313516 CO2 Removal From High Pressure Natural Gas With Hybrid Fixed-Site-Carrier Membranes: Experimental, Process Design and Feasibility Analysis

Thursday, November 7, 2013: 9:20 AM
Union Square 3 (Hilton)
Xuezhong He, Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim, Norway, Taek-Joong Kim, Sintef Materials and Chemistry, SINTEF, Trondheim, Norway and May-Britt Hägg, Chemical Engineering, NTNU, Trondheim, Norway

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 CO2 permeance 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 CO2 permeance 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 2nd–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 2nd 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

Parameters

Simulation results

Feed pressure: 1st / 2nd stage , bar

40/10

Sweet NG productivity, Nm3/h

4.67E+05

CH4 purity in sweet NG, %

96.00

CH4 losses, %

0.35

CO2 purity, %

95.19

CO2 recovery, %

62.50

Total membrane area, m2

2.62E+05

Specific power consumption, kWh/Nm3 sweet NG

4.11E-02

NG sweetening cost, $/Nm3 sweet NG

5.91E-03

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.

 ADDIN EN.REFLIST


Extended Abstract: File Not Uploaded
See more of this Session: Separations Needs for CO2 Capture I
See more of this Group/Topical: Separations Division