Application of PCS7 in ion-exchange membrane caustic soda projects

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The PCS7 system is a distributed, fully integrated control system designed for industrial processes, combining the technologies of DCS, PLCs, and field buses. The open network architecture, open operating system, and open interfaces enable optimization of the entire information system across the company, from the production level to the management level ； In a fully integrated automation environment, a unified automation platform is used to achieve automated control and management of the entire factory. Project Overview: This project is the 50,000-ton per year ion-exchange membrane caustic soda production facility of Meilan Chemical Plant, part of Jiangsu Meilan Group. Meilan Group is a large-scale comprehensive chemical enterprise whose products include several categories such as chlor-alkali, fluorine products, and refrigerants, totaling more than 30 different products. Figure 1: The main methods for preparing caustic soda in electrolyzers are the diaphragm process and the ion-exchange membrane process. Compared to the diaphragm method, the ion-exchange membrane method reduces energy consumption, yields products of high purity, causes less pollution, and is free from environmental hazards, making it more in line with environmental protection requirements. However, the ion-exchange membrane process is complex and requires high levels of control. The SIEMENS next-generation process control system PCS7 V6.0, selected by Meilan Chemical Plant and combined with PROFIBUS fieldbus technology, represents an excellent control system solution for the ion-exchange membrane caustic soda industry. System Overview The main sections of this project include: secondary brine, hydrogen chloride control, electrolysis, catholyte cooling and heating, dechlorination, high-purity water, high-purity acids, etc. The project uses two sets of AS417-4H systems, with 2,000 process I/O points, 2 redundant servers, 12 operator stations, 1 engineer station, and 40 ET200M units. One set of CPUs is used to control the electrolysis and hydrogen chloride treatment sections, while another set controls the auxiliary units. The secondary brine section is controlled by two sets of S7-300 PLCs, which are connected to the DCS via PROFIBUS-DP. System features: CPU redundancy, power supply redundancy, I/O redundancy, Ethernet redundancy, Profibus-DP redundancy, server redundancy. Composition of the control system: The system configuration is shown in Figure 2: Figure 2: System configuration diagram. ◆ Due to the complexity of the ion exchange membrane process, the large number of interlocks, and the high requirements for the reliability of the control system, redundant 417-4H CPUs, power supplies, Ethernet, PROFIBUS-DP communication networks, I/O cards, and a WinCC server are used. ◆ The 417-4H CPU features event synchronization, which reliably ensures the consistency of programs and data between the two CPUs. In the event of a failure in the main CPU, resulting in an abrupt shutdown, the CPU that is in monitoring mode immediately takes over as the new main CPU, assuming control of the functions previously performed by the original main CPU. ◆ Industrial Ethernet is a fiber optic ring network connected using OSM switches, with a communication speed of 100 Mbit/s. ◆ The PROFIBUS fieldbus technology enables DCS to abandon the traditional centralized I/O structure; the redundant PROFIBUS-DP communication method enhances the reliability of communications. The cost-performance ratio of ET-200M distributed I/O is **higher than that of centralized I/O. ◆ For the I/O points that are relatively important in the process, redundant card modules are used. When one of the cards fails, the system automatically reads from or outputs signals from the other card, without affecting the system’s ability to monitor or control the field devices. ◆ The power supply for the control cabinet consists of two redundant 220V AC circuits; in the event that one circuit loses power, the other can ensure the system continues to operate smoothly without any disruptions. ◆ The WINCC data server uses two IBM SERVERs. Data is exchanged with the CPU via industrial Ethernet. Figure 3 shows the hardware configuration diagram of station AS1. Figure 3: Hardware configuration diagram. Functions performed by the control system: PCS7 provides a unified platform for engineers to configure and program the system. Under the SIMATIC MANAGER platform, there are three views: the component view, the process view, and the process object view. In the electrolytic production of caustic soda using the ion exchange membrane process, there are many complex control loops, including control of the catholyte temperature, regulation of the pH value of the brine, control of the flow rates of brine/catholyte/pure water, and adjustment of the pressure differences for chlorine/hydrogen/chlorohydrine. The control effectiveness of these circuits directly affects the smooth operation of production. The ion membrane is installed in the electrolysis cell; during production, a direct current voltage is applied to the two poles of the electrolysis cell, and under the influence of this current, brine is electrolyzed to produce chlorine, hydrogen, and caustic soda. The conversion of alternating current to direct current is accomplished through a rectifier; therefore, the control of the rectifier is one of the key aspects in this project. The entire factory is equipped with three levels of safety interlocks: ◆ Rectifier load reduction: There are 11 interlock conditions, and if any one of them is met, the corresponding interlock is activated ； ◆ Single cell shutdown: There are 14 interlock conditions; if any one of them is met, the interlock is activated ； ◆ Plant shutdown: There are 20 interlock conditions; if any one of them is met, the interlock is activated. Figure 4: Taking the TIC-3125 loop temperature control loop TI-3125 on the WINCC screen as an example, it illustrates how to configure PCS7. Function description of the TI-3125 temperature control circuit: The TI-3125 control circuit is used to regulate the temperature of caustic soda circulating in the electrolysis unit. PV: Actual process value ； SP: The final temperature value set by the operator ； Setpoint rise/fall rate at 50°C ； The operator can choose among three operating modes: Cooling COOL: TV-3125.2 is in operation, while TV-3125.1 is turned off. After the operator switches the TIC-3125B/PID controller to “Auto” or “External Setpoint”, cooling begins. The external setpoint of TIC-3125B/PID reaches the final SP value from the current value at a set rate of decrease. STANDBY: Both TV-3125.1 and 2 are turned off, and the PID controller is set to “Manual”/“Internal Setpoint”. Heating HEAT: TV-3125.1 is in operation, TV-3125.2 is off. After the operator switches the TIC-3125A/PID controller to “Auto” or “External Setpoint”, heating begins. The externally set value of TIC-3125A/PID reaches the final SP value from the current value at a set rising rate. Figure 5: The TIC-3125 configuration screen requires a standby state before moving from heating to cooling, and vice versa. The CFC configuration is as follows: ◆ The control valve TV3125.1 is controlled by TIC-3125A. When programming with CFC, the system provides standard PID function blocks. ◆ The control valve TV3125.2 is controlled by TIC-3125B. Figure 6 is the CFC graph. ◆ The PID panel on the WINCC interface uses the standard PCS7 panel. After the configuration is completed, select the OS/COMPILE function in SIMATIC MANAGER to upload the tags to WINCC, and then establish a dynamic connection to complete the WINCC interface. Due to space constraints, they are not listed in detail here. Figure 6: Experience in using the TIC-3125B configuration interface. PCS7 combines the functions of traditional DCS systems and PLCs; it offers flexible programming and configuration options, boasts high reliability and strong scalability, and can meet various user requirements. In the ion-exchange membrane caustic soda industry, there are very high requirements for system stability and reliability, and the process is complex with numerous interlocks. The Siemens PCS7 system meets the needs of customers in this field very well. (end)