Abstract: Alkali recovery is currently the most effective and mature means of treating black liquor. This paper introduces a practical and effective DCS design scheme for alkali recovery based on ProfiBus fieldbus. Keywords: Alkali recovery ProfiBus DCS design 0 Introduction The pollution of the paper industry is one of the main sources of industrial pollution in the world. More than 80% of the pollution load of the pulp and paper industry comes from pulp black liquor. If black liquor cannot be effectively treated, it will not only seriously pollute the environment, but also cause a lot of waste of resources. Alkali recovery is currently the most effective and mature means of treating black liquor. It can not only effectively recover the alkali in the black liquor, but also use the heat generated by the combustion of black liquor to produce steam, and the steam and alkali can be reused in other production processes of the paper industry. Therefore, the alkali recovery system is an indispensable part of solving the black liquor pollution and resource recycling of pulp enterprises, and has good environmental protection and economic benefits. It can be seen that alkali recovery has a very important position and significance in the pulp and paper industry. [1] The alkali recovery workshop is generally divided into three sections: evaporation, combustion and causticization. The new alkali recovery system has been listed as one of the important frontier issues of the paper industry. Fieldbus technology is a newly emerging control technology. Currently popular fieldbus technologies include Profibus and Foundation Fieldbus (FF). Profibus is an international, open, and manufacturer-independent bus standard. It consists of three mutually compatible parts: ① Profibus S2DP: Master and slave stations communicate using a round-robin polling method, used for communication in device-level control systems and distributed I/O; ② Profibus 2PA: Power and communication data are transmitted in parallel through the bus, allowing sensors and actuators to be connected via a single bus, mainly used for unit-level and field-level communication; ③ Profibus 2FMS: Used for workshop-level monitoring networks, it is a token-structured, real-time multi-master network. This paper briefly introduces the computer-distributed control scheme implemented by the author in the alkali recovery section of a paper mill in Henan Province using Profibus fieldbus. 1 Process Flow and Control Strategy The main equipment in the evaporation section is the evaporator, which is connected in series to form an evaporation station. The evaporation station controlled in this design is a typical five-body, five-effect evaporation system. Besides the evaporator controls, it includes auxiliary evaporation equipment such as plate falling film evaporators, a warm water tank, a black liquor tank, a condensate flash tank, and a level tank. The system control objectives are to maintain stable concentrations of the finished black liquor, stable total effective temperature difference, and stable condensate tank levels. It can generally be divided into a steam subsystem, a black liquor subsystem, and a condensate system. The main control objectives of this section are to ensure the concentration of the finished black liquor, maintain a stable total effective temperature difference, and stabilize the condensate system. This section includes 11 pressure measurement points (including steam pressure and vacuum at each effect), 17 level measurement points (including black liquor levels at each evaporator effect, condensate flash tank and level tank, and black liquor storage tank), 18 temperature measurement points (including steam and black liquor temperatures), and 8 flow measurement points (including steam, black liquor, and clean water). The combustion section utilizes flue gas and waste heat to further concentrate the concentrated black liquor from the evaporation section to a concentration of 65%–70% for combustion. Its main equipment is the alkali recovery jet boiler, whose operation determines the overall production efficiency of the section. Its function is twofold: firstly, to recover the effective calorific value of a large amount of organic matter (delignification) from the black liquor to generate steam for power generation or paper drying; and secondly, to recover inorganic alkali salts from the black liquor. This section also includes other supporting equipment such as air heaters, disc evaporators, and electrostatic precipitators. It can generally be divided into a black liquor subsystem, an air supply subsystem, a water and steam feed system, and a green liquor subsystem. During combustion, these three systems are interconnected. The first step is to determine the fuel/air ratio; an appropriate ratio yields the best boiler efficiency. In this system, both the black liquor and air entering the furnace are metered, so a suitable ratio can be obtained after a period of commissioning. The induced draft volume is adjusted according to the furnace negative pressure. When the other two parameters mentioned above change, the induced draft volume must also respond and be adjusted accordingly. In this system, both the forced and induced draft volumes use frequency converters, allowing for precise adjustment. This section has 17 pressure measurement points, 11 liquid level measurement points, 21 temperature measurement points, and 8 flow rate measurement points. The main equipment in the causticizing section is the lime slagging and slag removal machine and the causticizer. Auxiliary equipment includes a white liquor clarifier, an emulsion clarifier, a white mud washing agent device, a vacuum filter, and a pre-coated filter. This section mainly controls the concentration and flow rate of the green liquor, the ratio of lime added, and the time and temperature of the causticizing reaction. This section has 1 pressure measurement point, 10 liquid level measurement points, 12 temperature measurement points, and 3 flow rate measurement points. 2 System Hardware Design The entire control system framework consists of three parts: a distributed process control device, an operation management unit, and a communication system. Its structure is shown in Figure 1. (1) The core of the distributed process control device adopts Siemens S7-300/400 series modules, including CPU414-2DP, CPU315-2DP, power supplies PS407 and PS307, communication modules CP443-1 and CP343-1, ET200M, SM331, SM332, SM321, and SM322 modules. The SM31 module mainly completes the data acquisition of temperature, pressure, flow rate, liquid level, and other analog quantities in the system. The SM332 module mainly completes the analog drive signal output of the actuator. The SM321 module mainly completes the acquisition of digital input signals in the system (such as motor status feedback, shut-off valve position feedback, etc.); the SM322 module mainly completes the output of digital signals in the system (such as motor start/stop, electric valve opening/closing drive signals, etc.). The specific module configuration is shown in Table 1. Table 1 Siemens Module Configuration Table (2) The operation management unit engineer station consists of a DELL laptop, and the programming platform adopts Siemens Step7 V5.3. It mainly completes the programming work of data acquisition, loop control, interlock control, flow accumulation, etc. The operator station consists of 5 DELL industrial control computers, of which 2 are allocated to the evaporation section and the combustion section, and 1 to the caustic section. The 5 computers are redundant and can exchange data with the three CPUs. The production status of each section of the alkali recovery can be monitored in real time on each computer, avoiding the blind spot where data cannot be exchanged and displayed between sections. The configuration software adopts Siemens WinCC V6.0 SP1. The system mainly includes the following functional screens: Control main screen. The control main screen displays the detection values ​​of each control link and the operating status of the equipment in the order of the process. At the same time, a pop-up control panel is designed for each control loop. The user can click on the corresponding control link in the main screen to pop up the control panel of the control loop. The operator uses the panel to perform on-site control, including changing the set value and manual operation. Historical curve display. The system displays historical curves for the main control variables, including liquid level, flow rate, temperature, and pressure. Alarm screen. The system will display corresponding alarm information for any faults that occur in the system. Fault information includes hardware faults and software faults. Hardware faults include communication faults between the computer and the PLC, motor failure to start, signal module faults in the DCS system, etc. Software faults are user-defined faults, including detection values ​​exceeding limits and adjustment time being too long. Parameter centralized display screen. This screen displays important process data in a table format according to the user's needs, making it easy for the user to quickly understand the on-site operation. Parameter setting screen. This screen can only be set by engineers and requires a password to enter. It is mainly used to modify important parameters such as PID control parameters. Report printing screen: This screen provides the cumulative production flow rate by shift and can be printed at any time. (3) Communication system CPU414-2DP and CPU317 each have two network communication ports, one MPI/DP integrated communication port and one DP dedicated communication port. Each workstation's CPU is connected to its respective EM200M slave station via a ProfiBus fieldbus, using a dedicated DP communication port with a communication rate of 1.5Mbps. The process control level and the operation management level are also connected via a ProfiBus fieldbus, using an MPI/DP integrated communication port configured for ProfiBus-DP communication with a communication rate of 1.5Mbps. Each CPU and EM200M slave station is assigned a ProfiBus-DP station address; each must be assigned a unique address to avoid duplication. Each operation management station is equipped with a CP5611 communication card, configured for ProfiBus-DP communication with a communication rate of 1.5Mbps, and assigned a unique station address. The ProfiBus networks of each workstation and the operation management level belong to the same network but to different network segments; therefore, bus communication issues within a workstation will not affect the normal operation of other workstations. The communication rate is significantly higher than that of an MPI (Multi-Point Interface) network, facilitating real-time data display. Siemens dedicated DP cables are used for network communication, ensuring the system's communication rate and distance. In addition to the main hardware framework mentioned above, the system hardware also includes a regulated power supply and a UPS for each subsystem. The evaporation section and the combustion section both use a 6KVA regulated power supply and a 6KVA/30min (effective load/effective power supply time) UPS, while the caustic section uses a 3KVA regulated power supply and a 3KVA/30min UPS. For digital input signals such as motor status feedback, opto-isolators are used to isolate the field signals from the signals in the control cabinet, avoiding the impact of abnormal field interference voltage on the operation of the Siemens module. For ordinary digital output signals, Omron intermediate isolators are used for isolation; while for the control signals of electric valve switches, zero-voltage (Z) SSRs (solid-state relays) are used for isolation, eliminating signal oscillations caused by frequent actions. [3] 3 System Software Design In view of the characteristics of the alkali recovery process, sampling, filtering, PID control, scaling transformation, alarm, and accumulation subroutines are compiled for each section. Since the structured programming method has the advantages of clear program structure hierarchy, generalization of some programs, standardization, easy modification, and simplified program debugging, we use this method to compile the control program. The following describes the basic functional blocks of the control system program. The system's control program is constructed by calling these basic functional blocks from organizational blocks such as OB1, OB32, OB33, OB34, and OB35 (with different control cycles). 1) Sampling Subroutine: Reads back the analog input and stores it sequentially into the data block. The starting address of the analog input module, the number of channels, the block number of the data storage block, and the storage location of the data in the data block are variable and can be determined during the call. 2) Filtering Subroutine: The sampling section samples each channel 8 times consecutively and stores them sequentially in the data block. It averages the 8 sampled values. When calling, the data block number, the starting address of the data storage, and the interval between two adjacent sampled values ​​in the data block need to be specified. 3) PID Control Program: STEP7's program library contains general PID control subroutines. Each subroutine must be assigned a background data block during the call. When called, it passes parameter values ​​to the logic block, and at the end of the call, it is used to save the output data of the logic block. 4) Scale Transformation: After analog input is converted by A/D converter, the nominal range of its digitized result is 0-27648. However, when setting the setpoint, operators use specific numerical values ​​or percentages for clarity. PID sampling values ​​have two forms: Word type or Float type. The Word type range is 0-27648, but it is not intuitive. Therefore, to make the setpoint and measured value comparable, the scales of the measured and setpoint values ​​should be consistent, which requires scale transformation. The values ​​of the two scales are one-to-one. 5) Alarm Subroutine: This subroutine judges whether the analog quantity exceeds the upper or lower limit. If so, it sets the over-limit flag. To prevent the over-limit flag output from jittering, a judgment dead zone is set. 6) Flow Accumulation Subroutine: To assess the efficiency of each shift, important flow rates (such as the flow rate of black liquor entering the work section) are usually accumulated. The flow sampling signal is time-accumulated, and the data is recorded separately for the morning, afternoon, and evening shifts, and the daily, yesterday's, and monthly sums are accumulated. 7) Motor Control: Motor start-stop control is the most basic control application of the system. When the ready signal is available, the motor can be started and stopped directly; when the ready signal is not available, the motor will automatically stop. An alarm will sound if the motor current is too high. Each section assigns different parameter values ​​when calling subroutines based on its actual conditions. For the control of the steam drum liquid level in the combustion section, simple PID control cannot be used; three-impulse control is generally adopted. The liquid level serves as the main impulse sent to the regulator, while the steam and feedwater flow signals serve as auxiliary impulses. When the boiler system is in material balance, the liquid level is stable. Since the feedwater and steam flow signals are equal in magnitude, the signs of the adders and subtractors are opposite, thus canceling each other out. Therefore, the control signal of the regulating system remains unchanged. When one of the auxiliary impulses suddenly changes (e.g., an increase in steam consumption), it disrupts the material balance. The difference between these two auxiliary impulses with opposite signs acts on the regulating system to promptly change the control signal, increasing the feedwater flow and restoring the boiler system's material balance. Therefore, the three-impulse regulating system can overcome fluctuations in steam or feedwater flow before they affect the drum level, thus reducing large fluctuations in the level and minimizing "false level phenomena." Air-tight valves are used for the boiler feedwater valves. In this case, the integral regulator is set to positive (+) operation, the level signal is positive (+), the steam flow signal is negative (-), and the feedwater flow signal is positive (+). Each section also has certain interlocking subroutines set according to process requirements. The steam pressure regulation in the evaporation section is interlocked with the first-effect black liquor circulation pump. When there is no operating feedback signal from the first-effect black liquor circulation pump, the steam pressure regulation is automatically switched to manual mode and the steam valve is closed. The upper drum level in the combustion section is crucial to the entire system; it is interlocked with the feedwater pump, primary, secondary, and tertiary air fans, and induced draft fan. When the steam drum liquid level exceeds the high limit, the feed water pump will automatically stop and an alarm will be triggered; when the steam drum liquid level exceeds the low limit, the induced draft fan, tertiary fan, secondary fan and primary fan will automatically shut down. 4 Conclusion This system was designed by the Microcomputer Application Research Institute of Shaanxi University of Science and Technology for the alkali recovery workshop of a paper mill in Henan. Its features are: (1) The functions of DCS have been updated and expanded. The traditional DCS has been supplemented with fieldbus, which not only maintains the stability of DCS, but also introduces the flexibility of fieldbus; at the same time, the number of hardware can be reduced, and the workload of installation and maintenance is reduced accordingly. (2) The sensitivity and accuracy of the system have been improved. Signals related to this fieldbus no longer need to be repeatedly converted between digital and analog/analog-to-digital during the sending and receiving process as in the traditional DCS, but can be directly communicated between CPUs, thereby improving the quality of signal acquisition and the control quality of the system. (3) The load of DCS has been reduced and the control quality of the system has been improved. As some of the adjustment tasks are assigned to the CPU of the field intelligent instrument or actuator, the load of the CPU in the relevant automatic processing unit in the DCS is reduced, and the adjustment quality of the relevant equipment is improved. (4) As the fieldbus has self-diagnosis and simple fault handling capabilities, and sends the relevant diagnostic and maintenance information to the control room through digital communication, users can query the operating status and diagnostic and maintenance information of the bus equipment. This system has low cost, is easy to use, has high reliability, strong communication capability, strong analog quantity calculation capability and digital logic processing function. It realizes the network monitoring of the evaporation, combustion and causticization production process control. At present, the system is running stably, the algorithm in the system is effective and feasible, and it has achieved satisfactory control effect and created good economic value. References [1] Wang Mengxiao et al. Pulping and papermaking process measurement and control system and engineering [M]. Beijing: Chemical Industry Press, 2003 [2] Cui Jian et al. Siemens Industrial Network Communication Guide [M]. Beijing: Machinery Industry Press, 2005 [3] Shao Huihe. Advanced Industrial Process Control [M]. Shanghai: Shanghai Jiaotong University Press, 1997.