Reactor system

Abstract

The present invention relates to a reactor system, and in particular provides reactor system comprising two loop reactors connected in series, each of the loop reactors having at least 8 vertical legs and having a rectangular footprint, thereby having two long sides and two short sides, wherein at least one of the long sides of the first loop reactor has at least two transfer lines which connect said long side of the first loop reactor to the second loop reactor.

Description

[0001] Case 00424(2) 1

[0002] Reactor System

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

[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. Slurry loop reactors 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 processes operate with two loop reactors in series. In such reactor systems slurry comprising polymer particles is withdrawn from the first loop reactor and passed to the second loop reactor via a transfer line. In the second loop reactor 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 each reactor, particularly with different molecular weight, in which case the product is referred to as bimodal or multimodal.

[0008] 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 loopCase 00424(2) 2

[0009] reactor without treatment. The withdrawal / transfer line may then simply comprise a line or pipe, with suitable valving, between the reactors.

[0010] 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 withdrawn from the first loop reactor before it is passed to the second loop reactor.

[0011] In a reactor system with two loop reactors in series, slurry comprising polymer particles is withdrawn from the second 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.

[0012] Separated components are usually cooled to condense, and then recycled to one or both of the two loop reactors.

[0013] Two common types of withdrawal from a loop reactor are practised.

[0014] 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 a 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.

[0015] 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.

[0016] (For avoidance of doubt, these types of withdrawal can be applied both to the withdrawal from the second loop reactor, but also to withdrawal from the first loop reactor of the slurry to be transferred to the second loop reactor.)

[0017] 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 ofCase 00424(2) 3

[0018] 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.

[0019] 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.

[0020] 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.

[0021] It has now been found, however, that in a reactor system with at least two loop reactors in series, with multiple vertical legs on each reactor and multiple withdrawals from the first loop reactor to the second loop reactor (“transfer lines”), then the relative orientation and positioning of the two loop reactors, and the relative locations of the withdrawals from the first loop reactor to the second loop reactor, can be selected to allow the optimisation of the layout of the overall reactor system.

[0022] Thus, in a first aspect the present invention provides reactor system comprising two loop reactors connected in series, each of the loop reactors having at least 8 vertical legs and having a rectangular footprint, thereby having two long sides and two short sides, wherein at least one of the long sides of the first loop reactor has at least two transfer lines which connect said long side of the first loop reactor to the second loop reactor.

[0023] In the present invention each of the loop reactors has at least 8 vertical legs and a rectangular footprint. The term “footprint” refers to the shape of a loop reactor as viewed from the top. 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. In the loop reactors the vertical legs of each loop reactor are connected by top and bottom sections to form aCase 00424(2) 4

[0024] continuous loop. (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.)

[0025] One of the long sides is marked “L” on the plan view of Figure 1, but of course there are two (parallel) long sides on a rectangular footprint as well as two (parallel) short sides, the short sides being perpendicular to the long sides.

[0026] Loop reactors generally have an even number of vertical legs. It will be appreciated that any such reactor having at least 8 legs can be configured to a rectangular configuration by two parallel rows of legs, but other rectangular configurations are possible. For example, a reactor with 10 legs can be formed by two rows of 5 legs (5 legs on long sides and 2 legs on short sides), or by having 4 legs on the long sides and 3 legs on the short sides.

[0027] We will now describe the individual loop reactors, followed by the preferred orientations.

[0028] First and Second Loop Reactors

[0029] The first and second loop reactors need not be the same size or have the same number of vertical legs, as long as both have at least 8 legs and both have a rectangular 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.

[0030] In the present invention 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.

[0031] (It will be apparent that the loop reactors should each have an even number of legs. In particular, for the 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.)Case 00424(2) 5

[0032] The first and second loop reactors, other than the locations of the transfer lines from the first loop reactor to the second loop reactor, are generally as known in the art.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] The total internal volume of each loop reactor is generally in the range 100 to 350m3. In the present invention we use the term “transfer line” to refer to a line which connects the first loop reactor to the second loop reactor. Each transfer line, as defined herein, can 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, at least with reference to the first loop reactor. We will generally use the term “transfer line” rather than withdrawal line to distinguish from product withdrawal lines on the second loop reactor, but nevertheless the transfer line may 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. Preferably, the transfer lines on the first loop reactor are each continuous take-off lines.

[0037] A key feature of the present invention is that at least one of the long sides of the first loop reactor has at least two transfer lines which connect said long side of the first loop reactor to the second loop reactor. Put another way, there are provided at least two transfer lines which connect the first loop reactor to the second loop reactor and at least two of the transfer lines are connected to the first loop reactor on the same long side of the first loop reactor. As already noted, each transfer line can provide withdrawal of polymer from theCase 00424(2) 6

[0038] first loop reactor for transfer to and introduction into the second loop reactor. Nevertheless, it should also be noted that although there are multiple transfer lines present not all transfer lines have to be “active” at the same time when the reactor is being operated.

[0039] The first loop reactor may comprise more than two transfer lines. In this case at least two are provided on the same long side of the first loop reactor. There may, for example, be three or more transfer lines. In preferred embodiments there may be at least n / 2 transfer lines which connect the first loop reactor to the second loop reactor, where n is the number of vertical legs of the first loop reactor (n being an even number, and being at least 8 in the present invention). It is also preferred that at least some transfer lines are provided on different legs, or, more preferably, on different bottom sections on the first loop reactor.

[0040] Whilst it is not precluded that some transfer lines may be provided on both long sides of the first loop reactor, it is generally preferred that a majority of them are provided on one long side of the first loop reactor. In some embodiments all transfer lines which connect the first loop reactor to the second loop reactor may be provided on one long side of the first reactor. Further, it is generally desirable that this is the long side of the first loop reactor which is closest to the second loop reactor. Thus, it is preferred that the long side of the first loop reactor which is closest to the second loop reactor has the at least two transfer lines which connect said long side of the first loop reactor to the second loop reactor. (Or more generally that the long side with the majority of the transfer lines connected thereto is the long side closest to the second loop reactor.) In this manner the length of the transfer lines can be minimised. (Even if some transfer lines to the second reactor are connected to the long side of the first loop reactor furthest from the second loop reactor, taking the majority from the long side closest to the second loop reactor enables to reduce the length of the majority of the lines.)

[0041] It is generally also preferred that transfer lines are connected to / located at lower ends of vertical legs or, and preferably, on the bottom sections of the first loop reactor which connect adjacent vertical legs. (Whichever long side they are on.)

[0042] Each transfer line can withdraw a slurry comprising polymer solids from the first loop reactor for transfer to the second loop reactor. The passage to the second loop reactor may be performed without any intermediate treatment (“direct transfer line”) or via an intermediate treatment (“indirect transfer line”). For example, in monomodal operation, where conditions in the first and second loop reactors are similar or the same, then theCase 00424(2) 7

[0043] 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.

[0044] (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).

[0045] 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.)

[0046] The first loop reactor may, and preferably does, comprise 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.

[0047] In this case, for example, one or more bottom sections of the first loop reactor may comprise direct transfer lines and one or more bottom sections may comprise indirect transfer lines.

[0048] It is also possible in this case that at least one bottom section may comprise both a direct 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 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).

[0049] As noted, each transfer line can withdraw a slurry comprising polymer solids from the first loop reactor and transfer to the second loop reactor. Thus, and independently ofCase 00424(2) 8

[0050] whether transferred “directly” or “indirectly” to the second loop reactor, there are also multiple connections on the second reactor for the transfer lines from the first reactor.

[0051] Whilst it is not precluded that some transfer lines may connect to both long sides of the second loop reactor, it is generally preferred that a majority of them connect to on one long side of the second loop reactor. Most preferably the majority, such as all, may connect to the long side of the second loop reactor closest to the first loop reactor.

[0052] It is also generally preferred that at least some transfer lines from the first loop reactor to the second loop reactor are connected to different legs, bottom sections or top sections on the second loop reactor, (i.e. not all to the same leg or section.)

[0053] The second loop reactor may also comprise multiple withdrawal lines. In the present invention the term “product withdrawal line” will be used to refer specifically to a withdrawal line by which a product stream comprising polymer solids is withdrawn from the second loop reactor, and in particular is 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 the second 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 line or lines on the second loop reactor are preferably each continuous take-offlines.

[0054] It is preferred that product withdrawal lines are located at lower ends of vertical legs or, and preferably, on the bottom sections of the second loop reactor which connect adjacent vertical legs.

[0055] As noted, it is typical that there are multiple product withdrawal lines on the second 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 the second loop reactor. This allows withdrawal from multiple different parts of the second 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.)

[0056] Each of the product withdrawal lines will 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 ofCase 00424(2) 9

[0057] the operation of slurry heaters can be found, for example, in WO 2009 / 127643 Al and WO 2009 / 127645 Al.

[0058] Where there are multiple product withdrawal lines on the second 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 the second loop reactor.

[0059] 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.

[0060] The locations of the product withdrawal lines on the second loop reactor are not especially critical, but typically where there are multiple product withdrawal lines on the second loop reactor then at least two product withdrawal lines are provided on the same long side of the second loop reactor.

[0061] Preferably at least two product withdrawal lines are provided on the long side which is most remote to the first loop reactor.

[0062] Generally, 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. (The term “withdrawal lines” encompassing “transfer lines” for the first loop reactor and “product withdrawal lines” for the second loop reactor.) An advantage of this is that withdrawal lines generally, and 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 product 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.

[0063] A further advantage to this can also arise where two or more of the transfer lines from the first loop reactor to the second loop reactor or two or more of the product withdrawal lines on the second loop reactor, withdraw the slurry of polymer solids initially to a hydrocyclone. In a typical hydrocyclone on the first loop reactor the initial slurryCase 00424(2) 10

[0064] withdrawn is concentrated prior to being passed to the second loop reactor (either directly or via an intermediate treatment as already described) whilst a stream of diluent from the hydrocyclone is recycled to the first loop reactor. In a typical hydrocyclone on the second loop reactor, the initial slurry withdrawn is concentrated prior to being passed to further product treatment (e.g. slurry heater and gas-solid separator), whilst a stream of diluent from the hydrocyclone is recycled to the second loop reactor. Minimising the number of withdrawals and returns on a single bottom section of the first loop reactor or of the second loop reactor minimizes the impact on local slurry flow / patterns in each bottom section. Preferred Relative Orientation and Location of the Loop Reactors

[0065] It is a feature of the present invention that each of the loop reactors has at least 8 vertical legs and a rectangular footprint, and that at least one of the long sides of the first loop reactor has at least two transfer lines which connect said long side of the first loop reactor to the second loop reactor.

[0066] As mentioned above, it is preferred that the long side of the first loop reactor which is closest to the second loop reactor has at least two transfer lines which connect said long side of the first loop reactor to the second loop reactor, and, more generally, that the long side with the majority of the transfer lines connected thereto is the side closest to the second loop reactor, so that the length of the transfer lines can be minimised.

[0067] In preferred embodiments the distance between the 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 any vertical leg on the long side of the first loop reactor which has the at least two transfer lines and any vertical leg of the second loop reactor. (In preferred embodiments where the long side of the first loop reactor which is closest to the second loop reactor has at least two transfer lines, the distance between the first and second loop reactors is then the distance between any vertical leg the long side of the first loop reactor closest to the second reactor and any vertical leg of the second loop reactor.)

[0068] In more preferred embodiments the longest distance between (1) any vertical leg on the long side of the first loop reactor which has the at least two transfer lines and which vertical leg either has a transfer line or is connected to a bottom section which has a transfer line and (2) the closest vertical leg of the second loop reactor to said vertical leg of the first reactor, is less than 50m, preferably less than 30m, such as less than 20m, or evenCase 00424(2) 11

[0069] less than 10m. (In this case, in the preferred embodiments where the long side of the first loop reactor which is closest to the second loop reactor has at least two transfer lines, then this is the longest distance between (1) any vertical leg on the long side of the first loop reactor closest to the second loop reactor and which vertical leg either has a transfer line or is connected to a bottom section which has a transfer line and (2) the closest vertical leg of the second loop reactor to said vertical leg of the first reactor.)

[0070] In most preferred embodiments the first and second loop reactors are orientated parallel to each other, which as used herein means that the planes coincident with the long sides of the first loop reactor are parallel to the planes coincident with the long sides of the second loop reactor.

[0071] More preferably neither of the planes coincident with the long sides of the first loop reactor intersect with the footprint of the second loop reactor, and vice versa. This means that, in preferred embodiments, the first and second loop reactor are off-set from each other in relation to their long sides. This off-set, measured as the shortest perpendicular distance between a plane coincident with a long side of the first loop reactor and a plane coincident with the long side of the second loop reactor is preferably less than 20m, and more preferably less than 10m. (And is greater than zero.)(This is further exemplified below with reference to the Figures.)

[0072] It is yet further preferred that a plane which extends from at least one short side of the first loop reactor either intersects with the footprint of the second loop reactor or does not intersect but is within a distance such that the shortest perpendicular distance from said plane to the second loop reactor is less than the length of the long sides of the first loop reactor. This means that the reactors are either aligned or are not too far “off-set” also in this direction from each other.

[0073] Preferred embodiments of the present invention are illustrated by reference to Figures 2 to 4 herein.

[0074] Figure 2 shows first (1) and second (2) loop reactors which are aligned with each other in relation to their shortest sides but off-set in relation to their longest sides. This offset is shown as the distance “y” in Figure 2 and, as already noted, is the shortest perpendicular distance between a plane coincident with a long side of the first loop reactor and a plane coincident with the long side of the second loop reactor, (i.e. the two closest planes of the respective loop reactors.) Also shown in Figure 2 are four transfer lines whichCase 00424(2) 12

[0075] connect the first loop reactor (1) to the second loop reactor (2), all of which are connected to the same long side of the first loop reactor (1). Also shown are two product withdrawal lines on the second loop reactor (2), by which product may be withdrawn and passed to one or more slurry heaters and gas-solids separation steps (not shown). It can be seen that this provides a compact overall footprint with multiple short product transfer lines. In particular, for loop reactors with multiple transfer lines from a first loop reactor (1) to a second loop reactor (2) (and in particular large scale reactors where multiple transfer lines are required), aligning the long sides and placing multiple transfer lines on one long side allows to minimise distances for the multiple transfer lines from different legs / lower sections.

[0076] (It can be noted that in Figure 2, the value “y” is both “the shortest distance which can be measured between any vertical leg on the long side of the first loop reactor which has the at least two transfer lines and any vertical leg of the second loop reactor” and also “the longest distance between (1) any vertical leg on the long side of the first loop reactor which has the at least two transfer lines and which vertical leg either has a transfer line or is connected to a bottom section which has a transfer line and (2) the closest vertical leg of the second loop reactor to said vertical leg of the first reactor”. This is specific for this and similar scenarios. It is not the case in Figures 3 and 4, for example.)

[0077] Figure 3 shows first (1) and second (2) loop reactors which are off-set from each other in two directions. The first off-set is the distance “y” between the closest long sides as also was present in Figure 2. The second off-set is shown as the distance “x” and represents the shortest perpendicular distance between a plane which extends from a short side of the first loop reactor (1) to the second loop reactor (2). As shown this distance is less than the length of the long sides of the first loop reactor (1). (Corresponding to “L” in Figure 1.) (It can also be noted that the distance between the first and second loop reactors in this case, defined as already noted as the shortest distance which can be measured between any vertical leg on the long side of the reactor which has the at least two transfer lines and any vertical leg of the second loop reactor, is (x2+ y2)05. As also already noted, this is preferably less than 30m, and more preferably less than 20m, such as less than 10m.) Also shown in Figure 3 is an intermediate separations step (3), with two transfer lines which connect the first loop reactor (1) to this separations step (3) and from there to the second loop reactor (2). As can be seen, this configuration (with an off-set “x” compared toCase 00424(2) 13

[0078] Figure 2) allows an intermediate separations step to be present whilst minimising the lengths of the transfer lines.

[0079] (Also present in practise, although not shown in Figure 3, or the following Figure 4, would be product withdrawal lines on the second loop reactor (2).)

[0080] Figure 4 shows a further option. This configuration again has a separation “y” between the long sides (although this is not labelled in this Figure), but in this case a plane (shown by a dashed line) which extends from the short side of the first loop reactor (1) intersects with the footprint of the second loop reactor (2). Figure 4 exemplifies a system with both direct and indirect transfer lines, and in particular shows two direct transfer lines by which product from the first loop reactor (1) can be transferred directly to the second loop reactor (2), and two indirect transfer lines by which product from the first loop reactor (1) can be transferred to the second loop reactor (2) via an intermediate treatment system (3). It will be apparent that the “incomplete off-set” of the two loop reactors enables short transfer lines for both options.

[0081] A particular advantage of Figure 4 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.

[0082] Polymerisation

[0083] 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.

[0084] As with the reactor system generally the process for polymerisation itself is largely as known in the art.

[0085] 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.

[0086] Thus, the product is preferably a polyethylene or a polypropylene.

[0087] Polymerisation of ethylene generally takes place in the presence of an inert diluent, typically an alkane, with isobutane being particularly preferred.Case 00424(2) 14

[0088] Polymerisation of propylene may also be performed in the presence of an inert diluent, although use of liquid propylene as diluent is also known.

[0089] 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.

[0090] 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 00424(2) 15Claims1. A reactor system comprising two loop reactors connected in series, each of the loop reactors having at least 8 vertical legs and having a rectangular footprint, thereby each having two long sides and two short sides, wherein at least one of the long sides of the first loop reactor has at least two transfer lines which connect said long side of the first loop reactor to the second loop reactor.

2. A reactor system as claimed in claim 1 wherein the long side of the first loop reactor which is closest to the second loop reactor has at least two transfer lines which connect said long side of the first loop reactor to the second loop reactor.

3. A reactor system as claimed in claim 2 wherein the vertical legs of each loop reactor are connected by top and bottom sections to form a continuous loop, and the long side of the first loop reactor which is closest to the second loop reactor has at least two transfer lines which are connected to different legs or different bottom sections on said long side of the first loop reactor.

4. A reactor system as claimed in claim 2 or claim 3 wherein the distance between the first and second loop reactors is less than 30m, preferably less than 20m, where this distance is the shortest distance which can be measured between any vertical leg on the long side of the first loop reactor which is closest to the second loop reactor and any vertical leg of the second loop reactor.

5. A reactor system as claimed in claim 4 wherein the longest distance between (1) any vertical leg on the long side of the first loop reactor closest to the second loop reactor and which vertical leg either has a transfer line or is connected to a bottom section which has a transfer line and (2) the closest vertical leg of the second loop reactor to said vertical leg of the first reactor is less than 50m, preferably less than 30m.

6. A reactor system as claimed in any one of the preceding claims wherein the first and second loop reactors are orientated parallel to each other such that the planes coincident with the long sides of the first loop reactor are parallel to the planes coincident with the long sides of the second loop reactor.

7. A reactor system as claimed in claim 6 wherein neither of the planes coincident with the long sides of the first loop reactor intersect with the footprint of the second loop reactor, and vice versa, such that the first and second loop reactor are off-set from each other in relation to their long sides.Case 00424(2) 168. A reactor system as claimed in claim 7 wherein this off-set, measured as the shortest perpendicular distance between a plane coincident with a long side of the first loop reactor and a plane coincident with the long side of the second loop reactor is less than 20m.

9. A reactor system according to any one of claims 6 to 8 wherein a plane which extends from at least one short side of the first loop reactor either intersects with the footprint of the second loop reactor or does not intersect but is within a distance such that the shortest perpendicular distance from said plane to the second loop reactor is less than the length of the long sides of the first loop reactor.

10. A reactor system as claimed in any one of the preceding claims wherein each loop reactor has at least 10 legs.

11. A reactor system as claimed in claim 10 wherein each loop reactor has 10-20 legs.

12. A reactor system as claimed in any one of the preceding claims wherein at least one of the long sides of the first loop reactor has at least at least three transfer lines which connect said long side of the first loop reactor to the second loop reactor and where said transfer lines are connected to different legs or different bottom sections on said long side of the first loop reactor.

13. 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.

14. A reactor system according to any one of the preceding claims wherein there are both direct and indirect transfer lines which connect the first loop reactor to the second loop reactor (and 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 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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