(396s) Facilitated Transport Membranes for Natural Gas Separation

Authors: 
Ansaloni, L., Alma Mater Studiorum - Università di Bologna
Zhao, Y., The Ohio State University
Ramasubramanian, K., The Ohio State University
Giacinti Baschetti, M., University of Bologna
Ho, W. S. W., The Ohio State University



Facilitated Transport Membranes for
Natural Gas Separation

 

Luca Ansaloni1,
Yanan Zhao2, Kartik
Ramasubramanian2, Marco Giacinti Baschetti1, W.S. Winston
Ho2,3

 

1Dipartimento di Ingegneria Civile, Chimica, Ambientale e
dei Materiali, Alma Mater Studiorum - Università di
Bologna, Bologna, Italy

2William G. Lowrie
Department of Chemical and Biomolecular Engineering,
The Ohio State University, Columbus, Ohio 43210, United States

3Department of Materials Science
and Engineering, The Ohio State University, Columbus, Ohio 43210, United States

Nowadays, natural gas is an important
source for energy production.  Depending on the original source, raw
natural gas contains various impurities in composition, which must be removed
before delivery to the pipelines.  Removal of acid gases, namely, CO2
and H2S, from natural gas is therefore an important industrial gas
separation process.

For CO2 removal from natural
gas, polymeric membranes compete against conventional separation processes,
such as amine absorption, due to superior advantages of lower costs, lower
energy consumption, and lower waste production.1  Several
attempts have been made to develop CO2/CH4 separation
membranes that match the pipeline specifications.  Glassy polymers, in
particular polyimides, have been reported to show the best performance for this
specific separation.2,3  The higher condensability and smaller
kinetic size make CO2 molecules more soluble and diffuse faster in
the membrane matrix than methane, resulting in good selectivity and permeability.
 Recently, polyimide-based polymer with intrinsic microporosity
(PIM)4 and cyclodextrine grafted
co-polyimides5 demonstrated very high CO2 permeability at
high pressure (10 atm) but low CO2/CH4
selectivity, less than 25.  In fact, at high pressures, these membranes
suffer considerably from the plasticization problem caused by high
concentration of the acid gas, resulting in decreasing membrane selectivity
significantly.  Crosslinking can help to
minimize the CO2-induced swelling; however, an increase in
separation factor of a solution-diffusion polymeric membrane always accompanies
a reduction in the penetrant flow due to the well-known trade-off relationship.6
 

Facilitated transport membranes hold
great potential for natural gas CO2 removal.  This type of
membrane is able to achieve high CO2 permeability accompanied with
high selectivities vs. CH4, H2,
or N2 due to the specific, prompt reaction between amine carriers
and CO2 molecules.7,8  This work exploited the
superlative mechanical property of multi-walled carbon nanotubes
(MWNTs) to reinforce the mechanical strength of amine facilitated transport
membranes, from which we successfully synthesized nanostructured
polymer-MWNT membranes by incorporating MWNTs (about 18 nm diameter by 180 nm)
into the crosslinked polyvinylalcohol-polysiloxane
network containing amine carriers.  

The synthesized membranes inherited the
facilitated transport mechanism and thus achieved high CO2
permeability and selectivity vs. CH4 at high pressures of at least
15 atm and high temperatures of more than 100°C.
 At 15 atm and 107°C using a feed gas of 50% CH4
and 50% CO2, which simulated the natural gas composition, the
membrane showed a high CO2/CH4 selectivity of about 100
accompanied with a CO2 permeability of about 350 Barrers.
 This lower permeability was due to the higher degree of carrier
saturation at the higher CO2 concentration.  It should be noted
that this permeability is still higher than that of commercial membrane, e.g.,
cellulose acetate (about 5 Barrers).

In this work, we also synthesized
acid-treated MWNTs and amino-functionalized MWNTs as advanced nanofillers for amine facilitated transport
membranes.  The acid-treated MWNTs were functionalized with hydrophilic
groups including carboxylic acid and hydroxyl groups by strong acid
oxidation.  The amino-functionalized MWNTs were chemically modified with
3-aminopropyltriethoxysilane.  The presence of these hydrophilic groups
was confirmed by FTIR spectroscopy.  The membranes incorporated with the
modified MWNTs demonstrated significant improvement in CO2/CH4
separation performance, especially in term of separation factor.  A CO2/CH4
selectivity of 180 along with CO2 permeability of around 1000 Barrers have been obtained at 15 atm
and 107°C for a feed gas composition of 59.8% CH4, 20.2% H2
and 20% CO2.  These improvements are attributed to the
modification of MWNTs.  With the hydrophilic surface groups, MWNTs were
compatible with the network of crosslinked polyvinylalcohol and polyamines.  Thereby, the MWNTs
were able to insert between polymer chains and disrupted polymer chain packing
more effectively, resulting in increased free volumes between polymer chains.
 In addition, the membrane performance at higher feed pressure up to 60 atm as well as the effects of membrane composition,
temperature and membrane thickness, has been investigated.

Keywords:
multiwalled carbon nanotube;
mixed matrix membrane; facilitated transport; carbon dioxide separation;
natural gas.

 

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