Membrane Separation
University of Illinois at Chicago
. (From “http://www.airproducts.no/index.pl?artID=Tech101&grID=3&Overgr=&grNiv=1&hgrID=&grID_niv3=&grNavn=Technology”)
Membrane Air Separation
Lab Prep Report
Unit Operations Lab 2
31 March, 2011
Group 4
Tien Diep
Kevin Estacio
Sebastian Iskra
Zack Labaschin
Kevin Thompson
Felix Velazquez
Unit Operations ChE-382 Group No. 4
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
03/31/2011
Membrane Separation
University of Illinois at Chicago
1. Introduction
Separation is one of the fundamental engineering principles necessary for a chemical
engineer. A separation is the transformation of one product stream into two or more dissimilar
streams. A chemical mixture is sometimes required to be separated through a separation process,
of which there are many. Because of the economical and chemical need to have a purified
concentrated product separation becomes very necessary.
The separation method used in this lab involves a membrane that uses the principle of
absorption. Adsorption involves a liquid or gas that is miscible in solid or liquid respectively.
The gas membrane separation can be used to separate mixtures that have properties which differ
enough for a selectively semi-permeable membrane to filtrate the components. Membrane
separations use a semi-permeable membrane to separate a mixture into two components in which
the two components have a reasonable physical size differential. Of the six different types of
membrane separation gas separation has been selected as the topic of study for this experiment.
Membrane separation is an applicable method of separation that is commonly used in
industry when a component is necessary but not needed in a high purity (McCabe, 855). This
occurs most notably in industry when removing CO2 from natural gas. The membrane commonly
used is a cellulose acetate membrane which is being replaced by polyimide as it has a greater
selectivity, thus advancing the technology of gas membrane separation.
In this lab air will be membrane separated into its nitrogen and oxygen components. The
mass transfer will take place because the membrane is made from a material that is selective
towards oxygen permeability. Because of the selectivity of the membrane this experiment differs
from Ultrafiltration because Ultrafiltration works solely by physical particle size. The apparatus
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
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Membrane Separation
University of Illinois at Chicago
(Section 3.1) has two configurations, series and parallel. In the series configuration the air will
pass through 2 modules in series before exiting the system. In the parallel configuration the feed
air will be split into two halves. Each part of the split will flow through one half of the apparatus.
To clarify in the series configuration the air stream will flow through both modules; while in the
parallel configuration feed 1 will flows through module 1, while feed 2 flows through module 2.
Fick’s Law of diffusion will be used to analyze the experimental data and Henry’s law of
sorption will also be referenced in order to draw conclusions (Wilson, 203).
2. Literature Review/Theory
As mentioned earlier, membrane air separation (or gas permeation) is a separation
process that takes advantage of the difference in rates of mass transfer between the components
of the solution. By using a non-porous semi-permeable membrane, a gaseous solution is
separated into permeate and non-permeate streams. Because this separation process uses a semipermeable membrane, it is explained using the laws of diffusion and solubility: Fick’s Law and
Henry’s Law.
The first physical property affecting membrane air separation is the diffusivity of each
gas, which follows Fick’s laws of diffusion. Fick’s second law of diffusion is expressed as:
𝑑𝑐𝐴
𝑑𝑡
= 𝒟𝐴𝐵 𝛻 2 𝑐𝐴
(1)
(Bird, Stewart, Lightfoot 2007, 585)
cA = concentration of A, moles / volume
t = duration of diffusion, time
𝒟AB = diffusivity of the gas, area / time
This equation can be simplified to:
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
03/31/2011
Membrane Separation
University of Illinois at Chicago
𝐽𝑖 =
𝐷𝑖
(𝐶 − 𝐶𝑖𝑛2 )
𝐿 𝑖𝑛1
(2)
(Lab Manual, 2)
Ji = flux of component i, moles / area time
Di = diffusivity of component i, area / time
L = thickness of membrane, length
Cin1 = concentration of component i inside membrane wall on feed side, moles / volume
Cin2 = concentration of component i inside membrane wall on permeate side, moles / volume
If the diffusivity is unknown, another way to find the total flux would be this equation:
𝐽𝑖 =
𝑄𝑖𝑝 𝜌
𝑛𝐴
(3)
Qip = volumetric flow rate of component i in the permeate, volume / time
ρ = permeate density, moles / volume
n = number of modules
A = membrane area
Solving for the flux in Equation 3 and substituting into Equation 2, the diffusivity can be
calculated for component i.
The second relationship involved in explaining membrane air separation is solubility.
This is explained by Henry’s law, expressed in the Membrane Air Separation Lab Manual as:
𝐶𝑖𝑚 = 𝑆𝑖 𝑝𝑖
(4)
Cim = concentration of component i inside the membrane wall, moles / volume
Si = solubility constant for component i, moles / volume pressure
pi = partial pressure of component i in the gas phase, pressure
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
03/31/2011
Membrane Separation
University of Illinois at Chicago
This equation can be used to solve for the solubility constant for component i. Once the
diffusivity is calculated from Equation 2, it can be used with the calculated solubility constant to
determine the membrane permeability of component i through the following equation:
𝑃𝑖 = 𝐷𝑖 𝑆𝑖
(5)
Pi = membrane permeability of component i, moles / volume time pressure
This process takes place under pressure; as such a pressure gradient for the system is
present. Figure 2.1 shows the pressure gradient of a two component equimolar mixture with one
component having twice the diffusivity of the other component.
Figure 2.1: Pressure Gradients in a Membrane for Gas Separation (McCabe, Smith, Harriott,
839)
As with all systems, it is important to calculate the efficiency and recovery in order to
help determine the economic viability of the process and whether or not it is worth carrying out.
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
03/31/2011
Membrane Separation
University of Illinois at Chicago
The efficiency of the system “is based on the different rates of permeation of the gas
components,” and is expressed by:
𝛼𝑖𝑗 =
𝑃𝑖
𝑃𝑗
(6)
(Lab Manual, 3)
αij = efficiency of the system
Pi = membrane permeability of component j, moles / volume time pressure
Another way to look at the efficiency of the system is done by quantifying the separation of the
components through a separation factor. The separation factors are calculated using mole
fractions of the components, able to be based on either the feed or the non-permeate
compositions. The non-permeate composition separation factor is defined by:
𝑥𝑖𝑝
𝑥𝑗𝑝
′
𝑎𝑖𝑗
= (𝑥 )
𝑖𝑟
𝑥𝑗𝑟
(7)
a'ij = separation factor based on non-permeate composition
xip, xir = mole fraction of component i in permeate, non-permeate respectively
xjp, xjr = mole fraction of component j in permeate, non-permeate respectively
The feed composition separation factor is defined by:
𝑥𝑖𝑝
𝑥𝑗𝑝
′′
𝑎𝑖𝑗
= (𝑥 )
𝑖𝑓
𝑥𝑗𝑓
(8)
a'’ij = separation factor based on feed composition
xif, xjf = mole fraction of component i, j respectively in feed
These experimental separation factors are easier to quantify due to gas sensors reporting
concentrations in mole percent.
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
03/31/2011
Membrane Separation
University of Illinois at Chicago
The recovery of the system can be determined by finding the recovery of each individual
component. The recovery of oxygen gas is defined by:
𝑅𝑂2 =
𝑄𝑝 𝐶𝑂2𝑝
𝑄𝑓 𝐶𝑂2𝑓
(9)
RO2 = recovery of oxygen gas
Qp, Qf = volumetric flow rate of permeate, feed respectively, volume / time
CO2p, CO2f = molar concentration of oxygen gas in permeate, feed respectively, moles / volume
Similarly, the recovery of nitrogen gas is defined by:
𝑅𝑁2 =
𝑄𝑟 𝐶𝑁2𝑟
𝑄𝑓 𝐶𝑁2𝑓
(10)
RN2 = recovery of nitrogen gas
Qr, Qf = volumetric flow rate of non-permeate, feed respectively, volume / time
CN2r = molar concentration of nitrogen gas in non-permeate, moles / volume
CN2f = molar concentration of nitrogen gas in feed, moles / volume
One final parameter that can be used to analyze the separation of the process is the stage cut,
which is determined by:
𝑆𝐶 =
𝑄𝑝
𝑄𝑝
=
𝑄𝑝 + 𝑄𝑟 𝑄𝑓
SC = stage cut
The efficiency, separation factors, recoveries, and stage cut can all be used in order to help
determine the overall efficiency of the system in order to find out whether or not it is functioning
adequately for the purpose of the company utilizing this separation process.
3. Experimental
3.1 Apparatus
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
03/31/2011
Membrane Separation
University of Illinois at Chicago
Figure 3.1 Pictorial image membrane air separation unit
Figure 3.1, shown above, displays the main equipment of the air separations unit module.
Air provided from tank 3.1.1 is released into the system and controlled via valves 3.1.2 – 3.1.4.
The pressure reader 3.1.7 must be monitored for appropriate pressure settings in the system,
listed in the procedure in section 5. The gas will travel through the stainless steel piping (3.1.15)
and into the separators (3.1.5). The gas flow meter and percentage oxygen meter (3.1.8 and
3.1.9) will allow the operator to analyze the success of the separators.
Unit Operations ChE-382 Group No. 4
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
03/31/2011
Membrane Separation
University of Illinois at Chicago
Figure 3.2 Schematic Representation Series Configuration
Figure 3.3: Parallel Configuration of apparatus
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
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Membrane Separation
No. Name
Air Tank
University of Illinois at Chicago
Purpose
Holds compressed air
Manufacturer
N/A
Measures pressure outside tank
AGA
Specifications
Compressed
gas, Nonflammable gas
2
Max 400 PSI
Measures pressure inside tank
AGA
Max 4000 PSI
Measures pressure of incoming
gas into system
Permeates gas through
membrane to separate oxygen
Converts pressure into analog
electric signal
Reads pressure of air through
pipes
Measures air flow through
system
ARO
Max 160 PSI
Permea
300 PSIG,
150°F
Range 13-26.7
bar
N/A
1
2
3
4
Pressure
gauge/valve
Pressure
gauge/valve
Pressure
gauge/valve
Separator
5
Pressure transducer
6
7
Digital pressure
reader
Air flow meter
N/A
ΩΕ
Sierra
InstrumentsTopTrak
8
Oxygen sensor
Measures amount of oxygen
present in gas
Turn valve
Controls direction of flowing
gas inside piping
Controls direction of flowing
gas inside piping
Controls flow to equipment No.
5 and 7
Controls flow to equipment No.
8
Controls flow to equipment No.
8
Moves gas through the system
9
10
Valve
11
12
Valve 1
Valve 3
13
Valve 2
14
15
Stainless steel
piping
Valve 4
16
17
Digital percent
oxygen reader
Controls flow to equipment No.
5 and 7
Output percent oxygen in gas
Teledyne
Analytical
Instruments
Whitey
Gas Air, Range
0-10, Units
SLPM, Max
Press 150 PSIG
Class I-17
N/A
Swagelok
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
%
Table 3.1 Apparatus Components
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
03/31/2011
Membrane Separation
University of Illinois at Chicago
3.2 Materials and Supplies
The only material needed for this lab is the supplied, closed system, air. The air will be run in the
closed system.
Materials
Description
Manufacturer
Specification
Compressed Air
Feed to membrane air
separation apparatus
--
--
Table 3.2 Materials needed for module
3.3 Procedure
Start Up
1) Power up the digital pressure reader (7), flow meters (8), and O2 sensors (17) by flipping
the power switch on the power strip into the ON position.
2) Open the valve (3) on the high-pressure air cylinder, and use the cylinder regulator to set
the outlet pressure to 140psig.
3) Calibrate the O2 sensors (17) by removing the transducers from their holders, and waving
them slowly in the air until the sensor display reads near 20.9%.
Parallel Configuration
4) Adjust pressure regulator on the apparatus to a pressure of 80 psig.
5) Turn valve A (11) to the left, valves B (14) and C (16) down, and valve D (13) up. This
is the correct positioning to run the system in a parallel configuration.
6) Adjust the flow rates of the air by turning the valve (10) located near the bottom flow
meter. Once the desired flow rates have been set, allow the system to reach steady state
by waiting a few minutes before logging data.
7) Repeat steps 4-6 increasing pressures in steps of 10 psig until 130 psig is reached.
Series Configuration
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
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Membrane Separation
University of Illinois at Chicago
8) Repeat the procedure from the parallel configuration, replacing step 5 with the following:
turn valve A (11) to the right, valves B (14), C (16), and D (13) up.
Shut Down
9) Close the valve on the high-pressure air cylinder.
10) Turn off the digital pressure sensor (7), flow meters (8), and O2 sensors (17) by flipping
the power switch on the power strip to the OFF position.
4. Anticipated Results
The purpose of the membrane separation lab is to separate a gaseous mixture with
differing particle size by having the smaller component diffuse through a semi-permeable
membrane and the mass transfer coefficients will be found. The mass separation is based upon
the properties of Fick’s law. The gas will be pressurized in order to drive it through the
membrane separator and the membrane material is selective about which component will be able
to permeate its boundaries. Oxygen is the permeate as it permeates through the membrane and
Nitrogen is the retentate as it is withheld by the membrane. Separations of this kind are a
function of solubility and diffusivity. If the mixture is extremely soluble in each other then
because of the attractive forces it may not be possible to separate even by membrane separation.
The solubility and diffusivity of nitrogen and oxygen are dissimilar enough to allow for an
appropriate separation. As long as the pressure is kept under control a moderate separation
should occur. Pressures too low will not be able to force the mixture through the semi permeable
membrane and be separated (Wankat, 546). Pressures that are too high are not only dangerous to
the membrane, but the excessive flux of feed will clog the membrane, thus not allowing for a
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
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Membrane Separation
University of Illinois at Chicago
proper separation. The clogging of the membrane with excess is important because the permeate
will not be able to have a proper contact area for and optimal diffusion to take place.
This will be tested by running the apparatus in series and parallel. When the apparatus is
run in series a single air stream will run through a two membrane modules to be separated. In the
parallel configuration of the experiment the feed air stream will be split and each split will run
through one of the 2 membrane modules to be separated. Pressure is the driving force of the
experiment and must be kept under control in order for separation conditions to be optimal. It is
anticipated that the series configuration at a controlled pressure will achieve the best separation.
Because the series configuration will have the air stream run through two filters before leaving
the system (See Figures 3.2 and 3.3) the separation is expected to be better for the series
configuration over the parallel configuration.
5. References
Air Products AS. Web. 05 Apr. 2011. <http://www.airproducts.no/index.pl?artID=Tech-101>.
"Membrane Air Separation." University of Illinois at Chicago - UIC. Web. 15 Mar. 2010.
<http://www.uic.edu/depts/chme/UnitOps/che382-2005f-frame.html>.
Bird, R. Byron, Warren E. Stewart, and Edwin N. Lightfoot. Transport Phenomena. New York:
J. Wiley, 2002. Print
McCabe, Warren L., Julian C. Smith, and Peter Harriott. Unit Operations of Chemical
Engineering. New York: McGraw-Hill, 1993. Print.
Wankat, Phillip C. Separation Process Engineering. Upper Saddle River, NJ: Prentice Hall,
2007. Print.
Wilson, Ian D., E. R. Adlard, Michael Cooke, and C. F. Poole. Encyclopedia of Separation
Science. San Diego: Academic, 2000. Print.
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
03/31/2011
Membrane Separation
University of Illinois at Chicago
6. Appendix I: Job Safety Analysis
What is the purpose of this experiment?
The purpose of this experiment is to learn about gas separation by using a semipermeable membrane. Air will be separated into components of oxygen and nitrogen gases with
the use of a selectively permeable membrane. From the experimental values observed the mass
transfer coefficient of Fick’s Law will be calculated in order to determine the effectiveness of the
experiment. Two differing apparatus setups will be used in the lab, one with the air flowing
through the system in series and the other with the feed being split into two in parallel streams.
What are the hazards associated with the experiment?
1. High pressure build-up throughout the system.
2. Electrical malfunction/failure.
3. Concentrated oxygen leakages, causing harm to the operator as well as being
combustible.
How will the experiment be conducted in a safe manner?
1. Wear safety goggles
2. Check that electrical components are properly secure and grounded.
3. Check tank cylinder supports
4. Check the air tank and components for leaks
5. Close airflow when experiment is completed
What safety controls are in place?
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
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Membrane Separation
University of Illinois at Chicago
1. There is a blowout valve on the tank in case it becomes too hot and releases pressure
from the tank.
2. There exists vents throughout the system near the valves which set the configuration
{(11), (12), (13), (14), Section3}of the apparatus, this allows excess pressure to be vented
from the system.
Describe safe and unsafe ranges of operation
1. The maximum safe pressure of the system is 140 psig, otherwise the pressure in the
membrane could cause it to rupture.
2. The flow rate in the system should be in the range of 10-6 m2/s since the module has a
membrane of area of 2.7 m2.
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.
Signatures:
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Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
Spring 2011
03/31/2011
Membrane Separation
Unit Operations ChE-382 Group No. 4
p. 16
Diep, Estacio, Iskra, Labaschin, Thompson, Velazquez
University of Illinois at Chicago
Spring 2011
03/31/2011