The Fabrication Of Serinol Modified Nanofiltration Membrane And Its Cleaning Technology For The Treatment Of Polymer Flooding Produced Water

Allowing for the current situation of water shortage,increasingly strict environmental emission standards and the fresh water consumption in polymer flooding process,reusing the polymer flooding produced water?PFPW?into the polymer flooding process after NF treatment becomes a reasonable choice,but the membrane fouling problem in this application process is still challenging.In order to control the membrane fouling and promote the virtuous cycle of PFPW,this study,which is oriented to the advanced treatment of PFPW,takes the membrane fouling mechanism,membrane preparation and membrane cleaning as an interdepended entity,and tries to design and fabricate a novel NF membrane with antifouling ability that is suitable for treating PFPW,and to develop membrane cleaning technology in consideration of this membrane material and water characteristics.A typical polyamide composite NF membrane?NF90?was adopted to treat the synthetic PFPW with different composition.After comparing the microstructure of the NF membrane and the flux attenuation characteristics under different fouling conditions,as well as analysing the influence of membrane fouling on the rejection performance,the membrane fouling mechanism of this process was revealed.It was found that salt ions,crude oil and anionic polyacrylamide?APAM?in PFPW can cause membrane fouling to different extent.However,APAM with higher concentration can firstly form a hydrophilic fouling layer on the membrane surface via hydrogen bonding and complex bridging induced by calcium ion.This APAM layer can shield crude oil fouling and mitigate inorganic fouling.Guided by the membrane fouling mechanism,it is believed that the preparation of NF membrane with antifouling ability should start from the following four aspects:Increasing the hydrophilicity of the membrane surface,thereby hindering the hydrophobic interaction between the hydrophobic crude oil component and the membrane surface;Reducing the membrane surface carboxyl group density,thereby reducing the binding force between the APAM molecules and the membrane surface induced by the complex bridging effect of calcium;Decreasing the membrane surface roughness,thereby avoiding the accumulation of foulants into the membrane surface valley;Controlling the salt rejection,thereby mitigating the concentration polarization of salt ions near the membrane surface.Guided by the antifouling theory of NF90 used in the treatment of PFPW,serineol,which contains both amino and hydroxyl groups,was used as the aqueous monomer for interfacial polymerization.N-methylaminopyridine?DMAP?was used as a phase transfer catalyst and acylation catalyst.A novel polyester amide composite NF membrane was prepared,and the membrane properties were regulated by optimising interfacial polymerization conditions.Meanwhile,various instruments were emloyed to analyse and compare the membrane physical and chemical properties.The results show that the optimum preparation conditions were as follows:1.0%?w/v?serinol,0.05%?w/v?TMC,DMAP adding amount was 10%of the mass of serinol,and the interfacial polymerization time was 70 s.The pure water flux of the NF membrane fabricated under the optimized conditions was 6.0 L/m²·h·bar,and the membrane MWCO was about 474 Da.The salt rejections for Na₂SO_4,MgSO₄,NaCl,and MgCl₂ were 96.27%,83.92%,58.68%and 28.76%,respectively.Compared with NF90,this novel NF membrane has stronger hydrophilicity,more membrane surface carboxyl groups,lower membrane surface roughness and lower salt rejection,which endowed the membrane with obvious antifouling ability.However,the water flux and divalent cation rejection of this membrane were relatively low.In order to overcome the drawbacks of the NF membrane prepared with serinol as the aqueous monomer,piperazine?PIP?was used as the aqueous monomer instead.The NF membrane with higher divalent ion rejection was prepared by regulating the reaction parameters.Meanwhile,the mechanism of the supporting membrane pore blocking?SMPB?was proposed,which could obviously affect the NF membrane performance.Finally,serinol was introduced onto the surface of the polyamide NF membrane by surface grafting,thereby exerting its antifouling ability.It was found that the SMPB can decrease the water flux by increasing the mass transfer resistance of water molecules.But on the other hand,it can produce a“mechanical interlocking effect”,which can enhance the adhesion between the polyamide layer and the support membrane,thus improving the membrane stability.When the PIP concentration was0.6%and the TMC concentration was 0.025%,a NF membrane with a higher divalent cation rejection can be obtained due to the high PIP/TMC mass ratio.When the interfacial polymerization time was 10 s,the heat treatment temperature was 50°C,and the heat treatment time was 15 min,the pure water flux of the corresponding NF membrane was 9.86 L/m²·h·bar,and the MgCl₂ rejection was 94.88%.The serinol molecule can be effectively grafted onto the NF membrane via the acylation reaction between the residual acid chloride group on the nascent membrane and the amino group in the serinol molecule.The grafting modification process can obviously enhance the hydrophilicity of the membrane surface and the pure water flux,and reduce the density of the carboxyl groups on the membrane surface while maintaining a smooth membrane morphology,stable salt rejections and MWCO.The pure water flux of the serinol grafting modified NF membrane?NF-PFPW?was increased to11.35 L/m²·h·bar.The smooth membrane surface structure,hydrophilic surface characteristics,lower residual carboxyl density,higher pure water permeability and lower NaCl rejection endowed the NF-PFPW membrane with a strong antifouling ability,and high rejection against Ca²⁺?higher than 93%,the effluent concentration was less than 0.5 mg/L?,Mg²⁺?higher than 96%,the effluent concentration was less than 3 mg/L?and SO₄^2-?higher than 97%,the effluent concentration was less than 2mg/L?,which can meet the recycling requirements of PFPW.Another membrane fouling control method was proposed from the perspective of membrane cleaning.Based on the treatment of PFPW with NF90,a chemical cleaning method suitable for this commercial NF membrane was developed.And the cleaning performance was systematically evaluated.Then,a more economical,green and effective chemical cleaning method suitable for the NF-PFPW membrane fabicated in Chapter 5 was further developed.For NF90,the optimized cleaning strategy included two steps:the first step was cleanning with the alkaline formulated cleaning reagent for 0.5 h.The alkaline formulated cleaning reagent consited of 0.05%EDTA,0.2%SDS and 0.2%SPP,and the pH of the solution was adjusted to 11 with NaOH;the second step was to clean with a hydrochloric acid solution of pH=2 for0.5 h.After the above cleaning process,the membrane flux can be completely recovered.Further,various methods were used to study the changes of other properties before and after cleaning.It was found that the desalting ability,membrane morphology,hydrophilicity and infrared spectrum of the NF membrane after cleaning were similar to those of the original membrane,indicating that the cleaning method was effective and non-destructive.When using the NF-PFPW with antifouling ability to treat PFPW,the membrane cleaning process was expected to be simplified.The optimization test shows that the fouled NF-PFPW can be effectively cleaned after cleaning with 100 mmol/L NaHCO₃ solution for 120 min.And a flux recovery of100.02%can be obtained.Allowing for the fouling mechanism and antifouling mechanism of NF-PFPW,it is inferred that the membrane cleaning mechanism of the NaHCO₃ solution includes the destruction of complex bridging induced by Ca²⁺,the destruction of hydrogen bonds,and the coiling of APAM molecules.