Product purge bin middle outlet as primary purge gas outlet control

Abstract

A process for recovering a polymer product from gas phase polymerization. The polymer product may be introduced into a purge vessel and contacted with a purge gas to provide a stripped polymer product. Hydrocarbons, such as unreacted monomers, can be removed and recycled to the polymerization and purge gas can be recovered and recycled to the purge vessel. By using the mid-vent stream from the purge vessel, hydrocarbon loss can be eliminated or made negligible. The concentration of the purge gas within the purge vessel may be controlled by selectively combusting the mid-vent stream.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 362,577, filed April 6, 2022, and entitled “Product Purge Bin Mid-Vent As Primary Purge Gas Vent Control,” which is incorporated herein by reference in its entirety. field

[0003] The present application relates to olefin polymerization processes. In particular, the present disclosure relates to methods of venting a gas phase olefin polymerization system.

[0004] background

[0005] Gas phase catalytic polymerization is the main technology used to produce polyolefin resins. The catalyst used in the process is contained in solid matrix particles from which the polymer chains grow. Gas phase olefin polymerization technology often uses a fluidized bed, in which the particles are fluidized by a gas stream that also contains reactants (e.g., one or more olefin monomers) and a carrier gas. The carrier gas is typically an inert gas such as nitrogen. This type of process is described in, for example, EP0475603A1; EP0089691A2; and EP0571826A3.

[0006] Pressure control is a key factor in any gas phase olefin polymerization system and is dominated by the demand for removing nitrogen (or other inert carrier gases). In order to control the total reactor pressure or monomer (ethylene) partial pressure, it is necessary to properly control nitrogen. Usually, nitrogen partial pressure is controlled to maintain stable reactor conditions by either adding or removing nitrogen. This can be achieved by directly discharging nitrogen from the reactor to a flare. However, doing so without separation facilities to reclaim the hydrocarbons entrained in the exhaust gas is economically unattractive and causes potential environmental problems.

[0007] In a gas phase polymerization process, polymer particles produced in a fluidized bed are usually discharged from a reactor continuously or discontinuously and pneumatically transported to a product recovery system. Polymer particles inevitably contain a small amount of unreacted monomers and other hydrocarbons added to the polymerization process or produced during the polymerization process. A product recovery system that generally includes a degassing or purge container is generally used to separate and remove unreacted monomers and heavier hydrocarbons from polymer particles by countercurrent contact with an inert gas (e.g., nitrogen). An inert gas stream produced by diluting unreacted monomers and heavier hydrocarbons is generally recovered from a purge container, and after separating hydrocarbon components, it is recycled as a conveying gas or as a part of a purge stream. A part of the effluent from the purge container is removed from the system and at the same time, because the concentration of unreacted monomers in the stream is too low to make it economically feasible to recover, the discharged stream is burned or used as a fuel. This not only represents a large loss of valuable monomers, but also causes regulated environmental emissions.

[0008] There remains a need for an improved degassing system for a gas phase olefin polymerization process in which the loss of unreacted monomer in the product effluent stream is reduced or eliminated.

[0009] Overview

[0010] Provide a method for recovering polymer products from gas phase polymerization. In at least one embodiment, a polymer product comprising polymer particles, one or more unreacted monomers, one or more other hydrocarbons and a conveying gas can be introduced into a purge vessel and contacted with a purge gas to remove any unreacted and / or entrained monomers and other hydrocarbons. An overhead stream comprising at least a portion of unreacted monomers, one or more other hydrocarbons and a purge gas and a bottom stream comprising polymer particles can be extracted from the purge vessel. The overhead stream can be separated into a hydrocarbon stream comprising at least 80% by weight of hydrocarbons in the overhead stream and a gaseous effluent stream comprising less than 20% by weight of the hydrocarbons. The gaseous effluent stream can be separated into a first permeate stream richer in hydrocarbons and a first residual stream poorer in hydrocarbons in a first membrane separator. The first residual stream can then be separated into a second permeate stream richer in hydrocarbons and a second residual stream poorer in hydrocarbons in a second membrane separator. The second permeate stream may be recycled to the purge vessel and the second residue stream may be recycled to the overhead stream or selectively combusted prior to separating the overhead stream into a hydrocarbon stream comprising at least 80 weight percent of the hydrocarbons in the overhead stream and a gaseous effluent stream comprising less than 20 weight percent of the hydrocarbons.

[0011] In one or more embodiments, a middle vent stream comprising at least 70 wt% purge gas can be withdrawn from the purge vessel and selectively combusted to control the concentration of the purge gas within the purge vessel. In one or more other embodiments, the second permeate stream can be selectively combusted to control the concentration of the purge gas within the purge vessel. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 A simplified flow chart is depicted for recovering polymer product from a gas phase polymerization according to one or more embodiments described herein.

[0013] Figure 2 When nitrogen / purge gas removal is performed by combustion from the membrane effluent according to one or more embodiments described herein, Figure 1 Degassing system.

[0014] Figure 3 Description When nitrogen / purge gas removal is performed by combustion from the middle vent stream according to one or more embodiments described herein, Figure 2 Degassing system.

[0015] Detailed description of implementation plan

[0016] The present disclosure relates to a degassing method and system for separating and recovering unreacted monomers, other hydrocarbons and purge gases from a solid polymer product. The solid polymer product may be or may contain a variety of polymer particles and / or microparticles. The solid polymer product may be produced by any olefin polymerization process. Suitable olefin polymerization processes may be or may include: (1) gas phase polymerization processes, including fluidized beds, horizontal stirred beds and vertical stirred bed reactors, (2) bulk processes, including liquid pools and loop reactors, and (3) slurry processes, including continuous stirred tanks, intermittent stirred tanks, loop and boiling butane reactors. However, the present disclosure is particularly useful for recovering unreacted monomers, other hydrocarbons and purge gases from gas phase polymerization processes. Illustrative monomers include ethylene, propylene and mixtures of ethylene and / or propylene, which contain or do not contain one or more C4-C8 α-olefins.

[0017] Figure 1A simplified flow chart describing the recovery of polymer product from gas phase polymerization according to one or more embodiments. The polymer product recovery system may include one or more product purge bins or vessels 110, compressors 130, coolers 140 and separators 145, 150, 160 to strip or purge entrained gases from the polymer product. The polymer product (stream 101) that has been discharged from the polymerization reactor may be entrained with various amounts of unreacted monomers, auxiliary gases or carrier gases, and one or more other hydrocarbons, such as one or more C4 to C6 alkanes that have been added to the polymerization reaction for gas phase polymerization. The term "hydrocarbon" as used herein refers collectively to unreacted monomer(s) and one or more other hydrocarbon(s) added to the polymerization reaction, unless otherwise specifically noted.

[0018] To aid in the removal of entrained gas(es) from the polymer product, a supply of fresh purge gas (which may be an inert gas such as nitrogen) may be added to the product purge vessel 110 via stream 103, and / or directly to the bottom cone of the purge vessel 110 via stream 105, to strip or purge entrained gases from the polymer product. The fresh nitrogen feed to the purge vessel may be less than 5 wt %, preferably less than 4 wt %, 3 wt %, 2 wt % or 1 wt % of the recycle purge gas flow. The operating conditions in the degassing vessel are not closely controlled, but typically include a temperature of 20° C. to 120° C., such as 65° C. to 85° C. and a pressure of 100 kPa-a to 200 kPa-a, such as 130 kPa-a to 165 kPa-a. As the polymer product flows downward through the purge vessel 110, unreacted monomers and other entrained hydrocarbons are stripped or desorbed from the polymer particles and leave the purge vessel 110 together with the purge gas via the top effluent stream 112. The top effluent stream 112 may contain at least 30 mol % of nitrogen and at least 40 mol % of hydrocarbons. The nitrogen content may be in the range of about 30 mol % to 50 mol %, or about 35 mol % to 55 mol %, or about 30 mol % to 40 mol %. The total hydrocarbon content (i.e., all hydrocarbons including unreacted monomers and other olefins / alkanes) in the top effluent stream 112 may be in the range of about 40 mol % to 69 mol %, about 50 mol % to 69 mol %, or about 55 mol % to 69 mol %. The stripped polymer particles fall and gather at the lower end of the purge vessel 110, where they may be removed as stream 126 via discharge valve 125 for further processing. The gas content of stream 126 is mostly, if not entirely, nitrogen.

[0019] The purge vessel 110 may further include one or more middle outlet streams 115. One or more middle outlet streams 115 may be located at any position along the height of the purge vessel 110. In certain embodiments, a single middle outlet stream 115 may be located near the lower end of the purge vessel 110, but above the fresh nitrogen feed stream 103 and above the bottom cone feed stream 105. The middle outlet stream will generally contain at least 95% by weight of inert gas and may contain at least 96% by weight, 97% by weight, 97% by weight or 99% by weight of inert gas. In some embodiments, the middle outlet stream is substantially composed of inert gas. As will be explained in more detail below, the middle outlet stream 115 can be used to better control the amount of purge gas (i.e. nitrogen) added to the purge vessel 110 and burned from the system. It has been surprisingly and unexpectedly found that controlling the flow of the gas through the middle outlet stream 115 can greatly reduce the amount of hydrocarbon loss, which means significant cost savings. When used, the middle vent stream 115 contains mostly, if not all, nitrogen. There may be very little or trace amounts of unreacted monomers or hydrocarbons in the middle vent stream 115.

[0020] The overhead stream 112 may be sent to a compressor 130 (shown as a two-stage compressor comprising a first stage 130A and a second stage 130B). Figure 1 ) and then passes through cooler 140 to condense at least a portion of the lighter hydrocarbons in effluent stream 112. Compressor 130 may have any number of stages depending on flow and / or pressure requirements; as noted, Figure 1146. The condensed hydrocarbons from the cooler 140 can be separated in one or more separators or collectors 145 and recovered via stream 146. Any recovered hydrocarbons can be recycled to the polymerization reactor. The hydrocarbon-containing stream 146 can contain at least 80% by weight, at least 85% by weight, at least 90% by weight, at least 92% by weight, at least 94% by weight or at least 96% by weight of the hydrocarbons (including unreacted monomers and other olefins / alkanes) in the top stream 112. The pressurized and cooled gas can leave the separator (one or more) 145 via stream 147. The gaseous effluent stream 147 can contain less than 20% by weight of the hydrocarbons in the top stream 112, for example, less than 15% by weight, 12% by weight, 10% by weight, 9% by weight, 8% by weight, 6% by weight or 5% by weight. The stream 147 can then be sent to one or more separation devices 150, 160 to further separate hydrocarbons from nitrogen. In certain embodiments, all or any portion of gas stream 147 may be recycled to the polymerization reactor via stream 148 for use as a transport gas and / or gas stream 147 may be sent to a flare (via stream 170) and disposed of. The transport gas is preferably an inert gas (e.g., nitrogen). In one or more embodiments, overhead effluent stream 112 may bypass the recovery section and go directly to the flare via bypass stream 113.

[0021] Any suitable type of nitrogen separator or separation device may be used to remove hydrocarbons from the nitrogen. For example, one or more membrane separators may be used. Figure 1 1, 160 are shown. In certain embodiments, the first membrane separator 150 can separate the cooled gas stream 147 leaving the separator 145 into a first portion 152 rich in hydrocarbons and depleted in nitrogen (compared to the hydrocarbon content of the cooled gas stream 147 entering the first separator 150), and a second portion 157 depleted in hydrocarbons and rich in nitrogen (compared to the hydrocarbon content of the cooled gas stream 147). The first portion 152 rich in hydrocarbons can be recycled to the front end of the compressor 130. The first portion 152 can be mostly nitrogen, with less than 50% by weight hydrocarbons (preferably 22% by weight or less, 23% by weight or less, 25% by weight or less, 27% by weight or less, 30% by weight or less, 35% by weight or less or 40% by weight or less). The nitrogen content can also range from a low of about 50%, 55% by weight or 60% by weight to a high of about 70%, 75% by weight or 80% by weight. The total hydrocarbon content can range from a low of about 20%, 25%, or 30% by weight to a high of about 35%, 42%, or 49% by weight. The unreacted monomer content in the first portion 152 can be from about 5, 10, or 15% by weight to about 20, 25, or 30% by weight.

[0022] The second portion 157, which is depleted in hydrocarbons, may be sent to a second membrane separator 160 for further separation and recovery. The second membrane separator 160 may further separate the second portion 157, which is depleted in hydrocarbons (relative to the hydrocarbon content of the cooled gas stream 147), into a third portion (via stream 162) that is depleted in hydrocarbons and a fourth portion (via stream 163) that is rich in hydrocarbons (relative to the hydrocarbon content of the second portion 157). The third portion (via stream 162) may be mostly nitrogen, having less than about 30 mole percent hydrocarbons (preferably less than 10 wt%, less than 8 wt%, less than 7 wt%, less than 6 wt%, less than 5 wt%, less than 4 wt%, less than 3 wt%, or less than 2 wt%) and may be recycled to the purge vessel 110 for use as a purge gas within the purge vessel 110. The fourth part (via stream 163) can also be mostly nitrogen, with less than about 30% by weight hydrocarbon (preferably less than 20% by weight, less than 18% by weight, less than 17% by weight, less than 16% by weight, less than 15% by weight, less than 14% by weight, less than 13% by weight or less than 10% by weight). The nitrogen content can also be in the range of about 75,80 or 90% by weight low to about 93,95 or 99% by weight high. The hydrocarbon content (i.e., the total hydrocarbon including unreacted monomers and other alkanes) can be in the range of about 0.5% by weight, 1% by weight or 5% by weight low to about 15% by weight, 18% by weight or 25% by weight high. Wherein, the content of unreacted monomers can be about 70% by weight, 75% by weight or 80% by weight to about 90% by weight, 95% by weight or 100% by weight. All or any part of the fourth part (stream 163) can be sent to the flare via stream 170. In certain embodiments, all or any portion of the fourth portion (stream 163) may be recycled to the front end of compressor 130 via stream 164. In certain embodiments, any portion of the fourth portion (stream 163) may be recycled to the front end of compressor 130 via stream 164, while any portion of the fourth portion (stream 163) may be sent to the flare via stream 170.

[0023] In operation, a series of control valves can be used to control the gas flow rate throughout the method. In one embodiment, three control valves 205, 210 and 215 can be used to control the purge system to significantly reduce the hydrocarbon loss in the system. For example, the first valve 205 can be used to control the gas flow from the purge vessel 110 through the middle outlet stream 115, via stream 170 to the flare. The second valve 210 can be used to control the third part (via stream 162) of the recycle leaving the second membrane separator 160 to the purge vessel 110. The third valve 215 can be used to control the gas flow leaving the second membrane separator 160, via stream 170 to the fourth part (stream 163) of the flare. At least, these three valve control systems can regulate the amount of purge gas required to effectively remove the entrained gas from the polymer product while minimizing the hydrocarbon loss via the flare. By "minimized" is meant that the flare stream contains less than 3 wt%, less than 2 wt%, less than 1 wt%, less than 0.5 wt%, less than 100 ppmw, less than 10 ppmw, or less than 1 ppmw hydrocarbon(s). In some embodiments, the flare stream will contain no hydrocarbons.

[0024] When the system is started up, the polymer product with one or more entrained gases can be introduced into the purge vessel 110 via feed stream 101. The polymer product can be contacted with a countercurrent of fresh purge gas (e.g., nitrogen) delivered to the vessel 110 via stream 103. The same or different purge gas can be added to the bottom or bottom cone of the purge vessel 110 via stream 105 to help prevent polymer particles from sticking and / or plugging the bottom of the vessel 110. This stream 105 can be a continuous or intermittent but typically continuous stream. A rotary valve 125 or the like can be used to remove the stripped polymer particles from the purge vessel 110 and transported away via stream 126.

[0025] The overhead stream 112 from the purge vessel 110, containing unreacted monomers, transport gas and purge gas (again, it is noted that the transport gas and / or purge gas are preferably inert gases, such as nitrogen), can be removed from the vessel 110 and compressed, cooled and finally separated into a recycle stream 162 to the purge vessel 110 and a fourth portion (stream 163) that leaves the second separator 160 and is sent to the flare. Some of this fourth portion / disposal stream 163 can be recycled to the front end of the compressor 130 via the recycle stream 164. This flow configuration can be continued until the pressure in the recovery system refrigerator 140 is high enough that the control valve 220 begins to open to remove gas to the flare. This means that the system is "full" and it is necessary to start removing molecules. Once the system capacity has been reached, the valve 205 on the middle outlet stream 115 can begin to open and allow gas to flow through the middle outlet stream 115 to the flare. When the middle vent valve 205 is open, the control valve 215 on the fourth portion (stream 163) leaving the second separator 160 can begin to close. When the middle vent gas flow through stream 115 exceeds the fresh purge gas flow to the purge vessel 110 (stream 105), the fresh purge gas flow 103 can be stopped. The system is now in a steady state or substantially in a steady state. The flow through the middle vent valve 205 can then be used to control the removal of purge gas from the system and significantly reduce hydrocarbon losses.

[0026] Figure 2 A method of removing inerts (i.e., nitrogen) and controlling the purge outlet by burning the second membrane stream effluent (i.e., the fourth portion) stream 163 is described. In this embodiment, inert removal and purge control can be performed by sending the second membrane stream effluent (i.e., the fourth portion) stream 163 to the flare. To achieve this removal method, as Figure 2 As described in, the membrane effluent valve 215 can be opened and the middle outlet control valve 205 can be closed. Table 1 below summarizes the simulated flow rates and stream compositions for this case.

[0027] Figure 3 Another way to remove inerts (i.e., nitrogen) and control the purge vent is illustrated by burning the middle vent stream 115 from the purge vessel 110. In this embodiment, the middle vent control valve 205 can be opened and the membrane effluent valve 215 can be closed. Table 2 summarizes the simulated flow rates and stream compositions for this case.

[0028] Table 1: Flow rates and compositions of streams for nitrogen removal through the second membrane effluent 163 to the flare.

[0029] Table 2: Stream flow rates and composition of mid-vent stream 115 to flare.

[0030] Referring to Tables 1 and 2, when the second membrane stream effluent (i.e., fourth portion) stream 163 and control valve 215 are used to control the purge vent, the amount of ethylene lost to the flare is about 0.08 T / hr (about 160 lb / hr) and the total hydrocarbon loss via stream 170 is 0.09 T / hr (about 180 lb / hr). In contrast, when using Figure 3 When the purge vent is controlled by the middle vent stream 115 (and control valve 205) described in , the amount of hydrocarbon loss is significantly lower. More specifically, the only hydrocarbon loss is ethylene (0.012 T / hr or about 24 lb / hr) via stream 115, as reported in Table 2.

[0031] Various terms have been defined above. When a term used in a claim is not defined above, it should be given the broadest definition given to that term by persons skilled in the art as reflected in at least one printed publication or issued patent. In addition, all patents, test procedures, and other documents cited in this application are fully incorporated by reference to the extent such disclosure is not inconsistent with this application and for all jurisdictions where such incorporation is permitted.

[0032] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the following claims.

Claims

1. A method for recovering a polymer product from a gas phase polymerization, comprising: introducing a polymer product comprising polymer particles, one or more unreacted monomers, one or more other hydrocarbons, and a transport gas into a purge vessel; contacting a polymer product with a purge gas in a purge vessel; withdrawing an overhead stream from the purge vessel, the overhead stream comprising unreacted monomer, one or more other hydrocarbons, a transport gas, and at least a portion of the purge gas; withdrawing a bottoms stream from the purge vessel, the bottoms stream comprising polymer particles; separating the overhead stream into a hydrocarbon stream comprising at least 80 weight percent of the hydrocarbons in the overhead stream and a gaseous effluent stream comprising less than 20 weight percent of the hydrocarbons; In a first membrane separator, separating the gas effluent stream into a first permeate stream richer in hydrocarbons and a first residual stream leaner in hydrocarbons; In a second membrane separator, separating the first residue stream into a second permeate stream richer in hydrocarbons and a second residue stream leaner in hydrocarbons; recycling the second residual stream to the purge vessel; recycling the second permeate stream to the overhead stream prior to separating the overhead stream into a hydrocarbon stream comprising at least 80 weight percent of the hydrocarbons in the overhead stream and a gas effluent stream comprising less than 20 weight percent of the unreacted monomers; withdrawing a middle vent stream from the purge vessel, the middle vent stream comprising at least 70 weight percent purge gas; and The center outlet stream is selectively burned to control the purge gas concentration within the purge vessel.

2. The process of claim 1 further comprising recycling the first permeate stream richer in hydrocarbons to the overhead stream before separating the overhead stream into a hydrocarbon stream comprising at least 80 weight percent of the hydrocarbons in the overhead stream and a gas effluent stream comprising less than 20 weight percent of the hydrocarbons.

3. The method of claim 1 or claim 2, further comprising selectively combusting the second permeate stream to control the purge gas concentration in the purge vessel.

4. The process of claim 1 or claim 2, wherein the second permeate stream richer in hydrocarbons is not combusted.

5. The process of claim 1 or any one of claims 2 to 5, wherein the second residue stream is enriched in purge gas.

6. The method of claim 1 or any one of claims 2 to 6, further comprising introducing a fresh purge gas feed into the purge vessel.

7. The process of claim 6 wherein the fresh purge gas feed to the purge vessel is less than 5 weight % of the recirculated purge gas flow rate.

8. The method of claim 1 or any one of claims 2 to 7, wherein the delivery gas is the same as the purge gas.

9. The method of claim 1 or any one of claims 2 to 8, wherein both the transport gas and the purge gas are nitrogen.

10. The process of claim 1 or any one of claims 2-9, wherein the polymer product further comprises one or more saturated hydrocarbons.

11. The process of claim 1 or any one of claims 2 to 10, wherein the central outlet stream comprises at least 95% by weight of inert gas.

12. The process of claim 1 or any one of claims 2 to 11, wherein the second residue stream recycled to the purge vessel is used as purge gas in the purge vessel.

13. The process of claim 1 or any one of claims 2-11, wherein the second residue stream recycled to the purge vessel is used as part of the purge gas added to the purge vessel.

14. A method for recovering a polymer product from a gas phase polymerization, comprising: introducing a polymer product comprising polymer particles, one or more unreacted monomers, and a transport gas into the purge vessel; contacting the polymer product with fresh purge gas in the purge vessel; withdrawing an overhead stream from the purge vessel, the overhead stream comprising unreacted monomer and purge gas; withdrawing a bottoms stream from the purge vessel, the bottoms stream comprising polymer particles; separating the overhead stream into a hydrocarbon stream comprising at least 80 weight percent of the hydrocarbons in the overhead stream and a gaseous effluent stream comprising less than 20 weight percent of the hydrocarbons; In a first membrane separator, separating the gas effluent stream into a first permeate stream richer in hydrocarbons and a first residual stream leaner in hydrocarbons; separating the first residue stream into a second permeate stream richer in hydrocarbons and a second residue stream leaner in hydrocarbons in a second membrane separator; and The second permeate stream is selectively combusted to control the purge gas concentration within the purge vessel.

15. The method of claim 14, further comprising withdrawing a middle vent stream from the purge vessel, the middle vent stream comprising at least 70 wt% inert gas, and selectively combusting the middle vent stream to further control the concentration of inert gas in the purge vessel.

16. The method of claim 15, further comprising limiting the flow of fresh purge gas to the purge vessel after the flow of the intermediate outlet stream exceeds the flow of fresh purge gas to the purge vessel.

17. The method of claim 15, further comprising restricting the flow of the second permeate stream to the flare and recycling the second residue stream to the purge vessel.

18. The process of claim 17, wherein the recycled second residue stream to the purge vessel is used as a portion of the purge gas within the purge vessel.

19. The method of claim 15, further comprising limiting the flow of fresh purge gas to the purge vessel after the flow of the intermediate outlet stream exceeds the flow of fresh purge gas to the purge vessel.

20. The process of claim 19, wherein the recycled second residue stream to the purge vessel is used as the purge gas within the purge vessel.

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