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A fractionation system for a polymerization reactor includes a membrane separation system designed to separate light components, such as unreacted monomer and inerts, from diluent. The membrane separation system may employ one or more membrane modules designed to separate hydrocarbons based on size,

A fractionation system for a polymerization reactor includes a membrane separation system designed to separate light components, such as unreacted monomer and inerts, from diluent. The membrane separation system may employ one or more membrane modules designed to separate hydrocarbons based on size, solubility, or combinations thereof. The fractionation system also may include a heavies fractionation column designed to separate heavy components, such as unreacted comonomer and oligomers, from the diluent.

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1. A method for recovering diluent from polymerization reactor effluent, the method comprising: separating effluent from a polymerization reactor into extracted polyolefin solids, a first stream including primarily diluent and heavy components, and a second stream including primarily the diluent and

1. A method for recovering diluent from polymerization reactor effluent, the method comprising: separating effluent from a polymerization reactor into extracted polyolefin solids, a first stream including primarily diluent and heavy components, and a second stream including primarily the diluent and light components;fractionating the first stream in a heavies fractionation column to produce a first discharge stream including primarily the heavy components and a second discharge stream including primarily the diluent; andseparating the second stream in a membrane separation system to produce a diluent enriched stream including primarily the diluent and a lights enriched stream including primarily the light components, wherein the membrane separation system is an integral top portion of the heavies fractionation column. 2. The method of claim 1, comprising polymerizing olefin monomer in a loop slurry polymerization reactor in the presence of catalyst suspended in the diluent to form the polyolefin solids and discharging the effluent from the polymerization reactor. 3. The method of claim 1, comprising separating the second stream in the membrane separation system using at least one membrane having a nonwoven fabric layer, a solvent resistant microporous support layer, and a nonporous selective layer. 4. The method of claim 3, wherein the solvent resistant microporous support layer and the nonporous selective layer are cross-linked to one another. 5. The method of claim 1, wherein separating the second stream in the membrane separation system comprises directing the second stream through one or more separation membranes configured to permeate the light components through the one or more separation membranes. 6. The method of claim 1, comprising separating the second stream in the membrane separation system using a first membrane module and a second membrane module. 7. The method of claim 6, wherein separating the second stream in the membrane system using the first membrane module and the second membrane module comprises: separating the second stream into a first lights enriched stream and a first isobutane enriched stream in the first membrane module, wherein the first lights enriched stream includes primarily light components , and wherein the first isobutane enriched stream includes primarily isobutane; andseparating the first lights enriched stream into a second lights enriched stream and a second isobutane enriched stream in the second membrane module, wherein the second lights enriched stream includes primarily light components, and wherein the second isobutane enriched stream includes primarily isobutane. 8. The method of claim 7, wherein separating the second stream into the first lights enriched stream and the first isobutane enriched stream comprises directing the second stream through a membrane of the first membrane module, the membrane being configured to permeate hydrocarbons with four or more carbon atoms and to substantially block hydrocarbons with three or fewer carbon atoms. 9. The method of claim 7, wherein separating the first lights enriched stream into the second lights enriched stream and the second isobutane enriched stream comprises directing the first lights enriched stream through a membrane of the second membrane module, the membrane being configured to permeate hydrocarbons with four or more carbon atoms and to substantially block hydrocarbons with three or fewer carbon atoms. 10. The method of claim 7, comprising combining the first isobutane enriched stream from the first membrane module and the second isobutane enriched stream from the second membrane module in an isobutane enriched flow path. 11. The method of claim 7, comprising conveying the first lights enriched stream from the first membrane module to the second membrane module via a first lights enriched flow path. 12. The method of claim 7, comprising: conveying the second lights enriched stream to a flare or a processing system; andconveying the second isobutane enriched stream directly to a separation unit. 13. The method of claim 1, comprising combining the second discharge stream from the heavies fractionation column with the second stream upstream of the membrane separation system. 14. The method of claim 1, comprising directing the lights enriched stream to a flare or to a monomer recovery unit. 15. A method for producing polyolefin solids, comprising: polymerizing an olefin monomer in a polymerization reactor to produce a slurry having polyolefin solids, isobutane, heavy components, and light components;discharging the slurry from the polymerization reactor as an effluent;producing a first stream, a second stream, and polyolefin solids from the effluent in an effluent treatment system;fractionating the first stream in a heavies fractionation column to produce a first discharge stream including primarily the heavy components and a second discharge stream including primarily the light components and the isobutane;separating the second stream in a membrane separation system to produce an isobutane enriched stream including primarily the isobutane and a lights enriched stream including primarily the light components;recycling at least a portion of the isobutane enriched stream directly to a separation unit of the effluent treatment system; andprocessing the polyolefin solids to produce polyolefin fluff. 16. The method of claim 15, wherein producing the first stream comprises separating a purge gas from a residual hydrocarbon stream to produce a purge gas recycle stream and the first stream in the separation unit. 17. The method of claim 15, wherein at least a portion of the second stream comprises the second discharge stream from the heavies fractionation column. 18. The method of claim 15, comprising separating the second stream in the membrane separation system through at least one membrane comprising a nonwoven fabric layer, a solvent resistant microporous support layer, and a nonporous selective layer, wherein the solvent resistant microporous support layer and the nonporous selective layer are cross-linked to one another. 19. The method of claim 15, wherein the membrane separation system is an integral top portion of the heavies fractionation column. 20. A method for producing polyolefin solids, comprising: producing a first stream, a second stream, and polyolefin solids from effluent discharged from a polymerization reactor in an effluent treatment system;fractionating the first stream in a heavies fractionation column to produce a first discharge stream including primarily heavy components and a second discharge stream including primarily light components and isobutane;separating the second stream in a membrane separation system to produce an isobutane enriched stream including primarily the isobutane and a lights enriched stream including primarily the light components;recycling at least a portion of the isobutane enriched stream directly to a separation unit of the effluent treatment system; andprocessing the polyolefin solids to produce polyolefin fluff.