Operation of multi-reactor polyolefin manufacturing process

Abstract

The invention relates to an improved process for manufacturing an olefin polymer composition, in particular polyethylene, that incorporates two or more reaction zones in an optimized configuration that ease product transitions and allows for improved reactor quality control.

1 . A process to manufacture a differentiated and/or non-differentiated olefin polymer comprising the steps of:
 (a) providing at least one olefin for continuous polymerization in a first reactor in the presence of a diluent, hydrogen and a catalyst in order to produce a slurry effluent comprising the diluent and solid particles of a first olefin polymer,   (b) introducing at least part of the slurry effluent from the first reactor comprising a suspension of particles of the first olefin polymer to a light gas removal system wherein the slurry is vaporized to remove light impurities and wherein overhead vapors are produced;   (c) recovering the overhead vapors from the light gas removal system and sending part of said recovered vapors to an analytical device capable of measuring light gas concentrations;   d) adjusting the levels of hydrogen and olefin being provided to the first reactor based on the measured light gas concentrations so as to maintain the desired properties of the first olefin polymer;   e) pumping part of the suspension of particles of the first olefin polymer to a second polymerization reactor zone where at least one second olefin is polymerized in order to form a second olefin polymer and to produce a second olefin polymer suspension comprising diluent and particles of an olefin polymer composition comprising both the first olefin polymer and the second olefin polymer; and   (f) withdrawing an olefin polymer suspension containing a differentiated or non-differentiated olefin polymer composition from the second polymerization reactor.   
     
     
         2 . A process according to  claim 1 , wherein the second olefin polymer suspension of step (e) additionally undergoes the following steps prior to the differentiated or non-differentiated olefin polymer composition being withdrawn:
 (i) sending the second olefin polymer suspension to one or more flash vessel(s) which operate at from about 50 to about 550 pound-force per square inch gauge less than the pressure within the second reactor zone, and wherein the pressure within at least one of said flash vessel(s) is less than 20 pound-force per square inch gauge to separate overhead vapors of hydrocarbon gases from the second olefin polymer suspension;   (ii) compressing the overhead hydrocarbon gases containing diluent, comonomer, and other reaction components from the overhead vapor to greater than 180 pound-force per square inch gauge;   (iii) sending at least part of the compressed hydrocarbon gases into one or more at least two fractionation columns operating at greater than 150 pound-force per square inch gauge to separate the components:   (iv) recycling at least the comonomer and diluent components back to one or more polymerization zones;   (v) sampling the recycled comonomer and diluent components and measuring their respective concentrations; and   (vi) adjusting one or more of the measured concentrations to control at least one or more conditions in the first reaction zone.   
     
     
         3 . A process according to  claim 2 , for the manufacture of a differentiated olefin polymer wherein the two fractionation columns in step iii are configured in series and a comonomer-rich diluent stream is separated out and recycled to a higher-molecular weight reactor zone and a comonomer-lean diluent stream is separated out and recycled to a lower-molecular weight reactor zone. 
     
     
         4 . A process according to  claim 3 , where the differentiated olefin polymer composition comprises a bimodal ethylene copolymer composition where from 30 to 70% of the total composition is a first block formed in the first reactor and said first block is a homopolymer or near homopolymer with a melt flow index MI2 of from about 5 to 1000 g/10 min, and where from 30 to 70% of the total composition is a second block formed in the second reactor and said second block is a copolymer of hexene and ethylene having a melt flow index MI5 of 0.01 to 2 g/10 min. 
     
     
         5 . A process according to  claim 2 , for the manufacture of a non-differentiated olefin polymer where the two fractionation columns are configured in parallel and the comonomer containing diluent stream is withdrawn from the base of each individual column and recycled to one or more polymerization reactors. 
     
     
         6 . A process according to  claim 2  wherein the polymerization reactors are transitioned from the production of a differentiated olefin polymer composition to production of a non-differentiated olefin polymer composition using the following steps:
 a) changing hydrogen flow to that required for a non-differentiated olefin polymer in the first reactor; 
 b) increasing pressure in the light-gas removal system from about 5 pound-force per square inch gauge per 20 minutes to about 20 pound-force per square inch gauge per 20 minutes; 
 c) simultaneously increasing the pressure within the second reactor to the same rate as that used for a non-differentiated product; 
 d) changing the hydrogen flow to that used for a non-differentiated olefin polymer in the second reactor; 
 e) changing the valve connections between the compressed recycle gas of step (iii) and the two fractionation columns in order to change the configuration of the columns from series to parallel; 
 f) adjusting comonomer flow rate in each reactor to that of non-differentiated product; and 
 g) adjusting ethylene flow rate to each reactor to achieve the ethylene concentration of the non-differentiated product within each reactor. 
 
     
     
         7 . A process according to  claim 6 , wherein the transition from differentiated olefin polymer composition to a non-differentiated olefin polymer composition is accomplished in from about 8 to about 12 hours. 
     
     
         8 . A process according to  claim 5  wherein the polymerization reactors are transitioned from the production of a non-differentiated olefin polymer composition to production of a differentiated olefin polymer composition using the following steps:
 a) adjusting ethylene flow rate to each reactor to achieve the ethylene concentration of the differentiated product within each reactor; 
 b) adjusting comonomer flow rate to each reactor to that of the differentiated product; 
 c) changing the valve connections between the compressed recycle gas of step (iii) and the two fractionation columns in order to change the configuration of the columns from parallel to series; 
 d) changing the hydrogen flow to that used for a differentiated olefin polymer in the second reactor; 
 e) decreasing the pressure in the light-gas removal system from about 5 pound-force per square inch gauge per 20 minutes to about 20 pound-force per square inch gauge per 20 minutes; 
 f) simultaneously decreasing the pressure within the second reactor at the same rate to that used for the differentiated product; and 
 g) changing the hydrogen flow to that required for a differentiated olefin polymer in the first reactor. 
 
     
     
         9 . A process according to  claim 8 , wherein the transition from non-differentiated olefin polymer composition to a differentiated olefin polymer composition is accomplished in from about 8 to about 12 hours. 
     
     
         10 . A process according to  claim 6 , further comprising the step of sampling the polymer product from each reactor at a frequency of from about 2 to about 4 hours before adjusting the ethylene or hydrogen flow rates. 
     
     
         11 . A process according to any prior  claim 2 , where the diluent is chosen from propane, isobutane, pentane, and hexane. 
     
     
         12 . A process according to  claim 11  where the diluent is isobutane. 
     
     
         13 . A process according to  claim 12  where the comonomer is hexene. 
     
     
         14 . A process according to  claim 1 , where from about 0.1% to about 50% of the recovered vapor in step (c) is sent to an in-line gas chromatograph analyzer where the concentration of at least one of the reaction components is measured and the feed flow of that reaction component to the first polymerization zone is adjusted based on the concentration measured. 
     
     
         15 . A process according to  claim 1 , further comprising the step of collecting a suspension of particles of the first polymer and sending from about 0.1% to about 20% of the suspension to a flake sampling system where a sample of the first polymer is removed, its properties measured and these measured properties are used to control at least one or more conditions in the first reaction zone. 
     
     
         16 . A process according to  claim 15 , where the flake sampling system comprises a sample filter vessel connected to the first reactor zone via an automated sequence of on-off valves, where the following steps are performed periodically:
 a) part of the slurry is collected in a volume between a first and second valves;   b) the second valve opens, sending the first polymer suspension to a sample filter that is vented to a pressure <20 psig;   c) a sample of polymer from the suspension is caught in the filter   d) diluent and other hydrocarbons present in the suspension are vaporized and removed   e) the sample filter vessel is flushed with inert gas and completely depressured to produce a degassed slurry and a degassed sample of polymer; and   f) the degassed sample of polymer is removed for analysis.   
     
     
         17 . A process according to  claim 1 , wherein the light gas removal system comprises:
 (1) a vessel having a vapor side and a non-vapor side and operating at an intermediate pressure of from about 50 pound-force per square inch gauge to about 300 pound-force per square inch gauge lower than the pressure of the first reaction zone where a concentrated suspension of particles of the first polymer is collected in the non-vapor side; and   2) a fractionation column having an overhead condenser, wherein the fractionation column is connected to the vapor side of the above vessel.   
     
     
         18 . A process according to  claim 17 , wherein the fractionation column consists of at least 3 stages. 
     
     
         19 . A process according to  claim 17 , wherein the fractionation column employs a spiral-flow overhead condenser. 
     
     
         20 . A process according to  claim 17  where the condenser is directly attached to the top of the fractionation column. 
     
     
         21 . A process according to  claim 17 , where part or all of the degassed slurry is pumped to the second polymerization reaction zone using one or more open-impeller centrifugal pumps. 
     
     
         22 . A process according to  claim 21 , where the total flow through the centrifugal pumps is controlled via the use of a recirculation line. 
     
     
         23 . A process according to  claim 21 , where the total head pressure produced by the series of at least two pumps is at least 250 psi. 
     
     
         24 . A process according to  claim 1 , where the withdrawn slurry effluent from the first reaction zone is sent to a concentrator where it is separated into two streams comprising:
 (1) a first separated stream comprising diluent (D) and fine particles of catalyst and/or polymer that is recycled to the first reaction zone under a controlled flow; and   (2) a concentrated suspension of particles of the first polymer that is sent on to the light gas removal system.   
     
     
         25 . A process according to  claim 1  where the catalyst used for polymerization is a catalyst of the Zeigler type. 
     
     
         26 . A process according to  claim 1  where a chromium oxide catalyst is used for polymerization. 
     
     
         27 . A process according to  claim 1  where a metallocene catalyst is used for polymerization. 
     
     
         28 . A process according to  claim 6  wherein the hydrogen flow rate in any reactor is changed from the original to the target flow rate required for desired hydrogen to olefin ratio set-point for the non-differentiated resin in less than one minute. 
     
     
         29 . A process according to  claim 8  wherein the hydrogen flow rate in any reactor is changed from the original to the target flow rate required for desired hydrogen to olefin ratio set-point for the differentiated resin in less than one minute. 
     
     
         30 . A process according to  claim 8 , further comprising the step of sampling the polymer product from each reactor at a frequency of from about 2 to about 4 hours before adjusting the ethylene or hydrogen flow rates.