Thin-film composite membrane contactors for desorption of CO2 from Monoethanolamine at elevated temperatures
Introduction
Membrane gas-solvent contactors are a hybrid approach that has been applied to carbon capture and storage (CCS) [1], [2]. The contactor consists of a permeable membrane separating the solvent and gas phases, which overcomes many of the limitations of existing solvent absorption and membrane technologies. The use of a solvent ensures high selectivity for CO2, while the membrane provides a high mass transfer area and the membrane module enables the flow regimes of the two phases to be separately controlled. The majority of the research on membrane contactors has focused on porous membranes for the absorption of CO2 into solvents, such as Monoethanolamine (MEA) [3], [4], [5]. Hence, membrane contactors have the possibility to replace traditional solvent absorber columns and correspondingly reduce the footprint of the carbon capture process [6]. The application of porous membrane contactors however, has suffered from the effects of pore wetting by the solvent and reduced mass transfer performance. Recent research has focused on thin film composite membrane contactors for the absorption of CO2 [7], [8], [9], [10], because the non-porous layer is believed to prevent pore wetting. Highly permeable polymeric membranes are required to ensure the mass transfer resistance of this non-porous layer is minimal. However, this approach has not always been successful at preventing pore wetting of the porous support layer, because of high water flux through the non-porous layer [8], [11]. The regeneration of CO2 loaded solvent, also known as stripping, through membrane contactors is an area that has been less studied, even though it is complementary to solvent absorption. Early work by Kosaraju et al. [11] studied CO2 stripping in both a porous polypropylene contactor with a non-volatile solvent as well as an ultra-thin dense asymmetric poly (4-methyl-1-pentene) membrane with MEA. They found that separation performance was strongly impacted by solvent volatility and its ability to wet the porous support layer. Simioni et al. [12] studied porous poly tetrafluoroethylene (PTFE) and a polyether sulfone membrane with a hydrophobic surface coating for the stripping of CO2 from potassium carbonate. They found that minimizing pore wetting through increasing the hydrophobicity of the membrane pores was necessary to ensure high mass transfer rates. Similar performance was observed by Khaisri et al. [13], [14] for porous PTFE membrane stripping CO2 from loaded MEA. Wang et al. [15] studied a range of amine-based solvents for the regeneration of CO2 through a vacuum regeneration process using a microporous polypropylene (PP) contactor. Importantly, they found that the regeneration through a membrane contactor was competitive with traditional solvent regeneration approaches. Porous ceramic membranes have also been applied to CO2 stripping from MEA [16]. The study of thin film composite membrane contactors for CO2 regeneration has been limited to the early study by Kosaraju et al. [11] with an asymmetric PMP membrane and Dibrov et al. [17], who studied a composite membrane consisting of non-porous poly(1-trimethylsilyl-1-propyne) (PTMSP) layer with MDEA solution. That study obtained high CO2 regeneration fluxes through the membrane, but the age dependence of PTMSP morphology meant that the flux of CO2 decreased substantially with time.
Here, three hollow-fiber thin-film composite membrane contactors are studied for the stripping of CO2 from loaded MEA solution at elevated temperatures from 70 to 105 °C. The three contactors consist of non-porous PTMSP, a non-porous Polymer of Intrinsic Microporosity (PIM-1) and non-porous perfluorinated polymer Teflon AF1600 on a porous polypropylene (PP) support, all of which have previously been investigated for solvent absorption [8]. The purpose of this investigation was to study the CO2 flux and mass transfer through the membrane contactors as a function of solvent flowrate and temperature. Furthermore, the rate limiting process is determined from mass transfer theory to provide insight into how the contactors operated under solvent stripping conditions. PTMSP and Polymer of Intrinsic Microporosity (PIM-1) were chosen because of their high CO2 permeances [18], [19] and Teflon AF1600 was chosen because of its unique solvent properties [20], [21]. In addition, the water flux through the three composite membrane contactors as a function of temperature was investigated.
Section snippets
Theory
The overall mass transfer across the membrane contactor consists of three mass transfer stages, the transfer of CO2 across the solvent boundary layer, the transfer of CO2 through the membrane and finally the transfer of CO2 across the gas boundary layer. These stages act as a resistance to mass transfer and are usually expressed as a series [2], [13]:where Kg is the overall mass transfer coefficient, kg, km and kl are the gas, membrane and liquid side physical mass transfer
Experimental
Thin film composite membranes consisting of a non-porous dense outer layer of Teflon AF1600 (DuPont, USA), PTMSP (Fluorochem Ltd, UK) and PIM-1 (the polycondensation product of ultrahigh purity monomers of 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobisindane (TTSBI) and 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN)) on porous polypropylene fibers (Membrana, USA) were produced through standard dip coating techniques, with the details reported elsewhere [8]. All fibers were vacuum
Gas permeability
The CO2 permeability for all three of the polymers used increases with temperature (Fig. 2), which is standard behavior and associated with increasing gas diffusivity [33]. PTMSP has the highest CO2 permeability, associated with the very high fractional free volume of the polymer [19]. The permeability of PIM-1 is ∼20% of the PTMSP permeability over the temperature range studied. The Teflon AF1600 CO2 permeabilities are an order of magnitude lower than those observed for PTMSP, due to the
Conclusion
Three thin-film composite membrane contactors, based on non-porous layers of PTMSP, PIM-1 and Teflon AF1600 on porous PP, were investigated for the stripping of CO2 from MEA, for solvent regeneration at temperatures 70 °C and greater. It was determined that the CO2 regeneration flux for all three membrane contactors increased with temperature, as well as with solvent Reynolds numbers. The overall mass transfer coefficients for the CO2 flux through the non-porous contactors were determined,
Acknowledgements
Colin Scholes would like to thank the Victorian State Government for funding. Colin Scholes, Sandra Kentish and Geoff Stevens would like to acknowledge funding provided by the Australian Government through its Cooperative Research Centre program as well as the Particulate Fluids Processing Centre and the Peter Cook Centre for Carbon Capture and Storage research at the University of Melbourne.
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