CA1068448A - Manufacture of high pressure polyethylene - Google Patents
Manufacture of high pressure polyethyleneInfo
- Publication number
- CA1068448A CA1068448A CA270,376A CA270376A CA1068448A CA 1068448 A CA1068448 A CA 1068448A CA 270376 A CA270376 A CA 270376A CA 1068448 A CA1068448 A CA 1068448A
- Authority
- CA
- Canada
- Prior art keywords
- pressure
- zone
- ethylene
- reaction mixture
- bars
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F10/00—Homopolymers and copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
- C08F10/02—Ethene
Abstract
ABSTRACT OF THE DISCLOSURE: A process for the high pressure polymeri-zation of ethylene at elevated temperatures in a polymerization zone followed by transfer of the reaction mixture, through a cooling zone, into the separation zone. In this process, the pressure at the cooling zone outlet is perodically reduced briefly, at recurring intervals, these brief pressure reductions being independent of the pressure reductions in the polymerization zone. Using this process, the con-version in the polymerization unit can be increased and a more homo-geneous ethylene polymer can be obtained.
Description
1~6844~ `
0.~. 31,853 MANUFACTURE OF HIGH PRESSURE POLYETHYLENE
The present invention relates to a process for the manufacture of ethylene polymersO
The ethylene polymers are obtained by homopolymerization of ethylene or copolymerization of ethylene with other compounds, copoly-merizable with ethylene, in a continuously operated polymerization system at pressures of from 1,000 to 4,000 bars and at from 100 to 450C, followed by transfer Or the reaction mixture through a cooling zone into a high pressure product separation zone, the pressure pre-vailing in the cooling zone being lower than in the polymerization zone and the pressure in the cooling zone being from 50 to 500 bars higher than in the high pressure product separation zoneO
Processes for the manufacture of ethylene polymers and copoly-mers by polymerizin~ ethylene, in the presence or absence of co-reac-tants, in polymerization syætems such as tubular reactors or autoclaves, in the presence Or initiators and in the presence or absence of chain transfer agents, stabilizers or solubilizers, at pressures above 1,000 bars and at from 100 to 450C have been disclosedO The subsequent isolation of the polymer from the reaction mixture is usually effected in a high pressure product separation zone, which is generally operated at pressures of from 200 to 350 bars, and in a downstream low pressure product separation zone, in which the pressure is a few bars.
The high pressure polymerization of ethylene is above all - 1 - ~
O.Z. 31,853 influenced by the strong exothermicity. In order that the reaction shall take place under favorable conditions, it is desirable to remove as much heat as possible from the polymerization zone, which consists of tubular reactors or autoclaves equîpped with stirrers, because the achievable output is virtually directly dependent thereon. Viewed theoretically, the amount Or heat which can be removed by means o~ a suitable coolant which ~lows over the reaction tubes in a jacketing tube is proportional to the heat conductivity of the ethylene reaction mixture, of the reactor tubes and of the coolantO In practice, how-ever, it is necessary to take into account that the ethylene polymer~ormed adheres as a deposit to the inside of the walls of the reactor and substantially reduces the heat conducted toward the tube wall. The deposit detaches from time to time, migrates through the reactor and in this way interferes with the course of the polymerization. In extreme cases, such deposits which detach at irregular intervals may cause blockages.Theselresult in severe and rapid pressure increases in the reactor, which in turn cause decomposition of the reaction mix-tureO
UOSo Patent 2,852,501 discloses a method of operation of tubular reactor~ which counteracts the above disadvantages~ In this method, the reactor is deliberately freed rrom wall deposits by periodically reducing the pressure by as much as 800 bars. In most cases this is done by reducing the pressure in the reactor from, for example, 2,000 bars to 1,200 bars at intervals of from 5 to 120 seconds. A pressure regulating valve, referred to as a let-down valve, located at the reactor outlet, is used for this purpose; through it, the pressure is briefly reduced by a fixed amount, i.e. the reaction zone is let down.
At the same time, the pressure at t~.e reactor inlet is measured. There-after, the actuator valve is shut until the reaction pressure has been re-established. The let-down valve is constructed as a regulating valve and, when the reaction pressure has been reached, holds the pressure constant until the next time that it is reducedO This method has essentially proved successfulO However, it also has some disadvan-
0.~. 31,853 MANUFACTURE OF HIGH PRESSURE POLYETHYLENE
The present invention relates to a process for the manufacture of ethylene polymersO
The ethylene polymers are obtained by homopolymerization of ethylene or copolymerization of ethylene with other compounds, copoly-merizable with ethylene, in a continuously operated polymerization system at pressures of from 1,000 to 4,000 bars and at from 100 to 450C, followed by transfer Or the reaction mixture through a cooling zone into a high pressure product separation zone, the pressure pre-vailing in the cooling zone being lower than in the polymerization zone and the pressure in the cooling zone being from 50 to 500 bars higher than in the high pressure product separation zoneO
Processes for the manufacture of ethylene polymers and copoly-mers by polymerizin~ ethylene, in the presence or absence of co-reac-tants, in polymerization syætems such as tubular reactors or autoclaves, in the presence Or initiators and in the presence or absence of chain transfer agents, stabilizers or solubilizers, at pressures above 1,000 bars and at from 100 to 450C have been disclosedO The subsequent isolation of the polymer from the reaction mixture is usually effected in a high pressure product separation zone, which is generally operated at pressures of from 200 to 350 bars, and in a downstream low pressure product separation zone, in which the pressure is a few bars.
The high pressure polymerization of ethylene is above all - 1 - ~
O.Z. 31,853 influenced by the strong exothermicity. In order that the reaction shall take place under favorable conditions, it is desirable to remove as much heat as possible from the polymerization zone, which consists of tubular reactors or autoclaves equîpped with stirrers, because the achievable output is virtually directly dependent thereon. Viewed theoretically, the amount Or heat which can be removed by means o~ a suitable coolant which ~lows over the reaction tubes in a jacketing tube is proportional to the heat conductivity of the ethylene reaction mixture, of the reactor tubes and of the coolantO In practice, how-ever, it is necessary to take into account that the ethylene polymer~ormed adheres as a deposit to the inside of the walls of the reactor and substantially reduces the heat conducted toward the tube wall. The deposit detaches from time to time, migrates through the reactor and in this way interferes with the course of the polymerization. In extreme cases, such deposits which detach at irregular intervals may cause blockages.Theselresult in severe and rapid pressure increases in the reactor, which in turn cause decomposition of the reaction mix-tureO
UOSo Patent 2,852,501 discloses a method of operation of tubular reactor~ which counteracts the above disadvantages~ In this method, the reactor is deliberately freed rrom wall deposits by periodically reducing the pressure by as much as 800 bars. In most cases this is done by reducing the pressure in the reactor from, for example, 2,000 bars to 1,200 bars at intervals of from 5 to 120 seconds. A pressure regulating valve, referred to as a let-down valve, located at the reactor outlet, is used for this purpose; through it, the pressure is briefly reduced by a fixed amount, i.e. the reaction zone is let down.
At the same time, the pressure at t~.e reactor inlet is measured. There-after, the actuator valve is shut until the reaction pressure has been re-established. The let-down valve is constructed as a regulating valve and, when the reaction pressure has been reached, holds the pressure constant until the next time that it is reducedO This method has essentially proved successfulO However, it also has some disadvan-
- 2 -1068448 o.z. 31,853 tages in that the greater part o~ the volume, or substantial amounts,of the ethylene introduced into the reaction chamber and as yet not containing any polymer are let down. This causes both energy losses and a lowering of the quality o~ the product ~ormed.
German Laid-Open Application DOS 2,047,290 describes a process which very substantially avoids the lowering in quality, and losses o~ energy, caused by reducing the pressureO In addition, it permit~
the manuracture o~ a product Or very high density and narrow molecular wei~ht distribution in a polymerization unit employing predetermined operatin~ conditions. In this process, the entire treatment zone, to be kept under pressure, of the reaction mixture is split at least once and brief pressure reductions are ef~ected over only a part-zone, which comprises a certain distance Or travel of the reaction mixture.
In a pre~erred arrangement ~or carrying out the process, pressure-retaining valves are provided as pressure-regulating devices at the points of subdivision of the treatment zone and these valves are brie~ly actuated, in the sense Or being closed, when the pressure is briefly reduced by means of a let-down valve located at the end of the reaction zone. According to a further characteristic o~ the pro-cess of German Laid-Open Application DOS 2,047,290, a polymerization unit possesses a plurality o~ pressure-reducing valves in order to maintain the pressure în several part-zones, each o~ these valves being allotted to one o~ a plurality of feed zones for the reaction mixtureO These pressure-reducing valves~ which~when the let-down valve is opened, are actuated in the opposite sense, iOe in the sense o~ being closed, ~urthermore advantageously respond with a short time delayO The pressure-retaining valve nearest the let-down valve is actuated ~irst. In this process, and in polymerization units with tubular reactors, constructed on this principle, advantages are achieved in various respects, eOgO a higher efrective pressure, better control of the course of the process, greater stability o~ the reaction, less pressure loss and improved product properties.
In the process of U.S. Patent 2,852,501 and o~ German Laid-Open 6 8 ~ .Z~ 31,853 Application ~,047,290 the reaction mixture is flashed through a let-down valve in a product separation zone where the pressure is normally from 200 to 350 bars (cfo ~hemical Engineering 73 (196~) (Dec.19) 113-120).
Another method Or product separation is described in German Published Application 2,120,6240 In this method, pressures of from 500 to 1,000 bars are set up in the separation zone which directly adjoins the let-down valve~ The separation zone consists of a heat exchanger, which acts as a coolin~ zone, and adjoining high pressure product separation zonesO Cooling can, however, al~o be efrected by direct addi~ion Or coolantsO The pressure in the separation zone is controlled by means Or a valve at the zone outletO
In all processes re~erred to above, the pressure in the reactor is reduced periodicallyO In the conventional processes, cooling devices in the separation zone decrease in efficiency as the period of operation of the reactor increases~ As a consequence, the temperatures Or the reaction mixture in the high pressure product separation zones rise continuouslyO In turn9 this results in the proportion Or ethylene converted to polymer decreasingO Accordingly 9 the conversion decreases ir the essential properties Or ~he e~hylene polymer, eOgO melt index, density and molecular wei~ht distribution, are to be kept constant.
This focuses on the essential disadvantage of the methods described.
It has also been disclosed that high pressure product separatîon zones can be placed downstream of autoclave reactors and that little or no pressure reduction is employed with autoclavesO Hence, autoclaves with downstream high pressure product separation zones also suf~er from undesirable temperature risesO
It is an object of the present invention to provide a method which avoids the undesirable temperature risesO
We have found that this object is achieved, according to the invention, by providing, at the outlet from the cooling zone to the high pressure product separation zone, a pressure control valve by means of which the pressure in the cooling zone is reduced periodically ~ 84~ oOzO 31,853 and recurrently within from 0~1 to 10 seconds, these brief pressure reductions being independent of the pressure reductions to which the reaction mixture is subjected in the polymerization zone.
According to a further advantageous embodiment Or the invention, the time interval between the end of one pressure reduction and the start of the next pressure reduction is in each case from 5 to 500 seconds.
The residence time o~ the reaction mixture in the cooling zone is advantageou31y from 5 to 200 seconds~
The term ethylene polymers embraces solid, waxy and oily poly-merqO
For the purposes of the invention, homopolymers and copolymers of ethylene are the ethylene homopolymers and copolymers which can be manufactured at pressures of from 1~000 to 4,000 bars and at from 100 to 450C. All polymerization initiators and chain transfer agents con-ventionally used for the high pressure homopolymerization and copoly-merization of ethylene may be employed for the purposes of the in-ventionO Su;table compounds copolymerizable with ethylene are all those which can be copolymerized with ethylene under the stated pressure conditions~ Examples of such compounds are vinyl esters, eOgO vinyl acetate and vinyl propionate, esters of acrylic acid, eOg. butyl acrylate9 esters of methacrylic acid, acrylonitrile, acrylamide, acrylic acid or vinyl ethers. Advantageous catalysts to use are oxygen, peroxides, eOg. benzoyl peroxide, or azo compounds, e~gO azo-isobutyro-nitrileO High energy irradiation may also be used to initiate the polymerizationO
The invention may be implemented using the conventional con-tinuously operated high pressure polymerization systemsO Polymerization zones are to be understood as being the conventional tubular reactors and stirred autoclavesO Tubular reactors are vessels whereof the ratio of length to diameter of the pressure-resistant tubes is from 10,000:1 to 60,00o : lo Autoclave reactors are pressure-resistant vessels whereof the length to diameter ratio is from 30 : 1 to 2.5 : 1.
~ 1068~ o~zO 31,853 In order to achieve thorough mixing of the reaction mixture and gooddistribution of the heat generated, the autoclaves are fitted with stirrersO Information on processes using tubular reactor~ or autoclave reactors is to be found, for example, in Ullmanns Encyklopadie der technischen Chemie, 3rd edition, volume 14 ~1963), pages 137-148.
Following the polymerization, the reaction mixture coming from the polymerization sys~em, i.e. from the tubular reactor or autoclave reactor, is flashed in a cooling zone. For this purpose, a let-down valve located at the reactor outlet is used; by means Or this valve the pressure in the polymerization system can be reduced briefly, i.e.
the reaction zone can be let down~ The cooling zone used is an apparatus which can be operated as a cooling device, iOe. an after-cooler. Prererably, this cooling zone is a pressure-resistant tube surrounded by a jacket through which passes a coolantO In this cooling tube, the reaction mixture which has left the reactor can be cooled to a temperature lower than the temperature prevailing in the reactor, i.e. in the polymerization zoneO In general, the temperature in the cooling zone, eOgO in the cooling tube described above, is from 200 to at most 320 CO The pressure in this cooling zone is lower than in the polymerization zone.
The reaction mixture is then passed into a high pressure product separation zone or high pressure separator, where the prevailing pressure is from 50 to 500 bars lower than in the upstream cooling zone (cooling tube)O The pressure in this separation zone is as a rule from 200 to 350 barsO In the separation zone, the ethylene polymer obtained in the reactor is separated from the unpolymerized monomers.
From the high pressure separator, the polymer, still containing small amounts Or monomer, is passed into a low pressure product separation zone or low pressure separatorO In the latter, the prevailing pressure is usually less than 10 barsO
According to the process of the invention, the pressure in the cooling zone is reduced periodically, at the outlet from the cooling zone to the high pressure separation zone, for a duration of at most ~06~4~ o.z. 31,853 10 seconds, these brief pressure reductions being independent of the pressure reductions to which the reaction mixture is subjected in the polymerization zone. For this purpose a control valve is provided at the outlet of the cooling zone which is located between the let-down valve at the reactor outlet and the high pressure product separation zone. Its function is to periodically reduce the pressure in the cooling zone. The pressure reduction at the coolin~ zone outlet is advantageously ~rom 50 to 500 bars, preferably from 100 to 150 bars, and may last in each case ~rom Ool to 10 seconds, prererably from 0.1 to lo 5 seconds. The duration of a pressure reduction is to be understood as the period, in seconds, for which a single pressure reduction is effected, iOeO for which the let-down valve at the end of the cooling zone, leading to the high pressure product separation zone, is openedO The decisive factor is that these brief pressure reductions are periodically repeatedO These time intervals between the end of one pressure reduction and the start Or the next pressure reduction amount in each case to from 5 to 500 secondsO
The residence time of the reaction mixture in the cooling zone is advantageously set to from 5 to 200 seconds, preferably from 10 to 20 secondsO The residence time is defined as the ratio of the through-put of reaction mixture, in unit volumes per unit time, to the volume of the cooling zone or of the cooling tubeO
It is an advantage of the process of the invention that by follo-wing this method the temperature in the cooling zone, and hence also in the high pressure product separation zone, io eO the high pressure separator, can be reduced significantlyO ~onsequently, the conversion in the polymerization unit can be increased and a more homogeneous product can be obtained.
The polymerization systems used in carrying out the process of the invention will now be described in relation to Figures 1, 2 and 3 of the accompanying drawingsO
Figure 1 shows a tubular reactor with a reaction zone.
The compre~sor I feeds a mixture of ethylene, with various O.Z0 31,853 1068~48 additives required for the polymerization, under high pressure intothe part II of the apparatus, which serves as a buffer vessel and pre-heater, and in which the pilsation caused by the compressor is damped and the reaction mixture is heated up. The reaction mixture pas~e~ to the starter reactor lII via an on-off valve 10. In this part-zone A of the total tubular reactor, the polymerization starts as a result of sufficient heat supplied by a heating mediumO The mixture then passes through the pressure-reducing valve 1~ to the actual reactor IV, which transmits its heat of reaction throu~h the tube wallO The pressure-reducin~ valve 12 operates in such a way as to keep the pressure inparts II and III of the apparatus constant at all timesO The reactor IV, in the part-zone B of the tubular reactor which is subjected to brief pressure reductions~ is cooled with a coolant, the temperature of which is below the temperature of the gas mixture at the inlet of the reactor I~o The let-down valve 13 normally keeps the pressure in the reactor IV constant at the desired valueO The brief pressure reductions form par~ of the functioning of the let-down valve 13. The reaction product leaving the reactor IV is flashed in the aftercooler IV (or cooling zone)O The latter is cooled by means of a coolant which is at a temperature below the temperature of the reaction mixture flowing through the cooling zone~ and cons;sts of pressure-resistant tubes surrounded by a jacket through which the coolant passes. It is provided with a pressure control valve 14 at its outlet, the function of this valve being to reduce the pressure in the aftercooler periodicallyO The pressure is controlled by means of the pressure-signalling device PC, which controls the valve 14 with the aid of the requisite control meansO From the aftercooler, the reaction mixture is let down, through this control valve, into the high pressure separa-tor 5. There, the polyethylene is separated from the unconverted ethyleneO The polymer, obtained as a melt, passes v;a the valve 15 to the low pressure separator 6 and from there is processed further by conventional methodsO The gas from the high pressure separator 5 is cooled and purified by means of a unit 7 designed for the purpose and 1 0 ~ ~ O.Z. 31,853 not shown in more detail, and is recycled to the compressor I. Heatingmedium or coolant is intrGduced at the inlets marked 1, 2, 3 and 4.
Figure 2 shows a polymerization reactor which has an autoclave A, equipped with a stirrer, in p~ace of the reactors III and IV and the valve 12 located between theseO The pressure-reducing valve 13 is located at the autoclave outlet and is rollowed by the aftercooler V, pressure-signalling device PC and control valve 14. All further parts o~ the installation, the high pressure product separator 5 and the low pressure product separator 6 are identical with those shown in iO F~gure 1. Heating medium and coolant are introduced at the inlets 1 and 40 Figure 3 shows a tubular reactor comprising 2 parts each Or type I, II, III and IV, marked I, IA, II, IIA, III, IIIA, IV~1 and IV/20 The corresponding valves are marked 10, 12, 13, 14, lOa and 12a.
Its essential difference from the reactor shown in Figure 1 will be described brierlyO The reaction mixture leaving the reaction zone IV/1 is mixed with ethylene which contains initiator and has been com-pressed and preheated as it travels through IA, IIA and IIIA. In the zone IV/2, a further polymerlzation reaction takes place as a result of the renewed addition of initiator, referred to aboveO The parts 13, V, PC, 14, 5, 15, 6 and 7 of the apparatus perform the functions already described in connection with Figure lo Heating medium and coolant are introduced at 1, laS 2, 2a, 3, 3a and 40 The following procedure ;s employed with the reactor shown in Figure lo The reactlon pressure in reactors II, III and IV may be set to a value from 1,000 to 4,000 barsO The pressure reductions effected by means of valve 13 are from 100 to 1,000 bars; they take place at intervals Or from 0.5 minute to 500 hours and last from Ool to 10 seconds. The pressure in the after-cooler V is set by means of the valve 14 so as to be from 50 to 500 bars above the pressure in the high pressure product separator 5 and is periodically reduced by means o~f valve 14 by rrom 500 to 50 barsO The pressure reductions take place at interval~ oP from 5 to 500 seconds and last from 0 1 to 10 seconds.
_ g _ 0.Z. 31,853 The residence time of the reaction mixture in the after-cooler is from 5 to 200 secondsO
In the reactor shown in Figure 2, the following conditions are employed. The pressure in autoclave A is set to from 1,000 to 4,000 bars. The pressure reductions effected by means of valve 13 are from 20 to 300 bars, they take place at intervals of from 0.5 minute to 500 hours. The pressure in the aftercooler V is set by means Or the valve 14 so as to be from 50 to 500 bars above the pressure in the high pres~ure product separator 5 and is periodically reduced by means ~ valve 14 by from 500 to 50 barYO The pressure reductions take place at intervals Or from 5 to 200 seconds and last from 0.1 to 10 seconds.
The residence time Or the reaction mixture in the aftercooler is from 5 to 200 seconds.
The conditions in the reactor shown in Figure 3 correspond to those of the reactor shown in Figure lo EXAMPLE
Per hour, 20,000 kg Or ethylene, containing 300 g Or oxygen and 44 kg Or propionaldehyde, are introduced continuously into a tubular reactorO The mixture is under a pressure Or 3,Q00 barsO At intervals Or 200 minutes, the pressure is reduced to 2,800 bars for a period Or ~!0 lo 5 secondsO After a residence time Or 60 seconds, the reaction mixture has reached a temperature Or 330Co After a further 20 seconds, 10,000 kg Or ethylene, containing 350 g Or oxygen and 20 kg of propionaldehyde, are addedO After a further 40 seconds, the reaction mixture, after having first returned to 330 C, is let down via the actuator valve into the aftercooler at 500 barsO After 15 seconds, the reaction mix-ture pas~es into the high pressure product separator, where it has a temperature Or 290C and a pressure of 280 barsO 8,000 kg of poly-ethylene having a melt index Or (MFI) 4 g/10 min (according to DIN 53,479) and a density of 0,~24 g/cm3 (according to DIN 53,735) are 30 obtainedO
The procedure followed is as in Example 1 but in contrast to the ~ ~448 oOz. 31,853 latter, the reaction mixture i5 let down to 500 bar~ in the after-cooler. This pressure is reduced to 400 bars ror a duration Or 0.3 second at intervals Or 10 seconds, by means of the valve at the aftercooler outlet. A~ter a residence time Or 15 seconds, the reaction mixture leaves the aftercooler via the valve and passes into the high pressure product separator where the temperature assumes a value of 271C at a pres~ure of 280 bars. 8~500 kg of polyethylene having a MFI of 4 g/10 min and a density Or 0~924 g~cm3 are obtained.
Per hour, 5,000 kg Or ethylene, 130 g oP tert.-butyl perpivalate, 180 g o~ tert.-butyl perisononanate and 10 kg o~ propionaldehyde are introduced continuously into an autoclave equipped with a stirrer. The pre~sure in the autoclave is kept at 2~000 bars and is periodically reduced to 1,970 bars for a duration of 0.3 second, at intervals of 300 minutesO The reaction temperature is 270Co Arter a residence time Or 60 seconds, the reaction mixture is let down into the after-cooler, at 500 bars, by means Or the let-down valveO After a residence time Or 20 seconds, it is let down to a pressure of 280 bars in the high pressure product separator. The temperature in the latter assumes a value Or 282C.
860 kg Or polyethylene having an MFI of 7 g~10 min and a density Or 0~ 922 g/cm3 are thùs obtainedO
Exactly the same conditions as those described in Example 3 are set up continuously in the autoclaveO However, in contrast to Example 3, different conditions are maintained in the aftercooler zone. The pressure is kept at 500 bars and is reduced to 400 bars for a duration Or 0.3 second at intervals Or 10 seconds by means of the valve fitted at the aftercooler outletO The mixture let down through this valve to 280 bars into the high pressure product separator is at 264Co 955 kg of polyethylene having an MFI of 6 8 g/10 min and a density of 0.922 g/cm3 are thus obtained.
` O.Z. 31,853 1068~48 Exactly the same conditions as those described in Example 3 are set up in the autoclave. In contrast to Example 3, the pressure in the aftercooler is kept constant at 280 bars. The residence time Or the reaction mixture in the artercooler is 20 seconds. The temperature in the high pressure product separator assumes a value Or 281C at 280 bars.
870 kg Or polyethylene having an MFI Or 7.2 g~10 min and a den-sity Or 0.9215 g~cm3 are thus obtained.
German Laid-Open Application DOS 2,047,290 describes a process which very substantially avoids the lowering in quality, and losses o~ energy, caused by reducing the pressureO In addition, it permit~
the manuracture o~ a product Or very high density and narrow molecular wei~ht distribution in a polymerization unit employing predetermined operatin~ conditions. In this process, the entire treatment zone, to be kept under pressure, of the reaction mixture is split at least once and brief pressure reductions are ef~ected over only a part-zone, which comprises a certain distance Or travel of the reaction mixture.
In a pre~erred arrangement ~or carrying out the process, pressure-retaining valves are provided as pressure-regulating devices at the points of subdivision of the treatment zone and these valves are brie~ly actuated, in the sense Or being closed, when the pressure is briefly reduced by means of a let-down valve located at the end of the reaction zone. According to a further characteristic o~ the pro-cess of German Laid-Open Application DOS 2,047,290, a polymerization unit possesses a plurality o~ pressure-reducing valves in order to maintain the pressure în several part-zones, each o~ these valves being allotted to one o~ a plurality of feed zones for the reaction mixtureO These pressure-reducing valves~ which~when the let-down valve is opened, are actuated in the opposite sense, iOe in the sense o~ being closed, ~urthermore advantageously respond with a short time delayO The pressure-retaining valve nearest the let-down valve is actuated ~irst. In this process, and in polymerization units with tubular reactors, constructed on this principle, advantages are achieved in various respects, eOgO a higher efrective pressure, better control of the course of the process, greater stability o~ the reaction, less pressure loss and improved product properties.
In the process of U.S. Patent 2,852,501 and o~ German Laid-Open 6 8 ~ .Z~ 31,853 Application ~,047,290 the reaction mixture is flashed through a let-down valve in a product separation zone where the pressure is normally from 200 to 350 bars (cfo ~hemical Engineering 73 (196~) (Dec.19) 113-120).
Another method Or product separation is described in German Published Application 2,120,6240 In this method, pressures of from 500 to 1,000 bars are set up in the separation zone which directly adjoins the let-down valve~ The separation zone consists of a heat exchanger, which acts as a coolin~ zone, and adjoining high pressure product separation zonesO Cooling can, however, al~o be efrected by direct addi~ion Or coolantsO The pressure in the separation zone is controlled by means Or a valve at the zone outletO
In all processes re~erred to above, the pressure in the reactor is reduced periodicallyO In the conventional processes, cooling devices in the separation zone decrease in efficiency as the period of operation of the reactor increases~ As a consequence, the temperatures Or the reaction mixture in the high pressure product separation zones rise continuouslyO In turn9 this results in the proportion Or ethylene converted to polymer decreasingO Accordingly 9 the conversion decreases ir the essential properties Or ~he e~hylene polymer, eOgO melt index, density and molecular wei~ht distribution, are to be kept constant.
This focuses on the essential disadvantage of the methods described.
It has also been disclosed that high pressure product separatîon zones can be placed downstream of autoclave reactors and that little or no pressure reduction is employed with autoclavesO Hence, autoclaves with downstream high pressure product separation zones also suf~er from undesirable temperature risesO
It is an object of the present invention to provide a method which avoids the undesirable temperature risesO
We have found that this object is achieved, according to the invention, by providing, at the outlet from the cooling zone to the high pressure product separation zone, a pressure control valve by means of which the pressure in the cooling zone is reduced periodically ~ 84~ oOzO 31,853 and recurrently within from 0~1 to 10 seconds, these brief pressure reductions being independent of the pressure reductions to which the reaction mixture is subjected in the polymerization zone.
According to a further advantageous embodiment Or the invention, the time interval between the end of one pressure reduction and the start of the next pressure reduction is in each case from 5 to 500 seconds.
The residence time o~ the reaction mixture in the cooling zone is advantageou31y from 5 to 200 seconds~
The term ethylene polymers embraces solid, waxy and oily poly-merqO
For the purposes of the invention, homopolymers and copolymers of ethylene are the ethylene homopolymers and copolymers which can be manufactured at pressures of from 1~000 to 4,000 bars and at from 100 to 450C. All polymerization initiators and chain transfer agents con-ventionally used for the high pressure homopolymerization and copoly-merization of ethylene may be employed for the purposes of the in-ventionO Su;table compounds copolymerizable with ethylene are all those which can be copolymerized with ethylene under the stated pressure conditions~ Examples of such compounds are vinyl esters, eOgO vinyl acetate and vinyl propionate, esters of acrylic acid, eOg. butyl acrylate9 esters of methacrylic acid, acrylonitrile, acrylamide, acrylic acid or vinyl ethers. Advantageous catalysts to use are oxygen, peroxides, eOg. benzoyl peroxide, or azo compounds, e~gO azo-isobutyro-nitrileO High energy irradiation may also be used to initiate the polymerizationO
The invention may be implemented using the conventional con-tinuously operated high pressure polymerization systemsO Polymerization zones are to be understood as being the conventional tubular reactors and stirred autoclavesO Tubular reactors are vessels whereof the ratio of length to diameter of the pressure-resistant tubes is from 10,000:1 to 60,00o : lo Autoclave reactors are pressure-resistant vessels whereof the length to diameter ratio is from 30 : 1 to 2.5 : 1.
~ 1068~ o~zO 31,853 In order to achieve thorough mixing of the reaction mixture and gooddistribution of the heat generated, the autoclaves are fitted with stirrersO Information on processes using tubular reactor~ or autoclave reactors is to be found, for example, in Ullmanns Encyklopadie der technischen Chemie, 3rd edition, volume 14 ~1963), pages 137-148.
Following the polymerization, the reaction mixture coming from the polymerization sys~em, i.e. from the tubular reactor or autoclave reactor, is flashed in a cooling zone. For this purpose, a let-down valve located at the reactor outlet is used; by means Or this valve the pressure in the polymerization system can be reduced briefly, i.e.
the reaction zone can be let down~ The cooling zone used is an apparatus which can be operated as a cooling device, iOe. an after-cooler. Prererably, this cooling zone is a pressure-resistant tube surrounded by a jacket through which passes a coolantO In this cooling tube, the reaction mixture which has left the reactor can be cooled to a temperature lower than the temperature prevailing in the reactor, i.e. in the polymerization zoneO In general, the temperature in the cooling zone, eOgO in the cooling tube described above, is from 200 to at most 320 CO The pressure in this cooling zone is lower than in the polymerization zone.
The reaction mixture is then passed into a high pressure product separation zone or high pressure separator, where the prevailing pressure is from 50 to 500 bars lower than in the upstream cooling zone (cooling tube)O The pressure in this separation zone is as a rule from 200 to 350 barsO In the separation zone, the ethylene polymer obtained in the reactor is separated from the unpolymerized monomers.
From the high pressure separator, the polymer, still containing small amounts Or monomer, is passed into a low pressure product separation zone or low pressure separatorO In the latter, the prevailing pressure is usually less than 10 barsO
According to the process of the invention, the pressure in the cooling zone is reduced periodically, at the outlet from the cooling zone to the high pressure separation zone, for a duration of at most ~06~4~ o.z. 31,853 10 seconds, these brief pressure reductions being independent of the pressure reductions to which the reaction mixture is subjected in the polymerization zone. For this purpose a control valve is provided at the outlet of the cooling zone which is located between the let-down valve at the reactor outlet and the high pressure product separation zone. Its function is to periodically reduce the pressure in the cooling zone. The pressure reduction at the coolin~ zone outlet is advantageously ~rom 50 to 500 bars, preferably from 100 to 150 bars, and may last in each case ~rom Ool to 10 seconds, prererably from 0.1 to lo 5 seconds. The duration of a pressure reduction is to be understood as the period, in seconds, for which a single pressure reduction is effected, iOeO for which the let-down valve at the end of the cooling zone, leading to the high pressure product separation zone, is openedO The decisive factor is that these brief pressure reductions are periodically repeatedO These time intervals between the end of one pressure reduction and the start Or the next pressure reduction amount in each case to from 5 to 500 secondsO
The residence time of the reaction mixture in the cooling zone is advantageously set to from 5 to 200 seconds, preferably from 10 to 20 secondsO The residence time is defined as the ratio of the through-put of reaction mixture, in unit volumes per unit time, to the volume of the cooling zone or of the cooling tubeO
It is an advantage of the process of the invention that by follo-wing this method the temperature in the cooling zone, and hence also in the high pressure product separation zone, io eO the high pressure separator, can be reduced significantlyO ~onsequently, the conversion in the polymerization unit can be increased and a more homogeneous product can be obtained.
The polymerization systems used in carrying out the process of the invention will now be described in relation to Figures 1, 2 and 3 of the accompanying drawingsO
Figure 1 shows a tubular reactor with a reaction zone.
The compre~sor I feeds a mixture of ethylene, with various O.Z0 31,853 1068~48 additives required for the polymerization, under high pressure intothe part II of the apparatus, which serves as a buffer vessel and pre-heater, and in which the pilsation caused by the compressor is damped and the reaction mixture is heated up. The reaction mixture pas~e~ to the starter reactor lII via an on-off valve 10. In this part-zone A of the total tubular reactor, the polymerization starts as a result of sufficient heat supplied by a heating mediumO The mixture then passes through the pressure-reducing valve 1~ to the actual reactor IV, which transmits its heat of reaction throu~h the tube wallO The pressure-reducin~ valve 12 operates in such a way as to keep the pressure inparts II and III of the apparatus constant at all timesO The reactor IV, in the part-zone B of the tubular reactor which is subjected to brief pressure reductions~ is cooled with a coolant, the temperature of which is below the temperature of the gas mixture at the inlet of the reactor I~o The let-down valve 13 normally keeps the pressure in the reactor IV constant at the desired valueO The brief pressure reductions form par~ of the functioning of the let-down valve 13. The reaction product leaving the reactor IV is flashed in the aftercooler IV (or cooling zone)O The latter is cooled by means of a coolant which is at a temperature below the temperature of the reaction mixture flowing through the cooling zone~ and cons;sts of pressure-resistant tubes surrounded by a jacket through which the coolant passes. It is provided with a pressure control valve 14 at its outlet, the function of this valve being to reduce the pressure in the aftercooler periodicallyO The pressure is controlled by means of the pressure-signalling device PC, which controls the valve 14 with the aid of the requisite control meansO From the aftercooler, the reaction mixture is let down, through this control valve, into the high pressure separa-tor 5. There, the polyethylene is separated from the unconverted ethyleneO The polymer, obtained as a melt, passes v;a the valve 15 to the low pressure separator 6 and from there is processed further by conventional methodsO The gas from the high pressure separator 5 is cooled and purified by means of a unit 7 designed for the purpose and 1 0 ~ ~ O.Z. 31,853 not shown in more detail, and is recycled to the compressor I. Heatingmedium or coolant is intrGduced at the inlets marked 1, 2, 3 and 4.
Figure 2 shows a polymerization reactor which has an autoclave A, equipped with a stirrer, in p~ace of the reactors III and IV and the valve 12 located between theseO The pressure-reducing valve 13 is located at the autoclave outlet and is rollowed by the aftercooler V, pressure-signalling device PC and control valve 14. All further parts o~ the installation, the high pressure product separator 5 and the low pressure product separator 6 are identical with those shown in iO F~gure 1. Heating medium and coolant are introduced at the inlets 1 and 40 Figure 3 shows a tubular reactor comprising 2 parts each Or type I, II, III and IV, marked I, IA, II, IIA, III, IIIA, IV~1 and IV/20 The corresponding valves are marked 10, 12, 13, 14, lOa and 12a.
Its essential difference from the reactor shown in Figure 1 will be described brierlyO The reaction mixture leaving the reaction zone IV/1 is mixed with ethylene which contains initiator and has been com-pressed and preheated as it travels through IA, IIA and IIIA. In the zone IV/2, a further polymerlzation reaction takes place as a result of the renewed addition of initiator, referred to aboveO The parts 13, V, PC, 14, 5, 15, 6 and 7 of the apparatus perform the functions already described in connection with Figure lo Heating medium and coolant are introduced at 1, laS 2, 2a, 3, 3a and 40 The following procedure ;s employed with the reactor shown in Figure lo The reactlon pressure in reactors II, III and IV may be set to a value from 1,000 to 4,000 barsO The pressure reductions effected by means of valve 13 are from 100 to 1,000 bars; they take place at intervals Or from 0.5 minute to 500 hours and last from Ool to 10 seconds. The pressure in the after-cooler V is set by means of the valve 14 so as to be from 50 to 500 bars above the pressure in the high pressure product separator 5 and is periodically reduced by means o~f valve 14 by rrom 500 to 50 barsO The pressure reductions take place at interval~ oP from 5 to 500 seconds and last from 0 1 to 10 seconds.
_ g _ 0.Z. 31,853 The residence time of the reaction mixture in the after-cooler is from 5 to 200 secondsO
In the reactor shown in Figure 2, the following conditions are employed. The pressure in autoclave A is set to from 1,000 to 4,000 bars. The pressure reductions effected by means of valve 13 are from 20 to 300 bars, they take place at intervals of from 0.5 minute to 500 hours. The pressure in the aftercooler V is set by means Or the valve 14 so as to be from 50 to 500 bars above the pressure in the high pres~ure product separator 5 and is periodically reduced by means ~ valve 14 by from 500 to 50 barYO The pressure reductions take place at intervals Or from 5 to 200 seconds and last from 0.1 to 10 seconds.
The residence time Or the reaction mixture in the aftercooler is from 5 to 200 seconds.
The conditions in the reactor shown in Figure 3 correspond to those of the reactor shown in Figure lo EXAMPLE
Per hour, 20,000 kg Or ethylene, containing 300 g Or oxygen and 44 kg Or propionaldehyde, are introduced continuously into a tubular reactorO The mixture is under a pressure Or 3,Q00 barsO At intervals Or 200 minutes, the pressure is reduced to 2,800 bars for a period Or ~!0 lo 5 secondsO After a residence time Or 60 seconds, the reaction mixture has reached a temperature Or 330Co After a further 20 seconds, 10,000 kg Or ethylene, containing 350 g Or oxygen and 20 kg of propionaldehyde, are addedO After a further 40 seconds, the reaction mixture, after having first returned to 330 C, is let down via the actuator valve into the aftercooler at 500 barsO After 15 seconds, the reaction mix-ture pas~es into the high pressure product separator, where it has a temperature Or 290C and a pressure of 280 barsO 8,000 kg of poly-ethylene having a melt index Or (MFI) 4 g/10 min (according to DIN 53,479) and a density of 0,~24 g/cm3 (according to DIN 53,735) are 30 obtainedO
The procedure followed is as in Example 1 but in contrast to the ~ ~448 oOz. 31,853 latter, the reaction mixture i5 let down to 500 bar~ in the after-cooler. This pressure is reduced to 400 bars ror a duration Or 0.3 second at intervals Or 10 seconds, by means of the valve at the aftercooler outlet. A~ter a residence time Or 15 seconds, the reaction mixture leaves the aftercooler via the valve and passes into the high pressure product separator where the temperature assumes a value of 271C at a pres~ure of 280 bars. 8~500 kg of polyethylene having a MFI of 4 g/10 min and a density Or 0~924 g~cm3 are obtained.
Per hour, 5,000 kg Or ethylene, 130 g oP tert.-butyl perpivalate, 180 g o~ tert.-butyl perisononanate and 10 kg o~ propionaldehyde are introduced continuously into an autoclave equipped with a stirrer. The pre~sure in the autoclave is kept at 2~000 bars and is periodically reduced to 1,970 bars for a duration of 0.3 second, at intervals of 300 minutesO The reaction temperature is 270Co Arter a residence time Or 60 seconds, the reaction mixture is let down into the after-cooler, at 500 bars, by means Or the let-down valveO After a residence time Or 20 seconds, it is let down to a pressure of 280 bars in the high pressure product separator. The temperature in the latter assumes a value Or 282C.
860 kg Or polyethylene having an MFI of 7 g~10 min and a density Or 0~ 922 g/cm3 are thùs obtainedO
Exactly the same conditions as those described in Example 3 are set up continuously in the autoclaveO However, in contrast to Example 3, different conditions are maintained in the aftercooler zone. The pressure is kept at 500 bars and is reduced to 400 bars for a duration Or 0.3 second at intervals Or 10 seconds by means of the valve fitted at the aftercooler outletO The mixture let down through this valve to 280 bars into the high pressure product separator is at 264Co 955 kg of polyethylene having an MFI of 6 8 g/10 min and a density of 0.922 g/cm3 are thus obtained.
` O.Z. 31,853 1068~48 Exactly the same conditions as those described in Example 3 are set up in the autoclave. In contrast to Example 3, the pressure in the aftercooler is kept constant at 280 bars. The residence time Or the reaction mixture in the artercooler is 20 seconds. The temperature in the high pressure product separator assumes a value Or 281C at 280 bars.
870 kg Or polyethylene having an MFI Or 7.2 g~10 min and a den-sity Or 0.9215 g~cm3 are thus obtained.
Claims (4)
1. A process for the continuous manufacture of an ethylene poly-mer by homopolymerization of ethylene or copolymerization of ethylene with one or more other monomers copolymerizable with ethylene, in a polymerization zone at from 1,000 to 4,000 bars and from 100 to 450°C, followed by transfer of the reaction mixture through a cooling zone into a high pressure product separation zone, the pressure prevailing in the cooling zone being lower than in the polymerization zone but being from 50 to 500 bars higher than in the high pressure product separation zone, wherein the pressure in the cooling zone is reduced periodically and recurrently for a duration of from 0.1 to 10 seconds at the outlet Prom the cooling zone to the high pressure product separation zone, these brief pressure reductions being independent of the pressure reductions to which the reaction mixture is subjected in the polymerization zone.
2. A process as claimed in claim 1, wherein the magnitude of the pressure reductions in the cooling zone is from 50 to 500 bars.
3. A process as claimed in claim 1, wherein the time interval between the end of one pressure reduction and the start of the next pressure reduction is in each case from 5 to 500 seconds.
4. A process as claimed in claim 1, wherein the residence time of the reaction mixture in the cooling zone is from 5 to 200 seconds.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19762606144 DE2606144C2 (en) | 1976-02-17 | 1976-02-17 | Process for the production of high pressure polyethylene |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1068448A true CA1068448A (en) | 1979-12-18 |
Family
ID=5970020
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA270,376A Expired CA1068448A (en) | 1976-02-17 | 1977-01-21 | Manufacture of high pressure polyethylene |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS5298790A (en) |
| AT (1) | AT350256B (en) |
| BE (1) | BE851487A (en) |
| CA (1) | CA1068448A (en) |
| DE (1) | DE2606144C2 (en) |
| FR (1) | FR2341599A1 (en) |
| GB (1) | GB1566509A (en) |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2852501A (en) * | 1954-10-05 | 1958-09-16 | Monsanto Chemicals | Polymerization of ethylene |
| GB1313836A (en) * | 1970-04-27 | 1973-04-18 | Mitsubishi Petrochemical Co | Process for the preparation of ethylene polymers |
| DE2047290C2 (en) * | 1970-09-25 | 1982-04-08 | Basf Ag, 6700 Ludwigshafen | Process and arrangement for regulating a tubular reactor for polymerizing ethylene to homopolymers |
-
1976
- 1976-02-17 DE DE19762606144 patent/DE2606144C2/en not_active Expired
-
1977
- 1977-01-21 CA CA270,376A patent/CA1068448A/en not_active Expired
- 1977-02-09 JP JP1259777A patent/JPS5298790A/en active Granted
- 1977-02-16 BE BE174979A patent/BE851487A/en not_active IP Right Cessation
- 1977-02-16 GB GB639777A patent/GB1566509A/en not_active Expired
- 1977-02-16 AT AT103477A patent/AT350256B/en not_active IP Right Cessation
- 1977-02-17 FR FR7704508A patent/FR2341599A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5298790A (en) | 1977-08-18 |
| FR2341599B1 (en) | 1981-02-20 |
| FR2341599A1 (en) | 1977-09-16 |
| AT350256B (en) | 1979-05-25 |
| JPS6114161B2 (en) | 1986-04-17 |
| DE2606144C2 (en) | 1983-02-03 |
| BE851487A (en) | 1977-08-16 |
| ATA103477A (en) | 1978-10-15 |
| GB1566509A (en) | 1980-04-30 |
| DE2606144A1 (en) | 1977-08-25 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4153774A (en) | Manufacture of high pressure polyethylene | |
| US4728701A (en) | Process for the polymerization of acrylates | |
| US4555384A (en) | Apparatus for producing styrenic/alkenylnitrile copolymers | |
| US4548788A (en) | Apparatus for producing styrenic/alkenylnitrile copolymers | |
| US4551309A (en) | Apparatus for producing styrenic/alkenylnitrile copolymers | |
| US4542196A (en) | Process for producing styrenic/alkenylnitrile copolymers | |
| US4551510A (en) | Process for producing styrenic/alkenylnitrile copolymers | |
| US4788265A (en) | Preparation of copolmers of ethylene with acrylic acid esters in a tubular reactor at above 500 bar | |
| EP2636689A1 (en) | Process and plant for manufacturing polyethylene or ethylene based copolymers | |
| US4416856A (en) | Process for the polymerization and copolymerization of ethylene, using a gas injector device | |
| EP0734289A1 (en) | Process for the production of polyolefins in an autoclave reactor | |
| WO2015082565A1 (en) | Process for separating components of a reaction mixture obtained by high-pressure polymerization of ethylenically unsaturated monomers | |
| EP0903175B1 (en) | Continuous process for polymerizing a vinyl monomer cooled by superheated water | |
| US4282339A (en) | Dual reactor process and apparatus for polymerizing ethylene | |
| US4229416A (en) | Dual reactor apparatus for polymerizing ethylene | |
| CA1068448A (en) | Manufacture of high pressure polyethylene | |
| US20230212337A1 (en) | Process for producing ethylene homo- and copolymers | |
| CA1285097C (en) | Amorphous polyalphaolefin process | |
| CA1077195A (en) | Manufacture of ethylene polymers in an autoclave reactor | |
| DE1520260C2 (en) | Process for the production of homopolymers or copolymers of ethylene | |
| EP0319622A1 (en) | Method for the polymerization of acrylics | |
| EP1951768B1 (en) | An apparatus and method for the production of polyethylene and ethylene copolymers | |
| NO141210B (en) | PROCEDURES FOR CONTINUOUS HIGH PRESSURE POLYMERIZATION OF ETHYLENE | |
| EP0499599A2 (en) | Reduction of polymerisation inhibitor in recycle streams | |
| US6245864B1 (en) | Copolymer of (a) ethylene and (b) and alkenoic acid or a derivative thereof a mixture of said monomers |