CA2421002C

CA2421002C - Membrane separation in a polymerization process

Info

Publication number: CA2421002C
Application number: CA2421002A
Authority: CA
Inventors: Samuel Chin Fu Mah; Patricio S. Herrera
Original assignee: Nova Chemicals Corp
Current assignee: Nova Chemicals Corp
Priority date: 2003-03-06
Filing date: 2003-03-06
Publication date: 2013-08-27
Anticipated expiration: 2023-03-06
Also published as: CA2421002A1

Abstract

In the present invention at least part of a liquid mixture comprising a mixture of alkane and alkenes and less than 10,000 ppm by weight of one or more polar components having a molecular weight less than 150 as a recycle stream from a solution polymerization process is contacted with a normally solid, semi-permeable, membrane at conditions which promote selective permeation of the polar components and the polar component reduced retentate is recycled as feed for the polymerization process.

Description

FIELD OF THE INVENTION
The present invention relates to membrane separation applied to recycle streams in polymerization processes. More particularly the present invention relates to the application of semi-permeable membrane separation to recycle streams in a solution polymerization process.
BACKGROUND OF THE INVENTION
Semi-permeable membrane separation has been known for a number of years. An early application of semi-permeable membrane separation was for desalination of water by reverse osmosis. Additionally, this technology has been used in the medical field for dialysis.
There is a significant amount of art relating to the formation of semi-permeable membranes. However, it does not disclose the specific application of the present invention.
The technology has been moving into other commercial fields.
The closest art to the present invention is United States Patent 4,623,704 issued November 18, 1986 to Dembicki et al., assigned to the Dow Chemical Company. The patent teaches passing a stream of gaseous ethane and ethylene from a solution polymerization through a semi permeable hollow fiber to remove ethylene from the feed and increase the concentration of ethylene in the permeate (i.e. alkene/alkane separation). The permeate is then recycled back to the polymerization reactor. The present invention is distinct over the reference in the following ways. The separation is not alkene/alkane separation but rather removal of polar species (i.e. water, oxygenates, CO) from a stream comprising primarily of either saturates (i.e. n-alkanes, isoparaffins or

2 Z \Trevor\TTResponse\9258canRDisclosurepages2and11 docx mixtures thereof), unsaturates (i.e. n-alkenes, branched alkenes), or mixtures of saturate/unsaturate. The feed to the semi-permeable membrane is liquid (i.e. the process is a pervaporation process ¨ liquid and vapour on opposite sides of the semi-permeable membrane). Further the membrane separates polar species (some high molecular weight polar species have been identified) from the feed so the retentate has a lower concentration of polar species.
United States Patent 5,034,134 issued July 23, 1991 to George et al., assigned to Union Carbide Chemicals and Plastics Technology Corporation discloses the use of semi-permeable membranes to separate impurities and/or additives from a liquid stream comprising ethylene glycol.
The patent does not teach separating polar impurities from a liquid stream of alkanes or alkenes.
The solution polymerization of alpha olefins, and particularly ethylene and C4_8 alpha olefins is known. After the polymerization the residual monomer(s) and solvent are separated from the polymer.
Additionally, in the separation and polymer work up various polar components such as water, alcohols and low molecular weight oxygenates may be generated or incorporated in the recycle streams. This leads to streams comprising one or more alkenes such as alpha olefins, one or more oxygenates such as alcohols. These streams may be treated using adsorption columns such as activated alumina and/or molecular sieves.
Acidic compounds such as alumina tend to isomerize alpha olefins (e.g. 1-octene may be isomerized to 2-octene). This may then lead to the need to M:\Trevor\TTSpec\9258can.doc 3 separate close boiling isomers which is expensive from both a capital and operating point of view.
The present invention seeks to provide a process to separate polar impurities from a liquid mixture comprising primarily of either saturates (i.e.
n-alkanes, isoparaffins or mixtures thereof), unsaturates (i.e. n-alkenes, branched alkenes), or mixtures of saturate/unsaturate by contacting such mixture with a semi-permeable membrane and extracting the polar impurities as permeate.
SUMMARY OF THE INVENTION
The present invention seeks to provide the improvement in a process for the solution polymerization of one or more C2-8 alpha olefins comprising:
(a) contacting at least part of a liquid mixture (Le. feed stream) comprising a mixture of alkane and alkenes and less than 10,000 ppm by weight of one or more polar components with a normally solid, semi-permeable, membrane at conditions which promote selective permeation of said one or more polar components having a molecular weight less than 150 through the membrane; and (b) recycling the polar component reduced alkane and alkene retentate as feed for the polymerization.
DETAILED DESCRIPTION
Typically in the solution polymerization of alpha olefins, and in particular ethylene with up to about 15 weight % one or more C4_8 alpha olefins as comonomers, the monomers are dissolved in a solvent. The solution of monomers is fed to a reactor, typically a stirred tank reactor MATrevonTTSpec\9258can.doc 4 (CSTR) at a pressure from about 4 to 40 MPa (about 5.8 x 102 psi to about 5.8 x 103 psi) and at a temperature from about 150 C to about 220 C in the presence of a catalyst (either Ziegler Natta or single site type catalyst).
Typically, the residence time in the reactor is short ranging for several seconds (e.g. not less than 5) to not more than 30 minutes. The resulting polymer solution is removed from the reactor and then in various orders (depending on the process):
the catalyst may be deactivated using a polar molecule (such as water or a low molecular weight alcohol);
(ii) the bulk of the solvent, residual monomer, and comonomer is removed from the solution typically by flashing;
(iii) the polymer is palletized;
(iv) the pelletized polymer may be subject to further stripping using methods such as steam or inert gas stripping.
The above process steps typically lead to one or more streams comprising a solvent, one or more residual monomers and one or more polar molecules.
The solvent is typically an alkane selected from the group comprising one or more C4-8 alkanes which may be straight chained or branched or cyclic. Some solvents include pentane, hexane, cyclohexane and mixed alkanes (e.g. a C4 cut, C8_9 cut such as lsopar E).
The residual monomer typically comprises one or more C2..8 alpha olefins. Typical alpha olefins include ethylene, propylene, butene, 4-methyl-1-pentene, hexene and octene.
MATrevor\TTSpec\9258can.doc 5 The weight polar molecules and or oxygenates typically are low molecular weight and have a molecular weight less than 150. Some of the polar molecules include water, carbon monoxide, carbon dioxide and lower order (i.e. C1_4) alcohols. Preferably the low molecular weight polar molecules have a molecular weight less than about 100. However, in some cases higher alcohols may be generated such as octanol. Typically, the polar molecules (impurities) are present in the mixed alkane/alkene stream (feed stream) in a total amount of not more than 5,000 ppm by weight (parts per million by weight), preferably less than 1,000 ppm by weight, most preferably less than 250 ppm by weight.
In the prior art the separation of the above components could have been accomplished in several manners. The components could have been separated using conventional distillation technology. However as some of the components are close boiling molecules (e.g. isomers of monomers) this would require a fairly significant distillation column. An alternate procedure is to pass the mixture through a fixed bed packed column containing an adsorbent such as alumina or a molecular sieve.
The alumina may cause some isomerization of alpha olefins. Packed bed adsorption is a semi-batch operation and requires two parallel trains, one in operation while the other in regeneration/stand-by mode in order to provide continuous service.
In accordance with the present invention a feed stream comprising a mixture of primarily either saturates (i.e. n-alkanes, isoparaffins or mixtures thereof), unsaturates (i.e. n-alkenes, branched alkenes), or MATrevor\TTSpec\9258can.doc 6 mixtures of saturate/unsaturate and polar molecules (preferably low molecular weight) is contacted with a semi-permeable membrane.
The semi-permeable membrane may be selected from the group consisting of:
(i) a zeolite or molecular sieve membrane;
(ii) a thin film of perfluorosulphonic acid polymer on a porous matrix of polytetrafluoroethylene. The protons may optionally be ion exchanged with a metal selected from the group consisting of sodium, potassium, calcium and silver;
(iii) a polymerized C28 alkene of the formula SO2N(R1)-AQX
where in R1 is a hydrogen atom or a C1_4 alkyl group, A is one or more C2_8 alkenes, Q is a quaternary ammonium compound and X is a negatively charged counter ion;
(iv) polyvinyl alcohol which is cross linked;
(V) C1-4 alkyl esters of polyvinyl alcohol;
(vi) polyacrylonitrile and polytetrafluoroethylene grafted with N-vinyl pyrolidone;
(vii) elastomeric membranes;
(viii) polyamides and polyimides;
(ix) polycarbonates; and (x) a C1.6 alkyl ester of cellulose.
The zeolite or molecular sieve membrane are generally inorganic compounds. The membrane may comprise a support of relatively larger particles and a surface layer of relatively smaller particles. The interstices between the smaller particles are the holes through which the polar MATrevorATTSpec\9258can.doc 7 molecules pass. Zeolites and molecular sieves are well known to those skilled in the art.
A class of perfluorosulphonic acid polymer is commercially sold under the trademark NAFION . It is an ion exchange resin with anionic sites. These negatively charged sites can be ion-exchanged with cations from the group consisting of sodium, potassium, calcium and silver.
The compounds of (iii) above are ion exchange resins.
Cross linked polyvinyl alcohol is prepared by hydrolysis of poly vinylacetate and subsequent cross linking with a difunctional agent as disclosed in U.S. Patent 4,798,674 issued January 17, 1989 to Pasternak et at., assigned to Texaco Inc. Some cross linking agents include dialdehydes, diacids, diacid halides, dihalogen compounds, epichlorohydrin and boric acid.
The lower alkyl (e.g. C1.6 preferably C1..4) esters of polyvinyl alcohol are suitable as semi-permeable membranes. A commercially available polymer is polyvinyl acetate.
Polyacrylonitrile (PAN) and polytetrafluoroethylene (e.g. TEFLON ) are known. These polymers may be grafted with N-vinyl pyrolidone to form a semi-permeable membrane.
Elastomeric membranes include various rubbers such as nitrile rubber (e.g. a copolymer of 60 to 40 weight % of one or more C4-5 conjugated diolefins (butadiene and isoprene, preferably butadiene) and from 40 to 60 weight % of acrylonitrile or methacrylonitrile (preferably acrylonitrile)), chlorosulphonated polyethylene, polyurethanes and styrene butadiene copolymers (e.g. copolymers of from 60 to 40 weight % of a MATrevor\TTSpec19258can.doc 8 C8_12 vinyl aromatic monomer (preferably styrene or alpha methyl styrene) and from 40 to 60 weight A3 of one or more C4.5 conjugated diolefins (e.g.
butadiene and isoprene, preferably butadiene).
Polyamides and polyimides are known as semi-permeable membranes.
Polycarbonates are well known polymers.
The C1_6 alkyl esters of cellulose may be the mono- di- or tri-esters of cellulose. The various acetate esters of cellulose are commercially available.
Generally the semi-permeable membrane is deposited upon a support membrane. Generally this type of structure (e.g. the semi-permeable membrane on a support) is referred to as an asymmetric semi-permeable membrane wherein the pores on one surface of the membrane have diameters several times smaller than the pores on the other surface of the membrane. The semi-permeable membrane typically has a pore size from about 1 to 15 microns, preferably about 3 to 8 microns, most preferably less than 5 microns. The membrane is usually deposited on a support layer. The membrane may have a thickness from about 0.5 to 15, typically about 1 to 5 microns thick.
All of the semi-permeable membranes having a polar functional group (e.g. acid or sulfonate group) may be treated with cations such as sodium, potassium, calcium and silver.
The support may be a rigid polymer in a porous form. That is the polymeric support needs to have sufficient porosity to permit the permeate to travel through the support and contact the semi permeable membrane.
M:\TrevotATTSpec\9258can.doc 9 The support may be a polymer selected from the group consisting of polyamides, polyimides, polysulphones and polycarbonates. The support may be from 5 to 50 microns thick or greater. The thickness of the support is relative to the mechanical strength required under the conditions of use and if required the support may be thicker.
The semi-permeable membrane may be in a number of configurations. The semi-permeable membrane could be in the form of a flat sheet or a plate, sometimes referred to frame and plate construction.
The semi-permeable membrane could be in the form of a spirally wound coil. The semi-permeable membrane could be in the form of a hollow fiber or a bundle of hollow fibers. The hollow fibers may have an inside diameter in the range from about 70 to about 130 microns and a wall thickness in the range from about 75 to about 110 microns. The semi-permeable membrane may be modular requiring several modules.
Generally the semi-permeable membrane needs to have a reasonable permeability for the low molecular weight polar molecules.
There are a number of different methods for measuring permeability. The semi-permeable membrane should have a permeability of at least 1, preferably greater than 10, most preferably greater than 100 when measured in units of mmol/h.m2 at a constant pressure drop across the membrane. The separation factor for the low molecular weight polar molecules relative to the stream of alkanes/alkenes should be at least about 2.2.
In a solution polymerization the stream of solvent and unreacted monomer(s) may be a single or combined stream of solvent and monomer MATrevor\TTSpec\9258can.doc 10 initially flashed off after the solution leaves the reactor and a stream generated by stripping the polymer. From a heat balance point of view the heat may be extracted from the stream. The stream is condensed to a liquid form and typically has a temperature from about 10 C to about 90 C, preferably from about 10 C to about 70 C, most preferably 10 C to 50 C.
The liquid is fed to one side of the semi-permeable membrane and the low molecular weight polar molecules are transported across the membrane.
The liquid may be at a pressure from about 100 to about 1,200 psi (6.89 x 102 kPa to 8.27 x 103 kPa). A vacuum may be applied to the downstream of the membrane. The vacuum may be as low as about 3 Torr (mm Hg ¨
about 400 pascals). In some cases it may be helpful to have a carrier stream or sweep gas pass over the downstream side of the semi permeable membrane (i.e. the permeate side of the membrane) to remove the permeate. Typically the sweep gas is an inert gas such as nitrogen and the like, or possibly air provided it doesn't harm the membrane.
Typically in the retentate the level of polar low molecular weight molecules should be reduced to less than 50, preferably less than 10 ppm by weight, most preferably less than 1 ppm by weight. The resulting retentate may then be returned to the polymerization process.
In some cases it may be desirable to treat the ethylene stream separately from the co monomer steam. If there is a flash zone after the reactor the ethylene and comonomer stream may come off together with some solvent. It may, in some cases, be desirable to do a primary separation typically by distillation to generate a stream rich in ethylene and a stream rich in comonomer (such as octene). Both streams may also Z \Trevor \TTResponse \9258canRDisclosurepages2and11 docx contain solvent for the process as well as small amounts of low molecular weight polar monomers.
MATrevor\TTSpec\9258can.doc 12

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for the solution polymerization of one or more C2-8 alpha olefins the improvement comprising.
(a) contacting at least part of a liquid mixture comprising a mixture of alkanes and alkenes and less than 10,000 ppm by weight of one or more polar components selected from the group consisting of water, carbon monoxide and C1-4 alcohols having a molecular weight of less than 150 with a solid, asymmetric, semi-permeable membrane consisting of a membrane of a semi-permeable component selected from the group consisting of (i) a zeolite or molecular sieve membrane, (ii) perfluorosulphonic acid polymers;
(iii) a compound of the formula SO2N(R1)-AQX wherein R1 is a hydrogen atom or a C1-4 alkyl group, A is a polymer of one or more C2-8 alkenes, Q is a quaternary ammonium compound and X is a negatively charged counter ion, (iv) polyvinyl alcohol which is crosslinked;
(V) C1-4 alkyl esters of polyvinyl alcohol, (vi) polyacrylonitrile and polytetrafluoroethylene grafted with N-vinyl pyrolidone;
(vii) elastomeric membranes; and (viii) polycarbonates;

deposited on a support membrane at conditions which promote selective permeation of said one or more polar components having a molecular weight of less than 150 through the membrane; and (b) recycling the alkanes and alkene retentate having said polar component reduced to an amount of not more than 1 ppm by weight as feed for the polymerization process.

2. The process according to claim 1, wherein the one or more polar components are present in said liquid mixture in an amount of less than 5,000 ppm by weight.

3. The process according to claim 2, wherein said liquid mixture contains one or more components selected from the group consisting of C4-9 alkanes and C2-8 alkenes.

4. The process according to claim 3, wherein the support membrane consists of a material selected from the group consisting of polyamides, polyimides, polysulphones and polycarbonates.

5. The process according to claim 4, wherein said one or more polar components have a molecular weight of less than 100.

6. The process according to claim 5, wherein the liquid mixture is at a pressure from 100 to 1,200 psi.

7. The process according to claim 6, wherein the temperature of the liquid mixture in contact with the membrane is in the range from 10°C to 90°C

8 The process according to claim 7, wherein the semi-permeable membrane has a separation factor for said one or more polar components relative to said mixture of said alkanes and alkenes of at least 2.2

9 The process according to claim 8, wherein the membrane comprises one or more bundles of hollow fibers, wherein each fiber has an inside diameter in the range from 70 to 130 microns and a wall thickness in the range from 75 to 110 microns

10. The process according to claim 9, wherein a sweep fluid is used on the permeate side of the membrane

11. The process according to claim 6, wherein the pressure on the permeate side is a vacuum of 3 Torr