Surface/Interface Engineering Of Polymer Membranes For Separation And Recovery Of Highly Viscous Chemicals

Polymer membranes have been widely applied in the separation of immiscible liquid/liquid system.Surface/interface engineering can be used to modify homogeneous membrane to obtain polymer membranes with multicomponent structures and thus multiple functions.In this thesis,commercially microfiltration membranes were modified to obtain composite membranes with different functional coatings via physical method.The resulting membranes can be applied to the separation and recovery of high value-added chemicals with high viscosity（e.g.,castor oil,ionic liquid（IL）and crude oil）,and remarkable separation effect has been achieved.These results could expand the application range of polymer membranes,and provide theoretical guidance and technical support for the development of separation materials for separation and recovery of high value-added and high-viscosity chemicals.The main contents are as follows:1.Continuous and synchronous synthesis and separation of hydrophobic ILs through Janus membrane reactor:A series of Janus hollow fiber membranes with IL-phobic inner surface/IL-philic outer surface and adjustable modification depth was prepared by vacuum-assisted solution coating of polydopamine/polyethylenimide.On the one hand,the charged/hydrophilic inner wall of Janus hollow fiber membrane provides a place for anion exchange reaction;on the other hand,the charged inner surface demulsifies the emulsified hydrophobic ILs in the reaction.After demulsification,the demulsified hydrophobic IL can be unidirectional transported from the inner surface to the outer surface,while the water phase is blocked within the inner surface of the Janus hollow fiber membrane.Thus,simultaneous synthesis and separation of hydrophobic IL was achieved.The Janus membrane reactor module,which was assembled by multiple fiber membranes,can be used to continuously and synchronously synthesize and separate hydrophobic ILs containing different length alkyl chains with scalability.First,the membrane reactor can improve the conversion ratio of anion exchange reaction.The ratio by membrane reactor for 5 min is higher than that of batch production for 30min.Second,the yield of hydrophobic ILs can be increased（up to 96%）by enhancing the packing density of membranes filaments.The hydrophobic ILs synthesized by membrane reactor has high purity.The water content is less than 1 wt%equivalent to that obtained by traditional method.The content of Li⁺reaches as low as 7 nmol·m L^-1,compared with that(2600 nmol·m L^-1)obtained by conventional methods.In addition,Janus membrane reactor shows stable performances in 8 cycles,indicating a long-term service ability.This part of work provides a membrane-based strategy for large-scale and continuous synthesis of ILs,and promotes the real application of ILs as“green”solvents in industrial production.2.Interfacial heating composite membrane is used for separation and recovery of high value-added and high-viscosity chemicals:Carbon nanofibers/carbon nanotubes were deposited on the surface of the polyvinylidene fluoride（PVDF）or polypropylene（PP）microfiltration membrane by vacuum filtration.Surface porosity,electrothermal effect and subsequent oil flux and interfacial heating function of such polymeric composite membranes were tailored by adjusting the swelling time of the substrate membrane and the ratio of CNFs/CNTs content.On the one hand,CNTs provide a major electrothermal effect at the liquid/membrane interface.On the other hand,the rod-like fiber structure of CNFs works as a supportive skeleton for the high-density CNTs.Both composite membranes superhydrophobic（～155°）and superoleophilic（～0°）surfaces.The interfacial electrothermal effect of the composite membranes significantly reduces the viscosity of viscous chemicals（e.g.,castor oil,IL（[C₈mim]PF₆）and crude oil）,and thus facilitates the separation and recovery of different viscous chemicals from water.The flux of the composite membrane（75/25-CNFs/CNTs@PVDF membrane）is twice of that with pristine PVDF membrane for the separation of water-in-castor oil emulsions,and the oil purity reaches above 99.5%.For the separation of[C₈mim]PF₆/water mixture,the flux of PP composite membrane（75/25-CNFs/CNTs@PP membrane）is 1.5 times of that with the pristine PP membrane,and the purity of IL reaches above 99.4%.For crude oil emulsions with higher viscosity,the pristine PVDF membrane lacks separation ability,while the composite membrane can effectively separate the water-in-crude oil emulsions.The fluxes of 50/50-and75/25-CNFs/CNTs@PVDF membranes can reach 28.9 and 49.8 L·m^-2·h^-1·bar^-1,respectively.This part of the work demonstrates that the interfacial heating function of polymer composite membranes can reduce the viscosity,demulsify the emulsions,and thus accelerate the mass transfer of viscous chemicals.It provides a universal structural design and regulation criteria for polymer membranes of high-viscosity chemical separation and recovery.