Multiwalled Carbon Nanotube Mixed-Matrix Membranes Containing Amines for Acid Gas Removal From Natural Gas

Multiwalled Carbon Nanotube Mixed-Matrix Membranes Containing Amines for Acid Gas Removal from Natural Gas

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:

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