Environ. Sci. Technol. 1992, 26, 1587-1593
Hydrolysis and Oxidation of Acetamide in Supercritical Water Dong-Soo Leet and Earnest
F. Gloyna**S
Environmental Health Englneering, Environmental & Water Resources Engineering Program, The University of Texas at Austin, Austin, Texas 777 12
This paper describes the development of acetamide destruction kinetics in supercritical water. The experimental conditions included temperatures between 400 and 525 "C and pressures between 230 and 335 atm. The empirical rate equations for the hydrolysis and the overall reaction (hydrolysis oxidation by hydrogen peroxide) were determined as follows: (hydrolysis) r = -( W2f 101.3) exp((-51000 k 560O)lRT)[a~etamide]~ and (overall reaction) r = -(103.6 f 10 .5) exp((48000- f 3800)/RT)[acetamide]', where the activation energies were in joules per mole and [acetamide] was in moles per liter. First-order kinetics with respect to the acetamide concentration properly described both the hydrolysis and overall reaction rates. The hydrolysis reaction proceeded more rapidly than the oxidation. Therefore, the role of supercritical water as an active reactant should be recognized. The residual byproducts formed from the hydrolysis or the overall reaction of acetamide were acetic acid and ammonia. The effects of hydrogen peroxide concentration and the reaction pressure on the reaction rate were negligible. Therefore, the use of an oxidant slightly above the stoichiometric demand may be sufficient in SCWO processes. The negligible pressure effect implies that, to minimize construction costs associated with containing high pressure, the pressure may be kept as low as possible. Also, the negligible pressure influence may add flexibility for controlling the retention time without loss of destruction efficiency.
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Introduction Supercritical water oxidation (SCWO), as applied to wastewater treatment and organic sludge destruction, is a relatively new concept that has considerable potential. Like most applications of supercritical fluid technologies, SCWO takes advantage of solvating characteristics of supercritical water (SCW). The physicochemical properties of SCW, especially density, are altered such that SCW behaves as a highly effective solvent for organic compounds. For example, benzene (I, 2), pentane, heptane, 2-methylpentane, and toluene (I)are known to be miscible with SCW in all proportions. Another important characteristic for the destruction of hazardous organic substances is that gases such as oxygen (3)and air (4), which are only sparingly soluble in ordinary water, are completely miscible with SCW. On the basis of these solvating characteristics of SCW, the oxidation of organic compounds is expected to proceed rapidly in a single, oxygen-rich phase. The efficient destruction of a number of hazardous organic compounds in SCW has been demonstrated (5-9). For example, the total organic carbon (TOC) destruction efficiencies for toxic chlorinated organics such as DDT, PCB-1242, l,l,l-trichloroethane, 1,2-ethylene dichloride, TCE, o-chlorotoluene, 1,2,4-trichlorobenzene, 4,4'-dichlorobiphenyl, and hexachlorocyclohexaneexceed 99.97 ?4 Permanent address: Dept. of Chemical Engineering, Soong Si1 University, Seoul, Korea. *Professorand Bettie Margaret Smith Chair. 0013-936X/92/0926-1587$03.00/0
at a temperature near 500 "C with a residence time of less than 4.5 min (9). The available kinetic investigations are limited to relatively simple compounds such as carbon monoxide (IO), methane ( I I , I 2 ) , methanol (13,14), p-chlorophenol(I5), hydrogen (16),ammonia (18,phenol (18),acetic acid (19), and phenol (19). The relatively limited kinetics data for SCWO of organic compounds motivated this study. Hence the objective of this research was to determine hydrolysis and oxidation kinetics of acetamide in SCW. Acetamide was chosen because it was subjected to both hydrolysis and oxidation. Hydrogen peroxide was used as the oxidant since less information is available on its use for SCWO of organic compounds (20,21). Experimental Method Reactor System. A tubular, continuous-flow reactor system was used to determine hydrolysis and oxidation kinetics for acetamide. An objective of the SCWO reactor design was to achieve turbulent plug flow while maintaining isothermal conditions. A schematic of the continuous-flow reactor system is shown in Figure 1. The tubular reactor consisted of a preheat section and an isothermal section. The preheat section was 0.305-cm i.d. (0.635-cm o.d.), 3.66 m long, and the isothermal section was 0.229-cm i.d. (0.476-cm o.d.), 6.1 m long. Both sections were constructed with stainless steel 316, and these sections were coiled to fit into an electric furnace (Thermolyne Model FA 1730-1,5.8 kW). Type-J thermocouples were used to measure temperatures at the inlet and outlet of the isothermal section. In most runs, the two temperatures differed by no more than 10 "C. The two temperatures were controlled within f 2 "C from desired values. Therefore, isothermality was assumed. Typical Reynolds numbers for flows in the isothermal section ranged from 4014 to 6055. Out of 52 hydrolysis tests, only 5 tests were conducted under laminar flow conditions for which the plug flow assumption was still used since the residence time distribution for coiled tubes is narrower than straight tubes (22). The system pressure was controlled within f1.5 atm accuracy by using a back-pressure regulator (Autoclave Engineers, Model 10VRMM). A standard test pressure gauge [3D Institute, 1.36-atm (20 psi) readability] was used. A safety rupture disk rated at 410 atm was installed. Hydrogen peroxide (32 w t % , Analytical Science) was used as the oxidant. Through a O.OBcm-i.d., 0.16-cm-0.d. tube, the oxidant was fed directly into the inlet of the isothermal section by an HPLC pump (Scientific Systems Inc., Model SSI 200-B). Acetamide concentrations in feed solutions were limited to a maximum of 0.0608 mol/L. The use of low concentrations minimized possible temperature changes resulting from the heat of reaction. The feed solution was pumped into the preheat section by a high-pressure pump (Haskel, DSF-72) and mixed with hydrogen peroxide at the inlet of the isothermal section. Therefore, the oxidation reaction was initiated at the inlet of the isothermal section. The effluent was quenched in a cooling loop (0.15-cm i.d.,
0 1992 Amerlcan Chemical Society
Envlron. Scl. Technol., Vol. 26, No. 8, 1992
1587
Electric F'urnace
temperature
lcontroiler Pressure
Safety Rupture
Liquid
Vapor Separator
Disk
Minipump Figure 1. Continuous flow reactor system.
0.32-em o.d., 3 m long), filtered (7-pm filter units) and controlled hy the hack-pressure regulator, and ultimately passed through a gas-liquid separator. Effluent analyses on acetamide were made after temperature and pressure were stahilized. The analyses indicated that steady-state conditions were achieved when the effluent volume exceeded more than 3 times the reactor volume. Therefore, in each test, an effluent sample was collected when the effluent volume reached 5 times the reactor volume. Also, to measure the extent of reaction occurring during the preheat period, one liquid sample per test was collected through a sampling tube (0.015-cm i.d., 0.16-cm ad.) prior to the isothermal section. Analytical Methods. Acetamide analyses were made using a gas chromatograph (GC) (Hewlett Packard, Model 5890A) equipped with a flame ionization detector (FID). A J&W, DB-5 fused-silica capillary column (0.32-mm i.d., 30 m long, 0.25mm fh thickness) was used. Calibration standards were prepared daily, and at least four standards were used to verify linearity of the detector response. Typically, each test sample was analyzed twice. Triple analyses were made for every eight test samples. Precisions within f8% were achieved. The main hyproducts such as acetic acid and ammonia, respectively, were identified by gas chromatograph/mass spectrometer (GC/MS) and ion chromatograph analyses. For the GC/MS analyses, the same GC and column were used. The column head pressure was 1.9 atm, and the carrier gas (He) flow rate was 3 mL/min. The oven temperature was increased from 70 to 150 "C at a rate of 10 OC/rnin. The m/e value ranged 35-100. Complete hyproduct search was beyond the scope of this study and was not attempted since the carbon and nitrogen balances based on acetamide, acetic acid, and ammonia were close to 1. An ammonia analyzer (Orion, Model 701A) and a pH meter (Orion, Model SA-720) were used. The permanganate titration method (23)was used to quantify hydrogen peroxide content. The measured content of hydrogen peroxide in the reagent solutions ranged from 32 to 33 wt %. The ferrieferricyanide spot test method (24) was used to monitor hydrogen peroxide in the effluent. Gas-phase analysis was not made since the flow rate was negligible (
