- Email: [email protected]
Internal staging for membrane gas separation
News and Views
Recovery of vinyl chloride monomer from PVC r e a c t o r v e n t s The US polyvinyl chloride {PVC) i n d u s t r y u s e s a b o u t 11 billion p o u n d s of vinyl chloride m o n o m e r (VCM} a n n u a l l y . A p p r o x i m a t e l y 1% of the VCM is not r e a c t e d in the m a n u f a c t u r i n g process. It is lost in the w a s t e s t r e a m a n d typically i n c i n e r a t e d , which l i b e r a t e s HCI acid. This m u s t t h e n be n e u t r a l i z e d by caustic. New e n v i r o n m e n t a l r e g u l a t i o n s a n d the need to i n c r e a s e p r o d u c t i o n yields for economic p u r p o s e s a r e driving PVC m a k e r s to c o n s i d e r technological a l t e r n a t i v e s for recovering u n r e a c t e d VCM. R e s e a r c h e r s in the USA have employed m e m b r a n e vapor s e p a r a t i o n technology in a PVC p l a n t to s e p a r a t e VCM from inert g a s e s in a r e a c t o r vent s t r e a m (Ind. Eng. Chem. Res., 32, pp 2236-2241). The p u r p o s e w a s to recover VCM which could be recycled a n d r e u s e d in the PVC reaction. Pilot a n d l a b o r a t o r y s t u d i e s d e m o n s t r a t e d t h a t the m e m b r a n e s y s t e m recovered 6 3 - 9 6 vol% of VCM from a vapor t h a t is routinely incinerated. C o m p u t e r s i m u l a t i o n s b a s e d on the e x p e r i m e n t a l d a t a s h o w e d t h a t VCM recoveries greater t h a n 99% c a n be acheived by i n c r e a s i n g the m e m b r a n e area. Benefits for u s i n g m e m b r a n e technology i n c l u d e recovered VCM cost savings, lower acid n e u t r a l i z a t i o n expenditures, and reduced incinerator maintenance.
Further informatlon from: Richard J. Lahiere, Vista Chemial Co, 900 Threadneedle, Houston, TX 77079, USA.
New water filter from Japan A filter c l a i m e d to be c a p a b l e of removing 99% of p a r t i c l e s a s s m a l l a s 2 m i c r o n s in d i a m e t e r from water or other liquid h a s b e e n i n t r o d u c e d b y T a n a b e Shoko Co. The device c o n s i s t s of a t u b e of pleated, e x t r e m e l y fine p o l y e s t e r gauze developed by Toray I n d u s t r i e s Inc. T a n a b e s a y s its filter is highly p e r m e a b l e a n d h a s a p p l i c a t i o n s in s e m i c o n d u c t o r - m a n u f a c t u r i n g p r o c e s s e s r e q u i r i n g e x t r e m e l y p u r e water, a s well as in recycling i n d u s t r i a l detergents. The ¥ 9 8 0 0 (US$92) e l e m e n t is d e s i g n e d to fit into existing filter cartridges.
Internal staging for m e m b r a n e g a s separation The s e p a r a t i o n of g a s e s by p o l y m e r i c m e m b r a n e s is now widely p r a c t i s e d in i n d u s t r y . One of the p r o b l e m s e n c o u n t e r e d d u r i n g this process, however, is t h a t the degree of s e p a r a t i o n achieved is often not high enough. This is b e c a u s e , for some g a s - p a i r s , the p e r m e a b i l i t y coefficients are not sufficiently different in m a g n i t u d e , r e s u l t i n g in poor selectivity. More efficient s e p a r a t i o n c a n be achieved b y the d e v e l o p m e n t of innovative s e p a r a t i o n s c h e m e s , of which the i n t e r n a l l y s t a g e d p e r m e a t i o n s c h e m e is of interest. In the i n t e r n a l l y s t a g e d p e r m e a t i o n s c h e m e two m e m b r a n e s of the s a m e type a r e m o u n t e d in the p e r m e a t o r . The p e r m e a t e e m e r g i n g from the first m e m b r a n e at a n i n t e r m e d i a t e p r e s s u r e would t h e n be a p p l i e d a s feed to the s e c o n d m e m b r a n e for the s e c o n d p e r m e a t i o n step. It c a n be viewed a s two s i n g l e - s t a g e p e r m e a t o r s c o u p l e d locally, as o p p o s e d to a c o n v e n t i o n a l t w o - s t a g e c a s c a d e , where the p e r m e a t o r s a r e c o n n e c t e d in series, a n d the p e r m e a t e from the first p e r m e a t o r is collected e x t e r n a l l y a n d t h e n fed into the s e c o n d p e r m e a t o r a s feed. A c o m p a r i s o n h a s b e e n m a d e b e t w e e n the p e r f o r m a n c e s of a n i n t e r n a l l y s t a g e d p e r m e a t o r , a two-stage c a s c a d e p e r m e a t o r a n d a c o n v e n t i o n a l s i n g l e - s t a g e p e r m e a t o r for the s e p a r a t i o n of CO2/N2 m i x t u r e s (Chemical Engineering Science, 48, 22, pp 3793-3803). Silicone r u b b e r was u s e d a s the m e m b r a n e m a t e r i a l a n d the s e p a r a t i o n w a s investigated e x p e r i m e n t a l l y a n d b y s i m u l a t i o n . The e x p e r i m e n t a l r e s u l t s o b t a i n e d from t h e s e p e r m e a t o r s a g r e e d well with t h e o r e t i c a l p r e d i c t i o n s b a s e d on perfect mixing models. A c o m p a r a t i v e s t u d y of the i n t e r n a l l y s t a g e d p e r m e a t o r with the two-stage c a s c a d e a n d the s i n g l e - s t a g e p e r m e a t o r i n d i c a t e d t h a t the i n t e r n a l l y s t a g e d p e r m e a t o r offered the b e s t CO2 e n r i c h m e n t p e r f o r m a n c e u n d e r perfect mixing a n d c o c u r r e n t flow conditions, b u t u n d e r the c o u n t e r c u r r e n t flow condition, the two-stage c a s c a d e w a s superior. Also, for the s a m e e n e r g y c o n s u m p t i o n , b o t h the i n t e r n a l l y s t a g e d p e r m e a t o r a n d the t w o - s t a g e c a s c a d e offered m u c h higher CO2 e n r i c h m e n t t h a n the s i n g l e - s t a g e p e r m e a t o r ; both r e q u i r e d a m u c h larger m e m b r a n e area.
Further information from: R. Hughes, Department of Chemical and Gas Engineering, University of Salford, Salford M5 4WT, UK.
(Source: The Nikkei Weekly, Japan, 11 October 1993) Further information from: Tanabe Shoko Co, Uchi-Kanda, Chiyoda-ku, Tokyo 101, Japan. Tel: +81 3 3252 2441.
M e m b r a n e T e c h n o l o g y No. 3 9
5
Recovery of vinyl chloride monomer from PVC r e a c t o r v e n t s The US polyvinyl chloride {PVC) i n d u s t r y u s e s a b o u t 11 billion p o u n d s of vinyl chloride m o n o m e r (VCM} a n n u a l l y . A p p r o x i m a t e l y 1% of the VCM is not r e a c t e d in the m a n u f a c t u r i n g process. It is lost in the w a s t e s t r e a m a n d typically i n c i n e r a t e d , which l i b e r a t e s HCI acid. This m u s t t h e n be n e u t r a l i z e d by caustic. New e n v i r o n m e n t a l r e g u l a t i o n s a n d the need to i n c r e a s e p r o d u c t i o n yields for economic p u r p o s e s a r e driving PVC m a k e r s to c o n s i d e r technological a l t e r n a t i v e s for recovering u n r e a c t e d VCM. R e s e a r c h e r s in the USA have employed m e m b r a n e vapor s e p a r a t i o n technology in a PVC p l a n t to s e p a r a t e VCM from inert g a s e s in a r e a c t o r vent s t r e a m (Ind. Eng. Chem. Res., 32, pp 2236-2241). The p u r p o s e w a s to recover VCM which could be recycled a n d r e u s e d in the PVC reaction. Pilot a n d l a b o r a t o r y s t u d i e s d e m o n s t r a t e d t h a t the m e m b r a n e s y s t e m recovered 6 3 - 9 6 vol% of VCM from a vapor t h a t is routinely incinerated. C o m p u t e r s i m u l a t i o n s b a s e d on the e x p e r i m e n t a l d a t a s h o w e d t h a t VCM recoveries greater t h a n 99% c a n be acheived by i n c r e a s i n g the m e m b r a n e area. Benefits for u s i n g m e m b r a n e technology i n c l u d e recovered VCM cost savings, lower acid n e u t r a l i z a t i o n expenditures, and reduced incinerator maintenance.
Further informatlon from: Richard J. Lahiere, Vista Chemial Co, 900 Threadneedle, Houston, TX 77079, USA.
New water filter from Japan A filter c l a i m e d to be c a p a b l e of removing 99% of p a r t i c l e s a s s m a l l a s 2 m i c r o n s in d i a m e t e r from water or other liquid h a s b e e n i n t r o d u c e d b y T a n a b e Shoko Co. The device c o n s i s t s of a t u b e of pleated, e x t r e m e l y fine p o l y e s t e r gauze developed by Toray I n d u s t r i e s Inc. T a n a b e s a y s its filter is highly p e r m e a b l e a n d h a s a p p l i c a t i o n s in s e m i c o n d u c t o r - m a n u f a c t u r i n g p r o c e s s e s r e q u i r i n g e x t r e m e l y p u r e water, a s well as in recycling i n d u s t r i a l detergents. The ¥ 9 8 0 0 (US$92) e l e m e n t is d e s i g n e d to fit into existing filter cartridges.
Internal staging for m e m b r a n e g a s separation The s e p a r a t i o n of g a s e s by p o l y m e r i c m e m b r a n e s is now widely p r a c t i s e d in i n d u s t r y . One of the p r o b l e m s e n c o u n t e r e d d u r i n g this process, however, is t h a t the degree of s e p a r a t i o n achieved is often not high enough. This is b e c a u s e , for some g a s - p a i r s , the p e r m e a b i l i t y coefficients are not sufficiently different in m a g n i t u d e , r e s u l t i n g in poor selectivity. More efficient s e p a r a t i o n c a n be achieved b y the d e v e l o p m e n t of innovative s e p a r a t i o n s c h e m e s , of which the i n t e r n a l l y s t a g e d p e r m e a t i o n s c h e m e is of interest. In the i n t e r n a l l y s t a g e d p e r m e a t i o n s c h e m e two m e m b r a n e s of the s a m e type a r e m o u n t e d in the p e r m e a t o r . The p e r m e a t e e m e r g i n g from the first m e m b r a n e at a n i n t e r m e d i a t e p r e s s u r e would t h e n be a p p l i e d a s feed to the s e c o n d m e m b r a n e for the s e c o n d p e r m e a t i o n step. It c a n be viewed a s two s i n g l e - s t a g e p e r m e a t o r s c o u p l e d locally, as o p p o s e d to a c o n v e n t i o n a l t w o - s t a g e c a s c a d e , where the p e r m e a t o r s a r e c o n n e c t e d in series, a n d the p e r m e a t e from the first p e r m e a t o r is collected e x t e r n a l l y a n d t h e n fed into the s e c o n d p e r m e a t o r a s feed. A c o m p a r i s o n h a s b e e n m a d e b e t w e e n the p e r f o r m a n c e s of a n i n t e r n a l l y s t a g e d p e r m e a t o r , a two-stage c a s c a d e p e r m e a t o r a n d a c o n v e n t i o n a l s i n g l e - s t a g e p e r m e a t o r for the s e p a r a t i o n of CO2/N2 m i x t u r e s (Chemical Engineering Science, 48, 22, pp 3793-3803). Silicone r u b b e r was u s e d a s the m e m b r a n e m a t e r i a l a n d the s e p a r a t i o n w a s investigated e x p e r i m e n t a l l y a n d b y s i m u l a t i o n . The e x p e r i m e n t a l r e s u l t s o b t a i n e d from t h e s e p e r m e a t o r s a g r e e d well with t h e o r e t i c a l p r e d i c t i o n s b a s e d on perfect mixing models. A c o m p a r a t i v e s t u d y of the i n t e r n a l l y s t a g e d p e r m e a t o r with the two-stage c a s c a d e a n d the s i n g l e - s t a g e p e r m e a t o r i n d i c a t e d t h a t the i n t e r n a l l y s t a g e d p e r m e a t o r offered the b e s t CO2 e n r i c h m e n t p e r f o r m a n c e u n d e r perfect mixing a n d c o c u r r e n t flow conditions, b u t u n d e r the c o u n t e r c u r r e n t flow condition, the two-stage c a s c a d e w a s superior. Also, for the s a m e e n e r g y c o n s u m p t i o n , b o t h the i n t e r n a l l y s t a g e d p e r m e a t o r a n d the t w o - s t a g e c a s c a d e offered m u c h higher CO2 e n r i c h m e n t t h a n the s i n g l e - s t a g e p e r m e a t o r ; both r e q u i r e d a m u c h larger m e m b r a n e area.
Further information from: R. Hughes, Department of Chemical and Gas Engineering, University of Salford, Salford M5 4WT, UK.
(Source: The Nikkei Weekly, Japan, 11 October 1993) Further information from: Tanabe Shoko Co, Uchi-Kanda, Chiyoda-ku, Tokyo 101, Japan. Tel: +81 3 3252 2441.
M e m b r a n e T e c h n o l o g y No. 3 9
5