The plant separates oxygen and nitrogen by cooling air to a very low temperature in a cold box. The boiling point of O2 and N2 are different at -186C and -196C respectively. The liquid O2 and liquid N2 are separated in a distillation column and purified.
Air is pre-filtered and preheated if required and compressed in the 3 stage Ingersoll Rand C80MX3-2SH compressor. This was built in 2000 and has a capacity of 4300 Nm3/hr and a final discharge pressure of 8bar. It has a 530KW electrical drive 400V, 50Hz. The compressor has water cooled interchangers on each stage. The air is then filtered and dryed in the refig dryer. This is a Donaldson dryer model DV550WP type 820W built in 2000. It uses R134a refrigerant at 16bar. The air then passes through one of the 2 adsorbers to purify the air. One is in operation and the other in regeneration. These were manufactured by GBM, Sirone LC Italy in 2000 and are 3000L capacity each and a design pressure of 10bar.
The purified air at approx. 7 bar and 26C is fed to the E01 heat exchanger which exchanges heat with the cold N2 and O2. Part of the cooled air at -158C is fed to the expander where it is further cooled to -167C. The 2 cooled air streams then remix to enter the air separation system.
The expander is a Cyrostar ECO90 unit built in 2000 operating at a design flow of 3300Nm3/hr and 19500rpm. Exit temperature of -167C and 4.5bar. The air is then fed to the K01 column which has 2 packed sections. The O2 rich stream is passed through the heat exchanger E04 to then enter the middle of the main air separation column K02. The N2 rich stream off K01 is further cooled in E02 at the bottom of the K02 column and also fed through the E04 heat exchanger to feed the top of the air separation column K02. Liquid N2 is used to assist the start up of the plant. Both K01 and K02 are installed inside the same exterior column.
A side stream of dilute N2 is used to regenerate the adsorbers. This is then vented.
There is a reheat system on the bottom of the column. The vapors/gases rise up the column to mix with the liquid stream to increase the O2 content of the liquid stream. At the top of the column most of the O2 has been removed and is composed mainly of N2.The N2 at 0.2bar is fed through the 2 vaporizers and then to the N2 compressor. The pure N2 is then pressurized to 28 bar in a Sulzer 4D 225-3A compressor rated for 2850m3/hr and fed to storage or to the N2 pipework network. The compressor has 3 stages with intercooling using cooling water. It is electrically driven with a 445KW motor 400V.
The maximum air feed rate to the air separation plant, based on SIAD technology is 4600Nm3/hr and the minimum is 3200Nm3/hr.
The pure O2 near the bottom of the column is also fed through the 2 vaporizers (E01/E04) initially at a pressure of 0.4bar. The O2 is then pressurized in the Oxygen Sulzer 3D 130-3A compressor to 30/40 bar rated at 675/595 Nm3/hr. The O2 is 99.5% O2 concentration. The inlet pressure to the O2 compressor is 0.15 bar. The compressor is electrically driven with a 118KW 400V motor. It has 3 stages with intercooling.
Any excess O2 or N2 above pipeline demand is vented.
Cooling water for the compressors is derived from the recycle cooling water system with a cooling tower and cooling water pumps (back up). The whole plant is controlled using a Siemans PLC system. The plant PLC system can be remotely accessed by engineers as there are no operators present during the nights/weekends.
There is also a smaller N2 compressor which is not used and is a Sulzer 4D200-3a 3 stage compressor rated for 1600Nm3/hr at 30 bar discharge pressure. There are several in line analyzers connected to the DCS system.
The Centac air compressor has made 187 starts from new and has a total running hours of 137,303 hrs.
The total power usage for the plant maximum of 945KWh at 400V 50 Hz.
The electrical energy consumption of the ASU is influenced by several factors, but mainly by the plant loading.
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