Properties And Mechanism Of Membrane Antifouling In Oilfield Produced Water Treated By Modified PVDF Ultrafiltration Membrane

This subject started from the characteristics of oil field produced water, based onthe theory of deconstruction and construction, through examination of themacro-phenomenon of the ultrafiltration process, supplemented by micro-liquid-solid interface interaction in-depth analysis to clarify the impact factors andmembrane fouling mechanism of this UF process. Moreover, membrane foulingmathematic models and corresponding pollution control measures were also carriedout. Thus, this study can provide an important theoretical basis and a technicalsupport for the practical application of TiO₂/Al₂O₃-PVDF ultrafiltration membranein the oilfield produced water.Firstly, the comparative study of operating pressure, feed concentration,temperature, pH, salinity and other conditions on the rejection rate of the mainorganic matter and permeate flux have been carried out. The results show that theO/W concentrations in permeate liquid was lower than0.5mg/L, which achieved thestandard of rejection in the low permeability layer of earth. Furthermore, theultrafiltration process operating conditions were optimized by the response surfacemethodology （RSM）, indicating that the operating pressure played a determined roleon the relative flux decline. Since then, the relative filtration flux prediction modelsfor different types of the modified membrane was investigated, which can provide atheoretical reference for the practical application of oil field produced water.Secondly, the adsorption performance of two kinds of membranes with two typesof different pollutants （APAM and O/W） was discussed. It was found that theTiO₂/Al₂O₃-PVDF membrane was polluted by the hydrophilic organic APAM wasmore serious than PVDF membrane, while the contrary results was got by O/W. Theadsorption isotherms of APAM-membrane and O/W-membrane adsorption systemwere Langmuir-Freundlich type and the Redlich-Peterson type, respectively.Moreover, the adsorption capacity of APAM by the two membranes was inverselyproportional to the pH value, while proportional to temperature. Cations have anobvious effect on this adsorption of APAM, and the relative capacity was: Na⁺<K⁺<Ca²⁺, but anion did not play a significant role. For the adsorption of O/W, theadsorption capacity of the O/W first increased and then decreased with pH value raising （between2.0and12.0）; while, decreased with temperature increasing;anions and cations did promote the O/W adsorption capacity on the membranes. Allin all, this indicating that, for some certain pollutants, this anti-fouling abilityincreased of modified membrane can be seen.Through comparative analysis of flux attenuation caused by the pollutants in thedifferent operating mode, the mathematical models on the major membraneresistance contribute to the ultrafiltration flux attenuation have been set up. Thenthe conclusions were drawn as: the membrane pore blocking occurs rapidly, lastinga very short time of about4min, but caused a significant flux decline; Gel layerformation consists of three phases, namely concentration polarization stage, the gellayer forming stage, and the stable gel layer stage, which needs about30min, andhas a significant contribution on flux attenuation; The adsorption fouling resistancemodel based on the static adsorption of the pollutants-membranes, can be used toexplain the slow pollution phenomenon of permeate flux decline process, and, thiskind of resistance plays a determined role in the late stage with a longer duration,nearly equal to the static adsorption equilibrium time. It was found that, theadaptability of the classic “Cake” model is getting worse with ultrafiltration timeextended when simulate the entire UF process. Thus, the "Buckingham" modelbased on the intrinsic relationship between the factors were established; although itis very simple, it is effective in simulating different wastewater UF process.Thirdly, on the basis of previous experiments, the form of a tubular membranemodule was used in oilfield produced water treatment process. The effects ofoperating pressure on the performance of the two ultrafiltration membranespermeable flux and water quality trends over time were investigated, and the resultsshow that high-pressure can only effect on the initial flux, while low-pressureoperation can reduce the membrane fouling, and maintain the relative stability ofthe membrane flux. After comparing the qualities by the two membranes, it wasfound that the water qualities by the two films has no obvious difference, theturbidity in the penetration was lower than0.5NTU, TOC value lower than170mg/L,O/W and APAM concentration was lower than0.6mg/L and0.2mg/L, respectively.Finally, the critical flux of oilfield produced water by dead-end ultrafiltrationprocess was investigated. The results show that the system has a relative low criticalflux with low pressure, neutral pH, temperature30°C, etc. The permeate water production was nearly the same between the system running with critical flux andun-critical flux with a long-running, however, membrane fouling slowed down andthe energy consumption decreased in critical flux model operation. For differentpollutants, different physical-chemical combination of cleaning methods werestudied, which, the effect of hydro washing, recoil, mechanical scrape was poor;while, cleaning by NaClO （1%）, followed by HCl （1%） and SDS （0.5%） has ansignificant effect. Moreover, ultrasonic （US） cleaning was not recommended. Withcertain cleaning method, the modified membrane flux recovery was significantlyhigher than the original membrane.