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I need to write the following parts for the lab report. Read the manual given and write thetheory pages (1 and 1/2 pages)results and discussion (3 pages)conclusion and recommendations ( 1/2 page)
Hollow Fiber Membrane Gas Separation for Ground-Based Inerting
The objective of this experiment is to design a hollow fiber membrane gas separation system
for the ground-based inerting of aircraft.
On July 17, 1996 TWA Flight 800 exploded after take-off from New York’s JFK Airport and
plunged into the Atlantic Ocean just south of Long Island. The Paris-bound 747 carried 230
passengers and crew members that died as a result of the accident. While the explosion was
initially blamed on a missile fired by a U.S. Navy ship, a National Transportation Safety Board
(NTSB) investigation  concluded the explosion probably resulted from a spark that ignited
vapors in the center wing fuel tank (CWT) – a living room sized metal box.
This accident, along with numerous others blamed on fuel tank explosions , spurred the
Federal Aviation Administration (FAA) to investigate technologies that could reduce or
potentially eliminate fuel flammability concerns . One such technology is ground-based
inerting (GBI). GBI involves displacing the vapor headspace (initially containing 21% oxygen
after refueling) above the liquid fuel with nitrogen enriched air (NEA, containing 1-5%
oxygen). As illustrated in Figure 1, NEA is forced into the vapor headspace to displace the air
initially present there. This headspace, referred to as the ullage, depends on the fuel tank size
and loading. Additionally, the ullage will change during flight as fuel is removed for
combustion in the engines.
Fuel to Engines
Figure 1. Displacement of air in the ullage during ground based inerting.
One process under consideration for producing the NEA uses hollow fiber membrane gas
separation technology. In the process, compressed air is fed to a hollow fiber gas separation
module. The module consists of a bundle of hollow fibers enclosed in a case. Each fiber is
approximately 200-400 microns in outer diameter and 100-300 microns inner diameter. The
wall of the fiber is more permeable to oxygen than nitrogen so as air flows through the fiber
lumen oxygen is removed and an enriched nitrogen stream is left behind. The case possesses
two external ports on either end of the fiber bundle, and two ports along the periphery of the
case. The latter two ports allow the unit to be run in counter-current or co-current mode,
depending on which valves are opened. A module with a single port along the periphery is
illustrated in Figure 2.
Figure 2. Hollow fiber membrane gas separation module in operation.
Commercially available modules can produce NEA at various flow rates and purities. Modules
are characterized by determining the product flow rate as a function of nitrogen purity.
Additionally, the fraction of the feed air that is recovered as NEA product is determined. The
ratio of product to feed flow rate, called the recovery, determines what compressor size is
required to provide compressed air to the module.
C. WORK STATEMENT
You have been asked to evaluate the number of modules, feed air requirements, and
compression requirements to inert the combined fuel tank ullage of a generic aircraft. Fuel tank
volumes for various generic aircraft are given in Table 1.
Table1. Generic aircraft fuel tank sizes.
Aircraft Type CWT Volume (Gal.) CWT + Wing (Gal.) CWT + Wing + Aux (Gal.)
For each plane type, the center wing fuel tank volume is provided as well as additional fuel
tank volumes in the left and right wings (Wing) as well as any auxiliary fuel tanks (Aux). Past
work  has shown that one can calculate the oxygen concentration in the ullage as a function
of time after initiating the NEA purge from the following equation,
y A − yU (t )
= 1 − exp −
y A − yNEA
where y is the oxygen mole fraction, the subscript A indicates the value for ambient conditions
(i.e., 0.21), the subscript U indicates the value in the ullage, the subscript NEA indicates the
NEA stream, Q is the NEA volumetric flow rate, V is the ullage volume, and t is elapsed time
since starting the NEA purge. The mole fraction ratio on the left of Equation (1) is referred to
as the “inerting ratio” while the ratio in the exp function on the right is referred to as the
“volumetric tank exchange” (VTE). The VTE indicates how many times the ullage volume has
been displaced by NEA.
You are to calculate the NEA composition and flow rate to inert the combined fuel tank ullage
of a generic aircraft using NEA. The tank is considered inerted when the oxygen concentration
drops below 8%. You may assume aircraft operate with fuel loadings of 60-90%. Additionally,
the time available to inert the tank is limited by aircraft turnaround times – the time between
when an aircraft arrives at the gate and the time it leaves for its next destination. Inerting must
be completed on or before the allotted turnaround time has elapsed. Typical turnaround times
are given in Table 2.
Table 2. Typical turnaround times for generic aircraft.
Aircraft Type Turnaround Time (min)
Additionally, you are to determine the number of hollow fiber gas separation modules required
to provide the NEA and to size a compressor to provide the required NEA flow rate. A sample
module has been provided for your testing to obtain the data required to do this.
Polymeric membranes in the form of sheets or fine hollow fibers can be used to separate gas
and liquid mixtures due to differences in permeation rates through the material. This makes
membrane process very useful for a wide range of separations. In this experiment, oxygen
permeates through the material faster than nitrogen, due to higher solubility and diffusivity in
the polymer, so one can produce a nitrogen-enriched and an oxygen-enriched stream with a
The separator in this experiment can be run in two different modes: counter-current flow and
co-current flow. These two modes are pictured in Figure 3.
a) Counter-current flow
b) Co-Current Flow
Figure 3. Counter-current and Co-current Flows
When deriving mathematical models of a gas separations unit, concentration gradients normal
to the surface of the membrane are considered negligible. This is justifiable because, unlike
liquids, gases form little or no boundary layer along a surface when it flows past. Therefore,
the resistance of the boundary layer relative to that of the membrane itself is very small and a
concentration gradient does not occur. On the other hand, there is a significant concentration
gradient as the gas moves from one end of the unit to the other because oxygen is constantly
being removed. How one accounts for this concentration gradients leads to different
mathematical models of module performance.
This model is based on the assumption that there is little or no concentration gradient within
the permeated gas axially along the fibers. Therefore, this case simplifies to a simple mass
balance of the oxygen within the unit. Equations for xo and yp can be derived from a component
mass balance on oxygen and an overall balance.
Equation 2 can be used to calculate the fraction of the feed that is recovered as NEA :
Am PO 2 / t ph xo − pl y p
= 1− = 1−
q f yp
where ph is taken to be the same as the inlet pressure and pl is taken as atmospheric pressure.
See Table 3 for a list of the nomenclature used in equation 2.
This model is significantly more difficult than the complete-mixing model because it requires
the solution of two coupled differential equations. Therefore, a Simulation Module created in
MS Excel will be provided for your use.
Table 3. Nomenclature.
Surface area of membrane (cm2)
Pressure (cm Hg)
Permeability of component A in membrane (cm2/s/cm Hg)
Flow rate (cm3/s)
Thickness of membrane (cm)
Mole fraction of component A in feed or reject
Mole fraction of component A in permeate
Separation factor = ( PA )/( PB )
Stage cut = qp/qf
Reject (NEA) flow
This experiment uses a tube fed Permea Model PPA-22AD air separator. Ambient air is fed
at ~80 psig through two Permea air filters in order to ensure no small particles enter the
separator. The necessary specifications for the air separator can be found in table 4.
Table 4. Permea Model PPA-22AD air separator specifications.
Active fiber length
Oxygen permeance = PO 2 t
8.3 x 10-12
Nitrogen permeance = PN 2 t
1.4 x 10-12
kg mol /m2/Pa/s
= ( PO
t )/( PN 2 t )=( PO 2 )/( PN 2
Number of Fibers
1 GPU (gas permeating unit) = 3.36 x 10-13 kg mol/(m2 Pa s) = 10-6 cm3 (STP)/(cm2 s cmHg)
F. EXPERIMENTAL PROCEDURE
This experiment is controlled using National Instruments LabVIEW software. If the control
program is not running, contact the laboratory instructor or TA.
You are to design an experiment with two objectives in mind: 1) You must determine which
model best predicts the performance of the Permea PPA-22AD model in both counter-current
and co-current flow. This will require a comparison of experimental measurements of as a
function of NEA concentration to predictions from equation (2) and the provided program. 2)
Design a membrane gas separation system for inerting a 747 aircraft (corresponding to a large
aircraft in Table 2). You are to specify the required NEA composition, NEA flow rate, feed
air flow rate, number of modules, and compressor size (assume compression to the same
pressure as used in the lab).
G. LABORATORY REPORT
Your laboratory report should be prepared using the guidelines provided by the instructor.
Additionally, you should address the following points:
1. Derive Equations (1) and (2).
2. Which operating mode (i.e., counter-current or co-current) is preferred? Why?
3. Which model provides the best prediction of for counter-current operation? for co-current
4. Provide the following information for your recommended design: NEA composition, NEA
flow rate, feed air flow rate, number of modules, and compressor size (assume compression
to the same pressure as used in the lab).
5. Would you recommend any changes to the design of the Permea PPA-22AD for use in
6. More than one design is possible. What are the trade-offs for different alternatives?
7. What safety and/or environmental concerns might you have for the use of a membrane
process for GBI?
H. LITERATURE CITED
1. “Aircraft Accident Report – In-Flight Breakup Over the Atlantic Ocean Trans World
Airlines Flight 800,” NTSB/AAR-00/03, National Transportation Safety Board, August
23, 2000, http://www.fire.tc.faa.gov/pdf/systems/AAR0003.pdf.
2. “A Benefit Analysis for Nitrogen Inerting of Aircraft Fuel Tanks Against Ground Fire
Explosion” DOT/FAA/AR-99/73, Federal Aviation Administration, December 1999,
4. “Inerting of a Vented Aircraft Fuel Tank Test Article With Nitrogen-Enriched Air,”
5. Geankoplis, Christie J. Transport Processes and Unit Operations. Englewood Cliffs, New
Jersey: PTR Prentice Hall, 1993.
6. Lemanski, J. and G.G. Lipscomb. “Effect of Shell-Side Flows on the Performance of
Hollow-fiber Gas Separation Modules.” Journals of Membrane Science 195 (2002): 215228.
7. Kaushik, Nath. “Membrane Separation Processes.” PHI Learning, 2008.
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