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 CO₂ permeance 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 CO₂ permeance 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 CO₂ permeance 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.

 ADDIN EN.REFLIST