Hydrogen generation from polyvinyl alcohol-contaminated wastewater by a process of supercritical wat

Available online at www.sciencedirect.comJOURNAL OFENVIRONMENTAL8 ScienceDirectSCIENCESISSN 1001-0742土LEJounal of Environmental Scieoces 19(2007) 1424-1429www.jeuc.c.cnHydrogen generation from polyvinyl alcohol-contaminated wastewater by aprocess of supercritical water gasificationYAN Bo, WEI Chao-hai, HU Cheng sheng, XIE Cheng, WU Jun-zhangColege of Environmental Science and Engineering, South China University of Teclnologs Guangdong 510640. China. E-mail: yanbo2007@ 163.comRecered 10 February 2007; revised 28 March 2007; accepted 4 April 2007Abstractreactor at 723- 873 K, 20- 36 MPa and residence time of 20- 60 s. The gas and liquid products were analyzed by GC/TCD, and TOCanalyzer. The main gas products were H2, CH, co and CO2. Pressure change had no significant infuence o gasification eficiency.Higher termperature and longer residence time enhanced gasification eficiency, and lower temperature favored the production of H2.The efcts of KOH catalyst on gas product composition were studied, and gasifcation eficiency were analyzed. The TOC removaleficieacy (Rroc), catbon gasifcation ratio (Rca) and hydrogen gasifcation ratio (RHo) were up to 96.00%, 95.92% and 126.40% at 873K and 60 s, respectively, which suggests PVA can be completely gasifiod in scw. The results indicate supercritical water gasificationfor hydrogen generation is a promising process for the treatment of PVA wastewater.Key words: bydrogen generation; supercritical water; gasification; polyvinyl alcohol wastewaterIntroductionnatural biomass such as lignin (Osada et al, 2004, 2006),cellulose (Minowa and Ogi, 1998; Sasaki et al., 2004),Supercritical water (SCW) is water at a temaperaturecornstarch (Antal et al, 2000), clover gTass (D'Jesis etand pressure higher tan its critical temperature (647.2 K)al, 2005;) in SCW has been investigated extensively. Theand pressure (22.1 MPa). It has characteristics different main reactions in the gasification of organic compoundsfrom either water in normal condition or steam within SCW include steam reforming (Reaction (1)), water-respect to density, dielectric constant, ion product, vis-gas shift reaction (Reaction (2)) and methanation reactioncosity, diffusivity, electric conductance, and solvent ability (Reactions (3) and (4)) (Boukis et al, 2003; Minowa(Savage, 1999). It is also known that SCW is miscible withInoue, 1999; Tang and Kitagawa, 2005).most of the organic compounds, and rapidly homogeneousCH2Om +(1 -m)H2O- +(n/2+1- m)H2+CO (1)organic chemical reactions are possible in SCW (Siskinand Katritzky, 1991; Poliakoff and King, 2001).CO+H2O -→CO2+H2AH298=-41 kJ/mol (2)With tbe rapid development of economy and technology,Co + 3H2→CH4+H2O0H298 = - 211 kJ/mol (3)more and more compounds are produced and emitted intoenvironment as industrial waste. At the same time, theCO2+4H2→CH4+2H2O0H298 = -223 kJ/mol (4)industrialization due to the depletion of fossil fuel and en-Alkali catalyst is one of the effective catalysts for thevironmental pllution during its combustion, which makes gasifcation of organic compounds and can promote theH2 a atractive altemative energy source (Cortright et al,water-gas shift reaction (Reaction (2)) to a greater extent2002). Some researchers (Knuse et al, 2005; Calzavara Elliot et al, 1983, 1986), so the molar fraction ofCO canet al, 2005; Matsumura et al, 2005) have suggested thatdecrease to zero and that H2 increases significantly. At theSCW gasification (SCWG) is a promising technology forsame time, gasifcation eficiency of organic compoundsconverting organic compounds to H2 due to the specialcan be enbanced greatly.characteristics of SCW. In the past few years, H2 genera-Polyvinyl alcohol (PVA, (CH2CH(OHI)n) is widely usedtion from simple molecules like glucose (Yu et al,1993;n the sizing process of the textile industry and paperLee et al, 2002; Hao et al, 2003; Sinag et al, 2004)coati_ood fexibili. highand methanol (Feng et al, 2003; Boukis et al, 2003) or therm中国煤化Isolubility and goodfilm-f, 2003). The globalProject supported by the National Natural Scieace Foundation of ChinaproduMHCNMH G va,. and toe lage(No, 20277010) and the Hi-Tech Research and Development Programamount of discharged PVA from industrial effuents has(863) of China No.2006AA062378). *Corresponding authotE-mail: ccbwei@ scu1.cdw.cn.caused significant environmental polution (Zhang andNo.12Hydrogen generation from polyvinyl alcobol-contaminated wastewater by a process of 8upercritical water gasificatioo1425Yu, 2004). The study of PVA wastewater treatment was a backpressure regulator in this system.focused in some oxidation processes (AOP) such as wet airPVA (A.R.) dissolved in de-ionized water was used tooxidation (WAO), Fenton oxidation and photo degradation simulate PVA-contaminated wastewater. In this study, the(Won et al, 2001; Giroto et al, 2006). These methods PVA concentration was 2 g/L. The homogenous catalystseither convert the pollutants to CO2 and H2O directly or were prepared by dissolving KOH (A.R.) into de-ionizedfunction as a pretreatment followed by a biotreatment.water.Generally, the direct decomposition is very expensive.1.2 Chemical analysisHowever, gasification of PVA in scw apparently bas notbeen reported.The gas products and total organic carbon (TOC) anal-In this study, the possibility of bydrogen energy obtained ysis of liquid efluents were described in the previousrom wastewater treatment was elucidated, as a model literature (Wei et al, 2006).macromolecule polymer organic compound, the charac-13 Terms and definitionsteristics of using SCwG for hydrogen generation fromdilute solution of PVA in water were investigated. The rela-Three parameters, TOC removal eficiency (Rroc), car-tionship of reaction pressure, temperature, residence time, bon gasification ratio (Rca) and hydrogen gasificationKOH catalyst with the gas molar fraction and gasification ratio (RHc), shall be discussed as the measure of organiceficiency was studied.compound destruction and gasfcation efficiency in theSCwG of PVA. The Rroc, Rca and RHG are defined as1 ExperimentalEqs.(1), (2) and (3).1.1 Apparatus and reagentsRroc =Mroc- MTOCmu x 100%(1MroceExperiments were performed in a continuous flow re-ctor. As shown in Fig.1, the apparatus includes feedwhere, Mroca is TOC of feed solution, Mroc e is TOC ofsystem, prebeater, reactor, condenser, gas-liquid separatoroutflow.and backpressure regulator. The preheater is a stainlesssteel coil tube (O.D. 8 mm, ID.4 mm, length 10 m).Rcc= CrO x 100%(2CocThe reactor is a stainless steel column (O.D. 80 mm,LD.30 mm, length 350 mm) and has a volume capacityRHG =Hrcx100%(3)of 250 ml. The gas-liquid scparator is a stainless steelHoccolumn with volume capacity of 1 L. The temperature and wbere, Cpe is the carbon anount in the gas product;Coc ispressure are cotrolled by four K-type thermocouples and carbon amount in organic componds, Hpa is the hydrogenDe-gas台epuredst .又多Iz|VV4|vwatar tamks FurmaceHigh pressare pumpD正Wet gas fiowmeterFurnaceV6Water outBack pressure regulatedGa-liquid B8好中甲中sparatorCondenserWater in中国煤化工盲Rupture diskLiquid