Europaisches Patentamt 0 372 683 3 European Patent Office © Publication number: A2 Office europeen des brevets EUROPEAN PATENT APPLICATION © Application number: 89308937.5 © int.ci«:C07C 29/76, B01D 61/36 © Date of filing: 04.09.89 © Priority: 05.12.88 US 279398 © Applicant: TEXACO DEVELOPMENT CORPORATION © Date of publication of application: 2000 Westchester Avenue 13.06.90 Bulletin 90/24 White Plains, New York 10650(US) © Designated Contracting States: @ Inventor: Reale, John, Jr. DE FR GB NL 8 Sidney Lane Wappinger Falls, N.Y. 12590(US) Inventor: Bartels, Craig Roger 19 Wildwood Drive Wappinger Falls, N.Y. 12590(US) © Representative: Brock, Peter William et al URQUHART-DYKES & LORD 91 Wimpole Street London W1M 8AH(GB) © Dehydration of organic oxygenates. © Aqueous charges containing organic oxygenates can be dehydrated by means of a non-porous layer of cast polyvinyl alcohol crosslinked with an aliphatic aldehyde having at least 3 carbon atoms, including those in the aldehyde group. A pressure drop is maintained across the layer with the aqueous charge on the high pressure side. Water containing a reduced proportion of the oxygenate is recovered on the low pressure side. A preferred oxygenate is isopropyl alcohol. Other alcohols, as well acids, esters, ethers, ketones and aldehydes (but excluding glycols) are examples of other oxygenates. CO 00 CO CM IS CO 0. Ill Xerox Copy Centre EP 0 372 683 A2 DEHYDRATION OF ORGANIC OXYGENATES This invention relates to the dehydration of organic oxygenates, such as isopropyl alcohol. More particularly it relates to a membrane technique for effecting separation of water from an aqueous charge containing isopropyl alcohol. As well known to those skilled in the art, it is possible to remove water from mixtures thereof with organic liquids by various techniques including adsorption or distillation. These conventional processes, particularly distillation, are however, characterized by high capital cost. In the case of distillation, for example, the process requires expensive distillation towers, heaters, heat exchangers (reboilers, condens- ers, etc.), together with a substantial amount of auxiliary equipment, typified by pumps, collection vessels, vacuum generating equipment, etc. 10 Such operations are characterized by high operating costs, principally costs of heating and cooling - plus pumping, etc. Furthermore the properties of the materials being separated, as is evidenced by the distillation curves, may be such that a large number of plates may be required, etc. when the material forms an azeotrope with water, additional problems may be present which for example, would require that separation be effected in 75 a series of steps (e.g. as in two towers) or by addition of extraneous materials to the system. These are also comparable problems which are unique to adsorption systems. It has been found to be possible to utilize membrane systems to separate mixtures of miscible liquids by pervaporation. In this process, the charge liquid is brought into contact with a membrane film; and one component of the charge liquid preferentially permeates the membrane. The permeate is then removed as 20 a vapor from the downstream side of the film - typically by sweeping with a carrier gas or by reducing the pressure below the saturated vapor pressure of the permeating species. Illustrative membranes which have been employed in prior art techniques include: Separating Layer References 25 - Nafion brand of - Cabasso and Liu perf 1 uorosulfonic acid J. Memb. Sci. 24, 30 101 (1985) - Sulfo.nated polyethylene - Cabasso, Korngold & Liu J. Pol Sc: 35 . Letters, 23, 57 (1985) 40 - Kluorinated polyether US-A-4,526,948 or CaVboxylic Acid fluorides 45 50 EP 0 372 683 A2 Separating Layer References - Selemion AMV - Wentzlaff brand of Asahi Glass Boddeker & Hattanbach cross-linked styrene J. Memb. Sci. 22,333 butadiene (with quaternary (1985) ammonium residues on a 10 polyvinyl chloride backing) - Cellulose triacetate - Wentzlaff, Boddeker 15 & Hattanback, J. Memb. Sci. 22/ 333 (1985) & 20 - Polyacrylonitrile - Neel, Aptel Clement Desalination 53, 297 (1985) 25 - Crosslinked EP-A-0,02S,33S Polyvinyl Alcohol 30 - Poly(maleimide- - Yoshikawa et al acrylonitrile) J. Pol. Sci. 22, 2159 (1984) 35 - Dextrine - - Chem. Econ. Eng. isophorodisocyanate Rev., 17, 34 (1985) 40 The cost effectiveness of a membrane is determined by the selectivity and productivity. Of the membranes commercially available, an illustrative membrane of high performance is that disclosed in EP-A- 0,096,339. 45 EP-A-0,096,339 discloses, as cross-linking agents, diacids (typified by maleic acid or fumaric acid); dihalogen compounds (typified by dichloroacetone or 1 ,3-dichloroisopropanol); and aldehydes, including dialdehydes, typified by formaldehyde. These membranes are said to be particularly effective for dehydra- tion of aqueous solutions of ethanol or isopropanol. This reference discloses separation of water from alcohols, ethers, ketones, aldehydes, or acids by use 50 of composite membranes. Specifically the composite includes (i) a backing typically about 120 urn in of thickness, on, which is positioned (ii) a microporous support layer of a polysulfone or a polyacrylonitrile about 50 urn thickness, on which is positioned (iii) a separating layer of cross-linked polyvinyl alcohol about 2 urn in thickness. Polyvinyl alcohol may be cross-linked by use of difunctional agents which react with the hydroxyl group 55 of the polyvinyl alcohol. Typical cross-linking agent may include dialdehydes (which yield acetal linkages), diacids or diacid halides (which yield ester linkages), dihalogen compounds or epichlorhydrin (which yield ether linkages) olefinic aldehydes (which yield ether/acetal linkages), boric acid (which yields boric ester linkages), sulfonamidoaldehydes, etc. EP 0 372 683 A2 See also J.G. Prichard, Polyvinyl Alcohol, Basic Properties and Uses, Gordon and Breach Science Publishers, New York (1970) 67 C.A. Finch, Polyvinyl Alcohol, Properties and Applications, John Wiley and Sons, New York (1973). Additional background information will be found in US-A-4,728,429, -4,067,805, -4,526,948, -3,750,735 5 and -4,690,766. Our prior European Patent Application 88113270.8 concerns a method of concentrating an aqueous solution of a glycol by a pervaporation process across a membrane of cast polyvinyl alcohol which has been cross-linked with an aliphatic polyaldehyde containing at least three carbon atoms, including those in the aldehyde groups. The glycols are defined as being compounds with at least two hydroxy groups on a w carbon backbone, and triols and glycol ethers are specifically mentioned as examples of glycols. We have now found that the membranes can be used for concentrated organic oxygenates in general. The present invention provides a method of concentrating an aqueous charge of an organic oxygenate (but excluding organic oxygenates which are glycols, triols or glycol ethers), which comprises passing said aqueous charge into contact with the high pressure side of a non-porous separating layer of cast polyvinyl 75 alcohol which has been cross-linked with an aliphatic polyaldehyde containing at least three carbon atoms including those in said aldehyde groups, and maintaining a pressure drop across said layer whereby at least a portion of said water and a lesser portion of organic oxygenate from said aqueous charge pass by pervaporation through said separating layer as an oxygenate-lean mixture containing more water and less organic oxygenate than are present in said aqueous charge, and leaving an oxygenate-rich liquid containing 20 less water and more organic oxygenate than are present in said aqueous charge. A preferred non-porous separating layer has a thickness of 1-10 urn and comprises cast polyvinyl alcohol of molecular weight Mn of 20,000 to 200,000 which has been cross-linked, in the presence of acid catalyst, with an aliphatic polyaldehyde containing at least three carbon atoms, including those in said aldehyde groups, and thereafter cured at 100 to 225° C. 25 The non-porous separating layer can form part of a multi-layer assembly which in the preferred embodiment preferably includes a porous carrier layer which provides mechanical strength and support to the assembly. 30 THE CARRIER LAYER This carrier layer, when used, is characterized by its high degree of porosity and mechanical strength. It may be fibrous or non-fibrous, woven or non-woven. In the preferred embodiment, the carrier layer may be 35 a porous, flexible, non-woven fibrous polyester. A preferred non-woven polyester carrier layer may be formulated of non-woven, thermally-bonded strands and characterized by a fabric weight of 80 ± 8 grams per square yard (96 ± 9.6 g/m2), a thickness of 4.2 ± 0.5 mils (106.7 ± 12.7 urn), a tensile strength (in the machine direction) of 31 psi (213.75 KPa) and (in cross direction) of 10 psi (68.95 KPa), and a frazier air permeability of 6 cuft/min/sq. ft. (182 40 cm3/min/cm2) @ 0.5 inches of water (125 Pa). THE POROUS SUPPORT LAYER 45 The porous support layer of this invention is preferably formed of a sheet of polysulfone polymer or less preferably of polyacrylonitrile. Typically the polysulfone may be of thickness of 40-80 urn, say 50 urn and of molecular weight M"n of 5,000 to 100,000, preferably 20,000 to 60,000 say 40,000. The polysulfone is preferably characterized by a pore size of less than 500A (50 nm) and typically about 200A (20 nm). This 50 corresponds to a molecular weight cut-off of less than 25,000, typically about 20,000. The sulfone polymers which may be employed may include those made from cumene containing isopropylidene groups in the backbone; e.g. 55 EP 0 372 683 A2 I II . -o-p-c-p-o-^-s-p- I IT C 0 10 These isopropylidene sulfones containing repeating units including ether-aromatic-isopropylidene- aromatic-ether-aromatic-sulfone-aromatic groups may typically have a molecular weight Wn of 15,000 - 30,000, a water absorption (at 20° C) of about 0.85w%, a glass transition temperature of 449° K, a density of 1.25 g/cm3, a tensile strength (at 20 °C) at yield of 10,000 psi (68.95 MPa), and a coefficient of linear ° thermal expansion of 2.6 x 10~5 mm/mm/ C.