WO2018204026A1

WO2018204026A1 - Methods and systems for recovering volatile volatile organic compounds from a purged polymer product

Info

Publication number: WO2018204026A1
Application number: PCT/US2018/026681
Authority: WO
Inventors: Joshuua P. OLSON; Giyarpuram N. Prasad; David J. Sandell
Original assignee: Exxonmobil Chemical Patents Inc.
Priority date: 2017-05-05
Filing date: 2018-04-09
Publication date: 2018-11-08

Abstract

Disclosed herein are methods and systems for recovering volatile organic compounds (VOCs) from a purged polymer product. The methods and systems are particularly useful in the recovery of VOCs from a purged polyethylene polymer product produced in a fluidized bed reactor.

Description

METHODS AND SYSTEMS FOR RECOVERING VOLATILE ORGANIC COMPOUNDS FROM A PURGED POLYMER PRODUCT

INVENTORS: Joshua P. Olson; Giyarpuram N. Prasad; David J. Sandell

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of Serial No. 62/501,942, filed May 5, 2017, the disclosure of which is hereby incorporated by referenced in its entirety.

FIELD OF THE INVENTION

[0002] This disclosure relates to methods and systems for recovering volatile organic compounds (VOCs) from a purged polymer product. The methods and systems are particularly useful in the production of polyethylene in a fluidized bed reactor.

BACKGROUND OF THE INVENTION

[0003] Polyolefin resins, including polyethylene, may be manufactured in various reactor systems, including systems comprising a fluidized bed reactor. In such processes, the polymer product discharged from the reaction zone comprises solid polymer granules and volatile non-polymer components, e.g., monomer, comonomer, and catalyst. These volatiles may be dissolved in, bound to, or otherwise attached to the polymer granules and/or in the vapor space external to the polymer granules. Heavy olefin monomers often used as comonomers in polyethylene polymerization processes, such as 1-hexene, are especially soluble in low density polyethylene. The process of removing these volatiles from the polymer product is referred to in the art as resin degassing or purging.

[0004] A polymer product may be purged by depressurizing the resin and stripping it with a light purge gas, such as nitrogen. In these processes, the polymer product is transferred to a lower pressure purge bin. The polymer product enters the upper portion of the vessel and is subjected to purge gas entering the vessel through ports or openings at the bottom of the vessel and possibly along the sides and other areas. It sweeps through the granular resin and exits the purge bin. The purged polymer product is discharged and conveyed to further downstream processes. A purge gas vent stream comprising the purge gas and purged volatiles, in particular volatile organic compounds (VOCs), is generally subjected to downstream processing in a recovery system to recover the VOCs, which may be recycled to the reactor, after which the remainder of the vent stream is flared. Background references for polymer purge and recovery systems include U.S. Patent Nos. 3,797,707; 4,286,883; 4,372,758; 4,731,438; 4,758,654; 5,292,863; 5,462,351 ; 8,470,082, U.S. Publication No. 2011/0201765, EP 2 172 494 A, and R.W. Baker and M. Jacobs, "Improve Monomer Recovery from Polyolefln Resin Degassing", Hydrocarbon Processing, Mar. 1996.

[0005] Optimizing the recovery of VOCs from the purged polymer product is challenging in view of operating equipment and VOC content constraints. For environmental and safety reasons, VOCs (e.g., unreacted monomer) present in the non-polymer components must be removed or reduced to an appropriate level in both the polymer product and flare gas before being exposed to the atmosphere. Additionally, it is economically advantageous to recover as much of the VOCs as possible to minimize the use of additional raw materials and compression and pumping energy.

[0006] Attempts have been made to manage the VOC content in poly olefin through improved purging methods and systems. For instance, U.S. Patent Nos. 7,957,947; 8,249,748; and 8,543,242 relate to techniques for reducing VOC content in polyolefin by constructing and implementing a purge model column to calculate or estimate the VOC content in the polyolefin exiting the purge column. U.S. Patent Application No. 15/258226 relates to control methods and systems for purging a polymer product of volatiles, particularly a polymer product comprising polyethylene produced in a fluidized bed reactor.

[0007] However, there remains a need for improved methods and systems for recovering VOCs from a purged polymer product. In particular, there is a need for robust methods and systems that enable optimizing various process parameters within operating equipment and VOC content constraints, such as maximizing polymer production rates, maximizing VOC recovery, and/or minimizing flaring.

SUMMARY OF THE INVENTION

[0008] Disclosed herein are methods and systems for recovering volatile organic compounds (VOCs) from a purged polymer product. The methods generally comprise the steps of i) producing a polymer product in a reactor, ii) transferring the polymer product from the reactor to at least one purge bin, injecting a purge gas into the at least one purge bin to create a purged polymer product and a purge bin vent stream including the purge gas and VOCs, iii) compressing the purge bin vent stream in a compressor, iv) cooling the compressed purge bin vent stream to create a condensed liquid stream and a non-condensable gas stream, v) passing the non-condensable gas stream across the surface of a first membrane to create a first permeate stream enriched in VOCs and depleted in purge gas, and a first residual stream depleted in VOCs and enriched in purge gas, and vi) determining two or more process parameters. For example, the methods preferably comprise determining X₀ / Xi and (S*P) / (G*H) for one or more species of VOCs purged from the polymer product. The parameter X₀ is the concentration in ppmw of the VOC species in the discharged purged polymer product and Xi is the concentration in wt% of the VOC species solubilized in the polymer product within the reactor. The parameter S is the production rate in Klb polymer per hour of the reactor, while P is the absolute pressure in psia within the purge bin, G is the mass flow rate in lb purge gas per hour in the purge bin, and H is the Henry's law constant for the VOC at the purging temperature in psia of the VOC species in the purge bin vent stream / wt% of the VOC species solubilized in the polymer product within the reactor. Generally, the methods further comprise determining a relationship between the two or more process parameters, determining a target value for at least one of the process parameters (i.e., a controlled process parameter), and adjusting at least one process parameter to maintain the controlled process parameter at or below its target value. For example, the methods preferably comprise determining a relationship between X₀ / Xi and (S*P) / (G*H) for one or more species of VOCs purged from the polymer product, determining a target value of X₀ / Xi and/or (S*P) / (G*H), and adjusting at least one of the parameters S, P, or G to maintain the value of X₀ / Xi at or below the target value of X₀ / Xi and/or to maintain the value of (S*P) / (G*H) at or below the target value of (S*P) / (G*H).

[0009] Preferably, the methods further comprise recycling at least a portion of the first permeate stream to the compressor. In such aspects, the methods generally comprise determining C_v and Sai, wherein C_v is the volumetric capacity (in actual m /hr) of the compressor and Sai is the area in m² of the surface of the first membrane. The methods generally further comprise determining a relationship between C_v and Sai, determining a target value of C_v, and adjusting Sai to maintain the value of C_v at or below the target value of

Cv.

[0010] Often, the methods further comprise passing the first residual stream across the surface of a second membrane to create a second permeate stream further enriched in VOCs and depleted in purge gas, and a second residual stream further depleted in VOCs, particularly heavy VOCs, and enriched in purge gas. Generally, at least a portion of the second residual stream is recycled to the purge bin. In such aspects, the methods generally comprise determining X_p and Sa2, wherein X_p is the concentration in mol% of C6+ VOC species in the second residual stream and Sa2 is the area in m² of the surface of the second membrane. The methods generally further comprise determining a relationship between X_p and Sa2, determining a target value of X_p, and adjusting Sa2 to maintain the value of X_P at or below the target value of X_p.

[0011] Also disclosed herein are systems for recovering VOCs from a purged polymer product. The methods and systems herein are particularly useful in the production of a polymer product comprising polyethylene in a fluidized bed reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 depicts a polymerization system suitable for use with the methods and systems of the invention disclosed herein.

[0013] FIG. 2 depicts a schematic representation of the VOC recovery system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] All numerical values within the detailed description and the claims herein are modified by "about" or "approximately" the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

Definitions

[0015] The term "volatile" as used herein refers to a component or compound that has a low relative boiling point compared with the components or compounds around it. The term "volatile organic compound (VOC)" as used herein refers to a volatile Ci-Cio hydrocarbon. VOCs can be saturated or unsaturated, inerts or non-inerts.

[0016] The term "purge" as used herein refers to the process of removing unwanted dissolved and undissolved gases, including VOCs and/or other volatile compounds, from a solid granular polymer resin that has interstitial space filled with gas. In addition to the interstitial gas, volatile compounds, e.g., VOCs, may be dissolved in the resin. The purging operation consists of creating a sufficient driving force to cause the absorbed volatile compound to diffuse from the resin.

[0017] The term "species", as used herein, is any one compound or group of compounds of volatiles, e.g., VOCs, within the polymer product that is less than the whole of all of the species of volatiles to be purged from the polymer product. For example, a species may be one compound, such as 1-hexene. A species, as used herein, may also refer to more than one compound or a group of compounds. The group of compounds may be related in some way or not related, but may be considered a species so long as the group is less than the whole of all of the species to be purged. For example, a species could be "C6 inerts," which would include all VOC species in the polymer to be purged having six carbon atoms that are inert to the polymerization process. The particular selection of species of VOCs to apply in the present methods that will be most useful in a particular process could be readily made by one of ordinary skill in the art in possession of this disclosure. Preferably, the selection of species includes the particular VOC species that have been determined to be most limiting to purge efficiency, which are often the heavier species present in a given process.

[0018] Useful methods for recovering VOCs from a purged polymer product have been discovered. The methods involve determining one or more defined process parameters relating to a purge bin and recovery system in a polymerization process. The methods further involve determining a relationship between two or more of the process parameters, determining a target value, e.g., a constraint, for at least one process parameter (i.e., a controlled process parameter), and adjusting one or more process parameters to maintain the controlled process parameter at or below its target value. These methods may be employed in the operating strategy of the polymerization process to yield robust methods and systems to maximize polymer production rates, maximize VOC recovery, and/or minimize flaring within VOC content constraints.

[0019] The VOCs in a polymer product may include unreacted monomer and comonomer and other impurities introduced to the reactor system with the monomer, comonomer, or other feed or produced as byproducts of the polymerization reaction. In a process to produce a polyethylene copolymer, for example, where a hexene comonomer is used, the polyolefin product will include unreacted ethylene and hexene, as well as other impurities that were introduced to the reactor system with the ethylene and hexene or other feed. Preferably, the methods of the present invention are particularly suitable in polyolefin production processes where the VOCs comprise one or more C6+ hydrocarbons, particularly Ce hydrocarbons. Examples of such Ce hydrocarbons include 1 -hexene, 2,3-dimethyl-l-butene, 2,3-dimethyl-2- butene, 2-ethyl-l-butene, 2-methyl-l-pentene, 2-methyl-2-pentene, 3,3-dimethyl-l -butene, 3- methyl- 1-pentene, 3-methyl-cis-2-pentene, 3-methyl-trans-2-pentene, 4-methyl-l -pentene, 4- methyl-cis-2-pentene, 4-methyl-trans-2-pentene, cis-2-hexene, cis-3-hexene, cyclohexane, trans-2-hexene, trans-3-hexene, and hexane.

[0020] Improved systems for recovering VOCs from a purged polymer product have also been discovered. The systems generally comprise a reactor system configured to produce a polymer product; at least one purge bin in fluid communication with the reactor, the at least one purge bin having at least one inlet configured to receive at least a portion of the polymer product from the reactor, at least one inlet configured to receive purge gas, at least one outlet configured to remove a purged polymer product, and at least one outlet configured to remove a purge bin vent stream; a compressor in fluid communication with the at least one purge bin, wherein the compressor is configured to receive and compress at least a portion of the purge bin vent stream to create a compressed purge bin vent stream; a condenser in fluid communication with the compressor, wherein the condenser is configured to receive and condense at least a portion of the compressed purge bin vent stream to create a condensed liquid stream and a non-condensable gas stream; a first membrane in fluid communication with the condenser, wherein the first membrane is configured to receive and preferentially separate at least a portion of the non-condensable gas stream to create a first permeate stream enriched in VOCs and depleted in purge gas and a first residual stream depleted in VOCs and enriched in purge gas. Preferably, the systems further comprise a second membrane in fluid communication with the first membrane, wherein the second membrane is configured to receive and preferentially separate at least a portion of the first residual stream to create a second permeate stream further enriched in VOCs and depleted in purge gas and a second residual stream further depleted VOCs and enriched in purge gas. The systems may further include one or more analyzers for measuring the concentration of one or more VOC species at various locations throughout the system. Preferred systems include a first analyzer for measuring the concentration of one or more VOC species in the purged polymer product. Additionally or alternatively, preferred systems include a second analyzer for measuring the concentration of at least one VOC species in a gas phase of the reactor system upstream of the purge bin.

[0021] Optionally, the systems may further include a screen for screening the purged polymer product. In such aspects, the first analyzer may be adapted to measure the total concentration of VOCs in the vapor space of the purged polymer product as the purged polymer product flows through the screen. Such systems are particularly useful where the reactor system comprises a fluidized bed reactor having a cycle gas loop. In such systems, the second analyzer may be adapted to measure the concentration of at least one VOC in the cycle gas of the fluidized bed reactor. The systems may further comprise a heater or heat exchanger for heating the polymer product, wherein the heater or heat exchanger is located between the reactor and the at least one purge bin.

[0022] An exemplary polymerization system, particularly for the production of poly olefin, e.g., polyethylene, is depicted in FIG. 1. FIG. 1 shows a fluidized bed reactor system 101 comprising a fluidized bed pressure vessel 102. A gas or gas/liquid mixture enters the fluidized bed pressure vessel 102 from an inlet 103, through a gas distributor 104, and exits the fluidized bed pressure vessel 102 through cycle fluid line 105. The fluidized bed pressure vessel 102 may be a reactor, a polymerization reactor, a vessel capable of holding a fluidized solid, or any pressure vessel from which a granular, powder, or particulate solid product may be removed. The cycle fluid line 105 exits the top of the reactor and is compressed in compressor 106 and then passed through heat exchanger 107, where heat is removed from the cycle fluid. After cooling, all or a portion of the cycle fluid line 105 can be returned to the reactor.

[0023] Polymer product is recovered from the reactor via line 108 and sent through the product discharge system 109. The product discharge system 109 can be any suitable system. Product discharge systems and operating methods particularly useful herein are disclosed in U.S. Patent No. 9,039,333. The polymer product exits the product discharge system 109 via line 110, and is fed to a purge bin 111. Although only one inlet 110 to the purge bin 111 is shown in FIG. 1, multiple inlets and outlets are possible, in any suitable configuration. The purge bin 111 may be any suitable vessel or bin, including multiple diameter purge bins having an upper zone, a lower zone, and optionally one or more intermediate zones, such as are described in U.S. Patent No. 4,758,654, which is herein in entirety incorporated by reference. Other purge bin designs suitable for use may include those disclosed in U.S. Patent No. 8,470,082, which is herein incorporated in entirety by reference. The purge bin 111 may include one or more inserts for gas distribution (not shown), which may be an inverted cone or other suitable design. The inverted cone may have any cross-sectional shape, such as round, oval, polygonal, or other, and may have a pointed tip, rounded tip, or square tip.

[0024] An optional fresh purge gas stream 112 is fed to the bottom of the purge bin 111 from purge gas source 113. Generally, the fresh purge gas stream 112 is a light, hydrocarbon free gas, preferably nitrogen or an equivalent thereof. A volatiles stream 114 is removed the bottom of the purge bin 111 and may be further processed or sent to a flare (not shown). This volatiles stream 114 may comprise light volatiles, such as VOCs comprising three or less carbon atoms, introduced into the purge bin from the use of recycled purge gas.

[0025] Purge gas sweeps up through the polymer product in the purge bin 111 and is removed as a purge bin vent stream via discharge line 115. It is sent to a recovery system 116. In the recovery system 116, a VOC rich stream comprising unreacted monomer exits the recovery system 116 in the liquid phase through liquid discharge line 118, after which it is preferably recycled to fluidized bed pressure vessel 102 (not shown). At least a portion of the remaining uncondensed portion of the purge bin vent stream is depleted of heavy VOCs, e.g., hydrocarbons comprising four, five, six, or more carbon atoms, and recycled back to the purge bin 1 11 through purge gas recycle line 117. Purge gas recycle line 1 17 is optionally supplemented with fresh purge gas. Alternatively, purge gas recycle line 1 17 is not supplemented with fresh purge gas, i.e., the purge gas consists or consists essentially of the recovered, heavy VOC depleted purge gas in purge gas recycle line 117. Preferably, the major components of the recovered purge gas in purge gas recycle line 117 are nitrogen and/or light VOCs, e.g., hydrocarbons having 3 or fewer carbon atoms, more preferably nitrogen and/or ethylene. Light VOCs may be separated and removed from the recovered purge gas in the bottom section of the purge bin 11 1. Additionally, some components of the used purge gas, particularly Ce VOCs, may be sent directly to a flare through flare line 1 19.

[0026] The purged polymer product exits the bottom of the purge bin 1 11 through polymer product discharge line 120. Optionally, the purged polymer product is fed through a screen 121 upon exit of the purge bin 1 11 , prior to being sent downstream for further processing via purged polymer product discharge line 122.

[0027] Optionally, a first analyzer may be adapted to measure the concentration of volatiles, particularly VOCs, in the purged polymer product. Often, the first analyzer may be an inline analyzer 123, providing real time data on the total concentration of volatiles (e.g., VOCs) in the vapor space as the purged polymer product flows through the screen. More preferably, the first analyzer is an offline analyzer. In such aspects, a sample of the purged polymer product may be taken from polymer sample 125 and sent to a laboratory for determination of the concentration in ppmw of the VOC species in the purged polymer product.

[0028] A second analyzer 124 may be adapted to measure the concentration of one or more VOC species in the cycle gas of the fluidized bed reactor. This second analyzer 124 is preferably an inline analyzer, providing real time data. Any suitable equipment may be used for the analyzers. For example, suitable inline analyzers may include a gas chromatograph or a mass spectrometer. A preferred suitable offline analyzer is a gas chromatograph.

[0029] Referring to FIG. 2, an exemplary recovery system 1 16 of the poly olefin production system of FIG. 1 is depicted. Stream 1 15 comprising purge gas and volatiles, e.g., VOCs, is compressed in compressor 126 to produce a compressed stream 127. Stream 127 is then condensed in condenser 128 to produce a condensed liquid stream 1 18 from FIG. 1 and a non-condensable gas stream 130. Stream 118, as described above, may be recycled to fluidized bed pressure vessel 102 (not shown). A portion of non-condensable gas from stream 130 may be separated to produce a second non-condensable gas stream 131. Stream 131 may be recycled and combined with the polymer product in product discharge system 109 (not shown). The remainder of stream 130 is passed across the surface of a first membrane 132. First membrane 132 is adapted to preferentially separate stream 130 to produce a first permeate stream 133 enriched in VOCs and depleted in purge gas and a first residual stream 134 depleted in VOCs and enriched in purge gas. Preferably, the first permeate stream 133 is combined with stream 115 and recycled to compressor 126. The first residual stream 134 is then generally passed across the surface of a second membrane 135. Second membrane 135 produces a second permeate stream 119 from FIG. 1, further enriched in VOCs and depleted in purge gas and a second residual stream 117 (supplying the purge gas recycle line 117 of FIG. 1) further enriched in purge gas and depleted in VOCs, particularly heavy VOCs, e.g., hydrocarbons comprising four, five, six, or more carbon atoms, more preferably hydrocarbons having six or more carbon atoms. As discussed above, the second permeate stream 119 may be further processed or sent to a flare (not shown).

VOC Recovery Methods

[0030] The methods of the present invention may be employed in the operating strategy of a polymerization process, such as that depicted in FIG. 1, to optimize various process parameters within operating equipment and VOC content constraints. Generally, VOC content constraints in a polymerization process include a maximum VOC content level in the polymer product, i.e., the granular residual VOC content. Typically, there is also a related maximum VOC content level, particularly a maximum C6+ VOC content level, in the second permeate stream 119. Preferably, the methods of the present invention enable maximizing polymer production rates, maximizing VOC recovery, and/or minimizing flaring within these constraints. The methods are particularly useful in the production of polyethylene in a fiuidized bed reactor.

I. Relationship Between X₀ / Xi and (S*P) / (G*H) or (S*P*M) / (G*H)

[0031] Preferably, the methods involve the determination of X₀ / Xi and (S*P) / (G*H) or (S*P*M) / (G*H) for one or more species of VOCs purged from the polymer product. The parameter X₀ is the concentration in ppmw of the VOC species in the discharged purged polymer product and Xi is the concentration in wt% of the VOC species solubilized in the polymer product within the reactor. Alternatively, the parameter Xi can be defined as the concentration in mol% of the VOC species in a gas phase of the reactor upstream of the purge bin. For example, in aspects wherein the reactor is a fiuidized bed reactor, Xi can be defined as the concentration in mol% of the VOC species in the gas phase at a point within the fluidized bed reactor. Additionally, in aspects wherein the reactor is a fluidized bed reactor with a cycle gas loop, Xi can be defined as the concentration in mol% of the VOC species in the cycle gas. The parameter S is the production rate in Klb polymer per hour from the reactor, while P is the absolute pressure in psia within the purge bin, M is the molecular weight of the purge gas, H is the Henry's law constant in psia / wt. frac. for the hydrocarbon being purged at purging temperature, and G is the mass flow rate in lb purge gas per hour in the purge bin. Where the purge gas is a different compound across product grades or systems, the methods generally comprise the determination (S*P*M) / (G*H) for one or more species of VOCs purged from the polymer product. Where the purge gas used is always the same molecular weight across product grades or systems (e.g. always nitrogen, ethylene, or predominantly nitrogen plus ethylene blends), the model may be simplified to exclude the parameter M. The methods further generally comprise determining a relationship between X₀ / Xi and (S*P) / (G*H) or (S*P*M) / (G*H) for one or more species of VOCs purged from the polymer product. In the presently disclosed methods, it has been found that X₀ / Xi is strongly correlated to (S*P) / (G*H) or (S*P*M) / (G*H).

[0032] Typically, the methods comprise determining a target value of X₀ / Xi, (S*P) / (G*H), and/or (S*P*M) / (G*H). Once the relationship between X₀ / Xi and (S*P) / (G*H) or (S*P*M) / (G*H) is understood for a given resin, at least one of the readily adjustable parameters S, P, M, or G can be adjusted to maintain the value of X₀ / Xi, (S*P) / (G*H), and/or (S*P*M) / (G*H) at or below its respective target value. For example, G, the mass flow rate of purge gas in the purge bin, can be increased in order to decrease X₀ / Xi and (S*P) / (G*H) or (S*P*M) / (G*H). Additionally or alternatively, S, the production rate of polymer in the reactor, and/or P, the absolute pressure in the purge bin, can be decreased in order to decrease X₀ / Xi and (S*P) / (G*H) or (S*P*M) / (G*H).

[0033] Generally, there is a maximum value of Xo, Xomax, at or below which the value of Xo must be maintained. The maximum value of X₀ is often imposed by environmental regulations. Generally, X₀max may range from a low of about 50 ppmw to a high of about 500 ppw, or from about 75 ppmw to about 200 ppmw, such as about 80 ppmw. Often, the methods further comprise determining a maximum value of X₀ / Xi, [Xo / XJmax, from X₀max and setting the target value of X₀ / Xi at or below [X₀ / XJmax. Additionally or alternatively, there is generally a maximum value of (S*P) / (G*H), [(S*P) / (G*H)]max, or (S*P*M) / (G*H), [(S*P*M) / (G*H)]max, at or below which the value of (S*P) / (G*H) or (S*P*M) / (G*H) must be maintained. Thus, the methods often further comprise determining the maximum value of (S*P) / (G*H), [(S*P) / (G*H)]max, or the maximum value of (S*P*M) / (G*H), [(S*P*M) / (G*H)]max, and setting the target value of (S*P) / (G*H) at or below [(S*P) / (G*H)]max or setting the target value of (S*P*M) / (G*H) at or below [(S*P*M) / (G*H)]max.

[0034] Particularly preferably, at least one of the parameters S, P, or G is adjusted to maximize the value of (S*P) / (G*H) or (S*P*M) / (G*H) while maintaining the value of (S*P) / (G*H) or (S*P*M) / (G*H) at or below its target value, preferably [(S*P* / (G*H)]max or [(S*P*M) / (G*H)]max. Additionally or alternatively, at least one of the parameters S, P, or G is adjusted to maximize the value of (S*P) / (G*H) or (S*P*M) / (G*H) while maintaining the value of X₀ / Xi at or below its target value, preferably [X₀ / Xi]max. Additionally, the mass flow rate of purge gas, G, is preferably maintained below a minimum fluidization velocity for the purge bin, which may be readily determined using any suitable method. Ideally, the value of S, i.e., the production rate of polymer in the reactor, is maximized while maintaining the value the (S*P) / (G*H) or (S*P*M) / (G*H) at or below its target value, preferably [(S*P) / (G*H)]max or [(S*P*M) / (G*H)]max, and while maintaining the value of X₀ / Xi at or below its target value, preferably [X₀ / XJmax.

II. Relationship Between C_v and Sai

[0035] As depicted in FIG. 2, often, at least a portion of the first membrane permeate stream is recycled to the compressor. In such aspects, the present methods further comprise determining C_v and Sai, wherein C_v is the volumetric feed rate in actual m /hr (am /hr) of the compressor and Sai is the area in m² of the surface of the first membrane. The methods further generally comprise determining a relationship between C_v and Sai. In the presently disclosed methods, it has been found that C_v is strongly correlated to Sai.

[0036] Typically, the methods comprise determining a target value of C_v. Preferably, once the relationship between Cv and Sai is understood, the surface area of the first membrane can be adjusted to maintain the value of C_v at or below its target value. For example, Sai can be decreased in order to decrease C_v.

[0037] Generally, the volumetric feed rate to the compressor must be held at or below a maximum value of C_v, CVmax, which generally corresponds to the maximum capacity rated by the equipment manufacturer. Generally, CVmax may range from a low of about 5,000 am /hr to a high of about 20,000 am /hr, or from about 7,500 am /hr to about 15,000 am /hr, such as about 10,000 am /hr. Thus, the methods often further comprise determining a maximum value of C_v, CVmax, and setting the target value of C_v at or below CVmax.

[0038] Particularly preferably, the value of Sai is maximized while maintaining the value of Cv at or below its target value, preferably Cvmax. Generally, maximizing the value of Sai results in a desirable maximization of the amount of VOCs recovered in the polymerization process.

III. Relationship Between X_P and S_a2

[0039] As depicted in FIG. 2, often, at least a portion of the second residual stream may be recovered and recycled to the purge bin. In such aspects, the present methods generally further comprise determining X_p and Sa2, wherein X_p is the concentration in mol% of C6+ VOC species in the second residual stream and Sa2 is the area in m² of the surface of the second membrane. The methods further generally comprise determining a relationship between X_p and Sa2. In the presently disclosed methods, it has been found that X_P is strongly correlated to Sa2.

[0040] Typically, the methods comprise determining a target value of X_P. Preferably, once the relationship between X_p and Sa2 is understood, the surface area of the second membrane can be adjusted to maintain the value of X_P at or below its target value. For example, Sa2 can be increased in order to decrease X_P.

[0041] Generally, there is a maximum value of X_P, X_Pmax, at or below which the value of X_p must be maintained. The maximum value of X_P is generally due to the limiting effect by heavier VOC species in the recovered purge gas present in the second residual stream on purging efficiency. For example, in a process to produce a polyethylene copolymer with a hexene comonomer, purging efficiency will often be limited by notable amounts of the heavier Cs and Ce inerts and non-inerts in the recovered purge gas, including alkanes, alkenes, alcohols, and other species. Generally, X_Pmax may range from a low of about 10 ppmw to a high of about 100 ppmw, or from about 25 ppmw to about 75 ppmw, such as about 50 ppmw. Thus, the methods often further comprise determining the maximum value of Xp, X_Pmax, and setting the target value of X_P at or below X_Pmax.

[0042] Particularly preferably, the value of Sa2 is minimized while maintaining the value of X_P at or below its target value, preferably X_Pmax. Generally, minimizing the value of Sa2 results in a desirable minimization of the amount of purge gas flaring from the polymerization process. [0043] The model(s) of the present disclosure can be applied to the recovery of VOCs from purged polymer products, including polyethylene copolymers, produced in a fluidized bed reactor. While specific units of measure are included herein for consistency and convenience, one of ordinary skill in the art would readily recognize that the determined process parameters, e.g., X₀ / Xi and (S*P) / G, and other calculations disclosed herein could be determined using other units of measure within the scope of the invention as claimed.

[0044] All documents described herein are incorporated by reference herein for purposes of all jurisdictions where such practice is allowed, including any priority documents and/or testing procedures to the extent they are not inconsistent with this text. As is apparent from the foregoing general description and the specific embodiments, while forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited thereby. For example, the compositions described herein may be free of any component, or composition not expressly recited or disclosed herein. Any method may lack any step not recited or disclosed herein. Likewise, the term "comprising" is considered synonymous with the term "including." And whenever a method, composition, element or group of elements is preceded with the transitional phrase "comprising," it is understood that we also contemplate the same composition or group of elements with transitional phrases "consisting essentially of," "consisting of," "selected from the group of consisting of," or "is" preceding the recitation of the composition, element, or elements and vice versa.

Claims

CLAIMS What is claimed is:

1. A method for recovering volatile organic compounds (VOCs) from a polymer product, the method comprising:

producing a polymer product in a reactor;

transferring the polymer product from the reactor to at least one purge bin;

injecting a purge gas into the at least one purge bin to create a purged polymer product and a purge bin vent stream including the purge gas and VOCs;

compressing the purge bin vent stream in a compressor;

cooling the compressed purge bin vent stream to create a condensed liquid stream and a non-condensable gas stream;

passing at least a portion of the non-condensable gas stream across the surface of a first membrane to create a first permeate stream enriched in VOCs and depleted in purge gas and a first residual stream depleted in VOCs and enriched in purge gas;

determining X₀ / Xi and (S*P) / (G*H) for one or more species of VOCs purged from the polymer product, wherein:

Xo is the concentration in ppmw of the VOC species in the discharged purged polymer product;

Xi is the concentration in wt% of the VOC species solubilized in the polymer product within the reactor;

S is the production rate in Klb polymer per hour of the reactor;

P is the absolute pressure in psia within the purge bin;

G is the mass flow rate in lb purge gas per hour in the purge bin; and

H is the Henry's law constant for the VOC species at the purging temperature in psia of the VOC species in the purge bin vent stream / wt% of the VOC species solubilized in the polymer product within the reactor;

determining a relationship between X₀ / Xi and (S*P) / (G*H) for one or more species of VOCs purged from the polymer product;

determining a target value of X₀ / Xi and/or (S*P) / (G*H); and adjusting at least one of the parameters S, P, or G to maintain the value of X₀ / Xi at or below the target value of X₀ / Xi and/or to maintain the value of (S*P) / (G*H) at or below the target value of (S*P) / (G*H).

2. The method of claim 1, further comprising determining a maximum value of X₀ / Xi, [Xo / Xi]max, and setting the target value of X₀ / Xi at or below [X₀ / XJmax.

3. The method of claim 1 or 2, further comprising:

determining (S*P*M) / (G*H) for one or more species of VOCs purged from the polymer product, where S, P, and G are as described in claim 1 and M is the molecular weight of the purge gas in lb/lb-mol;

determining a relationship between X₀ / Xi and (S*P*M) / (G*H) for one or more species of VOCs purged from the polymer product;

determining a target value of (S*P*M) / (G*H); and

adjusting at least one of the parameters S, P, M, or G to maintain the value of X₀ / Xi at or below the target value of Xo / Xi and/or to maintain the value of (S*P*M) / (G*H) at or below the target value of (S*P*M) / (G*H).

4. The method of any one of claims 1 to 3, further comprising:

i) determining a maximum value of (S*P) / G, [(S*P) / G]max, and setting the target value of (S*P) / (G*H) at or below [(S*P) / (G*H)]max; and/or

ii) determining a maximum value of (S*P*M) / (G*H), [(S*P*M) / (G*H)]max, and setting the target value of (S*P*M) / (G*H) at or below [(S*P*M) / (G*H)]max.

5. The method of any one of claims 1 to 4, further comprising maximizing the value of (S*P) / (G*H) and/or (S*P*M) / (G*H).

6. The method of any one of claims 1 to 5, further comprising recycling at least a portion of the first permeate stream to the compressor.

7. The method of claim 6, further comprising:

determining C_v and Sai, wherein:

Cv is the volumetric capacity (in actual m /hr) of the compressor; and

Sai is the area in m² of the surface of the first membrane;

determining a relationship between C_v and Sai;

determining a target value of C_v; and

adjusting Sai to maintain the value of C_v at or below the target value of C_v.

8. The method of claim 7, further comprising determining a maximum value of C_v, CVmax, and setting the target value of C_v at or below C_vmax.

9. The method of claim 7 or 8, further comprising maximizing the value of Sai.

10. The method of any of claims 1 to 9, further comprising passing the first residual stream across the surface of a second membrane to create a second permeate stream further enriched in VOCs and depleted in purge gas and a second residual stream further depleted in VOCs and enriched in purge gas.

11. The method claim 10, further comprising recycling at least a portion of the second residual stream to the purge bin.

12. The method of claim 11, wherein the purge gas consists of the recycled second residual stream.

13. The method of claim 11 or 12, further comprising:

determining X_p and Sa2, wherein:

Xp is the concentration in mol% of C6+ VOC species in the second residual stream; and

Sa2 is the area in m² of the surface of the second membrane;

determining a relationship between X_p and Sa2;

determining a target value of X_P; and

adjusting Sa2 to maintain the value of X_P at or below the target value of X_p.

14. The method of claim 13, further comprising determining a maximum value of X_P, X_Pmax, and setting the target value of X_P at or below X_Pmax.

15. The method of claim 13 or 14, further comprising minimizing the value of Sa2.

16. The method of any one of claims 10 to 15, further comprising flaring at least a portion of the second permeate stream.

17. A method for recovering volatile organic compounds (VOCs) from a polymer product, the method comprising:

producing a polymer product in a reactor;

transferring the polymer product from the reactor to at least one purge bin;

compressing the purge bin vent stream in a compressor;

recycling at least a portion of the first permeate stream to the compressor;

determining Cv and Sai, wherein:

Cv is the volumetric capacity (in actual m /hr) of the compressor; and

Sai is the area in m² of the surface of the first membrane;

determining a relationship between C_v and Sai;

determining a target value of Cv; and

adjusting Sai to maintain the value of C_v at or below the target value of C_v.

18. The method of claim 17, further comprising:

passing the first residual stream across the surface of a second membrane to create a second permeate stream further enriched in VOCs and depleted in purge gas and a second residual stream further depleted in VOCs and enriched in purge gas;

recycling at least a portion of the second residual stream to the purge bin;

determining X_p and Sa2, wherein:

X_P is the concentration in mol% of C6+ VOC species in the second residual stream; and

Sa2 is the area in m² of the surface of the second membrane;

determining a relationship between X_p and Sa2;

determining a target value of X_P; and

adjusting Sa2 to maintain the value of X_P at or below the target value of X_p.

19. The method of any one of claims 1 to 18, further comprising separating a portion of non-condensable gas from the non-condensable gas stream and combining the separated portion with the polymer product.

20. The method of any one of claims 1 to 19, further comprising recycling the condensed liquid stream to the reactor.

21. The method of any of claims 1 to 20, further comprising selecting the one or more VOC species to include one or more compounds from the group consisting of 1 -hexene, 2,3- dimethyl-l-butene, 2,3-dimethyl-2-butene, 2-ethyl-l -butene, 2-methyl-l-pentene, 2-methyl- 2-pentene, 3,3-dimethyl-l -butene, 3-methyl-l -pentene, 3-methyl-cis-2-pentene, 3-methyl- trans-2-pentene, 4-methyl-l -pentene, 4-methyl-cis-2-pentene, 4-methyl-trans-2-pentene, cis- 2-hexene, cis-3-hexene, cyclohexane, trans-2-hexene, trans-3-hexene, and hexane.

22. The method of any of claims 1 to 21 , wherein the purge gas comprises nitrogen, ethylene, or mixtures thereof.

23. The method of any of claims 1 to 22, wherein the reactor comprises a fluidized bed reactor.

24. The method of any one of claims 1 to 23, wherein the polymer product comprises polyethylene.

25. A system for recovering volatile organic compounds (VOCs) from a polymer product, the system comprising:

a reactor configured to produce a polymer product;

at least one purge bin in fluid communication with the reactor, the at least one purge bin having at least one inlet configured to receive at least a portion of the polymer product from the reactor, at least one inlet configured to receive purge gas, at least one outlet configured to remove a purged polymer product, and at least one outlet configured to remove a purge bin vent stream; a compressor in fluid communication with the at least one purge bin, wherein the compressor is configured to receive and compress at least a portion of the purge bin vent stream to create a compressed purge bin vent stream;

a condenser in fluid communication with the compressor, wherein the condenser is configured to receive and condense at least a portion of the compressed purge bin vent stream to create a condensed liquid stream and a non-condensable gas stream;

a first membrane in fluid communication with the condenser, wherein the first membrane is configured to receive and preferentially separate at least a portion of the non- condensable gas stream to create a first permeate stream enriched in VOCs and depleted in purge gas and a first residual stream depleted in VOCs and enriched in purge gas;

a second membrane in fluid communication with the first membrane, wherein the second membrane is configured to receive and preferentially separate at least a portion of the first residual stream to create a second permeate stream further enriched in VOCs and depleted in purge gas and a second residual stream further depleted VOCs and enriched in purge gas;

a first analyzer for measuring the concentration of at least one VOC species in the vapor space of the purged polymer product; and

a second analyzer for measuring the concentration of at least one VOC species in a gas phase of the reactor system upstream of the purge bin.