1 OBJECTIVE

The objective of this document is to describe the plant control principles. It contains a detailed description of the

process control loops, setpoint calculation based on operator!s production requests and load change strategy. It

contains as well cause-effect interlock tables of the general plant shutdown system.

2 GENERAL

2.1 Main characteristics of the plant

The Oxygen Plant consists of one Air Separation Unit (ASU). The process scheme is based on Mixing Column

Cycle (MCC).

Plant separation capacity is 880 MT/D of gaseous oxygen.

The ASU will deliver the following products:

27,035 Nm^3/hr of gaseous oxygen at 4.5 bar(a) with an oxygen content of at least 95%

27,000 Nm^3/hr of gaseous nitrogen at 11 bar(a) with a nitrogen content of at least 95%

The plant will consist of the following major equipments:

Air compressor (C01)

Air compressor aftercooler (C01E)

Steam Turbine (C01ST)

Nitrogen compressor (C50)

Nitrogen compressor aftercooler (C50E)

Air precooling with water cooler (E07)

Nitrogen water tower (E60)

Water chiller (X60)

Double bed (alumina/molecular sieves) air purification (water and CO2 removal) system (R01/R02)

Steam regeneration heater (E08)

Cold box containing the followings:

- Main exchanger (E01)

- Expansion turbine/turbine booster (ET01/ET01C)

- Booster aftercooler (ET01CE)

- Two liquid sub-coolers (E03 & E04)

- Main vaporizer (E02)

- Two LOX pumps (P02 1/2)

- LOX purge exchanger (E70)

- Three columns:

a. High Pressure Column (K01)

b. Low Pressure Column (K02)

c. Mixing Column (K04)

2.2 Process description

2.2.1 Air Compression

Atmospheric air is drawn through an inlet filter to remove dust and other particulates. It is then flowed into a three-

stage centrifugal compressor (C01) and compressed to a pressure level suitable for the downstream process. The

compressor is equipped with water-cooled intercoolers.

2.2.2 Air Precooling

Upon exiting the third stage of the compressor, the air is cooled against cooling water in an aftercooler (C01E).

Then, the air is further cooled in a chilled-water cooler (E07). The chilled water is first cooled in the nitrogen

chiller tower (E60) and then further cooled by a mechanical refrigeration unit (X60) before flowing into the chilled

water cooler. The water system between the chilled water cooler, the mechanical refrigeration unit, and the chiller

tower is a close loop system. The water circulation is achieved by the water chiller pumps (P60.1/2).

2.2.3 Air Purification

The air from the chilled-water cooler enters a dual vessel adsorption system (R01/R02) where carbon dioxide and

water are removed on a dual bed of alumina and molecular sieve. Each dual-bed adsorber operates sequentially in

adsorption and regeneration phases controlled by a timer. Waste nitrogen gas produced from the cold box is used to

regenerate the adsorbers. The air coming from the adsorbers is now suitable for cryogenic processing.

2.2.4 Cold Production

The air stream from adsorbers splits into three streams. One stream is compressed in the booster (ET01C) and

cooled in the aftercooler (ET01CE) before sending to the main exchanger (E01). This air stream is cooled against

the waste nitrogen and oxygen streams in the main exchanger and then expanded in the turbine (ET01). The

resulting cold low pressure air is sent to the low pressure column (K02) as feed. The power generated by the

turbine is used to drive the booster.

The second stream and the third stream are sent directly to the main exchanger to be cooled against the waste

nitrogen and oxygen streams. The second stream, which contains most of the air flow, is sent to the HP column

(K01) as feed. The third stream is sent to the mixing column (K04) as feed.

2.2.5 Air Separation

The air separation is achieved by three distillation columns.

2.2.5.1 HP Column (K01)

Air sent to the HP column is separated into oxygen-rich liquid at the bottom and impure nitrogen at the top. The

separation of the air is carried out in this column, as in any other rectification column, by the interaction of vapor

rising packing by packing with a descending stream of reflux liquid. The descending liquid gradually becomes

richer in oxygen, and at the same time, the vapor passing through each packing loses a corresponding small amount

of oxygen.

The gaseous impure nitrogen at the top of HP column enters the main vaporizer, where it is condensed by

vaporizing the liquid oxygen from the sump of LP column at the other side of the main vaporizer. The resulting

impure liquid nitrogen falls by gravity to the top of HP column where it acts as reflux. Part of this impure reflux is

withdrawn from the top of HP column and is subcooled against the waste nitrogen before sending to LP column as

reflux.

Rich liquid is withdrawn from the bottom of the HP column, subcooled against waste nitrogen, and expanded to LP

column as reflux.

2.2.5.2 LP Column (K02)

The rectification in LP column is same as that in HP column. Impure liquid nitrogen and oxygen-rich liquids are

fed into the appreciate level of the LP column. Liquid arriving at the bottom is approximately at 98% pure oxygen.

This liquid is pumped to the mixing column after being desubcooled in the mixing column subcooler (E04). A

small portion of this liquid oxygen (0.2% of the total air to the cold box) is sent to the LOX purge exchanger (E70)

to maintain a constant purging for the main vaporizer. This small amount of liquid oxygen is vaporized against

cooling down the air and then combined with the gaseous oxygen flow from the mixing column at the warm end as

product.

Waste nitrogen leaving the top of LP column is warmed in the subcooler against cooling down the impure liquid

nitrogen and rich liquid. It is further warmed in the main exchanger before leaving the cold box. This waste

nitrogen splits into three streams. One stream is sent to the purification unit for regeneration and the other stream is

sent to the chiller tower. The third stream is drawn by the product nitrogen compressor and compressed up to the

required delivery pressure.

2.2.5.3 Bath Type Main Vaporizer (E02)

This vaporizer is used as condenser for HP column. The gaseous nitrogen from the top of K01 is liquefied and sent

back to K01 as reflux. E02 is also used as reboiler for LP column. The liquid oxygen from the bottom of K02 is

partially vaporized to provide the reboil flow for K02. The latent heat of vaporization of low pressure oxygen is

used to liquefy the MP nitrogen from K01 side.

2.2.5.4 Mixing Column (K04)

Liquid oxygen from the sump of LP column is pumped and warmed in the subcooler before being sent to the top of

mixing column. The mixing column operates as a direct contact oxygen vaporizer with liquid oxygen falling from

the top and air flowing upwards through the column.

Very rich liquid oxygen is withdrawn from the intermediate location of the mixing column and then subcooled

before sending to LP column as feed.

Rich liquid is withdrawn from the bottom of mixing column, subcooled, and then sent to LP column as feed.

2.3 Flexibility

The load of the plant can be set within 70% to 100% of the nominal value in term of gaseous oxygen production.

3 CONTROL PRINCIPLES

3.1 General

The plant production rate will be automatically adjusted to meet the product demand.

The control principle is the following: according to requested GOX production flow rate, which is set by the

operator, a targeted air flow rate is calculated. The result is used with a variation velocity limitation (ramp) in order

to adapt smoothly the operation of the plant to the requested load.