oxygen separation with polymeric membranes
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Oxygen separation with polymeric membranes. Dr. Ari Seppälä Department of Energy Engineering Applied Thermodynamics. Structure of lecture. About membranes Basics Different type of membranes Membrane structures Mass transfer phenomena inside membranes - PowerPoint PPT PresentationTRANSCRIPT
Oxygen separation with polymeric membranes
Dr. Ari SeppäläDepartment of Energy Engineering
Applied Thermodynamics
Structure of lecture• About membranes
– Basics– Different type of membranes– Membrane structures– Mass transfer phenomena inside membranes
• Oxygen separation with polymeric membranes - Why separate oxygen and nitrogen? - Modeling boundary layers and gas separation in hollow fibre modules
• Based on fundamental differential balances of mass and momentum• Based on mass transfer coefficients
• Energy efficiency of oxygen separation - Effect of selectivity and pressure ratio - Pressurized vs vacuum mode - Comparision of different oxygen production methods
Maxwell’s demon (by James Clerk Maxwell)
p,T= constant ->
STH)A(G)B(GWm
Wm = work needed for separation of miscible
components of a mixture (excluding the expansion work)
membrane process
separation based on
Phases main applications global market, million $
Micro-filtration particle size L-L - disposable cartridge filters- water purification- municipal water treatment
2600 (year 2008)
Ultra-filtration particle size L-L - water purification- separation of oil and water- food and beverage processing
2300 (2008)
Nano-filtration particle size / differences in solubility and
diffusivity
L-L - water purification 90 (2006)
Dialysis particle size L-L - artificial kidneys 1600 (2008)
Electro-dialysis ions separated by electrical potential
difference
L-L - desalination of water- salt recovery
190 (2008)
membrane process
separation based on
Phases main applications
global market, million $
reverse osmosis .
differences in solubility and
diffusivity
L-L - desalination of water- drinking water purification
2500 (2008)
pervaporation differences in solubility and
diffusivity
L-L - dehydration of solvents- water purification- separation of organic mixtures
400 (2004)
gas separation differences in solubility and
diffusivity
G-GG-L
- natural gas purification - hydrogen and air separation
850 (2008)
membrane contactor
(numerous separation principles)
L-LL-G
- gas separation - oxygen transfer ∙ blood oxygenators ∙ dissolved O2
from water
Transport coefficients for gas separation with membranes
Permeability
where D= diffusivity, S= solubility
( ) ( )1
( ) ( )
volume flow rate membrane thicknessBarrer
membrane area pressure difference
310
2
( )10
cm STP cm
s cm cmHg
At a temperature of 20 oC 1 Barrer corresponds to 8.16 ·10-13 m2/s.
Note that the use of Barrer results in volumetric flux and m2/s in molar flux.
Selectivity
j
i
j
i
j
iij S
S
D
D
b
b
iii SDb
Polymeric membrane structures
dense, non-porous layerporous, porous, with a porous dense, symmetric asymmetric substructure homogeneous, non-porous
Mass transfer through membranes
convective +diffusive flow Knudsen diffusion molecular sieving and surface diffusion
macroscale microscale (gases) nanoscale (gases) phenomenon
Mass transfer through membranes Solution-diffusion (SD) model
dissolved free volume motion of the free a diffusion jump has penetrant element volume elements been performed
Solution-diffusion (SD) model and boundary layers
δ
Fm,iC
Pic
Pm,ic
Pm,i
Fm,i
i
CCDj
Solubility Si m,iim,i cSC
Pm,i
Fm,i
i
Pm,i
Fm,i
i
Pm,i
Fm,i
ii
pp
RT
b
pp
RT
DS
ccDSj
superscripts:F=feed, P=permeate (product), subscripts: m=membrane, s=shell, l= lumen
δδδδδδδδδδ
Fm,ic
Pm,iC
δ
Fic
ki,s ki,mki,l
Gas separation with membranes which transport is based on solution-diffusion model
• Partial pressure (pi) difference is the driving force
• Total pressure pF at feed side (F) must exceed the total pressure PP at the permeate side (P):
• Separation is based on different permeabilities of the species, i.e. selectivity must differ from α= 1.
PFFP
PPi
FFi
ii
ii
ii
ppyy
0pypyRT
bp
RT
bJ
pyp
Applications of oxygen-enriched air and nitrogen
Oxygen-enriched air Nitrogen
· combustion enhancement · controlled atmospheres
· enhancement of fuel cell processes - inerting of fuels and flammable
substances
· medical applications - protection of perishables
· underwater breathing - prevention of oxidation
· chemical industry, refineries · laboratory use
· fermentation and digestion processes · inflating tyres
· production of peroxides
· wastewater treatment
· welding
· glass production
Solving velocity and concentration boundary layers in fluids– fundamental method (model II)
0i i ij ut
r r
2 10
3 V
uu u u p u
t
rr r r r
0ut
r
2 0u
u u u pt
rr r r
0u r
0i i l i ic D c c ut
r
Gas phaseLiquid phase
Continuity of mass
Momentum equation
Weakly compressible Navier-StokesIncompressible Navier-Stokes
Continuity of species i
u=velocity of mixture,p=pressure, ω=mass faction,μ=viscosity,ρ=density, t=time, j=diffusion flux
+ SD-model +boundary and intial conditions
Solving boundary layers – correlation based approach (model I)
shell side permeate
lumen side feed
For detailed description and equations, see:
Meriläinen, Seppälä, Kauranen, Applied Energy 94 (2012) 285-294
residue
• axial flow direction: convection/advection>>diffusion -> diffusion can be neglected
• radial flow direction: serial resistance model accouning for transport through membrane and lumen and shell side boundary layers
Nagasep-module applied in experiments
manufacturer Nagayanagi
modelNagasep M60-
AS
membrane material silicone rubber
housing material polycarbonate
number of fibers Nf 3000
fiber inner diameter din 200 µm
fiber outer diameter dout 320 µm
shell inner diameter dm 2.5 cm
wall thickness δ 60 µm
active fiber length L 14.0 cm
membrane area Aeff 0.34 m2
specific membrane area Aeff / V 4900 m2 /
m3
packing factor f 0.49
-
Permeabilities of Nagasep module (at 26 oC)
gas symbol permeability
[Barrer]
nitrogen N2 250
carbon monoxide CO 300
helium He 310
oxygen O2 500
nitric oxide NO 530
argon Ar 530
hydrogen H2 570
methane CH4 830
carbon dioxide CO2 1530
xenon Xe 2280
nitrogen dioxide NO2 13300
carbon disulfide CS2 79300
Comparison of models and experiments
21
22
23
24
25
26
27
28
29
30
31
32
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
stage cut θ
per
mea
te o
xyg
en m
ole
per
cen
t
3 bar, experimental data
3 bar, simulation results (model I)
3 bar, simulation results (model II)
4 bar, experimental data
4 bar, simulation results (model I)
4 bar, simulation results (model II)
counter-current
Gas-Gas
Comparison of models and experiments
0,0
1,0
2,0
3,0
0 1000 2000 3000 4000 5000
volume flow rate of water [ml/min]
mas
s flo
w ra
te o
f oxy
gen
thro
ugh
mem
bran
e [m
g/m
in]
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9
average velocity of water vav g [m/s]
simulation results
experimental data
0,0
1,0
2,0
3,0
4,0
5,0
0 1000 2000 3000 4000 5000
volume flow rate of water [ml/min]am
ount
of d
isso
lved
oxy
gen
in tr
eate
d w
ater
[p
pm]
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9
average velocity of water vav g [m/s]
simulation results
experimental data
Deoxygenation of water with a vacuum pressure of 0.02 bar on the shell side
Gas-water
Boundary layers based on model II
nitrogen
oxygen
Molar fraction profiles of oxygen and nitrogen across the fibers at z = 0 in counter-current
flow. Stage cut θ = 0.07. pF = 3 bar, pP=1 bar.
Gas-Gas
In this case, boundary layer effect in both gas phases is
minimal ->
oxygen transfer is limited almost completely by
the resistance of the membrane!
Boundary layers based on model II
Oxygen concentration (mol/m3) in the three phases and streamlines illustrating oxygen flux in the shell of the module.
Stage cut θ = 0.70. pF = 3 bar.
Gas-Gas
feed air (3 bar) residue
permeate (1 bar)
Note! Oxygen concentration is smaller in the permeate than in feed because the total pressure of peremate is lower. Permeate is
enriched with oxygen.
water air
v = 2 cm/s 5 cm/s 10 cm/s 20 cm/s 50 cm/s
Gas-water
Boundary layers based on model II
Concentration profiles of dissolved oxygen in water in the fiber lumen with varying feed water velocities. Atmospheric air flows through the shell of the module and gases are transported through the membrane into water. T = 25 ºC.