Multicomponent Adsorption Isotherms on HydrogenMordenite

56 Multicomponent Adsorption Isotherms on Hydrogen-Mordenite

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JAMES I. JOUBERT and IMRE ZWIEBEL Chemical Engineering Department, Worcester Polytechnic Institute, Worcester, Mass. 01609

Adsorption isotherms of oxygen, nitrogen, carbon dioxide, and sulfur dioxide on hydrogen-mordenite were measured at several temperatures in the range of 0°-100°C. The SO and CO2 had considerably greater affinity for the adsorbent than the O and N . Using the pure-component data, multicomponent isotherms were predicted and compared with experimental results. The more strongly adsorbed species completely overwhelm the lesser adsorbed components ( e.g., SO vs. N ). Wherever 2 species of approximately equal affinity are adsorbed (e.g.,CO + SO ), the temperature sensitivity of the individual components influences the extent of the competition. 2

2

2

2

2

2

2

* ^ p h e literature contains a r e l a t i v e l y m e a g e r a m o u n t of m u l t i c o m p o n e n t •*· a d s o r p t i o n d a t a for i n o r g a n i c gases. T h e e a r l y literature was s u m marized by Young and Crowell (8),

a n d since t h e n , the b u l k of

the

p u b l i s h e d d a t a has dealt w i t h mixtures of o r g a n i c v a p o r s or i n o r g a n i c adsorbates at l i q u i d n i t r o g e n temperatures. M o t i v a t e d b y the a i r p o l l u t i o n c o m b a t , o u r r e s e a r c h w a s d i r e c t e d at the c o m p o n e n t s

m a k i n g u p a s i m u l a t e d exhaust gas, n a m e l y

nitrogen,

o x y g e n , c a r b o n d i o x i d e , a n d s u l f u r d i o x i d e . T h e experiments w e r e

re

s t r i c t e d to the 0 ° - 1 0 0 ° C r a n g e at 1 a t m absolute pressure. F i r s t , p u r e - c o m p o n e n t isotherms w e r e m e a s u r e d .

A s a consequence

of p r e l i m i n a r y screening i n w h i c h s u c h adsorbents as several c o m m e r c i a l grade 5 A sieves, 1 3 X sieve, H - m o r d e n i t e a n d a c t i v a t e d c a r b o n w e r e c o m p a r e d , the h y d r o g e n f o r m of m o r d e n i t e w a s selected as the a d s o r b e n t f o r the m u l t i c o m p o n e n t experiments.

This decision was based u p o n capacity,

selectivity, a n d d u r a b i l i t y c r i t e r i a , since p r a c t i c a l a p p l i c a t i o n u n d e r rather severe c o n d i t i o n s w a s one of the objectives. 209

In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

210

M O L E C U L A R SIEVE ZEOLITES

II

Experimental


T h e a p p a r a t u s u s e d is a m o d i f i c a t i o n of t h e e q u i p m e n t e m p l o y e d b y L e w i s et al. (6). F o r p u r e c o m p o n e n t s , i t is a c o n s t a n t - v o l u m e u n i t ; f o r gas m i x t u r e s , i t operates at constant t o t a l pressure ( n e a r l y constant v o l u m e ). It u t i l i z e s a n i n t e r n a l r e c i r c u l a t i o n d e v i c e to expose t h e adsorbent c o n t i n u o u s l y t o a gas m i x t u r e of a p p r o x i m a t e l y k n o w n c o m p o s i t i o n , a n d thus a v o i d l o c a l i z e d c o n c e n t r a t i o n v a r i a t i o n s w h i c h c o u l d arise as a result of differences i n t h e d i f f u s i v i t i e s of t h e v a r i o u s species. T h e p u r e - c o m p o n e n t isotherms w e r e d e t e r m i n e d b y a d m i t t i n g suc cessive i n c r e m e n t s of adsorbate i n t o t h e c h a m b e r a n d m e a s u r i n g t h e pressures. F o r t h e m u l t i c o m p o n e n t measurements, t h e system pressure is m a i n t a i n e d constant b y a manostat. W h i l e e q u i l i b r a t i o n takes p l a c e , the gas is c i r c u l a t e d i n a l o o p c o n t a i n i n g t h e a d s o r p t i o n c h a m b e r a n d a large reservoir w h e r e t h e c o n c e n t r a t i o n is a p p r o x i m a t e l y constant. A t the t e r m i n a t i o n of t h e e x p e r i m e n t , t h e a d s o r p t i o n c h a m b e r is i s o l a t e d , a n d t h e final c o m p o s i t i o n i n t h e reservoir is m e a s u r e d b y w i t h d r a w i n g a s a m p l e a n d a n a l y z i n g i t w i t h a mass spectrometer. T h e n , b y d e s o r b i n g , m e a s u r i n g t h e t o t a l a m o u n t of desorbate, a n a l y z i n g i t , a n d c o r r e c t i n g f o r t h e v o i d space gas content, t h e d e s i r e d a d s o r b e d phase q u a n t i t i e s are o b t a i n e d . T h e adsorbents are a c t i v a t e d in vacuo b e l o w 10" t o r r at 3 5 0 ° - 4 0 0 ° C f o r 16 h o u r s . D e s o r p t i o n s are c a r r i e d o u t b y r e p e a t e d l y c o n n e c t i n g t h e a d s o r p t i o n c h a m b e r to a n e v a c u a t e d vessel f r o m w h i c h t h e desorbate is c o l l e c t e d i n a m i x i n g c h a m b e r . D e s o r p t i o n s u s u a l l y are c a r r i e d o u t at the a d s o r p t i o n t e m p e r a t u r e , a n d completeness of r e m o v a l is c h e c k e d b y m a t e r i a l b a l a n c e ; i t is u s u a l l y w i t h i n 2 % . O c c a s i o n a l l y , to s p e e d u p t h e p r o c e d u r e , desorptions m a y b e c a r r i e d o u t at e l e v a t e d temperatures. 5

The hydrogen-mordenite (unit cell; hydrated H A l S i o 0 6 · 2 4 H 0 ) u s e d i n this s t u d y w a s p r o v i d e d b y t h e N o r t o n C o . , W o r c e s t e r , M a s s . , i n the f o r m of 1 / 1 6 - i n c h pellets f a b r i c a t e d w i t h o u t a b i n d e r . T h i s m a t e r i a l is c h a r a c t e r i z e d b y p a r a l l e l 1 2 - m e m b e r e d rings of s i l i c a - a l u m i n a tetrah e d r a f o r m i n g pores w i t h effective diameters of 7 - 9 A ; smaller cavities o c c u r i n t h e w a l l s of t h e large channels. M o r d e n i t e has r e p o r t e d B . E . T . surface area of 400 to 500 m / g r a m ( 3 ) ; synthesis a n d other characteris tics of this m a t e r i a l are d e s c r i b e d w e l l elsewhere ( I , 5 ) . 8

8

4

2

9

2

Correlations W h i l e u l t i m a t e l y t h e o b j e c t i v e of o u r project w i l l b e t h e t h e o r e t i c a l analysis of t h e b e h a v i o r of c o m p l e x m i x t u r e s , at this p o i n t w e o n l y c a n correlate o u r results w i t h p r e v i o u s l y d e v e l o p e d m u l t i c o m p o n e n t m o d e l s . W i t h the o b j e c t i v e of p r e d i c t i n g t h e t o t a l a m o u n t a d s o r b e d a n d t h e a d s o r b e d phase c o m p o s i t i o n f r o m n o m o r e t h a n p u r e - c o m p o n e n t d a t a , o n l y a f e w s p e c i a l l y selected systems h a v e b e e n u s e d i n p r e v i o u s e x p e r i m e n t a l p r o g r a m s to v e r i f y the v a r i o u s m o d e l s . H e n c e , these r e l a t i o n s h i p s , w h i c h usually were

derived f r o m theoretical

considerations,

have

not been

tested sufficiently y e t . T h u s , t h e y a l l c a n b e classified as e m p i r i c a l m o d e l s ,


56.

J O U B E R T AND

Multicomponent

zwiEBEL

Adsorption

Isotherms

211

e s p e c i a l l y as t h e y h a v e b e e n m o d i f i e d to m a k e t h e m r e a d i l y a p p l i c a b l e (2, 4,

6,8).

C o r r e l a t i o n s w i t h the m o d e l b a s e d d e v e l o p e d b y M y e r s a n d P r a u s n i t z (7),

on thermodynamic principles

hereafter r e f e r r e d to as the i d e a l

s o l u t i o n m o d e l , are p r e s e n t e d i n this p a p e r .

T h e m o d e l is b a s e d u p o n

the a s s u m p t i o n that R a o u l t ' s l a w is a p p l i c a b l e to the a d s o r b e d phase, a n d the s p r e a d i n g pressure of the m i x t u r e a n d those of the p u r e c o m p o n e n t s are set e q u a l at the same surface coverage.

U n l i k e some of the

other

m o d e l s , this one does not r e q u i r e that the p u r e c o m p o n e n t s a d h e r e to Downloaded by PRINCETON UNIV on September 30, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch056

a n y specific p u r e - c o m p o n e n t b e h a v i o r .

H e n c e , for e x a m p l e , the

S0 , 2

w h i c h w a s c o r r e l a t e d b y the L a n g m u i r e q u a t i o n , a n d C 0 , w h i c h best 2

fitted

the F r e u n d l i c h e q u a t i o n , c o u l d be c o m b i n e d r e a d i l y i n p r e d i c t i n g

m i x t u r e characteristics.

T h i s m o d e l c o u l d b e e x t e n d e d to h i g h e r cover

ages t h a n a n y of the other r e l a t i o n s h i p s ; h o w e v e r , the present w o r k is r e s t r i c t e d to the l o w coverage range. t i o n d a t a o n i n o r g a n i c adsorbates

F i n a l l y , this w o r k presents a d s o r p

w i t h w i d e l y different

characteristics

i n complex mixtures.

Results Pure Components. P u r e - c o m p o n e n t isotherms w e r e m e a s u r e d f o r 4 gases, o x y g e n , n i t r o g e n , c a r b o n d i o x i d e , a n d s u l f u r d i o x i d e . T h e first 3 e x h i b i t e d r e v e r s i b l e s o r p t i o n , w h i l e the

S0

2

s h o w e d a rather

broad

hysteresis l o o p . R e l a t i v e l y s p e a k i n g , the affinity of the h y d r o g e n - m o r d e n i t e is great est f o r the S 0 , t h e n i n d e c r e a s i n g o r d e r the C 0 , N , a n d 0 2

2

2

2

follow.

T h i s c a n b e e x p l a i n e d b y differences i n the electronic configurations of these adsorbates; n o " s i e v i n g " w o u l d be e x p e c t e d i n a n y of these systems since the m o l e c u l a r d i a m e t e r of these gas m o l e c u l e s is w e l l b e l o w the p u b l i s h e d p o r e diameters.

T h e s o r p t i o n of a l l the species is l i k e l y to b e

p h y s i c a l , since the c a l c u l a t e d isosteric heats are a l l b e l o w 5 k c a l / m o l e . T h e o x y g e n a n d n i t r o g e n e x h i b i t e d s i m i l a r characteristics; t h e y w e r e b o t h c o r r e l a t e d w e l l b y the F r e u n d l i c h i s o t h e r m . T h e i r s i m i l a r m o l e c u l a r structure m i g h t a c c o u n t for this, w i t h differences i n t h e i r p o l a r i z a b i l i t y b e i n g the major reason f o r t h e i r r e l a t i v e a d s o r b a b i l i t y . The C 0 respectively

2

a n d S 0 , h a v i n g c r i t i c a l temperatures of 3 1 . 1 ° a n d 1 5 7 . 2 ° C , 2

(vs.

—118.8°C

for 0

2

a n d —147.1 ° C f o r N ) , are 2

more

l i k e l y to adsorb b y the s o - c a l l e d m u l t i l a y e r m e c h a n i s m , a n d hence, i n the r e g i o n of the c u r r e n t experiments e x h i b i t c o n s i d e r a b l y h i g h e r capacities t h a n the 0

2

a n d N — a s m u c h as 15 times h i g h e r .

r e g i o n , the C 0

2

2

I n the e x p e r i m e n t a l

isotherms c o u l d b e c o r r e l a t e d b y the F r e u n d l i c h e q u a -


212

MOLECULAR

Table I.

SIEVE Z E O L I T E S

Correlation of Pure-Component Isotherms Ρ in torr, W in

mmol/gram

£5°

36°

56°

78°

100°

KP 7.44 0.909

3.31 0.943

2.62 0.946

1.52 0.970

1.42 0.924

0.824 0.950

Nitrogen; W = KP Κ Χ 10 17.4 m 0.841

7.56 0.870

5.28 0.891

2.55 0.936

1.78 0.919

0.836 0.977

KP 3.70 0.277

2.17 0.341

1.13 0.405

0.491 0.492

0.220 0.565

bP) 8.28 3.78

7.02 3.48

6.80 3.00

5.80 2.66

Temp., °C Oxygen; W = Κ Χ 10 m 4

II

0° m

m


4

C a r b o n D i o x i d e ; W == Κ X 10' 5.51 m 0.257

m

S u l f u r D i o x i d e ; W = WmbP/(l b Χ 10 11.95 9.13 Wm 4.18 3.90

+

3

S0

2

Figure 1.

PRESSURE S0

2

( torr)

adsorption isotherms

t i o n , a l t h o u g h there w e r e some d e v i a t i o n s

at l o w pressures.

d a t a s e e m e d to b e fitted best b y t h e L a n g m u i r e q u a t i o n .

The S 0

2

A t t e m p t s to fit

the o t h e r p u r e c o m p o n e n t s to t h e L a n g m u i r e q u a t i o n o r t h e B . E . T . e q u a t i o n w e r e n o t successful; h e n c e , the m i x e d L a n g m u i r m o d e l o r t h e B . E . T .


56.

JOUBERT AND zwiEBEL


0.51

ι

Multicomponent

\

1

ι

N Figure 2.

1

1

PRESSURE

2

N

adsorption

2

Adsorption

Isotherms

1

I

213

I

(torr) isotherms

m i x t u r e m o d e l w a s n o t u s e d i n this w o r k . T h e results of these p u r e c o m p o n e n t correlations are s u m m a r i z e d i n T a b l e I. M u l t i - C o m p o n e n t . O n l y a representative selection o f t h e m a n y pos sible c o m b i n a t i o n s of m u l t i c o m p o n e n t e q u i l i b r i a i n v o l v i n g t h e 4 gases u s e d i n t h e c u r r e n t p r o g r a m is p r e s e n t e d i n this p a p e r . F i r s t , t h e effects o f t h e l i g h t l y a d s o r b e d c o m p o n e n t o n t h e m o r e s t r o n g l y a d s o r b e d species are v i e w e d . N o significant effect w a s p r e d i c t e d , a n d this w a s c o n f i r m e d e x p e r i m e n t a l l y . F i g u r e 1 illustrates t h e a m o u n t of S 0

2

adsorbed.

pure S 0

The S 0 - N 2

2

b i n a r y system h a r d l y deviates f r o m t h e

i s o t h e r m . O n t h e other h a n d , a v e r y strong reverse effect w a s

2

p r e d i c t e d . A s s h o w n i n F i g u r e 2, t h e presence o f S 0 s i g n i f i c a n t l y r e d u c e s 2

the a m o u n t o f n i t r o g e n a d s o r b e d . T h i s is q u i t e p r o n o u n c e d e v e n at t h e v e r y l o w S 0 p a r t i a l pressures w h e r e t h e a m o u n t s a d s o r b e d of t h e 2 p u r e 2

c o m p o n e n t s is a p p r o x i m a t e l y e q u a l . S i m i l a r b e h a v i o r has b e e n o b s e r v e d in the C 0 - N 2

2

and C 0 - 0

The S 0 - C 0 2

2

2

2

b i n a r y systems.

system is a b i t m o r e interesting ( F i g u r e 1 ) ; h e r e t h e

c o m p e t i t i o n f o r a d s o r p t i o n sites is m o r e intense, a n d d e v i a t i o n s f r o m t h e p u r e - c o m p o n e n t curves is f a r m o r e significant. W i t h a n increase o f t e m perature, the selectivity f o r S 0

2

is greatly i n c r e a s e d , so m u c h so that

a r o u n d 30 ° C a r e v e r s a l occurs a n d S 0

2

b e c o m e s t h e p r e f e r r e d species.



214

M O L E C U L A R

0.51

1

1

1

1

1

1

SIEVE

1

ZEOLITES

II

r

900

N Figure 4.

PRESSURE

2

(torr)

N adsorption isotherms 2

atO°C

T h i s is a p p a r e n t l y because of t h e t e m p e r a t u r e d e p e n d e n c e of t h e v a r i o u s c o m p o n e n t s ; the C 0

2

is m u c h m o r e affected t h a n a n y of t h e other c o m

ponents tested.


56.

JOUBERT AND z w i E B E L

Multicomponent

Adsorption

Isotherms

215

It c a n b e n o t e d f r o m Y - X d i a g r a m s ( F i g u r e 3 ) that w i t h i n c r e a s e d temperatures t h e n i t r o g e n a d s o r p t i o n s e l e c t i v i t y is d e c r e a s e d w i t h respect to S 0 . S i m i l a r decrease i n s e l e c t i v i t y w a s n o t e d f o r t h e C 0 w i t h respect 2

2

to t h e S 0 . T h i s is e n c o u r a g i n g w i t h r e g a r d to a i r p o l l u t i o n c o m b a t , since 2

exhaust gases c o n t a i n l a r g e q u a n t i t i e s of C 0

2

and N

2

i n addition to the

undesirable S 0 . 2

F i n a l l y , F i g u r e 4 shows a n e x a m p l e of t h e 0 - N - C 0 2

2

t e r n a r y iso

2

therms at 0 ° C . T h e s o l i d c u r v e is p u r e N , t h e d a s h e d c u r v e is t h e b i n a r y 2

0 -N Downloaded by PRINCETON UNIV on September 30, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch056

2

2

system, w h i l e t h e r e m a i n i n g curves are t h e t e r n a r y cases w i t h t h e

amount of C 0

i n d i c a t e d . N o t e that t h e C 0

2

2

effect, e v e n at v e r y l o w

concentrations, is f a r m o r e significant t h a n t h e 0 tion. T h e 0 - N - S 0 2

2

C0 —S0 -0 , 2

2

2

2

2

effect o n t h e N

2

sorp

system behaves s i m i l a r l y ; t h e C 0 - S 0 - N , o r t h e 2

2

2

a n d t h e q u a t e r n a r y systems are n o t m u c h different f r o m

the S O ^ C O ^ b i n a r y system. T h i s is p r o b a b l y o w i n g t o t h e o v e r b e a r i n g effect of t h e 2 " a c t i v e " c o m p o n e n t s w i t h respect to 0

and N .

2

2

Conclusions E x p e r i m e n t a l l y m e a s u r e d p u r e - c o m p o n e n t a d s o r p t i o n characteristics of 0 , N , C 0 , a n d S 0 2

2

2

2

o n H - m o r d e n i t e w e r e c o r r e l a t e d to p r e d i c t t h e

b e h a v i o r of m u l t i c o m p o n e n t m i x t u r e of these gases. T h e s e correlations, based u p o n the relationships developed b y M y e r s a n d Prausnitz, were successfully s u b s t a n t i a t e d e x p e r i m e n t a l l y . T h e C 0

2

a n d S 0 , w h i c h are 2

the p r e d o m i n a n t l y a d s o r b e d c o m p o n e n t s , c o n t r o l l e d t h e fate of t h e m u l t i component

sorption.

T h i s p r e v a i l e d e v e n at the c o n c e n t r a t i o n

levels

w h e r e t h e p u r e - c o m p o n e n t d a t a i n d i c a t e c o m p a r a b l e affinity f o r b o t h the " s t r o n g l y " a n d t h e " w e a k l y " a d s o r b e d species.

H e n c e , i n d i c a t i o n s are

that a d s o r p t i o n m a y b e effectively u s e f u l i n exhaust gas c l e a n u p processes. T h e t e m p e r a t u r e sensitivity o f t h e p u r e c o m p o n e n t s c o n t r i b u t e s signifi c a n t l y to t h e s e l e c t i v i t y of t h e sieve f o r t h e v a r i o u s c o m p o n e n t s , a n d t h e d a t a o b t a i n e d i n d i c a t e that this also tends to f a v o r t h e d e s i r e d a p p l i c a tions i n p o l l u t i o n c o m b a t .

Nomenclature Ρ

Pressure, torr

W

A m o u n t adsorbed, m m o l e s / g r a m

X

M o l e f r a c t i o n o f a d s o r b e d phase

Y

M o l e f r a c t i o n of gas phase S u b s c r i p t s denote c o m p o n e n t s


216

MOLECULAR SIEVE ZEOLITES II

Literature Cited Barrer, R. M., Peterson, D. L., Troc. Roy. Soc. 1964, 280, 466. Danner, R. P., Wenzel, L. Α., A.I.Ch.E. 1969, 15, 515. Eberly, P. E.,J.Phys. Chem. 1963, 67, 2404. Hill, T. L., 7. Chem. Phys. 1946, 14, 268. Keough, A. H., Sand, L. B., J. Am. Chem. Soc. 1961, 83, 3526. Lewis, W. K., Gilliland, E. R., Chertow, B., Cadogan, W. P., Ind. Eng. Chem. 1950, 42, 1319. (7) Myers, A. L., Prausnitz, J. M., A.I.Ch.E.J.1965, 11, 121. (8) Young, D. M., Crowell, A. D., "Physical Adsorption of Gases," Butterworths, London, 1962.


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RECEIVED February 26, 1970.


Multicomponent Adsorption Isotherms on HydrogenMordenite

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