Ind. Eng. Chem. Res. 1994,33, 575-580
575
Autothermal Oxidation of Dilute Aqueous Wastes under Supercritical Conditions Dugan Kodra and Vemuri Balakotaiah' Department of Chemical Engineering, University of Houston, Houston, Texas 77204-4792
Analysis of the autothermal wet oxidation of dilute aqueous wastes reveals some important differences between the subcritical and supercritical operation of this process. The energy requirements are considerably higher for supercritical operation and are comparable to those for incineration. The calculations show that the efficiency of the heat exchanger in the near-critical region decreases significantly and using a regenerative heat exchanger for supercritical operation requires excessive heat-transfer area even for wastewaters with heating values around 1000 kJ.kgl. Better results are obtained a t higher pressures. This study demonstrates that autothermal operation of the supercritical wet oxidation process for dilute wastewaters is feasible only with the addition of auxiliary fuel.
Introduction Wet oxidation is an aqueous-phase oxidation process that starts spontaneously when organics containing water and oxygen (from air or pure) are mixed together at elevated temperatures and pressures. Economical considerations show that wet oxidation is especially suitable for the treatment of wastewaters that are too dilute (low heating values) to be economically incinerated and yet too concentrated to be treated by physical methods such as activated carbon adsorption or air stripping. Wet oxidation can be, depending on whether the reaction temperature and pressure are below or above the critical mixture temperature and pressure, performed under subcritical or supercritical conditions. For subcritical operation, the reaction temperature is between 200 and 325 "C and pressure is maintained to control evaporation (Copa and Gitchel, 1989). Subcritical wet oxidation has been successfully used in the US. and Europe since the early 1960s for the conditioning and oxidation of both municipal and industrial wastewaters. While the process may be carried out in a batch reactor, it is usually performed in a continuous-flow system. Supercritical wet oxidation is performed at temperatures between 400 and 650 OC and pressures around 250 bar (Modell, 1989). At present, no commercial installation has yet been built. The main reason for this is the extremely low solubility of inorganic salts in the supercritical water that can result in a severe scale formation at the reaction conditions (Bettinger, 1992). A majority of wastewaters already contain dissolved inorganic salts when entering a wet oxidation unit. The products of oxidation (such as carbon and sulfur dioxide) will also precipitate from the solution if a base is present in the reaction mixture. Currently, special reactor configurations that prevent the deposition of the precipitated salts are being studied. These configurations enable oxidation of the hazardous wastes and separations of inorganic salts in asingle step. High flow rates are utilized in these systems and the reaction temperature is around 620 "C in order to achieve 99.99 % destruction of the organics at a typical reaction time of 10 s or less (Barner et al., 1992). Wastewaters with a heating value sufficiently high to obtain the required reaction temperature can be treated without additional pretreatment. The heating value required depends on the reaction conditions, and the minimum values for subcritical and supercritical wet
* Author to whom correspondenceshould be addressed. E-mail address:
[email protected]. QSSS-5SS5/94/2633-0575$04.5QlQ
oxidation and incineration are listed in Table 1. These values are estimated from the difference between the enthalpy of pure water at the reaction conditions and the enthalpy of pure water at 25 OC and 1bar. The enthalpy values are obtained from steam tables (Haar et al.,1984). The minimum heating value required for subcritical wet oxidation without any pretreatment is approximately the heating value of wastewater containing 3 wt % benzene (heat of combustion of benzene at 25 "C is 41.8 MJ-kgl). The same heating value is expected in wastewaters containing approximately 10 wt % digested sludges (heat of combustion is between 10.0 and 14.0 MJDkgl of dry weight (Vesilind et al.,1985)). For supercritical oxidation, the heating value of wastewater containing at least 8 wt % benzene equivalent or nearly 28 wt % digested sludges is required for operation without any pretreatment. Wastewaters with a heating value that is below the minimum value require some form of pretreatment before entering a reactor. One possibility, which can be expensive especially in the treatment of very dilute wastes, is the addition of auxiliary fuel to make up the required heating value. In some cases, particularly when the wastewater contains solid particles (for instance sludges that result during the treatment of municipal wastewaters), additional dewatering can increase the heating value of the wastewater. This is, however, not always possible. Waste activated sludges, for example, are known to be notoriously difficult to dewater, and seldom thicken to solids concentration greater than 2 w t % (Vesilind et al., 1986). The most energy efficient method of pretreatment of diluted wastes is a combination of a regenerative heat exchanger and the oxidation reactor, as shown sghematically in Figure 1. This combination allows the heat of reaction to be directly utilized for preheating of the incoming reaction mixture. This lowers the minimum heating value of the feed stream necessary for operation of the reactor a t the required oxidation temperature and allows the system to operate autothermally even for very dilute wastes. The high energy efficiency of this combination is the reason for its wide use in commercial subcritical wet oxidation processes such as the Zimpro process for the treatment of hazardous wastes (Wilhelmiand Knopp, 1979; Copa and Gitchel, 1989). A step further in the design of the autothermal oxidation unit is the deep-well reactor technology that actually combines a heat exchanger and a reactor into a single unit that is suspended in a conventionally drilled and cased well (Smith, 1989). The necessary pressure in the subcritical deep-well reactor is 0 1994 American Chemical Society
576 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 Table 1. Minimum Heating Values Required for Operation without Any Pretreatment method (approximate heating value operating conditions) (kJ-kg') 1129.4 subcritical oxidation (280 "C and 100 bar) 3404.3 supercritical oxidation (600 O C and 230 bar) 4788.2 incineration (1100 O C and 1barP a
Rasmussen et al., 1989. Reactor
Regenerative Counter-Current Heat Exchanger
Figure 1. Schematic diagram of the reactor and heat exchanger system for autothermal wet oxidation of aqueous wastes.
provided mainly by the hydrostatic head, and therefore the length of the reactor needs to be around 1500 m. This means that there is a large heat exchange surface area available for the countercurrent heat exchange between the incoming feed and the outgoing reaction mixture. It was shown that wastewaters with a heating value as low as 100 kJ-kgl can be easily treated without any additional pretreatment (Lovo et al., 1990; Kodra and Balakotaiah, 1993). Energy requirements for the supercritical wet oxidation are much higher than for the subcritical operation. As shown in Table 1, the energy required to preheat the wastewater to the reaction temperature of 600 "C is more than three times that for the subcritical operation and equals over 70 7% of the energy required for preheating the wastewater to the incineration temperature. Autothermal operation is therefore an extremely important issue in the economics of the supercritical wet oxidation process. Although no commercial unit for supercritical wet oxidation has been built to date, it has been suggested that the combination of a regenerative heat exchanger and an oxidation reactor becomes a standard way of operating this process for dilute aqueous wastes (Modell, 1982,1985, 1989; Burleson, 1986; Titmas, 1986). Modell (1989) suggested that wastewaters with a heating value as low as 814 kJ.kgl could be efficiently treated in this way. In this work, we examine the feasibility of the suggested auththermal configuration for the treatment of dilute aqueous wastes under supercritical conditions. We do this by analyzing the behavior of an oxidative reactor and an external regenerative heat exchanger operating in the nearcritical region. We determine the effect of the heating value, the length of the countercurrent heat exchanger (the heat exchange surface area), and the operating pressure on the behavior of the system and the maximum temperatures that can be attained.
Adiabatic Temperature Rise Near the Critical Region We consider the system shown schematically in Figure 1 and assume that the process operates adiabatically. In addition, it is assumed that the reaction occurs only in the reactor and proceeds to completion. This means that the difference in the temperature of the stream entering and the reacted stream leaving the reactor equals the adiabatic temperature rise:
To= Ti+ AT, At constant pressure, the adiabatic temperature rise and the heating value of the reaction mixture are related by
-AH,= JTATaCpd T The adiabatic temperature rise thus depends on the value of the specific heat in the temperature range from Ti to Ti AT,. Figure 2 shows the variation of the specific heat of water in the near-critical region. The specific heat is signular at the critical point. At pressures higher than the critical pressure, a plot of the specific heat versus temperature (at constant pressure) exhibits a maximum. The temperature corresponding to this maximum is sometimes referred to as the pseudocritical temperature, and its value depends on the pressure (Kays and Perkins, 1985). The locus of the pseudocritical temperatures in the p-T plane is shown in Figure 3. This plot shows that the locus is a continuation of the gas-liquid equilibrium curve in the subcritical region. Figure 2 suggests that increasing the temperature of the reaction mixture above the pseudocritical temperature requires a considerable amount of energy. This energy is actually the remnant of the latent heat that existed at subcritical conditions. Although no phase change occurs, the density of water decreases rapidly when its temperature is increased above the pseudocritical temperature. This expansion is the main reason for the energy requirement of the supercritical wet oxidation being comparable to that for incineration. Figures 4 and 5 show the maximum adiabatic temperature rise of the reaction mixture as a function of pressure and initial temperature. The heating value used in the construction of these plots is 400 kJ+kg-' of the reaction mixture and is assumed to be independent of pressure and temperature. These plots show that, in the nearcritical region, the temperature rise strongly depends on the initial temperature and pressure. The temperature rise is practically negligible for the inlet temperatures that are below the pseudocritical temperature (Figure 3). As mentioned earlier, the energy transferred to the heated stream with initial temperature below the pseudocritical temperature is consumed for expansion and results only in a minimal temperature rise. The inlet temperature required for obtaining high reaction temperatures necessary for the effective destruction of organics must be above the pseudocritical temperature. In other words, the reaction mixture must be already expanded when entering the reactor. It is only in this case that the heating value commonly found in diluted wastewaters is sufficient for reaching the required high reaction temperature. This is further illustrated in Figure 6, which shows the energy requirements for heating the wastewater from 298.15 K (at 1.0 bar) to a temperature of 600 K, the pseudocritical temperature, and 900 K for the final pressure range between 225 and 325 bar. The plot shows that the change in enthalpy of water when the temperature is raised from 298.15 to 600 K (whichis at the upper limit of temperatures used in the subcritical wet oxidation) is approximately the same as the change in enthalpy when the temperature is raised from the psuedocritical temperature to 900 K. This means that even wastewaters that require little or even no pretreatment for subcritical wet oxidation must be preheated to above the pseudocritical temperature when treated at supercritical conditions.
+
Autothermal Operation in the Supercritical Region The autothermal arrangement shown in Figure 1 is feasible only if the heat exchanger is designed such that the temperature of the stream entering the reactor is above the pseudocritical temperature. In order to estimate the
Ind. Eng. Chem. Res.,Vol. 33, No. 3, 1994 577
"
inlcl tempram (K) Figure 4. Exit temperature eorreaponding to complete conversion of organics as a function of pressure and inlet temperature. Heating value of wastewater equals 400 kJ.kg'.
320 IW tempmure ( K )
Figure 2. Specific heat of water in the near-critical region. 3w
: cp. mar
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:
:- I 2 250
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200
.
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650
625
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675
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temperature ( K )
Figure%Lonrsofpwudoeritidtemperatures in thepTdiagram.
surface area (lengthof the heat exchanger) needed to meet this requirement, one must solve the following set of equations that describe the adiabatic operation of the countercurrent heat exchanger consistingof two concentric tubes: AdPdud
= A,PJJ,
=
(3)
inlet lempraiurc (K)
h,,,
72'1
61x1
Figure 5. Temperature rise correaponding to complete conversion of organics in an adiabatic reactor as a function of preasure and inlet temperature. Heating value of wastewater equals 400 kJ.kgl 4.0 9mK
The boundary conditions are as follows: Adpdud , C
2 = 0
z = L
Td
- PWT, - Td) = 0
(4)
0.0 225
Td = Tf
+
Tu= Ti AT,
(6)
1942):
Nuu = 0.02Re>8Pr>S(Du/Dd)o~sa
275
300
325
PMSUre
We assume that the pressure drop in the system can be neglected. The heat-transfer coefficient at the inner surface of the central tube is estimated by using the wellknown Dittus-Boelter equation (eq 7). A little-modified equation (eq8)issuggested for the heattransfer coefficient attheoutersurfaceofthecentral tube (MonradandPelton, Nud = 0.023Rep8Prp
250
(7) (8)
Theoverall heat-transfer coefficient Uis determined from the well-known relation for a composite cylindrical wall taking into account the heat-transfer resistance in the
FigureB. Energyrequirements for preheating wastewaterto900 K, pseudoeritical temperature (0, and 600 K an a function of pressure. For all three eases, initial conditions are TO= 298.15 K, p = 1 bar.
liquid on both sides of the central pipe as well as the h e a t transfer resistance in the pipe wall. The geometry of the heat exchanger, the thermal conductivity of the pipe wall, and the inlet conditions in our computations are given in Table 2. The thermodynamic and transport properties in eqs 3-5,7 and 8 are estimated using codes found in Haar et al. (1984). For the subcriticdoperation,an excellent approximation of the temperature increase in the heat exchanger may be obtained by assuming constant physical properties of the fluid as well as a constant value of the overall heat-transfer
578 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 1000
Table 2. Parameter Values Used in the Simulations 298.15 Tr (K) 1.0 vf (mas-1) 20.0 Xtub (Wsm-1.K-1) central tube 20.5 i.d. (cm) 22.0 0.d. (cm) 30.0 outside tube i.d. (cm)
g 5 B
e
h
1250
v
:
600
I
400
200 ot-----"---l 500 lo00 1500 2000 length (m) 2200
7
1750 1500
800
1\
WOK
g
3
1600 1400 1200
1
MOK
' ' '
500
' ' ' '
500
~~
250
150
4MK XI)K
0 225
250
215
300
325
I
' ' ' '
'
' ' ' ' J
lo00 1500 length (m)
2000
In
pressure (bar)
Figure 7. Preheat temperature, Ti,obtained in a countercurrent heat exchanger as a function of pressure and length of the heat exchanger. Heating value of wastewater equals 400 kJ.kg1.
coefficient U. In this case, the energy balances, eqs 4-6, may be combined to give the following expression for the temperature rise:
Ti - Tf = (UA/hC,) AT, = StAT,
(9)
where A (=PL) is the overall heat exchange surface area available, and St is the Stanton number. Equation 9 shows that, for constant fluidproperties, the preheat temperature rise is directly proportional to the heat exchange surface area. For supercritical operation, variations of thermodynamic and transport properties have to be accounted for and eqs 3-6 must be solved numerically.
Results We present here the results on the behavior of the autothermal process shown in Figure 1 in terms of the length of the countercurrent heat exchanger. Table 2 gives information on the flow rates and the geometry of the system used in our study. Similar results may be obtained for other geometries and flow rates. Figure 7 shows the temperature at the exit of the heat exchanger, Ti, as a function of the applied pressure and the heat exchanger length. The heating value of the inlet stream used in these computations is 400 kJ.kgl of wastewater. The plot shows that the exit temperature follows eq 9, Le., the temperature rise is proportional to the heat exchanger length for the exit temperatures up to approximately 600 K. A t this point, the exit temperature levels off (at lower pressures) and does not increase significantly even for a heat exchange surface area that is 3 or even 4 times larger. The first reason for this excessive heat-transfer area required is due to the fact that the exit temperature is close to the pseudocritical temperature (Figure 3) for the corresponding pressure and energy transferred to the heated stream is actually consumed for its expansion. The second reason for the slow temperature increase in this region is shown in Figure 8. The top diagram shows the temperature profile in the heat exchanger as a function
0
500
1000
1500
2000
length (m) Figure 8. Temperature profile and variation of overall heat-transfer coefficient and heat flux in the heat exchanger as a function of ita length. Pressure is fixed at 250 bar and heating value of wastewater equals 400 kJ.kgl.
of its length. (This diagram may also be looked upon as a plot of temperature at the exit of the heat exchanger, Ti, and temperature at the exit of the reactor, To, as a function of the length of the heat exchanger.) The pressure used in the calculations is 250 bar, and the heating value equals 400 kJ-kgl. The diagram shows that, for lengths below approximately 500 m, the temperature profiles in the heat exchanger are parallel which is the case when liquid properties are constant. For lengths above 500 m, the temperatures close to the exit of the heat exchanger start approaching the pseudocritical temperature. The temperature profiles level off and tbe temperature difference between the streams starts decreasing. This decrease in the temperature difference is only partially offset by an increase in the value of the heat-transfer coefficient(middle diagram), and the heat flux between the streams decreases significantly in this near-critical region (bottom diagram). This behavior is the reason for a low efficiency of the heat exchanger when operated in the near-critical region which results in an excessive heat-transfer surface area required for obtaining exit temperatures that are above the pseudocritical temperature. If the length of the heat exchanger is increased above approximately 1800m, the temperature difference between the two streams and the heat flux start increasing again toward the exit of the heat exchanger. At the same time, the value of the overall heat-transfer coefficient starts decreasing again. The temperatures of both streams are above the pseudocritical temperature, and further increasing of the length of the heat exchanger causes the temperature of both streams to start increasing quickly again (Figure 9). The profiles in this supercritical region of the heat exchanger qualitatively resemble those in the subcritical portion of the heat exchanger. If one increases the pressure in the heat exchanger to 300 bar, the profiles shown in Figure 9 are obtained. These
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 579 900 K) are considerably more difficult to obtain, even for
500
lOOO()'
"
" '
500
1000
length (m)
"
' I
"
1000
length (m)
2000
1500
"
'
"
1500
t
'
I
2000
I
u 1000 1500 500 length(m)
OO
2000
Figure 9. Temperature profile and variation of overallheat-transfer coefficient and heat flux in the heat exchanger as a function of ita length. Pressure is fixed at 300 bar and heating value of wastewater equals 400 kJ.kg-1. 2000 1750 1500
5
1250
750
500 250
0 , 0
260
460 Sdo 860 heating value (klkg-')
lob
Figure 10. Preheat temperature, Ti,obtained in a countercurrent heat exchanger as a function of heating value of wastewater and the length of the heat exchanger. Pressure equals 250 bar.
diagrams explain why the efficiency of a heat exchanger is better at higher pressures (Figure 7). The pseudocritical region of the heat exchanger (the region of low heat flux) is shorter and less pronounced at higher pressures. The reason for this is the lower maximum value of the specific heat at higher pressures (Figure 2). Therefore, the applied pressure in the system should be as high as practically possible. Figure 10shows the combined effect of the heating value and the heat exchange area on the temperature at the exit from the heat exchanger, Ti. The pressure used in these computations is fixed at 250 bar. This plot clearly shows that obtaining reaction temperatures up to 600 K is relatively easy for heating values above 400 kJ.kgl. Higher temperatures required for supercritical operation (around
heating values around 1000 kJ.kg1. In our analysis, we neglected heat losses from the system which means that the actual heating values required are higher than these shown here. As mentioned earlier, the main reason for the supercritical wet oxidation not being commercially available yet is the extremely low solubility of inorganic salts in the supercritical water that can lead to a severe scale formation. This can cause additional difficulties in operation of the heat exchanger. Many wastewaters contain a considerable amount of dissolved inorganic salts. The majority of these salts would precipitate in the part of the heat exchanger where the temperature of the heated stream approaches the pseudocritical temperature. This extremely low solubility of inorganic salts together with our analysis of the behavior of the heat exchanger in the near-critical region suggests that, for most wastewasters that contain dissolved salts, the only practical way of autothermal operation under supercritical conditions is preheating the reaction mixture to the temperature that is below 550 K. Temperatures below 550 K can be easily obtained in the countercurrent heat exchanger and are, at the same time, low enough to prevent excessive salt deposition from the heated stream. Also, in order to prevent deposition of salts on the heat exchanger walls from the cooled stream, the reactor configuration that allows separation of salts from the exit stream should be used. The minimum heating value required (the difference between the enthalpy of the reaction mixture at 900 and 550 K) for this type of operation is around 2300 kJ.kgl (Table 1)and is much higher than the heating values considered in the analysis presented here. This value is almost 3 times higher than the mininum heating value suggested in the prior literature (Modell, 1989). Addition of auxiliary fuel or external heating is required for wastewaters with a heating value that is below this value. It is possible to recover most of this energy in the form of steam.
Conclusions Autothermal operation of the wet oxidation process is an important issue in the economics of the treatment of dilute wastewaters. A simple analysis of the autothermal operation of the wet oxidation process presented here reveals some important differences between the subcritical and the supercritical operation of this process. For subcritical operation, reaction temperatures are below 600 K and this temperature is easily obtained by using a regenerative heat exchanger, even for wastewaters whose heating value is well below 1.0 wt % benzene equivalent (400 kJ0kg-l). The necessary heat exchanger performance required for any specific stream can be accurately estimated from eq 9. For supercritical operation, the reaction temperature is around 900 K and the analysis shows that the overallenergy requirements are comparable to those for incineration. The major part of the required energy is used for expansion of the reaction mixture that occurs in the vicinity of the pseudocritical temperature. This energy is the remnant of the latent heat that existed at subcritical conditions. The analysis presented here shows that preheating the reaction stream to a temperature that is above the pseudocritical temperature requires excessive heat-transfer surface area even for wastewaters with heating values around 1000 kJ.kg-l. The reason for this is in part due to low efficiency of the countercurrent heat exchanger when operated in the near-critical region. This efficiency depends on pressure with better results obtained at higher pressures.
580 Ind. Eng. Chem. Res., Vol. 33,No. 3, 1994
The behavior in the near-critical region described here is not limited to aqueous solutions. Similar behavior would be observed also for other liquids at near-critical conditions. The analysis presented here and the problems associated with scale formation in supercritical water suggest that preheating of the waste stream to a temperature that is below the critical temperature is the only viable option for most wastewaters that contain dissolved inorganic salts. This type of operation requires heating values of at least 2300 kJ.kgl. Dewatering, external heating, or addition of auxiliary fuel to make up the necessary heating value is required for wastewaters whose heating value is below this minimum value.
Acknowledgment This work was partially supported by the Gulf Coast Hazardous Substances Research Center and the Robert A. Welch Foundation. Nomenclature A = cross-sectional area (m2) C, = specific heat (Jakg-1.K-l) D = tube diameter (m) L = length of a regenerative heat exchanger (m) m = mass flow rate (kgs-l) Nu = Nusselt number p = pressure (bar) P = perimeter (m) Pr = Prandtl number Re = Reynolds number St = Stanton number T = temperature (K) U = overall heat-transfer coefficient (W-m-2.K-1) u = velocity (ms-l) Greek L e t t e r s X = thermal conductivity (W-m-1-K-') p =
density ( k ~ r n - ~ )
Subscripts d = entering (preheated) stream f = feed i = inlet to reactor o = exit from reactor u = exiting (cooled) stream
Literature Cited Barner, H. E.; Huang, C. Y.; Johnson, T.; Jacobs, G.; Martch, M. A.; Killilea, W. R. Supercritical Water Oxidation: An Emerging Technology. J . Hazard. Mater. 1992,31, 1. Bettinger, J. A. Supercritical Water. Chem. Eng. News 1992, 70,2. Burleson, J. C. Method and Apparatus for Disposal of Broad Spectrum of Waste Featuring Oxidation of Waste. US. Patent 4,564,458, 1986. Copa, W. M.; Gitchel, W. B. Wet Oxidation. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGraw-Hill: New York, 1989. Haar, L.; Gallagher, J. S.; Kell, G. S. NBSINRC Steam Tables; Hemisphere Publishing Corporation: New York, 1984. Kays, W. M.; Perkins, H. C. Forced Convection, Internal Flow in Ducts. In Handbook of Heat Transfer Fundamentals, 2nd ed.; Rohsenow, W. M., Hartnett, J. P., Ganic, E. N., Eds.; McGrawHill: New York, 1985. Kodra, D.; Balakotaiah, V. Two-Phase Model for Subcritical Oxidation of Aqueous Wastes in a Deep-Well Reactor. Submitted for publication in Hazard. Waste Hazard. Mater. 1993. Lovo, M.; Deans, H. A,; Balakotaiah, V. Modeling and Simulation of Aqueous Hazardous Waste Oxidation in Deep Well Reactors. Chem. Eng. Sci. 1990,45, 2703. Modell, M. Processing Methods for the Oxidation of Organics in Supercritical Water. U S . Patent 4,338,199,1982. Modell, M. Processing Methods for the Oxidation of Organics in Supercritical Water. U.S. Patent 4,543,190,1985. Modell, M. Supercritical-Water Oxidation. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGraw-Hill: New York, 1989. Monrad, C. C.; Pelton, J. F. Heat Transfer by Convection in Annular Spaces. Trans. Am. Inst. Chem. Eng. 1942,38, 593. Rasmussen, G. F.; Benedict, R. W.; Young, C. M. Fluidized-Bed Thermal Oxidation. In Standard Handbook of Hazardous Waste Treatment andDisposal;Freeman, H. M., Ed.; McGraw-Hill: New York, 1989. Smith, J. M. Deep-Shaft Wet-Air Oxidation. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGraw-Hill: New York, 1989. Titmas, J. A. Method and Apparatus for Conducting Chemical Reactions at Supercritical Conditions. U.S.Patent 4,594,164,1986. Vesilind, P. A.; Hartman, G. C.; Skene, E. T. Sludge Management and Disposal for the Practicing Engineer; Lewis Publishers: Chelsea, MI, 1986. Received for review April 26, 1993 Revised manuscript received October 22, 1993 Accepted November 16, 1993' @
Abstract published in Advance ACS Abstracts, February 1,
1994.
