Energy &Fuels 1994,8, 1126-1130
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Feedstream Preheating Effect on Supercritical Water Oxidation of Dissolved Organics Lilin Li and Nosa 0. Egiebor" Department of Mining, Metallurgical and Petroleum Engineering, University of Alberta, Edmonton, Canada T6G 2G6 Received January 31, 1994. Revised Manuscript Received May 4, 1994@
Supercritical and subcritical water oxidation of dissolved phenol and hexanol were carried out at temperatures of 315-426 "C under a pressure of 24.2 MPa in a tubular reactor. The effects of slow and fast preheating of the feed wastewater, and the concentration of dissolved phenol on the oxidation efficiencies, were studied. The results showed that with slow preheating, the destruction efficiencies were significantly lower than with fast preheating for both phenol and hexanol oxidation. Phenol was also observed to be more resistant to oxidation at supercritical water reaction conditions than hexanol. Higher concentrations of phenol in the wastewater sample results in higher destruction efficiencies, thus indicating variable oxidation kinetics which is concentration dependent.
Introduction Supercritical water oxidation (SCWO) is a promising oxidation process for the removal of dissolved organic contaminants from industrial wastewater. Above its critical temperature and pressure, water functions like a dense fluid with special properties such as high solubility of organics, complete miscibility with permanent gases, high diffusivity, low viscosity, and low solubility of inorganics. These properties enable the oxidation reactions of organics to proceed a t a high efficiency due to intimate contact between the organics and the oxidant. The resulting oxidation reactions can convert toxic organics t o harmless, simple compounds like C02 and HzO. Inorganics, if present in the wastewater, are precipitated due to the reduced solubility of salts under supercritical water conditions. The advantages of high efficiency in the destruction of refractory organic compounds and subsequent precipitation of inorganic salts from wastewater distinguishes supercritical water oxidation from the conventional wet oxidation which is generally unsatisfactory in treating some organic compounds. Growing interest in the application of this technology is evidenced by the significant number of research publications in recent years. Timberlake et a1.l designed and studied a SCWO system to treat spacecraft wastes, Tongdhamachart2has applied the method to the destruction of anaerobically digested municipal sludge, Rofer3 recently reported on the U.S. Department of Energy (DOE) and Air Force's programs in the treatment of DOE mixed wastes, propellants, and associated wastes using supercritical water oxidation technology. In these studies, the wastewater together with added oxidants were pressurized and heated to supercritical water conditions (22 MPa, 374 "C). The oxidation reactions were rapid, and the
dissolved organics were oxidized to simple compounds while the inorganics precipitated at the bottom of the reactor or were separated in a salts separator. Barner et al.4 recently published a comprehensive review of supercritical water oxidation technology. Due to the promise of this technology for wastewater treatment, fundamental studies are ongoing to understand process kinetics, fluid properties, and reactor design. For example, Lee and Gloyna5 have studied the supercritical water oxidation of acetic acid and 2,4-dichlorophenol using both hydrogen peroxide and oxygen as oxidants. They observed that higher destruction rates were achieved for 2,4-dichlorophenol than for acetic acid even at lower temperatures and shorter residence time with both hydrogen peroxide and oxygen. Subsequent studies by the same authors6 focused on the development of acetamide destruction kinetics in supercritical water. The results showed that, in the presence of hydrogen peroxide, acetamide undergoes both oxidation and hydrolysis reactions. The hydrolysis and overall (hydrolysis oxidation) reactions could be described by firstorder kinetics with respect to the acetamide concentration, but the hydrolysis reactions were reported to proceed more rapidly than the oxidation reaction. Helling and Tester7 and Webley and Testers studied the oxidation kinetics of carbon monoxide and methane in supercritical water. The overall disappearance of carbon monoxide was interpreted by two pathways: the oxidation of carbon monoxide, and the water gas shift reaction. Global rate expressions were applied to both pathways, and the predicted reaction order was first order for the oxidation of carbon and 0.5 order for the water gas shift reaction. However, the kinetic models failed t o predict the reaction rates, and this inconsis-
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* Author for correspondence. Abstract published in Aduance ACS Abstracts, July 1, 1994. (1)Timberlake, S. H.; Hong, G. T.; Simson, M.; Modell, M. SAE Tech. Pap. Ser. 1982,No.820872. (2) Tongdhamachart, C. Ph.D. Thesis, Department of Civil Engineering, The University of Texas a t Austin, i990. (3) Rofer, C. K. Waste Management 1991,No.1, 931. Ix
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(4) Barner, H. E.; Huang, C. Y.; Johnson, T.; Jacobs, G.; Martch,
M.A. J . Hazardous Mater.-1992, 3 1 , 1.
( 5 ) Lee, D-S.; Gloyna, E. F. J . Supercrit. Fluids 1990,3 , 249. ( 6 ) Lee, D-S.; Gloyna, E. F. Enuiron. Sci. Technol. 1992, 26, 1587. (7)Helling, R. K.; Tester, J. W. Energy Fuels 1987,1 , 417. ( 8 )Webley, P. A.; Tester, J. W. Energy Fuels 1991,5 , 411.
0 1994 American Chemical Society
Supercritical Water Oxidation of Dissolved Organics tency was attributed to the "cage effect" of supercritical water.7 In modeling methane oxidation in supercritical water, the same authors analyzed several key reactions leading to methane conversion but concluded that the rate of methane oxidation cannot be predicted accurately from elementary reaction models due to the uncertainty of other possible but undefined physical or chemical effects of supercritical water on the reactions.8 Preliminary studies in our laboratory have revealed that preheating of the feed water has a significant effect on the oxidation efficiency of the dissolved organics irrespective of the type of oxidant used. Similar studies by Webley and Testerg on the oxidation of methanol in supercritical water also showed that some pyrolysis of methanol occurred in the preheater tubing but that such pyrolysis was relatively negligible. However, while preheater pyrolysis may be negligible for relatively dilute solutions of organics in water, it could be significant at higher concentrations. Such pyrolysis reactions in the preheater may lead to the formation of other refractory organic products before oxidation reactions take place in the reactor proper. Consequently, the object of this communication was to investigate the effect of fast versus slow preheating of supercritical water oxidation feedstreams on the subsequent oxidation efficiency. Phenol and hexanol were chosen as the organic contaminants because phenol is a common aromatic organic pollutant in industrial wastewater. Hexanol was used as a nonaromatic straight-chain analogue for comparison. A secondary objective was to investigate the effect of phenol concentration on its oxidation efficiency under supercritical water conditions.
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Experimental Section Phenol from Anachemia and hexanol from BDH were used as received without further purification. The reagents were dissolved in distilled water to a total organic carbon (TOC) concentration of 3700 ppm for both phenol and hexanol, and the solutions were used as the feedstock for the study. The and 3.6 corresponding molar concentrations were 3.9 x x mom for phenol and hexanol, respectively. Air was used as the oxidant. Figure 1 shows a schematic diagram of the supercriticalwater oxidation reactor. Air was compressed by means of a compressor which drew air from a cylinder. The air pressure was controlled by a pressure regulator and the flow adjusted by a metering valve. The liquid feed was contained in a reservoir and pumped by an Eldex high-pressure metering pump. Air and feed were introduced into the reactor from separate lines. The air supply was preheated in a lls-in. stainless steel tubing before entering the reactor. The liquid feed was also preheated using either a fast preheater in a short (21cm) lI4in. 0.d. tubing line with minimum preheating time, or through a slow preheater in an extended (92 cm) lI4 in. 0.d. coil for an extended period of preheating time. The fast preheating rate was about 650 "CI min for 0.65 min, while the slow preheating was done at about 152 "Clmin for 2.8 min. Feed flow rates between 3.6 and 8.8 mumin, and air flow rates of 100-150 mumin were used. At these flow rates, oxygen was always in excess. Oxidation experiments were carried out at temperatures between 315 and 425 "C under a pressure of 24.2 MPa. Air and liquid feed were mixed at a Tee connector at the bottom of the reactor where a J-type thermocouple was (9) Webley, P. A.; Tester, J. W. Supercritical Fluid Science and Technology;ACS Symposium Series 406;American Chemical Society; Washington, DC, 1989, p 259.
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Figure 2. Effect of temperature and retention time on hexanol oxidation efficiency with rapid preheating at 3700 ppm TOC concentration. positioned and connected to a temperature controller. The temperature of the reactor furnace and preheater were controlled such that the predetermined reaction temperature was attained by the feed mixture at the Tee connector just before entering the reactor. The reactor consisted of l12 in. o.d., 12 cm long 316 stainless steel tubing. The reaction products were released from a side tubing on top of the reactor which was connected to the condenser. The product gases were released from the condenser, and the liquid was collected. All liquids were analyzed for total organic carbon using a Shimadzu Model 5050 TOC analyzer. The destruction efficiency of the organics was estimated from the following equation: destruction efficiency (%) = TOC in feed (ppm) - TOC in product (ppm) TOC in feed (ppm)
Results and Discussion Figures 2 and 3 present the effect of reactor retention or residence time on the organic destruction efficiency for hexanol and phenol respectively, at the indicated reaction temperatures. These runs were done with the fast preheating regime described earlier. At 315 "C,
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which is below the critical temperature of water, the destruction efficiencies are relatively low but comparable for both hexanol and phenol oxidation for retention times of 3.3 min and less. When the temperature was increased to near-critical (370 "C) or supercritical (426 "C) temperatures, the oxidation for hexanol is almost complete within 1.3 min with oxidation values of 99.8%, whereas for phenol the oxidation values at 1.3 min residence time are 76 and 95% for 370 and 426 "C, respectively. At 3 min retention time, the phenol oxidation values for 370 and 426 "C increases to 92 and 99.6%, respectively. All oxidation efficiency values reported here have experimental errors in the range of 33%. Since the starting TOC concentration for the two reactants were the same, the results indicate a significant difference in the oxidation reactivity of hexanol and phenol under supercritical or near-critical water conditions. This lower reactivity of phenol relative to hexanol can be attributed to the aromatic benzene ring in the phenol molecule. This phenomenon can be related to the mechanisms of the oxidation of organics in subcritical or supercritical water. One of the acceptable proposed mechanisms for the oxidation reactions is the freeradical reaction mechanism which involves the initial abstraction of hydrogen by oxygen to form hydroxyl radical, followed by subsequent propagation steps involving complete oxidation of organics to carbon dioxide and water.1° At subcritical temperatures, the comparable low reduction efficiencies for both hexanol and phenol are probably due to a mass-transfer limitation between the gas and liquid phases. When the temperature increases t o near-critical or supercritical conditions, the oxygen, organics, and water are completely mixed, and hence the reaction becomes chemically controlled. If the oxidation of organics involves the free-radical mechanism discussed above, the abstraction of hydrogen from the aromatic benzene ring, and the subsequent opening of the ring in phenol is chemically more resistant, and hence phenol destruction efficiency will be lower than that of hexanol. Irrespective of the operating mecha-
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(10)Li, L.;Chen, P.;Gloyna, E. F.MChE J. 1991, 37, 1687.
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Figure 5. Effect of temperature and retention time on phenol oxidation with slow preheating at 3700 ppm TOC concentration. nism, the complete oxidation of phenol will require the opening of the aromatic ring in phenol which should invariably provide additional oxidative refractoriness in comparison to hexanol. In the experimental data presented in Figures 2 and 3, the feed was preheated rapidly to near the reaction temperature at a heating rate of about 650 "Clmin and preheating retention time of 0.65 min, before entering the reactor. At the higher preheating temperatures, phenol is likely more susceptible to pyrolysis and polymerization reactions which may lead to the formation of secondary refractory organic products that are more difficult to oxidize. This phenomenon may be partly responsible for the lower reactivity of phenol toward supercritical water oxidation compared to hexanol. If this preheater pyrolysis effect is significant, a slow preheating of the feed should show a bigger difference between phenol and hexanol oxidation efficiencies. In the experiments leading to the data presented in Figures 4 and 5,a slow preheating regime was used t o perform the same experiments as in Figures 2 and 3 for hexanol and phenol oxidations. The slow preheater-
Supercritical Water Oxidation of Dissolved Organics retention time was about 3 min with a heating rate of 152 "C/min. It can be seen in Figure 4 that the hexanol reduction efficiencies a t reaction temperatures of 315, 370, and 426 "C are generally lower than those observed when fast preheating was employed in Figure 2. This suggests that extended preheating has a deleterious effect on oxidation efficiency, due to the formation of secondary pyrolysis and polymerization byproducts. For phenol oxidation as shown in Figure 5, the reduction eficiencies are also much lower with slow preheating than when fast preheating was used in Figure 3. Furthermore, the reduction efficiency for phenol at 315 "C is now observed to be slightly higher than at 370 "C at all retention times studied. The reason for this is that, at 315 "C, the preheating temperature is lower than the thermal pyrolysis and decomposition temperature of phenol, and hence only a limited pyrolysis can occur, whereas, at 370 "C, the preheating temperature is above the decomposition temperature of phenol and hence significant pyrolysis and secondary polymerization may occur in the preheater before oxidation occurs in the reactor. However, when the temperature was increased to the supercritical regime at 426 "C, the oxidation efficiency increased significantly. This is attributable to the increased oxidation power under supercritical conditions, leading to a higher oxidation efficiency, despite the deleterious pyrolysis reaction in the preheater. In general, the significantly lower oxidation efficiencies for both phenol and hexanol during slow preheating show that thermal pyrolysis of the organics is significant during preheating of organics to slightly below the supercritical water reaction temperatures. In the absence of oxidant in the preheater, the free-radical organic pyrolysis intermediates are subject to coking and polymerization to form products which are more difficult to oxidize. It was observed that the treated water samples from the slow preheating experiments contained small amounts of fine dark-colored carbonaceous char suspensions which where generally absent in the treated samples from the fast preheating runs. Although these suspensions were not analyzed, they were recovered and observed on fine filter paper. This observation is consistent with the formation of coke-like condensation products arising from the pyrolysis of the feed organics. Other investigatorsll have also reported that, in the absence of redox reactants, fast preheating could prevent the coking and polymerization of organic pyrolysis products from coal during liquefaction. The pyrolysis and hydrolysis of organics in supercritical water have also been reported by Helling and Tester12 in their test of possible pyrolysis or hydrolysis reactions of ethanol in supercritical water. They reported that the conversion of ethanol by pyrolysis and hydrolysis accounted for about 18%of the oxidation products. Penninger13also studied the effect of supercritical water on the decomposition pattern of di-n-butyl phthalate and observed conversions of 98% at 305 "C and 99%at 390 "C. However, further increase of temperature above 390 "C was reported to only (11)Neavel, R. C. In Coal Science; Gorbarty, M. L.,Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1982;Vol. 1, pp 12-17. (12) Helling, R. K.; Tester, J. W. Enuiron. Sci. Technol. 1988,22, 1319. (13) Penninger, J. M.L. Fuel 1988,67, 490.
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initiate secondary reactions which led to reduced oxidation efficiencies. In order to study the effect of higher organic concentrations on organic oxidation efficiencies, tests were conducted using water samples containing TOC concentrations of 9800 ppm for phenol as opposed to the 3700 ppm used in the data reported above. The results of this study are contained in Figures 6 and 7 for fast and slow preheating regimes, respectively. In Figure 6 where the fast preheating data are reported, the phenol destruction efficiencies are observed to be higher than that with lower phenol concentration (Figure 31, at 370 "C. At this temperature, a complete oxidation of phenol is observed at all retention times for the higher phenol concentration, as compared to only 76-92% for the lower phenol concentration runs reported in Figure 3. This result is contrary to expectations based on the discussion above regarding the effect of preheater pyrolysis. A higher concentration of phenol in the preheater will be expected to increase the possibility of the occurrence of deleterious secondary condensation reactions with both the fast and slow preheating regimes.
1130 Energy &Fuels, Vol. 8, No. 5, 1994 Consequently, the only reasonable explanation for this observation is that the reaction kinetics for the supercritical water oxidation of phenol at high concentrations is different from that at lower concentrations. These results are in agreement with those of Yang and Eckert14who reported that the oxidation of p-chloropheno1 in supercritical water is first order at low concentrations, and second order at high concentrations. This type of variable kinetics which depends on the phenol concentration in the feed water will explain the results observed above. The results of the slow preheating tests using the high phenol concentration sample are shown in Figure 7. The phenol oxidation efficiencies are significantly lower than those observed in Figure 6 at similar reaction temperatures. Again, the deleterious effects of slow preheating is clearly evident in these results in agreement with those discussed earlier at lower concentrations of phenol and hexanol. The slow preheating provides additional time for the organic thermal pyrolysis products to condense or polymerize to form secondary compounds which are refractory to oxidation.
Conclusions The results of this study have shown that phenol is generally more resistant to supercritical water oxidation (14)Yang,H. H.; Erkert, C. A. Ind. Eng. Chem. Res. 1988,27,2009.
Li and Egiebor than hexanol. This is attributable to the aromatic ring in phenol which is more difficult to oxidize. In the subcritical water state (i.e., 315 "C), phenol and hexanol show comparable oxidative reactivity, most likely due to the reaction being controlled by mass-transfer limitations between the gas and liquid phases, and the fact that secondary pyrolysis reactions are largely absent at this temperature. For both phenol and hexanol, it is shown that fast preheating leads t o a higher oxidation efficiency under near-critical and supercritical water temperatures. Slow preheating leads to the formation of secondary pyrolysis products which are more resistant to subsequent oxidation. Consequently, the industrial application of this technology to wastewater treatment will require that the preheating time be reduced to a minimum before oxidation reactions. For phenol, higher concentrations provide better oxidative destruction efficiencies, thus implying that the reaction kinetics vary with concentration.
Acknowledgment. This project was made possible by financial support from the Alberta Environmental Research Trust (AERT) and the National Sciences & Engineering Research Council (NSERC) of Canada. Additional equipment grant from NSERC for the purchase of a TOC Analyzer in support of this project is also gratefully acknowledged.
