A reactor assembly and method of depressurising a reactor assembly

Abstract

A Reactor Assembly and Method of depressurising a Reactor Assembly A reactor assembly for polymerising olefins and a method of depressurising a reactor assembly for polymerising olefins. The reactor, the assembly comprising a gas phase reactor for polymerising olefin to produce a polyolefin component, said gas phase reactor being a fluidized bed reactor and having a circulation gas inlet and a circulation gas outlet and a circulation line for circulating fluidisation gas, the circulation line fluidly connecting the circulation gas outlet to the circulation gas inlet and being configured to facilitate a flow of the fluidisation gas in a direction from the circulation gas outlet to the circulation gas inlet. The reactor assembly further comprising a compression unit disposed on the circulation line for pressurising the fluidisation gas and a depressurisation line for depressurising the fluidisation gas, the depressurisation line being fluidly connected to a connection section of the circulation line upstream of the compression unit. The depressurisation line extends from the connection section of the circulation line at an angle between 110 degrees and 170 degrees relative to a first section of the circulation line that is downstream of and adjacent to the connection section, thereby allowing a direction of flow of fluidisation gas along the depressurisation line at an angle of at least 90 degrees relative to a direction of flow of the fluidisation gas through the circulation line downstream of the connection section.

Description

[0001] A Reactor Assembly and Method of depressurising a Reactor Assembly

[0002] The invention relates generally to reactor assemblies. More particularly, the invention relates to a reactor assembly for polymerising olefins and a method for depressurising a reactor assembly.

[0003] BACKGROUND

[0004] It is known to provide reactor assemblies for polymerising olefins, such as polyethylenes or polypropylenes, that have a gas phase reactor. The gas phase reactor may be used for polymerising olefin to produce a polyolefin component. During use, depressurisation of the gas phase reactor may be required. In depressurisation, a gas stream of fluidisation gas, which typically has a high velocity, is directed to a flare system along a depressurisation line. This depressurizing gas stream can carry fine polymer particles from the reactor. When the velocity of the depressurizing gas stream exceeds the terminal velocity of the fine polymer particles, the depressurizing gas stream carries an increased number of fine polymer particles compared to when the velocity is less than the terminal velocity of the fine polymer particles. If a significant number of fine polymer particles enter the depressurisation line and the flare system, the particles can be detrimental to the operation of the reactor assembly. For example, the depressurisation line may be plugged by the polymer particles. Polymer particles that end up in the flare system may reduce the performance of and / or may plug the flare system. Additionally, polymer particles that end up in the flare system cause an increased need to empty the flare tanks.

[0005] As such, there exists a need to reduce the number of fine polymer particles entering the depressurisation line and which are carried by a depressurizing gas stream during depressurisation. The present invention provides a solution to this need. SUMMARY OF THE DISCLOSURE

[0006] According to a first aspect of the invention, there is provided reactor assembly for polymerising olefins. The reactor assembly comprising: a gas phase reactor for polymerising olefin to produce a polyolefin component, said gas phase reactor being a fluidized bed reactor and having a circulation gas inlet and a circulation gas outlet; a circulation line for circulating fluidisation gas, the circulation line fluidly connecting the circulation gas outlet to the circulation gas inlet and being configured to facilitate a flow of the fluidisation gas in a direction from the circulation gas outlet to the circulation gas inlet; a depressurisation line for depressurising the fluidisation gas, the depressurisation line being fluidly connected to a connection section of the circulation line; and wherein the depressurisation line extends from the connection section of circulation line at an angle of at least 90 degrees and less than 180 degrees relative to a first section of the circulation line that is downstream of and adjacent to the connection section, thereby allowing a direction of flow of fluidisation gas along the depressurisation line at an angle of at least 90 degrees and less than 180 degrees relative to a direction of flow of the fluidisation gas through the circulation line downstream of the connection section.

[0007] Orientating the depressurisation line at an angle of least 90 degrees relative to a direction of flow of the fluidisation gas through the circulation line may advantageously reduce the number of fine polymer particles that enter the depressurisation line with the fluidisation gas during depressurisation.

[0008] The reactor assembly may comprise a compression unit disposed on the circulation line for pressurising the fluidisation gas in the circulation line. Improved results may be achieved when the depressurisation line is fluidly connected to the circulation line upstream of the compression unit. The depressurisation line is preferably fluidly connected to the circulation line between the circulation outlet and the compression unit.

[0009] Optionally, the reactor assembly may be for polymerising olefins in a multistage polymerisation process configuration and may comprise an upstream reactor for polymerising olefin to produce a first polyolefin component. The gas phase reactor may be in fluid communication with the upstream reactor and the gas phase reactor may be for polymerising olefin to produce a second polyolefin component in the presence of the first polyolefin component.

[0010] The reactor assembly may comprise a flare unit. The depressurisation line may be in fluid communication with the flare unit. The depressurisation line may be configured to deliver fluidisation gas to the flare unit.

[0011] Preferably, the depressurisation line extends from the connection section of the circulation line at an angle between 110 degrees and 170 degrees relative to the first section of the circulation line. That is, the angle may in a range from 110 degrees to 170 degrees relative to the first section of the circulation line. Having the depressurisation line extend from the circulation line at an angle less than 170 degrees may improve the ease of connecting the depressurisation line to the circulation line. More preferably, the depressurisation line extends from the connection section of the circulation line at an angle between 120 degrees and 160 degrees relative to the first section of the circulation line. That is, the angle may in a range from 120 degrees to 160 degrees relative to the first section of the circulation line. It was observed that the number of fine polymer particles that enter the depressurisation line with the fluidisation gas during depressurisation may be further reduced when such angles are used. Optionally, the connection section of the circulation line may be orientated vertically. Therefore, the depressurisation line may be connected to part of the circulation line which is orientated vertically (i.e. parallel to the direction of gravitational force) and through which the flow of fluidisation gas is downwards (i.e. in the direction of gravitational force) during use. This arrangement improves the likelihood of polymer particles staying in the circulation line rather than entering the depressurisation line and this arrangement also causes any polymer particles which settle in the depressurisation line to fall, under gravity, back into the circulation line.

[0012] The reactor assembly may comprise a cooler disposed on the circulation line downstream of the compression unit.

[0013] In certain embodiments, the reactor assembly may comprise a control valve configured to control the flow of fluidisation gas along the depressurisation line. The reactor assembly may comprise a controller configured to operate the control valve to selectively cause fluidisation gas from the circulation line to flow along the depressurisation line.

[0014] The control valve may be a variable valve.

[0015] The controller may be further configured to operate the control valve to control a mass flow rate and a volumetric flow rate of the fluidisation gas along the depressurisation line such the mass flow rate is less than an upper mass flow rate limit and the volumetric flow rate is less than an upper volumetric flow rate limit.

[0016] A second aspect of the invention concerns a method for depressurising a reactor assembly for polymerising olefins, the assembly comprising a gas phase reactor having a circulation gas inlet and a circulation gas outlet; a circulation line for circulating a fluidisation gas, the circulation line fluidly connecting the circulation gas inlet to the circulation gas outlet; and a depressurisation line for depressurising the gas, the depressurisation line being fluidly connected to the circulation line, the method comprising:

[0017] (i) causing the fluidisation gas to flow from the circulation line into and along the depressurisation line to depressurise the gas phase reactor;

[0018] (ii) limiting a mass flow rate of the fluidisation gas along the depressurisation line to less than an upper mass flow rate limit; and

[0019] (iii) limiting a volumetric flow rate of the fluidisation gas along the depressurisation line to less than an upper volumetric flow rate limit.

[0020] The method may further comprise delivering fluidisation gas from the depressurisation line to a flare unit.

[0021] The method may comprise determining the mass flow rate of fluidisation gas along the depressurisation line; and determining the volumetric flow rate of fluidisation gas along the depressurisation line. The step of limiting the mass flow rate of the fluidisation gas along the depressurisation line may be based on the determined mass flow rate and the step of limiting the volumetric flow rate of the fluidisation gas along the depressurisation line is based on the determined volumetric flow rate.

[0022] Preferably, the method may comprise providing the reactor assembly according to the first aspect of the invention.

[0023] Preferably, the fluidisation gas may flow along the depressurisation line at an angle of at least 90 degrees and less than 180 degrees relative to the first section the circulation line.

[0024] Preferably, the fluidisation gas flows along the depressurisation line at an angle between 110 degrees and 170 degrees relative to the first section of the circulation line. More preferably, the fluidisation gas flows along the depressurisation line at an angle between 120 degrees and 160 degrees relative to the first section of the circulation line.

[0025] Optionally, the depressurisation line is connected to a section of the circulation line through which fluidisation gas flows downwards (i.e. in the direction of gravitational force). As such, the flow of fluidisation gas along the depressurisation line is at least partially against the direction of gravitational force. This improves the likelihood that polymer particles stay in the circulation line rather than enter the depressurisation line and also causes any polymer particles which settle in the depressurisation line to fall, under gravity, back into the circulation line.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures, in which:

[0028] Figure 1 schematically shows a reactor according to an embodiment of the invention;

[0029] Figure 2 schematically shows a part of the reactor assembly of Figure 1 ;

[0030] Figure 3 a graph illustrating an example of mass flow and volumetric flow of fluidisation gas during depressurisation of the reactor assembly of Figure 1.

[0031] DETAILED DESCRIPTION

[0032] Figure 1 shows a reactor assembly 1 according to an embodiment of the invention and Figure 2 shows a part of the reactor assembly 1 of Figure 1. In particular, Figure 2 shows a section of a depressurisation line and a circulation line if the reactor assembly as described further below. The reactor assembly 1 is for polymerising olefins.

[0033] The reactor assembly 1 comprises a gas phase reactor 2 for polymerising olefin to produce a polyolefin component. The gas phase reactor 2 may be a fluidized bed reactor. The longitudinal axis A of the gas phase reactor 2 may be orientated parallel to the direction of gravitational force. As such, the gas phase reactor 2 may be orientated vertically. The gas phase reactor 2 may comprise a bottom zone 4, a middle zone 6 and a top zone 8. The bottom zone 4 forms the lower part of the gas phase reactor 2. The bottom zone 4 may be at least partially conical in shape. The gas phase reactor 2 may comprise a gas distribution plate 10 between the bottom and middle zones 4, 6. During use, a fluidized bed may be located above the gas distribution plate 10. The gas distribution plate 10 may divide the flow of gas through the gas phase reactor 2 evenly through the cross-sectional area of the fluidized bed. The gas distribution plate 10 may cause gas to flow along the inner walls of the gas phase reactor 2. The middle zone 6 may comprise the fluidised bed. The middle zone 6 may be cylindrical in shape. The top zone 8 may form the upper part of the gas phase reactor 2. The top zone 8 of the gas phase reactor 2 may comprise a disengagement zone 12. In the disengagement zone 12, the diameter of the reactor may increase to reduce the gas velocity and allow the polymer particles in the gas phase reactor 2 that are carried upwards from the fluidized bed to settle back down to the bed. As such, at least part of the top zone 8 may be conical in shape.

[0034] The gas phase reactor 2 has a circulation gas inlet 14 and a circulation gas outlet 16. During use, fluidisation gas may enter the gas phase reactor 2 through the circulation gas inlet 14 and fluidisation gas may exit the gas phase reactor 2 through the circulation gas outlet 16. As shown in the embodiment in the Figures, the circulation gas inlet 14 may be in the bottom zone 4 of the gas phase reactor 2. The circulation gas inlet 14 may be positioned in the lowest part of the gas phase reactor 2. The circulation gas outlet 16 may be in the top zone 8 of the gas phase reactor 2. The circulation gas outlet 16 may be positioned above the disengagement zone to reduce the number of polymer particles present in fluidisation gas exiting the gas phase reactor 2 via the outlet 16. The circulation gas outlet 16 may be positioned at the highest part of the gas phase reactor 2. Therefore, during use, the flow of fluidisation gas through the gas phase reactor 2 is in an upwards direction, from the circulation gas inlet 14 to the circulation gas outlet 16. The net flow of fluidisation gas through the gas phase reactor 2 is parallel to the longitudinal axis A of the gas phase reactor 2. The gas phase reactor 2 may have further inlets and outlets in addition to the circulation gas inlet 14 and the circulation gas outlet 16.

[0035] As shown in the embodiment in the Figures, the gas phase reactor 2 may comprise a feed inlet 18. The feed inlet 18 may be in the middle zone 6 of the gas phase reactor 2. The feed inlet 18 may be positioned above the gas distribution plate 10. The feed inlet 18 may be connected to a feed line 20. The feed line 20 may comprise any suitable line, pipe or conduit. The feed line 20 and feed inlet 18 may, for example, be for delivering monomer, comonomer and / or polymer particles to the gas phase reactor 2. As shown in the embodiment in the Figures, the reactor assembly 1 may be for polymerising olefins in multistage polymerisation process configuration. The reactor assembly 1 may comprise an upstream reactor 22 for polymerising olefin to produce a first polyolefin component. The upstream reactor 22 may comprise a loop reactor. The gas phase reactor 2 may be in fluid communication with the upstream reactor 22. The feed inlet 18 of the gas phase reactor 2 may be fluidly connected to an outlet 24 of the upstream reactor 22. As such, the gas phase reactor 2 receives the output of the upstream reactor 22. The gas phase reactor 2 may be for polymerising olefin to produce a second polyolefin component in the presence of the first polyolefin component from the upstream reactor 22.

[0036] As shown in the embodiment in the Figures, the gas phase reactor 2 may comprise a product outlet 26. The product outlet 26 may be connected to a product line 28. The product line 28 may comprise any suitable line, pipe or conduit. The product outlet 26 and product line 28 are for withdrawing the product stream including, for example, a polyolefin component, from the gas phase reactor 2. The product outlet 26 may be in the middle zone 6 of the gas phase reactor 2. The product outlet 26 may be positioned above the distribution plate. The product stream may be delivered to, for example, a downstream gas phase reactor (not shown). The reactor assembly 1 has a circulation line 30 for circulating fluidisation gas. The circulation line 30 may comprise any suitable line, pipe or conduit for transferring gas. The circulation line 30 fluidly connects the circulation gas inlet 14 to the circulation gas outlet 16. As such, fluidisation gas exiting the gas phase reactor 2 from the circulation gas outlet 16 may be returned to the gas phase reactor 2 through the circulation gas inlet 14. Circulating the fluidisation gas is beneficial because during use, gas exiting the gas phase reactor 2 may include unreacted components which advantageously are returned to the gas phase reactor 2.

[0037] The reactor assembly 1 comprises a compression unit 32 disposed on the circulation line 30 for pressurising the fluidisation gas. The compression unit 32 may comprise one or more compressors 34 or fans suitable for compressing fluidisation gas flowing along the circulation line 30. The compression unit 32 compresses fluidisation gas as the gas flows along the circulation line. The compression unit 32 may comprise a motor 36 connected to the compressor or fan 34. The motor 36 may be configured to drive to compressor or fan 34 to compress the fluidisation gas. The compression unit 32 is downstream of the circulation gas outlet 16 of the gas phase reactor 2.

[0038] As shown in the embodiment in the Figures, the reactor assembly 1 may comprise a cooler 38 disposed on the circulation line 30. The cooler 38 cools fluidisation gas as the gas flows along the circulation line. The cooler 38 is downstream of the compression unit 32. As such, the fluidisation gas is cooled after being compressed and before being returned to the gas phase reactor 2. As shown in the embodiment in the Figures, the cooler 38 may be positioned between the compression unit 32 and the circulation gas inlet 14 of the gas phase reactor 2. The cooler 38 may comprise a heat exchanger or any suitable means for cooling the fluidisation gas.

[0039] Whilst not shown, the circulation line 30 may be configured to allow the addition of further components, such as propylene or hydrogen into the circulation line 30 and, consequently, into the fluidisation gas. During use of the gas phase reactor 2, fluidisation gas flows through the gas phase reactor 2 out of the circulation gas outlet 16 into the circulation line 30. The fluidisation gas in the circulation line is compressed by the compression unit 32 and then chilled by the cooler 38. The fluidisation gas is then returned to the gas phase reactor 2 through the circulation gas inlet 14 by the circulation line 30.

[0040] The reactor assembly 1 comprises a depressurisation line 40 for depressurising the fluidisation gas. The depressurisation line 40 may comprise any suitable line, pipe or conduit for transferring gas. In the embodiment shown in Figure 1 , the depressurisation line 40 is fluidly connected to the circulation line 30 upstream of the compression unit 32. As shown in the embodiment in the Figures, the depressurisation line 40 may be connected between the circulation gas outlet 16 of the gas phase reactor 2 and the compression unit 32. When the pressure in the gas phase reactor 2 needs to be reduced, fluidisation gas can be directed from the circulation line 30 along the depressurisation line 40 to depressurise the reactor assembly 1 . Fine polymer particles may be present in fluidisation gas that enters the circulation line 30 from the circulation gas outlet 16 of the gas phase reactor 2. For example, the disengagement zone 12 may reduce the number of polymer particles in the fluidisation gas but may not remove all the particles. As such, fine polymer particles may enter depressurisation line 40 from the circulation line 30.

[0041] The circulation line 30 comprises a connection section 46. The depressurisation line 40 extends from the connection section 46 of the circulation line 30. As shown in Figure 2, the connection section 32 is a section of the circulation line 30 which comprises an inlet 49 of the depressurisation line 40. Fluidisation gas can be directed from the circulation line 30 through the inlet 49 into the depressurisation line 40. As shown in the embodiment shown in the Figures, the connection section 46 may extend from the upstream side 49a of the inlet 49 to the downstream side 49b of the inlet 49. The circulation line 30 comprises a first section 47. The first section 47 is downstream of and adjacent to the connection section 46. As shown in Figure 2, the first section 47 may be adjacent to the downstream side 49b of the inlet 49. The first section 47 may begin at the downstream side 49b of the inlet 49 and extend in a downstream direction. As shown in the embodiment shown in the Figures, the orientation of the connection section 46 of the circulation line 32 and the orientation of the first section 47 of the circulation line may be the same as one another. Therefore, fluidisation gas flowing through the connection section 46 flows in the same direction as fluidisation gas flowing through the first section 47.

[0042] As shown in the embodiment in the Figures, the reactor assembly 1 may comprise a flare unit 42. The depressurisation line 40 may be in fluid communication with the flare unit 42. As shown in the embodiment in the Figures, a first end of the depressurisation line 40 may be fluidly connected to the circulation line 30 and a second end of the depressurisation line 40 may be fluidly connected to the flare unit 42. As such, gas may flow from the circulation line 30, along the depressurisation line 40 and into the flare unit 42. The flare unit 42 may comprise a gas combustion device configured to burn the fluidisation gas. As such, it is advantageous that the amount of polymer particles entering the depressurisation line 40 are minimised so that the polymer particles are not burnt in the flare unit 42.

[0043] To reduce the number of fine polymer particles entering the depressurisation line 40, the depressurisation line 40 extends from the connection section 46 circulation line 30 at an angle a of at least 90 degrees and less than 180 degrees relative to the first section 47 of the circulation line 30. This allows a direction of flow of fluidisation gas along the depressurisation line 40 to be at an angle a of at least 90 degrees and less than 180 degrees relative to a direction of flow of the fluidisation gas through the circulation line 30 downstream of the connection section 46. Figure 2 shows a geometry of the depressurisation line 40 and the circulation line 30 in greater detail. However, for simplicity, other features of the reactor assembly 1 are omitted in Figure 2. The direction of flow of fluidisation gas along the circulation line 30 is shown by a first arrow 44 in Figure 2. The second arrow 45 shows a direction of flow of fluidisation gas along the depressurisation line 40. As shown in Figure 2, the angle a between the direction 44 of flow of fluidisation gas along the circulation line 30 and the direction 45 of flow of fluidisation gas along the depressurisation line 40 is the same as the angle a of the depressurisation line 40 relative to the first section 47 of the circulation line 30. Typically, when the depressurisation is needed, the flow rate of the fluidisation gas in the circulation line 30 is greater that the terminal velocity of at least some of the polymer particles in the fluidisation gas. Orientating the depressurisation line 40 at an angle a of least 90 degrees relative to the first section 47 of the circulation line 30 reduces the number of fine polymer particles that enter the depressurisation line 40 with the fluidisation gas during depressurisation because the direction of flow of fluidisation gas along the depressurisation line 40 will be at an angle of at least 90 degrees relative to a direction of flow of the fluidisation gas through the connection section 46 of the circulation line 30.

[0044] Preferably, the depressurisation line 40 extends from the connection section 46 of the circulation line 30 at an angle a between 110 degrees and 170 degrees or at an angle a between 120 degrees and 160 degrees relative to the first section 47 of the circulation line 30.

[0045] Better results may be obtained when the connection section 46 and first section 47 are orientated vertically (i.e. parallel to the direction of gravitational force) and through which the flow of fluidisation gas is downwards (i.e. in the direction of gravitational force).

[0046] Whilst the angle a of the depressurisation line 40 relative to the first section 47 the circulation line 30 reduces the number of fine polymer particles that enter the depressurisation line 40, some particles may still enter the depressurisation line 40. These particles may drop out of the fluidisation gas in the depressurisation line 40 and may build up in the depressurisation line 40. However, it was observed that the build- up of particles is reduced when the depressurisation line 40 is connected to a vertical section of the circulation line 30 where the flow of fluidisation gas is downwards and at an angle a of at least 90 degrees relative to the downwards direction Therefore, connecting the depressurisation line 40 to a vertical section 46 of the circulation line 30 where the flow of fluidisation gas is downwards and having the depressurisation line 40 at the angle a of at least 90 degrees relative to first section 47 of the circulation line 30 reduces the chance of the depressurisation line 40 becoming plugged by fine polymer particles. Providing the depressurisation line 40 at an angle a of 110 degrees or 120 degrees relative to the first section 47 of the circulation line 30 may further reduce the number of fine polymer particles travelling along the depressurisation line 40.

[0047] As shown in the embodiment in the Figures, the reactor assembly 1 may comprise a control valve 48 to control the flow of fluidisation gas from the circulation line 30 along the depressurisation line 40. In the particular, non-limiting, embodiment of Figure 1 , the control valve 48 is positioned at or near the junction between the depressurisation and circulation lines 30, 40. Therefore, to flow along the depressurisation line 40, fluidisation gas must first flow through the control valve 48. The control valve 48 may be a variable control valve 48. The control valve 48 may comprise an opening (not shown) for fluidisation gas to pass through. The opening may be varied in size.

[0048] The reactor assembly 1 may comprise a controller 50. The controller 50 may be operably connected to the control valve 48. The controller 50 may be configured to operate the control valve 48 to control the flow of fluidisation gas to flow along the depressurisation line 40. The controller 50 may comprise one or more processors collectively configured to operate the control valve 48.

[0049] The controller 50 may be configured to operate the control valve 48 to selectively cause fluidisation gas from the circulation line 30 to flow along the depressurisation line 40. The controller 50 may be configured to close the opening of the control valve 48 to prevent fluidisation gas flowing along the depressurisation line 40. To close the opening of the control valve 48, the controller 50 may be configured to send a close signal to the control valve 48. The controller 50 may be configured to open the opening of the control valve 48 to allow fluidisation gas to flow along the depressurisation line 40. To open the opening of the control valve 48, the controller 50 may be configured to send an open signal to the control valve 48. The controller 50 may be configured to open the control valve 48 when depressurisation of the gas phase reactor 2 is required.

[0050] The control valve 48 may be used to control a mass flow rate and / or a volumetric flow rate of fluidisation gas along the depressurisation line 40. The controller 50 may also be configured to operate the control valve 48 to control the mass flow rate and / or the volumetric flow rate of fluidisation gas along the depressurisation line 40. The controller 50 may do this by varying the size of the opening of the control valve 48. Controlling the mass flow rate of fluidisation gas along the depressurisation line 40 may help to ensure that the capacity of the flare unit 42 is not exceeded during depressurisation. Controlling the volumetric flow rate of fluidisation gas along the depressurisation line 40 may help to reduce the number of fine polymer particles that enter the depressurisation line 40 and flare unit 42.

[0051] The controller 50 may be configured to operate the control valve 48 to control the mass flow of the fluidisation gas along the depressurisation line 40 such the mass flow rate is less than an upper mass flow rate limit. That is, during use, the mass flow rate maintained at a rate that is less than the upper mass flow rate limit. The upper mass flow rate limit may depend on the size of the reactor assembly 1 and the capacity of the flare unit 42.

[0052] To facilitate controlling the mass flow rate, the reactor assembly 1 may comprise a flow meter 52. The flow meter 52 may be operably connected to the controller 50. The flow meter 52 may be arranged to measure a mass flow rate of fluidisation gas in the depressurisation line 40. The controller 50 may be configured to operate the control valve 48 to control the mass flow of the fluidisation gas along the depressurisation line 40 such the mass flow rate is less than an upper mass flow rate limit based on the mass flow rate measured by the flow meter 52.

[0053] The flow meter 52 may be configured to output a mass measurement signal indicative of the measured mass flow rate through the depressurisation line 40. The controller 50 may be configured to receive the mass measurement signal from the flow meter 52 and to determine the mass flow rate based on the mass measurement signal. The controller 50 may be configured to determine the size of opening of the control valve 48 required for the mass flow rate of the fluidisation gas along the depressurisation line 40 to be less than the upper mass flow rate limit. The controller 50 may be configured to change the size of the opening to the determined size. The controller 50 may be configured to send a mass control signal to the control valve 48 to change the size of the opening to the required size. For example, if the mass flow rate needs to be reduced the mass control signal may cause the size of the opening to decrease.

[0054] The controller 50 may be configured to operate the control valve 48 to control a volumetric flow rate of the fluidisation gas along the depressurisation line 40 such the volumetric flow rate is less than an upper volumetric flow rate limit. That is, during use, the volumetric flow rate maintained at a rate that is less than the upper volumetric flow rate limit. The upper volumetric flow rate limit may depend on the size of the reactor assembly 1 and the capacity of the flare unit 42.

[0055] To facilitate controlling the volumetric flow rate, the reactor assembly 1 may comprise one or more sensors 54. For simplicity, the one or more sensor are illustrated by a single senor 54 in Figure 1. The sensors 54 may be operably connected to the controller 50. The sensors 54 may comprise a temperature sensor and a pressure sensor which may be arranged to measure a temperature and a pressure, respectively, in the depressurisation line 40. The controller 50 may be configured to receive a temperature measurement and a pressure measurement from the sensors 54. The controller 40 may be configured to determine the volumetric flow rate based on the mass flow rate measurement, the temperature measurement and the pressure measurement. As such, the volumetric flow rate may be measured.

[0056] The temperature sensor may be configured to output a temperature measurement signal indicative of the measured temperature in the depressurisation line 40. The controller 50 may be configured to receive the temperature measurement signal from the temperature sensor. The pressure sensor may be configured to output a pressure measurement signal indicative of the measured pressure in the depressurisation line 40. The controller 50 may be configured to receive the pressure measurement signal from the pressure sensor. The controller 50 may be configured to determine the volumetric flow rate based on the mass measurement signal, pressure measurement signal and the temperature measurement signal. The controller 50 may be configured to determine the size of opening of the control valve 48 required for the volumetric flow rate of the fluidisation gas along the depressurisation line 40 to be less than an upper volumetric flow rate limit. The controller 50 may be configured to change the size of the opening to the determined size. The controller 50 may be configured to send a volumetric control signal to the control valve 48 to change the size of the opening to the required size. For example, if the volumetric flow rate needs to be reduced the mass control signal may cause the size of the opening to decrease.

[0057] The controller 50 may be configured to operate the control valve 48 to control both the mass flow rate and the volumetric flow rate of the fluidisation gas along the depressurisation line 40 such that both the mass flow rate and the volumetric flow rate are less than their respective rate limits.

[0058] As described above, the controller 50 may be configured to determine the size of opening of the control valve 48 for each of the mass flow rate and the volumetric flow rate of the fluidisation gas along the depressurisation line 40 to be less than an upper mass and volumetric flow rate limits, respectively. As such, the controller 50 may determine two opening sizes: one for the mass flow rate; and one for the volumetric flow rate. The controller 50 may be configured to compare the two opening sizes to determine the smaller of the two sizes. The controller 50 may be configured to control the size of the opening of the control valve 48 based on smaller of the two determined opening sizes. That is, the controller 50 may change the size of the opening of the control valve 48 to be the same as the smaller of the two opening sizes. The controller 50 may therefore be configured to send either the volumetric control signal or the mass control signal to the control valve 48. The controller 50 sends the control signal that corresponds the smaller of the two determined opening size. For example, if the size opening required for the volumetric flow rate to be under the upper volumetric flow rate limit is smaller than the size of the opening for the mass flow rate to be under the upper mass flow rate limit, the controller 50 may be configured to control the opening of the control valve 48 based on the size required for the volumetric flow rate. As such, both the mass flow rate and the volumetric flow rate will be below their respective limits.

[0059] Figure 3 shows an example of depressurisation. The illustrative example shows how the pressure 56 in the gas phase reactor 2 and the mass flow rate 58 and volumetric flow rate 60 of fluidisation gas in the depressurisation line 40 change over time during depressurisation. At the start of depressurisation, the pressure 56 in the gas phase reactor 2 is high so the density of the fluidisation gas is also high. The mass flow rate 58 of the fluidisation gas through the depressurisation line 40 must be limited by the controller 50 to the upper mass flow rate limit 62 to stay within the capacity of the flare unit 42. The controller 50 limits the opening of the control valve to limit the mass flow rate 58 of the fluidisation gas through the depressurisation line 40. However, at the start of depressurisation, the volumetric flow rate 60 is well below the upper volumetric flow rate limit 64. As such, there is no need to limit the volumetric flow rate 60 of the fluidisation gas. As depressurisation continues, the pressure 56 inside the gas phase reactor 2 falls, the density of the fluidisation gas decreases and the velocity of fluidisation gas in the gas phase reactor 2 increases. Therefore, the mass flow rate 58 decreases and the volumetric flow rate 60 increases. As shown in the Figure, after a certain time and pressure, the mass flow rate 58 falls below the upper mass flow rate limit 62 and the volumetric flow rate 60 reaches the upper volumetric flow rate limit 64. From this time, the volumetric flow rate 60, rather than the mass flow rate 58, must then be limited by the controller 50. The controller 50 limits the opening of the control valve to limit the volumetric flow rate 60 of the fluidisation gas through the depressurisation line 40. As the density of the fluidisation gas decreases as described above, the velocity of fluidisation gas in the gas phase reactor 2 increases which increases the risk of fine polymer particles being carried by the fluidisation gas into the depressurisation line 40 and, consequently, the flare unit 42. Limiting the volumetric flow rate 60 to be below the upper volumetric flow rate limit 64 further reduces the number of fine polymer particles that enter the depressurisation line 40 and the flare unit 42.

[0060] Therefore, the orientation of the depressurisation line 40 relative to the circulation line disclosed herein and, when used, the control of the mass and volumetric flow rates of fluidisation gas through the depressurisation line 40, help reduce the number of fine polymer particles entering the depressurisation line 40 during depressurisation and, consequently, the flare unit 42.

[0061] Herein is also described a method for depressurising a reactor assembly 1 polymerising olefins according to an embodiment to the invention. The reactor assembly 1 shown in the embodiment in Figure 1 may therefore be used to implement the method.

[0062] As such, the method may comprise providing the reactor assembly 1 comprising a gas phase reactor 2 having a circulation gas inlet 14 and a circulation gas outlet 16; a circulation line 30 for circulating a gas, the circulation line 30 fluidly connecting the circulation gas inlet 14 to the circulation gas outlet 16; and a depressurisation line 40 for depressurising the gas, the depressurisation line 40 fluidly connected to the circulation line 30.

[0063] The method comprises causing fluidisation gas to flow from the circulation line 30 along the depressurisation line 40 to depressurise the gas phase reactor 2. Fluidisation gas flowing along the depressurisation line 40 may be called a depressurizing gas stream. The fluidisation gas entering the depressurisation line 40 flows along the depressurisation line 40 at an angle a of at least 90 degrees and less than 180 degrees relative to the first section 47 of the circulation line 30. The method step of causing fluidisation gas to flow along the depressurisation line 40 may comprise causing the controller 50 may send the open signal to the control valve 48 to cause fluidisation gas to flow from the circulation line 30 along the depressurisation line 40.

[0064] The method comprises limiting a mass flow rate of the fluidisation gas along the depressurisation line 40 to less than an upper mass flow rate limit and limiting the volumetric flow rate of the fluidisation gas along the depressurisation line 40 to less than an upper volumetric flow rate limit. The mass flow rate and volumetric flow rate of fluidisation gas along the depressurisation line 40 may be limited by the controller 50 limiting the size of the opening of the control valve 48.

[0065] The method may comprise measuring or determining the mass flow rate of fluidisation gas along the depressurisation line 40 and measuring or determining the volumetric flow rate of fluidisation gas along the depressurisation line 40. The mass flow rate may be measured by the flow meter 52. The volumetric flow rate may be measured indirectly by measurements taken by the flow meter 52 and the sensors 54. As described above, the controller 50 may receive a mass measurement signal indicative of the measured mass flow rate through the depressurisation line 40 from the flow meter 52 and receive a temperature measurement signal and a pressure measurement signal. The method step of limiting the mass flow rate and volumetric flow rate of the fluidisation gas along the depressurisation line 40 may be based on the measured mass flow rate and measured volumetric flow rate. The method may comprise determining the size of opening of the control valve 48 required for the mass flow rate of the fluidisation gas along the depressurisation line 40 to be less than an upper mass flow rate limit and determining the size of opening of the control valve 48 required for the volumetric flow rate of the fluidisation gas along the depressurisation line 40 to be less than an upper volumetric flow rate limit.

[0066] The method may comprise comparing the two opening sizes determined from the mass and volumetric flow rate measurements to determine the smaller of the two opening sizes. The method may comprise changing the size of the opening of the control valve 48 to be the same as the smaller of the two opening sizes. As such, both the mass flow rate and volumetric flow rate of fluidisation gas through the depressurisation line 40 may be kept below their respective upper limits. The method may comprise using the controller 50 to control the size of the opening of the control valve 48.

[0067] The method may comprise delivering fluidisation gas from the depressurisation line 40 to a flare unit 42. The method may comprise burning the fluidisation gas at the flare unit 42.

[0068] The skilled person will appreciate that various modifications can be made to the abovedescribed reactor assembly 1 and method without departing from the scope of the invention which is defined by the appended claims.

[0069] For example, in certain embodiments, the reactor assembly 1 may comprise a flush line (not shown). The flush line may be fluidly connected to the depressurisation line, preferably upstream of the control valve, i.e. between the control valve and the circulation line. The flush line may comprise any suitable line, pipe or conduit for transferring gas. The flush line may be configured to deliver a flush gas into the depressurisation line. The direction of flow of the flush gas may be from the depressurisation line into the circulation line. That is, in the opposite direction to the flow of fluidisation gas. The flush gas may therefore help expel fine polymer particles from the depressurisation line so that the particles are returned to the circulation line.

[0070] In the above-described embodiment, the flow meter 52 is configured to measure the mass flow rate in the depressurization line 40. The volumetric flow rate is determined from the measured mass flow rate, a temperature measured in the depressurization line 40 and a pressure measured in the depressurization line 40. As such, the volumetric flow rate is indirectly measured and the mass flow rate is directly measured. However, in an alternative embodiment, the flow meter may be configured to measured the volumetric flow rate and the mass flow rate may be determined from the measured volumetric flow rate, the temperature measured in the depressurization line 40 and the pressure measured in the depressurization line 40. In such embodiments, the flow meter may be configured to output a volumetric measurement signal indicative of the measured volumetric flow rate through the depressurisation line 40 to the controller 50. The volumetric flow rate may therefore be directly measured and the mass flow rate is indirectly measured.

[0071] In an alternative to the above-described embodiment, the reactor assembly may comprise two flow meters. One flow meter configured to measure the mass flow rate in the depressurization line 40 and the other flow meter configured to measure the volumetric flow rate in the depressurisation line 40. The flow meters may be configured to output a mass measurement signal indicative of the measured mass flow rate through the depressurisation line 40 to the controller 50 and output a volumetric measurement signal indicative of the measured volumetric flow rate through the depressurisation line 40 to the controller 50. Therefore, the volumetric flow rate and the mass flow rate may both be directly measured. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings). The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments falling within the scope of the claims. Each feature disclosed in this specification (including any accompanying claims and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. All of the features disclosed in this specification (including any accompanying claims and drawings) may be combined in any combination, except combinations where at least some of such features are mutually exclusive. The invention is defined by the appended claims.

Claims

23CLAIMS1. A reactor assembly (1 ) for polymerising olefins, the assembly (1 ) comprising: a gas phase reactor (2) for polymerising olefins to produce a polyolefin component, said gas phase reactor (2) being a fluidized bed reactor and having a circulation gas inlet (14) and a circulation gas outlet (16); a circulation line (30) for circulating fluidisation gas, the circulation line (30) fluidly connecting the circulation gas outlet (16) to the circulation gas inlet (14) and being configured to facilitate a flow of the fluidisation gas in a direction from the circulation gas outlet (16) to the circulation gas inlet (14); a compression unit (32) disposed on the circulation line (30) for pressurising the fluidisation gas; and a depressurisation line (40) for depressurising the fluidisation gas, the depressurisation line (40) being fluidly connected to the circulation line (30) at a connection section (46) arranged upstream of the compression unit (32); wherein the depressurisation line (40) extends from the connection section (46) of the circulation line (30) at an angle of at least 90 degrees and less than 180 degrees relative to a first section (47) of the circulation line (30) that is downstream of and adjacent to the connection section (46), thereby allowing a direction of flow of fluidisation gas along the depressurisation line (40) at an angle between 110 degrees and 170 degrees relative to a direction of flow of the fluidisation gas through the circulation line (30) downstream of the connection section (46).

2. A reactor assembly (1) according to claim 1 , wherein the reactor assembly (1) is for polymerising olefins in multistage polymerisation process configuration and comprises an upstream reactor (22) for polymerising olefins to produce a first polyolefin component; wherein the gas phase reactor (2) is in fluid communication with the upstream reactor (22) and the gas phasereactor (2) is for polymerising olefins to produce a second polyolefin component in the presence of the first polyolefin component.

3. An reactor assembly (1) according to claim 1 or 2, comprising a flare unit (42), wherein the depressurisation line (40) is in fluid communication with the flare unit (42).

4. An reactor assembly (1 ) according to claim 1 , wherein the depressurisation line (40) extends from the connection section (46) of the circulation line (30) at an angle between 120 degrees and 160 degrees relative to the first section (47) of the circulation line (30).

5. An reactor assembly (1 ) according to any one of the preceding claims, wherein the connection section (46) is oriented vertically thereby allowing the circulating fluidisation gas to flow downwardly through the connection section (46).

6. An reactor assembly (1) according to any one of the preceding claims, comprising a cooler (38) disposed on the circulation line (30) downstream of the compression unit (32).

7. An reactor assembly (1) according to any one of the preceding claims, comprising a control valve (48) configured to control the flow of fluidisation gas along the depressurisation line (40); and a controller (50) configured to operate the control valve (48) to selectively cause fluidisation gas from the circulation line (30) to flow along the depressurisation line (40).

8. An reactor assembly (1) according to claim 7, wherein the controller (50) is further configured to operate the control valve (48) to control a mass flow rate and a volumetric flow rate of the fluidisation gas along the depressurisation line(40) such that the mass flow rate is kept less than an upper mass flow rate limit and the volumetric flow rate is kept less than an upper volumetric flow rate limit.

9. A method for depressurising a reactor assembly (1) for polymerising olefins, the method comprising: providing a reactor assembly (1) according to any one of claims 1 to 8; causing fluidisation gas to flow from the circulation line (30) along the depressurisation line (40) to depressurise the gas phase reactor; limiting a mass flow rate of the fluidisation gas along the depressurisation line (40) to less than an upper mass flow rate limit; and limiting the volumetric flow rate of the fluidisation gas along the depressurisation line (40) to less than an upper volumetric flow rate limit.

10. A method according to claim 9, comprising delivering fluidisation gas from the depressurisation line (40) to a flare unit (42).

11. A method according to claim 9 or 10, comprising determining the mass flow rate of fluidisation gas along the depressurisation line (40); and determining the volumetric flow rate of fluidisation gas along the depressurisation line (40); wherein limiting the mass flow rate of the fluidisation gas along the depressurisation line (40) is based on the determined mass flow rate; and wherein limiting the volumetric flow rate of the fluidisation gas along the depressurisation line is based on the determined volumetric flow rate.

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