[0001 ] The present disclosure provides a process for polymerizing or copolymerizing ethyleni- cally unsaturated monomers at temperatures from 1 00 °C to 350 and pressures in the range of from 1 1 0 M Pa to 500 MPa in a continuously operated polymerization reactor.

BACKGROUND OF TH E INVENTION

[0002] Polyethylene is the most widely used commercial polymer and can be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the first method used to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE).

[0003] A common set-up of a production line for preparing low density polyethylene comprises a polymerization reactor, which can be an autoclave or a tubular reactor or a combination of such reactors, and additional equipment. For pressurizing the reaction components, usually a set of two compressors, a primary compressor and a secondary compressor, is used. At the end of the polymerization sequence, a production line for high-pressure polymerization normally further includes apparatuses like extruders and granulators for pelletizing the resulting polymer. Furthermore, such a production line generally also comprises means for feeding monomers and comon- omers, free-radical initiators, modifiers or other substances at one or more positions to the polymerization reaction.

[0004] A characteristic of the radically initiated polymerization of ethylenically unsaturated monomers under high pressure is that the conversion of the monomers is by far not complete. For every pass of the reactor or the reactor combination, only about 1 0% to 50% of the dosed monomers are converted in polymerizations in a tubular reactor and from 8% to 30 % of the dosed monomers are converted in polymerizations in an autoclave reactor. The resulting reaction mixture usually leaves the reactor through a pressure control valve and is then commonly separated into polymeric and gaseous components with the unreacted monomers being recycled. To avoid unnecessary decompression and compression steps, the separation into polymeric and gaseous components is usually carried out in at least two stages. The monomer-polymer mixture leaving the reactor is usually transferred to a first separating vessel, frequently called high- pressure product separator, in which the separation in polymeric and gaseous components is carried out at a pressure that allows for recycling of the ethylene and comonomers separated from the monomer-polymer mixture to the reaction mixture at a position between the primary compres- sor and the secondary compressor. At the conditions of operating the first separation vessel, the polymeric components within the separating vessel are in liquid state. The liquid phase obtained in the first separating vessel is transferred to a second separation vessel, frequently called a low- pressure product separator, in which a further separation into polymeric and gaseous components takes place at lower pressure. The ethylene and comonomers separated from the mixture in the second separation vessel are fed to the primary compressor where they are compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed and the joined streams are further pressurized to the pressure of the high-pressure gas recycle stream. [0005] The polymerization process in a LDPE reactor is carried out at high pressures which can reach 350 M Pa. Such high pressure requires special technology for the process to be handled in a safe and reliable manner. Technical issues in handling ethylene at high pressures are, for example, described in Chem. Ing. Tech. 67 (1 995), pages 862 to 864. It is stated that ethylene decomposes rapidly in an explosive manner under certain temperature and pressure conditions to give soot, methane and hydrogen. This undesired reaction occurs repeatedly in the high-pressure polymerization of ethylene. The drastic increase in pressure and temperature associated therewith represents a considerable potential risk for the operational safety of the production plants.

[0006] A possible solution for preventing a drastic increase in pressure and temperature of this type involves installing rupture discs or emergency pressure-relief valves. WO 02/01308 A2, for example, discloses a specific hydraulically controlled pressure relief valve which allows a particularly fast opening of the pressure relief valve in case of sudden changes in pressure or temperature. It is technically possible to handle such thermal runaways or explosive decompositions of ethylene within the polymerization reactor, however these situations are highly undesirable since thermal runaways or explosive decompositions of ethylene within the polymerization reactor lead to a shut-down of the polymerization plant with frequent emission of ethylene into the environment and loss of production.

[0007] Another threat to the operational safety of high-pressure polymerization plants is the occurrence of leaks. Due to the high pressure difference between the interior of the polymerization reactor and the surroundings, even small fissures in a wall of high-pressure equipment may lead to an exit of a considerably high amount of the reactor content resulting in locally high concentrations of combustible hydrocarbons in a short time period. On the other hand, in the case of larger leaks, the available time for reacting is extremely short.

[0008] Processes for polymerizing or copolymerizing ethylenically unsaturated monomers at pressures in the range of from 1 1 0 MPa to 500 M Pa accordingly place specific demands on a reliable detection of combustible or explosive gases, which may leak from the polymerization equipment. Depending on the size and the position of the leak, the leakage rate may be extreme- ly high or relatively low, with a risk of an accumulation of the leaked material. The leaked material may have a temperature in the range from 1 00 °C to 350 °C but can also be cold and may therefore sink to the ground and accumulate there. The concentration of leaked gas in a certain volume element in the vicinity of the polymerization plant can vary from substantially pure hydrocarbon to a very low concentration of combustible gas in air. Furthermore, the leakage can not only take place towards the atmosphere but the leakage can also occur at a section of the equipment which is covered by a cooling or heating jacket. Moreover, as such processes are not carried out in totally closed housings, also weather phenomena such as wind and rain may have an influence on the detection of leaked gases. [0009] Another difficulty with respect to processes for preparing ethylene polymers at high pressure is that the reaction mixture is a supercritical composition comprising monomer and polymer. After a leakage of such a reaction mixture into the atmosphere, small polymer particles are formed which are subject to electrostatic charging. Consequently, there is an enhanced probability for an ignition after an explosive gas cloud has developed after an escape of reaction mixture.

[0010] WO 2008/148758 A1 discloses a method of operating a high-pressure ethylene polymerization unit comprising a tubular reactor equipped with a cooling jacket, in which method the leakage of reaction mixture into the cooling jacket is controlled by monitoring the electrical conductivity of the aqueous cooling medium. Such a method, however, requires that at least one of the chemical substances in the reaction mixture changes the electrical conductivity of the aqueous cooling medium, and the method can only detect leakage at positions of the polymerization equipment which are covered by a cooling jacket.

[0011 ] The polymerization of ethylenically unsaturated monomers at pressures in the range of from 1 1 0 M Pa to 500 MPa in often carried out in tubular polymerization reactors because these reactors have a high surface for removing the liberated heat of polymerization. Typical tubular reactors have a length in the range from about 0.5 km to 5 km and are composed of thick-walled tubes of a length from 5 m to 25 m. The individual tubes of the tubular reactor are flanged together, either directly or via bends. Also pre-heaters or pre-coolers or post reactor coolers in produc- tion lines for preparing low density polyethylenes are often composed of thick-walled tubes of a length from 5 m to 25 m which are flanged together. Among the parts of a low density polyethylene production line which have a relatively high likelihood of a leakage are especially these flange connections because these sometimes need to be loosened and retightened for maintenance reasons. However, the leaking rate for a leakage at a flange connection may be relatively small and makes it difficult to quickly recognize the occurrence of such a leak.

[0012] Accordingly there is a need to overcome the disadvantages of the prior art and provide a process which allows for a fast recognition of a leakage in a flange of a production line for polymerizing ethylenically unsaturated monomers at pressures in the range of from 1 1 0 M Pa to 500 M Pa. SUMMARY OF TH E INVENTION

[0013] The present disclosure provides a process for polymerizing or copolymerizing one or more ethylenically unsaturated monomers at temperatures from 1 00 °C to 350 °C and pressures in the range of from 1 1 0 M Pa to 500 M Pa in a continuously operated polymerization reactor, wherein the polymerization is carried out in a production line in which the monomers are brought to the polymerization pressure by one or more compressors in a sequence of compression stages in which the compressed gas mixture is cooled after each compression stage by a compression stage cooler, the compressed monomers are optionally passed through a pre-heater or a pre- cooler and transferred into the polymerization reactor, which is optionally cooled by cooling jackets, a reaction mixture obtained by the polymerization is leaving the reactor through a pressure control valve, and optionally cooled by an post reactor cooler, the reaction mixture is separated into polymeric and gaseous components in two or more stages, where the gaseous components separated off in a first stage at an absolute pressure of from 15 M Pa to 50 M Pa are recycled to the one or more compressors via a high-pressure gas recycle line, and the gaseous components separated off in a second stage at an absolute pressure in the range of from 0.1 to 0.5 M Pa are recycled to the first stage of the sequence of compression stages via a low-pressure gas recycle line, and the polymeric components obtained by the polymerization are transformed into pellets, wherein at least one of the pre-heater or pre-cooler, the polymerization reactor and the post reactor cooler is composed of tubes of a length from 5 m to 25 m which are flanged together, either directly or via bends, and the flanges are covered by a chimney construction, and wherein air is conveyed through the chimney construction and the air exiting the chimney construction is moni- tored with respect to the hydrocarbon concentration.

[0014] In some embodiments, the polymerization reactor is a tubular reactor or a reactor cascade comprising a tubular reactor, and the production line comprises a pre-heater and a post reactor cooler, and the pre-heater, the polymerization reactor and the post reactor cooler are composed of tubes of a length from 5 m to 25 m which are flanged together, either directly or via bends, and the flanges are covered by a chimney construction, and air is conveyed through the chimney construction and the air exiting the chimney construction is monitored with respect to the hydrocarbon concentra- tion.

[0015] In some embodiments, the polymerization reactor is an autoclave reactor or a cascade of autoclave reactors and the production line comprises a pre-cooler or a post reactor cooler or a pre-cooler and a post reactor cooler, and the pre-cooler or the post reactor cooler or the pre-cooler and the post reactor cooler are composed of tubes of a length from 5 m to 25 m which are flanged together, either directly or via bends, and the flanges are covered by a chimney construction, and air is conveyed through the chimney construction and the air exiting the chimney construction is monitored with respect to the hydrocarbon concentration.

[0016] In some embodiments, at least 50% of the chimney constructions cover two or more flanges. [0017] In some embodiments, the chimney construction has openings at the bottom and at the top and the air is conveyed by natural convection.

[0018] In some embodiments, the air exiting a chimney construction is monitored by one or more I R open path detectors or by an IR point detector.

[0019] In some embodiments, the air exiting a chimney construction is monitored by an array of I R open path detectors which operate according to a voting logic.

[0020] In some embodiments, an emergency pressure release program is automatically start- ed when hydrocarbons are detected in the air exiting the chimney construction.

[0021 ] In some embodiments, further measurements for detecting a leakage of hydrocarbons in the surroundings of the production line are carried out. [0022] In some embodiments, an emergency pressure release program is automatically started when hydrocarbons are detected in the air exiting the chimney construction or a leakage of hydrocarbons in the surroundings of the production line is detected by the further measurements.

[0023] In some embodiments, the further measurements for detecting a leakage of hydrocar- bons in the surroundings of the production line are carried out by employing I R open path detectors, I R point detector or ultrasonic detectors, or combinations thereof.

[0024] In some embodiments, the further measurements for detecting a leakage of hydrocarbons in the surroundings of the production line include a monitoring of at least one of the cooling media which cool the compression stage coolers, the cooling jackets of the polymerization reactor, the post reactor cooler or a cooler within the high-pressure gas recycle line with respect to an occurrence of a leakage of monomers or of reaction mixture into the cooling medium.

[0025] In some embodiments, air is passed through the cooling medium and thereafter con- veyed to an I R point detector capable of detecting hydrocarbons. [0026] In some embodiments, the polymerization reactor and optionally further parts of the production line are installed within a protective enclosure and wherein a water based deluge system is started automatically in parallel with the emergency pressure release program and the water based deluge system provides droplets of a diameter in a range from 25 μιη to 20 mm to the enclosed area when a leakage of monomers or reaction mixture is detected and the droplets are provided with a minimum flow rate of 1 0 L/min per m ² of enclosed area.

[0027] In some embodiments, a steam based deluge system is started automatically in parallel with the emergency pressure release program.

BRI EF DESCRI PTION OF THE DRAWINGS

[0028] Figure 1 shows schematically a set-up for carrying out the process of the present disclosure.

[0029] Figure 2 shows schematically an arrangement of chimney constructions in a section of a tubular reactor.

[0030] Figure 3 shows schematically a chimney construction element covering one flange of a tubular reactor.

DETAILED DESCRIPTION OF THE INVENTION

[0031 ] The covering of the flanges of a production line for polymerizing ethylenically unsatu- rated monomers at pressures in the range of from 1 1 0 M Pa to 500 M Pa by a chimney construction allows for fast and reliable detection of a hydrocarbon leakage through flanges because leaking gas is confined to the volume within the chimney construction and a concentration above the detection limit will be reached more quickly than without the chimney construction. [0032] The present disclosure refers to a process for polymerizing or copolymerizing ethylenically unsaturated monomers in a continuously operated polymerization reactor, which can also be a combination of polymerization reactors, at temperatures from 1 00 °C to 350 °C and pressures in the range of from 1 1 0 M Pa to 500 M Pa. The present disclosure especially relates to such a process, in which the polymerization is carried out in a production line in which the monomers are brought to the polymerization pressure by one or more compressors in a sequence of compression stages in which the compressed gas mixture is cooled after each compression stage by a compression stage cooler, the compressed monomers are optionally passed through a pre-heater or a pre-cooler and transferred into the polymerization reactor which is optionally cooled by cooling jackets, a reaction mixture obtained by the polymerization is leaving the reactor through a pressure control valve and optionally cooled by an post reactor cooler, the reaction mixture is separated into polymeric and gaseous components in two or more stages, where the gaseous components separated off in a first stage at an absolute pressure of from 15 MPa to 50 M Pa are recycled to the one or more compressors via a high-pressure gas recycle line and the gaseous components separated off in a second stage at an absolute pressure in the range of from 0.1 to 0.5 M Pa are recycled to the first stage of the sequence of compression stages via a low-pressure gas recycle line, and the polymeric components obtained by the polymerization are transformed into pellets.

[0033] The high-pressure polymerization is preferably a homopolymerization of ethylene or a copolymerization of ethylene with one or more other monomers, provided that these monomers are free-radically copolymerizable with ethylene under high pressure. Examples of copolymeriza- ble monomers for use in the present technology are α,β-unsaturated C ₃ -C ₈ -carboxylic acids, in particular maleic acid, fumaric acid, itaconic acid, acrylic acid, methacrylic acid and crotonic acid, derivatives of α,β-unsaturated C ₃ -C ₈ -carboxylic acids, e.g. unsaturated C ₃ -C ₁₅ -carboxylic esters, in particular esters of C _r C ₆ -alkanols, or anhydrides, in particular methyl methacrylate, ethyl methacrylate, n-butyl methacrylate or tert-butyl methacrylate, methyl acrylate, ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, tert-butyl acrylate, methacrylic anhydride, maleic anhydride or itaconic anhydride, and 1 -olefins such as propene, 1 -butene, 1 -pentene, 1 -hexene, 1 -octene or

1 - decene. In addition, vinyl carboxylates, particularly preferably vinyl acetate, can be used as comonomers. Propene, 1 -butene, 1 -hexene, acrylic acid, n-butyl acrylate, tert-butyl acrylate,

2- ethylhexyl acrylate, vinyl acetate or vinyl propionate are particularly suitable as comonomer.

[0034] In the case of copolymerization, the proportion of comonomer or comonomers in the reaction mixture is from 1 to 50% by weight, preferably from 3 to 40% by weight, based on the amount of monomers, i.e. the sum of ethylene and other monomers. Depending on the type of comonomer, it can be preferred to feed the comonomers at more than one point to the reactor set-up. Preferably the comonomers are fed to the suction side of the secondary compressor.

[0035] For the purposes of the present disclosure, polymers or polymeric materials are sub- stances which are made up of at least two monomer units. The polymers or polymeric materials are preferably low density polyethylenes having an average molecular weight M _n of more than 20 000 g/mole. The term "low density polyethylene" includes ethylene homopolymers and ethylene copolymers. The process of the present disclosure can also be advantageously employed in the preparation of oligomers, waxes and polymers having a molecular weight M _n of less than 20 000 g/mole.

[0036] The process of the present disclosure is preferably a radical polymerization carried out in the presence of free-radical polymerization initiators. Possible initiators for starting the polymerization in the respective reaction zones are in general any substances that can produce radical species under the conditions in the polymerization reactor, for example, oxygen, air, azo compounds or peroxidic polymerization initiators. In a preferred embodiment of the disclosure, the polymerization is carried out by using oxygen, either fed in the form of pure 0 ₂ or as air. In case of initiating the polymerization with oxygen, the initiator is normally first mixed with the ethylene feed and then fed to the reactor. In such a case it is not only possible to feed a stream comprising monomer and oxygen to the beginning of the polymerization reactor but also to one or more points along the reactor creating two or more reaction zones. Initiation using organic peroxides or azo compounds also represents a preferred embodiment of the present disclosure. Examples of organic peroxides for use in the present technology are peroxy esters, peroxy ketals, peroxy ketones and peroxycarbonates, e.g. di(2-ethylhexyl) peroxydicarbonate, dicyclohexyl per- oxydicarbonate, diacetyl peroxydicarbonate, tert-butyl peroxyisopropylcarbonate, di-sec-butyl peroxydicarbonate, di-tert-butyl peroxide, di-tert-amyl peroxide, dicumyl peroxide, 2,5-dimethyl- 2,5-di-tert-butylperoxyhexane, tert-butyl cumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hex- 3-yne, 1 ,3-diisopropyl monohydroperoxide or tert-butyl hydroperoxide, didecanoyl peroxide, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, tert-amyl peroxy-2-ethylhexanoate, dibenzoyl peroxide, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate, tert-butyl peroxydi- ethylisobutyrate, tert-butyl peroxy-3,5,5-trimethylhexanoate, 1 , 1 -di(tert-butylperoxy)-3,3,5- trimethylcyclohexane, 1 , 1 -di(tert-butylperoxy)cyclohexane, tert-butyl peroxyacetate, cumyl perox- yneodecanoate, tert-amyl peroxyneodecanoate, tert-amyl peroxypivalate, tert-butyl peroxy- neodecanoate, tert-butyl permaleate, tert-butyl peroxypivalate, tert-butyl peroxyisononanoate, diisopropylbenzene hydroperoxide, cumene hydroperoxide, tert-butyl peroxybenzoate, methyl isobutyl ketone hydroperoxide, 3,6,9-triethyl-3,6,9-trimethyl-triperoxocyclononane and 2,2-di(tert- butylperoxy)butane. Azoalkanes (diazenes), azodicarboxylic esters, azodicarboxylic dinitriles such as azobisisobutyronitrile and hydrocarbons which decompose into free radicals and are also referred as C-C initiators, e.g. 1 ,2-diphenyl-1 ,2-dimethylethane derivatives and 1 , 1 ,2,2- tetramethylethane derivatives, may also be used. It is possible to use either individual initiators or preferably mixtures of various initiators. A large range of initiators, in particular peroxides, are commercially available, for example the products of Akzo Nobel offered under the trade names Trigonox® or Perkadox®. [0037] Preferred peroxidic polymerization initiators are 1 , 1 -di(tert-butylperoxy)cyclohexane, 2,2-di(tert-butylperoxy)butane, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-butyl peroxybenzoate, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butyl cumyl peroxide, di-tert-butyl peroxide and 2,5-dimethyl-2,5-di(tert-butylperoxy)hex-3-yne, and particular preference is given to using tert-butyl peroxy-3,5,5-trimethylhexanoate, di-(2-ethylhexyl)peroxydicarbonate or tert-butyl peroxy- 2-ethylhexanoate.

[0038] The initiators can be employed individually or as a mixture in concentrations of from 0.1 mol/t to 50 mol/t of polyethylene produced, in particular from 0.2 mol/t to 20 mol/t, in each reaction zone. In a preferred embodiment of the present disclosure the free-radical polymeriza- tion initiator, which is fed to a reaction zone, is a mixture of at least two different azo compounds or organic peroxides. If such initiator mixtures are used it is preferred that these are fed to all reaction zones. There is no limit for the number of different initiators in such a mixture, however preferably the mixtures are composed of from two to six and in particular of two, three or four different initiators. Particular preference is given to using mixtures of initiators which have differ- ent decomposition temperatures.

[0039] It is often advantageous to use the initiators in the dissolved state. Examples of solvents for use in the present disclosure are ketones and aliphatic hydrocarbons, in particular octane, decane and isododecane and other saturated C ₈ -C -hydrocarbons. The solutions comprise the initiators or initiator mixtures in proportions of from 2 to 65% by weight, preferably from 5 to 40% by weight and particularly preferably from 8 to 30% by weight.

[0040] In the high-pressure polymerization, the molecular weight of the polymers to be prepared can as usual be altered by the addition of modifiers which act as chain-transfer agents. Examples of modifiers for use in the present technology are hydrogen, aliphatic and olefinic hydrocarbons, e.g. propane, butane, pentane, hexane, cyclohexane, propene, 1 -butene, 1 -pentene or 1 -hexene, ketones such as acetone, methyl ethyl ketone (2-butanone), methyl isobutyl ketone, methyl isoamyl ketone, diethyl ketone or diamyl ketone, aldehydes such as formaldehyde, acetal- dehyde or propionaldehyde and saturated aliphatic alcohols such as methanol, ethanol, propanol, isopropanol or butanol. Particular preference is given to using saturated aliphatic aldehydes, in particular propionaldehyde or 1 -olefins such as propene, 1 -butene or 1 -hexene, or aliphatic hydrocarbons such as propane.

[0041 ] The high-pressure polymerization is carried out at pressures of from 1 10 M Pa to 500 M Pa, with pressures of from 1 60 M Pa to 350 M Pa being preferred and pressures of from 200 M Pa to 330 M Pa being particularly preferred for polymerization in a tubular reactor and with pressures of from 1 1 0 M Pa to 300 M Pa being preferred and pressures of from 120 M Pa to 280 M Pa being particularly preferred for polymerization in an autoclave reactor. The polymerization temperatures are in the range of from 1 00 °C to 350 ^ and preferably in the range of from 180 °C to 340 °C and more preferably from 200 °C to 330 °C for polymerization in a tubular reactor and preferably in the range of from 1 1 0 °C to 320 °C and more preferably from 120 °C to 31 0 °C for polymerization in an autoclave reactor.

[0042] The polymerization can be carried out with all types of high-pressure reactors appropri- ate for high-pressure polymerization. High-pressure reactors for use in the present technology are, for example, tubular reactors or autoclave reactors. Preferably the polymerization is carried out in one or more tubular reactors or one or more autoclave reactors or combinations of such reactors. In particularly preferred embodiments of the present disclosure, the polymerization reactor is a tubular reactor. [0043] Common high-pressure autoclave reactors are stirred reactors and have a length-to- diameter ratio in a range from 2 to 30, preferably from 2 to 20. Such autoclave reactors have one or more reaction zones, preferably from 1 to 6 reaction zones and more preferably from 1 to 4 reaction zones. The number of reaction zones depends on the number of agitator baffles which separate individual mixed zones within the autoclave reactor. I n production lines in which the polymerization or the first polymerization is carried out in an autoclave reactor, i.e. in production lines in which the only polymerization reactor is an autoclave reactor or in production lines in which the first reactor of a reactor cascade is an autoclave reactor, the reaction mixture coming from the compressors is usually first passed through a pre-cooler before entering the autoclave reactor.

[0044] Appropriate tubular reactors are basically long, thick-walled pipes, which are usually from about 0.5 km to 4 km, preferably from 1 km to 3 km and especially from 1 .5 km to 2.5 km long. The inner diameter of the pipes is usually in the range of from about 30 mm to 120 mm and preferably from 60 mm to 1 00 mm. Such tubular reactors have preferably a length-to-diameter ratio of greater than 1000, preferably from 10000 to 40000 and especially from 25000 to 35000. Preferably the tubular reactor is composed of tubes of a length from 5 m to 25 m, more preferably of a length from 1 0 m to 22 m, and especially of a length of from 15 m to 20 m. The individual tubes of the tubular reactor are preferably flanged together. The tubes can also be flanged to a bend, preferably to a 1 80 ° bend. Such 1 80 ° bends have preferably a small radius, i.e. have preferably a ratio R/d ₀ of 4 or less, with "R" being the radius of curvature of the bending and "d ₀ " being the outer diameter of the tube, for saving space. In a preferred embodiment of the present disclosure, the flanges are arranged such that groups of flanges are aligned on top of each other. Preferably such a group of flanges is arranged one atop of the other and has at least two flanges, more preferably from 3 to 1 00 flanges, and most preferably from 5 to 60 flanges.

[0045] Preferred tubular reactors have at least two reaction zones, preferably from 2 to 6 reaction zones and more preferably from 2 to 5 reaction zones. The number of reaction zones is given by the number of feeding points for the initiator. Such a feeding point can, for example, be an injection point for a solution of azo compounds or organic peroxides. Fresh initiator is added to the reactor, where the initiator decomposes into free radicals and initiates further polymerization. The generated heat of the reaction raises the temperature of the reaction mixture, since more heat is generated than can be removed through the walls of the tubular reactor. The rising temperature increases the rate of decomposition of the free-radical initiators and accelerates polymer- ization until essentially all of the free-radical initiator is consumed. Thereafter, no further heat is generated and the temperature decreases again since the temperature of the reactor walls is lower than that of the reaction mixture. Accordingly, the part of the tubular reactor downstream of an initiator feeding point in which the temperature rises is the reaction zone, while the part thereafter, in which the temperature decreases again, is predominantly a cooling zone. The amount and nature of added free-radical initiators determines how much the temperature rises and ac- cordingly allows for adjusting that value. Normally, the temperature rise is set to be in the range of from 70 °C to 170 °C in the first reaction zone and 50 °C to 130 °C for the subsequent reaction zones depending on the product specifications and the reactor configuration. Preferably, the tubular reactor is equipped with cooling jackets for removing the heat of the reaction. More prefera- bly, all reaction zones of the tubular reactor are cooled by cooling jackets.

[0046] The compression of the reaction gas composition to the polymerization pressure is carried out by one or more compressors in a sequence of compression stages, where preferably a primary compressor first compresses the reaction gas composition to a pressure of from 10 M Pa to 50 M Pa and a secondary compressor, which is sometimes designated as hyper compressor, further compresses the reaction gas composition to the polymerization pressure of from 1 1 0 M Pa to 500 MPa. Preferably the primary compressor and the secondary compressor are multistage compressors. It is further possible to separate one or more stages of one or both of these compressors and divide the stages into separated compressors. However, usually a series of one primary compressor and one secondary compressor is used for compressing the reaction gas composition to the polymerization pressure. In such cases, sometimes the whole primary compressor is designated as primary compressor. However, it is also common to designate the one or more first stages of the primary compressor, which compress the recycle gas from the low- pressure product separator to the pressure of the fresh ethylene feed, as the booster compressor and then only the one or more subsequent stages as the primary compressor, although the booster compressor and the subsequent stages are all part of one apparatus. After each of the compression stages, the compressed gas mixture is cooled by a compression stage cooler to remove the heat of compression. Usually the compression stage coolers are operated in way that the temperature of the compressed gas mixtures is below 130 °C.

[0047] In a preferred embodiment of the present disclosure, the production line comprises a pre-heater upstream of a tubular reactor for heating the reaction gas composition to a temperature capable of initiating the polymerization. The pre-heater is preferably composed of tubes of a length from 5 m to 25 m, more preferably of a length from 1 0 m to 22 m, and especially of a length of from 15 m to 20 m. The individual tubes of the pre-heater are preferably flanged together. The tubes can also be flanged to a bend, preferably to a 180 ° bend. Such 1 80 ° bends have preferably a small radius, i.e. have preferably a ratio R/d ₀ of 4 or less. In preferred embodiment of the present disclosure, the flanges are arranged such that groups of flanges are aligned on top of each other. Preferably such a group of flanges is arranged one atop of the other and has at least two flanges, more preferably from 3 to 50 flanges, and most preferably from 5 to 30 flanges.

[0048] In a preferred embodiment of the present disclosure the entire reaction gas composition provided by the secondary compressor is fed via a pre-heater to the inlet of the tubular reactor. In another preferred embodiment of the present disclosure only a part of the reaction gas composi- tion compressed by the secondary compressor is fed via the pre-heater to the inlet of the tubular reactor and the remainder of the reaction gas composition compressed by the secondary compressor is fed as one or more side streams to the tubular reactor downstream of the inlet of the tubular reactor. In such a set-up preferably from 30 to 90% by weight, more preferably from 40 to 70% by weight of the reaction gas composition provided by the secondary compressor are fed to the inlet of the tubular reactor and from 1 0 to 70% by weight, more preferably from 30 to 60% by weight of the reaction gas composition provided by the secondary compressor are fed as one or more side streams to the tubular reactor downstream of the inlet of the tubular reactor.

[0049] The production line for carrying out the polymerization of the present disclosure com- prises, beside the polymerization reactor, two or more gas recycle lines for recycling unreacted monomers into the polymerization process. The reaction mixture obtained in the polymerization reactor is transferred to a first separation vessel, frequently called high-pressure product separator, and separated into a gaseous fraction and a liquid fraction at an absolute pressure of from 15 M Pa to 50 M Pa. The gaseous fraction withdrawn from the first separation vessel is fed via a high-pressure gas recycle line to the suction side of the secondary compressor. In the high- pressure gas recycle line, the gas is usually purified by several purification steps for removing undesired components such as entrained polymer or oligomers. The liquid fraction withdrawn from the first separation vessel, which usually still comprises dissolved monomers such as ethylene and comonomers in an amount of 20 to 40% of weight, is transferred to a second separa- tion vessel, frequently called low-pressure product separator, and further separated, at reduced pressure, usually at an absolute pressure in the range of from 0.1 to 0.5 M Pa, in polymeric and gaseous components. The gaseous fraction withdrawn from the second separation vessel is fed via a so-called low-pressure gas recycle line to the primary compressor, preferably to the foremost of the stages. Also the low-pressure gas recycle line usually comprises several purification steps for purifying the gas from undesired components. The production line can further comprise additional separation steps for separating additional gaseous fractions from the reaction mixtures and additional gas recycle lines for feeding such additional gaseous fractions comprising not reacted monomers to one of the compressors, for example in-between the first separation step and the second separation step operating at an intermediate pressure.

[0050] Preferably, the recycled gas coming from the low-pressure gas recycle line is compressed by first stages of the primary compressor to the pressure of the fresh feed of ethylenically unsaturated monomers, preferably ethylene, thereafter combined with the fresh gas feed and the combined gases are further compressed in the primary compressor to the pressure of from 10 M Pa to 50 M Pa. Preferably the primary compressor comprises five or six compression stages, two or three before adding the fresh gas and two or three after adding the fresh gas. The secondary compressor has preferably two stages; a first stage, which compresses the gas from about 30 M Pa to about 120 M Pa, and a second stage, which further compresses the gas from about 120 M Pa to the final polymerization pressure. [0051 ] The pressure within the polymerization reactor is preferably controlled by a pressure control valve, which is arranged at the outlet of the polymerization reactor and through which the reaction mixture leaves the reactor. The pressure control valve can be any valve arrangement which is suitable for reducing the pressure of the reaction mixture leaving the reactor to the pres- sure within the first separation vessel.

[0052] In a preferred embodiment of the present disclosure, the production line comprises a post reactor cooler downstream of the polymerization reactor for cooling the reaction mixture. The post reactor cooler can be arranged upstream of the pressure control valve or the post reac- tor cooler can be arranged downstream of the pressure control valve. Preferably, the post reactor cooler is arranged downstream of the pressure control valve. The post reactor cooler is preferably composed of tubes of a length from 5 m to 25 m, more preferably of a length from 1 0 m to 22 m, and especially of a length of from 15 m to 20 m. The individual tubes of the tubular reactor are preferably flanged together. The tubes can also be flanged to a bend, preferably to a 1 80 ° bend. Such 1 80 " bends have preferably a small radius, i.e. have preferably a ratio R/d ₀ of 4 or less. In preferred embodiment of the present disclosure, the flanges are arranged such that groups of flanges are aligned on top of each other. Preferably such a group of flanges is arranged one atop of the other and has at least two flanges, more preferably from 3 to 80 flanges, and most preferably from 5 to 60 flanges.

[0053] The polymeric components obtained by the polymerization are finally transformed into pellets, normally by apparatuses like extruders or granulators.

[0054] Figure 1 shows schematically a set-up of a production line for polymerizing ethylenically unsaturated monomers in a production line comprising a continuously operated tubular polymerization reactor.

[0055] The fresh ethylene, which is usually under a pressure of 1 .7 M Pa, is firstly compressed to a pressure of about 30 M Pa by means of a primary compressor (1 ) and then compressed to the reaction pressure of about 300 M Pa using a secondary compressor (2). Chain transfer agents (CTA) may be added to primary compressor (1 ) together with the fresh ethylene. Comonomer may be added upstream of the secondary compressor (2) via line (3). The reaction mixture leaving the primary compressor (2) is fed to pre-heater (4), where the reaction mixture is preheated to the reaction start temperature of from about 120 ^ to 220 °C, and then conveyed to the inlet (5) of the tubular reactor (6).

[0056] The tubular reactor (6) is basically a long, thick-walled pipe with cooling jackets to remove the liberated heat of reaction from the reaction mixture by means of a coolant circuit (not shown). [0057] The tubular reactor (6) shown in Figure 1 has four spatially separated initiator injection points (7a), (7b), (7c), and (7d) for feeding initiators or initiator mixtures PX1 , PX2, PX3 and PX4 to the reactor and accordingly also four reaction zones. By feeding suitable free-radical initiators, which decompose at the temperature of the reaction mixture, to the tubular reactor the polymeri- zation reaction starts.

[0058] The reaction mixture leaves the tubular reactor (6) through pressure control valve (8) and passes a post reactor cooler (9). Thereafter, the resulting polymer is separated off from un- reacted ethylene and other low molecular weight compounds (monomers, oligomers, polymers, additives, solvent, etc.) by means of a first separation vessel (1 0) and a second separation vessel (1 1 ), discharged and pelletized via an extruder and granulator (12).

[0059] The ethylene and comonomers which have been separated off in the first separation vessel (1 0) are fed back to the inlet end of the tube reactor (6) in the high-pressure circuit (13) at 30 M Pa. In the high-pressure circuit (13), the gaseous material separated from the reaction mixture is first freed from other constituents in at least one purification stage and then added to the monomer stream between primary compressor (1 ) and secondary compressor (2). Figure 1 shows one purification stage consisting of a heat exchanger (14) and a separator (15). It is however also possible to use a plurality of purification stages. The high-pressure circuit (13) usually separates waxes.

[0060] The ethylene which has been separated off in the second separation vessel (1 1 ), which further comprises, inter alia, the major part of the low molecular weight products of the polymerization (oligomers) and the solvent, is worked up in the low-pressure circuit (1 6) at an absolute pressure of from about 0.1 to 0.5 M Pa in a plurality of separators with a heat exchanger being installed between each of the separators. Figure 1 shows two purification stages consisting of heat exchangers (1 7) and (19) and separators (1 8) and (20). It is however also possible to use only one purification stages or preferably more than two purification stages. The low-pressure circuit (1 6) usually separates oils and waxes.

[0061 ] The ethylene which has passed the low-pressure circuit (1 6) is fed to a booster compressor (21 ), compressed to a pressure of about 4 M Pa and then conveyed to primary compressor (1 ). Booster compressor (21 ) and primary compressor (1 ) are preferably part of one low- pressure compressor, i.e. of one apparatus powered by one motor. The gas mixtures com- pressed in the individual stages of the booster compressor (21 ), the primary compressor (1 ) and the secondary compressor (2) are cooled after every stage by heat exchanges (22), (23), (24), (25), (26), (27), and (28).

[0062] Different configurations for a tubular polymerization reactor in accordance with the pre- sent disclosure are of course also possible. It can be advantageous to add the monomers not only at the inlet of the reactor tube but to feed the monomers, preferably cooled, at a plurality of different points to the reactor. This is particularly preferably done at the beginning of further reaction zones and especially if oxygen or air is used as initiator, which is usually added to the monomer feed in the primary compressor.

[0063] The process of the present disclosure is characterized in that at least one of the pre- heater or pre-cooler, the polymerization reactor and the post reactor cooler is composed of tubes of a length from 5 m to 25 m which are flanged together, either directly or via bends, and the flanges are covered by a chimney construction, and wherein air is conveyed through the chimney construction and the air exiting the chimney construction is monitored with respect to the hydrocarbon concentration.

[0064] In a preferred embodiment of the present disclosure, the polymerization reactor is a tubular reactor or a reactor cascade comprising a tubular reactor and the production line compris- es a pre-heater and a post reactor cooler, and the pre-heater, the polymerization reactor and the post reactor cooler are composed of tubes of a length from 5 m to 25 m which are flanged together, either directly or via bends, and the flanges are covered by a chimney construction, and air is conveyed through the chimney construction and the air exiting the chimney construction is monitored with respect to the hydrocarbon concentration.

[0065] In another preferred embodiment of the present disclosure, the polymerization reactor is an autoclave reactor or a cascade of autoclave reactors and the production line comprises a pre-cooler or a post reactor cooler or a pre-cooler and a post reactor cooler, and the pre-cooler or the post reactor cooler or the pre-cooler and the post reactor cooler are composed of tubes of a length from 5 m to 25 m which are flanged together, either directly or via bends, and the flanges are covered by a chimney construction, and air is conveyed through the chimney construction and the air exiting the chimney construction is monitored with respect to the hydrocarbon concentration. [0066] Preferably all flanges of the production line are covered by a chimney construction. It is possible that each of the flanges is covered by a separate chimney construction. Preferably at least 50% of the chimney constructions cover two or more flanges and more preferably each chimney construction covers two or more flanges. It is especially preferred that each chimney construction covers from 2 to 1 00 flanges, more preferably from 3 to 80 flanges and especially from 5 to 60 flanges. Preferably a group of flanges, which are arranged one atop of the other, is covered by one chimney construction. Preferably the chimney construction is vertically orientated with openings at the bottom and at the top and the air, which is heated by the hot flanges, can rise by natural convention and exit the chimney construction at the top. It is also possible that one chimney construction has two or more "legs" which cover groups of vertically arranged flanges and, above the highest of the covered the flanges, the "legs" converge to one chimney. [0067] The chimney construction can have any suitable form that encloses the flanges and allows transportation of gas through the construction. Preferably the chimneys of the chimney construction are closed structures; however it is also possible that the chimney construction has not only one opening for the air to enter and one opening for the air to exit the chimney construc- tion but the chimney construction is also not closed at one side, for example has a U-shape and is open to an adjacent wall if the flanges are arranged close to this wall.

[0068] As the flanges of the production line have a higher temperature than the air coming from the environment, the air is heated and rises by natural convection. The earlier leaked gas reaches a detector the shorter is the time period for detecting a leakage. In a preferred embodiment of the present disclosure, the walls of the chimney construction are isolated in order to minimize heat losses to the exterior and maximize the uprising velocity of the air.

[0069] A further possibility to convey gas through a closed structure is to use fans or blowers. This allows for installing chimney constructions which are not vertically oriented. Preferably however, natural convection or natural convection in combination with fans or blowers is employed to convey the air through the chimney constructions and the chimney constructions are vertically oriented. When using fans or blowers for conveying the air or supporting the natural convection, situations where the amount of air blown through the chimney constructions is so large that leaked gas is so diluted that the concentration remains below the detection limit should be avoided.

[0070] Figure 2 shows a preferred arrangement of chimney constructions in which in a section (50) of a tubular reactor, five pairs of tubes (51 ) are arranged on top of each other and connected by bends (52). This section (50) of the tubular reactor is connected with other parts of the reactor by tubes (53). The flanges (54) are arranged on top of each other and covered by three chimney constructions (55) which are open at the bottom (56) and at the top (57) and in which air is conveyed by natural convection and exits the chimney constructions (55) at the top (57). [0071 ] The chimney construction has the purpose to guide leaked gas to a gas detection system and to minimize the dilution of the leaked gas cloud. Thus, the less space there is between the flanges and the chimney walls, the higher the concentration of ethylene is in the uprising air. A high concentration of the gas to be detected is favorable because a detection threshold will be surpassed more quickly than for a more diluted gas cloud. Therefore, a chimney design matching the flanges quite closely without obstructing the upward flow is preferred.

[0072] Figure 3 shows a preferred design of a chimney construction element (60) for one flange with a minimized internal volume. Chimney construction element (60) has a cylindrical form and covers a flange (61 ) for connecting two tubes (62). Chimney construction element (60) has two side walls (63) which each have a round opening of the diameter of the tubes (62) for the tubes (62) to pass. Chimney construction element (60) has further a rectangular opening (64) at the bottom and a rectangular opening (65) at the top. Chimney construction element (60) can by connected to identical construction elements arranged on top or below by rectangular connection elements (66) fitting into openings (64) and (65). Air can enter the chimney construction element (60) from below, leaves the chimney construction element (60) though opening (64) and can convey by natural convection through connection elements (66) into further chimney construction elements possibly installed above.

[0073] The monitoring of the air exiting the chimney construction can be carried out with all suitable detectors for detecting hydrocarbons. Preferably the air exiting the chimney construction is monitored by I R detectors such as I R point detectors or I R open-path detectors.

[0074] I R detectors are spectroscopic sensors which are often used as gas detectors. I R detectors operate on the principle that infrared (I R) radiation of a certain wavelength is differently absorbed by different materials such as different gases. Hydrocarbon gases are especially suited to be monitored by I R measurement because of a strong I R absorption at wavelengths in the range from 3000 to 2750 cm ^-1 . I R detectors commonly include an infrared source (lamp), an optical filter which selectively transmits I R radiation of different wavelengths and an infrared detector. IR radiation of a wavelength which is specific for the material to be detected is directed through a sample towards the detector, which measures the attenuation, typically with respect to a reference beam without absorption. Such I R detectors are frequently used to measure gas concentrations. For detecting combustible gases, two types of I R detectors are commonly employed, I R point detectors and I R open-path detectors. I R point detectors measure the attenuation of I R radiation within a sample chamber which is located within the I R point detector. I R open-path detectors have separated infrared source and detector and the beam of IR radiation travels typically from a few meters up to a few hundred meters before reaching the I R detector.

[0075] For monitoring the air exiting the chimney construction with respect to the hydrocarbon concentration, it is preferred that the air is monitored by one or more I R open path detectors or by an I R point detector.

[0076] When using I R open path detectors for monitoring the air exiting the chimney construction, it is possible that each chimney construction is equipped with one or more I R open path detectors designated for monitoring only air exiting one chimney construction. Preferably the I R open path detectors are arranged in a way that each I R open path detector monitors air exiting more than one chimney construction. More preferably the I R open path detectors are arranged in a matrix array in which at least 50% of the IR open path detectors monitor air exiting two or more chimney constructions and each stream of air exiting a chimney construction is monitored by at least three I R open path detectors. This allows for operating the I R open path detectors which monitor one chimney construction according to a voting logic. [0077] Operating a group of I R detectors according to a voting logic means in the context of the present disclosure that for having a confirmed detection of hydrocarbons, at least a predefined number of I R detectors of the respective group of I R detectors has to detect hydrocarbons. Preferably this predefined number is two. That means for a group of I R detectors having two detec- tors that both I R detectors have to detect the presence of hydrocarbons in order to have a confirmed detection of hydrocarbons; such a group operates consequently according to a two out of two (2oo2) logic. For a group of I R point detectors operating according to a 2oo2 logic, the I R point detectors are preferably arranged close to each other in a way that the distance between the two I R point detectors is at the most 3 meters. For a group of I R detectors having three detectors, this means that at least two of the three I R detectors have to detect the presence of hydrocarbons in order to have a confirmed detection of hydrocarbons; such a group operates consequently according to a two out of three (2oo3) logic. And for a group of I R detectors having N detectors, this means that at least two of N I R detectors have to detect the presence of hydrocarbons in order to have a confirmed detection of hydrocarbons; and the group operates consequently according to a two out of N (2ooN) logic. In case of larger groups of I R point detectors, it is further possible to operate with a 3ooN, a 4ooN or a XooN logic.

[0078] Employing I R open path detectors for monitoring the air exiting the chimney construction brings about the advantage that it is highly probable that a leakage is detected even if the chimney is damaged, for example by a large leak with gas escaping at a high velocity. This is so because the I R open path detectors not only measure the areas above the exits of the chimney constructions but along the whole detection path and a large gas cloud coming from a damaged chimney construction has a high probability of crossing such a path. [0079] Another option for monitoring the air exiting a chimney construction is employing an I R point detector. Preferably the I R point detector is supplied by an aspiration line, with air coming from a location close to the opening through which the air exits the chimney construction. An aspiration line is equipment which conveys air from a specific position to a detector such as an I R point detector. Preferably the air is conveyed through the aspiration line by an ejector or any oth- er kind of suitable pump. It is also possible to employ one I R point detector for monitoring two or more chimney constructions. It is possible to combine two or more aspiration lines and monitor the combined air with respect to the hydrocarbon concentration. It is further possible to monitor the air coming from two or more aspiration lines subsequently. Then, a multiplexer can be used which switches in predefined intervals between the aspiration lines coming from different chim- neys constructions. Employing an aspiration line for supplying the I R point detector with air coming from the chimney construction has the advantage that air is cooled down before arriving at the IR point detector. In a preferred embodiment of the present disclosure, the aspiration lines are equipped with water traps for removing water, which may have been introduced by condensation or cooling water leakage, from the air before the air arrives at the I R point detector. [0080] The monitoring of the air exiting the chimney construction with respect to the hydrocarbon concentration is carried out in order to detect the presence of hydrocarbons in this air after the occurrence of a leakage through one of the flanges. Detecting the presence of hydrocarbons means in the context of the present disclosure that the measured concentration of hydrocarbon rises above a pre-defined threshold value. Such threshold value can preferably be a certain proportion of the lower explosive limit (LEL), i.e. the lowest concentration of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source. A suitable threshold value can for example be 20% LEL. That means, for a group of I R point detectors operating according to a 2ooN voting logic, the detection of hydrocarbons is confirmed when the second I R point de- tector of the group of I R point detectors measures a hydrocarbon concentration above 20% LEL.

[0081 ] When detecting the presence of hydrocarbons in air exiting one of the chimney constructions, one possible reaction can be to give an alarm. Preferably, an emergency pressure release program is automatically started when hydrocarbons are detected in the air exiting the chimney construction. The emergency pressure release program is preferably a pre-implemented procedure in which the whole polymerization plant or only parts of the polymerization plant are depressurized or partly depressurized and the polymerization process is interrupted. For releasing the content of the reactor including pre-heater and post reactor cooler and the content of the secondary compressor including high-pressure gas recycle line and high-pressure product sepa- rator to the atmosphere, the production line is equipped with one or more emergency pressure release valves. These emergency pressure release valves are preferably installed along a tubular reactor or in the high-pressure gas recycle line or both along a tubular reactor and in the high- pressure gas recycle line. The depressurization of the polymerization reactor can occur via the one or more emergency pressure release valves or via the pressure control valve or a combina- tion of emergency pressure release valves and pressure control valve. Preferably the depressurization of the polymerization reactor occurs via the one or more emergency pressure release valves.

[0082] In a preferred embodiment of the present disclosure, the polymerization reactor and optionally further parts of the production line are installed within a protective enclosure. Such a protective enclosure, which is usually constructed from concrete and is frequently called a "reactor bay", surrounds the polymerization reactor and other parts of the production line which are subjected to a high pressure for safety reasons. The protective enclosure protects the surrounding areas inter alia against overpressure, radiations and missile effects in case of the ignition of an accidental release of ethylene or other hydrocarbons. Such protective enclosures are preferably open to the sky. Preferably not only the polymerization reactor but also the pressure control valve, the high-pressure product separator and, if present, the pre-heater or pre-cooler and the post reactor cooler are installed within the protective enclosure. [0083] A water based deluge system is preferably started automatically within the area enclosed by the protective enclosure when a leakage of monomers or of reaction mixture is detected. The water based deluge system provides to the enclosed area droplets having a diameter in a range from 25 μιη to 20 mm with a minimum flow rate of 1 0 L/min per m ² of enclosed area. That means the amount of liquid provided by these droplets having a diameter in the range from 25 μιη to 20 mm is at least 1 0 L/min per m ² of enclosed area. Preferably the water based deluge system provides effective water droplets having a diameter in the range from 200 μιη to 5 mm and more preferably effective water droplets having a diameter in the range from 300 μιη to 4 mm. [0084] Preferably the minimum flow rate of the effective water droplets is 14 L/min per m ² of enclosed area and more preferably 1 8 L/min per m ² of enclosed area.

[0085] For achieving an even distribution of the droplets within the protective enclosure, the droplets are provided to at least 1 0% of the enclosed area, preferably to at least 50% of the en- closed area and more preferably to at least 90% of the enclosed area. The droplets may be created by spraying through at least 1 nozzle per 1 0 m ² of enclosed area, more preferably through at least 3 nozzles per 1 0 m ² of enclosed area and especially through at least 5 nozzles per 10 m ² of enclosed area, into the protective enclosure. [0086] The diameter of the droplets provided to the area enclosed by the protective enclosure can for example be determined by laser-based disdrometers operating according to the extinction principle. For calculating the flow rate of droplets having a diameter in a chosen range, the total liquid flow rate is determined and multiplied with the ratio of the generated volume of droplets having a diameter in the chosen range to the volume of all droplets generated by spraying the liquid into the protective enclosure.

[0087] The curtain of droplets provided by the process of the present disclosure is able to effectively reduce the overpressure in case of an explosion. In the opinion of the present inventors, droplets having a diameter in the range from 25 μιη to 200 μιη, more preferably in the range from 25 μιη to 1 00 μιη, are especially suitable for mitigating the effect of an explosion. These small droplets possess a large surface to volume ratio. Therefore these small droplets will immediately evaporate when they are exposed to the temperature wave of the explosion. The evaporation of water requires a substantial amount of energy, which is thereby absorbed from the explosion and the hazardous potential is decreased. The overpressure within the protective enclosure and con- sequently the pressure acting on the enclosure walls are reduced.

[0088] Accordingly, to a preferred embodiment of the present disclosure, droplets having a diameter in the range from 25 μιη to 200 μιη are generated and provided to the enclosed area. For fire protection in buildings, high-pressure water mist systems are available. These systems require a pressure generating device, e.g. a pump, for delivering water with a pressure typically above 3.5 MPa to specially designed nozzles. These nozzles are designed such that very small droplets are created, typically having diameters from 25 μιη to 200 μιη.

[0089] According to another preferred embodiment of the present disclosure, droplets having a diameter in the range from 200 μιη to 20 mm are provided to the enclosed area. The present inventors are of the opinion that also such droplets are able to mitigate the consequences of an explosion and to absorb the energy of the overpressures. In the case of an explosion, droplets of more than 200 μιη in diameter are deformed by the pressure wave to such an extent that the droplets break up, whereby lots of small droplets with diameters smaller than 100 μιη are created. These small droplets have then the capability of effectively mitigating the consequences of an explosion.

[0090] According to a preferred embodiment of the present disclosure, the water based deluge system operates by providing water. Thus, water droplets having a diameter in the range from 25 μιη to 20 mm are sprayed to the enclosed area. Operating the water based deluge system by spraying water has the advantage that in principle simply a water connection is needed for setting up the system. Furthermore, using water avoids any contamination of the production line with undesirable materials. [0091 ] According to another preferred embodiment of the present disclosure, the water based deluge system operates by providing a solution of radical capturing salts. Thus, droplets of such a solution having a diameter in the range from 25 μιη to 20 mm are sprayed to the enclosed area. Examples of suitable radical capturing salts are potassium bicarbonate, sodium bicarbonate, sodium chloride or sodium carbonate. Preferably, the solution of radical capturing salts is an aque- ous solution of potassium bicarbonate or an aqueous solution of sodium bicarbonate. It is advantageous that the solution can be supplied by standard systems for providing droplets and that upon evaporation of the water droplets due to an explosion, fine powders of the radical capturing salt are formed. This combines two advantageous effects: a) the evaporation of the droplets takes out the heat of the explosion, thereby reducing the reaction rate of the radical chain reac- tions and b) the powder of the radical capturing salt is created, which further slows down the radical chain reactions. It is furthermore advantageous that the size of the powder can conveniently be controlled by the size of the droplets by choosing adequate nozzles and the mass fraction of the radical capturing salt in the aqueous solution. Moreover, using a solution of a radical capturing salt has the further advantage over employing such a salt as fine powder that no clumping can occur which could reduce the effectivity because of an increased particle size or, in the worst case, a fully blocked dosing system.

[0092] Preferably, the minimum flow rate of the droplets is achieved not later than 30 seconds after detecting a leakage of monomers or of reaction mixture. More preferably the minimum flow rate of the effective water droplets is achieved not later than 20 seconds after detecting a leakage of monomers or of reaction mixture.

[0093] For establishing a fully developed spray pattern, the water based deluge system needs about 10 to 15 seconds in which water pressure is built-up in the pipings and the droplet curtain is formed. The water based deluge system is therefore preferably combined with a faster starting system for minimizing the probability that a leaked gas cloud explodes and the negative effects of explosions which may possibly occur. Such a faster starting system is preferably a steam based deluge system. Thus, according to a preferred embodiment of the present disclosure, a steam based deluge system is started automatically in parallel with the water based deluge system.

[0094] The steam based deluge system can be operated by feeding steam having a pressure from 0.3 to 4 M Pa, preferably from 0.4 to 3 MPa. After being injected into the protective enclosure, the pressurized water vapor will expand, replace oxygen containing air and dilute the possi- bly explosive gas cloud.

[0095] Preferably, the steam based deluge system is operated by feeding pressurized water having a temperature from 140 °C to 220 ^< €, more preferably from 1 60 °C to 200 ^< €, into the protective enclosure. The pressurized water is above its boiling point at atmospheric conditions and will instantaneously evaporate when released.

[0096] Preferably, the steam based deluge system feeds pressurized water into the protective enclosure, which has been used as a cooling medium for removing the heat of polymerization from the reaction mixture, for example by circulating through cooling jackets. This water is availa- ble in sufficient quantities in a production line for the polymerization of ethylenically unsaturated monomers, where the pressurized water is used for taking away the liberated heat of polymerization.

[0097] In a preferred embodiment of the present disclosure, the pressurized water is provided by permanently circulating water having a temperature from 140 °C to 220 °C in a closed loop through a pipeline, which is installed from 2 to 20 m above the floor of the protective enclosure and which is equipped with nozzles for releasing the water when a leakage of monomers or of reaction mixture is detected. Such closed cycles may preferably be installed in different heights, for example one loop at the a height of 2 m and one loop at a height of 6 m. Alternatively, pipings which connect the cooling jackets of the polymerization reactor could be provided with fast opening valves for releasing the cooling medium.

[0098] In a preferred embodiment of the present disclosure, the steam based deluge system is shut down 15 seconds after the water based deluge system has become fully active. In this con- text, having become fully active means that the water based deluge system provides droplets with the minimum flow rate for at least 20 seconds, preferably at least 15 seconds and more preferably at least 1 0 seconds after the minimum flow rate has been reached. Shutting down the steam based deluge system after the water based deluge system has become fully active limits the amount of steam or hot pressurized water which needs to be kept ready and, if the pressurized water comes from the cooling system, saves resources for a timely restart of the production line.

[0099] According to a preferred embodiment of the present disclosure, not only the air exiting the chimney construction is monitored with respect to the hydrocarbon concentration but also further measurements for detecting a leakage of hydrocarbons in the surroundings of the produc- tion line are carried out. Preferably an emergency pressure release program is automatically started when hydrocarbons are detected in the air exiting the chimney construction or a leakage of hydrocarbons in the surroundings of the production line is detected by the further measurements. [0100] The further measurements for detecting a leakage of hydrocarbons in the surroundings of the production line can be carried out by employing I R open path detectors, I R point detector or ultrasonic detectors, or combinations thereof.

[0101 ] Ultrasonic detectors are sensors which measure the sound which is emitted by leaking of pressurized gas. Such a noise is usually created when the pressure difference between the pressured gas and the surroundings is more than 0.7 MPa. Ultrasonic detectors have the advantage that they are very fast as the measured signal travels by the speed of sound. However, ultrasonic sounds can also be created by other sources. Thus, according to a preferred embodiment of the present disclosure, the IR detectors are employed for starting automatically the emer- gency pressure release program while the ultrasonic detectors are used to give an alarm signal. The ultrasonic detectors may however also be installed in a production line in a way that they reliably indicate leakage of combustible gases. Thus, according to another preferred embodiment of the present disclosure, an emergency pressure release program is automatically started when an I R detectors detects the presence of hydrocarbons or an ultrasonic detector detects a gas leak. Ultrasonic detectors may especially be employed for monitoring the surroundings of the compressors or for monitoring non-jacketed pipelines such as supply lines for bringing ethylene to the production line.

[0102] According to a further preferred embodiment of the present disclosure, the further measurements for detecting a leakage of hydrocarbons in the surroundings of the production line include a monitoring of at least one of the cooling media which cool the compression stage coolers, the cooling jackets of the polymerization reactor, the post reactor cooler or a cooler within the high-pressure or the low pressure gas recycle line is monitored with respect to an occurrence of a leakage of monomers or of reaction mixture into the cooling medium. Preferably all these cooling media are monitored with respect to an occurrence of a leakage of monomers or of reaction mix- ture into the cooling medium. Preferably the monitoring of the cooling medium occurs by passing air through the cooling medium and thereafter conveying the air to an I R point detector capable of detecting hydrocarbons. [0103] As the solubility of ethylene in water is very low, leakages to the cooling medium will form small gas bubbles within the coolant stream. Preferably the cooling medium system is designed in a way that such bubbles are collected in designated regions. For example, a horizontal coolant piping section is equipped with a T-fitting which is introduced in such a manner that one arm of the fitting points upwards so that gas bubbles will rise into this section. Cooling medium from such a region is preferably conveyed to equipment which allows for the passing of air through the cooling medium, for example a small flash column. The air passing the cooling medium carries ethylene away from the cooling medium and a sample of the air is then led to an I R point detector. In a preferred embodiment of the present disclosure, cooling medium sampling, air sampling, and monitoring are carried out redundantly, i.e. more than one cooling medium sample is taken and passed by air and each of these air samples is conveyed to a different I R point detector. When the sampling is carried out redundantly, it is preferred that the I R point detectors for monitoring the air samples operate according to a voting logic.

[0104] It is further possible to monitor the cooling medium of cooling systems which operate at ambient pressure by use of a fume hood construction above open parts of cooling systems such as open cooling medium return lines. The air above the open part of the cooling system, which may comprise leaked ethylene, is preferably conveyed by natural convection to a detector. Depending on the dilution of the ethylene in the air, this detector can be an I R point detector or a Flame Ionization Detector (FI D).

[0105] In a most preferred embodiment of the present disclosure,

- the polymerization reactor is a tubular reactor which is composed of tubes which are flanged together and each flanges is covered by a chimney construction and the air exiting the chimney construction is monitored with respect to the hydrocarbon concentration by one or more IR open path detectors or by an I R point detector which is supplied with air exiting the chimney construction;

- the pre-heater, the polymerization reactor, the pressure control valve, the post reactor cooler and the high-pressure product separator are installed within a protective enclosure and the area within the protective enclosure is monitored by one or more I R point detectors, one or more IR open-path detectors or combinations thereof;

- the surroundings of the compressors is monitored by one or more IR point detector, one or more ultrasonic detectors or combinations thereof;

- the cooling media which cool the compression stage coolers, the cooling jackets of the polymerization reactor, the post reactor cooler and the coolers within the high-pressure gas re- cycle line are monitored by passing air is through the cooling medium and thereafter conveying the air to an I R point detector; and

- the pressure control valve is installed within a housing and the air within the housing is monitored with respect to the hydrocarbon concentration by an IR point detector which is supplied with air from within the housing by an aspiration line;

and an emergency pressure release program is automatically started when hydrocarbons are detected in the air exiting a chimney construction or a leakage of hydrocarbons in the surroundings of the production line is detected.