The design capacity of this blender (Baota Petrochemical) is 40,000T/a polypropylene, which is based on propylene separated from LPG fractionation. Intermittent liquid-phase propriety polymerization method is adopted. The main equipment for eight 12-cubic-meter polymerization reactor dosing system and eight flash mixer and other auxiliary equipment to form a set of intermittent production equipment, every four hours per reactor dosing system can be fed out of the material once per reactor dosing system can produce polypropylene powder about 3T.

1 Introduction

Due to the intermittent polypropylene production process of chemical reaction intermittent process characteristics are quite complex, with serious nonlinear and time-varying characteristics. At present in many intermittent industrial production, still using manual or semi-automatic operation, especially in the reaction period, the process control is more difficult, which requires the control strategy to adapt to the different characteristics of each time period.

2 process characteristics

The main body of this dosing equipment is A, B, C, D, E, F, G, H8 polymerization reactor weighing dosage, using intermittent liquid-phase propriety method of polymerization process, liquid-phase propylene as the raw material, using CS-1-type high-efficiency catalyst (Ticl4), triethylaluminum as an activator, and hydrogen as the molecular weight of the polymer regulator. Liquid-phase propylene is metered into the polymerization reactor, and the activator, catalyst and molecular weight adjuster are added to the polymerization reactor dosage in a certain proportion and order. After each material is added, hot water is passed to the jacket of the polymerization feeding system to warm up and pressurize the material in the reaction tank, and the temperature and pressure in the reaction kettle rise at this time. When the temperature rises to 55 ℃ or more, the pressure is about 2.4MPa when the reaction kettle began to react, release heat. As the heat released from the reaction will aggravate the reaction, so the heating should be stopped in time, open the circulating cooling water so that the temperature or pressure inside the reaction kettle rises at a certain rate, when the temperature inside the reaction kettle rises to 73 ℃ or so, the reaction kettle pressure rises to 3.55 MPa or so when the constant temperature and pressure reaction process. With the extension of the reaction time, the liquid phase propylene gradually decreases and the concentration of polypropylene particles increases. Finally, the reaction kettle liquid-phase propylene basically disappeared, the reaction kettle is mainly polypropylene solid particles and unreacted gas-phase propylene, that is, to reach the so-called "dry pot" state, the kettle pressure decreases, and at this time the reaction is considered to be over. Among them, the temperature and pressure stage is about 30min, constant temperature and pressure stage is about 4.5h.

Control idea

Combined with the process characteristics, the weighing and feeding system process can be divided into three stages. Temperature and pressure stage, transition stage, constant temperature and pressure stage.

1. Temperature and pressure stage. The purpose is to control the opening of the hot water valve in the appropriate time to add water to the jacket, so that the temperature, pressure in accordance with the more desirable rate of rise, rise too slow to affect the yield, too fast may make the late reaction is too intense, difficult to control. So generally use the way of bit control. According to the actual characteristics, in this stage, the weighing and dosing is divided into two stages to control, the pressure is less than 2.0Mpa, the hot water valve is open 100%, so that the kettle dosage system is fully warmed up and pressurized, the pressure is in the 2-2.4. when the hot water valve is gradually shut down from 100% to 50%, and keep it fully closed at 2.4-2.85. This not only shortens the warming time but also makes the late reaction get some relief.

2. Transition stage

Transition stage is the whole control of the difficulty, DCS weighing and feeding system using PID control, its purpose is to react according to the strength of the control of the corresponding valve, so that the temperature and pressure stage, transition stage, constant temperature and pressure stage. So that the pressure is controlled at about 3.55Mpa (because the pressure and temperature is a linear relationship). In this process, if the control is not good, there may be overpressure, causing high pressure recovery, and even the safety valve action, or vice versa, may cause "stiff reactor". At the same time, we also have to take into account the weak reaction caused by raw materials, so in the design of fuzzy at the same time, we have to consider the selection of appropriate parameters, so that the pressure in the pressure before 3.5Mpa can rise smoothly, that is to say, how to overcome the fuzzy control of the steady state error, after my own explorations, when the pressure is in the [3.3,3.5], in the rate of the interval of the [-0.1,0.1] introduction of the trigonometric sinX function to correct the fuzzy EC term while controlling the pressure quickly at 3.5-3.6Mpa. In accordance with the above requirements modeled on the experience of manual control, an intelligent two-dimensional fuzzy controller is designed, where the input becomes the rate of change of the pressure in the reactor and the mixing kettle pressure, and the output variable is the opening degree of the chilled water valve. The task of the fuzzy control is to realize a fast and smooth transition by replacing human manual operation with the fuzzy control with double input and single output in the stage of 2.85-3.6MPa of the transition process. Through the process law, you can get the fuzzy IF-THEN rule IF the pressure is too low, the rate is zero, THEN the valve is not open IF the pressure is low

(1) Select the set of words describing the input and output variables

By analyzing the historical data, take the input variable reactor dosage system pressure PT of the basic domain: [2.8,3.2,3.4.3.6] MPa four parameters respectively indicate that the pressure in the reactor dosage system is too low, low, slightly low and normal. Take the basic domain of pressure change PEC: [0,0.56,0.11,0.17]MPa/min 4 parameters indicate that the rate of pressure rise is zero, small, medium and large respectively. Take the chilled water valve opening (control volume) Y of the basic domain: [0,33,66,100] each parameter corresponding to the valve state are fully closed, small open, open, fully open.

(2) Define the fuzzy subset of each fuzzy variable

According to the manual strategy, the affiliation function adopts the form of isosceles triangle. From the curve of the affiliation function, the assignment table of each fuzzy variable on the thesis domain can be derived. By using the fuzzy control function block in DCS through the above thesis domain, a control scheme can be constructed, which proves to be very effective, it not only solves the problem of pressure overshooting, but also solves the problem of response strength. And shorten the response time.

3. PID control

The automatic batching machine adopts PID control. In the cut constant temperature and pressure, this time the reaction may still be relatively strong, so pure PID control, in this stage is not ideal. So rate feed forward is introduced. This also makes it possible to cut into the constant temperature system at an earlier time to be guaranteed. This enables a faster approach to the set target value. At the same time can make a fast response. Achieve better control accuracy.

4. Control of the internal cooling system

Internal cold system is to protect the overpressure of the powerful means, the general pressure in the 3.45Mpa, mainly in the rapid rise in pressure, and the split control valve has been fully open when the opening of the internal cold valve. In view of its characteristics, in the design of weighing and batching system, when the pressure is greater than 3.45 and the opening degree of the split cooling valve is greater than 75 degrees cut automatically. Timely input regulation.

Operation results

With the above control method, from the beginning of heating to the reaction kettle pressure rises to 3.5±0.1MPa, the time used is about 25-30min, the maximum over-regulation amount is ±0.03MPa, while the manual control, from the beginning of heating to the reaction kettle pressure rises steadily to 3.55±0.1MPa, it generally takes 40-60min, in the rapidity is obviously not as good as the intelligent composite control program. The rapidity is obviously inferior to the intelligent composite control program.

In addition, when manually controlled, in the range of 2.85 ~ 3.3MPa, the control is often not timely, easy to cause overshooting, especially for inexperienced operators more difficult. It affects the smoothness of the subsequent reaction, and also has a greater impact on the quality of the product and a greater chance of recovery. Adopting this feeding machine, the operator only needs to operate the start-up heating button after the automatic feeding is finished, and the program will control the hot water valve, cold water valve, and internal cold valve throughout the whole process, without manual intervention, until the material is discharged. In the past, one person was too busy to operate one kettle dosing system, after adopting the automatic, one person can easily operate 4 kettle systems.