Product performance regulation and control method suitable for slurry method polyethylene process

Abstract

The invention discloses a product performance regulation and control method suitable for a slurry method polyethylene process. Aiming at a slurry-method polyethylene production process adopting a slurry external circulation process, the method is characterized in that ethylene monomers are introduced into an external circulation pipeline, so that the concentration of ethylene in a circulation loop is effectively improved, and the molecular weight distribution of a polyethylene product is accurately regulated and controlled. And in combination with simulation data, a quantitative prediction model between the polymerization reaction parameters and the polyethylene product is constructed, and the polyethylene product performance corresponding to the current polymerization reaction parameters can be predicted based on the model or the polymerization reaction parameters meeting the performance requirements can be solved according to the required polyethylene product performance. According to the method, the performance of the polyethylene product can be regulated and controlled, and wide industrial application prospects and popularization values are shown.

Description

Technical Field

[0001] This invention belongs to the field of chemical engineering and relates to a method for regulating product performance in a slurry-based polyethylene process. Specifically, it involves a method that effectively increases the concentration of ethylene monomer in the circulating pipeline by introducing ethylene replenishment points at multiple locations within the slurry's external circulation pipeline, thereby adjusting the molecular weight distribution of the product. Background Technology

[0002] Polyethylene is the most widely used general-purpose resin in the modern petrochemical industry. To meet the market's urgent demand for both high capacity and low cost, various companies have added external circulation cooling systems to their existing slurry-process polyethylene plants. External circulation cooling refers to the process of removing the polymerization slurry through circulation pipelines for heat dissipation during the polymerization reaction, and then recirculating it back into the polymerization reactor. This method significantly improves the reaction load and plant capacity by enhancing the heat removal capacity of the reactor. However, a common problem in industrial production practice is that with the introduction of external circulation systems, the problems of adhesion and scaling on the reactor walls and circulation pipelines have become increasingly serious, forcing frequent shutdowns for cleaning and resulting in a significant shortening of the production cycle.

[0003] Studies have shown that the root cause of the above phenomenon lies in a series of adverse chain effects triggered by the low ethylene monomer concentration in the external circulation zone. Specifically, this manifests as follows: First, the catalyst activity gradually declines over time from the start of the process. The extra residence time in the external circulation system causes some catalyst to enter a low-activity state before returning to the reactor, resulting in the formation of a large amount of low-molecular-weight polymers in the pipeline. Second, the lack of ethylene replenishment in the external circulation section leads to a gradual decrease in ethylene concentration along the way, resulting in an increase in the hydrogen-ethylene molar ratio and an enhanced hydrogen regulation effect, further reducing the molecular weight of the polymer. In addition, the heat exchange in the external circulation section causes the local temperature to be significantly lower than that of the reactor body. The solubility of oligomers decreases at lower temperatures, making them more likely to precipitate and adhere to the pipe walls. Summary of the Invention

[0004] Given that existing slurry-process polyethylene production plants generally suffer from problems such as large monomer concentration gradients, increased oligomer formation, easy pipe adhesion and blockage, and shortened plant operation cycles after adopting slurry external circulation cooling systems, this invention proposes a product performance control method suitable for slurry-process polyethylene. This method is based on a multi-point ethylene supplementation control strategy to precisely control the ethylene concentration distribution in the polymerization system, adjust the properties of the polyethylene product, and thus improve product performance stability and plant operating efficiency.

[0005] The present invention provides a product performance control method applicable to slurry polyethylene process, comprising the following steps: reactants are subjected to slurry polymerization in a polyethylene reactor to produce polyethylene, the polyethylene reactor is cooled by external circulation of slurry, and at least one ethylene injection point is set in the external circulation pipeline, ethylene monomer is injected into the ethylene injection point, and the polymer molecular weight distribution and / or weight-average molecular weight are adjusted by controlling the operating parameters of the polymerization reaction; wherein, the polymerization reaction parameters include the external circulation injection rate of ethylene, the hydrogen injection rate, the external circulation injection rate of ethylene, and the reactor temperature.

[0006] The molecular weight distribution (PDI) and weight-average molecular weight (Mw) of the polymer obtained by the method of this invention satisfy the following relationship with the polymerization reaction operating parameters:

[0007] PDI = a1 + a2F + a3F 2 +a4R+a5T+a6x

[0008] +a7x 2 +a9U+a9U 2 +a 10 D+a 11 D 2

[0009] Mw=b1+b2F+b3F 2 +b4R+b5T+b6x

[0010] +b7x 2 +b8U+b9U 2 +b 10 D+b 11 D 2

[0011] In the formula, F is the molar ratio of ethylene injection rate in the external circulation of the slurry to the total ethylene injection rate in the reactor, R is the molar ratio of hydrogen injection rate at the reactor inlet to the total ethylene injection rate in the reactor, T is the temperature of the polymerization reactor, x is the ratio of the centroid of the ethylene injection point to the total length of the external circulation of the slurry, U is a dimensionless index measuring different ethylene injection methods, and D is the ratio of the distance between the first and last ethylene injection points in the external circulation to the total length of the external circulation; a i b i Let be undetermined coefficients, where i = 1, 2, ..., 11.

[0012] Furthermore, the product performance control method of the present invention can be specifically implemented in the following manner: based on the required molecular weight distribution (PDI) and weight-average molecular weight (Mw) of the target polymer, a feasible solution set of parameters F, H, T, x, U, and D is obtained by solving the relational formula. Based on the feasible solution set and the definition of the parameters, the required polymerization reaction parameters are calculated. The obtained polymerization reaction parameters are used for slurry polymerization to produce the required polyethylene product. Alternatively, the molecular weight distribution (PDI) and weight-average molecular weight (Mw) of the polymer product corresponding to the current polymerization reaction parameters can be directly predicted through the relational formula.

[0013] Compared to existing technologies, this invention arranges multiple ethylene injection points axially in the external circulation pipeline of polyethylene slurry, preferably 1-5, and injects ethylene monomers in segments according to a set flow rate. This makes the ethylene concentration in the circulating stream more uniform along the path, reducing the risk of side reactions caused by local monomer shortages, and thus achieving precise control over the polymerization rate and polymer structure. By adjusting the position and injection volume ratio of the injection points, key performance indicators such as molecular weight distribution (MWD) and dispersion index (PDI) of the product can be controlled in a targeted manner, while suppressing the formation of low molecular weight polymers, reducing the tendency of wall adhesion and deposition, and improving the controllability and stability of the equipment operation.

[0014] The method described in this invention can be directly integrated into existing slurry-process polyethylene production equipment without large-scale structural modifications, exhibiting excellent process compatibility and practicality. Multi-point ethylene replenishment effectively optimizes monomer distribution within the pipeline, maintains a suitable hydrogen-to-ethylene ratio, improves the controllability of the polymerization reaction, ultimately enhancing polyethylene product quality and extending the continuous operating time of the equipment.

[0015] Preferably, the present invention replenishes ethylene at at least one ethylene injection point in the external circulation pipeline of the slurry according to a set flow rate, so that the concentration of ethylene monomer in the circulating stream is maintained within the target range, thereby allowing for selective adjustment of key properties such as molecular weight distribution, melt index, and crystallinity of the product as needed.

[0016] In the method of the present invention, the raw material gas includes ethylene, hydrogen and / or comonomer.

[0017] In the method of the present invention, the comonomer is preferably at least one α-olefin with a carbon number greater than or equal to 3 and less than or equal to 10, selected from propylene, 1-butene, 1-pentene, 1-hexene, 1-octene and 1-decene, preferably 1-butene.

[0018] In the method of the present invention, the polymerization reactor is a slurry stirred reactor reactor, the external circulation mass flow rate is 0.1 to 50 times the feed mass flow rate, and the temperature after heat removal is 50 to 80°C.

[0019] In the method of the present invention, the solvent in the slurry reaction system is one or more of toluene, ethylbenzene, benzene, n-hexane, cyclohexane, n-heptane, and cyclopentane, preferably n-heptane or n-hexane.

[0020] In the method of the present invention, the slurry polymerization is carried out in the presence of a main catalyst and a co-catalyst, which can be added together with the feed gas; the main catalyst is selected from one or more of Ziegler Natta catalyst, FI catalyst, and Phillips catalyst, and the co-catalyst is selected from one or more of methylaluminoxane, triethylaluminum, n-butyllithium, triisobutylaluminum, trimethylaluminum, and trifluoroborane.

[0021] In the method of the present invention, the polymerization reaction temperature is 60–90°C, preferably 65–80°C. The polymerization reaction pressure is 0.1–10 MPa, preferably 0.1–4 MPa. The polymerization reaction pressure consists of the partial pressure of ethylene and the partial pressure of hydrogen, and the amount of hydrogen introduced is 0.01%–30% of the amount of ethylene introduced, preferably 0.1%–20%.

[0022] Compared with traditional technologies, the method described in this invention, by adjusting the spatial distribution of ethylene concentration, not only effectively controls the risk of oligomer formation and deposition, but also enables the controllable design of the structural performance of polyethylene products. This helps to improve product quality consistency, extend the production cycle, and promote the development of the slurry polyethylene production process towards high performance and high stability.

[0023] This invention addresses the industry pain points of relying on experience, iterative trial and error, and response lag in controlling the product dispersion index (PDI) and weight-average molecular weight (MW) in the slurry polyethylene production process. It proposes a multivariate nonlinear prediction model and its inverse method, which can solve the equations to obtain feasible solutions for key processes such as ethylene external circulation injection volume, hydrogen injection volume, and reactor temperature T, given the target PDI and MW, thus providing assistance for product property control. Attached Figure Description

[0024] Figure 1 The PDI variation trend under different external circulation injection ratios in Example 1;

[0025] Figure 2 This shows the trend of MW variation under different external circulation injection ratios in Example 1;

[0026] Figure 3 The PDI variation trend under different external circulation injection ratios in Example 2;

[0027] Figure 4 The variation trend of MW under different external circulation injection ratios in Example 2;

[0028] Figure 5The variation trend of MW under different external circulation injection ratios in Example 3;

[0029] Figure 6 The variation trend of MW under different external circulation injection ratios in Example 3;

[0030] Figure 7 This is a comparison of measured and predicted Mw values ​​in Example 1.

[0031] Figure 8 This is a comparison of measured and predicted PDI in Example 1. Detailed Implementation

[0032] 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.

[0033] This embodiment provides a method for controlling product performance in a slurry-based polyethylene process. The reactants are slurry-polymerized in a polyethylene reactor to produce polyethylene. The polyethylene reactor uses external slurry circulation for heat removal, and at least one ethylene injection point is provided in the external slurry circulation pipeline. Ethylene monomer is injected into the ethylene injection point. The polymer molecular weight distribution and / or weight-average molecular weight are adjusted by controlling the operating parameters of the polymerization reaction. The polymerization reaction parameters include the external ethylene circulation injection rate, the hydrogen injection rate, and the reactor temperature.

[0034] The molecular weight distribution (PDI), weight-average molecular weight (Mw), and polymerization operating parameters of the polymer satisfy the following relationship:

[0035] PDI = a1 + a2F + a3F 2 +a4R+a5T+a6x

[0036] +a7x 2 +a8U+a9U 2 +a 10 D+a 11 D 2

[0037] Mw=b1+b2F+b3F 2 +b4R+b5T+b6x

[0038] +b7x 2 +b8U+b9U 2 +b 10 D+b 11 D 2

[0039] In the formula, F is the molar ratio of ethylene injection rate in the external circulation of the slurry to the total ethylene injection rate in the reactor, R is the molar ratio of hydrogen injection rate at the reactor inlet to the total ethylene injection rate in the reactor, T is the temperature of the polymerization reactor, x is the ratio of the centroid of the ethylene injection point to the total length of the external circulation of the slurry, U is a dimensionless index measuring different ethylene injection methods, and D is the ratio of the distance between the first and last ethylene injection points in the external circulation to the total length of the external circulation; a i b i Let be undetermined coefficients, where i = 1, 2, ..., 11.

[0040] In this embodiment, the ratio x of the centroid of the ethylene injection location to the total length of the external circulation of the slurry is obtained by the following formula:

[0041]

[0042] Where: N is the number of external circulation ethylene injection points, F n F represents the injection volume at the nth ethylene injection point. total The total amount of ethylene injected into the external circulation system, L n L represents the distance between the nth ethylene injection point and the starting point of the external circulation system. loop This represents the total length of the outer loop.

[0043] In this embodiment, the dimensionless index U is obtained by the following formula:

[0044]

[0045] The probability density p(x) is defined by the following equation:

[0046]

[0047] r(x) is the instantaneous ethylene absorption rate curve measured along the outer circulation length L, obtained experimentally; where x and t are variables representing length.

[0048] Obtaining Undetermined Coefficients: The undetermined coefficients in this model can be obtained in various ways, including but not limited to: constructing a polymerization reaction simulation device with external slurry circulation, simulating the polymerization process under different operating conditions, collecting actual operating data of the polymerization process, fitting nonlinear relationships to obtain undetermined coefficients matching the operating conditions of the target device, substituting the undetermined coefficients into the relationships 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 to obtain a set of model parameters matching the operating conditions of the target device. 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.

[0049] It should be noted that the above undetermined coefficients are only valid within the fixed combination of "the same polymerization unit + the same ethylene grade". In general, if the catalyst system is changed, the external circulation pipeline or heat exchanger structure is modified, or even the equipment size is increased / decreased, the operating data should be collected again and the model parameters should be refitted to ensure the accuracy and reliability of the prediction.

[0050] Practical applications of the method and model of this invention: Based on the molecular weight distribution (PDI) and weight-average molecular weight (Mw) of the desired target polymer, feasible solution sets of parameters F, H, T, x, U, and D can be obtained by solving the relational formula. Based on the feasible solution set and the definition of the parameters, the required polymerization reaction parameters can be calculated. The obtained polymerization reaction parameters can then be used for slurry polymerization to produce the desired polyethylene product. Alternatively, the molecular weight distribution (PDI) and weight-average molecular weight (Mw) of the polymer product corresponding to the current polymerization reaction parameters can be directly predicted using the relational formula.

[0051] Example 1

[0052] In this embodiment, a Ziegler-Natta catalyst was used as the main catalyst, triethylaluminum as the co-catalyst, n-hexane as the solvent, and 1-butene as the comonomer. The reaction system underwent a slurry polymerization reaction at 82°C and 0.8 MPa. The reactor had two external slurry circulation streams; one stream was entirely refluxed back to the reactor, and the other was discharged at a reflux ratio (mass ratio) of 1:12.5, with the remainder returned to the reactor. The reaction residence time was 2.5 hours.

[0053] While maintaining a constant total ethylene injection rate into the reactor, an ethylene injection point is set at 0.071 times the total length of the external circulation (wherein, 0.071 times the total length of the external circulation means setting the ethylene injection point at a position 0.071 times the total length of the external circulation from the starting position. The definition of the ethylene injection point in subsequent embodiments is the same as that here and will not be repeated). The proportion of ethylene injection at this point is gradually increased.

[0054] The simulation results of the example are as follows Figure 1 and Figure 2 As shown, under the given ethylene grade, the dispersion coefficient of the product gradually decreases with the increase of the ethylene injection fraction at this point, indicating that the molecular weight distribution tends to be concentrated and the uniformity of the polymerization product is enhanced. Simultaneously, the weight-average molecular weight also shows a decreasing trend, indicating that the overall chain growth rate of the system is somewhat suppressed. The obtained experimental data were randomly divided into 90% and 10% portions, and the undetermined coefficients of the two relationships of this invention were fitted using the 90% portion of the experimental data:

[0055] PDI = a1 + a2F + a3F 2 +a4R+a5T+a6x

[0056] +a7x 2 +a8U+a9U 2 +a 10 D+a 11 D 2

[0057] Mw=b1+b2F+b3F 2 +b4R+b5T+b6x

[0058] +b7x 2 +b8U+b9U 2 +b 10 D+b 11 D 2

[0059] The fitting results of the undetermined coefficients in the relational formula corresponding to Example 1 are shown in Table 1.

[0060] Table 1: Fitting Results of Undetermined Coefficients in Example 1

[0061]

[0062]

[0063] We used 10% of the experimental data to verify the fitted relationship. Figure 7 and Figure 8The experimental data of Mw and PDI were compared between measured and predicted values. The results show that the predicted relationship obtained by the present invention is accurate and can be used to guide industrial production and realize the controllable design of the structural performance of polyethylene products.

[0064] Example 2

[0065] This embodiment is identical to Example 1 in terms of catalyst system, solvent type, comonomer, temperature, pressure, reflux structure, etc., with the only adjustment being the setting of the ethylene injection point.

[0066] In this embodiment, two ethylene injection points are set at 0.071 and 0.172 of the total length of the external circulation, respectively, forming a multi-point injection structure. While maintaining a constant total ethylene injection volume into the reactor, ethylene monomer is evenly distributed to the two injection points to investigate the impact of the injection configuration on product properties.

[0067] Numerical simulation results are as follows Figure 3 and Figure 4 As shown, the PDI reduction was more significant after adopting the multi-point injection method, the molecular weight distribution was further narrowed, and the monomer concentration within the polymerization system was more balanced. However, at the same time, the rate of decrease in weight-average molecular weight also accelerated.

[0068] Example 3

[0069] This embodiment maintains the same catalyst system, solvent and comonomer types, reaction temperature and pressure, reflux structure and other process conditions as Example 1, with the only adjustment being the ethylene monomer injection position.

[0070] In this embodiment, the ethylene injection point is set at 0.402 of the total length of the external circulation. Compared with the front-end injection method in Example 1, this case adopts a single-point injection strategy at the rear end to explore the influence of the injection position on the properties of the polymerization product.

[0071] Numerical simulation results are as follows Figure 5 and Figure 6 As shown, compared to Example 1, the decrease in PDI with increasing ethylene injection amount is significantly slower, indicating that the later-stage injection has a relatively weaker effect on molecular weight distribution regulation, and the local equilibrium ability of the reaction rate within the system is reduced. Simultaneously, the rate of decrease in weight-average molecular weight is also slightly slower than in Example 1, indicating that the chain growth termination trend is relatively gentle, and the polymerization reaction conditions tend to be milder.

[0072] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A method for regulating product performance in slurry polyethylene processing, characterized in that: The raw material gas undergoes slurry polymerization in a polyethylene reactor to produce polyethylene. The polyethylene reactor uses external slurry circulation for heat removal, and at least one ethylene injection point is set in the external slurry circulation pipeline. Ethylene monomer is injected into the ethylene injection point, and the polymer molecular weight distribution and / or weight-average molecular weight are adjusted by controlling the operating parameters of the polymerization reaction. The polymerization reaction parameters include the external ethylene circulation injection rate, the hydrogen injection rate, and the reactor temperature.

2. The method according to claim 1, characterized in that: The molecular weight distribution (PDI), weight-average molecular weight (Mw), and polymerization operating parameters of the polymer satisfy the following relationship: PDIa1+a2F+a3F 2 +a4R+a5T+a6x +a7x 2 +a8U+a9U 2 +a 10 D+a 11 D 2 Mw=b1+b2F+b3F 2 +b4R+b5T+b6x +b7x 2 +b8U+b9U 2 +b 10 D+b 11 D 2 In the formula, F is the molar ratio of ethylene injection rate in the external circulation of the slurry to the total ethylene injection rate in the reactor, R is the molar ratio of hydrogen injection rate at the reactor inlet to the total ethylene injection rate in the reactor, T is the temperature of the polymerization reactor, x is the ratio of the centroid of the ethylene injection point to the total length of the external circulation of the slurry, U is a dimensionless index measuring different ethylene injection methods, and D is the ratio of the distance between the first and last ethylene injection points in the external circulation to the total length of the external circulation; a i b i Let be undetermined coefficients, where i = 1, 2, ..., 11.

3. The method according to claim 1, characterized in that: The ratio x of the centroid of the ethylene injection location to the total length of the external circulation of the slurry is obtained by the following formula: Where: N is the number of external circulation ethylene injection points, F n F represents the injection volume at the nth ethylene injection point. total The total amount of ethylene injected into the external circulation system, L n L represents the distance between the nth ethylene injection point and the starting point of the external circulation system. loop This represents the total length of the outer loop.

4. The method according to claim 3, characterized in that: The dimensionless index U is obtained by the following formula: The probability density p(x) is defined by the following equation: r(x) is the instantaneous ethylene absorption rate curve measured along the external circulation length, obtained experimentally.

5. The method according to claim 2, characterized in that, Undetermined coefficient a i b i The following method was used to obtain a set of undetermined coefficients 'a' that match the operating conditions of the target unit: A polymerization reaction simulation device with external slurry circulation was constructed; the polymerization process was simulated under different operating conditions; actual operating data of the polymerization process was collected; and the relationship was fitted to obtain the coefficients 'a'. i b i .

6. The method according to claim 2, characterized in that: Based on the required molecular weight distribution (PDI) and weight-average molecular weight (Mw) of the target polymer, feasible solutions for the parameters F, H, T, x, U, and D are obtained by solving the relational formulas. Based on the feasible solutions and the definitions of the parameters, the required polymerization reaction parameters are calculated. The obtained polymerization reaction parameters are then used for slurry polymerization to produce the required polyethylene product.

7. The method according to claim 2, characterized in that: Based on the current polymerization reaction parameters, the molecular weight distribution (PDI) and weight-average molecular weight (Mw) of the polymer product corresponding to the current polymerization reaction parameters are predicted by using the relationship formula.

8. The method according to claim 1, characterized in that, The polymerization reaction temperature is 60–90℃; the polymerization pressure is 0.1–10 MPa.

9. The method according to claim 1, characterized in that, The solvent for slurry polymerization is one or more of toluene, ethylbenzene, benzene, n-hexane, cyclohexane, n-heptane, and cyclopentane, and the raw material gas includes polymerizing monomers, hydrogen, and / or comonomers.

10. The method according to claim 1, wherein the slurry polymerization is carried out in the presence of a main catalyst and a co-catalyst, wherein the main catalyst is selected from one or more of Ziegler Natta catalyst, FI catalyst, and Phillips catalyst, and the co-catalyst is selected from one or more of methylaluminoxane, triethylaluminum, n-butyllithium, triisobutylaluminum, trimethylaluminum, and trifluoroborane.

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