Cause analysis and countermeasure of blockage of pressure transmitter in LDPE reactor
Abstract: high pressure polyethylene (LDPE) device reaction pressure as high as 220 ~ 300 mpa (G), reaction temperature 270 ~ 330 ℃.To solve the problem of domestic a certain high pressure polyethylene plant reactor pressure transmitter often jams, seriously affecting the safe operation of the device's long period problem, through the comparison analysis of practical experience cause the clogging in the reactor pressure transmitter, the main reasons for the increase in operating high cycle separator row number of wax, reduce fouling factor and control the generation of preheater low polymer corresponding measures, such as slowing device reactor pressure transmitter of congestion, improve equipment operating cycle, provide certain reference for the similar equipment production.
The LDPE device described in this paper is a tubular reactor technology, the process flow of the device is based on ethylene as the main raw material, propylene and propylene aldehyde as the adjuster, the production of uniform polyethylene products, with peroxide as the initiator.Ethylene gas from the boundary area enters the reactor after being pressurized to 260MPa (G) by the primary and secondary machines. Peroxide is injected into different regions of the reactor in four points to form the reaction peak, peak, peak and peak.[1] the reactor pressure is controlled by a pressure pulse valve located at the outlet of the reactor, and the reactor pressure detection point is located at the inlet of the reactor. Three detection points are used to detect the reactor pressure, so as to improve the stability of pressure detection of the device. The intermediate value of the three detection points is taken as the actual value of the reactor pressure.Since the device was put into operation, the pressure transmitter of the reactor was often blocked by polymers, which led to the failure of the pressure transmitter detection, thus affecting the safe and stable operation of the device, and becoming one of the main factors restricting the long-term operation of the device.
1. Phenomenon and influence of blockage of reactor pressure transmitter
1.1 phenomenon
One of the three pressure transmitters is blocked.The transmitter test pressure will be with the other two transmitter detection, the deflection when the transmitter detection of pressure change rate and the remaining two rate deviation is greater than 5 MPa (G), reactor pressure pulse valve controller will automatically remove the testing point of testing pressure, no longer participate in the reactor pressure control, the remaining two testing point average values of reactor pressure as the actual values, participate in the reactor pressure control, DCS trend has been observed at this time will find that when a certain pressure testing point expires, the actual control of reactor pressure change,This in turn causes the pulse valve to open or close small.
Two of the three pressure transmitters are blocked.In the rest of the two pressure transmitter, when there is a testing point blocked again, the testing point pressure change rate and residual a rate deviation is greater than 5 MPa (G), the pressure testing point automatically removed by pulse valve controllers, the reactor pressure testing point only remaining a point as the only testing point of reactor pressure detection, the reactor pressure control increased risk of three pressure transmitter are blocked.When only the last pressure transmitter is blocked, the actual pressure of the reactor cannot be confirmed. At this time, the reactor is at risk of overpressure and overtemperature decomposition.
1.2 Impact
When one or two transmitters are blocked in the early stage, the pressure change rate of this pressure detection point will be more than 5 MPa (G) or less than 5 MPa (G) frequently, which means that this point frequently participates in the reference value of reactor pressure detection, resulting in frequent fluctuation of reactor pressure.In LDPE production process, pressure is a very important process control parameter to control product brand quality. Therefore, the pressure fluctuation will lead to the fluctuation of melting index and density of the product, resulting in the fluctuation of various performance indexes of the same batch of products.
When two transmitter blocked, this time the remaining a testing point of pressure in the reactor pressure control, increases the risk of the reactor pressure control at this time, when the last checkpoint appear jams, the reactor could not judge the actual pressure at this time, leading to the reactor pressure fluctuation is bigger, a product of melt index and density fluctuations.When the pulse valve frequently operates, it will increase the loss of the pulse valve and shorten the service life of the pulse valve. When the pulse valve suddenly opens, the pressure of high marks will rise rapidly, resulting in the fluctuation of high marks overpressure and high marks liquid level and the increase of high marks clamping amount.When the pulse valve is suddenly closed, the actual pressure of the reactor will rise rapidly, resulting in overpressure of the reactor, which will lead to interlocking shutdown or ethylene reaching the thermodynamic decomposition temperature. Carbon black generated by the decomposition reaction will adhere to the inner wall of the reactor. When starting the next time, it will lead to blackening of the starting material and increase the quantity of off-grade products.
2 Cause analysis of blockage of reactor pressure transmitter
2.1 there are residual polymers in the reactor during shutdown
During the shutdown of LDPE device, in order to fully remove the residual polymer in the reactor, after the polymerization reaction stops, the reactor pressure drops to a certain pressure (about 200 MPa (G)).Maintain the circulation of the primary and secondary machines for at least 30min, and fully bring the residual polymer in the reactor into the high-pressure product separator.If the cycle time is insufficient, residual polyethylene can be found in the reactor.In the nitrogen replacement process, if the high-pressure product separator is used as the nitrogen gas source for the reactor replacement, the residual polymer in the reactor will flow back to the reactor inlet, causing the polymer to enter the reaction pressure transmitter and block the transmitter.
2.2 insufficient separation of high-pressure circulating gas system
LDPE plant normal production process, due to high liquid level fluctuation, high level gauge failure or high score high level control leads to a large number of polymers with high cycle gas into the high pressure recycle gas cooling system, through the level 4 cooling and four high separation will recycle gas entrainment in the wax separated, after a separation of cooling process gas is mixed with a machine to fresh ethylene into the second machine.In the process, if the high cycle heat exchanger fouling severe or refrigerant system temperature control is too high, lead to high cycle gas entrainment in the wax is not fully separated, as the process gas into the second machine, and then through the reactor preheater entrance into the reactor, as the reaction progresses, high cycle for a long time into the second machine, led to more and more accumulation of polymer in the reactor pressure transmitter, thus cause the clogging in pressure transmitter.
2.3 the preheater produces oligomers
In the normal production process of LDPE device, the process gas compressed by the secondary machine enters the preheater, and the temperature of process gas entering the reactor is controlled by controlling the amount of LLPS, LPS and MPS passing through the preheater.When the reactor temperature control and high technology high active oxygen content in gas, don't expect the polymerization reaction in the preheater, producing oligomer, oligomer with process gas into the reactor inlet, long producing oligomer, oligomer slowly blocking reactor pressure transmitter, which leads to the reactor pressure control to fluctuate.
3 Countermeasures to reduce the blockage of pressure transmitter in reactor
3.1 reduce residual polymer in the reactor
During the shutdown of LDPE device, in order to fully remove the residual polymer in the reactor, the reactor pressure drops to a certain pressure after the polymerization reaction stops.Ensure that the primary and secondary machine circulation operation at least 30min, if the primary and secondary machine did not run for more than 30min, when the reactor needs to replace maintenance, ensure that the secondary machine inlet as the gas source of the reactor replacement, so as to prevent the residual polymer in the reactor back to the reactor inlet blocking the reactor pressure transmitter.
3.2 improve the high cycle heat transfer effect
The heat transfer process of hot and cold fluids passing through the interwall, the heat transfer rate equation is:
Q=KA train t (1)
Where Q is the heat transfer rate, unit W;For the total heat transfer coefficient K, the unit W/(m 2 ℃);A is the heat transfer area, unit m 2;Delta t m for the average temperature difference of two fluid ℃.How to determine K value correctly is an important problem in calculation of heat transfer process.[2] high cycle heat exchanger is A tubular heat exchanger, for the high cycle heat exchanger, formula (1) can be known, heat transfer area A is certain, the average temperature difference between the two fluids train t m is certain.Therefore, the total heat transfer coefficient K is the main factor influencing the heat transfer effect of high cycle heat exchanger.In the process of chemical operation, as the passage of time, the medium as a hot and cold fluid tends to scale on both sides of the interwall, the existence of such dirt will affect heat transfer, because the thickness of dirt and thermal conductivity is difficult to obtain, so in engineering, generally, a coefficient (fouling thermal resistance) is used to calculate the influence of fouling on heat transfer.[3] therefore, the wax in the high-cycle heat exchanger should be reduced as much as possible to improve the heat transfer effect of the high-cycle heat exchanger.The fouling coefficient can be reduced by the following measures to improve the total heat transfer coefficient.
(1) in the normal production of LDPE, by increasing the wax discharge times of each high-cycle separator, the polyethylene aggregation adhered to the inner wall of the high-cycle separator can be reduced
(2) reduce the control range of high resolution liquid level
(3) improve the control stability rate of the high resolution liquid level controller
(4) shorten the cycle of high cycle switching
(5) increase the heating temperature, constant temperature time and the number of constant temperature wax removal during the regeneration of high cycle heat exchanger
3.3 control the production of oligomers in the preheater
1) control the oxygen content in raw ethylene not exceeding the design index by connecting with the upstream device
(2) control the injection amount of technical gas to avoid excessive oxygen content due to the injection of more technical gas
(3) control the temperature of the preheater does not exceed 170 ℃
(4) increase the amount of ethylene returned and the amount of circulating gas purification
4. Countermeasures when the reactor pressure transmitter fails to detect
Through the collection of DCS data, the analysis and comparison showed that the three transmitter detection values (P1 ', P2 'and P3') for the outlet pressure detection of the secondary machine were relatively stable, but the pressure at these three points was higher than the reactor pressure (P1, P2 and P3) by a certain value.Through comparative analysis and addition of the modified values, it is proposed to use the revised test values of the three pressure transmitters at the outlet of the secondary machine as the test values of the reactor pressure and participate in the pressure control of the reactor.A pressure on-line switch is added into the configuration. When the detection failure occurs in the reactor pressure transmitter, the switch is adopted to switch the measured value of the reactor pressure to the outlet pressure detection of the secondary machine.The device has been successfully used when the pressure detection fails due to the blockage of two transmitters in the reactor. After the switch, the pressure control of the reactor is stable and the device operates smoothly, which improves the long-term operation rate of the device.FIG. 2 pressure online toggle switch.
5 Conclusion
Through the analysis of the cause of high pressure polyethylene plant reactor pressure transmitter jams, and cause a series of preventive measures, effectively slowed the transmitter of congestion, improved the device running cycle, reduce the unnecessary loss of parking, so as to achieve lower costs to increase yield, improve the economic benefit of enterprise.
Abstract: high pressure polyethylene (LDPE) device reaction pressure as high as 220 ~ 300 mpa (G), reaction temperature 270 ~ 330 ℃.To solve the problem of domestic a certain high pressure polyethylene plant reactor pressure transmitter often jams, seriously affecting the safe operation of the device's long period problem, through the comparison analysis of practical experience cause the clogging in the reactor pressure transmitter, the main reasons for the increase in operating high cycle separator row number of wax, reduce fouling factor and control the generation of preheater low polymer corresponding measures, such as slowing device reactor pressure transmitter of congestion, improve equipment operating cycle, provide certain reference for the similar equipment production.
The LDPE device described in this paper is a tubular reactor technology, the process flow of the device is based on ethylene as the main raw material, propylene and propylene aldehyde as the adjuster, the production of uniform polyethylene products, with peroxide as the initiator.Ethylene gas from the boundary area enters the reactor after being pressurized to 260MPa (G) by the primary and secondary machines. Peroxide is injected into different regions of the reactor in four points to form the reaction peak, peak, peak and peak.[1] the reactor pressure is controlled by a pressure pulse valve located at the outlet of the reactor, and the reactor pressure detection point is located at the inlet of the reactor. Three detection points are used to detect the reactor pressure, so as to improve the stability of pressure detection of the device. The intermediate value of the three detection points is taken as the actual value of the reactor pressure.Since the device was put into operation, the pressure transmitter of the reactor was often blocked by polymers, which led to the failure of the pressure transmitter detection, thus affecting the safe and stable operation of the device, and becoming one of the main factors restricting the long-term operation of the device.
1. Phenomenon and influence of blockage of reactor pressure transmitter
1.1 phenomenon
One of the three pressure transmitters is blocked.The transmitter test pressure will be with the other two transmitter detection, the deflection when the transmitter detection of pressure change rate and the remaining two rate deviation is greater than 5 MPa (G), reactor pressure pulse valve controller will automatically remove the testing point of testing pressure, no longer participate in the reactor pressure control, the remaining two testing point average values of reactor pressure as the actual values, participate in the reactor pressure control, DCS trend has been observed at this time will find that when a certain pressure testing point expires, the actual control of reactor pressure change,This in turn causes the pulse valve to open or close small.
Two of the three pressure transmitters are blocked.In the rest of the two pressure transmitter, when there is a testing point blocked again, the testing point pressure change rate and residual a rate deviation is greater than 5 MPa (G), the pressure testing point automatically removed by pulse valve controllers, the reactor pressure testing point only remaining a point as the only testing point of reactor pressure detection, the reactor pressure control increased risk of three pressure transmitter are blocked.When only the last pressure transmitter is blocked, the actual pressure of the reactor cannot be confirmed. At this time, the reactor is at risk of overpressure and overtemperature decomposition.
1.2 Impact
When one or two transmitters are blocked in the early stage, the pressure change rate of this pressure detection point will be more than 5 MPa (G) or less than 5 MPa (G) frequently, which means that this point frequently participates in the reference value of reactor pressure detection, resulting in frequent fluctuation of reactor pressure.In LDPE production process, pressure is a very important process control parameter to control product brand quality. Therefore, the pressure fluctuation will lead to the fluctuation of melting index and density of the product, resulting in the fluctuation of various performance indexes of the same batch of products.
When two transmitter blocked, this time the remaining a testing point of pressure in the reactor pressure control, increases the risk of the reactor pressure control at this time, when the last checkpoint appear jams, the reactor could not judge the actual pressure at this time, leading to the reactor pressure fluctuation is bigger, a product of melt index and density fluctuations.When the pulse valve frequently operates, it will increase the loss of the pulse valve and shorten the service life of the pulse valve. When the pulse valve suddenly opens, the pressure of high marks will rise rapidly, resulting in the fluctuation of high marks overpressure and high marks liquid level and the increase of high marks clamping amount.When the pulse valve is suddenly closed, the actual pressure of the reactor will rise rapidly, resulting in overpressure of the reactor, which will lead to interlocking shutdown or ethylene reaching the thermodynamic decomposition temperature. Carbon black generated by the decomposition reaction will adhere to the inner wall of the reactor. When starting the next time, it will lead to blackening of the starting material and increase the quantity of off-grade products.
2 Cause analysis of blockage of reactor pressure transmitter
2.1 there are residual polymers in the reactor during shutdown
During the shutdown of LDPE device, in order to fully remove the residual polymer in the reactor, after the polymerization reaction stops, the reactor pressure drops to a certain pressure (about 200 MPa (G)).Maintain the circulation of the primary and secondary machines for at least 30min, and fully bring the residual polymer in the reactor into the high-pressure product separator.If the cycle time is insufficient, residual polyethylene can be found in the reactor.In the nitrogen replacement process, if the high-pressure product separator is used as the nitrogen gas source for the reactor replacement, the residual polymer in the reactor will flow back to the reactor inlet, causing the polymer to enter the reaction pressure transmitter and block the transmitter.
2.2 insufficient separation of high-pressure circulating gas system
LDPE plant normal production process, due to high liquid level fluctuation, high level gauge failure or high score high level control leads to a large number of polymers with high cycle gas into the high pressure recycle gas cooling system, through the level 4 cooling and four high separation will recycle gas entrainment in the wax separated, after a separation of cooling process gas is mixed with a machine to fresh ethylene into the second machine.In the process, if the high cycle heat exchanger fouling severe or refrigerant system temperature control is too high, lead to high cycle gas entrainment in the wax is not fully separated, as the process gas into the second machine, and then through the reactor preheater entrance into the reactor, as the reaction progresses, high cycle for a long time into the second machine, led to more and more accumulation of polymer in the reactor pressure transmitter, thus cause the clogging in pressure transmitter.
2.3 the preheater produces oligomers
In the normal production process of LDPE device, the process gas compressed by the secondary machine enters the preheater, and the temperature of process gas entering the reactor is controlled by controlling the amount of LLPS, LPS and MPS passing through the preheater.When the reactor temperature control and high technology high active oxygen content in gas, don't expect the polymerization reaction in the preheater, producing oligomer, oligomer with process gas into the reactor inlet, long producing oligomer, oligomer slowly blocking reactor pressure transmitter, which leads to the reactor pressure control to fluctuate.
3 Countermeasures to reduce the blockage of pressure transmitter in reactor
3.1 reduce residual polymer in the reactor
During the shutdown of LDPE device, in order to fully remove the residual polymer in the reactor, the reactor pressure drops to a certain pressure after the polymerization reaction stops.Ensure that the primary and secondary machine circulation operation at least 30min, if the primary and secondary machine did not run for more than 30min, when the reactor needs to replace maintenance, ensure that the secondary machine inlet as the gas source of the reactor replacement, so as to prevent the residual polymer in the reactor back to the reactor inlet blocking the reactor pressure transmitter.
3.2 improve the high cycle heat transfer effect
The heat transfer process of hot and cold fluids passing through the interwall, the heat transfer rate equation is:
Q=KA train t (1)
Where Q is the heat transfer rate, unit W;For the total heat transfer coefficient K, the unit W/(m 2 ℃);A is the heat transfer area, unit m 2;Delta t m for the average temperature difference of two fluid ℃.How to determine K value correctly is an important problem in calculation of heat transfer process.[2] high cycle heat exchanger is A tubular heat exchanger, for the high cycle heat exchanger, formula (1) can be known, heat transfer area A is certain, the average temperature difference between the two fluids train t m is certain.Therefore, the total heat transfer coefficient K is the main factor influencing the heat transfer effect of high cycle heat exchanger.In the process of chemical operation, as the passage of time, the medium as a hot and cold fluid tends to scale on both sides of the interwall, the existence of such dirt will affect heat transfer, because the thickness of dirt and thermal conductivity is difficult to obtain, so in engineering, generally, a coefficient (fouling thermal resistance) is used to calculate the influence of fouling on heat transfer.[3] therefore, the wax in the high-cycle heat exchanger should be reduced as much as possible to improve the heat transfer effect of the high-cycle heat exchanger.The fouling coefficient can be reduced by the following measures to improve the total heat transfer coefficient.
(1) in the normal production of LDPE, by increasing the wax discharge times of each high-cycle separator, the polyethylene aggregation adhered to the inner wall of the high-cycle separator can be reduced
(2) reduce the control range of high resolution liquid level
(3) improve the control stability rate of the high resolution liquid level controller
(4) shorten the cycle of high cycle switching
(5) increase the heating temperature, constant temperature time and the number of constant temperature wax removal during the regeneration of high cycle heat exchanger
3.3 control the production of oligomers in the preheater
1) control the oxygen content in raw ethylene not exceeding the design index by connecting with the upstream device
(2) control the injection amount of technical gas to avoid excessive oxygen content due to the injection of more technical gas
(3) control the temperature of the preheater does not exceed 170 ℃
(4) increase the amount of ethylene returned and the amount of circulating gas purification
4. Countermeasures when the reactor pressure transmitter fails to detect
Through the collection of DCS data, the analysis and comparison showed that the three transmitter detection values (P1 ', P2 'and P3') for the outlet pressure detection of the secondary machine were relatively stable, but the pressure at these three points was higher than the reactor pressure (P1, P2 and P3) by a certain value.Through comparative analysis and addition of the modified values, it is proposed to use the revised test values of the three pressure transmitters at the outlet of the secondary machine as the test values of the reactor pressure and participate in the pressure control of the reactor.A pressure on-line switch is added into the configuration. When the detection failure occurs in the reactor pressure transmitter, the switch is adopted to switch the measured value of the reactor pressure to the outlet pressure detection of the secondary machine.The device has been successfully used when the pressure detection fails due to the blockage of two transmitters in the reactor. After the switch, the pressure control of the reactor is stable and the device operates smoothly, which improves the long-term operation rate of the device.FIG. 2 pressure online toggle switch.
5 Conclusion
Through the analysis of the cause of high pressure polyethylene plant reactor pressure transmitter jams, and cause a series of preventive measures, effectively slowed the transmitter of congestion, improved the device running cycle, reduce the unnecessary loss of parking, so as to achieve lower costs to increase yield, improve the economic benefit of enterprise.
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