High pressure polymerization process for producing low density polyethylene and copolymers thereof

Abstract

The present application relates to a process for the high pressure polymerization of ethylene, optionally with at least one comonomer thereof, to produce low density polyethylene, or an ethylene copolymer with at least one comonomer, preferably ethylene vinyl acetate (EVA) having a vinyl acetate (VA) content in the range of from 2 to 50 mass%, the process comprising: reacting ethylene, optionally with at least one comonomer thereof, in one or more reactors; optionally polymerized with at least one of its comonomers; reducing the pressure of the reaction effluent; in one or more injection devices, after the pressure of the reaction effluent has been reduced, reducing the temperature of the reaction effluent by an amount preferably varying in the range of 1 DEG C to 50 DEG C relative to the temperature obtained solely by throttling (isenthalpic expansion); separating the reaction effluent by at least three subsequent separation stages, the separation operating at different pressures, producing at least three different exhaust gas streams (high pressure exhaust gas, medium pressure exhaust gas, low pressure exhaust gas), each from a single separation stage, and recovering the low density polyethylene or optionally a copolymer thereof from the unreacted compound in each separation stage; recirculating a portion of the exhaust gas separated in the first stage (high pressure exhaust gas) and all the exhaust gas separated in the second stage (medium pressure exhaust gas) to one or more of said injection devices, recompressing said exhaust gas at the pressure of the first separation stage, thereby cooling the reaction effluent; and mechanically compressing the off-gas separated in the third stage (low-pressure off-gas) and the supplemental ethylene and optionally the supplemental at least one comonomer thereof to a reaction pressure.

Description

Technical Field

[0001] The present patent application relates to a process for producing low density polyethylene (LDPE) and / or its copolymers, such as ethylene vinyl acetate (EVA), using high pressure technology.

[0002] The process utilizes part of the waste gases generated in the process itself by appropriate recycling for cooling the reaction effluent and at the same time utilizes the enthalpy of the reaction effluent, thereby increasing the overall energy efficiency of the production cycle.

[0003] More specifically, the method described and claimed utilizes a cooling and energy recovery system that uses an injection device (equivalently referred to as an injection pump or injector). Background Art

[0004] In this patent application, all operating conditions reported herein must be understood as preferred conditions even if not explicitly stated.

[0005] For the purpose of this specification, the terms "comprise" or "include" also include the terms "consisting of" or "consisting essentially of.

[0006] For the purposes of this discussion, the definition of an interval always includes extreme values ​​unless otherwise stated.

[0007] For the purposes of this specification, values ​​expressed in "%" are always understood to be % by mass.

[0008] Plants for the production of LDPE polymers and copolymers using high-pressure technology are expensive both in terms of investment costs and operating costs. By high-pressure technology for the production of LDPE polymers and copolymers we mean processes in which the polymerization is carried out at a pressure of not less than 600 bar absolute (also called bar a).

[0009] The reaction effluent is in a supercritical phase because the reactor is operated at a pressure in the range of 1000 to 3000 bar absolute and a temperature in the range of 150° C. to 300° C. The mixture containing polymer species (e.g. polyethylene), copolymers and unreacted monomers (e.g. ethylene) must be separated, but in order to do this the pressure must first be reduced.

[0010] For this purpose, the effluent material is throttled at a pressure between 100 and 500 bar absolute, with the result that the temperature rises sharply, which creates two problems:

[0011] Product quality issues: Persistence of products at high temperatures, such as over 200°C in some EVA (ethylene vinyl acetate) products, can lead to gel formation, which deteriorates the optical properties and processability of the final product, especially for films and coatings.

[0012] Operational continuity issues: Continuous high temperatures in the high-pressure separator can lead to the formation of hot spots, which can trigger decomposition reactions of the mixture, resulting in shutdown of the production equipment and loss of production.

[0013] Therefore, the temperature control and pressure control of the reaction effluent are critical points in the ethylene polymerization equipment.

[0014] Various solutions for cooling polymers have been proposed in the prior art which provide for the use of chillers or other solutions for removing heat.

[0015] GB 1540894 discloses a method for homopolymerizing or copolymerizing ethylene (see FIG. 4 ), wherein ethylene or a mixture of monomers containing ethylene is reacted in a reactor (3) at a pressure greater than 1000 bar, and the resulting mixture (C) passes through a valve (4) for reducing the pressure and then enters a separator (6) at a pressure of 200-500 bar. The mixture of polymer and monomer (C) is cooled by injecting recycled monomer (D) through a jet pump (5) placed between the throttle valve (4) and the separator (6). The jet pump (5) operates at a lower pressure than the separator (6).

[0016] US 7,837,950 describes a high pressure polymerization process for ethylene and optionally its comonomers (see Figure 5). The monomers are polymerized in a high pressure reactor. The reaction effluent (C) flows through a reactor bottom valve (4) located downstream of the reactor (3) and is then sent to a system of separators (6, 11) from which waste gases are separated.

[0017] For example, the exhaust gas (E) of the second separator (11) is pumped upstream of the separator but downstream of the throttle valve (4) due to the jet pump (5). The reaction effluent flows through the jet pump (5) and acts as a driving fluid to create conditions for pumping the exhaust gas produced by the second separator (11) located downstream.

[0018] The reaction effluent (C) can also be sent to the cooling device (17), where a two-phase mixture is formed, one phase rich in polymer and the other phase poor in polymer. The two-phase mixture is sent to the jet pump (5) as a driving fluid, creating conditions for sucking the waste gas produced by the second separator (11).

[0019] US 8,048,971 discloses a method for high-pressure polymerization of polyethylene and its copolymers in a tubular reactor subjected to pulsation (pressure pulsation is caused by the partial opening of the reactor outlet valve for less than 1 second). A jet pump is used to pump a portion of the cooled exhaust gas from a first separator into the reaction product, thereby cooling it. A mixture consisting of the cooled reaction product and a portion of the cooled exhaust gas is sent to a separation device consisting of two exhaust gas separators. The method described in US Patent 8,048,971 does not provide for the use of an intermediate separator and the relative recovery of the exhaust gas compressed by a jet pump.

[0020] US 8,445,606 discloses a system for improving the productivity and / or reducing the specific energy consumption of existing olefin free radical polymerization equipment. The equipment includes a monomer compression section operating at a pressure of 1500-3500 bar, a reaction section (wherein a tubular reactor operating at 3000 bar is equipped with a jacket for cooling the polymerization reaction), and a downstream separation zone connected to a jet pump (which is used to cool the reaction with separated gas). The reaction products and unreacted compounds flow to three separation stages through a throttle valve. Part of the exhaust gas from the first separator and the second separator is cooled and pumped into the stream from the reactor through a jet pump after throttling. The application of periodic pressure pulsations ("shocks") is provided to keep the reactor clean, resulting in high mechanical stress on the equipment that is already highly stressed due to the high operating pressure.

[0021] US2013 / 171029 describes a system for producing polyethylene (LDPE) equipped with a cooling system using a jet pump. In fact, the system comprises a first compressor and a second compressor connected to each other, which feed a tubular polymerization reactor. Part of the supplementary monomer is sent to the jet pump from the first compressor. The jet pump first cools the reaction product and then separates the reaction product in a separation section formed by two waste gas separators. The method described in US patent 2013 / 171029 does not provide for the use of an intermediate separator and the relative recovery of the waste gas compressed by the jet pump. In addition, a regulating valve is provided between the primary compression and the secondary compression, which further complicates the process scheme.

[0022] In order to overcome the problems of the prior art, the applicant has proposed a high-pressure process for the polymerization of ethylene and / or its comonomers to produce low-density polyethylene (LDPE) and / or its copolymers, which uses one or more ejectors to cool the reaction effluent while restoring energy efficiency. In fact, during the process, waste gases are generated from the separation device of the reaction effluent, which are sent to the ejector to cool the reaction effluent by hot pressing.

[0023] The method comprises a step of reducing the pressure of the reaction effluent, followed by a cooling step with energy recovery by using one or more injection devices.

[0024] Due to the temperature control and pressure control of the reaction effluent and the polymer (which is achieved through the combination of technical features as described herein), the process is particularly suitable for the production of low density polyethylene for films and coatings (for example, films for greenhouses, blown films made from large-size blown, expanded / foamed products, coatings on paper or other substrates), and also for the production of copolymers of ethylene and vinyl acetate, the vinyl acetate content of which can be as high as 50%.

[0025] The subject of the present patent application is therefore a process for the high pressure polymerization of ethylene, optionally with at least one comonomer thereof, to produce low density polyethylene, or a copolymer of ethylene with at least one comonomer, such as EVA (ethylene vinyl acetate) having a vinyl acetate content in the range of 2-50%, comprising:

[0026] - high pressure polymerization of ethylene, optionally with at least one comonomer thereof, in one or more reactors;

[0027] Reduce the pressure of the reaction effluent;

[0028] in one or more injection devices, after reducing the pressure of the reaction effluent, reducing the temperature of the reaction effluent relative to the temperature obtained by throttling (isenthalpic expansion) alone, for example by an amount ranging from 1° C. to 50° C.;

[0029] separating the reaction effluent by means of at least three subsequent separation stages, said separations being operated at different pressures, producing at least three different off-gas streams (high-pressure off-gas, medium-pressure off-gas and low-pressure off-gas), each coming from a single separation stage, and recovering low-density polyethylene and / or optionally copolymers thereof from the unreacted compounds in each separation stage;

[0030] recirculating part of the offgas separated in the first stage (high-pressure offgas) and all of the offgas separated in the second stage (medium-pressure offgas) to one or more of the injection devices, recompressing the offgas at the pressure of the first separation stage, thereby cooling the reaction effluent; and

[0031] Mechanical compression of the offgas separated in the third stage (low-pressure offgas) and replenishment with ethylene and optionally with said at least one comonomer thereof to reach the reaction pressure.

[0032] The portion of the exhaust gas (high-pressure exhaust gas) separated in the first separation stage that is not supplied to one or more injection devices and is not recirculated in the first stage is supplied to a mechanical compression device: the portion of the exhaust gas includes not only the exhaust gas separated from the reaction effluent in the first stage, but also the exhaust gas that has been pre-thermally recompressed in the injection device at the pressure of the first separation stage.

[0033] In each separation stage, the reaction effluent from the previous stage is separated into two phases: a light phase (off gas) rich in unreacted monomers and / or comonomers and a heavy phase rich in polymer, which is then conveyed to the next separator operated at a lower pressure in order to obtain additional separation of the off gas from the heavy phase rich in polymer.

[0034] In this patent application, the term "low density polyethylene" is intended to mean polyethylene commonly known by the abbreviation LDPE, typically having a density in the range of 0.910 to 0.940 g / cm 3 Although this density range is not limiting for purposes of the present invention.

[0035] In this patent application, the term "throttling" refers to a transition that follows an isenthalpic behavior.

[0036] The density values ​​of LDPE given above are determined at 23°C and the density is usually measured by "ISO 1183-2:2004 method (determined at 23°C)".

[0037] In the present patent application, the term "copolymers thereof" refers to ethylene copolymers identified with at least one comonomer (eg, vinyl acetate, methyl acrylate, ethyl acrylate, n-butyl acrylate, methyl methacrylate, acrylic acid, methacrylic acid).

[0038] An example of the ethylene copolymer is an EVA (ethylene vinyl acetate) copolymer, preferably an EVA copolymer having a vinyl acetate (VA) content in the range of 2 to 50% by mass.

[0039] The polyethylene and / or copolymers thereof prepared by the high pressure polymerization process of ethylene and / or its comonomers as described above generally have a melt mass flow rate (MFR) in the range of 0.1 to 500 g / 10', preferably 0.15 to 110 g / 10', more preferably 0.2 to 65 g / 10', measured at 190°C / 2.16 kg according to ISO 1133:2005 standard, although the above MFR range is not limited for the purpose of the present invention.

[0040] The reaction effluent is in the gas phase and contains the ethylene polymer or ethylene copolymer, and also unreacted monomer and possibly unreacted comonomer.

[0041] To recover the polymer, the gaseous reaction effluent is subjected to several separation stages, producing high-pressure, medium-pressure and low-pressure offgases.

[0042] The applicant delivers part of the high-pressure exhaust gas (optionally cooled) and all of the medium-pressure exhaust gas to one or more injection devices (e.g., injection pumps). The medium-pressure exhaust gas and part of the high-pressure exhaust gas delivered to the injection pump serve as heat carriers to cool the reaction effluent, while the reaction effluent serves as a pressure carrier to compress the medium-pressure gas to the pressure of the first separator.

[0043] This solution simplifies the polymerization plant since it avoids the use of a cooling system designed to operate at high pressure, i.e. at a pressure equal to that of the reactor, and dedicated to cooling the reaction effluent, and avoids the installation of heat exchange means on the medium-pressure exhaust gas recirculation line.

[0044] Furthermore, due to the injection device, the compressor on the medium-pressure exhaust gas line requires less power and, overall, the entire compression line is simplified and reduced in size. All this has a positive effect in terms of reducing investment costs (CAPEX) and utility consumption (OPEX).

[0045] The use of this solution also makes it possible to increase the productivity of existing plants, since it allows increasing the conversion in the reactor and using existing compressors (which normally compress the offgas from the second separator in the prior art processes) to compress the make-up ethylene and / or its comonomers. The offgas from the second separator can be recompressed by the solution proposed by the applicant.

[0046] This solution also allows the recovery of the vapours leaving the second separation stage, which are usually provided in plants for the polymerisation of ethylene to polyethylene, thereby improving the overall energy efficiency of the process.

[0047] The ejection device, such as a jet pump or an ejector, allows the energy of the separated gas to be recovered by utilizing the thermal compression operated by the jet pump. The pressure of the reaction effluent compresses the medium-pressure waste gas and the high-pressure waste gas to the pressure of the separation device. At the same time, the medium-pressure waste gas and the high-pressure waste gas, which are lower in temperature than the reaction effluent, cool the reaction effluent as it passes through the jet pump. A first object of the present invention is to reduce specific energy consumption by recovering the energy generated in the production process of polyethylene. For example, a 100 kt / y plant can save a total of about 900 megawatt hours (MWh) of energy per year.

[0048] Another object of the present invention is to reduce the temperature of the fluid (e.g., effluent) flowing to the high-pressure separator. This reduction has a dual advantage: it allows the reduction or elimination of defects in the polymer; and it can reduce the possibility of uncontrolled reactions, thereby increasing the continuity of operation.

[0049] Furthermore, the invention described and claimed in this patent application allows precisely simplifying the equipment, since no heat exchange devices such as coolers and / or second separators at the reactor outlet are used, and allows the use of equipment of smaller size and power such as compressors.

[0050] These and other objects and advantages are obtained by the invention described and claimed in this patent application and will become more apparent from the following description and the accompanying drawings which are provided by way of non-limiting example only and represent preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 A preferred embodiment of the method according to the present invention is shown;

[0052] Figure 2 The enlarged form shows the details Figure 1 A cooling section with an ejector pump;

[0053] FIG. 3 shows a flow chart of the prior art, which does not provide for the installation of the injection pump.

[0054] FIG. 4 shows a second flow diagram of the prior art which provides for cooling of the exhaust gases leaving the medium-pressure separator by means of a heat exchange device.

[0055] FIG. 5 shows a third prior art flow diagram which provides a heat exchange device (cooler) for cooling the effluent.

[0056] Figure 1The system of the embodiment of the present invention shown has a primary mechanical compression system (booster) (1) and a secondary mechanical compression system (2); a polymerization reaction device (3), three throttle valves (4, 9, 12), a temperature control valve (8) of the high-pressure separator, a pressure control valve (10) of the medium-pressure separator, an injection device (injection pump (5)), a separation system operating in the absolute pressure range of 100-500 bar (high-pressure separator, 6), a separation system operating in the absolute pressure range of 2-100 bar (medium-pressure separator, 11), a separation system operating in the absolute pressure range of 0.01-5 bar (low-pressure separator, 13), a granulation device (14); two recycles of the gas phase: a high-pressure recycle (7) leaving the high-pressure separator (6) and a low-pressure recycle (15) leaving the low-pressure separator (13). In the schematic diagram shown, ethylene (A) and one of its comonomers (B) are compressed in a compression system (1, 2) and then reacted to produce a reaction mixture (C), which is throttled (4) and sent to a separator (6, 11, 13). The reaction mixture (C) leaving the reactor (3) is throttled (4) and constitutes the driving fluid for the injection device (jet pump, 5). The mixture leaving the jet pump (5) enters a high-pressure separator (6), where a heavy phase rich in polymer and a light phase rich in unreacted monomers and / or rich in comonomers are separated. After the first separation, the heavy phase is throttled, while the gas phase is cooled (7) and partly recycled to the secondary compression system and partly recycled to the jet pump (5). The heavy phase leaving the high-pressure separator (6) is throttled (9) and then enters the medium-pressure separator (11), in which the separated light phase is recycled to the jet pump (5) via a control valve (10) and the separated polymer-rich heavy phase is throttled (12) and then enters the low-pressure separator (13). The gas phase leaving the low-pressure separator (13) is cooled in the low-pressure recycle (15), and a condensed phase consisting of solvent, comonomer, compressor lubricant and very low molecular weight polymer is possibly separated and recycled to the primary compressor system (booster). DETAILED DESCRIPTION

[0057] Applicants now also refer to Figure 1 The method objectives of the present patent application are described in detail.

[0058] Ethylene and / or its comonomers (reactants) are mechanically compressed by one or more primary compression systems, preferably a booster / primary system (1) and a secondary compression system (2) (each system preferably comprising one or more compression stages, or one or more pumps). The reactants may be compressed to a pressure of up to 1000 bar absolute to 3000 bar absolute, preferably 1000 bar absolute to 2200 bar absolute, more preferably 1200 bar absolute to 1800 bar absolute.

[0059] Preferred comonomers may be selected from the following: vinyl acetate, methyl acrylate, ethyl acrylate, n-butyl acrylate, methyl methacrylate, acrylic acid, methacrylic acid.

[0060] A chain modifier may be added to the reactants; the modifier may preferably be selected from the group consisting of 1-butene, butane, propylene, propionaldehyde and propane.

[0061] Compressor systems, especially those started from low pressure, involve a large amount of energy consumption in terms of electricity. Once compressed, the reactants react in the reaction unit (3) according to chain-type, free radical and initiator-promoted polymerization reactions. The reaction unit is preferably an autoclave type reactor, sometimes also referred to as a container reactor. The reactor consists of a high-pressure vessel equipped with a stirring system that homogenizes the reactant mixture. The reactor can include one or more feeds of unreacted gas and one or more injections of one or more polymerization promoters. The autoclave type reactor can also provide an internal diaphragm that allows the reaction volume to be divided into smaller areas, within which a desired temperature profile can be formed to give the product (ethylene homopolymer and copolymer) specific optical and mechanical properties.

[0062] Autoclave reactors are particularly advantageous over tubular reactors because they operate at a constant pressure rather than with constant overpressure pulses (shocks), for example exceeding the reaction pressure by 700 bar every 40 seconds.

[0063] Furthermore, the use of autoclave reactors is advantageous because they show less fouling, lower pressures can be used to carry out polymerization than those used in tubular reactors, and autoclave reactors allow for the availability of polymer grades that are amenable to certain types of film and extrusion coatings.

[0064] Indeed, continuous stirring within the autoclave maintains the polymer mass at a uniform temperature and maintains a constant concentration of components (eg, one or more monomers, polymer, polymerization initiator, and chain transfer agent) within the reaction zone.

[0065] Furthermore, the lower temperatures and lower pressures available in autoclave reactors can positively influence polymer properties, such as optical properties (which is especially important for applications such as transparent films, films for greenhouses).

[0066] Preferred chain initiators are compounds which are unstable at the reaction temperature and which are capable of providing free radicals capable of promoting the polymerisation reaction. More preferred initiators are selected from organic peroxides and peresters and oxygen.

[0067] The polymerization reaction may be carried out at a temperature of 150°C to 300°C, preferably 170°C to 300°C, more preferably 170°C to 280°C.

[0068] The reaction pressure in the polymerization is generally from 1000 bar a to 3000 bar a, preferably from 1000 bar a to 2200 bar a, more preferably from 1200 bar a to 1800 bar a.

[0069] Even within the preferred range of the reaction pressure, the reaction pressure is consistent with the aforementioned compression pressure of the reactants.

[0070] Under these conditions of temperature and pressure, the reactant mixture is in a homogeneous phase known as the supercritical phase.

[0071] The reaction effluent (C) contains polyethylene and / or ethylene copolymers, unreacted monomers, such as ethylene and / or its comonomers.

[0072] Prior to entering the injection device, the pressure of the reaction effluent must be reduced.

[0073] A preferred method of reducing the pressure of the reaction effluent is to use a throttle valve (4) located at the bottom of the reaction unit, which is sometimes called a let-down valve. After the injection device (5), the reaction effluent (C) combined with the part gas (D) leaving the high-pressure recirculator (7) and the waste gas (E) of the medium-pressure separator (11) enters the high-pressure separator (6). The pressure that can be reached in the high-pressure separator can vary between 500 bar absolute pressure and 100 bar absolute pressure, more preferably between 300 bar absolute pressure and 150 bar absolute pressure, even more preferably between 290 bar absolute pressure and 200 bar absolute pressure.

[0074] The pressure measurement method is not binding for the purpose of the present invention and may be measured, for example, with a flow meter, however other methods known in the art may also be used without departing from the scope of the present invention.

[0075] Inside the high-pressure separator, the temperature is usually maintained in the range of 290÷180° C. Temperatures close to 290° C. are usually reached during the production of ethylene homopolymers, while temperatures close to 180° C. are usually maintained during the production of ethylene copolymers.

[0076] By reducing the pressure, the reactant mixture becomes two-phase, forming a polymer-rich phase and a polymer-poor phase. Starting from the high pressure conditions that occur inside the reaction unit (3), the pressure is reduced by throttling the reaction effluent, which is accompanied by a temperature increase. This temperature increase leads to two problems:

[0077] Product quality issues: Long-term exposure of products to high temperatures can lead to cross-linking of polymers and the formation of gels (or fish-eyes), which can reduce the optical properties of the product, especially the optical properties of film and coating grades.

[0078] • Operational continuity issues: Continuous high temperatures within the high-pressure separator can lead to the formation of hot spots, which can cause decomposition of the reactant mixture, resulting in equipment shutdown and loss of production.

[0079] After the pressure reduction, the reaction effluent flows into at least one ejector device (ejector pump, 5): the effluent (C) leaving the reaction device (3) is used as a driving fluid to compress the gas phase (waste gas, E) from the medium-pressure separator (11) to the pressure of the high-pressure separator (6).

[0080] The gas stream (E) leaving the medium-pressure separator ( 11 ) thus reenters the vapor circuit of the high-pressure separator ( 6 ).

[0081] At the same time, part of the offgas (D) leaving the high-pressure recirculation (7) is used to cool the reaction effluent (C) containing the produced polymer. This cooling is carried out in the ejector device (5) by mixing the reaction effluent (C) with the stream (D) leaving the high-pressure recirculation (7) before entering the first separator (6). The part of the offgas or recirculation gas leaving the first separator (6), i.e. the high-pressure offgas, which is conveyed to the ejector device (ejector pump, 5) to cool the reaction mixture leaving the reaction device (3), can vary between 1% and 30% relative to the total offgas, more preferably between 1% and 20% of the total offgas, even more preferably between 5% and 15% of the total offgas.

[0082] Two regulating valves (8) and (10) can be introduced to control the temperature of the high-pressure separator (6) by regulating the flow of cooling gas (D) leaving the high-pressure recirculation (7), in order to regulate the pressure in the medium-pressure separator (11).

[0083] The reaction effluent (C) leaving the reaction device (3) may contain a polymer fraction (i.e. all possible ethylene polymers and copolymers) varying in the range of 8% to 22% relative to the total mass of the effluent, more preferably in the range of 8% to 20% relative to the total mass of the effluent, even more preferably in the range of 10% to 18% relative to the total mass of the effluent. A separation is carried out in the high-pressure separator (6) of a heavy phase rich in polymer but still containing aliquots of monomers and / or possibly unreacted comonomers (which, after throttling in valve (9), will form stream G) and a light phase (offgas, I) poor in polymer and rich in unreacted monomers and / or optional comonomers. The portion of unreacted monomers contained in the heavy phase (which, after throttling in valve (9), will form stream G) may preferably vary between 20% and 60% by mass relative to the total mass of the polymer fraction. This stream G contains mainly polymers and / or optional copolymers, low molecular weight polymers and / or copolymers, unreacted monomers and / or comonomers, peroxide decomposition products, oils and solvents. The monomers and / or any unreacted comonomers (light phase, I) are cooled in a high-pressure recirculator (7) and partly sent to a secondary compression system (2).

[0084] The control valve (9) allows throttling of the heavy phase (which forms the material flow (G) after throttling) from the conditions of the high-pressure separator (6) to the conditions of the medium-pressure separator (11), and the control valve (9) can operate at a pressure in the range of 2 bar absolute pressure to 100 bar absolute pressure, more preferably in the range of 5 bar absolute pressure to 50 bar absolute pressure, and even more preferably in the range of 10 bar absolute pressure to 35 bar absolute pressure.

[0085] The gaseous effluent (E) leaving the head of the medium-pressure separator (11) is sucked in by one or more ejection devices (ejector pump, 5). The pressure of the medium-pressure separator (11) is regulated by a regulating valve (10).

[0086] The heavy polymer phase, which after throttling in valve (12) forms stream H, contains mainly polymer and / or possible copolymer and a portion of unreacted monomer and / or possible comonomer, oil and solvent. The portion of unreacted monomer and / or possible comonomer can vary between 1% and 10% by mass of the polymer fraction.

[0087] The control valve (12) allows throttling of the repolymerized phase (which forms stream H after throttling) from the conditions of the medium-pressure separator (11) to the conditions of the low-pressure separator (13), which throttling can be carried out at a pressure in the range of 0.01 bar absolute to 5 bar absolute, preferably in the range of 1.0 bar absolute to 2.5 bar absolute, even more preferably in the range of 1.25 bar absolute to 2.0 bar absolute.

[0088] The gases released in the low-pressure separator, which are released from the polymer, can be recovered in the reaction after being compressed.

[0089] After separation at low pressure, the polymer and any copolymer produced can be pelletized in an extrusion and cutting system (also referred to as a pelletizing system, 14). The pelletizing system can include one or more single screw extruders and / or one or more multi-screw extruders and / or one or more gear pumps (melt pumps) that press the molten polymer toward a die plate for subsequent cutting and cooling to form polymer pellets.

[0090] One or more injections of additives may be provided within the extrusion and cutting system (14) to impart specific properties to the product, such as resistance to hot or cold oxidation when exposed to air.

[0091] The polymer granules thus formed can be packaged and stored in various forms or transported to a conversion stage, after a possible homogenization stage of the polymer granules, which can consist, for example, in mixing granules produced at different times in order to obtain a product with homogeneous properties, and further degassing, which can consist in introducing air or nitrogen in order to remove any traces of monomers and / or comonomers still present in the polymer matrix.

[0092] The high-pressure ethylene polymerization process according to the invention is preferably carried out continuously and without applying periodic pulsations to the opening of the throttle valve (for example, instead using the impulse method of the process described in US 8,048,971 and US 8,455,606). In fact, it has been observed that due to the high operating pressures, the application of periodic pulsations does not bring technical advantages, but mechanically stresses the already highly stressed device.

[0093] It should be noted that the use of jet pumps in the applications described in US 8,048,971 and US 8,455,606 is easier to implement than that provided by the applicant in the present application, because the operating pressure of the tubular reactor subjected to pulsation is generally higher than the operating pressure of the autoclave reactor and reported in the present patent application, giving the driving fluid a greater driving force to compress the exhaust gas relative to the pressure of the reaction effluent from the autoclave reactor.

[0094] As regards the injection unit formed by one or more injection devices of the invention, the one or more injection devices are characterized in that they impart a sufficiently high momentum to the fluid (driving fluid) leaving the reactor (C) so that it can draw in the exhaust gases (D) and (E) entering the one or more devices.

[0095] In one embodiment, the use of at least two jet pumps (5) connected in series is provided, each jet pump (5) being equipped with two fluid inlets, wherein each ejector has only one entrained fluid: in practice, the effluent (C) of the reactor (driving fluid) and a portion of the high-pressure exhaust gas (D) (entrained fluid) enter the first jet pump, and the fluid leaving the first jet pump enters the second jet pump as driving fluid to suck in the medium-pressure exhaust gas (E).

[0096] In another embodiment, there is only one ejector, which is provided with two different inlets for the two entrained fluids: one or both entrained fluids may optionally be pressure-equalized before entering the ejector, for example by using one or more corresponding throttle valves, so that the pressures of the entrained fluids downstream of the valves become substantially the same, thereby ensuring that the lower pressure entrained fluid is directed to the outlet of the ejector pump instead of being discharged through the corresponding inlet.

[0097] In another embodiment, the use of a jet pump with an entrained fluid inlet is provided, wherein the entrained fluid is a mixture of two or more entrained fluids (exhaust gases) which have previously been combined into a single fluid: in this case, at least one throttle valve (not shown in the figure) can be provided upstream of the jet pump, which applies a higher pressure to the entrained fluid than the combined two fluids.

[0098] In order to better understand the present invention and its application scope, some embodiments are given below, but they are not intended to limit the scope of the present invention in any way.

[0099] These examples are to be understood as preferred embodiments of the method according to the invention.

[0100] Example

[0101] Three comparative examples 1, 2 and 3 are provided below for comparison with the results obtained in Examples 4, 5 and 6 according to the present invention.

[0102] These examples show the same flow rate and temperature at the outlet of the reaction system (3), and the comparison is mainly based on the following parameters:

[0103] οElectric energy consumption rate (unit consumption);

[0104] o the temperature (of stream F) in the high pressure separator (6), which is relevant in terms of maintaining product quality and continuity of operation; and

[0105] o From a thermodynamic point of view, thermodynamic irreversibility is advantageous as an indicator of the process, namely the compression and cooling process occurring in the injection device (5) (injection pump with three fluid inlets and upstream throttle valve for high pressure gas entry) as described in this patent application.

[0106] The comparative example has been specified, the flow rate of ethylene from the secondary compression (2) to the reaction system (3) is the same and equal to 72000 kg / h. In all examples, the temperatures entering and leaving the reaction system are considered the same and equal to 40°C and 280°C respectively, these two temperatures leading to a polymer production of 13,000 kg / h considering the adiabatic polymerization reaction.

[0107] For all examples, the make-up ethylene entering the system (stream A) was taken at a pressure of 71 bar.

[0108] The exhaust gas flow rate that must be compressed from the pressure of the low-pressure separator ( 13 ) (equal to 1.3 bar a) to the pressure of the medium-pressure separator ( 11 ) is the same for all embodiments and equal to 500 kg / h.

[0109] For the calculation of the power consumption rate, the following factors are considered:

[0110] ○Primary compression / boosting (compressing ethylene only), the power consumption rate is:

[0111] 0.0037 kW h / kg ethylene / bar pressure increment - for compression in the pressure range of 1.3 bar a to 31 bar a

[0112] 0.0006 kWh / kg ethylene / bar pressure increment - for compression in the pressure range of 31 bar a to 71 bar a

[0113] 0.00015 kW h / kg ethylene / bar pressure increment - for compression in the pressure range from 71 bar to the suction pressure at the secondary compression (assumed to be equal to the pressure of streams (D) and (J), i.e. equal to the pressure of the high pressure separator (6) minus 30 bar to take into account the pressure drop in the high pressure recycle (7))

[0114] o Secondary compression (compression of ethylene only) electrical energy consumption is 0.0000775 kWh / kg ethylene / bar pressure increment - for compression from the secondary compression suction pressure (assumed to be equal to the feed stream pressures (D) and (J), or equal to the pressure of the high-pressure separator (6) minus 30 bar to take into account the pressure drop in the high-pressure recycle (7)) to the outlet pressure of the secondary compression, which is equal to 1601 bar absolute (equal to the reaction pressure) and is kept the same in all examples.

[0115] o The electrical energy consumption rate for extrusion of LDPE polymer is assumed to be 100 kWh / 1000 kg of polymer produced.

[0116] In order to take into account any additional electrical energy consumption (eg cooling water circulation), an additional electrical energy consumption equal to 150 kW h / 1000 kg of polymer produced has been assumed in all examples.

[0117] For the calculation of the temperature including the high-pressure separator (6), the system is considered to be adiabatic and consists of: the reaction system (3), the throttle valve (4), the jet pump (5), the high-pressure separator (6), the throttle valve (9), the medium-pressure separator (11), the control valves (8) and (10) and the related interconnecting pipelines.

[0118] Thermodynamic irreversibility is calculated as the difference in entropy flow between the outgoing and incoming flows into a system consisting of a throttle valve (4) optionally combined with an injection device (ejector pump, 5), optionally ignoring the presence of a cooler (17).

[0119] The entropy flow is calculated as the product of the mass flow rate and the mass entropy of the relevant stream (calculated under the operating conditions shown in the tables (Tables 2 to 7)).

[0120] Similarly, the enthalpy flow is also calculated as the product between the mass flow rate and the mass enthalpy of the relevant stream (calculated under the operating conditions shown in the tables (Tables 2 to 7)).

[0121] The calculations of entropy and mass enthalpy consider only the fluid consisting of ethylene.

[0122] Thermodynamic properties have been calculated using the methods described in VV Sychev, AA Vasserman, EA Golovsky, AD Kozlov, GA Spiridonov, VA Symarny, “THERMODYNAMIC PROPERTIES OF ETHYLENE (National Standard Reference Data Service of the USSR)” (1987), ISBN 9783540176336—in particular, using the equation of state reported in Chapter 2 (pages 43 to 49) and the parameters of the equation of state and specific heats reported in Chapter 3 (pages 51-53, 56-57).

[0123] The first comparative example shows a configuration that does not include a jet pump device (as shown in FIG. 3 ).

[0124] The second comparative example follows the teaching of patent GB 1540894, in which only part of the exhaust gas (D) leaving the top of the high-pressure separator (6) and possibly cooled in the high-pressure recirculation (7) is taken to cool the reactor output product (C), see the configuration shown in Figure 4.

[0125] The third comparative example follows the teaching of patent US Pat. No. 7,837,950, in which a cooling device (cooler, 17) can be inserted between the reactor (3) and the jet pump (5) (structure shown in FIG. 5).

[0126] In the description of the following embodiments, reference will be made to the following numbers of the material streams involved.

[0127] Table 1

[0128]

[0129] Comparative Example 1 (C1)

[0130] The process flow of this comparative example is shown in FIG3 .

[0131] The equipment layout is consistent with the present invention and Figure 1 The difference shown is that no jet pump (5) is installed, and streams (D) and (E) and corresponding control valves (8) and (10) shown in Table 1 are also absent.

[0132] In addition, the device layout is consistent with the present invention and Figure 1 The difference shown is that the exhaust gas leaving the medium pressure separator must be cooled by a suitable heat exchange device (not shown in Figure 3) and recirculated to the booster / primary compressor suction port (part not shown in Figure 3) at a pressure lower than the secondary compressor suction pressure.

[0133] Stream (F) shows the increased temperature resulting from throttling by a valve which significantly reduces the pressure of stream (F).

[0134] Table 2 shows the mass and energy balance for this example.

[0135] Table 2

[0136]

[0137] Thermodynamic irreversibility of the system: 144633–130327=14306 W / °C (spontaneous process).

[0138] Electric energy consumption rate (per ton of polymer produced): 878kWh / t.

[0139] Comparative Example 2 (C2)

[0140] The process flow of this comparative example is shown in FIG4 .

[0141] The equipment layout is consistent with the present invention and Figure 1 The difference shown is that the stream (E) and the associated regulating valve (10) shown in Table 1 are not present.

[0142] In addition, the device layout is consistent with the present invention and Figure 1 The difference shown is that the exhaust gas leaving the medium-pressure separator must be cooled by a suitable heat exchange device (16) and recirculated to the suction port of the booster / primary compressor (1) at a pressure lower than the suction pressure of the secondary compressor (2).

[0143] Table 3 shows the mass and energy balance for this example.

[0144] Table 3: Case study of the system envisaged by the prior art (GB 1540894)

[0145]

[0146] Thermodynamic irreversibility of the system: 161134-(130327+15513)=15294W / °C (spontaneous process).

[0147] Electric energy consumption rate (per ton of polymer produced): 873kWh / t.

[0148] Comparative Example 3 (C3)

[0149] The process flow of Comparative Example 3 is shown in FIG5 .

[0150] In comparative example 3, there is an expensive high-pressure cooler (17) after the reactor (3) and after the throttle valve (4). The cooler (17) is followed by a jet pump (5): according to this solution, the jet pump (5) compresses the exhaust gas leaving the second separator (11), thereby improving the energy balance (see US 7,837,950).

[0151] Table 4 shows the mass and energy balance for this example.

[0152] Table 4: Reference case with a system similar to the solution provided by the patent (US 7,837,950) but without a product cooler

[0153]

[0154]

[0155] Thermodynamic irreversibility of the system: 153842-(130327+10031)=13484 W / °C (spontaneous process).

[0156] Electric energy consumption rate (per ton of polymer produced): 860kWh / t.

[0157] Example 4 (Ex 4)

[0158] With reference to the process flow of Example 4 Figure 1 and Figure 2 shown.

[0159] Ethylene (A) and any optional comonomer (B) are compressed by a booster / primary compression step (1) to the suction pressure of a secondary compressor (2), equal to about 261 bar a and 40° C. The secondary compressor (2) compresses the monomers (and any comonomers) to the reaction pressure, equal to about 1601 bar a. The monomers (and possible comonomers) compressed in the secondary compressor are conveyed to a reaction device (3). The reaction device (3) is a stirred autoclave type reactor, which consists of at least two reaction zones operated in an almost adiabatic state. A polymerization initiator is conveyed to the reactor. The polymerization initiator can be fed with the gas leaving the secondary compressor or added directly to the reactor via a dedicated injector. At the bottom of the reaction device (3) or after a longer or shorter pipeline section which may have a thermostat / cooling jacket, there is a throttle valve (4) (also called a pressure reducing valve). Expansion of the reactant mixture (C) having a composition of 18% polymer and 82% unreacted monomers leaving the reactor (3) is carried out via a throttle valve (4). This expansion is associated with the temperature increase caused by the high temperature and high pressure conditions in the reactor (3). The mixture leaving the reactor (stream (C) of Table 1) is cooled by the waste gas (D) leaving the high-pressure separator (6) and cooled in the high-pressure recycle (7) via the jet pump (5). The waste gas (stream I) leaving the high-pressure separator (6) is actually cooled in the recycle (7). A portion (J) of the stream (I) is sent to the secondary compressor (2), while another portion (D) is sent to the jet pump (5) (stream (J) consists of unreacted monomers and is equal to about 75% of stream (C)).

[0160] The stream leaving the reactor (3) (stream (C) in Table 1) is also used as a driving fluid to compress the waste gas leaving the medium-pressure separator (11) (stream (E) in Table 1; stream (E) consists of unreacted monomers and is approximately 7% of stream (C)) via a jet pump (5).

[0161] In a high-pressure separator (6) operating at about 291 bar a and 270° C., which receives the outlet stream from the jet pump (5), a separation occurs between a light phase consisting of unreacted monomers (stream (I) of Table 1; stream (I) is equal to about 79% of the input stream (F) and consists almost entirely of unreacted monomers) and a polymer-rich heavy phase (which, after throttling in valve (9), will form stream (G) of Table 1; stream (G) is equal to about 21% of the input stream (F) and consists of 71% of polymer and the remaining 29% of unreacted monomers).

[0162] The light phase leaves the top of the high-pressure separator (6) and is cooled in the recycle (7). The conditions at the outlet of the recycle (7) are indicatively 261 bar a and 40° C. A portion of this stream (about 14.5%) cools the stream leaving the reactor (3) via a control valve (8) and an ejector pump (5), the remainder of this stream (J) (about 85.5%) being recycled by suction to the secondary compressor (2). The polymer-rich phase, which forms stream G after throttling, passes through a throttling valve (9) and is expanded in a medium-pressure separator (11) operating at about 31 bar a and 220° C., wherein another portion of unreacted monomers (light phase) is removed from the top of the separator, equal to about 27.5% of the inlet stream and consisting mainly of unreacted monomers. The offgas (E) of the medium-pressure separator (11) is sent in its entirety via a regulating valve (10) to an ejector pump (5) and then to the inlet of the high-pressure separator (6). The (heavy) polymer phase, which after throttling forms stream H, emerges from the bottom of the device (11) and is throttled by a throttle valve (12) to expand in a low-pressure separator (13) which is operated at 1.5 bar a and 215°C.

[0163] In the low-pressure separator (13), further unreacted monomers (light phase) are removed, which constitute about 1.5% of the feed and mainly consist of unreacted monomers, which are cooled to 1.2 bar a and 40° C. in the low-pressure recycle (15) and returned by suction to the booster / primary compressor (1), which compresses this stream together with make-up ethylene (A) to the suction pressure of the secondary compressor (2).

[0164] The (heavy) polymer phase is discharged from the bottom of the low pressure separator (13) and fed to the extruder and cutting system (also called pelletizing, 14).

[0165] The polymer granules are then further degassed by introducing air or nitrogen and subjected to a possible homogenization phase which consists in mixing the granules produced at different times in order to obtain a product with homogeneous properties. The polymer thus produced can then be stored or packaged for transport to the consumer.

[0166] Table 5 shows the mass and energy balance for this example.

[0167] Table 5

[0168]

[0169] Thermodynamic irreversibility of the system: 171607-(130327+15513+11410)=14357W / °C (spontaneous process).

[0170] Electricity consumption rate (per ton of polymer produced): 854 kWh / t.

[0171] Two further examples according to the invention are reported below, highlighting different operating conditions of the system. These examples are based on the arrangement described in Example 4, they differ in the operating temperature of the high pressure separator (6) and the operating temperature and pressure of the medium pressure separator (11).

[0172] Example 5 (Ex 5)

[0173] Table 6 shows the mass and energy balance for this example.

[0174] Table 6

[0175]

[0176] Thermodynamic irreversibility of the system: 171824-(130327+15513+12102)=13883W / °C (spontaneous process).

[0177] Electric energy consumption rate (per ton of polymer produced): 848kWh / t.

[0178] Example 6 (Ex 6)

[0179] Table 7 shows the mass and energy balance for this example.

[0180] Table 7

[0181]

[0182] Thermodynamic irreversibility of the system: 171555-(130327+11291+15513)=14425 W / °C (spontaneous process).

[0183] Electricity consumption rate (per ton of polymer produced): 854 kWh / t.

[0184] The results are reported in Table 8.

[0185] Table 8: Comparison between the Examples of the present invention (Ex4 to Ex.6) and the Comparative Examples (C1 to C3)

[0186]

[0187] The present invention simplifies the process, eliminating the expensive cooler ((17) in Figure 5) and the smaller capacity primary compressor / booster (1) present in reference US 7,837,950, thereby reducing investment and operating costs because the flow rate of gas that must be recompressed by the machine is reduced.

[0188] Furthermore, as shown in Table 8, it was found that the high pressure separator (6) was operated at a lower temperature than Comparative Examples 1 and 3, and at a temperature almost equal to that shown in Comparative Example 2, ensuring excellent product quality, especially in terms of optical properties, cross-linked polymers, and point defects, especially in products intended for use in film products such as transparent films, greenhouse films (for agriculture).

[0189] Compared with Comparative Example 2, the temperature of the high-pressure separator (6) is the same, but a reduction in the power consumption rate is achieved.

[0190] On the other hand, in terms of irreversibility, a slight decrease was determined between Example 4 and Comparative Example 2, however, compared with Comparative Examples 1 and 2, the hot pressing method was advantageous.

[0191] Likewise, it can be seen from Table 8 how the invention allows equalization of the consumption rates achieved in comparative example 3. The advantage is given by the possibility of using the outlet stream of the reactor (3) as driving fluid to compress the offgas leaving the medium-pressure separator (11).

Claims

1. A high pressure polymerization process of ethylene, optionally with at least one of its comonomers, to produce low density polyethylene, or an ethylene copolymer with at least one comonomer, preferably ethylene vinyl acetate (EVA) having a vinyl acetate (VA) content in the range of 2 to 50% by mass, comprising: · high pressure polymerization of ethylene, optionally with at least one comonomer thereof, in one or more reactors; Reduce the pressure of the reaction effluent; in one or more injection devices, after the pressure of the reaction effluent has been reduced, reducing the temperature of the reaction effluent relative to the temperature obtained by throttling (isenthalpic expansion) alone, preferably by an amount ranging from 1° C. to 50° C.; separation of the reaction effluent by means of at least three subsequent separation stages, the separations being operated at different pressures, producing at least three different off-gas streams (high-pressure off-gas, medium-pressure off-gas, low-pressure off-gas), each of which comes from a single separation stage, and recovering low-density polyethylene or optionally a copolymer thereof from the unreacted compounds in each separation stage; recirculating part of the offgas separated in the first stage (high-pressure offgas) and all of the offgas separated in the second stage (medium-pressure offgas) to one or more of the injection devices, recompressing the offgas at the pressure of the first separation stage, thereby cooling the reaction effluent; and Mechanical compression of the off-gas separated in the third stage (low-pressure off-gas), supplementary ethylene and optionally supplementary at least one comonomer thereof to the reaction pressure.

2. The process according to claim 1, wherein the reaction apparatus consists of at least one autoclave type reactor.

3. The method according to claim 2, wherein the reaction apparatus consists of an autoclave type reactor.

4. The process according to any one of claims 1 to 3, wherein the reaction pressure is from 1000 bar a to 3000 bar a.

5. The process according to claim 4, wherein the reaction pressure is from 1000 bar a to 2200 bar a, preferably from 1200 bar a to 1800 bar a.

6. The process according to any one of claims 1 to 5, wherein the reaction temperature is 150°C to 300°C.

7. The method according to claim 6, wherein the reaction temperature is 170°C to 280°C.

8. The method according to any one of claims 1 to 7, wherein the first separation stage is carried out at a pressure of 100 bar absolute to 500 bar absolute, the second separation stage is carried out at a pressure of 2 bar absolute to 100 bar absolute, and the third separation stage is carried out at a pressure of 0.01 bar absolute to 5 bar absolute.

9. The method according to claim 8, wherein the first separation stage is carried out at a pressure of 150 bar absolute to 300 bar absolute, the second separation stage is carried out at a pressure of 5 bar absolute to 50 bar absolute, and the third separation stage is carried out at a pressure of 1.0 bar absolute to 2.5 bar absolute.

10. The process according to any one of claims 1 to 9, wherein the pressure of the reaction effluent is reduced by throttling (isoenthalpic expansion).

11. The method according to any one of claims 1 to 10, wherein the comonomer is selected from the group consisting of vinyl acetate, methyl acrylate, ethyl acrylate, n-butyl acrylate, methyl methacrylate, acrylic acid, methacrylic acid.

12. The method according to any one of claims 1 to 11, wherein the portion of the high-pressure exhaust gas flowing to the injection device varies between 1% and 30% of the total exhaust gas.

13. The method according to any one of claims 1 to 12, wherein the portion of the high-pressure exhaust gas flowing to the injection device varies between 1% and 20% of the total exhaust gas.

14. Method according to any of the preceding claims, wherein the one or more injection devices are one or more injection pumps (5).

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