Initiator injection line for high pressure polymerization

Abstract

The present invention relates to a process for preparing a polymer, preferably polyethylene, in a polymerization reactor system comprising an initiator injection line. The method includes operating a back pressure valve of the initiator injection line to maintain pressure upstream of the back pressure valve and downstream of the initiator pump. The invention also relates to an initiator injection line comprising a back pressure valve and a polymerization reactor system comprising said initiator injection line.

Description

Technical Field

[0001] The present invention relates to a method for producing a polymer, preferably a polyethylene polymer, in a polymerization reactor system. More particularly, but not exclusively, the method involves operating a backpressure valve in an initiator injection line to maintain pressure upstream of the backpressure valve and downstream of an initiator pump. The present invention also relates to an initiator injection line including a backpressure valve and a polymerization reactor system including the initiator injection line. Background Art

[0002] High pressure polymerization manufacturing method requires good temperature control. The production of polymers such as polyethylene (such as LDPE) is an exothermic process and therefore produces heat. When initiator is consumed and polymerization stops, the temperature in the reactor reaches a peak value. It is necessary to cool the reaction temperature to remain within a safe and required limit, and to reduce the temperature of the reaction mixture to a suitable initiation temperature to contact with the other initiator in the subsequent reaction zone. Reactor jacket water system can be used to remove heat from the polymerization reactor. In addition, a cooler monomer side stream can be introduced in the cooling zone between the reaction zones along the length of the reactor. Multiple initiator injection points can be provided along the length of the tubular reactor and be used to control the temperature distribution of the reactor.

[0003] When the temperature generated inside the reactor is not well controlled, the heat generated by polymerization may exceed the cooling capacity of the reactor jacket water system. In this case, the temperature inside the reactor may increase rapidly to undesirable levels. Temperatures exceeding the ideal polymerization temperature may potentially cause undesirable reactions, such as "runaway" or "reactor decomposition" reactions.

[0004] The properties of polymers produced in high pressure polymerization processes are affected by the reactor temperature profile. Therefore, good reactor temperature control is important for achieving the desired polymer properties.

[0005] Initiator (usually oxygen or organic free radical initiator) is injected into the reactor system to initiate polymerization. Usually, a mixture of different organic peroxides with different half-life temperatures is used according to the required reaction distribution. For safety and easier handling and metering reasons, the organic peroxide (mixture) is diluted in an inert organic solvent. Initiator (mixture) is injected into the reactor at one or more points to start polymerization. Injection is usually carried out by a high-pressure initiator injection pump. A pump usually equipped with a standby pump is used to inject initiator into the reaction zone through the injection point. Pump output (initiator flow) is controlled by the temperature measured in the reactor (single temperature or the temperature from the peak picker (a peak picker) in the reaction zone). See, for example, U.S. Patent Application No. 3,628,918 or U.S. Publication Nos. 2005 / 0192414, US 2019 / 0299182 A1, WO 2004 / 108271, WO 2007 / 018871, WO 2011 / 008 197 or WO 2013 / 154690. Other background references include EP 2 481 477 A and WO 2014 / 046835.

[0006] Under normal operating conditions of the reactor and initiator injection lines, the conditions within the reactor can be precisely controlled. However, disturbances in the initiator / monomer ratio within the reactor can cause elevated temperatures. If left unchecked, elevated temperatures can lead to product degradation and monomer and polymer decomposition.

[0007] Initiator injection line is easy to block or clog. Pressure accumulation in the initiator injection line upstream of this plug (a plug) may eventually eliminate obstruction, which may cause initiator to excessively enter the reactor, because the initiator accumulated is discharged into the reactor. Initiator is excessive and can cause the rapid rise of undesirable side reactions and reactor temperature. Moreover, the initiator injection line usually uses an oscillating pump (an oscillating pump), which causes the initiator to enter the flow rate (flow rate) of the reactor little (but significant). Expectation provides better control of the initiator flow rate entering the tubular reactor.

[0008] The initiator is delivered to the high-pressure polymerization tubular reactor in a solvent. Therefore, poorly controlled initiator injection into the reactor may also result in increased solvent injection into the reactor. The solvent can act as a chain transfer agent in the polymerization reaction and may affect the melt index properties of the polymer.

[0009] References of potential interest include WO 2017 / 106,940 A1; US 8,308,087; US 6,384,153; US2006 / 0167193 A1, US 8,217,124; WO 2018 / 127,533 A1; US 4,008,049; US 3,506,715; SU1,016,303 A; CN112973575.

[0010] The present invention seeks to alleviate the above problems.Alternatively or additionally, the present invention seeks to provide an improved initiator injection line and polymerisation process using the same, which provides a smooth and controlled flow rate of initiator to the reactor. Summary of the Invention SUMMARY OF THE INVENTION

[0011] The present invention provides, according to a first aspect, a method for producing a polymer in a polymerization reactor system, wherein the method is according to claim 1. The polymerization reactor system comprises a tubular reactor. A monomer source, a primary compressor, and a secondary compressor are provided upstream of the tubular reactor. The primary compressor is arranged to receive monomer from the monomer source. The secondary compressor is arranged downstream of the primary compressor. Thus, monomer flows from the monomer source to the primary compressor and from the primary compressor to the secondary compressor. The primary compressor and the secondary compressor pressurize the monomer feed before the monomer enters the tubular reactor. A pressure release valve is located downstream of the tubular reactor, and the product mixture exits the tubular reactor via the pressure release valve. The polymerization reactor system further comprises an initiator source. The initiator source is configured to supply initiator to an initiator pump of an initiator injection line. The initiator source can be located upstream of the initiator injection line. Alternatively, the initiator source can be located within the initiator injection line, for example, adjacent to the pump. As explained herein, the outlet of the initiator injection line is connected to the tubular reactor at an initiator injection point. The initiator injection line supplies initiator to the initiator injection point of the tubular reactor.

[0012] Preferably, the polymerization reactor system is a high-pressure polymerization reactor system. The tubular reactor can be a high-pressure tubular reactor, which operates at a pressure exceeding 1000 bar, optionally exceeding 2500 bar, for example, at a pressure of 2800 bar to 3200 bar. The pressure at which the tubular reactor is operated can be referred to as operating pressure. The tubular reactor can operate at a temperature of 150°C to 400°C, for example, 200°C to 300°C. Monomer is passed through a primary compressor and a secondary compressor so that the monomer is pressurized or compressed to a pressure of at least 1000 bar, for example, 2000 bar, optionally to the reactor pressure. After passing through the primary compressor and the secondary compressor, the monomer is introduced into the tubular reactor, where the monomer undergoes polymerization.

[0013] Optionally, the tubular reactor is used for high pressure polymerization of monomer and comonomer.When the process comprises polymerizing a monomer, preferably ethylene, with a comonomer, the polymerization reactor further comprises a comonomer source, and the process comprises introducing the comonomer into the tubular reactor.

[0014] The initiator injection line includes an initiator pump and an outlet located downstream of the pump. The outlet is connected to the tubular reactor at the initiator injection point. The outlet allows initiator to flow from the initiator injection line into the reactor at the initiator injection point. The outlet can be in the form of a sparger. The pump is configured to pump initiator from an initiator source (e.g., a storage tank for accommodating initiator) through the outlet and into the tubular reactor at the initiator injection point. The initiator pump can be an oscillating pump. The initiator pump can be a hydraulic piston pump. The total output of the initiator pump can be controlled to meet the desired reactor temperature distribution. In order to obtain a stable initiator supply, the initiator pump should be operated under its optimal output range. For example, too low a pump output may cause initiator injection instability (pressure pulsation) and subsequent reactor temperature fluctuations. The initiator concentration in the initiator mixture is selected so that the pump operates within the desired output range.

[0015] The initiator injection pipeline further includes a back-pressure valve located downstream of the initiator pump and upstream of the outlet. The back-pressure valve has a set pressure and is configured to maintain the pressure in the initiator injection pipeline upstream of the back-pressure valve and downstream of the initiator pump equal to the set pressure.

[0016] The method according to the first aspect of the present invention comprises the following steps: (i) pressurizing a monomer from the monomer source in the primary compressor and the secondary compressor, and introducing the monomer into a tubular reactor; (ii) operating the initiator pump so that the initiator from the initiator source flows through the backpressure valve and enters the tubular reactor at the initiator injection point; (iii) polymerizing the monomer in the tubular reactor to form a polymer; (iv) discharging a product mixture comprising the polymer and unreacted monomers from the tubular reactor via the pressure release valve; and (v) Isolation of polymer: Wherein step (ii) further comprises operating the back pressure valve at the set pressure, wherein the set pressure is in the range of 50 bar to 400 bar greater than the pressure of the tubular reactor at the initiator injection point.

[0017] It should be understood that the so-called pressure of a tubular reactor refers to the operating pressure of the tubular reactor. Therefore, the set pressure is directly related to the operating pressure of the tubular reactor. The operating pressure of the reactor is in the range of 1000 bar to 3500 bar. The pressure in the tubular reactor (e.g., at the initiator injection point) can fluctuate within the acceptable limits of the operating pressure during the reaction process. The set pressure is not adjusted in response to such fluctuations. However, if the operating pressure of the tubular reactor is adjusted, for example, to produce a specific polymer grade, the set pressure can be adjusted according to the new operating pressure.

[0018] Optionally, the method for preparing a polymer according to the first aspect of the invention is a continuous method. In a continuous method, two or more of the method steps (i) to (v) are performed simultaneously. Preferably, all method steps (i) to (v) are performed simultaneously.

[0019] Initiator is injected into the tubular reactor to initiate the polymerization reaction.Initiator is supplied to the initiator injection point(s) in the tubular reactor using at least one initiator injection line according to the present invention.

[0020] The product mixture comprises polymer and optional unreacted monomer. If comonomer is added to the reaction mixture, the product mixture may further comprise unreacted comonomer. The polymer mixture is discharged from the polymer, and the polymer is isolated from the mixture, for example, by passing the product mixture through at least one separation vessel.

[0021] The back pressure valve of the initiator injection line is a variable pressure regulating valve or a control valve that regulates the pressure upstream of the valve. The back pressure valve can be used to generate a higher pressure upstream of the valve by limiting the flow of the initiator through the opening in the valve. Alternatively, the back pressure valve has an adjustable opening for controlling the pressure upstream of the back pressure valve. The opening size of the valve can be continuously adjusted to regulate the pressure upstream of the back pressure valve so that the upstream pressure remains at a set pressure. The set pressure of the back pressure valve is in the range of 50 bar to 400 bar greater than the pressure at the initiator injection point of the tubular reactor. For example, the set pressure can be 200 bar greater than the pressure at the initiator injection point of the tubular reactor.

[0022] In operation, by changing the flow rate of the initiator passing through the opening of the back-pressure valve, the back-pressure valve maintains the pressure of the initiator injection line upstream of the back-pressure valve at a set pressure. If the pressure upstream of the valve exceeds the set pressure, the opening of the back-pressure valve is widened to reduce the upstream pressure. If the pressure upstream of the valve drops below the set pressure, the opening of the back-pressure valve is narrowed to increase the upstream pressure. The set pressure of the valve can be adjustable, for example, if the operating pressure of the tubular reactor is regulated. Different polymerization methods require different polymerization conditions (such as different operating pressures and temperatures) of the tubular reactor. The set pressure of the back-pressure valve can be adjusted according to the operating pressure of the reactor. It should be understood that the back-pressure valve maintains the pressure upstream of the valve within the acceptable limits of the set pressure. For example, within ± 20 bar of the set pressure, or within ± 10 bar of the set pressure.

[0023] The back-pressure valve can smooth the pulse output inherent in the initiator pump (e.g., oscillating pump). As explained above, the back-pressure valve keeps the pressure in the initiator injection line upstream of the back-pressure valve higher than the pressure downstream of the valve. The initiator is slightly compressed between the initiator pump and the back-pressure valve. The compressed initiator acts like a "capacitor" in the system. When the pump output drops momentarily, for example, when the pump cylinder reaches the end of its stroke and changes direction, the compressed initiator expands slightly, thereby compensating for the loss of the pump output. This ultimately results in a more constant and consistent flow rate for the initiator to enter the tubular reactor. Since the initiator concentration in the reactor controls the temperature in the reactor, a more stable peak reactor temperature can be achieved using the initiator injection line of the present invention.

[0024] The size of the opening of the back-pressure valve can be adjusted (preferably continuously) to control the pressure upstream of the back-pressure valve. The size of the opening of the back-pressure valve can be adjusted, for example, by a plug. The plug can be a tapered plug, wherein the plug is inserted into the opening to close the opening. Since the plug is tapered, the opening will not be completely closed until the diameter of the plug equal to the diameter of the opening has been inserted into the opening of the back-pressure valve. This allows fine control of the size of the opening and therefore allows pressure regulation upstream of the valve. It should be understood that the diameter of the opening itself is not adjusted, but the opening is blocked by the plug. Alternatively, in another embodiment, the diameter of the opening can be adjusted to change the size of the opening of the back-pressure valve.

[0025] The back pressure valve is set at a pressure in the range of 50 to 400 bar greater than the pressure of the tubular reactor at the initiator injection point. Optionally, the set pressure is in the range of 100 to 350 bar greater, or 150 to 300 bar greater, or 200 to 300 bar greater, or 225 to 275 bar greater than the pressure inside the tubular reactor at the initiator injection point. Possibly, such a pressure differential allows for smooth output of the initiator from the initiator injection line.

[0026] The optimal set pressure can be determined via empirical testing of the reactor system, but will be within the range of 50 bar to 400 bar greater than the reactor pressure at the initiator injection point. The back pressure valve can initially be set to any set pressure, for example, a set pressure within the range of 200 bar to 275 bar greater than the reactor pressure at the initiator injection point. The output of the initiator injection line can then be monitored to evaluate the flow velocity of the initiator entering the reactor. The set pressure can be adjusted according to the output of the initiator injection line. For example, if the output oscillates, the set pressure may increase, for example, increasing in increments of 20 bar, until the monitored output no longer oscillates, or at least reduces. On the contrary, if the flow of the initiator flowing out of the initiator injection line is insufficient, the set pressure can be reduced, for example, reducing in increments of 20 bar, until the flow velocity of the initiator entering the initiator injection line monitored reaches an optimum value.

[0027] Optionally, the polymerization reactor system also includes a control system that is configured to reduce the flow rate of the initiator through the outlet and therefore into the tubular reactor at the initiator injection point when the pressure in the initiator injection line between the initiator pump and the back pressure valve is equal to the first set point pressure. The first set point pressure is a pressure in the range of 100 bar to 200 bar greater than the set pressure of the back pressure valve. The first set point pressure can be selected according to the specific reactor setting (e.g., the operating pressure of the reactor). The control system can control the output of the pump to control the flow rate of the initiator through the outlet. Additionally or alternatively, the control system can control one or more valves downstream of the initiator pump to control the flow rate of the initiator through the outlet. The initiator injection line can include one or more pressure probes configured to measure the pressure in the initiator injection line. The pressure measured by the pressure probe can be converted into an analog signal to be input into the control system. Preferably, at least one pressure probe is located in the initiator injection line upstream of the back pressure valve and downstream of the initiator pump, so that the pressure between the initiator pump and the back pressure valve in the initiator injection line can be monitored using at least one pressure probe. At least one pressure probe can be a pressure transmitter, which is used to measure the incoming pressure and convert it into an analog signal for system control. Suitable pressure transmitters are known to those skilled in the art. Blockage or clogging of the initiator may form in the initiator injection line. Back pressure valves are particularly prone to blockage because the opening of the valve is usually the narrowest part of the initiator injection line. The reduction in the initiator flow rate through the outlet minimizes the further accumulation of initiator upstream of the plug. It can also reduce the risk of excessive initiator entering the reactor. Optionally, the control system includes an alarm system that can warn the operator via visual or auditory prompts that the pressure in the initiator injection line is equal to the first set point pressure, so there may be a blockage in the initiator injection line. For example, when the pressure in the initiator injection line between the initiator pump and the back pressure valve is equal to the first set point pressure, the alarm system can trigger an audible alarm, such as an alarm, and / or can generate a warning message in a graphical user interface of a computer (such as a human-machine interface (HMI) or a console connected to the control system).

[0028] The control system can be a distributed control system (DCS). As used herein, a distributed control system (DCS) refers to a control system for a process or plant in which control elements are distributed throughout the system. A DCS typically uses a custom-designed processor as a controller and communicates using proprietary interconnects and standard communication protocols. Input and output modules form an integral part of a DCS. The processor receives information from the input module and sends it to the output module. The input module receives information from the input instrument in the process (or field), and the output module transmits instructions to the output instrument in the field. The input and output can be continuously changing analog signals or discrete signals with two states of on or off. A computer bus or electrical bus connects the processor and modules via a multiplexer or demultiplexer. The bus also connects the distributed controllers to a central controller and ultimately to a human-machine interface (HMI) or control console. A DCS is used to control continuous or intermittent manufacturing processes. The DCS is connected to sensors (e.g., pressure probes) and actuators (e.g., valves) and uses set point control to control the flow of materials through the plant. One of the most common examples is a set point control loop consisting of a pressure sensor, a controller, and a control valve. Pressure or flow measurements are typically transmitted to a controller via signal-conditioning input / output (I / O) devices. When the measured variable reaches a certain point, the controller instructs a valve or actuator to open or close until the fluid flow process reaches the desired set point. A typical DCS consists of functionally and / or geographically distributed digital controllers capable of executing 1 to 256 or more regulatory control loops in a single control box. The input / output (I / O) devices can be integrated with the controller or remotely located via a field network. Today's controllers have extensive computing capabilities and can often perform logic and sequential control in addition to proportional, integral, and derivative (PID) control. DCSs are typically designed with redundant processors to enhance control system reliability. Most systems have displays and configuration software that enable the end user (operator) to configure the control system without requiring low-level programming. A DCS may utilize one or more workstations and can be configured at the workstation or by an offline personal computer. Local communications are handled by the control network, transmitted via twisted pair, coaxial, or fiber optic cables. Servers and / or application processors may be included in the system for additional computing, data collection, and reporting capabilities.

[0029] Optionally, the initiator injection line may further include a shut-off valve located upstream of the outlet and downstream of the back-pressure valve. The control system may be configured to close the shut-off valve when the pressure in the initiator injection line between the initiator pump and the back-pressure valve equals a second set-point pressure, wherein the second set-point pressure is a pressure in the range of 200 to 600 bar greater than the set pressure of the back-pressure valve. Thus, when the pressure in the tubular reactor upstream of the back-pressure valve and downstream of the pump exceeds the first set-point pressure and equals a higher second set-point pressure, the control system closes the shut-off valve to prevent initiator from entering the tubular reactor at the initiator injection point. This can prevent initiator from being excessively added to the tubular reactor. For example, when a blockage initially forms in the initiator injection line (e.g., at the back-pressure valve), a potential overdose may occur, which is then subsequently released due to increased pressure behind the blockage. If the shut-off valve is not closed, this will result in an overdose of initiator being added to the reactor. The second set-point pressure depends on the specific characteristics of the reactor and will be less than the set pressure of the bursting disc. The initiator injection line may include at least one additional shut-off valve located upstream of the aforementioned shut-off valve. The at least one additional shut-off valve may be used to isolate a section of the initiator injection line.

[0030] Optionally, the initiator injection line also includes an emergency discharge valve (dump valve) located upstream of the back pressure valve. The initiator injection line can also include a waste tank (waste tank) that is in fluid communication with the emergency discharge valve. Optionally, the control system is configured to open the emergency discharge valve to transfer initiator to the waste tank when the pressure in the initiator injection line between the initiator pump and the back pressure valve is equal to the second set point pressure. Therefore, the initiator in the initiator injection line can be transferred to the waste tank by opening the emergency discharge valve. Once the shut-off valve is closed, this allows initiator to escape the initiator injection line. In normal operation, the emergency discharge valve is closed. The emergency discharge valve is preferably opened after closing the shut-off valve. This prevents initiator or reaction mixture from flowing back from the reactor via the initiator injection point. Backflow may cause, for example, obstruction in a distributor or back pressure valve. Optionally, in order to minimize or eliminate backflow from the reactor, after closing the shut-off valve, for example, for more than 1 second, for example, for more than 2 seconds, more than 5 seconds, or more than 10 seconds after closing the shut-off valve, the emergency discharge valve is opened. Optionally, the emergency discharge valve is opened no more than 2 minutes, no more than 1 minute or no more than 30 seconds after closing the shut-off valve. For example, the emergency discharge valve can be opened at a time point in the range of 1 second to 30 seconds after closing the shut-off valve. When the supply of initiator to the reactor is stopped, the shut-off valve can also be closed to prevent the reaction mixture (gas and polymer) from flowing back into the initiator injection system.

[0031] As disclosed herein, a tubular reactor can include multiple initiator injection points distributed along the length of the reactor. Each initiator injection point defines the starting point of the reaction zone. It should be understood that the term "multiple" herein refers to two or more. The initiator injection points are supplied by at least one initiator injection line according to the present invention. It is possible that a single initiator injection line supplies all or most of the initiator injection points. It should be understood that if the initiator injection line supplies more than one initiator injection point, the initiator injection line will include more than one outlet. Each outlet can be paired with an initiator injection point so that the initiator injection point is connected to at most one outlet. It is possible that the initiator injection line includes a branch conduit located downstream of the backpressure valve. After the initiator has passed through the backpressure valve, the feed stream can be diverted into several fractions by the branch conduit. Each fraction can then travel down a single conduit to the outlet and enter the reactor at a single initiator injection point. In this way, initiator can be supplied to multiple initiator injection points. Thus, optionally, the initiator injection line has a plurality of outlets, and the tubular reactor comprises a plurality of initiator injection points, and each of the plurality of outlets of the initiator injection line is connected to any one of the plurality of initiator injection points, provided that each of the plurality of initiator injection points is connected to at most one of the plurality of outlets; and wherein step (ii) of the method comprises operating the initiator pump such that the initiator from the initiator source flows through the backpressure valve and enters the tubular reactor at the plurality of initiator injection points. Each outlet is connected to at most one initiator injection point.

[0032] The polymerization system can include two or more initiator injection lines according to the present invention. Optionally, when an initiator source is located at an initiator line upstream, at least two of the two or more initiator injection lines can be supplied by identical initiator sources. Optionally, the polymerization system comprises two or more initiator sources, wherein each initiator source only supplies a part in two or more initiator injection lines, preferably only one. The initiator source can be located at the upstream of the initiator injection line, or in the initiator injection line, for example, downstream of an initiator pump. Each initiator source can include different initiator mixtures so that each of the two or more initiator injection lines supplies different initiator mixtures to initiator injection point (one or more).

[0033] Optionally, a cooling zone is located downstream of at least one reaction zone. The cooling zone is a region of the tubular reactor where the temperature of the reaction mixture comprising the polymer and optional monomers and optional comonomers is reduced before the reaction mixture enters an additional reaction zone or exits the reactor. Optionally, the tubular reactor comprises a plurality of reaction zones spaced apart from the cooling zone.

[0034] Optionally, the polymerization reactor system further includes an autoclave reactor upstream of the tubular reactor, and the tubular reactor is a tubular tail reactor (a tubular tail reactor) configured to receive an outlet stream from the autoclave reactor, wherein the outlet stream comprises polymer and unreacted monomer. It should be understood that after polymerization in the autoclave reactor, the outlet stream discharged from the autoclave reactor comprises a mixture of polymer and unreacted monomer. If comonomer is present in the autoclave reaction mixture, the outlet stream may also comprise unreacted comonomer. It should be understood that unreacted monomer and unreacted comonomer are (co)monomers that have been exposed to polymerization conditions in the reactor but have not yet undergone polymerization. The tubular tail reactor is configured to receive unreacted monomer in the outlet stream of the autoclave reactor and polymerize the unreacted monomer. Suitable tubular tail reactors are known to those of ordinary skill in the art. Optionally, a plurality of tubular tail reactors can be arranged in series. Initiator is injected into the tubular tail reactor to initiate polymerization. Optionally, in addition to the outlet stream, fresh monomer and / or comonomer are injected into the tubular tail reactor.

[0035] Preferably, the monomer is ethylene and the polymer is a polyethylene polymer. If ethylene is the only monomer present, the polyethylene polymer will be a polyethylene homopolymer. For example, the polyethylene may be LDPE. Optionally, a comonomer may be used in the polymerization reaction, and the resulting polyethylene polymer may be a copolymer of ethylene and the comonomer.

[0036] According to a second aspect of the present invention, an initiator injection line for use in accordance with the method of the first aspect of the present invention is also provided. It should be understood that the initiator injection line of the second aspect of the present invention may include any of the features described relative to the first aspect of the present invention. The back-pressure valve of the initiator injection line has an adjustable opening for controlling the pressure upstream of the back-pressure valve. The size of the opening is adjustable to keep the pressure in the initiator injection line upstream of the back-pressure valve and downstream of the initiator pump equal to the set pressure of the back-pressure valve. Regulating the opening allows the back-pressure valve to maintain the pressure upstream of the valve at the set pressure. A back-pressure valve is a control valve, and suitable back-pressure valves are known to those skilled in the art.

[0037] Optionally, the outlet of the initiator injection line can be in the form of a distributor. The distributor allows for efficient mixing of the initiator in the reaction mixture within the reactor.

[0038] Optionally, the initiator injection line also includes at least one check valve (check valve) positioned at the initiator injection outlet upstream and the back pressure valve downstream. The initiator injection line can include multiple check valves, for example, 2 check valves. Optionally, at least one check valve is positioned at outlet upstream and is no more than 1000mm, for example, is positioned at outlet upstream and is no more than 500mm or is no more than 200mm. The at least one check valve prevents backflow into the initiator injection line. If obstruction occurs, a sudden pressure drop may be experienced in the initiator injection line. The check valve prevents initiator and / or reaction mixture from flowing back toward the back pressure valve. Suitable check valves are known to the technician.

[0039] Optionally, the initiator injection line also includes a three-way valve (three-wayvalve) positioned at the back pressure valve downstream and the outlet upstream. Suitable three-way valve is well known to those skilled in the art, and includes a first port for receiving the initiator, a second port for guiding the initiator to the outlet, and a third port for being connected to a waste tank. The three-way valve can operate with a first configuration and a second configuration, and in the first configuration, the flow direction is from the first port to the second port, and in the second configuration, the flow direction is from the first port to the third port. In the first configuration, the three-way valve allows the initiator to flow from the initiator pump through the initiator injection line to the initiator injection point in the reactor. In the second configuration, the initiator stream can be transferred to the waste tank. The waste tank can be a waste tank that is in fluid communication with the emergency discharge valve. The three-way valve can allow the initiator pipeline to be washed with a solvent, for example, to remove obstruction, particularly when the three-way valve is in the second configuration. The three-way valve can allow the initiator injection line to be flushed with a solvent before or after the initiator passes through the pipeline. It should be understood that the three-way valve can alternatively be two single valves (single valve), which are configured to assist the flushing of the pipeline in the same manner as the three-way valve. One of the two single valves is configured to control the flow from the initiator source to the outlet, and can be used to isolate the initiator injection line from the tubular reactor. The second of the two single valves allows flushing the initiator injection line. Solvent can be injected into the initiator injection line before the initiator mixture passes through the pipeline. For example, when the reactor is started, it may be necessary to send a minimum solvent flow to the reactor initiator injection point before the initiator is injected to confirm that there is no blockage before startup, and optionally, apply a minimum solvent flow to keep the injection point open when the initiator injection has not yet begun. Similarly, when the reactor is closed, it may be necessary to send a minimum solvent flow to the reactor initiator injection point after the initiator is injected. This may help prevent the blockage in the initiator injection line, such as the blockage in the distributor. Suitable solvents include, but are not limited to, isododecane, Isopar TM .

[0040] Initiator injection pipeline can have a total length of at least 6m, at least 10m, at least 20m, at least 50m or at least 100m. Initiator injection pipeline can have a total length of at most 200m, at most 140m or at most 100m. Initiator injection pipeline can have a total length of 6m to 200m. The length of initiator injection pipeline is measured from initiator pump to initiator outlet along initiator flow path. Initiator injection pipeline can have in the scope of 1mm to 10mm, optionally in the scope of 1mm to 3mm, for example, an internal diameter of 2mm.

[0041] Optionally, the initiator pump is within a range of 5 to 100 m upstream of the back pressure valve. Optionally, the initiator pump is within a range of 10 to 50 m upstream of the back pressure valve. Optionally, the initiator pump is at least 2 m, at least 5 m, or at least 10 m upstream of the back pressure valve. Optionally, the initiator pump is at most 100 m, at most 75 m, or at most 50 m, or at most 30 m upstream of the back pressure valve.

[0042] Optionally, the outlet is in the range of 1m to 100m downstream of the back pressure valve. Optionally, the outlet is in the range of 5m to 40m downstream of the back pressure valve. Optionally, the outlet is at least 1m, or at least 2m, or at least 5m downstream of the back pressure valve. Optionally, the outlet is at most 100m, at most 70m, at most 40m, at most 10m, or at most 5m downstream of the back pressure valve.

[0043] According to a third aspect of the present invention, there is provided a polymerization reactor system comprising an initiator injection line according to the second aspect of the present invention. The polymerization reactor system is suitable for use in the method according to the first aspect of the present invention. Preferably, the polymerization reactor system is used for the high pressure polymerization of ethylene to polyethylene.

[0044] Optionally, the polymerizer system also includes at least one autoclave reactor upstream of the tubular reactor, and wherein the tubular reactor is a tubular tail reactor, which is configured to receive an outlet stream comprising polymer and unreacted monomer from the autoclave reactor. When the polymerizer system also includes at least one autoclave reactor, the tubular reactor is a tubular tail reactor, which is configured to receive an outlet stream comprising polymer, unreacted monomer and optional unreacted comonomer from the autoclave reactor. The outlet stream of the autoclave reactor is transferred to the inlet of the tubular reactor. The polymerizer may include multiple autoclave reactors, such as at least two or at least three autoclave reactors. The polymerizer may include multiple tubular tail reactors, such as at least two or at least three tubular tail reactors. Optionally, at least one cooling zone is located between the autoclave reactor and the tubular tail reactor. The cooling zone may include a cooler, which reduces the temperature of the outlet stream of the autoclave to the inlet temperature required for the tubular tail reactor. Optionally, other monomer or optional comonomer is not introduced into the outlet stream so that fresh monomer and optional fresh comonomer (if present) are not introduced into the tubular tail reactor. Alternatively, in addition to the unreacted monomer and optional unreacted comonomer present in the outlet stream of the autoclave reactor, other (fresh) monomer and optional other (fresh) comonomer are introduced into the tubular tail reactor. Fresh monomer and optional comonomer can be added into the outlet stream. Additionally or alternatively, fresh monomer and optional comonomer can be directly introduced into the tubular tail reactor in a side stream. Initiator is introduced into the tubular tail reactor to initiate polymerization. The composition of the initiator solution introduced into the tubular reactor can be different from the initiator solution introduced into the autoclave reactor.

[0045] It will of course be appreciated that features described with respect to one aspect of the invention may be incorporated into other aspects of the invention. For example, the method of the invention may incorporate any features described with reference to the apparatus and system of the invention, and vice versa. Description of the drawings

[0046] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a polymerization reactor system utilizing a tubular reactor having at least three reaction zones in accordance with the present disclosure; Figure 2 is a schematic diagram of an initiator injection pipeline according to the present invention. A back pressure valve is located downstream of the initiator pump and upstream of the outlet. The back pressure valve maintains the pressure upstream of the valve; and Figure 3 is a flow chart of the steps of a method for preparing a polymer in a polymerization reactor system according to the present invention. Detailed Description of the Invention Polymerization reactor system

[0047] As referred to herein, "high pressure polymerization" refers to a highly exothermic polymerization reaction conducted in a reactor (such as a tubular reactor) at a high reactor operating pressure, for example, the reactor operating pressure in the high pressure polymerization process can be at least 1000 bar (100 MPa), or at least 2000 bar (200 MPa), or at least 3000 bar (300 MPa).

[0048] Will refer to Figure 1 The features of the polymerization reactor system according to the present invention are discussed. Preferably, the polymerization reactor system is used to polymerize ethylene. The polymerization reactor system includes a tubular reactor 102. The tubular reactor 102 has a length of about 200 meters to about 5,000 meters. The tubular reactor 102 has an internal diameter of about 20 mm to about 120 mm. It should be understood that the length and diameter of the tubular reactor can be selected based on the desired heat removal capacity, throughput, and the amount of turbulence required.

[0049] Main compressor 104 is in fluid communication with tubular reactor 102, and is in fluid communication with monomer source (not shown) (preferably ethylene source) via suction conduit 106. The suction pressure of main compressor 104 can be in the range of about 15 bar to about 100 bar. Main compressor 104 increases the pressure of monomer to the range of about 250 bar to about 350 bar. Main compressor 104 can be a single compressor that monomer is pressurized to the pressure of recycle stream separately, or it can be two or more compressors in series or parallel, which are combined to pressurize fresh monomer to the pressure of monomer recycle system. In addition to fresh monomer, main compressor 104 can receive monomer, preferably ethylene, which is recycled from each unit including product separation unit and / or from main compressor leakage system and auxiliary compressor leakage system (if available). Main compressor 104 can be in fluid communication with purge gas compressor 107 via recycle conduit 108 for recycled monomer from purge gas compressor 107 (a purge gas compressor). Main compressor 104 pressurizes monomer to the pressure of ethylene recycle system 124.

[0050] The monomer (preferably ethylene) discharged from the main compressor 104 can be divided into two streams, one stream is combined with the recycled monomer and discharged into the aspirator of the secondary compressor 110, and the other stream (not shown) can be injected into the monomer / polymer mixture downstream of the high-pressure relief valve, thereby providing rapid cooling of the monomer / polymer mixture before entering the product separation unit 118. The discharge pressure of the main compressor 104 is substantially equal to the pressure of the high-pressure monomer recycle system and can be, for example, in the range of about 240 bar to about 350 bar, optionally in the range of about 280 bar to about 320 bar.

[0051] A secondary compressor 110 upstream of the tubular reactor 102 is in fluid communication with the primary compressor 104. The monomer is cooled upon discharge from the primary compressor 104 and before being drawn into the secondary compressor 110. The secondary compressor 110 compresses the monomer to a pressure of at least 1,000 bar for supply to the tubular reactor 102. The pressure may be in the range of 1,000 bar to about 3,500 bar, optionally in the range of about 2,000 bar to about 3,500 bar. Operating at a pressure higher than 3,100 bar is feasible and may increase monomer conversion, but operating at such high pressures may increase costs. Those skilled in the art will select an appropriate pressure based on the operating configuration and the desired polymer properties. Similar to the primary compressor 104, the secondary compressor 110 may be driven by a single motor, or may include two or more compressors connected in series or in parallel and driven by independent motors. It should be understood that the compressor may be of any configuration as long as the configuration is suitable for compressing the monomer from the intermediate pressure (when the monomer leaves the primary compressor 104) to the desired reactor pressure, for example, about 1,000 bar to about 3,100 bar. The main compressor 104 can be operated, for example, at a gas throughput in the range of 30 to 120 tons / hour ("T / h"), in the range of 50 to 100 tons / hour, and in the range of 40 to 90 tons / hour. It should be understood that the relative capacities of the compressors are not critical to the polymerization process of the present invention.

[0052] Monomer, preferably ethylene, is discharged into the tubular reactor 102 via conduit 111 and heated to at least about 95° C., at least about 135° C., or at least about 160° C. to promote the decomposition of the initiator and start the polymerization reaction. Initiator is injected into the tubular reactor 102 at at least one initiator injection point. Preferably, the reactor 102 includes multiple initiator injection points, such as at least three different initiator injection points 112a, 112b, 112c along the length of the tubular reactor 102. Each initiator injection point defines a reaction zone. Thus, Figure 1 The reactor 102 includes three reaction zones. It should be understood that the tubular reactor according to the present invention can have one reaction zone or multiple reaction zones. The initiator can be transferred from the initiator source 114 to the initiator injection points 112a, 112b, 112c via at least one initiator injection line (not shown). At least one initiator injection line is an initiator injection line according to the present invention and can have any of the features disclosed herein.

[0053] For large tubular reactors, monomer may be discharged from the secondary compressor 110 via conduits via two or more discharge streams, one of which enters the front end of the tubular reactor 102 and the other stream(s) enter as side stream(s) 116a, 116b, 116c spaced longitudinally along the tubular reactor 102. Figure 1As shown, the plurality of monomer feed locations 116a, 116b, 116c may include at least three monomer feed locations. However, depending on the polymerization process, the tubular reactor 102 may have at least one, at least three, at least five, or at least ten monomer feed locations. Figure 1 As shown, monomers can be fed into multiple monomer feed positions 116a, 116b, and 116c on the tubular reactor 102 from multiple conduits connected to the secondary compressor 110. Before being discharged into the tubular reactor 102, the monomer side stream can be cooled to, for example, 10°C to 20°C in order to reduce the temperature of the reaction mixture. As described herein, in practice, the total conversion of monomer to polymer along the length of the tubular reactor is limited by the ability to cool the reaction mixture, so for a given tubular reactor, cooling the waste stream can allow an increase in conversion. If comonomers are present in the polymerization process, the comonomers can be introduced into the tubular reactor as part of the monomer feed. Alternatively, the comonomers can be introduced into the tubular reactor as a feed different from the monomers.

[0054] Also like Figure 1 As shown, multiple free radical initiator injection points (such as Figure 1 As shown, three free radical initiator points 112a, 112b, 112c) are also spaced apart in the length direction of the tubular reactor 102 to cause the monomer to be polymerized into a polymer in at least three reaction zones. The monomer can be converted into a polymer in at least three or at least six or more reaction zones. The tubular reactor 102 including three reaction zones includes a first reaction zone, a second reaction zone, and a third reaction zone. The tubular reactor 102 including six reaction zones includes a first reaction zone, a second reaction zone, a third reaction zone, a fourth reaction zone, a fifth reaction zone, and a sixth reaction zone. In the case of six reaction zones, the six monomer feed positions can be spaced apart longitudinally along the tubular reactor 102 (for example, in addition to Figure 1 116a, 116b and 116c in addition to the three feed positions 116a, 116b and 116c shown in FIG); and similarly, six free radical initiator injection points (e.g., in addition to Figure 1 The two free radical initiator locations (outside of the three points 112a, 112b, 112c shown in FIG) are also spaced apart along the length of the reactor 102. The initiator can be oxygen, peroxide, and similar agents. Suitable initiators are described herein.

[0055] Each initiator injection point 112 is associated with a reaction zone. Injecting initiator into the tubular reactor 102 causes an exothermic temperature rise, which is removed by cooling in the reaction zone and downstream of the reaction zone. Cooling occurs through the tube wall, optionally assisted by a coolant as a heat transfer medium and / or by feeding cold monomer added downstream. In addition, initiator can be added downstream to form another reaction zone for converting additional monomer into polymer. Thus, as the reaction mixture travels along the length of the tubular reactor, the temperature of the reaction mixture increases to a peak, then decreases until it reaches the next initiator injection point, at which point the process begins again. The reaction zone downstream of the initiator injection point where polymerization occurs is referred to as a "reaction zone." The tubular reactor 102 is typically equipped with at least one temperature-regulated heating / cooling jacket in each reaction zone. A cooling zone (not shown) may be located after each reaction zone. The cooling zone is provided to reduce the temperature of the reaction mixture before it enters the next reaction zone or leaves the reactor. The cooling zone may be equipped with a cooling jacket to control the temperature of the cooling zone. Additionally or alternatively, cold monomer may be injected into the tubular reactor in the cooling zone to reduce the temperature of the reaction mixture in the cooling zone.

[0056] Polymerization begins immediately downstream of this first reaction zone, causing the temperature of the reaction mixture to rise due to the exothermic nature of polymerization. As this temperature rises, the rate of initiator decomposition and polymerization increases, accelerating heat generation and causing the temperature to rise further. As the initiator is consumed, initiation and polymerization slow, and the temperature peaks and then begins to decline when the heat generation equals the heat removed from the reaction mixture.

[0057] In each reaction zone, monomer (preferably ethylene) is converted to polymer, therefore, having a greater number of reaction zones generally increases conversion. However, each reaction zone will generally necessitate an increase in the length of the tubular reactor, thereby increasing the pressure drop across the reactor. In the process of the present invention, initiator can be fed from an initiator source 114 comprising one or more initiator tanks and injected at multiple initiator injection points (e.g., at least 4, 5, or 6 different points along the tubular reactor 102), thereby creating at least 4, 5, or 6 different reaction zones. Typically, the peak temperature of the reaction zone can be in the range of about 170° C. to about 350° C.

[0058] The polymer composition produced by the polymerization process in the tubular reactor 102 can be discharged directly to the separation unit 118 or the product cooler (not shown). Figure 1 As shown, a mixture of polymer and unreacted monomer and optionally unreacted comonomer is discharged to separation unit 118. Figure 1As shown, separation unit 118 includes a high pressure separator 120 and a low pressure separator 122. However, if desired, product separation can be performed in a single stage. Figure 1 As shown, for multi-stage separation, in the first stage, polymer is separated from unreacted monomers such as ethylene. Unreacted ethylene gas can be fed to a high-pressure recycle gas system 124. In the second stage, the molten polymer is decompressed, and the separated ethylene gas can flow to a purge gas compression system 107. The pressure in the last stage of the separator unit 118 (or in the low-pressure separator 122) is in the range of about 1 to about 10 bar, or about 1 to about 3 bar.

[0059] like Figure 1 As shown, high-pressure separator 120 receives the monomer / polymer mixture discharged from tubular reactor 102. High-pressure separator 120 operates in the range of about 200 bar to about 350 bar. High-pressure separator 120 is connected to low-pressure separator 122 to further remove monomer. Molten polymer is discharged from low-pressure separator 120 to a polymer finishing section (not shown) with an extruder (not shown) via molten polymer conduit 126. High-pressure separator 120 discharges the separated volatile-rich monomer phase of unreacted monomer into a recycle gas system 124 at a pressure similar to that of the compressed gas discharged from main compressor 104. Unreacted monomer is discharged from recycle gas system 124 through recycle conduit 128 and merged with the monomer feed from main compressor 104 and discharged into secondary compressor 110. Unreacted monomer from low-pressure separator 122 is discharged into a pressure purge compressor 107 at a pressure higher than the suction pressure of main compressor 104. It will be appreciated that comonomer may be introduced at various points in the process in a manner similar to the introduction of monomer.

[0060] In a process for polymerizing a monomer (such as ethylene) in a tubular reactor, once the desired throughput of monomer through the secondary compressor 110 and into the reactor 102 is established, the pressure in the reactor is controlled by a high-pressure relief valve 130, through which the product mixture (polymer composition, unreacted monomer, and optionally unreacted comonomer) is discharged from the tubular reactor 102. Opening the valve 130 reduces the pressure in the tubular reactor 102; closing the valve 130 increases the pressure in the tubular reactor 102. In addition, there is a pressure drop along the length of the tubular reactor 102, which forces the reaction mixture to travel along the reactor at a desired rate (the term "reactor pressure" herein refers to the maximum pressure in the reactor 102, i.e., the pressure immediately downstream of the secondary compressor 110, unless another meaning is apparent from the context).

[0061] As described herein, chain transfer agent can be added at various locations in the process. Suitable chain transfer agents are described herein. Chain transfer agent can be added to each monomer feed. This can be achieved by mixing the chain transfer agent with the monomer feed before the monomer is compressed by the secondary compressor 110. Chain transfer agent can be added along the length of the tubular reactor 102, although it may be consumed unevenly, thereby potentially resulting in concentration variations along the tube. Figure 1 As shown, a source of chain transfer agent 132 can be fluidly connected to the primary compressor 104 so that, after passing through the secondary compressor 110, it is distributed to different monomer feeds 116a, 116b, 116c spaced along the tubes of the tubular reactor 102. Alternatively, the source of chain transfer agent can be fluidly connected directly to the secondary compressor 110 so that the chain transfer agent is supplied directly to the secondary compressor 110. A recycle conduit is in fluid communication with the low-pressure separator 122 and the purge compressor 106. The recycle gas from the high-pressure separator 120 can contain unconsumed chain transfer agent and can flow to the inlet of the secondary compressor 110. Thus, the chain transfer agent and monomer can form a single common gas stream having a desired chain transfer agent concentration for compression in the secondary compressor 110 and for supply to the various feed locations 116a, 116b, 116c along the tubes of the tubular reactor 102.

[0062] It should be understood that the present invention is also applicable to polymerization reactors comprising a combination of an autoclave reactor and a tubular tail reactor. Figure 1 In the embodiment of the present invention, the autoclave reactor is basically the same, except that the autoclave reactor is located upstream of the tubular tail reactor and downstream of the secondary compressor. The secondary compressor is fed into the autoclave reactor, rather than directly into the tubular tail reactor. The outlet stream of the autoclave reactor comprising polymer, unreacted monomer and optional unreacted comonomer is fed into the tubular tail reactor. The tubular tail reactor can only include one initiator injection point, and therefore includes a reaction zone. Alternatively, the tubular tail reactor can include multiple initiator injection points, and therefore can include multiple reaction zones. Tubular reactor

[0063] The reactor system according to the present invention comprises a tubular reactor. The tubular reactor is typically a continuous plug flow loop reactor. The tubular reactor typically has an initial section to which monomer(s) are fed from a secondary compressor and wherein they are heated to the desired reaction start temperature, typically to at least about 120° C., or at least about 130° C., or at least about 160° C. Once the desired temperature is reached, a polymerization initiator composition is injected at the initiator injection point in the tubular reactor to begin the reaction. The operating pressure of the tubular reactor is typically from about 1000 bar (100 MPa) to about 3500 bar (350 MPa).

[0064] Tubular reactors, such as tubular tail reactors, may include only one initiator injection point and therefore have only one reaction zone. Alternatively, a tubular reactor may include multiple initiator injection points that define multiple reaction zones. The injected initiator decomposes into free radicals, which initiate polymerization. Other points for injecting the initiator composition are located downstream along the length of the reactor. Optionally, the reactor has a total of at least two, preferably at least three, more preferably at least four different injection points, thereby producing at least two, at least three or at least four reaction zones, respectively. The number of initiator injection points and initiator zones may be up to six. In each reaction zone, polymerization is carried out as previously described herein. Preferably, a tubular reactor will typically be equipped with at least one temperature-regulated cooling jacket in each reaction zone. The reaction mixture in any reaction zone may be cooled by a cooling jacket through which water or another cooling fluid circulates, or a combination of the cooling jacket and a side stream for introducing cooling monomers.

[0065] The tubular reactor can also include a cooling zone located near and downstream of the reaction zone. Optionally, each reaction zone is followed by a cooling zone. The cooling zone allows the reaction mixture to be transferred to another reaction zone or to be discharged from the tubular reactor before the reaction mixture is cooled. The temperature in the cooling zone can be controlled by a jacket water system. Optionally or additionally, one or more cold monomer side streams can be introduced into the cooling zone along the length of the reactor to reduce the temperature of the reaction mixture in the cooling zone.

[0066] Optionally, the internal diameter of the tubular reactor can be in the range of 20mm to 120mm or 40mm to 80mm, for example, the internal diameter of the tubular reactor can be 85mm. The internal diameter of the reactor is limited by the tubular reactor wall. In this article, the term "internal diameter of the tubular reactor" refers to the diameter of the tubular reactor at the widest point along the length of the tubular reactor. In the polymerization reactor system in which a portion of monomer discharged from the secondary compressor enters the tubular reactor as a side stream, it may be desirable that the reactor has regions with different diameters, so that when the side stream enters, these regions are progressively increased along the length of the reactor. For example, for a method with a secondary compressor throughput of up to 180 tons / hour at 3000 bar (300MPa), 20% of which enters the front end of the tubular reactor and the remainder enters as a side stream. Initially, the tubular reactor can have a diameter within the range of 35mm to 40mm, and at the entry point of the first side stream, the diameter will increase, and this increase depends on the size of the side stream, and by analogy, until after the last side stream, the final diameter is approximately 75mm to 90mm. In such a cascade tubular reactor, the internal diameter of the tubular reactor is the final diameter since it is the widest point along the length of the tubular reactor. The internal diameter of the tubular reactor selected for any process according to the present invention will depend on the throughput of the secondary compressor, the output pressure of the secondary compressor and the length of the tubular reactor used, all of which are related to the pressure drop experienced over the length of the reactor.

[0067] Optionally, the tubular reactor has a length of 200m to 5000m, more preferably 1000m to 4000m, or 3000m to 4500m. It should be understood that the length of the tubular reactor is the total length of the reactor, including all reaction zones and all cooling zones of the tubular reactor.

[0068] In the method for polymerizing monomer (preferably ethylene) in a tubular reactor according to the present invention, once the monomer passes through the secondary compressor and enters the required flux of the reactor, the pressure in the reactor is controlled by a high-pressure release valve, and the product mixture leaves the reactor through the high-pressure release valve. Opening the valve reduces the pressure in the tubular reactor; closing the valve increases the pressure. In addition, there is a pressure drop along the length of the tubular reactor, which pushes the reaction mixture along the reactor at a desired speed (the term "reactor pressure" herein refers to the maximum pressure in the reactor, i.e., the pressure just downstream of the secondary compressor, unless another meaning is obvious from the context). The pressure drop on the reactor length depends on the condition that the pressure should not drop to below the point where the reaction mixture phase separates. The pressure drop for a given flux can be reduced by increasing the internal diameter of the tubular reactor. However, the increased tube diameter also makes the effective cooling of the reactor mixture more difficult.

[0069] The tubular reactor can be a tubular tail reactor. It should be understood that a tubular tail reactor is a type of tubular reactor connected to an autoclave reactor. The autoclave reactor is located upstream of the tubular tail reactor. The tubular tail reactor can have any of the features described herein for tubular reactors. The outlet stream comprises the polymer produced in the autoclave reactor, unreacted monomer, and optionally unreacted comonomer. The tubular tail reactor can have any of the features described herein for tubular reactors.

[0070] Optionally, the polymerization reactor system disclosed herein includes an autoclave and a tubular tail reactor. It is possible that more than one autoclave is located upstream of the tubular tail reactor. It is possible that more than one tubular tail reactor is located downstream of the autoclave reactor. A cold zone can be located between the autoclave and the tubular tail reactor, wherein the temperature of the outlet stream of the autoclave is reduced to a desired temperature before entering the tubular tail reactor.

[0071] The tubular reactor can be equipped with at least one thermocouple probe to measure the temperature within the reactor. Suitable thermocouple probes are known to those skilled in the art. The flow rate of the initiator and / or monomer and optional comonomer into the reactor can be adjusted in response to the temperature within the reactor. Initiator injection line

[0072] Now refer to Figure 2 The initiator injection line according to the present invention is described. The initiator injection line 200 includes an initiator injection pump (not shown, but connected to the conduit 202). The initiator cylinder 204 is located near the pump. The initiator pump moves the initiator from the initiator cylinder 204 to the outlet 206 of the initiator injection line. The outlet 206 is in the form of a distributor. The outlet 206 is fluidically connected to the initiator injection point of the tubular reactor 207. Therefore, the initiator flows from the outlet 206 into the tubular reactor 207 at the initiator injection point. Although Figure 2 The initiator injection line is shown as having one outlet 206 , but it should be understood that the initiator injection line may include multiple outlets 206 , each outlet 206 connected to a separate initiator injection point of the tubular reactor 207 .

[0073] The initiator injection pipeline 200 further includes a back pressure valve 208. The back pressure valve 208 is a control valve and has a set pressure. The back pressure valve 208 maintains the pressure in the initiator injection pipeline 200 upstream of the back pressure valve 208 and downstream of the pump at the set pressure.

[0074] In operation, initiator is pumped through initiator injection line 200 to outlet 206 and enters tubular reactor 207 at the initiator injection point. Initiator is injected into tubular reactor 207 to initiate the polymerization reaction. The initiator flows through the opening (not shown) of backpressure valve 208 located downstream of the initiator pump and upstream of outlet 206. Backpressure valve 208 has a set pressure that is selected to be within a range of 50 bar to 400 bar greater than the pressure inside tubular reactor 207 at the initiator injection point. Backpressure valve 208 is a dynamic control valve and adjusts the pressure upstream of backpressure valve 208 to be equal to the set pressure by changing the flow rate of initiator passing through backpressure valve 208. Backpressure valve 208 includes a variable opening (not shown) and maintains the pressure upstream of backpressure valve 208 by changing the size of the opening of backpressure valve 208. In operation, the pressure upstream of backpressure valve 208 is greater than the pressure downstream of backpressure valve 208. The pressure upstream of the back-pressure valve 208 is also greater than the pressure at the initiator injection point in the tubular reactor 207. The initiator is slightly compressed between the initiator pump and the back-pressure valve 208. The compressed initiator acts like a "capacitor" in the system. When the pump output drops momentarily, such as when the pump cylinder reaches the end of its stroke and changes direction, the compressed initiator expands slightly, thereby making up for the loss of pump output. This ultimately leads to a more constant and consistent flow rate of initiator into the tubular reactor 207 because the back-pressure valve 208 can smooth the inherent pulse output of the initiator pump. Since the initiator concentration in the reactor 207 controls the temperature in the reactor 207, a more stable peak reactor temperature can be achieved using the initiator injection line 200 of the present invention.

[0075] The initiator injection line 200 also includes a pressure transmitter 218 for measuring the pressure in the initiator injection line 200 upstream of the back pressure valve 208. The pressure transmitter 218 measures the pressure in the initiator injection line 200 upstream of the back pressure valve 208 and downstream of the pump and converts it into an analog signal for system control. When the pressure monitored by the pressure transmitter 218 exceeds the set pressure of the back pressure valve 208 within the range of 100 to 200 bar, a blockage in the initiator injection line 200 is indicated. The back pressure valve 208 is the narrowest part of the initiator injection line 200 and is therefore most susceptible to blockage. To warn the operator of a blockage, an alarm of an alarm system (not shown) can be triggered.

[0076] Shut-off valve 210 is located upstream of outlet 206 and downstream of back-pressure valve 208. In normal operation, shut-off valve 210 is open. However, shut-off valve 210 can preferably be closed by a control system to prevent initiator from entering tubular reactor 207, for example, in the event of a blockage in initiator injection line 200. Closing shut-off valve 210 when a blockage may exist in initiator injection line 200 prevents a surge of initiator from entering tubular reactor 207 if the blockage is removed.

[0077] An emergency drain valve 212 is located upstream of the backpressure valve 208 and is in fluid communication with a waste tank 214. The emergency drain valve 212 allows the initiator to be transferred to the waste tank 214. In normal operation, the emergency drain valve 212 is closed. However, if it is necessary to purge the initiator injection line 200 of initiator, for example, after closing the shut-off valve 210, the emergency drain valve 212 can be opened. Preferably, there is a time delay of at least 1 second and optionally no longer than 2 minutes between closing the shut-off valve 210 and opening the emergency drain valve 212.

[0078] The back pressure valve 208, the shut-off valve 210, the emergency drain valve 212, the connection to the waste tank 214 and the pressure transmitter 218 may all be located within a hot box.

[0079] The initiator injection line 200 further includes two check valves 216a, 216b for preventing backflow into the initiator injection line 200. The two check valves 216a, 216b prevent backflow from the tubular reactor 207 once the shut-off valve 210 is closed.

[0080] The initiator injection line 200 includes a three-way valve 217 located downstream of the backpressure valve 208 and upstream of the outlet 206 . Figure 2 A three-way valve 217 is shown located downstream of the two check valves 216a and 216b. The three-way valve 217 includes a first port for receiving initiator, a second port for directing the initiator to an outlet, and a third port connected to a waste tank 214 (connection not shown). The waste tank 214 is also fluidically connected to the emergency discharge valve 212, but it should be understood that multiple waste tanks 214 may be provided. In a first configuration of the three-way valve 217, the flow direction is from the first port to the second port of the three-way valve 217. In a second configuration, the flow direction is from the first port to the third port of the three-way valve 217. During normal operation, the three-way valve 217 is in the first configuration, and initiator flows from the initiator source through the initiator injection line to the initiator injection point of the tubular reactor 207. If a blockage occurs in the initiator injection line 200, the three-way valve 217 can be changed from the first configuration to the second configuration. This allows the initiator flow to be diverted to the waste tank 214. When the three-way valve 217 is in the second configuration, the initiator injection line may also be flushed with solvent by passing the solvent through the initiator injection line 200 and through the three-way valve 217 to the waste tank 214 .

[0081] The three-way valve 217 can also be replaced by two single valves configured to assist in flushing the line in the same manner as the three-way valve. One of the two single valves is configured to control the flow from the initiator source 202 to the outlet 206 and can be used to isolate the initiator injection line from the tubular reactor. The second of the two single valves allows the initiator injection line to be flushed and directed to the waste tank 214. Polymerization initiators and initiator compositions

[0082] Initiators are used to initiate free radical polymerization of monomers, preferably ethylene and optional comonomer(s). Suitable are organic peroxides. Typically, mixtures of different peroxides, so-called "peroxide mixtures," are used. Such mixtures of several peroxide initiators typically include peroxides with different half-lives: peroxides that are typically active at the lowest temperature required for a given reaction start temperature (about 120° C. to about 160° C.) and peroxides that are active at the desired maximum temperature (up to about 335° C.). The selection of an appropriate combination of different peroxides depends on the reactor setup and the desired reaction temperature profile along the length of the reactor and is within the knowledge of those skilled in the art.

[0083] Organic peroxides useful as polymerization initiators are well known in the art. Classes of peroxide initiators particularly useful in the present invention are, for example, the following: diacyl peroxides, dialkyl peroxydicarbonates, tertiary alkyl peroxyesters, 0-tertiary alkyl O-alkyl monoperoxycarbonates, di-tertiary alkyl peroxides, di(tertiary alkyl peroxy)ketals, tertiary alkyl hydroperoxides, and ketone peroxides.

[0084] Non-limiting examples of peroxides that can be used are, for example, the following: dibenzoyl peroxide, dilauroyl peroxide, succinic acid peroxide, diisononanoyl peroxide, dioctanoyl peroxide, tert-butyl peroxybenzoate, tert-butyl peroxyacetate, tert-butyl peroxymaleate, tert-butyl 2-ethylperoxyhexanoate, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-amyl 2-ethylperoxyhexanoate, 2,5-di(2-ethylhexanoyl-peroxy)-2,5-dimethyl-hexane, tert-butyl peroxypivalate, α-Cumyl peroxyneoheptanoate, 3-hydroxy-1,1-dimethylbutyl peroxyneodecanoate, 0-tert-butyl-O-(isopropyl)monoperoxycarbonate, 0-tert-amyl-O-(2-ethylhexyl)monoperoxycarbonate, ethyl-3,3-di(tert-amylperoxy)butyrate, n-butyl-4,4-di(tert-butylperoxy)valerate, 1,1-di(tert-butylperoxy)cyclohexane, 2,2-di(tert-butylperoxy)butane, 1,1-di(tert-amylperoxy)cyclohexane, 2 ,5-di-(tert-butylperoxy)-2,5-dimethyl-3-hexyne, 2,5-di-(tert-butylperoxy)-2,5-dimethyl-hexane, 2,5-di-(tert-butylperoxy)-2,5-dimethylhexane, 1,3(4)-bis(2-(tert-butylperoxy)-1-methylethyl)-benzene, di(tert-butyl)peroxide (DTBP), di(tert-amyl)peroxide, dicumyl peroxide, tert-butylcumyl peroxide, tert-butylperoxyisopropylcarbonate Peroxides of this type are commercially available, for example, from Nouryon under the trade name Trigonox. TM and Perkadox TM or by Arkema under the trade name Luperox TM Sale.

[0085] Preferred initiator mixtures, especially for tubular reactors, contain a minimum of one and a maximum of eight different types of initiators. Suitable mixtures of different organic peroxides, generally referred to as peroxide mixtures, are known to those skilled in the art.

[0086] Optionally, the initiator (or initiator mixture) can be injected into different reaction zones as the same initiator composition. The polymerization initiator composition used herein comprises at least one, preferably several, polymerization initiators as described above dissolved in an organic solvent, as further described below, and optionally one or more additional modifiers, also as further described below.

[0087] Suitable organic solvents may include, but are not limited to, one or more non-coordinating, inert liquids including, but are not limited to, straight and branched chain hydrocarbons such as propane, isobutane, butane, n-butane, pentane, isopentane, hexane, isohexane, heptane, octane, n-octane, dodecane, isododecane, and mixtures thereof; cyclic and alicyclic hydrocarbons such as cyclohexane, cycloheptane, methylcyclohexane, methylcycloheptane, and mixtures thereof such as those found commercially (Isopars® available from ExxonMobil); TM ); perhalogenated hydrocarbons such as perfluorinated C4-C10 alkanes, chlorobenzene, and aromatic and alkyl-substituted aromatic compounds such as benzene, toluene, mesitylene, and xylene. Suitable solvents also include liquid olefins that can serve as monomers or comonomers, including ethylene, propylene, 1-butene, isobutylene, 1-hexene, 1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 1-octene, and 1-decene. In certain embodiments, the initiator may include butane, n-butane, n-octane, or a mixture of one or more C9 to C12 alkanes.

[0088] The peroxide initiator (or mixture of initiators) may comprise from about 5 to about 50 weight percent of the polymerization initiator composition, preferably from about 5 to about 40 weight percent, and more preferably from about 10 to about 40 weight percent.

[0089] Optionally, where an additional modifier (other than the initiator solvent) is used, such additional modifier as described above may be added to the reactor along with the monomer feed or added to the reactor via one or more separate injection points. The amount of transfer agent may be up to the concentration in the reaction mixture required to control the melt index of the product to the desired specification.

[0090] Preferably, the organic solvent as described above comprises about 50 to about 95 weight percent of the total solution comprising the peroxide(s) in the organic solvent, more preferably about 65 to about 85 weight percent, and most preferably about 70 to about 85 weight percent.

[0091] The one or more polymerization initiators(s) comprise from about 30 to about 1500 ppm by weight, preferably from about 50 to about 1000 ppm by weight, based on the monomer feed, preferably ethylene and optional comonomer(s).

[0092] The organic solvent in which the initiator is dissolved may be used in an amount corresponding to about 100 to about 5000 ppm by weight, preferably about 250 to about 3000 ppm by weight, relative to the monomer feed.

[0093] The polymerization initiator composition according to the present invention may further contain conventional additives, such as free radical scavengers, in order to stabilize the initiator composition during storage. Chain transfer agent

[0094] The process of the present invention preferably includes the use of a chain transfer agent.The term "chain transfer agent," also used interchangeably herein as "modifier," refers to a component that can be added to a polymerization process to control the molecular weight of the polymer by promoting chain transfer.

[0095] Examples of modifiers may include, but are not limited to, tetramethylsilane, cyclopropane, sulfur hexafluoride, methane, tert-butanol, perfluoropropane, deuterated benzene, ethane, ethylene oxide, 2,2-dimethylpropane, benzene, dimethyl sulfoxide, vinyl methyl ether, methanol, propane, 2-methyl-3-butene-2-ol, methyl acetate, tert-butyl acetate, methyl formate, ethyl acetate, butane, triphenylphosphine, methylamine, methyl benzoate, ethyl benzoate, N,N-diisopropylacetamide, 2,2,4-trimethylpentane, n-hexane, n-butane, isobutane, dimethoxymethane, ethanol, n-Heptane, n-Butyl Acetate, Cyclohexane, Methylcyclohexane, 1,2-Dichloroethane, Acetonitrile, N-Ethylacetamide, Propylene, 1-Butene, n-Decane, N,N-Diethylacetamide, Cyclopentane, Acetic Anhydride, n-Tridecane, n-Butyl Benzoate, Isopropyl Alcohol, Toluene, Hydrogen, Acetone, 4,4-Dimethylpentene-1, Trimethylamine, N,N-Dimethylacetamide, Isobutylene, n-Butyl Isocyanate, Methyl Butyrate, n-Butylamine, N,N-Dimethylformamide, Diethyl Sulfide, Diisobutylene, Tetrahydrofuran, 4-Methylpentene-1, p-Xylene, p-Xylene alkane, trimethylamine, butene-2, 1-bromo-2-chloroethane, octene-1, 2-methylbutene-2, cumene, butene-1, methyl vinyl sulfide, n-butyronitrile, 2-methyl-butene-1, ethylbenzene, n-hexadecene, 2-butanone, n-butyl isothiocyanate, methyl 3-cyanopropionate, tri-n-butylamine, 3-methyl-2-butanone, isobutyronitrile, di-n-butylamine, methyl chloroacetate, 3-methylbutene-1, 1,2-dibromoethane, dimethylamine, benzaldehyde, chloroform, 2-ethylhexene-1, propionaldehyde, 1,4-dichlorobutene-2, tri-n-butylphosphine, dimethylphosphine, methyl cyanoacetate, carbon tetrachloride, trichlorobromomethane, di-n-butylphosphine, acetaldehyde, propionaldehyde and phosphine. Further details and other suitable transfer agents are described in Advances In Polymer Science, Vol. 7, pp. 386-448 (1970).

[0096] Optionally, the polyethylene produced by the apparatus described herein or according to the methods described herein contains one or more C2 to C12 unsaturated modifiers. The C2 to C12 unsaturated modifier contains at least one unsaturated portion, but may also contain multiple conjugated or non-conjugated unsaturated portions. In the case of multiple unsaturated portions, it is preferred that they are non-conjugated. Optionally, the unsaturated portion of the C2 to C12 unsaturated modifier may be di-substituted with one or more alkyl groups in the β position. Preferred C2 to C12 unsaturated modifiers include propylene, isobutylene, or a combination thereof. The amount of the modifier (one or more) may be in the range of about 0.001 wt%, 0.01 wt%, 0.1 wt%, 0.3 wt%, or 0.8 wt% to about 3.0 wt%, 6.0 wt%, or 10.0 wt% at a lower limit. Optionally, when the tubular reactor is a tubular tail reactor provided in combination with an autoclave reactor, there is less than 1 wt%, less than 0.1 wt%, or less than 0.01 wt% of the modifier. No modifier may be present.

[0097] The chain transfer agent can be added to the reaction mixture in any suitable manner. It can be included in the polymerization initiator composition. Alternatively, the modifier can be injected into the monomer feed, for example, into the inlet pipe of the feed sub-compressor. Since the modifier is not usually completely consumed during the first pass through the reactor, it is usually also present in a certain amount in the recycled ethylene returned to the sub-compressor. Monomers and comonomers

[0098] Preferably, the monomer is an olefin. Preferably, the monomer is ethylene, and the resulting polymer is a polyethylene polymer.

[0099] The method of the present invention can be used not only for the manufacture of ethylene homopolymers, but also for the manufacture of ethylene copolymers. Such comonomers (one or more) will be pressurized and injected into the main and / or secondary compressors and then fed into the polymerization reactor together with monomer (preferably ethylene).

[0100] Typical comonomers include, but are not limited to: vinyl ethers such as vinyl methyl ether, vinyl n-butyl ether, vinyl phenyl ether, vinyl β-hydroxy-ethyl ether, and vinyl dimethylamino-ethyl ether; olefins such as ethylene, propylene, butene-1, cis-butene-2, trans-butene-2, isobutylene, 3,3-dimethylbutene-1, 4-methylpentene-1, hexane-1, octene-1, and styrene; vinyl-type esters such as vinyl acetate, vinyl butyrate, vinyl pivalate, and vinylene carbonate; halogenated olefins such as Such as vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, vinyl chloride, vinylidene chloride, tetrachloroethylene and monochlorotrifluoroethylene; acrylic acid esters, such as methyl acrylate, ethyl acrylate, n-butyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, α-cyanoisopropyl acrylate, β-cyanoethyl acrylate, O-(3-phenylpropane-1,3-dinonyl)phenyl acrylate, methyl methacrylate, n-butyl methacrylate, tert-butyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate, methacrylic acid methyl methacrylate, glycidyl methacrylate, β-hydroxyethyl methacrylate, β-hydroxypropyl methacrylate, 3-hydroxy-4-methoxyphenyl methacrylate, N,N-dimethylaminoethyl methacrylate, tert-butylaminoethyl methacrylate, 2-(1-aziridinyl)ethyl methacrylate, diethyl fumarate, diethyl maleate and methyl crotonate; other acrylic acid derivatives such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, methyl hydroxy, maleate, itaconic acid, acrylonitrile, fumaronitrile, N,N -dimethylacrylamide, N-isopropylacrylamide, N-tert-butylacrylamide, N-phenylacrylamide, diacetoneacrylamide, methacrylamide, N-phenylmethacrylamide, N-ethylmaleimide and maleic anhydride; and other compounds such as allyl alcohol, vinyltrimethylsilane, vinyltriethoxysilane, N-vinylcarbazole, N-vinyl-N-methylacetamide, vinyldibutylphosphine oxide, vinyldiphenylphosphine oxide, bis-(2-chloroethyl)vinylphosphonate and vinylmethyl sulfide.

[0101] Examples of preferred comonomers are vinyl acetate, methyl acrylate, methacrylic acid, ethyl acrylate, n-butyl acrylate or acrylic acid, or mixtures thereof.

[0102] In some polyethylenes containing comonomers, the amount of comonomer is less than 10 wt %, but may also be 5 wt % or less, 3 wt % or less, or even 1.5 wt % or less. However, in other polyethylenes containing comonomers, the amount of comonomer may be 10 wt % or more, such as 15, 20, 30 or 40 wt % or more, depending on the desired end use of the polymer.

[0103] Typically, the purity of the ethylene feed suitable for the process according to the invention is provided by prior art steam crackers.In order not to interfere with the free radical initiated reactions, the oxygen content in the feed should be below 5 ppm. Aggregation conditions

[0104] As used herein, the terms "polymerization temperature" and "reactor temperature" are interchangeable. Reactor temperature refers to the peak temperature in the reactor, or if multiple reaction zones are present, the peak temperature in each reaction zone. It should be understood that the peak temperature in a reaction zone can be the same as or different from the peak temperature in another reaction zone.

[0105] In the method for producing polymers according to the present invention, high-pressure polymerization conditions include reactor temperatures of about 120°C to about 335°C. The peak temperature in each reaction zone can advantageously be in the range of 170°C to 350°C. For example, in at least one reaction zone, the peak temperature can be in the range of 170°C to 250°C, 280°C to 340°C, or 290°C to 315°C. It should be understood that the reaction zone temperature can be selected based on the specific grade of polymer being produced and the type of reactor. For example, in a tubular tail reactor, the peak temperature in the reaction zone can be no greater than 170°C. The temperature increase in the reaction zone is proportional to the amount of polymer produced in the reactor, so operating at high peak temperatures promotes high conversion. However, the kinetics of ethylene polymerization are such that as temperature increases, chain transfer relative to linear chain growth increases, and the polydispersity index increases, resulting in an increase in the haze value of the produced polymer. Therefore, the temperature can be selected based on the desired polymer product properties. Optionally, in each reaction zone upstream of the initiator injection point (i.e., in all reaction zones except the last reaction zone), the reaction mixture is cooled to at least 10°C, at least 20°C, more preferably at least 40°C, and most preferably at least 50°C below the peak temperature of that reaction zone before the reaction mixture reaches the next initiator injection point.

[0106] High pressure polymerization conditions include reactor operating pressures of about 1000 bar (100 MPa) to about 3500 bar (350 MPa). The ratio of the total monomers (whether in the front stream or as a side stream) that enter the tubular reactor and are converted into polymers before leaving the reactor is called conversion. In the method of the present invention, the conversion is at least 18%, for example at least 28%. The conversion obtained is partly related to the pressure of reactor operation, wherein a higher front pressure increases the polymerization rate and makes it possible to obtain a larger pressure drop in the reactor length. However, operating under high pressure also increases energy consumption by applying a higher strain to the secondary compressor, resulting in cost disadvantages. For these reasons, it may be desirable in some cases to operate at a pressure of 1000 bar (100 MPa) to 2800 bar (280 MPa) with a lower conversion (which can be, for example, approximately in the range of 18% to 32%). Alternatively, it may be desirable to operate at a high conversion (e.g., 28% to 37%) at a pressure of about 2300 bar (230 MPa) to 3100 bar (310 MPa). However, pressure is only one of the factors affecting conversion and generally, a conversion of about 30% to 40% is preferred, with a more preferred range of 30% to 37%. polymer products

[0107] It should be understood that the term "polymer" can be used to refer to homopolymers, copolymers, interpolymers, terpolymers, etc. A "polymer" has two or more identical or different monomeric units. A "homopolymer" is a polymer having identical monomeric units. A "copolymer" is a polymer having two or more monomeric units that are different from each other. The term "different" used in reference to monomeric units indicates that the monomeric units differ from each other by at least one atom or are isomerically different. As used herein, the terms "polyethylene," "ethylene polymer," and "ethylene copolymer" refer to a polymer or copolymer comprising at least 50 mol% ethylene units (preferably at least 70 mol% ethylene units, more preferably at least 80 mol% ethylene units, even more preferably at least 90 mol% ethylene units, even more preferably at least 95 mol% ethylene units or 100 mol% ethylene units (in the case of a homopolymer such as homopolyethylene)).

[0108] As used herein, when a polymer or copolymer is referred to as comprising an olefin, the olefin present in such polymer or copolymer is the polymerized form of the olefin. For example, when a copolymer is referred to as having an "ethylene" content of 45% to 65% by weight and a comonomer content of 35% to 55% by weight, it is understood that the monomer units in the copolymer are derived from ethylene in the polymerization reaction, and the derived units are present at 35% to 55% by weight, based on the weight of the copolymer. The copolymer can be a higher order copolymer, such as a terpolymer, a tetrapolymer, etc.

[0109] As used herein, unless otherwise indicated, mole percentages are expressed as "mol %" and weight percentages are expressed as "wt %." The comonomer content in a polymer composition is based on the total content of all monomers in the polymer.

[0110] Preferably, the initiator injection line, reactor system and method according to the present invention are suitable for preparing polyethylene compositions. The final polymer product (preferably homopolymer or copolymer of ethylene and one or more of the above-mentioned comonomers) produced by the apparatus described herein or according to the method described herein includes a wide range of low density polyethylene (LDPE) that can be prepared using a free radical initiated high pressure process using a tubular reactor or an autoclave reactor. Typically, for homopolymer LDPE, the density range is about 0.910 to 0.935 g / cm 3 , polydispersity of about 5 to about 50, melt index of about 0.1 to about 500 g / min, and haze value of about 1 to 20. If desired, high comonomer content, for example, up to about 40% by weight of comonomer, can be achieved. The molecular weight of the polymer can be modified by using modifiers of different types and concentrations. The polymer density can be affected by the type and amount of comonomer and the polymerization temperature. In addition, the haze can be affected by the reactor temperature, reactor pressure, and the choice of polymerization initiator. The main applications of these LDPE grades are films and extrusion coating. Optionally, the polymer contains 50% by weight or more of monomer, 70% by weight or more of monomer, 80% by weight or more of monomer, 95% by weight or more of monomer, or 100% by weight of monomer (in the case of homopolymers). For example, when the monomer is ethylene, the polymer can contain 50% by weight or more of ethylene, 70% by weight or more of ethylene, 80% by weight or more of ethylene, 95% by weight or more of ethylene, or 100% by weight of ethylene (in the case of homopolyethylene). It will be understood that the remaining weight percents of the copolymer will correspond to the weight percents of the comonomer(s).

[0111] In addition, the polyethylene composition can be a blend of LDPE and other polymers (e.g., additional polymers prepared from ethylene monomers). Exemplary additional polymers are LLDPE (e.g., LLDPE homopolymer and / or copolymers of ethylene and α-olefins), nonlinear LDPE, very low density polyethylene ("VLDPE"), medium density polyethylene ("MDPE"), high density polyethylene ("HDPE"), differentiated polyethylene ("DPE"), and combinations thereof. DPE copolymers include EVA, EEA, EMA, EnBA, and other specialty copolymers. Aggregation Method

[0112] The present invention provides a process for preparing polymers, preferably polyethylene polymers, in a polymerization reactor system also according to the present invention. Figure 3, method 300 includes a first step 301, wherein monomers and optional comonomers from a monomer source are pressurized in the main compressor and secondary compressor of a polymerization reactor system. The monomers and optional comonomers are then injected into a tubular reactor. In a second step 303, method 300 includes operating an initiator pump to pump initiator from the initiator source through a back pressure valve and into the tubular reactor at the initiator injection point. The back pressure valve operates at a set pressure, which is within the range of 50 bar to 400 bar greater than the pressure at the initiator injection point in the tubular reactor. The back pressure valve maintains the pressure in the initiator injection line upstream of the back pressure valve and downstream of the initiator pump at the set pressure. In a third step 305, monomers (preferably ethylene) are polymerized in the tubular reactor to form a polymer (preferably polyethylene). In a fourth step 307, a product mixture comprising polymer and unreacted monomers is discharged from the tubular reactor through a pressure release valve. In a fifth step 309, the polymer is separated from the product mixture, for example, by passing the product mixture through at least one separation vessel.

[0113] The method according to the present invention can be carried out as a continuous method. In a continuous method, at least some of steps 301, 303, 305, 307 and 309 are carried out simultaneously. Preferably, all steps 301, 303, 305, 307 and 309 are carried out simultaneously.

[0114] When reference is made in the foregoing description to wholes or units that have known, obvious or foreseeable equivalents, then these equivalents are also incorporated herein as if individually incorporated. Reference should be made to the claims for determining the true scope of the invention, which claims should be interpreted to the extent that any such equivalents are encompassed. The reader should also appreciate that wholes or features of the invention described as preferred, advantageous, suitable, etc. are optional and do not limit the scope of the independent claims. Furthermore, it should be understood that these optional wholes or features, while having possible benefits in some embodiments of the invention, may not be desirable and therefore may not be present in other embodiments.

Claims

1. A method for preparing a polymer in a polymerization reactor system comprising a tubular reactor, a monomer source, a primary compressor, a secondary compressor, a pressure relief valve, an initiator source, and an initiator injection line; Wherein the initiator injection pipeline comprises: an initiator pump and an outlet downstream of the pump, wherein the outlet is connected to the tubular reactor at an initiator injection point, and wherein the pump is configured to pump initiator from the initiator source through the outlet and into the tubular reactor at the initiator injection point; The initiator injection pipeline further includes a back-pressure valve located downstream of the initiator pump and upstream of the outlet, wherein the back-pressure valve has a set pressure and is configured to maintain a pressure in the initiator injection pipeline upstream of the back-pressure valve and downstream of the initiator pump equal to the set pressure; The method comprises the following steps: (i) pressurizing the monomer from the monomer source in the primary compressor and the secondary compressor, and introducing the monomer into the tubular reactor; (ii) operating the initiator pump so that the initiator from the initiator source flows through the backpressure valve and enters the tubular reactor at the initiator injection point; (iii) polymerizing the monomer in the tubular reactor to form a polymer; (iv) discharging a product mixture comprising the polymer from the tubular reactor via the pressure relief valve; and (v) isolating the polymer; Wherein step (ii) further comprises operating the back pressure valve at the set pressure, wherein the set pressure is in the range of 50 bar to 400 bar greater than the pressure of the tubular reactor at the initiator injection point.

2. The process of claim 1, wherein the set pressure is in the range of 100 bar to 300 bar greater than the pressure of the tubular reactor at the initiator injection point.

3. A method according to claim 1 or claim 2, wherein the polymerization reactor system further comprises a control system configured to reduce the flow rate of the initiator through the outlet when the pressure in the initiator injection line between the initiator pump and the back-pressure valve is equal to a first set point pressure, wherein the first set point pressure is a pressure in the range of 100 bar to 200 bar greater than the set pressure of the back-pressure valve.

4. A method according to claim 3, wherein the initiator injection pipeline further includes a shut-off valve located upstream of the outlet and downstream of the back-pressure valve; and wherein the control system is configured to close the shut-off valve when the pressure in the initiator injection line between the initiator pump and the back-pressure valve is equal to a second set point pressure, wherein the second set point pressure is a pressure in the range of 200 bar to 600 bar greater than the set pressure of the back-pressure valve.

5. The method of claim 4 , wherein the initiator injection line further comprises an emergency drain valve located upstream of the backpressure valve, and a waste tank; and wherein the control system is configured to open the emergency drain valve to transfer the initiator to the waste tank when the pressure in the initiator injection line between the initiator pump and the backpressure valve equals the second set point pressure, optionally, the control system is configured to open the emergency drain valve no less than 1 second after the shut-off valve is closed.

6. The process according to any of the preceding claims, wherein the initiator injection line has a plurality of outlets, and the tubular reactor comprises a plurality of initiator injection points, and wherein each of the plurality of outlets is connected to any one of the plurality of initiator injection points, provided that each of the plurality of initiator injection points is connected to no more than one of the plurality of outlets; and Step (ii) of the method further comprises operating the initiator pump so that the initiator from the initiator source flows through the back pressure valve and enters the tubular reactor at the plurality of initiator injection points.

7. The process of any of the above claims, wherein the polymerization reactor system further comprises an autoclave reactor upstream of the tubular reactor, and wherein the tubular reactor is a tubular tail reactor configured to receive an outlet stream from the autoclave reactor, wherein the outlet stream comprises polymer and unreacted monomer.

8. A process according to any preceding claim wherein the monomer is ethylene and the polymer is a polyethylene polymer.

9. The initiator injection line used in the method according to any one of claims 1 to 8, wherein the backpressure valve of the initiator injection line has an adjustable opening for controlling the pressure upstream of the backpressure valve; The size of the opening is adjustable to maintain the pressure in the initiator injection pipeline upstream of the back-pressure valve and downstream of the initiator pump equal to the set pressure of the back-pressure valve.

10. The initiator injection line according to claim 9, wherein the outlet of the initiator injection line is in the form of a distributor.

11. The initiator injection line of claim 9 or claim 10, wherein the initiator injection line further comprises at least one check valve located upstream of the initiator injection outlet and downstream of the backpressure valve, optionally wherein the at least one check valve is at most 1000 mm upstream of the outlet.

12. The initiator injection line according to any one of claims 9 to 11, further comprising a three-way valve located downstream of the backpressure valve and upstream of the outlet, wherein the three-way valve comprises a first port for receiving the initiator, a second port for directing the initiator to the outlet, and a third port connected to a waste tank, and wherein the three-way valve is operable in a first configuration and a second configuration, wherein in the first configuration, the flow direction is from the first port to the second port, and in the second configuration, the flow direction is from the first port to the third port.

13. The initiator injection line according to any one of claims 9 to 12, wherein the initiator pump is within a range of 5 to 100 m upstream of the back-pressure valve, optionally wherein the initiator pump is within a range of 10 to 50 m upstream of the back-pressure valve.

14. The initiator injection line according to any one of claims 9 to 13, wherein the outlet is in the range of 1 m to 100 m downstream of the back-pressure valve, optionally wherein the outlet is in the range of 5 m to 40 m downstream of the back-pressure valve.

15. A polymerization reactor system for use in the process according to any one of claims 1 to 8, comprising an initiator injection line according to any one of claims 9 to 14.

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