CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Patent Application 63/115,965 filed 19 November 2020 entitled POLYOLEFIN DISCHARGE PROCESS AND APPARATUS, the entirety of which is incorporated by reference herein.

FIELD OF THE INVENTION

[0002] Disclosed is an apparatus and process for discharging polyolefin granules generated within a gas phase reactor.

BACKGROUND

[0003] Polyolefins may be produced using gas phase polymerization processes. If the process is a gas-phase fluidized bed polymerization process, the process may include a gas stream including one or more monomers continuously passed through a fluidized bed of catalyst and growing polymer particles. As polymerization occurs, aj portion of the monomers are 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 may be recycled back to the reactor with additional monomers and additives. At certain intervals in the process, as granules of polyolefin are formed in the reactor, they must be removed or discharged in order to maintain a workable bed level as well as to obtain the desired commercial product. This is preferably accomplished in a cyclic fashion wherein batches of granules are discharged at once. Since a typical gas phase reactor operates under pressurized conditions, such as 20, 50, or 100 psig or more, the process to discharge the granules must be performed by transferring the granules to a lower-pressure environment for processing into a commercial product. This is a cyclic process that involves several steps, some of which can create bottlenecks in the discharge process.

[0004] Further, the granules, while preferably solid and thus highly flowable, may be of a nature that they become sticky and/or very fine and thus have a tendency to adhere to the inner walls of the various apparatus components and cause operational upsets.

[0005] What is needed is a method and apparatus to maximize gas phase polyolefin production rate by reducing the overall product discharge cycle time which allows for more drops (cycles) per hour. In particular, methods and apparatus to reduce the cycle time required to transfer granular polyolefin from a product chamber (high pressure) to a product blow tank (low pressure) are desired.

[0006] References of interest 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

[0007] Disclosed is a polyolefin discharge cycle comprising (or consisting of, or consisting essentially of) discharging an amount of polyolefin granules from a reactor to a product chamber, wherein the reactor is operated at a pressure of at least 20 psia; transferring the granules from the product chamber to a product blow tank, wherein the granules are transferred at a rate of at least 0.01 ton granules/second; displacing gas through an equalization line from the product blow tank into the product chamber as granules are transferred from the product chamber to the product blow tank; removing granules from the product blow tank; and repeating the cycle of discharging, transferring, displacing and removing of granules. The granules may in some embodiments be transferred from the product chamber to the product blow tank through a discharge valve having diameter 14 inches to 20 inches.

[0008] Furthermore, in certain embodiments, the discharge from the reactor to the product chamber may include opening a gas return/recycle line fluidly connecting the product chamber and reactor, such that pressure in the reactor and the product chamber comes to equalization during the discharge. In addition, this gas return/recycle line may include two valves, such that gas at reactor pressure can be trapped within the return/recycle line by closing both valves. In this way, loss of reactor gasses to downstream operation of the product blow tank (and/or processes downstream of that product blow tank) may be minimized.

BRIEF DESCRIPTION OF THE DRAWING

[0009] The Figure is a representation and flow diagram of the primary components of the inventive product discharge apparatus and an embodiment of the process associated therewith.

DETAILED DESCRIPTION

[0010] The inventors have found that gas phase polyolefin production rate can be maximized by reducing the overall product discharge cycle time which allows for more drops (cycles) per hour. This can be accomplished by improving the rate at which polyolefin granules are transferred or “dropped” between high and low pressure vessels, and ensuring that there is no fouling of the product discharge system with polyolefin granules or fines by detecting any residual or left over granules in the high pressure vessel (e.g., the product chamber) and ensuring that any polyolefin fines or granules are cleared out of the pressure equalization line leading from the low pressure vessel (e.g., product blow tank) to the high pressure vessel. These ends are accomplished by any combination of process and apparatus features as described herein.

[0011] It was found that increasing valve size, the “discharge valve,” between the product chamber and product blow tank to at least 14 or 16 inches or more can reduce the amount of time required to transfer granular polyolefins, especially polyethylene, from the product chamber into the product blow tank. The polyolefin granules produced in a gas phase reactor can be removed in discrete batches through the product discharge system via a series of automatic ball valves and pressure vessels. The production rate capacity is defined by the size of each batch, referred to as a “drop” or a “drop size,” and the number of batches that can be removed in a period of time, referred to as “drops per hour.” The overall cycle time is the sum of a number of process steps, one of them being the transfer of granules from the product chamber to the product blow tank. By reducing the time required for this step, the overall cycle time is reduced. Reducing the overall cycle time required to take a drop translates into an increase in the number of drops per hour, thus an increase in production rate capacity. The benefit of increasing the discharge valve size is surprisingly large, giving a “drops per hour” increase of as much as 15-40% for a relatively small increase in the cost of the system.

[0012] Another issue addressed by the present invention is the potential blocking of lines and/or plugging the flow of granules and/or gas. When polyolefin granules exit the reactor into the product chamber, they are still actively growing and exposed to a reactive environment. The granules become stagnant in the product chamber and if left too long under reactive conditions they can generate sufficient heat from the exothermic reaction to melt the polymer and fuse into an agglomerate. The cycle settings of the product discharge system cycle are designed to prevent this from happening. However, the risk escalates with higher productivity catalysts such as metallocenes and certain other catalysts. If the granules melt and form an agglomerate, the transfer from the product chamber to the product blow tank will not happen, which can lead to an accumulation of polyolefin in the product chamber and ultimately a shutdown of the whole product discharge system due to plugging. It is therefore desirable to know if all the polyethylene granules flow from the product chamber to the product blow tank leaving the product chamber empty. This is accomplished by installing a content detector such as a nuclear radiation level instrument on the bottom cone/nozzle of the product chamber. A radiation signal is emitted on one side of the cone and detected on the other. When the cone is empty this signal will be strong. When the cone is filled with granules or an agglomerate, the signal will be weaker indicating the product chamber is not fully empty. This is used to alert the operator of a potential problem in the product chamber, and to resolve it before it grows into something more difficult to manage.

[0013] During this transfer from the product chamber to the product blow tank, the polyolefin granules flow due to a pressure differential between the vessels in addition to gravity. The two vessels represent a single closed system. Volumetrically, the polyolefin granules displace the gas in the product blow tank up into the product chamber. A pipe, or equalization line, connects the two vessels to facilitate this gas flow, that is, to equalize the pressure in the two vessels. This equalization line has a valve, preferably an automatic ball valve, for controlling flow of gas. If this pipe gets plugged with polyolefin, either fines or agglomerates, the pressure equalization slows and results in slower transfer of granular polymer and thus reduced production capacity and possible agglomerate formation in the product chamber. To prevent such plugging of this equalization line, the line can be periodically purged to sweep any fine particles out. The purge involves in any embodiment opening the valve in this equalization line with no polyolefin in either the product chamber or product blow tank allowing gas flow from the higher pressure vessel into the lower pressure vessel. This is done between drops so there is no impact on cycle time and can be done manually or automatically. The preference is to do it automatically as part of the product discharge system logic.

[0014] As provided herein, after startup and upon reaching a steady state, the reactor can operate to perform polymerization using any of a variety of different processes including solution, slurry, or gas phase processes, but most preferably a gas phase process. For example, the polymerization reactor can be a fluidized bed reactor that is operated to produce polyolefin polymers by a gas phase polymerization process. Further, the polymerization reactor can be a staged reactor where two or more reactors are employed in series, where a first reactor can produce, for example, a high molecular weight component and a second reactor can produce a low molecular weight component. In operation, polymerization medium can be mechanically agitated or fluidized by the continuous flow of the gaseous monomer and diluent.

[0015] More specifically, in a continuous gas phase fluidized bed reactor, the polymerization reactor comprises a fluidized bed of dense phase material. At startup, the seed bed comprising polymer granules is loaded into the polymerization reactor. Liquid or gaseous feed streams of a primary monomer and hydrogen together with a liquid or gaseous comonomer are combined and then introduced into the fluidized bed, often via an upstream recycle gas line. The fluidized bed reactor for performing a continuous gas phase process typically comprises a reaction zone and a so-called velocity reduction zone. The reaction zone comprises a bed of growing polymer particles, formed polymer particles, and a minor amount of catalyst particles (collectively sometimes referred to herein as “dense phase material”) fluidized by the continuous flow of the gaseous monomer and/or comonomers and diluent to remove heat of polymerization through the reaction zone. Optionally, re-circulated gases (recycled gas) can be cooled and compressed to form liquids that increase the heat removal capacity of the circulating gas stream when readmitted to the reaction zone. This method of operation is referred to as “condensed mode.”

[0016] A suitable rate of gas flow into the fluidized bed reactor can be readily determined. The flow rates of monomer and circulating gas into the polymerization reactor is approximately equal to the rate that polymer product and unreacted monomer are withdrawn. In any embodiment, the cycle gas circulating rate (gas going to and leaving the bed) is within a range from 1000 ton/hour to 2500 ton/hour, such as 1500 ton/hour to 2200 ton/hour. The reactor production rate may in various embodiments be within a range from 20, or 40, or 50 ton/hour to 60, or 80, or 100 ton/hour. The composition of the gas passing through the reactor (and, hence, the bed) can be adjusted to maintain a steady state gaseous composition within the bed or ‘reaction zone’. Gas leaving the reaction zone is passed to the velocity reduction zone where entrained particles settle back into the dense phase zone. Gas is compressed in a compressor and passed through a heat exchanger wherein the heat of polymerization is removed, and the gas is returned to the reaction zone.

[0017] To maintain a constant reactor temperature, the temperature of circulating gas can be continuously adjusted up or down to accommodate any changes in the rate of heat generation due to the polymerization. The fluidized bed can be maintained at a constant height by withdrawing a portion of the bed at a rate equal to the rate of formation of particulate product. Polymer product can be removed semi-continuously via a series of valves into a fixed volume chamber, which is simultaneously vented back to the reactor for efficient removal of the product. At the same time, a significant portion of the unreacted gases are recycled into the reactor. Polymer product is purged to remove entrained hydrocarbons and can be treated with a small steam of humidified nitrogen to deactivate any trace quantities of residual catalyst. [0018] Furthermore, the reactor temperature of the fluidized bed reactor can range from 30, or 40, or 50°C to 85, or 90, or 95, or 100, or 120, or 150°C. In general, the reactor temperature is operated at the highest temperature that is feasible, taking into account the sintering temperature of the polymer product within the reactor. The polymerization temperature or reaction temperature typically must be below the melting or “sintering” temperature of the polymer to be formed. Thus, in an aspect, the upper temperature limit is the melting temperature of the polyolefin produced in the reactor.

[0019] As described herein, the reactor used in connection with the present methodologies can be operated to produce homopolymers of olefins, for example, ethylene or propylene, and/or copolymers, terpolymers, and the like, of olefins, particularly ethylene, and at least one other olefin. For example, the polymerization reactor can produce polyethylenes. Such polyethylenes can be homopolymers of ethylene and interpolymers of ethylene and at least one a-olefin wherein 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 utilized in the reactor are ethylene, propylene, 1 -butene, 1 -pentene, 1 -hexene, 1 -heptene, 1 -octene, 4-methylpent-l-ene, 1 -decene, 1- dodecene, 1 -hexadecene and the like. Also utilizable herein are polyenes such as 1,3 -hexadiene, 1,4-hexadiene, cyclopentadiene, di cyclopentadiene, 4-vinylcyclohex-l-ene, 1,5-cyclooctadiene, 5-vinylidene-2-norbomene and 5-vinyl-2-norbomene, and olefins formed in situ in the polymerization medium. When olefins are formed in situ in the polymerization medium, the formation of polyolefins containing long chain branching can occur.

[0020] In the production of polyethylene or polypropylene, comonomers can be present in the polymerization reactor. When present, the comonomer can be present at any level with the ethylene or propylene monomer that will achieve the desired weight percent incorporation of the comonomer into the finished granules.

[0021] In addition, hydrogen gas is often used in olefin polymerization to control the final properties of the polyolefin. For some types of catalyst systems, it is known that increasing concentrations (partial pressures) of hydrogen increase the melt flow (MF) and/or melt index (MI) of the polyolefin generated. The MF or MI can thus be influenced by the hydrogen concentration. The amount of hydrogen in the polymerization can be expressed as a mole ratio relative to the total polymerizable monomer, for example, ethylene, or a blend of ethylene and hexene or propene. The amount of hydrogen used in some polymerization processes is an amount necessary to achieve the desired MF or MI of the final polyolefin granule.

[0022] Thus, described more generally in any embodiment is a polyolefin discharge cycle comprising discharging an amount of polyolefin granules 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 granules from the product chamber to a product blow tank, wherein the granules are transferred at a rate of at least 0.01, or 0.05 ton granules/second, or within a range from 0.01 to 0.1, or 0.2, or 0.6, or 0.8 ton granules/second (wherein a ton is 1000 kg); displacing gas through an equalization line from the product blow tank into the product chamber as granules are transferred from the product chamber to the product blow tank; removing granules from the product blow tank; and repeating the cycle of discharging, transferring, displacing and removing of granules.

[0023] Also in any embodiment is a polyolefin discharge cycle comprising discharging an amount of polyolefin granules 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 granules from the product chamber to a product blow tank; displacing gas through an equalization line from the product blow tank into the product chamber as granules are transferred from the product chamber to the product blow tank; removing granules from the product blow tank; and repeating the cycle of discharging, transferring, displacing and removing of granules; wherein the equalization line is purged with gas before or after at least one cycle. Preferably wherein the equalization line is purged by allowing gas from either the product chamber or product blow tank to flow from the higher pressure vessel into the lower pressure vessel.

[0024] Also in any embodiment is a polyolefin discharge cycle comprising discharging an amount of polyolefin granules 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 granules from the product chamber to a product blow tank; displacing gas through an equalization line from the product blow tank into the product chamber as granules are transferred from the product chamber to the product blow tank; removing granules from the product blow tank; and repeating the cycle of discharging, transferring, displacing and removing of granules; wherein the product chamber comprises a content detector, preferably a nuclear density detector. [0025] As used herein, a “granule” is a flowable solid, preferably having a bulk density within a 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, a polyolefin or polyolefins, most preferably polyethylene or polypropylene.

[0026] In any embodiment the equalization line is purged with gas before or after at least one cycle. That gas may be a cycle gas made up of monomer, diluent and an inert gas such as nitrogen, or any combination of these gasses. In a typical “cycle,” polyolefin granules go from the product chamber to the product blow tank which contains gas, and that gas is displaced with polyolefin granules and flows up from product blow tank to the product chamber. The “purge” is an additional step. The purge can be done intermittently or on a regular basis. Typically, the purge is up-to-down (counter flow from normal cycle of polyolefin granules) because pressure is typically higher in the product chamber than the product blow tank, preferably by at least 50, or 80, or 100 psig or more in some portions of the discharge cycle. In any embodiment, the discharging, transferring and displacing steps described herein are sequenced events that form a cycle.

[0027] In any embodiment the equalization line is purged by allowing gas from either the product chamber or product blow tank to flow from the higher pressure vessel into the lower pressure vessel. Preferably, the equalization line is purged by allowing gas to flow from the product blow tank to the product chamber, wherein the pressure within the low pressure vessel, preferably the product blow tank, cycles between a range from 10 or 20 psig to 50 or 100 psig; and wherein the pressure within the high pressure vessel, preferably the product chamber, cycles between a range from 200, or 250 psig to 350, or 400 psig. In any embodiment the gas displaced through the equalization line from the product blow tank to the product chamber is under a gas pressure within a range from 10, or 20, or 30, or 40, or 50 psia to 100, or 200, or 300, or 400 psia.

[0028] In any embodiment the purging of the equalization line is manually controlled, or programmed for regular intervals between discharge cycles, or a combination of the two.

[0029] In any embodiment the product chamber comprises a content detector. Preferably the content detector is a nuclear density detector. An example of a nuclear detector is a Ohmart/VEGA™ nuclear density gauge which comprises a nuclear source and receiver. In any embodiment the content detector may be placed on an outlet flange at the lower portion of a conically shaped product chamber. The detector may be close to the bottom near a flange but on the cone of the product chamber. In any embodiment there may be a line from the bottom of the product chamber to the discharge valve where the content detector is fitted. Alternative content detectors include but are not limited to a level instrument, or ultrasonic source and detector. In any embodiment, the polyolefin granules are transferred from the product chamber to the product blow tank until the content detector finds substantially no granules present in the product chamber. By “substantially no granules” what is meant is that there are no granules that are detectable by the content detector. In any embodiment a timer is set to correspond to the time the valve should be opened to allow the product chamber to drain completely and consistently; the closing of the valve is not necessarily controlled on each drop based on the low level detection being reached. In any embodiment the low level detector is a check or guiding device for the operator and not the determining factor in opening or closing the discharge valve.

[0030] In any embodiment, the apparatus allowing the cycle(s), especially the line fluidly connecting the product chamber and the product blow tank, further comprises a discharge valve allowing granules from the product chamber to be held in the product chamber or transferred into the product blow tank at the desired rate. The discharge value is preferably sized to allow a desirable flow of polyolefin granules per unit time from the product chamber to the product blow tank, but in a particular embodiment the discharge valve has a diameter within a range from 14 inches to 16, or 18, or 20 inches, depending on the overall size of the product discharge system as well as other production factors.

[0031] In any embodiment the granules are transferred under gravity to the product blow tank, and optionally under gas pressure within a 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 substantially maintained throughout a cycle, meaning that the pressure does not change beyond a level that is more than 10%, or 5%, or 2% its average or steady state production pressure.

[0032] In any embodiment, the process described herein further comprises continuously combining one or more olefins with a polymerization catalyst in the reactor to form granules of polyolefin. Preferably, the reactor is a gas phase reactor comprising a bed of polyolefin granules. Also, preferably the combining takes place within the reactor at a pressure of at least 100, or 200, or 300, or 400, or 500 psig. In any embodiment the polymerization catalyst is selected from the group consisting of metallocene catalysts, Ziegler-Natta catalysts, chromium catalysts, atypical single-site catalysts (e.g., such as pyridyldiamide-transition metal catalysts, bis(2- pentamethylphenylamido)ethyl)amine-transition metal catalysts, Schiff base-transition metal catalysts, etc.), and combinations thereof. Finally, in any embodiment the olefins are selected from one or more of ethylene and C3 to C10 a-olefins, most preferably selected from ethylene, propylene, or a combination thereof. The preferred polyolefin granules that are generated from the polymerization process are polypropylene granules or polyethylene granules, wherein either may be a homopolymer of propylene-derived units or ethylene-derived units, or a copolymer comprising within a range from 0.1 to 10, or to 20 wt%, by weight of the polymer of a-olefin(s) units other than the major (propylene or ethylene) a-olefin.

[0033] Also disclosed herein is an apparatus suitable for product discharge from a reactor, preferably a polyolefin polymerization reactor. With reference to the Figure, in any embodiment is an apparatus 100 comprising (or consisting of, or consisting essentially of) a reactor 102 comprising granules of polyolefin; the reactor fluidly connected to a product chamber 112, the product chamber comprising a content detector 118; a product blow tank 114 fluidly connected 132 to the product chamber 112, wherein the fluid connection comprises a discharge valve 116; an equalization line 120 fluidly connected from the product blow tank 114 to the product chamber 112; and a takeoff line 122.

[0034] In any embodiment of the apparatus the discharge valve is sized to allow polyolefin granules to flow there through at a rate of at least 0.01 ton granules/second (as described above), or within a range from 0.01 to 0.1, or 0.2, or 0.6, or 0.8 ton granules/second. In a particular embodiment the discharge valve has a diameter within a range from 14 inches to 16, or 18, or 20 inches.

[0035] In any embodiment the product chamber has a top portion and a bottom portion, and wherein the product chamber has an inlet at the top portion fluidly connected to the reactor and an outlet at the bottom portion fluidly connected to the product blow tank, and wherein the content detector is positioned in the bottom portion of the product chamber. Preferably the content detector is a nuclear density detector.

[0036] An embodiment of the product discharge apparatus described herein is described in more detail with respect to the Figure. The Figure is a general flow diagram of a polymerization apparatus 100 comprising a gas phase reactor 102 and product discharge apparatus 104 and product discharge apparatus 106 fluidly connected thereto for allowing the cyclic removal, in timed intervals, of polyolefin granules from the gas phase reactor 102 which is under pressure. [0037] In any embodiment, lines in the Figure between 102, 112 and 114 represent fluid connections such as a hollow cylindrical steel or other metal piping that allows fluid solids, gasses and/or liquids to flow there through from one point to another, preferably along its length. Arrows on the lines represent the general flow of materials from one point to another that help exemplify the process of cycling described herein. Valves are symbolized by two inverted and connected triangles; valves allow the flow of materials to be variously stopped or slowed depending on how they are set by either a manual control or computerized system.

[0038] The product discharge apparatuses allow for the removal of polyolefin granules from the gas phase reactor without loss, or with minimal loss, of pressure, thus passing the polyolefin granules from a high pressure environment to a low pressure or atmospheric pressure environment for further finishing and processing steps. The polymerization apparatus is not herein limited to just two discharge apparatuses, but can include three, four, or more as is desired for the size of the gas phase reactor and polymerization apparatus manufacturing volume. For purposes of describing the primary components of the inventive product discharge apparatus and process, reference is made to the components of product discharge apparatus 104. The various valves are shown in the Figure to highlight the cyclic nature of the process associated with the product discharge process and apparatus, and the dual-pressure elements therein as well as the ability to control the flow of gases and polyolefin granules associated therewith, but it is understood that there can be any number of valves, or absence of valves, throughout a product discharge apparatus such as described with respect to apparatuses 104 and 106. That said, some valves are specifically noted below in connection with the description of the product discharge process.

[0039] In any embodiment, polyolefin granules such as polyethylene granules are produced within gas phase reactor 102 by the continuous combining of one or more olefins with a polymerization catalyst in the reactor to form granules of polyolefin, such granules maintained in a fluidized state in bed 110. The fluid state of the bed of polyolefin granules is maintained by flowing cycle gas from below the bed, preferably from the distributor plate (not shown) upwards through the bed to the expanded zone 108 to fluidize the polyolefin granules in bed 110. From time to time, an amount of polyolefin granules are discharged from the reactor 102 through a granule discharge line 136 to a product chamber 112, in a “drop,” as previously noted. The valve 136a in the product discharge line is opened to allow this drop through the discharge line 136, and concurrently, the valve 134a in the gas retum/recycle line 134 is opened, such that gas flows back from the product chamber 112 to the top of the reactor 102 (valve 134a may be referred to as a first gas retum/recycle line valve). As illustrated in The Figure, the valve 134a may be located along the gas return/recycle line 134 closer to the reactor 102 than to the product chamber 112. Some embodiments may also include at least a second valve 134b along the gas retum/recycle line 134, closer to the product chamber 112 end of the line 134 (the second valve 134b may, for instance, be along the retum/recycle line as illustrated in the Figure, or it may connect directly to the product chamber 112 on one end and the line 134 on the other end). Where a second valve 134b is present, it is also opened to ensure pressure equalization between product chamber 112 and reactor 102. This second valve 134b may be referred to as a second gas return/recycle line valve, and operation and function of this valve, per some embodiments, is described in more detail below.

[0040] In any case, after opening valve 134a and (if present) valve 134b, preferably, the pressure within the product chamber 112 is equal to the pressure within the reactor 102 at the end of the drop. In a preferred embodiment, the pressures are approximately equal (within 2, or 5% of one another) at the end of the drop to the product chamber 112. The product chamber 112 comprises an upper portion 124 having an inlet to allow polyolefin granules to flow therein, and a lower portion 126 having an outlet to allow polyolefin granules to flow to the next stage, a product blow tank 114. For flowing product to the product blow tank 114, the valves 134a (and 134b, if present) and 136a may be closed (e.g., upon completion of the desired drop from the reactor 102 to the product chamber 112), and the valve 116 between product chamber 112 and product blow tank 114 opened to allow product flow from the product chamber 112 to the product blow tank 114; valve 120a may also be opened to ensure pressure equalization between the product chamber 112 and product blow tank 114 via equalization line 120, facilitating the product transfer. The product chamber 112 also comprises a content detector 118, preferably at or near the lower portion 126 or at or near the outlet, or alternatively at line 132 that fluidly connects the product chamber 112 to the product blow tank 114 and comprises the discharge valve 116. The content detector 118 is desirably placed (taking into account the exact geometry of the product chamber 112 and lower portion 126) such that it can detect if and when the polyolefin granules have fully transferred from the product chamber 112 to the product blow tank 114. [0041] In some embodiments there may be no line 132; in which case the outlet flange of the product chamber 112 can be attached directly to inlet of valve 116 and the outlet flange of the valve 116 is attached directly to the inlet flange of the product blow tank 114.

[0042] Advantageously, where a second valve 134b is present, one can see that by closing both valve 134b and 134a for product transfer from the product chamber 112 to the product blow tank 114, the higher-pressure (i.e., at reactor pressure) gas is trapped in the gas recycle/return line 134; as compared to embodiments where only valve 134a (along the gas recycle/return line 134 nearer the reactor 102) is present, in which case the pressure in line 134 would decrease during the downstream transfer of product. By keeping the gas trapped in line 134, one can minimize downstream gas loss (e.g., passing through product blow tank 114 and further downstream with product) and maximize gas recycle to the reactor, such that when valves 134a and 134b are next opened to allow gas flow back to the reactor via gas return/recycle line 134, all of the trapped gas is also recycled back to the reactor.

[0043] As noted above, the maximum pressure in the product chamber 112 is approximately equal (within 2 to 5% of each other) to the reactor pressure at the end of the drop from reactor 102 to the product chamber 112. The lowest pressure is in the product blow tank 114 at the end of the drop transfer from the product blow tank 114 to the take-offline 122 and the purge bin downstream therefrom where pressure approaches zero psig (atmospheric pressure). The discharge of polyolefin granules from the product chamber 112 to the product blow tank 114 requires that pressure be allowed to drop, for instance within a range from 200 psi to 300 psi in the product chamber 112 to a pressure within a range from 10 to 50 psi. To facilitate such transfer, the polyolefin granules pass through a discharge valve 116 that is sized to allow for granules to be transferred at a rate of at least 0.01 ton granules/second, or within a range from 0.01 to 0.1, or 0.2, or 0.6, or 0.8 ton granules/second (a ton is 2000 kg). In any embodiment, this may mean having a discharge valve having a diameter within a range from 14 inches (35.5 cm) to 16 (40.6), or 18 (45.7), or 20 inches (51 cm).

[0044] In any embodiment, discharge valve 116 is a full port valve so effectively the valve has an inside diameter equal to the pipe / flange inside diameter (and nominally 14 in, 16 in, etc.).

[0045] Still referring to the Figure, the equalization line 120, previously mentioned in connection with product transfer from the product chamber 112 to the product blow tank 114, is fluidly connected from the product blow tank 114, preferably at the upper portion 128, back to the gas return/recycle line 134 connecting the product chamber 112 to the reactor 102. Preferably, the connection is below or “upstream” (with respect to flow from product blow tank 114 to the reactor 102 via return/recycle line 134) of a valve (e.g., valve 134a); and, where present, also upstream of the second valve 134b, as shown in the Figure. Thus, when valve 134a is closed (and valve 134b, if present, closed), the equalization line 120 may be used to equilibrate pressure between the product chamber 112 and product blow tank 114 as previously described. Further, when valve 134a (and 134b, if present) is/are opened, the equalization line 120 allows for pressure to be released back into the reactor 102 (along with return to the reactor of the trapped gas in line 134, if both valves 134a and 134b are present as discussed above). By returning gas from the product blow tank 114 ultimately to the reactor 102, this equalization line 120 also allows for a pressure drop in the product blow tank 114. From time to time, there may be a desire to clear the equalization line 120 of any polyolefin granules or fines that may have accumulated and thus potentially block the flow of gas from the product blow tank 114 to the gas return/recycle line 134. In that case, the equalization line is purged with gas such as nitrogen before or after at least one cycle as described herein. Note also that, although the Figure shows equalization line 120 connected to the gas return/recycle line 134, in other embodiments, the equalization line 120 may instead connect directly to the upper portion 124 of the product chamber 112, so that gas can flow (and pressure equalize) between the product blow tank 114 and product chamber 112; and if the gas return/recycle line 134 from top of product chamber 112 is also opened, then the same flow of gas from product blow tank 114 ultimately back to the gas phase reactor 102 is also enabled. In either case, the equalization line 120 alternately allows for pressure to be released back into the reactor 102 (and accompanying pressure drop in product blow tank 114); and/or for pressure to be equalized between the product blow tank 114 and product chamber 112.

[0046] In any embodiment the gas used for purging is reactor gas (mixture of all the components in the system such as ethylene, comonomer if used, hydrogen, diluent or condensing agent, nitrogen, etc.) that is present in the product chamber while it is empty of granules and waiting for a drop. In the normal cycle mode the product chamber 112 in standby mode is at approximately half the reactor operating pressure. Nitrogen alone is used as a purge gas only if the operator decides to use only nitrogen (or other gas mixture) to start purging or unplugging step.

[0047] In any embodiment, polyolefin granules within product blow tank 114, having been pressure equilibrated to a pressure at or near atmospheric pressure may then be passed via gravity feed and/or additional gas pressure from the product blow tank 114 having a lower portion 130 to a takeoffline 122 wherein the polyolefin granules can then be further processed such as by purging of excess monomer and other gases and blended with additives such as antioxidants and melt blended into pellets for transport and sale. When the drop from the product chamber 112 to the product blow tank 114 has equilibrated in pressure (after equalization) the product blow tank 114 will be at approximately 20 to 25 or 30% of the reactor pressure. This pressure is the driving force for conveying the granules downstream to the take-off line 112 and the purge bin downstream therefrom. At the end of the drop transfer from the product blow tank 114 to the downstream purge bin the product blow tank pressure approaches atmospheric pressure.

[0048] Still referring to the Figure, the product discharge system 104 may also include line 138 to remove excess monomer which can be recycled or flared through line 140. Such excess of monomer or other gasses may come from gases cycled up from product chamber 112 through gas return/recycle line 134, for instance.

[0049] All publications, patents and patent applications mentioned in this Specification are indicative of the level of skill those skilled in the art to which this invention pertains and are herein 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.

[0050] As used herein, the phrase “consisting essentially of’ means that there may be minor apparatus features present, such as valves, heaters, coolers, and pumps that facilitate the operation of the claimed apparatus or cycle but are not essential to the operation of such apparatus or cycle. Likewise, as it relates to a process claim, “consisting essentially of’ does not exclude minor features such as a valve operation, heating/cooling, and pumping of gases, fluids and/or solids that are not essential to the cycle as claimed: the transfer of solid granular polymer from a high pressure environment to a low pressure environment and/or maintaining clarity and flow within the fluid connections.

[0051] The invention being thus described, it will be apparent that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modification as would be apparent to one skilled in the art are intended to be included within the scope of the following claims.