https://doi.org/10.1016/j.seppur.2022.120786
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The microporous PTFE hollow-fibre membrane comprised a nominal pore size of 0.18 µm (Zeus Industrial Products, Ireland). However, mono-axial stretching of the PTFE tube under heat, created an isotropic wall with oval pores of median length 4.5 µm (Table 2). The polypropylene hollow-fibre membrane comprised a nominal pore size of 0.2 µm, with a dmax of 0.36 µm indicating a more circular pore geometry (Membrana GmbH, Wuppertal, Germany). Liquid entry pressure (Pa) was estimated according to [14]:(1)ΔPB.P.=-4BγLcosθdmaxwhere B (−) is the pore geometry coefficient, which takes a value between zero and unity (unity representing a perfectly spherical pore), γL(N m−1) is the liquid surface tension, θ (°) is the contact angle at the membrane-liquid interface, dmax is the maximum pore length (m). For each experiment, single hollow-fibres were fixed into a Perspex cell (L, 165 mm) comprising a 12 mm diameter channel (Fig. 1). To permit direct observation on the shell-side of the membrane, a viewing window was engineered into the recess within the upper section of the cell. A stereomicroscope was fixed above the viewing window and images captured via high resolution camera (0.5x lens magnification; Nikon SMZ-2T, Milton Keynes, UK). Hollow-fibres were potted in epoxy resin (Bostick Ltd., Stafford, UK) and sealed into the crossflow cell. Pure CO2 (99.8%, BOC gases, Ipswich, UK) was passed through the hollow-fibre lumen at a flow rate of 1000 ml min−1 using a laminar mass flow controller (0.01–1 L min−1, Roxspur Measurement and Control Ltd., Sheffield, UK). Solvent was pumped counter-current on the shell-side of the membrane at 200 ml min−1 with a peristaltic pump (520Du, Watson-Marlow Ltd., Falmouth, UK). Solvent temperature (5 or 20 °C) was fixed with a refrigerated bath (R1 series, Grant Instruments Ltd., Cambridge, UK), and fluid temperatures monitored with K-type thermocouples (Thermosense Ltd., Bucks, UK). For further experimental details of setup, see [2], [3].
Table 2. Dimensions and surface characteristics of the single membrane fibre.
| Empty Cell | PP | PTFE | |
|---|---|---|---|
| Fibre characteristics | |||
| Membrane material | – | Polypropylene (PP) | Polytetrafluoroethylene (PTFE) |
| Inner diameter | mm | 1.2 | 2.4 |
| Outer diameter | mm | 1.8 | 2.8 |
| Wall thickness | µm | 300 | 200 |
| Active length | mm | 165 | 165 |
| Surface areabb | m2 b | 9.33 × 10−4b | 1.44 × 10−3b |
| Porosity | % | 72 ± 2a | 60 ± 10a |
| ΔGhet / ΔGhom | % | 60 | 90 |
| Minimum pore size, (dmin) | µm | 0.2a | 0.18c |
| Maximum pore size, (dmax) | µm | 0.36c | 4.5c |
| Geometrical factor, B | – | 0.56 | 0.15 |
| Contact angle, (θ) | ° | 117db | 135db |
| Breakthrough pressure, (ΔPB.P.) | bar | 2.0eb | 0.3e |
| Lumen cross sectional area | m2b | 1.13 × 10−6b | 4.5 × 10−6b |
| Shell side characteristics | |||
| Height | mm | 5 | 5 |
| Width | mm | 12 | 12 |
| Shell cross sectional area | m2b | 6.0 × 10−5b | 6.0 × 10−5b |
| Priming volume | ml | 11.0 | 10.4 |
| aData provided by manufacturer; bBased on fibre outer diameter; cData statistically determined using log-normal distribution; d [6],eCalculated using Eq. (1)., based on geometric pore shape coefficient, and in contact with an ammonia solution. | |||
Fig. 1. (a) CO2 absorption into ammonia concentration using for PTFE membrane, with absorbent in single pass. Conditions: G/L 11, VG 0.2 m s−1; VL 1.4 × 10−3 m s−1, temperature 20 °C. Error bars indicate standard deviation; (b) evidence of CO2 bubbling at the solvent exit where is lower pressure drop exists; (c) close-up of CO2 bubbling into the solvent on the shell-side of the fibre using direct visual observation for real time in-situ observation.
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