Research On Treatment Of Oily Sludge By Supercritical Water Oxidation

The treatment and disposal of oily sludge represent major challenges for petroleum industries and are difficult problems in the world. Several conventional technologies for oily sludge treatment are confronted with many problems. It has been attracted widely attention for developing novel methods for harmless treatment of oily sludge. Supercritical water oxidation is a promising organic wastewater or wastes treatment process which can rapidly and perfectly convert toxic and refractory organics into mineral products of CO2 and H2O and so on. The treatment of oily sludge by means of supercritical water oxidation, catalytic supercritical water oxidation and supercritical water co-oxidation were performed innovatively using H2O2 as oxidant in a batch reactor, focusing on opening up a new way that can be followed for harmless treatment of oily sludge.It is the first time that effectiveness of oxidation of oily sludge in supercritical water was studied. Effect of reaction parameters on supercritical water oxidation of oily sludge was investigated. The analysis of the samples like exhaust gas, liquid effluent, and solid residues was performed. It was found that oil removal rate, TOC conversion, COD removal rate and carbon dioxide yield increase as the reaction time and temperature increase. Oxidation of oily sludge can produce final product of carbon dioxide and intermediate products of carbon monoxide and acetic acid in supercritical water. Intermediate acetic acid yields decreased with the increase of temperature, on the contrary, carbon monoxide yields increased with the increase of temperature. Pressure has no remarkable affect on the reaction. H2O2 is more effective by the direct addition of H2O2 without preheating. The rate of oxidation is independent of oxygen feed concentration above an O2 excess of 217%. COD removal rate increases as the initial COD increase. The highest COD removal rate is reached at pH of 10. Biodegradability of liduid effluents was improved obviously after treatment of oily sludge with supercritical water oxidation. The COD removal rate of 92% can be reached at reaction temperatures of 450℃in 10 min. The liquid effluent is clear and transparent. Gas does not contain other hazardous compounds except for CO of 2.64%. Organic matter content in solid residues is about 0.97wt %. Contents of heavy metals in solid residues are lower than limit values in"Control standards for pollutants in sludges from agricultural use"and third grade indexes in"Environmental quality standard for soils".The kinetics and possible pathways for supercritical water oxidation of oily sludge were studied systematically. The kinetics of supercritical water oxidation of oily sludge can be described by a global power-law rate expression. The results revealed that the reaction orders for oxidant of H2O2 and on COD are 0 and 1.405 respectively. The activation energy and the resulting pre-exponential factor are 213.13±1.33 kJ/mol and 8.99×1014 (mol·L–1) -0.405·s-1 respectively. The possible pathways for supercritical water oxidation of oily sludge were as follow: the oily sludge is decomposed firstly to acetaldehyde, acetone with conjugated double bond and carboxylic acids. The intermediate products of carbon monoxide and acetic acid may have been formed from a series of oxidation of the decomposed matter, then the final products of carbon dioxide and water are produced. Four-lumped kinetics model, comprising of oily sludge, carbon monoxide, acetic acid and carbon dioxide, was established. Six reaction rate constants and the activation energies of the model were determinated on the basis of experimental results. The results showed that the four-lumped kinetics model is reasonable.It was found that three catalysts containing alkali, transitional metal ions and transitional metal oxides could catalyze oxidation of oily sludge in supercritical water. The results showed that addition of Na2CO3 and NaHCO3 is propitious to COD removal from oily sludge in supercritical water oxidation. The addition of NaHCO3 is superior to that of Na2CO3. Optimal concentration of NaHCO3 is 100mg/L. Catalytic activity of NaHCO3 is higher at lower temperature than that at higher temperature. The addition of NaHCO3 appears in inhibition at higher temperature. The addition of NaHCO3 results in a decrease in yields of intermediate products of carbon monoxide and acetic acid, and is propitious to control of pH. It was found that the transitional metal ions catalysts show a significant activity for supercritical water oxidation of oily sludge. The overall order of catalytic activity of transitional metal salts is MnSO4 > Mn(NO3)2 > CuSO4 > Cu(NO3)2 > Fe2(SO4)3 > CoSO4 > Fe(NO3)3 >Ni(NO3)2>NiSO4. COD removal rate increases as the reaction time increases in the presence of Mn2+ and Cu2+. Increase in concentrations of metal ions results in no remakble incresing in catalytic activity when the concentrations of Mn2+ and Cu2+is above 50mg/L. MnO2/γ-Al2O3 and CuO/γ-Al2O3 present an advantage of catalytic activity over bulk phase transitional metal oxides such as V2O5, MnO2, TiO2, CuO, ZrO2 and ZnO. Catalytic activity of CuO-MnO2/γ-Al2O3 is higher due to a synergic action between CuO and MnO2. Optimum Cu:Mn atomic ratio is 1:10. COD removal rates are respectively 99.2% and 99.1% over MnO2/γ-Al2O3 and CuO/γ-Al2O3. Initial pH has an impact on catalytic activity of catalyts. Catalytic activity of the used catalysts decreases significantly.This paper proposed innovatively the idea that oily sludge was treated using supercritical water co-oxidation. The enhancements of labile, reactive species in supercriticai water oxidation of oily sludge were investigated. It was found that the addition of formaldehyde enhances significantly conversion of oily sludge. COD removal rate can reach 99.0% under the best condition.