News and Views 5ram across, more t h a n five times larger t h a n the
largest previously grown. Walter Klemperer, a chemist at the University, believes that the increased size of the crystals will permit better experiments to be u n d e r t a k e n into the extremely complicated structures. The growing process takes three to five days. Source: MRS Bulletin, J u l y 1992
Books Electrochemical Engineering and the Envlrol!ment '92, P.J. Mitchell (Editor), Hemisphere Publlshlng Corporation, 1992, c360 pp, clo'th, c£33.00, ISBN 1-56032-256-X. Proceedings of the third s y m p o s i u m with this title in the Institution of Chemical Engineers Series. Topics include water care a n d treatment processes, cell and process design for energy efficiency, sensors a n d control systems, and m e m b r a n e technology.
Computer-generated molecular model of the Cso fullerene polymer. The buckyballs are linked by hydrocarbon molecules (hydrogen atoms s h o w n in white).
xylylene molecules attached. When heated, the copolymer begins to slowly lose m a s s at 380"C b u t retains two thirds of its m a s s even at 1000"C. "This shows that the copolymer is relatively stable and that cross-linklng is slowing down its depolymerisation", says Douglas Loy of Sandla. Polymers capable of withstanding high temperature are obviously desirable for industrial processes. As yet the catalytic a n d other properties of the polymer r e m a i n u n k n o w n b u t Loy is looking further ahead. By u s i n g smaller a m o u n t s of fuilerene a n d more xylylene he hopes to create a material more like polyxylylene b u t which retains its C60 properties. Polyxylylene is c o m m o n l y used as a speciality coating. Further information from Douglas Loy, Sandla National Laboratories, Albuquerque, NM 87185, USA. Tel +1 505 844 4445. Fax +1 505 844 6367.
5mm zeolite crystals grown Researchers at the University of I11inois have developed a m e t h o d for growing zeolite crystals u p to
Polymers for Gas Separation, N. Toshlma (Editor), VCH Publishers, 1992, £58.00, ISBN 3.52728-234-3. The Membrane Handbook, H.W.S. Winston and K.K. Slrkar, Chapman & Hall, 1992, £87.00, ISBN 0-84930-I 77-7. Inorganic Membranes, R. Bhave, Chapman & Hall, 1992, £43.50, ISBN 0-44231-876-6. Unified S e p a r a t i o n ~ : i e n c e , J.C. Glddlngs, Wiley, 1991,346pp, £49.95, ISBN 0-47152-089-6. Solvent Recovery Handbook, I.M. Smallwood, Edward Arnold, In Press - - due February 1993, c320pp, c£75, ISBN 0-340-57467.4. Is being written to "provide a guide to the effective recovery of solvents a n d to the efficient operation of plants a n d processes in which they are used".
Polymer Applications for Biotechnology: Macromolecular Separation and Identification, D. Soane (Editor), Prentice Hall; 1992, 324pp, £85.60, ISBN 0-13-883251-2. Covers all of the major separation processes using polymers aimed at the biotechnology/pharmaceutical industries, including polymer m e m b r a n e s .
Science and Practice of Liquid-Liquid Extraction, J.D. Thornton (Editor), Oxford University Press, 1992, 978pp (2 volumes), £108, ISBN 0-19-856178-4.
P r o t o n exch mge m e m b r m l e fuel cell The ability to provide m u c h higher power densities h a s m a d e the proton exchange m e m b r a n e fuel cell the leading fuel ceil contender for power generation in the 5 to 500 kW range. The proton exchange
M e m b r a n e Technology
News and Views m e m b r a n e cell operates below I 0 0 °C a n d u s e s a solid film of immobilised fluorocarbon ion-exchange m e m b r a n e as electrolyte. This provides a more rugged unit a n d faster s t a r t - u p from room t e m p e r a t u r e t h a n other c u r r e n t types of fuel cell. One of the major impediments to the wider commercialisation of this fuel cell is not related to the m e m b r a n e technology, b u t to the high platinum content of the electrocatalyst u s e d to catalyse the hydrogen oxidation a n d oxygen reduction reactions - the p l a t i n u m cost h a s been over $300/kW. In conventional fuel cells, relying on the diffusion of the liquid electrolyte into the electrode, it is possible to utilise nearly all the platinum catalyst in the m a c r o p o r o u s electrode to catalyse the electromechanical reactions. Platinum utilisation with the 'solid' polymer electrolyte h a s been poor. Two recent develovments, one from the Los Alamos National Laboratol~I~ a n d the other from Physical Sciences Inc of M a s s a c h u s e t t s 121, in m e t h o d s for the preparation of low p l a t i n u m loading, high performance proton exchange m e m b r a n e fuel cell electrodes m a y lead to a b r e a k t h r o u g h in the commercialisation of this fuel cell system. The Los Alamos group have cast 'thin film catalyst' layers directly onto the solid electrolyte m e m b r a n e from inks consisting of c a r b o n - s u p p o r t e d platinum catalyst mixed with a solubilised form of Du Pont's Nation ion-exchange m e m b r a n e . The a p p r o a c h adopted by Physical Sciences Inc involves preparing a gas diffusion electrode from u n c a t a l y s e d c a r b o n into which the soluble form of Nation is impregnated. Finely dispersed platinum particles are then electrodeposited from a commercial platinum plating bath. (1) M.S. W i l s o n a n d S. G o t t e s f e l d . J. Electrochem. Soc., 139 (2), 1992, L28-30. (2) E . J . T a y l o r , E . B . A n d e r s o n a n d N . R . K . V i l a m b i .
J. Electrochem. Soc., 139 (2), 1992, L45--46.
Casting c e l l u l o s e acetate membranes The transport properties of fiat cellulose acetate m e m b r a n e s used in reverse osmosis separations can be affected quite markedly by the casting parameters. Much work h a s been done on this problem in the last twenty years a n d a recent paper [1) shows that the search for a predictive technique continues. The basic casting parameter is the casting solution composition, b u t even when this is c o n s t a n t the ambient conditions c a n lead to variations. As far as composition is concerned, Mr A.S. Bal of the Indian National Environmental Engineering Research Institute in Nagpur has correlated m e m b r a n e properties with a t e r n a r y p h a s e diagram which links polymer, additive a n d solvent contents in
the casting solution. Using this he h a s nominated a ' s t a n d a r d casting composition' for cellulose acetate RO membranes m 25% cellulose acetate, 30% formamide, 45% acetone - - a n d established the c o m p o s i t i o n z o n e which p r o d u c e s 'productive' m e m b r a n e s (Flux: 3 8 0 - 4 2 0 I/m2d; Rejection 85-93%). (I) A.8. Bit. Effect of casting p a r a m e t e r s o n m e m b r a n e t r a n s p o r t properties. Chemical Engineering World (India), 2 7 (I), J a n u a r y 1992, 25-30.
Composite m e m b r a n e for NH3/N2 separation An important step in a m m o n i a synthesis, with regard to overall process efficiency, is the recovery of a m m o n i a from the p r o d u c t stream. At 35 vol% NH3 for 800-1000 bar, reaction o u t p u t is low. More efficient removal of NH3 would favour a m m o n i a yield b y driving the synthetic step in the m a s s - l a w equation. NH3 can form labile coordination complexes with c o m p o u n d s like zinc halldes, a n d this behavlour raises the possibility of facilitated transport. The presence of a carrier would raise both permeability a n d selectivity, not only by the c a r r i e r - p e r m e a n t interaction yielding higher NH3 flux, b u t possibly also by the additional barrier effect of the carrier for the non-reacting N2. It is this consideration that h a s p r o m p t e d the investigation of m e m b r a n e technology for the separation problem. A n u m b e r of alternatives have been p u t forward since D.W. Brubaker a n d K. K a m m e r m e y e r first suggested a perm-selectlve m e m b r a n e for separating a m m o n i a in 1954 - - immobilized molten salts; an ion exchange m e m b r a n e ; a multilayer polymeric m e m b r a n e with poly(vinylammonlum thiocyanate) as a carrier. In a recent p a p e r ¢1~the use of a composite m e m b r a n e for NH3 separation is proposed. The authors, from the GKSS Research Centre in Germany, u s e d a m i c r o p o r o u s polyetherimlde (PEI) m e m b r a n e as support, a n d p r o d u c e d a P E / P E I composite m e m b r a n e by dip coating in a solution consisting of polyethylene dispersion (PE), 1 wt% glycerine a n d a zinc i o d i d e / a m m o n i a complex. The composite m e m b r a n e is s h o w n to permeate NH3 selectively from an NH3/N2 mlxture at ambient conditions, attaining a m a x i m u m selectivity of 480 at 1% NH3 feed concentration. The experimental data presented indicate a facilitated-transport-mode m e m b r a n e function. A search is now u n d e r w a y for a suitable polymer that can hold higher carrier quantities a n d yet yield a n integral separating layer as well as withstand real process temperatures. (I) S.K. S h u k l a a n d K.-V. P e l n e m a n n . A fixed-site carrier composite m e m b r a n e for NH3/N2 separation. Gas Separation & Purification, 6 (2), 1992, 79-81.