Split loop propylene polymerization plant and split loop propylene polymerization process

Abstract

Loop-loop-gas phase reactor polypropylene plant and process for producing polypropylene and polypropylene copolymers.

Description

[Technical field]

[0001] The present invention relates to a polymerization plant suitable for very large scale propylene polymerization. The present invention further relates to a propylene polymerization process using such a plant. [Background technology]

[0002] The combination of loop reactors and gas-phase reactors has been known for more than 20 years under the trademark Borstar® and can be found in virtually every textbook in the field of polyolefins. The basic process layout is described, for example, in WO 9858975 A1, which deals with the preparation of homopolymers and copolymers of propylene, which comprises the polymerization of propylene, optionally with comonomers, in at least one slurry reactor and at least one gas-phase reactor in the presence of a catalyst at high temperature and pressure, the polymerization product of at least one slurry reactor containing unreacted monomer, which is led directly to the first gas-phase reactor without substantially recycling the unreacted monomer to the slurry reactor.

[0003] Unfortunately, medium-sized polypropylene production plants have significantly higher operating and fixed costs per ton of product produced when compared to very large polypropylene plants. However, increasing the scale of production creates quality and operational issues. Increasing the capacity of the loop reactor can be done by adding additional legs to the reactor. In the industry, loop reactors have been scaled up from two legs to six legs, and even eight-leg loop reactors are not uncommon. Unfortunately, such modifications can result in drawbacks, such as significant reactant concentration gradients. For example, in polypropylene homopolymerization, the ratio of hydrogen to propylene can vary over the length of the loop reactor. In addition, in propylene copolymerization, the ratio of comonomer to propylene can cause significant gradients in the loop reactor. Naturally, properties such as randomness are affected when the comonomer / C3 ratio decreases over the length of the loop reactor. Furthermore, the olefin comonomer (i.e., ethylene) distribution is not evenly distributed both between and within the polymers, thus providing a higher organoleptic content. Organoleptic considerations refer to the phenomenon of organic compounds migrating from the polymer wrapping into the food product, which may alter the taste and / or odor of the food product. This is particularly problematic when single-site catalysts are used. Single-site catalysts are much more sensitive in terms of their hydrogen response and comonomer incorporation.

[0004] Thus, there remains a need for ultra-scalable processes and plants that at least partially avoid these and related problems.

[0005] WO 2021 / 045889 discloses a process for polymerizing alpha-olefin monomers in a slurry using a catalyst and hydrogen to produce low molecular weight polyolefins in a first step, hydrogen being released from the low molecular weight polyolefins in a second step, and the resulting intermediate being further polymerized in a gas phase reactor.

[0006] US Patent No. 6,455,643 relates to a process for preparing homopolymers and copolymers in the presence of a catalyst in a slurry reactor and a gas phase reactor. Russian Patent No. 2440842C2 relates to slurry circulation through an infinite circulation reactor system. WO 2017 / 032682 relates to a process and system for the continuous polymerization of one or more alpha-olefin monomers, comprising the steps of: a) introducing catalyst and / or polymer from at least one loop reactor into at least one second reactor; b) withdrawing fluid from the at least one second reactor; c) cooling fluids including the withdrawn fluids in a cooling unit; d) introducing the cooled fluids into a separator to separate at least a portion of the liquids from these fluids and form a liquid phase and a gas / liquid phase; e) introducing the gas / lower liquid phase into the reactor below the distribution plate; f) introducing the liquid phase into a settling tank to separate the liquid from fines that settle in the settling tank; g) introducing the liquid from the settling tank upstream of the cooling unit; and h) introducing a slurry including solid polymer particles from the settling tank into at least one loop reactor. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 9858975A1 Brochure [Patent Document 2] International Publication No. 2021 / 045889 Brochure [Patent Document 3] U.S. Patent No. 6,455,643 [Patent Document 4] Russian Patent No. 2440842C2 [Patent Document 5] International Publication No. 2017 / 032682 Brochure Summary of the Invention [Means for solving the problem]

[0008] The present invention relates to a plant for preparing a propylene homopolymer or a propylene copolymer, comprising: (i) a feed tank (1) for the single-site catalyst and a feed tank (2) for the optional cocatalyst, an optional activator and an optional external donor (3)(s); (ii) an optional pre-contacting unit (4) for catalyst mixing connected to said supply tank by supply lines (5, 5', 5"); (iii) a propylene supply tank (7); and (iv) a prepolymerization reactor (6) connected to the feed tanks (1, 2, 3) or to the precontacting unit (4) and to the propylene feed tank (7); (v) a first loop reactor (8) connected to the prepolymerization reactor; (vi) a second loop reactor (9) connected to the first loop reactor via a loop reactor connecting line (10), The first loop reactor has a volume of up to 40% by volume relative to the combined volume of the first loop reactor and the second loop reactor, the second loop reactor has a volume of at least 60% by volume relative to the combined volume of the first loop reactor and the second loop reactor, and the combined volume of the first loop reactor and the second loop reactor is between 50 and 150 m 3 a first loop reactor (8) and a second loop reactor (9) in the range of (vii) a single feed point for feeding propylene monomer to the first loop reactor (8), optional comonomer to the first loop reactor (8) (11), and hydrogen to the first loop reactor (8) (12), and a single feed point for feeding propylene monomer to the second loop reactor (9), optional comonomer to the second loop reactor (9) (11), and hydrogen to the second loop reactor (9) (12); (viii) a gas-phase reactor (13) comprising a gas circulation line (14), a circulation gas compressor (15) and a circulation gas cooler (16), said gas-phase reactor (13) being connected to said second loop reactor by a direct feed line (17); (ix) means (44) for feeding propylene monomer to said gas phase reactor (13), and / or means (18) for feeding comonomer to said gas phase reactor (13), and / or means (19) for feeding hydrogen to said gas phase reactor (13), preferably said means for feeding monomer and / or said means for feeding comonomer being suitable for feeding monomer and / or comonomer in condensed form; (x) optionally a product discharge vessel (20) connected to the gas phase reactor; (xi) optionally, a product outlet heater (21); (xii) an optional product receiver tank (22) connected to the product discharge vessel (20) or to the gas phase reactor (13); (xiii) means (25) for the recovery of propylene homopolymers or propylene copolymers, optionally including means (25) for homogenization, additives, and pelletization; The present invention provides a plant comprising:

[0009] The present invention further relates to a process for preparing propylene homopolymers and copolymers comprising the steps of: a) providing a single-site catalyst, an optional cocatalyst, and an optional activator in supply tanks (1, 2, 3); b) feeding said single-site catalyst, optionally said cocatalyst, and optionally said activator to a precontacting unit (4) to provide a mixed catalyst system, and feeding said mixed catalyst system to a prepolymerization reactor. or feeding said single-site catalyst, optionally said cocatalyst, and optionally said activator to a prepolymerization reactor (6); c) initiating prepolymerization by introducing propylene monomer and, optionally, comonomer, thereby providing a prepolymer; d) feeding said prepolymer into a first loop reactor (8) to polymerize propylene with an optional comonomer to obtain a first intermediate; e) feeding said first intermediate to a second loop reactor (9) via a loop reactor connecting line (10) for further polymerizing propylene, optionally with a comonomer, to obtain a second intermediate, f) the first loop reactor has a volume of up to 40% by volume relative to the combined volume of the first loop reactor and the second loop reactor, the second loop reactor has a volume of at least 60% by volume relative to the combined volume of the first loop reactor and the second loop reactor, and the combined volume of the first loop reactor and the second loop reactor is between 50 and 150 m 3 and there are single feed points for feeding monomer to the first loop reactor (8), for feeding comonomer to the first loop reactor (8) (11) and for feeding hydrogen to the first loop reactor (8) (12), and single feed points for feeding monomer to the second loop reactor (9), for feeding comonomer to the second loop reactor (9) (11) and for feeding hydrogen to the second loop reactor (9) (12), and feeding the second intermediate containing unreacted monomer(s) directly to a gas phase reactor (13) via a direct feed line (17); g) further polymerizing propylene and optional comonomers in said gas phase reactor (13) by feeding propylene and / or comonomers and / or hydrogen and circulating gas in an upward direction via a gas circulation line (14); h) optionally discharging the gas-phase reactor product to an optional product discharge vessel (20) and subsequently to a product receiver tank (22) via a connecting line (34) to provide a raw mixture; or discharging the gas-phase reactor product directly to a product receiver tank (22) to provide a raw mixture. The present invention provides a process (method) comprising:

[0010] In a further aspect the present invention relates to the use of a plant as described for preparing a propylene homopolymer or preferably a propylene copolymer.

[0011] In a still further aspect the invention relates to the use of a plant as described for reducing the comonomer / propylene molar ratio drift and / or the hydrogen / propylene molar ratio drift over the length of at least one of said loop reactors.

[0012] In contrast to conventional plants of this type, the plant according to the invention has a minimum flow rate of 50-150 m 3 The present invention further applies to a catalyst selected from the group of single-site catalysts, a class known for their sensitivity with respect to gradients with respect to monomer, comonomer and / or hydrogen.

[0013] Surprisingly, it has been found that the segregation phenomenon in gas phase reactors can be more easily controlled, solids flowability can be improved and agglomerate formation is reduced.

[0014] The plant according to the present invention preferably comprises: (xiv) at least one purge bin (23), preferably connected to the product receiver tank (22) via a purge bin supply line (48); (xv) at least one propylene nitrogen recovery unit (24), preferably connected to said at least one purge bin (23) via a feed line (51); (xvi) a column feed line (241) for supplying a hydrocarbon stream to the column (28); (xvii) a nitrogen resupply line (243) for resupplying the nitrogen-rich stream to said purge bin (23); (xviii) optionally a thermal oxidizer unit (43); (xix) an exhaust line (242) for optionally discharging an exhaust stream from said at least one propylene nitrogen recovery unit (24) to optionally said optional thermal oxidizer (43); (xx) a stripping gas processing unit (26) comprising at least one compressor (27), the column (28) and a reflux feed vessel (28a), the reflux feed vessel (28a) being connected to the gas circulation line (14) of the gas-phase reactor (13) via a stripping line (29); (xxi) a cooling circuit (30) for the circulating gas cooler (16), (xxii) a blowdown unit (31) including a high pressure blowdown bin (32) and a low pressure blowdown bin (33), preferably connected to the product receiver tank (22) via a connecting line (34); and (xxiii) a recovery supply line (35) connecting the recovery gas treatment unit (26) to the propylene supply tank (7); Contains one or more of the following:

[0015] The plant according to the present invention preferably comprises: (xxiv) knock out drum (38), (xxv) gas phase reactor area depressurization cyclone (39) (xxvi) Gas phase reactor flare knock out drum (40) The present invention further includes one or more of the following: Such a unit increases the safety of the plant according to the invention.

[0016] Preferably, in the plant according to the invention, the loop reactor connecting line (10) comprises a thickener, more preferably a hydrocyclone. Such thickeners, in particular hydrocyclones, and their configurations are described in WO 2017 / 097577, which is incorporated herein by reference. In particular, all configurations disclosed in WO 2017 / 097577 are adapted for a single feed in each of the two loop reactors, which configurations are incorporated herein by reference.

[0017] The described process preferably also includes a thickener, more preferably a hydrocyclone. All aspects described herein with respect to the plant are also intended to be held and disclosed for the process.

[0018] In the following, the invention will be explained with reference to the drawings. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 shows a plant according to the invention which can be used to carry out the process according to the invention. [Diagram 2] FIG. 2 shows the preferred connectivity of the loop reactor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The plant according to the invention is further explained with reference to FIGS.

[0021] The plant according to the invention for preparing propylene homo- and copolymers comprises a feed tank (1) for the single-site catalyst, a feed tank (2) for the optional cocatalyst and a feed tank(s) for the optional activator. Optionally, there is a pre-contacting unit (4) for the single-site catalyst mixing, connected to said feed tanks by feed lines (5, 5', 5"). In various embodiments and for various single-site catalyst systems, a pre-contacting tank is not necessary.

[0022] The plant according to the invention also comprises a prepolymerization reactor (6) connected to the feed tanks (1, 2, 3) or to the precontacting unit (4). Such prepolymerization is known in the art. The plant also comprises a propylene feed tank (7), a first loop reactor (8) connected to the prepolymerization reactor, a second loop reactor (9) connected to the first loop reactor via a loop reactor connection line (10), as well as means (11) for feeding comonomer to one or more of the first loop reactor (8), the second loop reactor (9) and / or the loop reactor connection line (10) between the loop reactors, and means (12) for feeding hydrogen to one or more of the first loop reactor (8), the second loop reactor (9) and / or the loop reactor connection line (10) between the loop reactors. As described above in [Means for Solving the Problems], the first and second loop reactors are 50 to 150 m 3 The first loop reactor has a combined volume of up to 40% by volume relative to the combined volume of the first and second loop reactors, and the second loop reactor has a volume of at least 60% by volume relative to the combined volume of the first and second loop reactors, as also explained above. These features ensure a favorable particle size distribution.

[0023] Apart from the loop reactor, the plant according to the invention also comprises a gas phase reactor (13) with a gas circulation line (14), a circulation gas compressor (15) and a circulation gas cooler (16), connected to at least a second loop reactor by a direct feed line (17). Means (44) for feeding monomer to the gas phase reactor (13) and / or means (18) for feeding optional comonomer to the gas phase reactor (13) and / or means (19) for feeding hydrogen to the gas phase reactor (13). In addition, the plant optionally comprises a product discharge vessel (20) connected to the gas phase reactor. Such a product discharge vessel contributes to the operational stability. Optionally, a product outlet heater (21) is present. Usually, the product outlet heater comprises several units. The plant according to the invention preferably also includes an optional product discharge vessel (20) or product receiver tank (22) connected to the gas-phase reactor (13), and at least one purge bin (23). In addition, the plant also includes at least one propylene nitrogen recovery unit (24) having a column feed line (241) for feeding the hydrocarbon stream to the column (28), a nitrogen re-feed line (243) for re-feeding the nitrogen-rich stream to the purge bin (23), and optionally a thermal oxidizer unit (43), as well as a vent line (242) for optionally discharging a vent stream to the optional thermal oxidizer (43). Optionally, there may be further feed lines, such as the nitrogen re-feed line (243), an external nitrogen feed line (245), a feed line for a catalyst deactivator, i.e., typically low pressure steam. Preferably, there is also an outlet (249) for oligomers.

[0024] The plant according to the invention also comprises means (25) for the recovery of propylene homopolymer or propylene copolymer, optionally comprising means for homogenization, additive and pelletization (25), as well as a recovery gas processing unit (26) comprising at least one compressor (27), said column (28) and a reflux supply vessel (28a), which reflux supply vessel (28a) is connected to the gas circulation line (14) of the gas-phase reactor (13) via a recovery line (29).

[0025] In addition to this there is a cooling circuit 30 for the circulating gas cooler 16. Such a circulating gas cooler contributes to a wide operating window.

[0026] Separate from these is a blowdown unit (31) which includes a high pressure blowdown bin (32), a low pressure blowdown bin (33), and which may be connected to the product receiver tank (22) via a connecting line (34).

[0027] The plant according to the invention further comprises a recovery feed line (35) connecting the recovery gas treatment unit (26) to the propylene feed tank (7). This essential recovery feed line (35) also allows the re-feed of propylene to the loop reactor, i.e. resulting in an integrated recovery system. EXAMPLES

[0028] Experimental Department Plants and processes according to the present invention are illustrated in the following examples, which are included for illustrative purposes and are not intended to be limiting of the invention.

[0029] Reference example 1 (RE1) A conventional Ziegler Natta catalyst was first introduced into a prepolymerization reactor with propylene at T=30°C and P=57 barg. After 30 minutes of polymerization, a propylene homopolymer was produced. The product was transferred to a single loop reactor with a length equal to 250 m and a diameter equal to 0.57 m, with a single feed point for propylene monomer, comonomer and hydrogen. 30 t / h of propylene, 0.45 t / h of ethylene and 10 kg / h of H were added. 2 was injected into the loop reactor through a single feed point and the polymerization was carried out at T=70° C., P=55 barg, with an average residence time equal to 1.5 hours. A propylene-ethylene copolymer was produced with a crystallinity equal to 50%, an average particle size of 950 μm, and the solids concentration in the reactor was 35% by weight. Table 1 shows the C2 / C3 and H2 / C3 ratios in the liquid phase at various reactor lengths.

[0030] [Table 1]

[0031] As can be seen, there is a 40% C2 / C3 molar ratio drift and a 21.5% H2 / C3 molar ratio drift along the reactor tube.

[0032] Reference example 2 (RE2) The procedure of Comparative Example 1 was repeated, except that two loop reactors were used: the first one having a length equal to 87 m and a diameter equal to 0.57 m, the second one having a length equal to 163 m and a diameter equal to 0.57 m. 10 t / h of propylene, 0.15 t / h of ethylene and 3.5 Kg / h of H 2 was injected into the first loop reactor (again via a single feed point) and 20 t / h of propylene, 0.30 t / h of ethylene and 6.5 Kg / h of H 2was injected (again via a single feed point) into the second loop reactor of the series. Polymerization conditions in the first loop were T=70° C., P=55 barg and average residence time equal to 45 min. Polymerization conditions in the second loop were T=70° C., P=53 barg and average residence time equal to 45 min. Table 2 shows the C2 / C3 and H2 / C3 ratios in the liquid phase at various reactor lengths in the two loop reactors.

[0033] [Table 2]

[0034] As can be seen, there is a 9.5% C2 / C3 molar ratio drift and a 5.25% H2 / C3 molar ratio drift along the reactor tubes in the first loop reactor, and a 20.5% C2 / C3 molar ratio drift and a 11.2% H2 / C3 molar ratio total drift from the inlet of the first loop reactor to the outlet of the second loop reactor. Example 1 shows the advantage of two loop reactors.

[0035] Comparative example 1 (CE1) The procedure of Reference Example 1 was repeated, except that a commercial single-site catalyst was used and the loop reactor was connected to a gas-phase reactor (GPR). More specifically, polymer particles characterized by d10=150 μm, d50=820 μm and d90=1750 μm (span=1.95, span=(d90-d10) / d50) were transferred to the GPR, where they were polymerized for 2.5 hours using operating conditions of T=85° C. and P=21 barg and a selected superficial gas velocity (SGV) equal to 0.55 m / s. In this reactor (GPR), a rubber fraction was produced, which was equal to 35% by weight, by adding ethylene as a comonomer in a molar ratio equal to C2 / C3=500 mol / kmol. During this test run, the segregation phenomenon in the gas-phase reactor was monitored. More specifically, the corresponding local fluidized bulk density values ​​were determined based on pressure differential measurements across the gas-solid fluidized bed reactor. The fluidized bulk density was therefore determined to be 280 kg / m 3 (near the bottom of the GPR) ~230kg / m 3 (central zone of GPR) and 195 kg / m 3 (upper zone of the GPR). Furthermore, the presence of agglomerates was detected and the quality of fluidization was poor (i.e., the temperature deviated by up to 5.0°C and the GPR operating pressure fluctuated by up to 25% of the set point value). The bulk density of the PP powder was 340 kg / m 3 , which reflected significant morphological problems (poor external morphology of the polymer particles imposed by agglomeration phenomena combined with segregation problems over the fluidized bed height). The GPR operation was discontinued after 7 days due to operability problems related to the segregation and agglomeration phenomena.

[0036] Comparative Example 2 (CE2) The procedure of Reference Example 2 (RE2) was repeated, except that a commercial single-site catalyst was used and the loop reactors were connected to a gas phase reactor (GPR). Loop reactor 1 had a volume of about 35% by volume, whereas loop reactor 2 had a volume of about 65% by volume, based on the combined volume. More specifically, the polymer particles characterized by d10=380 μm, d50=960 μm and d90=1600 μm (span=1.27) were transferred to a GPR where they were polymerized for 2.5 hours using operating conditions of T=85° C. and P=21 barg and a selected superficial gas velocity (SGV) equal to 0.55 m / s. In this reactor (GPR), a rubber fraction was produced, which was equal to 35% by weight, by adding ethylene as comonomer in a molar ratio equal to C2 / C3=500 mol / kmol. During this test run, the segregation phenomenon in the gas-phase reactor was monitored. More specifically, the corresponding local fluidized bulk density values ​​were measured based on pressure difference measurements across the gas-solid fluidized bed reactor. The fluidized bulk density was therefore 284 kg / m 3 (near the bottom of the GPR) ~278kg / m 3 (central zone of GPR) and 274 kg / m 3 (upper zone of the GPR). Furthermore, no agglomerates were detected and the quality of fluidization was very high (i.e., the temperature deviated by only a maximum of 1.0°C and the GPR operating pressure fluctuated by only a maximum of 8% of the set point value). The bulk density of the PP powder was 460 kg / m 3 , which reflected the very good morphology of the produced polymer particles. The operation of the GPR lasted for 15 days without any operability problems.

[0037] Based on Comparative Example 1 and Comparative Example 2, it can be concluded that the use of the split loop reactor configuration in the bulk stage of the process i) resulted in the production of polymer particles in the bulk polymerization stage with a narrower particle size distribution (lower span value), and ii) a significant reduction in segregation and agglomeration phenomena in the subsequent gas-phase polymerization reactor. The adoption of the split loop reactor configuration in the bulk stage of the multi-stage process contributes to smoothing the reactant concentration drift over the length of the bulk loop reactor, thus producing polymer particles with improved internal homogeneity (i.e., the comonomer is more evenly distributed within the polymer particle). This in turn results in polymer particles with well-distributed molecular properties (i.e., molecular weight, crystallinity, XS, amorphous fraction, etc.). All of the above contributes to the production of polymer particles that exhibit uniform local growth rates and uniform molecular properties at the particle level, resulting in narrow particle size distribution and less tendency for stickiness in the subsequent polymerization stages.

[0038] The results are shown in the table below.

[0039] [Table 3] [Explanation of symbols]

[0040] Reference Numbering in Figures 1 and 2 1. Supply tank for catalyst 2. Supply tank for optional cocatalyst 3 Supply tank(s) for optional activator and / or optional external donor 4 Optional pre-contact unit for catalyst mixing 5, 5', 5" Feed lines connecting the feed tank and the catalyst feed vessel or the feed tank connecting the prepolymerization reactor 6 Prepolymerization Reactor 7. Propylene Supply Tank 8 First Loop Reactor 9 Second Loop Reactor 10 Loop reactor connection line 101 Hydrocyclone (only shown in Figure 2) 11 Means for feeding monomer and optional comonomer 12 Means for supplying hydrogen 13 Gas Phase Reactor 14 Gas circulation line 15 Circulation gas compressor 16 Circulating gas cooler 17 Direct Supply Line 18 Means for supplying comonomer 19 Means for supplying hydrogen 20 Optional product discharge vessel 21 Product outlet heater 22 Product receiving tank 23 Purge Bin 24 Propylene Nitrogen Recovery Unit 25. An outlet line (for polymer powder) having means for recovery of propylene homopolymer or propylene copolymer, optionally including means for homogenization, additive and pelletization. 26 Gas recovery processing unit 27 Gas recovery compressor 28 Column 28a Return recovery supply vessel 29, 29' Recovery Supply Line 30 Cooling circuit for circulating gas cooler 31 Blowdown unit 32 High Pressure Blowdown Bin 33 Low Pressure Blowdown Bin 34 Connection Line 35 Recovery Supply Line 36 Further supply lines which may include dryer(s) 38 Knockout Drum 39 Gas Phase Reactor Zone Pressure Reducing Cyclone 40 Gas Phase Reactor Flare Knockout Drum 41, 41', 41" Propylene Supply Line 42 Means for catalyst deactivation (e.g., means for introducing low pressure steam) 43 Thermal Oxidizer Unit 44 Means for supplying liquid propylene monomer 48 Purge Bin Supply Line 49 Line connecting the product receiver tank (22) and the gas recovery compressor (27) 51 Supply line from purge bin (23) to propylene nitrogen recovery unit (24) 241 Column Supply Line 242 Exhaust Line 243 Nitrogen resupply line 244 External nitrogen supply line 245 Supply line for catalyst deactivator (usually low pressure steam) 246 Additional Nitrogen Supply Line 247 Nitrogen transport line Exit line for 249 oligomers

Claims

1. A plant for preparing a propylene homopolymer or a propylene copolymer, comprising: (i) a feed tank for the single-site catalyst (1), and an optional cocatalyst (2), an optional activator and an optional external donor (3); (ii) an optional pre-contacting unit (4) for catalyst mixing connected to said supply tank by supply lines (5, 5', 5"); (iii) a propylene supply tank (7); and (iv) a prepolymerization reactor (6) connected to said feed tanks (1, 2, 3) or to said precontacting unit (4) and to said propylene feed tank (7); (v) a first loop reactor (8) connected to the prepolymerization reactor; (vi) a second loop reactor (9) connected to the first loop reactor via a loop reactor connecting line (10), The first loop reactor has a volume of up to 40% by volume relative to the combined volume of the first loop reactor and the second loop reactor, the second loop reactor has a volume of at least 60% by volume relative to the combined volume of the first loop reactor and the second loop reactor, and the combined volume of the first loop reactor and the second loop reactor is between 50 and 150 m ３ a first loop reactor (8) and a second loop reactor (9) in the range of (vii) a single feed point for feeding propylene monomer to said first loop reactor (8), optional comonomer to said first loop reactor (8) (11), and hydrogen to said first loop reactor (8) (12), and a single feed point for feeding propylene monomer to said second loop reactor (9), optional comonomer to said second loop reactor (9) (11), and hydrogen to said second loop reactor (9) (12); (viii) a gas-phase reactor (13) comprising a gas circulation line (14), a circulation gas compressor (15) and a circulation gas cooler (16), said gas-phase reactor (13) being connected to said second loop reactor by a direct feed line (17); (ix) means (44) for feeding propylene monomer to said gas phase reactor (13), and / or means (18) for feeding comonomer to said gas phase reactor (13), and / or means for feeding hydrogen (19) to said gas phase reactor (13), preferably said means for feeding monomer and / or said means for feeding comonomer being suitable for feeding monomer and / or comonomer in condensed form; (x) optionally a product discharge vessel (20) connected to said gas phase reactor; (xi) optionally a product outlet heater (21); (xii) an optional product receiver tank (22) connected to the product discharge vessel (20) or to the gas phase reactor (13); (xiii) means (25) for the recovery of propylene homopolymer or propylene copolymer, optionally including means for homogenization, additive and pelletization (25); The plant includes:

2. (xiv) at least one purge bin (23), preferably connected to said product receiver tank (22) via a purge bin supply line (48); (xv) at least one propylene nitrogen recovery unit (24), preferably connected to said at least one purge bin (23) via a feed line (51); (xvi) a column feed line (241) for feeding a hydrocarbon stream to the column (28); (xvii) a nitrogen resupply line (243) for resupplying the nitrogen rich stream to said purge bin (23); (xviii) optionally a thermal oxidizer unit (43); and (xix) an exhaust line (242) for optionally discharging an exhaust stream from said at least one propylene nitrogen recovery unit (24) to said optional thermal oxidizer (43); (xx) a stripping gas treatment unit (26) comprising at least one compressor (27), the column (28) and a reflux supply vessel (28a), the reflux supply vessel (28a) being connected to the gas circulation line (14) of the gas phase reactor (13) via a stripping line (29); (xxi) a cooling circuit (30) for said circulation gas cooler (16); (xxii) a blowdown unit (31) including a high pressure blowdown bin (32), a low pressure blowdown bin (33), preferably connected to the product receiver tank (22) via a connecting line (34); (xxiii) a recovery supply line (35) connecting the recovery gas treatment unit (26) to the propylene supply tank (7); The plant of claim 1 further comprising:

3. below (xxiv) knockout drum (38), (xxv) gas phase reactor zone decompression cyclone (39); (xxvi) Gas Phase Reactor Flare Knockout Drum (40) 3. The plant of claim 1 or claim 2, further comprising one or more of:

4. The plant according to any one of claims 1 to 3, wherein the loop reactor connecting line (10) comprises a concentrator (101).

5. 5. The plant of claim 4, wherein the concentrator (101) is a hydrocyclone.

6. 1. A process for preparing a propylene homopolymer or a propylene copolymer, comprising the steps of: a) providing a single-site catalyst, an optional cocatalyst, and an optional activator in supply tanks (1, 2, 3); b) feeding said single-site catalyst, optionally said cocatalyst, and optionally said activator to a precontacting unit (4) to provide a mixed catalyst system, and feeding said mixed catalyst system to a prepolymerization reactor. or feeding said single-site catalyst, optionally said cocatalyst, and optionally said activator to a prepolymerization reactor (6); c) initiating prepolymerization by introducing propylene monomer and, optionally, comonomer, thereby providing a prepolymer; d) feeding said prepolymer into a first loop reactor (8) to polymerize propylene with optional comonomers to obtain a first intermediate; e) feeding said first intermediate through a loop reactor connecting line (10) to a second loop reactor (9) for further polymerizing propylene, optionally with a comonomer, to obtain a second intermediate; f) the first loop reactor has a volume of up to 40% by volume relative to the combined volume of the first loop reactor and the second loop reactor, the second loop reactor has a volume of at least 60% by volume relative to the combined volume of the first loop reactor and the second loop reactor, and the combined volume of the first loop reactor and the second loop reactor is between 50 and 150 m ３ and there is a single feed point for feeding monomer to said first loop reactor (8), for feeding optional comonomer to said first loop reactor (8) (11) and for feeding hydrogen to said first loop reactor (8) (12), and a single feed point for feeding monomer to said second loop reactor (9), for feeding optional comonomer to said second loop reactor (9) (11) and for feeding hydrogen to said second loop reactor (9) (12), feeding said second intermediate containing unreacted monomer directly to a gas phase reactor (13) via a direct feed line (17); g) further polymerizing propylene and optional comonomers in said gas phase reactor (13) by feeding propylene and / or comonomers and / or hydrogen and circulating gas in an upward direction via a gas circulation line (14); h) optionally discharging the gas-phase reactor product to an optional product discharge vessel (20) and subsequently to a product receiver tank (22) via a connecting line (34) to provide a raw mixture; or discharging the gas-phase reactor product directly into a product receiver tank (22) to provide a raw mixture. The method includes:

7. 7. The method of claim 6, wherein the first intermediate is subjected to a thickener, preferably a hydrocyclone.

8. 2. Use of a plant according to claim 1 for preparing propylene homopolymers or preferably propylene copolymers.

9. 2. Use of a plant according to claim 1 for reducing the comonomer / propylene molar ratio drift and / or the hydrogen / propylene molar ratio drift over the length of at least one of said loop reactors.

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