Ind. Eng. Chem. Res. 1993,32, 2259-2268
2259
Kinetics and Reaction Pathways of Pyridine Oxidation in Supercritical Water Neil Crain, Saadedine Tebbal, Lixiong Li, and E a r n e s t
F. Gloyna'
Environmental Health Engineering and Separation Research Programs, Center for Energy Studies, The University of Texas at Austin, Austin, Texas 78758
There is growing interest in applying supercritical water oxidation (SCWO) to the treatment of wastewaters and sludges. Existing mechanistic and kinetic data relate primarily to SCWO of simple compounds, hydrocarbons, and oxygenated hydrocarbons. Since many organic pollutants contain heteroatoms, the knowledge of SCWO reaction pathways, transition products, and kinetics for heteroatom-containing organic compounds is of critical design importance. This study focused on the kinetic and mechanistic aspects of pyridine oxidation in supercritical water using high-pressure oxygen gas. A laboratory-scale, continuous-flowreactor system was used. The experimental variables included temperatures varying from 426 to 525 "C, reactor residence times varying from 2.1 to 10.7 s, and oxygedpyridine molar feed ratios varying from 0 to 2.64. Pressure and feed flow rate, respectively, were maintained a t a nominal value of 27.6 f 0.4 MPa and 35 f 0.1g/min. The rate (mol/(L.s)) of pyridine oxidation in supercritical water was found to be 1013Ji1.@exp([-209.5 f 22.4 (kJ/gmol)l/RT) [Pyrl 1~~0~30[0210~20M~1s. Below 500 "C, the extent of pyridine hydrolysis was less than 1 % The largest hydrolysis impact corresponded to 4.276 of pyridine conversion, which occurred at 521 "C in less than 7 s. A number of transition products in both liquid and gaseous effluents from the pyridine oxidation experiments were identified. On the basis of these identified compounds, a network of simplified reaction pathways for pyridine oxidation in supercriticalwater was constructed.
.
Introduction Supercritical water oxidation (SCWO) is an innovative waste treatment process that can rapidly and completely convert organic compounds into mostly carbon dioxide and water. The SCWO process utilizes the unique properties of supercritical water. Above the critical point of water (374.2 "C and 221 bar), water may form a homogeneous phase with hydrocarbons (Connolly, 1966) and oxygen (Japas and Franck, 1985). As such, supercritical water can sustain a highly efficient oxidizing environment. The concept of applying SCWO for wastewater treatment appears to have been first disclosed in the late 1970s (Thiel et al., 1979). SCWO research and development activities accelerated in the mid 1980s. Although the early SCWO work demonstrated that a high destruction efficiency could be achieved for a wide range of organic pollutants (Modell et al., 1982;Tester et al., 1992),limited kinetic as well as mechanisticdata have become available. Furthermore, most kinetic or mechanistic studies have dealt with simple hydrocarbons or oxygenated hydrocarbons (Rofer and Streit, 1989;Webley and Tester, 1989; Thornton and Savage, 1992;Li et ai., 1991). Since many organic pollutants contain heteroatomssuch as nitrogen, halogens, sulfur, and phosphorus-the presence of heteroatoms can influence the physical characteristics of SCWO processes. Understanding the role of heteroatoms in the reaction mechanisms can prevent the formation of unwanted by-products and maximize process efficiencies. Therefore, the knowledge of SCWO reaction pathways as well as kinetics for heteroatomcontaining organic compounds is of critical design importance. Previously, gas-phase oxidation of pyridine was studied in the temperature range of 675-775 "C (Houser et al., 1982). The rate was found to be half-order in pyridine, first-order in oxygen, and half-order in oxygen consumption. The reported activation energy and preexponential factor were 226 f 16.3 kJ1mol and 1014.~0.9L/(mol*s),
respectively. The nitrogen compounds that were identified in the reactor effluent included N2, N20, NO,, and HCN. In addition, pyrolysis of pyridine' was also reported (Axworthy et al., 1978). These experiments were conducted in the temperature range of 950-1100 "C using helium as the carrier gas. Pyridine pyrolysis appeared to follow first-order kinetics. The reported activation energy and preexponential factor were 293 kJ1mol and 10l2-65-1, respectively. Benzonitrile, quinoline, acrylonitrile, acetonitrile, and hydrogen cyanide were found as nitrogencontaining products. However, little has been reported about reactions of nitrogen-containing organic compounds in supercritical water. An early study on reactivity of quinoline, isoquinoline, benzonitrile, carbazole, and aniline at supercritical water conditions (Tiffany et al., 1984) concluded that supercritical water had a profound effect on both the extent of reaction and reaction mechanism. Also, SCWO of simple compounds, such as ammonia and urea, has been reported (Timberlakeet al., 1982;Helling and Tester, 1988, Webley et al., 1991;Killilea et al., 1992). As a result of a jointly funded EPA, industry, and UT-Austin project, additional data have been developed for nitrogen-containing and chlorinated organic compounds. In a preliminary study involving a laboratoryscale,batch reactor system, pyridine was found to be highly refractory even at temperatures near 500 "C (Lee et al., 1991). This finding was in agreement with the observations reported by other researchers (Townsend et al., 1988; Houser et al., 1991) about the refractory nature of unsubstituted aromatic compounds in supercritical water. Following the preliminary tests, more detailed kinetic and mechanistic studies were conducted using a laboratoryscale, continuous-flow reactor system. The experimental variables included temperatures varying from 426 to 525 "C, reactor residence times varying from 2.1 to 10.7s, and oxygenlpyridine molar ratios varying from 0 to 2.64. Reaction pressure and feed flow rate, respectively, were maintained at a nominal value of 27.6 f 0.4 MPa and 35 f 0.1 glmin. These temperature, pressure, and flow rate conditions resulted in Reynolds numbers ranging from
0888-5885/93/2632-2259$04.QQJO 0 1993 American Chemical Society
2260 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993
High Pressure Oxygen Supply I O vent
Control
Gas Partitima
i
Figure 1. Flow diagram of laboratory-scale, continuous-flow SCWO reactor system.
7400 to 8200,suggesting an ideal plug-flowreactor. Reactor residencetimes were selected sothat they were long enough to obtain sufficient pyridine conversion for the support of kinetic studies, and they were short enough to preserve unstable reaction intermediates needed for the development of mechanistic studies. Various types of oxidants and oxidant injection techniques have been employed in previously reported SCWO kinetic studies. In one case, oxygen-saturated water was injected into the reactor (Helling and Tester, 1988; Rofer and Streit, 1989; Thorton and Savage, 1992). In another case, hydrogen peroxide aqueous solution was supplied (Lee, 1990; Wilmanns, 1990). Similarly, sodium nitrate/ nitrite aqueous solution (Dell'Orco et al., 1992) and compressed air (Ding and Abraham, 1992) were used. In the study reported herein, high-pressure oxygen was directly injected into the reactor. Experimental Section
A description of the SCWO test system, experimental procedure, sample analyses, and test materials is provided. A. SCWO Test Apparatus. The test apparatus used in this study was a laboratory-scale, continuous-flow SCWO reactor system. As shown in Figure 1, the major components of the system comprised a feed tank, feed pump, high-pressure oxygen supply and control subsystem, preheater (influent heat exchanger), coiled-tube reactor, fluidized sand bath, ice bath (effluent heat exchanger), back-pressure regulator, gas-liquid separator, and an online off-gas analyzer. The feed tank was a 20-gal polyethylene container fitted with three Teflon tube connections used for venting, helium purging, and feed supply. A diffuser was attached to the end of the purge line and placed in the bottom of the tank. The feed solution was purged with helium prior to each experiment to reduce the amount of dissolved oxygen and nitrogen. The feed tank was mounted on a magnetic stirrer (Bel Art Products Cool Stir, Model 14509-3). A 7.62-cm- (3-in.)long stir bar was used for mixing. A diaphragm metering pump (American Lewa, Model HLM-1) was used to pressurize the feed solution. The
throughput of this pump was rated 150 mL a t 69 MPa. An electronic interlock, preset a t 35 MPa, was connected to the discharge end of the pump. The pump was calibrated frequently with water at the test pressure by measuring the effluent volume over a selected time interval. All high-pressure connections after the pump discharge were Swagelok Stainless Steel (SS) 316 fittings. All highpressure plumbing, except the oxygen supply line, preheater, and reactor, was 6.35-mm (0.25-in.) outside diameter (0.d.) SS 316 tubing with 1.65-mm (0.065-in.) wall thickness. The pressurized feed solution was preheated prior to entering the sand bath. Two radiant heaters (Watlow, Model 9224C, 850 W each) were used to provide heat to the preheating section comprising 1.83-m- (6-ft)long, 12.7mm-o.d., and 2.1-mm wall thickness SS 316 tubing. The radiant heaters were controlled using two 120-V variacs. The preheater assembly was placed directly above the sand bath to minimize heat loss. The feed solution leaving the preheater flowed into an additional 6.07 m of 6.35-mm0.d. SS 316 tubing arranged in the fluidized sand bath (Techne, Model SBL-2D). The sand bath was rated for a temperature of 600 "C. Under the experimental conditions, the maximum steady-state reactor fluid temperature was found to be 520 "C. The preheated feed solution then flowed into a Swagelok 6.35-mm (0.25-in.) SS 316 crosslocated immediately above the fluidized sand surface. This arrangement served as a mixer. The reactor inlet was attached to the bottom port of the mixing cross. The coiled tube reactor was made of 6.35-mm-0.d. and 0.89mm wall thickness Hastelloy C-276 tubing. Reactors of different lengths (2.13,3.96, and 6.07 m, respectively) were used to provide variable residence times. The variation of the reactor length allowed Reynolds number to remain at about 8000 for the given flow rate (35 g/min) and temperatures. A thermocouple (typeK with Inconel 600 sheath,Omega, P/N KQIN) was inserted at the top port of the mixing cross and into the reactor tubing to determine the fluid temperature in the reactor inlet section. A similar thermocouple was inserted in the effluent end of the reactor. The length of both thermocouples varied from
Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 2261 46 to 122 cm. However, during most experiments, the length of both influent and efffluent thermocouples was 122 cm. The average of these two fluid temperatures was taken as the reaction temperature. The reactor effluent was given special attention. First cooling was accomplished by a heat exchanger consisting of a 6.07-m-long, 6.35-mm-0.d. SS 316 coiled tube which was submerged in an ice bucket. The cooled effluent then passed through a 0.5-pm SS 316 filter, and then to the back-pressure regulator (Tescom, Model 54-2122W24). The objectivewas to reduce the system pressure from about 27.6 MPa to a pressure slightly higher than atmospheric. This cooled and low-pressure effluent then flowed into a gas-liquid separator consisting of a 30.5-cm-high, 10.16cm-0.d. Lexan cylinder. This separator was filled with 1.43-cm-diameter glass pellets. The gaseous effluent was removed from the top of the separator and directed to the vent. A series of valves allowed the gaseous effluent to be redirected through a Fisher Hamilton gas partitioner for on-lineoff-gas analyses. The gas analyses included oxygen, nitrogen, carbon dioxide, carbon monoxide, and methane. The liquid effluent flowed from the separator into a container for disposal. Liquid effluent samples were collected from a sample port located in the effluent line from the gas-liquid separator. Oxygen was pressurized by an air-drivenbooster (Haskel, Model 27267) which elevated the pressure from about 3.5 MPa (500 psi), as supplied from bottled oxygen, to 31 MPa (4500 psi). The oxygen supply pressure limited the maximum reactor pressure to 27.6 MPa (4000 psi). The 3.4-MPa pressure differential was needed to control oxygen flow direction and flow rate using a metering valve (Badger Research Control, Model 1001GHT3635V) fitted with a P-14 trim. The oxygen mass flow rate was digitally displayed using a mass flow meter assembly (Brooks, Model 5860). The oxygen volumeric flow rate was calibrated using a soap-bubble flow meter. The high-pressure oxygen was preheated in a 3.66-mlong, 1.59-mm- (0.0625-in.) 0.d. SS 316 tubing submerged in the same sand bath. The preheated oxygen was mixed with the feed solution in the mixing cross. B. Experimental Procedure. Each experimental run began at the highest desired temperature and proceeded to the lowest. A detailed procedure was established for all experimental and analytical operations. A typical experiment began by preparing the feed solution consisting of distilled/deionized water and pyridine. The mixture was added to the feed tank 24 h in advance. The feed tank was agitated using the magnetic stirrer overnight and throughout the experiment. The next day helium was introduced to purge the feed solution. The helium purge was maintained throughout the 4-5-h preheat time. Prior to turning on the pump, the vent line to the feed tank was closed and a 10-12 psig pressure of helium was developed to help prevent oxygen from entering the feed tank and to provide adequate suction pressure for the pump. The dissolved oxygen concentration in the feed solution after a few hours of helium purging was less than 1 ppm, as measured by a dissolved oxygen probe (YSI, Model 51). A sample of the influent to the pump was collected at this time to determine the feed concentration. Meanwhile the system was thermally conditioned by the electric heaters and the fluidized sand bath. The fluid temperature at the preheater outlet was maintained between 350 and 360 "C for all experiments. The heat exchanger assembly (coiled tube and ice bucket) provided
sufficient cooling. The effluent temperature from the heat exchanger was 5-8 O C . After both the preheater and the sand bath reached the desired temperature set-points, the reactor was purged using high-pressurehelium for 5 min to reduce the residual oxygen in the reactor. After the purge, but before the pressure of the reactor system dropped below 50 psig, the feed pump was switched on. The feed flow rates were selected to meet the criteria for turbulent plug flow, i.e., Reynolds number 28000 for a tubular reactor with sufficient length/diameter ratio (Bird et al., 1960). When a steady liquid flow was established at the inlet to the gas-liquid separator, the back-pressure regulator was adjusted to maintain the system pressure at 27.6 MPa. Because of the quenching effect of the cold feed solution, an additional 30 min was required for all temperature readings to stabilize. During this 30-min period, the highpressure oxygen was introduced into the reactor system. Once the various temperature readings for the reactor system stabilized, the liquid collected in the gas-liquid separator was drained to remove reaction products collected under transient conditions. The gas-liquid separator was then allowed to refill. Once the liquid returned to its steady-state level, the effluent gas flow rate was measured using a soap-bubble flow meter. The gas flow rate was used to determine the time required to purge the gas sample lines connecting the reactor system to the gas chromatograph. At this point, sample collection for a particular set of experimental conditions began. First, the gas sample was injected into the gas chromatograph. Once the sample loop was closed to the reactor system, the gas flow was redirected to vent. Then an influent sample at the feed tank was collected, and a liquid sample was drawn from the bottom of the gas-liquid separator. All liquid samples were stored in glass vials without air space to minimize loss of volatile components. Immediately following collection, liquid samples were stored in arefrigerator (4 "C). C. Analytical Procedures. Ion chromatographic,gas chromatographic, gas chromatography/mass spectroscopic, and electrochemical techniques were used to analyze the test samples. Both precision and accuracy of the data produced from these analyses were within &lo%. Ion Chromatography(IC). Ionic transition products were analyzed using an ion chromatograph (Dionex, System 14). The IC was fitted with a conductivity detector and a Dionex HPIC-AS3 anion column. The eluent flow rate was 0.5 mL/min, and the operating pressure was 3.45 MPa. Two eluent solutions were used: (1)carbonate eluent was an aqueoussolution of 2.4 mM sodium carbonate (Na2C03) and 3.0 mM sodium bicarbonate (NaHC03); (2) borate eluent was an aqueous solution of 5 mM solution of sodium tetraborate (Na2B40,). Gas Chromatography (GC). The off-gasanalysis was conducted using agas partitioner (Fisher Hamilton, Model 29) equipped with two packed columns, a thermal conductivity detector (TCD), and an integrator (Hewlett Packard, Model 3392A). The first column was a 0.6-mlong and 6.35-mm-0.d. SS 316 tube packed with silica gel (Supelco Davidson 923, 100/200 mesh). This column separated the COz from the other gases. The second column was a 1.98-m-long and 3.18-mm- (0.125-in.) 0.d. SS 316 tube packed with a molecular sieve (Supelco 5A, 45/60 mesh). This column separated the 02,N2, CH4, and
2262 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 Table I. Experimental Data for Pyridine Hydrolysis in Supercritical Water influent [P,14 at room at reactor influent reaction res time conv sample temp [HzOl temp temp X ID no. (mol/L) (moVL) (mol/L) ("0 (8) 425 10.7 0.01 0.0254 0.004 00 8.60 518 450 8.83 0.01 7.11 511 0.003 33 0.0254 474 7.82 0.01 533 0.002 97 0.0254 6.30 501 7.05 0.01 0.002 70 0.0254 5.68 528 516 6.89 0.04 0.002 58 5.41 515 0.0254 521 6.75 0.04 0.003 86 5.33 540 0.0386 4 Influent pyridine concentration at room temperature representa the concentration of stock solutions prepared in the feed tank. Influent pyridine concentration at the reactor inlet equals influent pyridine concentration at room temperature multiplied by the water density at the reactor temperature. Water densities were obtained from National Bureau of Standards Reference Database 10-Therrnophysical Properties of Water by J. S. Gallagher and L. Haar,
0
0
g o
4
0
I
0 0 0
O f
0
I
I
I
I
2
4
6
8
Time (s)
Figure 2. Percentage of pyridine remaining during SCWO as a function of time at 450,475, and 500 "C (pressure = 27.6 m a , feed versus [Pyr] = 1.0-2.0). concentration = 1000-3000 mg/L, [02]
CO. The GC was operated isothermally a t 25 "C. The helium flow rate was 20 mL/min. The gas partitioner was calibrated using prescribed gas mixtures. (The compositions of these gas mixtures are provided in section D. Materials.) Averaged deviations for COz, CO, and CH4 were less than 4.5%, 4.4%, and 3.4%, respectively. The concentrations of pyridine and volatile transition products derived from pyridine oxidation were analyzed using a gas chromatograph (Hewlett Packard, Model 5895A). The H P GC was equipped with a flame ionizationd detector (FID). A 15-m-long capillary column (Supelco SPB5)was used. The temperature program used for these analyses began at 35 "C (4 min duration), and then ramped at 10 "C/min to 100 "C. Immediately upon reaching 100 "C, a rate of 30 "C/min was used until the terminal temperature of 190"C was achieved. The injector and detector temperatures, respectively, were 200 and 250 "C. The flow rate of helium was 60 mL/min. The H P GC was calibrated using o-cresolas the internal standard. For the development of calibration curves, the averaged deviation of internal standard responses was less than *7%.
Table 11. Experimental Data for Pyridine Oxidation in Supercritical Water
influent [Pyr]' at room at reactor sample temp temp IDno. (mol/L) (moUL) 0.0143 0.002 24 605 0.0386 0.005 97 502 658 0.0148 0.001 96 0.004 77 0.0363 703 719 0.002 09 0.0159 0.005 03 0.0386 529 0.0143 0.001 87 602 0.003 26 0.0252 526 0.001 61 0.0138 673 678 0.0446 0.005 21 542 0.0386 0.00451 0.0363 0.00423 718 0.001 73 0.0148 655 508 0.002 93 0.0252 0.002 93 0.0252 525 0.001 85 715 0.0159 0.0252 0.002 92 509 0.0252 52 0.002 92 0.0252 0.00292 520 0.002 92 0.0252 514 0.0143 0.001 66 604 0.0230 0.002 66 713 0.0280 0.003 02 685 0.0138 0.001 48 680 0.0148 0.001 59 666 0.0252 0.002 69 516 0.0386 0.00412 524 0.0143 0.001 53 535 0.0138 0.001 47 674 0.0138 0.001 47 681 0.0363 0.003 86 606 0.0230 0.002 44 700 0.0519 0.001 69 716 0.0159 0.001 68 651 0.0363 0.003 83 701 0.0363 0.00383 709 0.0363 0.003 77 671 0.0363 0.003 77 711 0.0363 0.003 76 708 0.0159 0.001 61 714 0.0159 0.001 61 754 0.0230 0.002 31 702 0.0143 0.001 44 537 0.0148 0.001 48 662 0.0138 0.01 38 682 0.0252 0.002 52 513 0.0386 0.003 85 541 0.0386 0.03 85 519 0.0446 0.004 42 668 0.001 36 684 0.0138 0.002 77 677 0.0280 0.001 36 660 0.0138 0.0446 661 0.00441 0.004 41 0.0446 676 4
influent reaction [Oa] temp (moVL) ("C) 426 0.0195 427 0.0467 449 0.0152 450 0.0644 0.0212 450 0.0384 451 0.0162 451 0.0329 452 0.0170 474 474 0.0258 474 0.0352 0.0564 475 0.0171 475 0.0107 475 0.0285 475 476 0.0183 0.0251 476 0.0214 476 0.0176 476 476 0.0140 0.0143 476 477 0.0374 0.0156 497 0.0075 498 0.0155 499 0.0259 499 0.0315 499 0.0130 499 0.0114 500 0.0154 500 0.0074 501 502 0.0343 0.0077 502 0.0017 503 0.0232 504 0.0501 504 0.0150 509 0.0073 509 0.0228 510 0.0161 517 0.0073 518 0.0323 520 0.0122 521 0.0142 522 522 0.0070 0.0241 522 522 0.0268 523 0.0291 525 0.0141 526 0.0106 526 0.0143 527 0.0141 527 0.0104 527 0.0217
ree time (a)
conv
X
10.50 0.03 10.60 0.03 4.16 0.00 2.84 0.02 2.84 0.02 8.78 0.06 8.78 0.07 8.72 0.14 4.61 0.11 4.61 0.18 7.82 0.22 2.50 0.09 5.49 0.13 7.79 0.16 7.78 0.25 2.47 0.09 7.75 0.16 7.75 0.24 7.75 0.24 7.75 0.13 7.75 0.24 2.49 0.06 4.22 0.36 4.17 0.34 4.16 0.33 7.05 0.66 7.05 0.75 7.05 0.62 4.15 0.34 4.15 0.37 2.27 0.08 2.26 0.21 2.26 0.22 2.25 0.24 2.25 0.23 2.25 0.26 2.21 0.20 2.21 0.12 2.20 0.22 2.16 0.44 2.15 0.40 2.14 0.43 6.75 3.89 3.89 6.59 6.59 6.57 3.86 3.85 3.85 3.84 3.84 3.84
0.80 0.67 0.60 0.74 0.94 0.95 0.53 0.74 0.60 0.81 0.55 0.68
See Table I.
Gas Chromatography/Mass Spectroscopy (GC/ MS). The unknown peaks found in the H P GC analysis were further identified using a second gas chromatograph (Varian, Model 3400) equipped with a mass spectroscope and an ion trap detector (Finnigan Mat 700). A SGE BP5 column was used. The temperature program began at 35 "C, held for 2 min, ramped at 10 "C/min to 170 OC, and held for 1min. Ammonia. The concentration of ammonia in the effluent samples was determined according to Standard Methods (4500-NH3 F) (Clesceri et al., 1989). A digital pH/mV Meter (Orion Research, Model 701A) equipped with an ammonia specific probe, was used. Orion Application Solution ammonia standards (1,10,100, and loo0
Ind. Eng. Chem. Res., Vol. 32,No. 10,1993 2263 Table 111. Global Kinetic Models for Pyridine in Various Reaction Media
pyridine
helium
flow
1
0
3.8 X 1OI2 293 950-1100
pyridined oxygen/helium stirred 0.5 1 7.9 X lo1' 226 flow pyridinec water flow pyridine oxygen/water flow 1 0.2 1.3 X 210
675-775
0.21 3.0 0.1
0.25-2.3
1.1 (950) 275 675
6.9X lo-'
Axworthyet al., 1978
1.3 X lo-' Houser et al., 1982
this work 0.25 5.1 X lo-' this work (525) pyridine ozone batch 1 1 25 0.1 0.014 2.0 2.8 X lo-' Andreozzi et al., 1991 (25) Kinetic parameters are defined 88 r = A eXP(EdRT)[Cdm~C~l", where Ea is in kJ/mol, T is in K, R = 8.314 J/(mol-K),and A =: l/e for first-order reaction, (L/mol)"+"/s for (m+ nlth-order reaction. The unite of rate constant, k, are the same as those of the preexponential factor, A. Initial reaction rates, ro, have been calculated using the listed rate constante and the highest concentration in each case. d r = A exp(EdRT) ~ C A ] ~ ~ C B ~where " ~ A8O= Z 0.5~and ~ the unit of A is L/(mol.s). e Insufficient data for calculation of kinetic parameters (see Table I). 1 Table IV. Transition Products of Pyridine Oxidation Identified by Ion Chromatography elution time 0.8 compound structure PK., PK, @in) Borate Eluent0 propanoic acid CHsCHzCOOH 4.87 5.0 L-lactic acid CHsCH(0H)COOH 3.08 5.0 acetic acid CH3COOH 4.75 5.0 0.6 glycolic acid HOCH&OOH 4.83 5.6 acrylic acid CH4HCOOH 4.25 6.8 V formic acid HCOOH 3.75 7.4 Carbonate Eluent0 0.4 monocarboxylic (mixture of the above 2.0 acids six compds) unknown A 3.4 nitrite NOz4.5 0.2 unknown B 4.8 maleic acid HOOCCH=CHCOOH 1.83 6.07 7.5 (cis) 0 See discussion in the text.
426525 27.6 426525 27.6
0.023-0.08 0.023-0.08
mi s
i
0
0
0.2
0.4
0.6
0.8
1
Observed Conversion Figure 3. Model-predicted conversion versus experimentally observed conversion for pyridine oxidation in supercritical water.
mg/L) were used. The detection limit for ammonia analysis was 0.1 mg/L. pH. A pH meter (Orion Research, Model SA 720)was used for the pH measurement. The pH meter was calibrated using Fisher Scientific pH Standards (pH 4,7, and 10). D. Materials. The test chemicals included pyridine (Aldrich,99% ), and oxygen and helium (Liquid Carbonic, zero grade). Chemicals used for calibration or identification purposes included formic acid (Fisher, 90% aqueous), methanol (Mallinckrodt, 99.9%), dimethylamine (anhydrous)(Kodak,reagent grade),acetone (EM Science, reagent grade), sodium tetraborate (EM Science, reagent grade), and sodium carbonate (anhydrous) (Spectrum, reagent grade). The following chemicals were obtained from Alrich: o-cresol(99+% ), acetic acid (99.7%),glycolic acid (99%), propanoic acid (99%), acrylic acid (99%), maleic acid (99%), and L-ldctic acid (sodium salt) (99% ). Distilledldeionized water was used to prepare the feed solution and analytical dilutions. GC calibration for off-gas analyses utilized two gas mixtures (Scotty Specialty Gases). Gas mixture A contained carbon dioxide (1.00% by volume), carbon monoxide (1.03%), oxygen (1.0751,and methane (1.00%) in nitrogen. Gas mixture B contained carbon dioxide (5.00%),carbonmonoxide(5.0%),carbondioxide(LOO%), nitrogen (4.8751,oxygen (5.1961, and methane (4.0%) in helium.
Results and Discussion
A total of 62 data points, including both hydrolysis and oxidation of pyridine, were generated. Controlled parameters included pyridine feed concentration, reactor residence time, reaction fluid temperature, and oxygen/ pyridine loading ratio. The kinetic and mechanistic aspects of hydrolysis and oxidation of pyridine in supercritical water were derived from these relationships. The results are discussed below. A. Kinetic Aspect. Experimental data for pyridine hydrolysis and oxidation in supercriticalwater are provided in Tables I and 11,respectively. The reaction temperature was an average of the reactor influent and effluent was defined as X = 1 temperatures. The conversion (X) - ([Pyrlf/[PyrIo). The terms, [Pyrlo and [Pyrlf, respectively, were the influent and effluent concentrations of pyridine. In this case, the influent pyridine concentration was the average of at least two samples collected during each run. The oxygen concentration, as calculated from the mass flow meter readings, represented the concentration of oxygen at the reactor inlet. The deviation of multiple influent concentration measurements was about 1% for all experiments. Most analyses were made within 2 days after the samples were generated. Hydrolysis of Pyridine. The extent of pyridine hydrolysis in supercritical water was investigated at five temperatures ranging from 425 to 516 "C and a fixed feed concentration of 0.0254 mol/L (2007 mg/L). One additional hydrolysis test was conducted at a temperature of 521 "C and a feed concentration of 0.0386 mol/L (3050 mg/L). The results from these tests are shown in Table
2264 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 Table V. Transition Products of Pyridine Oxidation Identified by Gas Chromatography and Gas Chromatography/Masr Spectroscopy
compound
structure
mol w t (g/mol) Liquid Effluent
reverse fit
retention time (min)
GCa methanol acetone unknown A dimethylamine unknown B
CH3OH CH3COCH3
32 58
(CHddH
45
dimethylamine formamide ethylamine
(CHa)zNH HCONHz CHaCHzNHz
formic acid acetic acid glutaconic acid maleic acid succinic acid glutaric acid malonic acid propionic acid oxalic acid
HCOOH CHaCOOH HOOCCHzCH4HCOOH (cis) HOOCCH=CHCOOH (cis)
0.63 0.78 0.80 0.86 0.97
GCIMSb 45 45 45
841
46 60 130 116 118 132 104 76 90 Gaseous Effluent
690 730 770 720 880 810 720 930 750
824 767
0.72 0.72 0.72
GUMSO
HOOCCHzCH2COOH HOOCCHzCHzCHzCOOH
HOOCCHzCOOH CHsCHzCOOH HOOCCOOH
7.6 8.1 28.8 23.8 23.9 29.5 24.4 32.0 26.2
GC carbon dioxide carbon monoxide methane
44 28 16
3.5 30 CHI 17 a Samples were analyzed without using solvent extraction. Samples were extracted using methylene chloride, and the solvent phase WBB analyzed. Samples were ether extracted and methyl esterified using boron trifluoride. c02
co
I. The largest extent of hydrolysis corresponded to 4.2% of pyridine conversion, which occurred at 521 "C in less than 7 s. No transition products were found by the GC and IC analyses. Samples collected at temperatures below 500 "C suggested that hydrolysis of pyridine was statistically insignificant. Furthermore, these results represented the combined pyridine hydrolysis in the preheater and reactor. The extent of pyridine hydrolysisthat occurred in the preheater should be relatively small, because of the low temperature profile. Therefore, the effect of pyridine hydrolysis during the heat-up period was negligible under most test conditions. Oxidation of Pyridine. Observed pyridine destruction during the oxidation experiments ranged from 2.3% to 95.2% . Figure 2 shows the conversion of pyridine as a and 500 function of time for three temperatures (450,475, "C). It should be noted that the multiple data entries for each temperature were obtained from pyridine feed concentrations ranging from 0.013 to 0.045 mol/L and oxygedpyridine ratios ranging from 1to 2. The data show a general trend of doubling the pyridine conversion for every 25 "C increase in temperature. The global kinetic expression for pyridine oxidation was derived from ideal plug-flow tubular reactor assumptions. Expressed in terms of pyridine conversion,X,the equation has two forms:
indicated that eq 1 failed to meet convergence requirements. However, eq 2 resulted in convergence at two slightly different sets of parameter values. First, fixing the reaction order with respect to HzO at zero yielded eq 3: r = 1.26 X 1013exp(-209.5/R~~Pyrl'[0z10~2(3)
where the units of the reaction rate (r)and the activation energy are mol(L.s) and kJ/mol, respectively. Second, fixing the reaction order with respect to HzO at 1.0yielded r = 9.33 X 10'' exp(-227.2/RT)[Pyrl'[0z10~2[HzO11 (4)
The values for the preexponential factor and the activation energy for both eq 3 and eq 4 were within the 95% confidence limits for either regression. The order with respect to oxygen and pyridine remained the same. The relative insensitivity of the rate expression to the water reaction order was a result of the small changes in water concentration over the reaction conditions. At a constant pressure, the concentration of water was a function of the mole fraction of the reactants and the temperature. The mole fraction of water was 0.99 or higher for all experiments. The change in the concentration of water caused by temperature was only about 60 % over the experimental x 1- [l- A exp(-E,/RT)(l range, limiting the sensitivity of the analysis for this parameter. Confidence intervals of 95% were calculated for the X = 1- exp[-A ~ X ~ ( - E , / R T ) C ~for ~ ~ a~ =C1~ (2) ~ ~ ~ I parameters by the regression program. The confidence interval for the pyridine reaction order was not calculated because eq 2 was valid only when a = 1.0. Confidence The data in Table I1 were treated using a multivariable limits for the pyridine reaction order were estimated by nonlinear regression technique to obtain optimized values fixing the value of a in eq 1a t values ranging from 0.5 to for the Arrenhius parameters A and E a (along with the reaction orders a, b, and c). The regression analysis results 1.5 and preforming a regression analysis. For pyridine
Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 2265
pyridine
H H@H
$/ 0 11 H C ' +~ NH2 ~ I H pOH CH3
O %OH
1
+ NH3
&
0 Glutaconic Ammonia*
Maleic Acid*
Maleic Acid*
I
7H2 0"C'OH
rH2 CH2 AH3
+
Malonic
Ethyl
Acid*
0'%'OH
0aC' OH
6
CH2
+ I
O'/ 'OH oxalic Acid*
CO2
+
7H3 OH
0
0
0
