(814d) CO2 Removal From High Pressure Natural Gas With Hybrid Fixed-Site-Carrier Membranes: Membrane Material Development

CO₂ removal from natural gas (NG) is mandatory to meet the
natural gas network grid specifications as CO₂ reduces the heating
value of natural gas, is corrosive, and easily forms hydrates that can clog the
equipment or damage the pumps. Processing of natural gas presents a large
industrial gas separation market owing to the increased worldwide natural gas
consumption. Traditional chemical (amine) absorption is well known and still
considered as the state-of-the-art technology. However, membrane systems show a
great potential for natural gas sweetening, even though it has only 5 % of the market
today. Recently, strong interest has been focused on the development of
different materials, especially for the composite membranes with a very thin
selective layer, to overcome the challenges of the commercial membranes in
natural gas sweetening. The novel fixed-site-carrier (FSC) membranes showed a good
performance for CO₂ / CH₄ separation in our previous work
[1-3]. However, high pressure still challenging the membrane operation due to
the membrane plasticization and compaction caused by the strong adsorption of
CO₂ and heavy hydrocarbon (BTEX) in the polymer matrix. The feasible
solutions are the crosslinking of the membrane materials and development of pressurization-resistant
membranes. Thus, the carbon nanotubes (CNTs) reinforced polyvinyl amine (PVAm) / polyvinyl alcohol (PVA) blend FSC membranes were
prepared using an optimized membrane condition in current work.

The casting solution was prepared by adding a certain amount of CNTs
into the PVAm (MW 340K) /PVA (72K) aqueous solution,
and completely mixed using an ultrasonic mixer. The casting solution was evenly
casted on the top of the polysulfone (PSf) supports following the procedure reported by Kim et
al. [4]. The membrane structure and morphology was characterized by scanning
electron microscopy (SEM), and a typical selective layer thickness was found to
be ~1 μm. The membrane separation performance was
characterized by gas permeation testing at 30 °C with a 10 % CO₂ -
90% CH₄ and a controlled feed flow rate. A lab-scale plate-and-frame
module (membrane area 19.6 cm²) and a small pilot-scale module
involves 3 sheets (each sheet is 110 cm²) were tested in high
pressure permeation rig.

The permeation testing results indicated that the membrane preparation
parameters such as heat treatment temperature, duration time and the pH of
casting solution will significantly influence the membrane performance, and an
optimal preparation condition is found to be: 95 °C - 0.75h ? pH10. Moreover, the
operating parameters such as feed pressure, feed CO₂ concentration, and
feed flow rate were also found to influence the membrane separation
performance, which should be optimized in a specific application.
The experimental results indicated that CO₂permeance decreases with the increase of feed pressure,
which may due to the less contribution from facilitated transport mechanism
(carrier saturation and low water vapor content at high pressure) and membrane
compaction. It was also found that both CO₂permeance and CO₂/CH₄ selectivity are
lower when the feed gas contains a high concentration of CO₂ 50 % compared
to a low concentration CO₂ at the same feed pressure. In addition, a
high feed flow rate presents a much high separation performance due to a better
flow pattern, but a relatively lower stage-cut. Therefore, a pilot-scale
membrane module with larger membrane area should be tested in future work to
document the membrane performance at a relatively high stage-cut which is
usually involved in an industrial process.

The dependence of the membrane separation performance on the
membrane area was tested with the small pilot-scale module, and the results
indicated that both CO₂permeance and CO₂/CH₄
selectivity decrease with the increase of membrane area, large membrane area
gives a high CH₄ purity in the retentate
but high CH₄ losses as well. Thus, a two-stage or multi-stage
membrane system may be needed to accomplish a specific separation requirement
(i.e., high CH₄ purity and low CH₄ losses). The developed
FSC membrane showed a nice potential application for CO₂ removal
from high pressure natural gas, but the process configuration and operating
condition should be well designed and controlled in a real process. The current
work can be well used to guide the process simulation to evaluate the process
feasibility of CO₂ removal from high pressure natural gas process.

Acknowledgements

 The authors acknowledge the NaGaMa project (partners: Norwegian Research Council,
Statoil and Petrobras) for the funding of this work.
The high pressure pilot-scale module designed by PHILOS is also acknowledged. The
authors also thank SHOWA DENKO K. K. company in Japan
to provide the carbon nanotubes for this work.

References

[1] Deng L, Kim T-J,
Sandru M, Hägg M-B. PVA/PVAm
Blend FSC Membrane for Natural Gas Sweetening. Proceedings of the 1st Annual
Gas Processing Symposium. Doha, Qatar; p. 247-55.

[2] Washim Uddin M, Hägg M-B. Natural
gas sweetening?the effect on CO2?CH4 separation after exposing a facilitated
transport membrane to hydrogen sulfide and higher hydrocarbons. J Membr Sci.
2012;423?424(0):143-9.

[3] He X, Hägg MB. Hybrid
Fixed-site-carrier Membranes for CO₂/CH₄ Separation.
Euromembrane 2012. London, UK.

  [4] Kim
T-J, Vrålstad H, Sandru M, Hägg M-B. Separation performance of PVAm composite membrane for CO2 capture at
various pH levels. J Membr Sci. 2013;428(0):218-24.

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