Abstract: This article describes in detail the implementation of automatic control for the molecular sieve purification system, waste nitrogen flow rate, and oxygen compressor start/stop, and introduces communication with other equipment. The implementation of the Quantum control system improves the reliability of the control system, simplifies the operation process, and ensures the long-term stable operation of the oxygen generator. Keywords: Quantum; control; oxygen generator; bridge 0 Overview With the continuous expansion of production capacity in recent years, Xinjiang Bayi Iron & Steel Company has seen a surge in demand for oxygen. Therefore, two 20,000 m³/h oxygen generator units were newly built to meet the needs of steel production. This project uses the Quantum series controller from Schneider Electric (France) to perform full-process data acquisition, process control, and emergency interlock control for the air separation section (molecular sieve system, air precooling system, air separation tower, etc.) and the oxygen compressor section. At the same time, the BM85 bridge communicates with the air compressor and nitrogen compressor in real time according to the Modbus communication protocol, realizing the monitoring, control, and operation of each production process. 1 System Composition and Hardware/Software Configuration 1.1 System Composition The structure of the distributed control system is shown in Figure 1, consisting of 2 control stations and 4 monitoring stations. Control station #1 employs a dual-cable hot standby configuration, while control station #2 uses a single-cable configuration. Within each station, remote I/O (RIO) is configured via coaxial cable. Communication between control stations is via Ethernet through a switch using shielded twisted-pair cable. Additionally, since real-time data from the air compressor and nitrogen compressor in the field devices also needs to be displayed in the distributed control system, control station #1 is externally connected to an MB+ network, which collects data from the air compressor and nitrogen compressor via a BM85 bridge. 1.2 Hardware and Software Configuration: The Quantum control system uses the Intel 586-based 140CPU53414A CPU with a storage capacity of up to 4MB. Its floating-point coprocessor significantly improves the processing speed for analog signal processing and loop regulation in process control. For safety reasons, the number of channels on each I/O module does not exceed 16 when selecting I/O modules. All I/O modules are hot-swappable for easy replacement during production. The Quantum control system can be programmed using Concept software under Windows. Concept provides various loop control functions such as analog signal processing, standard and cascade PID control, PID tuning, and function generator. It can be programmed using function block diagrams familiar to instrumentation and control personnel, and can also be programmed using high-level languages ​​for complex mathematical algorithms. Users can customize function blocks for specific applications and reuse them repeatedly, supporting function block nesting. 2. Process Flow (1) This unit uses a molecular sieve-purified air booster expander, a packed tower for the upper column, full distillation to produce argon, and an external compression process; (2) After dust and mechanical impurities are removed in the filter AF, the raw air enters the air turbine compressor, compressing the air to approximately 0.615 MPa, and then is sent to the air cooling tower AC for cleaning and pre-cooling. Air enters from the bottom of the air cooling tower and exits from the top. The water supply to the air cooling tower is divided into two sections: the lower section uses circulating water cooled by the user's water treatment system, while the upper section uses low-temperature water cooled by the nitrogen-water cooling tower WC, thus achieving an outlet air temperature of 19℃. A wire mesh demister is installed at the top of the air cooling tower to remove mechanical water droplets from the air. (3) The air exiting the air-cooled tower enters the alternating molecular sieve adsorber MS. There, impurities such as moisture, CO2, and C2H2 in the raw air are adsorbed by the molecular sieve. (4) The purified processing air is divided into two streams. One stream enters the main heat exchanger E1, where it exchanges heat with the returned portion of waste nitrogen and low-pressure nitrogen product before entering the lower tower C1 for rectification. The other stream is compressed in the first stage by an air booster and then divided into two streams. One stream, equivalent to the expansion volume, is pressurized at the booster end of the booster expander and then cooled by the gas cooler before entering the main heat exchanger E1. It is then extracted from the middle of the main heat exchanger and enters the expander ET, where it expands and enters the lower tower C1 for rectification. The other stream is compressed in the second stage by the air booster, then cooled in the main heat exchanger, and after throttling, enters the lower tower. After preliminary rectification in the lower tower, liquid air is obtained at the bottom of the lower tower, and pure liquid nitrogen is obtained at the top of the lower tower. (5) A certain amount of argon fraction is drawn from the middle of the upper column and sent to the crude argon column. The crude argon column is structurally divided into two sections. The reflux liquid at the bottom of the second section is pumped to the top of the first section as reflux liquid. After rectification in the crude argon column, crude argon with 98.5% Ar and 2ppm O2 is obtained. After liquefaction in the liquefaction unit, it is sent to the middle of the refined argon column. After rectification in the refined argon column, a portion of refined argon with a purity of 99.9997% Ar is obtained at the bottom of the refined argon column and extracted as product and sent to the storage tank. (6) Pure nitrogen gas is drawn from the top of the auxiliary column, reheated after passing through a cooler and the main heat exchanger, and then exits from the cold box for users. (7) Waste nitrogen gas is drawn from the top of the upper column, reheated after passing through a cooler and the main heat exchanger, and then exits from the cold box. It then enters the electric heater as molecular sieve regeneration gas. Excess gas is sent to the water cooling tower or vented. 3. Air Separation Control System The air separation control system involves multiple loop adjustments, including molecular sieve adsorber switching control, waste nitrogen flow control, expander interlock control, and argon system control. The quality of these adjustments directly affects the stable operation of the air separation system. Among these, molecular sieve adsorber valve switching and waste nitrogen flow control are the key and challenging aspects of the entire air separation system control. 3.1 Molecular Sieve Adsorber Switching Control This oxygen generator's molecular sieve purification system uses a horizontal double-bed structure purifier, employing adsorption to purify moisture, carbon dioxide, acetylene, and other hydrocarbons from the air. Adsorption involves using adsorbents such as activated alumina and molecular sieves to adsorb moisture, carbon dioxide, and other adsorbates from the air at room temperature. During heating regeneration, the adsorbent's adsorption capacity decreases at high temperatures, allowing for desorption of the adsorbates, thus achieving continuous air purification. The molecular sieve purifier switching control primarily controls the valve switching during the molecular sieve regeneration process. The molecular sieve regeneration process consists of five steps: depressurization, heating, cooling, pressurization, and switching. Depressurization: The molecular sieve purifier operates at a higher working pressure and undergoes desorption and regeneration at a lower pressure. When the purifier switches from adsorption to regeneration, the pressure inside the purifier is first reduced. At this time, the purifier bypass valve is slowly opened to depressurize the purifier. The heating stage can only begin when the purifier outlet pressure is less than 0.01 MPa. If the depressurization time is completed and the purifier outlet pressure is still greater than 0.01 MPa, the program is paused. Heating: After the heating stage begins, high-temperature waste nitrogen gas heated by the regeneration electric heater passes through the molecular sieve bed for further heating. During the heating stage, interlocking conditions for electric heater operation are set, including shutdown when the waste nitrogen flow rate is less than 10000 m³/h and shutdown when the electric heater temperature exceeds the interlocking value. The electric heater temperature interlocking value can be modified by the operator on the monitoring screen. Since not all electric heaters need to be turned on during the heating stage, a selection key is also provided on the monitoring screen. When the local control box handle is switched to DCS control to enter the heating stage, the electric heater selected by the operator is automatically activated, reducing the workload of the operator. Cold Blow: After cold blowing begins, the electric heater is turned off, and the waste nitrogen enters the purifier through a bypass, carrying away the heat from the bed and preparing it for reuse. At the end of the cold blowing process, the corresponding cold blowing valve needs to be closed, while the waste nitrogen vent valve is opened. The pressurization step can only begin after these two valves have switched. Since the total opening and closing time of these two valves is adjustable, the program uses the total cold blowing time minus the valve opening and closing time as the condition for starting the valve switching. Pressurization: At the start of pressurization, the purifier vent valve is closed, and the inlet valve is opened to introduce air from the pre-cooling system into the purifier, pressurizing it. Switching: The air inlet and outlet valves of the regeneration purifier are opened, putting both purifiers in operation and ensuring equal pressure between them. Switching between them does not cause system fluctuations. 3.2 Waste Nitrogen Flow Control To stabilize the pressure and composition within the column, the waste nitrogen flow rate FIC1201 for regeneration needs to be kept stable. However, during the molecular sieve valve switching process, the simultaneous operation of the heating regulating valve FIC1201A, the cold blowing regulating valve FIC1201B, and the venting regulating valve FIC1201C affects the stability of the nitrogen flow rate FIC1201. Therefore, there is a matching problem among these three regulating valves. The opening and closing states of the three regulating valves differ at different stages of molecular sieve switching. When one regulating valve is open, another must be closed. To maintain a constant waste nitrogen flow rate during this process, the program is designed so that the valve opening and closing process is divided into five steps, i.e., the valve opening and closing is stepped, and the five step values ​​are adjustable. A total opening and closing time is also set, allowing for a certain period of stabilization when the opening value changes at each step, reducing system disturbances. Through setting the five opening values ​​and the total opening and closing time, and through continuous experimentation, the valves are optimized to maintain a stable flow rate. When the regulating valve opens, it opens in a 5-step stepped manner over time. When the valve opening reaches the 5th step value, it indicates that the valve has reached a steady-state value, and then automatically switches to PID control. During the switching process, the valve output value OP is assigned to the PID output value, thus achieving a smooth switching when switching to automatic control. The PID control setpoint SP is a fixed value (19500 m³/h). When the regulating valve switches to automatic control, it immediately performs PID control based on the difference between the actual field value PV and the SP value. When the regulating valve closes, it automatically switches from PID control to manual control, simultaneously assigning the first step closing value to the valve's OP value, closing in a 5-step stepped manner over time, and finally setting the valve position output to 0. 4. Oxygen Compressor Control System The oxygen compressor配套的氧编编 (matching this oxygen generator) is a 2TYS100+2TYS76 oxygen turbine compressor manufactured by Hangzhou Oxygen Plant Group. Its design inlet pressure is 80 kPa, outlet pressure is 5 MPa, and flow rate is 25000 m³/h. 4.1 The main regulating loops of the automatic adjustment system of the oxygen compressor include: the intake pressure and discharge pressure regulation system, the intake nitrogen pressure regulation system, the differential pressure regulation between the mixed gas and oxygen, the differential pressure regulation between the shaft seal nitrogen and the mixed gas, and the differential pressure regulation between the shaft seal oxygen and the mixed gas. 4.1.1 Intake Pressure and Discharge Pressure Regulation System This system is designed to ensure constant intake and discharge pressures of the oxygen compressor. By detecting the intake pressure PIC3302, the opening of the oxygen compressor inlet guide vane is adjusted, thereby changing the flow rate of the oxygen compressor to maintain a constant intake pressure. If the oxygen supply to the oxygen compressor is insufficient, causing the inlet guide vane to close to its limit, and the intake pressure is still lower than the specified value, the high-pressure bypass valve V3303 will be automatically opened by program control, relying on backflow to maintain a constant intake pressure. The exhaust pressure PIC3309 and intake pressure PIC3302 are selected by the program and output as PV values ​​to control the high-pressure bypass valve V3303. Additionally, the exhaust pressure PIC3309 also serves as the PV value for the high-pressure vent valve V3304, participating in the regulation of the high-pressure vent flow. However, the SP values ​​for these two adjustments are different; backflow should be initiated before venting to ensure the exhaust pressure does not exceed the specified value and does not affect other regulation systems. 4.1.2 Intake Nitrogen Pressure Regulation System: This system is designed to ensure a constant intake pressure for the oxygen compressor during nitrogen operation. By detecting the intake nitrogen pressure PIC3313, the safety and test nitrogen inlet pressure regulating valve V3317 is adjusted, changing the amount of supplemental nitrogen to maintain a constant intake nitrogen pressure. 4.1.3 Mixed Gas and Oxygen Differential Pressure Regulation: The mixed gas and inlet oxygen differential pressure PDIC3302 serves as the PV value for the mixed gas and inlet oxygen differential pressure regulating valve V3309, allowing the regulating valve to maintain a constant differential pressure between the mixed gas and the intake oxygen by changing its opening value. 4.1.4 Differential Pressure Regulation of Shaft Seal Nitrogen and Mixed Gas: The differential pressure between the shaft seal nitrogen and the mixed gas is sent to the control system by a differential pressure transmitter. When the differential pressure deviates from the set value, the opening of the differential pressure valve V3312 is adjusted to change the amount of sealing nitrogen to maintain the set value. An alarm is triggered when the differential pressure drops to the lowest level (Level 1) due to a failure of the regulation system; a shutdown is triggered when the differential pressure drops to the lowest level (Level 2). 4.1.5 Differential Pressure Regulation of Shaft Seal Oxygen and Mixed Gas: The differential pressure between the shaft seal oxygen and the mixed gas is sent to the control system by a differential pressure transmitter. When the differential pressure deviates from the set value, the opening of the differential pressure regulating valve V3308 in the mixed gas and oxygen balance pipe is adjusted to change the amount of oxygen flowing from the high-pressure cylinder shaft seal oxygen chamber back to the low-pressure cylinder suction pipe to maintain the set value. An alarm is triggered when the differential pressure drops to the lowest level (Level 1) due to a failure of the regulation system; a shutdown is triggered when the differential pressure drops to the lowest level (Level 2). 4.2 Normal Start-up and Shutdown of Oxygen Compressor Normal start-up and shutdown of the oxygen compressor includes four steps: start-up preparation, start-up, shutdown preparation, and shutdown. Interlocked shutdown includes shutdown due to a major fault in the oxygen compressor or abnormal temperature of the casing or end seal. 4.2.1 Normal Start-up and Shutdown of the Oxygen Compressor When the selector switch on the local machine box of the oxygen compressor is set to the "automatic" position, the system is in automatic start-up and control mode. The operator clicks the "Start-up Preparation" button, and the system automatically completes the opening and closing of 11 valves, while simultaneously engaging the start-up interlock. After confirming that all valves are in the required start-up positions and that all process parameters are normal, the operator clicks the "Start" button. Ten seconds after the start-up alarm sounds, the main circuit breaker of the oxygen compressor closes, the start-up interlock is disconnected, and one minute later, the medium-pressure bypass valve closes, and the corresponding adjustments are put into automatic mode. The inlet pressure interlock and shaft seal differential pressure interlock are manually engaged by the operator. During normal shutdown, the operator clicks the "Shutdown Preparation" button, the oxygen vent valve fully opens, and the inlet pressure interlock and shaft seal differential pressure interlock are automatically disconnected. Open the high-pressure bypass valve, trip the main circuit breaker of the oxygen compressor, and close the oxygen compressor outlet valve and oxygen vent valve. After confirming the open/closed status of each valve, the operator clicks the "Stop" button, completing the normal shutdown procedure for the oxygen compressor. 4.2.2 Interlocked Shutdown of the Oxygen Compressor: During normal operation of the oxygen compressor, if a major fault occurs and the compressor stops, the main circuit breaker trips, the oxygen vent valve, high-pressure bypass valve, and medium-pressure bypass valve fully open, and the oxygen compressor outlet valve closes. The oxygen vent valve closes after being fully open for 5 minutes, and the oxygen inlet valve closes simultaneously. At this time, the operating display status changes from "Start" to "Stop". During normal operation of the oxygen compressor, if an abnormal temperature occurs and the compressor stops, the main circuit breaker trips, the oxygen inlet valve closes, the safety nitrogen valve opens, and emergency nitrogen is injected for 1 minute. The oxygen vent valve, high-pressure bypass valve, and medium-pressure bypass valve fully open, and the oxygen compressor outlet valve closes. After emergency nitrogen injection for 3 minutes, the oxygen vent valve closes, and the operating display status changes from "Start" to "Stop". 4.2.3 Nitrogen Test of Oxygen Compressor: One minute after the main circuit breaker of the oxygen compressor is closed, the medium-pressure bypass valve should be closed. During the nitrogen test, the closure of this valve is manually completed by the operator, and the subsequent steps are the same as the normal start-up of the oxygen compressor. 4.3 Automatic Switching of Two Oil Pumps in the Oxygen Compressor: The oxygen compressor lubrication system has two oil pumps, which serve as backups for each other. Pump #1 starts first. When the lubrication oil pressure is low, the system automatically starts pump #2. When the lubrication oil pressure is normal, pump #1 is stopped for maintenance. If pump #2 starts first, when the lubrication oil pressure is low, the system automatically starts pump #1. When the lubrication oil pressure is normal, pump #2 is stopped. This achieves "first-in, first-out, no distinction between main and backup," improving equipment utilization while ensuring safety and reliability. 5 Communication with Other Equipment According to design requirements, the real-time data of the air compressor and nitrogen compressor in the field equipment needs to be transmitted to this control system for monitoring. Therefore, the data is collected through the BM85 bridge according to the MODBUS industrial communication protocol. The MODBUS protocol is a universal language used in electronic controllers. Through this protocol, controllers can communicate with each other and with other devices via networks (such as Ethernet). It has become a universal industrial standard, allowing control devices from different manufacturers to be connected into industrial networks for centralized monitoring. The control system CPU is connected to field devices via a BM85 bridge. The communication transmission method, baud rate, parity check, number of data bits, and master-slave mode are configured in the BM85 bridge settings interface. Communication uses a master-request, slave-response model. That is, the master sends a command request, the slave responds, receives the data, analyzes it, and if the data meets the communication protocol, the slave responds. In the program, the MSTR function block is used to read the transmitted data, as shown in Figure 2. Data from the air compressor and nitrogen compressor can be read and written by setting the CONTROL and DATABUF parameters of the MBP_MSTR function block. The CONTROL parameter sets the read-only data (from the air compressor and nitrogen compressor), the length of the data to be read, the bridge's address in the MODBUS network, and the bridge's communication port. The parameter DATABUF specifies the starting address of the continuous data stored at the receiving end. Summary The two 20000m3/h oxygen generators of Baosteel were successfully put into operation at the end of March 2007. Since the system was put into operation, it has been running well and has achieved the expected results, and has been well received by users. References [1] Tao Wenhua, Li Xiaofeng. Application of Quantum control system in coking coal preparation system. Metallurgical Automation [2] Wu Hui. Application of distributed control system in Baosteel 72000m3/h oxygen generator. Cryogenic Technology [3] Liu Zhen, Zhang Jian. Application of fully automatic control system of oxygen compressor of 20000m3/h air separation equipment. Cryogenic Technology