Development of novel composite hollow fiber membranes for CO2 removal from biogas using membrane contactor

• Main Author: Xu, Yilin
• Other Authors: Wang Rong
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Environmental engineering
• Online Access: https://hdl.handle.net/10356/106556; http://hdl.handle.net/10220/48109
• Institution: Nanyang Technological University

Biogas is a clean and renewable energy source in a carbon-constrained world. Carbon dioxide (CO2) capture is essential for upgrading biogas to increase energy density. The gas-liquid membrane contactor (GLMC) has been proposed and investigated as a promising alternative to conventional CO2 absorption techniques such as packed towers and bubble columns. Owing to its desirable properties such as hydrophobicity, suitable pore size, high porosity, good mechanical strength, and high thermal stability, polyvinylidene fluoride (PVDF) membrane has been extensively used in GLMC processes. However, PVDF membrane is vulnerable to wetting, swelling and degradation by solvents under long-term operations, thus restricting its application for CO2 removal in membrane contacting processes. Therefore, developing novel membranes to improve the durability in the GLMC process is of significant importance. To achieve this goal, an inorganic-organic fluorinated titania-silica (fTiO2-SiO2)/PVDF composite membrane was fabricated via facile in-situ vapor-induced hydrolyzation method followed by hydrophobic modification. This low mass-transfer-resistance membrane, composing of a mesoporous layer deposited onto macroporous substrate, was designed for biogas upgrading in GLMC application. Surface hydroxylation was introduced to facilitate the bridging of TiO2-SiO2 nanoparticles and PVDF substrate, which resulted in a more coherent deposition of fTiO2-SiO2 layer onto the substrate. The surface microstructure was fine-tuned by controlling the amount of doped Si precursor, forming an integrated mesoporous fTiO2-SiO2 layer. The resultant fTiO2-SiO2/PVDF composite hollow fiber membrane exhibited a tighter pore size of ~25 nm and a desired water contact angle of ~124°, which effectively prevented membrane wetting. The high CO2 absorption fluxes were achieved due to lower mass transfer resistance, by using 1 M of monoethanolamine (MEA) and sodium taurinate as absorbents, respectively. The long-term stability test showed a good integrity between the fTiO2-SiO2 layer and the PVDF substrate after 31 days of GLMC operation. The main benefit is the robust fluorinated inorganic layer that exhibited strong chemical resistance and high hydrophobicity, thus preventing membrane damage and pore wetting. This study provides an insight into the preparation of high-performance inorganic/organic composite hollow fiber membranes for CO2 removal in GLMC applications. Considering the high corrosion and energy-intensity of using traditional amine-based absorbents for CO2 capture in GLMC processes, it is more appropriate to use environmentally-benign solvents for CO2 capture. A highly efficient biocatalytic carbonic anhydrase (CA)-polydopamine (PDA)/polyethylenimine (PEI)-PVDF composite membrane was designed and fabricated for CO2 conversion and capture using water as absorbent. The co-deposition of PDA/PEI with amino functional groups was employed to amine-functionalize a PVDF substrate as support for subsequent in-situ CA immobilization by cross-linking with glutaraldehyde. This enhances the enzyme stability and prolongs its lifespan, thus facilitating CO2 removal efficiency in the GLMC process due to the superb catalysis of CA enzyme for CO2 hydration. By using water as absorbent with a liquid velocity of 0.25 m·s−1 in a bench-scale GLMC setup, a high-efficiency CO2 absorption flux (2.5 × 10−3 mol·m−2·s−1) of the proposed biocatalytic membrane was obtained, which was ~165% higher than that of the non-biocatalytic membrane. A good long-term stability in terms of enzyme activity for CO2 hydration of the biocatalytic composite membranes was also achieved during 40 days of test duration. Overall, the results achieved in this work could provide promising insights into the development of biocatalytic membranes for extended GLMC applications using environmentally-benign absorbents to achieve a high-efficiency CO2 removal. Inspired by better gas selectivity along with tunable pore size and facile functionalization of metal-organic frameworks (MOFs), a highly efficient aminosilane-modified zeolitic imidazolate framework-8 (mZIF-8) based composite hollow fiber membrane with a dense skin layer was successfully fabricated for biogas upgrading in the GLMC process by dispersing mZIF-8 into dense polydimethylsiloxane (PDMS) matrix and depositing on a porous PVDF substrate. (3-aminopropyl)triethoxysilane was introduced to modify the ZIF-8 nanocrystals in order to cross-link with PDMS chains for further hydrophobicity enhancement. Compared with the pristine PVDF membrane, the newly developed composite membranes with a dense skin exhibited competitive hydrophobicity with a contact angle of 130°, ensuring its anti-wetting ability. The mZIF-8 based composite membrane realized a higher CO2 mass transfer efficiency as well as a higher selectivity of CO2/CH4 using water or aqueous amine absorbents in GLMC processes. A robust long-term stability of mZIF-8 based composite membrane was also achieved in a 15-day operation. This work provides a new strategy to improve the biogas upgrading performance using dense composite membranes with modified MOFs in GLMC applications.