Cryogenic air separation plant process description and flow sheet

Atmospheric air passes through the filter to remove the solid particles and dust by the help of air compressors. Filters are placed on the suction side of the compressors where they are filtered and pressurized. By the developed pressure, water vapor changes to liquid water and removed by the separators.
Water coolers decrease the temperature of the air, which have risen up due to compression. Turbo compressors operate in cascade and drive the clean air to Air Separation Unit at temperature of 45º C. The complete plant has three units

  • Air pre-cooling unit
  • Air purification unit
  • Air separation unit

Air pre-cooling unit:
The Air pre-cooling unit consists of the following equipment.

  • Air-H2O tower
  • Waste N2- H2O tower
  • Refrigeration unit

This pre-cooling unit is also called as the air refrigeration system. The purpose of pre-cooling unit is to cool air from 45 º C to 10º C. The Air-H2O tower uses H2O coming from general circuit which is cooled down in the N2- H2O tower by means of refrigeration contained in the waste N2 coming from the cold box. A refrigeration unit in this water circuit brings additional refrigeration make-up, so that the temperature of H2O is brought down to 5 º C before the water enters the Air-water tower.

Air purification unit:
The air purification unit removes moisture, CO2 and hydrocarbons from pre-cooled compressed air by adsorbents present in the ducts and then air goes to main exchanger of the unit, through post filters where adsorbent final particles are removed.
The Air purification unit consists of the following equipment.

  • Main Driers
  • Post Filters
  • Silencer

The pre-cooled compressed coming from Air-H2O tower enters into one of the driers which is in line while the other one is under regeneration. The driers consist of 2 bottles filled with activated alumina in the bottom and molecular sieves at the top, the air flow going from bottom to top. Moisture is adsorbed by Alumina, CO2 by molecular sieves. One of the two vessels is in operation, while the other is reactivated by waste N2 coming from the cold box. The total cycle is 10 hours with 10º C at the inlet of bottles, then the compressed air, dry and CO2 free, is filtered to eliminate dust particles it may carry by post filters and then enters the cold box.

Air separation unit:
This unit mainly consists of the following equipment.

  • Plate type heat exchanger
  • Medium pressure column
  • Low pressure column
  • Expansion turbine
  • Air liquefier
  • Rich liquid sub-cooler
  • Sub-cooler
  • Liquid Nitrogen separator
  • Liquid Oxygen separator

Purified air coming from the post-filters is sent directly to the main exchanger line. A part of the air is taken at an intermediate point of the exchangers and sent to centrifugal expansion turbines equipped with a brake generator to provide the necessary refrigeration to make-up for the thermal losses of the cold box. The adiabatic expansion drop the pressure down to approximately atmospheric pressure provides the main part of the refrigeration required by the plant.

The other part is cooled in and sent to the bottom of the medium pressure column to perform the 1st separation. In this column, the up-flowing gas becomes enriched with N2 by contact with the down-coming liquid. This liquid result from the nitrogen condensation in the vaporizer / condenser located at the top of the column.

From top to bottom, the medium pressure column gives the following products
→ Pure liquid N2
→ Liquid with low O2 content used as reflux on the top of the low pressure column after passing through sub-cooler and expansion in a valve.
→ Rich liquid at the bottom, a liquid containing about 40% O2 sent to the low-pressure column, after passing through exchanger and expansion in a valve.

The up flowing gas in the low pressure column is the gas vaporized in the vaporizer/ condenser is called main vaporizer and vaporizes liq.O2 at low pressure by condensation of medium pressure N2 in medium pressure column. The rich liquid sub-cooler makes it possible for the liquid oxygen to flow through filter by Thermo-siphon effect. This extra filtration permits to avoid abnormal accumulation of dangerous products in the O2 bath in case of accidental passage of impurities.

The low pressure column gives
→ At the bottom, liquid O2 at a purity of 99.5 %. Part of this liquid O2 is sent directly at the outlet of the cold box for storage. The other part is vaporized in the air liquefier and then heated in the main exchanger line.
→ At the top, waste N2 which is heated in the main exchanger line. The pure N2 column is located above the low pressure column and gives at its top, pure N2 at 99.9 % purity, which is also heated in the main exchanger line.

For recovery of Argon, a gaseous flow is picked up from the low pressure column at a point where Argon content is about the maximum possible. This fraction is taken to the bottom of the Crude Argon Column. This flow rises to the top of the column where it is partially condensed by vaporization of rich liquid in the condenser/ vaporizer. The liquefied fraction flows back down to the low pressure column. The gaseous fraction called crude Argon contains normally 2-3 % O2 and a small amount of N2. O2 is eliminated in an external warm section, and N2 is eliminated at low temperature.

Cryogenic air separation plant process flow sheet:

air separation done by cryogenic process

General design specifications of individual equipments in cryogenic air separation plant process:

Air -Water tower
Diameter/total height 2550/11990mm
Air flow (maximum) 592736.4 kg/hr
Air inlet/outlet temperature 45/10 oc
Water flow 120m3/hr
Waste Nitrogen-Water tower
Diameter /total height 2400/9550mm
Waste nitrogen flow (maximum) 154759.4 kg/hr
Water flow 60m3/hr
Water pumps
Pump motor power 45kw
Speed/voltage/frequency 1484rpm/440v/50Hz
Capacity 65m3/hr
Refrigeration unit
Operating temp 51.20/3.2 oc
Refrigeration capacity 19.7/5oc
Chilled water inlet/out let temp 73.6m3/hr
Refrigerant used Freon-12(CCl2F2)
Total charge of refrigerant 950 kg
Compressor
Discharge at operating point 2268 m3/hr
Refrigerant mass flow rate 12.135 kg/sec
Suction pressure /temperature 3.34 bar / 3.2oc
Discharge pressure /temperature 12.73bar/64oc
Driers
Diameter /length 4000 / 10300mm
Drier capacity 106000 nm3/hr (max)
Operating time 5hrs.20min
Activated alumina
Purpose To remove moisture
Quantity 13600kg
Size 2.5mm
Molecular sieves
Purpose To remove CO2&H2
Quantity 18000kg
Size 13*1/6 inches
Main Heat Exchangers
Type Plate type HX
Height 9000mm
Weight 35000kg
Expansion turbines
Air flow (maximum) 37500 Nm3/hr
Temperature (in/out) 135/178 oc
Pressure (in/out) 6.6/1.45 kg/cm2
Power 550kw
Height 1340mm
Crude Argon column
Working pressure 0.2 to 0.3 kg/cm2
No of trays 70
Material of construction stainless steel