Process

Abstract

The present invention provides a process for producing a polyolefin, the process comprising: a. Reacting one or more olefins in a polymerization reactor to produce a polyolefin, b. Withdrawing a polymerization effluent stream comprising polyolefin solids, c. Transferring the effluent to a first degassing vessel in which the polyolefin solids are degassed, the degassing vessel operating at a pressure P1, d. Opening the solids inlet of the connection container and transferring the polyolefin solids from the first degassing container via the solids inlet to the connection container until a volume of polyolefin solids is collected in the connection container, and wherein, once the volume of polyolefin solids is collected: i. Closing the solids inlet of the connection container, ii. Reducing the pressure in the connection vessel to a pressure P2 via the exhaust line of the connection vessel, iii. After reducing the pressure, opening the solids outlet of the connection vessel and transferring the polyolefin solids via the solids outlet to a second degassing vessel, and iv. After transferring the polyolefin solids to the second degassing vessel, closing the solids outlet, e. Degassing the polyolefin solids in a second degassing vessel, the second degassing vessel operating at a pressure P3, characterized in that i. The volume of the polyolefin solids collected in step (d) prior to steps (i)-(iv) is at least 50% of the volume of the connecting vessel, iii. P2 is at least 20% lower than pressure P1, iii. The connecting vessel has a cylindrical body, a top section is connected to the top of the cylindrical body and a bottom section is connected to the bottom of the cylindrical body, where the cylindrical body has H / D less than 2, H being the height of the container and D being the diameter, and iv. An exhaust line is located on the top section connected to the container or on the upper portion of the cylindrical body connected to the container, but above the solids of the collected volume.

Description

[0001] The present invention relates to the production of polyolefins, and in particular to a method for producing polyolefins involving serially connected degassing containers.

[0002] The production of polyolefins by the polymerization of olefin monomers (optionally olefin comonomers) in the presence of a polymerization catalyst is a well-known and widely commercially operated process. Different polymerization processes are known, including those operating in gas phase or slurry phase or a combination thereof, and with different types of reactors, including slurry loop reactors, stirred tank slurry reactors, horizontally or vertically oriented stirred bed reactors and fluidized bed reactors.

[0003] All methods need to separate the polyolefin product from unreacted monomer to a certain extent, and also need to separate from the inert components such as diluent or inert gas existing in the polymerization process usually.The component to be separated usually comprises the liquid phase or the gas phase on all sides of the polyolefin particles, but also comprises the component that may be initially absorbed on or in the polyolefin particles.The removal of this type of component is generally referred to as degassing.Many steps that can be considered as degassing steps are known in the art.These steps comprise decompression so that liquid evaporates or assists the desorption of absorbed components, and with gas flushing or purging so that gas and evaporation components are separated from polymer.

[0004] Degassing can (and often advantageously) be performed in several steps in series. For example, for a slurry polymerization process, a first degassing step can be performed in which the slurry is heated and depressurized to evaporate and remove most of the liquid components in the slurry for recycling. The polyolefin solids are then further depressurized and purged with a gas such as nitrogen to remove the remaining components.

[0005] WO 2005 / 003188 discloses a method for treating polyolefins discharged from a slurry polymerization process comprising two degassing steps using an intermediate concentrating vessel. The method involves a series of filling and transfers to minimize the transfer of hydrocarbon components to the downstream degassing step.

[0006] We have now discovered a method for further improving the transfer of polyolefin solids from a first degassing vessel to a second degassing vessel.

[0007] Therefore, the present invention provides a method for producing polyolefins, the method comprising:

[0008] a. reacting one or more olefins in a polymerization reactor to produce a polyolefin,

[0009] b. removing a polymerization effluent stream comprising polyolefin solids,

[0010] c. transferring the effluent to a first degassing vessel in which the polyolefin solids are degassed, the degassing vessel operating at a pressure P1,

[0011] d. opening the solids inlet of the connecting vessel and transferring the polyolefin solids from the first degassing vessel to the connecting vessel via the solids inlet until a certain volume of polyolefin solids is collected in the connecting vessel, and wherein, once the volume of polyolefin solids is collected, then:

[0012] i. Close the solid inlet of the connecting container,

[0013] ii. reducing the pressure in the connected container to pressure P2 via the exhaust line connected to the container,

[0014] iii. After decompression, open the solid outlet of the connecting container and transfer the polyolefin solid to the second degassing container via the solid outlet, and

[0015] iv. After transferring the polyolefin solids to the second degassing vessel, closing the solids outlet,

[0016] e. degassing the polyolefin solids in a second degassing vessel, the second degassing vessel operating at a pressure of P3,

[0017] It is characterized by

[0018] i. the volume of the polyolefin solids collected in step (d) prior to steps (i)-(iv) is at least 50% of the volume of the connected container,

[0019] ii. P2 is at least 20% lower than pressure P1,

[0020] iii. The connected container has a cylindrical body with a top section connected to the top of the cylindrical body and a bottom section connected to the bottom of the cylindrical body, wherein the cylindrical body has an H / D of less than 2, H being the height of the container and D being the diameter, and

[0021] iv. The vent line is located on the top section of the connecting vessel or on the upper portion of the cylindrical body of the connecting vessel, but above the collected volume of solids.

[0022] The present invention provides a method for producing polyolefins. The method comprises reacting one or more olefins in a polymerization reactor to produce the polyolefin. The method can be any suitable polymerization process, including fluidized bed gas phase processes, stirred bed gas phase processes, and slurry phase processes. The polyolefin is preferably polyethylene or polypropylene. The one or more olefins are selected accordingly. Thus, for the production of polyethylene, the one or more olefins will comprise ethylene, optionally with one or more other olefins as comonomers. For the production of polypropylene, the one or more olefins will comprise propylene, optionally with one or more other olefins as comonomers.

[0023] Other reactants and components of the reaction mixture, such as hydrogen, inert hydrocarbons, may be present as desired and as known in the art.

[0024] Most preferred are processes for producing polyethylene by reacting one or more olefins including ethylene.

[0025] The present invention is most preferably a slurry phase polymerization process, most preferably for the production of polyethylene, by reacting one or more olefins including ethylene.

[0026] A polymerization effluent stream comprising polyolefin solids is withdrawn from the reactor. In a gas phase process this typically comprises polymer solids and entrained gas, while in a slurry phase process this comprises polymer solids and a liquid diluent mixture.

[0027] In a most preferred embodiment, the process is a process for producing polyethylene comprising reacting ethylene and optionally one or more comonomers in one or more slurry loop polymerization reactors to produce polyethylene. The polymerization effluent in such a process typically comprises polyethylene solids in a liquid diluent, most preferably isobutane.

[0028] The method may comprise a single slurry loop polymerization reactor, or two or more slurry loop polymerization reactors in parallel or preferably in series. The example of a suitable polymerization process includes WO 2006 / 015807, WO 2005 / 003188 and WO 2013 / 135565. When there are two or more polymerization reactors in series, the effluent of step (b) is the effluent taken out from the last reactor in series. When there are two or more polymerization reactors in parallel, then in step (b), take out independent effluent from each reactor, and these effluents may preferably be merged before transferring to the first degassing vessel, or may be transferred to the first degassing vessel respectively and then merged therein.

[0029] The effluent is passed to a first degassing vessel, where the polyolefin solids are degassed. Degassing of polyolefins is well known. Specifically, the polyolefin solids are separated from entrained gases in a gas phase process, or from evaporated diluent in a slurry phase process. The effluent may be heated prior to the degassing vessel, for example, to aid evaporation of the diluent.

[0030] The degassing vessel is operated at a pressure P1. Preferably, the pressure P1 is at least 400 kPag, more preferably at least 500 kPag, such as at least 700 kPag. The pressure P1 is typically lower than the pressure in the polymerization reactor. P1 is typically less than 1500 kPag, more typically less than 1200 kPag.

[0031] In a slurry process, as is known in the art, it is preferred that the first degassing vessel is at a pressure high enough that the evaporated diluent can be condensed using cooling water without compression, which will enable efficient recycle to the reactor.

[0032] Generally, it is desirable to separate the majority of the components of the withdrawn effluent, other than the polyolefin solids, from the polyolefin solids in the first degassing vessel so that they can be recycled at the relatively high pressure therein. (As will be described below, the second degassing vessel removes the remaining or residual components (other than the solids) from the effluent at a lower pressure, which can increase recycling costs.)

[0033] In the method of the present invention, a connecting vessel is provided between the first and second degassing vessels. During the method of the present invention, the solids inlet of the connecting vessel is opened, and polyolefin solids are then transferred from the first degassing vessel to the connecting vessel via the solids inlet. This is done until a certain volume of polyolefin solids has been collected in the connecting vessel (which can be considered the "desired volume"). According to the present invention, this volume is at least 50% of the capacity of the connecting vessel.

[0034] Preferably the volume of polyolefin solids collected in step (d) prior to steps (i) to (iv) is at least 60%, such as from 60 to 85%, of the capacity of the connected vessels.

[0035] The connecting vessel is preferably located directly below the first degassing vessel. This allows the transfer of polyolefin solids to be performed using either the pressure P1 in the first degassing vessel or gravity. The latter, while not required, can help achieve high levels of polyolefin solids even after pressure equalization.

[0036] Once the desired volume of polyolefin solids has been collected, the solids inlet of the connecting vessel is closed and the pressure in the connecting vessel is reduced. This reduction in pressure is achieved by opening the vent line of the connecting vessel, particularly by opening a valve on the vent line, to release gas from the connecting vessel. This is done until pressure P2 is reached.

[0037] P2 is preferably at least 30% lower than P1, such as at least 50% lower than P1.

[0038] In absolute terms, the pressure is typically reduced to below 400 kPag, such as below 300 kPag. P2 is typically at least 100 kPag, such as at least 250 kPag.

[0039] In a preferred embodiment, the gas from the connected vessel can be vented to a second connected vessel in parallel via a vent line. Having two or more connected vessels in parallel allows the first degassing vessel to vent to one connected vessel, while another connected vessel vents to the second degassing vessel. This enables higher throughput.

[0040] In embodiments, particularly in the absence of parallel connected vessels, the gas from the connected vessels may be vented via a vent line, for example, to a vessel having a pressure lower than P1, and preferably to a second degassing vessel. In this embodiment, the vent line may be connected to the upper half of the second degassing vessel, and preferably connected to the top of the second degassing vessel or near the top.

[0041] In a particularly preferred embodiment, the vent line is designed to minimize entrainment of polyolefin solids during the venting step. For example, the vent line can have an inner diameter of 1.25 cm (0.5 in) to 7.5 cm (3 in), preferably 2.5 cm (1 in) to 5 cm (2 in), which is selected to provide relatively rapid degassing of the connected vessel without excessive linear gas flow rate (i.e., velocity) in the connected vessel.

[0042] In an embodiment, the connecting vessel may be provided with both a first vent line connected to a second connecting vessel in parallel, and a second vent line connecting the connecting vessel to a vessel having a pressure lower than P1, and preferably to a second degassing vessel (and these are as already described). The first vent line is preferably used for depressurization in step (d)(ii) of the present invention, i.e., after filling of the connecting vessel. The second vent line may be temporarily opened during filling of the connecting vessel, and optionally opened more than once, to allow gas to escape from the connecting vessel and reduce the pressure therein during filling.

[0043] In an embodiment, the connecting vessel may be provided with a high-pressure line (with a valve), and the high-pressure line connects the connecting vessel from typically its upper half, preferably its top or near its top, to the first degassing vessel, also typically its upper half, preferably its top or near its top. For the avoidance of doubt, the high-pressure line (when present) is provided in addition to the solids inlet and one or more exhaust lines of the connecting vessel. The high-pressure line allows gas to flow from the connecting vessel to the first degassing vessel when open. This can provide an equalization of pressure and / or provide an improved flow of solids from the first degassing vessel to the connecting vessel. In a preferred embodiment, the internal diameter of the high-pressure line may be from 5 cm (2") to 20 cm (8").

[0044] For the avoidance of doubt, the term "high pressure line" is not used to indicate a specific pressure requirement per se, but is merely used to reflect that the line is connected to a relatively high pressure vessel (ie the first degassing vessel, which is at pressure P1).

[0045] Typically, when both the first and second vent lines and the high-pressure line are present, the second vent line can be used at a relatively early stage (i.e., the valve on this line is opened) to reduce the pressure during the filling of the connecting container, while the high-pressure line can be used at a relatively late stage of filling. These lines should not generally be used at the same time (their valves are opened). Once the (required) volume of polyolefin solids has been collected in the connecting container, the solid inlet of the connecting container is closed. If the high-pressure line and the second vent line have not yet been closed, they should also be closed. The pressure in the connecting container is then reduced to a pressure P2 via the first vent line. In particular, once the volume of polyolefin solids has been collected, the pressure in the connecting container is substantially balanced with the pressure in the first degassing container and will be approximately equal to P1.

[0046] After decompression, the solid outlet of the connecting container is opened and the polyolefin solid is transferred to the second degassing container via the solid outlet. Once the polyolefin solid in the connecting container has been transferred to the second degassing container, the solid outlet is closed.

[0047] It should be noted that at this stage, the solids inlet of the connecting container can be opened again to the first degassing container and another cycle of filling and decompression is carried out. Preferably, before this happens, the connecting container is at least partially re-pressurized. In an embodiment with a second connected container in parallel, this can be performed by venting the parallel (second) connected container to the empty (first) connected container via an exhaust line. In particular, the pressure in the empty connected container can be increased by using the gas released by decompressing the filled parallel connected container (from P1 to pressure P2). Alternatively, and in particular in the absence of a parallel concentration container, re-pressurization can be obtained using gas from the first degassing container or elsewhere.

[0048] Partial re-pressurization allows the collection vessel to be checked for leaks before starting to fill it with solids.

[0049] More typically, the polyolefin solids are transferred to a second degassing vessel and then degassed therein. The second degassing vessel operates at a pressure P3. P3 is less than P1. The second degassing step is also generally known in the art.

[0050] Suitably, the pressure P3 is less than 100 kPag, more typically less than 50 kPag.

[0051] Degassing in the second degassing vessel may include removing residual hydrocarbons (from the effluent) using a purge gas, also as known in the art.

[0052] As mentioned above, the present invention is characterized in that the volume of the polyolefin solids collected in step (d) prior to steps (i) to (iv) is at least 50% of the capacity of the connected containers.

[0053] "Substantial" filling of the connecting vessel is advantageous because it means that less volume of the connecting vessel is filled with other components of the effluent. This will therefore minimize the volume of such components that are subsequently transferred to the second degassing vessel together with the polyolefin (and thus reduce the proportion of the transferred polyolefin).

[0054] However, increasing the amount of polyolefin solids generally increases the risk of polyolefin solids being carried over during depressurization of the connected vessel. We have found that this risk can be minimized by selecting a maximum H / D ratio for the connected vessel, particularly when the H / D ratio is less than 2. This finding may be surprising because, for a given "fill," e.g., 70% of the connected vessel, the height of the surface of polyolefin solids will be physically closer to the gas outlet in a vessel with a lower H / D ratio than in a vessel with a higher H / D ratio. However, we have found that, when the connected vessel is depressurized, the physical separation of the solids from the gas outlet is less important than the initial gas velocity, and a lower H / D ratio is advantageous.

[0055] In a preferred embodiment, H / D is less than 1.5, such as between 0.5 and 1.5.

[0056] Furthermore, by ensuring that the vent line (through which depressurization occurs) is located at or near the top of the connecting vessel, entrainment during depressurization to pressure P2 is minimized. In a preferred embodiment, the vent line is located on the top section, preferably as close to the top as possible. However, it can also be located on the upper portion of the cylindrical body, particularly in the top 10% of the cylindrical body, as long as it is above the collected solids volume. If a second vent line and / or a high-pressure line are present, the same preferred locations apply to these lines.

[0057] (It should be noted that the collected volume of solids does not usually have a flat surface, but rather is in the shape of a cone. The exhaust line (and other lines, if present) should be located above the surface of the cone on the side of the cylindrical body.)

[0058] More generally, the top section of the connecting container preferably has the approximate shape of a hemisphere or cone, with its maximum height at the center. The average angle between the top section and the horizontal plane (measured as the angle between a line drawn between the top at the center of the top section and the side of the top section connected to the cylindrical section and the horizontal plane) is preferably similar to the angle of repose of the polyolefin solid, particularly within 20° of the angle of repose. This allows the top section to be filled with polyolefin solids, if desired, while minimizing the free space between the surface of the collected polyolefin solids and the top of the connecting container.

[0059] In an embodiment, all or part of one or more of the first degassing vessel, the connecting vessel, and the second degassing vessel may be polished or lined, in particular a lining that provides low friction. In particular, polishing or using a lining can provide improved powder flow. Typically, for example, the bottom of the first degassing vessel will be in the form of a cone, and this can be polished or lined to help the polyolefin solids flow out of the first degassing vessel (and to the connecting vessel). Similarly, the bottom of the connecting vessel will typically be in the form of a cone, and at least this portion of the connecting vessel can be polished or lined (regardless of whether any portion of the first degassing vessel is polished or lined) to help the polyolefin solids flow out of the connecting vessel. Similarly, the bottom of the second degassing vessel will typically be in the form of a cone, and this can be polished or lined to help the polyolefin solids flow out of the second degassing vessel (to downstream processing or storage). (And this is independent of whether the first degassing vessel and / or the connecting vessel are polished or lined.)

[0060] Internal piping in connecting lines and components such as valves may also be polished or, where appropriate, lined to aid in the flow of the polyolefin solids.

[0061] As mentioned above, entrainment will be further minimized by careful selection of the diameter of the vent line. Thus, the inner diameter of the vent line is typically 1.25 cm (0.5 in) to 7.5 cm (3 in), preferably 2.5 cm (1 in) to 5 cm (2 in), which is selected to provide relatively rapid degassing of the connected vessel without excessive linear gas flow rate in the connected vessel. Example

[0062] This embodiment illustrates the method according to the invention, in particular using two connected containers arranged in parallel.

[0063] Ethylene was polymerized in the presence of isobutane diluent and a polymerization catalyst in a slurry polymerization system comprising two slurry loop reactors connected in series to produce high density polyethylene (HDPE). The reaction was carried out at about 4000 kPag.

[0064] A slurry stream containing HDPE and diluent is continuously withdrawn from the second polymerization reactor at a rate of 42,000 kg / hr for the HDPE phase and 53,000 kg / hr for the diluent phase. The slurry stream is depressurized and heated, then transferred to a first degassing vessel, which operates at a pressure P1 of 800 kPag. Degassing occurs in the first degassing vessel, and the evaporated diluent phase (including unreacted olefin monomer) is separated from the HDPE solids, with most of the evaporated phase discharged from the top of the first degassing vessel. The remaining portion of the HDPE and evaporated diluent phase is discharged into one of two connecting vessels (the "first" and "second" connecting vessels) arranged in parallel, through a solids inlet with a valve at the top of the connecting vessel. Each connecting vessel also has a solids outlet with a valve at its base and is connected to the second degassing vessel. The second degassing vessel operates at a pressure P3 of 30 kPag.

[0065] Each connecting container has a cylindrical section with a height to inner diameter ratio of 0.875 and (2) tapered heads on both ends of the cylindrical section. 3 Nominal volume.

[0066] A first exhaust line connects the two connecting vessels to each other. Specifically, the first exhaust line has an inner diameter of 5 cm (2") and connects the upper conical head of one connecting vessel to the upper conical head of the other connecting vessel. Each connecting vessel is also provided with: a second exhaust line having an inner diameter of 2.5 cm (1") and connected from the top conical head of the connecting vessel to the top of the second degassing vessel; and a 15 cm diameter (6") high-pressure line connected from the top conical head of the connecting vessel to the top of the first degassing vessel. (These are provided in addition to the solids inlet and solids outlet of the connecting vessels.)

[0067] The procedure for discharging HDPE from the first degassing vessel begins by opening the solids inlet valve of the (first) connecting vessel. Prior to opening the solids inlet, the connecting vessel is at a pressure of 250 kPag. The connecting vessel begins filling with solids. Five seconds later, the valve on the second vent line connecting the top of the connecting vessel to the inlet of the second degassing vessel is partially opened for three seconds to allow vapors to escape from the connecting vessel. After closing this valve, the valve on the high-pressure line connecting the top conical head of the connecting vessel to the top of the first degassing vessel is opened to allow the vapor spaces in the two vessels to equalize. Filling continues until the connecting vessel reaches a level of 60% full (i.e., the volume of polymer solids is 60% of the connecting vessel's capacity). At this point, the solids inlet valve of the connecting vessel is closed, as is the valve on the high-pressure line connecting the connecting vessel to the first degassing vessel. After filling, the pressure in the connecting vessel is 800 kPag (i.e., the same as in the first degassing vessel).

[0068] The pressure in the connecting vessel is reduced to a pressure P2 of 250 kPag by opening the first vent line connecting the connecting vessel to the parallel connecting vessel for 10 seconds. (The second connecting vessel is then also at a pressure of 250 kPag.)

[0069] (Note that in a system without two concentrating vessels connected in parallel, the vent line connecting the concentrating vessel and the second degassing vessel can be used as the vent line in this step, and the pressure in the connected vessels can be reduced by opening this line for a suitable period of time.)

[0070] After the pressure in the connecting vessel is reduced, the solid outlet valve of the connecting vessel is opened and the HDPE solid is transferred to the second degassing vessel via the solid outlet of the connecting vessel.

[0071] After 15 seconds, the solid outlet valve was closed. By this time, the HDPE solid in the connected container was empty and under a pressure of 30 kPag, which was the pressure of the second degassing container.

[0072] In the second degassing vessel, degassing of the HDPE solid was carried out at a pressure P3 of 30 kPag.

[0073] While the first connected vessel is emptying into the second degassing vessel, the inlet valve of the second connected vessel is opened so that it can be simultaneously filled with HDPE from the first degassing vessel. An identical procedure is followed, including venting the second degassing vessel to the first degassing vessel after the filling of the second degassing vessel (and the emptying of the first degassing vessel) is complete. Specifically, after closing the solids inlet of the second connected vessel and the solids outlet of the first connected vessel, the first vent line is opened to connect the second connected vessel to the first connected vessel. The pressure of the second connected vessel is reduced to 250 kPag, while the first connected vessel is re-pressurized to the same 250 kPag, ready for refilling.

[0074] The steps of filling and emptying the connected containers can then be repeated.

Claims

1. A method for producing polyolefins, comprising: a. reacting one or more olefins in a polymerization reactor to produce a polyolefin, b. removing a polymerization effluent stream comprising polyolefin solids, c. transferring the effluent to a first degassing vessel in which the polyolefin solid is degassed, the degassing vessel operating at a pressure P1, d. opening a solids inlet of the connecting vessel and transferring the polyolefin solids from the first degassing vessel via the solids inlet to the connecting vessel until a volume of polyolefin solids is collected in the connecting vessel, and wherein, once the volume of polyolefin solids is collected: i. closing the solid inlet of the connection container, ii. reducing the pressure in the connected container to a pressure of P2 via the exhaust line of the connected container, iii. After decompression, open the solid outlet of the connected container and transfer the polyolefin solid to the second degassing container via the solid outlet, and iv. After transferring the polyolefin solids to the second degassing vessel, closing the solids outlet, e. degassing the polyolefin solid in the second degassing vessel, the second degassing vessel operating at a pressure P3, It is characterized by i. the volume of the polyolefin solids collected in step (d) prior to steps (i)-(iv) is at least 50% of the volume of the connected container, ii. P2 is at least 20% lower than pressure P1, iii. The connected container has a cylindrical body with a top section connected to the top of the cylindrical body and a bottom section connected to the bottom of the cylindrical body, wherein the cylindrical body has an H / D of less than 2, H being the height of the container and D being the diameter, and iv. The vent line is located on the top section of the connecting vessel or on the upper portion of the cylindrical body of the connecting vessel, but above the collected volume of solids.

2. The process according to claim 1, wherein the process is a process for producing polyethylene, which comprises reacting ethylene and optionally one or more comonomers in one or more slurry loop polymerization reactors to produce the polyethylene.

3. A method according to claim 1 or claim 2, wherein the degassing vessel is operated at a pressure P1 of at least 500 kPag, such as at least 700 kPag.

4. The method according to any one of the preceding claims, wherein the connecting vessel is located directly below the first degassing vessel so that the transfer of the polyolefin solids can be performed using both the pressure P1 in the first degassing vessel and gravity.

5. A process according to any one of the preceding claims, wherein the volume of polyolefin solids collected in step (d) prior to steps (i) to (iv) is at least 60%, such as 60 to 85%, of the capacity of the connected vessel.

6. A method according to any one of the preceding claims, wherein the pressure in the connected vessel is reduced to a pressure P2 which is at least 50% lower than P1.

7. A method according to any one of the preceding claims, wherein gas from the connected vessel is exhausted to a second connected vessel in parallel.

8. The method according to any one of the preceding claims, wherein the exhaust line has an internal diameter of 1.25 cm (0.5 in) to 7.5 cm (3 in), preferably 2.5 cm (1 in) to 5 cm (2 in).

9. The method according to any one of the preceding claims, wherein after transferring the polyolefin solids in the connecting vessel to the second degassing vessel, the solids outlet is closed and the connecting vessel is at least partially re-pressurized.

10. The method according to claim 9, wherein the connecting container is connected to a parallel connecting container via the exhaust line, and the repressurizing is performed using gas released by depressurizing the filled parallel connecting containers.

11. A method according to any one of the preceding claims, wherein the pressure P3 is below 100 kPag, and more typically below 50 kPag.

12. A method according to any one of the preceding claims, wherein degassing in the second degassing vessel comprises using a purge gas to remove residual hydrocarbons.

13. A method according to any one of the preceding claims, wherein H / D is less than 1.5, such as between 0.5 and 1.

5.

14. The method according to any of the preceding claims, wherein the exhaust line is located on the top section of the connecting vessel or on the upper part of the cylindrical body of the connecting vessel.

15. The method according to any one of the preceding claims, wherein the top section of the connecting vessel is in the approximate shape of a hemisphere or cone with a maximum height at the center and an average angle with the horizontal plane within 20° of the angle of repose of the polyolefin solid.