Polymer monomer recovery method in shutdown process of polypropylene production system

Abstract

The invention relates to the field of polypropylene production, and discloses a polymeric monomer recovery method in a shutdown process of a polypropylene production system, which comprises the following steps: after production shutdown, propylene in a first reactor and a second reactor is respectively condensed into a first circulating tank and a second circulating tank through a first condenser and a second condenser under the action of pressure; when the internal pressure of the first reactor and the second reactor is reduced, the recovery efficiency of the first condenser and the second condenser is reduced, and the recovery effect is poor, at the moment, the second gas phase valve is closed, the first control valve is opened, and the trend of the polymeric monomer in the second reactor is changed; the gas phase between the first reactor and the second reactor is communicated by opening the gas lock system, so that polymeric monomers in the first reactor and the second reactor are pressurized to the second condenser for condensation through the recycle gas compressor, the pressure in the first reactor and the second reactor is reduced, and the condensation effect of the second condenser is improved; and the recovery efficiency and the recovery effect of the polymerization monomer are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of polypropylene production, and in particular to a method for recovering polymerization monomers during the shutdown process of a polypropylene production system. Background Art

[0002] The Horizone polypropylene process is a gas-phase polypropylene synthesis process, which consists of two horizontal stirred tanks, a first reactor and a second reactor, connected in series. It can produce homopolymer, random, impact copolymer polypropylene, and thermoplastic polyolefin elastomers. Using this process, products with high ethylene content and rubber content can be produced, and the balance between rigidity and toughness of the products is very good.

[0003] In the prior art, when the polypropylene production plant is shut down for maintenance, a large amount of polymerization monomers often remain in the plant. These polymers include components such as propylene and ethylene. In the face of these residual polymerization monomers, the condensation effects of the first condenser and the second condenser are limited by the gas pressure in the corresponding reactor. When the pressure in the reactor gradually decreases, the condensation recovery effect also decreases. A large amount of polymerization monomers still exist in each reactor and pipeline and cannot be recovered, resulting in a large amount of waste and an unsatisfactory recovery efficiency of existing polymerization monomers such as propylene. And without delaying the maintenance work, a large amount of polymerization monomers are often discharged into the flare for incineration, or the polymerization monomers are discharged into the fuel gas pipeline for use as fuel, wasting a lot of polymerization monomer propylene.

[0004] Moreover, among the above two treatment methods, discharging the polymerization monomers into the flare for incineration is likely to generate noise, and the incomplete combustion of the polymerization monomers will also generate a large amount of pollutants, thus causing environmental protection problems; while discharging the polymerization monomers into the fuel gas pipeline for use as fuel is likely to impact the fuel gas pipeline due to the differences in calorific value and pressure grade. And due to the limitations of the flare and the fuel gas pipeline receiving capacity, when multiple plants are shut down for maintenance at the same time, it is also necessary to queue up to discharge the process media in different plants, and the discharge process is slow, affecting the maintenance progress of the plant. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: how to better recover and reuse the polymerization monomer propylene during the shutdown process of polypropylene production, avoid waste of the polymerization monomer propylene and environmental pollution caused by it, and improve the recovery efficiency and recovery effect of propylene.

[0006] To solve the above technical problems, the present invention provides a method for recovering polymerization monomers during the shutdown process of a polypropylene production system. The polypropylene production system includes a first reactor, a first condenser, a first circulation tank, a discharge liquid refining unit, a second circulation tank, a second condenser, and a second reactor connected in sequence through pipelines; a first quenching liquid pump and a first discharge valve are sequentially arranged on the pipeline connecting the first circulation tank and the discharge liquid refining unit, and a second quenching liquid pump and a second discharge valve are sequentially arranged on the pipeline connecting the second circulation tank and the discharge liquid refining unit; the first reactor and the second reactor are connected through an air lock system; the polypropylene production system further includes a recovery circulation branch, both ends of the recovery circulation branch are connected to the second reactor and the second condenser respectively, and a first control valve and a circulation gas compressor are sequentially arranged on the recovery circulation branch; a first gas phase valve is arranged on the pipeline connecting the first reactor and the first condenser, and a second gas phase valve is arranged on the pipeline connecting the second reactor and the second condenser; and a first discharge branch for discharging propylene to a flare system is further connected to the first reactor, a first flare discharge valve is arranged on the first discharge branch, a second discharge branch for discharging propylene to the flare system is further connected to the second reactor, and a second flare discharge valve is arranged on the second discharge branch; based on this polypropylene production system, the method for recovering polymerization monomers includes the following steps:

[0007] Step S1: After the polypropylene production system is shut down, control the first gas phase valve, the second gas phase valve, the first discharge valve, and the second discharge valve to open, close the first flare discharge valve, the second flare discharge valve, and the air lock system, operate the first quenching liquid pump and the second quenching liquid pump, the circulation gas compressor circulates by itself. After the remaining gas components in the first reactor are condensed to a liquid state by the first condenser, they flow to the first circulation tank and are pumped to the discharge liquid refining unit by the first quenching liquid pump. After the remaining gas components in the second reactor are condensed to a liquid state by the second condenser, they flow to the second circulation tank and are pumped to the discharge liquid refining unit by the second quenching liquid pump;

[0008] Step S2: When the pressures in the first reactor and the second reactor drop below a first preset pressure value and the pressure difference between the first reactor and the second reactor does not exceed a threshold value, control the air lock system to open, close the second gas phase valve and open the first control valve. The remaining gas components in the first reactor and the remaining gas components in the second reactor are pressurized by the circulation gas compressor and then transported to the second condenser;

[0009] Step S3: When the pressures in the first reactor and the second reactor drop to the second preset pressure value, close the airlock system, the first control valve, and the first gas-phase valve, stop the operation of the recycle gas compressor, and control the opening of the first flare discharge valve and the second flare discharge valve to discharge the remaining gas components in the first reactor and the second reactor into the flare system;

[0010] Step S4: After all the liquid-phase propylene in the first recycle tank is transferred to the discharge liquid refining unit, close the first discharge valve and stop the first quench liquid pump; after all the liquid-phase propylene in the second recycle tank is transferred to the discharge liquid refining unit, close the second discharge valve and stop the second quench liquid pump.

[0011] Preferably, the polypropylene production system further includes a feed branch. One end of the feed branch is respectively connected to the first recycle tank and the second recycle tank, and the other end of the feed branch is connected to a propylene feed system. The step S1 includes:

[0012] S11: Before the shutdown of the polypropylene production system, by reducing the liquid level control target values of the liquid-phase propylene in the first recycle tank and the second recycle tank, the feed amount from the propylene feed system to the first recycle tank and the second recycle tank is reduced and adjusted.

[0013] Preferably, the polypropylene production system further includes a first recycle branch and a second recycle branch. The two ends of the first recycle branch are respectively connected to the first recycle tank and the first reactor, and a first recycle gas blower, a first start-up heater, and a third gas-phase valve are sequentially arranged on the first recycle branch; the two ends of the second recycle branch are respectively connected to the second recycle tank and the second reactor, and a second recycle gas blower, a second start-up heater, and a fourth gas-phase valve are sequentially arranged on the second recycle branch. The step S1 includes:

[0014] S12: Control the third gas-phase valve to open, and operate the first recycle gas blower. The first recycle gas blower pumps the gas-phase propylene in the first recycle tank to the first start-up heater for heating and then transports it into the first reactor; control the fourth gas-phase valve to open, and operate the second recycle gas blower. The second recycle gas blower pumps the gas-phase propylene in the second recycle tank to the second start-up heater for heating and then transports it into the second reactor.

[0015] Preferably, in the step S2, when the pressures in the first reactor and the second reactor drop below the first preset pressure value, close the first start-up heater and the second start-up heater; close the fourth gas-phase valve before closing the second gas-phase valve, and stop the second recycle gas blower and the first recycle gas blower.

[0016] Preferably, the polypropylene production system further includes a purge gas branch. The two ends of the purge gas branch are respectively connected to the propylene feed system and the airlock system. An inlet control valve, an airlock purge gas heater, and a purge program valve are sequentially arranged on the purge gas branch. The step S1 includes:

[0017] S13. When the polypropylene production system shuts down, control the opening duration and opening degree of the purge program valve to reduce the amount of propylene entering the first reactor through the airlock system.

[0018] Preferably, the polypropylene production system further includes a liquid pump flushing liquid branch. One end of the liquid pump flushing liquid branch is respectively connected to the first quench liquid pump and the second quench liquid pump through two flushing control valves. The other end of the liquid pump flushing liquid branch is connected to the propylene feed system through an inlet control valve. In step S4, when the first quench liquid pump and the second quench liquid pump stop, close the flushing program valves corresponding to the two quench liquid pumps respectively.

[0019] Preferably, the first condenser includes a first cooling water circulation pipeline, and a first circulating cooling water valve is arranged on the first cooling water circulation pipeline; the second condenser includes a second cooling water circulation pipeline, and a second circulating cooling water valve is arranged on the second cooling water circulation pipeline. The step S2 includes:

[0020] S21. When the pressure of the first reactor drops below the first preset pressure value, close the first circulating cooling water valve. When the pressure of the second reactor drops below the first preset pressure value, increase and adjust the opening degree of the second circulating cooling water valve.

[0021] Preferably, in the step S1, fully open the first discharge valve and gradually open the second discharge valve to transfer all the liquid-phase propylene in the first circulation tank to the discharged liquid refining unit; in the step S3, fully open the second discharge valve to transfer all the liquid-phase propylene in the second circulation tank to the discharged liquid refining unit.

[0022] Preferably, a bag filter is arranged between the first control valve and the recycle gas compressor. In the step S2, gradually increase the opening degree of the first control valve to control the pressure at the outlet of the first control valve within the design pressure range of the bag filter.

[0023] The beneficial effects of a method for recovering polymerization monomers during shutdown of a polypropylene production system provided in an embodiment of the present invention compared with the prior art are as follows:

[0024] In this embodiment, after the production shutdown, the gaseous propylene in the first reactor and the second reactor is condensed into the first circulation tank and the second circulation tank respectively through the first condenser and the second condenser under the action of the internal pressure. When the internal pressures of the first reactor and the second reactor decrease, the recovery efficiency of the first condenser and the second condenser decreases, and the recovery effect is also poor. At this time, the second gas-phase valve is closed and the first control valve is opened to change the flow direction of the gaseous propylene in the second reactor, and the gas connection between the first reactor and the second reactor is made through the opening of the gas lock system, so that the gaseous propylene in the first reactor and the second reactor is pressurized by the recycle gas compressor and condensed in the second condenser. When the pressures of the first reactor and the second reactor decrease, the condensation effect of the second condenser is improved, and thus the recovery efficiency and the recovery effect of propylene are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic diagram of the system structure process flow of the present invention;

[0026] Figure 2 is a system structure flow chart of the present invention.

[0027] In the figure:

[0028] 11. Receiving valve group; 12. Discharging valve group; 13. Discharging angle valve;

[0029] 2. First reactor; 21. First condenser; 211. First circulating cooling water valve; 212. First gas-phase valve; 22. First circulation tank; 221. First quench liquid pump; 222. First discharge valve; 223. Third gas-phase valve; 23. First recycle gas fan; 24. First start-up heater; 25. First flare discharge valve;

[0030] 3. Second reactor; 31. Second condenser; 311. Second circulating cooling water valve; 312. Second gas-phase valve; 32. Second circulation tank; 321. Second quench liquid pump; 322. Second discharge valve; 323. Fourth gas-phase valve; 33. Second recycle gas fan; 34. Second start-up heater; 35. Second flare discharge valve;

[0031] 4. Recycle gas compressor; 41. Bag filter; 42. First control valve;

[0032] 5. Gas lock system; 51. Gas lock purging gas heater; 52. Seal flushing gas heater; 53. Feed control valve; 54. Propylene feed system; 55. Flare system; 56. High-pressure nitrogen HN system; 57. Flushing control valve; 58. Emergency shut-off valve;

[0033] 6. Effluent refining unit. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] The specific embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. The following embodiments are used to illustrate the present invention, but are not used to limit the scope of the present invention.

[0035] As Figure 1 and Figure 2 shown, a method for recovering polymerization monomers during the shutdown process of a polypropylene production system is provided in a preferred embodiment of the present invention.

[0036] The polypropylene production system includes a first reactor 2, a first condenser 21, a first circulation tank 22, a discharge liquid refining unit 6, a second circulation tank 32, a second condenser 31, and a second reactor 3 that are connected in sequence through pipelines; a first quench liquid pump 221 and a first discharge valve 222 are sequentially arranged on the pipeline connecting the first circulation tank 22 and the discharge liquid refining unit 6, and a second quench liquid pump 321 and a second discharge valve 322 are sequentially arranged on the pipeline connecting the second circulation tank 32 and the discharge liquid refining unit 6; the first reactor 2 and the second reactor 3 are connected through an air lock system 5; the polypropylene production system further includes a recovery circulation branch, the two ends of the recovery circulation branch are respectively connected to the second reactor 3 and the second condenser 31, and a first control valve 42 and a circulation gas compressor 4 are sequentially arranged on the recovery circulation branch; a first gas phase valve 212 is provided on the pipeline connecting the first reactor 2 and the first condenser 21, and a second gas phase valve 312 is provided on the pipeline connecting the second reactor 3 and the second condenser 31; and a first discharge branch for discharging propylene to the flare system 55 is further connected to the first reactor 2, a first flare discharge valve 25 is provided on the first discharge branch, a second discharge branch for discharging propylene to the flare system 55 is further connected to the second reactor 3, and a second flare discharge valve 35 is provided on the second discharge branch;

[0037] In the original production process, after raw materials and catalysts are introduced into the first reactor 2 and the second reactor 3, the catalysts and the polypropylene powder polymerized in the first reactor 2 all enter the second reactor 3 for further reaction. The polypropylene powder in the first reactor 2 enters the second reactor 3 through the intermittent opening of the air lock system 5. The air lock system 5 can prevent the mutual mixing of the gas-phase materials in the first reactor 2 and the second reactor 3. By controlling the air lock system 5 and the feeding amounts of each component, the balance and stability of the material components in the first reactor 2 and the second reactor 3 are maintained. The polypropylene powder in the second reactor 3 is transported to the bag filter 41 through the discharge port for filtration. The filtered polypropylene enters the downstream device for treatment, and the filtered gas-phase propylene is transported to the second condenser 31 through the recycle gas compressor 4 and finally returns to the second reactor 3 to continue participating in the reaction. In addition, the second reactor 3 is also connected to the bag filter 41 through the first control valve 42. This line is not enabled during normal production. It is only enabled when there are problems such as blockage in the discharge port or pipeline in the second reactor 3 to avoid the interlock shutdown due to too low pressure in the bag filter 41 or too low inlet pressure of the recycle gas compressor 4. During the production process, reaction heat is generated in both the first reactor 2 and the second reactor 3. The reaction heat is absorbed by the vaporization of the quenching liquid pressurized by the first quenching liquid pump 221 and the second quenching liquid pump 321 and added to the reactor. Specifically, the first condenser 21 condenses the gas components in the first reactor 2. The condensed liquid-phase components enter the first circulation tank 22. Then, a part of the liquid-phase components enters the reaction bed of the first reactor 2 through the quenching liquid pump for heat removal, and the other part of the liquid-phase components goes to the discharge liquid refining unit 6 to take away inert components such as propane in the reactor, avoiding the accumulation of inert components such as propane in the reactor.

[0038] Based on the above polypropylene production system, the polymerization monomer recovery method includes the following steps:

[0039] Step S1: After the polypropylene production system is shut down, control the first gas-phase valve 212, the second gas-phase valve 312, the first discharge valve 222, and the second discharge valve 322 to open, close the first flare discharge valve 25, the second flare discharge valve 35, and the air lock system 5, operate the first quenching liquid pump 221 and the second quenching liquid pump 321, let the recycle gas compressor 4 operate in self-circulation. The remaining gas components in the first reactor 2 are condensed to liquid by the first condenser 21, flow to the first circulation tank 22, and are pumped to the discharge liquid refining unit 6 by the first quenching liquid pump 221. The remaining gas components in the second reactor 3 are condensed to liquid by the second condenser 31, flow to the second circulation tank 32, and are pumped to the discharge liquid refining unit 6 by the second quenching liquid pump 321;

[0040] Specifically, after the polypropylene production system shuts down, the first reactor 2 and the second reactor 3 are still filled with high-pressure gaseous propylene. Keeping the first gas-phase valve 212 and the second gas-phase valve 312 open, the gaseous propylene in the first reactor 2 and the second reactor 3 enters the first condenser 21 and the second condenser 31 respectively for condensation under the action of pressure and the first recycle gas blower 23 and the second recycle gas blower 33. Due to the high pressure in the first reactor 2 and the second reactor 3, the condensation efficiency of the first condenser 21 and the second condenser 31 is high. At this time, the gaseous propylene in the first reactor 2 and the second reactor 3 can be well condensed into liquid propylene and discharged into the first recycle tank 22 and the second recycle tank 32 by using the pressure range of this high-efficiency condensation. The first recycle tank 22 and the second recycle tank 32 respectively extract the liquid propylene through the first quench liquid pump 221 and the second quench liquid pump 321 and send it to the effluent refining unit 6 for pretreatment. The pretreated liquid propylene is then transported to the downstream device for recycling and refining to remove impurities for reuse again.

[0041] Step S2: When the pressure in the first reactor 2 and the second reactor 3 drops below the first preset pressure value and the pressure difference between the first reactor 2 and the second reactor 3 does not exceed the threshold, control the air lock system 5 to open, close the second gas-phase valve 312 and open the first control valve 42. The gaseous propylene in the first reactor 2 enters the second reactor 3 and is transported to the second condenser 31 after being pressurized by the recycle gas compressor 4.

[0042] Specifically, in the prior art, the gaseous propylene participating in condensation can only passively enter the condenser through the pressure in the reactor. When the pressures in the first reactor 2 and the second reactor 3 decrease, the pressure from the first reactor 2 and the second reactor 3 to the first condenser 21 and the second condenser 31 is insufficient, and the condensation efficiency of the first condenser 21 and the second condenser 31 will decrease. Due to reasons such as equipment maintenance shutdown, it is necessary to quickly discharge the gaseous propylene in the first reactor 2 and the second reactor 3. In the prior art, when the recovery efficiency of the first condenser 21 and the second condenser 31 is insufficient, more gaseous propylene can only be introduced into the flare system 55 and burned and consumed; in this step, when the condensation efficiency of the first condenser 21 and the second condenser 31 is insufficient, the charging valve group 11 and the discharging valve group 12 of the air lock system 5 are opened to connect the gas phases of the first reactor 2 and the second reactor 3, so that the residual gaseous propylene in the first reactor 2 can enter the second reactor 3 through the air lock system 5. At this time, the second gas-phase valve 312 is closed to cut off the connection between the second reactor 3 and the second condenser 31, and then the first control valve 42 is opened to connect the recycle gas compressor 4 with the second reactor 3, changing the passive condensation to active condensation. The gaseous propylene in the first reactor 2 and the second reactor 3 is extracted, pressurized by the recycle gas compressor 4 and then transported to the second condenser 31, improving the condensation efficiency of the second condenser 31, and naturally more gaseous propylene can be condensed and recycled to the discharge liquid refining unit 6 through the second recycle tank 32; in addition, the first reactor 2 is also connected to the second reactor 3 through a discharge angle valve 13. The discharge angle valve 13 is provided so that the polypropylene powder in the first reactor 2 can enter the second reactor 3 and be discharged faster during shutdown. During the process of propylene recovery, the discharge angle valve 13 can also be opened simultaneously to facilitate the better entry of the gaseous propylene in the first reactor 2 into the second reactor 3.

[0043] In some embodiments, the first preset pressure value is set to 2.0 MpaG. In addition, according to the differences in the gas-phase components of the first reactor 2 and the second reactor 3 and the temperature of the circulating cooling water, the optimal working ranges of the first condenser 21 and the second condenser 31 are also different. In one embodiment, when the temperature of the circulating cooling water is lower, the pressures in the first reactor 2 and the second reactor 3 can be reduced to below 1.8 MpaG or even below 1.60 MpaG through the first condenser 21 and the second condenser 31 to make full use of the high-efficiency range of the first condenser 21 and the second condenser 31, improving the propylene recovery efficiency while saving energy; generally speaking, according to the differences in the remaining gas-phase components in the reactor, the first preset pressure value is also different, and the range of the first preset pressure value is between 1.60 MpaG and 2.00 MpaG.

[0044] Further, in some embodiments, the threshold value of the pressure difference between the first reactor 2 and the second reactor 3 is 0.3 MpaG. When the pressure difference between the two reactors is greater than this value, the first reactor 2 and the second reactor 3 cannot be connected, otherwise the pressure difference will damage the accessories of the two reactors.

[0045] Step S3: When the pressures in the first reactor 2 and the second reactor 3 drop to the second preset pressure value, close the airlock system 5, the first control valve 42, and the first gas-phase valve, stop the operation of the recycle gas compressor 4, and control the opening of the first flare discharge valve 25 and the second flare discharge valve 35, and discharge the gaseous propylene in the first reactor 2 and the second reactor 3 to the flare system 55.

[0046] Specifically, when the pressures in the first reactor 2 and the second reactor 3 drop to the second preset pressure value, the remaining gaseous propylene inventory in the first reactor 2 and the second reactor 3 is not much. At this time, continuously operating the recycle gas compressor 4 to recover propylene is not efficient and will instead waste energy. The connection between the first reactor 2 and the second reactor 3 can be disconnected, and the first flare discharge valve 25 and the second flare discharge valve 35 can be opened. Through the back pressure of the flare system 55 and in cooperation with the pressure relief and replacement process of filling high-pressure nitrogen before maintenance, all the remaining propylene in the first reactor 2 and the second reactor 3 is replaced into the flare system 55 for treatment; in some embodiments, the second preset value is 0.03 MpaG.

[0047] Step S4: After all the liquid-phase propylene in the first recycle tank 22 is transported to the discharged liquid refining unit 6, close the first discharge valve 222 and stop the first quench liquid pump 221; after all the liquid-phase propylene in the second recycle tank 32 is transported to the discharged liquid refining unit 6, close the second discharge valve 322 and stop the second quench liquid pump 321.

[0048] Specifically, a part of the gaseous propylene in the first reactor 2 is transported to the effluent refining unit 6 through the first condenser 21 and the first circulation tank 22. Before closing the second gas-phase valve 312 to isolate the second reactor 3 and the second condenser 31, it is necessary to discharge all the liquid-phase propylene in the first circulation tank 22 to the effluent refining unit 6. After the first circulation tank 22 is emptied, the emergency cut-off valve 58 on the pipeline from the bottom of the first circulation tank 22 to the inlet of the first quench liquid pump 221 is closed and the reflux valve of the first quench liquid pump 221 is closed. Then, the liquid-phase propylene in the second circulation tank 32 is discharged and the two reactors are connected. This is mainly to avoid the vaporization of the residual liquid-phase propylene in the first circulation tank 22 under a low-pressure environment during the subsequent recovery process, which not only wastes energy, resulting in incomplete propylene recovery, but also causes frosting in the first circulation tank 22, leading to damage to the first circulation tank 22. When the pressures in the first reactor 2 and the second reactor 3 drop to the first preset pressure value, the first reactor 2 and the second reactor 3 are connected. The remaining gas-phase components in the first reactor 2 and the second reactor 3 are pressurized by the recycle gas compressor 4 and then sent to the second condenser 31 for condensation. Components such as ethylene and propylene in the gas-phase components are condensed into a liquid phase for recovery. At this time, a part of the polymerization monomers in the first reactor 2 enters the second reactor 3 and is finally condensed by the second condenser 31.

[0049] In some embodiments, the polypropylene production system further includes a feed branch. One end of the feed branch is connected to the first circulation tank 22 and the second circulation tank 32 respectively, and the other end of the feed branch is connected to a propylene feed system 54. During the production process, the propylene feed system 54 injects the propylene raw materials participating in the reaction into the first reactor 2 and the second reactor 3 through the first circulation tank 22 and the second circulation tank 32 respectively. Step S1 includes:

[0050] S11. Before the polypropylene production system shuts down, by reducing the liquid-level control target values of the liquid-phase propylene in the first circulation tank 22 and the second circulation tank 32, the feed amount from the propylene feed system 54 to the first circulation tank 22 and the second circulation tank 32 is reduced. In one embodiment, before shutdown, the liquid-level control target values of the two circulation tanks are automatically controlled to be reduced to 18% through the liquid-level feedback in the two circulation tanks, so that the liquid levels in the two circulation tanks are reduced. In another embodiment, the amount of liquid-phase propylene entering the two circulation tanks can also be directly reduced by manual control, thereby reducing the liquid levels in the two circulation tanks.

[0051] Specifically, during the normal production process, the liquid levels in the first circulation tank 22 and the second circulation tank 32 generally remain at about 30%. When shutdown is required, the feed rate of the propylene feed system 54 to the first circulation tank 22 and the second circulation tank 32 can be reduced in advance, thereby controlling the reduction of the liquid levels in the first circulation tank 22 and the second circulation tank 32, reducing the waste of propylene, and also reducing the recovery burden of the first circulation tank 22, the second circulation tank 32, and the discharge liquid refining unit 6 during subsequent recovery. This enables a larger space in the first circulation tank 22 and the second circulation tank 32 to participate in propylene recovery during shutdown recovery, improving the propylene recovery efficiency.

[0052] In some embodiments, the polypropylene production system further includes a first circulation branch and a second circulation branch. The two ends of the first circulation branch are respectively connected to the first circulation tank 22 and the first reactor 2. A first circulation gas blower 23, a first start-up heater 24, and a third gas-phase valve 223 are sequentially arranged on the first circulation branch. The two ends of the second circulation branch are respectively connected to the second circulation tank 32 and the second reactor 3. A second circulation gas blower 33, a second start-up heater 34, and a fourth gas-phase valve 323 are sequentially arranged on the second circulation branch. Step S1 includes:

[0053] S12. Control the third gas-phase valve 223 to open, and operate the first circulation gas blower 23. The first circulation gas blower 23 pumps the gaseous propylene in the first circulation tank 22 to the first start-up heater 24 for heating and then transports it into the first reactor 2. Control the fourth gas-phase valve 323 to open, and operate the second circulation gas blower 33. The second circulation gas blower 33 pumps the gaseous propylene in the second circulation tank 32 to the second start-up heater 34 for heating and then transports it into the second reactor 3.

[0054] In addition, in step S2, when the pressures in the first reactor 2 and the second reactor 3 drop below the first preset pressure value, turn off the first start-up heater 24 and the second start-up heater 34. Before closing the second gas-phase valve 312, close the fourth gas-phase valve 323, and stop the second circulation gas blower 33 and the first circulation gas blower 23 in advance.

[0055] Specifically, due to the easy liquefaction property of propylene, and the fact that the liquefaction of propylene in the first reactor 2 and the second reactor 3 is likely to cause danger, during shutdown, it is necessary to turn on the first start-up heater 24 and the second start-up heater 34. The first recycle gas blower 23 in the first recycle branch continuously extracts the gaseous propylene in the first recycle tank 22, heats the gaseous propylene through the first start-up heater 24 and then transports it back to the first reactor 2 to prevent the liquefaction of propylene in the first reactor 2 at the disadvantageous points. The function of the second recycle branch during the shutdown recovery process is the same as that of the first recycle branch; when the pressure in the first reactor 2 and the second reactor 3 drops to the first preset pressure value, the propylene in the two reactors is not easily liquefied at the disadvantageous points of the reactors. At this time, the first start-up heater 24 and the second start-up heater 34 can be turned off. Before closing the second gas-phase valve 312 and the fourth gas-phase valve 323 to isolate the second reactor 3 and the second condenser 31, just stop the second recycle gas blower 33 and the first recycle gas blower 23, which can avoid the surge of the recycle gas blower.

[0056] In some embodiments, the polypropylene production system further includes a purge gas branch. The two ends of the purge gas branch are respectively connected to the propylene feed system 54 and the gas lock system 5. An inlet control valve 53, a gas lock purge gas heater 51, and a program control valve are sequentially arranged on the purge gas branch. Step S1 includes:

[0057] S13. When the polypropylene production system is shut down, control the opening duration of the purge program valve and adjust the opening degree of the purge program valve to reduce the amount of propylene entering the first reactor 2 through the gas lock system 5. The purge program valve is located at the gas lock system. During the actual adjustment process, after gradually reducing the set target value of the inlet control valve 53 regarding the liquid levels of the two recycle tanks, the valve opening degree of the inlet control valve 53 automatically closes. A too large adjustment range will cause large fluctuations in the pressure and flow rate of the propylene feed system 54. After reducing the liquid level to the target value, then gradually adjust the valve opening degree of the inlet control valve 53 according to the opening degree. That is to say, after shutdown, the inlet control valve 53 not only needs to control the opening duration, but also needs to control the opening degree through the feedback of the liquid levels in the two recycle tanks.

[0058] Specifically, in the production process, the gas lock system 5 isolates the gas-phase components from the first reactor 2 to the second reactor 3. At this time, propylene needs to be introduced into the gas lock system 5 through the propylene feed system 54 to blow back the material components belonging to the first reactor 2 in the gas lock system 5 into the first reactor 2. During the discharging process of the gas lock system 5, the material components of the second reactor 3 are pressed into the second reactor 3 by high-pressure gaseous propylene, so that they do not enter the gas lock system 5 and the first reactor 2. The gas lock purging gas heater 51 is used to heat the propylene transported by the propylene feed system 54 to the gas lock system 5, increase the propylene temperature, and prevent propylene liquefaction; when the polypropylene production system is shut down, after terminating agents are injected into the first reactor 2 and the second reactor 3, since the reaction has not completely stopped and the polypropylene powder in the first reactor 2 and the second reactor 3 has not been completely emptied, during the subsequent discharging process, the polypropylene powder in the first reactor 2 also needs to be discharged into the second reactor 3 through the gas lock system 5. At this time, the propylene purging gas of the gas lock system 5 cannot be stopped, but the effect is limited. Then, the opening degree of the purging program valve is reduced to 15%, and at the same time, the opening time of the feed control valve 53 in a single purging action is reduced from 20 seconds to 5 seconds through program control to reduce the amount of propylene entering the reactor and improve the propylene recovery efficiency.

[0059] In some embodiments, the polypropylene production system further includes a liquid pump flushing liquid branch. One end of the liquid pump flushing liquid branch is respectively connected to the first quench liquid pump 221 and the second quench liquid pump 321 through two flushing control valves 57, and the other end of the liquid pump flushing liquid branch is connected to the propylene feed system 54 through the feed control valve 53. In step S4, when the first quench liquid pump 221 and the second quench liquid pump 321 stop, the flushing control valves 57 corresponding to the two quench liquid pumps are closed.

[0060] Specifically, based on the requirements of safe production, when the first quench liquid pump 221 and the second quench liquid pump 321 are working, propylene needs to be continuously introduced into the first quench liquid pump 221 and the second quench liquid pump 321 through the propylene feed system 54 as sealing gas for flushing and purging to protect the components in the first quench liquid pump 221 and the second quench liquid pump 321. Therefore, only when the propylene recovery is completed and the first quench liquid pump 221 and the second quench liquid pump 321 both stop working can the flushing control valve 57 be closed.

[0061] In some embodiments, the first condenser 21 includes a first cooling water circulation pipeline, and a first circulating cooling water valve 211 is provided on the first cooling water circulation pipeline; the second condenser 31 includes a second cooling water circulation pipeline, and a second circulating cooling water valve 311 is provided on the second cooling water circulation pipeline. Step S2 includes:

[0062] S21. When the pressure of the first reactor 2 drops below the first preset pressure value, close the first circulating cooling water valve 211. When the pressure of the second reactor 3 drops below the first preset pressure value and it is necessary to isolate the second reactor 3 from the second condenser 31 and the second circulating tank 32, increase the opening degree of the second circulating cooling water valve 311. The purpose of closing the first circulating cooling water valve 211 is to save energy and avoid unnecessary waste. Increasing the opening degree of the second circulating cooling water valve 311 is to further improve the condensation effect of the second condenser 31 to meet the condensation requirements of the recycle gas compressor 4.

[0063] Specifically, during the production process, the reaction pressure can be controlled by adjusting the flow rate of the circulating cooling water. When the opening degrees of the first circulating cooling water valve 211 and the second circulating cooling water valve 311 increase and the flow rate of the circulating cooling water becomes larger, the condensation effect is better, and more gaseous propylene is condensed into the first circulating tank 22 and the second circulating tank 32, and the pressures in the first reactor 2 and the second reactor 3 are lower. In addition, during actual production, the condensation area of the first condenser 21 is larger than that of the second condenser 31, and the condensation effect is also better. During the propylene recovery process of the present invention, since it is necessary to connect the first reactor 2 and the second reactor 3 in step S2, when the pressure difference between the first reactor 2 and the second reactor 3 is too large, connecting the first reactor 2 and the second reactor 3 rashly is dangerous and likely to damage the equipment. Therefore, after the pressure in the first reactor 2 drops to the first preset pressure value, control the opening degree of the first circulating cooling water valve 211 to 0%. When the pressure in the second reactor 3 also drops below the first preset pressure value and it is necessary to isolate the second reactor from the second condenser 31 and the second circulating tank 32, increase the opening degree of the second circulating cooling water valve 311. In some embodiments, the maximum opening degree of the cooling water valve of the second circulating tank 32 can be opened to 24% to fully improve the condensation effect of the second condenser 31 and improve the recovery effect after the subsequent connection of the first reactor 2 and the second reactor 3. Closing the first circulating cooling water valve 211 is because at this time the pressure in the first reactor 2 is low and the condensation effect is low, and the first circulating cooling water valve 211 can be closed to save water. The pressures in the first reactor 2 and the second reactor 3 have dropped to the first preset pressure value, and the risk of connection between the two sides is relatively low. Closing the first circulating cooling water valve 211 will not cause the problem of excessive pressure difference due to the inability to reduce the pressure in the first reactor 2, further improving the propylene recovery effect.

[0064] In addition, in some embodiments, the propylene feed system 54 is also connected to the first recycle gas blower 23 and the second recycle gas blower 33 through the seal flushing gas heater 52. Specifically, during production and shutdown recovery processes, the propylene feed system 54 needs to introduce propylene into the first recycle gas blower 23 and the second recycle gas blower 33 as seal flushing gas to provide seal protection for the mechanical components inside the first recycle gas blower 23 and the second recycle gas blower 33. In step S2, after the first recycle gas blower 23 and the second recycle gas blower 33 are shut down, the connection between the seal flushing gas heater 52 and the first recycle gas blower 23 and the second recycle gas blower 33 should also be isolated through valves. Instead, the valve between the high-pressure nitrogen HN system 56 and the first recycle gas blower 23 and the second recycle gas blower 33 is controlled to be half-open. At the same time, the inlet and outlet valves of the two recycle gas blowers are closed, and the high-pressure nitrogen is switched to conduct seal flushing for the first recycle gas blower 23 and the second recycle gas blower 33. Meanwhile, the high-pressure nitrogen and residual propylene inside the first recycle gas blower 23 and the second recycle gas blower 33 are introduced into the flare system 55 for pressure relief and replacement, and to avoid excessive internal pressure in the first recycle gas blower 23 and the second recycle gas blower 33 caused by continuous input of high-pressure nitrogen.

[0065] In some embodiments, in step S1, after fully opening the first discharge valve 222, the second discharge valve 322 is gradually opened to transfer all the liquid-phase propylene in the first recycle tank 22 to the discharge liquid refining unit 6; in step S3, the second discharge valve 322 is fully opened to transfer all the liquid-phase propylene in the second recycle tank 32 to the discharge liquid refining unit 6.

[0066] Specifically, during the actual shutdown and recovery process, due to the limited processing capacity of the effluent refining unit 6, if the first discharge valve 222 and the second discharge valve 322 are fully opened simultaneously in step S1, it is difficult for the effluent refining unit 6 to handle a large amount of liquid-phase propylene at the same time, and it is easy to be overloaded and damaged. The excess liquid-phase propylene can only be discharged and wasted through the flare system 55. Moreover, if the second discharge valve 322 is fully opened in advance to discharge the liquid-phase propylene in the second circulation tank 32, there will still be newly condensed liquid-phase propylene in the second circulation tank 32 in the subsequent step S2. Therefore, first fully open the first discharge valve 222 to empty the first circulation tank 22 faster within the processing capacity of the effluent refining unit 6, and gradually open the second discharge valve 322 so that the effluent refining unit 6 will not operate overloaded. In step S3, fully open the second discharge valve 322. At this time, the gaseous propylene in the first reactor 2 and the second reactor 3 has basically been fully recovered into the second circulation tank 32. Fully opening the second discharge valve 322 at this time can improve the recovery effect of propylene, and there is no risk of overloading the effluent refining unit 6. In fact, if all the liquid-phase propylene in the first circulation tank 22 is discharged in advance, the second discharge valve 322 can be fully opened so that the effluent refining unit 6 can continuously process propylene with a relatively high working load, thereby improving the recovery effect of propylene.

[0067] In addition, after the propylene in the first circulation tank 22 and the second circulation tank 32 is emptied, since the liquid level drops and continuing to operate the quench liquid pump at this time will cause cavitation of the pump and damage the equipment, it is necessary to stop the quench liquid pump. At this time, there is still a large amount of liquid-phase propylene remaining in the quench liquid pump inlet pipeline below the circulation tank. According to the conventional operation method, it is necessary to open the discharge valves at the inlet and outlet of the quench liquid pump to discharge the remaining liquid-phase propylene to the flare system 55. The flare system 55 is a low-pressure gas-phase pipe network system. Since a large amount of heat is absorbed when the liquid-phase propylene vaporizes during discharge, it will not only cause severe frosting of the quench liquid pump pipeline and may damage the pipeline, but also the discharge speed is very slow due to the low temperature effect, and the discharge process often takes more than 20 hours. In the present application, after the liquid-phase propylene in the first circulation tank 22 and the second circulation tank 32 is emptied, first close the emergency cut-off valve 58 below the circulation tank and the reflux valve of the quench liquid pump, and then introduce high-pressure nitrogen HN into the discharge pipeline at the outlet of the quench liquid pump. Since the water level of the discharge liquid refining unit 6 is relatively low, the discharge pipeline has a small diameter, a long length, many elbows, and a large fluid flow resistance. At this time, due to the density difference and resistance, the high-pressure nitrogen HN will flow to the top of the quench liquid pump inlet pipeline. Since the pressure of HN is higher than the back pressure of the discharge liquid refining unit 6, the remaining liquid-phase propylene at the inlet and outlet pipelines of the quench liquid pump will be discharged to the discharge liquid refining unit 6 under the action of pressure. When the liquid-phase propylene in the inlet and outlet pipelines of the quench liquid pump is emptied, the pressure of the discharge liquid refining unit 6 will rise rapidly, and at this time, it is necessary to close the HN valve in time. Using this optimized operation, the time for discharging the remaining pipeline at the inlet and outlet of the quench liquid pump is reduced from more than 20 hours originally to within 3 hours.

[0068] In some embodiments, a bag filter 41 is provided between the first control valve 42 and the recycle gas compressor 4. In step S2, the opening of the first control valve 42 is gradually increased step by step, and the pressure at the outlet of the first control valve 42 is controlled within the design pressure range of the bag filter 41 and above the minimum inlet pressure of the recycle gas compressor 4.

[0069] Specifically, by adjusting the opening of the first control valve 42 in real time, the second reactor 3 is flexibly connected to the recycle gas compressor 4, which not only avoids overpressure of the bag filter 41, but also efficiently uses the recycle gas compressor 4 to compress and transport the polymerization monomer propylene to the second condenser 31 for condensation and liquefaction. In one embodiment, when the pressure of the second reactor 3 is 2.0 MPaG, the opening of the first control valve 42 is initially opened by 3%, and the opening of the first control valve 42 gradually increases at a rate of about 3% every five minutes. Then, as the reactor pressure decreases, the opening of the first control valve 42 is gradually increased at a faster rate to control the pressure of the bag filter 41 within the range of 30 kPaG to 60 kPaG, so as to avoid both the recycle gas compressor 4 doing useless work due to too large an opening of the reflux valve caused by too low an inlet pressure and the waste or equipment damage caused by too high a pressure of the bag filter 41 resulting in the polymerization monomer being discharged to the flare system 55.

[0070] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and substitutions can be made, and these improvements and substitutions should also be regarded as the protection scope of the present invention.

Claims

1. A method for recovering polymerization monomers during the shutdown process of a polypropylene production system. The polypropylene production system includes a first reactor, a first condenser, a first circulation tank, a discharge liquid refining unit, a second circulation tank, a second condenser, and a second reactor that are connected in sequence through pipelines; a first quenching liquid pump and a first discharge valve are sequentially arranged on the pipeline connecting the first circulation tank and the discharge liquid refining unit, and a second quenching liquid pump and a second discharge valve are sequentially arranged on the pipeline connecting the second circulation tank and the discharge liquid refining unit; the first reactor and the second reactor are connected through an air lock system; the polypropylene production system further includes a recovery circulation branch, both ends of the recovery circulation branch are connected to the second reactor and the second condenser respectively, and a first control valve and a circulation gas compressor are sequentially arranged on the recovery circulation branch; a first gas-phase valve is arranged on the pipeline connecting the first reactor and the first condenser, and a second gas-phase valve is arranged on the pipeline connecting the second reactor and the second condenser; and a first discharge branch for discharging propylene to a flare system is also connected to the first reactor, a first flare discharge valve is arranged on the first discharge branch, and a second discharge branch for discharging propylene to the flare system is also connected to the second reactor, a second flare discharge valve is arranged on the second discharge branch; It is characterized in that the method includes the following steps: Step S1: After the polypropylene production system is shut down, control the first gas-phase valve, the second gas-phase valve, the first discharge valve, and the second discharge valve to open, close the first flare discharge valve and the second flare discharge valve, and close the air lock system. Operate the first quenching liquid pump and the second quenching liquid pump, and the circulation gas compressor operates in self-circulation. The remaining gas components in the first reactor are condensed to a liquid state by the first condenser and then flow to the first circulation tank, and are pumped to the discharge liquid refining unit by the first quenching liquid pump. The remaining gas components in the second reactor are condensed to a liquid state by the second condenser and then flow to the second circulation tank, and are pumped to the discharge liquid refining unit by the second quenching liquid pump; Step S2: When the pressures in the first reactor and the second reactor drop below a first preset pressure value and the pressure difference between the first reactor and the second reactor does not exceed a threshold value, control the air lock system to open, close the second gas-phase valve, and open the first control valve. The remaining gas components in the first reactor and the remaining gas components in the second reactor are pressurized by the circulation gas compressor and then transported to the second condenser; Step S3: When the pressures in the first reactor and the second reactor drop to a second preset pressure value, close the air lock system, the first control valve, and the first gas-phase valve, stop the operation of the circulation gas compressor, control the first flare discharge valve and the second flare discharge valve to open, and discharge the gaseous propylene in the first reactor and the second reactor to the flare system; Step S4: After all the liquid-phase propylene in the first circulation tank is transferred to the discharged liquid refining unit, close the first discharge valve and stop the first quench liquid pump; after all the liquid-phase propylene in the second circulation tank is transferred to the discharged liquid refining unit, close the second discharge valve and stop the second quench liquid pump.

2. The method for recovering polymerization monomers during the shutdown process of the polypropylene production system according to claim 1, wherein the polypropylene production system further includes a feed branch, one end of the feed branch is respectively connected to the first circulation tank and the second circulation tank, and the other end of the feed branch is connected to a propylene feed system. Characterized in that The step S1 includes: S11: Before the shutdown of the polypropylene production system, by reducing the liquid-level control target values of the liquid-phase propylene in the first circulation tank and the second circulation tank, so as to reduce and adjust the feed amount from the propylene feed system into the first circulation tank and the second circulation tank.

3. The method for recovering polymerization monomers during the shutdown process of the polypropylene production system according to claim 2, wherein the polypropylene production system further includes a first circulation branch and a second circulation branch. The two ends of the first circulation branch are respectively connected to the first circulation tank and the first reactor, and a first circulation gas blower, a first start-up heater and a third gas-phase valve are successively arranged on the first circulation branch; the two ends of the second circulation branch are respectively connected to the second circulation tank and the second reactor, and a second circulation gas blower, a second start-up heater and a fourth gas-phase valve are successively arranged on the second circulation branch. Characterized in that The step S1 includes: S12: Control the third gas-phase valve to open, operate the first circulation gas blower, and the first circulation gas blower pumps the gas-phase propylene in the first circulation tank to the first start-up heater for heating and then transports it into the first reactor; control the fourth gas-phase valve to open, operate the second circulation gas blower, and the second circulation gas blower pumps the gas-phase propylene in the second circulation tank to the second start-up heater for heating and then transports it into the second reactor.

4. The method for recovering polymerization monomers during the shutdown process of the polypropylene production system according to claim 3. Characterized in that In the step S2, when the pressures in the first reactor and the second reactor drop below the first preset pressure value, close the first start-up heater and the second start-up heater; close the fourth gas-phase valve before closing the second gas-phase valve, and stop the second circulation gas blower and the first circulation gas blower.

5. The method for recovering polymerization monomers during the shutdown process of the polypropylene production system according to claim 2, wherein the polypropylene production system further includes a purge gas branch, the two ends of the purge gas branch are respectively connected to the propylene feed system and the air lock system, and a feed control valve, an air lock purge gas heater and a purge program valve are successively arranged on the purge gas branch. Characterized in that The step S1 includes: S13: When the polypropylene production system shuts down, control the opening duration and opening degree of the purge program valve to reduce the amount of propylene entering the first reactor through the air lock system.

6. The method for recovering polymerization monomers during the shutdown process of the polypropylene production system according to claim 5, wherein the polypropylene production system further includes a liquid pump flushing liquid branch. One end of the liquid pump flushing liquid branch is respectively connected to the first quench liquid pump and the second quench liquid pump through two flushing control valves, and the other end of the liquid pump flushing liquid branch is connected to the propylene feed system through a feed control valve. Characterized in that, In step S4, when the first quench liquid pump and the second quench liquid pump stop, the flushing control valves connected to the respective quench liquid pumps are closed.

7. The method for recovering polymerization monomers during the shutdown process of the polypropylene production system according to claim 1, wherein the first condenser includes a first cooling water circulation pipeline, and a first circulating cooling water valve is provided on the first cooling water circulation pipeline; the second condenser includes a second cooling water circulation pipeline, and a second circulating cooling water valve is provided on the second cooling water circulation pipeline. Characterized in that, The step S2 includes: S21. When the pressure of the first reactor drops below the first preset pressure value, the first circulating cooling water valve is closed; when the pressure of the second reactor drops below the first preset pressure value, the opening degree of the second circulating cooling water valve is increased.

8. The method for recovering polymerization monomers during the shutdown process of the polypropylene production system according to claim 1. Characterized in that, In step S1, the first discharge valve is fully opened and the second discharge valve is gradually opened to transfer all the liquid-phase propylene in the first circulation tank to the discharge liquid refining unit; in step S3, the second discharge valve is fully opened to transfer all the liquid-phase propylene in the second circulation tank to the discharge liquid refining unit.

9. The method for recovering polymerization monomers during the shutdown process of the polypropylene production system according to claim 1, wherein a bag filter is provided between the first control valve and the recycle gas compressor. Characterized in that, In step S2, the opening degree of the first control valve is gradually increased at an accelerating rate to control the pressure at the outlet of the first control valve within the design pressure range of the bag filter.

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