WebField series DCS control for pressure swing adsorption hydrogen production

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1 What are the components of an automatic control system? 1.1 What is the development of pressure swing adsorption control systems? The pressure swing adsorption process for hydrogen production requires precise control of important parameters such as flow rate and pressure in the pressure swing adsorption system, as well as periodic switching of solenoid valves. In addition, there are a large number of pressure swing adsorption valves that operate frequently, and sometimes it is necessary to modify the process flow of pressure swing adsorption as well as various time parameters. This requires the control system to have good flexibility, reliability, stability, and environmental adaptability. The pressure swing adsorption control system has gone through development stages such as pneumatic and electric logic controllers, microcontroller controllers, and programmable controllers. In recent years, as the production scale in the nitrogen fertilizer industry has grown to medium and large sizes, there has been a demand for pressure swing adsorption production units and their control systems to evolve into DCS-integrated systems. The application of Zhejiang Zhejiang University Zhongkong WebField series DCS control systems (JX-300, JX-300X, ECS-100, etc.) in the PSA field is a result of the rapid development of this technology. ? 1.2 Control System Configuration? Below, we take the 8-tower process of the PSA hydrogen production unit at a factory in Shanxi as an example to illustrate the specific configuration of the WebField series DCS. The system features 1 control station and 2 operation stations (one of which also serves as an engineer station); through the plant’s control network, real-time exchange of important process data from units such as ammonia synthesis and urea production is enabled. Its configuration points are shown in Table 1. Table 1 System configuration parameters: http://www.nmtech.com.cn/jishuwang/upload/0602161405353517.jpg? 2 Control requirements and optimization for hydrogen production via pressure swing adsorption 2.1 Control of adsorption quality and adsorption time? Since the size of the adsorption tower and the amount of adsorbent used are fixed, under constant conditions of raw gas composition and adsorption pressure, the total amount of impurity gases that can be adsorbed by the tower in each cycle is also fixed. Therefore, the adsorption cycle plays a key role in the purity of the product in pressure swing adsorption hydrogen production units. A shorter adsorption cycle reduces the adsorption pressure and capacity of the molecular sieve, while a longer adsorption cycle allows impurity gases to pass through the adsorption bed, both of which affect the purity of H2. Additionally, the relationship between the adsorption cycle and H2 purity is influenced by many other factors as well. The relationship between H2 purity and adsorption cycle in the molecular sieve adsorption bed is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/0602161406312677.jpg Figure 1: Relationship between H2 purity and adsorption cycle. Currently, most control systems adjust the pressure swing adsorption cycle based on the flow rate of the feed gas and the purity of the product gas. If the flow rate of the feed gas is very stable and its composition changes little, the adsorption time can be adjusted manually; however, when the flow rate of the feed gas or its composition changes significantly, it is extremely difficult to achieve optimal control by manually adjusting the adsorption time. In this project, the advantages and resources of the DCS system were fully utilized by employing expert predictive control, which imitates the control skills and experience of experienced operators and control experts to adaptively handle various future behaviors in the process system, diagnose potential problems, and accordingly adjust the adsorption cycle continuously. ? In pressure swing adsorption, an expert system is used to determine the adsorption time, enabling fully automatic operation without human intervention; this improves the purity of the product gas as well as the recovery rate of the product. ? 2.2 Final charging adjustment for pressure swing adsorption The hydrogen extraction process via pressure swing adsorption is a cyclic process of adsorption and desorption; aside from the need to maintain a stable adsorption pressure during the adsorption phase, the system is in a state of pressure reduction or increase at other times. System pressure increase regulation, also known as final charging regulation, is used to control the rate at which the pressure of the product gas in the adsorption tower increases. The control requirement is to raise the pressure in the adsorption tower gradually to the operating pressure for adsorption within a specified time frame. The theoretical boost regulation curve is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload/0602161407247917.jpg Figure 2: Boost regulation curve. During the control process, the pressure increase should not be too rapid; too fast an increase can lead to the pulverization of the adsorption bed, while too slow an increase results in insufficient pressurization of the adsorption tower due to limited pressurization time. Currently, two methods are commonly used for this control loop: slope control or follow-up pressure regulation. ? 2.2.1 Slope adjustment? The control algorithm for slope adjustment is simple and easy to operate, but the control performance is poor. ? 2.2.2 Follow-up adjustment? This control method makes full use of the flexible programming and easy debugging capabilities of the DCS system, employing PID control with a variable setpoint in the control loop. Through segmented control, the control curve is made to approximate the theoretical boost curve. The control adjustment effect is shown in Figure 3. http://www.nmtech.com.cn/jishuwang/upload/0602161409575891.jpg Figure 3 Control and adjustment effects 2.3 Handling of pressure swing adsorption failures? Pressure swing adsorption hydrogen production units operate at high pressures, have a large number of programmable valves, and these valves operate frequently; as a result, failures in such equipment occur often, and the potential risks associated with these failures are significant. ? 2.3.1 Handling of abnormal operating pressures in pressure swing adsorption? During the operation of a pressure swing adsorption unit, the actual pressure variation curve of the adsorption tower should be similar to the ideal curve, but it will not be exactly the same. If the actual pressure variation curve of the adsorption tower differs significantly from the ideal curve shape mentioned above, it indicates that there is a problem with the operation of the device, and it is necessary to diagnose and address the adsorption failure. Figure 4 shows the theoretical operating pressure curve for hydrogen extraction by PSA at an inlet gas pressure of 2.5 MPa. http://www.nmtech.com.cn/jishuwang/upload/0602161410552953.jpg Figure 4: Theoretical operating pressure curve for hydrogen production using PSA. In the WebField series DCS control systems, batch processing of theoretical data is carried out using the SCX language provided by the system; during control operations, a comparison is made in real time between the theoretical and actual pressures. When the difference between the theoretical pressure and the actual operating pressure exceeds a set value, the system automatically activates the fault handling routine, issuing alarms and alerts. ? 2.3.2 Handling of Programmable Valve Failures? Many programmable valves are used in the control of pressure swing adsorption; failures of these valves can not only directly affect the purity of the product but may also lead to serious safety incidents. During programming, a dedicated device testing program was developed. Through device monitoring, the status of the programmable valve is detected in real time, and an alarm is issued when a fault occurs. ? 2.3.3 Faulty tower shutdown? A failure in pressure swing adsorption can affect the normal and safe operation of the subsequent processing stage. Therefore, when a malfunction occurs in the adsorption tower, an emergency shutdown of that tower is required to remove it. It is required that the removal of the faulty tower be carried out promptly, and at the same time, the removal process should cause minimal fluctuations in the entire adsorption bed. 3 Operation of the WebField DCS control system in pressure swing adsorption hydrogen production units. The WebField series of DCS control systems have been successfully used in hydrogen production within the fertilizer industry on many occasions; they are highly regarded by users for their stability, ease of operation, and the simplicity with which complex controls can be implemented. Through the optimization of control programs, the processing capacity of the production unit has been increased by 1.2 to 1.5 times compared to that of units of the same scale. In most current design institutes for pressure swing adsorption hydrogen production, the WebField series of DCS control systems is gradually replacing PLC series and other types of DCS, and is thus being widely used.