Preface: In the process of producing polypropylene by propriety method, the mixing reactor pressure is an important control parameter, which must be guaranteed to be within the range allowed by the process, otherwise, if the pressure is too high, bursting will occur, making the reaction process uncontrollable, and even explosions will occur. And kettle pressure is affected by temperature fluctuation, to maintain the kettle pressure at a constant value, must be realized by controlling the kettle temperature. Therefore, the dosage system chooses the pressure-temperature cascade adjustment system to solve this problem. At the same time, due to the polymerization reaction needs to absorb heat, and the reaction process and release a lot of heat, in order to maintain the kettle pressure must be removed from the reaction heat, there is a hot and cold water valves open alternately, for this reason, the metering and feeding system adopts a combination of remote control and split-range control control method. In addition, raw material propylene input determines the amount of catalyst, activator, etc., if the propylene input can not be accurately measured, will result in the catalyst, activator, hydrogen, etc., the amount of input is not allowed, which will cause the reaction quality deterioration, product quality is unstable, the consumption of higher, the production of accidents, therefore, the weighing and dosage system of propylene feeding metering control.

First, the chemical reaction process overview

Propylene is pumped into the mixed reactor by propylene pump after metering, and then catalyst, activator, hydrogen and so on are added respectively. After the feeding is completed, hot water is passed into the jacket of the mixed reactor for warming up, and when the temperature of the reactor reaches 70℃ and the pressure of the reactor reaches 3.5MPa, the polymerization reaction in the reactor releases heat, which makes the pressure of the reactor continue to rise. In order to maintain the kettle pressure constant, start to mix the reactor jacket and even the kettle water coil into the cold water, remove the reaction heat, stabilize the kettle pressure, until the end of the reaction.

Second, the determination of the control program

1、Automatic batching system for propylene feeding and measuring

Accurate measurement of the amount of propylene into the mixed reactor is the premise of polymerization reaction control. The traditional practice is the raw material propylene pump through the propylene pump into the propylene weighing kettle, through the plate level meter on the feeding measurement, through the booster into the mixing kettle, is an artificial control method. Since the recycled propylene contains certain polypropylene powder, it is very easy to cause the weighing reactor to contain powder or the level meter to be clogged. In addition, the calibration error of the scale of the liquid level meter of the weighing reactor is also very large, which makes it difficult to measure the feeding and must be improved. In fact, the weighing and feeding system is a typical batch loading system. The dosing machine completes this system by adopting the M200-DM2100 mass flow meter of SCHLUMBERGER Company of the United States, the S7-200 programmable controller of SIEMENS Company of Germany, and the ZSRX pneumatic three-way ball valve of SANZHOU AUTOMATION INSTRUMENTATION COMPANY, which realizes automatic control of the mixing kettle feeding quantity of the 12 mixing reactors. The system realized the automatic control of 12 mixing reactors for propylene feeding quantity. Due to the detection element of the mass flow meter is a straight tube, no resistance inside, and the vibration principle, it can completely solve the problem of propylene containing polypropylene powder.

The specific control process is as follows:

Normally, the propylene is circulated directly back to the propylene intermediate tank through the pneumatic three-way ball valve. When the operator through the feeding control button to send a feeding signal, the programmable controller to confirm the propylene thrown to which a mixing kettle and send a signal to open the kettle feed valve and pneumatic three-way ball valve, the propylene through the pneumatic three-way ball valve to the mass flow meter to accurately detect the input into the mixing kettle, the quality of the flow meter at the same time send a flow pulse signal to the programmable controller to feed the amount of calculations to achieve the specified amount of propylene after After reaching the specified amount of propylene, the control signal is sent to make the pneumatic three-way ball valve act, cut off the feeding, and wait for the next kettle to feed.

2、Mixing kettle temperature and constant temperature weighing and dosing

In the process of polymerization reaction, the main control index is kettle temperature. However, due to the hysteresis of the measurement of the temperature measuring element, and due to the measurement casing inserted into the kettle, the polymerization reaction of propylene occurs on the outer surface of the casing, and sometimes the polypropylene is covered on the surface of the casing, which can easily cause the false phenomenon of the kettle temperature. Therefore, in the process of temperature rise - constant temperature control, the choice of kettle pressure as the main control parameter is more reasonable, easier to achieve macro-control effect. However, the fluctuation of kettle pressure caused by temperature change can not be overcome in time, therefore, the kettle temperature or jacket water temperature is usually used as a sub-regulation parameter. In addition, due to the polymerization reaction of the temperature and constant temperature process is a chemical reaction of heat absorption and exothermic process, so the need for hot water and cold water switching alternately, so that in the initial stage of the reaction into the hot water heating so that the polymerization reaction is carried out, when the temperature rises to 70 ℃ or so, when the pressure reaches 3.5MPa or so, a large number of polymerization reaction begins, release a large amount of heat of the reaction, then you need to pass into the cold water to take away heat of the reaction to prevent Over-temperature and over-pressure. Conventional control program is kettle pressure - jacket water temperature series hot and cold water weighing dosage.

According to the conventional serial step-by-step control program, there are the following problems: Firstly, when the reaction is intense, a large amount of reaction heat released through the jacket cold water can not be taken away, the operator often open the kettle coil valve into the cold water in order to quickly take away the reaction heat, secondly, the use of conventional control instrumentation can not be realized by the automatic control of the heating process, can only be used by the remote way of warming up, thirdly, when the kettle is more, the use of a kettle corresponds to a cold water tank, a hot water tank way wasteful. Thirdly, when the number of kettles is large, it is wasteful to adopt one kettle corresponding to one cold water tank and one hot water tank, and it should adopt many kettles to share cold and hot water tanks in order to save investment. For this reason, the feeder has made the following improvements to the mixing kettle heating and constant temperature weighing and dosage system:

A, take kettle temperature - kettle pressure cascade control jacket cold water valve, kettle coil cold water valve, manual remote control of hot water valve way, a better solution to the kettle internal reaction heat is not easy to take away the drawbacks.

B. 12 sets of mixing kettles share one hot water tank and one cold water tank. In order to make sure that the cold water and hot water in the jacket return to each other and do not interfere with each other, the dosage system is designed with the interlocking of jacket outlet cold water and hot water quick shut-off valve and jacket inlet water cooling and hot water regulating valve. When the jacket into the hot water, hot water regulating valve action and interlocking hot water fast shut-off valve action, hot water back to the hot water tank; when the jacket into the cold water, cold water regulating valve action and interlocking cold water fast shut-off valve action, to ensure that the cold water back to the cold water tank, set up in the control room fast shut-off valve valve position indicator, monitor the water valve action position.

Third, the problem to explore

Polymerization reactor temperature and constant temperature shared a hot water tank, a cold water tank, mixed reactor jacket and water regulating valve to the pipeline in front of the water rapid shut-off valve are stored in a certain amount of cold water or hot water, in the process of switching between hot and cold water, due to the action of the water regulating valve, the corresponding water rapid shut-off valve action at the same time, resulting in the pipeline and the jacket of the existing hot and cold water strings into the opposite water tank, resulting in heat loss. Loss of heat. For this reason, should be in the water regulator valve action delayed for a certain period of time and then open the corresponding outlet quick shut-off valve to ensure that the pipe storage of hot and cold water is still able to return to the original place to avoid heat loss.

Conclusion

Polypropylene device mixed reactor automatic dosage system improved, the automatic control of the mixed reactor, whether it is feeding accuracy, or kettle temperature, kettle pressure adjustment quality, adjustment index, have met the requirements of the process of production, greatly reducing the labor intensity of the workers, reducing production consumption, truly automated to facilitate the production of easy to operate purpose.