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.