JPS5920682B2 - Reaction temperature control method in high pressure polyethylene tubular reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for controlling reaction temperature in a high-pressure polyethylene production process using a tubular reactor in order to stabilize its operation.

Ethylene or ethylene and a monomer copolymerizable with ethylene at a pressure of 500 to 9/Cd or more, 100 to 400
It is known to obtain ethylene polymers or copolymers by polymerization in the presence of a polymerization initiator at temperatures of .degree. When homopolymerizing ethylene, approximately 800 Kcal/9
of reaction heat is generated, a - portion of this reaction heat is absorbed by the reaction mixture and a portion is removed to the outside by cooling the reactor.

Polymerization is carried out in a tubular reactor or a tank reactor, but
When performing high-pressure polymerization reactions using a tubular reactor, it is known that the produced polymer deposits and adheres to the inner tube wall of the reactor, making the control of the pressure or reaction temperature inside the reactor extremely unstable. ing.

In extreme cases, deposits of the produced polymer on the reactor walls result in reactor blockage phenomena and local temperature increases in the reactor, which lead to decomposition reactions. In order to forcibly peel off the deposited polymer produced, the outlet valve of the reactor is opened intermittently, and the gas flow inside the reactor is pulsed at a flow rate higher than the normal flow rate (flow pulse). law) is also known. Furthermore, as described in Japanese Patent Publication No. 49-29301, under certain conditions, it is possible to prevent the produced polymer from adhering to the pipe wall without substantially opening and closing the outlet valve of the reactor. However, fluctuations in the reaction temperature distribution cannot be completely prevented even in the method of Japanese Patent Publication No. 49-29301, as well as in the case of the fluid pulse method.

Unlike a tank reactor, in a tubular reactor without a stirrer, the reaction temperature distribution occurs in the lengthwise direction of the tube, so the decomposition rate of the polymerization initiator is different, and the reaction temperature distribution fluctuates with small disturbances. Generally, the reaction temperature in high-pressure ethylene polymerization is often controlled by adjusting the amount of polymerization initiator injected so as to maintain a specified set reaction temperature. Points to keep in mind when using this reaction temperature control method are the time delay from the detection of a change in reaction temperature until the amount of polymerization initiator injection is changed, and the change in the amount of polymerization initiator injection. There is a time delay required for the reaction temperature to return to the set reaction temperature. Therefore, fluctuations in the reaction temperature distribution in the tubular reactor are not desirable for this reaction temperature control method.

This is because in a tubular reactor, the set reaction temperature is usually the reaction peak temperature, and the amount of polymerization initiator injected changes as the reaction peak temperature fluctuates. Unlike in other cases, in a tubular reactor, the injection point of the polymerization initiator and the position showing the reaction peak temperature are distant from each other, so that the time delay in the above-mentioned temperature control method appears significantly.

As a result, an excessive amount of polymerization initiator is likely to be injected into the reactor, which results in rapid generation of polymerization heat, leading to undesirable coloring of the produced polymer and, in extreme cases, decomposition reaction of ethylene. Careful consideration is required to control the amount of polymerization initiator injected. The present invention provides a novel reaction temperature control method that stabilizes the polymerization reaction by suppressing fluctuations in the amount of polymerization initiator injection even when the reaction temperature distribution fluctuates in high-pressure ethylene polymerization using a tubular reactor. This is what we provide.

FIG. 1 is an example of a time change in reaction temperature distribution in a tubular reactor.

FIG. 1 shows the reaction temperature distribution when a polymerization initiator is injected at two points along the length of the tube in a high-pressure polymerization method of ethylene using a tubular reactor.

In FIG. 1, the reaction temperature distribution repeatedly changes over time between reaction temperature distribution pattern 1 and pattern 2.

Therefore, the detection of the first and second reaction peak temperatures is
In the figure, for example, in the method of detecting the temperature at only one point Pl, P4, and controlling the injection amount of the polymerization initiator so that the temperature becomes the set reaction temperature, Pl, P4
The reaction temperatures at are T, ~T1, and T4~T', respectively.
4, the injection amount of the polymerization initiator will also vary greatly as the temperature changes.

As a result, as mentioned above, there is a risk of coloring of the produced polymer and a decomposition reaction of ethylene. However, the present inventors have found that the fluctuation in the reaction temperature distribution is due to the position of the reaction peak temperature moving along the length of the reactor tube, and that the fluctuation in the value of the reaction peak temperature itself is small. It is.

In the example of Figure 1, the reaction peak temperatures T1 (position P, ) and T4 (position P4) in reaction temperature distribution pattern 1 become T'2 (position P2) and T'3 (position P3) in pattern 2. They are simply moving, and the fluctuations in their reaction peak temperatures are small.

That is, the present invention provides a method for controlling the reaction peak temperature by the injection amount of a polymerization initiator in a high-pressure polyethylene polymerization process using a tubular reactor. This is a method for controlling the reaction temperature in a high-pressure polyethylene tubular reactor, characterized in that the highest reaction temperature among the reaction temperatures detected at a temperature detection point is controlled by the amount of polymerization initiator injected. According to the method of the present invention, the highest reaction temperature among the reaction temperatures detected at two or more temperature detection points along the length of the pipe near the reaction peak temperature (in FIG. 1, at temperature detection positions P1 and P2) Compare the detected reaction temperatures T1 and T'1 or T2 and T'!, and detect the higher one (the same applies to positions P3 and P4), and use the indicated value to determine the polymerization initiator. Since the injection amount is controlled, fluctuations in the injection amount of the polymerization initiator are extremely small, making it possible to control the reaction temperature distribution well.

The method of the present invention can be easily realized by combining measuring instruments, control equipment, electronic circuits, etc. that are normally used in industry, and a specific example of the configuration will be explained with reference to FIG. 2.

The supplied ethylene enters the compressor 2 from the pipe 1, and 5001<
g/CrA or more, preferably 10001<g/Crll
It is compressed to 4000 kg/CF7f or less and enters the reactor 4 through the pipe 3.

Although not shown, the outer wall of the reactor 4 is provided with a jacket through which a heating medium for preheating the supplied ethylene to the reaction starting temperature and a refrigerant for removing the heat of polymerization are passed. After the supplied ethylene is preheated to the polymerization starting temperature, the polymerization initiator is injected from the polymerization initiator supply tank 7 through the polymerization initiator injection pump 8 through the pipe 9 to start polymerization.

As the polymerization reaction progresses, the reaction temperature rises while partially exchanging heat with the refrigerant in the outer wall jacket, reaching the first reaction peak temperature at which the polymerization heat and cooling by the refrigerant are in equilibrium.
After that, it is cooled by the refrigerant in the outer wall jacket, so
The reaction temperature gradually decreases. Furthermore, when the polymerization initiator is injected from the polymerization initiator supply tank 13 through the polymerization initiator input pump 14 through the pipe 15, the reaction temperature gradually decreases after forming the second reaction peak temperature. The reaction temperature is 100°C or higher and 400°C or lower, preferably 150°C or higher and 350°C or lower. Ethylene entering the reactor may contain monomers copolymerizable with ethylene and chain transfer agents. When using oxygen as a polymerization initiator,
It can also be added to the ethylene stream entering compressor 2 as shown in FIG. The polymer produced in the reactor 4 and unreacted ethylene are discharged from the reaction system through a reactor outlet valve 5 and a pipe 6.

As shown in FIG. 2, temperature detection points (for example, thermocouples) are installed along the length of the reactor 4 to measure the reaction temperature.

In FIG. 2, temperature detection points are shown only near the reaction peak temperature position. Among them, the measurement signals detected at three temperature detection points along the pipe length near the respective reaction peak temperature positions are sent to the respective maximum temperature selection circuits via compensation conductors A, b, c or K, l, m. 10 and 16, only the measurement signal corresponding to the maximum reaction temperature is taken out and input to the respective maximum temperature control devices 11 and 17. The maximum temperature control devices 11 and 17 determine the necessary control operation amount (for example, the injection amount of polymerization initiator) from the input maximum temperature and set temperature, and send a control signal to the polymerization initiator injection pump via wiring 12 and 18. 8 and 14, respectively.

The rotation speed of the pumps 8 and 14 is changed, and the piping 9,
The injection amount of the polymerization initiator entering the reaction system through 15 is controlled. To determine the required control amount from the input maximum temperature and set temperature, various types of normal control such as P control (proportional control), PI control (proportional-integral control), PID control (proportional-integral-derivative control), etc. control method is applied, but PI control method, P
ID control method is preferred. If the polymerization reaction conditions of the ethylene polymer change, the position of the reaction peak temperature to be controlled also changes, so it goes without saying that the temperature detection points connected to the maximum temperature selection circuits 10 and 16 can be changed as desired. In order to effectively implement the method of the present invention, the spacing between the temperature detection points installed in the longitudinal direction of the reactor tube is very important.

If the spacing is narrowed and the number of temperature detection points is increased, the control of the reaction temperature distribution will be extremely good, but since a large number of temperature detection points will be installed in the reactor, the strength of the reactor used at ultra-high pressure will be affected. Problems occur. If the number of temperature detection points is small, the effects of the present invention cannot be expected much.

The position of the reaction peak temperature depends on the diameter and pipe length of the reactor, the amount of ethylene supplied to the reactor, the type and injection position of the polymerization initiator, the set reaction temperature, and the temperature of the heat medium flowing into the jacket of the reactor. However, a person skilled in the art can easily specify this. The spacing between the temperature detection points is 3 m to 507n1, preferably 5 m to 30 m. The number of temperature detectors near the individual reaction peak temperature positions to be connected to the maximum temperature selection circuit varies depending on the polymerization conditions, but is 2 or more, preferably 3 or more and 12 or less. As the polymerization initiator of the present invention, one type or a mixture of two or more of known oxygen, various organic peroxides, azo compounds, etc. can be used.

The polymerization initiator can be injected into the tubular reactor at one or more appropriate positions along the length of the tube. As chain transfer agents, paraffinic hydrocarbons such as ethane and propane, propylene, and butene are already known.
One or a mixture of two or more of α-olefins such as 1 and aldehydes such as acetaldehyde can be used.

The feed ethylene need not be introduced entirely at one end of the tubular reactor.

A portion of it can be introduced unheated at one or more points along the length of the pipe. The method of the invention can also be operated using the flow pulse method used to prevent blockage by polymers. The present invention is applicable not only to homopolymerization of ethylene, but also to copolymerization with monomers copolymerizable with ethylene, such as ethylene monovinyl acetate, ethylene-methyl acrylate,
It can also be used for copolymerization of ethylene-styrene, ethylene-glycidyl methacrylate, etc.

According to the method of the present invention, the control of the reaction temperature distribution becomes extremely good, and automatic control of the reaction temperature becomes easy.

Furthermore, since fluctuations in the injection amount of the polymerization initiator are reduced, there is an advantage that coloring of the produced polymer or decomposition reaction of ethylene caused by an excessive amount of the polymerization initiator can be prevented. Next, the present invention will be explained with reference to Examples, but the present invention is not limited thereto.

Example: Oxygen as a polymerization initiator and ethylene containing 0.6 mol% propylene were compressed to 2000 kg/d using a compressor and introduced into the inlet of a tubular reactor having a length of 600 m.

The amount of oxygen was adjusted so that the first reaction peak temperature was 270°C, but the concentration was approximately 3
It was 0 mol Ppm. The first reaction peak temperature appeared between 250 m and 300 m from the reactor inlet. Then, tert-butyl peroxybenzoate was further injected as a polymerization initiator at a position 450 m from the reactor entrance, and the second reaction peak temperature was adjusted to 270 °C.
The injection amount was approximately 2.41<g/Hr. The second reaction peak temperature appeared between 500 m and 550 m from the reactor inlet. The interval between the temperature detection points near the first and second reaction peak temperature positions was 10 m, and six detection points from the 50 m range of the reaction peak temperature positions were connected to each maximum temperature selection circuit by wiring.

Control of the first maximum reaction temperature selected by the selection circuit is performed by adjusting the flow rate of injected oxygen, and control of the second maximum reaction temperature is performed by adjusting the tertiary butyl peroxybenzoate, respectively, using a PID control method. did.

During this period, the fluctuations in the reaction temperature distribution were extremely small, and the maximum reaction temperature was easy to control. Both the first and second maximum reaction temperatures were in the range of 270 ± 1°C, and the produced polymer did not discolor even after 500 hours of long-term operation. It did not occur.

The concentration of oxygen is 26-33 mol relative to the ethylene supplied, and the injection amount of n1 tert-butyl peroxybenzoate is 1.

It was 9 to 2.6 kg/Hr.

The conversion rate of ethylene to polyethylene was 16% and the density was 0.
9221/Cfltl Melt Flow Index 2f/
Polymer was produced in 10 minutes. On the other hand, if the first and second set reaction temperatures were controlled at one point, 280 m and 530 m from the reactor inlet, the oxygen concentration would be 20 to 38 mol relative to the supplied ethylene due to fluctuations in the reaction temperature distribution. The injection amounts of Ppm and tert-butyl peroxybenzoate varied greatly from 1.4 to 3.2 kg/Hr, respectively.

With this change in injection volume, the maximum reaction temperature also increased from 272 to 2
The temperature varied between 85°C and 273-295°C, and the resulting polymer was often colored.

[Brief explanation of the drawing]

FIG. 1 is an example of a time change in reaction temperature distribution in a tubular reactor.

Claims (1)

[Claims] 1 In a method in which the reaction peak temperature is controlled by the injection amount of polymerization initiator in a high-pressure polyethylene polymerization process using a tubular reactor, two or more temperature detection points along the length of the tube near the reaction peak temperature position are used. A method for controlling a reaction temperature in a high-pressure polyethylene tubular reactor, characterized in that the highest reaction temperature among detected reaction temperatures is controlled by the injection amount of a polymerization initiator.