(396s) Facilitated Transport Membranes for Natural Gas Separation

Facilitated Transport Membranes for
Natural Gas Separation

Luca Ansaloni¹,
Yanan Zhao², Kartik
Ramasubramanian², Marco Giacinti Baschetti¹, W.S. Winston
Ho^2,3

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.

References:

1.     Baker, R.
W. & Lokhandwala, K. Natural Gas Processing with
Membranes:  An Overview. Industrial & Engineering Chemistry
Research 47, 2109?2121 (2008).

2.     Wang, L. et
al. Gas transport properties of 6FDA-TMPDA/MOCA copolyimides.
European Polymer Journal 44, 225?232 (2008).

3.     Wind, J.
D., Paul, D. R. & Koros, W. J. Natural gas
permeation in polyimide membranes. Journal of Membrane Science 228,
227?236 (2004).

4.     Ghanem, B. S., McKeown, N. B.,
Budd, P. M., Selbie, J. D. & Fritsch, D.
High-Performance Membranes from Polyimides with Intrinsic Microporosity.
Advanced Materials 20, 2766?2771 (2008).

5.     Askari, M., Xiao, Y., Li, P. & Chung, T.-S. Natural gas
purification and olefin/paraffin separation using cross-linkable
6FDA-Durene/DABA co-polyimides grafted with α, β, and γ-cyclodextrin. Journal of Membrane Science 390-391,
141?151 (2012).

6.     Robeson,
L. M. The upper bound revisited. Journal of Membrane Science 320,
390?400 (2008).

7.     Bai, H. & Ho, W. S. W. Carbon dioxide-selective
membranes for high-pressure synthesis gas purification. Industrial &
Engineering Chemistry Research 50, 12152?12161 (2011).

8.     Xing, R.
& Ho, W. S. W. Crosslinked polyvinylalcohol?polysiloxane/fumed
silica mixed matrix membranes containing amines for CO2/H2 separation. Journal
of Membrane Science 367, 91?102 (2011).