Supercritical Fluids - American Chemical Society

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Chapter 12

Solubility of meso-Tetraphenylporphyrin in Two Supercritical Fluid Solvents T. R. Bergstresser and Michael E . Paulaitis

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Department of Chemical Engineering, University of Delaware, Newark, DE 19716

Solubilities of meso-tetraphenylporphyrin (normal melting temperature = 444°C) in pentane and in toluene have been measured at elevated temperatures and pressures. Three-phase, solid-liquid-gas equilibrium temperatures and pressures were also measured for these two binary mixtures at conditions near the critical point of the supercritical-fluid solvent. The solubility of the porphyrin in supercritical toluene is three orders of magnitude greater than that in supercritical pentane or in conventional liquid solvents at ambient temperatures and pressures. An analysis of the phase diagram for toluene-porphyrin mixtures shows that supercritical toluene is the preferred solvent for this porphyrin because (1) high solubilities are obtained at moderate pressures, and (2) the porphyrin can be easily recovered from solution by small reductions in pressure. The enhanced solubility of solids in compressed gases at elevated temperatures and pressures was first noted more than one hundred years ago. Hannay and Hogarth (1) observed that the solubility of salts in compressed ethanol was considerably greater than expected based on the vapor pressure of the salts. Since this early investigation, numerous authors have discussed the phase behavior of solids in dense fluids at elevated temperatures and pressures. Review articles by Paulaitis et al. (2), Luks (3), Streett (4), and Rowlinson and Richardson (5) document many of the investigations that have been made. The phase behavior of solids in supercritical fluids has practical significance as well as academic interest. Since the mid1970's, it has been recognized that supercritical fluids can be useful as solvents for commercial-scale extractions. While a variety of applications are documented in the literature (2,6), supercritical-fluid (SCF) extraction has been particularly useful in upgrading petroleum fractions (7), extracting volatile components from coal (8), and deashing oil shale (9) and coal liquids (10). The 0097-6156/87/0329-0138$06.00/0 © 1987 American Chemical Society

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12.

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a p p l i c a t i o n o f SCF e x t r a c t i o n i n t h e s e p r o c e s s e s r e q u i r e s s o l u b i l i t i e s f o r t h e m i x t u r e s of i n t e r e s t i n SCF s o l v e n t s . S o l u b i l i t i e s can be d i f f i c u l t t o p r e d i c t , however, s i n c e t h e s e m i x t u r e s are h i g h l y complex i n t h e sense t h a t numerous c o n s t i t u e n t s are i n v o l v e d which d i f f e r s i g n i f i c a n t l y i n m o l e c u l a r s i z e , shape, s t r u c t u r e , and p o l a r i t y . E x p e r i m e n t a l s o l u b i l i t i e s on w e l l - d e f i n e d , model systems are r e q u i r e d t o develop b e t t e r p r e d i c t i v e methods. In t h i s p a p e r , we p r e s e n t t h e r e s u l t s of an e x p e r i m e n t a l study on t h e phase b e h a v i o r o f w e l l - c h a r a c t e r i z e d b i n a r y m i x t u r e s which r e p r e s e n t t h e more complex m i x t u r e s t h a t a r i s e i n SCF e x t r a c t i o n s of p e t r o l e u m r e s i d u a and c o a l l i q u i d s . These b i n a r y m i x t u r e s c o n s i s t of pentane and t o l u e n e w i t h m e s o - t e t r a p h e n y l p o r p h y r i η ( T P P ) . Porphyrins o c c u r n a t u r a l l y i n crude o i l s (11,12) and r e p r e s e n t an important c l a s s of high m o l e c u l a r - w e i g h t c o n s t i t u e n t s of t h e s e o i l s , i n c l u d i n g t h o s e which c o n t a i n heavy m e t a l s , such as n i c k e l and vanadium. Pentane and t o l u e n e were s e l e c t e d as SCF s o l v e n t s because t h e s e hydrocarbons have c r i t i c a l t e m p e r a t u r e s c o n v e n i e n t t o t h e d e s i r e d o p e r a t i n g t e m p e r a t u r e s f o r p r o c e s s i n g p e t r o l e u m r e s i d u a and c o a l liquids, respectively. S e v e r a l d i f f e r e n t measurements were made as a r e s u l t of the d i f f e r e n t phase b e h a v i o r observed f o r t h e two b i n a r y m i x t u r e s . G a s - s o l i d e q u i l i b r i u m was observed f o r m i x t u r e s of TPP and pentane at c o n d i t i o n s near t h e c r i t i c a l p o i n t of p e n t a n e . Hence, s o l i d s o l u b i l i t i e s f o r TPP i n s u p e r c r i t i c a l pentane were measured. However, f o r m i x t u r e s o f TPP and t o l u e n e , a t h i r d ( l i q u i d ) phase forms i n t h e p r e s e n c e o f t h e gas and t h e s o l i d , at p r e s s u r e s w e l l below t h e c r i t i c a l p r e s s u r e of t o l u e n e . At h i g h e r p r e s s u r e s , g a s - l i q u i d and s o l i d - l i q u i d e q u i l i b r i a were o b s e r v e d , r a t h e r than gas-solid equilibrium. T h u s , phase c o m p o s i t i o n s f o r g a s - l i q u i d e q u i l i b r i u m were measured f o r t h i s b i n a r y mixture t o g i v e TPP s o l u b i l i t i e s i n each o f t h e f l u i d p h a s e s . P r e s s u r e s and t e m p e r a t u r e s f o r t h r e e - p h a s e , s o l i d - l i q u i d - g a s e q u i l i b r i u m were a l s o measured f o r both b i n a r y m i x t u r e s . Experimental

Methods

The f o l l o w i n g e x p e r i m e n t a l t e c h n i q u e s were used t o measure t h e p r e s s u r e s and t e m p e r a t u r e s f o r s o l i d - l i q u i d - g a s e q u i l i b r i u m , phase c o m p o s i t i o n s (bubble and dew p o i n t s ) f o r g a s - l i q u i d e q u i l i b r i u m , and s o l i d s o l u b i l i t i e s in s u p e r c r i t i c a l pentane. Experimental procedures and t h e apparatus a r e d e s c r i b e d i n d e t a i l elsewhere ( 1 3 ) . S o l i d - l i q u i d - g a s (SLG) e q u i l i b r i u m t e m p e r a t u r e s and p r e s s u r e s were measured i n a c o n s t a n t - v o l u m e view c e l l . A known amount of p o r p h y r i n was loaded i n t o t h e c e l l , which was then a t t a c h e d t o t h e s o l v e n t d e l i v e r y system and heated t o t h e d e s i r e d o p e r a t i n g temperature. Once thermal e q u i l i b r i u m was o b t a i n e d , s o l v e n t was metered i n t o t h e c e l l u n t i l t h e p r e s s u r e w i t h i n t h e c e l l reached t h e SLG p r e s s u r e . E q u i l i b r i u m was o b t a i n e d when t h e p r e s s u r e s t a b l i z e d and a l l t h r e e phases c o u l d be o b s e r v e d . T h i s p r e s s u r e and t h e c o r r e s p o n d i n g t e m p e r a t u r e were r e c o r d e d as one p o i n t on t h e SLG e q u i l i b r i u m l i n e f o r the binary mixture. A d d i t i o n a l p o i n t s were o b t a i n e d by s e t t i n g a new temperature and r e p e a t i n g t h e p r o c e d u r e .

Squires and Paulaitis; Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Bubble p o i n t s f o r g a s - l i q u i d e q u i l i b r i u m were measured at c o n s t a n t t e m p e r a t u r e by o b s e r v i n g the p r e s s u r e at which the e q u i l i b r i u m gas phase d i s a p p e a r e d upon i n j e c t i o n of small amounts of s o l v e n t i n t o the view c e l l . The e q u i l i b r i u m c o m p o s i t i o n of the l i q u i d phase was o b t a i n e d from t h e known c o m p o s i t i o n i n the c e l l . Other p r e s s u r e s and c o r r e s p o n d i n g c o m p o s i t i o n s at t h i s temperature were o b t a i n e d by r e p e a t i n g t h e p r o c e d u r e f o r d i f f e r e n t p o r p h y r i n loadings. Dew p o i n t s were a l s o measured u s i n g t h e procedure d e s c r i b e d above, except t h a t the d i s a p p e a r a n c e of the l i q u i d phase was observed as s o l v e n t was added t o the view c e l l . Dew p o i n t measurements were l i m i t e d by low p o r p h y r i n c o n c e n t r a t i o n s i n the gas p h a s e , which r e q u i r e d l o a d i n g very small amounts of p o r p h y r i n i n t o t h e c e l l . Measurements of s o l i d s o l u b i l i t i e s i n s u p e r c r i t i c a l pentane were, i n p r i n c i p l e , i d e n t i c a l t o the bubble p o i n t or dew p o i n t measurements d e s c r i b e d above. The e q u i l i b r i u m p r e s s u r e and c o r r e s p o n d i n g s o l i d s o l u b i l i t y at a f i x e d temperature were determined from the measured p r e s s u r e and known m i x t u r e c o m p o s i t i o n i n the c e l l when the l a s t c r y s t a l of s o l i d d i s s o l v e d . These measurements were l i m i t e d at low s o l u b i l i t i e s by the low p o r p h y r i n l o a d i n g s , and at high s o l u b i l i t i e s by the dark p u r p l e c o l o r of the f l u i d phase which o b s c u r e d o b s e r v a t i o n of the s o l i d phase. M e s o - t e t r a p h e n y l p o r p h y r i η (>97% c h l o r i n f r e e ) was purchased from Man-Win Chemical Company and was used without f u r t h e r p u r i f i c a t i o n . Pentane and t o l u e n e (ACS c e r t i f i e d grade) were purchased from F i s c h e r S c i e n t i f i c Company and were degassed b e f o r e u s e . Experimental

Results

The a c c u r a c y of the p r e s s u r e and t e m p e r a t u r e measurements was v e r i f i e d by measuring the vapor p r e s s u r e curves and c r i t i c a l p o i n t s f o r pentane and f o r t o l u e n e . Vapor p r e s s u r e s were measured by o b s e r v i n g the f o r m a t i o n of a l i q u i d phase as pentane or t o l u e n e was i n j e c t e d i n t o the c o n s t a n t - v o l u m e view c e l l under i s o t h e r m a l conditions. The o b s e r v a t i o n of c r i t i c a l o p a l e s c e n c e was used t o determine the c r i t i c a l p o i n t . The measured vapor p r e s s u r e s and c r i t i c a l p o i n t s are given i n T a b l e I. Vapor p r e s s u r e s d e v i a t e from Table

Temperature (°C) 84.8 104.5 125.7 134.7 158.3 184.9 196.1

I.

Measured

Vapor P r e s s u r e s

Pentane Vapor P r e s s u r e (atm) 4.6 6.4 10.2 12.2 18.5 28.4 33.9 CP

CP = C r i t i c a l

for

Pentane and Toluene

Temperature (°C) 120.9 159.9 191.9 196.9 224.7 248.9 274.8 287.6 299.6 318.5

Toluene Vapor P r e s s u r e (atm) 1.4 3.4 6.3 7.0 11.1 16.1 23.1 27.6 32.1 40.3 CP

Point

Squires and Paulaitis; Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

12.

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Solubility of meso-Tetraphenylporphynn

141

l i t e r a t u r e values (14,15) by an average of 0.32 atm f o r pentane and 0.10 atm f o r t o l u e n e . C r i t i c a l temperatures and p r e s s u r e s are i n c l o s e agreement with l i t e r a t u r e values of 197.5°C and 33.6 atm f o r pentane ( 1 4 ) , and 3 1 8 . 5 7 ° C and 40.6 atm f o r t o l u e n e ( 1 5 ) . T h r e e - p h a s e , SLG e q u i l i b r i u m temperatures and p r e s s u r e s f o r b i n a r y mixtures of pentane and t o l u e n e with TPP are given i n T a b l e s II and III, respectively. A lower c r i t i c a l endpoint (LCEP) was observed f o r pentane-TPP m i x t u r e s , and i s a l s o denoted i n T a b l e II. Table

II.

Measured T h r e e - P h a s e SLG Temperatures and P r e s s u r e s f o r B i n a r y M i x t u r e s of Pentane

Temperature

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(°C) —

—

180.4 186.9 192.3 195.0 197.7 198.3 Table

III.

Pressure (atm) 23.0 25.6 28.3 30.8 32.1 33.4 33.6 LCEP

Measured T h r e e - P h a s e SLG Temperatures and P r e s s u r e s f o r B i n a r y M i x t u r e s of Toluene and TPP

Temperature (°C) 286.2 292.9 298.8 299.4 304.1 310.6 315.1 319.9 325.0 325.6 337.8 345.2 350.0

Pressure (atm) 22.2 24.2 24.9 25.1 26.2 27.3 27.7 28.5 29.6 29.5 30.2 31.0 31.1

T h i s LCEP i s a g a s - l i q u i d c r i t i c a l p o i n t i n the presence of t h e s o l i d phase. A LCEP was not observed f o r t o l u e n e - T P P m i x t u r e s at temperatures below 3 5 0 ° C . Measurements were not made at h i g h e r temperatures because of thermal d e g r a d a t i o n of t h e p o r p h y r i n . The r e s u l t s i n T a b l e s I—III a r e a l s o shown on PT p r o j e c t i o n s i n F i g u r e s 1 and 2. The m i x t u r e c r i t i c a l p o i n t s i n F i g u r e 2 a r e o b t a i n e d from F i g u r e 4. S o l i d s o l u b i l i t i e s f o r TPP i n pentane at temperatures of 207°C and 250°C are given i n T a b l e IV and p r e s e n t e d i n F i g u r e 3. The maximum e x p e r i m e n t a l u n c e r t a i n t i e s are 3 χ 10"5 f o r p o r p h y r i n mole f r a c t i o n , 1 atm f o r p r e s s u r e , and 1 degree C f o r t e m p e r a t u r e .

Squires and Paulaitis; Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Squires and Paulaitis; Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Figure

140

1,

170 Temperature

( C)

P r e s s u r e - T e m p e r a t u r e P r o j e c t i o n of Vapor P r e s s u r e Curve f o r Pentane and S o l i d - L i q u i d - G a s Equilibrium Curve f o r Pentane-TPP M i x t u r e s .

160

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200

η > r τπ r c

Η

73

o

70

c m

Κ)

4^

Squires and Paulaitis; Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

0)

(Ο

Ε

2.

270

Figure

260

290 Temperature

( C)

P r e s s u r e - T e m p e r a t u r e P r o j e c t i o n of Vapor P r e s s u r e Curve f o r Toluene and S o l i d - L i q u i d - G a s Equilibrium Curve f o r T o l u e n e - T P P M i x t u r e s .

280

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360

•a

"5"

I

ST

> c r >

SP

70

m

CO CO

m

Η 70

CO

ο

70

m

DO

Squires and Paulaitis; Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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saimj ivDiiiHDHHdns ο

PORPHYRIN

Mole Fraction

Squires and Paulaitis; Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

•

te-

4,

0.02

ta.

Figure

0.00

no

0.06

T" 0.10 Fraction

0.08 Mole

0.14 PORPHYRIN

0.12

O.K

—ι 0.18

at

1 0.20

Γ 0.22

Pts

3 5 0 C , Dew P t s

3 5 0 C, Bubble

Bubble and Dew P o i n t s f o r T o l u e n e - T P P M i x t u r e s 300 , 325 , and 3 5 0 ° C .

0.04

•

-Θ-

Pts

Pts.

C , Dew P t s

- A 525

5 0 0 C, Bubble 3 2 5 C, Bubble

-B--

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146

SUPERCRITICAL FLUIDS

Table

IV.

Measured S o l i d

Solubilities

of TPP i n Compressed Pentane Mole F r a c t i o n TPP χ 10*

Pressure (atm) T=207 C U

1.0 1.3 1.4 1.6

125 132 159 190

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T=250°C 89 115 137 169 220

1.1 2.1 2.7 3.0 3.1

The normal m e l t i n g p o i n t of TPP was measured u s i n g a d i f f e r e n t i a l s c a n n i n g c a l o r i m e t e r , and was found t o be 4 4 4 ° C . The e x p e r i m e n t a l r e s u l t s i n F i g u r e 2 i n d i c a t e , however, t h a t TPP w i l l melt at t e m p e r a t u r e s of 3 0 0 - 3 5 0 ° C i n the presence of compressed t o l u e n e at p r e s s u r e s of 20-30 atm (well below the c r i t i c a l p r e s s u r e of t o l u e n e ) . At h i g h e r p r e s s u r e s , g a s - l i q u i d and s o l i d - l i q u i d e q u i l i b r i a , r a t h e r than g a s - s o l i d e q u i l i b r i u m , w i l l be o b t a i n e d f o r t o l u e n e - T P P m i x t u r e s . Bubble p o i n t s and dew p o i n t s f o r t h i s b i n a r y m i x t u r e were measured at t e m p e r a t u r e s of 3 0 0 ° , 3 2 5 ° , and 3 5 0 ° , and p r e s s u r e s above t h e c o r r e s p o n d i n g SLG e q u i l i b r i u m p r e s s u r e s . The r e s u l t s are given i n T a b l e V and shown i n F i g u r e 4. The maximum T a b l e V.

Measured Bubble and Dew P o i n t s f o r T o l u e n e and TPP

T= 300°C Pressure (atm)

Mole Fraction TPPxlO

27 28 29 30 31 32 32

18.40 12.80 8.26 5.08 2.46 .70 .08

2

Binary Mixtures

of

T=325°C

T=350°C

Mole Fraction Pressure TPPxlO (atm) Bubble P o i n t s 32 18.80 34 15.20 35 13.90 37 11.60 39 8.78 40 7.36 41 4.78 41 3.31 42 1.77 43 .38 43 .15 43 .12 Dew P o i n t s .09 41 38 .04 38 .02

Mole Fraction Pressure TPPxlO (atm) Bubble P o i n t s 21.50 39 19.80 40 16.40 43 46 12.80 9.62 48 7.18 50 4.88 51 3.38 53 1.97 56 57 1.37 Dew P o i n t s 56 .79 .44 57 52 .30 .26 54 .24 51 .21 54

2

Squires and Paulaitis; Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2

12.

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Solubility of m^o-Tetraphenylporphynn

147

e x p e r i m e n t a l u n c e r t a i n t i e s f o r bubble and dew p o i n t mole f r a c t i o n s are 10% o f t h e r e p o r t e d v a l u e s . Maximum e x p e r i m e n t a l u n c e r t a i n t i e s are 1 atm f o r p r e s s u r e and 2 degrees f o r t e m p e r a t u r e at 3 5 0 ° C .

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Discussion The m e l t i n g b e h a v i o r f o r TPP i n t h e p r e s e n c e o f compressed pentane ( F i g u r e 1) i s c h a r a c t e r i z e d by an i n t e r r u p t e d t h r e e - p h a s e , SLG e q u i l i b r i u m l i n e which t e r m i n a t e s at a LCEP. This behavior i s c h a r a c t e r i s t i c o f a gas and a s o l i d with low mutual s o l u b i l i t y , and i s expected when t h e t r i p l e - p o i n t t e m p e r a t u r e of t h e s o l i d i s much g r e a t e r than t h e c r i t i c a l t e m p e r a t u r e of t h e gas ( 3 ) . At t e m p e r a t u r e s j u s t above t h e LCEP t e m p e r a t u r e , TPP does not melt i n t h e presence of compressed p e n t a n e , and g a s - s o l i d e q u i l i b r i u m i s o b s e r v e d at p r e s s u r e s up t o two hundred atmospheres ( F i g u r e 3 ) . The s o l u b i l i t y o f TPP i n s u p e r c r i t i c a l pentane at t h e s e e l e v a t e d p r e s s u r e s i s a l s o q u i t e l o w . At 2 5 0 ° C , t h e maximum s o l u b i l i t y i s a p p r o x i m a t e l y 2.7 χ 1 0 " mole f r a c t i o n u n i t s . 4

The m e l t i n g b e h a v i o r f o r TPP i n t h e p r e s e n c e o f compressed t o l u e n e i s s i g n i f i c a n t l y d i f f e r e n t ( F i g u r e 2 ) . The t h r e e - p h a s e , SLG e q u i l i b r i m curve does not show a LCEP, and p r o b a b l y c o n t i n u e s w i t h o u t i n t e r r u p t i o n t o t h e t r i p l e p o i n t o f TPP. T h i s b e h a v i o r i s c h a r a c t e r i s t i c o f b i n a r y m i x t u r e s with r e l a t i v e l y high mutual solubilities. F o r SLG e q u i l i b r i u m , t h e l i q u i d phase would have high TPP c o n c e n t r a t i o n s ; whereas t h e gas phase would be mostly t o l u e n e and t h e s o l i d phase would be e s s e n t i a l l y pure TPP. The p r e s s u r e c o m p o s i t i o n diagram d e p i c t i n g g a s - l i q u i d e q u i l i b r i u m f o r t o l u e n e - T P P m i x t u r e s ( F i g u r e 4) i n d i c a t e s t h a t l i q u i d - p h a s e c o n c e n t r a t i o n s of TPP can be on t h e o r d e r o f 20 mol % at 350°C and 40 atm. A comparison o f s o l i d s o l u b i l i t i e s f o r TPP i n s u p e r c r i t i c a l p e n t a n e , s u p e r c r i t i c a l t o l u e n e , and v a r i o u s l i q u i d s o l v e n t s i s given i n T a b l e V I . The s o l i d s o l u b i l i t y o f TPP i n t o l u e n e c o r r e s p o n d s t o t h e l i q u i d - p h a s e c o n c e n t r a t i o n f o r SLG e q u i l i b r i u m , e s t i m a t e d from t h e r e s u l t s i n F i g u r e s 2 and 4. S e v e r a l i m p o r t a n t c o n c l u s i o n s can be Table VI.

Solvent

Comparison o f S o l i d S o l u b i l i t i e s Solvents Mole F r a c t i o n TPP χ 1 0 2210.00 2.71 6.30 5.20 1.40 .01

of TPP i n V a r i o u s

Conditions

4

Toluene Pentane P y r i d i n e (16) Benzene (16) Acetone (17) E t h a n o l (17)

T=325°C,f>=3U atm

T=250°C,P=137 atm ambient ambient ambient ambient

drawn from t h i s c o m p a r i s o n . F i r s t , TPP s o l u b i l i t y i n s u p e r c r i t i c a l t o l u e n e i s t h r e e o r d e r s of magnitude g r e a t e r than t h a t i n s u p e r c r i t i c a l pentane and i n t h e l i q u i d s o l v e n t s at ambient t e m p e r a t u r e s and p r e s s u r e s . O b v i o u s l y , the higher temperature a s s o c i a t e d with s u p e r c r i t i c a l t o l u e n e must be a s i g n i f i c a n t f a c t o r . However, t h e chemical nature of t h e s o l v e n t i s a l s o i m p o r t a n t , as

American Chemical Society Library 1155 16th St., N.W. Washington, D.C. 20036 Squires and Paulaitis; Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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148

SUPERCRITICAL FLUIDS

demonstrated by the f a c t t h a t t h e enhancement f a c t o r (2) c a l c u l a t e d f o r TPP i n t o l u e n e i s about one o r d e r of magnitude g r e a t e r than t h a t c a l c u l a t e d f o r TPP i n s u p e r c r i t i c a l p e n t a n e . Second, the high s o l u b i l i t y i n t o l u e n e i s o b t a i n e d at a moderate p r e s s u r e , 30 atm, w e l l below the c r i t i c a l p r e s s u r e of t o l u e n e . This pressure c o r r e s p o n d s t o the SLG e q u i l i b r i u m p r e s s u r e at 3 2 5 ° C . T h i r d , the SLG p r e s s u r e of 30 atm r e p r e s e n t s t h e minimum p r e s s u r e f o r high TPP s o l u b i l i t i e s i n t o l u e n e at t h i s t e m p e r a t u r e — i . e . , the minimum p r e s s u r e f o r the e x i s t e n c e of the l i q u i d p h a s e . At lower p r e s s u r e s , o n l y the gas and s o l i d phases w i l l f o r m , c o r r e s p o n d i n g t o essentially complete s e p a r a t i o n of t o l u e n e and the p o r p h y r i n . In summary, the comparison i n T a b l e VI i n d i c a t e s t h a t s u p e r c r i t i c a l t o l u e n e would be t h e p r e f e r r e d s o l v e n t f o r m e s o - t e t r a p h e n y l p o r p h y r i η based upon the h i g h TPP s o l u b i l i t i e s , moderate o p e r a t i n g p r e s s u r e s , and the ease w i t h which the p o r p h y r i n can be r e c o v e r e d from s o l u t i o n by r e d u c i n g t h e p r e s s u r e below t h e SLG e q u i l i b r i u m p r e s s u r e . Acknowledgments F i n a n c i a l support 8000276) i s g r a t e f u l l y

from the N a t i o n a l acknowledged.

Science

F o u n d a t i o n (CPE

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August 27, 1986

Squires and Paulaitis; Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1987.