Supercritical Fluids - American Chemical Society

Chapter 11

Solubilities of Five Solid n-Alkanes in Supercritical Ethane

Downloaded by UNIV MASSACHUSETTS AMHERST on August 21, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch011

Iraj Moradinia and Amyn S. Teja School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100

The effect of solid structure on the solubilities of n-alkanes in supercritical ethane has been investigated at a temperature just above the critical point of ethane. Solubilities of nalkanes containing 28 to 33 carbon atoms in ethane at 308.15K and pressures up to 20 MPa are reported in this work. The enhancement factor is shown to exhibit a regular trend with the number of carbon atoms in the n-alkane, although different trends are exhibited by the odd and even members of the series. The n-alkanes are an interesting homologous series because they display great regularity in their behavior. Many of their fluid phase properties, for example, can be correlated with the number of carbon atoms in the molecules (J_>_2_) · In order to develop general relations for supercritical extraction, therefore, we have studied the solubilities of solid n-alkanes containing 28 to 33 carbon atoms in supercritical ethane. An additional reason for studying the n-alkane series is that even-numbered n-alkanes exhibit different trends in their solid phase properties (e.g. sublimation pressure, heat of fusion, etc.) than the odd-numbered members of the series (_3^6_)· This is shown for the heat of fusion in figure 1, and is a consequence of the different packing arrangements in the solid phase. It may be possible using supercritical extraction to exploit these differences in order to separate close-boiling members of the series. Any generalization for supercritical extraction behavior must, therefore, take account of these differences in behavior. The solubilities of solid n-Octacosane (n"*C28H58^' n-Nonacosane (n-C29H^Q), n-Triacontane (n*"C3oH62^ ' n " D o t r i a c o n t a n e (n"C32H66^' and n-Tritriacontane (n-C^H^g) in supercritical ethane are reported below. The solubilities of the even-numbered members of the series have been obtained from our previous work (7). 0097-6156/87/0329-0130$06.00/0 © 1987 American Chemical Society

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

MORADINIA AND TEJA

n-Alkane Solubility in Ethane


11.


131

SUPERCRITICAL FLUIDS

132 EXPERIMENTAL

The a p p a r a t u s used i n t h i s study was a s i n g l e - p a s s flow system shown s c h e m a t i c a l l y i n f i g u r e 2. I t i s s i m i l a r i n p r i n c i p l e to t h a t used by K u r n i k e t a l . ( 8 ) . The e x p e r i m e n t a l p r o c e d u r e has been o u t l i n e d i n a p r e v i o u s p u b l i c a t i o n (7_). SOURCE AND PURITY OF THE MATERIALS n

c

H

a n c

n

C

H

1 1 3 ( 1

a

s

t

a

t

e

d

The s o l i d n - a l k a n e s ( n - C ^ H ^ g , " 3 o 6 2 ' * ~ 32 66^ p u r i t y o f 99% o r b e t t e r and n~C29 60 bad a s t a t e d p u r i t y o f 98% o r better. The n - a l k a n e s were o b t a i n e d from W i l e y O r g a n i c s . S o l i d n33 68 ined from F l u k a C h e m i c a l Corp. and had a s t a t e d p u r i t y o f 97% o r b e t t e r . The s o l i d s were used w i t h o u t f u r t h e r purification. Ethane was f u r n i s h e d by the Matheson Gas Co. w i t h 99+% p u r i t y and was a l s o used w i t h o u t f u r t h e r p u r i f i c a t i o n .


H

C

H

w

a

s

0 D t a

RESULTS Our r e s u l t s f o r the b i n a r y C

H

+

n

c

H

2 6 " 30 62» t a b l e 1.

C

H

2 6

+

n

C

systems C Hg + ^"^28^58* a

n

d

C

H

2 6

+

n

T a b l e 1. E x p e r i m e n t a l R e s u l t s 3

y xl0 y xl0 y xl0 C H (1)+ C H (1)+ C H (1)+ n-C H (2) ηδ Η (2) nC H (2) 3

3

2

2

6

2 8

P(MPa) 6.57 10.10 12.02 13.64 16.67 20.20

DATA

2

2

2

5 8

1.89 3.38 6.43 7.53 10.80 15.18

2

6

2 9

6

3 0

6 0

6 2

0.549 1.24 1.45 1.71 2.24 3.20

2.32 4.32 8.29 9.91 14.20

c

H

" 33 68

r

e

S

l

v

n

e

"^29^6Q>

n

*

n

a t 308.15K 3

y xl0 C^H (1)+ 2

6

n C

a

+

^2^6

2

H

" 32 66»

H

32 66

( 2 )

3

y xl0 C H (1)+ nC H (2) 2

2

6

3 3

6 8

.371 .963 1.14 1.36 1.64 2.37

0.216 0.713 0.801 0.959 1.26 1.81

CORRELATION

E x p e r i m e n t a l data were c o r r e l a t e d u s i n g the P a t e l - T e j a (9) e q u a t i o n of state. For a pure solid phase i n equilibrium with a s u p e r c r i t i c a l gas phase, we may w r i t e ( 1 0 ) . s v 0

p

a

P

where I s the s u b l i m a t i o n (vapor) pressure o f the s o l u t e 2, i s the f u g a c i t y c o e f f i c i e n t o f the s o l u t e a t i t s s u b l i m a t i o n pressure, V i s the molar volume o f the pure s o l u t e , y i s the composition (solubility) of the s o l u t e i n the supercritical solvent, φ i s fugacity coefficient of the solute in the s u p e r c r i t i c a l s o l v e n t , Ρ i s the p r e s s u r e and a l l p r o p e r t i e s a r e evaluated a t the system temperature T. Since the sublimation pressure i s u s u a l l y very s m a l l , we may assume φ ~ 1 and the integral to be e v a l u a t e d from zero p r e s s u r e to the p r e s s u r e P. 2

ap

2

2

2

2

ν

Ρ

2



SUPPLY

ETHANE

VALVE

fc

Single-pass

VESSEL

r

TEST

apparatus.

METER

WET

flow

w

VENT

BATH

CELL

EQUILIBRIUM I—

CONSTANT TEMP.

HEAT EXCHANGER

supercritical

ROTAMETER

TEMPERATURE MEASUREMENT

MINIPUMP

SEPARATION

F i g u r e 2.

j — *

1

LIQUEFIER

r

®

BACK PRES. REG.


134


Also, since the s o l i d volume i s approximately constant p r e s s u r e , we can i n t e g r a t e and r e w r i t e eq. (1) as f o l l o w s : P EXP[PV!!/RT]

with

V a p


Q

Thus, i f the s o l i d phase p r o p e r t i e s ( d e n s i t i e s and sublimation pressures) a r e known, then the s o l u b i l i t y o f the s o l u t e i n the supercritical solvent a t any pressure and temperature can be calculated provided an e q u a t i o n o f s t a t e i s a v a i l a b l e f o r the c a l c u l a t i o n of φ^. The r e s u l t s f o r f i v e b i n a r y systems u s i n g P a t e l T e j a e q u a t i o n o f s t a t e shown i n f i g u r e s 3 and 4. The P a t e l - T e j a e q u a t i o n o f s t a t e i s a b l e to c o r r e l a t e the data f o r the b i n a r y systems r e a s o n a b l y well provided a binary interaction coefficient (k-π) i s included i n the c a l c u l a t i o n s . It is interesting to note that the b i n a r y interaction coefficients o b t a i n e d from c o r r e l a t i o n o f data f o r the odd members o f the s e r i e s a r e an o r d e r o f magnitude s m a l l e r than those o b t a i n e d f o r the even members o f the s e r i e s and t h a t they show r e g u l a r b e h a v i o r with carbon number. These d i f f e r e n c e s a r e due to d i f f e r e n c e s i n sublimation pressures and l e a d to the c o n c l u s i o n t h a t d i f f e r e n t c o r r e l a t i o n s w i l l be r e q u i r e d f o r odd and even numbered members o f any homologous s e r i e s . A p l o t o f enhancement f a c t o r (E) a l s o shows t h i s b e h a v i o r w i t h c a r b o n number ( F i g u r e 5 ) . Enhancement f a c t o r s a r e g i v e n i n T a b l e 2. S o l i d d e n s i t i e s and s u b l i m a t i o n p r e s s u r e s used i n the s o l u b i l i t y c a l c u l a t i o n s ( E q . 2) are given i n Table 3. The d e n s i t i e s were supplied by the m a n u f a c t u r e r s whereas the s u b l i m a t i o n p r e s s u r e s were e x t r a p o l a t e d u s i n g data on o t h e r n - a l k a n e s [ 7 ] . Table

n

C

2.

H

"* 28 58

Enhancement F a c t o r s a t Τ = 308.15K

n

C

n

H

" 29 60

C

H

"" 30 62

n

C

n

H

" 32 66

C

H

" 33 68

Ρ(MPa) 11 11 12 1.212x10 12 1.611x10 12 2.824x10 12 4.809x10

6, 57 10, 10 12.02 13.64 16.67 20.2

1.947x10

Table

3.

11 1.216x10 11 3.481x10 11 7.951x10 12 1.077x10 12 1.889x10

11 977x10 12 464x10 12 821x10 12 452xl0 * 13 037x10 13 1.788x10 A

12 6.926x10 13 3.514x10 13 4.699x10 13 6.364x10 14 .025x10 .784x10 14

S o l i d P r o p e r t i e s Required

i n Eqn. (2)

V a ï >

Substance n

n

n

n

n

" " ~ " ~

C

C

C

C

C

28 29 30 32 33

H

H

H

H

H

58 60 60 66 68

Lit./Mol. s ν. 0.4895 0.5058 0.5222 0.5550 0.5714

P

12 3.057x10 13 1.220x10 13 1.719x10 13 2.327x10 13 3.429x10 6.004x10 13

14 χ 10 MPa 308.15K 6.37619 12.53244 0.36151 0.02049 0.07973 i a

2



11.

MORADINIA AND TEJA


135

8 16 PRESSURE.MPa

Figure 3. Experimental and c a l c u l a t e d solubilities of even-numbered n-alkanes in supercritical ethane at 308.IK. · e x p e r i m e n t a l data for n-C H ( k , . = - ·0405); Δe x p e r i m e n t a l data ί θ Γ ( η - 0 Η ) —03067; o-experimental d a t a f o r n - C 3 H ^ ( k ^ = - · 0 3 8 3 ) . C a l c u l a t i o n s u s i n g the P a t e l T e j a e q u a t i o n or s t a t e a r e shown by the s o l i d l i n e s . 2 8

5 8

=

3 0

2

6 2

6

-3.0

10 15 PRESSURE.MPa

Figure 4. Experimental and c a l c u l a t e d solubilities of odd-numbered n-alkanes in supercritical ethane 308.1k. • e x p e r i m e n t a l data f o r n - C H Q ( k j j = - · 0 0 4 9 ) ; ο e x p e r i m e n t a l d a t a f o r n-C^Rt^ ( k ^ = - · 0 0 3 7 ) . C a l c u l a t i o n s u s i n g the P a t e l T e j a e q u a t i o n o f s t a t e a r e shown by the s o l i d l i n e s . 2Q

6




136

Figure 5. Enhancement ethane + n-alkane systems.

factor

( E ) V S . Carbon

number f o r


11.

MORADINIA AND TEJA


137


Literature Cited 1. Ambrose, D., National Physical Laboratory Report Chem. 57, December 1976. 2.

Chase, J. D., Chem. Eng. Prog., 1984, 80, 63-66.

3.

Morawetz, E., J. Chem. Thermo., 1972, 4, 139.

4.

Bondi, Α., J. Chem. Eng. Data, 1963, 8, 371.

5.

Broadhurst, M.G., J. Res. Nat. Bur. Stand. A, 1962, 66, 241.

6.

Muller, Α., Proc. Roy. Soc. Lond. A, 1929, 124, 317.

7.

Moradinia, I. and Teja, A. S. Fluid Phase Equil. in press 1986.

8.

Kurnik, R. T., Holla, S. J., and Reid, R. C., J. Chem. Eng. Data, 1981, 26, 47-51.

9.

Patel, N. C., and Teja, A. S., Chem. Eng. Sci. 1982, 37, 463468.

10. Prausnitz, J. M., "Molecular Thermodynamics of Fluid Phase Equilibria," Prentice Hall, Englewood-Cliffs N.J., 1969. RECEIVED

August

11, 1986


Supercritical Fluids - American Chemical Society

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