Patent Number: 060841478
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention is a decomposition process and system for decomposing organic wastes so that the volume and mass of the waste to be disposed of is greatly reduced from the initial volume and mass. Furthermore, those components of the processed waste that are released to the environment, gases and water vapor, are rendered harmless prior to release. The present process will be described in particular with respect to radioactive waste, and most particularly with respect to radioactive ion exchange resin, but any organic wastes can be processed in accordance with the following process and with the components of the system. The process is based on pyrolysis using steam supplemented with oxygen in a two-stage, fluidized bed reactor, and uses conventional off-gas treatment including wet scrubbers to treat the gaseous effluent. The solid residue from the processing of wastes, an inorganic, high-metal oxide content grit, is packaged for disposal or further treatment. The wastes that can be processed according to the present invention include not only ion exchange resins, but also steam generator cleaning solutions, solvents, oils, decontamination solutions, antifreeze, paper, plastics, cloth, wood, soils, sludge, nitrates, phosphates and contaminated waters. An ion exchange resin is made of organic materials, commonly styrene to which are grafted amino groups to make anion resins or to which sulfonic groups are grafted to form cation resins. As these resins are used in a nuclear reactor, they accumulate up to about 7% iron, calcium, silica and minute amounts of other metals and cations. Pyrolysis is the destruction of organic material using heat in the absence of a stoichiometric amount of oxygen. The presence of oxygen allows some oxidation to provide heat to offset the heat requirements of the pyrolysis or organic compounds, which is otherwise an endothermic reaction. In the present process, the organic component of the resin is destructively distilled by the steam from the inorganic components. When heated, the weak chemical bonds of the resin polymers break up into compounds with lower carbon numbers, including carbon, metal oxides, and metal sulfides, and pyrolysis gases, which in turn include carbon dioxide, carbon monoxide, water, nitrogen and hydrocarbon gases, typically called syngas (carbon monoxide, hydrogen, methane, etc.). The small volume of solid residue remaining after reformation contains the overwhelming majority of the radionuclides. Although pyrolysis can take place over a wide range of temperatures, the present process is a low temperature pyrolysis, generally around 550-700.degree. C. to prevent radioactive metals on the ion exchange resins from volatizing. These metals are retained in the reaction vessel residue. Consequently, the clean, low activity synthetic gases can then be converted at higher temperatures to carbon dioxide and water without concern for volatile radioactive metals such as cesium. Referring now to FIG. 1, there is shown a system according to the present invention and generally indicated by reference number 10. System 10 includes two stages of steam reforming reaction vessels 12 and 14. Waste passes through vessel 12 first and then to vessel 14 except for volatile gases from vessel 12 that are forwarded to a conventional gas handling system (not shown). Ion exchange resin 20 is slurried from a resin tank 22 to first stage reaction vessel 12 for drying and pyrolysis. Other waste forms are delivered to the reformer in other ways. For example, solid waste 40 that have been size reduced by shredding, grinding or chopping are delivered from a solid waste vessel 42 by screw auger 44 to vessel 12. Liquids and gases 30 are simply pumped or injected from their container 32 using a pneumatic pump 34 for example. In the first stage vessel 12, inert media 50 is used in the fluid bed. Media 50 is preferably silica or alumina, most preferably, amorphous alumina beads at least 200 and preferably up to 3000 microns in diameter, preferably between about 800 and 1300 microns. If acid gases are to be fed into the first stage, reactive media that will neutralize these gases is preferred, such as Na.sub.2 CO.sub.3, CaO or CaCO.sub.3 beads. These media are preferably made of a high density material to sustain a higher velocity of the fluidizing medium. Some choices of media will serve also as effective low cost catalysts for steam reforming, such as alumina beads. If the feedstock includes nitrates, then coal, charcoal and/or sugar can be added to it to facilitate oxidation heating and to create a highly reducing environment for direct reduction of nitrates to nitrogen. The use of carbon creates a highly reducing hydrogen and carbon monoxide atmosphere that strips oxygen from nitrates. The fluidizing medium can be an inert gas, but is preferably a reforming gas and an oxidizing gas in combination. Most preferably, the medium is superheated steam with oxygen. When the feedstock is aqueous, the steam content may accordingly be reduced and the oxygen content increased because of the increased heat requirements needed to evaporate the aqueous component of the waste. The fluidizing velocity can range from 1.0 feet per second or higher depending on the bed media, even as high as 400 FPS, preferably between about 1.25 and 5 FPS. The high fluidizing medium speed has several advantages. High fluidizing medium speed in a vertically oriented bed agitates the bed media to help break down the softer, friable feed. It speeds decomposition; it helps to carry fine particulate from vessel 12. The fluidizing medium can be distributed by any functionally appropriate design, however, for applications involving processing of radioactive wastes, distribution piping 56 is preferably made removable through the wall of first stage reactor vessel 12 so that it can be replaced or serviced without the need to remove the bottom of the vessel. After first stage reforming in vessel 12, the effluent is filtered in a filter separator 60 to remove carbon, metal oxides, and other inorganic compounds from the volatile organic materials and excess steam. The residue moving to the second stage reformer in reaction vessel 14 is again exposed to superheated steam to convert the fixed carbon to carbon monoxide that can then be exhausted to the offgas system. As an example of the mass and volume reduction obtained with the present system, beginning with 4910 pounds of resin, the residue from the second stage reformer is 73 pounds for a weight reduction factor of 67.3 and a volume reduction factor of 61.4. Furthermore, by keeping the temperature of the pyrolysis below 700.degree. C., the cesium carryover to the offgas system is held to less than 1%, which can be recovered using small, "polishing" ion exchangers on the scrubber water system rather than by incorporating more elaborate and expensive cesium traps. For starting the pyrolysis in both first stage reaction vessel 12 and second stage reaction vessel 14, electrical heaters 62 are needed. Heaters 62 may be internal or external to the vessel. Once at or near temperature, the addition of oxygen to the fluidizing medium permits oxidation to take place and thereby obviates the need for excess external heat and increases throughput rates. Heat exchange through the vessel walls is also preferable to reduce the heating requirements. In addition to oxygen injection and the use of electrical heaters 62 and heat exchange, co-reactants can be used to generate heat. These co-reactants can include coal, charcoal, methane, fuel oil, high-energy content wastes, etc. The operating temperature is preferably 425.degree. C. to 800.degree. C. for decomposing most organics. For radioactive feedstocks, the upper end of the temperature range is preferably 700.degree. C. to minimize corrosion, eutectic melting of salts, and the volatility of cesium, ntimony, technetium and ruthenium. The preferred pressure range is 10-45 psia, most referably 14-15 psia. In operation, the high velocity, fluidizing medium entrains fine, light waste residues including metal oxides, ash and salts and carries them out the top of reaction vessel 12 along with syngas and carbon. Heavier wastes that are not pyrolyzed, such as gravel, metals and debris are removed from the bottom 64 of vessel 12. To facilitate this separation, high fluidizing velocities are used in combination with larger, more dense bed media. The fluidizing gases are injected at speeds of at least 1.0 FPS and up to 400 FPS, preferably about 300 FPS. Bed media are preferably 200-3000 microns in diameter and made of a metal oxide such as alumina, or perhaps silica. Except for attrition losses, the bed media 50 of vessel 12 remains in vessel 12. The larger bed media also help to break up particles of softer, more friable waste. When wastes are removed from the bottom 64 of reaction vessel 12 of the first stage, the bed media 50 can frequently be separated from the waste residues and reused. Waste residues from the processing of ion exchange resins are primarily made of a magnetic form of metal oxide and therefore can generally be separated magnetically. Depending on the waste form fed to the first stage, the output can include light organic compounds, carbon dioxide, carbon monoxide, hydrogen gas, fixed carbon in the form of char, metals, oxides and other inorganics, and water (steam). After exiting the first stage reaction vessel 12, elutriated solids are removed from syngas by filter/separator 60. Filter/separator 60 is made of sintered metal or ceramic elements, and has a blowback capability to clean elements and heaters to assure that the temperature of filter/separator 60 is maintained above the dew point of the syngas stream. The solids collected by filter/separator 60 can be removed through the bottom 64 using cooled screw, lock valves or eductor and forwarded to second stage reaction vessel 14. The carbon, unpyrolyzed organics and other solids are then injected to the second stage reaction vessel 14 along with superheated steam and optional oxygen. The carbon is gasified on contact with steam and oxygen in vessel 14, unpyrolyzed organics are pyrolyzed and inert solids are carried out of vessel 14. Almost all solid residues will be separated, as with the first stage 12, by filtration in a second filter/separator 76 and added to the first filter/separator 74 in a disposable container 78. The operating conditions of temperature and pressure for second stage reaction vessel 14 may be the same as for the first. Bed media 72 and fluidizing gas are the same. However, because the bulk of the pyrolysis has already taken place in the first stage, the second stage can be used for partitioning the residues or otherwise placing them in modified chemical final form. For example, if nitrates are in the wastes received in the second stage, the presence of carbon in vessel 14 will reduce the nitrates to less harmful nitrogen gas, the nitrates dropping to less than 100 ppm at the gas outlet. Co-reactants introduced along with the fluidizing gas can be used to oxidize or reduce the wastes, changing an oxidation state to one that makes disposal more convenient, such as changing hazardous Cr+6 to non-hazardous Cr+3. This type of reaction is difficult to do in first reaction vessel 12 because the co-reactant may react with the excess steam or other pyrolysis gases. In reaction vessel 14, on the other hand, the processing can be more subtle. Some metals will volatize at lower temperatures than others and may be separated by the operating temperatures of the second stage. Zinc for example may be separated from cesium, antimony and ruthenium simply by selection of an operating temperature higher than the temperature at which cadmium volatizes and lower than that at which the others volatize. The second stage may also be operated as a calciner to convert CaCO.sub.3 to CaO, NaNO.sub.3 to Na.sub.2 O, and so on for use in the scrubbers of the offgas system. The syngas from the first and second stages is directed to the gas handling system where the gases are conditioned in one of several ways, all of which employ conventional technology: volatile organic gases are oxidized, hot gases are cooled, acidic gases are scrubbed and converted to stable salts, excess water vapor is condensed and removed, and the cooled, scrubbed gases are filtered prior to release. Gases are monitored prior to release to assure that applicable environmental release requirements are met. It will be apparent to those skilled in the art of decomposing wastes that many modifications and substitutions can be made to the preferred embodiments described above without departing from the spirit and scope of the present invention, which is defined by the appended claims.