Preparation And Properties Of Ultrafiltration And Nanofiltration Membrane For High Temperature Condensed Water Recycling

High temperature condensed water condensed from steam by heat exchange inthe petrochemical production process has a great value of recovery, for example, itcan be used as boiler makeup water. However, excess oil and iron contaminants incondensed water due to leakage and corrosion of production equipment andpipelines make it difficult to be reutilized directly. Therefore, purification of hightemperature condensed water is necessary for further usage. Moreover, such successwould increase both social and economic benefits in terms of reusage of water andenergy as well as reduction of pollutant discharge amount and the following sewagetreatment load. In this thesis, an ultrafiltration （UF） and nanofiltration （NF）combination separation process was established to remove oil, iron contaminantsand decrease hardness in high temperature condensed water. Meanwhile, athermostable UF membrane and a positively charged NF membrane were prepared.Thermostable resin poly（phthalazinone ether sulfone ketone）（PPESK） waschosen as the membrane material, and thermostable PPESK/TiO₂composite UFmembrane was prepared by using phase inversion method. The effects of castingsolution composition and membrane forming condition on the structure andproperties of UF membrane were investigated and optimized. The influences of TiO₂concentration on composite membrane pore structure, thermostability, hydrophilicity,mechanical properties and anti-fouling property were studied by SEM, XRD, AFM,TGA, electron-mechanical universal material testing machine, water contact angleand ultrafiltration experiments. The results show that the optimal membranepreparation condition was listed as follows: PPESK concentration was16wt.%, thebest additive was a water-soluble polyethylene glycol （PEG）4000with aconcentration of8wt.%, solvent evaporation time was20s and coagulation bathtemperature was25°C. With the addition of TiO₂, the penetrated finger-likestructure in sublayer is suppressed, while sponge-like structure begins to bedeveloped, and the skin layer thickness is increased. Thermostability, hydrophilicity,permeability and mechanical property of composite membranes are significantlyenhanced. The flux model, resistance in series model and membrane fouling index（MFI） were used to investigate membrane fouling in purifying simulated hightemperature condensed water by PPESK/TiO₂composite membranes. In comparisonwith parent membrane, the composite membrane showed enhanced anti-foulingproperty, decreased filtration resistance and MFI as well as higher flux recovery rate.When TiO₂concentration was2wt.%, PPESK/TiO₂composite membrane hasoptimal structures and properties, which can be effectively used for high temperature condensed water purification.To remove divalent and multivalent cation in high temperature condensed water,a positively charged NF membrane was prepared by cross-linking of hyperbranchedpolyethyleneimine （PEI） with P84polyimide UF membrane. The influences ofcross-linking time and PEI molecular weight on surface and cross-section porestructure, hydrophilicity, surface charge and separation performance of NFmembrane were studied by SEM, ATR-FTIR, XPS, water contact angle andnanofiltration experiments. The results show that the surface pore density and poresize of modified NF membrane decreases, the thickness of dense layer increases andporosity of sponge-like pore structure decreases. The hydrophilicity of modified NFmembrane is also improved. The optimal modification condition was confirmed: thecross-linking time is60min and PEI molecular weight is25000g·mol-1. Theprepared NF membrane under above condition behaves the smallest water contactangle of35±1.2°, a narrower pore size distribution and MWCO of436g·mol-1. TheNF membrane surface is positively charged when solution pH <10, and therejections of different salts show the sequence: MgCl2> MgSO4≈NaCl> Na2SO4.The surface charge intensity of modified NF membrane was affected by pH ofsolution, which could further influence the ions rejection of membranes. When pH <8, the membrane surface is positively charged, Na+rejetions keep constant; whilepH increases to10, Na+rejetion decreases due to the decrease of surface positivecharge intensity. Especially, the rejection of NaCl is the lowest at pH=10, ascribingto the sieving effect on neutral membrane surface.An UF and NF combination membrane separation process is established forremoval of hydrophobic oil droplets, iron corrosion products, Ca2+and Mg2+in hightemperature condensed water. The effects of five operation parameters in UF process（transmembrane pressure （TMP）, temperature and pH of feed water, initial oil andiron concentration） and four operation parameters in NF process （operating time,pressure, temperature and pH of feed water） on the permeate flux and removalefficiencies of contaminants were investigated in detail. The result reveals thathigher TMP aggravates membrane fouling, resulting in the increase of flux declinerate.0.1MPa is chosen as the optimal TMP. The temperature and pH of feed wateraffect ferric hydrolysate species. In strong acidic solution, the hydrolysis of ironions is inhibited, and the iron removal efficiency is almost zero. Hydrolysis of ironions is accelerated by high temperature and more polymeric species are formed. Thehigh polymeric species with a three-dimensional stereoscopic porous like structurecan sweep down the oil drops to form floccules. These floccules can be easily sweptaway by the hydraulic flush rather than be adsorbed on the membrane surface. Inthis process, the membrane fouling is mitigated. The composite membrane showshigh resistance towards oil and iron concentration. It can be seen that the optimal operating pressure is0.6MPa in NF process. The permeate flux increases with theincrease of temperature and pH of feed water. The NF membrane behaves a goodthermostability, and the removal efficiencies of Ca2+and Mg2+are not affected muchby the feed water temperature. When pH increase, a slight increase of Ca2+, Mg2+concentrations was found and this may be because of the lower positive chargedensity on the membrane surface.To remove cake layer pollution formed by accumulation and deposition of oildroplets and iron hydrolysate, four kinds of chemcial cleaning agents includingsodium hydroxide, ethanol, oxalic acid and hydrochloric acid were used, and thecontaminated UF membrane cleaning method was optimized. The result shows that0.01mol·L-1NaOH solution cleaning combination with30%ethanol solutioncleaning is the best method with over95%flux recovery rate. Actual hightemperature condensed water from refinery can be efficiently treated using UF andNF combined separation process. The turbidity is completely removed and thecontaminants concentrations in NF permeate solution satisfy the Quality Criterion ofWater and Steam for Steam Power Equipment （GB/T12145-2008, China）. The NFpermeate solution can be potentially used as boiler makeup water.Realization of UF and NF combination membrane separation process promisesa great many advantages for refinery high temperature condensed water purificationand recycling, like simple operation, small occupied area, high separation efficiency,low operational costs, energy saving and environmental protection and possibility toachieve automatic control and industrial extension. This study provides an effectivesolution for high temperature condensed water recycling for refinery enterprises.The presented thermostable PPESK/TiO₂composite UF membrane and positivelycharged NF membrane show comprehensive excellent performance, which isexpected to have wide application in other drinking water treatment and wastewatertreatment fields.