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Membrane Development and Fabrication for Reverse Osmosis and Ultrafiltration

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Membrane Development and Fabrication for Reverse Osmosis and Ultrafiltration MEMBRANE DEVELOPMENT AND FABRICATION FOR REVERSE OSMOSIS AND ULTRAFILTRATION PART 3 Report to the Water Research Commission by MJ. Hurndall and R.D. Sanderson Institute for Polymer Science University of Stellenbosch Stellenbosch 7600 WRC Report No 172 December 1987 MEMBRANE DEVELOPMENT AND FABRICATION FOR REVERSE OSMOSIS AND ULTRAFILTRATION by MJ HURNDALL and RD SANDERSON FINAL REPORT TO THE WATER RESEARCH COMMISSION BY THE INSTITUTE FOR POLYMER SCIENCE UNIVERSITY OF STELLENBOSCH PART 3: THE CHEMISTRY OF POLY-2-VINYLIMIDAZOLINE REVERSE OSMOSIS MEMBRANES WRC PROJECT: 172 DEC 1987 MEMBRANE DEVELOPMENT AND FABRICATION FOR REVERSE OSMOSIS AND ULTRAFILTRATION THIS PROJECT WAS FUNDED BY THE WATER RESEARCH COMMISSION AND CARRIED OUT AT THE INSTITUTE FOR POLYMER SCIENCE AT THE UNIVERSITY OF STELLENBOSCH THE REPORT IS PRESENTED IN FOUR PARTS: PART 1: EXECUTIVE SUMMARY TO THE FINAL REPORT EP JACOBS and RD SANDERSON PART 2: STATISTICAL AND NUMERICAL TECHNIQUES IN THE OPTIMIZATION OF MEMBRANE FABRICATION VARIABLES EP JACOBS and RD SANDERSON PART 3: THE CHEMISTRY OF POLY-2-VINYLIMIDAZOLINE REVERSE OSMOSIS MEMBRANES MJ HURNDALL and RD SANDERSON PART 4: TECHNOLOGY TRANSFER: THE DEVELOPMENT OF TUBULAR UF TECHNOLOGY FOR INDUSTRIAL USE (CLASSIFIED - NOT AVAILABLE FOR DISTRIBUTION) NKH STROHWALD, EP JACOBS and RD SANDERSON ACKNOWLEDGEMENT The Steering Comittee, co-opted members and referees for this project consisted of the following persons: Dr CF Schutte Water Research Commission (Chairman) Mr JP Barnard Bakke Industries (Pty) Ltd Prof CA Buckley University of Natal Mr NA Dowler University of Stellenbosch ProfWEngelbrecht University of Stellenbosch Dr PF Fuls National Institute for Water Research Dr OO Hart Water Research Commission Mr BA Hendry Bakke Industries (Pty) Ltd Dr MJ Hurndall University of Stellenbosch Dr EP Jacobs University of Stellenbosch Dr S Kuehl University of Stellenbosch Mr PE Odendaal Water Research Commission Dr C Patterson-Jones University of Cape Town Dr HS Pienaar Universiteit van Stellenbosch Dr LS Rayner University of Stellenbosch Dr RD Sanderson University of Stellenbosch Prof DF Schneider University of Stellenbosch Mr DB Smit Bakke Industries (Pty) Ltd Prof JJSmit University of Potchefstroom for CHE Mr PL Swart Bakke Industries (Pty) Ltd Dr AJ van Reenen University of Stellenbosch Mr PW Weideman Water Research Commission (Secretary) The financing of the project by the Water Research Commission and the contribution by the members of the Steering Committee are acknowledged gratefully. EXECUTIVE SUMMARY This investigation forms part of a research programme designed to contribute to the development of reverse osmosis (RO) technology. More specifically, it contributes to knowledge of the preparation, characterization and use of poly-2-vinylimidazoline-type polymeric precursors and of sulphonic acid chloride crosslinking agents for the fabrication of ultrathin-film (UTF) membranes for RO desalination. The technique of using membranes in pressure-driven separation operations is gaining ever-increasing acceptance, both in research and in various industrial applications. The first major breakthrough in the field of RO came in the late 1950s when a method for the making of effective and very thin CA membranes was discovered. This area of research then expanded rapidly into the field of UTF composite membranes. The use of these membranes in industry has become very important; much of the knowledge and many of the concepts regarding this young science are disclosed in patent literature and in government reports. Many materials have been screened for possible use as RO membrane materials. Of the numerous chemical systems used to create successful UTF membranes, those generally used have been the interfacially formed polymeric desalting barriers prepared from derivatives of polyethyleneimine (PEI), phenylenediamine or piperazine precursors, reacted with either a di- and/or tri- functional benzenecarboxylic acid chloride as crosslinking agent. Only during the last few years have sound scientific studies been directed at ascertaining why membranes perform the way they do and why they fail chemically. Much of this recent activity resulted from the availibility of sufficiently sophisticated analytical techniques, such as electron scanning calorimetric analysis (ESCA), which are suitable for the analysis of thin crosslinked polymeric networksC3). This followed from the premise that poly-2-vinylimidazoline (PVAM) can be synthesized inexpensively and simply from available reagents. A second reason for using such a chemical compound as a UTF membrane precursor was that the nitrogen atoms are not part of the main polymer chain. Upon reaction of homopolymeric PVAM with an acid chloride crosslinking agent, tertiary amide groups should be formed. Having any nitrogen-containing unreacted groups which might be susceptible to chlorination, causing subsequent deterioration of the membrane, in the polymer side chain, was thought to offer a possible way of increasing resistance of a membrane to degradation by chlorine. In the event of this study leading to the obtaining of efficient RO membranes, the novel chemical system was to be optimized in order to achieve formation of an ultrathin-film structure with high membrane performance. -11- The goals of the investigation was: i) to study the chemistry of PVAM; ii) to obtain insight into the reactions which this compound undergoes with aromatic acid chloride crosslinking agents in the fabrication of UTF-RO membranes and into the chemical composition of a final PVAM/SC1 deslating barrier; iii) to study certain selected membrane fabrication conditions and the effect of variations on the membrane performance; iv) to ascertain what the effect of various operating conditions may have a PVAM UTF membrane. The work proved the following: 1. Poly-2-vinylimidazoline (PVAM) can be used as precursor material in interfacial reactions to make efficient RO membranes. These could be made in flat-sheet and tubular forms. Two different polymer products, termed PVAM-OD and PVAM-FD, were obtained by using two different sets of reaction conditions. PVAM was characterized by 13C nmr spectroscopy and found to be a copolymer, comprising ring-closed (imidazoline) and its hydrolysed form of ring-opened (amide) repeat units. PVAM-CFD was obtained after partial purification of PVAM-FD. The use of PVAM, with its nitrogen atoms having one reactive hydrogen and being pendent to the hydrocarbon chain, resulted in some increase in chlorine-tolerance of the membranes. This chlorine-tolerance was greater than that of membranes made with aliphatic polymeric precursors, e.g. PEI (NS-100/1 membranes), but not as great as that of membranes made from aromatic precursors, e.g. phenylenediamine (FT-30 membranes). 2. The use of aromatic crosslinking agents containing sulphonylhloride groups, in addition to carboxylic acid chloride groups, illustrated the importance of the chemical nature of the reactive groups and of the effect which the structure of the reactants has on membrane structure, as reflected in the RO performance of the membranes. In terms of membrane performance, the use of the crosslinking agents 3-chlorosulphonylbenzoyl chloride (SCI) and 3,5-dichlorosulphonylbenzoyl chloride (C12S) gave flat-sheet membrane RO performances better than those of membranes prepared with the more generally used reagent, isophthaloyl chloride (IPC). For example: a) Use of SCI instead of IPC gave PEI UTF membranes which gave higher flux and rejection, as illustrated by the following RO performances: PEI/IPC membranes 98,4% rejection; 600 lmd flux PEI/SC1 membranes 98,7% rejection; 835 lmd flux (Membranes were tested with 5 000 ppm NaCl solution at 4 MPa) - Ill - (b) Use of SCI instead of IPC gave early PVAM UTF membranes which gave higher rejection, as illustrated by the following RO performances by flat-sheet membranes: PVAM/IPC membranes 72,7% rejection; 1 760 lmd flux PAM/SC1 membranes 84,6% rejection, 680 lmd flux (Membranes were tested with a 5 000 ppm NaCl solution at 4 MPa) (c) Use of the trifunctional reagent C12S gave PVAM UTF membranes which gave higher rejection and flux than those of membranes prepared with SCI, as illustrated by the following results: PVAM/SC1 membranes 84,6% rejection; 680 lmd flux PVAM/C12S membranes 96,4% rejection; 800 lmd flux (Membranes were tested with a 5 000 ppm NaCl solution at 4 MPa) 3. Numerous fabrication variables were found to be important in the preparation of efficient PVAM UTF membranes; slight variations led, in some cases, to large variations in membrane performance. Of the variables studied, those considered most important were: polysulphone (PS) substrate, chemistry of the crosslinking agent, concentration of crosslinking agent and of precursor, addition of acid acceptor to the precursor solution and heat-cure temperature. The best RO performance results obtained for PVAM-OD/SCl membranes in flat-sheet form were: 94.3 ± 1,2% rejection; 1 096 ± 50 lmd (tested for 5 000 ppm NaCl at 4 MPa) 97.4 ± 1,4% rejection; 400 ±22 lmd (tested for 2 000 ppm NaCl at 2 MPa) The optimum RO performances of PVAM-OD/SCl tubular membranes (developed during a parallel study) were: 97,1 + 0,1% rejection; 730 ± 24,1 lmd (tested for 2 000 ppm NaCl at 2 MPa) The optimum RO performances of PVAM-FD/SC1 tubular membranes (developed during a parallel study) were: 98,2 ± 0,7 rejection; 560 ± 150 lmd (tested for 2 000 ppm NaCl at 2 MPa) 4. Exposure of PVAM UTF membranes to chlorine is not advisable. It is, however, proposed that exposure of PVAM-FD/SC1 membranes to 5 ppm chlorine at pH 6 to 7 for a short period of time would be tolerated. Prolonged exposure to chlorine results in irreversible decline in rejection. - IV- Maximum performance of PVAM-FD/SC1 tubular membranes has been recorded in the pH 6,5 to 7 range. These membranes can be safely used over a pH range of 4,5 to 8,5. Exposure of membranes to solutions of pH 3 over a period of one month under dynamic conditions, led to a drop in baseline rejection performance and increase in flux.
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