Steel Plant

The main process flow, process design parameters and operation effect of NEWTEK 60000m/h air separation unit are summarized. The design features of the air separation project and the advanced technologies used are summarized. After the project was completed, the equipment operated safely and quickly reached the design indicators, reducing the energy consumption and operation costs of the equipment.

Keywords: air separation unit; oxygen production; design features; automation

The steel plant produces 8.1 million tons of hot metal, 8.5 million tons of crude steel and 8.1 million tons of hot-rolled steel annually. The long process route of metallurgical manufacturing is adopted. The main construction contents include 2 500m sintering machines, 2 5100m ironmaking blast furnaces, 4 210t converter steelmaking workshops, 1 2050mm hot-rolled strip production line, 1 3500mm furnace coil production line, 1 4300mm wide and thick plate production line, 1 800,000t push-pull pickling unit production line, 1 2030mm pickling combined production line, etc., as well as supporting wharf and railway transportation facilities, raw material preparation facilities, power stations and slag comprehensive utilization facilities and other public auxiliary facilities. In order to cooperate with the production of facilities, 2 x 60,000 m/h air separation units and supporting ancillary facilities were built at the same time.

2 Main facilities of air separation unit
This air separation unit is a fully low-pressure molecular sieve purification adsorption, air pressurization, oxygen and nitrogen compression in the pump, with a booster turbine expansion mechanism for refrigeration, and adopts a structured packing tower and a full distillation process for hydrogen-free argon production.

3 Air separation engineering design features

3.1 Process flow
1) The air separation unit adopts a process flow of full low-pressure molecular sieve purification adsorption, air booster turbine expansion mechanism refrigeration, full distillation hydrogen-free argon production, product oxygen internal compression, product nitrogen external compression, and argon internal compression. It has reliable operation, advanced process, convenient operation, reasonable equipment configuration, safety and low consumption.

2) The air precooling system uses dirty nitrogen and nitrogen cooling circulating water, which has good operation flexibility and makes full use of dry dirty nitrogen and excess nitrogen. The air cooling tower structure adopts necessary and reliable anti-liquid flooding measures to prevent mist free water from entering the molecular sieve adsorption system.

3) The molecular sieve adsorption system adopts a vertical activated alumina + molecular sieve double-layer structure molecular sieve adsorber with long-term switching. The adsorbent and switching valve have a long service life, the system switching loss is small, the bed resistance is small, and there are measures to prevent the molecular sieve from blowing over and possible blow-over treatment measures. The regeneration heater adopts an energy-saving steam heater (electric heater is spare).

4) The upper tower (low-pressure tower) and argon tower of the distillation tower adopt structured packing towers, which reduces the resistance of the tower and further improves the oxygen and argon extraction rates.
5) The turbo expander adopts a booster braking process, thereby reducing the amount of expanded air and making the upper tower of the distillation tower stable.
6) The recovery of vaporized argon gas from the atmospheric pressure liquid argon storage tank is considered when designing the air separation unit. The vaporized argon gas in the storage tank enters the argon condenser recovery device, and after being condensed by liquid nitrogen, it returns to the liquid argon storage tank as a liquid argon product; the vaporized nitrogen returns to the cold box dirty nitrogen pipeline to recover the cold capacity.

3.2 Design and selection of main equipment
1) The air separation equipment adopts full distillation hydrogen-free argon production technology, cancels the hydrogenation and deoxygenation process, greatly simplifies the layout of the side span plant in the main oxygen production plant in the factory design, and saves the plant area. Reliable operation, advanced process, convenient operation, reasonable equipment configuration, safety and low consumption.
2) Key equipment are all internationally and domestically renowned brands, the main air compressor is selected from Atlas, the air booster is selected from Siemens, the nitrogen compressor is selected from Atlas, and the oxygen booster is selected from Hangyang, which ensures the reliable operation of the equipment.
3) The motor power of the main air compressor is 2x30000 kW, using a variable frequency motor, and the others use soft start to reduce the impact on the main power grid. And the machine-side/centralized operation mode is adopted respectively, which can realize the remote start and stop control of the equipment and the operation status monitoring.
4) The oxygen booster adopts a turbine oxygen compressor, which is technically reliable and safe.
5) The molecular sieve adopts a vertical structure, and the pipeline adopts a two-ring layout. The height difference between the bottom ring pipeline and the top ring pipeline is 18m, and the temperature and pressure of the gas medium in the pipeline change alternately. The design uses CAESARII software to perform pipeline stress analysis, and set reasonable spring brackets and fixed brackets.

6) The circulating cooling water required by the motor adopts a closed-loop circulation system without external discharge. The living and cleaning water of various buildings in the plant area is centrally recovered and processed to achieve zero sewage discharge.
7) The main cooling and crude argon condensers in the device implement 1% liquid discharge to prevent the accumulation of dangerous impurities such as hydrocarbons.
8) The device has the ability to operate under variable conditions to achieve the most economical operating conditions of the device.

3.3 Automation design features
According to production and process requirements, one DCS system is set up for each of the two 60000 m/h air separation systems to complete the centralized monitoring and control of the compressor main plant and air separation system, circulating water system and external integrated pipeline process. The automation system consists of an operator station, DCS and I/O station2. The DCS and operator workstations are connected by Ethernet, and the DCS and I/O stations are connected by bus. The connection between the I/O station or DCS and the field components is connected by control cables. The operator station is concentrated in the oxygen production control room.

3.3.1 Operator station
The operator station and the field control station communicate with each other to achieve the following functions:

1) Display of production process parameters, flow chart screen, alarm screen, and historical trend curve display.

2) Selection of control operation mode: manual control at the machine, HMI manual control, and automatic control.

3) Modify the set value or directly operate the operation of the control equipment through human-computer dialogue.

4) Production report printing and alarm printing, etc.

3.3.2 DCS and I/O Station

The field control station is the core equipment for realizing process control. It provides I/O interface with the production process, performs process control, data collection, parameter calculation, etc., and then outputs the calculated control signal to the field actuator through the I/O module, thereby realizing PID control, sequence control, logical interlocking control, etc. of the production process. The control functions of the DCS of this project mainly include: collection and processing of process temperature, pressure, flow, level, analysis and other data; control of temperature, pressure, flow, liquid level, resistance, etc.; interlocking control and anti-surge control of air compressor; cooling tower control; timing control of molecular sieve purification; start and stop control of oxygen turbine compressor; interlocking control and anti-surge control of nitrogen compressor, etc.; operation control of each pump.

4 Operation effect
The equipment operates stably, and the air separation unit has not experienced any failure or shutdown since it was put into operation. The energy consumption of the equipment is reduced, and the equivalent unit oxygen production energy consumption (internal compression) is 0.55 kW·h/m. The operating cost is reduced, and the oxygen production plant has a fixed staff of 30 people.

5 Conclusion
By rationally designing the steel composition, nitrogen injection was used in the TSR furnace to carry out nitrogen alloying to develop 20Cr13N stainless steel. The production process is simple, low cost, high purity and stable composition. All performance indicators of the developed 20Cr13N hot-rolled steel strip meet the requirements of trial production. Through nitrogen alloying, the hardenability and corrosion resistance of the product are significantly improved.