CN115490786A - Method for recovering solvent and dehydrating by solution polymerization process - Google Patents
Method for recovering solvent and dehydrating by solution polymerization process Download PDFInfo
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- CN115490786A CN115490786A CN202211164564.XA CN202211164564A CN115490786A CN 115490786 A CN115490786 A CN 115490786A CN 202211164564 A CN202211164564 A CN 202211164564A CN 115490786 A CN115490786 A CN 115490786A
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Abstract
The invention provides a method for recovering solvent and dehydrating in a solution polymerization process, which comprises the steps of taking out a first polymer solution obtained by solution polymerization reaction, mixing the first polymer solution with a first polymerization reaction deactivator and then conveying the mixture to a first separator; withdrawing from the first separator a first vapor stream and a second polymer solution, the first vapor stream being recycled to the polymerization reactor; mixing the second polymer solution with a second polymerization reaction deactivator, conveying the mixture to a separation system formed by connecting a plurality of stages of gas-liquid separators in series, and conveying steam flows obtained by the gas-liquid separators of all stages to a first fractionator; obtaining a first overhead stream and a first bottoms stream from the first fractionator, the first bottoms stream portion being recycled to the polymerization reactor as a second recycle stream; the mass content of water in both the first recycle stream and the second recycle stream is less than 1ppm. The invention controls the consumption of the deactivator to timely inactivate the polymerization reaction so as to control the stable performance of the polymer product and lower the process cost.
Description
Technical Field
The invention relates to a method for recovering solvent and dehydrating by a solution polymerization process, in particular to a method for removing contained water from volatile hydrocarbon separated and recovered from a polymer solution by using water as a polymerization reaction deactivator.
Background
In olefin polymerization processes, the catalyst must be deactivated after the polymerization reaction to avoid continued reaction and thereby uncontrolled product properties, which is usually accomplished by the addition of deactivating agents, typically including alcohols such as methanol, isopropanol, and water in vapor or liquid form. The polymer solution has a certain residence time in the separator, and the deactivator is usually added before entering the separation system downstream of the reactor, i.e., before being removed from the polymer solution, and such an addition sequence may also result in the deactivator being contained in the recovered solvent, which is then refined accordingly before being reused to remove the deactivator.
Patent US 20110172375A1 discloses a method using methanol as a deactivator in a solution polymerization process, wherein methanol in a recovered solvent needs to be removed through a drying bed, and can be removed by using a 4A molecular sieve, and can also be removed by using a 3A molecular sieve to simultaneously remove water and methanol. Patent CN 1176257A proposes a method for terminating gas phase polymerization by adding carbon monoxide, carbon dioxide, oxygen, water, alcohols containing 1 to 6 carbons or aldehydes containing 1 to 6 carbons as a deactivator above a gas distribution plate, and the deactivator is added in a gaseous state. Patent CN 101998967B discloses a method of using a liquid deactivator including water/alcohol or a solid deactivator including sodium stearate/calcium stearate, and also mentions that if the deactivator has volatility, the amount to be added is determined according to the monomer, the recovery method of the solvent and the product characteristics, but the patent does not specify the method.
In addition to the method of deactivating by adding a deactivator, patent WO 2021136629A1 proposes a method of deactivating a catalyst by heating a polymer solution, the product of the heating temperature rise (. Degree. C.) of the polymer solution and the heating time (minutes) of the polymer solution being more than 0.05, preferably more than 0.1, more preferably more than 0.5. The problem of introducing a deactivator in the recovered solvent can be avoided by a method of deactivating the catalyst by heating the polymer solution. However, the properties of the polymer are greatly affected by the temperature, and heating the polymer solution may result in failure to obtain the product required for the process, or even thermal cracking of the polymer product. Meanwhile, heating the polymer solution also leads to an increase in energy consumption of the process, and is poor in economical efficiency.
The deactivation method by polymerization with addition of the deactivator is still a highly feasible method, but the addition method of the deactivator, the flow rate of addition, and the separation method of the deactivator still have room for improvement. The invention provides a method for using water as a deactivator, controlling the addition amount and the addition position of the deactivator and removing water by an azeotropic distillation tower, and a circulating solvent with the water content of less than 1ppm can be obtained.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a method for recovering a solvent and dehydrating by a solution polymerization process.
The technical scheme of the invention is as follows:
the invention firstly provides a method for recovering solvent and dehydrating by a solution polymerization process, which comprises the following steps:
1) Carrying out solution polymerization reaction in a polymerization reactor to obtain a first polymer solution; taking out the first polymer solution, mixing the first polymer solution with a first polymerization reaction deactivator, and conveying the mixture to a first separator;
2) Withdrawing from the first separator a first vapor stream and a second polymer solution, the first vapor stream being passed to a first heat recovery device to obtain a first recycle stream, the first recycle stream being passed to the polymerization reactor;
3) Mixing the second polymer solution with a second polymerization reaction deactivator, and conveying the mixture to a separation system formed by connecting a plurality of stages of gas-liquid separators in series, wherein in the separation system, each stage of gas-liquid separator obtains steam flow as a gas-phase product, and a liquid phase obtained by separation of the previous stage of gas-liquid separator is fed into the next stage of gas-liquid separator as a feed for gas-liquid separation;
4) The steam obtained by each stage of gas-liquid separator is conveyed to the first fractionator after heat recovery; the liquid phase obtained by the separation of the last stage of gas-liquid separator is output as a concentrated polymer solution;
5) Obtaining a first overhead stream and a first bottoms stream from the first fractionator, recovering at least a portion of the first bottoms stream as a second recycle stream and sending it to the polymerization reactor;
the first polymerization reaction deactivator and the second polymerization reaction deactivator are both water, the mass content of the water in the first recycle stream and the second recycle stream is less than 1ppm, and the first fractionator is an azeotropic distillation tower.
As a preferable embodiment of the present invention, the amount of the deactivator in the first polymerization reaction added per unit time is less than the total amount of the deactivator required for the catalyst and the cocatalyst added per unit time, preferably 50 to 90% of the total amount (by mass) of the deactivator required for the catalyst and the cocatalyst added per unit time;
the mass flow rate of the second polymerization deactivator added is less than 5%, preferably less than 2%, more preferably less than 1% of the mass flow rate of the second polymer solution.
In a preferred embodiment of the present invention, the solvent and water used in the solution polymerization reaction have a binary azeotropic phenomenon; the solvent is selected from C4-C12 chain alkane or cycloalkane, C6-C9 aromatic hydrocarbon or a mixed solvent thereof, preferably C5-C8 chain alkane or cycloalkane.
As a preferred embodiment of the present invention, the first polymerization deactivator is added before the polymer solution enters the first separator, or before the polymer solution enters a heat exchanger upstream of the first separator; the second polymerization deactivator is added before the polymer solution enters the separation system, or before the polymer solution enters a heat exchanger upstream of the separation system.
In a preferred embodiment of the present invention, a cooler and a collecting tank are arranged on the top of the first fractionator, the collecting tank is used for collecting condensate, and at least a part of materials in the collecting tank is conveyed back to the first fractionator.
As a preferred embodiment of the present invention, the first separator is a flash unit. The separation system is two-stage, and comprises a second separator as a front stage and a third separator as a rear stage, wherein the second separator is a flash evaporation unit, and the third separator is a deep devolatilizer, preferably a falling film devolatilizer or a wiped film devolatilizer.
As a preferred variant of the invention, the first separator is operated at a pressure of from 5 to 16bar, preferably from 8 to 12bar; (ii) a The second separator is operated at a pressure of 1 to 5bar, preferably 1 to 3bar; the third separator is operated at a pressure of 0.1 to 1bar, preferably 0.1 to 0.5bar.
As a preferable scheme of the invention, in the step 3), a stripping agent is also added into the second polymer solution, the second separator and the third separator are separated in the presence of the stripping agent, and the stripping agent is steam, water and CO 2 Nitrogen gas; the addition amount of the stripping agent is 0-5% of the mass flow of the polymer solution.
As a preferable scheme of the invention, the polymerization reactor is in the form of a plurality of combined reactors, including a series connection of double reactors and a parallel connection of double reactors.
(1) The invention inactivates the polymerization reaction in time by accurately controlling the dosage of the deactivator, and overcomes the problem of unstable performance of the polymer product caused by excessive deactivator or untimely addition of the deactivator in the prior art. (2) The invention overcomes the problems of the deactivation agent in multiple places of the process stream in the prior art by controlling the adding position and the dosage of the deactivation agent, thereby reducing the treatment amount of the deactivation agent. (3) The method removes the deactivator by azeotropic distillation, overcomes the problem of high cost of deactivator removal in the prior art, and reduces economic cost.
Drawings
FIG. 1 is a flow chart of a dehydration process for recovering a solvent in a solution polymerization process according to an embodiment of the present invention.
Detailed Description
The invention will be further illustrated and described with reference to specific embodiments. The described embodiments are merely exemplary of the disclosure and are not intended to limit the scope thereof. The technical characteristics of the embodiments of the invention can be correspondingly combined without mutual conflict.
Preferred embodiments of the present invention will be described in more detail below. It should be understood by those skilled in the art that the present invention should not be limited by the embodiments set forth herein and the scope of the present invention is not limited to the examples described below. These embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
The method of the present invention will be described with reference to the schematic representation of fig. 1, which fig. 1 shows an exemplary flow for using the method of the present invention. Additional lines and/or additional equipment may be added to the system as needed, such as additional reactors in series and/or in parallel, and/or other devices for operating the system, such as valves and pumps.
As shown in FIG. 1, the present invention provides a method for dehydrating a recovered solvent in a solution polymerization process, the method comprising the steps of:
1) Carrying out solution polymerization reaction in a polymerization reactor to obtain a first polymer solution; taking out the first polymer solution, mixing the first polymer solution with a first polymerization reaction deactivator, and conveying the mixture to a first separator;
2) Withdrawing from the first separator a first vapor stream and a second polymer solution, the first vapor stream being passed to a first heat recovery device to obtain a first recycle stream, the first recycle stream being passed to the polymerization reactor;
3) Mixing the second polymer solution with a second polymerization reaction deactivator, and conveying the mixture to a separation system formed by connecting a plurality of stages of gas-liquid separators in series, wherein in the separation system, each stage of gas-liquid separator obtains steam flow as a gas-phase product, and a liquid phase obtained by separation of the previous stage of gas-liquid separator is fed into the next stage of gas-liquid separator as a feed for gas-liquid separation;
4) The steam obtained by each stage of gas-liquid separator is conveyed to the first fractionator after heat recovery; the liquid phase obtained by the separation of the last stage of gas-liquid separator is output as a concentrated polymer solution;
5) Obtaining a first overhead stream and a first bottoms stream from the first fractionator, recovering at least a portion of the first bottoms stream as a second recycle stream and sending it to the polymerization reactor;
the first polymerization reaction deactivator and the second polymerization reaction deactivator are both water, the mass content of the water in the first recycle stream and the second recycle stream is less than 1ppm, and the first fractionator is an azeotropic distillation tower.
In one embodiment of the invention, the separation system is divided into two stages, comprising a second separator 6 and a third separator 8. Under the design, the system device adopted for implementing the method mainly comprises a feeding unit 1, a polymerization reaction unit 2, a first heat exchanger 3, a first separator 4, a second heat exchanger 5, a second separator 6, a third heat exchanger 7, a third separator 8, a first heat recovery device 9, a second heat recovery device 10, a third heat recovery device 11 and a first fractionator 12.
In fig. 1, ethylene monomer, alpha-olefin comonomer, solvent, catalyst, cocatalyst and hydrogen are introduced into feed unit 1, wherein monomer, comonomer and solvent are from a first recycle stream and also from fresh feed 13; subsequently, it is conveyed to the polymerization reaction unit 2 through the line 15; a first polymer solution is taken out of the polymerization reaction unit 2 and sent to the first separator 4 through the line 16, the first heat exchanger 3 and the line 17, and before entering the first heat exchanger 3, the deactivator is injected through the line 32 and mixed with the polymer solution; obtaining a first vapor stream and a second polymer solution (a first concentrated polymer solution) from the first separator 4; the first steam stream is sent to the first heat recovery device 9 via line 18, obtaining a first recycle stream; the first recycle stream is conveyed to the feed unit 1 via line 31; the second polymer solution from the first separator 4 is sent to the second separator 6 through line 19, the second heat exchanger 5 and line 20; before the second polymer solution enters the second heat exchanger 5, the deactivator is injected through the line 33 and mixed with the second polymer solution; obtaining a second vapour stream and a third polymer solution (second concentrated polymer solution) from the second separator 6; the second vapor stream is sent to the second heat recovery device 10 via line 21 to obtain a first condensate; the third polymer solution from the second separator 6 is sent to the third separator 8 via line 22, the third heat exchanger 7 and line 23; obtaining a third vapor stream and a fourth polymer solution (a third concentrated polymer solution) from a third separator 8; the third vapor stream is conveyed to the third heat recovery unit 11 via line 24 to obtain a second condensate; the first condensate and the second condensate are fed to the first fractionator 12 via lines 26 and 27, respectively; a first overhead stream and a first underflow stream are obtained from the top of the first fractionator, the first overhead stream being passed via line 28 to a downstream separation unit, at least a portion of said first underflow stream being recovered as a second recycle stream, the first underflow stream in this embodiment being recycled to the feed unit 1 via line 29 for at least 20% of the mass flow rate of the stream, and the remainder of the first underflow stream being passed via line 30 to the downstream separation unit.
As noted above, the feed of the present invention comprises essentially ethylene monomer, alpha-olefin comonomer, solvent, catalyst, cocatalyst and hydrogen. As is known in the art, the copolymerization of ethylene and alpha-olefins is a process for producing polymers having properties of different molecular weights, densities, etc., such as ethylene and 1-octene for producing Linear Low Density Polyethylene (LLDPE), ethylene and 1-butene for producing High Density Polyethylene (HDPE). The alpha-olefin may be an alpha-olefin containing 3 to 12 carbons and the comonomer is typically a single alpha-olefin or two or more alpha-olefins. Copolymerization of ethylene with various alpha-olefins can produce multipolymers. As known in the art, the solvent is inert to the catalyst system and reactants and is stable during the reaction, and the solvent is selected from C4-C12 chain alkane or cycloalkane, C6-C9 aromatic hydrocarbon or their mixed solvent, preferably C5-C8 chain alkane or cycloalkane. N-hexane, cyclohexane and Isopar E are well known in the art and are widely used solvents. As is known in the art, solution polymerization requires large amounts of solvent to maintain the ethylene monomer, alpha-olefin comonomer, catalyst, molecular weight regulator, and polymer product in a single phase state in the reactor.
As is known in the art, the polymerization of ethylene monomer and alpha-olefin comonomer is carried out in the presence of a catalyst, which may be any catalyst known in the art to be suitable for the solution copolymerization of ethylene with alpha-olefin comonomer, including metallocene catalysts and Z-N catalysts, such as the procatalyst being a Constrained Geometry Catalyst (CGC) and the cocatalyst being Methylaluminoxane (MAO). As is known in the art, the addition of small amounts of a substance having a high chain transfer constant can reduce the molecular weight of the polymer, and a variety of suitable chain transfer agents are available, such as hydrogen.
Before entering the separation system, the polymer solution needs to be heated through a heat exchanger to obtain a better separation effect and remove volatile components in the polymer solution. Patent CN 10956187B describes a process using a spiral heat exchanger as a preheater for polymer solutions. In the present process, the heat exchanger may be a shell and tube heat exchanger comprising means for intensifying the flow.
As is known in the art, polymer solution separation requires multiple separation and recovery steps, typically involving multiple flashing, rectification, recycling, extrusion, etc., with the primary objective of devolatilizing the polymer solution to obtain a polymer that meets product requirements. Patent CN 106414509A discloses a solution polymerization method including two-stage flash devolatilization, in which heated polymer solution from the outlet of a heat exchanger is fed to a first-stage flash drum for separation, a top gas phase stream is sent to a heat recovery unit, and a bottom liquid phase stream is sent to a second-stage flash drum for continuous separation, and temperature is adjusted by a heat exchanger before being sent to the second-stage flash drum unit. The first separator in the present invention operates at a pressure of 5 to 16bar, preferably 8 to 12bar; the second separator is operated at a pressure of 1 to 5bar, preferably 1 to 3bar; the third separator is operated at a pressure of 0.1 to 1bar, preferably 0.1 to 0.5bar. In the present invention, the polymer solution is heated by a heat exchanger before entering the first separator, the second separator and the third separator to provide the energy required for the separation. In addition, the polymer solution is injected with a deactivator before entering the heat exchanger upstream of the first separator and the second separator.
The process of the present invention is further described below by reference to examples and comparative examples, which should be construed as merely illustrative and not a limitation of the present invention.
Example 1, carried out according to the process of the invention, the reactor feed composition comprises 13313.6kg/h ethylene, 8.3kg/h methane, 592.2kg/h ethane, 7455.6kg/h 1-octene, 3655.2kg/h 2-octene, 500kg/h octane, 58448.5kg/h solvent n-hexane, 0.625kg/h procatalyst CGC, 4.89kg/h cocatalyst MAO, 2.3kg/h hydrogen as molecular weight regulator. The feeding stream is divided into two streams, one stream is adjusted to-28 ℃ by a cooler, the other stream is adjusted to 50 ℃ by a heater, and the mixed feeding stream reaches the set-25 ℃. The reactor is an adiabatic kettle type reactor, a stirring part is arranged in the reactor, the reaction pressure is 40bar, the retention time is 10min, and the reaction temperature is 145.7 ℃. The reactor polymer solution contained 12500kg/h of polymer, the mass fraction being 14.9%.
The separation system adopts a three-stage devolatilization mode, the first stage is medium-pressure flash evaporation, the pressure is 16bar, the polymer solution is mixed with 1.9kg/h of deactivator before entering the first-stage flash evaporation tank, and the temperature is raised to 200 ℃ through a heat exchanger; conveying the first steam flow obtained by the first-stage flash tank to a first heat recovery device to obtain a first recycle flow, and conveying the first recycle flow to a polymerization reactor; the second stage is low-pressure flash evaporation, the pressure is 3bar, the polymer solution is mixed with 20kg/h of deactivator and 980kg/h of stripping agent before entering the second stage flash evaporation tank, and the temperature is raised to 220 ℃ through a heat exchanger; the third stage adopts a falling film devolatilizer with the pressure of 1.3bar, the falling film devolatilizer maintains the temperature of 190 ℃, and 500kg/h of stripping agent is introduced into the falling film devolatilizer; after the three-stage devolatilization, the polymer solution enters an extruder, the pressure of the extruder is 0.1bar, and the temperature is 200 ℃; and extruding and granulating to obtain the polymer. In an example, the feed stream to the reactor was obtained by mixing a recycle stream with a fresh stream at a recycle stream mass flow rate of 70976.5kg/h, wherein the first recycle stream mass flow rate was 35117.8kg/h, the second recycle stream mass flow rate was 8946.4kg/h, the other recycle streams were from other plants of the separation unit, and the water content of all recycle streams was less than 1ppm.
In example 1, an azeotropic distillation column was selected as the first fractionator; the mass flow into the azeotropic distillation column was 36399.0kg/h, containing 77.3kg/h water. In example 1, the number of plates of the azeotropic distillation column was 15, the feed plate was 1 st, and the operating pressure was 4.5bar. The mass ratio of the distillate at the tower top to the feed is 0.04, a condenser and a liquid separation tank are arranged at the tower top, and the temperature of the condenser isAnd at the temperature of 40 ℃, the liquid separation tank is an oil-water separation tank, the oil phase with lower water content completely reflows to the azeotropic distillation tower, and the water phase with higher water content is conveyed to a wastewater treatment unit. The mass of the stream obtained from the bottom of the azeotropic distillation tower is 35786.0kg/h, and the mass fraction of the water content is 2.66x10 -8 。
Comparative example 1 the same procedure as in example 1 was used except that the mass flow ratio of the overhead of the azeotropic distillation column to the feed stream was 0.02, and the procedure resulted in a water content of the column bottoms of 0.0015 by mass fraction.
Comparative example 2 the same process as in example 1 was conducted except that the azeotropic distillation tower was not provided and water in the recovered solvent was removed by providing a dry bed.
Table 1 shows the bottoms water content, reboiler and condenser utilities consumption for the azeotropic distillation columns of example 1 and comparative example 1. The utility project used by the azeotropic distillation tower is 160 ℃ saturated steam, and the heat value is 2111.6kJ/kg. In example 1, the reboiler energy consumption per kg of water is 118875kJ, 56.30kg of steam is needed for removing 1kg of water, and 12.95 yuan is needed for removing 1kg of water per ton of steam of 230 yuan. Example 1 and comparative example 1 comparison, marginal thermal utility consumption per 1kg more water removed was
Only 5.7kg steam/kg H are required 2 O, is far less than the average consumption of the comparative example 1, namely, the example 1 can obtain better dehydration effect by increasing the ratio of distillate to feed, and the average energy consumption of dehydration is lower.
The desiccant used in comparative example 2 is a molecular sieve desiccant widely used in industry, and can absorb 0.04kg of water per kg of desiccant; the unit price of the desiccant is 14 yuan/kg, the average service life is 3 years, the desiccant needs to be regenerated once in average 3 months, and the desiccant can be used for 12 periods in total. That is, the maximum absorption per kg of desiccant was 1 × 0.04 × 12=0.48kg. The cost of the desiccant required to remove 1kg of water is 1/0.48 × 14=29 yuan, which is much higher than the cost of example 1.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is specific and detailed, but not to be understood as limiting the scope of the present invention. It will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
Claims (10)
1. A method for recovering solvent for dehydration in a solution polymerization process, comprising the steps of:
1) Carrying out solution polymerization reaction in a polymerization reactor to obtain a first polymer solution; taking out the first polymer solution, mixing the first polymer solution with a first polymerization reaction deactivator, and conveying the mixture to a first separator;
2) Withdrawing from the first separator a first vapor stream and a second polymer solution, the first vapor stream being passed to a first heat recovery device to obtain a first recycle stream, the first recycle stream being passed to the polymerization reactor;
3) Mixing the second polymer solution with a second polymerization reaction deactivator, and conveying the mixture to a separation system formed by connecting a plurality of stages of gas-liquid separators in series, wherein in the separation system, each stage of gas-liquid separator obtains steam flow as a gas-phase product, and a liquid phase obtained by separation of the previous stage of gas-liquid separator is fed into the next stage of gas-liquid separator as a feed for gas-liquid separation;
4) The steam obtained by each stage of gas-liquid separator is conveyed to the first fractionator after heat recovery; the liquid phase obtained by the separation of the last stage of gas-liquid separator is output as a concentrated polymer solution;
5) Obtaining a first overhead stream and a first bottoms stream from the first fractionator, recovering at least a portion of the first bottoms stream as a second recycle stream and sending it to the polymerization reactor;
the first polymerization reaction deactivator and the second polymerization reaction deactivator are both water, the mass content of the water in the first recycle stream and the second recycle stream is less than 1ppm, and the first fractionator is an azeotropic distillation tower.
2. The solution polymerization process for recovering solvent from dehydration according to claim 1, characterized in that the amount of deactivator per unit time added in the first polymerization reaction is less than the total deactivator amount required for the catalyst and cocatalyst added per unit time;
the mass flow of the second polymerization deactivator added is less than 5% of the mass flow of the second polymer solution.
3. The method for recovering solvent for dehydration according to the solution polymerization process of claim 1, wherein the solvent and water used in the solution polymerization reaction have a binary azeotropic phenomenon; the solvent is selected from chain alkane or cycloalkane of C4-C12, aromatic hydrocarbon of C6-C9 or a mixed solvent thereof.
4. The process of claim 1, wherein the first polymerization deactivator is added before the polymer solution enters the first separator or before the polymer solution enters a heat exchanger upstream of the first separator; the second polymerization deactivator is added before the polymer solution enters the separation system, or before the polymer solution enters a heat exchanger upstream of the separation system.
5. The method of claim 1 wherein the first fractionator is topped with a cooler and a collection tank for collecting condensate and transporting at least a portion of the collection tank back to the first fractionator.
6. The method of claim 1, wherein the first separator is a flash unit.
7. The method of claim 6, wherein the separation system is two-stage, comprising a second separator as a front stage and a third separator as a back stage, the second separator being a flash unit and the third separator being a deep devolatilizer.
8. The process according to claim 7, characterized in that the first separator is operated at a pressure of 5 to 16 bar; the second separator is operated at a pressure of 1 to 5 bar; the third separator is operated at a pressure of 0.1 to 1 bar.
9. The method according to claim 1, wherein in step 3), a stripping agent is further added to the second polymer solution, the second separator and the third separator are separated in the presence of the stripping agent, and the stripping agent is steam, water and CO 2 Nitrogen gas; the addition amount of the stripping agent is 0-5% of the mass flow of the polymer solution.
10. The method of claim 1, wherein the polymerization reactor is in the form of a single or multiple combined reactors, including a series of two reactors and a parallel of two reactors.
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