Tonnage Oxygen Plant (Pak. Steel)

  

Aqil Khan

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.