receivers at high pressure.
LD-Converter
steelmaking is a cyclic process whereas
air separation is a continuous steady state production process. Oxygen
storage is necessary to match the continuous output of an air separation plant
with the cyclic demand for oxygen in steel making department.
Since
the cost of electricity is the largest operating cost incurred in air
separation plants as heavy electric motors are used to drive the compression
equipment and also power is required for process heaters, instrumentation
systems and cooling systems.
Therefore
the issue of cost effective amount of liquid oxygen storage required for smooth
raw steel production at Steel Making Department, is resolved by mutual exchange
of production information between Steel Making Department and Oxygen plant.
Oxygen is delivered through pipelines to the Steel Making Department.
The
production capacity of the Oxygen plant of Pakistan Steel is as under.
Oxygen
|
Nitrogen
|
|
Output
(cubic meters/hour):
|
15200
|
9000
|
Purity:
|
99.5%
|
15ppm
|
Pressure
(bar):
|
0.15
|
0.80
|
The
gaseous Nitrogen produced is mainly used as a shielding & purging gas at
Cold Rolling Mill in the annealing & galvanizing units. Nitrogen is also
used in the hydraulic systems of the Cold Rolling and Hot Strip Mills and as an
inert gas at the Coke Dry Quenching Plant. Nitrogen is also used at purging
unit of steel making department.
High
purity Argon is used in production of high quality steel grades ( e.g
automotive grades).So provision of high purity Argon will be subjected to future steel quality demands. As production of high purity Argon is an high energy
intensive and cost effective process so it may be deemed as part of revamping package
of Pakistan Steel.
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