Loop reactor system

Abstract

The present invention relates to a reactor system, and in particular provides a reactor system for polymerisation of olefins, said reactor system comprising a loop reactor which has n vertical legs, n / 2 top sections and n / 2 bottom sections, the vertical legs, and the top and bottom sections being connected to form a continuous loop, n being an even number, characterised in the maximum number of legs in any vertical plane is less than n / 2.

Description

[0001] Case 00423(1) 1

[0002] Reactor System

[0003] The present invention relates to a reactor system, and in particular for the polymerisation of olefins in one or more slurry loop reactors.

[0004] The polymerisation of olefins, particularly to produce polyethylene and polypropylene, is a well known and commercially widely operated process. In one particular well-known process, often referred to as the slurry loop process, polymerisation takes place using one or more loop reactors. Pipe loop reactors generally comprise a series of legs, each being connected at each end to a further leg to form a continuous loop. Since the polymerisation of olefins is exothermic, the reactors are also generally provided with jackets around each leg through which a coolant can be passed. Water is most usually used as the coolant. Such reactors have been known for many years - US 3248179, for example, was filed in 1962 and exemplifies use of loop reactors for ethylene and propylene polymerisations.

[0005] Modem commercial loop reactors comprise at least 4 legs, usually more, which are orientated vertically. Each leg is connected at its top end to an adjacent leg and at its bottom end to a different adjacent leg. The connections at the top and bottom ends can be 180 degree elbows, or two 90 degree elbows can be connected to a horizontal straight section between the two vertical legs.

[0006] Polymerisation takes place in a diluent containing liquid phase, wherein olefin monomer polymerises in the presence of a catalyst to form polymer particles. The polymer particles form a slurry of polymer particles in the liquid phase circulating in the loop reactor.

[0007] Slurry, comprising polymer particles, is withdrawn from the reactor through a withdrawal line. In a “stand-alone” reactor or for the last reactor in a series of two or more, the slurry is generally treated to separate the polymer particles from the liquid phase.

[0008] Typically, slurry is subjected to heating and usually a reduction in pressure to vaporise the liquid phase, followed by separation of the polymer particles from the gaseous vaporised liquid phase in a gas-solids separation step. The polymer may then be further treated, for example with a purge gas and / or a further reduction in pressure, to remove residual liquid components.

[0009] Separated liquid components are usually cooled to condense, and then recycled.Case 00423(1) 2

[0010] There has been a general trend in slurry loop reactor technology of increasing capacity, namely producing increasing amounts of polymer per unit time. This has led to reactors of increasing internal volume. In particular, the main constraint on space time yield, which is the amount of polymer which can be produced per unit time per unit volume of a reactor is the rate at which the exothermic heat of reaction can be removed from the reactor. Increasing the diameter of the pipe which forms the loop reactor increases volume, but decreases surface area to volume ratio, which reduces the ease of cooling the reactor. Thus, whilst some increases in diameter can and have been made, generally increased capacity and reactor volume has been achieved by increasing the reactor length, and correspondingly the number of legs.

[0011] Typically slurry loop reactors are formed by providing two rows of legs, giving a rectangular footprint. In such configurations each long side of the rectangle has n / 2 legs where n is the total number of legs of the loop reactor.

[0012] It has now been found that for reactors with larger numbers of legs then other reactor footprints are not only possible, but can be advantageous.

[0013] Thus, in a first aspect the present invention provides a reactor system for polymerisation of olefins, said reactor system comprising a loop reactor which has n vertical legs, n / 2 top sections and n / 2 bottom sections, the vertical legs, and the top and bottom sections being connected to form a continuous loop, n being an even number , characterised in the maximum number of legs in any vertical plane is less than n / 2.

[0014] According to the present invention the maximum number of legs in any vertical plane is less than n / 2. This means no vertically orientated plane which can be considered will contain n / 2 (or more) of the vertical legs. Thus, the footprint of the reactor cannot be a rectangle with two rows of n / 2 legs. (Since in such a configuration the n / 2 legs on each “long side” of the rectangle fall within vertical planes.)

[0015] It can be noted that a rectangular configuration can have less than n / 2 legs in each plane. For example, a reactor with 10 vertical legs can be configured with a rectangular footprint, with 4 vertical legs along each long side and 3 along each short side. However, preferably the footprint of the reactor is not a rectangle. Similarly, preferably the footprint of the reactor is not a square.

[0016] For avoidance of doubt, the term “footprint” as used herein refers to the shape formed by the loop reactor as viewed from the top. In particular, Figure 1 shows an 8 legCase 00423(1) 3

[0017] loop reactor, which is configured with two rows of four vertical legs, in side view and in plan view. (This configuration is as known in the prior art and not according to the present invention.) The plan view shows the rectangular footprint of the reactor. (For avoidance of doubt in this and other plan views herein, a “top section” is shown as shaded, whilst a bottom section is shown not shaded. Vertical legs are present where each shaded section overlaps a non shaded section as can be seen by comparison of the plan view with the side view in this Figure 1.)

[0018] Preferably the loop reactor of the reactor system of the first aspect of the present invention has at least 8 vertical legs i.e. n is at least 8. There may be at least 10 vertical legs, such as 10 to 20 vertical legs. In embodiments, n is at least 12.

[0019] (The loop reactor has an even number of vertical legs. In particular, for the vertical legs to be connected into a continuous loop, each leg by which slurry would flow “up” in use must be connected at its top end to a subsequent leg by which slurry would flow “down” in use, and each leg by which slurry would flow “down” in use must be connected at its bottom end to a subsequent leg by which slurry would flow “up” in use.)

[0020] In embodiments the plan view of the reactor may be L-shaped. In other embodiments, the plan view of the reactor is H-shaped. In yet other embodiments, the plan view of the reactor is U-shaped. In other embodiments, the plan view of the reactor is T-shaped. And in yet further embodiments, the plan view of the reactor is cross shaped.

[0021] Some examples are shown in the Figures 2 to 5 herein. In particular:

[0022] Figure 2 shows an example of a reactor with 8 vertical legs, which is configured in an L-shape. (This is again shown in side and plan views.) The maximum number of vertical legs in any vertical plane is 3 - this occurring on both the top and right hand side as shown in the plan view.

[0023] Figure 3 shows two examples of a reactor with 10 vertical legs, which are configured in a T-shape and an L-shape respectively. The maximum number of vertical legs in any vertical plane is 4 on both of these Figures.

[0024] Figure 4 shows four examples of a reactor with 12 vertical legs. These are configured as T-shaped, U-shaped, L-shaped and cross-shaped respectively. The maximum number of vertical legs in any vertical plane is 4 in each of these Figures, although L-shaped or T-shaped configurations with 5 legs in a plane can also be envisioned for a reactor with 12 vertical legs.Case 00423(1) 4

[0025] Figure 5 shows two examples of a reactor with 16 vertical legs, which are configured in a U-shape and an H-shape respectively. The maximum number of vertical legs in any vertical plane is 4 on both of these Figures, this in fact occurring in multiple vertical planes.

[0026] Other than the configuration, the loop reactor, and its use for polymerisation are largely as known in the art.

[0027] The Loop Reactor

[0028] The loop reactor is, other than the footprint, generally as known in the art.

[0029] For example, typically each vertical leg generally comprises a straight pipe of at least 20m in length (height), such as 40 to 80m in length.

[0030] The top and bottom sections may be any suitable connecting pipework between adjacent vertical legs. As already described, for example the top and bottom sections can be 180 degree elbows or may comprise two 90 degree elbows can be connected to a horizontal straight section between the two elbows. It is preferred that the top and bottom sections are 180 degree elbow sections or that any horizontal straight section is relatively short, for example less than 5 metres. In particular, in general the top and bottom sections are not jacketed or otherwise directly cooled, so minimising the length of these minimises the proportion of the reactor which is not cooled.

[0031] The vertical legs and top and bottom sections are generally formed of pipe with an internal diameter, D, which is at least 500mm, such as in the range 550 to 750mm.

[0032] The total internal volume of the or each loop reactor is generally in the range 100 to 350m3.

[0033] The loop reactor generally comprises one or more withdrawal lines by which circulating slurry comprising polymer particles may be withdrawn.

[0034] The withdrawal from the loop reactor may be performed continuously or discontinuously, as known in the art. Preferably, the withdrawal lines are each continuous take-offlines.

[0035] For a “stand alone” reactor or for the final reactor where two or more reactors are provided in series (which is discussed further below), each of one or more product withdrawal lines will generally pass a product stream comprising polymer solids via a slurry heater to one of one or more gas-solid separators to thereby separate the polymer solids from the slurry diluent. The heating in a slurry heater before passage to a gas-solidCase 00423(1) 5

[0036] separator is also generally as known in the art. Examples of the operation of slurry heaters, as well as discussion of continuous and discontinuous withdrawal, can be found, for example, in WO 2009 / 127643 Al and WO 2009 / 127645 Al.

[0037] Where there are multiple product withdrawal lines from the loop reactor each product withdrawal line may have a separate dedicated slurry heater, or two or more product withdrawal lines may combine to feed a common slurry heater. There are preferably between p and p / 2 slurry heaters where “p” is the number of product withdrawal lines on the loop reactor.

[0038] In relation to the gas-solid separators, preferably the exits of multiple slurry heaters feed a single gas-solid separator. Most preferably there is present either a single gas-solid separator, connected to the exits of all slurry heaters present, or a maximum of two gassolid separators, each connected to a plurality of the exits of the slurry heaters present.

[0039] The locations of the product withdrawal lines on a loop reactor are not especially critical in the reactor system of the present invention.

[0040] It is preferred, however, that product withdrawal lines are located at lower ends of vertical legs or, and preferably, on the bottom sections of the loop reactor which connect adjacent vertical legs.

[0041] It is preferred that where there are multiple product withdrawal lines they are provided on different legs, or, more preferably, on different bottom sections on the loop reactor. This allows withdrawal from multiple different parts of the loop reactor. (Also it is also noted that not all such lines have to be “active” at the same time when the reactor is being operated.)

[0042] Series loop reactors

[0043] In embodiments of the present invention the reactor system may comprise two or more loop reactors where at least one reactor has a maximum number of legs in any vertical plane which is less than n / 2, where said n is the number of vertical legs on the respective loop reactors.

[0044] Most preferably, two or more loop reactors may be provided in series. Thus, in embodiments of the present invention the reactor system may comprise two or more loop reactors in series, where at least one reactor has a maximum number of legs in any vertical plane which is less than n / 2, where said n is the number of vertical legs on the respective loop reactors.Case 00423(1) 6

[0045] (For avoidance of doubt, where more than one loop reactor in a reactor system has a maximum number of legs in any vertical plane which is less than n / 2, then “n” may or may not be the same for each loop reactor.)

[0046] More specifically, and in a second aspect, the present invention provides a reactor system for polymerisation of olefins, said reactor system comprising first and second loop reactors connected in series, where

[0047] - the first loop has n vertical legs, n / 2 top sections and n / 2 bottom sections, the vertical legs, and the top and bottom sections being connected to form a continuous loop,

[0048] - the second loop reactor has m vertical legs, m / 2 top sections and m / 2 bottom sections, the vertical legs, and the top and bottom sections being connected to form a continuous loop,

[0049] where n and m are both even numbers, and characterised in at least one, optionally both, of the following apply:

[0050] - the maximum number of legs in any vertical plane of the first loop reactor is less than n / 2

[0051] - the maximum number of legs in any vertical plane of the second loop reactor is less than m / 2.

[0052] It will be apparent that this second aspect is an embodiment of the first aspect.

[0053] In this second aspect the preferred features of the first and second loop reactors individually e.g. number of legs, examples of the footprint shape, are as already described under “The Loop Reactor” for the individual reactor above, except:

[0054] (a) one of the loop reactors may be of a “conventional” footprint,

[0055] (b) in a series configuration the one or more withdrawal lines by which circulating slurry comprising polymer particles may be withdrawn from the first loop reactor do not pass the product stream comprising polymer solids via a slurry heater to one of one or more gas-solid separators to thereby separate the polymer solids from the slurry diluent, but instead pass (transfer) the slurry to the second loop reactor. In this second aspect, for example, preferably m is at least 8 and preferably n is at least 8.

[0056] In this second aspect, most preferably, m = n i.e. both reactors have the same number of vertical legs.Case 00423(1) 7

[0057] In relation to (a) above, whilst one of the loop reactors may be of a “conventional” footprint, this is not preferred. It is preferred, therefore that

[0058] - the maximum number of legs in any vertical plane of the first loop reactor is less than n / 2, and

[0059] - the maximum number of legs in any vertical plane of the second loop reactor is less than m / 2.

[0060] More particularly, it is further preferred that the footprint of each loop reactor is the same. For example, if the first loop reactor has an L-shaped footprint, then the second loop reactor also preferably has an L-shaped footprint, and most preferably the same footprint. (As used herein the same footprint includes a mirror image thereof.)

[0061] In relation to (b) above, in a series configuration of two loop reactors, generally the second loop reactor comprises one or more withdrawal lines by which the product stream comprising polymer solids is withdrawn and passed via a slurry heater to one of one or more gas-solid separators to thereby separate the polymer solids from the slurry diluent. This is therefore as already described.

[0062] However, for the first loop reactor in a series configuration the one or more withdrawal lines by which circulating slurry comprising polymer particles may be withdrawn from the first loop reactor do not pass the product stream comprising polymer solids via a slurry heater to one of one or more gas-solid separators to thereby separate the polymer solids from the slurry diluent, but instead pass (transfer) the slurry to the second loop reactor.

[0063] Thus, in the second aspect of the present invention the first loop reactor generally comprises at least one withdrawal line by which a product stream comprising polymer solids can be withdrawn from the first loop reactor and passed to the second loop reactor.

[0064] In this second aspect (i.e. where there are provided two reactors in series), then the term “transfer line” may be used to refer to a withdrawal line which connects the first loop reactor to the second loop reactor and hence provides this transfer. Each transfer line in such a case will provide withdrawal of polymer from the first loop reactor for transfer to and introduction into the second loop reactor. It will be appreciated therefore that a “transfer line” is a type of withdrawal line. We will generally use the term “transfer line” rather than withdrawal line however to distinguish from withdrawal lines which pass product via slurry heaters and gas-solids separation steps as already described (and whichCase 00423(1) 8

[0065] may be considered as “product withdrawal lines”). Nevertheless transfer lines have a number of features in common with other withdrawal lines. This would include, for example, that the actual withdrawal from the first loop reactor may be performed continuously or discontinuously.

[0066] Preferably, any transfer lines on the first loop reactor when two loop reactors in series are present are each continuous take-offlines.

[0067] The transfer from the first loop reactor to the second loop reactor may be performed without any intermediate treatment (“direct transfer line”) or via an intermediate treatment (“indirect transfer line”).

[0068] For example, in monomodal operation, where conditions in the first and second loop reactors are similar or the same, then the slurry withdrawn from the first loop reactor can be passed directly to the second loop reactor. The transfer line or lines in this case may be simply a pipe connecting the first loop reactor to the second loop reactor, with suitable valves to control the flow as needed. (Although in embodiments the transfer line can also include concentrators such as hydrocyclones if desired, such as described in Example 1 of US 2001 / 018499 Al.) We will, herein, use the term “direct transfer line” to refer to lines which provide such passage to the second loop reactor (even in the situation where they also comprise hydrocyclones).

[0069] However, in bimodal or multimodal operation, conditions in the first and second loop reactors are generally very different. In such operation the slurry withdrawn from the first loop reactor may be treated before passage to the second loop reactor. Examples of such treatment include where the first loop reactor comprises hydrogen but this is not desired, or is desired at a much lower concentration, in the second loop reactor. In these cases the slurry withdrawn from the first loop reactor may be treated to remove hydrogen before passage to the second loop reactor. Examples of such systems are described, for example, in US 2001 / 018499 Al or EP 2336200 Al. We will, herein, use the term “indirect transfer line” to refer to transfer lines which provide such passage to the second loop reactor. (It can also be noted that indirect transfer lines can also include concentrators, such as hydrocyclones, located before any subsequent treatment step or steps.)

[0070] Where two loop reactors in series are provided, the first loop reactor may, and preferably does, comprise both direct and indirect transfer lines by which a product streamCase 00423(1) 9

[0071] comprising polymer solids can be withdrawn from the first loop reactor and passed to the second loop reactor.

[0072] Relative Configuration of Loop Reactors

[0073] Particular advantages related to the shape of the loop reactor or reactors arise in the present invention when two (or more) loop reactors are provided. In particular, the shape or shapes can allow an advantageous overall configuration.

[0074] In preferred embodiments (and especially where the first and second reactors are provided in series, such as in the second aspect of the present invention) the distance between first and second loop reactors is less than 30m, preferably less than 20m, such as less than 10m, where this distance is the shortest distance which can be measured between the first loop reactor and the second loop reactor.

[0075] The present invention can be particularly advantageous in embodiments wherein there is provided a vessel which is fluidly connected to both first and second loop reactors. In such embodiments the reactors and the vessel may be configured to all be in relatively close proximity. For example, taking the shortest horizontal distance between the first loop reactor and the vessel as dl, the shortest horizontal distance between the second loop reactor and the vessel as d2 and the shortest horizontal distance between the first loop reactor and the second loop reactor as d3, at least one of the following may apply:

[0076] a. the sum of dl + d2 + d3, in absolute terms, is less than 60m, preferably less than 40m,

[0077] b. At least two of, and preferably each of, dl, d2 and d3, are less than 20m c. At least one of dl, d2 and d3 is less than 15m.

[0078] The “vessel” in these embodiments may be any suitable vessel that is fluidly connected to both the first and second loop reactors. For example, where first and second loop reactors are not in series, slurry may be separately withdrawn from each loop reactor and passed to product recovery steps, and the vessel may, for example, be a gas-solid separator, and in particular which is connected to product withdrawal lines from both the first and second loop reactors.

[0079] However, a particular advantage arises in reactor systems in which the first and second reactors are provided in series, and especially which are used for production of both monomodal and bimodal (or multimodal) polymers.Case 00423(1) 10

[0080] Thus, in embodiments where first and second reactors are provided in series, and in particular in the second aspect of the present invention, preferably, the vessel which is fluidly connected to both the first and second loop reactors is a vessel which is part of an intermediate treatment system as already discussed. (And in particular connected to the first loop reactor and the second loop reactor by one or more transfer lines for transfer of a polymer containing stream from the first loop reactor to the second loop reactor via the intermediate treatment system.)

[0081] This is perhaps best illustrated by reference to Figure 6 which shows a plant layout where two L-shaped reactors are provided. (As already described these are considered to have the same footprint, albeit one is a mirror image of the other.)

[0082] In particular, Figure 6 shows a first loop reactor (1), a second loop reactor (2), and an intermediate treatment system (3). As shown all three can be provided in close proximity. Also shown in Figure 6 are two “direct” product transfer lines, by which slurry is withdrawn from the first loop reactor (1) and passed to the second loop reactor (2) without intermediate treatment, and two “indirect” product transfer lines, by which slurry is withdrawn from the first loop reactor (1) and passed to the second loop reactor (2) via the intermediate treatment system (3).

[0083] A particular advantage of the L-shaped configuration in Figure 6 arises in reactor systems which are used for production of both monomodal and bimodal (or multimodal) polymers since there can be provided both a plurality of direct transfer lines and a plurality of indirect transfer lines, and the present invention allows both sets of lines to be optimally designed, and in particular to be relatively short.

[0084] Polymerisation

[0085] The reactor system according to the first and second aspects of the present invention is used for polymerisation of olefins. In a third aspect, the present invention provides a process for polymerisation, which process comprises polymerisation of one or more olefins in a reactor system according to the first aspect, and preferably according to the second aspect.

[0086] As with the details of the reactor system generally (other than the footprints) the process for polymerisation itself is largely as known in the art.

[0087] The polymerisation preferably comprises polymerisation of ethylene, optionally with a comonomer in one or more loop reactors, or of propylene, again optionally with aCase 00423(1) 11

[0088] comonomer in one or more loop reactors. It may also be noted that ethylene may be a comonomer for propylene polymerisations and vice versa.

[0089] Thus, the product is preferably a polyethylene or a polypropylene.

[0090] Polymerisation of ethylene generally takes place in the presence of an inert diluent, typically an alkane, with isobutane being particularly preferred.

[0091] Polymerisation of propylene may also be performed in the presence of an inert diluent, although use of liquid propylene as diluent is also known.

[0092] The polymerisation may be catalysed by any suitable polymerisation catalyst. Typical catalysts known in the art are commonly one of three types, commonly referred to as “chromium catalysts”, “Ziegler-Natta catalysts” and “metallocene catalysts”, and any such catalysts are suitable for the present invention.

[0093] Conventional other reaction components can include chain terminating agents, of which hydrogen is most common, as well as various other catalyst components, such as donors and co-catalysts.

Claims

Case 00423(1) 12Claims1. A reactor system for polymerisation of olefins, said reactor system comprising a loop reactor which has n vertical legs, n / 2 top sections and n / 2 bottom sections, the vertical legs and the top and bottom sections being connected to form a continuous loop, n being an even number, characterised in that the maximum number of legs in any vertical plane is less than n / 2.

2. A reactor system as claimed in claim 1 wherein the reactor, when seen in plan view, is not a rectangle or a square.

3. A reactor system as claimed in claim 1 or claim 2 wherein n is at least 8, such as at least 10, preferably in the range 10 to 20.

4. A reactor system as claimed in claim 3 wherein n is at least 12.

5. A reactor system as claimed in any one of the preceding claims wherein the plan view of the reactor is L-shaped.

6. A reactor system as claimed in any one claims 1 to 4 wherein the plan view of the reactor is H-shaped or U-shaped.

7. A reactor system as claimed in any one claims 1 to 4 wherein the plan view of the reactor is T-shaped or cross shaped.

8. A reactor system as claimed in any one of the preceding claims wherein the reactor system comprises two or more loop reactors.

9. A reactor system for polymerisation of olefins, said reactor system comprising first and second loop reactors, where- the first loop has n vertical legs, n / 2 top sections and n / 2 bottom sections, the vertical legs, and the top and bottom sections being connected to form a continuous loop,- the second loop reactor has m vertical legs, m / 2 top sections and m / 2 bottom sections, the vertical legs, and the top and bottom sections being connected to form a continuous loop,where n and m are both even numbers, and characterised in at least one, optionally both, of the following apply:- the maximum number of legs in any vertical plane of the first loop reactor is less than n / 2Case 00423(1) 13- the maximum number of legs in any vertical plane of the second loop reactor is less than m / 2.

10. A reactor system as claimed in claim 9 wherein m is at least 8 and n is at least 8, such as m and n both being least 10, preferably both in the range 10 to 20.

11. A reactor system as claimed in claim 10 wherein m=n.

12. A reactor system as claimed in any one of claims 9 to 11 wherein the plan views of both the first loop reactor and the second loop reactor are the same.

13. A reactor system as claimed in claim 12 wherein the plan views of both the first loop reactor and the second loop reactor are L-shaped.

14. A reactor system as claimed in any one of claims 9 to 13 wherein the first loop reactor comprises both direct and indirect transfer lines by which a product stream comprising polymer solids can be withdrawn from the first loop reactor and passed to the second loop reactor.

15. A process for polymerisation, which process comprises polymerisation of one or more olefins, preferably of ethylene, in a reactor system according to any one of claims 1

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