Online monitoring method for operation state of high-pressure polyethylene reaction kettle

Abstract

The invention discloses an online monitoring method for the running state of a high-pressure polyethylene reaction kettle, and belongs to the field of olefin polymerization. Based on a theoretical model, the friction coefficient in the stirring process is obtained through an input power signal in the operation process of a stirring shaft in the high-pressure polyethylene stirring kettle, and then the friction coefficient is compared with a theoretical value of the friction coefficient, and the operation state of the reaction kettle is judged. By adopting the method provided by the invention, the operation state of the high-pressure polyethylene reaction kettle can be conveniently monitored on line, and the calculation models involved in the invention are all theoretical models, so that the universality of the method provided by the invention is guaranteed.

Description

Technical Field

[0001] The present invention relates to the field of olefin polymerization, and particularly to an online monitoring method for the operating state of a high-pressure polyethylene reactor. Background Art

[0002] Polyethylene is an indispensable synthetic resin in production and life. There are two methods for preparing polyethylene: free radical polymerization and coordination polymerization. Among them, there are two processes for preparing polyethylene by free radical polymerization: the tubular method and the autoclave method. Among them, the polyethylene product obtained by the autoclave method has the advantages of wide molecular weight distribution, good melt fluidity, and easy infiltration and spreading on the surface of the substrate material, and has a broad market space. However, the autoclave ethylene polymerization process often needs to be carried out under high temperature (150-230 °C) and high pressure (150-200 MPa) conditions, which puts extremely high requirements on the dispersion and mixing of the reaction medium.

[0003] Due to the high temperature and high pressure conditions, it is possible to induce the decomposition of some monomers (such as ethylene decomposition to form hydrogen, carbon, and methane, etc.). Once local hot spots are formed due to uneven mixing during the reaction process, it is extremely easy to cause the reaction to get out of control, resulting in serious losses. The dispersion and mixing of the reaction medium in the reactor are realized by the high-speed rotation of the stirring paddle. Once the rotation of the stirring paddle is abnormal (such as local deformation, bending, or damage), it will lead to uneven mixing of the reaction medium, resulting in local high-concentration areas, and correspondingly bringing local hot spots, leading to decomposition. Therefore, real-time and accurate monitoring of the operating state of the high-pressure polyethylene reactor (with emphasis on the operating state of the stirring shaft) is of great significance for ensuring the stable operation of the reactor.

[0004] Chinese invention patent CN111913447A discloses an online data-driven non-linear chemical process monitoring method, aiming to establish a non-linear model for the chemical process using kernel learning techniques, and then optimize and search for the optimal projection transformation vector through online data-driven, so as to implement online process monitoring using the corresponding characteristic components. Chinese invention patent CN111913415A discloses a method for monitoring the operating state of a continuous stirred reactor based on time series data analysis, aiming to solve the problem of monitoring the operating state of the continuous stirred reactor by monitoring the abnormal changes in the time series of real-time sampling data. It is not difficult to find that most of the existing public methods choose to perform statistical regression on the reaction process signals and obtain a "black box" model to monitor the operating state of the reaction system. The advantage of this type of method is high accuracy, but the disadvantage is its poor universality. Summary of the Invention

[0005] The object of the present invention is to overcome the deficiencies of the prior art and provide an online monitoring method for the operating state of a high-pressure polyethylene reactor. This method is simple in form and easy to operate. By analyzing the power signal of the stirring shaft during the operation of the high-pressure polyethylene reactor in real time, the friction state of the stirring shaft is analyzed, and the operating state of the high-pressure polyethylene reactor is determined by judging the deviation between the actual friction state and the ideal state of the stirring shaft, so as to achieve timely operation and ensure the operating stability of the reactor.

[0006] The present invention is achieved by the following technical means.

[0007] An online monitoring method for the operating state of a high-pressure polyethylene reactor, which comprises the following steps:

[0008] (1) Use a power sensor to obtain the stirring electric power P1 during the operation of the high-pressure polyethylene reactor, calculate the stirring power P2 using the stirring power theoretical formula, and calculate the power difference ΔP between the two;

[0009] (2) Calculate the measured friction coefficient u1 of the stirring shaft based on the power difference ΔP, calculate the theoretical friction coefficient u2 based on the friction coefficient theoretical formula, and calculate the relative deviation RSDu between the measured friction coefficient and the theoretical friction coefficient;

[0010] (3) Compare the magnitude of RSDu with the discrimination threshold RSDt. If it is greater than the threshold, it indicates that the operating state of the stirring shaft of the reactor is poor and needs adjustment, otherwise it is in good operation.

[0011] According to a preferred embodiment of the present invention, the theoretical formula for the stirring power in step (1) is:

[0012]

[0013] where ρ f is the density of the reaction medium, kg·m –3 ; N is the rotational speed of the stirrer, r·s –1 ; d is the blade diameter, m; P 2 is the stirring power, W; Re is the stirring Reynolds number of the reaction medium in the reactor, and A, B, and C are constants.

[0014] According to a preferred embodiment of the present invention, the calculation formula for the measured friction coefficient in step (2) is:

[0015]

[0016] where m s is the mass of the stirring paddle, kg; V s is the volume of the stirring paddle, m 3 ; μ s is the viscosity of the reaction medium, Pa·s –1 ; dm is the nominal diameter of the bearing, in m; D, E, F, G, and H are constants.

[0017] According to a preferred embodiment of the present invention, the calculation formula for the theoretical friction coefficient in step (2) is:

[0018]

[0019] where υ is the kinematic viscosity of the reaction medium in the reactor, in units of m 2 ·s –1 , and I, J, and K are constants.

[0020] The undetermined constants in the above calculation formula can be obtained by regression from the cold model device similar to the industrial reactor structure, or the computational fluid dynamics simulation data or actual operation data of the industrial reactor.

[0021] The reaction temperature in the above polymerization reaction process is 10 - 300 °C, and the reaction pressure is 90 - 300 MPa.

[0022] Compared with the prior art, the method of the present invention is mainly based on a theoretical model. First, the theoretical stirring power is calculated using the theoretical formula for stirring power, and the difference between it and the actual stirring power of the high-pressure polyethylene reactor obtained by the power sensor is calculated to obtain the power difference. Based on the power difference, the measured friction coefficient u1 of the stirring shaft is calculated, and the theoretical friction coefficient is calculated based on the friction coefficient theoretical formula, so as to obtain the relative deviation RSDu between the two friction coefficients. According to the size of the obtained relative deviation RSD and the set threshold, the present invention can conveniently monitor the operating state of the high-pressure polyethylene reactor online. The whole method is fast and accurate, and the calculation models involved in the present invention are all theoretical models, which ensures the universality of the method proposed by the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a comparison diagram of the theoretical value and the measured value of the friction coefficient of the stirring shaft of the reactor.

[0024] Figure 2 is a graph showing the change of the operating state parameter RSDu of the stirring shaft of the reactor over time. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The present invention will be described in detail below with reference to the embodiments and the drawings. The embodiments and the drawings are only used for exemplary description of the present invention, and cannot constitute any limitation to the protection scope of the present invention. All reasonable transformations and combinations within the scope of the inventive concept of the present invention fall within the protection scope of the present invention.

[0026] An online monitoring method for the operating state of a high-pressure polyethylene reactor with universality in an embodiment of the present invention, more specifically, an online monitoring method for the operating state of the stirring shaft of a high-pressure polyethylene reactor. This method is mainly based on a theoretical model. By analyzing the power signal of the stirring shaft in real time during the operation of the high-pressure polyethylene reactor, the friction state of the stirring shaft is analyzed. By determining the deviation between the actual friction state and the ideal state of the stirring shaft, the operating state of the high-pressure polyethylene reactor is determined, so as to achieve timely operation and ensure the operating stability of the reactor.

[0027] This online detection method is applied to high-pressure polyethylene reaction. The copolymerization monomers in the polymerization reaction process can be selected from one or more of polar monomers such as vinyl acetate, butyl acrylate, etc. The reaction temperature in the above polymerization reaction process is 10-300 °C, and the reaction pressure is 90-300 MPa. The specific online detection method includes the following steps:

[0028] 1) Use a power sensor to obtain the stirring electric power P1 during the operation of the high-pressure polyethylene reactor, calculate the stirring power P2 using the stirring power theoretical formula, and calculate the power difference ΔP between the two;

[0029] The theoretical formula for stirring power is:

[0030]

[0031] Among them, ρ f is the density of the reaction medium, kg·m –3 ; N is the rotational speed of the stirrer, r·s –1 ; d is the blade diameter, m; P 2 is the stirring power, W; Re is the stirring Reynolds number of the reaction medium in the reactor, and A, B, and C are constants.

[0032] 2) Calculate the measured friction coefficient u1 of the stirring shaft based on the power difference ΔP, calculate the theoretical friction coefficient u2 based on the friction coefficient theoretical formula, and calculate the relative deviation RSDu between the measured friction coefficient and the theoretical friction coefficient;

[0033] The calculation formula for the measured friction coefficient is:

[0034]

[0035] Among them, m s is the mass of the stirring paddle, kg; V s is the volume of the stirring paddle, m 3 ; μ s is the viscosity of the reaction medium, Pa·s –1 ; d m is the nominal diameter of the bearing, m; D, E, F, G, and H are constants.

[0036] According to a preferred embodiment of the present invention, the calculation formula for the theoretical friction coefficient in step 2) is as follows:

[0037]

[0038] where υ is the kinematic viscosity of the reaction medium in the reaction kettle, with the unit of m 2 ·s –1 , and I, J, and K are constants.

[0039] 3) Compare the magnitudes of RSDu and the discrimination threshold RSDt. If it is greater than the threshold, it indicates that the operating state of the stirring shaft of the reaction kettle is not good and needs to be adjusted; otherwise, it is operating well. Among them, the relative deviation discrimination threshold RSDt is selected according to the specific reaction kettle or reaction type, generally being 0.1 - 0.2.

[0040] Among them, the method of the present invention can operate in real-time online or collect signals and monitor at intervals set in advance. When it is determined by using the method of the present invention that the operating state of the stirring shaft of the current reaction kettle is not good, it is necessary to stop the machine in time and perform maintenance on the device.

[0041] The calculation models involved in the method of the present invention are all theoretical models, which ensure the universality of the method proposed by the present invention. The undetermined constants in the above calculation formula can be obtained by regression from the calculation fluid dynamics simulation data or actual operation data of a cold model device similar to the industrial reaction kettle structure or an industrial reaction kettle. The following describes the method of the present invention in combination with specific embodiments:

[0042] Embodiment

[0043] In this embodiment, the diameter of the reaction kettle is 210 mm, the diameter of the stirring paddle is 190 mm, the diameter of the stirring shaft is 60 mm, and the height of the reaction kettle is 2050 mm. The experimental medium is an aqueous solution of sodium carboxymethyl cellulose (CMC). Before the experiment starts, start the centrifugal pump and adjust the opening degrees of each feed valve so that the volume flow rates of each feed in the stirring kettle all reach the set values. Gradually close the bottom outlet valve, and the liquid level in the stirring kettle slowly rises until it is completely filled with liquid. Then, turn on the frequency converter, increase the motor frequency to gradually increase the stirring speed N to 1000 rpm, stir for about 3 min after stabilization, and then lower the motor frequency until the stirring stops. Repeat this step 2 - 3 times until there are no obvious bubbles in the stirring kettle. Continue to adjust the stirring speed to the experimental value. After the stirring system is stable, start recording the readings of the torque meter, and stop recording after the readings are stable for a period of time. Lower the motor frequency until the stirring stops. Finally, turn off the centrifugal pump and the feed valve.

[0044] During the experiment, multi-sensors such as power sensors and mechanical sensors were used to comprehensively record state parameters such as input power, stirring power, and the force on the stirring shaft during the operation of the reactor. The undetermined coefficients (A: 2.48, B: 0.0386, C: 0, D: 5.8×10 -3 , E: 1.86, F: 0.970, G: -0.0301, H: 0, I: 0.954, J: 0.0302, K: 0) of the calculation formula for the stirring power and the friction coefficient were obtained by regression.

[0045] On this basis, the coefficients obtained above were substituted into the specific formula of the method of the present invention to evaluate the accuracy of the method of the present invention. One is to compare the difference between the predicted value and the measured value of the theoretical friction coefficient. As Figure 1 shown, the data in the figure shows the accuracy of the calculation formula for the theoretical friction coefficient proposed by the present invention. The other is to use the calculation formula with the undetermined parameters obtained to calculate the graph of the running state parameter RSDu of the stirring shaft changing with time during a polyethylene reaction process under a variable working condition. During the polyethylene polymerization process under this variable working condition, the stirring shaft was tightened in the first 120 minutes to make the stirring shaft rotate smoothly and ensure the smooth operation of the reaction. After 120 minutes of reaction, the position of the stirring shaft was finely adjusted to make the stirring shaft loose and eccentrically rotate to verify the overall accuracy of the method of the present invention. As Figure 2 shown, the data in the figure can be clearly divided into two stages. Among them, in the first stage (the first 120 min), the value of RSDu is stable and small, indicating that the reactor is operating well at this time and matches the actual operating conditions. In the latter stage, the value of RSDu shows an obvious step increase, exceeding the determination threshold of stable operation proposed by the present invention, indicating that the reactor is operating poorly at this time and needs to be shut down for maintenance, which matches the actual operating conditions, indicating that the method of the present invention can be applied to the on-line detection of the actual operating state of the reactor.

[0046] It should be noted that the above-described embodiments are only used to explain the present invention and do not constitute any limitation to the present invention. The description of the present invention by referring to typical embodiments should be interpreted as the words used therein being descriptive and explanatory words rather than limiting words. Modifications can be made to the present invention within the scope of the claims of the present invention as stipulated, and the present invention can be revised without departing from the spirit and scope of the present invention. Although the present invention described therein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed therein. On the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. An online monitoring method for the operating status of a high-pressure polyethylene reactor, characterized in that: The method comprises the following steps: (1) Using a power sensor to obtain the stirring electric power P1 during the operation of the high-pressure polyethylene reactor, using the stirring power theoretical formula to calculate the stirring power P2, and calculating the power difference ΔP between the two; (2) calculating the measured friction coefficient u1 of the stirring shaft based on the power difference ΔP, calculating the theoretical friction coefficient u2 based on the friction coefficient theoretical formula, and calculating the relative deviation RSDu between the measured friction coefficient and the theoretical friction coefficient; (3) Compare RSDu with the discrimination threshold RSDt. If it is greater than the threshold, it indicates that the stirring shaft of the reactor is not operating well and needs to be adjusted. Otherwise, it is operating well.

2. The online monitoring method for the operating status of a high-pressure polyethylene reactor according to claim 1, characterized in that: The theoretical formula of the stirring power in step (1) is: Among them, ρ f is the density of the reaction medium, kg·m –3 ; N is the stirrer speed, r·s –1 ; d is the blade diameter, m; P2 is the stirring power, W; Re is the stirring Reynolds number of the reaction medium in the reactor, and A, B, and C are constants.

3. The online monitoring method for the operating status of a high-pressure polyethylene reactor according to claim 1, characterized in that: The calculation formula of the measured friction coefficient in step (2) is: Among them, m s is the mass of the stirring paddle, kg; V s is the volume of the stirring paddle, m 3 ;μ s is the viscosity of the reaction medium, Pa·s –1 ;d m is the nominal diameter of the bearing, in m; D, E, F, G, and H are constants.

4. The online monitoring method for the operating status of a high-pressure polyethylene reactor according to claim 1, characterized in that: The calculation formula of the theoretical friction coefficient in step (2) is: Wherein, υ is the kinematic viscosity of the reaction medium in the reactor, unit: m 2 ·s –1 , I, J, and K are constants.

5. The online monitoring method for the operating status of a high-pressure polyethylene reactor according to claim 1, characterized in that: The relative deviation threshold RSD in step (3) is 0.1-0.

2.

6. The method for online monitoring of the operating status of a high-pressure polyethylene reactor according to claim 2, characterized in that: The constants A, B, and C are obtained by regression of computational fluid dynamics simulation data of a cold model device with the same structure as the high-pressure polyethylene reactor, or by regression of actual operation data of the high-pressure polyethylene reactor.

7. The method for online monitoring of the operating status of a high-pressure polyethylene reactor according to claim 3, characterized in that: The constants D, E, F, G, and H are obtained by regression of computational fluid dynamics simulation data of a cold model device with the same structure as the high-pressure polyethylene reactor, or by regression of actual operation data of the high-pressure polyethylene reactor.

8. The method for online monitoring of the operating status of a high-pressure polyethylene reactor according to claim 4, characterized in that: The constants I, J, and K are obtained by regression of computational fluid dynamics simulation data of a cold model device with the same structure as the high-pressure polyethylene reactor, or by regression of actual operation data of the high-pressure polyethylene reactor.

9. The method for online monitoring of the operating status of a high-pressure polyethylene reactor according to any one of claims 1 to 4, characterized in that: The polymerization reaction process is ethylene homopolymerization or ethylene copolymerization, wherein the comonomer of the ethylene copolymerization is selected from one or more of vinyl acetate and butyl acrylate.

10. The method for online monitoring of the operating status of a high-pressure polyethylene reactor according to any one of claims 1 to 4, characterized in that: The reaction temperature of the polymerization process is 10-300° C., and the reaction pressure is 90-300 MPa.

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