PRIORITY

[0001] This application claims priority to Provisional Application No. 61/535,586 (2011EM171) filed on September 16, 201 1, the disclosures of which are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

[0002] The present invention relates in general to continuous loop reactors, a method of enhancing the capacity of such reactors and a method of producing polyolefin homo- and copolymers in such reactors.

BACKGROUND

[0003] One of the most common processes for making polyolefin homopolymer and copolymer is based on what are commonly called loop reactors. In this process, a loop reactor is made up of a series of long, usually straight pipes (generally referred to as "legs") which are connected by bent pipes to form a continuous loop. Pumps are incorporated into this loop to circulate the polymer and liquid monomer slurry. Olefin polymerization is an exothermic reaction, so heat must be removed from the reactors. Typically, such exothermicity is a limiting factor in polyolefin production capacity.

[0004] One method of efficient heat removal is to provide jacketed reactor legs, such as described in U.S. Patent No. 7,678,341 to Smith, incorporated by reference herein, where water flows through the jackets to control the reactor temperature by removing heat from the reactor slurry.

[0005] U.S. Patent Nos. 3,318,857; 4,424,341; and 4,613,484 disclose conventional designs for polyolefin loop polymerization reactors.

[0006] U.S. Patent Nos. 7,632,899 and 7,820, 116 disclose processes for increasing production of polyolefin homo- and copolymers by controlling the Froude number of the polymer slurry circulating in a loop polymerization reactor.

[0007] U.S. Patent No. 7,723,446, incorporated by reference herein, discloses a process for producing polypropylene comprising: providing series reactors comprising a first and a second loop allowing flow of polypropylene, catalyst, hydrogen and propylene there between, wherein each loop comprises from six to eight legs, each leg having fluid connections there between; injecting into the first loop an amount of a catalyst, propylene and optionally hydrogen; withdrawing polypropylene, and unreacted propylene and optionally unreacted hydrogen from the first loop and injecting the polypropylene, and unreacted propylene and optionally unreacted hydrogen into the second loop; and providing a propylene/polypropylene separator fluidly connected to the second loop and isolating polypropylene therefrom. In one embodiment a recycle conduit is provided between the propylene/polypropylene separator and the second loop allowing the recycling of unreacted propylene and optionally hydrogen into the second loop.

[0008] One type of reactor design for high capacity polymerization lines uses two loop reactors operating in series. That is, catalyst and liquid monomers are fed to the first reactor and the slurry that leaves that reactor along with more liquid monomers is fed to the second reactor. Slurry leaving the second reactor is sent on for monomer separation and recovery. In this design the maximum capacity of the line may be determined by the heat transfer area available in these reactors. For new plants it is desired to increase plant capacity to reduce the capital and operating cost per unit of production. Various small increases have been achieved by increasing the reactor operating temperature, decreasing the temperature of the circulating cooling water and increasing the circulation rates of the water used to remove heat from the reactors.

[0009] Given the limitations on production capacity due to the exothermic nature of the olefin polymerization reaction, it would be advantageous to improve heat removal from the reactor(s). The inventors have solved these and other problems in aspects of the invention as described herein.

SUMMARY

[0010] One aspect of the invention is directed to a loop polymerization reactor comprising upflow and downflow legs that form a continuous loop, in which the number of downflow legs exceeds the number of upflow legs.

[0011] In another aspect, the invention is directed to a process for increasing the capacity of a loop polymerization reactor having upflow and downflow legs fluidly connected at the top and bottom thereof to form a continuous loop, comprising configuring said loop to have a number of downflow legs exceeding the number of upflow legs.

[0012] In another aspect, the invention is directed to a process for making polyolefins, comprising injecting at least one olefin monomer, a catalyst or polymerization initiator and optionally a diluent into at least one continuous loop reactor having upflow and downflow legs fluidly connected at the top and bottom thereof to form a continuous loop, in which the number of downflow legs exceeds the number of upflow legs and at least one of the downflow legs is jacketed and water flows through the jacket to control the reactor temperature by removing heat from the reactor; circulating said monomer, catalyst or polymerization initiator and optionally said diluent through said reactor loop; reacting said olefin monomer under flow, temperature and pressure conditions adequate to polymerize said monomer and to form said polyolefin; and recovering said polyolefin from said continuous loop reactor.

[0013] In another aspect, the invention is directed to a loop polymerization reactor comprising at least one upflow leg and at least one downflow leg forming a continuous loop, wherein the downflow leg is fluidly connected in with a second downflow leg with a Y-shaped section of tubing configured to ensure a predetermined distribution of polymer slurry between said downflow legs.

[0014] These aspects of the invention can be combined with the various embodiments disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a representation of the flowpath of a conventional slurry loop polymerization reactor.

[0016] Figure 2 is a representation of the flowpath of a slurry loop polymerization reactor according to one embodiment of the present invention.

[0017] Figure 3 is a representation of the flowpath of a slurry loop polymerization reactor according to another embodiment of the present invention.

DETAILED DESCRIPTION

[0018] The present invention proposes to increase the heat transfer area of a slurry loop polymerization reactor (hereinafter a "loop reactor") with a novel approach: rather than add additional reactor length, additional loop reactor legs are added (e.g., in parallel) to the legs used in a traditional configuration. This approach provides additional heat transfer area with the added benefit of reducing the overall head loss on the loop reactor pump (a smaller pump will be required). Normally, the new legs will be added in parallel to legs wherein the flow will be generally downward.

[0019] As used herein, "polyolefin" includes both homopolymers of ethylene and propylene, and copolymers of ethylene or propylene and another monomer-derived unit where the ethylene or propylene-derived units comprise at least 50 wt% of the polymer.

[0020] A "slurry loop" olefin polymerization reactor can generally be described as a loop- shaped continuous tube. In some instances, the reactor design may be generally "0" shaped. One or more fluid circulating devices, such as an axial pump, force the reactor constituents within the tube in a desired direction so as to create a circulating current or flow of the reactor constituents within the tube. The "flow" of materials can be also be assisted by pressure differentials and other means as is commonly known in the art, the flow of materials in certain embodiments achieved though fluid connections. The term "fluid connections" or "fluidly connected" includes piping, conduits and other means of allowing liquids and/or suspensions (e.g., a slurry of liquid olefin and solid polymer material) as is commonly known in the art to flow from one place to another without exposure to the atmosphere. Particularly, according to the present invention additional downflow leg(s) are fluidly connected with an adjacent downflow leg (e.g., in parallel). In an embodiment, the additional downflow leg(s) is connected using a Y-shaped section of tubing at both the top and bottom of the fluidly connected legs. The Y-shaped section of tubing can be configured to ensure a predetermined distribution of polymer solids between said downflow legs, such as, for example, a substantially equal distribution or even an unequal distribution if the design so warrants.

[0021] Desirably, the reactor is designed to provide high velocity of motion and a very intensive and well-defined mixing pattern of the reactor constituents. The reactor may be totally or partially jacketed with cooling water in order to remove heat generated by polymer polymerization.

[0022] One aspect of the present invention is directed to a loop polymerization reactor comprising upflow and downflow legs fluidly connected at the top and bottom thereof to form a continuous loop, in which the number of downflow legs exceeds the number of upflow legs. Embodiments of the invention include a series of such reactors comprising a first and a second loop reactor (or first or second "loop") allowing flow of polyolefin, catalyst, and monomer (and optionally a diluent) therebetween, wherein each loop comprises at least one upflow leg in fluid communication with a number of downflow legs exceeding the number of upflow legs. It has been discovered that increasing the number of downflow legs relative to the number of upflow legs is advantageous in increasing production capacity by increasing the capacity for removal of the heat of polymerization and reactor volume of such loop reactors, relative to conventional loop reactors having an equal number of upflow and downflow sections or an equal overall loop flow path length, even without increasing the overall number of loops. Even more advantageously, existing loop reactor systems can be readily retrofitted to this new design, thus reducing the need to increase the loop reactor pump capacity or to build entirely new reactor systems.

[0023] It will be understood by those of ordinary skill in the art that "upflow legs" means legs that allow the contents to flow in a generally upward manner (e.g., vertically upward or at an upward angle). Similarly, "downflow" means legs that allow the contents to flow in a generally downward manner (e.g., vertically downward or at a downward angle).

[0024] Accordingly, where the number of upflow legs is a number "n", the number of downflow legs may equal at least a number "n+1". For example, in a loop reactor having two upflow legs, the reactor is configured to have at least three downflow legs, or could have four downflow legs, such that the number of downflow legs is equal to "2n"; or a reactor having three upflow legs could be configured to have four to six downflow legs. However, the number of downflow legs relative to the number of upflow legs is not limited to 2n, and could be even greater.

[0025] The individual diameters of the upflow and downflow legs can be configured to be the same or different, such as where the upflow leg diameters are greater than, equal to or less than the diameters of the downflow legs. Preferably, the diameter of the upflow legs is greater than the diameter of the downflow legs. In addition, the diameter of the downflow legs can vary from one another, if desired. For example, some conventional loop reactors use 24-inch diameter legs throughout; but according to the present invention the upflow legs could be configured to 24-inch diameters, while the downflow legs could be configured to have only 20 inch diameters.

[0026] Likewise, the present invention is not limited as to the position of the downflow leg(s), and they can be positioned to optimize the path of cooling water supply and return lines, or the like.

[0027] Additionally, a polymerization reactor system can comprise a plurality, such as two or more of these enhanced-capacity, continuous loop reactors in series.

[0028] An embodiment of such a conventional loop reactor is shown in Figure 1, having four legs: two upflow legs 10 and two downflow legs 20, associated with a pump "P". Each leg is fluidly connected to another leg at each end by bent piping as apparent in Figure 1, to allow flow of liquid and/or slurry and/or gaseous material therebetween, and those skilled in the art will understand that the reactor loop will include one or more inlet conduits for monomer, catalyst and optionally diluent or other reactant materials (not shown), and at least one polymer slurry withdrawal conduit (not shown).

[0029] In an embodiment according to the present invention, Figure 2, the loop reactor is provided with two additional downflow legs 20a, connected in parallel with downflow legs 20. [0030] More preferably, as shown in Figure 3, the loop reactor comprises three upflow legs 10, and more than three downflow legs 20 and 20a, such as four, five or even six total downflow legs, with the optional downflow legs being indicted by the dashed lines.

[0031] In various embodiments, the heat of reaction can be removed by use of liquid flow, preferably water, through jackets surrounding the legs. In one embodiment, each jacket is independently supplied water, and in another embodiment, water flows through each jacket in series. The circulating slurry or liquid in the loop is kept at flow rates, temperatures and pressures sufficient to effect polymerization of the monomer(s) within the reactor, preferably at pressures from about 1 psig to about 1500 psig (0.0068 MPa to 10.3 MPa), more preferably between about 250 psig and about 1000 psig (1.72 MPa to 6.89 MPa), more preferably between about 500 psig and about 600 psig (3.44 MPa to 4.14 MPa) and temperatures of less than 130°C, or from 50°C to 125°C, or from 75°C to 115°C for producing polyethylene, or between about 400 psig to about 800 psig (2.76 MPa to 5.52 MPa), more preferably between about 450 psig and about 550 psig (3.45 MPa to 3.79 MPa) at temperatures of from 50°C to 90°C, more preferably between 60°C and 75°C for producing polypropylene. The flow rates include from about 20,000 gpm to about 40,000 gpm (4500 m3/hr to 9000 m ³ /hr), such as from about 28,000 gpm to about 33,000 gpm. In another embodiment, the circulating slurry or liquid in each loop is kept at a temperature below that which would cause the resulting polymer to dissolve in the monomer and/or diluent media.

[0032] The improved design and process described herein allows a series of loop reactors of the present invention the capacity to produce in excess of about 650 kTons per year, even in excess of about 800 kTons per year, and up to about 1000 kTons of polyolefins a year.

[0033] In any embodiments described herein, there may also be included in the fluid stream a diluent such as propane, isobutane or other hydrocarbon.

[0034] The type of polyolefin that can be made using the methods and apparatus of the invention are not limited. Preferably, an ethylene or propylene homopolymer is produced by the method of the invention. In another embodiment, an ethylene or propylene copolymer is produced wherein the ethylene or propylene derived units comprise at least 50 wt% of the polymer, more preferably at least 60 wt%, and most preferably at least 70 wt%. In one embodiment, the polyolefin (either homopolymer or copolymer) is bimodal in molecular weight. By "bimodal", what is meant is that two distinct peaks in a GPC fractionation of the polymer can be detected that indicates two distinct molecular weight polyolefin polymers being present and intimately blended into one polyolefin composition. In another embodiment, loop reactors that are connected in series are operated at optionally different conditions and optionally different monomer compositions and optionally different hydrogen concentrations to produce distinctly different products sequentially in the two reactors.

[0035] The polyolefin loop reactor(s) of the present invention can be used in series with any other type of reactor. For example, a fluidized bed gas phase reactor can be made part of a series reactor system in line with one or more of the present reactors, wherein polymer flow exiting a loop reactor is directed into a gas phase reactor. Other reactors can also be used such as stirred bed reactors, solution reactors and any other type of reactor suitable for producing polyolefins such as polyethylenes and polypropylenes. Such reactor designs are particularly suitable for making impact copolymers, for example, which is essentially a blend of a polypropylene and an ethylene-propylene rubber.

[0036] Embodiments of the present invention change the traditional reactor design to increase the number of downflow legs in a conventional four leg reactor (Fig. 1) from two to three or four (Fig. 2) and acts to increase the total heat transfer area by 50% allowing for an increase in plant capacity to a nominal 650 kTons per year with a traditional four leg reactor pump. Similarly, embodiments of the present invention change the traditional reactor design to increase the number of downflow legs in a conventional six leg reactor from three to four, five or six (Fig. 2) and acts to increase the total heat transfer area by up to 50%, allowing for an increase in plant capacity to a nominal 950 kTons per year with a traditional six leg reactor pump.

[0037] Accordingly, the various embodiments described herein may be combined with aspects of the invention. In one embodiment is provided:

1. A loop polymerization reactor comprising up flow and downflow legs that form a continuous loop, in which the number of downflow legs exceeds the number of up flow legs.

2. The loop polymerization reactor of 1, wherein the upflow legs and downflow legs are fluidly connected.

3. The loop polymerization reactor of 1, wherein the number of upflow legs equals n and the number of downflow legs is greater than or equal to n+1, or even 2n, or even more than 2n.

4. The loop polymerization reactor of any of 1-3, further comprising one or more inlet conduits for injecting monomer and catalyst into the loop, and at least one polymer slurry withdrawal conduit. 5. The loop polymerization reactor of any of 1-4, wherein the downflow leg(s) in excess of the upflow legs are fluidly connected to an adjacent downflow leg.

6. The loop polymerization reactor of any of 1-5, wherein the diameter of the upflow legs is greater than or equal to the diameter of the downflow legs.

7. The loop polymerization reactor of any of 1 -6, wherein the excess downflow leg is fluidly connected with an adjacent downflow leg with a Y-shaped section of tubing configured to ensure a predetermined distribution of polymer solids between said downflow legs, preferably a substantially equal distribution.

8. The loop polymerization reactor of any of 1-7, wherein the reactor has two upflow legs and three or four downflow legs, or three upflow legs and four to six downflow legs.

9. The loop polymerization reactor of any of 1-8, wherein one or more of the upflow legs and downflow legs and water flows through the jacket to control the reactor temperature by removing heat from the reactor.

10. The loop polymerization reactor of any of 1 -9.

1 1. A polymerization reactor system comprising a plurality of loop polymerization reactors according to any of 1- 10.

[0038] In another embodiment, the invention is directed to:

1. A process for increasing the capacity of a loop polymerization reactor having upflow and downflow legs that form a continuous loop, comprising configuring said loop to have a number of downflow legs exceeding the number of upflow legs.

2. The process of 1, wherein the excess downflow leg(s) increases the heat transfer capacity of the loop.

3. The process of 1 or 2, wherein the number of upflow legs equals n and the number of downflow legs is greater than or equal to n+1, or even 2n, or even more than 2n, such as where the reactor has two upflow legs and three or four downflow legs, or where the reactor has three upflow legs and four to six downflow legs.

4. The process of any of 1 -3, wherein the excess downflow leg is fluidly connected with an adjacent downflow leg with a Y-shaped section of tubing configured to ensure a predetermined distribution of polymer solids between said downflow legs, preferably a substantially equivalent distribution.

[0039] In yet another embodiment, the invention is directed to:

1. A process for making polyolefins, comprising: injecting at least one olefin monomer, a catalyst or polymerization initiator and optionally a diluent into at least one continuous loop reactor having upflow and downflow legs fluidly connected at the top and bottom thereof to form a continuous loop, in which the number of downflow legs exceeds the number of upflow legs, wherein at least one of the downflow legs is jacketed and water flows through the jacket to control the reactor temperature by removing heat from the reactor;

circulating said monomer, catalyst or polymerization initiator and optionally said diluent through said reactor loop;

reacting said olefin monomer under flow, temperature and pressure conditions adequate to polymerize said monomer and to form said polyolefin; and

recovering said polyolefin from said continuous loop reactor.

2. The process of 1, wherein said polyolefin is polyethylene or polypropylene homopolymer.

3. The process of 1 or 2, wherein the upflow and downflow legs are fluidly connected at the top and bottom thereof.

[0040] In another embodiment, the invention is directed to a loop polymerization reactor comprising upflow and downflow legs fluidly connected at the top and bottom thereof to form a continuous loop, in which the number of upflow legs exceeds the number of downflow legs.

[0041] In another embodiment, the invention is directed to a loop polymerization reactor comprising at least one upflow leg and at least one downflow leg forming a continuous loop, wherein the at least one downflow leg is fluidly connected with a second (e.g., adjacent) downflow leg with a Y-shaped section of tubing configured to ensure a predetermined distribution of polymer slurry between said downflow legs.

[0042] While aspects of the invention have been described herein, it will be apparent to one skilled in the art that the various embodiments can be combined or equivalent materials and means can be substituted for those described herein.