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0 Introduction my country’s methanol production has gradually formed a production process route using coal, natural gas, coke oven gas, etc. as the main raw materials. Especially in the past two years, the coke oven gas to methanol process has been adopted by more and more coking plants. Using coke oven gas as the raw material for synthesizing methanol has obvious economic advantages. Coke oven gas is a by-product of coking in coking plants. Coke oven gas is selected as the starting raw material for producing methanol. The methanol plant is built close to the coking plant, so there is no need for coke oven gas to be transported. It fundamentally solves the environmental pollution caused by the venting of coke oven gas and achieves the dual purpose of controlling the environment and turning waste into treasure. It provides an effective way for coking enterprises to resist market risks and improve profitability. Our company has provided DCS control systems and engineering services for a 100kt/a coke oven gas to methanol unit of a coking company in Xingtai, Hebei, a 100kt/a coke oven gas to methanol unit of a coking company in Tengzhou, Shandong, and a 100kt/a coke oven gas to methanol unit of a coking company in Zaozhuang, Shandong. This article analyzes and summarizes the control system configuration and implementation processes of these three companies, hoping to provide a reference for the future transformation of control systems of similar devices. 1 Overall design and characteristics of the control system The JX-300X control system of the methanol plant consists of 2 control stations, 1 engineer station, 6 operating stations (one of which is a remote operating station), 4 external cabinets and 1 set of process control network. The remote operating station and the control station are connected by 400m optical fiber. The system configuration diagram is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/0603031454119666.jpg Figure 1 System configuration diagram In the overall design, we focused on the following points and implemented them in the system. 1.1 Safe realization of the flammable, explosive and toxic characteristics of raw materials and products, as well as the occurrence of run-off, dripping and leakage during the production process, which will endanger production equipment and personal safety. Not only safety must be considered in production equipment and process operations, but also the response speed and effective solutions to unexpected events must be considered when selecting automation equipment and designing control plans, so as to ensure the safety of the production process in terms of hardware and software. In the three projects, we mainly made the following considerations and designs in terms of security. (1) The analog quantities between the control system and field equipment are isolated by safety barriers, and the switching input signals (DI) and output signals (DO) are isolated by relays. (2) During the software design, the operating parameters and operating behaviors were designed to limit operating behaviors and shield the operating interface, that is, operators of different levels correspond to different operating interfaces and are assigned different operating permissions. (3) Different alarm sounds are set in different work sections, and voice alarms are used for important process parameters. (4) Listing operation. When the instrument maintenance personnel perform equipment maintenance, the operator is reminded through software tagging that the loop (or measuring point) is under maintenance and the loop (or measuring point) is in a frozen state, that is, if it is a control loop, the loop is set to manual operation mode and only the output of the regulating loop can be changed. The remaining parameters remain unchanged from the values before freezing until the tagging is lifted. 1.2 Reliable operation During normal operation, if the system cannot perform accurate and reliable actions, it will cause production hazards. Especially when the production device has abnormal working conditions, it is even more necessary for the control system to perform reliable interlocking actions accurately and timely, otherwise it will cause parking and safety accidents. In this project, we made the following considerations and designs in terms of reliability. (1) For key field equipment, we design and select high-quality, high-precision equipment and components, such as flow meters, shut-off valves, and relays. (2) Double redundancy measures are adopted in the design of the main control card, data forwarding card, key card components, system power supply, network, system power supply, etc. of the control system. (3) The operating stations are designed in a peer-to-peer manner. Any operating station can be used as a hot redundant operating station for other operating stations. Even if one of the operating stations fails, the other operating stations can be used for operation at any time. 1.3 Fully realize automation (taking Hebei Xingtai methanol project as an example) The device has many detection points, and the actual number of points used is 544 ； There are many on-site instruments and equipment, including 200 detection, transmission and measurement devices and 55 pneumatic control valves. ； The degree of self-control is high, with a total of 55 control loops, mainly conventional control, with some cascade control, ratio control and three-impulse control. ； Comprehensive safety interlocking design. The I/O in the device is shown in Table 1. Table 1 Device I/O table I/O type conversion, compression, desulfurization and other methanol synthesis and distillation analog input AI (4 ~ 20mA) 11882 temperature RTD (thermal resistance) 3770 temperature TC (thermoelectric Yu) 5414 Analog output AO (4~20mA) 2834 Switching input DI (passive dry contact) 2332 Switching output DO (passive dry contact) 3022 Total 2902542 Implementation of the main control loop and resolution of practical difficulties 2.1 Coke oven gas compression/conversion interlock (1) Steam pressure ≤ 2.45MPa, or oxygen pressure in the working section ≤ 2.20MPa, or coke oven gas pressure ≤ 2.00MPa, Or when the steam drum liquid level is ≤200mm, or when the oxygen coke oven gas flow ratio is higher than 0.205 (oxygen is the compensated flow rate), the oxygen valve entering the reformer is closed and the steam valve is opened and the oxygen valve entering the reformer is closed. The interlocking logic is shown in Figure 2. (2) When the air inlet pressure of the preheating furnace is ≤0.001MPa, the fuel gas valve at the inlet of the preheating furnace is closed, and its interlocking logic is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload/0603031455391531.jpg Figure 2 Coke oven gas compression/conversion interlocking logic diagram 2.2 Three-impulse control of drum liquid level in methanol synthesis tower (shell-and-tube synthesis tower) Steam drum liquid level is the main process indicator that characterizes the production process, and it is also one of the main conditions to ensure the safe operation of equipment such as the synthesis tower and bubbles. If the liquid level is too high, the steam will be filled with liquid, which will reduce the output and quality of the steam. ； If the liquid level is too low, it will affect the water vapor balance at least, burn it dry at worst, and cause explosions and other accidents in serious cases. Therefore, water level is an extremely important controlled variable. The so-called "impulse" is actually a variable. The impulse in multi-impulse control refers to the measurement signal introduced into the system. In level control, the main impulse is water level. Auxiliary impulses are steam load and feed water flow, which are introduced to improve control quality. Figure 3 shows the three-impulse control scheme for the drum liquid level in the methanol synthesis tower. http://www.nmtech.com.cn/jishuwang/upload/0603031456326838.jpg Figure 3 Three-impulse control scheme for drum liquid level in methanol synthesis tower 3. Device operation status and future modification suggestions for similar devices. The 100kt/a coke oven gas to methanol device of a coking company in Xingtai, Hebei Province has been operating stably since it was put into operation in August 2005, and the control function meets the requirements of the process. It mainly has the following characteristics. (1) The control system has a reliable design, has detailed accident judgment and quick response capabilities, can handle various unexpected situations correctly and effectively, and plays a very good role in protecting process equipment and ensuring safety. (2) The system has been working normally since it was put into operation, and users are satisfied with the technical indicators of the system. (3) When the process is normal, the automatic control operation rate is above 93%. (4) The operation team is clearly segmented. * * It shortens the training time for operators and makes the system on-site debugging and operation time extremely short. The 100kt/a coke oven gas to methanol unit of a coking company in Tengzhou, Shandong is currently in the on-site installation and commissioning stage. The 100kt/a coke oven gas to methanol unit of a coking company in Zaozhuang, Shandong is currently in the engineering design stage.