Kinetics of Adsorption on A Zeolites. Temperature Effects

52 Kinetics of Adsorption on A Zeolites. Temperature Effects

Downloaded by PENNSYLVANIA STATE UNIV on June 18, 2012 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch052

JAMES D. EAGAN, BRUNO KINDL, and ROBERT B. ANDERSON Department of Chemical Engineering, McMaster University, Hamilton, Ontario

Maximum temperatures measured in the adsorbent during the adsorption of nitrogen on 4A and propane on 5A zeolite, both at —78°C, were 15° and 50°C above the bath temperature. Finite difference calculations, taking into account the generation and loss of heat and changes in diffusivity and equilibrium adsorption with temperature, reproduced the pertinent features of the rate and temperature data. When the temperature maximum occurs late in the adsorption process, the rate curve is drastically different from that expected for isothermal adsorption. Q o m e rate curves f o r the a d s o r p t i o n of gases o n z e o l i t i c m o l e c u l a r sieves ^

h a v e u n u s u a l shapes (5, 6).

A p o s s i b l e cause of some of these p h e

n o m e n a is the c h a n g i n g t e m p e r a t u r e of the s a m p l e d u r i n g o w i n g to the heat of a d s o r p t i o n ( 5 ) .

adsorption

A g r o u p of experiments has d e m o n

strated t h a t samples o v e r h e a t s u b s t a n t i a l l y d u r i n g k i n e t i c measurements. T e m p e r a t u r e s h i g h e r t h a n 15 ° C a b o v e the b a t h t e m p e r a t u r e h a v e b e e n measured d u r i n g kinetic experiments i n both volumetric and gravimetric apparatuses.

F i n i t e difference c a l c u l a t i o n s , w h i c h

approximate experi

m e n t a l results, i n d i c a t e that these t e m p e r a t u r e differences d o not c h a n g e the shape of the i n i t i a l p a r t of the rate c u r v e s u b s t a n t i a l l y , b u t i n some cases cause m a j o r changes at l o n g e r times. Experimental

and Computing

Methods

C o m m e r c i a l L i n d e 4 A a n d 5 A p o w d e r s w e r e u s e d after e v a c u a t i o n at 4 5 0 ° C f o r 15 h o u r s . E x p e r i m e n t s w i t h N

2

on 4 A powder were made

i n a c o n v e n t i o n a l glass v o l u m e t r i c system e q u i p p e d w i t h a g r a d u a t e d burette and a manostat ( 3 ) .

A 0.675-gram s a m p l e of 4 A p o w d e r w a s

c o n t a i n e d i n a s p h e r i c a l b u l b of i n s i d e d i a m e t e r 12 m m . I n s e r t e d i n the 164

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

52.

EAGAN

E T

AL.

Kinetics

of

165

Adsorption

p o w d e r was a 36-gauge c h r o m e l - a l u m e l t h e r m o c o u p l e that p a s s e d t h r o u g h the top of the s a m p l e t u b e at a p o i n t above the c o l d b a t h b y a c o m m e r c i a l g l a s s - m e t a l seal. T h e r e f e r e n c e j u n c t i o n was i m m e r s e d i n the c o o l i n g bath. Tests w i t h p r o p a n e o n 5 A w e r e p e r f o r m e d i n a C a h n R . G . electrobalance

system.

I n the g r a v i m e t r i c a p p a r a t u s , the i n i t i a l

disturbance

c a u s e d b y i n t r o d u c i n g the gas to the e v a c u a t e d s a m p l e p e r s i s t e d f o r a b o u t 6 seconds a n d a d s o r p t i o n measurements c o u l d not be m a d e i n this p e r i o d . Downloaded by PENNSYLVANIA STATE UNIV on June 18, 2012 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch052

I n separate experiments, 0.3- a n d 4 - m m layers of zeolite o n the b a l a n c e p a n w e r e u s e d a n d respective a d s o r p t i o n rate curves w e r e r e c o r d e d .

For

the t h i c k e r l a y e r i n another e x p e r i m e n t , the b a l a n c e p a n w a s s u p p o r t e d o n a 32-gauge t h e r m o c o u p l e , the j u n c t i o n of w h i c h w a s i m m e r s e d i n the zeolite; the temperatures w e r e d e t e r m i n e d d u r i n g a s i m i l a r a d s o r p t i o n e x p e r i m e n t i n w h i c h w e i g h t changes w e r e not m e a s u r e d .

In both ap

paratuses, the m e a s u r e d t e m p e r a t u r e was p r o b a b l y not e q u a l to the a c t u a l p a r t i c l e t e m p e r a t u r e b u t it seems reasonable to assume that t h e y d i f f e r o n l y s l i g h t l y . I n a l l experiments, the c o o l i n g b a t h was d r y i c e - a c e t o n e t h r o u g h w h i c h a s l o w stream of C 0 p h e r e of C 0

2

2

was b u b b l e d to m a i n t a i n a n atmos

under a l l conditions.

F i n i t e difference c a l c u l a t i o n s w e r e m a d e o n a C D C 6 4 0 0 c o m p u t e r to d e t e r m i n e the effect of the t e m p e r a t u r e rise of the p o w d e r o n the a d s o r p t i o n rate. H e r e the particles w e r e a s s u m e d to b e spheres of 1-mic r o n d i a m e t e r f o r c o n v e n i e n c e ; the a c t u a l s a m p l e consisted l a r g e l y of cubes w i t h w i d e v a r i a t i o n i n size, 0.1 to 6 m i c r o n s .

Calculations indi

c a t e d that the temperatures t h r o u g h o u t the p a r t i c l e c o u l d b e uniform.

H e a t w a s a s s u m e d to b e r e m o v e d f r o m the surface

particle b y natural convection.

assumed of

the

T h e largest heat transfer resistance

re

s u l t e d f r o m s l o w c o n d u c t i o n t h r o u g h the p o w d e r . T h u s , d u r i n g a d s o r p tion, temperature

gradients existed w i t h i n the s a m p l e b e d b u t it w a s

a s s u m e d to h a v e a u n i f o r m average t e m p e r a t u r e w h i c h c o u l d b e c o n s i d e r e d the t e m p e r a t u r e

of a n " a v e r a g e "

particle.

T h e heat

transfer

characteristics of the single p a r t i c l e w e r e m a d e e q u i v a l e n t to those

of

the a c t u a l s a m p l e b e d i n the f o l l o w i n g w a y : T h e t h e r m a l c o n d u c t i v i t y of the p o w d e r w a s a s s u m e d to b e the same as d i a t o m a c e o u s e a r t h a n d the t e m p e r a t u r e response

(4),

of the b e d was c a l c u l a t e d f o r a g i v e n

c h a n g e i n e x t e r n a l t e m p e r a t u r e . T h e heat transfer coefficient of the single p a r t i c l e was c a l c u l a t e d so that its c o o l i n g rate was the same as that of the average p a r t i c l e i n the p o w d e r . T h i s v a l u e , 5.0 Χ 10"

8

cal/sec-cm -°C, 2

was u s e d i n the finite difference c a l c u l a t i o n s . T h e p a r t i c l e w a s d i v i d e d i n t o 30 to 100 shells for the c a l c u l a t i o n a n d F i c k ' s L a w w a s a s s u m e d to h o l d .

H e a t g e n e r a t e d w a s t a k e n as rate of

a d s o r p t i o n times the heat of a d s o r p t i o n , a n d the latter q u a n t i t y was as s u m e d to b e i n d e p e n d e n t of a m o u n t a d s o r b e d .

W i t h i n c r e a s i n g tern-


166

M O L E C U L A R

Table I. Data in Figure


1 2 3

SIEVE

ZEOLITES

Π

Parameters for Computed Rate Curves

Heat of Adsorption, Kcal/Mole 5.0 8.0 5.0

Activation Energy, Kcal/Mole 5.5 3.0 5.5

D/R , Min~

H, Cal/Sec-Cm -°C

2

l

2

5.0 Χ 1 0 " 0.4 5.0 X 1 0 ~

3

3

5.0 X 1 0 ~ 5.0 X 1 0 ~ Variable

8 8

Figure 1.

Adsorption of nitrogen on 4A zeolite powder at —78°C and 760 torr. Open circles denote adsorption and solid circles the temperature. Solid lines are computed adsorption and temperature data. p e r a t u r e , t h e d i f f u s i v i t y increases

a c c o r d i n g to t h e A r r h e n i u s e q u a t i o n

b u t t h e e q u i l i b r i u m a m o u n t a d s o r b e d decreases.

T h e change i n e q u i

l i b r i u m a d s o r p t i o n w a s c a l c u l a t e d u s i n g the heat of a d s o r p t i o n a n d the F r e u n d l i c h i s o t h e r m w i t h pressure exponents

f r o m a d s o r p t i o n studies.

T h e e x p o n e n t o f t h e F r e u n d l i c h e q u a t i o n w a s a s s u m e d to v a r y l i n e a r l y w i t h temperature between experimental

temperatures.


52.

EAGAN

E T

AL.

Kinetics

of

167

Adsorption

T h e c o m p u t e r p r o g r a m calculates i n e a c h t i m e step a n e w c o n c e n t r a t i o n p r o f i l e , t e m p e r a t u r e , d i f f u s i v i t y , a n d a m o u n t a d s o r b e d at e q u i librium.

A t c o n v e n i e n t i n t e r v a l s , the c o n c e n t r a t i o n p r o f i l e is i n t e g r a t e d

u s i n g S i m p s o n s r u l e to g i v e the a m o u n t a d s o r b e d .

This quantity was

d i v i d e d b y the e q u i l i b r i u m a d s o r p t i o n at the b a t h t e m p e r a t u r e to g i v e the a p p r o a c h to e q u i l i b r i u m factor Z . A p p r o p r i a t e values of a c t i v a t i o n energies a n d heats of a d s o r p t i o n


f r o m o u r l a b o r a t o r y or f r o m the l i t e r a t u r e ( 2 ) w e r e c h o s e n , a n d d i f f u s i v i ties at — 78 ° C w e r e e s t i m a t e d f r o m o u r rate curves; these values are g i v e n i n T a b l e I. I n the present c a l c u l a t i o n s , w e h a v e a t t e m p t e d o n l y to o b t a i n rate a n d t e m p e r a t u r e curves r e a s o n a b l y a p p r o x i m a t i n g the e x p e r i m e n t a l d a t a , a n d n o a t t e m p t w a s m a d e to i m p r o v e the p r e d i c t e d results b y adjustment of constants.

Figure 2.

Adsorption of propane on 5A zeolite powder at -78°C. Open squares denote adsorption at 21 ton for an 0.3-mm layer. Open and solid circles denote adsorption and temperatures, respectively, at 30 ton in a 4-mm layer. Solid curves are calculated adsorption and temperatures.


168

MOLECULAR SIEVE ZEOLITES

Experimental

Π

Results and Discussion

F i g u r e 1 shows t h e rate o f a d s o r p t i o n o f N concurrent temperature

measurements.

2

on 4 A powder a n d

F i g u r e 2 gives t h e rate o f a d

sorption of propane o n 5 A w i t h a thin (0.3-mm) a n d a thick l a y e r o f z e o l i t e o n t h e b a l a n c e p a n ; temperatures

(4-mm)

were measured i n a

separate e x p e r i m e n t i n t h e t h i c k layer. F o r n i t r o g e n o n 4 A , F i g u r e 1, t h e n o n i s o t h e r m a l rate d a t a

despite


the h e a t i n g c o u l d b e r e p r e s e n t e d r e a s o n a b l y b y u s u a l i s o t h e r m a l F i c k ' s l a w equations ( I ) , i f D / R

2

is t a k e n as 7.5 Χ 10" m i n " . T h u s , t h e v a l u e 3

1

of D / R c a l c u l a t e d f r o m t h e i s o t h e r m a l e q u a t i o n w a s 5 0 % l a r g e r t h a n 2

that u s e d to d e r i v e t h e n o n i s o t h e r m a l c u r v e i n F i g u r e 1. H e r e t h e t e m p e r a t u r e m a x i m u m occurs at l o w amounts a d s o r b e d , a n d t h e i n c r e a s e d rate o w i n g to i n c r e a s e d d i f f u s i v i t i e s is n e a r l y c o m p e n s a t e d

b y the de

creased e q u i l i b r i u m a d s o r p t i o n at t h e o b s e r v e d temperatures.

TIME Figure

3.

.(MINT

2

Calculated adsorption data for different heat transfer coefficients


Propane

52.

Ε A GA N

E T

AL.

Kinetics

of

Adsorption

169

o n 5 A ( F i g u r e 2 ) was chosen as a n e x a m p l e of a r a p i d a d s o r p t i o n , w h e r e the rate a n d heat g e n e r a t i o n w e r e greater.

F o r the t h i c k s a m p l e ,

the

a m o u n t a d s o r b e d a p p r o a c h e d e q u i l i b r i u m s l o w l y c o m p a r e d w i t h the t h i n s a m p l e . T h e t e m p e r a t u r e m a x i m u m f o r the t h i c k s a m p l e o c c u r r e d at 0.7 of the e q u i l i b r i u m a d s o r p t i o n . I n this case, the a d s o r p t i o n is r a p i d at short times because of the i n c r e a s e d d i f f u s i v i t y , a n d e q u i l i b r i u m at the p a r t i c l e t e m p e r a t u r e is a p p r o a c h e d at a b o u t the t i m e c o r r e s p o n d i n g to the m a x i m u m t e m p e r a t u r e .

T h e a m o u n t a d s o r b e d t h e n increases

with


t i m e a n d the c o o l i n g of the samples—i.e., r e f l e c t i n g i n c r e a s i n g e q u i l i b r i u m adsorption. T h e c a l c u l a t e d curves i n F i g u r e s 1 a n d 2 r e p r o d u c e a l l of the p e r t i nent features of the e x p e r i m e n t a l rate a n d t e m p e r a t u r e d a t a , a l t h o u g h there are some q u a n t i t a t i v e differences.

F o r e x a m p l e , at l o n g e r t i m e s ,

e x p e r i m e n t a l measurements of a d s o r p t i o n l i e b e l o w c o m p u t e d curves for the single p a r t i c l e . T h e a c t u a l p o w d e r consisted of particles of different sizes, a n d at large times the s m a l l particles, h a v i n g a l r e a d y e q u i l i b r i u m , no l o n g e r c o n t r i b u t e to the rate.

reached

C a l c u l a t e d rate curves f o r

the same values of d i f f u s i v i t y at the b a t h t e m p e r a t u r e , a c t i v a t i o n e n e r g y , a n d heat of a d s o r p t i o n as for n i t r o g e n , b u t different heat transfer coeffi cients, are g i v e n i n F i g u r e 3. T h e s e curves a p p r o x i m a t e a l l of the types of rate curves d e s c r i b e d i n the present p a p e r . C h a n g i n g the heat transfer coefficient b y a factor of 50 p r o d u c e d p r o f o u n d effects o n the last p a r t of the rate c u r v e b u t has v i r t u a l l y n o effect o n the e a r l y p o r t i o n . O u r d a t a s h o w that large t e m p e r a t u r e measurements

of a d s o r p t i o n k i n e t i c s .

changes

can occur

during

W h e n the t e m p e r a t u r e m a x i m u m

occurs e a r l y i n the process, n o p r o n o u n c e d effect o n the rate c u r v e is o b s e r v e d , a n d the u n w a r y e x p e r i m e n t e r m a y c o n c l u d e that his d a t a w e r e obtained isothermally. Acknowledgment T h e authors are p l e a s e d to a c k n o w l e d g e f e l l o w s h i p a n d o p e r a t i n g f u n d s p r o v i d e d b y T h e N a t i o n a l R e s e a r c h C o u n c i l of C a n a d a .

Literature Cited (1) Anderson, R. B., Bayer, J., Hofer, L. J. E., Ind. Eng. Chem. Process Design Develop. 1965, 4, 167. (2) Eberly, P. E., Jr., Ind. Eng. Chem. Prod. Res. Develop. 1969, 8, No. 2, 140. (3) Harper, R. J., Stifel, G. R., Anderson, R. B., Can. J. Chem. 1969, 47, 4661. (4) Kreith, F., "Principles of Heat Transfer," 2nd ed., p. 594, International Textbook Co., Scranton, Pa., 1965. (5) Satterfield, C. N., Margetts, W. G., 62nd Annual Meeting, A.I.Ch.E., Washington, D. C., Nov. 1969, Reprint 56d. (6) Stifel, G. R., Master's thesis, McMaster University, 1967. RECEIVED February 4, 1970. Resubmitted September 1, 1970.


170

M O L E C U L A R

SIEVE

ZEOLITES

II

Discussion C . N . Satterfield ( Massachusetts Institute of T e c h n o l o g y , C a m b r i d g e , M a s s . 0 2 1 3 9 ) : R e c e n t e x p e r i m e n t a l results of M a r g e t t s ( S a t t e r f i e l d a n d M a r g e t t s , A.I.Ch.E. 1969)

i n press; p r e p r i n t , A . I . C h . E . M e e t i n g , N o v e m b e r

are consistent w i t h the observations a n d c a l c u l a t i o n s of A n d e r s o n .

M a r g e t t s s t u d i e d the gas phase s o r p t i o n of C H

4

or n - C H i 4

0

on N a -


m o r d e n i t e crystals at 25 ° C a n d pressures of a b o u t 10 to 80 torr.

The

c a l c u l a t e d effective diffusivities s h o w e d a m a x i m u m i n the g e n e r a l r e g i o n of Mi/Moo —

0.5.

C a l c u l a t i o n s a n d e x p e r i m e n t a l observations i n d i c a t e d

this was c a u s e d b y a t e m p e r a t u r e increase w i t h i n the s a m p l e , o c c u r r i n g d u r i n g the first f e w seconds. R. B. Anderson: T h a n k s for the s u p p o r t i n g e v i d e n c e . Y . H . M a ( W o r c e s t e r P o l y t e c h n i c Institute, W o r c e s t e r , M a s s . 01609) : W h a t is the d i f f u s i o n coefficient y o u o b t a i n e d ? rise be o w i n g to the a d i a b a t i c

C o u l d the

temperature

compression?

R. B. Anderson: T h e heat of a d s o r p t i o n was the cause of the t e m p e r a t u r e increase.

T h e a d i a b a t i c c o m p r e s s i o n of gas i n t r o d u c e d to the

e v a c u a t e d s a m p l e s h o u l d l e a d to a n e g l i g i b l e t e m p e r a t u r e

increase.

E . F. Kondis ( M o b i l R e s e a r c h & D e v e l o p m e n t C o r p . , P a u l s b o r o , N . J. 08066 ) : W e c o n f i r m e d some of the trends i n n o n i s o t h e r m a l results w h i c h y o u report.

O u r d a t a are i n a flow system a n d h e n c e d i s c o u n t the pos

s i b i l i t y of heat effects c a u s e d b y expansion of gas u s i n g y o u r C a h n instrument. R. B. Anderson: T h a n k s .


Kinetics of Adsorption on A Zeolites. Temperature Effects

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