Polyolefin discharge process and apparatus

Abstract

A polyolefin discharge cycle is disclosed that includes discharging an amount of polyolefin particles from a reactor to a product chamber, wherein the reactor is operated at a pressure of at least 20 psia; transferring the particles from the product chamber to a product discharge drum; displacing gas from the product discharge drum into the product chamber through a balance line as the particles are transferred from the product chamber to the product discharge drum; removing the particles from the product discharge drum; and repeating the cycle of discharging, transferring, displacing, and removing the particles; wherein the particles are transferred at a rate of at least 0.01 tons of particles per second; and / or wherein the balance line is purged with gas before or after at least one cycle; and / or wherein the product chamber includes a content detector, such as a nuclear density detector.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 115,965, filed November 19, 2020, entitled “Polyolefin Discharge Process and Apparatus,” the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0003] An apparatus and process for discharging polyolefin particles produced within a gas phase reactor is disclosed. BACKGROUND

[0004] Polyolefins can be produced using a gas phase polymerization process. If the process is a gas phase fluidized bed polymerization process, the process can include a gas stream containing one or more monomers that continuously passes through a fluidized bed of catalyst and grows polymer particles. As polymerization occurs, a portion of the monomers is consumed and the gas stream is heated in the reactor by the heat of polymerization. A portion of the gas stream exits the reactor and can be recycled back to the reactor along with additional monomers and additives. At certain time intervals in the process, as polyolefin particles are formed in the reactor, they must be removed or discharged to maintain a workable bed level and to obtain the desired commercial product. This is preferably done in a batch mode, where a batch of particles is discharged at one time. Since typical gas phase reactors are operated under pressurized conditions (e.g., 20, 50, or 100 psig or higher), the process of discharging the particles must be carried out by transferring the particles to a low pressure environment for processing into the commercial product. This is a cyclic process involving multiple steps, some of which can create bottlenecks in the discharge process.

[0005] In addition, while the particles are preferably solid and thus have high flowability, they can have tacky and / or very fine properties and thus tend to stick to the inner walls of various apparatus components and cause operational malfunctions.

[0006] There is a need for a method and apparatus that maximizes gas phase polyolefin productivity by reducing the total product discharge cycle time that allows for more blowdown (circulation) per hour. In particular, a method and apparatus for reducing the cycle time required to transfer particulate polyolefin from the product chamber (high pressure) to the product blowdown drum (low pressure) is desirable.

[0007] Noted references include US 10,035,864; US 8,835,576; US 7,891,527; US 4,543,399; US 4,588,790; US 5,028,670; US 5,317,036; US 5,352,749; US 5,405,922; US 5,436,304; US 5,453,471; US 5,462,999; US 5,616,661; and US 5,668,228. SUMMARY

[0008] A polyolefin discharge cycle is disclosed that includes (or consists of, or consists essentially of): discharging an amount of polyolefin particles from a reactor to a product chamber, wherein the reactor is operating at a pressure of at least 20 psia; transferring the particles from the product chamber to a product dump tank, wherein the particles are transferred at a rate of at least 0.01 tons of particles per second; displacing gas from the product dump tank into the product chamber through a balance line as the particles are transferred from the product chamber to the product dump tank; removing the particles from the product dump tank; and repeating the cycle of discharging, transferring, displacing, and removing the particles. In some embodiments, the particles can be transferred from the product chamber to the product dump tank through a discharge valve having a diameter of 14 inches to 20 inches.

[0009] Further, in certain embodiments, discharging from the reactor to the product chamber can include opening a gas return / recycle line that fluidly connects the product chamber and the reactor, such that the pressures in the reactor and the product chamber equilibrate during the discharge. Further, the gas return / recycle line can include two valves, such that gas at the pressure of the reactor can be trapped within the return / recycle line by closing both valves. In this way, reactor gas loss to downstream operations of the product dump tank (and / or processes downstream of the product dump tank) can be minimized. BRIEF DESCRIPTION OF DRAWINGS

[0010] The figure is a pictorial and flow diagram of an embodiment of the main components of the product discharge apparatus of the invention and the processes associated therewith. DETAILED DESCRIPTION

[0011] The inventors have discovered that the productivity of gas phase polyolefins can be maximized by reducing the total product discharge cycle time that allows for more drops (cycles) per hour. This can be accomplished by increasing the rate at which polyolefin particles are transferred or "dropped" between the high pressure vessel and the low pressure vessel, and by ensuring that the product discharge system is not fouled by polyolefin particles or fines by detecting any residual or carryover particles in the high pressure vessel (e.g., product chamber) and ensuring that any polyolefin fines or particles are purged out of the pressure equalization line leading from the low pressure vessel (e.g., product drop tank) to the high pressure vessel. These objectives are accomplished by any combination of the process and equipment features described herein.

[0012] It was discovered that increasing the size of the valve between the product chamber and the product drop tank ("discharge valve") to at least 14 or 16 inches or larger can reduce the time required to transfer particulate polyolefin (especially polyethylene) from the product chamber to the product drop tank. Polyolefin particles produced in a gas phase reactor can be removed through the product discharge system in discrete batches via a series of automated ball valves and pressure vessels. The productivity capacity is defined by the size of each batch (referred to as "drop" or "drop size"), as well as the number of batches that can be removed in a period of time (referred to as "drops per hour"). The total cycle time is the sum of multiple process steps, one of which is the transfer of particles from the product chamber to the product drop tank. By reducing the time required for this step, the total cycle time is reduced. The reduction in the total cycle time required for a drop translates into an increase in the number of drops per hour, and thus an increase in productivity capacity. The benefit of increasing the discharge valve size is surprisingly large, with "drops per hour" increasing by as much as 15-40% while the increase in system cost is relatively small.

[0013] Another problem addressed by the present invention is potential plugging of the line and / or clogging of the gas flow. As the polyolefin particles exit the reactor into the product chamber, they are still actively growing and exposed to the reaction environment. The particles can stagnate in the product chamber and if left there for too long under reaction conditions, they can generate enough heat from the exothermic reaction to melt the polymer and fuse into agglomerates. The cycle setup of the product discharge system is designed to prevent this from happening. However, the use of higher productivity catalysts, such as metallocenes and certain other catalysts, increases the risk. If the particles melt and form agglomerates, the transfer from the product chamber to the product dump drum does not occur, which can lead to the accumulation of polyolefin in the product chamber and eventually a shutdown of the entire product discharge system due to plugging. It is therefore desirable to know if all the polyethylene particles are flowing from the product chamber to the product dump drum to empty the product chamber. This is accomplished by installing a level detector, such as a nuclear radiation level gauge, on the bottom cone / nozzle of the product chamber. The radiation signal is emitted on one side of the cone and detected on the other side. When the cone is empty, this signal gets stronger. When the cone is full of particles or agglomerates, the signal will get weaker, indicating that the product chamber is not completely empty. This is used to alert the operator to a potential problem in the product chamber and to address it before it becomes more difficult to manage.

[0014] In this transfer from the product chamber to the product dump drum, the polyolefin particles flow due to the pressure difference between the vessels in addition to gravity. These two vessels represent a single closed system. The polyolefin particles displace the gas in the product dump drum upwards into the product chamber, volumetrically. A pipe or equalization line connects the two vessels to facilitate this gas flow, i.e. to equalize the pressure in the two vessels. This equalization line has a valve, preferably an automatic ball valve, to control the gas flow. If this pipe gets plugged with polyolefin, either as fine powder or as agglomerates, the pressure equalization will slow down and result in a slower transfer of the particulate polymer and thus a reduced productivity and possibly the formation of agglomerates in the product chamber. To prevent this plugging of the equalization line, the line can be purged periodically to sweep away any fine particles. In any embodiment, the purging comprises opening the valve in the equalization line without polyolefin in the product chamber or product dump drum, thus allowing gas to flow from the high pressure vessel into the low pressure vessel. This is done between blowdowns, thus having no impact on the cycle time and can be done manually or automatically. It is preferred to be done automatically as part of the product discharge system logic.

[0015] As provided herein, after start-up and upon reaching steady state, the reactor can be operated to carry out polymerization using any of a variety of different processes, including solution, slurry, or gas phase processes, but most preferably gas phase processes. For example, the polymerization reactor can be a fluidized bed reactor operated to produce a polyolefin polymer by a gas phase polymerization process. Further, the polymerization reactor can be a segmented reactor in which two or more reactors are used in series, where the first reactor can produce, for example, a high molecular weight component, and the second reactor can produce a low molecular weight component. In operation, the polymerization medium can be fluidized by a continuous flow of gaseous monomer and diluent or mechanical agitation.

[0016] More specifically, in a continuous gas phase fluidized bed reactor, the polymerization reactor includes a fluidized bed of dense phase material. At start-up, a seed bed containing polymer particles is charged into the polymerization reactor. Liquid or gaseous feed streams of the primary monomer and hydrogen are combined with liquid or gaseous comonomer and then introduced into the fluidized bed, typically via an upstream recycle gas line. Fluidized bed reactors used to carry out continuous gas phase processes typically include a reaction zone and a so-called velocity reduction zone. The reaction zone includes a bed of growing polymer particles, formed polymer particles, and a small amount of catalyst particles (sometimes collectively referred to herein as “dense phase material”) that is fluidized by a continuous flow of gaseous monomer and / or comonomer and diluent to remove polymerization heat throughout the reaction zone. Optionally, the recycle gas (recycled gas) can be cooled and compressed to form a liquid that, upon re-entry into the reaction zone, increases the heat removal capacity of the recycle gas stream. This method of operation is referred to as “condensed mode”.

[0017] The rate of the appropriate gas stream into the fluidized bed reactor can be readily determined. The flow rate of monomer and recycle gas into the polymerization reactor is approximately equal to the rate at which polymer product and unreacted monomer are withdrawn. In any embodiment, the recycle gas circulation rate (gas entering and leaving the bed) is in the range from 1000 tons / hour to 2500 tons / hour, such as 1500 tons / hour to 2200 tons / hour. In various embodiments, the reactor productivity can be in the range from 20, or 40, or 50 tons / hour to 60, or 80, or 100 tons / hour. The composition of the gas passing through the reactor (and thus through the bed) can be adjusted to maintain a steady state gas composition within the bed or “reaction zone”. The gas leaving the reaction zone is passed to a velocity reduction zone where entrained particles settle back into the dense phase zone. The gas is compressed in a compressor and passed through a heat exchanger where polymerization heat is removed, and the gas is returned to the reaction zone.

[0018] To maintain a constant reactor temperature, the temperature of the circulating gas can be adjusted continuously upward or downward to accommodate any changes in the rate of heat production due to polymerization. The fluidized bed can be maintained at a constant height by withdrawing a portion of the fluidized bed at a rate equal to the rate of particulate product formation. The polymer product can be moved into a fixed volume chamber semi-continuously via a series of valves while simultaneously being discharged back into the reactor to effectively remove the product. At the same time, a significant portion of the unreacted gas is recirculated into the reactor. The polymer product is purged to remove entrained hydrocarbons and can be treated with a small amount of humidified nitrogen steam to deactivate any trace amounts of residual catalyst.

[0019] Further, the reactor temperature range of the fluidized bed reactor can be from 30 °C, or 40 °C, or 50 °C to 85 °C, or 90 °C, or 95 °C, or 100 °C, or 120 °C, or 150 °C. Generally, the reactor temperature is run at the highest temperature feasible, taking into account the sintering temperature of the polymer product within the reactor. The polymerization temperature or reaction temperature must generally be below the melting or “sintering” temperature of the polymer to be formed. Thus, in one aspect, the upper temperature limit is the melting temperature of the polyolefin produced in the reactor.

[0020] As described herein, the reactor used in connection with the methods of the present application can be operated to produce homopolymers of an olefin (e.g., ethylene or propylene), and / or copolymers, terpolymers, etc. of an olefin, particularly ethylene, with at least one other olefin. For example, the polymerization reactor can produce polyethylene. Such polyethylene can be homopolymers of ethylene and interpolymers of ethylene with at least one a-olefin, where the ethylene content is at least 50%, or 60%, or 70%, or 80%, or 90%, or 95% by weight of the total monomers involved. Exemplary olefins that can be used in the reactor are ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 4-methylpent-1-ene, 1-decene, 1-dodecene, 1-hexadecene, and the like. Also useful herein are multiolefins such as 1,3-hexadiene, 1,4-hexadiene, cyclopentadiene, dicyclopentadiene, 4-vinylcyclohex-1-ene, 1,5-cyclooctadiene, 5-vinylidene-2-norbornene, and 5-vinyl-2-norbornene, and olefins formed in situ in the polymerization medium. When olefins are formed in situ in the polymerization medium, polyolefins containing long chain branching can be formed.

[0021] In the production of polyethylene or polypropylene, a comonomer can be present in the polymerization reactor. When present, the comonomer can be present at any level with the ethylene or propylene monomers to achieve the desired weight percent incorporation of the comonomer into the finished particulate.

[0022] Additionally, hydrogen gas is often used in olefin polymerization to control the final properties of the polyolefin. For certain types of catalyst systems, it is known that increasing the hydrogen gas concentration (partial pressure) increases the melt flow (MF) and / or melt index (MI) of the polyolefin produced. Thus, the MF or MI can be affected by the hydrogen gas concentration. The amount of hydrogen gas in the polymerization can be expressed as a molar ratio relative to the total polymerizable monomer (e.g., ethylene or a blend of ethylene with hexene or propylene). The amount of hydrogen gas used in some polymerization processes is the amount necessary to achieve the desired MF or MI of the final polyolefin particles.

[0023] Thus, in any embodiment is a polyolefin discharge cycle comprising discharging an amount of polyolefin particles from a reactor to a product chamber, wherein the reactor is operated at a pressure of at least 20, or 30, or 50, or 100, or 150, or 200 psia; transferring the particles from the product chamber to a product discharge drum, wherein the particles are transferred at a rate of at least 0.01, or 0.05 tons of particles per second, or in a range from 0.01 to 0.1, or 0.2, or 0.6, or 0.8 tons of particles per second (where a ton is 1000 kg); displacing gas from the product discharge drum into the product chamber through a balance line as the particles are transferred from the product chamber to the product discharge drum; removing the particles from the product discharge drum; and repeating the cycle of discharging, transferring, displacing, and removing particles.

[0024] Also in any embodiment is a polyolefin discharge cycle comprising discharging an amount of polyolefin particles from a reactor to a product chamber, wherein the reactor is operated at a pressure of at least 20, or 30, or 50, or 100, or 150, or 200 psia; transferring the particles from the product chamber to a product discharge drum; displacing gas from the product discharge drum into the product chamber through a balance line as the particles are transferred from the product chamber to the product discharge drum; removing the particles from the product discharge drum; and repeating the cycle of discharging, transferring, displacing, and removing particles; wherein the balance line is purged with gas before or after at least one cycle. Preferably, wherein the balance line is purged by allowing gas from the product chamber or product discharge drum to flow from a high pressure vessel into a lower pressure vessel.

[0025] Also in any embodiment is a polyolefin discharge cycle comprising discharging an amount of polyolefin particles from a reactor to a product chamber, wherein the reactor is operated at a pressure of at least 20, or 30, or 50, or 100, or 150, or 200 psia; transferring the particles from the product chamber to a product discharge drum; displacing gas from the product discharge drum into the product chamber through a balance line as the particles are transferred from the product chamber to the product discharge drum; removing the particles from the product discharge drum; and repeating the cycle of discharging, transferring, displacing, and removing particles; wherein the product chamber comprises a content detector, preferably a nuclear density detector.

[0026] As used herein, a "granule" is a flowable solid, preferably having a bulk density in the range from 300 or 350 kg / m 3 to 400 or 500 or 600 kg / m 3 Preferably, the granules comprise, or consist essentially of, or consist of, one or more polyolefins, most preferably polyethylene or polypropylene.

[0027] In any embodiment, the balance line is purged with a gas before or after at least one cycle. The gas can be the recycle gas consisting of monomer, diluent, and inert gas such as nitrogen, or any combination of these. In a typical "recycle", polyolefin granules enter the product discharge bucket containing gas from the product chamber, and the gas is displaced by the polyolefin granules and flows upward from the product discharge bucket to the product chamber. "Purging" is an additional step. Purging can be done intermittently or periodically. Typically, purging is upflowing (counter-current to the normal cycle of polyolefin granules) because the pressure in the product chamber is typically higher than the pressure in the product discharge bucket, preferably at least 50, or 80, or 100 psig or more, in some portion of the discharge cycle. In any embodiment, the discharge, transfer, and displacement steps described herein are sequential events that form a cycle.

[0028] In any embodiment, the balance line is purged by allowing gas from the product chamber or product discharge bucket to flow from the higher pressure vessel into the lower pressure vessel. Preferably, the balance line is purged by allowing gas to flow from the product discharge bucket to the product chamber, wherein the pressure in the lower pressure vessel (preferably the product discharge bucket) cycles in the range from 10 or 20 psig to 50 or 100 psig; and wherein the pressure in the higher pressure vessel (preferably the product chamber) cycles in the range from 200 or 250 psig to 350 or 400 psig. In any embodiment, the gas displaced from the product discharge bucket to the product chamber through the balance line is at a gas pressure in the range from 10, or 20, or 30, or 40, or 50 psia to 100, or 200, or 300 or 400 psia.

[0029] In any embodiment, the purging of the balance line is manually controlled, or programmed for regular intervals between discharge cycles, or a combination of both.

[0030] In any embodiment, the product chamber includes a level detector. Preferably, the level detector is a nuclear density detector. One example of a nuclear detector is an Ohmart / VEGA TMA nuclear density gauge, which includes a nuclear source and a receiver. In any embodiment, the level detector can be placed on the outlet flange at the lower portion of the conical product chamber. The detector can be near the bottom of the flange but above the cone of the product chamber. In any embodiment, there can be a line from the bottom of the product chamber to the discharge valve where the level detector is installed. Alternative level detectors include, but are not limited to, a level gauge or an ultrasonic source and detector. In any embodiment, the polyolefin particles are transferred from the product chamber to the product discharge tank until the level detector finds that substantially no particles are present in the product chamber. By "substantially no particles" is meant no particles that can be detected by the level detector. In any embodiment, a timer is set to correspond to the time at which the valve should be opened to allow the product chamber to be completely and consistently drained; it is not necessary to control the closing of the valve on each bleed based on the low level detection achieved. In any embodiment, the low level detector is a check or a guide for the operator, rather than a determinant of opening or closing the discharge valve.

[0031] In any embodiment, the apparatus that allows one or more cycles, especially the line that fluidly connects the product chamber and the product discharge tank, further includes a discharge valve that allows the particles from the product chamber to be retained in the product chamber or transferred to the product discharge tank at a desired rate. The discharge valve is preferably sized to allow the desired flow rate of polyolefin particles from the product chamber to the product discharge tank per unit time, but in particular embodiments, the discharge valve has a diameter in the range from 14 inches to 16, or 18 or 20 inches, depending on the overall size of the product discharge system and other production factors.

[0032] In any embodiment, the particles are transferred to the product discharge tank under the influence of gravity and, optionally, under a gas pressure in the range from 50, or 60, or 80, or 100 psia to 200, or 300, or 400, or 500 psia. In any embodiment, the pressure in the reactor is maintained substantially throughout the cycle, meaning that the pressure does not vary by more than 10%, or 5%, or 2% of its average or steady-state production pressure.

[0033] In any embodiment, the methods described herein further comprise continuously combining one or more olefins with a polymerization catalyst in a reactor to form polyolefin particles. Preferably, the reactor is a gas phase reactor comprising a bed of polyolefin particles. Further, preferably, the combining is conducted at a pressure of at least 100, or 200, or 300, or 400, or 500 psig within the reactor. In any embodiment, the polymerization catalyst is selected from the group consisting of metallocene catalysts, Ziegler-Natta catalysts, chromium catalysts, atypical single-site catalysts (such as, for example, pyridyl diamine-transition metal catalysts, bis(2-pentylphenylamino)ethyl)amine-transition metal catalysts, Schiff base-transition metal catalysts, and the like), and combinations thereof. Finally, in any embodiment, the olefins are selected from ethylene and C3 to C10 alpha-olefins, most preferably one or more of the alpha-olefins are selected from ethylene, propylene, or combinations thereof. The preferred polyolefin particles produced by the polymerization process are polypropylene particles or polyethylene particles, which can be homopolymers of propylene-derived units or ethylene-derived units, or copolymers comprising one or more alpha-olefin units other than the main (propylene or ethylene) alpha-olefin in a range of from 0.1 to 10, or to 20 wt% by weight of the polymer. 10 one or more of the alpha-olefins are selected from ethylene, propylene, or combinations thereof. The preferred polyolefin particles produced by the polymerization process are polypropylene particles or polyethylene particles, which can be homopolymers of propylene-derived units or ethylene-derived units, or copolymers comprising one or more alpha-olefin units other than the main (propylene or ethylene) alpha-olefin in a range of from 0.1 to 10, or to 20 wt% by weight of the polymer.

[0034] Also disclosed herein is an apparatus suitable for use in connection with a product discharge from a reactor, preferably a polyolefin polymerization reactor. With reference to the drawings, in any embodiment is an apparatus 100 comprising (or consisting of, or consisting essentially of) a reactor 102 comprising polyolefin particles; a reactor fluidly connected to a product chamber 112, the product chamber comprising a content detector 118; a product offloading tank 114 fluidly connected 132 to the product chamber 112, wherein the fluid connection comprises a discharge valve 116; a balance line 120 fluidly connected from the product offloading tank 114 to the product chamber 112; and an exit line 122.

[0035] In any embodiment of the apparatus, the discharge valve is sized to allow polyolefin particles to flow therethrough at a rate of at least 0.01 tons of particles per second (as described above), or in a range of from 0.01 to 0.1, or 0.2, or 0.6, or 0.8 tons of particles per second. In particular embodiments, the discharge valve has a diameter in a range of from 14 inches to 16, or 18, or 20 inches.

[0036] In any embodiment, the product chamber has a top and a bottom, and wherein the product chamber has an inlet fluidly connected to the reactor at the top and an outlet fluidly connected to the product offloading tank at the bottom, and wherein the content detector is located in the bottom of the product chamber. Preferably, the content detector is a nuclear density detector.

[0037] Embodiments of the product discharge apparatus described herein are described in greater detail with reference to the accompanying drawings. The drawings are a generalized schematic of a polymerization apparatus 100, which includes a gas phase reactor 102 and fluidly connected thereto product discharge apparatus 104 and product discharge apparatus 106 for allowing the cyclic removal of polyolefin particles at regular intervals from the gas phase reactor 102 under pressure.

[0038] In any embodiment, the lines between 102, 112, and 114 in the figures represent fluid connections, such as hollow cylindrical steel pipes or other metal pipes, which allow fluid solids, gases, and / or liquids to flow therefrom from one point to another, preferably along their length. Arrows on the lines represent the general flow of material from one point to another, which helps to illustrate the cyclic process described herein. Valves are represented by two inverted and connected triangles; valves can stop or slow the flow of material differently depending on how a manual control or computerized system sets them.

[0039] The product discharge apparatus allows the removal of polyolefin particles from the gas phase reactor without loss of pressure or minimal loss of pressure, thereby transferring the polyolefin particles from a high pressure environment to a low pressure or atmospheric pressure environment for further finishing and processing steps. The polymerization apparatus herein is not limited to just two discharge apparatuses, but can include three, four, or more, as desired for the size of the gas phase reactor and the volume of polymerization apparatus being manufactured. For the purpose of describing the main components of the product discharge apparatus and process of the present invention, reference is made to the components of product discharge apparatus 104. Various valves are shown in the figures to highlight the cyclic nature of the process and the ability to control the flow of gases and polyolefin particles associated with the product discharge process and apparatus, and the dual pressure assemblies therein, but it should be understood that there can be any number of valves, or no valves, throughout the product discharge apparatus as described with respect to apparatus 104 and 106. That is, the following description of the product discharge process specifically points out some of the valves.

[0040] In any embodiment, polyolefin particles, such as polyethylene particles, are produced within the gas phase reactor 102 by continuously combining one or more olefins with a polymerization catalyst in the reactor to form polyolefin particles. Such particles are maintained in a fluidized state in the bed 110. The fluidized state of the polyolefin particle bed is maintained by flowing a circulating gas from beneath the bed, preferably from a distribution plate (not shown), upward through the bed to the plenum 108 to fluidize the polyolefin particles in the bed 110. At times, an amount of polyolefin particles is discharged from the reactor 102 in the form of a "spill" to the product chamber 112 through a particle discharge line 136, as previously described. A valve 136a in the product discharge line is opened to allow this spill to pass through the discharge line 136, and at the same time, a valve 134a in the gas return / recycle line 134 is opened so that gas flows from the product chamber 112 back to the top of the reactor 102 (the valve 134a can be referred to as a first gas return / recycle line valve). As shown in the figure, the valve 134a can be positioned along the gas return / recycle line 134 closer to the reactor 102 than to the product chamber 112. Some embodiments can also include at least a second valve 134b along the gas return / recycle line 134 closer to the product chamber 112 end of the line 134 (for example, the second valve 134b can be along the return / recycle line as shown in the figure, or it can be connected directly to the product chamber 112 at one end and to the line 134 at the other end). Where a second valve 134b is present, it is also opened to ensure pressure equalization between the product chamber 112 and the reactor 102. This second valve 134b can be referred to as a second gas return / recycle line valve, and according to some embodiments, the operation and function of this valve are described in more detail below.

[0041] In any case, after opening valve 134a and, if present, valve 134b, preferably the pressure within product chamber 112 is equal to the pressure within reactor 102 at the end of the blowdown. In preferred embodiments, the pressure is approximately equal (within 2% or 5% of each other) at the end of the blowdown to product chamber 112. Product chamber 112 includes an upper portion 124 having an inlet to allow polyolefin particles to flow therein and a lower portion 126 having an outlet to allow polyolefin particles to flow to the next stage, product discharge tank 114. To allow product to flow to product discharge tank 114, valve 134a (and 134b, if present) and 136a can be closed (e.g., upon completion of the desired blowdown from reactor 102 to product chamber 112) and valve 116 between product chamber 112 and product discharge tank 114 opened to allow product to flow from product chamber 112 to product discharge tank 114; valve 120a can also be opened to ensure that the pressure between product chamber 112 and product discharge tank 114 is equalized via equalization line 120, facilitating product transfer. Product chamber 112 also includes a level detector 118, preferably at or near lower portion 126 or at or near the outlet, or alternatively at line 132 fluidly connecting product chamber 112 to product discharge tank 114 and including discharge valve 116. Level detector 118 is ideally placed (taking into account the precise geometry of product chamber 112 and lower portion 126) so that it can detect whether and when polyolefin particles have been completely transferred from product chamber 112 to product discharge tank 114.

[0042] In some embodiments, line 132 can not be present; in such cases, the outlet flange of product chamber 112 can be directly attached to the inlet of valve 116, while the outlet flange of valve 116 is directly attached to the inlet flange of product discharge tank 114.

[0043] Advantageously, in the presence of second valve 134b, it can be seen that by closing both valves 134b and 134a for the transfer of product from product chamber 112 to product discharge tank 114, gas at a higher pressure (i.e., at reactor pressure) is trapped in gas recirculation / return line 134; in contrast to embodiments where only valve 134a is present (along gas recirculation / return line 134 proximate reactor 102), in the latter case the pressure in line 134 will decrease during downstream transfer of product. By keeping the gas in line 134, downstream gas loss (e.g., through product discharge tank 114 and further downstream with the product) can be minimized and gas recirculated to the reactor maximized, such that when valves 134a and 134b are next opened to allow gas to flow back to the reactor through gas return / recirculation line 134, all trapped gas is also recirculated back to the reactor.

[0044] As noted above, the maximum pressure in the product chamber 112 is approximately equal to (within 2% to 5% of each other) the reactor pressure at the end of the blowdown from the reactor 102 to the product chamber 112. The minimum pressure is in the product discharge tank 114 at the end of the blowdown transfer from the product discharge tank 114 to the exit line 122 and downstream purge bin, where the pressure is near zero psig (atmospheric pressure). Discharging the polyolefin particles from the product chamber 112 to the product discharge tank 114 requires allowing the pressure to drop, for example, from a pressure in the product chamber 112 in the range from 200 psi to 300 psi to a pressure in the range from 10 to 50 psi. To facilitate this transfer, the polyolefin particles are passed through the discharge valve 116, which is sized to allow the particles to be transferred at a rate of at least 0.01 tons of particles per second, or in the range from 0.01 to 0.1, or 0.2, or 0.6, or 0.8 tons of particles per second (one ton is 2000 kg). In any embodiment, this can mean that the diameter of the discharge valve is in the range from 14 inches (35.5 cm) to 16 (40.6) or 18 (45.7) or 20 inches (51 cm).

[0045] In any embodiment, the discharge valve 116 is a full-port valve, so in effect the valve has an inner diameter equal to the inner diameter of the pipe / flange (and is nominally 14 inches, 16 inches, etc.).

[0046] Still referring to the figure, the aforementioned balance line 120 associated with the transfer of product from the product chamber 112 to the product discharge tank 114 is fluidly connected from the product discharge tank 114 (preferably at the upper portion 128) back to the gas return / recycle line 134 that connects the product chamber 112 to the reactor 102. Preferably, the connection is below or "upstream" of a valve (e.g., valve 134a); and, when present, also upstream of a second valve 134b, as shown in the figure. Thus, when the valve 134a is closed (and the valve 134b, if present, is closed), the balance line 120 can be used to balance the pressure between the product chamber 112 and the product discharge tank 114, as previously described. In addition, when the valve 134a (and 134b, if present) is open, the balance line 120 allows pressure to be released back into the reactor 102 (along with the gas trapped in line 134 returning to the reactor, if both valves 134a and 134b are present as discussed above). By returning the gas from the product discharge tank 114 ultimately to the reactor 102, the balance line 120 also allows for a pressure drop in the product discharge tank 114. At times, it can be desirable to purge any polyolefin particles or fines in the balance line 120 that can have accumulated and thus can be obstructing the flow of gas from the product discharge tank 114 to the gas return / recycle line 134. In that case, the balance line is purged with a gas (such as nitrogen) prior to or after at least one of the cycles described herein. Note also that while the balance line 120 is shown connected to the gas return / recycle line 134, in other embodiments the balance line 120 can instead be connected directly to the upper portion 124 of the product chamber 112 so that gas can flow (and pressure balance) between the product discharge tank 114 and the product chamber 112; and the same gas flow from the product discharge tank 114 ultimately back to the gas phase reactor 102 can also be enabled if the gas return / recycle line 134 from the top of the product chamber 112 is also opened. In either case, the balance line 120 alternately allows pressure to be released back into the reactor 102 (and a concomitant pressure drop in the product discharge tank 114); and / or to balance the pressure between the product discharge tank 114 and the product chamber 112.

[0047] In any embodiment, the gas used for purging is reactor gas (a mixture of all components in the system, such as ethylene, comonomer (if used), hydrogen, diluent or condensing agent, nitrogen, etc.) that is present in the product chamber when there are no particles in the product chamber and it is waiting to be drained. In the normal cycle mode, the product chamber 112 in standby mode is at about half the reactor operating pressure. Nitrogen is used as the purging gas alone only when the operator decides to start the purging or unplugging step with nitrogen (or other gas mixture) only.

[0048] In any embodiment, the polyolefin particles within the product discharge bucket 114 (which have been pressure balanced to atmospheric pressure or a pressure close to atmospheric pressure) can then be sent by gravity and / or additional gas pressure from the product discharge bucket 114 having a lower portion 130 to an exit line 122, where the polyolefin particles can then be further processed, such as by purging excess monomer and other gases and mixing with additives, such as antioxidants, and melt mixing into pellets for shipping and sale. When the blowdown pressure from the product chamber 112 to the product discharge bucket 114 has been balanced (post-balance), the product discharge bucket 114 will be at approximately 20% to 25% or 30% of the reactor pressure. This pressure is the driving force to transport the particles downstream to the exit line 122 and downstream to the purge bin. At the end of the blowdown transfer from the product discharge bucket 114 to the downstream purge bin, the product discharge bucket pressure is close to atmospheric pressure.

[0049] Still referring to this figure, the product discharge system 104 can also include a line 138 to remove excess monomer, which can be recycled through line 140 or burned. Such excess monomer or other gases can come from, for example, gases that are circulated upward from the product chamber 112 through the gas return / recycle line 134.

[0050] All publications, patents, and patent applications mentioned in this specification are indicative of the level of skill of those skilled in the art to which this application pertains, and are hereby incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference.

[0051] As used herein, the phrase "consisting essentially of means that minor equipment features, such as valves, heaters, coolers, and pumps, can be present which facilitate the operation of the claimed equipment or cycle, but are not essential to the operation of such equipment or cycle. Likewise, when it is associated with a method claim, "consisting essentially of does not exclude minor features, such as valve operations, heating / cooling, and pumping of gases, liquids, and / or solids, which are not essential to the claimed cycle of transferring solid particulate polymer from a high pressure environment to a low pressure environment and / or maintaining clarity and flowability within a fluid connection.

[0052] Having thus described the application, it will be obvious that the same can be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the application, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. A polyolefin emission recycling process, comprising: Discharging a certain amount of polyolefin particles from a reactor to a product chamber includes opening a gas return / recirculation line fluidly connecting the product chamber and the reactor, such that the pressure in the reactor and the product chamber is balanced during the discharge step; wherein the reactor operates at a pressure of at least 20 psia and the product chamber includes a content detector. The particles are transferred from the product chamber to the product discharge hopper, wherein the particles are transferred at a rate of at least 0.01 tons of particles per second. As the particles are transferred from the product chamber to the product discharge hopper, gas is displaced from the product discharge hopper into the product chamber through a balancing line; Remove the particles from the product discharge bucket; and The process cycle of emission, transfer, displacement, and removal of particles is repeated in such a way that the emission, transfer, and displacement steps are sequential events that form a cycle. The discharge process cycle further includes a discharge valve that allows particles from the product chamber to be retained in the product chamber or transferred at a desired rate to the product discharge hopper, wherein the diameter of the discharge valve is in the range of 14 inches to 20 inches. Furthermore, the gas return / recirculation line includes a first gas return / recirculation line valve and a second gas return / recirculation line valve. as well as Furthermore, transferring the particles from the product chamber to the product discharge hopper includes closing the first and second gas return / recirculation line valves, thereby trapping gas in the gas return / recirculation line between the first and second gas return / recirculation line valves. The balancing line is purged with gas before or after at least one cycle.

2. The process cycle as described in claim 1, wherein, The particles are transferred to the product discharge hopper under the influence of gravity and optionally under a gas pressure ranging from 50 psia to 500 psia.

3. The process cycle as described in claim 1, wherein, The gas displaced from the product discharge tank to the product chamber through the balancing pipeline is at a gas pressure ranging from 10 psia to 400 psia.

4. The process cycle as described in claim 1, wherein, The pressure in the reactor is maintained substantially throughout the cycle.

5. The process cycle of claim 1, further comprising continuously combining one or more olefins with a polymerization catalyst in the reactor to form polyolefin particles.

6. The process cycle as described in claim 5, wherein, The reactor is a gas-phase reactor containing a bed of polyolefin particles.

7. The process cycle as described in claim 5 or claim 6, wherein, The combination is carried out inside the reactor at a pressure of at least 100 psig.

8. The process cycle as described in claim 5 or claim 6, wherein, The polymerization catalyst is selected from the group consisting of: metallocene catalysts, Ziegler-Natta catalysts, chromium catalysts, atypical single-point catalysts and combinations thereof.

9. The process cycle as described in claim 5 or claim 6, wherein, The olefin is selected from one or more of ethylene and C3 to C10 α-olefins.

10. An apparatus for performing the process cycle according to any one of claims 1-9, comprising: Reactors containing polyolefin particles; The reactor is fluidly connected to a product chamber, which includes a content detector; A gas return / recirculation line fluidly connects the product chamber and the reactor, and further, the gas return / recirculation line includes a first gas return / recirculation line valve and a second gas return / recirculation line valve; A fluid connection is made to the product discharge tank of the product chamber, wherein the fluid connection includes a discharge valve sized to allow polyolefin particles to flow through it at a rate of at least 0.01 tons of particles per second and wherein the diameter of the discharge valve is in the range of 14 inches to 20 inches. The balancing pipeline connecting the product discharge tank to the product chamber; and Leave the pipeline.

11. The device as claimed in claim 10, wherein, The product chamber has a top and a bottom, wherein the product chamber has an inlet at the top that is fluidly connected to the reactor and an outlet at the bottom that is fluidly connected to the product discharge tank.

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