Innovations in Supercritical Fluids - American Chemical Society

Chapter 23

The Use of Rate Enhancers in Supercritical Water Oxidation Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 28, 2018 | https://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0608.ch023

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A. L. Bourhis , Κ. C. Swallow, G. T. Hong , and W. R. Killilea 1

MODAR, Inc., 14 Tech Circle, Natick, MA 01760 Department of Chemistry, Merrimack College, 315 Turnpike Street, North Andover, MA 01845

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The initiation of supercritical water oxidation was dramatically enhanced when small amounts of strong oxidizing agents, such as H2O2 and HNO , were added to a simulated waste stream. Tests were performed in a 6.2 meter pipe reactor where oxidation was initiated by mixing the cold aqueous/organic feed with a hot supercritical water/oxidant stream. Without rate enhancers, an inlet mix temperature of 362 C resulted in the incomplete oxidation of isopropanol. When the feed contained 0.75, 1.5 and 3.0 wt% H O oxidation was essentially completed at 5.1, 4.0 and 2.8 meters, respectively. When initiated at 380 C, without rate enhancers, isopropanol reacted completely in 5.6 meters. This length was reduced to ~ 2 meters when the feed contained 2.0 wt % HNO . 3

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Supercritical water oxidation (SCWO) is an emerging waste treatment technology that takes advantage of the unique properties of water above its critical point (374 C and 221 bar). Organic destruction efficiencies in excess of 99.99% are routinely reported with efficiencies exceeding 99.9999% not uncommon (7). Successful commercialization will require that SCWO be a cost effective method of waste disposal. Small amounts of strong oxidizing agents can help to minimize capital and operating costs by initiating reaction at lower temperatures and/or reducing the required residence time. Background A large body of theoretical and experimental work exists (2,3) describing plug flow reactor (PFR) analysis, design and scale-up. Pipe reactor benefits include their well defined residence time distributions, turbulent mixing of reactants, ease of obtaining and applying kinetic data, efficient use of reactor volume and mechanical simplicity.

0097-6156/95/0608-0338$12.00/0 © 1995 A m e r i c a n Chemical Society

Hutchenson and Foster; Innovations in Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 28, 2018 | https://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0608.ch023

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Pipe reactors do, however, have drawbacks^). Their high surface-to-volume ratios lead to high levels of corrosion for waste streams containing or producing inorganic acids (e.g. HC1, H2SO4 and H3PO4). These acids cannot usually be neutralized since the salts generated are generally "sticky" (e.g. NaCl, Na2SC>4, Na3PC>4 and Na2CC>3) and tend to plug the reactor. Due to the absence of back mixing in a PFR, the feed and oxidant must be heated to, or above, the critical point of water to initiate oxidation. Below the critical temperature, separate gas and liquid phases are generally present and reaction proceeds by the wet oxidation process (5). Wet oxidation typically requires tens of minutes or hours to achieve the same organic destruction that SCWO systems achieve in seconds. There are two general methods of raising the reactant temperatures; indirect and direct heating. Indirect feed preheating, the most common method, involves transferring energy across a solid boundary to the waste feed stream Regenerative heat exchange (e.g. feed/effluent heat exchangers, bayonet reactors, separate heat transfer fluid loops) and external heating (e.g. gas fired, resistive or radiant heating) are examples of indirect heating. Direct feed heating occurs when a cold (or partially preheated to < 250 C) aqueous waste stream is mixed with a hot (500 C to 650 C) SCW/oxidant stream to achieve an unreacted "mix" temperature at or above 374 C. This SCW is either recycled reactor effluent or a separate, indirectly heated water stream Regenerative heat exchange is especially amenable to treating low heating value feed streams, but it does have a number of drawbacks. Because the feed/effluent heat exchange surfaces experience the most corrosive environment in a SCWO system ((5), the heat exchangers can cost several times more than the reactor itself. The feed heat exchanger is also susceptible to organic char and oxide/salt fouling. By keeping the waste stream cold until it is injected into the reactor, direct feed heating avoids the highly corrosive 250 C to 400 C temperature range. Organic polymerization and charring are minimized. There are no feed heat exchange surfaces to foul. With the effluent no longer needed for regenerative heat exchange, corrosion can be further mitigated by quenching the effluent to below 300 C. In the case of a highly acidic effluent, the pH can be raised by quenching with a caustic solution. Salts generated by neutralization remain in solution. Initiating oxidation by recycling the reactor effluent does, however, require either a high temperature canned pump or an eductor. Likewise, an indirectly heated SCW stream requires an additional water pump and heater. This clean SCW stream, however, does not have the processing problems associated with preheating waste feeds. Figure 1 shows how the mix temperature of a simple cold water-hot water system varies as a function of the mass flow ratio. The cold water is assumed to be at 25 C and the hot water at 600 C. As the ratio of SCW to cold water is raised from zero to one, the mix temperature rapidlyrisesfrom25 C to roughly the critical point. Because of the near-critical heat capacity spike, there is little change in temperature as the ratio is increased from 1 to ~ 2.5. Beyond 2.5 the temperature again increases with increasing SCW feed rates, asymptotically approaching 600 C.



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Because higher heating value streams (-3500 J/g) can be treated, the reactor volume of a direct-heated system is the same as an equal capacity indirect-heated system, initiated at the same temperature. High initiation temperatures, needed for refractory compounds, normally would require large SCW flows. Beyond a certain point, the cost of providing this stream may outweigh the corrosion and processability benefits. The direct-heating method is most economical when initiation occurs near the critical point. Regardless of the initiation method, liquid oxygen and compressed air are the most commonly used oxidants, both in experimental and in proposed commercial units. No dramatic differences have been reported for reaction rates between the two. Oxidation is generally believed to occur via free radical mechanisms (7-9). Most theoretical and experimental SCWO studies conclude that the global reaction rates tend to befirstorder in organic concentration and zero, or lowfractional(< 1), order in oxygen concentration. Oxidants other than dioxygen (from liquid oxygen or compressed air) have occasionally been used in conventional wet oxidation and SCWO studies. Welch and Siegwarth (70) describe the use of oxidants in a liquid phase, for example aqueous solutions of hydrogen peroxide, ozone, or inorganic oxides, and suggest that the free radical species formed by peroxide decomposition should lead to vigorous oxidation. Lee and Gloyna (77) compared the effectiveness of hydrogen peroxide and dioxygen in oxidizing acetic acid and 2,4 dichlorophenol at temperatures between 400 C and 500 C. For both compounds, the peroxide gave a higher degree of oxidation at comparable reactor conditions. In a subsequent article by Li, Chen and Gloyna (72), however, the claim is made that hydrogen peroxide and dioxygen give comparable reaction rates. Thus, the comparison between these two oxidants requires further study. Rofer et al. (73) have carried out SCWO on propellant materials which inherently contain some or all of the oxygen needed for the oxidation reactions. The perchlorate ion of ammonium perchlorate and the nitro moiety of nitromethane both proved to be effective oxidizers. DelTOrco et al. (14) have also described the use of sodium nitrate as an oxidizing agent. The inconsistent hydrogen peroxide results may be due to the method by which the oxidant is introduced to the system Aqueous solutions of some oxidizing agents, hydrogen peroxide and ozone for example, are known to decompose irreversibly to less reactive dioxygen and water at temperatures well below the critical point of water. In much of the reported kinetic work, these oxidants are preheated to reactor conditions. The resulting oxidation rates may, therefore, be more indicative of those carried out with dioxygen. Nitric acid, in contrast, does not decompose irreversibly at SCWO conditions. Without a material to oxidize, nitric acid is recovered upon cooling and depressurizing the effluents to ambient conditions. Alternative oxidants are considerably more expensive than either liquid oxygen or compressed air. Hydrogen peroxide and nitric acid are roughly 30 and 6 times more expensive than liquid oxygen, respectively.


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Motivation of work The choice of pipe reactor initiation method depends on the specific feed stream Relatively clean, low heating value wastes are well suited to initiation by regenerative heat exchange and external heating. High heating value and corrosive waste streams are better suited to the direct injection initiation. Ideally, reaction would be initiated just above the critical temperature and the heat of reaction would quickly drive the temperature up to 500 C to 600C.,minimizing both corrosion and reactor size. An experimental study is presented where low concentrations of strong oxidizing agents and organics are fed into a pipe reactor cold and rapidly brought to initiation conditions by direct contact with a combined supercritical water and air stream The strong oxidizers, essentially intact at the reactor inlet, significantly enhance the initial organic destruction rate. Apparatus Figure 2 is a schematic of the MODAR bench unit as configured for use with a direct feed heated pipe reactor. Deionized water was pressurized to 234 bar and heated above the critical point. This water was mixed with air, which had also been pressurized and preheated. The combined, 500 C stream was introduced into the pipe reactor via the annulus of a co-annular nozzle. Air flows were chosen to provide stoichiometric excess oxidant; the effluent gas oxygen concentration varied between 2 and 5 volume percent. Simulated waste feeds, deionized water and - 1 2 wt% organic, were pumped up to 234 bar separately. The feed stream was fed into the pipe reactor cold, via the insulated core of the co-annular nozzle. The 6.2 m insulated pipe reactor has a 0.925 cm I.D.. No external heaters were used. The core feed tube had a 0.635 cm O.D. and a 0.305 cm I.D.. The mix temperature at the pipe reactor inlet was controlled by varying the ratio of core to annular flow rates (typically 1:6 on a mass basis). The main instrumentation used to assess the system performance was the set of thermocouples, consisting of three distinct groups; in-line,ribbonand bead. There were two in-line thermocouples, one located in the SCW/Air annulus of the nozzle, the other in the reactor exit (6.2 m). There were 6ribbonthermocouples secured onto the surface of the pipe with graphite cement (Thermon). Finally, there were originally 16 bead thermocouples wired to the surface of the pipe. Of the three sets, the in-line and theribbonthermocouples were the most accurate and were used to provide corrections for the more numerous bead thermocouples. The extent of reaction, along the length of the pipe reactor, was observed indirectly by measuring the temperaturerisedue to the exothermic nature of oxidation. Reaction initiators were introduced by switching the cold feed water from deionized water to deionized water plus initiators. The initiator concentrations were varied between 0 and 3 weight percent of feed water flow. The oxygen available from the initiators never exceeded 5 percent of the stoichiometric requirement. The effluent stream was first cooled to near ambient temperatures. The resulting two-phase flow was then letdownfrom234 bar to 103 bar through a control valve. After separating the vapor and liquid, a back-pressure regulator was used to


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Figure 1. Mix temperature versus SCW to core ratio - SCW at 600°C and core at 25°C.

Organic Feed

Pipe Reactor Cooldown Heat Exchanger Pressure | Letdown XD

Core Water - /'«) · SCW/ Air Annular Water

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Air Compressor

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mil Radiant Heaters

Effluent Gas • Low Pressure Liquid/ Vapor Separator ίο Effluent Liquid

Figure 2. MODAR bench scale pipe reactor - configured for direct injection feed heating.



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reduce the gas pressure to ambient. The liquid pressure was reduced to 4 bar by means of another control valve, leading to the evolution of CO2. A second phase separation was carried out in a low pressure liquid-vapor separator. The gas streams from both separators were mixed before sampling or venting. The flow rates ranged between 200 and 450 cc/min for SCW, 45 and 110 normal liters per minute (25 C., 1 bar) for air, 15 and 55 cc/min for cold water and 3 and 15 cc/min for organics. The resulting residence times ranged between 3 and 6 seconds. Reynolds numbers near 25,000 insured fully turbulent flow. Procedure No direct measurements of organic concentration were made either in the reactor or system effluent. The extent of reaction was monitored solely by the axial temperature rise resulting from the exothermic heat of combustion. The assumption that the temperature profile accurately indicated the extent of reaction was supported by carbon monoxide (CO) levels in the effluent gas. Historically effluent gas CO levels below 10 ppm (volume basis) in the bench system correspond to 99.99% overall organic destruction efficiencies. During these experiments, every run that exhibited a temperature maximum over 500 C had effluent CO levels below 10 ppm The tests were started by first feeding only SCW/Air to the reactor. Cold water was then fed through the core of the nozzle at flow rates equal to the combined water/fuel flows be used for the oxidation portion of the run. The resulting near nozzle temperature provided an estimate of the effective mix temperature. When the fuel flow was started, the cold water flow was reduced by an equal amount. The resulting temperatureriseindicated the reactivity of a given organic compound for the apparent mix temperature. In the majority of the runs, two or more organic compounds were compared head-to-head (constant SCW/Air flow, cold feed flows and mix temperatures) by simply switchingfromone feed tank to another. Results Figure 3 shows the sensitivity of isopropanol oxidation to the apparent mix temperature. At mix temperatures above 400 C., the isopropanol reacts completely in less than 1 m (1 sec). As the mix temperature is lowered towards the critical point, the temperature profiles take longer to develop until, finally, there is little or no oxidation in the reactor. The three profiles measured at the lowest mix temperature indicate the limits of resolving the mix temperature. Run 507d resulted in incomplete oxidation. In Figure 4 ethanol oxidation is compared to isopropanol. The isopropanol base case is the same poorly initiating profile described in the previousfigure(Run 507d). Two ethanol profiles are presented; one equi-molar and the other equienthalpic. In the equi-molar case the initial organic molar concentration is held constant, while in the equi-enthalpic case the overall heating value (J/g feed) is held constant. In both cases ethanol initiates more rapidly and reacts more completely that isopropanol. The dramatically faster reaction in the equi-enthalpic case is due to a combination of higher initial organic concentration and the "auto catalytic" effect of


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Figure 3. Sensitivity of isopropanol oxidation to mix temperature. 600

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Figure 4. Comparison of ethanol oxidation to isopropanol oxidation.


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the heat release as well as faster base kinetics. Even though the equi-molar case does not seem as dramatic an improvement as the equi-enthalpic case, it did result in essentially complete oxidation. The reactivity of some organic compounds, relative to isopropanol, can be summarized as follows: both ethanol and methyl-ethyl-ketone are much more reactive, acetone is slightly less reactive, methanol is less reactive, acetic acid and toluene are much less reactive. Even though the mix temperatures of all runs were at or above the critical temperature of water, finite mixing rates result in two phase flow at the reactor inlet. Liquid fuel and gaseous oxidant have to overcome mass transfer resistance in order to react. Liquid oxidants, in intimate contact with the organics, do not experience these resistances and initiate reaction closer to the nozzle. The dramatic effect of H2O2 on isopropanol oxidation is shown in Figure 5. The base isopropanol case does not react completely. When the core water flow is switched to a 0.75 wt% H2O2 solution, the reaction reaches completion in roughly 5 m As the concentration of H2O2 is increased to 3.0 wt%, the reaction rate increases. The improvement with increasing H2O2 concentration seems to display diminishing returns, suggesting that H2O2 is important only at the onset of the reaction. Once the reacting medium becomes homogeneous, oxidation by O2 (which makes up 95% of the oxidant) dominates. Figure 6 shows the effect of a 2 wt% HNO3 solution on isopropanol oxidation. Although the different mix temperatures makes a direct comparison with H2O2 impossible, it is clear that HNO3 also greatly increases the complete isopropanol oxidation rate. In this case, the residence time required to initiate isopropanol oxidation was cut by a factor of three. On a equivalent O2 basis, 2 wt% HNO3 corresponds to 2.7 wt% H2O2. Summary Axial temperature profiles provide valuable insights into the design and optimization of SCWO pipe reactors. The relative reactivity of various compounds is easily determined by comparing temperature profiles for a fixed mix temperature and mass throughput. The sensitivity of reaction initiation to mix temperature, for a given compound, is revealed by varying the core to annular ratio while keeping the overall and organic throughputs constant. The addition of small amounts of either hydrogen peroxide or nitric acid to the cold feed significantly enhanced the initial oxidation rates. These rate enhancers have the same effect as either raising the mix temperature or switching to a more reactive organic. The ability of the direct heating method to introduce these strong oxidizers into the reactor intact makes this initiation enhancement possible. The small quantities needed to effect this enhancement should allow this type of initiation to be economically viable for commercial SCWO pipe reactor applications.


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Figure 5. Enhancement of isopropanol oxidation by H 2 O 2 . 600

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Figure 6. Enhancement of isopropanol oxidation by H N O 3 .


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Literature cited (1) Tester, J. W., et. al., "Supercritical Water Oxidation: A Review of Process Development and Fundamental Research", ACS Symposium Series 518, 1993. (2) Levenspiel, O., Chemical Reaction Engineering, Second Ed., John Wiley and Sons, Inc., 1972. (3) Smith, J. M., Chemical Engineering Kinetics, Third Edition, McGraw-Hill Book Co., 1981. (4) Zimmermann, F. J., "Waste Disposal", United States Patent No. 2,665,249, Jan. 5, 1954. (5) Barnes, C. M., "Evaluation of Tubular Reactor Designs for Supercritical Water Oxidation of U. S. Department of Energy Mixed Waste", Contract No. DE-AC0794ID13223, Idaho National Engineering Laboratory, LITCO, Idaho Falls, ID, 1994. (6) Beller, J., K. Garcia and C. Shapiro, "Treatment of a Simulated Mixed Waste with Supercritical Water Oxidation", Contract No. DE-AC07-76ID01570, Idaho National Engineering Laboratory, LITCO, Idaho Falls, ID, 1993. (7) Helling, R K., Oxidation Kinetics of Simple Compounds in Supercritical Water: Carbon Monoxide, Ammonia and Ethanol, Doctoral Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1986. (8) Webley, P. Α., Fundamental Oxidation Kinetics of Simple Compounds in Supercritical Water, Doctoral Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1989. (9) Holgate, H. R , Oxidation Chemistry and Kinetics in Supercritical Water: Hydrogen, Carbon Monoxide and Glucose, Doctoral Thesis, Massachusetts Institute of Technology, Cambridge, MA., 1993. (10) Welch, J. F. and J. D. Siegwarth, "Method for the Processing of Organic Compounds", United States Patent No. 4,861,497, Aug. 29, 1989. (11) Lee, D-S. and E. F. Gloyna, "Efficiency of H O and O in Supercritical Water Oxidation of 2,4-Dichlorophenol and Acetic Acid", The Journal of Supercritical Fluids, 1990, 3, 249-255. (12) Li, L., P. Chen and E. F. Gloyna, "Generalized Kinetic Model for Wet Oxidation of Organic Compounds", AIChE Journal Vol. 37, No. 11, November 1991, pp. 1687- 1697. (13) Rofer, C. K., S. J. Buelow, R B. Dyer and J. D. Wander, "Conversion of Hazardous Materials Using Supercritical Water Oxidation", United States Patent No. 5,133,877, July 28, 1992. (14) Dell'Orco, P.D., B.R Foy, J.M. Robinson and S.J. Buelow, "Hydrothermal Reactions of Nitrate: Treatability of Hanford Tank Waste Constituents", Presented at the Summer National Meeting of the AIChE, Minneapolis, MN, August 1992. 2

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Innovations in Supercritical Fluids - American Chemical Society

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