Ind. Eng. Chem. Res. 2006, 45, 7977-7981
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APPLIED CHEMISTRY Results of Testing the Plant for Supercritical Water Oxidation of Nitroglycerin and Diethylene Glycol Dinitrate V. I. Anikeev,*,† N. S. Belobrov,‡ R. N. Piterkin,‡ R. Sh. Prosvirnin,‡ L. S. Zvolsky,‡ P. E. Mikenin,† and A. Yermakova† BoreskoV Institute of Catalysis, NoVosibirsk, Russia, and Federal Research and Production Center “Altai”, Biysk, Russia
In Russia, the first stationary supercritical water oxidation (SCWO) pilot plant for the oxidation of industrial wastes at an operating factory with a tubular flow reactor and a capacity of about 40 kg/h wastewater has been created on the basis of fundamental investigations. It allowed for the elimination of a mixture of nitroglycerin and diethylene glycol dinitrate in the wastewater with very high efficiency. Acetone was used as the fuel, and hydrogen peroxide or air was used as the oxidant. Introduction Wastewater from the manufacturing of industrial explosives commonly contains a mixture of 1000-1350 mg/L nitroglycerin (NGC) and 2000-2700 mg/L diethylene glycol dinitrate (DEGDN), sometimes 0.5-1.0% sodium carbonate, up to 2000 mg/L sodium nitrate, and up to 50 mg/L sodium sulfate. The usual treatment of this kind of wastewater at the Federal Research and Production Center (FRPC) “Altai” includes hydrolysis of nitrate ethers in alkaline medium at ca. 95 °C. Hydrolysis of NGC takes 1-2 h, whereas the removal of DEGDN lasts more than 10 h. However, this decreases the DEGDN concentration in the wastewater only to 40-50 mg/L and occasionally to 2-5 mg/L. It is known that the supercritical water oxidation (SCWO) of various organic substances by air oxygen or pure oxygen takes several minutes or less and yields water, carbon dioxide, and nitrogen as the main reaction products.1-7 Thus, it seems promising to apply SCWO to waste effluents containing nitrate ethers. Design of the main elements and construction of the SCWO plant was based on literature data,1-7 on data from mathematical modeling, and on kinetic experiments performed by the authors.8-10 Taking into account the absence of large amounts of inorganic salts in the wastewater, it was decided to choose a relatively simple version of a tubular reactor providing longterm operation in the case of insignificant salt deposits on its surface. The capacity of the plant is 30-45 L/h of wastewater. Acetone was used as the fuel, mixed with wastewater in the required proportions. Hydrogen peroxide (in the first stage) and air were used as oxidants. Flow Diagram of the SCWO Plant
and 2), each of volume 100 L, are equipped with turbine-type mixers and level gauges, which provides an efficient preparation of the acetone and hydrogen peroxide solutions in wastewater. A separate vessel for water (3) with a volume of 80 L is used for star-up. The metering pumps (4 and 5) have a maximum capacity of 63 L/h and a discharge pressure of up to 40 MPa. Electric heaters (6 and 7) provide heating of two liquids or one liquid and air. The coil electric heaters are made of a tube with an external diameter of 10 mm, a wall thickness of 2 mm, and a length of 48.4 m; the heat-exchanging surface comprises 1.52 m2. The electric heaters are equipped with standard heating cartridges, each of 0.8 kW power. The maximum power of each electric heater is 32 kW. The reactor (8) is a tube made of titanium alloy BT-9 with an external diameter of 89 mm, an internal diameter of 53 mm, and a length of 2.6 m; the reactor volume is 5.7 L. The reactor is equipped with two heating sections, each of 20 kW power. The cooler (9) is a spiral made of a tube with a diameter of 10 mm and a length of 53.5 m, immersed in water for cooling. The pressure-relief unit consists of a spiral tube (10) with an internal diameter of 2 mm and a length of 60 m that includes a valve (12). To feed the oxidant, air, a four-stage SVC 600/320 compressor (J. A. Becker & So¨hne, 14) was used; its capacity is 0.5 m3/min, and the maximum pressure is 40 MPa. During operation, the following parameters were controlled and measured: volumes of the liquids in the storage vessels; pressures after the pumps and cooler; temperature of the flow at the outlet of electric heaters 6 and 7; and temperatures of the reaction mixture in the reactor, at its outlet, and after the cooler. The control and recording system provides remote control of the plant and recording of process parameter at specified time intervals.
Figure 1 is a schematic representation of the plant constructed at the FRPC “Altai”. In this diagram, the storage vessels (1
SCWO Plant Operation
* To whom correspondence should be addressed. E-mail: anik@ catalysis.nsk.su. † Boreskov Institute of Catalysis. ‡ Federal Research and Production Center “Altai”.
Industrial waste containing a mixture of nitroglycerin and diethylene glycol dinitrate was used at the operating factory to test the SCWO plant. Prior to plant operation, the following solutions were prepared in the storage vessels: a solution of
10.1021/ie060918k CCC: $33.50 © 2006 American Chemical Society Published on Web 10/26/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006
Figure 1. Equipment-process diagram of the SCWO plant: 1, storage vessel for wastewater with acetone; 2, storage vessel for hydrogen peroxide; 3, vessel for water; 4 and 5, metering pumps; 6 and 7, electric heaters 1 and 2; 8, reactor; 9, cooler; 10, pressure-relief unit; 11, collector of purified effluents; 12 and 13, control valves; 14, compressor.
Figure 2. Process parameters at the plant during testing with hydrogen peroxide (Table 1, run 4): 1, volume of liquid in storage vessel 2; 2, volume of liquid in storage vessel 1; 3, fluid temperature after electric heater 2; 4, fluid temperature after the reactor.
acetone fuel in wastewater with a concentration of up to 10% and a hydrogen peroxide solution with a concentration of up to 30%. After power to the electric heaters and reactor was switched on, the liquid pump delivered water to the reactor from vessel 3. The required pressure in the reactor was attained using valve 12. Note that acetone and hydrogen peroxide are also the wastes of the operating factory. The average residence time in
the reactor was about 0.5-0.9 min when hydrogen peroxide was used. When the temperature of the water at the outlet of the electric heaters and in the reactor reached ca. 400 °C, wastewater, fuel, and oxidant were fed to the reactor instead of pure water. The working temperature in the reactor was attained in 4-5 min, and the reactor heating was switched off. The reactor temper-
Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006 7979 Table 1. Results of Testing the SCWO Plant Using Hydrogen Peroxide as Oxidant consumption, L/h
concentration, %
hydrogen peroxide acetone in hydrogen reactor pressure, wastewater with water wastewater peroxide temperature, °C MPa 1 2
24.3 23.0
27.4 26.0
5.0 7.3
16.0 29.4
3
24.0
26.5
6.1
23.2
4
23.0
25.9
4.4
22.4
a
582 650 690 710 590 670 650 520 590 635
25.6 26.0 25.0 22.8 24.0 24.0 24.0 24.0 24.8 26.5
content of nitroethers, mg/L before after 705 370 350 390
6.3 dla dl dl 0.39 dl 0.39 8.0 5.0 8.0
treatment efficiency, % 99.1 >99.9 >99.9 >99.9 99.9 >99.9 99.9 97.9 98.7 97.9
oxidizability with respect to bichromate, mg of O2/L before after 161 879 86 401 65 518
23.6 23.6 23.6 32.7 101.9 28.0 67.3 31.0 19.7
dl ) detection limit.
Figure 3. Process parameters at the plant during testing with hydrogen peroxide (Table 1, run 4): 1, temperature of flow in the reactor; 2, fluid temperature after electric heater 1; 3, pressure.
ature can be changed by varying the temperature of the flows at the electric heaters outlet, by switching on and off the reactor heaters, or by varying the fuel consumption. During the process, the liquid after the cooler was repeatedly sampled. A similar experimental procedure was performed with air delivered by the compressor instead of hydrogen peroxide. The air consumption was 30 m3/h during testing. The average residence time in the reactor was about 0.2-0.8 min. Pressure in the reactor was controlled using valves 12 and 13. Note that, in the case when hydrogen peroxide was used as the oxidant, the pressure was adjusted only by valve 12. The method of measuring the content of nitrate ethers in water was based on the reduction of nitroethers by ferrous iron sulfate, followed by determination of its excess using potassium bichromate. Results and Discussion In the first stage of the study, the selected organic compounds were oxidized with hydrogen peroxide. A goal was to find the optimal concentration of fuel (acetone) in the wastewater that would provide complete conversion of the chosen nitroethers. The efficiency of the SCWO process was estimated by the degree of nitroether oxidation and by the value of chemical
oxygen consumption, which actually characterizes the degree of acetone oxidation. Some data from the test runs are presented in Table 1. Figures 2 and 3 present diagrams with the main process parameter for run 4 (Table 1), showing the time when the feeding of wastewater and hydrogen peroxide flows started and the time when the reaction products were sampled. The actual consumption of the liquids was calculated from the changes in their amounts in the storage vessels presented in the diagram. One can see from Figure 3 that temperature 1 of the reaction mixture in the reactor increased continuously during the experiment because of the increasing temperature of the mixture at the heater outlet 2 (reactor inlet). This autothermal regime was chosen intentionally to study within a single experiment the effect of the inlet temperature on the conversion of certain compounds, other flows being equal. The concentration of hydrogen peroxide was chosen so as to provide its stoichiometric amount with respect to the total flow of acetone and nitroethers and ensure operation in the autothermal regime. However, such a ratio of oxidant to acetone and nitroethers was not always maintained because of the partial decomposition of hydrogen peroxide with time. Because of an oxygen deficiency, the complete oxidation of nitroethers was thus not reached in some experiments. Nevertheless, the data
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Table 2. Results of Testing the SCWO Plant Using Air as Oxidant inlet wastewater concentration of reactor consumpt ion, acetone in water, temperature, pressure, temperature, °C L/h % °C MPa wastewater air 1
22.9
8.0
2
21.8
5.8
3
21.7
5.8
4
28.5
6.5
a
414 639 657 580 705 611 676 683 666 632 636 650
24.7 23.7 27.0 24.0 24.3 28.3 26.3 27.4 29.1 23.9 28.8 25.6
361 344 344 357 374 374 378 366 391 347 366 359
622 580 544 534 514 534 529 529 529 531 529 533
oxidizability content of (chemical oxygen treatment degree of nitroethers, mg/L efficiency, minimum), mg of O2/L decomposition, before after % before after % 430 330 350 420
dla dl dl dl dl dl dl dl dl dl dl dl
>99.9 >99.9 >99.9 >99.9 >99.9 >99.9 >99.9 >99.9 >99.9 >99.9 >99.9 >99.9
107538 97001 106790 111240
3234.0 42.8 10.5 16.9 20.5 35.6 27.6 57.0 79.2 19.3 37.7 79.6
97.0 99.96 99.99 99.98 99.98 99.96 99.97 99.95 99.93 99.98 99.97 99.93
dl ) detection limit.
Figure 4. Process parameters at the plant during testing with air (Table 2, run 4): 1, temperature of flow in the reactor; 2, fluid temperature after electric heater 2; 3, pressure.
Figure 5. Process parameters at the plant during operation with air (Table 2, run 4): 1, volume of liquid in storage vessel 1; 2, fluid temperature after the reactor; 3, fluid temperature after electric heater 1.
Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006 7981
of Table 1 demonstrate the high efficiency of removal of nitroethers at reactor temperatures above 650 °C (runs 2 and 3). In this case, the degree of acetone oxidation always exceeded 99.9% (according to the value of chemical oxygen consumption). For a constant inlet temperature of the reagents and switchedoff heaters, the reactor temperature changed with the concentration of acetone, which provided the required heat by oxidation release. Commonly, the oxidation of acetone started at reactor temperatures above 400 °C; then, the temperature increased to 550-700 °C, depending on the acetone concentration in the wastewater. It was found that an acetone concentration of ca. 6% is optimal for the wastewater treatment presented in Table 1. This concentration of acetone provided a reactor temperature of ca. 650 °C without use of the reactor heaters. Acetone contents in the wastewater of less than 4% necessitate a constant heating of the reactor. The experiments showed the feasibility of reliable process control. The pressure ranged from 23 to 25 MPa. Fluctuations in the flow temperature were also observed at the outlet of the heat exchanger reactor depending on hydrogen peroxide decomposition. Because hydrogen peroxide is unstable in storage and flammable upon accidental contact with combustible materials, its use involves some inconvenience during SCWO plant operation. Therefore, the studies were continued using air as the oxidant, decreasing the concentration of acetone necessary to attain the required temperature in the reactor. The data on wastewater analysis and main parameter of the procedures are presented in Table 2. The values of the main process parameter for run 4 are shown in Figures 4 and 5. Compared to hydrogen peroxide, the use of air allowed the temperature at the inlet of reactor to increase to 540 °C, which made use of the heaters during reactor start-up unnecessary. The only disadvantage in the operation of the equipment was caused by periodic short-term shutting-down of the compressor for purging and removal of oil and moisture condensate, which is required by the compressor operation conditions. The interval between such shutdowns was 45 min, which led to a slight decrease in pressure and temperature (see Figures 4 and 5). After restarting the compressor, all parameters retrieved their levels without interference by an operator. As follows from Table 2, plant operation with productivity of up to 28.5 L/h of wastewater using air as the oxidant resulted in the complete oxidation of the nitroethers under consideration. At temperatures higher than 580 °C, the degree of acetone conversion was 99.93%. At the very low temperature of 414 °C (run 1), only 97% destruction efficiency was achieved. The time of continuous operation of the plant exceeded 3 h in most experiments, and the experiments were terminated because all liquid wastes from the storage vessel had been consumed.
Conclusion The presented results confirm the high efficiency of the SCWO of wastes discharged from the production of nitroethers and give cause to recommend the process and plant developed for the oxidation of a wide range of organic wastes that are difficult to destroy by other known methods. The authors plan experiments using the developed plant for the oxidation of more complex organic wastes formed at the pilot production facilities of the Federal Research and Production Center “Altai”. Further work on the SCWO of organic compounds using ammonium nitrate discharged by the operating factory as a main or additional oxidant is planned. Test results for the stationary SCWO plant also allowed the proposal of a new concept of a mobile SCWO plant and a compact reactor for the oxidation of a wide range of wastes. Acknowledgment This work was performed within the ISTC Project # 2383. Literature Cited (1) Cocero, M. J.; Alonso, E.; Torio, R.; Vallelado, D.; Fdz-Polanco, F. Supercritical Water Oxidation in a Pilot Plant of Nitrogenous Compounds: 2-Propanol Mixtures in the Temperature Range 500-700 °C. Ind. Eng. Chem. Res. 2002, 39, 3707-3716. (2) Abeln, J.; Kluth, M.; Bu¨ttcher, M.; Sengpiel, M. Supercritical Water Oxidation (SCWO) Using a Pipe and a Transpiring Wall Reactor: CFD Simulations and Experimental Results of Ethanol Oxidation. EnViron. Eng. Sci. 2004, 21 (4), 93-99. (3) Yermakova, A.; Anikeev, V. Thermodynamic calculations in modeling of multiphase processes and reactors. Ind. Eng. Chem. Res. 2000, 39, 1453-1472. (4) Cocero, M. J.; Alonso, E.; Fdz-Polanco, F. Supercritical Water Oxidation process under energetically self-sufficient operation. J. Supercrit. Fluids 2002, 24, 1, 37-46. (5) Crooker, P. J.; Anhwalia, K. S.; Fan, Z.; Prince, J. Operating Results from Water Oxidation Plants. Ind. Eng. Chem. Res. 2000, 39, 4865-4870. (6) Abeln, J.; Kluth, M.; Petrich, G.; Schmieder, H. Supercritical Water Oxidation (SCWO): A Process for the Treatment of Industrial Waste Effluents. High-Pressure Res. 2001, 20, 537-547. (7) Gidner, A.; Stenmark, L. Oxidation of De-inking Sludge in Supercritical Water. Presented at the Managing Pulp and Paper Process Residues Workshop, Barcelona, Spain, May 30-31, 2002. (8) Anikeev, V. I.; Yermakova, A.; Goto, M. Decomposition and oxidation of aliphatic nitrocompounds in supercritical water. Ind. Eng. Chem. Res. 2004, 43, 8141-8147. (9) Anikeev, V. I.; Yermakova, A.; Semikolenov, V. A.; Goto, M. Effect of supercritical water density on the rate constant of the aliphatic nitrocompounds decomposition. J. Supercrit. Fluids 2005, 33, 243-246. (10) Yermakova, A.; Anikeev, V. I. Modeling oxidation of organic compounds in supercritical water. Theor. Found. Chem. Technol. (Russ.) 2004, 38, 4, 355-363.
ReceiVed for reView July 15, 2006 ReVised manuscript receiVed September 22, 2006 Accepted September 27, 2006 IE060918K
