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Methanol, oxygen, and CO in the raw materials used for dimethyl ester production by Heihua Group all come from the urea system. Since the purity of CO is only 30%–40%, which does not meet the process requirements, pressure swing adsorption is used to increase the purity of CO. The DCS control system uses Zhejiang University Zhongkong’s custom configuration software JX300. This software features modular functionality, which facilitates maintenance and simplifies upgrades and optimizations; it meets the requirements for process control, as described briefly below. 1 Process flow: To obtain CO with a volume fraction of >98.5%, the crude desulfurized semi-water gas is compressed to 0.8 MPa and sent to the fine desulfurization stage; thereafter, it proceeds to the pressure swing adsorption process for CO purification. Pressure swing adsorption uses a three-tower two-equalization process, which serves to remove CO2, H2O, and trace amounts of S from the desulfurization feed gas. The adsorbent is regenerated through reverse pressure reduction and flushing, with the three adsorption towers alternating between adsorption and regeneration processes. The process flow for the pressure swing adsorption process is shown in Figure 1; the steps of the process for each adsorption tower are listed in Table 1, and the process flow diagram is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload1/081013942219371.jpg 2 Operation requirements: According to the process requirements, the following operations can be performed on the process flow diagram screen. (1) The operator can change the runtime for each step and the interval time between two steps (valve actuation time) online. (2) The operator can modify online the initial opening degrees and the coefficients for the valve-opening time of the 3 control valves, so that the valves can automatically adjust according to a certain functional relationship based on the actual process conditions in automatic mode. (3) According to the process requirements, 2 buttons are provided: ① Reset button, to return the entire system to its initial state ； ②Step button, to move the program to the next step. Set 2 switches: a run/pause switch and an auto/manual switch. Only in pause mode can other button switches be operated. In manual mode, the operator can adjust the solenoid valve switch and the opening degree of the control valve. In operational and automatic modes, the valve is automatically controlled in accordance with a programmed sequence. (4) Other conventional process operations can be carried out. At the same time, the flowchart also displays the current status of each adsorption tower, the remaining time for the current step, operation instructions, pause time, valve status, and a dynamic display of the pipelines, as well as other standard parameters such as temperature, pressure, liquid level, valve position, status displays, and operations. 3 Sequential Control: The DCS control system used for CO purification via pressure swing adsorption is the JX300. Based on the characteristics of the system’s configuration software and its corresponding functions, the implementation of all the requirements of this process is divided into several sub-processes; these sub-processes operate independently while also being interconnected, which facilitates the improvement and maintenance of the program. (1) Change in time parameters: Operators can modify the running time for each step online. When these time parameters change, it inevitably affects the overall operation cycle of the process. The control timing of the valve uses time as the switching condition and is a function of time, thereby affecting the entire control process. When time changes, the judgment program will invoke the relevant time-handling function to adjust the running time again. Given that the operator can make changes online, and considering that the system software runs once per cycle, the judgment program is placed at the beginning of the entire program; thus, the time period is adjusted before the entire valve control program is executed, in order to avoid errors resulting from inconsistent control. (2) Control of the solenoid valve: According to the requirements of the control process, the valve opens and closes automatically in a timed manner in automatic mode. Considering the time taken for each valve to operate as a condition for opening and closing it respectively, a time function for the operation of each valve is established, and valve control is achieved by adjusting these time values. Use the initial value set by time-based timing to achieve asynchronous startup ； The cessation of time, a pause in the process of occurrence ； Reset of time, resulting in a reset (stop) of the process. It greatly simplifies programming, with each valve control being independent of the others, facilitating maintenance. In manual mode, the manual operation switch is visible, allowing the operator to control it. (3) Control of the control valve: The purpose of installing a control valve is to prevent the packing inside the tower from being blown away due to high pressure differences and fast airflow during the pressurization and depressurization processes. In automatic mode, the control valve gradually opens over a specified time period according to a certain functional relationship based on the given initial opening degree and coefficients, thereby preventing the occurrence of the aforementioned problems. The purpose of setting the initial opening degree and coefficients is to enable operators, in light of the actual conditions at the production site and any future changes in process conditions, to adjust different parameters in order to meet the requirements of process control. In manual mode, the operator can make adjustments. To achieve program control, the control valve is implemented through a custom circuit; to prevent accidental operation by operators, the control mode can be locked to manual using specific commands. (4) Adsorption tower status display: As can be seen from Table 1, each adsorption tower has 8 operating states, and at any given time, the operating state of each tower is different. Taking advantage of the feature of using a custom 2-byte field to describe variable types, 1 global variable and 3 status variables for the adsorption towers are defined. As the program progresses to different steps, different values are assigned to global variables, and these global variables are directly passed (PASSED) to the status variables of the 3 adsorption towers. Depending on the different descriptive texts for the state variables of each tower at the time of definition, different states are displayed on the flowchart. The Zhejiang University Zhongkong JX300 features various editors in its custom configuration software; depending on the functions of each configuration editor and the convenience it offers for carrying out specific tasks, relevant program segments can be written using the FBD (Function Block Diagram) editor, LD (Ladder Diagram) editor, ST (Structured Text) language editor, and SCX language. They pass parameters to each other through internal system variables and custom data points. Given that each operation cycle of the program is executed from start to finish once, certain initialization statements should be locked to prevent incorrect operations. 4 Combination of operation commands and control programs: Once the control station receives an operation command, it operates according to a predetermined program to carry out the corresponding functions. The fulfillment of process requirements and control mechanisms must meet certain conditions or requirements before the next instruction can be executed or that instruction can be effective. The control program does not take into account the logical relationships between these instructions; it simply executes the corresponding procedure whenever an instruction is received. Therefore, in order to achieve process control as required, the switch buttons on the flowchart screen have certain interlock relationships. Under certain conditions, the relevant command switches and buttons are invisible or unintelligible for use. For example, in manual mode, the manual operation switch of the solenoid valve is visible, and the control valve can be operated ； In automatic mode, the manual operation switch for the solenoid valve is invisible, and the control valve appears gray and unusable. This approach can reduce the need to write control program statements, lower the computational load on the lower-level device, and enhance the independence of each functional program block, thereby facilitating changes to the functions. At the same time, the intuitive display of flowcharts enables operators to understand the conditions and requirements of the process at that time, helping to avoid operational errors and making the flowchart visuals more practical and attractive.