The following are some application cases of PLC control cabinets in chemical production processes:
Reactor control
Temperature control: In acrylic acid production, the S7-1200 series PLC control cabinet is used to control the reactor temperature. The temperature value is read through the PT100 temperature measuring element and the analog input module, and the temperature closed-loop control is realized using the PID instruction block. After fine tuning of the PID parameters, the integral (I) and differential (D) are first set to 0, and the proportional (P) value is gradually increased until the temperature oscillates slightly. The P value at this time is recorded and retreated by about 50%, and then the I value is slowly increased to eliminate the steady-state error. Finally, a small amount of D value is added and does not exceed one-quarter of I to avoid system instability, ensure temperature control accuracy, and ensure product quality.
Pressure control: In a high-pressure polymerization reactor, the PLC control cabinet is connected to a pressure sensor to monitor the pressure in the reactor in real time. When the pressure approaches the upper limit, the PLC controls the regulating valve to open slowly, releasing some gas to reduce the pressure; when the pressure is lower than the lower limit, the PLC controls the feed valve to open, replenishing the reaction raw materials to increase the pressure, maintaining the reaction within a stable pressure range, and preventing safety accidents such as explosions caused by excessive pressure or incomplete reactions caused by too low pressure, which affects product quality.
Powder conveying control
Conveying process monitoring: In the powder production line of chemical plants that produce fine talcum powder and ultra-fine calcium carbonate powder, the PLC control cabinet is used to control the separation, transportation, collection, sorting, packaging and shipment of mineral powder. The PLC receives the start and stop button signals from the operating console, the high and low pressure feedback signals of the pressure gauge, the operation feedback signals of each motor, the high and low speed feedback signals of the tachometer, and various fault input signals, etc., and realizes the automatic operation of the entire powder production line by controlling the sequential opening and closing of multiple motors, solenoid valves, and conveyor belts2.
Blockage and leakage prevention: In the powder conveying system of a chemical plant, a PLC intelligent control system is used to monitor the material level changes through material level sensors, detect pipeline pressure through pressure sensors, accurately measure the conveying volume through flow meters, and realize quantitative control through weight sensors. When the pressure sensor detects an abnormal increase in pressure, the PLC determines that a blockage may occur, immediately stops the operation of the relevant conveying motor, and starts the automatic cleaning program; when an abnormal drop in material level or an abnormal decrease in pipeline pressure is detected, the PLC determines that there may be a leak, immediately stops the conveying and sends an alarm signal to notify the operator to carry out maintenance.
Tank area management
Liquid level monitoring and management: In the solvent recovery project, S7-300 is used in conjunction with WinCC to build a tank area management system. Each storage tank is equipped with a radar level gauge to measure the liquid level and is connected to the PLC through the Modbus protocol. For irregularly shaped tanks, the piecewise linear interpolation method is used to accurately calculate the liquid volume in the tank according to the calibration data to achieve accurate monitoring and management of the tank area liquid level. When the liquid level reaches the upper or lower limit, the PLC control cabinet sends out an alarm signal and controls the corresponding feed or discharge valve to prevent overflow or lack of liquid.
Temperature and pressure monitoring: In the tank area where flammable and explosive solvents are stored, the PLC control cabinet not only monitors the liquid level, but also connects the temperature sensor and pressure sensor to monitor the temperature and pressure in the tank in real time. When the temperature or pressure exceeds the normal range, the PLC automatically starts the cooling or pressure relief device, and triggers the sound and light alarm at the same time to remind the operator to deal with it in time to ensure the safe operation of the tank area.
Distillation tower control
Parameter control: In the ethanol purification project, the S7-400 series PLC control cabinet is used in conjunction with the advanced process control software package provided by the system integrator to achieve optimized control of the distillation process. PLC is responsible for the underlying data collection, such as the collection of parameters such as temperature, pressure, flow, and control of pumps and valves. The advanced control algorithm runs on the host computer, calculates the optimal operating parameters, and transmits them to the PLC for execution through the OPC interface to achieve precise control of the distillation tower and improve the purity and production efficiency of the product.
Reflux ratio control: The PLC control cabinet adjusts the reflux ratio in real time according to the feed flow, composition, and quality requirements of the top and bottom products of the distillation tower. By controlling the speed of the reflux pump or the opening of the reflux valve, the reflux ratio is kept at the optimal state to ensure the stability of the distillation process and the consistency of product quality.
Centrifuge control
Speed control: In the pesticide production project, the S7-1500 series PLC control cabinet is used to control the large centrifuge. The inverter drives the motor, and the PLC communicates with the inverter through Profinet. The S curve acceleration is used at startup to avoid damage to the equipment caused by the impact at startup. At the same time, the encoder is used to feedback the speed of the centrifuge, and the PID closed-loop control is used to achieve precise adjustment of the speed to meet the requirements of different production processes for the speed of the centrifuge.
Determination of the end point of drying: During the drying process of the centrifuge, when the material is dried to a certain extent, the cake will become loose, and the speed will suddenly rise. The PLC control cabinet detects this feature by writing specific codes and uses it as the basis for judging the end point of the spin cycle. When an abnormal increase in speed is detected, the PLC controls the centrifuge to gradually slow down and stop running to prevent over-spinning or other abnormal situations.
