Development of the control system of the hottest t

2022-08-14
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Development of the control system of the tubular continuous cooking pulping production line

1 introduction

continuous cooking is a relatively advanced pulping technology at present. The tubular continuous cooking system can meet the continuous and large-scale production of the system, improve the uniformity of pulping, ensure product quality, realize the balance of steam and power consumption, and is gradually replacing the pulping production mode of intermittent cooking such as vertical pot and steam ball. Because the production of the whole system is a continuous process, with high temperature, high pressure and other working environment, it is difficult for people to access the production equipment, and it is difficult to realize it with the previous simple manual operation, so it must be equipped with advanced and reliable automatic control system

2 process and control requirements

the tubular continuous cooking system is mainly composed of wet material preparation system, continuous cooking system and washing water reuse system. According to different raw materials (wheat straw, reed, bamboo chips, etc.) and output, different specifications of main and auxiliary equipment can be matched.

2.1 control of wet material preparation system

raw material slices preliminarily purified by dry material preparation are sent to the silo through the conveyor belt. Through the discharging mechanism of the silo, the raw material slices come out of the silo and are sent to the wet purification device - hydraulic washer through the belt conveyor and drum washer. A belt scale and an electromagnetic iron removal device are equipped above the belt conveyor to make the wheat straw measure for the first time and remove the metal inclusion therein. The washed raw material slices are pumped into the inclined screw dehydrator, and the small impurities and moisture contained in the raw material slices are removed by the screening drum, meeting the requirements of the screw feeder of the cooking system. Its main control equipment includes: conveyor belt, horizontal conveyor, inclined lifting device, speed of straw pump, slag discharge of hydraulic washer, dehydration and slag discharge of screw dehydrator, water inflow of hydraulic washer, liquid level of hydraulic washer and circulating water tank, measurement of belt scale, and magnetic field strength of electromagnetic separator.

2.2 control of continuous cooking system

the purified raw material slices enter the continuous cooking system. Most of the raw material slices enter the pin drum meter through the feeding screw, and the remaining raw material slices are sent out from another outlet into the chute and back to the hydraulic washer. The drum metering device determines the processing capacity of the continuous cooking system and is also the basis for determining the temperature, pressure, time and other parameters of the cooking liquid. The raw material sheet measured by the distributor drum meter enters the preheating and pre steaming spiral, and the sheet is heated and softened by low-pressure steam, and part of the air in the sheet is removed to facilitate the immersion of chemicals. Its main control objects are: the speed of the pin drum gauge; Preheat the multi-point temperature and pressure of heated steam in the pre steaming screw; Speed of pre steaming screw.

the raw material sheet then enters the key equipment of the system - screw feeder. The main functions of the screw feeder are: first, push the material sheet into the T-shaped tube; The second is to compress the material sheet and form a tight material plug at the outlet, so as to seal the high-pressure steam in the cooking pipe and prevent the backflow of materials. The formation and propulsion of the plug is the key technology of the screw feeder. In order to prevent the steam and liquid medicine in the cooking pipe from flowing back through the feeder when the plug is not tight or the operation is abnormal, a pneumatic check valve is set between the feeder and the T-shaped pipe. Its main control objects are: automatic and manual control of the opening and closing of the check valve; Control of steam temperature and pressure; Pneumatic pressure control and pressure monitoring alarm of check valve.

the raw material plug leaves the plug tube and enters the T-shaped tube. It immediately expands and contacts with the liquid medicine and high-pressure steam added to the T-shaped tube. It begins to absorb the cooking liquid medicine and high-pressure steam and starts cooking. The flow rate of cooking liquid is closely related to the speed of drum metering device according to different products. The main control objects are: the flow of liquid medicine; The ratio of the speed of the pin drum meter to the liquid flow (commonly known as ratio pass); Multi point detection of steam pressure and temperature and control of gradient field. After absorbing the cooking liquid, the raw tablets enter the cooking tube. Under the action of saturated steam, the cooking liquid reacts with lignin to remove lignin. The number of cooking tubes shall be determined according to the design process requirements, and the spiral in each cooking tube has certain speed ratio requirements. After cooking, the slurry enters the intermediate pipe from the end of the cooking pipe. The intermediate pipe is connected with the vertical unloader, and the vertical unloader sends the slurry into the spray pot. The main control objects are: the rotating speed of the screw in the cooking pipe; The speed chain control of the spiral in each cooking tube; Temperature and pressure in each cooking pipe; Liquid level and temperature of vertical unloader; Black liquor flow; Liquid level indication of the spray pot; The flow rate and concentration out of the spray pot; Black liquor flow of diluted slurry.

the slurry ejected by the spray valve enters the spray pot through the spray pipe for storage, and then is transported to the next section. In addition, after entering the continuous steaming workshop, the automatic accumulation of the flow of saturated steam and clean water and the automatic adjustment of pressure, the automatic adjustment of compressed air, the flow measurement of alkali liquor, white liquor and black liquor and the automatic detection and control of the intermediate tank, the preparation and automatic batching of cooking liquid, the heat exchange control of black liquor, the heating control of alkali liquor, and the control of hot water tank are all the monitoring contents of the automatic control system. All drive motors in wet material preparation system and continuous cooking system shall have interlocking function, and their interlocking relationship can be input and unlocked as required. Multiple sets of interlocking schemes can be set in key parts according to process requirements, and any equipment can be controlled by manual mode instead of automatic control mode under abnormal conditions.

3 implementation of the control system

according to the actual requirements of continuous cooking, when selecting the scheme, we should focus on the reliability, safety, progressiveness, rationality, economy and other aspects of the control system. We choose the commonly used DCS control mode and use PROFIBUS-DP communication protocol to form a field communication network

The network structure is divided into production management level, automatic control level and field control level. The production management level uses the open industrial Ethernet to realize the information collection, system communication with the automatic control level, and send instructions to the automatic control level; The automatic control level (i.e. central control level) communicates with several distributed i/o field control stations with an open fieldbus, receives field sensing and detection signals, and controls the control object. Operating system Microsof Windows 2000 Chinese version monitoring software WinCC V5.1

configuration software SIMATIC STEP7 5.1 real time control PROFIBUS-DP 9.6kbps ~ 12mbps

the system is universal, the system can be flexibly configured, the control function is complete, the display operation set, the man-machine interface is friendly, the operation is safe and reliable, and it is helpful to improve the automation level of the production process, improve product quality It is of great significance to improve labor productivity. In terms of structure, DCS control system is to disperse the collection and control in multiple field control stations, and concentrate the operation and monitoring functions in one or more operation stations. It adopts standardized, modular and serialized products in terms of hardware, and has hot plug function, which is convenient for maintenance. All workstations on the system are connected through network interfaces, and the real-time control network of the system can realize the sharing of system resources. The application of communication bus makes the original complicated wiring work much simpler, and also saves a lot of wiring materials.

the control objects of the control system are various parameters in production. These parameters are finally sent to the nearest station by analog and digital signals through different transmitters, transmission devices, etc. Each station sends the relevant information to the programmable controller PLC of the automatic control master station in digital mode through the remote i/o station and through the field control bus PROFIBUS-DP mode with a baud rate of 9.6K ~ 12m. The main station programmable controller PLC will carry out logical or mathematical operations on the relevant information according to the process requirements and chain relationship, and then send the data through each station to each executing machine or display to adopt the lower mounted rotating spindle terminal.

in the pulping process, due to its many and complex control points, the PLC master station adopts a large controller to form an automatic central control system, which makes the system easy to expand and has various network communication capabilities; The distributed structure is adopted, and the remote intelligent interface provides digital and analog i/o units with various voltages and protection levels, which can be placed centrally or installed in the field remote i/o station dispersedly

frequency converter is used to drive the variable speed equipment in the pulping process; The transmitter converts the real-time data of liquid level, temperature, pressure, flow and concentration into 4 ~ 20mA standard signals and sends them to the remote i/o station to control the actuators such as valves, pumps and motors. For the convenience of operators, and considering the needs of process engineers and control engineers. Several touch screens or two upper monitoring computers are configured as ordinary operator stations or engineer stations to operate and control field equipment. Under normal circumstances, the operator can control the whole production line only in the control room to avoid the operator working in dangerous environments such as temperature and high steam pressure. At present, we mainly develop and realize a series of key technologies, such as the "ratio pass" function relationship between the output of continuous cooking and the flow of liquid medicine, the chain relationship between the load detection and control of the screw feeder drive and the reverse injection of the safety piston, the pressure balance control of the cooking pipe, the step-by-step control relationship of the rotating speed of the cooking pipe, the black liquor flow control of the vertical unloader and the automatic step discharge regulation control

4 quality assurance of control system

for a long time, the automation room of our institute has been engaged in the design, commissioning and manufacturing of automatic control system. Through continuous practice and summary, we have gained rich experience in software and hardware.

4.1 in terms of software

we have learned, mastered and applied new and advanced technical knowledge, making the design of the system more reasonable and scientific. The application of new technology has been tested and debugged repeatedly. Only after mastering its technology can it be used in production. Drawings need to be reviewed by special personnel before production. In the design, everyone's wisdom is gathered, and the applicability and reliability of the system are considered from many aspects. In this system, due to the production in the environment of high temperature and high steam pressure, some positions are inaccessible to people. In order to meet the needs of commissioning, production and maintenance, we adopt two three place operation modes of "remote/local". "Local" operation refers to the modified and high value-added utilization operation near the equipment, especially the vehicle engineering plastics. Considering that the remote operators cannot know the situation of the site during equipment maintenance, any blind operation will bring immeasurable losses, and even maintain the loading speed of the soil and hurt people's lives, so the "remote/local" choice can only be made in "local". In the preparation of the program, try to meet the habits of the operators according to the process requirements, so that the operators really feel the progressiveness of the system.

4.2 in terms of hardware

in order to ensure the quality of the complete set of equipment, not only the appearance of cabinets and desks and the absence of devices are strictly screened, but also experienced teachers are invited to connect the control cabinets and desks. Technicians shall check the wiring of cabinets and desks, and correct any problems found in time. Before leaving the factory, the power on test must also be carried out to ensure the quality of the factory equipment. Summary

5

material preparation and cooking is a continuous and systematic production process, and the level of automatic control is required to be high. The automatic control system is in improving product quality and quality

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