Vinyl chloride monomer is an indispensable raw material for the production of polyvinyl chloride. At present, most companies use the calcium carbide method to prepare vinyl chloride monomer. In the preparation of vinyl chloride monomer, the rectification of vinyl chloride is a crucial process. Its heat transfer and mass transfer processes are very complicated. The load of the rectification column is often changed due to production requirements, resulting in a distillation system. The pressure fluctuates greatly, the operation is difficult, the purity of the monomer is easily exceeded, and the quality of a large amount of resin is seriously affected. In response to the above problems, at the same time, in order to cooperate with the implementation of the technical expansion project of vinyl chloride expansion, our company adopted the JX-300XDCS control system and the corresponding control program of Zheda University, and it was put into operation in May 2003, and achieved good results. effect.
1 Process Crude vinyl chloride is fed into the full condenser through a compressor and cooled with industrial water to condense and liquefy crude vinyl chloride. The liquid vinyl chloride borrows poorly into the water separator and is continuously stratified by density into the low boiling tower. The uncondensed gas in the full condenser enters the tail gas condenser and returns as reflux to the top of the low boiling column. An autoclave at the bottom of the low boiler tower is heated by circulating the hot water in the converter to distill out the low boilers in the liquid phase running down the tray. The gas phase flows upward along the plate and exchanges heat and mass on the liquid in the plate. Finally, it is cooled at -10°C in the top condenser to be used as the reflux liquid at the top of the column. The noncondensable gas is also condensed at the top of the column. The device passes through the tail gas treatment, and the low-boiling-bottoms-removing low-boiling vinyl chloride is passed through the middle tank into the high-boiling tower through the middle tank. After the high-boiling tower is heated, the low-boiling vinyl chloride component is distilled out and passed through the finished condenser. Condensate into the monomer storage tank, high boilers discharged from the two valves at the bottom of the tower.
2 System Configuration and Control Scheme Implementation 2.1 System Composition The DCS control system for the distillation of vinyl chloride consists of two parts, a hardware system and a software system. In the hardware configuration, two operating stations and one control station are configured according to actual conditions. The operation station selected Advantech P4 IPC and communicated with the control station through the Scne process control network. The control station consists of hardware such as power supply, main control card, data forwarding card, and input/output card. In addition, one printer and one UPS uninterruptible power supply are also configured.
In the software configuration, the JX-300X system includes monitoring software and configuration software. The configuration software includes basic configuration software (SCKey), flowchart creation software (SCDraw), report creation software (SCForm), and programming language (SCLang )Wait. It is easy to realize the editing of various screens and the construction of complex controls on the configuration software. Monitoring software is mainly used by operators for operations and historical inquiries.
2.2 Implementation of Control Schemes The characteristic of vinyl chloride distillation process is that there are great correlations among various physical quantities, and the changes in crude vinyl chloride components, flow rate, and temperature entering the distillation system have great influence on the control of the tower. In response to this situation, by establishing a multivariable decoupling control system, the relevant variables are introduced as feedforward signals to eliminate disturbances, correlations and lags. According to the above situation, distillation tower control can be divided into low boiler towers and high boiler towers. Low Boiler Tower Controls Four Loops and Low Tower Temperature Differences, Pressure, etc. by Rear Pressure Control (PIRC-103), Feed Control (LIRC101), Low Boiler Heating Temperature Control (TIRC102), and Low Boiling Tower Return Control (TIRC101) Checkpoint composition. High Boiling Tower Control Testing Temperature difference, pressure and other detection points in the three loops of high boiler column liquid level control (LIRC201), high boiler column reflux control (TIRC201), finished product pressure control (PIRC201) and high tower rectification section and stripping section composition. For PIRC-103, TIRC102, TIRC101, TIRC201, PIRC201 are composed of a single loop.
A detailed analysis of the low boiler feed control (LIRC101) and high boiler column liquid level control (LIRC201) is also the most critical part of the control of vinyl chloride distillation.
2.2.1 Feed control of low boilers Crude vinyl chloride enters low boilers. Its composition, flow rate, and temperature are changed at any time depending on operating conditions. These changes can be made through the temperature difference between the low boiler (TDE101) and the low boiler bottom. The liquid level and the opening of the low boiling vessel valve are reflected. By analyzing the trend and logical relationship between these variables, the control scheme regards the liquid level as the control object, and the temperature difference and the heating valve opening degree as the disturbance quantity as the feedforward. Adding this control loop not only overcomes the interference but also improves the system's accuracy.
2.2.2 Liquid level control of high boilers The removal of low-boiler vinyl chloride passes through the intermediate tank into the high-boiling tower due to the difference in position. Since the middle tank is small in volume, it does not act as a buffer, and therefore the low-boiling tower and The correlation between the high boiling towers is lifted, which has a great influence on the control of the high boiling towers. In order to overcome this problem and ensure that the high boilers are within the quality index, a multivariable compensation control system is designed by analyzing each variable. The specific plan is as follows.
(1) The opening of the low boiling tower feed valve, the opening of the high boiler reflux valve, the opening of the high boiler liquid level valve, and the high boiler temperature compensation of the high boiling tower liquid level control loop: when several variables occur When changed, the level of the high boiling tower also changes accordingly. Therefore, the above variables are introduced into the high boiling tower liquid level control loop according to a certain compensation coefficient, and the high boiling tower liquid level control valve is used.
(2) The difference in the temperature difference between the high boiler tower pressure difference and the high-boiling tower stripping section compensates the high-boiling tower liquid level control loop: the difference between the high-boiling tower pressure difference and the high boiling tower stripping section temperature does not compensate at all times. Theoretical analysis and operating experience, first set a set value, the actual value of the measurement and the set value is compared to determine whether to introduce compensation. When the actual value is greater than the set value, the compensation works, and their corresponding compensation coefficients are introduced into the high boiling tower liquid level control loop. Otherwise, no compensation effect can be achieved.
Through continuous exploration and analysis, the compensation coefficients for the high boiling tower liquid levels for each variable are: LIC201BCIC-1, 0.24; LIC201BC-2, 0.1; LIC201BC-3, 0.7; LIC201BC-4, 0.1; LIC201BC-5, 0.15; LIC201BC-6, 0.1.
3Management function The system also has functions of display, historical data query, alarm, memory, printing, etc. It is especially pointed out that the system is connected with the company's ERP system through the information management network, and the key data is timely transmitted to the decision layer. The decision-making level feedbacks to the production in time according to the market conditions and constitutes the integration of management and control.
1 Process Crude vinyl chloride is fed into the full condenser through a compressor and cooled with industrial water to condense and liquefy crude vinyl chloride. The liquid vinyl chloride borrows poorly into the water separator and is continuously stratified by density into the low boiling tower. The uncondensed gas in the full condenser enters the tail gas condenser and returns as reflux to the top of the low boiling column. An autoclave at the bottom of the low boiler tower is heated by circulating the hot water in the converter to distill out the low boilers in the liquid phase running down the tray. The gas phase flows upward along the plate and exchanges heat and mass on the liquid in the plate. Finally, it is cooled at -10°C in the top condenser to be used as the reflux liquid at the top of the column. The noncondensable gas is also condensed at the top of the column. The device passes through the tail gas treatment, and the low-boiling-bottoms-removing low-boiling vinyl chloride is passed through the middle tank into the high-boiling tower through the middle tank. After the high-boiling tower is heated, the low-boiling vinyl chloride component is distilled out and passed through the finished condenser. Condensate into the monomer storage tank, high boilers discharged from the two valves at the bottom of the tower.
2 System Configuration and Control Scheme Implementation 2.1 System Composition The DCS control system for the distillation of vinyl chloride consists of two parts, a hardware system and a software system. In the hardware configuration, two operating stations and one control station are configured according to actual conditions. The operation station selected Advantech P4 IPC and communicated with the control station through the Scne process control network. The control station consists of hardware such as power supply, main control card, data forwarding card, and input/output card. In addition, one printer and one UPS uninterruptible power supply are also configured.
In the software configuration, the JX-300X system includes monitoring software and configuration software. The configuration software includes basic configuration software (SCKey), flowchart creation software (SCDraw), report creation software (SCForm), and programming language (SCLang )Wait. It is easy to realize the editing of various screens and the construction of complex controls on the configuration software. Monitoring software is mainly used by operators for operations and historical inquiries.
2.2 Implementation of Control Schemes The characteristic of vinyl chloride distillation process is that there are great correlations among various physical quantities, and the changes in crude vinyl chloride components, flow rate, and temperature entering the distillation system have great influence on the control of the tower. In response to this situation, by establishing a multivariable decoupling control system, the relevant variables are introduced as feedforward signals to eliminate disturbances, correlations and lags. According to the above situation, distillation tower control can be divided into low boiler towers and high boiler towers. Low Boiler Tower Controls Four Loops and Low Tower Temperature Differences, Pressure, etc. by Rear Pressure Control (PIRC-103), Feed Control (LIRC101), Low Boiler Heating Temperature Control (TIRC102), and Low Boiling Tower Return Control (TIRC101) Checkpoint composition. High Boiling Tower Control Testing Temperature difference, pressure and other detection points in the three loops of high boiler column liquid level control (LIRC201), high boiler column reflux control (TIRC201), finished product pressure control (PIRC201) and high tower rectification section and stripping section composition. For PIRC-103, TIRC102, TIRC101, TIRC201, PIRC201 are composed of a single loop.
A detailed analysis of the low boiler feed control (LIRC101) and high boiler column liquid level control (LIRC201) is also the most critical part of the control of vinyl chloride distillation.
2.2.1 Feed control of low boilers Crude vinyl chloride enters low boilers. Its composition, flow rate, and temperature are changed at any time depending on operating conditions. These changes can be made through the temperature difference between the low boiler (TDE101) and the low boiler bottom. The liquid level and the opening of the low boiling vessel valve are reflected. By analyzing the trend and logical relationship between these variables, the control scheme regards the liquid level as the control object, and the temperature difference and the heating valve opening degree as the disturbance quantity as the feedforward. Adding this control loop not only overcomes the interference but also improves the system's accuracy.
2.2.2 Liquid level control of high boilers The removal of low-boiler vinyl chloride passes through the intermediate tank into the high-boiling tower due to the difference in position. Since the middle tank is small in volume, it does not act as a buffer, and therefore the low-boiling tower and The correlation between the high boiling towers is lifted, which has a great influence on the control of the high boiling towers. In order to overcome this problem and ensure that the high boilers are within the quality index, a multivariable compensation control system is designed by analyzing each variable. The specific plan is as follows.
(1) The opening of the low boiling tower feed valve, the opening of the high boiler reflux valve, the opening of the high boiler liquid level valve, and the high boiler temperature compensation of the high boiling tower liquid level control loop: when several variables occur When changed, the level of the high boiling tower also changes accordingly. Therefore, the above variables are introduced into the high boiling tower liquid level control loop according to a certain compensation coefficient, and the high boiling tower liquid level control valve is used.
(2) The difference in the temperature difference between the high boiler tower pressure difference and the high-boiling tower stripping section compensates the high-boiling tower liquid level control loop: the difference between the high-boiling tower pressure difference and the high boiling tower stripping section temperature does not compensate at all times. Theoretical analysis and operating experience, first set a set value, the actual value of the measurement and the set value is compared to determine whether to introduce compensation. When the actual value is greater than the set value, the compensation works, and their corresponding compensation coefficients are introduced into the high boiling tower liquid level control loop. Otherwise, no compensation effect can be achieved.
Through continuous exploration and analysis, the compensation coefficients for the high boiling tower liquid levels for each variable are: LIC201BCIC-1, 0.24; LIC201BC-2, 0.1; LIC201BC-3, 0.7; LIC201BC-4, 0.1; LIC201BC-5, 0.15; LIC201BC-6, 0.1.
3Management function The system also has functions of display, historical data query, alarm, memory, printing, etc. It is especially pointed out that the system is connected with the company's ERP system through the information management network, and the key data is timely transmitted to the decision layer. The decision-making level feedbacks to the production in time according to the market conditions and constitutes the integration of management and control.
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