(640c) CO2 Removal From High Pressure Natural Gas With Hybrid Fixed-Site-Carrier Membranes: Experimental, Process Design and Feasibility Analysis

Natural gas (NG) is
becoming one of the most attractive growing fuels for world primary energy
consumption. However, the raw natural gas usually contains considerable amount
of CO₂ which should be removed to meet the natural gas network grid
specifications. Membrane systems possess of small footprint, low capital and
operating costs, being environmentally friendly, and exhibiting process
flexibility, which show a great potential for CO₂ removal from natural
gas even though it has only 5% of the market today. Commercial gas separation
membranes for natural gas sweetening are mostly made from cellulose acetate
(CA), cellulose triacetate (CTA) and polyimide (PI). The challenges of relatively
low separation performance (both CO₂permeance
and CO₂/CH₄ selectivity) and membrane plasticization for
these commercial polymeric membranes direct to the development of novel, high
performance composite membranes. Our previous work has already documented that
the carbon nanotubes (CNTs) reinforced PVAm /
polyvinyl alcohol (PVA) blend FSC membranes could improve the membrane
performance, especially at high pressure operation [1]. Thus, the high
performance hybrid FSC membranes were prepared using an optimized preparation
condition in this work.

Carbon nanotubes reinforced
PVAm/PVA blend FSC membranes were prepared to improve
the mechanical strength and reduce the compaction influence at high pressure
operation. A special casting procedure was employed to coat an
CO₂-selective layer with a pre-determined thickness as reported by
Kim et al. [2]. An optimized heat treatment procedure is conducted to enhance the
membrane mechanical strength, separation performance and stability. The size of
the prepared large flat-sheet hybrid FSC membrane is 30 cm ×30 cm.

Membrane separation
performance was characterized by high pressure gas permeation testing with the
synthetic mixed gases of 10% CO₂ - 90% CH₄ and 50% CO₂
- 50% CH₄. The high pressure pilot-scale module involves 3 sheets
(the membrane area of each sheet is 110 cm²) that can be operated in
parallel or individually. A constant feed flow of 3000 Nml/min
was humidified (RH 100%) and fed into the membrane module at 30 °C and different
pressures without sweep gas, and the developed FSC membranes showed a high CO₂permeance of 0.084 - 0.218 m³ (STP) / (m².h.bar)
with a CO₂/ CH₄ selectivity of 17.9 - 34.7 at different
feed pressures from 40 to 10 bar. Moreover, the membrane capacity was also investigated
by testing the whole module (area: 330 cm²) at 30 °C and 30 bar
using different feed flow rates. It was found that the CH₄ purity in
retentate can achieve the requirement (i.e., >96%)
using a single stage membrane unit at a low feed flow rate (high stage-cut), but the CH₄ losses
are quite high which indicated that part of methane should be recovered using a
2^nd?stage membrane unit- this could be used to guide the process
simulation.

Based on the experimental data, process
simulation was conducted by HYSYS integrated with an in-house membrane program
(ChemBrane) to evaluate the process feasibility of
membrane systems for CO₂ removal from high pressure natural gas. A
two-stage membrane system was designed for process feasibility analysis.
Process simulation was conducted on the basis of a natural gas feed flow of
5×10⁵ Nm³/h contains 10% CO₂ / 90% CH₄.
The minimization of specific power consumption and natural gas sweetening cost (economic
cost estimation using CAPCOST_2008 [3]) were applied to process optimization, and
the results are shown in Table 1. It can be found that membrane process with the
developed FSC membranes was feasible for CO₂ removal, even at a relatively
low CO₂ feed concentration (~10 %) in natural gas, and it is
possible to achieve > 96 % CH₄ purity in the retentate stream and
a low CH₄ loss < 2%. A minimum cost of 5.91e-3 $/ Nm³ sweet natural gas produced was found at a feed pressure
of 10bar in the 2^nd stage
to achieve the given separation requirements (i.e., CH₄ purity >
96%, CH₄ losses < 2% and captured CO₂ purity >
95%), which is lower compared to 6.40e-3 $/ Nm³ product
with amine absorption [4]. Thus, the developed CNTs reinforced hybrid FSC
membranes show great potentials for CO₂ removal from high pressure
natural gas.

Table
1 Simulation results

Acknowledgements

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

References

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

[2]       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.

[3]       Turton
R, Bailie RC, Whiting WB, Shaeiwitz JA. Analysis, Synthesis, and Design of
Chemical Processes, Third Edition. Prentice Hall, New Jersey. 2008.

[4]       Peters
L, Hussain A, Follmann M, Melin T, Hägg MB. CO2 removal from natural gas by
employing amine absorption and membrane technology?A technical and economical
analysis. Chem Eng J. 2011;172(2?3):952-60.

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