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VCM rectification process and characteristics
The VCM rectification process typically consists of a low boiling column system and a high boiling column system and is a typical two-stage cascade rectification process. The process flow is shown in Figure 1. The crude vinyl chloride from the compressor is first sent to the full condenser, so that most of the gas is condensed and liquefied. After the water separator removes water, it enters the low boiling tower, and the uncondensed gas enters the exhaust condenser, and the condensate is returned to the water separator. The low boiling tower reboiler is heated by hot water, and the low boiler separated by the low boiling tower is sent from the top of the tower to the full condenser to the tail gas condenser, and the uncondensed gas of the tail gas condenser is recovered by the tail gas adsorber. After that, the remaining inert gas is discharged. The tower kettle liquid flows into the high boiling tower through the intermediate tank, and the reboiler of the high boiling tower is heated by the hot water to evaporate the vinyl chloride, and is separated into fine vinyl chloride into the finished condenser through the tower body, and the condensed vinyl chloride is stored in the finished tank. In order to ensure the purity of the monomer, the material at the bottom of the high boiling column contains a certain amount of high boilers, and it is necessary to periodically discharge according to the working conditions.
Due to the continuity of production, the VCM rectification process is closely linked with the synthesis of the former section and the polymerization of the subsequent section. The discharge of the synthesis section is the feed of the VCM rectification section. Therefore, the VCM rectification of the low boiling tower feed flow rate and The feed components will change frequently, which is a major contributor to the VCM rectification section. The change in VCM feed flow changes both the material balance and the energy balance. Due to the long lag time of the crude vinyl chloride in the rectification column, the usual feedback adjustment cannot overcome the disturbance of the feed flow in time. The change of feed composition has great influence on the temperature of each tray and the distillate amount when it reaches a certain product purity, especially VCM rectification. The feed composition is complex and unmeasurable, which brings great difficulty to the smooth control of product quality. .
For high-boiling towers, the feed change is also very frequent, and the variation is relatively large, and the liquid level of the tower is difficult to control. At the same time, frequent changes in the liquid level of the high boiling tower will cause changes in the temperature of the column. The temperature of the column will affect the liquid level of the column and the temperature of the top of the high boiling column and the pressure of the column. The reflux and other factors will be combined with the tower. The liquid level and temperature are related to each other and affect each other. Therefore, there are complex coupling relationships between the operating parameters of the high boiling tower. Many factors work together to determine the operating condition of the device. There are certain constraints on the operation of the high-boiling tower unit. Each operating parameter has its own allowable operating area limitation. The control variables should be coordinated to ensure the overall control level of the high boiling tower and the final product quality. Therefore, the control problem of the high boiling tower is particularly prominent in the vinyl chloride rectification plant.
Principles and characteristics of Zhejiang University Central Control APC-PFC advanced control software
Basic principle
The core algorithm of APC-PFC control software technology is predictive function control (PFC), namely predictive model, rolling optimization, and feedback correction. PFC is a member of the model predictive control family and was proposed in 1986 by J. Richalet et al. of ADERSA, France. Different from the traditional model predictive control algorithm, PFC pays attention to the structural form of control variables, regards the control input as a weighted combination of basis functions, introduces error feedback correction, and performs online optimization. The basic principle of PFC is shown in Figure 2.
2. Main features of APC-PFC control software
1) The algorithm is simple, the amount of online calculation is small, and the accuracy of the model is low. In online control, only the dot product of the vector is calculated, and there is no complicated operation such as matrix inversion. Easy to implement and easy to integrate into any mainstream DCS;
2) In order to ensure the reasonableness of the future control input, under the premise that the controlled variable and the manipulated variable satisfy the position and speed hard constraints, each controlled variable can reach the set value according to the specified closed loop performance;
3) For the feedforward control of the measurable disturbance, suppress the disturbance in time;
4) The APC-PFC advanced control system has fewer adjustment parameters. The influence of each adjustment parameter on the control performance has its own emphasis. The parameters can be quickly adjusted according to the performance requirements, which greatly shortens the setting time. The relationship between the performance of the closed-loop system and the adjustment means is shown in Table 1;
The numbers 0, 1, and 2 in Table 1 indicate the degree of influence of the adjustment means on the control performance. The larger the number, the greater the degree of influence, and the 0 indicates no effect.
The VCM rectification prediction control scheme is designed to describe the variables of the VCM rectification process before the VCM rectification prediction control scheme is designed. The controlled variables, manipulated variables and disturbance variables of the low boiling tower and the high boiling tower are shown in Table 2.
It can be seen from Table 2 that the VCM rectification process is a multivariable control system with strong coupling between variables, and the presence of multiple disturbances that are measurable and unmeasurable, as well as the flow and composition of the feed, often occur in strength and persistence. Different time fluctuations, it is difficult to effectively consider the coupling characteristics between various control actions in the device by conventional control. Therefore, the control effect is not very good and needs to be corrected by manual experience.
Since there is an intermediate buffer tank between the low boiling column and the high boiling column, there is relative independence between the low boiling column and the high boiling column. Therefore, the controllers were designed for the low boiling tower and high boiling tower with the advanced control software of Zhejiang University Central Control APC-PFC. The block diagram of the VCM rectification advanced control system is shown in Figure 3. It is an advanced control system with a predictive function controller as the supervisory layer and a conventional PID control as the transparent control structure of the control layer. The control structure combines the reliability and stability of the PID control, and combines the good setpoint tracking performance and dynamic response of the PFC. In practical applications, the feedforward amount is added to the PFC controller to form a feedforward PFC controller. The feedforward amount is added to the PID controller through the adjustment and supervision of the PFC controller when the disturbance occurs. Adjust the corresponding set value in the PID controller in time, eliminate the residual, and make the output of the PID control close to the expected value, so as to overcome the disturbance effect of the disturbance variable on the controlled variable in time, so that the whole system can quickly reach the smooth control. .
Application effect
Since the application of VCM rectification predictive control software package in our company, the stability of production operation and product quality have been significantly improved. Figure 4 and Figure 5 are comparisons of the control effects of the APC-PFC predictive control software package before and after the operation of the low-boiling tower and the high-boiling tower.
It can be seen that the top temperature of the low boiling tower and the liquid level of the high boiling tower tower fluctuate greatly before the predicted control is put into operation, and the control effect is not satisfactory. After the predictive control software is put into operation, the smoothness of the process is significantly improved. Both variance and range are significantly reduced. Other parameters have been further improved and improved after the predictive control software is put into operation. In addition, in Figure 4, the predicted boiling point of the high-boiling tower after the control software is put into operation has two burrs. The two burrs are due to the liquid level disturbance caused by the intermittent discharge of high-boil, and the control package is explained from another angle. Has a strong anti-interference ability. After the above-mentioned predictive control software package was put into operation, the quality of the monomer has been significantly improved, and its purity is stable at 99.999%, which has fully met the requirements of PVC production.
Conclusion Zheda Central Control APC-PFC predictive control software package will not have any impact on the production process during installation and debugging.
Between Zhejiang University's central control APC-PFC predictive control software package and the original control system, the bumpless switching of conventional PID and predictive control can be realized with only one switch. Operators can also perform VCM rectification predictive control software packages with minimal training.
The adjustment parameters of Zhejiang University Central Control APC-PFC predictive control software package are few and easy to modify. System maintenance engineers can perform effective system maintenance with simple training.
Author: Xue Jian, Qinqiao Liang, Chen Dongmeng, Fan Minghua, Nantong Jiangshan Agrochemical & Chemical Co., Ltd. Source: Control Engineering China
Application of Predictive Control (PFC) in Vinyl Chloride Distillation Process
In view of the control problems in the VCM rectification process, our company used Zhejiang University Central Control Predictive Function Control Software (APC-PFC) to establish an advanced control system for the rectification process, which was successfully put into operation in March 2005, and VCM rectification uses advanced control. After greatly reducing the operator's work intensity, the forecasting model is used to predict the changing trend of the controlled variable, and the rolling control optimization and deviation correction are performed on the given value of the control loop from time to time to improve the control precision and stability of the key variables of each process. Thereby ensuring the product quality and maintaining the smooth operation conditions, and obtaining satisfactory control effects.