Polyolefin production pressure relief

Abstract

Disclosed is a plant for the production of polyolefins, particularly LDPE, comprising a polymerization reactor provided with a pressure relief system. The pressure relief system is designed such that after a pressure relief event, with discharging at least part of the reactor inventory into a pressure relief cooling and separation section, gaseous monomer is not vented into the atmosphere, but is collected, purified, and returned as a monomer feed to the reactor.

Description

[0001] P138012PC00

[0002] Title: POLYOLEFIN PRODUCTION PRESSURE RELIEF

[0003] Field of the invention

[0004] The invention is in the field of high pressure polyolefin production plants, and relates to preventing environmental consequences from pressure-relief processes in such plants. Particularly, the invention pertains to polyethylene production plants.

[0005] Background of the invention

[0006] Polyolefins, notably polyethylene, are produced by the polymerization, or co-polymerization, of ethylenically unsaturated monomers under conditions of high pressure (generally 500 bar to 4000 bar) and temperature (generally 150°C to 350°C). The polymerization reaction is exothermic, and reaction heat is removed rapidly from the polymerization reactor in order to prevent overheating of the reaction mixture, which could otherwise cause, also exothermic, decomposition of monomers and possibly polymers into carbon, hydrogen, and methane.

[0007] It is a fact of life that, despite all necessary control taking place in such polymerization plants, emergency programs serving to stop the processes in the polymerization reactor need be in place. Generally, such processes involve stopping the charging of the reactor with monomers, and removing the reactor inventory by opening applicable valves. The resulting pressure relief process as such is not without risk either. Particularly, it is known that, when the hot gas / product reaction mixture from a high- pressure polyolefin reactor is released into the atmosphere, said mixture is prone to ignition, resulting in explosion. Solutions to this problem have traditionally focused on preventing particles, i.e., polymer, from being released into the atmosphere, and on providing a water source in pipes and stacks through so as to cool down gas (i.e., monomers) before it is sent to the atmosphere.

[0008] A background reference in this respect is US 4,996.026. This is directed to a separation system, serving to reduce the emission of solids in pressure relief processes in high pressure polymerization reactors. The described system, which involves placing two separation containers in series. The first separator has 25% of its volume filled with water. The second separator is provided with a water injection system enabling quenching of the mixture. From the second separator gas is sent into the atmosphere, whilst the polymer particles are retained.

[0009] However, systems as disclosed in US4,996,026 will no longer sufficient. It is desired, for environmental reasons, to better control hydrocarbon emissions. Moreover, emergency venting, due to very high discharge velocities, generally goes with a noise that can reach extremely high levels, such as up to 125 dB at the ground, which would desirably be avoided. An industry goal is to entirely avoid the emission into the atmosphere of reactor inventories upon plant start-ups, shut-downs and, particularly, at emergency stops. This is at odds with conventional systems, which still result in gas emissions of monomers. Rather, it is further desired to be able to recycle monomers that would otherwise be emitted into the atmosphere.

[0010] Summary of the invention

[0011] In order to better address one or more of the foregoing desires, the invention provides, in one aspect, a plant for the production of polyethylene and / or polyethylene copolymers, said plant comprising a polymer synthesis section comprising a polymerization reactor provided with an inlet for monomer feed and an outlet for polymerization product, said reactor comprising a pressure relief system adapted to enable instantaneously emptying, partially or entirely, the reactor, said pressure relief system having an emergency outlet in fluid communication with an inlet of a pressure relief cooling and separation section configured to separate solids from gaseous monomers, said pressure relief cooling and separation section having a gas outlet in fluid communication with a gas storage section, said gas storage section having a gas outlet in fluid communication with a monomer recycle system.

[0012] In another aspect, the invention presents a process for preparing a polyethylene homopolymer or copolymer comprising subjecting ethylenic monomers to polymerization in a polymer synthesis section comprising a polymerization reactor, said reactor comprising a pressure relief system, wherein the process comprises, at the occurrence of a pressure relief event, discharging the contents of the reactor into a pressure relief cooling and separation section, subjecting said contents to separating off of gaseous monomers, sending said monomers to gas storage section, and recycling said monomers from said gas storage section.

[0013] In a further aspect, the invention pertains to a method of modifying a pre-existing plant for the production of polyethylene and / or polyethylene copolymers, said plant comprising a polymer synthesis section comprising a polymerization reactor provided with an inlet for monomer feed and an outlet for polymerization product, said reactor comprising a pressure relief system adapted to enable instantaneously emptying the reactor, said pressure relief system having an emergency outlet in fluid communication with an inlet of a pressure relief cooling and separation section configured to separate solids from gaseous monomers, the method comprising adding to the plant a gas storage section, said gas storage section having an outlet in fluid communication with a monomer recycle flow line.

[0014] Brief description of the drawings

[0015] FIG.l displays a schematic process flow diagram of a pressure relief system in accordance with the invention.

[0016] FIG.2 represents a customary set-up for a high-pressure LDPE plant, serving to illustrate a modification thereof with the pressure relief system of FIG. 1, resulting in a plant of the invention.

[0017] Detailed description of the invention

[0018] In a general sense the invention is based on the judicious insight to make use of a pressure relief system of a polyethylene polymerization reactor in recycling monomer reactants. Particularly, such pressure relief systems serve to discharge the contents of the reactor upon a pressure relief event, notably in an emergency situation, into a dedicated treatment and storage system. In order to prevent emission into the atmosphere of polymer particles, separation of solids from gaseous monomers is conventionally provided in such treatment systems. The inventors have realized the importance of such pressure relief cooling and separation for providing a system from which monomers that are separated off, do not need to be flared or sent into the atmosphere. This is accomplished by providing a gas storage system, wherein said gas storage system has an outlet in fluid communication with a monomer recycle flow-line. Said flow line serves particularly to connect the gas storage system to a monomer recycle system.

[0019] A monomer recycle system in the context of LDPE production generally is the train of equipment and controls that recovers unreacted ethylene from the reactor effluent, purifies it to polymerization-grade quality, and returns it to the high-pressure feed, while purging inerts / impurities so they don’t accumulate.

[0020] A normal set-up for a high-pressure LDPE plant, as shown in Fig.2, consists of a set of two compressors (Primary and Hyper compressor) supplying the monomer feed, a polymerization reactor (tubular or autoclave) with product cooler, a let-down valve, two separation units to separate the polymerization product from unreacted monomer (one high pressure separator with high-pressure recycle; one low-pressure separator with low- pressure recycle), with unreacted monomer is recycled back to the compressors. Generally present is further an initiator injection system for organic peroxides or oxygen and a modifier injection system to control the molecular weight of the product. More detailed information can be found in Ullmanns Encyklopadie der technischen Chemie 1980 / Bd. 19, page 169-178.

[0021] The pressure relief system of the present invention is different from the monomer recycle that can be found in LDPE production plants, generally. The latter is provided in fluid connection with the regular product exit (downstream of a product cooler). In the system of the invention, a not hitherto available treatment is provided in fluid connection with a pressure relief system, dedicated for an emergency emptying of the reactor inventory, i.e., terminating polymerization, when process upsets occur. Upon regular operation the pressure relief system is not activated. Upon normal operation, monomer is continuously converted into a polymer during its residence time in the reactor. The normal reactor output consists mainly of polymer (e.g. 30%) and unreacted monomer (e.g.70%). Upon emergency discharge, the polymer structure will generally be different on the molecular level. The main differences in reactor output from an emergency discharge as compared to normal operation, are caused by the fact that an emergency discharge typically occurs in the event that an ethylene decomposition has started in the reactor. This reflects a process upset with high temperature and pressure that triggers the emergency discharge. In comparison, the normal output exits the reactor at a pressure of, as a typical example, 3000 bar, is let down to about 300 bar, and is separated at elevated pressure in the HP separator. The separated gas remains at about 300 bar and can be sent to the suction side of the hyper compressor without being boosted again.

[0022] During an emergency discharge the complete inventory of the reactor is traditionally depressurized rapidly to atmospheric pressure while it is vented to the ambient. This is the only way to stop an ethylene decomposition. In the present invention, depressurizing can be limited, only to about 30 bar, which is sufficient to stop the decomposition.

[0023] The chemical composition of an emergency discharge will be different from the normal output. Because the ethylene decomposition started in the reactor, the emergency let-down mixture contains much more ethane, methane, CO and CO2. There are also significant amounts of hydrogen in the emergency let-down gas. The content of H2 in the normal output is negligible.

[0024] In the invention, preferably, a specific set of measures is provided allowing recycling monomers after a pressure relief event. This advantageously serves to avoid, or at least considerably reduce, hydrocarbon emissions into the atmosphere. By providing a closed loop, also the noise associated with emergency venting of gaseous monomers is advantageously avoided. Said closed loop can be part of the polyethylene plant itself. In a preferred embodiment, the gas storage section has a gas outlet in fluid communication with an inlet of a monomer purification section, said monomer purification section having a gas outlet in fluid communication with an inlet for monomers of the reactor, wherein the reactor, the pressure relief cooling and separation section, the gas storage section and the monomer purification section form a closed loop. Accordingly, in this embodiment a modified plant for the production of polyethylene and / or polyethylene copolymers is provided that can be described as follows. The plant comprises a primary compressor having an inlet for monomer feed and an outlet for primary compressed monomer, said outlet for primary compressed monomer being in fluid communication with a monomer inlet of a secondary compressor. The secondary compressor has an outlet for fully compressed monomer, said outlet for fully compressed monomer being in fluid communication with an inlet of a polymerization reactor. The polymerization reactor, preferably a tubular reactor, has an outlet for polymer product, said outlet for polymer product being in fluid communication with a first cooling and separation section. The first cooling and separation section comprising a product cooler and, downstream thereof, a first separator (generally referred to as a high- pressure separator), said first separator having a gas outlet and a polymer product outlet. Said gas outlet is in fluid communication with a second cooling and separation section comprising a high pressure recycle cooler and downstream thereof a high pressure recycle separator, which serves to separate off waxes and other oligomers.

[0025] Said high pressure recycle separator has a gas connection in fluid communication with a monomer inlet of the secondary compressor, said inlet typically being downstream of the primary compressor. The polymer product outlet of the first separator has an outlet for polymer product, in fluid connection with a second separator (generally referred to as a low- pressure separator), in which unreacted monomer is separated off, with condensable hydrocarbons typically becoming entrained therein, via agas outlet. The gas outlet of the second (low pressure) separator is in fluid communication with a third cooling and separation section comprising a low pressure recycle cooler and downstream thereof a low pressure recycle separator. The low pressure recycle separator separates off condensable hydrocarbons entrained in the unreacted monomer. The low pressure recycle separator has a gas connection in fluid communication with a monomer inlet of the primary compressor.

[0026] The reactor comprises a pressure relief system adapted to enable instantaneously emptying, partially or entirely, of the reactor. In the plant of the invention, i.e., a plant constructed or modified with a pressure relief system of the invention, said pressure relief system has an emergency outlet in fluid communication with an inlet of a fourth cooling and separation section configured to separate solids from gaseous monomers. Said fourth cooling and separation section, being the pressure relief cooling and separation section provided in accordance with the invention, has a gas outlet in fluid communication with a gas storage section. The gas storage section has a gas outlet in fluid communication with a monomer recycle system. Said monomer recycle system is adapted to purify ethylene from gaseous contaminants and typically comprises one or more distillation columns. The contaminants, mainly alkenes, carbon monoxide, carbon dioxide and nitrogen will be removed.

[0027] The monomer recycle system in turn has a gas outlet which is in fluid communication with a monomer inlet of the primary compressor.

[0028] The closed loop can also be provided by sending recycled monomers to a monomer production plant, i.e., typically a hydrocarbon cracker producing ethylene. In the latter event, the monomers can be recycled as such to a gas inlet of said monomer production plant. This gas inlet is preferably an inlet into a monomer purification section. In this event, the recycled monomers will ultimately become part of the monomer production output of the monomer production plant. Alternatively the recycled monomers or part of the recycled monomers can be utilized as fuel for the operation of said monomer production plant.

[0029] Where this disclosure speaks of emptying a reactor, this does not necessarily mean that the reactor thereby becomes fully empty. I.e., emptying the reactor refers to discharging at least part of the contents of the reactor into a dedicated treatment system as disclosed herein. It is noted that a polymer synthesis section as employed herein may comprise one or more reactors. Such reactor or reactors may comprise one or more reactor segments, e.g. different reactor layers in a tubular reactor. It may also comprise one or more autoclave reactors, e.g. multiple autoclave reactors installed in parallel or in series. In accordance with the invention, the emptying of the reactor can refer to emptying, entirely or partially, a single reactor, multiple reactors, or one or more segments or parts of a reactor, such as one or more layers of a layered tubular reactor. The emptying may include entirely or partly the hydrocarbon inventory of other parts of the polymer synthesis plant, e.g. compressors or separators.

[0030] The plant of the invention can generally be any plant for the polymerization of ethylene (IUPAC: ethene, CH2=CH2), optionally with ethylenically unsaturated co-monomers. This can thus be a plant configured to produce polyethylene homopolymers, copolymers, or both. Comonomers suitable for copolymerization with ethylene including vinyl ethers such as vinyl methyl ether and vinyl phenyl ether, olefins such as propylene, 1- butene, 1 -octene and styrene, vinyl esters such as vinyl acetate, vinyl butyrate and vinyl pivalate, halo olefins such as vinyl fluoride and vinylidene fluoride, acrylic esters such as methyl acrylate, ethyl acrylate, n- butyl acrylate or tert -butyl acrylate, other acrylic or methacrylic compounds such as acrylic acid, methacrylic acid, maleic acid, acrylonitrile and the acrylamides, and other compounds such as allyl alcohol, vinyl silanes, and other copolymerizable vinyl compounds.

[0031] In this disclosure reference is sometimes made to the normal operation of the polymerization plant. This generally refers to a process comprising feeding monomers to the polymer synthesis section, subjecting these monomers to polymerization in a polymerization reactor, and obtaining polymers from said reactor. Generally this normal operation will involve subjecting the polymer synthesis section to a pressure in a range of from 500 bar(g) to 4000 bar(g) and a temperature of 150°C to 350°C. Said normal operation is to be distinguished from a pressure relief event. A pressure relief event, in this disclosure, refers to any situation in which venting valves of the polymerization reactor are opened in order to empty at least part of the contents of the reactor into the pressure relief cooling and separation section. Although pressure relief events are not necessarily limited to emergency situations, their occurrence will mostly be in connection with an emergency shut-down of the polymerization reactor. In the course of a pressure relief event, the ethylene feed is stopped, as is charging the reactor with any comonomers, initiators and moderators. A pressure relief event generally results in depressurizing a polymerization reactor very quickly, such as from 3000 bar to 1000 bar within 12 seconds or less. This results in high flowrates, much higher than in the course of the plant’s regular operation, such as in a range of ~200kg / s to lOOOkg / s for a 400-450 kta plant. By comparison, a normal product stream of a 400-450kta plant has a flowrate of e.g. ~30kg / s (which is then also reflected in the HP recycle of gas).

[0032] Generally, as noted above, the composition of the mixture obtained from a polyethylene reactor upon an emergency discharge is different from the composition normally obtained from the outlet for polymerization product from the reactor. E.g., the emergency let-down mixture contains higher amounts of ethane, methane, CO and CO2. Typically, also significant amounts of H2 are present. The content of H2 in the process gas obtained from normal operation is generally negligible. The monomer recycle system present for normal operation is unsuitable to handle the higher concentration of impurities from an emergency discharge.

[0033] In its basic form, the specific combination of provisions herein disclosed, comprise a pressure relief cooling and separation section configured to separate solids from gaseous monomers, downstream thereof a gas storage section, and wherein said gas storage section has an outlet connected to a monomer recycle flow line.

[0034] Preferably the gas storage section is in fluid communication with a monomer purification section, which in turn is provided with a connection for purified monomers to the polymer synthesis section. The point at which this connection is linked to the regular recycle in an LDPE plant, will depend on the pressure to which the purified monomers from the system of the invention are brought. Preferably this connection is between the primary compressor and the secondary compressor, as shown in Fig.2.

[0035] The gas storage section serves to accommodate the inevitably large amounts of gas that are released from the reactor upon a pressure relief event. The purification section serves to ensure that the retrieved monomers can be recycled to the synthesis section in a sufficiently pure form. These two provisions work in concerted action. The purification is required to enable the recycling of monomers, but without a storage section the purification section would not be able to handle the one time burst amount of monomers that would become released upon a pressure relief event.

[0036] Preferably the plant is configured to have a purge gas circulating in the aforementioned closed loop during normal operation of the polymer synthesis section. Particularly, hereby said closed loop is configured to be kept at a set initial pressure. The purge gas comprises olefinic monomer and / or inert gas, such as nitrogen. Preferably the purge gas consists mainly of olefinic monomer, more preferably ethylene with a purity of typically more than 90% and methane, ethane, propane, propene, C4-C8 hydrocarbons, acetone, carbon monoxide and carbon dioxide as main contaminants. During normal operation of the polymerization plant, LDPE plant, such purge gas is sent to the closed loop preferably at a constant flowrate, e.g., It / h, entering at an upstream side of the pressure relief cooling and separation section. The set initial pressure of the closed loop, during normal operation of the polymerization plant, will generally be substantially below the pressures prevailing at a pressure relief event. Preferably, the closed loop is configured to be kept at a set initial pressure of 1 bar(g) to 10 bar(g), more preferably 3 bar(g) to 7 bar(g), such as 5 bar(g). Accordingly, during normal operation of the polymerization plant, the pressure in the closed loop will be kept at a pressure in the aforementioned ranges. Upon a pressure relief event, the pressure in the closed loop will increase, as a result of discharge of the reactor contents into the closed loop, typically up to about 10 bat(g) to 30 bar(g), such as 15 bar(g) to 25 bar(g).

[0037] In an embodiment, either or both of the pressure relief cooling and separation section and the gas storage section serve to achieve sufficient cooling and expansion in order reduce the pressure in the closed loop. Advantageously, however, an additional, active measure is taken to reduce the pressure, preferably allowing to restore said pressure to the set initial pressure. Accordingly, in a preferred embodiment the connection from the gas storage section to the monomer purification section is provided with a flow control system. This flow control system, preferably a flow control valve, is configured to regulate the gas flowrate such that after operation of the pressure relief system, the pressure in the closed loop is restored to the set initial pressure.

[0038] The reactor can be any reactor suitable for the production of polyolefins by means of radical initiated, polymerization at a pressure of >500 bar. A preferred type of reactor is a tubular reactor or an autoclave reactor. The skilled person is familiar with such reactors. Reference is made, inter alia, to W02007 / 01871. In a preferred embodiment, the reactor is a layered tubular reactor as disclosed in. US2019 / 0201865.

[0039] The reactor is provided a pressure relief system adapted to enable instantaneously emptying the reactor. To this end, said pressure relief system comprises an emergency outlet for the contents of the reactor. This can be a sacrificial outlet, such as a rupture disc. Preferably, with the recycle system of the present invention, the emergency outlet is a controllable outlet. The term “controllable” indicates that the outlet can be arranged to be in a closed state or in an opened state. In normal operation of the plant, the controllable outlet will be closed. Upon a pressure relief event, such as at an emergency shut-down of the plant, the controllable outlet can be opened in order to instantaneously empty the reactor. This can be a partial or complete emptying. The controls to open the outlet, can be subjected to human operation or can be computer-controlled operation, or an automatic operation depending on the circumstances in the reactor.

[0040] The controllable outlet is in fluid communication with an inlet of a pressure relief cooling and separation section configured to separate solids from gaseous monomers. This section generally comprises a blowdown vessel, and preferably at least two blowdown vessels in series. In these vessels the gaseous contents from the reactor are allowed to expand, and are subjected to cooling. The cooling is preferably provided by direct contact of the emptied reactor contents with water. This is preferably accomplished by having a level of water present in the vessel or by spraying water into the vessel, or by a combination of these measures.

[0041] The separation of gas (monomers) on the one hand, and solids and liquid (polymer and water) on the other hand, generally takes place by allowing the gas to exit at an upper side of a blown down vessel. The retained non-gaseous remainders are generally sent to a collection vessel, from which liquid / solid separation can take place.

[0042] The pressure relief cooling and separation section has a gas outlet in fluid communication with a gas storage section. Generally this comprises at least one storage vessel. If desired, a plurality of parallel storage vessels can be provided. Preferably, a single vessel should be sufficient.

[0043] In the event that the captured monomers are recycled as fuel, the gas storage section has a gas outlet to a recycle flow line that is connected to a fuel inlet of a monomer production plant. Preferably, the monomers are recycled via a monomer purification section. This section can be part of a a pre-existing monomer plant. It can also be added to the polyethylene plant, in which case the captured monomers can advantageously be recycled within the polyethylene plant itself.

[0044] The gas storage section thus preferably has a gas outlet in fluid communication with an inlet of a monomer purification section. As discussed above, the connection from the gas storage section to the monomer purification section is preferably provided with a flow control system. The monomer purification section generally comprises one or more distillation columns. The contaminants, mainly alkenes, carbon monoxide, carbon dioxide and nitrogen will be removed. Via a gas outlet which is in fluid communication with an inlet for monomers of the reactor, purified monomers can be returned to the reactor. Purification sections for ethylene polymers as such are familiar to the skilled person. Reference is made, e.g., to the chapter on Ethylene in Ullmann's Encyclopedia of Industrial Chemistry, 7th Ed., Vol.13, page 465-529.

[0045] In accordance with the invention, the reactor, the emergency outlet, the pressure relief cooling and separation section, the gas storage section and the monomer purification section form a closed loop. It will be understood that this closed loop is separate from the equipment and material flows during normal operation of the plant. In the plant modified or constructed with a pressure relief system of the invention as discussed hereinbefore, said closed loop will thus involve a recycle of monomer via the fourth cooling and separation section.

[0046] By virtue of providing said closed loop, the plant of the invention allows preventing emission of gaseous monomers in the event of an emergency-shut down of the plant.

[0047] In all of its embodiments, the plant of the invention is a plant for the production of LDPE (low-density polyethylene). In a further aspect, the invention pertains to a method of modifying a pre-existing plant for the production of polyolefins. This will generally be a plant provided with a pressure relief system allowing to separate of polymeric solids. Accordingly, the pre-existing plant comprises a polymerization reactor provided with an inlet for monomer feed and an outlet for polymerization product, and a pressure relief system adapted to enable instantaneously emptying the reactor. The pressure relief system in the pre-existing plant will normally have a controllable outlet in fluid communication with an inlet of a cooling and separation section configured to separate solids from gaseous monomers. In a preferred embodiment, the pressure relief system is as disclosed in US2019 / 0201865.

[0048] This refers to a typical tubular LDPE plant, reflecting the current standard in the art. Herein a polymerization unit has been provided with a plurality of emergency venting valves which are connected to a blowdown vessel. This blowdown vessel has a gas outlet connected to a vent stack. In adapting a pre-existing plant of this type in accordance with the present invention, the existing blowdown vessel is modified by replacing the connection tot he vent stack, or the vent stack altogether, with a flow line, e.g. a pipe, connecting to a further blowdown vessel, which will thus be added in accordance with the present invention.

[0049] Not all pre-existing LDPE plants, in particular older ones, necessarily have a blown down vessel. In such a case, the invention comprises the addition of both a first and a second blow down vessel.

[0050] The modification method of the invention further comprises adding to the plant a gas storage section and, preferably, a monomer purification section. This is done such that the pressure relief cooling and separation section is provided with a gas outlet in fluid communication with the gas storage section, the gas storage section is provided with a gas outlet in fluid communication with an inlet of the monomer purification section, and the monomer purification section is provided with a gas outlet in fluid communication with an inlet for monomers of the reactor.

[0051] The invention also pertains to a process for preparing a polyolefin, preferably polyethylene, most preferably LDPE. The process comprises subjecting olefinic (i.e., ethylenically unsaturated) monomers to polymerization in a plant in accordance with the invention as described hereinbefore, in all its embodiments. The process can serve to prepare homopolymers, copolymers, or both. Preferred ethylenically unsaturated monomers are selected from the group consisting of ethylenically unsaturated monomers and mixtures thereof, preferably ethylene (IUPAC: ethene, CH2=CH2).

[0052] In sum, a plant is disclosed for the production of polyolefins, particularly LDPE, comprising a polymerization reactor provided with a pressure relief system. The pressure relief system is designed such that after a pressure relief event, with discharging at least part of the reactor inventory into a pressure relief cooling and separation section, gaseous monomer is not vented into the atmosphere, but is collected, purified, and returned as a monomer feed to the reactor.

[0053] The invention will hereinafter be illustrated with reference to the following non-limiting example embodiments, and the accompanying figure.

[0054] Example

[0055] Reference is made to FIG.l, the reference signs in which have the following meaning. Shown is a pressure relief cooling and separation section adapted in accordance with the invention, containing the following equipment parts (101) to (107):

[0056] (101) first blowdown vessel;

[0057] (102) discharge vessel; (103) second blowdown vessel;

[0058] (104) separator;

[0059] (105) gas cooler;

[0060] (106) water collector;

[0061] (107) gas storage vessel;

[0062] (108) dedicated purification unit.

[0063] Flow lines numbered (1) to (16) are configured to carry the following process streams with reference to the aforementioned equipment parts:

[0064] (1) discharged contents of LDPE reactor (not shown) to the first blowdown vessel (101);

[0065] (2) gas from the first blowdown vessel (101) to the second blowdown vessel (103);

[0066] (3) polymer separated from the first blowdown vessel (101);

[0067] (4) gas from the discharge vessel (102) vented to flare;

[0068] (5) waste polymer and waste water

[0069] (6) polymer separated from the second blowdown vessel (103);

[0070] (7) gas from the second blowdown vessel (103) to the separator (104);

[0071] (8) waste polymer and waste water

[0072] (9) gas from the separator (104) to the gas cooler (105);

[0073] (10) cooling water to the gas cooler (105);

[0074] (11) steam generated in the gas cooler (105);

[0075] (12) cooled gas from the gas cooler (105) to the water collector (106);

[0076] (13) waste water;

[0077] (14) gas from the water collector (106) to the gas storage vessel (107); (15) gas from the gas storage vessel (107) to a monomer recycle system (not shown)

[0078] (16) purified monomer.

[0079] An LDPE plant is provided as described in US2019 / 0201865. The plant comprises an LDPE synthesis section comprising a layered tubular reactor (not shown) with a pressure relief compartment and a pressure relief cooling and separation section. Said pressure relief compartment consists of a blow-down vessel (101) with a cooling agent and vent.

[0080] Added to this plant is a second blow-down vessel (103) which is placed in series with the blow-down vessel (101) of the pressure relief compartment. This is done in such a way that the vent is replaced by a fluid connection (2) to the second blow-down vessel. Further added are a discharge vessel (102), a separator (104), a gas cooler (105), a water collector (106) and an ethylene storage vessel (107), i.e., an embodiment of a gas storage section according to the invention. The plant is designed in such a way that within a period of 32 hours, the available pressure release vents discharged during 2-3 emergency events can be collected in the ethylene storage vessel.

[0081] Downstream of the ethylene storage vessel (107) an ethylene purification unit (not shown) is installed to purify collected ethylene to a concentration of at least 99,5% ethylene, thus providing a monomer purification section in accordance with the applicable embodiment of the invention. The purified gas is sent back continuously as feed gas to the LDPE synthesis section, thereby establishing a closed loop comprising the reactor, the pressure relief cooling and separation section, the gas storage section and the monomer purification section form a closed loop.

[0082] The ethylene purification unit is working continuously. Purge gas from a Booster-Primary compressor (approx. It / h of process gas, mainly ethylene) is routed to the first blowdown vessel (101) and is transferred to the purification unit to allow a continuous operation. The ethylene purification unit is designed for a range of 1 to 5 t / h.

[0083] During an emergency shutdown, the reactor inventory is rapidly discharged through vent lines which are connected to the closed blow down, separation and storage system.

[0084] During normal operation of the LDPE plant, purge gas consisting mainly of ethylene is sent to the closed system at a constant flowrate of approximately It / h, entering at the first blow down vessel (101). The entire closed blowdown, separation and storage system is kept pressurized at 5 bar(g) by a flow control valve at the outlet of the ethylene storage vessel (107). During an emergency venting event up to about 20 t of ethylene discharged from the high pressure sections of LDPE are collected within several minutes and the pressure inside the entire system rises up to about 15 to 25 bar(g). Ethylene collected in said storage vessel is transferred through the flow control valve to the purification unit.

[0085] In case the emergency venting valves of the LDPE reactor system are opening, the released polymer / gas mixture at temperatures up to 350°C is routed through the vent lines into the first blow down vessel (101), which is partly filled with water. The water serves to separate the polymer from the discharged ethylene, to reduce the gas / polymer mixture temperature and to extinguish any smoldering particles that might become potential ignition sources. The water level is kept at about 200 mm below the vent lines inlet points. It is controlled by a level measurement acting on automatic valves in the overfilling line or water make-up line.

[0086] The separated gas from the first blow down vessel is routed from the top of the vessel into the second blowdown vessel (103) through one line, entering tangentially about 200 mm above the water level of this vessel. The water level in the second blow down vessel (103) again is controlled by a level measurement acting on automatic valves in the overfilling line or water make-up line. During a discharge event, water is injected from the top of the vessel by a spray nozzle water for better separation of fine particles and cooling of the gas / polymer mixture.

[0087] The partially cleaned gas from the second blow down vessel (103) is routed from the outlet at the top of the vessel into the separator (104). The connecting line (8) between the second blow down vessel (103) and the separator (105) has automatic double block and bleed valves to isolate the blowdown vessels from the downstream systems when dumping the content of the blow down vessels into the discharge vessel (102).

[0088] After completion of an emergency venting event the retained polymer / water mixture has to be removed from both vessels. From the bottoms of both vessels are routed 300 mm lines with automatic double block and bleed valves to the discharge vessel (102) to release separated polymer and to drain the water.

[0089] The discharge vessel (102) is designed in such a way, that the polymer can be separated from the dumped water by a sieve installed inside of the vessel. For maintenance purpose the vessel can be flushed with nitrogen. The discharge vessel is connected by flowline (4) to a flare, to release the gas pressure before any cleaning procedure. Separated polymer is removed from the sieve manually through a manhole. The separated water is discharged to the waste water system.

[0090] Gas leaving from the top of the second blow down vessel (103) is routed via a flowline (8) to the separator (104), to separate any water and small fines particles, that are entrained in the gas leaving the second blowdown vessel at high velocity. The separator is designed as an aero cyclone and the gas is entering tangentially. Separated water and polymer dust can be discharged from the bottom.

[0091] The cleaned gas is routed via a flowline (10) from the top of the separator (104) to the gas cooler (105) to reduce the gas temperature to 60- 80°C. Cooling water from a reservoir flows (11) through down comers into a circumferential water-jacket annulus. From here water enters several parallel tunnels, which distribute the water over the tube sheet and the individual tubes. The hot gas is cooled down by the water, producing saturated steam (12). Water flows through the tunnel system toward the opposite side at high velocity in intimate contact with the tube sheet, thereby preventing deposition of solids. The using of such heat exchanger is depending of the temperature outlet of the cyclone.

[0092] The cooled gas is after routed to the storage vessel (107) via a water collector (106). In case water is accumulating inside the storage vessel it can be drained to the waste water system.

[0093] From the storage vessel (107) the collected gas is sent by flow control to a dedicated purification unit (108), regulating a flowrate between 700 to 5000 kg / h until the initial pressure of 5 bar(g) is reached. The purification unit is consisting in the main part of distillation columns removing impurities from the recovered gas. The purified gas is ethylene of polymerization grade quality and sent back continuously to the LDPE plant as feedstock.

[0094] Reference is further made to FIG.2 in order to illustrate the position and connection of the pressure relief system of the invention, relative to a customary set-up for a high-pressure LDPE plant. Depicted is a flow scheme for an LDPE plant. Monomer enters the plant via an inlet of a prima compressor (201) and is subjected to further compression to reaction pressure in a secondary compressor (202), positioned in series downstream of the primary compressor. A modifier, typically a chain-transfer- agent, preferably propylene or propane, is added at the primary compressor as well. From the secondary compressor (202), pressurized monomer is led trough a polymerization reactor (203) - an autoclave reactor or, preferably, a tubular polymerization reactor - which is provided with at least one inlet for a polymerization initiator. Downstream of the polymerization reactor, an outlet for polymer product is in fluid connection via a let down valve (204) and, downstream thereof a product cooler (205), with a high pressure separator (206). In this high pressure separator, polymer product is separated from unreacted monomer. The latter is recirculated, via a high pressure recycle cooler (208a) and downstream thereof a high pressure recycle separator (208b), in which unreacted monomer is separated from waxes and other oligomers, to the monomer feed flow to the reactor, at a point downstream of the primary compressor (201) and upstream of the secondary compressor (202). The polymer product separated from the high pressure separator (206) is led to a low pressure separator (207). In this low pressure separator, polymer product is further separated from unreacted monomer, with condensable hydrocarbons typically becoming entrained in the unreacted monomer. The latter is recirculated, via a low pressure recycle cooler (209a) and downstream thereof a low pressure recycle separator (209b), in which unreacted monomer is separated from condensable hydrocarbons, to the monomer feed flow to the reactor, at a point upstream of the primary compressor (201).

[0095] The polymerization reactor (203) is provided with at least one outlet to a pressure relief system (210), serving to empty the reactor upon an emergency event. The pressure relief system is connected with a pressure relief system of the invention, see line (1) to blowdown vessel (101) in FIG.l, as indicated by the corresponding black arrow in either figure.

[0096] In a preferred embodiment, a purified monomer stream (16) resulting from treatment in the pressure relief system of the invention, is returned to the polymerization plant. This connection is from the dedicated purification unit (108) in FIG.l to the primary compressor (201) in FIG.2, as indicated by the corresponding open arrow in either figure.

Claims

23Claims1. A plant for the production of polyethylene and / or polyethylene copolymers, said plant comprising a polymer synthesis section comprising a polymerization reactor provided with an inlet for monomer feed and an outlet for polymerization product, said reactor comprising a pressure relief system adapted to enable instantaneously emptying, partially or entirely, the reactor, said pressure relief system having an emergency outlet in fluid communication with an inlet of a pressure relief cooling and separation section configured to separate solids from gaseous monomers, said pressure relief cooling and separation section having a gas outlet in fluid communication with a gas storage section, said gas storage section having a gas outlet in fluid communication with a monomer recycle system.

2. A plant according to claim 1, wherein the monomer recycle system comprises a gas flow line in fluid communication with a gas inlet of an ethylene monomer or polymer production plant.

3. A plant according to claim 2, wherein said gas inlet is an inlet for fuel.

4. A plant according to claim 2, wherein said gas inlet is an inlet into a monomer purification system.

5. A plant according to claim 4, wherein said monomer purification system is comprised in an ethylene monomer production plant.

6. A plant according to claim 4, wherein said monomer purification system has an outlet for purified monomer in fluid communication with an inlet of the polymerization reactor.

7. A plant according to claim 6, wherein the reactor, the pressure relief cooling and separation section, the gas storage section and the monomer purification section form a closed loop.

8. A plant according to claim 7, wherein said closed loop is configured to be kept at a set initial pressure during normal operation of the polymer synthesis section.

9. A plant according to claim 7 or 8, wherein the connection from the gas storage section to the monomer purification section is provided with a flow control system configured to regulate the gas flowrate such that after operation of the pressure relief system, the pressure in the closed loop is restored to the set initial pressure.

10. A plant according to any one of the preceding claims, wherein the plant is configured to have a purge gas circulating in the monomer recycle system during normal operation of the polymer synthesis section.

11. A process for preparing a polyethylene homopolymer or copolymer comprising subjecting ethylenic monomers to polymerization in a polymer synthesis section comprising a polymerization reactor, said reactor comprising a pressure relief system, wherein the process comprises, at the occurrence of a pressure relief event, discharging the contents of the reactor into a pressure relief cooling and separation section, subjecting said contents to separating off of gaseous monomers, sending said monomers to a gas storage section, and recycling said monomers from said gas storage section, the process preferably being carried out in a plant according to any one of the claims 1 to 10.

12. A process according to claim 11, wherein said recycling comprises controlling sending of monomers from said gas storage section to a monomer purification section, subjecting the monomers to purification, and sending purified monomers to the reactor.

13. A process according to claim 11 or 12, wherein the process is carried out in a plant according to claim 10, and comprises circulating a purge gas in the monomer recycle system during normal operation of the polymer synthesis section.

14. A method of modifying a pre-existing plant for the production of polyethylene and / or polyethylene copolymers, said plant comprising a polymer synthesis section comprising a polymerization reactor provided with an inlet for monomer feed and an outlet for polymerization product, said reactor comprising a pressure relief system adapted to enable instantaneously emptying the reactor, said pressure relief system having an emergency outlet in fluid communication with an inlet of a pressure relief cooling and separation section configured to separate solids from gaseous monomers, the method comprising adding to the plant a gas storage section, said gas storage section having an outlet in fluid communication with a monomer recycle flow line.

15. A method according to claim 14, wherein said monomer recycle flow line is in fluid communication with an inlet of a monomer purification section, and the monomer purification section is provided with a gas outlet in fluid communication with an inlet for monomers of the reactor.

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