Hybrid fixed-site-carrier membranes for CO2 removal from high pressure natural gas: Membrane optimization and process condition investigation

doi:10.1016/j.memsci.2014.07.016

Journal of Membrane Science

Volume 470, 15 November 2014, Pages 266-274

https://doi.org/10.1016/j.memsci.2014.07.016 Get rights and content

Highlights

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Development of high performance CNTs reinforced PVAm/PVA blend FSC membranes for high pressure CO₂/CH₄ separation up to 80 bar.
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Optimization of membrane preparation condition.
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Systematically investigate the influences of process operating parameters on the membrane performance.
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Small pilot-scale high pressure module design, installation and gas permeation testing.

Abstract

The hybrid fixed-site-carrier (FSC) membranes were prepared by coating the carbon nanotubes (CNTs) reinforced polyvinylamine (PVAm)/polyvinylalcohol (PVA) blend selective layer on the top of the polysulfone (PSf) ultrafiltration membranes. The influences of membrane preparation parameters of support, heat treatment condition and pH value of casting solution on the membrane separation performance were systematically investigated. The hybrid FSC membranes prepared under the optimal condition (PSf 20 K − 95 °C − 0.75 h – pH 10) showed high CO₂ permeance and relatively good CO₂/CH₄ selectivity based on high pressure gas permeation testing. Moreover, process operating parameters such as feed pressure, temperature, feed CO₂ concentration, and feed flow rate as well as water vapor content were found to significantly affect the membrane performance, which need to be optimized in the real application. Small pilot-scale modules with relatively large membrane areas (110–330 cm²) were additionally tested. The results could be used to guide process simulation and economic feasibility analysis of CO₂ removal from high pressure natural gas process with the developed FSC membranes in the future work.

Introduction

CO₂ removal from natural gas (NG) is mandatory to meet the specification of the natural gas network grid as CO₂ reduces the heating value of natural gas, is corrosive, and can easily form hydrates that may clog or damage the pumps or other equipment [1]. Processing of natural gas presents a large industrial gas separation application owing to a high worldwide natural gas consumption of 3169 billion m³ in 2010 [2]. Decision on which technology to use for carbon dioxide (CO₂) removal from natural gas is mainly dependent on the process condition and the crude natural gas composition as well as the plant location. Traditional chemical (amine) absorption is well known and implemented in the industrial processes, and is still considered the state-of-the-art technology. However, membrane systems possess many advantages such as small footprint, low capital and operating costs, being environmentally friendly, and exhibiting good process flexibility [3], which shows a great potential in natural gas sweetening even though it has only 5% of the market today. Membrane systems are preferred for high CO₂ concentration gas streams (e.g., enhanced gas recovery) and favorable to be used for processing small gas flows because of their simple flow schemes (typically in offshore platforms, <6000 Nm³/h) as reported by Baker et al. [4]. Some representative suppliers of membrane systems using different materials for CO₂ separation from natural gas are shown in Table 1. These commercial membranes are mostly made from cellulose acetate (CA, spiral wound and hollow fibers) and polyimides (PI, hollow fibers). CA membranes have relatively low CO₂/CH₄ selectivity under typical operating conditions in the field [5], [6], but have fair-to-good tolerance to contaminants such as benzene, toluene, ethylbenzene and xylene (BTEX) in the natural gas streams [7], [8]. While PI membranes have better intrinsic transport properties compared to the CA membranes, but are more sensitive to BTEX, which show lower performance in the field [9]. The use of membrane systems provides an economically and environmentally attractive solution compared to the traditional amine absorption even though there are still some challenges related to the polymeric membranes in natural gas sweetening:

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low CH₄ losses (<2%),
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high CO₂ flux and good CO₂/CH₄ selectivity (>30),
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tolerant to high operating pressure (i.e., good mechanical strength),
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durable to H₂S, higher hydrocarbon (HHC) and water,
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easy to fabricate, operate, and maintain,
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high operation stability and long lifetime.

One of the potential strategies to address these challenges is to develop high performance composite membranes with an active layer on the order of 1 μm in order to compete with other separation methods. The novel fixed-site-carrier (FSC) membranes showed a high performance for CO₂/CH₄ separation as reported by Deng et al. [10] and Kim et al. [11]. However, high pressure operation is still challenging to membrane systems in natural gas processing as membrane compaction and plasticization at high pressure operation are found to be a well-known phenomenon in most polymeric membranes due to the strong adsorption of CO₂ and heavy hydrocarbon (BTEX) in the polymer matrix [7]. The potential solutions to overcome membrane compaction and plasticization are crosslinking of the membrane materials and the development of the mechanical stronger membranes than can be resistant to compaction. Different heat treatment and cross-linking methods such as thermal annealing, formation of semi-interpenetration polymer networks, and ultraviolet (UV) radiation have been discussed by Wind et al. [12]. They concluded that most of the preceding crosslinking methods involve the procedures that would be difficult to be applied in the commercial membrane manufacturing processes. Ma et al. reported that ester-crosslinked hollow fiber membranes can maintain a high CO₂ permeance under highly aggressive feed pressures up to 55 bar for a 50/50 CO₂/CH₄ feed gas without CO₂ plasticization [13]. Deng et al. [14] and He et al. [15] used a physical cross-linking method (heat treatment) to increase the mechanical strength of their FSC membranes by controlling the heating temperature <120 °C, which can maintain the membrane morphology and minimize the production costs. Another potential approach to improve membrane separation performance is the development of mixed matrix membranes (MMMs) and hybrid membranes. Adams et al. prepared a 50% (vol.) Zeolite 4A/polyvinyl acetate (PVAc) MMMs for CO₂ separation from natural gas [16]. They found that the prepared MMMs can approach the Robeson CO₂/CH₄ upper bound. At low CO₂ partial pressures, CO₂ permeability roughly doubled with a nearly 50% increase in selectivity versus pure PVAc under the same conditions, while at high CO₂ partial pressures, CO₂ permeability remained effectively unchanged with a 63% increase in selectivity compared to pure PVAc membranes. He et al. reported carbon nanotubes (CNTs) reinforced polyvinylamine (PVAm)/polyvinyl alcohol (PVA) blend FSC membranes [15]. Although the membranes were not optimized, the improved separation performance was found from the high pressure CO₂/CH₄ permeation testing. These investigations can be used to guide the further development of membrane systems in high pressure natural gas sweetening. Hence, high performance FSC membranes were fabricated by optimization of the membrane preparation conditions in the current work. Moreover, it was also reported that carrier saturation and low water vapor content in the gas stream at high pressure may additionally reduce the CO₂ permeance and CO₂/CH₄ selectivity of the FSC membranes [15]. Thus, process operating parameters such as feed pressure, temperature, feed flow rate and composition as well as water vapor content were systematically investigated. Based on the optimal membrane preparation from lab-scale testing, the larger membranes (30 cm×30 cm) were prepared for small pilot-scale module testing without sweep gas in order to document the membrane performance in a more realistic way.

Section snippets

Materials

Commercial polysulfone (PSf) flat-sheet ultrafiltration membranes (MWCO 20K and 50K) were purchased from Alfa Laval. The commercially available high molecular weight polyvinylamine (PVAm, MW 340K) was kindly provided by BASF, and further purified in the lab using a specific procedure as reported by Sandru et al. [17]. 90+% hydrolyzed polyvinyl alcohol (PVA, MW 72K) was purchased from Merck Schuchardt. The nanoparticles of the carbon nanotubes (CNTs) VGCF-X (D/L, 15 nm/3 μm) were supplied from

SEM characterizations

Scanning electronic microscopy was used to characterize the structure and morphology of the prepared FSC membranes, and Fig. 2 shows the cross section images of the membrane Pilot-2. The thickness of the selective layer was estimated to be 2–3 μm. Although the selective layer can be further brought down to 1 μm or even thinner, the mechanical strength and long-term stability may decrease and cause the reduction of membrane separation performance at high pressure operation. Thus, the thickness of

Conclusions

CNTs reinforced FSC membranes have been prepared by coating a PVAm/PVA blend selective layer on the top of PSf ultrafiltration supports. The influences of different membrane preparation parameters such as support, heat treatment condition, as well as the pH value of polymer solution on the membrane separation performance were systematically investigated, and the optimal preparation condition was currently found to be: PSf 20K −95 °C −0.75 h – pH 10, while CNTs content can be further adjusted to

Acknowledgments

The authors acknowledge the NaGaMa project (partners: The Research Council of Norway (Grant no. 103840009, 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 for providing the carbon nanotubes for this work. We also thank Dr. Maria Teresa Guzman Gutierrez and Dr. Mohammad Washim Uddin and Dr. Marius Sandru for their help in SEM characterization and nice

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Citation Excerpt :
Matrimid hollow fiber membranes were reported to present a good performance with CO2/CH4 selectivity over 34 at 75 °C [10]. Recently, the fixed-site-carrier (FSC) membranes have been reported for CO2 removal from natural gas [9,11]. This type membrane showed a good CO2/CH4 separation performance at moderate pressures [9,11].
Subsea natural gas processing attracts increased interest due to the smaller environmental footprint. Natural gas (NG) dehydration and sweetening are the main processing steps to avoid pipeline plugging and corrosion caused by the presence of water and CO₂. Triethylene glycol (TEG) and amine absorption are the commercial technologies for these applications. However, membrane technology is considered as promising solutions for alternative subsea gas processing technologies, which provides unmanned operations without the requirements for rapidly periodical maintenance. In this work, a hybrid membrane process was designed for integrated dehydration and sweetening of a saturated natural gas containing 10 mol% CO₂, and the process operating parameters such as inter-stage feed and permeate pressures are investigated. The simulation results indicated that the optimal permeate pressure in the 2nd -stage unit is 4 bar, and the optimal 3rd-stage feed and permeate pressures are15bar and 2 bar, respectively. The minimum specific cost of <2.71 × 10⁻³ $/m³ sweet natural gas was estimated to achieve the separation requirement of <2.5 mol% CO₂ in purified NG together with captured high purity CO₂ (>95 mol%) for enhanced gas recovery. However, due to the relatively low water selectivity of the dehydration membranes at high pressure of 60 bar used in the simulation, the hydrocarbon loss is still quite higher. Thus, advanced membranes with high H₂O/CH₄ selectivity at high pressure should be pursued to promote the application of the designed membrane system for subsea natural gas dehydration and sweetening.

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Hybrid fixed-site-carrier membranes for CO2 removal from high pressure natural gas: Membrane optimization and process condition investigation

Highlights

Abstract

Introduction

Section snippets

Materials

SEM characterizations

Conclusions

Acknowledgments

Gas Sep. Purif.

J. Membr. Sci.

Gas Sep. Purif.

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

Proc. Eng.

J. Membr. Sci.

J. Membr. Sci.

Hybrid fixed-site-carrier membranes for CO₂ removal from high pressure natural gas: Membrane optimization and process condition investigation