SUPERCRITICAL FLUIDS
FOR
SEPARATION
NCHU-SF : Ch. 2-11
Objectives
What for
do we need phase equilibrium?
Design of solvent cycle (process)
NCHU-SF : Ch. 2-12
Objectives
Solubility of the compounds
Change of solubility with process conditions
Capacity of the solvent
Solubility of solvent in condensed phase
Distribution of compounds between the
phases (separation factor, selectivity)
Two-phase area
What do we need to know about
phase equilibrium?
NCHU-SF : Ch. 2-13
Objectives
For what type of systems (chemical
compounds) do we need phase
equilibrium?
CO2, C3H8, H2O, ....
(gases)
Paraffins, glycerides, vitamins, ...
Terpenes, sesquiterpenes, mono-, di-, tri....
....
NCHU-SF : Ch. 2-14
Extraction with Supercritical Fluids
Flow scheme of gas extraction (SFE)
The need for phase equilibria:
Solubility, K-factor, Separation factor, Two-phase region
NCHU-SF : Ch. 2-15
Solubility of quartz in water
Kennedy 1950
NCHU-SF : Ch. 2-16
Poynting-effect
Vapor pressure enhancement
E.U. Franck 1956
____ 25 oC, ------ 500 oC
 d P0 
V

  L .
 d P T VG
P02/P01


P 01 VL
ln 02 
P  P 01 ,
P
RT
NCHU-SF : Ch. 2-17
VL
Poynting-Factor
System: Ethanol - Water, 343 K
Ethanol
kPa
10
1000
10000
P
Water
02
P
1.0022
1.022
1.2425
NCHU-SF : Ch. 2-18
01
1.0006
1.0065
1.0667
Density CO2 (calc. Bender EOS)
NCHU-SF : Ch. 2-19
Vapour Pressure vs. Temperature
Sila n , Va p o r C u r v e
1 , E+0 2
1 , E+0 1
Pressure, bar
1 , E+0 0
1 , E- 0 1
1 , E- 0 2
1 , E- 0 3
1 , E- 0 4
1 , E- 0 5
75
125
175
Te m p e r a tur e , K
NCHU-SF : Ch. 2-20
225
275
Influences on Solute Concentration in Gaseous Phase
Density of supoercritical Fluid responsible for
solvent power
(increasing with pressure, decreasing with
temperature)
Hydrostatic pressure enhances effective
vapour pressure
(Poynting effect, small)
Temperature increases vapour pressure
(exponentially, big effect)
NCHU-SF : Ch. 2-21
Solubility of Caffeine in CO2
Gährs 1984
Ebeling, Franck 1984
Johannsen, Brunner 1994
NCHU-SF : Ch. 2-22
Solubility of Naphthalene in Ethylene
Solubility [g/l]
Tsekhanskaja 1964
NCHU-SF : Ch. 2-23
Oleic acid - ethylene
Brunner 1978
NCHU-SF : Ch. 2-24
Solubility of oleic acid in various gases
Brunner 1978
NCHU-SF : Ch. 2-25
Solubility of Triglycerides in sc Gases
Triglycerides of palm oil
Brunner 1978
NCHU-SF : Ch. 2-26
Solubility in sc CO2
NCHU-SF : Ch. 2-27
Solubility of Xanthines in dry sc CO2
Johannsen and
Brunner 1994
NCHU-SF : Ch. 2-28
P,T,x-diagram of a binary system
NCHU-SF : Ch. 2-29
P,x-diagram ethane - heptane
Kay 1938
NCHU-SF : Ch. 2-30
T,x-diagram ethane - heptane
Kay 1938
NCHU-SF : Ch. 2-31
Phase equilibria of a binary system
Melting point temperature of B higher than critical temperature of A
NCHU-SF : Ch. 2-32
Phase equilibrium in a binary system
Melting point temperature of B higher than critical temperature of A;
Interrupted critical curve.
NCHU-SF : Ch. 2-33
P,T- projections of phase boundary lines
NCHU-SF : Ch. 2-34
Phase behaviour of a ternary system
Temperature dependence
Pressure dependence
NCHU-SF : Ch. 2-35
CO2 - benzene - oleic acid
NCHU-SF : Ch. 2-36
Ethylene - DMF - C18 fatty acids
NCHU-SF : Ch. 2-37
Solubility Enhancement by a Modifier
NCHU-SF : Ch. 2-38
Schmitt 1984
Solubility Enhancement by Entrainer (Modifier)
NCHU-SF : Ch. 2-39
Solubility of Caffeine in CO2
Influence of nitrogen
Gährs 1984
NCHU-SF : Ch. 2-40
Solubility of Phenanthrene in CO2
Influence of propane
Prausnitz, Joshi 1984
NCHU-SF : Ch. 2-41
Phase Equilibrium in a Ternary System
Brunner 1983
NCHU-SF : Ch. 2-42
Solubility in a gas with a modifier
Influence of temperature
Brunner 1983
NCHU-SF : Ch. 2-43
Separation Factor
Influence of entrainer
Brunner 1983
NCHU-SF : Ch. 2-44
Separation Factor
Influence of
concentration
Brunner 1983
NCHU-SF : Ch. 2-45
Three Phase Equilibrium
NCHU-SF : Ch. 2-46
P,T-diagrams of Complex Mixtures
NCHU-SF : Ch. 2-47
Measuring Phase Equilibrium
NCHU-SF : Ch. 2-48
Measuring Phase Equilibrium
NCHU-SF : Ch. 2-49
Modeling of Phase Equilibrium with EOS
HPW modification of RK-EOS
NCHU-SF : Ch. 2-50
Thermodynamic Equilibrium
Gas-Liquid Phase Equilibrium
fi  fi
V
L
Two different ways:
A
fi  xi i fi
0
0
f i = standard fugacity
i
= activity coefficient: gE-models)
B
i
f i  xi i P
= fugacity coefficient: Equations of state
NCHU-SF : Ch. 2-51
Calculation of Phase Equilibrium
fi  fi
V
VLE-Calculation:
L
xi i f i  yi i P
0
V
With the simplification:
xi i f i
SAT
 yi i P
V
since
fi
SAT
 i
SAT
Pi
SAT
:
xi i i
NCHU-SF : Ch. 2-52
SAT
P
SAT
 yi i P
V
i
Phase Equilibrium: Non Ideal Solutions
High pressure
Low pressure
pi  Pyi   i xi p .
Pyii   i xi p .
i p
p
Ki 
.
P i
0
i
Ki 
0
i
P
 p
ij 
 p
0
i
o
i i
.
0
i i
0
j j
 ij 
NCHU-SF : Ch. 2-53
pi0 i j
p  j i
0
j
.
Concentration dependence of separation factor

P = 101 kPa
=1

=1
Methanol - water
Chloroform - acetone
NCHU-SF : Ch. 2-54
Concentration dependence of separation factor
NCHU-SF : Ch. 2-55
Phase Equilibrium
Limiting values of the partition coefficient K
f i s T 
Ki  s
1
lim
s
 reines i T Pi T 
xi 1
 i f i 0
H is
K i  v ,  v ,
lim
i P i P
x 0
i
xsf = solvent free concentrations in liquid phase
NCHU-SF : Ch. 2-56
T = const.
VLE
T = const.
VLE
xsf = const.
Phase Equilibrium
Limiting values of separation factor 
Binary system i-j in a multicomponent system
At low pressure
v ,
v ,

 T Pi T   j
j
s





P
T
ij
i
lim
v
 0


f
H jr
x 1
reines i
jr j
s
i
Pi s
 ij 
lim
H jr
xi 1
s
i
lim ij 
xi 0
 vj T , v v , y sf 
Binär:
 f

 j T , vl , x sf  f j0
 0
i
i
v ,
i
P1s
P1s
12 
  s
lim
H sf  2 P2
x1 1
lim 
x1 0
NCHU-SF : Ch. 2-57
12

 1 P1s
P2s
Murphy´s Law of Thermodynamics
THINGS GET WORSE
UNDER PRESSURE
NCHU-SF : Ch. 2-58