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Ch 9Gas Separation by Membranes
Membrane
Flow sheet of a membrane separation
Separation of solvent and solute in SFE-processes
Retentate
FeedPermeate
Problems: High pressure ( > 100 bar) Solution of Carbon Dioxide in Polymers Influence on glass transition point
Gas Circuit
CO2
OC
Permeate
Retentate
Membrane Process
GKSS-membrane (organic, active dense layer)
1.86 wt.-%
< 0.06 wt.-%
p = 2.0 MPa
active dense layer
1.5 mole CO2
kg/(m2 h)
P = 18 MPa, T = 323 K
Separation by membranes
Reference Membrane System T, P
Wagner (1986) RO, (Polyamid) Kaffein 473 K, 30 MPa
Semenova et al. (1992, ´93, ´94)
Kapton® (Polyimide)
Ethanol, Petroleum compounds
423 K, 15 MPa
Sarrade et al. (1996, ´97)
Composite with Nafion®
PEG, Triglycerides 333 K, 31 MPa
Nakamura et al. (1994)
Ceramic , NTGS-2100 (Silikon)
PEG 400-600 313 K, 20 MPa
Literature overview
Flat sheet membranes
ROMACO, high pressure RO, (Polyamide, Pall Rochem)
PAN-Fluorinated Polymer (FP), GP, (GKSS)
PEI-FP, GP, (GKSS)
PVDF-FP, GP, (GKSS)
6-FDA-4MPD/DABA 4:1 (Polyimide, crosslinked with ethylene glycol, University of Heidelberg)
Al2O3-TiO2, (Inocermic)
Membranes
N
O
O
N
O
O
COOH
CF3CF3
4 1
CF3CF3
N
O
O
N
O
O CH3
CH3
CH3
CH3
6FDA-4MPD/DABA 4:1
Membranes
Tubular membranes
Carbone membrane, ( 20 nm, Le Carbone-Lorraine)
ZrO2- TiO2, (Schuchmacher)
Al2O3-TiO2 -FP, (US-Filter, GKSS)
Membranes
porous membrane
nonporous membrane
carrier membrane
phase 1 phase 1phase 1phase 2 phase 2 phase 2
Mechanisms of membrane transport
Membranes
Table Fehler! Kein Text mit angegebener Formatvorlage im Dokument..1: Pore
diameter and thickness of inorganic gas separation membranes, after van Veen et al.
(1996).
Membrane systemPore diameter+ (thickness) Membrane system
Pore diameter+ (thickness)
Mesoporous alumina 4 nm(3 m)
Microporous carbon molecularsieve; hollow fibre
appr. 0.5 nm(6 m)
Mesoporous glass 4.5 nm(300 m)
Silicate on ceramic appr. 0.45 nm (5m)
Mesoporous carbon several nm(12–18 m)
Dense SiO2 by CVD modificationof microporous silica on alumina
dense(5 m)
Inorganic Membranes
Membranes
Pore diameter and thickness of inorganic gas separation membranes, after van Veen et al. (1996).
Table Fehler! Kein Text mit angegebener Formatvorlage im Dokument..1:Classification of ceramic membranes (Bonekamp, 1996).
Membrane Process Structure Average pore diameter Separation layerMicrofiltration 1 layer 5 m macroporous
2 layers 0.25 m macroporous3 layers 0.1 m macroporous
Ultrafiltration 4 layers 5 nm mesoporous
Gas separation,Nanofiltration,Pervaporation
5 layers 1 nm microporous
Membranes
Classification of ceramic membranes (Bonekamp, 1996).
Polymeric, Nonporous Membranes
amorphous semicrystalline crystalline
States of polymers
Membranes
v
T
vg
vc
vw
A
B
C
W
TmTg
freevolume
glassystate
rubberystate
Specific volume and free volume as a function of temperature for an amorphous polymer: A: specific volume of a liquid; B: specific volume of a glassy polymer; C: specific volume of a crystal solid; W: van der Waals volume; Tg: glass transition temperature; Tm: melting temperature.
Effect of Temperature on the Polymeric Structure
Effect of Temperature on the Polymeric Structure
Effect of Pressure on the Polymeric Structure
Swelling and Plasticization of Polymers
Aging of Polymers
Membranes
Influences on Membrane Properties
Gas Permeation through Membranes
l
ppP
l
ffP
A
nJ ee 2121
Steady state flux J :
Pe : effective permeability coefficient, (integral value over the whole membrane).
1
2
121
f
fe dfPffP
iii pf pfp
0
limwith
dpp
zp
i
0
1lnFugacity coefficient :
z: compressibility factor.
tApp
lVP
21
cmHgscm
cmSTPcm2
3
The permeability coefficient for ideal–gas conditions
Pressure - normalized flux Q, "membrane permeability":
l
PQ
Separation factor for a binary mixture of component A and B:
BA
BABA xx
yy/
Gas Permeation through Membranes
Gaspermeation:
P = D H: Permeationskoeffizient,D = Diffusionskoeffizient, H = Henry-Koeffizient.
Trennfaktor:
jP
zP P ( ).1 2
1Barrer = 10cm (STP) cm
cm s cmHg-10
3
2 .
ABA
B
P
P0 .
Stofftransport
Einheit von P:
Joule-Thomson effect
Definition of the Joule-Thomson coefficient:h
JT p
T
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 300
2
4
6
8
10
343.15 K
333.15 K
323.15 K
313.15 K308.15 K303.15 K J
T [K
MP
a -1]
pFeed [MPa]
Joule-Thomson coefficient of carbon dioxide
Gas Permeation through Membranes
E
F
B
A
C
D
A: Hagen-Poiseuille's flow,
B: Knudsen flow,
C: surface flow,
D: multilayer adsorption,
E: capillary condensation,
F: molecular sieving.
Gas Permeation through Porous Membranes
Transport mechanisms through porous membranes
Hagen-Poiseuille Flow
TRl
ppr
TRl
ppppr
TRl
pprJ SSmS
16168
22
21
22121
22
The term p/RT has to be replaced by the mean density rm resulting in the following relation for Hagen-Poiseuille's flow of carbon dioxide through mesoporous membranes:
28
2
COm
mS
Ml
prJ
28
2
COm
mS
Ml
rQ
Gas Permeation through Porous Membranes
Surface Diffusion and Capillary Condensation
Surface diffusion is a poorly understood phenomenon The total molar surface flux is calculated by:
dl
dqqDJ ss
with pore length l, porosity e, and density r of the solid. The surface diffusion coefficient Ds is a function of the amount of gas q adsorbed on the surface.
The effective surface coverage qe of gas can be described by adsorption isotherms, for monolayer adsorption: Langnuir isotherm:
pbpbq
q
sate 1
For multilayer adsorption BET isotherm:
re p 1
rrrr pCpp/pC 11
0pppr
Gas Permeation through Porous Membranes
1 2
0.0 0.2 0.4 0.6 0.8 1.0
Relative Mean Pressure pm/p0
0.4
0.6
0.8
1.0
Per
mea
bilit
y Q
x10
5(S
td C
.C.-cm
/cm
2–s
ec c
mH
g)
0.1
p/p0 T [K]
0.2
0.054
0.211
273
273
298
298
Permeability of carbon dioxide through vycor glass, after Rhim and Hwang, (1975). Maximum permeability: point where capillary condensation takes place.
Gas Permeation through Porous Membranes
The capillary condensation pressure (pt) can be predicted by the Kelvin equation:
2
0
cos2ln
r
tr
p
p
M
TR t
where t represents the thickness of the adsorbed layer. For non-cylindrical capillaries the term 2 cosq (r-t)/r2 has to be replaced by another relation.
Gas Permeation through Porous Membranes
Adsorption Isotherms at Sub- and Supercritical Conditions
0 5 10 150,0
0,1
0,2
0,3
0,4
0,5
Mas
s ad
sorb
ed C
O2 [g
g-1
Adsorb
ens]
Pressure [MPa]
Isotherms of the total amount of adsorbed carbon dioxide on two different silica gels. Silica gel with 10 nm mean pore diameter, -- 308.15 K, -- 318,15 K, data replotted from Bamberger (1996) silica gel with 1 nm mean pore diameter, -- 313,15 K, -- 333,15 K, data replotted from Ozawa and Ogino (1972).
Gas Permeation through Porous Membranes
The flux Ji of component i is given by Fick’s law:
dl
dcDJ Mi
ii,
2,1, MiMii
i ccl
DJ 2,1,
,ii
Giii pp
l
SDJ
Temperature dependence of the permeability coefficient P = D S:
TREexpPP p 0
Temperature dependence of diffusivity and solubility:
TREDD d exp0
TRHexpSS s 0
TR
EHexpSDP ds00
Gas Permeation Through Nonporous Membranes
TRH435.0SlogTSlog S
0
Sorption of gases in all types of amorphous polymers shows that the solubility of CO2 increases with decreasing temperature (van Krevelen, 1990):
Transport mechanism of penetrants through polymers differs below and above the glass transition of polymers.
The diffusion of penetrants through glassy polymers is a highly non-linear function depending on the state of the polymer.
Gas Permeation Through Nonporous Membranes
cPermeate
cMembrane
cBulkFeed
Boundary Layer
J cPermeate
J c
D dc/dx
x
Membrane
cPermeate
cMembrane= cGel
cBulkFeed
Boundary Layer
x
Membrane
Gel Layer
Concentration polarization at steady state conditions;
left: normal concentration polarization; right: gel-layer formation
Concentration Polarization
400 450 500 550 600 650 700 750 8000
2
4
6
8
10
12
14
16 T = 313 .15 K T = 333 .15 K T = 343 .15 K
Con
cent
ratio
n E
thyl
este
r [w
-%
]
C O
2 [kg m -2 ]
Solubility of fatty acid ethyl esters in carbon dioxide (Riha, 1996).
Concentration Polarization
A liquid layer forms on the retentate surface of the membrane when the solubility ís reached
1 phase
2 phases
Change in concentration when CO2 is removed by a membrane process
Membrane Test Cell
Membrane
Flat Sheet Test Cell
PI
P I
P I
TI
TI
TIC TIC
FI
FI
V1
S2
S1
P1
V2
V1
C 1P2
Experimental Set Up For Testing Flat Sheet Membranes
P l u gF e e d
R e t e n t a t e
M e m b r a n e M o u n t i n g
P e r m e a t e
M e m b r a n eO - R i n g
N u tC o o l i n g -H e a t i n gF l u i d
Tubular membrane test cell.
Experimenatal Membrane Test
TI
TI
P I
P I
P I
F I
TI
F I
d P
P I
P I
CO2
P I
P I
TCA
P C
2 80 b ar
1 20 b ar
1 20 b ar
K üh lwa sser
E V
TS H
LS
TE
TIC
2 50 b ar
2 50 b ar
2 50 b ar
N2
P I
V1
V2
V3
2 50 b ar
Experimental Set-Up
Pure Gas Permeation
0 2 4 6 8 10 12 140
2
4
6
8Q
CO
2 x 1
02 [km
ol m
-2 h
-1 M
Pa-1
]
Upstream Pressure [MPa]
TEOS, pressure-normalized CO2 flux vs. upstream pressure –– 34 °C, –– 49 °C,–– 66 °C –– 0.1 MPa 25 °C manufacturer, p = 0.3–1.9 MPa, 20-mm TC, membrane #2, values taken after 30 minutes.
Membranes
Inorganic Membranes: Titania-Alumina Composite Membranes
TiO2 (0,9 nm)
TiO2 (5 nm)
Al2O3 (macroporous)
approx. 1 mm
Schematic representation of titania-alumina membrane cross-sections
TEOSSubstructure: commercially available -Al2O3 membrane of 18 mm in diameter, mean pore diameter of 5 nm, porosity 50%, surface roughness appr. 0.2 m. Substructure modified by tetraethylorthosilicate (TEOS) treatment.
Polymeric Membranes
PEI-Teflon Membranes
N
O
O
ON
O
O
O
CH3
CH3
n
Chemical structure of polyetherimide (PEI), trademark Ultem by GE.
An intermediate ultrafiltration layer of polyetherimide (PEI) is applied to a polyester support fleece. The PEI-layer is then coated with a selective layer of poly (tetrafluoroethylene) (PTFE).
O O
F F
F3C CF3
n
F
F F
Fm
PDDn TFEm
Repeat unit of poly (2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole) [PDD] / PTFE, commercially available under the trademark (AF 2400 Du Pont)
1 Barrer = 10-10 (cm3 cm)/(cm Hg s cm2)
Gas Teflon® PTFE AF2400
CO2 2800 12O2 990 4.2H2 4100 #He 2700 #H2 2200 9.8N2 350 1.4CH4 340 #C2H4 350 #C2H6 180 #
CF2 CF2
CF3 CF3
O O
F F
TFE
PDD
Structure of Monomers of the AF2400-polymer
Gas Permeabilities of PTFE-Polymers
0 5 10 15 20
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
QC
O2 [k
mol
m-2 h
-1 M
Pa
-1]
Upstream Pressure [MPa]
TEOS membranes: pressure-normalized CO2 flux vs. upstream pressure, –– #1 increasing pressures, –– #1 decreasing pressures, –– #2 increasing pressures, p = 0.3–1 MPa; T = 50 °C, 20-mm TC.
Pure Gas Permeation
0 2 4 6 8 10 12 14 16 18 200.00.2
0.4
0.6
0.8
1.0
QC
O2 [k
mol
m-2 h
-1 M
Pa-1
]
Upstream Pressure [MPa]
PEI-TE10x pressure-normalized CO2 flux as a function of upstream and permeate pressure for increasing upstream pressures (solid symbols), and decreasing upstream pressure (open symbols): –– 1 MPa transmembrane pressure difference,–– downstream pressure at atmospheric pressure, T = 50 °C, 47-mm TC.
Pure Gas Permeation: Organic Membranes
0 1 2 3 4 5 60
1
2
3
4
5
J CO
2 [km
ol m
-2 h
-1]
p [MPa]
CO2 flux at different upstream pressures as a function of transmembrane pressure difference for PEI-TE1x, –– 7 MPa, –– 9 MPa, –– 12 MPa, –– 14 MPa, –– 15.9 MPa, –– 18.1 MPa, T = 50 °C, 47-mm TC.
Pure Gas Permeation: Organic Membranes
0 5 10 15 202,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
6,5
7,0
7,5 Neue Membran 2. Benutzung 3. Benutzung
Q C
O2
[km
ol m
-2h-1
MP
a-1]
Feeddruck [MPa]
Influence of Repeated Use
PAN-AF2400-Membran, 50 °C
0 5 10 15 200
1
2
3
4
5
6R
atio
QP
EI-
TE
1x / Q
PE
I-T
E10x [
-]
Upstream Pressure [MPa]
CO2-Permeate Flow of PEI-AF2400-1x Related to PEI-AF2400-10x
–– increasing retentate Pressure (Feed side),
–– decreasing retentate pressure (Feed side), T = 50 °C, 47-mm test cell.
0 5 10 15 200
50
100
150
200
PTFE, 50 °C, Muth, (1998)
PMMA, 50 °C, Muth, (1998)
PMMA, 58 °C, Liau, McHugh,(1985)
50°C 45°C
35°C
Teflon AF 2400, 25°CS
olub
ility
[cm
3 CO
2 (S
TP
) cm
-3
poly
mer ]
Pressure [MPa]
Solubility of CO2 in Teflon AF 2400
0 5 10 15 200
1
2
3
4
5
6
De, C
O2 x
105
[m2 h
-1]
Upstream Pressure [MPa]
Diffusion Coefficient of CO2
7-m Teflon AF 2400 layer on a PEI-AF2400-10x Membran, 50 °C, p = 1 MPa; 50 °C, p2 = atmospheric pressure; 35 °C, p2 = atmospheric pressure (after Merkel et al. (1999).
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6
3000
4500
600010500
12000
13500
15000
16500
18000
PExt = 23 MPaTExt = 60 °C
40 °C 70 °C 50 °C 80 °C 60 °C 85 °C
Per
mea
tflu
ss v
on C
O2
[mol
m-2 h
-1]
Druckdifferenz P [MPa]
CO2 Permeate Flow
2-HHU-1TEOS - membrane
Sartorelli 2001
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,20
5
10
15
20
25
30
35
40
40° C 70° C 50° C 80° C 60° C 85° C
Pe
rme
atf
luss
vo
n T
CA
[m
ol m
-2 h
-1]
Druckdifferenz P [MPa]
TEOS-Membrane: Feed: P = 23 MPa; T = 60° C
Sartorelli 2001
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0 b
PExt = 23 MPaTExt = 60 °C
40 °C 50 °C 60 °C 70 °C 80 °C 85 °C
Tre
nnfa
ktor
[-]
Druckdifferenz P [MPa]
TEOS-Membrane
Separation Factors
2-HHU-1
Sartorelli 2001
0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4900
1200
1500
1800
2100
2400
2700
3000
3300
3600 P
Ext T
Ext T
m
18 50 50 18 50 60 18 50 70 23 60 50 23 60 60 23 60 70
P
erm
eatflu
ss v
on C
O2 [
mol
m-2 h
-1]
DruckdifferenzP [MPa]
Permeate flows of CO2 at 50 °C, 60 °C and 70 °C,Exp. series: 2-FP-X10 (18 MPa/50 °C) and 3-FP-X10 (23 MPa/60 °C).
Permeate Flow
AF 2400 - Membrane
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2
1400
1600
1800
2000
2200
2400
2600
2800
3000
3200
3400
3600
316 K 323 K 333 K 343 K 353 K
PExt = 23 MPaTExt = 333 K
CO
2 P
erm
eate
Flu
x [
mol
m-2 h
-1]
Transmembrane Pressure P [MPa]
CO2 Permeate Flow: AF 2400 membrane
Sartorelli 2001
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8 316 K 323 K 333 K 343 K 353 K
PExt
= 23 MPaT
Ext = 333 K
Sep
arat
ion
Fac
tor
[-]
Transmembrane Pressure P [MPa]
LVCLVC
LVCLVCx100/x
y100/y
Separation Factor: AF 2400 membrane
Sartorelli 2001
0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,090,000
0,005
0,010
0,015
0,020
0,025
0,030
PExt = 23 MPaTExt = 60 °C
Y -
[J s
c-1]
X - [c Jv c-1]
TM = 43/44 °C TM = 50 °C TM = 60 °C T
M = 70 °C
TM = 80 °C
Determination of Transport Coefficients
Sartorelli 2001
Nanofiltration Membrane, AF 2400 coated
0 1 2 3 4 5 60
1
2
3
4
5
6
7 MPa 9 MPa 12 MPa 14 MPa 16 MPa 18 MPa
J C
O 2 [k
mol3
m-2
h-1]
p [MPa]
Pressure Difference, PEI-FP, 323
0 5 10 15 200
2
4
6
8 PAN-FP PVDF-FP PEI-FP
Q C
O2
[km
ol m
-2h-1
MP
a-1]
Feed Pressure [MPa]
Membrane Comparison, 323 K
0 2 4 6 8 10 12 14 16 18 200
1
2
3
4
5 Increasing Pressure 2.5 Decreasing Pressure 2.5 Increasing Pressure 10 Decreasing Pressure 10
Q C
O 2 [k
mo
l m-2
h-1M
Pa-1
]
Feed Pressure [MPa]
T = 323 K
P = 2 MPa
Hysteresis in the PEI-FP 2,5 and 10 Membranes
Membrane
Type
Q CO2
[kmol m-2 h-1 MPa-1]
? p
[MPa]
cFeed
[%]
LVC
[-]
FDA 1.25 0.85 1.94 0.9
ROMACO 12 0.65 0.83 1.02
PEI-FP 2.5 1.45 2.2 1.2 0.27
PEI-FP 10 0.34 2.2 1.94 > 0.1
Mixture Results Overview
0 1000 2000 3000 40000,00
0,02
0,04
0,06
0,08
0,10
[-
]
Operating Time [min]
T = 323 K
Pf = 18 MPa
P = 2 MPa
PEI-FP 10 Membrane
Scale Up: Plate and Frame Construction
Scale Up: Plate and Frame Construction
Scale Up: Plate and Frame Construction
PC
PC
RV1
RV2
WT1 WT2
B2
B1
P1WT3
1
18 MPa323 K
6 MPa323 K
273 K
Supercritical Fluid Extraction
PC
WT2
1
PC
P1
B1
RV3
PC
WT1 K1
RV1
RV2
M1
18 MPa323 K
P = 2 MPa
Coupling With a Membrane Separation Unit
53 kJ/ kgCO2
21 kJ/ kgCO2
7.6 kJ/ kgCO2
1
2
3
Wie in 2Like in 2
Energy For Different Solvent Cycles
Pump-Cycle
Compressor-Cycle
Membrane-Cycle
Sartorelli 2001