Fabrication Of Superhydrophobic PVDF Membrane And Its Use For CO₂ Absorption

Carbon dioxide?CO₂?is the main responsible for greenhouse effects and climate change.The enhancement of CO₂ concentration in atmosphere due to fossil fuel usage in industries.Therefore,CO₂ capture from flue gas seems a technically viable option for stabilizing or mitigating atmospheric CO₂ levels,having significance on reducing the risks of climate changes.Membrane gas absorption?MGA?process is a new hybrid technology that combines the advantages of membrane separation and conventional gas absorption process.Based on the wetting mechanism between2-aminoethanol?MEA?and poly?vinylidene fluoride?PVDF,the incorporation of graphene and hydrophobic modified SiO₂ nanoparticles?HMSNs?were promising approachs to obtain hydrophobic PVDF membrane for gas membrane absorption.Therefore,the modified membrane managed to resist the liquid intrusion and exhibited extraordinary anti-wettability in a long-term performance of CO₂ membrane absorption.The results from XPS FT-IR and TG analyses indicated that as the membrane was wetted by the MEA liquid absorbent,neither the chemical reaction between the PVDF membrane and MEA molecules nor dissolution of the hydrophobic components of PVDF membrane into the MEA solution occurred.However,the wetted membrane demonstrated that MEA molecules had intruded into the cross-linked network of PVDF membrane matrix,which indicated that the membrane swelling had occurred.FE-SEM images further testified that the membrane swelling was responsible for the membrane wetting process.The membrane wetted by water and MEA lead to the contact angles decreased from 89°to 82°and 65.4°respectively.The membrane flux of the wetted membrane decreased by 50%.It was concluded that developing new absorbents with high surface tension or improving the membrane hydrophobicity may be an alternative to smooth or eliminate the membrane wetting phenomenon.Graphene nanosheets were incorporated into PVDF membrane in order to improve the membrane hydrophobicity.The hybrid membrane was prepared using the non-solvent induced phase separation method,and used in a membrane contactor for CO₂ absorption.The incorporation of graphene induced the PVDF crystal transformation from?-form to?-form,and resulted in a more open and loose sub-layer structure.The hydrophobicity of the hybrid membrane was enhanced compared to the pristine PVDF membrane.However,the increase in the contact angle values with the graphene concentration only occurred for the bottom surface.The CO₂absorption performance with water as the absorption solution was dependent on the membrane orientation.When the bottom surface of high roughness faced the absorption solution,the hybrid membrane exhibited a faster CO₂ transfer.Due to the improvement in the membrane hydrophobicity,the wetting problem was significantly mitigated,and a long term run with high CO₂ absorption efficiency could be guaranteed.The in-situ embedment approach was adopted to hydrophobic surface-modify PVDF membranes using HMSNs.The different HMSNs concentration in ethanol bath resulted in PVDF precipitation dominantly by solid-liquid demixing,and the obtained membrane was uniformly skinless and composed of spherical microparticles.The deposition of HMSNs on the microspheres formed micro-and nanoscale structure,which was crucial for superhydrophobicity.When HMSNs concentration in bath was2 g·L^?1,the water contact angle of the modified PVDF membrane was higher than 160°and the surface free energy decreased to 0.9 mN·m^?1.Since the proposed modification allowed HMSNs migration into the membrane interior during the exchange between solvent and non-solvent,the hydrophobicity of internal surface of membrane pores and channels was also enhanced.This feature successfully inhibited the liquid intrusion into membrane pores,and promised a stable anti-wettability in long-term performance.The modified PVDF membrane was used in membrane contactor for CO₂ absorption.The wetting problem was markedly alleviated in an intense 20-day test even with a 12 wt%2-aminoethanol solution as absorption solution.Our results demonstrated that the incorporation of hydrophobic nanoparticles in coagulation bath is a promising approach to obtain superhydrophobic PVDF membrane for membrane gas absorption.The structure of PVDF/HMSNs hybrid membrane was controlled by changing the EtOH concentrate in the coagulation bath.It controlled the segregation of PVDF spherulites induced by HMSNs in the membrane preparation.And the microporous PVDF membrane with good performance and structure was prepared.With the increase of EtOH concentrate in the coagulation bath,the HMSNs in ethanol bath resulted in PVDF precipitation dominantly by solid-liquid demixing,and the obtained membrane was uniformly skinless and composed of spherical microparticles.With the increase of EtOH concentrate,the hydrophobicity of PVDF/HMSNs hybrid membrane increased gradually.In the pure EtOH coagulation bath,the surface water contact angle of the prepared PVDF/HMSNs hybrid membrane reached 154°.The modified PVDF membrane was used in membrane contactor for CO₂ absorption.The wetting problem was markedly alleviated in an intense 20-day test even with a 20 wt%MEA.The CO₂ flux of modified membrane was twice as pristine membrane.A systematic simulation has been carried out in membrane contactors to study CO₂capture by water under the wetted and the non-wetted operation modes.The prepared PVDF/Graphene hybrid membranes were used to conduct CO₂ absorption experiments.The corresponding experimental data were used to verify simulated results.Both experimental and theoretical study disclosed that the membrane wetting would result in a significant drop of CO₂ flux.The simulation results further revealed that for the physical absorption of CO₂ by water,the proportion of membrane phase resistance of pristine membrane in the overall mass transfer resistance increased to80%.With the introduction of an extra resistance caused by the membrane wetting,the mass transfer in the wetted membrane phase finally became the rate-controlling step.Moreover,approximate solution using conventional mass transfer measurement also validates the availability of the mathematical model.Theoretical results also indicate the anti-wetting property of hybrid membrane is much better than that of pristine membrane during the CO₂ absoption.