DENSITY POLYETHYLENE AND COPOLYMERS THEREOF"

Description

The present patent application relates to a process for the production of low-density polyethylene (LDPE) and/or copolymers thereof, for example, ethylene vinyl acetate (EVA) , with high-pressure technology.

Said process exploits, with a suitable recycling, part of the off-gases generated in the process itself for cooling the reaction effluent, and at the same time uses the enthalpy of the latter, improving the overall energy efficiency of the production cycle.

More particularly, the described and claimed process exploits a cooling and energy recovery system, which uses ejection devices (equivalently called jet-pumps or ejectors ) .

In the present patent application, all the operating conditions reported in the text must be understood as preferred conditions although not expressly stated.

For the purposes of the present description, the term "comprise" or "include" also includes the term "consist of" or "consisting essentially of" .

For the purposes of this discussion, the definitions of the intervals always include the extremes unless otherwise specified .

For the purposes of the present description, the values shown in "%" are always to be understood as % by mass.

The production plants of LDPE polymers and copolymers with high-pressure technology are expensive in terms of investment costs and operating costs. By high pressure technology for the production of LDPE polymers and copolymers we mean a process in which the polymerization takes place at a pressure of no less than 600 absolute bar (also referred to as bar a) .

The reaction effluent is in a supercritical phase because the reactor works at pressures ranging from 1000 absolute bar to 3000 absolute bar, and temperatures ranging from 150°C to 300°C. This mixture, which contains polymer mass (such as polyethylene) , copolymers and unreacted monomers (such as ethylene) must be separated, but in order to do this, the pressure must first be reduced.

For this purpose, the effluent mass is throttled at pressures comprised between 100 absolute bar and 500 absolute bar, with a consequent strong increase in temperature, which generates two problems:

• Product quality problems: the permanence of the product at high temperatures, e.g. over 200 °C in the case of some EVA (ethylene vinyl acetate) products, can lead to the formation of gels which worsen the optical properties and the workability in the transformation process of the finished product, especially products intended for films and coatings.

• Running continuity problems: the permanence of high temperatures inside the high-pressure separator can lead to the formation of hot spots, which can trigger a decomposition reaction of the mixture with consequent shutdown of the production plant and losses of production .

As a result, temperature and pressure control of the reaction effluent is a critical point in the ethylene polymerization plant.

Various solutions have been proposed in the state of the art for cooling the polymer, these solutions provide for the use of a cooler or other solutions for removing the heat . GB 1540894 discloses a process for the homopolymerization or copolymerization of ethylene (see Figure 4) , wherein ethylene or a mixture of monomers containing ethylene reacts in a reactor (3) at a pressure greater than 1000 bar a, the resulting mixture (C) passes through a valve (4) which reduces the pressure and then into a separator (6) which is at a pressure between 200 and 500 bar a. The mixture of polymer and monomers (C) is cooled by injection of recycled monomers (D) by means of a jet pump (5) placed between the throttling valve (4) and the separator (6) . The jet pump (5) works at a lower pressure than the separator ( 6 ) .

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

The off-gases (E) of the second separator (11) are pumped upstream of the separators but downstream of the throttling valve (4) thanks for example to jet pumps (5) . The reaction effluent flows through a jet pump (5) and acts as a driving fluid, creating the conditions for sucking off-gases produced by the second separator (11) located downstream. The reaction effluent (C) can also be fed to a cooling device (17) where a two-phase mixture is formed, wherein one phase is polymer-rich and one phase is polymer-poor. This two-phase mixture is fed to a jet pump (5) as driving fluid, creating the conditions for sucking the off-gases produced by the second separator (11) .

US 8, 048, 971 discloses a process for the high-pressure polymerization of polyethylene and copolymers thereof in a tubular reactor which is subjected to pulsations (the pressure pulsations are 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 off-gases from the first separator into the reaction product, thereby cooling it. The mixture consisting of the cooled reaction product and a portion of the cooled off-gases is sent to the separation consisting of two off-gas separators. The process described by US patent 8, 048, 971 does not provide for the use of an intermediate separator and the relative recovery of the off-gases by compression with a jet pump. US 8, 445, 606 discloses a system for increasing the capacity of an existing plant for the radical polymerization of olefins and/or reducing its specific power consumption. The plant includes a monomer compression section operating at a pressure of 1500-3500 bar, a reaction section in which the tubular reactors operating at 3000 bar are jacketed for cooling the polymerization reaction, and a downstream separation zone associated with a jet pump for cooling the reaction with the separated gases. The reaction products and unreacted compounds flow through a throttling valve to three separation stages. Part of the off-gases from the first separator and the second separator are cooled and pumped through a jet pump into the stream from the reactor after throttling. It is provided the application of periodic pressure pulsations ("bumping") to maintain the reactor clean, with the result of high mechanical stress on the devices which are already highly stressed due to the high operating pressure.

US 2013/171029 describes a system for the production of polyethylene (LDPE) equipped with a cooling system using jet pumps. In fact, the system comprises a first and a second compressor connected to each other, which feed a tubular polymerization reactor. From the first compressor part of the make-up monomer is sent to the jet pumps. The jet pumps cool the reaction product before it is separated in the separation section formed by two off-gas separators. The process described by US patent 2013/171029 does not provide for the use of an intermediate separator and the relative recovery of the off-gases by compression with a jet pump. Furthermore, a regulating valve is provided between the primary compression and the secondary compression, which further complicates the process scheme. To overcome the problems of the prior art, the Applicant proposes a high pressure process for the polymerization of ethylene and/or co-monomers thereof to produce low density polyethylene (LDPE) and/or copolymers thereof, which uses one or more ejection devices to cool the reaction effluent, and at the same time recover energy efficiency. Indeed, during the process off-gases are generated from the separation of the reaction effluent, which are sent to the ejection devices to cool the reaction effluent by thermal compression .

Said process comprises a step of reducing the pressure of the reaction effluent, followed by a step of cooling with energy recovery by the use of one or more ejection devices. Said process is particularly apt to produce not only low density polyethylene for films and coatings (e.g. films for greenhouses, blown films from blown of big size, expanded/f oamed products, coating on paper or other substrates) but also copolymer of ethylene and vinyl acetate whose content of vinyl acetate can be up to 50%, thanks to the control of temperature and pressure of the reaction effluent and of the polymer, which is performed by the combination of technical features as herein indicated .

Therefore, the subject of the present patent application is a high pressure polymerization process of ethylene, optionally with at least one co-monomer thereof , to produce low density polyethylene, or ethylene copolymers with at least one co-monomer, for example EVA (ethylene vinyl acetate) with a vinyl acetate content comprised in the range of 2 to 50%, which comprises:

• conducting a high pressure polymerization of ethylene, optionally with at least one co-monomer thereof, in one or more reactors;

• reducing the pressure of the reaction effluent;

• reducing the temperature of the reaction effluent, for example by an amount varying in the range from 1°C to 50°C, with respect to the temperature obtained by throttling alone (isenthalpic expansion) , after having reduced its pressure, in one or more ejection devices;

• submitting the reaction effluent to separation by means of at least three subsequent separation stages, operating at different pressures, generating at least three different off-gas streams (high pressure off-gas, medium pressure off-gas and low pressure off-gas) each from a single separation stage, and recovering in each stage of separation low density polyethylene and/or optionally copolymers thereof from unreacted compounds;

• recirculating part of the off-gas separated in the first stage (high pressure off-gas) and all the off-gas separated in the second stage (medium pressure off-gas) to one or more of said ejection devices, recompressing said off-gases at the pressure of the first separation stage and thus cooling the reaction effluent; and

• mechanically compressing the off-gases separated in the third stage (low pressure off-gas) and make-up ethylene and optionally make-up said at least one co-monomer thereof up to reaction pressure.

The part of the off-gas separated in the first separation stage (high pressure off-gas) which is not conveyed to the ejection device(s) , and which is not recirculated in the first stage, is conveyed to the mechanical compression: said part of off-gas includes not only off-gas separated from reaction effluent in the first stage but also offgases which have been previously recompressed thermally in said ejection devices at the pressure of the first separation stage.

In each of the separation stages it is performed the separation into two phase of the reaction effluent coming from the preceding stage: a light phase (off-gas) rich in unreacted monomers and/or comonomers, and a polymer-rich heavy phase which is then fed to the following separator operating under a lesser pressure in order to obtain an additional separation of the off-gases from the polymer- rich heavy phase.

In the present patent application, the term "low density polyethylene" is intended to identify a polyethylene conventionally known with the acronym LDPE, generally having a density comprised in the range which varies between 0.910 and 0.940 g/cm ³ , although this range of density is not limiting for the purposes of the present invention .

In the present patent application, the term "throttling" means a transformation that follows an isoenthalpic ( isenthalpic) behaviour.

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

In the present patent application, the term "copolymers thereof" means to identify the ethylene copolymers with at least one co-monomer such as for example vinyl acetate, methyl acrylate, ethyl acrylate, n-butyl acrylate, methyl methacrylate, acrylic acid, methacrylic acid.

Examples of ethylene copolymers are EVA (ethylene vinyl acetate) copolymers, preferably EVA copolymers having a vinyl acetate (VA) content in the range between 2 and 50% by mass.

Polyethylene and/or copolymers thereof prepared with the high pressure polymerization process of ethylene and/or co-monomers thereof as indicated above, generally have a melt mass-flow rate (MFR) in the range comprised between 0.1 and 500 g/10 ' , preferably between 0.15 and 110 g/10 ' , more preferably between 0.2 and 65 g/10 ' , measured at the condition 190°C/2.16 kg and according to the ISO 1133:2005 standard, although the aforementioned ranges of MFRs are not limiting for the purposes of the present invention.

The reaction effluent is in the gas phase and contains both ethylene polymers, or ethylene copolymers, and the unreacted monomer and possibly the unreacted comonomers.

In order to recover the polymer, the gaseous reaction effluent is subjected to various separation stages generating high-, medium- and low-pressure off-gases.

The Applicant conveys part of the high-pressure off-gas, optionally cooled, and all of the medium pressure off-gas into one or more ejection devices, for example jet pumps. The medium pressure off-gases and the part of the high- pressure off-gases conveyed to the jet pumps work as a heat carrier to cool the reaction effluent, while said reaction effluent works as a pressure carrier to compress the medium pressure gases bringing them to the pressure of the first separator.

This solution simplifies the polymerization plant because it avoids the use of a cooling system which is designed to operate at high pressures, i.e. at pressures equal to those of the reactor, and is dedicated to cooling the reaction effluent, and to avoid installation of a heat exchange device on the medium pressure off-gas recycling line.

Furthermore, thanks to the ejection devices, the compressors on the medium pressure off-gas line require less power and in general, the entire compression line is simplified and reduced in size. All of this has a positive impact in terms of reducing investment costs (CAPEX) and utility consumption (OPEX) .

It is also possible with this solution to increase the production capacity of existing plants since it allows to increase conversion in the reactor and to use existing compressors, which conventionally compress the off-gases from the second separator in the processes of the prior art, to compress make-up ethylene and/or comonomers thereof . The off-gases from the second separator can be recompressed by the solution proposed by the Applicant. This solution also allows the recovery of the vapors leaving the second separation stage, typically provided in a plant for the polymerization of ethylene to polyethylene, thus improving the overall energy efficiency of the process .

Ejection devices such as, for example, jet-pumps or ejectors, allow recovering energy of the separated gases by exploiting the thermal compression operated by the jet pumps. The pressure of the reaction effluents compresses medium and high-pressure off-gases to the pressure of the separation devices. Simultaneously medium and high- pressure off-gases, which are at a lower temperature than the reaction effluent, cool the latter as it passes through the jet pump. A first objective of the present invention is the reduction of specific energy consumption, due to recovery of the energy developed during the production of polyethylene. For example, a total of about 900 Megawatt hours (MWh) of energy can be saved per year for a lOOkt/y plant .

Another object of the invention is the reduction of the temperature of the fluid, e.g. effluent, to the high- pressure separator. This reduction allows a double advantage: it allows decreasing or eliminating defects in the polymer; and makes it possible to reduce probability of uncontrolled reactions with a consequent increase in operational continuity.

Furthermore, the invention described and claimed in the present patent application makes it possible to simplify the plant precisely thanks to the fact that no heat exchange devices are used, such as the coolers at the outlet of the reactor and/or the second separator and that allows the use of smaller size and power equipment such as compressors .

These and other objects and advantages are obtained with the invention described and claimed in the present patent application, and will appear more clearly from the following description and from the annexed figures, provided purely by way of non-limiting example, which represent preferred embodiments of the present invention. DESCRIPTION OF THE FIGURES

Figure 1 illustrates a preferred embodiment of the method according to the present invention;

Figure 2 illustrates in detail and in enlarged form the cooling section with jet pumps present in Figure 1;

Figure 3 illustrates a process diagram of the prior art which does not provide for the installation of the jet pump .

Figure 4 illustrates a second process diagram of the prior art which provides for the cooling of the off-gases leaving the medium pressure separator by means of heat exchange devices .

Figure 5 a third process diagram of the prior art which provides a heat exchange device (cooler) for cooling the effluent .

The system of an embodiment of the invention illustrated in Figure 1 has a primary mechanical compression system (booster) (1) and secondary mechanical compression system (2) ; one polymerization reaction device (3) , three throttling valves (4, 9, 12) , one temperature control valve (8) of high pressure separator, one pressure control valve of medium pressure separator (10) , an ejection device (jet pump (5) ) , a separation system operating in the pressure range from 100 to 500 bar absolute (high pressure separator, 6) , a separation system operating in the pressure range from 2 to 100 bar absolute (medium pressure separator, 11) , a separation system operating in the pressure range from 0.01 to 5 bar absolute (low pressure separator, 13) , a pelletizing device (14) ; two recycling of the gaseous phase: high pressure recycling (7) leaving the high pressure separator (6) and low pressure recycling (15) leaving the low pressure separator (13) . In the diagram illustrated, ethylene (A) and one of its comonomers (B) are compressed in the compression systems (1, 2) , then react to produce a reaction mixture (C) which is throttled

(4) and sent to the separators (6, 11, 13) . The reaction mixture (C) leaving the reactor (3) is throttled (4) and constitutes the driving fluid for the ejection device (jet pump, 5) . The mixture leaving the jet pump (5) enters the high-pressure separator (6) where a separation of heavy phase, rich in polymer, and a light phase, rich in unreacted monomers and/or co-monomers takes place. After the first separation, the heavy phase is throttled, while the gaseous phase is cooled (7) and partly recirculated towards the secondary compression system and partly towards the jet pump (5) . The heavy phase leaving the high-pressure separator (6) is throttled (9) before entering the medium pressure separator (11) where separation of a light phase is recycled through a control valve (10) to the jet pump

(5) and a polymer-rich heavy phase which is throttled (12) before entering the low pressure separator (13) . The gaseous phase leaving the low-pressure separator (13) is cooled in the low-pressure recycle (15) , with possible separation of a condensed phase consisting of solvent, comonomers, compressor lubricating oils and very low molecular weight polymer, and recirculated to the primary compressor system (booster) .

Detailed description.

The Applicant now describes in detail the process object of the present patent application also with reference to Figure 1.

The ethylene and/or comonomers thereof (reactants) are mechanically compressed through 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. Said reactants can be compressed up to a pressure comprised between 1000 bar absolute and 3000 bar absolute, preferably between 1000 bar absolute and 2200 bar absolute, more preferably between 1200 bar absolute and 1800 bar absolute.

Preferred comonomers can be selected from vinyl acetate, methyl acrylate, ethyl acrylate, normal butyl acrylate, methyl methacrylate, acrylic acid, methacrylic acid.

Chain modifiers can be added to the reactants; said modifiers can preferably be selected from 1-butene, butane, propylene, propionaldehyde and propane.

Compressor systems, especially those starting from lower pressures, involve a significant energy consumption in terms of electricity. Once compressed, the reactants are reacted in a reaction device (3) according to a chain-type, radical and initiator-promoted polymerization reaction. Said reaction device is preferably an autoclave-type reactor, sometimes also referred to as a vessel reactor. This reactor consists of a high-pressure vessel equipped with a stirring system, which allows the homogenization of the reactant mixture. The reactor can include one or more feeds of unreacted gas and one or more injections of polymerization promoter (s) . The autoclave-type reactor can also provide internal diaphragms, which allow the volume of reactions to be divided into smaller zones within which the desired temperature profiles can be formed to give the product (ethylene homopolymers and copolymers) particular optical and mechanical properties.

The autoclave reactor is particularly advantageous with respect to the tubular reactors since it works under constant pressure rather than under constant pulses of overpressure (bumping) , e.g. 700 bar over the reaction pressure every 40 seconds.

Moreover, the use of autoclave reactors is advantageous since they show less fouling, the polymerization can be carried out using pressures lower than those used in the tubular reactors, in addition to the fact that autoclave reactors allow to obtain grades of polymers apt to form some type of films and extrusion coatings.

In fact, the continuous stirring inside the autoclave keeps the polymerized mass at a uniform temperature and keeps the component concentrations constant inside the reaction zone, e.g. monomer (s) , polymer, polymerization initiator and chain transfer.

Moreover, the lower temperatures and pressures usable in the autoclave reactor can positively affect the polymer properties, e.g. optical properties particularly crucial for applications such as transparent films, films for greenhouses .

Preferred chain initiators are compounds labile at the reaction temperature and capable of supplying free radicals capable of promoting the polymerization reaction. More preferred initiators are selected from organic peroxides and peresters and oxygen.

The polymerization reaction can take place at temperatures comprised between 150°C and 300°C, preferably between 170°C and 300°C, more preferably between 170°C and 280°C.

The reaction pressure in the polymerization is generally between 1000 bar a and 3000 bar a, pref erably between 1000 bar a and 2200 bar a, more preferably between 1200 bar a and 1800 bar a.

The reaction pressure coincides, even in its preferred ranges, with the compression pressure of the reactants previously indicated.

At these temperature and pressure conditions, the reactant mixture is in a homogeneous phase called supercritical phase .

The reaction effluent (C) contains polyethylene and/or ethylene co-polymers, unreacted monomers, such as for example ethylene and/or co-monomers thereof .

The pressure of the reaction effluent must be reduced before entering the ejection devices.

A preferred way to reduce the pressure of the reaction effluent is to use a throttling valve (4) located in the bottom of the reaction device, sometimes referred to as a let-down valve. After the ejection devices (5) , the reaction effluent (C) combined with part of the gas (D) leaving the high pressure recycler (7) and with the offgases (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 and 100 bar absolute, more preferably between 300 bar absolute and 150 bar absolute, even more preferably between 290 bar absolute and 200 bar absolute. The pressure measurement method is not binding for the purposes of the present invention, and can be measured, for example, with stream gauges, although other methods known in the art can be used without thereby departing from the scope of the present invention.

Inside the high-pressure separator the temperature is generally kept in the range of 290 e 180 °C. Generally temperatures closer to 290°C are reached during the production of ethylene homopolymers while temperatures closer to 180°C are generally maintained during the production of ethylene copolymers.

By reducing pressure the reactant mixture becomes two-phase forming a phase rich in polymer and a phase less rich in polymer. The pressure reduction by throttling of the reaction effluent starting from the high-pressure conditions, which occur inside the reaction device (3) , is accompanied by a temperature rise. This increase can lead to two problems:

■ a product quality problem: the permanence of the product at high temperatures can lead to cross-linking of the polymer and the formation of gels (or fish-eyes) which worsen the optical properties of the product, in particular of film and coating grades.

■ a running continuity problem: the permanence of high temperatures inside the high pressure separator can lead to the formation of hot spots which can lead to the decomposition of the reactant mixture with consequent shutdown of the plant and loss of production.

Once its pressure has been reduced, the reaction effluent flows into at least one ejection device (jet pump, 5) : the effluent (C) leaving the reaction device (3) is used as a driving fluid to compress the gas phase (off-gases, E) from the medium pressure separator (11) up to the pressure of the high pressure separator (6) . The gas stream (E) leaving the medium pressure separator (11) thus re-enters the vapor cycle of the high-pressure separator ( 6 ) .

Simultaneously part of the off gases (D) leaving the high- pressure recycle (7) are used to cool the reaction effluent

(C) containing the polymer produced. This takes place inside the ejection device (5) by mixing the reaction effluent (C) with the stream (D) leaving the high-pressure recycle (7) before entering the first separator (6) . The portion of off-gas or recycle gas leaving the first separator (6) , i.e. high-pressure off-gas, which is conveyed into the ejection device (jet pump, 5) to cool the reaction mixture leaving the device of reaction (3) can vary between 1% and 30% with respect to the total offgases, more preferably between 1% and 20% of the total offgases and even more preferably between 5% and 15 % of total off-gases .

Two regulating valves (8) and (10) can be introduced in order to control the temperature of the high pressure separator (6) by regulating the flow rate of cooling gas

(D) leaving the high pressure recycle (7) and with the purpose of regulating the pressure in the medium pressure separator (11) .

The reaction effluent (C) leaving the reaction device (3) can contain a polymeric fraction (i.e. all ethylene polymers and copolymers possibly present) variable in the range between 8% and 22% with respect to the total mass of the effluent, more preferably ranging in the range between 8% and 20% with respect to the total mass of the effluent and even more preferably between 10% and 18% with respect to the total mass of the effluent. The separation of a heavy phase (which, after throttling in the valve (9) , will form the stream G) rich in polymer, but still containing an aliquot of monomers and/or possibly unreacted co- monomers, and a light phase (off-gas, I) poor in polymer and rich in unreacted monomers and/or optionally comonomers takes place in the high-pressure separator (6) . The aliquot of unreacted monomers contained in the heavy phase (which, after throttling in the valve (9) , will form the stream G) can preferably vary between 20% and 60% by mass with respect to the total mass of the polymeric fraction. This stream G mainly contains polymer and/or optional copolymer, low molecular weight polymer and/or copolymer, unreacted monomers and/or co-monomers, peroxide decomposition products, oils and solvents. The monomers and/or any unreacted co-monomers (light phase, I) are cooled in the high-pressure recycler (7) and sent, in part, to the secondary compression system (2) .

A control valve (9) allows throttling of the heavy phase (which, after throttling, forms the stream (G) ) from the conditions of the high pressure separator (6) up to those of the medium pressure separator (11) which can work at a pressure in the range comprised between 2 bar absolute and 100 bar absolute, more preferably in the range between 5 bar absolute and 50 bar absolute, still more preferably in the range between 10 bar absolute and 35 bar absolute.

The gaseous effluents (E) leaving the head of the medium pressure separator (11) are sucked in by the ejection device (s) (jet pump(s) , 5) . The pressure of the medium pressure separator (11) is regulated by the regulating valve (10) .

The heavy polymeric phase (which, after throttling in the valve (12) , forms the stream H) mainly contains polymer and/or possibly copolymer and an aliquot of unreacted monomers and/or possibly comonomers, oils and solvents. The aliquot of unreacted monomers and/or possibly comonomers can vary from 1% to 10% by mass of the polymeric fraction . A control valve (12) allows the throttling of the heavy polymeric phase (which, after throttling, forms the stream H) from the conditions of the medium pressure separator (11) up to those of the low pressure separator (13) which can take place at a pressure in the range between 0.01 bar absolute and 5 bar absolute, preferably in the range between 1.0 bar absolute and 2.5 bar absolute, even more preferably in the range between 1.25 bar absolute and 2.0 bar absolute.

The gas evolved in the low-pressure separator (which is released from the polymer) can be recovered in the reaction after being compressed.

After separation at low pressure, the polymer and any copolymers produced can be pelletized in an extruder and cutting system (also called pelletizing system, 14) . Said 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-pump) , which compress the melted polymer towards the die plate for subsequent cutting and cooling with formation of polymer pellets.

One or more injections of additives can be provided, inside the extrusion and cutting system (14) in order to give the product particular properties, e.g. resistance to cold and hot oxidation in case of exposure to air.

After a possible homogenization phase of the polymeric granules, which can consist, for example, in mixing the granules produced at various times in order to obtain a product with uniform characteristics and further degassing, which consists in introducing air or nitrogen in order to remove any traces of monomers and/or comonomers still present in the polymeric matrix, the polymer granules thus formed can be packaged in various forms and be stored or shipped to the transformation stages. The process for the high-pressure ethylene polymerization according to the present invention is preferably carried out continuously and without applying periodic pulsations to the openings of the throttling valves (for example, as instead carried out with the bumping method of the process described in US 8, 048, 971 and in US 8, 455, 606) . It has in fact been observed that the application of periodic pulsations does not lead to technical advantages but rather mechanically stresses devices that are already highly stressed, due to the high operating pressures.

It should be noted that the use of a jet pump for the application described in US 8, 048, 971 and in US 8, 455, 606 is easier to implement than that provided by the Applicant in the present application, since the working pressures of the tubular reactor subject to pulsations are normally higher than the working pressures for the autoclave reactor and reported in this patent application, giving the driving fluid a greater driving force to compress the off-gases with respect to that of a reaction effluent coming from an autoclave reactor.

With regard to the ejection unit formed by one or more ejection devices of the present invention, the feature of one or more ejection devices is to impart a sufficiently high momentum to the fluid leaving the reactor (C) (driving fluid) so that it can suck the off-gases (D) and (E) entering one or more devices.

In one embodiment, the use of at least two jet pumps (5) in series is provided, each equipped with two fluid inlets, wherein there is only one entrained fluid per ejector: in practice the effluent (C) of the reactor (driving fluid) and a portion (D) of the high pressure off-gases (entrained fluid) enter the first jet pump and the fluid leaving said first jet pump enters the second jet pump as the driving fluid to suck in medium pressure off-gases (E) . In another embodiment only one ejector is provided with two distinct inlets for two entrained fluids: one or both entrained fluids can optionally be equalized in pressure before entering the ejector, for example by using one or more respective throttling valves, so that the pressure of the entrained fluids downstream of the valves becomes substantially the same, thus ensuring that the lower pressure entrained fluid is drawn towards the outlet of the jet pump rather than pushed out through the respective inlet .

In another embodiment, the use of a jet pump with an inlet for the entrained fluid is provided where said entrained fluid is the mixture of two or more entrained fluids (offgases) which have previously been combined into a single fluid: in this case at least one throttling valve (not shown in the figures) can be provided upstream of the jet pump applied to the fluid entrained at a higher pressure than the two fluids being joined.

Some examples are given below for a better understanding of the invention and of the scope of application while not constituting in any way a limitation of the scope of the present invention.

The examples are to be understood as preferred embodiments of the process according to the present invention. EXAMPLES

Three comparative examples 1, 2 and 3 are provided below, to be compared with what is obtained according to the invention in examples 4, 5 and 6.

The examples are shown with the same flow rate and temperature at the outlet of the reaction system (3) and are compared mainly taking into consideration the following parameters : o specific consumption of electrical energy; o temperature in the high pressure separator (6) (current F) , related to aspects of maintaining product quality and operational continuity; And o thermodynamic irreversibility as an indicator to the described process, i.e. the compression and cooling process taking place in the ejection device (5) as described in the present patent application (jet pump with three fluid inlets and an upstream throttling valve of the entry of high pressure gases) is favored from a thermodynamic point of view.

The comparative examples have been elaborated taking a flow rate of ethylene from the secondary compression (2) fed to the reaction system (3) constant and equal to 72000 kg/h. The temperatures entering and leaving the reaction system were considered constant in all examples and equal to 40°C and 280°C respectively, which, considering the adiabatic polymerization reaction, lead to a production of 13, 000 kg/h of polymer.

The ethylene make-up (stream A) entering the system was taken at a pressure of 71 bar a for all the examples.

The off gas flow rate which must be compressed from the pressure of the low pressure separator (13) (equal to 1.3 bar a) up to the pressure of the medium pressure separator (11) , is constant for all the examples and equal to 500 kg/h .

For the calculation of specific electricity consumption, the following factors were considered: o Primary compression/booster (compression of ethylene only) taking a specific electricity consumption of :

• 0.0037 kW h/kg of ethylene/bar of pressure increase - for compression in the pressure range from 1.3 bar a to 31 bar a

• 0.0006 kWh/kg ethylene/bar of pressure increase - for compression in the pressure range from 31 bar a to 71 bar a • 0.00015 kW h/kg of ethylene/bar of pressure increase - for compression in the pressure range from 71 bar to the suction pressure at the secondary compression (assumed equal to the pressure of streams (D) and (J) , i.e. equal at the pressure of the high pressure separator (6) reduced by 30 bar to take into account the pressure drops in the high pressure recycle (7) ) o Secondary compression (compression of ethylene only) taking an electrical energy consumption of 0.0000775 kWh/kg ethylene/bar pressure rise - for compression from secondary compression suction pressure (assumed equal to stream pressure (D) and (J) , or equal to the pressure of the high pressure separator (6) reduced by 30 bar to take into account the pressure drops in the high pressure recycle (7) ) up to the outlet pressure from the secondary compression equal to 1601 absolute bar (equal to the reaction pressure) held constant in all the examples. o Extrusion of a LDPE polymer assuming a specific electricity consumption of 100 kWh/1000 kg of polymer produced .

In order to take into account any other electrical energy consumption (e.g. cooling water circulation) , in all the examples additional electrical energy consumption equal to 150 kW h/1000 kg of polymer produced has been assumed.

For carrying out the calculations including the temperature of the high pressure separator (6) , the system consisting of : reaction system (3) , throttling valve (4) , jet pump (5) , high pressure separator (6) , throttling valve

(9) medium pressure separator (11) , control valves (8) and

(10) and relative interconnection lines; was considered adiabatic .

The thermodynamic irreversibility is calculated as the difference of the entropic flows between the outgoing currents and the entering currents to the system consisting of the throttling valve (4) optionally combined with the ejection device (jet pump, 5) , optionally ignoring the presence of coolers (17) .

The entropic flows are calculated as the product of mass flow rate of the relevant stream and mass entropy (calculated under the operating conditions indicated in the tables (tables 2 to 7) ) .

Similarly, enthalpy flows are also calculated as the product between mass flow rate of the relevant stream and mass enthalpy (calculated under the operating conditions indicated in the tables (tables 2 to 7) ) .

Entropy and mass enthalpy are calculated considering flows composed of ethylene only.

The thermodynamic properties have been calculated with the methods indicated in the following book: VV Sychev, AA Vasserman, EA Golovsky, AD Kozlov, GA Spiridonov, VA Tsymarny, "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 (from page 43 to page 49) and the parameters of the equation of state and specific heats reported in chapter 3 (pages 51-53, pages 56 -57) .

The first comparative example shows a configuration (shown in Figure 3) that does not include the jet pump installation .

The second comparative example follows the teachings of the patent GB 1540894 where only an aliquot of the offgases (D) leaving the head of the high-pressure separator (6) and possibly cooled in the high-pressure recycle (7) is taken to cool the reactor output product (C) , see configuration shown in Figure 4.

The third comparative example follows the teachings of the patent US 7, 837, 950 where a cooling device (cooler, 17) can be inserted between the reactor (3) and the jet pump (5) (configuration illustrated in Figure 5) .

In the description of the examples below, reference will be made to the following numbering of the material flows involved .

Table 1

COMPARATIVE EXAMPLE 1 (Cl) The scheme of the process referring to this comparative example is shown in Figure 3.

The plant layout differs from that provided in the present invention and illustrated in Figure 1 in that the jet pump (5) is not installed and the currents (D) and (E) shown in Table 1 and the respective control valves (8) and (10) are not present .

In addition, the plant layout differs from that provided in the present invention and illustrated in Figure 1 in that the off-gases leaving the medium pressure separator (not illustrated in Figure 3) must be cooled with appropriate heat exchange devices and recycled into the booster /primary compressor suction at a lower pressure than the secondary compressor suction pressure (part not shown in Figure 3 ) .

The stream (F) shows an increased temperature generated by the throttling through the valve that has decreased noticeably the pressure of the stream (F) .

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

Table 2

Thermodynamic irreversibility of the system: 144633 -

130327 = 14306 W/°C (spontaneous process) .

Specific electricity consumption (per ton of produced polymer) : 878 kWh/t.

COMPARATIVE EXAMPLE 2 (C2)

The scheme of the process referring to this comparative example is shown in Figure 4.

The plant arrangement differs from that proposed in the present invention and illustrated in Figure 1 in that the current (E) shown in Table 1 and the relative regulating valve (10) are not present.

In addition, the plant layout differs from that proposed in the present invention and illustrated in Figure 1 in that the off-gases leaving the medium pressure separator must be cooled with appropriate heat exchange devices (16) and recycled in the suction to the booster /primary compressor (1) at a lower pressure than the suction pressure of the secondary compressor (2) .

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

Table 3: Case referring to the system as envisaged by the prior art (GB 1540894)

Thermodynamic irreversibility of the system: 161134- (130327+15513) = 15294 W/°C (spontaneous process) . Specific electricity consumption (per ton of polymer produced) : 873 kWh/t.

COMPARATIVE EXAMPLE 3 (C3)

The scheme of the process referring to the comparative example 3 is shown in Figure 5.

In the comparative example 3, an expensive high-pressure cooler (17) positioned after the reactor (3) and after the throttling valve (4) is present. The cooler (17) is followed by the jet pump (5) : according to this scheme, the jet pump (5) compresses the off gas leaving the second separator (11) , thus improving the energy balance (see US 7, 837, 950) .

Table 4 shows the mass and energy balance for this example. Table 4: Case referring to the system with a scheme similar to that provided by the patent (US 7, 837, 950) but without product -cooler

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

Specific electricity consumption (per ton of polymer produced) : 860 kWh/t.

EXAMPLE 4 (Ex 4)

The schemes of the process referred to example 4 are shown in Figure 1 and in Figure 2.

Ethylene (A) and any optional co-monomers (B) are compressed through the booster /primary compression step

(1) up to the suction pressure of the secondary compressor

(2) , equal to approximately 261 bar a and 40°C. The secondary compressor (2) compresses the monomers (and any co-monomers) up to the reaction pressure, equal to about 1601 bar a. The monomers (and possible co-monomers) compressed in the secondary compressor are fed to the reaction device (3) . The reaction device (3) is a reactor of the stirred autoclave type consisting of at least two reaction zones operating in an almost adiabatic regime. The polymerization initiators are fed into the reactor. The polymerization initiators can be fed either together with the gas leaving the secondary compressor or directly inside the reactor through special injectors. On the bottom of the reaction device (3) or after a more or less long section of pipe that may have a thermostat /cooling jacket, the throttling valve (4) also called Let-Down Valve is present. An expansion of the reactant mixture (C) leaving the reactor (3) , having a composition of 18% polymer and 82% unreacted monomers, takes place through the throttling valve (4) . This expansion is associated with a rise in temperature caused by high temperature and pressure conditions inside the reactor (3) . The mixture leaving the reactor (stream (C) of Table 1) is cooled by the off gases (D) leaving the high-pressure separator (6) and cooled in the high-pressure recycle (7) , by means of the jet pump (5) . The off-gases (current I) leaving the high-pressure separator (6) are in fact cooled in the recycle (7) . A part (J) of the stream (I) is forwarded to the secondary compression (2) while the other part (D) is sent to the jet pump (5) (the stream (J) consists of unreacted monomers and is equal to about 75% current (C) ) .

The stream leaving the reactor (3) (current (C) of table 1) also acts as a driving fluid to compress the off-gases leaving the medium pressure separator (11) (current (E) of table 1; the stream (E) consists of unreacted monomers and is approximately 7% of the stream (C) ) via the jet pump (5) .

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

The light phase leaves the head 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. Part of this stream (about 14.5%) cools the stream leaving the reactor (3) via the control valve (8) and the jet pump (5) , the remaining part (J) , about 85.5%, is recycled in suction to the secondary compressor (2) . The polymer-rich phase (which, after throttling, forms the stream G) passes through the throttling valve (9) and expands in the medium pressure separator (11) , operating at conditions of approximately 31 bar a and 220°C, where the removal of a further part of the unreacted monomers from the head of the separator takes place (light phase) equal to about 27.5% of the inlet stream and mainly consisting of unreacted monomers. The off-gases (E) of the medium pressure separator (11) are entirely sent to the jet pump (5) through the regulating valve (10) and then sent to the inlet of the high-pressure separator (6) . The (heavy) polymeric phase (which, after throttling, forms the stream H) comes out of the bottom of the apparatus (11) and is throttled by the throttling valve (12) to expand in the low pressure separator (13) which operates at conditions of 1.5 bar a and 215°C.

In the low pressure separator (13) there is a further removal of the unreacted monomers (light phase) , which constitute about 1.5% of the feed and which mainly consist of unreacted monomers, which are cooled in the low pressure recycle (15) up to conditions of 1.2 bar a and 40°C, return on suction to the booster /primary compressor (1) which compresses this current, together with the make-up ethylene (A) up to the suction pressure of the secondary compressor (2) .

The (heavy) polymeric phase exits from the bottom of the low-pressure separator (13) and feeds the extruder and cutting system (also called pelletizing, 14) .

The polymer pellets are then destined for further degassing, obtained by introducing air or nitrogen, and for a possible homogenization phase, which consists in mixing pellets produced at different times, in order to obtain a product with uniform properties. Subsequently, the polymer thus produced can be stored or packaged for shipment to customers.

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

Table 5

Thermodynamic irreversibility of the system: 171607- (130327+15513+11410) = 14357 W/°C (spontaneous process) . Specific electricity consumption (per ton of polymer produced) : 854 kWh/t.

Two other examples according to the invention are reported below, highlighting the different operating conditions of the system. The examples are based on the arrangement described in example 4 from which they differ in the working temperatures of the high pressure separator (6) and the working temperatures and pressures of the medium pressure separator (11) . EXAMPLE 5 (Ex 5)

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

Table 6 Thermodynamic irreversibility of the system: 171824- ( 130327+15513+12102 ) =13883 W/°C (spontaneous process) .

Specific electricity consumption (per ton of polymer produced) : 848 kWh/t. EXAMPLE 6 (Ex 6)

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

Table 7

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

Specific electricity consumption (per ton of polymer produced) : 854 kWh/t.

The results are reported in Table 8. Table 8: Comparison between Invention Examples (Ex4 to Ex.

6) and Comparative Examples (Cl to C3)

The invention allows a simplification of the process, with elimination of the expensive cooler ( (17) in Figure 5) present in the reference US 7, 837, 950 and a primary compressor /booster (1) of smaller capacity, and therefore lower investment and operating costs, as the flow rate of gas that has to be recompressed by this machine is reduced. Furthermore, as shown in table 8, the high pressure separator (6) is found to operate at a lower temperature than in comparative examples 1 and 3, and at a temperature almost analogous to that indicated in comparative example 2, guaranteeing an excellent product quality, especially as regards the optical properties, in terms of cross-linked polymer and punctual defects especially in products intended for applications for film products such as transparent films, greenhouse film (used in agriculture) .

Compared to Comparative Example 2, the temperature of the high-pressure separator (6) is the same but a reduction in the specific consumption of electric energy is achieved. As far as irreversibility is concerned, on the other hand, a slight decrease is determined between Example 4 and Comparative Example 2, however the thermal compression process is favored compared to Comparative Examples 1 and 2. Again, it can be seen from table 8 how the invention allows equalizing specific consumptions achieved with comparative Example 3. This advantage is given by the possibility of compressing the off-gases leaving the medium pressure separator (11) using the reactor (3) outlet stream as a driving fluid.