Humidified Gas Stream Separations at High Temperatures Using Matrimid 5218

Abstract

Most industrially relevant high temperature gas separations (≥150°C) of either carbon dioxide (flue gas) or hydrogen (syn-gas) must be performed in the presence of water vapor. At ambient temperatures, water vapor can permeate easily through most polymeric membranes and can influence the permeation of other gases through interaction with the polymer, such as swelling and clustering. At higher temperatures, water vapor can be destructive to polymer membranes by changing the polymer structure that can result in diminished gas separation performance. Little data has been reported on the influence of water vapor in gas separations at >100°C because most polymers are not stable at temperature. Many high performance (HP) polymers are able to endure high temperatures and aggressive chemical conditions. For example, polyimides are promising HP polymers that effectively separate permanent gases at temperatures higher than 150°C under dry conditions. In this report, the analysis of selected HP polymers in humidified gas streams (2–4 vol% water) shows that they can perform modest separations at ambient temperatures. In general, it was observed that water vapor permeability is greater than other tested gases. Additionally, the permeabilities of the analyte gases were somewhat influenced by the presence of humidity and their selectivities were significantly lower, as compared to corresponding experiments performed in the absence of water. To elucidate the role of water vapor in gas transport, energy of activation of permeation (E_p) values were obtained for Matrimid 5218 from 30–200°C in a humidified mixed gas stream, and it was found that the selectivities are nearly identical to dry gas streams at 150°C. This data suggests that water vapor functions as a gas and only slightly decreases selectivity of the other gases at elevated temperatures. As a result, economic wet gas separations may be possible using these materials if the gas stream is kept at higher temperature (≥150°C), which is assisted by the inherent stability of the membranes.

Keywords:

• gas permeation analysis
• high performance polymers
• high temperature
• polyimides
• water vapor

ACKNOWLEDGEMENTS

The work described in this paper was supported by Battelle Energy Alliance, LLC and Department of Energy – Energy Efficiency and Renewable Energy – Industrial Technology Program (DOE- EERE-ITP) through Contract DE-AC07-05ID14517. Also, the work was supported by National Science Foundation's Internal Research and Development (IR&D) Program and Laboratory Directed Research and Development (LDRD) Program at the Idaho National Laboratory.

Notes

^a D.W. Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use, John Wiley & Sons Inc., 1974. [Lennard-Jones (6–12) σ(Å) – minimum equilibrium cross-sectional diameter].

^b D.R. Lide, CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data, CRC Press, 2004.

^a Permeabilities measured in Barrers: [(10⁻¹⁰)((cm³(STP) • cm)/(cm² • sec • cmHg))].

^b Below 100°C, vapor-liquid equilibria as a function of pressure can lead to poor reproducibility.

^a Permeabilities measured in Barrers: [(10⁻¹⁰)((cm³(STP) • cm)/(cm² • sec • cmHg))].

^b 2–4 vol% Water Vapor Composition.

^c Below 100°C, vapor-liquid equilibria as a function of pressure can lead to poor reproducibility.

^a Permeabilities measured in Barrers: [(10⁻¹⁰)((cm³(STP) • cm)/(cm² • sec • cmHg))].

^b 2–4 vol% Water Vapor Composition.

^a 2–4 vol% Water Vapor Composition.