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GRAPHENE MEMBRANE FOR GAS SEPARATION
Group 3: Krista Melish, Phillip Keller,
James Kancewick, Micheal Jones
Gas Separation in Industry
Hydrogen separation○ From Nitrogen in ammonia plants○ From hydrocarbons in petrochemical applications
CO2 and water removal from natural gasNitrogen separation from airHydrogen Recovery From Tail Gases Air & natural gas drying Vapor removal Hydrocarbon Separations Helium recovery from natural gas
Pharmaceuticals
Food processing, packaging, and storing
Membranes for Gas Separation
Less waste produced
Less harm on environment
Lower industrial cost
Lower energy consumption
Limitations of Common Membranes Energy intensive Expensive Lack efficiency and
productivity Break easily The material plugs too
easily and becomes resistant to flow
Properties of a Good Membrane
High flux rate (permeability) High selectivity Ideal pore size High surface area Low manufacturing cost Small thickness Mechanically Stable
Flux Rate of Different Gases
Affected by:
• molecule size
• gas concentration
• pressure difference across the membrane
•the affinity of the gas for the membrane material
Mechanisms for Gas Separation in Membranes
Relationships Among Membranes
Fick’s Law
The Flux rate (J) is inversely proportional to membrane thickness (x)
Selectivity vs. Permeability
of Membranes
Graphene
Single layer of carbon atoms Densely packed Hexagonal pattern Sp2 bonded Crystal lattice One atom thick 2-D structure
Properties of Graphene
Tear-resistant Thermal conductor Very Thin Very stiff, but also flexible Mechanically Strong
Stronger than a diamond Electronically conducting
100 times faster than the silicon in computer chips Ductile
Graphene Becomes a Membrane
Graphene is impermeable to all gases due to the electron density of its Aromatic rings
In order to create a membrane, must create pores synthetically
http://www.physics.upenn.edu/~drndic/group/research.html
Two Methods for Creating Nanopores Bottom-up synthesis
chemical building blocks of functionalized phenyl rings "grow" into a 2-D structure on a silver substrate○ pore diameters of a single atom○ pore-to-pore spacing of less than a
nanometer
TEMPuncture
holes by removing carbon rings by electric beam
The unsaturated carbons are passivated by nitrogen○ Control pore
size
Graphene Membrane Thinnest possible membrane (1 atom thick)
Over 20,000 x thinner than other membranes Ideal pore size for separation
Improvement of 500x compared to other membranes
Large surface area(Up to areas of 1 mm ^2)
Resistant to oxidation (for temperature less than 450 celsius)
Very mechanically stable
POROUS GRAPHENE AS THE ULTIMATE MEMBRANE
FOR GAS SEPARATION
Research Article by the Chemical Sciences, Materials Science, and Technology Divisions of Oak ridge National Laboratory
(De-en Jiang, Valentino R. Cooper, and Sheng Dai)
Article taken from:
Nano Letters 2009
Volume 9
No. 12
Pages 4019-4024
All pictures not cited on slide are from this article and belong to the authors
Article Overview
Inspiration for Research No prior research on graphene as a separation membrane
○ Massive possible efficiency gains in the gas separation field
Goals Use first principles models to mathematically prove the viability
of graphene as the ultimate membrane for gas separation Encourage future research and experimentation
MethodDensity Functional Theory
Simulation Results Further Research and Experimentation Ideas
Research Inspiration Graphene first isolated in 2004
Although there has been a boom of graphene research lately, no efforts have been put into analyzing its usefulness as a gas separation membrane.
Gas separation is very energy intensive currently Huge opportunities to increase efficiency
Application to other fields Proton Exchange Membranes for fuel cells Carbon sequestration from flue gases Gas sensors in instrumentation
Research Goals Show Viability of graphene as a gas separation
membraneMathematical modeling from first principles
Inspire future research and experimentationNew nano-pore designsNew nano-pore construction methodsInnovative applications to new fields
Research Method Density Functional Theory based modeling using
Plane wave base○ 300 and 680 eV kinetic energy cutoffs
Periodic boundary conditions
Initial Static Calculations2 methods usedPerdew, Burke, and Erzenhoff functional form of the
generalized gradient approximation (PBE)Rutgers-Chalmers van der Waals density function for
exchange and correlation (vdW-DF)○ Good at evaluating strength of dispersion interactions
between neutral non polar molecules
Model: Nitrogen Functionalized
Hexagonal cell made of graphene15 H2 or CH4 molecules
placed inside the cell for dispersion calculations
One face of the cell contains the nano-poreNano-pore created by
removing two cells (a), leaving 8 dangling carbons
Functionalized with 4 hydrogens and 4 nitrogens (b)
Model: Nitrogen Functionalized
Resulting pore electron density isosurface (red) leaves a rectangular pore3.0 Angstroms by 3.5
AngstromsNitrogen slightly attractiveHydrogen slightly repulsive0.05 eV Barrier to H2 versus
0.33-0.41 eV for CH4
Nitrogen yellow, Hydrogen blue
Model: Nitrogen Functionalized Graph shows the
interaction energy between H2 and the nitrogen functionalized pore as a function of adsorption height and orientation of the molecule within the pore.
Red line and squares are calculated using vdW-DF method.
Black line and dots are calculated using PBE method.
Relatively flat curve shows little repulsion as molecule approaches the pore.
Model: Nitrogen Functionalized
Graph shows the interaction energy between CH4 and the nitrogen functionalized pore as a function of adsorption height and orientation of the molecule within the pore.
Red line and squares are calculated using vdW-DF method.
Black line and dots are calculated using PBE method.
Curvature shows the repulsion of the molecule as it approaches the pore
Model: Nitrogen Functionalized
Results Selectivity for H2 / CH4 with the
nitrogen functionalized pore is
108 (Arrhenius)
Selectivity is high compared to
traditional polymer membranes
and silica membranes with
selectivities ranging from 10-103
Graphene is also much more
resilient than other membrane
materials that are more
susceptible to Hydrogen
damage
Difficulties Such functionality will be
hard to specify during manufacture ○ i.e. The placement of the
Nitrogens and Hydrogens will be random around the edge of the pore
Much easier to functionalize the poor using only Hydrogen
Next calculations are for a Hydrogen only functionalized pore
Model: Hydrogen Functionalized
(a) Face of Hexagonal cell with nano-pore functionalized with only Hydrogen (blue)
Created by removing 2 neighboring rings from the graphene sheet like before.
(b) Pore-electron density isosurface showing effective pore size
Dimensions are now 2.5 Angstroms by 3.5 Angstroms
Now it will be harder for both species to pass through
Model: Hydrogen Functionalized
Graph shows the interaction energy between H2 and the hydrogen functionalized pore as a function of adsorption height and orientation of the molecule within the pore.
Red line and squares are calculated using vdW-DF method.
Black line and dots are calculated using PBE method.
Relatively flat curve shows little repulsion as molecule approaches the pore just like before.
Model: Hydrogen Functionalized Graph shows the
interaction energy between CH4 and the hydrogen functionalized pore as a function of adsorption height and orientation of the molecule within the pore.
Red line and squares are calculated using vdW-DF method.
Black line and dots are calculated using PBE method.
Curvature shows the repulsion of the molecule as it approaches the pore with values significantly higher than before.
Model: Hydrogen Functionalized
ResultsSelectivity for Hydrogen over methane raised to 1023
New barriers were 0.22 eV for H2 and 1.6 eV for CH4 which translates to a pass through frequency of 109 atoms of H2 per second at room temperature.
Conducted further research to judge the effect of inevitable errors in future manufacture such as removing three neighboring rings versus just 2 resulting in a width of 3.8 Angstroms.○ Found that this small error resulted in the pore becoming
useless (neither species impeded)○ Demands absolute precision in manufacture
Conclusions Although these are just mathematical models, they show the
viability of graphene as a new generation super membrane material.
This research applies universally to the separation of gaseous molecules based on size.
If findings can be reproduced in real life, this will seriously advance many industries including “green technologies” like fuel cells and carbon capture projects.
Next efforts should be focused into two main areas: 1. Further modeling to test new pores for more systems of gases 2. Experimentation to physically construct the pores being modeled.
Further Research As mentioned previously, further modeling and manufacture
processes need to be investigated.
Interesting Systems to model would be exhaust gases of common combustion engines, air separation, ethylene/ethane, and any other difficult distillation systems
Future manufacturing techniques using electron beams to punch holes into graphene need experiments focused on reducing the diameter of the beam to widths capable of targeting groups of 2-3 carbon atoms.
New functionalizing groups for liquids Desalination of sea water Wastewater treatment: community and industrial Biological screening
○ This would require functional group modeling that accounts for both mechanical and electrical interactions. Require many different equations for modeling, which will increase the time and computing
power needed.
Literature Cited
Article taken from:Nano Letters 2009
Volume 9
No. 12
Pages 4019-4024
All pictures not cited on slide are from this article and belong to the authors.
References
Graphene Source Sciencedirect Graphene Website