Process for the polymerization of olefins in loop reactor with variable slurry velocity

Abstract

A process and an apparatus for the slurry polymerization of one or more α-olefins in at least one loop reactor under circulation of the polymer slurry by means of an axial flow pump coupled to a variable speed drive, providing a substantially constant flow velocity of the polymer slurry variable from 1 to 11.5 m / s.

Description

PROCESS FOR THE POLYMERIZATION OF OLEFINS IN LOOP REACTORWITH VARIABLE SLURRY VELOCITY

[0001] The present invention relates to a process for the liquid-phase polymerization of olefins carried out in at least one loop reactor with variable velocity of the circulating slurry. The invention also relates to an apparatus for polymerizing olefins comprising at least one loop reactor provided with means to vary the slurry velocity.BACKGROUND OF THE DISCLOSURE

[0002] It is known to carry out the olefin polymerization under slurry conditions in one or more loop reactors. This type of reactor is widely used in the production of polyethylene and polypropylene. The obtained product consists of solid polymer particles in a suspension of liquid monomers (bulk polymerization), or alternatively, polymer particles in a suspension of liquid monomers mixed with an inert solvent, as the polymerization diluent. Propylene (co)polymers are typically produced by using liquid propylene as the polymerization medium, in order to maximize the rate of the polymerization reaction by providing a high monomer concentration.

[0003] The polymer slurry is continuously circulated in the loop reactor by a pump to maintain a homogeneous dispersion of the solid polymer in the liquid reaction medium. A polymer slurry is withdrawn from the reactor and may be concentrated so that the solid content at the reactor outlet is higher than the solid content inside the loop reactor. Traditionally, this has been done by using settling legs at the discharge from the loop reactor. Settling legs operate on a batch principle to recover the product. The concentrated polymer slurry is successively transferred to a flash tank, where most of the diluent and unreacted monomers are flashed off, so as to separate the solid polymer at the bottom of the flash tank.

[0004] It has also been known to use a second loop reactor that receives the polymer slurry from the first loop reactor to continue the polymerization reaction, possibly under different polymerization conditions, to produce olefin polymers and copolymers having desired composition and properties.

[0005] During the standard working of the polymerization apparatus, a predefined pressure gradient along the transfer line ensures a continuous transfer of polymer slurry between thefirst loop reactor and the second loop reactor.

[0006] WO 2009 / 027197 Al discloses a process for the slurry polymerization of one or more a-olefins in a sequence of at least two loop reactors interconnected by means of a transfer line, wherein a fraction of polymer slurry produced in the first loop reactor is discharged into said transfer line and transferred to said second loop reactor, and a fraction of polymer slurry withdrawn from said second loop reactor is continuously recycled back to it also by means of said transfer line.

[0007] When the process is carried in a loop reactor, however, it has been found that under certain operating conditions a slugging phenomenon can occur. Slurry slugging shows a segregation of the solid phase from the liquid phase, especially in the bends of the loop reactor, where a change of direction of the slurry flow occurs, possibly due to unbalanced energy exchange between the suspended polymer and the suspending liquid.

[0008] Slurry slugging causes an accumulation of some circulating polymer, which starts flowing in a concentrated form inside the loop reactor. This generates fluctuations and instability in the operating conditions, which may limit the concentration of solids that the loop reactor can manage.

[0009] In view of the foregoing, there is a need of improving the operation of the liquid-phase olefins polymerization process carried out in one or more loop reactors, so as to prevent or minimize the slugging of the slurry and the associated fluctuations in the operating conditions of the process, whereby ensuring a reliable working of the polymerization plant.SUMMARY OF THE DISCLOSURE

[0010] It is therefore an object of the present invention a process for the slurry polymerization of one or more a-olefins carried out in one or more loop reactors, wherein the polymer slurry is circulated in the loop reactor by means of an axial flow pump that provides a flow velocity of the circulating slurry variable from 1 to 11.5 m / s, preferably from 3 to 10.5 m / s, more preferably from 5 to 9.5 m / s, even more preferably from 6.0 to 9.0 m / s, wherein said velocity is kept substantially constant throughout the whole loop reactor, and wherein said axial flow pump is coupled to a variable speed drive whereby the rotational speed of said axial flow pump is adjusted to maintain the average circulating velocity of the polymer slurry at the desired value.

[0011] Another object of the invention is an apparatus for the slurry polymerization ofolefins, comprising at least one loop reactor, said loop reactor comprising at least two vertical legs joined each other by means of a top bend and a bottom bend, and an axial flow pump located at the bottom bend of said loop reactor, said pump being connected to a variable speed drive to adjust the flow velocity of said slurry circulating within said loop reactor.

[0012] The polymerization process of the present invention can be carried out in a single loop reactor or in more loop reactors connected in series by lines that transfer the polymer slurry from one reactor to the next. Each loop reactor may be formed by two or more legs, e.g., four legs or also eight legs.

[0013] In certain embodiments, the transfer lines operate also a continuous recycle of polymer between different zones of a loop reactor, as described in WO 2009 / 027197 Al.

[0014] In such embodiments, the transfer line is configured as a “belt connection” and, in case of two loop reactors, this arrangement allows to establish a recycle of the polymer slurry by withdrawing from the second loop reactor a fraction of polymer slurry which is continuously recycled back to the second reactor after having picked up a fraction of polymer discharged from the first loop reactor. The respective amounts of polymer slurry coming from the second and first loop reactor flow together in the transfer line, so that they are mixed together before reaching the outlet of the transfer line, arranged on the second loop reactor. The flow rates of the polymer slurry coming from the second loop reactor and entering the transfer line and the flow rate of the total polymer slurry including the fraction discharged from the first loop reactor and returning back to the second loop reactor are defined in WO 2009 / 027197 Al.

[0015] The olefin polymerization of the invention is carried out under slurry conditions, so that the polymer particles are suspended in a liquid polymerization medium. Slurry polymerization using the liquid monomers as the polymerization medium (bulk polymerization) allows to maximize the rate of the polymerization reaction by providing a high monomer concentration, and to simplify the process by eliminating the use of solvents or diluents that must be purified and recycled.

[0016] Loop reactors for slurry polymerization in liquid monomer are typically provided with jacketed walls, which provide a high ratio of cooling area to the reactor volume.

[0017] According to an aspect of the invention, the process for the polymerization ofone or more a-olefins is carried out in a loop reactor under circulation of the polymer slurry by means of an axial flow pump that provides a flow velocity of the circulating slurry variable from 1 to 11.5 m / s, preferably from 3 to 10.5 m / s, more preferably from 5 to 9.5 m / s, even more preferably from 6.0 to 9.0 m / s, wherein said velocity is kept substantially constant throughout the whole loop reactor.

[0018] It has been surprisingly found that when the velocity of the slurry is within the above ranges of velocity, no slugging phenomenon occurs, i.e., the solid phase does not segregate from the solid phase, as it can happen when the process is conventionally operated in loop reactors, with segregation of solids particularly in the bottom bends of the loop reactor.

[0019] To adjust the velocity of the circulating slurry at the desired value, use is made of an axial flow pump coupled to a variable speed drive, so that the rotational speed of the pump is adjusted in order to maintain the average circulating velocity of the polymer slurry at the desired value.

[0020] The variable speed drive varies the rotational speed of the pump by electrical means or by hydraulic means.

[0021] Variation of speed by electrical means is obtained by using an AC motor drive, which is a system incorporating a motor that controls speed and torque by varying the frequency of the input electricity (variable frequency drive or VFD). Power electronics technology has reduced VFD cost and size and has improved performance through advances in semiconductor switching devices, drive topologies, simulation and control techniques, and control hardware and software.

[0022] Variation of speed by hydraulic means is obtained by using a hydraulic speed variator that has a main circuit composed by primary and secondary pumps. Typically, both units are housed in the same case, and they are mounted on a fixed distributor shaft. Speed regulation is accomplished by adjusting the eccentricity of the primary pump and, therefore, the oil flow is sent to the secondary pump with fixed eccentricity. The latter, connected to the output shaft, drives a speed that is directly proportional to the received oil flow. The maximum eccentricity of primary pump corresponds to the maximum speed rotation of the output shaft, while the smaller eccentricity corresponds to a lower output speed rotation. These systems allow to vary the rotational speed of the axial flow pump, whereby the velocityof the slurry is varied and adjusted within the desired ranges of velocity, as described above.

[0023] To achieve a substantially constant flow velocity of the polymer slurry variable from 1 to 11.5 m / s or within the other preferred ranges, a variable speed drive capable to vary from 10 to 100% the nameplate rpm of the drive is used.

[0024] The polymerization catalysts used in the polymerization process of the invention are obtained from compounds of transition metals belonging to Groups IV, V or VI of the Periodic Table of the Elements, and aluminium alkyl compounds and / or alumoxane compounds, or are catalysts based on chromium oxide.

[0025] Preferred catalysts for the polymerization of propylene, optionally with other a-olefins, are the high-yield, high-stereospecificity Ziegler / Natta catalysts comprising solid components supported on active MgCh.

[0026] Hydrogen is preferably used as a chain transfer agent to adjust the molecular weight of the produced polyolefin. The monomers to be polymerized by the process of the invention are a-olefins of formula CH2=CHR, where R is hydrogen or a hydrocarbon radical having 1-12 carbon atoms. Preferably said a-olefin is propylene: in this case the propylene concentration is comprised between 60 and 100% by weight, preferably between 75 and 95%, based on the total amount of liquid present in the loop reactor. The remaining part of liquid may comprise an inert hydrocarbon, if present, and one or more a-olefin comonomers in case of copolymerization. The preferred comonomers are ethylene and 1 -butene.

[0027] The polymerization temperature in the loop reactors is comprised between 60°C and 95°C, preferably from 65 to 85°C, while the pressure ranges from 20 to 50 bar, preferably from 25 to 40 bar. The residence time of the slurry in the loop reactors is comprised between 10 min and 90 min, preferably between 20 min and 60 min.BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In the following of the description the invention is illustrated with reference to the attached drawings, which are simplified schemes representing preferred, but non-limiting, embodiments of the invention, wherein:

[0029] Fig. l is a diagrammatic representation of a slurry polymerization of olefins performed in a sequence of two loop reactors; and

[0030] Fig. 2 is an enlarged view of a detail of Fig. 1.DETAILED DESCRIPTION OF THE DISCLOSURE

[0031] The term “substantially constant” as used herein with respect to the flow velocity of the polymer slurry means that deviations from a baseline flow velocity do not produce meaningful changes in the density of the slurry. The term “substantially constant” in this respect includes deviations of approximately 10%, preferably 5%, from a baseline flow velocity. A flow velocity is considered substantially constant as long as it fluctuates no more than 10%, preferably no more than 5%, from the recited velocity.

[0032] With reference to Fig. 1, an olefin polymerization process according to an embodiment of the invention is carried out in two interconnected loop reactors 10 and a 20. Each reactor, as known in the field, comprises at least two vertical legs joined each other by means of a top bend and a bottom bend.

[0033] With reference to the first reactor 10, it comprises two vertical legs 11, I la, joined by a top bend 13 and a bottom bend 13a. The second reactor 20 has the same structure as the first reactor 10.

[0034] Catalyst components, co-catalyst, hydrogen as the molecular weight regulator, and monomers are introduced into the first loop reactor 10 via line 12. In the case of use of a Ziegler / Natta catalyst comprising a solid component supported on active MgCh, the solid component can be fed as such or preferably in a pre-polymerised form.

[0035] The monomers and / or the hydrogen concentrations in the second loop reactor 20 may differ from those in the first reactor 10, so as to produce final polymers or copolymers having a desired composition and physical properties.

[0036] The polymer slurry of the first reactor 10 and in the second reactor 20 is continuously recirculated inside the loop reactors by means of axial pumps 14, 24 driven by variable speed drives 16, 26, respectively. The arrows within the loop reactors show the direction of flow of the polymer slurry.

[0037] Transfer of polymer slurry from the first loop reactor 10 to the second loop reactor 20 is carried out by the transfer line 30.

[0038] More particularly, a fraction of polymer slurry is continuously discharged from the first loop reactor 10 via the discharge line 18 and enters the transfer line 30 connecting the two loop reactors, while a fraction of polymer slurry is continuously withdrawn from the second loop reactor 20 via the discharge port 22 and enters the transfer line 30. Said fraction of polymer slurry is preferably withdrawn from a zone located downstream the pumpingmeans 26 of the loop reactor 20.

[0039] The transfer line 30 conveys the polymer slurry withdrawn from the second reactor 20 to a zone in proximity of the first loop reactor 10, where the transfer line 30 bends and surrounds the lower portion of loop reactor 10, then the transfer line returns back to the second loop reactor 20.

[0040] The discharge line 18 is located at the bottom of loop reactor 10. The portion of the transfer line from the point of the discharge line 18 and the point of entry into the second loop reactor 20 is designated with 30a. In this portion 30a the fraction of polymer slurry discharged from the first and second loop reactor are mixed together before entering the second loop reactor 20, then these mixed portions are conveyed to the second loop reactor 20.

[0041] The obtained polyolefin slurry discharged from the loop reactor 20 via the discharge line 28 can be fed to a flash tank (not shown) for separating the solid polymer particles from the liquid phase.

[0042] With reference to Fig. 2, the axial flow pump 14 comprises an impeller 32 mounted on a shaft 34 connected to a variable speed drive 16, so that the rotational speed of the pump 14 is adjusted in order to maintain the average circulating velocity of the slurry at the desired value.

[0043] In the embodiment shown, the variable speed drive 16 is an electric motor connected to a speed controller 36. A speed sensor 38 is mounted on the shaft 34.

[0044] An operator of the process sets the desired speed of the motor, which can vary from 70 to 100% the nameplate rpm, to ensure the desired flow velocity of the slurry. Then, an automatic control of the system, comprising the speed controller 36 and the speed sensor 38, keeps the speed at the desired level, as mentioned above. In the embodiment of Figures 1 and 2 the rotational speed of the axial flow pump 14 was adjusted to ensure a flow velocity of the slurry of about 8 m / s.

[0045] In connection with the description of enclosed Fig.1 and Fig. 2, it is another object of the invention an apparatus for the continuous polymerization of olefins comprising at least a loop reactor 10 for the slurry polymerization of olefins, said loop reactor 10 comprising at least two vertical legs 11, 1 la, joined each other by means of a top bend 13 and a bottom bend 13a, and comprising an axial flow circulating pump 14 located at the bottom bend 13a of the loop reactor, said pump 14 being connected to a variable speed drive 16 toadjust the flow velocity of said circulating slurry within said loop reactor 10.

[0046] According to an aspect of the invention, the variable speed drive 16 varies the rotational speed of the pump 14 by electrical means or by hydraulic means.

[0047] According to an embodiment of the invention, the variable speed drive 16 is an AC motor drive incorporating an electric motor that controls speed and torque by varying the frequency of the input electricity.

[0048] According to another embodiment of the invention, the variable speed drive is a hydraulic speed variator that has a main circuit composed by primary and secondary pumps.

[0049] In both embodiments, the variable speed drive allows to vary the rotational speed of the axial flow pump 14, whereby the velocity of the slurry is varied and adjusted within the desired ranges of velocity, as described above.

[0050] According to another aspect of the invention, the variable speed drive 16 is connected to a speed controller 36 and a speed sensor 38 mounted on the shaft 34, whereby the speed of rotation of the pump 14 can be varied to ensure that the velocity of flow of the polymer slurry is set and kept within a desired range, too.

[0051] According to an aspect of the invention, the apparatus comprises a variable speed drive that can vary from 70 to 100% the nameplate rounds per minute (rpm), to ensure the desired speed of the axial flow pump and the resulting flow velocity of the slurry.

[0052] According to an aspect of the invention, the apparatus comprises an automatic control system comprising the speed controller 36 and the speed sensor 38, as mentioned above, whereby the speed of rotation of the pump (14) is detected and controlled, in particularly it is maintained at the desired level.

[0053] Concerning the loop reactor of the apparatus for the continuous polymerization of olefins, as known to the person skilled in the art, the polymerization loop reactors 10, 20 may be also formed by more than two vertical legs, joined each other by means of top and bottom bends, so as to generate a loop structure. According to a preferred embodiment, the polymerization loop reactors 10, 20 comprises from 4 to 8 vertical legs (not shown in Fig.1).

[0054] The bottom portion of the vertical legs of the loop reactors 10, 20 are provided with on-off emergency valves (not shown in Figures 1 and 2) for emptying the loop reactor in case of emergency or maintenance reasons. According to an embodiment, said on-off emergency valves may be placed on the bottom bends of the loop reactors 10, 20.

[0055] The following examples will further illustrate the present invention without limiting its scope.EXAMPLESExample 1

[0056] A polypropylene homopolymer is produced by liquid-phase polymerization in a sequence of two loop reactors interconnected each other by means of the transfer line, as shown in Figure 1. The liquid monomer is polymerized in the absence of any polymerization diluent (bulk polymerization), so as to form a slurry of polypropylene in the liquid monomer.

[0057] A Ziegler-Natta catalyst is used as the polymerization catalyst, comprising a titanium-based solid catalyst component supported on a magnesium chloride, triethylaluminium (TEAL) as the cocatalyst and an alkyl-alkoxysilane as an external donor.

[0058] The above catalyst components are fed to a pre-contacting vessel, in amounts such that the weight ratio TEAL / solid component is of 5, the weight ratio TEAL / extemal donor is of 3.5. The above catalyst components are pre-contacted at a temperature of 15°C for 10 minutes.Polymerization conditions 1stloop reactor a. Temperature: 73°C b. Pressure (upstream the circulation pump): 41 barPolymer concentration in the slurry: 50% by weight (the remaining is propylene)Polymerization conditions 2ndloop reactor c. Temperature: 73°C d. Pressure (upstream the circulation pump): 40.5 barPolymer concentration in the slurry: 50% by weight (the remaining is propylene)Operative conditions along the transfer line

[0059] The axial flow pump 24 provides the pressure head for the continuous circulation of the polymer slurry inside the second loop reactor 20 and also the pressure head needed to guarantee the flow of the polymer slurry along the transfer line 30.

[0060] 5400 t / h of polymer slurry are continuously circulated inside the first loop reactor 10 with a flow velocity of 8 m / s, and simultaneously 70 t / h of slurry are dischargedfrom this reactor via the discharge line 18. Likewise, 5400 t / h of polypropylene slurry are continuously circulated inside the second loop reactor 20 and 110 t / h of slurry are continuously discharged from the second loop reactor via the discharge line 28. Being the polymer concentration in the slurry of 50% by weight, the productivity P of the polymerization plant is of 35t / h of polypropylene.

[0061] The melt index MIL (ASTM-D 1238, conditions 230°C / 2.16 Kg) of the polypropylene particles discharged from the second loop reactor is about 6 g / 10’.

[0062] Each reactor 10, 20 was equipped with axial circulating pumps 14, 26, respectively. To obtain a circulating slurry with a flow velocity of 8 m / s, the axial flow pumps were set at a rotational speed of 1050 rpm. Each pump 14, 26 was actuated by an electric AC motor drive set at 1490 rpm. A speed control 36 associated to a speed sensor 38 ensured that the polymer slurry circulated with a flow velocity kept substantially constant of 8 m / s. No slugging phenomenon was observed in the reactors.Example 2 (Comparative)

[0063] The same polymerization conditions of Example 1 are set in the 1stand 2ndloop reactors to prepare the same polypropylene homopolymer of Example 1. The same operative conditions were adopted with the difference that in the first reactor 10 the slurry was circulated at the velocity of 13 m / s.

[0064] A segregation of the slurry was observed in the first reactor after about 3 hours of operation.

[0065] This example demonstrates that, when the slurry is circulated at a velocity higher than 11.5 m / s a slugging phenomenon occurs, with segregation of a solid phase in the bottom bend 13a of reactor 10, which caused fluctuations and operation instability.

Claims

CLAIMS1. Process for the slurry polymerization of one or more a-olefins carried out in at least a loop reactor under circulation of the polymer slurry by means of an axial flow pump, characterized in that said axial flow pump provides a flow velocity of the polymer slurry variable from 1 to 11.5 m / s, wherein said velocity is kept substantially constant throughout the whole loop reactor, and wherein said axial flow pump is coupled to a variable speed drive whereby the rotational speed of said axial flow pump is adjusted to maintain the average circulating velocity of the polymer slurry at the desired value.

2. Process according to claim 1, characterized in that said axial flow pump provides a flow velocity of the polymer slurry from 3 to 10.5 m / s.

3. Process according to claim 1 or 2, characterized in that said axial flow pump provides a flow velocity of the polymer slurry from 5 to 9.5 m / s.

4. Process according to any of claims 1 to 3, characterized in that said axial flow pump provides a flow velocity of the polymer slurry from 6.0 to 9.0 m / s.

5. Process according to any of claims 1 to 4, carried out in a sequence of at least two loop reactors interconnected by means of a transfer line, wherein by means of said transfer line the polymer is transferred from a first loop reactor to a second loop reactor and a fraction of polymer slurry is withdrawn from said second loop reactor and is continuously recycled back to it; characterized in that the flow velocity of the polymer slurry in at least said first loop reactor is variable from 1 to 11.5 m / s, preferably from 3 to 10.5 m / s, more preferably from 5 to 9.5 m / s, by means of said axial flow pump.

6. Process according to claim 5, characterized in that the flow velocity of the polymer slurry in both said first and second loop reactors is variable from 1 to 11.5 m / s, preferably from 3 to 10.5 m / s, more preferably from 5 to 9.5 m / s, by means of axial flow pumps located in each of said reactors.

7. Process according to any of claims 1 to 6, wherein the loop reactor comprises at least two vertical legs joined each other by means of a top bend and a bottom bend.

8. Apparatus for the slurry polymerization of olefins, comprising at least one loop reactor (10) comprising at least two vertical legs (11,11a) joined each other by means of a top bend (13) and a bottom bend (13a), and an axial flow pump (14) located at the bottom bend (13a) of said loop reactor, characterized in that said axial flow pump (14) isconnected to a variable speed drive (16) to adjust the flow velocity of said slurry circulating within said loop reactor.

9. Apparatus according to claim 8, characterized in that said variable speed drive (16) varies the rotational speed of the pump 14 by electrical means or by hydraulic means.

10. Apparatus according to any of claims8 to 9, characterized in that said variable speed drive (16) is connected to a speed controller (36) and a speed sensor (38) mounted on a shaft (34) of said pump (14), whereby the speed of rotation of the pump (14) is detected and controlled.

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