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Ind. Eng. Chem. Res. 2005, 44, 9014-9019
Supercritical Water Oxidation of Thiodiglycol Bambang Veriansyah,† Jae-Duck Kim,*,† and Jong-Chol Lee‡ Supercritical Fluid Research Laboratory, Clean Technology Research Center, Korea Institute of Science and Technology (KIST)sDepartment of Green Process and System Engineering, University of Science and Technology (UST), P.O. Box 131, Cheongryang, Seoul 130-650, Korea, and Agency for Defense Development (ADD), P.O. Box 35-1, Yuseong, Daejon, Korea
The total organic carbon (TOC) disappearance kinetics of the model organic sulfur heteroatom, thiodiglycol [TDG, (HOC2H4)2S], was investigated in an isothermal continuous tubular reactor under supercritical water oxidation (SCWO) conditions. The experiments were conducted at a temperature of 397-617 °C and a fixed pressure of 25 MPa, with a residence time that ranged from 9 s to 40 s. The initial TOC concentrations of TDG were varied from 1.75 mmol/L to 21.04 mmol/L and the oxygen concentrations were varied from 110% to 440% of the stoichiometric requirement. All the TOC conversions were >90% under the above experimental conditions. The major sulfur-containing products of the reaction were sulfuric acid and hydrogen sulfide, whereas the carbon-containing products included carbon monoxide, methane, and carbon dioxide. By taking into account the dependence of the oxidant and TOC concentration on the reaction rate, a global TOC disappearance rates expression was regressed from the data of 58 experiments, to a 95% confidence level. The resulting activation energy was determined to be 41.53 ( 1.65 kJ/mol, and the pre-exponential factor was (1.64 ( 0.55) × 102 L1.12 mmol-0.12 s-1. The reaction orders for the TOC and the oxidant were 1.02 ( 0.01 and 0.10 ( 0.01, respectively. Introduction Supercritical water oxidation (SCWO) provides a potential alternative for processing hazardous military waste without the concomitant production of noxious byproducts, as might be experienced with combustionbased technologies.1 SCWO uses supercritical water (Tc ) 374 °C and Pc ) 22.1 MPa) as a reaction medium and exploits the unique solvating properties to provide enhanced solubility of organic reactants and permanent gases (such as oxygen and carbon dioxide), a singlephase environment free of interphase mass-transfer limitations, faster reaction kinetics, and an increased selectivity to complete oxidation products.2-5 SCWO has most often been used to treat dilute organic waste streams that can be otherwise difficult to remediate. It already proved that SCWO is an environment-friendly waste treatment technology that produces disposable clean liquid (pure water), clean solids (metal oxides), and clean gases (CO2 and N2).6-14 Information about the kinetics and byproducts by oxidation of real aqueous pollutants is essential for the design, optimization, and control of reliable commercial SCWO reactors. Knowledge of the reaction kinetics allows one to calculate the residence times required for a desired destruction and removal efficiency in a commercial SCWO reactor.15 In the last two decades, many authors have studied kinetic parameters of SCWO for several real wastes and model compounds. However, almost all of these reports have concentrated on the kinetics of reactant disappearance, rather than the disappearance rate of the total organic carbon (TOC). Martino and Savage16 have demonstrated, for the kinetic studies of monosubstituted phenols in SCWO, * To whom correspondence should be addressed. Tel: +822-958-5873. Fax: +82-2-958-5879. E-mail:
[email protected]. † KISTsUST. ‡ ADD.
that the rate of TOC disappearance is always slower than that of reactant disappearance. It can be concluded that the kinetics of reactant disappearance is not sufficient for the control of reliable kinetic information. Therefore, the kinetics of TOC disappearance is needed and has significance. The present study examines the SCWO kinetics of the model organic sulfur heteroatom, thiodiglycol (TDG), based on TOC disappearance rate. TDG was chosen because it is a key refractory product in the hydrolysis of HD mustard, (ClC2H4)2S.17 It has the same C-S bond arrangement as HD mustard with a similar density.18 Quantitative information regarding the TDG oxidation kinetics in supercritical water (SCW) is important and pertinent for SCWO process design and optimization. There have been two previous experimental studies of TDG oxidation in SCW.19,20 Turner19 conducted oxidation at temperatures in the range of 425-525 °C, a pressure of 27.6 MPa, and residence time of 5-17 s to identify major byproducts and to establish a global pathway. The major products of the reaction were sulfuric acid (H2SO4) in the liquid phase and carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4) in the gas phase. Because of the problem associated with corrosion, a kinetic model was not developed. Transformation products were evaluated and CO and CH4 were identified as the rate-limiting intermediates. More recently, Lachance et al.20 performed the hydrolysis and oxidation of TDG in subcritical and supercritical water. The experiments were performed in the temperature range of 100-525 °C at a pressure of ∼25 MPa. In essence, Lachance et al.’s experiment focused on measuring the kinetics of TDG hydrolysis in subcritical water and finding a condition in SCW that would give only partial conversion. A global rate equation for TDG hydrolysis in subcritical water from Lachance et al.’s data yielded first-order behavior to TDG concentration. Oxidation rates could not be determined quanti-
10.1021/ie050482t CCC: $30.25 © 2005 American Chemical Society Published on Web 10/29/2005
Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 9015 Table 1. Experimental Conditions for Thiodigylcol (TDG) Oxidation in Supercritical Water experimental variable pressure, P temperature, T residence time, τ initial total organic carbon (TOC) concentrationa oxygen concentration
range of values 25 MPa 397-617 °C 9-40 s 1.75-21.04 mmol/L 110%-440% of the O2 stoichiometric requirement
a The TOC concentration at the reactor inlet was calculated from the feedstock concentration and flow rates of the feed streams with the process condition.
tatively; however, they suggest that complete conversion of TDG to CO2, H2O, and mostly elemental sulfur and sulfates occurs within a few seconds at temperatures >400 °C at 25 MPa. The primary goal of our work was to characterize the TOC disappearance kinetics of TDG oxidation in SCW at high conversion to represent more-practical treatment application. Thus, we explored factors that systematically affected TOC conversion, such as temperature, residence time, oxygen concentration, and TOC concentration under the experimental conditions shown in Table 1. A global TOC disappearance rates expression was obtained with the dependence on TOC and oxygen concentration with significantly lower uncertainty. Experimental Section Apparatus and Procedure. The experiments were conducted in a laboratory-scale, continuous-flow SCWO reactor system. A schematic diagram of the system for SCWO experimental apparatus is shown in Figure 1. The system involves two parallel sets of equipment that are almost identical: one for delivering the TDG solution and the other for the oxidant. All hot sections of the system were insulated in boxes of ceramic board, and the temperature was monitored directly using a thermocouple. The system temperature was controlled by a temperature controller (Hanyoung, model DX 7). Oxygen, which was the oxidant used in these experiments, was prepared by heating an aqueous solution of hydrogen peroxide (H2O2) so that the H2O2 decomposed to form oxygen gas (O2) and water (H2O). Complete conversion of H2O2 to O2 and H2O in the preheater line was verified experimentally.21,22 TDG and oxidant solution were pumped separately into the reaction system using high-pressure pumps (Thermo Separation Product Company). The oxidant solution was preheated in 6 m of 1/8 in. (3.175 mm) outer diameter (O.D.) stainless steel (SS316) tubing. TDG solution flowed directly into the reactor entrance without being preheated, to avoid the degradation of TDG. The solutions mixed at the reactor entrance in a Hastelloy C-276 cross and then entered the reactor, which was constructed by a 0.3-m length of 31.7 mm O.D. and 10 mm inside diameter (I.D.) Hastelloy C-276 tubing with an internal volume of 23.6 cm3. The flow rate ratios of TDG solution to the preheated oxidant solution were small, to keep the temperature of the mixed streams as close as possible to the reaction temperature. Upon leaving the reactor, the effluent was cooled rapidly in a shell and tube heat exchanger and, afterward, the solid particles were filtered out by a 0.5µm in-line metal filter before it was depressurized to ambient condition by a back-pressure regulator (Tescom
Figure 1. Schematic diagram of the continuous-flow reactor system for the supercritical water oxidation (SCWO) experiment.
Co., model 26-1721-24). After the fluid exited the regulator, it flashed to atmospheric pressure and the two-phase mixture was separated into two streams by the gas-liquid separator. The gas flow rate was measured using a soap bubble flow meter, while liquid flow rate was measured by recording the time required to fill a volumetric flask. The gaseous effluent was injected into the two gas chromatographs while the liquid samples were collected and injected onto a TOC analyzer and subjected to ion chromatography (IC). For further analysis, liquid samples were collected in glass sample vials, capped with zero headspace, and stored in a 4 °C refrigerator until analysis. For each experimental run, the equipment was handled precisely and accurately, to minimize errors that are related to control of the experimental conditions. The reactor was stabilized for at least an hour at each set of operating conditions, to ensure steady-state operation. Measurements were then taken over the course of another hour, at which point at least four liquid and three gas samples were drawn and at least eight liquid and gas flow rates were recorded. The inlet TOC of the TDG feed concentration was measured from at least four samplesstwo of each taken at the beginning and at the end of the set of runssto ensure that the feed concentration remained at the temperature, pressure, and flow rate in the system are constant and monitored continuously. Typically, the temperature was held constant within (2 °C over the length of the reactor, and the pressure was fixed at 25 ( 0.02 MPa as well as over the entire reaction time; the flow rate of oxidizer and TDG solution were each maintained to (2%. Material and Analytical Methods. TDG feed was prepared by making an aqueous solution of TDG (Aldrich, 99+% purity), which was used as received
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and dissolved in distilled and deionized (DDI) water. The H2O2 feed solution was prepared by diluting a 35% w/v aqueous solution H2O2 (Junsei Chemical Company) with DDI water to the desired concentration. The electrical resistance of the DDI water that was used in all experimental and analytical processes was ∼18 MΩ. All of the standard solutions for calibration and identification in liquid analytical methods were prepared using high-purity chemicals (Aldrich, ACS grade). Gas samples were analyzed using two HewlettPackard model 5890 Series II gas chromatographs with a thermal conductivity detector (TCD) and helium as the carrier gas. Both gas chromatographs had a Rheodyne single-mode injection valve installed in them, which provided on-line sample injection into the gas column. The first gas chromatograph was used to quantify hydrogen sulfide (H2S) and light hydrocarbons, such as CH4. This gas chromatograph was equipped with a 9 ft (2.74 m) × 1/8 in. (3.2 mm) SS Hayesep Q80100 column. The oven component of the gas chromatograph was operated isothermally at 90 °C with a helium carrier flow rate of 30 mL/min. The second gas chromatograph was used to identify the concentration of light gases, such as oxygen, nitrogen, CO, and CO2. The column was a 10 ft (3.05 m) × 1/8 in. (3.2 mm) Supelco Carbosieve S-II. A temperature ramp from 35 °C to 225 °C (at 32 °C/min) was used. The TOC concentration of the TDG feed and the liquid-phase reactor effluent were analyzed using a TOC analyzer (Shimadzu, model TOC-VCSH), which is based on the combustion catalytic oxidation method, and the highly sensitive nondispersive infrared (NDIR) detection, respectively. Samples were analyzed in triplicates. The relative standard deviation (RSD) for the TOC measurement falls between 0.2 and 2.5% and the averages are reported as result. TOC conversion used to evaluate the TOC disappearance efficiency by SCWO, X, was defined as
(
X) 1-
)
[TOC]f [TOC]i
(1)
where [TOC]i and [TOC]f are the concentrations at the reactor inlet and outlet, respectively. IC (Dionex DX-100) was used to determine concentration of sulfate in the effluent. The IC system was equipped with an anion column (Star-Ion A300) and an anion self-regenerating suppressor (ASRS-Ultra II). The signal output was recorded using a Hitachi integrator (model D-2500). Analysis of the filtered solids was performed using secondary ion mass spectrometry (PHI 7200 ToF SIMS) and an elemental analyzer (Leco CS600), to determine the elemental composition in the filtered solids. Results and Discussion Fifty-eight SCWO experiments at different temperatures, residence times, and initial concentrations of TOC and oxygen were conducted in an isothermal and isobaric flow reactor. A stoichiometric level of oxygen was determined using the assumption of the complete oxidation of TDG (as follows):
(HOC2H4)2S + 7O2 f 4CO2 + H2SO4 + 4H2O
(2)
Figure 2. Correlation between the thiodiglycol (TDG) and total organic carbon (TOC) concentrations.
The excess of oxygen is defined as
[O2]excess (%) )
[O2]initial - [O2]stoichiometric [O2]stoichiometric
× 100 (3)
The initial TOC and oxidant concentration is the concentration at the reactor entrance, as described in previous published work.23 Because the TOC concentration is used to develop the kinetic rate model, the correlation equation between TDG concentrations with TOC concentration may be useful in preparing the initial organic feed. Figure 2 shows the correlation between the TDG concentration and the TOC concentration from the data of 58 experiments. It compares well with the TDG concentration and TOC concentration, with a correlation coefficient of R2 ) 0.9996. The reaction products included gaseous, liquid components, and very few solid components. In all cases, the solid collected from the filter appeared as a blackbrown colored particles and the amount was too little for analysis. Thus, the particles from 58 experiments were combined to analyze the elemental composition using SIMS and an elemental analyzer. The result showed that nickel, chromium, and iron comprised most of the solid phase, which led us to believe that corrosion was occurred under all conditions. Upon quantification, sulfur accounted for ∼0.6 wt % of the solid phase and this observation indicated that, during the oxidation in SCW, only a small amount of sulfur is precipitated. A mass balance for both carbon and sulfur species were conducted for all experiments, based on the measured inlet TDG concentration and outlet product. The incoming carbon was based on carbon in the feed TDG, and the outgoing carbon was the sum of the carbon in the liquid and gas effluents. The detectable carbon-containing products and intermediates were CO, CH4, and CO2, all measured in the gas phase. Therefore, Henry’s law was used to calculate the dissolved CO, CH4, and CO2 concentrations in the liquid effluent, based on the concentration in the off-gas.24 The carbon balances had an average value of 100% ( 2.5%, with a range of 95%-106%. As for the sulfur balance, the incoming sulfur was based on sulfur in the feed TDG, and the outgoing sulfur was based on the sum of the sulfur in the liquid and gas effluents. The sulfur content in the solid components was excluded in the sulfur balance, because it was very
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Figure 3. Effect of temperature and residence time on TOC conversion.
small (small enough to be negligible). The detectable sulfur-containing products were H2SO4 and H2S. On the basis of this calculation, the sulfur balances had an average value of 98.4% ( 3.3%, with a range of 89%105%. The H2S concentration constituted less than 0.5% of the sulfur balance for all experiments, and it indicated that H2S was oxidized rapidly in all experiments, in good agreement with Turner’s experiment.19 TOC Conversion. The effects of temperature and residence time on TOC conversion were determined by a set of 11 experiments at three reaction temperatures (400, 450, and 500 °C), an oxygen excess of 188%, and an initial TOC concentration of 2.39 mmol/L. Figure 3 shows that the conversion of TOC increases with higher reaction temperature and longer residence time. TOC conversions were all >90%. The carbon fraction of the intermediates (CH4 and CO) and the final carbon containing product (CO2) are plotted in Figure 4 as a function of temperature at a residence time of 23 s. The carbon fraction for species i is defined as
species i carbon fraction ) moles of carbon in product i (4) total moles of carbon feed As shown in Figure 4, CO2 was the major oxidation product of TDG, whereas CO and CH4 is a minor product at high temperature (500 °C). Within these experimental temperature ranges, CO is a reactive intermediate and its fraction decreases with higher temperature, whereas CH4 is a refractory intermediate, because its fraction is relatively stable with temperature. The fraction of CO2 increased continually over the experimental temperature ranges to a maximum value of 0.96 at 500 °C, where TOC conversion was 98.72%. Effect of Oxidant and TOC Concentration. To investigate the effect of the oxidant and TOC concentrations individually on the TOC conversion, a series of experiments was performed in which one concentration was changed while the other remained constant during the experiment. Six experiments were conducted to determine the influence of the oxidant concentration on the TOC conversion at a temperature of 506 ( 1 °C and residence times of 14 s at a fixed pressure of 25 MPa. The TOC concentration at the inlet of the reactor was fixed at 7.83 mmol/L. In these experiments, the oxygen excess was varied from 85% to 253%. As can be seen in Figure 5,
Figure 4. Species carbon fraction of CH4, CO, and CO2 in the reaction outlet stream, as a function of temperature at constant residence time.
Figure 5. Effect of oxidant feed concentration ([O2]i) on TOC conversion and species carbon fraction.
increasing the oxidant concentration enhances the TOC conversions. This is an indication of the fact that the global reaction order for oxidant is greater than zero. For the carbon fraction, the CO2 carbon fraction increased as the oxidant concentration increased, while the amount of CO decreased and the amount of CH4 slightly decreased. The influence of the TOC concentration on the TOC conversion was determined by conducting six experiments at a temperature of 505 ( 1 °C and residence times of 13 s at a fixed pressure of 25 MPa. Figure 6 shows that the TOC conversion at a given oxidant concentration increases with higher TOC concentration of the feed. This implies that the global reaction order for TOC also greater than zero. For the carbon fraction, a slight increment was observed for the CH4 and CO carbon fraction under this condition while the CO2 carbon fraction decreased. From the study of Holgate et al.,25 it was known that oxidation rate of CO to CO2 has an oxygen dependence. Under this condition, as the TOC concentration is increased at the same initial oxidant concentration, the oxygen excess is decreased. Thus, the decrease in the CO2 fraction and the increase in the CO fraction indicate that oxygen is essential for CO destruction to CO2.
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Figure 6. Effect of TOC concentration on TOC conversion and species carbon fraction.
Rate Expression of TOC Disappearance. The global power-law rate reaction is used to predict the TOC disappearance rate of the TDG oxidation in SCW and can be described as follows:
rate ) -
d[Cn] ) k[Cn]a[O2]b[H2O]c dt
(5)
where [Cn] is the concentration of reactant (given in units of mmol/L); [O2] is the concentration of oxidant (mmol/L); [H2O] is the concentration of water; t is the reaction time; a, b, and c are the reaction orders of Cn, O2, and H2O, respectively. The rate constant (k) can be expressed in an Arrhenius equation as follows:
( )
k ) A exp -
Ea RT
(6)
where A is the pre-exponential factor and Ea is the activation energy. In this study, we assumed that the global oxidation of TOC is dependent only on the temperature, the reactant concentration, and the oxidant concentration. The water concentration was assumed to have no explicit effect on the reaction rate, as is the case in many reported SCWO kinetic studies; therefore, the global power-law reaction rate can be expressed as
d[Cn] rate ) ) k[Cn]a[O2]b dt
(7)
Substituting [Cn] with [TOC] and rearranging the equation, with respect to the TOC conversion X (defined by eq 1), the relationship obtained is
-
d(1 - X) (1 - X)a[O2]b ) k[TOC]a-1 i dt
(8)
Equation 8 can be solved analytically with the initial condition X ) 0 at reaction time t ) 0 to provide relationship between the TOC conversion and the experiment variables.
[O2]ib]1/(1-a) X ) 1 - [1 - (1 - a)kt[TOC]a-1 i (for a * 1) (9) A multivariable nonlinear least-squares technique was used to estimate the kinetic parameter A, the activation energy Ea, and the reaction orders. The bestfit values were obtained by minimizing the sum of
Figure 7. Comparison of the predicted and experimental conversions.
square of the differences of experimental and predicted conversion for all data points.26-28 The sum of squares (s2) can be described as follows: N exp
s2 )
∑i (X exp - Xpred)2
(10)
where Nexp is the number of experiments, Xexp the experimental conversion, and Xpred the model-predicted TOC conversion. The quality of data fitting was evaluated by R2 in an analysis of variance (ANOVA) routine.29 It has algorithms to estimate the 95% confidence interval on each parameter and on the predicted response. Using this procedure and considering all data points, the best-fit global rate expression of TOC disappearance during SCWO of TDG was obtained as
-
d[TOC] ) (1.64 ( 0.55) × dt -41.53 ( 1.65 × 102 exp RT [TOC]1.02(0.01 × [O2]0.10(0.01 (11)
(
)
Figure 7 shows a good comparison of the measured and the predicted conversions, with R2 ) 0.956. The dashed line, which indicates a deviation of (1% conversion from the 45° line (perfect match), contains all data points. This model fits our experimental data very well. Conclusion An experimental study for the total organic carbon (TOC) disappearance kinetic of TDG oxidation in SCW at high conversion values and under oxygen excess condition was performed in a continuous-flow reactor system. Experimental data showed that TOC conversions were all >90% at temperatures in the range of 397-617 °C in a fixed pressure of 25 MPa with a reactor residence time of 9-40 s. Especially, >99.99% of TOC conversion could be achieved at a temperature of 600 °C, a pressure of 25 MPa, an oxygen concentration of
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150% stoichiometric requirement, and residence times of
