Patent Publication Number: US-2017368498-A1

Title: Fluid separation process using membranes based on perfluorinated polymers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part .of U.S. application Serial No. 15/158,032, filed on May 18, 2016, which is a continuation of U.S. application Serial No. 14/330,714, filed on Jul. 14, 2014 and issued as U.S. Pat. No. 9,403,120 on Aug. 2, 2016, which is a continuation of 14/184,308, filed on Feb. 19, 2014 and issued as U.S. Pat. No. 8,828,121on Sep. 9, 2014, all of which are hereby incorporated by reference in their entireties. 
    
    
     FIELD 
     The embodiments described herein relate to membrane-based fluid separation processes. In particular, the embodiments relate to fluid separation processes using copolymer membranes containing perfluorodioxolane monomers. 
     BACKGROUND 
     Presented below is background information on certain aspects of the present invention as they may relate to technical features referred to in the detailed description, but not necessarily described in detail. The discussion below should not be construed as an admission as to the relevance of the information to the claimed invention or the prior art effect of the material described, 
     The search for a membrane for use in fluid separation applications that combines high selectivity with high flux continues. Current perfluoropolymer membranes., such as those made from Hyflpn® AD (Solvay), Teflon® AP (Du Pont), Cvtop® (Asahi Glass), and variants thereof, have excellent chemical resistance and stability. We reported earlier, in U.S. Pat. No. 6,361,583, membranes that are made from glassy polymers or copolymers, including Hyflon® AD, and are characterized by having repeating units of a fluoridated, cyclic structure. In general, the ring structures in these materials frustrate polymer chain packing yielding amorphous polymers with relatively high gas permeability. These developed membranes are also more resistant to plasticization by hydrocarbons than prior art membranes and are able to recover from accidental exposure to liquid hydrocarbons. 
     It is known that copolymerization of fluorinated cyclic monomers with tetrafluoroethylene (TFE) enhances the chemical resistance and physical rigidity of membranes, TFE is also known to improve processability and has the effect of lowering gas permeability and increasing size selectivity in Hyflon® AD and Teflon® AF. Therefore, combinations of TFE with other monomer units, in particular perfluorinated dioxoles, such as Teflon® AF and Hyflon® AD, that result in overall amorphous, yet rigid, highly fluorinated, copolymers are preferred for industrial membrane applications. However, a drawback to these membranes is that their selectivities are relatively low for a number of fluid pairs of interest, including H2/CH4, He/CH 4 , CO 2 /CH 4 , and N 2 /CH 4 . 
     Other than the commercially available perfluoropolymers, there is very limited fluid transport data available for fully fluorinated polymers. Paul and Chio, “Gas permeation in a dry Nafion membrane,” Industrial &amp; Engineering Chemistry Research, 27, 2161-2164 (1988), examined gas transport in dry Nafion® (an ionic copolymer of TFE and sulfonated perfluorovinyl ether) and found relatively high permeabilities and selectivities for several gas pairs (He/CH 4 , He/H 2  and N 2 /CH4) compared to conventional hydrocarbon-based polymers considered for membrane applications, Nafion® and related ionic materials are used to make ion exchange membranes for electrochemical cells and the like. Because of their high cost and need for carefully controlled operating conditions, such as adjusting the relative humidity of the feed gas to prevent polymer swelling and loss of performance, these ionic membranes are not suitable tor industrial gas separations. 
     Despite the improvements described above, there remains a need for better fluid separation membranes., and specifically for improved membranes combining high flux, high selectivity, and good chemical resistance. 
     Recently, there have been reports of a new class of non-ionic amorphous perfluoropolymers, U.S. Pat. Nos. 7,582,714; 7,635,780; 7,754,901; and 8,168,808, all to Yoshtyuki Okamoto, disclose compositions and processes for making perfluoro-2-methylene-1,3-dioxolane derivatives. 
     Yang et a.l., “Novel Amorphous Perfluorocopolymeric System: Copolymers of Perfluoro-2-methylene-1,3-dioxolane Derivatives,” Journal of Polymer Science; Part A; Polymer Chemistry, Vol. 44, 1613-1618 (2006), and Okamoto et. al., “Synthesis and properties of amorphous perfluorinated polymers,” Chemistry Today, vol. 27, n. 4, pp. 46-48 (July-August 2009), disclose the copolymerization of two dioxolane derivatives, perfluorotetrahydro2-methylene-furo[3,4-d][1,3]dioxolane and perfluoro-2-methylene-4-methoxymethyl-1,3-dioxolane, The copolymers were found to be thermally stable, have low refractive indices, and high optical transparency from UV to near-infrared, making them ideal candidates for use in optical and electrical materials. 
     U.S. Pat. No. 3,308,107, to Du Pont, discloses a similar dioxolane derivative, perfluoro-2methylene4-methyl-1,3-dioxolane. Homopolymers and copolymers of perfluoro-2-methylene-4-methyl-1,3-dioxolane with TFE are also disclosed. 
     U.S. Pat. No. 5,051,114, also to Du Pont, discloses the testing of poly-[perfluoro-2-methylene-4-methyl-1,3-dioxolane] for use in a membrane for gas separation. The results indicated that this material, exhibited gas permeabilities 2.5 to 40 times lower as compared to dipolymer membranes of perfluoro-2,2-dimethyl-1,3dioxole and TFE, but had higher selectivities. 
     To date, however, there have been no studies using copolymers of the perfluoropolymers described by Yang et al. and Okamoto et al. in membranes for fluid separation processes. 
     BRIEF SUMMARY 
     Embodiments of the present disclosure relate to a process for separating components of a fluid mixture whereby the fluid mixture is passed across an improved separation membrane having a selective layer formed from a copolymer of perfluorodioxolane monomers. 
     In a basic embodiment, the present disclosure provides for a process for separating two components, A and B, of a fluid mixture having a ratio (Rf) of A:B, comprising:
         (a) passing the fluid mixture across a separation membrane having a feed. side, and a permeate side, the separation membrane having a selective layer comprising a copolymer comprising at least two perfluorodioxolane monomers;   (b) providing a driving force for transmembrane permeation;   (c) withdrawing from the permeate side a permeate stream having a ratio (Rp) of A:B, where Rp&gt;Rf; and   (d) withdrawing from the feed side a residue stream having a ratio (Rr) of A:B, where Rr&lt;Rf.       

     Membranes previously developed for fluid separation processes have incorporated the use of amorphous homopolymers of perfluorinated dioxoles, dioxolanes, or cyclic acid ethers, or copolymers of these with tetrafluoroethylene. However, the use of TFE results in membranes that lack high selectivities for components of a fluid mixture. 
     To address these performance issues, particularly preferred materials for the selective layer of the membrane used to carry out the process described herein are perfluorodioxolane monomers selected from the group consisting of the structures found in Table 1, below: 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Perfluorodioxolane Monomers 
               
               
                   
               
             
            
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
         
       
     
     An important advantage of the present process is that use of perfluorinated dioxolane copolymers in the membrane can result in higher selectivity for desired fluids than can be obtained using prior art membranes that incorporate TFE or cyclic perfluorinated homopolymers. 
     In another embodiment, the present disclosure relates to a process for separating two components, A and B, of a fluid mixture, comprising:
         (a) passing the fluid mixture across a separation membrane having a feed side and a permeate side, the separation membrane having a selective layer comprising a copolymer       

     comprising a perfluorodioxolane monomer having the formula 
     
       
         
         
             
             
         
       
         
         
           
             and a perfluoro-2-methylene-1,3-dioxolane-based derivative; 
           
         
       
    
       1 (b) providing a driving force for transmembrane permeation;
         (c) withdrawing from the permeate side a permeate stream having a ratio (Rp) of A:B, where Rp &gt; Rf; and   (d) withdrawing from the feed side a residue stream having a ratio (Rr) of A:B, where Rr&lt;Rf.       
     Representative membranes having particularly high selectivity are those formed from perfluoro-2-methylene-1,3-dioxolane and perfluoro-2-methylene-4,5-dimethyl-1,3-dioxolane. Thus, a most preferred copolymer is one having the structure; 
     
       
         
         
             
             
         
       
         
         
           
             where m and n are positive integers. 
           
         
       
    
     In certain embodiments, the fluid separation membrane used in the process as described herein has a selective layer comprising copolymers having the following structure: 
     
       
         
         
             
             
         
       
         
         
           
             where m and n are positive integers. 
           
         
       
    
     In certain aspects, the copolymer is a copolymer containing at least 25 mol % or greater of perfluoro-2-methylene-1,3dioxolane. 
     Due to their advantageous properties, the membranes and processes disclosed herein are useful for many fluid separation applications. Examples include, but are not limited to the separation of various gases, for example, nitrogen, helium, carbon dioxide, and hydrogen from methane. 
     The fluid mixture may contain at least two components, designated component A and component B, that are to be separated from each other and optionally another component or components in the stream. The permeating desired fluid may be either a valuable fluid that is desired to retrieve as an enriched product, or a contaminant that is desired to remove. Thus, either the permeate stream or the residue stream, or both, may be the useful products of the process. 
     In certain aspects, the present disclosure provides for a process for separating two components, A and B, of a gas mixture wherein component A is hydrogen and component B is methane. Such a mixture may be found in a steam reforming process. For example, the process of the invention may be used to recover hydrogen from synthesis gas, to remove carbon dioxide from synthesis gas, or to adjust the ratio of hydrogen to carbon monoxide in synthesis gas. 
     In certain aspects, the present disclosure provides for a process for separating two components, A and B, of a gas mixture wherein component A is carbon dioxide and component B is methane. This process may be involved in carbon capture and storage or used in the separation of CO 2  from natural gas. 
     In other aspects, the present disclosure provides for a process for separating two components, A and B, of a gas mixture wherein component A is nitrogen and component B is methane. This process may be involved in removing nitrogen from nitrogen-contaminated natural gas. 
     In yet another aspect, present disclosure provides for a process for separating two components, A and B, of a gas mixture wherein component A is helium and component B is methane. This process may be useful for producing helium through natural gas extraction and subsequent purification. 
     In other aspects, the present disclosure provides for a process for separating two components, A and B, of a fluid mixture wherein component A is water and component B is either an alcohol, ketone, ether, or ester. In certain aspects, component B may be a solvent, such, as ethanol, bioethanol, propanol, acetone and the like. This process may be useful in the dehydration of aqueous solvent mixtures. 
     In certain aspects, the present disclosure provides for a process for separating two components, A and B, of a fund mixture wherein component A is an unsaturated hydrocarbon compound and component B is a saturated hydrocarbon compound. Such fluid separation processes include, but are not limited to, olefin/paraffin separations, which may be useful in recovering unreacted olefins in petrochemical operations. 
     In a further aspect, the present disclosure provides for a process for separating two components, A and B, of a fluid, mixture wherein component A is an aromatic hydrocarbon compound and component B is an aliphatic hydrocarbon compound. In another aspect, the invention is a process for separating two components, A and B, of a fluid mixture wherein component A is a first aromatic hydrocarbon compound and component B is a second aromatic hydrocarbon compound. Both of these processes may be useful as a low-energy alternative to distillation in petrochemical and refinery operations. 
     In other aspects, the present disclosure provides for a process for separation two components, A and B, of a fluid mixture wherein component A is a linear hydrocarbon compound and component B is a branched hydrocarbon compound. This process may be useful in refinery operations to enhance the octane rating of gasoline, for example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph showing pure gas H 2  permeance and H 2 /CH 4  selectivity as a function of Monomer H content for membranes with selective layers composed of copolymers of Monomers D and H (Polymers 443, 444, and 445). 
         FIG. 2  is a graph showing H 2 /CH 4  selectivity as a function of H 2  permeance for membranes made from commercial perfluoropolymers (Cytop®, Hyflon® AD, and Teflon® AF) and Polymers 443, 444, and 445. 
         FIG. 3  is a graph showing N 2 /CH 4  selectivity as a function of N 2  permeance for membranes made from commercial perfluoropolymers (Cytop®, Hyflon® AD, and Teflon® AF) and Polymers 443, 444, and 445. 
         FIG. 4  is a graph showing He/CH 4  selectivity as a function of He permeance for membranes made from commercial perfluoropolymers (Cytop®, Hyflon® AD, and Teflon® AF) and Polymers 443, 444, and 445. 
         FIG. 5  is a graph showing CO 2 /CH 4  selectivity as a function of CO 2  permeance for membranes made from commercial perfluoropolymers (Cytop®, Hyflon® AD, and Teflon® AF) and Polymers 443, 444, and 445. 
     
    
    
     DETAILED DESCRIPTION 
     The term “fluid” as used herein means a gas, vapor, or liquid. 
     The term “fluid separation” as used herein refers to molecular separations that can be carried out in three different modes: (1) gas separation (membrane is in contact with a gas or vapor phase on both sides of the membrane), (2) hydraulic .permeation (membrane is in contact with a liquid or supercritical phase on both sides of the membrane), and (3) pervaporation (membrane is in contact with a liquid or supercritical phase on one side of the membrane and with a gas vapor phase on the other side of the membrane). The membrane materials described herein can be used in any one of the fluid separation modes. 
     The term “polymer” as used herein generally includes, but is not limited to, homopolymers, copolymers, such as for example, block, graft, random and alternating copolymers, terpolymers, etc, and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term “polymer” shall include all possible geometrical configurations of the material. These configurations include, but are not limited to, isotactic and atactic symmetries. 
     The term “copolymer” as used for simplicity herein refers to all polymers having at least two different monomer units, and thus includes, terpolymers and any other polymers having more than two different monomer units. 
     The term “highly fluorinated” as used herein means that at least 90% of the total number of halogen and hydrogen atoms attached to the polymer backbone and side chains are fluorine atoms. 
     The terms “fully-fluorinated” and “perfluorinated” as used herein are interchangeable and refer to a compound where all of the available hydrogen bonded to carbon have been replaced by fluorine. 
     The term “membrane” as used herein refers to a thin selective layer supported on an integral or discrete support, such as an integral asymmetric membrane or a composite membrane. The membrane generally has a selective layer thickness of less than 10 μm, and more specifically less than 5 μm. 
     All percentages herein are by volume unless otherwise stated. 
     The present disclosure provides for a process for separating two components, A and B, of a fluid mixture, the fluid mixture having a ratio (Rf) of A:B. The separation is carried out by running a stream of the fluid mixture across a membrane that is selective for the desired component to be separated from another component. The desired component to be separated into the permeate may be either Component A or Component B. The process results, therefore, in a permeate stream having a ratio (Rp) of A:B, where Rp&gt;Rf, and a residue stream having a ratio (Rr) of A:B, where Rr&lt;Rf. 
     Thus, in a basic embodiment, the process includes the following steps:
         (a) passing the fluid mixture across a separation membrane having a feed side and a permeate side, the separation membrane having a selective layer comprising a copolymer comprising at least two perfluorodioxolane monomers;   (b) providing a driving force for transmembrane permeation;   (c) withdrawing from the permeate side a permeate stream having a ratio (Rp) of A:B, where Rp&gt;Rf; and   (d) withdrawing from the feed side a residue stream having a ratio (Rr) of A:B, where Rr&lt;Rf.       

     At least the selective layer responsible for the fluid discriminating properties of the membrane is made from a glassy copolymer. The copolymer should be substantially amorphous. Crystalline polymers are typically essentially insoluble and thus render membrane making difficult, as well as exhibiting generally very low fluid permeabilities. Crystalline polymers are not normally suitable for the selective layer, therefore. 
     The selective layer copolymer should be fluorinated, and generally the degree of fluoridation should be high to increase the chemical inertness and resistance of the material. By high, we mean having a fluorine:carbon ratio of atoms in the polymer of at least 1:1 , and more preferably greater than. 1.5:1. Most preferably, the polymer is perfluorinated, even if the perfluorinated structure has less than a 1:1 fluorine:carbon ratio, 
     Various materials may be used for the copolymeric selective layer to meet the characterizing requirements. These include copolymers comprising perfluorinated dioxolane monomers. 
     The perfluorinated dioxolane monomers as described herein are characterized by a 1,3-dioxolane ring, having the general form: 
     
       
         
         
             
             
         
       
     
     Preferred monomers may be selected from perfluoro-2-methylene-1,3-dioxolane or derivatives thereof containing various substituent groups at the fourth and fifth positions of the dioxolane ring. Non-limiting examples of these derivative monomers are represented by the structures found in Table  1 , above. 
     None of the structures in Table 1 are new monomers in themselves. Generally, dioxolanes can be prepared by socialization of aldehydes and ketalization of ketones with ethylene glycol. Formulations embracing those suitable for use in the invention, are described in, U.S. Pat. Nos. 3,308,107; 5,051,114; 7,754,901; 7,635,780; and 8,168,808, incorporated herein by reference. The homopolymers of the monomers in Table 1 may be prepared by direct fluorination of hydrocarbon precursors and polymerized using perfluoro dibenzoyl peroxide as a free radical initiator to yield a linear polymer. The resulting polymers are soluble, in fluorinated solvents, such as hexafluorobenzene, perfluoro-hexane, and fluorinated FC43 (3M™). Copolymerization of the monomers in Table 1 may also be carried out in bulk and in a hexafluorobenzene solution using perfluoro dibenzoyl peroxide. 
     In a preferred embodiment, the selective layer comprises a copolymer of the perfluorodioxolane monomers found in Table 1. Thus, the separation membrane may have a selective layer comprising a copolymer formed from a first perfluorodioxolane monomer and a second, different perfluorodioxolane monomer. Any combination of perfluorodioxolane monomers found in Table  1  may be used. 
     A homopolymer of perfluoro-2-methylene-1,3-dioxolane (Monomer H) is crystalline in nature, which was confirmed by Mike{hacek over (s)} et al., “Characterization and Properties of Semicrystalline and Amorphous Perfluoropolymer: poly(perfluoro-2-methylene-1,3-dioxolane),” Polymers for Advanced Technologies, v. 22, pp. 1272-1277 (2011). This crystallinity reflects the ability of the repeat unit in the homopolymer of Monomer H to pack-tightly, forming ordered structures. As a result, Monomer H does not dissolve in fluorinated solvents. However, as described herein, copolymerizing Monomer H with another dioxolane monomer from Table 1 in the appropriate amounts results in an amorphous structure, which is desirable for fluid separation membrane materials. 
     In other embodiments, the copolymer may comprise more than two perfluorodioxolane monomers. 
     Preferably, in some embodiments, the separation membrane has a selective layer comprising a copolymer formed from a first perfluorodioxolane monomer having the formula 
     
       
         
         
             
             
         
       
     
     and a second perfluorodioxolane monomer selected from Table 1, wherein the second perfluorodioxolane monomer is not Monomer B. 
     In one embodiment, the separation membrane has a selective layer comprising a copolymer formed from a first perfluorodioxolane monomer having the formula 
     
       
         
         
             
             
         
       
     
     and a second perfluorodioxolane monomer having the formula 
     
       
         
         
             
             
         
       
         
         
           
             where R and R′ are fluorine and/or alkylfluoro groups. 
           
         
       
    
     Unlike Monomer H, Monomer D is more bulky and frustrates polymer chain packing, yielding a selective layer with higher free volume and higher gas permeability. Thus, in a most preferred embodiment, the copolymer comprises monomers of perfluoro-2-methylene-1,3-dioxolane (Monomer H) and perfluoro-2-methylene-4,5dimethyl-1,3,-dioxolane (Monomer D). 
     When any pair of monomers is used, one will tend to be more densely packed and perhaps crystalline than the other, and the respective proportions of the two monomers will alter the membrane properties. As a representative, non-limiting example,  FIG. 1  shows the effect of the ratio of Monomers D and H on the performance of the resulting fluid separation membrane. An appropriate amount of Monomer D (or another monomer from Table 1 other than Monomer H) is required to yield an amorphous copolymer with Monomer H. However, an appropriate amount of Monomer H is required to yield copolymers with high selectivity; too much of Monomer D (or another monomer from Table 1 other than Monomer H) results in an amorphous copolymer with relatively low fluid selectivity. 
     Within the range of amorphous copolymers of D and H, there is a trade-off between permeance and selectivity. Relatively large proportions of D increase permeance at the expense of selectivity, and relatively large proportions of H increase selectivity at the expense of permeance. A preferred proportion of Monomer H is at least 25 mol %, more preferably at least 40 mol %, and most preferable at least 55 mol %. 
     Thus, the preferred copolymer has just enough of Monomer D, or in a different embodiment, another monomer selected from Table 1 other than Monomer H, such as Monomer B, to give an amorphous copolymer, but retains enough of Monomer H to yield high gas selectivity. 
     With the perfluoropolymers described herein, the bonding of the monomers occurs outside the main dioxolane ring. This process is different than dioxole polymerization, which polymerize by the opening of a double bond within a five-member ring. 
     Copolymerization of the perfluoromonomers is represented by the following exemplary formula: 
     
       
         
         
             
             
         
       
     
     where m and n are positive integers. 
     In a preferred embodiment, the copolymer is an ideal random copolymer. 
     In yet another embodiment, the selective layer of the separation membrane may comprise a copolymer formed from a perfluorodioxolane monomer selected from the group consisting of the structures found in Table 1 and a perfluorodioxole monomer, such as Teflon® AF and Hyflon® AD, or a polyperfluoro (alkersyl vinyl ether) monomer, such as Cytop®. 
     In yet a further embodiment, the selective layer of the fluid separation membrane may comprise a terpolymer formed from at least two perfluorodioxolane monomers, such as those structures found in Table 1, and a fluorinated or perfluorinated fluorovinyl monomer. Non-limiting examples of such fluorovinyl monomers may include trifluoroethylene, tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), perfluoro methyl vinyl ether (PFMVE), perfluoroethyl vinyl ether (PFEVE), perfluoropropyl vinyl ether (PFPVE), vinyl fluoride (VF), vinylidene fluoride (VDF), and perfluoromethoxy vinyl ether (PFMOVE). 
     The copolymer chosen for the selective layer can be used to form films or membranes by any convenient technique known in the art, and may take diverse forms. Because the polymers are glassy and rigid, an unsupported film, tube or fiber of the polymer may be usable in principle as a single-layer membrane. However, such single-layer films will normally be too thick to yield acceptable transmembrane flux, and in practice, the separation membrane usually comprises a very thin selective layer that forms part of a thicker structure. This may be, for example, an integral asymmetric membrane, comprising a dense skin region that forms the selective layer and a microporous support region. Such membranes were originally developed by Loeb and Sourirajan, and their preparation in flat sheet or hollow fiber form is now conventional in the art and is described, for example, in U.S. Pat. Nos. 3,133,132. to Loeb, and 4,230,463 to Henis and Tripodi. 
     As a further, and a preferred, alternative, the membrane may be a composite membrane, that is, a membrane having multiple layers. Modem composite membranes typically comprise a highly permeable but relatively non-selective support membrane, which provides mechanical strength, coated with a thin selective layer of another material that is primarily responsible for the separation properties. Typically, but not necessarily, such a composite membrane is made by solution-casting the support membrane, then solution-coating the selective layer. General preparation techniques for making composite membranes of this type are well known, and are described, for example, in U.S. Pat. No. 4,243,701 to Riley et al., incorporated herein by reference. 
     Again, the membrane may take flat-sheet, tube or hollow-fiber form. The most preferred support membranes are those with an asymmetric structure, which provides a smooth, comparatively dense surface on which to coat the selective layer. Support membranes are themselves frequently cast onto a backing web of paper or fabric. As an alternative to coating onto a support membrane, it is also possible to make a composite membrane by solution-casting the polymer directly onto a non-removable backing web, as mentioned above. In hollow-fiber form, multilayer composite membranes may be made by a coating procedure as taught, for example, in U.S. Pat. Nos. 4,863,761; 5,242,636; and 5,156,888, or by using a double-capillary spinneret of the type taught in U.S. Pat. Nos. 5,141,642 and 5,318,417. 
     A gutter layer may optionally be used between the support membrane and the selective layer, for example to smooth the support, surface and channel fluid to the support membrane pores. In this ease, the support membrane is first coated with the gutter layer, then with the perfluoro selective layer as described herein. 
     Multiple selective layers may also be used, 
     The thickness of the selective layer or skin of the membranes can be chosen according to the proposed use, but will generally be no thicker than 5 pro, and .typically no thicker than 1 μm. It is preferred that the selective layer be sufficiently thin that the membrane provide a pressure-normalized hydrogen flux, as measured with pure hydrogen gas at 25°C., of at least about 100GPU (where 1 GPU= 1 × 10   −6  cm 3 (STP)/cm 2 ·s·cmHg), more preferably at least about 200 GPU and most preferably at least about 400 GPU. In a preferred embodiment, the selective layer thickness is no greater than about 0.5 μm, and most preferably between about 0.3 μm and 0.5μm. 
     Once formed, the membranes exhibit a combination of good mechanical properties, thermal stability, and high chemical resistance. The fluorocarbon polymers that form the selective layer are typically insoluble except in perfluorinated solvents and are resistant to acids, alkalis, oils, low-molecular-weight esters, ethers and ketones, aliphatic and aromatic hydrocarbons, arid oxidizing agents, making them suitable for use not only in the presence of C3+ hydrocarbons, but in many other hostile environments. 
     The membranes of the invention may be prepared in any known membrane form and housed in any convenient type of housing and separation unit. We prefer to prepare the membranes in flat-sheet form and to house them in spiral-wound modules. However, flat-sheet membranes may also be mounted in plate-and-frame modules or in any other way. If the membranes are prepared in the form of hollow fibers or tubes, they may be potted in cylindrical housings or otherwise. 
     The membrane separation unit comprises one or more membrane modules. The number of membrane modules required will vary according to the volume of gas to be treated, the composition of the feed gas, the desired compositions of the permeate and residue streams, the operating pressure of the system, and the available membrane area per module. Systems may contain as few as one membrane module or as many as several hundred or more, The modules may be housed individually in pressure vessels or multiple elements may be mounted together in a sealed housing of appropriate diameter and length. 
     In some cases, of particular importance, the membranes and processes described herein are useful in applications for producing hydrogen or chemicals from hydrocarbon feedstocks, such as reforming or gasification processes followed by separation or chemical synthesis. Steam reforming is well known in the chemical processing arts, and involves the formation of various gas mixtures commonly known as synthesis gas or syngas from a light hydrocarbon feedstock, steam and optionally other gases, such as air, oxygen or nitrogen. Synthesis gas usually contains at least hydrogen, carbon dioxide, carbon monoxide and methane, but the exact composition can be varied depending on its intended use. 
     Plant design and process operating conditions thus differ in their details, but the steam reforming process always includes a basic steam/hydrocarbon reforming reaction step, carried out at high temperature and elevated pressure, and one or more subsequent treatments of the raw synthesis gas to remove carbon dioxide or make other adjustments to the gas composition. The processes of the invention are expected to be especially useful in carrying out such treatments. 
     In another aspect, the disclosure provides for a process for separating carbon dioxide from methane, especially if the mixture also contains C 3+  hydrocarbon vapors. Such a mixture might be encountered during the processing of natural gas, of associated gas from oil wells, or of certain petrochemical streams, for example. The processes of the invention are expected to be useful as part of the gas treatment train, either in the field or at a gas processing plant, for example. 
     In another aspect, the disclosure provides for a process for recovering helium from natural gas. Helium is a rare gas on Earth. Almost all of the commercial helium requirements are supplied by extraction from helium-containing natural gas by low temperature fractional distillation processes. The resulting helium rich gases are further purified or refined using additional cryogenic distillation steps or by pressure swing adsorption (PSA) processes which selectively remove other gases. These final refining steps result in commercial grades of helium in excess of 99.9%, The processes of the invention are expected to be useful in replacing or supplementing one or more of the unit operations in the helium recovery plant. 
     In yet another aspect, the disclosure provides for a process for separating nitrogen from natural gas. The goal will often be to reduce the nitrogen content of the natural gas to no more than about 4% nitrogen, which is an acceptable total inerts value for pipeline gas. In other circumstances, a higher or lower nitrogen target value, may be required. Once again, the processes of the invention are expected to be useful in field or plant equipment as stand alone or supplementary units to meet the desired nitrogen concentration target. 
     Additionally, in another aspect, the disclosure provides for a process for separating oxygen from nitrogen. Oxygen is used to enhance the combustion of all fuels, enabling improved burning zone control, and lowering emissions. The present process is expected to yield enriched oxygen that can be used advantageously in combustion processes, such as kilns, or when using low-grade fuels, where reduction in ballast nitrogen is beneficial. 
     In a further aspect, the disclosure provides for a process for the dehydration of aqueous solvent mixtures, The aqueous solvent mixture may include an alcohol, such as ethanol, bioethanol produced from natural sources, and propanol; or other solvents, such as acetone and the like. A major drawback to more economical use of bioethanol as a fuel is the energy used to grow the feedstock, to ferment it, and to separate a dry ethanol product from the fermentation broth, The present process is expected to be useful in lowering the energy costs associated with ethanol purification (dehydration). 
     In other aspects, the disclosure provides for a process for separating unsaturated hydrocarbon compounds from saturated hydrocarbon compounds. This type of process typically occurs in petrochemical operations and includes separations such as olefins from paraffins, such as propylene from propane, n-butene or isobutene from n-butanol or isobutanol, and styrene from ethylbenzene. 
     In certain aspects, the disclosure provides for a process for separating an aromatic hydrocarbon compound from an aliphatic hydrocarbon compound. Examples of such separations include the separation of benzene, toluene and xylene from octane, heptane, methylcyclohexane, and cyclohexane. Benzene, toluene, and xylene are feedstocks for nine of the top 50 chemicals produced in the United States and are produced at a rate of about 36 million tons/year. Thus, an energy savings of even 1,000 Btu/kg would save about 36 trillion Btu/year, 
     In another aspect, the disclosure provides for a process for separating a first aromatic-hydrocarbon compound from a second aromatic hydrocarbon compound. These separations include for example, benzene/ethylbenzene, benzene/toluene, and ethylbenzene/styrene. Distillation of such mixtures consumes about 80 trillion Btu/year of energy in the United States. The processes of the present invention are expected to result in a potential savings of about 20-50 trillion Btu/year. 
     The present process can also be used in refinery operations to enhance the octane rating of the gasoline pool by separating linear compounds from branched compounds. For example, n-butane, n-pentane, n-hexane, and n-heptane may be separated from 2,3-dimethylbutanol, iso-octane, 2,2-dimethyl butanol, iso-pentane, and iso-butane. 
     The process provided herein is now illustrated in further detail by specific examples. These examples are intended to further clarify the invention, and are not intended to limit the scope in any way. 
     EXAMPLES 
     Example 1: Membrane Preparation 
     Composite membranes were prepared using homopolymer and copolymer solutions prepared from the monomers found in fable 2. For Polymers 443-445, different compositions (mol %) of Monomers D and H were used. 
     The perfluoro selective layers were coated onto support membranes, either on a small coater or by hand coating, and the membranes were finished by oven drying. Samples of each finished composite membrane were then cut into 13.8 cm 2  stamps. 
     Example 2: Pure-Gas Testing of the Perfluoro Composite Membranes 
     The membranes were dried in order to remove any residual solvents and then tested in a permeation test-cell apparatus with pure gases at room temperature, 50 psig feed pressure, and 0psig permeate pressure. The gas fluxes of the membranes were measured, and the permeances and selectivities were calculated. 
     For comparative purposes, tests were also run with membranes having selective layers made from several formulations of Hyflon® AD, Cytop®, and Teflon®AF. 
     The results for the different homopolymers and copolymers tested are shown in Table 2, below: 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Pure-Gas Permeation Results 
               
            
           
           
               
               
               
               
            
               
                   
                 Monomer 
                 Pure-Gas Permeance (GPU) 
                 Pure-Gas Selectivity 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Sample 
                 Composition 
                 N 2   
                 H 2   
                 He 
                 CO 2   
                 N 2 /CH 4   
                 H 2 /CH 4   
                 He/CH 4   
                 CO 2 /CH 4   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Polymer 
                 43 mol % D/ 
                 28 
                 695 
                 1,370 
                 256 
                 5.3 
                 129 
                 258 
                 47 
               
               
                 443 
                 57 mol % H 
               
               
                 Polymer 
                 58 mol % D/ 
                 41 
                 704 
                 1,250 
                 328 
                 4.2 
                 72 
                 130 
                 34 
               
               
                 444 
                 42 mol % H 
               
               
                 Polymer 
                 74 mol % D/ 
                 54 
                 822 
                 1,410 
                 388 
                 3.2 
                 48 
                 82 
                 23 
               
               
                 445 
                 26 mol % H 
               
               
                 Hyflon ® 
                   
                 176 
                 1,730 
                 2,600 
                 1,330 
                 2.4 
                 23 
                 34 
                 18 
               
               
                 AD60 
               
               
                 Hyflon ® 
                   
                 33 
                 446 
                 1,120 
                 268 
                 2.7 
                 36 
                 90 
                 22 
               
               
                 AD40 
               
               
                 Cytop ® 
                   
                 18 
                 292 
                 788 
                 153 
                 3.0 
                 48 
                 130 
                 25 
               
               
                 Teflon ® 
                   
                 2,700 
                 10,500 
                 10,500 
                 13,000 
                 1.2 
                 4.6 
                 4.6 
                 5.8 
               
               
                 AF2400 
               
               
                   
               
            
           
         
       
     
     From Table 2, in most cases Polymer 443 has better selectivity performance for pure gas pairs than Hyflon® AD, Cytop®, and Teflon® AF. 
     Additionally, as can be seen in  FIGS. 2-5 , Polymer 443 performed well above the upper bound tradeoff line defined by the conventional perfluoropolymer membranes for hydrogen/methane, nitrogen/methane, helium/methane, and carbon dioxide/methane separations. 
     While Example 2 demonstrates that the membrane materials described herein are capable of performing gas separation, as discussed above, the membrane materials can also be used for hydraulic-permeation and pervaporation.