Carbon membranes for gas separation

Carbon membranes for gas separation

News T h e s y n t h e s i s a n d e v a l u a t i o n of p o r o u s m e m b r a n e s for g a s s e p a r a t i o n h a s m a d e g r e a t p r o g...

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T h e s y n t h e s i s a n d e v a l u a t i o n of p o r o u s m e m b r a n e s for g a s s e p a r a t i o n h a s m a d e g r e a t p r o g r e s s d u r i n g t h e l a s t 25 y e a r s . N o n - p o r o u s m e m b r a n e s for g a s s e p a r a t i o n c a n e x h i b i t v e r y h i g h s e l e c t i v i t y of s e p a r a t i o n for c e r t a i n c o m p o n e n t s of a g a s m i x t u r e . However, t h e r a t e s of t r a n s p o r t of t h e g a s e s t h r o u g h t h e s e m e m b r a n e s a r e u s u a l l y low d u e to t h e s o l u t i o n - d i f f u s i o n or s o l u t i o n - r e a c t i o n - - d i f f u s i o n m e c h a n i s m s of t r a n s p o r t . In g e n e r a l , p o r o u s m e m b r a n e s c a n p r o v i d e v e r y h i g h r a t e s of t r a n s p o r t for g a s e s , b u t t h e y e x h i b i t r e l a t i v e l y lower s e p a r a t i o n selectivity. Researchers in the USA have looked at n a n o p o r o u s c a r b o n m e m b r a n e s for t h e s e p a r a t i o n of g a s m i x t u r e s b y s e l e c t i v e s u r f a c e flow (J. Membrane ScL, 8 5 , p p 2 5 3 - 2 6 4 , 1993). T h e y d e v e l o p e d a novel t e c h n i q u e for p r e p a r i n g n a n o p o r o u s c a r b o n m e m b r a n e s s u i t a b l e for u s e in t h e s e p a r a t i o n of hydrogen-hydrocarbon mixtures by the surface d i f f u s i o n m e c h a n i s m . T h i s m e c h a n i s m involves selective a d s o r p t i o n of t h e m o r e s t r o n g l y a d s o r b e d components of a gas mixture onto the pore surface, followed b y s u r f a c e d i f f u s i o n of t h e a d s o r b e d molecules across the pore. The p u r e g a s p e r m e a b i l i t i e s t h r o u g h t h e c a r b o n membrane were measured using a volumetric m e m b r a n e a p p a r a t u s . In a d d i t i o n , p e r m e a b f l i t i e s of m i x t u r e s of h y d r o g e n a n d h y d r o c a r b o n s w e r e m e a s u r e d . It w a s f o u n d t h a t t h e a d s o r p t i o n of h i g h e r h y d r o c a r b o n s effectively b l o c k s t h e p e r m e a t i o n of H2 t h r o u g h t h e v o i d s p a c e in t h e p o r e s , d e m o n s t r a t i n g t h e ' h i n d r a n c e effect'. T h i s w a s s a i d to b e a m a j o r a d v a n t a g e of a n a n o p o r o u s m e m b r a n e r e l y i n g o n s u r f a c e diffusion.

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Separation mechanism in surface f l o w membrane.

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A p r a c t i c a l a p p l i c a t i o n w a s d i s c u s s e d . It w a s s u g g e s t e d t h a t t h e m e m b r a n e c o u l d b e u s e d for r e c o v e r i n g H2 f r o m r e f i n e r y w a s t e g a s e s . T h e hydrocarbon-rich membrane reject gas could be used a s a fuel. T h e H 2 - e n r i c h e d s t r e a m c o u l d b e c o m p r e s s e d a n d s e p a r a t e d i n a c o n v e n t i o n a l PSA s y s t e m to p r o d u c e u l t r a p u r e H2, a v a l u a b l e c h e m i c a l .

Carbon m e m b r a n e s for gas s e p a r a t i o n

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Further information from: S. Slrcar, Air Products and Chemicals, Inc, 7201 Hamilton Boulevard, Allentown, PA 18195, USA.

Enantioselective transport of amino acids Membrane-mediated separations are an attractive a l t e r n a t i v e to c h r o m a t o g r a p h i c t e c h n i q u e s for t h e r e s o l u t i o n of r a c e m i c a m i n o a c i d s . Of t h e p o s s i b l e m e m b r a n e p r o c e s s e s , l i q u i d m e m b r a n e s a p p e a r to b e promising. L i q u i d m e m b r a n e s y s t e m s a v a i l a b l e for t h e r e s o l u t i o n of r a c e m a t e s c a n b e c l a s s i f i e d i n t o two general categories: those containing carrier molecules and those combined with enzymatic reaction processes. A process based on stereoselective e x t r a c t i o n of o n e o p t i c a l i s o m e r b y m e a n s of a hydrophobic chiral organic phase has recently been developed. The membrane acted here as a separator which divided organic and water phases. The s u p p o r t e d c h i r a l l i q u i d m e m b r a n e (SCLM) s y s t e m is p o t e n t i a l l y u s e f u l d u e to t h e c o m p a c t n e s s of s e p a r a t i o n ( e x t r a c t i o n a n d r e e x t r a c t l o n a r e r e a l i z e d in o n e device). The e n a n t i o s e l e c t i v e t r a n s p o r t of a m i n o a c i d h y d r o c h l o r i d e s h a s b e e n i n v e s t i g a t e d (J. Membrane ScL, 8 5 , lap 2 2 1 - 2 2 8 , 1993). Two c h i r a l a l c o h o l s , n o p o l a n d ( 2 S ) - ( - ) - m e t h y l - I - b u t a n o l , i m m o b i l i z e d in t h e p o r e s of a p o l y e t h y l e n e • m w e r e u s e d in t h e e x p e r i m e n t s . T h e d e g r e e of s t e r e o s e l e c t i v i t y of t h e p e r m e a t i o n p r o c e s s w a s s h o w n to v a r y f r o m 0 . 3 9 to 1.52 d e p e n d i n g o n b o t h t h e t y p e of t h e c h t r a l m e m b r a n e p h a s e a n d t h e p r o p e r t i e s of t h e a m i n o acid. F r o m t h e i r r e s u l t s , t h e r e s e a r c h e r s s t a t e t h a t it s e e m s r e a s o n a b l e to s e p a r a t e u n d e r i v a t i z e d a m i n o a c i d s i n t o o p t i c a l i s o m e r s b y m e a n s of s u p p o r t e d chiral liquid membranes. Chiral alcohols used as the m e m b r a n e p h a s e offer e n a n t i o m e r s e p a r a t i o n c o m p a r a b l e to t h e c h r o m a t o g r a p h i c m e t h o d s . T h e a n a l y s i s of p o t e n t i a l m e m b r a n e s t e r e o s e l e c t i v i t y s h o w e d t h a t a m i n o a c i d a s y m m e t r y is t h e m a i n f e a t u r e involved i n d i s c r i m i n a t i o n . It w a s n o t e d t h a t t h e u s e of a m e m b r a n e p h a s e w h i c h h a s p r o p e r h y d r o p h o b i c i t y ( i n c l u d i n g p r e s e n c e of n - e l e c t r o n s ) , p o l a r i t y a n d c h i r a l i t y m a y s i g n i f i c a n t l y affect c h i r a l discrimination.

Further informatlon from: Marek Brtl]ak, Institute of Organic and Polymer Technology, Technical University of Wroclaw, 40-370 Wroclaw, Poland.

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