CN112920297B - Polymerization switching process for producing polyethylene - Google Patents
Polymerization switching process for producing polyethylene Download PDFInfo
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- CN112920297B CN112920297B CN202110120057.5A CN202110120057A CN112920297B CN 112920297 B CN112920297 B CN 112920297B CN 202110120057 A CN202110120057 A CN 202110120057A CN 112920297 B CN112920297 B CN 112920297B
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Abstract
The invention relates to the technical field of low-pressure gas-phase fluidized bed polyethylene process, and discloses a polymerization switching method for producing polyethylene, which comprises a process A for switching from producing PE-ML-63D082 polyethylene to producing PE-ML-57D075 polyethylene, and a process B for switching from producing PE-ML-57D075 polyethylene to producing PE-L-FB-20D20 polyethylene; 1-butene is introduced in the process A, and diethylaluminum monochloride is introduced in the process B, wherein the processes A and B are realized by adjusting components and parameters. Compared with the prior art, the method has the advantages of safe process, less transition material generation amount and small static fluctuation.
Description
Technical Field
The invention relates to the technical field of polyethylene processes, in particular to a polymerization switching method for producing polyethylene
Background
The low-pressure gas-phase fluidized bed polyethylene process has the advantages of low investment, low energy consumption, high safety and capability of producing full-density polyethylene, and is a process technology which is more used in polyethylene devices in China. In order to meet the diversified demands of the market, the low-pressure gas-phase fluidized bed polyethylene reactor is frequently required to switch the product grades. As the reactor has back mixing phenomenon, along with the increase of the scale of the device, in the process of switching the grade, when the reaction conditions are greatly different, the quantity of the transition materials is increased or the transition time is prolonged, thereby influencing the economic benefit of the device.
The existing method for switching PE-ML-63D082 polyethylene product to PE-L-FB-20D20 polyethylene product comprises a method for reestablishing composition after a reactor is stopped and a seed bed is replaced, a method for reestablishing composition after the reactor is deactivated, and the like.
The method for reestablishing the composition after the reactor is shut down and the seedbed is replaced causes a great deal of material loss because the circulating gas components of the reactor are discharged to a torch when the reactor is shut down and the torch is required to be discharged before the reactor is started to establish the components, such as ethylene, 1-butylene, hydrogen and the like. Meanwhile, a large amount of nitrogen, electricity and circulating water are consumed in the process of stopping and starting the vehicle, so that the energy consumption is increased. In addition, because the reactor is stopped for a long time, the extrusion granulator set is forced to stop due to material breakage, and ground materials and unqualified materials with standard-exceeding color grains are generated in the driving process, so that the product quality is influenced. Finally, the ethylene tank inventory rises during reactor shutdown, reducing polyethylene yield, affecting upstream and downstream material balance and polyethylene yield.
A method for reactivating and reestablishing components by a reactor requires that the reactor exit from a condensing mode for production, the components of the recycle gas are adjusted after deactivation, and then the load is increased to enter the condensing mode for production from a dry mode. On the one hand, the reactor needs to pass through a 'slurry' state in the process of exiting and entering a condensation mode, and the distribution plate is easy to be blocked. On the other hand, in the dry method mode, when 1-butene is introduced, due to the change of components and impurity content, the electrostatic indication in the reactor is easy to fluctuate greatly, so that the reactor is caked, and the reactor is required to be stopped and cleaned when the reactor is serious.
Therefore, there is a need for a polymerization switch-over process for producing polyethylene that is safe in process, produces low levels of transition materials, and has low static fluctuations.
Disclosure of Invention
In order to overcome the technical problems in the prior art, the invention provides a polymerization switching method for producing polyethylene, which has the advantages of safe process, less transition material generation amount and small electrostatic fluctuation.
The inventor of the invention discovers through research that the switching production of polyethylene product brands, particularly the process of switching from PE-ML-63D082 polyethylene products to PE-L-FB-20D20 polyethylene products, can be realized under the conditions that a reactor is not inactivated and the reactor is not stopped by skillfully designing a switching process route of polyethylene.
In order to achieve the above object, the present invention provides a polymerization switching process for producing polyethylene, comprising effecting a switch from producing PE-ML-63D082 polyethylene to producing PE-L-FB-20D20 polyethylene in a first polymerization stage and in a second polymerization stage; wherein the first polymerization stage comprises process A switching from producing PE-ML-63D082 polyethylene to producing PE-ML-57D075 polyethylene and the second polymerization stage comprises process B switching from producing PE-ML-57D075 polyethylene to producing PE-L-FB-20D20 polyethylene;
the process A comprises the following steps: in the presence of hydrogen, a catalyst and a first cocatalyst, ethylene is subjected to homopolymerization to produce PE-ML-63D082 polyethylene; adjusting the ethylene partial pressure and the hydrogen/ethylene molar ratio, introducing 1-butene and adjusting the 1-butene/ethylene molar ratio to ensure that the obtained polymer product-1 is switched to be polymerized to obtain PE-ML-57D075 polyethylene when the density and the melt index of the obtained polymer product-1 reach set values of-1;
the process B comprises the following steps: in the process of polymerizing to obtain PE-ML-57D075 polyethylene in the presence of hydrogen, ethylene, 1-butene, a catalyst and a first cocatalyst, introducing the second cocatalyst, adjusting the ethylene partial pressure, the 1-butene/ethylene molar ratio, the hydrogen/ethylene molar ratio, the polymerization temperature and the polymerization total pressure to switch to polymerization to obtain PE-L-FB-20D20 polyethylene when the density and the melt index of the obtained polymer product-2 reach set values of-2.
Compared with the prior art, the technical scheme provided by the invention at least has the following advantages:
(1) the method develops a polymerization process method of PE-ML-57D075 polyethylene;
(2) in a preferred embodiment of the invention, the temperature of the reactor inlet is lower than the dew point temperature, so that the invention can complete polyethylene switching in a condensed state mode, can effectively reduce the static fluctuation amplitude, can reduce the risk of reactor agglomeration in the polyethylene switching process, and is beneficial to the safe operation of the reactor in the ethylene polymerization reaction;
(3) in the method, the polyethylene switching process is carried out in a continuous production state, so that the material loss and the nitrogen consumption during the polyethylene switching can be reduced, and the yield loss of the polyethylene is avoided;
(4) in the process of the present invention, the amount of transition material in switching from the production of PE-M-63D082 polyethylene to the production of PE-L-FB-20D20 polyethylene is reduced by producing PE-ML-57D075 polyethylene.
Drawings
FIG. 1 is a schematic process flow diagram of one embodiment of the present invention.
Description of the reference numerals
1. Reactor 2, compressor 3, cooler
4. The first staying tank 5 and the second staying tank
Detailed Description
The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to give one or more new ranges of values, and these ranges of values should be considered as specifically disclosed herein.
The invention provides a polymerization switching method for producing polyethylene, which comprises a first polymerization stage and a second polymerization stage for switching from producing PE-ML-63D082 polyethylene to producing PE-L-FB-20D20 polyethylene; wherein the first polymerization stage comprises process A switching from producing PE-ML-63D082 polyethylene to producing PE-ML-57D075 polyethylene and the second polymerization stage comprises process B switching from producing PE-ML-57D075 polyethylene to producing PE-L-FB-20D20 polyethylene;
the process A comprises the following steps: in the presence of hydrogen, a catalyst and a first cocatalyst, ethylene is subjected to homopolymerization to produce PE-ML-63D082 polyethylene; adjusting the ethylene partial pressure and the hydrogen/ethylene molar ratio, introducing 1-butene and adjusting the 1-butene/ethylene molar ratio to ensure that the obtained polymer product-1 is switched to be polymerized to obtain PE-ML-57D075 polyethylene when the density and the melt index of the obtained polymer product-1 reach set values of-1;
the process B comprises the following steps: in the process of polymerizing to obtain PE-ML-57D075 polyethylene in the presence of hydrogen, ethylene, 1-butene, a catalyst and a first cocatalyst, introducing the second cocatalyst, adjusting the ethylene partial pressure, the 1-butene/ethylene molar ratio, the hydrogen/ethylene molar ratio, the polymerization temperature and the polymerization total pressure to switch to polymerization to obtain PE-L-FB-20D20 polyethylene when the density and the melt index of the obtained polymer product-2 reach set values of-2.
According to some embodiments of the invention, the conditions of the homopolymerization reaction may include: the temperature is 95-104 ℃, preferably 96-98 ℃; the pressure is 2300-; the ethylene partial pressure is 1150-1300kPa, preferably 1200-1250 kPa; the hydrogen/ethylene molar ratio is between 0.38 and 0.45mol/mol, preferably between 0.41 and 0.42 mol/mol; the flow rate of the catalyst is 8 to 10kg/h, preferably 8 to 8.5 kg/h.
According to some embodiments of the present invention, the catalyst may be selected from ziegler-natta slurry catalysts; wherein, the content of titanium element in the Ziegler-Natta slurry catalyst can be 2-3 wt%, preferably 2.2-2.5 wt%, and the content of magnesium element can be 6-8 wt%, preferably 6.5-7 wt%.
In the present invention, the Ziegler-Natta slurry catalyst can be prepared according to the conventional method in the field, for example, the Ziegler-Natta slurry catalyst can be obtained by firstly obtaining a powder catalyst through spray drying, and then preparing the powder catalyst into a mineral oil suspension with the solid content of 27-29 wt%.
According to a preferred embodiment of the present invention, the first co-catalyst may be tri-n-hexylaluminum.
In the present invention, the molar ratio of the first co-catalyst to tetrahydrofuran in the homopolymerization stage may be 0.15 to 0.19 mol/mol.
According to some embodiments of the invention, the setting is performed by a computerThe value-1 includes: the polymer product-1 had a density of 0.955 to 0.961g/cm3(ii) a Preferably 0.956 to 0.96g/cm3(ii) a The polymer product-1 has a melt index at 190 ℃ and a load of 2.16kg of 5-10g/10min, preferably 6-9g/10 min; wherein the density of said polymer product-1 means the powder density of said polymer product-1, and the melt index of said polymer product-1 means the powder melt index of said polymer product-1.
According to some embodiments of the invention, the conditions for the polymerization to obtain PE-ML-57D075 polyethylene may comprise: the temperature is 95-104 ℃, preferably 96-98 ℃; the pressure is 2300-; the ethylene partial pressure is 1000-1200kPa, preferably 1100-1150 kPa; the hydrogen/ethylene molar ratio is between 0.3 and 0.39mol/mol, preferably between 0.32 and 0.38 mol/mol; the 1-butene/ethylene molar ratio is between 0.01 and 0.018mol/mol, preferably between 0.012 and 0.016 mol/mol.
According to a preferred embodiment of the present invention, the second cocatalyst may be diethylaluminum monochloride.
According to some embodiments of the invention, the set-point-2 comprises: the density of the polymer product-2 is 0.917-0.923g/cm3(ii) a Preferably 0.918-0.922g/cm3(ii) a The polymer product-2 has a melt index at 190 ℃ and a load of 2.16kg of 1.5-2.5g/10min, preferably 1.7-2.3g/10 min; wherein the density of the polymer product-2 means the powder density of the polymer product-2, and the melt index of the polymer product-2 means the powder melt index of the polymer product-2.
According to some embodiments of the present invention, the conditions under which the polymerization results in PE-L-FB-20D20 polyethylene may include: the temperature is 85-89 ℃, preferably 85-87 ℃; the pressure is 2000-2300kPa, preferably 2100-2200 kPa; the ethylene partial pressure is 600-700kPa, preferably 650-690 kPa; the hydrogen/ethylene molar ratio is between 0.14 and 0.18mol/mol, preferably between 0.15 and 0.17 mol/mol; the 1-butene/ethylene molar ratio is between 0.25 and 0.35mol/mol, preferably between 0.29 and 0.33 mol/mol.
In the present invention, the molar ratio of the second cocatalyst to tetrahydrofuran at the stage of polymerization to obtain PE-L-FB-20D20 polyethylene may be set to 0.4 to 0.45 mol/mol.
According to some embodiments of the invention, the process a may further comprise: when the density and melt index of the polymer product-1 reach the set values of-1, the flow rate of the catalyst is adjusted so that the yield of the PE-ML-57D075 polyethylene obtained by polymerization is 35-45t/h, preferably 38-42 t/h.
According to some embodiments of the invention, the process a may further include: when the density and melt index of polymer product-1 reached the set point-1, the flow of isopentane was adjusted so that the dew point temperature was maintained at 50-60 deg.C, preferably 50-55 deg.C.
According to some embodiments of the invention, the B may further comprise: when the density and melt index of the polymer product-2 reach the set values of-2, the flow rate of the catalyst is adjusted so that the yield of the PE-L-FB-20D20 polyethylene obtained by polymerization is 35-45t/h, preferably 38-43 t/h.
According to some embodiments of the invention, the process B may further comprise: when the density and melt index of polymer product-2 reached the set point-2, the flow of isopentane was adjusted so that the dew point temperature was maintained at 45-55 deg.C, preferably 50-52 deg.C.
In the process A and the process B, the 1-butene concentration is established step by step, so that the rapid accumulation of the 1-butene impurity content can be effectively avoided, the operation risk that the 1-butene concentration is easy to have large electrostatic fluctuation when being established rapidly is reduced, and the reactor can run stably when polyethylene is switched and polymerized.
By producing the PE-ML-57D075 polyethylene product, the invention reduces the risk of electrostatic fluctuation caused by 1-butene introduction, and simultaneously reduces the amount of transition materials when switching from producing the PE-ML-63D082 polyethylene product to producing the PE-L-FB-20D20 polyethylene product. In the present invention, preferably, the process of switching from the production of PE-ML-63D082 polyethylene to the polymerization to obtain PE-ML-57D075 polyethylene can be carried out in a gas phase fluidized bed polyethylene plant, and the adjustment of the parameters can be carried out by stepwise adjustment, for example, the step a can comprise:
(A1) closing the regulating valve for discharging the circulating gas of the reactor to the degassing bin, opening the valve for introducing the nitrogen into the reactor, regulating the flow rate of the nitrogen to 400kg/h, reducing the ethylene partial pressure from 1250kPa to 1100 kPa 1150kPa at the descending speed of 20-100kPa/h, then closing the valve for introducing the nitrogen into the reactor, regulating the flow rate of discharging the circulating gas of the reactor to the degassing bin, and maintaining the ethylene partial pressure to 1100 kPa 1150 kPa; adjusting the hydrogen feed to control the hydrogen/ethylene molar ratio to be 0.38-0.42 mol/mol;
(A2) when the ethylene partial pressure is reduced to 1100-1150kPa, 1-butene is introduced into the reactor at an initial flow rate of 50-200kg/h, the flow rate of 1-butene is increased, and the 1-butene/ethylene molar ratio is controlled to be 0.009-0.013; wherein the initial flow rate is preferably 95-105 kg/h;
(A3) after the 1-butene is introduced in the step (A2), reducing the hydrogen flow entering the reactor to 1.5-2.5kg/h, and controlling the hydrogen/ethylene molar ratio to be 0.32-0.38 mol/mol;
(A4) analyzing the density and the melt index of a polymer product-1 obtained by polymerization, controlling the analysis frequency of the density to be 2-4 hours/time, and controlling the analysis frequency of the melt index to be 1-3 hours/time; and continuously adjusting the hydrogen/ethylene molar ratio and the 1-butylene/ethylene molar ratio until the density and the melt index of the polymer product-1 reach set values of-1, and switching to polymerization to obtain the PE-ML-57D075 polyethylene.
(A5) The flow of isopentane into the reactor was adjusted to maintain the dew point temperature at 50-55 ℃.
(A6) The flow rate of the catalyst is adjusted, the yield of the polyethylene is controlled to be 38-42t/h, and the inlet temperature of the reactor is controlled to be 39-45 ℃.
In the invention, after switching to polymerization to obtain PE-ML-57D075 polyethylene, entering a granulation stage of PE-ML-57D075 polyethylene products, and adding an additive of the PE-ML-57D075 polyethylene products; wherein the PE-ML-57D075 polyethylene product additive can comprise: beta- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionic acid octadecyl ester (short for antioxidant 1076) and zinc stearate.
In the present invention, preferably, the PE-ML-57D075 polyethylene product additive can be added in an amount of 0.1-0.5kg/t, preferably 0.3-0.4kg/t, based on the weight of the PE-ML-57D075 polyethylene powder; wherein, the weight ratio of beta- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionic acid octadecyl alcohol ester (antioxidant 1076 for short) and zinc stearate can be 1: (0.5-1.5), preferably 1: (0.8-1.2).
In the present invention, in the step (A2), after the 1-butene is introduced, it is preferable to stop the hydrogen feeding to the reactor, reduce the hydrogen/ethylene molar ratio to 0.37 to 0.38mol/mol, and then gradually resume the hydrogen feeding to make the hydrogen/ethylene molar ratio 0.32 to 0.38 mol/mol; meanwhile, the fluctuation range of the static electricity index of the reactor is ensured to be between minus 500V and plus 200V, if the fluctuation range is lower than minus 500V or higher than plus 200V, the introduction of 1-butene is stopped, the content of impurities in the 1-butene is analyzed, and the reason of large static electricity fluctuation is found.
In the present invention, it is preferred that in the step (A2), the rate of increase in the flow rate of 1-butene is 50 to 100kg/30 min.
In the present invention, preferably, the process of switching from the production of PE-ML-57D075 polyethylene to the polymerization to obtain PE-L-FB-20D20 polyethylene can be carried out in a gas-phase fluidized bed polyethylene device, and the adjustment of each parameter can be carried out by stepwise adjustment, for example, the step B can comprise:
(B1) fully opening nitrogen to enter a reactor valve, and controlling the flow of the nitrogen to be 360-440 kg/h; the dry gas seal of the circulating gas compressor is switched from ethylene to nitrogen, and the flow rate of the dry gas seal is controlled to be 200-300 kg/h; reducing the ethylene partial pressure from 1100-1150kPa to 850-900kPa at a descending speed of 20-100 kPa/h; closing a valve of hydrogen entering the reactor, controlling the flow of the butylene entering the reactor to be 100-plus 200kg/h, and controlling the molar ratio of 1-butylene/ethylene in the circulating gas to be 0.011-0.014 mol/mol; the flow rate of the recycle gas discharged to the degassing bin of the reactor is adjusted to be 200-600kg/h, so that the molar ratio of the hydrogen to the ethylene is 0.32-0.34 mol/mol;
(B2) modifying the mol ratio of the first cocatalyst to the tetrahydrofuran from 0.15-0.19mol/mol to 0.25-0.3 mol/mol;
(B3) 2-5h after step (B2) is performed, introducing a second cocatalyst, and setting the molar ratio of the second cocatalyst to tetrahydrofuran to be 0.4-0.45 mol/mol;
(B4) 3-5h after the step (B2) is carried out, reducing the polymerization temperature from 95-99 ℃ to 85-90 ℃;
(B5) when the polymerization temperature is reduced to 94-96 ℃ and the ethylene partial pressure is lower than 900kPa, the flow rate of 1-butene entering the reactor is increased to 500-1000kg/h at the speed of 300-600 kg/h;
(B6) reducing the total pressure set point of the reactor from 2300-2350kPa to 2150-2200kPa at a descending speed of 20-100kPa/10 min; increasing the flow rate of the reactor discharged to the degassing bin to 1500-; preferably the increasing speed is 1500-2000 kg/h;
(B7) increasing the reactor level set value from 14.8-15.2m to 15.5-16m at a speed of 0.09-0.12 m/h;
(B8) gradually increasing the opening of the guide vanes, and increasing the apparent gas velocity from 0.64-0.65m/s to 0.69-0.71 m/s;
(B9) adjusting the 1-butene/ethylene molar ratio according to the polymerization temperature so that the 1-butene/ethylene molar ratio is finally controlled to be 0.29-0.33 mol/mol;
(B10) when the hydrogen/ethylene molar ratio is reduced to 0.15-0.17mol/mol, the hydrogen feeding of the reactor is recovered, the nitrogen is closed to enter a valve of the reactor, the dry gas seal is switched from the nitrogen to the ethylene, and the reactor is closed to a degassing bin discharge valve; controlling the hydrogen/ethylene molar ratio to 0.15-0.17 mol/mol;
(B11) analyzing the density and the melt index of the polymer product-2, wherein the analysis frequency of the density is controlled to be 2-4 hours/time, and the analysis frequency of the melt index is controlled to be 1-3 hours/time; and continuously adjusting the hydrogen/ethylene molar ratio and the 1-butene/ethylene molar ratio until the density and the melt index of the polymer product-2 reach set values of-2, and switching to polymerization to obtain the PE-L-FB-20D20 polyethylene.
(B12) The flow of isopentane into the reactor was adjusted to maintain the dew point temperature at 50-52 ℃.
(B13) The flow rate of the catalyst is adjusted, the yield of the polyethylene is controlled to be 38-43t/h, and the inlet temperature of the reactor is controlled to be 39-45 ℃.
In the invention, after switching to polymerization to obtain PE-L-FB-20D20 polyethylene, entering a granulation stage of a PE-L-FB-20D20 polyethylene product, and adding a PE-L-FB-20D20 polyethylene product additive; wherein the PE-L-FB-20D20 polyethylene product additive may comprise: the addition proportion of beta- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionic acid octadecyl ester (antioxidant 1076 for short), phosphorous acid tri (2, 4-di-tert-butylphenyl) ester (antioxidant 168 for short) and zinc stearate is 0.25-0.30kg/t and antistatic agent 1800(N, N-dihydroxyethyl octadecyl amine).
In the invention, preferably, the addition amount of the PE-L-FB-20D20 polyethylene product additive can be 1-2kg/t, preferably 1.2-1.8kg/t, based on the weight of the PE-L-FB-20D20 polyethylene powder; wherein, the weight ratio of beta- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionic acid octadecyl ester (antioxidant 1076 for short), phosphorous acid tri (2, 4-di-tert-butylphenyl) ester (antioxidant 168 for short), zinc stearate and antistatic agent 1800(N, N-dihydroxyethyl octadecyl amine) can be 1: (1.5-2.5): (0.8-1.2): (0.8-1.2), preferably 1: (1.8-2.2): (0.9-1.1): (0.9-1.1).
In the present invention, in the step (B4), the step of lowering the polymerization temperature from 95 to 99 ℃ to 80 to 90 ℃ comprises: the polymerization temperature is reduced from 95-99 ℃ to 90-92 ℃ at the speed of 1.5-2.5 ℃/h, and then the polymerization temperature is reduced from 90-92 ℃ to 85-87 ℃ at the speed of 4-6 ℃/h.
According to some embodiments of the present invention, before step (B9), the method may further include: adjusting the flow rate of the reactor discharged to the degassing bin to ensure that the ethylene partial pressure is stabilized at 650-690 kPa; preferably, when the ethylene partial pressure of the reactor reaches 700-800kPa, the flow rate of the ethylene discharged to the degassing bin of the reactor is reduced to 500-700kg/h, and the ethylene partial pressure of the reactor is further reduced to 650-690 kPa; when the ethylene partial pressure of the reactor is reduced to 650-690kPa, the flow rate of the ethylene discharged to the degassing bin of the reactor is increased to 700-690 kg/h, and the ethylene partial pressure is maintained to be stable at 650-690 kPa.
According to some embodiments of the present invention, the step of step (B9) may comprise increasing the flow rate of 1-butene into the reactor to 1500-2000kg/h at a rate of 300-1000kg/h after the polymerization temperature is reduced to 89-90 ℃ so that the 1-butene/ethylene molar ratio is increased to 0.05-0.15 mol/mol; when the polymerization temperature is reduced to 87-88 ℃, the flow rate of 1-butene entering the reactor is increased to 2500-3500kg/h at the speed of 700-1500kg/h, so that the molar ratio of 1-butene/ethylene is increased to 0.15-0.25 mol/mol; when the polymerization temperature is reduced to 85-86 ℃, the flow rate of 1-butene entering the reactor is gradually increased to 4000-4500kg/h at the speed of 700-1500kg/h, so that the molar ratio of 1-butene/ethylene is increased to 0.3-0.33 mol/mol.
In the present invention, the hydrogen partial pressure can also be reduced to a minimum of 280kPa, preferably by producing the PE-ML-57D075 polyethylene product. The hydrogen partial pressure was about 505kPa for the normal production of PE-ML-63D082 polyethylene product and about 110kPa for the production of PE-L-FB-20D20 polyethylene product. Therefore, when the PE-ML-57D075 polyethylene product is switched to the PE-L-FB-20D20 polyethylene product, the requirement can be met by reducing the hydrogen partial pressure by 170kPa, which is far lower than the hydrogen partial pressure (395kPa) required to be reduced when the PE-ML-63D082 polyethylene product is switched to the PE-L-FB-20D20 polyethylene product; therefore, the invention further shortens the transition time and reduces the transition material amount by producing the polyethylene product with the middle grade.
In the present invention, preferably, in order to cope with the increase of the polyethylene transition material amount caused by the post-processing of the density analysis data, the following method can be adopted to assist in judging the product density and guide the switching of the bins in the granulation stage: adjusting the load of a granulator to ensure that the material level of the degassing bin is always maintained at 20-60% when the polymerization switching of polyethylene starts to finish; the amount of the powder of the buffered polyethylene product is 170-210t corresponding to 20-60% of the degassing bin material level; when the granulator is loaded at 37.5t/h, the analysis result of the density of the pelletized polyethylene product lags behind the analysis result of the density of the powder of the polyethylene product by about 4.5 to 5.5 h. Therefore, when the powder density of the polyethylene product reaches the set value of the invention, the switching of the storage bins after 4-5h can be assisted and guided.
In the invention, in order to deal with the increase of the transition material amount caused by the post-analysis of the analysis data, the following method can be used for assisting in judging the product quality and guiding the switching of the storage bins: when the temperature of the melting pump bearing of the reactor rises from 250-260 ℃ to 280-290 ℃, and the pressure in front of the template of the reactor rises from 3-5MPa to 7-9MPa, the property of the polyethylene product is close to that of the PE-L-FB-20D020 polyethylene product.
In the invention, by skillfully designing a polymerization switching strategy of polyethylene, a specific middle-grade polyethylene product (PE-ML-57D075 polyethylene) is used as transition, and the invention aims of less transition material generation amount, small static fluctuation and stable reactor in the process are finally realized by matching with the adjustment of the reactor temperature and the specific gradual adjustment method of parameters such as ethylene partial pressure, hydrogen/ethylene molar ratio, 1-butylene/ethylene molar ratio and the like in the two processes (process A and process B).
In the present invention, polyethylene product designations are made according to GBT 1845.1-1999 Polyethylene (PE) molding and extrusion materials part 1 naming system and classification base.
The present invention will be described in detail below by way of examples.
In the following examples, the test standards for each physical property index are:
and (3) determination of melt index: GB/T3682-2018;
and (3) density measurement: GB/T1033.2-2010;
determination of tensile yield stress: GB/T1040.2-2006;
and (3) measuring the impact strength of the simply supported beam opening: GB/T1043.1-2008;
tensile fracture nominal strain determination: GB/T1040.2-2006;
haze measurement: GB/T2410-2008;
fish eye determination: GB/T6595-.
Examples
The device adopted in the invention has a process flow diagram as shown in figure 1.
In the embodiment, ethylene and hydrogen are synthesized into polyethylene by a gas-phase fluidized bed reactor under the combined action of a catalyst and a cocatalyst; mainly comprising a first polymerization stage and a second polymerization stage;
the first polymerization stage comprises the following process a switching from producing PE-ML-63D082 polyethylene product to producing PE-ML-57D075 polyethylene product:
(A1) closing an adjusting valve for discharging the circulating gas of the reactor to a degassing bin, opening a valve for introducing nitrogen into the reactor, adjusting the flow rate of nitrogen to 300kg/h, reducing the ethylene partial pressure from 1250kPa to 1150kPa at a descending speed of 25kPa/h, then closing the valve for introducing nitrogen into the reactor, adjusting the flow rate of discharging the circulating gas of the reactor to the degassing bin, and maintaining the ethylene partial pressure to 1150 kPa; adjusting hydrogen feeding, and controlling the hydrogen/ethylene molar ratio to be 0.41 mol/mol;
(A2) when the ethylene partial pressure is reduced to 1150kPa, 1-butene is introduced into the reactor at an initial flow of 200kg/h, and the 1-butene/ethylene molar ratio is controlled to be 0.01 mol/mol;
(A3) after 1-butene is introduced in the step (A2), reducing the hydrogen flow entering the reactor to 2kg/h, increasing the flow of the 1-butene, and controlling the hydrogen/ethylene molar ratio to be 0.35 mol/mol;
(A4) analyzing the density and the melt index of a polymer product-1 obtained by polymerization, and controlling the analysis frequency of the density to be 3 hours/time and the analysis frequency of the melt index to be 2 hours/time; continuously adjusting the hydrogen/ethylene molar ratio and the 1-butylene/ethylene molar ratio at the same time until the density and the melt index of the polymer product-1 reach set values of-1, switching to polymerization to obtain PE-ML-57D075 polyethylene, and polymerizing to obtain PE-ML-57D075 polyethylene;
(A5) the flow of isopentane into the reactor was adjusted to maintain the dew point temperature at 50 ℃.
(A6) The flow rate of the catalyst is adjusted, the yield of the polyethylene is controlled to be 38-42t/h, and the inlet temperature of the reactor is controlled to be 45 ℃.
After switching to polymerization to obtain PE-ML-57D075 polyethylene, entering a granulation stage of PE-ML-57D075 polyethylene products, and adding additives of the PE-ML-57D075 polyethylene products; wherein, the PE-ML-57D075 polyethylene product additives comprise an antioxidant 1076 and zinc stearate, and the adding amounts of the antioxidant 1076 and the zinc stearate are respectively 0.15 kg/t.
The second polymerization stage comprises a switch from the production of PE-ML-57D075 polyethylene to Process B, which produces PE-L-FB-20D20 polyethylene;
(B1) fully opening nitrogen to enter a reactor valve, and controlling the flow of the nitrogen to be 400 kg/h; the dry gas seal of the circulating gas compressor is switched from ethylene to nitrogen, and the flow rate of the dry gas seal is controlled to be 250 kg/h; reducing the ethylene partial pressure from 1150kPa to 900kPa at a reduction rate of 50 kPa/h; closing a valve of hydrogen entering the reactor, controlling the flow of butylene entering the reactor to be 200kg/h, and controlling the molar ratio of 1-butylene/ethylene in the circulating gas to be 0.014 mol/mol; adjusting the flow rate of the circulating gas discharged to the degassing bin of the reactor to be 500kg/h, so that the molar ratio of hydrogen to ethylene is 0.33 mol/mol;
(B2) modifying the molar ratio of tri-n-hexylaluminum to tetrahydrofuran from 0.17mol/mol to 0.27 mol/mol;
(B3) introducing diethyl aluminum chloride 3h after the step (B2) is performed, and setting the molar ratio of diethyl aluminum chloride to tetrahydrofuran to be 0.4 mol/mol;
(B4) 4 hours after the step (B2) is carried out, reducing the polymerization temperature from 97 ℃ to 90 ℃ at the speed of 2 ℃/h, and then reducing the polymerization temperature from 90 ℃ to 86 ℃ at the speed of 5 ℃/h;
(B5) when the polymerization temperature had dropped to 95 ℃ and the ethylene partial pressure had fallen below 900kPa, the flow of 1-butene into the reactor was increased to 1000kg/h at a rate of 500 kg/h;
(B6) reducing the reactor total pressure set point from 2350kPa to 2150kPa at a rate of decrease of 50kPa/10 min; the flow rate of the reactor discharged to the degassing bin is increased to 2000kg/h at an increasing speed of 1000 kg/h;
(B7) increasing the reactor level set point from 15m to 15.8m at a rate of 0.1 m/h;
(B8) gradually increasing the opening of the guide vanes and increasing the apparent gas velocity from 0.64m/s to 0.69 m/s; adjusting the flow rate of the reactor discharged to the degassing bin to ensure that the ethylene partial pressure is stabilized at 650-690 kPa;
(B9) when the polymerization temperature is reduced to 90 ℃, the flow rate of 1-butene entering the reactor is increased to 2000kg/h at the speed of 1000kg/h, so that the molar ratio of 1-butene/ethylene is increased to 0.1 mol/mol; when the polymerization temperature is reduced to 88 ℃, the flow rate of 1-butene entering the reactor is increased to 3500kg/h at an increasing speed of 1500kg/h, so that the molar ratio of 1-butene/ethylene is increased to 0.2 mol/mol; after the polymerization temperature had dropped to 86 ℃, the flow of 1-butene into the reactor was increased to 4500kg/h at a rate of 1500kg/h, so that the 1-butene/ethylene molar ratio was increased to 0.33 mol/mol;
(B10) when the hydrogen/ethylene molar ratio is reduced to 0.17mol/mol, the hydrogen feeding of the reactor is recovered, the nitrogen is closed to enter a valve of the reactor, the dry gas seal is switched from the nitrogen to the ethylene, and the reactor is closed to a degassing bin discharge valve; controlling the hydrogen/ethylene molar ratio to 0.17 mol/mol;
(B11) analyzing the density and the melt index of the polymer product-2, wherein the analysis frequency of the density is controlled to be 3 hours/time, and the analysis frequency of the melt index is controlled to be 2 hours/time; continuously adjusting the hydrogen/ethylene molar ratio and the 1-butylene/ethylene molar ratio at the same time until the density and the melt index of the polymer product-2 reach set values of-2, and polymerizing to obtain PE-L-FB-20D20 polyethylene;
(B12) the flow of isopentane into the reactor was adjusted to maintain the dew point temperature at 50 ℃.
(B13) The flow rate of the catalyst is adjusted, the yield of the polyethylene is controlled to be 38-43t/h, and the inlet temperature of the reactor is controlled to be 45 ℃.
After the PE-L-FB-20D20 polyethylene is obtained by switching and polymerizing, the PE-L-FB-20D20 polyethylene product is added with the PE-L-FB-20D20 polyethylene product additive in the granulation stage; the PE-L-FB-20D20 polyethylene product additive comprises antioxidant 1076, antioxidant 168, zinc stearate and antistatic agent 1800; wherein, the addition amounts of the antioxidant 1076, the antioxidant 168, the zinc stearate and the antistatic agent 1800 are respectively 0.3kg/t, 0.6kg/t, 0.25kg/t and 0.25 kg/t.
In this example, the transition material amount 257t is generated by switching polyethylene.
The standards for the PE-ML-63D082 polyethylene product, the PE-ML-57D075 polyethylene product and the PE-L-FB-20D020 polyethylene product prepared in this example are shown in tables 1-3 respectively;
TABLE 1 PE-ML-63D082 polyethylene product Standard
TABLE 2 PE-ML-57D075 polyethylene product Standard
TABLE 3 PE-L-FB-20D020 polyethylene product Standard
The results show that the polymerization switching method for producing polyethylene and the stock bin switching strategy provided by the invention can be used for switching the polyethylene products in a continuous mode and a condensation mode, so that the transition material amount is reduced from about 600t to about 260t, the time required by the production switching of the polyethylene products is greatly shortened, and the manpower and material resources are saved.
The preferred embodiments of the present invention have been described above in detail, but the present invention is not limited thereto. Within the scope of the technical idea of the invention, many simple modifications can be made to the technical solution of the invention, including combinations of various technical features in any other suitable way, and these simple modifications and combinations should also be regarded as the disclosure of the invention, and all fall within the scope of the invention.
Claims (13)
1. A polymerization switch-over process for the production of polyethylene comprising effecting a switch-over from the production of PE-ML-63D082 polyethylene to the first and second polymerization stages of PE-L-FB-20D20 polyethylene; wherein the first polymerization stage comprises process A switching from producing PE-ML-63D082 polyethylene to producing PE-ML-57D075 polyethylene and the second polymerization stage comprises process B switching from producing PE-ML-57D075 polyethylene to producing PE-L-FB-20D20 polyethylene;
the process A comprises the following steps: in the presence of hydrogen, a catalyst and a first cocatalyst, ethylene is subjected to homopolymerization to produce PE-ML-63D082 polyethylene; adjusting the ethylene partial pressure and the hydrogen/ethylene molar ratio, introducing 1-butene and adjusting the 1-butene/ethylene molar ratio to ensure that the obtained polymer product-1 is switched to be polymerized to obtain PE-ML-57D075 polyethylene when the density and the melt index of the obtained polymer product-1 reach set values of-1;
the process B comprises the following steps: in the process of polymerizing to obtain PE-ML-57D075 polyethylene in the presence of hydrogen, ethylene, 1-butene, a catalyst and a first cocatalyst, introducing a second cocatalyst, adjusting the ethylene partial pressure, the 1-butene/ethylene molar ratio, the hydrogen/ethylene molar ratio, the polymerization temperature and the polymerization total pressure to switch to polymerizing to obtain PE-L-FB-20D20 polyethylene when the density and the melt index of an obtained polymer product-2 reach set values of-2;
the conditions of the homopolymerization reaction include: the temperature is 95-104 ℃; the pressure is 2300-2380 kPa; the ethylene partial pressure is 1150-1300 kPa; the hydrogen/ethylene molar ratio is 0.38-0.45 mol/mol; the flow rate of the catalyst is 8-10 kg/h;
the conditions for the polymerization to give PE-ML-57D075 polyethylene include: the temperature is 95-104 ℃; the pressure is 2300-2380 kPa; the ethylene partial pressure is 1000-1200 kPa; the hydrogen/ethylene molar ratio is 0.3-0.39 mol/mol; the 1-butene/ethylene molar ratio is 0.01-0.018 mol/mol;
the conditions for the polymerization to obtain PE-L-FB-20D20 polyethylene include: the temperature is 85-89 ℃; the pressure is 2000-2300 kPa; the ethylene partial pressure is 600-700 kPa; the hydrogen/ethylene molar ratio is 0.14-0.18 mol/mol; the 1-butene/ethylene molar ratio is 0.25-0.35 mol/mol;
the catalyst is selected from Ziegler-Natta slurry catalysts; wherein the content of titanium element in the Ziegler-Natta slurry catalyst is 2-3 wt%, and the content of magnesium element is 6-8 wt%;
the first cocatalyst is tri-n-hexyl aluminum;
the second cocatalyst is diethyl aluminum monochloride.
2. The method of claim 1, wherein the conditions of the homopolymerization reaction include: the temperature is 96-98 ℃; the pressure is 2300-2350 kPa; the ethylene partial pressure is 1200-1250 kPa; the hydrogen/ethylene molar ratio is 0.41-0.42 mol/mol; the flow rate of the catalyst is 8-8.5 kg/h.
3. The process of claim 1, wherein the catalyst is selected from the group consisting of ziegler-natta slurry catalysts; wherein the content of titanium element in the Ziegler-Natta slurry catalyst is 2.2-2.5 wt%, and the content of magnesium element is 6.5-7 wt%.
4. The method of any of claims 1-3, wherein the set point of-1 comprises: the polymer product-1 had a density of 0.955 to 0.961g/cm3(ii) a The polymer product-1 has a melt index at 190 ℃ and a load of 2.16kg of 5-10g/10 min.
5. The method of any of claims 1-3, wherein the set point of-1 comprises: the density of the polymer product-1 is 0.956 to 0.96g/cm3(ii) a The polymer product-1 has a melt index at 190 ℃ and a load of 2.16kg of 6-9g/10 min.
6. The process according to any one of claims 1 to 3, wherein the conditions for the polymerization to obtain PE-ML-57D075 polyethylene comprise: the temperature is 96-98 ℃; the pressure is 2300-2350 kPa; the ethylene partial pressure is 1100-1150 kPa; the hydrogen/ethylene molar ratio is 0.32-0.38 mol/mol; the 1-butene/ethylene molar ratio is 0.012-0.016 mol/mol.
7. The method of any of claims 1-3, wherein the set-point of-2 comprises: the density of the polymer product-2 is 0.917-0.923g/cm3(ii) a The polymer product-2 has a melt index at 190 ℃ and a load of 2.16kg of 1.5 to 2.5g/10 min.
8. The method of any of claims 1-3, wherein the set-point of-2 comprises: the density of the polymer product-2 is 0.918-0.922g/cm3(ii) a The polymer product-2 has a melt index at 190 ℃ and a load of 2.16kg of 1.7 to 2.3g/10 min.
9. The process of any one of claims 1-3, wherein the conditions under which the polymerization results in PE-L-FB-20D20 polyethylene comprise: the temperature is 85-87 ℃; the pressure is 2100-2200 kPa; the ethylene partial pressure is 650-690 kPa; the hydrogen/ethylene molar ratio is 0.15-0.17 mol/mol; the 1-butene/ethylene molar ratio is between 0.29 and 0.33 mol/mol.
10. The method of any of claims 1-3, wherein the process A further comprises: when the density and the melt index of the polymer product-1 reach set values of-1, adjusting the flow rate of the catalyst to ensure that the yield of the PE-ML-57D075 polyethylene obtained by polymerization is 35-45 t/h;
and/or, the process a further comprises: when the density and melt index of polymer product-1 reached the set point-1, the flow of isopentane was adjusted so that the dew point temperature was maintained at 50-60 ℃.
11. The method of any of claims 1-3, wherein the process A further comprises: when the density and the melt index of the polymer product-1 reach the set value of-1, adjusting the flow rate of the catalyst to ensure that the yield of the PE-ML-57D075 polyethylene obtained by polymerization is 38-42 t/h;
and/or, the process a further comprises: when the density and melt index of polymer product-1 reached the set point-1, the flow of isopentane was adjusted so that the dew point temperature was maintained at 50-55 ℃.
12. The method of any one of claims 1-3, wherein B further comprises: when the density and the melt index of the polymer product-2 reach set values of-2, adjusting the flow rate of the catalyst to ensure that the yield of the PE-L-FB-20D20 polyethylene obtained by polymerization is 35-45 t/h;
and/or, the process B further comprises: when the density and melt index of polymer product-2 reached the set point-2, the flow of isopentane was adjusted so that the dew point temperature was maintained at 45-55 ℃.
13. The method of any one of claims 1-3, wherein B further comprises: when the density and the melt index of the polymer product-2 reach set values of-2, adjusting the flow rate of the catalyst to ensure that the yield of the PE-L-FB-20D20 polyethylene obtained by polymerization is 38-43 t/h;
and/or, the process B further comprises: when the density and melt index of polymer product-2 reached the set point-2, the flow of isopentane was adjusted so that the dew point temperature was maintained at 50-52 ℃.
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