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Reaction and separation synergy through membrane technology
Feature
Reaction and separation synergy throug h membrane technology The drive t o w a r d s greater e c o n o m i c and e n v i r o n m e n t a l e f f i c i e n c y h a s r e s u l t e d in t h e d e v e l o p m e n t of p r o c e s s e s w i t h r e d u c e d overall environmental impact when compared with conventional technology. An I n n o v a t i o n in r e c e n t y e a r s h a s b e e n to create s y n e r g y t h r o u g h c o m b i n e d c a t a l y t i c and s e p a r a t i o n t e c h n o l o g i e s . Catalytic m e m b r a n e reactor o p e r a t i o n is a t y p i c a l e x a m p l e of s u c h a s y n e r g i s t i c combination involving a chemical reaction sequence with a membrane-based chemical separation. Here Dr Edward Gobina and Professor R o n a l d H u g h e s o u t l i n e research w o r k b e i n g carried o u t at t h e U n i v e r s i t y of Salford, UK. p r o c e s s i n g is possible. S u c h materials include porous ceramic (e.g. a l u m i n a , silica, zirconia, titania and magnesia) and m e t a l l i c m a t e r i a l s (e.g. s t a i n l e s s steel, b r o n z e a n d silver). In t h e o r y therefore, a l m o s t a n y c h e m i c a l r e a c t i o n c a n b e n e f i t from the i n c o r p o r a t i o n of a m e m b r a n e i n s o m e form. For e x a m p l e i n p a r t i a l o x i d a t i o n r e a c t i o n s w h e r e gas p h a s e oxygen h a s b e e n s h o w n to r e a c t with the d e s i r e d p r o d u c t to give total o x i d a t i o n p r o d u c t s , a m e m b r a n e c a n be u s e d to create a r e a c t i o n i n t e r f a c e b e t w e e n the h y d r o c a r b o n feed a n d oxygen. B e c a u s e the s t o i c h i o m e t r y of oxygen c a n n o w be c o n t r o l l e d w i t h i n t h e vicinity of the c h e m i c a l
I n a m e m b r a n e reactor, it is p o s s i b l e to c o m b i n e a c h e m i c a l reaction transformation with a membrane-assisted chemical s e p a r a t i o n . By t h i s m e a n s , it is p o s s i b l e t h e r e f o r e to i m p r o v e t h e efficiency of m a n u f a c t u r i n g via i n - s i t u p u r i f i c a t i o n . U n t i l very recently, catalytic m e m b r a n e s incorporated in chemical reactor s y s t e m s were m a i n l y a p p l i e d i n b i o c h e m i c a l r e a c t i o n s d u e to t e m p e r a t u r e l i m i t a t i o n s of t h e m o r e widely u s e d o r g a n i c m e m b r a n e s . Now, however, t h a n k s to r e c e n t a d v a n c e s i n materials science engineering r e s u l t i n g i n t h e d i s c o v e r y of newer materials, hightemperature membrane
t r a n s f o r m a t i o n , total o x i d a t i o n r e a c t i o n s c a n be e l i m i n a t e d . This h a s a n u m b e r of b e n e f t s i n c l u d i n g a v o i d a n c e of flammability constraints and hot spots. In a d d i t i o n , yields c a n b e optimized a s a r e s u l t of m o r e efficient r a t e control. A n o t h e r p r o c e s s w h e r e m e m b r a n e s c a n be of i m m e n s e b e n e f i t is i n selective r e m o v a l of a p a r t i c u l a r specie(s) in a n equilibrium limited reaction s e q u e n c e to i m p r o v e yield. A typical e x a m p l e is i n esterification r e a c t i o n s . In t h e s e r e a c t i o n s a n acid r e a c t s w i t h a n a l c o h o l to p r o d u c e a n e s t e r a n d water. However, t h e e s t e r a n d w a t e r produced can undergo a back r e a c t i o n : t h u s , t h e p e r c e n t yield is e q u i l i b r i u m limited. P e r v a p o r a t i o n c a n be u s e d to r e m o v e w a t e r during the reaction, a n d thus shift t h e e q u i l i b r i u m to i n c r e a s e yield.
The practical use of m e m b r a n e r e a c t o r s for industrial applications Although abundant opportunities do exist for the i n d u s t r i a l a p p l i c a t i o n of h i g h - t e m p e r a t u r e m e m b r a n e reactors, their implementation has until recently b e e n very limited. T h e m a j o r r e a s o n for this s t e m s from the difficulty i n c h a n g i n g a l r e a d y e s t a b l i s h e d priorities for n e w e r o n e s w i t h all t h e a s s o c i a t e d p r o b l e m s . Nowadays, however, m e m b r a n e separations are being r e c o g n i s e d a s e n e r g y efficient
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Figure 1: Schematic diagram of membrane reactor operation under cocurrent
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M e m b r a n e T e c h n o l o g y No. 76
Feature substitutes to conventional technology in n u m e r o u s industrial processes. This together with the development of more advanced processes for m e m b r a n e m a n u f a c t u r e s u c h as the radio frequency (r.f.) m a g n e t r o n sputtering technique and pulsed laser technology is increasing m e m b r a n e acceptability in industry. To explore the possibilities offered by the incorporation of s u c h reactors in existing a n d already established processes, researchers at the University of Salford, UK have studied b o t h by mathematical simulation and
experimentation the selective removal of by-product hydrogen during the catalytic dehydrogenation of alkanes using various types of high temperature membranes. Such reactors facilitate the m a n u f a c t u r e of the primary product, by driving the equilibrium towards high conversion. Figure I describes the major features of the m e m b r a n e reactor operation. Essentially, it describes the flow of reacting gas species through a m e m b r a n e tube which contains catalytic material h o u s e d within a stainless steel shell. Because the
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M E M B R A N E THICKNESS~um Figure 2: Reactor p e r f o r m a n c e a s a f u n c t i o n o f m e m b r a n e t h i c k n e s s f o r the d e n s e composite m e m b r a n e s y s t e m s . (Feed: EB = 5 0 mol%/N2 = 50 tool%; W / F A o = 5 x 105 g c a t . s / g m o l ; T = 873K).
M e m b r a n e T e c h n o l o g y No. 7 6
m e m b r a n e tube is permeable to one (dense membrane) or more (porous membrane) p r o d u c t species, the reaction rate and eventually the conversion can be affected as a result of equilibrium displacement. Sweeping the shell-side [annular space) with an appropriate gas can increase the driving force for permeation and thus e n h a n c e the conversion far beyond the equilibrium value. A comparative a s s e s s m e n t of the performance of two m e m b r a n e categories (dense and porous) and five m e m b r a n e types (Pd/Ag, silica, polyamide, Ru-impregnated porous and inert porous composites) h a s been carried out. This is shown in Figures 2 and 3 respectively, For the dense composite systems (Figure 2), the Pd/Ag composite does show good promise at small m e m b r a n e thicknesses. Pinholes and cracks are undesirable in applications where ultrapure hydrogen production is a major priority otherwise, the use of s u c h systems with a certain a m o u n t of pinholes and cracks does not cause too m u c h problems. The dense silica system, with its high hydrogen selectivity b u t poor permeability shows the least performance. However, this system is particularly useful in c i r c u m s t a n c e s where corrosive chemicals s u c h as HBr and H2S are present. For the porous systems (Figure 3), the Ru-d!spersed system shows good promise. With high dispersion of the active metal, a greater surface area of the material is exposed to hydrogen thus increasing its chemisorption. Membrane porosity is also an important parameter since this will determine how m u c h feed could be lost into the permeate stream. Increasing the porosity also increases the a m o u n t of ethylbenzene lost into the permeate. Table 1 shows the performance of dense and porous systems during lean and rich feed processing. While lean feeds would be advantageous in the dense Pd/Ag systems, there is no advantage to be gained in using s u c h compositions for the porous system. High t e m p e r a t u r e catalysis exploiting the structural and mechanical stability of s u c h
7
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Figure 3: Reactor p e r f o r m a n c e a s a f u n c t i o n o f m e m b r a n e t h i c k n e s s a n d porosity f o r the porous composite m e m b r a n e s y s t e m s . (Feed: E B = 5 0 mol%/N2 = 5 0 tool%; W / F A o = 5 x 105 g c a t . s / g m o l ; T = 873K).
Table 1. M e m b r a n e reactor p e r f o r m a n c e during lean a n d rich f e e d operations ( s w e e p g a s f l o w r a t e = 3 0 0 c m 3 (STP)/min; T = 873K).
Contact Time Parameter (WIFAo x 10 "s gcat.s/gmol)
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Ethylbenzene Conversion, % Lean Feed (EB = 10%/H2, = 5 % / N 2
= 85%)
Rich Feed 8%/N2 = 7%)
( E B = 85% /I-12 =
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Porous System
Pd-Ag System
Porous System
61.5 74.6 81.3 85.7 87.1
18.3 25.1 30.0 33.0 35.5
33.4 44.9 52.1 57.0 60.1
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M e m b r a n e T e c h n o l o g y No. 7 6
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MEMBRANE THICKNESS,~m Figure 4: Conversion e n h a n c e m e n t during n - b u t a n e d e h y d r o g e n a t i o n via kinetic coupling. (Feed: n - b u t a n e = 25 mol% / N2 = 75 mol %; W / F A o = 4.3 x lOO g c a t . s / g m o l . T = 6 7 0 K; s w e e p rate = 150 c m 3 (STP)/min). composite membrane systems c o u l d well f o r m t h e b a s i s for n e w c a t a l y t i c p r o c e s s e s in t h e chemical and petrochemical industry. The significant a d v a n t a g e s of w h i c h i n c l u d e a r e d u c t i o n in e n e r g y r e q u i r e m e n t s while enhancing product yields a n d selectivity. In t h e d e n s e Pd-Ag c o m p o s i t e m e m b r a n e , we h a v e s t u d i e d e x p e r i m e n t a l l y t h e p o s s i b i l i t y of further thermodynamic enhancement via kinetic coupling of t h e p e r m e a t e h y d r o g e n w i t h a r e a c t i v e s w e e p gas. F o r t h i s w e have employed the catalytic d e h y d r o g e n a t i o n of n - b u t a n e to b u t e n e in t h e t u b e - s i d e w h i l e oxygen and carbon monoxide was u s e d a s s w e e p g a s e s in t h e s h e l l - s i d e . T h i s is s h o w n in F i g u r e 4. W h i l e o n l y s m a l l c o n c e n t r a t i o n s of t h e r e a c t i v e c o m p o n e n t s is r e q u i r e d to s e c u r e maximum n-butane conversion, t h e u s e of o x y g e n is m o r e effective than carbon monoxide.
Membrane
T e c h n o l o g y No. 7 6
Process
economics
In t h e i n d u s t r i a l m a n u f a c t u r e of petrochemical feedstocks innovative practices are i m p l e m e n t e d to i n c r e a s e p r o d u c t yield a n d r e d u c e overall e n e r g y r e q u i r e m e n t . F o r e x a m p l e for a 190 m e t r i c t o n n e s p e r d a y styrene production plant (assuming 330 days per year operation), a 1% i n c r e a s e in ethylbenzene conversion r e p r e s e n t s a n i n c o m e g a i n of about £1M/year. Therefore using t h e levels of e n h a n c e m e n t s achieved represents an income g a i n r a n g i n g from £ 4 0 M for t h e Pd-Ag m e m b r a n e to £ 1 4 M for t h e d e n s e silica. F u r t h e r m o r e , in t h e dense systems, the hydrogen r e m o v e d c a n b e u s e f u l s i n c e it c o n t a i n s v e r y low i m p u r i t i e s . This can provide an additional i n c o m e of a b o u t £ 5 0 , 0 0 0 / y e a r assuming a 40% recovery rate of t h e p e r m e a t e h y d r o g e n . Therefore there are sufficient e c o n o m i c a d v a n t a g e s for membrane integration into existing process technology.
Conclusion The synergy between catalytic transformation and a membrane-based chemical s e p a r a t i o n d o e s offer n u m e r o u s a d v a n t a g e s a n d n e w p r o s p e c t s for carrying chemical reactions. Composite membrane systems ( d e n s e or p o r o u s ) will b e v e r y i m p o r t a n t in t h e f u t u r e a p p l i c a t i o n of m e m b r a n e c a t a l y s i s . E x t e n d e d s t u d y of t h e m e c h a n i s m of t h i n film f o r m a t i o n o n a v a r i e t y of s u b s t r a t e s a n d long t e r m s t a b i l i t y d o e s s e e m required. This together with o p t i m i z a t i o n of p r o c e s s e s for producing very thin but c o n t i n u o u s m e m b r a n e over larger areas with a high degree of r e p r o d u c i b i l i t y c o u l d well revolutionize established practices. For f u r t h e r information, contact: E d w a r d Gobina a n d Ronald Hughes, Department of Chemical Engineering, University o f Salford, M a x w e l l Building, Salford M5 4WT, UK. Tel: +44 161 745 5 0 8 1 ; f a x : +44 161 745 5999.
9
Reaction and separation synergy throug h membrane technology The drive t o w a r d s greater e c o n o m i c and e n v i r o n m e n t a l e f f i c i e n c y h a s r e s u l t e d in t h e d e v e l o p m e n t of p r o c e s s e s w i t h r e d u c e d overall environmental impact when compared with conventional technology. An I n n o v a t i o n in r e c e n t y e a r s h a s b e e n to create s y n e r g y t h r o u g h c o m b i n e d c a t a l y t i c and s e p a r a t i o n t e c h n o l o g i e s . Catalytic m e m b r a n e reactor o p e r a t i o n is a t y p i c a l e x a m p l e of s u c h a s y n e r g i s t i c combination involving a chemical reaction sequence with a membrane-based chemical separation. Here Dr Edward Gobina and Professor R o n a l d H u g h e s o u t l i n e research w o r k b e i n g carried o u t at t h e U n i v e r s i t y of Salford, UK. p r o c e s s i n g is possible. S u c h materials include porous ceramic (e.g. a l u m i n a , silica, zirconia, titania and magnesia) and m e t a l l i c m a t e r i a l s (e.g. s t a i n l e s s steel, b r o n z e a n d silver). In t h e o r y therefore, a l m o s t a n y c h e m i c a l r e a c t i o n c a n b e n e f i t from the i n c o r p o r a t i o n of a m e m b r a n e i n s o m e form. For e x a m p l e i n p a r t i a l o x i d a t i o n r e a c t i o n s w h e r e gas p h a s e oxygen h a s b e e n s h o w n to r e a c t with the d e s i r e d p r o d u c t to give total o x i d a t i o n p r o d u c t s , a m e m b r a n e c a n be u s e d to create a r e a c t i o n i n t e r f a c e b e t w e e n the h y d r o c a r b o n feed a n d oxygen. B e c a u s e the s t o i c h i o m e t r y of oxygen c a n n o w be c o n t r o l l e d w i t h i n t h e vicinity of the c h e m i c a l
I n a m e m b r a n e reactor, it is p o s s i b l e to c o m b i n e a c h e m i c a l reaction transformation with a membrane-assisted chemical s e p a r a t i o n . By t h i s m e a n s , it is p o s s i b l e t h e r e f o r e to i m p r o v e t h e efficiency of m a n u f a c t u r i n g via i n - s i t u p u r i f i c a t i o n . U n t i l very recently, catalytic m e m b r a n e s incorporated in chemical reactor s y s t e m s were m a i n l y a p p l i e d i n b i o c h e m i c a l r e a c t i o n s d u e to t e m p e r a t u r e l i m i t a t i o n s of t h e m o r e widely u s e d o r g a n i c m e m b r a n e s . Now, however, t h a n k s to r e c e n t a d v a n c e s i n materials science engineering r e s u l t i n g i n t h e d i s c o v e r y of newer materials, hightemperature membrane
t r a n s f o r m a t i o n , total o x i d a t i o n r e a c t i o n s c a n be e l i m i n a t e d . This h a s a n u m b e r of b e n e f t s i n c l u d i n g a v o i d a n c e of flammability constraints and hot spots. In a d d i t i o n , yields c a n b e optimized a s a r e s u l t of m o r e efficient r a t e control. A n o t h e r p r o c e s s w h e r e m e m b r a n e s c a n be of i m m e n s e b e n e f i t is i n selective r e m o v a l of a p a r t i c u l a r specie(s) in a n equilibrium limited reaction s e q u e n c e to i m p r o v e yield. A typical e x a m p l e is i n esterification r e a c t i o n s . In t h e s e r e a c t i o n s a n acid r e a c t s w i t h a n a l c o h o l to p r o d u c e a n e s t e r a n d water. However, t h e e s t e r a n d w a t e r produced can undergo a back r e a c t i o n : t h u s , t h e p e r c e n t yield is e q u i l i b r i u m limited. P e r v a p o r a t i o n c a n be u s e d to r e m o v e w a t e r during the reaction, a n d thus shift t h e e q u i l i b r i u m to i n c r e a s e yield.
The practical use of m e m b r a n e r e a c t o r s for industrial applications Although abundant opportunities do exist for the i n d u s t r i a l a p p l i c a t i o n of h i g h - t e m p e r a t u r e m e m b r a n e reactors, their implementation has until recently b e e n very limited. T h e m a j o r r e a s o n for this s t e m s from the difficulty i n c h a n g i n g a l r e a d y e s t a b l i s h e d priorities for n e w e r o n e s w i t h all t h e a s s o c i a t e d p r o b l e m s . Nowadays, however, m e m b r a n e separations are being r e c o g n i s e d a s e n e r g y efficient
HYDROGEN OR SPECIES PERMEATION i,'////
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Figure 1: Schematic diagram of membrane reactor operation under cocurrent
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M e m b r a n e T e c h n o l o g y No. 76
Feature substitutes to conventional technology in n u m e r o u s industrial processes. This together with the development of more advanced processes for m e m b r a n e m a n u f a c t u r e s u c h as the radio frequency (r.f.) m a g n e t r o n sputtering technique and pulsed laser technology is increasing m e m b r a n e acceptability in industry. To explore the possibilities offered by the incorporation of s u c h reactors in existing a n d already established processes, researchers at the University of Salford, UK have studied b o t h by mathematical simulation and
experimentation the selective removal of by-product hydrogen during the catalytic dehydrogenation of alkanes using various types of high temperature membranes. Such reactors facilitate the m a n u f a c t u r e of the primary product, by driving the equilibrium towards high conversion. Figure I describes the major features of the m e m b r a n e reactor operation. Essentially, it describes the flow of reacting gas species through a m e m b r a n e tube which contains catalytic material h o u s e d within a stainless steel shell. Because the
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M E M B R A N E THICKNESS~um Figure 2: Reactor p e r f o r m a n c e a s a f u n c t i o n o f m e m b r a n e t h i c k n e s s f o r the d e n s e composite m e m b r a n e s y s t e m s . (Feed: EB = 5 0 mol%/N2 = 50 tool%; W / F A o = 5 x 105 g c a t . s / g m o l ; T = 873K).
M e m b r a n e T e c h n o l o g y No. 7 6
m e m b r a n e tube is permeable to one (dense membrane) or more (porous membrane) p r o d u c t species, the reaction rate and eventually the conversion can be affected as a result of equilibrium displacement. Sweeping the shell-side [annular space) with an appropriate gas can increase the driving force for permeation and thus e n h a n c e the conversion far beyond the equilibrium value. A comparative a s s e s s m e n t of the performance of two m e m b r a n e categories (dense and porous) and five m e m b r a n e types (Pd/Ag, silica, polyamide, Ru-impregnated porous and inert porous composites) h a s been carried out. This is shown in Figures 2 and 3 respectively, For the dense composite systems (Figure 2), the Pd/Ag composite does show good promise at small m e m b r a n e thicknesses. Pinholes and cracks are undesirable in applications where ultrapure hydrogen production is a major priority otherwise, the use of s u c h systems with a certain a m o u n t of pinholes and cracks does not cause too m u c h problems. The dense silica system, with its high hydrogen selectivity b u t poor permeability shows the least performance. However, this system is particularly useful in c i r c u m s t a n c e s where corrosive chemicals s u c h as HBr and H2S are present. For the porous systems (Figure 3), the Ru-d!spersed system shows good promise. With high dispersion of the active metal, a greater surface area of the material is exposed to hydrogen thus increasing its chemisorption. Membrane porosity is also an important parameter since this will determine how m u c h feed could be lost into the permeate stream. Increasing the porosity also increases the a m o u n t of ethylbenzene lost into the permeate. Table 1 shows the performance of dense and porous systems during lean and rich feed processing. While lean feeds would be advantageous in the dense Pd/Ag systems, there is no advantage to be gained in using s u c h compositions for the porous system. High t e m p e r a t u r e catalysis exploiting the structural and mechanical stability of s u c h
7
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Figure 3: Reactor p e r f o r m a n c e a s a f u n c t i o n o f m e m b r a n e t h i c k n e s s a n d porosity f o r the porous composite m e m b r a n e s y s t e m s . (Feed: E B = 5 0 mol%/N2 = 5 0 tool%; W / F A o = 5 x 105 g c a t . s / g m o l ; T = 873K).
Table 1. M e m b r a n e reactor p e r f o r m a n c e during lean a n d rich f e e d operations ( s w e e p g a s f l o w r a t e = 3 0 0 c m 3 (STP)/min; T = 873K).
Contact Time Parameter (WIFAo x 10 "s gcat.s/gmol)
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Ethylbenzene Conversion, % Lean Feed (EB = 10%/H2, = 5 % / N 2
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Rich Feed 8%/N2 = 7%)
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Pd-Ag System
Porous System
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18.3 25.1 30.0 33.0 35.5
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M e m b r a n e T e c h n o l o g y No. 7 6
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MEMBRANE THICKNESS,~m Figure 4: Conversion e n h a n c e m e n t during n - b u t a n e d e h y d r o g e n a t i o n via kinetic coupling. (Feed: n - b u t a n e = 25 mol% / N2 = 75 mol %; W / F A o = 4.3 x lOO g c a t . s / g m o l . T = 6 7 0 K; s w e e p rate = 150 c m 3 (STP)/min). composite membrane systems c o u l d well f o r m t h e b a s i s for n e w c a t a l y t i c p r o c e s s e s in t h e chemical and petrochemical industry. The significant a d v a n t a g e s of w h i c h i n c l u d e a r e d u c t i o n in e n e r g y r e q u i r e m e n t s while enhancing product yields a n d selectivity. In t h e d e n s e Pd-Ag c o m p o s i t e m e m b r a n e , we h a v e s t u d i e d e x p e r i m e n t a l l y t h e p o s s i b i l i t y of further thermodynamic enhancement via kinetic coupling of t h e p e r m e a t e h y d r o g e n w i t h a r e a c t i v e s w e e p gas. F o r t h i s w e have employed the catalytic d e h y d r o g e n a t i o n of n - b u t a n e to b u t e n e in t h e t u b e - s i d e w h i l e oxygen and carbon monoxide was u s e d a s s w e e p g a s e s in t h e s h e l l - s i d e . T h i s is s h o w n in F i g u r e 4. W h i l e o n l y s m a l l c o n c e n t r a t i o n s of t h e r e a c t i v e c o m p o n e n t s is r e q u i r e d to s e c u r e maximum n-butane conversion, t h e u s e of o x y g e n is m o r e effective than carbon monoxide.
Membrane
T e c h n o l o g y No. 7 6
Process
economics
In t h e i n d u s t r i a l m a n u f a c t u r e of petrochemical feedstocks innovative practices are i m p l e m e n t e d to i n c r e a s e p r o d u c t yield a n d r e d u c e overall e n e r g y r e q u i r e m e n t . F o r e x a m p l e for a 190 m e t r i c t o n n e s p e r d a y styrene production plant (assuming 330 days per year operation), a 1% i n c r e a s e in ethylbenzene conversion r e p r e s e n t s a n i n c o m e g a i n of about £1M/year. Therefore using t h e levels of e n h a n c e m e n t s achieved represents an income g a i n r a n g i n g from £ 4 0 M for t h e Pd-Ag m e m b r a n e to £ 1 4 M for t h e d e n s e silica. F u r t h e r m o r e , in t h e dense systems, the hydrogen r e m o v e d c a n b e u s e f u l s i n c e it c o n t a i n s v e r y low i m p u r i t i e s . This can provide an additional i n c o m e of a b o u t £ 5 0 , 0 0 0 / y e a r assuming a 40% recovery rate of t h e p e r m e a t e h y d r o g e n . Therefore there are sufficient e c o n o m i c a d v a n t a g e s for membrane integration into existing process technology.
Conclusion The synergy between catalytic transformation and a membrane-based chemical s e p a r a t i o n d o e s offer n u m e r o u s a d v a n t a g e s a n d n e w p r o s p e c t s for carrying chemical reactions. Composite membrane systems ( d e n s e or p o r o u s ) will b e v e r y i m p o r t a n t in t h e f u t u r e a p p l i c a t i o n of m e m b r a n e c a t a l y s i s . E x t e n d e d s t u d y of t h e m e c h a n i s m of t h i n film f o r m a t i o n o n a v a r i e t y of s u b s t r a t e s a n d long t e r m s t a b i l i t y d o e s s e e m required. This together with o p t i m i z a t i o n of p r o c e s s e s for producing very thin but c o n t i n u o u s m e m b r a n e over larger areas with a high degree of r e p r o d u c i b i l i t y c o u l d well revolutionize established practices. For f u r t h e r information, contact: E d w a r d Gobina a n d Ronald Hughes, Department of Chemical Engineering, University o f Salford, M a x w e l l Building, Salford M5 4WT, UK. Tel: +44 161 745 5 0 8 1 ; f a x : +44 161 745 5999.
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