Supercritical Fluids - American Chemical Society

Chapter 13

Supercritical Fluid Adsorption at the Gas-Solid Interface Jerry W. King

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Downloaded by UNIV OF SYDNEY on March 27, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch013

CPC International, Moffett Technical Center, Argo, IL 60501

Adsorption of a supercritical fluid upon the surface of an adsorbent significantly alters the gas-solid interface thereby permitting the removal and migration of bound adsorbates. Such a process can contribute substantially to the regeneration of adsorbents and the chromatographic separation of solutes as reported in the scientific and patent literature. In this paper, we have characterized the adsorption of dense gases at solid interfaces in terms of their reported adsorption isotherms. Particular emphasis has been placed on the pressure at which maximum differential adsorption of the gas occurs and its importance in assuring rapid breakthrough of sorbates from packed columns. An experimental approach is presented for determining adsorbate breakthrough volumes as a function of gas compression using elution pulse chromatographic techniques. The derived retention volume data define distinct regions of solute breakthrough characteristics which are determined by the relative uptake of the compressed fluid on the sorbent surface. The results obtained in this study permit an estimation of the physical conditions required to effectively remove aliphatic and aromatic hydrocarbons from such sorbents as alumina and porous organic resins with minimal compression of the supercritical fluid phase. The utilization of supercritical fluids in conjunction with adsorbents and active solids is well documented in the technical literature. The most frequently cited applications involve the use of dense gases for the regeneration of adsorbents (1) and as mobile phases in supercritical fluid chromatography (2). Numerous 'Current address: Northern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 1815 North University Street, Peoria, IL 61604 0097-6156/87/0329-0150$06.50/0 © 1987 American Chemical Society

In Supercritical Fluids; Squires, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

13. KING

Supercritical Fluid Adsorption at the Gas-Solid Interface

p a t e n t s a l s o c o n t a i n examples o f the removal o f s p e c i f i c s o l u t e s , such as c a f f e i n e o r n i c o t i n e , from s u p e r c r i t i c a l f l u i d media by activated carbon (3, 4) and ion exchange resins (5, 6 ) . F r a c t i o n a t i o n o f s o l u t e s from dense gas streams has a l s o been demonstrated by B a r t o n and H a j n i k (7) u t i l i z i n g m o l e c u l a r s i e v e s as a s o r b e n t . L e s s w e l l known, b u t o f h i s t o r i c a l i m p o r t a n c e , i s the s o r p t i o n o f methane by c o a l ( 8 ) , an e q u i l i b r i a o f importance in p r e v e n t i n g mine e x p l o s i o n s . R e c e n t l y , Findenegg (9) has s u g g e s t e d t h a t r a d i o a c t i v e gases may be s t o r e d t o advantage on z e o l i t e s at elevated pressures. Regeneration o f a d s o r b e n t s was initially demonstrated in the mid-1970 s i n b o t h Japan (10) and the U n i t e d S t a t e s (11, 12). In r e s e a r c h sponsored by the E n v i r o n m e n t a l P r o t e c t i o n Agency (13-15), Critical Fluid Systems, Inc. ( A r t h u r D. Little) demonstrated the f e a s i b i l i t y of regenerating a c t i v a t e d carbon and o r g a n i c r e s i n s u s i n g s u p e r c r i t i c a l carbon d i o x i d e . Typical c o n d i t i o n s i n the above s t u d i e s u t i l i z i n g C 0 f o r the removal o f s e l e c t e d s o r b a t e s from a c t i v a t e d carbon a r e shown on a p l o t o f reduced s t a t e i n F i g u r e 1. F o r the e x t r a c t i o n s c i t e d , a wide range o f reduced temperature and p r e s s u r e have been employed depending upon t h e c h e m i c a l n a t u r e o f t h e compounds b e i n g d e s o r b e d . Presumably, t h e s e r e g e n e r a t i o n c o n d i t i o n s were s e l e c t e d e m p i r i c a l l y o r a r e based on independent measurements o f s o r b a t e ( s o l u t e ) s o l u b i l i t y i n the s u p e r c r i t i c a l f l u i d . More r e c e n t l y , Kander and P a u l a i t i s (16) have s t u d i e d the a d s o r p t i o n o f p h e n o l onto a c t i v a t e d c a r b o n and measured i t s sorption equilibria from dense C 0 . These r e s e a r c h e r s found that temperature c o n t r o l l e d the a d s o r p t i o n e q u i l i b r i a and that p h e n o l uptake was n e g l i g i b l y e f f e c t e d by changes i n the gas phase d e n s i t y . Such a r e s u l t i n d i c a t e s t h a t f a c t o r s o t h e r t h e n a s o l u t e ' s s o l u b i l i t y i n a dense gas p l a y a key r o l e i n d e f i n i n g the a d s o r p t i o n e q u i l i b r i u m which accompany such p r o c e s s e s . There i s a s y n e r g i s m between a d s o r p t i o n and c h r o m a t o g r a p h i c processes which is clearly demonstrated i n the supercritical fluid literature. Research i n s u p e r c r i t i c a l f l u i d chromatography can u s u a l l y be divided into analytical applications and the measurement o f p h y s i c o c h e m i c a l d a t a . Early a n a l y t i c a l separations methodology performed a t p r e s s u r e s c l o s e t o ambient conditions showed t h a t the e l u t i o n o r d e r o f i n j e c t e d solutes could be a l t e r e d by c h a n g i n g the n a t u r e o f the c a r r i e r gas (17-20). L a t e r , a d s o r b e n t s and c r o s s l i n k e d p o l y m e r i c p a c k i n g s were u t i l i z e d i n a n a l y t i c a l s t u d i e s t o c i r c u m v e n t the problem o f s t a t i o n a r y phase v o l a t i l i t y i n the h i g h l y compressed carrier gas. For example, S i e and R i j n d e r s (21, 22) demonstrated the advantages o f u s i n g such a d s o r b e n t s as alumina and m i c r o p o r o u s polymers f o r fractionating a wide v a r i e t y of mixtures i n c l u d i n g polycyclic aromatics hydrocarbons, alkaloids, and epoxy r e s i n o l i g o m e r s . R e s u l t s o b t a i n e d i n the above s t u d y i n d i c a t e d t h a t the s o l u t e distribution coefficients could be v a r i e d as a f u n c t i o n o f carrier gas p r e s s u r e i n a r a t h e r d r a m a t i c f a s h i o n and that e l u t i o n peak shape was s u b s t a n t i a l l y m o d i f i e d by the i n t e r a c t i o n o f the s u p e r c r i t i c a l f l u i d w i t h t h e s o r b e n t s u r f a c e . Kobayashi and co-workers (23-28) u s i n g t r a c e r p e r t u r b a t i o n chromatography have s u b s t a n t i a t e d the above f i n d i n g s and d e t e r m i n e d e q u i l i b r i u m K - v a l u e s , i s o t h e r m s , and thermodynamic f u n c t i o n s o f a d s o r p t i o n f o r l i g h t h y d r o c a r b o n s on s e v e r a l a d s o r b e n t s . 1


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SUPERCRITICAL FLUIDS

In t h i s p a p e r , we have u t i l i z e d l i t e r a t u r e d a t a i n tandem w i t h p u l s e c h r o m a t o g r a p h i c measurements t o d e f i n e t h e r o l e o f supercritical fluid adsorption a t the g a s - s o l i d i n t e r f a c e . I t s h o u l d be a p p r e c i a t e d t h a t t h e a d s o r p t i o n o f t h e s u p e r c r i t i c a l f l u i d on t h e a d s o r b e n t s u r f a c e i s a p p r e c i a b l e a t h i g h e r p r e s s u r e s leading t o the formation o f a condensed phase (29) i n t h e microporous adsorbents commonly used i n r e g e n e r a t i o n e x p e r i m e n t s . The f o r m a t i o n o f such a " l i q u i d - l i k e " f i l m a t t h e i n t e r f a c e c a n have a p r o f o u n d effect i n determining t h e uptake o f another adsorbate at the s o l i d surface or i n the c o n d i t i o n s r e q u i r e d f o r r e g e n e r a t i o n o f the adsorbent surface. The measurements r e p o r t e d in this study extend o u t t o 1600 atmospheres, a compression regime p r e v i o u s l y unexplored by g a s - s o l i d chromatography. Such experiments have permitted us t o c o r r e l a t e chromatographic retention volume measurements with adsorbate breakthrough c h a r a c t e r i s t i c s as a f u n c t i o n o f p r e s s u r e f o r s e v e r a l adsorbent/ adsorbate systems. The r o l e o f t h e s u p e r c r i t i c a l f l u i d a t t h e g a s - s o l i d i n t e r f a c e has been i n v o k e d t o e x p l a i n t h e s e b r e a k t h r o u g h c h a r a c t e r i s t i c s , t h e r e f o r e a b r i e f d i s c u s s i o n o f t h e fundamentals g o v e r n i n g t h e a d s o r p t i o n o f h i g h p r e s s u r e gases a t t h e a d s o r b e n t i n t e r f a c e w i l l be g i v e n . Fundamentals o f S u p e r c r i t i c a l F l u i d

Adsorption

Fundamental s t u d i e s on t h e a d s o r p t i o n o f s u p e r c r i t i c a l f l u i d s a t the g a s - s o l i d i n t e r f a c e a r e r a r e l y c i t e d i n t h e s u p e r c r i t i c a l fluid extraction literature. This i s most u n f o r t u n a t e since e q u i l i b r i u m s h i f t s i n d u c e d by gas phase n o n - i d e a l i t y i n m u l t i p h a s e systems c a n r a r e l y be t o t a l l y a t t r i b u t e d t o s o l u t e s o l u b i l i t y i n the s u p e r c r i t i c a l f l u i d phase. The p a r t i t i o n i n g o f an adsorbed s p e c i e between t h e i n t e r f a c e and gaseous phase c a n be governed by a complex a r r a y o f m o l e c u l a r i n t e r a c t i o n s which depend on t h e relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate a s s o c i a t i o n , the s o r p t i o n o f the s u p e r c r i t i c a l f l u i d a t t h e s o l i d i n t e r f a c e , and t h e s o l u b i l i t y o f t h e s o r b a t e i n the c r i t i c a l fluid. As we s h a l l demonstrate, competitive a d s o r p t i o n between t h e s o r b a t e and t h e s u p e r c r i t i c a l f l u i d a t the g a s - s o l i d i n t e r f a c e i s a s i g n i f i c a n t mechanism which s h o u l d be considered i n the proper design of adsorption/desorption methods which i n c o r p o r a t e dense gases as one o f t h e a c t i v e phases. I n g e n e r a l , a d s o r p t i o n o f a h i g h p r e s s u r e gas f o l l o w s a Type I I BET i s o t h e r m . The shape o f t h e i s o t h e r m i s p a r t l y dependent on t h e u n i t s chosen f o r t h e i s o t h e r m coordinates, however t h e r e i s a major d i f f e r e n c e i n t h e c h a r a c t e r o f t h e i s o t h e r m depending on whether a b s o l u t e o r Gibb's a d s o r p t i o n i s chosen f o r the a b s c i s s a . Whereas a b s o l u t e adsorption i s a measure o f t h e t o t a l mass o r volume o f t h e gas adsorbed onto t h e s u r f a c e , Gibb's a d s o r p t i o n ( o r t h e s u r f a c e e x c e s s ) measures t h e amount o f gas adsorbed on t h e s o l i d s u r f a c e i n excess o f t h a t corresponding t o t h e d e n s i t y o f gas i n t h e f l u i d phase a t t h e same temperature and p r e s s u r e . This l a t t e r d e f i n i t i o n provides some i n s i g h t as t o t h e m o l e c u l a r interactions taking place at the i n t e r f a c e as a f u n c t i o n o f gas d e n s i t y . A t y p i c a l a d s o r p t i o n isotherm i l l u s t r a t i n g b o t h t h e a b s o l u t e and d i f f e r e n t i a l (Gibbs) a d s o r p t i o n i s shown i n F i g u r e 2.



KING


REDUCED DENSITY

F i g u r e 1. Reported c o n d i t i o n s desorption o f s e l e c t e d adsorbates

f o r the s u p e r c r i t i c a l from a c t i v a t e d c a r b o n .

PRESSURE

•

fluid

··

F i g u r e 2. T y p i c a l a d s o r p t i o n i s o t h e r m f o r a s u p e r c r i t i c a l gas i n w h i c h a b s o l u t e and d i f f e r e n t i a l a d s o r p t i o n a r e e x p r e s s as a function o f pressure.



154


Menon has n o t e d i n h i s comprehensive review (30) t h a t an a d s o r p t i o n maxima o c c u r s as a s p e c i f i c gas p r e s s u r e , ρ , when the Gibbs a d s o r p t i o n i s p l o t t t e d v e r s u s t h e gas p r e s s u r e . These a d s o r p t i o n maxima a r e q u i t e pronounced when t h e s u p e r c r i t i c a l fluid i s adsorbed a t reduced temperatures approaching unity. F u r t h e r c o m p r e s s i o n o f t h e c r i t i c a l f l u i d beyond ρ results i n a d e n s i t y i n c r e a s e o f t h e adsorbed f i l m and a c o n c o m i t a n t d e c r e a s e i n t h e s u r f a c e e x c e s s as t h e gas phase approaches a density e q u a l t o o r e x c e e d i n g t h a t o f t h e adsorbed phase. As we s h a l l show l a t e r , a t t a i n m e n t o f t h i s s p e c i f i c p r e s s u r e v a l u e i s c r i t i c a l for s a t u r a t i n g t h e a d s o r b e n t s u r f a c e w i t h t h e compressed fluid and t h e r e b y l e a d i n g t o c o n d i t i o n s which f a v o r t h e d e s o r p t i o n o f the bound solute from the surface region. Menon (31) has c o r r e l a t e d the occurence o f ρ f o r v a r i o u s gases i n terras o f max their respective critical properties and has p r o p o s e d the f o l l o w i n g e m p i r i c a l e q u a t i o n which i s a p p l i c a b l e f o r a d s o r p t i o n on macroporous a d s o r b e n t s (pore d i a m e t e r s g r e a t e r t h a n 20 A) : Q

2

ρ = Ρ Τ *max c r

(1)

where Ρ = c r i t i c a l p r e s s u r e o f t h e gas T^ = reduced temperature a t which a d s o r p t i o n o c c u r s The above r e l a t i o n s h i p p r e d i c t s a monotonie i n c r e a s e i n P w i t h i n c r e a s i n g temperature as shown i n F i g u r e 3 f o r t h r e e d i f f e r e n t gases. Hence, t h e maximum amount o f gas (by t h e Gibbs d e f i n i t i o n ) adsorbed on t h e s u r f a c e o f t h e a d s o r b e n t c a n be a t t a i n e d a t a lower p r e s s u r e by o p e r a t i n g c l o s e t o t h e c r i t i c a l temperature o f the adsorbed gas. A p p l i c a t i o n o f even h i g h e r p r e s s u r e s then ρ will result i n a large increase i n the c o h e s i o n a l e n e r g y ( ? f t h e a d s o r b e d gas l e a d i n g t o t h e f o r m a t i o n of a l i q u i d - l i k e l a y e r on t h e s u r f a c e o f t h e porous a d s o r b e n t (27). m a x

ma

A d d i t i o n a l s t u d i e s by Menon (32) have i n d i c a t e d t h e P can o c c u r a t lower p r e s s u r e s t h e n t h o s e p r e d i c t e d by E q u a t i o n 1 depending on t h e p o r e s t r u c t u r e a s s o c i a t e d w i t h t h e a d s o r b e n t . E m p i r i c a l l y , adsorbents p o s s e s s i n g m i c r o p o r o s i t y e x h i b i t a that i s 0.6-0.8 o f t h e v a l u e p r e d i c t e d by E q u a t i o n 1. This o b s e r v a t i o n i s a t t r i b u t e d t o the o v e r l a p p i n g o f p o t e n t i a l f i e l d s i n t h e a d s o r b e n t p o r e s , t h e r e b y e n h a n c i n g s o r p t i o n o f t h e gas a t lower p r e s s u r e s . E x p e r i m e n t a l s t u d i e s by Ozawa (33) have v e r i f i e d t h i s t r e n d as shown i n F i g u r e 4 f o r t h e C 0 / a c t i v a t e d c a r b o n system. Here t h e a d s o r p t i o n maxima f o r t h e gas o c c u r s a t a lower p r e s s u r e t h a n t h e c r i t i c a l p r e s s u r e o f c a r b o n d i o x i d e . I t s h o u l d a l s o be n o t e d t h a t t h e amount o f gas adsorbed i s d e c r e a s e d at h i g h e r reduced t e m p e r a t u r e s and t h a t a d d i t i o n a l compression i s r e q u i r e d t o r e a c h a d e f i n e d a d s o r p t i o n maxima ( i . e . , a t v e r y high values of Τ i t i s sometimes d i f f i c u l t t o d i s c e r n a w e l l defined a d s o r p t i o n maximum). The above t r e n d has a l s o been found f o r other adsorbent/adsorbate systems, such as silica gel/C0 . m a x

2

2

The dependence o f gas a d s o r p t i o n on t h e pore s t r u c t u r e o f the s o r b e n t has been e x t e n s i v e l y s t u d i e d by Ozawa, Kusumi, and Ogino (34) and t h e o r e t i c a l l y c o r r e l a t e d by a p p l i c a t i o n o f t h e P i c k e t t e q u a t i o n . Data o b t a i n e d from t h i s s t u d y has been p l o t t e d




KING

F i g u r e 4. A d s o r p t i o n i s o t h e r m s f o r the c a r b o n d i o x i d e / a c t i v a t e d c a r b o n system. Reproduced w i t h p e r m i s s i o n from Ref. 33. Copyr i g h t 1972, Chemical S o c i e t y o f Japan.


155

156


in

Figure

5

where

the

ρ

v a l u e v e r s u s the mean pore diameter max of the adsorbent ( a c t i v a t e d c a r b o n ) i s shown f o r C 0 sorption. The a d s o r p t i o n maxima i n t h i s c a s e , i n c r e a s e s w i t h increasing pore diameter, a s y m p o t i c a l l y approaching a constant value at l a r g e pore diameters. The v a l u e p r e d i c t e d by Menon's r e l a t i o n s h i p ( E q u a t i o n 1) has a l s o been i n c l u d e d f o r the purpose o f comparison. Although i t would seem p r u d e n t t o u t i l i z e an a d s o r b e n t o f low p o r o s i t y t o a v o i d a d d i t i o n a l gas compression, m i t i g a t i n g f a c t o r s such as a d s o r p t i o n c a p a c i t y and s t e r e o - s e l e c t i v i t y a l s o p l a y a seminal role i n the f i n a l s e l e c t i o n o f a s u i t a b l e adsorbent. The pore s i z e dependence o f ρ a t v a r i o u s temperatures has a l s o been v e r i f i e d f o r above carlfons and shows a s i m i l a r dependence t o t h a t e x h i b i t e d i n F i g u r e 3. F u r t h e r d i s c u s s i o n on the t h e o r e t i c a l a s p e c t s o f s u p e r c r i t i c a l f l u i d a d s o r p t i o n on s o l i d s can be found by c o n s u l t i n g the papers of Findenegg (35-37). The importance o f the h i g h p r e s s u r e a d s o r p t i o n maximum as d i s c u s s e d above becomes a p p a r e n t i f we compare t y p i c a l ρ v a l u e s on a p l o t o f reduced s t a t e v a r i a b l e s with conditions a r e commonly employed f o r the r e g e n e r a t i o n of adsorbents ( F i g u r e 1). As shown i n F i g u r e 6, ρ values f o r C0 on typical carbonaceous a d s o r b e n t s occur at c o n s i d e r a b l y lower reduced p r e s s u r e s (p = 0.5-1.0) t h a n those u t i l i z e d f o r the d e s o r p t i o n o f common o r g a n i c s o l u t e s (14) o r p e s t i c i d e s ( 1 ) . T h i s r e s u l t i m p l i e s t h a t p o t e n t i a l l y much lower p r e s s u r e s and volumes o f s u p e r c r i t i c a l C 0 c o u l d be used t o d i s p l a c e adsorbed s o l u t e s from the a d s o r b e n t s u r f a c e by m a x i m i z i n g the c o m p e t i t i v e a d s o r p t i o n o f the dense C 0 on the s o l i d s u r f a c e . To provide some e x p e r i m e n t a l verification of the above h y p o t h e s i s , we have measured v i a p u l s e e l u t i o n chromatography, breakthrough volumes (the peak maximum retention volume in l i n e a r e l u t i o n chromatography c o r r e s p o n d i n g t o the 50% breakthrough volume i n f r o n t a l a n a l y s i s ) o f model a d s o r b a t e s on two d i f f e r e n t adsorbents over an extended pressure range u s i n g C 0 as the critical fluid. Our e x p e r i m e n t a l r e s u l t s s u g g e s t t h a t t h e r e a r e d i s t i n c t r e g i o n s i n which the s u r f a c e o f the a d s o r b e n t i s u n d e r g o i n g m o d i f i c a t i o n due t o the a d s o r p t i o n o f the s u p e r c r i t i c a l fluid c a r r i e r gas. The i m p l i c a t i o n s o f t h e s e r e s u l t s have p e r m i t t e d the i d e n t i f i c a t i o n o f d i s t i n c t p r e s s u r e ranges which can be used f o r the fractionation of d i s s o l v e d moieties i n the critical f l u i d , a minimum p r e s s u r e which s h o u l d be a t t a i n e d f o r commencement of adsorbent r e g e n e r a t i o n , and a upper p r e s s u r e l i m i t a t which sorbate breakthrough volume becomes constant with increasing pressure.


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2

2

2

2

Experimental

Measurements

The experimental apparatus utilized f o r these studies i s of s i m i l a r d e s i g n t o t h a t r e p o r t e d by G i d d i n g s , Myers, and King (38) as i s d e p i c t e d i n F i g u r e 7. Compression o f the mobile phase was accomplished u s i n g an Aminco a i r - a c t u a t e d diaphragm compressor which was kept i n a heated chamber above the c r i t i c a l temperature of the carrier gas. Pump p u l s a t i o n s and flow i r r e g u l a r i t i e s were m i n i m i z e d by s t o r i n g the compressed f l u i d i n a h i g h p r e s s u r e b a l l a s t chamber which s e r v e d as a s o u r c e o f m o b i l e phase. P r e s s u r e s up t o 3000 p s i g were a d j u s t e d by u s i n g



KING


F i g u r e 5. R e l a t i o n s h i p between t h e p r e s s u r e a t which maximum d i f f e r e n t i a l a d s o r p t i o n o f c a r b o n d i o x i d e o c c u r s and t h e mean pore diameter o f a c t i v a t e d carbons.


157



REDUCED DENSITY

F i g u r e 6. R e p o r t e d a d s o r p t i o n maxima f o r c a r b o n d i o x i d e / a d s o r b e n t system on a p l o t o f reduced s t a t e . ( I ) and ( I I ) r e p r e s e n t d i f f e r e n t t y p e s o f a c t i v a t e d carbon (AC) .



KING


CARY MODEL 31 RECORDER

•f

ELECTROMETER VOj

IfiRATED VALVE

SPLITTER

F i g u r e 7. E x p e r i m e n t a l h i g h p r e s s u r e gas a p p a r a t u s f o r r e t e n t i o n volume measurements.

chromatographic




160

an Apco b a c k p r e s s u r e r e g u l a t o r i n c o n j u n c t i o n w i t h a Bourdon tube gauge. R e g u l a t i o n o f p r e s s u r e s beyond 3000 p s i g was a c h i e v e d by i s o l a t i n g the low p r e s s u r e gauge from the main f l o w system and subsequently r e g u l a t i n g the e n t i r e chromatographic system w i t h a d u a l c o n t a c t e l e c t r i c a l c o n t r o l gauge ( m o d i f i e d Aminco Model 47-18330). The sampling chamber/valve and column were p l a c e d i n a h e a t e d , t h e r m o s t a t t e d oven h e l d a t 40°C (dashed l i n e i n F i g u r e 7) . Column design and preparation incorporated previously d e s c r i b e d methods r e p o r t e d i n the l i t e r a t u r e ( 3 9 ) . Two d i f f e r e n t a d s o r b e n t s were employed: a 100/120 mesh c r o s s l i n k e d s t y r e n e / divinylbenzene resin ( P o l y p a k P-Waters A s s o c i a t e s ) and a Woelm aniontropic activity grade alumina. These adsorbents were packed i n 300 and 94 cm. s t a i n l e s s s t e e l columns h a v i n g a 1 mm. i n t e r n a l diameter. P r e s s u r e drop a c r o s s the a d s o r b e n t bed was kept t o a minimum (

Supercritical Fluids - American Chemical Society

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