Abstract
The temperature detection point used by the Air Separation Unit is based on the principle of distillation gas-liquid equilibrium. Through the function that presents a gas-liquid equilibrium correspondence with the material components, it indirectly reflects the trend of material composition changes and directly and quickly responds to the actual process. This process has outstanding value. The control variables used can directly and effectively respond to the distillation conditions of the distillation tower, which helps to optimize the detection and execution results of the process control loop. It is a process automation control method with broad application value. This paper analyzes the cascade control scheme of the distillation tower temperature of the air separation unit.
I. Introduction to the Background Technology of Air Separation Unit
II. Interaction between the Main Distillation Tower and the Argon Distillation Tower
III. Implementation Process of the Control Scheme
IV. Design of the Main and Auxiliary Process Control Loops of the Main Distillation Tower Variables
V. Measures to Prevent Large Lag and Over-adjustment of Process Control
VI. Conclusion
Introduction to the Background Technology of Air Separation Unit
The Air Separation Unit mainly utilizes the different boiling points of oxygen, nitrogen and argon in the air to liquefy the air, and then partially evaporate and partially condense the air several times, thereby separating the oxygen, nitrogen and argon in the air. This method is called cryogenic distillation. The main distillation tower realizes the separation of oxygen and nitrogen in the air, and at the same time produces raw material components for argon extraction. The raw material will be transported to the argon distillation system to remove the oxygen and nitrogen in it. The method of argon extraction also adopts cryogenic distillation.
The core process unit of the Air Separation Unit is the distillation system, and the distillation tower is the core equipment. In operation, changes in the processing air flow and product structure will affect the distillation process. The operating status of the main distillation tower of the air separation unit is the main factor affecting argon distillation. Only by ensuring that the main distillation tower is in the best operating process state can the distillation of the argon extraction system and the normal operation of the crude argon condenser be ensured.
Interaction between the main distillation tower and the argon distillation tower
In the gas components of the raw material fraction of the argon distillation tower, if the nitrogen content exceeds the design value, it will cause abnormal heat exchange in the crude argon tower top condenser, causing a large process change in the flow rate of the argon fraction extracted from the main distillation tower, which will not only cause the argon extraction distillation tower to fail to work properly, but also cause abnormal distillation conditions of the main distillation tower if the working conditions deteriorate, affecting the separation of oxygen and nitrogen in the main distillation tower, and even causing the oxygen and nitrogen products of the device to be unable to be normally supplied to the outside, which has a negative impact on the stable production of the air separation unit.
The automatic control of the sensitivity temperature of the main distillation tower adopted in this paper is based on the need to stabilize the main distillation tower of the air separation, and technical improvements and developments are made to achieve effective control of the distillation conditions of the air separation unit. As the source material of the argon distillation system, once the nitrogen component of the raw argon fraction exceeds the standard, it will cause serious process fluctuations, which is called "nitrogen plug". Only by effectively controlling the distillation of the main distillation tower can the occurrence of abnormal working conditions of "nitrogen plugging" be avoided and preventive effect be played.
Implementation process of control scheme
The specific implementation process is:
Set the process cascade control loop through the automatic control system (DCS) Set the input of the main process control loop to the temperature detection point of the sensitive point of the main distillation tower Take the liquid nitrogen extraction flow of the main distillation tower as the control output variable Use the liquid nitrogen extraction flow as the given correction compensation of the auxiliary process control loop On the basis of the predetermined liquid nitrogen output, the liquid nitrogen extraction flow output by the main process control loop is compensated with the set liquid nitrogen product flow within a certain range so that the final liquid nitrogen product extraction amount has an effect on the liquid nitrogen reflux amount of the main distillation tower Change the reflux ratio of the main distillation tower and finally realize the precise control of the sensitivity temperature of the main distillation tower
Design of main and auxiliary process control loops for main distillation tower variables
The sensitive point of the main distillation tower is the position where the concentration gradient changes the most on all distillation packing layers of the distillation tower. The sensitive point temperature is the maximum point of concentration gradient change, and its corresponding temperature is the tower temperature on the distillation gas-liquid equilibrium phase diagram. The data of the temperature of the sensitive point of the main distillation tower detected by DCS is compared with the designed theoretical data, and the PID (proportional integral differential) control of the main process control loop inside the DCS can be used. The main process control loop is defined as TIC1717, and the corresponding PID output is the flow control of the liquid nitrogen product taken out of the upper tower. Based on the given value of the liquid nitrogen product FIC1630 extracted from the upper tower, combined with the compensation amount of the PID control output of TIC1717, the set value of FIC1630 is cascaded and compensated, which can realize the reflux ratio control of the main distillation tower.
Measures to prevent large lag and over-adjustment of process control
Measures to prevent large lag:
Convert the temperature data detected by the detection end of the TIC1717 control loop into thermodynamic temperature
Set the amplification factor to improve the sensitivity of the detection data. Select the liquid nitrogen output as the control variable because it affects the reflux ratio of the upper tower quickly and effectively. Measures to prevent over-adjustment:
Take a limited range for the output of the main process control loop TIC1717
Programming the control output range limitation module
Define the design range based on the liquid nitrogen output of the air separation unit product
Conclusion
In the control of industrial automation instruments and meters, by continuously optimizing the control path and improving the control accuracy, the potential of instruments and meters can be better exerted. This cascade control scheme comes from the ideas of senior process engineers. Technical personnel need to systematically analyze the process control requirements, explore the potential of the device equipment, and ensure that the equipment can still operate stably and reliably in a complex and changeable industrial production environment. Through technological innovation, automation instruments and meters can play a more important role in future industrial production.
