though it has only 5 % of the market today. Membrane ... zeolite. 4A/poly(vinyl acetate) (PVAc) MMM for CO2 separation from natural gas (Adams et al. 2011).
M
Membranes for Natural Gas Sweetening Xuezhong He Department of Chemical Engineering, Norwegian University of Science and Technology, Sør-Trøndelag, Trondheim, Norway
Natural gas sweetening (CO2 removal from natural gas) is mandatory to meet the specifications of natural gas grid. Decision on which technology to use for CO2 removal from natural gas is mainly dependent on process conditions and the crude natural gas composition. Conventional chemical absorption is well known and has been commercially used for CO2 removal in various processes and considered as a state-of-the-art technology. However, membrane system shows a great potential for natural gas sweetening since it possesses many advantages such as small footprint, low capital and operating costs, and environmental friendly and exhibits process flexibility even though it has only 5 % of the market today. Membrane unit cost and CH4 loss are two crucial parameters used for evaluation of membrane process efficiency, which are mainly dependent on the membrane performance and process design. Some representative suppliers of membrane systems using different materials for CO2 separation from natural gas are shown in Table 1. Although common polymer membranes for natural gas # Springer-Verlag Berlin Heidelberg 2015 E. Drioli, L. Giorno (eds.), Encyclopedia of Membranes, DOI 10.1007/978-3-642-40872-4_1334-6
sweetening are still using cellulose acetate/triacetate and polyimide, the novel, high-performance fixed-site-carrier (FSC) membranes showed a great interest for CO2/CH4 separation (Deng et al. 2009a). High-pressure operation is the main challenge for natural gas processing with membrane system. Membrane plasticization is found to be a well-known phenomenon in most polymer membranes which absorb a high content of CO2 at high pressure (Donohue et al. 1989; Wind et al. 2004), resulting to a significant decrease of CO2 permeance as well as selectivity of CO2/ CH4. The possible strategies to overcome membrane plasticization are cross-linking of membrane material (Wind et al. 2002) and fabrication of mechanical strength-enhanced membranes, e.g., mixed matrix membrane, by adding inorganic fillers into polymer matrix. Adams et al. prepared a 50 % (vol.) zeolite 4A/poly(vinyl acetate) (PVAc) MMM for CO2 separation from natural gas (Adams et al. 2011). They found that the prepared MMMs can approach Robeson CO2/CH4 upper bound, and CO2 permeability remained effectively unchanged with a 63 % increase in selectivity compared to pure PVAc. Their membranes showed a promising application in high-pressure natural gas sweetening. Deng et al. reported that carbon nanotubes (CNTs) reinforced PVAm/ PVA blend FSC membrane and presented a good CO2/CH4 separation performance (Deng et al. 2009b; He et al. 2014). It shows a much
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Membranes for Natural Gas Sweetening
Membranes for Natural Gas Sweetening, Table 1 Representative commercial membrane systems for CO2 removal from natural gas Membrane SeparexTM Cynara ® Prism ® Cytop Medal
Material Cellulose acetate Cellulose acetate Polysulfone Perfluoropolymers Polyimide
strong mechanical strength which could have possibility to maintain a good separation performance even at high pressure. Process design for CO2 removal by membrane system from natural gas depends on the membrane permeability and selectivity, CO2 concentration in feed stream, specific separation requirement, as well as the location of the plant. Peters et al. conducted process design, simulation, and optimization for CO2 removal from natural gas using HYSYS integrated with an in-house membrane program (ChemBrane) (Peters et al. 2011). They reported that a two-stage membrane system with a CO2 permeance 0.3 m3 (STP)/(m2 h bar) and a CO2/ CH4 selectivity 40 is comparable to that of amine process. Although the purity of sweet gas is a little low, it can still achieve the sales gas standards (
