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Membrane Air Separation University of Illinois

Membrane Air Separation

In this lab two membranes are used to separate oxygen from nitrogen in air. The two separators are used in series and in parallel to see which set-up will

result in the best separation.

1Unit Operations ChE 382 Group 5 Spring 2011 4/19/2011Damo, Duffy, Guerrero, Hsu, Kosak, Qamar, Tyska


Lab Prep Report

Unit Operations II Lab 6

April 19th, 2011

Group 5

Andrew Duffy

Daniyal Qamar

Jeff Tyska

Bernard Hsu

Ryan Kosak

Tomi Damo

Alex Guerrero



Contents

1. Introduction.........................................................................................................................................4

2. Literature Review/Theory....................................................................................................................5

3. Experimental.....................................................................................................................................10

3.1. Apparatus...................................................................................................................................10

3.2 Materials and Supplies.....................................................................................................................13

3.3 Experimental Procedure.............................................................................................................13

4. Anticipated Results............................................................................................................................15

5. References.........................................................................................................................................16

6. Appendix I: Job Safety Analysis........................................................................................................17



1. Introduction

The chemical industry relies heavily on the ability to separate streams into their constitute

components. Distillation, desalination, and membrane separation are among a few of the

methods used. This lab will demonstrate how membrane air separation can be used to achieve the

separation of nitrogen and oxygen from an air feed stream. This method of gas separation has an

industrial advantage of being moderately efficient yet inexpensive.

In this system an air stream will be feed through two permeable polymetric membranes.

Because oxygen and nitrogen each have different relative rates of diffusion through the

membranes, oxygen will selectively be transported through them at a different rate than nitrogen.

The membrane separators (Prism Separator Modules) consist of non-porous hollow fibers which

form a semi-permeable barrier for which the gas molecules must pass through to reach the

product stream. Oxygen is more permeable than nitrogen through the membranes thus oxygen

molecules will pass through at a faster rate creating a product stream rich in oxygen.

Four basic steps are involved in the movement of gas molecules through the membranes:

adsorption on to the membrane surface, dissolution through the membrane, elution from

membrane, and finally desorption from the membrane. The characteristic permeation rate of

transfer of the two gases is a function of the molecules ability to dissolve and diffuse through the

membranes. Fick’s Law of Diffusion and Henry’s Law will both be used to relate the diffusive

flux with diffusivity and concentration with solubility and pressure respectively.

Although oxygen and nitrogen pass through the membranes at different rates, both types of

molecules are permeable. This means that high levels of separation cannot be achieved using this



method. To determine the most efficient separation method the system will be run with the two

membranes arranged in both series and parallel, as well as differing pressures will be tested.

Separation efficiency can then be determined by taking the ration of the permeation of the two

gases.

2. Literature Review/Theory

Microporous membranes essentially perform as sieves and allow small molecules to

move through the pores while large molecules are blocked (Towler and Sinnot, 562). Membrane

separation works on the basis of relative permeation rates of the components being separated.

Each component transporting through the membrane has a permeation rate that is unique and is a

function of the ability of the component to dissolve and diffuse through the membrane. The two

main relationships for this separation are Fick’s Law (1) which relates diffusion and Henry’s

Law (3) which relates solubility. The gas being separated in this experiment is dry air. This fluid

is assumed to be an ideal binary perfect mixture of oxygen and nitrogen. The permeate in this

experiment is oxygen whereas nitrogen is the retentate. The first relation to show the diffusive

flux through the membrane is expressed by Fick’s Law in equation (1) below.

J i=Di

L∗(C¿1−C¿ 2) (1)

Where:

Ji = Flux of component i in mol/m2s

Di = Diffusivity of component i in m2/s

L = Thickness of membrane in meters



Cin1 = Concentration of component i inside membrane wall on feed side in mol/m3

Cin2 = Concentration of component i outside the membrane wall on permeate side in mol/m3

The total flux of component i, Ji, can then be calculated.

J i=Qip ρnA

(2)

Where:

Qip = Volumetric flow rate of species i in the permeate in m3/s

ρ = Density of permeate in mol/m3

A = Area of membrane, (in this case 2.7m2 per module)

n = Number of modules used, (in this case 2)

Using equation (1) and (2) the only unknown turns out to be the diffusivity of component i, Di,

which can be solved for when the two equations are equated through the calculated flux of

component i, Ji .

From Henry’s Law.

Cℑ=Si p i (3)

Where:

Cim = Concentration of component i inside the membrane wall in mol/m3

Si = Solubility constant for component i in the membrane in mol/m3Pa

pi = Partial pressure of component i in the gas phase in Pa ( Pascal)



Permeation through the membrane is a function of diffusivity and solubility therefore:

Pi=Di S i (4)

Where:

Pi = Component i membrane permeability in mol/m3sPa

Di = Diffusivity of component i in m2/s

Si = Solubility constant for component i in the membrane in mol/m3Pa

Figure. 1. General values on the permeability of species through different membranes

(Wankat, 545)

The graph above shows permeability’s for oxygen and nitrogen through four types of membranes

calculated at standard pressure and temperature. Using equation (3) and (4) along with the

collected data allows the calculation of the solubility constant for component i in the membrane,

Si.



Next the separation efficiency can be determined based off of different permeation rates

of the gas components.

α ij=Pi

P j(5)

Where:

αij = Separation factor

Pi = Component i membrane permeability in mol/m3sPa

An experimental separation factor is frequently used to quantify the separation of a binary

system using mole fractions.

α ij' =(

x ip

x jp

xif

x jf) (6)

α ij' '=(

x ip

x jp

x if

x jf) (7)

Where:

α’ij = Separation factor based on the non-permeate composition

α’’ij = Separation factor based on feed composition

Next the recovery of oxygen and nitrogen can be determined.8

Unit Operations ChE 382 Group 5 Spring 2011 4/19/2011Damo, Duffy, Guerrero, Hsu, Kosak, Qamar, Tyska


O2 Recovery=Qp CO2 p

Qf CO2 f(8)

N 2 Recovery=Qr CN 2 r

Qf CN 2 f(9)

Where:

Qp = Volumetric flow rate of permeate in m3/s

Qf = Volumetric flow rate of feed in m3/s

CO2f = Molar concentration of oxygen in feed in mol/m3

CO2p = Molar concentration of oxygen in permeate in mol/m3

CN2f = Molar concentration of nitrogen in feed in mol/m3

CN2r = Molar concentration of oxygen in permeate in mol/m3

Finally the stage cut can be determined which is the fractional amount of the total feed entering

the membrane that passes through as the permeate.

STAGECUT=Q p

Qp+Qf(10)

Where:

Qp = Volumetric flow rate of permeate in m3/s

Qf = Volumetric flow rate of feed in m3/s



3. Experimental

3.1. Apparatus

Apparatus:

The apparatus could be set up such that the membrane separators would operate in series or in

parallel. The apparatus is shown on the next page.


1



2

3

5

6 7 84

9

1

1

1

1

1

15 16


Part No.

Apparatus Use Make/Model Specification Error

1 Air Inlet to Apparatus

- - - -

2 Inlet Manometer Measures the pressure of the inlet air

ARO Max psig: 150Max Temp: 200 F

± 1 psi

3 Power Switch Turns apparatus On/Off - - -4 Membrane Air

separators Separates Oxygen from air Permea – Prism Alpha

Model # PPA-22A0Max psig: 300Max Temp: 150 F

-

5 Oxygen outlet Outlet to the separated oxygen - - -6 Pressure Reader Reads the pressure into the

apparatus, theoretically same pressure as 2

OMEGA - ±1 psi

7 Pressure Transducer

Turns pressure into an electrical signal to be read

- - -

8 Flow Meters Reads the air flow across each of the membranes

TOP TRAK Sierra Instruments Model# 822-13-0VI

Max Psig: 150Units: SLPMRange: 0-10

±.01

9 Oxygen Sensor Measures the concentration of oxygen in outlet airTop: Measures oxygen rich streamBottom: Measures Nitrogen rich stream

Teledyne analytical instruments S/N# 719940

Class I-17 sensor

10 Oxygen conc. reader

Displays the concentration measured by 9

GB300 Rev 2 Range: 0 - 100 % O2

Response time: 90% of final value in less than 10 s

±.01%

11 Valve A Control series/parallel flow through apparatusParallel: LeftSeries: Right

- - -

12 Valve B Control series/parallel flow through apparatusParallel: Down Series: Up

- - -

13 Valve C Control series/parallel flow through apparatusParallel: DownSeries: Up

- - -

14 Valve D Control series/parallel flow through apparatusParallel: UpSeries: Up

- - -

15 Manometer Air Tank

Measures air flow from air tank

US Gauge - ± 1 psi

16 Air tank Holds compressed air - - -



3.2 Materials and Supplies

Materials/Supplies Description SpecificationsCompressed Air Compressed air to see the

effects of membrane separation

-

3.3 Experimental Procedure

Start Up Procedure:

1. Turn the power switch on the power strip (3) to the ON position to power up the flow meters

(8), digital pressure reader (6), and oxygen analyzers (9).

2. Allow the oxygen analyzers several minutes to reach a steady reading. Also, the flow meters

may need resetting if there has been a ground fault.

3. Open the valve (near 15) on the compressed air cylinder. Using the cylinder regulator, set

outlet pressure to 140 psig.

4. Set the outlet of the second pressure regulator (near 2) to 100 psig. Do not let it exceed this

pressure.

5. At this point the oxygen analyzers should read a percentage by volume of oxygen near 20.9%

on the sensor display (10). If they do not, gently remove them and slowly wave them in the

air until the sensor displays 20.9%.

Parallel Configuration:

6. For a parallel configuration, position the valves as follows: turn valve A (11) to the left,

valves B (12) and C (13) down, and valve D (14) up.

7. Allow system to reach steady state and collect data on both oxygen analyzers.



8. Allow a few minutes between experimental runs for the system to reach a new steady state.

9. Repeat this procedure with various flow rates of air and various pressures (between 80-120

psig). Allow the system to reach steady state before resuming data collection on both oxygen

analyzers.

Series Configuration:

10. For a series configuration, position the valves as follows: turn valve A (11) to the right, and

valves B (12), C (13), and D (14) up.

Shut Down Procedure:

11. Close the valve on the compressed air tank completely.

12. Turn the power switch on the power strip (3) to the OFF position to turn off the flow meters

(8), digital pressure reader (6), and oxygen analyzers (9).

13. Close the valve for the outlet of the second pressure regulator.

14. Clean up the surrounding experiment area.

4. Anticipated Results

The procedure for running this experiment and changing the independent variables is very

simple. The inlet pressures from the air-gas cylinder are varied and the separation method for

the air can be run either in parallel or in series (separation cylinders). Because the purpose of

this experiment is to determine the mass transfer coefficients, the numbers are difficult to

anticipate without some sort of error. It can be anticipated however, that the separation of the 14

Unit Operations ChE 382 Group 5 Spring 2011 4/19/2011Damo, Duffy, Guerrero, Hsu, Kosak, Qamar, Tyska


oxygen from nitrogen will be greater for the series configuration than for the parallel

configuration. This is because the air will be passing through two different membranes, one after

the other, rather than in parallel, the feed stream will be split and each stream will pass through

the membranes only once, thus leading to lesser of separation as compared to the series

configuration.

While the configuration is one variable to be considered, pressure is a second variable that is

to be considered as well. For higher inlet pressures, the amount of oxygen in the permeate

stream will be greater and the amount of oxygen in the nonpermeate stream will be lower.

Because the driving force for this process is pressure, when pressure increases, more nitrogen

diffuses into the membrane.

The results that can be anticipated from this laboratory are based off of the two independent

variables for the experiment. Placing the membranes in a series configuration will lead to a

greater separation of oxygen from nitrogen due to the fact that a single stream will pass through

two membranes, while in parallel, the feed stream is split and two streams pass through only one

membrane. The second independent variable is that of inlet pressure, where when inlet pressure

is increased, the absorption of nitrogen and the concentration of oxygen in the permeate both

increase. In order to achieve the highest separation of oxygen from nitrogen as possible, a series

configuration with the highest allowable inlet pressure will be used. In order to achieve the

lowest separation of oxygen from this laboratory, a parallel configuration with low inlet pressure

will be used.



5. References

Bird, R. Byron, Warren E. Stewart, and Edwin N. Lightfoot. Transport Phenomena. New York: J. Wiley, 2007. Print.

Sinnott, Ray, and Gavin Towler. Chemical Engineering Design. Amsterdam: Elsevier, 2009. Print.

W.E. McCabe, J.C. Smith, and P. Harriott 2001. Unit Operations of Chemical Engineering, McGraw Hill, New York.



6. Appendix I: Job Safety Analysis

What is the purpose of this experiment?

The purpose of this lab is to separate ambient air into two streams concentrated in its two

major components; nitrogen and oxygen. This separation will be achieved using a semi-

permeable membrane in both a series and parallel setup. Data collected during this experiment

will be used to calculate the mass transfer coefficient used in Fick’s law. This value can be used

to determine how effective this apparatus is separating the two components.

What are the hazards associated with the experiment?

High pressure air will be used in this experiment along with sensitive electrical equipment.

Concentrated oxygen streams will be created which may be dangerous to operators due to pure

oxygen’s combustibility.

How will the experiment be conducted in a safe manner?

Eye protection will be worn at all times during this lab for protection from this apparatus and

the neighboring ones. The pressurized air cylinder will be safely secured to the wall and all

valves must be in proper working order. The apparatus must be inspected for any leaks or

damaged piping, fittings, or valves. The main cylinder valve will be shut off when the

experiment is not running. All electronic components will be checked for damage or exposed

(un-insulated) wiring.



What safety controls are in place?

The majority of this apparatus is cover with Plexiglas shielding to protect the operators. The

pressurized air cylinder is equipped with a blowout valve in case the pressure inside the take

becomes too great. Also, vents are built into the apparatus to exhaust excess pressure.

Describe safe and unsafe ranges of operations.

This system is rated to operate at a maximum pressure of 140 psig. Pressures above this

can damage the membrane. Gas flow rates should be set to around 10−6 square meters/second.

Higher flow rates can also damage the membrane used in this system.

I have read relevant background material for the Unit Operations Laboratory entitled:

Membrane Air Separation and understand the hazards associated with conducting this

experiment. I have planned out my experimental work in accordance to standards and acceptable

safety practices and will conduct all of my experimental work in a careful and safe manner. I

will also be aware of my surroundings, my group members, and other lab students, and will look

out for their safety as well.



Electronic Signatures:

Bernard Hsu

Daniya l Qamar

Jeff Tyska

Alex Guerrero

Tomi Damo

Ryan Kosak

Andrew Duffy


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