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Ind. Eng. Chem. Res. 2000, 39, 4734-4740
Ultrasound for Hydrothermal Treatments of Aqueous Wastes: Solution for Overcoming Salt Precipitation and Corrosion C. Aymonier,† M. Bottreau,† B. Berdeu,‡ and F. Cansell*,† Institut de Chimie de la Matie` re Condense´ e de Bordeaux (I.C.M.C.B), CNRS, Universite´ Bordeaux I, 33608 Pessac Cedex, France, and l’Electrolyse, Zone Industrielle, 33360 Latresne, France
A hydrothermal sonochemical reactor has been developed as a new approach to the hydrothermal treatment of aqueous wastes. At 2.8 MPa and 220 °C, acetic acid degradation reached 83% for a space time in the reactor of 10 min. The ultrasonic activation increased the yield of the acetic acid oxidation reaction by 40%. Furthermore, tests with an industrial waste containing salts and halogens showed the performance of the titanium liner of the sonochemical reactor for overcoming salt precipitation and corrosion. The hydrodynamic study of the reactor from the residence time distribution (Rtd) determination gave access to sonochemical reactor modeling from a battery of three continuously stirred tank reactors. Introduction Environmental regulations and the increasing cost of wastewater disposal lead to new concepts for the complete destruction of both toxic substances and sludges. Hydrothermal oxidation of wastes is being developed as an alternative technique in order to limit toxic end-product formation, waste final volume, and energy needs. Hydrothermal oxidation involves the oxidation reaction of organic matter in water under pressure and temperature. The end products are essentially water and mineral acids (HCl, H2SO4, H3PO4, etc.) in the liquid phase and carbon dioxide and molecular nitrogen in the gas phase.1,2 Wet air oxidation (WAO) and supercritical water oxidation (SCWO) are considered hydrothermal oxidation processes. WAO (2 e P e 20 MPa and 150 e T e 325 °C), developed since 1958, allows for a chemical oxygen demand (COD) reduction of 80-90% to be reached for a residence time in the reactor of approximately 1 h. The main limitations are the oxygen diffusion in the liquid phase (diphasic system) and the ammoniac produced as the end product. SCWO (P g 22.1 MPa and 374 e T e 600 °C) was introduced by Modell in the early 1980s. Under these conditions, both organic matter and oxidant (air, oxygen, or hydrogen peroxide) are completely miscible with water, which suppresses interfacial transport phenomena as in WAO. COD reduction greater than 99.99% is obtained in relatively short reaction times (few seconds). However, the solubility of the inorganic salts drops to the parts per million (ppm) range. Thus, inorganic salts precipitate and deposit on the reactor and heat exchanger surfaces up to plugging installation. Moreover, corrosion of the SCWO reactor materials by highly corrosive aqueous environment is also a major limiting factor, which must be taken into account for the development of SCWO processes. It was reported that several reactors have been designed to overcome salt precipitation and corrosion * Corresponding author. E-mail:
[email protected]. Fax: 33.5.56.84.27.61. † Institut de Chimie de la Matie ` re Condense´e de Bordeaux. ‡ l’Electrolyse.
in SCWO processes. One of the first developments concerned an aqueous-phase oxidizer and solid separator reactor, known as the Modar reactor.3 The Modar reactor is composed of two zones: an upper supercritical zone, in which the reaction takes place and the salts precipitate and fall with other solid particles into a lower subcritical zone, in which a brine containing salts and other solids is formed and removed from the reactor with the reaction products. Other reactors were designed from the Modar reactor concept to further reduce salt deposition on the reactor wall.4-6 Concerning corrosion, numerous tests have been performed but have concentrated on alloys containing mostly a large amount of nickel and chromium or ceramic.7-10 At that time, no material resistant to both pressure and corrosion had been found. As a consequence, reactors were designed with a decoupling of these parameters.11-13 In particular, reactors using a transpiring wall to provide a protective boundary layer against corrosion and salt deposition, thus preventing attack on the reactor wall, were proposed.14-18 Water was injected through a porous wall to form a nonreactive barrier between the wall material and the reactants. In parallel to the transpiring wall concept, titanium was shown to be resistant to corrosion by chloride, even in acidic hydrothermal conditions.19-21 To avoid plugging of the reactor or the heat exchanger and to minimize the corrosion phenomenon, a new reactor concept, based on the use of ultrasonics under cavitation conditions to speed up the reaction rate, was developed in our laboratory.22 Ultrasound effects have already been used in WAO processes in order to increase COD reduction23,24 and in supercritical extraction in order to improve mass transfer.25,26 In environmental protection, ultrasound effects are used in biological and chemical decontamination in more conventional processes.27-30 In this paper, we present the concept of a hydrothermal sonochemical reactor. The ultrasonic apparatus was used to accelerate the oxidation reaction rate in order to obtain the same conversion results at lower temperature and pressure than are used in standard SCWO operating conditions. The optimal working conditions of the reactor in terms of temperature and pressure are given for the degradation of acetic acid as a model
10.1021/ie0001568 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/28/2000
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4735
Figure 1. Hydrothermal sonochemical reactor.
molecule. The influence of the horn vibration amplitude on acetic acid conversion is also studied. Moreover, a study of the hydrodynamic behavior of the hydrothermal sonochemical reactor is experimentally performed in terms of the residence time distribution (Rtd), and a model is proposed. Finally, this reactor concept is validated on industrial wastewater containing salts and corrosive species. Experimental Section All experiments were performed at the continuous pilot plant facility, developed at ICMCB, which is able to treat 3 L h-1 of aqueous wastes in a pressure range from 0.1 to 50 MPa at temperatures up to 600 °C. This pilot plant has already been described elsewhere 2. A. Hydrothermal Sonochemical Reactor. The hydrothermal sonochemical reactor is depicted in Figure 1. It is a high-pressure vessel reactor (1) with a volume of 217 cm3 made of Pyrad 53NW (3.6 cm inner diameter × 210 cm length) and equipped with a titanium liner (2). The vessel was heated by an electric coil (3) and isolated by ceramic fibers (4). The feed solution (5) was brought into the vessel reactor by a plunger tube (6) in order to allow preheating of the fluid. Then, the reactive mixture left the reactor (7). Thermal regulation was performed by three thermocouples placed inside the vessel reactor (8). This thermal regulation allowed for the temperature to be kept constant throughout the system even if there was another source of heat linked to the operation of the ultrasonic apparatus. In this case, the power delivered by the electric coil decreased. The ultrasonic apparatus was composed of a generator (9), Branson PGB240A (4000 W, 20 kHz); a converter (10); a booster (11); and a titanium sonic horn (12) (19 mm diameter). The horn was fashioned in a stepped shape with an amplification factor equal to 3.2. The vibration amplitude of the tip was controlled by altering the power input to the transducer (0-64 µm). The vessel reactor volume was 185 cm3 after the adjustment of the ultrasonic apparatus. The mechanical fixing of the ultrasonic apparatus onto the hydrothermal reactor was performed at the horn nodal point (13). At the exit of
the hydrothermal sonochemical reactor, a high-pressure UV cell was positioned to determine the Rtd of the molecules in the reactor in order to model the reactor’s hydrodynamic behavior. B. High-Pressure UV Cell. The high-pressure UV cell was made of 316 stainless steel and was equipped with two quartz windows; the optical path length was 1.08 cm with a sample volume of 3.5 cm3. The highpressure UV cell was connected to the UV-visible spectrophotometer (Varian Cary EN61010-1) through optical fibers. Rtd determination is based on the injection of a tracer impulse at the reactor inlet and the measurement of the concentration evolution of this tracer at the reactor outlet. The tracer used was acetic acid because it was inert in the working conditions. The wavelength used for the UV determination of acetic acid was 220 nm. C. Analytical Materials and Methods. Organic matter concentrations in both the feed and the effluent liquid phases were determined by COD measurements (norm AFNOR T91/K). The sample was heated for 2 h with a powerful oxidant (potassium dichromate). Oxidable organic matter reduced Cr6+ ions to Cr3+ ions. The quantity of Cr3+ produced was measured by colorimetry with an accuracy of (5%. The liquid phase was also analyzed by high-performance ionic chromatography (HPIC, DX 120 Dionex) to determine the acetic acid concentration. The column system used was: Ion Pac AS12A 4 mm + Ion Pac AG12A 4 mm. Elution was performed at a flow rate of 1 mL min-1 with a solution at 2.7 mM of Na2CO3 and 0.3 mM of NaHCO3. The measured accuracy was evaluated at (2.5%. Furthermore, the feed and effluent liquid phases of the industrial waste were analyzed by inductive coupling plasma (ICP) to measure concentrations of ionic species (PO43-, SO42-, Na+, and Ca2+) and by HPIC to determine the NH4+ and Cl- concentrations. The composition of the gas effluent was analyzed online by gas chromatography (GC) (Varian star 3600CX). Helium was used as the carrier gas (He flow ) 30 mL min-1). The apparatus was equipped with a thermal conductivity detector. The temperatures of the injector, the column, and the detector were 150, 40, and 180 °C, respectively. Results and Discussion Commercially available sonic horns are proposed to work under standard conditions (atmospheric pressure and room temperature). Thus, the influence of temperature and pressure on the vibrational frequency of the sonic horn had to be studied first. A. Influence of Temperature and Pressure on the Sonic Horn Vibrational Frequency. The influence of temperature on the sonic horn vibrational frequency was studied at 25 MPa. The generator was equipped with a frequency self-adjustment system that can follow the evolution of the vibrational frequency of the sonic horn in a restricted frequency range (19.5020.50 kHz). When the sonic horn vibrational frequency is out of this range, the generator trips. This implies that the sonic horn only works in a defined temperature field. The sonic horn vibrational frequency decreased when the temperature increased, as classically observed. Indeed, the sonic horn vibrational frequency decreased regularly from 25 to 180 °C. At 180 °C, the frequency
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Figure 2. Operating conditions in the pressure-temperature diagram of pure water.
was below 19.50 kHz, and the generator tripped. Then, the generator tuned to a new sonic horn vibrational frequency at 200 °C (19.86 kHz), and the vibrational frequency dropped again with temperature up to 250 °C. Therefore, this sonic horn could work at 25 MPa from 25 to 150 °C and from 200 to 250 °C. Very little information has been published in the literature concerning the behavior of sonic horns under pressure, so the influence of pressure on the sonic horn vibrational frequency was studied. The sonic horn vibrational frequency was not significantly influenced by pressure. The average value was equal to 19.86 kHz between 2.5 and 25 MPa at 220 °C. This result may be correlated with low fluid density fluctuations, i.e., 0.840 and 0.859 kg m-3 for 2.5 and 25 MPa, respectively. Moreover, the sonic horn titanium density variation could be considered equal to zero in this pressure range. In conclusion, our sonic horn can efficiently work from room temperature to 150 °C and from 200 to 230-240 °C for pressures ranging from 0.1 to 25 MPa. B. Ultrasonic Activation in the P-T Phase Diagram. The propagation of acoustic waves induces compression and rarefaction zones in liquid media, which create local pressure variations. In the rarefaction zones, the formation of bubbles corresponds to an adiabatic expansion from the liquid phase to the gas phase. The bubbles are produced only if the difference between the macroscopic hydrostatic pressure and the vapor pressure of the liquid (at a given temperature) is lower than the microscopic variation in pressure induced by the acoustic wave propagation. In the compression zones, the bubbles drastically collapse and release energy into the medium. This cavitation phenomenon improves physical and chemical mechanisms. Physical effects are characterized by high rates of micromixing and cleaning of the reactor wall. Chemical effects, reaction activation, are also a consequence of the collapse. In fact, the “hot spot” theory31 predicts temperatures of many thousands of Kelvin and pressure of a few 10s of megaPascals inside the bubbles during the final compression. Figure 2 reports the phase diagram of pure water. Under supercritical conditions ([AB] in Figure 2, P g 22.1 MPa and T g 374 °C), the cavitation phenomenon cannot be expected because the medium remains monophasic. However, ultrasound effects have been shown in CO2 supercritical extraction,26 and mechanical
effects have been involved. In the homogeneous subcritical domain [BC] (T e 374 °C), cavitation can be observed if the local pressure variation generated by the sonic horn vibration is greater than the difference between the hydrostatic pressure and the vapor pressure of water at a given temperature (100 °C e T e 374 °C). If the local pressure variation generated by the sonic horn vibration is too low, then the hydrostatic pressure of the water must be reduced to obtain cavitation [C f D]. The local pressure variation induced by sonic wave propagation is linked with the sonic horn vibration amplitude. This domain of pressure and temperature is very interesting in order to overcome salt precipitation and corrosion. C. Ultrasonic Activation of Acetic Acid Hydrothermal Oxidation. In accordance with the acoustic characterization of the sonochemical reactor and the positioning of sonochemical activation in the pressuretemperature phase diagram of pure water, we first studied the effects of cavitation on the hydrothermal oxidation reaction in a pressure range from 2.7 to 25 MPa and a temperature range from 80 to 220 °C. Acetic acid was chosen as the model molecule. All experiments were performed with hydrogen peroxide as the oxidizer. The quantity of H2O2 used was in excess by 30% of the stoichiometric requirement according to the following equation:
CH3COOH + 4H2O2 f 2CO2 + 6H2O
(1)
All experiments were performed both without and with ultrasound and were called oxidation tests (without ultrasound) and sonooxidation tests (with ultrasound). The hydrothermal sonochemical reactor was controlled by different parameters: pressure, temperature, space time, and sonic horn vibration amplitude for tests performed with ultrasound. The space time (τ in s) is defined by32
τ)
FVR Qm
(2)
with F, the density in kg m-3; VR, the reactor volume in m3; and Qm, the mass flow in kg s-1. The space time represents the time required for a fluid volume equal to the reactor volume to pass through the reactor. The sonochemical reactor volume was 185 cm3, and the mass flow was an adjustable parameter in order to obtain the desired space time. For each sample, the acetic acid concentration and COD were measured. Only results concerning acetic acid concentration are presented because both methods lead to correlated results. There is almost no byproduct formation in the liquid phase in agreement with the literature.33 The acetic acid concentration was selected because the results obtained by HPIC ((2.5%) are more precise than those obtained by COD ((5%). The effects of ultrasound on the hydrothermal oxidation reaction of acetic acid at 25 MPa and 100, 150, and 220 °C were studied first. The results of acetic acid concentration reduction versus temperature are shown in Figure 3. The oxidation and sonooxidation tests were performed with a space time equal to 15 min. The rate of removal of acetic acid decreased with temperature for the oxidation and sonooxidation tests. The acetic acid concentration reduction reached 80% at 220 °C, and no ultrasonic activation effect was observed
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4737
Figure 3. Acetic acid concentration reduction versus temperature at 25 MPa (H2O2 1:1.3, τ ) 15 min, and A ) 38.4 µm).
Figure 4. Acetic acid concentration reduction versus pressure at 220 °C (H2O2 1:1.3, τ ) 10 min, and A ) 38.4 µm).
for these pressure-temperature conditions. Because the reduction in the acetic acid concentration was almost the same for the oxidation and sonooxidation tests, the local pressure variation generated by the propagation of acoustic waves was too low to induce cavitation. From these experimental results, the pressure was reduced to obtain the cavitation phenomenon ([C f D] on Figure 2). The temperature and space time in the reactor were fixed at 220 °C and 10 min, respectively. Concerning the liquid phase, the influence of pressure on the reduction of the acetic acid concentration at 220
°C and for a space time of 10 min is presented in Figure 4 for the oxidation and sonooxidation tests. For the oxidation tests, the reduction in the acetic acid concentration remained constant in the pressure range studied. The average concentration reduction was 60%, as determined classically. Under these operating conditions (2.7 e P e 25 MPa and T ) 220 °C), the reactants are soluble, and the reaction activation volume is not really influenced by pressure effects. For the sonooxidation tests, in the pressure range of 5-25 MPa, the results are close to those for oxidation: no cavitation phenomenon was observed. At 220 °C, the water vapor pressure is equal to 2.32 MPa. Finally, ultrasonic activation appeared at 2.8 MPa for pressures just above the water vapor pressure. Under these conditions, the acetic acid concentration reduction was greatly increased at 83%. Thus, ultrasonic activation increased the yield of the acetic acid oxidation reaction by approximately 40%. Concerning the gas phase, only oxygen and carbon dioxide were detected. From a general point of view, the acetic acid degradation obtained with the sonochemical reactor is better than that obtained with the WAO process at the same temperature without a catalytic system and with the SCWO process at 400 °C (Table 1). It is interesting to note that the sonochemical reactor at 2.8 MPa and 220 °C is more efficient than the process with KMnO4 at 29 MPa and 400 °C for the same reaction time. Thus, the sonochemical hydrothermal oxidation process allows for a very efficient treatment to be obtained, which reduces considerably the working conditions of the reactor (2.8 MPa, 220 °C). Under these conditions, the problems of salt precipitation and corrosion can be overcome according to the literature. Furthermore, by lowering the pressure and temperature, the investment costs (high-pressure pump, reactor material, heat exchanger, etc.) and the costs of the energetic power supply (compression cost, preheating cost,...) decrease. D. Influence of the Sonic Horn Vibration Amplitude on Ultrasonic Activation of Acetic Acid Hydrothermal Oxidation. One of the operating parameters of the sonochemical reactor is the sonic horn vibration amplitude. The influence of this parameter on ultrasonic activation was studied with tests at 38.4 and 64 µm for a space time of 10 min at 2.8 MPa and 220
Table 1. Comparison of Hydrothermal Sonochemical Reactor Performance with the Main Hydrothermal Oxidation Processes process
working conditionsa-c
hydrothermal sonochemical oxidation hydrothermal oxidation
P ) 2.8 MPa, T ) 220 °C, C0 ) 1.5 g L-1, H2O2 (1:1.3), τ ) 10 min P ) 2.8 MPa, T ) 220 °C, C0 ) 1.5 g L-1, H2O2 (1:1.3), τ ) 10 min T ) 247 °C, C0 ) 12.5 g L-1, Po2 ) 1 MPa, τ ) 60 min T ) 248 °C, C0 ) 12.5 g L-1, Po2 ) 1 MPa, τ ) 60 min, Co:Bi (5:1) at 20 mM P ) 27.6 MPa, T ) 400 °C, C0 ) 2.5 g L-1, H2O2 (1:1), τ ) 3.7 min P ) 25 MPa, T ) 505 °C, C0 ) 1.5 g L-1, H2O2 (1:1.3), τ ) 20 s P ) 24.6 MPa, T ) 437 °C, C0 ) 58.8 mg L-1, O2 (2:1), τ ) 7.8 s P ) 24.6 MPa, T ) 600 °C, C0 ) 58.8 mg L-1, O2 (2:1), τ ) 8.5 s P ) 29 MPa, T ) 400 °C, C0 ) 2.5 g L-1, KMnO4 (1:1), τ ) 10 min
wet air oxidation (WAO) catalytic wet air oxidation supercritical water oxidation (SCWO)
conversiond
ref
∆C ) 83%
this work
∆C ) 60%
this work
∆C ) 42%
34
∆C ) 99.9%
34
∆C ) 58%
35
∆C ) 99.2%
this work
∆C ) 8%
33
∆C ) 99.9%
33
∆C ) 76%
36
a C , the initial acetic acid concentration. b KMnO or H O or O (a:1) with a the excess of oxidizer with regard to stoichiometry. c τ, 0 4 2 2 2 the space time in the reactor. d ∆C, the acetic acid concentration reduction.
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Table 2. Influence of the Sonic Horn Vibration Amplitude on Electric Power Consumed by the Generator and Acetic Acid Concentration Reduction Aa in µm
Pb in W
Xc in %
0 38.4 64
0 500 1380
61.0 ( 1.5 83.1 ( 0.8 85.9 ( 0.6
Table 3. Rtd Function Analysis: Determination of τ, tsm, and σ2 without ultrasound with ultrasounds
τa in min
tsmb in min
σ2 c
9.825 9.858
8.991 9.783
44.4 47.4
a τ, the space time. b tsm, the mean residence time. c σ2, the mean deviation to the mean residence time.
a A, the sonic horn vibration amplitude. b P, the electric power consumed by the generator. c X, the acetic acid concentration reduction.
mean residence time. n
σ2 )
(ti - tsm)2E(ti)∆t ∑ i)1
(4)
The σ2 value for a real reactor is between σ2 for a plug flow reactor (0) and σ2 for a continuously stirred tank reactor (τ2). The values of τ, tsm, and σ2 are presented in Table 3. Experiments performed without and with ultrasound led to tsm values that were smaller than τ, indicating certainly the existence of dead zones (Vm) in the reactor.37 Vm is defined by
(
V m ) VR 1 Figure 5. Experimental and simulated Rtd function versus time (2.8 MPa, 220 °C, and τ ) 9.8 min).
°C. Results are shown in Table 2. The reduction of the acetic acid concentration was almost independent of the sonic horn vibration amplitude. However, the electric power consumed by the ultrasonic system depended on the sonic horn vibration amplitude. Thus, the optimal sonic horn vibration amplitude is the minimal sonic horn vibration amplitude for which the cavitation phenomenon can be observed. E. Hydrodynamic Behavior of the Hydrothermal Sonochemical Reactor. The hydrodynamic behavior of a real reactor can be characterized experimentally by the determination of the Rtd. Knowledge of the Rtd allows one to model the reactor hydrodynamic behavior from a combination of elementary modules. Experimental Rtd Determination. The acetic acid was injected as a tracer (0.484 cm3) at the reactor inlet. We have experimentally verified that the acetic acid impulse injected at the reactor inlet could be considered as a Dirac function. The high-pressure UV cell was disposed at the reactor outlet to follow the acetic acid concentration distribution. Six experiments were performed with pure water as the fluid at 2.8 MPa and 220 °C for a space time in the reactor of 10 min without and with ultrasound. The Rtd function, E(t), versus time is reported in Figure 5. The Rtd functions characterize the behavior of our reactor and allow us to calculate the mean residence time (tsm), which represents the statistical average of the residence times. The mean residence time, the firstorder moment of the Rtd function, is defined by n
tsm )
tiE(ti)∆t ∑ i)1
(3)
with ti, the time and ∆t ) ti+1 - ti. The second-order moment of the Rtd function, the variance σ2, represents the mean deviation from the
tsm τ
)
(5)
The dead volumes represented 8.5 and 0.8% of the reactor volume for experiments performed without and with ultrasound, respectively. This result was correlated with the calculated value of tsm. Indeed, tsm for experiments performed without ultrasound was smaller than that evaluated for experiments with ultrasound. Therefore, ultrasonic cavitation reduced the dead volume of the reactor and affected the hydrodynamic behavior of the reactor. However, hydrodynamic effects alone could not explain the difference in acetic acid destruction between runs performed without and with ultrasound. Ultrasonic cavitation really induced an effect on the oxidation reaction, speeding up its rate. Sonochemical Reactor Modeling. A reactor model was proposed to determine the reactor operating equation, to simulate the reactor behavior, and to scale-up new sonochemical reactors in the development phase of the process. Experimental investigations showed that the sonochemical reactor was neither a plug-flow reactor nor a continuously stirred tank reactor (CSTR). However, the experimental Rtd profiles seemed to be closer to those of a continuously stirred tank reactor battery than to those of a plug-flow reactor. Thus, we proposed an empirical model, constituted of a battery of three CSTRs of different volumes. The mathematical formalism is based on the calculation of the transfer function G(s) of the model defined by32
G(s) )
Laplace transform of the output signal Laplace transform of the input signal
(6)
For the proposed model, the transfer function is equal to the product of the three CTSR transfer functions (linear system). The transfer function of a CTSR is wellknown.34
G(s) )
1 1 + τs
(7)
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4739 Table 4. Fitted Values of Model Parameters without ultrasound with ultrasounds
Table 6. Salt Recovery at the Reactor Outlet in Percent
R
β
1-R-β
0.12 0.20
0.04 0.05
0.84 0.75
without ultrasound with ultrasounds
Table 5. Initial COD and Salt Concentration of Aqueous Waste in mg.L-1 PO43-
SO42-
Na+
Ca2+
Cl-
NH4+
COD
145
195
300
108
120
96
3000
Thus, the transfer function of our model is
G(s) )
1 (8) (1 + Rτs)(1 + βτs)[1 + (1 - R - β)τs]
From a table of Laplace operations, G(s) is transformed into a real function F(t) with the following form:
[
F(t) ) (-abc) -
]
(b - c)eat + (c - a)ebt + (a - b)ect (a - b)(b - c)(c - a) (9)
with a ) -1/Rτ, b ) -1/βτ, and c ) -1/(1 - R - β)τ In addition
Esimulation(t) )
F(t)
∫0 F(t) dt ∞
(10)
The model is composed of two parameters R and β, which are adjusted to fit the simulated Rtd to the experimental Rtd from the six experiments presented in the preceding section. The simulation results are presented in Table 4. The simulated Rtd functions are very close to experimental ones (Figure 5). From this modeling, we now work on a software development to simulate the sonochemical reactor behavior. After evaluation of the performance of ultrasonic cavitation in accelerating the oxidation reaction rate at relatively low temperature and pressure, the sonochemical reactor, developed to overcome salt precipitation and corrosion, was validated with an industrial waste of chemical industry. E. Sonochemical Reactor Validation with an Industrial Waste. The sonochemical reactor was tested with an industrial aqueous waste, whose characteristics are reported in Table 5. Experiments were performed for 4 h at 2.8 MPa and 220 °C for a space time in the reactor of 10 min, without and with ultrasound. For the oxidation experiments, a quantity of deposited organic matter (fibers) formed on the reactor wall and disturbed the COD reduction calculation. Therefore, the final COD reduction was obtained with poor accuracy and was not reported in this paper. For the experiments performed with ultrasound, the COD reduction was equal to 86%, and only carbon dioxide was detected in the gas phase. Most ions precipitated in the reactor for experiments performed without ultrasound. Only Na+ and Cl- were almost recovered at the reactor outlet. (See Table 6.) For the experiments performed with ultrasound, the percentage of salt recovery at the reactor outlet was greater than 90%. Thus, ultrasound considerably improved salt transport.
PO43-
SO42-
Na+
Ca2+
Cl-
6 92
45 95
86 90
22 94
95 97
Furthermore, no corrosion product was detected in the liquid effluent, and no corrosion trace was observed on the titanium liner. Therefore, our hydrothermal sonochemical reactor, protected by a titanium liner, is very efficient at allowing a COD reduction of 86% to be attained and salt precipitation and corrosion to be overcome. Conclusions The new concept of a sonochemical hydrothermal oxidation reactor was developed. This reactor, equipped with a commercial sonic horn, operated from room temperature to 150 °C and from 200 to 230 °C for pressures ranging from 0.1 to 25 MPa. The optimal operating conditions were determined to be 220 °C and 2.8 MPa. Under these conditions, the ultrasonic activation increased the yield of acetic acid oxidation reaction of 40%. Therefore, the performance of the sonochemical reactor is better than that of hydrothermal oxidation processes operating under subcritical conditions without a catalytic system. The hydrodynamic study of our reactor, from the Rtd determination, showed that ultrasonic cavitation actually induced an effect on the oxidation reaction, speeding up its kinetic rate. The hydrothermal sonochemical reactor was modeled from a battery of three continuously stirred tank reactors. The efficiency of the sonochemical reactor operation was validated with the treatment of a waste from the chemical industry. We now work on another new concept of hydrothermal reactor for overcoming salt precipitation and corrosion, based on an electrochemical process.38 This process can supply nascent oxidizer in situ to the reaction media and allow for a reduction in the operating conditions in comparison with those of SCWO. Literature Cited (1) Cansell, F.; Beslin, P.; Berdeu, B. Hydrothermal Oxidation of Model Molecules and Industrial Wastes. Environ. Prog. 1998, 17, 4, 240. (2) Aymonier, C.; Beslin, P.; Jolivalt, C.; Cansell, F. Hydrothermal Oxidation of a Nitrogen Containing Compound: The Fenuron. J. Supercrit. Fluids. 2000, 17, 45. (3) Hong, G. T.; Killilea, W. R.; Thomason, T. B. Method for Solids Separation in a Wet Oxidation Type Process. U.S. Patent 4,822,497, 1989. (4) Huang, C. Y. Apparatus and Method for Supercritical Water Oxidation. U.S. Patent 5,100,560, 1992. (5) Huang, C. Y.; Barner, H. E.; Albano, J. V.; Killilea, W. R.; Hong, G. T. Method for Supercritical Water Oxidation. Patent WO 92/21621, 1992. (6) Hazlebeck, D. A. Downflow Hydrothermal Treatment. Eur. Patent 0905090A2, 1998. (7) Boukis, N.; Landvatter, R.; Habicht, W.; Franz, G.; Leistikow, S.; Kraft, R.; Jacobi, O. First Experimental SCWO Corrosion Results of Ni-Base Alloys Fabricated as Pressure Tubes and Exposed to Oxygen Containing Diluted Hydrochloric Acid at T e 450 °C, P ) 24 MPa. First International Workshop on Supercritical Water Oxidation, Jacksonville, FL, 1995; Session VIII. (8) Latanision, R. M.; Mitton, D. B.; Zhang, S. H.; Cline, J. A.; Caputy, N.; Arias, T. A.; Rigos, A. The 4th International Symposium on Supercritical Fluids, Sendaı¨, Japan, 1997; C, p 865. (9) Kritzer, P.; Boukis, N.; Dinjus, E. Investigations of the Corrosion of Reactor Materials during the Process of Supercritical
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Received for review February 3, 2000 Revised manuscript received September 1, 2000 Accepted September 1, 2000 IE0001568
