Reactor system

Abstract

The present invention relates to a reactor system, and in particular provides a reactor system comprising first and second loop reactors connected in series, each of the first and second loop reactors having at least one longest side, wherein the plane or planes coincident with the longest side or sides of the first loop reactor are not parallel to any plane or planes coincident with the longest side or sides of the second loop reactor.

Description

[0001] Case 00425(1) 1

[0002] Reactor System

[0003] The present invention relates to a reactor system, and in particular for the polymerisation of olefins in two 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. Loop reactors for slurry polymerisation generally comprise a series of legs connected at each end to the next leg, and 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, usually water, to remove the heat of reaction. 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] Many slurry polymerisation processes operate with two loop reactors.

[0008] Two (or more) loop reactors can be operated separately to produce polymer product, with the product streams withdrawn from each reactor combined and passed to subsequent separation and treatment steps.

[0009] More commonly, two (or more) loop reactors are operated in series. In such reactor systems slurry comprising polymer particles is withdrawn from the first loop reactor and passed to the second loop reactor, in which further polymerisation occurs. Conditions in each reactor can be selected to produce the same polymer in each reactor, in which case the product is known as monomodal, or can be selected to produce different products in eachCase 00425(1) 2

[0010] reactor, particularly with different molecular weight, in which case the product is referred to as bimodal or multimodal.

[0011] Where the conditions in the two loop reactors are the same or similar then the slurry withdrawn from the first loop reactor can usually be passed directly to the second loop reactor without treatment. The transfer may then simply use a line or pipe, with suitable valving, between the reactors.

[0012] Where the conditions in the two loop reactors are significantly different the slurry withdrawn from the first loop reactor may be treated to remove components which are not desired or which are desired at lower concentrations in the second loop reactor. In some known processes, for example, hydrogen is removed from the slurry before it is passed to the second loop reactor.

[0013] For recovery of polymer product, slurry comprising polymer particles is withdrawn from a loop reactor through one or more withdrawal lines, and treated to separate the polymer particles from the liquid phase. Typically, slurry is withdrawn and 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.

[0014] Separated liquid components are usually cooled to condense, and then recycled to one or both of the two loop reactors.

[0015] Two common types of withdrawal from a loop reactor are practised.

[0016] One involves settling legs, which provide a “discontinuous withdrawal”. Circulating slurry collects above a closed valve in a settling leg, and in which the polymer particles settle by gravity. The settling concentrates the solids, and the valve is then opened temporarily to withdraw the slurry of concentrated solids from the leg. The valve is closed and the process repeats. The rate of withdrawal can be controlled by the timing of the valve opening and closing.

[0017] The second common type of withdrawal is a “continuous take off’ or “CTO”. A continuous take off generally is provided by a valve which is continuously open. The rate of withdrawal here can be controlled by varying the opening of the valve.

[0018] In both cases typically multiple withdrawal lines are provided on a loop reactor, either because multiple lines are required to obtain the total required product withdrawalCase 00425(1) 3

[0019] rate or to provide “spares” which can be opened if, for example, an operating line becomes blocked.

[0020] (For avoidance of doubt, these types of withdrawal can be applied both to the withdrawal from a loop reactor for passage to product recovery, but also to withdrawal from a first loop reactor where the withdrawn slurry is transferred to a second loop reactor.)

[0021] There has been a general trend to slurry loop reactors 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.

[0022] 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.

[0023] A single continuous take-off can generally remove significantly more slurry per unit time than a single settling leg, and generally the rate of continuous take-off can be increased by increasing the size of the valve and withdrawal line. Nevertheless, it is generally preferred to have more than one withdrawal line on large reactors. WO 2004 / 031245, for example, relates to a slurry loop reactor which is provided with a plurality of “active” continuous take-offs and at least one “inactive” continuous take-off. Figures 5 to 8 of this document in particular show different orientations for multiple takeoffs on one of the lower sections of a loop reactor.

[0024] It has now been found, however, that in a reactor system with at least two loop reactors, especially with multiple vertical legs and multiple withdrawals from the reactors, then the orientation of the two loop reactors being “non-parallel” can allow greater flexibility in the layout of the overall reactor system. This is especially so in relation to the transfer of the slurry from a first loop reactor to a second loop reactor provided in series and / or in connections to vessels which are connected to both loop reactors.

[0025] Thus, in a first aspect the present invention provides a reactor system comprising first and second loop reactors, each of the first and second loop reactors having at least oneCase 00425(1) 4

[0026] longest side, wherein the plane or planes coincident with the longest side or sides of the first loop reactor are not parallel to any plane or planes coincident with the longest side or sides of the second loop reactor.

[0027] The term “longest side” refers to the longest side of the reactor based on the reactor footprint. As used herein the reactor footprint is the external shape formed by the vertical legs of the loop reactor, as viewed from above the loop reactor. For example, loop reactors with 6 or 8 vertical legs commonly have two parallel rows of 3 or 4 vertical legs respectively, thus giving a rectangular footprint. In this or other rectangular footprints (e.g.

[0028] 2 x 5, 2 x 6 vertical legs) then there are two equal, and parallel, longest sides which correspond to the longest sides of the rectangular footprint. Examples are shown in Figures 1 and 3 herein. In particular, Figure 1 shows an 8 leg loop reactor in side view and in plan view, the latter showing 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.) One of the longest sides is marked “L” on the plan view. (Of course there are two parallel longest sides.)

[0029] In Figure 3, which is described further below, first (1) and second (2) loop reactors having 10 legs in a rectangular configuration (and in particular with 2 parallel rows of 5 legs) are shown.

[0030] A loop reactor with 8 or more legs need not have a rectangular footprint, however. It could, for example, be in an L-shape or other shape. In this case the longest side is that which provides the longest continuous straight line of vertical legs when viewed from above, or the two or more such sides if there are more than one such sides. Two examples are shown in Figure 2 herein, where one is a T-shape and one an L-shape. In both cases the longest sides are again marked “L”.

[0031] In one embodiment, therefore, at least one of the first and second loop reactors has a single longest side, and the plane coincident with this longest side is not parallel to the plane or planes coincident with the longest side or sides on the other loop reactor.

[0032] In a preferred embodiment, at least one of the first and second loop reactors has a rectangular footprint, and hence has two longest sides.Case 00425(1) 5

[0033] More preferably each of the first and second loop reactors has a rectangular footprint. In this case each has two longest sides. (And in particular, with the two longest sides of the first loop reactor being parallel to each other and the two longest sides of the second loop reactor being parallel to each other.) In this embodiment, the planes coincident with the two longest sides of the first loop reactor are not parallel to the planes coincident with the two longest sides of the second loop reactor.

[0034] Preferably, and whether there are single or multiple longest sides on each loop reactor, the plane or planes coincident with the longest side or sides of the first loop reactor are within 45 degrees of perpendicular to any plane or planes coincident with the longest side or sides of the second loop reactor.

[0035] Most preferably, and again whether there are single or multiple longest sides on each loop reactor, the plane or planes coincident with the longest side or sides of the first loop reactor are perpendicular to any plane or planes coincident with the longest side or sides of the second loop reactor, or, if not exactly perpendicular, are within at most within 10 degrees of perpendicular.

[0036] We will now describe the individual loop reactors, followed by the preferred orientations.

[0037] First and Second Loop Reactors

[0038] The first and second loop reactors are loop reactors for slurry polymerisation, and comprise a number of vertical legs.

[0039] The first and second loop reactors need not be the same size or have the same number of vertical legs. They also need not have the same footprint. Typically, however, both the loop reactors will have the same number of vertical legs, and usually the same reactor volume, as well as the same footprint formed by the vertical legs.

[0040] Preferably each of the first and second loop reactors has at least 8 vertical legs. More preferably each has at least 10 vertical legs. There is no particular upper limit on the number of vertical legs on each reactor, but usually it is no more than 20, and more usually no more than 16.

[0041] As already noted, in the loop reactors the vertical legs of each loop reactor are connected by top and bottom sections to form a continuous loop. (In particular each leg is connected at its top end to an adjacent leg and at its bottom end to a different adjacent legCase 00425(1) 6

[0042] using top and bottom sections respectively). Generally a loop reactor will have an even number of legs, n, with n / 2 top sections and n / 2 bottoms sections.

[0043] The first and second loop reactors, other than the orientation relative to each other, are generally as known in the art for loop reactors for slurry polymerisation.

[0044] 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. The vertical legs are generally provided with jackets around each leg through which a coolant can be passed, usually water, to remove the heat of reaction.

[0045] The top and bottom sections may be any suitable connecting pipework between adjacent 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.

[0046] 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.

[0047] The total internal volume of each loop reactor is generally in the range 100 to 350m3. Each loop reactor typically comprises multiple withdrawal lines.

[0048] In general, or in particular with reference to the second loop reactor when first and second loop reactors are provided in series, the term “product withdrawal line” will be used herein to refer specifically to a line by which a product stream comprising polymer solids is withdrawn from a loop reactor and fed via a slurry heater to one of the one or more gas-solid separators for recovery of polymer product. (For avoidance of doubt there may be other withdrawal lines on a loop reactor, for example for sampling or emptying of the reactor, but these are not encompassed within the term “product withdrawal line” as used herein.) The product withdrawal lines are preferably each continuous take-offlines.

[0049] It is preferred 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.Case 00425(1) 7

[0050] As noted, it is typical that there are multiple product withdrawal lines on a loop reactor. There may, for example, be 3 or more product withdrawal lines. 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 a loop reactor. This allows withdrawal from multiple different parts of a 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.)

[0051] Each of the product withdrawal lines pass a product stream comprising polymer solids via a slurry heater to one of one or more gas-solid separators. The heating in a slurry heater before passage to a gas-solid separator is generally as known in the art. Examples of the operation of slurry heaters can be found, for example, in WO 2009 / 127643 Al and WO 2009 / 127645 Al.

[0052] Where there are multiple product withdrawal lines on a 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 n and n / 2 slurry heaters where “n” is the number of product withdrawal lines on a loop reactor.

[0053] 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.

[0054] Where the first and second loop reactors are provided in series, then the “product withdrawal lines” as defined above are found on the second loop reactor. In a series configuration, the withdrawal lines on the first loop reactor by which polymer (slurry) is transferred to the second loop reactor, will be referred to herein as “product transfer lines”. (They still withdraw a polymer slurry from the reactor, but we will use the term "product transfer line” to distinguish from “product withdrawal line”, with the latter being defined explicitly as already noted.)

[0055] In series reactors, it is preferred that product transfer lines are located at lower ends of vertical legs or, and preferably, on the bottom sections of the first loop reactor which connect adjacent vertical legs.

[0056] It is typical that there are multiple product transfer lines which connect the first loop reactor to the second loop reactor. There may, for example, be 3 or more product transferCase 00425(1) 8

[0057] lines. It is also preferred that where there are multiple product transfer lines they are provided on different legs, or, more preferably, on different bottom sections on the first loop reactor. (Although it is also noted that not all such lines have to be “active” at the same time when the reactor is being operated.)

[0058] In relation to the product transfer lines, each withdraws a slurry comprising polymer solids from the first loop reactor for passage to the second loop reactor. The passage to the second loop reactor may be performed without any intermediate treatment (“direct product transfer line”) or via an intermediate treatment (“indirect product transfer line”).

[0059] For example, in monomodal operation of first and second loop reactors connected in series, conditions in the first and second loop reactors are similar or the same, and the slurry withdrawn from the first loop reactor can be passed directly to the second loop reactor. The product transfer line 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.

[0060] (Although in embodiments the withdrawal 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 product transfer line” to refer to lines which provide such passage to the second loop reactor (even in the situation where they also comprise hydrocyclones).

[0061] However, series loop reactors can also be operated in bimodal or multimodal operation, where conditions in the first and second loop reactors are generally significantly 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 2336200A1. We will, herein, use the term “indirect product transfer line” to refer to lines which provide such passage to the second loop reactor. (It can also be noted that indirect product transfer lines can also include concentrators, such as hydrocyclones, located before any subsequent treatment step or steps.)

[0062] In the present invention, where the first and second loop reactors are connected in series, then the first loop reactor may, and preferably does, comprise both direct andCase 00425(1) 9

[0063] indirect product 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.

[0064] In this case, for example, one or more bottom sections of the first loop reactor may comprise direct product transfer lines and one or more bottom sections may comprise indirect product transfer lines.

[0065] It is also possible in this case that at least one bottom section may comprise both a direct product transfer line by which a product stream comprising polymer solids can be withdrawn from the first loop reactor and passed to the second loop reactor and an indirect product transfer line by which a product stream comprising polymer solids can also be withdrawn from the first loop reactor and passed to the second loop reactor (via a treatment step).

[0066] However, and in particular in relation to both the first and second loop reactors, preferably no bottom section of either reactor comprises more than 3 withdrawal lines, preferably no more than 2 withdrawal lines. An advantage of this is that withdrawal lines generally (and for a series configuration specifically product 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 and / or product withdrawal lines on the second loop reactor), can be more easily located at the optimum position for withdrawal. This can be contrasted, for example, with WO 2004 / 031245, where multiple withdrawals are located on a single elbow. As issue with this is that the locations of the 2nd, 3rd, 4thor later withdrawals are constrained by those already present.

[0067] A further advantage to this can also arise where two or more product transfer lines or two or more product withdrawal lines withdraw a slurry of polymer solids initially to a hydrocyclone. In a typical hydrocyclone the initial slurry withdrawn (from either the first or second loop reactor) is concentrated whilst a stream of diluent from the hydrocyclone is recycled to the loop reactor. Minimizing the number of withdrawals and returns on a single bottom section of a loop reactor minimizes the impact on local slurry flow / pattems in each bottom section.

[0068] Relative Orientation of the Loop Reactors

[0069] The present invention provides a reactor system comprising first and second loop reactors where the plane or planes coincident with the longest side or sides of the first loopCase 00425(1) 10

[0070] reactor are not parallel to any plane or planes coincident with the longest side or sides of the second loop reactor.

[0071] As also already noted, most preferably, the plane or planes coincident with the longest side or sides of the first loop reactor are within 45 degrees of perpendicular, and preferably are perpendicular, to any plane or planes coincident with the longest side or sides of the second loop reactor.

[0072] Preferably the first loop reactor and the second loop reactor are in series (i.e. where slurry comprising polymer particles is withdrawn from the first loop reactor and passed to the second loop reactor).

[0073] As also has been noted above, the orientation of the two loop reactors being “nonparallel” can allow greater flexibility in the layout of the overall reactor system. This can arise in particular in relation to the transfer of the slurry from the first loop reactor to the second loop reactor in series configurations and / or in connections to vessels which are connected to both loop reactors. Specifically, the invention allows the layout of the reactor system to be configured in a way which allows to minimise distances between the loop reactors and other vessels

[0074] Thus, in a preferred embodiment, there is provided a vessel which is fluidly connected to both the first and second loop reactors, the reactor system being such that the length of the longest side or sides of the first loop reactor is L, the shortest horizontal distance between the first loop reactor and the vessel is dl, the shortest horizontal distance between the second loop reactor and the vessel is d2 and the shortest horizontal distance between the first loop reactor and the second loop reactor is d3, wherein the sum of dl + d2 + d3 is less than 3L.

[0075] In this embodiment all three of the vessel and the first and second loop reactors are relatively close together. The three are all fluidly connected, and this allows to minimise the distance the connecting pipework has to cover. (For avoidance of doubt, although the distances dl, d2 and d3 are defined as “horizontal distances”, the connecting pipework will generally not be solely horizontal. It may include non-horizontal sections, for example to bridge vertical differences in height e.g. there may be a height difference between a slurry withdrawal outlet on the first reactor and the corresponding slurry inlet on a second reactor or the corresponding inlet on the vessel.)Case 00425(1) 11

[0076] As defined above, the sum of dl + d2 + d3 is defined relative to the length of the longest side or sides of the first loop reactor, L, and in particular such that the sum is less than 3L.

[0077] The length, L, of a loop reactor generally depends on the number of vertical legs and the footprint. Typically, L is 10 to 40 metres, and more typically 15 to 30 metres.

[0078] In absolute terms, preferably the sum of dl + d2 + d3 is less than 60m. (And in an alternative embodiment, the sum of dl + d2 + d3 is less than 60m even if this is more than 3L.)

[0079] In most preferred embodiments one or more of the following may apply:

[0080] At least two of, and preferably each of, dl, d2 and d3, are less than the length L, At least one of dl, d2 and d3 is less than L / 2,

[0081] The sum of dl + d2 + d3 is less than 2L,

[0082] At least two of, and preferably each of, dl, d2 and d3, are less than 20m

[0083] At least one of dl, d2 and d3 is less than 15m,

[0084] The sum of dl + d2 + d3 is less than 40m.

[0085] The “vessel” in this embodiment may be any suitable vessel that is fluidly connected to both the first and second loop reactors. For example, where the first and second loop reactors are not in series, and the slurry withdrawn from each loop reactor is passed to product recovery steps, then 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.

[0086] However, a particular advantage of the present invention 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.

[0087] Thus, in a preferred embodiment the first and second reactors are provided in series and most 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. This is connected to the first loop reactor by one or more transfer lines for transfer of a polymer containing stream from the first loop reactor to the intermediate treatment system and connected to the second loop reactor by one or more transfer lines for transfer of a polymer containing stream from the intermediate treatment system to the second loop reactor.Case 00425(1) 12

[0088] In systems suitable for production of both monomodal and bimodal (or multimodal) polymers, production of monomodal polymers takes place with the same or similar conditions in the first and second loop reactors, as already noted. In such cases it is possible, and generally preferred, to transfer polymer slurry from the first loop reactor to the second loop reactor without any intermediate treatment. Thus, the product transfer can be via a direct product transfer line or lines. (And the shortest horizontal distance between the two loop reactors is d3 as defined above.)

[0089] In contrast, production of bimodal polymers takes place with different conditions in the first and second loop reactors. In such cases transfer of polymer slurry from the first loop reactor to the second loop reactor generally takes place via an intermediate treatment step. Thus, the product transfer is via an indirect product transfer line or lines. (And thus, in such embodiments, dl is the shortest horizontal distance between the first loop reactor and the intermediate treatment vessel and d2 is the shortest horizontal distance between the second loop reactor and the intermediate treatment vessel.)

[0090] Where there are both direct transfer lines and indirect transfer lines, the present invention allows the reactor system to be advantageously configured. In particular, in reactor systems which are used for production of both monomodal and bimodal (or multimodal) polymers, it is possible to locate direct product transfer lines on legs or bottom sections of the first loop reactor which are physically the closest to the second loop reactor, thereby minimizing the length of these lines between the reactors. In contrast, where a slurry of polymer solids is to be passed via an intermediate treatment, this can be located close to different legs or bottom sections of the first loop reactor, to minimize these lines also.

[0091] An Example is shown in Figure 3 which shows a first loop reactor (1) and a second loop reactor (2), both of which have 10 legs and a rectangular footprint. The first and second loop reactors are orientated such that the planes coincident with the longest sides of the first loop reactor are perpendicular to the planes coincident with the longest sides of the second loop reactor.

[0092] As shown in this Figure 3, direct product transfer lines are provided from each of two legs of the first loop reactor which are closest to the second loop reactor. Three indirect product transfer lines are also shown, these being connected to the second loop reactor via an intermediate treatment system (3). As can be seen, the perpendicular orientation of theCase 00425(1) 13

[0093] two reactors allows multiple lines of each type to be present whilst also allowing the lengths of the direct and indirect product transfer lines to be relatively short. A particular advantage of this is that there is a reduced risk of agglomeration is said lines. (Typically, these lines are not cooled but still comprise active catalyst and monomer reactants so reaction will continue during the transfer. Minimising the length of such lines minimises the residence time of active slurry in these lines, hence the amount of polymerisation and heat generation which will occur.)

[0094] (Of course, typically either the direct product transfer lines or the indirect product transfer lines, and not both sets, would be used at any one time, the selection of which is in use depending on whether producing a monomodal or a bimodal / multimodal product.) Figure 3 also shows three product withdrawal lines on the second loop reactor, which feed to a product separation system (4).

[0095] For avoidance of doubt, although the direct and indirect product transfer lines in Figure 3 are shown as aligned with individual legs of the first loop reactor, and the product withdrawal lines are shown as aligned with individual legs of the second loop reactor, in both cases this is only for the purposes of illustration, and the respective lines could, in practice, also be taken from bottom sections between two legs. It will also be appreciated that more than one product transfer line may be taken from a single leg or bottom section of the first loop reactor and / or that more than one product withdrawal line may be taken from a single leg or bottom section of the second loop reactor.

[0096] Polymerisation

[0097] The reactor system according to the first aspect present invention is used for polymerisation of olefins. In a second 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.

[0098] As with the reactor system generally, other than the orientation of the loop reactors the process for polymerisation is largely as known in the art.

[0099] The polymerisation preferably comprises polymerisation of ethylene, optionally with a comonomer in one or both reactors, or of propylene, again optionally with a comonomer in one or both reactors. It may also be noted that ethylene may be a comonomer for propylene polymerisations and vice versa.

[0100] Thus, the product is preferably a polyethylene or a polypropylene.Case 00425(1) 14

[0101] Polymerisation of ethylene generally takes place in the presence of an inert diluent, typically an alkane, with isobutane being particularly preferred.

[0102] Polymerisation of propylene may also be performed in the presence of an inert diluent, although use of liquid propylene as diluent is also known.

[0103] 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.

[0104] 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 00425(1) 15Claims1. A reactor system comprising first and second loop reactors, each of the first and second loop reactors having at least one longest side, wherein the plane or planes coincident with the longest side or sides of the first loop reactor are not parallel to any plane or planes coincident with the longest side or sides of the second loop reactor.

2. A reactor system according to claim 1 wherein there is additionally provided a vessel which is fluidly connected to both the first and second loop reactors and wherein the length of the longest side or sides of the first loop reactor is L, the shortest horizontal distance between the first loop reactor and the vessel is dl, the shortest horizontal distance between the second loop reactor and the vessel is d2 and the shortest horizontal distance between the first loop reactor and the second loop reactor is d3, wherein the sum of dl + d2 + d3 is less than 3L.

3. A reactor system according to claim 2 wherein at least one of the following applies:a. At least two of, and preferably each of, dl, d2 and d3 are less than the length L,b. At least one of dl, d2 and d3 is less than L / 2,c. the sum of dl + d2 + d3 is less than 2L,d. At least two of, and preferably each of, dl, d2 and d3, are less than 20m e. At least one of dl, d2 and d3 is less than 15m,f. The sum of dl + d2 + d3 is less than 60m, preferably less than 40m.

4. A reactor system according to any one of the preceding claims where the first and second loop reactor are connected in series.

5. A reactor system according to claim 2 or claim 3 wherein where the first and second loop reactor are connected in series and the vessel is part of an intermediate treatment system, connected to the first loop reactor by one or more transfer lines for transfer of a polymer containing stream from the first loop reactor to the intermediate treatment system and connected to the second loop reactor by one or more transfer lines for transfer of a polymer containing stream from the intermediate treatment system to the second loop reactor.

6. A reactor system according to any one of the preceding claims wherein each of the first and second loop reactors has a rectangular footprint and the planes coincident with theCase 00425(1) 16two longest sides of the first loop reactor are not parallel to the planes coincident with the two longest sides of the second loop reactor.

7. A reactor system according to any one of the preceding claims wherein the plane or planes coincident with the longest side or sides of the first loop reactor are within 45 degrees of perpendicular to any plane or planes coincident with the longest side or sides of the second loop reactor.

8. A reactor system according to any one of the preceding claims wherein the plane or planes coincident with the longest side or sides of the first loop reactor are perpendicular to any plane or planes coincident with the longest side or sides of the second loop reactor or, if not exactly perpendicular, are within at most 10 degrees of perpendicular, and most preferably where each of the first and second loop reactors has a rectangular footprint.

9. A reactor system according to any one of the preceding claims wherein both the loop reactors have the same number of vertical legs and the same footprint formed by the vertical legs.

10. A reactor system according to any one of the preceding claims wherein each of the first and second loop reactors has at least 8 vertical legs.

11. A reactor system according to claim 8 wherein each of the first and second loop reactors has at least 10 vertical legs.

12. A reactor system according to any one of the preceding claims wherein the first and second loop reactor are connected in series there are multiple withdrawal lines, also referred to as product transfer lines, on the first loop reactor.

13. A reactor system according to claim 12 wherein there are both direct and indirect product 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.

14. A reactor system according to any one of the preceding claims wherein there are multiple product withdrawal lines on the second loop reactor15. A process for polymerisation, which process comprises polymerisation of one or more olefins in a reactor system according to any one of claims 1 to 14.

16. A process according to claim 15 where the polymerisation comprises polymerisation of ethylene, optionally with a comonomer in one or both reactors.

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