The condensation vacuum is the fundamental condition for ensuring the normal operation of the entire production line. The smooth control of the vacuum directly affects the stability of the process and affects the quality of the product. In the polycondensation sprinkler system, the low-temperature EG shower will be trapped with the EG vapor and oligomers brought out by the vacuum, and at the same time, it will easily settle and adhere to a part of the oligomers on the alternating hot and cold walls. Therefore, in the more mature continuous production processes, there are scraper or barium facilities here so that a large number of oligomers adhering there can be cleaned at regular intervals. There is a scraper on the production line that can automatically run up and down regularly. After the device has undergone expansion and expansion at 150% load, due to the increased load and material flow rate, it will inevitably affect the vacuum condition.

During the production and operation after production, we found that the scraper operation program caused large fluctuations in the pre-shrinkage and final shrinkage vacuum during operation, which not only affected the stability of the vacuum control, but also affected the amount of material and the liquid in the reactor. The position control has a greater impact, and even affects the stability of the entire process in severe cases. The stability of vacuum control is a difficult point in production control.

1 Analysis of the status In the process of preshrinkage reaction, the vacuum pipeline is easy to be blocked during the vacuum process, and the process needs to periodically scrape off the blockage. In the very short time of the scraper movement, a large flow of EG is also set. rinse. Due to the operation of the squeegee and rinsing with EG, the degree of vacuum in the kettle is instantaneously changed. The entire process is: in the early stage of the scraper operation, the vacuum level drops, and the vacuum control valve opens large; when the scraper operation is terminated, the opening of the vacuum control valve is relatively large, and the vacuum in the autoclave rises rapidly. This phenomenon is more pronounced when the line load is relatively large. Because the vacuum control valve does not have the ability to adjust the sudden pressure change, the controllability is poor, and it takes 10 to 15 minutes to recover to the original vacuum value, and it also affects the vacuum of the final shrinking reactor, causing it to fluctuate, and the fluctuation amplitude is as high as 80Pa. , causing material flow instability, affecting the dynamic balance of the process. The scraper program is set according to the process requirements, and is run once every 30 minutes to control the viscosity and carboxyl content of the product.

2 Control System Analysis and Modification My plant production line uses the EMERSON process management DeltaV operating system, which is part of the process control implemented in Plantweb based on the site structure. The DeltaV system provides a wealth of configuration tools and very powerful screen editing and open OPC technology to more easily manage the entire process control process.

Pre-shrink vacuum regulation is a simple PID single-loop control where the scraper periodically cleans the lift gas line is a sequential control. We found that when the squeegee is periodically cleaned, the control valve is manually controlled to the opening before the squeegee action, and the vacuum value changes little. This gave us an inspiration. When the squeegee moves, the vacuum adjusts the PID function block to control the manual control of the regulating valve. After the squeegee moves, the regulating valve resumes automatic control and it should be able to maintain a relatively stable vacuum.

2.1 Vacuum regulation PID single loop control Vacuum regulation PID single loop control is to compare the vacuum value detected by the absolute pressure transmitter with the set value of the PID1 function block to obtain the deviation value, through the proportion, differential, integral of the PID1 function block. And the setting of the filter time, the output signal to the regulating valve, to maintain the control of the pre-steam vacuum.

2.2 Scraper sequence control flow and modification Scraper sequence control Main flow description: T1 set scraper operation start DOWN Scraper solenoid valve has electric action, scraper starts to run low T2 judge scraper run to low UP scraper solenoid valve In case of power loss, the squeegee starts to run high. T3 judges that the squeegee runs to the high DELAY time delay. The counter starts to time. The T4 counter time reaches the set value. Time T7 The end of the squeegee operation cycle We performed the following main flow of SFC sequence control as follows: In the process of modification, TRKVAL and TRKIND in the vacuum control PID1 control module are mainly used to achieve automatic and manual conversion; and after the manual conversion to automatic, the PID module automatically sets the automatic tracking setting. The following statement in the main flow of the Scraper (SFC sequence control) is a modified new statement.

Note: The new statement in DOWN is to judge the manual and automatic status in the vacuum adjustment PID control module. When the squeegee program runs and reaches the low position from top to bottom (only when the squeegee program assigns value to the PID control module), the PID control is converted to manual control, and the PID1 function block automatically gives the control valve a fixed opening. . The new sentence in DELAY is to judge the running of the squeegee program. When reaching the high position from bottom to top, the PID control module set value is automatically assigned, and the PID control is converted into automatic control.

After modifying the above program, online download the control configuration. By observing the vacuum curve at a later stage, it was found that the pre-shrinkage vacuum stability was significantly improved; the fluctuation of the material level of each reactor was reduced, the stability of the system material balance was enhanced, the controllability of the device was improved, and the average fluctuation range of the vacuum of the pre-shrink reactor was The range of values ​​has been reduced from the original (2.8 ± 0.19) kPa to the present (2.8 ± 0.07) kPa or less. The comparison of the pre-shrinkage vacuum curve before and after the transformation is shown in Figure 1.

3 Concluding remarks The technical improvement is the effective combination of SFC sequence control and traditional PID. It uses the special functions in the PID control module provided by the DeltaV system and the powerful programming platform in the SFC sequence control to better solve the existence of the production control process. The unstable factors optimize the control and stabilize the process.

Polyester worker This program is generally speaking, that is, the program PTA2 is set correspondingly after receiving the operation IC50.FL, these two parameters are logically operated in the logic point LOGIC50, and they are forced to open the non-online use (in the closed state). The feed valve of the batch bin starts to feed, closes the feed valve of the batch bin used online (in the open state), stops feeding, and opens and closes the corresponding loose gas to match the feed of the batch bin and Discharge. It should be noted that when the HS2782/HS27832 valve is closed, the switching operation will not be performed until the main operation intervention (that is, one of the valves is actively opened). The program works well in operation and satisfies the process production requirements.

3 Conclusion The system runs stably and is maintained with the opportunity of maintenance, which ensures the long-term stability of the system. The system is highly developed, can fully meet the requirements of the production process operation, and ensure the continuous and stable operation of the PTA device.

Section Steel Series

Stainless Steel Bar,Hexagonal Bar Steel Rod,Angle Steel Bar,H-Beam Steel

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