Oxidation of organic material in supercritical water and carbon dioxide

High Pressure Chemical Engineering Ph. Rudolfvon Rohr and Ch. Trepp (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

61

O X I D A T I O N O F O R G A N I C M A T E R I A L IN S U P E R C R I T I C A L WATER AND CARBON DIOXIDE H. Goldacker, J. Abeln, M. Kluth, A. Kruse, H. Schmieder and G. Wiegand Forschungszentrum Karlsruhe GmbH, Institut for Technische Chemie, Bereich Chemisch Physikalische Verfahren, Postfach 3640, D-76021 Karlsruhe Keywords: Supercritical Water, Supercritical Carbondioxide, Hazardous Organics, Oxidation, Separation 1. INTRODUCTION The industrial application of the SCWO process strongly depends on the solution of two major technical problems: corrosion of the reactor material when halogenated compounds are processed and precipitation of inorganic product salts plugging the reactor. Consequently, research programmes were established to investigate appropriate construction materials and modify reactor configurations. The principles of the most outstanding constructions are shown in figure 1: Fig. 1: Principles of the most outstanding SCWO reactor configurations: a) tubular reactor developed at the university of Austin, Texas /1/, commercialized by EWT, operated in Huntsville, Texas /2/, capacity 5 gpm; b) vessel reactor, MODAR Inc., Massachusetts/3/; c) transpiring wall reactor, Summit Research Corp., Santa Fe, New Mexico /4/; filmcooled coaxial hydrothermal burner developed at the ETH ZOrich, Switzerland/5/

Whereas the tank type and the transpiring wall type are experimentally operated in bench scale rigs, the tubular reactor with multiple feedpoints for oxygen and quenching water is already commercialized for the treatment of solutions, such as long-chain alcohols and amines, without the risks arising from salt formation and corrosive compounds.

62 The fourth type, the filmcooled coaxial hydrothermal burner/5/is under development at the ETH ZOrich and, to our knowledge, it is operated experimentally with model substances at pilot scale level. In 1991 a programme was started at the Research Center of Karlsruhe (FZK/ITC-CPV), comprising the oxidation of hazardous organic material in supercritical water/6/. For this purpose two continuously operated test facilities of different scale were constructed. Simultaneously a separation process of lubricating cooling oils from metal machining residues by means of supercritical CO2 extraction (SC-CO2-Extraction) was developed and presently has good chances to be commercialized/7/. In parallel, a liquid/fluid countercurrent extraction column is connected to the CO2 test rig and basic investigations concerning drop formation and mass transfer have been started.

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~ P r i n c i p l e s of the SCWO plants 1 kg/h and 10 kg/h water) operated in the FZK/ITC-CPV

Fig. 3: Principles of the SF-CO2 test rig, capacity 30 kg/h CO2/7/

The methods of supercritical extraction and of supercritical oxidation in carbondioxide can favourably be integrated in one process if the extracted solute has to be disposed. The VISCO code, developed at FZK/ITC-CPV in former times is now used both for process monitoring and for process modelling. First results will be presented at this conference/8/. 2. EXPERIMENTS IN THE SCWO LABSCALE PLANT/9/ Design data are: lkg water per hour, P<300 bar and T<600 ~ kept constant by a fluidized sandbath in which a 6 m tubular reactor coil with an inner diameter of 2 mm is submerged. 33 thermocouples measure the reaction temperature profiles. Water, organic material and the pressurized air can be preheated. Oxidation experiments started with ethanol and were continued with further model substances like n-hexane, cyclohexane, benzene, toluene and nitrobenze, etc. Investigated parameters include fluidized sandbath temperature levels of 350 ~ to 550 ~ the water throughput of 300 g/h to 1200 g/h and oxygen stoichiometry varying between 1.2 and 2.0. Organic waste concentration ranged between 2.5 and about 10 w%. In-line instrumentation in the product gas provides continuous data for the concentrations of CO, CO, and 02, whereas samples from the aqueous phase provide information about the residual TOC and soluble organic products de-

63 tected by GC-MS and IC. The resulting TOC values of 12 % in maximum show a clear dependency on bath temperature and contact time. The influence of the stoichiometry varied by a factor of nearly 2 is not clearly visible. Fig. 4 gives an example for the benzene and nitrobenzene oxidation at 240 bar and 550 ~ The dominating influence under these conditions of water throughput and residence time respectively is shown by the figure 4.

Fig. 4:

Residual TOC data for the oxidation of benzene (left) and nitrobenzene (fight) in the labscale plant. Temperature 550 ~ pressure 240 bar. Parameters are throughput of water (W), air (A) and organic material (O)

In preparation for an integrated separation and oxidation of organic wastes, preliminary oxidation tests with ethanol and toluene in supercritical CO2, substituting the water, were recently performed successfully/10/, figure 5. At a constant pressure of 240 bars, about 350 g/h CO2 loaded with 10 w% ethanol and a 1.5 fold stoichiometric excess of oxygen in air were fed to the system. The tests showed that TOC values of 0.01% can be obtained similiar to the SCWO process. It is remarkable that the reaction seems to continue for more than 2.5 hours at temperatures of the reaction tube substantially below 300 ~ This is in contrast to the ethanol oxidation in water only starting above 360 ~ In the near future these tests, investigating optimized reaction temperatures and the influence of water will be continued in the lab system.

Fig5.: Oxidation of ethanol in supercritical COz, 350 g/h CO2, 240 bar, 35 g/h ethanol, 1.5 fold stoichiometric excess of oxygen

64 3. EXPERIMENTS IN THE BENCHSCALE PLANT/11/ The benchscale plant principally follows the design scheme of the labscale plant, but is equipped with a tubular reactor coil made of Inconel 625, 15 metres in length, the 8 mm inner diameter of which is good for a throughput of 10 kg/h of water. Tests with model substances like ethanol and toluene showed results comparable to those of the labscale plant. In order to prepare experiments with real wastes, a 0.8 w% sodiumsulfate solution under various flow conditions was used to test the reactor performance in the presence of precipitations. It could be demonstrated that the system can be operated for a limited time with moderate salt concentrations in the effluent. Salt cakes accumulating to about 150 gramms in total and plugging the 8 mm tube were redissolved by flushing with water at temperatures below 100 ~ The results of first tests with industrial effluents are shown in Tab. 1. They comprise solutions such as pharmaceutical waste originating from the synthesis of penicillin and carrying low solid but high soluble salt concentrations, de-inking effluents from a paper mill and municipal sludge, both charged with mainly solid organics. As it could be expected, the system was operable only for a limited time after starting the feed at controlled bath temperatures. Several samples taken within one run proved steady state conditions. Especially the first two examples demonstrate that the carbon conversion depends on the temperature. The stoichiometric oxygen excess for the three runs was kept high at values of 8 and higher. The volumetric throughput varied between 6 and 10 kg/h. The mass flows resulted in residence times in the range of 25 to 50 seconds related to the bath temperatures and the whole reactor volume. The said operating conditions show that the carbon conversions are not optimized with respect to temperatures and residence times. However, the comparison of gaschromatographical data showed that no more feed compounds could be found in the product solutions. TAB. 1: Results with industrial wastes in the bench scale plant Waste type

Feed-TOC ppm

Conversion Temperature Salt-concentration % ~ %

Operating time h

Pharmaceutical waste water

7.000 20.000

83 97

410 550

1 3

2 (RP) 0.5

Paper mill sludge

2.000 2.000

98 99

450 500

0.1 0.1

2 (FE) 2

Sewage sludge

1.000

85

500

< 0.1

3 (PP)

RP: Reactor plugging, FE: Feed finished, PP: Pump valve plugging 4. CORROSION TEST P R O G R A M M E / 1 2 / In two test rigs materials like Inconel 625, Haynes 214, Hastelloy C-276, Nicrofer 5923 and 6025 were tested in aqueous solutions containing 0.5 mol/kg oxygen and 0.05 to 0.5 mol/kg HCI. One of the rigs offers the possibility to test five tubes in parallel at pressures between 240 to 400 bars and at temperatures up to 600 ~ Practically all types of material showed heavy local corrosion after some tens of hours. More encouraging results were obtained for oxide ceramics, small samples of which are presently under test. Fig. 6 gives an impression of the local corrosion depending on temperature.

65

Fig. 6: Corrosion in an Inconel 625 tube at 240 bar and 400 ~ operating temperature. Results after 150 hours in 0.5 mol 02 and 0.05 mol HC1 per kg of water, flowrate 60 ml/h,/12/

The attack at a pressure of 240 bar is very severe at temperatures between 300 and 390 ~ where the local gradient is steep. In the steady state section at 400 ~ the bite is comparatively moderate. Further corrosion tests indicate that at higher pressures the bite speeds up and that a higher steady state operating temperature has a minor influence on the corrosion rate. 5. FUTURE W O R K I N G P R O G R A M M E Salt precipitation and corrosion are the technical key problems of the economical solution which will make the SCWO process an important tool for the treatment of hazardous organic wastes. Fig. 7: Principles of the transpiring wall reactor configuration

66 The development of a corrosion resistant material e. g. may take a time too long for industrial implementation in a competitive market situation. Transpiring wall or filmcooled coaxial burner techniques have a high potential to solve said problems by design supported by a material test programme. If not, the SCWO process will be technically and economically restricted to special applications in a small waste section.In parallel to the described activities, a transpiring wall reactor configuration has been designed and is presently under construction, comp. Fig. 7. A concentrical porous tube separates the supercritical reaction volume from the pressure bearing containment. This porous tube can be made of sintered metal, what is presently intended, or of porous oxide ceramics. At the beginning the choice of materials will be limited to hardware on stock which can be changed for experimental reasons at comparatively low costs. First experiments will concentrate on the prevention of salt accumulation at the wall. Subcritical water is thought to remove precipitations under falling film conditions. Improved corrosion resistance is the second reason to follow this concept. Parameters for optimization are: Flow ratio of film water and feed, qualification of the sintered material, heat transfer between film and reacting bulk solution, energy input to start the SCWO reaction, output of gas charged salt brine. The system is planned to go into operation at the end of 1996.

6. R E F E R E N C E S

/1/ E. F. Gloyna, L. Li, and R. N. Brayer, Wat. Sci. Tech., Vol. 9 (1994) 1-10 /2/ ECO WASTE TECHNOLOGIES, Information Package 1995, 2305 Donley Dr., Suite 108, Austin, Texas 78758 /3/ H.E. Barner, C. Y. Huang, T. Johnson, G. Jacobs, M. A. Martch and W. R. Killilea, J. Hazardous Mat., 31 (1992) 1-17 /4/ Th. G. McGuinness, 1. Int. Workshop on SCWO, Jacksonville, Florida, Febr.6-9, 1995 /5/ H.L. La Roche, M. Weber, Ch. Trepp, 1. Int. Workshop on SCWO, Jacksonville, Florida, Febr. 6-9, 1995 /6/ H. Schmieder, H. Goldacker, G. Petrich, Interdiscipl. Science Reviews 18 (1993), 207 -215 /7/ J. Sch6n, N. Dahmen, H. Schmieder in chemie-anlagen + verfahren 12 (1994), 10-11 /8/ G. Petrich, J. Abeln, H. Schmieder, this symposium /9/ G. Wiegand, H.J. Bleyl, H. Goldacker, G. Petrich, H. Schmieder, 1 Int. Workshop on SCWO, Jacksonville, Florida, Febr. 6-9, 1995 /10/H.-J. Bleyl, J. Abeln, N. Boukis, H. Goldacker, M. Kluth, A. Kruse, G. Petrich, H. Schmieder, G. Wiegand, Colloquium Produktionsintegrierter Umweltschutz, Bremen 11.- 13. Sept. 1995 /11/J. Abeln, N. Boukis, H. Goldacker, M. Kluth, G. Petrich, H. Schmieder, GVC-Conference, Erlangen, 1996 /12/N. Boukis, R. Landvatter, W. Habicht, G. Franz, S. Leistikow, R. Kraft, O. Jacobi, 1. Int. Workshop on SCWO, Jacksonville, Florida, Febr. 6-9, 1995