2902
Ind. Eng. Chem. Res. 1999, 38, 2902-2910
Supercritical Water Oxidation of Methylphosphonic Acid Stephen Bianchetta,*,† Lixiong Li,‡ and Earnest F. Gloyna Environmental and Water Resources Engineering Program, Civil Engineering Department, The University of Texas at Austin, Austin, Texas 78712
Laboratory-scale, continuous-flow reactor tests were conducted to confirm the destruction efficiency of methylphosphonic acid (MPA) and the effect of sodium hydroxide on MPA destruction efficiency under supercritical water oxidation (SCWO) conditions. Oxygen was used as the oxidant. The reaction temperatures ranged from 400 to 594 °C; the reactor residence times varied from 3 to 83 s; and the oxygen concentrations varied from 110 to 200% of stoichiometric requirements. Fixed parameters included (1) a nominal pressure of 27.6 MPa (4000 psi), (2) a MPA feed concentration of 1000 mg/L, (3) a feed flow rate of 25 g/min, and (4) a NaOH to MPA molar ratio of 2:1. MPA destruction efficiencies (DE) of greater than 99% were achieved at a temperature of 550 °C, oxygen concentration of 200% stoichiometric requirements, and reactor residence time of less than 20 s. On the basis of data derived from 43 MPA experiments, kinetic correlations for the DE of MPA were developed. The model predications agreed well with the experimental data. Furthermore, data derived from 22 MPA/NaOH experiments indicated that NaOH did not affect the overall effectiveness of SCWO for the destruction of MPA under the conditions investigated. Introduction In compliance with the U.S. National Environmental Policy Act (NEPA) and international treaties,1 efforts are being made to develop means to safely and effectively destroy chemical weapon stockpiles. Among various existing and innovative demilitarization technologies2 that have been evaluated, supercritical water oxidation (SCWO) has been selected as an alternative method for treating and destroying chemical warfare agents.3 Recently, the U. S. Army Program Manager for Chemical Demilitarization (PMCD) has been supporting the development of a two-step process for destroying the chemical nerve agent VX.4 This detoxification process is accomplished by a low-temperature hydrolysis of VX using concentrated sodium hydroxide followed by SCWO of the VX/NaOH hydrolysate. The U. S. Army has also evaluated the treatment of the VX/NaOH hydrolysate with ultraviolet light and hydrogen peroxide.5 Removal efficiencies for this method are typically below 90% for reaction times on the order of hours, compared to removal efficiencies of 100% for reaction times on the order of minutes or seconds for SCWO. Advances in the decontamination and destruction of chemical warfare agents have been well documented.6-9 Because of safety and cost considerations, studies in reactions involving a particular chemical agent typically begin with its simulant, which is structurally similar to, but less toxic than, that of the chemical agent. For example, dimethyl methylphosphonate (DMMP) and thiodiglycol (TDG) were used as the simulants for Sarin (agent GB) and mustard gas (agent HD), respectively.10 SCWO destruction of several agent simulants was initially explored by Modell11 for the U. S. Army. More recently, both DMMP and TDG were studied in support of the Advanced Research Projects Agency (ARPA) * To whom correspondence should be addressed. † Current address: HDR/Simpson, 1100 NE Loop 410, San Antonio, TX 78209. ‡ Current address: Applied Research Associates, Bldg 1117, Tyndall AFB, FL 32403.
Figure 1. Proposed reaction pathways for hydrolysis/oxidation of VX in supercritical water.
SCWO Program.12-14 Since agents VX and GB have a similar functional group involving methylphosphonate, DMMP was also studied as one of the key model compounds for VX. However, it was found that DMMP could readily be hydrolyzed to MPA during the preheating stage of the SCWO process and the MPA was relatively refractory. Results from these earlier studies established the basis for the proposed VX hydrolysis/ oxidation reaction pathways in supercritical water, as shown in Figure 1.15 In this study, MPA was used as a rate-limiting model compound to evaluate the effective-
10.1021/ie990094p CCC: $18.00 © 1999 American Chemical Society Published on Web 07/14/1999
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 2903
Figure 2. Schematic flow diagram of continuous-flow SCWO reactor system. PI ) pressure indicator, TC ) thermocouple, and b ) TC measurement location.
ness of the SCWO process for treating VX/NaOH hydrolysate and its degradation products. The effect of sodium hydroxide on the rate of MPA destruction efficiency was also evaluated. Kinetic models were developed for the SCWO of MPA. Experimental Section SCWO tests were conducted using a laboratory-scale, continuous-flow reactor system. Experimental variables included temperature, oxygen concentration, and reactor residence time. Specifically, tests were conducted at six temperatures (400, 450, 500, 550, 574, and 594 °C) with 550 °C being the key condition. Although three oxygen concentrations were used (110, 150, and 200% of the stoichiometric oxygen required for complete oxidation of MPA), most experiments were conducted using the 200% stoichiometric oxygen concentration. The reactor residence time varied from 3 to approximately 83 s. The variation of reactor residence time was achieved by changing the reactor length while maintaining a constant feed rate. Turbulent flow was achieved at all conditions. Fixed parameters included (1) a pressure of 27.6 MPa, (2) an MPA feed concentration of 1000 mg/L, and (3) an MPA feed flow rate of 25 g/min. Several hydrolysis experiments were conducted to obtain baseline data for calculating the destruction efficiency (DE) of MPA. Apparatus and Procedures. As shown in Figure 2, the major components of the system consisted of an air-driven feed pump (Williams model no. CP205W300B316TG), a high-pressure oxygen supply and control subsystem, a feed heat exchanger, an external preheater (Watlow model 9224C), a preheater coil, a coiled-tube reactor, a fluidized sand bath (Techne, model SBL-2), a trim cooler, an in-line filter assembly, a back-pressure regulator, and a gas-liquid separator. All wetted parts of the system were made of stainless steel (SS) 316 tubings and fittings. Both the preheater coil and the coiled-tube reactor were made of tubings with 1/4-in. outside diameter (o.d.) and 0.049-in. wall thickness. By the use of the properties of water at the flow rate of 25 g/min, Reynolds numbers corresponding to this tubing size fell between 4000 and 4600 for all tests conducted.
Under these test conditions, the plug flow reactor assumption held.16 The preheater coil was 20-ft long. Four reactors with lengths of 4.5, 10, 19, and 40 ft were used to obtain different reactor residence times. Both of these coiled tubes were installed inside the fluidized sand bath. Pressure indicators (3-D Instruments, 2554536B14) and thermocouples (Type K, Omega P/N KQIN) were installed at various locations of the system. In particular, process temperature and pressure were measured at the reactor inlet and outlet. The reactor pressure was monitored with an accuracy of (10 psi in the pressure range from 3700 to 4200 psi. The temperature readings of the thermocouples were within (1 °C when the system was at steady-state conditions. All of these sensors were factory calibrated and used as received. Bottled oxygen (zero grade) was pressurized by an airdriven booster (Haskel, model 27267) to 33 MPa (4700 psi) and was stored in a high-pressure, 40-L capacity accumulator. Since the reactor pressure was controlled at 27.6 MPa and the oxygen consumption rate was small in these experiments, a relatively constant and positive pressure differential was maintained. The oxygen mass flow rate was controlled by a metering valve and digitally displayed by a mass flow meter (Brooks, model 5860). The oxygen volumeric flow rate was calibrated using a wet-test meter (Precision Scientific Co.) and a bubble meter (SKC UltraFlow). The same flow meters were also used to measure the off-gas flow rate. Since these gas flow rate measurements were critical for oxidation experiments and carbon balance calculations, gas samples were routinely collected using a 60-cm3 syringe. This measurement provided flow rates of both gaseous and liquid effluents at isolated time intervals with typical sampling time of about 1 min. Prior to each experiment, MPA feedstock solutions were prepared in 1-L polypropylene bottles. The nominal concentration was 1000-mg/L. Each feedstock was made by dissolving 1 g of MPA in 1 L of distilled and deionized (DDI) water. To start an experiment, the sand bath was heated to about 25 °C above the desired reaction temperature. Once the sand bath reached the set temperature, chilled water supply for the trim cooler was provided, the feed
2904 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999
pump was started to introduce DDI water, and the backpressure regulator was adjusted to build up the system pressure to 27.6 MPa. During this time, the feed flow rate was checked, and if necessary, the pumps were adjusted to achieve the desired flow rate. The total feed rate was kept constant at 25 cm3/min for all experiments. Also, the external electrical preheater was turned on. The Variacs controlling the preheater was adjusted as necessary to maintain the temperature of the preheater effluent at about 350 °C. After the reactor inlet and outlet temperatures stabilized at the set value, the oxygen delivery system was activated (i.e., opening the shut-off valve between the high-pressure oxygen source and the reactor system). The oxygen flow was controlled by the metering valve. A steady-state process condition was characterized by minimum changes in reactor conditions, i.e., temperature ((1 °C or (0.25%), pressure ((40 psi or (1%), and oxygen and liquid flow rates ((3 sccm or (2% and (0.5 cm3/min or (2%, respectively). After reaching a steady-state condition, the pumps were switched to the MPA feedstock. Temperature, pressure, and oxygen flow rate were recorded every 2 min for about 20 min. Before a sample was collected, the effluent sample valve was flushed for several minutes. A sample was then collected in a 60-cm3 syringe for a 1-min interval. This sampling procedure also provided flow rate readings for both liquid and gaseous effluents. The gaseous effluent portion was injected into the two gas chromatographs, while the liquid effluent portion was transferred into a glass vial. In most cases, the liquid samples were analyzed the same day as they were collected. In a few cases, when samples were collected late in the day, they were stored in a refrigerator (4 °C) overnight and analyzed the next morning. In addition, reactor influent samples were collected from each batch of the feedstock. Once a sampling procedure was completed, the set-points of either oxygen flow rate or sand bath temperature or both were adjusted to the next set of operating conditions. At three test temperatures (400, 500, and 550 °C), samples were collected at the reactor inlet (i.e., after preheating but prior to the contact with oxygen). These samples were used to establish thermal stability of MPA and the extent of MPA hydrolysis during heat-up. Furthermore, when conducting the SCWO tests of MPA with NaOH using the initial reactor setup, it was discovered that sodium methylphosphonate (SMP) precipitated in the preheat zone, thus leading to a decrease in MPA concentration at the reactor inlet and therefore an increase in oxygen/MPA molar ratios. To eliminate this organic salt accumulation in the preheat coil, an additional pump was used to inject a concentrated MPA/ NaOH feed solution directly into the reactor. The MPA concentration was 5000 mg/L, and the MPA/NaOH molar ration was 2:1. The injection rate was maintained at 5 cm3/min, while the main stream (DDI water) was delivered by the air-driven feed pump at a flow rate of 20 cm3/min through the preheat coil. The total flow rate and MPA concentration after mixing were held at 25 cm3/min and 1000 mg/L, respectively. This modification allowed the oxidation of MPA to occur during the mixing stage and minimized reactor clogging. This conclusion was based on the fact that the MPA concentration in the reactor wash water became negligible when the
modified experimental setup was used. Of the 22 MPA oxidation tests involving NaOH, 11 were conducted using this modified test setup. Analytical Methods. An ion chromatograph (IC) (Dionex system 500) equipped with an anion column (AS-11), ion pac columns (ATC-1 and AG-11), and an anion self-regenerating suppressor (ASRS-I) was used to determine concentrations of methylphosphonate and phosphate. The lower detection limit (LDL) for the IC analysis was 0.1-mg/L for MPA and 0.3-mg/L for phosphate. Total organic carbon (TOC) analyses were conducted using a Shimatzu analyzer (model 5050) and performed according to Standard Method 5310 C. Typically, four calibration curves were generated separately for the total carbon channel and the total inorganic carbon channel. The LDL for the TOC measurement was 1-mg/ L. Three gas chromatographs (GC) were used. The first GC (Fisher-Hamilton model 29) was used to quantify carbon dioxide. This GC was equipped with a 0.3-m long, 6.35-mm o.d. silica gel column and a thermal conductivity detector, and the signal output was recorded using a Hewlett-Packard integrator (model 3392A). The unit was operated isothermally at 25 °C with a helium carrier flow rate of 30 mL/min. The second GC (HewlettPackard model 5750) was used to establish the concentrations of carbon monoxide, oxygen, nitrogen, and methane. This GC was equipped with a 3.05-m long, 3.175-mm o.d. molecular sieve column (Supelco 5A) and a thermal conductivity detector. The column was maintained at 61 °C (isothermal), and the signal output was recorded using a Spectra-Physics integrator (model 4290). The detector was operated at 225 °C. The LDL for most gases analyzed by these procedures was 0.1 vol %. The molecular sieve was regenerated by performing a programmed baking procedure at the end of each experimental day. Both of these GCs were calibrated daily using three standard gas mixtures. Gas samples were analyzed within 15 min after each sample collection. Duplicate injections were made for each gas sample. The third GC (Hewlett-Packard model 5890A), which was equipped with a flame ionization detector, was used to quantify organic products other than MPA in the liquid effluent samples (For these experiments, methanol was the only other organic product detected in the liquid effluent). A Supelco NUCOL fused silica capillary column was used. The LDL for the methanol measurement was 0.5-mg/L. Materials. MPA (Aldrich, 98% purity) and NaOH (Aldrich, 99.99% purity) were used as received. The DDI water used in all experimental and analytical processes was produced in house (electrical resistance ≈10 MΩ). All TOC, GC, and IC standards for liquid effluent analyses were made from high-purity chemicals (Fisher, ACS grade). The IC eluent was prepared from the NaOH solution (50% w/w, certified Fisher SC). Helium (Wilson, zero grade) was used to operate IC and GC units. Air (Wilson, ultra zero grade) was used to operate the TOC unit. Standard gas mixtures (Scott) were factory calibrated and used as received. Results and Discussion A total of 65 MPA oxidation experiments, 43 MPA oxidation tests without NaOH and 22 MPA oxidation tests with NaOH, were conducted. In addition to dupli-
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 2905 Table 1. Results from the Oxidation of MPA in Supercritical Water temp (°C)
O2 stoic (%)
Res time (s)
MPA conv. (%)
TOC conv. (%)
P closure (%)
C closure (%)
CO2 (%)a
CO (%)a
CH4 (%)a
550 500 550 400 550 500 500 550 550 550 550 550 550 550 500 500 500 500 400 400 594 594 575 575 550 500 500 450 450 550 550 525 525 475 525 525 475 475 450 450 525 500 500
200 200 150 200 110 200 200 110 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 150 150 150
14.4 16.7 14.4 39.5 14.4 16.7 16.7 14.4 14.4 14.4 7.6 7.6 7.6 7.6 8.8 8.8 30.4 30.4 83.2 83.2 3.1 3.1 3.2 3.2 3.4 4 4 5 5 3.4 3.4 3.7 3.7 4.4 15.4 15.4 18.4 18.4 20.9 20.9 15.4 16.7 16.7
>99.9 79.7 >99.9 4.2 97 82.6 80.5 96.3 >99.9 >99.9 98.2 98.2 98.5 98.6 78.4 76.6 92.5 91.9 4.7 4.8 99.1 98.9 97.3 97.7 91.4 50.6 50.5 10.6 8.5 90.4 90.9 71.6 72.9 28 >99.9 97.3 76.4 75.8 49.8 47.1 76.8 88.2 90.7
99 80 >99 4 91 86 83 95 93 93 98 96 99 >99 78 77 97 94 ∼0 ∼0 97 94 93 97 91 51 50 8 4 93 92 76 75 26 >99 99 79 77 51 49 82 92 93
88 89 89 96 86 101 100 91 64 72 89 94 89 94 92 97 60 69 103 104 91 95 96 97 101 93 95 103 105 96 94 107 100 99 58 76 91 96 88 94 76 101 101
109 105 102 106 99 97 99 114 102 89
15.8 8.7 17 99 99 98.1 >99 99
CO2 (%)a
CO (%)a
CH4 (%)a
2.4 1.8 1.9 2.4 1.6 1.6 1.5 2.8 2.7 2.8 2.8
1
