Supercritical Fluid Extraction and Chromatography - American

Chapter 1 Physical Chemistry of Supercritical Fluids A

Tutorial

Downloaded by UNIV OF NEW ORLEANS on September 20, 2016 | http://pubs.acs.org Publication Date: March 17, 1988 | doi: 10.1021/bk-1988-0366.ch001

C. T . Lira Department of Chemical Engineering, Michigan East Lansing, M I 48824-1226

State University,

Solubility behavior of compounds in supercritical fluids is interpreted using a simplified solution model. Solid-fluid and liquid-fluid phase equilibria data are presented and discussions of the phase behavior are provided. Phase transitions and transport properties are briefly discussed.

Critical phenomena have f a s c i n a t e d many r e s e a r c h e r s . E a r l y s o l u b i l i t y experiments w i t h f l u i d s above t h e i r c r i t i c a l p o i n t s were p u b l i s h e d i n 1879 ( 1 ) . The s i m p l e c u b i c e q u a t i o n o f s t a t e t h a t v a n der Waals b r i l l i a n t l y c o n c e i v e d i n 1873 has been shown t o be c a p a b l e o f q u a l i t a t i v e l y d e s c r i b i n g v i r t u a l l y every type o f phase b e h a v i o r t h a t has been e x p e r i m e n t a l l y observed (2-3) , y e t phenomena i n h i g h p r e s s u r e f l u i d s a r e s t i l l f a s c i n a t i n g t o r e s e a r c h e r s over a c e n t u r y l a t e r . A l t h o u g h many advances have been made i n u n d e r s t a n d i n g o f h i g h p r e s s u r e b e h a v i o r , even q u a l i t a t i v e p r e d i c t i o n o f b i n a r y phase b e h a v i o r i s o f t e n d i f f i c u l t w i t h o u t some e x p e r i m e n t a l d a t a . T h i s paper e x p l o r e s t h e p h y s i c a l c h e m i s t r y o f f l u i d s near t h e i r c r i t i c a l p o i n t s . A l t h o u g h the term s u p e r c r i t i c a l denotes c o n d i t i o n s above c r i t i c a l temperature and p r e s s u r e , i n t e r e s t i n g b e h a v i o r o c c u r s throughout t h e c r i t i c a l r e g i o n . F o r t h i s d i s c u s s i o n , t h e term s u p e r c r i t i c a l w i l l r e f e r t o c o n d i t i o n s above the c r i t i c a l temperature and near the c r i t i c a l p r e s s u r e . This d i s c u s s i o n i s prepared t o provide i n t r o d u c t o r y i n f o r m a t i o n f o r r e s e a r c h e r s who a r e n o t a c t i v e l y i n v o l v e d i n s u p e r c r i t i c a l f l u i d r e s e a r c h , b u t who have i n t e r e s t i n f o l l o w i n g developments i n the f i e l d . While i t i s n o t p o s s i b l e i n one s h o r t c h a p t e r t o p r o v i d e a summary o f the work o f the l a s t c e n t u r y , s e v e r a l r e v i e w papers and symposia c o l l e c t i o n s have been p u b l i s h e d r e c e n t l y which provide e x c e l l e n t i n t r o d u c t o r y m a t e r i a l (4-9). I n a d d i t i o n , McHugh and K r u k o n i s have r e c e n t l y p u b l i s h e d an o u t s t a n d i n g s u r v e y book o f s u p e r c r i t i c a l f l u i d s t u d i e s (.10) . Through t h e l a s t c e n t u r y e x p e r i m e n t a l d a t a have been accumulated f o r a g r e a t many systems, b u t most d a t a a r e f o r systems w i t h c r i t i c a l p o i n t s w h i c h don't d i f f e r g r e a t l y . Much c u r r e n t

0097-6156/88/0366-0001$07.25/0 © 1988 American Chemical Society

Charpentier and Sevenants; Supercritical Fluid Extraction and Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


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SUPERCRITICAL FLUID E X T R A C T I O N AND

CHROMATOGRAPHY

i n t e r e s t d e a l s w i t h systems where the c r i t i c a l p r o p e r t i e s and, m o l e c u l a r i n t e r a c t i o n s o f the pure components d i f f e r g r e a t l y . Because o f l a r g e d i f f e r e n c e s i n m o l e c u l a r i n t e r a c t i o n s , the phase b e h a v i o r i s s u b s t a n t i a l l y d i f f e r e n t from t h a t u s u a l l y p r e s e n t e d i n t e x t b o o k s . A l t h o u g h much has been l e a r n e d , t h e s e m o l e c u l a r l y asymmetric systems a r e s t i l l n o t u n d e r s t o o d w e l l . Development o f s u p e r c r i t i c a l p r o c e s s e s w i l l e v o l v e as the fundamental d a t a base grows and r e s e a r c h e r s g a i n e x p e r i e n c e i n w o r k i n g w i t h t h e s e asymmetric systems. Chemical p r o c e s s i n g t e c h n i q u e s e v o l v e over p e r i o d s o f many y e a r s and i t seems u n l i k e l y t h a t s u p e r c r i t i c a l p r o c e s s e s w i l l r a p i d l y r e p l a c e d i s t i l l a t i o n , l i q u i d - l i q u i d e x t r a c t i o n , or a d s o r p t i o n i n most p r o c e s s e s . These u n i t o p e r a t i o n s a r e h i g h l y d e v e l o p e d and many fundamental r e l a t i o n s h i p s and e m p i r i c a l c o r r e l a t i o n s a r e used i n p r o c e s s d e s i g n . F u r t h e r , each p r o c e s s i s s e l e c t e d based on the p r o p e r t i e s o f the m i x t u r e t o be p r o c e s s e d . The r e c e n t growth i n a v a i l a b i l i t y o f h i g h p r e s s u r e equipment has g r e a t l y f a c i l i t a t e d an e x p a n s i o n i n the number o f fundamental s u p e r c r i t i c a l s t u d i e s , and the r a t e a t which t h e s e s t u d i e s c o n t r i b u t e t o our u n d e r s t a n d i n g s h o u l d c o n t i n u e t o a c c e l e r a t e . Thus, as we l e a r n more, s u p e r c r i t i c a l o p e r a t i o n s w i l l become i n c r e a s i n g l y i m p o r t a n t a l t e r n a t i v e s i n p r o c e s s i n g , s e p a r a t i o n , and a n a l y t i c a l techniques. F i g u r e 1 i l l u s t r a t e s a p o s s i b l e f l o w s h e e t f o r d e s i g n o f an a r b i t r a r y c h e m i c a l p r o c e s s . A f t e r c h a r a c t e r i z i n g the m a t e r i a l s i n s t e p 2, e x p e r i e n c e i n w o r k i n g w i t h many d i f f e r e n t p r o c e s s e s u s u a l l y p e r m i t s s e l e c t i o n o f a u n i t o p e r a t i o n (e.g. d i s t i l l a t i o n o r l i q u i d l i q u i d e x t r a c t i o n ) w i t h o u t f u r t h e r m o d e l i n g o r d e s i g n o f the p r o c e s s . Because s u p e r c r i t i c a l p r o c e s s e s a r e n o t u n d e r s t o o d w e l l , comparison o f these o p e r a t i o n s w i t h s u p e r c r i t i c a l p r o c e s s e s i s more complicated. Further a n a l y s i s of a s u p e r c r i t i c a l process i s n e c e s s a r y b e f o r e a comparison may be made. Steps 4 and 6 o f F i g u r e 1 u l t i m a t e l y determine the s u c c e s s o f a s u p e r c r i t i c a l p r o c e s s . However, w i t h o u t s t e p s 3 and 5 most o p t i m i z a t i o n must f o l l o w e x p e r i m e n t a l s t e p 7 which l e a d s t o slow and e x p e n s i v e p r o c e s s development. Steps 3 and 5 a r e w e l l c h a r a c t e r i z e d f o r most p r o c e s s e s , and a l t h o u g h the d e s i g n o f t e n i n v o l v e s e m p i r i c a l knowledge, we have enough fundamental u n d e r s t a n d i n g t o o p t i m i z e the p r o c e s s c o n d i t i o n s . As we develop u n d e r s t a n d i n g o f the r e l a t i o n s h i p s between s t e p s 3-5 f o r s u p e r c r i t i c a l systems, the development o f an i n c r e a s i n g number o f s u c c e s s f u l h i g h p r e s s u r e p r o c e s s e s w i l l emerge due t o improved p r o c e s s d e s i g n a t s t e p 6. Steps 1-3 a r e dependent on the s p e c i f i c m a t e r i a l s . The f o l l o w i n g d i s c u s s i o n f o c u s e s on s t e p s 4 and 5 t o p r o v i d e i n f o r m a t i o n t o the m a j o r i t y o f r e a d e r s who w i s h t o develop an u n d e r s t a n d i n g o f the t y p e s o f p r o c e s s i n g which may be a c h i e v e d and the e f f e c t s o f temperature and p r e s s u r e . The a p p l i c a t i o n o f s u p e r c r i t i c a l p r o c e s s e s w i l l always be i n t i m a t e l y c o u p l e d w i t h the h i g h p r e s s u r e phase b e h a v i o r and the p h y s i c a l c h e m i s t r y o f the system. S u p e r c r i t i c a l f l u i d s have r e c e i v e d much a t t e n t i o n because the s o l u b i l i t y o f a s o l u t e i s r e a d i l y i n f l u e n c e d by s m a l l v a r i a t i o n s i n p r e s s u r e o r temperature. I n b o t h c a s e s , the d e n s i t y o f the s u p e r c r i t i c a l f l u i d a l s o changes. The s o l u b i l i t y o f a g i v e n compound may be i n f l u e n c e d i n s e v e r a l ways. Both vapor p r e s s u r e and i n t e r m o l e c u l a r f o r c e s determine


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solubilities. The f o l l o w i n g d i s c u s s i o n s h o u l d be h e l p f u l i n u n d e r s t a n d i n g the e f f e c t s . S o l u b i l i t y of S o l i d s i n S u p e r c r i t i c a l F l u i d s The s i m p l e s t type o f phase b e h a v i o r to u n d e r s t a n d i s the s o l u b i l i t y o f a s o l i d s o l u t e , such as n a p h t h a l e n e , i n a s u p e r c r i t i c a l f l u i d . When the s o l u t e i s a c r y s t a l l i n e s o l i d , the s o l i d phase may be assumed to be pure and o n l y the s u p e r c r i t i c a l phase i s a m i x t u r e . Imagine s o l i d n a p h t h a l e n e i n a c l o s e d v e s s e l under one atmosphere o f c a r b o n d i o x i d e a t 40°C. The reduced temperature and reduced d e n s i t y o f C0 are 1.03 and 3.7x10 r e s p e c t i v e l y . At t h i s p r e s s u r e , the gas phase i s i d e a l and the n a p h t h a l e n e s o l u b i l i t y i s d e t e r m i n e d by i t s vapor p r e s s u r e . As the c o n t a i n e r volume i s d e c r e a s e d i s o t h e r m a l l y , the s o l u b i l i t y i n i t i a l l y d e c r e a s e s when the gas phase i s s t i l l n e a r l y i d e a l . As the p r e s s u r e i s i n c r e a s e d f u r t h e r , however, the gas phase d e n s i t y becomes i n c r e a s i n g l y n o n i d e a l and approaches the m i x t u r e c r i t i c a l d e n s i t y (near the c r i t i c a l d e n s i t y o f C0 because the gas phase i s s t i l l m o s t l y C 0 ) . The reduced d e n s i t y o f C0 i n c r e a s e s r a p i d l y near the c r i t i c a l r e g i o n as shown i n F i g u r e 2. The s o l v e n t power o f C0 i s r e l a t e d to the d e n s i t y w h i c h l e a d s to a r a p i d s o l u b i l i t y i n c r e a s e . A b r i e f d e s c r i p t i o n of intermolecular i n t e r a c t i o n s i s h e l p f u l i n understanding t h i s behavior. For t h i s d i s c u s s i o n , we w i l l c o n s i d e r a s o l u t e m o l e c u l e w i t h a d i p o l e moment and then g e n e r a l i z e the r e s u l t s . Imagine a m o l e c u l e w i t h a d i p o l e moment a t i n f i n i t e d i l u t i o n i n a n o n p o l a r supercritical fluid. The d i p o l e ' s f i e l d i n d u c e s a response o f the p o l a r i z a b l e f l u i d which r e s u l t s i n a net a t t r a c t i v e f o r c e . The p o l a r i z a b l e s o l u t e r e a c t s to the f l u i d ' s i n d u c e d f i e l d and f u r t h e r i n c r e a s e s the a t t r a c t i v e f o r c e . A s i m p l i f i e d m a t h e m a t i c a l model o f t h e s e i n t e r a c t i o n s i s h e l p f u l i n u n d e r s t a n d i n g the reasons f o r s o l u b i l i t y enhancement and the d e n s i t y e f f e c t . Consider a_^spherical p o i n t d i p o l e s o l u t e molecule w i t h a d i p o l e moment, μ, a t i n f i n i t e d i l u t i o n i n a s p h e r i c a l c o n t a i n e r o f supercritical fluid. W i t h a continuum assumption, the f l u i d ' s e l e c t r i c a l p r o p e r t i e s may be r e p r e s e n t e d by a homogeneous d i e l e c t r i c c o n s t a n t , e. The inhomogeneous f i e l d o f the d i p o l e p o l a r i z e s the f l u i d w h i c h r e a c t s and g i v e s r i s e to a f i e l d , R, a t the d i p o l e . R w i l l be p r o p o r t i o n a l to μ as l o n g as no s a t u r a t i o n e f f e c t s occur, 3


2

2

2

2

2

R - fμ

(D

where f i s c a l l e d the f a c t o r o f the r e a c t i o n f i e l d . mathematical a n a l y s i s provides f -

_J_ a

3

For our case a

2(6-1) (2e+l)

(2)

where a - radius of point d i p o l e e - d i e l e c t r i c constant of f l u i d The i n t e r a c t i o n energy o f a s p h e r i c a l n o n p o l a r i z a b l e i n i t s own r e a c t i v e f i e l d i s t h e n

point

dipole


4

SUPERCRITICAL FLUID EXTRACTION AND C H R O M A T O G R A P H Y

1. Selection of materials to be treated 2. Characterization of materials 3. Thermodynamic Properties of Components


4. Choice of solvent or solvent mixture

>

·>

5a. Modeling of Equilibrium Properties 5b. Modeling of Transport Properties

6. Process design

Optimization 7. Experimental verification

Final Process F i g u r e 1. S c i e n t i f i c

development

o f a chemical

1.0

process.

10.0

Reduced Pressure F i g u r e 2. Reduced d e n s i t i e s i n t h e c r i t i c a l r e g i o n a l o n g isotherms. (Reproduced w i t h p e r m i s s i o n from R e f . 4. C o p y r i g h t 1983. D. R e i d e l . )


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Γ - - 1 Z.Î - - 1 f S

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-

5

.

(3)

I n a more r e a l i s t i c c a s e , w i t h a p o l a r i z a b l e d i p o l e , the r e s u l t becomes 1


where

2

K

1-fa

}

a - p o l a r i z a b i l i t y of point dipole

Note t h a t t h i s i n t e r a c t i o n energy i s even more f a v o r a b l e s i n c e (1f a ) < l . D e r i v a t i o n s o f the r e s u l t s p r e s e n t e d h e r e are d i s c u s s e d by B o t t c h e r ( 1 1 ) . The d i e l e c t r i c c o n s t a n t o f a f l u i d may be r e p r e s e n t e d by the C l a u s i u s - M o s s o t t i f u n c t i o n (12-13).

τα ι - f where

V N^— α μ -

ë

500

û. Ο Ο

Q2

0.4

0.6

M O L E FRACTION

0.8

1.0

HEXANE

F i g u r e 13. B i n a r y phase b e h a v i o r o f C02~n-hexane, a type I system. (Reproduced w i t h p e r m i s s i o n from Réf. 10. C o p y r i g h t 1986 B u t t e r w o r t h s . )


SUPERCRITICAL FLUID EXTRACTION AND C H R O M A T O G R A P H Y


η - PROPYL ALCOHOL

H 0 2

F i g u r e 14. T e r n a r y phase b e h a v i o r o f n-propanol-water-C02 a t 15 °C and 750 p s i a . (Reproduced w i t h p e r m i s s i o n from Ref. 10. C o p y r i g h t 1986 B u t t e r w o r t h s . )

_

4 Ο Ο

-

Φ

υ 3 »

1

—ΓΤΤΤΙ 0.5

0.7

1

ι

I I I 3

2

Reduced Pressure (P ) r

F i g u r e 15. Reduced v i s c o s i t y (Data from Ref. 49.)

i n the s u p e r c r i t i c a l r e g i o n .


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r a t h e r t h a n o r d e r s o f magnitude. A t h i g h e r r e d u c e d t e m p e r a t u r e s the p r e s s u r e e f f e c t i s l e s s pronounced. Compare F i g u r e 15 w i t h F i g u r e 2 t o n o t e t h a t t h e v i s c o s i t y and d e n s i t y changes a r e coupled. Diffusion Coefficients. S e l f d i f f u s i o n c o e f f i c i e n t s f o r C 0 (5052.) , e t h y l e n e (53) , water ( 5 4 ) , and methane (55) a r e p r e s e n t e d i n F i g u r e 16. The c r i t i c a l d e n s i t i e s o f t h e s e f l u i d s a r e 10.6, 7.8, 17.9, and 10.1 mol/1, r e s p e c t i v e l y . F i g u r e 16 i s p r e s e n t e d f o r i l l u s t r a t i v e purposes o n l y and t h e r e f e r e n c e s p r o v i d e a d i s c u s s i o n o f t h e o r e t i c a l c o n s i d e r a t i o n s and m a t h e m a t i c a l r e l a t i o n s h i p s between d e n s i t y , v i s c o s i t y , and d i f f u s i o n . B i n a r y d i f f u s i o n c o e f f i c i e n t s a r e a v a i l a b l e f o r a few m i x t u r e s . The d i f f u s i o n v a l u e s a r e between t h e s o l v e n t s e l f d i f f u s i o n c o e f f i c i e n t s and normal l i q u i d phase d i f f u s i o n c o e f f i c i e n t s and t h e v a l u e s a r e summarized i n T a b l e I .


2

Table I .

Binary Diffusion Coefficients

i n Supercritical

Solvent (Ref.) C0

2

(56) C0

2

(57)

Solute

Temperature (K)

Density (mol/1)

Fluids

Approximate Range -log (D [cm /s]) 2

1 0

1 2

Benzene Phenol Naphthalene

313

7-18

3.5-4. 0

Benzene n- P r o p y l b e n z e n e 1,3,5-Trimethylbenzene

313

7-18

3.5-4.,0

C0 (58)

Benzoic a c i d 2-Naphthol

308-323

15-17

4.04-4..30

Benzoic a c i d Naphthalene

318-338

7.5-10

3.8-4,.08

(58)

523-548

4.0-5.3

3.4-3 .6

2

2,3-Dimethylbutane Benzene (59) Toluene Naphthalene Phenanthrene

I n g e n e r a l , mass t r a n s f e r i s n o t l i m i t e d by d i f f u s i o n r a t e s i n the s u p e r c r i t i c a l phase. I n f a c t , s i g n i f i c a n t buoyancy e f f e c t s i n s u p e r c r i t i c a l f l u i d s enhance mass t r a n s f e r r a t e s w i t h c o n v e c t i v e m i x i n g (58)· U s u a l l y , mass t r a n s f e r l i m i t a t i o n s w i l l o c c u r i n e i t h e r a l i q u i d o r s o l i d phase.


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SUPERCRITICAL FLUID EXTRACTION AND

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-1

^

-2-

4 -3H X

X

-4H Φ©

-5·

—Γ"

10

30

20

Density (mol/1) F i g u r e 16. S e l f d i f f u s i o n c o e f f i c i e n t s f o r s e v e r a l f l u i d s : C 0 , (313-318K, 1.03-1.05 Τ ), ( 5 0 ) ; (323K, 1.06 Τ ), C0 (51); • - C 0 (308K, 1.01 Ï ), ( 5 2 ) ; Δ - C2 H , (323K, 1.14 , (198K, 1.04 Τ J , (53) ; V - H 0, (673K, 1.04 Τ ) , ( 5 4 ) ; X CH. (54) . 2

o

2 l

J

2r

4

Γ

2

Γ


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Fluids

23


Conclusion T h i s c h a p t e r p r o v i d e s an i n t r o d u c t i o n t o s u p e r c r i t i c a l f l u i d b e h a v i o r . As a t u t o r i a l , q u a l i t a t i v e f l u i d b e h a v i o r i s s t r e s s e d r a t h e r than q u a n t i t a t i v e d e s c r i p t i o n . Solubilities in solid-fluid systems are i n t e r p r e t e d w i t h a s i m p l e f l u i d model. Enhancement f a c t o r s are i n t r o d u c e d t o demonstrate the importance o f r e p u l s i v e f o r c e s . I n t e r m o l e c u l a r i n t e r a c t i o n s i n c o s o l v e n t systems a r e d i s c u s s e d . L i q u i d - f l u i d phase b e h a v i o r and phase t r a n s i t i o n s i n l i q u i d - f l u i d and s o l i d - f l u i d systems a r e b r i e f l y p r e s e n t e d . T r a n s p o r t p r o p e r t i e s are b r i e f l y p r e s e n t e d t o s t r e s s t h e i r d e n s i t y dependence. Many i n t r o d u c t o r y t o p i c s have been o m i t t e d from t h i s d i s c u s s i o n , such as polymer b e h a v i o r and m i x t u r e f r a c t i o n a t i o n (see Ref. 10). R e a c t i o n s i n s u p e r c r i t i c a l f l u i d s are a l s o d i s c u s s e d elsewhere ( 6 0 ) . I n t e r e s t i n g a p p l i c a t i o n s o f s u p e r c r i t i c a l p r o c e s s i n g o f m a t e r i a l s a r e p u b l i s h e d , i n c l u d i n g powder m a n u f a c t u r i n g (10,60) and d r y i n g o f g e l s (62.) . A l t h o u g h the number o f a p p l i c a t i o n s c o n t i n u e s t o grow, each p r o c e s s r e q u i r e s s t e p s where an u n d e r s t a n d i n g o f phase b e h a v i o r f a c i l i t a t e s p r o c e s s o p t i m i z a t i o n . As an o v e r v i e w , t h i s c h a p t e r s h o u l d be h e l p f u l i n u n d e r s t a n d i n g more d e t a i l e d s t u d i e s .

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Supercritical Fluid Extraction and Chromatography - American

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