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Gas Membrane Separation

Separations & Reaction Engineering Lab

March 21, 2006

Kate CannadyChristopher Miller

Matt Mobily

Jennifer Pratt



Today’s Schedule

• Introduction & Applications

• Design Challenge

• Apparatus & Methods• Theory

• Preliminary Data, Results & Thoughts

on Scale Up

• Conclusions & Future Plans

• References



What is Gas Membrane Separation?

• Separation of a known component into

two product streams (known as the

permeate and reject, or retentate)through a semi-permeable polymeric

membrane – Permeate is oxygen rich (smaller)

– Reject is nitrogen rich (larger)



Industrial Uses?

• H2 Separation – H2/N2 separation in ammonia plants & H2/hydrocarbon

separation in petrochemical applications

• N2/Air Separation

• CO2 & H2O removal from natural gas

• Organic vapor removal from air or N2 streams• Inerting

– Chemical industry products stored in inert atmosphere

– Reduces risk by removing O2

• Blanketing – Uses N2 to ‘cover’ liquid

• Prevents vaporization

• Maintains atmosphere to reduce ignition potential

• Prevents oxidation or contamination by reducingexposure to atmospheric air



Advantages

• Separation units smaller than other types

– Small footprint = good for operations such as offshoregas-processing platforms

• Environmentally friendly (no waste product)

• Wide operating parameters (flexible)

• Requires less energy than other separation processes(no phase change)

• Very reliable

• Lacks mechanical complexity – no supervision required(low operating cost)

Disadvantages

• Membrane fouling - more frequent than other membranes due to is configuration (contaminated feed)

• Expensive - more so than other types available

(fabrication method)

• Lack of research - less research done compared to

other types of membrane

Hollow Fiber



Factors to Consider

• Properties of component

– Permeability

• Relative permeation rates

Slow: N2, Ar, CO

Medium: CO2, O2

Fast: H2O, H2, He – Diffusivity

– Selectivity

• Properties of membrane

– Material

– Estimated lifetime

– Size, shape & thickness

• Operating parameters

– Feed flow rate

– Pressure settings



Industrial Hollow Fiber Membrane

• Typically 300,000 – 500,000 individual fibers – OD ~ 300μm

– ID ~ 150μm

– FYI – diameter of a human hair is ~ 100μm

• Housing usually 6-12” diameter and about 40” long



Design Challenge

• Determine optimal conditions for

separation of an air stream into enriched

O2 & N2 streams using hollow-fiber membrane technology

• Size a membrane gas separator for a

selected application



Apparatus & Methods

• Initial calibration of flow

controlled and flow meter • Oxygen analyzer

calibrated to 21% in

ambient conditions &

probe placed in collection

trap

• Inlet flow set to desiredflow rate

• Pressure valves set to

desired levels

• Once steady state

achieved, oxygenconcentration recorded

for both permeate and

reject streams

gas separation unit

collection

trap

oxygen analyzer and probe

flow

controller

permeate and

reject pressure

controls

flow meters

flow inlet

• Procedure repeated varying flow rate and pressure settings

until desired data collected



Polysulfone - C27H22O4S

• Oxygen Permeability – P A = 1.38

• Nitrogen Permeability – PB = 0.239

• Selectivity – α = P A/PB

– α = 1.38/0.238 = 5.8



Theory

)(V

A

L

A A

A P y P xt

P N

])1()1[(V

A

L

A B

B P y P xt

P N

B A

B B

N N

N y

B A

A A

N N N y

B

A

B

A

N

N

y

y

Flux of A across film:

Flux of B across film:

x A = mole fraction of A on high pressure

side (reject)

y A = mole fraction of A on low pressure

side (permeate)

PL = reject pressure

PV = permeate pressure

P A = permeability of A

PB = permeability of B

t = membrane thickness

A variation of Fick’s…



More Theory…

B

A

P P

)( V

A

L

A A

A P y P xt

P N

])1()1[( V

A

L

A B

B P y P xt

P N

In terms of selectivity:

From before:

and

become )( V A

L A

A

A P y P x

t N

P

])1()1[( V

A

L

A

B B

P y P x

t N P

and



And More Theory…

])1()1[(

)(

V

A

L

A

B

V A

L A

A

P y P x

t N

P y P x

t N

t N

P y P x

P y P x

t N

B

V

A

L

A

V

A

L

A

A )1()1(*

)(

)(

)1()1(V

A

L

A

V

A

L

A

B

A

P y P x

P y P x

N

N

So…

Recall that B

A

B

A

N

N

y

y

)(

)1()1(V

A

L

A

V

A

L

A

B

A

P y P x

P y P x

y

y



Data & Results

• Highest O2 concentration at ΔPmax

• Conditions

– Reject Pressure = 80psi

– Permeate Pressure = 10psi

Flow Rate (mL/s)* O2 Concentration (%)

108 27.4

216 33.4

324 36.5

* Flow rates adjusted based on calibration (originally 100, 200 and 300 mL/s)



Data & Results

Pperm (psi) Prej (psi) % O2 perm O2 perm Flow Rate perm (mL/s) % O2 rej O2 rej Flow Rate rej (mL/s)

50 80 26.6 0.266 44.3 20.4 0.204 270.0

40 " 28.4 0.284 54.0 19.8 0.198 258.1

30 " 31.1 0.311 69.1 18.5 0.185 245.2

20 " 33.7 0.337 83.2 16.7 0.167 229.0

10 " 36.5 0.365 98.3 14.3 0.143 217.1

For calculating selectivity…

PL y A x APV

yB = (1-y A)

)(

)1()1(V

A

L

A

V

A

L

A

B

A

P y P x

P y P x

y

y



Data & Results

Selectivity Results for Collected Data:

α

Pperm (psi) Prej (psi) 108 (mL/s) 216 (mL/s) 324 (mL/s)

50 80 2.67 2.12 3.2440 " 3.71 2.43 3.14

30 " 5.60 2.82 3.67

20 " 8.61 4.06 4.1010 " 19.51 4.83 4.59

50 60 1.30 1.77 1.94

40 " 1.72 1.96 1.90

30 " 2.24 2.31 2.18

20 " 2.84 2.78 2.65

10 " 4.24 3.32 3.15

Recall αideal = 5.80



Conclusions & To Do List

• Conclusions – Highest O2 concentration at largest ΔP and at

higher flow rates

– Overall experimental selectivity is a bit lower

than ideal• Increases with ΔP, but a change in flow rate does

not appear to affect selectivity

• To Do

– Determine conditions for highest separationfactor

– More data analysis

– Scale-Up calculations



References

• Coker, D.T., Prabhakar, R. and Freeman, B. Gas Separation

Using Polymers. Chemical Engineering Education. Winter 2003.

60-67.

• Membranes For Gas Separation. Chemical & Engineering News.October 03, 2005. Volume 83: Number 40. 49-57.

• http://www.cheresources.com/blanketzz.shtml

• http://www.polymerlabs.com/elsd/images/membrane.gif



Brilliant

!

?

Brilliant!

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