Ind. Eng. Chem. Res. 2000, 39, 3707-3716
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Supercritical Water Oxidation in a Pilot Plant of Nitrogenous Compounds: 2-Propanol Mixtures in the Temperature Range 500-750 °C M. J. Cocero,* E. Alonso, R. Torı´o, D. Vallelado, and F. Fdz-Polanco Departamento de Ingenierı´a Quı´mica, Universidad de Valladolid, 47011-Valladolid, Spain
Supercritical water oxidation (SCWO) has been shown to be an effective method for the treatment of industrial wastes. Organic compounds containing nitrogen are very usual in industrial wastes, and therefore, it is necessary to study the oxidation behavior of such compounds in order to improve the applicability of this technology to wastewaters and sludges. In this paper, oxidation parameters of several nitrogen-containing compounds in supercritical water such as aniline, acetonitrile, pyridine, and the intermediate stable compound ammonia are studied, using 2-propanol as auxiliary fuel. SCWO of feedstreams containing 2-propanol and the nitrogenous compound was carried out using a pilot-plant scale, based on a continuous-flow reactor system. Results show that for these compounds suitable conditions for SCWO are the following: reaction temperature in the range 600-700 °C; stoichiometric amount of oxygen at residence time of 40 s. In these conditions, compound concentration in the effluent is below detection limits, TOC removal is greater than 99.97%, and N removal is greater than 97%. Introduction SCWO technology relies on the unique properties of water in supercritical conditions to create a useful and effective reaction medium. Critical conditions for pure water are 374 °C and 22.1 MPa. Although the presence of organics and dissolved gases in water can significantly affect the critical properties of the mixture, supercritical water oxidation is practically defined as oxidation occurring above the critical temperature and pressure of pure water.1 It is well-known that water above its critical point is an effective solvent for organic compounds. In addition, gases including oxygen are completely miscible in the fluid. Thus, a homogeneous single phase results when organic compounds and oxygen are dissolved in supercritical water, and the reaction process proceeds without interfacial mass transfer limitations.2 At these conditions, organic oxidation is initiated spontaneously and organics are destroyed rapidly with very high conversions into CO2 and low residence times. The SCWO system is capable of operating as a totally enclosed treatment facility, providing complete destruction of organic compounds. In all wastewater treatment processes the main controlled parameter is the quality of the effluents, either gaseous or liquids, and in this regard, as concerning to wastewaters containing nitrogenous compounds, emission of NOx gases and discharge of ammonia in the liquid or gaseous byproducts are especially controlled. Since many organic pollutants contain heteroatoms, such as nitrogen, halogens, sulfur, and phosphorus, it is necessary to know the oxidation behavior of such compounds for the supercritical water oxidation application into industrial wastewaters treatment. In this paper, the study of oxidation conditions in a pilot plant such as reaction temperature, amount of oxygen, and residence time is presented for wastewaters containing * Corresponding author. E-mail:
[email protected]. Tel.: +34 983 42 31 74. Fax: +34 983 42 31 66.
aniline, acetonitrile, pyridine, and ammonia, respectively. Previous works have been done about the SCWO of some of these compounds. Webley and co-workers1 determined the oxidation kinetics of ammonia and ammonia-methanol mixtures by oxygen in a packed and unpacked tubular reactor. Only 10% conversion of ammonia was achieved in the unpacked tubular reactor at 700 °C, 25 MPa, and a residence time of around 10 s. Higher conversion (up to 40%) was reached in the packed reactor under the same conditions. The presence of methanol did not noticeably affect the oxidation of ammonia. Killilea et al.3 achieved a 41% removal of ammonia (from urea feed) at 690 °C and 25 MPa and a residence time of 2 s. When ethanol was added to the urea feed stream, complete destruction was achieved at this temperature. At high temperatures, both with and without ethanol, ammonia was mainly converted to N2. At lower temperatures, with ethanol, however more N2O was produced than N2. This was explained by a kinetically limited N2O reduction. Crain et al.4 studied the SCWO of pyridine in a temperature range of 425-527 °C, and they found that conversion increased from 3% at 425 °C to 68% at 527 °C at a residence time of 10 s. Pyridine has been found to be a refractory compound, and high reaction temperature is required for its oxidation. In that sense, catalytic SCWO of pyridine has been studied with several catalysts (Aki and Abraham).5 Few works have been found in the literature6 about SCWO of aniline and acetonitrile. Most previous works with N-containing species have been done from a kinetic point of view, on a laboratory scale, and destruction of the compound was not always complete. In this paper, verification of SCWO operation with a pilot plant is presented for feedstreams containing aniline, acetonitrile, pyridine, and ammonia, and direct comparison with previous works is not possible. Operation parameters were studied in order to achieve complete oxidation of feedstream to CO2 and N2. Oxidation took place at high temperatures (500800 °C), and 2-propanol was used as auxiliary fuel.
10.1021/ie990852b CCC: $19.00 © 2000 American Chemical Society Published on Web 10/02/2000
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Figure 1. Flow diagram of the SCWO pilot plant.
Effect of temperature and oxygen excess on TOC and N-removal was studied. Experimental Section In this section, a description of the SCWO plant, experimental procedure, and sample analyses is provided. A. SCWO Plant. The experimental facility is a pilot plant with a continuous-flow, packed bed reactor with a feed capacity up to 25 kg of water/h at ambient conditions. A basic flow diagram of the plant is shown in Figure 1. As shown in Figure 1, the major components of the plant comprised a feed tank, high-pressure feed pump, high-pressure air compressor, preheaters, reactor, cooling system, a back-pressure regulator, gas-liquid separator, and the sampling ports. A piston metering pump (Dosapro, Model Milton Royal C) is used to pressurize the feed solution. Flowrate of the feedstream is determined by level measurement in the feed tank. The air, which is used as oxidant in the oxidation reaction, is compressed to the operation pressure by a four-stage compressor (Ingersoll-Rand H15T4). Both streams are mixed in a static mixer inside the reaction chamber as it is shown in Figure 1. Reactor design has been developed in the Chemical Engineering Department at the University of Valladolid (Valladolid, Spain).7 The aim of this reactor was to reduce the cost of such equipment, by reducing the amount of high-cost materials that are necessary to withstand high-pressure and oxidant atmosphere during the reaction. The reactor is composed by two concentric tubes; the inner one is made of Inconel 625, and the outer shell is made of SS 316. Oxidation reaction takes place inside the inner tube (reaction chamber). In the gap between both tubes, the pressurized feedstream is going down and cooling the reaction medium at the same time. In such way, the inner tube does not withstand any pressure at all, having the same pressure in one side than on the other, and the thickness of the inner tube (Inconel 625) can be reduced. The reaction chamber is of 0.135 m inner diameter and 1 m length, which means a reaction volume of 14.3 L. The reactor is filled with allumina spheres 4-mm diameter,
and the void volume was estimated by adding a known volume of water to a known volume of dry packed material. By measurment of the volume taken up by the water, a void fraction of 0.38 was obtained. Therefore, the effective volume of reaction is 5.4 L. Ceramic materials have been reported to be unstable under SCWO conditions; however this reactor has been working more than 1000 h without any problem. Analysis of Ni and Cr in the effluent did not reveal any corrosion in the reactor. The main feature of this reactor is that it can be operated without any external energy supply, due to the use of the released energy in the reaction for the preheating step. In that sense, the reactor can be defined as energetically self-sufficient. During the starting-up periods, two electrical heaters (3.9 kW each of them) can be used to preheat the feedstream and the air to operational conditions, and they are switched off when desired temperature is reached. The effluent reactor is cooled in two heat exchangers consisting of a 6.07-m-long, 6.35-mm-o.d. SS 316 coiled tube submerged in a bath where a tap water stream is used as cooling medium. Piping lines in the plant are 6.35-mm (1/4 in.) outside diameter (o.d.) SS 316 tubing, and all high-pressure connections are Swagelok Stainless Steel (SS) 316 fittings. The cooled effluent is then passed through the backpressure regulating valve where pressure is reduced to almost atmospheric pressure. This effluent is finally flowed to the gas-liquid separator where the gas phase is sampled or directly vented. Liquid samples are collected from a sample port located in the effluent line from the gas-liquid separator. Temperature is measured in the feedstream, air, and effluent streams. In the reaction chamber there are four thermocouples (type K Inconel 625) at different heights in order to identify the temperature profile, and three more are located in the outer wall of the reactor. The time progress of these data together with the pressure are recorded in a PC computer for their later study. B. Analytical Procedures and Materials. Feedstreams as well as effluents (gas and liquid) were
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characterized by analytical techniques. Samples were collected in triplicate for analysis. Total organic carbon (TOC) was characterized in the feedstream and in the liquid effluent in order to evaluate the efficiency of the oxidation. The equipment used was Shimadzu 5050 which uses combustion and IR analysis. The detection limit is 1 ppm. Nitrates and nitrites were characterized in the liquid effluent by ionic chromatography with a IC PAK A column of Waters. The detection limit is 1 ppm. Ammonia concentrations in both the liquid feed to the reactor and the liquid product from the reactor were determined by analysis with an ion-specific electrode (Orion Research model 95-12). The detection limit is 1 ppm. NOx in the gas effluent was analyzed with Dra¨gertube detectors, Lab Safety Supply CH29401 and CH31001. The NOx detection limits for these tubes ranged from 0.5 to 100 ppm. Analysis of organic compounds in the liquid effluent was done by gas chromatrography with headspace accessory, Hewlett-Packard model 5890. The detection limit is 1 ppm. The pH was measured potentiometrically using a glass electrode versus a reference electrode. C. Experimental Procedures. The aim of this work was to study the operational parameters, such as reaction time, reaction temperature, and oxidant excess, in the oxidation of wastewaters containing organicnitrogenous compounds. The effect of pressure has not been studied since, in previous works,8,9 it was found that above the critical pressure of water the effect of pressure on organic destruction efficiency is negligible. Most previous works have been made on a laboratory scale,1,2,3 but it is rare to find experiments at a bigger scale. In this paper, results of the oxidation of several nitrogen-containing organics in a SCWO pilot plant are presented. Selection of these compounds was made in order to study oxidation behavior on different kinds of molecules and different kinds of bonds between the nitrogen and the rest of the molecule. Efficiency of the process was determined by the analysis of nitrogen species in the effluents (gas and liquid streams) and organic matter elimination (TOC). Suitable conditions were chosen in terms of TOC elimination and the minimum presence of nitrogenous products in the effluent. The following definitions are used:
TOCREMOVAL )
O2 (excess) )
(TOC)in - (TOC)out (TOC)in
(O2)in - (O2)stoich
NREMOVAL )
(O2)in (N)in - (N)C (N)in
× 100
× 100
× 100
where (O2)stoich is the stoichiometric amount of O2 for the total oxidation of the organic compounds contained in the feedstream (2-propanol and nitrogenous compound). The error in TOC removal is (0.01% and for N removal (0.5%. In all the experiments feed contained 2-propanol as auxiliary fuel. The aim of adding 2-propanol to the feed
was to obtain the right enthalpy content in the feedstream in order to perform a self-sufficient operation in the reactor, avoiding in such a way a external energetic supply.10 Two sets of experiments were planned for each compound in the experimental work: (a) study of air excess effect; (b) effect of reaction temperature. Results and Discussion Oxidation of aniline, acetonitrile, pyridine, and ammonia in supercritical water was investigated over the temperature range 530-830 °C at a pressure of 25.0 MPa, for reactor residence times of 25-45 s. Feed contained a mixture of the nitrogen-containing compound and 2-propanol in different ratios. Table 1 provides a listing of the experiment conditions performed in the pilot plant. Tables 2 and 3 provide a detailed listing of tests conditions and results. N2O could be a gas product in these experiments; however, it has not been considered because its reduction to N2 takes place above 565 °C.11 At high temperatures, like in this work, N2 is the major gas product. This effect is described by Killilea et al.3 thermodynamically and kinetically. The pH of the liquid phase, measured in all the samples, was acid, and it was associated with nitrite and nitrate concentrations. (A) Experiments with Aniline. Aniline, C6H5NH2, is a member of the aromatic amines chemical family. Besides being a compound of the wastewater by itself (classified by the United States EPA as hazardous12), aniline can also be a product of the incomplete oxidation of different nitrogen-containing compounds, such as it was observed by Lee and co-workers during the decomposition of nitrobenzene13 and 4-nitroaniline.6 Therefore, the study of the oxidation behavior is very appealing. (1) Effect of Oxygen Concentration. The effect of oxygen excess on the SCWO of feedstreams containing aniline was studied at a constant reaction temperature of 650 °C, and results are shown in Figure 2. The efficiency of the process, based on TOC removal, is greater than 99.99% when oxygen excess is greater than -10%, and above this value, the effect of oxygen concentration on TOC removal is negligible. The effect of oxygen excess on nitrogen-containing products included nitrites, nitrates, and ammonia in the liquid stream, as well as NOx in the gas stream, is also shown in Figure 2. Due to relative low reaction temperature, in all the experiments, the presence of NOx is negligible, always below 10 ppm. In the liquid stream, the presence of ammonia is detected when the oxygen concentration is below the stoichiometric value, indicating that ammonia is an intermediate product in the oxidation of aniline, which is not completely oxidized in a deficit of oxygen. Nitrite concentration in the liquid product is always below 9 ppm, and there is not any significant variation with oxygen excess, whereas nitrates increase with the oxygen concentration up to 38 ppm at 49.3% oxygen excess. These results indicate that TOC removal above 99.99% was obtained for oxygen excess greater than -10%, and nitrogen-containing products are minimum for stoichiometric oxygen concentration (Figure 2). Therefore, for such kind of feedstream, the best elimination is reached when the stoichiometric amount of oxygen is used. (2) Effect of Reaction Temperature. Several aniline/2-propanol mixtures SCWO were investigated at 12 temperatures ranging from 550 to 700 °C. From
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Table 1. Set of Experiments Performed in the Pilot Plant (P ) 25.0 MPa)a
a Terms: [ ] , initial concentration of nitrogenous compound in the feedstream at ambient conditions; t , average residence time in 0 res the reactor calculated with the density of the mixture at the pressure and reaction temperature (Peng-Robinson).
Figure 2. Effect of oxygen concentration on SCWO of aniline/2propanol mixtures: P ∼ 25.0 MPa; T ∼ 655 °C; CAnil ∼ 1000 mg/ L; CisoprOH ∼ 6% w/w.
previous results, oxygen feed was fixed at the stoichiometric amount. Results are shown in Figure 3. Conversion of organic compounds above 99.95% (in terms of TOC removal) was obtained at temperatures greater than 570 °C, whereas for the run performed at 550 °C only 99.81% of TOC removal was reached, although residence time in the reactor was greater, 55 s. It is necessary to notice that residence time in the reactor
Figure 3. Effect of reaction temperature on SCWO of aniline/2propanol mixtures: P ∼ 25.0 MPa; CO2 ∼ stoichiometric; CAnil ∼ 1000 mg/L; CisoprOH ∼ 5% w/w.
was greater at lower temperatures (around 50 s), whereas it was nearly constant and lower (44 s) in the range 670-700 °C. There is a nearly constant TOC removal for temperatures greater than 570 °C, and higher temperatures, such as 700 °C, do not imply higher conversions. NOx in the gas product was always below the detectability limit of the Dra¨ger tube (
