Contents
Abstract
I. Introduction to Background Technology of Air Separation Unit
II. Design and Selection of Main and Auxiliary Process Control Loops for Variables of Main Distillation Tower in Air Separation Unit
III. Ways to Prevent Large Lag of Process Control and Selection of Process Control Output Variables
IV. Measures to Prevent Over-adjustment of Process Control
V. Conclusion
The air separation unit adopts temperature detection points based on the principle of distillation gas-liquid equilibrium. Through the function that corresponds to the gas-liquid equilibrium of the material components, it indirectly reflects the trend of material composition changes and can directly and quickly respond to the actual process. The control variables of this process can directly and effectively reflect the distillation working conditions of the distillation tower, which is helpful to optimize the detection and execution results of the process control loop, and is a process automation control method with broad application value. This paper analyzes the cascade control scheme for the temperature of the distillation tower in the air separation unit.
Keywords
Air separation unit; Gas-liquid balance principle; Cascade control
1. Introduction to the background technology of air separation unit
The air separation unit mainly uses the different boiling points of oxygen, nitrogen and argon in the air to liquefy the air, and then conducts multiple partial evaporations and partial condensations to separate oxygen, nitrogen and argon in the air. This method is called cryogenic distillation. The main distillation tower is responsible for separating 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 oxygen and nitrogen. Argon extraction also uses cryogenic distillation.
The distillation system is the core process unit of the air separation unit, and the distillation tower is the core equipment. During the working process, changes in the processing air flow rate and product structure will affect the distillation process. The operating state of the main distillation tower 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. If the nitrogen content in the gas component of the raw material fraction of the argon distillation tower exceeds the design value, the heat exchange of the crude argon tower top condenser will be abnormal, resulting in a large process change in the flow rate of the argon fraction extracted from the main distillation tower, which will not only make the argon extraction distillation tower unable to work normally, but also affect the oxygen and nitrogen separation of the main distillation tower if the working conditions deteriorate, and even cause the problem that the oxygen and nitrogen products of the device cannot be normally supplied to the outside, which has a negative impact on the stable production of the air separation unit. The main distillation tower and the argon distillation tower influence and promote each other. Only by optimizing the main distillation tower can the product extraction rate of the air separation unit reach an ideal state.
The automatic control of the main distillation tower sensitivity temperature adopted in this article is to stabilize the main distillation tower of the air separation, and to improve and develop it in technology to achieve effective control of the distillation working conditions of the air separation unit. As the source material of the argon distillation system, the raw argon fraction will cause serious process fluctuations, namely "nitrogen plugging" once the nitrogen component exceeds the standard. Only by effectively controlling the distillation of the main distillation tower can the occurrence of abnormal working conditions of "nitrogen plugging" be prevented.
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 as 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, and use the liquid nitrogen extraction flow as the given correction compensation of the auxiliary process control loop. That is, on the basis of the predetermined liquid nitrogen output, the liquid nitrogen extraction flow is output through the main process control loop, and the compensation calculation is performed 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, and the reflux ratio of the main distillation tower is changed, thereby realizing the precise control of the sensitivity temperature of the main distillation tower.
The core unit of the air separation unit is the distillation system. The sensitive point of the main distillation tower is the position where the concentration gradient changes the most on all the distillation packing layers of the distillation tower. Along the height of the distillation tower, on each distillation gas-liquid mass transfer interface, there is a distribution of concentration gradient change rates corresponding to the height of the distillation mass transfer unit, where the position with the largest concentration gradient change is the distillation sensitive point. The design data of this point is calculated by process simulation software, corresponding to the maximum point of the deviation between the liquid phase oxygen concentration and the gas phase oxygen concentration on the theoretical gas-liquid equilibrium phase diagram. From the theoretical analysis and principle, the sensitive point temperature is the tower temperature on the distillation gas-liquid equilibrium phase diagram corresponding to the maximum point of concentration gradient change.
The data of the sensitive point temperature of the main distillation tower detected by DCS is compared with the designed theoretical data. The main process control loop PID (proportional integral differential) inside the DCS is used to take this point as the detection point of the main process control loop, and compared with the sensitive point design temperature and perform PID operation. The measured design temperature is -187.5℃. The main process control loop is defined as TIC1717, and its corresponding PID output is the upper tower liquid nitrogen product removal flow control. On the basis of 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, thereby realizing the control of the reflux ratio of the main distillation tower. Through the reflux ratio, a key process operation variable, the reflux ratio is made consistent with the design, so that the gas-liquid balance of the upper tower distillation approaches the design value, and the composition of the raw material fraction required for the argon distillation extracted from the upper tower stripping section is accurately controlled. If the upper tower sensitive point temperature TI1717 is lower than the design temperature, the continuous PID control output is completed through the TIC1717 main process control loop to control the liquid nitrogen product output flow FI1630, so that the reflux ratio of the main distillation tower is maintained within the design range, and the precise control of TI1717 is realized. Since the raw material fraction required for argon distillation comes from the stripping section of the upper tower, the effective control of the sensitive point of the main distillation tower can control the concentration distribution of the stripping section of the main distillation tower within the design range, and realize the normal concentration distribution of nitrogen components in the stripping section.
3. Ways to prevent large lags in process control and selection of process control output variables
Based on the above process control principles, the key process control variable selection of the main process control loop of the main distillation tower introduced above can be adopted during the operation of the air separation process. The selection of the sensitive point temperature of the main distillation tower is based on process simulation calculation and gas-liquid equilibrium theory. The point with the largest concentration gradient change is the process data that can most quickly reflect the real process changes after the process load and working conditions change. The process sensitive point selected for the main distillation tower comes from the gas phase section of the distillation packing section corresponding to the part where the liquid air evaporates water vapor from the crude argon condenser into the main distillation tower. By solving the process control problems caused by pure process lag, the control quality can be effectively improved.
In order to improve the control quality of the main process control loop, the temperature data detected by the detection end of the TIC1717 control loop needs to be converted into thermodynamic temperature, and the amplification factor needs to be set to improve the sensitivity of the detection data, so as to achieve timely and effective process control.
From the process-related connection, the impact of the main distillation tower liquid nitrogen output on the upper tower reflux ratio is fast and effective, and the data change of the main distillation tower sensitivity TI1717 is the most direct and effective detection variable for the reflux ratio change. In addition, correcting and compensating the output of liquid nitrogen products within a certain range is the most controllable and effective measure for production operation and ensuring stable production of the main unit. The output of key gas products of the air separation unit is a key indicator to ensure subsequent customers. It cannot be adjusted at will during production. By adjusting the amount of liquid nitrogen products within a certain range, it can not only ensure the needs of subsequent gas customers, but also be the least costly way.
4. Measures to prevent process control over-adjustment
The main distillation tower sensitivity automatic control program is based on the compensation of liquid nitrogen output product flow. It is only an automatic compensation control mode of the air separation unit under process disturbance. It cannot perform process control when the air separation unit has a large range of load changes. Therefore, when the distillation tower sensitivity automatic control program outputs the liquid nitrogen output cascade correction compensation mode, it is necessary to adopt a certain limit range for the output of the main process control loop TIC1717. By programming a control output range limit module, and defining it based on the design range of the liquid nitrogen output of the air separation unit product, a certain control output limit is maintained to avoid overshoot.
Based on the data of the design capacity of the liquid nitrogen product, the output of the main process control loop is limited in a corresponding proportion, which can timely control and correct the process deviation when the process disturbance occurs, realize the real-time tracking and control of the key main distillation tower process control variables, and ensure the stable gas product output of the subsequent pipeline gas customers of the air separation unit. By using the automatic correction compensation method of liquid output, the distillation condition of the air separation unit can be automatically controlled in the optimal process state, and the key process control variables calculated by the design are used as the target control value to achieve the stable working condition of automated distillation.
In the control of industrial automation instruments and meters, continuously optimizing the control path and improving the control accuracy can enable the instruments and meters to better exert their potential and contribute more to improving the level of industrial production automation. In the field of industrial production, the DCS system provides a powerful platform for the intelligent implementation of various production processes. If process engineers and automation engineers work closely together on various new optimization schemes, the distillation automation of the distillation tower can be effectively realized. This cascade control solution is derived from the ideas of senior process engineers. Technical personnel need to systematically analyze process control requirements, tap the potential of equipment, and ensure stable and reliable operation of equipment in complex and changeable industrial production environments. At the same time, through technological innovation, automated instruments can play a more important role in future industrial production and lead enterprises to a more brilliant future.
