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: ~ ; i : : ' % L

ELSEVIERP I 1 : S 0 9 6 0 - 8 5 2 4 ( 9 8 ) 0 0 0 4 8 - 0

Bioresource Technology 66 (1998) 75-78© 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0960-8524/98 $19.00

S h o r t C o m m u n i c a t i o n

S e l e c t i o n o f A d s o r b e n t s t o b e u s e d i n

a n E t h a n o l F e r m e n t a t i o n P r o c e ss .

A d s o r p t i o n I s o t h e r m s a n d K i n e ti cs

Abstrac t

Adsorption equil ibrium assays were carried out in

order to f in d adsorben ts se lec t ive for e thano l to use in

a ferm enta t ion process . Three hydrophobic adsorben ts

- - s i l i ca l i te , ZSM-5 and CMS-5A - - were tes ted .

Resu l t s o f adsorp t ion equ i l ibr ium a t 298 K o f e thano l ,

g lucose , f ruc tose and g lycero l f rom aqueous so lu t ions

o f each s ingle solute are given. E tha no l adsorption

iso therms were descr ibed by Langmuir - type equa t ions .

Som e k inet ic assays o f e thano l adsorp t ion are al so

presen ted . The resu lt s ob ta ined appear to be use fu l for

pract ic ioners on the fermenta t ion-processes f i e ld .

© 1998 Elsevier Science Ltd. All r ights reserved

Key words: e t h an o l ad s o r p t i o n , C M S - 5 A , s i l i c a l i t e ,

Z S M - 5 .

I N T R O D U C T I O N

I n e t h a n o l f e r m e n t a t i o n r e l a ti v e l y h i g h c o n c e n t r a -

t i o n s o f e t h a n o l i n c r e a s e t h e p r o d u c t i n h i b i t i o n

e f f e c t ( M a i o r e l l a et al . , 1 9 8 4 ; L e t o u r n eau & V i l l a ,

1 98 7) . T h i s l i m i ts t h e a m o u n t o f e t h a n o l w h i c h c a n

b e p r o d u c e d p e r u n i t o f ti m e a n d v o l u m e o f

f e r m e n t o r . S e v e r a l m e t h o d s t o i m p r o v e t h e a l c o h o l

p r o d u c t i v i t y b y r e d u c i n g t h e e t h a n o l c o n c e n t r a t i o ni n t h e b r o t h h a v e b e e n s u g g e s t e d ; s u c h a s r e d u c t i o n

o f t h e p r e s s u r e i n th e r e a c t o r ( C y s e w s k i & W i l k e ,

1 9 7 7 ; F i n n & R am a l i n g am , 1 9 7 7 ) , o n - l i n e s o l v en t

e x t r a c t i o n ( M i n e r & G o m a , 1 9 82 ) o r s e l e c ti v e

e t h a n o l a d s o r p t i o n ( M i l e s t o n e & B i b b y , 1 9 8 3 ; L e e &

Wang , 1982 ; P i t t et al . , 1983; Lenck i et al . , 1983) .

C o n t i n u i n g o u r r e s e a r c h o n e t h a n o l i c f e r m e n t a -

t i o n ( B r av o & G o n z f i l ez , 1 99 1 ), w e a r e d ev e l o p i n g a

d e v i c e f o r a f e r m e n t a t i o n p r o c e s s i n a f l u i d i z e d b e d

b i o r e a c t o r w i t h i m m o b i l i z e d c e l l s o f S a cch a r o m yces

cerevisiae i n c a l c i u m - a i g i n a t e , t o g e t h e r w i t h s i m u l t a -

n e o u s e x t r a c t i o n o f t h e p r o d u c t e t h a n o l b y a d s o r p -

t i o n o n t o a n a d s o r b e n t . A n i n it ia l p h a s e o f t h is

s t u d y co n s i s t s o f s c r een i n g p o s s i b l e s o r b en t s . T h e

a i m o f t h e p r e s e n t w o r k w a s t o p r o v i d e r e s u lt s f o r

75

e t h a n o l a d s o r p t i o n o n t h r e e d i f f e r e n t a d s o r b e n t s a s

a n a i d t o w a r d s t h e a s s e s s m e n t o f t h e p o t e n t i a l o f

t h a t p r o ces s .

W e h a v e f o c u se d o n t h e h y d r o p h o b i c a d s o r b e n t s

s i l i c a l i t e , Z S M - 5 a n d C M S - 5 A b e c a u s e i t c o u l d b e

e x p e c t e d t h a t t h e y w o u l d s h o w h i g h e r s e l e c ti v it y f o re t h a n o l t h a n f o r w a t e r a n d t h e o t h e r c o m p o n e n t s o f

t h e f e r m e n t a t i o n b r o t h . N e v e r t h e l e s s , w e h a v e a l s o

t e s t e d t h e a d s o r p t i o n c h a r a c t e r i st i c s o f s u g a r s a n d

g l y c e r o l o n t o t h e s o li d s in c o n c e n t r a t i o n r a n g e s

u s u a l ly f o u n d f o r t h o s e c o m p o n e n t s i n e t h a n o l i c

f e r m e n t a t i o n p r o c e s s e s .

M E T H O D S

M a t e r ia l s

A d s o r b en t s

T w o s y n t h e t i c s i l i c a m o l ecu l a r s i ev es , s i l i c a l i t e

( U O P ) , Z S M - 5 ( M o b i l C h e m i c a l I n te r n a t i o n a l) a n d

a n a c t i v a t e d c a r b o n m o l e c u l a r s i e v e , C M S - 5 A

( T a k e d a C h e m i c a l I n d u s t r i e s ) , w e r e t e s t e d a s a d s o r -

b en t s . T h e s i ze o f th e p e l l e t i zed cy l i n d r i ca l s o l id s

w a s : 3 × 1 m m f o r p e l l e ts o f Z S M - 5 , 5 × 1 m m f o r

s il ic a l it e a n d 8 × 4 m m f o r C M S - 5 A . B e f o r e e a c h

r u n , Z S M - 5 a n d s i l i c a l i t e w e r e r e g e n e r a t e d b y

w a s h i n g r e p e a t e d l y w i t h u l t r a p u r e b o i li n g w a t e r a n d

d r y i n g a t 2 0 0 - 2 2 0 °C . C M S - 5 A w a s d i r e c t ly u s e d ,

a f t e r w a s h i n g , a s a w e t s o li d , b e c a u s e p r e l i m i n a r y

t e s ts s h o w e d t h a t i t s a d s o r p t i o n c a p a c i t y w h e n w e t

w a s s l i g ht ly h i g h e r t h a n w h e n d r i e d .

A d s o r b a t e s

E t h a n o l , g l u c o se , f r u c t o s e a n d g l y c e ro l w e r e u s e d i n

c o n c e n t r a t i o n r a n g e s l a r g e e n o u g h t o c o v e r a ll l ev e l s

w h i c h t y p i c a ll y a p p e a r i n f e r m e n t a t i o n b r o t h s .

S y n t h e t i c s o l u t io n s i n t h e r a n g e s 0 - 5 0 % ( w /w ) f o r

e t h a n o l - w a t e r m i x tu r e s, 0 - 2 - 1 5 % ( w /w ) fo r g l u c o s e -

w a t e r so l u ti o n s , 0 - 2 - 8 % ( w /w ) f o r f r u c t o s e - w a t e r

a n d 0 . 2 - 0 . 7 % ( w /w ) fo r g l y c e r o l - w a t e r w e r e

p r e p a r e d .

Kinet ic and equi l ibr ium exper iments

T h e e q u i li b ri u m a d s o r p t io n m e a s u r e m e n t s a t

2 5 _ 0 " 1 ° C a n d p H 4 . 0 + 0 . 1 w e r e c o n d u c t e d b y

i m m e r s i n g i n s t o p p e r e d f l a s k s k n o w n m a s s e s o f

a d s o r b e n t a n d s o l u t io n o f i n it ia l s o l u t e c o n c e n t r a -

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76 S h o r t c o m m u n i c a t i o n

t i o n C o. T h e s t i r r e d s o l u t i o n s w e r e a l l o w e d t o r e a c h

e q u i l i b r i u m a n d , a f t e r f i l tr a t io n , c l e a r s a m p l e s w e r e

t a k e n a n d a n a l y s e d i n o r d e r t o d e t e r m i n e t h e

c o n c e n t r a t i o n o f s o l u t e in s o l u t i o n a t e q u i li b r i u m ,

C e .

A s i m i l a r p r o c e d u r e w a s f o l l o w e d in s o m e e x p e r i -

m e n t s w i t h e t h a n o l - w a t e r s o l u t i o n s . I n t h i s c a s e ,

s a m p l e s o f s o l u t i o n w e r e i n t e r m i t t e n t l y t a k e n b y

m e a n s o f a h y p o d e r m i c s y r in g e t h ro u g h a r u b b e r

s e p t u m o n t h e t o p o f th e c l o s e d f la s k. T h e e v o l u t i o n

o f e th a n o l c o n c e n t r a t i o n w i th t i m e w a s d e t e r m i n e d

b y a n a ly s i n g e a c h s a m p l e .

A n a l y t ic a l m e t h o d s a n d m a s s b a l a n c e s

C o n c e n t r a t i o n s o f e th a n o l , c a r b o h y d r a t e s a n d

g l yc e r o l w e r e m e a s u r e d b y H P L C u s in g a n A m i n e x

I o n E x c lu s i o n H P X - 8 7 H B i o R a d c o l u m n a n d

0 "0 1 N H 2 5 0 4 s o l u t i o n a t 6 0° C a s m o b i l e p h a s e a n d

a f l ow r a t e o f 0 .7 c m 3 m i n - l . E l u t e d m a t e r i a l s w e r e

d e t e c t e d u s i n g a W a t e r s d i f fe r e n t ia l r e f r a c t o m e t e r

d e t e c t o r 4 10 ( W a t e r s C h r o m a t o g r a p h y D i v is i on /

M i l l ip o r e C o r p o r a t i o n , 3 4 M a p l e S t r e e t /M i l f o r d ,

M A 0 1 7 5 7 ) .

T h e a d s o r p t i o n u p t a k e C s , g a d s o r b e d s o l u t e / g

s o l id a d s o r b e n t , w a s f o u n d b y a m a s s b a l a n c e o n t h e

a d s o r b e d s p e c i e s, a s s u m i n g w a t e r w a s n o t s i g ni fi -

c a n t l y a d s o r b e d .

W h e n u s in g d r y a d s o r b e n ts , t h e m a s s b a l a n c e

y i e l d e d t o t h e e q u a t i o n :

M ( C o - - C ,, )C s - ( 1 )

S ( 1 O 0 - - C e )

M a n d S b e i n g t h e m a s s ( g ) o f i n i ti a l s o l u t i o n a n d

d r y a d s o r b e n t , r e s p e c t iv e l y .

W h e n u s i n g w e t a d s o r b e n t , t h e f o l l o w i n g e q u a t i o n

w a s u s e d :

M C o - - ( M + S w - - S ) C eCs = (2 )

S ( I O 0 - C e)

S w b e i n g t h e m a s s ( g ) o f w e t a d s o r b e n t .

R E S U L T S A N D D I S C U S S I O N

K i n e t i c s

K i n e t i c e x p e r i m e n t s o n s i li c a li te a n d C M S - 5 A w e r e

c o n d u c t e d a t t w o i n i t i a l e t h a n o l c o n c e n t r a t i o n s :

C o = 1 0 " 1 5 % a n d 5 . 0 9 % ( w / w ) .

F o r b o t h a d s o r b e n ts , t h e d e c r e a s e o f e t h a n o l

c o m p o s i t i o n i n s o lu t i o n w a s s l i g ht ly h i g h e r w h e n t h e

i n it ia l s o l u t io n w a s m o r e c o n c e n t r a t e d . I n g e n e r a l ,

a n i n i t ia l f a s t a d s o r p t i o n o f e t h a n o l f o l l o w e d b y as e c o n d s l o w s t a g e w a s o b s e r v e d . A s i m i l a r p e r f o r m -

a n c e w a s f o u n d w h e n c h e c k i n g th e a d s o r p t i o n o n

Z S M - 5 . A s s u g g e s t e d f r o m t h e k i n e t i c p r o f i l e s , t h e

k i n e t i c r e s u l t s w e r e a s s a y e d t o f i t a f i r s t - o r d e r k i n e t i c

m o d e l :

C t = C o e - k ' (3 )

C t ( % w / w ) a n d k ( m i n 1 ) b e i n g t h e e t h a n o l

c o n c e n t r a t i o n i n s o l u ti o n a t t im e t ( m i n ) a n d t h e

k i n e t i c r a t e c o n s t a n t , r e s p e c t i v e l y .

B y l i n e a r i z a t io n o f e q n ( 3 ), a n a c c e p t a b l e f it w a s

a c h i e v e d f o r t h e t w o s u c c e s s i v e f i r s t - o r d e r s t e p s .T a b l e 1 s h o w s t h e l i n e a r f i tt e d p a r a m e t e r s , t o g e t h e r

w i t h e x p e r i m e n t a l r e s ul ts . F o r e t h a n o l a d s o r p t i o n o n

s i l ic a l i te a t C o = 1 0 - 1 5 % , a l o w k - v a l u e r e s u l t e d f o r

t h e s e c o n d s t e p , t h i s f a c t s h o w i n g t h a t t h e o v e r a l l

a d s o r p t i o n o c c u r s i n t h e f i r s t s t a g e .

T able 1 . K ine t i c s o f e thano l adsorpt ion o nto s i l i ca li t e and C M S-5A : exper im enta l re su l t s and l inear param eters f it t ed toa tw o- s tage f i r s t order m ode l

S i l ica l i te S i l ica l it e CM S -5 A CM S -5 A

C t t C t t C t t C t t

10.15 0 5"09 0 10-15 0 5"09 09.92 5 4.92 8 10.05 5 4"85 59.75 15 4.81 23 9.93 10 4.71 209.73 35 4.78 39 9.63 50 4.63 359.75 70 4.75 57 9.51 65 4.53 529.79 80 4.74 72 9.50 85 4.49 839"78 110 4.73 90 9.46 105 4.48 1079"76 125 4.71 500 9"40 144 4.42 1379"73 500 9.36 500 4-32 500

C o n s t a n t s o f e q n ( 3 )S t e p 110.15 > C t > 9"75k = 2-55 x 1 0 -3

S t e p 2Ct < 9-75k = 1.26 x 10 .5

C o n s t a n t s o f e q n ( 3 )S t e p 1

5"09 > C t > 4"78k = 2"37 x 10 -3

S t e p 2Ct _< 4-78

k = 1.33 x 10 -4

C o n s t a n t s o f e q n ( 3 )S t e p 1

10.15 > C t > 9"70k = 2"02 x 10 3

S t e p 2Ct < 9-70

k = 2.70 x 10 4

C o n s t a n t s o f e q n ( 3 )S t ep 1

5.09 > Ct > 4.71k = 5.74 x 10 3

S t e p 2Ct < 4.71

k = 4 .80 x 1 0 -4

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Shor t communica t ion 77

T a b l e 2 . A d s o r p t i o n i s o t h e r m s o f e t h a n o l o n t o s i li c a l it e , C M S - 5 A a n d Z S M - 5 a t 2 9 8 K : e x p e r i m e n t a l r e s u l t s a n d

L a n g m u i r - ty p e a d s o r p t i o n p a r a m e t e r s

Silicalite CMS-5A ZSM-5

C~ C~ C~ C¢ Cs C~

1.274 0-016 0.322 0.032 0"365 0.0232-753 0.025 0.753 0.039 0.739 0"0364-700 0.034 2.144 0"055 1'641 0"0457"778 0"051 4.222 0.074 0.614 0.052

11.548 0.056 6.164 0.089 5"539 0'05614.609 0.064 9.705 0.107 7"576 0"05722-109 0.065 13.685 0" 116 13"692 0.05829-704 0.066 26.502 0.115 25"418 0.06440-484 0.072 30.323 0"116 28.921 0"06849-656 0.075 33"078 0.117 39.309 0-071

Langmuir parameters Langmuir parameters Langmuir parametersa = 0.015 a = 0"058 a = 0-095b = 0"179 b = 0"464 b = 1'437Norm = 0"008 Norm = 0-023 Norm = 0-009

Langmuir equation C~ = a'Ce/(1 + b'Ce).

E q u i l i b r i u m

A d s o r p t i o n o f e t h a n o l

Ethanol adsorption equilibrium results on to silica-

lite, ZSM-5 and CMS-5A from ethanol-water

mixtures are given in Table 2. The shape of the

isotherms displays the adsorption type favourable in

all cases. Therefore, a Langmuir-type equation was

used to represent equilibrium adsorption. Values of

model parameters, a and b, were determined by

fitting the experimental data to the proposed

equation, using the program SigmaPlot ScientificGraphing System. The magnitude of the norm is

also shown in Table 2.

The asymptotic uptakes at ethanol adsorption

equilibrium, a / b , increase in the order: 0-068

(ZSM-5)<0.084 (silicalite)<0.126 (CMS-5A). The

calculated saturation capacities of silicalite and

ZSM-5 are smaller than those given by Farhadpour

(Farhadpour & Bono, 1988) and Milestone

(Milestone & Bibby, 1983), respectively, for the

same systems at similar temperatures: 0-131g

ethanol/g microcrystalline silicalite and 0-12 g

ethanol/g microcrystalline ZMS-5. The discrepancy

might be due to the use of pelletized adsorbents,which diminishes the hyd rophobicity by the presence

of the agglomerant. The content of A1203 in the

adsorbent ZSM-5, even while low, could justify the

observed capacity difference between ZSM-5 and

silicalite.

Concerning the CMS-5A equilibrium adsorption

data, we have not found any references in the litera-

ture. For the purpose of comparison, Lee and Wang

(1982) found an adsorption capacity value of 0.13 g

ethanol/g adsorbent for the adsorption of ethanol on

to a conventional activated carbon.

A d s o r p t i o n o f g lu c o se , f r u c to s e a n d g l yc e ro l

An experimental adsorption study of the systems

glucose-water, fructose-water and glycerol-water

was made. No measurable adsorption of glucose,

fructose and glycerol on silicalite and ZSM-5 was

found. However, a maximal adsorption of glucose

(0"011g glucose/g adsorbent), fructose (0.010g

fructose/g adsorbent) and glycerol (0-014 g glycerol/g

adsorbent) on CMS-5A was found.

The non-adsorption of substrate, together with a

good selectivity for ethanol, make the use of silica-

lite or ZMS-5 as sorbents in removing ethanol from

fermentation broths attractive. The CMS-5A sieve

shows minimal adsorption of substrate. We alsochecked the multicomponent adsorption of ethanol,

glucose and fructose on to this adsorbent. The

results (not shown) indicated that the adsorption of

ethanol did not decrease significantly from the

binary to the multico mponent, while the ad sorption

of the substrates glucose and fructose in the multi-

component system was even lower than in their

respective binary systems. In conclusion, the adsor-

bents screened in this work seem to be promising to

use in ethanol fementation processes. Nevertheless,

we should stress that the results of this preliminary

study correspond to batch processes and these donot mimic the conditions which would be found in a

real fermentation. Further work would be needed to

examine aspects of mass transfer.

R E F E R E N C E S

Bravo, P. & Gonz~ilez, G. (1991). Continuous ethanolfermentation by immobilized yeast cells in a fluidized-bed reactor. J. Chem . Tech. Biotechnol. , 52, 127-134.

Cysewski, G. R. & Wilke, C. R. (1977). Rapid ethanolfermentat ions using vacuum and cell recycle. Biotechnol.Bioeng., 19, 1125-1143.

Farhadpour, F. A. & Bono, A. (1988). Adsorption fromsolution of nonelectrolytes by microporous crystallinesolids: ethanol-water/silicalite system.Journal of Colloidand Interface Science, 124, 209-227.

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A. Cart6n, G. Gonz~i lez Benito*,

J . A. Rey & M. de la Fuente

Department of Chemical Engineering,Faculty of Sc iences, University o f Valladolid,47011 Valladolid, Spain

(Rec e iv ed 1 4 Ju ly 1 9 9 7; r ev i sed v e r s io n r ece iv ed6 F eb ru ary 1 9 9 8 ; accep t ed 1 2 F eb ru ary 1 9 9 8 )

* A u t h o r t o w h o m c o r r e s p o n d e n c e s h o u l d b e a d d r e s s e d .