| The popularization and application of polymer flooding technology in Daqing oil-field in our country effectively improves the oil recovery, but the large amount of clean water consumption and wastwater output in this process has been bothering this water-deficient area. Between ultrafiltration and reverse osmosis, nanofiltration can intercept organic matter with small molecules and desalting to some extent. Therefore, lowering salinity of the oilfield produced wastewater(after ultrafiltration) and removing residual organic matter in it by advanced treatment with nanofiltration (so that meeting water quality requirement),and then reusing it to confect polymer solution can solve ecnomic and environment problems caused by clean water consumption and wastewater discharging. Early researchments of our laboratory show that oilfied produced wastewater after ultrafiltration pretreatment has the characteristics of high salinity,and otherwise a small amount of crude oil and anionic polyacrylamide are the mainly organic matter in it. In this paper, crude oil,APAM and salt are taken as target components in the wastewater.Nanofiltration treatment efficiency,mechanism of membrane fouling in the process and optimized cleaning scheme of contaminated nanofiltration membrane have been studied by treating simulated oilfied produced wastewater using a commercial nanofiltration membrane (NF90),so that providing technical guidance for the treatment of polymer flooding produced water.Based on the different combinations of target components in the feeding water, single factor experiment discussing influnces of the crude oil, anionic polyacrylamide (APAM) and salt ions on nanofiltration treatment efficiency has been studied by prepairing different types of nanofiltration feed water. Furthermore, the influences of operating pressure and temperature on nanofiltration process has been studied. Results showed that under different conditions, this kind of nanofiltration membrane’s conprehensive salt rejection could achieve85%above, bivalent cation rejection was more than95%, monovalent cations rejection was more than85%, anionic polyacrylamide rejection reached up to100%,crude oil concentration in the penetrants was lower than0.2mg/L;As operating pressure increased from0.6MPa to1.0MPa, flux,salt and crude oil rejections all grew, anionic polyacrylamide rejection remained stable, all reaching100%; As temperature increased from20℃to40℃, flux increased, salt and anionic polyacrylamide rejection remained stable, crude oil rejection reduced. By taking the flux attenuation characteristics in different water conditions as macro characterization of membrane pollution, together with the micro changes of membrane surface properties(AFM,SEM-EDS, contact angle, ATR-FTIR) before and after pollution as well as cleaning efficiency of different chemical agents, different role of salt, crude oil and polyacrylamide in membrane fouling,together with their interaction effects have been revealed. Results showed that during the process of treating simulated wastewater containing polymer,membrane flux decline was mainly due to the pollution of anionic polyacrylamide (APAM), the second was crude oil pollution,and the least was inorganic precipitation produced by salt ions. APAM was prior to combine with NF membrane, generating "shelding effect" against the organic pollution caused by the crude oil, salt ions reduced the solution viscosity, but exacerbated the APAM’s pollution against nanofiltration membrane by means of the electrostatic screenig and complexing bridging effects.Single factor experiment and orthogonal experiment have been conducted according to the fouling mechanism and properties of cleaning reagents,so that optimizing cleaning-scheme in allusion to contaminated nanofiltration membrane.In addition,evaluation of the selected cleaning-scheme’s cleaning efficiency has been perfomed from multiple perspectives. Final optimized cleaning-scheme is that: primarily clean the contaminated NF membrane with the optimum composite reagent confirmed by orthogonal experiment, and then wash again with HCl solution (pH=2). After this cleaning process, flux of the nanofiltration membrane could fully restore, and further experiments showed that the optimum cleaning-scheme could effectively remove all kinds of pollution on the membranes, meanwhile avoiding damage upon other properties of nanofiltration membrane,conmmendably achieving the goal of renewing the polluted nanofiltration membrane. |
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