Supercritical gasification for the treatment of o-cresol wastewater

ISSN 1001- 0742Journal of Enironmental Sciences Vol. 18, No. 4, pp. 644- 649, 2006CNII- 2629/XCO9AArticle ID: 1001-0742006)04-0644-06CLC number: X703 Document code: ASupercritical gasification for the treatment of o-cresol wastewaterWEI Chao-hai, HU Cheng-sheng, WU Chao-fei, YAN Bo(College of Environmental Science and Technology, South China University of Technology, Guangzhou 510640, China. E-mail: cechwei@scut.edu.cnAbstract: The supercritical water gasification of phenolic wastewater without oxidant was performed to degrade pollutants andproduce hydrogen-enriched gases. The simulated o-cresol wastewater was gasified at 440- 650 and 27.6 MPa in a continuousInconel 625 reactor with the residence time of 0.42- 1.25 min. The influence of the reaction temperature, residence time, pressure,catalyst, oxidant and the pollutant concentration on the gasification eficiency was investigated. Higher temperature and longerresidence time enhanced the o-cresol gasification. The TOC removal rate and bhydrogen gasification rate were 90.6% and 194.6%，respectively, at the temperature of 650C and the residence time of 0.83 min. The product gas was mainly composed of H, CO, CHand CO, among which the total molar percentage of H, and CH4 was higher than 50%%. The gasification fficiency decreased with thepollutant concentration increasing. Both the catalyst and oxidant could accelerate the hydrocarbon gasification at a lower reactiontemperature, in which the catalyst promoted H2 production and the oxidant enhanced CO2 generation. The intermediates of liquideffluents were analyzed and phenol was found to be the main composition. The esults indicate that the supercritical gasification is apromising way for the treatment of hazardous organic wastewater.Keywords: supercritical water; gasification; o-cresol; hydrogen-enriched gas; wastewater treatmentelectricity is required to supply high-pressure air.IntroductionWith the decreasing of fossil fuels, new ways ofPhenolic compounds are considered to bebiomass energy utilization have been studied all overhazardous waste. They are water soluble and usuallythe world in the last several years. Many researcherscontained in the wastewater of many industries, suchhave investigated the gasification of biomass t(as coke ovens, petroleum refineries, petrochemicalproduce hydrogen in the supercritical water conditionplants, phenolic resin plants, pharmaceutical plants,and a high gasification efficiency is achieved (Antal etand dyeing plants. The concentration of phenolical, 2000; Taylor et al, 2003; Yoshida et al, 2004). Incompounds in the wastewater is too low to behe supercritical water gasification， the reactionrecovered with general process. However, they aregenerally takes place at a temperature over 600C andhigh enough to severely cut down the effeciency of thea pressure above the critical point of water. When theconventional biological technology. Therefore,temperature is higher than 600C, water becomes aadvanced oxidation technologies prevail in thestrong oxidant, and ioxygen atoms can bphenolic wastewater treatment (Santos et al., 2005;transferred to the carbon atoms of hydrocarbons. As aZen et al, 2003).result, the carbon is oxidized into CO or CO2 And theSupercritical water oxidation(SCWO) is ahydrogen atoms of the water and the hydrocarbons arepromising technology in treating the wastewaterreleased as H2 and CH4. During the process, thecontaining hazardous or refractory organic com-organic carbons can be oxidized and converted intopounds. Its high efficiency in decomposing the stablegas without an oxidant. Since the product gas isorganic compound with the aromatic structure hashydrogen-enriched, a part of it can be burnt toatracted many researchers(Gopalan and Savage,maintain the high reaction temperature, and the1994; Martino and Savage, 1997a; Matsumura el al.,remainder may be served as a fuel used in fuel cells or2000; Perez et al., 2004). Phenolic compounds areother facilities (Calzavara et al, 2005). Therefore,used widely as the model pollutants in theirdeveloping a wastewater trcatment process which isresearches. The degradation in the SCWO is quickersimilar with the supercritical gasification of biomass,and more complete than other advanced oxidationand can degrade hazardous pollutants completely andtechnologies. However, the SCWO process requiresproduce hydrogen- enriched gases simultaneously, ishigher temperature and pressure (critical point: T=attractive. It can partially overcome the shortages of3749, P=22.1 MPa) which limits its wide applica-the SCwO and improve the application of thetion. A large amount of energy is needed to heat thesupercritical fluid technology in water treatment area.feed stream to the supercritical temperature. TheIn this study, o-cresol was chosen as the modelenergy may be compensated bycombusting the:- _he gasification oforganic compounds in the wastewater if theirphend中国煤化工water. The eft，concentration is high enough. As a result, moreof opl:YHC N M H Gure, time, pressureFoundation item: The National Natural Science Foundation of China (No. 20277010); *Corresponding authorNo.4Supercritical gasification for the treatment of o-cresol wastewater645and concentration) on the gasification efficiency waspressure was controlled by a back-pressure regulatorinvestigated. To moderate the reaction conditions andwhich operated from 0 to 34.4 MPa. After the pressureincrease the conversion efficiency, a small amount ofreduced, the two-phase effluent was separated in aoxidant and some homogeneous catalysts such asgas-liquid separator. The liquid and gas samples werealkali metal salts and transition metal salts were used.collected and analyzed after at least 60 min till theThe intermediates in the liquid effluents were alsosystem was stable.analyzed to understand the pathway of o-cresol1.3 Chemical analysisdegradation.The total organic carbon (TOC) concentration ofthe liquid stream was measured with a 1020A TOC1 Materials and methodsanalyzer (OI Analytical, USA). The composition of1.1 Materialsthe product gas was analyzed with a 6820 gas chroma-o-Cresol (99 % purity) was dissolved in ditilldtography (GC, Agilent Technology, USA.) containingwater to produce the simulated phenolic wastewater.a C-2000 column. The column was equipped with aA 27.5% wt solution of hydrogen peroxide was usedthermal conductivity detector that was calibrated withas the oxidant, and it was diluted to the desiredcalibrating gases. The intermediates of liquid effluentsconcentration assuming that one mole of hydrogenwere analyzed with a QP 2010 GC-MS (Shimadzu,peroxide provides half mole of oxygen. TheJapan) equipped with .a HP-5 column (30 m X 0.25homogenous catalysts were prepared by solvingmm X 0.25 μm).analytical reagents into distilled water.2 Results and discussion1.2 Apparatus and procedureThe simulated wastewater was gasified in aAll the experiments were repeated three times.continuous plug-flow reactor with the flow rate up toOnly four components (H2, CO2, CO and CH4) are6 Lh. The following experimental conditions wereconsidered in the product gases. In this paper,realizable in this plant. The maximal pressure andhydrogen gasification rate and TOC removal rate weretemperature were 31.0 MPa (4500 psi) and 650C,used to estimate the gasification efficiency. They arerespectively, and the residence time ranged from 0 tocalculated according to the following equations:5 min. The centerpiece of the plant was an electricallyheated 250 ml reactor made of Inconel 625. TheTHoRHosketch of the plant is shown in Fig.1.where ThxoR is the hydrogen gasification rate; Hg is thehydrogen amount in gas product; Hog is the hydrogenamount in initial organic material.TRR =TOC, - TOCetTOC。L°where, TIRR is the TOC removal rate; TOC。is the TOCin initial wastewater; TOCer is the TOC in liquidToGC←"Qrceffluent.2.1 Infuence of operation parameters on gasi-fication efficiency8示To investigate the influence of the reactiontemperature, the experiments of gasifying 0.02 mol/LLiquid sampleo-cresol solution were carried out under thFig. 1 Sketch of supercritical water gasification equipmentconditions: the pressure of 27.6 MPa, the residence1. ditilled water tank; 2. wastewater tank; 3. oxidant or catalystssolution tank; 4. high pressure pump; 5. preheater; 6. gasificationtime of 0.83 min and the temperatures of 440, 480,reactor; 7. condensator, 8. back pressure regulator; 9. separator;520, 560, 600 and 6509C, respectively. As shown in10. wet gas flowmeterFig.2, the THoR and TR increase rapidly with theThe procedure of the experiment was as thetemperature increasing from 440C to 650C. Whilefollows. The o-cresol solution was firstly pumped intothe THOR exceeds 100% at the temperatures abovethe pre-heater and heated up to the temperature near5609C, which indicates that the hydrogen atoms fromthe critical point. Then the fluid was mixed with thewater are released as H2 in the product gases.oxidant or the catalysts (if needed) before going to theMore中国煤化工atures are changed.product gases isreactor. In the reactor, the mixture was rapidly heateddifferup to the desired temperature and the gasificationWithC N M H Gthe contents of H2,reaction took place. The fluid was then cooled downCO2, and CH4 increase while that of CO decreases.in a condenser after leaving the reactor. The systemThat is because a water-gas shift reaction happens and646WEI Chao-hai et al.Vol.18it can be improved at a higher temperature (Sato et al,are not much different. It suggests that the pressure of2004). The equation of the water gas shift reaction is24 MPa is enough for supercritical water gasificationshown as follows:reaction. However, the influence of higher pressure onCO+H2O= H2+CO2the gasification efficiency can not be investigated inthe present for the limit of our apparatus.At the temperature of 6509C ，the molarpercentage of H2 and CO2 is 48.8% and 41.1% ，Table 1 Gasifcation eficiency at different pressuresrespectively, while that of CO is less than 3%.Dry gas compositin, %Pressure, MPam%10%CO OCH图CO2目H2▼- TRR -●- HGRCOCHCO2H50024.34:4..40.3 48.776.9 .156.46027.63.5.41.4 48.378.2162.6亏3(2031.02.68.40.546.978.7170.18 20Notes: Conditions: 600C, 0.83 min and 0.02 molLi 102.2 Influence of o-cresol concentration on gasi-fication efficiency440 480 520 560 600 650T. Che pollutant concentration is an importantparameter in the wastewater treatment process. ToFig.2 Gasification eficiency at dfferent reaction termperaturesConditions: 27.6 MPa, 0.83 min and 0.02 mol/Lfind out the influence of pollutant concentration on thegasification efficiency, experiments were carried outThe influence of residence time on theat the 0- cresol concentration of 0.01, 0.02, 0.04, 0.08,gasification efficiency is shown in Fig.3. The 0.020.12, and 0.20 molL, respectively. As shown in Fig.4,molL o-cresol solution was gasified at thethe ThkR and IRR decreased with the increasing of thetemperature of 600C and the pressure of 27.6 MPa.concentration. The reason may be that the wall ofWhen the residence time increased from 0.42 min toInconel 625 reactor acts as a heterogenous catalyst.1.25 min, the ThHR and TRR went up. While the contentsDifferent results were obtained when the supercriticalof H2 and CO2 slightly increased firstly and thengasification reaction was carried out in the reactorsdecreased. It may be that the reaction of COmade of different materials (Yuet al., 1993). In ourconverting to H2 and CO2 is dominant at a shortprevious work, more CH4 was produced and its molarresidence time. While, as the residence time gettingpercentage was more than 20% when o-Cresol waslonger，the amount of H2 consumed by thegasified in a 316 SS reactor. When o-cresolmethanation reaction is more than its output andconcentration was high, the wall of the reactor wastherefore its content decreases. The equation of theentirety occupied by the adsorbed o-cresol moleculesmethanation reaction is shown as follows:which prevent other molecules from reaching thCO +3H= CH4+ H,O(2)surface. Thus the degradation rate slowed down.CO2 + 4H2一CH4+ 2H2O(3)Basing on the analysis above, we can reach theTo investigate the influence of pressure,conclusion that increasing the residence time caexperiments were carried out at 24.3, 27.6 and 31.0improve the gasification; the experimental results alsoMPa, respectively. As shown in Table 1, the resultsprove the conclusion (Fig.3). Besides, the catalystsmay increase the TmR and THoR at a high organicconcentration. Because the catalyst can supply moreCo口CH, 8CO2 目H --TRR -●- HGR50active centers which can accelerate the degradation80rate of organic compounds. The efficacy of thecatalyst will be further discussed in Section 2.4. On160the other hand, the gasification efficiency reduction at3040;a higher concentration arises from the decreasing of2(120water and carbon ratio. The equilibrium of the100water- gas shift reaction is affected by the content of占10water (Rice et al, 1998). In Reaction (1), decreasingthe amount of water mav, shif the equilibrium to the0.420.670.831.25left中国煤化工，and CO, It is seen .Residence time, minin FMHCNMHGCO2 decrease whileliias withthe o-cresolFig.3 Gasifcation fficiency at diferent residence timeConditions: 600C, 27.6 MPa and 0.02 mol/Lconcentration growing.No.4Supercritical gasification for the treatment of o-cresol wastewater6472.4 Influence of catalysts on gasification efMi-CO口CH4 CO2目H2 -▼- TRR●HGR50180ciency160Compared with the supercritical water oxidationprocess, the supercritical gasification reaction needs a- 14higher temperature that makes it less practicable.30- 12Many works show that some catalysts can improve the20| 100water-gas shift reaction and accelerate the gasificationrate (Eillott and Sealock, 1983; Xu et al., 1996;启10Minowa et al., 1998; Watanabe et al., 2002). Thus,proper catalysts may reduce the gasification tempe-10rature. In this work, some transition mental nitratesCresol concentration, mol/Land alkali salts were chosen as the homogenousFig.4 Gasification eficiency at dfferent o-cresol concentrationscatalysts to study their effect on the gasificationConditions: 600C, 27.6 MPa and 0.83 mineffciency at a lower temperature. The concentrationof the o-cresol solution was 0.02 mol/L and the2.3 Influence of oxidant on gasification effi-reaction temperature was 520C. It is shown in Table 2that Na2CO3 and KOH lead to a drop of CO content inIn the Scwo process, the organic compound canthe gas phase and correspondingly an increase of H2be completely converted to CO2 and H2O at 400and CO2 content. The reason is that the water-gas shift500C within one minute with excessive oxidantreaction is improved in these cases. When the(Perez et al., 2004). Therefore, the oxidant can greatlyconcentration of KOH is up to 10 mmolL, the productincrease the conversion rate of the organic carbon at agas contains 54.3% (molar) H2 and a trace of CO. Andlower temperature. Experiments were carried out tothe rmR and nxcR is 70.7% and 175.8%, respectively.gasify 0.02 mol/L o-cresol solution with the oxidant atThe results are similar with those obtained in thedifferent oxygen equivalent ratio (0, 0.15, 0.36 andgasification reaction at 600C without a catalyst. It0.54 ) at 520C (Fig.5). With the oxygen equivalentindicates that higher gasification efficiency isratio increasing, the TInRR increased, while the rHcR didattainable in a more moderate condition with effectivenot go up and a decrease happened when the oxygencatalysts. For the transition metal salts, Mn(NO3)2 andequivalent rate was 0.54. The main composition of theCo(NO3)2 can improve the TOC removal but do notdry product gas is CO2 when the oxidant is added.affect the composition of the product gases. TheThat is because when the oxidant concentration isbehavior of Ni(NO3)2 is similar to KOH, but its effectlower, the organic carbon is preferentially oxidizedis weaker. Usually, it is regarded that nickel caninto CO. However, it is easier for the organic carboncatalyse the formation of CH in a hydrogenationto be oxidized into CO2 when the oxidantprocess (Gadhe and Gupta, 2005). But theconcentration is higher. Therefore, the content of COexperimental results show that CH content does notslightly goes up firstly and then decreases with theincrease while H2 and CO2 content go up whenoxygen equivalent ratio increasing. Since theNiNO)2 is added. Therefore, the increase of the gasi-concentration of CO2 increases while that of COfication efficiency is via the improvement of water-gasdecreases, the equilibrium of the water-gas reactionshift reaction with Ni(NO3)h catalysis.shifts to the left and the content of H2 drops rapidly. ItTable 2 Experimental results with different catalystscan be concluded that oxidant may raise the「RR, butnot the TIkR.CatalystsMe"t,Dry gas compositin, %Tro,% noa, %mmol/L co CH。 CO2 H■co口CH4图CO2 目H2 -▼- TRR -●- HGRNo catalyst.9.5 4.3 38.9 45.3 34.4 61.600F80Fe(NO)s0.4.6 39.9 46.2 37.6 69.0? 6070Cu(NO)h3.8 41.1 48.4 38.2 74.160只MnQNO)h0.4 8.4 3.9 403 46.2 4 | .774.9客40Co(NO3)20.4 10.2 5.5 37.8 45.2 43.4 79.83Ni(NO)20.4 2.8 5. I41 49.7 49.5 106.1占1Na.COr 50.51.4110.'KOH中国煤化工s1.52.3 112.70.150.360.54TYHCNMHGs257.2 129Oxygen equivalent ratioFig.5 Gasification fficicncy at different oxygen equivalent ratios10.0 0 4.6 40.1 54.3 70.7 175.8Conditions: 520C, 27.6 MPa, 0.83 min, and 0.02 mol/LNotes: Conditions. 520C, 27.6 MPa, 0.83 min and 0.02 mol/L648WEI Chao-bhai et al.Vol.182.5 Potential conversion pathway discussion0- Cresol |The intermediates of the liquid effluents wereanalyzed with GC-MS to deduce the potential[Dimers ]= =[ Phenol ←- Hyroxybenaldchydeconversion pathway. The experiments were carriedout at the temperatures of 440C, 5209 and 600C ,respectively, and the residence time was 0.42 min.Acids/: aldebydes ]As shown in Table 3， when the reactiontemperature is 440C, the main composition of theGasesliquid effluent is o-hydroxybenzaldehyde, phenol andundegraded o-cresol. At 520C, the intermediate isFig.6 Simplified conversion pathways of o-cresol gasificationmainly composed of phenol and several dimmers,such as dibenzofuran, biophenols and naphthols.degraded into the ring-opening product at higherWhen the temperature is up to 600C, the dimmers aretemperature.decomposed and single -ring aromatics are dominant.3 ConclusionsPhenol is the major composition of all the liquidproducts. Basing on the analysis above and otherIn this study, the supercritical gasification of thereports (Gopalan and Savage, 1994; Martino andwastewater containing o-cresol was investigated. TheSavage, 1997a, 1997b; Matsumura et al., 2000; Sinagexperimental results show that phenolic compounds inet al., 2003), a potential pathway of the o-cresolwastewater can be degraded rapidly and almostgasification is given in Fig.6. At first, o-cresol iscompletely in the supercritical gasification process.degraded in two parallel paths. Its methyl is oxidizedMeanwhile, a hydrogen-enriched gas is produced andnto o-hydroxybenzaldehyde or demethylated intoits main composition is H2, CO2, Co and CH4. Thephenol. Since hydroxybenzaldehyde is unstable in thetotal molar percentage of H2 and CH is higher thansupercritical water, it is converted to phenol or the50%. Increasing the reaction temperature and reside-ring opening product rapidly. And then the phenol isnce time can increase the gasification efficiency.While, the gasification efficiency is reduced when theTable 3 Main organic compounds in the liquid eMuent ofpollutant concentration increases. It is proved thato-cresol supercritical gasificationoxidant may increase the TRRo but decrease the HOB.Peak area ratio, %The TIRR and Thok increase at a lower temperature if thealkali salts catalysts such as Na2CO3 and KOH areCompoundStructure4409C 520C 600Cused. Meanwhile, the content of H2 also goes up. Theincrease of the gasification efficiency is via theimprovement of water-gas shift reaction with catalysts.Phenol13.72 39.85 44.26Compared with the SCwO process, the super-critical gasification process does not need an oxidant,o-Hydroxybenzalde-but its reaction temperature is slightly higher. In our17.65 0.39hydework, it is found that some catalysts can reduce thereaction temperature to a certain extent. Therefore,o-Cresol42.50 6.87more attention should be paid on developing bettercatalysts, which can greatly increase the gasificationIndanone.43 3.51efficiency at lower temperatures and higher hydro-carbon concentrations.References:1,I'- Biophenol2.69 5.94Antal Jr M J, Allen S G, Schulman D et al, 2000. Biomass gasificationin supereritical water[J].nd Eng Chem Res,339: 4040- 4053.3,3'-Biophenol<>-aom 1.83 3.63Calzavara Y, Joussot-Dubien C, Boissnnet G et al, 2005. Evaluationof biomass gasification in supercritical water process forhydrogen production[J]. 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