A method for regulating thermal stability for slurry process polyethylene loop reactor process

Abstract

The application discloses a kind of for slurry method polyethylene loop reactor process thermal stability regulation and control method.For the problem that slurry method loop reactor polyethylene production process, catalyst repeatedly injects slurry and produces hot spot in the same position, the application is by adjusting the injection frequency of catalyst in loop reactor, injection position, the circulation frequency of slurry, reduce the hot spot of catalyst in circulation loop, to improve the thermal stability in loop reactor.Combining simulation data, construct the quantitative prediction model between reactor condition parameters and reactor thermal stability, based on the model can predict the temperature change of reactor operation for a period of time under current conditions.The technology can realize the thermal stability regulation and control of slurry method polyethylene loop reactor, and has broad industrial prospect and popularization value.

Description

Technical Field

[0001] This invention belongs to the field of slurry polyethylene and relates to a method for controlling the thermal stability of a loop reactor process for slurry polyethylene. Background Technology

[0002] Polyethylene is currently the world's largest-produced and most widely used general-purpose plastic. To meet market demands for different types of polyethylene and to reduce costs, large-scale polyethylene chemical industries have adopted the loop reactor process, which is simple in structure and allows for rapid grade switching. The loop reactor is a closed-loop tubular reactor with a very high aspect ratio. The reactor wall is jacketed for rapid heat dissipation. The catalyst is added to the slurry intermittently, and the slurry circulates at high speed in the loop for polymerization, then is intermittently discharged. However, a common problem in actual industrial production is that repeated catalyst injection into the same location in the slurry leads to hot spots within the reactor, making temperature control difficult, reducing catalyst activity, increasing cooling load, and causing polyethylene agglomeration and adhesion to the reactor walls as the reactor's operating cycle increases. These problems seriously affect the thermal stability of the unit, shorten the operating cycle, and restrict the stable operation of the plant.

[0003] Studies have shown that the above phenomena originate from the complex operating conditions of the loop reactor process, characterized by continuous slurry flow, intermittent catalyst injection, and intermittent product discharge. Specifically, this manifests as follows: First, the catalyst enters the slurry system intermittently. When the ratio of the catalyst injection frequency to the circulation frequency is a rational number, the catalyst particles exhibit a periodic cumulative distribution on the loop, forming hot spots at fixed locations, leading to localized overheating, polymer agglomeration, and reduced catalyst activity. Second, due to intermittent discharge, a portion of the slurry is instantaneously discharged, causing a sudden drop in reactor temperature and resulting in significant pulses. This causes substantial temperature disturbances and affects the thermal stability of the reactor. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a method for controlling the thermal stability of polyethylene processes in slurry loop reactors. This invention controls the concentration distribution of the catalyst within the reactor by adjusting the catalyst feed frequency, location, circulation frequency, and discharge frequency, effectively reducing the generation of hot spots in the circulation pipeline, thereby improving the thermal stability of the reactor.

[0005] The present invention provides a method for controlling the thermal stability of a polyethylene loop reactor using a slurry catalyst, comprising the following steps: 1) Constructing the average reactor temperature T avg The relationship between time and the maximum temperature difference ΔT in the reactor. max The relationship that changes over time; 2) Based on the design parameters and simulation operation data of the polyethylene loop reactor process, the undetermined coefficients in all the relationships in step 1) are fitted to obtain the relationships with determined coefficients; 3) Based on the given target average peak temperature, target maximum temperature difference peak temperature and reactor operating cycle of the loop reactor, obtain the feasible solution set of operating parameters by solving the relationship in step 2). Run the polyethylene loop reactor process based on the operating parameters in the feasible solution to achieve thermal stability control.

[0006] Preferably, the average temperature T of the polyethylene loop reactor process is... avg The relationship that changes over time is:

[0007] in

[0008]

[0009]

[0010]

[0011] In the formula t Indicates the reactor running time. T 0 Indicates the initial temperature of the reactor. r This is the ratio of the catalyst injection frequency to the slurry circulation frequency in the reactor. p To measure the dimensionless number of catalyst injection sites, f out This refers to the slurry discharge frequency; C This indicates the steady-state reactor temperature after prolonged operation. α Indicates the rate at which the reactor temperature stabilizes; β A coefficient indicating the intensity of a temperature peak; c A coefficient representing the rate of decay of the peak temperature. c i 、a i 、b i 、d i Let be the coefficients to be determined, where i =0,1,2,3.

[0012] The maximum temperature difference ΔT in the polyethylene loop reactor process max The relationship that changes over time is:

[0013] in

[0014]

[0015]

[0016]

[0017] In the formula t Indicates the reactor running time. r This is the ratio of the catalyst injection frequency to the slurry circulation frequency in the reactor. p To measure the dimensionless number of catalyst injection sites, f out This refers to the slurry discharge frequency; l This indicates the maximum temperature difference caused by the hot spot; n This indicates the time required to reach the maximum temperature difference at the hot spot. f This indicates the background temperature difference within the reactor; m Indicates the rate at which the background temperature difference stabilizes; u i 、v i 、w i 、z i Let be the coefficients to be determined, where i =0,1,2,3.

[0018] To avoid hot spots, the actual catalyst injection frequency needs to be staggered from the slurry circulation frequency, leaving a sufficient safety margin. The ratio of the slurry circulation frequency to the catalyst injection frequency is expressed by the following formula:

[0019] In the formula r This is the ratio of the catalyst injection frequency to the slurry circulation frequency in the reactor; m This indicates the number of cycles the slurry circulates in the loop pipe. k Indicates in m Number of catalyst injections per cycle; d This represents the safety margin in the ratio of catalyst injection frequency to slurry circulation frequency. d The settings need to satisfy the following relationships:

[0020] In the formula Δ x To adjust the injection frequency, the offset distance of the catalyst's landing point in the slurry can be a preset value representing the minimum safe distance the catalyst injection point moves relative to the slurry each time. kIndicates in m Number of catalyst injections per cycle L This indicates the loop length of the loop reactor.

[0021] dimensionless number of injection site p We obtain it from the following formula:

[0022] In the formula x The distance between the catalyst injection point and the slurry feed point along the loop (the loop of the loop reactor is considered to be 0-) L In a one-dimensional straight pipeline, 0 refers to the slurry inlet, and the slurry flow direction is the positive direction. x (Increase).

[0023] Slurry discharge frequency f out We obtain it from the following formula:

[0024] In the formula or This represents the evacuation fraction of the settling legs at the discharge outlet of the loop reactor. i The time interval between consecutive opening of the settlement legs.

[0025] Obtaining Undetermined Coefficients: The undetermined coefficients in this model can be obtained in various ways, including but not limited to: constructing a loop reactor device, simulating the polymerization process under different operating conditions, collecting actual operating data of the polymerization process, fitting the nonlinear relationship to obtain undetermined coefficients matching the target device's operating conditions, substituting the undetermined coefficients into the relationship to obtain the model, or constructing a sample set based on detailed reaction kinetics and flow field simulations, and using nonlinear regression, global optimization, or mixed integer optimization methods to identify model parameters. Specifically, optimization strategies such as nonlinear least squares, genetic algorithm (GA), differential evolution algorithm (DE), and particle swarm optimization (PSO) can be used to fit the nonlinear relationships between variables, thereby obtaining a set of model parameters matching the target device's operating conditions. This process is not dependent on a specific experimental path, has good flexibility and generalization ability, and can update and optimize the model accuracy in real time according to actual operating conditions.

[0026] It should be noted that the above undetermined coefficients are only valid within the fixed combination of "the same loop reactor unit + the same ethylene grade". In general, if the catalyst system is changed, the structure of the loop reactor jacket is modified, or even the equipment model is changed, the operating data should be collected again and the model parameters should be refitted to ensure the accuracy and reliability of the prediction.

[0027] Step 3) Any feasible solution obtained from the set of feasible solutions should satisfy the following: When operating the polyethylene loop reactor process using the operating parameters corresponding to the feasible solution, the average temperature of the loop reactor is lower than the given target average peak temperature, and the maximum temperature difference of the loop reactor is lower than the given target maximum temperature difference peak temperature, throughout the entire given operating cycle. The average temperature and maximum temperature difference of the loop reactor within the given operating cycle are predicted by combining the relationships of the coefficients determined in Step 2) with the corresponding operating parameters.

[0028] Compared to existing technologies, this invention adjusts the ratio of catalyst injection frequency to slurry circulation frequency, slurry discharge frequency, and catalyst injection position in the loop reactor, resulting in a more uniform catalyst distribution in the reactor. This avoids repeated catalyst injection and accumulation in the same location in the slurry, preventing hot spots. Consequently, it reduces temperature fluctuations within the loop reactor, avoids problems such as agglomeration and reduced polymer quality caused by local overheating, lowers the risk of thermal runaway in the loop reactor, and improves the operational efficiency of the device.

[0029] The method described in this invention can be directly applied to existing slurry-based polyethylene loop reactor processes. Only adjustments to operating conditions are required; no large-scale reactor modifications are necessary. It offers excellent process compatibility, economy, and practicality. Controlling the catalyst injection frequency and slurry circulation frequency effectively controls the uniformity of catalyst concentration, reduces the number of hot spots within the loop reactor, ensures uniform temperature distribution within the reactor, and ultimately improves the reactor's thermal stability, enabling stable operation of the equipment.

[0030] Preferably, the present invention reduces hotspot formation by increasing the safety margin in the ratio of catalyst injection frequency to slurry circulation frequency, delaying the catalyst injection position, shortening the catalyst circulation path, and reducing hotspot formation; and by reducing the slurry discharge frequency, it avoids large temperature fluctuations caused by excessive discharge. This regulates the average temperature and maximum temperature difference of the reactor, thereby improving the reactor's thermal stability.

[0031] In the method of the present invention, the raw material gas includes ethylene, hydrogen and / or comonomer.

[0032] In the method of the present invention, the comonomer is at least one α-olefin with 3 or more carbon atoms and less than or equal to 10, selected from propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, preferably 1-butene. In the method of the present invention, the polymerization reactor is a slurry loop reactor, and the circulation speed of the slurry in the loop is 6-10 m / s.

[0033] In the method of the present invention, the dispersant in the slurry reaction system is one of isobutane, propane, n-pentane, n-heptane, and n-octane, preferably isobutane.

[0034] In the method of the present invention, the slurry polymerization is carried out in the presence of a metallocene catalyst, wherein the metallocene catalyst, the co-catalyst and the inert hydrocarbons form a slurry, which is intermittently injected into the reactor at high frequency; the active center metal of the metallocene catalyst includes zirconium, titanium and hafnium, preferably zirconium, and the co-catalyst is methylaluminoxane.

[0035] In the method of the present invention, the polymerization reaction temperature is 75-125°C, preferably 95-106°C, and the reaction pressure is 3-6.5 MPa, preferably 3.5-5.0 MPa. The polymerization reaction pressure consists of the partial pressure of ethylene and the partial pressure of hydrogen, and the hydrogen inlet rate is 0.1%-1.5% of the ethylene inlet rate, preferably 0.5%-1.2%.

[0036] Compared with traditional technologies, the method described in this invention, by adjusting the operating parameters of the loop reactor, not only effectively reduces hot spots and local high temperatures within the reactor, but also achieves efficient control over the thermal stability of the reactor. This helps to improve the overall stable operation of the device, extend the production cycle, and promote the development of the slurry loop reactor process towards a more efficient and stable direction.

[0037] This invention addresses the industry pain point of slurry loop reactor technology, where repeated addition of catalyst to the same location in the slurry leads to hot spot interference in reactor thermal stability, making accurate control difficult. It proposes a multivariate nonlinear prediction model that can accurately predict the thermal stability of the reactor at a given target average temperature. T avg and maximum temperature difference ΔT max Under these conditions, solving the equations yields feasible solutions for the operating parameters (the ratio of catalyst injection frequency to slurry circulation frequency, catalyst injection location, and discharge frequency) that ensure the thermal stability of the reactor under the target conditions, thus helping to ensure the stable operation of the unit. Attached Figure Description

[0038] Figure 1 For Example 1, the operating parameters were adjusted before and after the reactor's operating cycle was 10 minutes. T avg The changing trend; Figure 2 For Example 1, the operating parameters were adjusted before and after the reactor's operating cycle was 10 minutes. ΔT max The changing trend; Figure 3 After adjusting the operating parameters to make the reactor operating cycle in Example 1 10 minutes T avg Actual measurement vs. predicted comparison; Figure 4 After adjusting the operating parameters for a reactor operating cycle of 10 minutes in Example 1. ΔT maxActual measurement vs. prediction comparison. Detailed Implementation

[0039] The following examples provide those skilled in the art with guidance on how to manufacture and evaluate the invention. These examples are merely illustrative of the present disclosure and do not limit its scope. While every effort has been made to ensure accuracy regarding numerical values ​​(e.g., quantities, temperatures, etc.), some errors and deviations should be considered. Unless otherwise stated, temperatures are in °C or at ambient temperature, and pressures are at or near atmospheric pressure.

[0040] This embodiment provides a method for controlling the thermal stability of a polyethylene loop reactor using the slurry process. Specifically, to address the problem of hot spots caused by repeated catalyst injection into the same location in the slurry, the method increases the safety margin in the ratio of catalyst injection frequency to slurry circulation frequency, adjusts the injection location and discharge frequency, and staggers the catalyst injection locations under different circulation cycles. This reduces hot spots, regulates the average temperature and maximum temperature difference of the reactor, and improves the thermal stability of the reactor.

[0041] The thermal stability control method of the present invention includes the following steps: 1) Constructing the average reactor temperature T avg The relationship between time and the maximum temperature difference ΔT in the reactor. max The relationship that changes over time; The average temperature T in the polyethylene loop reactor process avg The relationship that changes over time is:

[0042] in

[0043]

[0044]

[0045]

[0046] In the formula t Indicates the reactor running time. T 0 Indicates the initial temperature of the reactor. r This is the ratio of the catalyst injection frequency to the slurry circulation frequency in the reactor. p To measure the dimensionless number of catalyst injection sites, f out This refers to the slurry discharge frequency; C This indicates the steady-state reactor temperature after prolonged operation. α Indicates the rate at which the reactor temperature stabilizes; βA coefficient indicating the intensity of a temperature peak; c A coefficient representing the rate of decay of the peak temperature. c i 、a i 、b i 、d i Let be the coefficients to be determined, where i =0,1,2,3.

[0047] The maximum temperature difference ΔT in the polyethylene loop reactor process max The relationship that changes over time is:

[0048] in

[0049]

[0050]

[0051]

[0052] In the formula t Indicates the reactor running time. r This is the ratio of the catalyst injection frequency to the slurry circulation frequency in the reactor. p To measure the dimensionless number of catalyst injection sites, f out This refers to the slurry discharge frequency; l This indicates the maximum temperature difference caused by the hot spot; n This indicates the time required to reach the maximum temperature difference at the hot spot. f This indicates the background temperature difference within the reactor; m Indicates the rate at which the background temperature difference stabilizes; u i 、v i 、w i 、z i Let be the coefficients to be determined, where i =0,1,2,3.

[0053] 2) Based on the design parameters and simulation operation data of the polyethylene loop reactor process, the undetermined coefficients in all the relationships in step 1) are fitted to obtain the relationships with determined coefficients; 3) Based on the given target average peak temperature, target maximum temperature difference peak temperature and reactor operating cycle of the loop reactor, obtain the feasible solution set of operating parameters by solving the relationship in step 2). Run the polyethylene loop reactor process based on the operating parameters in the feasible solution to achieve thermal stability control.

[0054] The present invention will be described in detail below with reference to a specific embodiment: Example 1 This embodiment employs a slurry loop reactor process. The loop reactor has a loop length of 150 meters, an initial reactor temperature of 60°C, and a hydrogen inlet rate of 0.75% of the ethylene inlet rate. The slurry circulation frequency is 0.057 Hz, and the discharge frequency is 0.0034 Hz. A zirconium-based metallocene catalyst supported on silica gel is used as the main catalyst, methylaluminoxane as a cocatalyst, isobutane as a dispersant, and 1-butene as a comonomer. The reaction system undergoes slurry polymerization at 106.5°C and 3.9 MPa, with a catalyst injection frequency of 0.0067 Hz. A catalyst injection point is set at 0.55 times the total loop length of the loop reactor (where 0.55 times the total external circulation length means setting the catalyst injection point at 0.55 times the total loop length along the positive direction of slurry flow from the slurry inlet position). Under operating conditions, after catalyst injection, the average temperature fluctuates greatly, and the maximum temperature difference in the reactor exceeds the safety threshold (10.2°C), indicating the presence of hot spots within the reactor.

[0055] Based on the design parameters of the device and existing actual operating data of the polymerization process, the relationships between the reactor's average temperature and time, and between the reactor's maximum temperature difference and time, were fitted to obtain undetermined coefficients that match the operating conditions of the target device. The fitting results of the undetermined coefficients in the relationships corresponding to Example 1 are shown in Table 1.

[0056] Table 1: Fitting Results of Undetermined Coefficients in Example 1

[0057] Substitute the undetermined coefficients from Table 1 into the relational formula. This embodiment uses a target average peak temperature of 110℃, a target maximum temperature difference peak temperature of 9.2℃, and a reactor operating cycle of 10 minutes as adjustment targets. Specifically, it requires that throughout the entire 10-minute operating cycle, the average temperature of the loop reactor remains below 110℃, and the maximum temperature difference of the loop reactor remains below 9.2℃. This embodiment obtains the relational formula by solving for the determined coefficients. r , p , f outThe feasible solution set of parameters is further converted to obtain the operating condition parameters that satisfy the thermal stability of the loop reactor. Specifically, in this embodiment, the catalyst injection frequency is increased to 0.00417, and the safety margin of the catalyst injection frequency and slurry circulation frequency is set to 0.05. The catalyst injection point is adjusted to 0.35 times the total length, the catalyst injection port is set closer to the slurry feed, and the discharge frequency is slightly increased to 0.0047.

[0058] The results of the examples are as follows Figure 1 and Figure 2 As shown, under the condition of this polyethylene grade, the dashed line represents the predicted result after adjusting the reactor operating parameters. With the increase of the operating cycle, the temperature fluctuation in the reactor decreases, the average temperature stabilizes faster, and the maximum temperature difference is reduced to within the safe threshold. This indicates that adjusting the reactor operating parameters can effectively avoid the generation of reactor hot spots and improve the thermal stability of the reactor.

[0059] Figure 3 and Figure 4 To adjust the operating parameters of the reactor T avg and ΔT max The results of the actual measurement-prediction comparison show that the relationship prediction results obtained by the present invention are accurate and can be used to guide industrial production and realize the control of thermal stability of the loop reactor.

Claims

1. A method for controlling the thermal stability of a slurry-process polyethylene loop reactor, characterized in that, Includes the following steps: 1) Constructing the average reactor temperature T avg The relationship between time and the maximum temperature difference ΔT in the reactor. max The relationship that changes over time; 2) Based on the design parameters and simulation operation data of the polyethylene loop reactor process, the undetermined coefficients in all the relationships in step 1) are fitted to obtain the relationships with determined coefficients; 3) Based on the given target average peak temperature, target maximum temperature difference peak temperature and reactor operating cycle of the loop reactor, obtain the feasible solution set of operating parameters by solving the relationship in step 2). Run the polyethylene loop reactor process based on the operating parameters in the feasible solution to achieve thermal stability control.

2. The method according to claim 1, characterized in that, The average temperature T in the polyethylene loop reactor process avg The relationship that changes over time is: ； in, ； ； ； ； In the formula t Indicates the reactor running time. T 0 This indicates the initial temperature of the reactor. r This is the ratio of the catalyst injection frequency to the slurry circulation frequency in the reactor. p To measure the dimensionless number of catalyst injection sites, f out This refers to the slurry discharge frequency; C This indicates the steady-state reactor temperature after prolonged operation. α This is a coefficient representing the overall temperature stabilization rate of the reactor. β A coefficient representing the intensity of a temperature peak; γ A coefficient representing the rate of decay of the peak temperature; c i 、a i 、b i 、d i Let be the coefficients to be determined, where i =0,1,2,3; The maximum temperature difference ΔT in the polyethylene loop reactor process max The relationship that changes over time is: ； in ； ； ； ； In the formula t Indicates the reactor running time. r This is the ratio of the catalyst injection frequency to the slurry circulation frequency in the reactor. p To measure the dimensionless number of catalyst injection sites, f out This refers to the slurry discharge frequency; λ This indicates the maximum temperature difference caused by the hot spot; ν This indicates the time required to reach the maximum temperature difference at the hot spot. φ This indicates the background temperature difference of the reactor; μ Indicates the rate at which the background temperature difference stabilizes; u i 、v i 、w i 、z i Let be the coefficients to be determined, where i =0,1,2,3.

3. The method according to claim 2, characterized in that, The ratio of catalyst injection frequency to slurry circulation frequency in the reactor is expressed by the following formula: ； In the formula m This indicates the number of cycles the slurry circulates in the loop pipe. k Indicates in m Number of catalyst injections per cycle; δ This represents the safety margin in the ratio of catalyst injection frequency to slurry circulation frequency; where, the safety margin... δ The settings need to satisfy the following relationships: ； In the formula Δ x This indicates the offset distance of the catalyst's landing point in the slurry after adjusting the injection frequency; L This indicates the loop length of the loop reactor.

4. The method according to claim 2, characterized in that, dimensionless number of injection site p We obtain it from the following formula: ； In the formula, The loop length of the loop reactor ，x This refers to the length of the loop between the catalyst injection location and the slurry feed point.

5. The method according to claim 2, characterized in that: Slurry discharge frequency f out We obtain it from the following formula: ； In the formula η This represents the evacuation fraction of the settling legs at the discharge outlet of the loop reactor. θ The time interval between consecutive opening of the settlement legs.

6. The method according to claim 2, characterized in that, In step 2), the undetermined coefficients c i 、a i 、b i 、d i 、u i 、v i 、w i 、 z i The following methods were used to obtain a set of undetermined coefficients that match the operating conditions of the target unit: A loop reactor simulation device was constructed, and the polymerization process was simulated under different operating conditions. Actual operating data of the polymerization process were collected, and the relationships were fitted to obtain the coefficients. c i 、a i 、b i 、d i 、u i 、v i 、w i 、z i .

7. The method according to claim 1, characterized in that, Step 3) Any set of feasible solutions obtained from the feasible solution set satisfies the following: when the polyethylene loop reactor process is operated using the operating parameters corresponding to the feasible solution, the average temperature of the loop reactor is lower than the given target average temperature peak value throughout the entire given operating cycle, and the maximum temperature difference of the loop reactor is lower than the given target maximum temperature difference peak value.

8. The method according to claim 1, characterized in that, The polyethylene loop reactor process has a polymerization reaction temperature of 75-125℃, a reaction pressure of 3-6.5MPa, and a circulation flow rate of 5-15m / s.

9. The method according to claim 1, characterized in that, The feed gas for the polyethylene loop reactor process includes polymeric monomers, hydrogen, and / or comonomers.

10. The method according to claim 1, characterized in that, The polyethylene loop reactor process is carried out in the presence of a metallocene catalyst. The metallocene catalyst, co-catalyst, and inert hydrocarbons form a slurry, which is intermittently injected into the reactor at high frequency.

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